In recent times, single frequency lasers with narrow bandwidth have long coherence length and this is an essential property for many applications in industry and science. One of the applications is remote laser sensing and in particularly laser guide star, where high power narrow band width laser at 589nm is required for excitation of sodium atoms. Another important application is coherent beam combining , where the power is scaled up by combining several lasers with the diffraction grating.
The narrow bandwidth (or line width) of a laser, mostly a single-frequency laser, is the width (sometimes used as narrow FWHM) of its optical spectrum. The linewidth of a single frequency laser is the full width at half maximum (FWHM) of the optical spectrum. More precisely, it is the width of the power spectral density of the emitted electric field in terms of frequency, wavenumber or wavelength. The narrow FWHM of a laser is closely related to the temporal coherence. A light field is called coherent when there is a fixed phase relationship between the electric field values at different locations or at different times.It is usually described in terms of the coherence time or coherence length. Phase fluctuations which are restricted to a small interval of phase values provoke a zero linewidth and some noise sidebands. This drifts can also promote to the linewidth and can make it dependent on the measurement time.
This shows that just the narrow band width doesn`t give full information on the spectral purity of laser light. One of the reasons of the noise is the negligent emission of excited atoms and ions. The unpredictable photons have random directions and random phases. Each negligent emission adds a random phase to the optical field. Some radiation arises by the spontaneous emission will amplify very nearly along the same direction as that of the stimulated emission and cannot be separated from it. The main consequence of the spontaneous emission noise is to make the laser output have a finite spectral width.
The spectral bandwidth (linewidth) of single-frequency lasers can be not only broad as several hundreds of megahertz but also narrow as several hundreds of hertz. Typical bandwidths of stable free-running single-frequency lasers are a few kilohertz range, while the linewidths of semiconductor lasers are often in the megahertz range. Much smaller linewidths, sometimes even below 1 Hz, can be reached by stabilization of lasers, which can be achieved using ultrastable reference cavity. Lasers with very narrow linewidth are required for various applications, e.g. as light sources for various kinds of fiber optic sensors, for spectroscopy (e.g. LIDAR), in coherent optical fiber communications, and for atomic trapping and cooling.
