Abstract:
A tunable optical filter is based on a conventional thin film optical filter. The thickness of the thin film optical filter determines the optical properties of the filter, specifically the pass band. The thin film optical filter is arranged within a hole extending through a layer of piezoelectric material such that the periphery of the thin film optical filter is secured to the periphery of the hole. Electrodes are provided on opposite surfaces of the piezoelectric layer such that different voltage levels can be applied. The resulting deformation of the piezoelectric layer causes a difference in the thickness of the thin film optical filter thereby permitting tuning of the optical filter.

Description:
FIELD OF INVENTION 
     The invention relates to an improved tunable optical filter and modulator. 
     BACKGROUND 
     Thin film bandpass filters are key components in wavelength-division multiplexing (WDM) systems, in particular density wavelength-division multiplexing (DWDM) systems. A typical thin film bandpass filter comprise a series of dielectric resonance cavities, with quarter wave thick layers of lower index material separating them. These filters are also sometimes referred to as dielectric multilayer interference filters. Many improvements in the performance of these filters, such as improved wavelength response and polarization independency, have been made. U.S. Pat. No. 6,018,421 which issued to Cushing on Jan. 25, 2000 provides a summary of known bandpass filters of this type along with disclosure of an improvement whereby the transition slopes of the filter are improved as well as removal of the ripple effect generally associated with these filters. 
     For WDM applications and, especially for DWDM applications, tunable optical filters are becoming useful. It is known to use thin film filters as tunable optical filters. Typically, such a tunable optical filter comprises a thin film filter mounted on a rotatable surface such that the angle of incidence of the incoming light on the filter can be altered by rotation of the surface. The change in the incident angle alters the pass band of the filter. However, these tunable filters have slow response time due to the mechanical rotation of the thin film filter. Further, because of the reflective angles created by the multilayers within the filter, the filter response becomes polarization dependent as the incident angle is altered. U.S. Pat. No. 5,481,402 which issued to Cheng et al on Jan. 2, 1996 provides an improved tunable optical filter of this nature wherein the light is passed through the filter, through a quarter wave plate and onto a reflective surface which reflects the light back through the quarter wave plate and through the filter a second time. Although this technique does improve the polarization dependency of the filter, the filter remains polarization dependent to a limited extent. Further, the tunable optical filter disclosed has the same slow response time as discussed above due to the mechanical tuning of the filter. 
     Other types of tunable optical filters known in the art are acoustic-optical tunable filters (AOTF) and liquid crystal tunable filters (LCTF). The AOTFs are undesirable for many applications because of the high insertion losses and because the passband of these filters is not sufficiently narrow. An example of a LCTF know in the art is disclosed in U.S. Pat. No. 5,321,539 which issued to Hirahayashi et al on Jun. 14, 1994. LCTFs are undesirable in certain circumstances due to slow response time and the polarization dependency of these filters. 
     Further, switches and modulators have become key components in modern optical communication systems. External modulation has become necessary for high bit rate systems, such as OC-192 . Mach-Zehnder interference modulators are well known in the art and commonly used as optical modulators or switches. These interference modulators consist of a single input waveguide, an input branching region for splitting the input light into two substantially equal portions into two branch waveguides and an output waveguide. By effecting a phase shift in one branch of the waveguide relative to the other, the output light power can be altered between zero and the input light power level, depending on the phase shift. The phase shifts are generally effected by means of electrodes disposed on the substrate of one or both of the branch waveguides. Although these modulators are effective, the costs of manufacturing these devices are high and the devices have high insertion losses, are polarization dependent and have poor thermal properties. 
     SUMMARY OF INVENTION 
     The invention is based on a conventional thin film optical filter. The thickness of the thin film optical filter determines the optical properties of the filter, specifically the pass band. The thin film optical filter is arranged within a hole extending through a layer of piezoelectric material such that the periphery of the thin film optical filter is secured to the periphery of the hole. Electrodes are provided on opposite surfaces of the piezoelectric layer such that different voltage levels can be applied. The resulting deformation of the piezoelectric layer causes a difference in the thickness of the thin film optical filter thereby permitting tuning of the optical filter. 
     Instead of a single layer of piezoelectric material a multi-layered arrangement of many thin layers of piezoelectric material could be used. In this case the layers are sandwiched between transparent substrates and electrodes. 
     Although the main embodiment suggested involves an optical filter completely surrounded by the piezoelectric material, in an alternative embodiment a narrow strip of thin film optical filter is provided with piezoelectric material only along the two longer sides. 
     The invention also provides a method of making the novel tunable optical filter. 
     The improved tunable filter can also be used as a modulator or a switch. As an essentially fixed wavelength filter, the invention provides a way of fine tuning the passband of conventional thin film filters such that variations in the filter due to changes in the surrounding environment, such as temperature, can be compensated for. As a tunable filter, although the dynamic range of a filter according to the invention is less then many conventional tunable filters, the tunable filter of the invention has a quicker response time and no polarization dependency. Further, when used in a DWDM system, it allows the density of the DWDM system to be significantly increased. The filter may also be used to replace several filters in multichannel signal devices as it can be switched to selectively transmit different wavelength signals corresponding to different channels. As a modulator, the invention is inexpensive compared to conventional modulators and provides improved performance characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described by way of example with reference to the attached drawings in which: 
     FIG. 1 is a side view of a conventional thin film optical filter; 
     FIG. 2 is a perspective view of a tunable optical filter according to one embodiment of the invention; 
     FIG. 3 is a side view of the tunable optical filter shown in FIG. 2 with a zero voltage applied across the filter; 
     FIG. 4 is a side view of the tunable optical filter shown in FIG. 2 with a voltage greater than zero applied across the filter; 
     FIG. 5 is a side view of the tunable optical filter shown in FIG. 2 with a voltage less than zero applied across the filter; 
     FIG. 6 is a schematic drawing illustrating the fabrication of the tunable optical filter shown in FIG. 2; 
     FIG. 7 is a diagram of transmissivity against wavelength to explain how switching between optical channels may be achieved using the invention; 
     FIG. 8 is a side view of an alternative arrangement of piezoelectric material used in the invention in which many thin layers of piezoelectric material are used; and 
     FIG. 9 is a perspective view of a tunable optical filter according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a thin film optical filter known in the art is shown. The thin film filter  1  comprises a transparent substrate  2  onto which multi-layers (perhaps as high as a few hundred layers) of dielectric films  3  are disposed to create a desired pass band wherein light with a wavelength within the pass band is transmitted through the filter while light with a wavelength outside the pass band is absorbed by the filter. Light is collimated onto the thin film filter  1  by an input waveguide  4  and a lens  5 . Light transmitted by the thin film filter  1  is focused by a lens  6  into an output waveguide  7 . 
     With reference to FIG. 2, a tunable filter  8  according to an embodiment of the invention is shown. The filter comprises a layer of piezoelectric material  10  disposed on a transparent substrate  12  such as amorphous Quartz. PZT or other known piezoelectric materials could be used. The piezoelectric material  10  has a hole  14  therein within which a conventional thin film filter  16  is disposed. The thin film filter  16  is secured around its periphery to the piezoelectric material  10  and has a cross-sectional area sufficiently large to permit transmission of a light beam for the application for which the tunable filter is being used. The thin film filter  16  can be secured to the piezoelectric material  10  by an adhesive or simply by the bonds created as a result of the technique used to dispose it within the hole  14 . Preferably, the piezoelectric material  10  is a stiffer material than the thin film filter  16 . Although the hole  14  in the piezoelectric material  10  shown in the drawings has a circular cross-section, it would be understood by those skilled in the art that different cross-sectional shapes could be used. 
     A first electrode  18  comprising a layer of conductive material is positioned between the piezoelectric material  10  and the transparent substrate  12 . Similarly, a second electrode  20  comprising a layer of conductive material is disposed on the opposing surface of the piezoelectric material  10 . The electrodes  18  and  20  could be made of any number of conductive materials known in the art such as Al or a Ti/Al combined film (thin Ti film plus over 95% Al film). However, the electrodes  18  and  20  must have holes therein such that they do not cover the thin film filter  16 . As shown in FIG. 3, a set of conductors  22  are connected to each of the layers of conductive material  18  and  20 . 
     In operation, a light source (not shown) is directed at the thin film filter  16 . As shown in FIG. 3, when no voltage is applied across the piezoelectric material, the thin film filter  16  acts as a conventional thin film filter. In this state, the thin film filter  16  permits light with a wavelength within a pass band to be transmitted through the filter. The pass band is set by the specific structure of the thin film filter  16  through conventional techniques. 
     The pass band of the thin film filter  16  is altered by varying the voltage applied across the piezoelectric material  10 . The effect of applying a voltage across the piezoelectric material  10  will now be described with reference to FIGS. 4 and 5. It would be understood by those skilled in that art that the effect of the voltage on the piezoelectric material  10  is dependent upon the piezoelectric coefficient of the material. It would also be understood that a piezoelectric material with a negative coefficient would response in a manner opposite to that of a material with a positive coefficient. For the purpose of illustration, the embodiment illustrated in FIGS. 4 and 5 has a piezoelectric material with a positive piezoelectric coefficient. 
     With reference to FIG. 4, when a positive voltage is applied across the piezoelectric material  10 , the piezoelectric properties of the piezoelectric material cause it to compress in the direction of the electric field created by the voltage applied. This results in the piezoelectric layer becoming thinner as is evident by comparing FIGS. 3 and 4. The compression of the piezoelectric material  10  in this direction results in the expansion of the piezoelectric material in directions orthogonal to the electric field thereby resulting in the cross-sectional area of the hole  14  being decreased. As a result, the thickness of the thin film filter  16  is increased, as can be seen on comparing FIGS. 3 and 4, by the stresses created about its periphery by the impinging portions of the piezoelectric material  10 . Accordingly, the filtering properties of the thin film filter  16  are altered such that the pass band of the filter is shifted to permit transmission of light with a higher wavelength. 
     Conversely, as shown in FIG. 5, when a negative voltage is applied across the piezoelectric material  10 , the piezoelectric material expands in the direction of the electric field thereby causing the piezoelectric material to compress in directions orthogonal to the electric field. Accordingly, in this state, the piezoelectric layer is thicker and the cross-sectional area of the hole  14  is increased thereby causing the thickness of the thin film filter  16  to decrease. Again, the filtering properties of the thin film filter  16  are altered. When the thickness of the thin film filter  16  is decreased, the pass band of the filter is shifted to permit transmission of light with a lower wavelength. Accordingly, the pass band of the thin film filter  16  can be adjusted by varying the voltage applied across the piezoelectric material  10 . 
     This embodiment of the invention can be used as a tunable filter, a switch or a modulator. As a true tunable filter, the pass band of the filter is varied by altering the voltage applied across the piezoelectric material  10 . The dynamic range of the tunable filter is not as large as conventional tunable filters; however, the response time of the embodiment of the invention is greater and it has no polarization dependency. Preferable, the embodiment of the invention is used simply to fine tune a fixed filter in situations where the dynamic range need not be large such as when compensation for changes, such as temperature, in the environment is required. 
     In operation as a switch, the pass band of the thin film filter  16  and the wavelength of the incoming light are selected such that the tunable filter  8  can be alternated between an on state and an off state. In the on state, wherein a first voltage is applied across the piezoelectric material  10 , the wavelength of the incoming light is such that it is within the pass band of the thin film filter  16  thereby permitting transmission of the light through the thin film filter. In the off state, wherein a second voltage is applied across the piezoelectric material  10 , the resulting change in the thickness of the thin film filter  16  is sufficient to shift the pass band of the thin film filter such that the wavelength of the incoming light is outside the pass band. In this state the light is completely absorbed by the filter. Accordingly, the tunable filter  8  can be operated as an optical switch by simply varying the voltage across the piezoelectric material  10  between the first and second voltage. If the voltage applied to the piezoelectric layer is large enough the centre wavelength of the filter could be switched from one WDM channel into another WDM channel. For example, the pass band could switch from i to i−1 or i+1 etc. as illustrated in FIG.  7 . 
     In operation as a modulator, the wavelength of the incoming light and the voltage to be applied across the piezoelectric material  10  are selected. However, the voltage to be applied is varied between a minimum voltage and a maximum voltage. The minimum voltage is selected such that pass band of the thin film filter  16  is positioned in the light spectrum so that a predetermined minimum intensity of light is transmitted through the filter. Similarly, the maximum voltage is selected such that pass band of the thin film filter  16  is positioned in the light spectrum so that a predetermined maximum intensity of light is transmitted through the filter. In operation, the voltage is varied within the range between the minimum voltage and maximum voltage such that the desired output light signal is achieved. For a high bit rate modulator it may be necessary to construct a “multi-layer sandwich” piezoelectric structure instead of a single piezoelectric layer. FIG. 8 illustrates such an arrangement from which it can be seen that on the substrate  12  are disposed a first layer  10 ′ of piezoelectric material with electrodes  18 ′ and  20 ′ disposed on either side of the layer  10 ′ followed by one or more sandwiches, comprising a thin transparent substrate  12 ′, further piezoelectric layer  10 ′ and further electrodes  18 ′ and  20 ′, stacked on top. The piezoelectric layers  10 ′ are all much thinner than the single layer  10  used in the first embodiment. This provides a faster response time as well as reducing the required signal voltage. For clarity, the thin film filter has been omitted from FIG.  8 . The thin film filter would be provided in exactly the same manner as shown in FIGS. 1-5 except that the single layer of piezoelectric material surrounding the optical filter would be replaced with the multi-layer arrangement of FIG.  8 . 
     FIG. 6 provides a schematic drawing illustrating the fabrication of a tunable optical filter in accordance with an embodiment of the invention. As shown in step  1 , the electrode  18  is deposited on the transparent substrate  12 . The electrode is deposited by a vacuum deposit or other known technique. The hole  14  is opened in the electrode  18  using a known photolithography process. As shown in step  2 , the piezoelectric material  10  is then deposited onto electrode  18  by a vacuum deposit or other known technique. The hole  14  is masked during the process so that no piezoelectric material is deposited therein. A second hole  26  is then opened in the piezoelectric material  10  using a photolithography process such that the electrode  18  is exposed through the piezoelectric material  10 . The hole  26  must be of a sufficient size to accommodate the welding of a conductor to electrode  18 . With reference to step  3 , the thin film filter  16  is deposited within hole  14  by a vacuum deposit or other known technique. The entire surface of the piezoelectric material  10  and the hole  26  are masked such that only the hole  14  is exposed to the process. If desired, an adhesive (not shown) can be placed around the periphery of hole  14  to improve the adhesion between the thin film filter  16  and the piezoelectric material  10 . As illustrated in step  4  the second electrode  20  is then deposited on the piezoelectric material  10  by vacuum deposit or other know technique. During this process, both hole  14  and hole  26  are masked. Conductors  22  are then welded to electrode  20  and electrode  18  through hole  26 . 
     Reference is made now to FIG. 9 which shows a novel tunable optical filter according to the invention which differs from the embodiment shown in FIGS. 2-5 in that, instead of the piezoelectric material completely surrounding the thin film filter the thin film filter is formed as a strip  16 ″ with the piezoelectric material  10 ″ provided on both long edges of the strip  16 ″. The short edges of the strip  16 ′ are not surrounded by piezoelectric material and are exposed. Otherwise the structure and operation is similar to the first embodiment with a transparent substrate  12 ″, first electrodes  18 ″ and second electrodes  20 ″. Of course, the multi-layer structure of FIG. 8 could be used for the piezoelectric material. 
     It will be understood by those skilled in the art that numerous alterations, modifications and variations to the above embodiments can be made without departing from the substance of the invention.