Fundamental mode amplification in 140 μm core diameter fiber

Abstract

Summary form only given. Many applications require high power good beam quality lasers with fiber delivered output. Very often, the necessary powers are above the damage threshold and nonlinear effects threshold of single mode (SM) fibers. This may be overcome by two approaches. A first approach is to use either large mode area fibers or photonic crystal fibers (PCFs), which have a very large and nearly SM cores. Nevertheless, this approach is limited by the achievable nearly SM core size, which is 40 μm for a flexible PCF. A second possible approach is to impose a Gaussian mode in a highly multimode (MM) step-index fiber with very large core diameter. In this approach, it was found that in passive highly MM fibers the fundamental mode can be maintained over short distances (~20 cm) in the case of straight fibers [1], or over longer distances in the case of coiled fibers [2]. Additionally, off-centered coupling of the fundamental mode into highly MM (100-400 μm core diameter), 2 m long, passive fibers was studied [4], along with its effect on the output beam quality [3]. Finally, SM excitation in active MM fibers, as means for achieving high quality output in an amplifier configuration, was also reported [2].Additionally, offcentered coupling of the fundamental mode into highly MM (100-400 m core diameter), 2 m long, passive fibers was studied [4], along with its effect on the output beam quality [3]. Finally, SM excitation in active MM fibers, as means for achieving high quality output in an amplifier configuration, was also reported [2]. Here we experimentally investigate propagation of the fundamental mode in highly MM passive and active fibers. We resorted to well controlled, free-space coupling of a Gaussian beam to the MM fiber, instead of tapering or other complex ways of mode coupling. The experimental setup in the case of a passive fiber consists of a 1 μm CW Gaussian seed source, coupling optics, and a commercially available highly MM passive fiber with 50 μm core diameter and 0.15 numerical aperture (NA). This fiber supports ~200 spatial modes and can handle almost 1 MW of peak power. The measured effect of the fiber length and coiling radius on the beam quality (M2) is shown in Fig 1 (left). As evident, the beam quality improves with larger coiling radius and with shorter fiber length. In addition, a clear minimum of the M2 parameter (<;1.2) is observed at a coiling radius of 7.5 cm for all fiber lengths. This demonstrates fundamental mode propagation in highly passive MM fiber for more than 15 m without significant degradation. In the case of an active double-clad fiber, the experimental setup is similar but includes a 2 m long fiber and a pump source with suitable coupling optics in a counter propagating pump configuration. In these experiments our active fiber is extremely MM (supports ~340 spatial modes), with 140 μm core diameter and 0.07 NA, which can handle up to 7.5 GW peak power. To the best of our knowledge, this is the largest active core diameter that was used for preservation and amplification of the fundamental mode. Our experimental results, which are presented in Fig. 1 (right), show successful preservation of the fundamental mode in this fiber with M2<;2. Additionally, the amplification results show some degradation in beam quality with increasing pump power that corresponds to the increase in amplified spontaneous emission (ASE). Since the fundamental mode field diameter is significantly smaller than the active core diameter, highly multimode ASE is generated, degrading the beam quality. A more detailed discussion as well as additional experimental results will be presented.