Comparative studies of eye-safe intracavity and extracavity optical parametric oscillators with an unstable telescopic cavity

Abstract

We have experimentally studied the lasing characteristics of an eye-safe optical parametric oscillator (OPO) with an unstable telescopic cavity when it is placed inside (intracavity OPO) and outside (extracavity OPO) the plane-parallel cavity of a pulsed, nearly single-mode KGW:Nd pump laser. We used a KTP crystal as the nonlinear medium for the OPO. We have shown that the intracavity OPO has the higher lasing efficiency. We have observed that the distribution of nonlinear losses introduced by the intracavity OPO, nonuniform over the cavity cross section, leads to an increase in the diameter and divergence of the radiation beam from the pump laser and a dependence of its temporal lasing dynamics on the transverse beam coordinate. We propose a physical model qualitatively explaining the spatial and temporal lasing dynamics of a radiation source with an intracavity OPO. Both OPO versions generate beams of radiation with about the same divergence. When the KGW:Nd laser has an electrical pumping energy of 7.3 J and a cavity length of 77 cm, the intracavity OPO and the extracavity OPO emit pulses with energies of 14.5 mJ and 12.0 mJ and duration 18 nsec and 13 nsec respectively. The divergence of the eye-safe radiation (λ = 1.578 µm) at 86.5% of the total pulse energy is no greater than 5.5 mrad for an OPO output beam diameter of ≈2 mm. Introduction.A widely used approach to designing eye-safe pulsed radiation sources is based on optical parametric oscillation in KTP crystals, pumped by the emission from nanosecond neodymium lasers. Due to the good nonlinear optical properties of the KTP crystal, an optical parametric oscillator (OPO) based on such a crystal makes it possible to efficiently convert the wavelength of emission from neodymium lasers to the eye-safe range of the spectrum (1.54-1.61 µm), enabling generation of nanosecond pulses with energies from a few millijoules to hundreds of millijoules. Eye-safe radiation sources based on KTP crystals are made with the OPO positioned both outside [1][2][3][4][5] and inside [6-13] the pump laser cavity. As we know [14], the basic idea of an intracavity OPO involves using high radiation intensity in the "closed" cavity of the pump laser, which makes it possible to easily achieve the high OPO lasing thresholds (especially for an OPO designed for generation of signal or idler waves: a single-cavity OPO). The indicated advantage of the intracavity position is directly pertinent to eye-safe OPOs based on KTP crystals, since in practice they are usually made in the single-cavity version due to strong absorption of idler wave radiation in the KTP

High-power eye-safe intracavity KTA OPO driven by a diode-pumped Q-switched Nd:YAG laser

“…High peak power eye-safe lasers (1.5-1.6 µm) have been widely used in many applications such as remote sensing, lidar systems, telemetry and range finders etc. Qswitched neodymium lasers [1][2][3][4][5][6][7][8][9][10][11][12][13][14] with high pulse peak power seem to be one of the best systems used as the pump source for such high peak power eye-safe lasers. In recent years, OPOs using KTP as the nonlinear crystals and pumped by the Q-switched Nd-doped lasers (such as Nd:YAG, Nd:YVO 4 , Nd:YAP,Nd:GdVO 4 , and Nd:KGW) have been extensively studied [6][7][8][9][10][11][12][13][14].…”

“…Qswitched neodymium lasers [1][2][3][4][5][6][7][8][9][10][11][12][13][14] with high pulse peak power seem to be one of the best systems used as the pump source for such high peak power eye-safe lasers. In recent years, OPOs using KTP as the nonlinear crystals and pumped by the Q-switched Nd-doped lasers (such as Nd:YAG, Nd:YVO 4 , Nd:YAP,Nd:GdVO 4 , and Nd:KGW) have been extensively studied [6][7][8][9][10][11][12][13][14]. As another member of the KTP family, KTA have many merits, such as high transparency range (0.35 -5.3 µm), high optical damage threshold, high nonlinear coefficient, and low temperature sensitivity [15].…”

Double-pass pumped extracavity optical parametric oscillator with passive optical feedback suppression

“…Using the ABCD matrix formalism [14] and ignoring the weak thermal lens in the Nd:KGW rod used [15], we have estimated the magnification m for the cavities used in experiments. Figure 2 presents the magnification m as a function of an intracavity telescope misadjustment Δz (changing of the telescope length).…”