Modelocked Thermal Frequency Combs for Ultrashort Chaotic Quantum Optics

Modelocked thermal frequency combs (MTCs) are generated by employing spectrally narrowed amplified spontaneous emission (ASE) seeded into an electro-optic frequency comb generator. The MTC emits 2-ps duration ultrashort pulses at a repetition rate of 10 GHz. Autocorrelation of the MTC pulses confirms a reduced coherence time, ${\tau }_c=213\pm 16$ ps, aligning with the narrowed bandwidth of the ASE seed. Intensity correlations of optically gated MTC pulses at a repetition rate of 250 MHz reveal nearly ideal thermal photon statistics with an experimental $g_{\mathrm{mtc}}^{\left(2\right)}=1.9564\pm 0.0004$, yielding an intrinsic $g_{\mathrm{int}}^{\left(2\right)}=1.9809\pm 0.0004$ after background noise removal. As a practical application, second harmonic generation (SHG) is performed utilizing the optically gated MTC pulses as a pump and experimental intensity correlations, $g_{\mathrm{sh}}^{\left(2\right)}$, are examined for the SH photons. An entire transition in $g_{\mathrm{sh}}^{\left(2\right)}$, continuously changing from six to two by increasing the pump strength, agrees with the single-mode analytical model. Furthermore, time-resolved pulse height correlations allow to simultaneously acquire power variations in SHG and third harmonic generation against the pump. With the maximum peak intensity, ${\overline{I}}_1^p\approx 1.6{\text{GW}\text{cm}}^{-2}$, realized in a periodically poled ${\mathrm{LiNbO}}_3$ waveguide for SHG, the demonstration highlights the potential for various applications in chaotic quantum optics experiments that necessitate ultrashort, high-intensity, single-spatiotemporal-mode thermal pulses.