Abstract:
Aspects of a method and system for feedback during optical communications are provided. In one embodiment, a system for optical communications comprises a digital-to-analog converter (DAC), a driver, and a transmit optical subsystem. The DAC is operable to receive a digital code of a plurality of digital codes and output an analog current signal having an analog current level of a plurality of analog current levels. The driver is operable to condition the analog current signal output from the digital-to-analog converter. The transmit optical subsystem is operable to generate an optical signal from the conditioned analog current signal. A digital modification of an input digital signal is dynamically controlled by a feedback path according to one or more characteristics of the optical signal. The one or more characteristics comprise a nonlinearity that may be temperature dependent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. provisional patent application Ser. No. 62/166,220, filed May 25, 2015, which is incorporated herein by reference as if fully set forth herein. 
     
    
     BACKGROUND 
       [0002]    Limitations and disadvantages of conventional and traditional approaches to optical communications will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    Systems and methods are provided for a transmit optical sub-assembly with local feedback, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims. 
         [0004]    These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       [0005]      FIG. 1A  shows a first example closed-loop optical communication system in accordance with aspects of this disclosure. 
         [0006]      FIG. 1B  shows a second example closed-loop optical communication system in accordance with aspects of this disclosure. 
         [0007]      FIG. 1C  shows another example closed-loop optical communication system with local transmit optical sub-assembly feedback in accordance with aspects of the disclosure. 
         [0008]      FIG. 2  is a flowchart illustrating operation of a closed-loop optical communication system with a local transmit optical sub-assembly feedback in accordance with aspects of this disclosure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]      FIG. 1A  shows a first example closed-loop optical communication system in accordance with aspects of this disclosure. The system  100  comprises an transmit and receive electrical subsystems  101  and  134 , transmit optical sub-assemblies (TOSAs)  112   a  and  112   b,  receive optical sub-assemblies (ROSAs)  118   a  and  118   b,  and optical fibers  116   a  and  116   b.    
         [0010]    Each of the subsystems  101  and  134  comprises a transmit digital signal processing circuit  102 , a receive digital signal processing circuit  126 , a digital-to-analog converter (DAC)  104 , an analog-to-digital converter (ADC)  124 , a PLL  108 , and a CPU  110  (where the different instances of each component are labeled ‘a’ and ‘b’, respectively). Each TOSA  112  comprises a laser diode driver  106 , and a laser diode  114 . Each ROSA  118  comprises a photodiode  120 , and a transimpedance amplifier  122 . The TOSA  112   a,  optical fiber  116   a,  and ROSA  118   a  are collectively referred to as ‘optical link A” and TOSA  112   b,  optical fiber  116   b,  and ROSA  118   b  are collectively referred to as “optical link B.” 
         [0011]    Each of the CPUs  110   a  and  110   b  is operable to manage operations of a respective one the electrical subsystems  101  and  134 . Such management may comprise, for example, each of the CPUs  110   a  and  110   b  receiving feedback via a respective one of the optical links and configuring its DSP  102 , DSP  126 , DAC  104 , and ADC  124  based on the received feedback. Each of the CPUs  110  may also generate a feedback signals based on output of its respective DSP  126 . 
         [0012]    Each PLL  108  is operable to generate one or more timing signals such as sample clocks for the DAC  104  and ADC  124 . 
         [0013]    Each DSP  102  is operable to receive one or more streams of data and process the data to generate a signal suitable for directly modulating a respective one of the TOSs  112 . 
         [0014]    Each DAC  104  is operable to convert the digital signal output by a respective one of DSPs  102  to generate an analog waveform. Example configuration and operation of the DACs  104  is described below with reference to  FIGS. 3C and 4 . 
         [0015]    Each driver  106  is operable to suitably condition the output of DAC  104   a  for application to a respective one of laser diodes  114 . 
         [0016]    Each laser diode  114  may comprise a semiconductor laser that is operable to generate a light beam having an intensity that is proportional to the current output by its respective driver  106  and at a wavelength that coincides with a minimum of dispersion in the optical fiber. The laser may be modulated with a data signal to be communicated via the optical fiber, where bandwidth limitations are reduced due to low dispersion and attenuation. The input current to output optical power of a typical laser diode may be highly nonlinear and vary greatly over temperature. Methods and systems for dealing with such nonlinearity and temperature dependence are further discussed below. 
         [0017]    Each photodiode  120  is operable generate an output current proportional to the intensity of light incident on it. 
         [0018]    Each transimpedance amplifier  122  is operable to convert the current output by a respective photodiode  120  to a voltage with a suitable range for input to a respective one of the ADCs  124 . 
         [0019]    Each ADC  124  is operable to convert the analog voltage present at its input to a corresponding digital value. 
         [0020]    Each DSP  126  is operable to perform various operations on the received signal output by its respective ADC  124 . Each DSP  126  may be operable to analyze a received signal to determine various characteristics of the optical link over which it was received. Such characteristics may include, for example: a nonlinearity of the optical link (e.g., coefficients of a Volterra series that models the link) and a temperature of the laser diode  114   a  of the optical link. The nonlinearity may be determined by, for example, comparing received signals (e.g., pilots or decoded data) with expected signals. The temperature may be indirectly determined based on known behavior of the optical components over temperature and/or determined directly from a temperature measurement reported by the optical components (e.g., on a control or “out-of-band” channel). 
         [0021]      FIG. 1B  shows a second example closed-loop optical communication system in accordance with aspects of this disclosure. The system  150  of  FIG. 1B  is similar to the system  100  of  FIG. 1A  except that electrical subsystem  101  is replaced by two discrete electrical subsystems  101   a  and  101   b  and electrical subsystem  134  is replaced by two discrete electrical subsystems  134   a  and  134   b.  In order to facilitate the feedback of the characteristics of the optical links, the electrical subsystems  101   a  and  101   b  comprise interface circuits  106   a  and  106   b  which are connected to each other via connection  138  and via which feedback about optical link A, received via optical link B, can be communicated to CPU  110   a  and used for configuring electrical subsystem  101   a.  Similarly, the electrical subsystems  134   a  and  134   b  comprise interface circuits  128   a  and  128   b  which are connected to each other via connection  136  and via which feedback about optical link B, received via optical link A, can be communicated to CPU  110   b  and used for configuring electrical subsystem  134   b.    
         [0022]      FIG. 1C  shows another example closed-loop optical communication system with local transmit optical sub-assembly feedback in accordance with aspects of the disclosure. As compared to the system  100  and  150 , the system  180  comprises a feedback path  309  directly from the TOSA  112   a  to the transceiver chip so that a feedback path/channel is not required from the opposite end of the optical fibers. The system  180  comprises a TOSA feedback path  309  via a monitor photodiode  311  that monitors the output of the TOSA laser  114   a  and communicates an electrical signal back into the transceiver circuitry, which includes a feedback TIA  313 , an ADC  315 , a model extraction module  317 , and a predistortion module  305 . 
         [0023]    The optical transceiver circuitry also includes a clock data recovery module  301 , a modulation and encoding module  303 , a pre-equalizer  307 , a continuous tile linear equalizer  319 , receiver ADC  124   a,  a speculative digital front end (DFE)  321 , a low-latency digital clock data recovery  323 , and a receive output demultiplexer  325 . 
         [0024]    In an example scenario, the monitor photodiode  311  comprises a backside monitor photodiode that monitors a back facet of the laser diode  114   a.  In another example scenario, an optical tap may be utilized to couple a portion of the optical output of the laser diode  114   a  to the monitor photodiode  311 . The monitor photodiode  311  may comprise a high-speed, high-bandwidth photodiode, i.e., on the order of the frequency of the optical signal, similar to the ROSA photodiode  120   b.  This is as compared to conventional backside monitor photodiodes that monitor laser output power changes with temperature, for example, which is a slow time-varying parameter. In this manner, the monitor photodiode  311  may directly measure high-frequency impairments from the laser diode  114   a  in the optical signal  116   a  and communicate this signal back to the driver circuitry  106   a.    
         [0025]    The feedback TIA  313  may amplify the received feedback electrical signal and an ADC  315  may convert this signal to a digital signal. The digitized signal may be input to the model extraction module  317  that may model the received signal and compare it to the desired digital signal. An output based on this comparison may be utilized by the predistortion module  305  to apply a predistortion signal to the signal communicated to the DAC  104   a  and pre-equalizer  301 . The predistortion may compensate for the impairments and non-linearities from the laser diode  114   a  thereby increasing output power and bandwidth into the optical fiber  116   a.    
         [0026]    Since the feedback is local, i.e., at one end of the optical link, this impairment/distortion suppression is independent of the type of modulation, whether it be OFDM, PAM 4, or NRZ, for example. In addition, this also means that out-of-band signaling is not needed to provide feedback from one end of the optical link to the other. 
         [0027]    Furthermore, since the monitor photodiode  311  is a high-frequency, high-bandwidth, i.e., on the order of the TOSA laser  114   a  and ROSA photodiode  120   b  (e.g., 25, 50, 100 GHz or higher), the suppression of high-frequency impairments and distortions is enabled. Furthermore, the monitoring may be continuous to change the predistortion as the distortion or non-linearity in the laser changes, or may be intermittent with a lower duty cycle, if the distortion or non-linearity is not constantly changing or stays within an acceptable level between monitoring periods, thereby reducing power usage. 
         [0028]      FIG. 2  is a flowchart illustrating the operation of a closed-loop optical communication system with a local transmit optical sub-assembly feedback in accordance with aspects of this disclosure. In block  202 , a first electrical signal is generated in the optical transceiver for data communication. In block  204 , the laser diode may be modulated with the data signal. 
         [0029]    In block  206 , the optical signal generated by the laser diode may be monitored by a high-speed, high-bandwidth photodiode. In block  208 , the output of the monitor photodiode may be amplified and converted to a digital signal. In block  210 , the digital signal may be communicated to a model extraction module. 
         [0030]    In block  212 , impairments, non-linearity, and/or other distortion may be determined by comparing the signal from the feedback path to the desired data signal in the model extraction module. In block  214 , the determined impairments, non-linearity, and/or other distortion may be utilized to generate a pre-distortion signal. In block  216 , the pre-distortion signal may be applied to the desired data signal to be communicated. 
         [0031]    In block  218 , the data plus predistortion signal may be utilized to modulate the laser diode for communication via the optical fiber, and the process may repeat on a constant basis or periodic basis, repeating the process from block  206  where the laser diode is monitored. 
         [0032]    Other embodiments of the invention may provide a non-transitory computer readable medium and/or storage medium, and/or a non-transitory machine readable medium and/or storage medium, having stored thereon, a machine code and/or a computer program having at least one code section executable by a machine and/or a computer, thereby causing the machine and/or computer to perform the processes as described herein. 
         [0033]    The present invention may be realized in hardware, software, or a combination of hardware and software. The present invention may be realized in a centralized fashion in at least one computing system, or in a distributed fashion where different elements are spread across several interconnected computing systems. Any kind of computing system or other apparatus adapted for carrying out the methods described herein is suited. A typical combination of hardware and software may be a general-purpose computing system with a program or other code that, when being loaded and executed, controls the computing system such that it carries out the methods described herein. Another typical implementation may comprise an application specific integrated circuit or chip. 
         [0034]    While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. 
         [0035]    As utilized herein the terms “circuits” and “circuitry” refer to physical electronic components (i.e. hardware) and any software and/or firmware (“code”) which may configure the hardware, be executed by the hardware, and or otherwise be associated with the hardware. As used herein, for example, a particular processor and memory may comprise a first “circuit” when executing a first one or more lines of code and may comprise a second “circuit” when executing a second one or more lines of code. As utilized herein, “and/or” means any one or more of the items in the list joined by “and/or”. As an example, “x and/or y” means any element of the three-element set {(x), (y), (x, y)}. In other words, “x and/or y” means “one or both of x and y”. As another example, “x, y, and/or z” means any element of the seven-element set { (x), (y), (z), (x, y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means “one or more of x, y and z”. As utilized herein, the term “exemplary” means serving as a non-limiting example, instance, or illustration. As utilized herein, the terms “e.g.,” and “for example” set off lists of one or more non-limiting examples, instances, or illustrations. As utilized herein, circuitry is “operable” to perform a function whenever the circuitry comprises the necessary hardware and code (if any is necessary) to perform the function, regardless of whether performance of the function is disabled or not enabled (e.g., by a user-configurable setting, factory trim, etc.).