JPH03144417A - Light modulation circuit - Google Patents

Abstract

PURPOSE:To perform external modulation independently from a main signal, to simplify configuration, and to improve reliability by arranging an optical isolator in the optical path of the light emitted by a semiconductor laser, and varying a magnetic field to be applied to a Faraday rotor according to an analog sub signal centering a DC component required for the rotation of a deflecting plane of around 45 deg.. CONSTITUTION:The circuit is comprised in such a way that the optical isolator 13 also used as a light modulator is arranged in the optical path of the light emitted by the semiconductor laser 11, and the external modulation of a laser light quantity by the analog sub signal SS is performed by varying magnetic field to be applied to the Faraday rotor around a DC magnetic field according to the signal SS. Therefore, an internal modulation system by the main signal can be separated completely and independently from an external modulation system by the sub signal. Thereby, it is possible to simplify the configuration of a laser driving circuit 12 for direct modulation, and to stabilize the operation of the semiconductor laser 11, and also, the fault of the system of the sub signal does not affect the system of the main signal, thus improving the reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulation circuit installed in an optical communication system or the like.

(Prior Art) Optical modulation circuits mainly using semiconductor lasers are installed in optical communication systems. As shown in FIG. 3, a typical optical modulation circuit includes a semiconductor laser 21, a laser drive circuit 22, and an optical isolator 23. The configuration is such that the light is incident on F.

The laser drive circuit 22 changes the amount of light emitted by the semiconductor laser 21 by changing the drive current of the semiconductor laser 21 according to the level of the main digital signal supplied from the input terminal MS. The optical isolator 23 includes a polarizer 23c, a Faraday rotator 23a, an analyzer d, and a permanent magnet 23b that applies a magnetic field to the Faraday rotator. Semiconductor laser 21
After passing through the polarizer 23c, the light emitted from the analyzer 2 passes through the Faraday rotator 23a while rotating its plane of polarization by 45 degrees.
3d and enters the optical fiber F. Further, a low-frequency analog sub-signal used for monitoring and controlling the optical communication system is supplied from the input terminal SS to the laser drive circuit 22, and the emission scene of the semiconductor laser 21 is changed in accordance with this sub-signal.

(Problem 8 to be Solved by the Invention) The conventional optical transmission circuit shown in FIG. 3 has a configuration in which a digital main signal and an analog sub-signal are combined within the laser drive circuit 12. Therefore, the configuration of the laser drive circuit becomes complicated, and there is a possibility that the operation of the semiconductor laser becomes unstable.

Furthermore, if a failure occurs in the sub-signal system, this is likely to spread to the main signal system, resulting in reliability problems.

(Means for Solving the Problems) The optical modulation circuit of the present invention includes an optical isolator that also serves as an optical modulator installed in the optical path of emitted light from a semiconductor laser, and a magnetic field applied to a Faraday rotator of the optical isolator. By changing the direct current component necessary for rotation of the plane of polarization by approximately 45 degrees according to the analog sub-signal, the optical level is multiplex modulated by the sub-signal using an external modulation system independent of the main signal. has been done.

Hereinafter, the operation of the present invention will be explained in detail together with examples.

(Embodiment) FIG. 1 is a block diagram showing the configuration of an optical modulation circuit according to an embodiment of the present invention, in which 11 is a semiconductor laser, 12 is a laser drive circuit, 13 is an optical isolator that also serves as an optical modulator, and F
is an optical fiber.

The laser drive circuit 12 changes the amount of light emitted by changing the drive current of the semiconductor laser 11 according to the level of the digital main signal supplied from the input terminal MS.

The optical isolator 13, which also serves as an optical modulator, has a polarizer 13C.
and a Faraday rotator 13a made of a magneto-optic crystal.
It is composed of a coil 13b that applies a magnetic field to the Faraday rotator, an analyzer 13d, and a current supply circuit 13e that supplies current to the coil 13b. Current supply circuit 1
3d is a DC component for generating a DC magnetic field that rotates the polarization plane of the laser beam passing through the Faraday rotator 13a by 45 degrees, and a polarization plane that maximizes the polarization plane according to the amplitude of the sub signal supplied from the input terminal SS. An alternating current component for generating an alternating magnetic field for rotation by ±δ degrees is supplied to the coil 13b. The polarization plane of the analyzer 13d is (45
+δ) degrees. Therefore, the amount of laser light proportional to the amplitude of the sub-signal passes through the isolator 13 and enters the optical fiber F. When the reflected light generated at the input end or inside of the optical fiber F passes through the optical isolator 13 again, its plane of polarization is further rotated by 45 degrees in the same direction within the Faraday rotator 13a, so that the reflected light is reflected by the polarizer 13C. The light enters this with a plane of polarization rotated by (90±26> degrees) from the plane of polarization, and is blocked from passing through.

FIG. 2 is a block diagram showing the configuration of an optical modulation circuit according to another embodiment of the present invention.

Components in FIG. 2 denoted by the same reference numerals as in FIG. 1 are the same components as already explained with respect to FIG. 1, and redundant explanations of these components will be omitted.

In this embodiment, the permanent magnet 13f applies a direct current magnetic field necessary for the function of the isolake to the Faraday rotator 13a, and an alternating current magnetic field corresponding to the sub-signal necessary for the function of the modulator is applied to the coil 13b and the current supply circuit 13e. The configuration is such that the voltage is applied to the Faraday rotator 13a by.

Therefore, there is no need to supply direct current to generate a direct current magnetic field necessary for the function as an isolator, and the power consumption of the entire circuit is reduced.

(Effects of the Invention) As explained in detail above, the optical modulation circuit of the present invention includes an optical isolator that also serves as an optical modulator installed in the optical path of the emitted light of a semiconductor laser, and a magnetic field applied to the Faraday rotator. The configuration is such that external modulation of the amount of laser light is performed by the sub-signal by changing the DC magnetic field around the DC magnetic field according to the analog sub-signal, so the internal (direct) modulation system for the main signal and the external modulation system for the sub-signal are completely independent. Become.

This simplifies the configuration of the laser drive circuit for direct modulation, stabilizes the operation of the semiconductor laser, and improves reliability because even if a failure occurs in the sub-signal system, it does not spread to the main signal system. This has the effect of improving.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an optical modulation circuit according to one embodiment of the present invention, and FIG. 2 shows another embodiment of the present invention. 11... Semiconductor laser, 12... Laser drive circuit,
13... Optical isolator that also serves as an optical modulator, 13a.
...Faraday rotator, 13b...coil, 13c...
...Polarizer, 13d...Analyzer, 13e...Current supply circuit, 13f...Permanent magnet, S - Digital main signal input terminal, SS - Analog sub signal input terminal.

Claims (1)

[Scope of Claims] A semiconductor laser; a laser drive circuit that changes the amount of light emitted by changing the drive current of the semiconductor laser according to the level of a digital main signal; an optical isolator that also serves as an optical modulator, comprising a polarizer, a Faraday rotator, an analyzer, and a magnetic field applying means for applying a magnetic field to the Faraday rotator, which are installed in the optical path of the optical isolator. The magnetic field applying means has a function of applying a DC magnetic field that rotates the plane of polarization by approximately 45 degrees to the laser beam passing through the Faraday rotator, and an AC magnetic field that changes according to an analog sub-signal having a lower frequency than the digital main signal. An optical modulation circuit characterized by having a function of applying a magnetic field.