Ultra‐large effective‐area, higher‐order mode fibers: a new strategy for high‐power lasers

Abstract

This paper describes the physics and properties of a novel optical fiber that would be attractive for building highpower fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses -a capability that is practically nonexistent in conventional fibers. Particularly interesting is the fact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.Mode image, canonical refractive index profile and mode profile of HOM fibers typically used in the LMA designs.

Optical vortices in fiber

“…One such rule, which has had ample experimental confirmation, providing for a degree of confidence, is that Δn eff > 10 -4 yields polarization maintaining (PM) fibers in which the orthogonal polarizations of the LP 01 modes remain stable for lengths scales exceeding 100 m. This rule has been successfully implemented in the context of designing large mode area fibers -a counterintuitive but universal phenomenon, discovered recently [35], reveals that Δn eff between LP 0m and LP 1m mode increases with mode order m, and thus higher order HOMs are actually more stable than lower order modes, yielding a scalable pathway for increasing the mode area of fibers, of utility in highpower applications. In these HOM experiments, it was experimentally confirmed that Δn eff > 10 -4 is a reasonable proxy for mode stability [36].…”

Break up of the azimuthal symmetry of higher order fiber modes

“…Such modes have earlier been investigated by e.g. Ramachandran et al [8]. We observe that for very high order Bessel-like modes the azimuthal symmetry is broken and the intensity profile of the mode assumes a bowtie shape, we denote these modes bowtie modes.…”

“…9. Modes of lower order propagate unstably in any fiber [8], and they are thus not included in the plot. It is observed that the diffraction free propagation distances are much like that of the ideal Bessel-like modes.…”

“…Finally, considering large mode area (LMA) fibers, the mode spacing between the LP 0X -modes and the nearest mode group, the LP 1X -modes, is an order of magnitude higher than what can be achieved when operating with the conventional LMA fiber for a given effective area. The increased mode spacing suggests significantly less mode coupling, with the mode spacing increasing with the mode order it allows for scaling the fiber to very large effective areas [8]. Recent applications employing the LP 0X -modes in LMA fibers include a chirped pulse amplification scheme [4], and high power four wave mixing conducted at 1 μm [9].…”

Femtosecond pulse written fiber gratings: a new avenue to integrated fiber technology

“…Since the grating crosssection is no longer determined by the dopant concentration, more elaborate fiber grating designs are possible: e. g. grating structures within the cladding or highly localized intracore modifications. Especially the latter enable new grating designs that tap the huge potential of higher-order fiber modes [166,167]. Specially polarized feedback can be obtained, e. g. a fully radially polarized beam [168].…”