Patent Publication Number: US-2006007969-A1

Title: Short pulse optical interconnect

Description:
FIELD OF THE INVENTION  
      The present invention relates to the field of optical communications. More specifically, the present invention relates to an optical interconnect using short pulses.  
     BACKGROUND  
      Optical communications usually involve modulating an optical carrier. A common optical carrier is a light beam from a laser. A laser beam can be modulated by turning the laser on and off, or by selectively redirecting the beam. By modulating a beam in particular patterns, a beam can be encoded with data to convey information.  
      A common example of optical communications is a fiber optic telephone network. At each end of a telephone conversation, sound can be captured and turned into an electrical signal. The electrical signal can be converted into an optical signal by modulating a laser beam. The modulated beam can be directed down an optical fiber. A photodetector at the other end of the optical fiber can convert the modulated light back into an electrical signal, and the electrical signal can be converted back into sound. The same basic process can be used to convey virtually any kind of information on virtually any scale, be it within a microchip, from microchip to microchip, from computer to computer, across the country, or around the world.  
      Optical communications can provide a number of advantages over electrical communications. For example, optical signals are largely immune to electric and magnetic interference, making for much “cleaner” signals. As a result, optical communications can be much faster than electrical communications because there is less noise or static to drown out the data. Even in optical communications though, the data usually start out as electrical signals. In which case, the biggest limiting factor to the speed and quality of optical communications often comes from the electrical equipment.  
      For instance, turning a laser on or off, or redirecting a laser beam, takes the electrical equipment a certain amount of time. That is, rather than crisp, instantaneous transitions between on and off, the amplitude of an optical carrier will ramp up and down over time as it is modulated by the electrical equipment. When the data rate is high compared to this transition time, the carrier amplitude may look like a sinusoid, drifting from one data state to the next. The more gradual and less distinct the slope of this transition is, the more the quality of the signal suffers. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      Examples of the present invention are illustrated in the accompanying drawings. The accompanying drawings, however, do not limit the scope of the present invention. Similar references in the drawings indicate similar elements.  
       FIG. 1  illustrates one embodiment of data and carrier signals.  
       FIG. 2  illustrates one embodiment of the present invention.  
       FIGS. 3-6  illustrate various embodiments of pulse train modulation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, those skilled in the art will understand that the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, well known methods, procedures, components, and circuits have not been described in detail. Parts of the description will be presented using terminology commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. Repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.  
      Embodiments of the present invention can provide crisp, almost instantaneous state transitions in optical communications for a wide range of data rates. Rather than using an optical carrier signal that is a continuous wave, embodiments of the present invention use an optical pulse train as a carrier signal. This pulse train carrier signal can be modulated with data by selectively passing pulses.  
      A pulse train can be created using a pulse laser, also called a mode-locked laser. By appropriately selecting the physical properties of a laser, the laser can be made to naturally generate highly uniform pulses of light at a highly uniform pulse frequency and duty ratio (ratio of light-to-no-light per cycle). By designing lasers with different physical properties, pulse trains can be achieved for a wide range of pulse frequencies and duty ratios. A pulse laser&#39;s energy for each cycle of the pulse train is concentrated into the pulse. So, for a given pulse frequency and laser power, the smaller the duty ratio, the higher the amplitude of the pulse. Very short pulses can be designed that approach the characteristics of an impulse function, with almost infinite slope.  
      Embodiments of the present invention can use these short pulses to achieve almost instantaneous state transitions. The electrical modulation of the carrier is only needed to selectively pass pulses. In other words, rather than relying on the electrical modulation of the optical carrier to provide the slope of a transition, the optical carrier itself can provide the slope. Using short pulses, embodiments of the present invention can not only improve signal quality, but can also reduce power compared to a continuous wave optical carrier.  
       FIG. 1  illustrates one embodiment of a digital data signal  110 , a corresponding electrical signal  120 , an optical continuous wave  130 , a corresponding modulated continuous wave  140 , a pulse train  150 , and a corresponding modulated pulse train  160 . Data signal  110  comprises a series of one&#39;s and zero&#39;s. The corresponding electrical signal  120  represents the one&#39;s with a higher voltage level and represents the zero&#39;s with a lower voltage level. In other embodiments, the electrical signal can represent the data in any of a number of ways.  
      Optical continuous wave  130  comprises an optical frequency signal. That is, the wavelength and frequency of the signal are based on the color of the laser light. Optical frequencies tend to be in the Terahertz range. Electrical signals tend to be in the Megahertz or Gigahertz range. In which case, each bit represented by an electrical signal may occupy many thousands of optical wavelengths.  
      Modulated continuous wave  140  illustrates how continuous wave carrier  130  might be modulated to represent data  110 . The electrical signal  120  can be combined with continuous wave  130 , forming an envelope filled by the continuous wave. As with the electrical signal, one&#39;s can be represented by high amplitudes of the carrier and zero&#39;s can be represented by low amplitudes of the carrier.  
      The modulation can be accomplished in any of a number of ways. For instance, the electrical signal  120  could be used to directly drive a laser die generating the carrier. Or, the electrical signal  120  could be used to drive a variable optical attenuator or a Mach-Zhender modulator to manipulate the amplitude of the carrier wave after it is generated by a laser die.  
      In each of these cases, the slope of a transition from one data state to another is determined by the slope of the electrical signal that controls the modulation. Moreover, noise on the electrical signal can change the shape of the carrier&#39;s envelope, degrading the quality of the optical signal.  
      Pulse train  150  illustrates one embodiment of a pulse train that may be generated by a pulse laser. Different laser designs can produce different kinds of pulse trains, with different pulse widths and different pulse frequencies. Pulses can be made quite short, having, for instance, on the order of 100 optical wavelengths per pulse.  
      Modulated pulse train  160  illustrates how pulse train  150  may be modulated to represent data  110 . The electrical signal  120  can be used to selectively pass pulses from pulse train  150 . By tuning the data rate of the electrical signal to the pulse rate of the pulse train, each cycle can represent one bit of data. One&#39;s can be represented by the presence of a pulse in a cycle, and zero&#39;s can be represented by the absence of a pulse in a cycle. Other embodiments may represent data with pulses in any of a number of ways, such as various patterns of pulses representing various types of data.  
      Any of a number of approaches can be used to selectively pass pulses. For example, the same variable optical attenuator or Mach-Zhender modulator that can be used to modulate a continuous wave carrier can also be used selectively pass pulses.  
      No matter what kind of modulation is used for the pulse train, the slope of a transition from one data state to another is likely to be the slope of the carrier&#39;s pulse. With this steep slope, transitions can be almost instantaneous. In which case, noise on the electrical signal is likely to have little or no effect on the quality of the optical signal.  
      In addition to improved signal quality, pulse train modulation can reduce the average power of optical communications. For example, a photodetector may require 100 milliwatts of optical power to register a logical one. In which case, a continuous wave carrier may need to operate at a continuous average power of over 100 milliwatts. The power from each cycle of a pulse train, however, is concentrated into the pulse. So, for instance, if the duty ratio of a pulse train is 1 to 100, the amplitude of the pulse may be about 100 times the amplitude of a continuous wave having the same average power. In which case, the average power of the pulse train could theoretically be reduced to just 1 milliwatt and still produce pulses of 100 milliwatts that can be detected by the photodetector.  
      Put another way, in the case of a 10 gigahertz data rate, each cycle is about 100 picoseconds long. A continuous wave carrier may generate a 100 milliwatt signal throughout all 100 picoseconds of each cycle. In contrast, a pulse train with a 1 to 100 duty ratio may generate a 100 milliwatt signal for just 1 picosecond of each cycle.  
       FIG. 2  illustrates one embodiment of the present invention. A pulse laser  210  generates a pulse train  230  that is received by modulator  220 . Modulator  220  also receives a data signal  240  which modulator  220  uses to selectively pass pulses from pulse train  230  to generate modulated pulse train  250 . These basic components can be used in a wide variety of configurations and applications. A few examples are shown below in  FIGS. 3 through 6 .  
       FIG. 3  illustrates one embodiment of pulse train modulation for on-chip and/or off-chip communications. Pulse laser  310  generates pulse train  335 . An optical conductor directs the pulse train to chip  320 . Any of a number of optical conductors can be used. In the illustrated embodiment, the optical conductor is either an optical fiber or a waveguide. Other embodiments may use a combination of both.  
      Chip  320  could be any of a number of devices, such as a microprocessor, a digital signal processor (DSP), a application-specific integrated circuit (ASIC), a programmable gate array (PGA), or the like. In any case, chip  320  includes modulators  345 . In alternate embodiments, one or both of the modulators could be discreet components separate from chip  320 .  
      Beam splitter  375  provides pulse train  335  to both modulators  345  through various on-chip waveguides  340 . Chip  320  generates or receives data signals (not shown) to drive the modulators to encode data onto the pulse trains by selectively passing pulses.  
      On-chip modulated pulse train  370  is received by photodetector  350 , which generates electric current in response to photons. Each pulse from modulated pulse train  370  can generate a current of electrons that can be interpreted as data by receiver  355 . Other embodiments may use any of a number of different approaches to capture or make use of the data, including various forms of pure optical processing.  
      Off-chip modulated pulse train  365  travels out of chip  320 . Modulated pulse train  365  could be used in any of a number of ways, and could travel to any number of other components.  
      In other embodiments, rather than using optical conductors, such as optical fibers and waveguides, the beams can simply travel through open space. Also, in other embodiments, additional beam splitters could be used to supply pulse train  335  to any number of individual or arrayed modulators, both in chip  320  as well as outside chip  320 . Any of a number of beam splitting devices could be used.  
       FIG. 4  illustrates another embodiment of pulse train modulation. In the illustrated embodiment, microprocessor  410  includes a number of laser drivers  420 . The laser drivers could each represent independent signals generated by microprocessor  410 , or an array of two or more of the laser drivers could operate together to drive a multi-bit bus.  
      In any case, electrical data signals  430  from drivers  420  are supplied to laser unit  440 . Laser unit  440  generates modulated pulse trains  460  on optical fibers or waveguides  450 . Laser unit  440  may comprise a number of modulators (not shown), one for each of the data signals  430 , and at least one pulse laser (not shown) to feed the modulators. The modulators may be arranged in an array or they may operate independently. In the case of an array, a single pulse laser may be adequate to feed all of the modulators. In the case of independently operated modulators, a separate pulse laser may be needed for each modulator. Laser unit  440  may comprise a single chip, or it may comprise a number of discreet components, or it may comprise some combination of one or more discreet components and one or more chips.  
       FIG. 5  illustrates another embodiment of pulse train modulation. Microprocessor  510  is coupled to printed circuit board  530 , and laser unit  540  is coupled to microprocessor  510 . The coupling  550  between the components could be, for instance, wave bonded or a flip chip package. Microprocessor  510  includes a number of laser drivers  520  which drive modulators (not shown) in laser unit  540 . Laser unit  540  includes at least one pulse laser (not shown) to feed the modulators. As in  FIG. 4 , the laser drivers, modulators, and pulse laser(s) can take any of a number of forms and configurations. In any case, laser unit  540  generates modulated pulse trains  560 .  
       FIG. 6  illustrates one embodiment of chip-to-chip pulse train communications. Chip  620  includes a modulator  630  that is fed by pulse laser  610  and driven by laser driver  640 . Chip  650  receives the modulated pulse train from chip  620 . Photodetector  660  can generate electric current for each pulse that is received. Receiver  670  can interpret the electric as data. In other embodiments, pulse laser  610  may be part of chip  620  and laser driver  640  may be separate from chip  620 .  
       FIGS. 2 through 6  illustrate a number of specific examples of the present invention. Alternate embodiments may arrange components differently, may include additional components, may include fewer components, and may combine one or more components.  
      Thus, a short pulse optical interconnect is described. Whereas many alterations and modifications of the present invention will be comprehended by a person skilled in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, references to details of particular embodiments are not intended to limit the scope of the claims.