This paper reports a second generation of radiation-balanced fiber laser and amplifier cooled internally using anti-Stokes fluorescence by pumping them at 1040 nm. In both devices the gain medium is a single-mode silica fiber with a core heavily doped with Yb3+, initially encapsulated in CaF2 nanoparticles, and co-doped with Al to reduce quenching and increase the cooling efficiency. After optimization of its length (4.1 m) and its output coupler reflectivity (3.3%), the 1065- nm continuous-wave fiber laser has a threshold of 160 mW and a radiation-balanced (no net heat generation) output power of 192 mW, or nearly 70% higher than the previous radiation-balanced fiber laser. At its radiation-balanced point, its optical efficiency is 56.8%. The single-frequency, single-mode fiber amplifier, constructed with the same fiber, was optimum with a length of 6.8 m, and it had a radiation-balanced gain of 20 dB: it amplified an 800-μW signal to 84.2 mW with 433 mW of input pump power. The significance of this result is underscored by the small diameter of the single-mode fiber core (7.8 μm), which makes cooling more challenging. This study further demonstrates the viability of achieving substantial gain and energy extraction in a small-core Yb-doped silica fiber while effectively utilizing anti-Stokes fluorescence to keep it cool.
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.
We propose a novel femtosecond laser writing approach allowing the fabrication of arbitrary long optical waveguides in coreless optical fibers, directly through the coating. In a silica fiber, we demonstrate a 4m-long single-mode waveguide operating at 1 µm. The refractive index distribution and the spatial mode are characterized. Using a cutback method, the propagation losses of the laser-inscribed waveguides are properly measured. This approach paves the way toward the fabrication of complex arrangement of cores in various optical fibers.
The recent reports of laser cooling in Yb-doped aluminosilicate fibers and silica preforms have opened up the field of optical refrigeration and radiation-balanced lasers to the enormous realm of silica fiber lasers and amplifiers. To increase the cooling efficiency achieved in these materials, it is critical to identify host compositions that improve the Yb3+-ion properties in the directions of low concentration quenching, short radiative lifetime, and a long-wavelength absorption tail that extends as far as possible above the zero-phonon line. In this on-going quest, nanoparticle-doped fibers offer a promising technique to modify the chemical environment of the Yb3+ ions and achieve some of these properties. In this work, three fibers in which the Yb3+ ions are initially encapsulated in CaF2, SrF2, or BaF2 nanoparticles were fabricated using a solution-doping technique, and their laser-cooling properties evaluated experimentally and analyzed. The CaF2 fiber and the SrF2 fiber were successfully cooled at atmospheric pressure when pumped with a continuous-wave laser at the near-optimum wavelength of 1040 nm. The measured maximum temperature change from room temperature was -26.2 mK for the CaF2 fiber at a pump power absorption level of 90 mW/m, and -16.7 mK at 66 mW/m for the SrF2 fiber. The BaF2 fiber did not cool, but it warmed only slightly, indicating that it was not far from cooling. Analysis of the measured dependence of the fiber temperature change on pump power with a model enabled extraction of the fiber’s critical quenching concentration and residual absorptive loss due to impurities. Comparison of these values to the values reported for an aluminosilicate fiber and fiber preforms that cooled shows that the CaF2 and SrF2 fibers faired as well as the fiber, and better than the preforms, in terms of quenching, but that they had a higher absorptive loss. This study establishes the significant research potential of nanoparticle-doped fibers in the search for efficient laser-cooling silica hosts.
This paper reports the first experimental observation of anti-Stokes cooling of fibers in which both the core and the cladding doped with Yb3+ to increase to number of Yb ions contributing to cooling and induce greater refrigeration. Two ZBLAN fibers were designed, fabricated by Le Verre Fluoré, and evaluated experimentally. Two cladding profiles were tested, both with asymmetric boundaries to induce greater mode mixing, and therefore better pump filling of the fiber and greater cooling. Temperature measurements showed that the fiber with a double-D cladding did not perform as well (it cooled to –78 mK for 240 mW of input pump power at 1025.5 nm) largely due to limited mode mixing. The octagonal cladding profile of the second fiber produced greater cooling, down to –1.3 K with 3 W. Fitting experimental results to a model showed good agreement with theory, and confirmed the high critical quenching concentration (Nc = 3.2x1027 Yb/m3), low absorptive background loss (40 dB/km), and good filling ratio (~38%) achieved in this second fiber. This study establishes that with straightforward improvement in mode filling, a cladding-pumped ZBLAN fiber can readily be cooled to ~10 K below room temperature at atmospheric pressure with only ~15 W of pump power.
We will report the first demonstration of an optically cooled fiber amplifier. The fiber was made of silica, had a core doped with Yb, and was core-pumped to achieve both gain and cooling in the core via anti-Stokes fluorescence. Gains larger than 10 dB were measured while maintaining a negative average temperature change along the fiber.
Fiber Bragg gratings (FBGs) with strong apodized index modulations behave like an in-line Fabry-Perot interferometer and exhibit a series of narrow resonances in the short-wavelength portion of their transmission spectrum. These resonances have proven invaluable for detecting extremely small strains (30-femtostrain/√Hz level) or temperature changes (millidegreeC/√Hz level). The sensitivity of these fiber sensors is limited by the linewidth and peak transmission of the resonance used to interrogate the sensor, which are themselves limited by the intrinsic loss of the grating. In this work, significantly narrower and stronger resonances are demonstrated by introducing a small amount of optical gain in the FBG to offset the intrinsic loss and create a resonator with a much smaller net internal loss. The fiber Bragg grating is written in an Er-doped single-mode fiber and optically pumped to provide the required gain. The device reported here is a 6.5-mm grating with an AC index modulation of 1.59×10-3. With only 30 μW of pump power absorbed by the grating (32.6 mW launched), the fundamental resonance of the FBG was observed to narrow from 737 fm in the absence of pump to a record linewidth of 8.5 fm. The measured peak transmission of the resonance improved from ~-37 dB to -0.2 dB. A new model that predicts the slow-light resonance spectrum of a slow-light grating in the presence of optical gain is presented. This model is in good quantitative agreement with the measured evolution of the resonance linewidth as the pump power and the power of the laser that probes the resonance lineshape are varied.
The experimental study of cooling by anti-Stokes fluorescence in a fiber or a radiation-balanced fiber laser necessitates the development of a sensor that can measure the temperature of the fiber core with an excellent temperature and spatial resolution, a large dynamic range, a small drift, a fast response, and a low absorptive loss. We report an in-situ slow-light fiber sensor written directly in a Yb-doped silica fiber using a femtosecond laser. The sensor has a spatial resolution of 6.5 mm, an excellent measured temperature resolution of 0.9 m°C/√Hz, and a measured drift as low as 20 m°C/min. One of the grating’s slow-light resonances is interrogated with a tunable 1.55-μm laser to measure the temperature-induced shift in the resonance wavelength when the fiber is optically pumped. The laser frequency is also modulated at 30 kHz to greatly reduce the detection noise. The sensor was pumped with 0.58 mW from a 1020-nm laser and measured a positive temperature change of 0.33 °C. The dominant source of heating is shown to be likely the photodarkening loss induced in the Yb-doped fiber when the FBG was written. The total FBG loss is predicted to be ~24 m-1 at 1020 nm and expected to reduce after annealing. Projections indicate that if the loss of the rare-earth doped FBG can be decreased to the level of the loss observed in slow-light FBGs written in SMF-28 fibers, these sensors can be used to measure ASF cooling.
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