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# Coupling Loss Measurement in Fiber Optic System

## Aim

To Study the Coupling Loss Measurement in Fiber Optic System

## Coupling Loss

In fiber optic system face some losses in communication, which losses accrued from material, cable length, cable bend, couple the more no of fiber optic cable, splicing the fiber cable etc., in coupling loss, fiber coupler will act as a loss in that Fiber Optic System. Fiber coupler used for couple two fiber cable. During this coupling process some light signal was loss in that Fiber Optic System, which is known as coupling loss.

## Optical fiber

The transmission medium in fiber-optic communications systems is an optical fiber. The optical fiber is the transparent flexible filament that guides light from a transmitter to a receiver. An optical information signal entered at the transmitter end of a fiber - optic communications system is delivered to the receiver end by the optical fiber.

## Model Diagram for Plastic Fiber cable

This is multi mode plastic fiber cable, in this fiber cable core diameter 1000 micron and fiber cladding index 1.402.

## Procedure

• Switch (sw1) ON the transmitter Module, CRO and receiver module (sw1).
• Set the sine wave output (Vin) 1 Vpp/ 2 KHz at P1.
• Connect P1 and P8, P9 and P10 using patch chord.
• Connect the 1 m Plastic fiber cable between transmitter module LED to receiver module Photo Diode.
• Connect P2 and P4, P5 and P6 using patch chord in receiver module.
• Vary the Amplifier pot meter and set output 3 Vpp.
• Note down the analog output on test point P7.
• Now we get the output voltage (VO1).
• Replace the Plastic fiber cable 1m into 3m.
• Now note down the analog output on test point P7 and will get output voltage (VO2).
• Calculate the Propagation loss ‘α3m’ for 3m fiber in dB/m, α3m= -10 log (VO2/Vin)
• Calculate the Propagation loss ‘α1m’ for 1m fiber in dB/m, α1m = α3m/ (L2 - L1) L1 - length of 1 meter fiber cable L2 - length of 3 meter fiber cable ie, α1m = α3m/ (3 - 1)
• Connect one end of 3m fiber cable into transmitter module, connect one end of 1m fiber cable into receiver module, and insert free ends of 1m and 3m fiber cable into coupling setup as close as possible (see Figure 1).

Figure 1 Coupling Setup

• Now note down the analog output on test point P7 and will get output voltage (VO3).
• Calculate the Coupling loss ‘αc’ in dB/m,
αc = -10 log (V
O3/Vin) – (total cable loss)
Total cable loss = α1m (L
2 + L1)
α
1m(3 + 1)
Coupling loss αc = -10 log (V
O3/Vin) – ( (3 + 1))