Optoelectronic switches using on-chip optical waveguides

Embodiments of the present invention are directed to optoelectronic network switches. In one embodiment, an optoelectronic switch includes a set of roughly parallel input waveguides and a set of roughly parallel output waveguides positioned roughly perpendicular to the input waveguides. Each of the output waveguides crosses the set of input waveguides. The optoelectronic switch includes at least one switch element configured to switch one or more optical signals transmitted on one or more input waveguides onto one or more crossing output waveguides.

TECHNICAL FIELD

Embodiments of the present invention are directed to optoelectronic devices, and, in particular, to optoelectronic switches.

BACKGROUND

Switch networks are employed to route data from output ports to input ports of various kinds of nodes, including processors, memory, circuit boards, servers, storage servers, external network connections or any other data processing, storing, or transmitting device. In large scale computer systems, scalable packet switch networks are used to connect ports. In order to build switch networks that can scale to a large number of ports, it is desirable for the basic switch component to have as many inputs and outputs as possible. This means that a switch network that can span all the ports and can be constructed with fewer stages. In switch networks with N log (N) growth characteristics, such as Clos networks, this is termed a high radix router, since a large switch component size reduces the logarithmic growth term in network complexity. Where electronic devices are used for switching, the overall external bandwidth of each switch component is constrained so that the system designer is forced to compromise between the number of channels on and off the switch, and the bandwidth of the channels. For example, the same silicon technology can implement a 64×64 switch with each channel operating at 40 Gbit/s or a 16×16 switch with each channel operating at 160 Gbit/s. This constraint arises for the maximum number of signal connections for a package and the data rate for the signals themselves. The signal data rate is determined by the power and signal integrity considerations.

Switch networks can often be a data processing bottleneck for computing environments. A typical switch network, for example, can limit the scope of a computing environment's ability to handle the ever increasing data processing and transmission needs of many applications, because many switch networks are fabricated to accommodate only the “port-rate of the day” and the “port-count of the day” and are not fabricated to accommodate larger bandwidths that may be needed to effectively accommodate future applications. In particular, the amount and frequency with which data is exchanged between certain ports can be larger for some ports than for others, and the use of low-latency, metal-signal lines employed by most switch networks have limited bandwidths. As a result, the amount of data that can be transmitted between ports may not be well matched to the data transfer needs of the ports employed by an application at each point in time, which often results in data processing and/or transmission delays. Switch networks have a large number of long signal line intra-chip connections arising from the need to connect any input to any output. These long signal lines consume significant amounts of power in the repeaters needed to overcome electronic transmission losses.

A number of the issues associated with electronic signals transmitted via signal lines can be significantly reduced by encoding the same information in particular wavelengths or channels of light transmitted via waveguides. First, the data transmission rate can be increased significantly due to the much larger bandwidth provided by waveguides. Second, degradation or loss per unit length is much less for light transmitted via waveguides than for electronic signals transmitted via signal lines. Thus, power consumption per transmitted bit is lower for light transmitted via waveguides than for transmitting the same data in electronic signals via signal lines.

Optical switch components have been constructed using a variety of different technologies such as micro-electro-mechanical systems, and magneto optic effects. However, these switches are all circuit switches, where configuring the switch is performed by a separate, generally electronic, control plane. A packet switch is distinguished from a circuit switch by the ability to make connections according to routing information embedded in the input data stream. A packet switch typically permits buffering of input data when the requested output is in use. Many electronic packet switches have been constructed. However, these network switches are limited in their ability to scale to meet demands of future higher performance processors. There are two limiting factors. First, the bandwidth on and off the router chips is limited, both in terms of the number of input/outputs (“IOs”), limited by packaging technology, and IO speed which is limited by signal integrity considerations. Second, the power required for the inter- and intra-chip communications grows significantly with higher IO counts and higher data rates.

Engineers have recognized a need for fast network switches that can accommodate data encoded light as a medium for transmitting massive amounts of data between various kinds of data processing, storing, or transmitting devices.

SUMMARY

Embodiments of the present invention are directed to optoelectronic network switches. In one embodiment, an optoelectronic switch includes a set of roughly parallel input waveguides and a set of roughly parallel output waveguides positioned roughly perpendicular to the input waveguides. Each of the output waveguides crosses the set of input waveguides. The optoelectronic switch includes at least one switch element configured to switch one or more optical signals transmitted on one or more input waveguides onto one or more crossing output waveguides.

DETAILED DESCRIPTION

Various embodiments of the present invention are directed to optoelectronic network switches. These embodiments greatly increase input and output bandwidth through the use of direct nanophotonic interconnects that need less power than electronic interconnects for high bandwidth chip-to-chip interconnections. In addition, embodiments of the present invention employ dense wave-division multiplexing (“DWDM”) to connect numerous optical signals to a device. DWDM is multiplexing optical signals of different wavelengths on a single waveguide. The network switches include switch elements that connect input waveguides with output waveguides and distribute optical signals to multiple ports. Embodiments of the present invention exploit the ability of optical signals to connect with many points across the switch to eliminate the need for long internal electronic connections. Above a certain distance threshold, optical on-chip communication is more efficient than electronic communication, as the lower transmission loss, for a given distance, of optical waveguides obviates the need for repeaters.

In describing embodiments of the present invention, the term “optical signal” refers to electromagnetic radiation of a particular wavelength that has been modulated or turned “on” and “off” to encode data. For example, high and low amplitude portions of an optical signal may correspond to the bits “1” and “0,” respectively, or “on” and “off” portions of an optical signal may correspond to the bits “1” and “0,” respectively. The “optical signals” are not limited to wavelengths that lie in just the visible portion of the electromagnetic spectrum but can also refer to classical and quantum electromagnetic radiation with wavelengths outside the visible portion, such as the infrared and ultraviolet portions. A number of structurally similar components comprising the same materials have been provided with the same reference numerals and, in the interest of brevity, an explanation of their structure and function is not repeated.

FIG. 1shows a schematic representation of an optoelectronic network switch100configured in accordance with embodiments of the present invention. The switch100includes a set of eight vertical input waveguides102-109, a set of eight horizontal output waveguides110-117, and a set of eight horizontal optical power waveguides118-125that are roughly parallel to the output waveguides110-117. The power waveguides118-125are optically coupled to a source waveguide126, which, in turn, is optically coupled to an optical power source127. The input waveguides102-109are oriented roughly perpendicular to the output and power waveguides110-125and each input waveguide crosses the output and power waveguides110-125and is optically coupled to each output waveguide via a switch element, such as switch element128, represented inFIG. 1by a dashed line box. As shown inFIG. 1, the switch100includes an 8×8 array of switch elements where each switch element includes an input waveguide crossing an output waveguide. For example, the switch element128includes the input waveguide107crossing the output waveguide113. Switch element embodiments are described in greater detail below with reference toFIGS. 2-3.

The waveguides102-126are each capable of carrying multiple optical signals using DWDM. The optical power source127outputs a number of continuous wave (“CW”) (i.e., unmodulated or roughly constant amplitude and wavelength) lightwaves onto the source waveguide126using DWDM, each lightwave having a different wavelength. A portion of each lightwave is coupled into each of the power waveguides118-125so that each of the power waveguides118-125carry the same set of lightwaves output from the optical power source127. The lightwaves are transmitted along the power waveguides118-125in the direction identified by directional arrow129. The input waveguides102-109are coupled separately to input ports132-139, respectively, and the output waveguides118-125are coupled separately to output ports140-147, respectively. Input optical signals are placed on the input waveguides102-109by the corresponding input ports132-139and transmitted in the direction identified by directional arrow130. Output optical signals are placed on the output waveguides110-117by the switch elements and transmitted in the direction identified by directional arrow131. The input and output optical signals are data encoded (i.e., amplitude modulated) optical signals. The input and output ports132-147can be connected to processors, memory, circuit boards, servers, storage servers, external network connections, other switches, or any other data processing, storing, or transmitting device.

The switch100can be operated as a circuit switch. Suppose the switch100is directed to transmit data from the input port137to the output port143. An external switch control (not shown) activates the switch element128. The input port137places input optical signals encoding the data onto the input waveguide107in the direction130. The switch element128extracts the input optical signals and the lightwaves transmitted along the power waveguide121in the direction129. The switch element128then encodes the data encoded in the input optical signals onto the extracted lightwaves by modulating or turning the lightwaves “on” and “off” to produce output optical signals that are transmitted in the direction131on the output waveguide113to the output port143.

Optoelectronic network switch embodiments are not limited to the square 8×8 network switch100. In other embodiments, the number of rows and columns of switch elements can scaled up or down as needed. In generals, embodiments of the present invention include N×N network switches, where N is a positive integer representing the same number of rows and columns of switch elements. In other network switch embodiments, the number of rows can be different from the number of columns. In general, network switches embodiments can be M×N, where M and N are positive integers representing the number of rows and columns of switch elements, respectively.

FIG. 2Ashows a schematic representation of a first circuit switch element200configured in accordance with embodiments of the present invention. The switch element200includes an input waveguide202, an output waveguide204, and a power waveguide206. The input waveguide202is optically coupled to six input resonators207-212, and the power waveguide206is optically coupled to six output resonators214-219, which are also optically coupled to the output waveguide204. The six input resonators207-212are optically coupled to detectors that are electronically coupled to a receiver220. For example, detector222is adjacent and optically coupled to the input resonator207and is electronically coupled to the receiver220. The detectors absorb input optical signals trapped in the input resonators207-212and generate corresponding data encoded electronic signals that are transmitted to the receiver220that transmits the electronic signals to a transmitter226via an electronic interconnect. The direct electrical connections are represented by directional arrows, such as directional arrow224. A switch state controller228holds the configuration data for the switch element and determines which input connects to which output.

The input resonators207-212and output resonators214-219are each electronically tunable and configured to have resonance with a particular wavelength of light propagating along an optically coupled waveguide when an appropriate voltage is applied. In this case, the resonator is said to be turned “on.” Each turned “on” resonator extracts via evanescent coupling at least a portion of the light from the waveguide and traps the extracted light within the resonator for a period of time. When the voltage is turned “off,” the resonance wavelength of the resonator shifts away from the wavelength of the light, and the light propagates undisturbed along the optically coupled waveguide past the resonator. In this case, the resonator is said to be turned “off.” The configuration and operation of the input resonators207-212and the output resonators214-219are described in greater detail below in the subsections “Microring Resonators and Ridge Waveguides” and “Photonic Crystals and Resonant Cavities.”

Operation of the switch element200is now described with reference to a particular example. In the following description, a lightwave of a particular wavelength is represented by λ, and a data encoded input or output optical signal of the same wavelength is represented byλ. In addition, all of the input optical signals are used to carry the data and all of the power signals are used to produce output optical signals encoding the same data. The input resonators207-212and the output resonators214-219are configured to have resonance with one of six different wavelengths λ1, λ2; λ3, λ4, λ5, and λ6, respectively, when turned “on.” As shown inFIG. 2A, the power waveguide206carries the six lightwaves output from an optical power source (not shown). The six resonators207-212are turned “on” and evanescently couple six input optical signalsλ1,λ2,λ3,λ4,λ5, andλ6, respectively, from the waveguide202. The six input optical signals encode data destined for an output port (not shown) connected to the output waveguide204. Corresponding detectors convert the six input optical signals resonating in the resonators207-212into six electronic signals that are sent to the transmitter226via direct electrical connections. Retiming logic230is used to synchronize the arrival of the electronic signals at the transmitter because input signals may have different phases when they extracted from the input waveguide202. The transmitter226encodes the data in the six lightwaves λ1, λ2, λ4, λ5, and λ6, to produce six output optical signalsλ1,λ2,λ3,λ4,λ5, andλ6that are transmitted along the output waveguide204.

Encoding data in the six lightwaves can be accomplished by turning the output resonators214-219“on” and “off” in accordance with the “0” and “1” bits of the electronic signals transmitted to the resonators214-219. For example, when the output resonator214is turned “on” for the time period corresponding to the bit “0,” the output resonator214evanescently couples at least a portion of the lightwaves λ1from the power waveguide206into the output waveguide204. When the output resonator214is turned “off” for the time period corresponding to the bit “1,” the lightwaves λ1passes the output resonator214undisturbed. The result232is an amplitude modulated or “on” and “off” output optical signalλ1encoding the data carried by the input optical signal.

Note that in certain embodiments, the wavelengths of the input optical signals can correspond to the wavelengths of the output optical signals, while in other embodiments, the wavelengths of the input optical signals do not have to correspond to the wavelengths of the output optical signals. For example, in certain embodiments, the data carried by the input optical signalλ2can be encoded on the lightwave λ2to produce the output signalλ2carrying the same data, while in other embodiments, the input optical signal can be encoded on the lightwave λ4to produce the output signalλ4carrying the same data.

FIG. 2Bshows a schematic representation of a second circuit switch element250configured in accordance with embodiments of the present invention. The switch element250is nearly identical to the switch element200except the switch element250includes an electronic crossbar252that is electronically coupled to the receiver220and the transmitter226. The electronic crossbar252is controlled by switch state controller228to route the electronic signals output from the receiver220to the transmitter226.

The switch element250can be used to route the electrical signals produced by data encoded on all six input optical signals to produce data encoded on all six output signals as described above with reference toFIG. 2A. In other embodiments, rather than receiving data on all six input optical signals and using all out optical signals, the switch element250can be used to receive data encoded on a certain number of input optical signals and output data on a different number of output optical signals. For example, an input port (not shown) places the input optical signalsλ2and λ4onto the input waveguide202. The two input optical signalsλ2andλ4are encoded with data intended for the output port (not shown) coupled to the output waveguide204. As shown inFIG. 2B, when the resonators208and210are turned “on,” the input optical signalsλ2andλ4are evanescently coupled from the input waveguide202. Corresponding detectors convert the input optical signalsλ2andλ4into electronic signals encoding the same data and transmit the electronic signals to the receiver220. The electronic crossbar224receives the electronic signals from the receiver220and reroutes the electronic signals to the transmitter226. Because the input optical signal transmission times may not be synchronized, the switch element200may include retiming logic230to synchronize the transmission of the electronic signals to the transmitter226. The transmitter226encodes the data in the three lightwaves λ1, λ3, and λ4to produce output optical signalsλ1,λ3, andλ4that are transmitted along the output waveguide204.

Note that the direct electronic interconnect of the switch element200and the electronic crossbar of the switch element250are just two of many different kinds of electrical interconnects that can be used to transmit electrical signals from the receiver220to the transmitter226.

In general, switch element embodiments can be configured to receive data on any number of input optical signals and output the data on any number of output optical signals. Unlike the example described above with reference toFIG. 2, in certain embodiments, the resonators of a switch element can be configured to receive input optical signals having one set of wavelengths and produce output optical signals having a different set of wavelengths. In addition, switch embodiments are not limited to six input and six output resonators. In other embodiments, any suitable number of input and output resonators can be used, and the number of input resonators can be different from the number of output resonators.

The switch100can also be operated as a data packet switch by configuring each switch element with a data packet buffer. Arbitration can be used to select which of multiple input packets is transmitted to a particular output port.FIGS. 3A-3Bshows a schematic representation of a packet switch element300configured to transmit data packets in accordance with embodiments of the present invention. The switch element300is nearly identical to the switch element250except the electronic interconnect252is replaced by a combined electronic interconnect and packet buffer302. The packet buffer can be additional memory space reserved for storing a packet awaiting transmission to an output port. In the first phase, as shown inFIG. 3A, the switch element is directed by arbitration304to turn “on” the resonator207. The optical signal λ1′ is evanescently coupled from the input waveguide202into the microring207, and the switch element300prepares to receive input optical signals by turning “on” the remaining resonators208-212. In the second phase, as shown inFIG. 3B, the input optical signalsλ1,λ2,λ3,λ4,λ5, andλ6are evanescently coupled into the resonators207-212, and the switch element300outputs the same data packet in the output optical signalsλ1,λ2,λ3,λ4,λ5, andλ6, as described above with reference toFIG. 2A. In other embodiments, the data packet can be sent on using certain input optical signals and output on different or the same output optical signals as described above with reference toFIG. 2B.

In certain embodiments, when the output port is not busy, the packet may be immediately routed to the output port in a technique called “cut-through.” Alternatively, when the output port is in use by another input port, the packet is stored in the packet buffer, and transmitted when the output port becomes available. The arbitration304is used to select between any of the possible switch elements requesting the packet.

Returning toFIG. 1, in certain switch embodiments, in order to reduce optical power consumption, data can be sent in two phases. In the first phase, each of the switch elements turns “on” a different resonator and waits to receive the corresponding optical signal within a first time interval. The switch elements all receive the same optical signal identifying the output port. However, the optical signal has resonance with a particular resonator of the switch element coupled to the selected output port. This switch element responds by preparing to receive the data encoded in a number of input optical signals during the second phase. Because the remaining switch elements did not turn “on” the resonators matching the resonance of the optical signal, these switch elements do not receive the optical signal during the first time interval and respond by turning “off” their resonators and wait for the data to be transmitted during the second phase. For example, initially, the eight different switch elements150-157each turn “on” a different resonator. The resonators can each correspond to one of eight different optical signals having wavelengths λ1, λ2, λ3, λ4, λ5, λ6, λ7, and λ8. Suppose the input port136is to transmit a data to the output port145. The input port136outputs a single optical signal, such as a pulse, of the wavelength having resonance with the resonator turned “on” by the switch element155. Upon receiving the optical signal, the switch element155responds by turning “on” its corresponding resonators and waits to receive input optical signals from the input port136, while the remaining switch elements150-154,156, and157turn “off” their corresponding input resonators. In the second phase, the input port136transmits the input optical signals, which are received by the switch element155and transmitted to the output port145.

In other switch embodiments, a single address optical signal can be used to activate the switch element coupled to the selected output port. For example, in the first phase, the output ports140-147can each be assigned a different address. All of the switch elements150-157can turn “on” the resonator having resonance with the wavelength of the address optical signal and wait to receive the address optical signal. The input port transmits the address of the output port145on the waveguide106in the address optical signal. The switch element155receives the address optical signal and prepares to receive input optical signals. The remaining switch elements150-154,156, and157also receive the address optical signal, but because the address does not match the address of their optically coupled output ports, the remaining switch elements150-154,156, and157respond by turning “off” their input resonators. In the second phase, the input port136transmits the input optical signals, which are received by the switch element155and transmitted to the output port145.

Optoelectronic network switch embodiments are not limited to employing a single switch element at each input and output waveguide crossing point. A hierarchical scheme in which short distance switching and communication is performed electronically can be applied to reduce the number of resonators, receivers, and transmitters while maintaining the same number of input and output waveguides.

FIG. 4shows a schematic representation of a second optoelectronic network switch400configured in accordance with embodiments of the present invention. The switch400includes the same waveguides102-126, optical power source127, and ports132-147as the switch100described above with reference toFIG. 1. Like the switch100, the switch elements of the switch400also switch input optical signals received on the input waveguides102-109into output optical signals carried by the output waveguides110-117. However, rather than employing a single switch element to perform switching of input optical signals carried by one input waveguides into output optical signals carried by an output waveguide, the switch400employs 2×2 switch elements to switch input optical signals carried by one of two input waveguides into output optical signals that can be carried by one of two output waveguides. For example, the 2×2 switch element402can receive input optical signals on the input waveguide104or the input waveguide105and place corresponding output optical signals on the output waveguide114or the output waveguide115.

FIG. 5shows a schematic representation of a 2×2 packet switch element500configured in accordance with embodiments of the present invention. The switch element500includes two input waveguides502and504and two output waveguides506and508. The input waveguides502and504are each optically coupled to a set of six resonators that are electronically coupled to receivers510and512, respectively. Output waveguides506and508are also each optically coupled to a set of six resonators that are optically coupled to power waveguides514and516, respectively, and are electronically coupled to transmitters518and520, respectively. The resonators are operated as described above with reference toFIG. 2. The switch element500includes a 2×2 electronic interconnect and packet buffer524that receives electronic packets from the receivers510and512, stores the data packets in the packet buffer, and transmits the packets to either the transmitter518or the transmitter520. The packets are encoded in lightwaves by the transmitters518and520as described above with reference toFIG. 2. In other embodiments, the 2×2 packet switch element500can be modified for circuit switches by eliminating the packet buffer and including retiming logic between each of the transmitters518and520and the 2×2 electronic interconnect and packet buffer524, and switch configuration state, as described above with reference toFIG. 2.

FIG. 6shows four 1×1 switch elements601-604and a single 2×2 switch element606in accordance with embodiments of the present invention. The four 1×1 switch elements601-604schematically represent four adjacent packet switch elements300described above with reference toFIG. 3. Note that each of the 1×1 switch elements601-604includes corresponding sets of input and output resonators, a receiver, a transmitter, and an electronic interconnect and packet buffer which totals four receivers, four transmitter, four electronic interconnects, and a total of 48 resonators. In contrast,FIG. 6also reveals a single 2×2 switch element606that schematically represents either a 2×2 circuit switch element or the 2×2 packet switch element500The single 2×2 switch element606can perform the same switching operation carried out by the four 1×1 switch elements601-604but with half as many resonators, receivers, and transmitters.

Switch element embodiments of the present invention are not limited to the 2×2 switch elements described above. In practice, the size of the switch element is determined by the crossover point in efficiency between optical and electronic intrachip communication. In other embodiments, switch elements can be scaled up to include 3×3, 4×4, 5×5 or large switch elements. In general, an M×N network switch has M×N receivers and M×N transmitters, and in the case of packet network switches, each arbiter needs to multiplex M inputs. By replacing an M×N network switch with a P×Q network switch for the same number of input and output waveguides, where M>P and N>Q such that P divides M and Q divides N, the number of receivers is reduced to N/Q and each output arbiter needs only to multiplex between M/P inputs. The total N×M network switch uses N×M/P receivers and M×N/Q transmitters. In the packet network switch, the use of a single electronic interconnect also permits sharing of buffer resources with the electronic interconnect reducing the M×N buffer requirement of the M×N network switch.

In certain optoelectronic network switch embodiments, the set of input waveguides and the set of output waveguides can be fabricated in two separate optical layers.FIG. 7Ashows an exploded isometric view of the switch element200formed in two separate optical layers in accordance with embodiments of the present invention. The input waveguide202and optically coupled resonators207-212are implemented in a first optical layer702, and the output waveguide204, the power waveguide206, and the output resonators214-219are implemented in a second optical layer704. As shown inFIG. 7A, the input, output, and power waveguides202,204, and206are implemented using ridge waveguides, and the input resonators207-212and the output resonators214-219are implemented using microring resonators described in greater detail below. In other embodiments, the set of input waveguides and the set of output waveguides can be implemented in a single optical layer.FIG. 7Bshows an isometric view of the switch element200formed in a single optical layer706in accordance with embodiments of the present invention. Little crosstalk occurs at the intersections708and710between the input waveguide202and the output and power waveguides204and206, respectively.

The switch embodiments of the present invention are capable of scaling to greater bandwidths and switch sizes than purely electronic switches through the use of integrated optical IO structures for inter-chip communication. These consume less power than equivalent electronic IOs operating at the same data rate. The use of hierarchical internal structures, using arrays of smaller electronic switches connected by optical on-chip interconnects, avoids the need for lengthy, on-chip, electronic interconnections, while optimizing the use of optical to electronic and electronic to optical converters. When compared to purely optical switches, the optoelectronic network switches of the present invention are more flexible due to the ability to implement packet switching and buffering, which is a requirement for many general purpose computing applications.

Microring Resonators and Ridge Waveguides

In certain system embodiments, the waveguides202,204, and206can be ridge waveguides, and the resonators can be microring resonators.FIG. 8Ashows an isometric view of a microring resonator802and a portion of an adjacent ridge waveguide804disposed on the surface of a substrate806and configured in accordance with embodiments of the present invention. Optical signals transmitted along the waveguide804are evanescently coupled from the waveguide804into the microring802when the optical signals satisfy the resonance condition:
neffC=Mλ
where neffis the effective refractive index of the microring802, C is the circumference of the microring802, m is an integer, and λ is the wavelength of an optical signal. In other words, optical signals with wavelengths that are integer multiples of the wavelength λ are evanescently coupled from the waveguide804into the microring802.

FIG. 8Bshows a plot of transmittance versus wavelength for the microring802and the waveguide804shown inFIG. 8A. Horizontal line808represents a wavelength axis, vertical line810represents a transmittance axis, and curve812represents the transmittance of optical signals passing the microring802over a range of wavelengths. The transmittance of an optical signal passing the microring802is defined by:

T=IoutIin
where Iinis the intensity of the optical signal propagating along the waveguide804prior to reaching the microring802, and Ioutis the intensity of the optical signal propagating along the waveguide804after passing the microring802. Minima814and816of the transmittance curve812correspond to zero transmittance for optical signals having wavelengths mλ and (m+1)λ and represent only two of many regularly spaced minima. These optical signals satisfy the resonance condition above, are said to have a “strong resonance” with the microring802, and are evanescently coupled from the waveguide804into the microring802. In the narrow wavelength regions surrounding the wavelengths mλ and (m+1)λ, the transmittance curve812reveals a steep increase in the transmittance the farther the wavelength of an optical signal is away from the wavelengths mλ and (m+1)λ. In other words, the strength of the resonance decreases, and the portion of the optical signal coupled from the waveguide804into the microring802decreases the farther an optical signal's wavelength is away from an integer multiple wavelength of λ. Optical signals with wavelengths in the regions818-820pass the microring802substantially undisturbed.

Because of the evanescent coupling properties of microring resonators, microring resonators can be used to detect particular optical signals transmitting along an adjacent waveguide, or microring resonators can be used to couple optical signals of a particular wavelength from one adjacent waveguide into another adjacent waveguide.FIG. 9Ashows the microring resonator802used as a photodetector in accordance with embodiments of the present invention. An optical signal having a wavelength that is resonant with the microring802is evanescently coupled from the waveguide804into the microring802and remains trapped for a period of time while circulating within the waveguide802. A detector902is disposed on the surface of the substrate806adjacent to the microring802. The detector902absorbs the optical signal circulating in the microring802and converts the optical signal into an electronic signal that can be transmitted over signal lines to electronic devices. The detector902can be comprised of germanium (“Ge”) or any other suitable light absorbing element or compound.FIG. 9Bshows the microring resonator802used to couple an optical signal from the waveguide804into a second waveguide904in accordance with embodiments of the present invention. An optical signal having a wavelength that is resonant with the microring802is evanescently coupled from the waveguide804into the microring802. The optical signal circulates with the microring802and is evanescently coupled into the waveguide904. Note that the optical signal is transmitted along the waveguide804in one direction and the optical signal coupled into the second waveguide904is transmitted in the opposite direction.

The microring802can be electronically tuned by doping regions of the substrate806surrounding the microring802and waveguide804with appropriate electron donor and electron acceptor atoms or impurities.FIG. 10shows a schematic representation and top view of doped regions surrounding the microring802and the ridge waveguide804in accordance with embodiments of the present invention. In certain embodiments, the microring802comprises an intrinsic semiconductor. A p-type semiconductor region1001can be formed in the semiconductor substrate interior of the microring802, and n-type semiconductor regions802and803can be formed in the semiconductor substrate806surrounding the outside of the microring802and on the opposite side of the waveguide804. The p-type region1001and the n-type regions1002and1003form a p-i-n junction around the microring802. In other embodiments, the dopants can be reversed in order to form an n-type semiconductor region1001in substrate interior of the microring802and p-type semiconductor regions1002and1003in the substrate surrounding the outside of the microring802.

The electronically tunable microring802can be configured to evanescently couple or divert light from an adjacent waveguide when an appropriate voltage is applied to the region surrounding the microring. For example, the electronic controlled microring802can be configured with a circumference C and an effective refractive index neff′ such that an optical signal with a wavelength λ propagating along the waveguide804does not satisfy the resonance condition as follows:
n′effC≠mλ
This optical signal passes the microring802undisturbed and the microring802is said to be turned “off.” On the other hand, the microring802can be formed with suitable materials so that when an appropriate voltage is applied to the microring802, the effective refractive index neff′ shifts to the refractive value neffand the optical signal satisfies the resonance condition:
neffC=mλ
The optical signal is now coupled from the waveguide804into the microring802and the microring802is said to be turned “on.” When the voltage is subsequently turned “off,” the effective refractive index of the microring802shifts back to neff′ and the same optical signal propagates along the waveguide804undisturbed.

Photonic Crystals and Resonant Cavities

In certain system embodiments, the optoelectronic network switch can be implemented using two-dimensional photonic crystals where the waveguides are photonic crystal waveguides and the resonators are resonant cavities. Photonic crystals are photonic devices comprised of two or more different materials with dielectric properties that, when combined together in a regular pattern, can modify the propagation characteristics of optical signals. Two-dimensional photonic crystals can be comprised of a regular lattice of cylindrical holes fabricated in a dielectric or semiconductor slab. The cylindrical holes can be air holes or holes filled with a dielectric material that is different from the dielectric material of the slab. Two-dimensional photonic crystals can be designed to reflect optical signals within a specified frequency band. As a result, a two-dimensional photonic crystal can be designed and fabricated as a frequency-band stop filter to prevent the propagation of optical signals having frequencies within the photonic bandgap of the photonic crystal. Generally, the size and relative spacing of cylindrical holes control which wavelengths of optical signals are prohibited from propagating in the two-dimensional photonic crystal. However, defects can be introduced into the lattice of cylindrical holes to produce particular localized components. In particular, a resonant cavity, also referred to as a “point defect,” can be fabricated to produce a resonator that temporarily traps a narrow wavelength range of optical signals. A waveguide, also referred to as a “line defect,” can be fabricated to transmit optical signals with wavelengths that lie within a wavelength range of a photonic bandgap.

FIG. 11shows a top view of a photonic crystal waveguide1102, a resonant cavity1104formed in a slab1106in accordance with embodiments of the present invention. Circles, such as circle1108, represent holes that span the height of the slab1106. A resonant cavity can be created by omitting, increasing, or decreasing the size of a select cylindrical hole. In particular, the resonant cavity1104is created by omitting a cylindrical hole. Photonic crystal waveguides are optical transmission paths that can be used to direct optical signals within a particular wavelength range of the photonic crystal bandgap. Waveguides can be fabricated by changing the diameter of certain cylindrical holes within a column or row of cylindrical holes, or by omitting rows or columns of cylindrical holes. The waveguide1102is created by omitting an entire row of cylindrical holes. The holes surrounding the resonant cavity1104and the waveguide1102form a two-dimensional Bragg mirror that temporarily traps optical signals in the frequency range of the photonic crystal bandgap. Networks of branching waveguides can be used to direct optical signals in numerous different pathways through the photonic crystal. The diameter of an electromagnetic signal propagating along a waveguide can be as small as λ/3n, where n is the refractive index of the slab, while a harmonic mode volume of a resonant cavity can be as small as 2λ/3n.

Waveguides and resonant cavities may be less than 100% effective in preventing optical signals from escaping into the area immediately surrounding the waveguides and resonant cavities. For example, optical signals within a frequency range in the photonic bandgap propagating along a waveguide also tend to diffuse into the region surrounding the waveguide. Optical signals entering the area surrounding the waveguide1102or the resonant cavity1104experience an exponential decay in amplitude in a process called “evanescence.” As a result, the resonant cavity1102is located within a short distance of the waveguide1102to allow certain wavelengths of optical signals carried by the waveguide1104to be evanescently coupled, as represented by directional arrow1110, from the waveguide1102into the resonant cavity1104. Depending on a resonant cavity1104Q factor, an extracted optical signal can remain trapped in the resonant cavity1104and resonate for a while.

FIG. 12Ashows a resonant cavity1202and portion of slab1204configured in accordance with embodiments of the present invention. The resonant cavity1202is created by omitting a cylindrical hole. The diameter of the resonant cavity1202and the pattern and diameter of cylindrical holes surrounding the resonant cavity1202, such as cylindrical hole1206, can be selected to temporarily trap a specific wavelength of an optical signal within the resonant cavity1202. The slab1204is located on top of a glass substrate1208. As shown inFIG. 12A, in certain embodiments, the slab1204may be comprised of an intrinsic layer1210sandwiched between a p-type semiconductor layer1212and an n-type semiconductor layer1214forming a p-i-n junction resonant cavity1202.

FIG. 12Bshows a cross-sectional view of a first electronically tunable resonant cavity configured in accordance with embodiments of the present invention. The resonant cavity1202is sandwiched between two electrodes1220and1222. The slab1204can be comprised of the p-i-n junction layers1210,1212, and1214or a single dielectric or semiconductor layer. Applying a voltage across the resonant cavity1202changes the effective refractive index of the resonant cavity1202, which can be shift the resonant cavity1202into or out of resonance with a particular wavelength of an optical signal propagating in a nearby waveguide (not shown).

FIG. 12Cshows a cross-sectional view of a second electronically tunable resonant cavity configured in accordance with embodiments of the present invention. The resonant cavity1202is sandwiched between on two electrodes1224and1226. The slab1204can also be comprised of the pin layers1210,1212, and1214or a single layer, such as a single dielectric or semiconductor layer. Applying a voltage across the resonant cavity1202changes the effective refractive index of the resonant cavity1202, which can shift the resonant cavity1202into or out of resonance with a particular wavelength of an optical signal propagating in a nearby waveguide (not shown).

In certain embodiments, a resonant cavity can be operated as an electronically tunable photodetectors by placing a detector, such as detector902described above, adjacent to the resonant cavity.

Note that system embodiments of the present invention are not limited to microring resonators and photonic crystal resonant cavities. In other embodiments, any suitable resonator that can be configured to couple with a particular wavelength of an optical signal propagating along the waveguide can be used.