Abstract:
An optical transmitter includes a laser diode drive circuit for driving a laser diode in accordance with a laser diode current that superimposes a pilot signal on a “0” logic level and a “1” logic level of a transmission signal, the pilot signal having a low-frequency compared with the transmission signal, so as to become an opposite phase each other, a monitor circuit for monitoring an optical output of the laser diode, a filter circuit for extracting a low-frequency component of the pilot signal from an output of the monitor circuit, a phase compare circuit for comparing a phase of the low-frequency component of the pilot signal extracted by the filter circuit with a phase of the pilot signal, and a deterioration judgment circuit for judging whether or not the laser diode deteriorates in accordance with a comparison result of the phase compare circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to optical transmitters, and more particularly, to an optical transmitter that performs deterioration detection of the optical transmitter using a laser diode (LD) and that controls an extinct ratio in accordance with a result of the deterioration detection.  
         [0003]     2. Description of the Related Art  
         [0004]     Optical transmitters using an LD as a light source are available for an optical transmission system capable of long-distance transmission. In order to make an optical transmission distance longer, optical transmitters control optical transmission power to a constant value. In addition, optical transmitters control an extinct ratio, which is a level ratio of a logical level “1” to a logical level “0” of an optical transmission signal, to be within a predetermined range.  
         [0005]      FIG. 12  is a diagram for explaining direct modulation of an LD. A peripheral circuit of an LD  101  is shown on the right side of  FIG. 12 , and an LD drive circuit including an amplifier (AMP)  102  is shown on the left side of  FIG. 12 . A transmission signal ( 2 ) is modulated in accordance with a modulation current Ip controlled based on an Ip control signal ( 4 ), and is applied to the LD  101  biased at an LD threshold current Ith or more in accordance with a bias current Ib controlled based on an Ib control signal ( 5 ). The LD threshold current Ith is a minimum current at which the LD  101  can oscillate.  
         [0006]      FIGS. 13A  to  13 C are diagrams for explaining an optical output in which a transmission signal is modulated.  FIG. 13A  shows a transmission signal.  FIG. 13B  shows an optical output signal in which a continuous wave (CW) of an LD that oscillates by being biased in accordance with a bias current Ib is modulated in accordance with a modulation current Ip, which is used for modulating the transmission signal. The optical output signal is generally represented as a waveform in which a continuous wave CW of the LD that uniquely oscillates is omitted, as shown in  FIG. 13C . In other words, the LD oscillates and generates the continuous wave CW by being biased at the LD threshold current Ith or more, the amplitude value of the continuous wave CW is determined in accordance with the received modulation current Ip, and the LD outputs an optical signal from which the transmission signal can be identified in accordance with the level of the continuous wave CW.  
         [0007]      FIG. 14  is a diagram for explaining LD conversion efficiency. An LD conversion efficiency curve ( 103 ) represents the relationship between an LD current and optical output power. An LD starts stimulated emission when the LD current exceeds a critical level. The critical level is represented as an LD threshold current Ith in  FIG. 14 . Generally, a bias current Ib and a modulation current Ip are set such that the LD current exceeds the LD threshold current Ith. Reference numeral ( 104 ) denotes an LD current when a transmission signal has a random pattern. Reference numeral ( 105 ) denotes an optical output of the LD that oscillates when the LD current ( 104 ) is applied. An operation point of the LD current is determined in accordance with the bias current Ib used for bias at the LD threshold current Ith or more, and a motion range of the LD current is determined in accordance with the modulation current Ip. An amplitude of the optical output is determined in accordance with the gradient of the LD conversion efficiency curve ( 103 ). Thus, the optical output power is controlled based on the bias current Ib and the modulation current Ip, and the extinct ratio is controlled based on the modulation current Ip.  
         [0008]     Direct modulation of the LD is described next with reference to FIGS.  12  to  14 . The transmission signal ( 2 ) is modulated in accordance with the modulation current Ip in the AMP  102 , and applied to the LD  101  biased in accordance with the bias current Ib set to the LD threshold current Ith or more. Thus, the LD current ( 104 ) to be applied to the LD  101  functions as a signal having amplitude of the modulation current Ip centered on the operation point biased in an LD current range of the LD conversion efficiency curve ( 103 ) in which the LD is capable of oscillating, and the optical output ( 105 ) is gained due to a characteristic of the LD conversion efficiency curve ( 103 ). Accordingly, the extinct ratio is affected by the gradient of the LD conversion efficiency curve ( 103 ) and the size of the modulation current Ip.  
         [0009]      FIG. 15  is a diagram for explaining control for the optical transmitter. A photodiode (PD)  111  monitors an optical output of the LD  101 . An LD drive circuit  112  is the circuit shown in  FIG. 12 . A control circuit  113  is a circuit that controls the LD drive circuit  112 .  
         [0010]      FIG. 16  is a diagram for explaining monitoring of an optical output. Reference numeral ( 106 ) denotes a PD current of a signal monitored by the PD  111 . Part of output light of the optical output ( 105 ) is received by the PD  111 , and is subjected to photoelectric conversion. Accordingly, the PD current ( 106 ) is acquired.  
         [0011]     The control circuit  113  receives a monitor current of the PD  111 , and controls the LD drive circuit  112  using the Ip control signal ( 4 ) and the Ib control signal ( 5 ) shown in  FIG. 12  such that the optical output of the LD  101  is constant power determined in advance. The control circuit  113  also controls the LD drive circuit  112  using the Ip control signal ( 4 ) in order to control the extinct ratio of the LD  101 .  
         [0012]      FIG. 17  is another diagram for explaining LD conversion efficiency.  FIG. 17  shows a case where the modulation current Ip increases from the value shown in  FIG. 14 . This represents that the extinct ratio increases in accordance with the LD conversion efficiency curve ( 103 ). In this case, although not shown in  FIG. 17 , when the bias current Ib increases, the operation point moves and the optical output power increases.  
         [0013]     Since LD conversion efficiency is susceptible to a temperature change and changes with age, the LD threshold current Ith and the gradient of the LD conversion efficiency curve ( 103 ) are not constant. Thus, there is a need to perform auto power control (APC) in order to maintain the optical output power constant and to perform auto modulation control (AMC) in order to maintain the extinct ratio within a predetermined range.  
         [0014]     A technology, as a control technology for the APC and the AMC, in which in order to control optical output power and an extinct ratio of a laser diode to be constant with respect to long-term deterioration, optical output power of the laser diode before deterioration is detected, the detected value is stored as a reference value, and feedback control is performed for the reference value is disclosed in Japanese Unexamined Patent Application Publication No. 11-135871.  
         [0015]     In addition, a technology in which an operation point of a bias current Ib and a motion range of a modulation current Ip of an LD conversion efficiency curve in the process of operation are compared with an operation point of the bias current Ib and a motion range of the modulation current Ip of the LD conversion efficiency curve set in advance and optimal optical output power and extinct ratio can be ensured by changing the bias current Ib and the modulation current Ip is disclosed in U.S. Pat. No. 6,414,974.  
         [0016]     LD conversion efficiency differs depending on the type of LD. In addition, even for LDs of the same type, LD conversion efficiency differs depending on the ambient temperature and depending on aged deterioration. Generally, deterioration increases by use in high temperature environment and with age.  
         [0017]      FIG. 18  is a diagram for explaining deterioration of an LD. Generally, when an LD deteriorates, the gradient of the LD conversion efficiency curve ( 103 ) decreases, as shown by LD conversion efficiency curves ( 103 ( 1 )), ( 103 ( 2 )), and ( 103 ( 3 )).  
         [0018]      FIG. 19  is a diagram for explaining LD conversion efficiency when an LD deteriorates. As shown in  FIG. 19 , when the LD deteriorates and the gradient of the LD conversion efficiency curve ( 103 ) significantly decreases, the extinct ratio can be increased by increasing the bias current Ib and the modulation current Ip. However, a larger bias current Ib and a larger modulation current Ip are required as the gradient of the LD conversion efficiency curve ( 103 ) decreases. In addition, when the LD deteriorates and the gradient of the LD conversion efficiency curve ( 103 ) reaches about zero, it is difficult to increase the extinct ratio even if the bias current Ib and the modulation current Ip increase. However, under the APC and AMC, as long as it is not recognized that the LD conversion efficiency is significantly degraded due to deterioration of the LD, feedback control for increasing the bias current Ib and the modulation current Ip is performed until a limiter for the APC and the AMC operates. Thus, unnecessary control is performed. Generally, in an optical transmitter using an LD, when a monitored LD bias current exceeds an allowable LD bias current set in advance, the optical transmitter issues a warning, and control for the optical transmitter is stopped.  
       SUMMARY OF THE INVENTION  
       [0019]     Accordingly, it is an object of the present invention to provide an optical transmitter using an LD that is capable of rapidly detecting deterioration of the LD. In addition, it is another object of the present invention to provide an optical transmitter capable of acquiring an optimal extinct ratio using a function to detect the deterioration of the LD and a method for controlling the optical transmitter.  
         [0020]     An optical transmitter according to an aspect of the present invention includes an LD drive circuit for driving a laser diode in accordance with a laser diode current that superimposes a pilot signal on a “0” logic level and a “1” logic level of a transmission signal, the pilot signal having a low-frequency compared with the transmission signal, so as to become an opposite phase each other; a monitor circuit for monitoring an optical output of the laser diode; a filter circuit for extracting a low-frequency component of the pilot signal from an output of the monitor circuit; a phase compare circuit for comparing a phase of the low-frequency component of the pilot signal extracted by the filter circuit with a phase of the generated pilot signal; and a deterioration judgment circuit for judging whether or not the laser diode deteriorates in accordance with a comparison result of the phase compare circuit.  
         [0021]     Accordingly, an optical transmitter using an LD that is capable of rapidly detecting deterioration of the LD can be provided.  
         [0022]     An optical transmitter according to another aspect of the present invention further includes a control circuit for controlling one of or both of optical output power of the laser diode and an extinct ratio of the optical output of the laser diode in accordance with a monitor current output by the monitor circuit. The control circuit maintains control for optical output power of the laser diode and the extinct ratio of the optical output of the laser diode when the deterioration judgment circuit judges the laser diode deteriorating.  
         [0023]     Accordingly, an optical transmitter that achieves an optimal extinct ratio using a function to detect deterioration of an LD can be provided.  
         [0024]     A method for controlling optical transmitter according to an aspect of the present invention includes the steps of driving a laser diode in accordance with a laser diode current that superimposes a bias current on a modulation current for modulating a transmission signal on which superimposes a pilot signal; extracting a low-frequency component of a pilot signal from an output of the monitor circuit; comparing a phase of the low-frequency component of the pilot signal extracted by the filter circuit with a phase of the generated pilot signal; judging whether or not the laser diode deteriorates in accordance with a comparison result of the phase compare circuit; controlling one of or both of optical output power of the laser diode and an extinct ratio of the optical output of the laser diode in accordance with a monitor current output by the monitor circuit; and maintaining control for optical output power of the laser diode and the extinct ratio of the optical output of the laser diode when it is judged that the laser diode deteriorates.  
         [0025]     Accordingly, a method for controlling optical transmitter capable of achieving an optimal extinct ratio using a function to detect deterioration of an LD can be provided.  
         [0026]     According to the optical transmitter and the method for controlling optical transmitter, in a state in which LD conversion efficiency deteriorates since an LD provided in the optical transmitter deteriorates due to a temperature increase or deteriorates with age, the deterioration of the LD can be rapidly detected. In addition, an optical transmitter that is capable of controlling a bias current Ib and a modulation current Ip to achieve an optimal extinct ratio in accordance with a result of comparison between phases of pilot signals and a result of detection of amplitude values of the pilot signals can be provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a diagram for explaining an example of the configuration of an optical transmitter according to an embodiment of the present invention;  
         [0028]      FIG. 2  is a diagram for explaining direction modulation of an LD according to the embodiment;  
         [0029]      FIG. 3  is a diagram for explaining an example of the operation of the optical transmitter according to the embodiment;  
         [0030]      FIG. 4  is a diagram for explaining an example of the operation of the optical transmitter according to another embodiment of the present invention;  
         [0031]      FIG. 5  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention;  
         [0032]      FIG. 6  is a diagram for explaining an example of the operation of the optical transmitter according to the embodiment;  
         [0033]      FIG. 7  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention;  
         [0034]      FIG. 8  includes diagrams for explaining an amplitude compare circuit;  
         [0035]      FIG. 9  is a diagram for explaining a deterioration judgement condition;  
         [0036]      FIG. 10  is a flowchart of a deterioration judgement and extinct ratio control process;  
         [0037]      FIG. 11  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention;  
         [0038]      FIG. 12  is a diagram for explaining direct modulation of an LD;  
         [0039]      FIGS. 13A  to  13 C are diagrams for explaining an optical output in which a transmission signal is modulated;  
         [0040]      FIG. 14  is a diagram for explaining LD conversion efficiency;  
         [0041]      FIG. 15  is a diagram for explaining control for an optical transmitter;  
         [0042]      FIG. 16  is a diagram for explaining monitoring of an optical output;  
         [0043]      FIG. 17  is another diagram for explaining LD conversion efficiency;  
         [0044]      FIG. 18  is a diagram for explaining deterioration of an LD; and  
         [0045]      FIG. 19  is a diagram for explaining LD conversion efficiency when the LD deteriorates. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Embodiments of the present invention will be described with reference to the drawings. In the descriptions below, the same or similar parts in the drawings are referred to with the same reference numerals.  
       First Embodiment  
       [0047]     It&#39;s explained by using FIGS.  1  to  3 .  
         [0048]      FIG. 1  is a diagram for explaining an example of the configuration of an optical transmitter according to an embodiment of the present invention. Reference numeral  101  denotes an LD, reference numeral  111  denotes a PD, reference numeral  10  denotes an LD drive circuit, reference numeral  11  denotes a phase compare circuit, reference numeral  12  denotes a filter circuit, and reference numeral  13  denotes a deterioration judgement circuit.  
         [0049]      FIG. 2  is a diagram for explaining direct modulation of the LD  101  according to this embodiment. Compared with the configuration shown in  FIG. 12 , a pilot signal ( 1 ) is superimposed on a modulation current Ip. By superimposing the generated pilot signal ( 1 ) on an Ip control signal ( 4 ), a transmission signal ( 2 ) is modulated in accordance with the modulation current Ip on which the pilot signal ( 1 ) is superimposed, and is applied to the LD  101  biased in accordance with a bias current Ib controlled based on the Ib control signal ( 5 ).  
         [0050]      FIG. 3  is a diagram for explaining an example of the operation of the optical transmitter according to the embodiment. An LD current ( 6 ) for driving the LD  101 , an LD conversion efficiency curve ( 8 ), an LD optical output ( 3 ), and a PD current ( 7 ) are shown in  FIG. 3 .  
         [0051]     The pilot signal ( 1 ) has a frequency sufficiently lower than that of the transmission signal ( 2 ). For example, when the transmission signal ( 2 ) has a frequency of 2.4 GHz, the pilot signal ( 1 ) has a frequency of about 2 KHz. Although, in the first embodiment, the pilot signal ( 1 ) has a constant cycle of logical levels “1” and “0”, the pilot signal ( 1 ) may have a unique and specified pattern that allows an opposing optical receiver in a transmission system including the optical transmitter to recognize a transmission destination.  
         [0052]     The generated pilot signal ( 1 ) is supplied to the LD drive circuit  10  and the phase compare circuit  11 . As shown in  FIG. 2 , in the LD drive circuit  10 , by superimposing the generated pilot signal ( 1 ) on the Ip control signal ( 4 ), the transmission signal ( 2 ) is modulated in accordance with the modulation current Ip on which the pilot signal ( 1 ) is superimposed. The LD current ( 6 ) is applied to the LD  101  biased in accordance with the bias current Ib controlled based on the Ib control signal ( 5 ).  
         [0053]     The Ip control signal ( 4 ) controls an amplitude value of the LD current ( 6 ). An amplification rate of the AMP ( 102 ) increases, and the LD current ( 6 ) grows by enlarging the Ip control signal ( 4 ). The Ib control signal ( 5 ) controls a center value of the LD current ( 6 ). The center value grows by enlarging the Ib control signal ( 5 ).  
         [0054]     So, a motion range of the LD current ( 6 ) is determined in accordance with the Ip control signal ( 4 ), an operation point is determined in accordance with the Ib control signal ( 5 ), as shown in  FIG. 3 .  
         [0055]     Since  FIG. 3  shows a case where the operation point and the motion range of the LD current ( 6 ) are set to the LD threshold current Ith or more, the LD optical output ( 3 ) is generated in accordance with the gradient of the LD conversion efficiency curve ( 8 ).  
         [0056]     The PD  111  monitors part of the LD optical output ( 3 ), and outputs to the filter circuit  12  the PD current ( 7 ) obtained by photoelectric conversion.  
         [0057]     The filter circuit  12  passes the PD current ( 7 ) in a frequency band equal to or lower than that of the pilot signal ( 1 ). Alternatively, the filter circuit  12  passes frequency components in the frequency band of the pilot signal ( 1 ). Since a pilot signal included in each of the LD optical output ( 3 ) and the PD current ( 7 ) that monitors the LD optical output ( 3 ) is superimposed on the modulation current Ip and modulated, the pilot signal has a positive phase on the side of logical level “1” and a negative phase on the side of logical level ( 0 ). Thus, when the LD optical output ( 3 ) is converted in accordance with the same gradient as the LD conversion efficiency curve ( 8 ), since the filter circuit  12  averages a pilot signal of a positive phase on the side of logical level “1” and a pilot signal of a negative phase on the side of logical level “0”, the filter circuit  12  does not output a pilot signal.  
         [0058]     The phase compare circuit  11  judges whether or not the filter circuit  12  outputs a pilot signal. When the filter circuit  12  outputs a pilot signal, the phase compare circuit  11  judges whether or not the pilot signal output from the filter circuit  12  has a positive phase or a negative phase by comparing the phase of the pilot signal output from the filter circuit  12  with the generated pilot signal ( 1 ).  
         [0059]     The deterioration judgement circuit  13  judges, in accordance with a comparison result of the phase compare circuit  11 , that the LD  101  is in the normal state, that is, the LD  101  does not deteriorate when the filter circuit  12  does not output a pilot signal.  
       Second Embodiment  
       [0060]      FIG. 4  is a diagram for explaining an example of the operation of the optical transmitter according to another embodiment of the present invention. Compared with the operation described with reference to  FIG. 3 , an operation of the optical transmitter when deterioration occurs in an LD conversion efficiency curve will be explained.  
         [0061]     For an optical output ( 13 ) obtained by converting the LD current ( 6 ), a pilot signal of a positive phase on the side of logical level “1” is completely suppressed, in accordance with an LD conversion efficiency curve ( 18 ) when the LD  101  deteriorates. Similarly, for a PD current ( 17 ) that monitors the optical output ( 13 ), a pilot signal of a positive phase on the side of logical level “1” is completely suppressed.  
         [0062]     Thus, the filter circuit  12  extracts only a pilot signal of a negative phase on the side of logical level “0”.  
         [0063]     The phase compare circuit  11  compares the phase of the pilot signal of the negative phase on the side of logical level “0” with the phase of the generated pilot signal ( 1 ), and reports that the pilot signal of the negative phase is detected.  
         [0064]     In accordance with the comparison result of the phase compare circuit  11 , the deterioration judgement circuit  13  judges that the gradient of the LD conversion efficiency curve ( 18 ) is abnormal, and determines that the LD  101  deteriorates.  
       Third Embodiment  
       [0065]      FIG. 5  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention. Compared with the configuration shown in  FIG. 1 , a control circuit  14  controls the LD drive circuit  10  in accordance with a deterioration judgement result of the deterioration judgement circuit  13  and a monitor signal of the PD  111 .  
         [0066]      FIG. 6  is a diagram for explaining an example of the operation of the optical transmitter according to this embodiment. Compared with the operation described with reference to  FIG. 3 , although the same LD conversion efficiency curve ( 8 ) is acquired, the bias current Ib is smaller. Thus, the operation point and the motion range of the LD  101  are lower, and part of the motion range of the LD  101  is less than the LD threshold current Ith.  
         [0067]     In the operation point and the motion range of the LD  101 , for an optical output ( 23 ) obtained by converting the LD current ( 6 ), a pilot signal of a negative phase on the side of logical level “0” is suppressed, in accordance with the LD conversion efficiency curve ( 8 ). Similarly, for a PD current ( 27 ) that monitors the optical output ( 23 ), a pilot signal in which a negative phase on the side of logical level “0” is suppressed is acquired.  
         [0068]     Since the filter circuit  12  averages a pilot signal of a positive phase on the side of logical level “1” and a pilot signal of a negative phase on the side of logical level “0”, the filter circuit  12  outputs a pilot signal of a positive phase on the side of logical level “1”.  
         [0069]     The phase compare circuit  11  compares the phase of the pilot signal of the positive phase on the side of logical level “1” with the phase of the generated pilot signal ( 1 ), and reports to the deterioration judgement circuit  13  that the pilot signal of the positive phase is detected.  
         [0070]     In accordance with the comparison result of the phase compare circuit  11 , the deterioration judgement circuit  13  judges that the operation point and the motion range of the LD  101  are lower, and determines that the LD  101  does not deteriorate.  
         [0071]     In accordance with the judgement result and the determination result of the deterioration judgement circuit  13 , the control circuit  14  controls the LD drive circuit  10  using the Ib control signal ( 5 ) such that the bias current Ib increases in order to achieve a higher operation point.  
       Fourth Embodiment  
       [0072]     With the configuration of the optical transmitter shown in  FIG. 5 , when the LD conversion efficiency curve ( 18 ) shown in  FIG. 4  is acquired, in accordance with judgement by the deterioration judgement circuit  13  that the LD  101  deteriorates, the control circuit  14  maintains control for the LD drive circuit  10  using the Ip control signal ( 4 ) and the Ib control signal ( 5 ). In addition, in a system using the optical transmitter, if allowable, the control circuit  14  may stop controlling the LD drive circuit  10  using the Ip control signal ( 4 ) and the Ib control signal ( 5 ).  
       Fifth Embodiment  
       [0073]      FIG. 7  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention. Compared with the configuration shown in  FIG. 5 , an amplitude compare circuit  15 , a deterioration judgement circuit  16 , a storage circuit  17  with deterioration judgement condition, and a control circuit  18  are provided.  
         [0074]     The amplitude compare circuit  15  compares the amplitude value of the pilot signal ( 1 ) with the amplitude value of a pilot signal obtained in the filter circuit  12  by averaging a pilot signal of a positive phase on the side of logical level “1” and a pilot signal of a negative phase on the side of logical level “0”.  
         [0075]      FIG. 8  includes diagrams for explaining the amplitude compare circuit. Each of FIGS.  8 A 1 ,  8 B 1 , and  8 C 1  represents a PD current focused on a pilot signal obtained by monitoring an optical output of the PD  111 . Each of FIGS.  8 A 2 ,  8 B 2 , and  8 C 2  represents an output of the filter circuit  12 , which is a pilot signal obtained by averaging a pilot signal of a positive phase on the side of logical level “1” and a pilot signal of a negative phase on the side of logical level “0”. Each of FIGS.  8 A 3 ,  8 B 3 , and  8 C 3  represents a comparison result of the amplitude compare circuit  15 , which includes a ratio of the filtered pilot signal shown in each of FIGS.  8 A 2 ,  8 B 2 , and  8 C 2  to the pilot signal ( 1 ) and a phase of the filtered pilot signal. FIGS.  8 A 1 ,  8 A 2 , and  8 A 3  show a case where the LD  101  does not deteriorate, as shown in  FIG. 3 . In this case, a comparison result of the amplitude compare circuit  15  is 0%. FIGS.  8 B 1 ,  8 B 2 , and  8 B 3  show a case where the gradient of the LD conversion efficiency curve ( 18 ) is not 0 and a pilot signal of a positive phase on the side of logical level “1” is half suppressed, as shown in  FIG. 4 . In this case, a comparison result of the amplitude compare circuit  15  is 50%, and a negative phase is detected. FIGS.  8 C 1 ,  8 C 2 , and  8 C 3  show a case where although the LD  101  does not deteriorate, the operation point and the motion range of the LD  101  are lower since the bias current Ib is small, and part of the motion range of the LD  101  is less than the LD threshold current Ith, as shown in  FIG. 6 . Thus, a pilot signal of a negative phase on the side of logical level “0” is suppressed by only 30%. In this case, a comparison result of the amplitude compare circuit  15  is 30%, and a positive phase is detected.  
         [0076]      FIG. 9  is a diagram for explaining a deterioration judgement condition. The deterioration judgement condition shown in  FIG. 9  is a condition used by the deterioration judgement circuit  16  to judge deterioration of the LD  101  and is stored in the storage circuit  17 . An amplitude value of a pilot signal represents an amplitude value of the pilot signal ( 1 ), and a judgement threshold represents a threshold for judging that the degree of deterioration is normal with respect to comparison results of the amplitude compare circuit  15  shown in FIGS.  8 A 3 ,  8 B 3 , and  8 C 3 .  
         [0077]      FIG. 10  is a flowchart of a deterioration judgement and extinct ratio control process. The process for judging deterioration of an LD and for controlling an extinct ratio using a phase comparison result, an amplitude comparison result, and a deterioration judgement condition is described next.  
         [0078]     In step S 1 , a result of phase comparison performed by the phase compare circuit  11  between the phase of the pilot signal ( 1 ) and the phase of the pilot signal filtered by the filter circuit  12  is judged. If a pilot signal does not appear in the filtered pilot signal, that is, if a pilot signal of a positive phase on the side of logical level “1” and a pilot signal of a negative phase on the side of logical level “0” are averaged and a pilot signal is not output, it is judged that it is in the normal state. For other cases, that is, if a pilot signal of a positive or negative phase appears in the filtered pilot signal, it is judged that it is not in the normal state.  
         [0079]     In step S 2 , if it is judged in step S 1  that it is not in the normal state, a result of amplitude comparison performed by the amplitude compare circuit  15  between the amplitude value of the pilot signal ( 1 ) and the amplitude value of the pilot signal filtered by the filter circuit  12  is judged. In accordance with the deterioration judgement condition shown in  FIG. 9 , the ratio of the amplitude value of the filtered pilot signal to the amplitude value of the pilot signal ( 1 ) is acquired. It is judged whether or not the acquired ratio is within a normal range with respect to a judgement threshold corresponding to the amplitude value of the pilot signal ( 1 ).  
         [0080]     In step S 3 , if the amplitude ratio is not within the normal range in step S 2 , it is determined that the LD  101  deteriorates and a warning is issued. The control circuit  18  may maintain control for optical output power of the LD  101  and the extinct ratio of the optical output of the LD  101 . In addition, the control circuit  18  may control the optical output of the LD  101  to stop.  
         [0081]     In step S 4 , if the result of phase comparison is normal in step S 1  or if the result of amplitude comparison is normal in step S 2 , it is judged whether or not to control the extinct ratio of the optical output of the LD  101 . If the present extinct ratio satisfies the characteristic of the optical transmission system including the optical transmitter to be controlled, it is judged that the extinct ratio is not to be controlled. If the extinct ratio is to be improved, it is judged that the extinct ratio is to be controlled.  
         [0082]     In step S 5 , if it is judged in step S 4  that the extinct ratio is not to be improved, the control circuit  18  controls the bias current Ib and the modulation current Ip to hold the present values.  
         [0083]     In step S 6 , if it is judged in step S 4  that the extinct ratio is to be improved, it is judged whether the result of phase comparison performed by the phase compare circuit  11  between the phase of the pilot signal ( 1 ) and the phase of the pilot signal filtered by the filter circuit  12  is a positive phase, a negative phase, or non-appearance of a pilot signal in the filtered pilot signal.  
         [0084]     In step S 7 , if the result of phase comparison in step S 6  is a positive phase, the control circuit  18  decreases the modulation current Ip. By decreasing the modulation current Ip, an operation of the pilot signal of the negative phase on the side of logical level “0” at the LD threshold current Ith or less is improved. The amount of change in the modulation current Ip is determined in accordance with control information stored in a control table (not shown) provided in advance in the control circuit  18 .  
         [0085]     In step S 8 , if the result of phase comparison in step S 6  is a negative phase, the control circuit  18  decreases the bias current Ib or the modulation current Ip. By decreasing the bias current Ib, an operation point in an LD conversion efficiency curve is moved to a smaller LD current side, and an operation at a curve point in the LD conversion efficiency curve of a pilot signal of a positive phase on the side of logical level “1” whose gradient is smaller than the gradient of the LD conversion efficiency curve of a pilot signal of a negative phase on the side of logical level “0” is improved. The amount of decrease in the modulation current Ip or the bias current Ib is determined in accordance with control information stored in a control table (not shown) provided in advance in the control circuit  18 .  
         [0086]     In step S 9 , if the result of phase comparison in step S 6  is that a pilot signal does not appear in the filtered pilot signal, the control circuit  18  increases the modulation current Ip. By increasing the modulation current Ip, the motion range in the LD conversion efficiency curve is increased, and the extinct ratio is increased.  
         [0087]     In step S 10 , after the processing in step S 7 , S 8 , or S 9  is performed, the pilot signal ( 1 ) is transmitted to judge whether or not the extinct ratio is improved by the processing. The processing in step S 10  must be performed when an optical transmitter that appropriately outputs the pilot signal ( 1 ) is used. If an optical transmitter that always outputs the pilot signal ( 1 ) is used, the processing in step S 10  can be omitted. By transmitting the pilot signal ( 1 ), the processing from step S 1  is performed, and an extinct ratio that satisfies the characteristic of the optical transmission system can be achieved.  
       Sixth Embodiment  
       [0088]      FIG. 11  is a diagram for explaining an example of the configuration of the optical transmitter according to another embodiment of the present invention. Compared with the configuration shown in  FIG. 7 , a pilot signal generation circuit  19  is provided. The pilot signal generation circuit  19  generates a pilot signal whose pattern, generation cycle, generation timing, and amplitude value are controlled by a control circuit  20 .