Abstract

High average output power femtosecond sources operating across the 1.5 µm telecommunication band are restricted mainly to colour-centre lasers, Kerr-lens modelocked Cr⁺⁴:YAG oscillators and optical parametric oscillators (OPOs). The need for cryogenic conditions limits the versatility of colour-centre-based souces and practical difficulties have made self modelocked operation of Cr⁻⁴:YAG lasers near the 1.4 µm water absorption band very difficult to sustain. In contrast, the OPO that we describe in this paper operates at room temperature and is continuouslv-tunable over 200 nm from 1.375 to 1.575 µm using only a single cavity mirror set. The OPO is based on periodically poled RbTiOAsO₄ (PPRTA)¹. The crystal used had a grating period of 30 µm and dimensions 5 mm long × 0.5 mm width × 5 mm high². The crystal was antireflection coated for a wavelength of 1.1 µm and we estimate that the loss of each surface at a wavelength of 1.5 µm to be ~1 %. The OPO was synchronously pumped by a modelocked Ti:sapphire laser at a repetition rate of 84 MHz. with output pulses of 100 fs duration and centred at a wavelength of 850 nm. We used a 90 mm focal-length lens to focus the pump beam in to the PPRTA crystal. The polarisation of the pump, signal and idler wavelengths were all horizontal in the cavity and parallel to the poling direction which lay along the crystalographic c axis. The dispersion-compensated OPO cavity consisted of two SF14 prisms, two curved mirrors (r = 100 mm) and one flat mirror as the output coupler. All the mirrors were high-reflecting at the central wavelength of 1.5 µm.

© 1998 IEEE