Abstract:
A light transmitter for preventing the degradation of the extinction ratio and the occurrence of pulse width distortion in the light output in spite of the variation of current to light conversion efficiency A of the laser diode. The transmitter uses the characteristics such that the light pulse width becomes narrower than the modulation current pulse width if the laser diode is modulated by a bias current smaller than the threshold level. The light transmitter includes mismatch detection circuit (14) which detects the decrease of the light pulse width. By the detected mismatch signal, the modulation current control circuit (15) limits to increase the modulation current. The modulation current control circuit (15) operates together with the control circuit which controls automatically the light output peak value to be constant.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a light transmitter for preventing the degradation of the extinction ratio of a light signal and the occurrence of pulse width distortion. 
     FIG. 3 is a block diagram of the prior transmitter disclosed in Japanese Patent publication 2-44420/1990. In FIG. 3, 1 is a transmitting data input terminal. 2 is a modulator connected to the transmitting data input terminal 1. 3 is a laser diode. 4 is a light receiving element for receiving a part of the output light from the laser diode 3. 5 is a capacitor connected in parallel to the light receiving element 4. 6 is a bias voltage source for supplying the voltage to the light receiving element 4. 7 is a current amplifier. 8 is a first reference current source. 9 is a second reference current source. 10 is a switching circuit connected to the output terminal of the second current source 9 for switching the second current source in response to the binary transmitted data. 
     The operation of the conventional light transmitter is described hereinafter. The output current from the reference current source 9 is added to the second output current by the switching circuit 10 in response to the binary transmitted data. The relation between the currents 101 outputted from the first current source, I O2  outputted from the second current source and the reference current I O  is obtained as follows. 
     
         I.sub.O =I.sub.O1 m·I.sub.O2                      ( 1) 
    
     where, 
     I O  : reference current 
     I O1  : output current of the first reference current source 
     I O2  : Output current of the second reference current source 
     m: mark rate of the digital signal 
     The output light of the laser diode 3 is radiated to the light receiving element 4. The current I PD  flowing the light receiving element 4 is in proportion to the output light of the laser diode as shown in the following equation. 
     
         I.sub.PD =m·D·L·P.sub.out       ( 2) 
    
     where, 
     I PD  : current flowing the light receiving element 
     D: pulse duty factor 
     L: current conversion efficiency from the laser diode to the light receiving element 
     P out  : peak value of the light output 
     The current difference (I 0  -I PD ) between the synthesized current 10 and the current I PD  flowing in the light receiving element 4 is amplified by the current amplifier 7 and applied to the laser diode 3. The current I PD  flowing in the light receiving element 4 is averaged in the capacitor 5 connected in parallel to the light receiving 4. The current I B  applied to the laser diode 3 is obtained as follows. 
     
         I.sub.B =β(I.sub.0 -I.sub.PD)                         (3) 
    
     where, 
     I B  : bias current sink current of the current amplifier) 
     β: amplification factor of the current amplifier 
     The current to light conversion characteristics of the laser diode 3 is shown in FIG. 4. From the FIG. 4, the peak value P out  of the light output of the laser diode is obtained in the following equation. 
     
         P.sub.out =A (I.sub.B +I.sub.op -I.sub.th)                 (4) 
    
     where, 
     I op  : modulation current (sink current of the modulator) 
     I th  : threshold current of the laser diode 
     A: current to light conversion efficiency of the laser diode 
     From the equations (1) to (4), the next equation is obtained. 
     
         P.sub.out ={A·β·I.sub.O +A (I.sub.op -I.sub.th)}/(1+A·β·m·D·L) (5) 
    
     The equations from (1) to (4) shows that the bias current I B  is controlled so that the peak value of the light output P out  may be constant if the threshold current I th  varies by some reasons. That is, if I th  increases to I th  &#39;, the bias current I B  is controlled automatically to increases to I B  &#39; in order to maintain the constant light output by the current amplifier as shown in FIG. 5. 
     For the initial condition, I B  is set as follows as shown in FIGS. 4, 5 and 7. 
     
         I.sub.B =I.sub.th                                          ( 6) 
    
     By setting I B  =I th  as shown in the equation (6), the equation (5) is satisfied so that P out  may be constant if the threshold current of the laser diode I th  varies. 
     Since the conventional light transmitter is constructed such as described above, when the current to light conversion efficiency A of the laser diode degrades, IB changes as shown in equation (7) even if I B  is set as I B  =I th  at the initial state as shown in the equation (6). The reason lies in the fact that the modulation current lop is always constant. 
     
         I.sub.B &gt;I.sub.th                                          ( 7) 
    
     As a result, the extinction ratio of the light signal decreases. 
     On the hand, if the current to light conversion efficiency A of the laser diode increases, I B  changes as follows. 
     
         I.sub.B &lt;I.sub.th                                          ( 8) 
    
     In this case, there are some problems such that pulse width distortion occurs in the light signal. 
     Therefore, it is a primary object of the present invention to provide a light transmitter for maintaining the peak value of the light output constant as shown in the equation (6) even if the current to light conversion efficiency A of the laser diode varies. 
     SUMMARY OF THE INVENTION 
     A light transmitter comprising a first current to voltage conversion circuit for converting the signal current flowing in the light receiving element to a voltage signal, a second current to voltage conversion circuit for converting the modulation current outputted from the modulator to a voltage signal and a mismatch detection circuit for detecting the mismatch between the current outputted from the first current to voltage conversion circuit and the current outputted from the second current to voltage conversion circuit, and a modulation current control circuit for limiting the modulation current by the detected output voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a circuit diagram of a first embodiment of the present invention 
     FIG. 2 is a circuit diagram of a second embodiment of the present invention 
     FIG. 3 is a circuit diagram of the prior art embodiment 
     FIG. 4 is a characteristic figure showing the relation between the light output and the laser diode current 
     FIG. 5 is a characteristic figure showing the relation between the light output and the laser diode current when the threshold level of the laser diode increases 
     FIG. 6 shows a timing chart between the modulation current, the light output and the mismatch output signal 
     FIG. 7 is a characteristics figure showing the relation between the light output and the laser diode current when the current to light conversion efficiency A varies. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 is a circuit diagram of a first embodiment of the present invention. In FIG. 1, 11 is a filter for averaging the current flowing in the light receiving element 4. 12 is a first current to voltage conversion circuit. 13 is a second current to voltage conversion circuit. 14 is a mismatch detection circuit for detecting the mismatch between the output of the first current to voltage conversion circuit 12 and the output of the second current to voltage conversion circuit 13. 15 is a modulation current control circuit for controlling the modulation current by the output current of the mismatch detection circuit 14. 16 is first current to voltage conversion resistor in the first current to voltage conversion circuit 12. 17 is an amplifier for amplifying the output of the first current to voltage conversion resistor. 18 is second current to voltage conversion resistor in the second current to voltage conversion circuit 13. 19 is a second amplifier for amplifying the output of the second current to voltage conversion resistor. Other elements of the first embodiment are the same as those having the same numerals in FIG. 3, and therefore the detailed descriptions are omitted. 
     The operation of the first embodiment of the present invention is described now. In the first embodiment, the circuit operates in the same manner as that of the conventional circuit for controlling the light output P out  to be constant. The difference from the conventional circuit is in that the first embodiment uses a filter 11 for averaging current I PD  flowing in the light receiving element 4. Resistor 16 is supplied to convert the current I PD  to a voltage signal without being averaged. The second current to voltage converter 13 equivalently converts the inverted modulated current to a voltage signal. The output voltages of the first current to voltage conversion circuit 12 and the second current to voltage conversion circuit 13 are applied to the mismatch detection circuit 14. The mismatch detection circuit 14 detects the mismatch of the pulse width of the two voltage output signals. 
     The laser diode has a characteristic in connection with the modulation current and the output light as shown in FIG. 6. As easily understood from FIG. 6, the rise time of the output light lags from the modulation current and then the pulse width of the light output becomes narrower at the condition of the equation (8). When the rise time of the output light lags at the condition of the equation (8), the mismatch detection signal is outputted from the mismatch detection circuit 14. The modulation current control circuit 15 controls to limit to increase the modulation current lop when the mismatch signal is generated. Therefore the bias current I B  decreases in order to maintain the constant light output P out  and then the condition shown in the formula (6) is maintained. Because, it is necessary to have the relation (I B  +I op  =constant) for maintaining the constant power P out . 
     The detailed explanation follows in connection with the operation of the modulation current control circuit 15. Assume that the current to light conversion efficiency A of the laser diode decreases to A&#39; as shown in FIG. 7. The bias current I B  increases to I B  &#39; for a short time by the auto light level control circuit (not shown in the figure) as shown in FIG. 7 in order to maintain the constant light output P out . Because the modulation current I op  can not change instantly. In this occasion (at the condition of the equation (7)), since the mismatch pulse is not outputted from the mismatch detection circuit 14, the modulation current I op  outputted from the modulation current control circuit 15 gradually increases up to I op  &#39;. During the modulation current I op  increases, the bias current I B  &#39; decreases, because (I op  +I B  &#39;) must be constant in order to maintain a constant light output P out . When the modulation current I op  becomes equal to I op  &#39;, the bias current becomes I B  and the formula (6) is satisfied. Therefore the system is stabilized. 
     On the contrary, assume that the current to light conversion efficiency A of the laser diode increases to A&#34; as shown in FIG. 7. The bias current I B  decreases to I B  &#34; for a short time as shown in FIG. 7 in order to maintain the constant light output P out , because the modulation current I op  can not change instantly. In this occasion (at the condition of the equation (8)), since the mismatch pulse is outputted from the mismatch detection circuit 14, the modulation current I op  outputted from the modulation current control circuit 15 gradually decreases down to I op  &#34;. During the modulation current I op  decreases, the bias current I B  &#34; increases, because (I op  +I B  &#34;) must be constant in order to maintain a constant light output P out . When the modulation current I op  becomes equal to I op  &#34;, the bias current becomes equal to I B  and the formula (6) is satisfied. Therefore the system is stabilized. 
     These operations satisfy the equations (4) and (6) at the same time. Therefore the following equation is obtained. 
     
         P.sub.out =A·I.sub.PD                             (9) 
    
     Second Embodiment 
     FIG. 2 is a circuit diagram of a second embodiment the present invention. In FIG. 2, 11 is a filter for averaging the modulated current outputted from the modulator. 22 is a first current to voltage conversion and averaging circuit. 23 is a second current to voltage conversion and averaging circuit. 24 is a comparison circuit for comparing the outputs between the output of the first current to voltage conversion and averaging circuit 22 and the output of the second current to voltage conversion and averaging circuit 23. 15 is a modulation current control circuit for controlling the modulation current by the output signal of the comparison circuit 24. 16 is first current to voltage conversion resistor in the first current to voltage conversion and averaging circuit 22. 17 is an amplifier for amplifying the output of the first current to voltage conversion resistor 16. 18 is second current to voltage conversion resistor for supplying the voltage to a second current to voltage conversion and averaging circuit 23. 19 is a second amplifier for amplifying the output of the second current to voltage conversion resistor. Other elements of the first embodiment are the same as those having the same numerals in FIG. 2, and therefore the detailed descriptions are omitted. 
     The operation of the second embodiment of the present invention is described now. In the second embodiment, the circuit operates in the same manner as that of the conventional circuit for controlling the light output P out  to be constant. The difference from the conventional circuit is in that the second embodiment uses the filter 11 for averaging the voltage converted by the second resistor 18 from the modulation current I op  outputted from the modulator 2. The first current to voltage conversion and averaging circuit 22 converts the current I PD  flowing in the light receiving element 4 to a voltage signal. In this circuit, the current flowing in the light receiving element 4 is averaged by the capacitor 5 connected to the diode 4 as shown in FIG. 2. The output voltages of the first current to voltage conversion and averaging circuit 22 and the second current to voltage conversion and averaging circuit 23 are applied to the comparison circuit 24. The comparison circuit 24 compares the two voltage signals and outputs a control signal for limiting to increase the modulation current I op  if the output of the second current to voltage conversion and averaging circuit 23 is greater than the output of the first current to voltage conversion and averaging circuit 22. 
     The laser diode has a characteristic as shown in FIG. 6 in connection with the modulation current and the output light. In FIG. 6, when the rise time of the light output lags the modulation current, then the pulse width of the light output becomes narrower and the averaged level decreases at the condition of the equation (8). The modulation current control circuit 15 controls to limit to increase the modulation current I op  when the averaged level decreases. Therefore the bias current I B  decreases in order to maintain the constant light output P out  and then the condition shown in the equation (6) is maintained, because, it is necessary to have the relation (I B  +I op  =constant) for maintaining the constant light output power P out . 
     A detailed explanation follows in connection with the operation of the modulation current control circuit 15. Assume that the current to light conversion efficiency A of the laser diode decreases to A&#39; as shown in FIG. 7. The bias current I B  increases to I B  &#39; for a short time as shown in FIG. 7 in order to maintain the constant light output P out , because the modulation current I op  can not change instantly. In this occasion (at the condition of the equation (7)), since the output level from the comparison circuit 24 is lower than the averaged output level of the modulated pulse current, the modulation current I op  outputted from the modulation current control circuit 15 gradually increases up to I op  &#39;. During the modulation current I op  increases, the bias current I B  &#39; decreases, because (I op  +I B  &#39;) must be constant in order to maintain a constant light output P out . When the modulation current I op  becomes equal to I op  &#39;, the bias current becomes I B  and the equation (6) is satisfied. Therefore the system is stabilized. On the contrary, assume that the current to light conversion efficiency A of the laser diode increases to A&#34; as shown in FIG. 7. The bias current I B  decreases to I B  &#34; for a short time by the auto light level control circuit (not shown in the figure) as shown in FIG. 7 in order to maintain the constant light output P out , because the modulation current I op  can not change instantly. In this occasion (at the condition of the equation (8)), since the output level from the comparison circuit 24 is higher thin the averaged output level of the modulated pulse current, the modulation current I op  outputted from the modulation current control circuit 15 gradually decreases down to I op  &#34;. During the modulation current I op  decreases, the bias current I B  &#34; increases, because (I op  +I B  &#34;) must be constant in order to maintain a constant light output P out . When the modulation current I op  becomes equal to I op  &#34;, the bias current becomes equal to I B  and the formula (6) is satisfied. Therefore the system is stabilized. These operations satisfy the equations (4) and (6) at the same time. Therefore the equation (9) is also obtained.