Abstract:
A gain voltage controller for use with a sampled grating distributed Bragg reflector (SGDBR) laser is presented. The controller for provides separate inputs to the laser including a front mirror current controlling a front mirror and a back mirror current controlling a back mirror to control the laser and a voltage monitor, coupled to a gain section of the laser for monitoring a gain voltage of the gain section and providing input of the gain voltage to the controller. The controller controls the front mirror current and the back mirror current to minimize the voltage monitored from the gain section of the laser.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of the following co-pending and commonly-assigned U.S. patent applications:  
         [0002]    Provisional Application Serial No. 60/215,739, filed Jun. 29, 2000, by Gregory A. Fish and Larry A. Coldren, entitled “OPEN LOOP CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.4-US-P1;  
         [0003]    Provisional Application Serial No. 60/215,170, filed Jun. 29, 2000, by Paul F. Crowder, entitled “POWER AND WAVELENGTH CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.5-US-P1, and  
         [0004]    Provisional Application Serial No. 60/215,742, filed Jun. 29, 2000, by Paul F. Crowder and Larry A. Coldren, entitled “GAIN VOLTAGE CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.6-US-P1,  
         [0005]    all of which applications are incorporated by reference herein.  
         [0006]    This application is a continuation-in-part patent application of the following co-pending and commonly-assigned U.S. patent applications:  
         [0007]    Utility application Ser. No. 09/848,791, filed May 4, 2001, by Gregory A. Fish and Larry A. Coldren, entitled “IMPROVED MIRROR AND CAVITY DESIGNS FOR SAMPLED GRATING DISTRIBUTED BRAGG REFLECTOR LASERS,” attorneys&#39; docket number 122.1-US-U1, which claims the benefit under 35 U.S.C. §119(e) of Provisional Application Serial No. 60/203,052, filed May 4, 2000, by Gregory A. Fish and Larry A. Coldren, entitled “IMPROVED MIRROR AND CAVITY DESIGNS FOR SGDBR LASERS,” attorneys&#39; docket number 122.1-US-P1;  
         [0008]    Utility application Ser. No. 09/872,438, filed Jun. 1, 2001, by Larry A. Coldren, Gregory A. Fish, and Michael C. Larson, entitled “HIGH-POWER, MANUFACTURABLE SAMPLED GRATING DISTRIBUTED BRAGG REFLECTOR LASERS,” attorneys&#39; docket number 122.2-US-U1, which claims the benefit under 35 U.S.C. §119(e) of Provisional Application Serial No. 60/209,068, filed Jun. 2, 2000, by Larry A. Coldren Gregory A. Fish, and Michael C. Larson, and entitled “HIGH-POWER, MANUFACTURABLE SAMPLED-GRATING DBR LASERS,” attorneys&#39; docket number 122.2-US-P1;  
         [0009]    Utility application Ser. No. ______, filed Jun. 11, 2001, by Gregory A. Fish and Larry A. Coldren, entitled “IMPROVED, MANUFACTURABLE SAMPLED GRATING MIRRORS,” attorneys&#39; docket number 122.3-US-U1, which claims the benefit under 35 U.S.C. §119(e) of Provisional Application Serial No. 60/210,612, filed Jun. 9, 2000, by Gregory A. Fish and Larry A. Coldren, entitled “IMPROVED, MANUFACTURABLE SAMPLED GRATING MIRRORS,” attorneys&#39; docket number 122.3-US-P1;  
         [0010]    Utility application Ser. No. ______, filed on same day herewith, by Gregory A. Fish and Larry A. Coldren, entitled “OPEN LOOP CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.4-US-U1, which claims the benefit under 35 U.S.C. §119(e) of Provisional Application Serial No. 60/215,739, filed Jun. 29, 2000, by Gregory A. Fish and Larry A. Coldren, entitled “OPEN LOOP CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.4-US-P1; and  
         [0011]    Utility application Ser. No. ______, filed on same day herewith, by Paul F. Crowder, entitled “POWER AND WAVELENGTH CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.5-US-U1, which claims the benefit under 35 U.S.C. §119(e) of Provisional Application Serial No. 60/215,170, filed Jun. 29, 2000, by Paul F. Crowder, entitled “POWER AND WAVELENGTH CONTROL OF SGDBR LASERS,” attorneys&#39; docket number 122.5-US-P1,  
         [0012]    all of which applications are incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0013]    1. Field of the Invention  
           [0014]    The present invention relates to gain voltage control for semiconductor lasers, and particularly, gain voltage control for Sampled Grating Distributed Bragg Reflector (SGDBR) semiconductor lasers.  
           [0015]    2. Description of the Related Art  
           [0016]    Diode lasers ate being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range, i.e. a widely tunable laser, is an invaluable tool. The number of separate channels that can utilize a given wavelength range is exceedingly limited without such a laser. Accordingly, the number of individual communications paths that can exist simultaneously in a system employing such range-limited lasers is similarly very limited. Thus, while diode lasers have provided solutions to many problems in communications, sensors and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be increased in order for optical systems to be realized for many future applications.  
           [0017]    For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can select any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths. Continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes.  
           [0018]    In addition, widely tunable semiconductor lasers, such as the sampled-grating distributed-Bragg-reflector (SGDBR) laser, the grating-coupled sampled-reflector (GCSR) laser, and vertical-cavity lasers with micro-mechanical moveable mirrors (VCSEL-MEMs) generally must compromise their output power in order to achieve a large tuning range. The basic function and structure of SGDBR lasers is detailed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, and entitled “MULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORS”, which patent is incorporated by reference herein. Designs that can provide over 40 nm of tuning range have not been able to provide much more than a couple of milliwatts of power out at the extrema of their tuning spectrum. However, current and future optical fiber communication systems as well as spectroscopic applications require output powers in excess of 10 mW over the full tuning band. Current International Telecommunication Union (ITU) bands are about 40 nm wide near 1.55 μm, and it is desired to have a single component that can cover at least this optical bandwidth. Systems that are to operate at higher bit rates will require more than 20 mW over the full ITU bands. Such powers are available from distributed feedback (DFB) lasers, but these can only be tuned by a couple of nanometers by adjusting their temperature. Thus, it is very desirable to have a source with both wide tuning range (&gt;40 nm) and high power (&gt;20 mW) without a significant increase in fabrication complexity over existing widely tunable designs. Furthermore, in addition to control of the output wavelength, control of the optical power output for a tunable laser is an equally important endeavor as optical power determines the potential range for the laser.  
           [0019]    Fundamentally, maximizing the output power, while stabilizing the output wavelength and the maximizing the side mode suppression ratio are very desirable objectives in the control of SGDBR lasers. Thus, there is a need in the art for devices and methods which maximize the power output. The present invention meets these objectives through a novel use of gain voltage control.  
         SUMMARY OF THE INVENTION  
         [0020]    A gain voltage controller for use with a sampled grating distributed Bragg reflector (SGDBR) laser is presented. The controller for provides separate inputs to the laser including a front mirror current controlling a front mirror and a back mirror current controlling a back mirror to control the laser and a voltage monitor, coupled to a gain section of the laser for monitoring a gain voltage of the gain section and providing input of the gain voltage to the controller. The controller controls the front mirror current and the back mirror current to minimize the voltage monitored from the gain section of the laser.  
           [0021]    The gain voltage control of the present invention uses feedback from the SGDBR Laser gain section, typically a voltage, to keep the mirrors aligned with the cavity mode of the laser. The feedback is used to align each mirror, and thereby minimizing the Laser gain section voltage, since the Laser gain section voltage minimum is where the cavity loss is a minimum. By minimizing the gain section voltage, the optical power output for a given operating point is maximized, the output wavelength is stabilized, and the side mode suppression ratio is increased.  
           [0022]    Gain voltage control is implemented in a Digital Signal Processor (DSP) by using either a numerical minima search, or a least mean squares (LMS) quadratic estimator, or can be done using analog circuits using a phase locker (PL) circuit.  
           [0023]    When gain voltage control is performed using a DSP, the Laser mirror currents are dithered while the laser gain section is monitored. The DSP then uses a numerical algorithm to align the mirrors by locating the minima of the Laser gain section voltage.  
           [0024]    To reduce the effects of noise in the sampled gain voltage signal, a LMS estimator is used to effectively filter the noise by using an array of data points to estimate the gain voltage surface. Use of the LMS promotes faster convergence to the gain voltage minima, as well as providing a smoother transition to the gain voltage minima than a straight minima search using only a minima search algorithm.  
           [0025]    In addition to the strictly digital approach using only a DSP, which are limited by analog-to-digital conversion rate and digital-to-analog conversion rate, along with the signal-to-noise ratio of the DSP circuitry, analog phase locking circuitry can be used to minimize these limitations. An analog phase locker (PL), which is a high speed, analog-locking loop is used in conjunction with the DSP, to dither the mirror current, measure the gain voltage with a tuned, narrowband amplifier, extract the phase difference between the stimulus and the measured signal, and drive an error amplifier to adjust the mirror current to the gain voltage. The PL error amplifier output is then measured by the DSP, which adjusts the mirror current values to reduce the error to zero. The DSP effectively operates as an integrator function.  
           [0026]    Once new currents to the various sections are established by locking to the external wavelength reference for a given channel, the look-up table can be updated so that the system is adapted to small changes in device characteristics as it ages. Also, by using a formula based upon the initial calibration characteristics, the currents for the other desired operating powers and wavelength channels stored in the look-up table can be updated as well. This insures that desired operating channels can always be accessed over the device&#39;s lifetime. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0028]    [0028]FIGS. 1A and 1B depict a typical multiple-section, widely-tunable laser as used in the invention;  
         [0029]    [0029]FIG. 2 is a block diagram of a typical embodiment of the invention;  
         [0030]    [0030]FIG. 3 illustrates an open loop control system of present invention;  
         [0031]    FIGS.  4 A- 4 B are flowcharts of the incremental and mirror reflectivity peak calibration processes;  
         [0032]    [0032]FIG. 5 is a block diagram of the current sources used in the controller;  
         [0033]    [0033]FIG. 6 illustrates a typical current source circuit of the present invention;  
         [0034]    [0034]FIG. 7 illustrates a typical current mirror circuit of the present invention;  
         [0035]    FIGS.  8 A- 8 C illustrate a typical closed loop power and wavelength control system;  
         [0036]    [0036]FIG. 9 illustrates the DSP gain voltage control block diagram;  
         [0037]    [0037]FIG. 10 illustrates the analog gain voltage control block diagram;  
         [0038]    [0038]FIG. 11 illustrates the analog phase lock circuit block diagram; and  
         [0039]    [0039]FIG. 12 illustrates the combined operation of analog gain voltage control circuits to correct the outputs to the two mirrors from the open loop digital controller. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, an embodiment of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0041]    1. Overview  
         [0042]    [0042]FIGS. 1A and 1B depict a typical multiple-section, widely-tunable laser  100  as used in the invention. A typical SGDBR laser  100  as used in the invention essentially comprises four sections that allow its unique tuning characteristics. The laser  100  is comprised of a gain section  102 , a phase section  104 , a back mirror  106  and a front mirror  108 . Below these sections is a waveguide  110  for guiding and reflecting the light beam, while the entire device is formed on a substrate  112 . In use, bias voltages are connected to the electrodes  114  on the top of the device and a ground is connected to a lower substrate  112 . When a bias voltage on the gain section  102  is above a lasing threshold, a laser output is produced from an active region  116 .  
         [0043]    The front and back mirrors  108 ,  106  are typically sampled grating mirrors that respectively include different sampling periods  118 ,  120 . The gratings behave as wavelength-selective reflectors such that partial reflections are produced at periodic wavelength spacings of an optical signal carried in the cavity. The front and back sampled grating mirrors  108 ,  106  together determine the wavelength with the minimum cavity loss through their effective lengths and grating differential; however, the lasing wavelength can only occur at the longitudinal modes of the optical cavity in the waveguide  110 . Therefore, it is important to adjust the mirrors  106 ,  108  and waveguide  110  modes to coincide, thereby achieving the lowest cavity loss possible for the desired wavelength and maximum mode stability. The phase section  104  of the device shown in FIG. 1 is used to adjust the optical length of the cavity in order to position the cavity modes.  
         [0044]    Optional back-side monitor  122  and front-side semiconductor optical amplifier (SOA) and/or optical modulator  124  sections are also indicated. Currents are applied to the various electrodes  114  of the aforementioned sections to provide a desired output optical power and wavelength as discussed in U.S. Pat. No. 4,896,325, issued Jan. 23, 1990, to Larry A. Coldren, and entitled “MULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORS”, which patent is incorporated by reference herein. As described therein, a current to the gain section  102  creates light and provides gain to overcome losses in the laser cavity; currents to the two differing SGDBR wavelength-selective mirrors  106 ,  108  are used to tune a net low-loss window across a wide wavelength range to select a given mode; and a current to the phase section  104  provides for a fine tuning of the mode wavelength. It should also be understood that the sections are somewhat interactive, so that currents to one section will have some effect on the parameters primarily controlled by the others.  
         [0045]    Currents and voltages are applied and/or monitored at the optional sections to monitor power or wavelength, or provide amplification or modulation as specified in commonly-assigned and co-pending applications, namely application Ser. No. 09/614,378, filed on Jul. 12, 2000, by Gregory Fish et al., and entitled “OPTOELECTRONIC LASER WITH INTEGRATED MODULATOR,”; application Ser. No. 09/614,377, filed on Jul. 12, 2000, by Larry Coldren, and entitled “INTEGRATED OPTOELECTRONIC WAVELENGTH CONVERTER,”; and application Ser. No. 09/614,375, filed on Jul. 12, 2000, by Beck Mason et al., and entitled “TUNABLE LASER SOURCE WITH INTEGRATED OPTICAL AMPLIFIER,” each of which claims priority to Provisional Application Ser. Nos. 60/152,072, 60/152,049 and 60/152,072, all filed on Sep. 2, 1999; all of which applications are incorporated by reference herein. The current invention operates under the same general principles and techniques as these background inventions.  
         [0046]    [0046]FIG. 2 is a block diagram of a typical control system  200  embodiment of the invention. In general, the controller  202  applies input signals  204  to the various sections of the laser  206  to operate it and produce a laser output  208  at a desired wavelength. Many factors may influence the laser output  208  and the controller  202  optimally stabilizes the laser output  208  over the life of the laser  206 . In closed-loop variants of the control system  200 , the controller  202  may monitor the laser  206  and its output via feedback signals  210  and adjust the various laser inputs  204  accordingly. For example, in one embodiment the laser  206  monitors the feedback signals  210  of the multiple-section, widely tunable laser gain section voltage, temperature, and an external reference  212 , such as a wavelength locket (e.g. a Fabry-Perot Etalon), via respective feedback signals  210 A- 210 C. The controller  202  adjusts the laser section currents and temperature to maintain a fixed optical power and wavelength. The Laser temperature is regulated with a cooling device  214 , such as a thermoelectric cooler (TEC), via a separate control loop. The laser  206  generates continuous optical output power.  
         [0047]    The controller  202  interfaces to the host over a system interface  216 , such as a serial or parallel interface. The host commands the operation of the controller  202 . The controller  202  regulates the laser optical output power and wavelength and may operate in one of the following control modes:  
         [0048]    A. Open loop control using fixed operating points.  
         [0049]    B. Power and wavelength control using open loop control&#39;s fixed operating points as initial operating points and regulating the optical power and wavelength to a reference thereafter.  
         [0050]    C. Gain voltage control using open loop control&#39;s fixed operating points as initial operating points and regulating the Laser mirror alignment with the cavity mode thereafter.  
         [0051]    D. Regulating power, wavelength, and gain voltage using open loop control&#39;s fixed operating points as initial operating points.  
         [0052]    Various embodiments of the control modes are detailed hereafter.  
         [0053]    2.0 Open Loop Control  
         [0054]    [0054]FIG. 3 illustrates an open loop control system  300  that sets the laser optical output  208  power and wavelength by setting the laser section current inputs  204  from values in an aging model stored in the controller  202 . The current inputs  204  may be applied, for example, to a back mirror (BM), phase (Ph), Gain (Gn), front mirror (FM), and optical amplifier (SOA) sections of the laser  304 . The controller  202  regulates the laser temperature to a fixed value by monitoring a sensor  308  and controlling the cooler  214  accordingly. The current input  204  settings or operating points of the various sections of the laser  304  are generated by a calibration routine. The settings are fixed over the lifetime of the product. The choice of the operating current inputs  204 , the current sources, and temperature regulator guarantees maximum stability of the optical output wavelength and power over operating lifetime and ambient environmental conditions.  
         [0055]    As previously mentioned, the integrated optical amplifier (SOA), like the integrated modulator, is optional and not included on all designs.  
         [0056]    2.1 Operating Points  
         [0057]    The laser operating points are determined by either an incremental calibration routine or a mirror reflectivity peak calibration routine.  
         [0058]    2.1.1 Incremental Calibration  
         [0059]    Incremental calibration steps and locks the laser to each channel, such as each ITU wavelength channel using a calibrated wavelength locker as a reference, such as a Fabry-Perot etalon. It steps to the next channel by adjusting the phase current and locking the mirrors to the cavity mode with gain voltage control. Once at the channel, it locks the Laser wavelength to the channel by adjusting the phase current using wavelength control and the laser power to a predetermined set point by adjusting the gain current with power control.  
         [0060]    Incremental calibration starts with the mirrors aligned at mirror reflectivity peak 0 and then searches for the next lower channel. At each cavity mode, it resets the phase current to its initial value and continues the search. At the end of each mirror tuning range, the mirror currents are reset to the next mirror reflectivity peak. Once the wavelength wraps around, the process is repeated at mirror reflectivity peak 0 by searching for the next upper channel.  
         [0061]    [0061]FIG. 4A is a flowchart of the incremental calibration process. The typical process may begin by setting the gain current at a nominal operation current at block  404 . The mirrors are set at the next reflectivity peak in a chosen direction (up or down) at block  406 . If the wavelength wrapped at block  402 , the chosen direction is changed at block  400  and the process begins again. If the wavelength did not wrap, the phase current is set at a minimum operation current at block  410  and the mirrors are locked to the cavity mode at block  412 . If the mirrors have reached the end of their tuning range at block  408 , the process resets to block  406  at the next reflectivity peak. If the tuning range has not been reached, the power and wavelength are locked at the channel and the mirrors are aligned at block  416 . The channel and corresponding input currents are recorded at block  418  and the laser is stepped to the next channel with the mirrors lock to phase at block  420 . If the cavity mode has been passed at block  414 , the process restarts at block  410  to reset the phase current. If the cavity mode has not been passed, power and wavelength are locked again at the new channel as the process resets to block  416 . This process will continue until a change in wavelength is indicated again at block  400 . At this point, the process ends.  
         [0062]    The following pseudo-code also describes the logic of the incremental calibration shown in FIG. 4A.  
         [0063]    For each wavelength direction about mirror reflectivity peak 0  
         [0064]    Until (wavelength wraps)  
         [0065]    Set gain current at nominal operational current  
         [0066]    Set mirrors at next reflectivity peak  
         [0067]    Until (end of mirror tuning range)  
         [0068]    Set phase current at minimum operational current  
         [0069]    Lock mirrors to cavity mode  
         [0070]    Until (passes cavity mode)  
         [0071]    Lock power and wavelength at channel and align mirrors  
         [0072]    Record channel and currents  
         [0073]    Step to next channel with mirrors locked to phase  
         [0074]    2.1.2 Mirror Reflectivity Peak Calibration  
         [0075]    Mirror reflectivity peak calibration determines the mirror reflectivity peaks, generates the mirror tuning efficiency curves, and uses the curves to set the mirror currents for each channel.  
         [0076]    [0076]FIG. 4B is a flowchart of the mirror reflectivity peak calibration process. The process may begin with sweeping the mirror with the cavity mode aligned to the mirror at block  424 . The gain voltage minima, which correspond to the mirror reflectivity peaks, are located at block  426 . The currents corresponding to the minima are recorded at block  428 . If the wavelength does not cross the 0 peak at block  422 , the process returns to block  424  to continue sweeping the mirror. Otherwise, a mirror tuning efficiency curve is generated from the reflectivity peaks at block  430 . Then at block  434  the mirrors are set to a channel using the mirror tuning efficiency curve. The phase section is aligned to the mirrors at block  436  and the wavelength is locked to the channel using wavelength control at block  438 . Finally, the power is locked to the set point using the power control at block  440  and the channel and input currents are recorded at block  442 . The process ends when the last channel has been located as checked at block  432 .  
         [0077]    The following pseudo-code also describes the logic of the mirror reflectivity peak calibration shown in FIG. 4B.  
         [0078]    Until (wavelength crosses mirror reflectivity peak 0)  
         [0079]    Sweep mirror with cavity mode aligned to mirror  
         [0080]    Locate the gain voltage minima, which is the corresponding mirror reflectivity peak.  
         [0081]    Record the currents  
         [0082]    Generate mirror tuning efficiency curve from reflectivity peaks  
         [0083]    Until (step through all channels)  
         [0084]    Set mirrors to channel using mirror tuning efficiency curve  
         [0085]    Align phase section to the mirrors  
         [0086]    Lock wavelength to channel using wavelength control  
         [0087]    Lock power to set point using power control  
         [0088]    Record the channel and currents  
         [0089]    2.2 Current Sources  
         [0090]    [0090]FIG. 5 is a block diagram of the current sources  500  used in the controller  202 . The Controller current sources  500  drive the phase, mirror, amplifier, and gain sections of the laser  100 . The current sources are comprised of a voltage reference  504 , individual 16-bit digital to analog converters  506  (DACs), and voltage to current (VI) amplifiers  508 . The DACs  506  connect to the digital signal processor (DSP) synchronous serial port  510  (SSP) through a programmable logic device  512  (PLD). The PLD  512  provides a logic interface between the DSP SSP  510  and the DACs  506 . The VI amplifiers  508  translate the DAC voltage outputs  514  to proportional current inputs  204  that drive the laser sections.  
         [0091]    2.2.1 Voltage to Current Converter  
         [0092]    [0092]FIG. 6 illustrates a typical current source circuit  600  of the present invention. The voltage to current amplifier is a modified Howland circuit source (MHCS). A current mirror  602  is added to the output stage of the amplifier  604  to increase the drive current beyond that of the amplifier  604  alone. A filter stage  606  is added at the load  608  to reduce noise.  
         [0093]    [0093]FIG. 7 illustrates a typical current mirror circuit  602  of the present invention. The current mirror inverts the output of the amplifier  604 , which requires the source, Vin, at the inverting node of the amplifier  604  of the current source circuit  600 .  
         [0094]    The current mirror operates at a fixed gain that is determined, primarily, by the ratio of the resistors  702  in the emitter leads of the transistors. An RC compensation network  704  is added to insure stability of the amplifier and current mirror. The gain of the current is variable up to a maximum ratio. The maximum ratio is determined by the additional drift introduced by heating of the transistor  706  and sense resistor  708  and the maximum thermal loss that can be sustained by the transistor  706  and sense resistor  708 . If additional gain is required, an additional Qmo and Rmo section can be added to the mirror  602 .  
         [0095]    3 Power and Wavelength Control  
         [0096]    FIGS.  8 A- 8 C illustrate a typical closed loop power and wavelength control system. FIG. 8A illustrates the control block diagram. Power and wavelength control  800  combines open loop control (as shown in FIG. 3) and feedback  210 A from an external wavelength locker (e.g., a Fabry-Perot Etalon) reference  212  to lock the laser optical output power and wavelength to the reference  212 . Power and wavelength control compensates for drift in the controller current sources  508  and the laser operating points over time and temperature.  
         [0097]    Once new currents to the various sections  304  are established by locking to the external wavelength reference  212  for a given channel, the aging model or lookup table can be updated so that the system is adapted to small changes in device characteristics as it ages. Also, by using a formula based upon the initial calibration characteristics, the currents for the other desired operating powers and wavelength channels stored in the aging model can be adjusted as well. For example, the currents for each section at any other channel are adjusted in proportion to the change in that section current at the operating channel.  
         dIgain=Igain,change/Igain,calibration [at operating channel] 
         change=(Igain,calibration+dIgain*Igain,calibration [at any other channel] 
         [0098]    This is done for each section current. This insures that desired operating channels can always be accessed over the device&#39;s lifetime.  
         [0099]    The power and wavelength controls may each operate independently or interdependently with other laser inputs.  
         [0100]    3.1 Independent  
         [0101]    [0101]FIG. 8B is a flow diagram of independent control of the power and wavelength. The least complex control algorithm is where the controls operate independently. Each control algorithm induces changes in one laser input, such as a current or temperature, independent of the other laser inputs. The control algorithms are classical proportional, integral control routines. The laser output is compared to the reference to identify whether a change in optical power and/or optical wavelength is indicated at block  810 . If a change in the optical power is indicated at block  812 , the optical power is adjusted by the gain current (Ign) or by the current to a SOA (if integrated into the Laser) at block  814 . If a change in the optical wavelength is indicated at block  814 , optical wavelength is adjusted by the phase current (Iph) or the submount temperature at block  818 . Of course, the order of the power or wavelength adjustment is unimportant. In addition, the aging model may be updated whenever a change (in power or wavelength) is indicated. Mirror currents are left fixed.  
         [0102]    3.2 Interdependent  
         [0103]    [0103]FIG. 8C is a flow diagram of interdependent control of the power and wavelength. The independent control algorithm is slow and marginally stable in its response to changes in the optical power output and optical wavelength. The mirrors and cavity mode become misaligned as the control algorithm adjusts the gain and phase currents from their predefined values. The quality of the optical output is reduced (decreased side mode suppression ratio) and the probability of a mode hop is increased (wavelength shift) as the mirrors and cavity mode become misaligned.  
         [0104]    The interdependent control algorithm induces primary changes in one laser input, such as a current or temperature, and corrects for secondary changes in at least one other laser input with an adaptive filter or estimator. This compensates for wavelength shifts or power changes and mirror misalignment induced when the control adjusts its primary variable. Here also, the laser output is compared to the reference to identify whether a change in optical power and/or optical wavelength is indicated at block  820 . If a change in the optical power is indicated at block  822 , the power is adjusted by the gain current (Ign) at block  824  and the wavelength is stabilized by adjusting the phase current (Iph) by an adaptive filter at block  826 . The mirror currents are realigned by a fixed estimator at block  828 . Following this, the aging model is updated at block  836 . If a change in the optical wavelength is indicated at block  830 , wavelength is adjusted by the phase current (Iph) or the carrier temperature at block  832 . The power is stabilized by adjusting the gain current (Ign) by an adaptive filter at block  834 . and the mirror currents are realigned by a fixed estimator at block  828 . Here too, the aging model is updated at block  836 .  
         [0105]    The interdependent controls provide more robust, stable, and faster convergence of the power and wavelength to its reference value.  
         [0106]    As outlined above, the aging model is then updated to reflect the new model coefficients whereby the currents from the aging model or look-up table are adjusted for a given desired wavelength and power. Also, the changes required for this particular channel can be used to estimate the changes required for all other channels.  
         [0107]    4.0 Gain Voltage Control  
         [0108]    Gain Voltage Control uses feedback from the Laser gain section voltage to keep the mirrors aligned with the cavity mode. It aligns the mirrors by minimizing the Laser gain section voltage. The Laser gain section voltage minimum is where the cavity loss is a minimum. It corresponds to maximum optical power output, wavelength stability, and side mode suppression ratio.  
         [0109]    Gain voltage control is implemented in the DSP using a numerical minima search or a least mean squares (LMS) quadratic estimator or in analog circuitry using a phase locker (PL) circuit.  
         [0110]    4.1 DSP Gain Voltage Control  
         [0111]    [0111]FIG. 9 illustrates the DSP gain voltage control block diagram. The DSP dithers the Laser mirror currents  902 ,  904  and monitors the Laser gain section voltage  906 . It uses a numerical algorithm to align the mirrors by locating the minima of the Laser gain section voltage.  
         [0112]    4.1.1 DSP Minima Search Algorithm  
         [0113]    The minima search algorithm uses three data points (mirror current, gain voltage) and estimates the slope of the gain voltage curve with respect to the mirror current. The algorithm steps towards the gain voltage minima and calculates the next data point and uses the new data point and the two best points to re-estimate the slope of the gain voltage curve. The algorithm continues the above step process, continually searching for the gain voltage minima.  
         [0114]    4.1.2 DSP LMS Estimator  
         [0115]    The minima search algorithm is susceptible to wandering around the gain voltage minima due to noise in the sampled gain voltage signal. The wandering is reflected as drift and noise on the optical signal. The LMS estimator reduces the wander and noise by using an array of data points to estimate the gain voltage surface, in effect, filtering the noise. The LMS estimator converges to the gain voltage minima faster and smoother than the minima search.  
         [0116]    For fixed phase and gain section currents, the gain section voltage can be modeled using a causal Volterra series expansion over 2 input signals, the front mirror and back mirror currents. For dithering signals in the sub-100 kHz regime, the analog circuitry and the device itself allow a memoryless model, so a 5-tap adaptive quadratic filter model will suffice.  
         [0117]    The LMS estimator can then be achieved using either of two classic adaptive filter update algorithms, a standard gradient descent adaptation (LMS or block LMS algorithm) or a (faster) recursive least squares adaptation (RLS algorithm—based on Newton&#39;s Method).  
         [0118]    The second approach is used to achieve faster convergence of adaptive linear filters when the signals driving the system do not have sufficient spectral flatness to allow a rapid gradient descent. However, in the case of adaptive linear filters, the gradient descent approach converges just as fast as the RLS approach when white noise can be used to drive the system. Recently published results indicate that comparable rates of convergence can be achieved with adaptive quadratic filters if a minor filter structure modification is used and (pseudo) Gaussian white noise can be used to drive the system.  
         [0119]    There are two advantages of this LMS estimator approach. First, an initial tap-vector can be stored along with the  4  drive currents in the laser calibration table in flash memory (resulting in much faster convergence). Second, the adaptation step size can be reduced as the system converges, reducing steady-state misadjustment in the mirror section currents.  
         [0120]    4.2 Analog Gain Voltage Control  
         [0121]    [0121]FIG. 10 illustrates the analog gain voltage control block diagram. The gain voltage  1002  is connected to analog phase lockers (PL)  1004 A,  1004 B for each mirror section  1006 A,  1006 B. The digital algorithms are limited in speed and accuracy by the analog to digital converters (ADC or A/D)  1008 A,  1008 B and digital to analog converters (DAC or D/A)  1010 A,  1010 B as well as the signal to noise ratio (SNR) of the circuit. The analog phase locker&#39;s speed and accuracy is limited by the SNR of the circuit.  
         [0122]    [0122]FIG. 11 illustrates the analog phase lock circuit block diagram  1100 . The analog phase locker is a high speed, analog-locking loop. It is realized by a phase lock loop (PLL) or RF dither locker. The PL works with the open loop control circuit. The output of the PL adds to the output of the open loop control current sources.  
         [0123]    The PL uses a high frequency narrowband stimulus  1102  to dither the mirror current. The gain voltage (Vg)  1104  is measured with a tuned, narrowband amplifier  1106 . The phase difference between stimulus and measured signal is extracted by a phase comparator  1108  and drives an error amplifier that adjusts the mirror  1110  current to the gain voltage minima and is sampled by an ADC  1112 .  
         [0124]    The PL error amplifier output is measured by the DSP. The DSP adjusts the mirror current values in the Open Loop Control aging model to reduce the error to zero. The DSP effectively operates as an integrator function.  
         [0125]    [0125]FIG. 12 illustrates the combined operation of analog gain voltage control circuits to correct the outputs to the two mirrors from the open loop digital controller. The digital memory/DSP  1200  can set a first approximation current and voltage from a table look up. The analog correction circuits  1004 A,  1004 B can provide feedback and correction signals to the device as described previously, and the digital controller then monitors the correction signals  1202 ,  1204  and readjusts the currents and voltages to have the feedback currents from the analog correction portions approach zero. The adjusted currents are used by the aging model to update the aging coefficients. This allows for correction of the laser controller over the life of the SGDBR laser, and accounts for changes in operating temperatures and conditions as well as changes in the operation of the SGDBR laser internal components.  
         [0126]    5 Power, Wavelength, and Gain Voltage Control  
         [0127]    Power, wavelength, and gain voltage control operates the power and wavelength control and gain voltage control simultaneously.  
         [0128]    6 Conclusion  
         [0129]    The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is not intended that the scope of the invention be limited by this detailed description.  
         [0130]    This concludes the description of the preferred embodiment of the present invention. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.