Optical switching fabric with an optical to electrical converter in the output plane

An optical switching fabric with an optical to electrical converter in the output plane. The optical switching fabric has an input plane with at least one input. The input plane provides at least a first optical signal (e.g., a first set of optical signals). The optical switching fabric has an output plane. The optical switching fabric has an interconnection mechanism for receiving the first set of optical signals and directing each received optical signal to a predetermined location in the output plane. The output plane has at least one optical to electrical converter (e.g., a plurality of optical to electrical converters) for receiving the directed optical signals and responsive thereto for generating a set of corresponding electronic output signals.

FIELD OF THE INVENTION

The present invention relates generally to optical switches, and more particularly, to an optical switching fabric with an optical to electrical converter in the output plane.

BACKGROUND OF THE INVENTION

There are two major prior art approaches to switching or re-arranging the connections between high-bandwidth signals. These existing solutions include the use of optical-electrical-optical (OEO) switches and the use of pure optical switches.

The optical-electronic-optical (OEO) approach converts incoming optical signals to electronic form, switches (i.e., rearranges) the signals electronically, and then converts the electrical signals back to optical form. The building blocks for such an approach are electronic crossbar switch integrated circuits (ICs), which are relatively inexpensive. However, the use of these electronic crossbar switch integrated circuits does have some disadvantages. First, the electronic crossbar switches have a limited capacity. The capacity is typically limited to the number of ports multiplied by the data rate per port. Second, the electronic crossbar switches consume significant amounts of power.

Moreover, high-capacity OEO switches typically require multi-stage networks because of the limited size of the building blocks. As can be appreciated, these multi-stage networks become physically large, consume large amounts of power, and once the networks reach a certain size require expensive optical interconnects between stages.

Consequently, at some capacity, the OEO switches lose viability and all-optical switches become attractive.

An all-optical switch is a switch that has optical inputs, optical outputs, and no intermediate optical-electronic-optical conversion. One example of an all-optical switch is a crossbar optical switch. The publication entitled, “Compact optical cross-connect switch based on total internal reflection in a fluid-containing planar lightwave circuit,” by J. E. Fouquet, paper TuM1, Conference on Optical Fiber Communications, OFC 2000, Baltimore Md., USA, pp. 204–206, describes the crossbar optical switch that employs a large number of very simple 1×2 and 2×2 switches. However, one significant disadvantage of the crossbar optical switch is that the optical loss scales linearly with the number of ports. Consequently, the crossbar optical switch is limited by the total number of ports required.

Another example of an all-optical switch is a “fan-out, fan-in” optical switch. The publication entitled, “1296-port MEMS transparent optical crossconnect with 2.07 Petabit/s switch capacity,” by R. Ryf et al., paper PD28, Conference on Optical Fiber Communications, OFC 2001, Anaheim Calif., USA, describes an exemplary “fan-out, fan-in” optical switch. “Fan-out, fan-in” optical switches have a plurality of single mode fibers as inputs and a plurality of single mode fibers as outputs. A 1×N optical switch that is associated with each input directs light to the N×1 optical switch associated with the desired output. Typically, these “fan-out, fan-in” optical switches employ optics to perform the interconnection in free space.

One disadvantage of these types of optical switches is that very precise alignment of the optical system is required to steer beams of light from a single-mode fiber input to a single-mode fiber output with acceptable loss. First, the beams of light must be steered to hit a very small target area of a single mode fiber output. Second, not only must the light hit the small target area, but the light must also arrive at the target within a particular range of angles. If either of these two conditions is not met, the loss may become unacceptable.

For example, typically a very sophisticated closed-loop control is required to achieve and maintain the needed optical alignment. The closed-loop control has components, such as an optical source and coupler for each input port, an optical detector and splitter for each output port, and a sophisticated electronic signal processing circuit for each connection. As can be appreciated, the need for this sophisticated closed-loop control approach increases the complexity of switch design that may result in higher costs to manufacture the switch and that may pose reliability concerns.

Consequently, it is desirable for there to be a switching fabric for use in optical cross-connects that simplifies the optical system needed and relaxes the optical alignment requirements, thereby reducing the complexity and costs associated with the manufacture of such a switch.

Based on the foregoing, there remains a need for an optical switching fabric with an optical to electrical converter in the output plane that overcomes the disadvantages set forth previously.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, an optical switching fabric with at least one optical to electrical converter in the output plane is described. The optical switching fabric has an input plane that receives at least one input signal. The input plane provides at least a first optical signal (e.g., a first set of optical signals). The switching mechanism has an output plane. The optical switching fabric also has an interconnection mechanism for receiving the first set of optical signals and directing each received optical signal to a predetermined location in the output plane. The output plane has at least one optical to electrical converter (e.g., an array of photo-detectors) for receiving the directed optical signals and responsive thereto for generating a set of corresponding electronic output signals.

DETAILED DESCRIPTION OF THE INVENTION

An optical switching fabric with an optical to electrical converter in the output plane is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention.

FIG. 1is a block diagram illustrating an optical switching fabric100according to one embodiment of the present invention. The optical switching fabric100routes one or more input channels to one or more corresponding output channels. The optical switching fabric100includes an input plane110for receiving at least one input (e.g., a plurality of inputs112), an output plane120for generating at least one output (e.g., a plurality of outputs128), and an interconnection mechanism130for switching the inputs112and the outputs128(i.e., for re-arranging the connections between the inputs112and the outputs128).

For example, optical switching fabric100is suitable for selectively switching high-bandwidth signals (e.g., signals of the order of a gigabit per second or greater). The optical switching fabric100is also capable of high switching rates and is suitable for current applications that require reconfiguration times of the order of milliseconds.

The optical switching fabric100can be utilized in any optical cross-connects in which at least one incoming optical signal is converted to electronic form at least once in the optical switching fabric100.

The output plane120includes at least one optical to electrical converter124(e.g., a photodetector). One aspect of the present invention is to provide an optical to electrical converter124in the output plane120instead of a single mode fiber. As compared to a single mode fiber, an optical to electrical converter124(e.g., a photodetector) in the output plane120has a larger target area and a greater angular tolerance for the light beams. Consequently, by utilizing optical to electrical converters124in the output plane120, the number and complexity of the optical components required to steer light beams to the output plane120, and specifically the optical to electrical converters in the output plane120, are greatly reduced.

The interconnection mechanism130can be any means for interconnecting a first set of input signals (e.g., P input signals) and a first set of output signals (e.g., P output signals). It is noted that the inputs112may be optical or electronic, and the outputs128are electronic, but may be converted into optical signals as described in greater detail hereinafter. The interconnection mechanism130performs the interconnection by optically steering the optical signals propagating in free space. In this embodiment, the interconnection mechanism130includes an imaging system150and a beam steering plane140.

The input plane110includes one or more input elements114. The input plane110can be, for example, a one-dimensional array of input elements114or a two-dimensional (2D) array of input elements114. Each input element114receives a corresponding input signal112.

The input signals112can be electrical input signals or optical input signals. When the input signals112are electrical, the input plane110can include a plurality of optical sources (e.g., a one dimensional array of lasers or a two dimensional array of lasers) that generate optical signals in response to electrical signals as described in greater detail with reference toFIG. 2. For example, each input element can include a light source that generates a light beam in response to an electrical input signal.

When the inputs112are optical, the input plane can include a plurality of optical waveguides (e.g., a one dimensional array of optical fibers or a two dimensional array of optical fibers) that receive the optical signals as described in greater detail with reference toFIGS. 3 and 4. For example, each input element can include an optical fiber the provides a light beam for a particular input channel.

The output plane120includes one or more output elements. The output plane120can be, for example, a one-dimensional array of output elements or a two-dimensional (2D) array of output elements. In one embodiment, the output plane120includes a plurality of optical to electrical converters224for converting the steered optical signals into corresponding electrical signals. For example, each output element can include an optical to electrical converter. The optical to electrical converters224can be, for example, photodetectors.

The imaging system150can include a plurality of optical elements250(e.g., lenses, curved mirrors, etc.) for focusing or collimating the light beams generated by the input plane110. For example, each optical signal leaving the input fiber or laser may have a corresponding optical element250for focusing the light beam onto a corresponding element in the beam steering plane140. The imaging system150may include a plurality of lenses and/or mirrors that can be formed as part of an array. The purpose of the imaging system150is to image the input fiber or laser through the beamsteering element onto the desired photodetector in the output plane.

The imaging system150may include other optical elements154(e.g., additional lenses, mirrors, optics, etc.) to aid alignment or reduce the size of the optical system150. These optical elements154may be shared between all input and outputs. An example of these optical elements154is a folding mirror or an imaging lens for manipulation of optical signals provided by the input plane110.

It is noted that the inputs to the optical switching fabric100can be optical or electronic. If the inputs are electronic, the inputs are first converted to optical signals by employing lasers. The optical signals are then directed to photodetectors in the output plane. The output signals from the optical to electrical converters124are by definition electronic. For applications that require optical outputs, lasers can be utilized to convert the electrical output signals into optical output signals.

Optical Switching Fabric200with Electrical Inputs

FIG. 2illustrates an optical switching fabric200with electrical inputs212in accordance with a second embodiment of the present invention. When the inputs to the optical switching fabric200are electronic signals212, each electronic signal is converted to a corresponding optical signal214. One manner to perform this conversion is by using an optical source218(e.g., a light source) that is responsive to the electrical signal212to generate the corresponding optical signal214. For example, each input element in the input plane can include a semiconductor laser for receiving the electrical signal and responsive thereto for generating a corresponding light signal. For example, the laser can use the electrical signal to modulate the optical output of the laser. The semiconductor laser can be, for example, a directly modulated vertical-cavity surface-emitting laser (VCSEL).

In one embodiment, the semiconductor lasers are arranged in a two-dimensional array (e.g., an N×M array of semiconductor lasers) so as to produce a two-dimensional array of optical signals that serve as the inputs to the interconnection mechanism130. In this embodiment, the output plane120is a two-dimensional (2D) array of photodetectors224(e.g., an N×M array of photodetectors).

The beam steering plane140may be implemented with a two dimensional (2D) array of beamsteering elements244that directs light from each element216(e.g., optical source or optical fiber) in the input plane110to a predetermined element226in the output plane120. The beamsteering element244may be a transmissive (e.g., a lens) or reflective (e.g., a mirror). Transmissive beamsteering elements are utilized in the switches illustrated inFIGS. 2 and 4and described with reference thereto. Reflective beamsteering elements are utilized in the switch illustrated inFIG. 3and described with reference thereto.

Each optical signal propagates through the switching fabric in free space. In one embodiment, the beamsteering plane140includes a beamsteering element for each optical input signal to steer the light from the input plane through space to arrive incident on a corresponding optical to electrical converter element in the output plane. For example, beamsteering element244corresponds to light source216and steers the light generated by the light source216and directs the light to a corresponding optical to electrical converter (e.g., photodetector226) that is associated with a predetermined output channel. The beamsteering elements (e.g., element244) can steer the beam in multiple dimensions (e.g., in two dimensions).

It is noted that the embodiments shown inFIGS. 2–4utilize input planes, beamsteering planes, and output planes that are configured as two-dimensional array of elements (e.g., N×M arrays). However, it is noted that for the input planes, beamsteering planes, and output planes, either N or M can be 1 for some applications. Specifically, one or more of the input planes, beamsteering planes, and output planes can be arranged as a one-dimensional array (e.g., 1×M array or N×1 array) of elements (e.g., optical sources, optical to electrical converters, etc.).

FIG. 3illustrates a optical switching fabric300with optical inputs312that employs micro-mirror beamsteering elements344in accordance with a third embodiment of the present invention. When the inputs to the optical switching fabric300are optical signals312, these optical signals312can, for example, arrive on a plurality of optical fibers314. The optical fibers314can be arranged in a two-dimensional array, so as to produce a two-dimensional array316of optical signals that serve as the inputs to the interconnection mechanism (e.g., the beamsteering plane340). Each input optical fiber314carries a single wavelength.

As described earlier, an imaging system150can be provided between the input plane310and beamsteering plane340, between the beamsteering plane340and the output plane320, or between both sets of planes (310and340,340and320). The imaging system150may be implemented with a microlens array and/or micromirrors. The micromirrors may be made non-flat to incorporate at least a portion of the imaging system150into the micromirror. The micromirrors may be manufactured with a silicon micro-machining process.

The beamsteering plane340is provided that has a plurality of reflective beamsteering elements344. The reflective beamsteering elements344that can be, for example, micro-mirrors that are manufactured by silicon micromachining. An output plane320receives the steering optical signals, converts the optical signals into corresponding electrical signals, and that provides the plurality of electrical output signals328.

Phase Hologram Beamsteering Elements

FIG. 4illustrates a optical switching fabric400with optical inputs that employs phase hologram beamsteering elements444in accordance with a fourth embodiment of the present invention. Similar to the switch ofFIG. 3, the inputs to the optical switching fabric400are optical signals412. These optical signals412can, for example, arrive on a plurality of optical fibers414. The optical fibers414can be arranged in a two-dimensional array, so as to produce a two-dimensional array416of optical signals that serve as the inputs to the interconnection mechanism130. Preferably, each input optical fiber414carries a single wavelength.

A beamsteering plane440is provided that has a plurality of beamsteering elements444. The beamsteering elements444can be transmissive beamsteering elements (as shown inFIG. 4) or reflective beamsteering elements.

The beamsteering elements444can be implemented with, for example, switchable phase hologram elements. In such an embodiment, one hologram is associated with each input. The hologram elements can each direct a beam from a first input channel to an output channel. For example, each hologram steers its beam through free space to the desired location in the output plane. The hologram may also perform imaging. A publication entitled, “Holographic Optical Switching: The ROSES Demonstrator,” by W. A. Crossland, I. G. Manolis, M. M. Redmond, K. L. Tan, T. D. Wilkinson, M. J. Holmes, T. R. Parker, H. H. Chu, J. Croucher, V. A. Handerek, S. T. Warr, B. Robertson, I. G. Bonas, R. Franklin, C. Stace, H. J. White, R. A. Woolley, and G. Henshall, Journal of Lightwave Technology, Vol. 18, No. 12, December 2000, pages 1845 to 1854, which is hereby incorporated by reference, describes in greater detail the construction and use of switchable phase holograms. An output plane420that generates a plurality of electrical output signals428is also provided.

By utilizing an optical core for performing the optical switching, the optical switching fabric of the present invention has a higher capacity than conventional OEO switches (e.g., scalable to higher capacities). Furthermore, the switch of the present invention is more compact than OEO switches and consumes less power.

In addition, the switch of the present invention is scalable to more ports than crossbar optical switches because optical loss does not increase linearly with number of ports for the switch of the present invention.

Furthermore, the switch of the present invention is simpler and less expensive than conventional optical switches for the following reasons. First, the optical switching fabric of the present invention employs an array of optical to electrical converters (e.g., photodetectors) instead of an array of fibers utilized by the prior art. As described earlier, photodetectors are insensitive to a first order to the incidence angle of the light the photodetectors detect. In contrast, single mode fibers have a small numerical aperture. Furthermore, a photodetector's active area may be larger than the core of a single mode fiber. For both these reasons, alignment tolerances are relaxed, and the required optical system is simplified for the optical switching fabric of the present invention.

Consequently, as compared to conventional optical switches that have single-mode fibers as outputs, the optical system required for the switch of the present invention has fewer elements (e.g., only one beamsteering array), has relaxed alignment tolerances (e.g., relaxed angular tolerances), and requires simpler control. Whereas the prior art optical switches typically have the need for very sophisticated closed-loop control to achieve and maintain the required optical alignment, the switch of the present invention needs either much simpler control or no control at all. Since sophisticated closed-loop control is expensive and raises reliability issues for prior art switches, the switch of the present invention obviates these expenses and reliability issues by employing optical to electrical converters in the output plane that require a simpler optical system and related control.

Applications of the Optical Switching Fabric of the Present Invention

One main application for the optical switching fabric of the present invention is in optical cross-connects. Optical cross-connects are elements used in optical telecommunications networks to provision (i.e., set up) new connections across the network, and to rearrange the network so as to mitigate the effects of equipment failures on existing connections. Optical cross-connects are described in greater detail hereinafter with reference toFIG. 5

Other applications that can utilize the switch of the present invention may include, but are not limited to, setting up new connections, protecting existing connections, restoring failed connections, and switching fabric functions in distributed routers, computers, and storage systems.

The electrical signals provided by the optical to electrical converters (e.g., photodetectors) may be utilized directly in certain applications. For example, the electrical signals can be provided as inputs to electronic switching and routing elements, such as an ATM switch, an IP router, or a SONET digital cross connect.

In other applications, the electrical signals provided by the optical to electrical converters (e.g., photodetectors) are first converted to optical signals and then utilized. For example, the optical signals can be launched into single mode fiber.

Exemplary Optical Telecommunications Network

FIG. 5illustrates an exemplary optical telecommunications network500having an optical cross-connects (OXCs)510in which the switch530of the present invention can be implemented. The network500includes a meshed optical network505, in which each node is connected to more than two other nodes. The network510also includes Dense Wavelength Division Multiplexing (DWDM) transmission systems540that connect the nodes and carry many wavelengths. Each node in the meshed network505is an optical crossconnect (OXC)510. At the OXC510, the wavelengths are spatially separated, and each wavelength is dropped to local users550or switched to an outgoing DWDM system

In an opaque network, built using opaque nodes, all wavelengths arriving at a node undergo optical-electrical-optical conversion at least once. This OEO conversion allows the signals to be regenerated, monitored bit-by-bit, and shifted in wavelength if necessary. The network500also includes a ring network515, in which the nodes are a particular type of OXC called an optical add-drop multiplexer (OADM)520. An OADM is an OXC connected to only two other nodes. The OADM520performs the same function as an OXC.