Self-heterodyne Linewidth Measurement | previous | next | feedback |
Definition: a technique for laser linewidth measurements, based on a beat note between the beam and a delayed version of itself
The self-heterodyne method [1] is a heterodyne technique which can be used to measure the linewidth of a laser, particularly a single-frequency laser. The basic setup is shown in Figure 1. One portion of the laser beam is sent through a long optical fiber which provides some time delay. Another portion is sent through an acousto-optic modulator (AOM), which shifts all the optical frequency components by some tens of megahertz. Both beams are finally superimposed on a beam splitter, and the resulting beat note (centered at the AOM frequency) is recorded with a photodetector (typically a photodiode). From this signal, the laser linewidth can then be calculated.
Figure 1: Setup for self-heterodyne measurement of a laser linewidth.
For sufficiently long 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 “sufficiently long” means in a specific case, can be a difficult question. The criterion which is usually used 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 [3], 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, shorter delays can be used, 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 [2].
Bibliography
| [1] | T. Okoshi et al., “Novel method for high resolution measurement of laser output spectrum”, Electron. Lett. 16, 630 (1980) |
| [2] | H. Tsuchida, “Simple technique for improving the resolution of the delayed self-heterodyne method”, Opt. Lett. 15 (11), 640 (1990) |
| [3] | P. Horak and W. H. Loh, “On the delayed self-heterodyne interferometric technique for determining the linewidth of fiber lasers”, Opt. Express 14 (9), 3923 (2006) |
See also: linewidth, coherence length, optical heterodyne detection, interferometers, recirculating fiber loops, lasers, single-frequency lasers, narrow-linewidth lasers
Categories: fluctuations and noise, methods, metrology
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
You may order the print version via Wiley-VCH.
