Fiber Optic Cable Propagation Experiment

A fiber optic cable propagation experiment measures how light signals attenuate as they travel through optical fibers, allowing calculation of propagation loss, numerical aperture, and acceptance angle.

Objective

The main goal of the experiment is to study the propagation of light through optical fibers, measure signal attenuation over different fiber lengths, and understand the factors affecting signal quality, such as fiber type, length, and connector quality .

Apparatus Required

• Fiber optic light source (LED or laser)
• Optical power meter or detector
• Fiber optic cables of different lengths (e.g., 1 m and 5 m)
• Function generator (for analog or digital signals)
• Oscilloscope
• Amplifier (for signal adjustment)
• Connectors and termination tools for plastic optical fibers (POF)
• Numerical aperture measurement jig and screen

Experimental Setup

1. Connect the function generator to the optical transmitter input to provide a 1 kHz sine wave or square wave signal .
2. Connect a short fiber optic cable (e.g., 1 m) between the transmitter and the detector.
3. Connect the detector output to an amplifier and then to an oscilloscope to observe the received signal .
4. Adjust the amplitude of the transmitted signal and record the received amplitude as V1.
5. Replace the fiber with a longer cable (e.g., 5 m) without changing other settings and record the new received amplitude as V2 .

Procedure

• Ensure proper termination of fibers using ST connectors with room-temperature epoxy and mechanical crimping to avoid signal loss .
• Measure the amplitude of the received signal for each fiber length.
• Calculate the propagation (attenuation) loss using the formula: $$\alpha =-\frac{\log (V1/V2)}{L1+L2}$$ where $L1$ and $L2$ are the lengths of the fibers used .
• Optionally, measure the numerical aperture (NA) and acceptance angle by projecting the light spot onto a screen and measuring the spot diameter and distance from the fiber end .

Theory

• Light propagates through the fiber via total internal reflection within the core.
• Each mode travels at a slightly different velocity, which can cause signal distortion over long distances .
• Attenuation occurs due to absorption, scattering, and connector losses.
• The numerical aperture defines the light-gathering ability of the fiber and is calculated from the acceptance angle and core-cladding refractive indices .

Observations and Calculations

• Record the transmitted and received signal amplitudes for each fiber length.
• Calculate attenuation in dB/km, numerical aperture, and acceptance angle.
• Compare results for different fiber lengths to understand how propagation loss increases with distance . This experiment provides hands-on understanding of fiber optic signal transmission, the impact of fiber length on signal strength, and the importance of proper fiber termination and alignment in optical communication systems .