Laser driver circuit and system

[0002] 1. Field

[0003] 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.

[0004] 2. Information

[0005] 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 P802.3ae and Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) published by the International Telecommunications Union (ITU). To transmit data in the optical transmission medium, a laser device typically modulates an optical signal using wave division multiplexing (WDM) in response to a data signal.

[0006] 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 device circuit may be formed in a single complementary metal oxide semiconductor (CMOS) device. The laser device 58 receives a modulation current 60 to power the transmission of an optical signal 62 in an optical transmission medium. 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 field effect transistor (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. Short channel lengths in such a CMOS device may introduce channel length modulation effects, causing nonlinearities in the output modulation current 60 (e.g., affecting the ratio of the reference current 52 to the modulation current 60). The resulting magnitude of the modulation current 60, therefore, may not be sufficiently accurately generated for high data rate communication in an optical transmission medium.

[0007] 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.

[0008] FIG. 1 shows a schematic diagram of a prior art laser driver circuit to provide a modulation current to a laser device.

[0009] 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.

[0010] 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.

[0011] FIG. 4 shows a schematic diagram of a laser driver circuit according to an embodiment of the physical medium dependent section shown in FIG. 3.

[0012] 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.

[0013] A “data bus” as referred to herein relates to circuitry for transmitting data between devices. A “multiplexed data bus” as referred to herein relates to a data bus that is capable of transmitting data among two or more devices coupled to the multiplexed data bus. A multiplexed data bus may transmit data messages to a device coupled to the multiplexed data bus according to an address associated with the device or a position on the multiplexed data bus where the device is coupled. However, this is merely an example of a multiplexed data bus and embodiments of the present invention are not limited in this respect.

[0014] 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 and which 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.

[0015] 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 signal in an optical transmission medium. For example, a laser driver circuit may provide a “modulation current” to a laser device to be used in modulating an optical signal. However, this is merely an example of a laser driver circuit and embodiments of the present invention are not limited in these respects.

[0016] An “operational amplifier” as referred to herein relates to a circuit or device to provide an output signal at an output terminal in response to a differential signal provided to input terminals. For example, an operational amplifier may provide an output voltage at the output terminal as a gain of a differential voltage applied to inverting and non-inverting input terminals. However, this is merely an example of an operational amplifier and embodiments of the present invention are not limited in this respect.

[0017] A “current mirror circuit” as referred to herein relates to a circuit to provide an output current in response to an input current. For example, a current mirror circuit may provide an output current having a magnitude that is substantially proportional to the magnitude of an input current. However, this is merely an example of a current mirror circuit and embodiments of the present invention are not limited in these respects.

[0018] Briefly, an embodiment of the present invention relates to a laser driver circuit to provide a modulation current. A current mirror circuit may generate the modulation current in response to a reference current. The current mirror circuit may comprise an operational amplifier providing an output signal to gates of transistors forming the current mirror circuit. However, this is merely an example embodiment and other embodiments of the present invention are not limited in these respects.

[0019] 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.

[0020] 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 in based upon a serial data signal provided by the clock and data recovery circuitry.

[0021] 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 P.802.3ae, 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/SDH) standard published by the International Telecommunications Union (ITU). 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.

[0022] 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 on 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.

[0023] 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 P.802.3ae, 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.

[0024] 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 modulate a modulation current 216 in response to a data signal from a PMA section. A laser device 208 may then modulate and power the transmitted optical signal 210 in response to the modulation current 216.

[0025] FIG. 4 shows a schematic diagram of a laser driver circuit 300 according to an embodiment of the laser driver circuit 222 shown in FIG. 3. The laser driver circuit 300 may be formed in a single complementary metal oxide semiconductor (CMOS) device. The laser device 308 may modulate an optical signal 324 an optical transmission medium in response to a modulation current 316. The modulation current 316 may be switched by FETs 330 and 332 in response to a data signal 328. The laser driver circuit 300 comprises an output stage including a field effect transistor (FET) 306 providing the modulation current 316 to the laser device at a desired current magnitude. However, this is merely an example of how a laser device may receive a modulation current and a data signal to modulate an optical signal, and embodiments of the present invention are not limited in these respects.

[0026] The FET 306 and an FET 304 provide a current mirror circuit to generate the modulation current 316 having a magnitude that is substantially proportional to the magnitude of a reference current 302. Accordingly, the magnitude of the modulation current 316 is substantially a scalar multiple of the reference current 302 where the scalar multiple is based, at least in part, on the physical characteristics and behavior of the FETs 304 and 306 (e.g., as formed in a CMOS implementation). The reference current 302 may be generated by, for example, a controlled voltage source (not shown) applied across an off-chip resistor (not shown). Given the scalar multiple of the current mirror (to generate the modulation current 316 in response to the reference current 302), the modulation current 316 may be set by selecting a particular resistance for the off-chip resistor. However, this is merely an example of determining a reference current input to a laser driver circuit, and embodiments of the present invention are not limited in this respect.

[0027] According to an embodiment, an operational amplifier 312 comprises an output terminal coupled to gates 314 of the FETs 304 to provide negative feedback from the drain terminals 320 and 316. The output signal of the operational amplifier 312 may cause the drain to source voltages for the respective FETs 304 and 306 (i.e., voltage between terminals 320 and 322 of FET 304, and voltage between terminals 316 and 318 of FET 306) to be maintained substantially the same. Accordingly, the resulting current mirror circuit may provide a substantially linear and predictable response to the reference current 302 in generating the modulation current 316, notwithstanding short channel lengths in a CMOS implementation of the current mirror.

[0028] The current mirror of driver circuit 300 comprises n-channel FETs 304 and 306. However, it should be understood that laser driver circuits according to other embodiments may comprise a current mirror comprising p-channel FETs without departing from the invention.

[0029] In the illustrated embodiment, the operational amplifier 312 receives inputs from the drain terminals 320 and 328 at non-inverting and inverting input terminals, respectively. A capacitor 326 may be coupled to the output of the operational amplifier 312, and the gates 314 of the FETs 304 and 306 to stabilize the negative feedback loop. The capacitance of the capacitor 326 may be selected to maintain any phase shift of the feedback loop to within a phase margin (e.g., 60 degrees or more) based upon the gain associated with the operational amplifier 312 and size of the FETs 304 and 306. However, this is merely an example of sizing a capacitor to stabilize a negative feedback loop and embodiments of the present invention are not limited in these respects.

[0030] At an initial condition, neither the FET 304 nor 306 may carry sufficient current to prevent a “lock-up” condition. According to an embodiment, a startup circuit (not shown) may be used to initialize the negative feedback loop until reaching steady state stability by applying a set minimum voltage to the gate terminals 314 of the FETs 304 and 306. Upon reaching steady state (e.g., at the gate voltage at gate terminals 314), the startup circuit may be disabled.

[0031] 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.