humidity sensor based fibre -optical

7/16/2019 Humidity sensor based fibre -optical

1/23

Review

Optical fibre-based sensor technology for humidity and

moisture measurement: Review of recent progress

L. Alwis , T. Sun, K.T.V. Grattan

School of Engineering and Mathematical Sciences and City Graduate School, City University London, London EC1V 0HB, UK

a r t i c l e i n f o

Article history:

Received 31 January 2013

Received in revised form 19 July 2013

Accepted 23 July 2013

Available online 31 July 2013

Keywords:

Optical fibre sensor

Humidity

Moisture

a b s t r a c t

Humidity and moisture sensing is becoming increasingly important in industry and

through a wide spectrum of applications and a review of research activity in the field across

a range of technologies was presented previously by some of the authors. Recognizing the

major developments in the last few years, especially in the field of fibre optic humidity and

moisture sensing, this paper aims to extend that approach to review and categorize recent

progress in the optical fibre field for the measurement of humidity and moisture and exam-

ine, as a result, the breadth of applications that now are being discussed.

2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4053

2. Humidity and moisture definitions and terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4053

2.1. Humidity/moisture measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4054

2.2. Calibration of humidity/moisture for sensing applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4054

3. Applications of humidity/moisture measurement in industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4055

3.1. Structural Health Monitoring (SHM) applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4055

3.2. Food process and storage applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4056

3.3. Medical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4056

3.4. Ecological applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4056

3.5. Agricultural applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4057

3.6. Mineral processing applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4057

3.7. Fuel applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4057

3.8. Aerospace applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4057

3.9. Applications underpinning human comfort. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40584. Fibre-optic techniques for humidity detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4058

4.1. Fibre grating sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4059

4.1.1. Fibre Bragg gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4059

4.1.2. Long period gratings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4063

4.2. Evanescent wave sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4064

4.3. Interferometric sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4065

4.4. Hybrid sensors (grating + interferometric). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4067

4.5. Absorbance sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4070

0263-2241/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.measurement.2013.07.030

Corresponding author. Tel.: +44 2070403641.

E-mail address:[email protected](L. Alwis).

Measurement 46 (2013) 40524074

Contents lists available at ScienceDirect

Measurement

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e/ m e a s u r e m e n t
http://dx.doi.org/10.1016/j.measurement.2013.07.030mailto:[email protected]://dx.doi.org/10.1016/j.measurement.2013.07.030http://www.sciencedirect.com/science/journal/02632241http://www.elsevier.com/locate/measurementhttp://www.elsevier.com/locate/measurementhttp://www.sciencedirect.com/science/journal/02632241http://dx.doi.org/10.1016/j.measurement.2013.07.030mailto:[email protected]://dx.doi.org/10.1016/j.measurement.2013.07.030http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.measurement.2013.07.030&domain=pdf


2/23

5. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4070

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4071

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4071

1. Introduction

The first moisture measurement scheme can be traced

as far as 179 BC when the Chinese made a humidity mea-

surement system using a balance type approach: a hang-

ing piece of wool, tied together on one end of a large pair

of scales where the weight of the wool would increase

when air becomes more humid and decrease when the

air tends to dry[1]. Many centuries later, in 1550, the de-

vice was improved by substituting a sponge for the wool

and various versions of the hygrometer, as it was known,

were developed subsequently, with the substitution of pa-

per, hair, nylon, and acetate. During the seventeenth and

eighteenth centuries, there were several opinions about

how water dissolves in air and by 1790, an important prin-ciple was established aqueous vapours have the proper-

ties of gases. It was also established that a relationship

exists between humidity and temperature [2]. In 1803,

L.W. Gilbert [1] claimed The degree of humidity depends

on the ratio of the vapour actually present to that which is

possible. Since then, the growth of both electronic and

optical fibre fields has enabled the establishment of differ-

ent types of humidity sensors and measurement tech-

niques. Today, the measurement of moisture and

Relative Humidity (RH) is an important factor in various

industries such as food process and storage[37], agricul-

ture[811], pharmaceutical[1213], biomedical[1418],

chemical[1921], SHM[2225], ecological[26,27], atmo-spheric weather conditions monitoring[2830]and vari-

ous others[3136].

A previous paper by Yeo et al. [37], reviewed the broad

field of mechanical, electrical/electronic and optical fibre-

based RH sensors and since then the field of optical and

optical fibre-based sensor methods has seen major pro-

gress and a number of new approaches and applications

have come to light. This paper aims to build on that work,

having a focus, however, only in the optical field and to re-

fer the interested reader to that previous paper for details

of mechanical and electrical/electronic sensors, a field

which has been relatively static since that paper was pub-

lished. Several interesting examples of applications wherehumidity and moisture sensors are of significant impor-

tance are presented below, ranging from food storage

applications to seeking to find evidence of life on Mars,

as well as the new technological developments which have

permitted these.

This review is structured as follows. Following the gen-

eral Introduction and definitions, the paper reviews the

measurement of humidity/moisture and the calibration of

humidity/moisture for sensing applications and, further,

examines methods using fibre-optic techniques for humid-

ity detection. This will include fibre grating sensors (both

Fibre Bragg Gratings (FBGs) and Long Period Gratings

(LPGs)) and also look at a range of approaches: evanescent

wave sensors; interferometric sensors; Hybrid sensors

(fibre gratings + interferometric) and absorbance sensors.This follows a review where a number of key applications

of humidity and moisture measurement are highlighted,

in areas such as Structural Health Monitoring (SHM); food

processing and storage; medicine; ecology; agriculture;

mineral processing; fuel quality control; aerospace and

other applications supporting human comfort. The paper

concludes with a tabular summary and overview of the

field and feature a list of topical and accessible references

to the key papers.

2. Humidity and moisture definitions and terminology

The term moisture refers to the content of water in aliquid or solid due to absorption or adsorption, while the

term humidityis reserved for the content of water vapour

in gases. Absolute humidity refers to the density of water

vapour, i.e. the mass of water vapour per unit volume of

gas. Since this is the same measurement for atmospheric

pressure, the term absolute humidityis generally not used.

The most commonly used terminology for humidity mea-

surement are expressed in terms of Relative Humidity

(RH), Dew/Frost point (D/F PT) and parts per million

(PPM) of moisture[38].

RH is the ratio of the actual vapour pressure of air at a

particular temperature, to the saturation vapour pressure

at the same temperature and is given by,

RH Pw

Pws 100%

wherePwis the partial pressure of water vapour, and Pwsis

saturated water vapour pressure at a given temperature.

The value of RH expresses the vapour content as a percent-

age of the concentration required to cause the vapour sat-

uration, that is, the formation of water droplets (dew) at

that temperature. Since RH is a function of temperature,

it is a relative measurement.

The Dew Point is the temperature (above 0 C) at which

water vapour in a gas condenses to liquid water. The Frost

Fig. 1. Correlation across the range of humidity units: Relative Humidity

(RH), Dew/Frost point (D/F PT), and parts per million by volume fraction

(PPMv)[38].

L. Alwis et al. / Measurement 46 (2013) 40524074 4053
http://-/?-http://-/?-http://-/?-


3/23

Point is the temperature (below 0 C) at which the vapour

condenses to ice. Dew/Frost Point is a function of atmo-

spheric pressure but is independent of temperature and

is therefore defined as absolute humidity measurement.

The use of the unit parts per million (PPM) represents

the water vapour content by volume fraction (PPMv) or,

if multiplied by the ratio of the molecular weight of water

to that of air, it is given as PPMw[38].Fig. 1shows the cor-

relation between the aforementioned units for humidity/

moisture measurement.

2.1. Humidity/moisture measurement

Humidity and moisture measurements can be made

by employing a range of methods that either probe the

fundamental properties of water vapour or use various

transduction methods that provide humidity-related

measurements. The long history of humidity sensing has

been highlighted in the introduction and over the years a

variety of schemes has been explored to obtain meaningful

and industrially-relevant humidity measurements. These

range from simple schemes involving the expansion and

contraction of materials such as human hair to much more

advanced techniques, such as using a miniaturised elec-

tronic chip or recently, the utilization of optical fibre tech-

nology. Some hygrometers that are in use widely are

illustrated inFig. 2, (these techniques have been discussedin some detail in the previous review by some of the

authors [37]). As discussed in this paper, the focus is

mainly on the methods and performance of RH and mois-

ture sensor utilizing the burgeoning technology that is

based on optics and especially optical fibres.

2.2. Calibration of humidity/moisture for sensing applications

The calibration of humidity sensors can be performed

by generating references to known and specific levels of

humidity in a controlled environment. To achieve this,

one of the techniques that is widely practised is the mixing

of dry (0% humidity) and steamed moist air (100% humid-

ity) in varying known proportions. Another widely used

method is the utilization of different salt solutions in a

closed chamber where salt with known saturating humid-

ity levels are mixed with water, placed in the enclosed

space (such as an air-tight box) and given time to saturate

to a particular (and known) humidity level. One such test

chamber made for the purpose of RH calibration in-situ is

shown in Fig. 3. The value of the RH generated depends

on the type of salt used and detailed investigation carried

out to work out the saturation RH of different salt solu-

tions, by Greenspan [39], is presented in Table 1. Recent

development in the field of RH calibration involves the

use of commercially-available environmental chambers

Fig. 2. Some conventional hygrometers that are currently in use in industry[19].

Fig. 3. Illustration of a small air-tight chamber containing Petri dishes

filled with salt solutions to achieve known levels of RH for sensorcalibration[106].

4054 L. Alwis et al. / Measurement 46 (2013) 40524074
http://-/?-


4/23

that can be configured to control both the RH and the tem-

perature and provide maximum flexibility as a result.

3. Applications of humidity/moisture measurement in

industry

Before considering the range of technologies by which

Relative Humidity (RH) and moisture levels can be made,

using advanced fibre optic techniques, it is important to

consider the wide range of applications where such mea-

surement is important. Below, several applications are

considered showing the breadth of industries where such

measurements are important, the need for clarity on issues

such as compatibility with use in extreme environments or

on human subjects and indeed the importance on making

such measurements in aerospace applications such as in

measurements on the Martian atmosphere. All this shows

the tremendous breadth that must be reflected effectively

in the system design, to achieve the required degree of rug-

gedness or biocompatibility, for example, and thus which

underpins the most effective selection of techniques for a

particular measurement in an individual circumstance.

3.1. Structural Health Monitoring (SHM) applications

One of the most widely used areas where RH and mois-

ture sensors find application is for SHM purposes. Over the

past few decades, the deterioration of civil infrastructure,

such as buildings, bridges and roadways have demon-

strated the need for high-performance sensing systems

that can be used effectively to monitor changes in struc-

tures, often occurring over many years. This has led to a ra-

pid growth in interest in SHM systems, which has the

potential to allow for real time monitoring and preventa-

tive maintenance within civil infrastructure.

Steel reinforcement bars (rebars) embedded in con-

crete are normally inherently protected against corrosion

by passivation of the steel surface due to the high alkalinity

of the concrete. With the rebars embedded into the con-

crete, this highly alkaline environment creates a very thin

but dense passivating oxide layer on the surface of the

rebar thus forming a protection barrier which reduces its

rate of corrosion to some insignificant extent [40]. Onemechanism that can trigger the corrosion process is the in-

gress of chloride ions into a reinforced concrete structure.

The presence of chlorides in the concrete most commonly

arises from the use of salt to melt ice and snow on roads

and bridges during the winter seasons, particularly in areas

that go through freezing temperature conditions [41]. As

illustrated inFig. 4, salt reacts with water from ice to liber-

ate chloride ions which will eventually end up in cracks

and other traps formed in civil structures, due to various

environmental events such as earthquakes and winter

freeze-thaw cycles. After this process occurs and the

effects accumulate over some time, the passivating oxide

layer can break down due to the drop of pH surroundingthe protective layer as a result of the carbonation process,

the rebar starts to corrode causing a volumetric expansion

on the rebar. Consequently this expansion on the rebar in-

duces pressure on the concrete which results in internal

damage to the structure. If this is left to build up over time,

it would lead to crack formation, spalling and delamination

in the concrete that may eventually lead to structural fail-

ure. The ingress of chloride ions hugely depends on the

presence of moisture to dissolve and carry chemical spe-

cies into the porous concrete. The moisture level within a

Table 1

Humidity fixed points for a series of saturated salt solutions from [39].

Temperature

(C)

Lithium

chloride

Potassium

acetate

Magnesium

chloride

Potassium

carbonate

Magnesium

nitrate

Sodium

bromide

Sodium

chloride

Strontium

chloride

Potassium

chloride

10 11.3 23.7 33.5 43.1 57.4 62.2 75.66 75.7 86.8

15 11.3 23.4 33.3 43.2 55.9 60.7 74.13 75.6 85.9

20 11.3 23.1 33.1 43.2 54.4 59.1 72.52 75.5 85.1

25 11.3 22.5 32.8 43.2 52.9 57.6 70.85 75.3 84.3

30 11.3 21.6 32.4 43.2 51.4 56.0 69.12 75.1 83.635 11.3 32.1 49.9 54.6 74.9 83.0

40 11.2 31.6 48.4 53.2 74.7 82.3

45 11.2 31.1 46.9 52.0 74.5 81.7

50 11.1 30.5 45.4 50.9 74.4 81.2

55 11.0 29.9 50.2 74.4 80.7

60 11.0 29.3 49.7 74.5 80.3

65 10.9 28.5 49.5 74.7 79.9

70 10.8 27.8 49.7 75.1 79.5

75 10.6 26.9 50.3 75.6 79.2

80 10.5 26.1 51.4 76.3 78.9

Fig. 4. Chemical process of the chloride induced steel corrosion [43].

http://-/?-http://-/?-http://-/?-


5/23

structure has a significant influence on the rate of carbon-

ation and corrosion, for example, the rate of carbonation in

concrete is observed to be the fastest between 60% and

80%RH and the rate of corrosion in reinforced concrete

with different concentration of chloride ions is influenced

by the internal RH level [42]. The corrosion in the rein-

forced steel bars affects the strength of the concrete struc-

ture in the long term. Thus, early detection of moisture (as

an important means by which chloride ions are delivered

into the structure) can save the reinforced concrete struc-

ture from severe damage resulting from a loss of structural

integrity and would allow appropriate action to be taken in

advance of major damage being caused and thus costs

saved.

3.2. Food process and storage applications

The loss of moisture due to transportation and storage

often limits the shelf life of fruit and vegetables. Fruits such

as bell peppers (capsicums), for example, are mostly placed

in cardboard boxes that are stored at a RH below 90%[3].

This results mainly in precocious desiccation of the peel,

shown as superficial shrivelling. Other products that dry

out too much during storage include pears, currants, and

avocado. Depending on the species and the storage condi-

tions, the environment can also be too humid. Incidence of

fungal diseases, such asBotrytisrot is common at such high

RH[44].Therefore it is of interest to measure the moisture

content or the RH of these products during transportation

and storage. In considering food storage potential, the

measurement of RH is more important than the moisture

content, as it measures the availability of water to micro-

organisms and hence gives an indication of the biological

activity, or potential activity, of the product as moulds will

develop rapidly during storage above 75%RH[4]. The den-

sity, porosity and expansion of extruded food products are

found to be dependent on feed moisture content, residence

time and temperature, and water is an essential reaction

partner in gelatinization and plays one of the major roles

in controlling extrudate expansion ratio[5]. For example,

the degree of expansion of high moisture imitation cheese

during microwaving was shown to increase with increas-

ing pre-expansion storage time and this phenomenon

was related to an increase in water mobility in the un-

heated cheese during storage prior to microwaving [6]. It

is therefore necessary to have a measurement of the mois-

ture/RH content of the food processing and storing

environment.

3.3. Medical applications

The monitoring of breathing is important during certain

imaging and surgical procedures where the patient needs

to be sedated or anesthetized[45]. Breathing airflow mon-

itoring has been widely applied to predict and detect respi-

ratory disorders and failures, such as hypopnoea and

apnoea, which may eventually develop into a life-threaten-

ing condition[14]. Also, some serious illnesses can be diag-

nosed by detecting alterations in breathing rates or

abnormal respiratory rate[45]. Monitoring of breathing is

also important to study the progression of a diagnosed

illness or to evaluate the health of a person. Electronic

breathing sensors are not recommended when patients

are, for example, in a magnetic resonance imaging (MRI)

system, or during any oncological treatment that requires

the administration of radiation or high electric/magnetic

fields since they can fail and also represent a burning haz-

ard to the patient[15]. In such cases, the utilization of opti-

cal fibre-based breathing sensors represents an important

alternative approach as breathing can be monitored by

placing the sensors close to the nose or mouth of the

patient.

Voice communication is the most familiar and com-

mon form of communication. Unfortunately, as a result

of hereditary or acquired impediment or due to other

reasons such as an accident, there are people with

speech/hearing impairment who find it difficult to con-

verse. Morisawa et al. [16]developed a language recogni-

tion system that focuses on the moisture included in

devoiced breaths as a method for communication support

in persons with speaking difficulties. Through the mois-

ture pattern formed in the pronunciation, the system

had a recognition rate of 93%. It was shown that by using

the optical fibre moisture sensor, a response representing

the moisture distribution pattern characteristic of a

breath corresponding to each devoiced vowel could be

obtained.

3.4. Ecological applications

In mountainous regions, stream water flow is often reg-

ulated by check dams, often made of concrete or wood-

logs, which decrease the water speed during storm events,

allow sediment to settle, and reduce erosion[26]. Timber

check dams have been successfully used for this purpose.

Biotic agents, especially wood-decaying fungi, grow and

spread when wood moisture is between 20% and 40% by

weight[46]. This wood decay causes an increase of poros-

ity that contributes to decrease material strength and in-

crease wood water storing capacity [47] and therefore,

measurements of the wood water content can provide

information about the degree of wood degradation. This

degradation will gradually alter the wood cells and struc-

tures generating micro/macro-pores where water can

move freely and currently electrical hygrometers are used

to measure the moisture content which measure either the

electrical conductivity or the electrical capacity at frequen-

cies under 10 MHz and these instruments typically have

two stainless steel electrodes that can either be inserted

into the wood samples at different depths according to

the measuring needs or kept in close contact with each

other[26]. Despite different conventional approaches used,

these methods cannot detect water flows out of the cellu-

lar walls, filling up the cell cavities, and leaking into the

vessels (raising the water content to over 30% when refer-

ring to the anhydrous wood)[26]. These measurements are

therefore only reliable in healthy timber. According to

Gambetta et al. [48], the development of more accurate

measurement techniques than traditional hygrometers

would be useful for watershed surveys since the water

content of wood is highly correlated with the level of

degradation.

http://-/?-http://-/?-


6/23

Another similar area of interest for humidity monitor-

ing is obtaining information relating to the fracture tough-

ness of geo-materials which is critical to the understanding

of tensile fracturing, and in particular in geological and

rock engineering projects that are subjected to elevated

moisture levels and recently Nara et al. [27]conducted a

comprehensive set of fracture toughness tests on a suite

of key rock types in air under different RH at constant tem-

perature in order to investigate the influence of RH on frac-

ture toughness. They found that the value of fracture

toughness decreases with increasing RH. In addition, it

was discovered that the decrease in fracture toughness

was more significant when a particular type of clay was in-

cluded in rock which expands in the presence of water and

therefore crack-growth resistance decreases at high RH

levels. It was concluded that crack growth in rock is af-

fected by humidity, and that clay content is an important

contributing factor to changes in fracture toughness and

subcritical stress intensity factor. Therefore sensing

schemes that could provide information on the RH varia-

tions within such environments would convey the nature

and fracture toughness of the geo-material.

3.5. Agricultural applications

The study of root distribution and its ability in recover-

ing water has also been a subject of considerable interest in

agriculture and ecology as it allows a better interpretation

of the behaviour of different crops under sub-optimal envi-

ronments which would help improve the quality of model-

ling root water uptake in hydrological and land use change

models [8]. One interesting investigation was conducted

by Mackay et al.[49]into the effect of soil moisture on corn

root growth and it was found that, as soil moisture was in-

creased, the total plant weight increased by 1343% and

the corn root length increased from 41% to 52% in 28 days.

As a consequence raising soil moisture content further, in

contrast, decreased the total plant weight by an average

of 13% and root length by an average of 16% in 21 days.

Therefore soil moisture content sensors are useful tools

for farmers and agricultural studies to improve the quality

of products, while saving farmers time to achieve opti-

mum conditions for satisfactory corn growth.

3.6. Mineral processing applications

Another area that can derive enormous benefit from soil

moisture content measurement is mineral processing

plants. The manual gravimetric drying moisture determi-

nation methods currently employed by most mineral pro-

cessing plants fail to provide timely and accurate

information required for automatic control[50]. The task

of moisture determination is still done by the classical

technique of loss in weight utilizing uncontrolled proce-

dures. Generally, it is acceptable to have ore concentrate

moisture content vary within a range of 79%, but control-

ling the moisture content below 8% is a difficult task with a

manually controlled system. On many occasions, delays in

achieving reliable feedback of the moisture content using

manual techniques, i.e. a delay between the humidity

variation and its measurement, results in a delay in the

corrective actions that would be necessary and therefore

by the time the corrective action is applied, the actual

moisture content has changed. Therefore, when consider-

ing the cash flow of a mining operation that is governed

by both the smelter contract, with moisture penalties

and the quantity and quality of the concentrates shipped,

an efficient method of on-line moisture content monitor-

ing such as a fibre-optic moisture sensor would be an ideal

tool for this application.

3.7. Fuel applications

Combustion of biomass for heat and power production

is expanding due to the search for renewable alternatives

to fossil fuels and an important parameter when using bio-

mass is the moisture content of the fuel, which often fluc-

tuates for biomass fuels[51]. The variation in its moisture

content results in an uncertainty in the energy content of

the fuel delivered to a plant. The fuel moisture-content in

a furnace may be determined either by direct measure-

ment on the entering fuel or by measuring the moisture

and oxygen contents of the flue gases deriving the mois-

ture content of the fuel. However, reliable methods of a

moisture sensor for small to medium-scale furnaces are

not readily available at present. An exception is if the fur-

nace is equipped with flue-gas condenser, which can be

used to estimate the moisture content of the flue gases. A

limitation of this method is, though, that not all furnaces

have flue-gas condensers and that the measured signal

has an inherent time delay. Therefore effective moisture

content sensors are needed to determine the moisture con-

tent of the fuel.

3.8. Aerospace applications

Recently there has been a growing interest in humidity

sensors to performin-situmeasurements in space and the

most spectacular of these has been the near-surface atmo-

spheric water content on Mars [5254]. Detailed atmo-

spheric temperature and RH data will result in an

improved understanding of the daily water vapour dynam-

ics and the stability of water near the surface [55]. The

water content of soils significantly influences their chemi-

cal and physical properties and is also needed for biological

processes to proceed. The amount of adsorbed water in the

soil is a function of water vapour density in the near-sur-

face atmosphere, the temperature, specific area per mass,

and the mineralogy [52]. An exciting example of this is

seen in space exploration using unmanned vehicles. The

thin layer of the upper millimetres of the Martian surface

is of particular interest since the soil interacts directly with

the varying atmospheric humidity, which can reach satura-

tion during night and early morning[56]. Therefore, near-

surface measurements of the atmospheric water vapour

content can allow the investigation into the interaction be-

tween the atmosphere and the adsorbed water, which is

deposited in the upper soil-layer. These studies have been

further enhanced by the discovery of methane on Mars

[57] which has increased interest concerning its origin

and destruction. The chemistry of methane production is

closely linked to the presence of water and detailed studies

http://-/?-http://-/?-http://-/?-


7/23

has been performed by Novak et al.[58]to measure water

vapour in Mars atmosphere and comparing their ratio to

that of the Earth (Fig. 5).

Liquid brines are of special interest to NASAs Mars

Exploration Program because they are essential to under-

stand the potential habitability, both past and present, of

the planet and a miniature microwave soil moisture sensor

capable of probing the shallow subsurface of Mars to mea-

sure the distribution of brines, without the need for a drill

was proposed by Renno et al. [59]. The Phoenix Mars Land-

er discovered salts which can form liquid solutions at Mars

current environmental conditions and found physical and

thermodynamical evidence for liquid brines at its landingsite[60]. A hypothesis has been made by Kok and Renno

[61] that water molecules from the precipitated ice be-

come available and diffuse into deliquescent salts in the

soil which would drive the salt concentration in the solu-

tion towards the eutectic value. If this is correct, liquid

brines could form almost anywhere where ground ice is

present near the surface of Mars [61] and if confirmed,

would have major implications for life on Mars. As can

be seen from these examples and other information from

the literature[62], it is evident that soil and atmospheric

moisture/humidity measurement in Mars is an interesting

new development in RH/moisture sensing technology that

would aid Martian explorations for habitability and otherrelated research.

3.9. Applications underpinning human comfort

Apart from the aforementioned applications, the mea-

surement of RH is important for human comfort such as

in air-condition monitoring and for achieving controlled

hygienic conditions. Hygrothermal analysis have become

more important in building design as moisture damages

has become one of the main causes of building envelope

deterioration [63]. Water and moisture can cause struc-

tural damage, reduce the thermal resistance, change the

physical properties and deform the building materials.

Hygrothermal analysis is needed to demonstrate the

acceptable performance of structures and to construct

healthy buildings with good indoor air quality and a fur-

ther example of RH sensing for health and hygienic condi-

tions is the monitoring of RH in hospitals. RH in operating

theatres are usually maintained between 40% and 60% as

humidity levels below 35% cause dry eyes, throat and skin,

and excessive thirst and evaporation is more rapid at low

humidity, increasing heat loss via sweat and body fluids

[19]. Above 50%RH, static build-up is minimized and high

humidity levels are uncomfortable. Pipeline and cylinder

gases must be dry, as moisture can act as a focus for bacte-

rial growth. Therefore, precise humidity and moisture

measurement and control is required in order to ensure

the health and comfort of the patients.

4. Fibre-optic techniques for humidity detection

There has been enormous growth in-fibre optic sensor

technology in the last few decades as new applications

open up and the technology matures. Previous reviews of

the underpinning technology of optical fibre sensors have

been published by some of the authors and are not further

reproduced here, but available to the interested reader

[64]. However to deal with the breadth of applications dis-

cussed in the previous sections and allow for the different

requirements of the sensors that have required develop-

ment to respond to those situations, a wide range of differ-

ent technological approaches to fibre optic humidity

sensors have been proposed in the literature. There is of

course no right answer when it comes to designing an

optical fibre sensor for humidity or moisture measurement

and no one technology offers superiority over anotherper

se. What is critically important is that there is a range of

effective sensors from which to choose, to tailor the sensor

and its response to a specific application and thus to allow

the engineer the maximum flexibility to make the mea-

surement needed. Fibre optics, as is shown below, play a

full role in providing that choice.

There are several key requirements that need to be ad-

dressed when designing a humidity sensor, whether it be

for general or specific applications. These include the opti-

mization of some or all of the following: the sensitivity,

precision or accuracy (depending on the circumstances in

which the sensor is used), response time, target humidity

range, reproducibility, hysteresis, durability, minimal tem-

perature or other chemical cross-sensitivity, structural

integrity, ease of operation and, of course, cost. The disad-

vantage of utilizing electrical/electronic sensors is often

their susceptibility to electromagnetic interference, cross-

sensitivity and their inability to be multiplexed or to be

employed in hazardous environments as well as a suscep-

tibility to becoming and remaining wet in use and thus

causing errors in the readings. On the contrary, optical fi-

bres possess a number of advantages over conventional

electrical/electronic sensors in general, such as immunity

to electromagnetic interference, chemical inertness, light

weight and low mass (which facilitates drying after use),

multiplexing capability, high thermal stability and remote

sensing ability, all of which make them well suited to both

Fig. 5. Modelled spectral transmittance of the atmosphere of the earth:

the black trace is composite synthetic transmittance resulting from a 50-

layer atmospheric model with parameters of a standard tropical

pressuretemperature profile and five infrared active atmospheric gases

(CO2, H2O, Ozone, Ethane and Methane). The red trace is the model for

water vapour alone[58].(For interpretation of the references to colour in

this figure legend, the reader is referred to the web version of this article.)

http://-/?-http://-/?-


8/23

general and remote sensing, making them ideal candidates

for measurement applications where conventional electri-

cal/electronic sensors are found to be inappropriate or sim-

ply would not function. With the development of optical

fibre sensors and the demand for humidity/moisture mea-

surement for a wide range of applications in industry,

there has been a major development in research in the field

(as is evidenced by the increasing number of papers pub-

lished) and a special focus on optical fibre-based tech-

niques for humidity/moisture sensing.

Due to the wide range of often competing techniques

available on the basis of which to realize a number of dif-

ferent optical fibre-based RH/moisture sensing schemes, in

this paper the different types of sensors reported have

been categorized under several general schemes that high-

light first of all the operating principles being used. Such

optical fibre-based sensing techniques include the use of

in-fibre gratings, evanescent wave techniques, interfero-

metric methods, hybrid approaches and absorption meth-

ods, as discussed in detail below. Table 2 has been

created as a reference to draw these together and provide

an overview of the results reported by a number of the

authors whose work is cited in this review, with results

published by various groups, in order to facilitate a simple

cross comparison on a quantitative basis using their pub-

lished data.

4.1. Fibre grating sensors

Ever since their development from the late 1970s, in-fi-

bre gratings have been extensively used for various optical

fibre sensing schemes. Fibre gratings are created by modu-

lating the RI of the fibre core either by physical deforma-

tion [6567] or by subjecting the photosensitive core to

intense radiation, usually in the UV part of the spectrum

[68,69]. Depending on the grating period achieved through

the modulation of the refractive index (RI) of the core, they

fall under two major categories: short-period or Fibre

Bragg Gratings (FBGs) or Long Period Gratings (LPGs). FBGs

have a very narrow reflection loss band resulting from the

typical grating periods used, usually being within 12 lm,

while the transmission of LPGs comprises a series of loss

bands resulting from relatively longer grating periods that

typically are in the region of several hundreds of microme-

tres. Both these grating types are sensitive to environmen-

tal parameters such as temperature and strain and, in

addition, LPGs also possess a sensitivity to external refrac-

tive index change which also enables them to be config-

ured as RI and species-specific sensors [7074]. A

detailed description of the key operating principles of the

grating-based sensors mentioned can be found in the liter-

ature[69,75,76].

4.1.1. Fibre Bragg gratings

Most of the FBG-based sensors that are discussed and

are available on the market at present are configured for

strain and temperature monitoring, as a group of FBGs

can conveniently be multiplexed with several FBGs (usu-

ally with different wavelength characteristic) placed in

each channel to provide a convenient configuration to

use [7779]. For humidity sensing purposes, the strain

sensitivity of the FBG is employed as the underlying sens-

ing mechanism where a polymer which expands in volume

due to a humidity change will apply a strain on the grating,

thus changing the resonance wavelength in a known and

reproducible way that can be calibrated against the humid-

ity change causing it. As the sensor relies on the secondary

strain effect induced on the fibre through the swelling of

the polymer coating, the following adapted expressions

are used to relate the shift in the Bragg wavelength to

the results of RH and temperature-related strains which

are induced on the fibre, as well as the influence of the

thermo-optic effect[80].

DkB

Dk 1 PeeRH 1 PeeT n DT

where eRH and eTrepresent strain induced on the fibre as a

result of polymer swelling due to moisture expansion and

thermal expansion of the materials (where xdenotes RH or

T) which is given below.

ex ApEp

ApEp AfEf apx afxDx

and, in addition, A is the cross-sectional area of the mate-

rial,E is Youngs modulus of the material, a the coefficient

of moisture expansion (CME) or the coefficient of thermal

expansion (CTE) and subscripts p and f represent the effect

on both polymer and fibre, respectively.

One example of the utilization of the strain effect to

realize an effective RH sensor is the work by Berruti et al.

[81]who conducted a feasibility analysis on the develop-

ment of a FBG-based humidity sensor that would with-

stand high energy ionizing radiation, in a series of

experiments conducted at the European Organization for

Nuclear Research (CERN). Polyimide (PI)-coated FBGs were

selected as a possible candidate for the primary sensor due

to the stringent requirements of the environment in light

of radiation hardness compliance and low temperature

operation. In this approach, two FBGs were coated with

layers of 22.5 lm and 9 lm of PI (specifically Pyralin

PI 2525) designated as sensors 1 (S1) and 2 (S2) respec-

tively. The sensors were analyzed in terms of their opera-

tion over the RH range 075% for three different

temperatures relevant to the operation, these being

15 C, 0 C and 20 C, both pre and post the ionization

radiation exposure. The pre- and post-test results on the

RH measurements carried out are shown in Fig. 6. It was

concluded that the PI coated FBG-based sensors were able

to perform RH measurements with high resolution in the

temperature range 15 to 20 C as well as in the presence

of ionizing radiation, at levels of up to 10 kGy and therefore

this work has demonstrated their potential as a robust and

valid alternative to currently used polymer-based elec-

tronic hygrometers in high energy applications the latter

suffering from the disadvantage of demonstrating no radi-

ation hardness capability toward the ionizing irradiation

used. It is important to note the different sensitivities of

the two sensors evaluated and as can be seen from Fig. 6,

the sensitivity of the FBG with the thicker coating (S1)

has demonstrated a higher sensitivity than that with the

thinner coating. This is due to a higher level of strain being

experienced by the underlying FBG from the effect on the

http://-/?-http://-/?-http://-/?-


9/23

Table 2

Humidity/moisture optical fibre-based sensor schemes proposed and discussed in the literature over the period 20082013.

Ref. Year Authors Sensing method Sensing material Range

(%RH)

Sensitivity and

response time

In-fibre grating (FBG) techniques

[111] 2012 Correia et al. Strain induced Bragg wavelength measurement Silica/di-ureasil 595 22.2 pm/%RH

[112] 2012 Zhang et al. Strain induced Bragg wavelength measurement of etched POF No coating (PMMA

polymer cladding)

3090 33.6 pm/%RH

7 min

[113] 2011 Berruti et al. Strain induced Bragg wavelength measurement PI 075 2.1 pm/%RH

[114] 2010 Ding et al. Strain induced Bragg wavelength measurement PI 3080 2 pm/%RH

[115]2009 Miao et al. Intensity variation measurement following external RI change of a

tilted FBG

PVA 2074 2.52 dB m/%RH

7498 14.9 dB m/%RH


10/23

thicker layer and the larger material volume and this result

is confirmed by research from other groups who also uti-

lized PI coated FBGs for RH measurement [82] (although

usually not under nuclear irradiation). There is, however,

usually a penalty with the response time of the sensor,

although this may not be a consideration in some applica-

tions[82].

A further example of utilizing a PI-coated FBG is in the

work by Sun et al. [23] who conducted an investigation

into the decay mechanisms and associated processes

occurring in masonry structures, with a view to achieve a

clearer understanding of the changing moisture and tem-

perature conditions that underpin decay and degradation.

In order to do so, several PI-coated FBG sensors were

Interferometric approach

[134] 2012 Liang et al. Etched PMF loop mirror PVA 2080 0.98 nm/%RH


11/23

developed and, prior to use in-the-field, their performance

was first assessed in the laboratory where they were char-

acterized under experimental conditions of controlled wet-

ting and drying cycles of limestone blocks. Sensors were

then employed to monitor an actual building stone in a

specially built limestone wall (using techniques similar to

those employed by the original constructors in past centu-

ries). The sensor design developed specifically for this work

can be seen inFig. 7(a). One of the advantages of this ap-

proach is the compact and minimally invasive nature of

the sensor, thus requiring much less damage to the wall

for its insertion, with only the drilling of one small pilot

hole required for the mounting of the sensor a key con-

sideration with historic structures. Another similar, but

uncoated FBG was also included in the sensor-head (serv-

ing as a temperature-only sensor) as can be seen from

Fig. 7(b), in order to eliminate the temperature induced

wavelength shift from the RH sensor (which also showed

a temperature sensitivity which needed to be eliminated).

One noticeable advantage is the faster response of the FBG

sensor compared to the commercial capacitance sensor.

The indication from the commercial (a conventional and

non-fibre optic design) RH probe was saturated at

100%RH, whereas the measurement by the FBG sensor var-

ied for RH between 90% and 93%, showing that the com-

mercial RH sensor element had saturated and hence the

RH measurements were unreliable during the drying of

the limestone structure reflecting a key drawback with

the conventional electrical sensors used which failed to

dry out properly, due to high mass when wet initially. By

Fig. 6. Bragg wavelength shift vs. relative humidity before and after the irradiation process for sensor (a) S1 and (b) S2 at the three considered temperature

[81].



12/23

contrast, the optical fibre RH sensor, due to its small sizeand low mass, has been able to follow the actual change

of the RH characteristics of the wall.

4.1.2. Long period gratings

The sensitivities of LPGs to environmental parameters

such as temperature and strain are much higher than those

of FBGs[75] although FBGs are popularly used owing to

their ease of production, handling and easier multiplexing

capability due to the simpler structure of their optical fea-

tures. In addition, the external RI sensitivity of LPGs has

been utilized to create species-specific sensors by coating

the sensor with a material that will interact well with

the target analyte. The RI and the coating thickness of thesensing material needs to be given careful consideration

in the sensor design using LPGs as the sensor response,

i.e. wavelength or intensity variation, will depend strongly

on these parameters [8385]. The RH sensitivity of LPGs

can be described by the equation below, where kres,0i is

the resonance wavelength of the ith mode, thoverlay is the

thickness of the overlay, RIoverlay and RIsur are the overlay

and surrounding refractive indices.

Dkres;0i @kres;0i

@RIoverlay DRIoverlay

@kres;0i

@thoverlayDthoverlay

@kres;0i

@RIsur DRIsur

Recently there has been a boom in the use of LPG-based

sensors in the fields of biomedical, SHM and chemical

sensing [86]. The most common method for LPG-based

sensing of a parameter such as RH is via the coating of

the sensor with a hydrophilic material, such as a polymer,

that will alter its physical or optical parameters in re-

sponse to the external stimulus, i.e. the variation of the

RI or causing an applied strain on the LPG as a result of

the coating-layer expansion, leading to a variation in the

target resonance band of the LPG. One such example is

the work by Bock et al. [87], who have demonstrated the

possibility of efficient distributed water ingress sensing

by the deposition of diamond-like carbon (DLC) on LPGs

and subjecting them to water ingress. This was achieved

by covering the sensor with a piece of Kimwipe paper

which was soaked with water before sliding the moist

Kimwipe piece along the grating and the spectral re-

sponse was measured at each length. Two sensors coated

with DLC, that have coating thicknesses and RIs of

181.6 nm, 2.02 (S1) and 285 nm and 2.07 (S2) respectively,

were tested. The test set-up and the results obtained are

shown inFigs. 8 and 9, where it can be seen that the two

coatings performed in a completely different manner, i.e.

S1 and S2 produced wavelength and intensity variations

in the resonance bands respectively. This effect is due to

the aforementioned coating thickness and RI differences

between the two coatings used. This phenomenon is a re-

sult of mode guiding in the overlay which has been stud-

ied by some groups recently[8890].

The results of the coating thickness investigations men-

tioned above have paved way to a mechanism that will in-

crease the sensitivity of the RH sensor. This is achieved by a

double-coating the LPG is initially coated with a thin

overlay of a material with a particular RI leading to an in-

crease in the sensitivity of the LPG to external RI variations.

Then a second overlay is deposited with the species-spe-

cific material targeting the particular analyte. Many theo-

retical and experimental investigations have been

undertaken to analyze this mechanism [91]. One such

example is in the work by Viegas et al. [92]where an initial

coating of PDDA/PolyR-478 was deposited on the LPG for

the sole purpose of increasing the total effective RI of the

coating, followed by the deposition of a humidity sensitive

coating of lower RI (PAH/SM30). The thickness and the RI

of the two overlays have been carefully designed for

Fig. 7. (a) Schematic diagram of the sensor design, (b) picture of the packaged sensor probe showing a coated grating as a relative humidity sensor and a

bare grating as a temperature sensor and (c) changes in RH at 30 mm depth of stone with drying of the stone block [23].

Fig. 8. Experimental set-up for measuring the response to water ingress

[87].

http://-/?-


13/23

maximum sensitivity. The results for varying the RH over

the range of 2080% at room temperature are presented

inFig. 10and it can be clearly seen that the initial coating

of the higher RI polymer layer has greatly increased the

overall sensitivity of the sensor to varying RH.

4.2. Evanescent wave sensors

Exponentially decaying evanescent fields surrounding

the cladding region of optical fibre may be utilized for

developing different types of intensity modulated Fibre

Optic Sensors (FOS). Through this method, evanescent

wave absorption in an external medium is obtained by

physically altering the fibre, such as the removal or etching

of the cladding, creating a taper or bending the fibre to al-

low interaction of the evanescent field with the target ana-

lyte. The main mechanism for evanescent wave sensing

has been detailed in the previous review by Yeo et al.

[37]. For the case of RH sensing, the physically deformed fi-

bre structures are then coated with a species-specific over-

lay that would react to an external measurand, in this case

to variations in humidity. A recent example of this type of

sensor for RH measurement has been proposed by Corres

et al. [93] who worked on a single-mode tapered fibre

coated with a [PDDA/Poly R-478] nanostructured overlay,

in such a way that the thickness of the overlay was

controlled in order to optimize the sensitivity of the sensor,

by stopping the deposition process at the maximum slope

of the transmitted optical power. The RH sensor structure

is presented inFig. 11. A variation of 16 dB in optical power

was achieved with a response time of 300 ms for changes

in RH from 75% to 100%. Due to the fast response (by com-

parison to many other RH sensors), high dynamic perfor-

mance and the low temperature cross-sensitivity, the

sensor was tailored to applications such as human breath-ing monitoring, the control of highly humidity dependent

chemical processes or weather prediction. The characteris-

tics of the sensor scheme for varying RH, together with the

results of a commercially available capacitive RH sensor,

are presented inFig. 12.

Recently, another evanescent wave-based sensor for

human breathing monitoring was proposed by Mathew

et al.[94]who utilized a buffer-stripped bent SMF where,

due to the coupling of the fundamental mode to cladding

modes, resonant peaks will occur in the transmission re-

sponse. This response is oscillatory with respect to the

bend radius and wavelength and also was seen to vary

Fig. 9. Spectral responses to water ingress for DLC-nanocoated LPG (a) S1 and (b) S2 [87].

Fig. 10. Resonant wavelength shift dependence with relative humidityfor both coatings[92].

Fig. 11. Taper humidity sensor structure with the ESA overlay[93].

Fig. 12. Relative humidity step response of tapered fiber sensor vs.commercial capacitive RH sensor[93].

http://-/?-http://-/?-http://-/?-


14/23

with ambient RI. If the surrounding RI of the bend fibre is

changed, it will lead to a change in the coupling conditions

and results in a shift in the wavelength of the resonant

peaks. The sensitivity to RH was achieved by coating the

bend with Polyethylene Oxide (PEO) which is a hydroscop-

ic polymer. In order to achieve improved sensitivity for the

humidity sensor, a high bend loss fibre (1060XP) with abend radius of 15 mm has been used. The experimental

setup and the coated fibre bed are shown in Fig. 13.

Although the sensor was tested against RH over the range

from 30% to 90%, it was observed that there was no mea-

surable wavelength attenuation band below 85%RH. This

was due to the RI of the coated PEO film being above the

RI of the cladding and therefore the PEO coating is acting

as an absorption coating. However, as the surrounding

RH increases above 85%RH resonant dips appeared in the

transmission spectrum due to mode coupling and experi-

ence a red shift with the increase of RH. To prove the fea-

sibility of using the sensor as a breath rate monitor it was

placed at a distance of about 2 cm from the tip of the noseand the resulting breath RH response of the sensor was re-

corded as shown in Fig. 14, for a time span of 60 s. An

agreeably fast recovery and repeatability of the sensor

was achieved for the target application. Another interest-

ing evanescent field RH sensor in a U-bend configuration

has been proposed by the same group[95]using humidity

sensitive Agarose coating on a SMF and the response of the

sensor for a step change in RH is shown inFig. 15.

4.3. Interferometric sensors

Optical fibre based interferometers use the interference

betweentwo beams that have propagated through different

Fig. 13. Experimental setup for studying the humidity response of the PEO coated fibre bend and the Poly(ethylene oxide) coated fibre bend[53].

Fig. 14. Continuous human breath response of the sensor[53].

Fig. 15. Time response of the Agarose coated fibre-bend sensor obtained

by applying a step change of humidity[95].

Opticalfibre

3-dBcoupler

3-dB

coupler

Reference

Sensing arm

tuptuOtupnI

3-dB coupler

PC

Sensing fibre

(b)

Optical

fibre3-dB

coupler

Mirror

Mirror

(a)

(b)

(c)

Fig. 16. Schematic of optical fibre-based (a) MachZehnder, (b) Michel-son and (c) Sagnac interferometer configurations.

http://-/?-


15/23

optical paths of a single fibre or twodifferent fibres in which

one of the optical paths could be engineered to be affected

by a specific external perturbation. The target measurand

can be determined by various means of detection in terms

of wavelength, intensity, phase and polarization, etc. Vari-

ous interferometer configurations such as the MachZehn-

der, Michelson, Sagnac and FabryPerot can be designed

for sensing applications[96100]as illustrated in Fig. 16.

Optical fibre interferometers have been very successful in

sensor application e.g. the Sagnac interferometer is used as

a rotation measurement for both civilian and military appli-

cations[101]. The current trend in fibre optic interferome-ters is to miniaturize them for micro-scale applications

and thus, traditional bulk optic components such as beam

splitters, combiners, and objective lenses have been rapidly

replaced by small-sized fibre devices that enablethe sensors

to operate on fibre scales. This innovation suits well for RH

monitoring applications and such being the case, in-line

structures such as that of fibres which have two optical

paths within its physical structure, offering easy alignment,

high coupling efficiency and high stability, are seen as ideal

for sensing applications.

Recently, Chen et al. [102]have proposed a RH sensor

based on a high-birefringence polarization-maintaining fi-

bre (PMF)-based Sagnac loop configuration as can be seen

fromFig. 17(a). The humidity sensing principle of the de-

vice discussed utilizes the inherent characteristics of the

Sagnac interferometer by coating the PMF with moisture-

sensitive Chitosan whose degree of swelling varies as a

function of RH leading to a secondary strain effect on thePMF. The strain effect induced on the PMF is seen to mod-

ulate its birefringence in a way that can be correlated with

the variation of RH. To optimize the response of the sensor,

a series of experiments was first conducted to evaluate the

effect of Chitosan concentration on the PMF, followed by

an investigation into the effects of chemically etched PMF

Fig. 17. RH response of the oxidized Chitosan with etched PM fiber and 1% Chitosan with etched polarization maintaining (PM) fiber[102].

Fig. 18. Schematic diagram of the Chitosan-coated FPI RH sensor and the wavelength shift of the sensor upon exposure to environment of varying relativehumidity [103].



16/23

with a modified Chitosan sensing film on the sensors per-

formance. The results obtained are shown in Fig. 17(b) and

as can be seen, the chemically modified Chitosan coating

combined with the etching has resulted in a good sensorperformance. The optimized sensor was reported to exhibit

a sensitivity of 81 pm/%RH for a humidity change over the

range from 20% to 95%.

Another interesting interferometric RH sensor proposed

by Chen et al.[103]involves splicing a section of hollow-

core fibre to a SMF and to coat the tip of the hollow core

fibre with Chitosan, thereby creating a FabryPerot Inter-

ferometer (FPI) sensor, as can be seen from the diagram

shown in Fig. 18(a). The sensing mechanism responds to

the swelling effect of Chitosan which then induces an opti-

cal path modulation when the external RH is changed

this can be monitored, as can be seen from Fig. 18(b).

The sensor exhibits a sensitivity of 0.13 nm/%RH for RHranging from 20% to 95% with a fast response time of

380 ms.

4.4. Hybrid sensors (grating + interferometric)

Several sensor designs have been reported which in-

volve a combination of both fibre grating and interferomet-

ric configurations to achieve more effective RH sensing

than through the use of either approach alone. As dis-

cussed in the in-fibre grating section, both FBGs and LPGs

written into conventional fibres are inherently sensitive

to temperature. Therefore in most cases where hybrid sen-

sors are considered, the involvement of the grating is for

the purpose of eliminating the temperature-induced mea-

surement error from the actual RH/moisture sensing re-sults. One such example is the work of Gu et al. [104]

who presented a RH sensor based on a thin-core fibre

modal interferometer with a FBG between, where poly

(N-ethyl-4-vinylpyridinium chloride) (P4VPHCl) and poly

(vinylsulfonic acid, a sodium salt) (PVS) are deposited on

the surface of the sensor for RH sensing. A schematic of

the sensor can be seenFig. 19(a). The FBG is used to com-

pensate temperature effects on the overall sensor perfor-

mance. The sensor described has been reported to be

able to detect RH changes with a resolution of 0.78%, oper-

ating over a large RH range at different temperatures. A lin-

ear, fast and reversible response has been experimentally

demonstrated, as can be seen inFig. 19(b).Another benefit of the hybrid design for interferomet-

ric-grating sensing is to improve the measuring technique,

i.e. to create a probe, and thus to achieve a better resolu-

tion in the detection system. A typical configuration

involving a single grating-based LPG sensor system fre-

quently has the disadvantage of the probe being used in

transmission mode. Further, the broad bandwidth of the

attenuation bands formed by the propagation mode cou-

pling between the core and the cladding modes constitutes

a difficulty when the device is used as a conventional sen-

sor probe. To overcome these limitations, a Michelson

Fig. 19. (a) Schematic configuration of the fiber-optic RH sensor and (b) the dynamic response of the fabricated RH sensor to the change in humidity[104].

Mirror

Mirror

cladding

LPG

core

(a)

(b)

(nm)

Intensity (dBm)

Intensity (dBm)

(nm)

Fig. 20. Light propagation in the SILPG (a) forward propagation path and (b) propagation path of the reflection[107].

http://-/?-http://-/?-


17/23

interferometer-type sensor configuration has been pro-

posed by Lam et al [41] using a LPG grating pair formed

by coating a mirror at the distal end of the LPG, i.e. termed

as Self interfering LPG (SILPG), as can be seen fromFig. 20,

in order to create a refractometer. This sensor configura-

tion is more convenient to use and is able to overcome

the limitations of the single LPG sensor due to the shifts

in the attenuation bands being more easily detectable.

(a)

(b)

Fig. 21. (a) Results for the PI coated LPG RH sensor probe and (b) comparison between the performance of PI and PVA coated LPG based RH sensor probes

[108].

Fig. 22. The system diagram of the humidity sensor and the detail structure of the fiber tip coated with sensitive thin-film. The test result of different

amount of CoClz in PYA/SiOz composite material [109].



18/23

Fig. 23. Experimental set-up for the absorption-based humidity sensing application reported by Estella et al. [110].

Table 3

Humidity/moisture application-specific sensors, over the period 20032013.

Ref. Year Authors Sensor application Sensing mechanism

Biomedical measurements

[15] 2012 Favero

et al.

Breathing sensor Reflection spectra measurement of SMF in-line with a PCF in both open and

closed end configurations

[17] 2011 Akita et al. Breathing sensor Light intensity variation of hetero-core fibre configuration coated with a

hydroscopic polymer

[16] 2008 Morisawaet al.

Recognition of devoiced vowels Light intensity measurement of moisture sensitive polymer coated de-clad multi-mode POF

[14] 2007 Kang et al. Breathing air-flow monitor Measurement of reflectance of polymer thin film coated the end-tip of fibre in an

optical cavity interferometric configuration

Climate/agricultural monitoring

[147] 2011 Bilro et al. Turbidity sensor Variation in the transmitted and scattered light collected via two fibres that are

made to be in and out-of-phase to each other

[148] 2008 Kuang

et al.

Flood monitoring Loss in intensity of a U-bend MMF due to surrounding refractive index change

[149] 2008 Clevers

et al.

Canopy water content sensor Reflection spectrum variation due to the water absorption by the target analyte.

[9] 2006 Eitel et al. Water stress detection of Poplar

plantation

Reflection spectrum variation due to the water absorption by the target analyte

[10] 2003 Sims et al. Water content sensor for

vegetation

Reflection spectrum variation in the NIR region due to the water absorption by

the target

Structural Health Monitoring (SHM)

[22] 2013 Kaya et al. Water detection in concrete Measurement of ring-down times of etched SMF embedded in concrete in a fibre

ring configuration due to change in the refractive index of the surrounding

[23] 2012 Sun et al. Building stone condition

monitoring

Bragg wavelength monitoring of PI coated FBG in SMF

[150] 2012 Mathew

et al.

Dew detection Power loss measurement of a PCF interferometer in line with SMF

[151] 2006 Yeo et al. Moisture absorption in concrete Bragg wavelength monitoring of PI coated FBG in SMF

Quality control applications

[152] 2011 Srivastava

et al.

Water content measurement in

ethanol

Wavelength shift measurement of gold coated de-clad MMF using SPR

[153] 2010 Xiong et al. Water content measurement in

ethanol

Measurement of evanescent field absorbance of a coiled optical fibre

[154] 2009 Puckett

et al.

Water detection in jet fuel Intensity and wavelength shift of a LPG coated with PAA/PDMA

(continued on next page)



19/23

The same configuration was applied to achieve a grating

based RH sensing reflective probe by Alwis et al. who

coated the LPGs (in such a configuration) with PI [105]

and PVA[106]respectively. Both the PVA and PI swell with

the increase of RH in its surroundings. The RI of PVA and PI

is 1.53 and 1.7 respectively. Since the cladding RI is around

1.44, PVA lies closer to the cladding RI than that of PI andtherefore it would experience a greater RI change than PI.

This latter material has been coated to create a moisture-

related strain induced RH sensor and PVA is used to induce

a RH related external RI variation on the LPG. A comparison

between the performance characteristics of these two dif-

ferent polymer-coated SILPGs are shown in Fig. 21. It can

be seen that PVA offers higher sensitivity of the two poly-

mers used, although its sensing region is limited and the

performance is non-linear, these being disadvantages that

may be overcome for certain applications. PI on the other

hand, offers a linear performance that is easy to process,

but with overall less sensitivity in most of the target range

compared to that achieved with PVA.

4.5. Absorbance sensors

Absorption-based methods have been familiar for opti-

cal fibre sensors since the beginnings of the sensors field

and absorbance-based optical fibre-based sensor measure-

ments are made by monitoring the intensity variation as a

result of absorption due to the interaction between the

chemical reagents involved and moisture (the main mech-

anism for absorbance-based optical fibre sensing has been

detailed in the previous review by Yeo et al.[37]). One such

RH sensor was proposed by Wang et al. [109]who coated a

moisture-sensitive film at the tip of a multi-mode fibre

(MMF). The film was synthesized by doping CoCl 2 into a

PVA/SiO2 composite solution. Based on the absorption

dependence of CoCl2 on humidity, the sensor was charac-

terized using the absorption at wavelength bands 550 nm

and 750 nm respectively. From the change of absorption,

RH changes over the range from 25% to 65% were detected.

The sensor showed good repetitive response with less than

2 min response time. The system and the test results are

shown inFig. 22.

Another absorption-based RH sensor design has been

proposed by Estella et al. [110] by using a porous silica

xerogel film synthesised by the solgel process as the sens-

ing element. The specific goals of the research were to

design and tune a measuring cell working under volumet-

ric static conditions and to evaluate the sensitivity, revers-

ibility and reproducibility of the sensor. The sensing

mechanism was based on the change in reflected optical

power when water molecules were adsorbed on the silica

xerogel film. The experimental set-up is shown in Fig. 23

and comprised an optical system, a measuring cell, a vac-uum and dosification system, and controllers for tempera-

ture and pressure. Light at port 4 is guided to the index

matching liquid (toluene), where no reflection occurs (to

avoid interference) while the signal in port 2 reaches the

xerogel film interface. Exposure of the xerogel film to

water vapour inside the measuring cell produces variations

in the reflected signal, which is reflected back to the cou-

pler and measured at the spectrometer (port 3).

5. Overview

The key feature of a review of this type has been to

present the key information required to the sensor user

which will allow the optimum choice of sensor to be made

for any particular application and for research to be stimu-

lated to enable the development of new sensors to tackle

unmet and likely challenging needs. Given the breadth of

applications and fibre optic-based technologies discussed

above, it is useful to tabulate the key features of these sen-

sors sensing method, sensing material, range, sensitivity

and response time (where known), as well as the date of

first publication and the group responsible for the develop-

ment as well as the source of further information (the

original reference in the literature) to help in that search.

To aim to do that, two tables are presented below:Table 2

presents an overview of the mentioned sensor schemes

and various other related sensor schemes available in the

literature. Table 2 particularly focus on work done in the

period 20082013 the last five years and thus aim both

to be highly topical and to build on the work tabulated in

our previous review [37] which dealt with the state-of-

the-art prior to 2008, and to which the interested reader

is referred for prior research in the field. Table 3provides

a few examples where optical fibre-based RH sensors have

been used for specific target applications.

Thus, in summary, this review has shown how well fi-

bre optic sensing technology has provided, and indeed con-

tinues to provide, an alternative and highly effective

approach to moisture and humidity sensing as it offers real

Table 3(continued)

Ref. Year Authors Sensor application Sensing mechanism

[155] 2009 Zhang

et al.

Water detection in jet fuel Wavelength shift measurement of FBG written into a POF where the cladding is

made of PMMA

[156] 2008 Rodreguez

et al

Humidity detection in oil-paper

insulation of electrical apparatus.

Intensity variation measurement of a U-bend PVA coated plastic MMF caused by

surrounding refractive index change.

Other applications

[31] 2012 Cho et al. Water leak detection Transmission loss causedby the bending of SMF attached toacrylate polymer that

swells with water

[32] 2011 Hsu et al. Water detection in optical fibre

splice enclosures

Reflection light measurement using OTDR technique incorporating the change in

refractive index (from air to water) when infiltrated

[33] 2008 Baldini

et al.

Dew detection inside organ pipes 1. Reflection spectra measurement of cladding removed U-bend MMF

2. Reflection spectra measurement of open-end MMF

http://-/?-http://-/?-


20/23

advantages over the use of conventional electrical-based

sensing methods. The explosive growth in both the tech-

nology and applications over the main focus period of this

review (20082013) is evident from the range of example

cited and compared and the breadth of applications seen.

The review has provided examples which signify the diver-

sity of applications and the demand for RH/moisture sen-

sors in industry today. A representative variety of optical

fibre-based sensing techniques available to perform the

measurement of humidity and moisture have been dis-

cussed, with a brief introduction to each optical fibre sens-

ing scheme. A detailed survey on each optical fibre sensing

scheme employed for RH and moisture detection in the lit-

erature has been presented.Tables 2 and 3have presented

an overview of the major work discussed in this review

using information from available literature with detailed

information on various optical fibre-based RH sensing

schemes that are proposed recently, over the past 5 years

(Table 2), that allow cross-comparison and the selection

of suitable sensing method for specific applications, nearly

all work and results obtained in a controlled laboratory

environment.Table 3has presented various optical fibre-

based sensing schemes that are in practice for RH and

moisture measurement for specific target applications.

The study has thus covered a wide variety of extrinsic

and intrinsic FOS schemes reported in the literature for

RH and moisture sensing as at present.

Acknowledgements

The authors would like to acknowledge the support

from the Engineering and Physical Sciences Research

Council (EPSRC) through various schemes.

References

[1] Q.A. Shams, C.G. Burkett, T.S. Daniels, G. Tsoucalas, T. Comeaux, B.S.Sealey, M.L. Fox, Characterization of Polymer-Coated MEMSHumidity Sensors for Flight Applications, NASA/TP-2005-213770.

[2] P.R. Wiederhold, Water Vapor Measurements, Marcel Dekker, 1997.[3] B.H. Dijkink, M.M. Tomassen, J.H.A. Willemsen, W.G. Doorn,

Humidity control during bell pepper storage, using a hollow fibermembrane contactor system, Postharvest Biology and Technology32 (2004) 311320.

[4] S.W. Pixton, S. Warburton, Moisture content/relative humidityequilibrium of some cereal grains at different temperatures,

Journal of Stored Products Research 6 (1971) 283293.[5] V. Stojceska, P. Ainsworth, A. Plunkett, S. Ibanoglu, The effect of

extrusion cooking using different water feed rates on the quality of

ready-to-eat snacks made from food by-products, Food Chemistry114 (2009) 226232.

[6] J.M. Arimi, E. Duggan, M. OSullivan, J.G. Lyng, E.D. ORiordan, Effectof moisture content and water mobility on microwave expansion ofimitation cheese, Food Chemistry 121 (2010) 509516.

[7] M.J. Mateo, D.J. OCallaghan, C.D. Everard, M. Castillo, F.A. Payne,C.P. ODonnell, Evaluation of on-line optical sensing techniques formonitoring curd moisture content and solids in whey duringsyneresis, Food Research International 43 (2010) 177182.

[8] A. Monti, A. Zatta, Root distribution and soil moisture retrieval inperennial and annual energy crops in Northern Italy, Agriculture,Ecosystems and Environment 132 (2009) 252259.

[9] J.U.H. Eitel, P.E. Gessler, A.M.S. Smith, R. Robberecht, Suitability ofexisting and novel spectral indices to remotely detect water stressin Populus spp, Forest Ecology and Management 229 (2006) 170182.

[10] D.A. Sims, J.A. Gamon, Estimation of vegetation water content and

photosynthetic tissue area from spectral reflectance: a comparison

of indices based on liquid water and chlorophyll absorptionfeatures, Remote Sensing of Environment 84 (2003) 526537.

[11] M. Ruiz-Altisent, L. Ruiz-Garcia, G.P. Moreda, R. Lu, N. Hernandez-Sanchez, E.C. Correa, B. Diezma, B. Nicola, J. Garca-Ramos, Sensorsfor product characterization and quality of specialty cropsareview, Computers and Electronics in Agriculture 74 (2010) 176194.

[12] M. Blanco, J. Coello, H. Iturriaga, S. Maspoch, M. Rovira,Determination of water in ferrous lactate by near infraredreflectance spectroscopy with a fibre-optic probe, Journal of

Pharmaceutical and Biomedical Analysis 16 (1997) 255262.[13] E. Shotton, N. Harb, The effect of humidity and temperature on the

cohesion of powders, Journal of Pharmacy and Pharmacology 18(1966) 175178.

[14] Y. Kang, H. Ruan, Y. Wang, F.J. Arregui, I.R. Matias, R.O. Claus,Nanostructured optical fibre sensors for breathing airflowmonitoring, Measurement Science Technology 17 (2006) 12071210.

[15] F.C. Favero, J. Villatoro, V. Pruneri, Microstructured optical fiberinterferometric breathing sensor, Journal of Biomedical Optics 17(2012) 037006.

[16] M. Morisawa, Y. Natori, T. Taki, S. Muto, Recognition of devoicedvowels using optical microphone made of multiple POF moisturesensors, Electronics and Communications in Japan 93 (9) (2010).

[17] S. Akita, A. Seki, K. Watanabe, A monitoring of breathing using ahetero-core optical fiber sensor, in: Proc. of SPIE, vol. 7981,79812W.

[18] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors,Analytical Chemistry 76 (2004) 32693284.

[19] J. Green, I. Dyer, Measurement of humidity, Anaesthesia andIntensive Care Medicine 10 (2009) 4547.

[20] M. Mkinen, M. Sillanp, A. Viitanen, A. Knap, J. Mkel, J. Puton,The effect of humidity on sensitivity of amine detection in ionmobility spectrometry, Talanta 84 (2011) 116121.

[21] R.S. Mahendran, L. Wang, V.R. Machavaram, S.D. Pandita, R. Chen,S.N. Kukureka, G.F. Fernando, Fiber-optic sensor design for chemicalprocess and environmental monitoring, Optics and Lasers inEngineering 47 (2009) 10691076.

[22] M. Kaya, P. Sahay, C. Wang, Reproducibly reversible fiber loopringdown water sensor embedded in concrete and grout for watermonitoring, Sensors and Actuators B 176 (2013) 803810 .

[23] T. Sun, K.T.V. Grattan, S. Srinivasan, P.A.M. Basheer, B.J. Smith, H.A.Viles, Building stone condition monitoring using specially designedcompensated optical fiber humidity sensors, IEEE Sensors 12 (2012)

10111017.[24] T. Venugopalan, T. Sun, K.T.V. Grattan, Structural concrete condition

monitoring using a long period grating-based humidity sensor, in:Proc. of SPIE, vol. 7004, 70045N.

[25] A. MacLean, W.C. Michie, S.G. Pierce, G. Thursby, B. Culshaw, C.Moran, N.B. Graham, Hydrogel/fibre optic sensor for distributionmeasurement of humidity and pH value, SPIE Proceedings 3330(1998) 134144.

[26] M. Previati, D. Canone, I. Bevilacqua, G. Boetto, D. Pognant, S.Ferraris, Evaluation of wood degradation for timber check damsusing time domain reflectometry water content measurements,Ecological Engineering 44 (2012) 259268.

[27] Y. Nara, K. Morimoto, N. Hiroyoshi, T. Yoneda, K. Kaneko, P.M.Benson, Influence of relative humidity on fracture toughness ofrock: implications for subcritical crack growth, International

Journal of Solids and Structures 49 (2012) 24712481.[28] A. Fujita, R. Kurose, S. Komori, Experimental study on effect of

relative humidity on heat transfer of an evaporating water dropletin air flow, International Journal of Multiphase Flow 36 (2010) 244247.

[29] Koster et al., Regions of strong coupling between soil moisture andprecipitation, Science 305 (2004) 11381140.

[30] Weckwerth et al., The effect of small-scale moisture variability onthunderstorm initiation, Atmospheric Technology Division,National Center for Atmospheric Research, 2000.

[31] T. Cho, K. Choi, D. Seo, I. Kwon, J. Lee, Novel fiber optic sensor probewith a pair of highly reflected connectors and a vessel of waterabsorption mat

humidity sensor based fibre -optical

Documents

measurement of humidity

fibre optical

sensing applications

humidity sensor

s e v i e r

optical fibre field

s u r e

storage applications