JPH10173290A - Light transmission circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light transmission circuit which can be reduced in size and cost and can be mass-produced and can appropriately transmit a burst signal by comparing the peak value of the monitor signal of a photodiode with a reference voltage and, when the peak value reaches the reference voltage, adjusting a bias current. SOLUTION: A peak detecting circuit section 106 detects the peak value of monitor signals outputted from a preamplifier section 105 and supplies the detected peak value to an operational amplifier section 107. The amplifier section 107 compares the peak value of the monitor signals with a reference voltage supplied from a reference voltage source circuit section 108 and, when the peak value of the monitor signals exceeds the reference voltage, provides negative feedback so as to make the peak value smaller than the reference voltage by adjusting a bias current supplied from a laser diode bias current supplying circuit section 104 in accordance with the exceeding amount. Therefore, a light transmission circuit which can be reduced in size and cost can be obtained, because the light output waveform of a light transmission circuit can be stabilized when the circuit transmits a high-speed burst signal and the number of adjusting spots of the circuit can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission circuit used in, for example, an optical communication device, and more particularly to an optical transmission circuit suitable for high-speed burst transmission.

[0002]

2. Description of the Related Art In recent years, optical signals have been used for transmitting signals via communication lines and the like. For example, in an optical communication apparatus, an electric signal input from various devices such as a computer and a telephone is transmitted by an optical transmission circuit. The optical signal is converted into an optical signal and transmitted to an optical line.

FIG. 2 is a block diagram showing a schematic configuration of a first example of a conventional optical transmission circuit, and FIG. 3 shows a driving current vs. optical output characteristic of a laser diode of the optical transmission circuit shown in FIG. .

In general, a laser diode LD, which is a light emitting element (photoelectric conversion element), has a large temperature dependence in the current versus light output characteristics as shown in the drive current vs. light output temperature characteristics in FIG. I have.

Therefore, in order to stabilize the optical output power with respect to a change in the installation environment temperature of the optical transmission circuit, the backward light (optical output power) of the laser diode LD is built in the same LD module unit 15. Monitoring is performed by the photodiode PD. Then, in order to automatically vary the drive current of the laser diode LD according to the monitored optical output power, an I / V conversion circuit (current / voltage conversion circuit) 20
5. A negative feedback circuit including an operational amplifier (differential amplifier) 207 and a reference voltage source circuit 208 is provided. Such a circuit system is called an APC (Automatic Power Control) circuit because it stabilizes the optical output power.

FIG. 5 is a block diagram showing a schematic configuration of a second example of the conventional optical transmission circuit, and FIG. 6 shows a driving current versus optical output characteristic of the laser diode of the optical transmission circuit shown in FIG. .

In the second example, as compared with the first conventional example, negative feedback is performed on the bias current Ib, the pulse modulation current Ip is made independent of the negative feedback loop, and the differential quantum of the laser diode LD is changed. Modulation current temperature compensation circuit section 502 as a temperature compensation circuit that follows the temperature characteristic of efficiency
Is provided. The modulation current temperature compensating circuit 502 allows the ON / OFF ratio of the optical transmission waveform (hereinafter, referred to as the ON / OFF ratio)
The temperature change coefficient of the modulation current Ip is adjusted to the current vs. light output temperature characteristics of each laser diode element so as to obtain a sufficient extinction ratio.

In general, when the laser diode LD is driven by a modulation current that exceeds the oscillation threshold current Ith of the laser diode LD, a light emission delay occurs at the rising edge of the light output waveform. The first conventional example cannot be used, and the second conventional example is usually employed.

[0009]

However, when the form of a signal to be transmitted occurs intermittently, that is, in the case of a so-called burst signal, both the first and second prior arts described above employ a negative feedback circuit. From the point of stability, it is necessary to set a large loop time constant. Therefore, as shown in FIG. 7, there is a problem that a stable optical output waveform cannot be obtained during the period of the time constant.

[0010] As an example of the conventional optical transmission circuit to cope with such burst signal transmission, there is an optical transmission circuit having a circuit configuration shown in FIG.

The optical transmission circuit shown in FIG. 8 is a method for driving the laser diode LD without using the APC circuit described in the above-mentioned conventional example, and performs temperature compensation for each of the bias current Ib and the modulation current Ip. The circuit (modulation current temperature compensation circuit units 802 and 805) is provided. However, the temperature coefficient of the differential quantum efficiency of the laser diode LD and the temperature coefficient of the oscillation threshold current of the laser diode LD are independent parameters, and compared with the temperature coefficient of the differential quantum efficiency of the laser diode LD, Since the temperature coefficient of the oscillation threshold current of the laser diode LD is large and its variation is large, the configuration of the temperature compensation circuit for the bias current Ib is particularly large and complicated, and the number of adjustment points is increased and complicated. There is a problem in that it is not only difficult to make an IC, but also for miniaturization, low cost, and mass production of the optical transmission circuit.

The present invention has been made in view of the above points, and is suitable for miniaturization, low cost, mass production, and capable of appropriately transmitting a burst signal. The purpose is to provide.

[0013]

In order to solve such a problem, in the present invention, an optical transmission circuit is constituted by the following components.

That is, a laser diode that converts an input signal into an optical signal and outputs the signal, a photodiode that receives an optical output of the laser diode and outputs a monitoring signal,
A bias current supply unit that supplies a predetermined bias current to the laser diode, a temperature compensation unit that compensates for temperature characteristics of the laser diode, and a peak value of a monitoring signal of the photodiode and a reference voltage are compared, and the peak value is compared. And adjusting means for adjusting the bias current supplied by the bias current supply means when the voltage reaches the reference voltage.

With this configuration, the bias current of the bias current supply means is adjusted by the negative feedback circuit of the adjustment means,
The temperature characteristics of the laser diode are compensated by the temperature compensating means, and the light output waveform is stabilized. For example, even when a high-speed burst signal is transmitted, the optical output waveform can be stabilized.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the optical transmission circuit according to the present invention will be described in detail with reference to the drawings. Here, FIG.
Is a block diagram showing the configuration of this embodiment.

In FIG. 1, the optical transmission circuit 1 of this embodiment is also a component of the conventional circuit shown in FIG.
Largely, the APC laser diode drive circuit (A
A PC type LD drive circuit section) 13 and a laser diode module section (LD module section) 15;
The APC laser diode drive circuit section 13 includes a laser diode drive circuit section (LD drive circuit section) 101,
A modulation current temperature compensation circuit section 102 as temperature compensation means,
It has an adder 103, a laser diode bias current supply circuit (LD bias current supply circuit) 104 as bias current supply means, an operational amplifier 107 as reference current adjustment means, and a reference voltage source circuit 108.

In addition, the optical transmission circuit 1 of this embodiment has a D flip-flop circuit 11, a preamplifier unit 105 and a peak detection circuit unit 106 provided in the APC laser diode drive circuit unit 13.

The details will be described below. The optical transmission circuit 1 of this embodiment has a data input terminal 11 as an input terminal.
1 and a clock input terminal 112, and a data signal and a clock signal are input to the D flip-flop circuit 11 via these input terminals. The data signal delayed (latched) by one clock in the D flip-flop circuit 11 is input to the laser diode driving circuit 101 of the laser diode driving circuit 13 to be connected, and further added to the laser The laser diode bias current supplied from the diode bias current supply circuit unit 104 is added, and the added laser diode bias current is supplied to the laser diode module unit 15 to drive the laser diode LD.

At this time, the laser diode driving circuit 1
The modulation current Ip output from 01 is adjusted by the modulation current temperature compensation circuit unit 102 so that the temperature change coefficient is adjusted so that a sufficient extinction ratio of the optical transmission waveform is obtained. The output light of the laser diode LD is output to an optical line or the like via the data output terminal 121, and the rear light thereof is received by a photodiode PD for optical monitoring, and a monitoring signal (current signal)
Is output to the preamplifier unit 105.

The preamplifier unit 105 is a transimpedance type preamplifier that amplifies the light receiving current Im from the monitoring photodiode PD and converts the voltage into a voltage. The peak of the monitoring signal (voltage signal) output from the preamplifier unit 105 is detected by the peak detection circuit unit 106, and the peak value is supplied to the operational amplifier unit 107.
The operational amplifier 107 compares the peak value of the monitoring signal with the reference voltage (peak detection level Vref) supplied from the reference voltage source circuit 108. When the peak value of the monitoring signal exceeds the reference voltage, the laser diode bias current supply circuit 1
By adjusting the bias current supplied from 04, negative feedback is applied so that the peak value of the monitoring signal does not exceed the reference voltage.

The bias current supplied from the laser diode bias current supply circuit section 104 is set to a current value slightly exceeding a threshold value for lighting (ON) of the laser diode LD. It should be noted that the light emission amount of the laser diode LD at this time only needs to be a very small light amount that can be stably detected by the monitoring photodiode PD, and therefore is normally close to the threshold current at which the laser diode LD emits light. Although the current value is set, it is needless to say that the current value is not limited to this.

Next, the operation of the optical transmission circuit 1 of the embodiment having the configuration shown in FIG. 1 will be described with reference to FIG.

First, the operation when the input data is "0" will be described. Since the input data is “0”,
Only the bias current from the laser diode bias current supply circuit unit 104 is directly supplied to the laser diode LD via the addition unit 103. This bias current is near the threshold current of the laser diode LD, and the laser diode LD slightly emits light. The monitoring photodiode PD that has received the slight light emission outputs the light receiving current Im to the preamplifier unit 105 as a monitoring signal.
Thereafter, the pulse voltage is converted by the preamplifier unit 105, and the peak level is detected by the peak detection circuit unit 106. When the peak level exceeds the peak detection level Vref, the negative feedback of the APC circuit is performed by the output of the operational amplifier unit 107. It is activated to stabilize the output power when the optical signal output is "0".

Next, the operation when the input data is "1" will be described. Since the input data is "1",
The pulse modulation current Ip from the laser diode driving circuit unit 101 is supplied to the adding unit 103. This adder 103
The laser diode bias current supplied from the laser diode bias current supply circuit 104 is added to drive the laser diode LD. That is, when the input data is “1”, the pulse modulation current Ip from the laser diode drive circuit unit 101 is supplied to the laser diode LD independently of the negative feedback loop, so that the temperature of the differential quantum efficiency of the laser diode LD is increased. A temperature compensation circuit that follows the characteristics is formed.

With such a configuration, the loop time constant can be set sufficiently large, and the optical signal output becomes "0".
In this case, the output power is always stabilized. That is, the output of the optical signal is always “0” other than the burst period, the output power is stabilized even in the non-burst period, and the output power is also stabilized when returning to the burst period.

The laser diode bias current supply circuit 104 outputs a laser diode bias current alarm output terminal 113 when an abnormality such as an excessively small bias current obtained by the negative feedback operation occurs.
, A function of notifying a transmission data generation source (not shown) of the occurrence of the abnormality is provided.

With the configuration described above, this embodiment activates the negative feedback when the input data is "0" and performs only the temperature compensation by the temperature compensating means when the input data is "1". It is possible to realize an optical transmission circuit having the characteristics of stabilizing the optical output waveform at the time of transmitting a burst signal, reducing the number of adjustment points, and easily integrating the circuit.

The present invention is suitable for application to an optical transmission circuit to which input data in a burst form is applied. However, the present invention is applicable to an optical transmission circuit to which input data is applied continuously between the start and end of communication. Can of course be applied.

[0030]

As described above, according to the optical transmission circuit of the present invention, there is provided the negative feedback circuit by the adjustment means for adjusting the bias current of the bias current supply means, and the temperature characteristic of the laser diode by the temperature compensation means. , Which stabilizes the optical output waveform when transmitting a higher-speed burst signal, reduces the number of adjustment points, and is suitable for miniaturization, low cost, and mass production.
It is possible to realize an optical transmission circuit having a feature that the configuration is easy.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of a first embodiment.

FIG. 2 is a block diagram showing a schematic configuration of a first example of a conventional optical transmission circuit.

FIG. 3 is a diagram illustrating drive current versus light output characteristics in a laser diode of the optical transmission circuit illustrated in FIG. 2;

FIG. 4 is a diagram showing a drive current versus optical output temperature characteristic in a laser diode of the optical transmission circuit shown in FIG. 2;

FIG. 5 is a block diagram showing a schematic configuration of a second example of a conventional optical transmission circuit.

FIG. 6 is a diagram illustrating drive current versus optical output characteristics in a laser diode of the optical transmission circuit illustrated in FIG. 5;

FIG. 7 is a diagram showing a waveform response to burst transmission in the optical transmission circuits of the first and second conventional examples.

FIG. 8 is a block diagram showing a schematic configuration of a conventional optical transmission circuit for transmitting a burst signal.

9 is a diagram showing a drive current versus optical output characteristic of a laser diode of the optical transmission circuit shown in FIG.

FIG. 10 is a timing chart for explaining an operation of the optical transmission circuit shown in FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical transmission circuit, 11 ... Flip-flop circuit, 13 ...
APC laser diode drive circuit section, 15 laser diode module section, 101 laser diode drive circuit section, 102 modulation current temperature compensation circuit section, 103
Adder, 104: laser diode bias current supply circuit, 105: preamplifier, 106: peak detector, 107: operational amplifier, 108: reference voltage source circuit, 111: data input terminal, 112: clock input terminal, 113: Laser diode bias current alarm output terminal, 121: Data output terminal.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/06 10/152 10/142

Claims (4)

[Claims] 1. A laser diode for converting an input electric signal into an optical signal and outputting the signal, a photodiode for receiving an optical output of the laser diode and outputting a monitoring signal, and supplying a predetermined bias current to the laser diode. Bias current supply means, temperature compensation means for compensating for the temperature characteristics of the laser diode, and comparing the peak value of the monitoring signal of the photodiode with a reference voltage, and when the peak value reaches the reference voltage, the bias current supply means And an adjusting means for adjusting a bias current supplied by the optical transmission circuit. 2. The adjustment means adjusts a bias current to a value that causes a laser diode to emit light slightly when an input data signal is “0”.
2. The optical transmission circuit according to claim 1, wherein when the input data signal is "1", the temperature characteristic of the laser diode is compensated. 3. The optical transmission circuit according to claim 1, wherein a bias current value supplied by said bias current supply means is near a threshold current value of said laser diode. 4. The optical transmission circuit according to claim 1, further comprising an amplification unit that amplifies a monitoring signal output from the photodiode and supplies the monitoring signal to the adjustment unit.