Laser cooling of a Yb doped silica fiber by 18 Kelvin from room temperature.

An ytterbium doped silica optical fiber with a core diameter of 900µm has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2±0.1%. The core of the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

Implementation of Laser-Induced Anti-Stokes Fluorescence Power Cooling of Ytterbium-Doped Silica Glass.

Laser cooling of a solid is achieved when a coherent laser illuminates the material, and the heat is extracted by annihilation of phonons resulting in anti-Stokes fluorescence. Over the past year, net solid-state laser cooling was successfully demonstrated for the first time in Yb-doped silica glass in both bulk samples and fibers. Here, we report more than 6 K of cooling below the ambient temperature, which is the lowest temperature achieved in solid-state laser cooling of silica glass to date to the best of our knowledge. We present details on the experiment performed using a 20 W laser operating at a 1035 nm wavelength and temperature measurements using both a thermal camera and the differential luminescence thermometry technique.

A path to high-quality imaging through disordered optical fibers: a review.

In this paper, we review recent progress in disordered optical fiber featuring transverse Anderson localization and its applications for imaging. Anderson localizing optical fiber has a transversely random but longitudinally uniform refractive index profile. The strong scattering from the transversely disordered refractive index profiles generates thousands of guiding modes that are spatially isolated and mainly demonstrate single-mode properties. By making use of these beam transmission channels, robust and high-fidelity imaging transport can be realized. The first disordered optical fiber of this type, the polymer Anderson localizing optical fiber, has been utilized to demonstrate better imaging performance than some of the commercial multicore fibers within a few centimeters transmission distance. To obtain longer transmission lengths and better imaging qualities, glass-air disordered optical fibers are desirable due to their lower loss and larger refractive index contrast. Recently developed high air-filling fraction glass-air disordered fiber can provide bending-independent and high-quality image transport through a meter-long transmission distance. By integrating a deep-learning algorithm with glass-air disordered fiber, a fully flexible, artifact-free, and lensless fiber imaging system is demonstrated, with potential benefits for biomedical and clinical applications. Future research will focus on optimizing structural parameters of disordered optical fiber as well as developing more efficient deep-learning algorithms to further improve the imaging performance.

Impact of spatial correlation in fluctuations of the refractive index on rogue wave generation probability.

The presence of refractive index fluctuations in an optical medium can result in the generation of optical rogue waves (RWs). Using numerical simulations and statistical analysis, we have shown that the probability of optical rogue waves increases in the presence of spatial correlations in the fluctuations of the refractive index. We have analyzed the impact of the magnitude and the spatial correlation length of these fluctuations on the probability of optical rogue wave generation.

Method for measuring the resonant absorption coefficient of rare-earth-doped optical fibers.

A nondestructive method for measuring the resonant absorption coefficient of rare-earth-doped optical fibers is introduced. It can be applied to a broad range of fiber designs and host materials. The method compares the side-collected spontaneous emission at two arbitrary locations along the fiber as a function of the pump wavelength to extract the absorption coefficient. It provides an attractive and accurate alternative to other available techniques. In particular, the proposed method is superior to the cutback method, which destroys the sample and is prone to inaccuracies due to the cladding mode contamination. Moreover, because it does not involve any mechanical movement, it can be used for fragile optical fibers.

Spectral selectivity in optical fiber capillary dye lasers.

We explore the spectral properties of a capillary dye laser in the highly multimode regime. Our experiments indicate that the spectral behavior of the laser does not conform to a simple Fabry-Perot (FP) analysis; rather, it is strongly dictated by a Vernier resonant mechanism involving multiple modes, which propagate with different group velocities. The laser operates over a very broad spectral range and the Vernier effect gives rise to a free spectral range, which is orders of magnitude larger than that expected from a simple FP mechanism. The theoretical calculations presented confirm the experimental results. Propagating modes of the capillary fiber are calculated using the finite-element method and it is shown that the optical path lengths resulting from simultaneous beatings of these modes are in close agreement with the optical path lengths directly extracted from the Fourier transform of the experimentally measured laser emission spectra.