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Reduction Of Laser Linewidth Noise



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.





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