Wavelength Division Multiplexing (WDM) is the prevalent solution for overcoming the bandwidth shortage problem in transmission facilities. With WDM, the capacity is increased by increasing the number of wavelength channels multiplexed over a single fiber. As the number of the wavelength channels grows, The channel spacing shrinks. This puts serious constraints on the absolute accuracy of the each individual wavelength channel. Ideally, the laser wavelength for each channel should be absolutely stabilized within a predetermined GHz range over its lifetime. However, the laser wavelength is prone to various instability, such as temperature sensitivity, acoustic motion sensitivity, bias current sensitivity and aging.
One prior art technique for stabilizing laser wavelength is to lock individual laser to a frequency discriminative device such as Fabry-Perot resonator or Bragg grating. Such a technique is described in the U.S. Pat. 5,706,301. A second approach is described in the article by T Mizuochi, et al., "622 Mbit/s-Sixteen-Channel FDM Coherent Optical Transmission System Using Two-Section MQW DFB-LDs", The transactions to the Institute of Electronics, Information and Communication Engineers of Jpa, B-I, Vol. J77-B-1. 5, pp.294-303, 1994. This approach uses a wide tuneable laser frequency which is swept, converting wavelength errors of each wavelength into time domain signal. Each individual pulse represents the difference of the laser wavelength and the center frequency of the Fabry-Perot resonator mode.
The first approach is undesirable because multiple resonators(filters) are needed which leads to a very expensive solution. In both of the prior approaches, the absolute accuracy of the laser wavelength is not guaranteed because the drift of the resonator (or filter) leads to the frequency drift of the laser. Although locking the resonator to an absolute optical frequency standard is suggested, an absolute frequency stabilized laser is still very expensive and not reliable.
What is needed is a cost effective and accurate technique for stabilizing the laser wavelengths used in a WDM system.