selfheterodyne

Self-heterodyne is a technique in which a signal is mixed with a delayed version of itself to produce a beat signal. It is used in both optical and radio frequency contexts to analyze spectra, measure linewidths, or implement downconversion without a separate local oscillator.

In optics, the conventional arrangement splits a coherent light source, such as a laser, with a beam splitter. One arm passes through a long delay line (often an optical fiber) before the two arms are recombined on a fast photodetector. The detector output contains a beat note corresponding to the optical frequency or a shifted frequency if phase or intensity modulation is applied. If the delay exceeds the source’s coherence time, the resulting beat spectrum broadens, and the measured width reflects the source’s spectral profile. A phase modulator may be used to shift the carrier away from direct current to improve measurement sensitivity. Delayed self-heterodyne is widely employed to determine laser linewidth and to characterize spectral purity.

In RF and microwave systems, self-heterodyne can refer to downconversion using a local oscillator derived from the received signal itself, typically by splitting and delaying the input and mixing it with the original signal. This approach can simplify hardware and support wideband operation, but it imposes requirements on phase stability and can introduce complexity in handling phase noise and aliasing.

Overall, self-heterodyne provides a compact method to access beat frequencies and spectral information without traditionally separate LO paths, at the cost of requiring careful control of delay lines and phase characteristics.