Deep-UV 236.5 nm laser by fourth-harmonic generation of a single-crystal fiber Nd:YAG oscillator
Résumé
We demonstrate a deep-UV laser at 236.5 nm based on extracavity fourth-harmonic generation of a Q-switched Nd: YAG single-crystal fiber laser at 946 nm. We first compare two nonlinear crystals available for second-harmonic generation: LBO and BiBO. The best results at 473 nm are obtained with a BiBO crystal, with an average output power of 3.4 W at 20 kHz, corresponding to a second-harmonic generation efficiency of 38%. This blue laser is frequency-converted to 236.5 nm in a BBO crystal with an overall fourth-harmonic generation yield of 6.5%, corresponding to an average output power of 600 mW at 20 kHz. This represents an order of magnitude increase in average power and energy compared to previously reported pulsed lasers at 236.5 nm. This work opens the possibility of LIDAR detection of dangerous compounds for military or civilian applications. The development of deep-UV solid-state lasers based on nonlinear frequency conversion has opened applications previously based on excimer lasers, such as material processing or spectroscopy applications. The most common approach is fourth-harmonic generation of the 1064 nm laser line of Nd:YAG, which has been a commercial product for many years. Unfortunately, important applications, such as manufacturing of Bragg gratings or waveguides, would be more efficient using wavelengths below 250 nm. Moreover, some particular spectroscopic applications, such as detection of dangerous compounds, require specific wavelengths in the deep-UV. One possible solution is to use a fundamental laser operating at a lower wavelength, such as the 946 nm laser line of Nd:YAG. Subsequent fourth-harmonic generation leads to a wavelength of 236.5 nm, which lies in the absorption band of molecules related to improvised explosive devices or nuclear, radiological, biological, and chemical components [1,2]. The challenge is to obtain enough pulse energy to increase the detection range and accuracy. Unfortunately, previous works at 236.5 nm only demonstrated a maximum energy of 500 nJ [3,4], with an average power of 20 mW. This energy limitation comes from the difficulty to obtain significant laser output in Nd:YAG at 946 nm, because the quasi-three-level transition induces strong thermal effects. Recently, we demonstrated that single-crystal fiber technology could significantly improve the available laser output at 946 nm in Nd:YAG, owing to an optimized thermal management allowed by the crystal geometry and packaging [5,6]. In this Letter, we propose to use a Nd:YAG single-crystal fiber Q-switched oscillator at 946 nm as a source for efficient fourth-harmonic generation. We show that this leads to an order of magnitude increase over previously reported results both in pulse energy and average power at 236.5 nm. The experimental setup is displayed in Fig. 1. The laser is a Q-switched oscillator at 946 nm based on a Nd:YAG single-crystal fiber [6]. It emits an average power of 9.2 W in a linearly polarized beam at a repetition rate of 20 kHz, with a pulse width of 45 ns. The beam profile is Gaussian with a measured beam quality of M 2 x 1.11 and M 2 y 1.13 (Fig. 3). The pulse energy is 460 μJ, corresponding to a peak power of 10.2 kW. Several crystals are commercially available to perform second-harmonic generation (SHG) to the visible range. We consider three crystals: lithium tetraborate LiB 3 O 5 (LBO), bismuth borate BiB 3 O 5 (BiBO), and periodically poled titanium phosphate (PPKTP). Their material and nonlinear properties are displayed in Table 1. LBO is the most commonly used nonlinear crystal for visible SHG. While its nonlinearity is low compared to the other two crystals (d eff 0.81 pm∕V), it has low walk-off, large angular acceptance, a very high damage threshold , and can be manufactured in large dimensions with very good optical quality. In contrast, BiBO has a large nonlinear coefficient (d eff 3.34 pm∕V), but suffers from a large walk-off and low angular acceptance, often resulting in elliptical output beam. This also lowers the conversion efficiency for tight focusing when compared to crystals with a lower walk-off value. Furthermore, BiBO is a hygroscopic crystal that cannot be exposed to ambient air for a very long period of time.
Domaines
Optique [physics.optics]
Origine : Fichiers produits par l'(les) auteur(s)
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