Laser control arrangement for tuning a laser

Abstract: In a laser control arrangement for tuning a laser a portion of the optical signal exiting a resonator is directed to the input of an optical frequency routing device. The routing device has a plurality of outputs each for receiving one of the frequencies supplied by the laser. Each output is coupled to a photodetector. When the laser emits a frequency of light corresponding to one of the resonant frequencies of the resonator, the resonant frequency will appear on one of the outputs of the routing device and will be detected by the photodetector coupled to that output.

Description: Technical Field This invention relates to optical communications systems. More particularly, this invention relates to a control arrangement for tuning a laser used in optical communications systems. Background of the Invention There are known arrangements for tuning a laser to operate at selected frequencies throughout a wide range of frequencies. Frequency-tunable semiconductor lasers are attractive devices for optical frequency division multiplex transmission systems. In a tunable arrangement, a distributed Bragg reflection laser provides a large tuning range when different values of injection or bias current are injected into the phase control and the distributed Bragg reflector regions of the laser. A known laser control arrangement is disclosed, for example, in U.S. Patent No. 4,914,666. This arrangement includes a laser tunable in response to different values of a control signal. A digital processor determines control signal values which produce different operating frequencies. The exact operating frequencies are defined by the resonances of a Fabry-Perot resonator. The control signal values are stored in the digital processor for ready retrieval. During operation, one of the control signal values, representing a desired operating frequency, is retrieved from storage and is applied to the single frequency laser to lock the laser at one of the resonances of the Fabry-Perot resonator. If the laser characteristic curve (i.e., laser frequency as a function of bias current) has drifted, a control signal error is determined by a control circuit part of the feedback loop and is added to the retrieved control signal value. The digital processor measures the value of the control signal error and derives a new total control signal value that is stored in place of the originally retrieved control signal value. One limitation of the laser control arrangement described above is that there is no provision for determining the Fabry-Perot resonant frequency to which the laser is being locked without using an external measuring device such as an optical frequency meter. JP-A-63 179587 discloses an oscillation frequency controller for a laser device. Not only a high finesse optical resonator but a low finesse optical resonator are simultaneously used as a frequency discriminator, the high finesse resonator stabilizes the frequency, and the low finesse resonator selects the frequency. Accordingly, the longitudinal mode of the high finesse resonator which cannot be discriminated solely by the high finesse resonator can be discriminated, and a laser device oscillation frequency is switched to be stabilized as one of the frequencies for providing the peak of the longitudinal mode of the high finesse resonator. The switching and the stabilizing of the frequency of a laser device emitting light can be simultaneously performed easiy by the combination of the two resonators. Roger C. Steele, in 'Stabilisation of lasers for FDM systems', IEE Colloquium on Sources for Coherent Optical Communication (Digest no 013), 18th January 1991, IEE, London, pages 13/1 - 13/6, discloses a system based on simple etalons made from commercial fibre connectors. These etalons form a passive optical comb onto which each channel can lock. In an experimental system FSK modulated lasers were successfully locked to an etalon passband. The control system was able to tune the lasers up and down to neighbouring etalon modes, under computer control if desired. The addition of a coarse resolution scanning etalon was able to overcome the ambiguity between different etalon modes, leaving the accurate stabilisation problem to the feedback electronics. Summary of the Invention A laser control arrangement according to the invention is set out in claim I. Preferred forms are set out in the dependent claims. In accordance with the invention, a portion of the optical signal exiting the resonator is directed to the input of an optical frequency routing device. The routing device has a plurality of outputs each for receiving one of the frequencies supplied by the laser. Each output is coupled to a photodetector. When the laser emits a frequency of light corresponding to one of the resonant frequencies of the resonator, the resonant frequency will appear on one of the outputs of the routing device and will be detected by the photodetector coupled to that output. A switch may be provided so that the user can select a desired laser frequency from among the resonant frequencies of the resonator. A frequency selector is used to selectively couple the switch to the photodetector which corresponds to the desired resonant frequency. A locking circuit is also provided for receiving an electrical signal from the photodetectors to lock the laser frequency at the desired resonant frequency that is selected. Brief Description of the Drawings FIG. 1 is a block diagram of a known laser control arrangement for tuning a laser. FIG. 2 is a block diagram of an example of the laser control arrangement in accordance with this invention. FIG. 3 is a diagram illustrating the details of the optical frequency routing device shown in FIG. 2. Detailed Description An example of a laser control arrangement 50 for tuning a laser to a desired resonant frequency selected from a range of frequencies is shown in FIG. 1. The desired frequencies coincide with the resonant frequencies of a Fabry-Perot resonator 56. Details of the laser control arrangement are disclosed in U.S. Patent No. 4,914,666, referenced above. Briefly, the control arrangement 50 includes a distributed Bragg reflector laser 51 which produces a tunable single frequency output signal that is coupled into an optical fiber or guide 52. An optical coupler 53 taps a fraction of the optical signal from the guide 52 to be applied to an opto-electronic arrangement 54 that includes the Fabry-Perot resonator 56. The opto-electronic arrangement 54 generates an error signal on lead 60 that is used by a locking circuit for locking the laser oscillator frequency to the desired operating frequency. The locking circuit employs a digital processor 75 for determining bias current values which produce the desired different operating frequencies corresponding to the set of Fabry-Perot resonant frequencies. Those current values are stored in the digital processor 75 for ready retrieval. During operation, one of the bias current values is retrieved and applied to the laser. If the laser characteristic curve has drifted, an incremental bias current is generated by an integrator 70 in response to the error signal 60 and is added to the retrieved bias current value. The resulting total laser bias current drives the laser 51 to oscillate at the desired Fabry-Perot resonant frequency. One limitation of the laser control arrangement described above is that there is no way to determine the Fabry-Perot resonant frequency to which the laser is being locked without using an external measuring device such as an optical frequency meter. Such a measurement is necessary because the laser control arrangement does not provide any means to determine which particular Fabry-Perot resonance is being used to lock the laser. FIG. 2 shows a simplified schematic diagram of a laser control arrangement similar to that shown in FIG. 1 in which an optical frequency routing device is used to unambiguously determine the particular Fabry-Perot resonant frequency to which the laser is tuned. In this arrangement a fraction of the optical signal exiting the Fabry-Perot resonator 56 is directed to the input 71 of an optical frequency routing device 72. The frequency routing device 72 directs different frequencies applied to its input to different ones of its output waveguides 18 1 , 18 2 , ..., 18 N . The output waveguides 18 1 , 18 2 , ..., 18 N are respectively coupled to photodetectors 20 1 , 20 2 , ..., 20 N , which in one example of the invention are photodiodes. The photodiodes may be formed from optically active sections of the waveguides and may be monolithicaly integrated with the frequency routing device on a single semiconductor wafer. The photodetectors 20 1 , 20 2 , ..., 20 N convert the detected optical signals into electrical signals. An electric switch 24 is connected to selectively enable the photodetectors to direct the control signal to a locking circuit. In operation, at any given time only one photodetector is coupled to the locking circuit via the switch 24 and hence only one electrical signal, which corresponds to one optical frequency, is transmitted to the locking circuit at any given time. A frequency selector 75 is used to selectively adjust the electronic switch so that it is coupled to a preselected one of the photodetectors 20 1 , 20 2 , ..., 20 N . The frequency selector 75 is also coupled to a power supply 26, which forms a part of the locking circuit, for supplying the tuning current to the laser. The frequency selector activates the power supply 26 so that tuning current is applied to the laser. The tuning current is varied until the laser frequency corresponds to the Fabry-Perot resonant mode that was preselected by coupling the switch 24 to the photodetector associated with that mode. Once this tuning current is achieved, a photocurrent from the photodiode activates the locking circuit so that the laser frequency is locked at that Fabry-Perot resonant mode. In the example of the invention shown in FIG. 2, the photocurrent is directed to an error signal generator 91 which forms a part of the electro-optic arrangement 54. The error signal generator 91 then produces the locking signal to lock the laser frequency. Additional details of one example of a locking circuit that may be employed with the tuning arrangement of the present invention is disclosed in U.S. Patent No. 4,914,666 referenced above. FIG. 3 shows the pertinent details of the frequency routing device employed in the invention. The frequency routing device contains a plurality of input waveguides 96 connected to a free space region 98. A plurality of output waveguides 30 extends from the free space region 98 and is connected to an optical grating 32. The optical grating 32 comprises a plurality of unequal length waveguides which provides a predetermined amount of path length difference to a corresponding plurality of input waveguides 34 connected to another free space region 46. The free space region 36 is connected to a plurality of output waveguides 38. These frequency routing devices operate as multiplexers and demultiplexers of optical frequencies. The details of their construction and operation are more fully described in U.S. Patent No. 5,136,671 to Dragone. In the case of the frequency routing device 72 seen in FIG. 2, the input waveguide 71 corresponds to one of the input waveguides 96 shown in FIG. 3 and the output waveguides 18 1 , 18 2 , ..., 18 N correspond to the output waveguides 38 shown in FIG. 3. When employed in the present invention, the frequency routing device 72 should be configured to have its passband centers aligned with the resonances of the Fabry-Perot resonator 56.

Claims: A laser control arrangement for tuning a laser, comprising: a resonator (56) and a laser (51) oscillating at a frequency, tunable via a control signal, to any of a set of resonant frequencies (F 1 ...F N ) of the resonator, the resonant frequencies being spaced apart over a range of frequencies; means (18,20,24,75) for selecting a desired resonant frequency from the set of resonant frequencies of the resonator, the means for selecting including an optical frequency routing device having passband centers aligned with the resonances of the resonator and an input (71) optically coupled to the resonator; and means (91) for locking the laser frequency of the laser at the desired resonant frequency. The laser control arrangement of claim 1 wherein the optical frequency routing device has a plurality of outputs and further comprising a plurality of photodetectors (20 1 ...20 N ) optically coupled to the plurality of outputs. The laser control arrangement of claim 2 wherein the selecting means comprises means (24,75) for selectively directing an output signal corresponding to the desired resonant frequency from one of the plurality of photodetectors to the locking means for driving the laser at the desired resonant frequency. The laser control arrangement of claim 3 wherein the selectively directing means comprises a switch (24) and a frequency selector (75) for selectively coupling the switch to one of the plurality of photodetectors corresponding to the desired resonant frequency. The laser control arrangement of claim 3 wherein the locking means (91) comprises means for generating the control signal to drive the laser at the desired resonant frequency. The laser control arrangement of claim I, wherein the optical frequency routing device comprises: a plurality of input waveguides (96); a first free space region (98) connected to the plurality of input waveguides; a plurality of output waveguides connected to the first free space region; an optical grating (32) connected to the plurality of output waveguides (30) comprising a plurality of unequal length waveguides; a plurality of input waveguides (34) connected to the optical grating; a second free space region (36) connected to the plurality of input waveguides connected to the optical grating; and at least one output waveguide (38) connected to the second free space region.

Inventors: GLANCE BERNARD, KAMINOW IVAN P, GLANCE, BERNARD, KAMINOW, IVAN P.
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: H01S3
IPC codes: H01S5, H04B10, H01S3, G02B6