JPH04191709A - Optical transmission method and its system - Google Patents

Abstract

PURPOSE:To make a transmission speed high by regulating position of a phase conjugate element to a position where wavelength dispersion arisen at the upstream side of the phase conjugate element and wavelength dispersion arisen at the downstream side of the phase conjugate element come to be equal. CONSTITUTION:As a modulation method in an optical transmission device 2, for example, it is possible to adopt a strength modulation method. In this case, even when direct modulation of a semiconductor laser which is easy to cause wavelength charping is carried out, it is possible to carry out optical transmission without being influenced by wavelength dispersion. As an optical transmission passage 2, a single mode optical fiber of a quartz group with which propagation loss of light becomes minimum at a wavelength 1.55mum can be used. In this case, it is possible to carry out long distance transmission by way of restraining the propagation loss small by setting the wavelength of an output light from the optical transmission device 2 at 1.55mum band.

Description

[Detailed Description of the Invention] Summary An object of the present invention is to provide an optical transmission method and system in which the chromatic dispersion of the optical transmission path is less than the total chromatic dispersion. 2
A phase conjugate element is provided at a position corresponding to 1/2 of the optical transmission path, and an optical signal is transmitted through the optical transmission path.

Industrial applications The present invention relates to an optical transmission method and system.

Optical fibers commonly used as optical transmission lines produce chromatic dispersion of a magnitude corresponding to the spread of the wavelength of transmitted light, which causes adverse effects such as waveform deterioration on the receiving side. In order to suppress the negative effects of chromatic dispersion, use a light source that oscillates at a nearly single wavelength on the transmitting side, or
Although it is required to use an optical fiber as an optical transmission line in which chromatic dispersion becomes zero at the relevant wavelength, it is difficult to realize this. Therefore, there is a need for an optical transmission method or system in which the adverse effects of wavelength dispersion are less likely to occur.

Problems to be Solved by the Prior Art and the Invention Economical optical transmission systems are required to have high transmission speeds and long transmission distances. The typical silica-based single mode optical fiber currently in practical use has a wavelength of 1.3 μm.
However, at a wavelength of 155 μm, where the optical propagation loss is minimum, a chromatic dispersion of about 17 ps/nm-km occurs.

When transmitting 1100k at 10 Gb/s using this optical fiber, a group delay of at least 1 oops occurs, making transmission virtually impossible. To solve this,
It has been proposed to use a dispersion-shifted fiber whose wavelength at which chromatic dispersion becomes zero is set to 1.55 μm, but in this case, the wavelength of the light source and the wavelength at which dispersion becomes zero must be precisely matched. Along with the difficulty of not having one or two people,
The use of conventional optical fibers, which are currently widely installed, results in considerable industrial and economic losses. Furthermore, in an attempt to minimize the negative effects of chromatic dispersion by optimizing the modulation method, etc. by suppressing the spread of the spectrum of modulated light as much as possible, the modulator O drive voltage is high enough to be practical. There are problems such as a lack of

The present invention was created in view of such technical problems, and aims to provide an optical transmission method and system that can overcome the adverse effects of chromatic dispersion without any great difficulty. There is.

Means for Solving the Problems The optical transmission method of the present invention provides a phase conjugate element at a position where the wavelength dispersion of the optical transmission line corresponds to one half of the total wavelength dispersion, and transmits an optical signal through the optical transmission line. It was designed to do so.

As the block diagram of the optical transmission system of the present invention is shown in FIG. The optical receiver 4 includes a phase conjugate element 3 provided at a position corresponding to one-half of the total chromatic dispersion, and an optical receiver 4 that receives an optical signal transmitted through an optical transmission line 2.

In the present invention, a phase conjugate element is provided in the middle of an optical transmission path, and the position of this phase conjugate element is determined so that wavelength dispersion occurring upstream of the phase conjugate element and wavelength dispersion occurring downstream of the phase conjugate element are different from each other. Since the positions are regulated to be equal, the electric field of the received optical signal received on the receiving side becomes the complex conjugate of the electric field of the transmitted optical signal transmitted on the transmitting side, despite the chromatic dispersion. . Therefore, the temporal change in the intensity of the optical signal is preserved as is, and when the intensity modulation/direct detection method is adopted, there is no consideration at all that the transmitted optical signal and the received optical signal are complex conjugates. , optical transmission can be carried out without being adversely affected by wavelength dispersion. Furthermore, even if a method such as a coherent optical transmission method that modulates the amplitude, frequency, phase, etc. of the electric field of light is adopted, the negative effects of chromatic dispersion can be avoided as long as the complex conjugate is taken into consideration. Optical transmission can be carried out without receiving any signal.

Example The present invention will be explained in detail below.

When implementing the system shown in FIG. 1, the modulation method in the optical transmitter l can be, for example, an intensity modulation method. In this case, even if a semiconductor laser, which is prone to wavelength chirping, is directly modulated, optical transmission can be performed without being affected by wavelength dispersion. When constructing a coherent optical transmission system, modulation methods such as ASK, FSK (for example, CPFSK
).

PSK (for example, DPSK) etc. can be adopted.

Transmission information is not limited to digital signals, but may also be analog signals.

As the optical transmission line 2, a quartz-based single mode optical fiber can be used, which has a minimum optical propagation loss at a wavelength of 1.55 μm. In this case, by setting the wavelength of the output light from the optical transmitter 1 to the 1.55 μm band,
Long-distance transmission becomes possible by minimizing propagation loss.

The phase conjugate element 3 can easily be implemented as one that performs phase conjugate conversion on an optical signal based on the phase of pump light having a constant frequency. As the phase conjugate element 3, one that does not use pump light can also be used.

As a configuration example of a phase conjugate element using pump light, one equipped with a nonlinear symmetric Matsuhatsu Ender interferometer using four-wave mixing is shown in FIG. In the same figure, 11, 12, 1
3.14 and 15.16 are optical couplers with a branching ratio of 50%, and 17.18 and 19°20 are third-order nonlinear media. The optical signal is branched by an optical coupler 11, and one of the branched lights is coupled with the pump light by an optical coupler 12 and is branched. After each of the branched lights passes through nonlinear media 17 and 18, they are coupled and branched by the optical coupler 13. One light branched by the optical coupler 13 is coupled with the other light branched by the optical coupler 11 and branched by the optical coupler 14. The light branched by the optical coupler 14 passes through the nonlinear medium 19 and 20, and then is coupled and branched by the optical coupler 15. The other light branched by the optical coupler 13 is coupled with the one light branched by the optical coupler 15 by the optical coupler 16, and this light is sent to the optical transmission line 2 on the optical receiver 4 side. The optical path lengths of all the optical paths from the optical coupler 11 to the optical coupler 16 are the same, and the optical path lengths of all the optical paths from the optical coupler 12 to the optical coupler 15 are also the same.

According to this configuration, the input optical signal can be phase conjugate-transformed using the phase of the pump light having a constant frequency as a reference, and then output.

Hereinafter, the state of change in the optical signal electric field from the optical transmitter 1 to the optical receiver 4 will be conceptually explained in the case where a phase conjugate element using pump light is employed. still,
Phase conjugate conversion of an optical signal using the phase of a pump light of a constant frequency as a reference means converting the incident light into light that is phase conjugate to the pump light when the frequency and phase of the pump light are taken as a reference. means.

Now, the pump light is exp [i. t] and the transmitted optical signal E. (1) Fourier component E o of frequency ω0−△ω
(Δω), the optical signal electric field changes as follows. Optical signal E o (Δω)exp[:i(ω.−△ω)
t] is added with a phase term exp Ci D(Δω)2] due to dispersion in the optical transmission path upstream of the phase conjugate element, and immediately before entering the phase conjugate element, E o (Δ
ω)ey, p Ci D(Δω)'shii(a+o+muω
) t ]. This is caused by phase conjugate transformation, E, (Δω biexp C−i D(Δω)”=i(ω.

−Δ)tE, and then due to dispersion in the optical transmission path downstream of the phase conjugate element, the phase term exp L ]
D(Δω)2] is added, so E,(Δω)”ex
p [i (ω0−Δω)t], and exp31cI
The Fourier component around Eo” (
−△ω). This is the received optical signal E. . (1), which is the complex conjugate of the transmitted optical signal EO(t), regardless of wavelength dispersion.

In addition, in the above explanation, ω. is the angular frequency of the pump light,
△ω is the angular frequency difference between the pump light and the optical signal, and D is the dispersion coefficient (
The unit is, for example, ps/nm), and * is a complex conjugate symbol.

As the phase conjugate element, an optical amplifier can also be used. In a Brilliant optical amplifier, an optical signal and pump light are emitted from one end of an optical fiber, and the backscattered light is extracted as output light. Become. Other examples of phase conjugate elements include Raman optical amplifiers, four-wave mixing optical amplifiers, and parametric optical amplifiers.

When carrying out the present invention, an ordinary optical amplifier that does not perform phase conjugate conversion can be inserted into the optical transmission line for the purpose of further increasing the transmission distance. Examples of this type of optical amplifier include an optical fiber amplifier using an optical fiber whose core is doped with a rare earth element such as Er, and a semiconductor laser type optical amplifier.

As described in detail, the present invention has the advantage that it is possible to provide an optical transmission method and an optical transmission system in which the adverse effects of chromatic dispersion are less likely to occur.

As a result, it becomes possible to use an optical fiber with large wavelength dispersion, or it becomes possible to transmit even light with large wavelength chirping, such as direct intensity modulation light from a semiconductor laser. Therefore, the present invention greatly contributes to increasing the transmission speed and increasing the transmission distance in optical transmission systems.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the optical transmission system of the present invention, and FIG. 2 is a diagram showing an example of the configuration of a phase conjugate element. DESCRIPTION OF SYMBOLS 1... Optical transmitter, 2... Optical transmission line, 3... Phase conjugate element, 4... Optical receiver.

Claims (1)

[Claims] 1. The wavelength dispersion of the optical transmission line (2) is one-half of the total wavelength dispersion
A method in which a phase conjugate element (3) is provided at a position corresponding to , and an optical signal is transmitted through the optical transmission line (2). 2. An optical transmission device (1) that sends out an optical signal, an optical transmission line (2) that transmits the optical signal, and a chromatic dispersion of the optical transmission line (2) that is equivalent to one half of the total chromatic dispersion. 1. An optical transmission system comprising: a phase conjugate element (3) provided at a position where the optical transmission path is a phase conjugate element; and an optical receiver (4) that receives an optical signal transmitted through the optical transmission line (2). 3. The optical transmission system according to claim 2, wherein the phase conjugate element (3) performs phase conjugate conversion on the optical signal based on the phase of pump light having a constant frequency.