What is a Wavemeter?

A wavemeter is a device used in fiber optics to measure the wavelength of laser beams. Also called a wavelength meter, a wavemeter tests the optical elements of products as they are being manufactured. Wavemeters are used when extremely precise measurements are necessary.

Wavemeters are interferometers used only for measuring wavelengths. Interferometers measure light waves by using interference, usually provided by mirrors, to split a light beam in two and then recombine it. By studying the resulting shape, measurements of the beam can be taken.

There are two main types of wavemeters: scanning and static. The first type is usually based on the Scanning Michelson Interferometer. These types of wavemeters consist of three mirrors, a light source, and a detector. Resembling a cross-shape, each piece of the Michelson Interferometer is located on one corner of the cross. The light source is positioned opposite one of the mirrors, and a second mirror and the detector are placed perpendicular to them. The third mirror, half-silvered, is positioned at an angle in the center. This is the mirror that actually splits the beam.

When the light source is activated, the laser will hit the half-silvered mirror and split, one half of the beam going straight through, and the other half moving perpendicular to the first. Both halves will hit the positioned mirrors behind and above the center mirror, rebound, and be recombined by the half-silvered mirror. The recombined beam will then move straight down into the detector, which will analyze the results.

The distance between the mirrors, or arms, can be changed to scan a range of lengths. The accuracy of these wavemeters can be as great as 0.01 nm. Several issues, however, such as imperfections in the beam, length drifts, and fluctuations in input power, could give less accurate results.

Most static wavemeters are based on the Static Fizeau Interferometer. These wavemeters do not have moving parts, but use the same principle of mirrored reflection. Less common static wavemeters are based on Fabry-Pérot interferometers, which are linear. These are more often used as optical spectrometers than wavemeters, however.

High accuracy for any type of wavemeter depends on the stability of the setup and the resolution of the display. A reference laser, whose wavelength is known, working in tandem with the laser being tested will also help to increase accuracy. In cases where extreme accuracy is vital, calibrating the machine as much as once a minute may be necessary.

Wavelengths can also be measured with spectrometers, but although spectrometers give more information about the components of the light beam, precision is sacrificed. Some types of wavemeters can also function as spectrometers, thus giving additional information without sacrificing accuracy.