JPH07120979B2 - Frequency shift keying optical transmitter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a frequency shift keying optical transmitter.

[Prior Art] Since the output light intensity of a semiconductor laser can be easily modulated by turning on / off an injection current, it is widely used as a light source for optical fiber communication and the like. Also, in this semiconductor laser, the oscillation frequency changes at the same time in response to minute changes in the injection electrode, so the so-called frequency shift keying method (FS
Signal transmission is also possible by the K method). In this case, if FSK signal transmission is performed by optical heterodyne type detection in particular, it is possible to construct an extremely effective communication system capable of long-distance transmission, etc. in combination with the effect of improving the optical reception sensitivity. Volume 32, Issue 12, 2173
−2184 (1982)).

By the way, in the FSK method, the frequency difference between each symbol of the signal must be a predetermined constant value,
The dependence of the semiconductor laser oscillation frequency (oscillation wavelength) on the modulation current is not necessarily uniform, and varies greatly depending on the structural parameters of the semiconductor laser, the bias current value, the modulation frequency, and the like. For example, it is known that a modulation current of 1 mA differs from 100 MHz to several GHz by 10 times or more. Therefore, in the signal transmitting section, it is necessary to frequency-modulate the semiconductor laser by controlling the frequency shift amount to a predetermined constant value.

[Problems to be Solved by the Invention]

However, conventionally, the light source and the drive circuit for the light source are only used, and the control has not been performed.

Therefore, an object of the present invention is to provide an optical transmitter that can output a frequency-modulated wave whose frequency deviation amount is controlled to a predetermined value as signal light.

[Means for Solving the Problems]

A frequency shift keying optical transmitter of the present invention outputs a first light source that outputs signal light that has been frequency shift keyed, and a wavelength that is substantially the same as that of the first light source and that has a frequency shift amount of at least twice. A second light source that is swept and modulated; and a photodetector that combines a part of the output from the first light source and the output from the second light source and then converts the light into an electric signal. A configuration is provided in which the frequency shift amount of the first light source is detected from the output time width of the beat output from the detector, and the modulation amount of the first light source is controlled so that the detected frequency shift amount becomes a predetermined value. is there.

[Action]

In the present invention, the beat output of a part of the frequency shift-modulated signal light and the reference light that is frequency-sweep-modulated is detected, and from the output time width of the beat output signal that enters the band of the detector electric circuit, The frequency shift amount of the signal light is detected, and the frequency shift amount is controlled to a predetermined value based on this detection output.

〔Example〕

FIG. 1 is a block diagram showing a first embodiment of a frequency shift keying optical transmitter obtained by the present invention, FIG. 2 (a),
(B), (c) is a figure which shows the output waveform and output spectrum of each part.

The binary pulse transmission signal 30 is sent to the drive circuit 25. The drive circuit 25 has a variable output peak value, and a drive current of a required amplitude is applied to the transmission semiconductor laser 8 to shift the binary frequency shift. The modulated signal light 26 is obtained. A part of the signal light 26 is branched by the optical branching device 4, and the reference semiconductor laser 1
The output light is multiplexed by the multiplexer 15. The reference semiconductor laser 1 is modulated by the sawtooth wave generator 2 and outputs the frequency sweep light 27. Frequency sweep light 27 and signal
The light 28 combined with 26 is guided to the photodetector 16 and converted into an electric signal. A beat signal 29 as shown in FIG. 2 (b) is detected from the obtained electric signal only when the beat frequency of the combined light 28 falls within the receiving circuit band. This beat signal 29
Is detected by the detection circuit 31, a pulse-like waveform as shown in FIG. 2 (c) is obtained. Therefore, this pulse width is detected by the pulse width detector 32, and the output amplitude of the drive circuit 25 is controlled so that the width becomes a constant value.

The transmission semiconductor laser 8 in this embodiment is a distributed feedback semiconductor laser that oscillates at a single wavelength, and outputs a transmission signal of 200 Mb / s with a frequency shift amount of 1 GHz. The drive current amplitude in this case was 7 mA. Further, as the reference semiconductor laser 1, a wavelength tunable semiconductor laser having a structure in which the injection electrodes are separated at three positions and having a wide wavelength tunable range is used. This wavelength tunable semiconductor laser is described in detail in Japanese Patent Application No. 165460/1983, so a detailed description thereof will be omitted, but a large wavelength shift amount can be obtained with a small injection current. In this embodiment, a sawtooth wave having a period of 200 Hz and an amplitude of 10 mA is applied to obtain a wavelength swept signal of 50 GHz. The band of the photodetector 16 is about 300 MHz, and the pulse width of the beat signal is
It becomes 100 μsec. Since the pulse width detector 32 for detecting this pulse width can be easily obtained by an ordinary digital logic circuit, its explanation is omitted here. In addition, the bias current of the reference semiconductor laser 1 is adjusted so that the bias current of the reference semiconductor laser 1 is located substantially at the center of the beginning and the end of the sawtooth wave of the beat signal.
Is automatically controlled.

In the frequency shift keying optical transmitter thus obtained,
The frequency shift amount can be suppressed to an error of 5% or less of the set value, and stable transmission light can be obtained regardless of operating conditions and the like.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention, and FIGS. 4 (a), (b) and (c) are diagrams showing the output spectrum of the main part. The second embodiment is one in which the present invention is applied to an apparatus for arranging the wavelengths of three transmitting light sources 8, 9, 10 at a predetermined wavelength interval. Regarding the operation for arranging the wavelength intervals of this device to a predetermined value, the same applicant as the present invention has filed a patent application.
Although detailed description is omitted because it is described in detail in the specification of 62-137269, the sweep light 27 of the variable wavelength reference semiconductor laser 1 whose output is modulated from the sawtooth wave generator 2 is output from the optical branching device 4. After splitting into 2 beams, one of them is combined with the etalon plate 5, and the other is combined with the output light of three distributed feedback semiconductor lasers 8, 9 and 10 for input to the photodetectors 6 and 16 respectively. I am letting you. Since the light beam passes through the etalon plate 5 only when the wavelengths of the resonance peak reference semiconductor laser 1 of the etalon plate 5 match, a pulse-like output is obtained from the photodetector at the free spectrum width period of the etalon plate 5. To be Also,
The beat signal is output in pulses from the photodetector 16 to which the multiplexed light is input only when the wavelengths of the swept light 27 and each signal light match. The bias control circuits 17, 1 are controlled by the interval control device 7 so that the pulse generation times from the photodetectors 6, 16 coincide with each other.
A semiconductor laser for transmission is controlled by controlling each of 8 and 19.
The 8,9,10 wavelength intervals are arranged in the interval of the free spectral width.

According to the second embodiment, since the wavelengths of a plurality of transmission light sources can be controlled by a single reference light source, not only the utilization efficiency is high, but also new functions such as wavelength interval control functions can be added at the same time. It will be possible.

In addition to the above embodiments, some modifications can be made in the present invention. Also in the first embodiment, as in the second embodiment, it is possible to improve the measurement accuracy of the pulse width by using the pulse output transmitted through the etalon plate 5 as the reference of the wavelength interval. In the second embodiment, the number of transmitting light sources is set to three, but the number is not limited to this and may be any time. As the semiconductor laser used, a wavelength tunable semiconductor laser and a distributed feedback semiconductor laser are exemplified.
It is not limited to this as long as it oscillates at a single frequency and has a wide frequency variable range.

〔The invention's effect〕

As described above, according to the present invention, it is possible to obtain an optical transmitter that outputs, as signal light, frequency-modulated light whose frequency shift amount is controlled to a predetermined value.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention, FIG. 2 is a diagram showing waveforms and spectrum shapes of main parts of the same embodiment,
FIG. 3 is a configuration diagram showing the second embodiment, and FIG. 4 is a diagram showing waveforms and spectrum shapes of main parts of the embodiment of FIG. 1 ... Reference semiconductor laser, 2 ... Sawtooth wave generator, 4
...... Optical branching device, 5 …… Etalon plate, 6,16 …… Photo detector,
7 ... Interval control device, 8, 9, 10 ... Transmission semiconductor laser,
15 …… Optical demultiplexer, 17,18,19 …… Bias control circuit, 20,2
1,22,23, ... Temperature control device, 25 ... Drive circuit, 26 ... Signal light, 27 ... Frequency sweep light, 28 ... Combined light, 30 ... Transmission signal, 31 ... Detection circuit, 32 ...... Pulse width control circuit.

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Claims (1)

[Claims] 1. A first light source that outputs a signal light that is frequency-shift modulated, and a second light source that outputs substantially the same wavelength as the first light source and that is frequency-sweep-modulated with at least twice the frequency shift amount. Light source, and a photodetector for converting a part of the output from the first light source and the output from the second light source into an electric signal and then converting the electric signal into an electric signal. Beat output from the photodetector The frequency shift modulation of the first light source is detected from the output time width of the first light source, and the modulation amount of the first light source is controlled so that the detected frequency shift amount becomes a predetermined value. Optical transmitter.