Patent Publication Number: US-2005123008-A1

Title: Multiple input/output ECDL cavity length and filter temperature control

Description:
FIELD OF THE INVENTION  
      Embodiments of the present invention relate to lasers and, more particularly, to tunable lasers.  
     BACKGROUND INFORMATION  
      Wavelength division multiplexing (WDM) is a technique used to transmit multiple channels of data simultaneously over the same optic fiber. At a transmitter end, different data channels are modulated using light having different wavelengths (colors) for each channel. The fiber can simultaneously carry multiple channels in this manner. At a receiving end, these multiplexed channels may be easily separated prior to demodulation using appropriate wavelength filtering techniques.  
      The need to transmit greater amounts of data over a fiber has led to so-called Dense Wavelength Division Multiplexing (DWDM). DWDM involves packing additional channels into a given bandwidth space. The resultant narrower spacing between adjacent channels in DWDM systems demands precision wavelength accuracy from the transmitting laser diodes.  
      Tunable lasers offer a flexible and cost-effective option for use in optical networking applications. A single tunable laser may replace anyone of hundreds of fixed wavelength lasers in a DWDM link and therefore offer a significant opportunity for cost reduction. They further allow precise control over the wavelength separation between lasers in the array. The ability to tune the lasing frequency also relaxes fabrication tolerances and makes for robust laser components that may be tuned to compensate for ambient temperature changes and drift due to the effects of aging. Tunable lasers further offer the advantage of permitting flexible network management as well as lending themselves well to reconfiguration. This lends to a more efficient bandwidth usage that can be readily adaptable to new customer services.  
      There is an increasing demand for tunable lasers for test and measurement uses, wavelength characterization of optical components, fiber optic networks and other applications. In dense wavelength division multiplexing (DWDM) fiber optic systems, multiple separate data streams propagate concurrently in a single optical fiber, with each data stream created by the modulated output of a laser at a specific channel frequency or wavelength. Presently, channel separations of approximately 0.4 nanometers in wavelength, or about 50 GHz are achievable, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.  
      DWDM systems have largely been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the International Telecommunication Union (ITU) wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels.  
      Continuously tunable external cavity lasers have been developed to overcome the limitations of individual DFB devices. Various laser-tuning mechanisms have been developed to provide external cavity wavelength selection, such as mechanically tuned gratings used in transmission and reflection. External cavity lasers should be able to provide a stable, single mode output at selectable wavelengths while effectively suppress lasing associated with external cavity modes that are within the gain bandwidth of the cavity. These goals have been difficult to achieve, and there is accordingly a need for an external cavity laser that provides stable, single mode operation at selectable wavelengths.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified:  
       FIG. 1  is a schematic diagram of a generalized embodiment of an external cavity diode laser (ECDL);  
       FIG. 2  is a diagram illustrating the effect modulating the optical path length of an ECDL laser cavity has on the frequency of the lasing mode and the modulation of the laser&#39;s output intensity;  
       FIG. 3  is a diagram illustrating how a modulated excitation input signal and a resulting response output signal can be combined to calculate a demodulated error signal;  
       FIG. 4  is a schematic diagram of an ECDL in accordance with an embodiment of the invention in which a Lithium Niobate block is employed as an optical path length adjustment element;  
       FIG. 5  is a block diagram illustrating a control scheme for controlling the temperatures of the various tunable elements;  
       FIG. 6  is a block diagram of a control scheme employing a coupler matrix for mathematically manipulating multiple inputs to produce multiple outputs for locking filter temperature to cavity length; and  
       FIGS. 7A-7C  include exemplary coupler matrices for relating an input signal matrix to an output signal matrix for achieving a desired tuning operation.  
    
    
     DETAILED DESCRIPTION  
      Embodiments of a servo or control technique and apparatus for performing wavelength locking that locks the cavity length of an external cavity diode laser (ECDL) to other tunable elements such as temperature controlled filters are disclosed. In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
      Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
      As an overview, a generalized embodiment of an ECDL  100  that may be used to implement aspects of the invention described below is shown in  FIG. 1 . ECDL  100  includes a gain medium comprising a diode gain chip  102 . Diode gain chip  102  comprises a Fabry-Perot diode laser including a partially-reflective front facet  104  and a substantially non-reflective rear facet  106  coated with an anti-reflective (AR) coating to minimize reflections at its face. Optionally, diode gain chip  102  may comprise a bent-waveguide structure on the gain medium to realize the non-reflective rear facet  106 . The external cavity elements include a diode intracavity collimating lens  108 , tuning filter elements  110 , a cavity-length modulating element  112 , and a reflective element  114 . In general, reflective element  114  may comprise a mirror, grating, prism, or other reflector or retroreflector which may also provide the tuning filter function in place of element  110 . The output side components include a diode output collimating lens  116 , an optical isolator  118 , and a fiber focusing lens  120 , which focuses an output optical beam  122  such that it is launched into an output fiber  124 .  
      The basic operation of ECDL  100  is a follows. A controllable current I is supplied to diode gain chip  102  (the gain medium), resulting in a voltage differential across the diode junction, which produces an emission of optical energy (photons). The emitted photons pass back and forth between partially-reflective front facet  104  and reflective element  114 , which collectively define the ends of the laser cavity. As the photons pass back and forth, a plurality of resonances, or “lasing” modes are produced. Under a lasing mode, a portion of the optical energy (photons) temporarily occupies the external laser cavity, as depicted by intracavity optical beam  126 ; at the same time, a portion of the photons in the external laser cavity eventually passes through partially-reflective front facet  104 .  
      Light comprising the photons that exit the laser cavity through partially-reflective front facet  104  passes through diode output collimating lens  116 , which collimates the light into output beam  122 . The output beam then passes through optical isolator  118 . The optical isolator is employed to prevent back-reflected light from being passed back into the external laser cavity, and is generally an optional element. After the light beam passes through the optical isolator, it is launched into the output fiber  124  by fiber focusing lens  120 . Generally output fiber  124  may comprise a polarization-preserving type or a single-mode type such as SMF-28.  
      Through appropriate modulation of the input current (generally for communication rates of up to 2.5 GHz) or through modulation of an external element disposed in the optical path of the output beam (not shown) (for 10 GHz and 40 GHz communication rates), data can be modulated on the output beam to produce an optical data signal. Such a signal may launched into a fiber and transmitted over a fiber-based network in accordance with practices well known in the optical communication arts, thereby providing very high bandwidth communication capabilities.  
      The lasing mode of an ECDL is a function of the total optical path length between the cavity ends (the cavity optical path length); that is, the optical path length encountered as the light passes through the various optical elements and spaces between those elements and the cavity ends defined by partially-reflective front facet  104  and reflective element  114 . This includes diode gain chip  102 , diode intracavity collimating lens  108 , tuning filter elements  110 , and cavity-length modulating element  112 , plus the path lengths between the optical elements (i.e., the path length of the transmission medium occupying the ECDL cavity, which is typically a gas such as air). More precisely, the total optical path length is the sum of the path lengths through each optical element and the transmission medium times the coefficient of refraction for that element or medium.  
      As discussed above, under a lasing mode, photons pass back and forth between the cavity end reflectors at a resonance frequency, which is a function of the cavity optical path length. In fact, without the tuning filter elements, the laser would resonate at multiple frequencies. For simplicity, if we model the external laser as a Fabry-Perot cavity, these frequencies can be determined from the following equation:  
             Cl   =       λ   ⁢           ⁢   x       2   ⁢   n               (   1   )             
 
 where λ=wavelength, Cl=Length of the cavity, x=an arbitrary integer—1, 2, 3, . . . , and n=refractive index of the medium. The number of resonant frequencies is determined from the width of the gain spectrum. Furthermore, the gain spectrum is generally shaped as a parabola with a central peak—thus, the intensity of the lasing modes on the sides of the center wavelength (commonly called the side modes) rapidly drops off. 
 
      As describe below in further detail, various techniques may be applied to “tune” the laser to produce an optical output signal at a frequency corresponding to a desired communication channel. For example, this may be accomplished by adjusting one or more tuning elements, such as tuning filter elements  110 , to produce a corresponding change in the cavity optical path length, thus changing the lasing mode frequency. The tuning filter elements attenuate the unwanted lasing modes such that the output beam comprises substantially coherent light having a narrow bandwidth.  
      Ideally, it is desired to maximize the power of the output beam over a frequency range corresponding to the various channel frequencies the ECDL is designed for. While an obvious solution might be to simply provide more drive current, this, by itself, doesn&#39;t work because a change in the drive current changes the optical characteristics (e.g., optical path length) of the diode gain chip. Furthermore, many diode gain chips only operate over a limited range of input current.  
      In accordance with aspects of the invention, one technique for producing a maximal power output is to perform “wavelength locking” through phase control modulation. Under this technique, a “dither” or modulation signal is supplied to cause a corresponding modulation in the optical path length of the laser cavity. This produces a modulated phase-shift effect, resulting in a small frequency modulation of the lasing mode. The result of this frequency modulation produces a corresponding modulation of the intensity (power) of the output beam, also referred to as amplitude modulation. This amplitude modulation can be detected using various techniques. In one embodiment, the laser diode junction voltage (the voltage differential across laser diode chip  102 ) is monitored while supplying a constant current to the laser diode, wherein the voltage is inversely proportional to the intensity of the output beam, e.g., a minimum measured diode junction voltage corresponds to a maximum output intensity. In another embodiment, a beam splitter is employed to split off a portion of the output beam such that the intensity of the split-off portion can be measured by a photo-electric device, such as a photodiode. The intensity measured by the photodiode is proportional to the intensity of the output beam. The measured amplitude modulation may then be used to generate a demodulated error signal that is fed back into a servo control loop to adjust the (substantially) continuous optical path length of the laser so as to produce maximal intensity.  
      The foregoing scheme is schematically illustrated in  FIG. 2 . The diagram shows a power output curve (P O ) that is illustrative of a typical power output curve that results when the lasing mode is close to a desired channel, which is indicated by a channel frequency centerline  200 . The objective of a servo loop that employs the phase-shift modulation scheme is to adjust one or more optical elements in the laser cavity such that lasing frequency is shifted toward the desired channel frequency. This is achieved through use of a demodulated error signal that results from frequency modulation of the lasing mode. Under the technique, a modulation signal is supplied to an optical element in the cavity, such as optical length modulation element  112 , to modulate the optical path length of the cavity. This modulation is relatively small compared to the channel spacing for the laser. For example, in one embodiment the modulation may have an excursion of 4 MHz, while the channel spacing is 50 GHz.  
      Modulated signals  202 A,  202 B, and  202 C respectively correspond to (average) laser frequencies  204 A,  204 B, and  204 C. Laser frequency  204 A is less than the desired channel frequency, laser frequency  204 C is higher than the desired channel frequency, while  204 B is near the desired channel frequency. Each modulated signal produces a respective modulation in the intensity of the output beam; these intensity modulations are respectively shown as modulated amplitude waveforms  206 A,  206 B, and  206 C. Generally, the intensity modulations can be measured in the manners discussed above for determining the intensity of the output beam.  
      As depicted in  FIG. 2 , the peak to valley amplitude of waveforms  206 A,  206 B, and  206 C is directly tied to the points in which the modulation limits for their corresponding frequency modulated signals  202 A,  202 B, and  202 C intersect with power output curve P O , such as depicted by intersection points  208  and  210  for modulated signal  202 A. Thus, as the laser frequency gets closer to the desired channel frequency, the peak to valley amplitude of the measured intensity of the output beam decreases. At the point where the laser frequency and the channel frequency coincide, this value becomes minimized.  
      Furthermore, as shown in  FIG. 3 , the cavity length error may be derived from:  
             Error   =         ∫     t   1       t   2       ⁢     ER   ⁢           ⁢     ⅇ     ⅈϕ   ⁡     (   ω   )         ⁢     ⅆ   t         ≈       ∑     i   =   1     n     ⁢       E   i     ⁢     R   i     ⁢     ⅇ     ⅈϕ   ⁡     (   ω   )                       (   2   )             
 
 wherein the non-italicized i is the imaginary number, φ represents the phase difference between the excitation input (i.e., modulated signals  202 A,  202 B, and  202 C) and the response output comprising the amplitude modulated output waveforms  206 A,  206 B, and  206 C, and ω is the frequency of modulation. The integral solution can be accurately approximated by a discreet time sampling scheme typical of digital servo loops of the type described below, as depicted by time sample marks  300 . 
 
      In addition to providing an error amplitude, the foregoing scheme also provides an error direction. For example, when the laser frequency is in error on one side of the desired channel frequency (lower in the illustrated example), the excitation and response waveforms will be substantially in phase. This will produce a positive aggregated error value. In contrast, when the laser frequency is on the other side of the desired channel frequency (higher in the example), the excitation and response waveforms are substantially out of phase. As a result, the aggregated error value will be negative.  
      Generally, the wavelength locking frequency of modulation ω should be selected to be several orders of magnitude below the laser frequency. For example, modulation frequencies within the range of 500 Hz-100 kHz may be used in one embodiment with a laser frequency of 185-199 THz.  
      In  FIG. 4 , an ECDL  400  is shown including various elements common to ECDL  100  having like reference numbers, such as a gain diode chip  102 , lenses  108 , 116 , and  120 , etc. A channel selection subsystem may include a wavelength selection control block  502 . It is noted that although the wavelength selection control block is shown external to controller  420 , the control aspects of this block may be provided by the controller  420  alone. Wavelength selection control block  502  provides electrical outputs  504  and  506  for controlling the temperatures of filters F 1  and F 2 , respectively. In one embodiment, temperature control element is disposed around the perimeter of a circular etalon, as depicted by TECs  508  and  510 . Heaters imbedded inside of the filters may also be used to control etalon temperature. Respective RTDs  512  and  514  are employed to provide a temperature feedback signal back to wavelength selection control block  502 .  
      Generally, etalons are employed in laser cavities to provide filtering functions. They function as Fabry-Perot resonators. The result of passing an optical beam through an etalon produces a set of transmission peaks (also called passbands) in the laser output. The spacing of the transmission peaks (in frequency, also known as the free spectral range) is dependent on the distance between the two faces of the etalon, e.g., faces  516  and  518  for filter F 1 , and faces  520  and  522  for filter F 2 . As the temperatures of the etalons change, the etalon material is caused to expand or contract, thus causing the distance between the faces to change. This effectively changes the optical path length of the etalons, which may be employed to shift the transmission peaks.  
      The effect of the filters is cumulative. As a result, all lasing modes except for a selected channel lasing mode can be substantially attenuated by lining up a single transmission peak of each filter. In one embodiment, the configurations of the two etalons are selected such that the respective free spectral ranges of the etalons are slightly different. This enables transmission peaks to be aligned under a Vernier tuning technique similar to that employed by a Vernier scale. In one embodiment, one of the filters, known as a “grid generator,” is configured to have a free spectral range corresponding to a communications channel grid, such as the ITU wavelength grid, and the peaks are aligned with ITU channel frequencies. This wavelength grid remains substantially fixed by maintaining the temperature of the corresponding grid generator etalon at a predetermined temperature. At the same time, the temperature of the other etalon, known as the channel selector, is adjusted so as to shift its transmission peaks relative to those of the grid generator. By shifting the transmission peaks of the filters in this manner, transmission peaks corresponding to channel frequencies may be aligned, thereby producing a cavity lasing mode corresponding to the selected channel frequency. In another embodiment, the transmission peaks of both the filters are shifted to select a channel.  
      Generally, either of these schemes may be implemented by using a channel-etalon filter temperature lookup table in which etalon temperatures for corresponding channels are stored, as depicted by lookup table  524 . Typically, the etalon temperature/channel values in the lookup table may be obtained through a calibration procedure, through statistical data, or calculated based on tuning functions or equations fit to the tuning data. In response to an input channel selection  444 , the corresponding etalon temperatures are retrieved from lookup table  524  and employed as target temperatures for the etalons using appropriate temperature control loops, which are well-known in the art.  
      ECDL  400  may further include a cavity optical path length modulating element  412  having a reflective rear face  414 . More specifically, the cavity optical path length modulating element comprises a Lithium Niobate (LiNbO3) phase modulator to which a back-side mirror is coupled. Optionally, a reflective material may be coated onto the backside of the phase modulator. Lithium Niobate is a material that changes its index of refraction (ratio of the speed of light through the material divided by the speed of light through a vacuum) when a voltage is applied across it. As a result, by providing a modulated voltage signal across the LiNbO3 phase modulator, the optical path length of the external laser cavity can be caused to modulate or “dithered”, thereby producing frequency modulated signals such as signals  202 A,  202 B, and  202 C discussed above.  
      The various optical components of the ECDL  400  are mounted or otherwise coupled to a thermally-controllable base or “sled”  416 . In one embodiment, one or more thermal-electric cooler (TEC) elements  418 , such as a Peltier element, are mounted on or integrated in sled  416  such that the temperature of the sled can be precisely controlled via an input electrical signal. Due to the expansion and contraction of a material in response to a temperature change, the length of the sled can be adjusted very precisely. Adjustment of the length results in a change in the distance between partially reflective front facet  104  and reflective element  414 , which produces a change in the optical path length of the laser cavity. As a result, controlling the temperature of the sled can be used to adjust the frequency of the lasing mode. In general, temperature control of the sled will be used for very fine tuning adjustments, while coarser tuning adjustments will be made by means of tuning filter elements  110 , as described in further detail below.  
      For wavelength-locking, a controller  420  generates a modulated or “dithered” wavelength-locking signal  422 , which is amplified by an amplifier  424 . For example, in one embodiment modulated wavelength locking signal  422  may comprise a sinewave having a constant frequency, such as a 2-volt peak-to-peak signal with a frequency of about 889 Hz. The amplified modulated wavelength locking signal is then supplied to a surface of the LiNbO3 phase modulator  412 , while an opposite surface is connected to ground, thereby providing a voltage differential across the LiNbO3 material. As a result, the optical path length of the modulator, and thus the entire laser cavity, is modulated at the modulation frequency (e.g. 889 Hz). In one embodiment, the 2-volt peak-to-peak voltage differential results in a frequency excursion of approximately 4 MHz.  
      This path length modulation produces a modulation in the intensity of output beam  122 , which in one embodiment is detected by a photodetector  426 . As depicted in  FIG. 4 , a beam splitter  428  is disposed in the optical path of output beam  122 , causing a portion of the output beam light to be directed toward photodetector  426 . In one embodiment, photodetector  426  comprises a photo diode, which generates a voltage charge in response to the light intensity it receives (hvdet). A corresponding voltage V PD  is then fed back to controller  420 . In an optional embodiment, the junction voltage across gain diode chip (V J ) is employed as the intensity feedback signal, rather than V PD . A cavity length error signal as discussed previously with reference to  FIG. 3  is then derived based on the amplitude modulation and phase of V PD  or V J  in combination with modulated wavelength locking signal  422 .  
      Controller  420  includes a digital servo loop that is configured to adjust the temperature of sled  416  such that the cavity length error signal is minimized, in accordance with the frequency modulation scheme discussed above with reference to  FIGS. 2 and 3 . In response to the error signal, an appropriate adjustment in temperature control signal  430  is generated. Adjustment of the sled temperature causes a corresponding change in the overall cavity length, and thus the lasing frequency. This in turn results in (ideally) a decrease in the difference between the lasing frequency and the desired channel frequency, thus completing the control loop. To reach an initial condition, or for controlling sled temperature, a resistive thermal device (RTD)  434 , or a thermister or thermocouple, may be used to provide a temperature feedback signal  434  to controller  420 .  
      When tuning a tunable laser to a target frequency (i.e., a new channel), both the tuning speed and frequency stability are very important to the operation. Embodiments of the invention provide a solution to improve both the speed and frequency stability.  
      As noted above, in general temperature control of the sled  416  may be used for very fine tuning adjustments of the ECDL, while coarser tuning adjustments may be made by means of tuning filter elements F 1  an F 2 . While two filter elements are shown in  FIG. 4 , it is understood that more or fewer elements may be employed without departing from the scope of the invention.  
       FIG. 5  shows an example control scheme for tuning the ECDL.  FIG. 4  shows the controller represented by two separate blocks, however; both controller  420  and the wavelength selection control  502  including the look-up table  425  may be embodied in the same module. Referring to  FIGS. 4 and 5 , for a particular channel the corresponding temperatures targets  602  and  604  for the tunable filters F 1  and F 2  are gleaned, for example, from the look-up table  524 . Signals representing the actual temperatures  606  and  608  for F 1  and F 2 , as measured by the RDTs  512  and  514 , are combined at  610  and  612  to produce error signals  614  and  616  for controlling the respective temperatures of the filter heaters or TECs  508  and  510 . The error signals may be processed for example by respective PID (proportional, integral and derivative) control blocks  618  and  620 , which are well known in the control system art. The PID blocks  618  and  620  produce a digital temperature command signal. The PID blocks may further include a digital/analog converter (DAC) as well as a current control circuit to control the direction of the current  624  passing through the respective TECs  508  and  510 . In accordance with Peltier device principles, if a current is driven one way, the TEC functions as a heating element, while reversing the current causes the device to act as a cooling element. Thus TECs  508  and  510  can be used to adjust the temperature of the filters F 1  and F 2  very rapidly.  
      Similarly, the demodulated error signal (Demod_Real)  626  as discussed with reference to  FIGS. 2 and 3  above is combined at  628  with a reference target signal  630  to produce an error signal  632  which is fed to PID controller  640 . In a manner as discussed above the PID controller  640  outputs a current  626  which is used to adjust the temperature of the sled TEC  418  to vary the length of the ECDL cavity through expansion and contraction of the sled  416 . Thus, the ECDL can be tuned by setting the filter temperatures F 1  and F 2  to select a particular channel and then use the sled temperature to lock the cavity length to match the wavelength set by the filters F 1  and F 2 . Generally, the setting of the filter temperatures is followed by setting the sled temperature to lock the cavity and these procedures are generally accomplished independently of one another. The bandwidth of this control scheme may be limited by the signal to noise ratio on the various RDT sensors. If there are sufficient temperature disturbances to the various temperature sensors it may be difficult to provide a stiff high bandwidth control loop for the filter temperature.  
       FIG. 6  shows an embodiment for a control scheme which relates the multiple input signals ( 614 ,  616  and  632 ) for controlling the temperatures of the Filters F 1  and F 2  and the sled  416  to achieve a fast and accurate lock sequence for the various components. In this case, the controller comprises a coupler matrix  700  that linearly relates the multiple input (error) signals  614 ,  616  and  632  to produce multiple output signals  702 ,  704 , and  706 . Each of these output signals are thereafter processed by a respective PID controller  708 ,  710 , and  712  for controlling the temperature of the sled TEC  418 , F 1  TEC  508  and F 2  TEC  510  as previously described.  
       FIGS. 7A, 7B , and  7 C show non-exhaustive examples of several coupler matrix configurations to achieve various results. In the example shown in  FIG. 7A , the coupler matrix  700   1  is an identity matrix which when crossed with the input matrix  800  comprising the various inputs ( 614 ,  616 , and  632 ) produces an output matrix  900  corresponding multiple output signals ( 704 ,  706 , and  702 ) for controlling the temperatures of F 1 , F 2 , and the sled TEC, respectively. Since the identity matrix comprises numeric “ 1 ” coefficients across the diagonal, this particular configuration accomplishes the same result as the control scheme shown in  FIG. 6 . That is: 
 Sled_PID_input=(1)( D mod_Real)+0+0    F   1 _PID_input=0+(1)(Filt 1 _error)+0    F   2 _PID_input=0+0+(1)Filt 2 _error  
      As shown in  FIG. 7B , the coupler matrix  7002  comprises various numerical coefficients relating the signal values in the input matrix  800  to produce signals in the output matrix  900  for controlling the various PID controllers  702 ,  704 , and  706 . In this case, the K 1  is a gain coefficient that may be selected to weight the filter temperature error signals more than the cavity length error signal or vice-versa. In this case, the “1s” and “−1s” in the lower right of the coupler matrix  700   2  compensates for the difference in the two filter temperatures F 1  and F 2 . While 1 and −1 are shown in this example, other additive inverses may also be used. Further, this coupler matrix  700   2  serves to lock the filter temperature are F 1  and F 2  to the cavity length as: 
 
Sled_PID_input=(0)( D mod_Real)+(0)(Filt 1 _error)+(0)(Filt 2 _error) 
 
 F   1 _PID_input=( K   1 )( D mod_Real)+(1)(Filt 1 _error)−(1)(Filt 2 _error) 
 
 F   2 _PID_input=( K   1 )( D mod_Real)−(1)(Filt 1 _error)+(1)(Filt 2 _error). 
 
      As a further example,  FIG. 7C  shows another coupler matrix used to lock the filter temperatures F 1  and F 2  to the cavity length. Again, the “1s” and “−1s” in the lower right of the coupler matrix  700   3  compensates for the difference in the two filter temperatures F 1  and F 2 . K 1  and K 2  are gain coefficients selected to weight the filter temperature values or the cavity length values. K 1  and K 2  may also be selected to include unit conversion factors since the filter error signals and the sled error signal may not be calibrated in the same units. Here, the control scheme with coupler matrix  700   3  serves to lock the filters F 1  and F 2  to the sled  416  rather than just commanding the sled  416  to keep the difference in the two filter temperatures constant. The control scheme of coupler matrix  7003  is set forth as: 
 
Sled_PID_input=(0)( D mod_Real)+( K   2 )(Filt 1 _error)+( K   2 )(Filt 2 _error) 
 
 F   1 _PID_input=( K   1 )( D mod_Real)+(1)(Filt 1 _error)−(1)(Filt 2 _error) 
 
 F   2 _PID_input=( K   1 )( D mod_Real)−(1)(Filt 1 _error)+(1)(Filt 2 _error). 
 
      When tuning a tunable laser to a target frequency (i.e., a new channel), both the tuning speed and frequency stability are very important to the operation. Embodiments of the invention provide a solution to improve both the speed and frequency stability.  
      When initially tuning the ECDL  400  to a new frequency (channel), the cavity length is on either side of the hill (P O ) as shown in  FIG. 2  and moves to reach to the peak of the transmission curve. This may be referred to as Temperature mode where, for example, look-up tables  524  or equations may be used to set the initial filter (F 1  and F 2 ) temperatures for a given frequency. Once the desired frequency is reached the controller switches to Cavity Mode where the coupler matrix  700  control scheme may be used to accurately lock the filter temperatures to the cavity length.  
      While embodiments have been described in terms of locking cavity length to filter temperatures for a tunable laser, the described techniques may also be applied to other types of tunable laser that uses different types of actuators to tune to a requested frequency.  
      The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
      These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.