JPH04335590A - Optical frequency deviation-amount stabilization method - Google Patents

Abstract

PURPOSE:To provide an optical frequency deviation-amount stabilization method which can suppress the fluctuation of the frequency deviation amount of a light source for optical FSK transmission use by using a constitution and a circuit which are comparatively simple. CONSTITUTION:The optical intensity modulation (AM) component of FSK signal light which is output from a semiconductor laser 3 is detected by using a photodetector 9; the magnitude of the amplitude of the AM component is detected by using a half-wave rectifier 15. The component is fed back to a variable attenuator 2 so that the output of the half-wave rectifier 15 becomes definite; the amplitude of a digital signal is adjusted. As a result, the magnitude of an optical intensity modulation is stabilized, and, at the same time, an optical frequency deviation amount is stabilized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency shift stabilizing device for a transmitting light source using an optical frequency shift keying method.

0002

[Background Art] Optical frequency shift keying (FSK)
In optical fiber communication using the frequency shift keying method, once the amount of frequency shift of the transmitting light source is determined, the optimal frequency discrimination characteristic of the receiver is usually determined. For example, in optical heterodyne detection reception using delayed detection, the frequency discrimination characteristic has a cosine wave-like periodic characteristic. When the frequency difference corresponding to the peaks and valleys of the periodic characteristic is equal to the frequency shift amount of the optical FSK signal light, the maximum demodulated signal can be obtained. However, if the frequency deviation of the transmitting light source fluctuates due to disturbances such as ambient temperature, the reception condition deteriorates. In order to suppress this deterioration, it is necessary to stabilize the amount of frequency deviation.

Conventionally, as a method for stabilizing the amount of frequency deviation, a method of controlling the amplitude of the applied digital signal so that the transmission period characteristic of the Mach-Zehnder interferometer matches the amount of frequency deviation (Japanese Patent Application Laid-open No. 2003-230002-1) has been proposed. 41043) is known.

0004

[Problem to be Solved by the Invention] However, in the conventional method, by multiplying the transmitted light reception signal of the Mach-Zehnder interferometer and the original digital signal using a mixer,
A control error signal was obtained by determining whether the received signal and digital signal were in phase or out of phase. This means that the phase difference is detected at a speed comparable to the bit rate of the digital signal, and it is expected that the higher the bit rate, the more difficult this phase detection will be. Also, the circuit used is required to be high-speed. As described above, in the conventional method of stabilizing the amount of optical frequency deviation, there is a need for a method that can simplify the circuit configuration, and there are problems that need to be solved.

An object of the present invention is to provide an optical frequency deviation stabilization method that can suppress fluctuations in the frequency deviation of an optical FSK transmission light source using a relatively simple configuration and circuit.

[0006]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides an optical frequency shift modulation type transmitter using a semiconductor laser as a light source that causes a shift in optical frequency according to an injected current. The amount of optical frequency deviation of the semiconductor laser is stabilized by controlling the amplitude of the digital signal applied to the semiconductor laser so that the amplitude of the optical intensity modulation component of the output signal light of the semiconductor laser is constant. It is characterized by

[0007]

[Operation] The present invention utilizes the characteristic that when optical frequency modulation is performed by modulating the current injected into a semiconductor laser, optical intensity modulation is also applied to the output of the semiconductor laser at the same time. That is, the output signal light of the semiconductor laser is converted into an electric signal by a photodetector, and only the AM component of the output of this photodetector is extracted. The magnitude of this AM component is determined by the optical frequency shift (
FM) component. The magnitude of the amplitude of this AM component is detected by a half-wave rectifier, and the amplitude of the digital signal applied to the semiconductor laser is controlled so that the output of the half-wave rectifier is constant. As a result, the FM component also becomes constant, so that the amount of optical frequency deviation of the output signal light of the semiconductor laser can be stabilized.

[0008]

[Examples] Examples of the present invention will be described in detail below. FIG. 1 shows an optical frequency shift keying transmitter implementing a first embodiment of the invention. In FIG. 1, a 1.5 μm band semiconductor laser 3 whose optical frequency is shifted by an injection current is oscillated by a bias current of 100 mA being applied by a bias current source 8. This semiconductor laser 3 has a threshold value of 20 mA and an FM modulation efficiency of 250 MHz/mA.
, has characteristics of AM modulation efficiency of 0.2 mW/mA. A 2.5 Gb/s digital electrical signal 1 to be transmitted is applied to a semiconductor laser 3 after being passed through a variable attenuator 2 whose attenuation can be changed depending on an external signal. Here, the injection current is adjusted by ±4 so that the optical frequency deviation amount is 2 GHz.
It is mA modulated. The output signal light 4 which has been optically frequency shift modulated and has been emitted from the semiconductor laser 3 is incident on the optical fiber 6 and is split into two by the optical splitter 5 . The branched output signal light 4 is received by a photodetector 9 and converted into an electrical signal. As shown in FIG. 2, the output of the photodetector 9 is a constant DC component (light intensity equivalent: 16 mW) and an AM component corresponding to the digital signal 1.
component (light intensity conversion ±0.8 mW). The output of the photodetector 9 is passed through a capacitor 10 to remove only the AM component.
This is amplified by an amplifier 11. In order to detect the amplitude of the output of the amplifier 11, the output of the amplifier 11 is connected to the diode 12.
, a resistor 13, and a capacitor 14. The output of the half-wave rectifier 15 is proportional to the AM component of the output of the photodetector 9. The output of this half-wave rectifier 15 is input to a differential amplifier 16, and the difference with the reference voltage of a reference voltage source 17 is calculated. The output of the differential amplifier 16 changes negatively when the AM component of the photodetector 9 becomes small, and changes positively when it becomes large. By feeding back the output of the differential amplifier 16 to the variable attenuator 2 and controlling the output of the differential amplifier 16 to zero, the magnitude of optical intensity modulation of the output signal light 4 is stabilized. As a result, the optical frequency shift amount of the output signal light 4 is also stabilized at the same time. As a result of performing control with the above configuration, the frequency deviation of 2 GHz was suppressed to ±10 MHz fluctuation, confirming the effectiveness of the present invention.

FIG. 3 shows an optical frequency shift keying transmitter implementing a second embodiment of the present invention. In the second example,
A photodetector 9 is built into a laser module 18. Output signal light 4 (
After passing through the lens 19 and the isolator 20, the signal is input into the optical fiber 6. Output signal light 4 emitted from the rear of semiconductor laser 3 is incident on photodetector 9 built into laser module 18 and converted into an electrical signal. The output of this photodetector 9 has a constant DC component and an AM component corresponding to the digital signal 1, just as in the first embodiment. The amplitude of this AM component is detected by a half-wave rectifier 15 and fed back to the variable attenuator 2 so that this value remains constant, thereby controlling the amplitude of the digital signal 1. As a result, not only the magnitude of the optical intensity modulation of the output signal light 4 is stabilized, but also the amount of optical frequency shift is stabilized at the same time. As a result of controlling with the above configuration, as in the first embodiment, ±10 MHz for a frequency deviation of 2 GHz.
The variation in z was suppressed, confirming the effectiveness of the present invention. Furthermore, since the photodetector 9 was built into the laser module 18, it was possible to make it compact. Furthermore, since the light from behind the semiconductor laser 3 is used, there is also the advantage that there is no loss of light.

As described above, since the present invention does not use a special optical interferometer, stabilization of the amount of optical frequency shift can be achieved simply and at low cost.

Two embodiments of the present invention have been described above.
It goes without saying that the present invention is not limited to these examples, and that various modifications and changes can be made within the scope of the present invention. For example, although a semiconductor laser is used as the transmission light source, any type of laser can be used as long as it is a laser device that modulates the optical frequency and shifts the optical frequency according to an external signal. Instead of a variable attenuator that changes the amplitude of the digital electrical signal, it is also possible to use a variable gain amplifier.

0012

[Effects of the Invention] As explained above, according to the present invention,
With a relatively simple configuration, fluctuations in the amount of frequency deviation of the optical FSK transmission light source can be suppressed.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an optical frequency shift keying transmitter that implements a first embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the current-light output characteristics of the semiconductor laser 3 and the output of the photodetector 9. FIG.

FIG. 3 is a configuration diagram showing another example of an optical frequency shift keying transmitter that implements the second embodiment of the present invention.

[Explanation of symbols]

1 Digital signal 2. Variable attenuator 3 Semiconductor laser 4,7 Output signal light 5 Optical demultiplexer 6 Optical fiber 8 Bias current source 9 Photodetector 10,14 Capacitor 11 Amplifier 12 Diode 13 Resistance 15 Half wave rectifier 16 Differential amplifier 17 Reference voltage source 18 Laser module 19 Lens 20 Isolator

Claims (1)

[Claims] 1. An optical frequency shift modulation transmitter using a semiconductor laser as a light source that causes a shift in optical frequency according to an injected current, wherein the amplitude of a light intensity modulation component of the output signal light of the semiconductor laser is constant. A method for stabilizing an amount of optical frequency deviation, characterized in that the amount of optical frequency deviation of the semiconductor laser is stabilized by controlling the amplitude of a digital signal applied to the semiconductor laser so that the amount of deviation in optical frequency is stabilized.