Coexistent optical parametric oscillation and stimulated Raman scattering in KTiOAsO4

Tunable KTA Stokes laser based on stimulated polariton scattering and its intracavity frequency doubling

This paper presents the tunable Stokes laser characteristics of KTiOAsO₄ (KTA) crystal based on stimulated polariton scattering (SPS). When the pumping laser wavelength is 1064.2 nm, the KTA Stokes wave can be discontinuously tuned from 1077.9 to 1088.4 nm with four gaps from 1079.0 to 1080.1 nm, from 1080.8 to 1082.8 nm, from 1083.6 to 1085.5 nm, and from 1085.8 to 1086.8 nm. When a frequency doubling crystal LiB₃O₅ (LBO) is inserted into the Stokes laser cavity, the frequency-doubled wave can be discontinuously tuned from 539.0 to 539.5 nm, from 540.1 to 540.4 nm, from 541.3 to 541.8 nm, from 542.7 to 542.9 nm and from 543.4 to 544.2 nm. With a pumping pulse energy of 130.0 mJ and an output coupler reflectivity of about 30%, the obtained maximum Stokes laser pulse energy at 1078.6 nm is 33.9 mJ and the obtained maximum frequency-doubled laser pulse energy at 543.8 nm is 15.7 mJ. By using the most probably coupled transverse optical modes obtained from the literature, the polariton refractive indexes, and the simplified polariton Sellmeier equations, the polariton dispersion curve is obtained. The formation of the Stokes frequency gaps is explained.

Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers

Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7-20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ((2)) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. The pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations P(NL) of the impulsively excited phonons and those of parametrically amplified waves.

THz-wave generation via stimulated polariton scattering in KTiOAsO4 crystal

A terahertz parametric oscillator based on KTiOAsO(4) crystal is demonstrated for the first time. With the near-forward scattering configuration X(ZZ)X + Δφ, the polarizations of the pump, the Stokes and the generated THz waves are parallel to the z-axis of the crystal KTA. When the incident angle θext of the pump wave is changed from 1.875° to 6.500°, the THz wave is intermittently tuned from 3.59 to 3.96 THz, from 4.21 to 4.50 THz, from 4.90 to 5.16 THz, from 5.62 to 5.66 THz and from 5.92 to 6.43 THz. The obtained maximum THz wave energy is 627 nJ at 4.30 THz with a pump energy of 100 mJ. It is believed that the terahertz wave generation is caused by the stimulated scattering of the polaritons associated with the most intensive transverse A(1) mode of 233.8 cm(-1). Four much weaker transverse A(1) modes of 132.9 cm(-1), 156.3 cm(-1),175.1 cm(-1), and 188.4 cm(-1) cause four frequency gaps, from 3.97 THz to 4.20 THz, from 4.51 to 4.89 THz, from 5.17 to 5.61 THz and from 5.67 to 5.91 THz, respectively.

Simultaneous operations at 1125 nm, 1161 nm, and 1567 nm from a single KTiOPO4 crystal pumped by a passively Q-switched Nd:GGG laser

A multiwavelength operation generated from an intracavity optical parametric oscillator (OPO) pumped by a passively Q-switched Nd:GGG laser with a Cr(4+):YAG saturable absorber is demonstrated. KTiOPO4 simultaneously worked as nonlinear OPO and Raman crystal. Maximum output powers of 302 mW at 1566.8 nm and 115 mW at 1124.9/1160.7 nm were obtained at a diode power of 10.5 W, corresponding to the optical-to-optical conversion efficiencies of 2.88% and 1.1%, respectively. The measured shortest pulse duration at 1566.8 nm was 1.61 ns, while the obtained minimum pulse duration at 1124.9/1160.7 nm was 2.88 ns.

Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm by coupled stimulated Raman scattering

Intracavity coupled Raman conversions in KTP and KTA driven by a laser diode (LD) pumped Nd:YAG/Cr(4+):YAG 1064 nm laser is demonstrated in this paper. Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm are achieved by balancing the Raman gain of KTP and KTA. Under the LD pump power of 8.1 W, the maximum average output powers at 1091 and 1095 nm are 170 and 150 mW, respectively. The corresponding pulse width and repetition rate are measured to be 3.3 ns and 11.2 kHz, with the pulse peak powers calculated to be 4.6 and 4.1 kW, respectively. The laser source with such small wavelength separation and orthogonal polarization provides the interest for terahertz generation in the 1 THz range. Our study provides a simple and flexible method to achieve orthogonally polarized dual-wavelength laser source by Raman-based intracavity coupled nonlinear frequency conversions.