JPS6218131A - Wavelength multiplex optical communication system - Google Patents

Abstract

PURPOSE:To relax remarkably the limitation in the transmission range and the operating wavelength region due to dispersion of an optical fiber by using an optical fiber bundle having a dispersion of opposite sign to that of the optical fiber so as to cancel the dispersion at each wavelength. CONSTITUTION:A signal light in wavelength lambda1-lambda5 irradiated from a semiconductor laser is collected and fed to optical fibers 11-15, subject to wavelength multiplex at a light synthesis circuit 5 and led to an optical fiber 2. A wavelength multiplex at a light synthesis circuit 5 and led to an optical fiber 2. A wavelength multiplex signal propagated through the fiber 2 is multiplexed into a signal light of wavelength lambda1-lambda5 and received by photodetectors 71-75. Then, a signal sent at each wavelength is extracted from electric signal output terminals 81-85. When the dispersion of the fiber 2 is MO(lambdai) and the length is LO, each dispersion Mi(lambdai) and each length Li of the fibers 11-15 are set to satisfy Mi(lambdai).Li=-MO(lambdai).Li. Thus, the dispersion of the fiber 2 is compensated b the fiber bundle 1.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a wavelength division multiplexing optical communication system, in particular, the limitations on transmission distance and wavelength range used due to dispersion of optical fiber transmission lines are significantly relaxed compared to the conventional technology. This invention relates to a wavelength division multiplexing optical communication system.

[Prior art and its problems]

With the recent development of high-performance single-axis mode semiconductor lasers and low-loss single-mode optical fibers, 1. Transmission speed 2Gb
High-speed, long-distance optical communications with transmission speeds of /s or more and transmission distances of 1,100 km or more have become possible at the laboratory level.

With the aim of further expanding transmission capacity by taking advantage of the broadband properties of optical fibers, studies are now being conducted on high-speed, long-distance wavelength-division multiplexing optical communication systems that multiplex and transmit multiple lights of different wavelengths. (Electronics Letters (Electron, Lett), No. 21
Vol., 1985, pp. 105-106).

On the other hand, optical fibers have a property, that is, dispersion, in which the fiber propagation time of light differs depending on the wavelength, so if the light source has spectral broadening, there is a problem that the pulse width of the light will expand due to fiber propagation. In order to minimize the spread of pulse width due to this dispersion, it is necessary to match the center wavelength of the transmitted signal light to the zero dispersion wavelength of the optical fiber. This is particularly important in high-speed optical communications, since the spectral broadening of semiconductor lasers becomes larger during high-speed modulation.

However, in wavelength division multiplexing optical communication in which multiple wavelengths of signal light are multiplexed and transmitted, although the signal light of one wavelength can match the zero dispersion wavelength of the optical fiber, the signal light of other wavelengths cannot match. I can't do it. As a result, in conventional wavelength division multiplexing optical communication systems, in signal light whose wavelength does not match the zero dispersion wavelength of the optical fiber,
In other words, the transmission distance is limited by the spread of the pulse width due to dispersion. Therefore, the dispersion of optical fibers has been a major impediment to increasing capacity using wavelength multiplexing.Furthermore, as the dispersion value increases, the pulse width expands, which limits the wavelength range that can be multiplexed. was.

[Purpose of the invention]

An object of the present invention is to provide a wavelength division multiplexing optical communication system which eliminates the drawbacks of the conventional system as described above and significantly reduces the limitations on transmission distance and usable wavelength range due to dispersion of optical fiber transmission lines compared to the conventional system. The purpose is to provide and provide.

[Structure of the invention]

The wavelength multiplexing optical communication system of the present invention combines N signal light sources each having a wavelength of λ+ (i=1.2...N, where N is a natural number of 2 or more) and light of at least N wavelengths. An optical multiplexing circuit that separates light of at least N wavelengths, an optical demultiplexer circuit that separates light of at least N wavelengths, N photodetectors that receive the separated light of each wavelength, and separates the light of N wavelengths from each other at a predetermined distance. , respectively, with dispersion Mj (λi) and length Li (+=1
．． 2. ... N) and dispersion M, (
λl).

an optical fiber having a length Lo, and the optical fiber east is ML(λi)·L,=Mo(λi)·(i=1.2
．． ...N) conditions for each wavelength λi (i=1
．． 2. ...N) is satisfied.

According to a preferred embodiment of the invention, the optical fiber bundle comprises:
The configuration is characterized in that it is inserted between the N signal light sources and the optical multiplexing circuit.

According to another preferred embodiment of the present invention, the configuration is characterized in that the optical fiber bundle is inserted between the optical demultiplexing circuit and the N photodetectors.

[Function/principle of the invention]

In this configuration, the influence of dispersion of the optical fiber that transmits wavelength-multiplexed light is canceled out at each wavelength by using the optical fiber east having dispersion of the opposite sign to that of the optical fiber. It is.

As a result, the pulse width of the signal light after propagating through the optical fiber bundle and the optical fiber can be made almost the same as the pulse width during transmission.

Therefore, according to the present invention, the transmission distance is determined only by three conditions: the power level of the transmitted light, the loss of the optical fiber, and the minimum light reception level given based on the noise characteristics of the light receiver, so that the ideal A wavelength division multiplexing optical communication system can be constructed.

The reason why the influence of dispersion can be offset by the present invention will be explained below.

Generally, the dispersion M(λ) of an optical fiber at wavelength λ is defined as follows. Here, L is the length of the optical fiber, and τ is the propagation time of light in the optical fiber of length L.

The pulse width △t after a signal light having a pulse width △tI'h at the time of transmission is transmitted through an optical fiber having such a dispersion M(λ) and a length L to a center wavelength λi and a spectral width 21 ,
ouL is △t1゜ut−(△j+”)2+(M(λi)・L・△
λi)2...■ ("Optical Fiber", Ohmsha (1981), p. 290).

Therefore, the dispersion Mo(λi), length. The dispersion M, (λl) + length of the optical fiber and the optical fiber east, and the pulse width Δ' of the signal light after propagating through the optical fiber.
17, o u L is △T□. u'-1...■.

Therefore, each wavelength λi (i=1.2...N.

N is a natural number greater than or equal to 2), and Mt(λi)・L+=M. (λi)・Lo(i=1.2
...N) ...■ If the optical fiber east is configured using N optical fibers that satisfy the condition, △T1゜ut-△t1"0(i=1
．． 2...N), so the broadening of the pulse width due to dispersion can be removed.

Furthermore, it can be seen that in the wavelength division multiplexing optical communication system according to the present invention, there is no restriction on the wavelength range to be used as long as there is an optical fiber chain that satisfies equation (2).

The dispersion of a -mode optical fiber can be changed by appropriately controlling the core diameter and relative refractive index difference, and its zero dispersion wavelength can be changed anywhere in the wavelength range from 1.3 μm to 1.7 μm. (Electronics Letters (El
ectron, Lett, ), Volume 15, 197
9, pp. 474-476). Therefore, it is possible to manufacture the above-mentioned optical fiber which satisfies the formula (2) in any wavelength range of 1.3 μm to 1.6 μm where the optical fiber has low loss.

Furthermore, from equation (2), the fiber length L* (i
=1.2. ...N) is (i=1.2...N)
・・・It is given by ■.

■As is clear from the formula, the optical fiber constituting the optical fiber east has a wavelength λi (i=1.2°...N).
If we use one with large variance in M, (λi) >
M o (λi), so each fiber length L
t (i=1.2...N) beam. It can be made shorter than .

〔Example〕

Next, the wavelength division multiplexing optical communication system of the present invention will be explained in detail with reference to the drawings.

FIG. 1 shows an example of the present invention, and FIG. 2 is a diagram showing the dispersion characteristics of all optical fibers used in this example. This embodiment is a 5-wavelength multiplexing optical communication system, and the optical fiber east 1 that compensates for the dispersion of the optical fiber 2 is
It is inserted between the signal light sources 31 to 35 and the optical multiplexing circuit 5.

In FIG. 1, signal light sources 31.32.33.34.3
5, each oscillation wavelength is )+=1.50At
m.

λ2=1.52μm, λ3=1.55μm, λ4=
1.57 μm.

I noG a set to λs = 1.60 p m
+-41A S I-YPy/InP distributed feedback single-axis mode semiconductor lasers are used, and they are each independently modulated by 2Gb/s binary code electric pulses input to the electric signal input terminals 41 to 45. has been done.

Signal lights of wavelengths λi to λ5 emitted from these semiconductor lasers are condensed and coupled to optical fibers 11 to 15, respectively. The optical fibers 11 to 15 are
It is connected to the optical multiplexing circuit 5, and the wavelength 21 to λi described above is connected to the optical multiplexing circuit 5.
The signal light is guided to the optical fiber 2 after wavelength multiplexing.

Further, the wavelength-multiplexed signal light propagated through the optical fiber 2 is demultiplexed by the optical demultiplexing circuit 6 into signal lights of wavelengths λi to λ5, and then received by the photodetectors 71 to 75. The signals sent by each wavelength are then extracted from electrical signal output terminals 81-85.

Here, the optical fibers 11 to 15 have a core diameter of 5 to 10 μm.
, relative refractive index difference 1. It is a single mode silica fiber with a Q~0.3% and a length of 2Qkm, and its dispersion characteristics are set as shown in Figure 2 by controlling its core diameter and relative refractive index difference. There is. Optical fiber 2 has the same core diameter and relative refractive index difference as optical fiber 15 (core diameter 5 μm).
, relative refractive index difference 1.0%), its zero dispersion wavelength is 1.60
μm, and the length is approximately 1100k. In addition, the optical multiplexing circuit 5
The optical demultiplexing circuit 6 is constructed using a diffraction grating and a lens, and the photodetectors 71 to 75 are 1nGaAs avalanche photodiodes (InGaAs-APD) with a receiving diameter of 50 μmφ. ing.

In this embodiment, the dispersion of the optical fibers 11 to 15 is M.

(λ+) (i-1, 2, 3, 4, 5), and the dispersion of optical fiber 2 is expressed as Mo (λ+) (i-1, 2, 3, 4, 5), these values are the second Since it is designed as shown in the figure, Mi λi) L1=-M. (λi)
・Lo(i=1.2.3,4.5)...■. For example, if 1=1, M, (λi-150
μm) = 14 ps/nm-km, L+ = 20 km.

Mo (λi=1.50μm)=-2,8pS/nm
-km.

Since it is Lo-100km, it can be seen that the condition of formula 0 is satisfied. The same applies to other wavelengths λ2 to λi. Therefore, in this embodiment, the wavelength λi (1=1.2
, 3, 4.5), no broadening of the pulse width due to optical fiber propagation was observed as expected.

Next, the performance of this system will be specifically explained.

The peak power of the signal light coupled to the optical fibers 11 to 15 is +ldBm at any wavelength. Furthermore, the transmission losses of the optical fibers 11 to 15 and the optical fiber 2 were greatest at the wavelength λi = 1.50 μm, and the values were approximately 0.21 dB/km and approximately 0.20 dB/km, respectively. Furthermore, the insertion losses of the optical multiplexing circuit 5 and the optical demultiplexing circuit 6 were each 3 dB. Therefore, the received peak power of the signal light at each wavelength was -29.2 dB m at a wavelength of 1.50 μm, and was approximately 1 to 2 dB higher than that at other wavelengths.

On the other hand, the power penalty due to optical fiber dispersion in this embodiment is less than 0.1 dB, and the bit error rate is 10-9.
The receiving sensitivity at each wavelength was approximately -30 dBm.

That is, in this system, the margin during 5-wavelength multiplexed 120 km transmission was 1 dB to 3 dB.

On the other hand, when the optical fiber 2 is set to 120 km and transmitted as in the conventional system, the wavelength is 1.50 μm.
A power penalty of approximately 3.5 dB occurred at m, and it was not possible to receive signal light at that wavelength with an error rate of 10-9 or less.

Although the wavelength division multiplexing optical communication system according to the present invention has been described above using one embodiment, the present invention is not limited to this embodiment, and several modifications can be made.

For example, in this example, 5 wavelengths are multiplexed in the 1.5 μm band, but the number of wavelength multiplexed channels may be any natural number as long as it is 2 or more, and the wavelength range used is 1. Other wavelength ranges such as the .3 μm band may also be used. In addition, in this embodiment, the entire length of the optical fiber east 1 is 2.
Although the length is set at 3 km, it goes without saying that the length of each optical fiber may be changed as long as the condition of the above-mentioned formula (2) is satisfied. In addition, optical multiplexing circuits and optical demultiplexing circuits may use interference film filters, and optical fibers may use GaO
An optical fiber having a core of 2 or P2O5, or a multimode optical fiber' may also be used.

Furthermore, the optical fiber bundle 1 that compensates for the dispersion of the optical fibers 2 may be inserted between the optical demultiplexing circuit and the optical receiver.

〔Effect of the invention〕

As explained above, in the wavelength division multiplexing optical communication system according to the present invention, the dispersion in the optical fiber that transmits the wavelength-multiplexed light is controlled by using another optical fiber having dispersion of the opposite sign to that of this optical fiber. Since each signal light wavelength is canceled out, there is an advantage that restrictions on the transmission distance and the wavelength range to be used due to dispersion of the optical fiber are significantly relaxed compared to the conventional method.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment according to the present invention, and FIG.
FIG. 3 is a diagram showing the dispersion characteristics of an optical fiber used in an example according to the present invention. 1・・・・・・・・・・・・・・・・・・・・・・・・
...Optical fiber east 11, 12. 13. 14.
15...Optical fiber 2...
・・・・・・・・・・・・Optical fiber 31, 32.3
3.34.35...Signal light source 41, 42.43.44
．． 45... Electric signal input terminal 5...
・・・・・・・・・・・・・・・・Optical multiplexing circuit 6・
・・・・・・・・・・・・・・・・・・・・・・・・
・Optical demultiplexing circuit 71, 72.73.74.75...Photodetector 81, 82.83.84.85...Electric signal output terminal agent Patent attorney Yoshiyuki Iwasa-To n dimension ω dimension + + Dimensions

Claims (3)

[Claims] (1) Each wavelength is λ_i (i=1, 2,...N
, N is a natural number of 2 or more), an optical multiplexer circuit that multiplexes light of at least N wavelengths, an optical demultiplexer circuit that demultiplexes light of at least N wavelengths, and an optical demultiplexer circuit that multiplexes light of at least N wavelengths; N photodetectors that receive light of each wavelength and dispersion M_i(λ
__i), an optical fiber bundle consisting of N optical fibers having length L_i (i=1, 2, . . . N), dispersion M_o (λ_i) for transmitting wavelength-multiplexed light, and length L_o. and the optical fiber bundle has the following formula: M_i(λ_i)·L_i=−M_o(λ_i)·L_
i (i=1, 2,...N conditions are set to each wavelength λ_
A wavelength division multiplexing optical communication system characterized in that i (i=1, 2, . . . N) is satisfied. (2) A wavelength multiplexing optical communication system according to claim 1, wherein the optical fiber bundle is inserted between the N signal light sources and the optical multiplexing circuit. Optical communication system. (3) In the wavelength division multiplexing optical communication system according to claim 1, the optical fiber bundle is inserted between the optical demultiplexing circuit and the N photodetectors. Wavelength multiplexing optical communication system.