Abstract:
A laser diode driver output stage for driving an associated laser diode device. In one aspect of the present invention, the laser diode driver output stage includes a driver circuit having at least one input node and an output node. The driver circuit is adapted to receive an input data signal at the at least one input node and provide an output signal at the output node in response to the data signal. The laser diode driver output stage further includes a transformer connected to the output node of the driver circuit. The transformer is adapted to receive the output signal at a first side and apply impedance compensation to the output signal to provide an output drive signal from a second side to drive the associated laser diode device.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to an improved laser diode driver integrated circuit, and in particular to a laser diode driver integrated circuit having an improved output stage. 
   BACKGROUND OF THE INVENTION 
   A majority of laser diode driver integrated circuits (ICs) for optical transmission can be broadly divided into two categories. The first category includes devices using direct modulation. This is generally used for short distance transmission in which a laser diode driver IC is used to directly drive a laser diode module with a drive current supplied by the laser diode driver IC. The second category includes devices which generally use external modulation. These are generally used for long-distance transmission in which, for example, an electroabsorption (EA) modulator driver IC is used to drive an electroabsorption (EA) modulator module. In both of these categories laser diode driver ICs can be found in both die forms in which the IC is assembled inside the laser module, and packaged forms in which the IC is assembled outside of the laser module and connected by a transmission line. 
   As a result of the operating frequencies of digital communication circuits, such as laser diode driver ICs, increasing beyond one gigahertz (GHz), lumped-element techniques for analyzing circuit behavior are no longer valid. As a result, scattering parameters, or S-parameters, have been developed for this purpose. S-parameters measurements become particularly important when the operating frequencies of laser diode driver integrated circuits are high enough that the size of circuit elements becomes a significant fraction, approximately one-tenth, of a wavelength of the transmitted signal. In addition, it is difficult to measure voltages and currents in order to obtain impedance measurements at frequencies in the microwave range. 
   In general, S-parameters are a measure of the ratio of reflected waves to incident waves delivered to a device to be measured. In particular, the S11 parameter, also referred to as the input return loss, is the measured ratio of the reflected wave from the device input to the incident wave on the device input. The S22 parameter, also referred to as the output return loss, is the measured ratio of the reflected wave from the device output to the incident wave on the device output. The S11 parameter can be related to the input impedance, and the S22 parameter can be related to the output impedance. Accordingly, S-parameters may be used to characterize the impedance properties of a device or transmission line. 
   Due to the need for ever increasing data transmission rates, the use of conventional laser diode driver integrated circuits results in numerous signal quality problems due to high frequency impedance effects. Thus there is a need for laser diode driver integrated circuits that provide improved output signal quality and reduced impedance mismatch at high frequencies of operation. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is directed to a laser diode driver output stage for driving an associated laser diode device. The laser diode driver output stage includes a driver circuit having at least one input node and an output node. The driver circuit is adapted to receive an input data signal at the at least one input node and provide an output signal at the output node in response to the data signal. The laser diode driver output stage further includes a transformer connected to the output node of the driver circuit. The transformer is adapted to receive the output signal at a first side and apply impedance compensation to the output signal so as to provide an output drive signal from a second side to drive the associated laser diode device. 
   Another embodiment of the present invention is directed to a method for providing an improved drive signal from a laser diode driver output stage to a laser diode device. The method includes the steps of receiving an input data signal at a driver circuit, providing an output signal from the driver circuit in response to the data signal, and receiving the output signal at an input to a first side of a transformer. The method further includes the steps of applying impedance compensation to the output signal to provide an output drive signal from an output of a second side of the transformer to drive the laser diode device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein: 
       FIG. 1  illustrates an output stage of a conventional packaged laser diode driver integrated circuit (IC) and associated laser diode module; 
       FIG. 2  illustrates an output stage of a laser diode driver integrated circuit (IC) in accordance with an embodiment of the present invention and an associated laser diode module; 
       FIGS. 3A &amp; 3B  illustrate a comparison of an S-parameter response of a conventional laser diode driver integrated circuit and a laser diode driver integrated circuit (IC) constructed in accordance with the principles of the present invention; and 
       FIGS. 4A &amp; 4B  illustrate a comparison of the output current pulse response of a laser diode driver integrated circuit (IC) in accordance with the principles of the present invention and a conventional laser diode driver integrated circuit (IC). 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference is now made to the Drawings wherein like reference characters denote like or similar parts throughout the various Figures. Referring now to  FIG. 1 , an output stage of a conventional packaged laser diode driver integrated circuit (IC) and associated laser diode module is illustrated. The conventional packaged laser diode driver IC  102  is connected to a laser diode module  104  through a transmission line  106 . The transmission line  106  typically consists of a microstrip line placed upon a printed circuit board. The conventional packaged laser diode driver IC  102  is comprised of an integrated circuit package  108  having a conventional laser diode driver integrated circuit output stage  110  therein. 
   The conventional laser diode driver integrated circuit output stage  110  includes a pre-driver circuit  112  and a conventional driver circuit  114 . During operation, the pre-driver circuit  112  provides differential input data signals (IN+,IN−), representative of the data signal that is desired to be transmitted by the laser diode module  104 , to the conventional driver circuit  114 . In response to the differential input data signals (IN+,IN−), the conventional driver circuit  114  provides a modulated output drive signal to an integrated circuit output pad  116 . The integrated circuit output pad  116  is connected to a package output pin  118  to provide the modulated output drive signal from the conventional driver circuit  114  to the exterior of integrated circuit package  108 . 
   Further, the package output pin  118  is connected to the transmission line  106  in order to provide the modulated output drive signal to the laser diode module  104 . In response to the modulated output drive signal, the laser diode module  104  generates an optical output data signal that is representative of the differential input data signals (IN+,IN−). 
   The conventional driver circuit  114  of the conventional laser diode driver integrated circuit output stage  110  typically comprises an output switch architecture in the form of a bipolar junction transistor (BJT) differential pair configuration. The conventional driver circuit  114  of  FIG. 1  includes a first transistor  120  and second transistor  122  whose respective emitter nodes are joined together and biased by a current source  128  having a current Io. The current source  128  is further connected to a low reference supply voltage (Vee). Each of the respective collector nodes of the first transistor  120  and second transistor  122  is connected to a high reference supply voltage (Vcc) through a termination resistor  124 . 
   During operation, the aforementioned differential input data signals (IN+,IN−) are provided from the pre-driver circuit  112  to the respective gate nodes of the first transistor  120  and the second transistor  122 . In addition, the current source  128  is typically modulated in synchronization with the voltage waveform of the input data signal. In response to the differential input data signals (IN+,IN−), the BJT differential pair configuration acting as a differential amplifier provides the modulated output drive signal from the collector node of the second transistor  122  to the integrated circuit output pad  116 . 
   The output pad  116  is connected to the package output pin  118  through a connection having a package inductance  136 , comprised of the sum of the wire bond inductance and the package lead inductance. The package output pin  118  is further connected to a cathode terminal of the laser diode module  104  through a transmission line  106 , while the anode terminal of the laser diode module  104  is connected to the high reference supply voltage (Vcc). 
   The conventional packaged laser diode driver IC  102  described in  FIG. 1  suffers from several disadvantages. The current of the output drive signal is typically required to be at a high level, on the order of several tens of milliamps (mA), to properly drive the laser diode module  104 . Additionally, in order to achieve high data transfer rates, such as 10 Gbps or more, the rising time and falling time (tr/tf) of the waveform transitions of the modulated output drive signal should be on the order of several tens of picoseconds (ps). The second transistor  122  possesses an inherent parasitic capacitance  126  associated with the collector node due to the capacitance of the semiconductor junction between the base and collector of the second transistor  122 . Due to the high drive current requirements of the laser diode module  104 , the inherent parasitic capacitance  126  associated with the collector node of the second transistor  122  can reach hundreds or even thousands of femtofarads (fF). Typically, the termination resistor  124  is matched with the characteristic impedance (Z 0 ) of the transmission line  106 . However, due to the parasitic capacitance  126  of the second transistor  122 , the integrated circuit output impedance (Zout) looking into the package output pin  118  from outside of the integrated circuit package  108  is capacitive and degrades at high frequency resulting in impedance mismatch. 
   The laser diode module  104  typically possesses a parasitic capacitance  130  of several picofarads (pF), as well as a resistance  132  and a cathode inductance  134 . The parasitic capacitance  130  causes the input impedance (Zin) of the laser diode module  104  to also be capacitive and degrade at high frequencies. If the signal propagation time of the transmission line  106  connecting the conventional packaged laser diode driver IC  102  and the laser diode module  104  is shorter than the rising time and falling time (tr/tf) of the transitions of the output signal, the output return loss (S 22 ) of the conventional packaged laser diode driver  102 , and input return loss (S 11 ) of the laser diode module  104 , will not substantially affect the drive waveform of the laser diode module  104 . However, in realistic module assembly conditions the propagation time of the transmission line  106  is over several picoseconds (ps), causing multiple reflections in the transmission waveform due to impedance discontinuities. Such impedance discontinuities result in overshoot/undershoot problems with the laser diode module drive signal waveform, which can be seen as small eye opening in an eye diagram, and an increase in deterministic jitter. 
   Referring now to  FIG. 2 , an output stage of a laser diode driver integrated circuit (IC) in accordance with an embodiment of the present invention and an associated laser diode module is illustrated. The packaged laser diode driver IC  202  is connected to a laser diode module  104  using a transmission line  106 . The packaged laser diode driver IC  202  is comprised of an integrated circuit package  208  having a laser diode driver integrated circuit (IC) output stage  210  therein. 
   The laser diode driver integrated circuit output stage  210  includes a pre-driver circuit  212  and a driver circuit  214  comprised of an output switch architecture. During operation, the pre-driver circuit  212  provides differential input data signals (IN+,IN−) to the driver circuit  214 . In the presently described embodiment, the driver circuit  214  of the laser diode driver integrated circuit output stage  210  is comprised of a bipolar junction transistor (BJT) differential pair configuration. The driver circuit  214  of  FIG. 2  includes a first switch transistor  222  and a second switch transistor  224  whose respective emitter nodes are joined together and biased by a current source  226  having a current Io. The current source  226  is further connected to a low reference supply voltage (Vee). In addition, the collector node of the first switch transistor  222  is connected to a high reference supply voltage (Vcc) through a first termination resistor  228 . 
   In accordance with the present invention, a transformer  220  is connected between an output node of the driver circuit  214  and an integrated circuit output pad  216  of the laser diode driver integrated circuit output stage  210 . It should be understood that the transformer  220  may be comprised of an on-chip transformer fabricated on the die of the laser diode driver integrated circuit output stage  210 . Alternately, the transformer  220  may be comprised of any other suitable transformer either external to or internal to the integrated circuit package  208 . 
   Still referring to  FIG. 2 , a negative terminal of a primary side (Lp) of the transformer  220  is connected to the collector node of the second switch transistor  224 , and a positive terminal of the primary side (Lp) of the transformer  220  is connected to the high reference supply voltage (Vcc) through a second termination resistor  230 . A positive terminal of a secondary side (Ls) of the transformer  220  is connected to the high reference supply voltage (Vcc) through the second termination transistor  230 , and a negative terminal of the secondary side (Ls) of the transformer  220  is connected to the integrated circuit output pad  216 . 
   Further, a primary side resistor  232  is connected in parallel between the positive and negative terminals of the primary side (Lp) of the transformer  220 , and a secondary side resistor  234  is connected in parallel between the positive and negative terminals of the secondary side (Ls) of the transformer  220 . 
   During operation, the differential input data signals (IN+,IN−) are provided from the pre-driver circuit  212  to the respective gate nodes of the first switch transistor  222  and the second switch transistor  224 . In response to the differential input data signals (IN+,IN−), the differential pair configuration acting as a differential amplifier provides an output signal from the collector node of the second switch transistor  224  to the primary side (Lp) of transformer  220 . In response to the output signal, an output drive signal is provided from the secondary side (Ls) of the transformer  220  to the integrated circuit output pad  216 . The integrated circuit output pad  216  is connected to a package output pin  218  through a connection having an inductance  242 . The package output pin  218  is in turn connected to a cathode terminal of the laser diode module  104  through a transmission line  106  having an impedance Zo, while the anode terminal of the laser diode module  104  is connected to the high reference supply voltage (Vcc). In addition, the laser diode module  104  possesses a parasitic capacitance  130 , as well as a resistance  132  and a cathode inductance  134 . 
   The inclusion of the transformer  220  in the laser diode driver IC output stage  210  as described in accordance with the principles of the present invention overcomes several disadvantages of conventional laser diode driver integrated circuits and provides an improved output drive signal. At low frequencies of data transmission from the laser diode driver integrated circuit  202 , the output return loss (S 22 ) of the output impedance (Zout) looking into the output pin  218  from outside of the integrated circuit package  208  is substantially determined by the value of the second termination resistor  230 . For example, if the second termination resistor has a value of 50 ohms, the output impedance (Zout) will be substantially equal to 50 ohms at low frequencies. 
   At high frequencies of data transmission, the inductance of the primary side (Lp) of the transformer  220  compensates for the parasitic capacitance  244  of the collector node of the second switch transistor  224  that arises from the capacitance of the semiconductor junction between the base and collector. This compensation occurs because the capacitance effects on the output signal due to the parasitic capacitance  244  of the second switch transistor  224  are suppressed by the inductance of the primary side (Lp). At still higher frequencies, the primary side resistor  232 , placed in parallel to the primary side (Lp) of the transformer  220 , functions to repress the component of the output impedance (Zout) caused by the primary side (Lp) of the transformer  220 . The secondary side (Ls) of the transformer  220  and secondary side resistor  234 , placed between the second termination resistor  230  and the integrated circuit output pad  216 , compensates for the impedance component due to the second termination resistor  230  looking from the Zout side. 
   Accordingly, the transformer  220  functions to improve the output return loss (S 22 ) of the output impedance (Zout), as well as reduce multiple reflections of the transmission waveform of the output drive signal due to impedance discontinuities. An additional advantage of the present invention is that the output drive current and/or output drive voltage, rising time and falling time (tr/tf) of the output drive signal, and the output return loss (S 22 ), can be optimized through the choice of appropriate values of the primary side resistor  232  and the secondary side resistor  234 . 
   It should be understood that numerous modifications may be made to the embodiment of  FIG. 2  without departing from the spirit of the present invention. Although the invention of  FIG. 2  is illustrated using a bipolar junction transistor (BJT) pair architecture as a driver circuit, any other suitable output switch architecture may be used, such as those using field effect transistors (FETs), CMOS devices, etc. In addition, although the driver circuit of  FIG. 2  is illustrated through the use of a single stage emitter-coupled differential amplifier, a cascaded configuration or other multi-stage differential amplifiers having a transformer in their output stages may be used without departing from the spirit of the invention. The principles of the present invention are also applicable to output stages having both single-ended and differential outputs and/or inputs, as well as laser diode driver output stages which include a bias current source to provide bias current to a connect laser diode module. 
   Although the embodiment of  FIG. 2  illustrates the use of a laser diode driver IC to drive a laser diode module, the principles of the present invention are equally applicable to an electroabsorption (EA) modulator driver IC used to drive an electroabsorption (EA) modulator module for long distance transmission. 
   Referring now to  FIGS. 3A &amp; 3B , a comparison of an S-parameter response of a conventional laser diode driver integrated circuit (IC) without a transformer in its output stage, and a laser diode driver integrated circuit (IC) with a transformer in its output stage in accordance with the principles of the present invention is illustrated. In particular,  FIG. 3A  illustrates a magnitude plot of output return loss (S 22 ) in decibels (dB) over a frequency range from 0 to 20 GHz. As illustrated in  FIG. 3A , the magnitude of the output return loss (S 22 ) of a laser diode driver IC having a transformer within its output stage  302  is significantly improved over the magnitude of the output return loss (S 22 ) of a conventional laser diode driver IC lacking a transformer within its output stage  304 . In particular, a significant improvement can be seen at frequencies above 10 GHz. 
     FIG. 3B  illustrates in Smith chart form the output return loss (S 22 ) of a laser diode driver IC in accordance with the principles of the present invention in comparison to the output return loss (S 22 ) of a conventional laser diode driver IC. As is known in the art, a Smith chart provides for a graphical mapping of the complex impedance plane to the reflection coefficient plane. It is often useful to plot s-parameters, such as the output return loss (S 22 ), using a Smith chart in order to illustrate the impedance properties of an electronic circuit over a range of frequencies. 
     FIG. 3B  illustrates a Smith chart plot of the output return loss (S 22 ) of a laser diode driver IC having a transformer within its output stage  306  in accordance with the principles of the present invention in comparison to a Smith chart plot of the output return loss of a conventional laser diode driver IC lacking a transformer within its output stage over a frequency range of 0 to 20 GHz. As illustrated in  FIG. 3B , the output return loss (S 22 ) of of a laser diode driver IC having a transformer within its output stage  306  is significantly improved over the output return loss (S 22 ) of a conventional laser diode driver IC lacking a transformer within the output stage  308 , in particular at higher frequencies. 
   Referring now to  FIGS. 4A &amp; 4B , a comparison of the output current pulse response of a laser diode driver integrated circuit (IC) with a transformer in its output stage in accordance with the principles of the present invention, and a conventional laser diode driver integrated circuit (IC) without a transformer in its output stage is illustrated.  FIG. 4A  illustrates the amplitude of an output current pulse of a laser diode driver IC having a transformer in its output stage looking at the laser diode module through a transmission line of an electrical length of 200 picoseconds (ps).  FIG. 4B  illustrates the amplitude of an output current pulse of a conventional laser diode driver IC looking at the laser diode module through a transmission line of an electrical length of 200 picoseconds (ps). As can be observed, the multiple reflections present on the output current pulse of the laser diode driver having a transformer in its output stage ( FIG. 4A ) are significantly reduced in both number and amplitude over those of a conventional laser diode driver IC ( FIG. 4B ). 
   Although a preferred embodiment of the method and apparatus of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.