JP2518403B2 - Light modulation circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical modulation circuit in optical communication, and more particularly to an optical modulation circuit using a branching interference type optical modulator (hereinafter abbreviated as an external modulator). It is a thing.

[Conventional technology]

An example of a conventional optical modulator circuit using an external modulator will be described with reference to FIG.

In the figure, 21 is a semiconductor laser, 22 is a lens, 23 is an external modulator, 24 is a lens, 25 is an optical fiber, and 26 is a bias supply circuit that applies a bias voltage to a main signal input to the external modulator 23 by AC coupling. , 27 is the main signal source.
27 is connected to the external modulator 23 via the capacitor 28. Reference numeral 29 is a bias supply circuit connected to the semiconductor laser 21.

In the optical modulation circuit thus configured, the modulated main signal is AC-coupled and a certain bias voltage is applied.

[Problems to be Solved by the Invention]

In the optical modulation circuit using the conventional external modulator described above,
Since the modulation main signal is AC coupled and a constant bias is applied, the mark ratio of the input modulation main signal information fluctuates, and when the DC bias level changes, the modulation level of the external modulator fluctuates. There was a problem of being lost. The power of the semiconductor laser output light input to the external modulator is controlled to be constant by monitoring the backward light of the semiconductor laser. With this configuration, when the optical system axis shift between the semiconductor laser and the fiber occurs, the optical system axis shift information cannot be detected, and the optical output power coupled to the fiber is reduced.

[Means for solving the problem]

The optical modulation circuit of the present invention is provided with a lens in an input / output section, an external modulator in which a main signal source is AC-coupled, an optical fiber branching section coupled to the external modulator, and an optical fiber branching section. One of the photodiodes to which the optical output is input, a peak value detection circuit that detects the peak value of the photocurrent of this photodiode, and the output voltage of this peak value detection circuit are input and are AC-coupled to the external modulator. A first bias supply circuit for applying a bias voltage to an input main signal, a semiconductor laser, a second bias supply circuit connected to the semiconductor laser, and an APC control connected to the second bias supply circuit. A circuit is provided, and the optical output power information monitored by the photodiode is fed back to the APC control circuit.

[Work]

In the present invention, by monitoring the modulated light output waveform, the feed back circuit is configured so that its peak value is constant, and the bias supply circuit is controlled by the change in the DC component with respect to the change in the mark ratio of the input main signal. To compensate.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical modulation circuit according to the present invention.

In the figure, 1 is a semiconductor laser, 3 is an external modulator in which lenses 2 and 4 are provided in the input / output sections, and the main signal source 10 is AC-coupled, 5 is an optical fiber branch section coupled to the external modulator 3, 6 is a photodiode to which one optical output is input from the optical fiber branching unit 5, 7 is a resistor, 8 is a peak value detecting circuit for detecting the peak value of the photocurrent of the photodiode 6, and 9 is this peak value detecting circuit. A bias supply circuit for applying a bias voltage to a main signal to which the output voltage of 8 is input and which is input to the external modulator 3 by AC coupling, 11 is a capacitor,
12 is a bias supply circuit connected to the semiconductor laser 1;
Is an APC (Automatic connected to this bias supply circuit 12
power control) control circuit.

The APC control circuit 13 is configured to feed back the optical output power information monitored by the photodiode 6.

 The operation of the embodiment shown in FIG. 1 will be described below.

First, the semiconductor laser 1 emits light by the bias supply circuit 12, and the semiconductor laser output light is input to the external modulator 3. Further, the signal from the main signal source 10 is input to the external modulator 3 via the capacitor 11, and the light intensity is modulated by the modulation voltage. The optical signal output from the external modulator 3 is input to the optical fiber branching unit 5 via the lens 4. One of the output lights of the optical fiber branching unit 5 is sent to the transmission line and the other is input to the photodiode 6.
Here, a reverse bias is applied to the photodiode 6 and a resistor 7 is connected. Then, a pulse voltage corresponding to the optical pulse input to the photodiode 6 is obtained.

Next, this pulse voltage is input to the peak value detection circuit 8 and a DC voltage corresponding to the peak value of the input pulse voltage is output. Then, this output voltage is input to the bias supply circuit 9, DC / DC converted, and applied to the external modulator 3. The pulse voltage is input to the APC control circuit 13, and the bias current of the semiconductor laser 1 is controlled by the bias supply circuit 12 so that the output power of the semiconductor laser 1 is constantly compensated.

〔The invention's effect〕

As described above, according to the present invention, the feed back circuit is configured so that the peak value thereof becomes constant by monitoring the modulated light output waveform, and the change of the DC component with respect to the change of the mark ratio of the input main signal is supplied to the bias supply circuit. There is an effect that it is possible to compensate by controlling the pulse width and output a stable optical pulse. At the same time, there is an effect that it is possible to compensate for a decrease in output light power due to axial misalignment of the optical system from the semiconductor laser to the optical fiber branch.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical modulation circuit according to the present invention, and FIG. 2 is a block diagram showing an example of a conventional optical modulation circuit. 1 ... Semiconductor laser, 2 ... Lens, 3 ... Branch interference type optical modulator, 4 ... Lens, 5 ... Optical fiber branching unit, 6
...... Photo diode, 8 ...... Peak value detection circuit, 9 ...
… Bias supply circuit, 10 …… Main signal source, 12 …… Bias supply circuit, 13 …… APC control circuit.

Claims (1)

(57) [Claims] 1. A branch interference type optical modulator in which a main signal source is AC-coupled with a lens provided in an input / output section, an optical fiber branch section coupled to the branch interference type optical modulator, and this optical fiber branch. Section, a photo diode to which one optical output is input, a peak value detection circuit that detects the peak value of the photo current of the photo diode, and the branch interference type optical modulation circuit to which the output voltage of the peak value detection circuit is input. A first bias supply circuit that applies a bias voltage to a main signal that is input to the device by AC coupling, a semiconductor laser, a second bias supply circuit that is connected to the semiconductor laser, and a second bias supply circuit. The APC control circuit connected to the APC
An optical modulation circuit characterized in that the control circuit is configured to feed back the optical output power information monitored by the photodiode.