JPS6317433A - Optical transmission line - Google Patents

Abstract

PURPOSE:To increase the insertion interval of an optical amplifier and to obtain a low-cost, high-reliability, and extremely-fast optical transmission line by connecting an optical fiber which has specific group speed dispersion and an optical amplifier which has a specific amplification degree alternately. CONSTITUTION:Optical transmission lines 1 and 2 consisting of many single- mode optical fibers with core diameters f1-f4 are connected through lenses 3 and 4 and the optical amplifier 5. The group speed dispersion kz of the optical fibers varies satisfying kzF<2>(kz)+alphaexp(-2gammaz)=0 in a light propagation direction (axial direction). Here, F(kz) is the core diameter of an optical as a function of kz, (z) the transmission-directional distance of the optical fiber, alpha a constant, and gamma a coefficient of light loss. The optical amplification degree of the optical amplifier 5 is so set that the peak power P of light at all incidence terminals of the plural optical fibers satisfies a relation 0.5Popt<P<1.5Popt. Here, Popt=n0 AE<2>/754; and n0 is the linear refractive index of a core, A is the core area, and E is expressed as shown by II.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical transmission line for an ultra-high-speed long-distance optical transmission system.

[Conventional technology]

Since optical solitons are ultrashort optical pulses whose width does not depend on the optical propagation distance, it has been proposed to utilize this property to realize ultrahigh-speed optical communication systems. However, when a conventional single mode optical fiber is used as an optical transmission medium, the optical soliton loses its properties as it propagates due to the optical loss of the optical fiber, and its width increases (for example, A, Hasegawa et al. , Y. Kodama, Proceedings of IEE, Volume 69, No. 9,
1145, 1981), this severely limits the achievable bit rate. Therefore, optical loss in optical fibers is compensated for by optical amplification means such as stimulated Raman amplification (see, for example, F. Mollenauer et al., Optics Letters, Vol. 10).

No. 5, page 229, 1985). Although this method solves the problem of optical loss, there is a major problem in that it is necessary to insert large-scale optical amplifiers into the optical transmission line at short intervals of about several km-10 ka+.

[Problem that the invention seeks to solve]

In this way, using a normal single mode optical fiber,
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 an optical amplifier, it is necessary to Since it is necessary to insert an optical amplifier into the optical transmission path, there are problems in that the optical communication system not only becomes very expensive, but also that the reliability of the optical communication system decreases.

SUMMARY OF THE INVENTION An object of the present invention is to provide an ultrahigh-speed optical transmission line that is inexpensive, highly reliable, and can greatly expand the insertion interval of optical amplifiers.

[Means for solving problems]

In the optical transmission line of the first aspect of the present invention, the group velocity dispersion kt (s2/cI11) in the optical fiber satisfies kg F'' (kz) + αexp (-2TZ) = 0 in the light propagation direction (axial direction). In an optical transmission line in which optical fibers and optical amplifiers are alternately connected, the peak power P of light at all the input ends of the plurality of optical fibers is 0.5 P, □< P The optical amplifier is characterized in that the optical amplification degree of the optical amplifier is set so as to satisfy the relationship: <1.5 P apt.

The optical transmission line of the second invention has an optical loss coefficient r (1/cI
An optical fiber transmission line is an optical fiber transmission line in which a plurality of optical fibers with the same I+) and different group velocity dispersions are connected, and the group velocity dispersion kz over the entire length is approximately kzFz(kz ) + ae Power P is 0.5 Popt<P<1. The optical amplifier is characterized in that the optical amplification degree of the optical amplifier is set so as to satisfy the relationship S P,, t.

[Effect]

The optical propagation characteristics of a single mode optical fiber are expressed by the wave equation % (for example, B. P. Nelson et al., Optics Communications, No. 48@, No. 4).

292 Hage, 1983). Here, the electric field strength of light E
is a(t, z) --E(t, z) e exp (
i (ωt−ko z) )+a plane wave of a complex conjugated ring. 2 is the coordinate representing the traveling direction of the light, t
is time, ω is optical frequency, ko, to, ,k.

are the phase velocity 9 group velocity and 1 group velocity dispersion of light, respectively.

Further, T is a light attenuation rate, and no and n2 are a linear refractive index and a nonlinear refractive index, respectively. The unit is 0g3.

Here, the Galilean coordinate transformation t' = (t-to, z)/r (2a
)z'=z/ζ (2b)F=
When formula (1) is transformed using ζT (3a) (3b), zl is obtained. 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 dispersion term). The second term is called the nonlinear term).

Here, the nonlinear term in equation (4) is exp (-2r'z')
This means that the effect of nonlinearity in the optical fiber due to the optical loss in the optical fiber decreases by -$i 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 as a result, its width increases. To avoid this problem, the dispersion term should also decrease at the same rate as the nonlinear term, that is, k, (z')-kzcz' = Q)exp ('
lr z'). 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 group velocity dispersion of an actual optical fiber is related to its core diameter (see, for example, Groge, Journal of Abstracts, Vol. 10, p. 2252, 1971).

Therefore, when the group velocity dispersion of an 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 nonlinearity changes similarly. Therefore, in consideration of this point, equation (5) is replaced with α. Here, a(z') is the effective core diameter of the optical fiber, and α is a constant specified later.

The effective core diameter is the effective core area of the optical fiber (for example, S, E
, Miller ed., Optical Fiber Telecommunications, Academic Press, p. 130), and is very close to the actual core diameter of an optical fiber. Now, a is generally! It can be approximated as a function of This relationship is expressed as a=F (kg)
(71, equation (6) is kz F" (kz) + αe xp (-2r z)
=0 (81.Here, we used real coordinates.In this way, the effective core diameter of an optical fiber can be approximated by a function of 2, such as a=F(k2), but a typical method is to use a polynomial. Therefore, equation (8) can be thought of as a polynomial equation of 2. Therefore, by solving equation (8) for each 2, the value of 2 at that 2 can be found. The effective core diameter of the optical fiber is determined using equation (7). α is automatically determined by determining 2 when 2 = O. The value of kt when 2 = 0 is usually takes the largest 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.

By the way, there is a limit to the range in which the value of 2 can be changed using the above method. This limits the achievable length of the optical fiber, which is actually about 150+a at most.

Therefore, it is conceivable to connect a large number of optical fibers whose group velocity dispersion changes in the transmission direction. On the other hand, in order to propagate an optical soliton, which is an ultrashort optical pulse, through an optical fiber without changing its width, the peak power at the input end must be (N. J. Dolan et al., IEE Journal of Quantum Electronics Magazine, Volume QB-19, No. 12, Page 1883.

(1983). However, no is the linearity of the optical fiber core.
The refractive index, A is the effective core area (cni) at the input end of the optical fiber, E (V/a++) is the full width at half maximum of the optical pulse propagating (s), ko (1/am) and kg (s
”/as) is the propagation constant and group velocity dispersion at the input end of the optical fiber, and n2 is the nonlinear refractive index (
cd/V”).

Therefore, as mentioned above, when connecting optical fibers, by inserting an optical amplifier at the connection point and adjusting the optical amplification, the relationship of equation (9) can be satisfied at the input end of the next optical fiber. It is necessary to adjust the peak power of the optical soliton. In reality, although equation (9) cannot be strictly satisfied, the peak power P can be calculated as 0.5 P apt < P < 1.5 P ops
If α, most of the energy of an optical soliton propagates as a soliton (J, Safsuma
In addition, it is more desirable to use subliment op progress op theoretical.

〔Example〕

Embodiments of the optical transmission line of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram of an embodiment of an optical transmission line.

As seen in Fig. 1, the optical transmission line of this example connects a large number of single-mode optical fibers with slightly different effective core diameters (in the figure, the core diameters of the single-mode optical fibers are
, b, , b, , b4...), prepare two identical optical fiber transmission lines 1゜2, and connect these optical fiber transmission lines 1 and 2 through an optical amplifier 5. It is connected. An optical lens 3.4 is arranged between the optical fiber transmission line 1.2 and the optical amplifier 5 to minimize coupling loss.

Here, the relationship between the length of the single mode optical fibers constituting the optical fiber transmission line 1.2 and their effective core diameters was determined as follows.

First, step-index type single-mode optical fibers 20 with different effective core radii (2 μm-5 μm)! Group velocity dispersion was measured for F-class and above. the result,
As shown in Figure 2, the group velocity dispersion and the effective core radius have an almost linear relationship, and it was confirmed that this is almost in agreement with the theory in previously reported literature [for example, Groge, Applied Observer. Chicks, Vol. 10, p. 2252, 1971 and S. Kobayashi et al., Proceedings of IOC (Tokyo, 1977), B8-3]. Therefore, the relationship in equation (7) is as follows. Here, 2 is the group velocity dispersion (s”/m), and is 2πC In the formula @, C is the speed of light in vacuum 3×10”
m/S, λv0 is the wavelength of light in vacuum, here 1.55
Let it be X10-6m. Substituting this into equation (8), k
,' +40 to kg" +400 g"k, +cx
'exp(-2yz)-0Q hot water.

Wavelength 1.55μ... then = 1.275 Xl0-"
(S”/m], and the group velocity dispersion at the input end of the fiber (z = 0) is +2, which is the largest among the fibers in hand.
0(ps/nm)/kffi [
k2=2.55X10-” (s”/m), ot
= -6,633xlO-'', and since the optical transmission loss of all fibers was about 0.2 dB/km, T = 2.3 Xl0-' (1/am).The above coefficients Using , find ! at each two points from the equation α,
Further, the change in the effective core diameter of the fiber determined from this result and equation Ql) is shown by a solid line in FIG.

Next, I cut the fibers of different diameters I had on hand into appropriate lengths, and connected them as closely as possible (as close as possible to the solid line in Figure 3).The resulting group velocity dispersion of the optical fiber transmission line The distance dependence of the effective core diameter is shown by the dotted line in Figure 3.The amount of optical loss at the connection point of the optical fibers is 0.05a because the fiber diameters are almost the same.
I was able to get it below B. The distance dependence of the group velocity dispersion of the optical fiber transmission line created in this way is different from the ideal one (solid line in Figure 3), and changes stepwise as shown by the dotted line in Figure 3. However, this effect can be ignored except when extremely short optical solitons are propagated. Needless to say, by connecting shorter optical fibers with an appropriate core diameter, a more ideal optical fiber transmission line can be constructed.

As described above, two almost identical optical fiber transmission lines 1.2 were prepared and connected via the optical amplifier 5 as shown in FIG. The optical amplifier 5 is a traveling wave laser diode with a large saturation output.

By the way, the peak power of the optical soliton at the input end of each optical fiber transmission line must satisfy the expression α, but if it satisfies the expression (9), it is more desirable. In this embodiment, almost the same optical fiber transmission line , the peak power at each input end is also approximately the same. Therefore, the optical amplifier 5 is required to have an optical amplification degree sufficient to compensate for the total optical loss in the optical fiber transmission line and the coupling loss in the optical fiber transmission line connection section. Optical loss of optical fiber transmission line is 20
dB, and since the coupling loss is about 3 dB, adjust the bias current of the traveling wave laser diode optical amplifier 5,
The optical amplification degree was set to 23 dB. This gives formula (9)
The relationship was satisfied almost exactly.

A soliton laser was prepared to test the performance of the optical transmission line of this example. Regarding soliton lasers, see Optics Letters, Volume 9, No. 1.

Since it was explained in detail by the inventor Nori, F., Molenaur et al. on page 13 in 1984, 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.
．． A 55 μ cocoon has a full width at half maximum of 17 ps and a peak intensity of about 1.
It was possible to continuously generate 5W optical pulses. However, in order for the optical pulse with a width of 17 pS at the input end of the optical transmission path to be a soliton held by the optical transmission path, its peak intensity must be about 155 mW. Therefore, considering the coupling loss between the soliton laser and the optical transmission line,
We decided to inject the output light of the soliton laser into the optical transmission line through an optical attenuator. When the width of the optical soliton thus injected after propagating through the optical transfer path was measured, it was confirmed that the width hardly changed.

Although one embodiment of the optical transmission line of the present invention has been described above,
The invention is not limited to this example. In this embodiment, the optical fiber transmission line is constructed using a step-index type single mode optical fiber, but an optical fiber having another refractive index distribution may be used. If an optical fiber with another structure is used, it is expected that the relationship between the group velocity dispersion and the effective core diameter will be different from that in this example, but even if such an optical fiber is used, basically It is possible to create an optical fiber transmission line using the same procedure as in this embodiment. Furthermore, optical fibers having different refractive index distributions may be combined. In addition, when considering the case of mass-producing optical fiber transmission lines, rather than combining fibers of different diameters as in this example, it is better to appropriately adjust the speed at which the optical fiber is drawn from the base material of the optical fiber when manufacturing the optical fiber. For example, an ideal optical fiber whose effective core diameter changes in a tapered manner can be obtained. Further, in this embodiment, a traveling wave laser diode optical amplifier is used as the optical amplifier, but a Raman amplifier that utilizes stimulated Raman scattering or various gas and solid state laser amplifiers may also be used. Finally, in this embodiment, an optical soliton with a width of t7 ps has been described, but the optical transmission line of this embodiment can also propagate an optical soliton with a width of 1 ps or less.

〔Effect of the invention〕

According to the present invention, it becomes possible to propagate an optical soliton, which is an ultrashort optical pulse, over a long distance without changing its width, and it becomes possible to significantly improve the transmission speed of an optical communication system.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an example of an optical transmission line, FIG. 2 is a diagram showing the relationship between the effective core radius and group velocity dispersion of the step-index type single-mode optical fiber used in the example, and FIG. The figure is a diagram showing the relationship between group velocity dispersion, effective core diameter, and length of an optical fiber transmission line. 1.2...Optical fiber transmission line 3.4...Lens 5...Optical amplifier Yoshiyuki Iwasa, patent attorney Figure 2 Figure 3

Claims (1)

[Claims] (1) When the group velocity dispersion k_2 in the optical fiber is expressed in the optical propagation direction (axial direction), k_zF^z(k_z)+αexp(-2γz)=0[
However, F(k_z) is the effective core diameter of the optical fiber expressed as a function of k_z, z is the distance in the transmission direction of the optical fiber, α is a constant determined by k_z when z=0, γ is the optical loss coefficient , ] In an optical transmission line in which optical fibers and optical amplifiers are alternately connected so as to satisfy the following, the peak power P of light at all the input ends of the plurality of optical fibers is 0.5P_o_p_t<P<1.5P_o_p_t [where P_o_p_t=(n_oA/754)E^2, where n_o is the linear refractive index of the core of the optical fiber, A is the effective core area at the input end of the optical fiber, and E is 0.57/T(2n_o｜k_z|/n_2k_o
)^-^1^/^2, T is the full width at half maximum of the optical pulse propagating in the optical fiber, k_o is the propagation constant at the input end of the optical fiber, k_z is the group velocity dispersion at the input end of the optical fiber, n_z is the nonlinear refractive index of the core of the optical fiber.] The optical transmission line is characterized in that the optical amplification degree of the optical amplifier is set so as to satisfy the following relationship. (2) In an optical fiber transmission line in which multiple optical fibers with similar optical loss coefficients γ and different group velocity dispersions are connected, the group velocity dispersion k_z over the entire length is approximately equal to the light propagation direction (axial direction). , k_zF^z(k_z)+αexp(-2γz)=0[
However, F(k_z) is the effective core diameter of the optical fiber expressed as a function of k_z, z is the distance in the transmission direction of the optical fiber, and α is a constant determined by k_z of the optical fiber at the input end of the optical fiber transmission line. ] In an optical transmission line in which optical fiber transmission lines and optical amplifiers are alternately connected that satisfy the relationship, the peak power P of light at all the input ends of the plurality of optical fiber transmission lines satisfies the following: 0.5P_o_p_t<P<1 .5P_o_p_t [where P_o_p_t=(n_oA/754)E^2, where n_o is the linear refractive index of the core of the optical fiber transmission line, A is the effective core area at the input end of the optical fiber transmission line, E is 0.57/T(2n_o｜k_z|/n_zk_o
)^-^1^/^2T is the full width at half maximum of the optical pulse propagating through the optical fiber transmission line, k_o is the propagation constant at the input end of the optical fiber transmission line, and k_z is the group at the input end of the optical fiber transmission line. An optical transmission line characterized in that the optical amplification degree of the optical amplifier is set so as to satisfy the following relationship: velocity dispersion, n_z is a nonlinear refractive index of the core of the optical fiber transmission line.