WO2003079069A2

WO2003079069A2 - Optical filter array and method of use

Info

Publication number: WO2003079069A2
Application number: PCT/US2003/007487
Authority: WO
Inventors: George Wildeman; Michael J Yadlowsky; Brian T Hart
Original assignee: Corning Incorporated
Priority date: 2002-03-15
Filing date: 2003-03-13
Publication date: 2003-09-25
Also published as: AU2003230630A8; AU2003230630A1; WO2003079069A3

Abstract

In accordance with an exemplary embodiment of the present invention, an optical apparatus includes a glass monolithic structure including a plurality of optical filter elements, and the glass monolithic structure is not an opticalfiber. The optical apparatus further includes a device which selectively aligns an optical input and an optical output to one of said plurality of optical filter elements and a method of adding/dropping a particular frequency from an optical signal. The alignment may be done by a translation stage which moves the optical filter rray. Filter elements may be detuned with respect to the center frequency of a passband.

Description

OPTICAL FILTER ARRAY AND METHOD OF USE

Cross-Reference to Related Applications

[0001] The present application is a continuation-in-part of U.S. Patent Application Serial Number 10/099,111, filed March 15, 2002, entitled "Optical Filter Array and Method of Use." The present application is a continuation-in-part of U.S. Patent Application Serial Number 10/099,089, filed March 15, 2002, entitled "Monolithic Filter Array." The present application is a continuation-in-part of U.S. Patent Application Serial Number 10/100,463, filed on March 15, 2002, entitled "Tunable Optical Filter Array and Method of Use." The inventions described in these applications are assigned to the assignee ofthe present invention, and the disclosures of these applications are incorporated by references herein and for all purposes.

Field of the Invention

[0002] The present invention relates generally to optical communications, and particularly to optical filter arrays and methods of their use.

Background of the Invention

[0003] Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. While the performance of optical communication systems continues to improve, there is increasing pressure on each segment ofthe optical communication industry to reduce costs associated with building and maintaining an optical network. [0004] One useful technology for improving performance and reducing the overall cost ofthe optical communication system is through the use of wavelength division multiplexing (WDM). As is well known, WDM pertains to the transmission of multiple signals (in this case optical signals) at different wavelengths down a single waveguide (e.g., optical fiber) with a channel being assigned to each wavelength, and each channel having a particular bandwidth. The nominal wavelength of a given channel is often referred to as the channel center wavelength. [0005] For purposes of illustration, according to one international Telecommunications Union (ITU) grid a wavelength band from 1530 nm to 1565 nm is divided up into a plurality of wavelength channels, each of which have a prescribed center wavelength and a prescribed channel bandwidth; and the spacing between the channels is prescribed by the ITU grid.

[0006] For example, one ITU channel grid has a channel spacing requirement of 100 GHz (in this case the channel spacing is referred to as frequency spacing), which corresponds to channel center wavelength spacing of 0.8 nm. With 100 GHz channels spacing, channel "n" would have a center frequency 100 GHz less than channel "n+1" (or channel n would have a center wavelength 0.8nm greater than channel n + 1). [0007] In WDM systems all ofthe channels are combined (multiplexed) at one end of the system, and separated (demultiplexed) at the other end for further use. The separation of individual wavelength channels may be carried out using optical filters. Currently, most multiplexing/demultiplexing schemes are based on fixed filters. However, there is a need in optical networks to provide flexibility that is not afforded by conventional fixed filter designs.

[0008] In addition to WDM systems, optical filters are useful in certain laser and amplifier applications. The lasers used in optical communication systems may be tunable. Moreover, erbium-doped fiber amplifiers (EDFA's) have been deployed widely in optical communication and sensor applications. Optical filters may be used to suppress broadband amplified spontaneous emission (ASE) around the signal from EDFA's and tunable lasers.

[0009] Accordingly, optical filter arrays serve a useful purpose in a variety of applications. What is needed is an optical filter array that overcomes the shortcomings of conventional optical filter arrays.

[0010] While the use of Bragg gratings and optical filters based on other technologies has shown promise from the perspective of performance and versatility in optical communication systems, there exist certain drawbacks in the known art. For example, the fabrication of an array of optical filters can be significantly hindered by a slight offset in the periodicity ofthe optical grating during manufacturing. This can result in a significantly reduced yield, and an overall increase in the cost ofthe final product.

[0011] What is needed, therefore, is an optical filter array which overcomes at least the drawbacks of conventional methods and apparati described above.

[0012] In WDM systems it may be useful to employ tunable optical filters in the demultiplexing process. For example, tunable optical filters may be useful in reconfigurable optical networks to facilitate a number of operations including demultiplexing. Moreover, the drive to reduce network costs and operation costs has placed a value on flexibility that has not previously existed; and that may be provided by tunable optical filters.

[0013] Unfortunately known tunable optical filters suffer certain implementation and performance drawbacks (e.g., suitably sharp cutoff outside ofthe passband ofthe filter; suitably low polarization dependent loss; and suitably low chromatic dispersion, some or all of which tend to degrade over the tuning range of conventional tunable filters).

[0014] Because the known tunable optical filters have unacceptable drawbacks, there exists a need for optical filter elements which overcome at least the drawbacks described above.

Summary of the Invention

[0015] In accordance with an exemplary embodiment ofthe present invention, an optical filter array includes a plurality of optical filter elements which are disposed in a glass monolithic structure, and the glass monolithic structure is not an optical fiber. [0016] hi accordance with another exemplary embodiment of the present invention, an optical apparatus includes a glass monolithic structure which includes a plurality of optical filter elements. The optical apparatus further includes a device which selectively aligns an optical input and an optical output to one of said plurality of optical filters.

[0017] hi accordance with another exemplary embodiment ofthe present invention, a method of adding/dropping a particular frequency from an optical signal includes providing a glass monolithic structure which further includes a plurality of optical filter filters. [0018] hi accordance with another exemplary embodiment ofthe present invention, a method of adding/dropping a particular frequency from an optical signal includes providing a glass monolithic structure which further includes a plurality of optical filter elements. The method further includes providing a device which selectively aligns an optical input and an optical output to at least one ofthe plurality of optical filters.

[0019] In accordance with another exemplary embodiment ofthe present invention, an optical apparatus, includes a bulk glass monolithic structure which includes a plurality of optical fiber elements.

[0020] In accordance with another exemplary embodiment ofthe present invention, an optical apparatus includes at least one monolithic structure formed in a photosensitive organic medium, the monolithic structure including a plurality of optical filters; and at least one device which selectively aligns an optical input and an optical output to one of said plurality of optical filters.

[0021] According to another exemplary embodiment ofthe present invention, an optical apparatus includes a monolithic optical filter array having a first optical filter element. The monolithic optical filter array also includes a second optical filter element proximate to the first optical filter element. The second optical filter element is detuned relative to the first optical filter element.

[0022] According to another exemplary embodiment ofthe present invention, an optical apparatus includes an input port. The optical apparatus further includes a monolithic optical filter array having at least one column comprising a nominal optical filter element, and at least a detuned filter element. The apparatus also includes a device for aligning the input port to a desired one optical filter ofthe monolithic optical filter array.

[0023] According to another exemplary embodiment ofthe present invention, a method of extracting light of a particular wavelength includes providing a monolithic optical filter array having at least one column which includes a nominal wavelength optical filter element and a detuned wavelength optical filter element. The method further includes providing an input port proximate to the optical filter array, and aligning the input port to a desired one ofthe optical filter elements ofthe monolithic optical filter array. [0024] According to an exemplary embodiment ofthe present invention, an optical apparatus includes an optical filter array which comprises a plurality of optical filter elements, wherein at least one ofthe optical filter elements is adapted for tuning to two or more wavelengths.

[0025] According to another exemplary embodiment ofthe present invention, an optical apparatus includes a monolithic optical filter array which further includes at least one tunable optical filter element. The optical apparatus also includes a tuning mechanism which tunes the tunable optical filter element to extract a signal of a particular wavelength from an optical signal which includes a plurality of wavelengths.

[0026] According to another exemplary embodiment ofthe present invention, a method for selectively extracting optical signals of particular wavelengths includes providing a monolithic optical filter array which further includes at least one tunable optical filter element. The method also includes tuning the tunable optical filter element to extract a signal of a particular wavelength from an optical signal which includes a plurality of wavelengths.

Brief Description of the Drawings

[0027] The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.

[0028] Fig. 1 is a perspective view of an exemplary embodiment ofthe present invention.

[0029] Fig. 2 is a graph of refractive index change versus anneal time for gratings fabricated in accordance with the present invention.

[0030] Fig. 3 is a perspective view of an exemplary embodiment ofthe present invention in which a translation stage is cooperatively engaged.

[0031] Figs.4-8 are perspective views of various input/output devices coupled to a monolithic filter array in accordance with exemplary embodiments ofthe present invention.

[0032] Fig. 9 is a schematic view of a lxN optical filter array according to an exemplary embodiment ofthe present invention. [0033] Figs.10 and 11 are schematic views of exemplary embodiments ofthe present invention in which wavelength channels are extracted from a multi-channel optical signal.

[0034] Fig. 12 is a schematic view of a stacked optical filter array.

[0035] Fig. 13 is a schematic view of an exemplary embodiment ofthe present invention.

[0036] Fig. 14. is a schematic view of an add/drop multiplexer in accordance with an exemplary embodiment ofthe present invention.

[0037] Fig. 15 is a graph of reflectivity versus wavelength for three optical filter elements of a monolithic glass optical filter array in accordance with an exemplary embodiment ofthe present invention.

[0038] Fig. 25 is a perspective view of an optical filter array of nominal and detuned optical filter elements in accordance with an exemplary embodiment ofthe present invention.

[0039] Fig. 17 is a graphical representation ofthe frequency response of optical filters showing channel spacing and detuning spacing in accordance with an exemplary embodiment ofthe present invention.

[0040] Fig. 18 is a two-port reconfigurable tunable filter array in accordance with an exemplary embodiment ofthe present invention.

[0041] Fig. 19 is a stacked optical array in accordance with an exemplary embodiment ofthe present invention.

[0042] Fig. 20 is a serial array of optical filters in accordance with an exemplary embodiment ofthe present invention.

[0043] Fig. 21 is a perspective view of a thermally tuned optical filter array in accordance with an exemplary embodiment ofthe present invention.

[0044] Fig. 22 is a graph ofthe 2 dB center wavelength versus temperature for an optical filter element in array in accordance with an exemplary embodiment ofthe present invention.

[0045] Fig. 23 is a perspective view of an angle-tuned filter array in accordance with an exemplary embodiment ofthe present invention.

[0046] Fig. 24 is a representative view of WDM signals over a tuning range incorporating a plurality of tunable filter elements in accordance with an exemplary embodiment ofthe present invention. [0047] Fig. 25 is a schematic view of a dual-fiber collimator suitable for use in the present invention.

[0048] Fig. 26 is a schematic view of a of a thermally tuned optical filter array operative as a wavelength blocker in accordance with an exemplary embodiment of the present invention.

[0049] Fig. 27 is a schematic graph of reflected intensity versus wavelength for a generalized optical filter.

[0050] Fig. 28 is a schematic view of a of a thermally tuned optical filter array operative as a wavelength add-drop multiplexer in accordance with an exemplary embodiment ofthe present invention.

[0051 ] Fig. 29 is schematic view of a of a thermally tuned optical filter array in accordance with an exemplary embodiment ofthe present invention.

[0052] Fig. 30 is a schematic view of a thermally tuned M x 2 optical filter array in accordance with an exemplary embodiment ofthe present invention.

Defined Term

[0053] As used herein the term "monolithic optical filter array" pertains to a plurality of optical filter elements formed in a common substrate.

Detailed Description

[0054] In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding ofthe present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit ofthe present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well- known devices, methods and materials may be omitted so as to not obscure the description ofthe present invention.

[0055] Briefly, the present invention is drawn to a glass monolithic optical filter arrays, apparati incorporating the glass monolithic filter arrays, and methods of use of the apparati. In accordance with an exemplary embodiment ofthe present invention, the glass monolithic optical filter array includes a plurality of optical filter elements. In this illustrative embodiment, each ofthe optical filters will extract a particular wavelength channel having a particular center wavelength from a plurality of wavelength channels. Advantageously, the glass monolithic optical filter array is fabricated on a common substrate, and by a method which facilitates large-scale production with improved yield and reduced cost when compared to conventional techniques. Finally, the glass monolithic optical filter array and its method of manufacture foster a great deal of versatility, enabling the manufacturer to tailor optical filter arrays for a specific use, without requiring significant variation in processing.

[0056] As will become clearer as the present invention proceeds, the optical filters in accordance with exemplary embodiments ofthe present invention may be reflective- type filters, transmissive-type filters or a combination of different reflection-type filters and/or transmissive-type filters.

[0057] It is noted that for purposes of facility of discussion, the disclosure ofthe present invention will focus on reflective-type filter, although it is to be understood that transmissive-type filters may be used as well. One salient feature ofthe optical filters in accordance with exemplary embodiments ofthe present invention is the capability of monolithic fabrication using various glass materials. [0058] It is further noted (again for clarity of discussion) that the present disclosure focuses primarily on the use of optical filters ofthe present invention in multiplexing/demultiplexing applications in optical communication systems. However, the optical filters ofthe present invention have utility in a variety of other applications.

[0059] According to one exemplary embodiment, the inventive optical filter arrays also could be used in EDFA applications where the amplifier operates over a relatively wide bandwidth. Additionally, the inventive optical filter arrays may be deployed to reduce broadband ASE around a signal channel. To this end, the optical filter elements ofthe optical filter arrays in accordance with an exemplary embodiment ofthe present invention exhibit an insertion loss versus frequency/wavelength that has both steep transition regions outside ofthe passband of the filter element and a relatively flat filter function (e.g., in a 50 GHz system, the insertion loss variation of an exemplary filter element is illustratively less than approximately 2 dB over the full width of 30 GHz, while having an extinction of greater than about 20dB over an 80 GHz full width). As a result, there is 'room' within the passband ofthe filter element for the laser signal to vary (e.g., approximately 10 GHz variation) without experiencing substantial attenuation. [0060] In accordance with another exemplary embodiment ofthe present invention, the optical filter elements are Bragg gratings which are chirped (linearly or non- linearly) for use as a chromatic dispersion compensator.

[0061] It is further noted that the above examples ofthe utility ofthe monolithic optical filter arrays ofthe present invention are merely illustrative of, and are intended to be in no way limiting. Clearly, other implementations ofthe glass monolithic optical filter array will be readily apparent to one of ordinary skill in the art who has had the benefit of applicants' disclosure.

[0062] Fig. 1 shows an optical apparatus 100 in accordance with an exemplary embodiment ofthe present invention. The optical apparatus 100 includes a lxN optical filter array 101 which is illustratively a glass monolithic optical filter array including a plurality of optical filter elements 102 fabricated in the glass substrate 103. In the presently described exemplary embodiment the optical filter array 101 includes N- filters for n-wavelength channels having center wavelengths , ..., λ_n .

For purposes of illustration, n and N may be 40, 80, 100, 200 or 400. Of course, this is merely illustrative and intended to be in no way limiting ofthe present invention. [0063] Illustratively, the optical filter elements 102 are reflective filter elements. For example, the optical filter elements 102 maybe Bragg gratings such as those described in detail in U.S. Patent Application Serial Number 09/874,721, entitled "Bulk Internal Bragg Gratings and Optical Devices," to Bhagavatula, et al., and filed on June 5, 2001. Moreover, the substrate 103, which is illustratively a bulk glass may be a glass material such as those taught in U.S. Patent Application Serial Number 09/874,352, entitled "UN Photosensitive Melted Germano-Silicate Glass," to Borrelli, et al., and filed on June 5, 2001; or may be one ofthe glass material as taught in U.S. Patent Application Serial Number 10/186,123 and entitled "Photosensitive UN Glasses" to Nicholas Borrelli, et ah, filed on even date herewith. The inventions described in the above referenced U.S. Patent Applications are assigned to the Assignee ofthe present invention, and the disclosures of these applications are specifically incorporated by reference herein and for all purposes. [0064] In one exemplary embodiment ofthe present invention, the substrate is formed from a meltable glass having a molecular hydrogen content of >10¹⁷ H₂ molecules/cm³. This meltable glass may be, for example, a germanosilicate glass. In another exemplary embodiment ofthe invention, the substrate is formed from a meltable photosensitive germanosilicate glass material having a hydrogen content less than approximately 10¹⁷ H molecules/cm³. In one embodiment ofthe invention, the substrate is formed from a glass material having a composition including approximately 40 mole % to approximately 80 mole % SiO , approximately 2 mole % to approximately 15 mole % GeO , approximately 10 mole % to approximately 36 mole % B₂O₃, approximately 1 mole % to approximately 6 mole % Al₂O , and approximately 2 mole % to approximately 10 mole % R₂O wherein R is an alkali. In another embodiment ofthe invention, the substrate is formed from a glass material having a composition including approximately 25 weight % to approximately 45 weight % SiO , approximately 3 weight % to approximately 22 weight % GeO₂, approximately 7 weight % to approximately 28 weight % B₂O₃, approximately 6 weight % to approximately 22 weight % Al₂O₃, approximately 6 weight % to approximately 25 weight % R₂O wherein R is an alkali, and approximately 3-11 weight % F.

[0065] The monolithic structure ofthe present invention may be formed from a variety of materials. As described above, the monolithic structure may be formed in a substrate ofthe glass materials taught in the above-referenced Borrelli et al. applications. Alternatively, other photosensitive glass materials may be used as the substrate material. For example, suitable materials include glasses that achieve an index change by thennally induced growth of crystals on light-induced nucleation centers, such as those described in U.S. Patent 4,514,053, and the photo-thermo- refractive glasses described in U.S. Patent Application Publication No. US 2002/0045104. Other desirable photosensitive glasses include doped porous glasses which are consolidated at a relatively high temperature. While the preferred embodiments given below are described with reference to a glass monolithic structure, the skilled artisan will recognize that the monolithic structure may be formed from other photosensitive materials.

[0066] It is further noted that the above referenced gratings and materials are intended to be illustrative of and in no way limiting ofthe scope ofthe present invention. In an exemplary embodiment ofthe present invention, photosensitive organic materials are used as the substrate in which optical filter elements may be formed. For example, materials such as dichromated gelatin and photosensitive polymeric materials may be used to form the monolithic structures ofthe present invention. Fluorinated polymeric materials are especially suitable for use in the present invention. Polymer- dispersed liquid crystal materials may also be suitable for use as the substrate in the present invention. According to one embodiment ofthe present invention, the monolithic structures ofthe present invention may be formed in polymeric materials having a shortest dimension of greater than about 100 μm, illustratively greater than about 400 μm.

[0067] Especially suitable polymers for use in the present invention are described, for example, in U.S. Patent Application Serial Numbers 09/745,076, 09/747,068, 09/912,827, and 10/067,669, which are incorporated herein by reference. Especially desirable polymeric materials for use in the present invention are cured products of energy curable compositions including two monomers having differing refractive index and rates of diffusion. One exemplary fluoropolymer material is the cured product of an energy curable composition including about 50 wt% 2,2,3,3,4,4,5,5- octafluorohexanediol diacrylate (UV-8), and about 50 wt% ofthe tetraacrylate of FLUOROLINK T, a tetrafunctional perfluoropolyether alcohol available from Ausimont USA. The tetraacrylate, known herein as UV-T, has the structure CH₂=CHC0₂CH₂CHCH₂OCH₂CF₂0-[(CF₂CF₂0)_m(CF₂O)_n]-CF₂CH₂OCH₂CHCH₂0₂CCH=CH₂

0₂CCH=CH₂ 0₂CCH=CH₂

The energy curable composition also includes about 1 wt% photoinitiator. [0068] In an exemplary embodiment ofthe invention, the monolithic filter elements ofthe present invention is fabricated by casting the energy curable composition described above into a mold having the desired dimensions. For example, the mold may have a parallelepiped shape with no interior dimension shorter than 100 μm. A grating is formed in the monolithic element using the method described in U.S. Patent 6,023,545, which is incorporated herein by reference. The energy curable composition is partially cured by brief (e.g. a few seconds) exposure to a suitable UN light source. The partially cured polymer is irradiated through a phase mask with UN radiation from an argon ion laser, forming the grating. The element is then exposed again to a suitable UN light source to fully cure the polymer, and the cured monolithic element is removed from the mold. [0069] Certain advantageous characteristics ofthe optical filter elements 102 are noted presently. One advantageous characteristic ofthe glass monolithic optical filter elements 102 in accordance with the presently described exemplary embodiments, is long-term reliability. It is desired that the gratings which comprise optical filter elements 102 remain substantially unchanged over time. To wit, as shown in Fig. 2, the refractive index change versus anneal time for gratings fabricated in a glass material referenced above is shown.

[0070] In addition to being reliable over time, the gratings which comprise the optical filter elements 102 are relatively large in volume (cross-sectional area times the length ofthe grating), for example relative to that of conventional fiber Bragg grating. This relatively large volume simplifies the optical coupling to an optical waveguide (e.g., an optical fiber) over the air gap necessary for spatial tuning. To fabricate such gratings, a relatively highly photosensitive medium is needed that is also relatively transmissive (low-loss) in the ultra-violet (UN) spectrum. These advantageous characteristics ofthe medium are provided, for example, by the melted glass materials ofthe inventions to Borrelli et al, referenced above, and by the fluoropolymeric materials referenced above.

[0071] The UN transmittivity enables the gratings to be written relatively deeply in the bulk glass material ofthe substrate 103. For purposes of illustration, a loss of approximately 5 dB/mm to approximately 2 dB/mm (or less) at the wavelengths at which the gratings are written is useful. The gratings are written in such low-loss glass materials at a wavelength in the range of approximately 220 nm to approximately 280 nm, illustratively at 248 nm and 257 mn; although it is noted that the wavelengths as great as 300 nm may be used to write the gratings. For purposes of illustration and not limitation, the substrate 103 has an index of refraction of 1.49; the gratings that comprise optical filter elements 102 have a length of 7mm, and induced refractive index change (An) of 2.8x 10^"4. The angle of incidence is 1.5 degrees and the beam size is 100-500 μm.

[0072] It is noted that the use of Bragg gratings as optical filter elements 102 is illustrative. Other filter elements including guided mode resonance (GMR) filters as well as holographic filters generally could be used in carrying out the invention. Finally, it is conceivable that the filter elements 102 are not based on the same filter technology. [0073] Finally, it is noted that the optical filter elements 102 may be fabricated using a variety of techniques. For example, the optical filter elements 102 may be fabricated using a plurality of phase masks, whereby one optical filter element (grating) may be written at a time. Alternatively, another type of interferometric device could be used to write the optical filter elements. Moreover, other techniques as well as variants ofthe techniques referenced above could be used. [0074] In the exemplary embodiment shown in Fig. 1, each ofthe optical filters 102 is designed to reflect an optical signal of a particular frequency/wavelength channel. Illustratively, an optical signal from an input collimator 104 is incident upon a first optical filter element 102. The optical signal is illustratively a WDM or dense WDM (DWDM) optical signal having a plurality of channels, each of which has a particular center wavelength/frequency.

[0075] The first filter 102 reflects wavelength channel 1 having center wavelength /-_! . To wit, the first filter element 102 reflects a wavelength band approximately corresponding to that of channel 1, which has a center wavelength λ_x , and prescribed channel bandwidth. (Likewise, the wavelength channel n is reflected by the n^th filter element, which reflects a wavelength band approximately conesponding to channel n, having a center wavelength λ„ and a prescribed channel bandwidth, and transmits all other wavelengths therethrough).

[0076] The reflected light from first filter element 102 is incident upon the first output collimator 105. All other wavelength channels are transmitted through the optical filter and are incident upon the second output collimator 106, which is optional in the presently described embodiment, h this manner, in the illustrative embodiment in which the optical signal is a WDM or DWDM optical signal, one wavelength channel may be separated (demultiplexed) from the other wavelength channels in the optical signal.

[0077] The other filter elements 108, 109, 110 and 111 reflect other wavelength channels ofthe WDM/DWDM input optical signal. The extraction of each particular optical channel from the optical signal merely requires the alignment ofthe input collimator 104, and first output collimator 105 to the particular one ofthe other optical filter elements 108 - 111, which reflects light having the wavelength conesponding to center wavelength ofthe particular wavelength channel desired. [0078] Alignment ofthe input collimator 104 and first output collimator 105 to a particular one ofthe optical filter elements 102 requires the relative motion ofthe input collimator 104 and first output collimator 105, and optical filter array 101. Illustratively, this may be carried out in a controlled manner through the use of a microcontroller which accesses a look-up table (neither of which are shown), and then commands a filter element selector 107 to effect the required relative motion ofthe optical filter anay 101 to the input collimator 104 and first output collimator 105. (Please refer to Fig. 3 in which an illustrative embodiment of a translation mechanism is described in further detail.)

[0079] Finally, it is noted that in the exemplary embodiment shown in Fig. 1, the second output collimator 106 maybe optically coupled to an input collimator of a second apparatus similar to that shown in Fig. 1. This cascaded structure would enable the extraction of further wavelength channels from the optical signal incident upon the second output collimator 106. Moreover, it is noted that the second output collimator may be completely forgone; and, alternatively, that the first output collimator 105 can be forgone. In the former case, the extraction of a single channel would be realized, while in the transmitted channels would be dropped. In the latter case, the reflected channel would be dropped. As will become more clear as the present description proceeds, it is possible to fabricate a channel add/drop device with the elements shown in the exemplary apparatus of Fig. 1.

[0080] Fig. 3 shows the optical apparatus 100 cooperatively engaging a translation stage 112 in accordance with an exemplary embodiment ofthe present invention. The translation stage enables one-dimensional motion (in this case in the ±x direction) enabling the selective alignment of input and output collimators (not shown in Fig. 3). The optical filter array 101, as well as optical filter elements 115, are identical in substance and function as those described in conjunction with the embodiment of Fig. 1. The translation stage includes a substrate 113 over which the optical filter anay 101 is disposed. The translation stage 112 illustratively includes a stepper motor 114 which is monitored by an encoder 116. The stepper motor 114 and the encoder 116 are disposed on a submount 117. Alternatively, the translational motion may be effected by using a mechanical device such as a D.C. motor or linear solenoid that moves the optical filter array 101 relative to the collimators. This mechanism may in fact be manually actuated (i.e. without a motor). [0081] It is noted that the individual optical filter elements are approximately 0.1mm to approximately 1.0mm in cross-section for typical WDM applications. The aligmnent tolerances for the optical apparatus should be roughly at least 10 times finer than this. This degree of tolerance is well within the capabilities of stepper motors, DC motors and linear solenoids discussed.

[0082] The control ofthe motion ofthe input/output collimator and output collimator is illustratively canied out as follows. A microcontroller (not shown) may access a look-up table which contains the wavelength band of each ofthe individual optical filter elements 115. The translation stage 112 illustratively moves either the input/output collimator (not shown) and output collimator 106, or the monolithic optical filter array 101 in the ±x direction so that selected one of filter elements 102 - 107 is properly aligned with the input/output collimator 109. [0083] Figs. 4-8 are perspective views of various input/output devices coupled to a monolithic optical filter array in accordance with exemplary embodiments ofthe present invention. It is noted that the various input/output schemes may be used in canying out the present invention as described through the exemplary embodiments ofthe present disclosure.

[0084] Fig. 4 shows a monolithic optical filter array 117 which includes a plurality of optical filter elements 118. A collimator 119 launches light at normal incidence to the optical filter. A circulator (not shown) well known to one having ordinary skill in the art is used to separate the incident light from the reflected light. [0085] Specular reflections from the front surface may result in unwanted cross talk due to their relatively broadband nature. To suppress specular reflections, an antireflection coating, again well known to one having ordinary skill in the art, may be provided on the surface of incidence ofthe monolithic filter anay 117. Alternatively, the surface of incidence ofthe monolithic filter array 117 maybe beveled. Again, this is a well-known technique to one having ordinary skill in the art. [0086] Fig. 5 shows an alternative technique to reduce specular reflections. In the exemplary embodiment shown in Fig. 5, the optical filter elements 118 maybe fabricated at an angle relative to the surface of incidence ofthe monolithic filter anay 117. Normally, whether providing a beveled surface to the monolithic filter anay 117, or orienting the optical filter elements 118 at an angle, the beveled angle is on the order of approximately 4° to approximately 8°. [0087] The above modifications improve the performance ofthe device, but may adversely impact the cost ofthe device. To reduce the cost ofthe device, it may be beneficial to avoid the need for a circulator. This is done by launching light at a small angle of incidence with respect to the axis ofthe optical filter element. To this end, as is shown in Figs. 6 and 7, a pair of collimators (e.g., 119, 120) or a multi-port fiber collimator (e.g., 122) may be used. The relatively small, but non-normal angle of incidence relative to the axis 121 of a particular filter element 118 needed will depend on several factors, including beam sizes used (e.g., beam waists of approximately 0.2 mm to approximately 0.5 mm) and the length ofthe grating needed to reach the target filter shape and dispersion. The angle of incidence may be calculated using known optical design techniques. It is noted that the two separate collimator design shown in Fig. 4 enables the separation ofthe reflected signal from the incident signal without the need for a separate circulator. It is further noted that a dual fiber collimator has nearly the same functionality as a pair of single fiber collimator but is more compact and generally more cost effective. Such a device could be used as an input/selected channel output collimator pair.

[0088] In contrast with thin-film interference filters, the reflectance ofthe Bragg grating filters ofthe present invention is distributed through the thickness ofthe filter. For example, as shown in Fig. 25, input beam 802 is incident upon Bragg grating filter 804 at a nonzero angle θ. Input beam is reflected over a substantial thickness of Bragg grating filter 804. The distributed reflectance serves to laterally shift the center ofthe reflected beam 806 from the point of incidence ofthe input beam 802, as well as anamorphically expand the size ofthe beam along the axis ofthe shift. In order to account for these effects, the dual fiber collimator shown in Fig. 25 may be used in conjunction with the Bragg grating filters ofthe present invention. The collimator 810 of Fig. 25 includes a collimating lens 812, an input fiber 814, and an output fiber 816. The fibers are held in a ferrule 818. The ends ofthe fibers are tilted at an angle θtiu to the axis ofthe collimating lens, and are symmetrically disposed around the axis ofthe collimating lens. The collimating lens 812 approximately a focal length away from the end ofthe input fiber and approximately a focal length from the surface of the filter 804. The tilt angle ofthe optical fibers may be approximated using the equation

where n is the average refractive index ofthe substrate material ofthe Bragg grating filter, T is the thickness ofthe Bragg grating filter at which half of the intensity ofthe input beam has been reflected, f is the focal length ofthe collimating lens, and D is the separation ofthe input and output optical fibers in the collimator. The skilled artisan may further optimize the tilt angle by using beam propagation techniques to maximize the overlap integral ofthe mode field ofthe output optical fiber and the field intensity ofthe focused beam at the end ofthe output optical fiber. In order to maximize the coupling ofthe reflected beam into the output optical fiber, it may be desirable to move the end ofthe output optical fiber somewhat away from the back surface ofthe collimating lens.

[0089] For some applications, better performance ofthe collimator may be achieved by using a waveguiding structure in place ofthe ferrule-held fibers described above. For example, the ends ofthe input and output fibers may be coupled to a planar waveguide configured with their ends having a proper tilt angle and separation. Waveguides with two dimensional guidance can be coupled to the fibers and formed to be close together near the focal plane ofthe collimating lens. This allows great flexibility with respect to the range of beam sizes and angles of incidence which can be achieved. For example, the small spacing possible between the ends ofthe input and output waveguides allows for small beam sizes and a small angle of incidence. Weak guiding or no guiding in one dimension (for example, by using a slab waveguide), or a tapered 'horn-like' structure can be used to introduce anamorphic expansion ofthe beam. The combination of waveguide separation control and anamorphic expansion provides considerable flexibility for matching the output waveguide mode shape with the reflected beam. Additionally, the waveguide surface can be polished to provide tilt to the wavefronts.

[0090] Finally, as shown in Fig. 8, the non-normal incidence and small angle of incidence approaches may be combined to optimize results. [0091] Fig. 9 shows a lxN optical filter anay 200 having optical filters 201 in accordance with another exemplary embodiment ofthe present invention. The optical filter array 200 is substantially identical to the optical filter anay 101 described in conjunction with the exemplary embodiment of Fig. 1. As such, the duplicative details ofthe optical filter anay 200 as well as optical filters 201 are forgone in the interest of brevity of discussion.

[0092] In the exemplary embodiment shown in Fig. 9, two sequential optical signals may be readily extracted. To this end, a first input collimator 203 inputs WDM/DWDM signal having a plurality of wavelength channels. The first input collimator 203 is illustratively aligned relative to a first optical filter 201, which reflects wavelength channel 1 having center wavelength λ_{ . As described in connection with the exemplary embodiment of Fig. 1, light ofthe wavelength channel 1 is reflected and is incident upon the first output collimator 204, which is suitably aligned to receive the reflected light. Light of all ofthe remaining wavelength channels ofthe optical signal is transmitted through the optical filter element, and is incident upon a second output collimator 205.

[0093] The second input collimator 206 is aligned with a second optical filter element 207 which is designed to reflect light of a wavelength channel 2 having a center wavelength λ₂. i a manner similar to that described immediately above, the light is reflected by the second filter element 207 and is incident upon a third output collimator 208, which is aligned to receive the reflected light. Finally, the unreflected optical signal having all remaining wavelength channels is transmitted through the optical filter, and is incident upon a fourth output collimator 209. [0094] In the exemplary embodiment shown in Fig. 9, if the input optical signals from first and second input collimators 203 and 206 are the same WDM or DWDM signal, by virtue ofthe optical filter anay 200 of Fig. 9, adjacent channels (e.g., channel 1 and chamiel 2) may be readily extracted. Moreover, as described in conjunction with the exemplary embodiment of Figs. 1 and 3, relative motion ofthe input and output collimators and the optical filter array 200 will allow the extraction of another two wavelengths. To this end, the optical filter elements (i.e. first optical filter element 201, second optical filter element 207, third optical filter element 210,..., Nth optical filter element 211) illustratively each reflect a different wavelength channel. Accordingly, by moving the optical filter anay 200 relative to the input and output ports, it is possible to align the respective input ports and output ports to another two ofthe optical filters, enabling the extraction of light of two other frequencies/wavelengths. Of course, this may be used to extract wavelength channels of a WDM or DWDM system as described immediately above. [0095] In the presently described exemplary embodiment, the optical filter elements 201, 207, 210, 211, etc., illustratively are designed to extract sequential optical wavelengths channels, although this is not necessarily the case. To wit, it may be that it is not desired to extract certain optical signals, or that the ordering ofthe optical filters be sequential. Because ofthe flexibility offered by the process for fabricating monolithic optical filter array according to the present invention, the optical filter elements may be fabricated in a plethora of combinations as the end user may require. Consequently, the fabrication of an anay of optical filter elements such as described in conjunction with the illustrative embodiment of Fig. 9 may be readily achieved by virtue ofthe present invention, thereby offering significant benefits from the perspective of large-scale manufacturability and cost. Moreover, while this advantage of flexibility of design afforded by the glass monolithic optical filter anay ofthe present invention has been described in connection with the illustrative embodiment of Fig. 9, it is noted that this certainly pertains to the other illustrative embodiments of the present invention described herein. Finally, it is again noted that in the exemplary embodiment in which the optical signal is a WDM or a DWDM system, there may be N- filters for n- wavelength channels having center wavelengths λ_λ , ..., λ_n . For purposes of illustration, N maybe 40, 80, 100, 200 or 400. Of course, this is merely illustrative and intended to be in no way limiting ofthe present invention. [0096] As is well known, it is often useful in optical communication systems to filter out a particular set of optical wavelengths/frequencies. For example, it may be useful to extract a particular set of WDM or DWDM channels from an optical signal containing channels 1, ..., n. In the exemplary embodiments shown in Figs. 10 and 11, wavelength channels 1 - 4 and wavelength channels 5 - 8, respectively, of a WDM/DWDM signal may be extracted from a multi-channel optical signal. The optical filter anay 300 illustratively is identical to the glass monolithic optical filter anays 200 and 101, as the optical filter elements therein. As such, the details ofthe filter elements and materials are not repeated in the interest of brevity and clarity. [0097] In the exemplary embodiment shown in Fig. 10, a first input collimator 302 is aligned to a first filter element 301 which illustratively reflects wavelength channel 1 having center wavelength λ ofthe WDM/DWDM signal from the first input collimator 302. The reflected light is incident upon a first output collimator 303, and the channel 1 is thereby extracted. Moreover, all remaining channels are transmitted and are incident upon second output collimator 304.

[0098] Similarly, wavelength channel 2 having center wavelength λ₂ is extracted from the optical signal from input collimator 305 by reflection from a second filter element 306 that selectively reflects wavelength channel 2. This reflected channel is incident upon a third output collimator 307, while all remaining optical channels incident from the second input collimator 305 are transmitted and incident upon a fourth output collimator 308. Likewise, channel 3 having center wavelength λ₃ is extracted from the input signal from a third input collimator 309 and is reflected a third optical filter element 310 which reflects wavelength channel 3 to the fifth output collimator 311. All remaining channels are transmitted to a sixth output collimator 312. Finally, channel 4 may be extracted from an optical signal of fourth input collimator 313, which is aligned with a fourth optical filter element 314 that reflects wavelength channel 4 having center wavelength . Channel 4 is extracted by reflection and is incident upon a seventh output collimator 315, while all remaining optical channels are transmitted through the chosen optical filter elements 314 to the eighth output collimator 315.

[0099] Turning to Fig. 11, a second optical filter anay 300 is useful in extracting optical channels 5 - 8 from a WDM/DWDM optical signal, h the interest of brevity, because the method of extraction ofthe optical channels using the optical filter anay 300 of Figs. 10 and 11 are identical, most details are forgone. Succinctly, a fifth optical filter element 316 reflects wavelength channel 5 having center wavelength λ₅; a sixth optical filter element 317 reflects wavelength channel 6 having center wavelength λe; a seventh optical filter element 318 wavelength channel 7 having center wavelength λ₇; and eighth optical filter element 319 reflects wavelength channel 8 having center wavelength λs . Of course, input and output collimators are aligned to the respective filter elements as shown to enable the extraction ofthe optical signal.

[00100] From the above exemplary embodiments described in connection with Figs. 9- 11, the number of wavelength channels extracted may be varied. Moreover, by simple relative motion ofthe optical filter anay and collimators, the optical filter array can be reconfigured to extract other channels. It is noted that optical signals may be input from either side ofthe filter anay, and, as shown in Figs. 10 and 11, the filter elements may be ordered in a non-sequential manner. Moreover, in the illustrative embodiments shown in Figs. 10 and 11, the non-sequential ordering ofthe filter elements enables the extraction of four sequential multiplexed channels, advantageously enabling an increased distance between collimators sets. Finally, it is noted that the filter elements may be cascaded, and channels not extracted by a first filter may be input to a second filter. This process of course may continue. As can be readily appreciated, cascading is useful in reducing the insertion loss if the through loss is less than the splitting loss ofthe conesponding 1 :N coupler. The ability to cascade also makes it possible to use the device as an add or drop filter in an add/drop multiplexer.

[00101] hi the exemplary embodiments described thus far, the filter elements for each WDM channel are located in a single optical filter anay. It is noted that it may be beneficial from the perspective of manufacturing, for example, to limit the number of optical filter elements in a single anay. Moreover, it may be useful to have multiple glass monolithic optical filter anays combined into a single device to provide an increased tuning range. Multiple glass monolithic optical filter anays may use more than two dimensions of translation to effect selective alignment ofthe collimators. Moreover, the optical filter anays may be placed serially, enabling one-dimensional translation of motion. Still, as described presently, an input/output collimator pair may be dedicated for each array.

[00102] Turning to Fig. 12, a stacked optical filter anay structure 400 is shown, hi the exemplary embodiment shown in Fig. 12, the stacked optical filter anay structure 400 includes a first monolithic optical filter anay 401, a second monolithic optical filter anay 402 and a third monolithic optical filter anay 403. Each ofthe first, second and third glass monolithic optical filter anays are virtually identical to those described in connection with the exemplary embodiments of Figs. 1, 9 10 and 11, and as such, repetition of these details is omitted in the interest of brevity and clarity of discussion. [00103] A first collimator pair 414, which is substantially identical to that described in conjunction with Fig. 3, is selectively aligned to one ofthe optical filter elements of the first monolithic optical filter array 401 for the selective extraction of a particular wavelength channel. In the present illustrative embodiment the first optical filter element 404 reflects a channel 1 having a channel center wavelength λ_x . As such, alignment ofthe first collimator pair 414 with first optical filter element enables channel 1 to be extracted from an WDM/DWDM optical signal. [00104] Similarly, a second collimator pair 415, may be aligned to one ofthe optical filter elements ofthe second monolithic optical filter anay 402. Illustratively a second optical filter 405 reflects channel 2, having channel center wavelength λ₂. As such, if the second collimator pair 415 is aligned to a second optical element 405 of the monolithic optical filter anay 402, chamiel 2 may be extracted. [00105] Likewise, a third collimator pair 416 which is substantially identical to first input collimator pair 414 may be selectively aligned to one ofthe optical elements of the third monolithic optical filter array 403. For example, if the third collimator pair 416 is aligned to a third filter element 406, which reflects channel 3 having a center wavelength λ₃ , channel 3 may be extracted.

[00106] By the translational motion in the ± x-direction 413, the second column of filter elements comprised of filter elements 407, 408 and 409 may be aligned with their respective optical collimator pairs for the extraction of channels 4, 5 and 6. Likewise, alignment of a third column of filter elements 410, 411 and 412 with their respective collimator pairs enables the extraction ofthe channels 7, 8 and 9 in the exemplary embodiment of Fig. 12.

[00107] In the exemplary embodiment shown in Fig. 12, translational motion (in the ±x direction 413) ofthe first monolithic optical filter anay 401 and the enables the selective alignment ofthe optical filter elements therein to the first input/output collimator pair 414. Similarly, the translational motion ofthe second monolithic optical filter anay 402 enables the selective alignment to the second input/output collimator pair 415; and the translational motion ofthe third monolithic optical filter anay 403 enables the selective aligmnent ofthe optical filter elements therein to the third input/output collimator pair 416. The translational motion may be effected and controlled using methods and apparati described above. Moreover, it is noted that the alignment ofthe input/output collimators 414, 415 and 416 to respective optical filters elements can be effected in a variety of combinations, enabling a plethora of demultiplexing schemes. Finally, it is note that the collimator pair could move to effect aligmnent.

[00108] Fig. 13 shows another exemplary embodiment ofthe present invention. A glass monolithic optical filter anay 500 has a plurality of optical filter elements 501. The optical filter anay, optical filter elements and collimators in the exemplary embodiment of Fig. 13 are virtually identical in substance to those described in connection with Figs. 10-12. As such, details which are duplicative are omitted in the interest of brevity.

[00109] In the exemplary embodiment shown in Fig. 5, a four-channel cascaded filter structure with reflective optical filter elements 501 is positioned to drop four WDM/DWDM channels, illustratively channels 1 - 4, of an optical signal containing channel 1, ..., channel N. To this end, an input collimator 502 illustratively inputs an optical signal having a plurality of WDM/DWDM optical channels. First optical filter element 501 reflects wavelength channel 1. This reflected light is incident upon a first output collimator 503, and thus channel 1 is extracted. The remaining channels ofthe optical signal are transmitted through the first optical filter element 501 and are incident upon a second output collimator 504.

[00110] A second input collimator 505 transmits the remaining channels ofthe optical signal to a second optical filter element 506 which reflects channel 2. The reflected wavelength channel is incident upon a third output collimator 507, while the remaining optical channels are transmitted through the second filter element 506 and are incident upon a fourth output collimator 507. The remaining channels are transmitted to a third input collimator 508, which is aligned to a third filter element 509 and which reflects wavelength channel 3.' This reflected light is incident upon a fifth output collimator 510, and channel 3 is thus extracted. The remaining channels are incident upon a sixth output collimator 511, and the optical signal containing these channels are transmitted to a fourth input collimator 512, which is in alignment with a fourth filter element 513, and which reflects wavelength channel 4. The reflected light is incident upon a seventh output collimator 514, and channel 4 is thus extracted. The remaining channels are transmitted through the fourth filter element 513 to an eighth output collimator 515.

[00111] As described previously, the relative motion of optical filter anay 500 and the input and output collimators enables the selective dropping of optical channels through the selective alignment ofthe input and output collimators to the 1 - N filter elements of optical filter anay 500.

[00112] hi accordance with an exemplary embodiment ofthe present invention a monolithic optical filter anay may have a plurality of rows of filter elements. Illustratively, this multiple row device could be used to form a passive reconfigurable optical add/drop multiplexer. Such an add/drop multiplexer is shown in an exemplary embodiment in Fig. 14. A glass monolithic optical filter anay 600 includes a first row of optical filter elements 601 and a second row of optical filter elements 602. The materials ofthe substrate, and the filter elements ofthe exemplary optical filter anay 600 are virtually identical in substance and function to those described in connection with the exemplary embodiments ofthe present invention discussed in connection with Figs. 1 and 9-13. As such, in the interest of brevity of discussion, details are omitted.

[00113] Each row 601, 602 contains filter elements 1 -N. In the exemplary embodiment shown, filter element 1 (e.g., 604, 610) is designed to reflect light having a first wavelength conesponding to the center wavelength of channel 1, while transmitting light of all other wavelengths. Likewise, filter element N is designed to reflect light having a wavelength conesponding to the center wavelength of channel N. In the exemplary embodiment shown in Fig. 1, an add/drop input collimator 603 illustratively transmits an optical signal having channels 1 - N. By reflection of first filter element 1 (604), channel 1 is dropped, and is incident illustratively upon a channel 1 drop collimator 605. All remaining channels are transmitted through filter element 1 (604) to output collimator 606. These remaining channels are then incident upon filter element 3 (607) via input collimator 608, and by similar technique, channel 3 is dropped. Through the principle of reciprocity of optics, the reverse of each ofthe described processes can be used to add a channel, in this case channel 1 and channel 3, using the same element referenced. To add channel 1, a channel 1 add collimator 609 is oriented relative to channel 1 filter element 610, such that channel 1 is reflected from channel 1 filter element 610, and is incident upon add/drop output collimator 611. Add/drop collimator 611 may include a WDM/DWDM signal received from the various combinations of collimators and filters of optical filter array 600. In this manner, channel 1 may be added to a WDM/DWDM optical signal. Likewise, from a review ofthe positioning and orientation ofthe various collimators and filter elements ofthe exemplary embodiment of Fig. 14, channels 3 and 5 maybe selectively added/dropped to/from WDM/DWDM optical signals in accordance with the present exemplary embodiment. Moreover, as can be readily appreciated, translation motion ofthe collimators relative to the optical filter anay enables the adding/dropping of other optical channels of a WDM/DWDM signal. [00114] It is noted that the above 2-row optical filter anay ofthe exemplary- embodiment of Fig. 14 is merely an illustrative application of a 2-row array. Clearly, other uses of such a multiple-row anay may be exploited. Such uses are within the purview of one having ordinary skill in the art having has the benefit ofthe present disclosure. It is further noted that in the exemplary embodiment shown in Fig. 14, the filter elements in first row 601 and second row 602 are contiguous. Of course, as described previously, this is not essential. As such, the ordering ofthe various filter elements may be tailored to the individual needs ofthe user. [00115] Figure 15 is a graph of the reflectivity versus wavelength for three optical filter elements of a monolithic glass optical filter anay in accordance with an exemplary embodiment ofthe present invention. The first filter element reflects an ITU wavelength channel having a center wavelength of 1543.73 nm. The second and third filter elements reflect second and third reflected wavelength channels, respectively having center wavelengths of 1544.13 nm and 1544.53, respectively. As described previously, an advantageous aspect ofthe optical filter elements of an exemplary embodiment ofthe present invention an insertion loss versus frequency/wavelength that has both steep transition regions outside ofthe passband of the filter element and a relatively flat filter function, as is shown in Fig. 15. [00116] In accordance with another exemplary embodiment ofthe present invention, the monolithic optical filter anay comprises a single row of nominal filters designed for the extraction of desired frequencies/wavelengths from an incoming optical signal which includes a plurality of frequencies/wavelengths. For example, the optical signal may be a WDM optical signal having n-wavelength channels with respective center wavelengths λ_x , ..., λ_n . To relax manufacturing accuracy, as well as to accommodate shifts in the transmission wavelengths of an optical emitter used in the WDM system, proximate to this row of nominal optical filter elements is one or more rows of optical filter elements that are detuned from the center wavelengths by some small but finite amount.

[00117] In a deployed optical communication system, input and output optical couplers may be selectively aligned to a particular filter element for the extraction of a desired wavelength. Illustratively, if it is desired to extract another wavelength channel, the input and output couplers would be moved to the appropriate filter. If the resonant wavelength of a particular nominal optical filter element does not match the frequency to be extracted due to some manufacturing defect or shift in wavelength ofthe transmitter, a positively or negatively detuned filter element may then be selected (as appropriate) to extract the desired wavelength band.

[00118] Fig. 25 shows a monolithic optical filter anay 1100 of nominal and detuned optical filter elements in accordance with an exemplary embodiment ofthe present invention. Nominal wavelength optical filter elements 1101 are illustratively shown in a first row in the anay. In the present exemplary embodiment negatively detuned wavelength optical filter elements 1102 are shown in a second row ofthe anay; and positively detuned wavelength optical filter elements 1103 are shown in a third row in the anay. In the exemplary embodiment presently described, each ofthe nominal wavelength optical filter elements 1101 is designed to extract a particular wavelength channel. Illustratively, the nominal and detuned wavelength optical filter elements 1101, 1102 and 1013 are Bragg gratings as described hereinabove. [00119] Moreover, the use of Bragg gratings as nominal and detuned wavelength optical filter elements 1101, 1102, and 1103 are illustrative. It is noted that other mterferometric filters such as holographic filters and guided mode resonance (GMR) filters may be used as nominal wavelength optical filter elements 1101, 1102 and 1103. In general, gratings that may be written in the substrate using interference effects or phase masks to form the optical filter elements ofthe monolithic optical filter array 1100 may be used in canying out the present invention. Moreover, other types of filters may be used including, but not limited to micro-electromechanical (MEMS) optical filter elements. Finally, it is conceivable that the nominal and detuned wavelength optical filter elements 1101, 1102 and 1103 are not based on the same filter technology; but rather on a combination of technologies. [00120] In accordance with the exemplary embodiment ofthe present invention shown in Fig. 25, the monolithic optical filter array 1100 includes columns 1104 of filter elements. Each column 1104 comprises a nominal wavelength optical filter element 1101, a negatively detuned wavelength optical filter element 1102 proximate the nominal wavelength optical filter element 1101, and a positively detuned nominal wavelength optical filter element 1103 also proximate the nominal wavelength nominal optical filter element 1101. [00121] In the presently described exemplary embodiment in which the monolithic optical filter anay is used in a WDM application, each nominal wavelength optical filter element will reflect one wavelength channel having a particular center wavelength and bandwidth and will transmit all other wavelength channels. For purposes of illustration an n^th nominal filter element 1101' reflects an n^l wavelength channel incident thereon having a center wavelength of λ_n from a WDM/DWDM input signal, and will transmit wavelength channels 1, ..., n-1, having respective center wavelength λ_x , ..., λ_n_ therethrough.

[00122] Each ofthe positively and negatively detuned wavelength optical filter elements (1102 and 1103) of each column 1104 reflects a wavelength band which has a center wavelength that is slightly offset relative to that of its proximate nominal wavelength filter. For example, in the exemplary embodiment shown in Fig. 25, column 1104' has a positively detuned optical filter element 103' and a negatively detuned optical filter 1102' . As referenced above, nominal filter element 1101' reflects wavelength chamiel n having a center wavelength λ_n . As such, the positively detuned optical filter element will reflect a wavelength band having center wavelength of λ_n + Aλ . Likewise, negatively detuned optical filter element 1102' will reflect a wavelength band having a center wavelength of λ_n - Aλ . hi the presently described exemplary embodiment, the 2 dB wavelength bandwidth is illustratively 0.24 nm (i.e., approximately 30 GHz), and the wavelength offset, Aλ , is illustratively 0.08nm (i.e. approximately 10 GHz).

[00123] As will become more clear as the present description proceeds, it is noted that the offset, Aλ , between a nominal filter element 1101, and the detuned optical filter elements 1102 and 1103 of a particular column 1104 is significantly less than the difference between the center wavelength, which are reflected by two adjacent nominal optical filter elements 1101. For example, in the exemplary embodiment shown in Fig. 25, the wavelength offset, Aλ , between nominal optical filter 1101' which reflects channel n having a center wavelength λ_n , and the differential between the center wavelength λ_n__λ of wavelength channel n-1 which is illustratively reflected by the nominal optical filter element 101 adjacent nominal optical filter element 1101' is significantly less. [00124] Fabrication ofthe nominal and detuned wavelength optical filter elements 1101, 1102 and 1103, regardless ofthe particular filter technology chosen or material used for substrate 1105, is illustratively canied out monolithically. Again, further details ofthe fabrication as well as the materials used may be found in the above referenced applications to Bhagavatula, et al, and Bonelli, et al., respectively. Beneficially, this fosters practical manufacturing and reduced cost when compared to conventional fabrication techniques. For example, in the fabrication of gratings such as Bragg gratings or holographic gratings, a plurality of masks could be used to fabricate the fixed frequency filters 1101, 1102 and 1103, with each mask tailored to fabricate a grating of a desired periodicity. Alternatively, a single phase mask could be used and the periodicity of each grating could be tailored by altering the angle of incidence ofthe grating and/or light source. Moreover, other interferometric techniques known to one of ordinary skill in the art may be used. Finally, it is noted that a combination ofthe illustrative fabrication techniques described immediately above could be used in fabricating the nominal wavelength optical filter elements 1101, 1102 and 1103.

[00125] It is further noted that the present invention as described in connection with the exemplary embodiment would benefit the task of accommodating any wavelength shift due to time, temperature, or tuning of an EDFA or laser device. [00126] From the above description sunounding column 1104' , in the presently described exemplary embodiment it is clear that the other columns 1104 each have a nominal optical filter element 1101 and detuned optical filter elements 1102 and 1103 in proximity thereto. However, this anangement is not essential to canying out the present invention. To this end, depending upon the desired application, it may be useful to anange the various optical filter elements 1101, 1102 and 1103 to tailor a need. For example, it may be that there are a few nominal wavelength optical filter elements 1101 sunounded by a plurality of detuned wavelength optical filter elements 1102 and 1103 of varying degrees. Moreover, it may be useful to have all ofthe detuned optical filter elements are positively detuned; or all are negatively detuned. Still other variations are possible, all of which are readily fabricated by virtue ofthe ease of manufacture afforded by the above referenced fabrication process. [00127] Fig. 17 shows the frequency spacing for nominal and detuned filter elements according to an illustrative embodiment ofthe present invention. To this end, the wavelength channel passbands 1201, 1202, 1203 and 1204 conespond to the reflected wavelength channels of four nominal wavelength optical filter elements in accordance with an exemplary embodiment ofthe present invention. Likewise, the passbands 1206 represent the wavelength passbands ofthe positively detuned optical filter elements in accordance with an exemplary embodiment ofthe present invention; and passbands 1207 represent the wavelength passbands of negatively detuned optical filter elements in accordance with an exemplary embodiment ofthe present invention. [00128] Focusing discussion momentarily on wavelength channel passbands 1203 and 1204, it can be readily appreciated from Fig. 17 that the spacing 1205 between passbands 1203 and 1204 is significantly greater than the spacing 1208 between the passbands ofthe positively detuned wavelength optical filter element and the spacing 1209 between the passband 1203 and the passband 1207 ofthe negatively detuned wavelength optical filter element. For purposes of illustration and certainly not limitation, in accordance with an exemplary embodiment ofthe present invention, the spacing 1205 between passbands 1203 and 1204 of nominal optical filters could conespond to the channel spacing of a WDM system. This channel spacing is illustratively 0.8nm, although it could be other frequency spacing such as are prescribed by the International Telecommunication Union (ITU) grids. In the exemplary embodiment in which the spacing 1205 is on the order of 0.8nm, the spacings 1208 and 1209, are on the order of approximately 0.16nm. [00129] As will become more clear as the present description proceeds, if it is desired to extract a wavelength channel passband 1203 in a demultiplexing application, a channel input comprising a plurality of optical channels would be aligned to the particular nominal wavelength optical filter element having the wavelength passband 1203. An output would be suitably aligned so that wavelength passband 1203 could be extracted from the plurality of frequencies ofthe channels. [00130] Illustratively, wavelength passband 1203 conesponds to a particular wavelength channel. Naturally, in accordance with exemplary embodiment ofthe present invention, tolerances as well as amplifier tuning and laser offset could result in the center wavelength ofthe particular desired channel being shifted to have a wavelength band conesponding to passband 1206, or conesponding to passband 1207. Aligmnent ofthe input and output devices to the particular detuned wavelength optical filter element would enable the extraction ofthe desired frequency/wavelength channel.

[00131] Fig. 18 shows a monolithic optical filter anay 1300 for use as a two-port reconfigurable tunable filter in accordance with an exemplary embodiment ofthe present invention. Practical applications of such a device include demultiplexing of desired multiplexed channels in a WDM system and adding/dropping channels in such a system. The monolithic optical filter anay 1300 includes a substrate 1311 which is of material in keeping with the materials described previously. A plurality of optical filter elements 1301 are used to extract a first wavelength channel having a first center wavelength, and second optical filter elements 1302 are used to extract a second wavelength channel having a second center wavelength. It is noted that for purposed of clarity of discussion, the first optical filter elements 1301 and second optical filter elements 1302 may be either the nominal wavelength optical filter elements, or the positively or negatively detuned wavelength optical filter elements as described previously. It is further noted that in accordance with the exemplary embodiment shown in Fig. 18, the nominal, positively detuned, and negatively detuned wavelength filters are monolithically formed on the substrate as previously described.

[00132] In accordance with the exemplary embodiment shown in Fig. 18, an input 1304 is aligned with one ofthe first optical filter elements 1301. The input illustratively includes a plurality of multiplexed optical signals such as those of a standard WDM optical system. A first optical filter element 1 301' is illustratively a nominal wavelength filter element that reflects a wavelength channel having a first center wavelength. This reflected signal is incident upon the output 1305. All other wavelength channels ofthe WDM signal from input 1304 are transmitted through to the output 1306.

[00133] If it is desired to extract another wavelength channel ofthe WDM signal, a number of options are available according to the exemplary embodiment ofthe present invention. First, simple translational motion such as shown at 1307 enables the alignment ofthe input 1304, outputs 1305 and 1306 to another ofthe first optical filter elements 1301 and 1302. For example, it may be desired to extract the second wavelength channel through the use of one ofthe second optical filter elements 1302. This is carried out in accordance with an exemplary embodiment ofthe present invention using a second input 1308 which may be aligned to one ofthe second optical filter elements 1302. The extracted wavelength channel having the second frequency is output to output 1309, and the remaining WDM channels are output to the other output 1310.

[00134] Accordingly, the relative motion ofthe monolithic optical filter anay 1300 and the inputs and outputs enables the chosen alignment of a particular input to a particular fixed-frequency filter. It is noted that the exemplary embodiment as shown in Fig. 18 can be readily expanded and/or modified. To this end, the anay 1300 could include a plurality of filters, each designed to reflect a particular wavelength channel center frequency. It is further noted that the anay 1300 could include the nominal and positively and negatively detuned filters for all channels in a particular passband. As such, there could be 40, 80 or 100 nominal filter elements each having respective detuned elements proximate thereto.

[00135] To effect the extraction of a particular wavelength channel, the relative motion ofthe anay can be canied out properly align the input and output ports to a particular fixed- frequency filter. This may be readily carried out by filter control circuitry (not shown) which incorporates a look-up table to recall the position of a filter element which reflects a desired frequency. Moreover, the look-up table can retain the nominal, positively detuned, or negatively detuned filter elements chosen at a particular time of calibration to be used for each channel setting. As such, if a particular filter does not reflect the required wavelength channel due to a manufacturing defect or drifting ofthe optical emitter ofthe system, alignment ofthe input and output ports can be effected via the look-up table and filter control circuitry. Further details ofthe structure and electronics for canying out this relative motion may be found hereinabove.

[00136] It is noted that in the illustrative embodiments described thus far, the optical filter elements are contiguously arranged. It is noted that it is not required that the optical filter elements be distributed contiguously. To this end, all elements, nominal optical filters as well as positively and negatively detuned optical filter elements may be written in a single linear anay in any order. To wit, it is not required that the progression of resonant wavelengths/frequency be sequential, as the look-up table and filter control circuitry can be readily modified to accurately determine the position of a particular filter, regardless if its particular resonant wavelength frequency is sequential in the optical filter anay. This enables the user to tailor a particular system for a particular intended use. Moreover, enors in manufacturing can be readily mitigated. To this end, if there is an enor in the fabrication of a particular filter causing a break in a particular filter sequence, the filter anay would not be lost to scrap. Instead, a slight modification in a look-up table can account for the break in the sequence. Finally, the anays described have been rectangular with regular rows and columns. However, this is not essential. For example, circular or elliptical arrangements of filters may be effected in keeping with the present invention. [00137] Figs. 20 and 21 show stacked and serial filters anays, respectively, in accordance with exemplary embodiments ofthe present invention. The NxM optical filter anays may be as described hereinabove. A first substrate 1401 and a second substrate 1402 have a plurality of nominal filter elements 1403 and 1404, respectively. Positively detuned elements 1405 and 1406, as well as negatively detuned elements 1407 and 1408 complete the anay. The stacked nature ofthe first and second anays 1408 and 1409 ofthe illustrative embodiment shown in Fig. 19 enables a reduction in the complexity of fabrication. To wit, by fabricating a particular anay to reflect a first number of wavelength channels and another anay to reflect another number of wavelength channels, a full passband can be accommodated, but with less complexity in fabrication. In accordance with the exemplary embodiment shown in Fig. 19, it is merely necessary to have the capability of aligning input and output ports by motion in the x-direction (1410) as well as in the y-direction (1411). Again, a look-up table and filter control circuitry would be used to guide the input and output ports to a particular filter so that a desired wavelength could be extracted. Similarly, as shown in Fig. 20, a first anay 1501 and a second anay 1502 could be fabricated and motion in the x-direction (1503) and y-direction (1504) enables the alignment to any ofthe elements of either anay. Finally, it is noted that the NxM optical filter arrays may be accessed using one-dimensional motion, using a method described hereinabove.

[00138] In accordance with other exemplary embodiments ofthe present invention described herein, the present invention relates to optical apparatus comprising a monolithic structure which includes a plurality of tunable optical filter elements, an apparatus for extracting optical signals including the optical apparatus, and a method for extracting optical signals using the optical apparatus. [00139] Advantageously, the optical filter array in accordance with exemplary embodiments described herein enables substantially continuous tuning over a wavelength range using coarse and fine tuning as is described in detail below. [00140] Illustratively, the monolithic structure includes a plurality of optical filters, wherein at least one ofthe optical filters is adapted for tuning at two or more wavelengths. In its use as an apparatus for extracting optical filters, the optical filter anay includes a tuning mechanism which selectively tunes at least one tunable optical filter enabling the separation of a particular wavelength channel from an optical signal that includes a plurality of wavelength channels. Moreover, a method of use ofthe apparatus enables selective extraction of a particular wavelength channel from an optical signal which includes a plurality of wavelength channels. Illustratively, the optical signal is a wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) signal.

[00141] The optical apparatus in accordance with an exemplary embodiment ofthe present invention could be a tuneable dispersion compensator. To this end, at least one ofthe optical filter elements ofthe monolithic optical filter anay would illustratively would be a chirped grating, such as a chirped Bragg grating. The grating could be linearly or non-linearly chirped, and could be finely tuned by techniques described herein. Moreover, a plurality of such gratings could be used in which coarse tuning and fine tuning could be carried out by techniques described herein. [00142] It is further noted that the above examples ofthe utility ofthe monolithic optical filter anays ofthe present invention are merely illustrative, and are intended to be in no way limiting ofthe present invention. Clearly, other implementations ofthe monolithic optical filter anay will be readily apparent to one of ordinary skill in the art who has had the benefit of applicants' disclosure.

[00143] Fig. 21 shows an exemplary embodiment ofthe present invention in which a thermally tuned optical filter apparatus 2100 includes a monolithic optical filter anay 2101 which further includes aplurality of optical filter elements. The monolithic optical filter anay 2101 comprises N optical filter elements (N=integer) 2102, 2103, 2104, 2105, 2106, 2107 for n-wavelength channels having center wavelengths λ_x , ..., λ_n . For purposes of illustration, n and N may be 40, 80, 100, 200 or 400. Of course, this is merely illustrative and intended to be in no way limiting ofthe present invention. Illustratively, the N optical filter elements 2102 - 2107 are reflective filter elements as described hereinabove.

[00144] In accordance with the exemplary embodiment shown in Fig. 21, a thermal element 2108 is disposed proximate to filter elements 2102 - 2107. Thermal element 108 is illustratively a thennoelectric cooler (TEC), such as Peltier effect thermoelectric cooler, and is used to modulate the temperature ofthe optical filter elements 2102 - 2107 in a manner described herein. Of course, the selection of a TEC for the thennal element 2108 is merely illustrative, and other thermal elements

) could be used to effect thermal tuning ofthe optical filter elements while keeping within the purview ofthe present invention. For example, thin-film heating elements may be used in this capacity.

[00145] It is further noted that in the exemplary embodiment shown in Fig. 21, all filter elements are tuned using a single thermal element 2108 disposed beneath the monolithic optical filter anay 2101. This is also merely illustrative. To wit, the thermal element 2108 could comprise a plurality of individual elements, each of which thermally tunes a certain number (i.e. two or more) ofthe optical filter element 2102 - 2107. Moreover, the thermal element, or a plurality of thermal elements as referenced immediately above, could be disposed over the top surface ofthe monolithic optical anay. This placement of such a thennal element(s) could be instead of or in addition to thermal element 2108 shown disposed beneath the monolithic optical filter anay 2101 in Fig. 21. Finally, it is noted that it is possible to have individual thermal elements for each optical filter element. [00146] hi operation, an optical signal from an input/output collimator 2109 is incident upon a selected one ofthe filter elements 2107. The input optical signal from input collimator 2107 includes a plurality of optical channels. For example, the input optical signal could be a WDM or a DWDM optical signal having channels 1, ..., n, which have respective center wavelengths λ_x , ..., λ_n . In the exemplary embodiment shown in Fig. 21 wherein the filter elements 2102 - 2107 are illustratively reflective filter elements, a selected one of said channels is reflected by the selected optical filter element 2107 and is incident upon input/output collimator 2109. The remaining optical channels are transmitted through the chosen filter element, and are incident upon an output collimator 2110. It is noted that a variety of input/output devices may be used for input/output collimator 2109 and output collimator 2110. Moreover, certain techniques may be used to reduce specular reflection. Further details of these input/output devices as well as techniques to reduce specular reflections may be found hereinabove It is further noted that these referenced input/output devices and techniques may be used in conjunction with other exemplary embodiments described herein.

[00147] As will become more clear as the present description proceeds, each ofthe filter elements 2102 - 2107 is designed for thermal tuning over two or more wavelength channels. As such two or more wavelength channels may be reflected by the particular optical filter element chosen depending upon the shifting ofthe effective optical periodicity by thermal effects ofthe thennal element 2108. To wit, the thennal variance may change the refractive index and/or the physical periodicity ofthe Bragg grating ofthe optical filter element, thereby changing its resonant wavelength. It is noted that the thermal tuning results in the fine tuning ofthe optical apparatus 2100.

[00148] Coarse tuiϋng is effected by the one-dimensional motion 2111 and alignment ofthe input/output collimator 2109 and the output collimator 2110 to a particular optical filter element. Consequently, the coarse tuning ofthe optical apparatus involves the selection of a particular optical filter element which will reflect a particular number of channels. Reflection ofthe desired one particular wavelength channel (i.e., fine tuning) entails the thermal tuning described above. [00149] To effect the motion ofthe input/output collimator 2109 and the output collimator 2110, an electronically controlled mechanical device such as a stepper motor could be used. Further details of such a device may be found hereinabove [00150] The control ofthe motion ofthe input/output collimator and output collimator is illustratively carried out as follows. A microcontroller (not shown) may access a look-up table which contains the reflection wavelength band of each ofthe individual filter elements over a particular temperature range. A translation stage illustratively moves either the input/output collimator 2109 and output collimator, or the monolithic optical filter anay 2101 in one direction 2111 so that selected one of filter elements 2102 - 2107 is properly aligned with the input/output collimator 2109. Thereafter thermal tuning may be effected to fine tune the optical filter element to reflect a desired frequency/wavelength channel. [00151] In accordance with the exemplary embodiment of Fig. 21, coarse (mechanical) and fine (thermal) tuning are carried out enabling wavelength channel selection over a prescribed passband in a manner which affords significant advantage over conventional methods/apparati.

[00152] For example, the illustrative embodiment enables adjustment ofthe wavelength of each filter element to accommodate for manufacturing induced variations in the center wavelength of a wavelength channel. Moreover, the present invention as described with the illustrative embodiments enables continuous or nearly continuous tuning over a relatively wide range (e.g. 30nm - 80nm). [00153] Furthermore, because each individual filter requires a relatively small amount of tuning , tight control is not necessary to ensure that the filter device is at a target wavelength within a prescribed absolute tolerance. Hence, the control system can be simplified. Typically, the application ofthe filter device dictates the accuracy with which the filter must be set (e.g., WDM systems illustratively require 5 GHz of filter de-tuning). The broader the range over which the filter device must operate, the more difficult it becomes to maintain particular absolute accuracy. [00 54] By virtue of the present invention the tuning range illustratively may be may be reduced from approximately 40nm for a conventional single tunable filter to approximately 0.4nm to approximately 1.2nm for the optical filter elements (e.g., filter elements 2102 - 2107) ofthe present invention. To this end, 0.4nm is the approximate range for a 50 GHz system (i.e., 0.4mn at 1550nm) if a tuneable optical filter elements are targeted to be nominally between the 50 GHz channels. Then the adjacent channels are spaced approximately 25 GHz apart for a total of approximately 50 GHz or 0.4 nm.

[00155] Of course, the 0.4nm tuning range pertains to two channels in the present illustrative embodiment. Three channels could be reached by targeting a particular tuneable optical filter element for a specific channel and tuning up or down 50 GHz (i.e. over 100 GHz), requiring a tuning range of 0.8 nm. To go to four channel tuning per tuneable optical filter element, it is necessary to tune 25 GHz to the adjacent chamiel plus 50 GHz to the next channel for a total of 75 GHz in each direction. The total range of 150 GHz requires a tuning range of 1.2 nm. [00156] Turning to Fig. 22, a graph of an illustrative fine tuning range for an illustrative optical filter element of an embodiment ofthe present invention using thermal tuning is shown. To this end, the 2 dB center wavelength versus temperature for an optical filter element of a monolithic optical filter array formed in a glass substrate according to an exemplary embodiment is shown. The temperature tuning in the present example is 0.013 mn per °C. For 0.4nm tuning a temperature change of approximately 31 °C is needed; for the illustrative optical filter element to be tuneable over three channels would require a temperature change of 62 °C; and tuning over four channels would require a temperature change of 93°C. [00157] It is noted that the above described data shown in Fig. 22 is illustrative, and other glass materials may exhibit different tuning characteristics over temperature. Moreover, it is noted that materials other than glass may have different tuning characteristics. For example, polymer materials generally exhibit a greater temperature sensitivity (for example, on the order of 0.3 nm/°C); thus, a smaller temperature variation will result in a greater wavelength frequency change compared to the glass material referenced in connection with Fig. 22.

[00158] Beneficially the above described illustrative tuning range ofthe tunable optical filter elements of an exemplary embodiment ofthe present invention results in a significant reduction ofthe required accuracy ofthe control system. [00159] Moreover, the fine tuning may be used to reduce the center wavelength tolerance ofthe optical filter elements. A temperature offset could be stored in the controller, and used to conect the nominal temperature to which the filter element is set for a particular target wavelength.

[00160] Finally, as previously discussed, coarse tuning is effected by the motion ofthe input/output collimator 2109 relative to the monolithic optical filter anay. This may be achieved by methods described above. The individual optical filter elements 2102 - 2107 are approximately 0.1mm to approximately 1.0mm in cross-section for typical WDM applications. The alignment tolerances for the optical apparatus should be roughly 10 times finer than this. This degree of tolerance is well within the capabilities of stepper motors, DC motors and linear solenoids discussed in the referenced application.

[00161] Fig. 23 shows an angle tuned filter apparatus 2200 according to an exemplary embodiment ofthe present invention. A filter element anay 2201 is disposed in close proximity to a filter selector 2202. A rotation stage 2203 is selectively rotated by a rotation mechanism (not shown) to orient the angle of incidence of light from an input collimator 2204 upon the frequency selector 2202, and therefore to a particular filter element 2207 in the optical filter anay 2201. The optical signal from the input collimator 2204 includes a plurality of wavelengths. Illustratively, the optical signal is a WDM or DWDM optical signal which may have 40, 80, 100 (or more) wavelength channels, with each wavelength channel having a center wavelength. In this illustrative embodiment, one ofthe center wavelengths is reflected from the selected filter element and is incident upon a first output collimator 2205. All remaining wavelengths are transmitted through the filter element 2207, and are incident upon an output collimator 2206. These transmitted signals may be further demultiplexed by a similar teclmique, and using a cascaded apparatus which is similar to the angled tuned filter apparatus ofthe illustrative embodiment shown in Fig. 23. [00162] The optical filter anay 2201 in accordance with an exemplary embodiment shown in Fig. 23 illustratively includes two optical filter elements 2207. Each filter element 2207 is designed to be tunable over a defined portion ofthe frequency/wavelength band ofthe input optical signal from input collimator 2204. Moreover, it is noted that the use of two filter elements 2207 in the optical filter anay 2201 is merely illustrative, and is no way limiting ofthe invention. To this end, depending upon the application, as well as the tunability ofthe filter elements, more than two filter elements could be used to form the filter anay. It is further noted that the anay could have a series of rows and columns, and that a plurality of individual substrates could be stacked 'or sequentially ananged.

[00163] The use of individual substrates usefully relieves manufacturing yields since the failure of an optical filter element wastes fewer collocated filter elements. Further details ofthe fabrication of a monolithic optical filter anay having rows and columns of filter elements; and ofthe anangement of a plurality of individual optical filter arrays in a stacked anangement are described hereinabove. [00164] It is noted that angled tuned filter apparatus 2200 ofthe exemplary embodiment shown in Fig. 23 has a coarse tuning capability and a fine tuning capability. To this end, coarse tuning refers to the alignment ofthe input collimator 2204 to one ofthe filter elements 2207. This coarse tuning enables the selection of one ofthe optical filter elements 2207 from the plurality thereof, which may be tuned to reflect more than one wavelength channel. Fine tuning is illustratively carried out by rotation ofthe rotation stage so that light from the input collimator 2204 is oriented at a particular angle of incidence relative to the chosen filter element 2207. By choosing the suitable angle of incidence, the reflection ofthe particular desired wavelength channel is effected.

[00165] In operation, a microcontroller (not shown) may be used to control the rotation ofthe rotation stage 2203. Specifically, the microcontroller may access a look-up table winch stores coarse tuning and fine tuning information. To this end, the look-up table could store specific rotational positions ofthe rotation stage 2203 for each optical filter element 2207 and the conesponding wavelength channels the filters element 2207 will reflect at particular angles of incidence ofthe input collimator 204. [00166] For purposes of illustration, if the input signal from input collimator 204 includes wavelength channels 1, ..., 5 with conesponding wavelengths λ_x -λ_i , and it is desired to reflect wavelength channel 3 having center wavelength λ₃ to the first output collimator 2205, the microcontroller can look up the desired rotational position ofthe rotation stage 2203 relative to one ofthe two filter elements 2207 and command the rotation stage 2203 to move to that position. Extraction of any other wavelength chamiel having a particular center wavelength could be similarly effected. Finally, it is noted that precise rotation ofthe rotation stage to effect orientation ofthe input collimator 2204 and filter selector relative to a particular filter element 2207 of the optical filter anay 2201 may be carried out using a stepper-motor, DC motor or similar device. Moreover an encoder may be used to provide feedback. [00167] Turning to Fig. 24, a representative view ofthe frequency (F) for four filter elements 2301, 2302, 2303 and 2304. Illustratively, these filter elements are identical to those described in connection with the exemplary embodiment of Fig. 23. In this rather straight forward but illustrative example ofthe present invention, the thermal tuning range of each element 2301 - 2304 may be engineered to be slightly greater than one full channel spacing. To wit, the first filter element 2301 may be designed to reflect a first wavelength chamiel having a frequency 2305 and a second wavelength channel having a frequency 2306. Likewise, the second filter element 2302 would reflect a third frequency 2307 and a fourth frequency 2308. By design, the first frequency 2305 could be chosen to conespond to a center wavelength of a first channel, while the second frequency 2306 could be chosen to conespond to a center wavelength of a second wavelength channel. Likewise, third frequency 2307 and fourth frequency 2308 would conespond to third and fourth wavelength channels of the optical signal. Similarly, the frequencies of third filter 2303 and fourth filter 2304 would conespond to other wavelength channels.

[00168] As can be readily appreciated, by virtue ofthe thermally tuned filter elements in accordance with the present exemplary embodiment ofthe invention, the number of filter elements needed would be approximately one-half of the total number of channels. In conventional thermally tuned filters, the filters are engineered so that the full temperature tuning range is equal to the full desired tuning range. As practical temperature ranges are within approximately 50°C to approximately 100°C, this requires a large change in filter center wavelength per degree Celsius. This in turn requires exceedingly great temperature stability and resolution. [00169] However, in accordance with the present exemplary embodiment ofthe invention, with the tuning range being equal to only one channel spacing, the required resolution to tune the filter is greatly relaxed. The filter also becomes less sensitive to thermal transients and manufacturing variations. Finally, it is noted that a representative diagram ofthe frequency selectivity ofthe tunable optical filters which are shown in Fig. 24 and described in connection with thermal tuning may also be readily be ascertained for the angle tuned filter apparatus 2100 ofthe exemplary embodiment of Fig. 21. Again, advantageously, the angle tuning enables the reduction in the number of filters required to accommodate extraction of frequency/wavelength across a particular range by having tunable optical filters which can be tuned to reflect more than one frequency.

[00170] Another exemplary embodiment of the present invention is shown in Fig. 26. In this embodiment ofthe invention, the device includes an input port 2402, and an output port 2404. Input port 2402 and output port 2404 define an optical path 2405. The device further includes a monolithic filter anay 2406. The filter elements 2408- 2413 ofthe filter anay 2406 are disposed in series along the optical path. In this device, a multiplexed optical signal may encounter each filter element in sequence while propagating from the input port to the output port. Illustratively, at least one of the optical filter elements is adapted to be selectively tunable independent from the remaining optical filter elements. For example, each ofthe optical filter elements may be adapted to be selectively tunable independent from the remaining optical filter elements. As used herein, an optical filter element is selectively tunable independent from the remaining optical filter elements if the optical filter element can be tuned without affecting the tuning of any other filter element. Illustratively, at least one of the filter elements ofthe monolithic filter anay is coupled to a unique tuning device. For example, each ofthe filter elements may be coupled to a unique tuning device. As used herein, a unique tuning device is configured to substantially tune one and only one filter element. A single substrate having a MxN anay of filter elements may be used to fabricate, for example, M devices, each having a separate input port and output port with N filter elements therebetween.

[00171] In the embodiment ofthe invention of Fig. 26, the device is configured as a wavelength blocker. The input port of this device is at the input end of input collimator 2416, while the output port ofthe device is at the output end of output collimator 2418. Input and output optical fibers (2420 and 2422) are coupled to the input and output ports, hi this exemplary device, the filter elements are formed to be slightly detuned from the center wavelengths ofthe wavelength channels ofthe optical signal. For example, for a 50 GHz channel spacing, the filter elements may be detuned to be about 25 GHz more or less than the center wavelengths ofthe wavelength channels. Alternatively, the filter elements may be detuned to be entirely out ofthe wavelength band of interest. Each filter element is coupled to a unique tuning device 2424-2429. As described above, the tuning devices may be, for example, thermoelectric coolers or thin film heaters. The filter elements are aligned to reflect optical signals out ofthe optical path. In use, one or more ofthe filter elements may be tuned to the wavelength of a wavelength channel, reflecting that wavelength out ofthe optical path, thereby "blocking" that wavelength. Any non- blocked wavelengths continue along the optical path and are coupled through the output port into the output optical fiber. As the skilled artisan will appreciate, a similar device may be constructed in which the filter elements are formed to be resonant with the wavelengths ofthe wavelength channels, and tuned to become non- blocking.

[00172] By tuning the filter elements to be only partially blocking, the wavelength blocker configuration described in connection with Fig. 26 may also be used to perform dynamic spectral equalization. In this embodiment ofthe invention, at least one ofthe filter elements is adapted to be selectively tunable to variably reflect a wavelength channel. As the skilled artisan will recognize, most filters have a filter function similar to that shown in Fig. 27. The exemplary reflective filter of Fig. 27 has a center wavelength of λ_c, but is somewhat reflective over a range of wavelengths λ_c-δ to λ_c+δ. The filter has its highest reflectivity at λ_c, and becomes gradually less reflective as the wavelength moves away from λ₀. When used as a dynamic spectral equalizer, each filter element may be fabricated to be essentially non-reflective for a given wavelength channel. In use, each filter may be tuned so that it is variably reflective for its wavelength channel. For example, an optical signal may have a first wavelength channel that is 3 dB weaker than a second wavelength channel. A first filter may be tuned to allow a first wavelength channel to pass through substantially unreflected, while a second filter may be tuned to provide about 3 dB of reflection to the second wavelength channel, thereby equalizing the intensities ofthe two channels. In this embodiment ofthe invention, the filters may be designed by the skilled artisan to have a relatively wide range of tunability.

[00173] In another embodiment ofthe present invention, shown in Fig. 28, the device may be configured as a reconfigurable add-drop multiplexer. The device of Fig. 28 is similar to the device of Fig. 26, including a monolithic filter anay 2606 disposed between an input port 2602 and an output port 2604. The monolithic filter anay includes a plurality of filter elements 2608-2613, each coupled to a unique tuning device 2624-2629. In the embodiment of Fig. 28, the filter elements are aligned so that they reflect optical signals along the optical path. In this embodiment ofthe invention, the input port is one port of an input circulator 2640, and the output port is one port of an output circulator 2650. An optical signal entering input port 2602 is coupled through circulator 2640 to the input collimator 2616, and encounters filter elements 2608-2613 en route to the output collimator 2618. Light exiting the output collimator is coupled through circulator 2650 to the output port 2604. As described above in connection with Fig. 26, the filter elements may be detuned (e.g. by ± 25 GHz) from the center wavelengths ofthe wavelength channels. In use, one or more of filter elements may be tuned to the wavelength of a wavelength channel, reflecting that wavelength back along the optical path, through input collimator 2616, through circulator 2640 and out of drop port 2642, thereby dropping that wavelength. Any wavelength chamiels to be added enter the device at add port 2652 of output circulator 2650, are coupled through the output collimator to filter anay, and are reflected along the optical path by an appropriately tuned filter element. Any non-dropped wavelengths and any added wavelengths are coupled through the output collimator 2618, through circulator 2650, and exit the device at the output port 2604. [00174] As the skilled artisan will appreciate, the add-drop multiplexer ofthe present invention may be constructed using two-port collimators in place of circulator-single collimator architectures, as described hereinabove.

[00175] Another exemplary embodiment ofthe present invention is shown in Fig. 29. In this embodiment ofthe invention, the device 2800 includes an input port 2802, an output port 2804, and a monolithic filter array including a first filter element 2810 and a second filter element 2812 ananged in series in the optical path defined by the input port and the output port. Each filter element is adapted to be selectively tunable independent from the other filter element. Illustratively, the filter elements have different tuning ranges. The combined tuning range ofthe two filter elements is selected to just exceed the wavelength band of interest. In use, one ofthe filter elements is tuned to reflect the desired channel wavelength, while the other filter element is tuned to be transmissive for all chamiels (for example, by tuning the filter element to a wavelength between channels, or by tuning the filter element out ofthe wavelength band of interest.) Filter elements constructed from polymeric materials may be adapted to have wide tuning ranges (e.g. 30-40 nm), and are therefore suitable for use in this embodiment ofthe invention.

[00176] Another exemplary embodiment ofthe present invention is shown in FIG. 30. hi this embodiment ofthe invention, the monolithic filter anay 2906 includes an M x 2 anay of filter elements 2908. The combined tuning ranges ofthe filter elements are chosen to cover the wavelength band of interest. Each column of filter elements is coupled to a thermal tuning device 2910. As described above, motion ofthe filter anay relative to the collimators 2902 and 2904 is used to select a row ofthe anay. The tuning technique described with reference to Fig. 30 is used to select a channel from the tuning range ofthe filter elements in the selected row. As there may be tens of filter elements in the device of Fig. 30, an high range of tunability for each filter element is not necessary; for example, each filter element may be tunable through a range of just over one channel spacing, as described above.

[00177] It is noted that in addition to the reflective and transmissive filters referenced in conjunction with the exemplary embodiments described above, the filter elements may be tunable micro-electromechanical (MEMS) based filters. Moreover, it is noted that in the illustrative embodiments described thus far, the optical filter anays are comprised of filter elements based upon the same technology. It is noted that this is not necessarily the case, as a variety of such elements based on more than one ofthe referenced technologies may be incorporated into the same substrate to form an optical filter anay.

[00178] The invention having been described in detail in connection through a discussion of exemplary embodiments, it is clear that modifications ofthe invention will be apparent to one having ordinary skill in the art having had the benefit ofthe present disclosure. Such modifications and variations are included in the scope ofthe appended claims.

Claims

In the Claims:We Claim:

1. An optical apparatus, comprising: a glass monolithic structure which includes a plurality of optical filter elements, wherein said glass monolithic structure is not an optical fiber.

2. An optical apparatus, comprising: at least one glass monolithic structure which includes a plurality of optical filters; and at least one device which selectively aligns an optical input and an optical output to one of said plurality of optical filters.

3. An optical apparatus as recited in claim 2, further comprising: a plurality of said glass monolithic structures each of which include an MxN array of optical filter elements, and said plurality of glass monolithic structures are ananged to form a JxN anay of said optical filter elements, where J, M and N are integers.

4. An optical apparatus as recited in claim 3, wherein each of said plurality of monolithic glass structures is disposed proximate a respective collimator, pair; and each of said collimator pairs is selectively aligned by a respective one of said devices to a selected one of said optical filter elements by translational motion.

5. A method of adding/dropping an optical signal, comprising: providing at least one glass monolithic structure which includes a plurality of optical filters elements; providing at least one optical input and at least one optical output; and selectively aligning the optical input and the optical output to one of said plurality of optical filters elements.

6. A method as recited in claim 5, further comprising: a plurality of said glass monolithic structures each of which include an MxN array of optical filter elements, and said plurality of glass monolithic structures are ananged to form a JxN anay of said optical filter elements, where J, M and N are integers.

7. An optical apparatus, comprising: a bulk glass monolithic structure which includes a plurality of optical filter elements.

8. An optical apparatus as recited in claim 1 or claim 7, wherein said optical filter elements are arranged in an MxN array, where M and N are integers.

9. An optical apparatus as recited in claim 1 or claim 7, wherein the apparatus further comprises: a plurality of said glass monolithic structures, each of which has an MxN anay of said optical filter elements; and said plurality of said glass monolithic structures are ananged to form an JxN anay of said optical filter elements, where J, M and N are intergers..

10. An optical apparatus as recited in claim 8 or claim 9, wherein said optical filter elements of said MxN anay each reflect one of a plurality wavelength channels 1, ..., n.

11. An optical apparatus as recited claim 10, wherein said optical filter elements are ananged to reflect contiguous wavelength channels.

12. An optical apparatus as recited in claim 10, wherein said optical filter elements are not ananged to reflect contiguous wavelength channels.

13. An optical apparatus, comprising: at least one monolithic structure formed in a photosensitive organic medium, the monolithic structure including a plurality of optical filters; and at least one device which selectively aligns an optical input and an optical output to one of said plurality of optical filters.

14. An optical apparatus as recited in claim 2 or claim 13, wherein said device effects dimensional motion of said monolithic structure.

15. An optical apparatus as recited in claim 2 or claim 13, wherein said device effects motion of said optical input and output.

16. An optical apparatus as recited in claim 2 or claim 13, wherein said input and said output are a collimator pair.

17. An optical apparatus as recited in claim 13, wherein the photosensitive organic material is selected from the group consisting of photosensitive polymers and polymer-dispersed liquid crystals.

18. An optical apparatus as recited in claim 13, wherein the photosensitive organic material is a fluorinated polymeric material.

19. The invention as recited in any one of claims 2, 5 or 13, wherein an output collimator is selectively aligned with one of said plurality of optical filter elements to receive an optical signal which is transmitted through said optical filter element.

20. The invention as recited in claim 17, wherein said output collimator is optically coupled to an input of another optical apparatus, fonning a cascaded structure.

21. An optical apparatus, comprising: a monolithic optical filter anay which includes a first optical filter element, and a second optical filter element proximate to the first optical filter element, wherein said second optical filter element is detuned relative to said first optical filter element.

22. An optical apparatus as recited in claim 21, further comprising a third optical filter element proximate to said first optical filter element, and which is detuned relative to said first optical filter element.

23. An optical apparatus as recited in claim 21, wherein said second optical filter element is positively detuned relative to said first optical filter element.

24. An optical apparatus as recited in claim 22, wherein said third optical filter element is negatively detuned relative to said first optical filter element.

25. An optical apparatus as recited in claim 22, wherein a fourth optical filter element is disposed proximate to said first optical filter element, and said first and said fourth optical filter elements are nominal wavelength optical filter elements.

26. An optical apparatus as recited in claim 22, wherein a plurality of said first optical filter elements forms a first row, a plurality of said second optical filter elements forms a second row, and a plurality of said third optical filter elements fonns a third row.

27. An optical apparatus as recited in claim 26, wherein said monolithic optical filter array further includes a plurality of columns, and each of said columns includes one of said first optical filter elements, one of said second optical filter elements, and one of said third optical filter elements.

28. An optical apparatus as recited in claim 27, wherein each of said first optical filter elements of said rows is a nominal wavelength filter element.

29. An optical apparatus as recited in claim 27, wherein each of said second optical filter elements is a positively detuned wavelength optical filter element.

30. An optical apparatus as recited in claim 27, wherein each of said second optical filter elements of said columns is a negatively detuned wavelength optical filter element.

31. An optical apparatus, comprising: a monolithic optical filter array which includes at least one column comprising a nominal wavelength optical filter element and a detuned wavelength optical filter element; an input port proximate to said monolithic optical filter array; and a device for aligning said input port to a desired one of said optical filter elements of said monolithic optical filter array.

32. An optical apparatus as recited in claim 31, further comprising another detuned wavelength optical filter element in said at least one column.

33. An optical apparatus as recited in claim 31 or claim 32, further comprising a plurality of said columns.

34. An optical apparatus as recited in claim 32, wherein said detuned wavelength optical filter element is positively detuned, and said another detuned wavelength optical filter element is negatively detuned.

35. An optical apparatus as recited in claim 31, further comprising an output port which is also aligned to a desired one of said optical filter elements by said device.

36. A method of extracting light of a particular wavelength, comprising: providing a monolithic optical filter anay having at least one column which includes a nominal wavelength optical filter element and a detuned optical filter element; providing an input port proximate to said optical filter anay; and aligning said input port to a desired one of said optical filter elements of said monolithic optical filter anay.

37. A method as recited in claim 36, further comprising: providing another detuned wavelength optical filter element in said at least one column.

38. A method as recited in claim 36, further comprising a plurality of said columns.

39. A method as recited in claim 37, further comprising a plurality of said columns.

40. A method as recited in claim 37, wherein said detuned wavelength optical filter element is positively detuned, and said another detuned wavelength optical filter element is negatively detuned.

41. A method as recited in claim 38, wherein said monolithic optical filter array further comprises N rows and M columns, wherein one of said N rows comprises a plurality of said nominal wavelength optical filter elements.

42. A method as recited in claim 36, further comprising providing an output port proximate to said optical filter anay; and aligning said output to a desired one of said optical filter elements of said monolithic optical filter anay.

43. An optical apparatus, comprising: a monolithic optical filter anay which includes a plurality of optical filter elements wherein at least one of said plurality of optical filter elements is adapted for selective tuning at two or more wavelengths.

44. An optical apparatus as recited in claim 43, wherein said plurality of optical filters are reflective-type filters.

45. An optical apparatus as recited in claim 43, wherein said plurality of optical filters are transmissive-type filters.

46. An optical apparatus as recited in claim 43, wherein said selective tuning enables extraction of a desired wavelength from an optical signal which is comprised of a plurality of optical wavelengths.

47. An optical apparatus as recited in claim 43, wherein said selective tuning is effected by altering an angle of incidence of light upon a selected one of said optical filter elements.

48. An optical apparatus as recited in claim 43, wherein said selective tuning is effected by thermal variation of a selected one of said optical filter elements.

49. An optical apparatus as recited in claim 46, wherein said optical signal is a WDM signal, and said desired wavelength further comprises a wavelength channel.

50. An optical apparatus as recited in claim 46, wherein the optical apparatus is a dispersion compensator.

51. An optical apparatus as recited in claim 46, wherem said optical signal is an output of a laser, and said plurality of wavelengths are the output wavelengths of said laser.

52. An optical apparatus as recited in claim 43, wherein said monolithic optical filter anay further comprises an NxM array of said optical filter elements, where N and M are integers.

53. An optical apparatus as recited in claim 52, wherein N=l, and M=40.

54. An optical apparatus as recited in claim 43, wherein each ofthe plurality of optical filter elements has a unique tuning device associated therewith.

55. An optical apparatus as recited in claim 43 wherein the apparatus further comprises an input port and an output port, the input port and the output port having an optical path therebetween; and wherein the plurality of optical filter elements are ananged in series along the optical path.

56. An optical apparatus as recited in claim 55, wherein at least one ofthe plurality of optical filter elements is adapted to be selectively tunable independent from the remaining optical filter elements.

57. An optical apparatus as recited in claim 55, wherein each ofthe plurality of optical filter elements is adapted to be selectively tunable independent from the remaining optical filter elements.

58. An optical apparatus as recited in claim 55, wherein at least one ofthe optical filter elements is adapted to be selectively tunable to variably reflect a wavelength channel.

59. An optical apparatus as recited in claim 55, wherein the apparatus is configured as a device selected from the group consisting of a tunable filter, a dynamic spectral equalizer, a wavelength blocker, and a reconfigurable add/drop module.

60. An optical apparatus, comprising: a monolithic optical filter anay which includes at least one tunable optical filter element; and a tuning device which tunes said at least one tunable optical filter to extract a signal of a particular wavelength from an optical signal which includes a plurality of wavelengths.

61. An optical apparatus as recited in claim 60, wherein said monolithic filter includes an anay of said optical filter elements.

62. An optical apparatus as recited in claim 60, wherein said tuning device further comprises a coarse tuning device, and a fine tuning device.

63. An optical apparatus as recited in claim 62, wherein said coarse tuning device moves an input/output device in one dimension thereby aligning said input/output device with one of said at least one optical filter elements.

64. An optical apparatus as recited in claim 63, wherein said fine tuning device further comprises a device which alters a temperature of said one of said at least one optical filter elements so that said one optical filter element reflects light of a selected wavelength.

65. An optical apparatus as recited in claim 64, wherein said selected wavelength is a wavelength channel of a WDM optical signal.

66. An optical apparatus as recited in claim 60, wherein each of said at least one optical filter elements may be tuned by said fine tuning device to reflect one of at least two wavelengths.

67. An optical apparatus as recited in claim 60, wherein said fine tuning device further comprises an angle tuning device which selectively aligns said input/output device to receive reflected light of said selected wavelength.

68. An optical apparatus as recited in claim 60, wherein the optical apparatus is a tunable dispersion compensator.

69. An optical apparatus as recited in claim 60, wherein each ofthe plurality of optical filter elements has a unique tuning device associated therewith.

70. An optical apparatus as recited in claim 60, wherein the apparatus further comprises an input port and an output port, the input port and the output port having an optical path therebetween; and wherein the plurality of optical filter elements are ananged in series along the optical path.

71. A method for selectively extracting optical signals of particular wavelengths, comprising: providing a monolitliic optical filter array which includes at least one tunable optical filter; and tuning said at least one tunable optical filter anay to extract a signal of a particular wavelength from an optical signal which includes a plurality of wavelengths.

72. A method as recited in claim 71, wherein said tuning further comprises: aligning an input/ouput device to one of said optical filters; and tuning said one optical filter to reflect light of said particular wavelength.

73. A method as recited in claim 71 wherein said tuning further comprises coarse tuning and fine tuning.

74. A method as recited in claim 73, wherein said coarse tuning further comprises moving an input/output device in one dimension to aligning said input/output device with one of said at least one optical filter elements.

75. A method as recited in claim 73, wherein said fine tuning further comprises altering a temperature of said one of said at least one optical filter elements so that said one optical filter element reflects light of a selected wavelength.

76. A method as recited in claim 73, wherein said fine tuning further comprises angle tuning said input/output device to receive reflected light of said selected wavelength.

77. The invention as recited in any one ofthe preceding claims, wherein said optical filter elements are chosen from the group consisting of: Bragg gratings; holographic filters; and guided mode resonance filters.

78. The invention as recited in any one ofthe preceding claims, wherein said optical filter elements are interferometric optical elements.

79. The invention as recited in any one ofthe preceding claims, wherein said glass monolithic structure is a melted photosensitive glass substrate.

80. The invention as recited in claim 79, wherein said melted photosensitive glass substrate includes a germanosilicate glass.

81. The invention as recited in claim 80 wherein the germanosilicate glass comprises approximately 40 mole % to approximately 80 mole % SiO₂, approximately 2 mole % to approximately 15 mole % GeO₂, approximately 10 mole % to approximately 36 mole % B₂O₃, approximately 1 mole % to approximately 6 mole % Al₂O₃, and approximately 2 mole % to approximately 10 mole % R₂O wherein R is an alkali.

82. The invention as recited in claim 80 wherein the germanosilicate glass comprises approximately 25 weight % to approximately 45 weight % SiO₂, approximately 3 weight % to approximately 22 weight % GeO₂, approximately 7 weight % to approximately 28 weight % B₂O , approximately 6 weight % to approximately 22 weight % Al O , approximately 6 weight % to approximately 25 weight % R O wherein R is an alkali, and approximately 3 weight % to approximately 11 weight %

83. The invention as recited in claim 80 wherein said photosensitive glass substrate has a molecular hydrogen content of less than approximately 10 17 - H₂ molecules/cm .

84. The invention as recited in 79 wherein said photosensitive glass substrate has a molecular hydrogen content of greater than approximately 10¹⁷H₂ molecules/cm³ and a fluorine content of approximately 6% weight percent or less of fluorine.

85. The invention as recited in any one ofthe preceding claims, wherein the monolithic optical filter anay is fonned in an organic photosensitive material.

86. The invention as recited in claim 85 wherein the organic photosensitive material is a fluorinated polymeric material.