Fiber Optic Instrumentation System

Fiber Optic Instrumentation System

1. 1ntroduction

The communication engineers need the fiber characteristics to design the optical fiber link with an efficient waveguide without any loss or dispersion. Similarly, the fiber manufactures ned the fiber characteristics for further development. Generally, the fiber attenuation measurement are used to determine repeaters spacing and light source power dispersion measurements are used to determine the maximum bit rate. Refractive index profile measurement are to know the number of modes propagating the fiber and to determine its numerical aperature.

2. Measurement of attenuation (by cut back method)

Light from a halogen lamp or white light source is couple into the experimental fiber having length about 1 km. The lens placed in front of the source focuses the light on to the interference filter or monochromatic prism or grating. The light with a given wavelength is incident on the chopper which is used to convert d.c light into square pulses of light (a.c). It also sends the reference signal to the lock in amplifier. Monitor is used to view the intensity of the optical beams. The cladding mode strippers are connected at the input end and output end of fiber. The cladding mode stripper is used to remove the cladding light or cladding modes. Then the jacket fiber is placed in an index matching liquid whose refractive index is slightly higher than that of cladding.

This arrangement is called cladding mode stripper which will attenuate the light propagating through the cladding. After travelling through the fiber of 1Km length, the given height reaches the index matched photodetector whose output is given to the lock amplifier. The lock amplifier delivers a output to the recorder or nanovoltmeter. Then the fiber is cut back, leaving typically 2m of the fiber and the experiment is repeated. In this case the output power is noted P_{r (}λ) is noted. This procedure is repeated for other wavelength also. Thus the fiber attenuation at a given wavelength ‘λ’ is given by,

Where L is the length of the fiber cut back in Km. In the case of multimode fibers, there are mode scrambler used to get the uniform intensity distribution among all the modes and order sorting filter acting as a mode selector to determine the fiber loss for each mode.

Advantage:

This method is very accurate and simple.

Drawback

i)                   This method cannot be utilized to find the fiber attenuation in a working fiber optic link.

ii)                It is a destructive testing method.

3. Optical domain reflectometers:

The OTDR is the instrument which is used both in laboratory and field measurements for determining fiber attenuation ,joint losses and detecting fault losses. When the fiber attenuation varies with distance, then the OTDR is the only instrument which can measure the fiber attenuation along the fiber optics link. The OTDR measurement is a non-destructive measurement.

Principle:

This method is often called the both scatter method. It is based on the measurement and analysis of the fraction of light which is reflected back within the numerical aperture of the fiber due to Rayleigh scattering.

Construction and working:

A light pulse from a pulsed laser is launched into the fiber through a directional coupler. The back scattered light from the fiber is received by a photo detector like APD, through the directional coupler. A box car integrator is mainly used to improve S/N ratio by taking arithmetic average over a number of measurements taken at one point within the fiber. The signal from the integrator is fed to the logarithmic amplifier and its output is given to the recorder in DB. The recorder will display the averaged measurements for successive points within the fiber .The intial

peak is caused by the reflection at the fiber end. The reflection from the input coupler is as small increase in the reflected power. There is a long tail caused by Rayleigh scattering of the input pulse as it travels through the fiber link in the forward direction. Due to presence of a fault in the fiber link. There is a sudden decrease of reflected power. Next peak is caused by splice or joint. Finally there is a peak due to Fresnel reflection of the fiber end where the reflected power is more than that of splice.

4. Fiber scattering loss Measurement:

Usually a high power laser source like He-Ne laser or Nd-YAG laser is used to provide sufficint input optical power to the fiber. The focusing lens focuses the light into the input end of the fiber having short length. Before and after the scattering cell or integrating sphere, the cladding mode strippers are used to avoid the light propagating in the cladding so that the scattering measurement is taken only for the light guided by the fiber core. Further the output end of the fiber is in index matched liquid to avoid reflections contributing to the optical signal within the integrating sphere. The light scattered from the fiber core is detected by the series solar cell in the integrating sphere. The integrating sphere also contains the index matching liquid surrounding the fiber. The detected signal by the series of solar cell gives the measurement of the scattered signal. The detected signal is given to lock in amplifier an to then to the recorder or nano voltmeter.

5. Fiber Absorption Measurement:

Fiber absorption measurement will give the impurity level in the filter.

Fiber absorption loss(bB/km)= Fiber attenuation loss (dB/Km)- Fiber scattering loss(dB/km)

Thus the fiber absorption lass istrhe difference between fiber attenuation loss and scattering loss.

Principle: Amount of light energy absorbed by the fiber= Heat energy developed in the calorimeter

Construction:Here there are two fibers one is the fiber under measurement and other is the dummy fiber. The dummy fiber is meant for compensation of any radiation loss of heat energy developed. These two fibers are mounted separately in silica capillary tubes surrounded by the low refractive index liquid like methanol in the calorimeter for good electrical contact. The light from the laser source is well focused on the fiber under measurement. The dummy fiber is not connected with light input. Then the fiber guided light is inserted into the cladding mode stripper which removes the light propagated in the cladding of the fiber. After passing through the capillary tube, The fiber with light is immersed in the index matching liquid to avoid reflections contributing to the optical signal within the capillary tube.

Procedure;

When the light enters the fiber under measurement there is a temperature rise in the capillary tube containing the fiber with light. The temperature rise due to absorption tube containing the fiber with light. The temperature rise due to absorption of energy by the fiber is measured for every 10 seconds by a thermocouple which is spirally around the silics tubes. The hot junction of the thermocouple and the cold junction of the thermocouple are connected with a nanovoltmeter. Electrical calibration is done by placing a thin wire instead of fiber such that and passing known amount of current such that

mST=I²RT=VIt

6. Fiber dispersion measurements:

Dispersion is measured in terms of pulse broadening. There are two types of fiber dispersions. One is intermodal dispersion and the other is intra nodal (or) chromatic dispersion. Both dispersion measurements can be performed using the the same exceptthe light source. Internodal dispersion measurement is made by the monochromatic laser with narrow spectral width. This intermodal dispersion is dominant in the multimode fibers. The intra nodal dispersion measurement is made by the injection laser whose frequency or line width increases with respect to time.

The laser with driver circuit gives short narrow pulses of light. The laser light is focused onto the beam splitter. The beam splitter is used for triggering the oscilloscope and for input pulse with measurement. One of the beams passing through the beam splitter is again focused into the fiber under measurement. Normally its length is 1 km .The focused output laser beam is incident on the avalanche photodiode and it gives the output pulses. The input pulse and output pulse are displayed separately on the screen of sampling oscilloscope and they are in Gaussian shape.

7. End reflection method:

The light from the lambertian source is focused onto the entrance end of the fiber having a length 2 metre. The magnified image of the output end of the fiber is obtained by a lens arrangement and is then passed through chopper. The near field of the output of the chopper is scanned transversly by a p-i-n detector . The detector output is amplified by a preamplifier. The chopper and the preamplifier are linked with the lock in amplifier. So the phase sensitive detected signal is further amplified and plotted directly on a X-Y recorder. For a graded index fiber, the display appears in the form of a Gaussian curve and for a step index fiber it appears in the form of a rectangular shape curve

Limitation of this method

1.     There should not be any contamination on the fiber surface

2.     The fiber surface should be optically plane.

3.     During scanning proper alignment of the fiber is necessary.

8. Near field scanning techniques:

Theory

When a lambertian source like tungsten filament lamp or LED is used to excite all the guided modes then P® is the near field optical power at a distance’r’from the core axis and p(0) is the optical power at the centre of the core.

Measurement of numerical aperture of the fiber:

The lambertianthe numerical aperture of the fiber from the far end pattern. The lambertian source gives the angled visible light. It is then focused onto the test fiber of length 1m. The far field patteren from the fiber is displaced on the screen which is at a distance ‘D’ from the output end of the fiber. The test fiber is aligned so that there is maximum intensity of light on the screen. The pattern size on the screen is measured as Ametre.

For a graded index fiber

N.A(r)=sinθa (r) = (n₁² (r)-n₂²)^1/2