Abstract:
A system for matching a optical filter characteristic of a first filter tunable in wavelength with an optical first signal comprises a modulator for modulating at least a part of the first signal with a modulation signal before being applied to the first filter. An analyzing unit derives a control signal for tuning the first filter by analyzing the modulated first signal after passing the first filter in conjunction with the modulation signal.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to tunable filters. 
   Tunable filters, such as Fiber Fabry-Perot tunable filters (FFP-TFs), can be used in connection with laser sources e.g. for suppressing source spontaneous emission (SSE) noise representing an undesired optical power in parts of the spectrum other than the lasing frequency itself. However, the central wavelength in transmission spectrum of the tunable filter must well match with the wavelength of the laser signal in order not to unduly decrease optical power of the laser signal. The filter is therefore tuned in wavelength to ensure that the maximum of the tunable filter pass band is fixed to the frequency of the peak lasing intensity. 
   Beside an initial adjustment of the filter maximum to the laser peak frequency, a continues adjustment might be required in order to correct for mismatches between filter and laser caused e.g. by tuning the laser frequency, thermal variations in the filter, or frequency drift in the laser. 
   In U.S. Pat. No. 5,552,919, the central wavelength in the transmission spectrum of the tunable filter is periodically and minutely modulated. An error signal is generated for use in tracking the central wavelength of the filter to the wavelength of the laser by analyzing the modulated filter output. 
   Although no details of an “Automatic Laser Tracking Filter—ALTF”, as published by Micron Optics, Inc. in its “2001 Test Instruments” brochure, see http://www.micronoptics.com/altf.htm, are disclosed, it appears that the ALTF makes use of the same principle as disclosed in the aforementioned U.S. Pat. No. 5,552,919, i.e. dithering the filter characteristics for deriving a control signal to displace the filter characteristics. After passing the tunable filter, a small fraction of the optical power is tapped off and sent to a detector. The electrical signal from the detector is fed to a scan and lock circuitry. A phase lock loop (PLL) ensures that the maximum of the tunable filter pass band is fixed to the frequency of peak lasing intensity. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an improved adjustment for tunable filters. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims. 
   According to the present invention for matching an optical filter characteristic of an (optical) first filter tunable in wavelength with a first optical signal, at least a part of the first optical signal is modulated with a modulation signal before being applied to the first filter. For deriving a control signal for tuning the first filter, the modulated first optical signal after passing the first filter is analyzed in conjunction with the modulation signal. Thus, instead of modulating or dithering a filter characteristic as provided by the aforementioned U.S. Pat. No. 5,552,919, the invention derives the control signal for the tunable first filter by modulating the first optical signal, or at least a part of it, before being applied to the first filter. 
   The first optical signal is preferably frequency or wavelength modulated, preferably in a frequency/wavelength range of at least a few percent of the FWHM (Full Width Half Maximum) frequency span of the underlying first filter characteristic. 
   Preferably, a second optical signal is applied to be filtered by the first filter. 
   In a first preferred embodiment, an initial optical signal is split up, divided or otherwise separated into the first signal and the second (preferably a part in a range of 95% of the initial optical signal) signal. The second signal is travelling to the first filter, and the first signal will be modulated with the modulation signal by a modulation unit before being applied to the first filter. 
   While the modulated first signal can be provided to the first filter having the same propagation direction as the second signal, it is preferred to direct the modulated first signal to the first filter with propagation direction opposite to the second signal. In the latter case, adequate direction controllers, such as circulators or beam-splitters, are provided, preferably before and after the first filter (with respect to the propagation direction of the second signal). Thus, the modulated first signal can be passed through the first filter with opposite propagation direction as the second signal, and can be coupled out after passing the first filter without adversely affecting the source(s) of the optical signal(s). 
   Such separation of propagation directions for the second signal and the modulated first signal allows that the second signal will not be affected by the modulation, thus avoiding the disadvantage in the aforementioned U.S. Pat. No. 5,552,919 that the (main) optical signal is also modulated after passing the first filter. Preferably when coupling out only a small fraction for the first signal, the second signal substantially representing the optical signal will only be filtered as desired but not be subjected to additional noise as resulting from the modulation. 
   In one embodiment, the incoming optical signal is split up by a first beamsplitter into the second signal directly applied to the first filter and the first signal to be modulated. A second beam splitter is provided after the first filter (with respect to the second signal) in a way that the filtered second signal will be coupled out by the second beam splitter while the modulated first signal provided at a different input of the second beamsplitter than the filtered second signal, so that the modulated first signal will be applied to the first filter with opposite propagation direction than the second signal. After passing the first filter, the filtered modulated first signal will be coupled out by the first beamsplitter. 
   In a second embodiment, the first and second beamsplitters are provided as polarization dependent beamsplitters, so that the first and second signals travel with different states of polarization. This allows clearly separating the signals, so that e.g. an unwanted coupling back of the filtered modulated first signal to the source of the incoming optical signal can be efficiently avoided. 
   In a further embodiment wherein a wavelength or frequency modulation (in the following referred to as only frequency modulation) is applied, the filtered modulated signal is converted into an electrical signal, preferably by detecting the optical power e.g. using a photodetector. The electrically converted filtered modulated signal together with the modulation signal are provided to an analyzer unit for deriving the control signal. Various algorithms or methods for further signal processing, as known in the art, can be applied for achieving the control signal therefrom. Preferably, a Lock-In Amplifier can be applied for generating the desired control signal. 
   By modulating the optical signal (instead of modulating the first filter characteristic), the invention allows to achieve significantly higher control speed for tuning the first filter. This is due the fact that in such control circuits as preferably applied the lower limit of the response time of the control signal is given by the inverse of the modulation frequency. Since modulation frequencies for optical signals can easily be in the order of a few GHz, the method allows for faster response times compared to systems, where the modulation is attained by the motion of moveable parts. 
   In a second preferred embodiment, the first optical signal (to be modulated) and the second optical signal (to be filtered only) are substantially independent from each other. This is in contrast to the first preferred embodiment wherein the first and second signals are both derived from an initial optical signal. In both cases, however, the first signal represents the optical signal to adjust the characteristic of the first filter to, so that the characteristic of first filter will be adjusted to the first signal. 
   In one embodiment of the second preferred embodiment, the first and second signals are first filtered by a preset filter preset to a desired wavelength, before being modulated and/or filtered by the first filter. Preferably, the first and second signals are provided to the preset filter in opposite propagation directions. Thus, the first filter will ‘automatically’ follow the wavelength setting of the preset filter. This setup allows e.g. for setting up an optical spectrum analyzer, where the unwanted transverse modes of optical filters can be strongly suppressed, if the filter characteristics of the two optical filters are chosen to have a different transverse mode spectrum. It is clear that the invention, in particular for deriving the control signal, can be 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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). 
       FIG. 1  illustrates a first preferred embodiment according to the present invention. 
       FIGS. 2A and 2B  show preferred embodiments for the optical path as of  FIG. 1 . 
       FIG. 3  illustrates a preferred algorithm for deriving the control signal. 
       FIG. 4  illustrates a second preferred embodiment according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , an incoming optical signal  10  is split a by a splitting device  20  into a second optical signal  30  and a first optical signal  40 . The second optical signal  30  is directed via a direction controller  50  to a first filter  60 , and a filtered second optical signal  30 ′ is applied to a second direction controller  70  and coupled out as an output signal  80 . 
   The first optical signal  40  is directed to a modulation unit  90  providing a frequency modulation thereto. A modulated first optical signal  100  is directed via the second direction controller  70  to the first filter  60 , however, travelling through the first filter  60  with opposite propagation direction then the second optical signal  30 . A filtered modulated first optical signal  110  is directed by the first direction controller  50  to a photodetector  120  converting the filtered modulated first optical signal  110  into an (electrical) modulated signal  130 . 
   A modulation signal  140  (generated e.g. by a local oscillator  142 ), as provided to the modulation unit  90  for modulating the first optical signal  40 , is received together with the modulated signal  130  by an analysis unit  150 . An optional phase shifting device  145  can be applied for phase adjustment of the modulation signal  140 . The analysis unit  150  derives a control signal  160  by analyzing the modulation signal  140  together with the modulate signal  130 . The control signal  160  is then applied to the first filter  60  for tuning its filter characteristic in wavelength, so that a maximum in the filter characteristic of the first filter  60  matches with the wavelength of the optical signal  10  or the therefrom derived second optical signal  30 . 
   The term “wavelength of the optical signal” has to be interpreted here in its broadest sense and shall cover in particular a spectral peak of or in the optical signal. However, when applying adequate algorithms for deriving the control signal, the first filter always adjusts its center wavelength to the spectral peak. 
   In  FIG. 2A , the splitter  20  and the first direction controller  50  of  FIG. 1  are embodied as a first beamsplitter  250 A. Correspondingly, the second direction controller  70  is embodied as a second beamsplitter  270 A. Thus, the optical signal  10  is split by the first beamsplitter  250 A into the second optical signal  30  and the first optical signal  40 . The returning filtered modulated first optical signal  110  is split by the first beam splitter  250 A into the beam  110 A directed towards the photodetector  120  but also into a portion  110 B propagating into the direction of the source of the optical signal  10 . The second beamsplitter  270 A splits the modulated first optical signal  100  into the beam  100 A directed towards the first filter  60  and a beam  100 B. Correspondingly, the filtered second optical signal  80  is split by the second beamsplitter  270 A into the beams  80 A and  80 B, with beam  80 B propagating towards the modulation unit  90  with opposite propagation direction than the beam  100 . 
   In  FIG. 2B , the beamsplitter  250 A and  270 A are replaced by polarization dependent beamsplitters  250 B and  270 B in order to clearly separate the modulated and the unmodulated optical signals. The polarization of the incoming optical signal  10  is adjusted with respect to the s (indicated as a point e.g. for the first beam  40 ) and p (indicated as an arrow e.g. for the output beam  80 ) polarization orientation of the beamsplitter  250 A that the desired fraction ratio between the intensities of the signals  30  and  40  is obtained. Beamsplitter  270 A can now be oriented to couple a desired fraction of signal  100  back into the first filter  60 , respectively a desired fraction of signal  80  into the output. 
   A preferred mathematical model for deriving the control signal from the modulation signal and the filtered modulated signal shall now be illustrated in detail. 
     FIG. 3  shows a schematic first filter curve represented by the function A(ω), where T is the transmission and ω is a frequency in the optical range. If a monochromatic lightwave, having frequency ω, is tuned across the first filter characteristic and subsequently detected by a photodetector, the detector signal S D  is given by the following equation:
 S D (ω)= k·A (ω),  (1) 
where k is a constant, describing the detector properties. The detector response is calculated for frequency modulated light, which is modulated with a frequency Ω and a modulation amplitude Δ m . For this type of light the time dependent optical frequency is:
 ω( t )=ω 0 +Δ m  cos(Ω t ),  (2) 
   The function A(ω) then exhibits a time dependency and can be expanded in a Taylor series, which can be truncated after the first order for small modulation amplitudes Δ m : 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             A 
                             ⁡ 
                             
                               ( 
                               
                                 ω 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                           ≈ 
                           
                             
                               A 
                               ⁡ 
                               
                                 ( 
                                 
                                   ω 
                                   0 
                                 
                                 ) 
                               
                             
                             + 
                             
                               
                                 ∂ 
                                 A 
                               
                               
                                 ∂ 
                                 ω 
                               
                             
                           
                         
                          
                       
                       
                         ω 
                         0 
                       
                     
                     · 
                     
                       Δ 
                       m 
                     
                   
                   ⁢ 
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         Ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         t 
                       
                       ) 
                     
                   
                 
                 + 
                 
                   … 
                   ⁢ 
                   
                       
                   
                   . 
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   Inserting equation (3) into equation (1) and allowing for a general phase φ in the detector response leads to the approximated time varying detector signal: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         S 
                         D 
                       
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                     = 
                     
                       
                         k 
                         · 
                         
                           
                             [ 
                             
                               
                                 A 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     ω 
                                     0 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   ∂ 
                                   A 
                                 
                                 
                                   ∂ 
                                   ω 
                                 
                               
                             
                              
                           
                           
                             ω 
                             0 
                           
                         
                         · 
                         
                           Δ 
                           m 
                         
                       
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             
                               Ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               t 
                             
                             = 
                             φ 
                           
                           ) 
                         
                       
                     
                   
                   ] 
                 
                 . 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Now this signal can be mixed with a reference signal of the form
 
 S   Reƒ   =B ·cos(Ω t ),  (5)
 
where B is an arbitrary amplitude. The mixing result of eqs. (4) and (5) is as follows:
 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         S 
                         D 
                       
                       · 
                       
                         S 
                         
                           Re 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           f 
                         
                       
                     
                     = 
                     
                       
                         B 
                         · 
                         k 
                         · 
                         
                           
                               
                             
                               
                                 
                                   [ 
                                   
                                     
                                       
                                         A 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             ω 
                                             0 
                                           
                                           ) 
                                         
                                       
                                       · 
                                       
                                         cos 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             Ω 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             t 
                                           
                                           ) 
                                         
                                       
                                     
                                     + 
                                     
                                       
                                         1 
                                         2 
                                       
                                       ⁢ 
                                       
                                         
                                           ∂ 
                                           A 
                                         
                                         
                                           ∂ 
                                           
                                             ω 
                                             0 
                                           
                                         
                                       
                                     
                                   
                                    
                                 
                                 
                                   ω 
                                   0 
                                 
                               
                               ⁢ 
                               
                                 Δ 
                                 m 
                               
                               ⁢ 
                               
                                 cos 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       2 
                                       ⁢ 
                                       Ω 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       t 
                                     
                                     + 
                                     φ 
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 1 
                                 2 
                               
                               ⁢ 
                               
                                 
                                   ∂ 
                                   A 
                                 
                                 
                                   ∂ 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                              
                           
                           
                             ω 
                             0 
                           
                         
                       
                       ⁢ 
                       
                         Δ 
                         m 
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       φ 
                     
                   
                   ] 
                 
                 . 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   Filtering the mixing signal of equation (6) by means of a low pass filter, which transmits only frequencies smaller than Ω, one obtains 
   
     
       
         
           
             
               
                 
                   
                     
                       S 
                       Filter 
                     
                     = 
                     
                       
                         B 
                         · 
                         
                           
                             k 
                             [ 
                             
                               
                                 1 
                                 2 
                               
                               ⁢ 
                               
                                 
                                   ∂ 
                                   A 
                                 
                                 
                                   ∂ 
                                   
                                     ω 
                                     0 
                                   
                                 
                               
                             
                              
                           
                           
                             ω 
                             0 
                           
                         
                       
                       ⁢ 
                       
                         Δ 
                         m 
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       φ 
                     
                   
                   ] 
                 
                 . 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   Since (7) is proportional to the first derivative of A with respect to ω, it can be readily used as control signal  160  for the optical first filter  60 . Appropriate adjustment of the phase φ allows for setting the sign of the signal, respectively allows for maximizing it. 
     FIG. 4  illustrates a second embodiment of the present invention. Whereas FIGS.  1 – 2  derive the first optical signal  40  from the initial signal  10 , the first optical signal  40  in the embodiment of  FIG. 4  is independent from the initial signal  10 . In any case, the first optical signal  40  represents the optical signal to adjust the characteristic of the first filter  60  to. Circuit  400  as the right part of  FIG. 4  therefore substantially corresponds with  FIG. 1 , however already starting with the first and second optical signals  40  and  30 . 
   The circuit  400  also works in accordance with  FIG. 1 , so that the characteristic of first filter  60  will be adjusted to the first optical signal  40  (i.e. the characteristic of the first filter  60  matches with the first optical signal  40 ) as illustrated above. 
   Circuit  410 —as the left part of FIG.  4 —illustrates a specific example for generating the first and second optical signals  40  and  30  to be applied then to the right part of  FIG. 4 , as explained above. In the circuit  410 , a preset filter  420  is preset to a desired wavelength λ D . The optical input signal  10  is filtered according to the filter characteristics of the preset filter  420 . The reverse path through the preset filter  420  is illuminated by a beam  430 , preferably provided by an optical broadband source. The filtered beam  430  represents the first optical signal  40  and exhibits the characteristics of the preset filter  420 . Direction controllers  440  and  450  can be applied as explained above. 
   The first optical signal  40  is now frequency modulated and can be fed through the reverse path of the first filter  60 . The detection scheme then just follows the procedure already discussed above. Thus, first filter  60  will ‘automatically’ follow the wavelength setting λ D  of the preset filter  420 . 
   This setup of  FIG. 4  allows e.g. for setting up an optical spectrum analyzer, where the unwanted transverse modes of optical filters can be strongly suppressed, if the filter characteristics of the two optical filters  420  and  60  are chosen to have a different transverse mode spectrum. 
   In case of an application of  FIG. 4  as optical spectrum analyzer, the optical input signal  10  represents the signal to by analyzed. The beam  430  is provided by an optical broadband source in order to ensure that the first optical signal  40  with wavelength setting λ D  will be present (even if the optical input signal  10  does not contain that setting λ D ).