Frequency-resolved optical gating in periodically-poled lithium niobate waveguide devices
Frequency-Resolved Optical Gating (FROG) is a well-established and widely-employed technique for the intensity and phase characterisation of ultrashort optical pulses. Essentially, FROG involves mixing an ultrashort optical pulse with its time-delayed replica, or another pulse, in a nonlinear material or device to yield a two dimensional data set called a spectrogram, from which the electric field of the ultrashort pulse can be retrieved by an iterative algorithm. The most commonly used configuration is based on second-order nonlinear interactions in bulk materials, mainly because of its high efficiency compared to other schemes based on third-order nonlinear interactions. The research work in this thesis led to the first successful implementation of an integrated Lithium Niobate for the FROG device, based on sum-frequency generation. We demonstrated simultaneous complete characterisation of two ultrashort pulses of durations 4-25 ps in the 1.55 μm-band with a coupled energy of 430 fJ in a 26mm long PPLN waveguide device. The temporal walk-off between the interacting pulses in this interaction resulted in an acceptance bandwidth of 0.75 nm, limiting the measurable pulse duration to ~ 4.5 ps. In order to overcome this limitation, we proposed and demonstrated a novel FROG configuration based on cascaded second-harmonic and difference-frequency generations. Theoretical and numerical analyses of this configuration revealed its robustness against the temporal walk-off effect, resulting in improved temporal resolutions. This was experimentally verified by characterising a 2.1 ps pulse train with a coupled average power (energy) of 72 μW (29! fJ) i n the PPLN waveguide device previously mentioned.