Optical cooling in Yb-doped silica fibers using anti-Stokes fluorescence has become a subject of great interest in the fiber laser community. This paper provides an update on the development of silica fibers designed specifically to enhance their cooling properties. This growing list includes a new, nearly single-mode fiber with a borophosphosilicate core that produced –65 mK of cooling with only 260 mW of 1040-nm pump power. The silica compositions that have now been successfully cooled at atmospheric pressure by anti-Stokes fluorescence by our team include aluminosilicate, aluminofluorosilicate, borophosphosilicate, and aluminosilicate doped with one of three different alkali-earth nanoparticles (Ba, Sr, and Ca). By fitting the measured temperature dependence of the cooled fiber on pump power, two key parameters that control the degree of cooling are inferred, namely the critical quenching concentration and the absorptive loss due to impurities. The inferred values compiled for the fibers that cooled indicate that the extracted heat is highest when the Yb concentration is 2 wt.% or more (to maximize heat extraction), the Al concentration is ~0.8 wt.% or greater (to reduce quenching), and the absorptive loss is below approximately 15 dB/km, and ideally below 5 dB/km (to minimize heating due to pump absorption). Only two of the reported fibers, an LaF3-doped and an LuF3-doped nanoparticle fiber, did not cool, because their Yb and Al concentrations were not sufficiently high. This analysis shows that through careful composition control (especially the Al and Yb concentrations) and minimization of the OH contamination, a new generation of Yb-doped silica fibers is emerging with higher Yb concentrations, greater resistance to quenching, and lower residual loss than commercial Yb-doped fibers. They can be expected to have a significant impact not only on optically cooled devices but also on a much broader range of fiber lasers and amplifiers.
Nonlinearities are a persistent obstacle to power scaling in fiber lasers. Approaches to their management are often highly complex and difficult to implement. They can even introduce new problems that give rise to tradeoffs in system design. As a result, there is a continued need for complementary ways to eliminating them. This talk will focus on materials-based approaches to managing nonlinearities in high power fiber systems. The role of novel multicomponent glasses in scaling high power fiber systems will be envisaged.
Power demands from high beam quality fiber-based lasers have reached levels where nominally weak optical nonlinearities now limit continued scalability. Amongst the parasitic nonlinearities, transverse mode instability (TMI) is especially problematic because it is the dominant scaling limitation. To manage optical nonlinearities, the fiber laser community has singularly focused on large mode area (LMA) designs to spread the optical power out over a larger cross-sectional area and reduce the effective intensity to increase nonlinear thresholds. Such LMA designs are necessarily multimode and, so, TMI, while not necessarily predictable, was not surprising in hindsight. This paper will focus on an alternative and complementary approach; one where nonlinearities are managed materially through understanding and judicious design of the glass compositions from which the fiber is comprised. Indeed, optical nonlinearities are fundamentally light-matter interactions and so attacking them through the ‘matter’ component is the purest approach. A further benefit of a materials approach to mitigating nonlinearities is that multiple nonlinearities, e.g., SBS and TMI, can be simultaneously reduced while permitting a much simpler fiber design, which aids in manufacturability and cost. In other words, discussed here, is a materials approach to larger core, simple step-index fibers bypassing TMI. This paper highlights several material approaches to specifically mitigating TMI including < 1% quantum defect fiber laser compositions, power-scaling in intrinsically low thermo-optic core fibers, and novel fullypassive “thermally self-single moding” LMA fibers that intrinsically become single moded as the fiber lases and reaches its operating power and temperature.
Thermal processes are key limiting factors to power scaling in high-power fiber lasers. Heat generation can arise from the quantum defect, partly manageable through judicious selection of pumping and lasing wavelengths. Heat can also result from nonradiative processes, presently believed to be the main obstacle to efficient anti-Stokes fluorescence cooling in silicates. Therefore, it is meaningful and necessary to evaluate the quantum conversion efficiency (QCE), which is defined to be the fraction of pump photons that undergo the desired radiative process. Here, an accurate and sensitive Brillouin-based method to characterize the QCE in Yb-doped optical fiber is proposed and evaluated.
An optical fiber-based microheater is described. The fiber, a highly Yb-doped (23.4wt% Yb2O3) silicate glass, can produce thermal power densities in excess of 10 W/nL via optical pumping at 976nm. No evidence of luminescence is observed, indicating efficient conversion from optical to thermal energy. Demonstrated are two applications for this microheater. The first is an all-optical-fiber Pirani thermal vacuum gauge, which uses a dual-fiber configuration. The second is an all-optically-driven, all-optical-fiber, Mach-Zehnder-based modulator. The phase delay, introduced by inserting the microheater into one interferometer arm, is a function of its temperature and can be actively controlled by the pump power.
Anti-Stokes fluorescence cooling in a silica-based fiber is reported for the first time. The fiber had a core with a 20-μm diameter doped with 2.06 wt.% Yb and co-doped with 0.86 wt.% Al and 0.88 wt.% F. Core-pumping the fiber with 1040- nm light, temperature changes as large at -50 mK were measured at atmospheric pressure. Temperature measurements were performed at 12 pump wavelengths, and the measured dependence of the temperature change as a function of pump wavelength was in excellent agreement with a previously reported model. With this model, the absorptive loss in the fiber was inferred to be less than 15 dB/km, and the critical quenching concentration to be ~15.6 wt.% Yb. This combination of low loss and high quenching concentration (a factor of 16 times higher than the highest reported values for Yb-doped silica) is what allowed the observation of cooling. The temperature measurements were performed at atmospheric pressure using a custom slow-light fiber Bragg grating sensor with an improved thermal contact between the test fiber and the FBG. The improved method involves isopropanol to establish a good thermal contact between the two fibers. This eliminated a source of heating and enabled more accurate measurements of the cooled-fiber temperature. This improved temperaturemeasurement set-up also led to a new cooling record in a multimode Yb-doped ZBLAN fiber at atmospheric pressure. When pumped at 1030 nm, the fiber cooled by -3.5 K, a factor of 5.4 times higher than the previous record.
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