Abstract:
A thin film electrical optical filter includes a thin film electrooptical dielectric having a voltage controlled electrooptical refractive index that is disposed between opposing stacks of alternative layers having differing refractive indices for transmitting received incident light through a target passband. An applied voltage to the electrooptical dielectric can serve to alter the actual passband to the target passband for compensating for thin film manufacturing tolerances for producing controllable optical filters with precise passbands.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of thin film optical devices. More particularly, the present invention relates to transparent electroactive cavity thin film optical filters. 
     BACKGROUND OF THE INVENTION 
     Highly accurate optical interference filters can be manufactured using thin film deposition processes. These devices are typically built by depositing alternating layers of transparent materials where one layer possesses a much larger refractive index relative to the other layer. Theoretically, the proper choice of composition, thickness and quantity of layers could result in a device with any desired transmission spectrum. Among the simplest devices is the single cavity bandpass filter, that is the thin-film form of an etalon. This device consists of three sets of layers. The first stack is a dielectric mirror. This is followed by a thicker layer that forms the spacer. This is followed by another stack that forms a dielectric mirror. The mirror stacks are typically fabricated by depositing alternating transparent materials that have an optical thickness that is one quarter of the optical wavelength of light. To achieve the theoretical optical performance, each layer must possess a precise and specific physical thickness and refractive index. Any nonuniformity in the deposition of the layers can affect the spectral placement and transmission or reflection characteristics of the device. A design that requires very tight manufacturing tolerances over large substrate areas could result in the costly rejection of many devices. Given these manufacturing limits, it would be desirable to analyze the devices after construction and alter the devices that do not meet a predetermined optical transmission or reflection specification by some electrical or mechanical means. For example, if the peak transmission wavelength of a manufactured optical bandpass cavity filter was slightly out of tolerance, it would be desirable to have a means for shifting the peak back to the desired spectral location. It is also desirable that the optical filters have precise rejection bands and passbands that are electrically or mechanically selectable. 
     Mechanical methods of achieving a variable transmission spectrum device are well known. This includes changing a prism or grating angle, or altering the optical spacing between mirrors of an etalon. To overcome the performance, size and cost disadvantages of using mechanical schemes, many have conceived of electrical methods for varying a transmission spectrum. For example, U.S. Pat. No. 5,150,236 issued Sep. 22, 1999 discloses a tunable liquid crystal etalon filter. The liquid crystal fills the space between dielectric mirrors. Electrodes on the mirrors are used to apply an electric field, which changes the orientation of the liquid crystal that changes the optical length for tuning. The change in the optical length corresponds to a change in the location of the passband. In addition, U.S. Pat. No. 5,103,340 issued Apr. 07, 1992 discloses piezoelectric elements placed outside the optical path, which are used to change the spacing between cascaded cavity filters. Furthermore, U.S. Pat. No. 5,799,231 discloses a variable index distributed mirror. This is a dielectric mirror with half of the layers having a variable refractive index that is matched to other layers. Changing the applied field increases the index difference that increases the reflectance. The mathematics that describe the transmission characteristics of multilayer films composed of electro-optic and dielectric materials are well known. 
     Another electrically actuated thin film optical filter uses a series of crossed polarizers and liquid crystalline layers that allow electrical controls to vary the amount of polarization rotation in the liquid by applying an electric field in such a way that some wavelengths are selectively transmitted. However, these electrically actuated thin film optical filters have the characteristic that the light must be polarized and that the frequencies of light not passed are absorbed, not reflected. Another electrically actuated thin film optical device is the tunable liquid crystal etalon optical filter. The tunable liquid crystal etalon optical filter uses a liquid crystal between two dielectric mirrors. U.S. Pat. No. 5,710,655 issued Jan. 20, 1998 discloses a cavity thickness compensated etalon filter. 
     The common cavity filter, such as the etalon optical filter, is an optical filter with one or more spacer layers that are deposited in the stack and that define the wavelength of the rejection and pass bands. The optical thickness of the film defines the placement of the passband. In the tunable liquid crystal etalon optical filter, an electric field is applied to the liquid crystal that changes the optical length between the two mirrors so as to change the passband of the etalon. Still another tunable optical filter device tunes the passband by using piezoelectric elements to mechanically change the physical spacing between mirrors of an etalon filter. Bulk dielectrics are made by subtractive methods like polishing from a larger piece; whereas thin film layer are made by additive methods like vapor or liquid phase deposition. A bulk optical dielectric, for example, greater than ten microns, disposed between metal or dielectric mirrors suffer from excessive manufacturing tolerances and costs, the bulk material providing unpredictable, imprecise, irregular, or otherwise undesirable passbands. These electrical and mechanical optical filters disadvantageously do not provide precise rejection bands and passbands that are repeatably manufactured. These and other disadvantages are solved or reduced using the invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a tunable optical interference filter. 
     Another object of the invention is to provide a tunable optical filter with stable rejection and passbands having electrooptic medium. 
     Yet another object of the invention is to provide tunable optical filter with stable rejection and passbands manufactured using conventional thin film deposition processes. 
     Still another object of the invention is to provide tunable optical filter with stable rejection and passbands that is polarization insensitive. 
     A further object of the invention is to provide a tunable optical filter with stable rejection with stacked cavities made of transparent electroactive material forming an optical cavity. 
     Yet a further object of the invention is to provide a tunable optical filter with stable rejection and passbands using a series of optical cavities made of transparent electroactive materials forming an optical device having a refractive index that changes the resonant wavelength with an applied voltage to provide a tunable passband shift. 
     The invention is directed to a thin film tunable optical filter that provides highly accurate passbands yet manufactured using conventional manufacturing processes. After fabrication, the optical properties of the filter are stable with a degree of tunability. A tuning voltage is applied to an optical interference stack that shifts spectral features such as the transmission passband by a predetermined amount. The optical filter includes the use of a solid state electrooptic medium manufactured in a single chamber using conventional thin film deposition processes that grow transmissive material in layers of up to ten microns. Passband tuning occurs without moving any of the elements so that the optical filter is robust and vacuum compatible. The optical filter is polarization insensitive and reflects energy not transmitted with many applications including multiplexing and demultiplexing of optical signals. 
     The optical filter uses a tunable spacer. One or more spacer transparent electroactive material spacer layers are deposited in the stack to define the resonant wavelength of the device. The use of multiple electrooptic spacer layers as coupled cavities serve to widen and narrow the optical passband. The use of materials compatible with the single chamber deposition process provides a manufacturable monolithic device. The optical thickness of the film spacer layers defines the spectral placement of the passband. The solid state electrooptic material enables changes in the optical thickness of the spacer layers to shift the passband wavelength. The use of multiple spacers function to electrically alter multiple coupled cavities and thus shift the passbands. The optical filter can be controlled to electrically narrow or broaden the spectral width of the passband. When the passbands overlap, one spacer layer can define the long wavelength band edge and another spacer layer can define the short wavelength edge. The refractive index changes in response to changes in an applied tuning voltage producing changes in the resonant wavelength to shift the passband. 
     The electroactive cavity is disposed between a transparent conductor like indium tin oxide or very thin metal to deliver the charge. The conductive layer can be patterned so that the filter is divided into pixels that are individually addressable over the surface of the filter. Patterning the transparent conductor provides an addressable array of tunable filters. Pixelization is useful in improving uniformity over a large optical area. When applied in conjunction with a detector array, pixelization allows proximal detector elements to be illuminated with different spectra of light. 
     Parallel production of monolithic optical filters significantly reduces cost. The layers are deposited as a repeatable multilayer stack on a large wafer that can then be diced into very small pieces. Each piece can be tested for wavelength placement and band shape. With repeatable fabrication processes identical filters can be used for multiple channel application. Each filter may have a slightly different voltage due to small manufacturing tolerances that can be compensated by changing the applied field. A set of filters can be manufactured as a single stage demultiplexer that could be tuned to any channel desired to eliminate the need for the whole filter set. The compensating applied voltage can be programmed to compensate for temperature dependence as well. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a tunable solid state thin film optical filter. 
     FIG. 2 is a graph of a refractive index profile of the optical filter. 
     FIG. 3 is a graph of transmission spectral through the optical filter. 
     FIG. 4 is a graph of an actual and target transmission spectra of a three cavity optical filter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a substrate  10  is used to support a lower dielectric stack  12 , an electrooptic layer  14  and an upper dielectric stack  16 . A voltage source  18  is externally applied across the electrooptic layer  14 . A lower conductive layer  20  is disposed between the electrooptic layer  14  and the lower dielectric stack  12 . An upper conductive layer  22  is disposed between the electrooptic layer  14  and the lower dielectric stack  16 . The tunable optical filter receives incident light  24 , and reflects back reflected light  26  while passing transmitted light  28 . 
     The lower and upper dielectric stacks  12  and  16  preferably contain alternative dielectric layers made of materials having respective high and low refractive indices. As such, each stack contains a plurality of sets of layers with each set, in the preferred form, consisting of two layers each having a respective refractive index. The stacks  12  and  16  may comprise any number of sets of layers, but preferably include five or more sets. Each set of layers may have the same number of identical layers. The plurality of layers in each set is two or more, and is preferably two so that both of the stacks  12  and  16  have only two alternating layers with two respective refractive indices. The preferred lower stack  12  has low and high alternating refractive indices  30  and  31 , and the preferred upper stack  16  could have the same low and high alternating refractive indices  35  and  34 . The electrooptic dielectric has an electrooptic refractive index  32  that can be controlled by the voltage source  18  by an applied voltage. 
     Referring to all of the Figures, the voltage source  18  is varied to apply the applied voltage so as to change the electrooptic refractive index  32 . When changing the electrooptic refractive index, the passband of the filter shifts, for example, from a low passband having low voltage transmission spectrum  40 , to medium passband having medium voltage transmission spectrum  42 , and to a high passband having a high voltage transmission spectrum  44 . By varying the applied voltage, the thin film optical filter functions to select a desired passband. 
     In another aspect, the thin film electrooptical filter can be used to correct for unacceptable tolerances in conventional thin film processes. The thin film electrooptical filter is conventionally designed using standard semiconductor processes. The processes are set so that the thin film electrooptical filter has a target passband having a target spectrum  50 . However, due to variances in the deposition process, yields of the thin film electrooptical filter having the target passband are often not achieved due to unacceptable high process tolerances, and produce poorly functioning thin film electrooptical filters. The applied voltage can be set to a correction voltage to compensate for these fabrication tolerances by shifting the actual passband to the target passband. The passband shift is relatively small but is sufficient in a functioning filter to correct for the fabrication tolerances where the actual passband does not substantially overlap the target passband. 
     The thin film optical filter functions to receive the incident light  24  that becomes either transmitted light  28  or reflected light  26  with insignificant light absorption. The optical filter transmits the transmitted light  28  within the passband and reflects the reflected  26  light outside of the passband. The external voltage source provides the applied voltage for selecting the desired passband. The electrooptical dielectric  14  has a thin film of predetermined thickness and an electrooptical refractive index that is controlled by the applied voltage that establishes a desired electric field through the electrooptical dielectric  14  between the two conductive layers  20  and  22 . The electrooptical refractive index is selected by the applied voltage to select the desired passband. The optical filter then communicates the transmitted light  28  within the passband and reflects the reflected light  26  outside the passband. 
     The upper stack of thin film layers  16  is disposed on the electrooptical dielectric  14  that is in turn disposed on the lower stack of layers  12  that is in turn disposed on the substrate as an integrated thin film device. The layers of the upper stack- 16  have respective thicknesses and respective refractive indices so that each of the uppers layers reflect an upper portion of the incident light. Likewise, the lower stack  12  of thin film layers also preferably have similar respective thicknesses and refractive indices so that each layer also reflects lower portion of the incident light. The electrooptical refractive index of the electrooptical dielectric  14  is controlled by the applied voltage. The electrooptical dielectric optically couples the reflections in phase sensitivity so that reflected lower portion from the lower stack  12  cancels in a desired amount the reflected upper portion of reflected light in the upper stack  16  so as to control the amount of reflected light  26 . The optical filter increases the transmitted light  28  within the target passband when the electrooptical dielectric  14  is suitably controlled by the applied voltage during transmission when the electrooptical refractive index is adjusted to adjust the phase of the incident light  24  passing through the electrooptical dielectric  14 . The phase difference between the lower portion of the reflected light  26  and the upper portion of the reflected light  26  can be increased for increased or decreased cancellation of the upper portion of reflected light  26 . The phase difference applied voltage control is for increasing or decreasing the amount of transmitted light  28  within the passband while concomitantly decreasing the amount of reflected light  26  within the passband. Hence, the applied voltage shifts the passband to increase or decrease the amount of transmitted light  28  with the selectably controlled applied voltage. In the case where the incident light  24  is monochromatic, that is, consisting of a narrow distribution of wavelengths, the passband can be controlled so that in the presence of the first applied voltage, the passband transmits all of the incident light  24  as the transmitted light  28 , and in a second applied voltage, to reflect all of the incident light  24  as the reflected light  26 . 
     The thin film optical filter can be used in the telecommunications industry that uses narrow, fixed wavelength filters to demultiplex signals on each optical fiber using dense wavelength division multiplexing. One of the limits to the number of channels available per fiber is the ability to manufacture very narrow, accurate filters. Each channel is determined by a carrier wavelength of light. The thin film electrooptical filter can be used for demultiplexing the different optical channels. The channels can be selected using one selecting thin film electrooptical filter having a passband shift using a varying controlled applied voltage. The channels can equivalently be selected using a series of bandpass filters with each filter is controlled by a static applied voltage to select and hence pick off a designated wavelength of light and route that the channel of light to another device. 
     The thin film electrooptical filter could also be used in the manufacture of lasers. The thin film electrooptical filter can be equipped with a mirror where the passband could be modulated providing an alternative way to mode-lock a laser. The thin film electrooptical filter could also be used for remote sensing for imaging through very narrow filters that can be tuned around the IR absorption bands of interesting compounds. The thin film electrooptical filter could also be used for rocket plume detection to detect an IR signature of a launch from space. The thin film electrooptical filter can provide flexibility to shift the optical system transmission band farther into the atmospheric absorption band to optimize for different atmospheric conditions. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.