Invited Paper
Fiber designs for higher power lasers Ming-Jun Li, Xin Chen, Ji Wang, Anping Liu, Stuart Gray, Donnell T. Walton, A. Boh Ruffin, Jeffrey Demeritt, Luis Zenteno Science and Technology Division, Corning Incorporated, SP-AR-02-2, Corning, NY 14831 Tel: 607-974-3099, E-mail:
[email protected]
ABSTRACT This paper reviews different fiber design approaches for high power lasers. First, we discuss the conventional step index profile design and methods for achieving single mode operation in high power lasers such as bending, helical core fibers and Yb dopant profile designs. Then we present new design approaches for reducing the SBS through profile and glass composition designs. Finally, we describe fiber designs to achieve single polarization and at the same time to mitigate the SRS effect. Keywords: Fiber laser, fiber design, stimulated Brillouin scattering, stimulated Raman scattering, large mode area fiber
1. INTRODUCTION There have been significant efforts on developing high power fiber lasers with narrow line-width and good beam quality [1]. Such high power lasers pose significant challenges on fiber designs to reduce nonlinear effects such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), and achieve simultaneously single mode operation. Neither SBS nor SRS are desirable for high power lasers because they limit the amount of output optical power. One well known way to reduce the SBS and SRS effects is to make fibers with large mode area (LMA) [2]. However, when the core size is too large, the fiber becomes multimoded. Modal discrimination techniques, such as bending, are then required to achieve single mode operation. But for large core size, bending deforms the mode field distribution and reduces the mode area, which puts a limit on the benefit of using large core. Nonlinear effects can also be managed through design approaches other than increasing the effective area. It has been found that SBS can be reduced by decreasing the overlap between the optical and acoustic fields [3-5]. This is achieved through fiber profiles designs or fiber material designs to confine the optical and acoustic filed in different regions in the core. SRS effect can be mitigated by using fibers that have a cutoff wavelength for the fundamental mode [6], for example single polarization fibers [7-8]. In this paper, we review design concepts for achieving single mode operation in high fiber power lasers. We also discuss different fiber design approaches for managing SBS and SRS effects in high power fiber lasers.
2. FIBER DESINGS FOR SINGLE MODE LASER OPERATION To reduce the nonlinear effects, it is desirable to design a fiber with an effective area as large as possible. In a step index profile, the mode effective area is increased by lowering the core refractive index and increasing the core diameter. This can be seen in Figure 1, which shows how the effective area changes with the core diameter for different numerical aperture (NA) values of a step index fiber. For example, by lowering the NA to 0.05, and increasing the core diameter to 30 µm, the effective area is increased by a factor of 10 compared to standard single mode fiber (SMF) with NA of 0.12 and core diameter of 8 µm. However, for a given NA value, if the core diameter is too large, the fiber becomes multimoded. This is illustrated in Figure 2 where the fiber core diameter is plotted as a function of the NA for the cutoff wavelength of 1060 nm. With the core size larger than 20 µm, the fiber becomes multimoded even with NA as low as 0.05. A modal discrimination technique is then required to strip the higher order modes.
Optical Components and Materials IV, edited by Shibin Jiang, Michel J. F. Digonnet, Proc. of SPIE Vol. 6469, 64690H, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.714018
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Figure 1. Effective area changes with core diameter for different NA values.
Figure 2. Core diameter as a function of NA for cutoff wavelength of 1060 nm.
One way to strip the higher order modes is to bend the fiber [9]. When a fiber is under bending conditions, the bend losses of the higher order modes increase more rapidly than the fundamental mode. By choosing a right bend radius, it is possible to have the differential bend losses between the higher order modes and the fundamental mode high enough such that the fiber behaves like a single mode fiber. However, this approach has its limitations. For a given NA, the larger the core size the smaller the bend radius is required to strip the higher order modes. This is illustrated in Figure 3, where the bending losses of LP01 and LP11 modes are plotted versus the bend radius for 20 and 30 µm core fibers. The NA value for both fibers is 0.06. To create a bend loss of several dB/m, the bend radius is about 10 cm for the 20 µm core fiber, and 5 cm for the 30 µm core fiber, respectively. For a fiber with a core larger than 30 µm, the bend radius is even smaller than 5 cm. One drawback of using a small bend radius is that the mode field is deformed and the effective area is reduced. To illustrate this phenomenon, Figure 4 shows the effective area changes as a function of bend radius for the fibers with 20 and 30 µm core diameters. For the fiber with a 20 µm core, the effective area change is negligible for the bend radius down to 5 cm. On the other hand, for the fiber with a 30 µm core, the effective area drops quickly when bend radius decreases. For the bend radius of 5 cm, the effective area is reduced by more than 25%. For a core size larger than 30 µm, the effective area is expected to drop even quicker and the benefit of larger core size disappears. Bases on the results in Figures 3 and 4, we conclude that, for LMA fibers, the largest core size that can be used with the bending method to achieve single mode operation is about 25 µm. 500
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Figure 4. Effective area changes with the bend radius for the fibers with 20 and 30 mm core.
Another approach to strip the higher order modes is to make a fiber with helical a core [10-11]. A helical core fiber in a pump cladding is shown schematically in Figure 5. The structure can be described by two parameters: one is the offset from the center Doff, the other one is the spin pitch, Λ, defined as the distance along the length of the fiber that completes one full cycle of the rotation. The radius of curvature of the helix can be calculated from the following equation 2 + (Λ /(2π )) 2 Doff (1) R=
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The helical structure has a constant curvature so that it has a similar effect to bending a fiber. The advantage of a helical core fiber is that the bending is built into the fiber when the fiber is made and no bending is required in the fiber deployment. By changing the two parameters of the offset and the spin pitch, different bending radii can be obtained. Figure 6 shows calculated effective bending radii as a function of spin pitch for different offset values. By replacing the bend radius with the curvature radius of the helix, the bending loss of the helical fiber can be obtained from the conventional bending loss methods. Single mode operation on a 30 µm diameter core and a numerical aperture of 0.087 has been demonstrated yielding 60.4 W of output at 1043 nm in a beam with M2
