JP2009044098A - Gain photonic crystal optical fiber, laser amplification system, and setting method for gain photonic crystal optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain amplified spontaneous emission or parasitic oscillation in a codoped optical fiber. <P>SOLUTION: In a codoped optical fiber, an additive to function as a sensitizer is excited by the energy injected by a laser beam so that energy is applied to an additive to function as an activator based on the excitation level. In the case the refractive index guide type photonic crystal optical fiber (PCF) is used in a bent state, its loss is increased to a shorter wavelength side. Utilizing the propagation characteristics, the PCF is used in a radius of curvature so that the loss is small with a wavelength in a gain spectrum of the activator and the loss is large with a wavelength in a gain spectrum of the sensitizer (<the wavelength of the activator). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a photonic crystal optical fiber.

For the purpose of responding to the increased output of laser light, optical fibers to which a plurality of types of active ions are added have been studied. In such an optical fiber, a dopant that functions as a sensitizer is excited to an upper level, and its energy is transferred to a dopant that functions as an activator. In the example shown in FIG. 1, Yb ³⁺ is an ion that functions as a sensitizer and Er ³⁺ functions as an activator. Yb ³⁺ absorbs light near 975 nm and is excited to the upper level, and its energy shifts to Er ³⁺ . As Er ³⁺ moves to the lower level, light near 1550 nm is emitted.

  In the co-addition system of a plurality of active ions used in such a laser medium, it is ideal to transfer the total energy of the sensitizer ion excited to the upper level using the energy transfer between ions to the activator ion. However, in reality, the upper level of the sensitizer ion has a finite lifetime, so there is always a path that relaxes to the ground state without energy transfer. This relaxation path often leads to undesirable operations such as parasitic oscillation and reduced amplification efficiency and stability.

For example, the Er: Yb co-doped glass system in FIG. 1 is a material system having a gain in the 1.5 μm band due to the energy transfer from Yb to Er by adding Yb at a high concentration to the Er system. Unlike Er, Yb does not cause concentration quenching, so it can be added at a high concentration, and its absorption cross section is an order of magnitude larger. Therefore, high gain can be obtained with a short fiber length, which is extremely important as a medium for high-power amplifiers and high-power lasers. Therefore, research is progressing as a high-power amplifier in high-density wavelength multiplexing communication. In order to efficiently transfer energy, the lifetime of the ⁴ I _11/2 level of Er is shortened by adding P and changing the crystal field, or Yb is clustered so that it surrounds Er Efforts such as controlling the concentration ratio of substances and the method of addition have been made for many years.

  However, it is generally difficult to add active ions to glass, and clustering results in an inhomogeneous crystal field. Therefore, it is not possible to ideally control the interaction between ions without leaving the field of empirical engineering. Therefore, there are problems such as reduced efficiency due to amplified spontaneous emission (ASE) / parasitic oscillation, end face destruction due to giant pulse generation, output limit, and reduced stability, which have not been solved yet.

Non-patent documents 1 and 2 are given as examples of the Er / Yb co-doped optical fiber laser.
JK Sahu et al, “A 103 W erbium-ytterbium co-doped large-core fiber laser”, Optics Communications, 2003, Vol. 227, p.159-163 PK Cheo et al, “Clad-Pumped Yb: Er Codoped Fiber Lasers”, IEEE Photonics Technology Letters, March 2001, Vol. 13, No. 3, p.188-190

In general, the transition energy of the above-described relaxation path of the sensitizer ion is larger than the transition energy of the activator ion used, and therefore, within the gain spectrum of the activator used as the wavelength of the signal amplified by the optical fiber or the oscillation wavelength of the laser. Light is emitted on the shorter wavelength side than the wavelength (wavelength λ _A ). This light emission (light emission of wavelength λ _S in the gain spectrum of the sensitizer) is further amplified by taking the energy of the upper level sensitizer, resulting in a decrease in amplification efficiency and stability as ASE and parasitic oscillation, and high output operation limit give.

In order to suppress this inductive amplification process, it is desirable to give a wavelength-dependent loss that cuts light in the vicinity of the wavelength λ _S in the gain spectrum of the sensitizer while amplifying it in the vicinity of the wavelength λ _A. However, in a long continuous medium such as a fiber, it is difficult to insert a large number of dielectric multilayer filters.

  In the following, means for solving the problem will be described using the numbers used in [Best Mode for Carrying Out the Invention] in parentheses. These numbers are added to clarify the correspondence between the description of [Claims] and [Best Mode for Carrying Out the Invention]. However, these numbers should not be used to interpret the technical scope of the invention described in [Claims].

  In order to solve the above-described problems, a gain optical fiber according to the present invention is arranged around a core portion (4) to which an activator as a first dopant and a sensitizer as a second dopant are added, and around the core portion. A refractive index guide type photonic crystal optical fiber (2) comprising a cladding portion (6), wherein the photonic crystal optical fiber has a positive gain at a wavelength within the gain spectrum of the first dopant; And a loss portion having a radius of curvature determined to increase loss at a wavelength at which ASE or parasitic oscillation occurs in the gain spectrum of the two dopants.

  Preferably, the gain optical fiber according to the present invention is a single mode photonic crystal optical fiber. The loss part has a larger loss as the wavelength is shorter.

  The laser amplification system according to the present invention comprises a gain photonic crystal optical fiber according to the present invention and a laser for supplying excitation energy for exciting the second dopant. This laser amplification system is used to amplify signal light input to a gain photonic crystal optical fiber and having a wavelength within the gain spectrum of the first dopant. Alternatively, the laser amplification system further comprises a resonator. The light stimulated and emitted by using the gain photonic crystal optical fiber to which the first dopant is added as a laser medium resonates in the resonator and oscillates.

A method for setting a gain photonic crystal optical fiber according to the present invention includes a step of providing a refractive index guide type photonic crystal optical fiber having a positive gain at a predetermined laser or amplification wavelength λ _A , and a wavelength λ _A And a step of setting and fixing the radius of curvature of the photonic crystal optical fiber so that the loss of the photonic crystal optical fiber is larger than a predetermined reference at the short co-doped ASE or parasitic oscillation wavelength.

In a refractive index-guided photonic crystal fiber (PCF), the loss increases as the wavelength decreases when a curvature is applied. This is because the effective refractive index of the cladding portion increases as the wavelength becomes shorter, and the effective refractive index difference from the core mode decreases. Using this propagation characteristic, a periodic void structure is applied to the clad portion of the multi-ion co-doped gain fiber to form a PCF, and when a curvature is applied, there is a large loss near the wavelength λ _S , while almost no near the wavelength λ _A. A gain photonic crystal optical fiber having propagation characteristics without loss can be produced, and the above-described problems can be solved.

  The present invention is expected to be particularly effective for high-power Er: Yb co-doped fiber amplifiers and lasers that have been actively studied in recent years for the purpose of high-density wavelength multiplexing communications and eye-safe applications. It is possible to reduce the noise by increasing the output, increasing the efficiency, and reducing the emission noise from Yb, and provide an idea leading to a next-generation amplifier.

  The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 2 shows a cross section of a refractive index guide type PCF in this embodiment. The PCF 2 includes a core 4 that extends in the extending direction of the optical fiber, and a clad 6 that is disposed around the core 4. A number of holes 8 are formed in the cladding 6. These holes 8 are formed in parallel with the core 4, that is, extending in the extending direction of the optical fiber. In the cross section perpendicular to the extending direction as shown in FIG. 2, the plurality of holes 8 are periodically formed in a triangular lattice arrangement with a pitch Λ. Such PCF 2 guides light along the core 4 based on the difference in optical properties between the clad 6 and the core 4 resulting from the formation of the holes 8.

  The core 4 is co-doped with Er (erbium) ions and Yb (ytterbium) ions. Yb functions as a sensitizer, Er functions as an activator, and has a gain in the 1.5 μm band of Er. Unlike Er, Yb can be added at a high concentration because concentration quenching does not occur, and the absorption cross section is also an order of magnitude larger. By absorbing excitation light and transferring energy with Yb, high gain can be obtained with a short fiber length. it can. The Er: Yb co-doped fiber is extremely important as a medium for high-power amplifiers and high-power lasers, and research is progressing as a high-power amplifier in high-density wavelength multiplexing communication.

As the sensitizer ion / activator ion, instead of Yb / Er in this embodiment, Tm ³⁺ / Ho ³⁺ , Yb ³⁺ / Pr ³⁺ , Er ³⁺ / Tm ³⁺ , Nd ³⁺ / Yb ³⁺ , Cr ³⁺ / Nd ^{3+ and the} like Can be used. The same effect can be expected when applied to a co-added PCF containing three or more additive substances containing these dopant pairs.

  In this embodiment, the PCF 2 is in a single mode, the core 4 diameter = 35 μm, the mode field diameter MFD = 26 μm, the hole 8 pitch Λ = 22 μm, the hole 8 diameter / pitch ratio d / Λ = 0. 54. The calculation result of the bending loss in the case of various curvature radii of this PCF2 is shown in FIG. If the radius of curvature is smaller than 10 cm, the loss increases in the 1 μm band. FIG. 4 shows the result of actually measuring the core mode transmission spectrum of white light. It can be seen that a loss of about 1 μm occurs when the curvature radius is 9 cm or less.

  FIG. 5 shows an experimental diagram of the fiber amplifier. The fiber has an air clad and is clad pumped by a laser diode (LD). Amplification is performed using an ultrashort pulse laser (12 ps, 200 mW, 47 MHz@1.557 μm) as a seed. Although the following effects can be obtained even if the bending region bent with the radius of curvature set as described above is a part of the co-added PCF2, it is desirable that the bending region is almost the entire region. For example, when the optical fiber is long, the above radius of curvature can be applied to the entire region of the optical fiber by winding a plurality of turns.

  When the radius of curvature is 16 cm and the excitation power exceeds 15 W, parasitic oscillation from Yb ions occurs near 1.030 μm, exceeding the power of the signal light (see FIG. 6A). This parasitic oscillation occurs as an unstable giant pulse, and the fiber end face is destroyed, so that the excitation power cannot be increased any more. This limit gives the power limit. However, when the radius of curvature is 9 cm, the parasitic oscillation is completely suppressed (see FIG. 6B), the 1.557 μm seed light can be stably amplified, and the pump power limit can be improved.

As described above, in the method of attenuating parasitic oscillation by actively bending and installing the PCF with an appropriately adjusted curvature radius, it is important to design an appropriate hole structure and bending diameter. In the case of a general triangular lattice structure of PCF as shown in FIG. 2, the diameter d of the holes 8, the pitch Λ of the holes 8, and the refractive index of the fiber material are n. When the effective mode area of this PCF is A _eff and the V parameter is V _PCF , the loss α (λ) [dB / m] at the wavelength λ at the radius of curvature R _{bend can} be expressed by the following equation.

と書ける。実施例のΛ＝２２μｍ、λ_Ｓ＝１．０３μｍ、ｎ＝１．４４４（シリカ）を代入すれば、Ｒ_ｃ〜８ｃｍとなる。このように、寄生発振波長におけるＰＣＦの損失が所定の基準を満たす大きさとなるように曲率半径を計算し、その曲率半径となるように光ファイバーを固定して使用することによって寄生発振を抑圧できる。 Here, A _eff and V _PCF are determined by d and Λ, and must be obtained by numerical calculation such as the finite element method, but can be regarded as constants because they are less dependent on the wavelength. Usually, it is used in the vicinity of V _PCF ˜π, so it is placed in this way for simplicity. The coefficient of loss depends on A _eff and the wavelength, but the characteristic loss increase due to bending is determined by F (x). Therefore, if the wavelength to be cut is λ _S , and x˜2 is the loss increase point, the critical curvature Radius R _c is

Can be written. Substituting Λ = 22 μm, λ _S = 1.03 μm, and n = 1.444 (silica) in the example, R _c ˜8 cm. As described above, the radius of curvature is calculated so that the loss of the PCF at the parasitic oscillation wavelength is large enough to satisfy a predetermined standard, and the parasitic oscillation can be suppressed by using the optical fiber with the curvature radius being fixed.

  As an application, a twisted core fiber in which a core is formed in a helical shape in the PCF can be considered. FIG. 7 shows a schematic diagram thereof. The core 12 is formed in a spiral shape with the extending direction of the PCF 10 as the central axis. A large number of holes are periodically formed around the core 12 in parallel with the core 12 to form a PCF cladding. The additive is a sensitizer and activator similar to those illustrated with reference to FIG. By precisely designing the offset from the center and the period of the helix, it is possible to maintain propagation characteristics with a predetermined wavelength-dependent loss without depending on macro bending, and for amplifiers and lasers where energy loss from activator ions is suppressed It can be used as a gain fiber.

で表される。このＲが臨界曲率半径Ｒ_ｃであるとき、（２）式を用いれば、

となる。実施例の場合、Ｐ^２／ρ＝３．１６ｍとなり、例えばＰ＝１０ｍｍとするとρ＝３２μｍのオフセットが必要となる。 In the twisted core fiber shown in FIG. 7, when the period of the spiral in the axial direction of the core 12 is P and the offset from the central axis is ρ, the curvature radius is P >> ρ.

It is represented by When this R is the critical radius of curvature R _c ,

It becomes. In the embodiment, P ² /ρ=3.16 m. For example, when P = 10 mm, an offset of ρ = 32 μm is required.

The energy level diagram in an Er / Yb co-added glass system is shown. It is sectional drawing of PCF. The calculation result of the relationship between the radius of curvature and loss is shown. The core mode transmission spectrum of white light when the radius of curvature is changed is shown. An experimental diagram of a fiber amplifier is shown. The output characteristics when the radius of curvature is changed are shown. A twisted core PCF is shown.

Explanation of symbols

2 Photonic crystal optical fiber (PCF)
4 Core 6 Clad 8 Hole 10 PCF
12 cores

Claims (5)

A refractive index-guided photonic crystal optical fiber comprising a core portion to which a first dopant and a second dopant are added, and a clad portion disposed around the core portion,
The photonic crystal optical fiber has a positive gain at a wavelength in the gain spectrum of the first dopant;
The curvature of the photonic crystal optical fiber is determined such that the loss is larger at a wavelength at which amplified spontaneous emission or parasitic oscillation occurs in the gain spectrum of the second dopant than at a wavelength in the gain spectrum of the first dopant. A gain photonic crystal optical fiber having a loss portion having a radius. A gain photonic crystal optical fiber according to claim 1,
Gain photonic crystal optical fiber that is single mode. A gain photonic crystal optical fiber according to claim 1 or 2,
The loss part is a gain photonic crystal optical fiber having a larger loss at shorter wavelengths. A refractive index-guided photonic crystal optical fiber comprising a core portion to which a first dopant and a second dopant are added, and a cladding portion disposed around the core portion;
A laser for supplying excitation energy for exciting the second dopant,
The photonic crystal optical fiber has a positive gain at a wavelength in the gain spectrum of the first dopant;
The curvature of the photonic crystal optical fiber is determined such that the loss is larger at a wavelength at which amplified spontaneous emission or parasitic oscillation occurs in the gain spectrum of the second dopant than at a wavelength in the gain spectrum of the first dopant. Laser amplification system with a lossy part with radius. Providing a co-doped index-guided photonic crystal optical fiber having a positive gain at a predetermined wavelength;
The radius of curvature of the photonic crystal optical fiber is set so that the loss of the photonic crystal optical fiber is larger than a predetermined reference at a wavelength shorter than the predetermined wavelength at which the amplified spontaneous emission or parasitic oscillation of the co-added system occurs. A method for setting a gain photonic crystal optical fiber comprising: a fixing step.

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