Presented By,Er. Swapnil V. Kaware,svkaware@yahoo.co.in

Topic:- Optical Sources, Detectors and Optical Fibre measurements

Absorption and Dispersion

• The phase speed c = λν, which is defined as the product of wavelength and frequency, is reduced in comparison to speed of light in a vacuum, c0, when the electromagnetic wave travels through a medium with an index of refraction n > 1.

• The reduced value is c = c0 /n. We will show that the frequency dependency of n leads to a dispersion, which can be described using a classical model.

• It will be also shown that the imaginary part of a complex index of refraction describes the damping of an electromagnetic wave.

Absorption and Dispersion

Absorption and Dispersion

Laser• Light Amplification by Stimulated Emission of

Radiation– “Stimulated emission ”antonym of “spontaneous emission”

– optical transition stimulated by the effect of electric field of light wave on the contrary usually emission occur spontaneously without help of electric field.

– Light amplification by stimulated emission of radiation, or laser in short, is a device that creates and amplifies electromagnetic radiation of specific frequency through process of stimulated emission.

– In laser, all the light rays have the same wavelength and they are coherent; they can travel long distances without diffusing.

Laser

Laser• Light amplification by stimulated emission of radiation, that is--

lasers, have become an important tool in chemistry.

• Lasers are ideal light sources for spectroscopy, chemical kinetics studies, and light scattering studies of molecular motion.

• High-powered lasers are finding use as light sources for photochemical synthesis, yielding products not available from other techniques.

• Dye lasers are an important class of lasers because they can be tuned to a range of wavelengths.

Laser

Laser

LED and LD

• LED is light emitting diode• LD is laser diode

– Diode is a semiconductor device which has an effect of rectification

– Both LED and LD are semiconductor diode with a forward bias. Both emit light

– LED emits light by spontaneous emission mechanism, while LD has an optical cavity which enables multiplication of photon by stimulated emission

LED and LD

• LD (laser diode) works as LED if the operating current does not exceed the threshold value.

Threshold currentSpontaneous emission

Laser actionWith stimulated

emission

Forward bias current Wavelength

Spontaneous emission

Laser actionWith stimulatedemission

Ligh

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tens

ity

(a) (b)

Development of Communication

• To meet with the growing need for large capacity information exchange, optical fiber communication system has been developed.

Light wave network

WDM

EDFA

ADSL

FTTH

Dat

a ca

rryi

ng c

apac

ity (

bps)

Development of Optical Communication

Total Reflection

critical anglec

medium 2

medium 1

ic

ic

evanescent field

total reflection and evanescent wave

Transmission Loss of Fiber

Attenuation and dispersion in optical fiber

• Attenuation: reduction of light amplitude

• Dispersion: deterioration of waveform

Principle of Photodetection• When a photon with an energy greater than the band gap

i.e. Eg is incident on the semiconductor, it is absorbed and it generates an electron-hole pair, that is an electron in the conduction band and a hole in the valence band.

• If the pair is created within the space charge region, the electric field in the junction separates the charges and drifts them to the neutral regions.

• The carrier drift generates a photocurrent in the external circuit that provide an electrical signal.

Photodetectors• Photodetectors (or photosensors ) are transducers that alter one of their

characteristics when light energy impinges on them. In this category, photoresistors alter their ohmic resistance, rods and cones of the retina neurons of the eye alter their electrochemical response, and chlorophyll in plant leaves alters the rate of converting CO2 to O2.

• Some other photodetectors alter the flow of electrical current or the potential difference across their terminals.

• Photodetectors with sufficiently fast response that provide a measurable output for a small amount of light, are easily reproducible, and are economical are worth investigating for applications in high-speed optical communications.

• This category includes avalanche photodiodes (APDs) and positive intrinsic negative photodiodes.

Photodetector Characteristics• Spectral response relates the amount of current produced with wavelength,

assuming that all wavelengths are at the same level of light.

• Photosensitivity is the ratio of light energy (in watts) incident on the device to the resulting current (in amperes).

• Quantum efficiency is the number of generated electron-hole pairs (Le., current) divided by the number of photons.

• Dark current is the amount of current that flows through the photodiode in the absence of any light (dark), when the diode is reverse-biased.

• This is a source of noise when the diode is reverse-biased.

Photodetector Characteristics• Forward-biased noise is a (current) source of noise that is related to the

shunt resistance of the device. The shunt resistance is defined as the ratio voltage (near oV) to the amount of current generated. This is also called shunt resistance noise.

• Timing response of the photodetector is the time for the output signal to climb from 10% to 90% of its amplitude (rise time) and to drop from 90% to 10% (fall time).

• Frequency bandwidth is the frequency (or wavelength) range in which the photodetector is sensitive.

• Cutoff frequency is the highest frequency (wavelength) at which the photodetector is sensitive.

Avlanche Photodiode• The APD is a semiconductor device that, when reverse-biased,

creates strong fields in the junction region.

• When a photon causes an electron-hole pair, the pair flows through the junction.

• Because of the strong fields in the junction, the electron gains enough energy to cause secondary electron-hole pairs, which in turn cause more.

• Thus a multiplication (or avalanche) process takes place (hence the name), and a substantial current is generated from few initial photons.

Avlanche Photodiode

Wavelength Measurement• Optical wavelength detection in sensing can be generally

categorized into two types: passive detection schemes and active detection schemes.

• In passive schemes there are no power driven components involved.

• A passive detection scheme refers to those that do not use any electrical, mechanical or optical active devices in the optical part of the system.

• Most of the passive devices are linearly wavelength dependent devices such as bulk edge filters (Mille etal., 1992), biconical fiber filters (Ribeiro et al., 1996), wavelength division couplers (Davis & Kersey, 1994), gratings (Fallon et al., 1999), multimode interference couplers (Wang &

Wavelength Measurement

Wavelength Measurement

• The simplest way to measure the wavelength of light is to use a wavelength dependent optical filter with a linear response.

• This method is based on the usage of an edge filter, which has a narrow linear response range with a steep slope or a broad band filter, which has a wide range with less steep slope.

• In both cases, the wavelength interrogator is based on intensity measurement, i.e., the information relative to wavelength is obtained by

• monitoring the intensity of the light at the detector. • For intensity based demodulators, the use of intensity referencing is necessary

because the light intensity may fluctuate with time.• This could occur not only due to a wavelength change but also due to a power

fluctuation of the light source, a disturbance in the light-guiding path or the dependency of light source intensity on the wavelength.

• References:-

(i). Optical Fibers by, T. Okoshi,, Academic Press, San Diego, CA, 1982.(ii). Optical Waveguide Theory by, A.W. Snyder and J. D. Love,,

Chapman & Hall, London, 1983.(iii). Single-Mode Fiber Optics by, L. B. Jeunhomme,, Marcel Dekker,

New York, 1990.(iv). Optical Fiber Communications by, T. Li, Ed, Academic Press, San

Diego, CA, 1985.(v). Optical Fibers: Materials and Fabrication by, T. Izawa and S. Sudo,,

Kluwer Academic, Boston, 1987.(vi). Fundamentals of Optical Fiber Communications D. B. Keck, in, M. K.

Barnoski, Ed., Academic Press, San Diego, CA, 1981.