JP6306637B2 - Fiber that provides gain with increased cladding absorption while maintaining single mode operation - Google Patents

Description

Cross-reference of related applications This application is a US Patent No. 69 from DOUBLE CLAD, GAIN PRODUCING FIBERS WITH INCREASED CLADDING ABSORPTION WHILE MAINTINGING SINGLE MODE OPERATION No. 69, filed on August 29, 2012. It claims priority. In addition, this application is also filed with the application number _ (T.F.T.F.T.F.) Which was also named “DOUBLE-CLAD, GAIN-PRODUCING FIBERS WITH INCREASED CLADDING ABSORPTION WHILE MAINTAINING SINGLE-MODE OPERATION”. is there.

  The present invention relates to a fiber (GPF) that provides gain that supports either a single signal mode or a minority signal mode, and more particularly to increase the cladding absorption of pump light while maintaining single signal mode operation. It relates to the designed GPF.

Single mode GPFs with double clad fiber (DCF) designs are commonly used in high power fiber optic lasers and amplifiers that require superior beam quality. FIG. 7 shows a known DCF 70, which comprises a core region 70.1, an inner cladding region 70.3 surrounding the core region, and an outer cladding region 70.4 surrounding the inner cladding region. The waveguide formed by the core region and the inner cladding region is mainly configured to support and guide the propagation of signal light in a single mode, that is, preferably a fundamental (LP ₀₁ ) mode.

  In order to provide gain when properly pumped, the core region relies on the wavelength of the signal light to be amplified or the laser light to be generated, depending on the species that provides the gain, usually one or more rare earth elements (e.g. Er, Yb, Tm, Nd, Ho) or one or more non-rare earth elements (eg Cr, Bi). Alternatively, gain may be generated by utilizing the Raman effect of glass (eg, silica) fiber.

  Multimode pump light that is coupled (emitted) to (through) the core region via the inner cladding region is reflected from the interface 70.5 between the inner and outer cladding regions and passes through the fiber axis. When propagating down, the pump light traverses the core region and is absorbed by a particular dopant (ie, a species that provides gain) in the core region. In this way, the energy of the pump light amplifies the signal light propagating (mainly) simultaneously down the core region of the fiber.

The effect of amplification process (i.e. energy transfer from the pump light to the signal light) is partly determined by given by parameter known as the (pumping light) cladding absorption (alpha _clad),
α _clad = α _d (A _d / A _clad ) (1)
In this equation, α _d is the material absorption of the pump light in a portion of the fiber doped with a species that provides gain (eg, core region 70.1 in FIG. 7, hereinafter referred to as gain region), and A _d is a transverse cross-sectional area of the gain region, a _clad is the (πD _{ic ^2/4} for the inner cladding region having, for example circular ^{cross-section)} the entire lateral cross-sectional area of the inside of the inner cladding region 70.3 . The material absorption is given by
α _d = N _d σ _d (2)
In this equation, N _d is the volume concentration of the species that provides the gain in the gain region, and σ _d is the absorption cross section of the dopant in the gain region matrix (eg, quartz or glass) at the pump light wavelength.

  The ability to increase the cladding absorption of pump light would be advantageous. For fiber optic lasers and amplifiers, for a given output power, increased pump light absorption means increased gain, resulting in shorter amplifier fiber length and shorter laser cavity resonators. The length means that the desired output power can be achieved. Also, shorter fiber lengths can reduce the occurrence of nonlinear effects such as stimulated Raman scattering (SRS), which can be beneficial in improving fiber laser output stability and long-term reliability.

Equation (1) and (2), of the fiber, the core region 70.1, doped region A _d and more precisely, that it is possible to increase only the cladding absorbs increasing the seed concentration N _d bring gain It may seem to suggest. However, for certain rare earth element species that are commercially important (especially Yb), the concentration of the species that provides gain by pump light-induced photodarkening is limited, thereby achieving only by increasing the concentration. The cladding absorption that can be done is limited.

  An alternative approach to increase cladding absorption would be to simply increase the area of the gain region, for example by increasing the core region diameter. However, increasing the core region diameter too much adversely affects the ability of the fiber to continue operating in single signal mode, i.e., higher order signal modes (HOMs) can propagate. HOM excitation can cause output instabilities that can be very harmful and damage the fiber laser or amplifier architecture. This constraint on the size of the core region also improperly limits the achievable mode field diameter (MFD) of conventional DCF.

  Thus, there is a need to achieve an increase in cladding absorption of a double-clad GPF without having to increase the concentration of seeds that provide gain inside the gain region of the fiber while maintaining single mode operation.

  There is also a need for a double clad GPF that can support MFDs on the order of about 16-20 μm.

  On the other hand, not all GPFs are DCFs for cladding pumping, and some pumps are pumped by alternative mechanisms such as core pumping design, which also increases cladding absorption while maintaining single mode operation. There are benefits from applying the principles of the invention.

US Pat. No. 5,864,644 US Pat. No. 6,826,335 US Pat. No. 7,916,386 US Pat. No. 5,907,652 US Pat. No. 5,937,134

P. A. Belanger, Optical Engineering, Vol. 32, no. 9, pp. 2107-2109, (1993) Specialty Optical Fibers Handbook, ed. Mendez and Moore, Ch. 7.2.3, “Rare Earth Fibers”, DiGiovanni et. al. , Academic Press (2007)

  In accordance with one aspect of the present invention, a GPF with increased cladding absorption is provided without the need to increase the concentration of species that provide gain in the gain region. This design allows the core region diameter to be increased while maintaining MFD and single mode operation. Single mode operation means that the fiber supports only a single signal mode (preferably a fundamental mode) or several signal modes (preferably a fundamental mode plus at most about 1st to 4th order HOMs). Means. Preferably, the fiber operates only in the fundamental mode.

  Therefore, according to one embodiment of the present invention, the GPF includes a core region having a longitudinal axis and a cladding region surrounding the core region. The core region and the cladding region are configured to support the propagation of the fundamental transverse mode signal light and lead (mainly) in the axial direction in the core region. The signal light to be amplified (or the generated laser light) generally has a wavelength of about 1000 nm or more. The cladding region includes a trench region that surrounds the core region, an inner cladding region that surrounds the trench region, and an outer cladding region that surrounds the inner cladding region. In some embodiments (eg, DCF), the outer cladding region has a refractive index that is lower than the refractive index of the inner cladding region, and the inner cladding region is between the refractive index of the outer cladding region and the refractive index of the core region. The trench region has a refractive index lower than that of the inner cladding region. At least the core region is signal light when appropriate pumping energy is applied to the fiber, for example when multimode pump light is coupled (emitted) to the core region through the inner cladding region. Including species that provide at least one gain. In the core region and the cladding region, the fundamental mode is mainly guided in the core region, and as a result, the absolute value of the difference in refractive index between the trench region and the inner cladding region is the refraction between the core region and the inner cladding region. It is configured to be smaller than the difference in rate. This state represents a large number of GPF designs that utilize several designs with Yb-doped core regions, Tm-doped core regions, and Er-doped core regions (particularly the ratio of core radius to MFD). Is less than about 0.6).

  On the other hand, in other GPF embodiments utilizing an Er-doped core region, the core region and the cladding region are again configured such that the fundamental mode is primarily guided in the core region, resulting in the trench region and The absolute value of the refractive index difference between the inner cladding region is larger than the refractive index difference between the core region and the inner cladding region. In these embodiments, the ratio of core radius to MFD is greater than about 0.6.

  Calculations of some embodiments of the present invention show single mode GPF designs with increased cladding absorption (eg, greater than about 30%). This calculation also shows that some embodiments of such GPF designs can reduce the optical loss induced by light blackening for a given cladding absorption.

  In addition, other calculations show that some embodiments of such GPF designs can operate in single mode without increasing bending losses.

  According to another aspect of the present invention, a method for increasing the cladding absorption of a fiber that provides gain includes: (a) forming a trench region between a core region and an inner cladding region; and (b) a core region, a trench. And (ii) the refractive index of the trench region is lower than the refractive index of the inner cladding region, and (ii) the absolute value of the refractive index difference between the trench region and the inner cladding region is (C) the core region, the trench region, and the inner cladding region are supported by the fundamental mode of signal light, and the mode is mainly configured to be smaller than the absolute value of the difference in refractive index between the cladding regions. Directing in the core region and further configured to couple (release) pump light into the core region.

  The present invention can be readily understood along with various features and advantages of the present invention from the following more detailed description, taken in conjunction with the accompanying drawings, in which:

1 is a schematic cross-sectional view of a double clad GPF according to an exemplary embodiment of the present invention. 2 is a schematic diagram of an exemplary refractive index profile of the GPF of FIG. FIG. 4 shows a refractive index profile graph 30 of a step index core (SIC) fiber according to a conventional design, and a refractive index profile graph 31 of a GPF having a trench region according to an exemplary embodiment of the present invention. (The fiber shown in FIG. 3 is both a double-clad GPF, but the outer cladding region is not shown for ease of explanation.) Graphs 32 and 33 in FIG. Also shown is the calculated fundamental mode output for fiber and GPF. Compared to FIG. 2, FIG. 3 shows only the refractive index profiles of the first and fourth quadrants, but the corresponding profiles of the second and third quadrants are the first and fourth quadrants, respectively. It is understood that this is a mirror image of the profile. The index depression 34 is due to a particular process artifact that can be used to fabricate the fiber, ie, but not limited to, partial evaporation of common dopants in the core region, such as GeO ₂ or P ₂ O _5. Note that this is a fire-off effect. It is a graph which shows the light (signal) loss induced by absorption of pump light in light blackening, ie, the pump light wavelength of the range of 600 nm to 1100 nm. The results for a standard SIC fiber (refractive index profile 30 in FIG. 3) are shown by curve 40 and the results for the GPF of the present invention (profile 31 in FIG. 3) are shown by curve 41. 1 is a schematic block diagram illustrating an exemplary fiber optic amplifier utilizing GPF according to an exemplary embodiment of the present invention. FIG. 1 is a schematic block diagram illustrating an exemplary clad pump optical fiber laser utilizing GPF according to an exemplary embodiment of the present invention. FIG. 1 is a schematic cross-sectional view of a double clad GPF according to a normal (prior art) design.

  The various figures described above are shown schematically in that they are not to scale, and / or for the sake of simplicity and clarity of illustration, all of the actual optical fiber or product details shown. Is not included.

Glossary Bending: Macro bending is generally referred to simply as bending, and the fiber is bent, wound or curled so that the curvature of the fiber is relatively constant along at least a portion of its length. Sometimes happens. In contrast, microbending occurs for a particular fiber when there is a significant curvature change on a scale within the adiabatic length range (eg, on the order of 1 millimeter or shorter fiber length). Such microbending is formed, for example, in a standard microbending test by pressing the fiber against sandpaper.

  Center wavelength: Throughout the discussion herein, reference to a wavelength is intended to mean the center wavelength of a particular emission, and all such radiation has a well-known wavelength range above and below the center wavelength. It is understood that the characteristic line width is included.

  Glass fiber: An optical fiber of the type described herein is typically made of glass (eg, silica) and the refractive index of the core region and the refractive index of the cladding region are well known in the art. As such, it is controlled by the amount and type of one or more dopants (eg, P, Al, Ge, F, Cl), or hollow voids incorporated during the manufacture of the fiber. These refractive indices as well as the thickness / diameter of the core / cladding region determine important operating parameters, as is well known in the art. Such glass fibers may also provide one or more gain species (rare earth species (eg, Er, Yb, Tm, Dy, Ho, etc.) or non-gain to provide gain when properly pumped. Doped with rare earth elements (eg Bi, Cr).

  Refractive index: The term refractive index and indices shall mean refractive index and refractive indices. In designs where certain regions (eg, cladding regions) include microstructures (eg, filled or unfilled pores (eg, low refractive index gas, liquid or solid) or unfilled pores (eg, pores)) The refractive index of such a region is intended to mean the average refractive index found in light propagating within that region.

  Refractive Index Profile: The approximate refractive index profile of FIGS. 2-3 shows the average of the actual fine changes in refractive index observable in an optical fiber. In addition, various regions of the refractive index profile may be shown as rectangular, but the boundaries of such regions need not be horizontal or vertical, and one or more may be inclined, for example, The region may be trapezoidal or triangular.

LMA: In high power applications, the fibers of large mode area (LMA), is defined as having an effective area of about 90Ramuda ² or more fundamental mode, lambda is the wavelength of the signal. For example, at a wavelength of 1060 nm (1.06 μm), the large mode area is composed of an effective area of about 100 μm ² or more, and at a wavelength of 1550 nm (1.55 μm), the large mode area is composed of an effective area of 216 μm ² or more. The

The optical properties of the M ² : LMA fiber are sensitive to details of its lateral refractive index profile. According to common wisdom, a desirable LMA fiber has a fundamental mode with an M ² of almost 1.0, which means that the transverse index profile inside the core region is essentially uniform, ie, refractive Under the assumption that the rate profile is essentially uniform inside the cross section of the core region, it means that the light field of the fundamental transverse mode is almost Gaussian. M ² evaluates the similarity between the mode field and the true Gaussian function. More specifically, M ² = 1.0 for modes with a Gaussian shape and M ² > 1.0 for all other mode field shapes.

M ² is P.M., which is incorporated herein by reference. A. Belanger, Optical Engineering, Vol. 32, no. 9, pages 2107 to 2109, (1993), which defines the similarity of the fundamental transverse mode of the fiber to an ideal Gaussian function. (This paper defines M ² for the LP ₀₁ fundamental mode of stepped optical fibers, but this definition is valid for all optical fibers described herein.) In theory, M ² can be of any size, but in practice, M ^{2 for} GPF is typically in the range of approximately 1 <M ² <10. Moreover, for example, M ² of about 1.06 is generally considered small in the sense of M ^{2 of} about 1.0, but for example M ² of about 1.3, M ² >> 1 It is considered large in the sense of .0.

If M ² is very close to 1.0, the beam emerging from the fiber can be efficiently collimated to the diffraction limit point or can be strictly focused.

M ² is therefore a well-known measure of what is commonly referred to as beam quality.

  Mode: The term mode (s) refers to an electromagnetic wave (eg, signal light that includes signal light that is amplified in the case of an optical amplifier, or signal light that includes stimulated emission in the case of a laser. ) Transverse mode (s).

Mode size: The size of the light mode is characterized by the effective area A _{eff of the} light mode given by:

この式で、Ｅはモードの電界の横方向の空間的包絡線であり、積分はファイバの横方向の断面積にわたって遂行されるものと理解される。モード・フィールドの形状が軸対称（すなわち、ファイバの回転の前後軸のまわりで対称）のガウス関数に近いとき、モード・フィールドの直径（ＭＦＤ）は、モードの直径の適切なメトリックであり、次式で表されてよく

In this equation, E is understood to be the lateral spatial envelope of the mode electric field, and the integration is performed over the transverse cross-sectional area of the fiber. When the mode field shape is close to an axisymmetric (ie, symmetric about the longitudinal axis of fiber rotation) Gaussian function, the mode field diameter (MFD) is an appropriate metric for the mode diameter, and May be represented by a formula

この式で、ｒは動径座標である。モード・フィールドの形状が、線対称のガウス関数に正確に等しければ、Ａ_ｅｆｆ＝π×ＭＦＤ^２／４となる。

In this equation, r is a radial coordinate. The shape of the mode field, if exactly equal to Gaussian _{axisymmetric,} the ^{_{A eff = π × MFD 2/}} 4.

  Light blackening: Light blackening is the reversible generation of color center absorption in an optical fiber. For example, when a Yb highly doped fiber is exposed to strong pump radiation, the signal degrades over time. This photodarkening is usually due to the formation of color centers in the glass and appears as strong absorption at visible wavelengths with strong absorption around 975 nm. Specialty Optical Fibers Handbook, ed. Mendez and Moore, Ch. 7.2.3, “Rare Earth Fibers”, DiGiovanni et. al. , Academic Press (2007), incorporated herein by reference.

  Radius / Diameter: The use of the term radius and the term diameter in the previous discussion (and the discussion below) means that the cross-sections of the various regions (eg, core, trench, cladding) are circular and / or annular. In practice, however, these regions may be non-circular, for example, rectangular, elliptical, polygonal, irregular, or other more complex shapes. Nevertheless, generally in the art, the terms radius and / or diameter are used for simplicity and clarity.

  Signal propagation: When signal light propagates along a fiber, it may actually intersect the longitudinal axis, but in the art, the general propagation direction is the longitudinal axis (eg, axis 10. It is fully understood that it is properly stated to be in line with 5).

  Single Mode: Reference to single transverse mode light propagation is intended to include propagation that is essentially or effectively single mode, ie, in a substantial sense, complete of all other modes. Suppression may not always be possible. However, single mode means that the intensity of other such modes is low or insignificant for the intended application. Thus, the term single mode fiber refers to a fiber that operates in only one mode (preferably the fundamental mode) or several modes (ie, the fundamental mode and at most about the first to fourth order higher order modes (HOM)). means.

  Suppressed HOM: The degree to which the HOM must be controlled depends on the particular application. In many applications, no total or complete suppression is required, which means that the continued presence of relatively low strength HOMs can be tolerated. In many instances, it may be sufficient to provide a high degree of HOM attenuation compared to fundamental mode attenuation. This suppression is referred to as relative suppression or selective suppression. In any case, suppressing HOM improves system performance by improving, for example, beam quality, reducing total insertion loss, reducing signal mode noise, and reducing microbend loss.

  Cross section: The phrase cross section refers to the cross section of the fiber in a plane perpendicular to the longitudinal axis of the fiber.

  Undoped: The term undoped or unintentionally doped is unintentionally added to the region of the fiber or the starting tube used to form such a region during manufacture, or Although meant to include uncontrolled dopants, the term does not exclude low levels of background doping that may inherently be incorporated during the manufacturing process.

Gain-Producing Fiber Design Embodiments of the present invention show a gain-providing fiber (GPF) design with increased cladding absorption without the need to increase the species / dopant concentration that provides gain. Specifically, this fiber design includes a trench region that allows for an increase in core diameter, which results in increased cladding absorption, as shown in equation (1). That is, α _clad = α _d (A _d / A _clad ). For single mode operation, the _core control Δn _core should be less than about 8 × 10 ⁻³ . (Note that Δn _core > 8 × 10 ⁻³ means a correspondingly smaller core region diameter in order to maintain single mode operation and therefore also reduce cladding absorption.)

In contrast, as a general rule, at an operating wavelength of about 1080 nm, Δn _core is about 2.0 × 10 ^{−3 to} 2.2 × 10 ⁻³ (to achieve LP ₁₁ with a cutoff wavelength of about 1030 nm). And LMA fiber with MFD≈11 μm is used. However, the scope of the present invention is not only LMA fibers that provide a gain with a large MFD, in some embodiments 16-18 μm and in other embodiments 20 μm, but also fibers with a smaller MFD, for example 6 μm. Include. Turning now to the drawings, FIG. 1 shows a double clad GPF 10 according to an exemplary embodiment of the present invention. GPF 10 includes a core region 10.1 having a longitudinal axis 10.5 and a cladding region comprising regions 10.2, 10.3, and 10.4 surrounding core region 10.1. The core region and the cladding region are configured to support the propagation of basic transverse mode signal light and to guide mainly in the direction of the axis 10.5 in the core region. In addition, the cladding region is configured to support and guide the propagation of multimode pump light.

  The cladding region includes a trench region 10.2 surrounding the core region 10.1, an inner cladding region 10.3 surrounding the trench region 10.2, and an outer cladding region 10.4 surrounding the inner cladding region 10.3. .

  The outer perimeter 10.6 of the inner cladding region 10.3 is shown as a circle for simplicity of illustration only. In practice, it is well known that this boundary is generally non-circular in order to prevent the formation of a helical mode of pump light and increase the overlap to the core region of all pump modes.

As shown in FIG. 2, the various refractive index difference [Delta] n is defined for the refractive index [Delta] n _ics of the inner cladding region 10.3, therefore, the refractive index of the inner cladding region 10.3 is zero and labeling Yes. Accordingly, the outer cladding region 10.4 has a refractive index Δn _oc lower than the refractive index of the inner cladding region 10.3, and the inner cladding region 10.3 has a refractive index of the outer cladding region 10.4 and the core region 10. has a refractive index between the first refractive index (Δn _core), the trench region 10.2 has a lower refractive index [Delta] n _tr than the refractive index of the inner cladding region 10.3. (In some embodiments utilizing a core pumping design, the refractive index profile (not shown) will typically exhibit Δn _ic <Δn _core and Δn _tr <Δn _ic ).

  At least the core region 10.1 is in the signal light when appropriate pumping energy is applied to the fiber, ie when coupled to the core region 10.1 through the inner cladding region 10.3. Includes at least one species that provides gain, which provides gain. The pump light propagates in a multimode manner across the core region (absorbed by the species providing the gain) inside the inner cladding. The choice of species that provides the gain depends on the wavelength of the signal light to be amplified or the wavelength of the laser light that is generated. This type of GPF can provide gain at signal wavelengths above about 1000 nm (eg, about 1000 nm for Yb, about 1600 nm for Er, and about 1900 nm for Tm). Alternatively, gain may be generated by using a fiber that exhibits the well-known Raman amplification effect.

  The pump light may be coupled (released) to the fiber 10 using various techniques / devices well known in the fiber optic art. Typically, these techniques couple the pump light to the core region 10.1 through (through) the inner cladding region 10.3 and the trench region 10.2. Illustratively, a tapered fiber bundle (TFB) coupler may be used. TFB couplers are typically of the type described in DiGiovanni and Stentz, US Pat. No. 5,864,644 (1999), which is incorporated herein by reference. Alternatively, an evanescent coupler may be used in which the pump fiber and the GPF are in contact with each other and are disposed along the entire length of the GPF. As described in Grudinin et al. US Pat. No. 6,826,335 (2004), which is also incorporated herein by reference, the pump fiber and GPF may be arranged parallel to each other. , May be twisted together. On the other hand, the GPF may also be core pumped. Illustratively, in a core pumping design, a high intensity pump source is coupled to the GPF core to co-propagate (or back-propagate) the amplified signal light. GPFs pumped in this manner also benefit from applying the principles of the present invention that cladding absorption increases while maintaining single mode operation. One such core pumping mechanism is described in US Pat. No. 7,916,386 to DiGiovanni and Headley, which is incorporated herein by reference.

  The core region 10.1 and the cladding regions 10.2, 10.3, 10.4 are configured such that the fundamental mode is guided primarily in the core region (ie, the bottom of the fundamental mode extends outside the core region). Spans the trench region 10.2). For this purpose, the diameter of the core region can be increased without introducing a large number of HOMs, especially with the fiber design of the present invention including the trench region 10.2. As defined above, the design of the present invention is believed to achieve single mode operation as long as it supports fundamental mode and at most about first to fourth order HOMs.

  As discussed in more detail below, calculations of some embodiments of the present invention show that the GPF design of the present invention has an increased cladding absorption (eg, greater than about 30%). Calculations also show that some embodiments of such GPF designs have less optical loss induced by photodarkening.

  Illustratively, GPF 10 is made of silica, core region 10.1 is doped with Yb as a species that provides gain, and is also doped with, for example, Ge, Al and P together. The trench region 10.2 is doped with a species that lowers the refractive index, such as F, or any dopant or combination of dopants that lowers the refractive index of the trench region below that of the core region. The inner cladding region 10.3 may be undoped, and the outer cladding region 10.4 typically includes a low refractive index polymer, down-doped silica, or DiGiovanni and Windeler, which is incorporated herein by reference. US Pat. No. 5,907,652 (1999), selected from the group consisting of air cladding structures of the type described.

  The outer cladding region 10.4 illustratively has an NA of at least 0.22.

  Alternatively, the inner cladding region may be doped with a dopant that increases the refractive index, such as Al or Ge, where the refractive index of the core region 10.1 and the trench region 10.2 is relative to the inner cladding region 10.3. It rises while maintaining the difference in refractive index in the same manner. Similarly, the inner cladding can be down-doped with a corresponding change in the refractive index with a dopant that lowers the refractive index, such as F or B.

Alternative GPF Design In the alternative embodiment shown in FIG. 2, the shaded portion is such that the species providing the gain is located only in the core region 10.1 where most of the fundamental mode energy of the signal light is confined. Indicates. However, the bottom of this mode extends to the trench region 10.2. However, since the trench region contains no species that provides gain, this tail is not amplified, thereby reducing the efficiency of the amplifier or laser in which the GPF is incorporated. (One such inefficiency is the generation of amplified spontaneous emission (ASE).) To mitigate this effect, the portion of trench region 10.2 adjacent to core region 10.1 ( That is, the skirted portion) may also be doped with seeds that provide gain.

Method of Increasing Clad Absorption From the foregoing description of the optical fiber design of FIGS. 1 and 2, another aspect of the invention is that it is a method of increasing the clad absorption of a double clad GPF while maintaining single mode operation. Obviously, this method comprises the steps of (a) forming a core region 10.1 having a core diameter greater than the core diameter (D _core ) of the corresponding double clad GPF without the trench region; .1 and the inner cladding region 10.3, forming a trench region 10.2; (c) a core region 10.1, a trench region 10.2, and an inner cladding region 10.3; (i) The refractive index of the trench region is lower than the refractive index of the inner cladding region 10.3, and (ii) the absolute value of the refractive index of the trench region 10.2 and the inner cladding region. The difference [Delta] n _tr between the refractive index of 10.3, and a step of configuring to be smaller than a difference [Delta] n _core between the refractive index and the refractive index of the inner cladding region 10.3 of the core region 10.1, (d ) Core region 10.1, trench region so that the fundamental mode of signal light can be supported and guided mainly in core region 10.1 and pump light can be coupled to core region 10.1. 10.2 and configuring inner cladding region 10.3.

Calculated GPF Design An example of an embodiment of the invention provided describes the design of a double clad GPF of the type shown in FIGS. The performance and design calculations for this particular embodiment show that the core region is doped with seed Yb that provides gain, and Ge, Al, and P are also doped together, the trench region 10.2 is down-doped with F, and the inner Assume that the cladding region is undoped. However, these calculations are independent of the material used to form the outer cladding region 10.4 as long as the outer cladding region has a lower refractive index than the inner cladding region.

  Various materials, dimensions and operating conditions are provided as examples only and are not intended to limit the scope of the invention, except where explicitly stated otherwise.

  More specifically, the refractive index profile of this example of the double clad GPF 10 of the present invention is shown by curve 31 in FIG. 3 and the refractive index profile of a standard double clad GPF shown by curve 30 in FIG. It is juxtaposed. At a given wavelength (eg 1080 nm), the GPF design of the present invention can increase the core diameter compared to a standard double clad GPF by including a trench region for a given MFD. An example of this is shown in Table I below.

The refractive index difference between the core region and the trench region is by definition relative to the refractive index of the inner cladding region. In both cases, the core region is doped with Yb so that at least 65% of the total refractive index difference is provided by Yb, and is also doped with Ge, Al, P oxide and silicon fluoride. For example, Yb (1-2 mol%), GeO ₂ (<0.5 mol%), AI ₂ O ₃ (3.5 mol%), and P ₂ O ₅ (<0.5 Mol%) and SiF ₄ (1 mol%). In both cases, the inner cladding region was assumed to be undoped. In the case of the GPF of the present invention, it was assumed that the trench region was down-doped with F. However, any combination of dopants (Ge, Al, P, F, B) may be appropriate as long as the refractive index difference is configured as described above across the fiber cross section.

The standard GPF described above and the GPF of the present invention have the same core NA (0.08), the same material absorption of the pump light (at a wavelength of about 915 nm), and the same HOM (LP _{11 at} a wavelength of about 1030 nm). ) Had a shut-off.

The GPF of the present invention has many superior optical properties compared to standard DCF, i.e., some embodiments exhibit a lower percentage of photodarkening (i.e., standard If the cladding absorption is the same in the DCF and the DCF of the present invention, the larger D _core that the trench region allows allows for lower material absorption resulting in lower light blackening) Other embodiments exhibit increased cladding absorption (ie, lower material blackening due to the larger D _core that the trench region allows if the material absorption is the same in the standard DCF and the DCF of the present invention). (But rather, higher cladding absorption). Thus, some embodiments of the GPF design of the present invention may provide a volume concentration N of species that provides such a gain required to achieve a given α _clad for a given cladding absorption. _{As d} decreases, the level of photodarkening observed in species that provide some gain (eg, Yb) can be reduced. The lower the photodarkening rate, the better the efficiency and power, and in the case of a clad pumped fiber laser (eg Figure 6), there is a greater end of life margin for this pump laser. Increases reliability. The details of the light loss induced by photodarkening over the wavelength range from 600 nm to 1100 nm are shown in FIG. Curve 40 shows the loss induced by photodarkening for a standard GPF (refractive index profile shown by curve 30 in FIG. 3), and curve 41 shows the GPF of the present invention (shown by curve 31 in FIG. 3). The corresponding lower loss at all wavelengths.

  On the other hand, when the cladding absorption increases, the SRS threshold value increases and the output can be increased. This also reduces the design footprint (GPF length) and facilitates mounting. In this example, the cladding absorption increased by more than 30%, from about 0.88 dB / m for the standard GPF to about 1.2 dB / m for the GPF of the present invention.

  As mentioned above, a particularly preferred feature of the GPF of the present invention is that larger core diameters can be used while still maintaining the ability to achieve single mode operation. In the specific examples of Table I, the core diameter was about 11.8 μm for the standard GPF and about 14.0 μm for the GPF of the present invention, an increase of about 2.2 μm. For clad pumped fiber lasers (eg, FIG. 5), larger cores increase clad absorption (eg, 36%), thus making the laser shorter (eg, 36% shorter double clad GPF) and more implemented. It means it will be easy.

The larger the core size, the greater the overlap between the fundamental mode of the signal light and the gain region (ie, core region 10.1 for the GPF of the present invention). Therefore, the efficiency of energy transfer to the signal light is improved. This feature is characterized by the LP ₀₁ output curve 33 (FIG. 3) for the fundamental signal mode of the GPF of the present invention having a refractive index profile of curve 31 and the LP for the fundamental signal mode of a conventional SIC GPF having the refractive index profile of curve 30. When compared to the ₀₁ output curve 32, this is shown by comparing the overlap between them.

Various designs of GPF doped with ytterbium (Yb) (DCF with trench region) of the present invention are shown with standard Yb-doped GPF (trench width column abbreviation N / A) Compared to the DCF) design without the trench region, it is shown in Table II below. Table II shows the MFD at a wavelength of 1080 nm for various parameters of the core and trench regions: core radius (R _core ), core refractive index difference (Δn _core ), trench refractive index difference (Δn _tr ), and specific As a function of the minimum trench width (W _tr ) to maintain single mode operation for a given core radius. For each inventive GPF of a particular MFD, Table II also shows the resulting coefficient of increase in core area and the resulting percent increase in cladding absorption (α _clad ).

For the inner and outer cladding regions of the designs listed in Table II, the refractive index difference of the inner cladding region is zero because it is a specified criterion, and the refraction of the outer cladding region (which guides the pump light) The rate difference is at least −15.5 × 10 ⁻³ . The inner cladding region has a thickness of at least 50 μm and the outer cladding region has a thickness of at least 10 μm.

In summary, Table II shows that an MFD of about 6-18 μm can be achieved with the inventive design doped with Yb when the core region and the cladding region are configured to increase the cladding absorption by about 26-69%. It is shown that. In addition, when the MFD is about 6-8 μm, the radius of the core region is about 2.9-4.1 μm, and the refractive index difference Δn _core of the core region is about 4.3 × 10 ⁻³ -7. 8 × a ^{10 -3,} the refractive index difference [Delta] n _tr of the trench region be from about -0.5 × ^{10 -3} -4.0 × ^{10 -3,} the minimum width of the trench region from about 1.5 7.0 μm. On the other hand, when the MFD is about 10 to 18 μm, the radius of the core region is about 4.6 to 11.0 μm, and the refractive index difference Δn _core of the core region is about 0.8 × 10 ^{−3 to} 2.8 ×. 10 ⁻³ , the refractive index difference Δn _tr of the trench region is about −0.15 × 10 ⁻³ to −0.8 × 10 ⁻³ , and the minimum width of the trench region is about 2.5 to 10. 0 μm.

  Similarly, Tables III and IV below show standard GPF (DCF without trench region), core region erbium (Er, for operation at about 1530-1630 nm) or thulium (Tm, about 1940-2050 nm). The corresponding parameters for comparison with alternative inventive designs (DCF with trench regions) doped with either

In summary, Table III shows that an Er-doped design of the present invention can achieve an MFD of about 9-20 μm when the core and cladding regions are configured so that cladding absorption is increased by about 76-98%. It is shown that. In addition, when the MFD is about 9 to 20 μm, the radius of the core region is about 4.3 to 13.0 μm, and the refractive index difference Δn _core of the core region is about 1.0 × 10 ⁻³ to 4. 5 × 10 ⁻³ , and the refractive index difference Δn _tr of the trench region is about −1.0 × 10 ⁻³ to −6.0 × 10 ⁻³ , and the minimum width of the trench region is about 2.0 to 8.0 μm.

An interesting feature of Er-doped designs is the relative value of Δn _core and Δn _tr , ie, in some Er-doped designs, the absolute value of Δn _tr is greater than the absolute value of Δn _core. Can be. More specifically, this condition has been found to be effective for a subset of Er-doped core region designs with an R _core / MFD ratio of about 0.6 or greater. Illustratively, R _core /MFD≧0.6 is in the approximate range of 0.61 to 0.67.

A theoretical explanation for the behavior of Er doped GPFs above and below R _core / MFD ≧ about 0.6 is currently not possible. However, from a practical standpoint (ie, for ease of manufacture and / or utilization), the GPF of the present invention preferably has a relatively shallow trench. In fact, trenches are typically fabricated by adding rapidly diffusing fluorine during typical thermally promoted bonding operations, so that deeper trenches make it more difficult to bond GPFs. .

In contrast, for all Yb-doped designs in Table I and for all Tm-doped designs in Table III, the opposite is true, ie, in absolute values, Δn _tr <Δn _core is there.

In summary, Table IV shows that an MFD of about 8-20 μm can be achieved with a Tm-doped design of the present invention when the core region and the cladding region are configured to increase the cladding absorption by about 41-53%. It is shown that. In addition, when the MFD is about 8 to 20 μm, the radius of the core region is about 3.5 to 11.0 μm, and the refractive index difference Δn _core of the core region is about 2.35 × 10 ^{−3 to} 16. 0 × 10 ⁻³ , the refractive index difference Δn _tr of the trench region is about −1.0 × 10 ⁻³ to −7.0 × 10 ⁻³ , and the minimum width of the trench region is about 3.0 to 6.0 μm.

  It should be understood that the foregoing mechanism is merely illustrative of the many possible specific embodiments that can be devised to represent the application of the principles of the present invention. In accordance with these principles, many different other mechanisms may be devised by those skilled in the art without departing from the spirit and scope of the present invention. Specifically, the bending sensitivity of the GPF of the present invention is reduced by the presence of the trench region, and the bending-induced effects are mitigated when the fiber is wound, for example, in an amplifier or laser package.

Application Example: High Power Fiber Amplifier and Laser One application example of the present invention is shown in FIG. 5, in which a high power fiber amplifier 230 is a GPF optically coupled to an optional pigtail fiber 235p. 235a. GPF 235a is optically coupled to coupler 233, and pigtail fiber 235p is optically coupled to utilization device 234. GPF 235a is designed according to the present invention and is typically wound with pigtail 235p inside an amplifier package. In a typical commercial amplifier package, the wound GPF fiber 235a and pigtail 235p are mounted on a well-known plate or mandrel (not shown).

  For relatively high power applications, the coupler 233 is known as a pump coupler and couples the output of the optical input signal source 231 and the optical pump source 236 to the GPF 235a. The pump coupler 233 may be formed by well-known bulk optical components, or, as mentioned above, a tapered shape of the type described by DiGiovanni and Stentz in US Pat. No. 5,864,644 (1999). These may be formed by fiber bundles, or by evanescent couplers of the type described by Grudinin et al. In US Pat. No. 6,826,335 (2004), or by other known methods, Both of which are hereby incorporated by reference. Input signal source 231 generates an optical input signal of a first wavelength, which is coupled to the input of coupler 233 via a conventional fiber 232 or bulk optical component (not shown), while pump source 236. Generates an optical pump signal of the second wavelength, which is coupled to another input of the coupler 233 by a typical fiber 237, typically multimode. (The coupler 233 eventually couples the signal light to the core region of the GPF 235a and passes the pump light to the core region of the GPF 235a via the inner cladding region, as described above with reference to FIGS. Combine directly or directly.)

  As is well known in the art, the pump signal produces an inverted distribution in GPF 235a, which amplifies the input signal from input source 231. The amplified input signal propagates along the GPF 235a (through the pigtail 235p, if present) to the utilization device 234. For high power applications, the utilization device 234 includes a wide variety of well-known devices or apparatuses such as, for example, another optical amplifier, beam collimator, lens system, workpiece to be cut or welded. It's okay.

  Illustratively, the input source 231 is a laser that generates a relatively low power optical input signal at a wavelength in the amplification range of the GPF 235a, and the pump source 236 is preferably a semiconductor laser, but optionally semiconductor light emitting. It can also be an array of diodes (LEDs). In either case, the pump source 236 generates a relatively high optical power (eg, greater than about 150 mW, or even greater than 100 W) pump signal at shorter wavelengths resulting in the desired amplification of the input signal. In a preferred embodiment, when GPF 235a is doped with ytterbium, signal source 231 generates an input signal having a wavelength of about 1080 nm, and pump source 236 has a wavelength of about 915 nm or alternatively about 975 nm. A pump signal is generated at a wavelength of.

  The amplifier 230 of FIG. 5 shows a typical co-propagating pump configuration (ie, the pump signal and input signal propagate in the same direction through the GPF), but a counter-propagating configuration (ie, pump signal). And the input signal propagates in the reverse direction through the GPF). In addition, various amplifiers may be configured in tandem, which is a scheme for increasing the overall gain of high power multi-stage systems well known in the art. The pump energy may alternatively be coupled transversely to the amplifier.

  If the amplification device is configured to operate as a laser, the signal source 231 is omitted, and the signal light described above is equivalent to the stimulated emission generated internally by the laser.

  A schematic embodiment of such a laser 200 is shown in FIG. The laser 200 is designed to provide a high power 215. This basic design follows that described by DiGiovanni in US Pat. No. 5,937,134 (1999), which is also incorporated herein by reference. However, as discussed above with reference to FIGS. 1-3, in this case, the active medium of laser 200 comprises a wound, single-mode, double-clad GPF fiber 211 of the design according to the present invention.

  A portion of the rolled GPF 211 is shown cut out at position 213 to indicate a substantial length. The fiber length of such a laser structure is typically about 1 meter to several tens of meters. Since the diameter of the coil is illustratively about 15 cm, the fiber 211 exhibits many turns. (Similar annotations apply to the wound amplifier GPF 235a of FIG. 5, and like such an amplifier, the wound GPF 211 of laser 200 is typically a well-known plate or mandrel (not shown). Wrapped around

  The active medium of the laser is located inside the cavity resonator formed by Bragg gratings 212 and 213, which are illustratively disposed at (or near) the ends 217 and 219 of GPF 211, respectively. It is. These gratings or reflectors are typically formed by light-induced refractive index changes in the core region of GPF 211.

  The active medium is end-pumped by the optical pump source 214, thereby generating pump light that is coupled to an inner cladding region (not shown) at one end 217 of the GPF 211. Although source 214 is shown separate from end 217, it will be apparent to those skilled in the art that end 217 is typically attached to source 214 via a suitable coupler (not shown). I will. Pump light from the source 214 is absorbed in the core region of the GPF 211 (ie, by the species that provides the gain there), resulting in an inversion distribution to produce the laser light output 215, which is the opposite end of the GPF 211. Come out of 219.

  Typically, the pump source 214 comprises a semiconductor laser diode (or an array of such diodes). Illustratively, the active region of the laser diode includes AlGaAs that generates pump light at a wavelength of about 980 nm, or InGaAs or InGaAsP for generating pump light at longer wavelengths in the range of 1000-1600 nm. Alternatively, the pump source 214 may comprise other types of solid state lasers, such as T sapphire, Nd glass, or may be a fiber laser.

Claims (13)

An optical fiber providing gain,
A core region having a longitudinal axis and having a circular cross section in a direction perpendicular to the axis;
A cladding region surrounding the core region, wherein the core region and the cladding region are configured to support the propagation of signal light in a basic transverse mode and guide it in the direction of the axis,
The cladding region includes a trench region surrounding the core region, an inner cladding region surrounding the trench region, and an outer cladding region surrounding the inner cladding region, wherein the inner cladding region has a refractive index lower than a refractive index of the core region. The trench region has a refractive index lower than the refractive index of the inner cladding region,
At least the core region includes at least one species that provides gain to the signal light when appropriate pumping energy is applied to the fiber;
In the core region and the cladding region, the fundamental mode is mainly guided in the core region, and a refractive index difference Δn _tr between the trench region and the inner cladding region is an absolute value, and the core region and the inner region are It is configured to be smaller than the difference in refractive index Δn _core between the cladding regions, so that the single-mode operation of the signal light is maintained, but the trench region is not present but has a similar layer configuration and similar refraction. Both the fiber core region diameter and the fiber cladding absorption, as compared to a fiber that provides a corresponding gain with a rate profile, a similar core region and a similar cladding region,
At least the core region is doped with Tm, the fundamental mode is characterized by a mode field diameter (MFD) of 8-20 μm, and the core region and the cladding region are similar layers without the trench region A fiber configured to increase the cladding absorption by 41 to 53% compared to a fiber providing a corresponding gain having a configuration, a similar refractive index profile, a similar core region and a similar cladding region.   The fiber of claim 1, wherein the species that provides the gain provides gain to the signal light at a signal wavelength of 1000 nm or greater. The radius of the core region is 3.5 to 11.0 μm, the Δn _core is 2.3 × 10 ^{−3 to} 16.0 × 10 ⁻³ , and the Δn _tr is −1.0 × 10 ^−3. To −7.0 × 10 ⁻³ with a minimum trench region width (W _tr ) in the range of 3.0 to 6.0 μm, and for a given MFD, W _tr is a single mode The fiber of claim 1 , wherein the fiber has the minimum trench area width required to maintain operation.   The fiber of claim 1, wherein at least a portion of the trench region also includes a species that provides at least one gain.   The gain-providing fiber comprises a double-clad fiber, wherein the core region, trench region and inner cladding region comprise silica, and the outer cladding region comprises a low index polymer, down-doped silica, and an air cladding structure The fiber of claim 1 selected from the group consisting of:   The fiber according to claim 1, wherein the core region and the cladding region are configured to support and guide only the fundamental mode of the signal light.   The fiber according to claim 1, wherein the core region and the cladding region are configured to support and guide the fundamental mode and at most first to fourth order modes of the signal light.   The gain-providing fiber comprises a double-clad fiber, the pumping energy is supplied by a source of multimode pump light, and the core region and the clad region are configured such that the pump light passes through the inner clad region. The fiber of claim 1, wherein the fiber is configured to be coupled to a core region. The optical fiber according to any one of claims 1 to 8 , for amplifying the signal light coupled to the core region in response to pump light applied to the core region.
An optical device, wherein the source of pump energy comprises an optical fiber coupled to the core region via the inner cladding region.   The apparatus of claim 9, wherein the pump source of pump energy further comprises a combiner for providing multimode pump light and coupling the pump light and the signal light to the core region.   The apparatus of claim 10, wherein the fiber, the source, and the coupler are configured as an optical amplifier. A cavity resonator,
The optical fiber according to any one of claims 1 to 8, which is disposed inside the resonator;
An optical device comprising: a coupler for coupling pump energy to the core region via the inner cladding region, thereby generating the signal as stimulated emission within the core region. The optical device according to claim 12, wherein the fiber, the resonator and the coupler are configured as a laser .

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