Responsivity and Noise of Self-Mixing Photodetection Schemes

Responsivity and noise properties of the self-mixing (SM) detection process are analyzed. Starting with the photodiode and SM laser scheme as a detector of weak optical echoes from remote targets, we find equivalence to a coherent homodyne detection of the returning signal. In particular, in the VIS-NIR wavelength range, echoes can be detected down to about -90 dB of relative amplitude, typically. Then we consider the laser-diode voltage self-mixing (LV-SM) detection and find it is also a coherent homodyne scheme, though noise performance is limited by shot-noise of the bias current and by the relatively small signal supplied as an output. Theoretical results are finally compared to recent experimental data, finding a substantial agreement and confirming that the LV-SM is an attractive alternative to conventional photo-detection, especially for THz waves and other spectral ranges where good low-noise detectors may be difficult to employ.