fbg & circulator_2

8/6/12 4. Characterisation of FBG and Circulator

1/6file:///C:/Users/Adib/Desktop/Li Stuff/426/LAB Manuals/files/4.HTM

4. Characterisation Of FBG And Circulator

FIBER BRAGG GRATING

Fiber Bragg Grating (FBG) is an optical fiber with a periodic variation of core refractive index along the

fiber length (see Fig. 4.1). An FBG acts as a highly wavelength selective reflector, with a high reflectivity at

a given central wavelength and the reflectivity dropping to very small values close to the central wavelength

(see Fig. 4.2). The central wavelength, the peak value of reflectivity and the bandwidth of the reflection

spectrum depends on the period of the refractive index modulation, on the index modulation of the grating

and the length of the grating.

If L is the period of index modulation, L the length of the grating and Dn the index modulation, then the

central wavelength lc, peak reflectivity R and the bandwidthDl(the spectral width over which the

reflectivity is high) are given approximately by

(9)

(10)



Here ne is the effective index of the fundamental mode of the fiber.

As an example, let us consider an FGB with a period of 0.53 mm, an index modulation of 10-4 and a length

of 5 mm made in a single mode fiber having a mode effective index of 1.45. Using the above set of

equations we obtain the following characteristics of the grating: lc = 1537 nm, R= 59% and Dl = 0.36 nm.

By measuring lc, R and Dl of a grating, the values of L, L and Dn can be estimated from Eqs(9-11).

FBGs find wide applications in optical fiber communication systems. They are used in laser transmitters for

single frequency oscillation in optical add drop multiplexers, fiber lasers, and fiber optic sensors.

In the experiment described here, the primary measurement is the peak reflectivity of the grating. In order to

measure other quantities, additional equipments are required. For example for measuring the central

wavelength and the spectral bandwidth an optical spectrum analyzer or a tunable optical filter would be

required. Using these additional instruments it is also possible to show effects of strain and temperature on

the grating performance and thus demonstrate its application to sensing.

OPTICAL CIRCULATOR

An optical circulator is a multiport device with non reciprocal transmission characteristics. In Fig 4.3 we

show a three port optical circulator. When light enters port 1 of the circulator, it exits from port 2. If light

enters port 2 of the circulator, instead of its emerging from port 1 it now emerges from port 3 (showing

non-reciprocity). Such a device finds wide applications in many areas such as dispersion compensation

using FBGs, add/drop multiplexers etc. For example if an FBG with a central wavelength of l1 is placed at

port 2 and if light at wavelengths l1, l2, and l3, are incident on port 1of the circulator, then out of the three

wavelengths exiting from port 2, FBG reflects back wavelength l1. This wavelength propagates back

towards port 2 of the circulator and exits from port 3 while the wavelengths l2, and l3 continue to

propagate along port 2. Thus this acts as a drop filter for wavelength l1.

The most important characteristics of a circulator are insertion loss and cross talk. These are defined as

follows: If the power entering port 1 is P1 and if the output at port 2 is P2, and that at port 3 is P3 then the

insertion loss is defined as

and the cross talk is defined as

(11)

(12)



Apart from these characteristics other important characteristics of a circulator include its polarization

dependence in terms of polarization dependent loss (PDL) and polarization mode dispersion (PMD), the

wavelength of operation and the power handling capacity. With additional equipment these characteristics

can also be measured using the kit.

EXPERIMENT

AIM

a. To determine the reflectivity of the given Fiber Bragg Grating at four different wavelengths and verify its

wavelength selectivity.b. To measure the insertion loss and cross talk of a 3-port circulator at various wavelengths.

COMPONENTS REQUIRED

1. FBG-12. Fiber Optic Circulator - 1

3. C-Band Lasers - 44. InGaAs Photodetector-1

FORMULA

Reflectivity of FBG = (P1-P2)/P1

P1 is the input power to an FBG and P2 is output power from the FBG.

Insertion Loss of Circulator = -10 log (P2/P1),

(13)



P1 is the input power and P2 is the output power at port 2

Cross talk in Circulator = 10 log (P3/P2)

P3 is the output power at the port 3 due to an input power at Port 1

PROCEDURE

After setting up LIGHT RUNNER as per the instructions given in page vi, the 1510nm laser is switched on

and its power is coupled to photodetector PD1 using a patchcord. The power level of 1510nm laser is

adjusted to be below saturation (refer to page ix) and measured as P1. The patchcord is then disconnected

for PD1 and connected to 3dB coupler as shown below. One of the other ends is connected to FBG and

the other to PD1. An ideal reflector with 100% reflectivity in lieu of FBG will amount to 1/4 of P1 at PD1.

The power at PD1 is is again measured as P2. Therefore the ratio of 4P2 to P1 will give the reflectivity of

FBG at the given wavelength. The experiment is repeated for all the other C-band wavelengths.

One of the C-band Lasers, say 1550nm is connected to an InGaAs photodetector, PD3 using a patchcord

and the laser power is measured as P1. The patchcord end is disconnected from the photodetector and it is

connected to Port 1 of the given Optical Circulator. Another patchcord is used to connect Port 2 of

Circulator to the same photodetector, PD3 and the power measured is P2 (see Fig 4.5). Patch cord at Port

2 is disconnected and then connected to Port 3 for determination of optical power, P3 coming there. The

given formulas are used to determine the insertion loss and cross talk in the circulator. It may be noted that

light travelling from Port 1 to Port 2 is termed as forward propagation. In a similar manner described above,

insertion loss and cross talk for backward propagation can also be determined by changing Port 1 to Port

2, Port 2 to Port 3.



OBSERVATION

Reflection Efficiency of FBG

Laser Wavelength (nm) Input Optical Power, P1 Reflected Optical power from FBG, P2 Reflectivity = (4P2/P1)*100%

1510

1530

1550

1570

Forward Insertion Loss (Channels 1 to 2) and Cross Talk (Channels 1 to 3) of Optical Circulator

Wavelength

(nm)

Input Power at Port 1 of

Circulator, P1 (mW)

Output power at Port 2 of

circulator P2 (mW)

Optical Power at

Port 3, P3 (mW)

Insertion Loss in dB

= -10 log (P2/P1)

Cross Talk in dB =

10 log (P3/P2)

1510

1530

1550

1570

Backward Insertion Loss (Channels 2 to 3) and Cross Talk (Channels 2 to 1) of Optical Circulaor

Wavelength

(nm)Input Power at Port 2

of Circulator, P1 (mW)

Output power at Port 3

of circulator P2 (mW)

Optical Power at

Port 1, P3 (mW)

Insertion Loss in dB

= -10 log (P2/P1)

Cross Talk in dB

= 10 log (P3/P2)

1510

1530

1550

1570

RESULTS

It is shown that the reflectivity of an FBG is a strong function of wavelength. The insertion loss of each port

of optical circulator and cross talk between ports for various wavelengths are determined.

FURTHER EXPLORATION

1. Find out if the reflectivity of the FBG depends on the direction of light launching to the FBG.

2. A combination of FBG and Circulator could be used to realize Add/Drop filter. This is described in

Experiment 12.

NOTE

The 50/50 coupler given along with LIGHT RUNNER can be used in the insertion loss of FBG/Circulator

experiment for simultaneously observing and measuring the input power and output power from the

FBG/Circulator. This can be done by connecting the COM port of the 50/50 coupler to the laser and

connecting one of the output arms to a photodetector and the other to the FBG/Circulator Port 1/Port 2.

Before doing this connection please find out the splitting ratio of the 50/50 coupler.

fbg & circulator_2

Documents