Multiwavelength ultrafast LiNbO3 Raman laser Aravindan M. Warrier,* Jipeng Lin, Helen M. Pask, Andrew J. Lee, David J. Spence MQ Photonics Research Centre, Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia *
[email protected]
Abstract: We present a multiwavelength ultrafast Raman laser based on lithium niobate which uses polariton scattering in combination with Raman scattering to selectively generate new wavelengths from a nanojoule-scale picosecond pump laser. Pumped by a 1064 nm pump laser, the system generates 1123 nm by stimulated polariton scattering (SPS) and 1140 nm by stimulated Raman scattering (SRS). Cascading of these intracavity fields generates 1155 nm and 1174 nm, as well as generating THz output. ©2015 Optical Society of America OCIS codes: (140.3550) Lasers, Raman; (240.5420) Polaritons; (190.5890) Scattering, stimulated.
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#246556 © 2015 OSA
Received 27 Jul 2015; revised 14 Sep 2015; accepted 15 Sep 2015; published 21 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025582 | OPTICS EXPRESS 25582
1. Introduction Wavelength conversion of nanojoule-scale picosecond pulses from unamplified oscillators is often achieved using synchronous pumping of an external cavity, where the wavelengthshifted pulse is resonated with the same roundtrip-time as the pump oscillator, with repetitive amplification driven by each pulse in the pump train. Synchronous pumping is well established for both χ(2) optical parametric oscillators [1–3] and χ(3) Raman lasers [4–10]. Lithium niobate (LiNbO3), a negative uniaxial crystal, is one of the most-studied materials for nonlinear frequency conversion due to its large χ(2) and χ(3) nonlinear coefficients and its transparency over a wide wavelength range (0.4-5.5 µm). Lithium niobate has been most widely used for frequency doubling and optical parametric oscillator applications in the visible and infrared [11–13] but has also been used as a Raman-active material [14–16] to generate wavelengths in the IR and THz regions.
Fig. 1. Dispersion curve of lithium niobate calculated using data from [17] with phase matching curves for small angle scattering of a 1064 nm pump laser.
Lithium niobate has two strong vibrational modes with wavenumbers 248 cm−1 and 628 cm [17]. These vibrations of polar bonds produce coupled electromagnetic-vibrational modes of mixed photon-phonon character called polaritons. Stimulated polariton scattering (SPS) is an inelastic process that involves both χ(2) and χ(3) interactions. Unlike stimulated Raman scattering (SRS), in SPS the frequency shift is a function of the angle between the pump and Stokes fields, determined by the intersection of the phase matching curves and the polariton dispersion curve shown in Fig. 1. The polariton associated with the 248 cm−1 vibrational mode generates THz radiation for angles of a few degrees between the pump and Stokes fields; for angles ≥7° the polariton is predominantly vibrational in nature and tends to a resonant frequency of 248 cm−1. The polariton associated with the 628 cm−1 vibrational mode has a resonant frequency of 497 cm−1 for co-propagating pump and Stokes; for angles ≥7° the polariton is predominantly vibrational in nature and tends to a resonant frequency of 628 cm−1. In this paper we present a synchronously-pumped Raman laser that uses these different scattering frequencies to switchably or simultaneously convert the pump light to a set of new wavelengths. -1
2. Laser operation The experimental setup comprised a synchronously-pumped ring resonator as shown in Fig. 2. We used a 20 mm x-cut 5-mol% MgO-doped LiNbO3 crystal (HC Photonics Corp.) with
#246556 © 2015 OSA
Received 27 Jul 2015; revised 14 Sep 2015; accepted 15 Sep 2015; published 21 Sep 2015 5 Oct 2015 | Vol. 23, No. 20 | DOI:10.1364/OE.23.025582 | OPTICS EXPRESS 25583
antireflection-coated end faces (R
