Diode-end-pumped solid-state ultraviolet laser based on intracavity third-harmonic generation of $\text{1.06}\text{μm}$ in YCa₄O(BO₃)₃ crystal

Abstract

Intracavity type-I sum-frequency mixing of $\text{1.06}\text{μm}$ and $\text{532}\text{nm}$ with a (θ,ϕ)=(106°,77.2°)-cut YCOB crystal was performed in a compact laser-diode-pumped solid-state laser. Three type-II phase-matching KTP crystals with different length were used to generate $\text{532}\text{nm}$ light by frequency-doubling of $\text{1.06}\text{μm}$. The $\text{355}\text{nm}$ output power was measured with the three KTP crystals for Q-switched and continuous-wave (CW) operation, respectively. The maximum ultraviolet output power of $\text{1305}\text{μW}$ was obtained with a $\text{15}\text{mm}$ KTP crystal for CW operation, while the maximum ultraviolet average output power of $\text{124}\text{mW}$ was obtained with a $\text{10}\text{mm}$ KTP crystal for Q-switched operation.

Introduction

In recent years, short wavelength coherent radiation in ultraviolet (UV) region has attracted wide attention for a variety of applications such as machining, spectroscopy, optical data storage, laser printing, undersea communication and medical treatment. Nonlinear optical (NLO) conversion of solid-state lasers operating in the near-infrared range is currently a very effective method for UV light generation. Compared to traditional ultraviolet gas lasers, diode-pumped solid-state ultraviolet lasers have many advantages, including compactness, high efficiency, long lifetime, high stability and all solid-state construction. Nowadays LiB₃O₄ (LBO) and β-BaB₂O₄ (β-BBO) are the most widely used nonlinear optical crystals for UV laser-beam generation. However, these crystals suffer from some sort of limitations, such as the difficulty of growth, hygroscopy, small acceptance angle and large walk-off. Therefore it is very necessary to search for new NLO crystals which can mitigate these limitations.

During the last few years, newly developed NLO crystals of ReCa₄O(BO₃)₃ (ReCOB) (Re=Y,Gd) have attracted much attention due to their good optical properties [1], [2], [3], [4], [5], [6]. They possess many advantages such as non-hygroscopy, large nonlinear coefficient which is comparable to that of LBO, high damage threshold, wide transmission band, large phase-matching range and good mechanical properties allowing easy polishing. What is more, since they melt congruently, they can be grown rapidly and easily to large-size single crystals with high optical quality by using conventional Czochralski technique. While other famous NLO crystals, such as KTP, KDP, LBO and β-BBO, do not possess this advantage. So ReCOB has been recognized as a promising NLO crystal in the frequency-conversion domain.

Until now, most of the research interest involving ReCOB has been focused on the aspects of second-harmonic generation (SHG) and self-frequency doubling (SFD). In 1997, Iwai found that YCOB crystal can reach type-I phase-matching for third-harmonic generation (THG) of $\text{1064}\text{nm}$ by means of sum-frequency mixing (SFM) $\text{(1064+532→355}\text{nm}\text{)}$, while GdCOB cannot [1]. The transmission limit of YCOB can be as short as $\text{200}\text{nm}$, while for GdCOB there are several sharp absorption peaks in the region of 200–$\text{320}\text{nm}$. So YCOB is more suitable for UV light generation than GdCOB. Recently, our group have fitted the spatial distribution curve of d_eff (effective NLO coefficient) for the THG of $\text{1064}\text{nm}$ in YCOB according to its second-order NLO susceptibilities. By the calculating and the extracavity THG experiments, we found that the largest d_eff for THG of $\text{1064}\text{nm}$ is near the direction of (θ,ϕ)=(106°,77.2°), [7].

In this paper, we report a compact diode-pumped solid-state ultraviolet laser, in which a (106°,77.2°)-cut YCOB crystal has been used for type-I sum-frequency mixing of $\text{1.06}\text{μm}$ and $\text{532}\text{nm}$ to generate $\text{355}\text{nm}$ light. The $\text{532}\text{nm}$ light was generated by SHG of $\text{1.06}\text{μm}$ with type-II phase-matching KTP crystals. Two kinds of NLO process, SHG and SFM, occurred in one resonant cavity, which was very complicated. We investigated the $\text{355}\text{nm}$ output power with three KTP crystals with different length as frequency-doubler for continuous-wave (CW) and Q-switched operation, respectively.