JPH0238026B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a laser diode drive circuit using a transistor or the like, and particularly to a reduction in the occurrence of distortion due to nonlinearity of the light emission characteristics of the laser diode.

[Prior art]

As shown in the static characteristic diagram of FIG. 1, a laser diode oscillates when a bias current exceeding a predetermined threshold current is applied thereto, and its light emitting output power is proportional to the difference between the bias current and the threshold current. However, its linearity cannot be said to be sufficiently good, and nonlinear distortion is a problem in optical transmission by analog light intensity modulation using a laser diode. As a method to improve this distortion, a conventional method has been used to reduce distortion by detecting a part of the laser diode's light emission output with a light receiving element, amplifying the signal, and feeding it negative feedback to the laser diode. There is.

FIG. 2 is a wiring diagram showing an example of a conventional laser diode drive circuit. In the figure, 1 is a laser diode, 2 is a DC power supply that supplies power to this laser diode, etc., and 3 and 4 are laser diode 1.
5 is a modulation input terminal for applying a modulation input to the base of the transistor 3 through a resistor 6 and a capacitor 7; 8 is a modulation input terminal for applying a modulation input to the base of the transistor 3 through a resistor 6; This is a light-receiving element that receives light through the

In the laser diode drive circuit configured as described above, the laser diode 1 is supplied with a bias current equal to or higher than its threshold current from the transistor 4, and performs a light emitting operation according to a modulation signal input from the modulation input terminal 5. A part of the light emission output of the laser diode 1 is transmitted to and received by the light receiving element 8 via the optical waveguide 9.

The light receiving element 8 outputs a light receiving current proportional to the optical output signal sent out by the laser diode 1, and both its DC component and modulation signal component are input as control signals to a current amplifier made up of transistors 3 and 4. Since the control signal causes negative feedback, the nonlinearity of the laser diode 1 is compensated,
Modulation waveform distortion of the optical output signal is improved.

However, in the conventional laser diode drive circuit, since the modulation signal input from the modulation input terminal 5 is coupled by the capacitor 7, the magnitude of the DC component included in the modulation signal input is input to the laser diode 1. The peak value of the current fluctuates, and as a result, the modulation region of the laser diode fluctuates, causing the signal to become distorted in the non-linear portion near the threshold of the laser diode shown in FIG. 1, resulting in waveform distortion.

The above drawbacks are particularly problematic when transmitting baseband video signals. That is, due to changes in APL (Average Prcture Level) of the baseband video signal, the synchronization level in the case of a positive modulation method and the white level in the case of a negative modulation method are subjected to waveform distortion.

[Summary of the invention]

This invention was made to eliminate the drawbacks of the conventional circuits described above, and it reproduces the DC component of the modulated signal input, and uses a differential amplifier to DC-couple both the signal and the bias to the laser driver, thereby generating the modulated input signal. It is an object of the present invention to provide a laser diode drive circuit that can obtain constant low waveform distortion regardless of the state of the laser diode.

[Embodiments of the invention]

FIG. 3 is a wiring diagram showing one embodiment of the present invention, and 1 to 6 and 8 to 9 indicate the same or corresponding parts as the same reference numerals in FIG. 2. In the figure, 20 is a clamp circuit, 21 is a differential amplifier, and 22 is a bias setting terminal.

In the laser diode drive circuit configured as described above, the signal input to the modulation input terminal 5 is a baseband video signal and the modulation method is negative modulation.

In the embodiment shown in FIG. 3, the function of distortion compensation by negative feedback is the same as that of the circuit shown in FIG. 2, so a description thereof will be omitted, and only the waveform distortion improvement function by direct DC connection will be described here.

The baseband video signal input to the modulation input terminal 5 has its pedestal level fixed to DC by the clamp circuit 20, and is then connected to the inverting input terminal of the differential amplifier 21, and outputted to the resistor 6 as a negative modulation signal, and then the laser diode 1 is supplied to

On the other hand, the DC bias current that drives the laser diode 1 controls the positive phase input terminal of the differential amplifier 21 by setting the voltage of the bias setting terminal 22, and applies the DC bias voltage superimposed on the modulation signal to the resistor 6. Then, the Darlington-connected transistors 3 and 4 are driven to set the DC light emission level of the laser diode 1. FIG. 4 is a waveform diagram of each part of the drive circuit in FIG. 3. FIG. The modulated signal waveform inverted by the amplifier 21 and superimposed with a DC bias, FIG. 4c, is the modulated optical output waveform.

In the above laser diode drive circuit, the baseband video signal is within a constant range of the drive current-emission output characteristic (I-P curve) of the laser diode 1 (range d to c in Figure 4c) regardless of APL fluctuations. As a result, a low distortion laser diode drive circuit can be realized.

In the above embodiment, the modulation input signal is a baseband video signal and the modulation method is negative modulation. However, the same effect can be obtained even with a positive modulation method by changing the input signal of the differential amplifier 21 to the positive phase. . Furthermore, even if the modulation input signal is a digital signal, by converting the clamp circuit 20 into a pulse clamp circuit, a negative feedback laser diode drive circuit that controls the pulse peak light constant regardless of the pulse duty or mark rate can be created. can get.

〔Effect of the invention〕

As described above, the present invention has the advantage that by providing a clamp circuit and a bias setting terminal for a differential amplifier, the drive level of the laser diode can be kept constant at high speed and a laser diode output with low distortion waves can be obtained.

[Brief explanation of drawings]

Fig. 1 shows the static characteristics of a laser diode, Fig. 2 is a wiring diagram showing an example of a conventional laser diode drive circuit, Fig. 3 is a wiring diagram showing an example of the present invention, and Fig. 4 is the same as that shown in Fig. 3. FIG. 3 is a waveform diagram of each part of the laser diode drive circuit. In the figure, 1 is a laser diode, 3 and 4 are
NPN type transistor, 5 is a modulation input terminal, 6 is a resistor, 7 is a capacitor, 8 is a light receiving element, 9 is an optical waveguide, 20 is a clamp circuit, 21 is a differential amplifier,
22 is a bias setting terminal. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] 1 A Darlington circuit consisting of multiple stages of transistors for supplying the DC bias and modulation signal current of the laser diode from the collector of the final stage transistor, and a part of the output of the laser diode is received through an optical waveguide and its output is detected. In a laser diode drive circuit, the laser diode driving circuit is equipped with a photodiode that outputs a photodiode, and a negative feedback circuit that connects the output of the photodiode in negative feedback to the modulation signal input terminal of the first stage transistor base of the Darlington circuit. 1. A laser diode drive circuit comprising: a DC level fixing circuit that fixes a DC level; and a DC coupling circuit that superimposes a variable DC level on the output of the DC level fixing circuit.