CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/319,907, filed Jan. 27, 2003. 


   BACKGROUND OF INVENTION 
   1. Technical Field 
   The present invention relates generally to systems for generating and using coherent light, and more particularly to determining and controlling the frequency of light used in such systems. It is anticipated that a primary application of the present invention will be in telecommunications, but the present invention is also well suited for use in laboratory measurement and other fields. 
   2. Background 
   The Fabry-Perot etalon has long been used for stabilizing laser frequencies, and the confocal etalon is now starting to see similar use. Due to its wide use, the Fabry-Perot etalon is used in the examples here, but it should be appreciated that the scope of what we present here is not limited to only that device. It should also be noted that the terms “frequency” and “wavelength” are used interchangeably in the following discussion. 
   In a typical fiber optics application, the spacing between the two reflectors of a Fabry-Perot etalon assembly is fixed and the resonant spectrum (transmissive or reflective) coincides with the ITU grids, which come in increments of 200 GHz, 100 GHz, 50 GHz, . . . , etc. This type of arrangement is termed a “wavelength locker” in the telecommunications industry. 
     FIG. 1  (background art) is a block diagram that conceptually shows the structures of two Fabry-Perot etalons that are commonly used in wavelength lockers for fixed wavelength applications. The first of these is etalon  10 , an “air spaced etalon” as discussed below. It comprises two light transmissive plates  12  each having a partially reflective surface, i.e., reflectors  14 . The reflectors  14  are separated apart a distance L 1  by two spacers  16 , thus defining a chamber  18  that contains a medium with a refractive index, n 1 . 
   The second Fabry-Perot device in  FIG. 1  is etalon  20 , a “solid etalon” as also discussed below. This comprises one light transmissive block  22  having two partially reflective surfaces, reflectors  24 , separated apart a distance L 2 . The material of the block  22  is a medium having a refractive index, n 2 . 
   Numerous variations of Fabry-Perot etalons such as those in  FIG. 1  are possible, for manufacturing convenience, etc. For example, the shape of the structure supporting the reflective surfaces can be either rectangular or round, and either two rectangular bars or a single cylinder can be used to separate the reflective surfaces. 
     FIG. 2  (background art) is a graph  30  showing a typical transmissive spectrum  32  of a wavelength locker. The relationship of frequency verses transmission intensity is depicted with a peak-valley curve  34 , wherein adjacent peaks  36  define a free spectral range (FSR  38 ). For instance, 50 GHz. 
     FIG. 3  (background art) is a graph  40  showing, in simplified manner, the principle of a conventional wavelength locker using a Fabry-Perot etalon (e.g., etalons  10 ,  20 ). 
   When laser light is injected into the etalon its frequency falls somewhere on a peak-valley curve  42  that is characteristic for the particular etalon. The etalon is normally pre-calibrated so that this occurs in a shoulder region  44 , typically centered about the 50% point with respect to amplitude on the ITU grids  46 . The laser frequency of the wavelength locker is then normally adjusted to this 50% point, termed a lock point  48 , and kept there by use of a servo control circuit. The laser frequency will thereafter remain stable as long as the peak-valley curve of the etalon does not drift. 
   In the etalons  10 ,  20  of  FIG. 1 , the spacings L 1 , L 2  between the respective reflectors  14 ,  24  determine the FSR  38  of the resonant (transmissive or reflective) spectrum according to the equation:
 
 FSR=c /(2* n*L )  EQ. 1
 
   where c is the speed of light in vacuum and n (n 1  or n 2  as the case may be in  FIG. 1 ) is the refractive index of the medium between the respective set of two reflectors  14 . 
   In the case of the air spaced etalon  10 , when the medium between the reflectors  14 ,  24  is vacuum, n=1 and the only parameter that affects the FSR  38  is the spacing L 1 . When the medium between the paired reflectors  14  is air, n 1 ˜1.000273 and the refractive index follows the Edlen equation (EDLEN, B., “The Refractive Index of Air,” Metrologia, 2, 71–80, 1966). An etalon of this type is generally called an “air spaced etalon,” regardless of whether the medium is vacuum, air, or some other gas mixture. 
   In the case of the solid etalon  20 , when the block  22  is glass, n˜1.5 and the Fabry-Perot etalon effectively consists of a single piece of sold glass having both reflectively coated reflectors  24  parallel to each other. An etalon of this type is generally called a “solid etalon,” and the term “glass” may loosely mean any transparent solid medium. 
   The spacing L 1 , L 2  between the reflectors  14 ,  24  is maintained constant so that the FSR  38  does not change during usage. This is achieved by using a material for the spacers  16  or block  22  (i.e., a medium) that has a low thermal expansion coefficient. Materials with such expansion coefficients are currently commercially available from Corning Glass™ in the U.S. and from Schott Glass™ in Germany (e.g., Zerodur™). These glass materials exhibit nearly zero thermal expansion in the environment typically required for telecommunications. 
   In addition to maintaining the spacing L constant, a process to keep the refractive index n constant has also been invented by Fibera, Inc. of Santa Clara, Calif. This process makes the wavelength locker “a thermal” and provides superior functionality throughout a very wide temperature range. 
   Such fixed spacing arrangements are fine, so long as the laser frequency does not have to be varied to achieve the underlying application. However, there are applications that require tuning the laser frequency through the ITU grids in a steady fashion, while also maintaining the frequency stability of the laser. A “tunable” wavelength locker would therefore be very useful for providing both frequency stabilization and tunability. 
   From in EQ. 1 it can be appreciated that tuning a wavelength locker can be achieved by varying either “L” or “n” in a controlled manner. First, consider tuning the FSR by varying “n.” This can be accomplished by changing conditions present in the wavelength locker package. From Eden&#39;s work, noted above, it is known that the refractive index of air is a function of pressure, humidity, and temperature. One of these parameters can therefore be precalculated and used to implement tuning. In actuality, however, this is not an easy process to accomplish. For example, the presence of a pressure adjusting device is usually not possible in the field. 
   Next, consider mechanically tilting the Fabry-Perot with respect to the incident laser beam, that is, effectively changing L. By doing this the optical path between the reflective surfaces is changed so that tuning is also achieved. However, this approach requires a motive means (e.g., a motor) to perform the tilting, and the addition of such a means to the wavelength locker is also undesirable in the field. For example, in the telecommunications field the constraints on space, with respect to both footprint and volume, can be quite severe. Recently there has been significant progress in MEMS technology, and tilting an etalon with a MEMS motor might be possible in the near future, but his does not address present needs. 
   Accordingly, there remains a need for a system to provide both frequency stabilization and tunability. 
   SUMMARY OF INVENTION 
   Accordingly, it is an object of the present invention to provide systems to provide both frequency stabilization and tunability. 
   Briefly, one preferred embodiment of the present invention is a system for tuning the wavelength of a laser beam emitted by a tunable laser. A tunable etalon assembly is provided that includes a Fabry-Perot (“FP”) etalon and a thermal unit. The FP etalon has paired reflectors to receive and wavelength filter the laser beam. The thermal unit thermally effects the separation of the paired reflectors in response to an etalon tuning signal. A photodetector receives the laser beam after filtering by the FP etalon and generates a detected signal based on the transmitted intensity. A generates the etalon tuning signal, and receives the detected signal and generates a laser tuning signal based on it. 
   Briefly, another preferred embodiment of the present invention is a system for determining how much the wavelength of a laser beam emitted by a tunable laser has been tuned. A beamsplitter receives and splits the laser beam into first and second beam portions. A tunable etalon assembly is provided that includes a first Fabry-Perot (“FP”) etalon and a thermal unit. The first FP etalon has paired reflectors to receive and wavelength filter the first beam portion. The thermal unit thermally effects the separation of the paired reflectors in response to an etalon tuning signal. A first photodetector receives the first beam portion after filtering and generates a first detected signal based on transmitted intensity. A second FP etalon receives and wavelength filters the second beam portion. A second photodetector receives the second beam portion after filtering and generates a second detected signal based on transmitted intensity. A controller generates the etalon tuning signal, receives the detected signal and generates a laser tuning signal based on it, receives the second detected signal and counts peak-valley cycles therein. 
   Briefly, another preferred embodiment of the present invention is a system for determining the difference in wavelengths of first and second laser beams emitted by first and second tunable lasers. A first beamsplitter receives and splits the first laser beam into first and second beam portions. A coupler alternately receives and redirect either of the second beam portion and the second laser beam as a tuning beam portion. A first Fabry-Perot (“FP”) etalon receives and wavelength filters the first beam portion. A first photodetector receives the first beam portion after filtering and generates a first detected signal based on transmitted intensity. A tunable etalon assembly is provided that includes a tuning FP etalon and a thermal unit. The tuning FP etalon has paired reflectors to receive and wavelength filter the tuning beam portion. The thermal unit thermally effects the separation of the paired reflectors in response to an etalon tuning signal. A tuning photodetector receives the tuning beam portion after filtering and generates a tuning detected signal based on transmitted intensity. A controller receives the first detected signal, generates a first tuning signal based thereon to tune the first tunable laser to emit the first laser beam at a specific known wavelength, and controls the first tuning signal to servo lock the first laser beam to the known wavelength. The controller also generates the etalon tuning signal such that the tuning detected signal is at a known point on a peak-valley curve for the tuning FP etalon. The controller records a first value for the etalon tuning signal when the tuning beam portion comes from the first laser beam and the tuning detected signal is at the known point. The controller also records a second value for the etalon tuning signal when the tuning beam portion comes from the second laser beam. The controller generates the etalon tuning signal such that the second value matches the first and reports on the first and second tuned values via an output link. 
   The objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings. 


   
     BRIEF DESCRIPTION OF DRAWINGS 
     The purposes and advantages of the present invention will be apparent from the following detailed description in conjunction with the appended figures of drawings and table in which: 
       FIG. 1  (background art) is a block diagram that conceptually shows the structures of two Fabry-Perot etalons that are commonly used in wavelength lockers for fixed wavelength applications. 
       FIG. 2  (background art) is a graph showing a typical transmissive spectrum of a wavelength locker. 
       FIG. 3  (background art) is a graph showing, in simplified manner, the principle of a conventional wavelength locker using a Fabry-Perot etalon. 
       FIG. 4  is a block diagram showing a tunable wavelength locker in accord with the present invention. 
       FIG. 5  is a graph showing, in simplified manner, the principle employed by the tunable wavelength locker of  FIG. 4 . 
       FIGS. 6   a  and  6   b  are perspective views of two etalon assemblies that may be used in the tunable wavelength locker of  FIG. 4 , wherein  FIG. 6  depicts an etalon assembly including an etalon wrapped with a heating element and  FIG. 6   b  depicts an etalon assembly including an etalon mounted on a thermal-electric cooler that provides a thermal effect. 
       FIG. 7  is a stylized diagram showing application of a tunable wavelength locker in accord with the present invention in a tunable wavelength spectrum monitor. 
       FIG. 8  is a stylized diagram showing application of a tunable wavelength locker in accord with the present invention in a relative wavelength measurement system. 
   


   And TBL. 1 is a table presenting an example set of ITU frequency verses etalon temperature and length look-up data that is suitable for use in embodiments of the invention. 
   DETAILED DESCRIPTION 
   Preferred embodiments of the present invention are a tunable wavelength locker, a tunable wavelength spectrum monitor, and a relative wavelength measurement system. Briefly, the invention uses a thermal effect, either heating or cooling, to vary the spacing between the reflective surfaces of an etalon used in a wavelength locker, wavelength spectrum monitor, or relative wavelength measurement system. 
     FIG. 4  is a block diagram showing a tunable wavelength locker  100  in accord with the present invention. A tunable laser  102  is used to produce a laser beam  104  having a particular wavelength. The laser beam  104  is passed through a tunable etalon assembly  106  (described presently) to a photodetector  108 . Based upon the energy then remaining in the laser beam  104 , the photodetector  108  provides a detected signal  110  to a controller  112 . The controller  112  stylistically depicted here generally represents all of the control functionality of the tunable wavelength locker  100 . It adjustably provides an etalon tuning signal  114  to the tunable etalon assembly  106 ; it adjustably provides a laser tuning signal  116  to the tunable laser  102 ; and it can receive or provide input and output via an I/O link  118  (represented with an arrowed line) with an outside system (not shown). 
   The adjustable characteristic of the etalon tuning signal  114  can be any that permits controllably changing the spacing of the reflective surfaces (reflectors) of the tunable etalon assembly  106 . For most applications this can simply be electrical current. The etalon tuning signal  114  is based, all or in substantial part, on input provided in the I/O link  118 . 
   The adjustable characteristic of the laser tuning signal  116  can be any that permits changing the particular wavelength of the laser beam  104  which the tunable laser  102  produces. This can be electrical current, voltage, a combination of these, or yet some other characteristic. The laser tuning signal  116  is based, all or in substantial part, on the detected signal  110 . 
   The adjustable characteristic of the input from the I/O link  118  may be merely a manual adjustment by a user (e.g., operating switches or turning a rheostat), or it may be a complex electrical or optical signal provided by the application employing the tunable wavelength locker  100 , or it may come from an outside system entirely. In some embodiments the detected signal  110  and the laser tuning signal  116  can be the same, bypassing the controller  112  entirely if desired. All of this is largely a matter of design choice and is well within the capabilities of one of ordinary skill in the art when all of the teachings herein are appreciated. 
     FIG. 5  is a graph  150  showing, in simplified manner, the principle employed by the tunable wavelength locker  100  of  FIG. 4 . The frequency verses intensity relationship of the laser beam  104  after the tunable etalon assembly  106  is represented by a peak-valley curve  152 . In the tunable wavelength locker  100  the peak-valley curve  152  can be “shifted” in a controlled manner by tuning the tunable etalon assembly  106  with the etalon tuning signal  114 . In actuality the peak-valley curve  152  does not simply shift. Rather, the free spectral range (FSR) increases or decreases. 
   When the tunable laser  102 , the photodetector  108 , the detected signal  110 , and the laser tuning signal  116  are used in a servo-control manner the frequency of the laser beam  104  shifts accordingly. Thus, when the frequency of the laser beam  104  is locked at a designated lock point  154 , varying the etalon tuning signal  114  “moves” the lock point  154  along a line  156 . 
     FIGS. 6   a  and  6   b  are perspective views of two etalon assemblies that may be used in the tunable wavelength locker  100  of  FIG. 4 .  FIG. 6   a  depicts an etalon assembly  170  including an etalon  172  wrapped with a heating element  174  (say, heating tape or wire, for example). In this arrangement the etalon  172  operates in conventional manner and the heating element  174  provides a thermal effect. The heating element  174  is connected to a conventional power source (e.g., within the controller  112 ), and straightforward adjustment of the power in the etalon tuning signal  114  controls the shift of the peak-valley curve  152  of the etalon  172  in the manner shown in  FIG. 5 . 
     FIG. 6   b  depicts an etalon assembly  190  including an etalon  192  mounted on a thermal-electric cooler (TEC  194 ) that provides a thermal effect here. TECs are quite flexible, and the term “cooler” the industry uses can be somewhat misleading. A TEC can be used to cool, to heat, or to alternately cool and head, as required. The TEC  194  here is also connected to a conventional power source (e.g., within the controller  112 ), permitting straightforward adjustment with the power in the etalon tuning signal  114  to controllably shift the peak-valley curve  152  of the etalon  192  in the manner shown in  FIG. 5 . 
   In the actual employment of these etalon assemblies  170 ,  190  a conventional temperature sensor (not shown) can be installed, preferably adjacent to the etalon optics, and used to monitor and report the temperature to the controller  112 . The controller  112  can then control the power in the etalon tuning signal  114  So that the proper thermal effect is achieved to tune the etalon assembly  170 ,  190  to a desired frequency. 
   Continuing with  FIG. 6   a , in use the etalon  172  can first be warmed up to a predetermined temperature and allowed to cool down at a desired rate when the power in the etalon tuning signal  114  is reduced. This permits controlled, stable setting of the etalon  172  initially, and then enables adjusting in an ongoing manner, either by heating it up (by increasing the power) or by cooling it down (by decreasing the power). The free spectral range of the etalon  172  can therefore be increased or decreased at will. 
   The embodiment in  FIG. 6   b  operates similarly, only even more flexibly. Here the etalon  192  is first brought to a predetermined temperature, by heating or cooling. Then its free spectral range is increased or decreased, as desired, by changing its temperature up or down, as needed. Unlike the embodiment in  FIG. 6   a , where heating above the ambient temperature is typically needed to provide both increasing and decreasing the free spectral range of the etalon  172 , the expected average ambient temperature may be chosen as the predetermined temperature here, thus tending to minimize the power needed and also tending to minimize any thermal influence on surrounding elements or systems. 
     FIGS. 6   a  and  6   b  both depict tunable etalon assemblies  170 ,  190  having air-spaced etalons  172 ,  192 , but this is not a requirement. For a solid etalon, the thermal effect can be applied to the solid glass in essentially similar manner. When a solid etalon is used, however, the refractive index of the glass medium is not linearly proportional to temperature variation and a more sophisticated algorithm is required to control the heating or cooling. For example, a look-up table that contains values of the refractive index verses the temperature of the glass can be provided to supply values for when the solid etalon reaches a certain temperature (see e.g., TBL. 1). 
   In contrast to the conventional practice in etalon construction of using spacers or glass mediums with low thermal expansion, the inventors prefer to use materials that have higher thermal expansion coefficients. In this manner, a small change in the heat energy applied or removed can cause an appropriate elongation or shrinkage of the spacer or glass medium. The materials used desirably have good stiffness and thermal conductivity. An appropriate stiffness maintains component alignment and good thermal conductivity shortens the time required to perform tuning. All of this can also help avoid over heating or over cooling the etalon. The inventors have identified several materials that have high thermal expansion coefficients with good stiffness and thermal conductivity. Some examples, without limitation, include PTFE (Teflon ™), Derlin, and ABS. 
   Turning now to a “real world” example using an air spaced etalon, the typical tuning range in a telecom application is the gain bandwidth of the laser medium, which is approximately 4 nm. For the 50 GHz ITU grid, the spacer length is 3 mm (EQ. 1). The condition for a standing wave to exist in a Fabry-Perot etalon is:
 
 L=m*□/ 2
 
   where L is the spacer length, m is an integer, and □ is the resonant wavelength. One can then calculate the required length change for the spacer (by using the center wavelength (1,544.33 nm) of the ITU grid as an example). Using □ 0 =1,544.33 nm and the same value for “m” (i.e., the same order), it follows that:
 
 L   1   /L   0 =□ 1 /□ 0  where □ 1 =□ 0 *(1+4 nm/1544.33 nm)=□ 0 *(1+2.59*10^−3).
 
   This means that the maximum length-wise elongation (or shrinkage) of the spacer is 3 mm*2.59*10^−3. The thermal expansion coefficient of PTFE is 16*10^−5. Thus, to change the length of a PTFE spacer by 2.59*10^−3 the temperature needs to be changed by:
 
□ L =(2.5 9*10^−3)/(16*10^−5)=16.2° C.
 
   This is a relatively mild change in temperature. 
   During tuning it can be critical to keep track of the amount the frequency is tuned. As was discussed with respect to  FIG. 2 , a fixed-spacing etalon produces peaks and valleys as the laser wavelength is varied. By counting the number of these peaks or valleys, one can tell how far a laser wavelength has been tuned. 
     FIG. 7  is a stylized diagram showing application of a tunable wavelength locker in accord with the present invention in a tunable wavelength spectrum monitor  200 . Here the tunable wavelength locker and a fixed-spacing Fabry-Perot etalon in combination allow scanning a laser wavelength by a known amount. Since this combination performs not only laser wavelength locking, but also widerange frequency tuning, it becomes a “tunable wavelength spectrum monitor.” The tunable wavelength spectrum monitor  200  includes a tunable laser  202  that outputs a laser beam  204  into a source optical fiber  206 . A beam splitter  208  receives the laser beam  204  from the source optical fiber  206  and outputs a portion of it into a tuned channel  210 . 
   The tuned channel  210  includes a tuned channel optical fiber  212  that receives a portion of the laser beam  204  from the beam splitter  208 , and passes it through a tuned channel collimator  214 . This portion of the laser beam  204  is then passed through a tunable etalon  216  to a tuned channel photodetector  218 . Based upon the energy in the portion of the laser beam  204  reaching it, the tuned channel photodetector  218  then provides a tuned channel signal  220 . Up to this point the apparatus described roughly corresponds to the tunable wavelength locker  100  of  FIG. 4 . 
   A fixed channel  230  is also provided. It includes a fixed channel optical fiber  232  that also receives a portion of the laser beam  204  from the beam splitter  208 , and that passes it through a fixed channel collimator  234 . This portion of the laser beam  204  is then passed through a fixed spaced etalon  236  to a fixed channel photodetector  238 . Based upon the energy in the portion of the laser beam  204  reaching it, the fixed channel photodetector  238  then provides a fixed channel signal  240 . 
   A controller  250 , stylistically depicted simply as a block in  FIG. 7 , represents the control functionality of the tunable wavelength spectrum monitor  200 . The controller  250  receives the tuned channel signal  220 , the fixed channel signal  240 , and input via an I/O link  252 . Based on these, the controller  250  provides a etalon tuning signal  254  to the tunable etalon  216  and a laser tuning signal  256  to the tunable laser  202 . 
     FIG. 7  includes a graphical depiction wherein a peak-valley curve  260  having a lock point  262  (e.g., corresponding with a 50% amplitude) represents the tuned channel signal  220 . As the tunable etalon  216  is tuned (e.g., by heating or cooling), the peak-valley curve  260  will shift (a few possible positions are depicted with ghost outline in  FIG. 7 ) and the lock point  262  will move accordingly, i.e., move only within the frequency domain (the set of possible positions are depicted by line  264 ). 
   The tuned channel  210  thus may operate similarly to the tunable wavelength locker  100  of  FIG. 4 . By setting the etalon tuning signal  254  to a specific value and setting the laser tuning signal  256  such that the tuned channel signal  220  coincides with the lock point  262 , the tunable wavelength spectrum monitor  200  can be set to a specific frequency. With appropriate servo-control based on the tuned channel signal  220 , the frequency of the tunable laser  202  can be locked to this frequency. Additionally, the tunable etalon  216  may now be tuned (e.g., by heating or cooling) so that the peak-valley curve  260  and the lock point  262  controllably shift, and with ongoing servo-control based on the tuned channel signal  220  the laser tuning signal  256  will change the frequency of the tunable laser  202  accordingly. In this manner the tunable wavelength spectrum monitor  200  can be scanned across a frequency range. 
     FIG. 7  further includes a graphical depiction wherein a peak-valley curve  270  having multiple peaks  272  represents the fixed channel signal  240 . As the frequency of the tunable laser  202  changes the peaks  272  can be detected by the fixed channel photodetector  238  and counted by the controller  250 . The tunable etalon  216  and the fixed spaced etalon  236  are preferably arranged to have the lock point  262  on the peak-valley curve  260  coincide with a peak  272  on the peak-valley curve  270  when the tunable etalon  216  is set to its middle range. 
   The combination of the tuned channel  210  and the fixed channel  230  (and the other components described) thus provides the tunable wavelength spectrum monitor  200  with the ability to be tuned to any frequency within and scanned across a large range of frequencies, typically a large multiple of the FSR of the fixed spaced etalon  236 . 
     FIG. 8  is a stylized diagram showing application of a tunable wavelength locker in accord with the present invention in a relative wavelength measurement system  300 . This can be useful in a system having two light sources where the wavelength of one is known and finding the wavelength of the other is desired. Here a tunable Fabry-Perot etalon and two fixed-spaced Fabry-Perot etalons are used. 
   Briefly, the known wavelength light source is locked to the known wavelength and the unknown wavelength light source is locked so it does not change. The tunable etalon is then set to match its peak with the known wavelength. The spacer length of the tunable etalon is then changed (tuned) until the unknown wavelength is also at the peak. The amount of tuning required for this is the separation between the known and unknown wavelengths. 
   The relative wavelength measurement system  300  includes a first laser system  302  that has a first tunable laser  304 , a first fixed spaced etalon  306 , and a first photodetector. In combination these permit servo locking the first tunable laser  304  to emit light at a specific wavelength. Similarly, a second laser system  312  has a second tunable laser  314 , a second fixed spaced etalon  316 , and a second photodetector  318  that permit servo locking the second tunable laser  314  to emit light at a specific wavelength. 
   A tunable etalon assembly  320  and a third photodetector  322  are further provided, as well as a first beamsplitter  324 , a second beamsplitter  326  and a coupler  328  to deliver part of the light from the first laser system  302  or the second laser system  312  to the tunable etalon assembly  320 . 
   In practice, the first photo detector  308  generates a first detected signal  332 , the second photodetector  318  generates a second detected signal  334 , and the third photodetector  322  generates a third detected signal  336 . An etalon tuning signal  338  is also provided, by a controller (not shown). The first laser system  302  is locked to emit light at a specific known wavelength using the first detected signal  332  and the second laser system  312  is locked to emit light at an unknown wavelength using the second detected signal  334 . 
   When light from the first laser system  302  is coupled into the tunable etalon assembly  320  the etalon tuning signal  338  is adjusted to bring the third detected signal  336  to a particular point on the peak-valley curve, say, the peak. The value of the etalon tuning signal  338  is now recorded. Then light from the second laser system  312  is coupled into the tunable etalon assembly  320  and the etalon tuning signal  338  is changed as needed to bring the third detected signal  336  back to the same point on the peak-valley curve. Note, this is a phase adjustment, since the light from the respective laser systems  302 ,  312  will usually have different amplitudes in the third detected signal  336 . The amount of change needed for the etalon tuning signal  338  represents the difference in the wavelength of the first laser system  302  and the second laser system  312 . 
   To further know the amount of tuning used for this, the cavity length of the Fabry-Perot etalon can be calibrated against a group of known wavelengths (e.g., the 80 channels of ITU grids), for corresponding etalon temperature settings. A look-up table of wavelengths vs. etalon temperatures is then constructed and any unknown wavelength within the ITU grids can be found by checking against this look-up table. TBL. 1 is a table presenting an example set of ITU frequency verses etalon temperature and length look-up data that is suitable for use in this manner. 
   While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
A system for tuning the wavelength of a beam from a tunable laser. A tunable etalon assembly includes a Fabry-Perot etalon with paired reflectors to filter the laser beam. The tunable etalon also includes a thermal unit to thermally adjust the separation of the paired responsive to an etalon tuning signal. A photodetector receives the laser beam after filtering the etalon and generates a detected signal based on intensity. A controller generates the etalon tuning signal, and receives the detected signal and generates a laser tuning signal based on it. Optionally, additional Fabry-Perot etalons, photodetectors, and one or more beamsplitters permit extending wavelength range and determining relative wavelength difference with a beam from a second laser.