Large-mode-area (LMA) fiber
An optical fiber with an unusually large, low-NA core, typically 20–30 µm across with a numerical aperture near 0.06, that spreads the guided light over a large area to raise nonlinear thresholds in high-power fiber lasers. A 25 µm, NA 0.06 core at 1064 nm has an effective area of about 370 µm², roughly nine times that of a conventional 6 µm core.
A large-mode-area (LMA) fiber is a fiber whose core is made as large as possible while the output still behaves as a single, near-Gaussian mode. Typical step-index LMA fibers have cores of 20–30 µm and a core numerical aperture of about 0.06, against roughly 6 µm and 0.12 for a conventional fiber at 1 µm. The large core lowers the intensity at a given power, which raises the thresholds for stimulated Raman scattering, stimulated Brillouin scattering and self-phase modulation, and raises the pulse energy a core can hold before it is damaged. Most high-power ytterbium fiber lasers and pulsed fiber amplifiers use LMA fiber, usually in a double-clad form so that the pump travels in a much larger inner cladding.
Effective area and the single-mode limit
The benefit is measured by the effective area of the fundamental mode, since nonlinear thresholds scale with it. Computed from the exact LP₀₁ field of a step-index core at 1064 nm, a 6 µm core with NA 0.12 has an effective area of about 39 µm²; a 25 µm core with NA 0.06 has about 370 µm², about 9.4 times larger. In the Smith estimate for the SRS threshold, , a 10 m fiber with m/W reaches about 6.2 kW for the LMA core against a few hundred watts for a small one.
The cost is the mode count. The V-number,
must stay below 2.405 for strictly single-mode guidance, so the largest single-mode core diameter is
At 1064 nm and NA 0.06 that is 13.6 µm. A 20 µm core at the same NA has V = 3.54 and also guides LP₁₁; a 25 µm core has V = 4.43 and guides LP₁₁, LP₂₁ and LP₀₂ as well. Lowering the NA enlarges the single-mode core in proportion, to 27 µm at NA 0.03, but the index step falls with the square: about 1.2 × 10⁻³ at NA 0.06 and 3 × 10⁻⁴ at NA 0.03. Steps that small approach the index variations of a doped preform, and a weakly guided mode is easily distorted by bending, so NA values near 0.05–0.06 are close to the practical lower limit.
Bend-induced mode filtering
LMA fibers are therefore slightly multimode by design and are made to operate in one mode by differential loss. Coiling the fiber to a suitable diameter raises the bend loss of LP₁₁ and higher modes, which extend further toward the cladding and have smaller effective-index margins, by orders of magnitude more than that of LP₀₁. Koplow, Kliner and Goldberg showed in 2000 that a coiled 25 µm multimode amplifier could deliver a diffraction-limited beam this way. A matched launch into the fundamental mode and gain concentrated near the core center help further.
Bending also changes the fundamental mode. A tight coil tilts the index profile, pushes the mode toward the outside of the bend and shrinks its effective area, substantially for the largest cores, so an LMA fiber's effective area under its operating coil is smaller than the straight-fiber value. Designs that push the core size beyond about 30 µm therefore avoid bending: rod-type and photonic crystal fibers of 40–100 µm cores are held straight, and leakage-channel and chirally coupled core designs strip higher-order modes by structure instead of by coiling.
Measurement and practical use
The output mode is checked by beam quality measurement, with M² below about 1.1–1.3 usually taken as effectively single-mode, and by spatially and spectrally resolved imaging or the S² method, which detects small fractions of higher-order-mode power through their interference with the fundamental.
LMA fibers are spliced to small-core components through mode-field adapters, since a direct splice from a 10 µm mode to a 20 µm mode excites higher-order modes and loses power. At high average power, LMA amplifiers show transverse mode instability: above a threshold, a thermally induced index grating couples power from LP₀₁ into LP₁₁ and the beam quality fluctuates on millisecond time scales. That threshold, rather than nonlinearity, now limits the average power of many single-mode fiber amplifiers.
Common questions
Is a large-mode-area fiber single-mode?
Usually not strictly. Most LMA fibers have V between 3 and 5 at their operating wavelength and guide a few modes, but bend loss, a matched launch and gain confinement keep almost all the power in the fundamental mode, so the output is effectively single-mode.
Why not simply use a larger core with a lower NA?
Below an NA of about 0.05 the index step is comparable to fabrication variations and the mode becomes very sensitive to bending, so the effective area gained on paper is lost to mode distortion and bend loss in a coiled fiber.
How is LMA fiber different from multimode fiber?
A conventional multimode fiber of 50 µm or more with an NA of 0.2 guides hundreds of modes and is used to carry power without regard to beam quality. An LMA fiber guides only a few modes and is designed so that one dominates.
References: J. P. Koplow, D. A. V. Kliner and L. Goldberg, "Single-mode operation of a coiled multimode fiber amplifier," Opt. Lett. 25, 442 (2000); R. G. Smith, "Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering," Appl. Opt. 11, 2489 (1972); D. J. Richardson, J. Nilsson and W. A. Clarkson, "High power fiber lasers: current status and future perspectives," J. Opt. Soc. Am. B 27, B63 (2010); G. P. Agrawal, Nonlinear Fiber Optics, 6th ed. (Academic Press, 2019).