Large Mode Area Fibers | previous | next | feedback |
Definition: optical fibers with relatively large mode areas and a single transverse mode or only a few modes
For some applications, it is desirable to use optical fibers with large effective mode areas (LMA fibers) – often with single-mode guidance. Due to the reduced optical intensities, such fibers effectively have lower nonlinearities and a higher damage threshold, which makes them suitable for, for example, the amplification of intense pulses or single-frequency signals in fiber amplifiers, or in the case of passive fibers for delivery of such light. Whereas standard single-mode fibers have an effective mode area below 100 μm2, large mode area fibers reach values of hundreds or even thousands of μm2.
Design Approaches and Limiting Factors
A straightforward design approach to obtain large mode areas is to decrease the numerical aperture, i.e., the refractive index difference between the core and the cladding, for a step-index fiber design. However, there are severe limitations: the guidance (waveguiding) then becomes rather weak, and significant losses can arise from small imperfections of the fiber or from bending (→ bend losses). To achieve relatively robust single-mode guidance at larger mode areas, there are several more refined design approaches with specially optimized refractive index profiles, which allow for mode areas up to the order of 1000 μm2. This is an order of magnitude higher than for ordinary single-mode fibers.
There are additional difficulties in applying this concept to rare-earth-doped fibers. Relatively high concentrations of additional dopants are often required, e.g. for reducing certain quenching effects, and these dopants often increase the numerical aperture. Even if the refractive index contrast can be reduced in some way, the precision of refractive index control may be decreased, and this affects the ability to realize very large mode areas.
There are also large mode area photonic crystal fibers of various designs, such as leakage channel fibers, which reach even a few thousand μm2. In this regime, diffraction becomes rather weak, and the guidance (waveguide effect) also becomes weak, making the fiber sensitive to disturbing effects such as microbends; no method is known which removes this fundamental limiting effect.
Note that bending not only introduces losses, but also can reduce the effective mode area. This is particularly true for large mode area step-index fibers. For a fair comparison of fiber types, this effect definitely has to be taken into account [4]. It turns out that some fiber designs have a large mode area without bending, but a much reduced mode area with bending, whereas there are other designs (e.g. with a parabolic index profile) where the mode area starts with a somewhat smaller value but is much less sensitive to bending.
Larger mode areas can be achieved if one allows for multimode propagation. It may then still be possible to guide light dominantly in the fundamental mode, so that the output e.g. of a fiber amplifier is close to diffraction-limited [1, 3]. Limitations arise from the more critical launch conditions and from mode mixing in the fiber.
An interesting concept described recently [6] is first to couple light from the fundamental mode to a particular higher-order mode, using a long-period fiber Bragg grating, then to propagate the light in this higher order mode in the amplifying fiber, and then finally to convert the light back to the fundamental mode with another fiber Bragg grating. The claimed advantage of using a higher-order mode is twofold: such modes have larger effective mode areas, and they are claimed to exhibit a weaker coupling to other modes. The losses associated with coupling to and from this higher-order mode can be small, and the fiber design can be optimized for a broad bandwidth for this coupling. However, difficulties can arise from the very uneven intensity distribution. This can lead to fiber damage even in a regime where the overall nonlinearity is moderately strong, so the approach may solve problems with nonlinearities but not those with damage. Also, the mode field significantly extends into the cladding (the inner cladding in the case of a double-clad fiber), which is not ideal for amplification.
Bibliography
| [1] | M. E. Fermann, "Single-mode excitation of multimode fibers with ultrashort pulses", Opt. Lett. 23 (1), 52 (1998) |
| [2] | N. G. R. Broderick et al., "Large mode area fibers for high power applications", Opt. Fiber Tech. 5, 185 (1999) |
| [3] | J. P. Koplow, D. A. V. Kliner, and L. Goldberg, "Single-mode operation of a coiled multimode fiber amplifier", Opt. Lett. 25 (7), 442 (2000) |
| [4] | J. M. Fini, "Bend-resistant design of conventional and microstructure fibers with very large mode area", Opt. Express 14 (1), 69 (2006) |
| [5] | J. M. Fini, "Bend-compensated design of large-mode-area fibers", Opt. Lett. 31 (13), 1963 (2006) |
| [6] | S. Ramachandran et al., "Light propagation with ultralarge modal areas in optical fibers", Opt. Lett. 31 (12), 1797 (2006) |
| [7] | L. Dong, J. Li, and X. Peng, "Bend-resistant fundamental mode operation in ytterbium-doped leakage channel fibers with effective areas up to 3160 μm2", Opt. Express 14 (24), 11512 (2006) |
| [8] | Y. Tsuchida et al., "Design of single-moded holey fibers with large-mode-area and low bending losses: the significance of the ring-core region", Opt. Express 15 (4), 1794 (2007) |
| [9] | O. Schmidt et al., "Single-polarization ultra-large-mode-area Yb-doped photonic crystal fiber", Opt. Express 16 (6), 3918 (2008) |
See also: fibers, effective mode area, bend losses, numerical aperture, fiber core, fiber amplifiers, master oscillator fiber amplifier, photonic crystal fibers, Spotlight article 2006-08-10, Spotlight article 2006-10-04, Spotlight article 2006-12-03, Spotlight article 2006-12-03, Spotlight article 2007-04-28
Category: fibers and other waveguides
