To determine the losses in optical fibers in dB due to macro bending of the fiber
Step index F.O. Cable of lengths (a) 1 m (b) 2 m (c) 3 m (d) 4 m (e) 5 m; digital multimeter, Adaptors, D.C. power supply, Fiber optic trainer module, mandrel etc.
The schematic diagram of the fiber optics trainer module is shown in figure1.
The circuit comprises of three parts.
The E/O converter converts an input voltage to an optical output, Po, by driving the fiber optic light emitting diode (Fo – LED) current linearly using a negative feedback operational amplifier circuit. Direct current LED (DC – LED) current setting is done by rotating the knob SET / Po. The optical power is coupled to the optical fiber through the connector. The LED current can be measured by monitoring the voltage with a digital multi-meter. A.C. input is given to the Vin (Vr / 100) given the LED current in milli amperes.
The optical to electrical converter accepts the input optical power (Pin) from the optical fiber connected through the connector and provides an output voltage (Vo). For D.C. measurements, a multi-meter may be used. For A.C. measurements are made.
The optical power meter converts the optical power coupled to it through an SMA terminated optical fiber and facilitates display of the optical power Po in Db. The optical power in dB, is given by the (multi-meter reading / 10) in dB referred to 1 mw.
As in the case of any transmission medium, an optical fiber also suffers from transmission loss as signal propagates through it. Losses in an optical fiber is a result of a number of effects, like fiber to fiber joints, splicing due to axial displacement of fibers, angular displacement of fibers, mismatch of core diameters of fibers, mismatch of N.A.s, improper cleaving and cleaning at the ends of the fibers, macro and micro bending losses, connector losses etc. However, in the present experiment, we confine to the study of the attention in a fiber due to macro bending of the fiber and the adaptors connecting the two fiber patch cords.
If ‘Po’ represents the power launched at the input of a fiber of length ‘L’, then the power at the output end of the fiber is given by Bouger’s Law.
P L= P 0 e -αL-----------------------------(1)
Where α is the attenuation coefficient in nepers pre unit length , in practice the attenuation is expressed in dB / km and is defined as
???? = 10 ???????????? 10 [P 0/P L]/-----------------------------(2)
P L = P 0 10e[-α /10]/-----------------------------(3)
The typical attenuation coefficient value for a step index fiber used in the experiments is 0.3 dB per meter for light at a wave length of 660 mm.
Loss in fibers expressed in decibels is given by:
???????????????? = − 10/L Log{P 0/P F}-----------------------------(4)
Where ‘P 0= Power launched
‘PF’ = Power at the far end of the fiber
The loss at each connector junction may vary from 0.3 dB to 0.8 dB.
The above two losses are to be determined in the present experiment.
The schematic diagram of the optical fiber loss measurement is shown in figure 2. One end of the one meter fiber optic cable is connected to the ‘P 0’ and the other end is connected to the ‘P inof the loss measurement module. The output power is calibrated in terms of the milli-volts. Hence, the two wires of the power output is connected to the digital multi-meter (DMM). The DMM is set into 2000 mv range. The input of the device is now connected to the A.C. mains. The SET P 0 knob is set to a suitable value, say -15 dBm (the DMM is calibrated to read 150 mv). This value is noted as P 01
To determine the bending losses
| S.No. | Output power without bending PF1 dB | Output power with bending PF2 dB | Loss= P F1- P F2 |
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| 2 | |||
| 3 | |||
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| 6 |
The transmission loss in the given optical fiber cable is ………dB.
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