JP2004128323A - Control method for light transmitter and its light transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the control method of a light transmitter capable of easily executing table preparation, and excellently facilitating countermeasures to the changes over aging. <P>SOLUTION: In a light transmitter for impressing modulating currents to a light source 1 by AC connection to modulate light output strength, a modulating current value(amplitude) is scanned at each surrounding temperature under an environment where the change of a surrounding temperature is preliminarily applied from the outside, light source lights are received by an internal light sensor 7, a mean light output level is measured to retrieve an inflection point at which the mean light output level starts to change, a modulating current value at the inflection point is defined as the modulating current value whose extinction rate is the maximum, and the modulating current value is stored in an internal table 11 so as to be made to correspond to the surrounding temperature measured by the internal temperature sensor 9. Afterwards, the modulating currents are controlled based on the modulating current value whose extinction rate is the maximum obtained by referring to the table 11 at the surrounding temperature measured by the temperature sensor 9 to adjust the extinction rate. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmitter for transmitting a signal by intensity modulation, and more particularly to a method for controlling an optical transmitter which is easy to create a table and is excellent in coping with a change with time, and to the optical transmitter.
[0002]
[Prior art]
The optical transmitter includes a light source that outputs light at an intensity corresponding to the bias current and the modulation current, and a modulation circuit that generates a modulation current based on the transmission signal, and includes a transmission signal provided from an external device (here, a transmission signal). A digital signal having a voltage corresponding to logic "0" or "1" and a signal obtained by inverting the digital signal, and a laser diode as a light source. (Hereinafter referred to as LD), an optical transmission signal whose optical output intensity changes between 0 level and 1 level corresponding to the logic "0" and "1" of the transmission signal is obtained.
[0003]
In an optical transmitter using an LD as a light source, a change in light output intensity due to a change in oscillation threshold current and a change in light output waveform (a change in extinction ratio) due to a change in modulation efficiency occur due to a change in temperature of the LD. Therefore, it is necessary to control the bias current and the modulation current.
[0004]
For controlling the bias current to avoid a change in the light output intensity, APC (auto power control) that monitors the light output and feeds back the bias current is used.
[0005]
It is known to measure the ambient temperature of the light source using a thermistor or the like and control the modulation current according to the ambient temperature in order to avoid changes in the optical output waveform, and the output of the thermistor is sent to a modulation circuit. A method in which the modulation current is input and variably controlled in an analog manner, and an appropriate modulation current value for each temperature is recorded in a table provided on a recording medium such as a non-volatile memory, and a microcomputer or the like is operated according to the output of the thermistor. A method of controlling a modulation current for each temperature by reading a record by a digital control circuit and applying the read record to a modulation circuit is used.
[0006]
[Problems to be solved by the invention]
The method in which the modulation current is variably controlled in an analog manner by the output of the thermistor has a problem that it is difficult to compensate for the characteristic variation of each LD. In addition, there is a problem that it is difficult to maintain characteristics optimally over the entire operating temperature range of the optical transmitter.
[0007]
In the method of controlling the modulation current by reading an appropriate modulation current value for each temperature, a table can be provided for each individual LD, and fine control can be performed for each temperature. Can be However, to create the table, change the ambient temperature to the entire range of the operating temperature of the optical transmitter, and adjust the modulation current value at each ambient temperature while checking the waveform (or extinction ratio) with an observation device such as an oscilloscope. Therefore, there is a problem that it takes time and effort to determine an appropriate modulation current value.
[0008]
In addition, as can be said in both of the above two methods, a temporal change of the optical output intensity (aging) can be detected by the above-mentioned optical output monitor. Extinction ratio change). This is because the monitor of the light output can detect the average of the light output intensity, but the peak speed cannot be detected because the signal speed is too fast than the response speed of the monitor.
[0009]
Therefore, an object of the present invention is to solve the above-mentioned problems, to provide a control method of an optical transmitter which is easy to create a table, and is excellent in coping with a change with time, and to provide the optical transmitter.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a method for controlling an optical transmitter according to the present invention is directed to an optical transmitter that modulates optical output intensity by applying a modulation current to a light source by AC coupling in an environment in which ambient temperature is changed from the outside in advance. The light source light is received by the internal light sensor while the modulation current value (amplitude) is swept at each ambient temperature, and the average light output level is measured. A search is made, and the modulation current value at this inflection point is set as the maximum modulation current value of the extinction ratio, and this modulation current value is stored in an internal table in correspondence with the ambient temperature measured by the internal temperature sensor, and thereafter The extinction ratio is adjusted by referring to the table based on the ambient temperature measured by the temperature sensor and controlling the modulation current based on the obtained modulation current value having the maximum extinction ratio.
[0011]
When a bias current is applied to the light source and an APC control function is provided to feed back the measured average light output level to the bias current to make the average light output level constant, the APC is performed before the inflection point search is performed. After performing the control to obtain a bias current having a desired average light output level, the APC control may be interrupted to fix the bias current, and then the inflection point search may be performed.
[0012]
The modulation current may be controlled to a value obtained by subtracting the maximum extinction ratio modulation current value obtained from the table by a predetermined ratio.
[0013]
A modulation current value at which a desired extinction ratio is obtained at an arbitrary ambient temperature is obtained, and a ratio between the modulation current value at the maximum extinction ratio at the ambient temperature and the modulation current value at which the desired extinction ratio is obtained is calculated and stored. After that, the modulation current control value may be obtained by multiplying the modulation current value having the maximum extinction ratio obtained by referring to the table by the ratio.
[0014]
By measuring the average light output level while sweeping the modulation current value at an arbitrary ambient temperature, an inflection point at which the average light output level starts to change is searched for, and the modulation current value at this inflection point (maximum extinction ratio) By comparing the modulation current value with the maximum extinction ratio obtained by referring to the table at the ambient temperature measured by the temperature sensor, and thereby diagnosing a change in the modulation efficiency with time.
[0015]
The diagnosis may be activated at power-on.
[0016]
Further, in the optical transmitter control method of the present invention, in an optical transmitter that modulates optical output intensity by applying a modulation current to a light source by DC coupling together with a bias current, in an environment in which ambient temperature is changed externally in advance. At each ambient temperature, a threshold current at which the light source starts emitting light is obtained by measuring the light output level by receiving the light source light with the optical sensor while sweeping the bias current in a state where no modulation current is applied, and then obtaining the threshold current. With the current fixed to the bias current, the modulation current value (amplitude) is swept and the modulation current value that reaches the desired average optical output level is determined. This modulation current value corresponds to the ambient temperature measured by the internal temperature sensor. Then, the temperature is stored in an internal table, and thereafter, the table is referred to by the ambient temperature measured by the temperature sensor, and is changed based on the obtained modulation current value. By controlling the current, and adjusts the extinction ratio.
[0017]
The optical transmitter includes a light source that outputs light at an intensity corresponding to a bias current and a modulation current, a modulation circuit that generates a modulation current based on a transmission signal, an amplitude control unit that controls the modulation amplitude, and a bias current. , A light sensor that monitors and receives the light source light, an average light output level measurement circuit that measures the average light output level of the light source light, a temperature sensor that measures the ambient temperature, and a modulation current value ( A search unit that searches for a modulation current value at which a specific average light output level is obtained by measuring the average light output level of the light source light while sweeping the amplitude), and measures the searched modulation current value with the temperature sensor. And a quench control unit that controls a modulation current based on a modulation current value obtained by referring to the table based on the ambient temperature measured by the temperature sensor. It is obtained by a regulating unit.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[0019]
As shown in FIG. 1, an optical transmitter according to the present invention includes a light source (LD) 1 that outputs light at an intensity corresponding to a bias current and a modulation current, and a transmission signal provided from an external device (not shown). A differential / buffer amplifier 2 for differentially / buffer-amplifying the signal, a modulation circuit 3 for generating a modulation current to be applied to the light source 1 by switching two transistors based on the transmission signal, and the modulation circuit 3 A current source 4 for applying a current to the two transistors, an amplitude control unit 51 for controlling the current source 4 to control the modulation amplitude in the modulation circuit 3, a current source 6 for applying a bias current to the light source 1, A bias control unit 52 for controlling the bias current by controlling the current source 6, an optical sensor (PD) 7 for directly monitoring and receiving the light source light from the light source 1, and an average of the light source light by integrating the output of the PD 7. An average light output level measuring circuit 8 for measuring an output level, a temperature sensor (thermistor) 9 for measuring an ambient temperature, a constant current source 10 for the temperature sensor 9, and a light source while sweeping a modulation current value (amplitude). A search unit (an inflection point) that searches for an inflection point at which the average light output level starts to change as a modulation current value at which a specific average light output level is obtained by receiving light with the light sensor 7 and measuring the average light output level. A table (memory) 11 for storing the modulation current value at the searched inflection point as the maximum modulation current value of the extinction ratio in association with the ambient temperature measured by the temperature sensor 9; An extinction ratio adjusting unit 54 that adjusts the extinction ratio by controlling the modulation current based on the maximum extinction ratio modulation current value obtained by referring to the table 11 at the ambient temperature measured by the sensor 9; And the average optical output level fed back to the bias current and a APC control unit 55 for the average light output level constant.
[0020]
The microprocessor 5 is shown enlarged on the left side of the figure. The amplitude control unit 51, the bias control unit 52, the inflection point search unit 53, the extinction ratio adjustment unit 54, and the APC control unit 55 are configured as programs executed by the microprocessor 5. The microprocessor 5 stores not only these programs but also programs necessary for the operation described later. Further, D / A converters 12 and 13 or A / D converters 14 are provided between the microprocessor 5 and the current source 4, the current source 6, the average light output level measuring circuit 8 and the temperature sensor 9, respectively. , 15 are provided. The functions of the D / A converters 12 and 13 and the A / D converters 14 and 15 are self-evident and will not be described in the following description of operation.
[0021]
A capacitor 16 for AC coupling is inserted between the light source 1 and one transistor of the modulation circuit 3. That is, in this embodiment, the modulation current is applied to the light source 1 by AC coupling.
[0022]
Next, a procedure for creating a table by the optical transmitter will be described.
[0023]
In a factory or the like, a table creation process is performed immediately after the manufacture of the optical transmitter until the shipment. In the table creation processing, the optical transmitter is put into a thermostat, a test signal is provided instead of the transmission signal, and the optical transmitter is set in the table creation mode.
[0024]
To set the optical transmitter in the table creation mode, a command is issued from the outside of the optical transmitter to the microprocessor 5 via a communication line not shown in FIG. As a test signal, a continuous pulse signal having a duty ratio of 50%, or logic "0" or "1" within a time shorter than the reciprocal (= time) of the cutoff frequency determined by the capacity of the capacitor 16 used for AC coupling. It is preferable to use a signal whose appearance frequency of the indicated voltage is 1: 1. The test signal may be provided from the outside of the optical transmitter, or a pseudo random binary sequence (PRBS) signal generator may be provided inside the optical transmitter, and the PRBS signal is supplied from the signal generator as a test signal. You may do so.
[0025]
In the constant temperature bath, the temperature required for recording on the table is slowly changed from a low temperature to a high temperature. The term “slow” refers to a speed at which the temperature can be regarded as substantially constant during execution of steps (1) to (3) described later.
[0026]
In step (1), the optical transmitter obtains a bias current at which the average light output level measured by the average light output level measuring circuit 8 becomes the target output level while performing the APC control by the APC control unit 55. It is assumed that the target output level is set in the memory 11 in advance. When the average optical output level reaches the target output level, the optical transmitter fixes the bias current value commanded from the bias control unit 52 to the current source 6, and stops the APC control by the APC control unit 55.
[0027]
In step (2), the optical transmitter operates the inflection point searching unit 53 to sweep the modulation current value. That is, the modulation current value commanded from the amplitude control unit 51 to the current source 4 is gradually increased from a sufficiently small value. As shown in FIGS. 2A to 2C, when the modulation current value is small, the amplitude of the optical output intensity (the drop between the 0 level and the 1 level) is small, but when the modulation current value is large, the optical output is large. The amplitude of the intensity increases. At the same time, the optical transmitter measures the average light output level by the average light output level measuring circuit 8. Since the bias current is constant and the modulation current is applied by alternating current, the average light output level is kept constant from FIG. 2A to FIG. 2B, and the light output intensity is averaged regardless of the amplitude. It swings equally above and below the light output level. However, as shown in FIG. 2C, when the modulation current value is further increased, the light output intensity at one level increases, but the light output intensity at zero level cannot fall into a minus region. Not (clipped). As a result, the average light output level increases as indicated by the arrow. At this time, if the APC control is performed, the bias current value is controlled in a direction to lower the average light output level, so the APC control is stopped.
[0028]
2A shows a state where the extinction ratio is small, FIG. 2B shows a state where the extinction ratio is large, and FIG. 2C shows a state where the extinction ratio is infinite. The state immediately before the transition from FIG. 2B to FIG. 2C (state where the 0 level is almost zero light intensity) is the state where the extinction ratio is maximum. Therefore, the extinction ratio becomes maximum at the inflection point at which the average light output level starts to increase from a constant. Therefore, the inflection point search unit 53 of the optical transmitter always starts to increase the average optical output level by comparing the average optical output level input from the average optical output level measurement circuit 8 with the immediately preceding value. Detect that. Thus, the inflection point has been captured.
[0029]
FIG. 3 shows the relationship between the modulation current value and the average light output level. When the modulation current value is increased at a certain temperature, the average light output level is constant while the modulation current value is small, but increases at a certain modulation current value (this is called the inflection point). (Characteristic 31). When the modulation current value is larger than the inflection point, the extinction ratio becomes infinite. When the temperature increases, the characteristics change due to a decrease in the differential efficiency of the light source 1. Therefore, the modulation current value serving as an inflection point increases (characteristic 32). As the temperature further increases, the modulation current value at the inflection point further increases (characteristic 33). Repeating the sweep of the modulation current value while slowly increasing the ambient temperature as described above is nothing less than obtaining FIG.
[0030]
In step (3), the optical transmitter sets the modulation current value commanded by the amplitude control unit 51 at the moment when the inflection point is captured as the modulation current value with the maximum extinction ratio, and calculates the ambient temperature measured by the temperature sensor 9. And stored in the table 11.
[0031]
In step (4), the optical transmitter waits until the ambient temperature measured by the temperature sensor 9 rises to a temperature one step above the temperature interval of the table 11. When the ambient temperature reaches the target temperature, APC control is started again, and the process returns to step (1).
[0032]
The optical transmitter repeatedly executes steps (1) to (4) while the constant temperature bath slowly changes from a low temperature to a high temperature. Thus, the table 11 records all the modulation current values having the maximum extinction ratio at each ambient temperature for each temperature step. When the recording of all the columns of the table 11 is completed, the optical transmitter automatically ends the table creation mode. By making the memory constituting the table 11 non-volatile, the contents of the table 11 are retained even after the power of the optical transmitter is turned off. Therefore, the optical transmitter to be shipped is an optical transmitter that has been temperature-compensated including the characteristic variation of each LD used in the light source 1.
[0033]
In the conventional technology, a function of arbitrarily interrupting / restarting the APC control, a function of sweeping a modulation current value, a function of detecting a start of a change in an average optical output level, a function of writing a modulation current value to a table, and an increase in ambient temperature Since there was no function to repeat the search while waiting for, the APC control was externally interrupted and restarted, the modulation current value was externally applied, the waveform was observed with an observation device, and the modulation current value was externally written. It was necessary for a person to perform the work of slapping. On the other hand, in the present invention, as long as the table creation mode is set, the optical transmitter automatically creates the table only by changing the ambient temperature, so that there is no trouble. Even when a large number of optical transmitters are simultaneously subjected to the table creation processing, the time and labor remain unchanged.
[0034]
Next, the operation of the optical transmitter during normal use will be described.
[0035]
When the optical transmitter has already created the table 11 and is operating in the normal mode, the extinction ratio adjusting unit 54 refers to the table 11 with the ambient temperature measured by the temperature sensor 9. As a result, the modulation current value with the maximum extinction ratio at the current temperature is obtained. Therefore, if the amplitude control unit 51 controls the current source 4 of the modulation circuit 3 with this modulation current value as a target, it is possible to output a light intensity modulation signal having the maximum extinction ratio.
[0036]
However, here, it is assumed that the transmission is performed with the extinction ratio smaller than the maximum. The reason is that when the extinction ratio is increased, if the modulation is performed based on a high-speed transmission signal, the oscillation (light emission) of the light source 1 hardly follows the change in the modulation current, and the optical output waveform is broken. is there. If the extinction ratio is reduced, this waveform problem is solved, but it becomes difficult for the receiving side to identify the signal. Therefore, the extinction ratio should not be increased to the maximum extinction ratio, but should be an extinction ratio exceeding the lower limit specified in the communication standard, and an extinction ratio that does not cause distortion of the optical output waveform. Therefore, the extinction ratio adjusting unit 54 controls the modulation current to a value obtained by subtracting the modulation current value having the maximum extinction ratio by a predetermined ratio. This makes it possible to output a light intensity modulation signal whose extinction ratio is smaller than the maximum by a predetermined ratio. Even if the ambient temperature changes, a fixed ratio set in advance is applied, so that the transmission can be performed with the extinction ratio smaller than the maximum by a constant ratio regardless of the ambient temperature.
[0037]
When it is desired to control the ratio to a desired value, the following is performed.
[0038]
At a factory or the like, an extinction ratio setting process is performed after the above-described table creation process. In the extinction ratio setting process, the same test signal as in the table creation process is given at an arbitrary ambient temperature, and the optical transmitter is set to the extinction ratio setting mode. To set the optical transmitter to the extinction ratio setting mode, a command is issued from the outside of the optical transmitter to the microprocessor 5 via a communication line not shown in FIG.
[0039]
In this state, the optical output of the LD 1 is observed with an observation device such as an oscilloscope, and the waveform (or extinction ratio) is obtained. Since the optical transmitter controls the modulation current value so that the extinction ratio becomes the maximum according to the already created table 11 regardless of the ambient temperature, the observed extinction ratio is the maximum. In the extinction ratio setting mode, an extinction ratio setting value is commanded to the microprocessor 5 from outside the optical transmitter through a communication line. Since the optical transmitter controls the modulation current based on the extinction ratio setting value, the observation device observes the result and confirms whether the extinction ratio is as set. When the desired extinction ratio is obtained, the extinction ratio setting mode ends. At that time, the optical transmitter stores the extinction ratio set value given from the outside when the desired extinction ratio is obtained instead of the preset extinction ratio in the memory 11.
[0040]
Thereafter, in the normal mode, the optical transmitter multiplies the modulation current value having the maximum extinction ratio obtained by referring to the table 11 by the extinction ratio setting value extracted from the memory 11 to obtain a modulation current control value. As a result, a modulation current value having a desired extinction ratio is obtained at each ambient temperature, so that a light intensity modulation signal having a desired extinction ratio can be output. Instead of storing the extinction ratio set value, all the modulation current values having the maximum extinction ratio stored in the table 11 are rewritten to appropriate modulation current values multiplied by the extinction ratio set value. Alternatively, the appropriate modulation current value may be directly read from 11 and used for control.
[0041]
Next, diagnosis of a temporal change in modulation efficiency will be described.
[0042]
At any time after the optical transmitter has created the table 11, the same test signal as in the table creation processing is given to perform a diagnosis process of a change with time. The diagnosis process of the change with time may be performed by instructing the microprocessor 5 to execute a diagnosis mode via a communication line from outside of the optical transmitter, or by performing initialization performed by the microprocessor 5 first when the power of the optical transmitter is turned on. It may be performed as part of the program.
[0043]
In the aging diagnosis processing, steps (1) and (2) in the table creation processing described above are performed. As a result, the modulation current value with the maximum extinction ratio at the current time at the ambient temperature measured by the temperature sensor 9 is obtained. On the other hand, when the table 11 is referred to based on the ambient temperature measured by the temperature sensor 9, a modulation current value having the maximum extinction ratio at the time of creating the table is obtained. If the two modulation current values at the current time and the table creation time are the same or slightly different, it can be determined that no deterioration has occurred or that the deterioration has not occurred. If there is a significant difference between the two modulation current values, it can be determined that the optical output waveform is degraded (the extinction ratio is degraded) due to the degradation of the modulation efficiency, which leads to a failure. By storing the judgment value in the optical transmitter in advance, the optical transmitter determines whether or not the difference between the two modulation current values exceeds the judgment value, and determines deterioration, and the deterioration proceeds to a significant level. If so, it can notify the host device or issue a warning to the user using a display or the like.
[0044]
Next, an embodiment in which the modulation current is applied to the light source by DC coupling will be described. As shown by a broken line in FIG. 1, DC coupling is realized by short-circuiting between the light source 1 and one transistor of the modulation circuit 3. In this case, the procedure for creating the table is as follows.
[0045]
In a factory or the like, a table creation process is performed immediately after the manufacture of the optical transmitter until the shipment. In the table creation process, the optical transmitter is placed in a thermostat, and the optical transmitter is set in the table creation mode. Although a test signal is supplied, the modulation current may be stopped as described later, so that there is substantially no test signal.
[0046]
In the constant temperature bath, the temperature required for recording on the table is slowly changed from a low temperature to a high temperature. The term “slow” refers to a speed at which the temperature can be regarded as substantially constant during execution of steps (1) ′ to (3) ′ described later.
[0047]
In step (1) ′, the optical transmitter stops the APC control by the APC control unit 55 and controls the current source 4 by the amplitude control unit 51 to stop the modulation current. The bias current is swept by the bias control unit 52 without applying the modulation current. The bias control unit 52 gradually increases the bias current value commanded to the current source 6 from a very small value to a large value. At the same time, the optical transmitter measures the average light output level by the average light output level measuring circuit 8. Since the current applied to the light source 1 is only the bias current, when the bias current value is very small, the light source 1 does not emit light, and the measured average light output level is zero. When the light source 1 starts emitting light, the average light output level increases. The bias current value at this time is the threshold current value. The optical transmitter fixes the bias current value to a threshold current value.
[0048]
In step (2) ', the optical transmitter operates the search unit 53 to sweep the modulation current value. That is, the modulation current value commanded from the amplitude control unit 51 to the current source 4 is gradually increased from a sufficiently small value. As a result, the amplitude of the light output intensity increases. However, since the modulation current is applied to the light source by DC coupling, the bias current value does not increase while keeping the average light output level constant as shown in FIGS. Based on the current value, the 1 level increases while the 0 level keeps the light intensity near zero, and the average light output level increases accordingly. The average light output level input from the average light output level measuring circuit 8 is constantly compared with a preset table creation set value to detect that the average light output level has reached the table creation set value. I do. In other words, it means that the modulation current value at which a specific average light output level is obtained is searched.
[0049]
In step (3) ', the optical transmitter sets the modulation current value commanded by the amplitude control unit 51 to the ambient temperature measured by the temperature sensor 9 at the moment when the average light output level reaches the set value for table creation. The corresponding information is stored in the table 11.
[0050]
In step (4) ', the optical transmitter waits until the ambient temperature measured by the temperature sensor 9 rises to a temperature one step above the temperature interval of the table 11. When the ambient temperature reaches the target temperature, the modulation current is stopped and the process returns to step (1) '.
[0051]
While the constant temperature bath slowly changes from low temperature to high temperature, the optical transmitter repeatedly executes steps (1) 'to (4)'. Thus, the table 11 records all the modulation current values at which a specific average light output level is obtained at each ambient temperature for each temperature step.
[0052]
Thereafter, at an arbitrary ambient temperature, the APC control target value is set. In the APC control target value setting process, the same test signal as in the table creation process is given at an arbitrary ambient temperature, and the optical transmitter is set to the APC setting mode. In order to set the optical transmitter to the APC setting mode, the microprocessor 5 may be instructed from outside the optical transmitter via a communication line not shown in FIG. 1, or the table 11 is completed in the table creation mode. At this time, the mode may shift to the APC setting mode as it is.
[0053]
In this state, the optical output of the LD 1 is observed with an observation device such as an oscilloscope, and the waveform (or extinction ratio) is obtained. Since the optical transmitter controls the modulation current value according to the already created table 11 regardless of the ambient temperature, the average light output level is equal to the table creation set value. In the APC setting mode, an APC control target value is commanded to the microprocessor 5 from outside the optical transmitter through a communication line. Since the optical transmitter controls the bias current based on the APC control target value, the result is observed and confirmed by the observation device. When the desired average light output level is obtained, the APC control target value given from the outside is stored in the memory 11, and the APC setting mode ends.
[0054]
Next, the operation of the optical transmitter during normal use will be described.
[0055]
When the optical transmitter has already created the table 11 and is operating in the normal mode, the extinction ratio adjusting unit 54 refers to the table 11 with the ambient temperature measured by the temperature sensor 9. As a result, a modulation current value such that a specific average optical output level is obtained when the bias current value at the current temperature is the threshold current value is obtained. The amplitude controller 51 controls the current source 4 of the modulation circuit 3 with the modulation current value as a target. On the other hand, the APC control section 55 controls the bias current so that the average light output level measured by the average light output level measurement circuit 8 becomes the APC control target value stored in the memory 11.
[0056]
In diagnosing the temporal change in the modulation efficiency in the case of DC coupling, steps (1) 'and (2)' are performed, and the modulation current value searched at that time (that is, a specific average light output level at the present time is obtained). Then, based on the ambient temperature measured by the temperature sensor 9, the modulation current value obtained by referring to the table 11 (the modulation current value at which a specific average light output level is obtained at the time of creating the table) is compared. The method of determining the deterioration and the method of coping with the deterioration are the same as in the case of the AC coupling.
[0057]
【The invention's effect】
The present invention exhibits the following excellent effects.
[0058]
(1) Since the optical transmitter automatically sweeps the modulation current value to search for an inflection point and automatically stores the modulation current value in a table, no effort is required when creating the table.
[0059]
(2) Maintenance is easy because the optical transmitter automatically detects the deterioration by comparing the modulation current value with the maximum extinction ratio between the time when the table is created and the current time.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of an optical transmitter showing one embodiment of the present invention.
FIG. 2 is a waveform diagram showing a relationship between an optical output intensity and a modulation current value in an optical transmitter.
FIG. 3 is a characteristic diagram showing a relationship between a modulation current value and an average light output level.
[Explanation of symbols]
1 Light source (LD)
3 Modulation circuit
5 Microprocessor
7 Optical sensor (PD)
8 Average light output level measurement circuit
9 Temperature sensor (thermistor)
11 Table (memory)
51 Amplitude controller
52 Bias control unit
53 Search Unit (Inflection Point Search Unit)
54 Extinction ratio adjuster

Claims (8)

In an optical transmitter that modulates the optical output intensity by applying a modulation current to the light source by AC coupling, the light source light is swept while sweeping the modulation current value (amplitude) at each ambient temperature in an environment where the ambient temperature is changed from the outside in advance. Is detected by an internal optical sensor and the average light output level is measured to search for an inflection point at which the average light output level starts to change, and the modulation current value at this inflection point is calculated as the modulation current having the maximum extinction ratio. The modulation current value is stored in an internal table corresponding to the ambient temperature measured by the internal temperature sensor, and then the table is referred to by the ambient temperature measured by the temperature sensor. A method for controlling an optical transmitter, characterized by adjusting an extinction ratio by controlling a modulation current based on a modulation current value having a maximum extinction ratio. When a bias current is applied to the light source and an APC control function is provided to feed back the measured average light output level to the bias current to make the average light output level constant, the APC is performed before the inflection point search is performed. 2. The method according to claim 1, wherein after performing control to obtain a bias current having a desired average light output level, APC control is interrupted to fix the bias current, and then the inflection point search is performed. Control method of optical transmitter. 3. The control method of an optical transmitter according to claim 1, wherein the modulation current is controlled to a value obtained by subtracting a modulation current having a maximum extinction ratio obtained from the table by a predetermined ratio. A modulation current value at which a desired extinction ratio is obtained at an arbitrary ambient temperature is obtained, and a ratio between the modulation current value at the maximum extinction ratio at the ambient temperature and the modulation current value at which the desired extinction ratio is obtained is calculated and stored. The optical transmission according to any one of claims 1 to 3, wherein the modulation current value obtained by referring to the table is multiplied by the ratio to obtain a control value of the modulation current. How to control the vessel. By measuring the average light output level while sweeping the modulation current value at an arbitrary ambient temperature, an inflection point at which the average light output level starts to change is searched for, and the modulation current value at this inflection point (maximum extinction ratio) By comparing the modulation current value with the maximum extinction ratio obtained by referring to the table at the ambient temperature measured by the temperature sensor, thereby diagnosing a temporal change in modulation efficiency. The method for controlling an optical transmitter according to claim 1. 6. The control method for an optical transmitter according to claim 5, wherein the diagnosis is started when power is turned on. In an optical transmitter that modulates the optical output intensity by applying a modulation current to the light source together with a bias current by a DC coupling, the bias current is not applied at each ambient temperature in an environment where the ambient temperature is changed from outside in advance. The light source light is received by the optical sensor while the light source is swept, and the light output level is measured to determine a threshold current at which the light source starts emitting light. Amplitude) while sweeping the amplitude, to obtain a modulation current value that reaches a desired average light output level, and stores the modulation current value in an internal table corresponding to the ambient temperature measured by an internal temperature sensor. By referencing the table with the ambient temperature measured by the temperature sensor, and controlling the modulation current based on the obtained modulation current value, the extinction ratio can be calculated. Method of controlling an optical transmitter, characterized in that the sections. A light source that outputs light at an intensity corresponding to the bias current and the modulation current, a modulation circuit that generates a modulation current based on the transmission signal, an amplitude control unit that controls the modulation amplitude, and a bias control unit that controls the bias current , An optical sensor that monitors and receives light from a light source, an average light output level measuring circuit that measures the average light output level of the light from the light source, a temperature sensor that measures ambient temperature, and a light source that sweeps a modulation current value (amplitude). A search unit that searches for a modulation current value at which a specific average light output level is obtained by measuring the average light output level of light, and associates the searched modulation current value with the ambient temperature measured by the temperature sensor. And an extinction ratio adjustment unit that controls a modulation current based on a modulation current value obtained by referring to the table at an ambient temperature measured by the temperature sensor. Optical transmitter according to claim and.

Images