Patent Publication Number: US-2005135444-A1

Title: Laser driver circuit and system

Description:
RELATED U.S. PATENT APPLICATIONS  
      The subject matter disclosed herein relates to U.S. pat. appl. Ser. No. 10/442,829, filed on May 21, 2003. 
    
    
     BACKGROUND  
      1. Field  
      The subject matter disclosed herein relates to data communication systems. In particular, the subject matter disclosed herein relates to transmitting data in an optical transmission medium.  
      2. Information  
      Data transmission in an optical transmission medium such as fiber optic cabling has enabled communication at data rates of 10 gigabits per second and beyond according to data transmission standards set forth in IEEE Std. 802.3ae-2002, Synchronous Optical Network/Synchronous Digital Hierarchy (SONET) protocol as indicated in a set of standards provided by the American National Standards Institute (ANSI T1.105.xx) or Synchronous Digital Hierarchy (SDH) as indicated in a set of recommendations provided by the International Telecommunications Union (e.g., ITU-T G.707, G.708, G.709, G.783 and G.784). To transmit data in the optical transmission medium, a laser device typically modulates an optical signal in response to a data signal. The laser device typically modulates the optical signal using wave division multiplexing (WDM) in response to the data signal.  
       FIG. 1  shows a schematic diagram of a prior art laser driver circuit  50  to provide a modulation current  60  to a laser device  58 . The laser driver circuit  50  may be formed in a single complementary metal oxide semiconductor (CMOS) device. The laser device  58  receives a bias current  62  combined with a modulated power signal to power the transmission of an optical signal in an optical transmission medium. The modulated power signal is generated by a switch transistor  66  formed as a field effect transistor (FET). As such, the switch transistor  66  selectively transmits the modulation current I MOD  to be combined with the bias current  62  based upon a voltage applied to a gate terminal of the switch transistor  66 . The laser device  58  typically also modulates the optical signal in response to a data signal. The laser driver circuit receives a reference current  52  generated by, for example, a controlled voltage source applied across an off-chip resistor. A diode coupled FET  54  and FET  56  form a current mirror to generate the modulation current  60  at a magnitude that is substantially proportional to the magnitude of the input reference current  52 .  
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
      Non-limiting and non-exhaustive embodiments of the present invention will be described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified.  
       FIG. 1  shows a prior art laser driver circuit.  
       FIG. 2  shows schematic diagram of a system to transmit in and receive data from an optical transmission medium according to an embodiment of the present invention.  
       FIG. 3  shows a schematic diagram of physical medium attachment and physical medium dependent sections of a data transmission system according to an embodiment of the system shown in  FIG. 2 .  
       FIG. 4  shows a schematic diagram of a laser driver circuit according to an embodiment of the physical medium dependent section shown in  FIG. 4 .  
    
    
     DETAILED DESCRIPTION  
      Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase “in one embodiment” or “an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in one or more embodiments.  
      An “optical transmission medium” as referred to herein relates to a transmission medium capable of transmitting light energy in an optical signal which is modulated by a data signal such that the data signal is recoverable by demodulating the optical signal. For example, an optical transmission medium may comprise fiber optic cabling coupled between a transmitting point and a receiving point. However, this is merely an example of an optical transmission medium and embodiments of the present invention are not limited in this respect.  
      A “laser device” as referred to herein relates to a device to transmit a light signal in response to a power source. For example, a laser device may transmit a light signal in an optical transmission medium which is modulated by a data signal. A laser device may comprise a laser diode to transmit a light signal in response to a current. However, these are merely examples of a laser device and embodiments of the present invention are not limited in these respects.  
      A “laser driver circuit” as referred to herein relates to a circuit to provide power to a laser device to be used for transmitting a light signal in an optical transmission medium. For example, a laser driver circuit may provide a controlled current signal to provide power for transmitting the light signal. However, this is merely an example of a laser driver circuit and embodiments of the present invention are not limited in these respects.  
      A laser driver circuit may provide a current signal to a laser device having a “bias current” component combined with a “data current” component which is modulated by a data signal. The data current may be generated by modulating a “modulation current” with the data signal. The modulation current may determine an extent to which the magnitude of the current signal may deviate from the bias current component. However, these are merely examples of a bias current and modulation current, and embodiments of the present invention are not limited in these respects.  
      A “reference modulation current” as referred to herein relates to a current signal having a magnitude that approximates a magnitude of a desired modulation current. For example, a reference modulation current may have a magnitude that is tailored to provide a data current signal according to specific characteristics of a laser device and a desired intensity of a light signal to be generated by the laser device to represent a data signal. However, this is merely an example of a reference modulation current and embodiments of the present invention are not limited in this respect.  
      A “transistor” as referred to herein relates to an active solid state device to generate an output current having a magnitude that is based upon an input signal. A “bipolar transistor” as referred to herein relates to a transistor that generates an output current having a magnitude that is based upon a current applied to a base terminal of the transistor. A “field effect transistor” (FET) as referred to herein relates to a transistor that generates an output current having a magnitude that is based upon a voltage applied to a gate terminal of the transistor. However, these are merely examples of a transistor, bipolar transistor and FET, and embodiments of the present invention are not limited in these respects.  
      A “photodiode” as referred to herein relates to a device that provides an output current in response to light energy collected on a surface. For example, a photodiode may provide an output voltage or an output current in response to charge collected at a photodiode gate. However, this is merely an example of a photodiode and embodiments of the present invention are not limited in this respect.  
      Briefly, an embodiment of the present invention relates to a laser driver circuit comprising a bipolar transistor for transmitting a modulated power signal to the laser device. The bipolar transistor may generate the modulated power signal in response to a modulation current and a base current representative of a serial data signal. The laser driver circuit may further comprise a circuit to combine a replica of the base current with a reference modulation current to provide the modulation current. However, this is merely an example embodiment and other embodiments are not limited in these respects.  
       FIG. 2  shows a schematic diagram of a system to transmit in and receive data from an optical transmission medium according to an embodiment of the present invention. An optical transceiver  102  may transmit or receive optical signals  110  or  112  in an optical transmission medium such as fiber optic cabling. The optical transceiver  102  may modulate the transmitted signal  110  or demodulate the received signal  112  according to any optical data transmission format such as, for example, wave division multiplexing wavelength division multiplexing (WDM) or multi-amplitude signaling (MAS). For example, a transmitter portion (not shown) of the optical transceiver  102  may employ WDM for transmitting multiple “lanes” of data in the optical transmission medium.  
      A physical medium dependent (PMD) section  104  may provide circuitry, such as a transimpedance amplifier (TIA) (not shown) and/or limiting amplifier (LIA) (not shown), to receive and condition an electrical signal from the optical transceiver  102  in response to the received optical signal  112 . The PMD section  104  may also provide to a laser device (not shown) in the optical transceiver  102  power from a laser driver circuit (not shown) for transmitting an optical signal. A physical medium attachment (PMA) section  106  may include clock and data recovery circuitry (not shown) and de-multiplexing circuitry (not shown) to recover data from a conditioned signal received from the PMD section  104 . The PMA section  106  may also comprise multiplexing circuitry (not shown) for transmitting data to the PMD section  104  in data lanes, and a serializer/deserializer (Serdes) for serializing a parallel data signal from a layer 2 section  108  and providing a parallel data signal to the layer 2 section  108  based upon a serial data signal provided by the clock and data recovery circuitry.  
      According to an embodiment, the layer 2 section  108  may comprise a media access control (MAC) device coupled to the PMA section  106  at a media independent interface (MII) as defined IEEE Std. 802.3ae-2002, clause 46. In other embodiments, the layer 2 section  108  may comprise forward error correction logic and a framer to transmit and receive data according to a version of the Synchronous Optical Network/Synchronous Digital Hierarchy (SONET) protocol as indicated in a set of standards provided by the American National Standards Institute or Synchronous Digital Hierarchy (SDH) as indicated in a set of recommendations provided by the International Telecommunications Union. However, these are merely examples of layer 2 devices that may provide a parallel data signal for transmission on an optical transmission medium, and embodiments of the present invention are not limited in these respects.  
      The layer 2 section  108  may also be coupled to any of several input/output (I/O) systems (not shown) for communication with other devices in a processing platform. Such an I/O system may include, for example, a multiplexed data bus coupled to a processing system or a multi-port switch fabric. The layer 2 section  108  may also be coupled to a multi-port switch fabric through a packet classifier device. However, these are merely examples of an I/O system which may be coupled to a layer 2 device and embodiments of the present invention are not limited in these respects.  
      The layer 2 device  108  may also be coupled to the PMA section  106  by a backplane interface (not shown) over a printed circuit board. Such a backplane interface may comprise devices providing a 10 Gigabit Ethernet Attachment Unit Interface (XAUI) as provided in IEEE Std. 802.3ae-2002, clause 47. In other embodiments, such a backplane interface may comprise any one of several versions of the System Packet Interface (SPI) as defined by the Optical Internetworking Forum (OIF). However, these are merely examples of a backplane interface to couple a layer 2 device to a PMA section and embodiments of the present invention are not limited in these respects.  
       FIG. 3  shows a schematic diagram of a system  200  to transmit data in and receive data from an optical transmission medium according to an embodiment of the system shown in  FIG. 2 . An optical transceiver  202  comprises a laser device  208  to transmit an optical signal  210  in an optical transmission medium and a photo detector section  214  to receive an optical signal  212  from the optical transmission medium. The photo detector section  214  may comprise one or more photodiodes (not shown) for converting the received optical signal  212  to one or more electrical signals to be provided to a TIA/LIA circuit  220 . A laser driver circuit  222  may provide a current signal  216  to the laser device  208  in response to a data signal from a PMA section  205 . The laser device  208  may then transmit optical signal  210  in response to the current signal  216 .  
       FIG. 4  shows a schematic diagram of a laser driver circuit  400  according to an embodiment of the laser driver circuit  222  shown in  FIG. 3 . According to an embodiment, the laser driver circuit  400  may be formed using a BiCMOS process to enable the formation of bipolar and field effect transistors on the same semiconductor device. Bipolar transistors may enable increased current switching speed over the use of field effect transistors. A bipolar transistor Q 2  may generate a data current in response to a data signal (e.g., data signal  218  as shown in  FIG. 3 ) received at the base terminal of bipolar transistor Q 2 . The data signal may be received at the base terminals of both bipolar transistors Q 1  and Q 2  such that the bipolar transistor Q 2  is turned on to generate a current for a “1” and bipolar transistor Q 1  is turned on to transmit a current to ground for a “0.” The data current generated by bipolar transistor Q 2  may be additively combined with a bias current I BIAS  to generate a power signal for powering a laser device  405 .  
      A reference modulation current Irefmod may be applied to the emitter terminals of bipolar transistors Q 1 , Q 2  and Q 3 . According to an embodiment, the output data current of the bipolar transistor Q 2  (in response to a data signal of “1”) to be combined with bias current I BIAS  has a magnitude that is substantially equal to the magnitude of the reference modulation current Irefmod. Accordingly, a transistor M 5  may generate a current that is substantially equal to a base current loss from the base terminal of bipolar transistor Q 2 .  
      According to an embodiment, the bipolar transistors Q 1 , Q 2  and Q 3  may be formed substantially identically and behave substantially the same in response to process, temperature and power supply variations. Transistors M 3 , M 4  and M 5  are mirror coupled such that they generate the same current in response to a gate voltage. The current at the base terminal of transistor bipolar transistor Q 3  is substantially equal to the current at the base terminal of bipolar transistor Q 2 . This current at the base terminal of bipolar transistor Q 3  is then measured and mirrored by transistors M 1 , M 2  and M 3  to feedback the base current loss to mirror coupled transistors M 4  and M 5 . Accordingly, the M 5  current provides the base loss current back to the emitter terminal of bipolar transistor Q 2 .  
      While there has been illustrated and described what are presently considered to be example embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.