JPH08163046A - Optical transmitter - Google Patents

Abstract

PURPOSE: To make an optical output and an optical output waveform stable by supplying a current to drive a laser diode from a modulator only and changing a ratio of length of times of a modulation current. CONSTITUTION: Let a mean value of an output waveform from an input buffer 11 be V1 , when a threshold voltage given from a variable voltage source 13 to a limiter 12 is equal to the voltage V1 , a duty factor of the waveform outputted from the limiter 12 is 50%. On the other hand, when the threshold voltage is set to be a voltage V2 smaller than the voltage V1 , the duty ratio is more than 50% with the assumption that the rise time and the fall time of the output waveform from the input buffer 11 are equal to each other. Thus, ratio of levels 1, 0 in the optical output waveform is adjusted to 1:1 by increasing the duty ratio of the output current waveform from a modulator 2 more than 50%. That is, the optical output waveform whose duty factor is 50% without distortion is obtained by adjusting the threshold voltage given to the limiter 12.

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. 8 is a block diagram of a conventional optical transmitter shown in, for example, Japanese Patent Publication No. 2-44420.
Is an input terminal, 2 is a modulator connected to the input terminal 1, 3 is a laser diode driven by the modulator 2, 4 is a photodiode for receiving a part of the output light of the laser diode 3, and 5 is a photodiode 4 Bias power source for applying a bias voltage to the diode, 6 a capacitor connected in parallel to the photodiode 4, 7 a current amplifier for supplying a bias current to the laser diode, 8 a first current source, 9 a second current source, 10 Is a switch circuit that switches the current from the second current source 8 in accordance with the binary transmission data input from the input terminal 1.

Next, the operation will be described. The output of the first current source 8 is output as a current turned on / off by the switch circuit 10 in accordance with the transmission data, and is added and synthesized with the output of the second current source 9. When this is mathematically expressed, it becomes as shown in Expression 1. _{I o = I o2 + mI o1} ······ (1) I o: adding composite current I _o1: first current source of the output current I _o2: a binary signal: second output current m of the current source Mark rate

Next, a part of the output light of the laser diode 3 is incident on the photodiode 4, and a current proportional thereto flows through the photodiode 4. When this is mathematically expressed, it becomes as shown in Expression 2. I _PD = m · D · L · Pout (2) I _PD : Photodiode current D: Pulse occupancy ratio L: Optical output and photodiode current conversion efficiency Pout: Optical output power

A difference between a current obtained by averaging the output current of the photodiode 4 by the capacitor 6 and the reference current is amplified by the current amplifier 7 and applied to the laser diode 3. When this is mathematically expressed, it becomes as shown in Expression 3. I _B = β (I _o −I _PD ) ... (3) I _B : Current amplifier output current β: Current amplifier multiplication factor

Current-light conversion characteristics of the laser diode 4 are
This is as shown in FIG. Therefore, the optical output power value Pout can be expressed by Expression 4. _{Pout = A (I B + I} op -I th) ······ (4) A: a laser diode current-light conversion efficiency I _op: modulator output current I _th: laser diode threshold current

Equation 5 is derived from Equations 2-4.

[0008]

[Equation 1]

This is the threshold value I _th of the laser diode 3.
Shows that the bias current I _b follows so as to keep the optical output power Pout constant, even when _f . First I _th and I
_{If b} is set to be equal, even if I _th changes, Po
It is set so that ut is constant and I _B is equal to I _th .

[0010]

Since the conventional optical transmitter is constructed as described above, it is controlled so as to be constant by changing the bias current I _b . It takes time and effort to adjust the extinction ratio determined by the level ratio. There is a problem that the consumption current increases due to the bias current I _b .

The present invention has been made to solve the above problems, and an object thereof is to obtain an optical transmitter using a laser diode which does not require a bias current circuit for the laser diode. The purpose is to stabilize the output and optical output waveforms.

[0012]

In the optical transmitter according to the first embodiment of the present invention, the current for driving the laser diode is supplied only by the modulator, and the ratio of the time length of 1 and 0 of the modulation current is changed. It was done like this.

The optical transmitter according to the second embodiment is such that feedback is applied so that the modulation current is controlled by the average value of the monitor current of the photodiode that receives a part of the optical output of the laser diode.

The optical transmitter according to the third embodiment is such that feedback is applied so as to control the modulation current at the peak value of the monitor current of the photodiode that receives a part of the optical output of the laser diode.

In the optical transmitters according to the fourth and fifth embodiments, the temperature is monitored, and the ratio of the time length of 1 and 0 of the modulation current is adjusted by the monitored value.

[0016]

The optical transmitter according to the first embodiment of the present invention is
The current for driving the laser diode is only the modulation current, and the waveform distortion caused by the characteristics of the laser diode is 1 of the modulation current.
Compensate by adjusting the ratio of 0 and 0.

The optical transmitter according to the second embodiment can stabilize the optical output by monitoring the average value of the optical output and feeding it back so as to control the modulation current.

The optical transmitter according to the third embodiment can stabilize the optical output by monitoring the peak value of the optical output and feeding it back so as to control the modulation current.

In the optical transmitters according to the fourth and fifth embodiments, the optical waveform is stabilized by adjusting the ratio of 1 and 0 of the modulation current depending on the temperature.

[0020]

【Example】

Example 1. Embodiment 1 of the present invention will be described below with reference to the drawings. In FIG. 1, 11 is an input buffer for outputting input transmission data with a constant amplitude, and 12 is an input buffer 1.
A limiter that receives the output of 1 and outputs the transmission data to the modulator 2, and a variable voltage source 13 that applies a threshold voltage to the limiter 12.

Next, the operation will be described. FIG. 6 shows the input buffer 11, the limiter 12, and the modulator 2 in the first embodiment.
2 shows the output waveform of the. The transmission data input to the input buffer 11 has a constant amplitude and is output as a continuous waveform of 0 and 1 having a certain rise / fall time. At this point, the ratio of the length of 1 to 0 is 1: 1 and the duty is hereinafter referred to as 50%. Input buffer 11
The average value of the output waveform when the threshold voltage given by the variable voltage source 13 to the limiter 12 When V ₁ is equal to V ₁ was the duty of the waveform output from the limiter 12 is a 50%, V ₁ If the threshold voltage is set to a smaller value of V _2, it is assumed that the rise time and the fall time of the output waveform of the input buffer 11 are equal.

[0022]

[Equation 2]

And becomes larger than 50%. Here, it is assumed that the limiter 12 has an infinitely large gain and the output waveform of the limiter 12 is changed so that the rising time and the falling time are almost zero. The output of the limiter 12 is input to the modulator 2, and the modulator 2 outputs a current waveform having a duty greater than 50%, like the output of the limiter 11.

FIG. 7 shows the relationship between the output current of the modulator 2 and the optical output waveform of the laser diode 3. Since the laser diode 3 hardly emits light until the threshold current I _th , the duty of the current output waveform of the modulator 2 is 50%.
In the case of, the length of the laser diode 3 from the beginning to the end of the light emission, that is, the length t ₃ corresponding to 1 of the transmission data.
And the ratio of the length of non-emission of the laser diode, that is, the length t ₄ corresponding to 0 of the transmission data, is 1: 1 when the current output waveform has a rising and falling time as shown in FIGS.
However, the duty becomes smaller than 50%. Therefore, by increasing the duty of the output current waveform of the modulator 2 to more than 50%, it becomes possible to adjust the ratio of 1 and 0 of the optical output waveform to 1: 1. That is, by adjusting the threshold voltage applied to the limiter 11, it is possible to obtain an undistorted optical output waveform with a duty of 50%, and it is possible to realize a transmitter that does not require a bias current circuit.

Example 2. In the first embodiment, only the duty of the output current of the modulator 2 is adjusted, but it is conceivable that the optical output of the laser diode 3 is monitored by the photodiode 4 and the output is fed back in order to stabilize the optical output. In FIG. 2, 14 is a resistor for converting the monitor current of the photodiode 4 into a voltage, 1
Reference numeral 5 is a current-voltage converter which gives the modulator 2 a voltage proportional to the current flowing through the resistor 14.

The monitor current output from the photodiode 4 is averaged by the capacitor 6 to become a direct current, and a voltage proportional to the average value of the optical output of the laser diode 3 is applied across the resistor 14. This voltage is multiplied by the current-voltage converter and given to the modulator 2, and the modulation output current is controlled so that the average value of the optical output of the laser diode 3 becomes constant.

Embodiment 3 FIG. Although the resistor 14 and the capacitor 6 are used to convert the monitor current of the photodiode 4 into a voltage in the second embodiment, a peak detector composed of the diode 16 and the capacitor 6 may be used. In this case, the laser diode 3 is calculated by the average value of light as in the second embodiment.
The light output of is controlled not by the peak value of the light waveform.

Example 4. The threshold I _th of the laser diode 3 varies depending on the temperature such as 0 ° C., 25 ° C. and 50 ° C. as shown in FIG. In the fourth embodiment shown in FIG. 4, the variable voltage source 13 is controlled by a signal proportional to the temperature output from the temperature sensor 18 and the fluctuation of the duty of the optical output due to the change of the threshold value is given to the limiter 12. Correct by adjusting the threshold voltage. Further, by using the method of Example 2, the optical output is kept constant at the average value of the monitor current.

Embodiment 5 FIG. In Example 4, the method of Example 2 was used to keep the light output constant, but the method of Example 3 may be used.

[0030]

As described above, according to the first embodiment of the present invention, the current for driving the laser diode is limited to the modulation current, and the distortion of the optical output waveform caused by the current-optical conversion characteristic of the laser diode is suppressed. By compensating by changing the ratio of the time lengths of 1 and 0, there is an effect that an optical transmitter that does not require a bias circuit can be obtained.

According to the second and third embodiments, the effect of stabilizing the light output is obtained by detecting the monitor current of the photodiode for monitoring the light output of the laser diode and applying the feedback so as to control the modulation current. be able to.

According to the fourth and fifth embodiments, the effect of stabilizing the optical output waveform can be obtained by changing the length of time of 1 and 0 of the modulation current depending on the temperature.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the present invention.

FIG. 4 is a block diagram showing a fourth embodiment of the present invention.

FIG. 5 is a block diagram showing a fifth embodiment of the present invention.

FIG. 6 is a diagram showing a transmission data waveform of each unit.

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

FIG. 8 is a block diagram showing a conventional optical transmitter.

FIG. 9 is a diagram showing a relationship between a drive current of a laser diode and an optical output waveform in a conventional optical transmitter.

[Explanation of symbols]

1 input terminal, 2 modulator, 3 laser diode, 4
Photodiode, 5 bias power source, 6 capacitor, 7 current amplifier, 8 first current source, 9 second current source, 10 switch circuit, 11 input buffer, 12
Limiter, 13 variable voltage source, 14 resistance, 15 current-voltage converter, 16 diode, 17 peak detector, 1
8 Temperature sensor.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 10/02 10/18 // H01S 3/096

Claims (5)

[Claims] 1. A laser diode as a light emitting device, a modulator for driving the laser diode, a limiter for giving transmission data to the modulator, a variable voltage source for giving a threshold voltage to the limiter, and a constant for the limiter. An optical transmitter, comprising: an input buffer for giving transmission data of amplitude. 2. A photodiode for receiving a part of the transmitted light output of the laser diode, a capacitor connected in parallel with the photodiode, a resistor connected to the cathode of the photodiode, and a resistor flowing to the resistor. The optical transmitter according to claim 1, further comprising a current-voltage converter that converts a current into a voltage and supplies the modulator as a control voltage. 3. A peak detector comprising the photodiode, which receives a part of the transmitted light output of the laser diode, a diode connected to the cathode of the photodiode and the capacitor, and a peak detector of the peak detector. 2. The optical transmitter according to claim 1, further comprising the current-voltage converter that converts an output current into a voltage and supplies the modulator as a control voltage. 4. The optical transmitter according to claim 2, further comprising a temperature sensor for giving a signal for controlling a threshold output voltage of the variable voltage source. 5. The optical transmitter according to claim 3, further comprising the temperature sensor for giving a signal for controlling a threshold output voltage of the variable voltage source.