The power available from a single semiconductor laser is limited. In many applications, power levels in excess of one watt are required. For example, communication applications at wavelengths of about 1.5 micrometers (μm) can require more power than the power that can be provided by a single laser diode.
In principle, the light from a number of laser diodes of the same type can be combined to provide a light source with an output power level greater than the power level that can be obtained with a single laser. Unfortunately, the individual laser diodes emit light at slightly different wavelengths, and hence, the resulting light source has an emission spectrum that is too broad for many applications.
To overcome this problem, some form of servo loop could, in principle, be utilized to tune each of the individual laser diodes to the desired frequency. In such a system, a portion of the output laser light from the laser diode to be regulated is directed into a cell that has a gas with an atomic or molecular absorption line at the desired wavelength. Such gas will be referred to as absorption gas. There will be a maximum in the absorption level when the wavelength of the laser is at the center of the wavelength of the absorption line. In some special methods, e.g., saturation spectroscopy, there will a local minimum in the absorption level when the wavelength of the laser is at the center of the wavelength of the absorption line. The output wavelength of the laser is continuously adjusted to maintain the absorption at the desired level.
The output wavelength of the individual laser diodes can be controlled by varying the current through the laser diode or by controlling the temperature of the laser diode. A servo loop based on the absorption of the laser light by a gas cell can be used to servo the temperature and/or current through the laser. For example, hydrogen cyanide and acetylene have molecular absorption bands at wavelengths suitable for use in a feedback loop to lock the lasers at wavelengths around 1.5 μm. In this type of system, the laser wavelength is adjusted to maintain the absorption of the laser light in the cell at a predetermined level.
Unfortunately, the wavelength at which the cell has an absorption maximum depends on a number of factors in addition to the type of absorption gas in the cell. The location of the absorption maximum depends on the pressure, temperature, and electric and magnetic fields on the cell, and impurities in the absorption gas within the cell. Since these additional factors can vary from cell to cell, this type of servo mechanism presents problems when multiple cells are used to control multiple lasers.
In addition, servo mechanisms in which the laser wavelength is dithered to detect the location of the wavelength relative to the absorption maximum further increase the effective line width of the laser source. In such systems, the output wavelength is intentionally changed back and forth between two different wavelengths and the absorption measured at each wavelength to determine if the laser is currently adjusted to the correct wavelength. This constant dithering broadens the output spectrum even when the laser is correctly adjusted. To avoid this spectrum broadening, the frequency dithering can be implemented using a frequency modulator or a phase modulator. However, this approach increases the cost of the servo loop.
Furthermore, the servo mechanism must be insensitive to fluctuations in the power output of the laser and fluctuations in the polarization of the laser output. The portion of the laser output light that is delivered to the gas cell is typically provided on an optical fiber. If the fiber moves, the polarization of the light delivered to the cell changes.