All-dielectric subwavelength metasurface focusing lens.

We have proposed, designed, manufactured and tested low loss dielectric micro-lenses for infrared (IR) radiation based on a dielectric metamaterial layer. This metamaterial layer was created by patterning a dielectric surface and etching to sub-micron depths. For a proof-of-concept lens demonstration, we have chosen a fine patterned array of nano-pillars with variable diameters. Gradient index (GRIN) properties were achieved by engineering the nano-pattern characteristics across the lens, so that the effective optical density of the dielectric metamaterial layer peaks around the lens center, and gradually drops at the lens periphery. A set of lens designs with reduced reflection and tailorable phase gradients have been developed and tested, demonstrating focal distances of a few hundred microns, beam area contraction ratio up to three, and insertion losses as low as 11%.

Semi-guiding high-aspect-ratio core (SHARC) fiber amplifiers with ultra-large core area for single-mode kW operation in a compact coilable package.

A new class of optical fiber, the SHARC fiber, is analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined-gain dopants. The high-aspect-ratio (~30:1) rectangular core allows mode-area scaling well beyond 10,000 µm2, which is critical to high-pulse-energy or narrow-linewidth high-power fiber amplifiers. While SHARC fibers offer modally dependent edge loss at the wide "semi-guiding" edge of the waveguide, the inclusion of gain filtering adds further modal discrimination arising from the variation of the spatial overlap of the gain with the various modes. Both methods are geometric in form, such that the combination provides nearly unlimited scalability in mode area. Simulations show that for kW-class fiber amplifiers, only the fundamental mode experiences net gain (15 dB), resulting in outstanding beam quality. Further, misalignment of the seed beam due to offset, magnification, and tilt are shown to result in a small (few percent) efficiency penalty while maintaining kW-level output with 99% of the power in the fundamental mode for all cases.

Er-doped high-aspect-ratio core rectangular fiber producing 5 mJ, 13 ns pulses at 1572 nm.

We have produced 5 mJ, 13 ns pulses using a very large-mode-area rectangular fiber containing a high-aspect-ratio core with dimensions of ~30 µm×580 µm. The rectangular fiber design retains a thin cladding dimension ~0.5 mm, which is compatible with a compact coiled package. We have developed a theoretical model that achieves good agreement with the experimental data. This model indicates that an optimized fiber design can be scaled to pulse energies of several tens of millijoules. We also discuss a strategy for maintaining good beam quality in both transverse dimensions of this fiber.

Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package.

A new class of optical fiber is presented that departs from the circular-core symmetry common to conventional fibers. By using a high-aspect-ratio (~30:1) rectangular core, the mode area can be significantly expanded well beyond 10,000 µm2. Moreover, by also specifying a very small refractive-index step at the narrow core edges, the core becomes "semi-guiding," i.e. it guides in the narrow dimension and is effectively un-guiding in the wide mm-scale dimension. The mode dependence of the resulting Fresnel leakage loss in the wide dimension strongly favors the fundamental mode, promoting single-mode operation. Since the modal loss ratios are independent of mode area, this core structure offers nearly unlimited scalability. The implications of using such a fiber in fiber laser and amplifier systems are also discussed.

Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering.

We present, for the first time to our knowledge, an explicit experimental comparison of beam quality in conventional and confined-gain multimode fiber lasers. In the conventional fiber laser, beam quality degrades with increasing output power. In the confined-gain fiber laser, the beam quality is good and does not degrade with output power. Gain filtering of higher-order modes in 28microm diameter core fiber lasers is demonstrated with a beam quality of M(2)=1.3 at all pumping levels. Theoretical modeling is shown to agree well with experimentally observed trends.