Optical space switches using multiport couplers

Details Optical space switches using multiport couplers Optical space switches using multiport couplers

1999-02-19

2001-06-26

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Switch

C385S024000

Reexamination Certificate

active

06253000

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical switches for fiber-optic communications systems and, in particular, to an optical space switch wherein all possible combinations of output routing of the input signals can be effected by an appropriate setting of the switch.
BACKGROUND OF THE INVENTION
Space-division switches are central to telecommunications networks. A combination of space-division switches and time-division switches, for example, provide the kernel to the AT&T ESS™ electronic switches used in the North American Telecommunications network.
With the advance of fiber-optic communications and the attendant rapid growth in the carrier bit-rate and typical cable fiber-counts, as well as the increased level of network complexity, there has been a growing interest in space-division switches that operate in the optical-domain, routing optical signals from a set of inputs to a set of outputs without intermediate conversion of the optical signals to electronic form. A good introduction to these optical space-division switches is provided by Hinton et al. [Ref.
4
]. (Numbered references are fully cited in the References section at the end of this disclosure. All cited references are incorporated herein by reference).
As described by Hinton et al, a space-switch of large order, i.e. one with a large number of inputs, is typically constructed of smaller elemental space-switches interconnected in a prescribed manner. As the order of the switches increases, it is also seen that the high-order switch rapidly comes to comprise a very large number of switch elements. Also, that the number of crossings of the signal paths increases. Both these effects hinder the practical realization of the switch which become increasingly more difficult as the switch order increases.
To date, practical space switches of order 16×16 have been realized in the Lithium Niobate material system [Ref.
7
], and also in the doped silica waveguide material system [Ref.
8
] The former reference describes a 16×16 switch that comprises
23
modules connected in a three-stage network architecture and contains in total 448 2×2 directional coupler waveguide switch elements with
308
waveguide cross-overs and
42
crossings in the module connection fabric. The latter reference describes a 16×16 switch which uses a 16x16 matrix of double Mach-Zehnder 2×2 switching elements on a single silica waveguide substrate and employs a total of 512 MachZehnder elements with 512 waveguide cross-overs.
Practical considerations determine that a space-division switch should be realized with a minimum number of switch elements and a minimum complexity of the routing fabric that interconnects them. In order to realize the highest-order switches, the switch fabric is advantageously composed of individual switch modules that are then inter-connected to provide the overall switching. This alleviates the severe packaging challenge that would occur if the entire switch fabric were realized as a single module, with the attendant necessarily lower chip yield and the increased complexity and restrictions of the required module electrical and thermal management. The switch modules themselves generally comprise a number of switch elements built up into a higher-order switch fabric and may also contain several such independent higher-order switches. The art of a good overall switch design lies to a large extent in determining the optimum trade off between a complexity of the switch fabric that occurs within an individual switch module and the complexity of the interconnection that occurs between the switch modules. Increasing the complexity of the switch fabric within a module and on a single waveguide substrate increases the complexity of the attendant electrical and thermal management and reduces the anticipated wafer yield. However, it alleviates the number of interconnections that must then be provided between the switch modules to form the overall switch fabric. Reducing the order or complexity of the switching function provided within the modules eases the module electrical and thermal packaging constraints but increases the complexity of the interconnection fabric.
An example of this trade off is provided by the 16×16 Extended Generalized Shuffle (EGS) network switch described by Murphy et al [Ref.
7
]. This 16×16 switch fabric comprises 448 of the basic 2×2 switch elements realized in
39
switch modules of a three-stage network; 16 modules each containing two 1×8 switches (comprising seven 2×2 switch elements) provide the first column of the network, 7 modules each providing a 16×16 switch functionality (comprising thirty two 2×2 switch elements in a 4column Banyan architecture) compose the center column, and a final 16 modules each containing two 1×8 switches form the third column. The fiber network interconnecting these 39 modules is relatively simple to provide, being 112 connections between the first and second stage of the network and another 112 connections between the second and third stages, all arranged in a simple geometric fashion. The waveguide switch fabrics of the 7 center switch modules each contain 44 waveguide crossovers, and the waveguide switch fabrics of the modules in the first and third network columns contain no waveguide cross-overs at all. The vast majority of the signal cross-overs required for this 16×16 EGS switch fabric, however, are provided by the fiber connections that link the switch modules. This is highly advantageous as the ‘cross-over’of the fibers are loss-free and with negligible cross-talk, whereas signal crossings in waveguides on a waveguide switch-bearing substrate is always accompanied by some signal loss and also some signal cross-talk. The 16×16 EGS switch fabric has thus been partitioned into a modest number of switch modules so as to gain the advantage of integrating many switch elements onto the same waveguide substrate while the waveguide interconnection complexity and potential consequent performance degradation has been limited by using fiber to interconnect between the switch modules.
In consideration of the arrangement of the switch fabric or fabrics that occurs on a single substrate and may be packaged either on its own or with others as a module of the overall switching network, good design seeks to minimize the number of active switching elements, the number of elements through which the signals may pass, and the number of signal waveguide crossings that occur on-chip.
The 16×16 space switches described above were built up of individual 2×2 switch elements. It is evident that if a basic switch element of a higher order were available, fewer of such elements would be required to form the overall switch. In the case of Goh [Ref.
8
], for example, the 16×16 switch matrix array uses 256 2×2 switch units (each comprising two 2×2 Mach-Zehnder interferometers, doubled-up in order to obtain a high extinction ratio on switching) but would only comprise 64 switch elements if the basic switch element were a 4×4 switch. In the case of Murphy [Ref.
7
], availability of a 1×8 basic switch element would reduce the number of switch elements employed in the first and last columns of the fabric from 224 to 32, and the availability of a basic 4×4 switch element would reduce the number of elements required for the center column from 224 to 70 (14 4×4 and 56 2×2 elements), for a total element count of 102, reduced from the present 448. It is clear that use of higher-order elemental switch units provides a very significant reduction in the total number of elements required.
A form of multi-port waveguide device, known as the multi-mode interference (MMI) coupler, first noted in 1975 [Ref.
9
], has over the past few years received considerable attention arising from its properties of self-imaging [Refs.
10
,
11
,
12
]. This is the property whereby an o

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