JP2000278217A - Optical transmission equipment - Google Patents

Abstract

(57) [Problem] To provide an optical transmission device capable of increasing an unnecessary optical signal suppression degree with a simple configuration. SOLUTION: A wavelength division multiplexed optical signal 15 having two optical signals generated from the same light source 1 and having different optical frequencies is split into respective optical signals 16 and 18 by an optical multiplexer / demultiplexer, and one of the split lights is provided. The signal 16 is optically modulated by the transmission signal 7, the optically modulated optical signal 17 and the other optical signal 18 are multiplexed, and the multiplexed wavelength-division multiplexed optical signal 19 is transmitted. In an optical transmission apparatus which is capable of receiving a modulated signal of the transmission signal having a center frequency of an optical frequency difference between two optical signals by detection, the one optical signal 17 and the other optical signal which are optically modulated are provided. 18 are reflected, respectively, and returned to the optical multiplexer / demultiplexer 5 where the original wavelength-division multiplexed optical signal 15 is demultiplexed. The two optical signals 17 and 18 returned by the optical multiplexer / demultiplexer 5 are multiplexed. The wavelength-division multiplexed optical signal 19 is The light is separated from the original wavelength multiplexed optical signal 15 by the curator 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device for optically transmitting a high-frequency signal such as a millimeter wave, and more particularly to an optical transmission device capable of suppressing unnecessary optical signals with a simple configuration.

[0002]

2. Description of the Related Art Conventionally, in a millimeter wave band communication system, a waveguide is used as a transmission line. However, since transmission loss is large and long-distance transmission cannot be performed, use of an optical fiber having a small transmission loss has been studied. I have. However, when an optical signal is directly intensity-modulated by a millimeter-wave band signal of several tens of GHz and transmitted through an optical fiber, the optical spectrum spread of the modulated optical signal due to the intensity modulation is large, and is greatly affected by waveform distortion due to optical fiber chromatic dispersion. Transmission distance is limited. Therefore, FIG.
An optical transmission device as shown in FIG.

In the optical transmission apparatus shown in FIG. 3, a single-oscillation optical signal output from a light source 1 is input to an optical intensity modulator 3 and intensity-modulated by a millimeter-wave signal 2 having a frequency of F / 2. The optical spectrum of the intensity-modulated optical signal becomes an optical spectrum having two sidebands at an optical frequency separated by ± F / 2 with respect to the oscillation optical frequency. A wavelength-division multiplexed optical signal 1 composed of two optical signals whose optical frequency interval (optical frequency difference) is F
5 is input to the optical multiplexer / demultiplexer 13 and separated into optical signals having optical spectra in the respective sidebands. One of the separated optical signals 16 is input to the optical modulator 6 and optically modulated by the transmission signal 7. This optically modulated optical signal 17
Is input to the optical multiplexer / demultiplexer 14. The other optical signal 1
8 is input to the optical multiplexer / demultiplexer 14 as it is. The optical multiplexer / demultiplexer 14 multiplexes the optically modulated optical signal 17 and the other optical signal 18. This wavelength multiplexed optical signal 19 is transmitted to the receiving station.

In the receiving station, the wavelength multiplexed optical signal 1
By inputting 9 to an optical receiver and performing heterodyne detection, it is possible to receive a millimeter-wave modulated signal that appears as a beat signal at a frequency F corresponding to the frequency difference between two optical signals.

FIG. 4 shows an optical spectrum of a wavelength multiplexed optical signal 19 output from the optical multiplexer / demultiplexer 14, and FIG. 5 shows a spectrum of an electric signal after heterodyne detection.

In the wavelength-division multiplexed optical signal 15 generated by performing intensity modulation at the frequency F / 2, the optical frequency difference of the sidebands appearing on both sides of the optical spectrum is F. The two sidebands having the frequency difference F are once separated separately by the optical multiplexer / demultiplexer 13, only the optical signal 16 in one sideband is optically modulated by the transmission signal, and the optically modulated optical signal 17
Is recombined with the other unmodulated sideband optical signal 18 to output a wavelength multiplexed optical signal 19 having only two sidebands as shown in FIG. The main carrier appearing at the center of the modulated optical spectrum is removed. When the wavelength-division multiplexed optical signal 19 is heterodyne-detected by one optical receiver, a beat signal is received around the frequency F as shown in FIG. Since the sideband optical signal 18 is not modulated, the beat signal includes the sideband optical signal 17.
Only the modulation component of appears. That is, it becomes the millimeter wave band modulated signal 20 of the center frequency F modulated by the transmission signal 7.
According to this transmission method, only the modulated optical signal 17 is affected by dispersion, and the unmodulated optical signal 18 is not affected by fiber dispersion. Since the frequency of the transmission signal 7 is sufficiently lower than the center frequency F of the millimeter-wave band modulated signal 20, this transmission method is a method of directly modulating the intensity of an optical signal in the millimeter wave band and transmitting it through an optical fiber. In comparison, the influence of dispersion is reduced.

It is known that when heterodyne detection is performed, the phase noise characteristics of two optical signals are problematic. However, in this system, two sideband optical signals generated from one light source and having the same phase noise are generated. Since the signal is used, the phase noise is canceled. Therefore, the received signal is not affected by the phase noise.

Here, a method of generating a millimeter wave from the difference frequency between the two sideband spectra has been described.
There is also a method using output light of a mode-locked laser that generates short pulse light. The output light of the mode-locked laser is an optical spectrum having a plurality of sidebands at equal optical frequency intervals, and two optical spectra at an optical frequency interval corresponding to a desired millimeter wave frequency are extracted from the optical spectrum by an optical multiplexer / demultiplexer. After separating and modulating only one in the same manner as described above, a multiplexed optical transmission and heterodyne detection can receive a millimeter-wave modulated signal.

The optical multiplexer / demultiplexer 13 used in such an optical transmission device must optically demultiplex two optical signals having an optical frequency difference corresponding to a millimeter wave frequency.
The millimeter wave frequency of 0 Hz is a signal light wavelength of 1550 nm.
Is equivalent to about 0.5 nm when converted into a wavelength difference of light, and this wavelength difference is a very narrow value as a wavelength difference for optical demultiplexing. Therefore, it is difficult to sufficiently suppress unnecessary optical signals other than the desired optical signal to be split. Therefore, it is necessary to connect two or three optical multiplexer / demultiplexers having the same transmission wavelength characteristic to increase the degree of suppression.

Further, an external modulator for modulating an optical signal has a large insertion loss, and when an optical multiplexer / demultiplexer is connected in multiple stages,
Since the total insertion loss also increases, insertion compensation using an optical amplifier is required.

[0011]

As described above, when two sideband optical signals are demultiplexed by an optical multiplexer / demultiplexer, the optical multiplexer / demultiplexer is connected in multiple stages to increase the degree of suppression of unnecessary optical signals. There is a need. Since the optical signal having a narrow wavelength interval is demultiplexed, the transmission wavelength characteristic of the optical multiplexer / demultiplexer is steep. Even if the transmission center wavelength is slightly shifted, a large loss occurs. However, it is very difficult to exactly match the transmission wavelengths of all the optical multiplexer / demultiplexers connected in multiple stages and stabilize the transmission wavelengths for a long period of time.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide an optical transmission device which can increase the degree of suppression of unnecessary optical signals with a simple configuration.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a method for producing a light source, comprising:
A wavelength division multiplexed optical signal having two optical signals is demultiplexed into respective optical signals by an optical multiplexer / demultiplexer, one of the demultiplexed optical signals is optically modulated by a transmission signal, and this optically modulated optical signal and the other are modulated. By combining this optical signal and transmitting the combined wavelength-division multiplexed optical signal and performing heterodyne detection on the receiving side,
In an optical transmission device capable of receiving a modulation signal of the transmission signal having a center frequency of an optical frequency difference between two optical signals, one of the optically modulated optical signals and another of the optical signals are reflected. The original wavelength multiplexed optical signal is returned to the demultiplexed optical multiplexer / demultiplexer, the two optical signals returned by the optical multiplexer / demultiplexer are multiplexed, and the multiplexed wavelength multiplexed optical signal is returned to the original optical circulator. It is separated from the wavelength multiplexed optical signal.

Each of the optical signals may be optically amplified between the optical circulator and the optical multiplexer / demultiplexer by the excited rare earth-doped optical fiber.

[0015] A reflection film may be formed at the output end of the optical modulator for optically modulating the one optical signal.

The polarization state of the reflected optical signal may be orthogonal to the polarization state of the incident optical signal.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

As shown in FIG. 1, the optical transmission device of the present invention comprises a light source 1 for outputting a single oscillation optical signal, and intensity modulation of the single oscillation optical signal by a millimeter wave signal 2 having a frequency of F / 2. Accordingly, a light intensity modulator 3 for generating a wavelength multiplexed optical signal 15 having two optical signals having an optical frequency difference F, three terminals A, B, and C, and a terminal connected to the light intensity modulator 3 A to terminal B and terminal C connected from terminal B to the transmission destination
Optical circulator 4 having a forward transfer characteristic to the optical circulator 4, and connected to terminal B of the optical circulator 4,
5 is connected to a first output terminal of the optical multiplexer / demultiplexer 5 for separating the optical signal 5 into an optical signal having an optical spectrum of each sideband. An optical modulator 6 for optical modulation, an optical reflector 8 that reflects the output of the optical modulator 6 and returns the optical modulator 6 to the optical multiplexer / demultiplexer 5, and is connected to the second output terminal of the optical multiplexer / demultiplexer 5 and separated from the other. An optical reflector 9 reflects an optical signal 18 and returns it to the optical multiplexer / demultiplexer 5.

With this configuration, a single oscillation light signal output from the light source 1 is input to the light intensity modulator 3 and the frequency F /
The intensity is modulated by two millimeter-wave signals 2. The optical spectrum of the intensity-modulated optical signal 15 becomes an optical spectrum having two sidebands at an optical frequency separated by ± F / 2 around the emission frequency. That is, the optical frequency difference between the two sidebands is F. The wavelength multiplexed optical signal 15 composed of the two optical signals is input from the terminal A of the optical circulator 4 to the optical multiplexer / demultiplexer 5 through the terminal B. The optical signal 16 and the optical signal 18 are separated by the optical multiplexer / demultiplexer 5. One of the separated optical signals 16 is input to the optical modulator 6 and optically modulated by the transmission signal 7. The optically modulated optical signal 17 is reflected by the optical reflector 8 and returned to the first output terminal of the optical multiplexer / demultiplexer 5 via the optical modulator 6. The other optical signal 18
Is reflected by the optical reflector 9 and returned to the first output terminal of the optical multiplexer / demultiplexer 5. The optical signal 17 and the optical signal 1 are output by the optical multiplexer / demultiplexer 5.
8 are multiplexed, and the wavelength multiplexed optical signal 19 is input to the terminal B of the optical circulator 4. Optical circulator 4
The wavelength multiplexed optical signal 19 output from the terminal C is transmitted to the receiving station.

In the receiving station, the wavelength multiplexed optical signal 1
By inputting 9 to an optical receiver and performing heterodyne detection, it is possible to receive a millimeter-wave modulated signal that appears as a beat signal at a frequency F corresponding to the frequency difference between two optical signals.

Here, the modulated optical signal 17 output from the optical modulator 6 is reflected by the optical reflector 8 and is again input to the optical modulator 6, so that it is doubly modulated. In general, when the modulation is performed twice, the waveform distortion of the modulation signal may occur. However, the interval between the optical reflector 8 and the optical modulator 6 is sufficiently short, and the optical signal output from the optical modulator 6 is output. There is no problem if the delay time from when the signal 17 is reflected and input again is sufficiently smaller than the wavelength of the transmission signal 7.

In the above operation, the optical multiplexer / demultiplexer 5 functions as a demultiplexer and a demultiplexer, and two optical signals transmitted to the receiving station are each transmitted twice for convenience. Therefore, the use of one optical multiplexer / demultiplexer 5 can double the degree of suppression of the unnecessary optical signal. In addition, a control circuit for reducing the number of optical multiplexers / demultiplexers as compared with the case of using two optical multiplexers / demultiplexers as in the prior art and for matching the transmission center wavelengths of the two optical multiplexers / demultiplexers. Becomes unnecessary, and the cost and reliability of the optical transmission device can be reduced.

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

As shown in FIG. 2, this optical transmission device comprises a rare earth-doped optical fiber 1 between a terminal B of an optical circulator 4 and an input end of an optical multiplexer / demultiplexer 5 in the optical transmission device of FIG.
0 is inserted, and the pumping light from the pumping light source 12 is injected into the rare-earth-doped optical fiber 10 via the pumping light multiplexer 11 to constitute an optical amplifier.

In this configuration, the input and output optical isolators required to prevent oscillation in a normal optical amplifier are:
Since the optical circulator 4 used for multiplexing / demultiplexing the input / output optical signals 15 and 19 is substituted, an optical isolator is not required, and an optical amplifier is inserted at low cost by adding a small number of components. The optical signal 15 input to the optical multiplexer / demultiplexer 5 and the return optical signal 1 output from the optical multiplexer / demultiplexer 5
9 can be given an amplifying action, so that the same characteristics as when two optical amplifiers are inserted can be obtained.

In the optical transmission device shown in FIGS. 1 and 2,
The light reflector 8 can be formed by forming a reflection film on the output end of the light modulator 6. In this way, the configuration can be simplified, and the delay time due to reflection can be minimized.

In the optical transmission device shown in FIGS. 1 and 2,
It is preferable that the light reflectors 8 and 9 make the polarization state of the reflected optical signal orthogonal to the polarization state of the incident optical signal. Specifically, by configuring the optical reflectors 8 and 9 with Faraday rotator mirrors, it is possible to reflect an optical signal in a polarization state orthogonal to the polarization state of the incident optical signal. Thus, the optical signal passing through the optical component between the terminal B of the optical circulator 4 and the optical reflectors 8 and 9 propagates toward the optical reflectors 8 and 9 and to the optical circulator 4. Since the polarization states are orthogonal to each other, instability such as output fluctuation of an optical signal due to polarization dependent loss of these optical components can be suppressed.

[0028]

The present invention exhibits the following excellent effects.

(1) Since one optical multiplexer / demultiplexer is used as a multiplexer / demultiplexer, the degree of suppression of an unnecessary optical signal can be increased without increasing the number of optical multiplexer / demultiplexers.

(2) There is no need for a control circuit for matching the transmission center wavelengths of the optical multiplexer / demultiplexer, and the cost and reliability of the optical transmission device can be reduced.

(3) An optical amplifier can be configured at low cost by adding a small number of components.

(4) By configuring the optical reflector with a Faraday rotator mirror, it is possible to suppress the output fluctuation of the optical signal due to the polarization dependent loss.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical transmission device according to an embodiment of the present invention.

FIG. 2 is a block diagram of an optical transmission device according to another embodiment of the present invention.

FIG. 3 is a block diagram of a conventional optical transmission device.

FIG. 4 is an optical frequency-gain characteristic diagram showing an optical spectrum of an output from the optical transmission device.

FIG. 5 is an electric frequency-gain characteristic diagram showing a spectrum of an electric signal after heterodyne detection in a receiving station.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source 3 light intensity modulator 4 optical circulator 5 optical multiplexer / demultiplexer 6 optical modulator 8 optical reflector 9 optical reflector 10 rare-earth-doped optical fiber 11 excitation light multiplexer 12 excitation light source

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/02 10/18

Claims (4)

[Claims] 1. A wavelength division multiplexed optical signal having two optical signals generated from the same light source and having different optical frequencies is split into respective optical signals by an optical multiplexer / demultiplexer, and one of the split optical signals is transmitted as a transmission signal. Multiplexes the optically modulated optical signal and the other optical signal, transmits the multiplexed wavelength-division multiplexed optical signal, and performs heterodyne detection on the receiving side to obtain two optical signals. In the optical transmission apparatus capable of receiving the modulated signal of the transmission signal having the optical frequency difference of the center frequency as the center frequency, one of the optically modulated optical signals and the other optical signal are respectively reflected to reflect the original wavelength multiplex. The optical signal is returned to the demultiplexed optical multiplexer / demultiplexer, and returned by this optical multiplexer / demultiplexer.
An optical transmission apparatus, comprising: combining two optical signals; and separating the combined wavelength-multiplexed optical signal from an original wavelength-multiplexed optical signal by an optical circulator. 2. An optical signal is amplified between the optical circulator and the optical multiplexer / demultiplexer by an excited rare-earth-doped optical fiber.
An optical transmission device according to claim 1. 3. A reflection film is formed on an output end of an optical modulator for optically modulating one of the optical signals.
An optical transmission device according to claim 1. 4. The optical transmission device according to claim 1, wherein the polarization state of the reflected optical signal is orthogonal to the polarization state of the incident optical signal.