JP3025544B2 - Optical communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system, and more particularly, to a multiplexed optical communication system for multiplexing and transmitting a large number of signals using one optical transmission line. .

[0002]

2. Description of the Related Art In recent years, with the advancement of optical communication technology, various methods of transmitting more information through one optical transmission line have been studied and put to practical use. For example, the use of an optical transmission line is time-divided, and each terminal compresses and transmits a signal at an assigned time, and each transmitting terminal transmits a signal using light of a different wavelength. A wavelength division multiplexing method in which a receiving terminal selectively receives and demultiplexes a signal light having a required wavelength from the transmitted light is implemented as a suitable multiplexing method.

Further, as a method of increasing the capacity of a communication path and increasing the communication distance, a method called a coherent communication system has been realized. This uses light as a wave instead of transmitting information by modulating the intensity of light as performed in conventional optical communication, and transmits signals by optical frequency modulation, optical phase modulation, etc. On the receiving side, this signal is received by the heterodyne method of causing interference with locally oscillated light beams having wavelengths close to each other.

[0004]

However, the above-mentioned prior art has the following problems. First, in the time division multiplexing optical communication system, a control terminal for controlling communication is required to allocate a time slot to each transmitting terminal,
Further, in order to perform complicated communication control between the control terminal and each transmitting terminal, a complicated and expensive circuit must be provided. Further, in the time-division multiplexing method, information is compressed and transmitted in a time slot allocated to the transmitting terminal, so that the transmitting bit rate must be higher than the bit rate of the information generated by the transmitting terminal. Absent. For example, in order to multiplex 10 transmission terminals that generate 100 Mb / s information, 1 Gb / s can be obtained even if the guard time is ignored.
Must be transmitted at a transmission rate of Transmission at such a high bit rate also complicates the transmission circuit and causes an increase in cost.

In the wavelength division multiplexing optical communication system, if the number of multiplexes is increased, the wavelength intervals must be reduced, so that the width of the oscillation spectrum of the semiconductor laser on the oscillation side is reduced or the wavelength variation with temperature is reduced. It is necessary to suppress or further reduce the wavelength fluctuation due to chirping at the time of direct modulation, which raises the cost. Conversely, if the wavelength interval is increased in order to increase the tolerance for these problems, there arises a problem that a large number of wavelength multiplexes cannot be obtained.

In the coherent communication system, a semiconductor laser is used as a local oscillation light source on the receiving terminal side, and the oscillation wavelength of this semiconductor laser must almost completely coincide with the oscillation wavelength on the transmitting terminal side. In addition, there is a problem that strict temperature control must be performed in both the transmitting and receiving terminals in order to suppress wavelength fluctuation due to temperature change.

Therefore, an object of the present invention is to provide a complicated time division control device, a high bit rate transmission circuit,
Another object of the present invention is to provide an optical communication system having a configuration in which strict temperature control, strict suppression of wavelength fluctuation, and the like are not required and multiplicity can be increased.

[0008]

In an optical communication system according to the present invention which achieves the above object, part of light from at least one light source is modulated by a transmission signal, and the rest or another part is assigned to the light source. Modulated with a high-frequency signal having a specific frequency (a signal that is separated from the frequency domain of the transmission signal in a higher frequency direction so that a signal obtained by modulating the transmission signal with the high-frequency signal can be sufficiently demodulated). An optical transmitter provided with a unit for combining light and transmitting the transmission light to a transmission line, and the same as a unique frequency assigned to the light source after converting the modulated signal light transmitted to the transmission line into an electric signal. And an optical receiver provided with means for detecting and receiving a signal having the center frequency described above.

More specifically, a plurality of the optical transmitters and the optical receivers are provided, and the plurality of optical transmitters and the optical receivers perform communication via at least one optical transmission line. The detecting and receiving means provided in the optical receiver has a function of tuning to a specific frequency assigned to the light sources of the plurality of optical transmitters, selecting and receiving the light, or modulating the light with the transmission signal. Light to be modulated by a high-frequency signal having a specific frequency assigned to the light source is light output to both sides of a laser resonator as a light source, respectively, or in the optical receiver, the light receiver, by a photodetector After converting the modulated signal light into an electric signal, the electric signal is filtered by a band-pass filter having substantially the same transmission center frequency as the frequency of the high-frequency signal assigned to the light source of the optical transmitter that the terminal wants to receive. To do Therefore and receives the desired signal.

According to the above configuration, a complicated time-division control device and a high bit rate transmission circuit are not required, and strict temperature control and severe wavelength fluctuation are not required since the receiving terminal does not use a local oscillation light source. Suppression becomes unnecessary,
Nevertheless, the multiplicity can be increased.

[0011]

1 and 2 are schematic diagrams showing a first embodiment of the present invention. FIG. 1 shows a transmitting side and FIG. 2 shows a receiving side. In the figure, T1, T2,...
When they are collectively referred to, they are denoted by Ti, and the other codes are indicated in the same manner.) Indicates the transmitting terminal by R1, R2,.
, RN denote the receiving terminal, and 11, 12,.
.., 1N is a semiconductor laser, 21, 22,..., 2N is an optical branching element, 311, 312,.
Is an external light modulator, 4 is an optical converging element, 5 is a photodetector, 61
, ..., 6N are band-pass filters, 71, ...
, 7N denotes a detection circuit, and 8 denotes an optical fiber transmission line. Also, for simplicity, in FIGS. 1 and 2, the location where light propagates is indicated by a double line and an arrow indicating the direction, and the location where electricity propagates is indicated by a solid line with an arrow.

As the semiconductor laser 1i in FIG. 1, a laser having a narrow spectral line width of oscillation light in a longitudinal single mode, such as a DFB (distributed feedback) laser, is preferably used. A fiber fusion type or waveguide type optical branching / combining device is used for the optical branching device 2i and the optical combining device 4, and an A / O modulator (using an acousto-optic effect) is used for the external light modulators 3I1 and 3I2. Do) or E
A deflection type optical modulator such as an I / O modulator (using the electro-optic effect), an absorption type optical modulator using the electric field dependence of the light absorption spectrum of a semiconductor, or a propagation position of light branched into two An interference type optical modulator utilizing interference due to a phase difference is preferably used. Further, as the photodetector 5, a pin photodiode or an avalanche photodiode having nonlinearity between the input light intensity and the output current is suitable. Further, as the optical fiber transmission line 8, a polarization-preserving non-dispersion optical fiber or the like having no speed difference due to the wavelength difference is used.

In FIG. 1, for example, when light is output from a semiconductor laser 12 provided in a transmitting terminal T2, this light is split into two by a splitter 22 and one of the modulator 32
1 modulates according to the transmission signal S2. The other light is transmitted to the semiconductor laser 12 by the modulator 32 2.
High-frequency signal f2 having a unique frequency assigned to
Modulated by Similarly, in the N transmitting terminals Ti, the light from the semiconductor laser 1i provided at each terminal is branched into two, one of which is transmitted by the transmission signal Si and the other of which is transmitted to the semiconductor laser 1i of each terminal. It is modulated by a high frequency signal fi having the assigned frequency. These modulated signals are combined by the optical combining element 4, transmitted through the optical transmission line 8, and converted into an electric signal by the photodetector 5.

As described above, the photodetector 5 determines the input light intensity and the output.
There is a non-linearity between the force currents, which is usually
It can be approximated by a number. Here, the input light is temporally A (t).
Frequency ω modulated at₁(Wavelength) light A (t) co
sω₁t and the frequency ω modulated by B (t)_TwoLight B
(T) cosω_TwoLet t be incident on the photodetector 5 at the same time.
Then, the output current becomes I (t) = ｛A (t) cosω₁t + B (t) cosω_Twot｝^Two = ｛A (t) cosω₁t｝^Two  + ｛B (t) cosω_Twot｝^Two + 2A (t) B (t) cosω₁t ・ cosω_Twot = ｛A (t) cosω₁t｝^Two+ ｛B (t) cosω_Twot｝^Two + A (t) B (t) ｛cos (ω₁−ω_Two) T + cos (ω₁+ Ω_Two ) T｝ can be written.

The term A (t) B (t) c in the last equation
os (ω ₁ −ω ₂ ) t corresponds to the beat signal of the two lights, and in the present invention, the signal is detected by suitably using the signal represented by this term. I have. That is, as described above, the light incident on the photodetector 5 is obtained by multiplexing the output light of the semiconductor laser 1i provided at each transmission terminal Ti, which is modulated by the transmission signal Si and the high-frequency signal fi. Is transmitted. These lights are separated from each other by the photodetector 5 as shown in the above equation.
Since two lights output and transmitted from the same semiconductor laser 1i, for example, 12, which are mixed due to the nonlinearity and generate a beat signal, are light from the same light source, so that ω ₁ = ω _{2 in the} above equation. Therefore, the beat signal component is represented by A (t) · B (t), that is, S2 · f2. This represents a waveform obtained by modulating a high-frequency signal having the frequency f2 by the transmission signal S2. Similarly, each transmitting terminal T1, T
, TN, the beat signal components of each light transmitted from TN are S1 · f1, S2 · f2,..., SN · f
N.

On the other hand, the beat signal components of two lights from different semiconductor lasers, for example, the light from the semiconductor laser 11 modulated by the transmission signal S1 and the light from the semiconductor laser 12 modulated by the high-frequency signal f2 are If the frequency difference between the output lights of these semiconductor lasers is ω ₁ −ω ₂ = Δω, it is expressed as S 1 · f 2 · cos (Δω). The frequency difference Δω (wavelength difference) of light from different semiconductor lasers is
Unless a special processing such as mode lock is applied to the semiconductor laser, the signal does not substantially fall to the frequency band realized by the electric circuit.
cos (Δω) is not transmitted as an electric signal to the receiving terminal R2.

Therefore, to the receiving terminals R1,..., RN, the mixed electric signals of the beat signals S1.f1, S2.f2,. Only the signal is transmitted, and is filtered by a band-pass filter 6i having the center frequency of the high-frequency signal fi at each receiving terminal Ri, so that S1.f1, S2.f2
,..., SN · fN can be discriminated.

Therefore, at each receiving terminal Ri, the frequency f1
,..., FN, it is possible to receive a signal from an arbitrary transmitting terminal Ti by providing means for selecting, tuning and receiving an arbitrary wave from the high frequency signals of fN. When it is sufficient to receive a signal from one specific terminal, a band-pass filter 6i that transmits any one of the high-frequency signals of frequencies f1,..., FN may be used.

As described above, in the present embodiment, communication multiplexed on one optical transmission line 8 between a plurality of transmitting terminals and a plurality of receiving terminals is possible. Here, the wavelength difference of each semiconductor laser is Δω when converted into the above-mentioned frequency difference Δω.
However, it is only necessary that the distances are so large that they cannot be received by the electric circuit, so that a large multiplex number can be obtained. Further, since the beat signal of light from the same semiconductor laser is used, strict temperature control and stabilization of the oscillation wavelength are not required.

FIGS. 3 and 4 are views schematically showing a second embodiment of the present invention, in which the same parts as those in FIGS. 1 and 2 are denoted by the same reference numerals. Usually, the semiconductor laser 1i has a resonator, and laser light is output to both sides of the resonator. Conventionally, only one of the outputs of a semiconductor laser is used, and the other is discarded or used as a reference light for stabilizing the output intensity. Both outputs are used for transmission. For example, in the transmitting terminal T1, the semiconductor laser 1
One of the lights output from 1 to both sides of the resonator is modulated by the modulator 311 with the transmission signal S1 and the other by the modulator 312 with the high frequency signal of the frequency f1. Similarly, at the other transmitting terminals Ti, the lights output to both sides of the resonator are modulated by the transmission signal Si and the high-frequency signal fi, respectively, and are combined by the optical combining element 4 and transmitted.

According to this embodiment, since both of the two outputs of the semiconductor laser are used, the intensity of the transmission light can be increased as compared with the first embodiment, and transmission over a longer distance becomes possible. The other points are substantially the same as the first embodiment.

Although the concept of the present invention has been described in detail with reference to the first and second embodiments, the application of the present invention is not limited to only these embodiments. First, the light source, the light branching / combining element, the modulator, the photodetector, and the like can be applied to the present invention as long as they have the same functions as those shown in the embodiments. Further, in the above-described embodiment, in each optical receiving terminal, the configuration in which the high-frequency signal is filtered and then detected by the band-pass filter is described.
For example, the configuration is not limited as long as it is a receiving method used in a normal receiver such as a heterodyne receiving method.

Further, in the above embodiment, the configuration is shown in which the photodetector converts the transmitted light into an electric signal and then transmits the electric signal to each optical receiving terminal. The configuration may be such that the optical signal distributed to each receiving terminal by the branching element is converted into an electric signal. Optical transmission terminal,
Even a configuration in which an optical amplifier is provided in at least one of the optical transmission line and the optical receiving terminal does not depart from the scope of the present invention.

In the above embodiment, an optical communication system in which light transmitted from a plurality of optical transmitting terminals propagates in one direction on a transmission line and is transmitted to a plurality of optical receiving terminals has been described. The present invention can be effectively implemented in a bidirectional optical communication system in which transmission / reception terminals are provided on both sides of a road or an optical communication network such as a bus type.

[0025]

As described above, according to the present invention, each optical transmitting terminal has a part of light from the same light source assigned to a transmission signal, and the rest or other part assigned to the light source. Each of the optical receiving terminals is configured to receive and select and tune, or filter, the beat signal of the light from the same light source electrically, after modulating and multiplexing with a unique high-frequency signal. An optical communication system capable of multiplexing and transmitting a large number of signals with a simple configuration can be realized, and strict temperature control and wavelength stabilization are not required.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating an optical transmitting side according to a first embodiment of the present invention.

FIG. 2 is a diagram schematically illustrating an optical receiving side according to a first embodiment of the present invention.

FIG. 3 is a diagram schematically illustrating an optical transmission side according to a second embodiment of the present invention.

FIG. 4 is a diagram schematically illustrating an optical receiving side according to a second embodiment of the present invention.

[Explanation of symbols]

 T1,..., TN transmission terminal R1,..., RN reception terminal 71 to 7N detection circuit 8 Optical fiber transmission line

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁷ Identification code FI H04J 14/02

Claims (5)

(57) [Claims] 1. modulating a portion of light from at least one light source with a transmission signal and modulating the remainder or another portion with a high frequency signal having a unique frequency assigned to the light source;
An optical transmitter provided with a unit for combining these modulated signal lights and transmitting the modulated signal light to a transmission path, and a unique transmitter assigned to the light source after converting the combined modulated signal light and transmitting the modulated signal light to an electric signal. An optical communication system comprising: an optical receiver provided with means for detecting and receiving a signal having the same center frequency as the frequency of the signal. 2. The apparatus according to claim 1, wherein a plurality of said optical transmitters and a plurality of optical receivers are provided, and said plurality of optical transmitters and said optical receiver communicate with each other via at least one optical transmission line. The optical communication system according to claim 1. 3. The detecting and receiving means provided in the optical receiver has a function of tuning to, selecting and receiving a specific frequency assigned to a light source of a plurality of optical transmitters. Item 3. The optical communication system according to item 1 or 2. 4. The light to be modulated by the transmission signal and the light to be modulated by a high-frequency signal having a specific frequency assigned to a light source use light output to both sides of a laser resonator as a light source, respectively. 4. The method according to claim 1, wherein
An optical communication system according to claim 1. 5. The optical receiver according to claim 1, wherein after converting the modulated signal light into an electric signal by a photodetector, the terminal has substantially the same frequency as a high-frequency signal assigned to a light source of the optical transmitter desired to be received. 4. A desired signal is received by filtering the electric signal with a band-pass filter having a transmission center frequency.
Or the optical communication system according to 4.