JP2002111120A - Optical transmission module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of the transmission characteristic of an optical transmission module caused by the extinction ratio deterioration, etc., of a laser diode and the disturbance of the I waveform of reproduced signals on the receiving side of optical communication caused by the fluctuation of a bias current by controlling the ambient temperature. SOLUTION: The optical transmission module is provided with a laser diode 1 for optical communication, a light receiving element 2 which receives the monitor light of the diode 1 and converts the light into electric signals, and a laser driving circuit 23 which receives inputted modulated signals and supplies the bias current and an amplitude-modulation current to the diode 1. The module is also provided with a capacitor 14 which averages the response of a monitor voltage obtained by converting the monitor current of the light receiving element 2, and an operational amplifier 20 which inputs the averaged voltage and a reference voltage. In addition, the module is also provided with an integration circuit 24 which supplies the voltage that controls the bias current of the diode 1 to the laser driving circuit 23, and a temperature detecting circuit 26. The module controls the driving circuit 23 to perform temperature compensation on the modulated current of the diode 1 by using the output voltage of the temperature detecting circuit 26.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission module for optical communication, and more particularly, to a circuit for compensating a temperature characteristic of a laser diode in a control circuit for controlling an optical output of the laser diode to be constant.

[0002]

2. Description of the Related Art An optical transmission module for optical communication is a combination of a driving circuit for a semiconductor laser diode (hereinafter, referred to as a laser diode) for optical communication and performs optical communication via an optical fiber.

FIG. 4 shows a general current versus light output characteristic curve using the ambient temperature of a laser diode for optical communication as a parameter. From this figure, it can be seen that when the drive current of the laser diode is constant, the light output of the laser diode changes with the temperature of the environment.

When a laser diode is used continuously for a long period of time, the threshold current of the laser diode deteriorates due to wear and the threshold current increases. When the bias current and the drive current of the laser diode are used constant, the light output decreases due to the increase in the threshold current. This decrease in the optical output affects the sensitivity of the optical communication receiving side, and if the input light is small, there is a high possibility that the transmission characteristics will deteriorate. Therefore, in order to keep the light output of the laser diode for optical communication constant, a control circuit using a light receiving element for monitoring the light output is used together with the laser diode.

FIG. 5 shows a configuration example of a conventional optical transmission module.

Here, 1 is a laser diode (LD) for optical communication, 2 is a light receiving element (PD for monitoring) for monitoring optical output, 30 is a laser driving IC for driving LD1,
31 is APC (Auto power c) for keeping the optical output of LD1 constant.
ontrol) circuit.

[0007] The optical transmission module usually also incorporates an optical system (not shown) such as a lens system for condensing optical output and an optical fiber. Further, as a means for keeping the temperature of LD1 constant, a Peltier element and its control circuit may be incorporated.

[0008] In addition, the LD1 usually extracts laser oscillation light by a resonator, and can extract optical signals from at least two surfaces. One optical signal is used as an optical output, and the other optical signal is used as an optical signal. Used as monitor light output.

By the way, the LD1 has a threshold current as shown in FIG. 4, and the bias current of the LD1 is usually used by controlling it near the threshold current. The bias current of LD1 is
It is supplied from the bias current terminal 102 of the laser drive IC 30. The modulation signal used for optical transmission is input to the modulation signal input terminal 106 of the laser drive IC 30, converted into a current for driving the LD 1 internally, output from the modulation current terminal 101, and superimposed on the bias current. .

The monitoring PD2 converts the monitoring light output signal of the LD1 into an electric signal. The APC circuit 31 is a monitor PD
A signal is supplied to the bias current control terminal 103 of the laser drive IC 30 so that the output of the second electrical signal is constant.

The laser drive IC 30 has a bias current control terminal
The bias current of LD1 is controlled by the signal supplied to 103, and the modulation current of LD1 is controlled by the signal supplied to modulation current control terminal 104.

Therefore, LD1 → PD2 for monitor → APC
The closed circuit of the circuit 31 → laser drive IC30 → LD1 forms an APC loop.

The modulation current control terminal 104 of the laser driving IC 30 may be controlled by applying a voltage or may be controlled as a current mirror circuit. In the former case, a closed circuit is formed by monitoring the modulation current and adopting the same configuration as the amplitude APC loop so that the modulation current becomes constant. In the latter case, a resistor is connected to the modulation current control terminal 104, and control is performed so that the mirror of the current flowing therethrough becomes the modulation current.

As described above, since the bias current of the LD1 is controlled so that the amplitude current is constant and the electric signal of the monitoring PD2 is constant, the optical output is constant. When the temperature of LD1 is controlled to be constant by a Peltier element or the like, the circuit configuration of FIG. 5 is functionally sufficient.

However, the Peltier element and its control circuit consume large power and require a dedicated power supply, which complicates the system and increases the cost. Therefore, when advanced control is not required or when the cost of the system is to be reduced, an optical transmission module that does not use a Peltier element is required.

When the Peltier element is not used, the temperature of the LD1 is not constant, and as shown in FIG. 4, the threshold current and the light output change depending on the environmental temperature.
The bias current of LD1 controlled by 31 also changes,
Since the light output of LD1 is constant, the desired purpose has been achieved.

However, the current dependence (slope efficiency) of the light output of the LD1 also changes depending on the environmental temperature, so that the bias current of the LD1 greatly exceeds or falls below the threshold depending on the environmental temperature.

If the bias current of LD1 greatly exceeds the threshold current, the extinction ratio deteriorates and the dynamic range of the receiver cannot be obtained, so that the transmission characteristics deteriorate.

When the bias current of the LD1 is lower than the threshold current, a pattern effect occurs due to the carrier lifetime of the LD1, so that the pattern jitter increases and as a result, the transmission characteristics deteriorate. Pattern jitter is particularly noticeable at high frequencies of several hundred MHz or higher.

The APC circuit 31 is often configured mainly using an operational amplifier (operational amplifier), and the output of the operational amplifier generally does not operate up to the power supply voltage. For example, when the power supply voltage is +5. It saturates around 5V. On the other hand, many laser drive ICs 30 used to drive the LD 1 require an input up to near the power supply voltage.

FIG. 6 shows an input voltage-output current characteristic of the laser driving IC 30 in FIG.

From this figure, it can be seen that when an input voltage higher than the saturation voltage of the operational amplifier is required for the laser drive IC 30, a desired current value cannot be taken out from the laser drive IC 30. Transmission characteristics deteriorate due to an increase in pattern jitter, and it becomes impossible to obtain good transmission characteristics.

On the other hand, the light receiving element responds to an optical signal as can be seen from the fact that the light receiving element is also used for communication other than monitoring. If the time constant of the APC circuit 31 for controlling the output signal of the monitor PD2 to be constant is small, the APC circuit
31 cannot average the response signal of the monitor PD2,
The output of the APC circuit 31 fluctuates, and the bias current of LD1 fluctuates. For this reason, the waveform of the signal reproduced on the optical communication receiving side (a so-called eye waveform) is disturbed, and the standard is not satisfied.

[0024]

As described above, in the conventional optical transmission module, the extinction ratio of the laser diode deteriorates due to the environmental temperature, and the transmission characteristics deteriorate, or the pattern jitter increases to deteriorate the transmission characteristics. There was a problem of doing. Also, the output signal of the APC circuit for controlling the output signal of the monitoring light receiving element to be constant cannot be averaged, and the output fluctuates, and the bias current of the laser diode fluctuates.
There is a problem that the eye waveform of the signal reproduced on the receiving side of the optical communication is disturbed, and the standard is not satisfied.

The present invention has been made to solve the above problems, and the extinction ratio of a laser diode deteriorates due to the environmental temperature to deteriorate the transmission characteristics, and the pattern jitter increases to deteriorate the transmission characteristics. It is an object of the present invention to provide an optical transmission module which can prevent the reproduction signal from being disturbed and the eye waveform of the reproduced signal on the optical communication receiving side from being disturbed by the fluctuation of the bias current of the laser diode.

[0026]

An optical transmission module according to the present invention comprises a semiconductor laser diode for optical communication, a light receiving element for receiving monitor light of the semiconductor laser diode and converting the monitor light into an electric signal, and a modulation signal input. A laser drive circuit for receiving and supplying a bias current and a modulation amplitude current to the semiconductor laser diode; a current-voltage conversion circuit for converting a monitor current of the light-receiving element into a monitor voltage; and a current-voltage conversion circuit. A capacitor for averaging the response of the monitor voltage; and a first operational amplifier to which a voltage averaged by the capacitor and a predetermined reference voltage are input, and a voltage for controlling a bias current of the semiconductor laser diode is generated. And a first integration circuit for supplying the laser drive circuit with an ambient temperature, and detecting a voltage corresponding to the detection result. ; And a temperature detection circuit for force, and controls the laser driving circuit to the temperature compensation of the modulation current of the semiconductor laser diode with an output voltage of the temperature detection circuit. Here, the first
It is preferable that an operational amplifier whose voltage output can be changed to a power supply voltage be used as the operational amplifier.

A second operational amplifier to which an output voltage of the temperature detecting circuit is used as a reference input and to which a monitor voltage of a modulation current of the semiconductor laser diode is input, a control for temperature-compensating the modulation current of the semiconductor laser diode; A second integration circuit for generating a voltage and supplying the voltage to the laser driving circuit may be provided. Here, as the second operational amplifier, it is desirable to use an operational amplifier whose voltage output can be changed to a power supply voltage.

[0028]

Embodiments of the present invention will be described below in detail with reference to the drawings.

<First Embodiment> FIG. 1 shows a circuit configuration of an optical transmission module according to a first embodiment of the present invention.

Here, 1 is a laser diode (LD) for optical communication, 2 is a light receiving element (for example, photodiode; PD) for monitoring optical output, 3 to 12 are resistors or resistance elements, and 13 to 16 Is a capacitor, 17 is a variable resistor, 18 is a thermistor, 19 and 20 are operational amplifiers (ICs), 21
Is a constant voltage circuit (IC), 22 is an inductor, 23
Is a laser drive IC for driving LD1, 24 is a first integration circuit, 25 is a second integration circuit, and 26 is a temperature detection circuit.
In the laser driving IC 23, 101 is a modulation current output terminal, 10
2 is a bias current output terminal, 103 is a bias current control terminal, 104 is a modulation current control terminal, 105 is a modulation current (amplitude current) monitor terminal, and 106 is a modulation signal input terminal.

In addition to the printed wiring board on which the above-mentioned components are mounted, a lens system for condensing light output, optical components such as an optical fiber and an optical isolator (not shown), and a laser drive IC 23 are provided. The optical transmission module is configured by a package (for example, a CAN type) for incorporating circuit components such as an input / output circuit, a monitor circuit, and a digital input / output circuit, and the above-described components.

As a means for keeping the temperature of LD1 constant, a Peltier element and a control circuit for the Peltier element are sometimes incorporated. The power supply of elements used at a high frequency such as the laser drive IC 23 and LD1 is decoupled ( (Not shown). The data signal input to the laser drive IC 23 may be either a single-ended input or a differential input.

In addition, the LD 1 normally extracts laser oscillation light with a resonator, and can extract an optical signal from at least two surfaces. One optical signal is used as an optical output, and the other optical signal is used as an optical signal. Used as monitor light output.

The constant voltage circuit 21 is for preventing the reference voltage from fluctuating due to fluctuations of the power supply voltage due to external factors, and uses an IC using a zener diode or a band gap reference circuit. Thus, the temperature dependency of the constant voltage output can be reduced.

The modulation current output terminal 101 of the laser drive IC 23
Is coupled to LD1 via a waveform shaping resistor 3, and a bias current output terminal 102 is coupled to LD1 via a bias current supply resistor 5, an inductor 22, and the resistor 3. . Between the modulation current output terminal 101 and the ground terminal, a waveform compensation circuit composed of a series circuit of a resistor 4 and a capacitor 13 is connected. Thus, LD
1 transmits an optical signal and performs desired optical communication by being supplied with a bias current and a modulation current from the laser drive IC 23.

The monitoring PD2 receives the optical signal on the monitor side of the LD1 and converts it into a current.
It is converted to a voltage by 17. Further, by adjusting the resistance value of the variable resistor 17, a desired bias current of the PD2 can be obtained.

The signal converted into a voltage by the variable resistor 17 is averaged by the capacitor 14. The parasitic capacitance of the PD2 for monitoring is generally several pF to several tens of pF.
If a long modulation signal pattern is input to the input terminal 2, a response is made. Therefore, the capacity of the capacitor 14 is preferably large.

The first integrator circuit 24 includes the first operational amplifier 20
The reference voltage from the constant voltage circuit 21 is supplied to the (+) input terminal of the laser drive IC, and the voltage signal averaged by the capacitor 14 is input to the (-) input terminal. Input to the current control terminal 103. In this case, (-) the capacitor 16 and the resistor between the input terminal and the output terminal
Eleven parallel circuits are inserted, and have an incomplete integration characteristic in which the integrated output is saturated with respect to the input on the large input side within the input range.

As the operational amplifier 20, an amplifier capable of outputting up to the power supply voltage, for example, a rail-to-rail (registered trademark of Japan Motorola) circuit is used, and the gain thereof is desirably large in order to detect a small change. .

LD1 → PD2 for monitoring → (variable resistor
17, capacitor 14) → integration circuit 24 → laser drive IC 23 → LD
The closed circuit of 1 forms an APC loop to keep the optical output of LD1 constant.

The modulation current monitor terminal 105 of the laser drive IC 23
Is often directly connected to a modulation current supply source or has a current mirror configuration, so that a resistor 15 for converting a current into a voltage is connected. The modulation monitor signal converted into a voltage by the resistor 15 has a constant amplitude if there is no temperature change. However, an integration circuit for compensating (cancelling) the amplitude fluctuation due to the temperature change by the output of the temperature detection circuit 26. Supplied to 25. The temperature detection circuit 26 includes a resistor 7,
8 and a thermistor 18. Thermistor 18
Has a negative temperature characteristic whose resistance changes with temperature.

The second integrating circuit 25 is provided with a second operational amplifier 19
The reference voltage from the constant voltage circuit 21 is supplied to the (+) input terminal of the laser drive IC 23 via the temperature detection circuit 26, and the modulation monitor signal is input to the (-) input terminal.
To the modulation current control terminal 104. In this case, (-)
A parallel circuit of a capacitor 15 and a resistor 10 is inserted between the input terminal and the output terminal, and has an incomplete integration characteristic in which the integrated output is saturated with respect to the input on the large input side within the input range. In this case, like the operational amplifier 20, the operational amplifier 19 uses, for example, a rail-to-rail circuit that can output up to the power supply voltage, and its gain is desirably large in order to detect a small change.

The reference voltage supplied to the integration circuit 25 changes with temperature, and the output voltage of the integration circuit 25 changes with temperature. As a result, it is possible to compensate for the amplitude fluctuation of the modulation monitor signal due to the temperature change, and it is possible to compensate the temperature characteristics of the optical output of the LD1.

FIG. 2 is a graph showing an example of the temperature characteristic of the modulation amplitude current of the LD1 due to the use of the temperature detection circuit 26 in FIG.

As can be seen from this figure, LD1 depends on the temperature.
The modulation output current of LD1 changes, the optical output of LD1 changes, the monitor current of PD2 changes accordingly, and the APC loop controls the bias current. In this case, it is desirable that the temperature change of the modulation amplitude current corresponds to the temperature change of the optical output of LD1. However, by adjusting the gain of the integration circuit 25, it is possible to absorb the mismatch between the temperature fluctuation of the LD1 and the temperature fluctuation of the temperature compensation circuit including the temperature detection circuit 26.

According to the optical transmission module of the first embodiment, the response of the PD 2 monitoring the signal light of the LD 1 is averaged over time by the capacitor 14. As a result, fluctuations in the input waveform of the integration circuit 24 at the next stage can be reduced, and output fluctuations of the circuit can be reduced. As a result, the fluctuation of the bias current of the LD1 output from the laser driving IC 23 can be reduced, and a good eye waveform can be obtained as the waveform of the signal reproduced on the optical communication receiving side.

Further, the modulation current supplied to LD1 can be given a temperature characteristic by the temperature detection circuit 26. As a result, the influence of the fluctuation of the threshold current and the optical output due to the temperature characteristic of LD1 can be reduced. it can. As a result, it is possible to prevent deterioration of the transmission characteristics due to deterioration of the extinction ratio and pattern jitter, and to obtain good transmission characteristics. The above effects are temporally averaged by using the integration circuit 25, so that unintended fluctuations can be suppressed, and more stable control can be performed.

Although the integrating circuit 24 has the resistor 11 connected in parallel to the capacitor 16, the resistor 11 is omitted, and the integrating circuit has a complete integration characteristic in which the integrated output is linearly proportional to the input range. It is desirable from a characteristic point to change to use. Similarly, the integrating circuit 25 includes a resistor
Although 10 is connected in parallel, it is desirable in terms of characteristics to omit the resistor 10 and change to an integration circuit having complete integration characteristics.

Further, since each of the integrating circuits 24 and 25 is configured using the operational amplifiers 20 and 19 whose voltage output can be changed to the power supply voltage, there is no need to pull up or pull down the voltage output. Therefore, there is no need to provide a redundant circuit, and the reliability is increased. Further, the amplitude range of the integrated output can be widened, and the entire control input voltage range of the laser drive IC 23 can be used. The PD1 and the laser drive are operated by the buffer function of the operational amplifier circuits 20 and 19.
It is possible to separate the ICs 23 and suppress the influence of noise generated from each other.

Further, since the temperature detecting circuit 26 uses the thermistor 18 having a negative temperature characteristic, the associated circuit can have a simple configuration of, for example, only two resistors 7 and 8.
Also, by optimizing the constants of resistors 7 and 8, the LD
It is possible to supply a modulation amplitude current suitable for the temperature characteristic of 1.

<Second Embodiment> In the first embodiment, the laser driving IC 23 is a high-speed type IC using a GaAs substrate and usually does not include an APC circuit. However, a relatively high-speed laser driving IC that can be realized at low cost using an Si substrate may be used, and an example thereof will be described in a second embodiment.

FIG. 3 shows a circuit configuration of the optical transmission module according to the second embodiment of the present invention.

The optical transmission module shown in FIG. 3 is different from the optical transmission module described with reference to FIG.
Is omitted and the output of the temperature detection circuit 26 is directly connected to the modulation current control terminal 104 of the LD drive IC 27,
Others (such as bias current control) are the same,
The same reference numerals as in the figure are used.

The modulation current monitor terminal 105 of the LD drive IC 27 is
In many cases, integration is impossible only by converting current to voltage, and a capacitor may be provided outside the LD drive IC 27 to perform this integration. However, since the gains of the bias current control signal and the modulation current control signal are adjusted inside the LD drive IC 27, it is impossible to adjust the gain of the modulation current monitor signal. Therefore, an APC circuit may be provided outside the LD drive IC 27 to control the bias current. However, an APC circuit in which the APC circuit is built in the LD drive IC 27 can also be used.

The modulation current control terminal 104 of the LD drive IC 27 is constituted by a current mirror, and the modulation amplitude is controlled by the current of the modulation current control terminal 104. Modulation amplitude control terminal
104 is connected to the temperature detection circuit 26, and when the current flowing through the temperature detection circuit 26 changes according to the temperature, the modulation amplitude current also changes. Desirably, this change in current matches the temperature characteristics of LD1, as in the first embodiment. As a result, similarly to the first embodiment, temperature fluctuations in the extinction ratio and the optical output can be minimized, and good transmission characteristics can be obtained.

[0056]

As described above, according to the optical transmission module of the present invention, the extinction ratio of the laser diode is degraded due to the environmental temperature, and the transmission characteristics are degraded, or the pattern jitter is increased to degrade the transmission characteristics. This can prevent the disturbance of the eye waveform of the reproduction signal on the optical communication receiving side due to the fluctuation of the bias current of the laser diode.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an optical transmission module according to a first embodiment of the present invention.

FIG. 2 is a graph showing an example of a temperature characteristic of a modulation amplitude current of a laser diode when the temperature detection circuit in FIG. 1 is used.

FIG. 3 is a circuit diagram showing an optical transmission module according to a second embodiment of the present invention.

FIG. 4 is a characteristic diagram showing general current-optical output characteristics using the ambient temperature of a laser diode for optical communication as a parameter.

FIG. 5 is a circuit diagram showing a configuration example of a conventional optical transmission module.

6 is a characteristic diagram showing an input voltage versus an output current of the laser driving IC in FIG.

[Description of Signs] 1… Laser diode for optical communication, 2… Light receiving element (PD for monitoring), 3… 12… Resistor 13… 16 Capacitor 17… Variable resistor 18… Thermistor 19… Second operation Amplifier 20 first operational amplifier 21 constant voltage circuit 22 inductor 23 laser driving IC 24 first integration circuit 25 second integration circuit 26 temperature detection circuit 101 modulation current output terminal 102 bias current Output terminal 103: Bias current control terminal 104: Modulation current control terminal 105: Modulation current monitor terminal 106: Modulation signal input terminal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/06

Claims (5)

[Claims] 1. A semiconductor laser diode for optical communication, a light receiving element for receiving monitor light of the semiconductor laser diode and converting the monitor light into an electric signal, receiving a modulation signal input, and supplying a bias current and a modulation amplitude to the semiconductor laser diode A laser driving circuit for supplying a current, a current-voltage conversion circuit for converting a monitor current of the light receiving element into a monitor voltage, and a capacitor for averaging a response of the monitor voltage obtained by the current-voltage conversion circuit. A first operational amplifier to which a voltage averaged by the capacitor and a predetermined reference voltage are inputted, a first voltage for controlling a bias current of the semiconductor laser diode being generated and supplied to the laser drive circuit; An integration circuit; and a temperature detection circuit that detects an environmental temperature and outputs a voltage corresponding to the detection result. Optical transmission module and controls the laser driving circuit to the temperature compensation of the modulation current of the semiconductor laser diode with an output voltage of the detection circuit. 2. The optical transmission module according to claim 1, wherein the first operational amplifier uses an operational amplifier whose voltage output can be changed up to a power supply voltage. 3. A control for temperature-compensating the modulation current of the semiconductor laser diode, comprising a second operational amplifier to which an output voltage of the temperature detection circuit is used as a reference input and a monitor voltage of a modulation current of the semiconductor laser diode is input. The optical transmission module according to claim 1, further comprising a second integration circuit that generates a voltage and supplies the voltage to the laser driving circuit. 4. The optical transmission module according to claim 3, wherein the second operational amplifier uses an operational amplifier whose voltage output can be changed up to a power supply voltage. 5. The optical transmission module according to claim 1, wherein the temperature detection circuit includes a thermistor and a resistor.