JPS624688B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical fiber used in the field of optical communications and a method for optical amplification using such an optical fiber as an amplification medium.

[Prior Art] FIG. 2 shows the loss-wavelength characteristics of a silica-based optical fiber widely used for optical communications. In Figure 2, relatively steep loss peaks can be seen near wavelengths of 1.25 μm and 1.39 μm, but these loss peaks are caused by impurities mixed into the optical fiber.
This is due to the OH group. Absorption due to such impurities has a relatively steep loss peak, but
The wavelength is determined by the material itself. Therefore, it is not possible to cause a large loss at any wavelength.

Therefore, it has been impossible to cause a large loss only at a specific wavelength in conventional silica-based optical fibers.

On the other hand, in optical amplification using stimulated Raman scattering of an optical fiber, it is possible to amplify signal light that matches the nth-order Stokes wavelength, but the degree of amplification is highest when using the first-order Stokes light. It is. FIG. 3 shows the relationship between the gain coefficient and pump light power in this case.

As can be seen from FIG. 3, there is a tendency for the gain coefficient to become saturated as the pump light power increases. This is because the pump light power contributes to the second-order Stokes oscillation with a longer wavelength via the first-order Stokes light, and the pump light power at which the gain coefficient begins to saturate and the critical pump power of the second-order Stokes is almost equal. Therefore, in order to obtain a large gain coefficient by linearly changing the gain coefficient under high pump light input, it is necessary to increase the oscillation critical pump light power of the second-order Stokes light. However, conventionally, there was no method for realizing this.

Furthermore, in optical amplification, it is desirable to be able to maintain the first-order Stokes gain over as long a distance as possible. However, in normal optical fibers, second-order Stokes occurs, so when the fiber length exceeds a certain value, the first-order Stokes gain decreases rapidly, and conversely, the amplified signal light is affected by this decrease in gain. tend to lose energy. Therefore, even if the pump light power is increased, the gain is limited.

[Objective] The present invention was made in view of the above circumstances, and an object of the present invention is to provide an optical fiber having a configuration suitable for optical amplification.

Another object of the present invention is to provide a method for performing optical amplification using the above-mentioned optical fiber.

[Configuration of the Invention] The optical fiber of the present invention includes a cladding, a main core buried in the cladding and forming a main waveguide, a sub-core arranged in parallel with the main core and coupling only light of a specific wavelength, and a loss imparting section disposed along the longitudinal direction in the vicinity of the sub-core.

The optical amplification method of the present invention includes a cladding, a main core buried in the cladding and forming a main waveguide, a sub-core placed in parallel with the main core and coupling only light of a specific wavelength, and a sub-core located in the vicinity of the sub-core. A pump light and a signal light are simultaneously guided into an optical fiber having a loss imparting section disposed along the longitudinal direction, and the wavelength of the first Stokes light of stimulated Raman scattering and the wavelength of the signal light are determined with respect to the wavelength of the pump light. The waveguide mode of the main core is coupled with the waveguide mode of the sub-core only at the wavelength of the second-order Stokes light of stimulated Raman scattering, and optical energy is transferred from the main core to the sub-core by mode coupling. , by attenuating the transferred optical energy by the loss imparting section, the second stimulated Raman scattering mode of the main core waveguide mode is
Only the loss of light with a wavelength corresponding to the wavelength of the Stokes light is increased, and the signal light is optically amplified by stimulated Raman scattering in the main core.

[Example] Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of the configuration of the optical fiber of the present invention, where 1 is a cladding, 2 is a core (main core) buried in the cladding 1 and forming a main waveguide;
Numeral 3 is a core (sub-core) placed in parallel with the main core 2 in the cladding 1 and couples only light of a specific wavelength to form a sub-waveguide. This is a loss imparting section arranged in parallel.

Optical fibers with such a structure are placed inside a quartz tube using the rod-in-tube method using the VAD method or
It can be manufactured by inserting a core base material, a clad base material, and a loss imparting part base material produced by the MCVD method, and drawing these base materials while heating and integrating them.

For the loss imparting base material, silica glass doped with impurities such as B ₂ O ₃ can be used for the purpose of imparting loss.

Although two cores are provided in the embodiment shown in the first drawing, the optical fiber of the present invention is not limited to this, and it goes without saying that it may be configured to include three or more cores and a plurality of loss imparting parts. It is.

Now, in the optical fiber shown in _FIG .
m, λ _s2 = 1.4954 μm), sufficient coupling is generated between the waveguide modes of both cores 2 and 3, and the pump light wavelength λp = 1.318 μm and the first Stokes light wavelength λ _s1 = 1.401 μm. The coupling can be suppressed sufficiently to achieve wavelength-selective coupling. That is, such coupling can be realized by setting the core diameters of the two cores and the refractive index difference between the core and the cladding to different values.

Figure 4 shows the relationship between the normalized propagation constant β/k of the lowest-order waveguide mode LP ₀₁ mode propagating through cores 2 and 3 in Figure 1 and the wave number k (=2π/λ, λ is the wavelength). To give an example, the second Stokes wavelength λ=λ _s2
The propagation constants of both modes are equal, and λp
= 1.318 μm and λ _s1 are different. In this case, the relative refractive index difference Δ between the core and the cladding is the main core 2
0.4% for sub core 3, 0.204% for sub core 3, core radius is 3.42 μm for main core 2, sub core 3
The diameter was set to 8.635 μm.

At this time, if a light wave of λ = λ _s2 is incident on the main core 2, after propagating for a certain distance, it will be caused by mode coupling.
100% power is transferred to sub-core 3, but λ=λ
The coupling at p, λ _s1 can be sufficiently suppressed by appropriately selecting the relative refractive index difference and the core diameter.
Incidentally, in this numerical example, the coupling at λ=λp and λ _s1 was as small as about 5% or less. Since the light wave of λ=λ _s2 transferred to the main core 2 or the sub-core 3 is absorbed by the loss imparting part 4, the coupling apparently contributes to the loss at λ=λ _s2 when viewed from the main core 2. Become.

That is, as shown in FIG.
The loss wavelength characteristic of LP ₀₁ mode is λ=λ _s2 (=1.4954μ
The losses in m) are superimposed.

In addition, as the material of the loss imparting part 4 shown in the first diagram,
For example, when using a base material of quartz doped with B ₂ O ₃ , the B ₂ O ₃ doped glass has a
(dB/Km) or more, by arranging the loss adding section 4 adjacent to one side of the sub-core 3, the light waves with a wavelength of 1.4 μm or more propagating through the sub-core 3 can be transferred to the loss adding section 4. can be almost attenuated by

Figure 6 shows that at λ _s2 = 1.4954 μm, degeneracy occurs in the LP ₀₁ mode of the main core and sub-core, and the coupling becomes 100%.
The optimum relative refractive index difference and core radius to minimize the coupling at λ= _{λp with} This is what is shown. Here, the relative refractive index difference Δ ₁ of the main core 2 is Δ
₁ = 0.4%, λp = 1.318μm, λ _s2 =
δβ=0 at 1.4954 μm. In this case, in order to set _the V value ratio V _p2 /V _p1 to ₂ from FIG. Relative refractive index difference Δ
It turns out that ₂ can be set to 0.19.

FIG. 7 shows an example in which the optical amplification method using the optical fiber of the present invention is applied to an optical transmission line. Here, 5 is a pump light, 6 is a signal light, 7 is a total reflection mirror, 8 is a dichroic mirror, 9 is a condensing lens, 10 is an optical fiber for optical amplification according to the present invention, and 11 is a demultiplexer. .

The pump light 5 and the signal light 6 are combined by a dichroic mirror 8, condensed by a lens 9, and then input into an optical fiber 10 for optical amplification. Within the optical fiber 10, the signal light 6 is amplified by stimulated Raman scattering caused by the strong pump light 5, and is split again at the output end of the optical fiber 10 to form the pump light 5' and the signal light. 6'.

As is generally known, the Stokes critical gain coefficient in stimulated Raman scattering increases in proportion to the loss, so as shown in this example, λ
=λ _s2 By using the optical fiber 10 with a large loss for optical amplification, the second-order Stokes critical gain coefficient g ^(s2) _th becomes large.

For example, as shown in FIG. 8, the relationship (curve) between the gain coefficient and pump light power of the optical fiber of the present invention does not exhibit the saturation characteristic as shown in FIG.
The maximum gain can be increased compared to the case of a normal optical fiber (curve). In FIG. 8, the length of the optical fiber is 10 km, and the signal light power is 1 mW. Critical gain coefficient g shown in the figure
^(s2) _th indicates the case of a normal optical fiber with no loss at λ=λ _s2 , and according to the optical fiber of the present invention, the critical gain coefficient increases as shown by the arrow.

FIG. 9 compares the relationship between gain and fiber length between an optical fiber of the present invention (curve) and a conventional optical fiber (curve). Here, the pump light power was 20 W and the signal light power was 1 mW. As can be seen from Figure 9, in a normal optical fiber,
While the gain decreases rapidly at a fiber length of about 10 km due to the occurrence of second-order Stokes, in the optical fiber of the present invention, the gain decreases in accordance with the linear optical loss of the fiber, so it can be used over extremely long distances. It can be seen that high profits can be maintained.

[Effects] As explained above, the optical fiber of the present invention has the following effects:
In addition to the main core, a sub-core is provided that causes a large loss only at a specific wavelength. According to the optical amplification method of the present invention using such an optical fiber, it is possible to avoid problems that have been avoided in optical amplification using stimulated Raman scattering. It is possible to suppress the phenomenon of gain saturation that would otherwise have occurred.
A gain coefficient proportional to the pump light power can be obtained.
Moreover, in the optical fiber of the present invention, gain saturation does not occur even if the fiber length is increased, so that the gain can be increased. Therefore, the upper limit of the gain of approximately 30 dB obtained with conventional optical amplification can be increased to a level where the optical fiber end face is not destroyed by the pump light power, and the gain at that time is 100 dB or more. It is estimated that This makes it possible to eliminate repeaters in ultra-long-distance optical transmission systems such as submarine systems. For example, when an amplification gain of 100 dB is obtained with the F-400M system, it is possible to achieve repeater-free transmission over a distance of about 500 km, which is practical. The effect is large.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing an embodiment of the optical fiber of the present invention, Fig. 2 is a loss wavelength characteristic diagram of a normal optical fiber, and Fig. 3 is the relationship between gain coefficient and pump light power in a conventional optical amplification method. FIG. 4 is a characteristic diagram showing the dispersion curve of the LP ₀₁ mode of the optical fiber of the present invention, FIG. 5 is a characteristic diagram showing an example of the loss wavelength characteristic of the optical fiber of the present invention, and FIG. A characteristic diagram showing the relationship between fiber parameters and V _p2 /V _p1 , FIG. 7 is a diagram showing an example of the optical system of the optical amplification method of the present invention using the optical fiber of the present invention, and FIG. 8 is a diagram showing the relationship between the gain coefficient and Characteristic diagram showing the relationship with pump light power,
FIG. 9 is a characteristic diagram showing the relationship between gain coefficient and fiber length, 1...Main core, 2...Sub-core, 3...Loss imparting section, 4...Clad, 5...Pump light, 6... Signal light, 7... total reflection mirror, 8... dichroic mirror, 9... condensing lens, 10... optical fiber for optical amplification, 11... splitter.

Claims (1)

[Claims] 1. A cladding, a main core buried in the cladding and forming a main waveguide, and a main core arranged in parallel with the main core, and a signal light and a first order of stimulated Raman scattering in the main core. a sub-core that couples only light at a secondary Stokes light wavelength when wavelength pump light whose Stokes light wavelength matches the signal light wavelength is incident; and a loss disposed along the longitudinal direction in the vicinity of the sub-core. An optical fiber characterized by comprising a provision section. 2. A cladding, a main core buried in the cladding and forming a main waveguide, and a main core disposed in parallel with the main core, in which the signal light and the first Stokes light wavelength of stimulated Raman scattering are transmitted to the main core. A sub-core that couples only light having a secondary Stokes light wavelength when pump light having a wavelength that matches the optical wavelength is incident, and a loss imparting portion disposed along the longitudinal direction in the vicinity of the sub-core. simultaneously guiding a pump light and a signal light into an optical fiber, matching the wavelength of the first-order Stokes light of stimulated Raman scattering with respect to the wavelength of the pump light and the wavelength of the signal light, and changing the waveguide mode of the main core to Only at the wavelength of the second-order Stokes light of stimulated Raman scattering is coupled with the waveguide mode of the sub-core, optical energy is transferred from the main core to the sub-core by mode coupling, and the transferred optical energy is transferred to the loss. By attenuating it with the imparting section, only the loss of light of a wavelength corresponding to the wavelength of the second-order Stokes light of the stimulated Raman scattering is increased among the waveguide modes of the main core, and An optical amplification method characterized by optically amplifying signal light by stimulated Raman scattering.