Aim of the Experiment
The aim of the experiment is to study the characteristics of an intensity modulation laser and fiber optics system:

Basic Principles and Definitions
The intensity modulation/demodulation system uses the Laser Diode Module and the Laser Detector Module linked through an optical fiber. We use the 2-meter PMMA fiber cable.

The laser carrier power, Po is set by adjusting the SET Po knob in the middle laser region. Selection of optimum carrier power is essential to minimise distortion. Limiting depth of modulation also ensures distortion free transmission. The bandwidth of the system of the present case is limited by the photodetector. We operate in the ACC mode only to obtain optical output proportional to the modulating signal Vin.

An ideal IM transmission system will have the relationship VO=GVin, where G is a factor dependent on the LD conversion efficiency, loss in the optical transmission path and the laser photodetector conversion efficiency. Distortion results from the LD being biased in the non-linear region. Bandwidth is limited by the slowest device in the system; in this case the phototransistor. Speed can be increased by using a PIN diode, which is inherently a faster device.