FSK Heterodyne Detection
FSK transmission systems are more promising than ASK or PSK. Optical FSK transmission and heterodyne detection experiments at 560-Mbit/s and 1-Gbit/s are conducted at a wavelength of 1497 nm. Receiver sensitivities of -39 dBm at 560 Mbit/s and -37 dBm at 1 Gbit/s are obtained.
Heterodyne Detection for FSK:
- By considering FSK heterodyne synchronous detection in the shot or quantum noise limit.
- The two angular frequencies for the transmitted 1 and 0 bits are assumed to be ω1 and ω2 so that
Where ISH is defined as
- And φ, which is a function of time, represents the phase noise associated with the semiconductor laser.
- This is neglected because we are concerned with shot-noise-limited detection
- It is assumed that the signal IS(t) is received using two receivers tuned to ω1 and ω2 and that the output voltages from receivers 1 and 2 are V1 and V2 respectively.
- It is assumed that the two receivers exhibit ideal frequency selectivity such that there is no crosstalk between ω1 and ω2 and therefore any additional voltages are generated by shot noise effects only.
- It is possible to just consider the time slot when a 1 bit (ω1) is transmitted without losing generality.
- We assume the local oscillator output power to be much higher than that of the incoming signal so that the total noise current is approximately equal.
- The noise output from receiver 2 can therefore be written as:
- As an error occurs when V2 > V1, then the probability of error P (e) is equivalent to the probability that V1 − V2 < 0. Hence:
- Changing the limits of the integration gives
- Comparison with the definition for the complementary error function allows Eq. (6) to be written as