Radiation-balanced lasers (RBLs) is an innovation that merges the dual roles of pump light to serve both as the gain medium for lasing and as a cooling mechanism to regulate the device’s temperature. This unique characteristic eliminates the thermal distortions and lensing often associated with traditional laser systems, paving the way for stable, high-power laser operation. The concept of RBLs has generated interest due to the potential for high efficiency and low heat generation. Such lasers could have applications in various fields, including telecommunications, materials processing, and scientific research. One of the central challenges in developing RBLs lies in the efficiency of cooling using the anti-Stokes fluorescence method. This cooling process can be hindered by the presence of background impurities that absorb the pump light but do not contribute to the refrigeration process. Researchers have explored various methods to address this challenge, including crystal growth using highly purified ingredients or leveraging intense pump light to saturate background absorption. In the pursuit of developing efficient RBLs, one crucial factor is the level of background absorption in the gain medium. In relatively impure materials, such as Yb3+:KYW, it was initially challenging to achieve radiation balance until the concept of saturation was taken into account. This realization marked a turning point in the quest for RBLs, as it expanded the range of materials suitable for this technology.
In a new study published in the Journal Optics Express by PhD candidate Long Cheng, Dr. Laura Andre, and led by Professor Stephen Rand from the University of Michigan, and Daniel Rytz from FEE GmbH explores RBLs and their potential to advance laser technology. The research team provided essential experimental insights into the world of RBLs. In their work, they present two distinct RBLs, each highlighting the significance of background absorption saturation in the analysis. The first RBL employed a relatively pure Yb3+:YAG crystal, serving as a benchmark for comparing theoretical predictions with experimental measurements. The agreement between the two was remarkably close, validating the model’s accuracy. Thermal imagery was used to confirm the achievement of radiation balance, with temperature variations within the gain medium being minimal. The second RBL experiment conducted involved a relatively impure Yb3+:KYW crystal, a material not previously associated with RBL operation. This experiment was a testament to the importance of saturation in achieving RBL functionality. Despite the challenges posed by high background impurity absorption, the authors demonstrated that saturation could make RBLs viable in materials previously deemed unsuitable. The theoretical advantages of Yb3+:KYW as an RBL medium were also highlighted, suggesting the potential for high-efficiency RBLs with further improvements in crystal quality, optical coatings, and polishing.
To provide a more detailed understanding of the experimental work, the research team involved two distinct types of crystals, 3% Yb3+:YAG and 2% Yb3+:KYW. The Yb3+:YAG crystal, with known impurity characteristics, was inserted into a laser cavity with high-quality mirrors. The crystal was thermally isolated using an aerogel disk, and all measurements were conducted at room temperature. The RBL experiment with the Yb3+:YAG crystal reached radiation balance at an input power of 1.8 W, with temperature variations well below 0.1 K along the gain medium. In the case of Yb3+:KYW, the high background impurity absorption presented additional challenges. The crystal was carefully selected, and the experimental setup included specialized optics. Despite these challenges, the 2% Yb3+:KYW crystal demonstrated radiation-balanced laser action, reaching radiation balance at an input power of 1.92 W. The larger temperature gradient observed in Yb3+:KYW compared to Yb3+:YAG was attributed to differences in thermal conductivity.
According to the authors, the introduction of radiation-balanced laser operation in 2% Yb3+:KYW, a material previously unexplored in the context of RBLs, expands the horizons of this technology. The demonstration of RBL functionality in relatively impure materials, with the help of saturation, challenges preconceived limitations and opens the door to more versatile applications of RBLs. The theoretical advantages of Yb3+:KYW for RBL operation further highlight the potential for high-efficiency RBLs, with room for improvement through crystal quality enhancements, advanced optical coatings, and precise polishing. In conclusion, the study by Professor Stephen Rand and colleagues underscores the importance of understanding and addressing background absorption and saturation effects in the development of radiation-balanced lasers. The successful demonstration of RBL operation in previously unexplored materials paves the way for further innovations in this domain. With ongoing advancements in crystal growth techniques and optical technologies, the future holds great promise for the development of high-efficiency radiation-balanced lasers.
Cheng L, Andre LB, Rytz D, Rand SC. Radiation-balanced lasing in Yb3+:YAG and Yb3+:KYW. Opt Express. 2023 ;31(7):11994-12004. doi: 10.1364/OE.486469.