JPS59169239A - Optical modulating circuit - Google Patents

Abstract

PURPOSE:To stabilize and improve the optical output of a light emitting element for an optical modulating circuit containing a light emission output stabilizing function, by transmitting a part of the modulated signal through a delay circuit before supplying it to a difference signal detecting circuit. CONSTITUTION:A delay circuit 10 containing an optical connecting circuit consisting of a semiconductor laser 12 and a photodetecting element 13 is set on a signal route before a part of the modulated signal supplied from a terminal 1 is supplied to a difference signal detecting circuit 7. Therefore the output signal of the circuit 7 produces no undesired error signal due to a difference of delay time by applying a signal waveform showing the optical output of a semiconductor laser 4. Thus the optical output of the laser 4 can be extremely stabilized and improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical modulation circuit VC having a light emission output stabilizing function useful for modulating semiconductor light emitting devices and the like.

[Technical background of the invention and its problems 3 When a semiconductor light emitting device such as a light emitting diode or a semiconductor laser is used as a light source for pulse communication, the bias current value of the semiconductor light emitting device in the vicinity of the threshold current value, especially the star configuration, is usually Alternatively, even when a semiconductor laser is used as a light source in an optical network having a bus configuration, a bias current is normally applied to the semiconductor laser in a standby state.

Therefore, in such an original network, the pipe light generated by the bias current applied to the semiconductor lasers of multiple stations other than the one transmitting the signal is added up and increased in the optical network, and the light emitted from the pipe at the receiving station that receives the signal increases. This increased scattering noise (shot noise) and caused significant ψ deterioration.

In order to reduce this dielectric deterioration, it is preferable to use a zero-bias driving method in which a pulse current is directly applied without applying a bias current to the semiconductor laser.

However, in order to suppress the temperature dependence of the output of a semiconductor laser, a commonly used method has been used, namely, applying a bias current and detecting the bias light caused by this bias current, making it possible to control the optical output of the semiconductor laser. The method becomes ineffective in the zero-bias driving method because no bias current is applied.

For this reason, a high-speed output stabilization circuit (hereinafter abbreviated as FA8T-APC) has been devised, which is composed of elements that increase the speed of the stabilization loop for the optical output of a semiconductor laser.

This FAST-APC has a terminal (1
) A part of the input variable Hq No. 18 is input to the adder (2), and the output signal of this adder (2) is further input to the drive circuit (3), which causes the semiconductor laser (4) to fully emit light. There is.

In addition, a part of the optical output of the semiconductor laser (4) is received and detected by the photodetecting element (5), and this detection signal is passed through the amplifier (6), and then a difference signal is detected together with the other part of the modulation signal which is the original signal. Input to circuit (7).

Further, an error signal obtained by comparing a difference signal detection circuit (force output signal, that is, a detection signal and a modulation signal) is input to the addition circuit (2) and fed back to the semiconductor laser (4).

In principle, this circuit configuration allows semiconductor lasers (4
) can be made into a similar waveform that is faithful to the modulation signal.

However, as mentioned above, since the semiconductor laser (4) uses a zero-bias driving method in which no bias current is applied, the detection signal of the photodetector (5) is delayed relative to the modulation signal, which is a pulse signal. arise.

The quantitative expression regarding the delay of the optical output with respect to the input cabling signal of the semiconductor laser is well known and is given by p.

Here, T is the pulse delay time, Ts is the carrier lifetime in the semiconductor laser, p is the input pulse current value, and I
th is the threshold current value of the semiconductor laser.

In this case, for example, I th = 50 ma, T
If s=5 ns and 'I-0 mA, then T=9 ns, that is, in a semiconductor laser with a threshold current value of 50 m h, the pulse delay time is about 9 ns.

Furthermore, the photodetector also causes a slight delay, although not as much as the above-mentioned semiconductor laser.

Next, a description will be given with reference to FIG. 2 showing signal waveforms at each part of this FAsT-APC.

In FIG. 2, one point (■) of the feedback loop in FIG. 1 is opened. The signal waveforms (FIGS. 2(a) to 2(e)) of each part in a so-called open loop state are shown as time charts.

First, the modulated signal input from terminal (1) (Fig. 2 (a)
), the output signal of the drive circuit (3) (FIG. 2(b)) is transmitted with almost no delay and causes the semiconductor laser (4) to emit light.

However, the optical output of the semiconductor laser (4) is delayed by, for example, the above-mentioned pulse delay time α), and the detection signal of the photodetector (5) (Fig. 2 (C)) is delayed by the output signal of the drive circuit (3) ( Second
The waveform is delayed by T compared to the waveform shown in FIG.

Next, in the difference signal output circuit (7), FIGS. 2(a) and (
As shown in bJ, even if the output peak value of the semiconductor laser (4) is the same as the predetermined peak value to be set, the output signal (FIG. 2(d)) is a signal with a large error.

Since the output signal (Fig. 2(d)) having this error is applied to the adder (2), the output signal of this adder (2) will fluctuate unnecessarily (Fig. 2(e)). Laser (4)
The output stabilization was reduced.

Furthermore, since no bias current is applied, it is not possible to suppress the temperature dependence of the semiconductor laser output by bias light detection, and therefore the above-mentioned pulse delay time fluctuates and the output of the semiconductor laser (4) becomes unstable. This caused further reduction in chemotaxis.

[Purpose of the invention]

The present invention has been made in consideration of the above-mentioned problems, and its object is to provide an optical modulation circuit having a light emission output stabilizing loop suitable for zero-bias driving of a light emitting element.

[Summary of the invention]

The present invention is an optical modulation circuit having a light emission output stabilization loop that receives and detects a part of the light output of a light emitting element by a photodetector and feeds back this detection signal negatively to a drive section of the light emitting element,
A modulation signal for driving a light emitting element is delayed by a delay circuit section having an optical coupling circuit including a light emitting element and a photodetecting element having characteristics similar to those of the light emitting element and the photodetecting element,
Furthermore, the output signal and the detection signal of this delay circuit section are added to a difference signal detection circuit, the output signal of this difference signal detection circuit and the modulation signal are inputted to an addition circuit, and the output signal of this addition circuit is used to control the light emitting element. This is an optical modulation circuit configured to drive.

〔Effect of the invention〕

According to the present invention, it is possible to obtain an optical modulation circuit that is suitable for zero-bias driving of a light emitting element, compensates for the effects of delay characteristics and temperature characteristics of the light emitting element and the photodetecting element, and has a stable and good optical output of the light emitting element.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to Box 3 and FIG.

FIG. 3 is a configuration diagram showing an embodiment of the optical modulation circuit according to the present invention.

In FIG. 3, the same parts as those in the circuit shown in FIG. 1 are given the same reference numerals.

In this embodiment, in the circuit configuration diagram shown in FIG. 1, part of the signal of the modulation input from terminal (1) is the difference signal detection circuit (
7) On one word path before inputting to
) tl-provided. This delay circuit section (10)
The configuration consists of a drive circuit (u), a semiconductor laser (6) driven by the drive circuit (u), a photodetector (c) that detects the optical output of the semiconductor laser, and a photodetector (c) that amplifies the detected signal. The amplifier 0 → further comprises a waveform shaping circuit (to) that shapes the signal waveform that has passed through the amplifier αΦ.

Therefore, this delay circuit section 10) has an optical coupling circuit consisting of a semiconductor laser @ and a photodetecting element C13.

Figure 4 + a shows the signal waveforms of various parts in an open loop state where one point (0) of the feedback loop of such an optical modulation circuit is open.
) to (d).

That is, this means that the optical output of the semiconductor laser (4) for external output is detected by the photodetector (5) in response to the modulated signal (Fig. 4(a)).
Although the detected signal waveform (Fig. 4 (b)) is delayed, a part of the modulated signal (Fig. 4 (a)) is detected by the semiconductor laser (Fig. 4 (a)) using the semiconductor laser @ and the photodetector μ peak. 4
) is delayed in the same way as the output waveform (FIG. 4(b)) is delayed to obtain a delayed signal (FIG. 4(C)).

Therefore, the signal waveform (
When the delay signal (Fig. 4(b)) and the delayed signal (Fig. 4(c)) are applied to the difference signal detection circuit (force), the output code of this difference signal detection circuit (7) (Fig. 4(d)) is obtained. In this case, unnecessary error signals due to delay time differences are no longer generated, and this optical output stabilization loop can perform its original function.

Delaying a part of the above-mentioned modulation signal (Fig. 4(a)) in the same way as the output waveform of the semiconductor laser (4) (Fig. 4(b)) is delayed by delaying the delay circuit section tloJ. This can be realized by an optical coupling circuit composed of a semiconductor laser (6) and a photodetector aΦ having characteristics similar to those of the semiconductor laser (4) and the photodetector (5).

Furthermore, the temperature characteristics of the semiconductor laser 04 and the photodetection element (to) of the delay circuit unit U@ are the same as those of the semiconductor laser (4) and the photodetection element (5), so the semiconductor laser +4), (12 and photodetector t5)
, u3 The delay time changes due to the temperature of each optical coupling part of the semiconductor laser @ and the photodetection element (l
(2) Temperature compensation can be performed.

The delay circuit section JO) is mainly composed of an optical coupling circuit and a waveform shaping circuit (G), and this waveform shaping circuit Q5
In order to accurately delay only a part of the signal waveform of the modulated signal (Fig. 4(a)), the waveform or peak value of the output signal of the amplifier α→ in the delay circuit section (ICjI) is This is to make the waveform or peak value equal to that shown in FIG. 4(a).

As described above, the semiconductor laser (6) and the photodetector element q have the same characteristics as the semiconductor laser (4) and the photodetector element (5).
The influence of the delay characteristics and temperature characteristics of the semiconductor laser (4) and the photodetector element (5) can be compensated for by the circuit configuration having the delay circuit section (do) using a diode.

The optical output of the semiconductor laser (4) can be made stable and good.

The equation regarding the pulse delay time T of a semiconductor laser was explained earlier, and this pulse delay time T is as follows.

In detail, it also depends on the bias current Id when applied, and the relational expression is as follows.

That is, if the temperature characteristics of the semiconductor laser are constant, the pulse delay time is greatest at zero bias and becomes zero at the threshold current value.

Therefore, if the delay times of the semiconductor laser (4) and the photodetector (5) are slightly different from the delay time caused by the delay circuit section 110), or if a delay time difference occurs due to other factors, the bias current of the semiconductor laser in the delay circuit section By fine-tuning the delay time, the delay time can be adjusted and matched.

Furthermore, the output stabilization loop generally includes a band-limiting factor, and the input signal from the amplifier (6) to the difference signal detection circuit is not perfectly rectangular. On the other hand, the signal passing through the delay circuit unit uO) is almost not subject to band limitation, so that a nearly rectangular signal can be obtained.

Therefore, it is possible to compensate for such an imperfect rectangular signal in the output stabilization loop by inserting a band-limiting factor equivalent to that in the output stabilization loop, for example, in the waveform shaping circuit αυ. can.

This makes it possible to extremely accurately compensate for the effects of the delay characteristics and temperature characteristics of the semiconductor laser (4) and the photodetector (5), making the optical output of the semiconductor laser (4) extremely stable and good. be able to.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram showing a conventional output stabilization circuit, and Figure 2 (
a) to (e) are diagrams showing signal waveforms of each part of the output stabilization circuit shown in FIG. 1, FIG. 3 is a circuit configuration diagram showing an embodiment of the present invention, and FIG. 4 is shown in FIG. FIG. 3 is a diagram showing signal waveforms at various parts of the circuit.

Claims (1)

[Scope of Claims] (1) A driving section for driving a first light emitting element, a first light detecting element for receiving and detecting a part of the output light of the first light emitting element, and a delay circuit section that extracts a part of the modulation signal that drives the light emitting element and delays the extracted modulation signal; an output signal of the delay-tread section and the first
a difference signal detection circuit that detects a difference between the output signal of the photodetector and an adder circuit that adds the output signal of the difference signal detection circuit and the modulation signal and outputs the result to the drive section of the first light emitting element. an optical modulation circuit, characterized in that the delay circuit section has an optical coupling circuit comprising a second light emitting element and a second photodetecting element that receives and detects the output light of the second light emitting element. . (2) The second light emitting element and the second photodetecting element have substantially the same delay characteristics as the first light emitting element and the first photodetecting element, as set forth in claim 1. optical modulation circuit. (3j) The optical modulation circuit according to claim 1, wherein the first and second light emitting elements are semiconductor lasers. (4) The delay circuit section outputs the output from the second photodetecting element. 2. The optical modulation circuit according to claim 1, wherein the optical modulation circuit has a tPf characteristic that includes a circuit that shapes the waveform of a signal.