Abstract:
A method of operating an optical waveguide for transmitting an optical signal input to the optical waveguide with a first frequency. The optical waveguide includes a plurality of modulator circuits configured along an optical transmission channel. Each modulator circuit includes at least one resonant structure that resonates at the first frequency when the modulator circuit that includes the at least one resonant structure is at a resonant temperature. Each modulator circuit has a different resonant temperature.

Description:
This application is a divisional of U.S. application Ser. No. 14/153,342, filed Jan. 13, 2014, now U.S. Pat. No. 8,090,000, which is a divisional of U.S. application Ser. No. 13/117,844, filed May 27, 2011, now U.S. Pat. No. 8,644,649, the entire disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The embodiments of the invention relate generally to the field of silicon optical waveguides and, more particularly, to optical modulating circuits in silicon optical waveguides. 
     BACKGROUND OF THE INVENTION 
     Silicon-based integrated circuits have long been used as a platform for microelectronic applications. For example, microprocessors in computers, automobiles, avionics, mobile devices, control and display systems and in all manner of consumer and industrial electronics products are all traditionally based on a silicon platform that facilitates and directs the flow of electricity. As processing requirements have increased, the design of silicon-based integrated circuits has adapted to accommodate for faster processing times and increased communication bandwidths. Primarily, such performance gains have been the result of improvements in feature density, meaning that technologies have been developed to crowd ever-increasing numbers of features such as transistors onto a silicon chip. While efforts to increase feature density continue, alternative methods for increasing processing speeds and bandwidth on silicon-based platforms are also being developed. One such method is known as silicon photonics. 
     The term “silicon photonics” relates to the study and application of photonic systems that use silicon as an optical medium. Thus, instead of or in addition to using silicon to facilitate the flow of electricity, silicon is used to direct the flow of photons or light. While the speed of electricity and the speed of light are the same, light is able to carry data over a wider range of frequencies than electricity, meaning that the bandwidth of light is greater than that of electricity. Thus, a stream of light can carry more data than a comparable stream of electricity can during the same period of time. Accordingly, there are significant advantages to using light as a data carrier. Furthermore, using silicon as a preferred optical medium allows for application of and tight integration with existing silicon integrated circuit technologies. Silicon is transparent to infrared light with wavelengths above about 1.1 micrometers. Silicon also has a high refractive index of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers, thus facilitating integration with current nanoscale semiconductor technologies. Thus, silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip. 
     In practice, silicon photonics are implemented using silicon-on-insulator, or SOI, technology. In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer on which they are fabricated, it is necessary to have an intervening material. This is usually silica, which has a much lower refractive index of about 1.44 in the wavelength region of interest. This results in total internal reflection of light at the silicon-silica interface and thus transmitted light remains in the silicon. 
     A typical example of data propagation using light is illustrated in  FIG. 1 .  FIG. 1  illustrates an optical transmission system  100  that includes, for example, a silicon waveguide  110 . The silicon waveguide may make up the entirety of the optical transmission system  100  or just one or more portions of the system  100 . The system includes multiple data input channels  120 , where each channel  120  transmits data in the form of pulses of light. In order to simultaneously transmit the data carried on the multiple data channels  120 , the light in each channel  120  is modulated by a frequency modulator  130 . The modulated light from each channel  120  is then combined into a single transmission channel  150  using an optical multiplexer  140 . The multiplexed light is then transmitted along the single transmission channel  150  to an endpoint (not shown) where the light is de-multiplexed and demodulated before being used by an endpoint device. 
     Transmission of light in an optical waveguide is, however, affected by temperature. In general, changes in temperature can result in changes in the device dimensions (due to thermal expansion) and refractive indices of the materials used in the optical waveguide. More particularly, changes in temperature can affect the operation of the optical frequency modulators  130  illustrated in  FIG. 1 . Resonant photonic modulators are designed to only modulate received frequencies that are at or close to specific known frequencies. To only allow the modulation of the specific known frequencies, the modulators include resonant structures that act to filter out all but the known frequencies which are to be modulated by the modulators. Thus, the known frequencies are resonant frequencies of the resonant structures. Unfortunately, because the refractive indices of the resonant structures tend to change according to temperature, the specific frequencies that are modulated (i.e., the resonant frequencies) tend to deviate from the known frequencies as the temperature changes. Therefore, there is a need for silicon optical waveguides with modulator circuits that are tolerant of changes in temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an optical transmission system with a silicon optical waveguide. 
         FIG. 2  illustrates a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIG. 3  illustrates a frequency/intensity graph for a ring resonator in accordance with a disclosed embodiment. 
         FIG. 4  illustrates a method of operating a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIG. 5  illustrates a frequency/intensity graph for a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIG. 6  illustrates a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIG. 7  illustrates a frequency/intensity graph for a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIG. 8  illustrates a method of operating a silicon optical waveguide in accordance with a disclosed embodiment. 
         FIGS. 9A and 9B  illustrate silicon optical waveguides in accordance with disclosed embodiments. 
         FIG. 10  illustrates a processor system in accordance with a disclosed embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Because silicon-based integrated circuits are used in a variety of products and circumstances, silicon-based integrated circuits are likely to be exposed to a wide range of temperature conditions. In silicon-based optical waveguides, however, temperature fluctuations can result in decreased performance of included optical frequency modulators. Therefore, in order to enable a silicon optical waveguide to be more robust to temperature changes, an improved silicon optical waveguide with optical frequency modulators is herein disclosed. 
     One embodiment of an improved silicon optical waveguide  210  is illustrated in  FIG. 2 . The illustrated portion of the improved waveguide  210  includes an optical transmission channel  220  and two frequency modulator circuits  230 T 1 ,  230 T 2 , each serially coupled to the waveguide  210 . While only two frequency modulator circuits (referred to generally as  230 ) are illustrated, the improved silicon optical waveguide  210  could include any number of frequency modulator circuits  230 , as will become clear in the following explanation. In  FIG. 2 , each modulator circuit  230  includes two switches (e.g., switches  240 AT 1 ,  240 BT 1 ) and a modulator (e.g., modulator  250 T 1 ). The switches (referred to generally as  240 ) are coupled to the waveguide  210  so as to allow optical signals of a specific frequency to be shunted from the waveguide  210  to a modulator (referred to generally as  250 ) which is configured in parallel with the waveguide  210 . Thus, because the switches  240  are tuned to allow specific frequencies of optical signals access to the modulators  250 , the switches  240  act like band-pass filters that provide filtered signals to the modulators  250 . Optical signals that are not of the specific frequencies are allowed to continue without obstruction along the waveguide  210 . 
     In each modulator circuit  230 , one switch (e.g., switch  240 AT 1 ) is designated as an input switch (referred to generally as input switch  240 A). The other switch in the modulator circuit  230 , e.g., switch  240 BT 1 , is designated as an output switch (referred to generally as output switch  240 B). The input switch  240 A couples optical signals from the optical transmission channel  220  to the modulator  250 . The output switch  240 B couples optical signals from the modulator  250  back to the optical transmission channel  220 . 
     The switch frequency response is a result of the resonant properties of the switch  240 . Resonant optical switches are switches that only fully pass or allow transmission of signals that have frequencies that match the switch resonant frequency. For example, a ring resonator switch is essentially a looped optical waveguide whose circumference allows for constructive interference of a desired frequency. An optical ring resonator whose circumference is equal to an integer-multiple of an optical signal&#39;s wavelength (e.g., λ, 2λ, 3λ, etc.) that corresponds to a desired frequency will fully pass or transmit a signal with the desired frequency because the signal experiences constructive interference as it travels around the optical ring resonator. Conversely, the same optical ring resonator will fully block an optical signal where the ring resonator&#39;s circumference is equal to an odd-numbered integer-multiple of one-half of the optical signal&#39;s wavelength (e.g., (1/2)λ, (3/2)λ, (5/2)λ, etc.) due to the destructive interference that is generated. The optical ring resonator will only partially pass other frequencies. 
     The frequency-pass characteristics of a ring resonator are illustrated in the graph  300  of  FIG. 3 . For a given temperature T 0 , a ring resonator will fully pass a signal at the ring&#39;s resonant frequency ω 0 . This is evidenced in the graph  300  by the deep trough at frequency ω 0 , which indicates the ring resonator is significantly more sensitive to signals at frequency ω 0  than at other frequencies. Signals at frequencies that are far away from frequency ω 0  are essentially blocked while signals at frequencies near frequency ω 0  are only partially blocked. However, if the temperature changes to temperature T 1 , then the resonant frequency of the ring resonator is shifted to frequency ω 1 . Thus, the ring resonator acts as a temperature-dependent band-pass filter for the ring&#39;s resonant frequency. 
     Returning to  FIG. 2 , the ring resonator switches  24  provide filtered access to the optical modulators  250 . The optical modulators  250  may be resonant modulators or any other type of frequency modulator. Like the switches  240 , a resonant modulator is tuned to function at a specific temperature. Thus, as an example, a resonant modulator in series with a resonant switch is generally tuned to function at a temperature T 0  that corresponds with the temperature T 0  at which the switch passes a resonant frequency ω 0 . The optical modulators may also be of a non-resonant type. Irregardless, the resonant modulators  250  are driven by a common signal  260  to modulate the received frequency ω 0  received via input switch  240 A. The common signal  260  functions to inject charge into the modulators  250 , thus altering the index of refraction of the modulators  250  in order to effectuate a frequency modulation. The modulated frequency is then coupled back onto the optical transmission channel  220  via output switch  240 B. 
     In  FIG. 2 , each modulator circuit is tuned to a specific temperature. In other words, the switches  240  and modulator  250  within each modulator circuit  230  are selected and/or designed to filter and modulate a specific frequency at a specific temperature. In order to compensate for changes in temperature, each modulator circuit  230  is tuned to a temperature that is different from the tuned-temperature of the other modulator circuits  230 . Thus, when one modulator circuit is inactive because the temperature is different from its tuned temperature, another modulator circuit whose tuned temperature corresponds with the actual temperature is active. In this way, the waveguide  210  is designed to accommodate frequency modulation at a variety of temperatures. 
     A method  400  of operation of the waveguide  210  of  FIG. 2  is illustrated in  FIG. 4 . Initially, a laser input of a given frequency ω 0  is input to the waveguide (step  410 ). The input frequency ω 0  is to be modulated using one or more modulator circuits, depending on the waveguide temperature T. The modulator circuits are each tuned to modulate frequency ω 0  at different temperatures. Thus, for example, modulator circuit  230 T 1  is tuned to modulate frequency ω 0  at temperature T 1 . Modulator circuit  230 T 2  is tuned to modulate frequency ω 0  at temperature T 2  which differs from temperature T 1 . Additional modulator circuits  230 TN may be included that each modulate frequency ω 0  (step  430 ) at respective temperatures TN (step  420 ). 
     An intensity versus temperature graph  500  showing the response of all of the modulator circuits  230  at frequency ω 0  is illustrated in  FIG. 5 . The graph illustrates that for a given frequency ω 0 , each modulator circuit is active within a different temperature range. For example, at temperature T 1 , modulator circuit  230 T 1  is fully active and no other modulator circuit is active. At temperature T 2 , modulator circuit  230 T 2  is fully active and no other modulator circuit is active. Similarly, at temperature TN, modulator circuit  230 TN is fully active. At temperatures in between temperatures T 1  and T 2 , both modulator circuits  230 T 1  and  230 T 2  are only partially active. 
     Graph  500  also illustrates the modulation depth or degree of modulation provided by the waveguide  210  at different temperatures T. For example, at temperature T 1 , the illustrated modulation depth is approximately −20 dB. At temperature T 2 , the illustrated modulation depth is also approximately −20 dB. However, at a temperature in between temperatures T 1  and T 2 , the modulation depth provided by any one modulator circuit  230  is substantially less than −20 dB. Nevertheless, because of the overlap in modulator circuit activity, at temperatures in between temperatures T 1  and T 2 , both modulator circuits  230 T 1  and  230 T 2  provide some modulation. The total modulation depth provided is thus the sum of overlapping modulation depths provided by individual modulator circuits  230 . 
     It is possible to design a modulator array with a variable frequency response versus temperature graph so that overlapping of modulation depths only involves a few devices at any given temperature. Thus, during operation of the waveguide, if the waveguide temperature T is equal to temperature T 1 , modulator circuit  230 T 1  is active in modulating the received frequency ω 0  while other modulator circuits  230 T 2 ,  230 TN are not active. If the waveguide temperature T changes and equals temperature T 2 , modulator circuit  230 T 2  becomes active in modulating the received frequency ω 0  while the other modulator circuits  230 T 1 ,  230 TN are not active. If the waveguide temperature T changes and equals a temperature in between temperatures T 1  and T 2 , both modulator circuits  230 T 1  and  230 T 2  become partially active in modulating the received frequency ω 0  at a reduced modulation depth, though the modulator circuits  230 T 1  and  230 T 2  may be designed and configured so that the sum of modulation from both modulator circuits  230 T 1 ,  230 T 2  may be approximately equal to the maximum modulation depth of any individual modulator circuit  230 . This is the result when the modulation ranges of neighboring modulator circuits  230  overlap at a point where each modulator circuit&#39;s modulation depth is approximately one-half of the circuit&#39;s maximum modulation depth. Alternatively, some variance in modulation depth may be tolerated. For example, depending on the waveguide system&#39;s noise tolerance, a modulation depth of seventy-percent of the maximum modulation depth may be tolerated. 
     Thus, the optical waveguide system facilitates frequency modulation within a range of temperatures, where the temperature range is dependent upon the number of modulator circuits placed in series in the waveguide and the characteristics (e.g., the frequency/temperature response) of the modulator circuits. 
     In another embodiment, the resonant switches are removed and only resonant ring modulators are provided in series with the optical waveguide.  FIG. 6  illustrates this “switchless” embodiment of an optical waveguide  610 . In the embodiment of  FIG. 6 , two or more modulators (referred to generally as modulators  650 ) are positioned in series along the waveguide  610 . The modulators  650  are selected and/or designed to be resonant at a frequency ω 0  at different temperatures. Or, in other words, for a given temperature T, each modulator has a different resonant frequency. The resonant frequencies of neighboring modulators  650  are offset such that modulation overlap between the neighboring modulators  650  occurs with a modulation depth for each modulator  650  equal to approximately one-half their greatest modulation depth, as illustrated in  FIG. 7 . Thus, at a given temperature, T 1 , the optical circuit is designed such that a first modulator  650 T 1  is resonant. At a temperature T 2 , the first modulator  650 T 1  is no longer resonant, but a second modulator  650 T 2  is resonant. At a temperature T 3  in between temperatures T 1  and T 2 , both the first and second modulators  650 T 1 ,  650 T 2  are partially resonant. In this way, by cascading multiple modulators  650  in series with the optical transmission channel  220 , the optical waveguide  610  is made to be more robust against fluctuations in temperature. The number of modulators  650  used in the waveguide  610  is not limited except by considerations of cost, space and overall need. 
     A method  800  of operation of the waveguide system of  FIG. 6  is illustrated in  FIG. 8 . Initially, a laser input of a given frequency ω 0  is input to the waveguide (step  810 ). The input frequency ω 0  is to be modulated using one or more modulators, depending on the waveguide temperature T. The modulators are each tuned to modulate frequency ω 0  at different temperatures. Thus, for example, modulator  650 T 1  is tuned to modulate frequency ω 0  at temperature T 1 . Modulator  650 T 2  is tuned to modulate frequency ω 0  at temperature T 2  which differs from temperature T 1 . Additional modulators  650 TN may be included that each modulate frequency ω 0  (step  830 ) at respective temperatures TN (step  820 ). 
     During operation of the waveguide, if the waveguide temperature T is equal to temperature T 1 , modulator  650 T 1  is active in modulating the received frequency ω 0  while other modulators  650 T 2 ,  650 TN are not active. If the waveguide temperature T changes and equals temperature T 2 , modulator  650 T 2  becomes active in modulating the received frequency ω 0  while the other modulators  650 T 1 ,  650 TN are not active. If the waveguide temperature T changes and equals a temperature in between temperatures T 1  and T 2 , both modulators  650 T 1  and  650 T 2  become partially active in modulating the received frequency ω 0  at a reduced modulation depth. Both modulators are driven from the same signal, and hence both can work in conjunction to encode the signal on the received frequency ω 0 . 
     The waveguides  210 ,  610  may additionally be modified as illustrated in  FIGS. 9A and 9B . In  FIGS. 9A and 9B , waveguides  910 A and  910 B, respectively, are modified by the addition of a temperature sensor  920  and a control circuit  960 . In the waveguides  910 A,  910 B, operation of the modulators  250 ,  650  is optimized by using a temperature sensor  920  whose output enables a control circuit  960  to actively drive the modulators  250 ,  650 . For example, a control algorithm could be used to use the sensed temperature of the optical waveguide to drive specific modulators at specific sensed temperatures. In this way, specific modulators may be driven to provide greater modulation depth for given frequencies than the modulation depth provided by a purely passive modulation circuit. Additionally, the sensed temperature information may be used to help generate specific wavelengths for transmission along the waveguide so that the generated wavelengths correspond to those that the other side of the communications link or waveguide expects to receive. 
     The improved optical waveguides may be fabricated as part of an integrated circuit. The corresponding integrated circuits may be utilized in a typical processor system. For example,  FIG. 10  illustrates a typical processor system  1500  which includes a processor and/or memory device employing improved silicon optical waveguides such as optical waveguides  210 ,  610 ,  910 A,  910 B in accordance with the above described embodiments. A processor system, such as a computer system, generally comprises a central processing unit (CPU)  1510 , such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device  1520  over a bus  1590 . A memory device  1400  communicates with the CPU  1510  over bus  1590  typically through a memory controller. The memory device may include RAM, a hard drive, a FLASH drive or removable memory for example. In the case of a computer system, the processor system may include peripheral devices such as removable media devices  1550  which communicate with CPU  1510  over the bus  1590 . If desired, the memory device  1400  may be combined with the processor, for example CPU  1510 , as a single integrated circuit. 
     Any one or more of the components of the processor system  1500  may include one or more of the silicon optical waveguides described above. For example, CPU  1510 , I/O device  1520  and memory device  1400  may include silicon optical waveguides. In addition, communication between two or more of the processor system components via bus  1590  may be via silicon optical waveguides  210 ,  610 ,  910 A,  910 B. 
     The above description and drawings should only be considered illustrative of exemplary embodiments that achieve the features and advantages described herein. Modification and substitutions to specific process conditions and structures can be made. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.