WO2015188058A1

WO2015188058A1 - Cascaded beam combiner

Info

Publication number: WO2015188058A1
Application number: PCT/US2015/034402
Authority: WO
Original assignee: Bribbla Dynamics Llc
Priority date: 2014-06-06
Filing date: 2015-06-05
Publication date: 2015-12-10

Abstract

A cascaded beam combiner can receive a plurality of incident beams, including first and last incident beams. The incident beams can have overlapping wavelength spectra that decrease in wavelength from the first incident beam to the last incident beam. Bandpass filters spectrally narrow the incident beams to form respective spectrally narrowed beams inside the cascaded beam combiner. The spectrally narrowed beams can have non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam. Long wavelength pass filters sequentially combine components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam. The cascaded beam combiner can form a multi-wavelength beam from the cascaded beam. The multi-wavelength beam can have a wavelength spectrum that includes the non-overlapping wavelength spectra of the spectrally narrowed beams.

Description

CASCADED BEAM COMBINER

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. Provisional Application No. 62/008,960, titled "CASCADED BEAM COMBINER", and filed on June 6, 2014, which is hereby incorporated by reference in its entirety.

BACKGROUND

[0002] This disclosure relates to a cascaded beam combiner that receives incident broadband beams, spectrally narrows the incident broadband beams to form spectrally narrowed beams, and sequentially combines the spectrally narrowed beams to form a single output beam having a multi- wavelength spectrum.

[0003] In some optical systems, it can be desirable to produce a beam having a relatively broad wavelength spectrum. Such a beam can be useful for optical systems that perform measurements at more than one wavelength. For instance, an optical measurement system can direct multi- wavelength light onto a sample, collect light reflected and/or scattered from the sample, direct the collected light onto a spectrometer, and analyze the properties of the sample as a function of wavelength.

[0004] In some of these optical systems, it may not be practical or possible to use broadband light produced by a single source. For instance, a wavelength spectrum for a particular beam may be too broad to produce with a single source.

[0005] Accordingly, there exists a need for a beam combiner that combines light from different portions of the wavelength spectrum into a single beam.

SUMMARY OF THE DISCLOSURE

[0006] A cascaded beam combiner can receive a plurality of incident beams, including first and last incident beams. The incident beams can have overlapping wavelength spectra that decrease in wavelength from the first incident beam to the last incident beam. Bandpass filters spectrally narrow the incident beams to form respective spectrally narrowed beams inside the cascaded beam combiner. The spectrally narrowed beams can have non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam. Long wavelength pass filters sequentially combine components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam. The cascaded beam combiner can form a multi-wavelength beam from the cascaded beam. The multi- wavelength beam can have a wavelength spectrum that includes the non-overlapping wavelength spectra of the spectrally narrowed beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the following detailed description of example embodiments of the invention, reference is made to the accompanying drawings which form a part hereof, and which is shown by way of illustration only, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0008] FIG. 1 is a schematic drawing of an example of an optical system that includes a cascaded beam combiner.

[0009] FIG. 2 is a functional block diagram of an example of a cascaded beam combiner.

[0010] FIG. 3 is a side-view drawing of an example of a cascaded beam combiner.

[0011] FIG. 4 is a side-view drawing of an example of a first optical element.

[0012] FIG. 5 is a side-view drawing of an example of an intermediate optical element.

[0013] FIG. 6 is a side-view drawing of an example of a last optical element. [0014] FIG. 7 is another side-view drawing of the last optical element of FIG. 6, showing rays along an exiting path from the last optical element.

[0015] FIG. 8 is a side-view drawing of another example of a cascaded beam combiner.

[0016] FIG. 9 is a side-view drawing of another example of a cascaded beam combiner.

[0017] FIG. 10 is a side-view drawing of another example of an optical system having tunable bandpass filters in its incident beam paths.

[0018] FIG. 11 is a plot of transmission versus wavelength for an example of an idealized bandpass filter.

[0019] FIG. 12 is a plot of transmission versus wavelength for an example of an idealized long wavelength pass filter at a non-zero incident angle.

[0020] FIG. 13 is a flow chart of an example of a method for forming a multi- wavelength beam.

DETAILED DESCRIPTION

[0021] FIG. 1 is a schematic drawing of an example of an optical system 100 that includes a cascaded beam combiner 106. The optical system 100 of FIG. 1 is but one example; other suitable optical systems can also be used.

[0022] Several light sources 102A-C are arranged to produce respective incident beams 104A-C having different wavelength spectra. Each wavelength spectrum is relatively high within a particular wavelength range. Each wavelength spectrum falls to a relatively low level, relatively slowly, outside of the particular wavelength range. For the purposes of this document, wavelength values are denoted by letters A through F, with wavelength B being greater than wavelength A, wavelength C being greater than wavelength B, and so forth. Incident beams 104A, 104B, and 104C have respective wavelength spectra in ranges extending between wavelengths E and F, C and D, and A and B, respectively, with tails of the wavelength spectra extending beyond the respective ranges.

[0023] The cascaded beam combiner 106 receives the incident beams 104A- C, spectrally narrows each incident beam to remove the wavelength spectrum tails that are below a low-wavelength threshold and above a high-wavelength threshold, and combines the spectrally-narrowed beams into a single multi- wavelength output beam 108. The cascaded beam combiner 106 can also enhance a polarization state of the output beam 108 (discussed in detail below).

[0024] The output beam 108 has a wavelength spectrum extending between wavelengths A and B, between C and D, and between E and F. The wavelength spectrum is suppressed between wavelengths B and C and between D and E. Such a spectrum can be useful for optical systems in which the output beam 108 is directed onto a sample, light reflected and/or scattered from the sample is collected, and the collected light is analyzed using a monochromator or other device that can spectrally disperse the collected light. The breaks in the spectrum can eliminate an ambiguity in deciding whether light at a particular monochromator pixel arises from a particular light source, or from a spectrally adjacent source. In addition, because broadband sources typically have wavelength spectra with relatively gradual rolloffs, it can be difficult or impossible to simply combine the light from different broadband sources to produce an output spectrum having relatively sharp edges, such as at wavelengths A, B, C, D, E, and F in FIG. 1.

[0025] The optical system 100 includes three light sources 102A-C; in other examples, more or fewer light sources can also be used, including two, four, five, six, seven, eight, or more than eight. Each light source 102A-C can include one or more light -producing elements. Examples of suitable light sources can include a single semiconductor laser, multiple semiconductor lasers having the same wavelength, multiple semiconductor lasers having different wavelengths, a single light emitting diode, multiple light emitting diodes having the same wavelength, multiple light emitting diodes having different wavelengths, one or more quantum cascade lasers, one or more superluminescent light sources, one or more amplified spontaneous emission sources, any combination of the above, or other suitable light sources. In some examples, each light source can vary or modulate the intensity of its output, optionally independently of the other light sources. In some examples, an incident optical path includes one or more wavelength- selective elements, such as a filter or grating, so that the optical system 100 system can perform spectroscopic measurements of a sample. In some examples, the sample can produce light wavelengths other than the incident wavelength(s), such as through fluorescence or Raman scattering; for these examples, the incident path can also include a suitable wavelength- selective element. Each light source 102A-C can optionally include one or more collimating or focusing elements, which can be made integral with the light- producing elements or can be made separately and attached to the light- producing elements. In some examples, the incident beams 104A-C are collimated.

[0026] In some examples, the cascaded beam combiner 106 can be configured to be compatible with an external mount or housing. For instance, an external housing may include three light sources, such as LEDs or optical fibers, each with a respective collimating lens, to produce three output beams. The three output beams may be parallel and/or coplanar, and may be aligned to fiducials or one or more surfaces of the external housing. Alignment of the system can be simplified if the cascaded beam combiner 106 can be aligned and/or mechanically coupled to the external housing, so that the beams can pass from the external housing to the cascaded beam combiner 106 with little or no alignment.

[0027] In some examples, the incident beams 104A-C are parallel to one another. For the purposes of this document, the term parallel is intended to signify that the beams are made to be as parallel as is practically possible, to within typical manufacturing and alignment tolerances that can vary from process to process or from system to system, so that any non-parallelism can be due to unintentional and unavoidable alignment and manufacturing tolerances. For instance, the light sources can be mounted in a housing that includes parallel channels for securing collimating lenses, and so forth. The housing itself can have manufacturing tolerances on its various surfaces. The mounts can be threaded or press-fit, which can include additional angular tolerances. In general, the stacked tolerances on the incident beams can ensure that the beams are parallel to within a particular angle, such as 10 degrees, 5degrees, 3 degrees, 2 degrees, 1.5 degrees, 1 degree, 0.5 degree, 0.2 degrees, 0.1 degree, or another suitable value. [0028] In some examples, the incident beams 104A-C are coplanar.

Similarly, for the purposes of this document, the term coplanar is intended to signify that the beams are made to be as coplanar as is practically possible, to within typical manufacturing and alignment tolerances that can vary from process to process or from system to system, so that any non-coplanarity can be due to unintentional and unavoidable alignment and manufacturing tolerances.

[0029] In other examples, the incident beams 104A-C can be non-parallel and/or non-coplanar. For instance, the light sources can alternate on opposite sides of the cascaded beam combiner. As another example, the light sources can precess helically around a single longitudinal axis, and can direct their respective light outputs toward the longitudinal axis. In still other examples, the light sources can have any suitable angular orientations with respect to the longitudinal axis, such as out of the page in FIG. 1 , so that suitably oriented reflecting elements in the cascaded beam combiner 106 can combine the light beams onto a single optical path.

[0030] The optical system 100 can include circuitry 110 to dynamically electrically control the output levels of the light sources 102A-C. In some examples, the circuitry 1 10 can switch particular light sources on and off at specified times, and/or set particular light sources to specified intensity levels. In some examples, the circuitry 110 can modulate particular light sources independently. In some examples, the circuitry 110 can modulate the light sources at unique assigned frequencies, so that the optical system can use a lock- in amplifier to link a detected signal with a particular wavelength that produces the signal. The circuitry 1 10 can be included in a computer system that includes hardware, firmware and software. Examples may also be implemented as instructions stored on a computer-readable storage device, which may be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transitory mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a computer-readable storage device may include readonly memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. In some examples, computer systems can include one or more processors, optionally connected to a network, and may be configured with instructions stored on a computer-readable storage device.

[0031] FIG. 2 is a functional block diagram of an example of a cascaded beam combiner 206, such as cascaded beam combiner 106 from FIG. 1.

Elements numbered 2xx between 200 and 210 in FIG. 2 are identical in structure and function to elements number lxx from FIG. 1. The cascaded beam combiner 206 of FIG. 2 is but one example; other suitable cascaded beam combiners can also be used.

[0032] The cascaded beam combiner 206 directs three incident beams 204A-C through respective bandpass filters 210A-C. In some examples, each incident beam 204A-C has a wavelength spectrum that overlaps with that of an adjacent incident beam.

[0033] The bandpass filters 210A-C spectrally narrow the respective incident beams 204 A-C to form respective spectrally narrowed beams 212 A-C. The bandpass filters perform the spectral narrowing by blocking wavelengths shorter than a low-wavelength threshold, such as wavelength E for incident beam 204 A, transmitting wavelengths between the low-wavelength threshold and a high- wavelength threshold, such as between wavelengths E and F for incident beam 204A, and blocking wavelengths greater than high-wavelength threshold, such as wavelength F for incident beam 204 A. The wavelength spectrum of each spectrally narrowed beam 212A-C is a product of the wavelength spectrum of the respective incident beam 204A-C, which is relatively broad with slowly decreasing tails, with a top hat transmission function of the bandpass filter, which truncates the tails. The wavelength spectrum of each spectrally narrowed beam 212A-C is narrower than that of the corresponding incident beam 204A-C.

[0034] A total internal reflection 214 redirects the spectrally narrowed beam 212A to initiate a cascaded beam 218A. The cascaded beam, at location 218A, has the same wavelength spectrum as the spectrally narrowed beam 212 A.

[0035] The cascaded beam 218A and spectrally narrowed beam 212B both strike a long wavelength pass filter 216A. The cascaded beam 218A, having wavelengths greater than the cutoff wavelength of the long wavelength pass filter 216A, transmits through the wavelength pass filter 216A to cascaded beam 218B. The spectrally narrowed beam 212B, having wavelengths less than cutoff wavelength, reflects off the long wavelength pass filter 216A, to join cascaded beam 218B. The cascaded beam, at location 218B, includes the summed wavelength spectra from spectrally narrowed beams 212A and 212B.

[0036] Similarly, long wavelength pass filter 216B joins cascaded beam 218B with spectrally narrowed beam 212C to form cascaded beam 218C. The cascaded beam, at location 218C, can optionally be redirected to form the output beam 208.

[0037] The long wavelength pass filters 216A-C also clean up the polarization states of the reflected spectrally narrowed beams 212B-C; this is addressed in detail below with regard to FIG. 5.

[0038] FIG. 3 is a side-view drawing of an example of a cascaded beam combiner 306. Elements numbered 3xx between 300 and 318 in FIG. 3 are identical in structure and function to elements numbered 2xx from FIG. 2. The cascaded beam combiner 306 of FIG. 3 is but one example; other suitable cascaded beam combiners can also be used.

[0039] The cascaded beam combiner 306 includes a plurality of transparent or partially transparent optical elements 322A-C attached sequentially, such as along a longitudinal direction (e.g., left-to-right in FIG. 3). The plurality includes a first optical element 322A at a first longitudinal end of the cascaded beam combiner 306 and a last optical element 322C at a second longitudinal end of the cascaded beam combiner 306. The optical elements 322A-C, together with any filters disposed on or between the optical elements 322A-C, can be referred to as a body 320.

[0040] The optical elements 322A-C of the plurality include respective first surfaces 334A-C. The first surfaces 334A-C are positioned to receive a respective plurality of parallel, coplanar incident beams 304 A-C having wavelength spectra that decrease in wavelength from the first optical element 322A to the last optical element 322C. In some examples, the first surfaces 334A-C of the optical elements 322A-C are all coplanar and parallel to the longitudinal direction (e.g., left-to-right in FIG. 3).

[0041] The first surfaces 332A-C can include respective bandpass filters 310A-C disposed thereon. Each bandpass filter 310A-C spectrally narrows a respective incident beam 304A-C to form a respective spectrally narrowed beam 312A-C inside the respective optical element 322A-C. The spectrally narrowed beams 312A-C have non-overlapping wavelength spectra.

[0042] In some examples, one or more of the bandpass filters 310A-C can be formed as a thin film stack disposed on the respective first surface 334A-C. In some examples, one or more of the bandpass filters 310A-C are air-incident, and are configured to operate at normal incidence or within 1 , 2, 3, 4, or 5 degrees of normal incidence. In other examples, one or more of the bandpass filters 310A-C can be manufactured as a separate element and attached to the respective first surface 334A-C. In still other examples, one or more of the bandpass filters 310A-C can be manufactured as a separate element and can be separated from the respective first surface 334A-C. For these cases, the respective first surface 334A-C can be anti-reflection coated.

[0043] The cascaded beam combiner 306 further includes long wavelength pass filters 316A-B. In some examples, the long wavelength pass filters 316A-B are disposed between adjacent optical elements 322A-C of the plurality. The long wavelength pass filters 316A-B sequentially combine components of the spectrally narrowed beams 312A-C from the first optical element 322A to the last optical element 322C to form a cascaded beam 318A-C. The long wavelength pass filters 316A-B can sequentially combine s-polarized components, p-polarized components, or fractions of both s- and p-polarized components.

[0044] In some examples, the spectrally narrowed beams 312A-C and the cascaded beams 318A-B all strike the long wavelength pass filters 316A-B at the same incident angle. The incident angle is defined as the angle between an incident ray propagation direction and a local surface normal. Such a geometry can ensure that the reflected optical path from a long wavelength pass filter 316A-B, after reflection, coincides with a transmitted optical path through the long wavelength pass filter 316A-B. In some examples, the incident angles are all specified to be 45 degrees. For optical elements 322A and 322C having refractive indices greater than 1.414, and surfaces 314 and 324 being devoid of optical coatings, the 45 degree condition can provide for total internal reflection at surfaces 3 Hand 324. [0045] In some examples, one or more of the long wavelength pass filters 316A-B can be formed a thin film stack disposed on one or both of the opposing surfaces of adjacent optical elements. In some of these examples, the long wavelength pass filters 316A-B can be an immersed thin film stack (e.g., having its incident medium be the same material as the exiting material). In other examples, one or more of the long wavelength pass filters 316A-B can be manufactured as a separate element and attached to the respective surfaces of the optical elements.

[0046] In some examples, the cascaded beam 318A-C can be redirected, such as through total internal reflection at surface 324, to form an exiting beam 326. The cascaded beam 318A-C, or the exiting beam 326, exits the cascaded beam combiner 306 with a wavelength spectrum that includes the non- overlapping wavelength spectra of the spectrally narrowed beams 312A-C.

[0047] In some examples, the plurality of transparent optical elements 322 A-C includes at least one intermediate optical element 322B between the first and last optical elements 322A, 322C. In some examples, the optical elements 322A-C of the plurality have the same size, shape, and orientation in the cascaded beam combiner 306.

[0048] FIGS. 4-7 show the progression of the various beams in the cascaded beam combiner 306 of FIG. 3. FIGS. 4-7 show incident beams, propagating through the various optical elements of the cascaded beam combiner, to form an output beam.

[0049] FIG. 4 is a side-view drawing of an example of a first optical element 422A, such as element 322A from the cascaded beam combiner 306 of FIG. 3. Elements numbered 4xx between 400 and 434 in FIG. 4 are identical in structure and function to elements numbered 3xx from FIG. 3. The first optical element 422A of FIG. 4 is but one example; other suitable first optical elements can also be used.

[0050] The first optical element 422A can have a cross-sectional shape that includes a first surface 434, a second surface 436 adjacent to the first surface 434 and forming an acute angle with the first surface 434, a third surface 438 adjacent to the second surface 436 and forming an obtuse angle with the second surface 436, and a fourth surface 440 adjacent to both the first 434 and third 438 surfaces and parallel to the second surface 436. In some examples, the third surface 438 is parallel to the first surface 434. In some examples, the acute and obtuse angles are 45 degrees and 135 degrees, respectively. This cross-sectional shape is but one example; other suitable shapes, optionally with more or fewer than four sides, can also be used.

[0051] A first bandpass filter 410A can be disposed on the first surface 434 of the first optical element 422A. The first bandpass filter 410A can be a thin film stack disposed on the first surface 434, or can be a separate element attached to the first surface 434. The first bandpass filter can be air-incident. The first bandpass filter 410A is configured to spectrally narrow a first incident beam 404 A to form a first spectrally narrowed beam 412A inside the first optical element 422A.

[0052] The first spectrally narrowed beam 412 A totally internally reflects off the second surface 414 of the first optical element 422A to form a first cascaded beam 418 A inside the first optical element 422A. The second surface 436 can be considered to be an incident total internal reflection surface, which can be devoid of a thin film coating, and can be arranged to totally internally reflect the respective spectrally narrowed beam 412A and thereby initiate the cascaded beam 418 A inside the first optical element 422A along the longitudinal direction (e.g., left-to-right in FIG. 4).

[0053] The first cascaded beam 418A exits the first optical element 422A at the fourth surface 440 of the first optical element 422A. Upon exiting the first optical element 422A, the first cascaded beam 418A strikes a long wavelength pass filter 416A, which is described in detail with regard to FIG. 5.

[0054] FIG. 5 is a side-view drawing of an example of a second optical element 422B, such as element 322B from the cascaded beam combiner 306 of FIG. 3. Elements numbered 5xx between 500 and 540 in FIG. 4 are identical in structure and function to elements numbered 4xx from FIG. 4. The second optical element 522A of FIG. 5 is but one example; other suitable second optical elements can also be used. The second optical element 522B can be considered to be an intermediate optical element; the cascaded beam combiner can include one or more intermediate optical elements in succession, such as attached along the longitudinal direction (e.g., left-to-right in FIG. 5). [0055] The second optical element 522B can have a cross-sectional shape that includes a first surface 534, a second surface 536 adjacent to the first surface 534 and forming an acute angle with the first surface 534, a third surface 538 adjacent to the second surface 536 and forming an obtuse angle with the second surface 536, and a fourth surface 540 adjacent to both the first 534 and third 538 surfaces and parallel to the second surface 536. In some examples, the third surface 538 is parallel to the first surface 534. In some examples, the acute and obtuse angles are 45 degrees and 135 degrees, respectively. This cross-sectional shape is but one example; other suitable shapes, optionally with more or fewer than four sides, can also be used.

[0056] The second surface 536 of the second optical element 522B is attached to the fourth surface of the first optical element (440 in FIG. 4). The surfaces can be optically contacted with no adhesive, can be affixed together with an adhesive, can be attached to another element but not attached to each other directly, or can otherwise be positioned to be adjacent to each other and, optionally, parallel to each other. In some cases, the surfaces can surround the long wavelength pass filter 516A, so that the long wavelength pass filter 516A is immersed, and the optical material of the first and second optical elements forms the incident and exiting medium for the long wavelength pass filter 516A.

[0057] A second bandpass filter 510B is disposed on the first surface 534 of the second optical element 522B. The second bandpass filter 510B is configured to spectrally narrow a second incident beam 504B to form a second spectrally narrowed beam 512B inside the second optical element 522B.

[0058] The second incident beam 504B has a wavelength spectrum

(extending beyond the range of wavelengths C to D) at wavelengths less than that of the first incident beam 404A (FIG. 4; extending beyond the range of wavelengths E to F) and overlapping with that of the first incident beam 404A (FIG. 4, overlaps between wavelengths D and E).

[0059] The second spectrally narrowed beam 512B has a wavelength spectrum (spectrally narrowed to the range of wavelengths C to D) at wavelengths less than that of the first spectrally narrowed beam 404A (FIG. 4; spectrally narrowed to the range of wavelengths E to F) and non-overlapping with that of the first spectrally narrowed beam 404A (FIG. 4; overlap eliminated between wavelengths D and E).

[0060] A first long wavelength pass filter 516 A is disposed between the fourth surface 440 (FIG. 4) of the first optical element 422A (FIG. 4) and the second surface 536 of the second optical element 522B. The first long wavelength pass filter 516 A has a high transmission (and low reflection), for s- polarized light, for wavelengths greater than a cutoff wavelength. In this example, the cutoff wavelength is between wavelengths D and E. The first long wavelength pass filter 516 A has a low transmission (and high reflection), for s- polarized light, for wavelengths less than the cutoff wavelength.

[0061] In some examples, one or more of the bandpass filters 310A-C can be formed as a thin film stack disposed on the respective first surface 334A-C. In some examples, one or more of the bandpass filters 310A-C are air-incident, and are configured to operate at normal incidence or within 1 , 2, 3, 4, or 5 degrees of normal incidence. In other examples, one or more of the bandpass filters 310A-C can be manufactured as a separate element and attached to the respective first surface 334A-C.

[0062] The first long wavelength pass filter 516A combines the first cascaded beam 518 A with an s-polarized component of the second spectrally narrowed beam 512B to form a second cascaded beam 518B inside the second optical element 522B. The second cascaded beam 518B has a wavelength spectrum extending between wavelengths C and D and between wavelengths E and F. The wavelength spectrum between wavelengths D and E is suppressed.

[0063] Additionally, the first long wavelength pass filter 516A can be used to clean up the polarization state from the second incident beam 504B. The second spectrally narrowed beam 512B can be largely s-polarized, with some residual p-polarized light. For wavelengths between C and D, the first long wavelength pass filter 516 A has a relatively high reflectivity for s-polarized light and a relatively low reflectivity for p-polarized light. As a result, the s-polarized light is reflected and is directed into the second cascaded beam 518B, while the p-polarized light is transmitted as residual light 532 and is absorbed, reflected, or is otherwise directed out of the cascaded beam. [0064] The second cascaded beam 518B exits the first optical element 522B at the fourth surface 540 of the second optical element 522A. Upon exiting the second optical element 522B, the second cascaded beam 518B strikes a second long wavelength pass filter 516B, which is described in detail with regard to FIG. 6.

[0065] FIG. 6 is a side-view drawing of an example of a third optical element 622C, such as element 322C from the cascaded beam combiner 306 of FIG. 3. Elements numbered 6xx between 600 and 640 in FIG. 6 are identical in structure and function to elements numbered 5xx from FIG. 5. The third optical element 622A of FIG. 6 is but one example; other suitable third optical elements can also be used. The third optical element 622C can be considered to be a last optical element, sequentially following the intermediate optical element(s) and the first optical element.

[0066] The third optical element 622C can have a cross-sectional shape that includes a first surface 634, a second surface 636 adjacent to the first surface 634 and forming an acute angle with the first surface 634, a third surface 638 adjacent to the second surface 636 and forming an obtuse angle with the second surface 636, and a fourth surface 640 adjacent to both the first 634 and third 638 surfaces and parallel to the second surface 636. In some examples, the third surface 638 is parallel to the first surface 634. In some examples, the acute and obtuse angles are 45 degrees and 135 degrees, respectively. This cross-sectional shape is but one example; other suitable shapes, optionally with more or fewer than four sides, can also be used.

[0067] The second surface 636 of the third optical element 622C is attached to the fourth surface 540 (FIG. 5) of the second optical element 522B (FIG. 5).

[0068] A third bandpass filter 610C is disposed on the first surface 634 of the third optical element 622C. The third bandpass filter 6 IOC is configured to spectrally narrow a third incident beam 604C to form a third spectrally narrowed beam 612C inside the third optical element 622C.

[0069] The third incident beam 604C has a wavelength spectrum (extending beyond the range of wavelengths A to B) at wavelengths less than that of the second incident beam 504B (FIG. 5; extending beyond the range of wavelengths C to D) and overlapping with that of the second incident beam, 504B (FIG. 5, overlaps between wavelengths B and C).

[0070] The third spectrally narrowed beam 618C a wavelength spectrum (spectrally narrowed to the range of wavelengths A to B) at wavelengths less than that of the second spectrally narrowed beam 504B (FIG. 5; spectrally narrowed to the range of wavelengths C to D) and non-overlapping with that of the second spectrally narrowed beam 504B (FIG. 5; overlap eliminated between wavelengths B and C).

[0071] The second long wavelength pass filter 616B is disposed between the fourth surface 540 (FIG. 5) of the second optical element 522B (FIG. 5) and the second surface 636 of the third optical element 622C. The second long wavelength pass filter 616B has a high transmission (and low reflection), for s- polarized light, for wavelengths greater than a cutoff wavelength. In this example, the cutoff wavelength is between wavelengths B and C. The second long wavelength pass filter 616B has a low transmission (and high reflection), for s-polarized light, for wavelengths less than the cutoff wavelength.

[0072] The second long wavelength pass filter 616B combines the second cascaded beam 618B with an s-polarized component of the third spectrally narrowed beam 612C to form a third cascaded beam 618C inside the third optical element 622C. The p-component of the third spectrally narrowed beam 612C is discarded as residual light 632.

[0073] FIG. 7 is another side-view drawing of the third optical element 622C of FIG. 6, showing rays along an exiting path from the third optical element 622C. Elements numbered 7xx between 700 and 740 in FIG. 7 are identical in structure and function to elements numbered 6xx from FIG. 6.

[0074] The third cascaded beam 618C totally internally reflects off the fourth surface 740 of the third optical element 722C to form a redirected beam 712C inside the third optical element 722C. The fourth surface 740 can be considered to be an exiting total internal reflection surface, which can be devoid of a thin film coating, and can be arranged to totally internally reflect the third cascaded beam 718C to form an exiting beam 712C inside the third optical element. The exiting beam 712C passes through the third surface 738, which can be anti-reflection coated, to form output beam 708. [0075] The geometry of FIGS. 3-7 can be advantageous for a number of reasons. For example, the optical elements can all have the same size and shape, so that they can be manufactured in relatively high volumes, with the same tooling, and without the need for multiple configurations. As another example, there is only one optical element per incident beam, rather than multiple optical elements, which can reduce the number of parts to align and assemble. The geometry of FIGS. 3-7 is but one example; other suitable geometries can also be used.

[0076] For example, FIG. 8 is a side-view drawing of another example of a cascaded beam combiner 806. In FIG. 8, the long wavelength pass filters 816A, 816B are formed on the hypotenuses of cubes 850B/852A and 850C/852B, where the cubes are attached to one another and to end pieces 850A and 852C are attached at interfaces 854A-C. Bandpass filters 810A-C are formed on outward-facing surfaces of the cubes and end pieces. The cascaded beam combiner can include incident 814 and exiting 824 total internal reflection surfaces, or can optionally omit the end pieces and couple the first incident beam in and the exiting beam out along the longitudinal direction (e.g., left-to-right in FIG. 8.)

[0077] FIG. 9 is a side-view drawing of another example of a cascaded beam combiner 906. In FIG. 9, the long wavelength pass filters 916A, 916B are formed on the hypotenuses of cubes 950B/952A and 950C/952B, where the cubes are attached to one another and to end pieces 950A and 952C are attached at interfaces 954A-C. Bandpass filters 910A-C are formed on outward-facing surfaces of the cubes and end pieces. The cascaded beam combiner can include incident 914 and exiting 924 total internal reflection surfaces, or can optionally omit the end pieces and couple the first incident beam in and the exiting beam out along the longitudinal direction (e.g., left-to-right in FIG. 9.)

[0078] Unlike the geometry in FIG. 8, the elements in the cascaded beam combiner 906 are arranged to accept incident beams that are not parallel. For the example of FIG. 9, the elements are arranged to accept incident beams from alternating sides of the cascaded beam combiner 906. In other configurations, elements 905A and 950B can be formed as a single element, and elements 952A and 952B can be formed as a single element. The cascaded beams and spectrally narrowed beams all strike the long wavelength pass filters 916A-B at the same incident angle. In the example of FIG. 9, the incident angle is 45 degrees, although other suitable incident angle values can also be used.

[0079] Other geometries are possible as well. For example, some geometries can accommodate incident beams that are not co-planar (e.g., one or more incident beams propagates out of the plane of the page, in side views such as FIG. 9). As another example, some geometries can bend the cascaded beam at one or more wavelength pass filter, so that the cascaded beam path follows one or more reflection, rather than all transmissions. As another example, some geometries can include a wedge between the second and fourth surfaces. As still another example, one or more short wavelength pass filters can be used instead of long wavelength pass filters, where the wavelength spectra of the incident beams can be chosen suitably.

[0080] FIG. 10 is a side-view drawing of another example of an optical system having tunable bandpass filters 1060A-C in its incident beam paths. Elements numbered lOxx between 1000 and 1054 in FIG. 10 are identical in structure and function to elements numbered 8xx from FIG. 8. Such tunable bandpass filters can augment or replace one or more monochromators between the light sources and the cascaded beam combiner, which can be useful for optical systems that characterize a sample as a function of wavelength, such as spectrometers. The tunable filters 1060A-C are but one example of tunable elements in the incident beam paths; other suitable tunable elements can also be used. The tunable filters 1060A-C can also be used with any suitable configuration for the cascaded beam combiner, including the configurations of FIGS. 3-7, FIG. 8, and others.

[0081] Each tunable bandpass filter 1060A-C has a transmission band that lies within the transmitted wavelength range of the corresponding bandpass filter 1050A-C. For instance, bandpass filter 1050C transmits wavelengths between A and B, as discussed above. Tunable bandpass filter 1060C transmits a relatively small portion of the wavelength range between A and B at any one time, but can be tuned to move the transmitted portion to cover most or all of the wavelength range between A and B as needed. [0082] Each tunable bandpass filter 1060A-C can be formed as a thin film coating on a pivotable, transparent or partially transparent substrate. Tuning can be performed by pivoting the substrate, such as with an electrically controlled micromirror or other suitable actuator. As the substrate pivots, the incident angle of the incident beam varies, and the transmitted wavelength range shifts upward or downward in wavelength.

[0083] In the example of FIG. 10, tunable bandpass filter 1060C has a transmission band centered between wavelengths A and B for an incident angle of 45 degrees. When the tunable bandpass filter 1060C is pivoted to reduce the incident angle (e.g., to bring the tunable bandpass filter 1060C closer to normal incidence), the transmission band shifts toward a longer wavelength. In the example of FIG. 10, when tunable bandpass filter is pivoted to reduce the incident angle to 30 degrees, the transmission band shifts toward longer wavelength B. Similarly, when the tunable bandpass filter 1060C is pivoted to increase the incident angle (e.g., to bring the tunable bandpass filter 1060C closer to grazing incidence), the transmission band shifts toward a shorter wavelength. In the example of FIG. 10, when tunable bandpass filter is pivoted to increase the incident angle to 60 degrees, the transmission band shifts toward shorter wavelength A.

[0084] In general, the range of incident angles is selected so that the transmission band shifts within the within the transmitted wavelength range of the corresponding bandpass filter 1050A-C, and does not shift to values outside the transmitted wavelength range of the corresponding bandpass filter 1050A-C. An example of a suitable range of incident angles is between 30 and 60 degrees; this is but one example, and other suitable examples can also be used.

[0085] In some examples, the range of incident angles can be selected to include regions having high sensitivity, so that a change in incident angle can produce a relatively large change in transmitted wavelength. In some examples, the range of incident angles excludes angles close to 0 degrees (e.g., normal incidence), because the sensitivity drops to zero at 0 degrees.

[0086] In some examples, one or both surfaces of the tunable bandpass filters can be anti-reflection coated. Such anti-reflection coatings are relatively simple for most incident angles, but can be difficult to design or manufacture at incident angles close to 90 degrees. As a result, in some examples, the range of incident angles excludes angles close to 90 degrees (e.g., grazing incidence), because reflections off the surfaces of the tunable bandpass filters can become difficult to reduce with anti-reflection coatings.

[0087] In some examples, all of the tunable bandpass filters 960A-C are pivoted in unison, so that the incident angles for the tunable bandpass filters 960A-C vary together. In other examples, one or more tunable bandpass filters 960A-C are pivoted so that one or more incident angles differ, at a given time, from one or more other incident angles.

[0088] In some examples, the tunable bandpass filters 960A-C are positioned so that incident light is s-polarized. In the example of FIG. 9, the tunable bandpass filters 960A-C pivot within the plane of the page, and the s- polarized light incident on the tunable bandpass filters 960A-C has its electric field vector oscillating into and out of the plane of the page. For the configuration of FIG. 9, s-polarized light at the tunable bandpass filters 960A-C is also s-polarized light at the long wavelength pass filters 916A-B. Using s- polarized light at the tunable bandpass filters 960A-C can produce a narrower range of transmitted wavelengths through the tunable bandpass filters 960A-C, although p-polarized light can also be used.

[0089] In some examples, when the tunable bandpass filters 960A-C are present, the bandpass filters 950A-C can be omitted. In those examples, the bandpass filters 950A-C can be replaced with suitable anti-reflection coatings.

[0090] FIG. 11 is a plot of transmission versus wavelength for an example of an idealized bandpass filter, such as those shown in FIGS. 2-7.

[0091] The transmission of the bandpass filter is relatively high, typically around 100%, for wavelengths 1108 between a low wavelength cutoff 1116 and a high wavelength cutoff 1118. The transmission of the bandpass filter is relatively low, typically around 0%, for wavelengths 1 102 less than the low wavelength cutoff 1116 or wavelengths 1114 greater than the high wavelength cutoff 1118.

[0092] There is a region of transition 1120 at the low wavelength cutoff 1116, and a region of transition 1122 at the high wavelength cutoff 1118. In most cases, it is desirable to make these regions of transition as small as possible, so that the transmission changes from low to high or high to low relatively quickly, with respect to wavelength.

[0093] Such a bandpass filter can be designed and fabricated as a stack of thin films, typically of varying thicknesses less than one wavelength, and often including one or more blocks of layers having alternating high- and low- refractive indices. There is commercially available software that can simulate performance of a particular thin film stack, can run Monte Carlo analyses of performance as a function of incident angle, incident wavelength, layer thickness, refractive index, and the tolerances associated with these quantities, and can optimize layer thicknesses to reach a performance goal and reduce sensitivity to particular parameters. In general, a skilled coating designer can locate the low 1116 and high 1118 cutoff wavelengths to a desired precision, can reduce the regions of transition 1120, 1122 to suitably small values, and can reduce ringing on either sides of the regions of transitions, such as at wavelength 1104, 1106, 1110, and 1112, to suitably low levels. For example, the telecommunications industry, operating at wavelengths around 1.55 microns, has developed thin film coating design and manufacturing capability that can routinely design thin film stacks having regions of transition 1 nm wide, or narrower. This is but one numerical example; other suitable numerical values can also be used.

[0094] FIG. 12 is a plot of transmission versus wavelength for an example of an idealized long wavelength pass filter, such as those shown in FIGS. 3-7. The long wavelength pass filters can be designed and fabricated as a stack of thin films, similar to the processes used for the bandpass filters, and well-known to one of ordinary skill in the art.

[0095] Unlike the bandpass filters, which are used at normal or near-normal incidence, the long wavelength pass filters are used at relatively high angles of incidence, such as 45 degrees, and are used in an immersed manner, where the incident and exiting material are both the optical material of the optical elements. As a result, the long wavelength pass filter exhibits different performance for p- and s-polarized light. The cascaded beam combiners make use of the different p- and s- performance to enhance a polarization extinction ratio for reflected components, as discussed above. [0096] For s-polarized light, the transmission of the long wavelength pass filter is relatively low, typically around 0%, for wavelengths 1202 below an s- polarized wavelength cutoff 1220, and relatively high, typically around 100%, for wavelengths 1208, 1210, 1218 above the s-polarized wavelength cutoff 1220.

[0097] For p-polarized light, the behavior is similar, but the p-polarized wavelength cutoff 1224 is at a greater wavelength than the s-polarized wavelength cutoff 1220. For p-polarized light, the transmission of the long wavelength pass filter is relatively low, typically around 0%, for wavelengths 1202, 1212 below the p-polarized wavelength cutoff 1224, and relatively high, typically around 100%, for wavelengths 1218 above the p-polarized wavelength cutoff 1224.

[0098] The long wavelength pass filter also has regions 1226, 1228 of transition, which can be made suitably small, and areas of ringing, such as wavelengths 1204, 1206, 1214, 1216, which can also be made suitably small.

[0099] For the design of long wavelength pass filter 416A (FIG. 4), wavelengths D and E (FIG. 4) can be located at or near wavelength 1222, between wavelengths 1220 and 1224, so that long wavelength pass filter 416A (FIG. 4) reflects s-polarized wavelengths less than D, transmits p-polarized wavelengths between C and D, and transmits s-polarized wavelengths greater than E.

[00100] FIG. 13 is a flow chart of an example of a method 1300 for forming a multi- wavelength beam. The method 1300 is suitable for use in the cascaded beam combiners of FIGS. 3-9, as well as other suitable beam combiners. The method 1300 is but one example; other methods can also be used to form a multi-wavelength beam.

[00101] At 1302, method 1300 receives a plurality of incident beams. The plurality can include first and last incident beams. The incident beams can have overlapping wavelength spectra that decrease in wavelength from the first incident beam to the last incident beam. At 1304, method 1300 spectrally narrows each incident beam to form a respective spectrally narrowed beam. The spectrally narrowed beams can have non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam. In some examples, method 1300 can direct each incident beam through a respective bandpass filter disposed on an outward-facing surface of a cascaded beam combiner to form a respective spectrally narrowed beam inside the cascaded beam combiner. At 1306, method 1300 sequentially combines components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam. In some examples the components are s-polarized components. In some examples, method 1300 can direct the spectrally narrowed beams to respective long wavelength pass filters that direct s-polarized components of the spectrally narrowed beams sequentially into the cascaded beam inside the cascaded beam combiner. At 1308, method 1300 forms a multi-wavelength beam from the cascaded beam. The multi-wavelength beam can have a wavelength spectrum that includes the non-overlapping wavelength spectra of the spectrally narrowed beams. In some examples, method 1300 can direct the cascaded beam out of the beam combiner to form the multi-wavelength beam.

[00102] The description of the invention and its applications as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible, and practical alternatives to and equivalents of the various elements of the embodiments would be understood to those of ordinary skill in the art upon study of this patent document. These and other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.

Claims

What is claimed is:

1. An optical system, comprising:

a cascaded beam combiner configured to receive a plurality of incident beams, the incident beams having wavelength spectra that decrease in wavelength from a first incident beam to a last incident beam;

the cascaded beam combiner further including a bandpass filter for each incident beam, each bandpass filter spectrally narrowing a respective incident beam to form a respective spectrally narrowed beam inside the cascaded beam combiner, the spectrally narrowed beams having non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam; and

the cascaded beam combiner further including a plurality of long wavelength pass filters positioned to sequentially combine components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam inside the cascaded beam combiner, the cascaded beam exiting the cascaded beam combiner with a wavelength spectrum that includes the non-overlapping wavelength spectra of the spectrally narrowed beams.

2. The optical system of claim 1 , further comprising a plurality of light sources arranged to produce the incident beams.

3. The optical system of claim 2, wherein plurality of light sources are configured such that each incident beam has a wavelength spectrum that overlaps with that of an adjacent incident beam.

4. The optical system of claim 2, further comprising circuitry configured to dynamically control the output levels of the light sources of the plurality.

5. The optical system of one of claims 1-4,

wherein the light sources produce light having wavelength spectra that decrease in wavelength from a first light source to a last light source; and wherein each light source produces light having a wavelength spectrum broader than the respective bandpass filter.

6. The optical system of claim 5, wherein the spectrally narrowed beams and the cascaded beam all strike the long wavelength pass filters at the same incident angle.

7. The optical system of claim 6,

wherein the incident beams are collimated, parallel, and co-planar; wherein the bandpass filters are positioned parallel to one another; and wherein the long wavelength pass filters are positioned parallel to one another.

8. The optical system of claim 7,

wherein the cascaded beam combiner further includes a plurality of optical elements;

wherein each bandpass filter is disposed on a respective optical element; and

wherein each long wavelength pass filter is disposed between adjacent optical elements.

9. The optical system of claim 8, wherein the optical elements have the same size, shape, and orientation in the cascaded beam combiner.

10. The optical system of claim 8, wherein the bandpass filters and the long wavelength pass filters are thin film coatings disposed on respective surfaces of the optical elements.

11. The optical system of claim 5, wherein the cascaded beam exits the cascaded beam combiner with a wavelength spectrum that extends fully across the wavelength spectra of the bandpass filters.

12. The optical system of claim 6, wherein the long wavelength pass filters sequentially combine s-polarized components of the spectrally narrowed beams.

13. A cascaded beam combiner, comprising:

a plurality of optical elements configured to receive a plurality of incident beams, the incident beams having wavelength spectra that decrease in wavelength from a first incident beam to a last incident beam;

a plurality of bandpass filters disposed on respective optical elements, each bandpass filter spectrally narrowing a respective incident beam to form a respective spectrally narrowed beam inside the respective optical element, the spectrally narrowed beams having non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam; and

a plurality of long wavelength pass filters disposed between adjacent optical elements, the long wavelength pass filters being positioned to sequentially combine components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam inside the optical elements, the cascaded beam exiting the optical elements with a wavelength spectrum that includes the non-overlapping wavelength spectra of the spectrally narrowed beams.

14. The cascaded beam combiner of claim 13, wherein the spectrally narrowed beams and the cascaded beam all strike the long wavelength pass filters at the same incident angle.

15. The cascaded beam combiner of claim 14,

wherein the incident beams are collimated, parallel, and co-planar;

wherein the bandpass filters are positioned parallel to one another; and wherein the long wavelength pass filters are positioned parallel to one another.

16. The cascaded beam combiner of claim 15, wherein the optical elements have the same size, shape, and orientation in the cascaded beam combiner.

17. The cascaded beam combiner of claim 16, wherein the optical elements have a cross-sectional shape that includes a first surface, a second surface adjacent to the first surface and forming an acute angle with the first surface, a third surface adjacent to the second surface and forming an obtuse angle with the second surface, and a fourth surface adjacent to both the first and third surfaces and parallel to the second surface.

18. The cascaded beam combiner of claim 13, wherein the bandpass filters and the long wavelength pass filters are thin film coatings disposed on respective surfaces of the optical elements.

19. The cascaded beam combiner of claim 13, wherein the cascaded beam exits the optical elements with a wavelength spectrum that extends fully across the wavelength spectra of the bandpass filters.

20. The cascaded beam combiner of claim 13, wherein the long wavelength pass filters sequentially combine s-polarized components of the spectrally narrowed beams.

21. An optical system, comprising:

a plurality of tunable bandpass filters positioned to receive a respective plurality of incident beams, the incident beams having wavelength spectra that decrease in wavelength from a first incident beam to a last incident beam, each tunable bandpass filter spectrally narrowing a respective incident beam to form a respective spectrally narrowed beam; and

a cascaded beam combiner configured to receive the spectrally narrowed beams, the spectrally narrowed beams having non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam;

22. The optical system of claim 21,

wherein each tunable bandpass filter includes a thin film coating disposed on a pivotable substrate; and

wherein each tunable bandpass filter is tuned by pivoting the substrate and thereby varying an incident angle of the respective incident beam.

23. The optical system of claim 22, wherein the incident beams are s- polarized with respect to the tunable bandpass filters.

24. The optical system of claim 22, wherein the substrates pivot in unison.

25. The optical system of claim 21, further comprising:

a plurality of light sources configured to produce the respective plurality of incident beams; and

circuitry configured to dynamically control the output levels of the light sources of the plurality.

26. A method for forming a multi-wavelength beam, comprising:

receiving a plurality of incident beams, the plurality including first and last incident beams, the incident beams having wavelength spectra that decrease in wavelength from the first incident beam to the last incident beam;

spectrally narrowing each incident beam to form a respective spectrally narrowed beam, the spectrally narrowed beams having non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam;

sequentially combining components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam; forming a multi-wavelength beam from the cascaded beam, the multi- wavelength beam having a wavelength spectrum that includes the non- overlapping wavelength spectra of the spectrally narrowed beams.

27. The method of claim 26, wherein each incident beam has a wavelength spectrum that overlaps with that of an adjacent incident beam.

28. The method of claim 26, wherein spectrally narrowing each incident beam to form a respective spectrally narrowed beams comprises:

directing each incident beam through a respective bandpass filter disposed on an outward-facing surface of a cascaded beam combiner to form a respective spectrally narrowed beam inside the cascaded beam combiner.

29. The method of claim 28, wherein sequentially combining components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam comprises:

directing the spectrally narrowed beams to respective long wavelength pass filters that direct s-polarized components of the spectrally narrowed beams sequentially into the cascaded beam inside the cascaded beam combiner.

30. The method of claim 29, wherein forming a multi-wavelength beam from the cascaded beam comprises:

directing the cascaded beam out of the beam combiner to form the multi- wavelength beam.

31. A method for forming a multi-wavelength beam, comprising:

spectrally narrowing each incident beam with a respective tunable bandpass filter to form a respective spectrally narrowed beam, the spectrally narrowed beams having non-overlapping wavelength spectra that decrease in wavelength from a first spectrally narrowed beam to a last spectrally narrowed beam, the non-overlapping wavelength spectra being tuned in time;

sequentially combining components of the spectrally narrowed beams from the first spectrally narrowed beam to the last spectrally narrowed beam to form a cascaded beam;

forming a multi-wavelength beam from the cascaded beam, the multi- wavelength beam having a wavelength spectrum that includes the non- overlapping wavelength spectra of the spectrally narrowed beams.

32. The method of claim 31, further comprising:

tuning each tunable bandpass filter to vary the transmitted wavelengths of each spectrally narrowed beam within a respective non-tunable wavelength window, the wavelength windows being non-overlapping.

33. The method of claim 32,

34. The method of claim 32, wherein the incident beams are s-polarized with respect to the tunable bandpass filters.

The method of claim 32, wherein the substrates pivot in unison.