2.12. Homodyne Detection

A variant of heterodyne detection is homodyne detection, where the local oscillator frequency equals the signal frequency. For optical homodyne measurements, both waves are virtually always derived from the same laser source. Optical homodyne detection improves the receiver sensitivity and eases the receiver bandwidth requirement. The homodyne technique is phase-sensitive in the sense that the power of the heterodyne signal depends on the relative phase of signal and local oscillator, and may even totally vanish. A useful modification is balanced homodyne detection, where two photodiodes are used after a beam splitter with precisely 50% reflectivity, and the sum and difference of photocurrents are electronically obtained. If the signal port is blocked, the difference of photocurrents exhibits the shot noise level of the local oscillator beam, even if the actual noise level of that beam is different. The latter noise level can be obtained in addition by taking the sum of the photocurrents. That sum exhibits the same noise as for direct detection of the local oscillator beam with a single photodiode. The homodyne setup provides the difference signal in addition, which allows one to conveniently compare the actual noise with the shot noise limit. Hence in the optical homodyne detection the optical phase difference b/w the incoming signal and the local oscillator signal is detected by the photo detector prior to being fed back to correct the local oscillator frequency and phase. An optical PLL configuration for homodyne detection is shown above. A photo detector (used as the phase detector), a laser LO and a VCO are used. The carrier signal together with the incoming signal is combined in a 3dB fiber directional coupler and then detected using a balanced receiver. The o/p signal from the difference amplifier is a function of the phase error which is used for phase locking through the loop filter to the optical local oscillator which works as the VCO. Thus the optical phase difference b/w the incoming signal and the local oscillator signal is detected by the photo detector prior to being fed back to correct the local oscillator frequency and phase.