We have studied an optical parametric oscillator (OPO) with an unstable telescopic cavity, placed inside the cavity of an actively Q-switched multimode Nd 3+ :KGW pump laser. We used a KTP crystal as the nonlinear medium for the OPO. We have compared the emission characteristics of OPOs with unstable telescopic and planar cavities. We have established that compared with the planar cavity, the unstable cavity reduces the OPO beam divergence and improves the spatial distribution of the radiation energy in the far wave zone.Based on our investigations, we have designed a compact eye-safe (λ = 1.578 µm) laser source with natural cooling, emitting (for electrical pumping energy 7.3 J) pulses with pulse energy 22 mJ and pulse duration 6 nsec. The FWHM beam divergence for the source is no greater than 3.5 mrad.Introduction. One of the most effective methods for generating eye-safe radiation is based on an optical parametric oscillator using crystals in the KTP family, pumped by radiation from Nd 3+ -containing lasers. KTP and KTA crystals have high radiation resistance, high nonlinear optical coefficients, and ensure lasing under noncritical phase matching conditions. Because of the indicated properties, they make it possible to rather efficiently, using single-cavity OPOs, to convert the wavelength of emission from Nd lasers to the wavelength range 1.54-1.61 µm. Single-cavity OPOs based on crystals of the KTP family are constructed using Nd:YAG, Nd:YLiF, and Nd:YVO 4 lasers with lamp and diode pumping [1][2][3][4].With pumping by Q-switched Nd lasers, OPOs based on KTP crystals can generate nanosecond pulses with energies of tens and hundreds of millijoules [5-10]. Such good power characteristics make KTP OPOs promising for many practical applications. However, the OPO beam divergence and field structure are no less important characteristics of eye-safe radiation (for example, for detection and ranging). Previously, high-power OPOs were generally constructed using linear planar cavities which, when the pump beam is matched with an eigenmode of the OPO cavity, ensure a sufficiently large waist (a few millimeters) and thus make it possible to operate below the damage threshold of the crystals. However, the divergence of the radiation from an OPO with a planar cavity is relatively large, since for short cavities it is determined more by the geometric aperture of the pumped volume than by the pump beam quality, if the pump beam divergence exceeds the diffraction limit [11]. A decrease in the divergence of the OPO radiation is facilitated by use of a ring cavity [10]. However, in order to achieve high radiation energy, this approach requires using several KTP crystals in the OPO, which increases the cost of the source.As shown by recent theoretical [12] and experimental [13,14] investigations, the divergence of the OPO radiation can be reduced by using an unstable telescopic cavity. The advantage of such a cavity over a planar cavity has been demonstrated for extracavity OPOs, pumped by laser beams with divergence close to...

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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

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