Development and characterisation of a near-infrared femtosecond optical parametric oscillator frequency comb

Abstract

This thesis describes a 280 MHz MgO:PPLN-based optical parametric oscillator (OPO) synchronously pumped by a 50 fs Ti:sapphire laser to produce ultrafast pulses in the near-infrared. The OPO tuned over a wavelength range from 1450 - 1550 nm and 1624 - 1750 nm for the signal and idler respectively. The carrier-envelope-offset (CEO) frequency of the signal pulses was stabilised to a 10 MHz reference frequency without f-2f self-referencing, with an RMS phase variation of 0.56 rad over an observation time of 1 second. The relative intensity noise was measured for the CEO-stabilised OPO over an observation time of 64 seconds as 0.04%. The repetition frequency of the OPO was stabilised to 280 MHz using a frequency synthesiser at the eighth harmonic (2.24 GHz). This locking loop had an RMS phase variation of 0.98 mrad over a 1 second observation time. The CEO- and repetition frequencies were then locked simultaneously to a synthesiser referenced to a Rb-disciplined source, to generate a fully stabilised 1.5 μm frequency comb with an absolute uncertainty in comb mode position of 110 Hz. The upper limit for the fractional instability for a comb mode at 200 THz was found to be 2 x 10-11, limited by the stability of the Rb reference. A five-fold increase in comb mode spacing to 1.4 GHz was demonstrated with the stabilised frequency comb. This was achieved using a passive filter cavity, stabilised to a transmission peak via dither locking. The FWHM bandwidth of the optical spectrum for the filtered frequency comb was reduced to 8 nm, however no increase in comb linewidth was observed. An additional experiment was carried out where an external cavity diode laser was frequency-stabilised to a saturated absorption peak in Rb at 780.2 nm using dither locking, providing an optical frequency reference for future OPO frequency combs.