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Temporal Beam Self-Cleaning in Second-Harmonic Generation

Siyu Chen, Jun Ye, Lei Du, Wenwen Cheng, Jiangming Xu, Rongtao Su, Pu Zhou, Zongfu Jiang

TL;DR

The paper addresses temporal instability in laser sources and demonstrates that second-harmonic generation can induce temporal self-cleaning, stabilizing the fundamental wave and producing a more coherent residual wave across single, dual, and multi-longitudinal-mode regimes. Using a fiber-ring seed laser and a 25 mm PPLN SHG stage, the study combines detailed temporal statistics, autocorrelation analysis, and RF spectra with numerical SHG simulations to reveal the mechanism. The main finding is that SHG preferentially attenuates high-peak pulses while preserving phase, leading to reduced fluctuations (lower Std and PV) and stronger temporal correlations in the residual wave, most notably in MLM. This SHG-based approach offers a new route to high-stability, spectrally clean laser sources with potential benefits for coherent beam combining and precision metrology.

Abstract

The spatial-temporal beam quality of laser sources is crucial for applications such as nonlinear spectroscopy and master oscillator power amplification systems. However, the temporal stability remains challenged by issues like line-width broadening and high-power demand in efforts to improve it. In this work, we investigate the effect of the second-harmonic generation process on the laser characteristics under three longitudinal mode regimes: single-longitudinal-mode, dual-longitudinal-mode, and multi-longitudinal-mode. The results demonstrate that the second-harmonic generation process effectively stabilizes the temporal characteristics of the laser and enhances its correlation, leading to a temporally clean output beam. The physical mechanism of the observed temporal stabilization effect can be attributed to a high-peak-pulse attenuation effect, jointly induced by nonuniform longitudinal-mode depletion and phase preservation in the residual fundamental wave. Statistical analysis indicates that at the maximum fundamental-wave power in the multi-longitudinal-mode regime, the standard deviation and peak-to-valley values derived from the normalized temporal profile decrease from 0.6122 and 5.6846 for the fundamental wave to 0.189 and 0.8847 for the residual fundamental wave. Meanwhile, the background level of the intensity auto-correlation function rises from ~0.72 to ~0.96, revealing its evolution toward a more coherent state. To the best of our knowledge, this research presents the first demonstration of laser temporal stabilization and correlation enhancement via second-harmonic generation. It not only deepens the comprehension of second-harmonic generation mechanisms, but also opens up a new avenue for realizing temporal beam self-cleaning of light.

Temporal Beam Self-Cleaning in Second-Harmonic Generation

TL;DR

The paper addresses temporal instability in laser sources and demonstrates that second-harmonic generation can induce temporal self-cleaning, stabilizing the fundamental wave and producing a more coherent residual wave across single, dual, and multi-longitudinal-mode regimes. Using a fiber-ring seed laser and a 25 mm PPLN SHG stage, the study combines detailed temporal statistics, autocorrelation analysis, and RF spectra with numerical SHG simulations to reveal the mechanism. The main finding is that SHG preferentially attenuates high-peak pulses while preserving phase, leading to reduced fluctuations (lower Std and PV) and stronger temporal correlations in the residual wave, most notably in MLM. This SHG-based approach offers a new route to high-stability, spectrally clean laser sources with potential benefits for coherent beam combining and precision metrology.

Abstract

The spatial-temporal beam quality of laser sources is crucial for applications such as nonlinear spectroscopy and master oscillator power amplification systems. However, the temporal stability remains challenged by issues like line-width broadening and high-power demand in efforts to improve it. In this work, we investigate the effect of the second-harmonic generation process on the laser characteristics under three longitudinal mode regimes: single-longitudinal-mode, dual-longitudinal-mode, and multi-longitudinal-mode. The results demonstrate that the second-harmonic generation process effectively stabilizes the temporal characteristics of the laser and enhances its correlation, leading to a temporally clean output beam. The physical mechanism of the observed temporal stabilization effect can be attributed to a high-peak-pulse attenuation effect, jointly induced by nonuniform longitudinal-mode depletion and phase preservation in the residual fundamental wave. Statistical analysis indicates that at the maximum fundamental-wave power in the multi-longitudinal-mode regime, the standard deviation and peak-to-valley values derived from the normalized temporal profile decrease from 0.6122 and 5.6846 for the fundamental wave to 0.189 and 0.8847 for the residual fundamental wave. Meanwhile, the background level of the intensity auto-correlation function rises from ~0.72 to ~0.96, revealing its evolution toward a more coherent state. To the best of our knowledge, this research presents the first demonstration of laser temporal stabilization and correlation enhancement via second-harmonic generation. It not only deepens the comprehension of second-harmonic generation mechanisms, but also opens up a new avenue for realizing temporal beam self-cleaning of light.
Paper Structure (4 sections, 2 equations, 5 figures)

This paper contains 4 sections, 2 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Experimental setup
  3. Results and discussion
  4. Conclusions

Figures (5)

  • Figure 1: (a) Experimental setup for the impact of SHG process on laser’s characteristics. LD, laser diode; WDM, wavelength division multiplexer; YDF, Yb-doped fiber; CIR, circulator; SA, saturable absorber; FBG, fiber Bragg grating; ISO, isolator; OC, optical coupler; PPLN, periodically poled lithium niobate; PD, photodetector. Schematic diagrams of the normalized temporal profiles for the (b) residual FW and the (c) FW. (d) Variation of residual FW power with FW power.
  • Figure 2: Characteristics of FW and Res. FW for (a-c) SLM and (d-f) DLM regimes. Normalized temporal profiles of the FW and Res. FW in the range of 200 $\mu$s (left) and 100 ns (right) for (a) SLM and (d) DLM regimes. Standard deviations ($\sigma$) and peak-to-valley ($PV$) values of the FW and Res. FW varies with FW powers for (b) SLM and (e) DLM regimes. RF spectra of the FW and Res. FW for (c) SLM and (f) DLM regimes. The inset of (c) and (f) shows the detailed RF spectra spanning the 0-300 MHz band.
  • Figure 3: Characteristics of FW and Res. FW for MLM regime. (a) Normalized temporal profile of the FW and Res. FW in the range of 200 $\mu$s (left) and 100 ns (right) under different FW powers. (b) Standard deviations and peak-to-valley values of the FW and Res. FW varies with FW powers. (c) RF spectra of the FW and Res. FW. The inset of (c) shows a zoom-in view around the fundamental repetition rate.
  • Figure 4: Impact of SHG process on the statistical characteristics of the laser source. (a) PDFs and (b) intensity ACFs of FW and Res. FW for SLM (left), DLM (middle), and MLM (right) regimes.
  • Figure 5: Simulation results regarding the influence of SHG process on the MLM laser source. (a) Normalized temporal profile of the FW and Res. FW in the range of 4 $\mu$s (left) and 100 ns (right) at the FW power of 5.34 W. (b) Standard deviations and peak-to-valley values of the FW and Res. FW varies with FW powers. (c) PDFs and (d) intensity ACFs of FW and Res. FW at 5.34 W FW power. (e) Detailed temporal profiles, (f) spectra, and (g) phase of the FW and the Res. FW.