Optics & Laser Technology 39 (2007) 973–979 Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals Liviu Neagu, Constantin Ungureanu, Razvan Dabu à , Aurel Stratan, Constantin Fenic, Laurentiu Rusen National Institute for Lasers, Plasma and Radiation Physics, Bucharest, Romania Received 22 November 2005; accepted 7 June 2006 Available online 1 August 2006 Abstract We report on compact eye-safe nanosecond laser sources emitting in the 1.5 mm wavelength range based on non-critically phase- matched parametric interaction in optical parametric oscillators (OPOs) with KTP and periodically poled KTP (PPKTP) crystals, pumped by the fundamental frequency of Nd:YAG lasers. As much as 250 mJ signal pulse energy at 1.5 mm wavelength, 6.5 ns FWHM pulse-width, has been obtained in a PPKTP-OPO, extracavity pumped by a Nd:YAG microlaser oscillator–amplifier at 650 mJ pump pulse energy, 8 ns pulse-width. A single signal pulse of 2.7-mJ output energy at 1.57 mm wavelength, less than 5 ns pulse-width, was generated in a KTP-OPO, intracavity pumped by a passively Q-switched Nd:YAG laser. r 2006 Elsevier Ltd. All rights reserved. Keywords: Eye safe laser radiation; Nanosecond optical parametric oscillator; Non-critical phase-matching 1. Introduction Within the eye-safe band near 1.5 mm wavelength, radiation is absorbed mainly in the frontal part of the eye, especially in the ocular fluid. This absorption greatly increases the effective optical damage threshold of the eye, given mainly by the retina injury. For this reason, eye- safety is an essential requirement for laser application in populated areas: laser radar and ranging, obstacle avoid- ance, remote sensing of aerosols, and communications. Due to their compactness, robustness, long life-time operation, and relatively low cost, nanosecond optical parametric oscillators (OPOs) were developed [1,2] as an alternative to the currently used eye-safe light sources like laser pumped high pressure gas Raman cells and Er,Yb:- glass lasers [3,4]. Nanoseconds OPOs based on KTP and periodically poled KTP (PPKTP) crystals, pumped by near-infrared lasers, are high efficiency sources of laser radiation in the 1.50–1.58 mm range, generated by a non- critically phase-matched parametric interaction [5,6]. In this paper, we describe two types of compact nanosecond OPOs emitting eye-safe radiation. A PPKTP OPO, extra-cavity pumped by a Nd:YAG microlaser oscillator–amplifier system, is described in the Section 2, whereas in the Section 3 a KTP-OPO, intra-cavity pumped by a passively Q-switched Nd:YAG laser, is presented. 2. Extra-cavity pumped PPKTP-OPO Quasi-phase-matching (QPM) is an alternative technique to birefringent phase-matching for compensating phase velocity dispersion in frequency conversion applications. One advantage of the QPM lies in its ability to accomplish the phase-matching for waves of the same polarization using the largest available nonlinear coefficients of the crystals. At the same time, engineerable phase-matching conditions provide flexibility in generating the desired parametric wavelengths. As the phase-matched interaction is non-critical, the degradation of the signal’s beam quality owing to Poynting vector walk-off is avoided. For a first- order QPM, the nonlinear coefficient is modulated inside the periodically poled crystal with a period L twice the coherence length l c of the interaction to offset the ARTICLE IN PRESS www.elsevier.com/locate/optlastec 0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.06.004 à Corresponding author. Tel./fax: +40 21 4575066. E-mail address: razvan.dabu@inflpr.ro (R. Dabu).