Abstract:
The output of a dedicated control laser element in a laser array, which closely tracks the output of the array, is directed to a discriminator and feedback control means through a closed loop feedback-based system to provide both wavelength stability and power control of the array without reducing the optical power output from the array. The output of a dedicated control laser element in a laser array is also directed to a discriminator and synchronous detection means and is frequency modulated with a local oscillator to reduce the electronic drift and stabilize the wavelength of the dedicated laser source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to optoelectronics and laser technology, and more specifically to methods employed in industrial and communication applications using a control laser element and laser array source for sensing and stabilizing laser array power and wavelength, without reducing array output power (i.e. lossless), and reducing drift in a wavelength stabilized laser source. 
     2. Description of the Prior Art 
     Wavelength stability in the optical output signal of the light source is necessary in many sensor systems and telecommunication systems using optical fibers. Depending upon the system, a high precision of wavelength stability, in parts per million, is required. Similarly, a high degree of stabilization is also necessary in the lasers used in fiber optic telecommunication systems. Further limiting the influence of noise and drift in these sensitive systems would enable additional traffic utilization in these systems. 
     Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today&#39;s optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiber optic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV, and ISDN switching services will be dependent on and be substantial to users of such systems. 
     Devices representative of existing technology for implementing fiber optic networks, which are known in the industry, include waveguide division multiplexers (WDMs), fiber amplifiers such as erbium doped fiber amplifiers (EDFAs), and add/drop networks. These devices, as well as other components of a fiber optic network, contribute to or are affected by power level variances in the independent channels of a fiber optic network and may also contribute to noise and drift in the system. Therefore, the maturation of economically feasible and technically satisfactory fiber optic networks for the multiple users and diverse uses previously described, is dependent upon the stabilization of power levels in the independent channels of a fiber optic network, the reduction of wavelength spacing between adjacent channels, and the reduction of drift in a stabilized optical source. 
     Lasers are employed in numerous applications, particularly within fiber optic networks. Often, due to constraints in design or by way of limitations of devices in the system, it is important to stabilize and control the laser output wavelength, i.e. locking the wavelength to a reference wavelength. Similarly, because a single semiconductor laser may not generate sufficient power for the system, high power semiconductor laser sources such as semiconductor laser arrays, generating high optical power, are utilized. These high power laser arrays are made of single stripes of semiconductor lasers and may also be stacked to form two dimensional laser arrays. As is often the practice, each element of a laser array may be of a different wavelength than another element. Controlling and stabilizing the wavelength and output of the arrays, while minimizing losses to or splitting of the output power is critical to system performance. 
     In particular, in a wavelength division multiplex communication system, there typically exists a high number of independent channels in a transmission line. These channels are independent of each other and are well-suited for multimedia and multi-data transmission and communication. Wavelength division multiplexed (WDM) communication systems have further advanced to dense wavelength division multiplexed (DWDM) systems, the DWDM systems being point-to-point systems designed to increase the capacity of installed fiber. DWDM systems currently provide up to 400 GBps capacities and beyond over a single strand of fiber, and provide transmission capabilities four to eight times those of traditional time division multiplexed (TDM) systems. DWDM systems require 0.1-20 ppm frequency stabilization over their anticipated life spans which are estimated at 25 years. 
     A DWDM system typically includes at least one optical amplifier having two key elements: an optical fiber that is doped with the element erbium and the amplifier. Typically a laser is employed to energize the erbium with light at a specific wavelength and the erbium thereby acts as a gain medium that amplifies the incoming optical signal. The gain of the optical amplifier is dependent upon the optical power in the signal channel. The strength of the incoming signal is desired to be optimal. If the power of the optical signal is degraded, as may occur due to phase modulations, the signal may be insufficient to meet demands of the communication system, often due to too little power resulting in not enough gain from the amplifier. 
     In a WDM and DWDM communication systems, many channels can be secured by narrowing the channel gaps and transmitting the signals in the same tunable range for each channel. In order to keep the gaps smaller than a change in width due to drift of wavelength of a semiconductor laser, control is necessary to reduce the influence of the drift of wavelength. In order to reduce the influence of drift of wavelength it is necessary to stabilize the wavelength absolutely or relatively. 
     The wavelength of light emitted from a laser source varies as a function of the operating temperature, and of the current applied to the energy source for excitation. Controlling the wavelength output in an optical multichannel system with narrow channel spacing is further complicated by the fact that laser output wavelength is influenced by other factors such as acoustic vibrations as an example. 
     Critical to these systems is the ability to reduce wavelength spacing between adjacent channels, and thereby increase the number of channels available to be utilized within any particular waveband. Employing a method for wavelength stabilization is necessary to compensate for the effects of temperature and current variation to obtain a reasonable degree of stabilization. Employing a method for wavelength stabilization without reducing power of the optical light sources, for a plurality of optical light sources, is also desired. Employing a method which utilizes high power optic sources and has a means for stabilizing the wavelength of the source, without dramatically reducing the output of the source, is especially desired. 
     A number of light source stabilization approaches are known for application to discrete laser devices. The following three patent applications referred to following are hereby incorporated by reference into the present application. For example, U.S. Pat. No. 4,842,358 discloses optical signal source stabilization using an interferometer forming optical beams through a birefringent crystal interferometer, wherein the beams have similar intensities at a desired source frequency. The difference between the intensities of each beam generates an error signal which is directed to a means for altering the drive current of the source to produce an optical output signal which minimizes the error signal. 
     U.S. Pat. No. 5,167,444 discloses optical signal source stabilization, wherein the optical output signal from the source is stabilized by adjusting its frequency to maintain a selected optical transmission through a Fabry-Perot interferometer. The interferometer has a “split-level” gap whereby the gap is split into two discrete portion of different widths. A photodetector associated with each portion generates an electric signal indicative of the beam intensity or power transmitted through that portion of the gap. At a desired wavelength of the optical signal incident in the interferometers, the transmission of the light through the gap will be approximately equal in both gap portions, resulting in substantially equal amplitudes of the electrical signals from the two photodetectors. The two photodetectors&#39; signals are then fed into a drive current of the optical source so as to minimize the amplitude error signal. This minimum error will occur at the desired wavelength. 
     U.S. Pat. No. 5,428,700 discloses the stabilization of a wavelength of a light source by introducing a collimated beam from the source into a Fabry-Perot cavity having a continuously decreasing width along at least one axis that is normal to the axis of beam propagation. Photodetectors at the output side of the etalon produce signals which are employed in a feedback loop, whereby the wavelength of the light source is controlled to minimize the amplitude difference between the photodetector output signals. 
     Similarly, other methods, known in the art, employ variations of the splitting of discrete laser light output whereby laser light may be split prior to an optical fiber, or laser light is split from the back side of the laser, between the discriminator and the photodetector used for power control. Each of the known approaches have certain limitations. U.S. Pat. No. 4,842,358 requires relatively expensive components such as birefringent crystals made to precisely controlled tolerances. U.S. Pat. No. 5,167,444 is relatively difficult to adapt to multiple light source applications, such as arrays. U.S. Pat. No. 5,428,700 splits the power from the laser package output fiber and send a portion of the light to the wavelength discriminator thereby reducing the output of the laser light by 5-10% or more. Splitting laser light from the back side of the laser further reduces an already low power source, as the back side of a typical communications laser is generally 5%-25% of the power of the front side, and may further limit the available output power causing an inability to control the laser optical power output and wavelength. Further, each of the prior art systems discloses wavelength stabilization techniques for discrete laser sources which are insufficient for certain communication systems demanding higher power optic sources or bundled light sources for higher output power. 
     The optical amplification in fiber links is a necessity in long-haul communication systems using optical fibers, and also in distribution systems involving large numbers of subscribers. Optical amplifiers enable the optical power in a fiber path to be maintained at sufficiently high levels such that the signal-to-noise ratio (SNR) degradation due to signal shot noise and receiver noise is nearly inconsequential. However, the optical amplifier introduces a dominant noise source to such systems. This dominant noise source, being amplified spontaneous emission (ASE), occurs when the amplifiers are operated at “true” direct current (e.g. microhertz to hertz ranges) and is inversely related to the frequency. Similarly, drift of the electronics and the drift of the laser directly impact performance of the optical systems as longer integration times are required to accurately extract signals from the drift and noise components. It is therefore desirable to filter out noise and drift factors related to the wavelength and system electronics in a communication system. 
     As reducing output power and utilizing discrete lasers is contrary to the rising communication systems demands, there remains an unsatisfied need for a system of high power light source wavelength stabilization that is more easily and economically fabricated than the prior art systems, which also possesses a convenient and ample output power and wavelength source for achieving future communication demands. 
     The subjects of the present invention provide methods for stabilizing the output wavelength of an optical laser source, and for reducing the drift in a wavelength stabilized laser source. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for stabilizing, to a stability generally in the range of 0.1 ppm to 20 ppm, or better, the wavelength of an array of optical output signals from an optical source array, using a control laser element of the array, wherein the present invention comprises a discriminator, through which a pair of optical beams from the source are propagated; photodetection means for generating an electrical signal in response to each of the beams propagated through the discriminator; a temperature sensing means, for monitoring the temperature of the control laser element; a temperature controlling means, for adjusting the temperature of the control laser element to change the resulting output wavelength; and controlling means for (a) generating an error signal in response to the difference in amplitudes of the two electrical signals generated by the photodetection means, and (b) feeding the error signal to the optical source through the temperature controller to control the output wavelength of the control laser element so as to minimize the amplitude of the error signal. 
     It is another object of the present invention to provide a method for reducing drift in a wavelength stabilized laser source by frequency modulating with an alternating current a dedicated element of the laser array and filtering noise through a synchronous detection system, having a phase-sensitive detection means. 
     It is a further object of the present invention to provide a method for utilizing a high power laser diode driver array, comprised of semiconductor or solid-state laser elements, as an optic source for stabilizing the wavelength without reducing output power of the array. 
     It is still a further object of the present invention to provide a method which can be effectively employed with laser arrays having many individual laser elements, with at least one dedicated laser element of the array generating the optical source to the discriminator. 
     It is another object of the present invention to provide such a method which is efficient and inexpensive. 
     As will be more readily appreciated from the detailed description that follows, the present invention offers a number of advantages not previously achieved in the prior art. For example, the present invention offers both wavelength and power stabilization, with the ability to reduce drift in a wavelength stabilized source, in an efficient and inexpensive manner. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a system for laser wavelength and power stabilization, in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is an enlarged perspective view of a diode laser array which may be employed in the method of the present invention. 
     FIG. 3 is a schematic diagram of a variation of the controller, in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a schematic diagram for of a system for reducing drift in a wavelength stabilized laser source, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a stabilization of the optical output of a light source in accordance with a preferred embodiment of the present invention. The light source  100  in applications of interest will be an array  110  of semiconductor lasers or solid state lasers, and will be referred to hereinafter as an “array”. The array  110  is comprised of a plurality of lasers  120  which are adaptively fabricated such that each of the laser elements in the array exhibit characteristics which may be similar to other elements or which may be different intentionally. The laser elements independently output light sources in a range of wavelengths, wherein in a preferred embodiment, the output light  130  from the laser array is propagated through waveguides  140 . Output from the waveguides  140  is preferably directed along a fiber optic path  141 . Within the array  110 , at least one of the lasers, known hereafter as a dedicated element of the array, is a control laser element  150  which has an output  160  representative of the output of the entire array  130 . The control laser element  150  is preferably fabricated to track and generate an output light source  160  representative of the power and wavelength of the entire array. Preferably, the laser is a distributed feedback laser (DFB). The laser, by way of example and not of limitation, may also be a DBR laser, a high-power or a low-power laser. The control laser element  150  outputs a light source  160  at a predetermined power and wavelength. The temperature of the control laser element  150  is monitored at predetermined intervals through the temperature sensor  190 . The temperature of the control laser array  150 , as monitored by the temperature sensor  190 , is directed to a controller  710  for evaluation. 
     The light source  160  is emitted from the control laser element  150 . The emitted beam  200  is then directed as an optical input signal to the input side of a discriminator  300 , which may preferably be an interferometer of the etalon or Mach-Zender or Fabry-Perot type, a high-pass filter, a low-pass filter, a band-pass filter, or an interference filter. Additionally, the discriminator  300  may be integrated onto the Planar Lightwave Circuit (PLC)  900 . The discriminator  300  generates a pair of ideally equi-power optical beams  400 ,  500 . Then, advantageously but not necessarily, each of the two beams  400 ,  500  may then be directed into one of a pair of collimating lenses  410 ,  510  to produce a pair of substantially parallel, collimated light beams  420 ,  520 . 
     Emergent beams from the interferometer are preferably but not necessarily first respectively reflected off adjustable turning mirrors  430 ,  530  which may be preferably etched into the PLC  900  which houses the array  110 . The emergent beams are then detected by a matched pair of photodetectors  440 ,  540 , which may be conventional photodiodes. In another embodiment of the present invention, the photodetectors  440 ,  450  are cofabricated as a single unit. Each of the photodetectors detects one of the two emergent optical beams and generates an electrical signal  450 ,  550  indicative of the amplitude of the respective emergent beam, wherein the amplitude is a function of wavelength and power for the non-reference path having optical beam  400 . The reference path emergent beam amplitude is a function only of power. The electrical output signal of each of the photodetectors is inputted into a conventional amplifier  460 ,  560 . The amplifier amplifies the inputted electrical signal from the photodetectors, increasing the SNR of each of the signals. The amplified electrical signals outputted from each amplifier are then inputted into a closed feedback loop  700 , along with temperature data of the control laser element  151  from the temperature sensor  190 , wherein the feedback loop  700  has a controller  710 , which evaluates differences in the input signals and produces a control signal  720  indicative of valuation differences between the two amplified electrical signals. The control signal is communicated to the temperature controller  800  through the closed feedback loop  700 , wherein the temperature controller adjusts the control laser element  150  to output  160  at the desired wavelength. Optionally, the feedback loop  700  may comprise a microprocessor, Application Specific I.C. (ASIC), field programmable gate array (FPGA), digital signal processor (DSP), digital circuits and analog circuits, by way of example and not of limitation. 
     FIG. 2 illustrates details of a diode laser array  110 . The array is comprised of a plurality of laser elements  120 , each of which includes a number of individual output facets  170 , where it is to be understood that any convenient number of laser elements  120  and output facets  170 . The array has a control laser element  150  which has an output  160  that may be separated from the output of the rest of the array  130  as a dedicated optical source. 
     In FIG. 3, detail of a preferred embodiment of controller of the present invention is depicted. The controller  710  may preferably be comprised of a pair of analog-to-digital converters (ADC)  730 ,  740  and a microprocessor  750  having a digital-to-analog converter (DAC)  760 . Optionally, the ADCs  730 ,  740  and the DAC  760  are integrated with the microprocessor  750 . The output signals from the amplifiers are applied directly to the respective ADCs. The microprocessor  750  then generates a digital version of the control signal  720 . The control signal is then converted to analog through the DAC  760  for communication to the temperature controller  800 . Advantageously but not necessarily, other variations to the controller  710  are herein incorporated including the use of active embedded software, a software-based control signal generating device, a differential amplifier, and similar. Further variations of the present invention which are beneficial may also include microprocessor-based iterative and algorithmic evaluations of the control signal with collected control laser element temperature data. 
     FIG. 4 is a schematic diagram of a system for reducing drift in a wavelength stabilized laser source. The direct current source  311  is used to drive the laser array  111  to generate sources of optical output to a fiber optic path  131 . A second current source  312  having both alternating and direct current is used to drive the control laser element  151 . The current source  312  has a direct current (dc) component which energizes the control laser element  151 , and an alternating current (ac) component which is controlled by a modulation controller  321 . The modulation controller  321  modulates the optical frequency of the signal delivered by the control laser element  151 . Optionally, the ac component further modulates the laser array  111 . The control laser element output  161  is emitted from the control laser element  151  wherein the emitted beam  201  is directed as an optical input signal to a discriminator  301 . The discriminator  301  may be an interferometer of the etalon or Mach-Zender or Fabry-Perot type, a high-pass filter, a low-pass filter, a band-pass filter, or an interference filter, which generates a pair of optical beams of equal wavelength. Advantageously but not necessarily, a wavelength selecting filter  331  may be used along the non-reference path to selectively filter the optical signal from the discriminator  301  and thusly improve wavelength stabilization. The output signals from the discriminator  301  are detected by a pair of photodetectors  441 ,  541 , which may be conventional photodiodes or which may be cofabricated photodetectors. Each of the photodetectors detects one of the two emergent optical beams from the discriminator and generates an electrical signal  451 ,  551 . The electrical signal output of the one photodetectors  451  of the non-reference path is passed through an ac coupled amplifier-filter-amplifier combination  361 , and is mixed with a dithering signal  351  from the local oscillator  399 , generated at a particular dither frequency, at a mixer  371  within a phase sensitive detector (PSD)  381 . The electrical signal out of a second photodetector  551 , of the reference path, is passed through an ac coupled amplifier-filter-amplifier combination  362 , and is mixed with a dithering signal  351  from the local oscillator  399 , generated at a particular N dither frequency, at a mixer  372  within a phase sensitive detector (PSD)  382 . 
     The non-reference path PSD  381 , a synchronous detection system, is used to provide a control DC output voltage, as a DC signal, that is linearly proportional to the amplitude and phase modulation at a predetermined frequency, f. The combination of the photodetectors and the PSDs perform optical to electrical conversion of the sampled optical pulses and outputs electrical signals. The PSD has very low bandwidth (on the order of 1 Hz), filters the drift and out-of-band noise terms and generates SNR of the order of 1:1000. In the PSD, only signals at or near a predetermined frequency or one of the harmonics of the predetermined frequency is detected. The PSD is preferably further comprises a low pass filter. The output from the local oscillator  399  is directed to the PSD  381 . The local oscillator  399  generates an output signal  391  which is directed to the modulating controller  321  for coupling. The local oscillator may be comprised of either a microcontroller or an analog circuit. Preferably, the DC signal from the PSD is inputted to an amplifier  373  which is further inputted to an ADC  731  to produce binary or another digital output type signal representative of the value of the DC signal. Similarly, output from the local oscillator  399  is directed to the reference path PSD  382 . DC output from the reference path PSD  382  is inputted to an ADC  732  and the ADC output is inputted to the microcontroller  398 . The output from the ADC is input to a microcontroller  398  adapted to provide a pulse width modulator output signal in relation to the received digitally converted DC signal. The modulated output is communicated to the modulation controller  321  for modulation of the ac current. Output signals  790  are directed to the Temperature Controller  800  which adaptably adjusts and controls the temperature of the control laser element  151 . The temperature of the control laser element is monitored by the temperature sensor  190 , which preferably is a thermistor, and a temperature signal  192  is fed back to the microcontroller  398 .