JP2753101B2 - Coherent optical transmission equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coherent optical transmission device that electrically modulates a signal to be transmitted in advance and optically transmits the signal by an optical frequency modulation heterodyne detection method.

2. Description of the Related Art As an optical transmission device of this type, a coherent optical transmission device disclosed in Japanese Patent Application No. 1-69702 has been proposed. This is shown in FIG. In FIG. 5, 1 is a semiconductor laser, 2
Is a semiconductor laser for local oscillation, 3 is an optical coupler, 30 is an input signal, 40 is an output signal, 50 is a modulation circuit, 60 is a light receiving circuit, 70 is
FM demodulation circuit, 80 is a demodulation circuit, 101 is a transmitter, 201 is a receiver. Hereinafter, the operation will be described.

In a transmitter 101, a signal 30 to be transmitted is modulated in advance by a modulation circuit 50. Then, the modulation signal is added to the driving current of the semiconductor laser 1, the output light of the semiconductor laser 1 is optically frequency-modulated, and the optical signal is transmitted. In the receiver 201, first, the optical signal of the local oscillation semiconductor laser 2 is combined with the optical signal transmitted from the transmitter 101 by the optical coupler 3, and the beat signal of both is converted into an electric signal by the light receiving circuit 60. . Then, the beat signal is FM-demodulated by the FM demodulation circuit 70, and furthermore, the demodulation circuit 80
Demodulates a signal that is electrically pre-modulated in the transmitter.

Problems to be Solved by the Invention In the conventional coherent optical transmission device described above, the receiver needs two demodulators, an FM demodulator and a demodulator for demodulating premodulation.

When an optical signal having a wavelength of 1.55 μm is transmitted over a long distance over a normal single-mode fiber having a zero-dispersion wavelength of 1.3 μm, the speed of the signal propagating in the optical fiber due to the chromatic dispersion of the optical fiber is increased by the optical frequency. Depends on Therefore, the signal sent to the receiver is distorted.

The present invention has been made in consideration of the above problems, and has as its object to provide a coherent optical transmission device that has a simple configuration of a receiver and is not affected by chromatic dispersion of an optical fiber.

Means for Solving the Problems In order to solve the above-mentioned problems, the following means were performed.

In the transmitter, the signal to be transmitted is subjected to FM or pulsed FM in advance, and this modulation signal is added to the drive current of the semiconductor laser to optically modulate the output light from the semiconductor laser, and the receiver transmits from the transmitter. When the optical signal of the local oscillation semiconductor laser is combined with the optical signal obtained and the beat signal of the two optical signals is detected, and the minimum carrier frequency of the beat signal is fmin and the maximum value is fmax from this signal. To (fmin + fma
It was configured to extract and demodulate frequency components near x) / 2.

Operation With the above configuration, the receiver does not need two demodulators, the FM demodulator and the demodulator for demodulating premodulation, and can be configured with a filter and a demodulation circuit. Further, since only a signal component of a specific frequency is used, there is no influence of dispersion of the optical fiber.

FIG. 1 shows an embodiment of the present invention.

In FIG. 1, 1 is a semiconductor laser, 2 is a local oscillation semiconductor laser, 3 is an optical coupler, 10 is an FM circuit, 20 is a light receiving circuit, 21 is a filter, 22 is a demodulation circuit, 30 is an input signal, and 40 is an input signal. The output signal, 100 indicates a transmitter, and 200 indicates a receiver. Hereinafter, the operation will be described.

The input signal 30 shown in FIG. 2A enters the FM circuit 10 and is frequency-modulated. The output signal waveform is shown in FIG.
When this signal is applied to the semiconductor laser 1, the frequency of the light emitted from the semiconductor laser 1 shifts according to the magnitude of the signal current. That is, frequency modulation of light is applied. FIG. 2C shows the change in the optical frequency of the output light.
The optical signal from the semiconductor laser 1 is transmitted from the transmitter 100 to the receiver 2
Propagate to 00.

The optical signal transmitted to the receiver 200 enters the optical coupler 3. In the optical coupler 3, the optical signal of the local oscillation semiconductor laser 2 is combined with the optical signal from the transmitter 100. The two optical signals synthesized by the optical coupler 3 enter the light receiving circuit 20. Here, the beat signals of the two optical signals are detected.

Next, a process of demodulating the original signal 30 from the output signal of the light receiving circuit 20 will be described.

Now, as shown in FIG. 3 (a), the output signal of the light receiving circuit 20 is such that the minimum value of the carrier frequency of the output signal is fmin, the maximum value is fmax, and the frequency is f1 = (fmin + fmax) / 2. It is assumed that the waveform is changing. This signal has the frequency f1
When input to the bandpass filter 21 that passes only the components of
The output has a waveform as shown in FIG. If this is input to the demodulation circuit 22 and smoothed, FIG.
An output signal 40 as shown in FIG. The demodulation circuit 22 includes, for example, an envelope detection circuit and a low-pass filter (LPF). The original signal 30 is obtained as described above.

Next, an embodiment of the present invention will be described.

According to a second aspect of the present invention, the FM circuit 10 of the first embodiment is replaced with a PFM circuit. Semiconductor laser 1 when input signal 30 has waveform shown in FIG. 4 (a)
FIG. 4 (d) shows a change in the optical frequency of the optical signal output from the optical disk. The demodulation method is the same as that of the first embodiment.

According to the present invention, there is an advantage that the original signal 40 can be obtained without reproducing the digital signal by discriminating the frequency of the output signal of the light receiving circuit 20. An advantage of the second aspect of the invention over the first aspect is that the frequency of the beat signal can be easily stabilized. In the case of the second aspect of the invention, the optical signal from the transmitter has two types of light frequencies, and alternates with time. The spectral distribution is two peaks, each centered on two frequencies. Therefore, if the optical frequency of the output light of the local oscillation semiconductor laser is controlled so that the peak of the mountain has a predetermined frequency, the frequency fluctuation of the beat signal can be suppressed.

As described above, the present invention can simplify the configuration of the receiver, and can demodulate even if the optical frequency of the optical signal from the transmitter or the optical signal of the local oscillation semiconductor laser of the receiver fluctuates somewhat. There are excellent effects such as possible and not affected by dispersion of optical fiber.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a coherent optical transmission apparatus according to a first embodiment of the present invention, FIG. 2 is a signal waveform diagram of each part of the transmitter 100 in FIG. 1, and FIG. FIG. 4 is a signal waveform diagram of each part of the receiver 200 in FIG. 1, FIG. 4 is a signal waveform diagram of each part of the transmitter in the coherent optical transmission apparatus according to the second embodiment of the present invention, and FIG. FIG. 2 is a block diagram illustrating a coherent optical transmission device. 1 ... semiconductor laser, 2 ... semiconductor laser for local oscillation,
3 ... Optical coupler, 10 ... FM circuit, 20 ... Light receiving circuit, 21 ...
... Filter, 22 ... Demodulation circuit, 30 ... Input signal, 40 ...
Output signal, 50: Modulation circuit, 60: Light receiving circuit, 70: FM
Demodulation circuit, 80 ... demodulation circuit, 100, 101 ... transmitter, 20
0, 201 ... Receiver.

Continuation of the front page (56) References JP-A-61-114624 (JP, A) JP-A-63-198426 (JP, A) JP-A-63-64418 (JP, A)

Claims (2)

(57) [Claims] A transmitter comprises at least an FM circuit for frequency-modulating (FM) a signal to be transmitted, and a semiconductor laser, and an output light of the semiconductor laser is a signal current output from the FM circuit and has an optical frequency. The modulated signal is transmitted to the receiver, and the receiver includes at least a local oscillation semiconductor laser, a combiner that combines the optical signal transmitted from the transmitter and the optical signal of the local oscillation semiconductor laser, and the combiner. A light receiving circuit for converting a beat signal of two optical signals output from the light receiving device into an electric signal, and a carrier wave frequency of an output signal of the light receiving circuit is fmin when the minimum value is fmin and fmax when the maximum value is fmax.
x) / 2: a coherent light comprising: a filter for extracting a frequency component near 2); a demodulation circuit for demodulating an output signal of the filter; and the demodulation circuit comprising an envelope detection circuit and a low-pass filter. Transmission equipment. 2. A transmitter comprising at least a PFM circuit for pulsating FM (PFM) of a signal to be transmitted and a semiconductor laser, and an output light of the semiconductor laser is a signal current output from the PFM circuit. The frequency-modulated and transmitted to the receiver, the receiver is at least a local oscillation semiconductor laser, a combiner that combines the optical signal sent from the transmitter and the optical signal of the local oscillation semiconductor laser, A light receiving circuit that converts a beat signal of two optical signals output from the combiner into an electric signal, and a carrier wave frequency of the output signal of the light receiving circuit is fmin when the minimum value is fmin and fmax when the maximum value is fmax.
x) / 2: a coherent light comprising: a filter for extracting a frequency component near 2); a demodulation circuit for demodulating an output signal of the filter; and the demodulation circuit comprising an envelope detection circuit and a low-pass filter. Transmission equipment.