JPH02168746A - High frequency superimposition type light emitting drive circuit - Google Patents

Abstract

PURPOSE:To make the circuit scale small by devising an error amplifier such that positive feedback is applied for high frequency self-oscillation when an optical output of a semiconductor laser is at a prescribed level or over and negative feedback is applied when the optical output of the semiconductor laser is less than a prescribed level. CONSTITUTION:When an input pulse signal is at an H level, an error amplifier 10 is in positive feedback operation and oscillated in itself at a prescribed frequency, resulting that a high frequency signal is used as a drive signal and fed to a laser diode 4, from which a high frequency optical pulse signal is outputted. On the other hand, when the input pulse signal is at an L level, a reference current Iin is decreased, then a drive current of the laser diode 4 decreases below the threshold level Ith. Thus, the error amplifier 10 stops self-oscillation, resulting that the high frequency optical output is not generated from the laser diode 4. Thus, a high frequency oscillator, an amplitude modulator and a current adder are not required, the circuit constitution is simplified more and the size is made small.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a high frequency superimposition type light emission drive circuit used when driving a semiconductor laser to emit light by a high frequency superposition method in, for example, an advance device. Regarding.

(Prior Art) When a video signal is directly intensity-modulated and optically transmitted through a multimode optical fiber, modal noise is generated on the transmission path, leading to S/N deterioration of the video signal. on the other hand,
Even in digital transmission (pulse transmission), when optical transmission is performed by connecting a multimode optical fiber and a single moto optical fiber using existing optical fibers, significant spatial filtering occurs, so modal There are cases where the noise increases and significantly degrades the bit error rate.

Therefore, conventionally, a high frequency superimposition method, for example, has been adopted as a drive method to reduce the generation of modal noise. Fifth
The figure shows an example of the configuration of a light emitting drive circuit to which this type of system is applied (Japanese Patent Application No. 58243579). In this circuit, an input pulse signal a is smoothed by a smoothing circuit 1 and then introduced into an error amplifier 2. At the same time, the optical output level of a laser diode 4 is monitored and detected by a monitoring light receiving element '3 made of a photodiode, for example. After the signal is smoothed by a smoothing circuit 5, it is introduced into an error amplifier 2. Then, the error amplifier 2 detects the difference in current value between the human pulse signal and the monitor detection signal, and this difference current C
is fed back to the laser diode 4 as a bias current. That is, this causes the laser diode 4
The APC (Auto Power
r Control) Perform the operation. The light emission drive circuit also includes an oscillator 6 and an amplitude modulator 7. The amplitude modulator 7 generates an oscillator 6.
The high frequency oscillation output of 1.1 is amplitude-modulated using the input pulse signal Is, and the modulated high frequency signal d is superimposed on the bias current C by the current adder 8 and is applied to the drives 154 e and 1. The laser diode 4 is then supplied to the laser diode 4. Therefore, the laser diode 4 generates a high-frequency optical output as indicated by f in the figure in response to the high frequency of the drive signal e. By generating the high frequency optical signal f from the laser diode 4 in this manner, the coherency of the laser diode 4 is low (11), thereby reducing the generation of modal noise on the transmission path. Furthermore, since the high frequency optical signal described in Section 2 is subject to the 8) range restriction of the transmission line and the band restriction of the optical receiver, after receiving it! [;, it is equalized and becomes the original pulse waveform that does not contain high frequency components.

(Problems to be Solved by the Invention) However, such a conventional high-frequency superimposed group light emission drive circuit requires an oscillator 6, an amplitude modulator 7, and a current adder 8 in order to generate and superimpose a high-frequency signal. . For this reason,
The disadvantage is that the configuration becomes complicated and the circuit size becomes large.

The present invention focuses on this point and provides a high-frequency superimposition group light emission drive circuit that eliminates the need for an oscillator, amplitude modulator, etc. for performing high-frequency superposition, thereby simplifying the circuit configuration and reducing the circuit scale. The purpose is to

``Structure J of the Invention (Means for Solving the Problems) The present invention provides a high frequency superimposed group light emitting drive circuit that drives a semiconductor laser to emit light using a drive signal obtained by superimposing a high frequency signal on an input signal. A reference current generation circuit that generates a reference current, a light receiving element that monitors and detects a part of the optical output of the semiconductor laser, and a current difference between the reference current and the monitor current of the light receiving element r that is spread to the semiconductor laser. 1) An error amplifier that performs frequency feedback, and when the optical output of the semiconductor laser described in 1. It is configured to perform negative feedback operation when the optical output level is less than a certain value.

(Function) As a result, according to the present invention, a high frequency drive signal is generated from the error amplifier itself in accordance with the optical output level of the semiconductor laser, and the semiconductor laser is driven to emit light by this signal. Therefore, a circuit for generating a high-frequency drive signal such as a high-frequency oscillator or an amplitude modulator is not required, which allows the circuit configuration to be simplified and the circuit scale to be reduced accordingly.

(Embodiment) FIG. 1 is a schematic configuration diagram of a high frequency superimposed group light emission drive circuit in an embodiment of the present invention. In addition, in the same figure, the fifth
The same parts as those in the figure are given the same reference numerals.

This circuit consists of a light emitting module consisting of a laser diode 4 and a monitoring light receiving element 'J''-3, a reference current generating circuit 9 that generates a reference current Ijn proportional to the level of an input pulse signal a, and a broadband feedback amplifier. The error amplifier 10 is composed of:
The difference between the reference current 1in and the monitor current Im obtained by the monitor light receiving cable B is calculated, and the difference signal 1 ou
t is used as a drive signal and is supplied to the CC radar diode 4.

This error amplifier]0 is A"I out / I in It has a current amplification factor of 1. ing.

By the way, now the driving current Io of laser diodes 1 and 4 is
If the differential conversion efficiency of the monitor current rm with respect to ut is ηm, the drive current 10 and the monitor current MLzn are rout = A (in-Im) Im = η
m I out ・-(1) and Table 4
will be taken. I wanted to, 1011 years old 1 out = A-I
tn/(1+nm A) = -(2), and further ηm A>], which is 1out' = (1/η river) Ii mouth
...-(3) can be obtained.

Here, if co+ηmA>0, the error amplifier 10 enters a negative feedback operation state, and the current amplification factor in the open loop at this time I out /
The frequency characteristic of I in is, for example, curve 2 in FIG.

In addition, since a feedback amount of 20 log (]+nm A) is generated in the closed loop, the frequency characteristic of the current amplification factor is as shown by the broken line in FIG. In this case, since feedback is not applied up to the second ball, the circuit enters a negative feedback operation state. This operation is performed in the region of differential conversion efficiency η1, which is generally in the one-noser oscillation state, on the drive current versus first output characteristic of the one-nother diode shown in FIG.

On the other hand, for example, if we change the frequency characteristics of the current amplification factor and set it to the characteristics shown in the curve @ in Figure 2, feedback will be applied to the second ball, so the phase will be 180 degrees at the rotational frequency f . Oscillation occurs at In other words, the error amplifier 10 enters a self-oscillation state. Therefore, in this state, the high frequency signal e' is input from the error amplifier 0 and is supplied to the laser diode 4 as a drive signal. Therefore, the laser diode 4 emits a high frequency optical signal such as f' in FIG.

On the other hand, if the reference current l1nO value decreases in this state and is set to be below the threshold value of the laser diode 4,
The differential conversion efficiency of the laser diode 4 is η2 shown in Fig. 3.
become. For this reason, the frequency characteristic of the current amplification factor changes as shown in (■) in FIG. 2, and as a result, the error amplifier 10 stops its self-oscillation operation. Therefore, at this time, only the bias current is output from the error amplifier ]0, and thereby the laser diode F'4 stops its light emitting operation.

FIG. 4 shows a specific circuit configuration of the drive circuit shown in FIG. 1. That is, the reference current generation circuit 9 connects the emitter of the transistor Q1 to the power supply V2 via the resistor R1.
The input pulse signal a is input to the base of the transistor Q1, and the collector current is the reference current Ii.
It is output to the error amplifier 10 as n. error amplifier]
0 is the transistor Q2. Q3. Q4, resistance R2, R3
゜R+: A parallel circuit of a resistor R' and a capacitor Cr is connected as shown in the figure to constitute a parallel feedback type negative feedback amplifier, and the difference signal current between the reference current Iin and the monitor current Il'N is transferred to the transistor. Q2 and resistor Rf
It is converted into a voltage signal by

Then, this voltage signal is converted into a current signal by the transistor Q4 and its emitter resistor Rt=, and this current signal is supplied to the laser diode 4 as a drive current 1 out.

By the way, in order to set the open-loop frequency characteristic to the curve @ shown in FIG. 2 in this circuit, the following procedure may be performed. That is, the first pole of the frequency characteristic is determined by the input impedance and the capacitance of the capacitor CF, and the second pole is determined by the other circuits or the light receiving element 3 for monitoring.
Determined by the response speed of Therefore, in order to change the frequency characteristics of the circuit from ■ to @, the capacitance of the capacitor C'' may be reduced and adjusted as appropriate, for example.

With such a configuration, the error amplifier 1o will have open-loop characteristics as shown by the curve @ in FIG.

Therefore, when the input pulse signal is at the "H" level, the error amplifier 1o enters a positive feedback operation state and self-oscillates at a predetermined frequency. As a result, the high frequency signal or drive signal is supplied to the laser diode 4, and thereby The laser diode 4 ] 0 outputs a high frequency optical pulse signal. On the other hand, when the input pulse signal is at the "L" level, the reference current fin decreases, and the driving current of the laser diode 4 decreases to below the threshold value 11)+, so that the loop including the laser diode 4 decreases. The characteristics are as shown by curve (■) in FIG. Therefore, the error amplifier 10 stops self-sustained oscillation, and as a result, the laser guide 4 stops generating high-frequency optical output.

As described above, in this embodiment, the error amplifier 10 performs self-sustained oscillation on/off operation according to the logic level of the input pulse signal, and as a result, the laser guide 4 outputs a high frequency optical signal, which is different from the conventional method. The high frequency oscillator needed
Amplitude modulators and current adders can be eliminated,
Accordingly, the circuit configuration can be simplified and made smaller.

Incidentally, in the circuit of this embodiment, since a tuning circuit is not included in the feedback loop, the oscillation frequency is somewhat unstable. but,
Generally, the frequency required to reduce the coherency of a laser diode using the high frequency superimposition method may be approximately 200 MHz or higher. 1] For example, if the self-oscillation frequency fo of the error amplifier 10 is set to 500 MHz, even if this oscillation frequency fluctuates by about ±100 MHz, it will have little effect on the coherency reduction effect of the laser guide. However, it has no practical effect at all.

Note that the present invention is not limited to the above embodiments. For example, in the above embodiments, the case where a semiconductor laser is driven by a digital signal has been described as an example, but the invention can also be applied to a case where a semiconductor loop is driven with an analog signal or a case where a direct current is driven. Moreover, it may be applied not only to fiber-optic communication but also as a light source for accessing optical recording media such as video disks. If applied to such applications, the coherency of the semiconductor laser can be reduced, thereby reducing the influence of reflected light. In addition, various modifications may be made to the specific circuit configuration, semiconductor loop, type of monitoring light receiving element, use, etc., without departing from the gist of the present invention.

2 [Effects of the Invention] As detailed above, according to the present invention, in a high frequency superimposition type light emitting drive circuit that drives a semiconductor laser to emit light using a drive signal obtained by superimposing a high frequency signal on an input signal, a reference current generating circuit that generates a reference current proportional to , a light receiving element that monitors and detects a part of the optical output of the semiconductor laser, and a current difference between the reference current and the monitor current of the light receiving element that is applied to the semiconductor laser. The error amplifier is equipped with an error amplifier that performs broadband feedback, and when the optical output level of the semiconductor laser is above a certain level, the error amplifier performs positive feedback operation and performs high-frequency self-oscillation, and when the optical output level of the semiconductor laser is below a certain value, In some cases, by configuring it to operate with negative feedback, it is possible to eliminate the need for an oscillator or amplitude modulator for superimposing commercial waves, thereby simplifying the circuit configuration and reducing the circuit size. A superimposed light emission driving circuit can be provided.

[Brief explanation of the drawing]

Figure 1 is a circuit block diagram showing a schematic configuration of a high-frequency type in an embodiment of the present invention] 3. Figure 2 is a frequency characteristic diagram of a small current amplification factor in an error amplifier. A diagram showing an example of the optical output characteristics with respect to the driving current of a laser diode, Fig. 4 is a diagram showing a specific circuit configuration example of the circuit shown in Fig. 1, and Fig. 5 is a diagram showing a specific example of the circuit configuration of the circuit shown in Fig. 1. FIG. 2 is a block configuration diagram. 3... Light receiving element for monitor, 4... Laser guide, 9... Reference current generating circuit, ]0... Error amplifier.

Claims (1)

[Claims] A high frequency superimposition type light emitting drive circuit that drives a semiconductor laser to emit light using a drive signal obtained by superimposing a high frequency signal on an input signal includes a reference current generation circuit that generates a reference current proportional to the input signal level; A light receiving element that monitors and detects a portion of the light receiving element, and an error amplifier that feeds back a difference current between the reference current and the monitor current of the light receiving element to the semiconductor laser over a wide band, and the error amplifier is configured to maintain a constant optical output of the semiconductor laser. A high frequency superimposed light emitting drive circuit characterized in that when the optical output level of a semiconductor laser is above a certain level, positive feedback operation is performed to perform high frequency self-sustained oscillation, and when the optical output level of a semiconductor laser is less than a certain value, negative feedback operation is performed.