JP2000269897A - Optical transmitter - Google Patents

Abstract

(57) [Problem] In a conventional optical transmitter, a ratio of one level to zero level of an optical digital signal, that is, an extinction ratio is deteriorated due to a change in current-optical output characteristics of a laser diode as a light emitting element. There was a problem. SOLUTION: A change in an extinction ratio is detected by a change in output amplitude of a preamplifier whose gain changes according to an input level connected to a photodiode as a light receiving element, and a bias current applied to a laser diode is controlled accordingly. I made it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter for optical communication using an optical fiber as a transmission line.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing a configuration of a conventional optical transmitter. In FIG. 6, 1 is a DATA input terminal, 2
Denotes a laser driver, 3 denotes a laser diode as a light emitting element, and 4 denotes a bias current source.

Next, the operation will be described. The digital signal input to the DATA input terminal 1 is a laser driver 2
, And is output as a modulation current Imod. The laser diode 3 generally has a current-to-light characteristic in which the light output sharply rises from a certain threshold current value Ith1 as shown in FIG. Since the bias current Ib supplied to the laser diode 3 from the normal bias current source 4 is set to be substantially equal to the threshold current value Ith1, the digital optical signal output from the laser diode 3 is set to 0 as shown in FIG. The ratio of one level to the level, that is, the extinction ratio is sufficiently large and has a good waveform.

[0004]

It is generally known that a laser diode fluctuates current-optical characteristics at ambient temperature. However, since a conventional optical transmitter is configured as described above, for example, the ambient temperature is reduced. Rises and the laser diode 3
When the threshold current changes to Ith2 in FIG. 7, there is a problem that the 0 level of the output digital optical signal increases and the ratio to the 1 level decreases, that is, the extinction ratio deteriorates. In order to solve such a problem, the light output is stabilized by a light output stabilizing circuit using a light output power detection photodiode, and the bias current Ib and the modulation current Imo of the laser diode 3 are changed according to the ambient temperature.
Although a method of compensating for d has been adopted, since the deterioration of the extinction ratio was not directly detected, the accuracy of the extinction ratio control was poor.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide an optical transmitter capable of accurately controlling the extinction ratio even when the current-optical output characteristics of a laser diode change. It is assumed that.

[0006]

In the optical transmitter according to the first aspect of the present invention, the fluctuation of the extinction ratio is detected by the output amplitude fluctuation of a preamplifier having a different current-to-voltage conversion magnification depending on the amount of current flowing through the photodiode connected to the photodiode. , And the bias current is controlled accordingly.

In the optical transmitter according to the second invention, the bias current is controlled so as to keep the average value of the current flowing through the photodiode constant, and the fluctuation of the extinction ratio is controlled by the amount of current flowing through the photodiode connected to the photodiode. Detection is performed based on output amplitude fluctuations of preamplifiers having different current-voltage conversion magnifications, and the modulation current is controlled accordingly.

In the optical transmitter according to the third aspect of the present invention, the bias current is controlled so as to keep the average value of the current flowing through the photodiode constant, and the variation of the extinction ratio is controlled by the amount of current connected to the photodiode and flowing through the photodiode. The modulation current is detected by detecting the output amplitude fluctuation of the preamplifiers having different current-voltage conversion magnifications, and controlling the modulation current so that the ratio between the detected output and the average value of the current flowing through the photodiode becomes constant.

In the optical transmitter according to the fourth aspect of the present invention, the bias current is controlled so as to keep the average value of the current flowing through the photodiode constant, and the fluctuation of the extinction ratio is controlled by the amount of current connected to the photodiode and flowing through the photodiode. Detection is performed based on output amplitude fluctuations of preamplifiers having different current-voltage conversion magnifications, and the modulation current is controlled accordingly. Further, the current-voltage conversion magnification change point of the preamplifier is adjusted according to the optical output power, so that a wide optical output level can be adjusted.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a block diagram showing Embodiment 1 of the present invention, in which 1 is a DATA input terminal, 2 is a laser driver, 3 is a laser diode as a light emitting element, 4 is a bias current source,
Is a photodiode as a light receiving element, 6 is a preamplifier,
7 is a peak detector, and 8 is a bias current controller.

Next, the operation will be described. The digital signal input to the DATA input terminal 1 is converted into a modulation current Imod by the laser driver 2, and drives the laser diode 3 together with the bias current Ib supplied from the bias current source 4. Part of the optical signal output from the laser diode 3 is input to the photodiode 5 and converted into a current signal. This current signal is converted into a voltage signal by the preamplifier 6, and the peak detector 7 holds the voltage amplitude value by the capacitive load and outputs a voltage value proportional to the voltage amplitude value. The output voltage of the peak detector 7 is input to a bias current controller 8, and the bias current controller 8 controls the magnitude of the bias current Ib output from the bias current source 4 so that the input voltage becomes constant.

FIG. 2 shows (a) input / output characteristics of the preamplifier 6, (b) an optical input waveform, and (c) a voltage output waveform of the preamplifier 6. Here, for the sake of explanation, FIG.
In (a), it is considered that the light level input to the photodiode 5 is proportional to the value of the current flowing through the photodiode 5, and the light level input to the photodiode 5 is changed instead of the current signal input to the preamplifier 6. The horizontal axis is used. The current-voltage conversion magnification of the preamplifier 6 is set so as to have a large value R1 in a small light input region and a small R2 value in a large light input region from the changing point Pth. Here, when the 0 level of the optical signal input from the laser diode 3 to the photodiode 5 is Pi0 and the 1 level is Pi1, the output voltage amplitude of the preamplifier 6 is set to V1. Next, when only the 0 level is slightly increased to Pi2 in FIG. 2, that is, when the extinction ratio is slightly deteriorated, the output voltage amplitude of the preamplifier 6 becomes V2 as shown in FIG. This is because the change in the 0 level of the signal is emphasized more than the change in the 1 level due to the non-linear current-voltage conversion characteristics of the preamplifier 6 as described above. On the other hand, since the one level is in the region R2 where the current-voltage conversion magnification is small, even if the one level fluctuates, the output amplitude fluctuation of the preamplifier 6 is small. Therefore, the fluctuation of the extinction ratio can be detected by detecting the output amplitude value of the preamplifier 6 by the peak detector 7. In order to keep the output voltage of the peak detector 7, that is, the input voltage to the bias current controller 8, constant, the bias current Ib supplied from the bias current source 4 to the laser diode 3 is reduced according to the amount of decrease in the voltage value. By controlling the bias current controller 8, the deterioration of the extinction ratio can be compensated. For example, as shown in FIG. 7, when the threshold current of the laser diode 3 changes from Ith1 to Ith2 and the extinction ratio deteriorates, the bias current value Ib can follow the threshold current change.

Embodiment 2 FIG. FIG. 3 is a block diagram showing Embodiment 2 of the present invention. 9 is a photodiode 5
Is a current detector that outputs a voltage proportional to the average value of the current flowing through the bias current controller 8. The output voltage is input to the bias current controller 8. Reference numeral 10 denotes a modulation current controller that controls the modulation current Imod output from the laser driver 2 so that the voltage input from the peak detector 7 becomes constant. In the first embodiment, extinction ratio compensation is possible, but the optical output power is not controlled. On the other hand, in the second embodiment, when the voltage value decreases, the bias current Ib output from the bias current source 4 is increased so that the voltage output from the current detector 9 becomes constant, and the voltage value increases. In this case, the bias current controller 8 controls the bias current to reduce the bias current, thereby keeping the optical output power output from the laser driver 3 constant. When the voltage input from the peak detector 7 to the modulation current controller 10 decreases, the modulation current controller 10 modulates the modulation current Imod output from the laser driver 2 so that the output amplitude of the peak detector 7 becomes constant. , It is possible to prevent the extinction ratio from deteriorating as in the first embodiment.

Embodiment 3 FIG. 4 is a block diagram showing Embodiment 3 of the present invention. Reference numeral 11 denotes a value of a modulation current Imod output from the laser driver 2 by comparing the output voltage of the current detector 9 and the output voltage of the peak detector 7. Is an extinction ratio controller that controls In the third embodiment, similarly to the second embodiment, the bias current Ib is set so that the output voltage of the current detector 9 becomes constant.
The optical output power is kept constant by controlling the power.
In the first and second embodiments, there is no function of setting the value of the extinction ratio only by compensating for the deterioration of the extinction ratio. However, in the third embodiment, the output voltage of the peak detector 7 and the The extinction ratio controller 11 controls the modulation current Imod output from the laser driver 2 so that the output voltage ratio, that is, the ratio between the AC component and the DC component of the optical output signal, becomes a certain set ratio. The extinction ratio can be set accurately while compensating for the deterioration of the extinction ratio.

Embodiment 4 FIG. 5 is a block diagram showing a fourth embodiment of the present invention. Reference numeral 12 denotes a value of the bias current Ib output from the bias current source 4 according to the output of the current detector 9 and a light-to-voltage conversion magnification of the preamplifier 6. Change point P
An optical output controller that controls th. In the fourth embodiment, similarly to the second embodiment, the optical output controller 12 controls the bias current Ib so that the output voltage of the current detector 9 becomes constant, thereby making the optical output power constant, and the peak detector 7 The modulation current Imod output from the laser driver 2 is controlled by the modulation current controller 10 so that the output voltage value becomes constant, thereby compensating for the deterioration of the extinction ratio. Further, in the fourth embodiment, the current-voltage conversion magnification change point Pth of the preamplifier 6 is controlled according to the set optical output power. For example, in FIG. 2, the optical output power is set to be small and the 0 level of the input optical digital signal is
When both the levels become lower than the change point Pth, both the 0 level and the 1 level are converted into voltages by the same gain R1, so that the non-linear effect of the current-voltage conversion characteristics of the preamplifier 6 does not appear, and the extinction as described above occurs. No specific deterioration can be detected. Therefore, when the optical output power is set to a low output, the output voltage of the current detector 9 decreases, and when one level of the input optical digital signal becomes smaller than the change point Pth,
By controlling the one level to be equal to or higher than Pth, it is possible to compensate for the deterioration of the extinction ratio for the light output power in a wider range as in the second embodiment.

[0016]

According to the first aspect of the invention, the fluctuation of the extinction ratio is detected by the fluctuation of the output oscillation of the preamplifier having a different current-to-voltage conversion magnification depending on the amount of current flowing through the photodiode connected to the photodiode. By controlling the current, it was possible to accurately compensate for the deterioration of the extinction ratio even when the current-light output characteristics of the laser diode changed.

Further, according to the second invention, the optical output power is stabilized by controlling the bias current so that the average value of the current flowing through the photodiode is constant, and the fluctuation of the extinction ratio is connected to the photodiode. The output current of a preamplifier with a different current-to-voltage conversion magnification is detected based on the amount of current flowing through the photodiode, and the modulation current is controlled accordingly. The deterioration of the ratio could be compensated with high accuracy.

According to the third aspect of the present invention, similarly to the second aspect of the invention, the bias current is controlled so that the average value of the current flowing through the photodiode is constant, thereby stabilizing the optical output power, and The extinction ratio is accurately compensated for by maintaining a constant ratio between the output amplitude value of the preamplifier and the average value of the current flowing through the photodiode, which is different in current-voltage conversion magnification depending on the amount of current flowing through the photodiode connected to the The extinction ratio was set with high accuracy.

According to the fourth aspect of the present invention, similarly to the second aspect of the present invention, the bias current is controlled so as to keep the average value of the current flowing through the photodiode constant, thereby stabilizing the optical output power and increasing the extinction ratio. Is detected by the output amplitude fluctuation of the preamplifier with different current-to-voltage conversion magnifications based on the amount of current flowing through the photodiode connected to the photodiode, and controlling the modulation current in response to this. , The deterioration of the extinction ratio could be compensated with high accuracy. Further, by adjusting the current-voltage conversion magnification change point of the preamplifier according to the optical output power, a wide adjustment range of the output power could be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing Embodiment 1 of an optical transmitter according to the present invention.

FIG. 2 is a diagram showing a relationship between an input waveform and an output waveform of a preamplifier.

FIG. 3 is a block diagram showing Embodiment 2 of an optical transmitter according to the present invention.

FIG. 4 is a block diagram showing Embodiment 3 of an optical transmitter according to the present invention.

FIG. 5 is a block diagram showing Embodiment 4 of an optical transmitter according to the present invention.

FIG. 6 is a block diagram showing a conventional optical transmitter.

FIG. 7 is a diagram showing a relationship between an input waveform and an output waveform of a laser diode.

[Explanation of symbols]

1 DATA input terminal, 2 laser driver, 3 laser diode, 4 bias current source, 5 photodiode, 6 preamplifier, 7 peak detector, 8 bias current controller, 9 current detector, 10 modulation current controller, 11 extinction ratio controller, 12 Light output controller.

Claims (4)

[Claims] A laser diode as a light emitting element;
A laser driver that applies a modulation current to the laser diode, a bias current source that applies a bias current to the laser diode, a photodiode that receives a part of the optical output of the laser diode and converts it into a current signal, A preamplifier for converting a current signal into a voltage signal, the current / voltage conversion magnification of which varies depending on the amount of current of the photodiode, and detecting the amplitude of the preamplifier output signal; and outputting a bias output from the bias current source in accordance with the detected value. An optical transmitter, comprising: a bias current controller for controlling a current. 2. A laser diode as a light emitting element;
A laser driver that applies a modulation current to the laser diode, a bias current source that applies a bias current to the laser diode, a photodiode that receives a part of the optical output of the laser diode and converts it into a current signal, A preamplifier which converts a current signal into a voltage signal, and a current / voltage conversion magnification of which varies depending on the amount of current of the photodiode; and detects the amplitude of the preamplifier output signal, and modulates a current output from the laser driver in accordance with the detected value. Current controller for controlling a bias current, a current detector for detecting an average value of a current signal of the photodiode, and a bias current for controlling a bias current output from the bias current source in accordance with an output value of the current detector An optical transmitter comprising a controller 3. A laser diode as a light emitting element;
A laser driver that applies a modulation current to the laser diode, a bias current source that applies a bias current to the laser diode, a photodiode that receives a part of the optical output of the laser diode and converts it into a current signal, A preamplifier that converts a current signal into a voltage signal and a current / voltage conversion magnification varies depending on the amount of current of the photodiode, a peak detector that detects the amplitude of the preamplifier output signal, and detects an average value of the current signal of the photodiode A current detector, a bias current controller that controls a bias current output from the bias current source according to an output value of the current detector, and a laser that compares outputs of the current detector and the peak detector. An extinction ratio controller that controls the modulation current output from the driver Optical transmitter, characterized in that a roller. 4. A laser diode as a light emitting element;
A laser driver that applies a modulation current to the laser diode, a bias current source that applies a bias current to the laser diode, a photodiode that receives a part of the optical output of the laser diode and converts it into a current signal, A preamplifier which converts a current signal into a voltage signal, and a current / voltage conversion magnification varies depending on the amount of current of the photodiode; detects a voltage amplitude of the preamplifier output signal; and modulates a voltage output from the laser driver in accordance with the detected value. A modulation current controller for controlling a current, a current detector for detecting an average value of the current signal of the photodiode, a bias current output from the bias current source according to an output of the current detector, and a current of the preamplifier. An optical output controller that controls the voltage conversion magnification Optical transmitter, characterized in that was e.