WO2001029599A1

WO2001029599A1 - Mems based 1xn fiber optics switch

Info

Publication number: WO2001029599A1
Application number: PCT/US2000/028402
Authority: WO
Original assignee: Laor, Herzel
Priority date: 1999-10-15
Filing date: 2000-10-13
Publication date: 2001-04-26
Also published as: AU1210701A; WO2001029584A2; AU1203201A; WO2001029584A3

Abstract

A fiber optic 1xN switch (200) is constructed using a MEMS mirror (204) to accomplish switching. The fiber ends are magnified onto the mirror (204) to reduce optical power concentration at the mirror. The switch (200) has low sensitivity to polarization, due to near normal reflectance at the mirror (204). In addition, the enlarged beam diameter at the mirror surface and the correspondingly enlarged mirror also relaxes manufacturing tolerances to provide a more practical design.

Description

MEMS Based 1xN Fiber Optics Switch

Field of the Invention The present invention relates in general to 1xN optical switches and, in particular, to 1xN switch designs employing dispersive or magnifying optics for reduced optical density at a switch mirror surface.

Background of the Invention Optical 1xN switches are used in a variety of applications including for example, display devices, communications networks and digital signaling. As used herein, 1xN switches include switches where an optical signal transmitted from a first fiber can be controllably directed to one or more output ports such as output fibers. Notably, such switches include simple on/off switches as well as 1x2 switches, each of which have a broad range of applications. Moreover, 1xN switches include dual 1xN switches, a species of 2x2N switches where the two input fibers are switched in concert, and other multiples of 1xN switches. Specific implementations of such switches are discussed in more detail below.

Conventionally, optical 1xN switches have been implemented by moving the input and/or output fibers to accomplish the switching function or by moving a conventional mirror to selectively direct the optical signal from the input fiber to one or more output fibers. However, in either case, the response time of the switch is limited due to the need to move elements that have significant mass. Also, construction of such switches is difficult due to the precise tolerances required in order to provide for proper alignment of the transmitted optical signal relative to the output fiber(s). In this regard, it is noted that the cores of single mode optical fibers typically have a diameter of about 10 microns.

Microelectromechanical Systems (MEMS) technology provides the potential for construction of 1xN switches having a faster response time.

MEMS technology involves the use of small mechanical devices fabricated on a Silicon or other substrate together with electronic circuitry for actuating motion of the mechanical device. In the context of 1xN switching, the use of MEMS technology has been proposed for construction of a moveable mirror for executing the switching function. Because of the small size and mass of MEMS mirror designs, it is anticipated that faster 1xN switches may be achieved. Existing MEMS mirrors range in size from 10 microns to 100 microns. An example of such a MEMS mirror is the Digital Light Processing (DLP) chip manufactured by Texas Instruments, which may include up to one million mirrors. These chips are commonly used in display devices. Electrostatic forces actuate movement of each mirror.

Attempts at using MEMS technology for fiber optics switching are exemplified by U.S. Patent No. 5,199,088 by Magel. That patent discloses a switch including a Spatial Light Modulator implemented using MEMS mirror technology. An optical signal from an input fiber is directed onto the mirror such that the beam incident on the mirror has the same diameter and, therefore, optical density as the beam has upon exiting the input fiber. Such construction is problematic in the context of modern communications system equipment. Specifically, today's optical transmission equipment may utilize optical signals having a power of up to one watt with a core diameter of only approximately 10 microns. Thus, designs such as Magel's result in a high concentration of optical energy on the mirror, e.g., on the order of 1 megawatt per square centimeter (1 MW/cm²), which may damage the mirror. In addition, the Magel design utilizes grazing incidence mirrors. Such mirrors have a reflectance that is dependent on the polarization of the optical signals, which may be problematic in fiber communication equipment. Moreover, the Magel design involves dimensioning and tolerances that may be difficult to achieve in practical manufacturing environments.

Summary of the Invention

The present invention is directed to an optical 1xN switch design where in the input optical signal is controllably dispersed relative to a MEMS mirror utilized to implement the switching function. In this manner, the optical density of the signal incident on the MEMS mirror can be significantly reduced to accommodate the high power optical signals of modern fiber communication equipment with reduced risk of mirror damage, thereby potentially extending mirror life. The increased signal diameter also relieves the required manufacturing tolerances to provide for more practical designs. In accordance with one aspect of the present invention, focusing optics are used to provide a magnified signal diameter at the MEMS mirror surface. Focusing optics include lenses or the like configured to form a converging output beam; that is, an output beam contained within an optical envelope that progressively narrows as the signal travels away from the optics. An associated optical switch includes an input optical fiber, at least one output port, a MEMS mirror and focusing optics disposed between an end of the input fiber and the MEMS mirror, where the fiber end, focusing optics and MEMS mirror are configured such that the signal as incident on the MEMS mirror is magnified relative to the signal as emitted from the input fiber end. It will be appreciated that the MEMS mirror is suitably dimensioned to receive the magnified optical signal. As noted above, magnification reduces the likelihood of mirror damage and facilitates switch construction. However, unduly enlarging the mirror size can slow the response time of the switch. Thus, in accordance with the present invention, the optical signal incident on the MEMS mirror is preferably magnified by a factor of between about 2x and 10x and, more preferably, by between about 4x and 6x, relative to the signal diameter at the input fiber end. For example, a magnification of 5x will result in a signal spot diameter of about 50 microns on the MEMS mirror surface for a typical single mode input fiber. Substantially, all of the signal energy may be received by a mirror having a width or diameter of about 60-70 microns. Thus, the 5x magnification reduces the incident optical density by a factor of 25 while allowing for the use of a mirror that can achieve acceptable switching response times for many applications. Such dimensioning also facilitates practical switch manufacturing.

In accordance with another aspect of the present invention, substantially collimating optics are used to provide an enlarged signal diameter at the MEMS mirror. Collimating optics include lenses and the like configured such the light emitted from a point source exits the optics such that all rays are directed along substantially parallel paths. That is, the optics are configured and positioned so that the exiting rays are focused substantially at infinity. For sources having a finite dimension, such as an input optical fiber end, collimating optics provide an output signal that defines a dispersive optical envelope that enlarges in diameter with increased distance from the optics. Substantially, collimating optics can be used in conjunction with MEMS mirrors having a width or diameter of about 500 microns to 1 millimeter in accordance with this aspect of the invention. Such designs can achieve certain advantages as discussed above, although perhaps with increased response times relative to particular focused beam implementations.

In accordance with a further aspect of the invention, a multiple 1xN switch is implemented using MEMS technology. The switch includes at least a first input fiber associated with a first output fiber and a second input fiber associated with a second output fiber. A MEMS mirror is operative for concerted direction of first and second optical signals from the respective first and second input fibers relative to the first and second output fibers to achieve dual switching functionality. It will be appreciated that more complicated designs involving more than two input fibers can be implemented in accordance with the present invention. Such multiple 1xN designs may be useful in a variety of contexts such as for reducing the required number of mirrors in certain display devices.

Brief Description of the Drawings For a more complete understand of the present invention and further advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the drawings, in which:

Fig. 1 is a perceptive view showing a common design of a conventional MEMS mirror;

Fig. 2 is a cut-away view of a 1x2 switch in accordance with the present invention;

Fig. 3 shows one switching position of the switch of Fig. 2;

Fig. 4 is a perspective view, partially cut-away, of the switch of Figs.2-

3;

Fig. 5a is a bottom view showing the lens plane of the switch of Figs. 2- 4;

Fig. 5b is a bottom view showing the lens plane of an alternative embodiment of a 1x2 in accordance with the present invention;

Fig. 6 is a bottom view showing the lens plane of a dual 1x2 switch in accordance with the present invention; and Fig. 7 shows various perimeters of a magnifying beam configuration in accordance with the present invention.

Detailed Description of the Invention The present invention is directed to MEMS-based 1xN switch designs employing a dispersed or magnified optical beam for reducing the optical density incident on the mirror and simplifying construction by relaxing positioning tolerances. It will be appreciated that such designs can be beneficially used in a variety of 1xN switching contexts. In the following description, the invention is set forth in the context of certain exemplary embodiments relating to on/off and 1x2 switch implementations.

Fig. 1 shows a common design for a MEMS mirror system 100. A Silicon chip 102 or other substrate is coated with successive layers of material, typically including metallic and other sacrificial material layers, that are later etched or otherwise configured to leave the mirror 104, hinges 108 and posts 106. It will be appreciated that many different mirror and hinge configurations are possible using conventional MEMS fabrication processes. The mirror 104 may rotate about the torsion hinges 108 until one side rests on the chip surface. Alternatively, other bi-stable, multi-positionable or analog mirrors may be deflectable without contacting the chip surface so as to avoid stiction concerns. An electrostatic force can be applied to actuate deflection of the mirror 104. The electrostatic actuating mechanism is not shown. The size of the chip 102 is approximately 1 mm x 1 mm. The mirror 104 may be between 10 and 100 microns on a side (the drawing is not to scale). Fig. 2 shows a cut-away of a 1x2 switch 200 built according to the present invention. Fibers a, b and c are each terminated with a lens 202. The lenses 202 focus the respective fiber core on the MEMS mirror 204 to a magnified spot. Thus, the optics 202 effectively provide an enlarged image of the fiber core on the mirror 204. As noted above, it is desirable to magnify the beam to reduce the optical intensity incident on the mirror 204 and simplify construction. However, unduly increasing the mirror dimensions may slow the response time of the switch 200. To address both of these considerations, the optical beam is preferably magnified such that the beam diameter at the mirror 204 is between about 2 times and 10 times as great and, more preferably, between about 4 times and 6 times as great as the diameter of the beam exiting the fiber. The lenses 202 may be common Graded Index (GRIN) lenses as manufactured by Nippon Sheet Glass Company, and bonded to the fiber. The fibers may be laser welded to a lens 202, or otherwise adhesively bonded to the lens 202. The fiber-lens combinations are assembled to the lens carrier by glue, laser welding or soldering. Laser welding or soldering allows for hermetic sealing of the mirror surroundings, extending mirror life and enabling operation in harsh environment.

A magnification of the beam by, for example, 5 times enables easy assembly, since the optical spot only needs to be aligned to the center of the mirror 204 within an accuracy of few microns. All three fiber-lens assemblies are aligned to the center of the mirror 204. A 5x magnification also results in a reduction of the Numerical Aperture (N.A.) at the mirror 204, essentially creating a beam angle smaller by factor of five at the mirror 204. This allows for a longer path length between the lenses 202 and the mirror 204, and smaller angles between the lenses 202 as seen from the mirror 204. Smaller angles between the mirror 204 result in less polarization dependent reflection by the mirror 204, as desired for communications applications, because reflection is near the mirror normal. When the illustrated mirror 204 is not energized, light from a fiber is reflected back to the same fiber. When the mirror 204 is actuated to tilt to a first deflected position, as shown in figure 3, fibers a and b are connected, so that photons transmitted from fiber a are received by fiber b and vice-versa. If the mirror 204 is tilted the other way (not shown), fibers a and c will be connected. Some designs of MEMS mirrors provide a bi-stable mirror with the rest position in one of the tilted positions described above or having a mirror that latches to the last tilted position. Such MEMS mirrors may be used to create bi-stable, latching switches or switches that return to a pre-defined connection when electrical control is lost. Fig. 4 is an isometric view of the 1x2 switch 200 of Figs. 2-3. The electrical wiring required to operate the mirror 204 is not shown. Fig. 5 shows the switch 200 as seen from the lenses 202. In figure 5a the position of an axis perpendicular to the mirror surface at its center is shown at its intersection with the plane of the lenses 202. The axis 500 is shown for each of the two switched positions. In order to avoid reflecting light from fiber a back to itself when the mirror is not energized, the offset configuration of the figure 5b may be used.

Fig. 6 shows a construction of dual 1x2 switch 600, viewed from the perspective of the lenses 602. Fibers {a,b,c} constitute one 1x2 switch and fibers {d,e,f} constitute the second 1x2 switch. One mirror with axis positions as shown serves both switches so that switching is accomplished in concert. More complex structures may be assembled involving multiple 1x2 switches serviced by one MEMS mirror. Simpler designs may utilize only 2 fibers for an on-off fiber switch.

Fig. 7 illustrates the optical magnification of the lens 700. The lens 700 is drawn as a thin lens, and a thin lens approximation will be used in the following analysis. For greater optical modeling accuracy a ray trace may be performed as will be understood by skilled artisans. The following definitions apply for the formulas set forth below:

d Beam waist diameter (1/e² power points) at fiber.

D Beam waist diameter (1/e² power points) at mirror.

L Beam waist diameter (1/e² power points) at lens. f Lens focal length.

U Fiber to center of lens distance

(the distance to the first nodal point in a real lens)

V Mirror to center of lens distance

(the distance to the second nodal point in a real lens) N.A._f Numerical aperture of the fiber.

N.A._m Numerical aperture at the mirror.

M Magnification.

The applicable formulas use a thin lens approximation: 1/f = 1/V + 1/U L = 2 ^* U ^* tan (sin-1 (N.A._f))

M = V / U L = 2 ^* V * tan (sin-1 (N.A._m))

M = D / d M = N.A_f/ N.A._m Common single-mode fibers used in telecommunications have a core diameter of approximately 9 microns and d equals approximately 10 microns. D may be designed to be approximately 50 microns with M=5. Since N.A._f is approximately 0.13 in such fibers, N.A._mwill be about 0.26. The N.A._m defines the minimum angular separation of the lenses relative to the mirror, and the minimum angular movement of the mirror. In such designs, the actual mirror is preferably between about 60-70 microns to provide for minimum light loss without unnecessarily increasing the mirror size. In this regard, a mirror of about 65 microns in diameter will reflect 99% of the light. If a large mirror is required due to high power transmitted in the fiber, D could be made equal to L. With common existing lenses, D may be between 500 microns and 1 mm. Since the mirror in such a design is reflecting nearly collimated light, a very flat mirror is required. Such mirrors are available, but are much slower to operate compared to the smaller mirrors. These mirrors are usually operated electromechanically due to the large size, requiring larger energy to operate then the smaller mirrors.

While various embodiments of the present invention have been described in detail, it is apparent that further modifications and adaptations of the invention will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.

Claims

1. An optical switch comprising: an input optical fiber for transmitting an optical signal, said optical fiber having a fiber end; a microelectromechanical system (MEMS) mirror device for receiving said optical signal and directing said optical signal on an optical pathway associated with an output port, said MEMS mirror device including one of a moveable mirror surface and an actuating assembly for moving said mirror surface, fabricated in a successive layer deposition process on a substrate; and magnification optics, disposed between said fiber end and said MEMS mirror device, for magnifying said optical signal, wherein said optical signal has a first cross-section diameter at said fiber end and a second cross- section diameter at said moveable mirror surface, said second cross-section being at least twice said first cross-section diameter.

2. An optical switch as set forth in Claim 1 , wherein said magnification optics, fiber end and a moveable mirror surface are configured to substantially image said fiber end onto said moveable mirror surface.

3. An optical switch as set forth in Claim 1 , wherein said magnification optics are configured to magnify said optical signal transmitted from said fiber end such that said second cross-section diameter is between about 2 times and 10 times as great as said first cross-section diameter.

4. An optical switch as set forth in Claim 1 , wherein said magnification optics are configured to magnify said optical signal transmitted from said fiber end such that said second cross-section diameter is between about 4 times and 6 times as great as said first cross-section diameter.

5. An optical switch as set forth in Claim 1 , wherein said input optical fiber is a signal mode fiber and said mirror surface has a dimension of between about 20 microns and 100 microns.

6. An optical switch as set forth in Claim 1 , wherein said input optical fiber is a signal mode fiber and said mirror surface has a dimension of between about 60 microns and 70 microns.

7. An optical switch as set forth in Claim 1 , wherein said output port comprises an output fiber.

8. An optical switch as set forth in Claim 7, wherein said output fiber is associated with optics for receiving said optical signal directed by said mirror surface and concentrating said optical signal on an end of said output fiber.

9. An optical switch as set forth in Claim 1 , wherein said switch further includes first and second output fibers and said mirror is operative for selectively directing said optical signal to either said first and second output fibers.

10. An optical switch as set forth in Claim 9, wherein said mirror device has a resting position associated with the absence of any driving signal for positioning said mirror device, said optical signal being directed to a location separate from said first and second output fibers when said mirror is in said resting position.

11. An optical switch as set forth in Claim 1 , wherein said switch further comprises: a second input optical fiber for directing a second optical signal to said mirror device; and a second output port for receiving said second optical signal once said second optical signal is directed to said second port by said mirror device.

12. An optical switch comprising: an input optical fiber for transmitting an optical signal, said optical fiber having a fiber end; a microelectromechanical system (MEMS) mirror device for receiving said optical signal and selectively directing said optical signal on an optical pathway associated with an output port, said MEMS mirror device including one of a moveable mirror surface and an actuating assembly for said mirror surface fabricated in a successively or deposition process on a substrate; and collimating optics, disposed between said fiber end and said MEMS mirror device, for collimating said optical signal, wherein said optical signal has a first cross-section diameter at said fiber end and a second cross-section diameter at said moveable mirror surface, wherein said second cross-section diameter is greater than said first cross-section diameter.

13. An optical switch as set forth in claim 12, wherein said fiber is a single mode fiber and said mirror surface has a dimension of between about

500 microns and 1 millimeter.

14. An optical switch as set forth in Claim 12, wherein said output port comprises an output fiber.

15. An optical switch as set forth in Claim 14, wherein said output fiber is associated with optics for receiving said optical signal directed by said mirror surface and concentrating said optical signal on an end of said output fiber.

16. An optical switch as set forth in Claim 12, wherein said switch includes a first and second output fibers and said mirror device is operative for selectively directing said optical signal to either of said first and second output fibers.

17. An optical switch as set forth in Claim 16, wherein said mirror device has a resting position associated with the absence of any driving signal for position said mirror device, said optical signal being directed to a location separate from said first and second output fibers once said mirror is in said resting position.

18. An optical switch as set forth in Claim 12, wherein said switch further comprises: a second input optical fiber for directing a second optical signal to said mirror device; and a second output port for receiving said optical signal once the second optical signal is directed to said second port by said mirror device.