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Definition: devices for precise measurements of laser wavelengths

Categories: optical metrology, photonic devices

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A wavemeter (or wavelength meter) is a kind of interferometer which is used for precise wavelength measurements on laser beams. There are different variants, including scanning wavemeters and static devices with no moving parts.

Scanning Michelson Interferometer

scanning Michelson wavemeter
Figure 1: Scanning Michelson wavemeter.

An often used type of wavemeter is based on a Michelson interferometer. Figure 1 shows the principle setup. Light from the optical source under test is sent into the Michelson interferometer, while the length of one of interferometer arms is scanned through a certain range. The period of the output power recorded by the photodetector with respect to the arm length changes then reflects the wavelength. The control of the measurement process and the data analysis are usually accomplished with a microprocessor device.

The principle of such a wavemeter can be extended for the measurement of the optical spectrum of non-monochromatic sources (→ spectrometers). Essentially, the spectrum is obtained by applying a Fourier transform to the detected power versus arm length difference. This method is called Fourier transform spectroscopy.

Various types of errors can affect the measurement accuracy:

Depending on the quality of the device, the achieved wavelength accuracy can be 0.01 nm

Static Fizeau Interferometer

static Fizeau wavemeter
Figure 2: Static wavemeter based on a Fizeau interferometer.

A Fizeau interferometer (Figure 2) uses two plane reflecting surfaces with a slight deviation from exact parallelity. For example, it can contain a glass wedge with a small angular mismatch of a few arcseconds, for example, where the front surface is partially reflecting and the back surface is fully reflecting. There are also implementations with discrete mirrors.

Effectively, two copies of the input beam are superimposed with a slight relative angle, leading to an interference pattern, the period of which depends on the wavelength. Usually, the input beam first gets through some spatial filter, and a collimated beam with large diameter is sent onto the Fizeau interferometer.

The shape of the interference pattern is measured with a CCD array, for example, and the data are processed using a microprocessor.

Aspects for Selecting a Wavemeter

Wavemeters of different types can differ in many respects:

Alternative Measurement Techniques

Other types of wavemeters can be based on Fabry–Pérot interferometers, for example.

The accuracy of wavelength measurements is affected by a number of factors such as wavefront distortions of the beam. A much higher precision can be achieved with optical frequency measurements (→ frequency metrology), which are immune to such effects.

Using a wavemeter is usually more precise than measuring a wavelength with a spectrometer. The advantage of a spectrometer, however, is that it also delivers information on relative powers of different spectral components. There are also wavemeters which can act as spectrometers, thus offering both functions with high accuracy.


[1]M. B. Morris et al., “Fizeau wavemeter for pulsed laser wavelength measurement”, Appl. Opt. 23 (21), 3862 (1984)
[2]R. Castell et al., “The accuracy of laser wavelength meters”, Appl. Phys. B 38 (1), 1 (1985)
[3]D. F. Gray et al., “Simple compact Fizeau wavemeter”, Appl. Opt. 25 (8), 1339 (1986)
[4]C. Reiser and R. B. Lopert, “Laser wavemeter with solid Fizeau wedge interferometer”, Appl. Opt. 27 (17), 3656 (1988)
[5]P. J. Fox et al., “A reliable, compact, and low-cost Michelson wavemeter for laser wavelength measurement”, Am. J. Phys. 67 (7), 624 (1999)
[6]J. J. Snyder and T. W. Hänsch, “Laser Wavemeters”, in Topics in Applied Physics: Dye Lasers (ed. F. P. Schäfer), Springer, Berlin (1973)

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See also: interferometers, spectrometers, frequency metrology, spectroscopy
and other articles in the categories optical metrology, photonic devices

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