JPH0626266B2 - Light emitting element drive - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is applied to an optical communication device. For example, it is used in an apparatus for optical-amplitude-modulating a baseband television signal and transmitting it to an optical fiber. In particular, the present invention relates to a driving device of a device that amplitude-modulates a light emission output intensity by an input signal.

[Conventional technology]

The output light intensity of the light emitting element is generally non-linear with respect to the magnitude of the drive current, and the output intensity does not increase proportionally as the drive current increases. Conventionally, in order to compensate for this, a drive circuit as shown in FIG. 5 has been used. This circuit amplifies the signal input to the input terminal Si by the transistor Q and sends the amplified signal to the output terminal So, so that the light emitting element ED
Is a circuit that drives a plurality of i diodes in parallel to the emitter circuit, and is configured to reversely compensate the nonlinearity of the light emitting element. That is, the input terminal Si
When the signal level of is extremely low, all the diodes D _{1 to} Di are in the conductive state, and the gain of the transistor Q is limited to a small value. However, when the signal level of the input terminal Si becomes high, the diodes are sequentially turned off. It is configured such that the transistor Q becomes conductive and the gain of the transistor Q gradually increases.

This circuit can compensate for the non-linearity to some extent, but it is necessary to individually adjust the characteristics of the light emitting element ED, and the number of manufacturing steps is large. Further, since it is not a closed loop control, there is a drawback that the stability is lacking.

FIG. 6 is another block diagram of the conventional example. In this example, the signal at the input terminal Si is amplified by the drive circuit DR and appears at the output terminal So, causing the light emitting element ED to emit light. A part of this light emission is received by the light receiving element PD, its signal is amplified by the amplifier AMP, and negative feedback connection is made to the input of the drive circuit DR.
Since this circuit is a closed loop control circuit as a whole, its operation is stable and absorbs the characteristic variations of the elements to a considerable extent, but it requires the light receiving element PD, and therefore the optical signal is branched. There are drawbacks such as the need for an optical circuit to operate and the mechanism of this optical circuit being more unstable than an electric circuit.

[Problems to be solved by the invention]

An object of the present invention is to improve the above and to provide a driving device which is stable in operation and which can compensate the non-linearity of a light emitting element having a simple mechanism.

[Means for solving problems]

The present invention amplifies the signal at the input terminal and sends it to the output terminal,
In a light emitting element drive device equipped with a drive circuit for driving a light emitting element connected to this output terminal with a substantially constant power source impedance, a voltage detection circuit with a high input impedance that inputs the voltage of the output terminal, and this voltage detection circuit And a circuit for negatively feedback coupling the output of the circuit to the input of the drive circuit.

[Action]

When a semiconductor is used for the light emitting element, the impedance of the light emitting element itself becomes non-linear in a region where the relation between the current and the light emission intensity becomes non-linear, and this effect appears in the terminal voltage. It is considered that this is because the amount of change in the terminal voltage of the semiconductor element is closely related to the carrier density in the active layer of the semiconductor. Therefore, when the light emitting element is driven by the driving circuit having a substantially constant power source impedance, the non-linear characteristic of the light emitting characteristic can be considerably improved by negatively feedback coupling the terminal voltage.

〔Example〕

FIG. 1 is a basic block diagram of the apparatus of the present invention. A signal given to the input terminal Si (a baseband television signal as an example) is amplified by the drive circuit DR and appears at the output terminal So. The output terminal n is connected to the light emitting element ED of a semiconductor laser. Here, the feature of the present invention resides in that the voltage detection circuit DET is connected to the output terminal So and the output thereof is negatively feedback-coupled to the input of the drive circuit DR. The input impedance of the voltage detection circuit DET is set sufficiently higher than the output impedance of the drive circuit DR and the impedance of the light emitting element ED so that the terminal voltage of the light emitting element ED can be detected without affecting its operation. Is configured.

In this circuit, the terminal voltage that varies according to the light emission characteristics of the light emitting element ED is negatively fed back to the input of the drive circuit DR to realize linear driving.

FIG. 2 is a concrete circuit diagram of this embodiment circuit. The drive circuit DR, the voltage detection circuit DET, and the light emitting element ED are indicated by broken lines in the figure. That is, the signal of the input terminal Si is amplified by the _two transistors Q ₁ and Q ₂ and supplied to the light emitting element ED from the output terminal So connected to the collector of the transistor Q ₂ . The voltage at the output terminal So is applied to the field effect transistor Q by the capacitor Co.
₃ is supplied to the gate of the field effect transistor Q ₃
The drain output of is feedback-connected to the input terminal Si.

With this configuration, the entire driving device can form a negative feedback loop to perform stable driving with good directness.

3 and 4 are diagrams showing the test results of the apparatus of this embodiment. FIG. 3 shows that the light emitting element ED is driven in a state (open loop state) in which the negative feedback loop of this device is disconnected by disconnecting the connection of the capacitor Co shown in FIG. FIG. 6 is a signal waveform diagram of a result of receiving light from a light receiving element and analyzing the frequency component with a spectrum analyzer. FIG. 4 shows the second circuit with the capacitor Co connected.
The signal waveform diagram of the result of having driven the light emitting element ED in the state of the Example of this invention shown in the figure (open loop state), and having analyzed the frequency component of the optical output signal with the spectrum analyzer is shown.

The signal at the input terminal Si is a 600 kHz sine wave. 3 and 4, the horizontal axis represents frequency (linear) and the vertical axis represents amplitude (relative value, dB). In FIGS. 3 and 4, fo is the same as the input signal, 600 kHz, and 2fo is the second harmonic of the input signal, 12
00kHz and 3fo indicate the third harmonic 1800kHz.

As can be seen from this figure, the second and third harmonics are observed when driven in the open loop state without using the negative feedback loop, but using the negative feedback loop improves linearity and You can see that the waves can no longer be observed.

In this example circuit, the total loop gain (μ
β) was about 16 dB, and the amount of emission distortion improvement in the closed loop state and the open loop state was about 14 dB. That is, in the device of the present invention, the light emitting element ED is outside the negative feedback loop, and although the non-linearity of the light emitting element itself is not improved by this negative feedback loop, this test shows that most of the light emission distortion is on the drive circuit side. It has been found that this can be improved by applying effective negative feedback to.

〔The invention's effect〕

As described above, according to the present invention, since the negative feedback loop absorbs the variation of the elements, individual adjustment is not necessary. Since the negative feedback loop is used, its operation is stable. Moreover, since the optical circuit is not included in the loop, the configuration is simple. The present invention is extremely effective when applied to a system for transmitting an analog television signal to an optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. FIG. 2 is a concrete circuit diagram of the device of the present invention. FIG. 3 and FIG. 4 are oscillographic waveform diagrams of signals showing the test results of the apparatus of the present invention. 5 and 6 are configuration diagrams of a conventional example device.

Claims (2)

[Claims] 1. A light emitting element drive device comprising a drive circuit for amplifying a signal from an input terminal, sending the amplified signal to an output terminal, and driving a light emitting element connected to the output terminal with a substantially constant power source impedance. 2. A light emitting element drive device comprising: a voltage detection circuit having a high input impedance, which receives the voltage of FIG. 3 as an input, and a circuit for negatively feedback coupling the output of the voltage detection circuit to the input of the drive circuit. 2. The light emitting element drive device according to claim 1, wherein the light emitting element is a semiconductor laser.