Abstract

Recently, a continuous-wave (cw) diode-pumped intra-cavity frequency-doubled Nd:YLF ring laser oscillating on the σ-polarized transition (λ_ω=1314nm) and employing a temperature-tuned periodically-poled KTP (ppKTP) nonlinear crystal has been reported to generate up to 0.92W single-frequency red output power [1]. When the ppKTP was strongly phase-mismatched, evidence of passive cascaded Kerr-lens mode locking (KLM) was inferred from a broad phase-coherent spectrum analyzed by a confocal Fabry-Perot interferometer. In this work, we demonstrate indeed that complex cascaded second-order nonlinear processes occurring inside the ppKTP crystal are responsible of the KLM operation of a π-polarized (λ_ω=1321nm) Nd:YLF/ppKTP ring laser (Fig. 1), over the full range of the ppKTP temperature tuning (15°C<T<75°C). While the ppKTP phase-matching temperature for the gain center wavelength occurs at T~25–30°C (with a FWHM temperature bandwidth ΔT_PM=10°C and spectral bandwidth Δλ_PM=0.8nm for L=10mm), significant broadband (Δλ_2ω≤1nm) red average power (<100mW) is measured from intra-cavity SHG over the above full temperature variation. The preliminary analysis of the laser emission versus the ppKTP temperature or versus the intra-cavity power demonstrates unambiguously the tendency to cascaded KLM effects, which threshold occurs at about 5W of diode pump power. Mid-IR spectra recorded with an optical spectrum analyzer (OSA) reveal emission bandwidths greater than BW=3nm (Fig. 2) under the pulsing temporal regime, in correlation with a broadband background transmission from the scanning Fabry-Perot interferometer. Time traces recorded with a 10-GHz bandwidth InGaAs photodetector yield a periodic train of bi-colour sub-300ps pulses (instrumentation-limited) at the laser free-spectral range (c/L_cav~420 MHz or τ_rep~2.41ns). Further analysis of the pulsing features is under way. While passive KLM mode-locking due to pure cascaded second-order nonlinear effects was first reported in a GdVO₄/ppKTP standing-wave cavity operating close to its stability region [2], our unidirectional ring laser operates well within the cavity stability range and is rather designed for cw operation. Such a mode-locked behaviour is never observed with a much broader spectral phase-matching bandwidth SHG crystals such as BiBO (Δλ_PM=3.9nm) or LBO (Δλ_PM=47nm).

© 2009 IEEE