Generation of spectrally narrow, phase-stable pulse trains using birefringent crystal array

We generate stacked pulses using a birefringent crystal array consisting of five SiO₂ plates of increasing thickness, producing a phase-stable 32-subpulse train spanning ~1.3 ps. In addition to characterizing its temporal structure, we investigate its spectral response through second-harmonic generation (SHG). Measurements reveal that the SHG spectrum exhibits a narrow bandwidth with a full width at half maximum of ~0.37 nm, despite the complex spectral structure of the fundamental field. This narrow SHG spectrum indicates that the birefringent array produces a temporally extended field suitable for long-duration gating of optical free-induction-decay (FID) signals. Cross-correlation frequency-resolved optical gating (XFROG) measurements and numerical simulations reveal weak side lobes and a low FROG error of 0.9%. Furthermore, we show that the stacked pulse generated by the birefringent crystal array provides a fully passive and stable alternative for gating in optical FID detection.