JPH0659141A - Large-input optical cable - Google Patents

Abstract

PURPOSE:To provide the large-input optical cable which performs unrepeated transmission with a longer span than usual and has low loss and high reliability by rationally connecting optical fibers which differ in Brillouin shift frequency. CONSTITUTION:Lengthwise adjacent optical fibers of the optical cable constituted by connecting optical fibers which have a Brillouin shift frequency A (MHz) and Brillouin gain band half-value overall width 2B (MHz) roughly satisfy relations Ai-1+Bi+1<=Ai-Bi and Ai+Bi<=Ai+1>=Ai-1-Bi+1 (where i=1, 2, 3... show an (i)th fiber).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large input optical cable suitable for long-distance communication, and more particularly to improvement of its connection structure.

[0002]

2. Description of the Related Art In an optical communication system using an optical cable, an optical signal emitted from a light source is transmitted to a remote place using an optical fiber and detected by a light receiver.

The power of the optical signal propagating in the optical fiber is large, the optical fiber as the transmission medium has a low loss,
Moreover, the higher the sensitivity of the light receiver, the longer the distance can be transmitted.

However, in the conventional optical cable, the light source power incident on the optical fiber is reflected in the fiber by the stimulated Brillouin scattering phenomenon (hereinafter referred to as SBS: stimulated Brillouin scattering). However, there is a drawback in that an upper limit occurs in the optical power that can be transmitted through the optical fiber (hereinafter referred to as the transmission power).

As a solution to this drawback, as disclosed in Japanese Patent Application No. 3-25242 filed by the same applicant, in a cable structure in which strain is distributed in the longitudinal direction of the optical fiber, the Brillouin gain bandwidth is expanded. By,
This transmission power can be increased.

[0006]

However, in this structure, there is a possibility that new defects may occur such as deterioration of mechanical strength of the optical fiber and increase of transmission loss of the optical fiber due to expansion of strain. .

Therefore, the present invention has been made in view of the above points, and by reasonably connecting optical fibers having different Brillouin shift frequencies, it is possible to perform unrepeatered transmission with a longer span than conventional. The purpose of the present invention is to provide a large input optical cable with low loss and high reliability.

[0008]

[Means and Actions for Solving the Problem] Japanese Patent Application No. Hei 3
As disclosed in Japanese Patent No. 25242, the Brillouin gain bandwidth can be expanded by distributing the strain in the longitudinal direction of the optical fiber.

FIG. 1 shows a case where the distortion is changed in a sine wave shape.
The linear scale shows the calculation result of the relation between the distortion amplitude and the bandwidth. The scale of the vertical axis is arbitrary. In the calculation, the bandwidth without distortion was set to 50 MHz. Further, the following equation was used to calculate the Brillouin shift frequency V.

V = 12680xCxε (1) However, ε is the optical fiber strain, and the constant C = 4.6 It can be seen from FIG. 6 that the gain peak decreases according to the strain amount. If the bandwidth at which the peak height is halved is defined as the Brillouin gain bandwidth, the bandwidth can be expanded to about 900 MHz, or 18 times the initial value, by adding a distortion with an amplitude of about 0.8%.

Assuming that the critical optical input that causes stimulated Brillouin scattering is proportional to the Brillouin gain bandwidth, it is possible to input 18 times, that is, 12.5 dB of optical power, so that the loss of the optical fiber is 0.18 dB / If the distance is km, the transmission distance can be expanded to about 69 km.

In this method, it is necessary to double the distortion amount of the optical fiber in order to roughly double the gain bandwidth.

On the other hand, FIG. 2 shows (1) an optical fiber in which the distortion is distributed in a sinusoidal shape with an amplitude of 0.4%, and (2) the average distortion is + with an amplitude of 0.2%.
The calculated Brillouin gain waveforms (3) when two types of optical fibers distributed in a 0.2% and -0.2% sinusoidal shape are connected in tandem are shown.

As can be seen from FIG. 2, the Brillouin gain bandwidths of (1) and (2) are equal at about 440 MHz. Thus, even if the individual strain amplitudes are small, by combining optical fibers having different Brillouin shift frequencies, it is possible to obtain the same bandwidth as when a large strain amplitude is applied.

Therefore, when a plurality of optical fibers having a Brillouin shift frequency A (MHz) and a Brillouin gain band full width at half maximum 2B (MHz) are connected in a tandem (longitudinal direction) as shown in FIG. Optical fibers that were next to each other 1,2,
3 ... i-1, i, i + 1 ... each _{A i-1 + B i-} 1 ≦ A i -B i, A i + B i ≦ A i + 1 -B i + 1 ... the relationship (2) It is necessary to make connections so as to satisfy the requirements so that the Brillouin gain frequencies of the respective optical fibers do not overlap. However, i = 1,2,3 ... indicates the i-th optical fiber connected in tandem. Further, by connecting the optical fibers in the longitudinal direction so that the relationship of the expression (2) is almost satisfied, the strain can be reduced.

[0016]

EXAMPLE FIG. 4 shows an experimental result of Brillouin shift frequency when a light source having a wavelength of 1.550 μm is used.

FIG. 5 shows the parameters of the optical fiber used. 4 and 5, PSF is a pure silica core optical fiber, SM is a 1.3 μm zero-dispersion optical fiber, and DSF is a 1.55 μm zero-dispersion optical fiber.

The G _e O ₂ molar concentration of these optical fibers is about 0%, about 3.7% and about 8% (central portion 10%, peripheral portion 2%), and the _Ge O ₂ molar concentration is 0-10. It can be seen that the Brillouin shift frequency decreases in proportion to the amount of G _e O _{2 in} the range of%.

Further, the Brillouin shift frequencies of these three types of optical fibers are about 3 each centering on SM from FIG.
You can see that they are 30MHz apart.

FIGS. 6 and 7 show the calculated values of the Brillouin gain waveform when the DSF or PSF optical fiber and the SM optical fiber are connected, using the distortion amplitude as a parameter.

From this, it can be seen that the peak of the combined waveform of the Brillouin gain can be minimized by applying the distortion of 0.26% to 0.28%.

By connecting these three types of optical fibers with the SM optical fiber as the center, the Brillouin gain band can be expanded to about 3 times 330 MHz, that is, 1 GHz.

This corresponds to 20 times (13 dB) the Brillouin gain bandwidth of 50 MHz of a normal optical cable.

The applied strain in this case is the applied strain in FIG.
It is only 1/3 of 0.8%, which makes it possible to significantly reduce the amount of strain.

Based on the above calculation results, the strain holding optical unit 10 shown in FIG.
A prototype cable 100 was prepared by forming a double spiral structure using No. 1, and a sinusoidal strain distribution with an amplitude of 0.27% was added to this. FIG. 9 shows the dimensions of the prototype cable 100 and the strain holding unit 101. The cable material used is the same as the optical cable described in Japanese Patent Application No. 3-25242.

That is, the strain maintaining optical unit 101 has an outer diameter
Six optical fiber center lines 103 each having a 0.4 mm outer diameter and a UV urethane coating are arranged around a central body 102 having a 0.4 mm outer diameter and a 0.5 mm outer diameter, and a Z twist. The first and second adhesive resin layers 10
4, 105 are coated.

In the prototype cable 100, six strain holding optical units 101 as described above are arranged on the circumference of the circle.
Tensile strength member 106, slot 107, water absorbing tape 108,
Consists of stainless steel tape 109 and jacket 110,
It has a dimensional relationship as shown in FIG.

FIG. 10 shows the transmission test result of the prototype cable 100. The length of this prototype cable is 1km, and two of the six optical units are drawn from the same preform and the carbon coated optical fiber is used to make the optical fiber of PSF, SM and DSF of 12km length. A fiber line was constructed and these were fusion-spliced for a total length of 36km.

The increase in optical loss in the cabling process is greatest in SM optical fibers, but at a maximum wavelength of 1.55 μm, 10/100
It was less than 0 dB / km.

A wavelength of 1.55μ amplified by an optical amplifier on this line
m, maximum 20 dBm of CW light was incident.

In the case of the conventional optical cable, stimulated Brillouin scattering occurs at an optical power of about 7 dBm or more, and a sudden increase in reflected light is confirmed. However, in the prototype cable, the reflected light does not increase sharply up to the maximum input. An improvement effect of 13 dB was confirmed as designed. In this test, a 1 km-long cable was prototyped in order to confirm the effect of the invention, but in the actual submarine transmission line to which the present invention is applied, in order to reduce the connection loss of the optical fiber and save the cable connection cost. Needless to say, the single length of the optical fiber is about 20 km, and the single length of the optical cable is about 30 to 70 km.

Therefore, in one piece of the optical cable, two to four pieces of optical fibers in which the Brillouin gain frequency is not superimposed are connected in tandem.

[0033]

As described above, according to the present invention, the amount of strain applied to the optical fiber required for suppressing the stimulated Brillouin scattering can be reduced to about 1/3 of that of the conventional one. It is possible to realize an optical cable for long-span transmission with little loss increase and excellent long-term reliability.

[Brief description of drawings]

FIG. 1 is a diagram showing a calculation result of a relation of a strain range width when strain is changed in a limiting wave shape.

[Fig. 2] (1) Optical fiber in which strain is distributed in a sinusoidal shape with an amplitude of 0.4%, and (2) Average strain is + 0.2% and -0.2% at a strain amplitude of 0.2%.
FIG. 6 is a diagram showing a calculated value (3) of a Brillouin gain waveform when two types of optical fibers distributed in a sine wave are connected in tandem.

FIG. 3 is a diagram for explaining the configuration and principle of the present invention.

FIG. 4 is a diagram showing a test result of dependence of a Brillouin shift frequency on an optical fiber according to an embodiment of the present invention.

FIG. 5 is a diagram showing parameters of an optical fiber used in one embodiment of the present invention.

[FIG. 6] DSF or PSF optical fiber and SM according to the present invention
The figure which shows the calculated value of the Brillouin gain waveform of the cable which connected the optical fiber.

FIG. 7 is a diagram showing a calculated value of a Brillouin gain waveform of a cable in which a DSF or PSF optical fiber according to the present invention and an SM optical fiber are connected.

FIG. 8 shows a distortion maintaining optical unit and 2 according to an embodiment of the present invention.
Sectional drawing which shows a heavy spiral structure cable.

FIG. 9 is a diagram showing a transmission test result of a prototype cable according to the present invention.

10 is a diagram showing a dimensional relationship between the cable and the strain holding unit of FIG.

[Explanation of symbols]

 1, 2, 3 ... i-1, i, i + 1 ... Optical fiber A ... Brillouin shift frequency B ... Brillouin gain band half width 100 ... Prototype cable 101 ... Strain holding optical unit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeyuki Imai 1-1-6, Saiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical cable constructed by connecting a plurality of optical fibers having a Brillouin shift frequency A (MHz) and a Brillouin gain band full width at half maximum 2B (MHz) in tandem,
Adjacent optical fibers in the longitudinal direction are A _i-1 + B
_i-1 ≤ A _i -B _i , A _i + B _i ≤ A _{i + 1} -B _{i +1} (where i = 1,2,3 ... indicates the i-th optical fiber) is approximately satisfied. Large input optical cable featuring. 2. The large input optical cable according to claim 1, wherein the molar concentration of G _e O _{2 in} the optical fiber core portion is set to 0 to 10%. 3. A pure silica core optical fiber, a 1.3 μm zero dispersion optical fiber (core portion G _e O ₂ molar concentration: about 3%), and
Large input optical cable according to claim 1, characterized in that connecting the three optical fibers of 1.55μm zero dispersion optical fiber (core portion G _e O ₂ molar concentration of about 8%). 4. A large input optical cable according to claim 1, wherein a strain-holding type optical unit is used, and the optical fibers are assembled into a double spiral to expand the Brillouin gain bandwidth.