HK1114908A - Multi-conjugate liquid crystal tunable filter - Google Patents

Abstract

A hyper-spectral imaging filter has serial stages (33, 35) along an optical signal path, each stage having angularly arranged retarder elements (45, 47) and one or more polarizers (42, 44). The retarders can include tunable (such as abutted liquid crystals tuned in unison), fixed and/or combined tunable and fixed birefringences, and can be arranged in Sole, Lyot, Evans or similar configurations. Each stage has a periodic transmission characteristic with periodic bandpass peaks (52) spaced by free spectral range bandpass gaps. Distinctly different retardations are employed in cascaded stages, causing some stages to pass narrow bandpass peaks and other stages to have widely spaced bandpass peaks (large free spectral range). The transmission functions of the serial stages are superimposed, providing a high finesse ratio and good out-of- band rejection. Preferably at least some stages have tunable liquid crystals for at least part of their retardation, and are controlled to selectively align respective bandpass eaks.

Description

Multi-conjugate liquid crystal tunable filter

Technical Field

The invention relates to an optical filter with a tunable wavelength passband for use in hyperspectral imaging. An optimized design is employed to achieve a high filter precision ratio with multiple stages while maintaining a high optical power transfer ratio.

The inventive tunable conjugate filter has a plurality of birefringent filter stages arranged in series, each stage having a number of stacked retarder elements. Preferably, each stage of stacked retarder elements has a Solc-type filter structure for that stage. A Solc structure represents a stack of equally birefringent retarders between the polarizers. The retarder elements have a particular rotational relationship with each other. The retarders may be present as liquid crystal tunable birefringent stacking elements, or by stacking elements with fixed retarders adjacent to the tunable birefringent liquid crystals. In other embodiments, the individual retarders are formed from two or more adjacent liquid crystal elements. Several Solc stages can be applied, each having a succession of Solcs with birefringent elements. The number of polarization filters is limited by the polarizers between Solc stages. The transfer functions of the stages are superimposed, each stage contributing to further narrow the bandpass, or to increase the free spectral range between the bandpass. The result is a high level of detail suitable for hyperspectral images. The filter is tunable to the separate bands. The filter is particularly useful for Raman (Raman) chemical imaging and other suitable applications requiring multiple tuning functions and a narrow passband.

Background

Optical bandpass filters that rely on birefringence are known in a number of different configurations. Birefringence is a property of a crystal in which orthogonal light components, aligned along the respective fast and slow axes of the crystal, differ in optical index. If a plane-polarized incident optical signal is aligned at a 45 ° angle with respect to the indicator (reference axis) of a birefringent crystal, the effective optical path length through the crystal is not equal for the relative component perpendicular to the indicator and the vector component parallel to the indicator. The crystal causes differential phase retardation and wavelength retardation.

Differential time delays can cause changes or rearrangements in the polarization state of light propagating through the crystal. For a given distance along the light propagation path, a larger phase angle is achieved if the wavelength is shorter, and a smaller phase angle is achieved if the wavelength is longer. If a birefringent crystal causes a differential time delay, the delay corresponds to a larger phase angle or a smaller phase angle, respectively, for short or long wavelengths.

The polarization state of light relates to the phase relationship between the orthogonal light components among other variables. Differential time delays in birefringence can cause rotational polarization realignment as a function of differential phase angle retardation. Thus, birefringence provides a basis for distinguishing particular wavelengths. For example, a planar polarization filter disposed at the output of a birefringent filter has an input with a normal polarization alignment that distinguishes wavelengths in the output aligned with the polarizer.

In summary, an optical signal passing through a birefringent crystal will have a differential time delay. The differential delay changes the polarization arrangement of the optical signal by an angle that is a function of wavelength to place one or more polarizers along the optical path. The wavelength or vector component that appears aligned with the polarizer is passed.

The stacked bandpass filter elements may be arranged along an optical signal path, the principle being that each stage should make the filter more discriminating. However, there are complex situations. For example, if the individual stages are not precisely aligned, in particular if tuned, light will be blocked from passing. Also, each stage may cause some transmission loss. Thus, by stacking multiple stages, the wavelength bandpass can become more discriminative, but at the desired wavelength, the ratio of light energy passed to light energy input can be unacceptably low.

Each polarizer has an inherent loss and reduces the light energy of the transmitted beam, even though the beam is plane polarized and aligned with the polarizer. The specific loss varies with wavelength and polarizer used, but is likely to be, for example, 12%. The level of light energy passing through the filter may also be low if many stages are required to provide a high degree of discrimination or a very narrow bandpass. Low transmission rates may require collection of light energy over a relatively long period of time to obtain an image or measurement.

The discrimination of specific wavelengths by changing the polarization state produces a wavelength-phased effect. For example, if the differential delay is 2 π radians or an integer multiple thereof, the effect is the same as without the delay. With respect to the plane polarizer, if the polarization state is changed due to a differential phase delay of integral multiples of pi radians (180 °), the wavelength retarder and the plane polarization filter pass aligned light of two wavelengths.

Birefringent interference filters with multiple stages have been developed, using different combinations of wavelength retardation and polarization filtering to distinguish wavelengths. In a particular configuration, the stacked birefringent filters may provide very narrow spectral resolution early stacked birefringent crystal filters were used for viewing the daylight lugs and were configured with crystal thickness and rotational direction to emit certain very specific, narrow and precisely defined spectral lines of the spectrum through the daylight lugs. In other applications, it is desirable that such filters be tunable.

Sub angstrom spectral resolution is said to be obtained using filters developed by b. A basic Lyot filter comprises several filter stages arranged in succession along the optical path. (see Yariv, A. and Yeh, P. (1984) light waves in crystals, Chapter 5, John Wiley and Sons, New York). Each stage has birefringent crystal elements (a wavelength retarder) between parallel polarizers. The exit polarizer of one element may act as the input polarizer of the next element.

The Lyot birefringent crystal has an optical axis parallel to the surface and is rotated 45 degrees with respect to the input polarization direction, thus equally dividing the light from the input polarizer into two components between the fast and slow axes of the birefringent crystal. During propagation through the crystal, the component in the slow axis becomes sluggish relative to the component in the fast axis. The polarization direction of the light is also changed. At output, the exit polarizer at 45 degrees to the previous crystal maintains equal proportions of the slowed and un-slowed components, but passes only the wavelengths that pass through the relative alignment of the change in polarization angle behind the crystal and the input and output polarizers (or differ by an integer multiple of 180 degrees).

A Lyot-type filter has a repeating arrangement of crystals between polarizers, each crystal being aligned at 45 degrees relative to their polarizers. The induced phase difference in Lyot is due in part to the difference in thickness of each stacked birefringent crystal element. Both thickness and birefringence contribute to retardation. In the Lyot structure, the wavelength delay caused by the crystal in each stage is exactly twice the wavelength delay of the crystal in the previous stage. The bandpass wavelength is related to the thickness and birefringence of the crystal.

The thicknesses of successive stages (e.g., 1d, 2d, 4d, 8d, etc. for Lyot) and the relative rotational alignment of successive stages are selected to provide an arithmetic, geometric, or other mathematical progression. The operation of the stages may be mathematically modeled and measured empirically. Multi-stage crystal devices have exhibited 0.1 angstrom resolution (title, a.m. and Rosenberg, w.j.opt.eng.20, 815 (1981)). To achieve such a resolution, spatial accuracy is required, making such filters expensive. In general, resolution is improved by simply increasing the number of consecutive cells, sometimes using a very large number of consecutive cells. This has the disadvantage that the ratio of transmitted light to filtered light is reduced. Such a filter is suitable for astronomy applications, where the filter is tuned to the daylight lug specific spectral lines, where the light source, e.g. the sun, is very bright.

Another variation of the stacked crystal filter is a Solc-type filter. Solc filters, like Lyot, use multiple birefringent crystals in the stack of plates, but unlike Lyot, Solc filters do not require polarizers between each birefringent crystal retarder. The Solc structure better preserves visible light energy. The Solc structure uses birefringent crystals of the same thickness. Multiple interference wavefronts are obtained by varying the direction of rotation of each successive birefringent crystal.

Solc filters provide high precision and high transmission using series of identical phase retarders without the need for polarizers between each retarder. For example, Solc-type filters are described in Solc, J.Opt.Soc.Am.55, 621, (1965).

Solc filters are an example of a wider class of filters. In Harris et al, j.opt.soc.am.54, 1267, (1964) it is believed that in principle any filter transfer function can be produced by a stack of suitably oriented retarders having the same direction. Other researchers have created filter designs using network synthesis techniques in conjunction with standard signal processing methods on this premise. These designs have sought high resolution in a limited spectral range to combat a wide spectral range. The filter holds the retarder. When tuning is considered, all the wavelength delays need to be varied in unison.

In the Solc configuration, the relative rotation angle between each birefringent crystal and the next preceding or succeeding crystal is a fraction of the rotation angle between the entrance and analyzer polarizers before or after the stack. A Solc sector filter structure has N identical crystals with an angle of rotation theta, 3 theta, 5 theta. The folded structure of Solc has N identical crystals at an angle of + -theta to the incident polarization, where theta is the angle of the optical axis between the crystal and the transmission axis of the entrance polarizer. The folded design has crossed polarizers.

For example, there may be four retarder elements and parallel polarizers in a Solc "fan" arrangement. In this Solc "fan" arrangement of four crystals (N ═ 4), the first crystal is rotated 11.25 degrees with respect to the input polarizer. Successive crystals are rotated 22.5 degrees relative to the next preceding crystal. The output or analyzer polarizer is parallel to the entrance polarizer. In contrast, a four retarder Solc "folded" arrangement has four stacked crystals, staggered with respect to the polarizer, arranged at clockwise and counterclockwise rotation angles, such as +11.25 degrees, -11.25, +11.25, etc.; and the analyzer polarizer is perpendicular to the entrance polarizer. Other variations that may be used are different values of N, theta and orientation of the polarizer.

The Solc structure has advantages and disadvantages. Since all crystals have the same thickness, a large number of stages will be mounted on a stack that is longitudinally shorter than a Lyot structure. On the other hand, having more crystals in the stack increases the number of different crystals, which requires precise dimensions and alignment.

Multistage crystal filters depend on their geometric accuracy. Dimensional changes due to manufacturing errors and temperature, as well as relative position, may appear to be small, but in light wavelengths measured in nanometers, physical dimensional changes should be taken into account. Therefore, stacked crystal filters require temperature control to ensure that alternative operating temperatures are reached, resulting in further increases in cost.

In the present invention, the birefringent stacked filter is applied to a spectral imaging apparatus, such as a high performance Raman spectral imaging system. Such imaging applications require very narrow band-passes and generally high finesse (defined as the ratio of free spectral range to bandwidth: FSR/FWHM). The filter also needs to be tunable to select the wavelength passband.

The fixed retarder provides a phase retardation determined by the crystal birefringence and the crystal thickness along the propagation axis. Electrically tunable birefringent filters are recommended which use liquid crystals as tunable elements. Typically, the birefringence of a liquid crystal is a function of the amplitude of the electric field applied to the crystal. The variation in birefringence of the liquid crystal produces an effective retardation similar to the variation in thickness of a fixed retarder. In a Solc-type stacked element filter, the structure is such that the birefringent crystal elements in the stack have the same thickness and/or the same birefringence. The tunable elements are tuned in unison. The stacked elements are aligned sequentially in a particular rotational arrangement, either in a progressive angular sequence or in an interactive "folded" configuration. In a further example, the geometric approach (including birefringence and thickness) is to align the polarization state of a selected wavelength passing through each stacked element in an equal manner to the next element. An input polarizer establishes a reference alignment. Any number of elements may be stacked in the filter body, with a larger number of elements generally providing higher resolution than a fewer number of elements. The output polarizer passes only wavelengths emerging from the stack having a selectable predetermined polarization alignment.

Several wavelength band pass filter configurations may be considered for a given application. However, limitations can affect the selection, including performance considerations such as bandpass resolution and accuracy. A high transmission rate may be required to obtain acceptable signal strength, signal-to-noise ratio, or image acquisition speed. Very large capacity structures may not be suitable for desktop and portable applications. Another important way is that it is very costly.

It would be desirable to provide a structure that takes into account most or all of the assertions of various options, and provides a high degree of precision in setup, and has reasonable cost and other performance.

Disclosure of Invention

It is an object of the present invention to provide a wavelength band-pass filter with good optical and operational characteristics, suitable for raman imaging and other potentially demanding spectral imaging applications, and at a reasonable cost. The term "spectral imaging" is understood to include, but is not limited to, the development of spatially accurate wavelengths to account for a two-dimensional image of an object in a selected wavelength image.

Another object is to provide an optimized selection of elements to maximize accuracy while also ensuring a high transmission rate. One technique for achieving high transmission is to use a Solc filter structure that requires only an input polarizer and an output polarizer, plus a folded stack or continuous angle birefringent stage.

Another technical approach involves placing Solc filter stages in series such that the dilution of precision from each successive stage is doubled to provide the transfer function of the conjugate filter as a whole. It is worth noting that the series arrangement is a continuum of all Solc filters, not just birefringent retarder elements as typical conventional Solc filter arrangements. Each successive stage has a stack of an input polarizer, an output polarizer, and a number of identical birefringent elements therebetween. According to an advantageous aspect, the output polarizer of a given stage acts as the input polarizer of the next stage, the total number of polarizers being only one more than the number of stages.

The birefringent elements in each stage provide the same retardation (same birefringence and typically the same thickness), which is an aspect of the Solc structure. According to yet another inventive aspect, the retardation and/or thickness of the different steps are different. Similarly, different numbers of retarder elements may be included in the stack of successive stages. By varying the stages in this way, the effect of the stages can be superimposed, with only ordinary precision if they are separate, but with very high precision if they are put together. For example, a stage with good free spectral range FSR values (that is, a long range between periodically repeating bandpass peaks) is provided, although the stage may have relatively poor bandpass resolution (disadvantageously large FWHM values) at its peaks. In one or more next successive stages, stages with advantageously narrow FWHM are provided, although it is possible to have a relatively short free spectral range FSR. By coordinating the stages as described above, in particular by superimposing the passbands of the two stages, the conjugate filter comprises two stages with narrower FWHM and longer FSR. The ratio of these values for the two stages is precision, doubled to provide a conjugate precision value.

The conjugate filter described above is preferably tunable. The filter may comprise one or more tunable stages using only liquid crystal birefringent elements, and/or stages using fixed retarders with liquid crystals connected to add controllable retardation to the fixed retardation. It is also possible to include stages some of which are tunable and others of which are not, in any case, to provide a series superposition of the wide free spectral range and the narrow selected bandpass FWHM value therein.

In an operational embodiment, the conjugate filter need not have an entirely continuous free spectral region, but rather has tunable selectable peaks, the conjugate tuning by the stage being operable to select a plurality of bandpass peaks that do not include all possible continuous center wavelengths, but include a large number of closely spaced selectable bandpass wavelengths within the entire tunable region.

These and other aspects will be apparent from the following discussion and detailed description, which are intended to illustrate non-limiting examples of the invention, the scope of which is defined in the following claims.

Drawings

FIG. 1 is a schematic diagram of a multi-conjugate liquid crystal filter for spectral imaging and similar applications in accordance with the present invention;

FIG. 2 is a schematic view of a single stage Solc sector configuration with multiple elements;

FIG. 3 is a graph showing an exemplary spectral transmission of a six-element Solc filter transfer function;

FIG. 4 is a spectral transmission diagram corresponding to FIG. 3, wherein two six-element Solc stages are arranged in series along the optical transmission path;

FIG. 5 is a schematic diagram showing how Solc filters can be arranged in series in accordance with the present invention, in this case one with a high free spectral range and the other with a narrow bandpass, coupled to provide high accuracy and good transmission rate;

FIG. 6 is a schematic diagram of a tunable element that is a controllable birefringence in the embodiment of FIGS. 1 and 2;

fig. 7 is a schematic diagram of a two-stage Solc filter according to the present invention having fixed and tunable retardation, respectively, with cooperating elements.

FIG. 8 is a schematic of three stages in a further amplifiable architecture.

Detailed Description

In some spectral imaging applications, it is advantageous for a filter to have very fine resolution and also be capable of being stably tunable. At the same time, it is desirable to have a strong signal level in the final photo-responsive sensor element so that there is a good signal-to-noise ratio when obtaining relatively fast images. Meeting these objectives at the same time is difficult, thus resulting in design compromises. According to one aspect of the present invention, a preferred arrangement is provided to achieve tight bandwidth discrimination, high transmission rates and tunability over at least a set of selectable wavelength bands. This is achieved by connecting multiple tunable Solc wavelength bandpass filter stages in series.

Any one Solc stage can have a medium value of precision. The accuracy is the ratio of the free spectral range FSR (i.e., the wavelength span between bandpass peaks) to the full width at half maximum of the peak bandwidth (FWHM, full width measurement at half maximum).

According to one aspect of the invention, the stages are arranged in series, wherein one stage comprises elements having a birefringence that is substantially different from the birefringence of the elements of the other stage. Thus, the order is different for the corresponding contribution of the free spectral region between periodic peaks (which needs to be larger) compared to the bandpass width of the individual peaks (which needs to be narrower). The others are the same, with the stage with the greater birefringence (typically having a greater retarder thickness) having a advantageously narrower bandpass peak than the other comparable stage with the lesser birefringence. However, this stage (a retarder with thicker or greater birefringence) has an undesirably short interval between peaks. Conversely, a thin retarder stage (with lower birefringence) has a broad bandpass peak (peak with poorer resolution), but the peaks are more widely spaced apart.

The transfer function of the serially arranged filters is incrementally applied to the passing optical signal. The transfer function is doubled. The overall precision ratio of the multistage filter is the mathematical product of the precision ratios of the stages. By applying the transfer function of a low birefringence level successively to the output with a higher birefringence level (or vice versa), one bandpass peak can be selected by the action of the lower birefringence level from the sequence of closely spaced narrow peaks provided by the higher birefringence level in subsequent neighbourhood. The doubled transfer function advantageously produces narrow peaks with a wide free spectral range between the peaks.

Preferably, each of the two stages (and more if any) is tunable, so that the band pass is adjusted so that it is tuned to any desired wavelength in the tuning band. The lower birefringence element is tuned so that a discrete narrow bandpass peak can be selected from the transfer function of the higher birefringence element. If the lower birefringence element is sufficiently tunable, a narrow band can be selected in the transfer function of the higher birefringence element using the tunable peak.

Fig. 1 illustrates the aforementioned arrangement in a simplified schematic diagram. Such as a light input signal formed by aligning microscope optics of a laser illuminated sample (not shown) to produce a light signal 30, schematically illustrated as a single light beam. An exception setting is also possible. The optical signal 30 is collected in a photodetector 39 through a plurality of filter stages 33, 35, typically also connected in series to a digitizer (not shown) or device for processing the signal.

Each of the stages 33, 35 is a functional Solc-structured wavelength band-pass filter. As shown in fig. 2, a Solc structure has an entrance polarizer 42, an exit polarizer or analyzer 44, and successive retarder elements 45, 47, etc. The entrance polarizer 42 and the exit polarizer 44 are planar polarizers having a reference angle associated with a predetermined rotation angle between zero and pi radians.

In the examples of FIGS. 1 and 2, the entrance and exit polarizers of each Solc stage are rotated by π/2 or 90 relative to each other. This angle is merely an example. Other angles are also possible. In this example, assuming that the light pillar 30 is plane polarization aligned in alignment, or at least includes a vector component aligned with the entrance polarizer 42, the polarization component parallel to the polarizer reference angle passes through the entrance polarizer. In the example of a 90 relative rotation, those components would be precisely misaligned with the exit polarizer 44 and no light would pass. However, several retarder elements 45, 47 are arranged between the polarizers 42, 44. Each retarder element 45, 47 contributes incremental retardation due to birefringence. Each retarder element thus rearranges the polarization state of the beam.

Each retarder produces a phase delay between orthogonal components due to birefringence, i.e., the crystal properties of the optical index of light passing through the retarder are different for light aligned along orthogonal fast and slow axes of the crystal. Effectively, the optical propagation path is longer for one component and shorter for the other component. However, polarization is only part of the problem of the phase relationship between orthogonal light components, the phase retardation of which produces a rotational change in polarization alignment. The polarization state is twisted by a certain angle determined by the birefringence of the retarder 45 or 47.

A given wavelength delay distance is equal to a larger phase angle for shorter wavelengths and a smaller phase angle for longer wavelengths. Thus, the twist angle due to the retardation varies as a function of wavelength. In the embodiment shown in fig. 2, the respective retarders are dimensioned and aligned at a specific rotation angle Φ, which is known as a folded Solc configuration. In an alternative embodiment, the Solc configuration may be further such that each retarder is further angularly rotated at a particular angle than the previous retarder to oppose the back-and-forth alternation shown. The settings are the same anyway: for a particular wavelength (and a particular wavelength of periodic relevance), the sum of the rotational angle differences from one retarder 45 to the next 47, by this process, adds up to the rotational angle between the input and output polarizers 42, 44. Assuming a range of wavelengths are present in the input signal 30, only a certain wavelength is wavelength-retarded by the angle required to pass through the exit polarizer 44.

The free spectral range is the difference between successive periodically related wavelengths passed by a stage of the filter. In general, a larger free spectral range can be obtained by using a smaller retardation. The degree of retardation is determined by the birefringence of the retarder material and its thickness. As shown in fig. 1, one aspect of the invention is the use of multiple Solc filter stages, providing different retarders in the different stages (retarders with thicknesses "d" and "2 d" as shown).

The accuracy of the filter is the ratio of FSR to FWHM, that is, the bandwidth of the bandpass peak through which the free spectral region passes. The pass bandwidth FWHM can be made small by using a large number of retarders 45, 47, etc. in the stack of series retarder elements in the respective Solc stages 33 or 35, etc. Again, given a range of wavelengths, the Solc filters provide successive stacked elements of the same wavelength delay. Each element selects the desired wavelength from the output of the previous element since the same wavelength delay and optional alignment of the desired wavelength is also selected.

In general, the FSR value of a Solc filter is inversely related to the degree of birefringence, proportional to thickness in a fixed retarder. The Solc filter structure requires the same birefringence for each retarder, and the same rotation angle (or further twist angle) that equally divides the position angle from the entrance polarizer to the exit polarizer in equally birefringent retarders. The requirement for equal birefringence and/or equal thickness retarders in Solc structures may prevent the use of different thicknesses or birefringence. However, according to one aspect of the invention, the thickness and/or birefringence remains the same within each stage (thus maintaining the Solc structure), but is completely different between successive polarizers of different stages.

An electrically tunable Solc filter can be envisioned. The difficulty is that the liquid crystal retarder elements need to be aligned accurately and their birefringence needs to be the same. However, operational limitations due to liquid crystal thickness often result in a limited tuning range.

According to one aspect of the invention, at least one, and optionally all, of the Solc filter stages 33, 35, etc., comprises a stack of retarders, wherein the birefringence of the individual retarders in the stack is provided in part by a fixed retarder and in part by a tunable liquid crystal retarder, the tunable retarder being selectively coupled to the fixed retarder.

According to a particular embodiment, at least one stage of the Solc filters may have only tunable liquid crystal retarders. Also, the liquid crystal retarders may be defined as retarder elements each disposed in series at a different rotation angle Φ (see fig. 2). Alternatively, the retarder may include two or more tunable and equally sized liquid crystal elements at respective corners, abutting each other to provide a multiple of the total thickness. One pair (or three, or four, etc.) of consistent controls.

Since the filtering process of each stage is applied to the filtering process of the previous stage or stages in an overlapping manner, the effect of successive Solc filter stages is doubled. One or more thin retarder Solc stages with a large bandpass (large FWHM) and large free spectral range (high FSR) can be applied together with one or more thicker retarder Solc stages with a narrow bandpass and small Free Spectral Range (FSR). If the bandwidth of the former Solc stage is narrow enough to effectively select one of the periodic peaks of the latter Solc stage, the result is that the stage precision of the filter is doubled.

In this manner, in accordance with the present invention, a conjugate filter may be provided having multiple stages 33, 35, etc., where one or more of the stages, e.g., stage 33 in FIG. 1, has retarders of the same but lesser birefringence and the other stages, e.g., stage 35 in FIG. 1, have retarders of greater birefringence, although the same in that stage. By multiplying the transfer functions of the Solc filter stages with greater and lesser birefringence, the accuracy of the stages is also doubled. Since the stages 33, 35 are Solc stages (see also FIG. 2), only one output side polarizer is required to distinguish the selected wavelengths.

Fig. 3 and 4 illustrate aspects of improving filter accuracy, particularly by providing aligned spectral peaks for multiple series Solc filter stages to improve or narrow spectral resolution. Fig. 3 shows the transmission spectrum of a given Solc filter stage. Fig. 3 shows the transmission spectra of two similar structural levels arranged in a series sequence. As shown in fig. 3, the transmission spectrum of a stage has a given FWHM bandpass width at any periodic peak 52, separated by free spectral range FSR. The spectrum of the stage typically has side lobes 55 which are generally disadvantageous in the filter as it tends to widen the effective FWHM bandpass peak and thus reduce accuracy. As shown in fig. 4, two (or more) successive filter stages should have the same or similar transmission spectra, with the combined effect of narrowing the FWHM bandpass, including reducing the side lobe amplitude values.

Fig. 5 shows another aspect of improved accuracy in which two (or more) spectrally aligned filter stages are provided, where stage one and stage two in this example produce a bandpass peak over a particular free spectral region (the next peak of stage one and stage two is not within the labeled range of the figure). The peak may have certain side lobes and may also have a wider FWHM width for ease of illustration. However, when the spectrum is doubled by a third order spectrum, in this example with a narrow passband and disadvantageously short FSR, the conjugate filter result is characterized by a narrow bandpass and wide FSR, i.e., improved accuracy.

As mentioned above, the spectral imaging filter of the present invention comprises at least two spectral filter stages 33, 35 connected along the optical signal path. Each filter stage has a periodic transfer characteristic with bandpass peaks 52 separated by free spectral bandpass gaps FSR (i.e., as shown in fig. 3). In one embodiment, the filter stages are Solc stages, which improve accuracy by simply narrowing the bandpass peaks in succession. In another embodiment, the stages of the two or more filters comprise at least one stage having a larger free spectral range than the other stage, i.e. having a larger gap between bandpass peaks on one of the filter stages, and the stages comprise at least one overlapping passband. Thus, when multiple filter stages are conjugated or caused to double their spectra in successive transmissions, the accuracy of the conjugate pair includes bandpass peaks in the overlap band.

The accuracy can be improved by applying filter stages of similar structure as shown in fig. 3, 4 in succession, since the product of two identical transmission peaks is a narrow peak at the same central wavelength.

Substantial accuracy improvements (and thus polarizer requirements) can also be provided by configuring the filter stages as Solc stages without unduly reducing the overall transmission rate, and with one of the conjugate stages having a longer free spectral range than the other of the two filter stages, which has a narrower bandpass peak. Assuming that the bandpass peaks of the at least two filter stages overlap in the operational state of the filter, the overall transmission characteristic of the spectral imaging filter is characterized by an optimal performance of each stage, i.e. a larger free spectral range of one stage uses a narrower bandpass peak of the other stage.

In one embodiment, the bandpass peak center wavelength of one or more stages is determined by using a fixed retarder as a birefringent element in that filter stage. Preferably, however, at least one filter stage is tunable to an operational state, wherein the bandpass peaks of the at least two filter stages overlap. Referring to fig. 5, for example, by providing a tunable structure in which the bandpass peak wavelengths of stage one and stage two are selectively shifted within a certain range, the bandpass peak can effectively select one of the peaks in stage three, although stage three need not be tunable.

In another arrangement, stage three may also be tunable, so that tunable selection of the periodic bandpass wavelengths of two stages in a conjugate filter comprising successive stages in series may be made in a coordinated manner by tunably shifting the bandpass peaks of the transmission spectra of the stages to enable substantially continuous selection of any wavelength within a tuning range.

In any case, one or preferably more of the filter stages may be tuned to an operational state in which at least one bandpass peak in the transmission spectra of all of the plurality of filter stages overlap. The conjugate filter passes overlapping frequency bands.

In fig. 1 and 2, at least two filter stages 33, 35 comprise a Solc filter structure having a set of birefringent retarders 45, 47, etc. disposed and rotationally aligned between two polarizers 42, 44. The retarders within each stage provide the same retardation as the other retarders within the same stage. Preferably, however, the retarders within different stages produce completely different retardations. Thus, the spectral responses of the multiple stages are typically different with respect to the balance between the FSR and FWHM values, e.g., characterized by the stage one-stage two spectrum versus the stage three spectrum in fig. 5.

One solution for providing a tunable birefringent retarder is to use tunable liquid crystal cells as a source of birefringence. A liquid crystal typically comprises support plates of fused silica or the like (glass) separated by mechanical spacers to define a gap for the liquid crystal material. Between the plates and the liquid crystal is a transparent conducting layer for the application of control voltages, and an alignment layer for determining the static alignment of the typically elongated liquid crystal molecules. The transparent conductive layer typically comprises Indium Tin Oxide (ITO). Various alignment layers are possible, for example SiO_xTreatment with brush or ion bombardment or similar methods may be used to establish crystal orientation. The application of the control voltage changes the birefringence of the liquid crystal. In effect, increasing birefringence produces an additional phase delay in the light component vector along the normal and extraordinary axes, the latter being the axis affected by the applied control voltage.

Liquid crystals comprise certain specific chemical compounds capable of exhibiting one or more liquid crystal phases, wherein the molecules of the compound are movably aligned. The material is birefringent when the molecules are aligned, and the degree of alignment is variable to vary the birefringence.

In a preferred construction, the liquid crystal cell used in the multiconjugate filter of the present invention is an Electrically Controlled Birefringence (ECB) liquid crystal cell with parallel rubbing on the top and bottom substrates to establish the molecular orientation. Other liquid crystal forms may be used, such as a vertically aligned nematic liquid crystal cell, a pi cell, an OCB cell, or a bend cell. In another structure, two of the above liquid crystal cells may be doubly stacked on each other to obtain better viewing angle characteristics.

Such a liquid crystal can be used as the retarder 45 or 47, etc. described above, and further has the advantage of providing an electrically controlled birefringence, which is very similar to the controllable retarder thickness. In accordance with a further aspect of the present invention, a controlled birefringence liquid crystal cell is attached to the fixed retarder to provide a predetermined retarder thickness and birefringence, wherein the controlled liquid crystal cell provides additional retardation at the control voltage used, as shown in FIG. 6. A lithium niobate (LiNO) may be used₃) The material acts as a retarder. Preferably, however, the retarder includes Bromine Borate (BBO) to enable approximate parameter matching with glass and/or to serve as a support plate for the liquid crystal structure shown in fig. 6.

In FIG. 6, a quartz plate 62 is mechanically separated from a fixed retarder 72 by a spacer 66 and sandwiches a liquid crystal layer 64. The ITO layer 82 allows the application of a control voltage V_CTL. The alignment layer 83 determines the orientation of the liquid crystal director and thus the fast and slow axes. The degree of retardation is determined by the controllable birefringence of the liquid crystal.

In the illustrated embodiment, the fast and slow axes of the liquid crystal 64 are aligned with the fast and slow axes, respectively, of the fixed retarder 72. The liquid crystal thus provides a controllable additional retardation between the polarization components of the same orthogonal vector passing through the fixed retarder 72.

In accordance with one aspect of the present invention, as shown in FIG. 6, the liquid crystal 64 and the fixed retarder 72 are preferably substantially optically matched. For this purpose, the fixed retarder preferably comprises bromine borate. This material has an optical index in the visible wavelength range of about 1.5 to 1.7, which is similar to that of glass. By using optical fingers having a similar appearance to glassSeveral retarders may be used without an anti-reflective coating when reducing the reflection at the interface between the retarder and the glass plate. If the optical indices do not match at this interface, multiple reflections may occur thereby reducing the transmission. Other birefringent materials, e.g. LiNO₃Have a higher index (e.g., 2.0 or 2.1) and may be used in the present invention, but should be used with an anti-reflective coating or the like. Other birefringent materials, such as calcite crystals, may also be used and may be compared to LiNO₃More compatible with glass and may be applicable but not preferred because of the difficulty in their manufacture. However, birefringent materials have different optical indices along different axes that can only be approximately matched to adjacent isotropic materials such as glass. This rough index matching reduces reflections.

The aforementioned materials and optical indices are concentrated in a hyperspectral filter in the visible wavelength spectrum. The invention is fully applicable to other spectra such as near infrared or ultraviolet light and is equally useful in a variety of chemical imaging applications.

The retarders 45, 47 in the respective stages may provide different retardations, either by materials with different birefringence characteristics, or by differences in thickness along the optical signal path, using a single retarder element or aligned and adjacent thin elements to form thicker elements, or by tunable additional birefringence, or by any combination of these different features. Preferably, the retarders in at least one stage comprise a liquid crystal tunable birefringent element. The liquid crystal element may be included in the retarders in one or more stages, that is, only liquid crystal is used for generating birefringence so that the retarders are fixed by default. Preferably, one or more of the stages includes a retarder employing both a fixed retarder and a liquid crystal. More preferably, however, the liquid crystals in these combined fixed and liquid crystal retarders are interconnected to form a liquid crystal tunable birefringent element, wherein the fixed retarder provides partial wavelength retardation.

According to the invention, there are many different construction possibilities. Figure 1 shows a simple schematic. Figure 2 shows one stage and shows that an infinite number "n" of wavelength delay elements can be applied in a Solc-structured filter. Fig. 7 shows a two-stage arrangement in which each retarder in each stage comprises a liquid crystal section and a fixed retarder section, the retardation being the same (and controllable in unison) within a given stage, but different from stage to stage, either by differences in thickness or by birefringence control.

In fig. 8, at least three spectral filter stages are connected along the optical signal path. Each stage is separated by a polarizer so that the output polarizer of the preceding stage serves as the input polarizer of the subsequent stage. Thus, the number of polarizers is limited to the number of stages plus one. In fig. 8, all the retarder elements include liquid crystal, and the retarder elements of a given rotational alignment state of the card include more than one liquid crystal.

In the advantageous arrangement shown in fig. 8, the thickness of the retarders (which differ in different stages) is determined by stacking different numbers of liquid crystal controlled birefringence elements in the retarders for the different stages as a way of achieving birefringence and thickness from stage to stage. The first stage in fig. 8 thus has two rotationally wobbled retarder elements, each having a liquid crystal element. The second stage has two aligned and stacked liquid crystal elements for each rotationally wobbled retarder. The third stage stacks three liquid crystals for each element. The use of multiple controllable but very thin liquid crystals in the stack increases the tuning speed of the filter, which is superior to using thicker liquid crystals to achieve the same total birefringence. Further, assuming that the tunable liquid crystal elements are of the same single thickness, this arrangement results in multiple stages with retarder thicknesses d, 2d, 3d, etc. in successive stages (where "d" is the thickness of one element).

In the embodiments that have been described, the cooperating retarders include one or more stages having retarder elements that are all or partially fixed retarders adjacent to the controllable liquid crystal, using an index matching fixed retarder material such as bromine borate to maintain a high transmission rate that is generally obtained using the stacked Solc structured filter stages of the stacked retarders described above, that is, by reducing the intermittent reflection during the progression of the optical index along the optical path.

Although generally applicable for high resolution wavelength discrimination, a preferred application of the filter of the present invention is as a tunable spectral filter for raman images. Each filter stage comprises a plurality of identical retardation, rotationally distributed retarders leading to an output polarizer (i.e., Solc-structured stage), with the output polarizer of the preceding stage serving as the input polarizer of the subsequent stage. The number of stages, and the number and thickness of the respective retarders in the stages, are selected to provide a free spectral range from about 500 to 650 nm, and a FWHM bandpass peak width of about 0.25 nm. This requires a filter accuracy of at least 600 (i.e. 150 η m FSR and 0.25 η m FWHM). In that case, the required accuracy can be obtained by limiting the number of spectral filter stages so that a reasonably good transmission is obtained when the selected wavelength passes through the spectral filter.

The transmission loss from the polarizer is a function of the polarizer material, the wavelength of the optical signal, and the like. In the visible spectrum, a typical polarizer may have a typical transmission of about 88% (i.e., 12% loss) for polarized light aligned with the polarizer. Under this assumption, a Solc stage with an input polarizer and an output polarizer has a 77% transmission rate, due only to the associated loss of the polarizers. In the multi-stage Solc structure of the present invention, only one polarizer is added per additional stage. Thus, three polarizers (two stages) allow a transmission of 68%, three stages 59%, four stages 52%, five stages 46%, six stages 40%, etc., which may be attributed to the polarizers. As already described, the precision of the stages is doubled and the necessary precision 600 can be exceeded with six stacked stages at a transmission rate of about 40% if each stage has a moderate precision ratio of about 3.

One characteristic of the described birefringent filter is that the operable wavelength range of the filter is largely determined by the transmission spectrum of the polarizer used in the filter. Fixed retarders and liquid crystal materials typically have high transmission properties over a very wide range of the wavelength spectrum. However, the transmission spectrum of the polarizer may be limited to, for example, the Ultraviolet (UV), Visible (VIS), Near Infrared (NIR) or mid-wave infrared (MWIR) ranges. For the multiconjugate filter of the present invention, the filter can be operated in the corresponding UV, VIS, NIR or MWIR wavelength ranges by selecting different types of polarizers. A typical VIS polarizer may be the NPF series polarizer of NITTO DENKO. Edmund optics with high contrast UV and NIR polarizers can cover the 365 nm to about 1700 nm wavelength range. The ColorPol series UV-to-NIR polarizers from CODIXX may cover the 350 to 2500 nm wavelength range.

This is advantageous for many hyperspectral imaging applications where the collection of light signals in a broad wavelength spectrum is required.

The following set of tables is provided to illustrate a particular embodiment of the multistage conjugate filter of the present invention. By way of explanation of the abbreviations, these embodiments illustrate that the multi-stage conjugate filter (MCF) comprises a multi-stage Liquid Crystal Tunable Filter (LCTF) in a Solc configuration, where the rotational position angle between two polarizers along the optical signal path is distributed to a set of identical birefringent retarder elements. Thus, a predetermined wavelength (which may be tunably selected) is continuously rotated or twisted to a polarization state, passing through each birefringent element, so as to be aligned with the second or exit polarizer. Other wavelengths are blocked.

In one embodiment, each LCTF has a liquid crystal birefringent element, a fixed retarder plate, and a polarizer. In other embodiments (or in stages of a given embodiment), the retardation is provided only by the liquid crystal tunable element. In an LCTF embodiment, liquid crystals may be stacked.

Each stage has a Solc filter structure, but according to an aspect of the invention, the retarders of fixed and/or variable retarders have the same retarder within a stage and have specific differences between successive stages to meet performance goals. In particular, the retardation provided by some stages is significantly greater than that of others, typically by employing fixed or variable retarders in some stages that are significantly thicker than those of others. In the embodiments discussed above, the difference may be at least one of two factors. The examples in the following table illustrate other factors and the properties they can achieve.

To implement the present invention, each stage of the multistage conjugate filter preferably has a precision ratio (that is, FSR/FWHM) greater than 4>4). More preferably, the accuracy is greater than 5. Also preferably, the ratio of the free spectral range of any two stages in the filter is greater than 2, and more preferably greater than 2.5 (that is to say the FSR is₁/FSR₂>2.0 to 2.5). For a given retarder material, the FSR varies as a function of retardation or birefringence and thickness. Thus, the ratio of the free spectral range as a whole produces a clear difference in the thickness of the retarder plates in the Solc filter structure of at least two stages.

The individual retarders in a stage may comprise a single birefringent layer, incorporate a fixed birefringent filter and preferably index-matched with a liquid crystal cell, or multiple birefringent layers with fixed retarders and liquid crystal cells. The filter structure may be a Solc folded structure with corners, as shown in FIG. 2, or alternatively the stages of the filter may be Solc fan-out structures, Solc Gaussian (Guassian), Solc Linear, Solc sine (Sinc), or other structures. In addition, the multipole conjugate filter of the present invention may have stages of different configurations, both in terms of the use of fixed and tunable retarders in the respective stages, and in terms of the co-operating stages in which the various possible Solc filter configurations described may be used.

The reason for adopting the structure of the Solc filter stages is that the number of polarizers is limited to the polarizers provided between the successive stages, and the output polarizer of the preceding stage can serve as the input polarizer of the subsequent adjoining stage. This results in an optimally high transmission at the filter bandpass wavelength. However, in an embodiment where the transmission rate is not very important, it is also possible to provide different conjugate stages with a structure such as a Lyot filter structure or an Evans (Evans) variant.

A preferred embodiment comprises a Solc filter stage. In a stage with one or more fixed retarders, the fixed retarder may comprise BBO or. alpha-BBO material having a refractive index close to that of glass. This is advantageous in reducing or eliminating the need for anti-reflective coatings between elements.

In this embodiment, each fixed retarder is followed by a liquid crystal birefringent element along the optical signal path. The liquid crystal has a maximum retardation of less than 5 microns (5 μm).

According to one aspect of the invention, tuning the filter to include the full operating spectral range does not require shifting the peaks of each individual stage covering the full spectral range. The periodically repeating peaks are tunable sufficient to provide a range of tunably selected wavelengths covering a spectral range in which the bandpass peaks are aligned and thus provide an entry for the same tunably selected wavelength to pass through a continuous train of filter stage peaks. Typically, any particular periodic peak needs to be tunable and movable over only a small portion of the spectral range of the multiconjugate filter. The ability to selectively align the different peaks from the periodic pattern of repeating peaks of each stage (where each peak may be tunable only over a medium free spectral range) ensures proper structural control to set the stage to a tuned state where the periodic peaks of a given stage are selectively aligned with the other peaks when tuned to a desired bandpass wavelength.

Multiple LCTF tunable stages can achieve high out-of-band rejection and narrow spectral bandwidth around the tuned bandpass wavelength in this manner. Multiple stages may use tunable and fixed retarder stages, but differences in retardation from one stage to the next may also be achieved by using stacked liquid crystals with multiple abutting elements (or a larger number of multiple abutting elements) for other stages that require greater thickness to achieve greater retardation.

The invention can use tunable LCTF wavelength delay elements that include relatively low birefringence or low dispersion liquid crystal materials, but still achieve the desired Free Spectral Range (FSR) when the multi-conjugate filter is integrated. Individual stages with low birefringence retarder elements can be used to exploit their larger free spectral range. This may lead to the result of a discrimination of the sacrificial spectrum (bandwidth wide), but by repeating the secondary and by relying on a narrow spectral discrimination of the other typically thicker element stages, the conjugate filter achieves all the purposes: out-of-band filtering, tunability, and wavelength bandwidth (FWHM) at the tuned bandpass wavelength.

The number of examples, including examples, that appear in the various embodiments of the present description are detailed in tables 1 through 6 below. These embodiments have at least two stages, preferably from three to six stages, each stage having at least two, preferably four or more, retarders. In other arrangements, six filter stages are provided, each stage being provided with at least four retarders.

For the purpose of describing the stages, the following table illustrates, with simplified symbols, various possible variations of the number of retarder elements distributed within a discrete fold or corner (determining the corner) between polarizers within a stage, and also the number of tunable liquid crystal elements used to make up each element. Only MCF in any LC structure can be expressed in this way.

It is assumed that each individual level can be expressed as:

(i_k，m_k，θ_k) Where the subscript K denotes that the stage is the K-th stage in the filter, i_kIs the total number of elements in the K-th stage, m_kRepresents how many repeated LC elements adjoin at the same angle of rotation in the K-th stage (as shown in FIG. 8), and θ_kRepresenting the rotation angle of the Solc elements in the K-th stage.

From these simplifications, the FluoMCF001 in Table 2, having a 2-stage folded 6-element Solc structure with a corner of 7.5 °, 6 elements in the first stage, and 18 elements in the second stage (and at the same corner of 7.5 °), can be expressed in simplified form as: (6, 1, 7.5 °) - (18, 3, 7.5 °).

FluoMCF002 in Table 2, having a 3-stage fold of 6 elements Solc, has a rotation angle of 7.5. The first stage is a 6 element, the second stage 12 element, and the third stage 12 element, all with a rotation angle of ± 7.5 °, which can be expressed as: (6, 1, 7.5 °) - (12, 2, 7.5 °) - (12, 2, 7.5 °).

The following is a general expression for different structures of a multiconjugate filter with LC-only stages,

the two-stage filter is (i)₁，m₁，θ₁)-(i₂，m₂，θ₂)；

The three-stage filter is (i)₁，m₁，θ₁)-(i₂，m₂，θ₂)-(i₃，m₃，θ₃) (ii) a And

a four-stage filter of (i)₁，m₁，θ₁)-(i₂，m₂，θ₂)-(i₃，m₃，θ₃)-(i₄，m₄，θ₄)。

Here I_k2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, etc.;

M_k1, 2, 3, 4; and the number of the first and second groups,

Θ_k＝22.5°，11.25°，7.5°，5.625°，4.5°，3.75°，3.21°，2.81°

there are many possible arrangements in which a given set of retarders in a Solc configuration are used to subdivide the angle of rotation between the input and output polarizers. There are also a number of possibilities for how many components are tunable relative to the fixed. Finally, any particular element may be composed of one or more birefringence that can be tuned consistently. The use of several relatively thin adjacent liquid crystal elements constituting a relatively thick retarder element results in a filter that tunes faster than a filter using a thicker tuning element.

The retarders in at least one stage comprise liquid crystals connected to a common tuning controller. The birefringence of the retarder is caused to vary the same for all retarders within a given stage. The effect of tuning the stage is therefore the same as varying the thickness of a set of equally sized and equally birefringent retarders within the stage.

In another example, the retarders within the at least two stages may include liquid crystals connected to a tuning controller for independently varying birefringence of the retarders of a respective one of the at least two stages. However, the retarders are identically tuned within a given stage, preferably by changing two or more adjacent liquid crystal elements, or alternatively wherein at least a subset of the retarders connected to the tuning controller comprise fixed retarders connected to tunable liquid crystals.

Table 1: example of a fixed retarder multipole conjugate filter

RamanMCFD001	5 stage fold, 6 elements Solc, angle of rotation + -7.5 °
		RamanMCFD002	6 fold, 6 elements Solc, angle of rotation + -7.5 °
RamanMCFD011	4-fold, 8-element Solc, angle of rotation + -5.625 °
		RamanMCFD012	5 stage fold, 8 element Solc, angle of rotation + -5.625 °

RamanMCFD021	3-10 fold, 4 elements Solc, angle of rotation + -11.25 °
		RamanMCFD031	2-10 fold, 10 elements Solc, angle of rotation + -11.25 °
RamanMCFD042	2-10 fold, 12 elements Solc, angle of rotation + -11.25 °

Table 2: examples of LC filters only

FluoMCF001	2-stage folding 6 elements Solc with a rotation angle of +/-7.5 degrees, a first-stage 6 element, a second-stage 18 element with a rotation angle of +/-7.5 degrees and a (1-3)/6 structure.
		FluoMCF002	The 3-stage folding 6-element Solc has a rotation angle of +/-7.5 degrees, a first-stage 6-element structure, a second-stage 12-element structure, a rotation angle of +/-7.5 degrees, a third-stage 12-element structure, a rotation angle of +/-7.5 degrees and a (1-2-2)/6 structure.
Additional structure with LC elements of the same size as a retarder	(1-m)/k (1-m-n)/k (1-m-n-o)/km 2 to 4n 2 to 4o 2 to 4k 4 to 16 wherein "k" is the number of retarder elements in a stage; "m" is a multiple of element k in the next stage, "n" is a multiple of k in the third stage; "0" is in the fourth stage, and so on.

Table 3: examples of combining L and fixed retarder structures

RamanMCFD101	5-stage folding 6 elements Solc, +/-7.5 DEG	The first stage is an LC-only stage
			RamanMCFD102	6-stage folding 6 elements Solc, ± 7.5 °	The first stage is an LC-only stage
RamanMCFD1ll	4-stage folding 8-element Solc, ± 5.625 °	The first stage is an LC-only stage

RamanMCFD112	5-stage folding 8-element Solc, ± 5.625 °	The first stage is an LC-only stage
			RamanMCFD121	3-10 stage folding 4 elements Solc, +/-11.25 DEG	The first stage is an LC-only stage
RamanMCFD131	2-10 stage folding 10 elements Solc, +/-11.25 DEG	The first stage is an LC-only stage
			RamanMCFD142	2-10 stage folding 12 elements Solc, +/-11.25 DEG	The first stage is an LC-only stage

Table 4: comparison of Performance

	RamanMCFD101	RamanMCFD102
			Number of stages	5	6
Fixed retarder material	Quartz, alpha-BBO	Quartz, alpha-BBO
			Liquid crystal element	MLC6080，d＝6.5um	MLC6080.d＝6.5um
Free spectral range	500 nm to 650 nm	500 nm to 650 nm
			Secondary design	6 element Solc, +/-7.5 ° fold	6-element Solc + -7.5 DEG fold
FWHM at 500 η m	0.15ηm
			FWHM at 575 η m	0.21ηm	0.16ηm
FWHM at 650 η m	O.28ηm
			Out of band rejection	2*10-2	1.2*1O-3
Transmission ratio of polarizer at 550 nm	About 45 percent	About 38 percent

Table 5: ramamcfd 101-material, thickness and alignment

Stage	Design of	Component(s) of	Material	θ	d(μm)
						1	6 element Solc, +/-7.5 ° fold	LC cell	MLC6080	7.5	6.5
2	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	Quartz	7.5	600

		LC cell	MLC6080	7.5	6.5
						3	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	Quartz	7.5	1800
LC cell	MLC6080	7.5	6.5
				4	6 elements Solc, 7.5 ° fold			Fixed wavelength retarder	α-BBO	7.5	450
LC cell	MLC6080	7.5	6.5
						5	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	α-BBO	7.5	1350
LC cell	MLC6080	7.5	6.5

Table 6: ramamcfd 102-material, thickness and alignment

Stage	Design of	Component(s) of	Material	θ	d(μm)
						1	6 elements Solc, 7.5 ° fold	LC cell	MLC6080	7.5	6.5
2	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	Quartz	7.5	600
						LC cell	MLC6080	7.5	6.5
		3	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	Quartz					7.5	1800
LC cell	MLC6080					7.5	6.5
				4	6 elements Solc, 7.5 ° fold			Fixed wavelength retarder	α-BBO	7.5	450
LC cell	MLC6080	7.5	6.5
						5	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	α-BBO	7.5	1350

		LC cell	MLC6080	7.5	6.5
						6	6 elements Solc, 7.5 ° fold	Fixed wavelength retarder	α-BBO	7.5	1350
LC cell	MLC6080	7.5	6.5

The foregoing tables illustrating the embodiments of operation should be considered as non-limiting examples.

Patents and publications are available in the background and detailed description, and their teachings and further citations are deemed to be incorporated by reference into this disclosure.

Many modifications and variations of the present invention are possible in light of the above teachings, which are disclosed and illustrated by way of example. It is to be understood that the invention is not to be limited to the specific embodiments used as examples, and reference should be made to the appended claims for an assessment of the scope of the exclusive rights claimed.

Claims (18)

1. A spectral imaging filter comprising:

at least two spectral filter stages connected along an optical signal path, wherein each of the two spectral filter stages has at least a periodic transmission characteristic with bandpass peaks separated by a free spectral bandpass gap;

wherein one of the at least two filter stages has a larger free spectral range between bandpass peaks of one of the filter stages than the other of the two filter stages;

wherein the other of the two filter stages has a narrower bandpass peak than the one of the two filter stages;

wherein the bandpass peaks of the at least two filter stages overlap in an operational state of the filter, whereby the transmission characteristic of the spectral imaging filter is generally characterized by the larger free spectral region and the narrower bandpass peaks.

2. The spectral imaging filter of claim 1, wherein at least one of said at least two filter stages is tuned to said operational state wherein the bandpass peaks of said at least two filter stages overlap.

3. The spectral imaging filter of claim 2, wherein a plurality of said at least two filter stages are tunable to said operational state, and wherein bandpass peaks of said plurality of filter stages overlap.

4. The spectral imaging filter of claim 1, wherein at least two filter stages comprise a filter structure having a set of birefringent retarders and polarizers having a retardation and a relative angular orientation for determining the periodic transmission characteristics of the at least two filter stages.

5. The spectral imaging filter of claim 1 wherein said stages are arranged according to one of the group consisting of Lyot, Solc and Evans type structures.

6. The spectral imaging filter of claim 5, wherein the retarder comprises at least two different retardations by having a difference in each of at least one of retarder material, thickness along the optical signal path, and tunable difference.

7. The spectral imaging filter of claim 5, wherein the retarders in at least one stage comprise liquid crystal tunable birefringent elements.

8. The spectral imaging filter of claim 5, wherein the retarder comprises at least two liquid crystal tunable birefringent elements, and further comprising a controller for tuning the tunable birefringent elements in a coordinated manner.

9. The spectral imaging filter of claim 7, wherein the retarders in at least one stage comprise fixed retarders connected to a liquid crystal tunable birefringent element.

10. The spectral imaging filter of claim 9, wherein the fixed retarder is index matched to at least a portion of the liquid crystal tunable birefringent element.

11. The spectral imaging filter of claim 10, wherein the fixed retarder comprises bromine borate.

12. The spectral imaging filter of claim 1, comprising at least three stages of said spectral filter connected along an optical signal path, each stage leading to an output polarizer, whereby the output polarizer for a preceding stage acts as the input polarizer for a subsequent stage, and the number of polarizers is limited to the number of stages plus one.

13. The spectral imaging filter of claim 4, wherein the stages of each filter comprise a plurality of rotationally oriented wavelength retarders and polarizers, the orientation being such that the output polarizer for a preceding stage serves as the input polarizer for a subsequent stage.

14. The spectral imaging filter of claim 4, wherein the number of stages, and the number of retarders within the stages and their respective thicknesses, are selected to provide a Free Spectral Range (FSR) from about 500 to 650 η ι, and a full width half maximum bandpass peak width (FWHM) of about 0.25 η ι.

15. The spectral imaging filter of claim 4, wherein the number of spectral filter stages is limited such that the transmission of selected wavelengths through the spectral filter is at least about 40%.

16. The spectral imaging filter of claim 4, comprising at least three filter stages, wherein each stage has at least four retarders.

17. The spectral imaging filter of claim 16, wherein the retarders in at least one stage comprise liquid crystals connected to a common tuning controller for co-tuning the retarders.

18. The spectral imaging filter of claim 17, wherein at least a subset of the retarders connected to the tuning controller comprise fixed retarders connected to a liquid crystal.