JPH05249416A - External modulation device - Google Patents

Abstract

PURPOSE:To obtain an external modulation device whose output light beam does not distort even when the peak voltage of a modulation signal exceeds the half wavelength voltage of a Mach-Zehnder type optical modulator. CONSTITUTION:In the external modulation device where a laser beam 1 outgoing from a light source is modulated in light intensity by being inputted into the Mach-Zehnder type optical modulator applied with an electrical modulation signal 3, the modulation signal 3 is changed to the modulation signal 3' whose peak voltage does not exceed the half wavelength voltage of the Mach-Zehnder type optical modulator 2 by adding the modulation signal 3 to the Mach-Zehnder type optical modulator 2 through a voltage compressor 5 compressing the peak voltage so as not to exceed the half wavelength voltage of the Mach-Zehnder type optical modulator 2 and thus, the laser beam 4' without distortion is outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external modulator using a Mach-Zehnder type optical modulator.

[0002]

2. Description of the Related Art Conventionally, an external modulator using a Mach-Zehnder type optical modulator, which is used by being incorporated in an optical transmitter of an optical fiber transmission system, is known.

FIG. 2 shows an example of a conventional external modulator of this type, in which a laser beam 1 emitted from a light source (not shown) is input to a Mach-Zehnder type optical modulator 2.
An electric modulation signal, for example, a frequency-division-multiplexed modulation signal 3 is applied to the Mach-Zehnder interferometer type optical modulator 2, and its optical loss is changed to output a laser beam 4 to which optical intensity modulation is applied. Was there.

FIG. 3 shows the relationship between the modulated signal and the output light in the apparatus of FIG. 2, which is a Mach-Zehnder type optical modulator 2.
It is shown that the laser light 4 having the light intensity corresponding to the voltage value of the modulation signal 3 is output according to the curve A showing the applied voltage vs. light loss, that is, the output light intensity characteristic.

[0005]

By the way, the above curve A
Shows a substantially linear characteristic within a predetermined voltage range (hereinafter referred to as a half-wave voltage of a Mach-Zehnder type optical modulator), but exhibits a reverse characteristic when exceeding this range. Therefore, in the above device, when the peak voltage of the modulation signal 3 exceeds the half-wave voltage of the Mach-Zehnder type optical modulator 2 as shown at points 3a and 3b in FIG. 3, the output waveform of the laser light 4 is shown in FIG. There was a problem of distortion as in points 4a and 4b.

In view of the above-mentioned conventional problems, it is an object of the present invention to provide an external modulator in which output light is not distorted even when the peak voltage of a modulation signal exceeds the half wavelength voltage of a Mach-Zehnder optical modulator. .

[0007]

In the present invention, in order to achieve the above-mentioned object, a laser beam emitted from a light source is input to a Mach-Zehnder type optical modulator to which an electric modulation signal is added to modulate the light intensity. In the external modulator to
We propose an external modulator that applies a modulation signal to a Mach-Zehnder type optical modulator via a voltage compressor that compresses the peak voltage so that it does not exceed the half-wave voltage of the Mach-Zehnder type optical modulator.

[0008]

According to the present invention, the peak voltage of the modulation signal is compressed by the voltage compressor so as not to exceed the half-wavelength voltage of the Mach-Zehnder type optical modulator. The intensity-modulated laser light is output.

[0009]

1 shows an embodiment of an external modulator of the present invention. In the figure, the same components as those of the conventional example are represented by the same reference numerals. That is, 1, 4'denotes a laser beam, 2 a Mach-Zehnder type optical modulator, 3, 3'a modulated signal, 5 a voltage compressor.

In the above structure, the laser light 1 emitted from the light source (not shown) is input to the Mach-Zehnder type optical modulator 2 as in the case of the conventional example.
For example, the frequency-division-multiplexed modulated signal 3 is input to the voltage compressor 5.

The voltage compressor 5 has a peak voltage that does not exceed the half-wavelength voltage of the Mach-Zehnder interferometer type optical modulator 2 with respect to an input electric signal, and that the waveform near the peak voltage changes gently. Compress and output.

FIG. 4 shows the relationship between the input signal and the output signal of the voltage compressor 5. According to the curve B showing the input voltage-output voltage characteristic of the voltage compressor 5, the Mach-Zehnder type of the modulation signal 3 is shown. Points exceeding the half-wave voltage of the optical modulator 2, for example, points 3a and 3b are respectively point 3a '.
And 3b ', the compressed modulation signal 3'is output.

The modulated signal 3'output from the voltage compressor 5 is applied to the Mach-Zehnder type optical modulator 2, and its optical loss is changed to modulate the optical intensity of the laser beam 1 to produce a laser beam 4 '. Output.

FIG. 5 shows the relationship between the modulated signal and the output light in the apparatus of FIG. 1, which is a Mach-Zehnder type optical modulator 2
According to the curve A indicating the applied voltage vs. output light intensity characteristic in, the laser light 4 ′ having the light intensity corresponding to the voltage value of the modulation signal 3 ′ obtained by compressing the modulation signal 3 is output, and the modulation is performed. It can be seen that no distortion occurs in the points 4a 'and 4b' of the laser beam 4'corresponding to the points 3a and 3b of the signal 3 that exceed the half-wavelength voltage of the Mach-Zehnder optical modulator 2, respectively.

[0015]

As described above, according to the present invention, the laser light emitted from the light source is input to the Mach-Zehnder type optical modulator to which an electrical modulation signal is added to modulate the light intensity. In the device, the modulation signal is added to the Mach-Zehnder type optical modulator through a voltage compressor that compresses the peak voltage so that it does not exceed the half-wave voltage of the Mach-Zehnder type optical modulator. Is applied with a modulation signal whose peak voltage does not exceed its half-wavelength voltage, whereby a laser light whose intensity is modulated without distortion can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an external modulator of the present invention.

FIG. 2 is a block diagram showing an example of a conventional external modulator.

3 is an explanatory diagram showing a relationship between a modulated signal and output light in the apparatus of FIG.

FIG. 4 is an explanatory diagram showing a relationship between an input signal and an output signal in the voltage compressor in FIG.

5 is an explanatory diagram showing a relationship between a modulated signal and output light in the device of FIG.

[Explanation of symbols]

1 ... Laser light, 2 ... Mach-Zehnder type optical modulator, 3, 3
′ ... Modulation signal, 5 ... Voltage compressor, 4 ′ ... Laser light whose light intensity is modulated.

Claims (1)

[Claims] 1. An external modulator for modulating light intensity by inputting a laser beam emitted from a light source into a Mach-Zehnder type optical modulator to which an electric modulation signal is added, wherein a peak voltage is a Mach-Zehnder type optical modulator. An external modulation device characterized in that a modulation signal is applied to a Mach-Zehnder type optical modulator through a voltage compressor that compresses the half-wave voltage so as not to exceed the half-wave voltage.