Optical fiber for amplification and optical fiber amplifier

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an optical fiber for optical amplification and an optical fiber amplifier.

【0002】

[Conventional technology]

The optical fiber amplifier photoamplifies the signal light in order to compensate for the loss when the signal light propagates in the optical transmission path in the optical communication system, and includes an optical fiber for optical amplification and an excitation light supply means. That is, when excitation light having a predetermined wavelength is supplied to the optical amplification optical fiber by the excitation light supply means and signal light is input to the optical amplification optical fiber, the input signal light is photoamplified in the optical amplification optical fiber. Is output. Further, as such an optical fiber amplifier, an optical fiber in which a rare earth element is added to an optical waveguide region is used as an optical fiber for optical amplification (hereinafter referred to as "rare earth element added optical fiber amplifier"), and Raman amplification is used. There is something to do (hereinafter referred to as "Raman amplifier").

【0003】

Raman amplifiers are described, for example, in Reference 1 "S. Hamidi, et al., Electronics Letters, Vol. 28, No. 18, pp. 1768-1770 (1992)" and Reference 2 "E. Desurvire, et al., Electronics. Letters, Vol.19, No.19, pp.752-753 (1983) ", and Reference 3" Y.Aoki, et al., Electronics Letters, Vol.19, No.16, pp.620-622 ( 1983) etc. Further, the optimization of the refractive index profile of the optical fiber for optical amplification is described in, for example, Document 4 "MJ Holmes, et al., Electronics Letters, Vol.26, No.22, pp.1873-1874 (1990)". Has been done.

【0004】

[Problems to be Solved by the Invention]

However, the inventor of the present application has found that the conventional optical fiber amplifier has the following problems. That is, the optical fiber for optical amplification used in the conventional optical fiber amplifier is a single mode and a base mode (LP) at both the wavelength of the signal light (for example, 1.55 μm) and the wavelength of the excitation light (for example, 1.48 μm).₀₁Propagate only mode). However, the effective cross-sectional area of the optical fiber for optical amplification at the excitation light wavelength (corresponding to the power distribution of the excitation light in the base mode) is the effective cross-sectional area at the signal light wavelength (the power distribution of the signal light in the base mode). Smaller than the corresponding one).

【0005】

Therefore, in the conventional optical fiber amplifier, the gain of optical amplification is not sufficient due to the difference in the effective cross-sectional area between the two wavelengths of the optical fiber for optical amplification. In particular, in a region that contributes to optical amplification and is far from the center, the signal light may not be photoamplified or may be absorbed because the excitation is not sufficient. Further, since the efficiency of optical amplification is high and the signal optical power increases near the core center of the optical fiber for optical amplification, a nonlinear optical phenomenon is likely to occur, and from this point as well, there is a limit in increasing the gain of optical amplification. Further, it is difficult to increase the output of the semiconductor laser light source (excitation light source) that outputs only the excitation light in the base mode, and from this point as well, there is a limit in increasing the gain of optical amplification. It is conceivable to increase the power of the excitation light supplied to the optical fiber for optical amplification by using a plurality of semiconductor laser light sources, but in this case, the optical fiber amplifier becomes expensive.

【0006】

The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical fiber amplifier having a high gain of optical amplification and an optical fiber for optical amplification preferably used for this optical fiber amplifier. And.

【0007】

[Means for solving problems]

The optical amplification optical fiber according to the present invention is an optical amplification optical fiber capable of photoamplifying signal light by supplying excitation light, and has a cutoff wavelength shorter than the wavelength of the signal light and longer than the wavelength of the excitation light. It is characterized by having. This optical fiber for optical amplification can propagate only the basal mode for signal light and not only the basal mode but also at least a secondary mode for excitation light. By adding the power distribution of the higher-order mode of the excitation light to the power distribution of the base mode of the excitation light, it can be made close to the power distribution of the base mode of the signal light, and the effective cross-sectional area for the signal light and the excitation light can be obtained. Since the difference from the effective cross-sectional area is effectively reduced, the gain of optical amplification can be increased. Further, it is possible to excite not only the vicinity of the center of the core of the optical fiber for optical amplification but also the periphery of the core that contributes to optical amplification, which is also suitable for increasing the gain of optical amplification.

【0008】

Further, the optical fiber for optical amplification according to the present invention is characterized in that the group velocity of the signal light in the base mode is equal to or lower than the group velocity of the excitation light in the base mode and equal to or higher than the group velocity of the secondary mode of the excitation light. And. In this case, even if there is a difference in group velocity between the base mode of the signal light and the base mode of the excitation light, the difference in the group velocity between the base mode of the signal light and the higher-order mode of the excitation light. If is small, the gain of optical amplification can be increased by excitation of the excitation light by the higher-order mode.

【0009】

Further, the optical fiber for optical amplification according to the present invention is characterized in that the excitation light can propagate only in the basal mode and the secondary mode. Under the condition of ensuring single-mode operation in which only the base mode is propagated for the signal light, the higher-order mode of the third-order or higher-order of the excitation light usually has a large attenuation. Therefore, the gain of optical amplification can be increased by not propagating the third-order or higher-order mode of the excitation light.

【0010】

Further, the optical amplification optical fiber according to the present invention is characterized by being a polarization-retaining optical fiber that maintains the polarization state at the time of propagation of each of the signal light and the excitation light. In this case, the gain of optical amplification can be increased by matching the polarization states of the signal light and the excitation light with each other.

【0011】

Further, the optical fiber for optical amplification according to the present invention is characterized in that it is Raman amplified. In this case, the power of the signal light that is photoamplified while propagating through the optical fiber for optical amplification is small enough that a nonlinear optical phenomenon does not occur, and the utilization efficiency of the excitation light is high.

【0012】

Further, the optical fiber for optical amplification according to the present invention is characterized in that a substance that increases the Raman gain coefficient is added to a region where the refractive index is maximized. In this case, the gain of optical amplification can be further increased.

【0013】

The optical fiber amplifier according to the present invention supplies the optical fiber for optical amplification according to the present invention and the excitation light having a wavelength shorter than the cutoff wavelength of the optical fiber for optical amplification to the optical fiber for optical amplification. It is characterized by having means. According to this optical fiber amplifier, the excitation light supplied to the optical fiber for optical amplification by the excitation light supply means includes not only the basal mode but also at least the secondary mode. Then, the optical fiber for optical amplification can propagate only the ground mode for the signal light and propagate not only the ground mode but also at least the secondary mode for the excitation light. Therefore, in the optical fiber for optical amplification, by adding the power distribution of the higher-order mode of the excitation light to the power distribution of the base mode of the excitation light, it can be made close to the power distribution of the base mode of the signal light. Since the difference between the effective cross-sectional area for the signal light and the effective cross-sectional area for the excitation light is effectively reduced, the gain of the optical fiber amplifier is increased. Further, since it is possible to excite not only the vicinity of the center of the core of the optical fiber for optical amplification but also the periphery of the core that contributes to optical amplification, it is suitable for increasing the gain of optical amplification from this point as well.

【0014】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate description will be omitted.

【0015】

FIG. 1 is a schematic configuration diagram of the optical fiber amplifier 1 according to the present embodiment. The optical fiber amplifier 1 includes an optical fiber 10 for optical amplification, excitation light sources 21 and 22, optical fibers 31 and 32 for supplying excitation light, and optical couplers 41 and 42.

【0016】

The optical fiber 10 for optical amplification can amplify signal light by supplying excitation light. The cutoff wavelength λc of the optical fiber 10 for optical amplification is shorter than the wavelength λs of the signal light and longer than the wavelength λp of the excitation light. That is, between these three wavelengths λp <λc <λs ... (1) The relationship is established. Therefore, at the signal light wavelength λs, the ground mode (LP) is set in the optical fiber 10 for optical amplification.₀₁Only the mode) propagates, and at the excitation light wavelength λp, at least the ground mode (LP) is inside the optical fiber 10 for optical amplification.₀₁Mode) and secondary mode (LP)₁₁Mode) propagates.

【0017】

The refractive index profile of the optical fiber 10 for optical amplification is arbitrary as long as the above equation (1) holds. For example, a step type having a simple core and a clad, a first core (refractive index n1), a second core (refractive index n2), and a clad (refractive index n3) in order from the center, and the magnitude relationship of the refractive index is Dual shape type with n1> n2> n3, with 1st core (refractive index n1), 2nd core (refractive index n2), 3rd core (refractive index n3) and clad (refractive index n4) in order from the center. Segment type in which the magnitude relationship of the refractive index is n1> n3> n2 n4, the first core (refractive index n1), the second core (refractive index n2), the third core (refractive index n3), and the cladding in order from the center. Depressed segment type with (refractive index n4) and the magnitude relationship of the refractive index is n1> n3> n4> n2, with the first core (refractive index n1) and the second core (refractive index n2) in order from the center. It is a ring type that has a clad (refractive index n3) and the magnitude relationship of the refractive index is n2> n3> n1 or n2> n1 n3.

【0018】

When the optical fiber amplifier 1 is a rare earth element-added optical fiber amplifier, the optical fiber 10 for optical amplification has rare earth elements (for example, Er, Tm, Pr, Nd, etc.) added to the optical waveguide region. .. When the optical fiber amplifier 1 is a Raman amplifier, the optical fiber 10 for optical amplification is in a region where a substance (for example, Ge, Al, P, etc.) that increases the Raman gain coefficient has a maximum refractive index. It is preferable that it is added. In this case, the gain of optical amplification can be further increased. The region where the refractive index is maximized in the optical fiber 10 for optical amplification is a fixed region including the peak position of the power distribution in the basal mode, and in the case of the step type, it is the central portion of the core, and is a dual shape type. In the case of the segment type and the depressed segment type, it is the first core region, and in the case of the ring type, it is the second core region.

【0019】

Here, the signal light wavelength λs is generally preferably around 1.55 μm, which has the smallest transmission loss in a quartz-based optical fiber. At this time, when the optical fiber amplifier 1 is a rare earth element-added optical fiber amplifier, the excitation light wavelength λp is 1.48 μm or 0.98 μm. When the optical fiber amplifier 1 is a Raman amplifier, the excitation light wavelength λp is a wavelength in the range of 1.40 μm to 1.50 μm.

【0020】

The cut-off wavelength λc of the optical fiber 10 for optical amplification is generally the secondary (LP) in which the optical fiber 10 for optical amplification having a length of 2 m is loosely wound once with a radius of 140 mm.₁₁) Defined as the mode cutoff wavelength (ITU-G.650). When the optical fiber amplifier 1 is a rare earth element-added optical fiber amplifier and the length of the optical fiber 10 for optical amplification is several meters to several hundreds of meters, it is preferable to use the cutoff wavelength λc of this definition. is there.

【0021】

On the other hand, when the optical fiber amplifier 1 is a Raman amplifier and the length of the optical fiber 10 for optical amplification is several tens of kilometers to several hundreds of kilometers, the cutoff wavelength λc of the optical fiber 10 for optical amplification is set to Leff = (1-exp (-αL)) / L ... (2) It is preferably defined for the effective length Leff of the optical fiber 10 for optical amplification represented by the following equation. Here, α is the transmission loss at the excitation optical wavelength of the optical amplification optical fiber 10, and L is the total length of the optical amplification optical fiber 10. Further, the effective length Leff of the optical fiber 10 for optical amplification is an effective length at which the excitation light can contribute to optical amplification.

【0022】

Each of the excitation light sources 21 and 22 outputs the excitation light to be supplied to the optical amplification optical fiber 10, and for example, a semiconductor laser light source is preferably used. Further, each of the excitation light sources 21 and 22 outputs not only the base mode of the excitation light but also at least a secondary mode. The excitation light supply optical fiber 31 guides the excitation light (including the base mode and the two modes) output from the excitation light source 21 to the optical coupler 41. The excitation light supply optical fiber 32 guides the excitation light (including the ground mode and the two modes) output from the excitation light source 22 to the optical coupler 42.

【0023】

The optical coupler 41 inputs the signal light input to the input terminal 1a to the optical amplification optical fiber 10, and also receives the excitation light arriving from the excitation light source 21 via the excitation light supply optical fiber 31 to the optical amplification optical fiber 10. To input to. The optical coupler 42 outputs the signal light output from the optical amplification optical fiber 10 to the output terminal 1b, and also outputs the excitation light arriving from the excitation light source 22 through the excitation light supply optical fiber 32 to the optical amplification optical fiber 10. To input to.

【0024】

In the optical fiber amplifier 1, it is also preferable to provide an optical isolator that propagates light only in the forward direction. The optical isolator is provided between the incident end 1a and the optical coupler 41, or between the optical coupler 42 and the output end 1b.

【0025】

In the optical fiber amplifier 1 configured in this way, the excitation light (including the ground mode and the two modes) output from the excitation light source 21 passes through the excitation light supply optical fiber 31 and the optical coupler 41 and is emitted from the forward direction. It is supplied to the amplification optical fiber 10. The excitation light (including the ground mode and the two modes) output from the excitation light source 22 is supplied to the optical amplification optical fiber 10 from the opposite direction via the excitation light supply optical fiber 32 and the optical coupler 42. The signal light input to the input terminal 1a is input to the optical fiber 10 for optical amplification via the optical coupler 41, is optically amplified by the optical fiber 10 for optical amplification, and is output to the output terminal 1b via the optical coupler 42.

【0026】

In the optical fiber 10 for optical amplification, the relationship of the above equation (1) holds between the cutoff wavelength λc, the signal light wavelength λs, and the excitation light wavelength λp. Base mode (LP₀₁Mode) only propagates. On the other hand, for the excitation light, at least the ground mode (LP) in the optical fiber 10 for optical amplification₀₁Mode) and secondary mode (LP)₁₁Mode) propagates.

【0027】

FIG. 2 is a diagram illustrating a light power distribution in an optical fiber for optical amplification. FIG. 3A shows the powers of the signal light base mode (solid line), the excitation light base mode (broken line), and the excitation light secondary mode (dashed line) in the optical fiber 10 for optical amplification according to the present embodiment. Shows the distribution. FIG. (B) shows the power distributions of the signal light base mode (solid line) and the excitation light base mode (dashed line) in the conventional optical fiber for optical amplification. These show the power distribution of light on a straight line orthogonal to the optical axis of the optical fiber.

【0028】

In the conventional optical fiber for optical amplification, since the cutoff wavelength λc is smaller than both the signal light wavelength λs and the excitation light wavelength λp, only the ground mode can propagate for the signal light, and only the ground mode can be propagated for the excitation light. Can be propagated. Further, in general, even in the basal mode, the shorter the wavelength, the smaller the width of the light power distribution and the smaller the effective cross-sectional area. That is, as shown in FIG. 2 (b), the effective cross section of the excitation light having a shorter wavelength than this signal light is smaller than the effective cross section of the signal light with respect to the base mode. Therefore, as already described, the optical fiber amplifier using such a conventional optical fiber for optical amplification has a gain in optical amplification due to the difference in the effective cross-sectional area between the two wavelengths of the optical fiber for optical amplification. Not enough. Further, since the efficiency of optical amplification is high and the signal optical power increases near the core center of the optical fiber for optical amplification, a nonlinear optical phenomenon is likely to occur, and from this point as well, there is a limit in increasing the gain of optical amplification.

【0029】

On the other hand, in the optical fiber 10 for optical amplification according to the present embodiment, the signal light is in the base mode (LP).₀₁Only mode (mode) propagates, at least base mode (LP) for excitation light₀₁Mode) and secondary mode (LP)₁₁Mode) propagates. In this case as well, the effective cross section of the excitation light with respect to the basis mode is smaller than the effective cross section of the signal light with respect to the basis mode. However, in the present embodiment, the secondary mode of the excitation light can also propagate through the optical fiber 10 for optical amplification. Then, by adding the power distribution of the secondary mode of the excitation light to the power distribution of the base mode of the excitation light, it can be made close to the power distribution of the base mode of the signal light (Fig. 2 (a)). From this, in the optical fiber amplifier 1 according to the present embodiment, the difference between the effective cross section for the signal light and the effective cross section for the excitation light (including the basal mode and the two modes) is effectively reduced, so that the optical amplification is achieved. Gain can be increased. Further, it is possible to excite not only the vicinity of the center of the core of the optical fiber 10 for optical amplification but also the periphery of the core that contributes to optical amplification, which is also suitable for increasing the gain of optical amplification. is there.

【0030】

When the optical fiber optical amplifier 1 is a Raman amplifier and the excitation light is supplied in the forward direction, if the difference between the group velocities of the signal light and the excitation light is large, the gain of Raman amplification becomes small. Further, even if the wavelength is the same, the group velocity differs between the basal mode and the higher-order mode, and the group velocity usually becomes slower as the order is higher due to the mode dispersion. Therefore, even if there is a difference in group velocity between the base mode of signal light and the base mode of excitation light, if the difference in group velocity between the base mode of signal light and the higher-order mode of excitation light is small, , The gain of optical amplification can be increased by excitation of the excitation light by the higher-order mode. Therefore, when the group velocity of the ground mode of the signal light is Vs, the group velocity of the ground mode of the excitation light is Vp1, and the group velocity of the secondary mode of the excitation light is Vp2, Vp2 Vs Vp1 ... (3) It is preferable that the relational expression is established.

【0031】

Further, under the condition of ensuring the single mode operation in which only the base mode is propagated for the signal light, the high-order mode of the third or higher order of the excitation light usually has a large attenuation, so that the optical fiber 10 for optical amplification is excited. It is preferable that light can propagate only in the basal mode and the secondary mode.

【0032】

Further, the gain of Raman amplification is high when the polarization states of the signal light and the excitation light are in agreement with each other. Therefore, the optical amplification optical fiber 10 is preferably a polarization-retaining optical fiber that maintains the polarization state at the time of propagation of each of the signal light and the excitation light.

【0033】

[Effect of the invention]

As described in detail above, the optical amplification optical fiber according to the present invention is an optical amplification optical fiber capable of photoamplifying signal light by supplying excitation light, and is excited shorter than the wavelength of the signal light. It has a cutoff wavelength longer than the wavelength of light. This optical fiber for optical amplification can propagate only the basal mode for signal light and not only the basal mode but also at least a secondary mode for excitation light. By adding the power distribution of the higher-order mode of the excitation light to the power distribution of the base mode of the excitation light, it can be made close to the power distribution of the base mode of the signal light, and the effective cross-sectional area for the signal light and the excitation light can be obtained. Since the difference from the effective cross-sectional area is effectively reduced, the gain of optical amplification can be increased.

【0034】

Further, in the optical fiber for optical amplification according to the present invention, it is preferable that the group velocity of the signal light in the base mode is equal to or lower than the group velocity of the excitation light in the base mode and equal to or higher than the group velocity of the secondary mode of the excitation light. Is. In this case, even if there is a difference in group velocity between the base mode of the signal light and the base mode of the excitation light, the difference in the group velocity between the base mode of the signal light and the higher-order mode of the excitation light. If is small, the gain of optical amplification can be increased by excitation of the excitation light by the higher-order mode.

【0035】

Further, it is preferable that the optical fiber for optical amplification according to the present invention can propagate only the basal mode and the secondary mode for the excitation light. Under the condition that only the base mode is propagated for the signal light, the higher-order mode of the third-order or higher of the excitation light usually has a large attenuation, but the higher-order mode of the third-order or higher of the excitation light is used. By not propagating, the gain of optical amplification can be increased.

【0036】

Further, the optical amplification optical fiber according to the present invention is preferably a polarization-retaining optical fiber that retains the polarization state at the time of propagation of the signal light and the excitation light. In this case, the signal light and The gain of optical amplification can be increased by matching the polarization states of the excitation lights with each other.

【0037】

Further, it is preferable that the optical fiber for optical amplification according to the present invention is Raman-amplified. In this case, the power of the signal light that is photoamplified while propagating through the optical fiber for photoamplification is nonlinear optics. It is small enough that the phenomenon does not occur, and the utilization efficiency of the excitation light can be improved.

【0038】

Further, in the optical fiber for optical amplification according to the present invention, it is preferable that a substance that increases the Raman gain coefficient is added to the region where the refractive index is maximized. In this case, the gain of optical amplification is further increased. Can be enhanced.

【0039】

The optical fiber amplifier according to the present invention supplies the above-mentioned optical fiber for optical amplification according to the present invention and excitation light having a wavelength shorter than the cutoff wavelength of the optical fiber for optical amplification to the optical fiber for optical amplification. Provide means. According to this optical fiber amplifier, the excitation light supplied to the optical fiber for optical amplification by the excitation light supply means includes not only the basal mode but also at least the secondary mode. Then, the optical fiber for optical amplification can propagate only the ground mode for the signal light and propagate not only the ground mode but also at least the secondary mode for the excitation light. Therefore, in the optical fiber for optical amplification, by adding the power distribution in the higher-order mode of the excitation light to the power distribution in the base mode of the excitation light, the power distribution should be effectively close to the power distribution in the base mode of the signal light. Therefore, the difference between the effective cross-sectional area for the signal light and the effective cross-sectional area for the excitation light becomes small, so that the gain of the optical fiber amplifier becomes high.

[Simple explanation of drawings]

[Figure 1]

It is a schematic block diagram of the optical fiber amplifier which concerns on this embodiment.

[Figure 2]

It is a figure explaining the power distribution of light in an optical fiber for optical amplification.

[Explanation of symbols]

1 ... Optical fiber amplifier, 10 ... Optical fiber for optical amplification, 21,22 ... Excitation light source, 31,32 ... Optical fiber for excitation light supply, 41,42 ... Optical coupler.