Abstract:
The present invention is to provide a laser module stably operable with less jitter in high frequencies. The laser module of the invention provides a semiconductor laser diode and a current-shunting device that shunts the current flowing in the LD by responding the input modulation signal. A path where the current flows puts a serial circuit comprised of an inductor and a compensation circuit to compensate a ripple in the frequency response of the module. The resonance frequency of the compensation circuit corresponds to a frequency of a dip or a peak in the frequency spectrum of the module.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor laser module, in particular, the invention relates to a laser module installed with an Ld-driver. 
         [0003]    2. Related Prior Art 
         [0004]    Some prior documents have disclosed a semiconductor laser module with a shunt-driving configuration, where the current to be supplied with the semiconductor laser diode (Ld) is switched by a field-effect-transistor (FET) connected in parallel to the Ld. For example, a Japanese Patent Application published as JP-2005-033019A has disclosed in  FIG. 4C  thereof, in which the Ld is connected in parallel to an n-type FET. The cathode of the Ld and the source of the FET commonly connect to the ground, while, the anode of the Ld and the drain of the FET are biased with the Vcc through an inductor L. The gate of the FET receives a modulation signal. When the FET turns on by the modulation signal, the current from the power supply Vcc flows primarily in the FET, while, the current flows in the Ld when the modulation signal becomes the low level because the FET turns off. Thus, this type of the driver for the Ld is called as the shunt-configuration because the modulation signal supplied with the gate of the FET determines whether the current flows in the FET or the Ld. 
         [0005]    The laser module mentioned above generally provides an inductor in the power supply line for isolating the module from high-frequency signals (RF), that is, the power supply Vcc is provided through the inductor. However, a laser module with a CAN-type package does not have enough space to install electronic components within the package. Accordingly, electronic components installed within the package are necessary to have a small size. A small-sized inductor is unable to show a large inductance, accordingly, the isolation of the high-frequency signal becomes unsatisfactory, which increases a jitter in the optical output signal. 
         [0006]    Accordingly, the present invention is to provide a laser module that enables to emit light with less jitter. 
       SUMMARY OF THE INVENTION 
       [0007]    A semiconductor laser module according to the present invention, which has a configuration of the shunt-driving, includes a semiconductor laser diode, an electronic device, a first inductor and a compensation circuit. The electronic device switches a current to be supplied to the laser diode by responding to a modulation signal input thereto. The electronic device is connected in parallel to the laser diode in the shunt-driving configuration. The first inductor is put on a path for supplying the current to the laser diode. The compensation circuit, which is connected in serial to the first inductor, has a characteristic to compensate a frequency response of the current with respect to the modulation signal provided to the electronic device. In the present invention, the compensation circuit is serially connected to the inductor, and thus serially connected inductor with the compensation circuit is further connected in serial to the laser diode and to the electronic device. 
         [0008]    The laser module of the present invention may provide a CAN package that includes a stem with a block and first and second lead pins. The stem mounts the laser diode, the electronic device, the first inductor and the compensation circuit. The first lead pin, which is connected to the path to supply the current, and the second lead pin is connected to the electronic circuit. The laser diode, the electronic device and the compensation circuit may be mounted on a side of the block. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a laser module according to an embodiment of the present invention; 
           [0010]      FIG. 2  is an equivalent circuit diagram of the laser module shown in  FIG. 1 ; 
           [0011]      FIG. 3  is a frequency response, S 21 , of a laser module without a compensation circuit; 
           [0012]      FIG. 4  is an impedance characteristic of the compensation circuit; 
           [0013]      FIG. 5  is a frequency response, S 21 , of a laser module with the compensation circuit according to an embodiment of the present invention; and 
           [0014]      FIGS. from 6A to 6D  show various eye-diagrams. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    Next will describe embodiments of the present invention as referring to accompanying drawings. In the description below, the same numerals or the symbols will refer to the same elements without overlapping explanations. 
         [0016]      FIG. 1  is a perspective view of a laser module according to an embodiment of the present invention, while,  FIG. 2  is an equivalent circuit diagram of the laser module shown in  FIG. 1 . The laser module  10  has a CAN-type package with a disk-shaped stem  11  and a plurality of lead pins, four pins in the present embodiment,  12  to  15 . Three of pins,  12  to  14  pass through respective holes in the stem  11 . Seal glass  16  fills a gap between the pin and the stem  11  to hermetically seal the package. The last lead pin  15  is resistance-welded or brazed to a back surface of the stem  11  to secure an electrical contact with the stem  11 . The lead pin  15  functions as a ground pin. Here, the primary surface of the stem  11  mounts the Ld and electronic components, while, the back surface is opposite thereto. 
         [0017]    The primary surface  11   a  of the stem  11  installs the Ld  20 , an electronic device  21 , an inductor  24  and a compensation circuit  26 . The heat sink  19  made of electrically conductive material mounts the Ld  20  and the compensation circuit  26 . The cathode of the Ld  20  is connected to the mount  17  through the heat sink  19 . A ceramics substrate  47 , which is fixed on the heat sink  19 , mounts the compensation circuit  26 . The electronic device  21  provides an FET  22  thereon. As shown in  FIG. 1 , the mount  17  is protruded on the primary surface  11   a  of the stem  11 , and the side of the mount  17  attaches the heat sink  19  and the electronic device  21  thereto. The mount  17  is grounded via the stem  11  and the lead pin  15 . 
         [0018]    The gate of the FET  22  connects with the lead pin  13 , the source thereof connects with the mount  17 , while, the drain connects with the anode of the Ld with respective bonding wires. The source of the FET  22  and the cathode of the Ld  20  are commonly grounded through the mount  17 , the stem  11  and the lead pin  15 . Thus, the FET  22  is coupled in parallel to the Ld  20 . The drain of the FET  22  also connects with the compensation circuit  26  with a bonding wire. The other terminal of the compensation circuit  26  connects with the inductor  24  with another bonding-wire. 
         [0019]    On the primary surface  11   a  of the stem is disposed with a photodiode (Pd)  23  through a sub-mount  49 . One of the cathode and the anode of the Pd connects with the lead pin  14  with a bonding wire, while, the other directly connects with the stem  11  with another bonding wire. 
         [0020]    In the circuit diagram shown in  FIG. 2 , elements  32  and  33  show parasitic inductance of the lead pin  12 , which is 0.0742 nH and 0.287 nH, respective, and the other element  34  denotes parasitic capacitance between the lead pin  12  and the stem  11 , which is 0.404 pF. The elements,  35  and  36 , are the parasitic inductance of the lead pin  13 , 0.557 nH and 0.337 nH, respectively, while the element  37  denotes the parasitic capacitance between the lead pin  13  and the stem  11 , which is 0.436 pF. The elements,  41  to  43 , are the parasitic inductance attributed to the bonding wire, 0.2 nH, 0.2 nH and 2 nH, respectively, while, the element  46  is the junction capacitance of the Ld  20 . The reason why the lead pins  12  and  13  have parasitic inductance different from each other is that a diameter of the lead pin  13  is smaller than that of the other lead pins to increase the impedance thereof. 
         [0021]    When the laser module operates, the external power supply Vcc provides the DC current  45  to the module through the lead pin  12 . The current  45 , passing through the inductor  24  is provided with a parallel circuit comprised of the Ld  20  and the FET  22 . On the other hand, the lead pin  13  provides the modulation signal Vs including high-frequency components to input in the gate of the FET  22 . The FET  22 , responding to this modulation signal Vs, switches the current supplied to the Ld  20 . That is, when the modulation signal is high level, the FET  22  turns on to flow the primarily portion of the DC current  45  in the Ld  22 , while, when the modulation signal becomes low level, the FET  22  turns off to flow the current  45  in the Ld  22  to emit light. 
         [0022]    A termination resistor  23  with a resistance of 50Ω, which is not shown in  FIG. 1 , is connected between the gate and the source of the FET  22  to match the impedance of a path from the lead pin  13  to the gate of the FET  22  with the transmission impedance. The electronic device  21  may integrate this termination resistor  23 . To match the input impedance of the module with the transmission impedance may suppress the attenuation and the reflection of the signal at the input, which enhances the quality of the modulation signal, thus, that of the optical signal. 
         [0023]    The inductor  24 , put on the path to flow the current  45  to the Ld  20 , cuts the signal with high-frequency components, which suppresses the degradation of the optical signal due to noises with high-frequency components generated in the power supply Vcc. The inductor  24  may be a chip inductor with a type of ferrite beads inductor with laminated ceramics, or a type of coiled inductor. As shown in  FIG. 1 , the present embodiment connects one of terminals  24   a  of the inductor  24  connects the compensation circuit with a bonding wire through a metal chip  25  stacked thereon, while the other terminal  24   b  connects with the lead pin  12  with the conductive adhesive. 
         [0024]    The modulation signal Vs, input in the lead pin  13 , is influenced by the parasitic inductance,  35  and  36 , and the parasitic capacitance  37  attributed to the lead pin  13 . Because the bonding wire from the lead pin  13  to the gate of the FET  22  is quite short, the parasitic inductance attributed to this bonding wire is merely 0.2 to 0.3 nH. Because the source of the FET  22  is grounded to the mount  17  with a plurality of bonding wires, the parasitic inductance attributed to these bonding wires may be considered to be quite small. 
         [0025]    The compensation circuit  26  is connected, as a load circuit of the FET  22 , in serial to the inductor  24  on the pass  45  for the current to the Ld  20 . This compensation circuit  26  is a parallel resonant circuit including an inductor  27 , a capacitor  29  and two resistors,  28  and  30 . Two resistors,  28  and  30 , operate to relax the Q-value of the compensation circuit. The capacitor  29  connects one of the resistors  28 , while, the inductor  27  connects in parallel the other of the resistor  30 . The inductance of the inductor  27 , the capacitance of the capacitor  29 , and the resistance of the resistors,  28  and  30 , may be 1 nH, 2 pF, 10Ω and 40Ω, respectively. Because the resistor is unfavorable to be connected in series with the inductor, the resistor  30  is connected in parallel to the inductor  27 . The inductor may be a thin film inductor with a spiral metal pattern. The capacitance  29  may be a MIM (Metal-Insulator-Metal) capacitor where two metal plates put the insulating material therebetween. Two resistors,  28  and  30 , may be general metal resistor with metal thin film. 
         [0026]    Next will describe advantages of the laser module  10  shown in  FIGS. 1 and 2 .  FIG. 3  shows a frequency response of a laser module, where the compensation circuit shown in  FIG. 2  is removed. The frequency response shown in  FIG. 2  illustrates the ratio of the current I L  flowing in the Ld  20  to the modulation signal Vs supplied to the gate of the FET  22  without the compensation circuit  26  in  FIG. 2 , which corresponds to the high frequency response of the module without the compensation circuit  26 . In  FIG. 3 , the vertical axis denotes the response in the unit of dB, while the horizontal axis denotes the frequency by the linear scale. When the horizontal axis is taken by the logarithmic axis to investigate the frequency response in low frequency regions, the response shown in  FIG. 3  shows the attenuation by a slope −20 dB/oct in the high frequency region. 
         [0027]    As shown in  FIG. 3 , the response appears a dip  50  around 4 GHz. The reason why the dip  50  appears is that: the pass to flow the current provides the inductor to cut the high frequency components. However, as mentioned earlier, the module with the CAN package may not retain an enough space to install a large-sized inductor. The small-sized inductor does not show enough performance to cut the high-frequency components, which results on a resonance caused by the parasitic inductance of the lead pin, the parasitic capacitance due to the seal glass of the lead pin and the parasitic capacitance attributed to the wiring patterns disposed outside of the modules. This resonance makes the dip  50  appeared around 4 GHz in the response shown in  FIG. 3 . 
         [0028]    As well as the resonance described above, various factor may cause ripples in the response of the module, such as the parasitic capacitance due to the inductor  24  to cut the high frequency components, the fluctuated ground, the junction capacitance of the Ld, the life time of the carrier in the Ld and the relaxation time of the Ld  20 . Here, the relaxation time is a period from time when a photon to be a seed light is generated within the cavity of the Ld to a situation when the stimulated light becomes coherent light by reiterating within the cavity. From an electrical viewpoint, the relaxation time is denoted as a period from the supplement of a pulsed signal to the Ld to obtain the laser light, which generally corresponds to a frequency with a few giga-hertz. 
         [0029]    Because the phase drastically varies in the region where the dip appears, the dip becomes a primarily reason for causing the jitter in the optical output from the Ld  20 . Although a large sized inductor, such as an inductor to cut the alternating current in relatively low frequencies, may suppress the generation of the dip, it is unfavorable to bring a large-sized package. Although an additional resistor to compensate the lack of the inductance may be disposed within the package, it is also unfavorable to increase the power dissipation, in particular, the bias voltage to provide the same bias condition to the Ld  20 . 
         [0030]    Therefore, the optical module according to the present embodiment provides the compensation circuit  26  serially connected with the inductor  24  to compensate the dip  50  in the frequency spectrum.  FIG. 4  illustrates the impedance of the compensation circuit  26 . The inductor  27  is regarded as that has an infinite impedance, while, the capacitor  29  is short-circuited in high frequencies. Accordingly, the composite impedance of the compensation circuit  26  becomes a parallel circuit of the resistor  28 , 10Ω, and the other resistor  30 , 40Ω, that is, the impedance thereof becomes 8Ω at high frequencies. On the other hand, in low frequencies, the capacitance of the capacitor  29  is regarded as infinite, while the inductor  27  is short-circuited; the composite impedance of the compensation circuit  26  becomes nearly zero, which is minus infinite in a decibel scale. In moderate frequencies, the composite impedance becomes the maximum at a frequency of ω=1/√LC, which is the resonance frequency of the inductor  27  and the capacitor  29 . The resonance frequency  52  where the composite impedance becomes the maximum is about 3 GHz in the present embodiment. 
         [0031]    Thus, to put the compensation circuit  26 , whose resonance frequency corresponds to that of the dip to be compensated, in the current path may relax the frequency undulation, such as the dip  50 , because the insufficient inductance may be compensated by the circuit  26 .  FIG. 5  illustrates the frequency response, the current I L  flowing in the LD against the input modulation signal, I L /V S , of the laser module  10  according to the present embodiment. The dip  50  appeared around 4 GHz in the frequency response of the conventional module shown in  FIG. 3  clearly disappear. 
         [0032]    The compensation circuit  26  according to the present embodiment may reduce the jitter in the output optical signal of the LD  20 .  FIGS. 6A to 6D  show eye-diagrams of the optical output from various modules, including the modules with the compensation circuit  26  and that without the circuit  26 , for the input modulation signal of 10 Gbps.  FIG. 6A  is the eye-diagram of the optical output from the LD  20  without the compensation circuit  26 ,  FIG. 6B  is the case where the module provides the compensation circuit  26 . These diagrams may be calculated by solving the rate equation based on the current flowing in the LD  20 . The frequency response of the current respects the frequency response of the module shown previously.  FIGS. 6C and 6D  are eye-diagrams each obtained by receiving the optical output shown in  FIG. 6A  or  6 B by a PD and by passing the output from the PD with a Bessel-Thomson filter. Comparing  FIG. 6A  with  FIG. 6B , and  FIG. 6C  with  FIG. 6D , respectively, the module with the compensation circuit  26  may clearly reduce the jitter in the optical output. The Bessel-Thomson filter may reduce the jitter of about 12 ps to about 3 ps. 
         [0033]    Thus, the present invention is described based on embodiments and referring to accompany drawings. The invention is not restricted to those embodiments or arrangements illustrated in the drawings. It will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. 
         [0034]    For instance, although the embodiment above compensates the dip in the frequency response, the present invention may compensate the peak. Moreover, the embodiment provides the compensation circuit  26  independent of the FET  22 . However, the electronic device  21  may integrate the compensation circuit  26  with the FET  22 . Such electronic device  21  becomes a size of about 0.7 mm×0.7 mm. A multi-fingered configuration for the FET  22  may shrink the size of the FET  22 , while, the electronic components within the compensation circuit  26  may be built within the electronic device  21  as they are. Moreover, the FET may be replaced with a bipolar transistor or other active devices for shunting the current flowing in the LD. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims.