JP2001237804A - Optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical communication system that can realize prevention of an optical beat noise and improvement of signal quality at a low cost thereby increasing the number of signals that can be multiplexed from transmission stations and be applied to all electric modulation signals independently of whether or not a modulation component exists in the amplitude of the signal. SOLUTION: This invention of the optical communication system where the transmission stations 1A, 1B,..., 1N convert electric modulation signals with different subcarrier frequencies fa, fb,..., fn into optical signals, the optical signals from each transmission station are multiplexed into a signal that is sent through an optical fiber 4 to a reception station 5, is characterized in that an optical switch 3 is used to sequentially switch the optical signals from the transmission stations 1A, 1B,..., 1N to sample the optical signals into optical pulses, the optical pulse signals are multiplexed in a way of not being overlapped on a time base, the multiplexed signals are transmitted through the optical fiber 4 to the reception station 5, and the reception station uses a single photoelectric converter (optical detector) to detect them altogether.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for converting an electric modulation signal having a different subcarrier frequency into an optical signal at each transmitting station, multiplexing the optical signal from each transmitting station onto one optical fiber, and multiplexing the received optical signal at a receiving station. The present invention relates to an optical communication system for transmitting data up to a maximum.

[0002]

2. Description of the Related Art Conventionally, in an optical communication system using an optical fiber, an electric modulation signal having a different subcarrier frequency is converted into an optical signal in an uplink, and an optical signal from each transmitting station is converted into an optical signal. A so-called optical sub-carrier multiple access system, in which a multiplexed signal is transmitted to one optical fiber using a coupler (optical power combiner) and transmitted collectively by a single optical detector, is known. .

[0003]

However, in the above-described optical subcarrier multiple access system using an optical coupler, a plurality of optical signals are simultaneously detected at the time of optical detection. Beat noise occurs, deteriorating signal quality. This occurs because a frequency difference of the same order as the sub-carrier frequency occurs between the optical carriers used by each transmitting station. Therefore, the number of multiplexed optical signals is limited, and the number of transmitting stations is limited.

In order to prevent optical beat noise and suppress deterioration of signal quality, the wavelength of laser light used in the electro-optical converter of each transmitting station may be controlled and stabilized with high accuracy. It is necessary to equip the transmitting station with an advanced and high-cost control device, which increases the network construction cost.

[0005] To convert an electric modulation signal into an optical signal, there has been proposed a so-called overmodulation method in which a laser beam is modulated with an excessive injection current. It is possible to suppress optical beat noise (THWood, et.al. "Opera
tion of a Passive Optical Network with Subcarrier
Multiplexing in the Presence of Optical Beat Inter
ference ", Journal of Light. Tech., vol. 11, No. 10, Oct.
ober1993)

However, in this overmodulation method, although optical beat noise generated at the time of collective detection of received light can be suppressed, it cannot be eliminated in principle.
Also in this case, the number of multiplexed optical signals is limited, and the number of transmitting stations is also limited.

In the above overmodulation method, a large nonlinear distortion is caused in the amplitude of the electric information signal superimposed on the optical signal. Therefore, the overmodulation method is effective for an electric modulation signal having no information in the amplitude. Value quadrature amplitude modulation (QAM)
It cannot be applied to an electric modulation signal in which the amplitude has a modulation component such as tude modulation.

[0008] The present invention has been proposed in view of the above,
The optical beat noise can be prevented and the signal quality can be improved without increasing the cost, so that the number of multiplexable transmitting stations can be increased. It is another object of the present invention to provide an optical communication system that can be applied to any electric modulation signal.

[0009]

In order to achieve the above object, according to the first aspect of the present invention, each transmitting station converts an electric modulation signal having a subcarrier frequency different from each other into an optical signal, and transmits each of the signals. In an optical communication system in which an optical signal from a station is multiplexed on one optical fiber and transmitted to a receiving station, the optical signal from each of the transmitting stations is sequentially switched using an optical switch, and is optically sampled to form a pulse. The optical signals are multiplexed so as not to overlap on the time axis, transmitted to one optical fiber and transmitted to a receiving station, and the receiving station collectively detects the signals with a single photoelectric converter.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the optical communication system of the present invention. In the figure, an optical communication system according to the present invention comprises transmitting stations 1A, 1B,.
Up to the transmission line 1
A, 1B,..., 1N convert electric modulation signals having different subcarrier frequencies into optical signals, and multiplex each optical signal obtained by the conversion into one optical fiber transmission line 4 by the optical sampling multiplexer 3. This is a system for transmitting data to the receiving station 5. This optical communication system includes a plurality of transmitting stations 1A,
, 1N, an optical sampling multiplexing device 3, and a receiving station 5. An optical fiber transmission line 2 is provided between the transmission stations 1A, 1B,.
A, 2B,..., 2N, and the optical sampling multiplexing apparatus 3 and the receiving station 5 are connected by one optical fiber transmission line 4.

The transmitting station 1A includes a modulator 11A for converting transmission information into an appropriate electric modulation signal, and a frequency converter 12 for converting the electric modulation signal into an appropriate subcarrier frequency fa.
A and an electro-optical converter 13A for converting an electric modulation signal having the sub-carrier frequency fa into an optical signal.

The transmitting stations 1B,..., 1N correspond to the transmitting station 1 described above.
As in the case of A, a modulation device, a frequency converter, and an electro-optical converter are provided, and have substantially the same configuration. And transmitting station 1
A, 1B,..., 1N frequency converters 12A, 12B,
, 12N, the frequency converter 12A converts the electric modulation signal into an electric modulation signal having the subcarrier frequency fa as described above, and the frequency converter 12B converts the electric modulation signal into an electric modulation signal having the subcarrier frequency fb. , Frequency converter 12
N converts the electric modulation signal into an electric modulation signal having a subcarrier frequency fn, and so on, into electric modulation signals having different subcarrier frequencies.

The electro-optical converters 13A, 13B,..., 13N
Are the different sub-carrier frequencies fa, fb,
, Fn are converted into optical signals. The optical signals are transmitted through optical fiber transmission lines 2A, 2B,.
The signal is transmitted to the optical sampling multiplexer 3 via 2N.

The optical sampling and multiplexing device 3 includes:
The optical signals from A, 1B,..., 1N are sequentially switched using an optical switch and optically sampled to form a pulse, and the pulsed optical signals are multiplexed so as not to overlap on the time axis. That is, the optical sampling and multiplexing device 3 includes, for example, a high-speed optical switch that performs a rotary type operation using the electro-optic effect, and as illustrated schematically in FIG. 1, connects the optical path 3Z to the optical fiber transmission paths 2A and 2B. ,..., 2N are sequentially switched to time-division multiplexing, and the optical signal S obtained by the time-division multiplexing is transmitted to one optical fiber transmission line 4 to the receiving station 5. Transmit.

The receiving station 5 includes an opto-electric converter 6, band-pass filters 7A, 7B,..., 7N, and demodulators 8A, 8B,.
8N. The optical signal S time-division multiplexed as described above is transmitted through the optical fiber transmission line 4 to the receiving station 5.
When the optical signal S enters, the optical signal S is first detected by the photoelectric converter 6 at a time and converted into an electric signal. This electric signal passes through the bandpass filters 7A, 7B,..., 7N, so that the sub-carrier frequencies fa, f at the transmitting stations 1A, 1B,.
b,..., fn are electric modulation signals having the same frequency as the demodulators 8A, 8B,
.., 8N. Demodulators 8A, 8B,
.., 8N demodulate the electric modulation signal, and
B,..., 1N, take out the original information signal. Also, if the demodulation device has a function of directly demodulating and converting the input subcarrier frequency modulation signal into an intermediate frequency band without converting it into an intermediate frequency band, the band filter at the preceding stage is not necessarily required. unnecessary.

As described above, in the optical communication system of the present invention, the optical signal S time-division multiplexed using the optical sampling multiplexing device 3 is input to the receiving station 5. The detector 6 instantaneously detects only one optical signal, and can greatly suppress or eliminate optical beat noise that has conventionally been caused by simultaneously detecting a plurality of optical signals. Therefore, optical beat noise can be suppressed without controlling the wavelength of laser light on the transmitting side with high precision, and optical beat noise can be prevented and signal quality can be improved without increasing costs. can do. Further, since it is not necessary to control the wavelength of the laser beam on the transmitting side with high accuracy, the transmitting stations 1A, 1
B,..., 1N can be reduced in size and configured at low cost.

Since the optical beat noise can be suppressed, the number of multiplexable optical signals can be greatly increased, the number of transmitting stations can be increased, and a low-cost optical communication network can be constructed. it can.

Further, according to the present invention, the transmitting stations 1A, 1
When converting the electric modulation signal into an optical signal in B,..., 1N, the linear modulation region of the electric modulation signal is used, so that no amplitude distortion occurs in the information signal restored after detection,
Therefore, it can be applied to any electric modulation signal regardless of whether or not the amplitude has a modulation component.

The above invention can be applied to, for example, a subscriber access system using a wired optical fiber access network. In that case, the transmitting stations 1A, 1B,.
Corresponds to a subscriber station, and an information signal inputted by each subscriber is transmitted from the subscriber station to the receiving station 5 and reproduced with high quality.

The above-mentioned invention can be applied to, for example, an uplink in an optical-wireless fusion communication system.
In this case, the transmitting stations 1A, 1B,..., 1N correspond to radio base stations, and the information signals received by the antennas of the radio base stations have already been modulated. Input to the container. Then, the information signal is sent to the control station including the receiving device,
It will be reproduced with high quality.

[0022]

As described above, in the optical communication system according to the present invention, the optical signals from the respective transmitting stations are sequentially switched using the optical switches and optically sampled to form pulses.
The pulsed optical signals are multiplexed so that they do not overlap on the time axis and are sent out to one optical fiber and transmitted to the receiving station, so when detecting at the receiving station, only one optical signal is instantaneous. Is detected, and the optical beat noise which has conventionally been generated by simultaneously detecting a plurality of optical signals can be greatly suppressed or eliminated.
Therefore, optical beat noise can be suppressed without controlling the wavelength of laser light on the transmitting side with high precision, and optical beat noise can be prevented and signal quality can be improved without increasing costs. can do. In addition, since it is not necessary to control the wavelength of the laser beam on the transmitting side with high precision, the transmitting station can be reduced in size and configured at low cost.

Also, since the optical beat noise can be suppressed, the number of optical signals that can be multiplexed can be greatly increased, the number of transmitting stations can be increased, and a low-cost optical communication network can be constructed. it can.

Further, according to the present invention, when the electric modulation signal is converted into an optical signal at the transmitting station, the linear modulation region of the electric modulation signal is used, so that any amplitude distortion is generated in the information signal restored after detection. Therefore, it can be applied to any electric modulation signal regardless of whether or not the amplitude has a modulation component.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an optical communication system according to the present invention.

[Explanation of symbols]

1A, 1B, ..., 1N Transmitting stations 11A, 11B, ..., 11N Modulators 12A, 12B, ..., 12N Frequency converters 13A, 13B, ..., 13N Electro-optical converters 2A, 2B, ..., 2N Optical fiber transmission lines 3 Optical sampling and multiplexing device 3A, 3B,..., 3N Output terminal of optical fiber transmission line 3Z Optical path 4 Optical fiber transmission line 5 Receiving station 6 Photoelectric converter 7A, 7B,..., 7N Band filter 8A, 8B, .... 8N demodulator

Claims (1)

[Claims] An optical communication system in which each transmitting station converts an electric modulation signal having a different subcarrier frequency into an optical signal, multiplexes the optical signal from each transmitting station onto one optical fiber, and transmits the multiplexed signal to a receiving station. In the above, the optical signals from each of the transmitting stations are sequentially switched using an optical switch and optically sampled to form a pulse, and the pulsed optical signals are multiplexed so as not to overlap on the time axis and transmitted to one optical fiber. An optical communication system, wherein the signal is transmitted to a receiving station, and the receiving station collectively detects the signal with a single photoelectric converter.