Intense ultraviolet-visible-infrared full-spectrum laser

Optical spectroscopy spanning the ultraviolet (UV), visible (Vis), and infrared (IR) regions is crucial for probing microscopic physical, chemical, and biological phenomena. Traditional methods rely on multiple individual coherent light sources, leading to complexity and limitations. Broadband light sources covering multiple absorption bands are essential for simultaneously resolving dynamic processes in various materials. While laser technology and nonlinear optics have advanced spectral coverage, generating a high-brightness UV-Vis-IR full-spectrum white laser remains a challenge. Two primary approaches exist for supercontinuum white laser generation: one based on third-order optical nonlinearities (3rd-NL) like self-phase modulation (SPM) in various media, and another employing second-order nonlinearities (2nd-NL) such as high harmonic generation (HHG) in nonlinear crystals. The former often suffers from limitations in spectral quality (bandwidth, flatness, and energy), while the latter faces challenges in spectral and power scaling due to factors like limited pump bandwidth and conversion efficiency. This research aims to overcome these limitations by combining both 2nd-NL and 3rd-NL effects in a novel cascaded system.

Previous research on supercontinuum generation has explored various methods. Third-order nonlinearity-based approaches utilizing microstructured optical fibers, hollow-core fibers, and multiple thin plates have shown promise but face limitations in achieving high pulse energy and spectral flatness. Second-order nonlinearity-based methods using various harmonic generation techniques in nonlinear crystals have also been investigated. However, these methods are limited by the narrow pump bandwidth and the limited QPM working bandwidth, leading to poor spectral and power scaling. The current work aims to leverage the advantages of both approaches by combining them in a synergistic way.

This study demonstrates an intense, four-octave-spanning UV-Vis-IR full-spectrum laser source (300 nm to 5000 nm at −25 dB from the peak) using a cascaded HCF-LN-CPPLN optical module pumped by an intense mid-IR femtosecond pulse laser. The system integrates both 2nd-NL and 3rd-NL effects. The process begins with a 3.3 mJ, 3.9 µm pump laser sent through a gas-filled hollow-core fiber (HCF) and a bare lithium niobate (LN) crystal. This creates a one-octave-wide mid-IR supercontinuum (2500–5000 nm) with 1.15 mJ pulse energy via SPM. This broadened mid-IR pulse then pumps a specially designed chirped periodically poled lithium niobate (CPPLN) crystal. The CPPLN crystal is engineered to exhibit multiple broadband reciprocal-lattice vector (RLV) bands, enabling simultaneous 2nd–10th HHG. The design of the CPPLN crystal is crucial; it features six continuous RLV bands with varying Fourier coefficients, facilitating efficient broadband QPM interactions across a wide spectral range. The synergetic action of 2nd-NL HHG and 3rd-NL SPM effects within the CPPLN broadens the spectrum further, leading to a flat and smooth multi-octave supercontinuum. The experimental setup includes a home-built 3.3 mJ, 3.9 µm optical parametric chirped pulse amplification (OPCPA) system as the pump source. The output from the HCF-LN module is then directed into the CPPLN crystal. The resulting full-spectrum laser is characterized using optical spectrum analyzers to measure the spectral characteristics of the laser output at each stage (HCF, LN, and CPPLN).

The researchers successfully generated an intense full-spectrum laser spanning 300–5000 nm with a 25 dB bandwidth, achieving a pulse energy of 0.54 mJ. The cascaded HCF-LN-CPPLN architecture proved essential for this achievement. The HCF and LN stages broadened the initial mid-IR pump laser into a one-octave supercontinuum, providing a suitable pump source for the CPPLN crystal. The CPPLN crystal, with its specifically designed poling structure, efficiently generated high-order harmonics through QPM, with the synergetic effects of 2nd-NL and 3rd-NL processes leading to the final ultrabroadband spectrum. The spectral characteristics were thoroughly analyzed at each stage. The HCF stage significantly broadened the spectrum through SPM, while the LN stage further improved its flatness and balance. The final output from the CPPLN crystal displayed remarkable spectral flatness and smoothness across the entire UV-Vis-IR range. The high pulse energy (0.54 mJ) of the output laser is a significant improvement over previous supercontinuum sources, which usually yield much lower pulse energies, particularly those using photonic optical fibers. The successful generation of a 4-octave spanning, flat, and high-energy full-spectrum laser represents a significant advancement in laser technology. The measured spectrum extends to at least 5000 nm, but the possibility of even longer wavelengths is suggested, limited only by the capabilities of the available spectrometers.

The results demonstrate the successful implementation of a novel cascaded HCF-LN-CPPLN system for generating an intense, ultrabroadband, full-spectrum laser. This system overcomes the limitations of previous approaches by combining the advantages of both 3rd-order and 2nd-order nonlinear processes in a synergistic manner. The high pulse energy and spectral flatness of the output laser are particularly significant and open up new possibilities for various applications. The flatness of the spectrum is crucial for applications in spectroscopy, as it ensures uniform excitation across a wide range of wavelengths. This approach offers a promising route for creating high-performance, full-spectrum lasers, which are increasingly sought after in diverse fields. Further research could focus on optimizing the design of the CPPLN crystal to extend the spectral range even further and enhancing the output energy, pushing the boundaries of what's achievable with current technology.

This research successfully demonstrates the generation of an intense, four-octave spanning UV-Vis-IR full-spectrum femtosecond laser source using a novel cascaded HCF-LN-CPPLN architecture. The system leverages the synergistic effects of 2nd-order and 3rd-order nonlinearities to achieve a remarkably flat and smooth spectrum with high pulse energy (0.54 mJ). This breakthrough technology opens exciting new avenues in ultrafast and full-spectrum optical spectroscopy and promises diverse applications across various scientific and technological domains. Future work may focus on further optimization of the system for even broader spectral coverage and higher energy output.

The current study is limited by the spectral range of the available detection equipment, preventing a complete characterization of the full spectral extent of the generated laser light beyond 5000 nm. Further investigation is necessary to accurately determine the exact upper limit of the output spectrum. Additionally, the long-term stability and reproducibility of the laser source require further evaluation to ensure reliable performance in practical applications.