JPH06123902A - Lossless dispersion compensation optical fiber and lossless dispersion compensation device using the same - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a lossless dispersion compensating fiber that compensates for dispersion and reduces the loss of the dispersion compensating fiber itself, and a lossless dispersion compensator using the same. According to the lossless dispersion compensating optical fiber of the present invention, a signal light is emitted in a state where a core portion of an optical fiber having a large negative dispersion is doped with a rare earth element and the fiber 4 is pumped by using a pumping light source 2. Dispersion is compensated by transmitting. Further, the lossless dispersion compensator using the above-mentioned optical fiber has a large negative dispersion value and has a length for compensating for a desired transmission line cumulative dispersion value, and for optical amplification in a desired wavelength band. It comprises an optical fiber 4 doped with a rare earth element, a pumping light source 2 for generating pumping light, and an optical coupler 3 for making the pumping light and the signal light enter the optical fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lossless dispersion compensating optical fiber, which is used to extend a transmission distance when the transmission distance is limited by dispersion.

[0002]

2. Description of the Related Art In a transmission system using an optical fiber,
The transmission distance is mainly determined by the loss and dispersion of the optical fiber. As shown in Fig. 1, the transmission distance when transmitting a signal light in the 1.5 µm band using an optical fiber designed to have a wide band in the 1.3 µm band is a loss of up to about 5 Gb / s. The fiber loss is less than the permissible loss at a transmission rate of 6 Gb / s or more,
Due to the band limitation, it is not possible to transmit over the distance determined by the loss limitation. That is, the transmission distance is determined by the dispersion limit. Therefore, in order to increase the transmission distance, it is necessary to reduce the dispersion. For this reason, a fiber is used that has zero dispersion at the wavelength used.

However, in such a fiber,
Since the dispersion is controlled, there is a drawback that the fiber loss is large. In addition, in practice, the dispersion is not completely zero at the wavelength used, so that the transmission distance is limited by the dispersion when the transmission speed increases. Also, by reducing the spectral width of the light source to reduce the effects of dispersion,
A method of extending the transmission distance is also used. Further, in order to compensate the dispersion of the entire transmission line, a fiber having a dispersion characteristic opposite to that of the fiber used has been considered. However, in this case, the loss in the fiber itself that compensates for dispersion also increases, and there is a drawback that the transmission distance is shorter than the transmission distance determined by the loss of the fiber used for transmission. Further, in the dispersion compensating fiber, a special structure is required to control the dispersion as compared with the ordinary fiber, and therefore the loss cannot be reduced so much.

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and uses a lossless dispersion compensating fiber for compensating for dispersion and reducing the loss of the dispersion compensating fiber itself to zero. It is to provide a lossless dispersion compensator.

[0005]

In the lossless dispersion compensating optical fiber of the present invention, a core portion of an optical fiber having a large negative dispersion is doped with a rare earth element, and the fiber is excited by a pumping light source. , Dispersion is compensated by transmitting signal light. Further, a lossless dispersion compensator using the lossless dispersion compensating optical fiber of the present invention,
An optical fiber having a large negative dispersion value and having a length that compensates for a desired transmission line cumulative dispersion value, in which a rare earth element is added for optical amplification in a desired wavelength band, and a laser which generates pumping light. It comprises a light source and optical means for making the excitation light and the signal light incident on the optical fiber.

[0006]

According to the present invention, the transmission distance, which has been conventionally limited by dispersion, can be extended to the distance limited only by loss. 17 to 19 ps / km at a wavelength of 1.55 μm
Considering the case of using a 1.3 μm band zero dispersion fiber having a dispersion value of / nm for transmission at a wavelength of 1.55 μm, the relationship between transmission distance and transmission speed due to loss limitation is shown in FIG.

The calculation here is obtained based on the following values. Fiber loss is 0.2 dB / km at wavelength 1.55 μm
Then, the minimum light-receiving level of the receiver at fiber input power of 10 dBm and transmission speed of 2 Gb / s and 10 Gb / s is -36 dBm and -28 dBm, respectively. To approximate.

[Equation 3]

FIG. 1 also shows the result of calculation of the transmission restriction due to dispersion using the following equation.

[Equation 4]

Where L is the transmission distance and D (λ) is the wavelength λ
, B is the transmission speed, c is the speed of light, and γ is the transmission parameter, which is usually 0.4 to 1.5. Here, γ = 0.9 is used. The dispersion value is 17ps / km / nm
Was used. From FIG. 1, it can be seen that the transmission distance is limited by the loss at the transmission rate of about 5 Gb / s, but when the transmission rate is 6 Gb / s or more, the transmittable distance is determined by the dispersion limitation. Therefore, at a transmission speed of 6 Gb / s or higher, the transmission distance can be extended by compensating for the dispersion.

Therefore, FIG. 2 shows the amount of dispersion to be compensated for with respect to the transmission rate. In Fig. 2, the broken line is the dispersion amount that should compensate all the dispersion of the transmission line when transmitting the 1.3 μm band fiber to the distance due to loss limitation, and the solid line is the minimum dispersion amount that should be compensated at each transmission speed. Show. Therefore, the area enclosed by these two lines shows the amount of dispersion to be compensated for at each transmission rate. For example, for transmission rates up to 10 Gb / s, the minimum amount of dispersion that should be compensated for is approximately −2200.
ps / nm, and up to 20 Gb / s is -2600 ps / nm. That is, at a transmission rate of up to 20 Gb / s, in order to compensate for dispersion with one type of dispersion compensating fiber, approximately -2600ps /
It is necessary to design a fiber with a wavelength of nm. In this way, there is a feature that the dispersion can be compensated and the transmission distance can be extended if there is no loss in the transmission line itself.

The dispersion compensating fiber of the present invention can be designed as follows. Since the amount of compensation for dispersion is quite large, it is not possible to compensate for dispersion using a normal fiber. Also, the dispersion per 1km of fiber is -100ps
Even if designed to / nm, 26km of fiber would be needed to compensate for 2600ps / nm. Further, since the fiber itself causes a loss, the transmission distance cannot be dramatically increased. Therefore, the dispersion compensating fiber of the present invention is an optical fiber having a large negative dispersion for compensating for dispersion, and has a structure of a distributed fiber amplifier doped with Ε _r for compensating for loss. The doping amount of Ε _r is relatively small.

First, the dispersion D of an optical fiber can be generally expressed by the following equation.

[Equation 5]

Here, c and λ are the speed of light in vacuum and the wavelength of the light source, respectively. k (= 2π / λ) is the wave number, and β is the propagation constant of the HE ₁₁ mode which is the fundamental mode. Given the refractive index distribution of the optical fiber, the propagation constant β can be obtained by solving the wave equation. By such a method, the variance can be calculated.

The core radius and the wavelength of 1.55 for the step type and square distribution type fibers of the dispersion compensating fiber of the present invention.
The relationship with the dispersion value D (ps / km / nm) in μm is shown in FIG. It can be seen from FIG. 3 that the larger the relative refractive index difference Δ, the larger the absolute value of the dispersion.

FIG. 4 shows the variation dD / da of the dispersion with respect to the variation of the core radius. The solid line is shown for the step type fiber, and the broken line is shown for the square distribution type fiber. From Fig. 4, the sensitivity of the dispersion value to the core radius is much larger than that of a normal fiber (several tens (ps / km / nm / μm). Therefore, as the core radius of the fiber, the variation of dispersion dD / da
It is effective in manufacturing to select one in which is zero.

In the actual design, it is necessary to consider the bending characteristics. FIG. 5 shows the relationship between the allowable bending radius and the relative refractive index difference. In order to keep the bending characteristics equal to or higher than that of the conventional single mode optical fiber, the allowable bending radius R ^* needs to be 3 cm or less. Therefore, the relative refractive index difference needs to be 2.% or more.

Next, the design of the fiber parameters in the case of the distributed EDFA will be considered. A rigorous analysis of the distributed EDFA is performed by Desurvire. Based on the analysis technique, normalized input excitation power q to compensate for fiber loss in the case of unidirectional excitation ₀ ⁺ = _{P p} ⁱⁿ / P
_p ^th (P _p ⁱⁿ : excitation power, P _p ^th : excitation light threshold) can be described by the following equation.

[Equation 6]

Here, when the physical quantities at the signal wavelength and the pump wavelength are represented by subscripts s and p, α _i (i = s,
p) represents the intrinsic absorption loss due to the rare earth element. α ′
_i (i = s, p) mainly represents the scattering loss of the host material, and can be evaluated from the scattering characteristics of each host material. η _i (
i = s, p) indicates the ratio between the absorption cross section and the stimulated emission cross section. Further, α _s and α _p are given by the following equation as a ratio of the stimulated absorption cross section between the signal wavelength and the excitation wavelength.

[Equation 7]

Therefore, the equation (3) is an equation which depends only on α _s . That is, given α _s , the required normalized input excitation power is determined. Here, the input excitation power is preferably as small as practical, so if the condition of the following equation is added,
α _s is uniquely determined.

[Equation 8]

That is, if the structural parameters of the fiber and the host material are determined by the above equation (3), α _s for making the fiber lossless is determined with respect to the fiber length. Once α _s is determined, the optimum erbium doping amount can be obtained from the following equation.

[Equation 9]

Here, Γ _s represents the superposition ratio of the electric field distribution and the Ε _r- doped distribution at the signal wavelength. Above (3) ~
The dispersion-compensating optical fiber amplifier designed from equation (9) enables high-speed transmission and long-distance transmission. In addition, the above-mentioned design method has different fiber dispersions, different fiber requirements due to different transmission restriction conditions,
When the refractive index distribution of the dispersion compensating fiber is various (step type,
It is also applicable to (triangular type, segment type, etc.).

[0022]

Embodiments of the present invention will now be described in detail with reference to the drawings. Example 1 FIG. 6 is a schematic diagram showing the configuration of a lossless dispersion compensating apparatus using a lossless dispersion compensating optical fiber according to an example of the present invention. In FIG. 6, the incident pulse 1 and the pumping light 2 pass through the optical coupler 3 and enter the lossless dispersion compensating fiber 4.
As the incident pulse 1 propagates through the fiber 4, the pulse is narrowed due to the inverse dispersion of the fiber, and at the same time, the rare earth element in the fiber 4 is excited by the excitation light generated from the excitation light source 2, and the fiber itself is amplified. Since it becomes a medium, the peak value of the pulse is reduced by the background loss of the fiber 4, and an output pulse 5 having a narrow pulse width is obtained at the output end.

Embodiment 2 As another embodiment of the present invention, a 1.3 μm zero dispersion optical fiber is used.
A lossless dispersion compensating fiber for enabling transmission up to 20 Gb / s in the 1.55 μm band is shown. Considering the case where a 1.3 μm band zero-dispersion fiber is used for 1.55 μm band transmission, the relationship between transmission distance and transmission speed due to loss limitation is shown in FIG. Further, from FIG. 2, in order to compensate for dispersion with one type of dispersion compensating fiber at a transmission speed of up to 20 Gb / s, a fiber of about −2600 ps / nm is required. Considering the case where the core is a square distribution type as a shape that is relatively easy to make as a fiber, the relative refractive index difference of the fiber is 2.
At 5%, the core radius is 1.22 μm, and the dispersion value at the wavelength of 1.55 μm is −71.37 ps / km / nm. Therefore, the fiber length is about 37 km. In this case, as a rare earth element for compensating for the loss in the 1.5 μm band,
_r ³⁺ element. In this example, Ε _r ³⁺ is doped.

FIG. 7 shows the relationship between the amount of Ε _r doping and the fiber length and the relationship between the pump input power and the fiber length. The background loss was calculated using the Rayleigh scattering coefficient of GeO ₂ -doped fiber. Other physical property values are shown in [Table 1].

[Table 1] From FIG. 7, E _r doping amount in the case of fiber length 37km is 0.
It becomes 15wtppm, and the excitation power becomes about 23mw. Therefore, with the fiber of the present invention designed in this way, 20 Gb
It is possible to transmit up to a transmission speed of / s and a transmission distance up to a distance determined by loss.

[0025]

As described above, according to the lossless dispersion compensating optical fiber of the present invention and the lossless dispersion compensating apparatus using the same, it is possible to transmit up to a desired transmission speed and a distance within which loss can be transmitted. . Furthermore, by reducing the loss of the optical fiber, the loss-limited transmission distance can be expanded, and the fiber of the present invention can be connected to increase the capacity. It is difficult to reduce fiber loss and control dispersion,
It is relatively easy to reduce only the loss without considering the dispersion. Therefore, it is possible to construct a long distance and large capacity transmission system by combining the dispersion compensating fiber amplifier and the low loss optical fiber.

[Brief description of drawings]

FIG. 1 is a diagram showing transmission restrictions due to loss and dispersion.

FIG. 2 is a diagram showing a relationship between a required dispersion compensation amount and a transmission rate.

FIG. 3 is a diagram showing the relationship between core radius and dispersion at a wavelength of 1.55 μm.

FIG. 4 is a diagram showing a variation dD / da of dispersion with respect to a variation of a core radius.

FIG. 5 is a diagram showing a relationship between an allowable bending radius and a relative refractive index difference Δ.

FIG. 6 is a schematic diagram showing the configuration of a lossless dispersion compensator according to an embodiment of the present invention.

FIG. 7 is a diagram showing the relationship between the amount of Ε _r doping and the fiber length, and the relationship between the pump input power and the fiber length.

[Explanation of symbols]

 1 incident pulse 2 pumping light source 3 optical coupler 4 lossless dispersion compensating fiber 5 output pulse

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H01S 3/10 Z 8934-4M 3/17 8934-4M

Claims (3)

[Claims] 1. Transmission is possible up to a loss-limited transmission path length.
Therefore, a part of the amount of dispersion accumulated in the transmission line τ (ps / nm)
An optical fiber for compensating,
A and the cladding refractive index n₁And n₂And the core radius
Is a and the wavelength used is λ_sDispersion value at D (ps / km / nm)
, The total length L is determined by L = τ / D, and the core radius a is
Determined under the condition of dD / da = 0, the relative refractive index difference Δ (= (n₁ ²
−n₂ ²) / 2n₁ ²) Is a bending loss of 0.01 dB / km or less
Compensate for loss by setting the bending radius to 3 cm or less
In order to do so,Concentration ρ₀Is doped with the rare earth element of
Γs at is λ_sOptical field distribution and Ε_rDope
Is the superposition ratio of the distribution, σ_a(λ_s) Is λ_sAbsorption in
Is the cross-sectional area, and the quantities at the signal and pump wavelengths are
Expressed with subscripts s and p, it is unique to rare earth elements.
Absorption loss and background loss α_i, α ′_i(I = s, p) and
And α in Eq. (9)_sIs the background loss α ′_i, Rare earth
Elementary loss α _i, Stimulated emission cross section σ_i ^e, Absorption cross section σ
_i ^aUsing (i = s, p),It is uniquely determined from the five expressions
Lossless dispersion compensating optical fiber. 2. The lossless dispersion compensating optical fiber according to claim 1, wherein silica glass, fluoride glass and multi-component glass are doped with Ε ³⁺ , Nd ³⁺ , Pr ³⁺ etc. as rare earth elements. A lossless dispersion compensating fiber comprising a fiber. 3. An optical fiber doped with a rare earth element for optical amplification in a desired wavelength band, which has a large negative dispersion value and has a length that compensates for a desired transmission line cumulative dispersion value, and pumping. A lossless dispersion compensator comprising: a laser light source for generating light; and an optical means for causing the pumping light and the signal light to enter the optical fiber.