There are two types of transponders
- non coherent transponders
- coherent transponders
non coherent transponders:
These transponders involve IM/DD (Intensity Modulation/Direct Detection) technique also known as OOK method for transmission of signal. In IM/DD the intensity, or power, of the light beam from a laser or a light-emitting diode (LED) is modulated by the information bits and no phase information is needed. Due to this nature, no local oscillator is required for IM/DD communication, which greatly eases the cost of the hardware.
coherent transponders:
The basic idea behind coherent detection consists of combining the optical signal coherently with a continuous-wave (CW) optical field before it falls on the photodetector. The CW field is generated locally at the receiver using a narrow line width laser, called the local oscillator (LO). With the mixing of the received optical signal with the LO output can improve the receiver performance.