JP2008263245A - Optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve pumping efficiency of a cladding pumped optical amplifier using an optical fiber that includes: a core doped with a rare earth metal; an inner cladding provided around the core and having a refractive index lower than the refractive index of the core; and an outer cladding provided around the inner cladding and having a refractive index lower than the refractive index of the inner cladding. <P>SOLUTION: The cladding pumped optical amplifier 20 is provided with an optical fiber grating for combining a guided mode contributing to pumping the rare earth metal with a guided mode not contributing to pumping the rare earth metal out of a plurality of guided modes of pumped light entering the amplifier 20. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical amplifier composed of a rare earth-doped optical fiber having a mechanism for guiding pumping light to a clad, and more particularly, to improve pumping efficiency by using an optical fiber grating.

  In recent years, the dramatic improvement in transmission capacity in the field of optical communications is largely due to the advent of optical amplifiers. FIG. 6 is a schematic configuration diagram showing an example of a conventional general optical amplifier. As shown in this figure, an optical amplifier is generally configured to guide signal light and pumping light in parallel to the core of a silica-based optical fiber 1 doped with rare earth metal, and is pumped by pumping light. Amplifying action is obtained by stimulated emission of the rare earth metal by signal light. In the figure, reference numeral 2 denotes an excitation light source, and 3 denotes an isolator. Recently, the rare earth-doped optical fiber 1 used in an optical amplifier has a double clad structure having two layers of clads 12 and 13 as shown in FIG. Technologies have been developed to further increase the amplification characteristics and reduce the noise. In the figure, reference numeral 11 denotes a core, and 13 denotes an outer clad portion. The core diameter is, for example, 4 μm, the inner cladding diameter is, for example, 30 μm, and the outer cladding diameter is, for example, 125 μm.

  Thus, an optical amplifier having the rare earth doped optical fiber 1 having a double clad structure, that is, an optical amplifier having a structure in which pump light is guided also to the inner clad portion 12 (hereinafter referred to as a clad pump optical amplifier) has a high output pump. Since a light source can be used, the output characteristics can be dramatically improved by increasing the output of the excitation light source. However, there is a disadvantage that it is significantly inferior to the conventional optical amplifier in terms of pumping efficiency with respect to pumping light power. That is, since most of the electric field of the signal light incident on the cladding pump optical amplifier is present in the core 11, the rare earth metal contributing to the amplification action is also mainly present in the core 11. Therefore, most of the pumping light incident on the cladding pumping optical amplifier contributes to the optical amplification is the pumping light propagating in the core 11, and the pumping light propagating in the inner cladding part 12 is almost light. It will not be used for amplification, so the excitation efficiency will be low. FIG. 8 shows the electric field intensity distribution (indicated by a broken line in the figure) when the excitation light is incident on the clad excitation optical amplifier and the electric field intensity distribution after 100 m of wave guiding (indicated by a solid line in the figure). . From this figure, the excitation light propagating in the core 11 is absorbed and attenuated for the excitation of the dopant while being guided 100 m, whereas the excitation light propagating in the inner cladding portion 12 is almost attenuated. It can be seen that it is hardly used for dopant excitation. It can be seen that the amount of light absorbed for excitation is small relative to the power of the entire incident excitation light, and the excitation efficiency is poor.

  Here, the reason why the pumping efficiency in the clad pumping optical amplifier is low will be described. The pumping light propagating in the clad pumping optical amplifier actually has a plurality of waveguide modes, and the incident pumping light in FIG. 8 is represented by these couplings. Among these guided modes, there is a mode having almost no electric field component near the core, such as the primary mode shown in FIG. 9, and such a guided mode hardly contributes to the excitation of the dopant. On the other hand, there is also a waveguide mode having a strong electric field component near the core, as in the 0th-order mode shown in FIG. Therefore, when the light absorptance in the core 11 of the rare earth-doped optical fiber having the double clad structure shown in FIG. 7 is α, the electric field strength of the light propagating in the rare earth doped optical fiber is defined by the following formula (I). Then, the change in electric field intensity was simulated for each waveguide mode with α = 1.0 × 10 −1 (unit: 1 / m). P = P0 × exp [−αZ] (I) (where P is the power at Z, P0 is the initial power, and Z is the propagation distance.) FIG. 11 is the zero-order mode, and FIG. FIG. 13 shows the simulation result of the secondary mode, FIG. 14 shows the simulation of the quartic mode, and FIG. 15 shows the simulation of the quintic mode. As shown in these figures, the 0th-order mode is attenuated during propagation, and is about 2 to 3% of the electric field strength at the time of incidence, for example, when it propagates 100 m. From this, it can be seen that the zero-order mode is absorbed by the core and excites the dopant efficiently. On the other hand, in the first-order mode, second-order mode, fourth-order mode, and fifth-order mode, 50% or more of the electric field strength at the time of incidence remains even after 100 m propagation, and hardly contributes to the excitation of the dopant. Recognize.

  As described above, in an optical amplifier of a type that propagates pumping light only to a conventional core, in order to improve pumping efficiency, a total reflection mirror is provided at the output end opposite to the pumping light incident end, and the output light is transmitted. It has also been proposed to make it enter again into the optical amplifier. However, in the clad pumping optical amplifier, the power that contributes to pumping is small relative to the power of the entire pumping light as described above, so even if the pumping light that has once passed through the optical amplifier enters the optical amplifier again. The effect of improving the excitation efficiency is small.

  The present invention has been made in view of the above circumstances, and an object thereof is to improve the pumping efficiency in a clad pumping optical amplifier.

An optical amplifier according to a first aspect of the present invention includes a core, an inner clad provided around the core and having a lower refractive index than the core, and an inner clad provided around the inner clad and having a lower refractive index than the inner clad. An optical amplifier comprising an optical fiber having an outer cladding of a rate, wherein the core is doped with rare earth metal,
An optical fiber grating in which periodic perturbation by microbending is formed is provided for the optical fiber constituting the optical amplifier, and the rare earth element is selected from a plurality of waveguide modes of pumping light incident on the optical amplifier. The waveguide mode that contributes to the excitation of the metal is combined with the waveguide mode that does not contribute to the excitation of the rare earth metal.

An optical amplifier according to a second aspect of the present invention is the optical amplifier according to the first aspect, wherein the periodic perturbation period due to the microbending is Λ1, and among the plurality of wave modes existing in the pumping light incident on the optical amplifier, When the propagation constant of the 0th-order mode contributing to the excitation of the rare earth metal is B0, and the propagation constant of a specific higher-order mode other than the 0th-order mode is Bn,
(B0−Bn) = 2π / Λ1
It is characterized by satisfying the relationship.

  The optical amplifier according to claim 3 of the present invention is characterized in that, in claim 2, the period of periodic perturbation by microbending is Λ1, which is a long period of 150 μm or more and 300 μm or less.

According to a fourth aspect of the present invention, there is provided an optical amplifier comprising: a core; an inner clad provided around the core and having a lower refractive index than the core; and an inner clad provided around the inner clad and having a lower refractive index than the inner clad. An optical amplifier comprising an optical fiber having an outer cladding of a rate, wherein the core is doped with rare earth metal,
An optical fiber grating in which a periodic perturbation is formed in an optical fiber having the same core diameter as that of the inner cladding is connected to an output end opposite to the excitation light incident end of the optical amplifier, and the optical amplifier Among the plurality of waveguide modes of excitation light incident on the waveguide, the waveguide mode that contributes to the excitation of the rare earth metal is coupled with the waveguide mode that does not contribute to the excitation of the rare earth metal. And

The optical amplifier according to claim 5 of the present invention is the optical amplifier according to claim 4, wherein the periodic perturbation period formed in the optical fiber connected to the output end is Λ2, and is present in the excitation light incident on the optical amplifier. Among the plurality of dynamic wave modes, the propagation constant of the 0th-order mode contributing to excitation of the rare earth metal is B0, the propagation constant of the reflected light is -B0, and the propagation constant of a specific higher-order mode other than the 0th-order mode is When Bn
(Bn − (− B0)) = (Bn + B0) = 2π / Λ2
It is characterized by satisfying the relationship.

  The optical amplifier according to claim 6 of the present invention is characterized in that, in claim 5, the period of the periodic perturbation by the microbending is Λ2 is a short period of 300 nm or more and 350 nm or less.

  According to the optical amplifier of the present invention, among the pumping light incident on the clad pumping optical amplifier, the optical power that is not used as it is for pumping the dopant is used as the waveguide mode used for pumping the dopant using the optical fiber grating. Since it can be used effectively for the excitation of the dopant, the excitation efficiency of the optical amplifier can be improved. Also, if the optical fiber grating is provided by forming periodic perturbations in the optical fiber constituting the optical amplifier, the optical amplifier will not be enlarged by providing the optical fiber grating. It is preferable in order to make it easier. On the other hand, if the optical fiber grating formed by periodically perturbing the optical fiber is connected to the output end of the optical fiber constituting the optical amplifier separately from the optical fiber constituting the optical amplifier, the optical amplifier And an optical fiber grating can be produced and connected to each other, so that an advantage of simple manufacturing can be obtained.

  The present invention will be described in detail below. FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention. The optical amplifier of this embodiment is different from the conventional clad pump optical amplifier in that a periodic perturbation is formed in the optical fiber constituting the optical amplifier 20, and this optical fiber has a grating function. . In FIG. 1, reference numeral 2 is an excitation light source, and 3 is an isolator. As the excitation light source 2, for example, a laser diode (LD) having an oscillation wavelength of 980 nm is used. The optical amplifier 20 of the present embodiment is made of an optical fiber having a double clad structure as shown in FIG. That is, the optical fiber constituting the optical amplifier 20 is provided with a core 11 having a high refractive index at the center, and an inner cladding 12 having a lower refractive index than the core 11 around the core. Further, an outer cladding having a lower refractive index than that of the inner cladding 12 is provided around the inner cladding 12. The refractive index difference between the core 11 and the inner cladding 12 is about 0.2 to 2%, and the refractive index difference between the inner cladding 12 and the outer cladding 13 is about 0.2 to 10%. The core 11 is doped with rare earth metal. As the rare earth metal, a conventionally known one can be used as a dopant in an optical amplifier such as erbium.

  And the optical fiber which comprises the optical amplifier 20 has the periodic perturbation formed in the length direction, and is also provided with the function as an optical fiber grating. As a method of forming a periodic perturbation in the optical amplifier 20, for example, germanium is added to the core 11 and the inner cladding 12 in advance using a known photolifting effect, and the phase is masked from the side of the optical fiber. By irradiating with ultraviolet light, a method of periodically changing the refractive indexes of the core 11 and the inner cladding in the optical fiber length direction can be used. Alternatively, for example, as shown in FIG. 2, a part of the optical fiber constituting the optical amplifier 20 is sandwiched between two blocks 25 and 26 having irregularities on the contact surface with the optical fiber, whereby the optical fiber is It is also possible to adopt a method of holding the meandering wave. In this case, the meandering portion of the optical fiber becomes a grating, and the periodic microbend (small bend) causes a periodic change in the electromagnetic field distribution and the refractive index distribution, and this perturbation causes coupling between specific modes. . A grating can also be formed by the method disclosed in Japanese Patent Application Laid-Open No. 7-333453. In this method, as shown in FIG. 3A, first, a plurality of notches 27 are formed at predetermined intervals in the length direction on the surface of the optical fiber constituting the optical amplifier 20. In this state, the diameter of the optical fiber is reduced at the notch 27, and the fiber axis indicated by the dashed line in the drawing is linear. Next, when the entire optical fiber is heated and softened, as shown in FIG. 3B, the surface of the optical fiber becomes smooth due to the effect of the surface tension of the glass, and the fiber axis meanders in a substantially sinusoidal shape. In this state, a grating by periodic microbending is formed.

  The perturbation period Λ1 formed in the optical amplifier 20 is the propagation constant of the 0th-order mode (fundamental mode) that contributes to the excitation of the rare earth metal among the plurality of waveguide modes existing in the excitation light incident on the optical amplifier 20. When B0 is set and Bn is a propagation constant of a specific high-order mode other than that, (B0−Bn) = 2π / Λ1 is set. By setting the perturbation period Λ1 in this way, the specific higher-order mode (propagation constant Bn) can be coupled to the zero-order mode (propagation constant B0). Further, when a grating is formed in the optical amplifier 20 as in this embodiment, it is more efficient that the propagation direction of the fundamental mode and the propagation direction of the light of a specific higher-order mode coupled to the same direction are the same. Therefore, it is preferable that B0 and Bn have the same sign. Therefore, the difference (B0−Bn) between these two propagation constants is small, and the preferable value of the period Λ1 is a long period of about 150 μm to 300 μm. In the present embodiment, the length of the rare earth-doped optical fiber constituting the optical amplifier is preferably about 1 to 80 m, and the grating is formed in part or all of it.

  According to the optical amplifier 20 of the present embodiment, an optical fiber grating is formed in the optical fiber constituting the optical amplifier 20, and the perturbation period Λ1 is appropriately set to propagate through the inner cladding 12. The coupling between the higher-order mode of the excitation light and the fundamental mode of the excitation light propagating through the core 11 occurs. In the optical amplifier 20, since the fundamental mode propagating through the core 11 is absorbed by the core and contributes to the excitation of the dopant, it attenuates during propagation. As a result, the fundamental mode of the core 11 changes from the higher order mode of the inner cladding 12. Bonding to occurs. That is, according to the present embodiment, of the pumping light propagating through the optical amplifier 20, the waveguide mode that cannot contribute to the excitation of the dopant as it is can be selectively coupled to the fundamental mode that effectively contributes to the excitation of the dopant. it can. As a result, the optical power used for pumping the dopant in the pumping light power is increased, and the pumping efficiency of the optical amplifier 20 is improved.

  FIG. 4A is a schematic configuration diagram showing a second embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. The optical amplifier 30 of this embodiment is different from the conventional clad pumping optical amplifier in that an optical fiber grating 40 is provided on the side of the optical amplifier 30 opposite to the pumping light source 2. The optical fiber constituting the optical amplifier 30 in the present embodiment has the same configuration as the optical fiber constituting the optical amplifier 20 in the first embodiment, except that no grating is formed. That is, the optical amplifier 30 has a double clad structure as shown in FIG. 7, and is composed of an optical fiber in which the core 11 is doped with a rare earth metal such as erbium.

  The optical fiber grating 40 in the present embodiment has a core diameter that is the same as the diameter of the inner cladding 12 in the optical fiber constituting the optical amplifier 30, and is formed in an optical fiber that is not doped with a rare earth metal in its length direction. It is preferably configured by forming a periodic perturbation along, and is connected to an emission end of the optical amplifier 30 opposite to the incident end of the excitation light. By making the diameter of the inner cladding 12 in the optical amplifier 30 and the core diameter of the optical fiber grating 40 equal, the connection loss due to these connections can be reduced. The perturbation period Λ2 in the optical fiber grating 40 is the propagation constant of the 0th-order mode (fundamental mode) that contributes to the excitation of the rare earth metal among the plurality of waveguide modes present in the excitation light incident on the optical amplifier 30. Then, the propagation constant of the reflected light in this basic mode is (−B0). When the propagation constant of a specific higher-order mode other than the fundamental mode is Bn, the perturbation period Λ2 is set to be (Bn − (− B0)) = (Bn + B0) = 2π / Λ2. By setting the perturbation period Λ2 in this way, a specific higher-order mode (propagation constant Bn) can be coupled to the reflected light of the fundamental mode (propagation constant-B0) as shown in FIG. 4B. . If the perturbation in the optical fiber grating 40 is, for example, a periodic core refractive index perturbation and a slant periodic perturbation in which the direction of the perturbation is tilted with respect to the optical fiber length direction, the basic A particular higher order mode coupled to the reflected light of the mode can be an odd mode. When the perturbation has no inclination with respect to the length direction of the optical fiber, the specific higher-order mode gathered by the reflected light of the fundamental mode is an even mode. In this embodiment, the directions of the two lights coupled in the grating are opposite to each other, and therefore the signs of the propagation constants of the two are different signs. Therefore, the difference between these two propagation constants (Bn − (− B0)) is larger than that in the first embodiment, and the value of the period Λ2 is as short as approximately 300 nm to 350 nm.

  As a method of manufacturing such an optical fiber grating 40 having a short period, for example, by utilizing a known photolift attractive effect, an optical fiber in which germanium is added to a core in advance is subjected to ultraviolet rays from a side surface through a phase mask. A method of periodically changing the refractive index of the core in the optical fiber length direction by irradiating light can be used. Further, in order to obtain the optical fiber grating 40 having the slant type periodic perturbation, it is formed on one surface of the phase mask 43 made of quartz glass, for example, as shown in FIG. It is only necessary to irradiate the optical fiber 40a with ultraviolet light through the phase mask 43 in a state where the periodic gratings 43a, 43a,... Are set obliquely with respect to the axial direction of the optical fiber 40a.

  According to the optical amplifier of the present embodiment, by appropriately designing the period of perturbation of the optical fiber grating 40, it does not contribute to pumping among the pumping light that propagates through the optical amplifier 30 and is emitted from the optical amplifier 30. Coupling between a specific higher-order mode and reflected light of a fundamental mode that contributes to excitation occurs efficiently. In the optical amplifier 30, since the fundamental mode propagating through the core 11 is absorbed by the core 11 and contributes to the excitation of the dopant and is attenuated during propagation, the reflected light of the fundamental mode in the optical fiber grating 40 is a higher-order mode. Small compared to propagating light. Therefore, as a result, in the optical fiber grating 40, the excitation light emitted from the optical amplifier 30 after propagating through the core 11 from the higher order mode of the excitation light emitted from the optical amplifier 30 after propagating through the inner cladding 12. Coupling to the reflected light in the fundamental mode occurs. That is, according to the present embodiment, by providing the reflection type optical fiber grating 40 on the emission side of the optical amplifier 30, the excitation light emitted after propagating through the optical amplifier 30 contributes to the excitation of the dopant as it is. An incapable guided mode is a fundamental mode that effectively contributes to the excitation of the dopant, and is selectively coupled to a mode that propagates from the exit side to the entrance side of the optical amplifier 30 so as to enter the optical amplifier 30 again. be able to. As a result, the optical power used for pumping the dopant in the pumping light power is increased, and the pumping efficiency of the optical amplifier 30 is improved.

1 is a schematic configuration diagram illustrating a first embodiment of an optical amplifier according to the present invention. It is explanatory drawing of an example of the manufacturing method of the optical fiber grating used with the optical amplifier of FIG. It is explanatory drawing of the other example of the manufacturing method of the optical fiber grating used with the optical amplifier of FIG. It is a schematic block diagram which shows 2nd Embodiment of the optical amplifier of this invention. It is explanatory drawing of an example of the manufacturing method of the optical fiber grating used with the optical amplifier of FIG. It is a schematic block diagram which shows the example of the conventional optical amplifier. It is the front view which showed the example of the optical fiber which comprises a clad excitation optical amplifier. It is a graph which shows the attenuation state of the excitation light in a clad excitation light amplifier. It is a graph which shows the primary mode of the pumping light which propagates a clad pumping optical amplifier. It is a graph which shows the 0th mode of the pumping light which propagates a clad pumping optical amplifier. It is a graph which shows the propagation state of the 0th mode of the excitation light in a clad excitation light amplifier. It is a graph which shows the propagation state of the primary mode of the excitation light in a clad excitation light amplifier. It is a graph which shows the propagation state of the secondary mode of the excitation light in a clad excitation light amplifier. It is a graph which shows the propagation state of the 4th mode of the excitation light in a clad excitation light amplifier. It is a graph which shows the propagation state of the 5th mode of the excitation light in a clad excitation light amplifier.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 2 ... Excitation light source, 11 ... Core, 12 ... Inner clad, 13 ... Outer clad, 20, 30 ... Optical amplifier, 40 ... Optical fiber grating.

Claims (6)

A core, an inner clad provided around the core and having a lower refractive index than the core, and an outer clad provided around the inner clad and having a lower refractive index than the inner clad. An optical amplifier comprising an optical fiber doped with
An optical fiber grating in which periodic perturbation by microbending is formed is provided for the optical fiber constituting the optical amplifier, and the rare earth element is selected from a plurality of waveguide modes of pumping light incident on the optical amplifier. An optical amplifier comprising a waveguide mode that contributes to excitation of a metal and a waveguide mode that does not contribute to excitation of the rare earth metal. The period of the periodic perturbation by microbending is Λ1, the propagation constant of the 0th mode contributing to the excitation of rare earth metal among the plurality of wave modes existing in the excitation light incident on the optical amplifier is B0, and the 0 When the propagation constant of a specific higher order mode other than the next mode is Bn,
(B0−Bn) = 2π / Λ1
The optical amplifier according to claim 1, wherein:   3. The optical amplifier according to claim 2, wherein the period of the periodic perturbation by microbending is a long period of [Lambda] 1 of 150 [mu] m or more and 300 [mu] m or less. A core, an inner clad provided around the core and having a lower refractive index than the core, and an outer clad provided around the inner clad and having a lower refractive index than the inner clad. An optical amplifier comprising an optical fiber doped with
An optical fiber grating in which a periodic perturbation is formed in an optical fiber having the same core diameter as that of the inner cladding is connected to an output end opposite to the excitation light incident end of the optical amplifier, and the optical amplifier Among the plurality of waveguide modes of excitation light incident on the waveguide, the waveguide mode that contributes to the excitation of the rare earth metal is coupled with the waveguide mode that does not contribute to the excitation of the rare earth metal. An optical amplifier. The period of periodic perturbation formed in the optical fiber connected to the output end is Λ2, and the 0th order which contributes to the excitation of rare earth metal among the plurality of wave modes existing in the excitation light incident on the optical amplifier When the propagation constant of the mode is B0, the propagation constant of the reflected light is -B0, and the propagation constant of a specific higher-order mode other than the 0th-order mode is Bn,
(Bn − (− B0)) = (Bn + B0) = 2π / Λ2
The optical amplifier according to claim 4, wherein:   6. The optical amplifier according to claim 5, wherein the period of the periodic perturbation by microbending is a short period of [Lambda] 2 of 300 nm or more and 350 nm or less.

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