In an article recently published in Light | Science & Applications, researchers from Beihang University in Beijing, China, introduced an innovative method to improve the efficiency and compactness of laser-sustained plasma (LSP) sources. They developed an orthogonal LSP design that increases conversion efficiency and brightness, making it a promising choice for high-speed inspection and spectroscopy.
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Laser-Sustained Plasma Technology
LSP sources are known for their high brightness and broad spectral range. They are used in various scientific and industrial applications. These sources generate plasma by focusing laser beams on high-pressure noble gases, like xenon or argon, sustaining the plasma without electrical current.
However, traditional LSP systems have low conversion efficiencies, typically below 10 %, due to the plasma's negative lensing effect, which deflects the laser and lowers power density. This inefficiency limits the compactness and utility of LSP systems, creating a need for more efficient designs.
Innovative Orthogonal LSP Design
The research addressed the limitations of conventional LSP sources by introducing an orthogonal LSP design to counter the negative lensing effect and improve conversion efficiency. The design uses orthogonal laser paths to reduce deflection in the laser transmission and increase power density within the plasma.
The precision tuning of the off-focal crossing point optimizes beam overlap and plasma absorption to create a clear connection between the pumping optics' design and plasma performance. This configuration is expected to boost efficiency and brightness at relatively low laser power, making it ideal for compact systems.
Experimental Setup and Methodology
The researchers built an experimental setup to test their orthogonal LSP design. A 1080 nm continuous laser was split into two beams using a 50/50 beam splitter. These beams were focused orthogonally at the center of a xenon lamp, with the focal points separated to create a symmetrical plasma.
The plasma emission spectrum was measured with a spectrometer, and its power was recorded using a power meter. A two-dimensional refractive index model was also used to analyze the laser's path within the plasma.
Key Findings and Insights
The results demonstrated that the orthogonal LSP design significantly enhanced both conversion efficiency and brightness. Using a 90 W laser, the orthogonal LSP achieved a spectral radiance of 210 mW/(mm²·sr·nm) in the ultraviolet (UV) range (300-370 nm), the highest UV spectral radiance recorded for LSP sources to date.
Conversion efficiency from absorbed laser power to UV emission exceeded 20 %, while total efficiency from laser power to UV emission surpassed 15 %. These improvements were attributed to the reduction of the negative lensing effect, which increased power density and enhanced plasma absorption.
The orthogonal design also shortened the plasma length and reduced its cross-sectional area compared to single-path LSPs, further mitigating the negative lensing effect. This led to higher plasma temperatures and greater efficiency, with the electron temperature of the orthogonal LSP being around 1000 K higher than in the single-path design, boosting power absorption and UV emission.
Applications
The improved efficiency and brightness of the orthogonal LSP make it useful for high-speed inspection and spectroscopy. The authors showed substantial improvements in spectral single-pixel imaging, with a 4 dB increase in contrast-to-noise ratio (CNR) compared to xenon lamps of the same power. The high brightness and low temporal coherence of the orthogonal LSP are particularly beneficial for imaging systems, reducing interference fringes and speckles.
This efficient design could significantly improve throughput in semiconductor processing, reduce integration times in spectral imaging, and enhance performance in broadband detector calibration, biomolecular monitoring, and nano spectroscopy. The orthogonal LSP’s high brightness and low power consumption also make it a strong option for sensor calibration, metrology, microscopy, and imaging applications.
Conclusion and Future Scope
The novel orthogonal LSP design represents a significant step forward in laser-sustained plasma technology. By overcoming the limitations of traditional LSPs, the researchers achieved unprecedented efficiency and brightness, making the orthogonal design a promising option for many scientific and industrial uses. The findings emphasized optimizing laser power density and plasma absorption to enhance LSP performance.
Future work should further improve the brightness and efficiency by exploring higher-pressure chambers and advanced beam-shaping techniques. Additionally, the development of compact, air-cooled fiber lasers could enhance the practicality and integration of orthogonal LSP sources across various applications.
Overall, this design holds significant potential for advancing high-speed inspection, spectroscopy, and other fields that require bright, efficient broadband light sources.
Discover More: Advanced Optical Emission Spectroscopy in Plasma Systems
Journal Reference
Shi, Z., et al. (2024). Bright compact ultrabroadband source by orthogonal laser-sustained plasma. Light Sci Appl. DOI: 10.1038/s41377-024-01602-2, https://www.nature.com/articles/s41377-024-01602-2
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