The signal to noise ratio in optical heterodyne receivers may be improved by using balanced receivers that allow subtraction of the intensity noise while doubling the heterodyne signal. Intensity noise, as described in this application, includes both the traditional intensity noise resulting from the beating between the spectral component of the local oscillator (LO) or the measured signal, and the purposely induced intensity modulation of the optical signal as means for the transmission of information. In the receiver of the heterodyne optical spectrum analyzer the measured quantity is the optical spectrum contained within the heterodyne beat signal. Therefore, the heterodyne beat signal has to be distinguished from the intensity noise, which may appear in the same frequency range.
FIG. 1 shows balanced receiver 101 having two photodiodes 105 and 110 which are used at the outputs of coupler/combiner 115. The intensity noise of the laser under test (not shown) is split and appears at both photodiodes 105 and 110 with the same phase. The intensity noise can be a characteristic of the laser under test (random modulation resulting from the beating of the spectral components) or the result of intensity modulation (induced modulation that achieves the transmission of information). Similarly, the intensity noise (random modulation) of the local oscillator is split and appears at both photodiodes 105 and 110 with the same phase. The heterodyne signal resulting from the mixing of the laser under test and the local oscillator (not shown) at the frequency difference ωo−ωs. Because of the phase shift experienced by the cross-coupled waves in coupler/combiner 115 the heterodyne signals incident on detectors 105 and 110 are out of phase. Information about the spectrum is contained in the heterodyne signal so it is advantageous to suppress the intensity noise terms. This may be accomplished by subtracting the signal received by photodiode 105 from the signal received by photodiode 110.
Ideally, subtraction cancels the intensity noise terms while doubling the heterodyne signal which has opposite phase at photodiodes 105 and 110. In practice however, complete cancellation of noise does not take place because the splitting ratio of coupler/combiner 115 is not fifty-fifty, the responses of photodiodes 105 and 110 are not identical along with other aspects related to the two signal paths. Furthermore, the electrical signals from photodiodes 105 and 110 are amplified by preamps 130 and 135 whose amplitude and phase response is not identical. Therefore, it is desirable to condition the electrical signals from preamps 130 and 135 to compensate for the path differences and enhance the effect of the intensity noise subtraction. Additionally, it is desirable to compensate for any drift in the system due to wavelength, polarization state and other changes.