High repetition-rate coherent femtosecond sources of tunable near-IR radiation are widely used for time resolved spectroscopy and optical microscopy. We have developed a compact femtosecond optical parametric oscillator (OPO) for the near-IR in a novel configuration based on MgO doped periodically poled LiNbO3 (MgO:PPLN) synchronously pumped by a Kerr-lens-mode-locked (KLM) Ti:sapphire laser at 76 MHz. The singly-resonant OPO in a four-mirror X-cavity arrangement incorporates an 86-cm-long single mode fibre with one silver coated facet functioning as an end mirror. By coupling the resonant signal into the fibre comprising 63% of the total cavity length, the total OPO footprint is kept highly compact, while maintaining the repetition rate at 76 MHz. Moreover, the use of the optical fibre drastically alters the cavity dispersion profile compared to a free-space OPO, with the zero-dispersion wavelength shifted to ~1.3 μm. At a pump wavelength of 800 nm, the signal wavelength is tunable across 1128–1243 nm in the normal dispersion regime, and 1358–1600 nm (degeneracy) in the anomalous dispersion regime, by variation of cavity length and the crystal grating period. Tuning curves are compared to those predicted using the Sellmeier equations for MgO:PPLN and fused silica, confirming close agreement with experiment. Using a maximum pump power of ~850 mW, up to 50 mW of average signal power is extracted at 1468 nm using a 13% output coupler, and the oscillation threshold is measured to be ~600 mW. The output power is limited by an estimated coupling loss of 2.2 dB at the fibre coupler, and imperfect cavity mode-matching. Excellent passive spectral stability is observed, and a power stability of 1.1% rms is recorded. The signal beam is of high spatial quality with a TEM00 profile. Signal pulses are characterised using intensity autocorrelation, yielding pulse durations of ~130 fs at 1192 nm, very similar to those of the pump. Spectra in the anomalous dispersion regime are well-defined single peaks, while those in the normal dispersion regime exhibit a rich variety of features including symmetrical sideband formation and evidence of optical wave breakup. These features are modelled and verified using simulations based on the nonlinear Schrodinger equation. The wavelength offset of sidebands of order N is seen to follow a N0.5 dependence, analogous to those observed in the spectrum of a mode-locked soliton laser operating in the anomalous dispersion regime. To the best of our knowledge, this represents the first fibre-feedback OPO configured using a fibre facet as an end mirror; and the first time tuning in both dispersion regimes has been achieved. The combination of high intracavity dispersion and a broad phase matching bandwidth enables rapid continuous tuning across the near-IR using cavity delay, and excellent spectral stability. With suitable optics, the idler beam across 1.6–3 μm could also be extracted. This OPO represents a promising compact source for high repetition-rate ultrafast spectroscopic applications.
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