Abstract:
A wavemeter ( 30 ) comprises a first wavelength determination unit ( 40 ) having a substantially periodic wavelength dependency and being adapted for providing a reference wavelength dependency ( 100 ) over a reference wavelength range. A second wavelength determination unit ( 50 ) has a substantially periodic wavelength dependency and is adapted for providing a second wavelength dependency ( 140 ) over a second wavelength range ( 120 ). An evaluation unit ( 60 ) compares the second wavelength dependency ( 140 ) with the reference wavelength dependency ( 100 ) for adjusting ( 160 ) the second wavelength dependency ( 140 ) in wavelength.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to determining the wavelength of optical signals.  
           [0002]    Determining the wavelength of optical signals is common object in optical applications. A description of the most common principles is given in EP-A-1099943 and the teaching thereof is incorporated herein by reference.  
           [0003]    Accuracy and wide wavelength application range represent contravening requirements to wavemeters. Generally, wavemeters either provide high-accuracy over a limited wavelength range, such as absolute wavelength references (e.g. gas cells) as disclosed e.g. in U.S. Pat. No. 5,780,843, or wavemeters show a wide applicable wavelength range with limited accuracy, such as EP-A-875743.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the present invention to provide an improved wavemeter. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.  
           [0005]    A wavemeter according to the present invention comprises two interferometric wavelength determination units and an evaluation unit. In a first operation mode, the first wavelength determination unit is employed to determine a reference interferogram over a reference wavelength range. The reference wavelength range preferably represents the entire applicable wavelength range for the wavemeter or covers at least one known wavelength event. The reference interferogram is preferably provided with reduced wavelength accuracy in order to limit the data volume required.  
           [0006]    In a second operation mode, each of the first and second wavelength determination units will be employed to provide an interferogram over a measurement wavelength range. The measurement wavelength range is preferably selected to be smaller than the reference wavelength range in order to limit the volume of the data acquisition.  
           [0007]    In a preferred embodiment, the interferogram of the first wavelength determination unit is obtained with substantially the same accuracy as the reference interferogram, while the second wavelength determination unit will obtain an interferogram with higher (preferably substantially higher) accuracy.  
           [0008]    The evaluation unit receives the interferograms from the first and second wavelength determination units and compares the first interferogram from the first wavelength determination unit with the reference interferogram in order to determine a phase offset therebetween. The determined phase offset is then employed to adjust the second interferogram provided by the second wavelength determination unit. Thus, the higher accuracy interferogram provided by the second wavelength determination unit can be calibrated by the reference interferogram as provided by the first wavelength determination unit.  
           [0009]    In a preferred embodiment, a wavelength reference point is employed for determining the phase offset between the first and the reference interferogram. The wavelength reference point identified in both the first and the reference interferograms thus leads to the phase offset therebetween. The wavelength reference point can be e.g. a start point of the wavelength range of the first interferogram or any other identifiable wavelength point. Preferably, the wavelength reference point is already known (e.g. provided by a signal source providing the wavelength signal as input for the wavemeter), but might also be determined by the wavemeter preferably employing a coarse wavelength determination unit as disclosed in the European Patent Application No. 0117607.2.  
           [0010]    In a preferred embodiment, the reference interferogram is calibrated by at least one absolute wavelength feature covered in the reference wavelength range. In this case, the higher relative accuracy interferogram provided by the second wavelength determination unit can be absolutely calibrated by the reference interferogram. This is preferably accomplished making use of an absolute wavelength reference unit, such as a gas cell, as disclosed e.g. in the European Patent Application No. 01109135.2 by the same applicant. The teaching of that document with respect to the calibration of measurement results making use of absolute wavelength references shall be incorporated herein by reference.  
           [0011]    Thus, the generally limited applicable wavelength range of absolute wavelength reference units can be expanded virtually to and is limited only by the accuracy of the first wavelength determination unit.  
           [0012]    In a preferred embodiment, the first operation mode (providing the reference interferogram) and the second operation mode (providing the first and second interferograms in the measurement wavelength range) are performed within such time interval wherein the environmental conditions for both measurements can be regarded as substantially equal or constant.  
           [0013]    In another preferred embodiment, the first and second operation modes are combined as one wavelength sweep. Thus, the reference interferogram and the first interferogram coincide, and the measurement wavelength range coincides with the reference wavelength range. In this case, no phase offset has to be determined (or in other words, the phase offset is automatically zero), and the second interferogram is automatically adjusted to the reference interferogram.  
           [0014]    The wavemeter of the present invention can be employed e.g. for monitoring a wavelength sweep of a light source tunable in wavelength or for determining discrete wavelength values. In the former case, the wavemeter preferably determines the variation of wavelength over the time, which might be employed for correcting measurement results as disclosed in detail in the aforementioned European Patent Application No. 01109135.2 by the same applicant. The teaching of that document with respect to measurement correction shall be incorporated herein by reference. In the latter case, a discrete wavelength value can be received by using a start value for determining the period covering the wavelength value and then detecting the actual wavelength value within that period as described in detail e.g. in the aforementioned EP-A-875743.  
           [0015]    It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. In particular the signal processing as provided by the evaluation unit can be embodied using software. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
         [0017]    [0017]FIG. 1 shows an improved embodiment of a wavemeter according to present invention in a measurement setup.  
         [0018]    [0018]FIG. 2 illustrates the wavelength determination according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    In FIG. 1, a tunable laser source  10  provides a laser signal λ(t) with varying wavelengths over time. The laser signal λ(t) is provided to a device under test (DUT)  20  as well as to a wavemeter  30 . The wavemeter  30  comprises first and second wavelength determination units  40  and  50 , each receiving the laser signal λ(t) and providing an output to an evaluation unit  60  of the wavemeter  30 . A measurement unit  70  receives a response signal from the DUT  20  on the laser signal λ(t) as well as an output signal from the evaluation unit  60  (which also represents an output of the wavemeter  30 ).  
         [0020]    In operation (as illustrated in FIG. 2), the tunable laser source  10  provides a wavelength sweep λ(t) over a wide wavelength range (in the example of FIG. 2, over a wavelength range from 1520 to 1620 nm). The first wavelength determination unit  40  provides a reference interferogram  100  having a period p r  over the swept wavelength range. In the example of FIG. 2, the first wavelength determination unit  40  provides the interferogram with the period p r  of roughly 4 pm and a wavelength increment of about 1 pm.  
         [0021]    In a preferred embodiment, the reference interferogram  100  is calibrated making use of absolute wavelength features  110  as provided e.g. by a gas cell  115 . Details of such calibration process are disclosed in detail in the co-pending European Patent Application No. 01109135.2 by the same applicant. The wavelength sweep is preferably selected to cover the gas cell spectrum  110  and a desired wavelength span  120  for later testing the DUT  20 . The period p r  and the phase of the reference interferogram  100  can be calculated very accurately and calibrated by the absolute gas cell features  110 . Thus, the wavelength behavior of the reference interferogram  100  can be very accurately known also within the DUT span  120 .  
         [0022]    A second wavelength sweep is performed, preferably directly after the first wavelength sweep in order to make sure that environmental conditions possibly affecting the period of the first wavelength determination unit  40  remains substantially constant. In this second wavelength sweep, the tunable laser source  10  will provide a wavelength sweep λ(t) over the DUT span  120 . The DUT span  120  represents such measurement range for actually measuring wavelength properties of the DUT  20 . During the second wavelength sweep, the first wavelength determination unit  40  provides a first measurement interferogram  130 , and the second wavelength determination unit  50  provides a second measurement interferogram  140 .  
         [0023]    The first and second measurement interferograms  130  and  140  are preferably sampled with increased accuracy with respect to the first wavelength sweep for determining the reference interferogram  100 . In the example of FIG. 2, the wavelength increment for detecting the interferograms  130  and  140  are provided with about 0.05 pm (in contrast to about 1 pm for the reference interferogram  100 ). Accordingly, only the second measurement interferogram  140  might be sampled with increased accuracy with respect to the reference interferogram  100 .  
         [0024]    While the period p r  of the first wavelength determination unit  40  remains substantially the same during both wavelength sweeps (interferograms  100  and  130 ), a period p 2  of the second wavelength determination unit  50  is preferably selected to be smaller than the period p r  of the first wavelength determination unit  40  in order to provide improved measurement accuracy. In the example of FIG. 2, the period p 2  is roughly 0.2 pm (in contrast to the period p r  of roughly 4 pm).  
         [0025]    Differences in the period p r  between the two wavelength sweeps might arise from different measuring conditions during the wavelength sweeps and might be compensated or simply neglected dependent on the required accuracy.  
         [0026]    The evaluation unit  60  receives the measurement results of both wavelength determination units  40  and  50  for both wavelength sweeps. In case that a wavelength start point  150  of the DUT span  120  is not already known (e.g. as provided by the tunable laser source  10 ), the wavemeter  30  might further comprise a coarse wavelength determination unit  200  for unambiguously determining the wavelength of the signal λ(t) and thus the wavelength start point  150  of the tunable laser source  10 . Preferably, the coarse wavelength determination unit  200  determines the wavelength with lower accuracy then the first and second wavelength determination unit  40  and  50 , and might be embodied as disclosed in the aforementioned EP-A-1099943 (which teaching with respect to the coarse wavelength measurement shall be incorporated herein be reference).  
         [0027]    The wavelength start point  150  preferably represents the first (valid) wavelength point of the DUT span  120 . However, different wavelength points can be applied accordingly.  
         [0028]    The evaluation unit  60  will then identify the wavelength start point  150  inside one period of the first measurement interferogram  130  as well as inside one period of the reference interferogram  100 . The evaluation unit  60  then determines a phase offset as the phase difference between the first measurement interferogram  130  and the reference interferogram  100  at the wavelength start point  150 . This determined phase offset then allows adjusting the second measurement interferogram  140  in wavelength (i.e. along the wavelength axis in FIG. 2 as indicated by arrow  160 ).  
         [0029]    The evaluation unit  60  and the measurement unit  70  are preferably synchronized, so that the measurement unit  70  can associate to each received response signal from the DUT  20  a corresponding wavelength value of the stimulus laser signal.  
         [0030]    In case that the reference interferogram  100  has been calibrated making use of the absolute wavelength features  110 , the high accuracy of the absolute wavelength feature  110  can be transferred via the first and second wavelength determination units  40  and  50  also to the DUT spectrum range  120 . Thus, the second wavelength determination unit  50  can provide a relative error smaller than 0.02 pm and an absolute error smaller than 0.4 pm resulting in a total error smaller than 0.5 pm in the DUT spectrum range  120 . This can be achieved with the first wavelength determination unit  40  having a relative error smaller than 0.2 pm and an absolute error smaller 0.2 pm as calibrated by an HCN gas cell (with spectrum  110 ), whereby the tunable laser source  10  provides the wavelengths start point  150  with a relative error of 3-4 pm and an absolute error of 3 pm.