Definition: technique for laser linewidth measurements, based on beat note between the beam and a delayed version of itself

Fig.1: Setup for self-heterodyne measurement of a laser linewidth.

The self-heterodyne method (Ref. Okoshi 1980) is an interferometric technique which can be used to measure the linewidth of a laser, particularly a single-frequency laser . The basic setup is shown in the figure below. One portion of the laser beam is sent through a long optical fiber which provides some time delay, while another portion is sent through an acousto-optic modulator (AOM), which shifts all the frequency components by some tens of megahertz. Both beams are finally superimposed on a beamsplitter, and the resulting beat note (centered at the AOM frequency) is recorded with a photodetector (typically a photodiode ). From this signal, one may then calculate the laser linewidth.

For long enough delays, the superimposed beams are essentially uncorrelated, and the output spectrum becomes a simple self-convolution of the laser output spectrum, from which the laser linewidth is easily retrieved. However, what "long enough" means in a concrete case, can be a difficult question. The usually used criterion is that the delay length must be larger than the coherence length , and the latter is derived from the (measured) linewidth itself. This is correct for white frequency noise, but not in situations with higher (e.g. 1/f) low-frequency noise (Ref. Horak 2006), as can occur e.g. for narrow-linewidth fiber lasers . If such issues are not completely understood, the measured linewidth values may be wrong (often too low).

For lasers with a very narrow linewidth (long coherence length), it may not be practical to have a long enough time delay to obtain uncorrelated beams. In that case, one may also work with shorter delays, but the data processing then has to be substantially more sophisticated. Another possibility is to use an extension of this method, based on recirculating fiber loops (Ref. Tsuchida 1990).