Abstract:
A temperature compensated optical filter assembly including a plurality of thin films having temperature dependent indices of refraction which are deposited on a glass substrate so as to form a conventional interference filter thereon. The glass substrate is adhesively coupled to a metal holder such that the deposited thin film interference filter is interposed between the glass substrate and an adhesive layer distributed along a mounting surface of the holder. Thus, a first thermal mismatch stress is applied by the glass substrate onto an inner layer of the interference filter and a second mismatch stress is applied by the holder onto an outer layer of the interference filter, wherein the first and second mismatch stresses depend on the temperature of the filter assembly. The glass substrate, the adhesive, and the holder are preferably formed of materials having mechanical properties such that the first and second mismatch stresses compensate for the effects of the temperature dependent indices of refraction of the thin films so as to uniformly maintain the spectral performance of the filter assembly in response to a change in temperature. In one embodiment, an additional degree of freedom is obtained by electro/magneto-strictively prestressing the thin film stack during the formation thereof. In another embodiment, an active stress management system is utilized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical filters and, in particular, relates to temperature compensated optical filter assemblies having a thin film interference filter sub-assembly deposited therein. 
     2. Description of the Related Art 
     Optical filters are commonly used in science and industry to selectively attenuate optical signals according to frequency. For example, communication systems which transmit multiplexed optical signals having a plurality of frequency components along a single optical fiber often rely on optical filters to demultiplex the transmitted signal. In particular, an optical filter adapted to substantially attenuate or reflect all but a narrow frequency band, commonly known as a band pass filter, allows the corresponding frequency component to be isolated from the transmitted optical signal so that the information carried by the isolated frequency component can be subsequently processed in a non-interfering manner. 
     A typical optical filter assembly comprises a glass substrate, an interference filter deposited on the glass substrate and a holder that supports the substrate. In particular, the glass substrate structurally supports the interference filter, which comprises a plurality of thin films deposited in a layered manner over a surface of the glass substrate. Furthermore, the holder couples with another surface of the glass substrate, wherein the two surfaces are on opposite sides. 
     When an input broadband optical signal is directed so as to be incident upon the interference filter, the interference filter selectively attenuates the signal by exploiting the well known principles of reflection, refraction, and interference. In particular, the input signal is initially subdivided into multiple broadband constituents as the signal undergoes reflection and refraction at each layer of the interference filter. Thus, each constituent travels along a unique optical path length, defined hereinbelow as the product of the physical path length times the index of refraction of the refractive medium, so that the frequency components of each constituent undergo frequency dependent phase changes. Furthermore, after traveling through the varying optical path lengths within the refracting medium of the filter, the subdivided constituents that eventually exit the rear layer recombine in an interfering manner to produce a transmitted filtered output signal. Likewise, the light energy that exits the front layer recombines to form a reflected filtered output signal. 
     Thus, the filtering aspects are determined by the thickness and index of refraction of each of the thin films of the interference filter, and the incident angle of the input signal with respect to the interference filter. Consequently, the interference filter may be adapted to perform virtually any specific filtering operation, such as band pass filtering or band rejection filtering, using appropriately dimensioned thin films having appropriate refractive indices. Moreover, the interference filter may operate both as a reflecting device as well as a transmitting device such that the reflected and transmitted signals are complementary to each other. 
     However, known optical filter assemblies are often sensitive to a change in temperature. In particular, because variations in temperature alter the properties of the thin films, the indices of refraction and the thicknesses of the thin films typically vary in response to a change in temperature. Furthermore, because the glass substrate and the interference filter usually have different coefficients of thermal expansion, the glass substrate usually exerts a thermal mismatch stress onto the deposited thin films that often causes the thin films to experience further temperature dependent changes in thickness. Thus, because the filtering characteristics depend on the indices of refraction and thicknesses of the thin films, a change in temperature often changes the filtering characteristics of the filter. 
     Consequently, known filter assemblies having substantial temperature dependencies may limit the performance of optical systems that rely on such devices. In particular, the temperature dependent filtering characteristics of known filter assemblies may limit their ability to consistently transmit one signal having a first frequency range while consistently attenuating or reflecting another signal having a second frequency range. Because these devices are often placed in environments having substantially changing temperature conditions, substantial allowances may be required in the design of optical systems that utilize such devices to compensate for the foregoing temperature dependency. 
     For example, in the case of the multiplexed fiber optic communications system mentioned above, the required frequency spacing between each of the frequency components of the transmitted signal may need to be relatively large so as to accommodate the temperature dependent spectral performance of the filter assembly. Because the maximum number of simultaneous signals that can be transmitted along a single optical fiber is directly related to the minimum frequency spacing, the temperature dependent filter assembly will likely limit the number of simultaneous signals that can be transmitted through the fiber optic cable. 
     The typical solution used in the industry to reduce the forgoing problem of temperature dependency is to deposit the interference filter on a compensating glass substrate. In particular, the material of the glass substrate is chosen so that the thermal mismatch stress exerted by the substrate onto the thin films induces the thicknesses of the thin films to change such that the filtering characteristics of the filter have a reduced sensitivity to a change in temperature. However, although this approach can be used to reduce the thermal dependency of the filtering characteristics, substantial thermal dependencies often remain. Furthermore, because the compensating glass substrate is typically formed of relatively expensive glass materials, such optical filters are relatively expensive to produce. 
     Therefore, from the foregoing, it will be appreciated that there is a need for an optical filter assembly having a spectral response that is less affected by a change in temperature. To this end, there is a need for an optical filter assembly that is able to further reduce thermally induced changes in the optical pathlengths of the filter. Furthermore, there is a need for the device to be constructed in a simple manner so that it can be inexpensively produced. Moreover, there is a need for the device to be formed with a small size so as to be usable in space constrained fiber optic systems. 
     SUMMARY OF THE INVENTION 
     The aforementioned needs are satisfied by the optical apparatus of the present invention. According to one aspect of the invention, the optical apparatus comprises an optically transmissive substrate, a holder, and an interference filter deposited on a surface of the substrate and bonded to a surface of the holder. Where, the substrate and holder each have different coefficients of thermal expansion selected to apply compensating stresses on said filter over a range of temperatures. In one embodiment, the holder and filter are bonded together by an adhesive, said adhesive partially isolating said filter from stresses caused by thermally induced dimensional changes of the holder. In another embodiment, the filter is comprised of layers of electro/magneto-strictive material. 
     Another aspect of the invention includes an optical apparatus comprising an input port for receiving light along a light path and an interference filter comprised of layers of electro/magneto-strictive material. The filter is disposed in said path such that light impinges on said layers. An electric/magnetic field generator is further included for applying an electric/magnetic field to said material. Preferably, a controller is included for controlling the electric/magnetic field generator. 
     Yet another aspect of the invention comprises a method of stabilizing the filtering characteristics of an interference filter. The method comprises applying plural thermally dependent stresses to said interference filter so as to reduce the deviation of said interference filter from a desired filtering characteristic in response to a change in temperature. 
     In one embodiment, the method of stabilizing the filtering characteristics of an interference filter further comprises creating the plural stresses mechanically. In another embodiment, the method further comprises creating at least one of the plural stresses utilizing an electric/magnetic field. 
     The optical apparatus of the preferred embodiments filters an input optical signal such that the filtering characteristics are substantially unaffected by a changing temperature. These and other advantages of the preferred embodiments will become more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a side elevation view of a temperature compensated filter assembly of an embodiment of the present invention; 
     FIG. 1B is a magnified view of the filter assembly of FIG. 1A; 
     FIG. 2 is a side elevation view of a filter assembly of an embodiment of the present invention that utilizes electro/magneto-strictive materials; 
     FIG. 3 is a side elevation view of a fiber optic junction assembly that utilizes the filter assemblies of FIGS. 1A and 2; 
     FIG. 4 is a side elevation view of a filter assembly of an embodiment of the present invention that includes an active stress management control system; and 
     FIG. 5 is a block diagram which schematically illustrates the control system of the filter assembly of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made to the drawings wherein like numerals refer to like parts throughout. In particular, FIG. 1A illustrates an optical apparatus  30  that provides improved filtering characteristics in response to a changing temperature. Specifically, the apparatus  30  selectively attenuates an input optical signal  32  according to a frequency dependent attenuation curve such that the attenuation curve remains substantially unchanged while the apparatus  30  undergoes a change in temperature. As will be described in greater detail below, improved thermal compensation is provided by the application of a first and second thermal mismatch stress on an interference filter  46  of the optical apparatus  30 . 
     As shown in FIG. 1A, the optical apparatus  30  comprises a holder  34  having a mounting surface  36 , an optically transmissive substrate  40  having opposed first and second surfaces  42  and  44 , and an interference filter  46  interposed between the substrate  40  and the holder  34 . The first surface  42  of the substrate  40  acts as an input port such that light entering therein is selectively attenuated by the optical apparatus  30 . The interference filter  46 , which provides a substantial portion of the filtering capabilities of the optical apparatus  30 , is deposited on the second surface  44  of the substrate  40  and adhesively bonded to the mounting surface  36  of the holder  34  so as to couple the substrate  40  with the holder  34 . Thus, as will be described in greater detail below, the temperature dependent first and second thermal mismatch stresses are respectively exerted on the interference filter from the substrate  40  and from the holder  34 . 
     As shown in FIG. 1A, the optical apparatus  30  is preferably positioned so as to receive the input optical signal  32  at the first surface  42  of the substrate  40  such that the input signal  32  impinges on the first surface  42  along a path substantially perpendicular to the surface  42 . The input signal  32  then passes through the substrate  40  and exits the substrate  40  at the second surface  44 . The input signal  32  then enters the interference filter  46 , wherein the input signal  32  is converted into a reflected output signal (not shown) that returns into the substrate  40  and a transmitted output signal  54  that exits away from the substrate  40 . 
     However, it will be appreciated that the optical apparatus  30  would function in a similar manner if it were rotated by  180  degrees. In particular, the optical apparatus  30  could be positioned so that an input optical signal having a direction opposed to that of the input signal  32  of FIG. 1A impinges directly on the interference filter  46  prior to entering the substrate  40 . Thus, in this alternative configuration, a reflected output signal would be directed away from the substrate  40  and a transmitted output signal would enter the substrate  40 . 
     As shown in FIG. 1A, the holder  34  is formed with an opening  68  which extends from the mounting surface  36  so as to enable the output signal  54  to pass therethrough, as indicated by the dashed lines in FIG.  1 A. Moreover, the holder  34  may be pivotally mounted to any suitable mounting structure so as to provide a method of adjusting the frequency response of the optical apparatus  30 . 
     As shown in FIG. 1B, the interference filter  46  is comprised of a plurality of optically transmissive thin film layers  60  deposited over the optically transmissive substrate  40  in a manner known in the art. In particular, the plurality of thin film layers  60  includes a front film layer  62  deposited on the second surface  44  of the substrate  40 , a rear film layer  64  deposited distally from the front layer  62 , and inner film layers  66  deposited therebetween. Moreover, since the substrate  40  and filter  46  have different coefficients of thermal expansion, the first thermal mismatch stress is exerted by the substrate  40  onto the front layer of the filter  62 . 
     As shown in FIGS. 1A and 1B, an adhesive layer  70  is flushly interposed between the rear layer  64  of the interference filter  46  and the mounting surface  36  of the holder  34 . In particular, the adhesive layer  70  is chosen so as to securely couple the interference filter  46  with the holder  34 . Thus, since the thermal coefficient of expansion of the holder  34  is different from that of the filter  46 . the holder  34 , indirectly, exerts the second thermal mismatch stress onto the rear layer  64  of the filter  46  through the adhesive  70 . 
     As mentioned previously, some prior art interference filters realize a degree of temperature compensation by adapting a single thermal mismatch stress applied by a supporting substrate onto the filter so that thermally induced changes in the optical pathlengths of the filter are reduced. However, this requires that the substrate be formed of material having a narrowly defined thermal coefficient of expansion. Thus, this method often results in insufficient temperature compensation and often requires the substrate to be formed of relatively expensive material. 
     However, in the preferred embodiment, improved thermal compensation is realized by applying the second mismatch stress in combination with the first mismatch stress. In particular, the apparatus  30  is adapted so that the second mismatch stress further modifies the thicknesses of the films  60  of the filter  46  so as to further reduce thermally induced changes in the optical pathlengths of the films  60 . Thus, the additional degree of freedom provided by the second stress allows the apparatus  30  to be formed from materials having less narrowly defined thermal coefficients of expansion, thereby enabling the apparatus  30  to be constructed at a reduced cost. 
     In one embodiment, the holder  34  is formed of material having a thermal coefficient of expansion that provides the holder  34  with an appropriate dimensional change in response to a change in temperature. Furthermore, since the second stress is developed through the adhesive  70 , the physical dimensions and elastic properties of the adhesive  70  also determine the magnitude of the second stress. Thus, for example, a highly elastic thick adhesive  70  would result in a relatively small second stress being applied on the filter  46 , whereas a non-elastic thin adhesive  70  would result in a relatively large second stress. Therefore, by forming the holder  34  with the appropriate thermal coefficient of expansion and by forming the adhesive  70  with an appropriate thickness and elastic modulus, the holder  34  can be adapted to develop the second stress onto the filter  46  through the adhesive  70  so that the thermally induced variations in the filtering characteristics of the filter  46  are relatively small. Thus, since the second stress is defined by the combination of the coefficient of thermal expansion of the holder  34  and the physical dimensions and elastic properties of the adhesive  70 , the second stress can be developed with a temperature dependent magnitude that falls within a substantially large range of values. 
     In one embodiment of the optical apparatus  30  of FIG. 1A, the substrate  40  is poorly matched with the interference filter  46 . In particular, the substrate  40  exerts the first stress onto the interference filter so as to adversely affect the interference filter in response to a change in temperature. However, in this embodiment, the holder  34  and the adhesive  70  are adapted so that they develop the second stress in such a way as to counteract the effects of the first stress. 
     In another embodiment, an additional method of stress compensation is utilized. In particular, inherent restrictive properties of some filter materials are exploited so as to provide an additional degree of freedom in the effort to regulate the filtering characteristics of the optical apparatus. Specifically, it is well known in the art that some materials exhibit restrictive properties such that the dimensions of these materials may be altered when exposed to an appropriate field. For example, electro-strictive materials are affected by an externally applied electric field whereas magneto-strictive materials are affected by an externally applied magnetic field. 
     In this application, electro/magneto-strictive materials are referred to hereinbelow as those materials that exhibit electro-strictive properties and those materials that exhibit magneto-strictive properties. Furthermore, an electric/magnetic field is referred to hereinbelow as an electric field, in the case that the electro-strictive properties are exploited, or a magnetic field, in the case that the magneto-strictive properties are exploited. 
     Since it has been determined that some refractive materials exhibit electro/magneto-strictive properties, it is possible to form an interference filter from such materials. Thus, as will be described in greater detail below, such an interference filter can be adapted with filtering characteristics that are influenced by an externally applied electric/magnetic field. 
     Reference will now be made to FIG. 2, which generally illustrates one embodiment of an optical apparatus  130  that utilizes electro/magneto-strictive materials, such as electro/magneto-strictive oxides, to further regulate its filtering characteristics. In particular, the optical apparatus  130  is substantially similar to the optical apparatus  30  of FIG. 1A except that layers  160  of an interference filter  146  are formed of electro/magneto-strictive oxide material. 
     In one embodiment, an adhesive  170  is prestressed during the formation of the optical apparatus  130 . In particular, subsequent to the formation of the interference filter  146 , the interference filter  146  is exposed to a suitable electric/magnetic field so as to modify the elongated dimensions of the interference filter  146 . The adhesive layer  170  is then deposited and cured in a UV process. The electric/magnetic field is then removed so that the elongated dimensions of the interference filter  146  substantially return to their initial values. Thus, the adhesive  170  experiences a corresponding dimensional change that results in the adhesive  170  being prestressed. 
     In one embodiment, the filter  146  is continuously exposed to an electric/magnetic field  180 . In particular, the optical apparatus  130  further comprises an electric/magnetic field generator  182  that is adapted to produce the electric/magnetic field  180 . Thus, in addition to the first and second mismatch stresses respectively applied by the substrate  40  and holder  134  onto the interference filter  146 , the electro/magneto-strictively induced dimensional changes provide another compensating mechanism for thermally compensating the filter  146 . 
     As indicated in FIG. 2, the electric/magnetic field generator  182  is preferably positioned within the holder  134 . However, in another embodiment, the field generator  182  could be positioned at another location, provided that the electric/magnetic field generated therefrom is sufficient to effectively manipulate the electro/magneto-strictive interference filter  146 . Furthermore, the electric/magnetic field generator  182  may comprise a plurality of charged plates so as to produce an appropriate electric field or a current carrying wire so as to produce an appropriate magnetic field. 
     In one embodiment, the optical apparatus further includes an active control system  184  that comprises a controller  186  and a temperature sensor  188  as indicated in FIG.  2 . The controller  186  is adapted to control the electric/magnetic field generator  182  in a manner known in the art. Furthermore, the controller  186  is adapted to monitor the temperature sensor  188  so that the controller  186  can vary the electric/magnetic field in a temperature dependent manner so as to more effectively maintain uniform filtering characteristics. 
     Reference will now be made to FIG. 3 which illustrates a thermally compensated fiber optic filter apparatus  190 . In particular, the filter apparatus  190  may comprise either of the thermally compensated filter assemblies  30  and  130  of FIGS. 1 and 2 so as to enable an input signal  191  from a first optical fiber  196  to be reliably converted into a filtered output signal  193  which is transmitted along a second optical fiber  198 . 
     As shown in FIG. 3, the apparatus  190  further comprises a first light guide  192  that optically couples with the first fiber  196  and a second light guide  194  that optically couples with the second fiber  198 . The light guide  192  includes a graded index lens  195  that focuses the input signal  191  into a central region  199  of the apparatus  190 . Furthermore, the light guide  194  includes a graded index lens  197  that focuses the output signal  193  as it exits the central region  199 . 
     As shown in FIG. 3, the filter assembly  30 ,  130  is positioned in the central region  199  of the apparatus  190  so as to be interposed between the first and second light guides  192  and  194  so that the input signal  191  from the first fiber  196  is directed toward the filter assembly  30 ,  130 . In particular, the holder  34 ,  134  is mounted to the second light guide  194  so that the first surface  42  of the substrate  40  faces the first light guide  192 . Moreover, the transmitted output signal  193  from the filter assembly  30 ,  130  is directed through the second light guide  194  so that it continues into the second fiber  198 . 
     As shown in FIG. 3, the apparatus  190  further comprises an inner housing member  200  and an outer housing member  202 . The inner housing member  200  encloses the first and second light guides  192  and  194  so as to maintain the first and second light guides  192  and  194  in a preferred alignment. Furthermore, the outer housing member  202  encloses the inner housing member  200  as well as the terminating ends of the first and second fibers  196  and  198 . 
     Reference will now be made to FIG. 4 which illustrates an embodiment of an optical apparatus  230 . The optical apparatus  230  is substantially similar to the actively controlled optical apparatus  130  of FIG. 2 except that an interference filter  246  is deposited on a first surface  242  of a substrate  240  so as to be spaced from a holder  234 . Furthermore, the electro/magneto-strictive induced stress mechanism and the first mismatch stress applied by the substrate  240  are combined as will be described in greater detail below. 
     In one embodiment, the optical apparatus  230  is adapted so that variations in the filtering characteristics in response to a change in temperature are relatively small. In particular, the controller  286  directs the electric/magnetic field generator  282  to modify the electric/magnetic field  280  so that the electro/magneto-strictive induced stress mechanism combines with the first thermal mismatch stress to thermally compensate the apparatus  230 . 
     In another embodiment, the optical apparatus  230  is adapted with a control system  284  that enables the filtering characteristics to vary between a first filter characteristic and a second filter characteristic. In particular, the center wavelength can be adjusted between a first center wavelength and a second center wavelength, as will be described in greater detail below. 
     As indicated in FIG. 4, the control system  284  of the apparatus  230  is substantially similar to the control system  184  of FIG.  2 . In particular, the control system  284  includes the controller  286  and the temperature sensor  288 . Furthermore, the control system  284  of the optical apparatus  230  further comprises a user input device  248 , such as a potentiometer. Moreover, the controller  286  is adapted to receive input from the user input device  248  so as to enable a user to change the filtering characteristics of the optical apparatus. Thus, according to the signal provided by the user input device  248 , the controller  286  directs the electric/magnetic field generator  282  to modify the electric/magnetic field  280  that provides the interference filter  246  with the requested filter characteristic. 
     Reference will now be made to the block diagram of FIG. 5 which generally illustrates the control systems  184  and  284  of the actively controlled optical apparatus  130  and  230  of FIGS. 2 and 4. As shown in FIG. 5, the controller  186 ,  286  is adapted to receive an input signal S a  from the temperature sensor  188 ,  288  that is indicative of the temperature of the temperature sensor  188 ,  288 . Furthermore, the signal S a  is directed along a communication path P a  that links the temperature sensor  188 ,  288  with the controller  186 , 286 . 
     As shown in FIG. 5, the controller  286  is further adapted to receive a control signal S b  from the user input device  248  which is indicative of the requested filtering characteristics. In particular, the signal S b  is directed along a communication path P b  that links the user input device  248  with the controller  286 . 
     As shown in FIG. 5, the controller  186 ,  286  is adapted to transmit a control signal S c  to the electric/magnetic field generator  182 ,  282 . In particular, the control signal S c  is transmitted along a communication path P c  that links the controller  186 ,  286  with the electric/magnetic field generator  182 ,  282 . Furthermore, upon receiving the control signal S c , the electric/magnetic generator  182 ,  282  generates the electric/magnetic field  180 ,  280  having a field strength that corresponds to the signal S c . 
     It will be appreciated that, in the embodiment of FIG. 1A, the effects of the temperature dependent indices of refraction of the thin film layers  60  of the interference filter  46  are substantially reduced by exposing the filter  46  to both the first and second mismatch stresses. In particular, the thermal expansion properties of the holder  34  and the elastic properties of adhesive  70  are chosen so that the second mismatch stress combines with the first mismatch stress to provide improved thermal compensation. 
     Thus, it will be appreciated that, in one embodiment, the optical apparatus  30  could be formed from a relatively inexpensive and commonly available optical filter having relatively poor thermal compensation. In particular, such a filter, comprised of the substrate  40  and the interference filter  46  deposited on the substrate, could have filtering characteristics that are substantially sensitive to a change in temperature. However, by mounting the optical filter to the holder  34  in the manner described previously in connection with FIGS. 1A and 1B so as to provide the filter  46  with the compensating second mismatch stress, the optical apparatus  30  can be adapted with filtering characteristics that are less sensitive to a change in temperature. 
     It will also be appreciated that, in another embodiment, the electro/magneto-strictive nature of the interference filter  146 ,  246  is exploited to provide another degree of freedom in the effort to thermally compensate the filter  146 ,  246 . In particular, the electro/magneto-strictive filter  146  may be prestressed during the manufacturing process. Alternatively, the filter  146 ,  246  may be actively stressed by continually exposing the filter to a controllable electric/magnetic field. Furthermore, the actively managed electric/magnetic field may be controlled by the controller  186 ,  286  in communication with the temperature sensor  188 ,  288 . 
     It will also be appreciated that, in yet another embodiment, the electro/magneto-strictive properties of the filter  246  are exploited to provide variable filtering characteristics. In particular, depending on the signal provided by the user interface  248 , the controller  286  directs the electric/magnetic field generator  282  to produce an electric/magnetic field which provides the filter  246  with the requested filter characteristic. 
     Although the preferred embodiment of the present invention has shown, described and pointed out the fundamental novel features of the invention as applied to this embodiment, it will be understood that various omissions, substitutions and changes in the form of the detail of the device illustrated may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing description, but should be defined by the appending claims.