Optical spectrum analyzers (OSA) can divide a lightwave signal into its constituent wavelengths. This means that it is possible to see the spectral profile of the signal over a certain wavelength range. The profile is graphically displayed, with wavelength on the horizontal axis and power on the vertical axis. In this way, the many signals combined on a single fiber in a dense wavelength division multiplexing (DWDM) system can be taken apart to perform per-channel analysis of the optical signal and its spectral interaction with the other wavelengths.

Applications for optical spectrum analyzers include testing devices lasers and LED light sources for spectral purity and power distribution, and testing other optical devices for transmission quality.  Optical spectrum analyzers are ideal instruments for laser modes analysis, very high-resolution spectroscopic measurements, telecommunication device and system tests and other applications.

There are three categories of optical spectrum analyzers.  These are diffraction grating based optical spectrum analyzers, Fabry-Perot interferometer based optical spectrum analyzers, and Michelson interferometer based optical spectrum analyzers.

Diffraction grating based optical spectrum analyzers are used for measuring the spectra of lasers and LEDs.  The resolution of these devices typically ranges from 0.1 nm to 10 nm.  These types of optical spectrum analyzers use monochromators with diffraction gratings as tunable optical filters.  The monochromator separates the different wavelengths of light, and allows only specific wavelengths to which the optical spectrum analyzer is tuned to reach its photodetector.  The result is better wavelength resolution.

Fabry-Perot interferometer based optical spectrum analyzers have a fixed, narrow resolution, typically in a specified frequency between 100 MHz and 10 GHz.  Due to their narrow resolution, they can be used for measuring laser chirp (an abrupt change of the center wavelength of a laser, caused by laser instability.), but the range they can cover is smaller than that of the diffraction grating style.  Due to their narrow resolution range, these devices may allow many wavelengths to pass through their filter at any one point, presenting an interference issue.  Placing a monochromator in cascade with the optical spectrum analyzer can lessen this effect.

Michelson interferometer based optical spectrum analyzers are used for direct coherence-length measurements as well as very accurate wavelength measurements.  Other types of optical spectrum analyzers cannot make direct coherence-length measurements.