JPS62178220A - Optical transmission line - Google Patents

Abstract

PURPOSE:To improve sharply the transmission speed of an optical communication system by changing the group velocity dispersion of optical loss factors from an optical transmission direction (axial direction) so that a specific equation can be satisfied. CONSTITUTION:The optical intensity of an optical soliton is reduced by the optical loss of an optical fiber in accordance wit its transmission through the optical fiber and the influence of the non-linearity of the fiber is reduced. As a result, only the influence of the group velocity dispersion of the fiber is exerted upon the soliton and its width is expanded. To prevent the width from being expanded, the group velocity dispersion is arranged to be varied also with the transmission distance correct the reduction in the non-linearity of the fiber which may be generated in accordance with the transmission of the soliton. Namely, the soliton can be transmitted in a long distance without changing its width by using the fiber for changing the group velocity dispersion K2 from the optical transmission direction (axial direction) at the optical loss factor gamma(1/cm) so that the equation 1 can be satisfied. Consequently, sharply improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical transmission line for an ultrahigh-speed long-distance optical transmission system.

(Prior Art) Since an optical soliton is an optical ultrashort pulse whose width does not depend on the optical propagation distance, it has been proposed to utilize this property to realize an ultrahigh-speed optical communication system. However, when a conventional single mode is used as an optical transmission rod, the optical soliton loses its soliton properties as it propagates due to the influence of optical loss in the optical fiber, and its width increases (for example, A,
Hasegawa, Y., Kodama, Proceedings of IEE, Volume 69, No. 9, Page 1145, 1981.
Year). As a result, the achievable bit rate is severely limited. Therefore, the optical loss of the optical fiber is compensated for by optical amplification means such as stimulated Raman amplification (for example, see F. Molenaur et al., Optics Letters, Vol. 10, No. 5, p. 229). (1985). Although this method solves the above-mentioned optical loss problem, it
There is a major problem in that it is necessary to insert large-scale optical amplifiers into the optical transmission line at intervals of m to ioom.

(Problems to be Solved by the Invention) As described above, when a normal optical fiber is used, the width of the optical soliton increases due to the optical transmission loss of the optical fiber, which limits the achievable transmission speed. In order to solve this problem by using
Since it is necessary to insert optical amplifiers into the optical transmission line at intervals of 0 km, there is a problem that not only does the optical communication system become very expensive, but also that the reliability of the system decreases.

An object of the present invention is to provide an optical transmission line that can propagate optical solitons over long distances without using an optical amplifier.

(Means for solving the problem) The optical soliton in the optical fiber balances the force that tries to compress the soliton due to the nonlinearity of the optical fiber and the force that tries to spread the soliton due to the group velocity dispersion of the optical fiber. As a result, it propagates over long distances without changing its width.

However, as described above, since optical fibers inherently have optical loss, the optical soliton loses its optical intensity as it propagates through the optical fiber, and is therefore less affected by the nonlinearity of the optical fiber. As a result, the optical soliton is greatly influenced only by the group velocity dispersion of the optical fiber, and its width increases. Therefore, the optical transmission line of the present invention has a structure in which the group velocity dispersion also changes with the propagation distance in order to correct the decrease in the nonlinearity of the optical fiber accompanying the propagation of the optical soliton. In other words, the optical loss coefficient is y (1/am), and its group velocity dispersion is 2, which changes in the optical propagation direction (axial direction) so that it satisfies 2F2 (k2) + aexp (-2yz) = 0. The structure of the optical fiber is as follows.

However, 2 is the group velocity dispersion (s/cm2), 2 is the transmission direction distance of the optical fiber (am), F(k2) is the effective core diameter of the optical fiber (am) expressed as a function of 2, and α is a constant determined by 2 when z=0.

Another type of optical transmission line is that the optical loss coefficient is approximately (1/cm) by connecting multiple optical fibers with different dispersions, so that the dispersion over the entire length can be approximated in the optical propagation direction (axial direction). This is an optical transmission line characterized by satisfying the following relationship.

(Function) The optical propagation characteristics of a single mode optical fiber are well expressed by a wave equation (for example, B. P. Nelson et al., Optics Communications, Vol. 48, No. 4, 2).
92 pages, 1983). Here, the electric field strength of light is e(
t, z) = −E(t, z)exp[i(ωt −kz
)] is a plane wave with ten complex conjugate terms. 2 is a coordinate representing the traveling direction of light, t is time, ω is optical frequency, and kQ is 1.
and 2 are the phase velocity, group velocity, and group velocity dispersion of light, respectively. Further, Y is a light attenuation rate, and nQ and n2 are a linear refractive index and a nonlinear refractive index, respectively. The unit is cgs.

Here, Galilean coordinate transformation t'=(1z to t-)8 (
2a) z'=z/ζ
(2b) r=kuγ
(3
When formula (1) is transformed using a), we obtain. In equation (4), the first term on the right side represents the influence of the group velocity dispersion of the optical fiber (hereinafter referred to as the dispersion term), and the second term represents the influence of the nonlinearity of the optical fiber (hereinafter referred to as the nonlinear term). Here, the nonlinear term in equation (4) is exp(-2r
z'), which means that the effect of the nonlinearity of the optical fiber due to the optical loss in the optical fiber decreases as 2 increases (i.e. as the optical soliton propagates through the optical fiber) . As a result, the optical soliton is strongly influenced only by the dispersion term, and its width increases. In order to avoid this problem, the dispersion term may also be reduced at the same rate as the nonlinear term. In other words, 2(z')=k2(z'==0)exp(-
2rz') (5). As a result,
It can be easily confirmed by numerical calculation that the width of the optical soliton does not change at all even after propagation through the optical fiber.

However, the dispersion of an actual fiber is related to its core diameter (
For example, Gloge, Ablide Optics, Vol.
Volume 0, page 2252, 1971). Therefore, when the dispersion of the optical fiber is made to have distance dependence as shown in equation (5), the core diameter of the optical fiber also becomes distance dependent. When the core diameter of the optical fiber changes, the photon density within the optical fiber changes in proportion to the reciprocal of the square of the core diameter, and therefore the strength of the nonlinearity also changes. Therefore, taking this point into consideration, equation (5) is changed to 2(z')=--exp(-
2rz') (6) Replace with α2(z'). Here, α(2') is the effective core diameter of the optical fiber, and α is a constant specified later. The effective core diameter is the diameter that corresponds to the effective core area of the optical fiber (e.g., S, E, Miller Polarization, Optical Fiber Telecommunications, Academic Press, page 130), and is very close to the actual core diameter of the optical fiber. close. Now, a can generally be approximated as a function of 2. This relationship can be expressed as a = F(k2) (
7), then equation (6) becomes 2F2(k2) + aexp(-2yz) = 0
(8) becomes. Here, real coordinates were used. As mentioned above, the effective core diameter of the optical fiber is α=
It can be approximated by a function of 2 such as F(k2), but a typical method is to use a polynomial. Therefore, equation (8) can be considered as a polynomial equation of 2. Therefore, the formula (8
) is solved for each 2 to find the value of 2 for that 2. Then, the effective core diameter of the optical fiber is determined from Equation (7). Note that α is automatically determined if 2 is determined when z=O. The value of 2 when z=0 is usually the maximum possible value. As described above, by using an optical fiber whose effective core diameter changes in two axial directions (light propagation directions), it is possible to propagate an optical soliton over a long distance without changing its width.

(Example) An example of the optical transmission line of the present invention will be explained using the drawings. FIG. 1 is a schematic diagram of an embodiment of an optical transmission line. As shown in the figure, the optical transmission line of this embodiment is one in which a large number of single mode optical fibers having slightly different effective core diameters are connected. The relationship between the length of each of these single mode optical fibers and their effective core diameter was determined as follows.

First, we measured the dispersion of more than 20 types of step-index type single-mode optical fibers with different effective core radii (2 pm to 511 m). As a result, as shown in Figure 2, the relationship between variance and effective core was found to be roughly linear, and this was confirmed to be almost in agreement with the theory in the general report literature (for example, Groge, Applied - Optics Magazine, Vol. 10, p. 2252, 1971 and S. Kobayashi et al., Proceedings of IOS (Tokyo, 1977), B8-3). Therefore, the relationship in equation (7) is as follows. Here, 2 is the group velocity dispersion (s
2/m), C is 3 x 10"cm, and λvac is the wavelength of light in vacuum, here 1.55 x 10-6 m. Substituting this into equation (8) gives 23+ 40xk22+
400x2 to 200'exp (-2γz) = 0 (11)
becomes. At wavelength 1.5511m, x = 1.275
X 10-27 [s2/m], and fiber input end (Z
The group velocity dispersion of =O) is the largest in the available fiber. -20 (ps/nm) km, [k
2 = 2.55 x 1O-26 (s2/m)]
.

]α=-6.633xlO-7. Furthermore, since the optical transmission loss of all fibers was about 0.2 dB/km, γ=2.3×1O−7 (1/cm). Using the above coefficients, 2 at each two points is determined from equation (11), and the change in the effective core diameter of the fiber obtained from this result and equation (9) is shown by a solid line in FIG. Next, the fibers with different diameters were cut into appropriate lengths, and the fibers were combined and connected as shown in FIG. 1 so as to be as close to the solid line in FIG. 3 as possible. The distance dependence of the group velocity dispersion and effective core diameter of the soliton optical fiber obtained as a result is shown by the dotted line in FIG. Note that the amount of optical loss at the connection point of the optical fibers was able to be reduced to 0.05 dB or less, partly because the fiber diameters were almost the same. The distance dependence of the group velocity dispersion of the soliton optical fiber made in this way differs from the ideal one (solid line in Figure 3), and changes in a stepwise manner as shown by the dotted line in Figure 3. , this effect can be ignored except when propagating extremely short optical solitons. Needless to say, a soliton laser was also prepared to test the performance of the dual optical transmission line, which can create a more ideal soliton fiber by connecting shorter optical fibers with appropriate core diameters. The soliton laser has been explained in detail by the inventor Nori, F., Molenaur et al. in Optics Letters, Vol. 9, No. 1, Page 13, 1984, so a brief explanation will be given here. A soliton laser is a color center laser that has an external injection locking mechanism using an optical fiber.By appropriately selecting the length and core diameter of the optical fiber, it can be set to any width within the range of the number of modes that can be synchronized with the output of the color center laser. It can generate light pulses with an intensity of . Therefore, when the length of the optical fiber was adjusted appropriately, the wavelength was 1.55 pm and the full width at half maximum was 17 ps.
It was possible to continuously generate optical pulses with a peak intensity of about 1.5 W. However, in order for the optical pulse having a width of 17 ps at the input end of the optical transmission line to be a soliton held by the optical transmission line, its peak intensity must be about 155 mW. Therefore, considering the coupling loss between the soliton laser and the optical transmission line, it was decided to inject the output light of the soliton laser into the optical transmission line through an optical attenuator. When we measured the width of the optical soliton injected in this way after it propagated through the optical transmission path, we found that although the optical intensity was significantly attenuated due to optical loss, the width did not change for the most part. confirmed.

Although one embodiment of the optical transmission line of the present invention has been described above,
The invention is not limited to this example. Although a step-index type single mode optical fiber is used in this embodiment, an optical fiber having another refractive index distribution may be used. When using optical fibers with other structures,
Although the relationship between the group velocity dispersion and the effective core diameter is expected to be different from that in this example, even if such an optical fiber is used, the optical transmission line can be basically constructed using the same procedure as in this example. It is possible to create. Furthermore, optical fibers having different refractive index distributions may be combined. Furthermore, when considering mass production of optical transmission lines, rather than combining fibers of different diameters as in this example, it is possible to suitably adjust the speed at which the optical fiber is pulled from the base material of the optical fiber when manufacturing the optical fiber. An ideal optical transmission path in which the effective core diameter changes in a tapered manner can be obtained.

Finally, although this embodiment has been described with respect to an optical soliton with a width of 17 ps, it should be noted that the optical transmission line of this embodiment is capable of propagating an optical soliton with a width of 1 ps or less.

(Effects of the Invention) The present invention makes it possible to propagate optical solitons, which are ultrashort optical pulses, over long distances without changing their width, making it possible to significantly improve the transmission speed of optical communication systems. be.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an embodiment of an optical transmission line. In the figure, the relationship between the width and length of the optical fiber is different from the actual one. FIG. 2 shows the relationship between the effective core diameter and group velocity dispersion of the step-index type single-mode optical fiber used in the examples. FIG. 3 shows the relationship between group velocity dispersion, effective core diameter, and length of an optical transmission line. In the figure, the solid line indicates an optical transmission line using an optical fiber whose core diameter changes in a tapered manner, and the dotted line indicates an optical transmission line connecting single mode fibers having different core diameters.

Claims (2)

[Claims] (1) The optical loss coefficient is _γ (1/cm), and its group velocity dispersion k_2 is k_2F^2 (k_2) + aexp (-2γz) = 0 in the light propagation direction (axial direction).
However, k_2 is the group velocity dispersion (s/cm^2), z is the transmission direction distance of the optical fiber (cm), F(k_2) is the effective core diameter of the optical fiber (cm) expressed as a function of k_2,
And a is a constant determined by k_2 when z=0. ] An optical transmission line characterized by comprising an optical fiber that has been modified to satisfy the following. (2) By connecting multiple optical fibers with different dispersions with optical loss coefficients of the order of _γ (1/cm), the dispersion over the entire length is approximately K_2F^2 (k_2 )+aexp(-2γz)=0[
However, k_2 is the group velocity dispersion (s/cm^2), z is the transmission direction distance of the optical fiber (cm), and F(k_2) is the effective core diameter of the optical fiber (cm) expressed as a function of k_2.
, and a is a constant determined by k_2 of the optical fiber at the input end of the optical transmission line. ] An optical transmission line characterized by satisfying the following relationship.