RU2468399C2 - Method of compensating for differential modal delay in multimode fibre-optic line in low-mode signal transmission - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: main multimode optical fibre transmission line is connected in series to a compensating multimode optical fibre whose parameters are selected depending on the parameters of the main multimode optical fibre transmission line. The refraction index profile and the length of the compensating multimode optical fibre are selected based on the condition of minimisation of the value

where M is the number of directed modes; ν_g(j) is the value of the group velocity of the j-th mode LP_lm ^(j) of the main multimode optical fibre;

is the value of the group velocity of the mode LP_lm(j) of the same order in the compensating multimode optical fibre.

j = 1…M, where ν_g(max), ν_g(min) are the maximum and minimum value of the group velocities of M mode components of the low-mode signal in the main fibre of the transmission line; Q = L_lin/L_comp is the compensation parameter which determines the length of the compensating multimode optical fibre.

EFFECT: higher bandwidth of the transmission line.

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Description

The invention relates to fiber-optic communication technology and can be used to compensate for the differential mode delay of a multimode fiber-optic line in the mode of transmission of small-mode signals.

Known methods [1, 2] to increase the bandwidth of a multimode fiber optic transmission line, which consists in the fact that sequentially the main multimode optical fiber transmission lines include a compensating multimode optical fiber, the gradient profile of the refractive index of which has a gap in the center of the core. By choosing the parameters of this dip, they achieve a decrease in the differential mode delay and an increase in the transmission line bandwidth. The application of these methods is limited to transmission lines with first-generation multimode optical fibers with gradient refractive index profiles having a defect in the form of a dip in the central region of the fiber core.

A known method [3] compensation of the mode dispersion of a multimode fiber optic transmission line, which consists in the fact that sequentially the main gradient multimode optical fiber transmission line include compensating multimode optical fiber, the refractive index profile of which is selected depending on the parameters of the main multimode optical fiber transmission line, namely α _comp parameter describing a power gradient refractive index profile of a multimode compensating wholesale Cesky fibers selected parameter α with respect to the ground _ling multimode optical fiber transmission line, whose value lies in the range 0,8≤α _lin <2.1, as follows: to compensate for modal dispersion of a multimode optical fiber transmission path in the wavelength region λ = 850 nm, the parameter α _{comp of the} profile of the compensating fiber must satisfy the inequality α _lin ≤ α _comp ≤8; while to compensate for the mode dispersion of a multimode fiber-optic transmission line in the wavelength region λ = 1300 nm, the parameter α _{comp of the} profile of the compensating fiber is selected from the range 0.8 ≤ α _comp ≤ α _lin . To subsequently determine the optimal value of the parameter α _comp from the specified range, as well as the length L _{comp of the} compensating fiber, a transition is made from series-connected main and compensating fibers to some equivalent multimode fiber, the gradient profile of the refractive index of which is described by the parameter α _equiv , which is directly related profiles of α _lin and α _comp and the length of L _lin and L _{comp of the} main and compensating fibers in the following ratio:

.

.

Then, using the appropriate technique, the optimal value of the parameter α _{equiv (opt) of} the refractive index profile of the equivalent multimode optical fiber is calculated, at which its maximum passband is achieved in the given wavelength range, and, accordingly, the optimal value of the parameter α _{comp (opt) of} the compensating fiber profile is determined providing the desired α _{comp (wholesale).} The length of the compensating fiber L _comp is estimated by the formula

.

.

The scope of this method is limited to compensating for the mode dispersion of gradient multimode optical fibers of a transmission line with a classical power-law profile of the refractive index. In this case, the profile shape of the compensating multimode optical fiber is also limited by the classical power-law profile of the refractive index. This, given that the refractive index profiles of real optical fibers differ from ideal power fibers, limits the scope of this method.

A known method [4] compensation of the mode dispersion of a multimode fiber optic transmission line, which consists in the fact that sequentially multimode optical fiber transmission line include compensating multimode optical fiber, the refractive index profile of which is selected depending on the parameters of the main multimode optical fiber transmission line so that the ratio was fulfilled:

Δ _comp (r) = Δ _opt + R · [Δ _opt (r) -Δ _lin (r)],

where Δ _comp (r) is the value of the profile height parameter of the compensating multimode optical fiber at a distance r from the center of the core; Δ _lin (r) is the value of the profile height parameter of the main multimode optical fiber of the transmission line at a distance r from the center of the core; Δ _opt (r) is the value of the height parameter of the optimal profile of the gradient multimode optical fiber at a distance r from the center of the core, which ensures maximum throughput; R is a constant (compensation parameter). Here, by analogy with [3], we also consider an equivalent multimode optical fiber with an optimal gradient profile of the refractive index, which corresponds to the maximum bandwidth of a multimode fiber in a given wavelength range, realized by sequentially connecting the main and compensating fibers. However, unlike [3], when constructing the profile of the refractive index of the compensating multimode fiber, local deviations of the main fiber profile from the optimal are taken into account, while the length of the compensating fiber is determined from the relation:

where L _comp - the length of the compensating multimode optical fiber; L _lin is the length of the main multimode optical fiber transmission line; a _comp is the radius of the core of the compensating fiber; a _lin is the radius of the core of the main fiber. The scope of this method is the compensation of mode dispersion, as well as differential mode delay. However, the profile formation of a compensating multimode optical fiber is limited only by the selection of parameters relative to the local deviations of the profiles of the main multimode fiber of the transmission line from the optimal one, while the entire mode composition propagating mainly in the multimode fiber is conditionally divided into two groups of lower and higher order modes in terms of their path propagation - closer to the center of the core or, conversely, closer to the core / shell interface, and by the inclusion of a compensating fiber in line, taking into account the deviation of the main profile from the optimal one, which corresponds to the maximum bandwidth and the minimum difference in the speeds of these two groups of modes of lower and higher orders, their speed is equalized.

The division of the mode composition into two groups of modes is true if the optical fiber operates in a multimode mode, the signal is transported by a large (at least more than 100) number of directed modes, while the mode dispersion manifests itself as an increase in the duration of the optical pulse without significant changes in its shape. However, when a coherent radiation source — a laser — is connected to a multimode optical fiber, a limited number of guided modes are excited in the fiber (usually less than 30) and, as a result, a low-mode signal propagates. In other words, the line operates in a mode of transmitting low-mode signals (low-mode). The number of excited modes depends on the profile of the refractive index of the optical fiber, the characteristics of the radiation source, and the input conditions. Moreover, from the point of view of the low-mode regime, the difference in the velocities of the individual mode components is significant, even if these modes belong to the same group in the multimode mode, and manifests itself in the form of the differential mode delay effect, which leads to significant distortions of the optical pulse shape, in particular, the presence of several local highs. The refractive index profile that is optimal from the point of view of the multimode transmission mode may not be such from the point of view of the low-mode transmission mode. In turn, the optimal, from the point of view of compensating for the differential mode delay when transmitting a low-mode signal under conditions of uniform excitation of the mode composition by a laser source, the profile of the refractive index of the compensating fiber may not be such under conditions of strictly coaxial input of the signal at which the vast majority of the radiation power arriving from the laser output, it is redistributed between a much smaller, compared with uniform excitation, number of guided modes. Thus, the capabilities of the method [4] for compensating the mode dispersion of a multimode fiber optic transmission line are limited in terms of compensating for the differential mode delay. It is applicable only when the profile of the refractive index of the main multimode optical fiber of the transmission line is close to power and does not allow taking into account the features of the input, in particular, strictly coaxial input.

The essence of the invention is the expansion of the scope.

This essence is achieved by the fact that according to the method for compensating the differential mode delay of a multimode fiber optic line in the mode of transmission of small-mode signals, which consists in the fact that in series with the main multimode optical fiber, the transmission lines include a compensating multimode optical fiber, the parameters of which are selected depending on the parameters of the main multimode optical fiber transmission line, while the profile of the refractive index and the length of the compensating multimode op nical fibers are selected from minimization condition value F, calculated by the formula:

,

,

- значение групповой скорости моды LP_lm ^(j) этого же порядка, распространяющейся в компенсирующем многомодовом оптическом волокне, которое определяется по формуле:where M is the number of guided modes that carry the power of a low-mode optical signal in the main multimode optical fiber of the transmission line for which the normalized amplitude is at least 0.1; ν _{g (j)} is the group velocity value of the jth mode LP _lm ^(j) propagating in the main multimode optical fiber of the transmission line;

- the value of the group velocity of the LP mode _lm ^{(j) of} the same order, propagating in a compensating multimode optical fiber, which is determined by the formula:

; j=1…M,

; j = 1 ... M,

where ν _{g (max)} , ν _{g (min)} is the maximum and minimum value of the group velocities M of the mode components of the low-mode signal (j = 1 ... M) propagating in the main multimode optical fiber of the transmission line; Q = L _lin / L _comp - compensation parameter that determines the length of the compensating multimode optical fiber.

Figure 1 presents the structural diagram of a device for implementing the proposed method. Figure 2 (a) shows the gradient profile of the refractive index of the main multimode optical fiber transmission line with a characteristic technological defect in the center of the core in the form of a dip, figure 2 (b) presents the profile of the refractive index of the compensating optical fiber to compensate for the differential mode delay of 24 mode components of the LP signal _lm (l = 0 ... 3; m = 1 ... 6), excited in the main fiber by a laser (wavelength λ = 850 nm) at an axial displacement of 10 ... 15 μm. Figure 3 (a) shows the gradient profile of the refractive index of the main multimode optical fiber transmission line with a characteristic technological defect in the center of the core in the form of a peak, figure 3 (b) presents the profile of the refractive index of the compensation signal obtained for it by minimizing the indicated value F optical fiber to compensate for the differential mode delay of 24 mode components of the LP signal _lm (l = 0 ... 3; m = 1 ... 6), excited in the main fiber by a laser (wavelength λ = 850 nm) at an axial displacement of 10 ... 15 μm. Figure 4 (a) shows the gradient profile of the refractive index of the main multimode optical fiber transmission line, optimized for working with lasers, figure 4 (b) presents obtained by minimizing the profile of the refractive index of the compensating optical fiber to compensate for differential mode delay 24 mode components of the LP signal _lm (l = 0 ... 3; m = 1 ... 6), excited in the main fiber by a laser (wavelength λ = 850 nm) at an axial displacement of 10 ... 15 μm, and Fig. 4 (c) shows minimized uu indicated value F refractive index profile of the compensating optical fiber to compensate for differential-mode delay 12 modal components LP _lm signal (l = 0 ... 3; m = 1 ... 3) excited mainly fiber laser (wavelength λ = 850 nm) under the conditions coaxial input.

The device contains the main multimode optical fiber 1 of the transmission line 2, which is connected in series with the compensating multimode optical fiber 3 (junction 4).

The device operates as follows.

When a coherent radiation source is connected in a multimode optical fiber, a limited number of guided modes are excited, a low-mode mode is implemented, and the number of mode components of the optical signal is determined by the input conditions and the mode composition of the radiation, which is formed directly at the source output, which in turn depends on the type of laser and its characteristics. For example, the radiation at the output of a single-mode laser diode with a Fabry-Perot resonator (operating wavelength λ = 1310 nm) contains one main mode LP ₀₁ , while the signal from the output of a laser with a vertical volume resonator contains (working wavelength λ = 850 nm) 6 mode components. In this case, the number of guided modes excited in a multimode fiber under coaxial input conditions, for which the value of the normalized amplitude is more than 0.1, is almost half as compared to uniform excitation. Thus, for a given type of laser and corresponding input conditions in the main multimode fiber of the transmission line, most of the optical signal power is carried by a limited set of M guided modes LP _lm , conditionally numbered as LP _lm ^(j) (j = 1 ... M), for example, in ascending order of azimuthal and radial orders. Based on the introduced numbering, one can construct a distribution diagram of the group velocities of the guided modes of the main multimode fiber of the transmission line, the values of which ν _{g (j) are} completely determined directly by the shape and parameters, as well as by local defects of the gradient profile of the refractive index of the main fiber. Accordingly, by choosing the shape and profile parameters of a multimode optical fiber, it is possible to achieve the desired type of distribution diagram of group velocity values of a given number of guided modes of a certain order. The shape and profile parameters of the refractive index of the compensating fiber are selected in such a way as to provide a reversible reproduction of the distribution diagram of group velocity values M of the guided modes of the main multimode fiber of the transmission line, which allows to reduce (compensate) the differential mode delay in the serial connection of such fibers under given excitation conditions and characteristics source. Thus, in a multimode fiber optic transmission line 2, in which the main 1 and the compensating fiber 3 are connected in series, the transmission line bandwidth is increased by compensating for the differential mode delay.

In contrast to the known method for compensating mode dispersion, which is a prototype and in which the parameters of the compensating multimode optical fiber are selected from the point of view of correcting the shape of the profile of the refractive index relative to local deviations of the profile of the main fiber from some optimal profile, at which the maximum bandwidth should be achieved, the proposed method of compensating for differential mode delay the shape and profile parameters of the refractive index of the compensation of the multimode fiber are selected based on the initial values of the group velocities of only those guided modes of the main multimode fiber of the transmission line that transfer the main power of the low-mode signal under given input conditions and the type of coherent radiation source, thereby providing the deepest suppression of the differential mode delay in the low-mode transmission. The proposed method, unlike the prototype, allows you to take into account the input conditions, which expands the scope of its application.

LITERATURE

1. US 4723828.

2. RU 2264638 C1.

3. WO 99/22471.

4. US 2006/0034573 A1.

Claims (1)

A method for compensating the differential mode delay of a multimode fiber optic line in a mode of transmitting low-mode signals, namely, that the main multimode optical fiber of the transmission line is sequentially equipped with a compensating multimode optical fiber, the parameters of which are selected depending on the parameters of the main multimode optical fiber of the transmission line, characterized in that the profile of the refractive index and the length of the compensating multimode optical fiber is selected from the conditions for minimizing the value of F, calculated by the formula:

where M is the number of guided modes that carry the power of a low-mode optical signal in the main multimode optical fiber of the transmission line for which the normalized amplitude is at least 0.1;
ν _{g (j)} is the group velocity value of the jth mode LP _lm ^(j) propagating in the main multimode optical fiber of the transmission line;

- the value of the group velocity of the LP mode _lm ^{(j) of} the same order, propagating in a compensating multimode optical fiber, which is determined by the formula:

; j = 1 ... M,
where ν _{g (max)} , ν _{g (min)} is the maximum and minimum value of the group velocities M of the mode components of the low-mode signal (j = 1 ... M) propagating in the main multimode optical fiber of the transmission line; Q = L _lin / L _comp - compensation parameter that determines the length of the compensating multimode optical fiber.

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