Travelling-wave 1.57-µm optical parametric oscillator driven by a pulsed multimode Nd3+:YAG laser

Abstract

The generation properties of an optical parametric oscillator (OPO) with a three-mirror ring resonator have been investigated under conditions of a pulsed pump by multimode radiation from a Nd 3+ :YAG laser. KTP crystals were used as the OPO nonlinear medium. At pump energies up to 100 mJ, decreasing the diameter d of the Nd 3+ :YAG laser beam causes a decrease in OPO radiation divergence and an increase in the generation efficiency at λ = 1.571 µm despite a decrease in the differential efficiency. At d = 2.25 mm and a KTP crystal total length of 40 mm, the efficiency of the pump conversion to the signal-wave pulse reaches 36.5%. Based on the traveling-wave OPO, a compact, highly effective, eye-safe laser source radiating pulses of up to 35 mJ in energy, 11 nsec in duration, and 10 Hz in repetition rate at electrical pump energy of ≤ 8 J is developed. At the 86.5% level of the total pulse energy, the source-beam divergence does not exceed seven diffraction limits.Introduction. One method for producing eye-safe (ES) radiation is based on the use of an optical parametric oscillator (OPO) of radiation in KTP (KTiPO 4 ) crystals pumped by radiation from Nd 3+ lasers (λ g ~ 1 µm). In view of the need to construct wholly solid-state sources, the use of OPO based on KTP crystals is an effective alternative to direct generation of ES radiation using Er-containing lasers [1, 2] and generation of ES radiation based on Raman transformation of Nd 3+ laser radiation operating at the 4 F 3/2 → 4 I 13/2 (λ g ~ 1.35 µm) transition [3][4][5]. Operation under non-critical phase synchronization and generation of ES radiation in the nanosecond range with energies of tenths and hundredths of millijoules can be achieved with pumping by Nd 3+ -radiation with amplitude modulation of an OPO in KTP crystals [6][7][8][9]. Owing to the high energy characteristics, these OPO are promising for many practical applications, several of which, however, require very reliable OPO operation and relatively low radiation divergence.One of the pathways for meeting these requirements is to construct an OPO based on an unstable telescopic resonator. The leakage of signal radiation from the axial region of the resonator to the periphery that is typical of such a resonator leads for several outputs of the resonator to a correlation of the phases for all points of the output cross section of the source and thereby reduces the divergence of the generated radiation. Furthermore, unstable resonators typically have a rather homogeneous distribution of the generation field in the transverse cross section. The advantages of the OPO with an unstable telescopic resonator have been demonstrated [10-13] for instances of extracavity and intracavity location of the OPO relative to the Nd 3+ -laser that generates both multimode pumping beams and beams for which the divergence is close to the diffraction limit.It has been shown [14] that the divergence of the radiation generated by an OPO under critical phase synchronism conditions can be decreased by combining effects o...

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

“…Optical parametric oscillators (OPOs) employing crystals of the KTiOPO 4 (KTP) family in a non-critically phase-matched configuration are known to provide efficient eye-safe wavelength conversion from 1 μm (neodymium lasers) to 1.5-1.6 μm [1][2][3][4][5][6]. Compact, robust, and highly reliable eye-safe emitters with defined performances are always of great interest due to the wide range of their applications including pollution detection, free space communication, range finders, target indicators, etc.…”