Abstract:
A laser diode driver circuit is disclosed that uses a cascode output stage having high-impedance load and a matching network for reducing mismatch interference. Due to the high-impedance load, on-chip dummy current is less than that required for laser diode drivers with matched loads. Accordingly, the number of transistors of said output stage can be reduced. Moreover, with an AC-coupled active load circuit replacing the matched resistance of the matching network, the power efficiency is improved. A laser diode driver circuit, in accordance with the present invention, can be applied to an optical transmitter with low power requirement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from the following U.S. provisional Patent Application, the disclosure of which is incorporated by reference in its entirety for all purposes: U.S. Provisional Patent Application Ser. No. 60/288,752, Cheh-Ming Jeff Liu and Neng-Haung Sheng entitled, “LASER DIODE DRIVER WITH HIGH POWER EFFICIENCY,” filed May 4, 2001 
    
    
     BACKGROUND—FIELD OF THE INVENTION 
     The present invention relates to laser diode drivers and modulator drivers for fiber-optic communication applications and more particularly to laser diode drivers with output matching networks to improve power efficiency. 
     BACKGROUND—DESCRIPTION OF PRIOR ART 
     U.S. Pat. No. 5,721,456 to N. Kebukawa, assigned to Mitsubishi Denki Kabushiki Kaisha, discloses an optical transmitter in which a light-emitting device is driven by a differential circuit. FIG. 1 shows an optical transmitter with a reflection absorbing circuit  107  coupling between a common-emitter differential amplifier and a laser diode  105 . The impedance of the differential amplifier is approximately that of an open circuit while the impedance at the laser diode end is that of a very low resistor, when the laser diode  107  is activated by the biasing circuit  106 . There exists a significant mismatch at the interface resulting in considerable reflection. The reflection absorbing circuit  107  can absorb the reflection due to the mismatch between the high impendence differential amplifier and the low impedance laser diode  105 . This approach provides a high efficiency means of delivering the modulation current to the laser diode  105 . 
     However, in most modern laser diode modules, there is a matched termination as well as a RF choke for external biasing. Moreover, the integration of an optical transmitter is normally comprised of a laser diode module, a laser diode driver, which is in die format or in package format, and a matched transmission line at the interface. Therefore, with the typical configurations, there is no mismatch at the laser module or at the transmission line. Conventional laser diode drivers are designed with their output impedance matched to the transmission line and the laser diode module. Nevertheless, there is considerable power consumption associated with the matched load inside the laser diode driver. A laser diode driver with a high output impedance can increase the efficiency of delivering modulation current to the matched laser diode module at the expense of mismatch interference between the laser diode driver and the transmission line. 
     In the article “A 10-Gb/s Laser Diode/Modulator Driver IC With a Dual-Mode Actively Matched Output Buffer”, IEEE Journal of Solid-State Circuits, vol. 36, No. 9, Sept. 2001, pp. 1314-1320, H. Ransijn, et al., disclose an active circuit that can be employed to reduce mismatching at the interface as demonstrated illustrated in FIG. 2. A unity-gain matching amplifier  202 , MA, with an output impedance  204 , R m , is coupled between the output  206 , V o , with a modulation current  208 , I o , driving the load  210 , Z o , and a dependent source  212 , V o ′, which is a combination of a fractional current source  214 , I o /k, and a multiple impedance  216 , kxZ o . Also shown in FIG. 2 is an offset  218 , V D , and a terminating impedance  220 , R L . Depending on the applications, the driver with this actively matched buffer can provide a DC-coupled back termination to optimize the matching condition at the interface between the output of the laser diode driver and laser diode module. Due to the matching amplifier  202 , MA, the mismatch at the interface can be minimized without sacrificing the transmitting power. Therefore, the power efficiency of the output buffer is virtually twice that a conventional driver. However, because of the unknown external offset voltage of the laser diode, this approach requires a DC control loop to be robust to the variation in laser turn-on voltage. This DC control loop results in extra components and less output swing. 
     SUMMARY OF THE INVENTION 
     To achieve high-speed application, a differential amplifier with cascode configuration is utilized as an output stage of the laser diode driver. In order to reduce the power consumption, a high-impedance resistor is employed as a load for output transistors. The value of the load resistor is higher than the matched resistance of the laser diode module. The high-impedance transistor load ensures most of the modulated current from the transistors flows through the laser diode rather than through the load itself. Nevertheless, without additional circuitry, the mismatch of impedance can induce significant interference at the interface. Instead of a direct connection between the laser diode module and the transistor output, a matching network is employed. The matching network can be a combination of lumped elements, e.g., resistors and capacitors, as well as distributed elements, e.g., transmission lines. Because the mismatch interference is significant at high frequencies, the matching condition between the laser diode and the transistor output can be optimized to reduce high-frequency mismatch by introducing a capacitor in series with a resistor. The in-series resistor is selected to be close to matched resistance of the laser diode module. The matching network with a high DC load, a matched RF load in series with a capacitor, and distributed elements, can significantly reduce the mismatch interference between the high-impedance at the output of a cascode stage and the matched impedance at the laser diode module. 
     Furthermore, as opposed to Ransijn, et al., an AC-coupled active load is utilized to replace the RF matched resistor and thereby further increasing the power efficiency as compared to a passive matching network. The AC-coupled active matching network obviates the need for a DC control loop and can potentially increase the output swing experienced with the teachings of Ransijn, et al. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a prior art optical transmitter with a reflection absorbing circuit. 
     FIG. 2 illustrates a prior art DC-coupled active load circuit that can be employed to reduce interface mismatching. 
     FIG. 3 is a circuit drawing illustrating the laser diode driver output stage of the preferred embodiment of the instant invention. 
     FIGS. 4A and 4B show respectively a frequency response of the driving-point impedance of the circuit driver and the return loss response sensitivity to transmission line effects. 
     FIG. 5 is a circuit drawing illustrating the laser diode driver output stage of an alternative embodiment of the instant invention with an AC-coupled active load circuit. 
     FIG. 6 is a circuit drawing providing further detail of the fractional amplifier of the alternative embodiment of the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 3, the driver circuit  300 , also termed a laser diode driver or LDD, is used to drive a laser diode module  330 . A bond wire  331  connects the bond pad  319  and the transmission line  332 . The laser diode  333  is connected to the opposite end of the transmission line  332  through a matching resistor  334 . The laser diode  333  is biased by the bias circuit  336  through the RF choke  335 . The matched resistance, Z o , of the laser diode module  330  is a combination of the matching resistor  334  and the turn-on resistance of the laser diode  333 . 
     Transistors  310  and  311  form a differential pair with emitter degeneration resistors  324  and  325  connected to a current generator  320 . Common-base transistors  322  and  323 , in conjunction with common-emitter transistors  310  and  311 , form a cascode stage. A high-resistance resistor  313  and module  312  are the loads of the differential pair. A first transmission line  314  is the extension to a matching network  302 , which is comprised of a second high-resistance resistor  317  in parallel with a resistor  315 , the resistor  315  in parallel with the second high-resistance resistor  317  being in series with a capacitor  316 . The second transmission line  318  is an extension between the matching network  302  and the bond pad  319 . 
     Common-base transistors  322  and  323  increase amplifier bandwidth by reducing the increased capacitances of transistors  310  and  311  generated by the Miller effect. Moreover, due to the reduced capacitive coupling, the cascode stage offers a better isolation and improves the matching condition at high frequencies as opposed to a bare common-emitter differential amplifier as in FIG.  1 . High-resistance resistors  313  and  317  operate to ensure most current passes through the matched-resistance laser diode module. High-resistance resistors  313  and  317  also provide the DC path in the absence of the laser diode  333 . 
     The mismatch interference is determined by the matching condition between the high impedance transistor output and the matched impedance bond pad  319 . The overall matching condition can be optimized by the matching network  302  functioning in association with the transmission lines  314  and  318 . Thus, in a larger sense, the matching network can be considered as comprising the resistive and capacitive elements, as disclosed, as well as the transmission lines and/or other distributed elements. Since the mismatch interference occurs significantly only at high frequencies, it follows that matching condition optimization is better achieved at high frequencies. This optimization can be obtained by introducing an optimizing resistor  315  in series with a capacitor  314 , both being in parallel with the second high-resistance resistor  317 . The resistance value of resistor  315  is selected to be much smaller than the high-resistance resistors  313  and  317  and close to the matched resistance, Z o , of laser diode module  330 . The reciprocal of the time constant, that is the inverted multiplicative product of the optimizing resistor  315  and its in-series capacitor  316 , yields the corner frequency  401 , ω c  (FIG.  4 A), above which the matching network  302  is approximately equivalent to the optimizing resistor  315 . 
     FIG. 4A shows a frequency response of the driving-point impedance of the circuit driver  300 . At frequencies lower than ω z    402 , the impedance of the matching network  302  is equivalent to a high-resistance resistor  313  that is in parallel with the second high-resistance resistor  317 . In order to ensure most of the transistor current is delivered to the laser diode  333 , the high-resistance resistors  313  and  317  are designed to be much larger in impedance than the matched resistance, Z o , of the laser diode module  330 . Mismatch interference is negligible at low frequencies. At frequencies higher than the cutoff frequency  401 , ω c , the driving-point impedance of circuit driver  300  is approximately equivalent to the optimizing resistor  315 , where the resistance of the resistor  315  is selected to be close to the matched resistance, Z o , of the laser diode module  330  and thereby improve the matching condition at high frequencies. 
     The second transmission line  318  plays a role as an impedance transformer and is used to modify the impedance response of matching network  302  as shown in FIG.  4 B. Without the second transmission line  318 , the first return loss response  403  is higher than the return loss response  404  achieved with the use of the second transmission line  318  at the interesting frequencies. 
     FIG. 5 illustrates an alternative embodiment of the invention. An active load circuit is used to replace the optimizing resistor  315 . The active load circuit is comprised of a fractional amplifier  503  and an emitter-follower transistor  561  biased by a current source  560 . The driving-point impedance, Z x , of the active-load circuit can be adjusted by the bias current  560 . FIG. 6 shows an alternative embodiment as a detailed circuit for the fractional amplifier  503 . Fundamentally, in order to achieve high power efficiency, the bias current  540  of the fractional amplifier  503  is designed to be a fraction 1/k (k&gt;1) of the bias current source  520 . Referring to FIG. 6, resistors  654 - 657  are selected to make the fractional amplifier  503  have virtually the same gain response as the buffer circuit  301 . The emitter follower transistor  561  acts as a unity-gain amplifier. By properly adjusting the bias current  560  and selecting the size of emitter follower transistor  561 , the driving-point impedance Z x  can be designed close to the matched resistance Z o  of the laser diode module  330 . Therefore, with the AC coupling capacitor  316 , the active load circuit provides matching conditions at high frequencies. Finally, due to the AC coupling, the laser diode driver  300  does not require a DC control loop to compensate the turn-on voltage variation of the laser diode  333 . 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. 
     In addition to the equivalents of the claimed elements, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.