N×N non-blocking optical switch

Details N×N non-blocking optical switch N×N non-blocking optical switch

1999-01-11

2001-09-18

Font, Frank G. (Department: 2877)

Optical waveguides

Temporal optical modulation within an optical waveguide

C385S017000

Reexamination Certificate

active

06292597

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to optical communication systems and the like. In particular, the present invention relates to optical switches having a greater number of switching states than a conventional Generalized Mach-Zehnder interferometer (GMZI). One embodiment of this invention relates to an N×N non-blocking optical switch.
BACKGROUND OF THE INVENTION
As we move towards the realization of widespread fiber optic networks, it is becoming increasingly important to provide optical switching at the optical network nodes. Optical switching is expected to become increasingly important as wavelength division multiplexing expands the number of optical paths available. By using integrated optical components to perform network node routing functions, advantages in terms of functionality, size, speed, and efficiency are achievable.
The integrated optical multimode interference (MMI) coupler has been the subject of much attention and research in recent years, see for example: L. B. Soldano, et al. in a paper entitled “Planar Monomode Optical Couplers Based on Multimode Interference Effects,”
J Lightwave Technol.,
vol. 10, no. 12, pp. 1843-1849, 1992; M. Bachmann, et al. in a paper entitled “General self-imaging properties in N×N multimode interference couplers including phase relations,”
Appl. Opt.,
vol. 33, no. 18, pp. 3905-3911, 1994; and L. B. Soldano et al., in a paper entitled “Optical multi-mode interference devices based on self-imaging: principles and applications,”
J Lightwave Technol.,
vol. 13, no. 4, pp. 615-627, April 1995. All references in this document are herein incorporated by reference. This passive device has been shown to possess a host of desirable qualities such as low excess loss, small size, fabrication tolerant behavior, and relative polarization and wavelength insensitivity. It has also been shown that MMI couplers can be used in a generalized Mach-Zehnder interferometer (GMZI) configuration to actively route and switch optical signals, as detailed by: L. B. Soldano et al., in a paper entitled “Optical multi-mode interference devices based on self-imaging: principles and applications,”
J Lightwave Technol.,
vol. 13, no. 4, pp. 615-627, April 1995; and R. M. Jenkins, et al., in a paper entitled “Novel 1×N and N×N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides,”
Appl. Phys. Lett.,
vol. 64, no. 6, pp. 684-686, February 1994.
An N×N GMZI has a limited switching capacity. The N×N GMZI has N possible switching states. In view of this, there are many desired switching states that are not accessible. Indeed, once a route has been chosen for light launched into a particular input port of the N×N GMZI to emerge from a selected output port, routes for light launched into all remaining input ports are fixed. For example, if light is switched from a first input port to a fourth output port in a 4×4 GMZI, light can only be switched from: a second input port to a second output port; a third input port to a third output port; and a fourth input port to a first output port. This demonstrates blocking switching capacity provided by an isolated N×N GMZI. 15 Switches have been proposed that use a plurality of Mach-Zehnder interferometers, see, for example, M. Bachmann, et al., “Compact Polarization-Insensitive Multi-Leg 1×4 Mach-Zehnder Switch in InGaAsP/InP,” in
Proc.
ECIO, Firenze, Italy, pp. 519-522, 1994, in which a number of independently controlled 1×N GMZI switches are used. While this design is a strictly non-blocking optical switch, it requires 4N MMI couplers, 2N
2
phase shifters, and numerous waveguide crossings, resulting in a large and complex switch with complicated control requirements. The waveguide crossings have specific geometrical tolerances that have to be met. If the specific geometrical tolerances are not met, “cross-talk” increases substantially and often increases attenuation. Control of a 4×4 switch using Bachmann's design requires 16 MMIs and 32 phase shifters. It will be appreciated by one skilled in the art that both manufacture and control of such a device is not a simple matter.
There is a need for reliable switches that are not overly complicated to manufacture and operate.
OBJECT OF INVENTION
Thus, in an attempt to overcome limitations of known prior art devices, it is an object of this invention to provide a new N×N optical switch.
SUMMARY OF THE INVENTION
A new generation of optical switches is provided. The optical switches provide any switch states. A switch state is a specific series of routing connections formed by outing light from one set of ports to another set of ports in a particular optical switch.
The terms input port and output port are used primarily to define structure. Non-blocking optical switches commonly function in reverse and those described herein are no exception. The terms output port and input port do not signify any restriction in direction of light flow in the optical switches of the present invention. For any singular switch state, if a beam of light is launched into an input port to emerge from a particular output port, a beam of light when launched into the output port will emerge from the input port. The optical switches defined in this document function in both routing directions. However, a particular direction of flow may be preferred for some switching applications. Obviously, the devices disclosed are able to be made uni-directional such that they operate in one or the other direction of flow by incorporation of other components. As used herein, each of the terms “first ports” and “second ports” refers to either input ports or output ports or input/output ports.
As used herein, the expression “non-blocking optical switch” refers to a “wide sense non-blocking optical switch” which describes a device with input ports and output ports. A signal from any input port is routed to any unoccupied output port. A new path is provided from any input port to any output port; however, to provide the new path, existing connections of the optical switch are disrupted. A “wide sense non-blocking optical switch” is distinct from a “strictly non-blocking optical switch”. A “strictly non-blocking optical switch” describes a device with input ports and output ports. In a “strictly non-blocking optical switch” a signal from any input port is capable of being routed to any unoccupied output port, and the new path is capable of being provided from any input port to any output port without disrupting existing connections other than those necessary to make the change.
The terms “power” and “intensity” as used herein are interchangeable as one killed in the art will appreciate power is time dependent intensity.
In accordance with the invention there is provided an optical switch comprising: n integrated optical N×N Mach-Zehnder interferometer, N being greater than 2, the integrated optical Mach-Zehnder interferometer comprising a first N×N multimode interference coupler having N first ports and N second waveguides, and a second N×N multimode interference coupler having N first waveguides and N second ports, wherein N second waveguides of the first N×N multimode interference coupler are optically coupled with N first waveguides of the second N×N multimode interference coupler thereby forming N waveguide arms; optical path length changers for changing an optical path length of at least 2 of the N waveguide arms; and a P×P optical switch having P second ports and P first ports, where P is at least 2 and less than N, at least 2 of the P second ports optically coupled to at least 2 of the N first ports.
In a preferred embodiment of the optical switch N is greater than 3 and the switch comprises a U×U optical switch having U second ports and U first ports, where U is at least 2 and less than N, at least 2 of the U second ports optically coupled to at least 2 of the N first ports. In another preferred embodiment the optical switch further compri

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