Abstract:
In one embodiment, the invention provides an optical interconnect comprising a transmitter for generating and transmitting an optical signal, a receiver for receiving the optical signal from the transmitter and for converting the received optical signal to an electrical signal, and a pre-transmitter distort circuit for applying a pre-transmitter distort signal to the transmitter to adjust the shape of the optical signal generated by the transmitter. Distortions are introduced into the optical signal when the optical signal is generated, transmitted to the receiver, and converted to the electrical signal. As a result of the signal applied to the transmitter by the pre-transmitter distort circuit, the optical signal generated by the transmitter has distortions to compensate for the distortions introduced into the optical signal, wherein the electrical signal, into which the optical signal is converted, has a desired shape.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No.: MDA972-03-3-0004 awarded by Defense Advanced Research Projects Agency. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to fiber and waveguide optics, and more specifically, to optical interconnects in fiber optic transmissions. 
     Fiber optics and optical interconnects are used in a wide variety of applications. The use of optical fibers as a medium for transmission of digital data (including voice data) is becoming increasingly more common due to the high reliability and large bandwidth available with optical transmission systems. In addition, optical interconnects provide for a significant increase to the available bandwidth of board-level interconnects. 
     For example, short-reach interconnects over multi-mode optical fiber (MMF), on the order of 100 m, are widely used in computer systems, data centers, and campus networks. For these data links, the optical channel contributes relatively little signal degradation for 10-40 G data rates. In addition, generations of fiber with optimized modal bandwidth continue to be developed that minimize this primary source of signal degradation. The biggest challenge for short-reach interconnects is to produce active components, namely optical transmitters and receivers, that provide sufficient bandwidth to operate at high data rates (&gt;20 Gb/s) while maintaining low power consumption to maximize the power efficiency, commonly expressed in mW/Gb/s or pJ/bit: the energy required to transmit a bit of information. 
     The typical approach to implementing an optical link is to separate the transmitter and receiver and design them separately. The transmitter (TX) is optimized to make its digital optical output as ideal as possible, while the receiver (RX) is designed to receive the transmitted optical signal and convert it to an electrical signal with high sensitivity (to operate with as little optical power as possible at a given bit-rate), and with minimum added jitter. Both TX and RX are designed to meet their specifications while consuming a minimum amount of power. 
     The most straightforward method for building optical TXs and RXs is to utilize high-speed analog drivers and receiver amplifiers that have sufficient bandwidth to faithfully convert the electrical signal to an optical signal and back again with minimal distortion. However, as data rates have increased beyond 10 Gb/s, it is difficult to realize the multimode optical devices (lasers and detectors) and amplifier circuits with sufficient raw bandwidth with acceptable power consumption. To achieve the highest data rates, the lasers must be operated at high current densities and the circuits typically consume high amounts of electrical power to deliver the required speed performance. 
     BRIEF SUMMARY 
     Embodiments of the invention provide a method and system for compensating for distortions introduced into an optical signal, in the operation of an optical interconnect, as the optical signal is generated, transmitted to a receiver, and converted into an electrical signal. Generally, embodiments of the invention pre-distort the optical output of an optical transmitter such that when the optical signal is received by a receiver and converted into an electrical signal, the overall link performance is improved. The link improvement is quantified through multiple parameters: data rate capability, power efficiency, sensitivity or link margin, and timing margin improvement or reduced jitter. 
     In one embodiment, the invention provides an optical interconnect comprising a transmitter for generating and transmitting an optical signal, a receiver for receiving the optical signal from the transmitter and for converting the received optical signal to an electrical signal, and a pre-transmitter distort circuit connected to the transmitter for applying a pre-distort signal to the transmitter to adjust the shape of the optical signal generated by the transmitter. In the operation of this interconnect, distortions are introduced into the optical signal when the optical signal is generated, transmitted to the receiver, and converted to the electrical signal. As a result of the pre-distort signal applied to the transmitter from the pre-transmitter distort circuit, the optical signal generated by the transmitter has particular distortions that compensate for the above-mentioned distortions introduced into the optical signal as that signal is generated, transmitted and converted. As a result of this the electrical signal, into which the optical signal is converted, has a desired shape. 
     In an embodiment, the pre-transmitted distort circuit includes an equalizer circuit located in parallel with the main driver stage for the transmitter. In one embodiment, the equalizer circuit includes a time delay control and a signal strength control. The time delay control is used for controlling a time delay of a feed-forward component of the pre-distort signal applied to the transmitter, and the signal strength control is used for controlling the strength of said feed forward component of the pre-distort signal applied to the transmitter. 
     In one embodiment, the pre-transmitter distort circuit includes a main circuit portion and a feed forward circuit portion, and the time delay control and the strength control are located in series in said feed forward circuit portion. In an embodiment, the time delay control is continuously variable over a given time range, and the strength control is continuously variable over a given strength range. 
     In an embodiment, the equalizer circuit includes a main buffer on the main circuit portion of the equalizer circuit to buffer a main portion of the signal applied to the transmitter, and a tap buffer on the feed forward circuit portion of the equalizer circuit to buffer said feed forward component of the signal applied to the transmitter. 
     Embodiments of the invention apply equalization to overcome the limited performance of the optical components and the circuits that comprise the optical link, not to overcome signal impairments due to the fiber transmission. Equalization has been previously demonstrated to compensate for signal distortion arising from fiber bandwidth limitations and is widely used in long-distance optical communication systems in the form of fiber dispersion compensators. The approach in the current invention is very different. The output current or voltage of the transmitter circuit is pre-emphasized, possibly to the point of visible distortion. 
     The optical device connected to the transmitter circuit, in one embodiment a Vertical Cavity Surface Emitting Laser (VCSEL), converts the pre-emphasized/distorted output of the circuit into a pre-emphasized/distorted optical signal, but when the transmitter optical output is presented to a bandwidth-limited receiver, the data recovered by the receiver is dramatically improved. The present invention uses transmitter equalization or pre-distortion to optimize the total link performance, that is the electrical output of the receiver and not to improve the signal quality at an intermediate point such as the transmitter output or receiver input. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a typical optical data link. 
         FIG. 2  shows the optical output of a transmitter and the electrical output of a receiver in a link operated in a convention manner. 
         FIG. 3  shows the optical output of a transmitter and the electrical output of a receiver utilizing transmitter pre-distortion. 
         FIG. 4  is a schematic diagram of a feed forward equalizer circuit used in an embodiment of the invention to generate a pre-distorted transmitter output. 
         FIG. 5  shows eye diagrams that illustrate results of using the equalizer circuit of  FIG. 4  without and with pre-emphasis of the transmitter output. 
         FIG. 6  shows examples of the main buffer output and the tap buffer output of the circuit of  FIG. 4 , and the output of the circuit itself. 
         FIG. 7  illustrates a transmitter frequency response output and a receiver frequency response output of an optical interconnect using an embodiment of the invention. 
         FIG. 8  shows an N-tap feed forward equalizer that may be used in an embodiment of the invention. 
         FIG. 9  depicts an N-tap analog feed forward equalizer that may be used in an embodiment of the invention. 
         FIG. 10  illustrates an N-tap mixed digital and analog feed forward equalizer that may be used in an embodiment of the invention. 
         FIG. 11  shows an embodiment of the invention including a performance monitor. 
     
    
    
     DETAILED DESCRIPTION 
     A block diagram of a typical optical data link is shown in  FIG. 1 . A laser diode driver  12  (LDD) converts an incoming data stream into a modulation current that is used to directly modulate the optical output of a laser  14 . The laser driver is typically divided into a pre-driver stage  16  followed by an output stage  20  that directly interfaces with the laser. The modulated optical output of the laser is routed through a channel  22 , typically optical fiber or optical waveguides, and coupled into an optical receiver  24 . The optical receiver  24 , in the embodiment shown in  FIG. 1 , is a photodiode that converts the incident optical signal into a small electrical current. The photodiode is connected to a transimpedance amplifier (TIA)  26  that converts the photodiode current into a voltage with some amount of gain. The TIA is followed by one or more amplifier stages  30  that provide additional gain to increase the output amplitude of the receiver to a useful level (typically 0.1-1V). 
       FIG. 1  also shows logic devices  32  and  34  and buffer  36 . Logic device  32  applies digital data and logic control signals to the laser drive  12 , and logic device  34  receives the digital signal from amplifier  30  and converts that signal into a digital format. Buffer  36  is located between amplifier  30  and logic device  34  to drive the electrical link between the amplifier and the logic device. 
     Typically the optical link is designed such that the transmitter optical output is as clean as possible, meaning that the logical “1”/“0” transitions are as fast as possible, that the “1” and “0” levels are uniform with minimal over/undershoot, and that the jitter is as low as possible. All of these goals are captured in viewing an eye diagram that overlays many digital transitions over each other in time. An example of an optimized transmitter eye diagram at 20 Gb/s is shown in  FIG. 2  at  42 . The problem is that when the optimized TX signal is received, the eye diagram  44  of the receiver&#39;s electrical output is completely closed: none of the bits of information are successfully transferred through the optical link. 
     In accordance with an embodiment of the invention, the optical output of the optical transmitter is purposefully pre-distorted with significant over- and under-shoot and jitter. However, when the optical signal is presented to the optical receiver with limited bandwidth, the quality of the data recovery performed by the receiver is dramatically improved. This effect is shown in  FIG. 3  in which the pre-distorted TX output  46  shows significant degradation in terms of over- and under-shoot and jitter. However, when the pre-distorted optical signal is received, the receiver produces an open eye-diagram  48 , indicating that the bits of information are being successfully transferred through the optical link. 
     This pre-distortion technique has been demonstrated to be effective in enabling optical links to work at significantly higher data rates, as illustrated in the comparison between the receiver outputs in  FIGS. 2 and 3 . In addition, by enabling operation at higher data rates, the power efficiency of the link is improved in terms of pJ/bit since more bits can be transmitted for a given amount of power consumed. The link margin is also improved by a significant amount: at high data rates the pre-distortion technique can improve the receiver sensitivity (the minimum power required to operate at a given bit error ration) by &gt;6 dB (4×). In addition, the power consumption required to operate the pre-distortion circuit is very low, on the order of 5% of the total circuit power consumption. Therefore all of the benefits of increased data rate, power efficiency, and link margin come at the cost of a very minimal increase in raw power consumption (the per-bit power consumption is decreased). 
     A variety of circuits may be used to implement the transmitter pre-distortion described above. For example, as shown in  FIG. 4 , embodiments of the invention may use a feed-forward equalizer (FFE)  50  preceding a final driver stage directly connected to a VCSEL. The FFE amplifier has two controls, VBDELAY  52  and VBTAP  54  that control the time-delay and strength of the fed-forward portion of the signal that is subtracted from the main signal. These two signal portions make the shape and amount of the pre-distortion continuously variable, from no distortion to maximum distortion.  FIG. 5  shows, as example, an eye diagram  56  of a transmitter output with no distortion, and an eye diagram  58  of a transmitter output with heavily pre-emphasized transmitter output. 
     Circuit  50  includes a main circuit portion  60  and a tap or feed forward portion  62 . The main signal portion through circuit  50  parses from the circuit input, through linear amplifiers  64 , through main buffer  66 , and then to the circuit output. A portion of the main signal is tapped off the circuit portion  60  and is conducted through feed forward portion  62 . Circuit portion  62  includes time delay device  68 , and buffer  70 . Time delay control  52  is applied to time delay device  68 , and strength control  54  is applied to buffer  70 . 
       FIG. 6  illustrates the currents outputs of the main buffer  66 , tap buffer  70 , and of the equalizer circuit  50 . As can be seen, the main buffer  66  and the tap buffer  70  introduce different time delays, and the strength of the tap buffer output adjusted by strength control  54 . The main buffer output and the tap buffer output are combined to produce the FFE circuit output. 
     The pre-distortion technique is particularly effective for VCSELs and receivers with relatively low bandwidth. The pre-emphasis (pre-distortion) shown in  FIG. 4  emphasizes the high-frequency components of the signal compared to the lower frequency components. The VCSEL rise/fall times are improved and the receiver output is dramatically affected since as the signal traverses the receiver, which acts as a low-pass filter, the higher frequencies in the pre-distorted signal are attenuated. This results in a flat frequency response with a greater bandwidth compared to the case where the input signal is not pre-distorted. The effect of pre-distortion is illustrated through the example frequency response shown in  FIG. 7 . 
     The present invention may be used with a feed-forward equalizer having plural or multiple taps, and provides a digital, analog, or a mixed digital and analog implementation of the pre-transmitter distort signal.  FIG. 8  shows, for example, a feed forward equalizer  80  having N taps  82 . Each of these taps is connected in parallel with the transmitter and the driver stage for the transmitter. 
       FIG. 9  shows an N-tap analog feed-forward equalizer  90  having input x(t), a series of analog devices  92 , a summer  96 , and an output y(t). With reference to  FIGS. 8 and 9 , input x(t) would be received from the output of linear amplifier  64 , and output y(t) would be applied after buffer  66 . 
       FIG. 10  shows an N-tap mixed signal feed-forward equalizer  100  having input x(n), a series of digital devices  102 , an analog device  104 , a summer  108 , and an output y(n). With reference to  FIGS. 8 and 10 , input x(n) would be received from the output of linear amplifier  64 , and output y(n) would be applied after buffer  66 . 
     In one aspect, the invention provides a method of looking at the final received data in order to adjust the pre-distortion to an optimum point.  FIG. 11  illustrates an embodiment of this aspect of the invention. In particular,  FIG. 11  shows a transmitter  112 , receiver  114 , pre-distortion control  116 , receiver performance monitor  118 , optical signal channel  120 , and back-communication channel  122 . 
     The pre-distortion may be adjusted by monitoring the jitter or the bit-error rate of the receiver output. This adjustment could be either an initial calibration step of a monitoring circuit on the receiver side with a back-communication channel  122  to the transmitter. For a given link with a set bandwidth, the calibration setting for one part may be applicable to a large set of similar parts using the same components. 
     The fact that embodiments of the invention show the greatest improvement when applied to optical links with low-bandwidth VCSELs and receivers makes it extremely valuable. VCSEL speed and reliability are often inversely related: the bandwidth of a VCSEL improves as its bias current increases, but the lifetime is concurrently reduced. For receivers, the bandwidth and sensitivity are always linked and inversely proportional. Therefore, a receiver can be purposefully constructed with a bandwidth lower than that required to operate at a given data rate, but with higher sensitivity at lower data rates. This would mean that the receiver would not operate at the target date rate without transmitter pre-distortion, but the performance of the receiver, and therefore the complete optical link can be restored at the target data rate through transmitter pre-distortion. 
     Another example of where a lower bandwidth receiver may be very useful is to reduce packaging cost. The primary factor that determines the bandwidth of the front-end of an optical receiver is the capacitance of the photodiode. Therefore, as data rates increase, photodiodes always get smaller to contribute a smaller capacitance. But smaller photodiodes are more expensive to package since they are more sensitive to mechanical misalignment and so require very precise and costly assembly tools. Embodiments of the invention can help to alleviate this situation: a relatively large photodiode can be used that reduces the bandwidth of the receiver but the receiver (and link) can be successfully operated at a data rate beyond that expected based on the receiver bandwidth through the application of transmitter pre-distortion. 
     Embodiments of the invention significantly improve the sensitivity, power efficiency, and jitter of optical links at data rates higher than can be achieved without using embodiments of the invention. Although one implementation is to improve the speed and power efficiency of VCSEL based links, embodiments of the invention are widely applicable to a variety of optical interconnects utilizing different transmitter technologies, such as directly-modulated distributed feedback (DFB) lasers, electro-absorption (EA) or Mach-Zehnder (MZ) modulators and for Si-Photonic integrated modulators (MZ, ring-based, or EA). 
     Embodiments of this invention have a wide range of applications in all optical communication systems. Embodiments of the invention are particularly suited for applications in multimode optical interconnects where the VCSEL, photodiode, and circuits all have relatively low bandwidths due to their low-cost. Embodiments of the invention are also well suited for highly integrated silicon photonics designs, where power-efficient modulators and circuits may also be bandwidth limited. The demonstrated improvements in speed capability, power-efficiency, link margin and timing jitter make embodiments of this invention well suited to a wide variety of optical interconnects. 
     While it is apparent that the invention herein disclosed is well calculated to fulfill the objectives discussed above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.