Patent Publication Number: US-6339603-B1

Title: Tunable laser with polarization anisotropic amplifier for fabry-perot filter reflection isolation

Description:
BACKGROUND OF THE INVENTION 
     Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme, although semiconductor optical amplifiers are also used in some situations. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes. 
     The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link is shared by all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system. 
     Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine the channels into, and separate the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end. 
     Minimally, the optical signal generators, e.g., the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength control. Especially in systems with center-to-center wavelength channel spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any wander impairs the demodulation of the wandering signal at the far end receiver since the wavelength is now at a wavelength different than expected by the corresponding optical signal detector, and the wandering signal can impair the demodulation of spectrally adjacent channels when their spectrums overlap each other. 
     In addition to wavelength stability, optical signal generators that are tunable are also desirable for a number of reasons. First, from the standpoint of manufacturing, a single system can function as the generator for any of the multiple channel wavelength slots, rather than requiring different, channel slot-specific systems to be designed, manufactured, and inventoried for each of the hundreds of wavelength slots in a given WDM system. From the standpoint of the operator, it would be desirable to have the ability to receive some wavelength assignment, then have a generator produce the optical signal carrier signal into that channel assignment on-the-fly. Finally, in higher functionality systems such as wavelength add/drop devices, wavelength tunability is critical to facilitate dynamic wavelength routing, for example. 
     SUMMARY OF THE INVENTION 
     Tunable laser systems have been proposed in which a gain media, such as a semiconductor optical amplifier (SOA) chip, is installed in a laser cavity along with a Fabry-Perot tunable filter or cavity. These systems address the issue of preventing off-resonant light, i.e., light rejected (reflected) by the Fabry-Perot tunable filter, from re-entering the SOA chip and being amplified. Specifically, these proposed systems place a discrete polarizer filter between the Fabry-Perot filter and the SOA chip along with a Faraday rotator. Essentially, the polarizer filter blocks FP-rejected light from entering the chip since it has been rotated 90 degrees by the rotator. The problems exist, however, with these systems insofar as they are complex and costly to implement. 
     According to the present invention, an SOA chip is selected that is polarization anisotropic. As a result, it generates substantial light at only one polarization. This polarization is based on the strain, i.e., compressive or tensile, characteristics of the quantum well and thus generates either TE or TM polarized light. As a result, because of this configuration, it will not amplify light at the orthogonal polarization. Thus, by selection of such an SOA chip, the rejected light can be simply rotated to be orthogonal to the light generated by the SOA chip. This light will propagate through the SOA chip and not be amplified. Further, because of the SOA chip&#39;s relatively low reflectivity front facet or equivalent front reflector, most of this off-resonant light will be dissipated out the front of the laser cavity, and thus, the SOA will not amplify the rejected light. 
     In general, according to one aspect, the invention features a semiconductor tunable laser system. It comprises a front reflector and a back reflector defining a laser cavity. A semiconductor laser chip is installed within this cavity that is polarization anisotropic and thus only amplifies light at an amplification polarization. A filter is further provided in this laser cavity that has a tunable passband for selecting a wavelength of operation of the laser system. A first polarization rotator is located between the semiconductor chip and the tunable filter. This first rotator that rotates the polarization that is emitted from the semiconductor chip and is outside of a passband of the tunable filter such that a polarization of the rejected light is substantially orthogonal to the amplification polarization upon returning to the semiconductor chip. Further, a second polarization rotator is utilized in the laser cavity that rotates a polarization of light that is emitted from the semiconductor chip and is within the passband of the tunable filter such that the polarization of the pass light is substantially parallel to the amplification polarization upon returning to the semiconductor chip. 
     Depending on the implementation, various systems can be used for the first polarization rotator and the second polarization rotator. In one embodiment, Faraday rotators are used. In other embodiments, since the polarization of the light emitted from the SOA chip is known and invariant due to the polarization anisotropy, quarter-wave plates or sub-wavelength gratings are used for the first and second polarization rotators. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a perspective view of a tunable laser system of the present invention; 
     FIG. 2 is a perspective, close-up view showing the wavelength locker system of the present invention; 
     FIG. 3 is a perspective view showing the details of the multi-element detector of the wavelength locker system of the present invention; 
     FIG. 4 is a block diagram/schematic view showing the stepped etalon, multi-element detector, and the electronics for controlling the semiconductor laser system of the present invention; 
     FIG. 5 is a spectral plot of power as a function of wavelength (arbitrary units) showing the multiple spectral filtering characteristics of the differential wavelength filter system and the relationship to a wavelength of operation of the laser system; 
     FIG. 6 is a block/schematic view of another embodiment of the wavelength locker system utilizing a coarse and fine etalon system of the present invention; 
     FIG. 7 is a close-up view showing the semiconductor optical amplifier portion of the tunable laser of the present invention and the laser cavity of the present invention; 
     FIG. 8 is a perspective view showing the tunable Fabry-Perot cavity and the cavity length modulator of the present invention; 
     FIG. 9 is a perspective view showing the integration of the tunable Fabry-Perot cavity and cavity length modulator according to the present invention; 
     FIG. 10 is a schematic block diagram illustrating the electronic control scheme according to the present invention for the tunable laser; 
     FIG. 11 is a spectral plot of power as a function of wavelength (arbitrary units) showing the relationship between the wavelength of operation or passband of the Fabry-Perot filter and the cavity modes of the laser cavity of the tunable laser prior to compensation by the cavity length modulator; and 
     FIG. 12 is a plot of free spectral range in GHz as a function of cavity optical length in millimeters. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a tunable laser system  100 , which has been constructed according to the principles of the present invention. 
     Generally, the tunable laser system  100  comprises a hermetic package  110 . Typically, this hermetic package is a standard pin package, such as a butterfly package or dual, in-line (DIP) package. Presently, a standard sized packaging platform is used, which has a hermetic cavity of less than 2.0 centimeters (about 0.75 inches) in length and less than 1.5 centimeters (cm) (approximately 0.5 inches) in width. 
     An optical bench or submount  105  is installed within the hermetic package  110 . The optical components of the tunable laser system  100  are installed on this bench. In the preferred embodiment, the bench is constructed from a mechanically and temperature stable substance, such as aluminum nitride, silicon, silicon oxide, or beryllium oxide in various implementations. The bench is typically installed in the package on a thermoelectric cooler. 
     An optical fiber pigtail, such as a length of single mode optical fiber,  112 , enters the hermetic package  110  through a fiber feed-through  114 . Typically, the pigtail  112  passes through a ferrule in the feed-through. In one implementation, single mode polarization-maintaining fiber is used. 
     The termination or endface  122  of the fiber pigtail  112  is installed or attached to the bench  105  using a fiber mounting and alignment structure  210 . This mounting structure  210  connects the fiber pigtail  112  such that the endface  122  is secured in a rigid connection to the bench  105  and supported above the bench. The optical signal is coupled into this fiber pigtail  112  via the fiber endface  122  and transmitted outside of the tunable laser system  110 . Preferably, deformable mounting structures are used to enable active or passive alignment during system manufacture or calibration after an in-service period. 
     The optical components of the tunable laser system  100  are divided into three subsystems. The tunable semiconductor laser  116  generates the optical signal in the form of an output beam. A wavelength locker subsystem  118  determines a wavelength of the optical signal using a portion light, preferably from the output beam, from the tunable laser  116 . The output coupling section  120  couples the remainder, or main portion, of the optical signal beam into the fiber pigtail  112  for transmission out of the system  100 . An isolator  117  of the tunable laser  116  is preferably installed between the laser and the output coupling section  120 /locker  118 . This isolator prevents backreflections into the laser  116  to help stabilize its operation. 
     FIG. 2 shows the details of the wavelength locker system  118  of the tunable laser system  100 . Specifically, beam splitter  310  splits the output beam  410  from the tunable laser system  116  such that a portion  312  of the beam is provided to the locker system  118  as its input beam. 
     The input beam  312  is filtered by what is termed a differential wavelength filter system. This system applies multiple spectral filtering characteristics to input beam  312 . 
     In the preferred embodiment, the differential wavelength filter system  314  is a stepped etalon system that has bulk material  318  providing the general free spectral range of the etalon. Further, one end of the etalon has stepped sections that have different heights with respect to each other. These steps have the effect of fine-tuning the net free spectral range of the filter and yield the multiple spectral characteristics of the system. 
     The filtered beam  320  from the differential wavelength filter system is then received by a multi-element detector  316 . This detector is aligned with the differential wavelength filter to detect the magnitude of the beam, and specifically, different portions of the beam to which the different spectral filtering characteristics have been applied. 
     FIG. 3 shows the multi-element detector  316 . Specifically, in the present implementation, the multi-element detector  316  is a quad-detector with the individual detector elements  322 ,  324 ,  326 , and  328  being commonly mounted on a single substrate-carrier that is mounted in an orthogonal relationship to the plane of bench  105 . Specifically, detector  316  comprises four discrete, photosensitive elements  322 ,  324 ,  328 , and  326  that are laid out in square, grid fashion on the carrier  329 . Each element  322 - 328  of the quad detector has a corresponding bond pad electrode  330 ,  332 ,  334 ,  336  with connecting wire traces formed on the substrate (see reference  327 , for example). In this way, the magnitude of the optical energy impinging upon each element of the quad detector is separately, electrically detected via wire-bonding to the four bond pads  330 - 336 . 
     FIG. 4 illustrates the operation of the stepped etalon  314  and wavelength locker system opto-electronics. Specifically, the beam  312  from the tunable laser system  116  enters the stepped etalon  314 . One face  340  of the etalon  314  has a standard, partially reflecting surface. The beam propagates through the bulk portion  318  of the etalon to the other partially reflecting surface of the etalon, which is stepped. Specifically, there are, in the illustrated embodiment, four separate faces  342 ,  344 ,  346 ,  348  at the other end of the etalon, each at a different optical distance from the first face  340 . 
     Each one of surfaces  342 ,  344 ,  346 ,  348  corresponds to a different effective length etalon. Specifically, the etalon length difference between surface  342  and  344 , in one embodiment is approximately ⅛ of the wavelength of the center operational wavelength of the system (⅛γ). The distance between the second surface  344  and third surface  346  is ⅛γ, and the distance between the third surface  346  and the fourth surface is ⅛γ. As a result, the stepped etalon provides multiple spectral filtering characteristics. 
     The magnitude of the light transmitted through the separate filter characteristics is then detected by the elements  322 - 328  of the quad detector. Specifically, the signals from the quad detector elements are amplified by an amplifier bank  350  and provided to either separate analog-to-digital (A/D) converters or one shared AID converter circuit  352 . The magnitude of the signals from the elements is then received by a wavelength controller  354 . 
     The wavelength controller  354  compares the absolute and relative magnitudes of the signals from the detectors  322 - 328 . The wavelength controller  354  determines the wavelength of operation by reference to a quad-detector to wavelength mapper  356 . 
     In one embodiment, this mapper  356  is an equation or translation function that relates the detector element response to a wavelength of operation of the tunable wavelength circuit  116 . This equation or translation function was determined for the system at the time of manufacturing or later during a calibration or recalibration process. In another embodiment, the mapper  356  is implemented as a look-up table (LUT), which similarly records the quad detector element response to wavelength relationship. Based on this information, the wavelength controller then tunes the wavelength of operation of the tunable laser  116 . In a simple embodiment, this can be accomplished by adjusting injection current or current to the thermoelectric cooler to thereby control the tunable laser&#39;s wavelength of operation. As described later, in the present embodiment, it is also used to tune a cavity element of the tunable laser  116 . 
     In other embodiments, the sum signal from the elements of the quad-detector is used to determine amplitude, allowing the controller to further provide amplitude feedback control. This information is also used to tune the cavity length. 
     FIG. 5 is a spectral plot illustrating the operation of the wavelength locker  118 . Specifically, the differential wavelength filter  314 , in its stepped etalon configuration, applies essentially four separate filtering characteristics or transfer functions  360 ,  362 ,  364 ,  366  corresponding to the endfaces  342 ,  344 ,  346 , and  348  of the stepped etalon  314 . These multiple spectral filtering characteristics repeat themselves spectrally in increments of the free spectral range  368  determined by the bulk  318  of the etalon  314 . As a result, each of the detector elements detects a different filtered version of the input beam. 
     Specifically, in the illustrated case, if the wavelength of the input signal  312  is as illustrated, detector  322  receives a relatively strong signal since the emission from the tunable laser  312  is near the peak of its corresponding filtering characteristic  364 . Detector element  328  receives a smaller signal since the emission wavelength is a greater spectral distance from the peak of its corresponding filtering characteristic  366 . The other detectors receive relatively small signals. 
     The wavelength controller  354  then compares the relative intensities of the signals from the various detector elements and then uses the wavelength mapper  356  to determine the wavelength of operation of the semiconductor laser system  116 . Generally, for the wavelength controller to resolve the wavelength of the signal, it must receive a sufficient signal from at least two detectors since a given voltage from a single detector can be mapped to two distinct wavelengths as illustrated by the fact that line  322  intersects characteristic  364  at two different wavelengths. 
     FIG. 6 illustrates another embodiment of the wavelength locker system  118 . In this configuration, the input signal  312  from the tunable semiconductor laser  116  is again divided by a second beam splitter  610  of the locker and then redirected by an optional fold mirror  612  so that two discrete beams enter the differential wavelength filter system  314 . This second embodiment differential filter system, however, comprises two stepped etalons  618 ,  614 , and two separate multi-element quad detectors  616  and  316 . 
     In this embodiment, etalon  614  corresponds to a coarse filtering etalon. Specifically, it is again an etalon with a multiple steps at its output facet. The bulk material, however, has a free spectral range corresponding to a relatively wide or coarse spectral band. In the present embodiment, it has a free spectral range of approximately 50 to 150 nm. Specifically, it is anticipated that its free spectral range exceeds 100 nm. This allows it to have a free spectral range that covers the entire wavelength band associated with common WDM systems. Such system bands stretch from about 1500 to 1600 nm. In this way, the corresponding coarse multi-element detector  616  can be used to determine a general or coarse wavelength of operation of the tunable laser  116 . 
     The fine detecting etalon  618  has a much smaller spectral range. Typically, its spectral range is less than 10 nm and most commonly less than one nm. This fine filtering etalon in combination with the fine multi-element detector  316  allows for fine wavelength detection of the beam  312  from the tunable laser  116 . Specifically, because of its much narrower free spectral range, it can more precisely determine the wavelength of operation. 
     As a result, the coarse and fine multi-element detectors are used in combination to first determine the general wavelength of operation with the coarse multi-element detector, and thereby determine which mode of the fine-element detector etalon is being excited. Then, the fine multi-element detector is then used to control precisely the wavelength to better than 0.1 nm accuracy. 
     In other embodiments, one stepped coarse etalon and one standard, non-stepped, fine etalon are used in a cascade arrangement. One quad detector detects the output of the first etalon, and the other detects the signal after propagating through both etalons 
     FIG. 7 shows the specifics of the preferred embodiment of the tunable laser system  116 . Generally, the tunable laser  116  comprises a semiconductor gain medium  422 . In the preferred embodiment, the gain medium is, for example, a semiconductor optical amplifier (SOA) chip constructed from InGaAsP/InP, for example. As is common, the chip  422  has an anti-reflection coated rear facet. The front facet is coated to define the front reflector of the laser cavity. In one embodiment, the front facet coating provides between 5 and 15% power reflectivity. The tunable laser further comprises a tunable Fabry-Perot filter or cavity  410  and a laser cavity length modulator  412  with polarization rotators material, such as quarter-wave plates or Faraday rotators, to prevent amplification of rejected light from the filter in the chip  422 . 
     According to the preferred implementation, the SOA chip  422  is polarization anisotropic. As such, it only amplifies light that is either TM or TE polarized. As is commonly known, this is accomplished by controlling the strain in the quantum well layers of the chip during wafer-stage epitaxial growth to either have tensile strain (TM) or compressive strain (TE). 
     Utilizing a polarization anisotropic SOA chip runs contrary to the typical design of SOA&#39;s. When used as amplifiers, it is typically desirable to amplify light regardless of its polarization. In the current invention, the polarization anisotropy, however, is used to avoid the amplification of light rejected from the tunable filter  410  as described below. 
     Light is coupled out and into the SOA chip  422  by front and rear facet focusing lenses  418 ,  416 , which are supported on respective mounting structures  420  and  414 . These mounting structures allow for the x-, y- and z-axis alignment of the lenses  418  and  416  relative to the facets of the chip  422  after construction of the system to improve the coupling efficiency both into and out of the SOA chip  422 . 
     FIG. 8 illustrates the details of the cavity length modulator. The modulator is constructed on a rotator-material substrate  436 , which is attached to bench  105 . On the back surface of the substrate, a highly reflecting membrane  430  is fabricated. In the preferred embodiment, this membrane is a flexing membrane constructed from a metal such as aluminum or semiconductor material such as silicon. 
     This membrane is translated axially, or in the direction of the z-axis by applying an electrical potential between the membrane  430 , via an electrical connection to pad  436 , and a stationary membrane electrode  432 , via pad  434 . As a result, by controlling the electrostatic voltage level between the membrane  430  and the stationary membrane electrode  432 , the membrane is translated in a controlled fashion. Since the membrane is highly reflecting or HR coated and defines the cavity length, the length of the laser cavity is thus modulated. 
     FIG. 9 illustrates the tunable Fabry-Perot filter  410  and its relationship to the cavity length modulator  412 . Specifically, the tunable Fabry-Perot (FP) filter or cavity  410  is constructed between two rotator material substrates  440 ,  436 . The FP cavity comprises a front partially reflecting surface, which is constructed from a movable partially reflecting membrane  442 . This membrane is supported on support posts  450  in proximity to a stationary electrode  444 , which is fabricated or deposited on the proximal side of the second rotator material substrate  436 . The stationary electrode also functions as the second partially reflecting surface of the FP cavity. The membrane is preferably constructed from silicon, for example. The posts also function as stand-offs with respect to the two rotator substrates  440 ,  436  to thereby define the length of the FP cavity demarcated by the membrane  442  and the electrode  444 . 
     By applying a voltage between bond pad  448  and bond pad  446 , an electrostatic field is generated between partial reflecting membrane  442  and the electrode  444 . This field flexes the membrane to provide z-axis movement of the reflector electrode  442  to change the distance between the membrane  442  and the partial reflector-electrode  444  to thereby change the filtering characteristic or passband and thus tune the Fabry-Perot filter  410 . This controls the spectral location of the laser cavity&#39;s gain. 
     In the illustrated embodiment, the bond pads  448 ,  446  also function as x-axis alignment stubs for facilitating construction of the Fabry-Perot cavity  410 /cavity length modulator  412 , typically prior to installation on the bench  105 . Specifically, they facilitate the attachment of the rotator material substrates to each other by allowing the height alignment is between substrates  436 ,  440 . 
     The rotator material substrates  440  and  436  co-act to prevent rejected light from the FP cavity from being amplified in the SOA chip  422 , whereas passband light is amplified. 
     Specifically, in the preferred embodiment, the material and thickness of the substrates  440 ,  435  is selected such that they function as Faraday rotators that rotate the polarization of transmitted light by  45 °. Specifically, light emitted from the rear facet of the SOA chip  422  is polarized either orthogonally or parallel, depending on the strain characteristics of the chip&#39;s quantum well(s). The light is subsequently rotated  45 ° before entering the FP cavity  410  by the rotator material substrate  440 . 
     The FP cavity divides the light into the resonant and off-resonant components. That is, light outside the passband of the FP cavity is reflected to return to the SOA chip via a second pass through the first rotator substrate  440 . This rotates the light a further  45 °. Since the light rejected by the FP cavity is now orthogonal to the light emitted by the chip  422  it passes through the chip without amplification because of the designed polarization anisotropy. The low reflectivity of the chip front facet prevents reflection of most of the light back into the laser cavity and consequently the lasing in the double round-trip configuration. 
     In contrast, light at the FP cavity&#39;s passband, i.e., the resonant light, passes through the FP cavity and enters the second rotator material substrate  436  and is reflected by the membrane  430  to return. As a result, it is rotated  45 ° twice in the first substrate  436  and  45 ° again in the second substrate  440  after passing back through the FP cavity. Thus, this returning passband light, in contrast to the rejected light, is polarized parallel to the light generated in the SOA chip and this is amplified, resulting in the tunable laser functionality. 
     In one embodiment the rotator material substrates are constructed from latched garnet crystal material as sold by Mitsubishi Gas Chemical Co., Inc. 
     In another embodiment, quarter-wave plates are used as the substrates  440  and  436 . Quarterwave plates can be used since the polarization of the light from the SOA chip is known and invariant. Specifically, in this embodiment, light passes through the first quarterwave plate polarization rotator  440  and is converted to circularly polarized light. Rejected light from the filter passes back through the first polarization rotator to be converted to light that is orthogonal in polarization to the light from the SOA chip, and thus it is not amplified. In contrast, passband light passes through tunable filter to the second quarterwave plate polarization rotator  436  and converted to orthogonally polarized light and reflected. As it returns, it is converted to a parallel polarization after successively passing through the second and first quarterwave plates. 
     In still another embodiment, subwavelength period grating substrates are used as polarization rotators. Such gratings, as described in  Applied Physics Letters  42 (6), Mar. 15, 1983, page 492, do not diffract the light, but instead operate as a homogenous birefringent material to rotate the polarization of the beam. In one implementation, the gratings are etched, or otherwise formed, onto a side of the bulk substrates  440 ,  436  to a depth required for quarterwave operation. In still other embodiments the polarization rotation is performed by liquid crystal, preferably in photopolymerizable polymer utilizing photoalignment. 
     FIG. 10 illustrates the electronic control scheme for the tunable laser  100 . Specifically, a locker system  118  is used to generate a signal indicative of the wavelength of operation of the tunable laser section  116 . In one implementation, this locker is configured as described previously herein. In any case, however, the information from the wavelength locker system  118  is then fed to the wavelength control system  354 . 
     More specifically, in one embodiment, the wavelength controller  354  comprises a microprocessor  718 . The microprocessor  718  controls the tunable filter  410  and the cavity length modulator  412  via a filter driver  714  and length modulator driver  716 . In one implementation, each of these drivers  714 ,  716  comprises a digital-to-analog converter that converts a digital voltage setting from the microprocessor  718  into an electrostatic drive voltage for the tunable filter  410  and the cavity length modulator  412 . 
     In one embodiment, the wavelength controller further comprises a capacitance proximity detector  710  and capacitance-to-wavelength look-up table or mapper  712 . Specifically, the proximity detector detects a capacitance between the membrane  442  of the Fabry-Perot filter and the stationary electrode  444 . This detected capacitance is then converted to a current wavelength of operation of the tunable filter by using the detected capacitance as an address into the look-up table or mapper  712 . The mapper contains the relationship between the Fabry-Perot filter&#39;s passband the membrane-electrode capacitance. By detecting the capacitance, the current passband of the tunable filter is found without an optical reference signal. This information is passed to the controller. 
     Of course, the mapper can alternatively be implemented as an equation or function instead of a look-up table. This is a more compact implementation especially where the relationship is linear or near linear. 
     FIG. 11 illustrates the operation of the semiconductor tunable laser system and the control of the tunable Fabry-Perot cavity  410  and the cavity length modulator  412  by the microprocessor  718 . Specifically, the desired wavelength of operation γ 0  is achieved by tuning the Fabry-Perot cavity such that the filtering characteristic  1110  of the Fabry-Perot cavity has a peak at the desired wavelength γ 0 . This is achieved in one embodiment by reference the capacitance to wavelength look-up table or mapper  712 . Optical reference signals are used in other implementations. 
     As is known, the laser cavity has discrete longitudinal modes of operation  1112 . The modal separation is related to the cavity length of the laser: 
     Δγ=γ 0   2 /(2nL) where L is the length of the cavity and n is the refractive index. 
     According to the present invention, the Fabry-Perot cavity  410  is first tuned to the desired wavelength of operation, then the cavity length modulator  412  is driven to vary the physical length of the laser cavity by an amount that is typically less than one wavelength of light at the operational wavelength. The operational wavelength is typically less than  1610  nm. Minimally, to render a mode coincident with the desired wavelength of operation, the length should only need to be adjusted by one half of a wavelength either larger or smaller. Typically, this one-half wavelength distance is less than 1000 nm. 
     In FIG. 11, no mode is aligned with the peak of the Fabry-Perot cavity filtering characteristic  110 . This can result in either mode-hopping between modes for which the laser system has gain, or suboptimal power output because of the difference between the peak of the transfer function of the passband of the Fabry-Perot cavity and the next closest cavity mode. 
     This fine intercavity mode tuning is achieved in one embodment by reference to the absolute wavelength as detected by the locker  118 . In another embodiment, the cavity length is adjusted until the output power is maximized, which will occur when a cavity mode is centered at the center wavelength of the FP filter  410  and/or there is no mode-hopping between two or more competing modes within the passband. In either case, one of the cavity modes  112  is placed to reside at the peak of the passband of the Fabry-Perot cavity  410 . In this way, the present invention makes the necessary correspondence between the pass wavelength of the tunable Fabry-Perot cavity and a longitudinal mode placement of the laser cavity. 
     The present invention is most applicable to tunable lasers having short laser cavities. According to the present invention, the length of the laser cavity (See distance “L” in FIGS. 1 and 10) is preferably less than three centimeters. In the preferred embodiment, it is less than one centimeter. This length yields a modal separation of greater than a 3 GigaHertz (GHz). Preferably, the spacing is greater than 10 GHz, with a spacing of better then 20 to 30 GHz being ideal. 
     FIG. 12 illustrates the relevance of the short laser cavity L to modal spacing. Laser cavities of three centimeters and longer have free spectral ranges of approximately 3 GHz and less. With such close modal spacing, the need for cavity length adjustment can be avoided. Specifically, modem dense WDM (DWDM) systems contemplate channel spacings of 100 GHz with reference to the L α , C α , and S α  bands in the ITU 100 GHz grid. Denser systems contemplate 50 GHz channel separations as specified in the L β , C β , and S β  band of the 50 GHz offset ITU grid. At these channel separations, transmitter frequency must be specified to typically about +/−5 GHz. The modal spacing in a four centimeter or longer cavity can inherently accommodate these close channel spacing, but short cavities of one centimeter or less as preferred in the present invention yield mode spacings of greater than 10 GHz. Thus, the short cavity lasers, such as the instant invention, necessitate cavity length compensation as achieved with the cavity length modulator  412  of the present invention. 
     The preceding analysis suggests that longer cavity lasers are in fact desirable, since they avoid the need for cavity length adjustment in DWDM systems. Two factors, however, advocate for shorter cavity configurations: 1) device size; 2) an FP filter finesse requirements. 
     DWDM systems comprising up to one hundred channels or more require a corresponding number of tunable transmitters. In such complex systems, individual device sizes become critical and there is a need to have small transmitter form factors. In short, smaller is typically better from the standpoint of integration. 
     Higher finesse Fabry-Perot filters are required as cavity mode spacing decreases. The bandwidth of the Fabry-Perot filter must be narrow enough to select and restrict the tunable laser to single mode operation. When modes are tightly spaced, high finesse Fabry-Perot filters are required to avoid mode hopping or multi-longitudinal mode operation. 
     In contrast, when a shorter cavity is used, a much lower finesse or “quality” Fabry-Perot filter can be used that has a wider bandwidth end to 100 GHz. The ability to use a lower finesse Fabry-Perot filter decreases alignment tolerances between the SOA  422  and the filter, increases the gain of the laser cavity, and the avoids the requirement to fabricate very thick, multi-layer dielectric coatings on the surfaces of the Fabry-Perot filters. These factors substantially reduce cost and manufacturing complexity of the device. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.