JPH07111355A - Optical transmitter - Google Patents

Abstract

PURPOSE:To obtain an optical transmitter which produces a constant optical output at a constant extinction ratio independent of a change in ambient temperature of a laser diode. CONSTITUTION:An output change difference between the optical output of a laser diode 5 and the back optical output received by a photodiode 6 to a change in an ambient temperature is previously set by calculates 10 and 12 in an optical transmitter. The ambient temperature is detected by a temperature sensing device 7. The optical transmitter is so constituted as to compensate for a change in the optical output of the laser diode 5 to a temperature change, whereby the optical so that it produces a constant optical output at a constant extinction ratio.

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 shows, for example, Japanese Patent Laid-Open No. 2-44420.
2 is a block diagram showing a conventional optical transmitter shown in FIG. 1, in which 1 is a pulse or analog signal input terminal, 2
Is a modulator connected to the input terminal 1, 3 is a bias circuit that supplies a DC bias to the laser driver, 4 is a laser driver that drives the laser diode, 5 is a laser diode that is driven by the laser driver 4, and emits light, and 6 is a laser A photodiode that receives a part of the back light of the diode 5 and converts the light into a current, 9 is a bias power supply that biases the photodiode 6, and 11 is an integrating circuit that converts the output current of the photodiode 6 into an averaged voltage. Is.

Next, the operation when a pulse is applied as an input signal will be described. The output of the bias circuit 3 is a combined current of the current corresponding to the pulse and the bias current corresponding to the output of the integrating circuit 11. When formulated, I ₀ = I ₀₂ + I ₀₁ ... (1) I ₀ : Summing combined current I ₀₁ : Current corresponding to pulse I ₀₂ : Bias current m: Mark ratio of pulse signal.

Next, a part of the back light of the laser diode 5 is incident on the photodiode 6, and a current proportional thereto flows through the photodiode 6. When formulated, I _PD = mD L P _out (2) I _PD : photodiode current D: pulse occupancy ratio L: optical output and photodiode current conversion efficiency P _out : optical output It becomes electric power.

The output current of the photodiode 6 is converted into a voltage averaged by the integrating circuit 11. The difference between this average voltage and the reference voltage of the bias circuit 3 is the laser driver 4
The laser diode 5 is driven by the current amplification. When formulated, I _B = β (I ₀ −I _PD ) ... (3) I _B : Laser diode output current β: Laser diode amplification factor.

The current-light conversion characteristics of the laser diode are shown in FIG.
It is as shown in (a). Therefore, the optical output power value P
_out is _{P out = A (I B +} I OP -I th) ··· (4) A: a laser diode current-light conversion efficiency I _OP: modulator output current I _th: a laser diode threshold current formula (2) The following equation is derived from the equation (4). P _out = {A · β · I ₀ + A (I _OP −I _th )} / (1 + A · β · m · D · L) (5)

This indicates that the bias current I _b follows the optical output power P _out to be constant even if the threshold current of the laser diode 3 fluctuates. First I _th
If it is set to be equal to _b , even if I _th changes, P
_{Out is} constant and I _B = I _th ... (6) can be satisfied.

[0008]

Since the conventional optical transmitter is constructed as described above, the current-light conversion characteristic of the laser diode 5 shown in FIG. 8 is equal to or lower than the current I _th shown in FIG. 3 (a). However, when it changes so as to have a slope, the input current waveform of the laser diode 5 shown in FIG.
As shown in the optical output waveform of the laser diode 5 shown in FIG. 2, there is a problem that the extinction ratio is deteriorated due to the existence of an optical output of a certain value or more with respect to the bias current I _b of I _th or less. Since it is controlled by the output current of the photodiode 6, there is a problem that a constant light output cannot be obtained over the entire temperature range of use.

The present invention has been made to solve the above problems, and compensates for fluctuations in current photoelectric characteristics of output light including back light with respect to temperature of a laser diode and fluctuations of photoelectric current characteristics of a photodiode. The purpose is to obtain a constant light output.

[0010]

In the optical transmitter according to the present invention, the back light of the laser diode is detected by the photodiode and the temperature of the laser diode is detected by the temperature sensitive element to compensate for the temperature fluctuation of the optical output of the laser diode. In order to achieve this, a laser is provided by an integrator that integrates the current of the photodiode, a first calculator that calculates the resistance value change of the temperature-sensitive element, and a second calculator that calculates using the integrator and the output of the first calculator as inputs. It controls the bias current of the diode. The output voltage of the second arithmetic unit and the pulse or analog signal of the input signal are used as inputs, and the third arithmetic unit is used as an input signal of the modulator to control the modulation current. Further, the bias current and the modulation current are controlled by using a ROM that stores the correction functions of the optical output of the laser diode and the current output of the photodiode with respect to the temperature, corresponding to each laser module.

[0011]

In the optical transmitter according to the present invention, the back light of the laser diode detected by the photodiode does not match the optical output over the entire temperature range. Therefore, the temperature of the laser diode is detected by the temperature sensitive element and A constant light output with a good extinction ratio is obtained by correcting the difference in back light.

[0012]

【Example】

Example 1. An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 7 is a temperature sensitive element for detecting the temperature of the laser diode 5, 8 is a laser module in which the laser diode 5, the photodiode 6 and the temperature sensitive element 7 are arranged on the same stem, and 10 is the temperature sensitive element 7 as an input. A first calculator that calculates the temperature as a resistance and a second adder 12 that receives the output voltage of the integration circuit 11 and the output voltage of the first calculator 10 as an input and adds them.

FIG. 2A is a diagram showing current photoelectric conversion characteristics of the laser diode 5 of FIG. 1 at temperatures of 0 ° C., 25 ° C. and 70 ° C., and FIG. 2B is a bias of the laser diode 5 of FIG. FIG. 2 (c) is a diagram showing the optical output waveform of the laser diode 5 at 25 ° C., showing the current I _b and the input pulse modulation current I _0b .

FIG. 3 shows the laser diode 5 shown in FIG.
It is a figure which shows the current photoelectric conversion characteristic which showed the detail of the threshold current part in 5 degreeC and 70 degreeC, the bias current _Ib , the modulation current _{I0b of an} input pulse, and an optical output waveform.

A slope D in FIG. 4 is a line (called a tracking error) showing the temperature variation of the optical output and the back light photoelectric conversion characteristic,
The slope E indicates the compensation line of the second calculator 12, and the slope F indicates the tracking error after compensation.

Next, the operation will be described. As shown in FIGS. 2 and 3, the laser diode 5 has such characteristics that the threshold current I _th for emitting laser light increases as the temperature rises and the slope of the current photoelectric conversion characteristic becomes gentle. As a result, a varied optical output with a poor extinction ratio results. As described in the prior art example, the normal photodiode 6 receives a part of the back light of the laser diode 5, converts it into a current signal, and controls the bias current so that the light output becomes constant. Hold on. Since the photodiode 6 does not directly receive the optical output of the laser diode 5 as shown by the slope D in FIG. 4, there is usually a difference between the output light and the back light with respect to the temperature change, and the light is received. The photoelectric conversion characteristic of the photodiode also varies with temperature. Therefore, the temperature fluctuation of the actual light output with respect to the photodiode current based on 25 ° C. is called tracking error.
It shows the characteristics of.

A temperature sensitive element 7 detects the temperature fluctuation of the laser diode 5. The temperature sensitive element 7 detects a temperature change as a change in resistance value and is calculated as an input resistance of the first calculator 10. The photoelectric current of the photodiode 6 is the integration circuit 11
Is converted into an averaged voltage. The second computing unit 12 outputs the output voltage corresponding to the temperature of the first computing unit and the integrating circuit 11
The output voltage corresponding to the optical output of is added as an input to perform addition calculation. The calculation output of the second calculator 12 has a slope E which is the inverse characteristic of the slope D in FIG. The bias current of the bias circuit 3 is controlled by this arithmetic output to obtain a constant light output without tracking error shown in the slope F of FIG.

Example 2. Hereinafter, another embodiment of the present invention will be described with reference to the drawings. In FIG. 5, 13 is an input terminal 1
Is a third arithmetic unit for inputting the pulse from and the output voltage of the second arithmetic unit 12 and outputting the calculation result to the modulator 2.

Next, the operation will be described. The third computing unit 13 controls the modulator 2 by adding the computation output of the second computing unit 12 having the slope E in FIG. 4 and the pulse from the input terminal 1. Taking the high temperature of 70 ° C. in FIG. 4 as an example, the tracking error is −
The output of the second computing unit 12 is 2 dB and the optical output is 2 dB smaller than the specified value, and the output is +2 dB because of the slope E. The third calculator 13 adds +2 dB to the input pulse and transmits the output signal to the modulator 2. This signal controls the laser driver 4. The current of the laser driver 4 increases so that the optical output of the laser diode 5 becomes 2 dB higher. Since the laser driver 4 drives the modulation current of the laser diode 5, the magnitude of the modulation current I _0b pulse is controlled and the bias current I _b is not controlled, as shown in FIG. Therefore, the DC light emission P _out 2 due to the bias current maintains the same constant value as the value at 25 ° C., so that the extinction ratio does not deteriorate at high temperature and a constant light output with a good extinction ratio can be obtained.

Example 3. The third embodiment of the present invention will be described below with reference to the drawings. In FIG. 6, reference numeral 13 denotes a third arithmetic unit which inputs the pulse from the input terminal 1 and the output voltage of the second arithmetic unit 12 and outputs the arithmetic result to the bias circuit 3 and the modulator 2.

Next, the operation will be described. The third computing unit 13 adds the computation output of the second computing unit 12 having the slope E in FIG. 4 and the pulse from the input terminal 1 to generate a modulation current for the modulator 2 and a bias current for the bias circuit 3, respectively. Control. Taking the high temperature of 70 ° C. in FIG. 4 as an example, the tracking error is −2 dB, and the output of the second computing unit 12 whose optical output is 2 dB smaller than the specified value has the slope E, and therefore the output is +2 dB. The fourth calculator 14 is +2 dB for the input pulse.
Are added and the output signal is transmitted to the modulator 2 and the bias circuit 3. This signal controls the laser driver 4 through the modulator 2 and the bias circuit 3. Both the modulation current and the bias current of the laser driver 4 increase the optical output of the laser diode 5 by 2 dB. The extinction ratio tends to worsen if the increase in the bias current is large and the increase in the modulation current is small, and it becomes good if the increase in the bias current is small and the increase in the modulation current is large. be able to.

Example 4. Another embodiment of the present invention will be described below with reference to the drawings. In FIG. 7, 15 is an A / D converter for converting the output voltage of the first arithmetic unit 10 into a digital signal to access the ROM 16, 16 is a ROM for storing a compensation function for the tracking error, and 17 is this RO.
It is a D / A converter that converts the stored digital output of M into an analog signal and outputs the analog signal to the second arithmetic unit 12. The output of the integrating circuit 11 is input to the bias circuit 3.

Next, the operation will be described. The temperature fluctuation signal output from the first arithmetic unit 10 is the A / D converter 15
Is converted into a digital signal to access the ROM 16. ROM 16 is a laser module 8 to be used in advance
A function (for example, the slope E in FIG. 4) for compensating for the tracking error is stored. The accessed ROM 16 outputs the compensation value as a digital signal. Digital signal is D /
It is converted into an analog signal by the A converter 17 and input to the second arithmetic unit 12. The advantage of using the ROM 16 is that even if there are variations in the characteristics of the individual laser modules 8 to be used, the compensation value can be set for the laser module 8 to be used and even if the compensation value with respect to temperature is complicated In point.

[0024]

As described above, according to the present invention, the difference between the light output of the laser diode and the back light output with respect to the temperature is set in advance by the calculator or the ROM, and the temperature is detected by the temperature sensing element. Since the optical output with respect to temperature is always compensated, there is an effect that a transmitter having high accuracy in optical output and extinction ratio can be obtained.

[Brief description of drawings]

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

FIG. 2 is a diagram showing characteristics of laser diodes according to Examples 1, 2, 3, 4 of the present invention.

FIG. 3 is a diagram showing temperatures and thresholds of laser diodes according to Examples 1, 2, 3 and 4 of the present invention.

FIG. 4 is a diagram showing tracking error and compensation of a laser module according to Examples 1, 2, 3 and 4 of the present invention.

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

FIG. 6 is a block diagram showing an optical transmitter according to a third embodiment of the present invention.

FIG. 7 is a block diagram showing an optical transmitter according to a fourth embodiment of the present invention.

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

[Explanation of symbols]

 1 Input Terminal 2 Modulator 3 Bias Circuit 4 Laser Driver 5 Laser Diode 6 Photodiode 7 Temperature Sensing Element 8 Laser Module 9 Bias Power Supply 10 First Computing Unit 11 Integrating Circuit 12 Second Computing Unit 13 Third Computing Unit 15 A / D converter 16 ROM 17 D / A converter

Claims (4)

[Claims] 1. A laser diode as a light emitting element, a photodiode for receiving back light of the laser diode and converting the light into an electric current, and a temperature sensing device which is located near the laser diode and detects the temperature of the laser diode. Element,
A laser module in which the laser diode, the photodiode and the temperature sensitive element are arranged on the same stem, a laser driver for driving the laser diode, a modulator for supplying a pulse or analog modulation signal to the laser diode, and an input. A bias circuit for supplying a DC bias to the laser driver with a pulse or an analog signal from a terminal, a first calculator using the resistor of the temperature sensitive element as an input resistance, and an average value of the output current of the photodiode are calculated. A second integrating circuit for converting the voltage into a voltage; a second adding circuit for adding an output voltage of the integrating circuit and an output voltage of the first computing unit; and applying the added output voltage to an input terminal of the bias circuit.
An optical transmitter, comprising: 2. A laser diode which is a light emitting element, a photodiode which receives back light of the laser diode and converts the light into an electric current, and a temperature sensor which is located near the laser diode and detects the temperature of the laser diode. Element,
A laser module in which the laser diode, the photodiode and the temperature sensitive element are arranged on the same stem, a laser driver for driving the laser diode, a modulator for supplying a pulse or analog modulation signal to the laser diode, and an input. A bias circuit that receives a pulse or analog signal from a terminal and supplies a DC bias to the laser driver, a first calculator that uses the resistor of the temperature sensitive element as an input resistance, and an average of the output currents of the photodiodes. From an integrating circuit for converting a value to a voltage, a second computing unit for adding an output voltage of the integrating circuit and an output voltage of the first computing unit, an output voltage of the second computing unit and the input terminal Input each pulse or analog signal of
An optical transmitter, comprising: a third arithmetic unit that supplies the arithmetic result to the modulator. 3. The optical transmitter according to claim 2, wherein the output voltage of the third arithmetic unit is supplied to the bias circuit. 4. A laser diode as a light emitting element, a photodiode for receiving back light of the laser diode and converting the light into an electric current, and a temperature sensing device located near the laser diode for detecting the temperature of the laser diode. Element,
A laser module in which the laser diode, the photodiode and the temperature sensitive element are arranged on the same stem, a laser driver for driving the laser diode, a modulator for supplying a pulse or analog modulation signal to the laser diode, and an input. A bias circuit that receives a pulse or analog signal from a terminal and supplies a DC bias to the laser driver, a first calculator that uses the resistor of the temperature sensitive element as an input resistance, and an average of the output currents of the photodiodes. An integrating circuit that converts the value into a voltage and outputs the voltage to the bias circuit, a memory that stores a compensation function for the temperature of the laser diode optical output of the laser module and the current output of the photodiode, and the output of the first computing unit. A to convert the voltage into a digital signal and access the memory
/ D converter, a D / A converter for converting the digital output of the memory into an analog signal, the analog signal and a pulse or an analog signal from the input terminal are added, and the addition result is added to the modulator and the bias. An optical transmitter, comprising: a second arithmetic unit that supplies the circuit.