# Patent

## Title
Bragg grating elements for optical devices

## Abstract

Disclosed herein is a method for manufacturing three-dimensional Bragg grating elements. The method for manufacturing three-dimensional Bragg gratings may include forming a first Bragg grating element using a pair of recording beams. The method may also include using a single recording beam to replicate the first Bragg grating element to form a second Bragg grating element.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of U.S. patent application Ser. No. 11/407,742, filed Apr. 19, 2006, which is a division of U.S. patent application Ser. No. 10/390,521, filed Mar. 17, 2003, now U.S. Pat. No. 7,125,632, which claims benefit under 35 U.S.C. §119(e) of provisional U.S. patent application No. 60/365,032, filed Mar. 15, 2002.


BACKGROUND
Light wavelength selectivity of thick periodic structures was, historically, studied first in x-ray diffraction on crystalline solids. It was recognized that such selectivity arises due to the coherent addition of the light energy diffracted by individual layers forming precisely spaced stacks, such as that of the atomic layers of a crystalline lattice. The name of phenomenon, “Bragg diffraction,” was given in recognition of the studies of it performed by Bragg.
Later, largely the same behavior was observed during the diffraction of light at optical wavelengths on the acoustic waves of the appropriate frequencies created inside optically transparent solid media. Acoustic waves create a periodic modulation of the index of refraction of a dielectric material via perturbation of its density. As a result, an acoustic wave can be used to manipulate light based on its wavelength. Thus, it functions as a wavelength filter.
Acoustic perturbation, however, is of a temporal nature, and relaxes completely after its source is extinguished and with it disappears the filter. Long-lasting Bragg gratings were first utilized, perhaps, with the invention of full-color holography. It employed relatively thick films of dichromatic gelatins (DCG) for holographic recording of color-realistic images of 3-D objects by using lasers of different colors. Subsequent reconstruction of images with conventional white light sources became possible due to the wavelength selectivity property of volume Bragg gratings. However, to the inventors' knowledge, even though the wavelength selectivity of the volume Bragg gratings was the underlying mechanism that enabled white-light reconstruction of thick-layer DCG display holograms, their utility for separating, combining, or otherwise manipulating specific wavelengths of light with the intention of achieving practical device functionality has not been recognized.
Use of volume Bragg gratings (VBG) recorded in doped lithium niobate photorefractive crystals for filtering light at optical wavelengths was adopted in construction of solar and lidar filters used to isolate light at a particular wavelength from the broad band background. A principal issue, however, is that recording of such filters must be performed at the same wavelength at which the filter will subsequently operate. As a result, the use of these filters is limited to a very limited range of wavelengths where sufficiently powerful lasers exist. Furthermore, the list of appropriate recording materials is confined to two or three narrow classes of photorefractive materials, which often have physical properties that are unsuitable for their intended mode of operation. For example, no material is known to the inventors that would allow construction of practical functional fiber-optic devices that would utilize volume Bragg grating filters recorded at wavelengths in the range of about 800-1650 nm.
This drawback can be partially overcome in photorefractive lithium niobate crystals when a VBG filter is recorded through a different surface than that used for its operation. By using this approach, filters can be constructed in lithium niobate that can operate at wavelengths that are useful for practical photonic devices, such as, for example, fiber-optic devices. Nonetheless, this approach is still rather limited due to a number of factors. First, the usable wavelength range is limited to λop>n*λrec on the one side, and the near infrared absorption edge of the lithium niobate on the other. Also, for practical devices, the bandwidth of the filter Δλ is limited by the maximum refractive index modulation achievable in that material (or its dynamic range, Δn): Δλ<(λop)*Δn/2n. This factor substantially limits the usefulness of this type of filter. This approach also requires the use of at least two (and typically four) polished surfaces that are orthogonal to each other, which increases the complexity of the filter manufacturing process and its cost. Additionally, the wavelength of the filter is substantially fixed to the value determined by the angle between the recording beams in the holographic setup. As a result, the wavelength must be controlled precisely for any practical device and is, therefore, unique for a particular wavelength or information-carrying “channel” of light, which complicates the issues in manufacturing of these elements.
SUMMARY
A method for manufacturing a three-dimensional holographic element may include placing a virgin recording wafer directly behind a recorded master hologram, and directing a reference beam onto the master hologram such that a replica of the master hologram is recorded on the virgin wafer. The reference beam may cause a transmitted reference wave and a reconstructed object wave to be formed. The transmitted reference wave and the reconstructed object wave may interfere behind the master hologram to cause the replica of the master hologram to be recorded on the virgin wafer. The reference beam may be directed onto the master hologram in the same fashion as during recording of the master. The replica of the master hologram may be recorded through a first side of the holographic element, and read from a second side of the holographic element that is different from the first side.
The virgin recording wafer may be a three-dimensional bulk of sensitized silica glass. The master hologram may be a three-dimensional Bragg grating, which may be recorded holographically in a bulk of photorefractive material. The master hologram may be formed by a surface profile on a transparent substrate, which may include a glass, a polymer, or a crystal, for example. The master hologram may be formed in a bulk of photosensitive glass, photorefractive crystal, dichromated gelatin, or polymer, for example.
A method for manufacturing a three-dimensional Bragg grating element may include forming a first Bragg grating element using a pair of recording beams, the first Bragg grating element having a three-dimensional Bragg grating recorded therein, and using a single recording beam to replicate the first Bragg grating element to form a second Bragg grating element having recorded therein a replica of the Bragg grating recorded in the first Bragg grating element. A virgin recording medium may be placed directly behind the first Bragg grating element, and the single recording beam may be used to replicate the first Bragg grating to form the second Bragg grating in the virgin recording medium, thereby forming the second Bragg grating element. The single recording beam may interfere with a reconstructed object wave behind the first Brag grating to form the replica in the virgin recording medium.
Forming the first Bragg grating element may include directing at least one of the pair of recording beams onto a first recording medium. During replication, the single recording beam may be directed onto the first Bragg grating element in the same fashion as the at least one of the pair of recording beams was directed onto the first Bragg grating element during the formation of the first Bragg grating element.



BRIEF DESCRIPTION OF THE DRAWINGS
Certain preferred embodiments of the invention will now be described in detail with reference to the figures. Those skilled in the art will appreciate that the description given herein with respect to the figures is for exemplary purposes only and is not intended in any way to limit the scope of the invention.
FIGS. 1A and 1B depict reflective and transmissive VBGs, respectively.
FIG. 2 demonstrates the transparency property of a VBG.
FIG. 3 is a schematic of a device according to the invention for combining a plurality of optical inputs into a single optical fiber output.
FIG. 4 is a perspective view of a preferred embodiment of a device according to the invention.
FIG. 5 depicts the interior of a device such as shown in FIG. 4.
FIG. 6 is a perspective view of another preferred embodiment of a device according to the invention.
FIG. 7 is a schematic diagram of a device such as shown in FIG. 6.
FIG. 8 is a perspective view of another preferred embodiment of a device according to the invention.
FIG. 9 is a schematic diagram of a DWDM multi-source combiner according to the invention.
FIG. 10 is a schematic diagram of an optical add-drop multiplexer according to the invention.
FIG. 11 is a schematic diagram of a multi-channel wavelength monitor according to the invention.
FIG. 12 depicts chain cascading of a plurality of VBGs in a fiber optic device according to the invention.
FIG. 13 depicts lamination cascading of a plurality of VBGs in a fiber optic device according to the invention.
FIG. 14 depicts a multiple path device according to the invention.
FIG. 15 depicts a method according to the invention for fabricating VBGs.
FIGS. 16A-C depict another method according to the invention for fabricating VBGs.
FIG. 17 depicts yet another method according to the invention for fabricating VBGs.
FIG. 18 depicts an integrated VBG “chip” according to the invention.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Using Sensitized Silica Glasses for Manufacturing of VBG Filters
One of the major problems in developing and using any kind of permanent VBG filters for practical applications has been the unavailability of a material or a class of materials possessing physical properties that are adequate for the practical applications. For example, the photorefractive electro-optic crystals, in which much of the research was conducted on the subject of VBGs, among other problems, are incapable of providing truly permanent, stable recording across a wide temperature range. Furthermore, these crystals are strongly anisotropic, which limits their usage substantially. For these reasons, an entire range of applications of VBG filters in general has not been substantially explored. In fact, to the inventors' knowledge, there is not a single photonic device now in the market that uses VBG elements.
According to the invention, a previously unexplored class of materials, the silicate photorefractive glasses (PRG), can be used to enable the design and manufacturing of practical devices based on VBGs, with special emphasis on photonic devices for fiber-optic applications. This type of materials substantially overcomes all of the above-mentioned drawbacks of the previously studied materials and possesses all the required properties to manufacture devices for demanding applications exemplified by the fiber optics. These properties include, but are not limited to, the following: a) optical transparency in the entire optical window from UV to mid-infrared; b) outstanding longevity of the recorded gratings; c) outstanding thermal stability (>200 C); d) adequate dynamic range; e) excellent optical quality, including the achievable polishing quality of the elements made of this material; d) low manufacturing costs; e) ability to be formed and processed in the adequate shapes and sizes (e.g., flat disks or wafers); f) refractive index isotropy. Compositions and processes for manufacturing such PRGs are described in U.S. Pat. No. 4,057,408, the disclosure of which is hereby incorporated herein by reference in its entirety.
Manufacturing of VBG Elements in Silica Glasses by Recording Holographically at a Specific Wavelength and Using them in Fiber-Optic Devices at an Arbitrary Wavelength
As described in the literature on the theory of Bragg diffraction in thick holograms (see, e.g., Kogelnik, H., “Coupled wave theory for thick hologram gratings,” The Bell System Technical Journal, November 1969, 48(9), 2909-2947), there are two basic types of the VBGs—transmission and reflection, which are different in their mode of operation (see Kogelnik FIG. 4).
FIG. 1A depicts a reflective VBG 102 having a grating wave vector, A, in the horizontal direction as shown. An input light beam 104 composed of light of a plurality of wavelengths λ1, . . . λN is directed toward the VBG 102 at a first angle α to the input face 102A of the VBG 102. The VBG 102 is formed such that it is transparent to all but one of the wavelengths λ1, . . . λN. That is, the light beam propagates through the grating relatively unaffected, except that the light having a certain wavelength, λ1, is filtered out of the beam. As a result, only that light 106 having wavelengths λ2, . . . λN continues through the VBG 102 and exits the VBG 102 at a second angle β to the output face 102B of the VBG 102. Preferably, the VBG 102 is fabricated so that the angle β at which the beam exits the VBG 102 is as near as possible to the angle α at which it entered the VBG 102 (i.e., the beam continues along in a generally straight line). Light 108 having wavelength λ1, however, is reflected back at an angle γ from the input face 102A of the VBG 102 because of the holography within the VBG 102. That is, the VBG 102 is fabricated such that the index of refraction varies within the VBG 102 to allow light having wavelengths λ2, . . . λN to continue through the VBG 102, and light having wavelength λ1 to be reflected back at a known angle. Methods for fabricating such VBGs are discussed in detail below.
FIG. 1B depicts a transmissive VBG 112 having a grating wave vector, A, in the vertical direction as shown. An input light beam 114 composed of light of a plurality of wavelengths λ1, . . . λN is directed toward the VBG 112 at a first angle α to the input face 112A of the VBG 112. The VBG 112 is formed such that it is transparent to all but one of the wavelengths. That is, the light beam propagates through the grating relatively unaffected, except that the light having a certain wavelength, λ1, is filtered out of the beam. As a result, only that light 116 having wavelengths λ2, . . . λN continues through the VBG 112 and exits the VBG 112 at a second angle β to the output face 112B of the VBG 112. Preferably, the VBG 112 is fabricated so that the angle β at which the beam 116 exits the VBG 112 is as near as possible to the angle α at which it entered the VBG 112 (i.e., it continues along in a generally straight line). Light 118 having wavelength λ1, however, exits the VBG 112 at a third angle γ to the output face 112B because of the holography within the VBG 112. That is, the VBG 112 is fabricated such that the index of refraction varies within the VBG 112 to allow light having wavelengths λ2, . . . λN to continue relatively straight through the VBG 112, and light having wavelength λ1 to be deflected as it passes through the VBG 112 such that it exits the VBG 112 at a known angle β to the output face.
Wavelength filtering properties of transmission and reflection VBGs are different primarily in the width of the filter that can be constructed in an element of practical size. Generally, reflection thick volume holograms have very narrow wavelength bandwidth, with the upper limit determined by the dynamic range of the material, as described above in connection with the example of lithium niobate VBG filters. Conversely, transmission thick volume holograms generally have wider bandwidth, which, historically, has precluded their use for the generation of white light color display holograms.
Nonetheless, when recorded in a sufficiently thick slab of a transparent material (e.g., >1 mm), a method can be devised to record transmission VBGs that can achieve bandwidths sufficiently narrow for practical photonic devices (e.g., bandwidth of 30 nm or less).
Another principal difference between reflection VBGs and transmission VBGs is that the transmission type allows tuning of the central wavelength of the filter by adjusting the incident angle of light upon the VBG. For that reason, a VBG filter can be recorded at one wavelength (e.g., in the UV range where silicate PRGs are sensitive) and operate at another (e.g., in the 850 nm to 1650 nm range typically employed in various fiber-optic devices). This can be achieved without the limitations of recording through an orthogonal side of the element, described above for the case of the lithium niobate VBG filters. This means that: a) the range of the usable wavelengths is practically unlimited; b) wider bandwidths are readily available; c) there is no need for polishing additional surfaces.
The use of permanent transmission VBGs as band-pass filters for manipulation of wavelengths in photonic devices, exemplified by the fiber-optic active and passive components, has not been explored so far probably for one or more of the following reasons: a) strong anisotropy of the material (e.g., inorganic electro-optic photorefractive crystals); b) impossible to manufacture in sufficiently thick layers (>1 mm, e.g., DCG); c) impossible to achieve sufficient optical quality of the bulk material and/or polishing quality of the surfaces (e.g., photo-polymers); d) insufficient temperature stability.
FIG. 2 demonstrates the transparency property of a VBG 200 in which an input light beam 202 composed of light of a plurality of wavelengths λ1, . . . λN is directed toward the VBG 200, through a lens 204, along an optical axis, x, of the device. As shown, the input light beam 202 can be emitted from an optical fiber 210. The VBG element 200 is fabricated such that the index of refraction varies within the VBG 200 to allow light 206 having wavelengths λ3, . . . λN to continue relatively straight through the VBG 200, through a lens 208, and into a receiver 212, which can be another output optical fiber, for example, as shown. Light 214 having wavelength λ1, however, is reflected back at a first angle α from the input face 200A of the VBG 200. Similarly, light 216 having wavelength λ2 is reflected back at a second angle β from the input face 200A of the VBG 200 because of the holography within the VBG 200.
FIGS. 3-5 depict a preferred embodiment of a fiber optic device 300 according to the invention for combining a plurality of optical fiber inputs 311-314 into a single optical fiber output 310. As shown, the device 300 includes four optical inputs 311-314, which can be optical fibers, for example. Each optical input 311-314 carries light 301-304 of a different wavelength λ1-λ4. The device 300 also includes three VBG elements 330, 332, 334. Light 301 from the first input 311, having wavelength λ1, is transmitted into the interior of the device 300, where it is deflected via a first deflector 320 (such as a mirror, for example) such that it enters the first VBG element 330 at a known angle. As shown, the light travels along the optical axis of the device, and enters the VBG 330 at an angle of 90° to the input face of the VBG 330. The first VBG 330 is transparent to light having wavelength λ1, so the light having wavelength λ1 exits the first VBG 330 at an angle of 90° with the output face of the VBG 330.
Light 302 from the second input 312, having wavelength λ2, is transmitted into the interior of the device 300, where it is deflected via a second deflector 322 such that it enters the first VBG element 330 at a known angle. The first VBG 330 deflects the light having wavelength λ2 such that the light having wavelength λ2 exits the first VBG 330 at an angle of 90° with the output face of the VBG 330 and, therefore, is combined with the light having wavelength λ1.
Similarly, light 303 from the third input 313, having wavelength λ3, is transmitted into the interior of the device 300, where it is deflected via a third deflector 324 such that it enters the second VBG element 332 at a known angle. The second VBG 332 deflects the light having wavelength λ3 such that the light having wavelength λ3 exits the second VBG 332 at an angle of 90° with the output face of the second VBG 332. The second VBG 332 is transparent to light having wavelength λ1 or λ2. Consequently, the light having wavelength λ3 is combined with the light having wavelength λ1 and λ2.
Similarly, light 304 from the fourth input 314, having wavelength λ4, is transmitted into the interior of the device 300, where it is deflected via a fourth deflector 326 such that it enters the third VBG element 334 at a known angle. The third VBG 334 deflects the light having wavelength λ4 such that the light having wavelength λ4 exits the third VBG 334 at an angle of 90° with the output face of the third VBG 334. The third VBG 334 is transparent to light having wavelength λ1, λ2, or λ3. Consequently, the light having wavelength λ4 is combined with the light having wavelength λ1, λ2, and λ3.
Thus, an output light beam 306 composed of light have wavelengths λ1, λ2, λ3, and λ4 can be formed using a plurality of VBG elements. The output light beam 306 is received by an optical receiver 310, such as an optical fiber. It should be understood that, by reversing the direction of the light flow, a device as shown in FIG. 5 can be used to generate a plurality of output light beams, each having a known wavelength, from an input light beam composed of light having a plurality wavelengths.
FIGS. 6 and 7 depict a preferred embodiment of a triplexer bi-directional transmitter/receiver 600 according to the invention. As shown, the device 600 includes an optical input 613, two optical outputs 611, 612, and a bi-directional optical carrier 614, each of which can be an optical fiber, for example. The bi-directional carrier 614 carries light 604 having wavelengths λ1 . . . λ3 as shown. The first output 611 carries light 601 of wavelength λ1 to a first receiver 621. The second output 612 carries light 602 of wavelength λ2 to a second receiver 622. The optical input 613 carries light 603 of wavelength λ3 from a source 623.
The VBG 610 is fabricated such that it is transparent to light having wavelength λ3, which is transmitted to the VBG 610 via the optical input 613. The VBG 610 can also be fabricated such that it deflects light 601 having wavelength λ1 and light 602 having wavelength λ2. The light 601 having wavelength λ1 can be received by a first optical receiver 621, and the light having wavelength λ2 can be received by a second optical receiver 622. The bi-directional carrier 614 carries light 604 having wavelength λ1 and wavelength λ2 in a first direction (toward the VBG) and wavelength λ3 in a second direction (away from the VBG).
FIG. 8 depicts a preferred embodiment of a Xenpak form-factor CWDM transmitter 800 according to the invention. As shown, the device 800 includes an optical input 811 and four optical outputs 812-815, each of which can be an optical fiber, for example. The optical input carries light having wavelengths λ1 . . . λ4. The first output 811 carries light of wavelength λ1; the second output 812 carries light of wavelength λ2; the third output 813 carries light of wavelength λ3; and the fourth output 814 carries light of wavelength λ4.
FIG. 9 is a schematic diagram of a DWDM multi-source combiner 900 according to the invention. As shown, the multi-source combiner 900 includes four optical inputs 911-914, such as optical fibers, for example. Each optical input 911-914 carries light of a different wavelength λ1, . . . λ4 as shown. The device 900 also includes two VBG elements 932, 934. Light 901 from the first input 911, having wavelength λ1, is transmitted, preferably through a lens 921, such that it enters the first VBG element 932 at a first known angle α. The first VBG 932 is fabricated such that the light 901 having wavelength λ1 is deflected from the first VBG 932 along the optical axis x of the device 900. Similarly, light 902 from the second input 912, having wavelength λ2, is transmitted, preferably through a lens 922, such that it enters the first VBG element 932 at a second known angle β. The first VBG 932 is fabricated such that the light 902 having wavelength λ2 is also deflected from the first VBG 932 along the optical axis x of the device 900.
Light 903 from the third input 913, having wavelength λ3, is transmitted, preferably through a lens 923, such that it enters the second VBG element 934 at a third known angle γ. The second VBG 934 is fabricated such that the light 903 having wavelength λ3 is deflected from the VBG 934 along the optical axis x of the device 900. Similarly, light 904 from the fourth input 914, having wavelength λ4, is transmitted, preferably through a lens 924, such that it enters the second VBG element 934 at a fourth known angle δ. The second VBG 924 is fabricated such that the light 904 having wavelength λ4 is also deflected from the second VBG 924 along the optical axis x of the device 900. The first VBG 932 is transparent to light having wavelength λ3 and λ4. Thus, light beams having respective wavelengths λ1, λ2, λ3, and λ4 can be combined into a single optical beam 905, which can then be transmitted, preferably through a lens 925, to an optical receiver 915, such as an optical fiber, for example.
FIG. 10 is a schematic diagram of a free-space optical add-drop multiplexer (OADM) 1000 according to the invention. As shown, the OADM 1000 includes an optical input 1011, such as an optical fiber, for example, that carries light 1001 having wavelengths λ1, . . . λN as shown. As shown in FIG. 10, the OADM 1000 includes a VBG element 1020 that is fabricated to reflect light 1002 having wavelength λ1. The VBG 1020 is transparent to light having wavelengths λ2, . . . λN. The light beam 1001 from the first input 1011 is incident onto the VBG 1020 at a first angle α to a first face 1020A of the VBG element 1020. Consequently, a light beam 1002 having wavelength λ1 is deflected at a second angle β from the face 1020A of the VBG element 1020. The light beam 1002 having wavelength λ1 is thus “dropped” from the input signal, and can be directed to an optical receiver 1012, such as another optical fiber, for example.
The OADM 1000 also includes an additional input 1013, which can be an optical fiber, for example, that carries a light beam 1003 having wavelength λN+1. The light beam 1003 having wavelength λN+1 is incident onto the VBG 1020 at an angle γ to a second face 1020B of the VBG 1020. The VBG element 1020 is fabricated to reflect light having wavelength λN+1 from the second face 1020B such that the light 1003 from the additional input 1013 is combined with the light from the first input to form an output light beam 1004 having wavelengths λ2, . . . λN+1. The output light beam 1004 can be directed to an optical receiver 1014, such as another optical fiber, for example.
FIG. 11 is a schematic diagram of a multi-channel wavelength monitor 1100 according to the invention. As shown, the multi-channel wavelength monitor 1100 includes an optical input 1115 that carries light 1005 having wavelengths λ1, . . . λN. The monitor 1100 also includes two VBG elements 1132, 1134. The input light beam 1105 is transmitted, preferably through a lens 1125, such that it enters the first VBG element 1132 at a first known angle (preferably, along the optical axis x of the device 1100, that is, 90° to the face 1132A of the first VBG 1132). The first VBG 1132 is fabricated such that the light 1101 having wavelength λ1+Δ is deflected from the first VBG 1132 at a first angle α, and light 1102 having wavelength λ1−Δ is deflected from the first VBG 1132 at a second angle β. The first VBG 1132 is transparent to the rest of the wavelengths in the input beam 1105. Light 1101 having wavelength λ1+Δ may be received by an optical receiver 1111, and light 1102 may be received by an optical receiver 1112.
The light beam is then transmitted to the second VBG element 1134, which is fabricated such that the light 1103 having wavelength λ2+Δ is deflected from the second VBG 1134 at a first angle γ, and light 1104 having wavelength λ2−Δ is deflected from the second VBG 1134 at a second angle δ. Light 1103 having wavelength λ2+Δ may be received by an optical receiver 1113, and light 1104 having wavelength λ2−Δ may be received by an optical receiver 1114. The second VBG 1134 is transparent to the rest of the wavelengths in the beam. The output beam 1106 can then be received, preferably through a lens 1126, by an optical receiver 1116, which can be another optical fiber, for example.
Methods for Packaging Devices with Large Channel Counts Using VBG Filters
One of the main advantages of VBG filters and, indeed, their unique property is the ability to record multiple filters sharing the same volume of the material. This allows for the fabrication of devices of very small size and unique functionality. Nevertheless, the number of gratings that can share the same volume, known as the multiplexing number, or the M/#, in the holographic memory field, is limited by the dynamic range of the material. For that reason, for practical materials suitable for manufacturing of VBG filters, that number will typically be rather limited (a realistic estimate is around 4 filters for a 4 mm thick element). Furthermore, fabrication of VBG filters with a larger number of gratings becomes progressively more complex, while at the same time reducing the flexibility in packaging them in a device. In addition, when sharing the same volume, the combined effect of the VBGs can be obtained via the coherent addition of the effects of the individual gratings, which results in the appearance of cross-terms, leading sometimes to undesirable side effects. It is, therefore, desirable to have a practical method for manufacturing devices with sufficiently large channel count. According to one aspect of the invention, fiber optic devices can be fabricated which can have a basically unlimited number of channels while using very simple VBG elements as building blocks.
FIG. 12 depicts a device according to the invention in which any number of transmissive VBG filters can be combined to construct a device with an arbitrary channel count and for an arbitrary set of wavelengths. Preferably, the VBG filters are identical, thereby reducing the cost of fabrication. In this approach, the individual VBG elements are positioned on the main optical axis of the device and their tilt angles are adjusted individually in order to tune it to the peak wavelength of the desired channel. This approach may be referred to as “chain cascading.”
As shown in FIG. 12, an optical input 1214 carries light 1204 having wavelengths λ1, . . . λN. Light 1204 is incident on a first VBG element 1232 along the optical axis x of the device. The first VBG 1232 is fabricated such that the light 1201 having wavelength λ1 is deflected from the first VBG 1232 at a first angle α to the exit face 1232B of the VBG 1232. As shown, the VBG 1232 is positioned such that its grating vector A and exit face 1232B are perpendicular to the optical axis x of the device. Thus, the light 1201 having wavelength λ1 is deflected from the first VBG 1232 at an angle 90-α to the optical axis x of the device. The device may include a first optical receiver 1211 that receives the deflected beam 1201. The first VBG 1232 is transparent to the rest of the wavelengths λ2, . . . λN in the input beam 1204, such that a transmitted beam 1205 having wavelengths λ2, . . . λN is transmitted through the VBG 1232 along the optical axis x.
The transmitted beam 1205 is incident on a second VBG element 1234. The second VBG 1234 may be fabricated, like the first VBG 1234, such that light having wavelength λ1 would be deflected from the second VBG 1234 at a first angle α to the exit face 1234B of the VBG 1234. As shown, the VBG 1234 is positioned such that its grating vector A and exit face 1234B are at a known angle θ (>90°) to the optical axis x of the device. Light 1202 having wavelength λ2 is deflected from the second VBG 1234 at a known angle β to the exit face 1234B of the VBG 1234 (and, therefore, at a known angle to the optical axis x). The device may include a second optical receiver 1212 that receives the deflected beam 1202. The second VBG 1234 is transparent to the rest of the wavelengths λ3, . . . λN in the transmitted beam 1205, such that a second transmitted beam 1206 having wavelengths λ3, . . . λN is transmitted through the VBG 1234 along the optical axis x.
The transmitted beam 1206 is incident on a third VBG element 1236. The third VBG 1236 may be fabricated, like the first VBG 1232, such that light having wavelength λ1 would be deflected from the third VBG 1236 at a first angle α to the exit face 1236B of the VBG 1236. As shown, the VBG 1236 is positioned such that its grating vector A and exit face 1236B are at a known angle φ (<90°) to the optical axis x of the device. Light 1203 having wavelength λ3 is deflected from the third VBG 1236 at a known angle γ to the exit face 1236B of the VBG 1236 (and, therefore, at a known angle to the optical axis x). The device may include a third optical receiver 1213 that receives the deflected beam 1203. The third VBG 1236 is transparent to the rest of the wavelengths λ4, . . . λN in the transmitted beam 1206, such that a third transmitted beam 1207 having wavelengths λ4, . . . λN is transmitted through the VBG 1236 along the optical axis x. The device may include a fourth optical receiver 1215 that receives the transmitted beam 1207.
FIG. 13 depicts a device according to the invention that includes a complex VBG filter element that has been fabricated from a number of simple, possibly identical, VBG elements. In function it is similar to the device described above in connection with FIG. 12 but instead of being positioned and adjusted individually in the package, the elements can be properly positioned in a suitable fixture in direct physical contact with one another and then permanently bonded together, using suitable bonding materials that are well known in the art, thus creating a single compounded element with complex functionality.
The positioning of the individual VBG elements with respect to one another in such an arrangement can be important to the usefulness of the assembly. Methods of exercising such control can include: a) proper surface preparation of the wafers of the recording material, such as polishing, parallelism of the surfaces etc.; b) proper rotational orientation of the elements with respect to each other during the bonding procedure; c) use of calibrated spacers to the adjust relative angle between the individual VBG elements; d) precise control of the tilt angle of the wafer with respect to the recording laser beams during the holographic recording process.
This approach, referred to as “lamination cascading,” enables the achievement of the same density of the grating packing in the same package volume as the direct multiplexing of the filters during the recording process, but without the need of multiple exposures and without the physical overlap, and thus interference, of the individual filters in the bulk of the material.
As shown in FIG. 13, an optical input 1314 carries light 1304 having wavelengths λ1, . . . λN. Light 1304 is incident on a series of VBG elements 1332-1334 along the optical axis x of the device. The VBG elements 1332-1334 are fabricated such that light 1301 having wavelength λ1 is deflected from the third VBG 1334 at a first angle α to the exit face 1334B of the VBG 1334, light 1302 having wavelength λ2 is deflected from the third VBG 1334 at a second angle β to the exit face 1334B of the VBG 1334, and light 1303 having wavelength λ3 is deflected from the third VBG 1334 at a third angle γ to the exit face 1334B of the VBG 1334. The device may include a first optical receiver 1311 that receives the deflected beam 1301, a second optical receiver 1312 that receives the deflected beam 1302, and a third optical receiver 1313 that receives the deflected beam 1303. The VBGs 1332-1334 may be transparent to the rest of the wavelengths λ4, . . . λN in the input beam 1304, such that a transmitted beam 1305 having wavelengths λ4, . . . λN is transmitted through the VBGs 1332-1334 along the optical axis x. The device may include a fourth optical receiver 1314 that receives the transmitted beam 1304.
FIG. 14 depicts a device according to the invention in which any simple, individual VBG element can be used for processing several wavelength channels by allowing multiple paths through it in different directions. In its simplest form, the so-called “double pass configuration” functions as follows:
A series of simple individual VBG elements is positioned in line as described above in connection with FIG. 12. A mirror is placed at the end of the chain of the elements, which reflects the transmitted light back onto the same elements. This has the effect of folding the chain of the elements back onto itself. The mirror angle is adjusted slightly, so that the angle of the back-reflected light is somewhat different than the forward-propagating light. This angle is adjusted in such a way as to tune the center wavelength of the VBG filters to the desired value.
In such an implementation, the method allows using each of the VBG elements more than once, thus effectively increasing the number of filters without increasing the number of VBG elements, and thereby enabling the overall size of the package to remain practically the same. Multiple path folding is also possible if an additional mirror is used in the beginning of the chain of the VBG elements, slightly offset from the axis in angle and space.
As shown in FIG. 14, an optical input 1415 carries light 1405 having wavelengths λ1, . . . λN. Light 1405 is incident on a first VBG element 1432 along the optical axis x of the device. The first VBG 1432 is fabricated such that light 1401 having wavelength λ1 is deflected from the first VBG 1432 at an angle α to the exit face 1432B of the first VBG 1432. The first VBG 1432 is transparent to the rest of the wavelengths λ2, . . . λN in the input beam 1405, such that a transmitted beam 1406 having wavelengths λ2, . . . λN is transmitted through the VBG 1432 along the optical axis x.
The transmitted beam 1406 is incident on a second VBG element 1434. The second VBG 1434 is fabricated such that light 1402 having wavelength λ2 is deflected from the second VBG 1434 at an angle β to the exit face 1434B of the second VBG 1434. The second VBG 1434 is transparent to the rest of the wavelengths λ3, λN in the transmitted beam 1406, such that a transmitted beam 1407 having wavelengths λ3, λN is transmitted through the VBG 1434 along the optical axis x.
The transmitted beam 1407 is directed toward a mirror 1420, which is disposed at an angle φ to the optical axis x of the device. The reflected beam 1408 is incident on the second VBG 1434 at an angle φ to the exit face 1434B. The second VBG 1434 is fabricated such that light 1403 having wavelength λ3 is deflected from the second VBG 1434 at an angle γ to the entrance face 1434A of the second VBG 1434. The second VBG 1434 is transparent to the rest of the wavelengths λ4, . . . λN in the reflected beam 1408, such that a reflected beam 1409 having wavelengths λ4, . . . λN is transmitted through the VBG 1434.
The reflected beam 1409 is incident on the first VBG 1432 at an angle φ to the exit face 1432B. The first VBG 1434 is fabricated such that light 1404 having wavelength λ4 is deflected from the first VBG 1432 at an angle δ to the entrance face 1432A of the first VBG 1432. The first VBG 1432 is transparent to the rest of the wavelengths λ5, . . . λN in the reflected beam 1409, such that a reflected beam 1410 having wavelengths λ5, λN is transmitted through the VBG 1432.
The device may include a first optical receiver that receives the deflected beam 1401, a second optical receiver that receives the deflected beam 1402, and a third optical receiver that receives the deflected beam 1403, and a fourth optical receiver that receives the deflected beam 1404. The device may also include a fifth optical receiver that receives the reflected beam 1410.
It should be understood that any of the techniques described above can be optimized to take maximum advantage of VBG properties such as: transparency to all but one wavelength, angular tunability, functionality distributed over the volume of a thick material, material rigidity and dimensional stability, excellent polishing qualities, and the like.
Methods for Economically Manufacturing VBG Elements
In the manufacturing of VBG elements, it is typically desirable to minimize the costs of production of such elements. For that reason, holographic recording of each element individually is likely to be cost-prohibitive for most or all of the high-volume applications. A number of methods according to the invention for cost-effective production of such elements will now be described.
A first such method, depicted in FIG. 15, exploits the unique property of a hologram, whereupon each fractional piece of the recorded hologram possesses full and complete information about the recorded object. When applied to the VBG filters recorded on the PRG plates, it means that each piece of such plate, or wafer, should have the same filtering properties as the wafer in whole. For that reason, a large-size wafer 1500 can be diced, using a suitable cutting device, such as a saw, for example, into a large number of relatively small individual VBG elements 1502, each with complete filter functionality. In following this process, one could significantly reduce the number of recording and testing operations, thereby reducing the manufacturing costs of the VBG elements.
A second cost-reduction method according to the invention applies to the repetitive fabrication of the filter with identical properties. Such an approach is particularly suitable for high-volume production environments. In such circumstances reproduction of a filter with a complex shape, which may require, for example, multiple exposure steps to achieve the complete control over its spectral shape, may result in a prohibitively long and complex manufacturing operations. However, since holography allows true and complete reconstruction of the recorded wavefront, it is, therefore, possible to record a hologram of the reconstructed wavefront, rather than the true original, to achieve the same result.
This approach includes: a) placing a “virgin” recording wafer directly behind a recorded “master” hologram; and b) directing the reference beam onto the master hologram in exactly the same fashion as during the recording of the master. The transmitted reference wave and the reconstructed object wave interfere again behind the master hologram. Consequently, a new hologram is recorded on the virgin wafer, which is an exact replica of the master.
The advantages of this method include but are not limited to the following: a) better stability (not sensitive to the phase fluctuations); b) simpler setup (no filter shape control required); c) no polishing on the virgin wafer is required, if it is placed in direct contact with the master and an index matching fluid is used on the interface; and d) shorter cycle times (higher throughput).
Methods to Control the Filter Response Function
When used in practical applications such as in fiber-optic devices, for example, the spectral shape of a filter can be used to manipulate the wavelengths of light in a desired fashion. The filter shape can determine such device parameters as adjacent channel isolation, cross-talk, suppression ratio, etc. The ability to control the spectral shape of the VBG filters, therefore, can make the difference between a practically usable device and a practically useless one.
As follows from the general theory of Bragg diffraction (see Kogelnik), the spectral shape of the filter created by a VBG is related via a Fourier transform to the amplitude and phase envelope of the VBG along the general direction of propagation of the affected light wave. It is, therefore, desirable to be able to control both in order to create a filter with a desired spectral shape.
A method according to the invention for controlling the spectral shape of a VBG filter relates to the use of the Fourier transform property of a lens and the phase capturing ability of the holographic recording method. As depicted in FIG. 16A, a method 1600 for creating a VBG filter with any desired spectral shape can be performed as follows. A mask 1602 representing the desired filter shape is placed in the front focal plane of a lens 1604 situated in the object arm of the holographic recording setup. The recording media sample 1606 (e.g., a glass wafer) is placed in the back focal plane of the same lens. The plane-wavefront reference beam of the holographic recording setup overlaps with the object beam on the sample, subtending it at an angle required by the target operational wavelength of the VBG filter being recorded.
When positioned as described, the lens creates a true Fourier transform of the mask directly on the recording medium. Via a coherent interference with the plane reference wave, both the amplitude and the phase of the Fourier transform are transferred to the amplitude and phase envelope of the VBG imprinted on the recording material. When reconstructed, or “read,” with a light beam nearly normal to the recorded grating planes, the spectral response of the VBG filter thus recorded will take the shape of the masks placed in the front focal plane of the lens.
This method allows for a single exposure recording of a filter with practically arbitrary complexity of the shape of the spectral response function and is referred to as the “parallel method” or the “holographic filter imprinting method.”
A holographic filter imprinting method according to the invention can be similarly applied to the task of shaping the filter response function of transmission VBG filters. It may be accomplished by choosing a proper orientation of the apodizing mask relative to the direction of the grating planes and, similarly, by choosing the proper entrance and exit faces on the VBG element.
Exemplary methods asks for creating transmissive and reflective VBGs are depicted in FIGS. 16B and 16C respectively. As shown, a mask 1612 having a slit 1614 can be placed in the front focal plane of the lens. Light shone through the mask will generate a square wave 1616. As the light passes though the lens, the Fourier transform 1618 of the square wave will be imprinted on the sample 1620. When reconstructed, the spectral response of the pattern 1622 recorded on the VBG filter 1620 will take the shape of a square wave 1624. Depending on the orientation of the mask relative to the grating plane, the VBG can be made reflective (as shown in FIG. 16B) or transmissive (as shown in FIG. 16C).
Method for Controlling the Shape of Transmissive VBG Filters
Furthermore, when dealing with VBG filters functioning in the transmission geometry, a different approach can be taken in order to manipulate the spectral shape of the filter. In this case, the method, which is depicted in FIG. 17, comprises multiple, sequential exposures of the same volume of the recording material. Each exposure would produce a simple plane VBG, but after recording multiple gratings a filter of an arbitrary shape will be constructed via coherent addition of the recorded VBGs. Volume Bragg gratings are physical representations of sinusoidal waves, and, therefore, their coherent sum is a Fourier transform of an envelope function.
For that reason, a close representation of an arbitrary amplitude and phase envelope function can be constructed via a series of holographic exposures, provided appropriate control is exercised over both the amplitude and the relative phase of the gratings recorded in such series of exposures. Such control can be achieved via employing techniques for active measurement and stabilization of the phase of the recorded VBGs.
As shown in FIG. 17, a virgin sample 1702 is subjected to a first pair of incident beams 1712 and 1714. Beam 1712 is incident on the entrance face 1702A of the virgin sample 1702 at an angle α relative to the entrance face 1702A (and, as shown, relative to the grating vector A). Beam 1714 is incident on the entrance face 1702A of the virgin sample 1702 at an angle β relative to the entrance face 1702A (and, as shown, relative to the grating vector A). Thus, a first holographic sample 1704 is formed having a first holographic image 1722.
The first holographic sample 1704 is then subjected to a second pair of incident beams 1716 and 1718. Beam 1716 is incident on the entrance face 1704A of the first holographic sample 1704 at an angle γ relative to the entrance face 1704A (and, as shown, relative to the grating vector A). Beam 1718 is incident on the entrance face 1704A of the first holographic sample 1704 at an angle δ relative to the entrance face 1704A (and, as shown, relative to the grating vector A). Thus, a second holographic sample 1706 is formed having a second holographic image 1724.
VBG Chip
FIG. 18 depicts an integrated VBG “chip” according to the invention. As shown, the VBG chip 1800 is a monolithic glass structure into which a plurality of holographic images or “gratings” have been recorded. An optical input 1815 carries light 1805 having wavelengths λ1, . . . λN. Light 1805, which may be collimated, is incident on a first grating 1822. Grating 1822 is recorded such that light 1801 having wavelength λ1 is deflected at an angle such that it is received by grating 1832. Grating 1832 is recorded such that it deflects the light 1801 out of the chip 1800 toward an optical receiver 1811. Grating 1822 is transparent to the rest of the wavelengths λ2, . . . λN in the input beam 1805, such that a transmitted beam 1806 having wavelengths λ2, . . . λN is transmitted through the grating 1822.
The transmitted beam 1806 is incident on grating 1824, which is recorded such that light 1802 having wavelength λ2 is deflected at an angle such that it is received by grating 1834. Grating 1834 is recorded such that it deflects the light 1802 out of the chip 1800 toward an optical receiver 1812. Grating 1824 is transparent to the rest of the wavelengths λ3, . . . λN in the beam 1806, such that a transmitted beam 1807 having wavelengths λ3, . . . λN is transmitted through the grating 1824.
Similarly, the transmitted beam 1807 is incident on grating 1826, which is recorded such that light 1803 having wavelength λ3 is deflected at an angle such that it is received by grating 1836. Grating 1836 is recorded such that it deflects the light 1803 out of the chip 1800 toward an optical receiver 1813. Grating 1826 is transparent to the rest of the wavelengths λ4, . . . λN in the beam 1807, such that a transmitted beam 1808 having wavelengths λ4, . . . λN is transmitted through the grating 1826.
The transmitted beam 1808 is incident on grating 1828, which is recorded such that light 1804 having wavelength λ4 is deflected at an angle such that it is received by grating 1838. Grating 1838 is recorded such that it deflects the light 1804 out of the chip 1800 toward an optical receiver 1814. Grating 1828 is transparent to the rest of the wavelengths λ5, . . . λN in the beam 1808, such that a transmitted beam 1809 having wavelengths λ5, . . . λN is transmitted through the grating 1828. The transmitted beam 1809 is directed toward an optical receiver 1815. As shown, each of the optical receivers 1811-1814 and 1816 can be an optical fiber, for example. Any or all of the optical receivers 1811-1814 and 1816 can be bundled together to form an optical fiber ribbon, for example.
A VBG chip as shown can be made according to the following method. One or more incident beams are directed toward a first location of a virgin sample (to form grating 1822, for example). Then, the beams are turned off, and either the sample or the source of illumination is positioned (e.g., the sample may be moved laterally and/or rotationally as necessary) such that the incident beam(s) may now be directed toward a second location on the sample (to form grating 1824, for example). This process is repeated until all desired gratings have been recorded.
Thus, there have been described fiber optic devices comprising volume Bragg gratings and methods for fabricating the same. Those skilled in the art will appreciate that numerous changes and modifications can be made to the preferred embodiments of the invention, and that such changes and modifications can be made without departing from the spirit of the invention. Examples of devices that can be made in accordance with the invention include, without limitation, 1×N laser source combiners, multi-channel transmit/receive modules (including triplexers), optical add-drop multiplexers, terminal multiplexers, network monitors, wavelength lockers, tunable filters, tunable gain equalizers, dispersion compensators, and the like.

## Claims

1. A method for manufacturing a three-dimensional Bragg grating element, the method comprising:
forming a first Bragg grating element using a pair of recording beams, the first Bragg grating element having a three-dimensional Bragg grating recorded therein; and
using a single recording beam to replicate the first Bragg grating element to form a second Bragg grating element having recorded therein a replica of the Bragg grating recorded in the first Bragg grating element.

2. The method of claim 1, further comprising:
placing a virgin recording medium directly behind the first Bragg grating element; and
using the single recording beam to replicate the first Bragg grating to form the second Bragg grating in the virgin recording medium, thereby forming the second Bragg grating element.

3. The method of claim 1, wherein the single recording beam interferes with a reconstructed object wave behind the first Brag grating to form the replica in the virgin recording medium.

4. The method of claim 1, wherein forming the first Bragg grating element comprises directing at least one of the pair of recording beams onto a first recording medium, the method further comprising:
directing the single recording beam onto the first Bragg grating element in the same fashion as the at least one of the pair of recording beams was directed onto the first Bragg grating element during the formation of the first Bragg grating element.