Abstract:
An optical transmitter is disclosed in which the wavelength deviation occurred at the turning on from the disabled state to the enabled state by the negating of the Tx_Disable command is suppressed. The optical transmitter includes a semiconductor laser diode (LD) and an automatic temperature controller (ATC) circuit to drive the thermoelectric cooler (TEC). When the transmitter receives the Tx_Disable to start up the operation of the LD, a pulsed signal is generated in synchronizing with the transition of the Tx_Disable signal to momentarily enhance the cooling capacity of the TEC in order to compensate the increase of the temperature of the LD by the self heating, which prevents the output wavelength of the transmitter from deviating.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical transmitter, in particular, an optical transmitter with an automatic temperature controller for a semiconductor laser diode to make an output wavelength constant. 
         [0003]    The wavelength division multiplexing (WDM) system for an optical communication is to increase a transmission capacity by multiplexing a plurality of signal channels each having a specific wavelength different from each other. An optical transmitter applied to the WDM system is necessary to selectively output a signal with a wavelength unique to the channel thereof among channels in the WDM system. However, a laser diode, which is generally used in the optical transmitter for the WDM system, shows a strong dependence on a temperature in an output wavelength thereof. Thus, the optical transmitter is inevitable to control the temperature of the LD precisely. 
         [0004]      FIG. 8  is a block diagram on a conventional optical transmitter  100  that includes an optical module comprising an LD  102 , a thermistor that senses a temperature of the LD  102 , and the thermo-electric cooler (TEC)  106  to control the temperature of the LD  102 . This optical module  108  often called as a transmitter optical sub-assembly (TOSA). The optical transmitter  100  further comprises a TEC driver  112  to provide driving current to the TEC  106 , a processing unit (CPU)  114  to generate a target signal ST 0  corresponding to a target temperature of the LD  100 , a digital-to-analog converter (D/A-C)  116  that converts the digitally target signal ST 0  to an analog signal, and a comparator  118  to generate a differential signal STD between the target signal ST 0  and the sensed signal ST 1 . The TEC driver provides the driving current to the LD  102  based on the differential signal STD. The closed loop of the LD  102 , the thermistor  104 , the processing unit  114 , the D/A-C  115 , the comparator  118  and the driver  112  constitute an automatic temperature control (ATC) circuit  108  for the LD  102 , which maintain the temperature of LD constant, because the ATC circuit operates so as to equalize the sensed signal ST 1  with the target signal ST 0  by an enough voltage gain of the comparator  118 . 
         [0005]    The optical transmitter  100  further provides an LD driver  110  to provide a driving current to the LD  102 . The LD driver  110  receives a transmitting signal Tx and a command Tx_Disable from the outside of the transmitter  100 . The command Tx_Disable enables or disables the emission of the LD  102 . When the transmitter  100  receives the assertion of the Tx_Disable, the LD driver  110  fully shuts down the current to the LD  102 . 
         [0006]    The United States Patent, the U.S. Pat. No. 5,978,395, has disclosed an optical transmitter with a function to shut the driving current down when the LD temperature deviates from a target value to prevent the failure in the wavelength from affecting the neighbor channels. A Japanese Patent Application published as JP-2002-101556A has disclosed an optical transmitter in which the LD emits signal light after the temperature thereof reaches the target value specific to the signal channel and becomes stable thereat. 
         [0007]    When the optical transmitter changes its mode from the disable state to the enable state by the command negating the Tx_Disable, the self-heating by the current provided to the LD causes the deviation of the temperature of the LD, until the automatic temperature control practically operates, even it is kept stable at the target temperature. This causes the deviation of the output wavelength of the LD and possibly affects the operation in the neighbor channel in the WDM system. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is to provide the subject above that the output wavelength of the LD temporarily deviates from the target value when the LD is initially operated even the temperature of the LD is kept at the predetermined value. 
         [0009]    One feature of the present invention relates to an optical transmitter that has a function to enable or to disable an optical output in synchronized with a command provided from the outside of the transmitter. This optical transmitter includes a semiconductor laser diode, a feedback loop to control a temperature of the laser diode automatically, which is often called as an automatic temperature control (ATC) loop, and a unit to suppress a temperature deviation of the laser diode, when the optical transmitter is enabled by the external command, by superposing a pulsed signal momentarily on the feedback loop. The ATC loop may include a thermoelectric cooler (TEC) that mounts the laser diode thereof, a temperature sensor to sense the temperature of the laser diode, and a processing unit that compares the monitored temperature with a reference and provide a driving current to the TEC based on the comparison. 
         [0010]    The pulsed signal superposed on the feedback loop momentarily increases a current supplied to the TEC which may compensate the temperature deviation of the laser diode due to the self-heating by the current provided to the laser diode in synchronized with the enable command. The unit to suppress the temperature deviation may include an attenuator to attenuate the external command and a differentiator to form the pulsed signal, while the processing unit within the ATC loop may include a comparator to compare the sensed temperature with a reference and a driver to provide a current to the TEC based on the comparison by the comparator. The pulsed signal may be superposed either on the reference or the sensed temperature, or on the comparison result. 
         [0011]    Another aspect of the present invention relates to a method to control the temperature of the laser diode, in particular, the method relates to compensate the temperature deviation of the laser diode when the laser diode start to operate and raise the temperature thereof by the self heating due to the current provided thereto. The method may comprise steps of, first setting the temperature of the laser diode to be a preset value that corresponds to a specific wavelength of the optical output by the thermo-electric cooler, second receiving an external command to enable the laser diode, and third, in responding to the external command, concurrently providing a current to the laser diode and a pulsed current to the thermo-electric cooler to compensate a self-heating of the laser diode due to the current provided thereto. Thus, the wavelength deviation at the start of the operation due to the driving current may be compensated by momentarily enhancing the cooling capacity of the TEC by the pulsed current, even the temperature of the laser diode is kept to the preset value in a condition with no driving current. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a block diagram of an optical transmitter according to an embodiment of the present invention; 
           [0013]      FIG. 2  specifically illustrates a suppressing unit according to the embodiment of the invention; 
           [0014]      FIG. 3  shows time charts of the disable command Tx_Disable, the optical output, the LD temperature, and the output wavelength obtained in the conventional optical transmitter without any suppressing unit; 
           [0015]      FIG. 4  is time charts of the disable command Tx_Disable the optical output, the output of the attenuator, the output of the differentiator, the TEC driving current, the LD temperature and the output wavelength according to the present embodiment; 
           [0016]      FIG. 5  shows the wavelength deviation at the start of the operation in the conventional optical transmitter; 
           [0017]      FIG. 6  shows the wavelength deviation at the start of the operation according to the present embodiment; 
           [0018]      FIG. 7  is a block diagram of a modified optical transmitter according to the present embodiment of the invention; and 
           [0019]      FIG. 8  is a block diagram of a conventional optical transmitter without any suppressing unit. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    Next, embodiments of an optical transmitter according to the present invention will be described as referring to accompanying drawings. In the description of drawings, the same elements will be referred by the same symbols or the same numerals without over lapping explanations. 
         [0021]      FIG. 1  is a block diagram of an optical transmitter  1 A according to an embodiment of the present invention. The optical transmitter  1 A provides an optical module  16 , which is often called as a transmitter optical sub-assembly (TOSA), that installs a laser diode (LD)  10 , a thermistor  12 , and a thermo-electric controller (TEC)  14 . The thermistor  12  senses a temperature of the LD  10  to generate a monitoring signal Sa that decreases when the sensed temperature increase. The TEC  14 , which mounts the LD  10  thereon, lowers and raises a temperature of the LD  10  depending on the driving current Ip in the magnitude and the direction thereof. 
         [0022]    The optical transmitter  1 A further provides a control unit  20  to provide the driving current Ip to the TEC  14 . This control unit  20  includes the TEC driver  22 , the CPU  24 , the digital-to-analogue converter (D/A-C)  26  and the amplifier  28 . The CPU  24  and the D/A-C  26  generates a signal Sb corresponding to the target temperature of the LD  10 . This target temperature is equivalent to a channel wavelength assigned to the optical transmitter  1 A in the WDM communication system, and is held in the CPU  24 . The amplifier  28  is configured to receive the monitoring signal Sa in the non-inverting input thereof, while, the signal Sb in the inverting input, to compare these two signals and to generate a resulting signal Sc, which is substantially equal to a difference between two signals, Sa and Sb. The TEC driver  22  generates the driving current Ip corresponding to the resultant signal Sc and provides this current Ip to the TEC  14 . 
         [0023]    The optical transmitter  1 A further provides an input terminal  44  to receiver a transmitting signal Tx to modulate the LD  10 , a terminal  40  to receiver a disable signal Tx_Disable, and a laser driver  18  to generate a current Id to provide it to the LD  10 . The disable signal, Tx_Disable, forcibly turns off the LD  10 . The LD driver  17  provides the current Id only when the disable signal, Tx_Disable, is inactive, while, it fully stops the provision of the current Id to the LD  10 . 
         [0024]    The optical transmitter  1 A provides a unit  30 A to suppress the fluctuation of the output wavelength. This unit  30 A increases the driving current Ip for the TEC  14  when the disable signal Tx_Disable turns from the disable state to the enable state. The unit  30 A comprises a buffer amplifier  36  whose input terminal receives the signal Tx_Disable, an attenuator (ATT)  32  and a differentiator  34 . 
         [0025]      FIG. 2  illustrates a circuit diagram of the unit  30 A. Referring to  FIG. 2 , the ATT  32  includes to resistors,  32   a  and  32   b,  one of which  32   a  is connected to the buffer amplifier  36 , while the other resistor  32   b  is connected between the former resistor  32   a  and the ground. The differentiator  34  includes a resistor  34   a  and a capacitor  34   b.  The resistor  34   a  is inserted between the output of the D/A-C  26  and the non-inverting input of the amplifier  28 , while, the capacitor  34   b  is connected between the ATT  32  and the non-inverting input of the amplifier  28 . Thus, the transition of the disable signal Tx_Disable maybe reflected in the non-inverting input of the amplifier  28 , that is, the disable signal Tx_Disable may be superposed by a pulsed form to the signal Sb corresponding to the target temperature, which varies the driving current Ip for the TEC  14 . 
         [0026]    Next will explain the subjects appeared in the conventional optical transmitter as comparing the function of the present invention.  FIG. 3  shows time charts of the conventional optical transmitter shown in  FIG. 8 , where  FIGS. 3A to 3D  show a disable signal Tx_Disable, the output power of the LD, the temperature of the LD, and the output wavelength of the LD, respectively. The disable signal Tx_Disable turns to the enable state at the time A in the figure, the LD begins to emit light. Although the LD is practically modulated by the modulation current,  FIG. 3  illustrates the output power of the LD as the constant value P 0 . 
         [0027]    Synchronizing with the beginning of the emission, the power consumption by the LD abruptly increases, in other words, the temperature of the LD temporarily increases even though it stably keeps the target temperature T 0  until the beginning of the emission. Subsequently, the thermistor detects the this temperature increase, and the TEC driver controls, be receiving the output of the thermistor, the driving current Ip so as to set the temperature of the LD to be equal to the target value T 0 . However, there is substantial delay from the increase of the temperature to the stable state at the target value T 0 . Accordingly, the output wavelength of the LD fluctuates from the preset value λ 0  to a longer wavelength λ 1 . 
         [0028]      FIG. 4  shows the time charts of the signals, the disable signal Tx_Disable, the output power of the LD, the output from the ATT  32 , the output from the differentiator  34 , the driving current Ip, the temperature of the LD, and the output wavelength of the LD, respectively, in the optical transmitter  1 A according to the present invention. Similar to the conventional setup, the disable signal Tx_Disable turns to the enable state at the instance A and the LD begins to emit. 
         [0029]    The disable signal Tx_Disable also enters the ATT  32  of the suppressing unit  30 A to suppress the wavelength shift. The ATT  32  attenuates the disable signal Tx_Disable. This attenuated signal enters the differentiator  34  and the differentiator  34  generates a signal pulse D with a width of B and a peak height Vp. The leading edge of the pulse D traces the output of the ATT  32  and the falling edge thereof is primarily determined by a time constant of the resistor  34   a  and the capacitor  34   b,  while, the height Vp may be determined by the attenuation factor of the ATT  32 , and the time constant above. This pulsed signal D superposed on the target signal Sb enters the non-inverting input of the amplifier  28 . Accordingly, the output Sc of the amplifier includes a component corresponding to this pulsed signal D, which temporarily increases the driving current Ip synchronized with the negation of the Tx_Disable. 
         [0030]    Thus, the capacity of the TEC  14  to cool down the temperature of the LD  10  momentarily increases just after the timing A, which suppresses the temperature increase of the LD  10  due to the raised power consumption thereof and keeps the temperature around the target value T 0 . The output wavelength of the LD  10  may be also maintained at the predetermined value λ 0 , as shown in  FIG. 4 . 
         [0031]    The suppressing unit  30 A for the wavelength shift may provide the ATT  32  to attenuate the disable signal Tx_Disable and the differentiator  34  to generate the single pulsed signal synchronized with the leading or falling edge of the disable signal. This two step process for the disable signal Tx_Disable may optionally vary the width and the height of the pulsed signal, and the driving current Ip for the TEC may be widely arranged by superposing this pulsed signal on the target signal Sc. The buffer amplifier  36  interposed between the input terminal  40  and the ATT  32 , which is an inverter in the embodiment explained above, may select the synchronization of the pulsed signal with the leading edge or the falling edge of the disable signal Tx_Disable. 
         [0032]    Next, results according to the suppressing unit  30 A of the present invention will be specifically explained.  FIG. 5  illustrates the wavelength fluctuation appeared in the conventional optical transmitter without any suppressing unit, while,  FIG. 6  shows the behavior of the output wavelength practically observed in the optical transmitter  1 A of the present invention. An exemplarily LD consumes the power of 50 mW when it is driven by a condition of the driving current 50 mA and the bias 1V. Based on this condition, the conventional optical transmitter shifts the output wavelength thereof by about 55 pm synchronized with the negation of the disable signal, as shown in  FIG. 5 . The dense WDM system (DWDM system) with a grid interval of 100 GHz, which is equivalent to a wavelength interval of about 800 pm, often implements a band passing filter with a narrower bandwidth of 60 to 70 GHz, equivalent to about 480 to 600 pm. When an optical transmitter applied to the DWDM system implements a function to detect a deviation in the signal wavelength, the transmitter may issue an alarm even when the signal wavelength shifts only by 10% of the grid interval. Therefore, the fluctuation of about 55 pm occurred in the conventional transmitter becomes a subject of the alarm. 
         [0033]    On the other hand, the optical transmitter according to the present invention may provide the inverter  36  with an output swing following the CMOS logic level, namely, between 3.3 V and nearly ground level (0 V), the ATT  32  with two resistors,  32   a  and  32   b,  whose resistance are 33 kΩ and 100 Ω, respectively, and the differentiator  34  with the resistor  34   a  of 50 kΩ and the capacitor of 4.7 μF, then the output signal shown in  FIG. 5  may be obtained and be added to the target signal Sb. The driving current Ip momentarily increases that enhances the capacity to cool down the LD  10  and compensates the increase of the power consumption by the LD  10 , which may effectively suppress the fluctuation of the output wavelength of the LD  10  as shown in  FIG. 6 . 
         [0034]    (First Modification) 
         [0035]      FIG. 7  is a block diagram of an optical transmitter  1 B modified from the embodiment described above. The optical transmitter  1 B, compared to those configurations shown in  FIG. 1  for the first transmitter  1 A, modifies the node to which the output of the suppressing unit  30 A is provided. The optical transmitter  1 B varies, not the target signal Sb, the control signal Sc corresponding to a different between the target signal Sb and the monitored signal Sa by the output of the differentiator  34 . 
         [0036]    (Second Modification) 
         [0037]    The suppressing unit  30 A of the optical transmitters,  1 A and  1 B, may be modified so as to include a non-inverting buffer when the disable signal Tx_Disable is negated by a transition from the L level to the H level, opposite to the disable signal explained above. Thus, the suppressing unit  30 A of the present invention may choose an inverting buffer and a non-inverting buffer depending on the logic level of the disable signal Tx_Disable. 
         [0038]    While, this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. For instance, although the embodiments above superposes the output of the suppressing unit on the non-inverting input or the output of the amplifier to vary the driving current momentarily, the output of the suppressing unit may be led to the inventing input of the amplifier. In this case, the suppressing unit is necessary to output a negative single pulse synchronized with the negation of the disable signal Tx_Disable to decrease the sensed signal Sa, that is, the negative pulsed signal from the suppressing unit  30 A simulates the increase of the LD temperature. Further, although the suppressing unit in the embodiments first attenuates the disable signal Tx_Disable and next differentiates the attenuated signal, the suppressing unit may be comprised of, first forming a pulsed signal by a one-shot multi-vibrator that outputs a signal pulse in synchronized with the edge of the disable signal Tx_Disable, and second attenuating an output of this one-shot multi vibrator. 
         [0039]    When the ambient temperature of the LD  10  is less than the target temperature, the LD temperature may be set to the target one by reversing the direction of the driving current Ip, which heats up the LD. In this case, the pulsed output of the suppressing unit  30 A functions to decrease the capacity to raise the temperature, which may compensate the increase of the power dissipation by the LD to suppress the wavelength deviation of the LD  10 . It is therefore intended that the appended claims encompass any such modifications or embodiments.