One of the major aspects to any fiber optic transmitter is its power level. It is obvious that the fiber optic transmitter should have a sufficiently high level of light output for the light to be transmitted along the fiber optic cable to the far end.
The type of light produced is also important. Light can be split into two categories, namely coherent and incoherent light.
Essentially, coherent light has a single frequency, whereas incoherent light contains a wide variety of light packets all containing different frequencies, i.e. there is no single frequency present.
This is in contrast to radio communication where term coherent refers to detection techniques in which the carrier is generated locally at the receiver for detection of signal.
Often fiber optic systems will operate around a given wavelength, so we go for coherent optical communication.
In optical fiber communication the term coherent refers to any technique which employs non linear mixing between two optical waves.
To send the information signal it can be modulated either by amplitude, frequency or phase of the optical carrier. Therefore modulation techniques used are ASK, FSK, PSK.
In the direct detection system the electrical signal coming into the transmitter, amplitude- modulates the optical power level of the light source.
Thus the optical power is proportional to signal current level.
At the receiver the incoming optical signal is converted directly into a demodulated electrical output.
Thus the directly detected current is proportional to the intensity of the optical signal.
At the receiver in coherent optical systems, the receiver first adds a locally generated optical wave to the incoming signal and then detects the combination.
There are four demodulation formats depending on how optical signal is mixed with local oscillator (which gives homodyne or heterodyne detection) and how electrical signal is detected (either synchronously or asynchronously).