Abstract:
A wavelength monitor is provided based on the transmission response of an optical filter. The monitor provides feedback to the laser enabling it to lock to any given wavelength within its tuning range. The invention is also a process for integrating the wavelength monitor directly on chip with a variety of tunable semiconductor lasers. The invention also comprises a method for controlling the wavelength of a tunable laser by using a wavelength monitor to measure the output light and provide feedback to a control system. The laser and wavelength monitor are integrated together on a single indium phosphide chip. The wavelength monitor comprises a filter with a wavelength dependent transmission function and a pair of detectors. One detector is illuminated with light that has passed through the filter and the other provides a reference to measure the input intensity. Taking the ratio of the filtered light level to the unfiltered light provides a wavelength dependent wavelength. The filter is designed such that the transmission function is monotonic and varies from a minimum at one extent of the laser&#39;s tuning range to a maximum at the other extent.

Description:
RELATED APPLICATIONS 
     The present application is related to U.S. Provisional Patent Application, serial No. 60/122,194, filed on Mar. 1, 1999. 
    
    
     This invention was made with Government support under Grant No. N00014-96-1-6014 awarded by the Office of Naval Research. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of semiconductor lasers, and in particular to tunable semiconductor lasers. 
     2. Description of the Prior Art 
     There are a variety of semiconductor laser designs in which the wavelength can be tuned by current injection into a section which has some spectral filtering characteristic. Examples of these are moderately tunable devices such as the distributed Bragg reflector laser, which consists of a gain section, a phase control section and a grating mirror, and widely tunable lasers that employ Vernier effect tuning like the sampled grating distributed Bragg reflector lasers. In all of these devices precise control of the tuning current is required to achieve the desired output wavelength. The amount of current required to achieve a given wavelength can vary with the device temperature and also with aging. Over time as the laser ages changes in the leakage current and the carrier lifetime in the tuning sections cause the wavelength to drift. This variation in the tuning characteristics requires the use of some sort of feedback control system in order for such tunable lasers to be used in applications where precise wavelength control must be maintained such as dense wavelength division multiplexed (WDM) communications systems. 
     Single wavelength devices such as distributed feedback lasers are also subject to wavelength drift. For these devices simple wavelength lockers are used with temperature or current feedback to control the wavelength of the laser. However these wavelength lockers are only capable of maintaining the laser wavelength on a single channel which makes them useless as multiple-channel tunable lasers. 
     Tunable semiconductor lasers are important components for next generation dense wavelength division multiplexed fiber optic networks. 
     What is needed is frequency stable, tunable laser module whose output frequency does not change over time. The device should be integrated on a chip, compact and cost effective. It should have a wide tuning range and be usable in high speed data transmission under direct modulation in multichannel dense WDM networks. 
     BRIEF SUMMARY OF THE INVENTION 
     What has been developed is a wavelength monitor based on the transmission response of an optical filter for use in controlling the wavelength of a tunable laser. This monitor can provide feedback on the laser&#39;s wavelength over a wide tuning range enabling it to also lock the device to any given wavelength or channel within its tunable range. It can also be used in combination with conventional external lockers to more precisely tune to a particular wavelength within a given channel. 
     The invention is also a process for integrating the wavelength monitor directly on chip with a variety of tunable semiconductor lasers. This results in a significantly more compact and cost effective device. 
     The invention comprises a method for controlling the wavelength of a tunable laser by using a wavelength monitor to measure the output light and provide feedback to a control system. In the preferred embodiment the laser and wavelength monitor are integrated together on a single indium phosphide chip. The technique is useful for control of lasers that are tuned by current injection into sections which have a bandgap energy, which is greater than the lasing wavelength. 
     The wavelength monitor comprises a wavelength filtering element and a pair of detector elements. In one embodiment these elements are combined by using detectors of different bandgap arranged in series. In others a separate filter with a wavelength dependent transmission function is placed in front of the detectors. In some of these embodiments this separate filter is designed as a wavelength dependent splitter to provide an output beam that translates laterally from one detector to the other as a function of wavelength (much like a prism); in other embodiments the laser output is first split into two beams, and a wavelength dependent transmission filter is placed only in one of these prior to illuminating the detectors. In all cases, taking the difference between in the photocurrents in the two detectors, and normalizing by dividing by the sum, provides a measure of the integrated laser&#39;s wavelength. In all cases, the net filtering function is designed such that the normalized difference current is monotonic and varies from a minimum at one extent of the laser&#39;s tuning range to a maximum at the other extent. This approach can provide channel identification across this entire band of wavelengths, and it can supplement or replace the function of existing external wavelength lockers that only lock within the span of one channel. 
     More specifically, the invention is an apparatus comprising a tunable laser and a wavelength-dependent optical device to produce two optical signals from light sampled from the laser. The laser is semiconductor laser and wherein the wavelength-dependent optical device is integrally fabricated with the semiconductor laser on a common chip. The two optical signals are wavelength dependent and are distinguished from each other by a different dependence on wavelength. A first and second detector are provided to detect the two optical signals to generate two corresponding electrical detection signals. A processor is coupled to the to first and second detector to generate a control signal from the two corresponding electrical detection signals by which the tunable laser is tuned. 
     In one embodiment, the wavelength-dependent optical device comprises a two mode interference waveguide. A wavelength-dependent Y-branch splitter is coupled between the two mode interference waveguide and the first and second detector. The first and second detector comprises a segmented detector and further comprises a flared waveguide coupling the two mode interference waveguide and the segmented detector. 
     In another embodiment, the a wavelength-dependent optical device comprises a wavelength-dependent optical filter. In one embodiment, the wavelength-dependent optical device comprises a wavelength-dependent optical coating. In another embodiment, the a wavelength-dependent optical device comprises a wavelength-dependent optical grating. 
     In some embodiments, wavelength-dependent optical device comprises one of the two detectors, and is termed a monitoring detector. The monitoring detector has a higher bandgap absorber therein than the other one of the first and second detectors. In one embodiment, the monitoring detector is coupled inline between the tunable laser and the other one of the first and second detectors. 
     In still another embodiment the apparatus further comprises an attenuator and a filter. The monitoring detector is coupled inline with the other one of the first and second detectors which is coupled to the tunable laser. The attenuator and the filter are coupled in line between the monitoring detector and the other one of the first and second detectors. 
     In still another embodiment, the apparatus further comprises a splitter. The splitter directs a first portion of light from the laser to the first detector and a second portion of light from the laser to the coating. The second detector detects light reflected from the coating. 
     In one embodiment, the apparatus further comprises a diffractor. The diffractor directs a first portion of light from the laser into the coating such that the first portion is totally internally reflected and a second portion of light from the laser into the coating such that the second portion of light from the laser is reflected through the coating to the second detector. 
     The invention is also defined as a method of operating a tunable laser comprising the steps of sampling light from the laser and producing two optical signals from light sampled from the laser. The two optical signals are wavelength dependent and distinguished from each other by a different dependence on wavelength. The two optical signals are detected to generate two corresponding electrical detection signals. A wavelength dependent control signal is generated from the two corresponding electrical detection signals. The control signal is fed back to the tunable laser to control wavelength of light generated by the laser. 
     Again in one embodiment the step of producing two optical signals is performed by creating a wavelength dependent, beat pattern between two modes of light. 
     In another embodiment the step of producing two optical signals is performed by monotonically filtering the light as a function of wavelength. For example, the step of producing two optical signals is performed by monotonically filtering the light as a function of wavelength by transmission through a grating or by reflection from a grating. In the latter case, the two optical signals are produced by monotonically filtering the light as a function of wavelength by reflecting the light off a back facet of the tunable laser through a wavelength dependent coating disposed on the back facet. 
     The tunable laser has a tuning range, and the degree of filtering of the intensity of the light is varied between extremums corresponding to the tuning range of the laser. 
     The step of creating a wavelength dependent, beat pattern between two modes of light is performed for several beat lengths of the two modes of light and then comprises splitting the two modes light into first and second paths and thereafter detecting the two modes of light in the first and second paths. The step of splitting the sampled light into a first and second path comprises splitting the two modes of light into the first and second path with a wavelength dependent splitting ratio. 
     The method further comprises determining total power output from the laser by summing the two corresponding electrical detection signals. The wavelength dependent control signal is generated from a ratio of intensity of the light detected corresponding to the two optical signals by normalizing the control signal with the total power output. 
     In one embodiment, the step of splitting the sampled light comprises transmitting the two modes of light to two segmented detectors. Detection of the two optical signals comprises detecting the light in the two segmented detectors to provide two complementary sinusoidal detection signals. The step of generating a wavelength dependent control signal comprises generating the control signal as a ratio between the difference of the two complementary sinusoidal detection signals and the sum of the two complementary sinusoidal detection signals. 
     In some embodiments the steps of producing and detecting the two optical signals comprises detecting the light with a first detector with an absorber having a bandgap energy low enough to provide substantially full absorption of the light, and detecting the light with a second detector with an absorber having a bandgap energy slightly greater than the shortest wavelength of the light so that absorption in the second detector varies with wavelength of the light. 
     In another embodiment the step of producing two optical signals comprises diffracting the light back onto a back facet of the laser so that a portion is totally internally reflected and a portion reflected back from a wavelength dependent coating disposed on the back facet. 
     In another embodiment the step of producing two optical signals comprises splitting the light into two detectors in which at least one of the detectors detects the light in a wavelength dependent manner. For example, the step of producing two optical signals comprises transmitting the light through a first detector into a second detector in which at least one of the detectors detects the light in a wavelength dependent manner. The step of producing two optical signals comprises transmitting the light first through the first detector and then into the second detector in which at least the first detector detects the light in a wavelength dependent manner. Alternatively, the step of producing two optical signals comprises transmitting the light first through the first detector, then through an attenuator and filter in optical series, and then into the second detector in which at least the second detector detects the light in a wavelength dependent manner. 
     The invention can also be characterized as a method of operating a tunable laser comprising the steps of sampling light from the laser, and generating two signals from the light sampled from the laser, which two signals are wavelength dependent and distinguished from each other by a different dependence on wavelength. Two detected signals corresponding to the two signals are generated. The two detected signals are combined to produce a wavelength dependent control signal. The laser is tuned by feeding back the wavelength dependent control signal to the tunable laser. 
     Although the method has been described above for the purposes of grammatical ease in terms of steps, it is to be expressly understood, that the claims are not to be limited by “means” or “steps” restrictions based on a construction under 35 USC 112. The invention having been summarized can be better visualized by turning to the following drawings wherein like elements are referenced by like numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective schematic of a tunable laser fabricated according to the invention. 
     FIG. 2 is a graph of wavefunction intensity showing the overlap of two mode interference waveguide modes with a laser mode in a TMI waveguide section as a function of lateral dimension. 
     FIG. 3 a  is a side cross-sectional view of an active section of the laser. 
     FIG. 3 b  is a side cross-sectional view of a passive section of the laser. 
     FIG. 4 is a perspective schematic of another embodiment of a tunable laser fabricated according to the invention in which a buried heterostructure is provided. 
     FIG. 5 is a graph of the normalized detector response as a function of wavelength for the embodiment of FIG. 4 showing sinusoidal wavelength dependence. 
     FIG. 6 is a graph of fiber launched power and detector current as a function of drive current for the embodiment of FIG. 4 with fixed tuning currents. 
     FIG. 7 a  is a schematic of another embodiment of a tunable laser including series connected detectors. 
     FIG. 7 b  is a graph of the photocurrents from two detectors shown as a function of wavelength. 
     FIG. 8 is a schematic diagram of another embodiment in which the wavelength monitor is implemented with external or nonintegrated components. 
     FIG. 9 is a top plan schematic view of still another embodiment of the invention using an external wavelength monitor implementation of a dielectric filter in a planar lightwave circuit platform. 
     FIG. 10 is a schematic diagram of an embodiment in which reflection from a wavelength dependent dielectric film is used to implement the wavelength monitor. 
     FIG. 11 is a schematic diagram of an embodiment in which reflection from the wavelength dependent dielectric film is used to implement the wavelength monitor as in the case of FIG. 10, but without the use of a beam splitter. 
     FIGS. 12 a - 12   f  are block diagrams summarizing various embodiments of the invention. 
    
    
     The invention and its various embodiments can be better understood by turning to the following detailed description of the illustrated embodiments. It is to be expressly understood that the invention as defined in the following claims is not to be limited by the illustrated embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Tunable semiconductor lasers are important components for next generation dense wavelength division multiplexed fiber optic networks. The wide tuning range, and ability for high speed data transmission under either direct or external modulation, of sampled grating, distributed Bragg reflector (SGDBR) lasers makes them especially attractive for use in multichannel dense WDM networks. These lasers tune over wide wavelength ranges by employing Vernier effect tuning. Wavelength control in these devices requires the alignment of reflection peaks from the front and back mirrors with the cavity mode at the desired wavelength. This entails control of four separate tuning currents to achieve complete wavelength coverage over the entire tuning range. Using feedback on the output wavelength and intensity, it is possible to lock the device at the optimum operating point for a given wavelength. 
     Existing devices for monitoring the wavelength and output power of a tunable laser are too complicated and expensive for practical application. What has been devised is a compact integrated wavelength monitor that can be fabricated on chip with a tunable laser to provide feedback on the wavelength and output power. In another embodiment there is an external version of the same monitor which is low cost and highly stable. 
     In the preferred embodiment the apparatus of the invention is comprised of a SGDBR laser coupled to a wavelength dependent splitter and a pair of integrated detectors. A schematic of the device is shown in FIG.  1 . The tunable laser, generally denoted by reference numeral  10 , is a 3 μm wide ridge waveguide device which is comprised of four separate elements. These include two sampled grating DBR mirrors  12   a  and  12   b  and sections for gain control  14  and phase control  16 . By controlling the injection current into gain control  14  and phase control  16 , laser  10  can be made to tune over more than 40 nm with continuous wavelength coverage. At the output  18  of the back mirror section  12   b,  the laser waveguide is asymmetrically coupled into a two mode interference (TMI) waveguide  20 . At the end of the TMI section  20 , there is a Y-branch splitter  22  that divides the light into a pair of output detectors  24   a  and  24   b.  This ratiometric design results in a compact easily fabricated wavelength monitor, generally denoted by reference numeral  26 . 
     The generalized transverse active device structure as shown in side cross-sectional view of FIG. 3 a  is comprised of a thick higher bandgap energy waveguide layer  28  with an offset multiquantum well active region  30 . Waveguide  30 , which in the illustrated embodiment is a 400 nm, 1.4Q waveguide, is disposed on a 300 nm n-type buffer layer  32 . The thick high bandgap waveguide layer  28  is necessary for good carrier-induced index change in the tuning sections, which are comprised of sampled grating DBR mirrors  12   a  and  12   b  and sections for gain control  14  and phase control  16 . In the case of the active section of FIG. 3 a,  an InP etch stop layer  34  is disposed on waveguide layer  28  on which strained quantum well active layer  30  is in turn disposed. In the case of a passive section, layers  34  and  30  are omitted. Nonabsorbing passive elements as shown in the side cross-sectional view of FIG. 3 b  are formed by selectively removing the quantum wells from on top of the waveguide  28 . A p-type InP ridge  36  is then disposed on quantum well layer  30  in the case of an active section as shown in FIG. 3 a  or on waveguide  28  in the case of a passive section as shown in FIG. 3 b.  A 100 nm InGaAs contact layer  38  is then disposed on top of ridge  36 . The use of the offset quantum wells  30  allows the formation of active and passive sections in a single waveguide without having to perform a butt joint regrowth. This allows the device to be fabricated with only two chemical vapor deposition (MOCVD) growth steps. 
     Light coupled from the laser  10  into the TMI section  20  excites the two lowest order lateral modes of the waveguide as shown in the graph of FIG.  2 . These modes propagate with different phase velocities which leads to a characteristic beating in a modal superposition pattern. The beat length is dependent on the difference in the propagation constants for the two modes which each depend on the wavelength of the light. At the end of the TMI section  20 , a Y-branch splitter  40  splits the light into a pair of output waveguides  42   a  and  42   b  with a wavelength dependent splitting ratio. By making the TMI section  20  long enough to contain several beat lengths, the splitting ratio becomes increasingly sensitive to the wavelength of the laser  10 . The total power out of the back facet  18  of the laser  10  can be determined by summing the currents from both detectors  24   a  and  24   b.  The wavelength dependent signal is normalized to this sum to remove the power dependence. This results in a sinusoidal output signal that varies with wavelength. The TMI waveguide  20  is wide at its end to enhance the optical coupling and tapers quadraticly along its central interference section to increase the modal dispersion. The splitter  40  employs a Y-branch design to divide the incoming optical power into two output waveguides  42   a  and  42   b.  The output waveguides  42   a  and  42   b  contain the photodetectors  24   a  and  24   b  respectively which measure the divided light intensity. Large detector sizes are used to prevent saturation of their absorption by the high optical power levels that are experienced in the photonic integrated circuit. 
     The detector signals can then be coupled to a conventional processor or controller circuit (not shown) where the control signal is generated from the ratio of the detection signals, normalized if needed, and coupled to laser  10  to provide the appropriate tuning control. The manner in which the data or signals are processed according to the teachings of the invention is conventional and therefore will not be further illustrated in detail. It is contemplated that the processor or controller circuit will be integrated or assembled into the chip or may be externally provided as shown in FIG. 8 below. 
     A key advantage of the TMI-based wavelength monitor  26  is that it can be fabricated using many of the steps required for tunable lasers  10 , making it relatively easy to integrate on chip. There are eight main steps in the fabrication procedure for the SGDBR laser with the integrated wavelength monitor. First, a base structure  44  is grown using near atmospheric MOCVD with tertiarybutlyphosphine and tertiarybutylarsine for the group V sources. The transverse structure for the active and passive sections are shown in FIGS. 3 a  and  3   b  respectively. Passive sections are formed by selectively etching off the cap layer (not shown) and then the quantum wells  30 . The gratings  12   a  and  12   b  are then formed for the laser mirrors using a dry etch process. 
     In the second MOCVD step, a 2 μm InP upper cladding layer  36  and a 100 nm InGaAs contact layer  38  are regrown. The ridge waveguides are etched to within 200 nm of the waveguide layer using a methane-hydrogen-argon MHA RIE etch. A wet etch (HCL:H 3 PO 4 ) is then used to remove the last 200 nm and stop on the waveguide  28 . The dry etch is done to maintain the straight side walls in the curved waveguide section, while the final wet etch compensates for the nonuniformity in the MHA RIE and provides for consistent etch depth. A SiN x  layer (not shown) is used to passivate the surface with self-aligned openings on the ridge tops for the Ti/Au p-contacts  13 . Isolation between the adjacent laser sections and between the detectors is achieved by etching off the contact layer  38  and performing a deep proton implant (not shown). This step is essential for providing good isolation between the reversed biased detectors  24   a  and  24   b  and the forward biased laser sections  10 . In the final steps, the device is lapped to 100 μm thick, and a backside contact (not shown) is deposited before cleaving and mounting. 
     It is also possible to fabricate this device with a buried heterostructure. In this case the Y-branch splitter and detector pair of FIG. 1 can still be used, but in FIG. 4 an alternative embodiment is shown in which a tapered waveguide  46  with two detectors  48   a  and  48   b  placed on each side are used in place of the Y-branch splitter  40 . The output mode from the TMI waveguide  20  is then imaged on the region between the detectors  48   a  and  48   b  to provide a position sensitive output difference circuit. The fabrication is similar to the ridge process except that the ridge is etched prior to regrowth and the etch proceeds until it has cut completely through the waveguide  28 . Then in the regrowth InP material is grown up around the ridge completely burying it. The contact layer  38  is then patterned so that it only covers the area directly over the buried ridge  36  and a proton implantation step is also used to prevent lateral current spreading. 
     When the wavelength of the laser  10  is tuned, the two detectors  48   a  and  48   b  provide complementary sinusoidal responses for wavelength monitoring. Taking the ratio of the difference between the detector currents to the sum of the detector currents provides a single monotonically varying signal which is wavelength dependent and power level independent as shown in the graph of FIG.  5 . The sum of the two detector currents can be used to monitor the laser output power as shown in the graph of FIG.  6 . The current from detector  48   a  is shown by line  64 , the current from detector  48   b  is shown by line  66 , and their sum is shown by line  68 . Dots  70  show the measured launched power in the output fiber. It is important for both this and the operation of the wavelength monitor  26  that the photodiode response is as close to linear as possible and that both detectors  48   a  and  48   b  are well matched. Again the detection signals are processed by a signal processor or controller (not shown) according to conventional means consistent with the teachings of the invention. 
     Another potential embodiment for the wavelength monitor  26  would replace the TMI-Y branch wavelength-dependent splitter with a simple wavelength-independent Y-branch splitter and combine this with a wavelength-dependent detection in one branch. The wavelength-dependent detection is accomplished either by integrating a waveguide filter in front of one of the two detectors, say detector  24   b,  or by using a higher bandgap absorber material in detector  24   b  so that it has an inherent wavelength dependence. In both cases, the filtering is designed to provide a monotonically varying detection current from detector  24   b  across the tuning range of the tunable laser. The operation is similar to the external implementations to be illustrated below in FIGS. 8 and 9. 
     In the case of the separate filter element, this can be formed by etching a grating filter across the waveguide leading to detector  24   b  at the same time the gratings are etched for the mirrors of the tunable laser. It is desirable to use a different period grating to provide for the desired monotonic response as well as angle it with respect to the waveguide axis so that the rejected light does not reflect back into the laser. Alternatively, for this and other embodiments, a wavelength independent absorber can be placed between the output of the laser and the Y-branch splitter to isolate the wavelength monitor from the laser. In the case of a detector which is given a wavelength dependent absorption, an absorber with higher bandgap energy and a relatively short length is used to limit the absorption of lower energy photons. This is created by either modifying the originally grown material by such processes as quantum-well intermixing or by employing selective area growth. Both techniques are well known to those skilled in the art. 
     Reference detector  24   a  would retain the lower bandgap material of the tunable-laser gain section so that all light reaching it would be fully absorbed, thereby making its output current proportional to the total optical power produced by the laser. Thus, the output of detector  24   a  serves as a wavelength independent reference. Dividing the current from wavelength dependent detector  24   b  by that from  24   a  provides the desired normalized wavelength monitoring signal. These currents are processed by a signal processor or controller (not shown) accordingly to conventional means consistent with the teachings of the invention. 
     Another potential embodiment for the wavelength monitor  26  would employ a pair of detectors  49   a  and  49   b  with different bandgaps placed in series along a single passive waveguide continuing from the output  18  of laser  10  as illustrated in FIG. 7 a.  As can be seen, this embodiment eliminates the need for the Y-branch splitter as well as the dispersive TMI dispersive waveguide section. The back detector  49   b  is highly absorbing across the entire band while the front detector  49   a  has an absorption edge beginning near the shorter wavelength side of the tuning range as illustrated in FIG. 7 b.  This front detector  49   a  is relatively short, so that it only partially absorbs the incident light, thus transmitting about half of it in the center of the laser&#39;s tuning range. Thus, the net photocurrent would appear as in FIG. 7 b  for the two detectors. Therefore, as in other cases an unambiguous wavelength readout can be obtained by using the ratio of the difference to the sum of the two currents. An advantage of this embodiment is that it is much shorter than ones requiring the TMI and Y-branch sections. 
     It is also possible to implement the wavelength monitor  26  using an external device as shown in the schematic diagram of FIG.  8 . In this embodiment a dielectric filter  50  is used with a wavelength dependent response and a pair of detectors  52   a  and  52   b.  A 10/90 fiber optic splitter  54  is used to tap some of the output light from the laser  10  on optic fiber  57  which is split using a 3 dB coupler  56  into two separate optic fibers  58   a  and  58   b.  One fiber  58   b  is coupled directly to a reference detector  52   b.  Detector  52   b  monitors the laser power. The other fiber  58   a  has a collimator  60  with the filter element  50  in front of it. This gives it a wavelength dependent response. The signal from detector  52   a  is divided in a controller circuit  62  coupled to laser  10  by the signal from the reference detector  52   b  to provide a power independent signal. 
     The external device illustrated in FIG. 8 can also be implemented in a very compact manner using a hybrid technology shown in FIG.  9 . The laser is mounted accurately onto a planar lightwave circuit (PLC) platform  72  such that the power emitted by either the back  74  or front  76  facet is coupled into the PLC waveguide  78 . If the front facet  76  used, the light from the laser  10  is passed first through a wavelength independent 5/95 tap  80  such that the majority of the light can be used as output. Light from back facet  78  is transmitted entirely to monitor  26  without tapping. The monitored or tapped portion of light is then passed though a wavelength independent 3 dB splitter  82  which couples the pair of photodetectors  84   a  and  84   b  also bonded to the PLC platform  72 . Before detector  84   b  is a wavelength filtering element  86  is inserted. This can be implemented by a either a waveguide coupler filter or the waveguide can be cut through and the filter described in FIG. 8 is placed in the groove with an index matched epoxy. Again the detection signals are processed by a signal processor or controller (not shown) according to conventional means consistent with the teachings of the invention. 
     Another embodiment for the wavelength monitor  26  is diagrammatically shown in FIG.  10 . This embodiment employs a filter coating  88  deposited on the back facet  74  of the laser  10 . A Y-splitter  90  is used to divide the light into two paths. One path  94   a  is incident on a reference detector  92   a  for measuring the light intensity and the other path  94   b  reflects off of the back facet  74  at an angle of incidence less than the critical angle. Thin film facet coating  88  with a wavelength dependent reflectivity is used. The intensity of the reflected light varies with the wavelength providing a signal which is used for wavelength monitoring in detector  92   b.  The operation is, thus, similar to the operation of the external circuits of FIGS. 8 and 9. Again the detection signals are processed by a signal processor or controller (not shown) according to conventional means consistent with the teachings of the invention. 
     This method is also used in the embodiment schematically shown in FIG. 11 without a Y-splitter  90  by letting the light diffract so that a portion of the light is incident on the facet  74  at an angle greater than the critical angle, θ c , and a portion is incident at an angle less than the critical angle, θ c . The laser output may be curved somewhat to facilitate separation of these two portions and to prevent feedback into the laser section. Light from back facet  74  is reflected by wavelength dependent facet coating  88  at an angle to facet  74 . The portion of light which was subject to total internal reflection  98  is collected by a reference detector  100   a  and the portion which is subject to a wavelength dependent reflection is collected by a second detector  100   b.  The function of facet coating  88  might also be accomplished by an integrated grating similar to those formed in the tunable laser. The operation is again similar to the external circuits of FIGS. 8 and 9. 
     In summary, the primary embodiments of the invention are symbolically depicted in the block diagrams of FIGS. 12 a - 12   f.  FIG. 12 a  symbolizes the embodiments of FIGS. 1 and 4 where tunable laser  10  is coupled through a wavelength dependent splitter  20 ,  22  to detectors  24   a  and  24   b  or  48   a  and  48   b  respectively in which the reference signal is the sum of two detector currents. 
     FIG. 12 b  symbolizes the embodiment of FIG. 8 which is illustrated there diagrammatically as a discrete implementation, but could be implemented in a monolithic design as well. In FIG. 12 a  splitter  22  is wavelength independent and wavelength dependent filter  50  is placed between splitter  22  and monitoring detector  49   b.  The reference signal is the current output from detector  49   a.    
     FIG. 12 c  symbolizes the embodiments of FIGS. 10 and 11 in which splitter  22  is wavelength independent and a wavelength dependent reflection filter  74  is placed between splitter  22  and monitoring detector  92   b  and  100   b  respectively. The reference signal is the current output from detector  92   a  and  100   a  respectively. 
     FIG. 12 d  symbolizes the embodiments of FIGS. 1 and 4 in which detector  24   b  and  48   b  respectively is itself wavelength dependent by reason of a higher bandgap absorber in wavelength dependent detector  24   b  and  48   b  respectively and in which TMI waveguide  20  is omitted. Splitter  22  is wavelength independent. The reference signal is the current output from detector  24   a  and  48   a  respectively. 
     FIG. 12 e  symbolizes the embodiment of FIG. 7 in which there is no splitter  22 , but an inline monitor comprised of wavelength dependent detector  49   b,  which is itself wavelength dependent by reason of a higher bandgap absorber in detector  49   b  is placed between tunable laser  10  and detector  49   a.  The reference signal is the sum of the current output from detectors  49   a  and  49   b.    
     FIG. 12 f  symbolizes the embodiment of FIG. 7 in which an inline attenuator  102  and filter  104  have been added between detector  49   a  and wavelength dependent detector  49   b.  The reference signal is the current output from detector  49   a.    
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more of different elements, which are disclosed in above even when not initially claimed in such combinations. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.