Abstract:
A laser diode drive circuit and an optical transmission system that are capable of achieving compensation for degradation occurring as a result of a rapid change in the ambient temperature or an operation performed over an extended period of time are provided. The laser diode drive circuit having a temperature compensation circuit  117  further comprises a device that stores in memory a signal from a monitor photodiode  125  as light output power data and a device that implements automatic control on degradation compensation or temperature compensation for a laser diode  112  by using the light output power data as a reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims the priority benefit of Japanese Patent Application No. 2001-6160, filed Jan. 15, 2001, the entire disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a laser diode drive circuit and an optical transmission system and, more specifically, it relates to a laser diode drive circuit having a temperature compensation circuit and an optical transmission system.  
           [0004]    2. Description of the Related Art  
           [0005]    In a standard laser diode drive circuit in the conventional art, the operation is performed by adopting one of the two methods described below. The following is an explanation of laser diode drive circuits in the conventional art. One type of laser diode drive circuit adopts the zero bias drive method achieved without supplying a preliminary DC bias current to the laser diode, and another type of laser diode drive circuit adopts the bias drive method achieved by supplying a preliminary DC bias current to the laser diode.  
           [0006]    In a laser drive circuit adopting the zero bias drive method in which no preliminary DC bias current is supplied, an input signal is re-timed at a D-flip-flop to turn on/off a current switch circuit and a drive current Ip is made electrically continuous to either the laser diode or a resister R. As a result, light emission/extinction occurs at the laser diode and an optical signal corresponding to the input signal is output.  
           [0007]    The level of the drive current Ip, which determines the level of the light output power from the laser diode is controlled by varying the base potential at a transistor Tr connected to the current switch.  
           [0008]    In addition, the control on the base potential is achieved through a temperature compensation circuit. Namely, the present ambient temperature is first detected by a temperature sensor, digital data matching the ambient temperature are called up from an internal data storage unit, in which data are stored in correspondence to ambient temperatures and the digital data thus extracted are converted to an analog voltage value through D/A conversion.  
           [0009]    This analog voltage value is input to a non-inversion input terminal of an operational amplifier and is used as the base potential of the transistor Tr. The emitter potential VE of the transistor Tr becomes lowered by VBE and the lowered potential is input to an inversion input terminal of the operational amplifier. Through the negative feedback loop constituted by the operational amplifier and the transistor Tr, the modulation current Ip of the laser diode becomes fixed at a constant level in correspondence to a reference voltage VE and the value at the resister R and, as a result, the light output power, too, becomes fixed at a constant level. Through this temperature compensation control method, the modulation current Ip corresponding to the ambient temperature is made electrically continuous to the laser diode.  
           [0010]    In addition, when it is necessary to supply a preliminary DC bias current Ib in correspondence to the type of laser diode in use, a method similar to the control implemented on the modulations side may be adopted.  
           [0011]    However, if the ambient temperature fluctuates, the values of the modulation current Ip and the DC bias current Ib may become incorrect due to the difference between the temperature at the laser diode and the temperature detected by the temperature sensor in the conventional art. If the value of the modulation current Ip is incorrect, the level of the light output power will become deviated from the standard value range stipulated in the specification, whereas if the value of the DC bias current Ib is incorrect, a problem related to the extinction ratio will occur.  
           [0012]    Furthermore, due to the deterioration occurring over time in the threshold current and the efficiency in the light emission differentiation at the laser diode, the data stored in advance become inapplicable. This leads to a problem in that degradation occurs caused by a rapid change in the ambient temperature or an operation performed over an extended period of time for which sufficient compensation cannot be achieved.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, an object of the present invention is to provide a new and improved laser diode drive circuit and a new and improved optical transmission system, that are capable of achieving compensation for degradation caused by a rapid change in the ambient temperature or an operation performed over an extended period of time.  
           [0014]    In order to achieve the object described above, in a mode that represents the present invention, a laser diode drive circuit having a temperature compensation circuit, which further comprises a device that stores in memory a signal output from a monitor photodiode as light output power data and a device that implements automatic control on degradation compensation or temperature compensation for the laser diode by using the light output power data as a reference voltage, is provided.  
           [0015]    Since the level of the drive current Ip for the laser diode is controlled in correspondence to the differential voltage from the data storage unit and the monitor photodiode, stable light output power is obtained even when the laser diode becomes degraded over time in addition to achieving temperature compensation. As a result, the performance of the optical transmission system improves. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The above and other features of the invention and the concomitant advantages will be better understood and appreciated by persons skilled in the field to which the invention pertains in view of the following description given in conjunction with the accompanying drawings which illustrate preferred embodiments. In the drawings:  
         [0017]    [0017]FIG. 1 is a block diagram of the laser diode drive circuit achieved in a first embodiment;  
         [0018]    [0018]FIG. 2 is a block diagram of the laser diode drive circuit achieved in a second embodiment;  
         [0019]    [0019]FIG. 3 is a block diagram illustrating the structure adopted in the optical transmission system achieved in a third embodiment; and  
         [0020]    [0020]FIG. 4 is a flowchart of the operation achieved in the optical transmission system in the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The following is a detailed explanation of the preferred embodiments of the present invention, given in reference to the attached drawings. It is to be noted that in the following explanation and the attached drawings, the same reference numerals are assigned to components achieving identical functions and structural features to preclude the necessity for a repeated explanation thereof.  
         [0022]    (First Embodiment)  
         [0023]    The following is an explanation of the first embodiment given in reference to FIG. 1, which is a block diagram of the laser diode drive circuit achieved in the embodiment.  
         [0024]    First, as illustrated in FIG. 1, the laser diode control circuit in the embodiment is constituted by providing a monitor photodiode  124 , a current/voltage conversion amplifier  125 , an A/D conversion circuit  126 , a first mode selector circuit  128 , a data storage unit  131 , a differential amplifier  133 , an adder circuit  121  and the like, in addition to the components of the drive circuit achieved in the conventional art.  
         [0025]    Now, the operation achieved in the laser diode drive circuit structured as described above is explained.  
         [0026]    An operation to achieve mode setting 1 is performed at the first mode selector circuit  128 . First, the first mode selector circuit  128  is set so as to short a first wiring  127  and a second wiring  129 . Likewise, a second mode selector circuit  135  is set so as to open a third wiring  134  and a fourth wiring  136 . Following this modes setting, a temperature compensation unit  117  in the laser diode drive circuit is set.  
         [0027]    By using specific data stored in correspondence to a given temperature level at a data storage unit  119  in the temperature compensation circuit  117  provided in the laser diode drive circuit, a specific level of light output power is obtained. This operation is referred to as operation 1. The operation 1 is explained below.  
         [0028]    During operation 1, after achieving re-timing at a D-flip-flop  112 , data and a clock are input to a current switch unit  113 . During this process, an operational amplifier  122  is driven via a D/A converter  120  by the data stored in the data storage unit  119  and, as a result, a electric current flows to a transistor Tr  116 . By storing correct data at the data storage unit  119 , a specific level of light output power is obtained at a laser diode  114 .  
         [0029]    The light output from the laser diode  114  is transmitted to the transmission path via an optical fiber. At the same time, the back light of the laser diode  114  is input to the monitor photodiode  124 . This back light generates a photocurrent Im, which is then converted to a voltage at the current/voltage conversion amplifier  125 , and the corresponding data are stored at the data storage unit  131  via the A/D conversion circuit  126 .  
         [0030]    This operation 1 may be executed at a predetermined temperature setting (e.g., room temperature). Since a specific photocurrent can be obtained as data as long as a specific light output is achieved at a given temperature within the temperature setting range, it is not necessary to detect the temperature by employing a temperature sensor at the data storage unit  131 . When operation 1 is completed, mode settings 2 is implemented.  
         [0031]    During mode setting 2, the first mode selector circuit  128  is set so as to open the first wiring  127  and the second wiring  129  and the second mode selector circuit  135  is set so as to short the third wiring  134  and the fourth wiring  136 , by reversing mode setting 1 explained earlier. Thus, operation 2 is enabled in the laser diode drive circuit. It is to be noted that operation 2 refers to a normal operation.  
         [0032]    During operation 2, the laser diode  114  is driven to output light. In addition, back light is input to the monitor photodiode  124 . The photocurrent Im generated by the back light is input to the differential amplifier  133  as a voltage Vpd via the current/voltage conversion amplifier  125 .  
         [0033]    The data having been stored through operation 1 are detected from the data storage unit  131 , the detected data undergo analog conversion at a D/A converter  132  and the converted data are then input as Vref to the differential amplifier  133 . This voltage Vref is used as a reference voltage at the differential amplifier  133 .  
         [0034]    The differential amplifier  133  amplifies the difference between the reference voltage Vref and Vpd and outputs the amplified difference as a voltage Vo. It is to be noted that the gain at the differential amplifier  133  is determined in conformance to the characteristics of the laser diode and the monitor photodiode.  
         [0035]    The output voltage Vo is input to the adder circuit  121 . The voltage Vo input to the adder circuit  121  is added to a voltage VP resulting from the conversion performed at the D/A converter  120  in the temperature compensation unit  117 .  
         [0036]    It is to be noted that the reference voltage used at the adder circuit  121  at this time is Vref. A voltage Vr representing the sum obtained through the addition is input to the non-inversion input terminal of the operational amplifier  122 . As a result, the modulation current Ip supplied to the laser diode  114  changes until the value of Vpd at the differential amplifier  133  becomes equal to the Vref value.  
         [0037]    Through operation 2 explained above, the difference between Vpd and Vref is input to the adder circuit, Vr input to the operational amplifier is changed accordingly and the modulation current Ip is changed until the light output power from the laser diode achieves the initial value (the value set through operation 1). As a result, the light output power from the laser diode automatically sustains its initial value and maintains stability.  
         [0038]    In the embodiment, in which the level of the drive current Ip for the laser diode is controlled by using both the data stored in the data storage unit and the differential voltage provided from the monitor photodiode, achieves stable light output power even when the laser diode becomes degraded over time in addition to achieving temperature compensation. As a result, the performance of the light transmitter improves.  
         [0039]    (Second Embodiment)  
         [0040]    The following is an explanation of the second embodiment given in reference to FIG. 2. FIG. 2 presents a block diagram of the laser diode drive circuit achieved in the second embodiment.  
         [0041]    The laser diode drive circuit in the embodiment is constituted by providing a gain variable amplifier, a current/voltage conversion amplifier that supplies a reference voltage to the gain variable amplifier, a buffer amplifier, a bottom detection circuit, a peak detection circuit, an operational amplifier and the like in addition to the components of the drive circuit in the first embodiment. In this circuit, the DC current which is supplied in advance to the laser diode can be controlled as well.  
         [0042]    First, mode something 1 is achieved by engaging mode selector circuits  260 ,  267 ,  264  and  271 . A first mode selector circuit  260  is set so as to short a first wiring  272  and a second wiring  273 , and the second mode selector circuit  267  is set so as to short a third wiring  274  and a fourth wiring  275 . Likewise, the third mode selector circuit  264  is set so as to open a fifth wiring  276  and a sixth wiring  277  and the fourth mode selector circuit  271  is set so as to open a seventh wiring  278  and an eighth wiring  279 .  
         [0043]    When mode setting 1 is completed, operation 1 similar to that implemented in the first embodiment is executed to store data in a data storage unit  244  in a temperature compensation unit  242  and the level of the light output power from a laser diode  239  is set. Likewise, the level of a preliminary bias DC current Ib supplied to the laser diode  239  in advance is set.  
         [0044]    The back light from the laser diode  239  is detected by a monitor photodiode  252 , thereby generating a photocurrent Im. The correct signal is converted into a voltage signal at a current/voltage conversion amplifier  253  and the voltage signal is input to a gain variable amplifier  255  together with the output from another current/voltage conversion amplifier  254  which is used as a reference voltage source.  
         [0045]    The bottom value VB and the peak VP of the output signal from the gain variable amplifier  255  are detected by a bottom detection circuit  257  and a peak detection circuit  258  via a buffer amplifier  256 . The detected bottom value undergoes A/D conversion at an A/D converter  259  and the converted data are stored in a data storage unit  261 . At the same time, a voltage VPB representing the difference between the peak value and the bottom value is extracted at a differential amplifier  265 , and the data obtained by implementing A/D conversion on the voltage VPB at an A/D converter  266  are stored in a data storage unit  268 . This operation may be executed at a predetermined temperature as in the first embodiment. When operation 1 is completed, operation 2 is enabled through mode setting 2.  
         [0046]    Mode setting 2 is achieved by engaging the various mode selector circuits  260 ,  267 ,  264  and  271 . The first mode selector circuits  260  is set so as to open the first wiring  272  and the second wiring  273 , and the second mode selector circuit  267  is set so as to open the third wiring  274  and the fourth wiring  275 . Likewise, the third mode selector circuit  264  is set so as to short the fifth wiring  276  and the sixth wiring  277  and the fourth mode selector circuit  271  is set so as to short the seventh wiring  278  and the eighth wiring  279 . Thus, operation 2 is enabled in the laser diode drive circuit. It is to be noted that operation 2 corresponds to a normal state.  
         [0047]    As the laser diode  239  is driven and its back light is input to the monitor photodiode  252 , the photocurrent Im is generated. The bottom value VB and the peak value VP of the photocurrent Im are detected by the bottom detection circuit  257  and the peak detection circuit  258  via the current/voltage conversion amplifier  253 , the gain variable amplifier  255  and the buffer amplifier  256 .  
         [0048]    The digital data having been stored through operation 1 in the data storage unit  261  are converted to an analog value at a D/A converter  262  and the converted data are input to a differential amplifier  263  as a reference voltage VBREF, together with the bottom value VB. A voltage VOB representing the difference between the reference voltage VBREF and the bottom value VB is extracted and input to an adder subtractor circuit  248 . It is to be noted that the adder circuit  248  uses VBREF as the reference voltage.  
         [0049]    The difference between the peak value and the bottom value is extracted at the differential amplifier  265  and the difference is input as a voltage VPB at one phase of a differential amplifier  270  provided at the following stage. The data having been stored in the data storage unit  268  through operation 1 are input at the other phase of the differential amplifier  270  as a voltage VPREF via a D/A converter  269 . At the differential amplifier  270 , a voltage VOPB representing the difference between VPB and the voltage VPREF is extracted and then input to an adder circuit. The adder circuit uses VPREF as a reference voltage.  
         [0050]    VOB and VOPB having been input to the adder subtractor circuits  248  and  247  are added to the value resulting from a D/A conversion  246  and the value resulting from a D/A conversion  245  respectively until the individual voltages become equalized at the differential amplifiers  263  and  270 . Thus, the light output power is allowed to sustain its initial value and thus maintains stability by changing the DC bias current and the modulation current supplied to the laser diode.  
         [0051]    As described above, advantages similar to those in the first embodiment are achieved with regard to the extinction ratio of the DC bias current in addition to the advantages realized in the first embodiment, to further improve the performance of the light transmitter.  
         [0052]    (Third Embodiment)  
         [0053]    Next, in reference to FIGS. 3 and 4, the third embodiment is explained. FIG. 3 is a block diagram illustrating the structure adopted in the optical transmission system achieved in the third embodiment.  
         [0054]    In the optical transmission system in the embodiment, the mode selector circuits and the data storage units in the laser diode drive circuit in the first or second embodiment are controlled by an external CPU. In the embodiment, the data in data storage units  385  and  386  are regularly updated. In addition, the mode selector circuits, too, are switched by the CPU through automatic control.  
         [0055]    Next, the flow of the operation achieved in the optical transmission system in the embodiment is explained in reference to FIG. 4. FIG. 4 is a flowchart of the operation achieved in the optical transmission system in the embodiment.  
         [0056]    As shown in FIG. 4, when operation 1 is completed, the system shifts to operation 2 which is a normal operation, and after a specific cycle, the data in the data storage units are updated.  
         [0057]    The laser diode drive circuit according to the present invention may be adopted in An optical transmission system having An optical transmission circuit that converts an electrical signal to a light signal and transmits the converted signal. In addition, application of the present invention is not limited to an optical transmission system and may be adopted to achieve temperature compensation and degradation compensation for any circuit that converts an electrical signal to a light signal by using a laser diode as well as in An optical transmission system.  
         [0058]    While the invention has been particularly shown and described with respect to preferred embodiments thereof by referring to the attached drawings, the present invention is not limited to these examples and it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit, scope and teaching of the invention.  
         [0059]    Since the level of the drive current Ip for the laser diode is controlled by using both the data in the data storage units and the differential voltage supplied from the monitor photodiode, light output power which remains stable in spite of degradation occurring in the laser diode over time is achieved as well as achieving temperature compensation, to contribute to an improvement in the performance of a light transmitter.