Laser wavelength and Spectral width

The measurement of spectral width (also referred to as line width) is important because the pulse broadening due to chromatic dispersion is proportional to the spectral width.

In a single channel optical transmission, an accurate measurement of the wavelength of laser is not that crucial as the fiber loss is a slowly varying function of the wavelength. However, in a WDM system, the precise measurement of the laser wavelength is necessary.

In principle, the wavelength and the spectral width can be measured by passing the light through a tunable filter and measuring the output light intensity as function of wavelength. This not very easy.

Generally, a system called the Optical Spectrum Analyzer (OSA) is used to measure the spectral characteristics of the light. The OSA displays the optical power as a function of wavelength. OSAs with spectral resolution of 0.001nm and power density of -70dBm are readily available.