Coherent light generators – Particular beam control device – Mode locking
Reexamination Certificate
1999-02-25
2001-05-29
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Mode locking
C372S032000
Reexamination Certificate
active
06240109
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to Wavelength Division Multiplexed (WDM) systems and, more particularly, to a method of and apparatus for wavelength stabilization of WDM channels.
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
In accordance with the method and apparatus of my invention, laser wavelengths used in a WDM system are stabilized using a known accurate electrical frequency standard. The laser wavelengths are stabilized using a first frequency locking circuit which individually locks each laser wavelength to a different resonant frequency of an optical interferometer and a second frequency locking circuit which locks a different resonant frequency of the optical interferometer to an accurate electrical frequency standard signal. The known accurate electrical frequency standard prevents the resonator from drifting and the resonator prevents the WDM system lasers from drifting.
More particularly, apparatus is disclosed for stabilizing lasers used in a wavelength division multiplexed (WDM) system comprising (1) a first frequency locking circuit for locking each of the WDM laser frequencies to a different resonant frequency of an optical interferometer, the optical interferometer having a free spectral range (FSR) which is a fraction of the separation between the WDM laser frequencies; and (2) a second frequency locking circuit for locking a different resonant frequency of the optical interferometer to an accurate electrical frequency standard signal.
According to one aspect of my invention, the first frequency locking circuit includes (1) dither means for modulating each WDM laser frequency using a unique electrical dither signal; (2) an optical multiplexer for multiplexing together each of the dithered WDM laser frequencies into a multiplexed signal; (3) the optical resonator receiving the multiplexed signal and generating an optical frequency offset error signal for each WDM laser frequency; (4) an optical detector for detecting the optical error signals for each laser and for generating a multiplexed signal including an electrical error signal for each laser frequency; and (5) a wavelength control circuit for comparing each electrical error signal with the electrical dither signal and in response thereto generating a separate feedback control signal for adjusting the frequency of each of the WDM lasers.
According to another aspect of my invention, the second frequency locking circuit includes (1) a first and second auxiliary laser frequencies signals; (2) a coupler for combining the first and the second auxiliary laser frequency signals; (3) an optical detector for detecting the first and the second auxiliary laser frequency signals and generating therefrom an electrical difference frequency signal; (4) a comparator circuit for comparing the accurate electrical frequency standard signal with the electrical difference frequency signal and for generating therefrom an error control signal for controlling the resonance frequencies of the optical resonator so as to minimize the error signal.
According to a feature of my invention, the optical interferometer may be selected from a group including a Fabry-Perot resonator and a Mach-Zehender interferometer. According to another feature, the electrical accurate clock standard signal is selected from a group including a stratum clock signal, a national frequency standard, and a global positioning system (GPS) distributed frequency standard.
REFERENCES:
patent: 4835782 (1989-05-01), Kaede et al.
patent: 5631758 (1997-05-01), Knox et al.
patent: 5706301 (1998-01-01), Lagerström
patent: 6111681 (2000-08-01), Mizrahi et al.
Caccuro John A.
Davie James W.
Lucent Technologies Inc
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