Abstract:
In a semiconductor laser manufacturing method, a GaN single-crystal substrate is formed by slicing a GaN bulk crystal, grown on a c-plane, parallel to an a-plane which is perpendicular to the c-plane. In this substrate, crystal defects extending parallel to the c-axis direction do not readily exert an influence, and degradation of element characteristics due to crystal defects can be suppressed. Further, because the a-plane is a nonpolar plane, improved light emission efficiency and longer wavelengths can be achieved compared with the c-plane, which is a polar plane. Hence a semiconductor laser manufacturing method of this invention enables further improvement of the element characteristics of the semiconductor laser to be fabricated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical transmitter with a plurality of transmitter units each emitting signal light with specific wavelengths controlled by adjusting a temperature of a semiconductor laser diode. 
         [0003]    2. Related Background Art 
         [0004]    Some optical transmitter applied in the wavelength division multiplexing (WDM) communication system installs a plurality of optical transmitter units each emitting signal light whose wavelength is specific to one of the signal channels in the WDM system. Such a transmitter unit often provides a Peltier device to adjust a temperature of the semiconductor laser diode (hereafter denoted as LD) in the target temperature. The United States Patents issued as U.S. Pat. No. 5,515,682 and U.S. Pat. No. 5,920,583, and Japanese Patent Application published as JP-H10-141781 have disclosed a driver circuit to control such Peltier devices. However, when the transmitter provides a plurality of TEC drivers each corresponding to one Peltier device, the size of the transmitter must be large to install all components. Moreover, as shown in the second prior art, JP-H10-335724A, an ordinary Peltier device shows a resistance of about 1Ω and has a limited current to operate it. The maximum current allowable to the ordinary Peltier device is limited from −1 A to 1 A. Accordingly, in the case that the TEC driver down converts the DC power supply of 5 V, or occasionally 3.3 V, to a voltage necessary to control the Peltier device, where the absolute value is 1 ampere in an example above, an unsatisfactory efficiency of the power conversion is available. We can not overlook this situation from a viewpoint of the low power consumption. 
         [0005]    Noting the load efficiency of the electronic power of the transmitter, the efficiency increases as the load impedance becomes larger as shown in  FIG. 4 . This figure shows the efficiency of the transmitter whose configuration obeys those illustrated in the second prior art, JP-H10-141781A, as varying the load impedance. A reduced loss of the driver for the Peltier device is available for the larger load impedance, which is equivalent to the higher efficiency in the driver circuit. 
       SUMMARY OF THE INVENTION 
       [0006]    A feature of the optical transmitter according to the invention is that, in an optical transmitter comprising a plurality of transmitter units each providing a Peltier device, a laser diode and a thermistor, and a driver to determine a driving current for the Peltier device; these Peltier devices are connected in series with respect to the controller. Accordingly, the load impedance viewed from the driver becomes large compared with a case where one driver independently controls one Peltier device and, accordingly, the power conversion efficiency may be enhanced. Moreover, only one driver controls a plurality of Peltier, devices, the transmitter may save a space to install components and may avoid a large sized package. 
         [0007]    The controller may set the driving current such that a center value of the sensed temperatures becomes a target temperature, or that an average value of the sensed temperatures becomes the target temperature. 
         [0008]    Each transmitter units may further provide a bypassing unit connected in parallel to the Peltier device, and the driver may provide a bypass control unit that controls bypassing units installed in each transmitter unit. This bypassing unit may bypass a portion of driving current flowing depending on the temperature sensed by the thermistor installed also in the same transmitter unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  schematically illustrates a block diagram of the optical transmitter according to the first embodiment of the present invention; 
           [0010]      FIG. 2  schematically illustrates a block diagram of the optical transmitter according to the second embodiment of the present invention; 
           [0011]      FIG. 3  is an exemplarily circuit diagram of the bypassing unit included in the transmitter unit of the second embodiment; 
           [0012]      FIG. 4  compares a power conversion efficiency of two cases of the load impedance viewed from the driver; and 
           [0013]      FIG. 5  schematically illustrates a block diagram of a conventional optical transmitter that provides drivers individually corresponding to respective transmitter units. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Next, preferred embodiments according to the present invention will be described as referring to accompanying drawings. 
       First Embodiment 
       [0015]      FIG. 1  schematically illustrates a block diagram of an optical transmitter according to the first embodiment of the invention. The optical transmitter  1 , which is installed in a optical communication system for the wavelength division multiplexing (WDM) standard, comprises a plurality of transmitter units  10 , three units,  10 A to  10 C are illustrated in  FIG. 1 , and a TEC driver  20 . Each transmitter unit,  10 A to  10 C, includes a laser diode (hereafter denoted as LD)  11 , a Peltier device  14  and a thermistor  13 . The LD  11  emits signal light with a wavelength specific to the transmitter unit by supplying a current from an LD driver, which is not shown in  FIG. 1 . 
         [0016]    The transmitter unit  10  provides a Peltier device  14  that heats up or cools down a temperature of the LD  11  by receiving a driving current from the TEC driver  20 . Peltier devices  14  in each transmitter units,  10 A to  10 C, are connected in series with respect to the TEC driver  20 . That is, one current input terminal TEC+ of the transmitter unit  10  in the first transmitter unit  10 A is connected with one output terminal of the TEC driver  20  to receive the driving current I_tec+, while, the other current input terminal TEC− of the transmitter unit  10  is connected with one current input terminal TEC+ of the transmitter unit  10  in the second transmitter unit  10 B. The other current input terminal TEC− of the second transmitter unit  10  is connected with one current input terminal TEC+ of the transmitter unit  10  in the third transmitter unit  10 C, while, the other current input terminal TEC− of the third transmitter unit  10  is connected with the other output terminal of the TEC driver  20  to return the driving current I_tec−. Moreover, the thermistor  13  in each transmitter unit  10 , which is arranged close to the LD  11  to sense the practical temperature of the LD  11 , outputs the sensed signal Temp_mon to the TEC driver  20 . 
         [0017]    The TEC driver  20  controls the Peltier devices  14  in respective transmitter units,  10 A to  10 C. This TEC driver  20  includes a unit  21  to set the driving current, where the magnitude of the driving current supplied to the Peltier devices  14  connected in series based on the temperature sensed signal Temp_mon output from each thermistor  13 . Specifically, the unit  21  sets the driving current such that, when the target temperature of the LD  11  is T 0  and the maximum and the minimum values of the sensed signal are A and B, respectively, the unit  21  sets the driving current so as to satisfy the condition of (A+B)/2=T 0 , that is, the center value of sensed temperatures becomes the target temperature T 0 . The TEC driver provides thus determined driving current to the Peltier device  14 . In another embodiment, the TEC driver  20  provides the driving current such that an average of sensed temperatures becomes the target temperature T 0 . 
         [0018]    The optical transmitter  1  of the present embodiment, because the Peltier devices  14  in respective transmitter units  10  are connected in series with respect to the TEC driver  20 , a total load resistance viewed from the TEC driver becomes a substance value even when each Peltier device  14  shows a relatively small impedance. As shown in  FIG. 4 , the larger load impedance for the TEC driver  20  may enhance the conversion efficiency of the TEC driver  20 . 
         [0019]    Comparing the optical transmitter  1  of the present invention with a conventional transmitter shown in  FIG. 5 , in which each transmitter unit,  10 A to  10 C, accompanies with an independent TEC driver,  20 A to  20 C. In this case, the load impedance of each TEC driver,  20 A to  20 C, becomes 1Ω when the practical resistance of each Peltier device  14  is 1Ω. When each TEC driver  20 A to  20 C is connected with a power supply whose voltage is 5 V and the current of 1 A is necessary to control the Peltier device  14 , the TEC driver  20 A to  20 C must provide a function equivalent to a voltage converter generating 1 V down from 5 V. 
         [0020]    In contrast, the total load impedance of the Peltier devices  14  viewed from the TEC driver  20 , where three devices are connected in series, becomes 3Ω in the transmitter  1  of the present invention. Consequently, the TEC driver  20  is necessary to have a function equivalent to generate a voltage of 3 V converting from the power supply of 5 V. Thus, the power conversion efficiency of the TEC driver  20  may be enhanced in the present invention, which enables to reduce the power consumption of the transmitter  1  even when a plurality of transmitter units,  10 A to  10 C, is installed. Moreover, because one TEC driver  20  controls a plurality of the Peltier devices  14 , the optical transmitter  1  may avoid a large-sized scale. 
       Second Embodiment 
       [0021]    Next, another embodiment according to the present invention will be described.  FIG. 2  schematically illustrates a block diagram of an optical transmitter  31  of the second embodiment of the invention. The optical transmitter  31  has a feature that each transmitter unit  10 A to  10 C provides a bypassing unit  40 . That is, the bypassing unit  40  in each transmitter unit  10  is connected in parallel to the transmitter unit such that the driving current is bypassed from the Peltier device  14  in the transmitter unit  10 . Moreover, the TEC driver  20  further provides a bypass control unit  22 . 
         [0022]    The bypassing unit  40 , as shown in  FIG. 3 , provides a switching device  41 , a resistive element  42 , and a pair of low-passing filters,  43  and  44 . The switching device  41  may be an active device, such as power MOSFET, whose conductive state is controlled by the signal, Bypass_set_n, output from the TEC driver  20 . The duration of ON state of this switching device  41  may be finely adjustable by the protocol of the signal, Bypass_set_n, such as the pulse width modulation or the pulse count modulation the signal. 
         [0023]    The resistive element  42  is connected in one side of the switching device  41  and the resistance of this resistive element is set to be about 50Ω but may be removed. The low-passing filters,  43  and  44 , are connected so as to put the switching device  41  and the resistive element  42  therebetween, and these filters,  43  and  44 , absorb the noise caused by the turning on or turning off of the switching device  41 . 
         [0024]    The bypass control unit  22  generates the signal Bypass_set_n that is provided to respective bypassing unit  40  based on the sensed signal Temp_mon supplied from the thermistor in respective transmitter unit  10 . When a Peltier device  14  in one transmitter unit  10  is in an excess controlled state, in which the temperature of the LD in this transmitter unit  10  is off from the target temperature T 0 , then, the bypass control unit  22  outputs the control signal Bypass_set_n to the bypassing unit  40  of this transmitter unit  10 . A portion of the driving current I_tec+ supplied from the TEC driver  20  bypasses the Peltier device  14  under controlled, and the temperature of the LD  11  may close to the target temperature T 0 . 
         [0025]    In the optical transmitter  31  of the second embodiment, similar to the aforementioned transmitter  1  of the first embodiment, because the Peltier devices  14  in respective transmitter units,  10 A to  10 C, are connected in series, the power conversion efficiency of the TEC driver  20  may be enhanced and, consequently, the power consumption may be reduced even when the transmitter  1  installs a plurality of transmitter units each providing the Peltier device. Moreover, one TEC driver  20  may control a plural Peltier devices, the transmitter may avoid the large-sized scale. 
         [0026]    Further, when a temperature of the LD  11  in one transmitter unit becomes off from the target temperature T 0  by the excess control of the Peltier device  14 , the TEC driver  20  of the second embodiment may precisely and independently adjust the temperature of this LD  11  by bypassing a portion of the current supplied to the Peltier device under controlled. Accordingly, respective LDs installed within the optical transmitter  31  may be controlled in the temperature thereof, namely, the emission wavelength of respective transmitter unit  10 . 
         [0027]    The present invention is thus described by exemplarily embodiments; however, the present invention is not restricted to those embodiments. For instance, the switching device  41  provided in the bypassing unit may be a mechanical relay switch, or in the case of the semiconductor switching device, the control signal Bypass_set_n may be a DC signal, in which the ON resistance of the switching device  41  may be adjusted by the level of this DC signal. Moreover, one of the low-passing filters,  43  and  44 , may be removed.