Patent Grant
9304237
This disclosure relates to materials, components, and methods directed to the fabrication and use of tunable band-pass filters.
Wavelength bands of light or other radiation can be isolated in a system. For example, a system with a detector can be configured such that a detector receives as input only a narrow range of wavelengths, while eliminating, as input to the detector, as much light at other wavelengths as possible, where such other light can be considered “noise.” Optical components that isolate one or more bands of wavelengths are called “band-pass filters.” The width of a pass-band for conventional near-UV, visible, and near-IR optical band-pass filters can range from less than 1 nm to a few nm (or less than about 1% of the center wavelength of the pass-band) for so-called “narrowband” filters to several tens of nm (about 1 to 10% of the center wavelength) for most band-pass filters—such as those used in fluorescence detection and imaging systems. The width of some band-pass filters can be several tens of percent of the center wavelength wide.
A thin-film interference filter conventionally referred to as a “narrow-band-pass” (i.e., NBP) filter can be constructed from a series of “quarter-wave” layers of material that alternate between a high-index-of-refraction material and a low-index-of-refraction material. A “quarter-wave” layer is a thin layer of material with an optical thickness that is equal to ¼ λ (or an odd-integer multiple of ¼ λ), where λ is the wavelength associated with a center of the transmission band. (The “optical thickness” of a layer of material with an index of refraction n and a geometrical thickness t is the product n×t.) A series of quarter-wave layers can create a “stop band” that is approximately centered about the wavelength λ by which the quarter-wave layers are calibrated (where the “quarter-wave” layers can have an optical thickness of (2z+1) (¼ λ) where z can be 0, 1, 2, . . . ). The “stop band” is a region of low transmittance, which is created due to destructive interference between internally-reflected and incident light through the layers of material. An NBP filter can be constructed from a series of layers by interspersing, within a stack of quarter-wave layers, resonant “cavity” layers. A “cavity” layer in an NBP filter can be an integral multiple of half-wave layers (i.e., its optical thickness is equal to: ½ λ; 1 λ; 1½ λ; etc.) of the λ associated with the quarter-wave layers. The presence of cavity layers interspersed within quarter-wave layers (where the quarter-wave layers are responsible for a stop band) can cause a relatively narrow portion of the stop band to transmit light rather than block light. A quarter-wave stack with more than one resonant cavity is referred to as a “multi-cavity” filter. The presence of multiple cavity layers can have the effect of increasing both the steepness of the cut-on edge of a transmitting region of the stop band and the steepness of the cut-off edge of the transmitting region of the stop band.
Graph 250-2 depicts the relatively narrow transmission band for s-polarized light, located approximately in the center of the stop band, and associated with the added cavity layers 110-1, 110-2, etc. The transmission curve 210 has an associated cut-on wavelength 220 for s-polarized light within the stop band and a cut-off wavelength 225 for s-polarized light within the stop band. Although the transmission curve 210 at the cut-on wavelength 220 and the cut-off wavelength 225 that is depicted in
Each transmission curve 310-X (where, as depicted, “X” can take on the values of “1,” “2,” and “3”) has an associated cut-on wavelength 420-X and a cut-off wavelength 425-X. Moreover, each depicted transmission curve 310-X has an associated full-width-half-maxima (“FWHM”) value 430-X in the narrow-pass-band region. In each graph 350-X, the regions immediately outside of the FWHM values 430-X are part of the stop band associated with the plurality of quarter-wave layers 120-x and 130-x as depicted in
Graph 350-1 in
Graph 350-2 depicts an exemplary transmission curve 310-2 for s-polarized light where the incident s-polarized light 303-2 strikes the multi-cavity band-pass filter 100-2 at an angle-of-incidence 302-2 between normal incidence and approximately 60 degrees from normal incidence. The transmission curve 310-2 depicts a cut-on wavelength 420-2 and a cut-off wavelength 425-2 and a FWHM value 430-2.
Graph 350-3 depicts an exemplary transmission curve 310-3 for s-polarized light where the incident s-polarized light 303-3 strikes the multi-cavity band-pass filter 100-3 at an angle-of-incidence 303-3 at approximately 60 degrees from normal incidence. The transmission curve 310-3 depicts a cut-on wavelength 420-3 and a cut-off wavelength 425-3 and a FWHM value 430-3.
As depicted in
In one aspect, the present disclosure is directed to an optical filter configured to provide a wavelength-tunable pass-band to incident s-polarized electromagnetic radiation. The optical filter can include a first stack of layers and a second stack of layers. The first stack of layers can include a plurality of quarter-wave layers and at least two half-wave cavity layers, where an optical thickness value of each quarter-wave layer in the first stack of layers can be approximately an odd-integer multiple of one-fourth of a first wavelength value. An optical thickness value of each half-wave cavity layer in the first stack of layers can be approximately an integer multiple of one-half of the first wavelength value. In addition, the first stack of layers can be configured to provide a cut-on edge for s-polarized radiation incident on the first stack of layers at a second wavelength value and a cut-off edge for s-polarized radiation incident on the first stack of layers at a third wavelength value, the second wavelength value and the third wavelength value being functions of angle-of-incidence for a set of non-normal angle-of-incidence values, and the third wavelength value being greater than the second wavelength value at the set of non-normal angle-of-incidence values. The second stack of layers can be configured to provide a cut-off edge for s-polarized radiation incident on the second stack of layers at a fourth wavelength value, the fourth wavelength value being a function of angle-of-incidence for the set of non-normal angle-of-incidence values. Further, the fourth wavelength value can lie substantially between the second wavelength value and the third wavelength value at the set of non-normal angle-of-incidence values. Further still, a pass-band determined by the second wavelength value and the fourth wavelength value can have a set of associated full-width half-maximum values as a function of angle-of-incidence for the set of non-normal angle-of-incidence values, the set of associated full-width half-maximum values substantially defining at least one of: a FWHM curve as a function of angle-of-incidence and a FWHM line as a function of angle-of-incidence. Furthermore, the at least one of a FWHM curve and a FWHM line can define a first derivative with respect to angle-of-incidence values at at least one angle-of-incidence value of the set of non-normal angle-of-incidence values that is at least one of: substantially positive and substantially zero.
In another aspect, the present disclosure is directed to an optical filter, wherein the optical filter includes a surface with a normal vector at an angle-of-incidence value to a direction of incident s-polarized electromagnetic radiation. The optical filter can also include a multi-cavity band-pass filter and at least one other filter. The multi-cavity band-pass filter can be configured to provide a tunable cut-on edge for s-polarized radiation at a first wavelength value as a function of the angle-of-incidence value, and a tunable cut-off edge for s-polarized radiation at a second wavelength value as a function of the angle-of-incidence value. The at least one other filter can be configured to provide a tunable cut-off edge for s-polarized radiation at a third wavelength value as a function of the angle-of-incidence value. Moreover, for a set of non-normal angle-of-incidence values, the third wavelength value can lie substantially between the first wavelength value and the second wavelength value. Further still, a tunable pass-band determined by the first wavelength value and the third wavelength value can have a set of associated full-width half-maximum values as a function of the angle-of-incidence value, the set of associated full-width half-maximum values substantially defining at least one of: a FWHM curve as a function of the angle-of-incidence value and a FWHM line as a function of the angle-of-incidence value. Furthermore, the at least one of a FWHM curve and a FWHM line can define a first derivative with respect to the angle-of-incidence value at at least one angle-of-incidence value of the set of non-normal angle-of-incidence values that is at least one of: substantially positive and substantially zero.
In a further aspect, the present disclosure is directed to a method of making an optical filter configured to provide a wavelength-tunable pass-band to incident s polarized electromagnetic radiation. The method can include depositing a first stack of layers on a structure, where the first stack of layers can include a plurality of quarter-wave layers and at least two half-wave cavity layers. The method can also include depositing a second stack of layers. An optical thickness value of each quarter-wave layer in the first stack of layers can be approximately an odd-integer multiple of one-fourth of a first wavelength value. An optical thickness value of each half-wave cavity layer in the at least two half-wave cavity layers can be approximately an integer multiple of one-half of the first wavelength value. In addition, the first stack of layers can be configured to provide a cut-on edge for s-polarized radiation incident on the first stack of layers at a second wavelength value and a cut-off edge for s-polarized radiation incident on the first stack of layers at a third wavelength value, the second wavelength value and the third wavelength value being functions of angle-of-incidence for a set of non-normal angle-of-incidence values, and the third wavelength value being greater than the second wavelength value at the set of non-normal angle-of-incidence values. The second stack of layers can be configured to provide a cut-off edge for s-polarized radiation incident on the second stack of layers at a fourth wavelength value. Further, the fourth wavelength value can lie substantially between the second wavelength value and the third wavelength value at the set of non-normal angle-of-incidence values. Further still, a pass-band determined by the second wavelength value and the fourth wavelength value can have a set of associated full-width half-maximum values as a function of angle-of-incidence for the set of non-normal angle-of-incidence values, the set of associated full-width half-maximum values substantially defining at least one of: a FWHM curve as a function of angle-of-incidence and a FWHM line as a function of angle-of-incidence. Furthermore, the at least one of a FWHM curve and a FWHM line can define a first derivative with respect to angle-of-incidence values at at least one angle-of-incidence value of the set of non-normal angle-of-incidence values that is at least one of: substantially positive and substantially zero.
In another aspect, the present disclosure is also directed to a method of making an optical filter, wherein the optical filter includes a surface with a normal vector at an angle-of-incidence value to a direction of incident s-polarized electromagnetic radiation. The method can include depositing a first stack of layers on a structure, the first stack of layers including a multi-cavity band-pass filter. The method can also include depositing a second stack of layers, the second stack of layers including at least one other filter. The multi-cavity band-pass filter can be configured to provide a tunable cut-on edge for s-polarized radiation at a first wavelength value as a function of the angle-of-incidence value, and a tunable cut-off edge for s-polarized radiation at a second wavelength value as a function of the angle-of-incidence value. The at least one other filter can be configured to provide a tunable cut-off edge for s-polarized radiation at a third wavelength value as a function of the angle-of-incidence value. Moreover, for a set of non-normal angle-of-incidence values, the third wavelength value can lie substantially between the first wavelength value and the second wavelength value. Further still, a tunable pass-band determined by the first wavelength value and the third wavelength value can have a set of associated full-width half-maximum values as a function of the angle-of-incidence value, the set of associated full-width half-maximum values substantially defining at least one of: a FWHM curve as a function of the angle-of-incidence value and a FWHM line as a function of the angle-of-incidence value. Furthermore, the at least one of a FWHM curve and a FWHM line can define a first derivative with respect to the angle-of-incidence value at at least one angle-of-incidence value of the set of non-normal angle-of-incidence values that is at least one of: substantially positive and substantially zero.
Additional features and advantages will be set forth in part in the description which follows, being apparent from the description of or learned by practice of the disclosed embodiments. The features and advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the scope of the embodiments, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments and, together with the description, serve to explain the features, advantages, and principles of the disclosed embodiments.
Reference will now be made in detail to the one or more embodiments, characteristics of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
As discussed above in connection with the structure of
Where a filter includes a series of materials layers that exhibit thicknesses that deviate from a quarter wave-length value, the stop band edges can be configured to exhibit certain characteristics as a function of the angle at which light is incident on the filter.
Introduction of mismatch can be a useful, constrained way of allowing layer thicknesses to deviate from a quarter-wave thickness while maintaining the same position of the stopband with respect to the quarter-wave reference wavelength. For example, mismatch can be introduced in an optical filter design to intentionally create a second-order stopband (a harmonic of a fundamental stopband which occurs at approximately one half of the fundamental stopband wavelength) where a strictly quarter-wave stack will conventionally not exhibit harmonic stopbands with even orders. Consistent with this disclosure, mismatch can be used here to modify a dependence of the change in wavelength of the edges of the stopband on the angle-of-incidence.
The entire structure of
Graph 750-1 in
As depicted in
The multi-cavity band-pass filter 905 includes a half-wave cavity layer 910-m, which is adjacent to a quarter-wave layer 920-(n+1), which is adjacent to a quarter-wave layer 930-n, which is adjacent to a quarter-wave layer 920-n, etc. The variables “n” and “m” as used in connection with
The short-wave pass filter 985 includes plurality of mismatched layer pairs. In particular, mismatched layer pair 980-j includes first material layer 960-j and second material layer 970-j. Each of the first material layer 960-j and the second material layer 970-j can exhibit a total thickness which deviates from a quarter wave-length value. For example, where the reference wavelength is λd, then each of the first material layer 960-j and the second material layer 970-j can exhibit a thickness which is different from approximately (2z+1) (¼ λd) (where z can be 0, 1, 2, . . . ). Consistent with this disclosure, the optical thickness of any first material layer (such as 960-j, 960-(j−1), 960-(j+1), etc.) and the optical thickness of any second material layer (such as 970-j, 970-(j−1), 970-(j+1), etc.) can take on values different from a quarter-wave thickness of a reference wavelength λd, with the constraint that the combined optical thickness of a mismatched pair of a first material layer and a second material layer is approximately ½ λd (or an integer multiple of ½ λd). Consistent with this disclosure, the optical thickness of a first material layer 960-(j+1), that is adjacent to the first material layer 960-j, can be different from both the optical thickness of the first material layer 960-j and approximately (2z+1) (¼ λd), where z can be 0, 1, 2, . . . . Likewise, the optical thickness of a second material layer 970-(j+1), that is adjacent to the second material layer 970-j, can be different from both the optical thickness of the second material layer 970-j and approximately (2z+1) (¼ λd) where z can be 0, 1, 2, . . . ). However, the combined optical thickness of the first material layer 960-(j+1) and the second material layer 970-(j+1) (to the extent they are components of a mismatched layer pair 980-(j+1)) will be approximately an integer multiple of ½ λd. A further design constraint associated with the short wave pass filter 985 will be discussed in connection with
Graph 1050-1 in
Graph 1050-2 depicts exemplary transmission curves 1010-2 and 1065-2 for s-polarized light where incident s-polarized light strikes the tunable band-pass filter 906 at between normal incidence and approximately 60 degrees from normal incidence. The pair of transmission curves 1010-2 and 1065-2 depict a cut-on wavelength 1020-2 and a cut-off wavelength 1015-2 and a FWHM value 1035-2.
Graph 1050-3 depicts exemplary transmission curves 1010-3 and 1065-3 for s-polarized light where incident s-polarized light strikes the tunable band-pass filter 906 at approximately 60 degrees from normal incidence. The pair of transmission curves 1010-3 and 1065-3 depict a cut-on wavelength 1020-3 and a cut-off wavelength 1015-3 and a FWHM value 1035-3.
As depicted in
Consistent with this disclosure, the cut-off wavelength 1015-X of the short wave pass curves 1065-X for s-polarized light will also tend to shift to lower wavelengths as the angle of incidence progresses from approximately normal to approximately 60 degrees from normal. Consistent with this disclosure, each mismatched layer pair 980-j (which includes the first material layer 960-j and the second material layer 970-j) can be configured such that the “rate” (as a function of angle of incidence) of the tendency of the cut-off wavelength 1015-X to shift to lower wavelengths more closely matches the “rate” (as a function of angle of incidence) of the tendency of the cut-on wavelength 1020-X to shift to lower wavelengths. By placing the cut-off wavelength 1015-1 at normal incidence of the short wave pass filter 985 within the pass-band of the multi-cavity filter 905 at normal incidence (i.e., by placing the cut-off wavelength 1015-1 between the cut-on wavelength 1020-1 and the cut-off wavelength 1025-1, as depicted in
This is further illustrated in
Graph 1150-1 in
Graph 1150-2 in
Graph 1150-3 in
As discussed above in connection with
Tables 1 and 2 below provide a design for an exemplary multi-cavity band-pass filter 905 (Table 1) and an exemplary short wave pass filter 985 (Table 2) consistent with this disclosure.
As compared with the corresponding graphs 500-1 and 500-2 associated with prior art filter 100, the curve 1300-2 (which depicts a set of FWHM values of s-polarized light as a function of the angle-of-incidence) exhibits a region of non-normal angle of incident values with a minimum—that is, there is a region of non-normal angle of incident values where the first derivative of the curve 1300-2, describing the FWHM as a function of angle-of-incidence values greater than zero degrees and less than sixty degrees, goes from the negative value of the prior art curve 500-2, to a value that is substantially zero (or is otherwise substantially flat), and then to values that are substantially positive (i.e., the region between 40-50 degrees in curve 1300-2). Moreover, the second derivative of the curve 1300-2, describing the FWHM as a function of angle-of-incidence values greater than zero degrees and less than sixty degrees, is substantially zero as the first derivative of the FWHM curve changes sign from negative to positive values in the region of non-normal angle of incidence values. As used herein, the term “angle of incidence” is used exclusively to refer to positive angle-of-incidence values (rather than angle-of-incidence values than might extend to the left of the “0 degrees” axes in
As can be derived from the curves 1300-1 and 1300-2 in
As depicted in
Further still, although the embodiments associated with
The multi-cavity band-pass filter 1505 includes a half-wave cavity layer 1510-m, which is adjacent to a quarter-wave layer 1520-(n+1), which is adjacent to a quarter-wave layer 1530-n, which is adjacent to a quarter-wave layer 1520-n, etc. As with
The multi-cavity band-pass filter 1585 includes a half-wave cavity layer 1518-q, which is adjacent to a quarter-wave layer 1528-(r+1), which is adjacent to a quarter-wave layer 1538-r, which is adjacent to a quarter-wave layer 1528-r, etc. The variables “q” and “r” as used in connection with
Graph 1650-1 in
Graph 1650-2 depicts exemplary transmission curves 1610-2 and 1665-2 for s-polarized light where incident s-polarized light strikes the tunable band-pass filter 1506 at between normal incidence and approximately 60 degrees from normal incidence. The pair of transmission curves 1610-2 and 1665-2 depict a cut-on wavelength 1620-2 and a cut-off wavelength 1615-2 and a FWHM value 1635-2.
Graph 1650-3 depicts exemplary transmission curves 1610-3 and 1665-3 for s-polarized light where incident s-polarized light strikes the tunable band-pass filter 1506 at approximately 60 degrees from normal incidence. The pair of transmission curves 1610-3 and 1665-3 depict a cut-on wavelength 1620-3 and a cut-off wavelength 1615-3 and a FWHM value 1635-3.
As depicted in
Consistent with this disclosure, the multi-cavity band-pass filter 1585 can be configured such that the “rate” (as a function of angle of incidence) of the tendency of the cut-off wavelength 1615-X to shift to lower wavelengths more closely matches the “rate” (as a function of angle of incidence) of the tendency of the cut-on wavelength 1620-X to shift to lower wavelengths. By placing the cut-off wavelength 1615-1 at normal incidence of the multi-cavity band-pass filter 1585 within the pass-band of the multi-cavity filter 1505 at normal incidence (i.e., by placing the cut-off wavelength 1615-1 between the cut-on wavelength 1620-1 and the cut-off wavelength 1625-1, as depicted in
This is further illustrated in
Graph 1750-1 in
Graph 1750-2 in
Graph 1750-3 in
As discussed above in connection with
As depicted in
Consistent with a further embodiment,
For example, graph 2250 of
Graph 2350 of
Graph 2450 of
Graph 2550 of
Furthermore, graph 2650 of
To understand how the tuning range 2861-1 can increase over the tuning range 2861-2, consider that Snell's Law governs the refraction of light at an interface between media of two different indexes of refraction. Accordingly, the angle-of-incidence of light within a set of thin-film coating layers can be smaller than the angle-of-incidence at the air interface with the thin-film coating layer. (That is, where a surface of a thin-film coating layer is flush with air.) For example, when light is incident upon a tunable filter at an angle-of-incidence in air of θ, within the thin-film coating layers the angle will be approximately θeff=sin−1(sin θ/neff) where neff represents an effective index of refraction within the thin-film coating layers. Where an effective index of refraction within the thin-film coating layers is approximately neff=1.84, and if the angle of incidence with air is θ=60 degrees, then θeff=28 degrees.
In contrast, if the air-interface to a tunable filter is replaced with an interface to a medium comprising a material other than air (such as a liquid or glass), then the angle-of-incidence within the layers can be larger, leading to a greater wavelength tuning sensitivity. For example, where the tunable filter is embedded and/or immersed in a material other than air (an “embedded geometry” such that a surface of the tunable filter where incident light enters or exits is flush with a material other than air), such as a liquid or glass, where the liquid or glass has an index of approximately n=1.5, then the angle inside the layers can be approximately θeff=sin−1(n sin θ/neff), or θeff=45 degrees for an incident angle on a tunable filter embedded and/or immersed in glass of θ=60 degrees. The effect that such a change in the internal angle of incidence can have on a “ray” path of light is approximately depicted in
A further advantage of this increased sensitivity is that the tunable filter coating area does not need to be as large in the embedded and/or immersed case for a given wavelength tuning range as it does for the same coating in air on a glass plate. This latter effect is depicted in
Generally, when the incident medium has an index of n, the tuning dependence changes to
In other words, it is as if the effective index neff has changed from neff to neff/n. Thus, for an exemplary tunable filter a new effective index can be approximately 1.84/1.5=1.23. Consistent with an embodiment, accordingly, there can also be an increased wavelength tuning range for such an embedded geometry.
Thus, relative to a tunable filter coating in air, there can be a larger change in wavelength value for a given change in angle-of-incidence for the case of a tunable filter coating “immersed” and/or “embedded” in a liquid or glass.
Consistent with an embodiment,
Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
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