JPH05264934A - Optical modulator driving circuit - Google Patents

Abstract

PURPOSE:To reduce the power consumption by allowing most of currents for flowing through a transistor to flow to an optical modulator side. CONSTITUTION:A DC current path of an FET 1 is formed by connecting in series the FET 1 for executing power amplification of an input electric signal 101, a coil L11 of 100nH to a drain terminal of the FET 1 and a resistor R11 of 50OMEGA in which the other terminal is grounded. Also, to the drain terminal, a capacitor C11 of 0.1muF, an optical modulator 2 of a progressive form, a bias circuit 3 and a terminal resistance RL of 50OMEGA are cascaded, and the optical modulator 2 is driven. According to such a constitution, a low frequency current 102 of <=30kHz flows through the path of the coil L11 and the resistance R11, and a component of >=30kHz being a modulating signal flows through the optical modulator 2 side. When this driving circuit is operated at a transmission rate 10Gb/s, and current f is allowed to flow through the FET 1 in amplitude of 120mA, as for a high frequency component, that is, the modulating signal, its amplitude value becomes 110mA and becomes a value which can drive enough the optical modulator 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator drive circuit for an optical transmitter in an optical communication system.

[0002]

2. Description of the Related Art Recently, as an optical communication system for transmitting a large amount of information, an optical communication device having a transmission speed of a gigabit range has been developed. As an optical modulator of an optical communication unit used in such a large capacity optical communication system, a lithium niobate (niobium) that has a small spectrum spread even during modulation and is capable of long-distance transmission without being affected by dispersion of an optical fiber. There is a Mach-Zehnder type optical modulator using lithium oxide).

However, in the Mach-Zehnder type optical modulator using lithium niobate, since there is an operating point drift (DC drift) of the optical modulation characteristic with respect to the modulation voltage, a bias circuit for applying an appropriate DC bias to the optical modulator. Is required. As a driving circuit of such a Mach-Zehnder type optical modulator, a method of superposing a DC bias on an electric signal to be modulated has been proposed. For example, the circuit shown in FIG. (See Kuwata et al. “Study on automatic bias control circuit for Mach-Zehnder type optical modulator” 19)
90th IEICE Spring National Convention B-976).

In this drive circuit, a load resistor R31 is connected to the output side of the FET1 driven by an input electric signal 301, and an optical modulator 2 is connected in parallel to the load resistor R31. The modulated signal 303 is supplied to this. That is, the modulation signal 303 flows to the drive circuit of the optical modulator 2 through the DC blocking capacitor C31, but the DC bias voltage VB is applied from the bias circuit 3 from the opposite side. The current value of the modulation signal 303 is set by the load resistor RL. In this drive circuit, for example, a proper DC bias voltage is superimposed on an electric signal at a transmission rate of 2.5 Gb / s to obtain a good optical transmission waveform.

[0005]

As described above, the Mach-Zehnder type optical modulator can be driven in a good state by superimposing the DC voltage on the modulation signal by the bias circuit. However, in the above-mentioned method, since the capacitor for blocking the direct current component is used in the bias circuit, it is impossible to flow all the current flowing through the transistor of the drive circuit to the optical modulator.

That is, in the drive circuit of the optical modulator, the load resistance of the transistor is provided in the drive circuit, and
The C bias condition is stabilized, and the modulation signal is applied to the optical modulator through the bias circuit. In this case, the load seen from the transistor is a parallel circuit of the load resistance in the drive circuit and the load on the optical modulator side, so that the current flowing through the optical modulator is about 50% of the current flowing through the transistor. That is, there is a drawback that the power supplied to the optical modulator is only 50% of the power generated by the driving transistor and the power consumption increases.

An object of the present invention is that most of the current flowing through the transistor flows to the optical modulator side, and the power usage efficiency is almost 1%.
An object of the present invention is to provide an optical modulator driving circuit improved to 00%.

[0008]

The optical modulator driving circuit of the present invention comprises a transistor driven by a modulation input signal,
A coil for high frequency blocking connected to the output side of the transistor, a resistor for stabilizing the operating point of the transistor connected in series to the coil, and a parallel to the coil and the resistor. A low-frequency blocking capacitor connected to, an optical modulator connected in series to the capacitor, and a bias circuit for applying a DC bias to the optical modulator connected in series to the optical modulator, A load resistor for setting a drive current of the optical modulator connected in series to the bias circuit. Also,
A low frequency device and a high frequency device may be used instead of the coil and the capacitor, respectively.

[0009]

According to the present invention, the load of the transistor of the drive circuit has a frequency characteristic, the DC component necessary for stable operation of the transistor is passed to the coil or the low frequency pass filter side, and the high frequency signal as the modulation signal is fed to the optical modulator side. It is intended to improve the efficiency of the power flow to the drain.

[0010]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 shows a circuit diagram of a first embodiment of the present invention. Figure 1
In order to amplify the electric power of the input electric signal 101,
ET1, a coil L11 of 100 nH at the drain terminal of FET1 and a resistor R1 of 50Ω whose other end is grounded are connected in series to form a direct current path of FET1.
Further, a 0.1 μF capacitor C11, a progressive type optical modulator 2, a bias circuit 3 and a terminating resistor RL of 50Ω are connected in cascade to the drain terminal to drive the optical modulator 2.

With the above configuration, the low-frequency current 102 of 30 kHz or less flows along the path of the coil L1 and the resistor R1, and the component of the modulation signal 103 of 30 kHz or more flows through the optical modulator 2 side. This drive circuit is connected at a transmission speed of 10 Gb
When operated at / s and a current with an amplitude of 120 mA is passed through the FET 1, the high frequency component, that is, the modulation signal 103 has an amplitude value of 110 mA, which is a value capable of sufficiently driving the optical modulator 2.

Further, when input light is made incident on the optical modulator 2, it is possible to obtain an output having a good modulated waveform.

Although the first embodiment of the present invention has been described above, the active element used in the drive circuit is not limited to the FET, but a bipolar transistor may be used. Further, the circuit type of the drive circuit is not limited to the operation type, but may be a single end type. Further, the positions of the resistor R1 and the coil L1 may be exchanged.

FIG. 2 is a circuit diagram of the second embodiment of the present invention. 2, in order to supply the high-frequency component to the optical modulator 2 side more efficiently than in FIG. 1, the coil L11 in FIG.
Instead of, the low frequency pass filter (LPF) 4 is adopted. Here, the LPF 4 has a cutoff frequency of, for example, 3k.
A fifth-order low-frequency pass filter composed of Hz coils L23, L24, L25 and capacitors C23, C24. On the other hand, a high frequency pass filter (HPF) 5 is used instead of the capacitor 11, and as the HPF 5, for example, capacitors C25, 26, 27 having a cutoff frequency of 3 kHz are used.
And a coil L26, 27 to form a fifth-order high-frequency pass filter.

The second embodiment of the present invention has been described above. However, either one of LPF4 and HPF5 is adopted, and LPF4 and HPF5 are not limited to the five-stage configuration shown in the present embodiment, but the number of stages can be changed. The one that changed
A Butterworth type or a lag lead filter using a resistor may be adopted. Furthermore, the connection order of the HPF 5 connected to the cascode, the bias circuit 3 of the optical modulator 2, and the terminating resistor RL may be changed.

[0016]

As described above, according to the present invention,
Since it is possible to efficiently supply the modulation signal to the optical modulator, an extra load is not applied to the drive transistor, which has an effect of reducing power consumption.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional example.

[Explanation of symbols]

 1 FET 2 Optical modulator 3 Bias circuit 4 LPF 5 HPF 101,201 Input electric signal 102,202 Low frequency current 103,203 Modulation signal

Claims (2)

[Claims] 1. A transistor driven by an input electric signal, a high frequency blocking coil connected to the output side of the transistor, and an operating point of the transistor connected in series to the coil for stabilizing the operating point. A resistor, a low frequency blocking capacitor connected in parallel to the coil and the resistor, an optical modulator connected in series to the capacitor, and a serial connection to the optical modulator. An optical modulator comprising: a bias circuit for applying a DC bias to the optical modulator; and a load resistor connected in series with the bias circuit for setting a drive current of the optical modulator. Drive circuit. 2. The optical modulator driving circuit according to claim 1, wherein a low-frequency device and a high-frequency device are used instead of the coil and the capacitor, respectively.