Abstract:
The present invention provides an apparatus for mechanically stabilizing tunable lasers by mechanically grounding laser components. Lasers stabilized using mechanical grounding of the laser components exhibit phase synchronous tuning, reduced mode hop and increased wavelength stability across the entire tuning range. Additionally, lasers stabilized using mechanical grounding exhibit improved resistance to thermal and physical shock. The robust compact design of mechanically grounded lasers makes them suitable for a broad range of applications including optical signal generators and optical multimeters.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Provisional Application Nos. 60/099,901, entitled “Modulation/Continuous Wave Constant Power Control Circuit”; 60/100,055, entitled “Drive Train Passive Thermal Compensation”; 60/099,839, entitled “Phase Continuous Tuning in An Extended Cavity Diode Laser Using Dispersion Compensation Together With Mechanical Grounding”; 60/099,865, entitled “Drive Train Flexure”; and 60/099,831, entitled “Passive Thermal Compensation of External Cavity Diode Laser”, all filed Sep. 11, 1998. Each of the above-cited applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to optical multimeters and more particularly to signal generating portions thereof. 
     2. Description of the Related Art 
     The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation. 
     The T-1 standards committee ANSI has provided a draft document, “ANSI T1.105-1988”, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS 0  and DS 1  signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished. 
     Currently this expansion of bandwidth is being accomplished by what is known as “wavelength division multiplexing” (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. The data structure of a basic SONET signal at a typical data rate of 51.84 Mbps, a.k.a. an STS-1 signal, has 9 rows of 90 columns of 8 bit bytes at 125 μs frame period. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes that are used for various control purposes. The remaining 87 columns of bytes constitute the STS-1 synchronous payload envelope (SPE). As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and coherency correction by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are detected by photodetectors separated into channels, framed and decoded. 
     As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for optical multimeters, e.g. signal generators and detectors, which can be used to test the various components of an optical, e.g. SONET, network. What is needed is a tunable optical signal generator that does not require the complex control systems relied on by prior art devices. Those control systems utilize closed loop feedback of wavelength or position to select the output wavelength of the optical signal generator. As a result they are expensive and exhibit a large form factor. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for mechanically stabilizing tunable lasers by mechanically grounding laser components. Lasers stabilized using mechanical grounding of the laser components exhibit phase synchronous tuning, reduced mode hop and increased wavelength stability across the entire tuning range. Additionally, lasers stabilized using mechanical grounding exhibit improved resistance to thermal and physical shock. The robust compact design of mechanically grounded lasers makes them suitable for a broad range of applications including optical signal generators and optical multimeters. 
     In an embodiment of the invention, a tunable laser with a base, gain medium, pivot arm, and first and second feedback device is disclosed. The base has an upper and lower surface and an aperture extending therethrough. The gain medium couples to the upper surface of the base. The first feedback device couples to the upper surface of the base to provide feedback of a selected wavelength to the gain medium. The pivot arm has a proximal and a distal end. The proximal end of said pivot arm pivotally attaches to the lower surface of the base at a first pivot axis normal to the upper surface of the base. The distal end of the pivot arm extends through the aperture. The actuate displacement of the pivot arm about the first pivot axis lies in a first plane substantially parallel to the upper surface. The second feedback device couples to the distal end of the pivot arm to provide feedback of the selected wavelength to the first feedback device. The second feedback device, together with the first feedback device and gain medium, defines a resonant cavity substantially parallel to the upper surface of the base. The second feedback device is responsive to the arcuate displacement of the pivot arm to vary the selected wavelength. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 shows an optical multimeter, according to the current invention, coupled to an optical network. 
     FIG. 2 is a hardware block diagram of an embodiment of the optical multimeter according to the current invention. 
     FIG. 3 is an isometric view of the signal generator portion of the optical multimeter shown in FIG. 2 which incorporates a tunable laser. 
     FIG. 4 is a top plan view of the tunable laser shown in FIG.  3 . 
     FIG. 5 is an exploded isometric view of a tunable laser shown in FIGS. 2-4. 
     FIG. 6 is an assembled view of the tunable laser shown in FIG.  5 . 
     FIG. 7 is an exploded isometric view of the drive portion of the tunable laser shown in FIGS. 3-4. 
     FIG. 8 is an assembled view of the drive portion of the laser shown in FIG.  7 . 
     FIG. 9 is an isometric view showing the laser and actuator portions of the tunable laser shown in FIGS. 3-4. 
     FIG. 10 is a hardware block diagram showing a manufacturing setup configuration for programming and calibrating the signal generator portion of the optical multimeter. 
     FIGS. 11A-D are plan views of hardware associated with thermally stabilizing the optical pathlength of the laser cavity. 
     FIG. 12 is a top view of the resonant cavity portion of the tunable laser shown in FIGS. 2-3 with compensating elements for thermally stabilizing the optical pathlength. 
     FIG. 13A is a top plan view of a prior art drive train for mechanically activating the tuning element of a tunable laser. 
     FIGS. 13B-D are top plan views of alternate embodiments of hardware for thermally stabilizing the tuning element of a mechanically tuned laser in accordance with an embodiment of the current invention. 
     FIG. 14A is an isometric view of a mounting system for attaching both intermediate and optical elements of a tunable laser to a base. 
     FIG. 14B is a cross-sectional side view of the mounting system shown in FIG.  14 A. 
     FIG. 15 is a detailed circuit diagram of an embodiment of a modulation circuit for driving the signal generator shown in FIG.  2 . 
     FIGS. 16A and 16B show modulated waveforms generated by the signal generator portion of the optical multimeter. 
     FIG. 17 shows a data lookup table utilized to configure the signal generator to output a beam at a selected wavelength. 
     FIG. 18 shows an embodiment of the processes associated with generating the lookup table. 
     FIG. 19 shows an embodiment of the processes associated with selecting an output wavelength for the signal generator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an optical multimeter for use in calibrating and testing the various components associated with an optical network, e.g. SONET. The optical multimeter includes an analog/digital signal generator for delivering an optical output beam which can be modulated over a wide range of frequencies, duty cycles and amplitudes with very precise definition of the rising and falling edges of the waveform. Circuitry is also provided for modulation of an analog modulation signal onto the optical output. The signal generator includes a tunable laser that is thermally stabilized as to optical path length, as well as tuning angle of the tuning element. This substantially reduces thermally induced mode hops as well as thermally induced variations in the output wavelength. The tunable laser exhibits a small form factor due in part to a novel wavelength control process which utilizes an open loop system to maintain precise output wavelength control without the requirement of either a wavelength or position feedback device. Additionally, the tunable laser incorporates an inexpensive modulator circuit which combines a low frequency closed loop power control with a separate digital modulator. A novel mounting mechanism is disclosed which simplifies device fabrication by allowing precise positioning of optical and intermediate elements of the laser to a base. 
     FIG. 1 shows an optical multimeter  100  coupled via a network access device  102  to the various components of an optical network  120 . The Synchronous Optical Network (SONET) standard defines a networking approach for high speed data communication at data rates from 51.8 Mbps to 2.48 Gbps. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems. 
     A plurality of central offices/switching centers  104 - 106  are shown coupled to an optical network  120 . Datastreams are multiplexed using wavelength division multiplexing (WDM) in different portions of the optical spectrum. The network itself typically includes: Erbium doped line amplifiers  122 - 124  to maintain signal strength, circulators  126  with Bragg filters to maintain the coherence of the signals, and optical switches to route the traffic between appropriate data terminals. At the central office, the line cards  108 - 112  handle the transmission and reception of datastreams. On the transmit side, each line card includes semiconductor lasers each tuned to a specific wavelength within the range of 1525-1575 nm. Optical modulators inject datastreams into the output beams of these lasers which are collimated in a single fiber optic line for transmission across the network. On the receive side, each card includes photodetectors and demodulators to convert the received data into a format suitable for transmission across fiber subscriber lines  130  or copper subscriber lines  132  to data terminals  114 - 116  or to a traditional analog telephone  118 . All of these components need to be tested and calibrated across a range of frequencies and power levels with signals that may be analog or digital. The high precision optical multimeter of the current invention includes a high precision optical signal generator and optical detector which allows these components to be tested on site or on the lab bench. 
     FIG. 2 shows an exterior isometric view of the optical multimeter housing as well as a hardware block diagram of the components within the optical multimeter. The optical multimeter  100  includes: display  200 , user inputs  202 , I/O interface  204 , processor  206 , memory  208 , modulation circuit  222 , overload sensor  242 , temperature sensor  246 , power-detector  270  and the signal generator  250 . The signal generator includes: gain medium  224 , tunable cavity  226 , output  228 , actuator  230  and start condition detector  240 . The memory  208  includes program code  210  and lookup table  212 . 
     The I/O interface couples the display  200  and user inputs  202  to the system bus  216 . The memory  208  is coupled to the processor  206  and the system bus. The system bus also couples to the power detector  270 , modulation circuit  222 , start condition sensor  240 , overload sensor  242 , and temperature sensor  246 . Within the signal generator the actuator  230  drives a tuning element within the tunable cavity  226 . The start condition detector  240  couples either directly to the actuator or to the tunable element within the tunable cavity to detect a starting point thereof. 
     On the transmit side, the signal generator  250  generates an output beam  260 . The output beam can be tuned to any of a number of center wavelengths associated with, for example, each channel in the IEEE-ITU standard. Selection of a center wavelength is accomplished by an open loop control system which utilizes the lookup table  212  to drive the actuator to the selected wavelength. Unlike prior art optical signal generators which require a continuous feedback across the tuning range of either position of the tuning element or wavelength of the output, no feedback is required to select output wavelength. Instead, an open loop control system is implemented, thus reducing the cost and form factor of the signal generator. To fabricate a precision tunable signal generator without either a wavelength feedback apparatus or position sensor, there must be a precise and repeatable correlation between the control signals supplied by the processor to the actuator  230  and the output wavelength  260 . This in turn requires that the hardware be optically stable across a range of temperatures, where optical stability includes stability of both the optical pathlength as well as the tuning angle of the tuning element within the tunable cavity. Each signal generator includes processes for utilizing a unique lookup table, the records of which are generated during the manufacture of the device to correlate drive signals with output wavelength. This calibration involves ramping the tunable laser through a range of frequencies, and using a wavelength sensor, recording the correlation between output wavelength and the drive signals supplied to the actuator. This information is recorded in a wavelength_vs._drive signal lookup table  212  which is stored in memory  208  during the assembly of the device. Responsive to a user request for an output signal centered within a specific channel, the processor  206 , using this table, generates the required number of actuator signals to tune the laser to the requested channel. 
     Environmental effects on laser output wavelength must be accounted for. Temperature is one of the primary environmental factors which affect output wavelength. The center wavelengths associated with neighboring channels are narrowly separated, i.e. less than 1 nm apart. These wavelength variations could easily be produced by thermal expansion/contraction of the tuning mechanism for the tunable cavity  226  or by variations in the optical pathlength. Two techniques may be used singly or in combination to substantially reduce the effect of temperature variations on the wavelength stability/accuracy of the output beam. One technique involves actively adding or removing energy from the cavity to maintain a constant thermal state, thus avoiding thermal contraction and expansion by stabilizing the temperature in the tunable cavity. The other involves fabricating the tunable laser in a manner which allows thermal expansion and contraction without, however, inducing temperature-related variation in the output wavelength from the tunable laser. Although either approach is suitable for use with the current invention, the latter passive approach set forth in FIGS. 11-14, has the advantage of lower cost and form factor since no thermal generator, monitor, and control circuitry is required. 
     In operation, the optical multimeter may be used singly or in combination with other multimeters to test optical devices on the bench or across a network connection. One method for testing an optical device involves coupling the multimeter output beam  260  to a device under test (DUT) and monitoring the DUT output  262  at the power detector  270 . For a DUT such as an Erbium doped optical amplifier, the output signal  260  can be injected into the optical amplifier, and the resultant output  262  signal from the amplifier may be coupled to the receive side of the optical multimeter. On the receive side, an optical signal  262  received via power detector  270  is digitally sampled and passed to the processor via system bus  212 . The processor executing program code  210  stored in memory  208  analyzes the received signal according to parameters input by the user on input  202 . Additionally, the processor passes the signal via I/O interface  204  for presentment to the user on display  200 . Because the output signal  260  is precisely controlled, the processor  206  may compare the received signal with the known parameters of the transmitted signal in order to characterize various parameters of the DUT such as: power level, gain, rise and fall time, etc. 
     In an alternate embodiment of the invention both the signal generator and the power detector as well as other modules would each implement plug-and-play technology with dedicated master processor resident in the multimeter mainframe. 
     FIG. 3 as an isometric view of a hardware embodiment of the signal generator  250  shown in FIG.  2 . The base  300 , fiber mount  302 , fiber coupling  304 , motor bracket  310 , laser diode housing  330 , diffraction grating  340 , grating mount  342 , retroreflector  350 , compensating element  352 , pivot bracket  354 , actuator  370 , drive train  376  and start condition sensors  390 - 392  are shown. In an embodiment of the invention the signal generator incorporates a tunable laser in a Littman-Metcalf configuration. In this configuration, the laser diode within housing  330 , the diffraction grating  340  and the retroreflector  350  are laid out in a generally triangular arrangement. The laser housing  330  is affixed to base  300  at a grazing angle with respect to the diffraction grating  340 , such that a reflection from the diffraction grating passes to the fiber coupling  304  where it is coupled to a fiber optic (not shown). The diffraction grating is coupled to the grating mount  342 , which is in turn fastened to the base  300 . The fiber coupling  304  is fastened to the fiber mount  302 , which is in turn coupled to base  300 . The laser beam is also diffracted from the diffraction grating  340  striking the retroreflector  350 . The return beam from the retroreflector strikes the diffraction grating and returns through an anti-reflective coating on the front facet of the laser diode within housing  330  to select the output wavelength of the laser. The retroreflector  350  is coupled to a compensating element  352 , which is in turn coupled to the pivot bracket  354 . The pivot bracket is coupled to the base  300  at a pivot point which allows tuning of the laser by combined rotation and translation of the retroreflector with respect to the diffraction grating (See FIGS.  5 - 6 ). The pivot point may be selected to provide the requisite combination of rotation and translation so as to maintain a constant integer number of half-wavelengths in the cavity, thus reducing mode hopping. This pivot point may be selected in accordance with the teachings of U.S. Pat. No. 5,319,668, issued on Jun. 7, 1994, entitled “Tuning system for External Cavity Diode Laser” and having in common with the current invention the Assignee New Focus Inc., of Santa Clara, Calif. 
     In an embodiment of the invention, a pivot bracket and attached retroreflector is used to tune the laser. The motion of the pivot bracket is brought about by a linear translation of the drive train  376  coupled to a pivot arm to which the pivot bracket is attached. The motion of the pivot arm results from the actuator  370 . The actuator  370  is coupled to the motor bracket  310 , which is in turn coupled via a fastener placed within coupling  312  to the base  300 . In this embodiment of the invention, the actuator is a rotary stepper motor. Other actuators may be used with equal advantage, including, but not limited to: linear stepper motors, piezo-electric stacks, bi-metallic elements, AC/DC motors, etc. As will be obvious to those skilled in the art, the actuator  370  could be bolted directly to the base  300  without departing from the scope of the invention. The stepper motor operates under control of the processor  206  (See FIG.  2 ). In an embodiment of the invention, start condition sensors  390 - 392  are used to determine a starting position for the drive train by making a linear and arcuate readout of the drive train. These sensors, in combination with the wavelength lookup table  212 , allow the processor to control the actuator so as to select output wavelengths for the tunable laser (See FIG.  9 ). 
     FIG. 4 shows a top plan view of the tunable laser embodiment shown in FIG.  3 . The base  300  with attached laser diode housing  330 , diffraction grating  340 , and fiber coupling  304  is shown. The actuator  370  is coupled to the base  300  via motor bracket  310  and strap  440 . The individual components of the drive train  376  are visible and include: drive shaft  400 , hub and rim  402 - 404 , rotary flex member  406 , compensating element  410 , translation unit  412 , cylindrical nut  414 , lead screw  418 , and linear flex member  420 . 
     The drive train  376  comprises rotary, linear, and arcuate portions. Generally the drive shaft converts the rotary motion of shaft  400  to linear movement of compensating block  410  and finally to arcuate movement of the tip  430  of the pivot arm to which the bracket  354  and associated retroreflector  350  are attached (See FIG.  5 ). This provides for the tuning of the output beam of the laser. 
     The rotary portion of the drive train includes: shaft  400 , rim  404 , rotary flex member  406  and cylindrical nut  414 . In the embodiment shown, the actuator  370  is a rotary actuator and specifically a stepper motor. As will be obvious to those skilled in the art, suitable alternate actuators include: piezo-electric stacks, AC/DC motors, linear stepper motors, etc. The output shaft  400  of the stepper motor is coupled via the hub and rim  402 - 404  to the rotary flex member  406 , which is in turn coupled to the cylindrical nut  414 . The cylindrical nut includes a threaded interior portion. The rotary flex member  406  is placed intermediate the cylindrical nut and the drive shaft  400  in order to de-couple the cylindrical nut from any misalignments of the stepper motor shaft  400 . These misalignments can arise, for example, due to non-parallelism between the axes of the lead-screw assembly and motor, or run-out and wobble in the motor-shaft, nut and screw. The rotary flex member is relatively compliant in all directions except longitudinally. The torsional compliance of the driveshaft introduces hysteresis into the system. This is overcome by driving the motor to approach all target positions from the same direction. In this way the “wind-up” of the driveshaft becomes a constant, rather than a variable. The rim  404  passes through the start condition optical switch  392  and is encoded (See FIG.  9 ), so as to allow the switch to sense an arcuate starting point for the actuator shaft. After registering that starting location during the initialization of the signal generator, no further detection is required for the switch(s). 
     The linear portion of the drive train includes translation unit  412 , compensating element  410 , and lead screw  418 . The lead screw  418  includes a threaded portion which engages the interior threaded portion of the cylindrical nut. The head of the cylindrical nut is coupled to the distal end of the compensating element  410 . The compensating element  410  is in turn coupled to the linear translation unit  412 . The linear translation unit  412  is coupled to the motor bracket  310 . Thus, rotation of the stepper motor shaft  400  results in a linear movement of the lead screw  418  toward, or away from, the cylindrical nut with which it is threadably engaged. The movement of the lead screw is linearized with respect to the base by means of the attachment of the nut to the base via the compensating element  410  and translation unit  412 . As will be obvious to those skilled in the art, the placement of the lead screw and nut could be reversed without departing from the scope of the invention. In that alternate embodiment of the invention, the rotary member would have an external thread, i.e. lead screw, and the cylindrical nut would be attached to the compensating element. In still another embodiment of the invention, the linearization of the lead screw and compensating element could be achieved by the positioning of the head of the lead screw within a complementary opening of the base, thereby linearizing the motion of the lead screw with respect to the base. In an alternate embodiment of the invention the lead screw is rotationally driven and axially constrained as shown in FIGS. 13B-C. 
     The arcuate portion of the drive train includes the linear flex member  420 , fasteners  422 - 424 , and the tip  430  of the pivot arm. In the embodiment shown, the flex member  420  is a spring metal strip, the cross-sectional profile of which is rectangular. In alternate embodiments of the invention, the linear flex member may include square or round, cross-sectional profiles. The linear flex member allows conversion of the linear motion of the compensating element into an arcuate motion of the tip  430 . In an alternate embodiment, the linear flex member comprises part of the tip  430  of the pivot arm. 
     FIG. 5 is an exploded isometric view of the tunable laser shown in FIGS. 3-4, in which the actuator and drive train assembly have been omitted. The relationship of the primary components of the tunable laser to a common base or ground plane is shown. The laser diode housing  330  couples to mounting holes  504  within base  300  via fasteners  500 - 502 . The diffraction grating  340  couples to the mount  342 . This coupling may be by means of an adhesive fastener, soldered, welded or integral with the base. The mount  342  couples to mounting holes  510  within base  300  via fasteners  506 - 508 . The pivot member  550  is rotatably coupled to the base  300  at thru-hole  532 , the center of which is aligned with the pivot axis  530 . In a preferred embodiment of the invention, the pivot axis location with respect to the laser diode, diffraction grating and retroflector is determined in accordance with the teachings embodied in the &#39;668 Patent. Significantly, the pivot point location takes into account the effect of the dispersion of the laser and other optical elements in the system on the cavity length. This pivot point is selected so as to provide an internal cavity length (See FIGS. 11-12) which is substantially a constant integer number of half-wavelengths throughout all wavelengths within the tuning range. Bearing post  540  is fit into the thru-hole from the bottom side of the base  300 . The base portion  552  of the pivot member  550  includes a cylindrical bearing  560 . The bearing is fit over the post on the bottom of the base  300 , thereby providing precise rotation of the pivot member in a plane parallel to the lower surface of the base plate  300 . Attached to an intermediate portion  554  of the pivot member is the above-mentioned pivot bracket  354 . This extends from the bottom of the base to an exposed position on the top side of the base. The pivot member  550  is secured to the lower portion of the base  300  via mounting plate  570  and fastening members (not shown). In the assembled position (See FIG.  6 ), the compensating element  352  and retroreflector  350  are coupled to the pivot bracket from the top side of the base  300 . The fiber coupling  304  and fiber mount  302  are fastened to the base via fasteners (not shown) positioned within mounting holes  520  defined within the base. 
     An advantage to the embodiment of the tunable laser shown in FIGS. 3-5 is that all components are coupled to a common base. Consistent with the teachings on the current invention, the locations of each of the components can be precisely calculated. Thus, as is the case in the prior art, it is not necessary that adjustment features be provided for any of the components. Instead, the laser diode, diffraction grating, and retroreflector are either absolutely or rotatably fixed to a common base, thereby greatly improving the stability of the output signals generated by the tunable laser. In alternate embodiments of the invention, the apparatus for coupling the pivot member  550  to the base includes: rotary bearing, needle bearing, journal bearings, flexural bearings, rotary flexural bearings, etc. 
     FIG. 6 is an assembly isometric view of the tunable laser shown in FIG. 5 in which the actuator and drive train assembly have been omitted. The laser diode housing  330 , diffraction grating  340 , and fiber coupling  304  are shown coupled to the base  300 . The pivot bracket  354  extends partially above the top surface of base  300 . The proximal end of the compensating element  352  is attached to the pivot bracket by fasteners (not referenced). The retroreflector  350  is coupled to the distal end of the compensating element. In the embodiment shown the retroreflector is fastened by means of an adhesive, solder, weld, etc. Finally, the tip  430  of the pivot member  550  is shown beneath the top of the base and extends into the upper portion of the base in which the drive train assembly will be located. 
     FIG. 7 is an exploded isometric view of the drive train portion of the tunable laser shown in FIGS. 3-4. Specifically, the rotary and linear portions of the drive train are shown with the arcuate portion omitted. The actuator  370 , motor bracket  310 , start condition detectors  390 - 392 , and drive train assembly  376  are shown. In the exploded view, the linear translator  412  is shown with a lower portion  740  coupled to the base of the motor bracket  310  in an orientation which provides for linear movement along the longitudinal axis defined by the drive assembly of the linear translator. This axis, as will be discussed in the following FIG. 13, is generally tangent to the arc swept out by the tip of the pivot arm. The start condition detectors  390 - 392  are shown coupled to the motor bracket. The strap  440  and mounting holes  312  provide two fastening points by which the drive assembly and actuator are rigidly coupled to the base  300 . In an alternate embodiment of the invention, the actuator and translation unit may be directly coupled to the base  300 . The rotary portion of the drive assembly  376  includes the actuator shaft (not shown), hub  402 , rim  404 , rotary flex member  406  and cylindrical nut  414 . A notch  720  on the rim  404  is shown. When assembled, the rim rotates within opposing arms of the rotary start condition sensor  392 , that notch is optically detected, thereby accurately gauging an arcuate starting position of the actuator  370 . 
     The lead screw  418  threadably couples at a proximal end to the interior threaded portion of the cylindrical nut  414 . Thus, as the cylindrical nut is rotated by the actuator, the lead screw is retracted or extended within the cylindrical nut. The lead screw is affixed at a distal end to a distal end  710  of the compensating element  410 . The attachment of the compensating element to a translation unit  412  both linearizes the movement of the compensating element, as well as prevents the rotation of the lead screw. This limits the lead screw and compensating element to the desired linear motion along the longitudinal drive axis. 
     The linear flex member  420  is a strip of spring metal generally rectangular in cross-section and with lower and upper portions  700 - 702 , respectively. At a proximal end, the lower portion is attached via fasteners  422  to the compensating element  410 . The point of attachment is precisely determined at a distance from the distal end  710  of the compensating element. At a distal end the flex member is coupled via fasteners  424  to the tip  430  (not shown) of the pivot member  550  (See FIG.  5 ). The thermal expansion of the compensating element is calculated so as to thermally pacify the drive assembly and prevent steady state motion of the pivot arm  550  and retroreflector (See FIG. 13) as a result of temperature variations. 
     FIG. 8 is an assembled view of the drive assembly  376 , motor bracket  310  and actuator  370  shown in FIG.  7 . The drive assembly is shown attached to the actuator. The rim  404  is positioned within start condition detector  392  to detect rotational orientation of the actuator. The compensating element  410  is fastened to the linear translator  412 , which is in turn coupled to the motor bracket  310 . The compensating element is constrained to linear motion with respect to the base along a line tangent to the arc swept by the tip of the pivot arm  550  (See FIG. 5) during tuning of the laser. The upper portion  702  of the linear flex member  420  is positioned within start condition sensor  390  such that the start position of the compensating element may be detected. 
     FIG. 9 is an assembly view of the embodiment of the tunable laser discussed above in connection with FIGS. 3-8. The base  300 , actuator  370  and motor bracket  310  are not shown. The laser diode housing  330  is positioned above the base  552  of the pivot arm. The pivot bracket, located at an intermediate portion of the pivot arm, is shown with the retroreflector  350  attached thereto via the compensating element  352 . The retroreflector  350  is caused to undergo a combination of rotation and translation with respect to the diffraction grating  340  (shown in phantom view) by means of arcuate motion of the tip  430  of the pivot arm. The distal portion of the lead screw  418  (See FIG. 4) and compensating element  410  have been removed in order to view the lower portion  700  of the flex member. The lower portion of the linear flex member is coupled via fasteners  424  to the tip  430  of the pivot arm. The upper portion  702  of the linear flex member is shown positioned within the linear start condition detector  390 . Rotation of the drive shaft of the actuator results in rotation of the cylindrical nut  410 . This results in linear movement of the lead screw and the compensating element to which it is attached. This linear motion in turn arcuately displaces the pivot arm through the coupling of the tip of that arm to the compensating element via the linear flex member. The linear flex member is sufficiently rigid so as to overcome any friction in the pivot arm bearing  560  (See FIG.  5 ), thus assuring that for each unique linear displacement of the compensating element, a unique pivot arm angle is also defined. This retroreflector coupled to the pivot arm is thereby caused to undergo both rotation and translation with respect to the diffraction grating. 
     In operation, a laser diode within housing  330  emits a beam  900  through the front facet which intersects at a grazing angle the diffraction grating  340 . The diffracted beam  902  from the grating strikes the retroreflector  350 . A portion of the diffracted beam having a specific wavelength determined by the orientation of the retroreflector with respect to the grating is reflected back to the grating and injected back into the laser diode, thus selecting a cavity mode that supports the desired output wavelength. The reflection  904  of the laser beam from the diffraction grating provides a potential source for the optical output  260  (See FIG. 2) of the signal generator of which the tunable laser is a part. An alternate output signal source is provided by beam  906  from the back facet of the laser. This optional beam results when the back facet of the laser diode is partially transmissive. 
     Output Wavelength Determination 
     In order for the tunable laser to be controlled with an open loop system, which does not require closed loop feedback of, for example, wavelength or position, several requirements must be met in embodiments of the invention in which the laser is mechanically tuned. First, the actuator which drives the tuning element must be capable of incrementally moving the tuning element, e.g. retroreflector, diffraction grating, etalon, etc., from one position to the next across the tuning range so that narrowly separated center wavelengths can be selected. Second, there must be some way of correlating control/activation signals supplied to the actuator with output wavelength. Third, in the absence of wavelength or position feedback, there must be some means of maintaining the correlation between control/activation signals and the output wavelength, even in the presence of environmental variations. Temperature variations, for example, cause the drive train, base, pivot arm, and other components within the tunable laser to expand/contract, thereby varying the output wavelength. 
     The first of these requirements is fulfilled by the combination of a rotary actuator, such as a stepper motor, with the de-amplification provided by a cylindrical nut and a finely pitched lead screw. The pitch of the lead screw determines the amount of linear movement produced that will resolve from each rotation of the stepper motor. As finer pitched thread is utilized on the lead screw  418 , the wavelength resolution of the system will increase. In an alternate embodiment of the invention, the wavelength resolution may be increased by increasing the length of the pivot arm. 
     The second requirement is fulfilled by the combination of the start condition sensors  390 - 392 , the actuator  370 , and the lookup table  212 . Start conditions sensors may be used to determine a base location for one or more of: the tuning element; the pivot arm; the arcuate, linear or rotary portions of the drive train; or the actuator. In the embodiment shown, the start condition sensors each have a small cavity with a beam of light emitted from one side which is detected on the other side. Interrupting the beam changes the state of the sensor. When the processor  206  (See FIG. 2) initializes the system, the actuator is caused to turn until the upper portion  702  of the flex member either interrupts or clears the light beam of linear sensor  390 . If the system exhibits hysteresis, then the direction in which the actuator makes a final approach at the starting point will be the same each time, thus removing the effect of hysteresis. The linear sensor may be positioned on any part of the tuning system, e.g. the drive assembly, pivot arm, tuning element, etc. 
     Where the accuracy of the linear start condition sensor alone is insufficient to indicate a unique starting condition, the rotary start condition sensor  392  may be used in combination with the linear sensor. Unlike the linear sensor, the rotary sensor does not have a unique start condition where the actuator output shaft makes more than one rotation across the tuning range. Thus, when used in combination, the linear and rotary sensors operate sequentially, with the linear sensor required to give a first indication of a start condition, and the rotary sensor providing a subsequent indication. In this embodiment, the processor actuates the stepper motor in a predefined direction, i.e. clockwise or counterclockwise, until the linear sensor is triggered. Subsequently, the stepper motor is backed off in the reverse direction, and then energized in the forward direction until the rotary sensor  392  changes state. The predefined direction for triggering the change of state of sensor  392  assures that backlash/hysteresis is removed from the drive assembly. Sensors other than linear or rotary may be used to signal the start condition. In an alternate embodiment of the invention, the start condition sensor(s) may be electrically coupled to the actuator to sense an overload current/voltage level thereof. When the actuator moves the drive train to a mechanical endpoint, the increase in the drive voltage/current level resulting from the increased load on the actuator could be used to signal the start condition. Alternately, responsive to a unique output wavelength, an inexpensive optical sensor could be used to signal the start condition. In still another embodiment of the invention, microswitches, capacitative sensors inductive sensors, magnetic read switches, etc. could be utilized to signal the start condition. 
     Once the base condition has been indicated, no further signaling from either the linear/rotary or other start condition sensor is required during the selection of output wavelengths for the device. Instead, an open loop control system is utilized in which the processor using the lookup table determines the type/quantity of drive signals relative to the base state that are required to move the tuning element to the selected output wavelength and drives the actuator accordingly. The actuator accepts drive signals, and responsive thereto produces incremental movements, e.g. actuate displacements from the base state. Where a high degree of accuracy is required, the lookup table is unique to each device. The processes associated with generating the lookup table are set forth in FIGS. 10 and 18. The processes for generating selected output wavelengths are set forth in the following FIG.  19 . Although satisfaction of both the first and second requirements is a necessary condition for implementing an open loop control system for the signal generator, alone or in combination, they are not a sufficient condition where high degrees of wavelength accuracy and resolution are required. The signal generator must be environmentally stable as well. 
     One of the primary factors affecting both accuracy and repeatability of the combined drive unit and laser is temperature. Small changes in the angle of the tuning element, induced not by the actuator but by thermal expansion, can vary the output wavelength from one to another of the narrowly separated output wavelengths. Thus, a signal generator without feedback of position or wavelength may not exhibit a unique/repeatable output wavelength in response to a given drive signal sequence unless the signal generator is thermally stable. FIGS. 11A-B and FIG. 12 show embodiments of the invention for thermally stabilizing optical path length. FIG. 13 shows an embodiment of the invention for thermally stabilizing a mechanically actuated tuning element of an external cavity laser. 
     Generating a Lookup Table 
     FIG. 10 shows an embodiment of the invention for generating a lookup table. The tunable laser discussed above is superimposed on the multimeter hardware layout shown in FIG.  2 . An input of a wavelength meter  1000  is shown connected to the output beam  260  from the signal generator  250 . The output from the signal generator is coupled through the I/O interface to CPU  206  and memory  208 . During the assembly of each signal generator or groups thereof, the signal generator is hooked up to an external multimeter in a final stage of the assembly process. Next, the processor  206 , using program code  210  in memory  208 , energizes the actuator  230  and monitors the start condition detectors  240  until a start condition is indicated. Then, the wavelength measured by the wavelength meter is recorded as the first/base record in the database/lookup table  212 . Next, the processor sends a known combination/amount/type of activation signals to the actuator  230  which results in the tuning of the laser to a next wavelength level. The combination/amount/type of activation signals is recorded along with the wavelength measured by the wavelength meter in the database/lookup table  212  as the next record therein. The process is repeated to generate subsequent records. Next, additional records may be generated in the lookup table/database by interpolation between existing records. When the population of the lookup table is complete, the table is downloaded/stored in the memory  208  of the multimeter. 
     In an alternate embodiment of the invention, the lookup table is generated using an external processor and memory in combination with the external wavelength meter. The lookup table is generated in a manner substantially similar to that discussed above. The processor drives the actuator; the wavelength meter indicates the output wavelength of the output beam  260 . The processor records the correlation between wavelength and actuator drive signals and stores the results in the lookup table. Then, after the signal generator is assembled into the optical multimeter, the lookup table for the signal generator portion of the multimeter is downloaded to the memory  208 . Further details on the processes associated with generating the lookup table/database  212  are set forth in FIG.  18 . 
     Thermally Stabilizing the Optical Path Length 
     Temperature changes affect the overall cavity length and index of refraction of the cavity, which in turn result in variations in output wavelength as well as mode hops. As the optical length of the laser cavity varies with respect to temperature, the integral number of half-wavelengths that may be supported in the cavity varies. The optical path length of a cavity is a function of the physical thickness of each element, optics and air included in the cavity, and the refractive index of the element. Two elements with identical thickness and different indices of refraction will each support a different number of half-wavelengths along their thickness since the speed of light varies inversely with refractive index. Thus an element with a higher refractive index, e.g. glass, supports a greater number of wavelengths over an identical physical length than an element, such as air, with a lower refractive index. 
     Once an output wavelength is selected, any variations in the optical path length in the cavity result in discontinuities, a.k.a. “mode hops”, in the output beam brought about by variations in the integral number of half-wavelengths in the cavity. These variations may be brought about by a combination of physical path length variations and/or variations in the indices of refraction of the elements within the cavity, including: optics, gain medium, and any gas such as air. 
     FIGS. 11A-D show alternate embodiments of a tunable laser with a compensating element for passively stabilizing the optical path length of a laser cavity during variations in temperature. FIGS. 11A-C are elevation views of variations on the Littman-Metcalf configuration. FIG. 11D is an elevation view showing the Littrow configuration. Each incorporate compensating elements. The compensating element(s) work by expanding/contracting along the optical axis by an amount sufficient to offset any temperature related contraction/expansion in the optical path length, to thermally stabilize the optical path length. In FIGS. 11A-B a tunable external cavity diode laser with fixed proximal and distal ends and an intermediate tuning element is shown. In FIG. 11A-B, a compensating element attaches an optical component to the base of the laser in a manner which respectively decreases and increases the optical path length during expansion of the compensating element. In FIG. 11C an external cavity diode laser with a fixed tuning element, e.g. diffraction grating, and a variably positioned proximal and/or distal end(s) is shown with a compensating element which decreases the optical path length during expansion. In FIG. 11D an external cavity diode laser with a fixed gain medium and a variably positioned tuning element is shown with a compensating element which decreases the optical path length during expansion. 
     The tunable laser of FIG. 11A includes: foundation  1100 , gain medium  1120 , optical elements  1128 , tuning element  1130  and a retroreflector  1126 . The optical elements, tuning element and retroreflector provide a retroreflective tuning device which tunes the laser by providing feedback of a selected wavelength to the gain medium. In an embodiment of the invention, the gain medium is a laser diode with front and rear facets  1124 - 1122 , respectively. In various embodiments of the invention, the optical elements  1128  include lenses and filters. In various embodiments of the invention, the tuning element  1130  includes an interference filter, an Etalon, a diffraction element, and a grating. In these embodiments, tuning is accomplished by rotation and/or translation of the tuning element. In other embodiments of the invention, the tuning element includes an optical crystal the wavelength absorption/transmission of which varies with an applied current or voltage. In various embodiments of the invention, the retroreflector includes a mirror, a corner cube and a dihedral prism. A resonant cavity is formed with a length L Opl  between the rear facet  1122  of the laser diode  1120  and the retroreflector  1126 . The resonant cavity includes an internal cavity between the rear and front facets  1122 - 1124  of the laser diode and an external cavity between the front facet  1124  of the laser diode and the retroreflector  1126 . 
     At the proximal end, the laser diode  1120  is fixed to the foundation  1100  at pad  1102 . At the distal end of the cavity, the retroreflector is fastened to a compensating element  1118 . At one end, the compensating element is coupled to the base  1100  at pad  1104 . At the opposing end, the compensating element fastens to the retroreflector. Pad  1104  is positioned outside the optical path, beyond the retroreflector. Thus, as the compensating element expands, the retroreflector is pushed into the cavity reducing the length of the cavity. As the temperature of the foundation increases, the separation between pads  1102 - 1104  changes, typically for most materials, increasing as well. The compensating element  1118  offsets this physical expansion of the base by expanding in an amount which maintains a constant optical path length L opl . As will be obvious to those skilled in the art, the compensating element may be positioned elsewhere in the cavity, for example, joining the gain medium to the base, without departing from the scope of the invention. In still another embodiment of the invention, there may be more than one compensating element positioned between, for example, the retroreflector-base and gain medium-base connections. 
     The compensating element should be designed to maintain an optical pathlength which does not vary with temperature. Satisfaction of this requirement assures that instances of thermally induced mode hopping or variations in output wavelength will be substantially reduced. As shown in FIG. 11A, the optical pathlength L Opl  may be expressed as the sum of the optical paths through the individual components of the tunable laser including: the diode  1124 , the optical element(s)  1128 , the tuning element  1130  and the air gaps La 1 ,La 2 ,La 3  between the various elements. The optical path length through the diode is L d . The optical path length through the optical element(s) is L l . The optical path length through the tuning element is L t . The optical path length through the air gap between the laser and optical element(s) is La 1 . The optical path length through the air gap between the optical and tuning element(s) is La 2 . The optical path length through the air gap between the tuning element and the retroreflector is La 3 . Since all elements are directly or indirectly coupled in a fixed or pivoting manner to the base  1100 , their relative physical separation will typically increase as the temperature of the base increases. This may in turn vary the optical pathlength of the cavity. 
     The optical pathlength of an element is equal to the product of its refractive index and its dimension along the optical path. The optical pathlength of the cavity of the tunable laser is the sum of the products of index of refraction and thickness along the optical path for all elements, including air, within the cavity. This requirement is expressed in the following Equation I, in which n i  is the index of refraction of each element and l i  the physical thickness of the element along the optical path. 
       L   Opl   =Σn   i   ·l   i   Equation I 
     The lower case “l” will reference the physical dimension of an element and the upper case “L” the optical dimension. The integer number of half-wavelengths supported by an element with fixed endpoints increases as the refractive index of the element increases, as predicted by Huygens principle. This results from the observation that light travels slower in media of higher index of refraction, and the wave peaks are more closely packed. Thus, over an identical distance, an element with a higher index of refraction supports a greater number of wavelengths. Thus, the optical path length rather than physical pathlength is a more accurate measure of the integral number of half-wavelengths which a cavity can support. 
     Nevertheless, as a first order approximation, the thermal expansion required by the compensating element  1118  is that required to maintain the physical pathlength dimension of the cavity, i.e. l Opl  constant. That requirement would be met provided dl F1 /dT=dl C /dT for the configuration shown in FIG.  11 A. Given the physical distance between attachment points  1102 - 1104  and the coefficient of thermal expansion α F  of the base  1100 , the required combination of material and thickness between pad  1104  and retroreflector  1126  could be determined so as to hold the physical distance between the cavity endpoints  1122 , 1126  constant. There would several sources of error in the first order approximation. First, optical and physical pathlength are not equivalent as discussed above. Instead, for each segment of the optical path, e.g. L d , L l , L t , L a1 -L a3 , the refractive index of each element must be considered in order to hold the integer number of half-wavelengths in the cavity constant. Second, in determining the number of wavelengths each element can support, the expansion of the element must be calculated. Expansion of each element varies depending on its coefficient of thermal expansion and cross-sectional thickness along the optical path. Additionally, during temperature variations, some cavity elements may expand while others contract, thus varying the average weighted refractive index of the cavity. The average weighted refractive index being the sum of the products of the physical length and refractive index of each element divided by the physical length of the cavity. For example, during a temperature increase, the air gap L a3  may decrease due to the rapid inward expansion of the compensating element while the optical element(s) increase in thickness. Thus the average weighted refractive index may vary as a result. A third source of error results from the fact that the refractive index of each element varies with temperature and by different amounts. What is needed is a way of incorporating all these variables into the choice of material and thickness for the compensating element(s) so that the cavity is optically stable over a broad temperature range. 
     A more accurate way of determining the combination of material and thickness for the compensating element(s)  1118  is provided in the following Equation II in which the temperature related variation in optical path length both due to changes in the physical length of each element as well as the change in the index of refraction of each element is expressed.              0   =                L   Op1            T       ≡     ∑                       (       n   i     ·     l   i       )            T           =     ∑                  (         n   i     ·     a   i       +            n   i            T         )     ·   li                 Equation                 II                                
     In Equation II, the requirement that the rate of change of the optical pathlength L Opl  with respect to temperature be zero satisfies the condition that the optical pathlength be temperature invariant. The optical path length is expressed as the sum of the derivatives of the product of the refractive index “n i ” of each element, the thermal expansion coefficient “α i ” of each element and the physical length “l i ” of each element. As stated above, the elements of the cavity include: laser, optics, filters, and gasses, such as air, in the optical path. 
     The optical path of the laser shown in FIG. 11A is the sum of the optical length of the individual segments of which it is composed including the columns of air/gas separating the elements. This relationship is expressed in the following solution EI-1a to the above mentioned Equation I. 
     
       
           L   Opl   =L   d   +L   l   +L   t   +L   a123   =n   d   l   d   +n   l   l   1   +n   t   l   t   +n   a   l   a123   Solution EI-1a 
       
     
     Now the last term, i.e. the air gap length l a1-3 , is affected by expansion and contraction of the base  1100  as well as the compensating element  1118 . The air gap length can be expressed in terms of the dimensions of the base l F1  and compensating element l c . The appropriate substitutions have been made in the following Solution EI-1b. 
     
       
           L   Opl   =n   d   l   d   +n   i   l   i   +n   t   l   t   +n   a ( l   F1   −l   d   −l   l   −l   t   −l   c )  Solution EI-1b 
       
     
     Next the terms are rearranged in Solution EI-1c to express the optical path length in terms of: L F1  the optical length of the base, L O  the additional optical length produced by the optical elements in the cavity, and L C  the optical length of the compensating element(s). 
     
       
           L   Opl   =+[n   a   l   F1 ]+[( n   d   −n   a ) l   d +( n   l   −n   a ) l   l +( n   t   −n   a ) l   t   ]−[n   a   l   c   ]″=L   F   +L   O   −L   C   Solution EI-1c and equivalent expression 
       
     
     Then the derivative of L Opl  is found and set equal to zero, as indicated in Equation II. This provides a solution for the derivative of the optical length of the compensating element(s)  in terms of the sum of the derivative  of the optical length of the base and  the additional optical length produced by the optical elements in the cavity as set forth in the following Solution EII-1d. The coefficients of thermal expansion α c , α F , α d , α l  for: the compensating element, the base, the gain medium, e.g. laser diode, the lens, and the tuning element, respectively will be utilized in solving the derivative. In addition, the indices of refraction n a , n d , n l , and n t  for air, the diode, the optical elements and the tuning element will be utilized in solving the following derivative.          Solution                 EII        -        1      d                 and                 equivalent                     expression        
     [       n   a          l   c       ]     ′       =       +       [       n   a          l   F       ]     ′       +       [         (       n   d     -     n   a       )          l   d       +       (       n   l     -     n   a       )          l   l       +       (       n   t     -     n   a       )          l   t         ]     ′                 L   C   ′     =       L   F   ′     +     L   O   ′                              
     The derivatives in solution EII-1d may be solved for to produce a solution for the product of the coefficient of thermal expansion and length of the compensating element(s). 
     FIG. 11B shows a different compensating block to base geometry than that of FIG.  11 A. In FIG. 11B, the compensating element attaches the optical component to the base of the laser in the opposite manner to that discussed above. Expansion of the compensating element  1118  in FIG. 11B increases the optical path length during expansion of the compensating element. As in FIG. 11A, the optical path of the laser shown in FIG. 11B is the sum of the optical length of the individual segments of which it is composed. This relationship is expressed in the following solution EI-2a to the above mentioned Equation I. 
     
       
           L   Opl   =L   d   +L   l   +L   t   +L   a124   =n   d   l   d   +n   l   n   l   +n   t   l   t   +n   a   l   a124   Solution EI-2a 
       
     
     As before, the air gap length l a1−3  is affected by expansion and contraction of the base  1100  as well as the compensating element  1118 , however, in this case the expansion of the compensating element has the opposite effect. The air gap length can be expressed in terms of the dimensions of the base l F1  and compensating element l c . The appropriate substitutions have been made in the following Solution EI-2b. Only the sign of the last term has changed from that of Solution EI-1b to reflect the fact that the optical element expansion adds to the cavity length. 
     
       
           L   Opl   =n   d   l   d   +n   l   l   l   +n   t   l   t   +n   a ( l   F1   −l   d   −l   l   −l   t   +l   c )  Solution EI-2b 
       
     
     Next, the terms are rearranged in Solution EI-2c to express the optical path length in terms of: L F1  the optical length of the base, L O  the additional optical length produced by the optical elements in the cavity, and L C  the optical length of the compensating element(s). 
     
       
           L   Opl   =+[n   a   l   F1 ]+[( n   d   −n   a ) l   d +( n   l   −n   a ) l   l +( n   t   −n   a ) l   t   ]−[n   a   l   c   ]″=L   F   +L   O   +L   C   Solution EI-2c and equivalent expression 
       
     
     Then, the derivative of L Opl  is found and set equal to zero, as indicated in Equation II. This provides a solution for the derivative of the optical length of the compensating element(s)  in terms of the sum of the derivative  of the optical length of the base, and  the additional optical length produced by the optical elements in the cavity as set forth in the following Solution EII-2d. The coefficients of thermal expansion α c , α F , α d , α l  for: the compensating element, the base, the gain medium, e.g. laser diode, the lens, and the tuning element, respectively will be utilized in solving the derivative. In addition, the indices of refraction n a , n d , n l , and n t  for air, the diode, the optical elements and the tuning element will be utilized in solving the following derivative.            Solution                 EII        -        2      d                 and                 equivalent                 expression          
     -       [       n   a          l   c       ]     ′       =         +       [       n   a          l   F       ]     ′       +       [         (       n   d     -     n   a       )          l   d       +       (       n   l     -     n   a       )          l   l       +       (       n   t     -     n   a       )          l   t         ]     ′          
     -     L   C   ′       =       L   F   ′     +     L   O   ′                                
     This in turn may be solved to produce a solution for the product of the coefficient of thermal expansion and length of the compensating element(s). 
     The tunable laser of FIG. 11C also includes: foundation  1100 , gain medium  1120 , optical elements  1128 , tuning element  1130  and a retroreflector  1126 . The optical elements, tuning element and retroreflector provide a retroreflective tuning device which tunes the laser by providing feedback of a selected wavelength to the gain medium. In an embodiment of the invention, the gain medium is a laser diode with front and rear facets  1124 - 1122 , respectively. In various embodiments of the invention, the optical elements  1128  include lenses and filters. In various embodiments of the invention the tuning element includes an interference filter, an Etalon, a diffraction element, and a grating. A resonant cavity is formed with a length L Opl  between the rear facet  1122  of the laser diode  1120  and the retroreflector  1126 . The resonant cavity includes an internal cavity between the rear and front facets  1122 - 1124  of the laser diode and an external cavity between the front facet  1124  of the laser diode and the retroreflector  1126 . 
     In various embodiments of the invention, the retroreflector includes a mirror, a comer cube and a dihedral prism. In these embodiments, tuning may be accomplished by rotation/translation of the retroreflector  1126  which is pivotally fastened to the base at pivot point  1112  via compensating element  1118  and pivot arm  1110 . In alternate embodiments of the invention, the tuning may be accomplished by rotation/translation of the gain medium with respect to the base. 
     As the temperature of the foundation increases, the separation between pads  1102 - 1104  changes, typically for most materials, increasing as well. The compensating element  1118  offsets this physical expansion of the base by expanding in an amount which maintains a constant optical path length L opl . As will be obvious to those skilled in the art, the compensating element may be positioned elsewhere in the cavity, for example joining the gain medium to the base, without departing from the scope of the invention. In still another embodiment of the invention there may be more than one compensating element positioned between, for example, the retroreflector-base and gain medium-base connections. 
     FIG. 11D shows a Littrow configuration of an external cavity diode laser with a fixed gain medium and a variably positioned tuning element, e.g. a diffraction grating  1150 . The optical elements  1128  and tuning element  1150  provide a retroreflective tuning device which tunes the laser by providing feedback of a selected wavelength to the gain medium. Tuning is accomplished by rotation/translation of the tuning element, e.g. grating  1150  which forms the distal end of the cavity. The grating is pivotally fastened to the base at pivot point  1112  via compensating element  1118  and pivot arm  1110 . In alternate embodiments of the invention, the tuning may be accomplished by rotation/translation of the gain medium with respect to the base. A resonant cavity is formed with a length L Opl  between the rear facet  1122  of the laser diode  1120  and the tuning element  1150 . The resonant cavity includes an internal cavity between the rear and front facets  1122 - 1124  of the laser diode, and an external cavity between the front facet  1124  of the laser diode and the tuning element  1150 . 
     As the temperature of the foundation increases, the separation between pads  1102 - 1104  changes, typically for most materials, increasing as well. The compensating element  1118  offsets this physical expansion of the base by expanding in an amount which maintains a constant optical path length L opl . As will be obvious to those skilled in the art, the compensating element may be positioned elsewhere in the cavity, for example joining the gain medium to the base, without departing from the scope of the invention. In still another embodiment of the invention there may be more than one compensating element positioned between, for example, the retroreflector-base and gain medium-base connections. 
     FIG. 12 is a top plan view of the resonant cavity portion of the tunable laser signal generator  250  (See FIG.  2 ). The laser is tuned by a retroreflective tuning device which tunes the laser by providing feedback of a selected wavelength to the gain medium, e.g. laser diode  332 . The tuning device includes diffraction grating  340  and retroreflector  350 . The relative physical location of the laser components is affected by the expansion of the base and further by expansion of any intermediate elements, e.g. housings, or mounting blocks, which may be used to fasten the laser components to the base. Laser components in FIG. 12 include: laser diode  334 , diffractor  340 , retroreflector  350 , as well as any lens or filters that may be present. Housing  330 , diffraction mount  342  and compensating element  352  are intermediate elements used to fasten the corresponding laser component to the base. A resonant cavity is formed with a length L Opl1 +L Opl1  between the rear facet  334  of the laser diode  332  and the tuning element  350 . The resonant cavity includes an internal cavity between the rear and front facets  1122 - 1124  of the laser diode and an external cavity between the front facet of the laser diode and the tuning element  350 . 
     Absent intermediate members, the relative physical separation between optical components will increase with temperature since all components are attached in a fixed or pivotal manner to a common base which expands with an increase in temperature. Intermediate members may be used to either increase or decrease the relative physical separation between optical components during a temperature-induced expansion of the base. In the embodiment shown, all intermediate members, i.e. housing  330 , diffraction mount  342  and compensating element  352 , make contact with the base at locations outside the optical path. Laser diode  332  is coupled via housing  330  to the base. The housing contacts the base at contact line  1200 , which is displaced outside the optical path by distance l Cd . The laser housing is fastened to the base by fasteners along a centerline  1204 . Thus, expansion of the housing reduces the length of air gap l al  between the front facet of the laser diode and the diffraction grating. Diffraction grating  342  is coupled via mount  340  to the base. For purposes of simplifying the solution set that follows, it is assumed that the diffraction mount  342  and base have identical coefficients of expansion and that the expansion coefficient of the diffraction grating is zero. In this specific case the intermediate component, i.e. mount  342 , does not create relative expansion/contraction of the diffraction grating surface with respect to the base. Were this not the case, the solution set that follows would take into account the reduction in length of both optical path segments L Opl1  and L Opl2  resulting from differential expansion of the surface of the diffraction grating and the base. Retroreflector  350  is coupled via compensation element  352  to the pivot bracket  354 , which is in turn pivotally coupled to the base  300 . The compensation element contacts the pivot bracket at contact line  1202 , which is displaced outside the optical path by distance l Cr . Thus, expansion of the compensation element reduces the length of air gap l a2  between the front face of the retroreflector and the diffraction grating. 
     As will be obvious to those skilled in the art, intermediate members may be fabricated in different lengths of different materials, with varying coefficients of expansion less than, or greater than that of the base. If they have higher coefficients of thermal expansion than the base to which they are attached, then their expansion tends to decrease the physical separation between components and may be used to counteract or completely offset expansion of the base. Conversely, were the intermediate components rearranged to make contact with the base at locations within the optical path, they would have the opposite effect, i.e. increasing the relative separation between optical components beyond what would be the case, were the optical components attached directly to the base. Thus, one or more intermediate members may be used with a base and laser components to thermally induce separations between optical components which either vary directly/inversely with temperature. This capability will be relied on to fabricate a thermally stable signal source. 
     As discussed above in Equation II, the requirement of a thermally stable optical pathlength is met when the rate of change of the optical pathlength L Opl  with respect to temperature is zero. In the embodiment shown in FIG. 12, the optical path of the laser is folded to include two distinct segments L Opl1  and L Opl2 , between the laser diode  332  together with the diffraction grating  340 , and the diffraction grating together with retroreflector  350 , respectively. The total optical pathlength L Opl12  is the sum of the optical length of all optical components within each of the segments including the columns of air/gas separating the elements. This relationship is expressed in the following solution EI-3a to the above-mentioned Equation I. 
     
       
           L   Opl12   =L   d   +L   r   +L   a12   =n   d   l   d   +n   r   l   r   +n   a   l   a12   Solution EI-3a 
       
     
     Now the last term, i.e. the air gap length l a12  is affected by expansion and contraction of the base  300  as well as the compensating element  352  and housing  330 . The air gap length can be expressed in terms of the dimensions of the base l F1-2 , compensating element l Cr  and diode housing l Cd . The appropriate substitutions have been made in the following Solution EI-3b. 
     
       
           L   Opl12   =n   d   l   d   +n   r   l   r   +n   a ( l   F1   −l   Cd   −l   d   +L   F2   −l   Cr )  Solution EI-3b 
       
     
     Next the terms are rearranged in Solution EI-3c to express the optical path length in terms of: L F12  the optical length of the base, L O  the additional optical length produced by the optical elements in the cavity, and L C  the optical length of the compensating element(s). 
     
       
           L   Opl12   =+[n   a ( l   F1   +l   F2 )]+[( n   d   −n   a ) l   d   +n   r   l   r   ]−[n   a ( l   Cd   +l   Cr )]″=L F   +L   O   −L   C   Solution EI-3c and equivalent expression 
       
     
     Then the derivative of L Opl12  is found and set equal to zero, as indicated in Equation II. This provides a solution for the derivative of the optical length of the compensating element(s)  in terms of the sum of the derivative  of the optical length of the base and  the additional optical length produced by the optical elements in the cavity as set forth in the following Solution EII-3d. The coefficients of thermal expansion α Cd , α Cr , α F12 , α d ,α r  for: the laser housing, compensating element, base, diode, retroreflector respectively will be utilized in solving the derivative. In addition, the indices of refraction n a , n d , and n t  for air, the diode, and the tuning element will be utilized in solving the following derivative. Additionally, where a collimating lens is positioned at the output of the laser diode the index of refraction and thermal expansion coefficient for that element would appear as well in the following equation.          Solution                 EII        -        3      d                 and                 equivalent                     expression        
     [       n   a          (       l   Cd     +     l   Cr       )       ]     ′       =       +       [       n   a          l   F12       ]     ′       +       [         (       n   d     -     n   a       )          l   d       +       n   r          l   r         ]     ′                 L   C   ′     =       L   F   ′     +     L   O   ′                              
     This in turn may be solved to produce a solution for the product of the coefficient of thermal expansion and length of the compensating element(s). 
     In an alternate embodiment of the invention the length of the compensating elements can be obtained experimentally by measuring the wavelength of the composite cavity and using this information to determine the length of the compensating element(s). 
     Thermally Stabilizing the Drive Train 
     Thermal variations in a mechanically tuned laser affect not only the optical pathlength but also the angle of the tuning element. Both optical pathlength variations, as well as changes in the tuning angle, contribute to thermally induced mode hop and wavelength variations in the output beam. Typically, thermal stabilization of the optical pathlength as discussed above, is a necessary but not sufficient condition for reducing thermally induced mode hop and wavelength variations in the output beam. FIGS. 13B-D show embodiments of the invention for passive thermal stabilization of the angle of a tuning element in a mechanically tuned laser. FIG. 13A shows a prior art design in which variations in the tuning angle may be thermally induced. 
     FIG. 13A shows a prior art design for a mechanical drive train to move the tuning element of an external cavity laser. A base  1300 , pivot arm  1302 , tuning element  1310 , lead screw  1340  and threaded block  1320  are shown. The pivot arm is fastened to the base  1300  at pivot point  1304 . The tuning element  1310 , e.g. retroreflector, grating, etalon, etc., is attached to the pivot arm such that arcuate movement of the pivot arm, induced by the lead screw  1340 , tunes the laser (not shown). The lead screw is flexibly attached to the tip of the pivot arm  1342 . The lead screw has an elongated threaded portion extending from the tip of the pivot arm through a threaded opening in the threaded block to a drive end  1344  of the lead screw. The threaded block is fixed to the base  1300 . As the lead screw is rotated by an actuator (not shown), it moves linearly along a line tangent to the tip of the pivot arm. The arcuate motion of the pivot arm induced, thereby tunes the laser by rotating the tuning element to a specific angle with respect to the base. This in turn selects a specific output wavelength for the laser. At any selected output wavelength, the angle must be held constant during temperature variations in order to avoid variations in the output wavelength. In the prior art case shown in FIG. 13A, this requirement is met only when the thermal expansion coefficient α Dt  of the lead screw  1340  and base α B  are identical. In the unique case where this condition is met, the expansion of the drive train, e.g. lead screw, along length D Dt  will equal that of the base D B  over the distance separating the tip of the pivot arm from the centerline of the threaded block  1320 . In a practical implementation, this condition will typically not be met since the base is typically fabricated from a very hard, thermally inert material such as a nickel-steel alloy, and the lead screw of a soft, easily machined material with a relatively high coefficient of expansion, such as brass. Therefore, in the typical case, the prior art drive train design is not thermally stable since the differential expansion of the drive train exceeds that of the base. Thus, prior art tunable lasers are subject to temperature induced tuning of the laser, i.e. “thermal tuning”, which creates undesirable variations in the output wavelength of the laser and/or mode hopping. Therefore, what is needed is a way to use materials suitable for the drive train and base without the requirement that they have identical expansion coefficients. 
     FIGS. 13B-D show various embodiments of the invention for thermally stabilizing the drive train. A compensating element is provided to offset the differential expansion between the base and the drive assembly. In the embodiments shown, the compensating element is linked to the drive train in a geometry which offsets the differential expansion thereby enhancing the thermal stability of the tuning element at any selected output wavelength. 
     FIG. 13B shows a drive train similar to that discussed above in connection with FIG. 13A, with the exception of compensating element  1322  which couples the drive train to the base. The compensating element is U-shaped with a threaded opening in the base and with a rim which is affixed to the base. The compensating element is laid out on its side with the lead screw passing through the opening in the rim and through a threaded opening in the base of the compensating element to a point of termination at the drive end  1344  of the lead screw. The compensating element typically has an overall thermal expansion greater than that of the base  1300  by an amount sufficient to compensate for differential expansion of the base and lead screw. That relationship is expressed in the following Equation III, where d B  is the length of the base from the tip of the pivot arm to the fastening point for the rim  1324 , d c  is the length of the compensating element, and d dt  is the lead screw length from the tip of the pivot arm to the base of the compensating member. The thermal expansion coefficients for the compensating element, base, lead screw are: α c , α dt , α b  respectively. 
     
       
         +α c   d   c =α dt   d   dt −α b   d   b   Equation III 
       
     
     FIG. 13C shows an alternate embodiment of the compensating element for thermally stabilizing the mechanical drive train. In this embodiment, the lead screw is stationary and is rotatably fastened at opposite ends to the base via pillow blocks  1330  and  1328  on either side of pivot arm  1304 . Shoulders on the lead screw on either side of pillow block  1328  maintain a fixed relationship between the lead screw and that pillow block. Expansion of the lead screw exhibits itself at pillow block  1330  in which the lead screw is free to move linearly. Movement of the tip of the pivot arm results from the threaded attachment at an intermediate point on the lead screw of a threaded portion of the base of compensating element  1350 , with the rim of that element attaching to the tip  1342  of the pivot arm. As the lead screw rotates in a clockwise or counterclockwise direction, the threaded base of the compensating element is caused to undergo linear translation along a line tangent to the tip. This movement produces arcuate movement of the tip to tune the laser. The compensating member  1350  offsets the differential expansion between the drive train assembly, e.g. lead screw  1340 , and the base by expanding in a direction opposite to the expansion of the lead screw so as to maintain the pivot arm in a fixed position. That relationship is set forth in Equation III above. 
     As will be obvious to those skilled in the art, the thermal stabilization provided by the compensating element is equally applicable to laser drive trains such as: piezo-electric actuators, solenoids, linear stepper motors, etc., without departing from the scope of the invention. 
     FIG. 13D is a top plan view of the embodiment of the tunable laser discussed above in connection with FIGS. 3-9. The base  300 , drive train  376 , retroreflector  350 , diffraction grating  340 , pivot arm tip  430  and motor attachment bracket  310  are shown. The hole  532  about which the pivot arm rotates is shown. In the embodiment shown, the drive shaft including stepper motor output shaft, rotary flex member, cylindrical nut, and lead screw, has a length of d Dt . The compensation element  410  is coupled to the end of the drive shaft to the head of the lead screw. As the drive shaft expands along length d Dt , the compensating element expands in the opposite direction over the length d c  to the point at which one end of the linear flex member  420  is coupled to the compensating element. The compensation element will typically have a coefficient of expansion greater than either the drive train or the linear flex member. It is dimensioned such that its expansion offsets the difference between the expansion of the drive shaft together with the linear flex member from that of the base. In the embodiment shown, for purposes of simplification, the faceplate of the stepper motor at which the drive shaft originates is assumed to be fixed to the base at location  1380 . The base expands over the distance d B  measured from the tip  430  of the pivot arm to the origin of the actuator drive shaft. These parameters are set forth in accordance with Equation III in the following solution EIII-1a. 
      +α c   d   c =(α dt   d   dt +α B   d   B) −α b   d   b   Solution EIII-1a 
     In this embodiment of the invention, passive thermal compensation of the drive train achieves the effect of maintaining a stable angle between the tuning elements, i.e. retroreflector  350  and the diffraction grating  340 . This assures that the output wavelength will remain temperature invariant on any output channel/frequency. In combination, passive thermal pathlength compensation and thermal compensation of the drive train also substantially reduce mode hopping. As will be obvious to those skilled in the art, it is evident that the roles of the lead screw and cylindrical nut may be reversed without departing from the scope of the invention. In an alternate embodiment of the invention, the length of the compensating element(s) can be obtained experimentally by measuring the wavelength of the composite cavity and using this information to determine the length of the compensating element(s). 
     Accurate Positioning of Components 
     Thermal path length compensation requires accurate positioning of the laser components. In addition to accurate positioning, the line/point of contact between each component of the laser system, as well as any intermediate elements necessary to fasten them to the base, must be determined. In order to properly dimension compensating elements, such as the laser housing, it is preferable that they frictionally contact the base along a narrow and well-defined line of contact. From this line of contact, expansion and contraction calculations necessary for determining the length and material combination for the intermediate compensating components may be calculated. 
     FIGS. 14A-B show respectively an isometric exploded view and a side cross-sectional view of pads which are used, in an embodiment of the invention, to position the laser components with respect to one. These pads improve the precision of the relative thermal expansion calculations necessary to dimension the laser components and intermediate elements properly so as to thermally stabilize the optical path length (See FIGS. 11A-D and  12 ). They do so by reducing the contact area between the attached objects, e.g. a laser component or intermediate element and the base. Additionally the pads serve to provide three points of contact or contact along a line together with a point of contact to level the device as well as accurately position it. Typically two or more pads will be utilized between the attached objects. Where two pads are utilized, the first, a contact pad, will typically provide a narrow line of contact from which expansion calculations are performed while the second, a leveling pad, provides a low friction surface area to level the attached component or intermediate element. The line of contact provided by the contact pad will typically be orthogonal to the optical path. The contact pad will typically have a triangular or narrow rectangular cross-section to increase frictional contact between it and the objects between which it is sandwiched. The leveling pad will typically have a broad rectangular cross-section with a smooth surface to allow the objects on either side to move relative to one another during thermal expansion/contraction. Fasteners between the contact and leveling pad will be utilized to apportion the loading on each. Typically, the greater loading will be placed on the contact pad to increase the friction between it and the objects between which it is sandwiched. A reduced loading on the leveling pad allows relative movement between the objects on either side. The pads may be separate from the corresponding attached object or part of either of them. In the absence of these contact pads, thermal expansion calculations would be made from the centerlines of the fasteners used to couple laser components or intermediate elements to the base. This latter technique may lack the precision provided by the contact pads due, for example, to the slop between fasteners and the thru and threaded holes of the attached objects. 
     In FIG. 14A, a three pad fastening system is shown for the attachment of the laser housing  330  to the base  300 . There are two contact pads  1400 - 1402  and one leveling pad  1404 . Within the base are defined the fastening holes  504 ,  510  and  520  for fastening respectively the laser housing  330 , diffraction mount  342  and fiber mount  302  to the base (See FIGS.  3 - 5 ). Pads  1400 - 1404  are positioned between the base  300  and the laser housing  330 . The two contact pads  1400 - 1402  are aligned with one another along contact line  1200  which is generally orthogonal to the optical path. These two pads provide the frictional contact with the housing from which thermal path length calibration will be calculated. The remaining leveling pad  1404  is laid out on axis  1410  and serves to level the housing and has a light enough contact with the housing so that the housing is slidably positioned with respect to this pad. The laser housing  330  is brought into contact with the pads by fasteners  500 - 502  which threadably engage holes  504  having a centerline  1204  within the base  300 . 
     FIG. 14B shows a cross-sectional side elevation view of the base  300  and laser housing  330 . The housing is shown contacting both contact pad  1402  and leveling pad  1404 . Fasteners  500  are shown positioned at a distance d 1  , from the contact pad  1402 , and a distance d 2  from the leveling pad  1404 . The contact force between the laser housing and the contact pad is F×d 2 /(d 1 +d 2 ) where “F” is the fastening force. As d 1  decreases, the force on the leveling pad, i.e. F×d 2 /(d 1 +d 2 ), decreases as well. The separation l cd  between the contact pad  1402  and the rear facet  334  of the laser diode  332  (See FIG. 3.) is selected in combination with the thermal expansion coefficient of the laser housing material to provide, in combination with the other components and compensating elements of the tunable laser, a thermally stable optical pathlength as discussed above in connection with FIGS. 11A-B and  12 . In alternate embodiments of the invention the contact pads may be integral with either of the elements being fastened, or may be fastened between them. The contact pads may have varying cross sectional profiles with the contact pad(s) typically having a narrow high friction profile to prevent relative movement of the objects being fastened. The leveling pad by contrast has typically a planar surface to minimize friction and allow relative expansion between the objects fastened. 
     Active Thermal Compensation 
     In an embodiment of the invention, active thermal stabilization may be utilized alone or in combination with the passive techniques discussed above to maintain wavelength stability and avoid mode hopping. Active thermal compensation avoids temperature related wavelength variations and mode hopping by maintaining the tunable laser components at a constant temperature. By actively adding or removing energy from the cavity responsive to feedback from temperature/energy monitors, a relatively constant thermal state can be maintained for the tunable laser. This approach requires heaters/coolers as well as closed loop feedback sensors and circuitry. In an alternate embodiment of the invention a less expensive approach to active thermal stabilization may be implemented. In this approach there is no active feedback, relying instead on maintaining a temperature in the tunable laser that is significantly above or below the ambient condition so as to reduce external environmental effects on the laser. To avoid additional components such as heaters/refrigerators, it is advantageous to utilize the existing components in the system where possible to provide the requisite energy input. The actuator holds such potential. In a stepper motor, for example, energy is consumed in moving the tuning element from one to another output wavelength. By designing the stepper motor control to output a constant power level at any pole, or phantom pole, and even in a locked condition, the overall thermal variations in the tunable laser may be kept at a relatively constant temperature. 
     FIG. 15 shows a detailed circuit diagram of an embodiment of the modulation circuit  222  discussed above in connection with FIG.  2 . The circuit provides, as is shown in the following FIG. 16, a range of analog and digital modulation which is suitable for testing the various optical components associated with an optic network (See FIG.  1 ). The circuit provides a relatively low-frequency feedback loop for maintaining a stable output power that operates in combination with a relatively high frequency open loop switched threshold voltage source  1510  and a laser power shunt to inject a digital modulation signal onto a selected peak output power of the optical signal generator. The modulating circuit includes a setpoint module to generate a fixed output current/voltage, a first modulation module  1510  to switchably connect the output of the setpoint module to an input of the feedback module  1520 ; and a second modulation module  1580  to switchably connect the laser diode/gain medium  1584  to a current source  1566  and a control unit. The laser diode/gain medium  1584  is part of the tunable laser, e.g. gain medium  224  in FIG. 2 or laser diode  332  in FIG.  4 . 
     In the embodiment shown, the set point module comprises an analog to digital converter  1502  and a voltage controlled current source  1504 . The analog to digital converter  1502  drives the voltage controlled current source  1504  to a specific output current/voltage which is provided as an input to the first modulation module  1510 . The first modulation module  151   0  includes a transistor switch  1512 , pull down resistor  1514 , and resistor bridge  1516 - 1518 . In a first position, switch  1512  couples the input from the set point module to the pull down resistor  514 , which is in turn coupled to ground. In a second position, the switch  1512  couples the output of the set point module to the node formed between resistors  1516 - 1518 . This raises or lowers the voltage at the node of the resistor bridge. The resistor bridge is coupled at one end to a ground within the first set point module  1510 , and at the alternate end provides an input to the low frequency feedback module  1520 . 
     The low frequency feedback module accepts inputs from the laser diode  1584  and the first modulation module  1510 , and provides an output to the analog modulator  1562  and the voltage controlled current source  1566  to which it is attached. The feedback module includes: a beam splitter  1544 , a photodetector  1542 , a summer, and an integrator. In the embodiment shown, the summer is an op amp  1524  with a bridging resistor between the negative input and output. The positive output of the op amp is coupled via resistor  1522  to ground. The negative input of the op amp couples both to the output resistor  1518 , which is part of the first set point module  1510 , as well as to the photodetector  1542  via intermediate resistor  1540 . Thus, at the negative input, the op amp sums the current provided by the first modulation module with the current withdrawn by the photodetector  1542 . The output of the summer is coupled via resistor  1528  to the input of the integrator. The integrator includes: op amp  1532 , bridging capacitor  1534 , and resistor  1536 , which couple between the negative input of the op amp and the output. The positive input of the op amp is coupled via resistor  1530  to ground. At the output, the integrator couples via analog modulator  1562  to the voltage controlled current source  1566 . Within the feedback loop, the beam splitter  1544  accepts as an input the output beam  1546  provided by laser diode  1584 . This beam is split into an output portion  1548  and a feedback portion  1550 . The feedback portion  1550  drives the photodetector  1542 . When the system is in equilibrium, the amount of current withdrawn by the photo diode  1542  would be equivalent to the current provided by the first modulation module  1510  at the negative input of the op amp  1524 . In this steady state condition, the amount of current provided by current source  1566  will be that required to drive the laser diode  1584  at a power level determined by the output level of the set point module  1500 . Any variations in the set point module will result in more or less current provided by the current detector  1566 . 
     The laser diode  1584  couples to the second modulation module  1580 . The second modulation module includes a switch  1582  and a pull up resistor  1586 . The switch switchably couples either the laser diode  1584  or the pull up resistor  1586  to the current source  1566 , which is in turn connected via resistor  1564  to ground. The current detector  1568  monitors the current through laser diode  1584 . 
     The control module provides a control input to the set point module  1500  and specifically the analog to digital converter  1502  therein. The control module also provides control inputs to both switches  1512  and  1582  in, respectively, the first and second modulation modules. Additionally, the control module provides an input to analog modulator  1562 . The control module accepts input from the current detector  1568 . The control module  1560  is in turn coupled via system bus  216  to the processor  206  (See FIG.  2 ). 
     In operation, the user selects an output channel/wavelength for the optical signal generator which is tuned to that wavelength via the actuator and drive train assembly, lookup table and processor as discussed above and in the following FIG.  19 . Next, a specific power level, digital modulation frequency, and duty cycle are selected. Responsive to the power selection, the control module  1560  generates a signal to the analog to digital converter  1502  within the set point module which results in the appropriate current being delivered by current source  1504  to the first modulation module  1510 . Then, responsive to the user-selected modulation frequency and duty cycle, the control unit  1560  generates signals which cause switches  1512  and  1582  within the first and second modulation modules to switch between poles at a rate and duty cycle proportional to the inputs from the control module. Switches  1512 - 1582  are operated substantially synchronously such that in the first position switch  1512  shunts the output of the setpoint module via resistor  1514  to ground and switch  1582  in the first position couples the pull up resistor  1586  to the current source  1566 . Thus, in the first position, no input is provided from the first modulation module to the feedback unit  1520 , and no current is delivered to the laser  1584 . In the second position, switch  1512  couples the output of the set point module to the resistor bridge  1516 - 1518  which provides control input to the feedback module  1520 , and specifically the summer thereof. In the second position, switch  1582  couples the laser diode  1584  to the current source  1566 . By virtue of the substantially synchronous operation of switches  1512  and  1582 , the relatively low frequency feedback circuit  1520  is not required to engage in digital modulation, seeking instead a relatively constant peak output state that can be maintained across any range of duty cycles and modulation frequencies which can be implemented by switches  1512 - 1582 . 
     An additional feature of the modulation circuit  222  is that analog modulation capability is provided. At a constant DC power level or during digital modulation, control module  1560  can provide an analog input to analog modulator  1562 . In an embodiment of the invention, analog modulator  1562  is a pull down resistor which adds or removes current from the line connecting the output of the integrator to the voltage controlled current source. An additional feature of the modulation circuit is the provision of overload current protection provided by detector  1568 . Detector  1568  provides a signal proportional to the current through the laser diode  1584 . The signal is provided to the control module  1560  which, in conjunction with the processor  206  to which it is coupled, causes the switch  1582  to de-couple the laser diode  1584  from the current source  1566  when an overload condition is detected. 
     As will be obvious to those skilled in the art the modulator may be implemented using either analog or digital circuits or software, singly or in combination without departing from the scope of the invention. In one digital embodiment of the circuit an integrator within the error detector/feedback circuit  1520  would integrate only in the on state when the laser diode was coupled to the current source. 
     The modulating circuit shown in FIG. 15 may be utilized with equal advantage in numerous lasers including: distributed feedback lasers, YAGG lasers, gas lasers, tunable semiconductor lasers, distributed Bragg reflectors, etc., without departing from the scope of the invention. In fact, the modulating circuit may be utilized in any laser in which modulation of output beam intensity can be accomplished. 
     FIGS. 16A and 16B are a graph and an enlarged view  1620  of a portion of the graph, respectively, showing some of the various optical output signal profiles which can be generated by the optical signal generator  250  (See FIG. 2) under the control of the modulation circuit  222  (See FIGS. 2,  15 ). The laser output beam may be modulated across a range of duty cycles, frequencies and power levels. Signal sequences  1600 - 1604  are shown. In signal sequence  1600 , the set point module  1500  (See FIG. 15) provides a fixed output current/voltage while the first and second modulation module alternately de-couple and couple the set point module and laser diode from the feedback circuit  1520  across a range of duty cycles at a fixed frequency and a power level P 1 . In signal sequence  1602 , the set point module  1500  (See FIG. 15) provides a second power level P 2  while the first and second modulation modules couple and de-couple the laser diode and set point module with the feedback circuit across a range of frequencies at a fixed duty cycle. In signal sequence  1604 , both the frequency and the duty cycle of the first and second modulation modules is fixed, and the set point module  1500  delivers a voltage/current sufficient to drive the current source  1566  (See FIG. 15) at a third power level P 3 . In signal sequence  1608 , a fixed duty cycle and frequency is provided by the control module to the first and second modulation modules  1510 - 1580  (See FIG.  15 ), while the set point module  1500  is ramped from the first to the third power level. Enlarged signal diagram  1620  shows a portion of signal sequence  1604  in which the analog modulator  1562  under the control of the controller  1560  (See FIG. 15) injects an analog signal onto the output beam  1548  by modulating the current source  1566 . This analog sequence is injected only on the positive going digital modulation sequence since only during that portion of the signal sequence is the laser diode  1584  coupled to the current source  1566 . 
     FIG. 17 shows an embodiment of the data structure associated with the lookup table  212  utilized during open loop operation of the signal generator  250  (See FIG.  2 ). During open loop operation, the processor  206  (See FIG. 2) responds to the user selection of a specific output wavelength by implementing processes (See FIG. 19) which in conjunction with the lookup table result in the appropriate drive signals being delivered to the actuator (See FIG. 2) so as to cause the laser to emit an output beam at the selected wavelength. Database  212  comprises a number of wavelength records, each of which contains a wavelength field  1704  and a drive signal/pulse field  1702 . A first of the wavelength records, i.e. the base record, additionally contains a flag  1700  indicating that it is the starting point for further calculations. In an embodiment of the invention, this flag would be the beginning of file (BOF) or end of file (EOF) indicator or a specific starting address within the database in which the lookup table  212  was contained. In the embodiment shown, the first record has entries of “0” for a pulse count and a wavelength of 1525 nm. The second record has a pulse count of 4 and a wavelength of 1525.5 nm. The third record has a pulse count of 8 and a wavelength of 1526 nm. In the embodiment shown, the pulses are total cumulative pulses required to move the actuator from the base wavelength to the wavelength associated with the cumulative number of pulses. The following process flow FIG. 18 shows the processes associated with generating the lookup table. 
     FIG. 18 is a process flow diagram showing the processes associated with generating the lookup table  212  (See FIG.  10 ). Processing begins at start block  1800 , in which the system for driving the signal generator  250  and for measuring the output wavelength from the wavelength meter  110  and storing that wavelength in a lookup table, are initiated. Control then passes to process  1802 . In process  1802  the actuator  230  is gradually swept from a starting position until, in the following decision step  1804 , a signal is received from a first start condition sensor indicating that the base state has been reached. In an embodiment of the invention, that sensor, e.g. sensor  390  and/or  392  (See FIGS. 3-9) indicates that the start/base position for the pivot arm has been reached. Control then passes to process  1820 . 
     In an embodiment of the invention which implements a combined linear and rotational sensor such as that shown in FIG. 9, control may alternately pass from decision process  1804  to processes  1806 - 1812  for a base state determination by a second sensor. In process  1806 , any backlash is removed from the drive system by sending appropriate activating pulses to the actuator. Control is then passed to process  1810  in which the actuator is energized. Control then passes to decision process  1812  in which a determination is made as to when a second sensor, e.g. sensor  392  (See FIG. 3) indicates that the base position has been reached. When this determination is made, control is passed to process  1820 . 
     In process  1820 , the measurement of the output wavelength at the base position is obtained from the wavelength meter  1000  (See FIG.  10 ). Control is then passed to process  1822 . In process  1822 , the wavelength measurement is stored in the first record in the database along with the drive signal sequence/amount associated with the base position, e.g. a pulse count of “0”. Control is then passed to process  1824 . In process  1824  the processor  206  (See FIG. 10) or its equivalent sends a fixed sequence/type/number of activation signals to the actuator  230  which results in the tuning of the laser to a next wavelength level. Control is then passed to process  1826 . In process  1826  the pulses generated in process  1824  are added to the previous amount to generate a cumulative pulse count. Control is then passed to process  1828 . In process  1828  the wavelength measurement made by the wavelength meter  1000  (See FIG. 10) is obtained. Control is then passed to process  1830 . In process  1830  the wavelength obtained in process  1828  and the cumulative pulse count obtained in process  1826  are combined into a single record which is stored in a database  212  (See FIG.  2 ). Control is then passed to decision process  1832 . In decision process  1832  a determination is made as to whether the last wavelength obtained in process  1828  lies at the end of the operating range of the signal generator. In the event that determination is in the negative, control returns to process  1824  for the next increment of the actuator. Alternately, if in decision process  1832  an affirmative is reached, that the signal generator has reached the end of the operating range, control is then passed to decision process  1834 . In decision process  1834  a determination is made as to whether additional records will be generated by interpolation. If that determination is negative, then control is passed to process  1838 . If the determination is affirmative, control passes to process  1836 . In process  1836  an interpolation is performed using existing records in the database, and additional records corresponding to interpolations between the initial records in the database are added to the database. These additional interpolated records each have a pulse count offset from the base and a wavelength. Then control passes to process  1838 . In process  1838  the completed database with records correlating pulse count and wavelength is stored in memory  208  (See FIGS.  2 - 10 ). In an alternate embodiment of the invention multiple traces, averages, curve fitting may be used to generate additional records. In still another embodiment of the invention, measurements of drive signals and output wavelengths may be made across the tuning range to establish a functional relationship between wavelength and drive signals. In this embodiment, the lookup table would contain the single function correlating wavelength with drive signals rather than a plurality of records. 
     FIG. 19 is a process flow diagram showing the processes associated with operation of the signal generator portion of the multimeter  100 . 
     Processing begins at start block  1900  in which the signal generator is initialized. Control then proceeds to process  1902 . In process  1902  the CPU  206  (See FIG. 10) outputs drive pulses to the stepper motor causing it to initiate a slow sweep from a start position. Control is then passed to decision process  1904 . In decision process  1904  a determination is made as to whether a start condition sensor has signaled the base position. In the event that determination is in the affirmative, control may optionally be passed to additional processes  1906 - 1912  for the confirmation of a second sensor as to the base condition. Alternately, control is passed directly to process  1920 . 
     In optional process  1906  any backlash in the drive train is removed and control is passed to process  1910 . In process  1910  the processor outputs drive signals to the stepper motor. Control is then passed to decision  1912 . In decision process  1912 , a determination is made as to when a signal is received from the second sensor, e.g. sensor  392 , indicating a base position. Control is then passed to process  1920 . 
     In process  1920  the wavelength and pulse count for the base position are read from the lookup table  212  (See FIGS. 2,  10 ). These are stored in the history register. Control is then passed to process  1922 . In process  1922  the wavelength value read from the base record in the lookup table may be displayed on display  200  (See FIG.  2 ). Control is then passed to decision process  1924 . In decision process  1924  a determination is made as to when the next output channel/center wavelength has been selected. Selection may result from a number of input sources. These sources include entries from the user via user inputs  202  (See FIG. 2) or from program code stored in memory  208  and having a specific operating regime for the signal generator. In either event, once a determination is made that the next wavelength/channel has been indicated, control is then passed to process  1926 . In process  1926  a lookup is performed on the lookup table/database using the wavelength obtained in decision process  1924 . If the next wavelength corresponds to that of a wavelength record in the database, then that record including the associated cumulative pulse count is read by the processor  206 . Alternately, if the target wavelength does not match any of the records of the database, then the two closest records in the database are obtained and an interpolation of the pulse count stored in each is performed to generate a cumulative pulse count or drive signal profile which lies in between the two records. Control is then passed to process  1928 . In process  1928 , the pulse count stored in the history register in process  1920  is subtracted from the pulse count obtained in process  1926 . Control is then passed to decision process  1930 . In decision process  1930  a determination is made as to whether the difference obtained in decision process  1928  has a positive or negative value. If the value is positive, indicating that movement of the actuator in the same direction is appropriate to achieve the next output wavelength, then control is passed directly to process  1940 . Alternately, if the difference obtained is negative, control is passed to intermediate process  1932 . In intermediate process  1932  appropriate pulses are output, e.g. amounting to the difference obtained in process  1928  plus an additional backlash value. Control is then passed to process  1934  in which the backlash is reversed. Control is then passed directly to process  1944  in which the wavelength obtained in decision process  1924  is displayed to the user. 
     Alternately, if in decision process  1930  a determination is made that the difference is positive then control is passed directly to process  1940 . A positive determination as discussed above indicates that there is no backlash/hysteresis to remove since the movement to the next wavelength selected is in the same direction as was utilized in the previous measurement. In process  1940  the pulse difference obtained in process  1928  is output by the processor to the actuator. Control is then passed to decision process  1944  in which the desired wavelength is displayed to the user. Then control returns to decision process  1924  for the processing of the next selected output wavelength. 
     The many features and advantages of the present invention are apparent from the written description, and thus, it is intended by the appended claims to cover all such features and advantages of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.