Self-heterodyne Linewidth Measurement

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

The self-heterodyne method [1] is a heterodyne technique which can be used to measure the linewidth (width of the optical spectrum) 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 is driven with a constant frequency (typically some tens of megahertz) and shifts all the optical frequency components by that frequency. 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.

For sufficiently long delays, the superimposed beams are essentially uncorrelated, and the output spectrum becomes a simple self-convolution of the laser output spectrum [1], 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 [5], 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).

The delay time in the self-heterodyne setup can be interpreted as a finite measurement time, which implies that the impact of noise frequencies below that inverse measurement time is low [2]. A consequence of that is that measurements with shorter delay time yield smaller linewidth values if the instantaneous frequency fluctuations exhibit substantial <$1 / f$> noise, as is the case for many laser diodes.

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 [3].

More to Learn

Encyclopedia articles:

• linewidth
• coherence length
• optical heterodyne detection
• interferometers
• recirculating fiber loops

Bibliography

(Suggest additional literature!)

Suppliers

The RP Photonics Buyer's Guide contains 16 suppliers for linewidth measurement equipment. Among them:

TOPTICA Photonics

The HighFinesse Linewidth Analyzers (LWAs) are very versatile, robust and compact instruments for measuring, analyzing and controlling frequency and intensity noise of lasers. This makes them ideal equipment for exact laser characterization in real time.

The HighFinesse wavelength meters are the unsurpassed high-end instruments for wave-length measurement of pulsed or continuous laser sources. They deliver the superb absolute and relative accuracy required by cutting edge scientific research, as well as industrial and medical applications.

HighFinesse

HighFinesse linewidth analyzers are the ultimate high-end instruments for measuring, analyzing and controlling frequency, linewidth and intensity noise of laser light sources. The product series covers a measurement range from 450 nm up to 1625 nm and enables the analysis of both very narrow laser lines down to 1 kHz as well as broader spectra up to 100 MHz. Thanks to the included software, the instrument delivers real-time analysis, with no need for further calculation. The analyzers feature an extremely high resolution and accuracy and are ideal for optimizing the stability of laser setups.

Questions and Comments from Users