The first self-cooling laser made from quartz fibers
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2021-03-24
Solid-state optical cooling of liquids and solids can be achieved through a process known as anti-Stokes fluorescence. When these materials are optically excited within an appropriate wavelength range, they emit fluorescence whose average energy exceeds that of the pump photons, enabling energy to be extracted from the material in the form of phonons—thus leading to cooling without significant heat dissipation. This effect is primarily observed by doping trivalent rare-earth elements into carefully selected crystalline or amorphous host matrices, whose compositions are engineered to minimize detrimental heat-generation mechanisms that would otherwise overshadow the ASF cooling effect.
Using the anti-Stokes fluorescence mechanism, the first solid-state cooling system achieving double-digit cooling performance—reaching temperatures as low as 91K—has been demonstrated in highly Yb³⁺-doped, purified YLiF₄ crystals. ASF cooling is particularly beneficial for high-power fiber-based lasers and amplifiers. Traditionally, external coolers are employed to dissipate heat from these devices by circulating air or water around the laser gain medium. However, such coolers often introduce thermal gradients and vibrations within the laser, leading to increased noise levels and degrading both the temporal coherence and spatial quality of the laser output beam. Alternatively, thermoelectric coolers based on the Peltier effect are commonly used—but while they are typically vibration-free—they can still cause asymmetrical cooling of the optical fiber, resulting in undesirable thermal gradients. In contrast, ASF cooling offers a compelling solution that sidesteps all these challenges altogether. Moreover, optically cooled fibers open up promising applications in cooled detectors, where they help reduce thermal noise; they can also serve as a stable reference cavity for ultra-stable lasers, minimize dark current in spaceborne IR and X-ray sensors, and enable precise point cooling of microelectronic devices.
Figure 1. Recording the cooling results
▲ After about 12 minutes of pumping, the crystal temperature (blue) reached 91°K, while the flip-chip temperature remained at approximately 265°K.
So far, brief studies on ASF cooling in optical fibers have been conducted primarily in tellurite-based glasses, though most demonstrations have been carried out in fluoride hosts. It has been reported that Yb-doped ZBLANP fibers placed under vacuum exhibited a temperature change of -65 K, while recently, Yb-doped ZBLAN fibers tested at ambient pressure showed a temperature shift of -0.65 K. Compared to traditional silica glass, fluoride glasses demonstrate significantly greater resistance to concentration quenching, allowing them to incorporate higher concentrations of non-quenched rare-earth ions. This translates into more ions—acting as "heat engines"—capable of extracting heat per unit volume, thus offering enhanced cooling potential. However, practical applications of fluoride materials remain limited compared to silica. Most notably, fluoride fibers are typically fabricated by melting powders and casting them into bulk glass, from which the fibers are then drawn. As a result, these fibers often contain impurity levels that are about one order of magnitude higher than those achieved with quartz glass produced via chemical vapor deposition. Additionally, fluoride glasses are more brittle and notoriously harder to polish, cut, or splice seamlessly with other fibers. They also exhibit lower optical damage thresholds, making it more challenging to write reflective gratings directly into their structure. Furthermore, their weaker absorption and emission cross-sections pose additional hurdles for efficient laser cooling applications. From a technical standpoint, therefore, developing silica-based compositions capable of enabling laser cooling remains a critical priority.
The quartz fibers evaluated in this work were designed to feature low loss, minimizing absorptive heating, which scales linearly with pump power. Additionally, since cooling efficiency increases proportionally with the number of heat-generating elements, the fibers were also engineered with a high Yb concentration. However, as mentioned earlier, this approach may lead to some undesirable side effects, such as enhanced concentration quenching and reduced non-radiative lifetimes. To mitigate these effects, the silica core was doped with fluorine and aluminum oxide (Al₂O₃).
Figure 2. Experimental setup used to measure temperature changes in Yb-doped quartz fibers. As the temperature varies within the doped fiber and the FBG, the spectral shift induced in the FBG is tuned to match one of the FBG’s resonance peaks, which is probed by a detection laser.
The figure below shows an example of temperature measurements taken on a fiber in air. The pump (solid red curve) remained off for 20 seconds before being suddenly turned on for 40 seconds. The steady-state temperature change is defined as the difference between the average temperature variation during the initial 20 seconds (typically well below 5 mK) and the average temperature variation during the subsequent 20 seconds (represented by the two gray areas). To establish the relationship between temperature change per unit length and the absorbed pump power, as shown in Figure 4, a thermal power meter was used to measure the pump power at the output end of the doped optical fiber. By combining the measured output power with known values for saturation power, small-signal absorption, and the distance between the FBG and the fiber output, it was possible to calculate the pump power absorbed per unit length at the specific temperature-measurement location.
▲ Figure 3. Time trace of temperature changes recorded in a Yb-doped quartz optical fiber pumped at 1040 nm, with 110 mW coupled into the fiber core. The gray shaded area indicates the portion of data averaged to calculate the steady-state temperature variation.
▲ Figure 4. Dependence of fiber temperature on pump power absorbed per unit length at three wavelength measurement points, along with the fit provided by the ASF model.
Since Jennifer Knall of the University of California, along with her collaborators Magnus Engholm from Uppsala University in Sweden, John Ballato from Clemson University, Martin Bernier and Tommy Boilard from Laval University, and Peter Dragic and Yu Nanjie from the University of Illinois at Urbana-Champaign, first achieved this groundbreaking discovery, researchers have since identified two additional self-cooling quartz fiber components. Knall has now used the top-performing candidate material to create a cooling fiber amplifier, enabling her to amplify lasers by more than 40 times while maintaining a consistently negative average temperature change along the length of the fiber. Although cooling tests confirmed that laser cooling is indeed possible in silica, this type of fiber amplifier has proven undeniably practical and highly effective in real-world applications.
Currently, researchers have managed to extract about 4% of the energy injected into the fiber. While this makes it unlikely to use the fiber for high-power applications without first addressing its inherent inefficiency, the team has identified numerous promising opportunities for highly stable lasers in low-power applications—such as ultra-precise metrology or measurement science.
▲ Figure 5. Temperature variation of Fiber 1 as a function of pump power at measurement locations corresponding to three different wavelengths
To establish a new baseline for ASF cooling in silica, the fiber exhibiting the best performance (Fiber 1) was subjected to more comprehensive characterization. Additionally, the temperature dependence of pump power was measured at two other pump wavelengths: 1020 nm and 1040 nm. Since these parameters were constrained to remain consistent across all pump wavelengths, incorporating this additional data allowed for more accurate determination of absorption losses and the critical quenching concentration. As expected, the fiber heated up when pumped at 1020 nm, while it displayed the strongest cooling effect at 1040 nm (Figure 3). At 1020 nm, although pump absorption was relatively high, the average energy difference between pump and fluorescence photons was smaller, leading to insufficient energy extraction—primarily due to concentration quenching and increased absorption losses—to counteract the heating. In contrast, at 1040 nm, the trade-off between pump absorption and the energy gap between pump and fluorescence photons approached an optimal balance. At the measurement point, with a pump power of 80 mW, the fiber achieved cooling down to -70 mK (corresponding to an absorbed pump power of 170 mW/m). This represents a heat-extraction rate per unit length that is twice as efficient as previously reported in the literature.
▲ Graduate student Jenny Knall stands next to the experimental setup for self-cooling fibers. The computer displays the measured fiber temperature changing over time—showing a noticeable drop as soon as Knall turns on the laser pump.
"Digonnet said, 'The extent to which we can adopt this technology depends on how far researchers can push the boundaries of materials science. And this is just the tip of the iceberg.'"
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