We report for the first time cooling by anti-Stokes fluorescence (ASF) of a single-mode fiber, and cooling of a fiber at atmospheric pressure. This demonstration and the ability of our model to accurately predict cooling are crucial steps towards the development of radiation-balanced fiber lasers and were the primary focus of this work. We also experimentally investigated the effects of pump power and wavelength, the core size, and the dopant concentration on ASF cooling, in order to maximize this process. Experiments were performed on two Yb-doped ZBLAN fiber from Le Verre Fluoré: a single-mode fiber doped with 1 mol% Yb and a multimode fiber doped with 3 mol% Yb. The maximum temperature change achieved in the two fibers was -5.2 mK and -0.64 K, respectively, confirming that cooling scales with doped area. However, we also discuss limitations to this scaling, namely the absorptive loss, concentration quenching, and the mode profile of the pump. We use our previously reported model to quantify these scarcely reported parameters. For the multimode fiber, comparison between the experimental data and the model gave an inferred absorptive loss of 45 dB/km and a critical quenching concentration of 3.57x1027 m-3. In addition to these parameters, accurate modeling also requires precise knowledge of the absorption and emission cross-sections. To this end, we propose a method to obtain spectra that obey the McCumber relation and accurately represent the material under investigation. Finally, we report on the cooling efficiencies achieved in the single-mode (2.0%) and multimode (0.85%) fibers and show that the efficiency decreases with increasing pump power due to absorptive loss.
Several silicate fibers doped with 0.5-6 wt.% Yb were evaluated experimentally for optical cooling, namely three nanoparticle-doped fibers (LaF3, BaF2, and YbF3) and three fibers with network modifiers (borophosphosilicate, fluorosilicate, and aluminosilicate). Their performance was compared to that of a commercial Yb-doped silica fiber. Simulations were first carried out to investigate the influence of absorptive loss and concentration quenching on the relationship between temperature change and pump power. This analysis provided a method for inferring the values of these two quantities from the measured dependence of the temperature change on pump power. For fair comparison of the cooling performance of different fibers, we show that the temperature change exhibited by the fibers must be compared at the same pump power absorbed per unit length. Although none of the fibers exhibited negative temperature changes, this metric was used to show that nanoparticles and network modifiers effectively reduce heating and increase the cooling efficiency. The borophosphosilicate and BaF2 nanoparticle fibers performed best, exhibiting 92.7% and 93.9% less heating than the silica fiber. Based on this result, we propose a borophosphosilicate fiber design with a lower Yb concentration and a larger core that is predicted to produce cooling at atmospheric pressure by -12 mK for 100 mW of 1020-nm pump.
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.
Thermodynamic phase noise in passive fiber devices is generally so weak that in most devices, in particular fiber sensors, it has only been observed in fiber lengths in the range of 1 meter or much longer. Here we present a passive fiber strain sensor only 4.5 mm in length in which the noise in the frequency range of 1 kHz to ~12 kHz is limited by thermal phase noise in the fiber. The phase noise could be measured in such a short fiber by utilizing a slow-light fiber Bragg grating (FBG) resonator in which the phase noise is magnified by the resonator's slowing-down factor ng/n ≈ 370, where ng is the group index. At the same time, the usually dominant laser frequency noise was brought below the level of the phase noise by using a short fiber and a low-noise laser with a linewidth under 200 Hz. At 4 kHz, the total measured noise expressed in units of strain is 110 fε/√Hz, and the phase noise accounts for 77% of it. This sensor resolves a single-pass thermodynamic length fluctuation of only 5 x10-16 m/√Hz. These measurements provide experimental support for the dependencies of the phase noise on the fiber resonator length and group index predicted by a recent model.
We report a new generation of slow-light FBG strain sensor with a strain resolution (or minimum detectable strain) as
low as 30 fepsilon/√Hz at 30 kHz, which is one order of magnitude lower than the record held by the previous generation. This
sensor has an ultra-stable output (no drift in 4 days) and is capable of resolving an absolute strain of ~250 attostrains by
integrating its output for ~8 hours, which is also a new record for an FBG fiber sensor. These improvements were
accomplished by first maximizing the slope of the slow-light resonances, and hence the strain sensitivity. To this end the
apodized FBG was written in a deuterium-loaded fiber with a femtosecond infrared laser, then thermally annealed. The
three main sources of noise in the sensor system were also carefully reduced. The dominant source of noise, laser
frequency noise, was reduced by interrogating the FBG with an ultra-stable laser (linewidth under 200 Hz) with a low
intensity noise. The phase noise was minimized by selecting the proper FBG length (~25 mm). When used as an acoustic
sensor, the same grating had a measured average pressure resolution of 50 μPa/√Hz between 3 kHz and 6 kHz, one order
of magnitude lower than the previous lowest reported value for an FBG sensor.
This paper reports the generation of record low group velocities, large group delays, and high optical confinements in strong apodized fiber Bragg gratings (FBGs). The gratings were fabricated in deuterium-loaded fiber using an 806-nm femtosecond laser and a phase mask to produce strong apodized index-modulation profiles and low internal loss, followed by annealing to reduce residual losses. In a first FBG of this type with a length of ~25 mm and a non-saturated index modulation we measured a group delay–transmission product of 10.4 ns, the highest ever reported. In a stronger, shorter FBG (12.3 mm in length), a group delay of 42 ns was observed, corresponding to a group velocity of only ~290 km/s and a group index of 1020. In a still shorter and therefore lower loss device (~5 mm) we were able to observe the fundamental mode, and infer a Purcell factor as high as 25.5. These exceptional features are made possible in part by the gratings’ strong index modulation (~2x10-3) and ultra-low single-pass loss (~0.01–0.015 dB/cm).
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