Graph 9008-044 demonstrates a technique for reducing the
noise contaminating effects of finite laser linewidth. For the particular laser profile
shown (approx. 200 kHz), if the baseband modulation frequency is 0.5 MHz, the SNR will be
severely limited by the noise wings of the Lorentzian profile. In fact, for the system
modelled, the SNR cannot be greater than a few dBs. This occurs whatever form of
modulation is used, i.e, intensity, polarization or frequency modulation.
If sufficient power is transmitted so that the basic CNR at Earth is above the
threshold of an F.M receiver (approx. 10 dB) in a much larger bandwidth, then the
intelligence (data) to be transmitted can be placed on a higher frequency subcarrier,
e.g., 5 MHz. In this way it would be placed outside the region where Lorentzian noise from
the carrier (or heterodyned I.F.) is significant. If the data frequency modulates the
subcarrier, then the Lorentzian noise on the subcarrier and its data sidebands cancels out
when the F.M. subcarrier signal is demodulated. This is because the intensity noise on the
subcarrier is eliminated by the F.M. limiter, and linear superposition applies to phase or
frequency modulation, such that the phase or frequency jitter caused by the Lorentzian
noise on the subcarrier and its sidebands also cancels out. For this technique to work,
the noise in the subcarrier bandpass filter must not be so great as to drive that
subcarrier demodulator below its F.M. threshold.