Examples of spectral filters for tuning an oscillator
Tunable lasers

Spectral filters are first and foremost components whose spectral bandwidth enables the selection of one wavelength from all the emission possibilities offered by the amplifying medium. The spectral width of the filter's transmission peak is smaller than the range of maximum tunability Δν on the line under consideration.

Spectral filters can be passive, i.e. they can select a wavelength once and for all. They can be active, capable of shifting their transmission peak according to an external control parameter (displacement, rotation, temperature change, control voltage...). If the speed of transmission change is high (ms or even μs), we speak of frequency-agile filters.