Abstract:
Disclosed are digitally-switchable bandpass filters combining non-tunable retarder stacks with switchable liquid crystal cells. The disclosed filter embodiments function like a filter wheel with no moving parts that may provide faster switching, better image registration, compact size, and lower electrical power consumption. These benefits are attractive in portable handheld devices, such as bio-hazard sensors or glucose monitors.

Description:
TECHNICAL FIELD  
       [0001]     Disclosed embodiments relate to bandpass filters for optical systems, and more particularly to digitally-switchable liquid crystal bandpass filters.  
       BACKGROUND  
       [0002]     A tunable filter uses electrically controlled liquid crystal (LC) elements to transmit specific wavelengths of light through the filter by exploiting the variable retardation associated with certain LC modes as a means of shifting a spectral feature. One type of LC tunable filter is a Lyot (or Lyot-hybrid) polarization interference filter. With a Lyot filter, a bandpass profile is synthesized through multistage filtering using geometric relationships between retarder stack films. The polarization analyzer of one stage forms the input polarization for a subsequent stage, such that (N+1) polarizing films are used for an N-stage filter. In other word, 2 polarizing films are used for 1 filter stage, 3 polarizing films for 2 filter stages, and so forth. The overhead associated with calibrating a fully tunable Lyot bandpass filter to provide acceptable spectral characteristics can be significant. Additionally, each polarizing film has approximately a 10% transmission loss. Consequently, tunable Lyot filters with high finesse and acceptable dynamic ranges are not only bulky and expensive, but also have poor peak transmission.  
         [0003]     A Solc or Solc-like filter, on the other hand, can be synthesized using only two polarizers bonded to a single retarder stack. Also, the Solc filter can, in principle, be customized to reduce side-lobe levels. Presently known bandpass-tuning Solc filter approaches, however, require that each multi-order retarder stack be fully tunable. In practice, there is no significant improvement in throughput because the insertion loss of a polarizer is traded for the additional LC cell loss. Furthermore, the construction of a Solc filter is more challenging than that of a Lyot filter, in that precise alignment of many interleaved active and passive elements must be done before system calibration can commence. Consequently, if at any point an error is made in building such an assembly, a large amount of high value material may have to be scrapped.  
       SUMMARY  
       [0004]     Described are digitally-switchable bandpass filters with multiple polarizers and enhanced functionality within each stage of the filter. The switchable filters contain fixed elements, such as retarder stacks having pre-determined sets of available spectral profiles, and digital quasi-achromatic polarization switches, such as liquid crystal (LC) cells for selecting the particular spectral output profiles from the retarder stacks.  
         [0005]     The presently disclosed filter embodiments function like a filter wheel with no moving parts. The bandpass filter structure permits selection among predefined sets of spectral profiles. Additionally, there are several unique advantages of the presently disclosed digitally-switchable bandpass filter relative to an electro-mechanical device. The lack of moving parts translates into: (a) potential for faster switching, (b) potential for better image registration, (c) compact size, and (d) lower electrical power consumption. These benefits are attractive in portable handheld devices, such as bio-hazard sensors or glucose monitors. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a general block diagram illustrating a presently disclosed digitally tuned filter embodiment;  
         [0007]      FIG. 2  is an example of a retarder stack;  
         [0008]      FIG. 3  illustrates a spectral profile that may be encoded by retarder stack films;  
         [0009]      FIGS. 4A-4B  illustrate a bandpass spectrum and the complementary notch spectrum for an exemplary filter according to the present disclosure;  
         [0010]      FIG. 5  illustrates four transmission tuning spectra of a single stage filter incorporating a twisted-nematic liquid crystal cell;  
         [0011]      FIG. 6  is a graph illustrating the minimum spectra separation criteria;  
         [0012]      FIG. 7  is a graph illustrating the spectral shift of a uniaxial bandpass filter;  
         [0013]      FIG. 8  is a graph illustrating resolution versus number of retarder films at different bandpass wavelengths; and  
         [0014]      FIG. 9  is a block diagram of a digitally tuned filter embodiment in a portable handheld device application. 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 1  is a general block diagram  100  illustrating a digitally-switchable liquid crystal (LC) cell  106  as a quasi-neutral polarization switch for accessing pre-defined sets of spectral profiles from a retarder stack  104 . Similar to a Lyot filter, each block 102 represents an independent filter stage. Several stages  102   a ,  102   b ,  102   c  of this digitally tuned filter  100  are illustrated, with each stage having a polarizer  108 , a retarder stack filter  104 , a LC polarization switch  106 , and another polarizer  110 . Neighboring filter stages  102   a ,  102   b ,  102   c  may share polarizers  108 ,  110 . For example, polarizer  110   a ,  110   b  is a part of stages  102   a ,  102   b , while polarizer  108   b ,  108   c  is a part of stages  102   b ,  102   c . The polarizers  108 ,  110  may have alternate orientations from each other. If the polarization orientations alternate, then each stage  102  will have crossed polarizers  108 ,  110 .  
         [0016]     The optical elements  104 ,  106 ,  108 ,  110  may be coupled with a water-clear transparent index-matched adhesive. The task of generating a high-quality bandpass profile in this embodiment is mostly confined to a single element: the retarder stack  104  within each stage  102 . Localizing the critical filtering function to each stage  102  improves manufacturability, minimizes or eliminates calibration, and reduces cost. Although only three filtering stages  102  are illustrated, there may be an arbitrary number (N) of filter stages  102 .  
         [0017]      FIG. 2  illustrates a retarder stack  104 , which is a multi-layer laminate of bulk transparent-stretched polymer retarder films  105 . These films are ideally laminated using a solvent bonding process. Through a suitable selection of films&#39;  105  in-plane angles (or optic axis orientations) “θs”  107 , arbitrary spectral profiles  200  can be encoded as polarization information, such as illustrated in  FIG. 3 . The illustrated angles of θ and 3θ and their relationship to respective axes of the retarder stack layers, as well as the resulting profile of  FIG. 3  are all solely for purposes of illustration and shall not be viewed as limiting in any way.  
         [0018]     When a retarder stack  104  is positioned between neutral polarizers  108 ,  110  as previously described, moderate finesse bandpass profiles  200  with OD3 dynamic ranges may be produced. “OD3” is defined as “Optical Density 3,” where optical density is expressed by log 10(1/T), where T is transmittance, and where log 10(1/T)=−30 dB at the OD3 “filtered” level; in other words, the OD3 level of optical filtering is an optical filtering with three orders of magnitude between the unfiltered and filtered levels. This dynamic range allows a high degree of filter functionality in a single low-cost component (the retarder stack  104 ) that will, in principle, have lower signal loss. Presently, retarder stacks  104  with 12 to 18 layers may be mass-produced for the optical projection industry at a mean price of only a few dollars per square inch. The features and functions of retarder stacks  104  are further described in a commonly assigned U.S. Pat. No. 6,452,646 entitled “Optical retarder stack formed of multiple retarder sheets,” which is incorporated herein by reference in its entirety for all purposes.  
         [0019]     When a neutral LC polarization switch  106  is adjacent to a retarder stack  104  and between polarizing films  108 ,  110  (see  FIG. 1 ), the outputted spectral profile can be electronically switched between a bandpass spectrum  200  and the complementary notch spectrum  300 , as illustrated in  FIGS. 4A-4B , respectively. An exemplary digitally tuned filter  100  with N-stages according to the presently disclosed embodiment can generate each bandpass profile  200  as the product of a single bandpass function with (N−1) notch spectra  300 . The notch spectra  300  can serve to incrementally improve resolution and dynamic range, but has relatively little impact on the bandpass profile 200. As such, there is little coupling between stages  102 , which again improves manufacturability.  
         [0020]     In principle, any polarization interference filter containing N digitally-switchable LC cells  106  is capable of providing 2 N  distinct spectral outputs  200 . This scaling is attractive from the standpoint of minimizing the number of LC cells  106  and filter stages  102 , but generally involves a high degree of spectral coupling between stages  102 , which hampers performance robustness and detracts from manufacturability. One aspect of the present disclosure is to use a scheme in which the number of output bands  200  scales linearly with the number of LC cells  106 . Potential benefits include creating the desired independence between stages  102 , and using the isotropic state of the LC cell  106  for generating the critical bandpass profile  200 .  
         [0021]     It is generally the case that nematic LC cells  106  operating as digitally-switchable elements have one voltage state that is substantially more or less chromatic than the alternate state. For instance, a 90° twisted-nematic LC cell  106  has a self-compensation feature, such that the driven state is very nearly isotropic. It is not unusual for a driven twisted-nematic LC cell  106  between crossed polarizers  108 ,  110  to have a light leakage below 0.1% at normal incidence. Conversely, it requires substantial effort to design a twisted-nematic LC cell  106  that provides a wavelength independent conversion of input linearly polarized light to the orthogonal polarization. In practice, twisted-nematic LC cells  106  with reasonable switching speeds have relatively high degrees of chromaticity to their polarization conversion spectrum. This has the effect of compromising the performance of the retarder stack  104 , most notably by reducing the dynamic range. In a preferred embodiment, the driven state is preferably used to generate the bandpass profile  200  to minimize compromising the spectrum.  
         [0022]     In one design, and in a particular example a twisted nematic design, the retarder stack  104  is designed and positioned such that each layer of the retarder stack has a retarder angle that is acute (less than 45 degrees) relative to the rub direction of an alignment layer of one of the transparent electrodes of the LC cell  106 . In this design, the light is introduced with a state-of-polarization that is parallel to the rubbing direction of that transparent electrode.  
         [0023]     In practice, a twisted-nematic LC cell  106  can be designed to be more achromatic in the converting state than other devices (e.g. a π-cell). This translates into reduced sensitivity to cell gap non-uniformity, which translates into spatial non-uniformity of the transmission state. As cells are not completely uniform most of the time, this is another reason for assigning the zero-conversion state of the bandpass output. In a first-minimum twisted-nematic LC cell  106 , one fringe does not represent significant transmission non-uniformity.  
         [0024]     Given the above, the low voltage state of the twisted-nematic LC cell  106  can generate a notch filter spectrum  300  with minimally compromised performance. The degree of degradation increases with the spectral coverage of the filter (e.g. 420-680 nm for visible switching). This typically manifests itself as leakage at the notch center wavelength  302  or as throughput loss outside of the notch  304 . In another embodiment, the low-voltage state of the twisted-nematic LC cell  106  can be tuned in order to position the wavelength of ideal polarization conversion to correspond to the center wavelength of the selected bandpass. This first-order analog correction to the low voltage state is relatively tolerant, such that 3 bits of voltage level is generally adequate to insure high throughput of the bandpass.  
         [0025]     Referring again to  FIG. 1 , the application of voltages to the LC cells  106  of the several filter stages  102  is shown. Multiple sets of input voltages can be applied to the voltage inputs of the LC cells  106  in order to achieve desired filter characteristics. The voltages may be analog voltages generated by conversion of digital signals from a microcontroller  120  applied to digital-to-analog conversion circuitry (not shown) or the voltages may be otherwise generated to be applied to the voltage inputs. By this configuration, the optical filter characteristics can be flexibly tuned under digital control according to system design needs and flexibly adjusted according to changing performance needs.  
         [0026]      FIG. 5  shows peak transmission tuning spectra  500  of a single stage (12-layer bandpass stack with OD3 blocking) first-minimum twisted-nematic LC cell  106  between crossed polarizers  108 ,  110 . Four spectra are illustrated corresponding to different voltage levels  500   a ,  500   b ,  500   c ,  500   d . This spectra sampling sufficiently limits the loss in the 400 to 700 nm band to below 1%. Additional levels may be required to, for instance, reduce losses further, or to compensate for temperature effects.  
         [0027]     In addition to twisted-nematic LC cells  106 , the general principle may apply to other LC modes. A parallel-aligned nematic (electrically controlled birefringence, or ECB) LC cell  106  and π-cell  106  do not provide self-compensation, which can give residual retardation at any high-voltage level. A film compensator may be used to produce the preferred isotropic state for generating the bandpass spectrum. For a vertically-aligned nematic LC cell  106  with reversible voltage states, a potential benefit is that the cell may have low in-plane retardation at zero volts and low pretilt, and may avoid compensation. If not, however, a film compensator can also be used to eliminate any residual retardation. Other LC cells, such as ferroelectric LC cells  106  may also be achromatized with film compensators. In-plane LC switches, such as ferroelectric LC cells  106 , offer the unique feature that both the zero-conversion and 90° conversion states can be quasi-achromatic. Furthermore, ferroelectric LC cells  106  are bistable and therefore may not require analog tuning of the converting states for maximum throughput. However, ferroelectric LC cells  106  are relatively uncommon and expensive, and are not as mechanically or thermally stable as nematic LC cells  106 .  
         [0028]     According to the presently disclosed embodiments, the selection of a center wavelength for each bandpass spectra of a composite filter is arbitrary until the profiles begin to overlap. There are benefits to the dynamic range by close packing of bandpass spectra. However, there exists maximum bandpass packing, which depends upon the width and shape of the bandpass profile. In general, polarization interference filter bandpass spectra are the result of a compromise between the number of films and finesse (ratio of separation between the periodic spectra to the full width of the profile). The result is that the normal incidence bandpass is relatively smooth, unlike the steep edge-functions characteristic of dichroic or holographic filters. Consequently, there is often a spectral tail associated with a notch profile, which if positioned too close to an adjacent bandpass, can produce a significant throughput loss. The characteristics of the tail depend on the degree of apodization. As such, there is a limitation to the spectral sampling interval imposed by the retarder stack filter profile. The described effects will be better illustrated in subsequent figures and discussion.  
         [0029]      FIG. 6  is a graph illustrating the minimum spectra separation criteria  600  using a 12-layer design, where an OD3 maximum side-lobe level design is used (−30 dB). The maximum spectral sampling is a function of resolution, subject to the requirement that throughput is not too much affected by adjacent notches. The criterion used is that the first minima  604   a ,  604   c  of adjacent bandpass stages  602   a ,  602   c  correspond to the center wavelength  606  of the central band  602   b . This is equivalent to requiring zero loss of the notch spectra at the bandpass center wavelength  606 . The figure shows that the central bandpass spectrum  602   b  and the first null  604   a ,  604   c  of adjacent bandpass spectra  602   a ,  602   c  occur at around 531 nm. Mathematically, an estimate of the separation for this case is given by: Δλ=( 4/3) (FWHM), where Δλ is the separation or free spectral range, and where FWHM is the full width at half maximum  608  of a transmission peak  602 .  
         [0030]     Based on the formula, the overlap point occurs at roughly 25-30% at this side-lobe level. There are resolution benefits to densely packing the spectra. The adjacent spectra  602   a ,  602   c  will tend to narrow the base of the bandpass. With a 12-layer bandpass design centered at 535 nm, the 1% width of the profile is 53 nm. When notch filters of the same design are densely packed on either side of this profile, the 1% base width becomes 47 nm. Moreover, the notch provides enhancement of the dynamic range over a small range of wavelengths.  
         [0031]     In another embodiment, preferred retarder stacks  104  may be designed based on small angle solutions. These designs are based on films  105  with multi-order half-wave retardation at the center wavelength of the bandpass profile. Using this mode, a relatively small portion of the spectrum is converted to the orthogonal state, giving a bandpass between crossed polarizers  108 ,  110 . A preferred embodiment is to use an optimized bandpass design with a particular number of retarder films  105 , which can achieve the selected dynamic ranges (or stop-band ripple) with minimum bandpass width.  
         [0032]     There are several benefits to the small-angle retarder stack design. First, the geometrical yield of the film cut from the roll stock is maximized. Second, and perhaps more importantly, the spectral performance of a manufactured stack may be more consistent with theoretical prediction, subject to the real-world issues of retardation statistics, optic axis statistics, and the influence of the lamination process. In addition, by using small-angle designs, the center wavelength spectral shift of a uniaxial bandpass filter versus incidence angle (in air) will be quite similar to that of a single multi-order retarder oriented along zero as illustrated in  FIG. 7 . Like a positive uniaxial retarder, there is a negative shift in the plane of the optic axis  700   a , a positive shift in the orthogonal plane of similar magnitude  700   b , and almost no shift in the ±45° azimuth  700   c . For square parts, the latter should correspond to the corners of the part, where the angle of incidence is the largest.  
         [0033]     Bandpass designs are preferably apodized, such that peak side-lobe levels remain below 0.1% (OD3). In order to converge to this level of performance in manufacturing, spatial statistics of the retarder film  105  must be maintained to a very tight level. Ideally, the standard deviation for a cross-web measurement is approximately ±1-2 nm (spectral shift) and ±0.2°, and is fairly smooth. In a well-controlled stretching process, down-web statistics are relatively slow-varying so the statistics are relatively stable over the span corresponding to the layout of a single mother sheet of retarder-stack material.  
         [0034]     With these constraints, there is generally a small-angle retarder stack design  104 , such that the optic axis angles  107  are clustered about an input polarization direction. In the following exemplary stack design  104 , the angles  107  do not deviate by more than ±7° and the side-lobe levels in the stop band do not exceed 0.1% between crossed polarizers  108 ,  110 .  
                                                                                                                                                                   Design                α 1     α 2     α 3     α 4     α 5     α 6     α 7     α 8     α 9     α 10     α 11     α 12     α 13                          BP12   −1.7   1.5   −3.3   4.1   −5.2   6.1   −5.8   5.8   −4.0   3.8   −1.5   1.9   N/A       BP13   1.8   −0.9   3.4   −2.8   5.4   −4.5   6.4   −4.6   5.6   −3.0   3.6   −1.1   1.9                  
 
         [0035]     Preferred stack designs convert the narrowest possible bandwidth to the orthogonal polarization (per number of films) with prescribed dynamic range (e.g. OD3). This is a relative of the Solc filter, though preferably with improved apodization. Using a stack  104  composed of films  105  with identical retardation (giving a real impulse response with N+1 terms), the bandpass has symmetric behavior in the frequency domain with respect to the half-wave wavelength. A stack  104  consisting of an odd number of films  105  (BP 13 ) is known to behave as a compound half-wave retarder at the half-wave wavelength, while a stack  104  consisting of an even number of films  105  (BP 12 ) behaves as a pure rotator at this wavelength. Because the transmission band is quite narrow, the behavior of the stack  104  over the spectral range can be fairly uniform.  
         [0036]     Such is not the case on the unconverted band, where small-angle designs generate significant compound retardation along the input polarization. The compound retardation is frequently a significant fraction of the total retardation in the stack  104 . Therefore, there is a critical orientation alignment of the retarder stack  104  with respect to the polarizers  108 ,  110  in order to insure that the dynamic range is not degraded. An orientation error can significantly raise side-lobe levels, as this gives a projection of the electric field along both axes of the compound retarder.  
         [0037]     A preferred set of solutions has retarder angles  107  that are symmetric with respect to a midpoint. When the number of retarders  105  is odd (BP 13 ), the preferred stack design conforms to the angle sequence (α 1 , α 2 , α 3 , . . . α N , α O , α N , . . . α 3 , α 2 , α 1 ). When the number of retarders  105  is even (BP 12 ), the preferred stack design conforms to the angle sequence (α 1 , α 2 , α 3 , . . . α N , −α N , . . . −α 3 , −α 2 , −α 1 ). As previously discussed, these angles  107  are preferably smaller than ±7°. When a bandpass design uses an odd number of films  105  (BP 13 ), the preferred set of solutions has pure half-wave retardation in the converted band. When a bandpass design uses an even number of films  105  (BP 12 ), the preferred set of solutions has pure rotation in the converted band.  
         [0038]     Apart from the relationship between throughput and spectral overlap, the digitally tuned filter  100  can permit independent selection of spectral profiles from retarder stacks  104 . An example of this is managing the wavelength dependent resolution of polarization interference filters based on retardation dispersion. For instance, a conventional tunable polarization interference filter using polycarbonate dispersion with 18 nm FWHM resolution at 440 nm has a resolution of 33 nm at 655 nm. Using a digitally tuned filter  100  of the presently disclosed embodiment, the number of retarder films  105  can be selected in order to provide constant resolution throughout the operating band. At a fixed resolution (FWHM), the number of retarder films  105  needed in a 443 nm bandpass  802  is fewer than the number of retarder films  105  in a 601 nm bandpass  806  as illustrated in  FIG. 8 .  
         [0039]     Additionally, film-based digitally tuned filters  100  allow a significant range in resolution while maintaining an acceptable number of layers. If the range of free spectral range is large, then 1.5-wave retarder films  105  can be used. If the free spectral range is small, but requires greater resolution, then films  105  with 2.5-waves of retardation can be used. Additional waves of retardation may also increase resolution.  
         [0040]     Digitally tuned filters  100  of the presently disclosed embodiment may be used in a number of applications. In applications involving electronic sensors, such as silicon detectors of CMOS/CCD detector array, the filters  100  can be used to control the spectrum of light illuminating a scene. Alternatively, the scene can be illuminated with natural light and the filters  100  can be placed directly adjacent to the sensor. Filters  100  can be placed before the imaging optics, frequently relaxing the field-of-view and cosmetic requirements, but increasing the aperture size. Filters  100  placed between the imaging optics generally are smaller, but the optical quality and stability of the transmission spectrum with incidence angle is more critical.  
         [0041]     Exemplary applications of the systems above include an image projection system in which the optical components are used with the digital filter in order to scan through wavelength spectra to be projected on an image plane. In another possible application, the digital filter is to scan through light focused by the optical components onto a CMOS or CCD detector array.  
         [0042]      FIG. 9  illustrates a digitally tuned filter  912  in a fluorescence or other spectrometry system  900 . Light  902  from a lamp  904  in connection with a bandpass excitation filter  905  or light from a laser is filtered and reflected from a beam splitter  906 , and focused onto a sample  908 . The beam splitter  906  may also comprise a narrow-band reflecting coating at its reflecting surface. The sample  908  emits a fluorescence signature  910 , which passes through the beam splitter  906 . This signature light  910  is filtered by the digitally tuned filter  912 , which cycles through a sequence of bandpass spectra, and is focused/imaged onto a sensor  914 . Through this wavelength scanning, which also might be performed in the optical path before impinging upon the sample  908 , the spectral profile of the signature light  910  can be measured. Control and sensing may be achieved by a digital controller  120  positioned to provide voltage control signals to the LC cells  106  as illustrated in  FIG. 1 .  
         [0043]     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein.  
         [0044]     Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.