Variation of linewidth with tuning in external cavity semiconductor lasers

A linewidth of a few tens of kHz and stable single-mode operation can be achieved by coupling a semiconductor laser, with one of its facets anti-reflection coated, to a diffraction grating which then provides strong, frequency-selective feedback into the laser. In such a configuration the lasing wavelength can be tuned (discontinuously) over a wide range by rotation of the grating. In addition, the lasing wavelength may also be tuned continuously, over a smaller range, by the rotation of a transparent optical flat placed within the external cavity. The authors have carried out a systematic investigation of the variation of linewidth as a function of both types of tuning for such an external cavity semiconductor laser (ECSL) and report the two main new results obtained. The first is the rebroadening of the output linewidth when the ECSL is detuned, towards shorter wavelengths, well away from the laser gain peak and the second is a periodic modulation of the linewidth when the output wavelength is fine tuned using the intercavity tuning plate