Abstract:
To provide a transmitter optical subassembly that provides an enhanced high-frequency response without increasing the power consumption within a CAN-type package. The transmitter optical subassembly includes a semiconductor laser diode, an auxiliary circuit, and a package for installing the semiconductor laser diode and the auxiliary circuit. The auxiliary circuit generates a transient signal synchronized with the leading edge or falling edge of a driving signal for the semiconductor laser diode in an electrode of the semiconductor laser diode for a very short period in order to boost the response speed of the semiconductor laser diode. The auxiliary circuit operated intermittently so that it is not necessary to consider heat caused by the auxiliary circuit even in case the auxiliary circuit is installed in the package.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to a transmitter optical subassembly and a transmitter optical module installing the same.  
         [0003]     2. Related Prior Art  
         [0004]     In general, it is known, as configurations to connect a device for switching a current supplied from a current source among the semiconductor laser diode (LD) driver systems, a series configuration where switching devices are inserted in series to a light-emitting device and a shunt configuration where switching devices are inserted in parallel to a light-emitting device. A Japanese Patent application published as JP-2001-015854A discloses a typical example of series configuration. The shunt configuration has advantageous in that it uses a smaller number of devices than the series configuration and allows high-speed operation, while the shunt configuration has a drawback that the degree of modulation permitted is small and the controllability of a current is poor.  
         [0005]     A Japanese Patent application published as JP-2004-179591A discloses another LD driver. This driver reflects the switching status of the reference current, which flows in the reference current path of the current mirror circuit, by toggling the switch connected to the reference current path, to the current supplied to the LD connected to the other current path of the current mirror circuit. In order to make steep the leading edge and the falling edge of the mirrored current supplied to the LD, a circuit is added that generates a pulse current synchronous with the leading edge and the falling edge of a signal to select the switching state. With this additional circuit, the LD driver increases/decreases the transient currents on the edges of the signal to perform high-speed LD switching.  
         [0006]     Optical communications in recent years reaches a transmission speed exceeding 10 Gbps. In the high-speed transmission range over 10 Gbps, the frequency response of the wiring connecting a light-emitting device and a driver, that is, the transmission impedance becomes important. Especially, a waveform may be degraded by the parasitic inductance of a lead pin in a transmitter optical subassembly. Or, the leading edge or the falling edge of a signal may be delayed by a parasitic capacitance.  
         [0007]     To solve these problems, a package with good controllability of the transmission impedance, for example, a butterfly package is used as a package for a transmitter optical subassembly. However, the butterfly package is expensive and is not effective in terms of high-speed transmission when the light emitting device is directly driven. For indirect modulation system that uses a MZ (Mach-Zender modulator) or an EA modulator (Electro-absorption modulator), the termination of a signal line may be easily and reliably carried out. For a system for directly modulating a light-emitting device, the internal resistance of the light-emitting device becomes 3 to 30 ohms in the case of a laser diode (LD). When a resistor of 1 to 40 ohms is serially inserted to the LD to terminate the transmission line, the inserted resistor and the junction capacitance of the light-emitting device constitute an integrator, which causes a problem in the high-speed transmission.  
         [0008]     It is known, what is called, a CAN-type package as another form for the optical subassembly. The CAN-type package is less expensive than the butterfly package. A problem with the CAN-type package is that impedance matching of a lead pin is hard to ensure and the high-speed modulation is difficult. Another problem is the heat dissipation characteristic of a driver when a driver is installed within the CAN-type package. The CAN-type package has less tolerance freedom in terms of its shape and specifications including its diameter, the number of pins and LD installation method than the butterfly package. When a driver is installed within the CAN-type package, it is difficult to efficiently dissipate the heat of the driver out of the package. Large quantity of heat generated in a transmitter optical subassembly increases the temperature of the LD, which degrades the light emission characteristic and the reliability of the optical assembly.  
         [0009]     The invention relates to a transmitter optical subassembly that may improve a high-frequency performance without increasing the power consumption of the transmitter optical subassembly and an optical data link installing the same.  
       SUMMARY OF THE INVENTION  
       [0010]     An aspect of the invention relates to a transmitter optical subassembly. The transmitter optical subassembly comprises a light-emitting device including an anode electrode and a cathode electrode, a CAN-type package including a plurality of lead pins, and an auxiliary circuit. The light-emitting device and the auxiliary circuit are installed within a package and a driving signal is supplied through a lead pin. The auxiliary circuit according to the invention generates a transient current synchronous with either the leading edge or falling edge of the driving signal in the anode electrode or cathode electrode of the light-emitting device.  
         [0011]     The transient current synchronous with the transition timing of the driving signal is supplied to a light-emitting device so that a quicker response is achieved in the light-emitting device. The auxiliary circuit includes a differentiator (low-frequency cut-off filter) for differentiating a driving signal and a transistor for generating a transient current by receiving the output of the differentiator. According to the invention, the auxiliary circuit is installed within the CAN-type package, however, current flows through the auxiliary circuit only for a short period of time while the driving signal is transmitted and thus heat generation by the current is not necessary to be considered.  
         [0012]     Another aspect of the invention relates to a transmitter optical module. The transmitter optical module includes a driver and a transmitter optical subassembly. The driver is provided outside the transmitter optical subassembly and receives an electrical signal and outputs a driving signal. The transmitter optical subassembly includes a semiconductor laser diode, an auxiliary circuit, and a CAN-type package installing the semiconductor laser diode and the auxiliary circuit. The package has a plurality of lead pins and the driving signal is supplied to the semiconductor laser diode through one of the lead pins as well as to the auxiliary circuit. The auxiliary circuit generates a transient current in the anode electrode of the semiconductor laser diode in synchronous with the leading edge of the driving signal.  
         [0013]     In the transmitter optical module according to the invention, a driver may be separated to a first driver and a second driver, and the output of the first driver or a first driving signal may be supplied to the anode electrode of the semiconductor laser diode installed within the transmitter optical subassembly through the first lead pin and the output of the second driver or a second driving signal may be supplied to the auxiliary circuit installed within the transmitter optical subassembly through the second lead pin. Because the first and the second drivers are used, an optimum condition can be set for each of the semiconductor laser diode and the auxiliary circuit.  
         [0014]     In the transmitter optical module according to the invention, a current source for generating a bias current may be provided outside the transmitter optical subassembly. By supplying the first driving signal superimposed with the bias current to the anode electrode of the semiconductor laser diode, it is possible to bias the semiconductor laser diode without increasing the number of lead pins in the transmitter optical subassembly.  
         [0015]     The first driving signal may be supplied to the cathode electrode of the semiconductor laser diode and the auxiliary circuit may bypass the current supplied to the semiconductor laser diode in synchronous with the leading edge of the second driving signal. Further, a photodiode for monitoring the optical output from the semiconductor laser diode may be installed within the transmitter optical subassembly according to the invention and the output of the photodiode may be extracted through the second lead pin for inputting the second driving signal. Any of the new features of the transmitter optical subassembly are implemented without increasing the number of lead pins. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows the circuit configuration of a transmitter optical subassembly and a transmitter optical module according to a first embodiment.  
         [0017]      FIG. 2  is a time chart illustrating the features of an auxiliary circuit in the transmitter optical subassembly shown in  FIG. 1 .  
         [0018]      FIG. 3  is a circuit diagram showing a variation of the first embodiment.  
         [0019]      FIG. 4  shows the circuit configuration of a transmitter optical subassembly and a transmitter optical module according to a second embodiment.  
         [0020]      FIG. 5  is a circuit diagram showing a variation of the second embodiment.  
         [0021]      FIG. 6  is a circuit diagram showing another variation of the second embodiment.  
         [0022]      FIG. 7  is a circuit diagram showing still another variation of the second embodiment. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     Preferred embodiments of the transmitter optical subassembly according to the invention will be detailed referring to the drawings. In the description of drawings, the same or corresponding elements are given the same symbol and duplicated description is omitted.  
       First Embodiment  
       [0024]      FIG. 1  shows the configuration of a transmitter optical subassembly  3  as a first embodiment of the invention.  FIG. 1  also shows a transmitter optical module  1  having the transmitter optical subassembly  3  and converting a high-frequency driving signal input from outside to an optical signal and outputting the resulting optical signal.  
         [0025]     The transmitter optical subassembly  3  comprises an LD (light-emitting device)  2 , a CAN-type package PKG. installing the LD  2 , and an auxiliary circuit including an n type FET  5 . The transmitter optical module  1  includes the transmitter optical subassembly  3  and a driver  4  arranged outside the package PKG.  
         [0026]     The driver  4  receives a signal S IN  input from outside the transmitter optical module  1  and outputs a driving signal S 1 . The driving signal S 1  is supplied to the cathode electrode of the LD  2  in the package of the transmitter optical subassembly  3  and the auxiliary circuit  6  through a lead pin P 1  provided in the package PKG of the transmitter optical subassembly  3 .  
         [0027]     The auxiliary circuit  6  includes the n type FET  5 , a differentiator  9  configured with a resistor  7  and a capacitor  8  connected between the gate of the FET  5  and the input of the auxiliary circuit  6 , and a diode  10  that is connected to the source of the FET  5  and generates a self bias in the gate of the FET  5 . The drain of the FET  5  serves as the output of the auxiliary circuit  6  and is connected to the anode electrode of the LD  2 . The differentiator  9  generates a pulse signal S 3  synchronized with the leading edge and the falling edge of the signal S IN  (detailed later) and its time constant is determined by the transmission rate of the signal S IN  and the value of a terminator of a transmission line including the lead pin P 1 . In this embodiment, the cut-off frequency is set to approximately 8 GHz assuming that the resistor  7  is 20 Ω and the capacitor  8  is 1 pF.  
         [0028]     Operation of the transmitter optical module  1  explained above will be described referring to  FIG. 2 .  
         [0029]     When the signal S IN  is input to the driver  4  ( FIG. 2A ), the output S 1  becomes an inversion of the signal S IN . The input S 3  of the FET  5  is obtained by differentiating the signal S 1  as shown in  FIG. 21 . That is, a negative and positive pulses are generated in synchronous with the leading edge and the falling edge of the output S 1 , respectively. Since the FET  5  is self-biased by the diode  10 , the drain current of the FET  5  is not influenced in the stable state except the leading edge and the falling edge of the signal S 1  and by a negative pulse from the differentiator. Only when a positive pulse is applied to the gate of the FET  5 , that is, only on the falling edge of the input signal S IN , the drain current of the FET  5  increases in the shape of a pulse ( FIG. 2C ).  
         [0030]     For the LD  2 , the current supplied from power supply Vcc flows through the LD  2  and is sunk into the output of the driver  4  through the lead pin P 1  in response to the output of the driver  4 . In the driving, influenced by the junction capacitance of the LD  2  and the inductance of the lead pin, the waveform I LD  supplied to the LD  2  shows its falling edge delayed compared with the leading edge as shown in the dotted line in  FIG. 2D . In this invention, the auxiliary circuit  6  is installed within the package PKG and the auxiliary circuit is driven by the output S 1  of the driver  4  so that a drain current ID is generated in a pulse shape on the FET  5  of the auxiliary circuit  6  only on the leading edge of the output S 1  (on the falling edge of the input signal S IN ) as shown in  FIG. 2C . The drain current ID bypasses a portion of the current I LD  supplied from the power supply to the LD  2 . Thus, the falling edge of the current I LD  supplied to the LD becomes steeper as shown by the solid line in  FIG. 2D .  
         [0031]     According to the above transmitter optical subassembly  3  and the transmitter optical module  1 , even when the falling edge of the current I LD  flowing through the LD  2  is delayed due to the influence of the interconnection connecting the driver  4  and the package PKG, especially due to the inductance component at the lead pin on the anode electrode of the LD  2  and the junction capacitance of the LD  2 , a pulse current is generated on the FET  5  of the auxiliary circuit  6  and this current bypasses a portion of the current supplied to the LD  2  thereby making steeper the falling edge of the current flowing through the LD  2 . It is thus possible to obtain the high-speed configuration of the transmitter optical subassembly  3  by only installing a single active device (FET  5 ) within the package PKG. The current flowing through the FET  5  is a pulse current that occurs only at the transition of the driving signal so that the power consumption in the package PKG is not increased.  
         [0032]     The capacitor  8  has a capacitance of a some picofarads at most and small dimensions. When the current supplied to the LD  2  is on the order of 60 mA, a small-sized FET  5  will be sufficient. The resistor in the differentiator  9  also serves as a terminator of the transmission line viewed from the output of the driver  4  so that it may be chosen to a small value of several tens of ohms, thus being less influenced by a parasitic capacitance. The influence of the parasitic capacitance is represented by a circuit parallel to the resistor so that the time constant of the parallel circuit corresponds to a much higher frequency. The auxiliary circuit  6  does not require a power source thus no additional lead pins are required.  
         [0033]      FIG. 3  shows the configuration of a transmitter optical subassembly  13  and a transmitter optical module  1   a  installing the transmitter optical subassembly  13  according to a variation of the first embodiment. The variation differs from the circuit shown in  FIG. 1  in terms of the configuration of the driver  4 . This variation has a first driver  4   a  and a second driver  4   b . The first driver  4   a  supplies an in-phase output S 1  with the driving signal S IN  to the LD  2  through a lead pin P 1 . The second driver  4   b  supplies an out-phase output S 2  with the driving signal S IN  to the auxiliary circuit  6  through a lead pin P 2 . The configuration of the package PKG is similar to that shown in  FIG. 1  except that separate lead pins are provided respectively for the in-phase signal S 1  and the out-phase signal S 2 .  
         [0034]     In this variation, the driving signal S 2  of the auxiliary circuit  6  and the driving signal S 2  of the LD  2  are provided separately so that the freedom to design the auxiliary circuit  6  increases. Specifically, in the auxiliary circuit  6  shown in  FIG. 1 , the resistor  7  in the differentiator  9  works as a terminator of a transmission line. The transmission line also serves as a transmission line to the LD  2 . The terminator viewed from the transmission line is determined in a complex fashion by the resistor  7 , operating resistance of the LD  2 , and the input resistance of the FET  5 . The output of the driver  4  is directed to the LD  2  and the auxiliary circuit  6 . When the optimum driving conditions are not fully consistent with those of the auxiliary circuit  6 , the operation of the transmitter optical subassembly  3  could be unstable.  
         [0035]     In this variation, it is possible to set individually optimum conditions for the LD  2  and the auxiliary circuit  6 . The load on the second driver  4   b  is provided by a parallel circuit of the resistor  7  and the input resistance of the FET  5 . The input resistance is much larger than that of the resistor  7  because the input of the PET  5  is self-biased (inversely biased) by the diode  10 . Thus, the value of a terminator may be substantially determined by the resistor  7  alone. When the resistor  7  is determined, the value of the capacitor  8  is automatically determined from the value of a time constant required for the differentiator  9 . For the output transmission line of the first driver  4   a , only the LD  2  is connected. When the operating resistance of the LD  2  is below the characteristic impedance of the transmission line, it is possible to satisfy the impedance matching conditions simply by serially inserting a resistor into the LD  2 .  
       Second Embodiment  
       [0036]     A second embodiment of the invention is described below.  
         [0037]      FIG. 4  shows the configuration of a transmitter optical module  1   b  including a transmitter optical subassembly  23  according to the second embodiment of the invention. The transmitter optical module  1   b  according to this embodiment differs from the variation of the first embodiment in that a driver  24   a  to drive an LD is a voltage-driving type,  
         [0038]     That is, the driver  24   a  receives a signal S IN  and generates a voltage signal S 21  as a driving signal to drive the LD  2 . The driver  24   a  is connected to the anode electrode of the LD  2  and the drain of an FET  5  through a coupling capacitor  38  and the lead pin P 1  of a CAN-type package PKG. Further, the lead pin P 1  is connected to a current source  37  for supplying a bias current  1   b  to the LD  2  outside the package PKG. Same as the first embodiment, the second driver  4   b  receives a driving signal S IN  and supplies a signal S 2  phase-inverted from the driving signal S IN  to the auxiliary circuit  6  through a lead pin P 2 . The LD  2  is connected between the lead pin P 1  and the ground in the package PKG.  
         [0039]     In the circuit shown in  FIG. 4 , the current source  37  may be connected to the anode electrode of the LD  2  through another lead pin P 3 . That is, while the output of the first driver  24   a  is superimposed with the current  1   b  from the current source  37  and supplied to the anode electrode of the LD  2  through the lead pin P 1  in  FIG. 4 , a lead pin P 3  for supplying a bias current may be provided separately from the lead pin P 1  and the both pins may be connected together in the package PKG to supply the bias current  1   b  superimposed with the signal S 21  to the anode electrode of the LD  2 . In this case, it is preferable to interpose an inductor or a resistor between the new lead pin P 3  and the anode electrode of the LD  2  in order to weaken the influence of the current source  37  on the transmission line from the first driver  24   a.    
         [0040]     The operation of this embodiment will be described. When a signal S IN  is input to the first driver  24   a  and the second driver  4   b , a driving signal S 21  in-phase with the signal S IN  is generated in the first driver  24   a  and a signal S 2  out-phase with the signal S IN . is generated in the second driver  4   b . The signal S 21  and the signal S 2  are voltage signals. When the signal S 2  is input to the auxiliary circuit  6 , a pulse signal  33  is generated in synchronous with the leading edge and the falling edge of the signal S 2 . When the pulse signal S 3  is input to the FET  5 , a pulse-shaped drain current is generated corresponding to the pulse signal synchronized with the leading edge of the signal S 2  (equivalent to the falling edge of the signal S 21 ) because the FET  5  is self-biased by the diode  10 . As a result, same as the first embodiment, the leading edge of the supply current to the LD  2  appears faster.  
         [0041]     As the drivers  24   a ,  24   b  of the transmitter optical module  1   b , a differential circuit may be used. That is, the cathode electrode of the LD  2  is grounded through a resistor or an inductor instead of the direct grounding and the in-phase output (one output) of the differential circuit is connected to the anode electrode of the LD  2  through the lead pin P 1  while the out-phase (the other output) of the differential circuit is connected to the gate of the FET  5  and the cathode electrode of the LD  2 . With this circuit configuration, it is possible to independently control the amplitudes of the in-phase output and the out-phase output of the differential circuit. This allows the control of the FET  5  of the auxiliary circuit  6  at the same time with driving of the LD  2 .  
         [0042]     Preferable embodiments of the invention are described referring to drawings. Note that the invention is not limited to the circuits illustrated in a series of drawings but various changes can be made without departing from the spirit of the invention. FIGS.  5  to  7  show such variations.  
         [0043]     While the transition from the lighting state to the non-lighting state is boosted by discharging the anode electrode of the LD  2  to a ground potential in the first embodiment, the transition from the lighting state to the non-lighting state is boosted by artificially bypassing the anode electrode and cathode electrode of the LD  2  in a transmitter optical subassembly  43  and a transmitter optical module  1   c  according to the variation shown in  FIG. 5 . In the circuit of  FIG. 5 , a signal out-phase with the driving signal S IN  is input to a p-FET  5   a  to bypass a current flowing through the LD  2 . While the cathode electrode of the LD  2  is grounded by an inductor  11  and a driving signal is supplied to the cathode electrode of the LD  2  through a coupling capacitor  48  in  FIG. 5 , the cathode electrode of the LD  2  may  5  be directly coupled to the first driver to current drive the LD  2 . This example provides a current source  47  for supplying the  5  bias current of the LD  2  outside the package PKG.  
         [0044]      FIG. 6  is a circuit diagram showing a transmitter optical module  1   d  including an optical transmitter  23 , the module providing an FET for shunt driving the LD  2  outside the package PKG according to a variation of the second embodiment. It is preferable to install within the package PKG an inductor or a resistor  39  between the anode electrode of the LD  2  and a bias current source  57 . This is to prevent the degradation of the transmission characteristic of a driving signal supplied to the LD  2  in the frequency response of the power supply Vcc.  
         [0045]     While a driving FET is arranged outside the package PKG in this variation, the driving FET may be installed within the package. In this case, a signal lead pin may be shared by two driving systems. Specifically, a first and second drivers  54   a ,  54   b  are used in common and the outputs are brought into the package PKG through the lead pin P 1 . In the package PKG, one output is input to the gate of the driving FET and the other to the auxiliary circuit  6 . The FET of a differentiator  9  is self-biased by the diode  10  while the source of a driving FET  58  is grounded, so that the independent bias conditions may be also set to two FETs in this variation.  
         [0046]     A monitor PD (photoreceptor device) for monitoring the intensity of optical output from the LD  2  may be installed within the transmitter optical package PKG.  FIG. 7  is a circuit diagram of an optical module  1   e  including a monitor PD  60  and an optical transmitter  63  in the example of  FIG. 5 . To the wiring from the lead pin P 2  connected to the gate terminal (input terminal) of the FET in the auxiliary circuit  6   a  is connected the anode electrode of the PD  60 . Also, to the lead pin P 2  is connected the output of the second driver  64   b  through ED the coupling capacitor  69  and an APC circuit  70  for detecting the monitor current output from the PD  60 .  
         [0047]     With this configuration, the lead pin P 2  is simultaneously applicable to more than one usage, that is, to the output of a monitor current from the PD  60  and to a signal to a differentiator  9  thus implementing a new feature without increasing the number of lead pins. The monitor PD  60  is used to control the average optical output power of the LD  2  and its operational speed is sufficiently lower than the transmission rates which eliminates the interference to the transmission signal.  
         [0048]     Further, it is possible to provide a transistor for generating a pulse signal synchronized with the leading edge of the supplied current to the LD  2  such as the FET  5   a  in the auxiliary circuits  6 ,  6   a , in place of a transistor for generating a pulse current synchronized with the falling edge of the supplied current to the LD  2 , or in addition to such a transistor.