What is optical spectrum?

 

The optical spectrum (or emission spectrum) of a light source or some beam contains information on how the optical energy or power is distributed over different wavelengths. Usually, it is presented in the form of a diagram where some spectral quantity is plotted as a function of the wavelength or optical frequency.

Optical spectrum analyzers (OSA) are precision instruments which are used for measuring optical spectra, based on which a further analysis is often possible. Some typical applications are:

characterization of light sources such as lasers and light emitting diodes (LEDs)

• testing of optical systems, for example wavelength division multiplexing systems in optical fiber communications networks, where one may need to test the optical powers of the different wavelength channels and measure signal-to-noise ratios.
• measurements of the wavelength-dependent transmissivity or reflectivity of optical systems or devices by comparing spectra with and without the device
• characterization of fiber amplifiers (e.g. telecom erbium-doped fiber amplifiers) in terms of wavelength-dependent gain and noise figure

Specialized and All-purpose Spectrum Analyzers

Some spectrum analyzers are specialized for certain applications:

• There are devices for use in optical communications, which are restricted to one or more telecom wavelength bands (e.g. the C band, or the full range from 1250 to 1700 nm).
• Some high-resolution spectrum analyzers are made for specific applications in spectroscopy, for example in Raman spectroscopy and fluorescence spectroscopy. Some of them work far in the infrared spectral region.
• Other instruments are made for application mainly with visible light, for example spanning the wavelength range from 400 to 1000 nm.

Other spectrum analyzers are versatile laboratory instruments; such equipment can offer many features:

• They can cover wide wavelength ranges, possibly using different photodetectors.
• They often offer a variable wavelength resolution (RBW = resolution bandwidth), e.g. between 0.1 nm and 5 nm.
• The sweep rate can automatically be determined based on the width of the scanned wavelength range, the resolution and possibly an additional sensitivity setting.
• A spectrum analyzer may either take single wavelength sweeps, e.g. every time when receiving some external trigger signal, or continuously take sweeps, updating the display regularly (e.g. with a rolling average).
• The operation parameters of an analyzer are set with controls on a front panel. There may be the option to save device settings in order to retrieve them with a single button.
• There is normally a built-in digital display, where different display modes can be chosen – for example, a linear or logarithmic scale for the power spectral density (see below).
• An interface of an OSA to a computer, e.g. via a USB, GPIB or network connection, can also be useful. One can transfer your the measured spectra to a computer in order to store, process and display them there. Also, one may automate measurements through a computer which may also control other devices.