Optical filter assembly

Abstract

Various embodiments of an optical filter assembly are disclosed. In one embodiment, the optical filter assembly includes a single pair of optical filters including a first filter having a first optical density, a first bandwidth, and a first center wavelength, and a second filter having a second optical density, a second bandwidth, and a second center wavelength. The first and second center wavelengths may be the same or different. The first filter is configured to reflect a first in-band portion of an incoming optical signal and allow a first out-of-band portion to be transmitted or absorbed. The second filter is configured to reflect a portion of the first in-band portion as a second in-band portion to the first filter, and allow a second out-of-band portion to be transmitted or absorbed. Successive reflections between the two filters result in high in-band transmission and high out-of-band blocking by the optical filter assembly.

Description

Docket Number: 00784-WOOPTICAL FILTER ASSEMBLYCross-Reference to Related Applications

[0001] The present application claims priority to U.S. provisional application Serial No. 63 / 735,334 filed on December 18, 2024, entitled “Optical Filter Assembly” the contents of which are incorporated herein.Technical Field

[0002] The following disclosure generally relates to optical filter assemblies and in particular to optical filter assemblies using counter-facing reflective notch filters that have high optical densities and use multiple reflections to result in very high attenuation of out-of-band wavelengths while maintaining high throughput of in-band wavelengths.Background

[0003] Optical filter assemblies are used in a wide variety of applications such as gas detection, air pollution monitoring, water treatment, germicidal treatment, semiconductor inspection, spectroscopy, and the like. Many of these applications require UV light. These assemblies use multiple optical filters to control the wavelength or spectrum of light that interacts with chemical samples. While these prior art filter assemblies have proven useful in the past, a number of shortcomings have been identified. For example, optical transmission filters often have transmission throughput that is relatively low for the intended application, so high power light sources are required, increasing the cost of the instrument and created additional heat which must be managed. Optical filter assemblies that use multiple reflection filters can provide improved optical filtering of non-desired wavelengths, but each filter must be precisely aligned to the other filters. This results in increased device complexity, size, and cost, depending on the number of filters required. In light of the foregoing, there is an ongoing need for an optical filter assembly that provides high throughput of the desired wavelength bands and high attenuation of undesired wavelength bands at a lower cost than prior art filter assemblies.Brief Description of the Drawings

[0004] Various embodiments of an improved optical filter assembly will be explained in more detail by way of the accompanying drawings, wherein:Docket Number: 00784-WO

[0005] FIG. 1 shows a sketch of a prior art optical filter assembly with four individual reflective filters.

[0006] FIG. 2 shows a plot of out-of-band attenuation and in-band throughput relative to the number of reflections of light from an optical filter.

[0007] FIG. 3 shows a plot of the region of peak optical intensity (throughput) from FIG. 2.

[0008] FIG. 4 shows a ray tracing diagram of an embodiment of an optical filter assembly that uses two filters and multiple reflections per filter.

[0009] FIG. 5 shows an embodiment of an optical filter assembly that uses two filters and multiple reflections per filter.

[0010] FIGS. 6 and 7 show views of an embodiment of a structure module of a optical filter sub-assembly.

[0011] FIG. 8 shows a plot of attenuation relative to the number of reflections of light from the optical filter used in the embodiment shown in FIG. 5.

[0012] FIG. 9 shows a plot of attenuation relative to the number of reflections of light from optical filters used in the embodiment shown in FIG. 5 having different center wavelengths.

[0013] FIG. 10 shows an embodiment of an optical filter assembly that uses two pairs of filters, each pair of filters having two filters and multiple reflections per filter.SUMMARY

[0014] Multiple embodiments of optical filter assemblies are disclosed. In one embodiment, the optical filter assembly includes a single pair of optical filters including a first optical filter and a second optical filter, wherein the first optical filter has a first optical density, a first bandwidth, and a first center wavelength. The first center wavelength may be substantially equal to or different from the second center wavelength. The first optical filter is configured to reflect a first reflected portion of an incoming optical signal and allow at least one first out-of-band portion to be transmitted through or absorbed by the first optical filter. The second optical filter has a second optical density, a second bandwidth, and a second center wavelength,Docket Number: 00784-WO wherein the second optical filter is configured to reflect a portion of the first reflected portion as a second reflected portion to the first optical filter, and allow a second out- of-band portion to be transmitted or absorbed by the second optical filter.

[0015] A portion of the second reflected portion is reflected by the first optical filter to form a third reflected portion, a portion of the third reflected portion is reflected by the second optical filter to form a fourth reflected portion, a portion of the fourth reflected portion is reflected by the first optical filter to form a fifth reflected portion, and a portion of the fifth reflected portion is reflected by the second optical filter to form a sixth reflected portion as an outgoing optical signal.

[0016] Each reflected portion includes an in-band portion and an out-of-band portion. The out-of-band portions of the reflected portions are transmitted and / or absorbed by the respective first or second filters. More specifically, a portion of the second reflected portion is transmitted or absorbed by the first optical filter as a third out-of-band portion, a portion of the third reflected portion is transmitted or absorbed by the second optical filter as a fourth out-of-band portion. A portion of the fourth reflected portion is transmitted or absorbed by the first optical filter as a fifth out-of- band portion, and a portion of the fifth reflected portion is transmitted or absorbed by the second optical filter as a sixth out-of-band portion. The optical filter assembly may further include at least one first absorbing body configured to absorb at least a portion of at least one of the first out-of-band portion, the third out-of-band portion, and the fifth out-of-band portion. The optical filter assembly may further include at least one second absorbing body configured to absorb at least a portion of at least one of the second out-of-band portion, the fourth out-of-band portion, and the sixth out-of-band portion.

[0017] In other embodiments, the optical filter assembly includes a first optical filter and a second optical filter, wherein the first optical filter and the second optical filter each include a reflective surface at least partially facing the reflective surface of the other filter, wherein each of the first optical filter and the second optical filter are sized to allow at least a portion of an incoming optical signal to reflect therefrom a minimum of three times. In other embodiments, portions of the incoming signal may reflect from the optical filters fewer than or more than three times.Docket Number: 00784-WO

[0018] In various embodiments, the first optical filter and the second optical filter are reflective notch filters. The first and second optical filters may be configured with a variety of center wavelengths. In one embodiment, the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 100 nanometers and 280 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength below 190 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 190 nanometers and 250 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 200 nanometers and 280 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 250 nanometers and 315 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 280 nanometers and 315 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength above 315 nanometers. In another embodiment, the first optical filter and the second optical filter each have a center wavelength above 400 nanometers.

[0019] In another embodiment, the optical filter assembly includes a single pair of optical filters including a first optical filter having a first center wavelength and a first bandwidth, and a second optical filter having a second center wavelength and a second bandwidth. The single pair of optical filters is configured to receive an incoming optical signal having an in-band portion and an out-of-band portion and output an outgoing optical signal, wherein the single pair of optical filters has a combined optical density for the out-of-band portion of the optical signal, and wherein the single pair of optical filters is configured to have a throughput for the in- band portion of the optical signal.

[0020] In one embodiment, the combined optical density is greater than 3 and the throughput for the in-band portion of the optical signal is greater than 85%. In another embodiment, the combined optical density is greater than 5 and the throughput for the in-band portion of the optical signal is greater than 80%. In another embodiment, the combined optical density is 6 or greater and the throughputDocket Number: 00784-WO for the in-band portion of the optical signal is greater than about 50%. In yet another embodiment, the combined optical density is 6 or greater and the throughput for the in-band portion of the optical signal is greater than about 60%. In still another embodiment, wherein the combined optical density is greater than 6 and the throughput for the in-band portion of the optical signal is greater than about 70%.

[0021] In various embodiments, the center wavelengths of the optical filters are provided in a variety of ways. In one embodiment, the first center wavelength and the second center wavelength are substantially equal. In another embodiment, the first center wavelength and the second center wavelength are different. In one embodiment, the second center wavelength is shifted approximately 5% of the first bandwidth so that the second center wavelength is lower than the first center wavelength. In another embodiment, the second center wavelength is shifted approximately 10% of the first bandwidth so that the second center wavelength is lower than the first center wavelength. In another embodiment, the second center wavelength is shifted approximately 5% of the first bandwidth or the second bandwidth so that the second center wavelength is higher than the first center wavelength. In another embodiment, the second center wavelength is shifted approximately 10% of the first bandwidth or the second bandwidth so that the second center wavelength is higher than the first center wavelength.Detailed Description

[0022] Example embodiments are described herein with reference to the accompanying drawings. Unless otherwise expressly stated, in the drawings the sizes, positions, etc., of components, features, elements, etc., as well as any distances therebetween, are not necessarily to scale, and may be exaggerated for clarity. In the drawings, like numbers refer to like elements throughout. Thus, the same or similar numbers may be described with reference to other drawings even if they are neither mentioned nor described in the corresponding drawing. Also, even elements that are not denoted by reference numbers may be described with reference to other drawings.

[0023] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless otherwise defined, all terms (including technical and scientific terms) used herein have theDocket Number: 00784-WO same meaning as commonly understood by one of ordinary skill in the art. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Also, the terms “at least one”, “at least a”, and “one or more” may are intended to include both the singular and plural forms, depending on the context. It should be recognized that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Unless indicated otherwise, terms such as “first,” “second,” etc., are only used to distinguish one element from another. For example, one member could be termed a “first member” and similarly, another member could be termed a “second member”, or vice versa.

[0024] Unless indicated otherwise, spatially relative terms, such as “below,” “beneath,” “lower,” “above,” and “upper,” “opposing,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element or feature, as illustrated in the FIGS. It should be recognized that the spatially relative terms are intended to encompass different orientations in addition to the orientation depicted in the FIGS. For example, if an object in the FIGS, is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. An object may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly. A set of reference axes (e.g., X, Y, Z), directions, or coordinates, and the rotation around them (e.g., 9X, 0Y, 0Z) may be included in the FIGS, for the purpose of orienting the reader to facilitate understanding of the FIGS, and the specification, and do not necessarily indicate that any particular feature or element is aligned with, or is orthogonal to, any other feature or element.

[0025] The paragraph numbers used herein are for organizational purposes only, and, unless explicitly stated otherwise, are not to be construed as limiting the subject matter described. It will be appreciated that many different forms, embodiments and combinations are possible without deviating from the spirit and teachings of thisDocket Number: 00784-WO disclosure and so this disclosure should not be construed as limited to the example embodiments set forth herein. Rather, these examples and embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the disclosure to those skilled in the art.

[0026] Unless stated otherwise, the terms “approximately”, “about” and “substantially” are used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and within ±2% in some embodiments. The terms “approximately” and “about” can also include the target value.

[0027] As used herein, the term “in-band” refers to ranges of wavelengths that are desired for a particular purpose (such as an optical instrument). Conversely, the term “out-of-band” refers to ranges of wavelengths that are not desired for a particular purpose. The term “through-device transmission” refers to the amount or percentage of in-band light that makes it all the way through an optical filter or optical filter assembly. The terms “attenuated”, “blocked”, or “rejected” are related to the amount or percentage of out-of-band light energy that does not make it all the way through the optical filter or optical filter assembly. The design of optical filter assemblies often strives to maximize through-device transmission of in-band light and to minimize through-device transmission of out-of-band light.

[0028] The embodiments described herein are directed to a novel optical filter assembly that achieves high through-device transmission at desired wavelengths and very high blocking of undesired wavelengths of the electromagnetic spectrum. These embodiments can be customized, tailored, or configured to work with a variety of wavelengths or wavelength ranges emitted by a variety of optical light sources. In some embodiments, these sources emit light in the UV-A band (approximately 315- 400 nanometers, also referred to herein as “near-UV”), the UV-B band (approximately 280-315 nanometers, also referred to herein as “near-UV”), or the UV-C band (approximately 100-280 nanometers). The wavelength band from 200- 280 nanometers, also referred to herein as “deep UV” is a sub-spectrum of the UV-A band. The embodiments of optical filter assemblies described herein may be configured to work with laser light. Examples of these laser sources include but are not limited to Argon (Ar2) excimer lasers at 126 nm, Krypton (Kr2) excimer lasers atDocket Number: 00784-WO146 nm, fluorine (F2) excimer lasers at 157 nm, argon-fluoride (ArF) excimer lasers at 193 nm, 214 nm, or 224 nm, krypton-fluoride (KrF) excimer lasers at 248 nm, and a wide variety of other excimer lasers. Other laser sources include solid state lasers such as Nd:YAG, Nd:YV04, Nd:YLF, Yb:Yag at a variety of wavelengths. The optical filter assemblies described herein may also be configured to work with nonlaser (non-coherent) light sources selected for particular wavelengths or wavelength ranges. Examples of these sources include deuterium lamps at 112-900nm (with a continuous spectrum of 180 nm-370 nm), zinc lamps at 202.5 nm, 206.2 nm, 213.9 nm, 275.6nm, 277.1 nm, or 280.1 nm, and cadmium lamps at 214.4 nm, 226.5 nm, 228.8 nm, 283.7, 288.0 nm). In other embodiments, the filter assemblies disclosed herein may operate in the extreme UV (EUV) band of wavelengths.

[0029] One application for optical filter assemblies is air pollution monitoring, including specific detection and measurement of pollutants such as sulfur dioxide (SO2) in ambient air. SO2 molecules strongly absorb UV light at 214 nm, so an instrument used to measure the concentration of SO2 requires high through-device transmission of light in a bandwidth centered at or near 214 nm and high blocking of light in wavelengths outside of this bandwidth.

[0030] Further, absorption within such filters, based on the materials used in their construction increases with higher numbers of coating layers and / or added coating layer thickness. Traditional coating materials such as Hafnium Oxide (HfO2) are problematic as both reflection and transmission are reduced due to increasing material absorption at wavelengths below 250 nm. The amount of absorption is directly related to both decreasing wavelength and increasing material thickness (of both the coating and the filter substrate). Depending on the end-use application, minimizing the number of coating layers can result in lower filter cost and increased through-device transmission of in-band light.

[0031] While some of the embodiments of the optical filter assemblies described herein are used for the UV spectrum, in other embodiments, these optical filter assemblies may also be configured or designed for use in the visible (~380-750 nanometer) range and the infrared ranges (>750 nanometers (NIR), MWIR, LWIR)) of the electromagnetic spectrum.Docket Number: 00784-WO

[0032] For a reflective optical filter, the terms “attenuated”, “blocked”, or “rejected” are related to the amount or percentage of out-of-band light that does not reflect from the filter. Reflective filters do reflect some percentage of the out-of-band light.Optical Density or “OD” is a dimensionless number used as a measure of attenuation or blocking of the out-of-band light. OD is the logarithm of the ratio of the intensity of incident light to the intensity of light attenuated rejected by the filter. Expressed as a formula OD = -logw(T) or OD = 1 / logi o(T), where T is the fraction of out-of-band light reflected. This is expressed as the table below.

[0033] Those skilled in the art will appreciate that optical filters may have optical densities that are not discrete integer values.

[0034] FIG. 1 shows a diagram of a prior art optical filter assembly 10 that uses four separate optical filters to provide attenuation of undesired wavelengths of an input beam to form an output beam directed to portions of an optical instrument. The filter assembly 10 has four optical filters 12, 14, 16, and 18 mounted on respective filter mounts A, B, C, and D. The input beam is reflected once by the first filter 12, once by the second filter 14, once by the third filter 16, and finally once by the fourth filter 18, propagating out of the filter assembly as an output beam. If the instrument requires precise orientation of the output beam relative to other optical devices in the instrument, each of the filters 12, 14, 16, 18 must be precisely aligned by rotating the respective filter mounts A, B, C, and D. This can require significant complexity and cost of the filtering assembly 10.Docket Number: 00784-WO

[0035] FIGS. 2 and 3 show plots of various aspects of optical filtering performance of the filters used in an embodiment of an optical filter assembly discussed below with respect to FIGS. 6 through 9. FIG. 2 shows an example plot of out-of-band attenuation and in-band reflection relative to the total number of reflections of light, wherein each of the filters has an optical density of approximately 1. The plot includes individual traces for 1 , 2, 3, 4, 5, and 6 reflections. As those skilled in the art will appreciate, optical filters often do not have exactly the same optical performance at each wavelength in a particular range of wavelengths. In FIG. 2, this is shown as the undulating or wavy nature of each of the traces. The horizontal axis is the wavelength of the optical signal, measured in nanometers (nm). In this embodiment, the optical filter is a reflective notch filter having a center wavelength (CWL) of approximately 220 nm and a bandwidth of about 26 nm (a wavelength range of about 212 nm to about 228 nm) measured at the full-width half-maximum (FWHM) of the optical intensity. As such, in this embodiment, the range of “in-band” light is from about 212 nm to about 228 nm. Conversely, out-of-band light is light having wavelengths outside of the “in-band” range, in this embodiment above about 246 nm and below about 207nm. In the case of a reflective notch filter, the out-of- band light is either absorbed by the filter or transmitted through the filter and absorbed elsewhere (e.g., within the filter’s housing, such as a filter mount, a beam dump, or by some other absorptive or absorbing body). Those skilled in the art will appreciate that the optical filter may have any center wavelength and any bandwidth. While this plot shows a wavelength range of 200 nm to 450 nm, in other embodiments, the wavelength range can be any range of wavelengths.

[0036] The vertical axis is the logarithmic intensity of light that is reflected by the optical filter normalized, so that a value of OD = 1 represents 100% reflection. For example, in one embodiment, the optical filter has an OD = 1 , so that approximately 90% of the out-of-band light (also referred to herein as an “out-of-band portion”) is transmitted through or absorbed by the filter, and approximately 10% of the out-of- band light is reflected by the optical filter. If an optical device uses multiple of these filters with optical densities of 1 , then each reflection transmits or absorbs 90% of the out-of-band light. The example trace in FIG. 2 labeled “1 reflection” is the intensity of an optical signal reflected once from the filter of OD = 1 . The intensity of the light in the out-of-band wavelengths after six reflections off this optical filter of OD = 1 isDocket Number: 00784-WO0.000001 (equivalent to “OD6” (also known as “6OD) as shown in the horizontal line) times the optical intensity of the out-of-band light in the incident optical signal. Those skilled in the art will appreciate that a filter with a “nominal” OD = 1 may not have an OD of exactly 1 throughout an entire wavelength range. This is shown as the wavy nature of the traces on the plot out-of-band attenuation at wavelengths between about 250 nm to about 400 nm as shown in FIG. 2. For example, in reference to FIG. 2, one embodiment, an example filter may have an OD = 1.017 at about 390 nm, so after six reflections, the total OD = 6.100. The same filter may have an OD = 1 .109 at about 380 nm, so after six reflections the total OD is about 6.656. The same filter may have an OD = 1 .15 at about 262 nm, so after six reflections, the total OD is about 6.90. In the specific embodiment shown in FIG. 2, the optical density of six reflections varies between about 6.84 at 262 nm and about 6.09 at 391 nm (an average of about 6.47 ±0.38 nm. For this embodiment, for the purposes of product specifications, an optical assembly having this range of optical density for six reflections would be specified as having an OD equal to the nearest minimum integer value, OD=6.

[0037] In other embodiments, a filter with a nominal OD = 1 may have an optical density of less than 1 at certain wavelengths. In some embodiments, the optical density may be in a range between 0.95 and 1 .00 for a given wavelength range. In other embodiments, the optical density may be between 0.90 and 0.96 for a given wavelength range.

[0038] For the purpose of this disclosure, for a single filter, the in-band reflection may also be referred to herein as “throughput” of the in-band light reflected from that filter. For multiple filters or multiple reflections from the same filter, the throughput is the amount of in-band light remaining after these multiple reflections (or the amount of in-band light in the output signal relative to the in-band light in the input signal). For an incoming optical signal incident on such a filter, a portion of the incoming optical signal (also referred to herein as “the reflected portion”) is reflected from the filter. If, for example, the OD of the filter equals 1 , the reflected portion contains about 10% of the incident out-of-band light with the reflected in-band light. So in this example, for a first reflection, 90% of the out-of-band light (i.e. , a “first out-of-band portion”) of the incoming signal is transmitted through the filter, but 10% of the out-of- band light of the incoming signal is reflected. For this first reflection, a majority (e.g.,Docket Number: 00784-WO96%) of the in-band light (i.e. , a “first in-band portion”) is reflected. Applying this to the example filter performance from FIG. 2, in a single reflection, the filter reflects 96% of the in-band light of incoming optical signal and allows 90% (an OD = 1) of the out-of-band light to be transmitted through it. So, in this example, throughput of the filter is approximately 96%.

[0039] FIG. 3 shows a detailed view of the region of peak optical intensity of in- band reflection as a function of wavelength, as shown in FIG. 2. This represents the amount of reflected in-band light of the embodiment of an optical filter assembly described above with respect to FIG. 2. Because reflective optical filters do not have 100% reflection of in-band light, the intensity of reflected in-band light becomes lower with subsequent reflections. Also, in this embodiment, the bandwidth becomes narrower with each reflection. In this embodiment, the filter has in-band reflection of about 96% of the incident light, so the intensity of the in-band portion of the incident optical signal reflected by the filter is about 96% of the intensity of the in-band light of the incoming optical signal. So, successive reflections further reduce the in-band reflection down to 0.96 to the sixth power (about 78% of the intensity in-band light in the incoming optical signal) for six reflections, for a throughput of 78%.

[0040] FIG. 4 shows a ray tracing diagram of an embodiment of an optical filter assembly 200 that uses two filters and multiple reflections per filter. In this embodiment, the filter assembly 200 includes a first filter 220 and a second filter 230. A bundle of rays 210 is incident on the first mirror 220 and is reflected to the second mirror 230, then back and forth between the filters 220 and 230, becoming a bundle of rays 240 exiting the filter assembly 200. In this embodiment, each ray in the bundle of rays is reflected three times from the first filter 220 and three times from the second filter 230. In another embodiment, the bundle of rays 210 may be reflected two times from each filter 220, 230. In another embodiment, the bundle of rays 210 may be reflected four times from each filter 220, 230. Those skilled in the art will appreciate that the bundle of rays 210 may be reflected any number of times from each filter 220 and 230.

[0041] FIG. 5 shows an optical diagram of an example embodiment of an optical filter assembly 300 (also referred to herein as “the filter assembly 300”) configured to provide significant attenuation of out-of-band light using only a single pair of filters.Docket Number: 00784-WOIn the illustrated embodiment, the filter assembly 300 includes at least one body or housing 302 with a first port 304 and a second port 306, an inner surface 308, and a single pair of optical filters 305 (also referred to herein as “the filter pair 305”), wherein the single pair of filters 305 includes a first reflective notch filter 310 (also referred to herein as “the filter 310”) and a second reflective notch filter 320 (also referred to herein as “the filter 320”).

[0042] The filter pair 305 is configured to receive an incoming optical signal 330 having an in-band portion and an out-of-band portion and output an outgoing optical signal 342, wherein the filter pair 305 has an in-band throughput measured as the ratio of the intensity of the in-band portion of the outgoing optical signal 342 and the intensity of the in-band portion of the optical signal incoming 330. At least in some embodiments, the filter pair 305 has a total (also referred to herein as “combined”) optical density for out-of-band light (also referred to herein as the out-of-band portion of an optical signal” approximately equal to the optical density of the first filter 310 times the number of reflections from it, plus the optical density of the second filter 320 times the number of reflections from it. Also, together, at least in some embodiments, the filter pair 305 has a throughput for the in-band light (also referred to herein as the “in-band portion of an incoming optical signal”) that is approximately the addition of the throughout of each filter times the number of reflections.

[0043] In the illustrated embodiment, the filter 310 includes at least one filter body 312 with at least one first surface 314 (also referred to herein as “the surface 314” or “the front surface 314”). In the illustrated embodiment, the first surface 314 has at least one optical coating 316 applied thereto. In the illustrated embodiment, the filter body 312 also includes a second “back” surface on the opposing side of the filter body 312 from the surface 314. The back surface of the filter 310 may also have at least one optical coating applied thereto, wherein this back surface optical coating is the same or different from the coating 316.

[0044] In the illustrated embodiment, the filter 320 includes at least one filter body 322 having at least one first surface 324 (also referred to herein as “the surface 324” or “the front surface 324”). In the illustrated embodiment, the first surface 324 has at least one optical coating 326 applied thereto, with the first surface 324 generally facing the surface 314 of the filter 310. In the illustrated embodiment, the filter bodyDocket Number: 00784-WO322 also includes a second “back” surface on the opposing side of the filter body 322 from the surface 322. The back surface of the filter 320 may also have at least one optical coating applied thereto, wherein this back surface optical coating is the same or different from the coating 326. In this embodiment, the coatings 316 and 326 are multi-layer dielectric coatings. Those skilled in the art will appreciate that some portions of the surfaces 314, 324 may not be coated (e.g., in regions where the surfaces 314, 324 are mounted to some structure). Those skilled in the art will appreciate that the optical coatings applied to the back surfaces of the filters 310, 320 may also provide additional reflection of in-band light and additional attenuation (or transmission therethrough) of out-of-band light, or to achieve any of a variety of other optical effects desired or beneficial for the optical filter assembly 300.

[0045] The “rear” or “back” surfaces of the filters 310, 320 generally face other components or elements of the housing 302, heat sinks in thermal communication with the back surfaces, or beam dumps in optical communication with the back surfaces. For example, the filter assembly 300 may include absorbing bodies 370 and 372 in optical and thermal communication with the first filter 310 and the second filter 320, respectively. During operation, the absorbing bodies 370 and 372 may absorb thermal energy generated within each of the filters 310, 320 and transfer this thermal energy away from the respective filters. Also, the absorbing bodies 370 and 372 may absorb the optical energy of any light that is propagates through the filters 310, 320. The absorbing bodies 370, 372 may be provided as desired or beneficial. In some embodiments, the absorbing bodies 370, 372 may be formed from silicon carbide, silicon nitride, tungsten carbide, or any of a variety of materials having high heat capacity, high heat absorption, and high thermal conductive properties. The absorbing bodies 370, 372 may be in thermal communication with the housing 302 or with heat sinks or other heat transfer devices associated with the filter assembly 300. In some embodiments, the filters 310, 320 may be mounted to the absorbing bodies 370, 372 respectively. In FIG. 5, the absorbing bodies 370, 372 are shown both immediately adjacent to their respective filters 310, 320 and spaced apart from their respective filters 310, 320 in order to clearly show that light (e.g., portions 350, 352, 354, 356, 358, and 360 described below) that propagates through the filters 310, 320 is incident on the absorbing bodies 370, 372. Those skilled in the art willDocket Number: 00784-WO appreciate that even when the absorbing bodies 370, 372 are located immediately adjacent to the filters 310, 320, they still absorb such light.

[0046] The filter 310 and the filter 320 are spaced apart and aligned (as shown by the arrow-arcs A and B in FIG. 5) so that an incoming optical signal 330 is reflected from the filters 310 and 320 multiple times before leaving the optical assembly 300 as outgoing optical signal 342 (also referred to herein as “device-transmitted optical signal 342” or “signal 342”). In some embodiments, the incoming optical signal 330 and the outgoing optical signal 342 may comprise pulsed, continuous wave, or quasi-continuous wave laser light, whether or not that laser light is contains information encoded thereon. In other embodiments, the incoming optical signal 330 and the outgoing optical signal 342 may comprise non-coherent light, whether pulsed or not pulsed. In some embodiments, the filter 310 and the filter 320 require alignment relative to each other so that the device-transmitted signal 342 propagates along a path that is parallel or substantially parallel to the path of the incoming signal 330. In other embodiments, the alignment of the filters 310 and 320 may not require that the device-transmitted signal 342 propagates along a path that is parallel to the path of the incoming signal 330. This alignment can be done in multiple ways. In one embodiment, the filters 310 and 320 may be mounted to adjustable optical mounts that enable positioning of the filters in multiple degrees of freedom (e.g., X,Y,Z, pitch, roll, and yaw). In another embodiment, only one of the filters may be mounted to an optical mount. Optical mounts enable “active” alignment of the filters 310, 320, wherein the paths of the incoming signal 330 and the device-transmitted signal 342 are measured while adjustments to the optical mount(s) are made, until the paths of the signals 330, 342 are sufficiently aligned and the optical mount(s) are locked in place. While adjustable optical mounts enable very precise alignment of the filters, they do introduce mechanical complexity and cost to the optical assembly 300. In addition, such optical mounts can increase the overall size of the optical assembly 300. In another embodiment, the filters 310, 320 may be secured to surfaces machined into the body 302, wherein these surfaces are precisely machined so that the filters 310, 320 are sufficiently aligned by mechanical placement means. In other embodiments, the filters 310, 320 may be secured to non-adjustable optical mounts that are aligned to each other and then fixed in place (e.g., by screws, adhesives, or welding). In other embodiments, as described belowDocket Number: 00784-WO with respect to FIGS. 6 and 7, the filters 310 and 320 may be aligned and fixed relative to each other to form a module that is separate from the housing 302, then placed within and secured to the housing 302. In other embodiments, the filters 310, 320 may be secured to surfaces machined into the body 302, to the adjustable or non-adjustable optical mount(s) by their back surfaces, their edges, or by their front surfaces or faces (at portions of the front surfaces outside the regions where light is incident).

[0047] During operation of an example embodiment of the optical assembly 300 wherein the filter 310 has an in-band reflection of about 96% and an OD = 1 , an incoming signal 330 is incident on the filter 310, resulting in a first reflected portion 332 (having about 96% in-band light from the signal 330 and about 10% out-of-band light from the signal 330) being reflected by the filter 310 to the filter 320, and a first out-of-band portion 350 (having about 90% of the out-of-band light of the signal 330) being transmitted through or absorbed by the filter 310.

[0048] The first reflected portion 332 is incident on the filter 320, resulting in a second reflected portion 334 (having about 92% of the in-band light from the signal 330 and about 1 % of the out-of-band light from the signal 330) being reflected to the filter 310, and a second out-of-band portion 352 (having about 9% of the out-of-band light from the signal 330) being transmitted through or absorbed by the filter 320.

[0049] The second reflected portion 334 is incident on the filter 310, resulting in a third reflected portion 336 (having about 88% of the in-band light from the signal 330 and about 0.1 % of the out-of-band light from the signal 330) being reflected to the filter 320, and a third out-of-band portion 354 (having about 0.9% of the out-of-band light from the signal 330) being transmitted through or absorbed by the filter 310.

[0050] The third reflected portion 336 is incident on the filter 320, resulting in a fourth reflected portion 338 (having about 85% of the in-band light from the signal 330 and about 0.01 % of the out-of-band light from the signal 330), being reflected to the filter 310, and a fourth out-of-band portion 356 (having about 0.09% of the out-of- band light from the signal 330) being transmitted through or absorbed by the filter 320.

[0051] The fourth reflected portion 338 is incident on the filter 310, resulting in a fifth reflected portion 340 (having about 82% of the in-band light from the signal 330Docket Number: 00784-WO and about 0.001% of the out-of-band light from the signal 330), being reflected to the filter 320, and a fifth out-of-band portion 358 (having about 0.009% of the out-of- band light from the signal 330) being transmitted through or absorbed by the filter 310.

[0052] Finally, the fifth reflected portion 340 is incident on the filter 320, resulting in a sixth reflected portion (the output optical signal 342, having about 78% of the in- band light from the signal 330 and about 0.0001 % of the out-of-band light from the signal 330) exiting the optical assembly 300 through the port 306, and a sixth out-of- band portion 360 (having about 0.0009% of the out-of-band light from the signal 330) being transmitted through or absorbed by the filter 320.

[0053] In various embodiments, depending on the choice of the optical properties of the filters 310 and 320, the filter pair 305 may have a variety of combined optical densities and a variety of throughputs. In one embodiment, the combined optical density of the filter pair 305 is greater than 3 and the throughput for the in-band portion of the incoming optical signal 330 is greater than 85%. In another embodiment, the combined optical density of the filter pair 305 is greater than 5 and the throughput for the in-band portion of the incoming optical signal 330 is greater than 80%. In another embodiment, the combined optical density of the filter pair is 6 or greater and the throughput for the in-band portion of the incoming optical signal 330 is greater than about 50%. In yet another embodiment, combined optical density of the filter pair 305 is 6 or greater and the throughput for the in-band portion of the incoming optical signal is greater than about 60%. In still another embodiment, combined optical density of the filter pair is greater than 6 and the throughput for the in-band portion of the incoming optical signal is greater than about 70%. Those skilled in the art will appreciate that the combined optical density of the filter pair 305 may not have an integer value (e.g., when the filters 310, 320 have optical densities that are not integer values). In addition, the filter pair 305 and the angle of incidence may be configured to result in 7, 8, 9, or more reflections.

[0054] FIGS. 6 and 7 show views of a filter sub-assembly 380 that includes the filters of the filter pair 305 aligned to each other and secured relative to each other. The sub-assembly 380 may be used as a drop-in component of the optical assembly 300. In the illustrated embodiment, the sub-assembly 380 includes at least oneDocket Number: 00784-WO structure 382 that the filter pair 305 are secured to. As shown in FIG. 6, the subassembly 380 can be configured to accept an incoming signal 384 incident on the first filter 310 at an angle of incidence A , where it is reflected to the second filter 320, then back and forth between the first filter 310 and the second filter 320 until the light exits as an outgoing optical signal (or “device-transmitted light”) 386 in similar fashion as described above with respect to FIG. 5. FIG. 7 shows the filter subassembly 380 with the structure 382 rotated (as shown by the arrow-arc “C”) relative to the incoming light 384 so that the incoming light is incident on the first filter 310 at a different angle of incidence Ah. Adjusting the angle of incidence can have multiple effects on the device transmitted light 384. As is known in the art, changing the angle of incidence on a reflective dielectric filter such as the first filter 310 can shift the center wavelength of the reflected light. So, when the sub-assembly 380 is installed in the optical assembly 300, rotating the sub-assembly 380 can tune the center wavelength of the device-transmitted light 342. The sub-assembly 380 can be fully configurable with different filter lengths, different filter spacings, different filter specifications. The sub-assembly 380 may be mounted on a rotating optical mount or other device that allows for adjustment of the angle of incidence.

[0055] The embodiment of the optical filter assembly 300 described above uses a total of six filter reflections so that the filter assembly 300 has a combined optical density of 6 when the first filter 310 and the second filter 320 each have an optical density of approximately 1 for the out-of-band light of optical signals. In other embodiments, the filter assembly 300 may be configured to use more than six reflections in order to increase out-of-band attenuation. In still other embodiments, the first filter 310 and the second filter 320 may have optical densities other than 1 , in order to customize the performance of the filter assembly 300 for particular applications or equipment.

[0056] In still other embodiments, the filter assembly 300 may include only one reflection or two reflections from each filter. When configured as such, these filter assemblies can be used as “building blocks” in optical assemblies, that would be configurable based on the desired amount of in-band light through the device and the desired amount of optical blocking in out-of-band wavelength ranges.Docket Number: 00784-WO

[0057] FIG. 8 shows the optical intensity plots for the embodiment of the filter assembly shown in FIG. 5 wherein the first filter 310 and the second filter 320 have the same optical characteristics, in this case a CWL of about 220 nm and a bandwidth (for two reflections) of about 23 nm, and a bandwidth (for six reflections) of about 15 nm. Also, for this example, the filters 310, 320 are selected to have in- band reflection of about 96% and an OD = 1. Because the in-band reflection is 96%, after two reflections, the intensity of in-band light has dropped to about 92% and after six reflections the intensity of in-band light has dropped to about 78%. When configured as such, the filter assembly 300 has a combined optical density of 6 so the out-of-band blocking is 1x106. Those skilled in the art will appreciate that the reflection of a filter may be more or less than about 96% and the throughput may therefore be higher or less than about 78%. Those skilled in the art will appreciate that the filters 310, 320 in the filter assembly 300 can have different center wavelengths and bandwidths.

[0058] FIG. 9 shows optical intensity plots for an embodiment of the two-filter device 300 shown in FIG. 5, wherein the first filter 310 and the second filter 320 have different optical characteristics such as different center wavelengths. In this example embodiment, the first filter 310 has a CWL of about 220 nm and a bandwidth (BW2) for two reflections) of about 23 nm, and the second filter 320 has a CWL of 230 nm (shifted about 10 nm relative to the first filter 310), but the same bandwidth as the first filter 310. In this embodiment, the shifted CWLs may result in additional in-band attenuation, so the throughput after six reflections may be about 0.70 (70% of the in- band portion of the incoming signal 330). Also, for this example, the filters 310, 320 have the same optical density of 1 . Using two filters with center wavelengths shifted relative to each other results in a combined bandwidth (BW2) of about 17 nm for two reflections and a combined bandwidth (BWe) of about 9 nm for 6 reflections relative to the bandwidth of 23 nm for each individual filter. So, when configured as such, the optical assembly 300 can have a throughput of 70%, a combined optical density of 6, and a narrower bandwidth than for an optical assembly 300 with filters having the same CWL. In other embodiments, the shift in center wavelength of the second filter 320 relative to the first filter may be a percentage (e.g., between 5% and 50%) of the bandwidth of the first filter 310.Docket Number: 00784-WO

[0059] Those skilled in the art will appreciate that the optical characteristics of the filters 310, 320 can be selected to result in a wide variety of optical performance and any of a wide variety of wavelength ranges or spectra. For example, in one embodiment the first optical filter and the second optical filter have center wavelengths in a wavelength band between 100 nm and 280 nm. In another embodiment the first optical filter and the second optical filter have center wavelengths below 190 nm. In another embodiment the first optical filter and the second optical filter have center wavelengths in a wavelength band between 190 nm and 250 nm. In another embodiment the first optical filter and the second optical filter have center wavelengths in a wavelength band between 200 nm and 280 nm. In another embodiment, the first optical filter and the second optical filter have center wavelengths in a wavelength band between 250 nm and 315 nm. In another embodiment the first optical filter and the second optical filter have center wavelengths in a wavelength band between 280 nm and 315 nm. In yet another embodiment the first optical filter and the second optical filter have center wavelengths above 315 nanometers.

[0060] FIG. 10 shows an optical diagram of an example embodiment of an optical filter assembly 400 (also referred to herein as “the filter assembly 400”) configured to provide additional attenuation of out-of-band light using two pairs of filters. In the illustrated embodiment, the filter assembly 400 includes at least one body or housing 402 with a first port 404 and a second port 406, an inner surface 408, and a first pair of optical filters 305 (also referred to herein as “the filter pair 305”). In this embodiment, the filter pair 305 is configured the same as the filter pair 305 described above with respect to FIG. 5. As shown in FIG. 10, the filter assembly 400 also includes at least one second filter pair 500 similar to, but in this embodiment, a mirror image of the filter pair 305. As such, the second filter pair 500 includes a first reflective notch filter 510 (also referred to herein as “the filter 510”) and a second reflective notch filter 520 (also referred to herein as “the filter 520”).

[0061] In the illustrated embodiment, the filter 510 includes at least one filter body 512 with at least one first surface 514 (also referred to herein as “the surface 514” or “the front surface 514”). In the illustrated embodiment, the first surface 514 has at least one optical coating 516 applied thereto. In the illustrated embodiment, the filter body 512 also includes a second “back” surface on the opposing side of the filterDocket Number: 00784-WO body 512 from the surface 514. The back surface of the filter 510 may also have at least one optical coating applied thereto, wherein this back surface optical coating is the same or different from the coating 516.

[0062] In the illustrated embodiment, the second filter 520 includes at least one filter body 522 with at least one first surface 524 (also referred to herein as “the surface 524” or “the front surface 524”). In the illustrated embodiment, the first surface 524 has at least one optical coating 526 applied thereto. In the illustrated embodiment, the filter body 522 also includes a second “back” surface on the opposing side of the filter body 522 from the surface 524. The back surface of the filter 520 may also have at least one optical coating (not shown) applied thereto, wherein this back surface optical coating is the same as or different from the coating 526. The “rear” or “back” surfaces of the filters 510, 520 generally face other components or elements of the housing 402 (such as the interior surface 408), heat sinks in thermal communication with the back surfaces, or beam dumps in optical communication with the back surfaces. For example, the filter assembly 400 may include absorbing bodies 570 and 572 in optical and thermal communication with the first filter 510 and the second filter 520, respectively. The absorbing bodies 570 and 572 are configured similar to and operate similar to the absorbing bodies 370 and 372 described above with respect to FIG. 5.

[0063] The second filter pair 500 is configured to receive the outgoing optical signal 342 (having the sixth in band portion reflected from the mirror 320) and further attenuate it using a seventh in-band reflection 530, an eighth in-band reflection 532, a ninth in-band reflection 534, a tenth in-band reflection 536, an eleventh in-band reflection 538, and a twelfth in-band reflection 540 (also referred to herein as the “outgoing optical signal 540”) that exits the second port 406. An advantage of the configuration of the filter assembly 400 is that the filter pairs 305 and 500 are oriented in a mirror-image configuration so that an optical axis of the outgoing optical signal 540 and an optical axis of the incoming optical signal 330 can be aligned so that the optical filter assembly 400 can be more easily integrated into end-use equipment. While the out-of-band portions are not shown in FIG. 1 for the sake of clarity and simplicity, the out-of-band portions of the first filter pair 305 are the same as those of filter pair 305 shown in FIG. 5. The out-of-band portions for the second filter pair are consistent with those of the out-of-band portions of the first filter pairDocket Number: 00784-WO305 shown in FIG. 5. In other embodiments, the filter pairs 305 and 500 can be sized or otherwise configured so that each filter pair provides three in-band reflections instead of six. In other embodiments, the filter pairs 305 and 500 may have any number of in-band reflections. Those skilled in the art will appreciate that in other embodiments, there may more than two filter pairs. This allows for significant customization of the optical filter assembly 400 to provide any amount of attenuation of the out-of-band portions of incoming optical signals.

[0064] The foregoing is illustrative of embodiments and examples of the invention, and is not to be construed as limiting thereof. Although a few specific embodiments and examples have been described with reference to the drawings, those skilled in the art will readily appreciate that many modifications to the disclosed embodiments and examples, as well as other embodiments, are possible without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications to the subject matter described herein are intended to be included within the scope of the invention as defined in the claims. For example, skilled persons will appreciate that the subject matter of any sentence, paragraph, example or embodiment can be combined with subject matter of some or all of the other sentences, paragraphs, examples or embodiments, except where such combinations are mutually exclusive. The scope of the present invention should, therefore, be determined by the following claims, with equivalents of the claims to be included therein.

Claims

Docket Number: 00784-WOClaims1 . An optical filter assembly, comprising: a single pair of optical filters including a first optical filter (310) and a second optical filter (320, wherein the first optical filter (310) has a first optical density, a first bandwidth, and a first center wavelength, wherein the first optical filter (310) is configured to reflect a first reflected portion (332) of an incoming optical signal (330) and allow at least one first out-of- band portion (350) to be transmitted through or absorbed by the first optical filter (310); and wherein the second optical filter (320) has a second optical density, a second bandwidth, and a second center wavelength, wherein the second optical filter (320) is configured to reflect a portion of the first reflected portion (332) as a second reflected portion (334) to the first optical filter (310), and allow a second out-of-band portion (352) to be transmitted or absorbed by the second optical filter (320), wherein the first center wavelength is substantially equal to the second center wavelength.

2. The optical filter assembly of claim 1 , wherein the first center wavelength is different than the second center wavelength.

3. The optical filter assembly of claim 1 , wherein:(a) a portion of the second reflected portion (334) is reflected by the first optical filter (310) to form a third reflected portion (336);(b) a portion of the third reflected portion (336) is reflected by the second optical filter to form a fourth reflected portion (338);(c) a portion of the fourth reflected portion (338) is reflected by the first optical filter to form a fifth reflected portion (340); and(d) a portion of the fifth reflected portion (340) is reflected by the second optical filter (320) to form a sixth reflected portion (342) as an outgoing optical signal (342).Docket Number: 00784-WO4. The optical filter assembly of claim 3, wherein:(a) a portion of the second reflected portion (334) is transmitted or absorbed by the first optical filter (310) as a third out-of-band portion (354);(b) a portion of the third reflected portion (336) is transmitted or absorbed by the second optical filter (320) as a fourth out-of-band portion (356);(c) a portion of the fourth reflected portion (338) is transmitted or absorbed by the first optical filter (310) as a fifth out-of-band portion (358); and(d) a portion of the fifth reflected portion (340) is transmitted or absorbed by the second optical filter (320) as a sixth out-of-band portion (360).

5. The optical filter assembly of claim 4, further comprising at least one body or housing (302) configured to absorb at least a portion of at least one of the first out-of- band portion (350), the second out-of-band portion (352), the third out-of-band portion (354), the fourth out-of-band portion (356), the fifth out-of-band portion (358), and the sixth out-of-band portion (360).

6. The optical filter assembly of claim 4, further comprising at least one first absorbing body (370) configured to absorb at least a portion of at least one of the first out-of-band portion (350), the third out-of-band portion (354), and the sixth out- of-band portion (360).

7. The optical filter assembly of claim 4, further comprising at least one second absorbing body (372) configured to absorb at least a portion of at least one of the second out-of-band portion (352), the fourth out-of-band portion (356), and the sixth out-of-band portion (360).

8. An optical filter assembly, comprising: a first optical filter (310) and a second optical filter (320), wherein the first optical filter (310) and the second optical filter (320) each include a reflective surface (314 / 324) at least partially facing the reflective surface of the other filter, wherein each of the first optical filter (310) and the second optical filter (320) are sized to allow at least a portion of an incoming optical signal (330) to reflect therefrom a minimum of three times.Docket Number: 00784-WO9. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter are reflective notch filters.

10. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 100 nanometers and 280 nanometers.

11. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength below 190 nanometers.

12. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 190 nanometers and 250 nanometers.

13. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 200 nanometers and 280 nanometers.

14. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 250 nanometers and 315 nanometers.

15. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength in a wavelength band between 280 nanometers and 315 nanometers.

16. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength above 315 nanometers.

17. The optical filter assembly of claim 8, wherein the first optical filter and the second optical filter each have a center wavelength above 400 nanometers.

18. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) have an out-of-band optical density of approximately 1 .

19. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) have an out-of-band optical density of less than 1 .

20. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) have an out-of-band optical density of more than 1.Docket Number: 00784-WO21. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) each have a different optical density.

22. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) are aligned to each other using passive alignment.

23. The optical filter assembly of claim 8, wherein the first optical filter (310) and the second optical filter (320) are aligned to each other using active alignment.

24. The optical filter assembly of claim 8, wherein: the first optical filter (310) is configured to reflect a first reflected portion (332) of an incoming optical signal (330) and allow at least one first out-of-band portion (350) to be transmitted through or absorbed by the first optical filter (310); and the second optical filter (320) is configured to reflect a portion of the first reflected portion (332) as a second reflected portion (334) to the first optical filter (310), and allow a second out-of-band portion (352) to be transmitted through or absorbed by the second optical filter (320), wherein:(a) a portion of the second reflected portion (334) is reflected by the first optical filter (310) to form a third reflected portion (336);(b) a portion of the third reflected portion (336) is reflected by the second optical filter to form a fourth reflected portion (338);(c) a portion of the fourth reflected portion (338) is reflected by the first optical filter to form a fifth reflected portion (340); and(d) a portion of the fifth reflected portion (340) is reflected by the second optical filter (320) to form a sixth reflected portion (342) as an outgoing optical signal (342).

25. The optical filter assembly of claim 24, wherein:(a) a portion of the second reflected portion (334) is transmitted or absorbed by the first optical filter (310) as a third out-of-band portion (354);(b) a portion of the third reflected portion (336) is transmitted or absorbed by the second optical filter (320) as a fourth out-of-band portion (356);(c) a portion of the fourth reflected portion (338) is transmitted or absorbed by the first optical filter (310) as a fifth out-of-band portion (358); andDocket Number: 00784-WO(d) a portion of the fifth reflected portion (340) is transmitted or absorbed by the second optical filter (320) as a sixth out-of-band portion (360).

26. An optical assembly, comprising: a single pair of optical filters (305) including a first optical filter (310) having a first center wavelength and a first bandwidth, and a second optical filter (320) having a second center wavelength and a second bandwidth, wherein the single pair of optical filters (305) is configured to receive an incoming optical signal (330) having an in-band portion and an out-of-band portion and output an outgoing optical signal (342); wherein the single pair of optical filters (305) has a combined optical density for the out-of-band portion of the optical signal (330), and wherein the single pair of optical filters (305) is configured to have a throughput for the in-band portion of the optical signal (330).

27. The optical assembly of claim 26, wherein the combined optical density is greater than 5 and the throughput for the in-band portion of the optical signal is greater than 80%.

28. The optical assembly of claim 26, wherein the combined optical density is 6 or greater and the throughput for the in-band portion of the optical signal is greater than about 50%.

29. The optical assembly of claim 26, wherein the combined optical density is 6 or greater and the throughput for the in-band portion of the optical signal is greater than about 75%.

30. The optical assembly of claim 26, wherein the first center wavelength and the second center wavelength are substantially equal.

31. The optical assembly of claim 26, wherein the first center wavelength and the second center wavelength are different.Docket Number: 00784-WO32. The optical assembly of claim 26, wherein the second center wavelength is shifted between approximately 5% and approximately 10% of the first bandwidth so that the second center wavelength is lower than the first center wavelength.

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