Abstract:
The invention describes devices and methods for determining the wavelength of coherent optical radiation such as is emitted from a laser source. The apparatus is monolithic with no moving parts and consists of optical components that generate signal periodic in the optical frequency of the coherent radiation detected by the component. Each optical component generates a signal with a different period. Differences between the periods of the signals generated by the optical frequency-dependent optical components provides a means of measuring optical wavelengths over a range far exceeding the free spectral range limitations of conventional interferometers. The method of the present invention allows for measurement of optical frequency with an uncertainty of much less than the period of the optical frequency-dependent optical components forming the apparatus.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/312,502, filed Aug. 14, 2000, which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to wavelength measurement devices (e.g. wavelength meter, optical spectrum analyzer or optical channel monitor), instruments that map the wavelength of a tunable laser, and devices installed within a laser to monitor optical wavelength.  
           [0004]    2. Description of the Related Art  
           [0005]    Laser frequency monitoring and locking is an essential technology for the emerging telecommunications market and will be useful in a number of other technological fields in the future. In state-of-the-art telecommunications systems, each of a plurality of laser signal sources is tuned in frequency to a distinct channel, allowing a plurality of signals to be simultaneously transmitted down a single optical fiber. The communications channels are defined on a grid with equal frequency spacing in a band near 194 THz (ITU grid). Each telecommunication laser must be stabilized or monitored to ensure it remains tuned to the proper communications channel.  
           [0006]    The high sensitivity of telecommunication components to wavelength deviations also requires that the test equipment possess a high wavelength accuracy and precision. Test lasers whose wavelength is swept to measure the wavelength dependence of a parameter require wavelength information at the picometer level. The associated devices that monitor optical components or network channels also require high accuracy. Measurement speed is extremely important for high sweep rates of optical wavelength, or monitoring of rapid changes in the optical wavelength (e.g due to noise). Finally, equipment must be compact so as to occupy minimal space and fit directly into existing devices (such as telecom laser packages) as well as portable or handheld instruments.  
           [0007]    High speed and accuracy of wavelength measurement is essential to the monitoring and control of existing and next generation optical networks Minimizing downtime in optical networks is already an operational and economic necessity that requires constant care and monitoring of the optical wavelength and power of the communication channels. The need for monitoring will only increase as networks use tunable lasers to add network flexibility and responsiveness. Accurate, compact, rugged and inexpensive optical channel monitoring is imperative.  
           [0008]    A simple form of wavelength monitoring is a system shown in FIG. 1. A first beamsplitter  2 , positioned in a portion of a first optical beam  4  from a laser source, generates a second optical beam  6  that is detected by an optical power detector  8  to provide a power reference for the laser. A second beamsplitter  10 , joined to first beamsplitter  2 , reflects a third optical beam  12  of the first optical beam into a resonator  14 . The light  16  transmitted through the resonator is detected by a second optical power detector  18 . As illustrated in FIG. 2, resonator  14  may be a Fabry-Perot (FP) Interferometer (or etalon) whose optical transmission varies periodically with optical frequency, where the period is called the free spectral range  20  (FSR). FPs and etalons are excellent wavelength discriminators and references for optical sources whose optical frequency resides on the high slope region  22  of a transmission fringe. When the FSR of the resonator equals the frequency spacing of the ITU grid the device can be used as a monitor of optical communication channels, or as a wavelength locker. Such a monitoring device is ideal for telecommunications laser packages because the device is very compact and rugged. The response of the device, limited only by the photodiode and associated electronics, can provide very fast wavelength information.  
           [0009]    Devices of FIG. 1 are only able to accurately measure the optical frequency of light modulo the FSR, for two reasons. The first limitation to accuracy is shown in FIG. 2: Each peak in optical transmission of the device is indistinguishable from the next. The second limitation to accuracy is that the measurement precision depends on the optical frequency. Again referring to FIG. 2, the measurement resolution of wavelength is a maximum on the high slope region  22  where the transmitted intensity varies rapidly with changing frequency. At the peaks and valleys of the transmission spectrum, the precision is greatly reduced because the signal changes very little as the optical frequency changes (e.g. low slope region  24 ). When the high slope region  22  is aligned with the ITU grid the wavelength measurement device provides sufficient precision to monitor and control a communications laser. If low slope region  24  is aligned with the grid, the device using a single FP interferometer cannot generate the necessary signals for laser control. Among other limitations, devices of FIG. 1 cannot identify specific ITU channels, adapt to revisions of the ITU channel spacing, or measure arbitrary wavelengths within the telecommunication bands.  
           [0010]    Another way to monitor the frequency of the laser is to build a wavelength meter: a device that measures the wavelength of light independently of the ITU grid. A description of a sophisticated and very high precision wavelength measuring device is found in J. Hall et al (J. L. Hall and S. A. Lee, Applied Physics Letters, 29, 367 (1976)). A first disadvantage of this device is the very slow update rate due to the physical motion of the interferometer arm. A second disadvantage is the large size and lack of portability, which makes it impractical to include with each telecommunication laser, or even a laser in a test instrument. A third disadvantage is the cost and environmental sensitivity of the reference laser needed for high measurement accuracy.  
           [0011]    The current needs of wavelength measurement in the telecommunication industry exceed the capabilities of the prior art. There is a need for picometer wavelength accuracy over the entire telecommunication bandwidth. There is a need for high speed measurement. There is a need for a robust, compact and inexpensive wavelength meter device for measurement of optical wavelength. There is a need for a single wavelength meter device with all these features.  
         SUMMARY OF THE INVENTION  
         [0012]    Accordingly, an object of the present invention is to provide a wavelength meter device capable of measuring one optical wavelength with an accuracy better than about 10 parts per million over a range of optical wavelengths greater than about 50 nm.  
           [0013]    Another object of the invention is to provide a rugged, compact and relatively inexpensive wavelength meter device.  
           [0014]    Another object of the invention is to provide a wavelength meter device with measurement rates much faster that 10 per second.  
           [0015]    A further object of the invention is to provide a wavelength meter device that measures more than one optical wavelength in an optical beam.  
           [0016]    A further object of the invention is to provice a optical spectrum analyzer device capable of measuring an optical spectrum of an optical beam with more than one optical wavelength.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic of a single-etalon wavelength measurement scheme in the prior art.  
         [0018]    [0018]FIG. 2 is an optical transmission spectrum of a Fabry-Perot Interferometer showing wavelength periodicity and the Free Spectral Range (FSR).  
         [0019]    [0019]FIG. 3 a  is a schematic of an wavelength meter device including partial reflectors, positioned serially in at least part of an optical beam from an optical source.  
         [0020]    [0020]FIG. 3 b  is a schematic of the wavelength meter device including partial reflectors, positioned in series and in parallel in at least part of an optical beam from an optical source.  
         [0021]    [0021]FIG. 3 c  is a schematic of the optical wavelength meter device including a grouping of optical components positioned without partial reflectors in at least part of an optical beam from an optical source.  
         [0022]    [0022]FIG. 4 a  shows an optical component, of a wavelength meter device made of an interferometric optical element (IOE) and optical power detector (OPD).  
         [0023]    [0023]FIG. 4 b  shows an optical power spectrum of an optical beam input to the IOE.  
         [0024]    [0024]FIG. 4 c  shows an optical power spectrum of an optical beam transmitted by the IOE.  
         [0025]    [0025]FIG. 5 a  shows an optical component, of a wavelength meter device made of an IOE and a photodiode.  
         [0026]    [0026]FIG. 5 b  illustrates a plurality of IOEs and an array of photodiodes.  
         [0027]    [0027]FIG. 5 c  illustrates a plurality of IOEs and a multi-segment photodiode.  
         [0028]    [0028]FIG. 6 a  shows an OPD including photodiode and optical integrating sphere.  
         [0029]    [0029]FIG. 6 b  shows an OPD including photodiode and substantially hollow optical integrating sphere.  
         [0030]    [0030]FIG. 6 c  shows an OPD including photodiode and substantially solid optical integrating sphere.  
         [0031]    [0031]FIG. 7 shows a wavelength meter device including partial reflectors (PR i ), etalons (E i ), OPDs and a signal processor.  
         [0032]    [0032]FIG. 8 shows a typical optical transmission of a three-etalon wavelength meter device with differential path lengths in each etalon.  
         [0033]    [0033]FIG. 9 illustrates an algorithm for determination of optical frequency of an optical beam in a wavelength meter device.  
         [0034]    [0034]FIG. 10 shows an air-spaced etalon including two optical elements, each with partial reflectivity coatings separated by a spacer.  
         [0035]    [0035]FIG. 11 shows a wavelength meter device including three air-spaced etalons formed of thin film layers deposited on a single optical surface.  
         [0036]    [0036]FIG. 12 shows a wavelength meter device including three air-spaced etalons formed of two equal-thickness thin film layers deposited on two optical surfaces.  
         [0037]    [0037]FIG. 13 shows a wavelength meter device including partial reflectors (PR i ), solid etalons (SE i ), OPDs and signal processor.  
         [0038]    [0038]FIG. 14 shows a wavelength meter device including partial reflectors (PR i ), electro-optic elements (EOE i ), OPDs and signal processor.  
         [0039]    [0039]FIG. 15 shows prior art of an optical component including a birefringent element and a polarizer.  
         [0040]    [0040]FIG. 16 shows a waveguide resonator with optical tap grating.  
         [0041]    [0041]FIG. 17 shows a wavelength meter device including three waveguide resonator planes.  
         [0042]    [0042]FIG. 18 shows a wavelength meter device including three waveguide resonators, optical tap gratings and detectors on a single planar substrate.  
         [0043]    [0043]FIG. 19 shows a wavelength meter device including a 1×3 waveguide splitter coupling to three waveguide resonators.  
         [0044]    [0044]FIG. 20 shows a wavelength meter device including a polarization controller (PC).  
         [0045]    [0045]FIG. 21 shows a wavelength meter device including a polarization scrambler (PS).  
         [0046]    [0046]FIG. 22 illustrates a wavelength locking device including, a wavelength meter device made of optical components (OC i ) and a coupler coupling optical frequency readout to an optical source.  
         [0047]    [0047]FIG. 23 illustrates a wavelength locking device including, a wavelength meter device made of etalons (E i ) and a coupler coupling optical frequency readout to an optical source.  
         [0048]    [0048]FIG. 24 illustrates a wavelength locking device including, a wavelength meter device made of air-spaced etalons (ASE i ) and a coupler coupling optical frequency readout to an optical source.  
         [0049]    [0049]FIG. 25 illustrates a wavelength locking device including, a wavelength meter device made of solid etalons (SE i ) and a coupler coupling optical frequency readout to an optical source.  
         [0050]    [0050]FIG. 26 illustrates a wavelength locking device including, a wavelength meter device made of electro-optic elements (EOE i ) and a coupler coupling optical frequency readout to an optical source.  
         [0051]    [0051]FIG. 27 illustrates a wavelength locking device including, a wavelength meter device made of polarization controller (PC) and air-spaced etalons (ASE i ); and a coupler coupling optical frequency readout to an optical source.  
         [0052]    [0052]FIG. 28 illustrates a wavelength locking device including, a wavelength meter device made of polarization scrambler (PS) and air-spaced etalons (ASE i ); and a coupler coupling optical frequency readout to an optical source.  
         [0053]    [0053]FIG. 29 shows a wavelength meter device including a tunable optical filter element (TOFE).  
         [0054]    [0054]FIG. 30 is a schematic of a multi-wavelength measurement scheme.  
         [0055]    [0055]FIG. 31 is an algorithm for determination of more than one optical frequency in an optical beam in a wavelength meter device.  
         [0056]    [0056]FIG. 32 shows a wavelength meter device including a tunable optical filter element (TOFE) and an optical power detector (OPD).  
         [0057]    [0057]FIG. 33 is an algorithm for determination of the optical frequency spectrum of an optical beam in a wavelength meter device.  
         [0058]    [0058]FIG. 34 shows a wavelength meter device including a polarization controller (PC) and tunable optical filter element (TOFE).  
         [0059]    [0059]FIG. 35 shows a wavelength meter device including a polarization scrambler (PS) and tunable optical filter element (TOFE).  
         [0060]    [0060]FIG. 36 shows a wavelength locking device including a multiple-wavelength meter device and a coupler coupling optical frequency readouts to optical sources.  
         [0061]    [0061]FIG. 37 shows a wavelength locking device including a polarization-mitigated multiple wavelength meter device and a coupler coupling optical frequency readouts to optical sources.  
         [0062]    [0062]FIG. 38 shows a wavelength meter device including an optical power detector (OPD) and optical components (OC i ).  
         [0063]    [0063]FIG. 39 shows a wavelength meter device using air-gapped etalons, formed of a reflection coated monolithic beam splitter and a pattern coated substrate.  
         [0064]    [0064]FIG. 40 shows a wavelength meter device using an air-spaced etalon made from two optical elements.  
         [0065]    [0065]FIG. 41 shows a wavelength meter device using an air-spaced etalon made from two optical elements each having a thin film coating.  
         [0066]    [0066]FIG. 42 shows a wavelength meter device including an optical power detector (OPDO) and solid etalons (SE i ).  
         [0067]    [0067]FIG. 43 shows a wavelength meter device including 1×4 beamsplitter, optical power detector and solid etalons.  
         [0068]    [0068]FIG. 44 shows a wavelength meter device including an optical power detector and electro-optic elements (EOE i )  
         [0069]    [0069]FIG. 45 shows a polarization-insensitive wavelength meter device including three solid etalons joined to a fourth component with a clearance hole.  
         [0070]    [0070]FIG. 46 shows a polarization-insensitive wavelength meter device including three solid etalons joined in an L-shaped profile.  
         [0071]    [0071]FIG. 47 shows a polarization-insensitive wavelength meter device including optics for asymmetric expansion of an input optical beam.  
         [0072]    [0072]FIG. 48 shows a wavelength locking device including a wavelength meter device made of an optical power detector (OPD) and optical components (OC i ); and a coupler coupling optical frequency readout to an optical source.  
         [0073]    [0073]FIG. 49 shows a wavelength locking device including a wavelength meter device made of an optical power detector (OPDhd  1 ) and etalons (E i ); and a coupler coupling optical frequency readout to an optical source.  
         [0074]    [0074]FIG. 50 shows a wavelength locking device including a wavelength meter device made of an optical power detectors (OPD 1 ) and air-spaced etalons (ASE i ); and a coupler coupling optical frequency readout to an optical source.  
         [0075]    [0075]FIG. 51 shows a wavelength locking device including a wavelength meter device made of an optical power detector (OPD 0 ) and solid etalons (SE i ); and a coupler coupling optical frequency readout to an optical source.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0076]    Various embodiments of the present invention are illustrated by FIGS. 3 a - 3   c . Referring now to FIG. 3 a , optical source  26  generates an optical beam  28  that is received by wavelength meter device  29 . A plurality of partial reflectors  30 ,  32 ,  34  and  36  are positioned serially and at least partially in optical beam  28 . Partial reflectors  30 ,  32 ,  34  and  36  produce optical beams  38 ,  40 ,  42 , and  44 . Optical components  46 ,  48 ,  50  and  52  receive optical beams  38 ,  40 ,  42  and  44  and generate signals  54 ,  56 ,  58  and  60 . Each signal is periodic in the optical frequency of the light detected by each optical component. Signals  54 ,  56 ,  58  and  60  couple to signal processor  62 . Signal processor  62  implements an algorithm, detailed below, which calculates the optical frequency of optical beam  28 . A similar embodiment is shown in FIG. 3 b , wherein partial reflectors  64 ,  66 ,  68  and  70  and mirrors  72 ,  74 ,  76  and  78  generate optical beams  80 ,  82 ,  84  and  86  that are detected by the optical components. FIG. 3 c  illustrates a third embodiment. Optical beam  88  is incident on optical components  90 ,  92 ,  94  and  96  that are positioned at least partially in optical beam  88  without additional partial reflectors or mirrors. Optical beam  88  may be prepared by expanding it with a magnifying, collimation optic or other suitable optics.  
         [0077]    As shown in FIGS. 4 a , optical components in the above embodiments may consist of an interferometric optical element  90  (IOE) and optical power detector  92  (OPD). A portion of an optical beam  94  enters IOE  90 . IOE  90  generates optical beam  96 , which is detected on OPD  92 . As shown in FIG. 4 b , optical beam  94  has a nominally flat dependence  98  of optical power on the optical frequency. As illustrated in FIG. 4 c , optical beam  96  has a periodic dependence  100  of transmitted optical power on optical frequency.  
         [0078]    OPD  92  may include any number of optical-to-electrical converters, including but not limited to photodiodes, avalanche photodiodes, PMTs and the like. FIG. 5 a  illustrates one embodiment of a single IOE  102  producing an IOE beam  104  detected by a single photodiode (PD)  106 . Referring to FIG. 5 b , another scheme is to use an array of photodiodes, such that a plurality of IOEs  108 ,  110 ,  112  and  114  generate a plurality IOE beams  116 ,  118 ,  120  and  122 , and the IOE beams are detected on a photodiode array  124 . Similarly, FIG. 5 c  shows a quadrant photodiode, hybrid CCD array or similar multi-segrnent photodiode  126 , which detects IOE beams  128 ,  130 ,  132  and  134 .  
         [0079]    A concern with OPD  92  is the sensitivity to the state of polarization of the detected light. As shown in FIG. 6 a , various embodiments of an optical power detector  136  include an integrating sphere  138  to mitigate polarization sensitivity, as well as to increase the maximum light level that may be detected on photodiode  140  prior to saturation of the photodiode output. As illustrated in FIG. 6 b , integrating sphere  142  may include a hollow cavity  144 . As illustrated in FIG. 6 c , integrating sphere  146  may include a solid interior  148 .  
         [0080]    Referring to FIG. 7, one embodiment of a wavelength meter device  29  uses etalons and optical power detectors in place of optical components  38 ,  40 ,  42  and  44 . Etalon refers to a device made of two surfaces separated to form a resonant cavity for light, each surface coated with metal, dielectric or other layer to provide partial reflectivity. Etalons  150 ,  152 ,  154  and  156  generate etalon optical beams  158 ,  160 ,  162  and  164  relative to the optical frequency of the first optical beam  28 , and the finesse and FSR of each etalon. Each etalon optical beam is detected by optical power detectors (OPD 1 , OPD 2 , OPD 3 . . . OPD n )  166 ,  168 ,  170  and  172 . Optical power detectors generate signals  54 ,  56 ,  58 ,  60 . Signal processor  62  reduces the signals, via the algorithm explained below, to an optical frequency with uncertainty of less than the FSR of the etalons.  
         [0081]    The FSR of the etalons and the difference in FSR between the plurality of etalons is chosen such that, at any optical frequency within the measurement range, the relative transmission orders of the etalons remains unchanged, although the absolute order of transmission of each etalon may be different. The result is a device that exhibits a progression in the transmission characteristics over the range of operation. At the beginning of the optical frequency range  174 , the transmission peaks for the etalons cluster together (which we refer to as the cluster frequency). At the end of range  176  of optical frequencies, the transmission peaks for the etalons are separated. The difference in optical phase between the etalons is 0&lt;φ&lt;π (modulo π) over the measurement range. Mathematically this is not the only ordering of the optical phase of the transmission signals. For example, π&lt;φ&lt;2π also satisfies the operational requirements for the device (the clustering of the transmission signals has a different characteristic to that shown in FIG. 8). Each measurement of optical frequency requires a set of three optical power signals with a unique relationship between the optical path lengths that identifies the exact transmission order of the etalons leading to a unique and precise measurement of the optical frequency.  
         [0082]    In order to use the three or more optical power signals, wavelength meter device  29  removes variations in the signals due to variations in the optical power of the optical source  26 . In various embodiments of wavelength meter device  29 , only signals  54 ,  56 ,  58  and  60  periodic in optical frequency are generated by wavelength meter device  29 . Wavelength meter device  29  does not measure the optical power of optical source  26 . Signals  54 ,  56 ,  58  and  60  may be normalized by a signal external to wavelength meter device  29  that is proportional to the optical power of optical beam  28 . Information about the optical power of optical beam  28  may be already available from optical source  26  (e.g., from the power setting or from a power meter elsewhere in the system). Correct normalization of signal  54 ,  56 ,  58  and  60 , to the same reference signal level requires accounting for insertion losses and reflection ratios along each path to optical components  46 ,  48 ,  50  and  52 . Embodiments without internal optical power measurement have the advantage of fewer components, simpler construction, and lower overall cost.  
         [0083]    Once signals  54 ,  56 ,  58  and  60  are normalized, a method illustrated in FIG. 9 uniquely determines the optical frequency of an optical source. In the discussion below we employ the focus on the case of a wavelength meter device  29  consisting of optical components made of IOE  90  and OPD  92 . For clarity, we refer to IOEs as etalons although generally the IOE may be some form of interferometric device such as a Michelson or Mach-Zehnder Interferometer. The analysis is presented in the frequency domain. An optical frequency can be expressed as (n+e)*FSR, where n represents the integer etalon order of transmission and e represents a fractional etalon order based on the relationship between transmission and frequency as shown in FIG. 2. With a single etalon, n is not known but the fractional order e can be measured very precisely if it occurs in high slope region  22  of FIG. 2. In a wavelength meter device  29  consisting of only three etalons, where each etalon has a different FSR, three different expressions for the optical frequency can be written for the three etalons. The fractional order e can be computed with high accuracy for each etalon based on an initial calibration of the etalon finesse and FSR (2nd Step  178  and 3rd Step  180 ). For the invention described here, the FSR of the etalons have a relationship that enables identification of the integer orders from a measurement of the three fractional orders.  
         [0084]    For a single etalon, the difference between two frequencies is written  
         ƒ−ƒ 0 =( n−n   0   +e−e   0 )* FSR    
         [0085]    where ƒ is the unknown frequency, n is the etalon order and e is the fractional etalon order. The fractional etalon order is a number between 0 and 1 and resolves the unknown frequency to a fraction of the FSR. In the wavelength meter device  29 , although n and n 0  may be different for each etalon, the etalons are constructed so that the etalon order difference n-n 0  is the same for every etalon. As described lo below, the fractional order is measured for each etalon. A calibration of the reference frequency ƒ 0  provides the etalon orders n 0  and e 0 . Thus, the expression for frequency, in terms of a single etalon, reduces to a single equation in two unknowns, m and ƒ:  
         ƒ−ƒf 0 =( m+e−e   0 )* FSR    
         [0086]    where m is the etalon order difference common to all of the etalons. Taken together with a similar expression for either of the remaining two etalons we have a system of two equations in two unknowns, from which m and ƒ are calculated. The three-etalon scheme described here ensures that the frequency ƒ is in the high slope region of the transmission spectra of at least two etalons, which provides the resolution to determine the order difference m and the frequency ƒ.  
         [0087]    The fractional etalon order is determined by comparing the etalon transmission signal to the Airy transmission function, and by use of a decision tree (4 th  Step  182  and 5 th  Step  184 ). By comparing the normalized transmission signal versus the theoretical transmission curve, the location of the unknown frequency may be found modulo an uncertainty of one-half of a FSR. The uncertainty arises because the transmission function is periodic. Thus, for a time-independent transmission signal through one etalon it is not possible to locate which side of a transmission peak the signal comes from. Fortunately, since a known ordering exists between the etalons in this invention, the side of the peak from which the signal arises for each etalon can be resolved with a decision tree. In the decision tree, a FSR for one etalon is partitioned according to the set of fractional etalon orders of all three etalons. For example, FIG. 8 illustrates how the ambiguity in the middle etalon is resolved by comparing the magnitudes of the signals from the other two etalons. A similar pattern of logic and concomitant self-consistency uniquely identifies the fractional order for each etalon.  
         [0088]    Once the algorithm deduces the fractional etalon order for each of the etalons, the algorithm can obtain the common etalon order (6 th  Step  186 ). In principle, the frequency equations of the three etalons over-determine the solution for our two unknowns, m and ƒ. In practice, a transmission signal for one of the etalons comes from a peak or valley. The lack of slope near the extrema renders the calculation of the fractional order useless for that etalon. Because the FSRs differ, the transmission signals for the remaining two etalons will occur in regions of high slope (and hence, high accuracy). The problem reduces to two equations from which the etalon order difference and optical frequency are found (7 th  Step  188 ).  
         [0089]    The choice of the difference in the FSRs of the etalons in wavelength meter device  29  is an important aspect of the present invention. The size of the FSR difference determines the frequency range for accurate measurement of the frequency. The effect may be understood as a breakdown of the validity of the decision tree. At or near the cluster frequency, there is no measurable difference between transmission signals and no means to determine the fractional etalon orders. The cluster frequency defines the low-end of the optical frequency measurement range. As optical frequency increases, the spacing of the etalon transmission signals increase by one FSR difference for each etalon order. The decision tree resolves the fractional etalon orders. The signals also generally occur in regions of high slope such that the fractional etalon order may be known to high precision. Ultimately, when the frequency increases enough that the etalon transmission signals are 180 degrees out of phase, the decision tree is unable to resolve the ambiguity in fractional etalon order. The optical frequency when this occurs defines the high-end of the optical frequency measurement range. The present invention extends the measurement range of the wavelength meter device by suggesting devices of three or more etalons, which increases the range of optical frequencies by insuring at least two etalons exhibit transmission in the high slope regions everywhere in the measurement region.  
         [0090]    The length differences between etalons are at the scale of the wavelength of the light with the exact length dependent on the measurement range desired. In one embodiment, the path length differences are identical. In this case, the optical phase difference between any two etalons may be kept below π if and only if the length difference between etalons are odd multiples of one-sixteenth of the central wavelength of the measurement range (defined as the midpoint between the wavelength at the cluster frequency, and the wavelength at an optical path length difference of π between the shortest and longest etalon). The invention is not fundamentally changed by choosing non-identical path length differences, but rather the details of the algorithm for calculating optical frequency are more complicated. In the three-etalon device, the reflectivity of the etalons is optimally near 0.25, which implies a finesse of about 2.  
         [0091]    Returning now to an embodiment of a wavelength meter device of three etalons, the following parameters are typical. For the desired part per million (ppm) precision, over a frequency range of about 6 THz or more, the FSR will typically be on the order of 100-200 GHz. The actual FSR depends on the signal-noise ratio (SNR) and the operation speed or bandwidth. Since the measurement is a two-step process, first determining the partial etalon order and then the integer etalon order, a 1 ppm resolution requires each step to attain a resolution of approximately one part per thousand. For example, over the optical telecommunication frequencies in the C and L-bands (15 THz) this implies a FSR near 200 GHz.  
         [0092]    For devices employing larger numbers of etalons, the reflectivity may be substantially higher and the precision of the wavelength meter device may be greatly increased. Using high reflectivity is an advantage because it increases the precision of the optical frequency measurement. On the other hand, the larger the reflectivity, the smaller are the regions of optical frequency over which the etalon&#39;s transmission spectrum has high slope. By adding additional etalons of larger or smaller optical path length, the range of optical frequencies covered by at least two etalons with high slope is increased. Therefore, using more than three etalons with finesse greater than 2 can obtain even higher precision in the optical frequency measurement than achieved with only three etalons, yet operate over a similar range of optical frequencies.  
         [0093]    The previous language of etalons, explaining the constraints on etalons in the wavelength meter device, is readily generalized to optical components that generate signal periodic in optical frequency. The FSR is merely the period of the optical transmission (or reflection) generated by an etalon. The above discussion also obtains for optical components that generate signal periodic in the optical frequency, simply by replacing “FSR” with “period of the signal generated by the optical component”. When the etalon discussion refers to different optical path lengths, a more general device requires optical components with different periods in the generated signals. Although the concept of reflectivity is not directly transferable, the finesse does transfer since it is defined as the ratio of signal period to half-width at half maximum of the periodic features.  
         [0094]    In various embodiments of a wavelength meter device  29 , the device may be constructed as a single unit using optical contacting techniques. Partial reflectors  30 ,  32 ,  34  and  36  may take the form of a single, monolithic beamsplitter such as a 1 by 4 beamsplitter optic with 45 degree reflectors. As shown in FIG. 10, IOEs  90  may be air-spaced etalons wherein two surfaces, coated with partial reflectivity coatings  190 ,  192 ,  194  and  196  are separated by spacer  198 . The air-spaced etalons may be constructed in a variety of configurations. Referring to FIG. 11, three air-spaced etalons  199  may be constructed in a monolithic architecture in which first optical beam  28  is partitioned by partial reflectors  200 ,  202  and  204  into optical beams  206 ,  208  and  210 . Optical beams  206 ,  208  and  210  enter three air-spaced etalons  199  made from a first optical element  212  with partial reflectivity coating  214 ; a second optical flat  216  with a two-layer thin-film pattern coating (layers  218  and  219 ) and partial reflectivity coating  220 ; and a spacer  221  creating three air gaps  222 ,  223 ,  224  of different lengths between optical elements  212  and  216 . Coatings  218  and  220  may consist of a material similar to the material of optical elements  212  and  216 , but in general may be made of a non-identical material. Coating  220  covers about ⅓ of the surface area of element  216 , and coating  218  covers about ⅔ of the surface area of element  216  such that the three regions of optical path length occupy about equal areas of optical elements  212  and  216 . Etalon optical beams  226 ,  228  and  230 , generated relative to the optical frequency of first optical beam  28 , the reflectivities of coatings  214  and  222 , and the optical path lengths, are detected by optical power detectors (OPDs)  232 ,  234  and  236 . The three OPDs generate signals  238 ,  240  and  242  that couple to signal processor  62  that employs the algorithm of FIG. 9 to determine the optical frequency.  
         [0095]    [0095]FIG. 12 illustrates wavelength meter  243  in which three etalons are formed from a single run of thin film pattern-coating applied to first and second optical elements  212  and  216 . Thin film coating  219  covers about ⅓ of the surface area of optical element  212 , while the coating  218  covers about ⅔ of the surface area of optical element  216 . Partial reflectivity coatings  214  and  222  cover the patterncoated optical elements  212  and  216 . Overlapping the pattern-coated flats creates three different etalons with substantially equal path-length difference ΔL  244 . The path-length differences in the three air-spaced etalons  199  are very uniform because the uniformity of coating thickness is much better in a single run of coating than between multiple coating runs.  
         [0096]    In another embodiment of the wavelength meter device, the etalons are made of solid material with polished end surfaces and partial reflectivity coatings. As before, first optical beam  28  is partitioned by a plurality of partial reflectors (PRs)  30 ,  32 ,  34  and  36  that generate optical beams  38 ,  40 ,  42  and  44 . Referring now to FIG. 13, the optical beam interact with solid etalons (SEs)  246 ,  248 ,  250  and  252 . The solid etalons generate optical beams  254 ,  256 ,  258  and  260  relative to the frequency of first optical beam  28 , and the length of and optical coating on the SEs. The optical beams are detected by optical power detectors (OPDs)  262 ,  264 ,  266  and  268 . A plurality of signals  270 ,  272 ,  274  and  276 , which are generated by the OPDs, couple to signal processor  62  that implements the optical frequency measurement algorithm of FIG. 9.  
         [0097]    The calculation of the measurement algorithm of FIG. 9 is performed in signal processor  29 , which includes but is not limited to a digital signal processor (DSP). The calculation may be refined to account for wavelength dependencies of partial reflectors  30 ,  32 ,  34 ,  36  (and  64 ,  66 ,  68  and  70 ) and OPDs  92 . Accounting for wavelength dependencies of components may require a second iteration of the calculation. The wavelength dependencies are small by the very design of the monolithic structure as in three air-spaced etalons  199  and the first pass calculation provides a very good value for the wavelength that, in many applications, will be sufficient. Within the scheme of the algorithm of FIG. 9, the initial value for the optical wavelength provides the appropriate correction factors from an initial calibration lookup table or parameterized equation.  
         [0098]    The facility of signal processor  62  to perform successive approximations and computations also allows an alternative embodiment of wavelength meter device  29  consisting of solid etalons  246 ,  248 ,  250  and  252 . The algorithm of FIG. 9 must be augmented to account for the temperature dependence and dispersion properties of the solid etalons in the determination of the optical wavelength. A simple and effective calculation method is to store information on the wavelength and temperature dependencies of the glass in the form of a lookup table in signal processor  62 . Iterative calculation then corrects for wavelength and temperature dependence through calculation of successive approximations  
         [0099]    The flexible computation capabilities of signal processor  62  also allows use of very general electro-optic components in wavelength meter device  29 . A power signal processor  62  maybe required to make general corrections for temperature, wavelength and other systematics in the signals from electro-optic components. As shown in FIG. 14, the wavelength meter device  29  may be constructed from a plurality of electro-optic elements (EOEs)  278 ,  280 ,  282  and  284  that generate signals  286 ,  288 ,  290  and  292  periodic in the optical frequency of first optical beam  28 . As shown in the prior art of FIG. 15, an EOE might consist of a birefringent material  294  and polarizer  296 . For an optical beam with a linear state of polarization  298 , birefringent medium  294  creates a rotation in the state of polarization. The rotation depends on the phase shift of light in birefringent medium  294 ; the phase shift is periodic in the optical frequency. Polarizer  296  transforms the periodicity in phase shift into an amplitude modulation of output optical beam  300  that is periodic in optical frequency.  
         [0100]    Another class of possible EOEs for use in a wavelength meter device are waveguide resonators. Referring to FIG. 16, a first optical beam  302  enters a planar substrate  304  consisting of an optical tap grating  306  that couples a small fraction of optical beam  302  into a waveguide  308 . Tap grating  306  has a period of one-half the wavelength of first optical beam  302  to generate a diffracted optical beam along the surface of the substrate and into the waveguide  308 . Optical power coupled into waveguide  308  is detected by optical power detector  310 . A second tap grating  312  couples light from waveguide  308  into waveguide resonator  314 . A third tap grating  316  couples light in resonator  314  into waveguide  318  where it is detected by optical power detector  320 . The ratio of the difference to the sum of signals from detectors  310  and  320  is a normalized signal that is periodic in optical wavelength.  
         [0101]    A wavelength meter device may be formed from several combinations of the waveguide resonator EOEs. A first embodiment consists of a plurality of resonator planes  322 ,  324  and  326  stacked as illustrated in FIG. 17. As shown in FIG. 18, a second embodiment consists of tapping a single waveguide  328  with a plurality of waveguide resonators  330 ,  332  and  334  in series on a single substrate. As shown in FIG. 19, a third embodiment consists of dividing a light input  336  among a plurality of waveguide resonators  338 ,  340  and  342  by a waveguide beamsplitter  344 .  
         [0102]    For various embodiments of a wavelength meter device  29  device of the present invention, an accurate measurement of wavelength require an accurate initial calibration of the optical components  46 ,  48 ,  50  and  52 . For instance, one objective of the calibration is to provide accurate values for the FSR&#39;s of each SE  246 ,  248 ,  250  and  252  along with an absolute wavelength reference. A second objective of initial calibration is to provide an accurate description of the finesse (transmission line shape) of each SE so that signals  270 ,  272 ,  274  and  276  may be interpreted accurately as partial orders of transmission of the respective etalons. A third objective of the calibration is to provide information on wavelength dependencies of partial reflectors  30 ,  32 ,  34  and  36  and detectors  262 ,  264 ,  266  and  268 . If solid etalons are used as in optical components  46 ,  48 ,  50  and  52 , a fourth objective of the calibration is to measure the wavelength dependencies of the refractive index of etalon materials.  
         [0103]    Monolithic construction of various embodiments of wavelength meter device  29  ensures long-term mechanical stability and ruggedness. Three etalons  199  move in lockstep due to variations in the operating environment of the device. Optical contacting and absence of adhesives ensures long-term precision of the device by maintaining long-term stability of optical paths  222 ,  223  and  224 , and surface reflectivities  214  and  220 . As a result, recalibration of the device would be unnecessary under most conditions. In the event that recalibration is necessary or desirable, the procedures required are greatly simplified by the construction technique. Absolute wavelength recalibration may be accomplished with a single point measurement of three partial orders e 01 , e 02 , e 03  of the three etalons  199  at a known wavelength. A two-point calibration may be used to update the FSR of each of the three etalons  199  for use in extreme temperature environments, or to check self-consistency.  
         [0104]    A potential problem for the previously mentioned embodiments of wavelength meter device  29  is the reflectivity of partial reflectors  30 ,  32 ,  34  and  36  depend on the state of polarization (SOP) of optical beam  28 . Hence, the calibration of the wavelength meter device must account for variations with SOP. In environments where the SOP is well-known and controlled, such as free-space propagation within a laser package or along a polarization-maintaining fiber, polarization dependence is not an issue. However, for light transmitted through a single-mode fiber to wavelength meter device  29 , the SOP will change over time. As illustrated in FIG. 20, a first way of mitigating polarization effects in wavelength meter device  29  is to employ a polarization control (PC) device  346  that produces a well-defined output SOP for any input SOP. A PC device ensures that the polarization effects in wavelength meter device  29  do not change with time, allowing for a single, well-known correction for polarization systematics in the optical frequency calculation of FIG. 9. The Coming Acrobat or General Photonics polarization controllers used in feedback mode are examples of PC devices. Alternatively, an optic of fixed, polarization-insensitive transmission may suffice (e.g. a polarizer or other optic generating a single, known SOP). A second embodiment for mitigating polarization effects removes polarization effects in wavelength meter device  29  by using a polarization scrambler (PS) device. As illustrated in FIG. 21, PS  348  changes the SOP more quickly than the response rate of the detection electronics thereby averaging-out polarization effects. PS  348  may comprise devices with a spatial gradient in birefringence to create a random SOP; or, for optical features with linewidths on the order of about a GHz or more, devices employing recirculating loops (e.g. Alliance Fiber Optics&#39; All fiber Optical Depolarizer) or Lyot filters; or, pseudo-randomizing devices (e.g. ILX Lightwave&#39;s PSC-8420).  
         [0105]    An improvement of the present invention is to use precise measurement of optical frequency to feedback to the optical source and control the optical frequency. Devices that control or regulate the optical source to a specific optical frequency (or wavelength) are called wavelength lockers. Each of the aforementioned embodiments of a wavelength meter device may be coupled to an optical source, and the information about the optical wavelength may be used as a feedback mechanism to control the source. The feedback bandwidth (and the optical frequency measurement bandwidth) of a wavelength locker device must exceed the bandwidth of noise on optical source  26 . Using a DSP as signal processor  62  to perform the optical frequency calculations, more than a thousand, and as many as ten thousand or more, optical frequency measurements may be possible per second. The timescale of monitoring is suited to corrective action on many of the parameters that change the optical frequency of a source. For example, in the case of laser diodes used in telecommunication systems, laser diode current and temperature may two fundamental control parameters that may change on the timescale of tens of milliseconds to seconds. In addition, laser diode performance changes as the diode ages over the timescale of months. All of these parameters may be controlled with an optical wavelength measurement with an update rate of 1 KHz or faster.  
         [0106]    One embodiment of a wavelength locking device  349  is described in FIG. 22. Wavelength locker  349  combines wavelength meter device  29  and a coupler  350  coupling optical frequency readout  352  to the optical source  354 . Readout  352  may consist of an optical frequency, possibly read by the source through an interface format such as PXI, GPIB or RS-232. Alternatively, the optical frequency readout  352  may simply communicate deviations from a set-point optical frequency at which source  354  must remain locked. Communicating the change may prove faster than communicating the absolute value of the optical frequency. The communication of change might take the form of a number communicated by interface, or as a voltage proportional to the change that is a simple analog input to the optical source.  
         [0107]    A myriad of wavelength locking devices  349  are possible based on the wavelength meter device embodiments of the present invention. FIG. 23 shows a wavelength locking device consisting of a wavelength meter device made of etalons (E i )  356 ,  358  and  360  and optical power detectors (OPD i )  362 ,  364  and  366  wherein coupler  350  couples optical frequency readout  352  to optical source  354 . FIG. 24 shows a wavelength locking device consisting of a wavelength meter device made of air-spaced etalons (ASE i )  368 ,  370  and  372 , and OPDs. FIG. 25 shows a wavelength locking device consisting of a wavelength meter device made of solid etalons (SE i )  374 ,  376 ,  378  and  380  and OPDs  382 ,  384 ,  386  and  388 . FIG. 26 shows a wavelength locking device consisting of a wavelength meter device made of electro-optic elements (EOE i )  390 ,  392 ,  394  and  396 . FIG. 27 shows a wavelength locking device consisting of a wavelength meter device made of a polarization controller (PC)  346  and optical components (OC i )  46 ,  48 ,  50  and  52  that generate signals periodic in optical frequency. FIG. 28 shows a wavelength locking device consisting of a wavelength meter device made of a polarization scrambler (PS)  348  and optical components (OC i )  46 ,  48 ,  50 ,  52  that generate signals periodic in optical frequency.  
         [0108]    Each of the embodiments of wavelength meter  29 , which measure a single optical wavelength, may form the basis for a wavelength meter device capable of measuring multiple optical wavelengths. FIG. 29 illustrates a multi-wavelength meter device  397 . Optical source  398  generates first optical beam  400 , which enters a tunable optical filter element (TOFE)  402 . TOFE  402  generates a second optical beam  404 . A wavelength meter device  29  is positioned at least partially in second optical beam  404 . The TOFE restricts the measurement wavelength range to the transmission region of TOFE  402 . FIG. 30 illustrates the operating principle. A transmission region  406  of TOFE  402  is shown in transmission spectrum  408 , assures resolution of the smallest spectral feature. The TOFE scans from a calibrated start wavelength or scans at a calibrated rate. Note that the accuracy of the TOFE scan does not limit the accuracy of the multi-wavelength measurement. Optical spectrum  410  illustrates the optical frequencies present in optical beam  400 . Optical frequencies that are outside the transmission region  406  of TOFE  402  are suppressed and only an optical frequency within transmission region  406 , at a specific moment in the scan, transmits to wavelength meter device  29  to make a single, highly accurate and precise measurement. The resultant optical spectrum from convolving the optical spectra  408  and  410  is shown in optical spectrum  412 . An example of a TOFE is available from Micron Optics.  
         [0109]    The algorithm for measuring multiple optical frequencies is shown in FIG. 31. The multi-wavelength measurement algorithm consists of the previously described algorithm for wavelength measurement in the three-etalon device (Steps  178 ,  180 ,  182 ,  184 ,  186  and  188 ) and a loop step  414  for iterating the TOFE to scan multiple optical frequency regions and to assign a single frequency to the light in said optical frequency scan segment. Compiling the results of multiple scan segments (8 th  Step  416 ) allows for the accurate determination of optical spectrum  410  with precision of single-wavelength meter device  29 , an ability to resolve nearby wavelengths determined by the width transmission region  406  of TOFE  402 , a measurement time determined by the scanning time TOFE  402 , and a wavelength range dictated by the calibrated scan range of TOFE  402  and the operating range of wavelength meter device  29 .  
         [0110]    When combined with accurate power measurements with wide dynamic range, multi-wavelength meter device  397  becomes an optical spectrum analyzer (OSA) or optical channel monitor (OCM). A particularly important application is in monitoring spectra of a DWDM system, in which many laser sources, each with narrow spectral linewidth, are multiplexed in a single optical fiber. One optical spectrum analyzer device  417 , shown in FIG. 32, is suited to measuring the wavelengths and optical power of individual optical channels in a DWDM system. Referring to FIG. 32, optical source  398  generates first optical beam  400 . TOFE  402  is positioned at least partially in first optical beam  400 , generating a second optical beam  404  with a narrowed optical frequency spectrum (as discussed above). A sequence of partial reflectors (PR 1 , PR 2 , . . . PR n )  418 ,  30 ,  32 ,  34  and  36  generate optical beams  420 ,  422 ,  424 ,  426  and  428 . An optical power detector (OPD)  430  is arranged in optical beam  420  and generates a signal  432  in proportion to the optical power of second optical beam  404 . Optical beam  420  need not be generated prior to any other of the optical beams  422 ,  424 ,  426  and  428 . Optical components (OC 1 , OC 2  . . . OC n )  46 ,  48 ,  50  and  52  are arranged in optical beams  422 ,  424 ,  426  and  428 , generating signals  434 ,  436 ,  438  and  440 . Signals  432 ,  434 ,  436 ,  438  and  440  couple to signal processor  442  for determination of the optical spectrum of optical beam  400 .  
         [0111]    An algorithm for measuring the optical frequency spectrum (FIG. 33) builds upon the multi-wavelength measurement algorithm. One embodiment of an optical spectrum analyzer  417  first measures optical power (0 th  Step  444 , FIG. 33). Optical power measurement  444  may be used to normalize the signals from optical components  46 ,  48 ,  50  and  52 . The algorithm then determines the optical frequency with the bandwidth of TOFE  402 , and assigns the optical power measured in Step  444  with the optical frequency measured in 1 st  through 7 th  Steps  178 ,  180 ,  182 ,  184 ,  186  and  188 . Scanning TOFE  402  and measuring a sequence of optical frequency segments in loop  414  allows for construction of the optical spectrum over the entire scan range of TOFE  402  (8 th  Step  446 , FIG. 33).  
         [0112]    In one embodiment, the width of the transmission region  406  of TOFE  402  is narrower than the channel spacing in the monitored optical system, which allows discrimination of one wavelength from another. The present embodiment requires a large dynamic range of power measurement  444  to monitor the possibly large attenuation of specific wavelengths within a DWDM system. Automatic gain switching in the electronics is one method of generating a large dynamic range of the power measurement of at least about 30 dB.  
         [0113]    A set of embodiments of the OSA  417  address the need to mitigate the polarization dependence in the wavelength meter device  29  and power measurements  444 . Problems arise, in part, because the responsivites of OPD  430  and OPDs in optical components  46 ,  48 ,  50  and  52  are polarization sensitive and because the SOP of each wavelength in a DWDM system is generally different and changes at different rates and amounts. One embodiment of a polarization-mitigated multiple-wavelength meter device  447 , shown in FIG. 34, uses either an optical element that permits transmission of a single state of polarization, regardless of the input state or wavelength (a polarization homogenizer or polarization controller PC  346 ). Another embodiment (FIG. 35) uses a polarization scrambler (PS)  348  to randomize the input state of polarization on a timescale of the measurement of each optical frequency in a multi-frequency measurement. A polarization controller that actively controls the SOP of each optical frequency may also mitigate polarization effects. A multi-channel polarization controller is not the preferred polarization mitigation scheme because of the cost and the difficulty controlling the SOP of multiple optical frequency channels. Current techniques require de-muxing the multi-frequency optical beam, individually controlling the SOP of each channel, and re-muxing the optical frequencies into a single beam. The preferred solutions, a polarization scrambler or polarization homogenizer, may be placed prior to the TOFE  402  (1 st  location option  448 ) or between the TOFE  402  and the wavelength meter device  405  (2 nd  location option  450 ). Another method of reducing polarization effects is to remove polarization dependence from the optical power detectors (OPDs), as in FIGS. 6. One embodiment combines an integrating sphere  138  with OPD  140 . The integrating sphere may contain a hollow cavity  144  or consist of a solid interior  148  that randomizes the state of polarization of light through multiple reflections off a diffusive reflection surface inside the sphere.  
         [0114]    Multiple-wavelength meter device  397  may be used to feedback optical frequency information to an optical source, or sources, and thereby control the optical frequencies. Referring to FIG. 36, one embodiment of a multi-wavelength locking device  451  uses coupler  452  to couple optical frequency readouts  454  from a multiple-wavelength meter device  397  to an optical source or sources  458 . Considering the importance of polarization effects, FIG. 37 shows one embodiment of a multiple-wavelength locking device  459  wherein optical frequency readouts  460  couple from a polarization-mitigated multiple-wavelength meter device  447  to optical source or sources  458 . Polarization effects are mitigated by a polarization controller (PC)  346  or polarization scrambler (PS)  348  positioned either before or after TOFE  402 .  
         [0115]    An improved set of embodiments monitor optical power within wavelength meter device  29  as a means of normalizing power fluctuations from signals  54 ,  56 ,  58  and  60 . Signal variations due to changes in the optical power of the optical source may be reduced or removed by dividing said frequency-dependent signals with a signal in proportion to the optical power of the optical source. This Second Class of wavelength meter devices have no reliance on an external power measurement and achieve simplicity of self-reliance at a minimum of additional cost.  
         [0116]    A general embodiment of a wavelength meter device with optical power detection  463  is shown in FIG. 38. Optical source  26  generates optical beam  28 . A sequence of partial reflectors (PR 1 , PR 2 , PR 3  . . . PR n )  464 ,  466 ,  468 ,  470  and  472  positioned at least partially in optical beam  28 , generate optical beams  474 ,  476 ,  478 ,  480  and  482 . Optical power detector (OPD)  484  generates a signal  486  in proportion to the optical power of optical beam  28 . Optical components (OC 1 , OC 2 , OC 3  . . . OC n )  488 ,  490 ,  492  and  494  generate signals  496 ,  498 ,  500  and  502 . Signals  486 ,  496 ,  498 ,  500  and  502  couple to signal processor  504  for determination of the optical frequency of optical beam  28 .  
         [0117]    The algorithm for calculating the optical frequency is identical to the method of FIG. 5, with the exception that the optical power signal is derived from OPD  484 . Proper normalization using OPD signal  486 —which achieves a cancellation of common-mode noise and optical power variations—requires a careful accounting for the various insertion losses and partial reflectivity into optical components  488 ,  490 ,  492  and  494 , in addition to possible wavelength and temperature dependencies.  
         [0118]    [0118]FIG. 39 illustrates an embodiment of a wavelength meter device with optical power detection  463  consisting of three air-spaced etalons  505 . Input optical beam  28  enters beamsplitter structure  506  through entrance surface  508 , and is split into four substantially equal parts by first beamsplitter  510 , second beamsplitter  512 , third beamsplitter  514  and fourth beamsplitter  516 . Remaining optical power exits through an antireflection coated exit window  518 . Optical beam  520  reflected from first beamsplitter  510  is directed to OPD  522  to monitor the optical power of beam  28 . Optical beam  524  reflected from second beamsplitter  512  is directed towards a first etalon  525 , entering through reflecting surface  526 , exiting through reflecting surface  527  on the optical element  528  and is detected by OPD  530 . Surfaces  526  and  527  may consist of a partial reflectivity coating. Optical beam  532  enters second air-spaced etalon  533 . Optical beam  534  enters third etalon  535 . Optical beams  536  and  537 , generated by etalons  533  and  535  respectively, are detected by OPDs  538  and  539 . The first reflective surface  526  is separated from second reflective surfaces  526  by a spacer element  540 . Thin film pattern coatings  542  and  544 , of thicknesses ΔL 1  and ΔL 2 , are layered on optical element  528 . The layer thicknesses ΔL 1  and ΔL 2  may be identical, but may also differ according to certain criteria as discussed below. Layer  542  covers about ⅔ of element  528 . Layer  544  covers about ⅓ of element  528 . Layers  542  and  544  are overlapped such that three regions are formed with about the same area, but consisting of three different optical path lengths making up etalons  525 ,  533  and  535 . Those skilled in the art will notice that a variety of beam splitting ratios can be used in the case where only a small portion of the total laser power is used or when the reflected signals from the etalons rather than the transmitted signals are used in an alternative embodiment.  
         [0119]    An alternative embodiment of the wavelength meter device consisting of optical power detection  463  is shown in FIG. 40. An optically flat element  548 , made of a surface coating  526  with partial reflectivity, creates the first surface of the three air-spaced etalons  199 . The separation of the beamsplitter from the etalon optical surfaces may make the wavelength meter device  463  more manufacturable by placing the precision reflectivity coating on element  548 , rather than directly on the four beamsplitter structure  506 .  
         [0120]    In both embodiments illustrated in FIGS. 39 and 40, the precise path length differences between the air-spaced etalons  525 ,  533  and  535  are created by thin film deposition of layers  542  and  544  onto the surface of element  528  inside the airspace. The length differences ΔL 1  and ΔL 2  may be equal to an odd multiple of one-sixteenth of the central operating wavelength to ensure that frequency ƒ is in the high slope region of the transmission spectra of at least two etalons over the wavelength range of wavelength meter device  463 . The invention is not fundamentally changed by choosing non-identical path length differences, but rather the details of the algorithm for calculating wavelength are altered. Two of the various embodiments of the invention in FIGS. 39 and 40 use identical path-length differences ΔL 1 =ΔL 2  =ΔL. Reflectivity of surface coatings  526  and  527  should optimally be around 0.25. Among the various embodiments, etalon spacer  540  is optically flat and the etalon is constructed by optically contacting spacer  540  to beamsplitter  506  and the pattern-coated element  528 .  
         [0121]    Identical path length differences in air-spaced etalons  525 ,  533  and  535  may be more easily achieved in an embodiment of wavelength meter device  463  shown in FIG. 41. Two pattern coatings of non-identical length, but identical thickness are applied to optical elements  528  and  548  during the same coating run. As in wavelength meter device  243 , layer  544  applied to element  548  is substantially ⅓ the length of the portion of element  548  making the three etalons. Layer  542  on element  528  is substantially ⅔ of the portion of element  528  making the air-spaced etalon. Alternatively, layer  544  may be applied to element  528  and layer  542  may be applied to element  548 . Partial reflectivity coatings  526  and  527  are applied over thin film coating  542  and  544  and the surfaces of elements  528  and  548  that face inside the air-gapped etalons  525 ,  533  and  535 . Overlapping the two substrates, as in FIG. 41, three etalons are created with equal length differences ΔL. Spacer  540  separates elements  528  and  548 .  
         [0122]    Embodiments of wavelength meter device  463 , consisting of air-spaced etalons  525 ,  533  and  535 , may be constructed with a variety of techniques. The temperature sensitivity of the etalon lengths may be reduced by constructing spacer  540  from a material of low thermal expansion coefficient such as Zerodur, ULE or others. In the case of Zerodur, the desired wavelength accuracy of wavelength meter device  463  can be maintained over a range of several degrees without other temperature compensation. Structure  505  may be robustly constructed by joining the optical components (such as beamsplitter  506 , spacer  540 , and elements  528  and  548 ) with optical contacting methods. By optical contacting we refer to a variety of techniques including, but not limited to, wafer bonding, ringing, adhesion through optical contact, anodic bonding and diffusive bonding.  
         [0123]    Constructing wavelength meter device  463  from solid etalons may prove more easy to manufacture than three air-spaced etalons  505 . Moreover, solid etalons may be 50% smaller when composed of fused silica, for example. Referring to FIG. 42, optical source  550  generates a first optical beam  552 . Optical beam  552  enters wavelength meter device  553 . Partial reflectors (PR 1 , PR 2 , PR 3 . . . PR n ,)  554 ,  556 ,  558 ,  560  and  562  positioned at least partially in first optical beam  552 , generate optical beams  564 ,  566 ,  568 ,  570  and  572 . Optical power detector  570  generates a signal  572  in proportion to the optical power of optical beam  552 . Solid etalons  574 ,  576 ,  578  and  580  (SE 1 , SE 2 , SE 3 . . . SE n ) generate optical beams  582 ,  584 ,  586 ,  588  relative to the frequency of optical beam  552  and the length, finesse and material properties of solid etalons  574 ,  576 ,  578  and  580 . Optical power detectors  590 ,  592 ,  594  and  596  generate signals  598 ,  600 ,  602  and  604 . Signals  598 ,  600 ,  602  and  604  couple to signal processor  606  for determination of the optical frequency of optical beam  552 . A calculation algorithm substantially similar to the schematic of FIG. 9, and accounting for the dispersion of solid etalons, calculates the optical frequency.  
         [0124]    One of various embodiments of a wavelength meter device  553  is shown in FIG. 43. Beamsplitter  608 , consisting of four partially reflective surfaces  610 ,  612 ,  614  and  616 , is positioned at least partially in optical beam  552 . Partial reflectors  610 ,  612 ,  614  and  616  generate optical beams  618 ,  620 ,  622  and  624 . Optical beam  618  is detected by optical power detector (OPD)  626 , which generates a signal  628  in proportion to the power of optical beam  552 . Solid etalons  630 ,  632  and  634  optically contacted to beamsplitter  608 , are positioned at least partially in the path of optical beams  620 ,  622  and  624 . The solid etalons generate three optical beams  636 ,  638  and  640  relative to the lengths of solid etalons  630 ,  632  and  634 , their index of refraction (for example about 1.44 for fused silica) and their finesse (of about 2 or greater). The etalon lengths differ by equal amounts ΔL, where ΔL is an odd multiple of one-sixteenth of the central wavelength of the operating range of the wavelength meter device. OPDs  642 ,  644  and  646  detect optical beams  636 ,  638  and  640 , respectively, generating signals  648 ,  650  and  652 , respectively. Signals  628 ,  648 ,  650  and  652  couple to signal processor  654 . Signal processor  654  employs a calculation algorithm substantially similar to the schematic of FIG. 9, with additional accounting for the dispersion of solid etalons, to calculate the optical frequency.  
         [0125]    An embodiment of a wavelength meter device  463  consisting of electro-optical elements  278 ,  280 ,  282  and  284  in place of optical components  488 ,  490 ,  492  and  494  is shown in FIG. 44. Optical source  550  generates first optical beam  552 . Partial reflectors  656 ,  658 ,  660 ,  662  and  664  (PR 1 , PR 2 , PR 3  . . . PR n ), positioned at least partially in optical beam  552  generate optical beams  666 ,  668 ,  670 ,  672  and  674 . An optical power detector  676  generates signal  678  in proportion to the optical power of optical beam  552 . Electro-optic elements  278 ,  280 ,  282  and  284  (EOE 1 , EOE 2 , EOE 3  . . . EOE n ) generate signals  286 ,  288 ,  290  and  292  relative to the optical frequency of optical beam  552  in response to passage of the optical beams through the EOEs. The EOEs may take the form of birefringent material  294  with polarizer  296  or planar waveguide resonator planes  326 ,  324  and  322 . Generally the EOEs consist of a component that generates a signal periodic with the optical frequency of detected light. Signals  678 ,  286 ,  288 ,  290  and  292  couple to signal processor  696  to determine the optical frequency of optical beam  552 , using the algorithm of the schematic of FIG. 9.  
         [0126]    Another embodiment of wavelength meter device  463  reduces or eliminates polarization effects by removing polarization sensitive components such as partial reflectors  464 ,  466 ,  468 ,  470  and  472  and 1 by 4 beamsplitter  506 . Referring now to FIG. 45, a first optical beam  698  is incident, at least partially, on etalons  700 ,  702  and  704  and a fourth optical path  706  without etalon. The etalons may be air-spaced or solid, and are substantially similar to the etalons of previously mentioned embodiments. Fourth optical path  706  may consist of a glass substrate with a clearance aperture that joins to three etalons. Light transmits through etalons  700 ,  702  and  704  and fourth optical path  706  and is detected, respectively, by optical power detectors  708 ,  710  and  712  and  714  on detector array  716 . The clearance hole of fourth optical path  706  allows for unobstructed detection of at least a portion of the optical power of first optical beam  698 . The optical power detected by detector  714  serves as a power reference for the device. In another of various embodiments of a polarization insensitive wavelength meter device (FIG. 46), first optical beam  698  is incident upon three etalons  700 ,  702  and  704  that join together to form an L-shaped cross section. The open crux of the L-structure creates fourth optical path  717 .  
         [0127]    [0127]FIG. 47 illustrates yet another of various embodiments of a polarization insensitive wavelength meter device  717 . An input fiber  718  is the source of a first optical beam  720 . Optical beam  720  is expanded and collimated by optics  722  that generate optical beam  724 . Beam elongation optics  726  expand optical beam  724  asymmetrically to form an elongated or elliptical beam  728 . Three etalons and an unobstructed optical path  730  are positioned at least partially in optical beam  728 . Optical beams generated by  730  are detected by photodiode array  732 . Array  732  generates photodiode signals  734  that couple to digital signal processing  736  for calculation of optical frequency of optical beam  720 .  
         [0128]    In each of the embodiments of FIG. 45, 46 and  47 , the light transmitted through four paths (etalons  700 ,  702 ,  704  and optical reference path  706 , or structure  730 ) is detected by an array of photodetectors  716  and  732  such as a quadrant photodiode  126 , photodiode array  124 , or the like. The etalons may be optically contacted together to form a monolithic structure. Since no beam splitters are used, the polarization dependence of the light along each path is greatly reduced. Beyond these designs, further reduction of the polarization dependence may be achieved by using integrating sphere  138  with photodiode arrays  716  and  732 .  
         [0129]    Another of the various aspects of the present invention is a wavelength locking device  737  consisting of wavelength meter device  463  and a coupler  738 . The strategy is substantially similar to the wavelength locker  349 . In contrast to previous embodiments, the wavelength stability of the lock is improved by a normalization signal derived from optical power measurement. An embodiment, shown in FIG. 48, relies upon the wavelength meter device  463 , consisting of optical power detector  484 , frequency-dependent optical components  488 ,  490 ,  492  and  494  (OC i ), and a coupler  738  coupling optical frequency readouts  740  (or readout of deviation from a set-point optical frequency) to the optical source  742 . The coupling comprises substantially similar designs as discussed in the wavelength locking device  349 . One embodiment shown in FIG. 49 includes power measurement in optical power detector  742  (OPD i ), etalons (E i )  744 ,  746 ,  748  and  750  and optical power detectors (OPD i )  752 ,  754 ,  756  and  758  that detect optical beams  760 ,  762 ,  764  and  766  generated by the etalons. Another embodiment shown in FIG. 50 includes air-spaced etalons (ASE i )  766 ,  768 ,  770  and  772  with optical power detectors  774 ,  776 ,  778  and  780 . Yet another embodiment shown in FIG. 51 includes solid etalons (SE i )  782 ,  784 ,  786  and  788  and optical power detectors  790 ,  792 ,  794  and  796 .