JPS62275204A - Low dispersion optical transmission line - Google Patents

Abstract

PURPOSE:To reduce wavelength dispersion in a transmission wavelength range by interposing a dispersion compensation line which cancels wavelength dispersion generated on an existent line in a transmission wavelength range different from specific wavelength in the middle of the existent line which is so set as to has no dispersion only to the specific wavelength. CONSTITUTION:The wavelength dispersion generated on the existent line 6 of length L(km) is m0L(ps/nm), where m0 is the wavelength dispersion coefficient of the existent line 6 of transmission wavelength (ps/km/n); and the dispersion compensating line 7 made of a single mode fiber which satisfies m0L+ml=0 is interposed to eliminate the wavelength dispersion of the total transmission line, i.e. transmission line between the terminal of the transmitter 6 on the existent line 6 and the terminal of the receiver 5. Here, (m)(ps/km/nm) is the wavelength dispersion coefficient of the dispersion compensating line 7 corresponding to the transmission wavelength and l(km) is the substantial length of the single mode fiber constituting the dispersion compensating line 7. Consequently, even if the wavelength dispersion of the transmission wavelength range occurs on the existent line 6, this dispersion is canceled by the dispersion compensating line 7 and the output side, i.e. receiver 5 is not affected by the wavelength dispersion at all.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention Overview For example, in the middle of an existing line that is set to have zero dispersion only for a specific wavelength, an existing line is installed in a transmission wavelength range different from the specific wavelength. A low-dispersion optical transmission line that makes it possible to reduce chromatic dispersion in the transmission wavelength range by inserting a dispersion compensation line that cancels out the chromatic dispersion that occurs in the transmission wavelength range.

INDUSTRIAL APPLICATION FIELD The present invention relates to a low dispersion optical transmission line that reduces chromatic dispersion in a desired transmission wavelength range.
The present invention relates to a low-dispersion optical transmission line that reduces chromatic dispersion that occurs when transmitting other wavelength ranges using an existing line that is set to have zero dispersion for a specific wavelength.

In optical communication systems using optical fibers as transmission paths, single-mode fibers, which are essentially broadband, are considered advantageous in order to increase the number of transmission customers and extend the relay spans over long distances. That is, since single mode fiber does not produce the multimode dispersion seen in multimode fiber, it is extremely useful for widening the transmission line. However,
When using this single mode fiber, the influence of chromatic dispersion caused by the dependence of the group velocity of the transmission mode on the wavelength of light cannot be ignored. Therefore, in order to further widen the band, it is desired to reduce chromatic dispersion in the transmission wavelength range.

BACKGROUND OF THE INVENTION FIG. 7 shows the wavelength dependence (a) of the group delay time and the associated wavelength dependence (b) of the chromatic dispersion coefficient in a typical silica-based single mode fiber. From the same figure, it is clear that the wavelength that gives zero wavelength dispersion, that is, the wavelength where the wavelength differential coefficient of the group delay time is zero, is around 1.3 μm, but this zero dispersion wavelength depends on the type of dopant. It is said that it is difficult to make large changes by controlling the concentration. Furthermore, it is known that transmission loss at a wavelength of 1.3 μm is relatively small in a silica-based single mode fiber. Therefore, in the past, the transmission wavelength was set in the vicinity of 1.3 .mu.m to match the zero dispersion wavelength originally distributed by the fiber, in order to further widen the band.

Problems to be Solved by the Invention Incidentally, it is said that the wavelength range with the least transmission loss in a silica-based single mode fiber is the 1.55 μm band, and in recent years, light transmitting and receiving means that are sufficiently durable for practical use in this wavelength range have been developed. As a result, the transmission wavelength of long-distance, high-capacity optical transmission lines is shifting from the 1.3μ band to the 1.55μ band.

However, when the above-mentioned transmission in the 1.55 .mu.m band is carried out using a conventional transmission line having a zero dispersion wavelength of 1.3 .mu.mats, the following problems occur. (This problem is nothing but the effect of chromatic dispersion itself, but I will explain it in detail to aid understanding.) As shown in FIG. An optical communication system is assumed that includes a receiver 2 and a receiver 2 connected via a transmission path 3. Here, the transmitter 1 is configured using a laser diode (LD) having an oscillation spectrum with a center wavelength at 1.55 μTrL (λ.) as shown in FIG. 9, and the transmission line 3 is
It is assumed that the fiber is made of a single mode fiber having the group delay time and wavelength dispersion characteristics shown in FIG. When using this system to transmit, for example, an optical pulse signal, on the transmitter 1 side, if we pay attention to the oscillation modes up to the second order of the LD's oscillation spectrum, we can see the oscillation modes shown in FIG. 10(a). so that the wavelength λ. It can be regarded as a single pulse P in which a pulse with a wavelength λ0+Δλ and a pulse with a wavelength λ0+Δλ are superimposed in the same phase, but on the receiving side, as shown in FIG. The pulse of +Δλ is Δτ=mo ・Δλ・L for the pulse of wavelength λ0
As a result, these superimposed pulses p are distorted, which prevents widening of the band. Here m. is the wavelength λ of the transmission line 3. Let it represent the chromatic dispersion coefficient at .

The above-mentioned problem is caused by single-mode fibers designed to have zero chromatic dispersion in the 1.55 μm band, or in a wide wavelength band that includes both 1.3 μm and 1.55 μm wavelengths. Although it is possible to solve this problem by providing the following, a large industrial economic loss is expected because the existing 1.3 μm band zero dispersion transmission line cannot be used.

The present invention was created in view of these circumstances, and provides a low-dispersion optical transmission line that can prevent an increase in chromatic dispersion even if the transmission wavelength of an existing transmission line (existing line) is changed. is intended to provide.

Means to solve problems FIG. 1 shows an explanatory diagram of the principle of the low dispersion optical transmission line of the present invention.

In the figure, 4 is a transmitter, 5 is a receiver, and 6 is an existing line connecting the transmitter 4 and receiver 5, and this existing line 6 is made of a single mode fiber having uniform wavelength dispersion characteristics in the longitudinal direction. .

At least one location along the existing line 6 is provided with a dispersion compensating line 7 made of a single-mode fiber having a chromatic dispersion coefficient that is uniform in the longitudinal direction and whose sign is opposite to the chromatic dispersion coefficient of the existing line 6 at the transmission wavelength. Interposed.

The dispersion compensation line 7 is set to a length such that the chromatic dispersion of the existing line 6 and the chromatic dispersion of the dispersion compensation line 7 cancel each other out.

Operation Since the optical transmission line is configured as described above, even if chromatic dispersion occurs in the transmission wavelength range in the existing line 6, this dispersion is canceled out by the dispersion compensation line 7, and the output side, that is, the receiver 5 side is Not affected by wavelength dispersion.

In other words, in FIG. 1, if the chromatic dispersion coefficient of the existing line 6 at the transmission wavelength is m 0 (ps/lv/nm), then
The wavelength dispersion occurring in the existing line 6 with length L (lv) is m
L (Its/nl), and by inserting the dispersion compensation line 7 made of a single mode fiber that satisfies moL + m No. The wavelength dispersion of the transmission line between the ends can be made zero.

Note that m (ps/km/no+) is the chromatic dispersion coefficient at the transmission wavelength of the dispersion compensation line 7, and! (km) is the actual length of the single mode fiber that constitutes the dispersion compensation line 7.

In this case, since the existing line 6 and the dispersion compensation line 7 have uniform wavelength dispersion characteristics in the longitudinal direction, the dispersion compensation line 7 can be easily designed from the relationship between the parameters and definitions described above.

Example EXAMPLES The present invention will be described in more detail below with reference to Examples.

In FIG. 1, the existing line 6 is a zero-dispersion single mode fiber with a wavelength of 1.31 .mu.m, which has been used practically in the past, and the transmitter 4 and receiver 5 are of a wavelength 1.55 .mu.m door specification.

FIG. 2 is a graph showing the chromatic dispersion characteristics of the existing line 6, where the horizontal axis is the wavelength (μm) and the vertical axis is the chromatic dispersion coefficient (Ds
/lv/nm). The unit of chromatic dispersion coefficient (ps/+I/rv) is when using a light source such as an LD that has a wavelength spectrum width of 1 nm (10-9 m) with a single mode fiber of 1 KIR length. 1p
This shows that waveform expansion (wavelength dispersion) occurs in units of s (10 s). Furthermore, in FIG. 2, the + (plus) side of the chromatic dispersion coefficient indicates that if the wavelength deviates by +Δλ with respect to the corresponding wavelength (λ), the optical pulse waveform will be delayed, deviated, and spread.

Therefore, if the dispersion compensation line 7 is not inserted, the existing line 6 having a length of, for example, 40 stories will have a chromatic dispersion of about 800 as/rv at a transmission wavelength of 1.55 μm. Therefore, a dispersion compensating line 7 made of a predetermined length of a single mode fiber having a chromatic dispersion coefficient of a - (minus) sign is inserted into the existing line 6 to cancel out the chromatic dispersion. The insertion position of this dispersion compensation line 7 is not necessarily in the middle of the existing line 6, but for example, it is provided at the end of the existing line 6. In other words, the connection between the existing line 6 and the transmitter 4 or receiver 5 is 7 may also be used.

FIG. 3 shows the chromatic dispersion characteristics of the dispersion compensation line 7 used in this embodiment, and as is clear from the figure, the chromatic dispersion coefficient of this dispersion compensation line 7 is approximately -7 in a relatively wide band.
0 (ps/kIIl/nm), which is larger than the absolute value of the chromatic dispersion coefficient at the transmission wavelength of 1.55 μm of the existing line 6 and has a chromatic dispersion coefficient with an opposite sign. Therefore, at a transmission wavelength of 1.55 μm, for example, 800 (ps
/nm), the length of the dispersion compensation line 7 should be set to 800 (ps/ns) / 7.
0 (as/km/nm) = 1 1, 4
By making fine adjustments before and after 1, as shown in Figure 4(b),
It is sufficient to make the total wavelength dispersion of the transmission path zero at the transmission wavelength of 1.55 μm. In addition, in the figure, the solid line A shows the chromatic dispersion of the existing line 6, the dotted line B shows the chromatic dispersion of the dispersion compensation line 7, and the dashed line A+8 shows the chromatic dispersion of the total transmission line. In a), group delay time characteristics corresponding to each of these chromatic dispersions, that is, group delay time characteristics that are the cause of each of these chromatic dispersions are shown.

By the way, when the dispersion compensation line 7 is inserted into the existing line 6, the total transmission path length increases, and the transmission loss increases accordingly. Therefore, the absolute chromatic dispersion coefficient in the transmission wavelength range of the dispersion compensation line 7 It is desirable to set the value to be as large as possible compared to the absolute value of the chromatic dispersion coefficient in the transmission wavelength range of the existing line 6 to minimize increase in transmission loss.

The dispersion compensation line 7 having the above characteristics can be achieved by winding a high dispersion single mode fiber around a suitable member having a diameter such that bending loss does not become a problem. Let us briefly explain this example.

FIG. 5 is a diagram showing the shape of the refractive index distribution in the radial direction within the cross section of the fiber, where the horizontal axis represents the distance in the radial (r) direction from the center (0) of the cross section, and the vertical axis represents the distance in the radial (r) direction from the center (0) of the cross section. The relative refractive index difference Δ (%) is taken.

Now, in the illustrated refractive index distribution shape, the relative refractive index difference of the core portion 10 at r<rl with respect to the cladding portion 11 at r>r2 is set as Δ1(x), and the relative refractive index difference of the intermediate layer 12 at rl<r<r2 is assumed to be Δ1(x). The relative refractive index difference with respect to the cladding part 11 is Δ2
〈%), the characteristics shown in Figure 3 are as follows: Core diameter 2r1 = 2μ door.

Outer diameter of the intermediate layer 2r2 = 4 μm.

Δ1 (%) = 2.0%.

Δ2 (%) = -0.3%.

This can be obtained by setting .

The single mode fiber having the above-described structure is generally referred to as a W-type optical fiber, and can be manufactured, for example, by the following method. First, a silica glass layer doped with germanium oxide or phosphorus pentoxide, which is to become a cladding part, is deposited on the inner surface of a pure quartz tube by the MCVD method.

Next, fluorine-doped silica glass, which will become an intermediate layer, is deposited on its inner surface. Finally, a preform is created by depositing silica glass doped with germanium oxide [V, which is to become the core part, and drawn in the usual manner to create a W-type optical fiber having a refractive index distribution as shown in Figure 5. is obtained. The above-mentioned fiber parameters such as the relative refractive index difference in the refractive index distribution and the diameter of each part are designed by numerical calculation according to the desired dispersion characteristics, and then the doping level of the additive, the amount of each deposit, It is set by controlling the area reduction rate, etc. during line drawing.

FIG. 6 shows a low-dispersion optical transmission line having the above-mentioned operational configuration, with a wavelength of 1.
The transmission capacity and relay span characteristics (solid line A) when transmitting a 55 μm optical pulse signal, for example, are compared with the characteristics when the dispersion compensation line 7 is not inserted (single-dot chain @B). The transmission capacity (bits/sec) and the relay span (KIR) are plotted on the Kl axis on a logarithmic scale. It is generally said that the transmission capacity is determined by the dispersion limit of the transmission line, and the relay span is determined by the loss limit of the transmission line. Although the relay span characteristics are slightly deteriorated due to the increase in the total transmission path length, the transmission capacity characteristics are improved due to the reduction in chromatic dispersion, and a large capacity of the transmission path is achieved.

Effects of the Invention As detailed above, according to the present invention, even if the transmission wavelength of the existing line is changed and chromatic dispersion occurs,
Since this chromatic dispersion is offset by the dispersion compensation line to reduce dispersion, it is possible to increase the transmission capacity.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the principle of the present invention. FIG. 2 is a diagram showing the chromatic dispersion characteristics of an existing line in an embodiment of the present invention. FIG. 3 is a diagram showing the chromatic dispersion characteristics of a dispersion compensation line in an embodiment of the present invention. 4(a) and 4(b) are diagrams for explaining cancellation of chromatic dispersion in the transmission wavelength range. FIG. 5 is a dispersion layer tα line (W-type optical fiber) in an embodiment of the present invention. FIG. 6 is a diagram showing the transmission capacity and relay span characteristics in an example of the present invention, and FIGS. 7(a) and (b) are diagrams showing the refractive index distribution shape of a general silica single mode fiber A diagram showing the wavelength dependence of the group delay time (a) and the wavelength dependence of the chromatic dispersion coefficient (b), FIG. 8 is a schematic configuration diagram of a general optical communication system, and FIG. 9 is a diagram showing the wavelength dependence of the wavelength dispersion coefficient (b). Figures 10(a) and 10(b) showing the oscillation spectrum of a diode (LD) are diagrams for explaining distortion of optical signal pulses. 1.4...Transmitter, 3...Transmission line, 2.5...
・Receiver, 6... Existing line, 7... Dispersion compensation line. Fig. 1 Fig. 2 Fig. 3
0 group delay time (ns
) 10: Core part 11: Clad part 12: Intermediate layer refractive index distribution of dispersion compensation line Figure 5 A: Actual example of the present invention B: Existing line only The transmission capacity and relay span characteristics of the embodiment of the present invention are compared to the existing line only. Figure 6 comparing the case and baboon? Figure 7: Wavelength dependence of group delay time and chromatic dispersion coefficient in a typical silica-based single mode fiber Figure 7: Oscillation spectrum of an LD with a general optical communication system Figure 9

Claims (1)

[Claims] At least one end or intermediate portion of the existing line (6) made of a single-mode fiber having uniform wavelength dispersion characteristics in the longitudinal direction, the chromatic dispersion of the existing line (6) at the transmission wavelength. A dispersion compensation line (7) made of a single-mode fiber with a uniform wavelength dispersion coefficient in the longitudinal direction and with opposite signs is inserted, and the chromatic dispersion of the existing line (6) and the chromatic dispersion of the dispersion compensation line (7) are A low-dispersion optical transmission line, characterized in that the length of the dispersion compensation line (7) is set so that the lines cancel each other out.