Abstract:
High-speed driver circuits interface with and enhance the performance of vertical-cavity surface-emitting laser (VCSEL) diodes used within fiber optic communication systems, according to embodiments of the invention. A reference diode is formed from a first transistor, and a first current mirror sets a modulation current through the reference diode for driving a light-emitting device. A second current mirror sets a reference biasing current to maintain a constant bias of the reference diode. A first capacitor, charged by a differential amplifier and coupled to the reference diode, supplies complementary charge to the reference diode during differential input voltage transitions. Components of the driver circuit provide asymmetrical operation of the laser, allowing rapid light-emitting-device turn-on and turn-off.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority under 35 U.S.C. §119 to United States Provisional Patent Application No. 60/073,540, filed Feb. 3, 1998, which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to fiber optic communication, and more particularly, to high-speed drivers for interfacing with and enhancing the performance of vertical-cavity surface-emitting laser (VCSEL) diodes used within fiber optic communication systems, as well as in other applications.  
           [0004]    2. Description of Related Art  
           [0005]    Optical transmission systems have three general components: the light source, the transmission medium, and the detector. Light sources for an optical transmission system are typically either Light Emitting Diodes (LEDs) or lasers. (Semiconductor lasers have distinct advantages over LEDs, including higher data rates and longer distance transmission capabilities.) Typically, a pulse of light from the light source indicates a one bit and the absence of light indicates a zero bit. The transmission medium is commonly ultra-thin glass fiber. The detector generates an electrical pulse when light falls upon it.  
           [0006]    Low-cost, high-performance, highly integrated fiber optic interface circuits are becoming increasingly necessary to meet the demands of high-speed digital data communication. With the advent of gigabit Ethernet systems, for example, fiber optic technology has become increasingly preferred. A fiber optic transmission line preferably uses a VCSEL diode as the light source to transmit optical data. In contrast to edge-emitting lasers, VCSELs have a vertical optical cavity that is perpendicular to the epitaxial growth direction. Beams emitting from an edge-emitting laser are highly astigmatic, making them less desirable in high-speed digital data communication applications. VCSELs typically emit a circularly symmetric Gaussian beam which is very conducive to high-efficiency coupling into optical fiber.  
           [0007]    The high-speed nature of fiber optic communication necessitates that the VCSEL diodes operate quickly, accurately and efficiently. To enhance the operation of the VCSEL diodes, new driver circuits designed specifically for the VCSEL diodes are needed. These new driver circuits need to address the speed demands of fiber optic communication systems, including the need for higher edge rates and integrated edge enhancement circuitry, low-voltage differential signaling input interfaces (LVDS), low overhead current requirement, good power supply rejection, low power supply requirement, optimization for common-cathode VCSEL connection, and multiple parallel driver integration.  
           [0008]    A typical prior art driver circuit for a laser diode appears in FIG. 1. Transistors T 10  and T 15  make up the differential input circuit, using input ports  10  and  15 . Included also are two current mirrors, the first being formed from transistors T 20  and T 25 . This current mirror sets the first reference current, IRef 1 , from current source  20 . The second current mirror, formed by transistors T 30  and T 35 , sets the second reference current, IRef 2 , from current source  25 .  
           [0009]    As a result of this prior art configuration, the dc bias current is required to flow even when the laser is off. This requires a high overhead current supply, which is undesirable. Because burst-mode optical transmitters require lasers to be off more than they are on, this constant current consumption is inefficient. The constant current draw also discourages driving multiple common cathode connected VCSELs or integrating multiple parallel drivers for use with VCSEL arrays. The relatively high current demands of such configurations do not integrate well with the present low-power fiber optic communications systems, which typically incorporate LVDS interfaces.  
           [0010]    Another limitation of the prior art circuit in FIG. 1 is that it contains no integrated edge-rate enhancement circuitry and no precise current controls. In FIG. 1, transistor T 40  switches the drive current to the laser diode. Gate-drain parasitic capacitance of transistor T 40  typically results in voltage spiking that can drive the laser to emit light longer than it should, causing “overshoot” and thus undesirably limiting the efficiency and speed with which the VCSEL can operate.  
           [0011]    Yet another limitation of the prior art circuit in FIG. 1 is that it has a narrow range of available driving current, which undesirably restricts its adaptability to drive VCSELs having different current requirements. This limitation further restricts using a plurality of common cathode connected VCSELs and further limits multiple parallel driver integration.  
           [0012]    Still another limitation of previous laser driver circuits is common anode connection. Common anode driving of VCSELs can be undesirable because contact with a common anode-connected VCSEL can cause a harmful electrical discharge. Not only can this pose a safety risk, but also damage to the VCSEL and related circuitry may result.  
           [0013]    There is a need in the industry for a driver circuit for a VCSEL diode that is cost-effective, safe, and preferably fabricated with complementary metal oxide semiconductor (CMOS) technology, while at the same time addresses the problems of typical driver circuits as outlined above.  
         SUMMARY OF THE INVENTION  
         [0014]    Driver circuits according to the embodiments of the invention substantially meet the above-described needs of the industry.  
           [0015]    An object of the present invention is to provide a light transceiver circuit having a driver, fabricated in low-cost CMOS integrated circuit technology, for interfacing with and enhancing the performance of a VCSEL diode.  
           [0016]    Another object of the present invention is to provide a driver circuit with an improved driver edge-rate, preferably less than 250 pS.  
           [0017]    Still another object of the present invention is to provide a VCSEL diode driver circuit responsive to an LVDS input interface.  
           [0018]    Yet another object of the present invention is to provide precision on-chip control of the modulation and bias currents.  
           [0019]    Another object of the present invention is to provide a driver circuit that requires low overhead supply current, preferably less than 10 mA.  
           [0020]    Still another object of the present invention is to provide a driver circuit that requires a low supply voltage, preferably less than five volts.  
           [0021]    Another object of the invention is to provide integrated adjustable negative charge peaking, and integrated edge rate enhancement circuitry to promote fast laser turn-off and turn-on.  
           [0022]    Yet another object of the invention is to provide common cathode connection for the light-emitting device with a single ground plane for safety, and a more reliable design.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a schematic diagram of a typical prior art driver circuit.  
         [0024]    [0024]FIG. 2 is a schematic diagram of a driver circuit for use with a VCSEL diode according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 3 is a block diagram showing an array of VCSEL drivers driving a common-cathode VCSEL array.  
         [0026]    [0026]FIG. 4 is a digital sampling oscilloscope image of an output waveform from a preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    Referring to FIG. 2, one embodiment of a driver circuit  30  for use with VCSEL diodes according to the present invention may be appreciated. A differential input, typically representing data to be transmitted, is received at ports InP and InN of differential amplifier  35 . The non-inverted output  40  of amplifier  35  is connected to the gate of transistor T 45 . The inverted output  45  of amplifier  35  is connected to inverter  50 . Inverted output  45  also charges capacitor C 1 .  
         [0028]    The source of transistor T 45  is connected to the drain of transistor T 50 , while the drain of transistor T 45  is connected to the drain of transistor T 55 . The gate of transistor T 50  is connected to Imod current source  55  and to the drain of transistor T 60 , while the source of transistor T 50  is connected to ground. Current source  55  is also connected to the gate of transistor T 60 , and the source of transistor T 60  is connected to ground. Transistors T 50  and T 60  make up current mirror  60 , the purpose of which is to generate a stable and predictable dc reference current for biasing various transistors in the circuit.  
         [0029]    The source of transistor T 55  is connected to voltage supply  65 , to the source of transistor T 65 , and to the source of transistor T 70 . The gate of transistor T 65  is connected to ground. The gate of transistor T 55  is connected to the gate of transistor T 70  and to the drain of transistor T 75 . The source of transistor T 75  is connected to ground, and the gate of transistor T 75  is connected to Ibias current source  70  and the drain of transistor T 80 . Ibias current source  70  is further connected to the gate of transistor T 80 . The source of transistor T 45  is connected to current mirror  60 .  
         [0030]    Transistors T 75  and T 80  make up current mirror  75 . Current mirror  75  maintains a constant bias of transistor T 55  and transistor T 70 . The drain of transistor T 65  is connected to inverter  50 , as is the drain of transistor T 85 . The source of transistor T 85  is connected to ground, and the gate of transistor T 85  is connected to voltage supply V+65. Capacitor C 1  is connected between the input of inverter  50  on one side, and the source of transistor T 55  and drain of transistor T 60  on the other. The output of inverter  50  is connected to capacitor C 2  and the gate and the source of transistor T 90 . Capacitor C 2  is also connected to the drains of transistors T 70  and T 90  and to current output, Iout. Capacitor C 2  is further connected to the source and drain of transistor T 90  and to the anode of diode  80 . The cathode of diode  80 , as well as the sources of transistors T 50 , T 60 , T 75 , T 80 , and T 85 , are connected directly to single ground plane  85 .  
         [0031]    The operation of driver circuit  30  may be described as follows. The differential input is received at ports InP and InN of amplifier  35 , whereby a voltage output is produced. This voltage output is converted to modulation current, Imod, by switching of a reference current, set by the current mirror  60 , through the reference threshold voltage of a reference diode formed of transistor T 55 . The reference threshold voltage of the reference diode (transistor T 55 ) is formed when the gate of transistor T 55  is connected to drain. Imod current is coupled through transistor T 70  to diode  80 , which is preferably a VCSEL diode.  
         [0032]    A substantially constant Ibias current is set through current mirror  75  and maintains a substantially constant bias of transistors T 55  and T 70 . Such biasing improves the turn-on speed of driver circuit  30 . Capacitor C 1  couples complementary charge into reference threshold voltage of the reference diode formed of transistor T 55  when the differential input transitions from high-to-low or low-to-high, to improve the driver edge-rate. Further edge-rate improvement is provided through energy coupled through the combination of capacitor C 2  and transistor T 90 . These devices provide an asymmetrical coupling of charge directly into light-emitting device  80 . Positive current is coupled through transistor T 90  and charges capacitor C 2 .  
         [0033]    The charge in capacitor C 2  is drained on the negative-going edge when the modulation current, Imod, is cut off. This asymmetrical operation provides the negative edge peaking to ensure more rapid laser turn-off, thereby reducing the effect of a turn-off “tail” that is often exhibited by VCSEL diodes in prior art circuits. Note that the speed and magnitude of the negative edge peaking is preferably controlled through adjustment of the effective resistance of transistors T 85  and T 65  as they enter the triode region of operation.  
         [0034]    The combination of the edge and charge coupling provided by capacitors C 1  and C 2  and transistor T 90  provides excellent edge characteristics in the laser light output of the VCSEL diode, opening the eye of the laser output waveform (see FIG. 4, described below) and advantageously reducing bit error rates for data transmission with bitrates in the Gbit/s range of operation. Driver circuit  30  advantageously accomplishes the desired edge characteristic without requiring external response-shaping circuitry that past drivers have required. Integration of the design ensures correct edge alignments and summing of the various charge and peaking effects without external adjustments upon assembly. Integration resolves many inherent matching problems encountered when constructing the circuit with discrete components.  
         [0035]    Temperature, threshold, and slope efficiency compensation of diode  80  is accomplished with precision, preferably by setting bias and modulation current values with simple resistor and thermistor devices.  
         [0036]    Embodiments of driver circuit  30  preferably are implemented entirely with a low-cost integrated circuit (IC) process. Ideally, driver circuit  30  is implemented using complementary metal oxide semiconductor (CMOS) technology. CMOS technology is a relatively low-cost IC process due to its present use in high-volume computer applications. Integration substantially ensures optimum time alignment of the edge rate enhancement features described above.  
         [0037]    Driver circuit  30  preferably is optimized to allow both single and multiple common cathode connected VCSELs to be driven. This common cathode configuration is promoted by the single ground plane  85  configuration, as shown in FIG. 2.  
         [0038]    [0038]FIG. 3 is a block diagram of one preferred embodiment of the invention, showing an array of VCSEL drivers according to the invention, driving a common-cathode VCSEL array. Multiple data input lines  90  match to the individual driver circuits  93  in driver array  95 . Driver output lines  97  match to a corresponding common-cathode VCSEL  99  in VCSEL array  100 . Light emissions  105  result, matching the corresponding data input. With the growing popularity of VCSELs and the ability to construct VCSEL arrays, the ability to construct an array of drivers linked to an array of VCSELs is one important feature of the invention, among others.  
         [0039]    Low overhead current and good power supply rejection of driver circuit  30  allow integration of multiple driver circuits  30  for use with VCSEL diode arrays, as shown in FIG. 3. Differential amplifier  35  (FIG. 2) provides good power supply rejection, reduced signal-to-noise ratios (SNR) and favorable slew rates.  
         [0040]    [0040]FIG. 4 is a digital sampling oscilloscope image of an output waveform  110  from a preferred embodiment of the present invention superimposed about an ideal waveform  111 . The “open eye” waveform is a histogram of waveform data superimposed on each other to yield the “open eye”. It is desired that the output waveform  110  be open and conform as closely as possible to the ideal waveform  111 . The more open the eye, the better the data transmission. Advantageously, output  120  represents a lack of ringing. At waveform output portion  130 , desired edge peaking is depicted, indicating clean laser turn-off. Waveform portion  125  illustrates the desirable high-edge rate obtained from the invention, needed for high-speed optical communication systems.  
         [0041]    Each of the transistors used and described above may be a field-effect transistor (FET) such as a MOS transistor, a bipolar transistor, a gallium arsenide (GaAs) FET or other similar transistor, as long as it is capable of the corresponding function as described herein.  
         [0042]    The specification is intended to be illustrative of the many variations and equivalents possible according to the invention. Various modifications in and changes to the above-described devices and methods will be apparent to those of ordinary skill. Though driver circuits were described with particularity for uses directed at high-speed fiber optic communications, other uses for such driver circuits are readily apparent to one of ordinary skill reading the specification. For example, VCSEL diodes have uses in myriad devices, such as bar code scanners, encoders, proximity sensors, laser printers, and laser range finders, among others.