CA2310853A1

CA2310853A1 - A modular, expandable and reconfigurable optical switch

Info

Publication number: CA2310853A1
Application number: CA002310853A
Authority: CA
Inventors: Alberto Leon-Garcia
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-06
Filing date: 2000-06-06
Publication date: 2001-12-06

Abstract

A method and apparatus is disclosed for a modular and expandable optical switch that transfers optical signals arriving at a multiplicity of input ports to distinct output ports. A
basic switching unit is disclosed that transfers n input optical signals to n distinct output signals under the control of digital electronic signals. Apparatus and methods are disclosed for building said n by n basic switching units using electrooptic wafer beam deflectors. Methods are disclosed for building larger N input by N output optical switches by interconnecting and controlling arrays of basic switching units.
Finally, methods are disclosed for building even larger optical switches by using wavelength-division multiplexing and demultiplexing at the input and output ports of an N
x N
optical switch.

Description

A Modular, Expandable and Reconfigurable Optical Switch Abstract A method and apparatus is disclosed for a modular and expanda lcal switch that transfers optical signals arriving at a multiplicity of inpu its to distinct output ports. A
basic switching unit is disclosed that transfers n i optical signals to n distinct output signals under the control of digital electrons 'gnats. Apparatus and methods are disclosed for building said n by n bas' itching units using electrooptic wafer beam deflectors. Methods are disclo or building larger N input by N output optical switches by interconne g and controlling arrays of basic switching units.
Finally, methods are disc d for building even larger optical switches by using wavelength-division plexing and demultiplexing at the input and output ports of an N x N
opt' switch.
Field of invention The present invention in general relates to optical switches used in telecommunications and computer networks to switch and route optical signals arnving in one or a plurality of input ports to one or a plurality of output ports.
Optical transmission technologies have increased the information-carrying capacity of a single optical fiber to more than 1 Terabit per second. Future switches must therefore be able to transfer aggregate information rates in the many Terabits per second.
Electronic switches that can handle these information rates are extremely difficult to build because of the relatively-limited information-carrying capacity of electronic systems.
Optical switches that transfer information in optical form can avoid the bottleneck inherent in electronic switches.
Optical switches differ in terms of the type of optical signals they can transfer and in terms of the rate at which the switch interconnection pattern can be reconfigured. Some optical switches can transfer optical signals spanning a broad range of wavelengths, while other optical switches are limited to transferring individual wavelengths.
Most optical switches are reconfigured infrequently and require relatively long time periods to reconfigure.
More specifically, the present invention relates to optical switches that are:
1. modular in design and can be built from small to large number of port counts; 2. flexible in the type of optical signals that can be carried, from single wavelengths, to bands of wavelengths, to broad regions of optical spectrum; 3. rapidly reconfigurable. The present invention can be used in optical switches and routers in telecommunications and computer networks.

Discussion of Previous Art The transmission of information over optical fiber systems provides the advantages of extremely high transmission rates (measured in bits per second) and extremely low bit error rates. The design of electronic switches to transfer information among optical fiber systems is very challenging because of the extremely large volumes of information that must be handled electronically. All-optical switches transfer information among optical fiber systems without converting the information streams into electronic form, and hence avoid the electronic bottleneck inherent in electronic switches. In this patent we disclose a novel optical switch that is constructed from multiple basic switching units and that possesses the desireable properties of low loss, high switching speed, and modular expandibility.
The design of an optical switch involves the routing of an incoming optical signal along a desired path. This routing can be accomplished in a number of ways. Mechanical force can be used to move the incoming optical fiber so that it is aligned with the desired outgoing optical fiber [Personick 1987, pg. 6]. Mechanical force can also be used to control the incidence angle between an incoming light beam and a mirror in order to reflect the beam to a desired output optical fiber. This approach is used in micro-electromechanical systems [Stern 1999, pg. 228].
Electro-optic effects are also used to control the routing of an optical signal. The index of refraction of a substrate such as lithium niobate can be controlled through the application of an electric field created by a voltage applied to across a slab of material.
Regions of higher index of refraction in a substrate can be created by an ion exchange process. A 2x2 optical crosspoint can be produced by creating regions of higher refractive index in the shape of two channels or optical waveguides. Voltage control signals can then be used to direct two incident optical signals to the desired output ports [Personick 1987, pg. 6]. Larger n input by n output optical switching fabrics can be constructed from elementary 2x2 crosspoints using a crossbar arrangement [Nakajima 1999]. Even larger NxN optical switching fabrics can be constructed from nxn basic switching fabrics using Clos and Benes multistage switch constructions [Hui 1990, pp.
70-77].
The feasibility of constructing large switches from elementary components such as 2x2 waveguide-based crosspoints is determined by several factors. The loss in signal power incurred in traversing each component determines the maximum number of stages that can be traversed without amplification. The crosstalk that results when the power in one optical signal leaks into another signal affects the integrity of the information that traverses the fabric. The time required to reconfigure each component determines the rate at which the overall switch fabric can be reconfigured.
Typically, electromechanical switching systems control crosstalk by controlling the distance between different optical signals so very low crosstalk levels can be achieved.
However the reconfiguration rate of electromechanical systems is typically in the order of 1 kHz. Electro-optic effect systems can be reconfigured at much faster rates, 1 MHz or

2 higher. Unfortunately electro-optic waveguide-based systems can suffer from significant crosstalk impairment and/or significant loss levels.
A recent approach to routing optical signals using an electro-optic effect involves the forming of a series of prisms in a segment of a substrate [Chiu 1995], [Li 1996], [Stancil, 199x]. Reversing the ferroelectric polarization in triangular-shaped regions in a substrate forms prisms. The value of an applied voltage across the substrate controls the deflection angle of a light beam as it propagates through the substrate, and hence determines the point at which the beam exits the substrate. We refer to this novel switching component as an "electrooptic wafer beam deflector." The speed at which the exit point of a light beam in this component can be reconfigured is limited essentially by the speed of the control voltage signal. The optical signal undergoes very low loss in traversing the component [Li 1996]. The electrooptic wafer beam deflector component provides a preferred embodiment for the optical switch designs disclosed in this document.
Several methods have been developed for the construction of optical switch fabrics from splitter and combiner components. [Spanke, 1987] proposed a multiple-substrate n x n optical switch architecture based on 1 x n splitters and n x 1 combiners which has good properties in terms of signal-to-noise ratio across a fabric constructed from waveguide-based switch components. [Dial 1988] proposed an n x n optical switch involving passive splitters, passive combiners, and spatial light modulators that control the transfer of the optical signals from inputs to outputs.
Current optical transmission systems have an inherent information transmission capacity in excess of 1 Terabit per second, but the digital modulation systems currently available can only handle information in the tens of Gigabits per second. In order to use the large inherent capacity of optical fiber, wavelength division multiplexing (WDM) systems combine multiple independently modulated optical signals of different wavelengths into a single combined optical signal that can be transmitted in a single optical fiber. WDM
thus provides a means of packing extremely high volumes of information transfer in small regions in space. The present invention discloses methods that use WDM
to exploit the inherently high information transfer capability of optical switches.
Summary of Invention The present invention provides a method and apparatus for switching and routing optical signals arriving in one or a plurality of input ports to one or a plurality of output ports.
An optical switch with N inputs and N outputs is constructed from multiple basic optical switching units. Said basic optical switching units consist of an arrangement of lxn splitter and nxl combiners. A preferred approach for constructing said splitters and combiners uses electrooptic wafer beam deflector component. An array of electronic control signals determines the configuration of the NxN switch. Finally an even larger NW by NW optical switch is constructed using wavelength-division multiplexing and demultiplexing at the input and output ports of the said N x N optical switch.

3 Brief Description of the Drawings Figure 1 lxn active splitter Figure 2 1x2"' sputter using binary control signals Figure 3 active combiner Figure 4 nxn basic switching unit with active combiners Figure 5 nxn basic switching unit with passive combiners Figure 6 16x16 Benes Switch using identical basic switching units Figure 7 16x16 Benes Switch using multiple size basic switching units Figure 8 Switch fabric and associated control unit Figure 9 Clos strictly non-blocking switch Figure 10 Expanded optical switch using wavelength division multiplexing Detailed Description of the Invention Figure 1 shows the block diagram of an active lxn splitter 100, according to the present invention, that is used to route an incident optical beam 10 to one of a number of egress optical fibers 11. Using an electrooptic wafer beam deflector component, a voltage 12 is applied to the prism segment 13 to produce a deflection angle that determines the exit point of the optical beam 14. As shown in [Li 1996] the deflection angle is proportional to the applied voltage. A collimator 15 is placed so that it directs the exiting optical beam to a corresponding optical fiber 16.
The system in Figure 1 is used as a 1x2 splitter by applying a voltage control signal that is either 0 volts or V volts. The time to switch the optical beam from one position to the other position can be made very small because of the binary nature of the control signal.
Figure 2 shows how a lx2m active splitter 200 can be formed by concatenating in a substrate multiple prism segments 20, each under an independent binary control signal 21. The binary nature of the independent control signals enables the 1x2"' splitter to have fast transition times.
Figure 3 shows an nxl active combiner 300 which takes a single optical signal that arrives at one of n possible input optical fibers 30 and directs it to the single output fiber 33. A consequence of the reciprocity principle [Stern 1999 pp 228] is that an active combiner is obtained by operating an active splitter in reverse. A single optical signal arnves in one of n input fibers 30 and is incident at the substrate at a certain point 31. A
voltage signal 32 then directs the signal to the single output fiber 33. An active combiner is more efficient than an ordinary passive combiner in directing the energy in the optical beam to the output fiber [Spanke, 1987].

4 Figure 4 shows a 4x4 example of an nxn basic switching unit 400 constructed using n 1 xn active splitters 200 and n nxl active combiners 300. A single output fiber 40 from each active splitter is connected to an input 41 of each of the active combiners.
The control voltage 42 in each active sputter directs the input optical signal to the desired output fiber 40 and thereafter the optical signal propagates to the corresponding active combiner. The active combiner directs the single arriving optical signal to the output fiber 45 under the control of a voltage signal 46.
A consistent set of control voltage signals is required in the nxn basic switching system in Figure 4 to direct each of the n input optical signals to a distinct set of n output ports.
The nxn basic switching unit is then equivalent to a crossbar switch in the sense that it can direct any of n input signals to any output port that is not already in use.
Figure 5 shows a 4x4 example of an nxn basic switching unit 500 constructed using n lxn active splitters 200 and n nxl passive combiners 350. A single output fiber 50 from each active sputter is connected to an input 51 of each of the active combiners.
The control voltage 52 in each active sputter directs the input optical signal to the desired output fiber 53 and thereafter the optical signal propagates to the corresponding passive combiner.
The passive combiner combines all arnving optical signals and a portion of the energy in the arriving optical signal appears at the output fiber 55. The system in Figure 5 provides an acceptable basic switching unit as long as the output signals have an adequate signal-to-noise ratio.
Benes formulated a general method for constructing large switching fabrics from smaller switching fabrics [Hui 1990 pg. 72]. Figure 6 shows an example of how a 16x16 optical switching fabric 600 can be constructed from three stages of 4x4 basic optical switching units 61. Each stage in this Benes construction consists of 4 rows of individual 4x4 basic switching units. The ith output fiber 62 from the jth switching unit 63 in the first stage is connected to the jth input 64 of the ith basic switching unit 65 in the second stage. The interconnection pattern between the second and third stages is the reflection of the interconnection pattern between the first and second stage: the ith input fiber 66 into the jth switching unit 67 in the third stage is connected to the jth output 68 of the ith basic switching unit 69 in the second stage. More generally, given an nxn basic switchin~ unit constructed as shown in Figure 4 or in Figure 5, it is possible to construct and n2 x n larger switching fabric using a three-stage construction using the interconnection approach described above. In general an n2 x n2 three-stage Benes construction requires 3n basic switching units.
A five-stage n3 x n3 Benes construction for a large switch is obtained as follows. The first stage consists of n2 rows of nxn basic switching units. The center stage consists of n "central" switches of dimenstion n2 x n2, with the ith output from the jth basic switch in the first row connected to the jth input of the ith switch in the second stage. Moreover, each n2 x n2 central switch can in turn be decomposed into a three-stage array of n rows of nxn basic unit switches. In general an n3 x n3 five-stage Benes construction requires 5n2 basic switching units. More generally, an n'' x nk (2k-1)-stage Benes construction requires (2k-1)nk-1 basic switching units.
A preferred embodiment of the present invention involves the construction of n2 x n2 and n3 x n3 Benes constructions of optical switching fabrics using the basic nxn switching units shown in Figure 4 and Figure 5. The corresponding three- and five-stage switches are feasible because of the low loss property of the basic switching units constructed using electrooptic wafer beam deflector components.
The Benes method also allows the construction of large optical switching fabrics from smaller basic switching units of several sizes. Figure 7 shows a three-stage 16x16 optical switch fabric 700 constructed from first and third stages consisting of 8 2x2 basic switching units 71 and a central stage consisting of 2 8x8 basic switching units 72. In general, an N=mn switch can be constructed in three stages using first and third stages of m nxn basic switching units and a central stage of n mxm basic switching units. Five-stage Benes constructions of dimension N=mnk, where m, n, and k are positive whole numbers. The first and last stages are constructed using mn kxk basic switching units;
the Benes method is the applied to each of the k mn x mn central switching units.
A preferred embodiment of the present invention involves the construction of three and five stage Benes optical switching fabrics of dimension N=mn or N=mnk using basic switching units shown in Figure 4 and Figure Sof sizes n x n, m x m, and/or k x k. The corresponding three- and five-stage switches are feasible because of the low loss property of the basic switching units constructed using electrooptic wafer beam deflector components.
All the Benes switch fabric constructions described above are "non-blocking"
in the sense that they can realize any interconnection pattern of any N inputs to any N distinct outputs [Hui 1990 pg 70]. The addition of a new connection to an existing set of fewer than N existing connections may require the re-arrangement of all connections.
For this reason Benes switching fabrics are said to be rearrangeably non-blocking.
Various algorithms have been developed for determining the pattern of interconnections within each basic switching units to realize a given overall interconnection pattern in a Benes networks [Paull 1962], [Opferman 1971]. Figure 8 shows an example of a switching fabric and its associated fabric control unit. The figure only shows the basic switching units and their associated control signals. Requests for interconnection patterns are received from elsewhere in the system. The connection matrix request pattern is examined by the fabric control and an algorithm is executed to determine the interconnection pattern within the basic switching units in the overall switching fabric required to realize the given request pattern. A set of digital control signals c;~ is then applied to the ij basic switching unit to execute the desired interconnection patterns.
These control signals are converted to voltage levels that cause the optical beams in each basic switching unit to be routed to the appropriate output. The requested interconnection pattern is maintained as long as is necessary by applying the appropriate control voltage signals.

[Clos 1953] developed a method for constructing non-blocking multi-stage fabrics that do not require rearrangement of existing connections when a new connection is set up. The basic Clos construction for an N=pk switch consisting of three-stages. The first and third stages consist of k rows of pxm basic switching units, and the central stage consists of m k x k basic switching units. The ith output of the jth switch in the first row is connected to the jth input of the ith central switch. It is well-known that if m=2p-1, then the Clos fabric is strictly non-blocking in the sense that existing connections do not need to be rearranged to establish a new connection from an available input to an available output. Figure 9 shows an example of an 8x8 non-blocking Clos switch 800 constructed from 2x2 and 4x4 basic switching units. In this example, p=2, k=4 and m=2p-1=3.
A pxm basic switching unit can be constructed by simply using p of the inputs in an m x m basic switching unit. A preferred embodiment of the present invention is a three-stage arrangement of a Clos switching fabric in which the basic switching units are constructed using electrooptic wafer beam deflector components.
The electrooptic wafer beam deflector component can route optical signals and maintain high signal quality even when the optical signals are composite and consist of multiple wavelength signals. Consequently, the above disclosed optical switches constructed using electrooptic wafer beam deflector components have the capability of transferring composite optical signals. Figure 10 shows the use of WDM multiplexers and demultiplexers to concentrate multiple optical signals that occupy non-overlapping wavelengths into a single optical signal that can be switched across the NxN
optical switch. The structure of the switch constrains all components of the composite signal to be switched to the same output port. T'he composite signal can then be either decomposed into individual components, or the entire composite can be transmitted from the switch and onto an outgoing optical transmission link.

Claims

1. 1 xn active sputter using electroopic wafer beam deflector

2. nxn basic switching unit using active sputter and active combiner

3. nxn basic switching unit using active splitter and passive combiner

4. multistage optical switch using basic switching units constructed using electrooptic wafer beam deflector

5. multistage Benes construction using identical basic switching units constructed using electrooptic wafer beam deflector

6. multistage Benes construction using basic switching units of different size constructed using electrooptic wafer beam deflector

7. multistage non-blocking Clos construction using basic switching units of different size constructed using electrooptic wafer beam deflector

8. WDM-expanded port multistage optical switch using basic switching units constructed using electrooptic wafer beam deflector

9. WDM-expanded port multistage Benes construction using identical basic switching units constructed using electrooptic wafer beam deflector

10. WDM-expanded port multistage Benes construction using basic switching units of different size constructed using electrooptic wafer beam deflector

11. WDM-expanded port multistage non-blocking Clos construction using basic switching units of different size constructed using electrooptic wafer beam deflector