Optical switch using risley prisms

An optical switch using Risley prisms and rotary microactuators to independently rotate the wedge prisms of each Risley prism pair is disclosed. The optical switch comprises an array of input Risley prism pairs that selectively redirect light beams from a plurality of input ports to an array of output Risley prism pairs that similarly direct the light beams to a plurality of output ports. Each wedge prism of each Risley prism pair can be independently rotated by a variable-reluctance stepping rotary microactuator that is fabricated by a multi-layer LIGA process. Each wedge prism can be formed integral to the annular rotor of the rotary microactuator by a DXRL process.

BACKGROUND OF THE INVENTION

The optical switch of the present invention comprises Risley prisms (i.e., pairs of wedge prisms) to redirect light beams from an array of input ports to an array of output ports. The invention further comprises a stepping rotary microactuator for independent rotation of each wedge prism of a Risley prism pair to redirect the light beams.

As the demand for network capacity grows, telecommunications are being increasingly constrained by the need for more bandwidth. Optical fiber is a transmission medium capable of meeting this demand, having the potential in combination with dense wavelength division multiplexing (DWDM) to provide carrying capacity in a single fiber of hundreds of trillions of bits (terabits) per second, far greater than other means suggested for long-distance communications.

However, network transmission speeds and equipment costs are currently severely limited by the requirement of slow and complex electronic switching for signal routing—converting an optical (i.e., photonic) signal from an input optical fiber into an electronic signal, switching the lower speed electronic signal, converting the electronic signal back to an optical signal, and redirecting the optical signal through an output optical fiber. In particular, it is unlikely that such optoelectronic switching will be able to accommodate the large increase in network bandwidth that will accompany the full implementation of DWDM. To fully exploit an optical fiber's full bandwidth will likely require integrating the transmission, combination, amplification, and switching of optical signals in an all-optical network without optoelectronic switching. Furthermore, efficient switching of terabit optical signals from an input optical fiber array to an array of output optical fibers may require optical cross-connect switches with 256-input×256-output ports or more.

A number of technologies have been proposed to provide an all-optical switch for telecommunications. These include micromachined tilting mirrors, liquid crystals, bubbles, holograms, and thermo- and acousto-optics. However, none of these technologies are likely to satisfy a wide range of applications, as the requirements for optical fiber array size, scalability, switching speed, reliability, optical loss, cost, and power consumption differ greatly depending on the functionality desired.

In particular, microelectromechanical systems (MEMS) technology has been proposed for optical cross-connects whereby arrays of micromirrors are built on a silicon wafer using surface micromachining fabrication similar to that used in making integrated circuits. These MEMS optical switches use micromirrors to redirect light beams from as many as 256-input to 256-output ports. Each micromirror can be less than 1 millimeter in diameter. However, such a MEMS switch is complex. Furthermore, switching times can be slow and the long-term reliability of moveable parts is a concern. Additionally, the spatial resolution of the MEMS switch may need improvement for some applications. In a large cross-connect switch, the mirrors must be capable of a large range of angular motion, yet be able to accurately move an incident light beam through small tilt angles in order to redirect the incident light beam to a particular output optical fiber and achieve low optical throughput loss. Finally, this MEMS switch requires tightly controlled cleanroom fabrication and contaminant-free switch operation.

Particularly for cross-connect applications, there remains a need for a reliable, scalable, low loss, fast, and low cost optical switch.

SUMMARY OF THE INVENTION

According to the present invention, one or more incident light beams from the array of input ports (i.e., input optical fibers) are collimated into an associated array of input Risley prisms (hereinafter termed a Risley prism pair). By independent rotation of the first wedge prism relative to the second wedge prism of each input Risley prism pair, the light beam exiting from any input Risley prism pair can be selectively redirected to any one of an array of output Risley prism pairs. In similar fashion, the wedge prisms of each output Risley prism pair can be independently rotated to direct the light beam into an associated output port (i.e., the output optical fiber) of the optical switch.

Each wedge prism of each input and output Risley prism pair is rotated independently by a rotary microactuator. The rotary microactuator can comprise a cylindrically symmetric electromagnetic stator and an annular soft ferromagnetic rotor that can be patterned with magnetically salient, variable-reluctance pole faces suitable for small angle stepping to provide precise, independent rotation of each wedge prism of a Risley prism pair.

The rotary microactuator can be fabricated by a seven-layer LIGA process (LIGA is a German acronym that stands for lithography, electroplating, and molding), described hereinafter. The seven-layer LIGA process comprises forming sets of stator coil bottoms on a substrate, forming bond pads on the substrate, bonding stator core suspensions to the bond pads, bonding stator coil columns to the stator coil bottoms, bonding stator coil tops to the stator coil columns to form a stator assembly, and bonding a rotor assembly to the substrate within the stator assembly. The rotor assembly is fabricated by forming a torsional spring on a sacrificial substrate and bonding the annular rotor to the torsional spring.

The wedge prisms can be fabricated, integral to the annular rotor of the rotary microactuator, by a deep X-ray lithography (DXRL) process. Using batch-processing techniques, an array of such rotatable wedge prism assemblies can be fabricated on the substrate.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises an optical switch comprising two or more Risley prism pairs for selectively switching an incident light beam from any input fiber of a plurality of input optical fibers to any output fiber of a plurality of output optical fibers. The optical switch uses independent rotation of each wedge prism of input and output Risley prism pairs to redirect the incident light beam to a selected output fiber. The invention further comprises a rotary microactuator that can be formed integrally with each wedge prism for precise rotation thereof. The rotary microactuator can be fabricated by a seven-layer LIGA process. The integral wedge prism can be fabricated by a DXRL process. The optical switch using Risley prisms and a method for fabricating the optical switch and the rotatable wedge prism assembly are described below.

Optical Design of the Optical Switch

A prism can be used to change the direction of propagation of an incident beam of light. As shown inFIG. 1, a wedge prism1having an optical axis3, can be used when a relatively small deviation of an incident light beam4is desired. An exiting light beam5will emerge from the wedge prism1having been deflected from its incident direction by an angular deviation given by the angle Δ. Using Snell's Law and geometric considerations for small prism apex angles, α, the angular deviation, Δ, is given by
Δ=α(n−1)
where n is the index of refraction of the prism material relative to the surrounding medium. Furthermore, the exiting beam5can be deflected azimuthally by rotating the wedge prism1around the optical axis3by an azimuthal angle, θ, relative to the optical axis normal7. This allows the incident light beam4to be deflected anywhere upon a conical surface determined by the angles Δ and θ, with Δ further being determined by the prism apex angle between the optical surfaces of the wedge prism1.

Risley prisms comprise a pair of generally identical wedge prisms that can be independently rotated to redirect the beam in two dimensions. As shown inFIGS. 2A and 2B, when a Risley prism pair11a,11bis placed with the optical axis of the first wedge prism1parallel to or in line with the optical axis of the second wedge prism2, the total angular deviation, Φ, of the Risley prism pair11a,11bdepends on the relative orientation of the first and second wedge prisms1,2about the common optical axis3. As shown inFIG. 2A, when the first and second wedge prisms1,2of the Risley prism pair11aare oriented the same way (i.e., θ1=θ2=0°) the beam deviation of each wedge prism1and2is additive so that the total angular deviation, Φ, of the exiting beam5afrom the Risley prism pair11ais a maximum and is equal to the sum of the angular deviations of each wedge prism (i.e., Φ=2Δ). As shown inFIG. 2B, when the first and second wedge prisms1,2are counterrotated at 180° relative to one another (e.g., θ1=0° and θ2=180°) their beam deviating effects cancel and the total angular deviation of the beam5bexiting from the Risley prism pair11bis zero (i.e., Φ=0). In general, the total angular deviation Φ of the exiting beam from a Risley prism pair,11, is
Φ=2Δcos(θ1−θ2)/2
and the total azimuthal angle, Θ, of the Risley prism pair,11aor11b, is
Θ=(θ1+θ2)/2

FIG. 3shows a schematic cross-sectional view of opposing arrays11,21of Risley prism pairs configured to form an optical switch10according to the present invention. The optical switch10is designed to selectively switch light beams5from one or more input optical fibers12to one or more output optical fibers22by appropriate orientation of each input and output Risley prism pair arrays,11and21, respectively. An array of input collimating lenses13can be used to collimate the incident light beam4from each input optical fiber12into an input Risley prism pair11. Conversely, optical output lenses23focus the switched light beams6from each output Risley prism pair21into an output fiber22.

To center the cone of deflection of each Risley prism pair in the input array11about the output array21, the input collimating lenses13can be configured so that the source beams5are pre-aligned to a center detector24when the first and second wedge prisms1,2of a Risley prism pair are counterrotated by 180° (e.g., θ1=0° and θ2=180°). This is shown inFIG. 3for the light beam5bfrom the counterrotated Risley prism pair11b. The same pre-alignment can be done for the output focussing lenses23with respect to an opposing center detector14by providing backward-directed light beams through each of the output fibers22. The pre-alignment can be provided by a lens mounts15,25wherein the lenses13,23are held and arrayed as shown in FIG.3. The lens mounts15,25can also be used to align the fibers12,22to the lenses13,23.

A Risley prism pair11on the input side of the optical switch10can be oriented so that a light beam5passing therethrough is redirected to any selected optical fiber22of the switch10. When the first and second wedge prisms1,2are oriented with the optical axis normal7(i.e., θ1=θ2=0°), as shown by Risley prism pair11ainFIG. 3, the beam5ais deflected upward in the plane ofFIG. 3by the maximum angular deviation, Φ=+2Δ, to address the uppermost optical fiber22a. If the wedge prisms1,2are now counterrotated equally in opposite directions, movement about a line is generated with the light beam5being deflected up or down in the plane ofFIG. 3about a centerline13. Conversely, when both wedge prisms are rotated 180° (θ1=θ2=+/−180°), the Risley prism pair11cwill deflect the beam5cdownward with the maximum angular deviation, Φ=−2Δ, to address the lowermost optical fiber22c. The Risley prism pair in the output array21processing a particular redirected beam5is oriented oppositely to the corresponding Risley prism pair in the input array11that produced that particular redirected beam5, so as to direct the switched beam6into the output optical fiber22associated with the output Risley prism pair through which the deflected beam5passes.

The wedge prisms of an input Risley prism pair can also be counterrotated to change the total azimuthal angle of the beam (i.e., with Θ not equal to zero, to bring the beam out of the plane of FIG.3). If rotated about its optical axis3, a single wedge prism1will rotate the deflected beam5in a cone (see FIG.1). A second, identical wedge prism2of a Risley prism pair can double the angle of the beam rotation and generate a cone twice as large (see FIG.2A).

FIG. 4shows an end-on view of an optical switch10comprising an output array21having 36 output Risley prism pairs configured in a circular array according to one embodiment of the present invention. As shown inFIG. 4, if the first and second wedge prisms1,2of an input Risley prism pair of the input array11are initially oriented so that the deflected beam5is initially directed to a centerline detector24(e.g., θ1=−135 and θ2=45°), a small dashed-line circle A can be addressed by rotation of the second wedge prism2through 360° while the first wedge prism1of the input Risley prism pair is held fixed. When the second wedge prism2is aligned in the same orientation as with the first wedge prism1(e.g., θ1=θ2=−135°) a large dashed-line circle B can be addressed that is twice the diameter as the small circle A. By changing the relative orientations of the first and second wedge prisms1,2of the input Risley prism pair, the deflected beam5can be directed to access any one of the output Risley prism pairs of the circular output array21. Other, non-circular, configurations of the Risley prism pair arrays11,21are possible.

For Risley prisms of a particular aperture size, the size of the optical switch10and the distance between the input and output arrays11,21can be estimated from simple optics. For example, for Risley prism pairs having a clear aperture of 1.5 millimeter diameter, the input/output array separation can be on the order of 10 inches. For a 256-input ×256-output port circular array optical switch10with a separation between the axes of adjacent Risley prism pairs of about 2.5 mm, to provide space for the rotary microactuators, the overall radius of each array,11and21, can be about one inch. The maximum total angular deviation required of each Risley prism pair for such arrays11,21is thus 0.09 radians. With each wedge prism of the Risley prism pair having an index of refraction, n=1.5, the required prism apex angle is about 5°.

Fabrication of the Rotary Microactuator for Rotating Each Wedge Prism of a Risley Prism Pair

An example of a rotary microactuator useful for rotating each wedge prism of a Risley prism pair is now described with reference toFIGS. 5-14. In particular, three-dimensional (3D) microstructures possible with LIGA fabrication enable electromagnetic rotary microactuators capable of generating significant driving torque with low driving impedance.

FIG. 5shows a schematic cross-sectional perspective view of a rotatable wedge prism assembly for rotating a wedge prism1of a Risley prism pair that can be used in the optical switch10of the present invention. The rotatable wedge prism assembly comprises a rotary microactuator30and an integral wedge prism1. The rotary microactuator30, can be fabricated by a LIGA process. The integral wedge prism1can be fabricated by a DXRL process. The rotary microactuator30comprises a rotor assembly40and a segmented stator assembly50. The stator assembly50further comprises a plurality of electrically conducting stator coils51surrounding soft ferromagnetic stator cores55that enhance the magnetic field generated by the stator coils51in response to the electrical current flowing in the stator coils51. A soft ferromagnetic annular rotor41is rotatable in response to the magnetic field produced by the electrical currents provided to the stator coils51. The rotor41and stator cores55have magnetically salient, variable-reluctance pole faces42,56, respectively, that define a working gap and enable small angle stepping of the rotor41for precise rotation and control of the integral wedge prism1. The opposing pole faces42,56can comprise a plurality of axially aligned teeth to provide the magnetic saliency. This magnetic saliency presents a variable reluctance to the magnetic circuit created when the stator coils are electrically excited that depends on the positioning of the rotor and stator teeth. Stepping of the rotor41is thereby driven by a tangential force generated by the phased magnetic field between the pole faces42,56, with the magnetic field tending to align the rotor with the stator teeth therein to minimize magnetic circuit reluctance.

A variety of means, such as gears, bearings, springs, or other guides can be used to rotatably support the rotor41within the stator assembly50. However, because the optical switch10can require switching frequencies of tens of kHz or more, minimization of sliding and rolling contacts is preferred for reliable, repetitive operation. A restoring rotor torsional spring43, as shown inFIG. 5, can enable rotation of the rotor41without sliding or rolling contact, while inhibiting radial and axial misalignment of the rotor41within the stator assembly50. Wobble of the rotor41is further inhibited by the axial variable reluctance of the annular rotor41within the stator pole faces56.

The rotary microactuator30shown schematically inFIG. 5can be fabricated by a LIGA process as described hereinafter. LIGA is particularly well suited for the present invention because of its ability to produce precision, large aspect ratio microstructures. LIGA combines DXRL with thick film deposition, typically by electroplating, to produce prismatic geometries with several hundred microns feature heights and with a lateral run-out of generally less than 0.1 micron per 100 micron feature thickness. Microstructures comprising polymers, glasses, metals, ceramics, and composites can be fabricated by LIGA. Furthermore, modern batch processing enables the parallel fabrication of multiple, complex LIGA microstructures on a common substrate.

With batch LIGA processing, arrays of rotary microactuators30can be fabricated to independently rotate the wedge prisms1,2of each Risley prism pair in the input and output arrays11,21of the optical switch10. A seven-layer LIGA process is described below for fabricating a six-pole, three-phase variable-reluctance stepping rotary microactuator30. With LIGA, the rotary micro-actuator30can have a working gap of 5 microns, a diameter at the working gap58of 1.75 millimeters, and a pole face42,56height of 250 microns. As indicated above, rotary microactuators of other dimensions are readily obtained with LIGA. A wedge prism1can also be integrally fabricated by a DXRL process. Although fabrication of a single rotary microactuator30with an integral wedge prism1will be described, those skilled in the art will understand that an array of rotatable wedge prism assemblies, as shown inFIG. 5, can be similarly formed on a common substrate by modern batch processing.

The first layer of the seven-layer rotary microactuator fabrication process comprises forming the bottoms52of a plurality of square cross-section helical stator coil windings51on an electrically insulating substrate31. The coil winding bottoms52can be formed by conventional LIGA processes, whereby a substrate is coated with a thick photoresist, the photoresist is selectively exposed to collimated radiation through a patterning mask, the exposed polymer is removed by a suitable developer to produce a mold, a complimentary microstructure is formed by electroplating of the structural material in the void spaces of the mold, and the mold is removed by stripping.

FIG. 6shows a set of six coil bottoms52for a six-pole stator as formed on the substrate31. The substrate31can be a flat, precision substrate of a structural material that can withstand the process temperatures (<500° C.). Furthermore, the substrate31can be an optically transparent material, such as glass, silicon, or ceramic composite, to allow the light beam to pass through the annulus32of the rotary microactuator30that defines the optical aperture of the integral wedge prism1. Alternatively, the substrate31can have a shaped opening (not shown) therethrough to provide the optical aperture. The substrate31can be blanket coated with a thin, electrically conducting precursor layer (e.g., 500 angstroms of copper) to facilitate subsequent build-up of the stator coil bottoms52and bond pads57by electroplating.

InFIG. 6, the stator coil bottoms52are fabricated by patterning an electrically conductive layer, such as copper, on the substrate31. For a 25-turn stator coil51, the copper coils can have a square cross-section of about 25 microns on a side. The substrate31can be initially coated with a thick photoresist (not shown), such as polymethylmethacrylate (PMMA). The photoresist can be lithographically exposed through a patterning mask to produce a latent image of the stator coil bottoms in the photoresist. Standard UV lithography can be used to pattern the first layer, since the stator coil bottoms52can be only about 25 microns thick. The photoresist is developed to remove the exposed photoresist corresponding to the latent image and thereby produce a mold, into which the coil material (e.g., copper) can be electroplated. The upper surface of the electroplated coil bottoms can be planarized, by precision diamond lapping, to achieve the substrate parallelism necessary to maintain actuator tolerances and the surface planarity necessary for subsequent good diffusion bonding of the stator coil columns53to the stator coil bottoms52. The mold photoresist is then removed to yield a set of six-pole stator coil bottoms52on the substrate31, as shown in FIG.6.

InFIG. 7, a second layer for building up the structure of the rotary microactuator30defines a plurality of bond pads57at each end of the stator coil bottoms52. In addition to providing bonding sights for each stator core suspension55(see FIG.8), the bond pads57provide attachment sites for the rotor bond extensions44(see FIG.11).FIG. 7shows the twelve paired bond pads57for the six stator core suspensions55, for the six-pole stator50, formed on the substrate31. The bond pads57can be of the same material (e.g., PERMALLOY) as the stator core suspensions55. Since the bond pads57provide risers for the core suspensions55, the bond pad layer should preferably have a thickness sufficient (e.g., 40 microns) to provide electrical isolation of about 15 microns of the stator core suspensions55from the stator coil bottoms52. The bond pads57can be formed on the substrate31by a LIGA process, as described above. The bond pad material can be electroplated through the developed openings in a patterned photoresist layer (not shown). After deposition of the bond pad material, the bond pads57can be planarized for subsequent bonding of the stator coil columns53and the rotor bond extensions44to the bond pads57. After planarization, the photoresist layer can be removed. The precursor layer can then be selectively removed by etching to avoid inter-coil electrical shorting of the stator coils51and shorting of the stator to coils51to the stator cores55and rotor41.

Subsequent coil and core layers can be built up from the stator coil bottoms52and bond pads57by conventional multi-layer processes, such as additive deposition or electroplating processes, or by diffusion bonding of separately fabricated subassemblies or subassembly arrays. A wafer-scale micromachine assembly method using diffusion bonding is disclosed, for example, in U.S. patent application Ser. No. 09/761,359 to Christenson, which is incorporated herein by reference. The assembly method disclosed by Christenson has been shown to produce integrated multi-layer micromachines by aligned layer-to-layer diffusion bonding of micromachine subassemblies at temperatures that are substantially less than one-half the melting temperature of the bonded materials. The method of Christenson which is applicable to the present invention comprises forming a first micromachine subassembly having a planar mating surface on a foundation substrate (e.g., by a LIGA process); forming a second micromachine subassembly having a planar mating surface on a substrate having a sacrificial surface release layer (i.e., a sacrificial substrate); aligning the foundation and sacrificial substrates with the mating surfaces of the micromachine subassemblies facing; diffusion bonding the mating surfaces together; and releasing the sacrificial substrate, thereby exposing the released surface of the second micromachine subassembly. The planarized mating surfaces are typically prepared immediately prior to bonding by a plasma cleaning and ammonium hydroxide treatment, in the case of nickel for example, to remove the surface oxide. This layer-to-layer method can be repeated as many times a needed to build up the structure of the multi-layer micromachine. Precision alignment tolerances of below one micron have been achieved with this method. The excellent bond strength and electrical conductivity resulting from the low-temperature bonding process is due to the high purity and small grain size of the electroplated material and the high flatness and low surface roughness (i.e., better than 1 microinch) achieved by modern planarization techniques, such as precision diamond lapping. Thus, the low-temperature diffusion bonding method can enable batch assembly of an array of rotary microactuators30with submicron intercomponent tolerances.

InFIG. 8, a third layer is used to build up a plurality of stator core suspensions55attached to the paired bond pads57and suspended above the stator coil bottoms52. As described above, the stator core suspensions55and bond pads57can be a soft (high permeability) ferromagnetic material, such as electroplated PERMALLOY (78/22 Ni—Fe) or an electroplated nickel-iron-cobalt alloy. The stator core suspensions55can have a cross-section of about 250 microns on a side. The stator core suspensions55can be patterned to form magnetically salient, variable-reluctance pole faces56. The stator pole face geometry can further comprise axial square teeth with a tooth pitch, for example, of about 10 microns.

InFIG. 8, the core suspensions55can be formed on a core sacrificial substrate (not shown) and diffusion bonded to the bond pads57ofFIG. 7according to the layer-to-layer method describe above. The core sacrificial substrate can be a precision substrate (e.g., silicon or alumina) having an electrically conductive sacrificial surface release layer, such as 0.5-1 micron thickness of titanium. The core suspensions55are formed by coating the core sacrificial substrate with a photoresist, lithographically exposing the photoresist (e.g., PMMA) through a patterning mask, developing of the photoresist to form a mold, and electroplating of the core material into the photoresist mold. DXRL is preferred for patterning of the photoresist to form the stator core suspensions55because of the greater thickness (e.g., 250 microns) of the stator core suspensions55and the precise tolerances desirable for the stator-rotor working gap and the stator pole face geometry. The exposed mating surface of the stator core suspensions55is then planarized, for subsequent bonding of the stator core suspensions55to the bond pads57, and the photoresist is removed to yield the stator core suspensions55on the core sacrificial substrate.

The substrate31and core sacrificial substrate can be inverted and aligned with the aid of precision gauge pins press fit into one of the substrates with the opposing substrate having complimentary matching holes. The substrates can then be press-fit together, and the stator core suspensions55can be diffusion bonded to the bond pads57. For Ni and Ni—Fe cores, the diffusion bonding can be done at about 475° C. and 5000 psi for 1 hour in a vacuum hot press. The core sacrificial substrate can then be released from the stator core suspensions55by removing the release layer (e.g., titanium) with a suitable etchant (e.g., hydrofluoric acid and/or hydrogen peroxide) to yield an array of stator core suspensions55bonded to the bond pads57on the substrate31as shown in FIG.8.

InFIG. 9, a fourth layer used to build up the structure of the rotatable wedge prism assembly comprises a plurality of stator coil columns53. The stator coil columns53can have a height of 280 microns to provide an electrical isolation of 15 microns of the stator coil tops54from the stator core suspensions55. The stator coil columns material can be the same material as that used for the stator coil bottoms52(e.g., electroplated copper). The stator coil columns53can be formed on a coil columns sacrificial substrate (not shown) and then diffusion bonded to the stator coil bottoms52by a layer-to-layer method as described above. Prior to bonding, the stator coil columns53are planarized, the mating surfaces are prepared as described previously, and the coil columns sacrificial substrate is aligned to the substrate31using gauge pins. Because of the relatively large height of the stator coil columns53(e.g., 280 microns) and the need for precise definition of the inter-coil spacings, DXRL is preferred for patterning of the stator coil columns53. For copper stator coils51, the stator coil columns53can be diffusion bonded to the stator coil bottoms52at about 400° C. and 5000 psi for one hour in a vacuum hot press. The coil columns sacrificial substrate can then be released from the stator coil columns layer, yielding an array of stator coil columns53bonded to the stator coil bottoms52on the substrate31.

The stator assembly50, shown inFIG. 9, is completed by bonding a fifth layer, comprising the tops54of the stator coils51, onto the stator coil columns53. The stator coil tops54are formed on a stator coil tops sacrificial substrate (not shown) by a LIGA process, as described previously. The mating surfaces are planarized and prepared, the coil tops sacrificial substrate is inverted and aligned to the substrate31, the stator coil tops54are diffusion bonded to the stator coil columns53, and the coil tops sacrificial substrate is released from the stator coil tops54to yield the completed stator assembly50on the substrate31, as shown in FIG.9.

The rotor assembly40, shown inFIG. 11, can be fabricated by two-layer LIGA process on a spring sacrificial substrate45. The rotor assembly40comprises a soft ferromagnetic annular rotor41that is rotatably attached to a plurality of interleaved rotor torsional springs43that are in turn attached to rotor bond extensions44which can be diffusion bonded to the bond pads57on the substrate31. The rotor torsional springs43allow for rotation of the rotor41within the stator assembly50.

The rotor torsional springs43are shown in greater detail in FIG.10. For this example of the rotary microactuator30of the present invention, the spring constant about the rotational axis can be about 2 Newtons per meter (N/m) to enable the rotor41to rotate through +/−180°. The spring material preferably has a yield strength near 1 Gigapascal (Gpa) and can be electroplated Ni/Fe, Ni/P, or Ni/Co. Each rotor torsional spring43can have a spring flexural element with a rectangular cross section having a height (e.g., 250 microns) much greater than its width (e.g., 20 microns). The high aspect ratio of the flexural element enables azimuthal rotation of the rotor41within the stator assembly50, yet has sufficient stiffness in the axial direction to inhibit axial misalignment of the rotor41and stator50during prism1rotation. Additionally, the variable reluctance forces of the magnetic field acts to stiffen the spring response in the axial direction and thereby maintain rotor and stator axial alignment. The rotor torsional spring43can be formed on the spring sacrificial substrate45by the LIGA process previously described. DXRL patterning of a PMMA photoresist can be used to obtain a high aspect ratio of the rotor torsional springs43.

Referring toFIG. 11, the soft ferromagnetic annular rotor41and rotor bond extensions44can be formed on a rotor sacrificial substrate (not shown) by a LIGA process. The rotor material can be a soft ferromagnetic material, such as electroplated PERMALLOY. The rotor cross-section can have a width of115microns. Because the height of the rotor pole face42can preferably match that of the stator pole face56(i.e., 250 microns), DXRL can be used to pattern the photoresist used to form the rotor41. The rotor41is further patterned with a magnetically salient, variable-reluctance pole face42suitable for small angle stepping. The rotor pole face42can comprise axial teeth that compliment the stator pole face geometry. Stepping in increments of 0.25° can be achieved for the three-phase stator, as shown inFIG. 9, using a rotor tooth pitch of 10 microns. After the mating surfaces of the rotor41, rotor bond extensions44, and rotor torsional springs43are planarized and prepared, as described previously, and the rotor and spring sacrificial substrates aligned, the rotor41and rotor bond extensions44can be diffusion bonded to the rotor torsional springs43. The rotor sacrificial substrate can then be released, yielding the rotor assembly40on the spring sacrificial substrate45, as shown in FIG.11.

Finally, the completed rotor assembly40can be diffusion bonded to the stator assembly50, as shown in FIG.12. The mating surfaces of the rotor bond extension44and the corresponding bond pads57are prepared and the spring sacrificial substrate45is aligned to the substrate31. After diffusion bonding of the rotor bond extensions44to the bond pads57, the rotor spring sacrificial substrate is released to yield the completed rotary microactuator30on the substrate31. Because of the alignment accuracy enabled by DXRL and multi-layer diffusion bonding fabrication, the stator-rotor working gap can be about 5 microns or less. As discussed above, an array of such rotary microactuators30can be batch fabricated on the substrate31.

The stepping rotary microactuator30is energized by connecting the stator coils51to a three-phase power supply (not shown). The required drive electronics can be standard three-phase variable reluctance or brushless DC type circuitry, as is known in the art. Rotor rotation is produced by sequential phased excitations of the stator coils51. Stator poles belonging to one-phase are located opposite each other to drive the rotor41symmetrically. With a stator coil winding excitation at 0.5 volts and 5 milliamperes, a working magnetic gap flux density of 3000 gauss and a tangential rotor force per phase of about 5 mN can be generated with the rotary microactuator30of the present invention.

An even higher drive torque and greater efficiency are possible with a hybrid stepping actuator having a permanent magnet in the rotor41or stator55. This can be done by a DXRL process as described in copending U.S. patent application Ser. No. 09/452,321 to Christenson, which is incorporated herein by reference.

An alternative to a magnetic drive is to use an electrostatic stepping microactuator and drive the microactuator electrostatically with a high voltage. An example of an electrostatic stepping microactuator fabricated by LIGA processes is disclosed byWallrabe, et al., “Design rules and test of electrostatic micromotors made by the LIGA process” in J. Micromech. Microeng. 4 (1994). Because the variable-reluctance stepping rotary microactuator30of the present invention can be formed using electromagnetic stator coils to drive the rotor41rather than electrostatic forces, much lower voltages can be used as compared to typical electrostatic microactuators. A drive voltage of 200 volts or more may be required for an equivalent electrostatic drive with a 5 micron rotor-stator gap.

Fabrication of the Integral Wedge Prism

An integral wedge prism1of a Risley prism pair can fabricated in situ within the rotary microactuator30to complete the fabrication of a rotatable wedge prism assembly, as shown in FIG.5. The wedge prism1can be fabricated by lithographically patterning the optical surface of the wedge prism1in a photoresist, as disclosed in copending U.S. patent application Ser. No. 09/742,778 to Sweatt and Christenson, which is incorporated herein by reference. Formation of the wedge prism1comprises coating a substrate with a photoresist (e.g., PMMA); positioning a patterning mask above the photoresist; exposing the photoresist to a collimated beam of radiation through the mask to code a latent profile defining the optical surface in the photoresist; and developing the photoresist to produce the wedge prism I on the substrate. With DXRL, wedge prisms1having structural heights in excess of one millimeter, RMS optical surface smoothness better than 10 nanometers, and dimensional tolerances of less than 0.1 micron can be produced.

FIG. 13shows an example of a particular exposure geometry whereby a wedge prism1can be fabricated by DXRL integral to the rotor41within the rotary microactuator30. If a transparent substrate31is used, the rotor annulus32(i.e., the inner diameter of the rotor41) can be filled with a photoresist prism material. The photoresist prism material can be an injection-molded polymer or an in-situ polymerized casting resin, such as PMMA. Alternatively, the substrate31can have circular openings (not shown) to define the optical apertures therethrough and the openings can be plugged prior to formation of the wedge prism1. The angled optical surface34of the wedge prism1can be produced by DXRL. This can be done by exposing the photoresist prism material at the prism apex angle, α, to a collimated beam of high-energy x-rays35through a patterning mask36. If desired, second masked exposure (not shown) can then be made parallel to the optical axis to better define the outer circumference of the wedge prism1. The exposed photoresist is then removed by a suitable developer. As shown inFIG. 13, this process yields a rotatable wedge prism assembly comprising a wedge prism1mounted with the rotor41of the rotary microactuator30. As indicated inFIG. 13, an array of wedge prisms1can be batch fabricated by the DXRL process using an appropriate patterning mask36.

Alternatively, one or more wedge prisms1can be separately fabricated by DXRL, or other process for fabricating micro-optical elements, and subsequently inserted into the rotor annulus32and affixed thereto. Alternative embodiments of the present invention can produce prisms1comprising other optical materials, such as glass. As disclosed in the copending U.S. patent application Ser. No. 09/742,778 to Sweatt and Christenson, a metal mold for forming the prism array can be produced by LIGA. Glass, or other suitable optical material, can be introduced into the mold by melting, sol-gel processing, or other suitable method. The exposed surface of the molded optical material can be polished to form the angled optical surface34, the wedge prism1aligned and bonded to the annular rotor41, and the mold removed. Again, arrays of such wedge prisms1can be fabricated by suitable batch processing methods.

Assembly of the Optical Switch

The optical switch10, as shown in the example ofFIG. 3, can be used to switch light beams from a plurality of input ports to a plurality of output ports. The optical switch10comprises an array of input Risley prism pairs11for redirecting the light beams5from the plurality of input ports to an array of output Risley prism pairs21that similarly direct the switched light beams6to the plurality of output ports. The arrays of input and output Risley prism pairs11,21each comprise an array of first wedge prisms1paired with an array of associated second wedge prisms2, as shown schematically in FIG.3. The first and second wedge prism arrays can be fabricated on separate substrates31, according to the LIGA and DXRL processes described above, with reference toFIGS. 5-13.

The input port can comprise a collimating lens array13, or other means known to those in the art, for collimating the incident light beams4from a plurality of input optical fibers12into the array of input Risley prism pairs11. Similarly, the output port can comprise a focussing lens array23, or other means known to those in the art, for focussing the switched light beams6from the output Risley prism pairs21into an array of output optical fibers22. The input and output ports can be fabricated according to methods known to those in the art. The entire optical switch10ofFIG. 3can be enclosed within a hermetic package or other container with fiber optic connectors at each end for attaching the optical fibers12,22.

It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.