Optical switch

Details Optical switch Optical switch

2001-06-27

2003-06-10

Phan, James (Department: 2872)

Optical waveguides

With optical coupler

With alignment device

C385S016000, C385S031000, C385S032000, C385S017000

Reexamination Certificate

active

06577793

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to optical communications, and more particularly to all-optical switching of fiber networks.
2. Description of the Related Art
A critical technology in enhancing speed and bandwidth in communication systems is All-Optical switching, a primary goal of the telecommunication industry. Optical cross-connects are the enabling devices for the planned all-optical communication networks. They connect high-capacity fiber optic communication links coming into a particular hub with any of hundreds of outgoing channels. In doing so they solve two major problems. First, they provide controlled connections among numerous intermediate links to create a continuous optical pathway between endpoints anywhere in the network, optimizing the stream of data and reducing the cost of service. Secondly, they protect the network in the event of catastrophic failure of an intermediate link by instantaneously re-routing a circuit. An all-optical network will be easier to manage and more reliable while reducing the cost of bandwidth.
There are several types of All-Optical switches known in the art. The classification of optical switches is presented in FIG.
1
. Among them there are switches based on light birefringence phenomenon, switches utilizing light polarization in liquid crystals, switches utilizing bubbles in capillaries, electromechanical switches, and mirror-based switches.
Many 1×N switch architectures are based on a combination of two-state gates in a tree like structures. For N input channels N similar structures are required. It is clear that N×N switch requires N
2
gates. Moreover, in such a switch each of N output channels requires additional couplers and therefore increases both cost and optical losses in this switch architecture.
The operation of birefringent switches, typically based on lithium niobate or titanium niobate crystals, is polarization sensitive, and thus these switches require polarization-preserving optical fibers, and also require careful input/output waveguide mode matching in the optical system. Lithium niobate based switches have relatively large insertion loss and provide only a moderate degree of channel isolation. Besides, such switches require complicated fabricating processes. Examples of such switches can be found in the U.S. Pat. Nos. 4,976,505 and 5,946,116.
Liquid Crystal Optical Switches offer relatively high on/off ratios and relatively low optical insertion losses. But they require polarized light. Additionally, liquid crystal switches have certain environmental limitations including limited operating temperature range and environmental degradation. It is generally agreed upon that the technology lends itself only for small-size switching arrays. Examples of such switches can be found in publication Bawa et al., “Miniaturized total-reflection ferroelectric liquid-crystal electro-optic switch,” Appl. Phys. Lett., vol. 57, No. 15, pp. 1479-1481, Oct. 8, 1990 and in the U.S. Pat. No. 5,132,822.
Another architecture, based on waveguides and gas bubbles in fluid media, is described in the U.S. Pat. No. 6,055,344. At each switching point an input waveguide intersects an output waveguide at a fluid-filled trench. If the intersection is filled by liquid then the light passes straight through the intersection. When a gas bubble is placed in the intersection then light reflects to the output waveguide. It is obvious that an N×N channel switch also requires N
2
gates. Gas bubble based switches have certain environmental limitations including operating temperature range and environmental degradation. Insertion loss for such switches greatly depends on optical path and can vary many times within one switch. A similar architecture, based on waveguides and mirrors, is described in the U.S. Pat. No. 5,960,132.
Optical switch utilizing thermo-optical attenuators as the gates is described in “Silica-based optical-matrix switch with intersecting Mach-Zehnder waveguides for larger fabrication tolerances” by M. Kawachi et al, Conference OFC/IOOC '93, Feb. 21-26, 1993, San Jose, Calif. (U.S.A.), paper TuH4. Each input guide splits on two guides. After splitting each guide will have a gate, which can either open or close the guide. It can be shown that the total number of required gates for an N×N switch is 2 N
2
.
Another technology is based on a sliding mirror between two or three fibers, which can potentially be used as a variable optical attenuator or as an optical switch in small-size switching arrays. See U.S. Pat. No. 6,031,946.
Another group of optical switches utilizes multi-state switching elements. One of the great advantages of open space architecture is that the light beams can physically cross each other without interference of the signals transmitted by both beams. The light beams carrying information are transparent to each other. This is a unique property of light, which allows building switches with absolutely different architecture not possible in the electrical wire world.
The majority of current open space optical switching technologies are based on MEMS micro-mirrors. Schematically this principle is shown in
FIG. 2
The light beams
10
from the input fibers
12
are focused with collimators
14
on the first set of mirrors
16
, where they are redirected, as shown in
18
, onto a second set of mirrors
20
, which in their turn are redirecting the beams
22
into required output collimators
24
and then to the fibers
26
. N×N optical switch based on this architecture requires
2
N mirrors. Optical attenuation is in the range of 5 to 10 dB and they require at least two major optical alignments: between the transmitting array and the first mirror array and between the second mirror array and the receiving array. This architecture is complicated mechanically, optically and electronically.
Some of these MEMS micro-mirror arrays are based on surface micromachining technology. These devices have few disadvantages. The reported switching time is relatively slow. The optical losses are high. A large portion of these losses is inherent to this technology. For example, a non-flatness of the mirror is one of the sources of optical losses.
Other technologies use micro-mirrors based on bulk silicon micromachining. Bulk micro-machined mirrors with Gimbals suspension are inherently extremely fragile due to the relatively large mass of the mirrors, which are suspended by very thin beams. This results in low yield, high cost, and low reliability. See U.S. Pat. No. 5,629,790 incorporated fully herein by reference.
In another approach the switching or channel selection is achieved by means of a prism. Optical losses are moderate but the architecture and structure of the switch is complicated. See U.S. Pat. Nos. 5,999,669 and 6,005,993.
Another approach of redirecting the light beams between the transmitting and receiving arrays is based on lateral movement of the micro-lenses in front of collimators. However, it requires large space around the lens and the efficiency of the real estate utilization in the array is very low. See, for example: H. Toshiyoshi, Guo-Dung J. Su, J. LaCosse, M. C. Wu, “Microlens 2D Scanners for Fiber Optic Switches”, Proc. 3
rd
Int'l Conf. On Micro Opto Electro Mechanical Systems (MOEMS99), Aug. 30-Sep. 1, 1999, Mainz, Germany, pp. 165-167.
In electromechanical optical switches the input optical fibers are moving relative to the output optical fibers. Electromechanical switches don't require mirrors and therefore, don't require corresponding optical alignments and have smaller optical losses. However, macro actuators, for example step motors, are usually used in electromechanical switches as actuators. As an alignment of the fibers is critical in such systems, providing this precise and reproducible alignment with the motors is a big challenge. Another limitation of the electromechanical switches is that it is difficult to move simultaneously and independently more than one input fiber with respect to N output

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