Abstract:
A method for tuning a dielectric filter comprises determining filter characteristics of a filter including a cut-on wavelength and a cut-off wavelength taken at a selected gain level. The filter is rotatably attached to a collimator assembly and a light source having a wavelength approximately equal to the cut-on frequency is applied to the filter and rotated to adjust the filter to the cut-on wavelength, and the spectral performance is measured and compared with a cut-on rating value. The method further includes applying a light signal having a wavelength set to a cut-off wavelength, to the filter rotating the filter relative to the collimator assembly, measuring spectral performance, and comparing the measured spectral performance with a cut-off rating value. The light source having a wavelength set to the cut-on wavelength is reapplied to the filter, and the difference in measured spectral performance with the rating values for each of the low cut-on wavelength and high cut-off wavelength are compared to determine if the difference is within an acceptable range. The steps are repeated until the difference is within an acceptable range and the filter is thus fixedly attached to the collimator assembly to complete the resultant filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to optical filters, and more particularly to a technique for tuning a dielectric filter in a collimator assembly. 
     2. Technical Background 
     Data communication systems increasingly are employing light signals and optical fibers for the transmission of information. The use of optical fibers allows the transmission of large amounts of data at high speeds and with low signal loss over long distances. To increase the data transmission capacity on an optical fiber, a plurality of light signals at different wavelengths are commonly multiplexed with wavelength division multiplexing for transmission through a single optical fiber so that the information is transmitted on multiple wavelengths, i.e., channels. In a wavelength division multiplexed system, the plurality of light signal sources have different wavelengths corresponding to different signal channels. After the multiple channels are transmitted through the optical fiber, the multiple wavelength signals are separated at a receiving end with the use of a demultiplexer to separate the individual signal channels. Select wavelength band filters may be used as one technique to select specific channel wavelengths. 
     In order to maximize utilization of the available bandwidth, the channel wavelength band and any wavelength separation between adjacent channels should be minimized. Accordingly, to minimize the wavelength band for each signal channel, a narrow band filter is used for accurately selecting the narrow band channel. Such filters are generally fixed, although other filters are tunable to a selected frequency. 
     One common filter tuning approach is to tune a dielectric filter in a collimator package to resolve a single spectral feature, such as a peak value. The single spectral feature is generally correlated to a specified waveform and serves as the target wavelength for resolving the angle of incidence for that filter. One disadvantage to this approach is that the accuracy for resolving the target wavelength based on a single spectral feature is generally limited such that the per channel wavelength band of the filter is limited. Inaccuracy in resolving the target wavelength may induce significant error for optimal spectral alignment of the dielectric filter. Accordingly, the accuracy of the dielectric filter&#39;s initial spectral measurement and optimizing the resulting target wavelength currently limits precise spectral alignment to a collimator assembly&#39;s optical axis. For this reason and others, it is desirable to provide a technique and structure for tuning a tunable filter with a high degree of accuracy. 
     SUMMARY OF THE INVENTION 
     The present invention provides a tunable filter and method of spectrally tuning a dielectric filter with precise tuning accuracy. To achieve this and other advantages, and in accordance with the purpose of the present invention as embodied and described herein, the present invention provides for a method for tuning a dielectric filter comprising the steps of determining filter characteristics including a cut-on wavelength and a cut-off wavelength at a selected gain level. The filter is aligned with a collimator assembly, and a light source having a wavelength approximately equal to the cut-on frequency is applied as an input to the filter. The collimator assembly is rotated relative to the filter to adjust the filter to the determined cut-on wavelength, the spectral performance is measured, and the measured spectral performance is compared with a cut-on rating value. The method further includes the steps of applying as an input to the filter a light signal having a wavelength set to a cut-off wavelength, rotating the collimator assembly relative to the filter, measuring spectral performance, and comparing the measured spectral performance with a cut-off rating value. The difference in measured spectral performance with the rating values for each of the cut-on wavelength and cut-off wavelength are compared to determine if the difference is within an acceptable range. If the difference is not within an acceptable range, the steps of applying a light signal at the cut-on and cut-off wavelengths and rotating the collimator assembly is repeated until the difference is within an acceptable range. The method advantageously tunes a dielectric filter by utilizing with the cut-on wavelength and a cut-off wavelength to provide precise filter tuning and a resultant filter with a high degree of accuracy. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings. 
     It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a filter measurement system used in the present invention for measuring wavelength characteristics of a filter; 
     FIG. 2 is a power versus wavelength diagram of one example of a target waveform for a dielectric filter; 
     FIG. 3 is a schematic and block diagram of a filter tuning system according to the present invention; 
     FIG. 4 is an enlarged schematic view of the collimator assembly shown in FIG. 3; and 
     FIGS. 5A and 5B is a flow diagram illustrating steps employed for tuning a dielectric filter according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a filter measurement system  10  is illustrated for measuring the filter characteristics of a dielectric filter  30 . The measurement system  10  includes a broadband light source  12 , such as an amplified spontaneous emission (ASE) light source, for generating a variable wavelength light signal. The measurement system  10  also includes an input Grin lens collimator  14  and an output Grin lens collimator  16  disposed on the respective input and output sides of a dielectric filter  30 . An optical fiber  13  couples light source  12  to the input of collimator  14 . The output of collimator  14  focuses the light signal onto the filter  30 , while collimator  16  receives and collects the output light beam passing through the filter  30  and transmits the signal onto optical fiber  17 . 
     A conventional optical spectrum analyzer  18  is connected to optical fiber  17  for receiving the light signal and spectrally analyzing the wavelength spectrum of the received light signal. Measurement system  10  further includes a computer  20  coupled to optical spectrum analyzer  18  for processing the measured signal data and storing the data in memory. The computer  20  may include a general purpose, commercially available computer with standard interface circuit and a digital controller and/or analog circuits to process the information. 
     The dielectric filter  30  is disposed between the input collimator  14  and output collimator  16  at a predetermined angle θ of incidence. According to the preferred embodiment, the dielectric filter  30  is arranged at an angle of incidence in the range of 1.7 to 3.3 degrees, and more preferably at an angle of approximately 1.8 degrees relative to an axis perpendicular to the output light beam of input collimator  14 . With the filter  30  arranged in place between the Grin lens collimators  14  and  16 , the broadband light source  12  is varied in wavelength through a predetermined spectrum, while the optical spectrum analyzer  18  measures the gain power in decibels (dB) for the wavelength spectrum of analysis. 
     In addition, the computer  20  has a specified target waveform stored in memory. The computer  20  receives the measured power spectrum from the optical spectrum analyzer  18  and compares the measured power spectrum to the target waveform stored in memory. Computer  20  further determines the insertion loss error function (ILEF) as the difference between the target waveform and the measured spectrum of the filter  30 . In addition, the measurement system  10  measures the −15 dB cut-on and cut-off transmission points and the center wavelength which are used as primary targets for tuning the filter&#39;s spectrum in a collimator assembly package. While the filter characteristics are measured with the measurement system  10  shown and described herein, it should be appreciated that the filter characteristics could otherwise be measured without departing from the teachings of the present invention. 
     Referring now to FIG. 2, one example of a target waveform is shown for a typical 3-cavity gain flattening filter (GFF). The gain flattening filter is specified to have a target waveform  22  as shown by the thickened solid line. The target waveform  22  is generally dependent on the application for which the filter is intended to be used. The target waveform  22  has a −15 dB cut-on wavelength at point  26  and a −15 dB cut-off wavelength at point  28  as selected at a −15 dB gain level  24 . The target waveform  22  also has a center wavelength point  27  midway between the cut-on and cut-off points  26  and  28 . The cut-on and cut-off points  26  and  28  are taken at wavelengths where the slope of gain is substantially constant to define a wavelength band therebetween. It should be appreciated that the measured waveform generally will exhibit somewhat similar characteristics, but is limited in accuracy due to the limitations of the optical spectrum analyzer. The difference between the measured power spectrum waveform and the target waveform provides the insertion loss error function which is minimized by the spectral tuning methodology of the present invention. The measured power spectrum is shifted linearly to minimize the insertion loss error function and the spectral shift is used to define whether the filter is compliant to the maximum insertion loss error function allowable. If the insertion loss error function is too great, the filter  30  is preferably not employed in the tuning methodology of the present invention, since the corresponding filter does not have characteristics desirable for tuning to within an acceptable level. 
     The accuracy of the measurement system  10  used to define the spectral characteristics of a dielectric filter is generally limited to the measurement equipment&#39;s resolution. The measured filter characteristics may be measured with the measurement system  10  by the manufacture of the dielectric filter at a location remote from where the filter tuning occurs. Accordingly, the measurement system  10  provides the measured filter characteristics in a readable form so that a user may enter the characteristics into the filter tuning system or download the characteristics via memory card. According to one example, a printout of the measured filter characteristics is provided so that a user can enter the characteristics into the tuning system. The spectral resolution of a system employing an erbium-doped amplified spontaneous emission source and an optical spectrum analyzer as explained herein may have a resolution of approximately 0.050 nanometers. While one example of a measurement system  10  and a gain flattening filter are described herein, it should be appreciated that the present invention may employ other measurement systems, and is not limited to tuning a gain flattening filter, as the filter may include other types of dielectric filters including, but not limited to, narrow band wavelength division multiplexing (WDM) filters. 
     Referring to FIG. 3, a filter tuning system  32  is illustrated for tuning a dielectric filter  30  with a collimator assembly  38  to achieve acceptable filter characteristics. The filter tuning system  32  includes a wavemeter  34  and a tunable laser source  36 . The tunable laser source  36  provides a light beam that may be adjusted to select wavelengths in response to the wavelength set by the wavemeter  34  controlled by a computer  46 . The collimator assembly  38  receives the laser light signal which is focused onto the filter  30 . The output of filter  30  is coupled to an optical detector  42  to receive the filtered light output. A power multimeter  44  is coupled to an output of the optical detector  42  and provides an output reading of the power rating in decibels (dB) which corresponds to the wavemeter  34 . The computer  46  enables a user to more easily select the desired wavemeter setpoints to select the cut-on and cut-off wavelengths. The wavemeter setpoints may be entered by a user by keying in filter characteristics provided on the printout provided from the measurement system  10 . Alternately, a memory card containing filter characteristics may be downloaded into computer  46 . Computer  46  is a commercially available computer with a digital controller or suitable analog circuits. 
     The collimator assembly  38  is shown in detail in FIG.  4 . Attached to the output end of the collimator assembly  38  is a filter holder  40 . The filter  30  is fixedly bonded in place to the filter holder  40  to be arranged at an angle θ (FIG. 3) to the perpendicular direction of the light beam output of the collimator assembly  38 . Angle θ is selected to be in the range of from about 1.7 to about 3.3 degrees and, preferably is set to approximately 1.8 degrees. The Grin lens collimator  38  has an inherent output field angle of from about 0.5 to about 1.0 degrees. As the filter holder  40  is rotated about the collimator assembly  38 , the filter  30  rotates relative to the collimated source to shift the filter&#39;s center wavelength, resulting in changing the angle of incidence. By rotating the filter  30  on the filter holder  40 , the filter  30  may be optimally tuned to provide the desired filter characteristics as explained herein. Once the filter  30  is tuned to achieve filter characteristics within an acceptable tolerance, the filter  30  and filter holder  40  are fixedly bonded to the collimator  38  to provide a tuned filter and collimator assembly that may be used in various applications to filter light signals. 
     Referring to FIGS. 5A and 5B, the method  50  of tuning a dielectric filter is provided according to the present invention is illustrated. Filter tuning method  50  includes step  52  of measuring filter characteristics including the low −15 dB cut-on wavelength, the high −15 dB cut-off wavelength, and the center wavelength. The filter characteristics may be measured with the measurement system of FIG. 1 by the filter manufacturer. It should be appreciated that the measurement of the −15 dB cut-on and cut-off wavelengths provides that the power rating corresponding thereto will be taken from a region that is substantially linear. While a −15 dB power cutoff is used herein, it should be appreciated that other cutoff powers may be used without departing from the present invention. 
     Once the cut-on, center, and cut-off wavelengths are determined, step  54  retrieves the specified target waveform spectrum analyzer  18  (FIG.  1 ), which waveform may be specific to the type of application intended for use by the filter. The method then proceeds to step  56  to determine, using computer  20  and a conventional algorithm, the insertion loss error function (ILEF) which is equal to the difference between the measured filter characteristics and the specified target waveform. Once the filter characteristics and ILEF are determined, the determined values are stored in computer memory in step  58 . 
     Once the filter characteristics and ILEF have been measured and stored in memory, the filter  30  is bonded in place, preferably with epoxy, to the filter holder as shown by in step  60 . The various components used in the filter tuning setup, including the filter and filter holder assembly, are then aged as shown by step  62  to stress relieve the epoxy bond prior to the alignment and bonding of the filter to the collimator assembly. The aging step  62  may include subjecting the components or subassemblies to ten temperature cycles with a temperature variation of between −50° C. to +85° C. According to one example, the temperature aging step  62  starts at a temperature of +85° C. for 30 minutes, then ramps down at a rate of 3 degrees per minute to a temperature of −40° C., remains at −40° C. for 30 minutes, and then repeats the temperature variation for the next cycle. The temperature cycling minimizes spectral shift of the optimized center wavelength due to stress induced alignment changes in the collimator/filter. Thereafter, the filter  30  is assembled onto the collimator assembly as shown by step  64 . At this point, the filter  30  is ready to be tuned as now described. 
     The filter tuning method  50  includes step  66  of setting the tunable laser to the low cut-on wavelength. In step  68 , the collimator assembly is then rotated relative to the filter. Each incremental amount of rotation preferably is less than ten arc seconds per degree of rotation. Rotation of the collimator assembly relative to the filter typically will be accomplished by rotating the filter holder and filter mounted thereto relative to the stationary collimator. At each incremental angle of rotation, the power is measured by multimeter  44  and compared to the cut-on power rating. The comparison can be made by the person performing the tuning process, with or without the aid of a computer, such as computer  46 . Decision block  72 , of which is performed by the tuning person or program for computer  46 , checks to see if the −15 dB power rating has been reached and, if not, returns to step  68 . If the −15 dB power rating has been reached, method  50  proceeds to step  74  to set the tunable laser to the upper cut-off wavelength. 
     With the laser set by wavemeter  34  to the cut-off wavelength, the filter is again rotated relative to the collimator assembly in step  76 . The rotation in step  76  is preferably in increments of less than ten arc seconds per degree of rotation. At each incremental angle of rotation, the power is measured by multimeter  44  and compared to the cut-off power rating in step  78 . Decision block  80  checks to see if the −15 dB power rating has been reached and, if not, returns to step  76 . If the −15 dB power rating has been reached, method  50  proceeds to return the tunable laser to the low cut-on wavelength in step  82 . 
     Next, decision block  84  checks the measured data to see whether the difference between each of the cut-on wavelength power and cut-off wavelength power and the corresponding power ratings are within an acceptable range. Accordingly, method  50  checks both the low cut-on wavelength and high cut-off wavelength at the same time to see if both power measurements are within an acceptable range. An acceptable range may include ±0.5 dB, for example, for a gain flattening filter or ±0.25 dB for a narrow band WDM filter. If either the cut-on or cut-off wavelength measurements are not within the acceptable range, method  50  returns to step  68  to repeat the rotation and measurement of power at each of the low cut-on and high cut-off wavelength. Once the difference between each of the low cut-on and high cut-off wavelength measurements and the corresponding power ratings are within the acceptable range, the filter is adequately tuned and method  50  proceeds to step  86  in which the filter holder is bonded preferably with an epoxy to the collimator assembly. Once bonded in position, the filter holder with filter and the collimator assembly are fixed relative to each other. 
     Following the bonding step  86 , the bonded filter and collimator assembly is aged in step  88  to mechanically stabilize the package. The aging step  88  may include subjecting the assembly to ten temperature cycles by varying the temperature between 50° C. to 85° C. For example, the aging step  88  may include starting at +85° C. for 30 minutes, ramping down at a rate of 3 degrees per minute to −40° C., remaining at −40° C. for 30 minutes, and repeating the next cycle. The temperature cycling minimizes spectral shift of the optimized center wavelength due to stress induced alignment changes in the collimator/filter subassembly. Following the aging step  88 , method  50  proceeds to measure the ILEF of the filter, including measuring the low −15 dB cut-on wavelength, the high −15 dB cut-off wavelength, and the center wavelength in step  90  to determine whether the bonded assembly and filter is thermally stable following the aging process. Following step  90 , the filter construction is completed and is ready for use as indicated by block  92 . 
     The field angle of the collimator  38 , the slope of the measured power spectrum taken at the −15 dB cut-off points, and resolution of the measuring equipment generally define the accuracy of the tuning methodology. The output power in terms of spectral change at −15 dB may be approximately ±0.03 dB/picometer for a 3-cavity filter design according to the example provided herein. If the spectral resolution of the measurement system is 0.005 nanometers, the resulting accuracy for tuning the filter to the target center wavelength may be less than 0.010 nanometers for a gain flattening filter. The filter tuning methodology  50  of the present invention advantageously tunes a dielectric optical filter  30  to within an acceptable range with enhanced tuning accuracy, as compared to conventional measurement systems, for tuning the filter to the target. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.