Deep ultraviolet dual-comb from a thin-disk laser

Thin-disk single-cavity dual-comb lasers (TDSCs) offer a promising alternative to traditional dual-comb systems, simplifying the setup and enabling power scaling. Their shared resonator components ensure high coherence between the two mode-locked pulses, a key advantage over traditional approaches. While current dual-comb systems predominantly operate in the visible, near-infrared, and mid-infrared ranges, exploration of the ultraviolet (UV) and deep ultraviolet (DUV) regimes has been lacking.  Recently, the research group led by Prof. Hongwen Xuan at the GBA branch of the Aerospace Information Research Institute, collaborating with Prof. Jinwei Zhang’s group in Huazhong University of Science and Technology, achieved the first DUV dual-comb using a high-power TDSC with frequency conversion. This work, demonstrating the application of a TDSC Yb:YAG laser for precise ranging, highlights the potential of this platform for future development of extreme ultraviolet (XUV) and terahertz dual-comb systems.

Overview of the study

Optical frequency combs provide a direct, measurable link between optical and microwave frequencies, bridging the gap between these two domains and revolutionizing fields like timekeeping and spectroscopy. Dual-comb systems, utilizing two optical frequency combs with slightly different repetition rates, have become a powerful tool for high-resolution sensing, precise ranging, and other applications.  By employing fast optical heterodyne techniques, they offer significant advantages over traditional single-comb methods, including increased measurement speed, larger unambiguous range, and relaxed detector bandwidth requirements. The TDSC simplifies dual-comb setups by eliminating the need for complex locking electronics and offers high output power, facilitating efficient direct frequency conversion to the DUV with similarly high power.

Noise performance is a crucial metric for evaluating optical frequency combs.  The relative intensity noise (RIN) of the free-running TDSC was measured. Figures 3(a) and (b) show the RIN characteristics of Laser A and Laser B, respectively. The integrated RIN for both beams was approximately 0.6% (integrated from 1 Hz to 100 MHz).  Figure 3(c) depicts the jitter when the dual-comb repetition rate difference was set to 3 kHz, demonstrating a standard deviation of 0.634 Hz over a 30-minute period.

Frequency conversion employs LBO and BBO crystals to convert infrared dual-comb to DUV at 258 nm with an output power exceeding 340 mW for each beam.  Beat operation was performed on the DUV dual-comb, yielding the beat signal and the interference beat signal, as shown in Figures 4(b) and (c).  This compact and high-power DUV dual-comb offers a promising light source for applications such as time-resolved transient plasma research.

This technology holds promise for extension to other spectral regions, including the extreme ultraviolet (XUV) and terahertz (THz).  As a prospect, Figure 5(a) illustrates a potential pathway for XUV dual-comb generation using the TDSC platform.  The process involves boosting the dual-comb power via a chirped-pulse amplifier (CPA), compressing the pulses, and subsequently generating XUV radiation through high harmonic generation (HHG) within an external enhancement cavity.  Analogously, THz dual-combs can be realized by amplifying and compressing the TDSC dual-comb output, followed by frequency conversion using nonlinear crystals such as LiNbO3 or GaP.

Summary

This research presents the first demonstration of a DUV dual-comb system based on a TDSC, providing a valuable and simplified light source for DUV spectroscopy, including applications in time-resolved transient plasma research.  Furthermore, the team is exploring the potential of this TDSC platform for generating XUV or terahertz dual-combs, which could significantly impact fields such as precision spectroscopy, quantum metrology, and magnetic confinement fusion.