JPH10209538A - Optical transmitter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical transmitter circuit wherein a stable output in a temperature changing state and a small distortion waveform are compatible with each other by generating a driving current capable of absorbing a wide- range LD individual deviation with a simple circuit. SOLUTION: A modulation current source and a bias current source are constructed of two or more independent current sources 31, 32 and 41, 42 which change by different functions depending on temperature respectively, and the sum of the currents from these current sources is permitted to be an output current. These four or more current sources are constructed of a simple circuit respectively, and are made adjustable so that necessary characteristics are realized and simple construction suitable for integration can be achieved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser diode (hereinafter referred to as an LD) in a transmission circuit for an optical communication system.
Abbreviation) relates to the circuit system of the drive circuit.

[0002]

2. Description of the Related Art FIG. 2 shows a relationship between an LD drive current and an LD light output current in a general LD using temperature as a parameter. In FIG. 2, Ith0, Ith25, and Ith80 denote oscillation threshold values at temperatures of 0 ° C., 25 ° C., and 80 ° C., respectively, and η0, η25, and η80 similarly denote luminous efficiencies (over the oscillation threshold value) at each temperature. Of the change in the optical output power with respect to the change in the LD drive current (corresponding to the slope in FIG. 2).

As shown in FIG. 2, as the temperature rises, the oscillation threshold value increases (Ith0 <Ith25 <Ith80), and the luminous efficiency decreases (η0>η25> η80).

On the other hand, as important performances of the optical transmission circuit, it is desired that the optical waveform and the optical output power are constant regardless of the temperature. It is known that this can be achieved by driving with a modulation current corresponding to a change in the luminous efficiency and a bias current according to a change in the oscillation threshold. Ith is approximately expressed by the functions shown in the following (Formula 1) and (Formula 2).

Η ≒ η0 ÷ exp {(T-T0) / T0｝ (Equation 1) Ith ≒ Ith0 × exp {(T-T0) / T0｝ (Equation 2) where T is temperature and η0 is Η and Ith0 when T = T0 represent Ith when T = T0.

Conventionally, as an optical transmission circuit, a system shown in FIG. 3 is generally used. That is, an input circuit 1 that receives an input voltage signal 91 and shapes the waveform, a differential current switch 2 that receives an output signal 92 of the input circuit 1 and generates a modulation current 93 for driving the LD 5, The modulation current source 3 adjusts and supplies the amplitude according to the characteristics of the LD, and the bias current source 4 adjusts and supplies the bias current 94 according to the characteristics of the LD. At this time, as shown in the following equation (3), the bias current 94
Is set to a value smaller than the oscillation threshold value, and the modulation current 93 is set as shown in the following equation (4), whereby the waveform distortion of the optical output signal can be minimized.

0.5 × Ith <IB <1.0 × Ith (Equation 3) IM + IB = (1 / η) + Ith (Equation 4) Here, IM represents a modulation current, and IB represents a bias current.

From the equations (1) to (4), in the conventional optical transmission circuit, the temperature dependence of the modulation current 93 and the bias current 94 is represented by temperature on the horizontal axis and LD drive current on the vertical axis. FIG. 4 shows a linear scale. As shown in FIG. 4, the temperature dependency has an exponential function characteristic.

On the other hand, since the oscillation threshold value and the luminous efficiency of a general LD greatly differ from each other, it is necessary to finely adjust both the modulation current and the bias current during mass production. Therefore, as shown in FIG. 4, in order to be able to change with an exponential function with respect to temperature and to make fine adjustments, it is conventionally configured by a device which cannot be integrated such as a thermistor or a complicated circuit such as an exponential amplifier. I was Regarding this type of circuit, for example, Japanese Patent Publication No. 59-3
No. 1996 (light output stabilization method).

[0010]

In the above-mentioned prior art, the modulation current and the bias current need to be exponential and temperature-dependent currents. Alternatively, since an element that cannot be integrated is required, there has been a problem that the circuit scale is increased and integration is difficult.

SUMMARY OF THE INVENTION An object of the present invention is to provide a drive circuit capable of absorbing a wide range of LD individual deviations with a simple circuit, and to achieve an integrated optical transmission that achieves both a stable optical output with respect to temperature fluctuations and a waveform with little distortion. It is to realize a circuit.

[0012]

In order to achieve the above object, according to the present invention, at least one of a modulation current source and a bias current source is provided with two or more independent functions each of which varies with a different function depending on temperature. And a sum of those currents as an output current. The above problem can be solved by configuring these current sources with simple circuits suitable for integration and making them adjustable.

According to a first aspect of the present invention, when only a modulation current source is constituted by two or more independent current sources that vary with different functions depending on temperature, a second aspect of the present invention includes only a bias current source. When configured in the same manner as described above, claim 3
The case where both the modulation current source and the bias current source are configured in the same manner as above is shown.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described with reference to FIG. In this embodiment, an input circuit 1 receives an input voltage signal 91 and shapes the waveform, and a modulation current 9 that receives an output signal 92 of the input circuit 1 and drives the LD 5.
3, a current source 31 for generating a stable current 311 (hereinafter abbreviated to IMSTB) with respect to a temperature change, and a current 321 (hereinafter, referred to as a current 321) which increases almost in proportion to a temperature change. A current source 32 for generating an IMDEV, and a current 411 (hereinafter referred to as I
A current source 41 that generates BSTB, and a current 421 (hereinafter, IBDE) that increases almost proportionally to a temperature change.
V (abbreviated as V). The current sources 31, 32, 41, and 42 are connected to the control voltage sources 312, 322, 412, 422 and the transistor 3 respectively.
13, 323, 413, 423 and adjustment resistors 314, 3
24, 414 and 424.

IMSTB 311, IMDEV 321, I
The BSTB 411 and the IBDEV 412 are connected as shown in FIG.
(B) is set to have a temperature dependency as shown in FIG.
Adjustment resistors 314, 324, 414, 42 according to D5
By setting 4, it is possible to generate a current having a temperature dependence with a characteristic close to an exponential function, and to realize an optical transmission circuit with stable and low distortion output. The control voltage sources 312 and 41
2 can share some or all of the circuits and further simplify the circuits. Similarly, the control voltage source 322
And 422 can also be shared.

Next, a second embodiment will be described with reference to FIG. In the present embodiment, an input circuit 1 for receiving an input voltage signal 91 and shaping a waveform, and an output signal 92 of the input circuit 1 are provided.
A differential current switch 2 that generates a modulation current 93 that drives the LD 5 in response to the current, a current source 31 that generates an IMSTB 311, a current source 32 that generates an IMDEV 321,
A photodiode 6 (hereinafter abbreviated as PD) for monitoring the optical output of the LD 5; an error amplifier 7 for receiving and converting the output current of the PD 6 into a control voltage signal; and a bias current for receiving the output of the error amplifier 7 And a current source 8 for generating 94. The current source 8 includes a transistor 83
And an adjustment resistor 84. In addition, the error amplifier 7
Performs negative feedback control on the bias current 94 so that the optical output of the LD 5 is always constant. By adopting such a configuration, the circuit scale is slightly larger than that of the first embodiment, but the effect is large, and a more stable and low distortion optical output can be obtained. In the present embodiment, the configuration in which the bias current is subjected to the negative feedback control has been described.
A similar effect can be obtained by a method in which the bias current source is constituted by a plurality of current sources.

In the above two embodiments, the current source is independent of the temperature change and the current source increases almost proportionally, and the sum of both currents is output. Even greater effects can be obtained by combining the functions with various functions such as a quadratic function, a logarithmic function, and an exponential function so as to have a temperature dependency and output the sum of the currents.

In the above two embodiments, each of the modulation current source and the bias current source is constituted by two current sources. However, even when only one of them is constituted by a single current source, the above embodiment is not limited to the above. An effect close to is obtained. If both or one of the current sources is composed of three or more current sources, the effects of the above-described embodiment can be obtained.

In the above two embodiments, the NPN transistor is used.
The same effect can be obtained even if the ESFET is used alone or in combination.

In the above two embodiments, the circuit for driving the LD has been described. However, the circuit has an exponential dependence on temperature, such as an electro-absorption type optical modulator and a waveguide type optical modulator. Similar effects can be obtained in various driving circuits for driving the elements.

As described above, the circuit of the present invention does not require a non-integratable element such as a thermistor, and can be realized with a simple circuit configuration. Therefore, it is suitable for integration.
It can be easily realized as C.

[0022]

According to the present invention, with a simple circuit, a drive current capable of absorbing a wide range of individual LD deviations is generated, and optical transmission stable with respect to temperature fluctuation and having a waveform with little distortion is achieved. A circuit can be realized, which is particularly effective when integrated.

[Brief description of the drawings]

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

FIG. 2 is a characteristic diagram illustrating a characteristic example of a general LD.

FIG. 3 is a circuit diagram showing an example of the related art.

FIG. 4 is a characteristic diagram showing a characteristic example of a general LD drive current.

FIG. 5 is a characteristic diagram showing characteristics of an LD drive modulation current and a bias current according to the present invention.

FIG. 6 is a circuit diagram showing a second embodiment of the present invention.

[Description of Signs] 1 ... Input circuit 2 ... Differential current switch 3 ... Modulation current source 4 ... Bias current source 5 ... Semiconductor laser diode 6 ... Photodiode 7 ... Error amplifier 8 ... Current source 31 for generating bias current 94 ... Current 311 (IMST) stable against temperature change
Current source 32 for generating B) current 32 that increases almost proportionally to a temperature change
Current source 41 for generating 1 (IMDEV) 41 Current 411 (IBST) stable against temperature change
A current source 42 for generating B): a current 42 that increases almost proportionally to a temperature change
Current source for generating 1 (IBDEV)

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/06

Claims (3)

[Claims] An input circuit for receiving an input voltage signal and shaping the waveform, a current switch for receiving a signal output from the input circuit to generate a modulation current for driving a semiconductor laser diode, and adjusting a modulation current amplitude of the current switch. A modulation current source that adjusts according to the characteristics of the semiconductor laser diode;
In a light transmitting circuit including a semiconductor laser diode driving circuit including a bias current source that adjusts and supplies a bias current in accordance with the characteristics of the semiconductor laser diode, wherein the modulation current sources each have a different function depending on temperature. An optical transmission circuit comprising two or more independent current sources that change, and using the sum of the currents as a modulation current output. 2. An input circuit for receiving an input voltage signal and shaping a waveform, a current switch for receiving a signal output from the input circuit to generate a modulation current for driving a semiconductor laser diode, and adjusting a modulation current amplitude of the current switch. A modulation current source that adjusts according to the characteristics of the semiconductor laser diode;
In an optical transmission circuit including a semiconductor laser diode driving circuit including a bias current source that adjusts and supplies a bias current in accordance with characteristics of the semiconductor laser diode, wherein the bias current sources each have a different function depending on temperature. An optical transmission circuit comprising two or more independent current sources that change, and using the sum of those currents as a bias current output. 3. An input circuit for receiving an input voltage signal and shaping the waveform, a current switch for receiving a signal output from the input circuit to generate a modulation current for driving a semiconductor laser diode, and adjusting a modulation current amplitude of the current switch. A modulation current source that adjusts according to the characteristics of the semiconductor laser diode;
In an optical transmission circuit including a semiconductor laser diode driving circuit including a bias current source that adjusts and supplies a bias current according to characteristics of the semiconductor laser diode, both the modulation current source and the bias current source are different from each other. It is composed of two or more independent current sources that vary depending on the temperature as a function, and the sum of the currents of the plurality of modulation current sources is used as a modulation current output, and the sum of the currents of the plurality of bias current sources is calculated. An optical transmission circuit characterized in that a bias current output is obtained by the following.