The present invention relates to external cavity lasers.
An external cavity laser comprises a cavity that provides an optical path so that a laser beam, stimulated by a gain or active medium, can resonate. The optical spectrum of an external cavity laser is limited by the spectral region over which the gain medium yields optical gain. Depending on the geometry of the cavity, the laser radiation has certain allowed spectral modes, also referred to as longitudinal modes. The spectral modes correspond to different distinct wavelengths or frequencies of the light. The allowed radiation spectrum in a laser cavity thus is a discrete distribution of modal frequencies limited by the gain curve. Examples of external cavity lasers can be found in U.S. Pat. No. 5,331,651, EP-A-0 552 394 and in EP-A-1 202 409.
U.S. Pat. No. 5,331,651 discloses a tunable Fabry Perot etalon to be used in an external cavity laser comprising a semiconductor laser chip and adjustable diffraction grating. The adjustable diffraction grating is used for a coarse measurement of the laser output wavelength and the etalon is used for the fine adjustment.