Abstract:
An optical wavelength meter for measuring wavelength of an optical beam includes two periodic out of phase fine optical filters ( 44, 45 ), using, for example Fabry Perot etalon filters, Fizeau filters, fibre Bragg gratings or photonic crystals. The phases of the periodic responses are arranged such that a peak ( 5109  or trough ( 511 ) of one response coincides with a slope ( 522 ) of the other response so that a slope portion of a response may always be chosen for measurement. A coarse filter ( 43 ) is provided to unambiguously define on which cycle of the periodic response of the fine filters a measured wavelength lies. Synchronized clock signals are provided to measure output of the filters using, for example, photodiodes ( 421, 422, 423, 424 ), at a rate of (1,000 to 10,000) wavelength measurements per second.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is derived from PCT application PCT/EP2004/003010 filed Mar. 22, 2004 and claims priority from UK patent application GB 0306724.6 filed Mar. 24, 2003. 
   BACKGROUND OF THE INVENTION 
   1.Field of the Invention 
   This invention relates to an optical wavelength meter. 
   2.Description of Related Art 
   Interferometer optical wavelength meters are known for measuring wavelengths in the optical C-band (1525-1565 nm) and optical L-band (1568-1610 nm) at a rate of approximately 10 measurements/sec. The use of linear and periodic optical filters for wavelength measurement is known from laser characterisation measurements. However, it is desirable to make faster optical wavelength measurements within these wavebands, for example, in VCSEL (vertical cavity surface emitter laser) production wafers having 20,000 devices need to be tested and in mass production of other active or passive optical components. 
   Referring to  FIGS. 1 to 3 , the measurement of optical wavelength using coarse and fine wavelength dependent filters is known from, for example, GB 0223448.2 for characterisation of tuneable lasers. A known arrangement  10  is shown in  FIG. 1 , in which a laser beam  11  from the tuneable laser  12  is split by a branched waveguide to direct the light beam under test to four pathways  131 , 132 , 133 , 134  leading respectively to: a reference photodiode  141 ; a coarse, monotonic or linear spectral filter  15  for the wavelength band of interest with an output to a second photodiode  151 ; and two periodic transmission filters  16 , 17 , the respective outputs of which vary periodically with wavelength across free spectral ranges, having outputs to third and fourth photodiodes  161 , 171 , respectively. Typically the periodic filters  16 , 17  are Fabry Perot etalon filters having 50 GHz and 5 GHz free spectral range respectively, as shown in  FIG. 2 . 
   The course feature extraction filter  15  is a dielectric multilayer coating on a glass substrate with a monotonic frequency response across The C-band; while the medium and fine frequency identification filters  16 ,  17  are, for example, each formed from two parallel 95% dielectric minors with flat response. 
   Characteristics of the filters are shown in  FIG. 2 , in which a first plot  21  shows a linear response of the coarse filter  15 , a second plot  22  shows a periodic response of the first periodic filter  16  with a free spectral range of 50 GHz and a third plot  23  shows a periodic response of the second periodic filter  17  with a free spectral range of 5 GHz. 
   In use, the wavelength of an emitted beam is determined approximately with the linear filter  15  by determining transmissivity of the filter at the given wavelength by comparing the power of the reference beam, as measured with the first reference photodiode  141 , with the power of the beam transmitted through the filter  15  as measured by the second photodiode  151 . The wavelength is determined with sufficient precision, e.g. is found to be in the range  24  shown in  FIG. 2 , to determine on which peaks  221 ,  231  of the periodic filter transmission curves  22 ,  23  the measured wavelength lies. As shown in  FIG. 3 , knowing on which peaks the wavelength lies, the wavelength  241  may be determined unambiguously from the outputs of the third and fourth photodiodes  161 ,  171 . 
   It is an object of the present invention at least to ameliorate the aforesaid deficiency in the prior art. 
   BRIEF SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, there is provided an optical wavelength meter for measuring wavelength of an optical beam comprising: coarse optical filter means and first optical power measurement means for measuring output of the coarse optical filter means and second optical power measurement means for measuring an unfiltered reference beam for coarse wavelength measurement; fine optical filter means comprising two periodic out-of-phase optical filters and third and fourth optical power measurement means for measuring output of the two out-of-phase optical filters for fine wavelength measurement, respectively; synchronized clock signal measurement means for synchronized measurement of the output of the first, second, third and fourth optical power measurement means; and processing means for determining the optical wavelength of the optical beam from the first and second optical power measurement means for coarse wavelength measurement and at least one of the third and fourth optical power measurement means for fine wavelength measurement. 
   Preferably, the optical wavelength meter further comprises beam splitting means for splitting the optical beam between the unfiltered reference beam and the coarse and fine optical filter means. 
   Advantageously, the coarse optical filter is a linear spectral filter. 
   Conveniently, the coarse optical filter comprises a dielectric multilayer coating on a glass substrate. 
   Preferably, the periodic optical filters comprise at least one of a Fabry Perot filter, a Fizeau filter, a fibre Bragg grating and a photonic crystal. 
   Advantageously, the two periodic out-of-phase optical filters are substantially 90 ° out of phase, i.e. in quadrature, such that peaks and troughs of a first of the filters coincide with substantially linear ranges between peaks and troughs of a second of the filters. 
   Conveniently, a phase offset between the two out of phase periodic optical filters is tuned by angle, temperature or pressure using a piezoelectric transducer. 
   Preferably, reflectivity of the periodic optical filters is substantially 25%. 
   Advantageously, the periodic optical filters have a finesse of substantially 2. 
   Conveniently, the periodic optical filters have a free spectral range of substantially 100 GHz. 
   Alternatively, the periodic optical filters have a free spectral range of substantially 50 GHz. 
   Conveniently, the periodic optical filters are parallel or wedge quartz etalons. 
   Advantageously, the optical wavelength meter further comprises calibration filter means and calibration filter output power measuring means. 
   Conveniently, the calibration filter means comprises an etalon filter. 
   Preferably, the etalon filter has precisely set or controllable free spectral range. 
   Advantageously, the free spectral range of the etalon filter is controllable and preset by rotation adjustment or temperature. 
   Preferably, a free spectral range of the calibration etalon filter differs just sufficiently from the free spectral range of the fine measurement filters that the calibration etalon filter is in phase only at top, middle and bottom wavelengths of a range of measurements of interest to obtain co-incident or Vernier-like maximum power at those wavelengths. 
   Conveniently, at least one of the optical power measurement means comprises a photodiode. 
   Advantageously, the synchronised clock signal measurement means comprises master module and slave modules to trigger measurement and read output of the power measurement means. 
   Preferably, the synchronised clock signal measurement means enables 40,000 points on each channel to be read in 2.5 seconds. 
   Advantageously, the synchronised clock signal measurement means enables 1,000 to 10,000 wavelength measurements/second. 
   Advantageously, the optical wavelength meter has a precision of substantially 2 picometers or substantially 250 MHz. 
   Preferably, the optical wavelength meter is arranged to make wavelength measurements in at least one of optical C-band, optical L-band and optical S-band. 
   Advantageously, the optical wavelength meter further comprises temperature control means for stabilising the optical components. 
   Conveniently, the temperature control means comprises a thermistor or thermocouple and fan cooling or Peltier temperature elements. 
   Advantageously, the optical wavelength meter is adapted for external triggering for synchronisation with external instrumentation. 
   According to a second aspect of the invention, there is provided a method of determining wavelength of an optical beam comprising: splitting the optical beam into first, second, third and fourth sub-beams; presenting the first sub-beam to reference first photodetector means; presenting the second sub-beam to linear filter means having an output to second photodetector means; presenting the third sub-beam to the first fine filter having an output to third photodetector means; presenting the fourth sub-beam to the second fine filter having an output to third photodetector means; using synchronized clock signal measurement means to read outputs from the first, second, third and fourth photodetector means; identifying from the first and second photodetector means outputs a limited range of wavelength in which the wavelength of the optical beam lies, to determine which of the first fine filter and the second fine filter has a greater sensitivity to wavelength in that limited range; and using the corresponding third or fourth photodetector means output, corresponding to the fine filter means having the greater sensitivity, to determine the wavelength of the optical beam. 
   Preferably, the first fine filter and the second fine filter are two periodic out of phase optical filters which are substantially 90° out of phase, i.e. in quadrature, such that peaks and troughs of a first of the filters coincide with substantially linear ranges between peaks and troughs of a second of the filters. 
   Conveniently, the method comprises the further steps of: providing a calibration etalon filter with conventional Airy function transmitting only at reference wavelength for calibration having a common maximum with the fine filter at a limited number of wavelengths within range; providing a broadband light source; and calibrating the processed readout from fine filter to the reference wavelength of calibration etalon filter. 
   Alternatively, the method comprises providing a tuneable laser tuned to the reference wavelength. 
   Conveniently, the method additionally comprises providing a fourth Airy etalon in ratio with the third Airy etalon to provide a common maximum at the limited number of wavelengths for a more defined optical transmitted bandwidth. 
   According to a third aspect of the invention, there is provided a computer program comprising code means for performing all the steps of the methods described above when the program is run on one or more computers. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of a known wavelength measuring arrangement for laser characterisation; 
       FIG. 2  shows plots of transmissivity versus wavelength or frequency for filters included in the arrangement of  FIG. 1 ; 
       FIG. 3  shows in more detail two of the plots of  FIG. 2 ; 
       FIG. 4  is a schematic diagram of optical components of a wavelength meter according to the invention; 
       FIG. 5  is a transmissivity plot of a periodic filter of the wavelength meter of  FIG. 4 ; 
       FIG. 6  is a transmissivity plot of a known periodic filter for comparision with the plot of  FIG. 5 ; 
       FIG. 7  shows two transmissivity plots of two periodic filters of the wavelength meter of  FIG. 4 , which are mutually 90° out of phase; 
       FIG. 8  shows sections of the plots of  FIG. 7  used to obtain wavelength readings with the wavelength meter of the invention 
       FIG. 9  is a schematic diagram of an experimental wavelength meter according to the invention; 
       FIG. 10  is a schematic diagram of one of the photodetectors and associated circuitry of  FIG. 9 ; 
       FIG. 11  is a schematic diagram showing interconnections of a wavelength meter according to the invention with an external personal computer; 
       FIG. 12  is a schematic diagram of electronic components of a wavelength meter according to the invention; 
       FIG. 13  is a schematic side view of packaging of the wavelength meter of  FIG. 12 ; and 
       FIG. 14  is a schematic plan view of the packaging of the wavelength meter of  FIG. 13 . 
     In the Figures, like reference numerals denote like parts. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 4 , which shows optical components  40  of a wavelength meter according to the invention, a beam splitter  41  directs a light beam under test to four pathways  411 ,  412 ,  413 ,  414  that incorporate respectively a reference photodetector  421 ; a coarse, monotonic or linear spectral filter  43  for the wavelength band of interest, an output of which is optically connected to a second photodetector  422  and two periodic transmission filters  44 ,  45  that vary in transmission across a free spectral range of typically 100 GHz. Output of the periodic filters  44 ,  45  are optically connected to photodetectors  423 ,  424  respectively. The function of the coarse feature extraction filter  43  is again provided by a dielectric multilayer coating on glass substrate with monotonic frequency response across the C-band; while the fine frequency identification filters are, for example, Fabry Perot (FP) or Fizeau type, created by, for example, parallel or wedge quartz etalons. Typically these FP etalon filters have 100 GHz down to 5 GHz free spectral range. The selection of 100 GHz allows for 250 MHz reading precision with the detector scheme described. More precisely, for optical C-band angle-tuned filters with 90 degree relative-phase offset, it is found that a fixed free spectral range of 100 GHz with a peak at 194 THz (mid C-band) has peaks offset by &lt;267 MHz at top and bottom of C-band (196 and 192 THz). These values are input to a look up table to compensate for any resultant error. The optical components may be interconnected by pigtail optical fibres the linear filter having a first input lens  461  and the periodic filters  44 ,  45  having second and third input lenses  462 ,  463  respectively. Similarly, the second, third and fourth photodetectors have respective photodetector input lenses  471 ,  472 ,  473 . Alternatively, the optical components may be interconnected by, for example, planar waveguides as in planar lightwave optics. 
   The periodic filters have a low reflectivity of substantially 25% compared with the conventionally used 95% reflectivity and have a finesse of about 2, so that the etalon response  51 , shown in  FIG. 5 , is closer to sinusoidal than a response  61  of a conventional Airy function, shown in  FIG. 6 , of a Fabry Perot etalon, where the finesse of the etalon is defined as the ratio of the free spectral range (FSR) to the width of the peaks, i.e. the fill width at half maximum height (FWHM). This means that effectively the etalon is similar to a two-beam interferometer while retaining the compactness and ruggedness of a glass substrate. Referring to  FIGS. 7 and 8 , the 90° offset of two periodic filter responses  51 ,  52  means that the peaks  510  and troughs  511  of a first filter, where slope is low, coincide with a steep-sloped substantially linear portions  522 ,  523  of a second filter, so that by use of the substantially linear portions  512 ,  522 ,  513 ,  523  of one filter which coincide with the substantially constant response portions  510 ,  610 ,  511 ,  611  of the other filter, good resolution is achieved at all wavelengths by suitable choice of periodic filter from which measurements are taken. This use of portions of the response curves of alternate filters effectively divides each of the 40 ranges of the linear response of the coarse filter, each range corresponding to a cycle of the periodic response of the fine filters, into four sub-ranges corresponding to portions of the responses  512 ,  522 ,  513 ,  523 , as best seen in  FIG. 8 . 
   A precise offset of the two periodic filters is achieved by, for example, angle-tuning, temperature-tuning, or pressure-tuning but angle-tuning is preferred. If the middle of the measurement range, for example C or L band, is chosen for this setting, deviations from 90° at extreme ends of the range do not deteriorate performance. With a frequency scale range of 100 GHz, and angle tuning of 25 GHz, to provide the 90° offset, a precision of better than 250 MHz or 2 picometer has been achieved in wavelength measurement. 
   A fifth etalon filter, not shown, may be provided for calibrating the wavelength meter. The fifth filter is chosen to have a response with peaks co-incident with either of the other etalons  44 ,  45  only at, say, every nineteenth peak transmission wavelength. Since with a frequency scale range of 100 GHz there are approximately 40 peaks of the periodic filters in the C-band, co-incident or Vernier-like maximum power from such a pair may be arranged to appear only at top, mid and bottom of the C-band in this example, that is forty 100 GHz periodic cycles occupy the 4 THz span of the C-band. 
   A block-diagram layout of an experimental wavelength meter according to the invention is shown in  FIG. 9 . This uses a PXI-card, PXI being PCI (Peripheral Component Interconnect bus) extensions for Instrumentation, an open industry instrumentation specification. Sequencing of the photodetector output power measurements is achieved by the use of a PXI feature of a very precise electrical clock signal. The designed filter responses are used to compute wavelength from the four photodetector outputs in a time faster than the actual measurement. 
   The wavelength meter includes the first, second, third and fourth  421 ,  422 ,  423 ,  424  photodetectors connected to respective photodetector circuits  911 ,  912 ,  913 ,  914  for reading outputs of the photodetectors. Each of the photodetector circuits  911 ,  912 ,  913 ,  914  is controlled by a respective programmable gain control  921 ,  922 ,  923 ,  924 . Outputs from the photodetector circuits  911 ,  912 ,  913 ,  914  are input to a multiplexer  93  and are sequenced at typically 5 microsecond delay. The wavelength meter also includes a microcontroller unit  94 , a flash RAM  95  and an analog/digital converter  96 . 
   Referring to  FIG. 10 , the photodetector circuit  911 , which is typical of all four photodetector circuits, supports an input range of 10 nanoA-100 milliA and an output range 0 to 2.5V. The circuit supports auto-ranging though digital I/O gain control  921  in the following seven steps: 
   
     
       
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Min 
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               Stage1 
               10 
               nanoA 
               100 
               nanoA 
             
             
                 
               Stage2 
               100 
               nanoA 
               1 
               microA 
             
             
                 
               Stage3 
               1 
               microA 
               10 
               microA 
             
             
                 
               Stage4 
               10 
               microA 
               100 
               microA 
             
             
                 
               Stage5 
               100 
               microA 
               1 
               milliA 
             
             
                 
               Stage6 
               1 
               milliA 
               10 
               milliA 
             
             
                 
               Stage7 
               10 
               mA 
               100 
               mA 
             
             
                 
                 
             
           
        
       
     
   
   The circuit also supports dark current and gain calibration requirements. Data input intervals are 100 micro-seconds and a channel to each photodetector  421 ,  422 ,  423  and  424  is independent. 
   Referring to  FIG. 11 , the wavelength meter may be controlled by a personal computer  111  using a PXI interface, that is the personal computer  11  is provided with a first PCI interface card  112  connected to a second PCI interface card  113  in the wavelength meter. A source (master) module  114  outputs 40,000 points to each channel  1151  on slave module  115 . The slave module  116  simultaneously read inputs from each of the photodetectors. Any one of the modules can act as master and all others are synchronised with the master module. This means that the slave module  116  needs to have a capability of storing a minimum of 160,000 data points. This measurement process takes approximately 2.5 seconds from triggering the source of 40,000 points to reading 40,000 points on each channel. In order to carry out the operation, the modules are configured for triggering with one master and multiple slaves. Memory is reserved on the slave modules RAM prior to execution. For example the source could be set-up as the master module while all other modules are slaves. The PXI mainframe into which cards are slotted using edge connectors may have an independent PC chip set. The operation is executed as follows:
         Initialise slave modules  115 ,  116 .   Slave modules  115 ,  116  are activated to accept trigger.   Master module  114  or separate triggering module initiates the operation.   Slave modules  116  start reading inputs for each channel.   Slave modules  116  store readings in ‘module’ memory  95 , i.e. 160,000 points per operation.   The slave module  116  sets a status bit indicating completion of the operation.   An application on the personal computer  111  polls the status bit until the operation is complete.   Once completed data from the slave module  116  is read from the onboard RAM  95  to the personal computer application. 2 Mbyte of RAM is provided to store data tables for stimulus and measurement data.       

   The gain of each photodetector circuits  911 ,  912 ,  913 ,  914  is automatically adjusted, by the associated programmable gain control  921 ,  922 ,  923 ,  924 , determined by a detected range in which output from the respective photodetector falls. To enable such auto-ranging, the maximum slope resolution of the photodetector circuit is 30 dB per picometer. Each measurement interval is 100 microseconds within which the card auto-ranges for the input data and reads the value through the analog to digital converter. An alternative option is to auto-range first via a log-amplifier for initial signal level assessment. 
   Conversion of photodetector power measurements to wavelength may be achieved using one or more look-up tables which store the filter characteristics or by using a mathematical function to represent filter responses. 
   The wavelength meter provides fast wavelength measurements spanning the optical ‘C’ band and/or possibly the optical ‘L’ band and/or possibly the optical S-band. In particular, the invention may be applied to visible and near IR radiation. Alternatively the wavelength meter may span some other optical band of order 4 THz. An embodiment of the instrument is based on the PXI modular instrumentation standard and occupies two PXI slots. 
   The key advantage of the invention is speed of measurement. Traditional interferometer-based wavelength measurement techniques are accurate but relatively slow (˜10 measurements/sec), the wavelength meter is less accurate but capable of much higher speed (1,000 to 10,000 measurements/sec) and has no moving parts or complicated features such as a sine-drive. 
   In one embodiment, shown in  FIG. 12 , electronics for the wavelength meter are based on an improvement on an existing PXI two-channel optical power meter (PX2000-306) available from PX Instrument Technology Ltd., Ireland. Because the wavelength meter optics require four channels of optical power to be measured simultaneously, the two channel unit requires modification to four channels. 
   The optics used for the wavelength meter require temperature control. This is provided as a plug-on module, not shown in  FIG. 12 . This will allow the wavelength meter module according to the invention to be deployed independently as a single slot module. 
   The modified design of optical power meter uses an additional PIC microprocessor  1252  and an additional programmable logic device (PLD)  124  to control the additional two channels. A 2MB memory  1263  on the existing module is shared between the 4 channels. Inputs to the modified quad optical power meter are from four InGaAs photodiodes, or other diode type suited to the waveband of interest,  1211 ,  1212 ,  1213 ,  1214  fitted in an optics box. 
   Referring to  FIG. 12 , the photodiodes, their associated analog measurements circuits and analog to digital converters are located in a shielded enclosure  1291 . Considering the first channel, the other three channels being identical to the first channel, the InGaAs photodiode  1211  has an input alternately switchably connectable to earth or a bias voltage. An output of the photodiode is alternately switchably connectable to a high current measurement circuit  1221  and a low current measurement circuit  1222 . Outputs of the high current measurement circuit  1221  and the low current measurement circuit  1222  are switchably connectable to an input of a analog digital converter  1231 . The inputs of the photodiode  1211  may be connected to earth and the output to low current measurement for measurement of the dark current and the input connected to the bias voltage and to the high current measurement to make a reading. Dark currents are measured before each reading or before each batch of readings whose size/interval is programmable. If the output level is below 1 microwatt a sequence of ten measurements is taken and averaged; this slows the measurement rate to 1000 wavelengths per second. If the output level is above 1 milliwatt a higher bias voltage is switched in to ensure linearity. 
   Outputs from the first and second channel analog digital converters  1231 ,  1232  are connected by a first serial connection to a first PIC microprocessor  1251  and outputs from the third and fourth channel analog digital converters  1233 ,  1234  are connected by a second serial connection to a second PIC microprocessor  1252 . 
   There is also a parallel data interconnection between the outputs of the analog-digital converters  1231 ,  1232 ,  1233 ,  1234  and a second programmable logic device (PLD)  124 , and parallel digital interconnections between the second PLD  124  and the first PIC microprocessor  1251  and the second PIC microprocessor  1252  respectively. 
   The first PIC microprocessor  1251  and the second PIC microprocessor  1252  are both serially connected to a first PLD  126 . The first PIC microprocessor  1251  and the second PIC microprocessor  1252  are also interconnected by a parallel data bus to the first PLD  126  and to a RAM array  1263 . The first PLD  126  includes a status/control register  1261  and a RAM address generator/control module  1262 . The first PLD  126  has an address bus output and a control bus output to the RAM Array  1263 . The RAM array is also interconnected by a parallel data bus to a 2× latch  1264  which has a data bus switch control output to the photodiode, current measurement and analog/digital converter circuits. The first PLD  126  has a first parallel bus output to local bus relays  1272  and a second parallel bus output to trigger bus buffers  1273 . The local bus arrays  1272  have serial connections to a PXI bus  1282  and the trigger bus buffers have a parallel bus connection to the PXI bus  1282 . The first PLD  126  also has a parallel bus input/output  1292  to an external trigger. 
   The first PLD  126  is interconnected by a parallel data bus to a PCI bus interface micro-controller chip (PLX 9052)  1271  for controlling data transfer via a bus. The first PLD  126  also has address bus and control bus inputs from the PLX 9052 and the PLX 9052 has a parallel bus interconnection to a PCI bus  1281 . 
   A Trigger Mode is provided to set the module into a mode in which an external trigger input is enabled, via the external trigger bus  1292 . An external trigger can be connected to the ‘Trigger In’ (pin 8) input on a 9-way ‘D’ connector on a front panel. With the external trigger mode enabled, holding this signal in a low state causes the module to wait in a measurement cycle, driving it high allows the measurement sequence to continue. This feature is used to allow synchronization with external instrumentation. 
   A side view of packaging of the wavelength meter, including the optical components shown in  FIG. 4  and the electronics components shown in  FIG. 12  is shown in  FIG. 13  and a plan view in  FIG. 14 , from which it can be seen that a can  131  containing the electronics circuit shown in  FIG. 12  is mounted on a substrate  130 . Mounted on the substrate  130  proximate one end of the can  131  is a TEC controller  132  for temperature control feedback from a thermistor to a Peltier device. Proximate an end of the TEC controller remote from the can  131  there is a first PXI edge connector  133  mounted on the substrate  130  for external connection to the analog/digital converters  1231 ,  1232 ,  1233 ,  1234 . Mounted above the first PXI connector  133  on a sub-substrate  134  is a second PXI connector  135  for external connection to an optics box  136 . The optics box  136  is mounted above the can  131  partially covering the TEC controller. The optics box  136  contains the micro-optics components shown in  FIG. 4 , namely a beam splitter, etalon filters, a coarse filter each in its own mini-package with fibre in/out; and the photodiodes. The optics box  136  is surmounted by a thermal environment substrate  139  spaced from the optics box by a thermal spacer  138 . 
   In use procedural steps to measure wavelength are as follows: 
   [1] The beam is split and the four photo-detector currents are measured rapidly. 
   [2] The linear filter output first identifies that range in the total wavelength band in which the beam wavelength lies. There are typically forty FSR ranges with four sub-ranges each in a spectral band of a C-band wavelength meter. The sub-range with its slope, rising  512 ,  522  or falling  513 ,  523  ( FIG. 8 ), is identified. 
   [3] Output from the appropriate one of the pair of in-quadrature etalon detectors is measured and the wavelength is output from that etalon&#39;s look-up table for power to wavelength conversion. 
   The procedural steps for calibration of the wavelength meter are: 
   [1] A calibration third etalon is provided having high reflectivity to give a conventional Airy function response. The third etalon also has an FSR that is in a ratio with the other etalons to provide a common maximum throughput at only three, for example, points in the spectral band of the meter. An Airy function response, as shown in  FIG. 6 , which is sharper than the responses of the fine filters, produces a more defined optical bandwidth that transmits only at the reference wavelength. 
   [2] A broadband light source provides a maximum common transmission at this reference wavelength and the pair of quadrature etalon circuits are re-calibrated to read that wavelength. 
   Alternatively, a narrowly-tunable laser may be in-built with this option that is thermally or electrically fine-tuned to the common filter reference value to provide a calibration signal for the instrument. 
   A further, more expensive, refinement incorporates a calibration fourth etalon, also of Airy type, that has an FSR that is in a ratio with the other Airy etalon to provide a common maximum throughput at only three, for example, points in the spectral band of the meter. The sharper Airy function pair produces an even more defined optical bandwidth that transmits only at a further reduced bandwidth reference wavelength. 
   Depending on the combination of three or four etalons the detectors readings are appropriately sequenced and processed from the relevant look-up tables. 
   Although the wavelength has been described using periodic filters with free spectral range of 100 GHz, where a higher resolution of the wavelength measurement is desired, 50 GHz or lower FSR etalon filters may be used, in which case the total number of sub-ranges in the spectral band of the meter, and look-up table data, will increase accordingly. 
   In order to stabilize the optics, the filter housings are maintained in close-fitting metal enclosures that have thermal sensing electronics (e.g. a thermistor) with feedback to a fan-assisted environment. An enclosed environment with Peltier temperature elements is another variation. 
   The invention provides the advantages of a measurement speed 1,000 to 10,000 faster than conventional instruments and without moving parts. The optics components include rugged etalon filters of low finesse. Using a wavelength meter according to the invention, VCSELs (vertical cavity surface emitting lasers), for example, can be calibrated at wafer stage during volume production where a wafer may carry 20,000 or more lasers. There are many other single frequency optical signals that require fast measurement, e.g. monitoring dense WDM networks, which would also benefit from the invention.