Second-harmonic Autocorrelation Research Articles

Stimulated generation of deterministic platicon frequency microcombs

Dissipative Kerr soliton generation in chip-scale nonlinear resonators has recently observed remarkable advances, spanning from massively parallel communications, to self-referenced oscillators, and to dual-comb spectroscopy. Often working in the anomalous dispersion regime, unique driving protocols and dispersion in these nonlinear resonators have been examined to achieve the soliton and soliton-like temporal pulse shapes and coherent frequency comb generation. The normal dispersion regime provides a complementary approach to bridge the nonlinear dynamical studies, including the possibility of square pulse formation with flattop plateaus, or platicons. Here we report observations of square pulse formation in chip-scale frequency combs through stimulated pumping at one free spectral range and in silicon nitride rings with + 55 fs 2 / mm normal group velocity dispersion. Tuning of the platicon frequency comb via a varied sideband modulation frequency is examined in both spectral and temporal measurements. Determined by second-harmonic autocorrelation and cross correlation, we observe bright square platicon pulse of 17 ps pulse width on a 19 GHz flat frequency comb. With auxiliary-laser-assisted thermal stabilization, we surpass the thermal bistable dragging and extend the mode-locking access to narrower 2 ps platicon pulse states, supported by nonlinear dynamical modeling and boundary limit discussions.

Simple ultrashort light pulse chirp measurement device

A simple device, Frequency Tracer (FT), for simultaneous measurement of ultrashort light pulse duration and chirp is presented. FT operation is based on the two-dimensional image analysis (time versus frequency) of the non-colinear secondharmonic autocorrelator beam, and FT enables one to evaluate the temporal phase variation over the femtosecond pulse duration. The spectral information of a fs pulse in FT originates from the angular divergence of a second-harmonic signal beam, and there is no need to use the spectral apparatus. Femtosecond pulses duration and chirp measurements of the Ti:sapphire laser system multi-pass amplifier (MPA) during adjustment of the compressor pair of gratings were made.

Single-shot autocorrelator based on a Babinet compensator

A compact, single-shot, second-harmonic autocorrelator for ultrashort laser pulse measurements is presented. The autocorrelator uses a Babinet compensator to split the pulse to be measured into two replicas with a relative delay between them varying across the beam. It consists of a few optical elements only, requires no sensitive alignment, and offers a robust diagnostics tool for low repetition rate femtosecond laser amplifiers.

A far-infrared broadband (8.5–37 μm) autocorrelator with sub-picosecond time resolution based on cadmium telluride

A background-free autocorrelator has been developed for measuring the duration of far-infrared laser pulses in the spectral range from 8.5 to 37 μm by using an 840-μm-long wedged cadmium telluride crystal as the second-harmonic generator. Typical intensity second-harmonic autocorrelation traces are given for the wavelengths of 19.7 and 37 μm, indicating FWHM pulse duration of 0.90 and 1.5 ps respectively. An interferometric autocorrelation trace at 18.2 μm has been measured for the first time, and the distortion of autocorrelation traces due to the absorption and re-emission of ambient water vapor is shown at 28 μm.

Tracing the phase distortion of a single ultrashort light pulse from angularly resolved second-harmonic autocorrelation

We implement for the first time the frequency tracer based on the second-harmonic autocorrelator able to measure pulse duration and instantaneous frequency of a single ultrashort light pulse. This tracer is used to characterize femtosecond pulses from a Ti:sapphire CPA system. The data on chirp and pulse duration are in good agreement with the estimations of dispersion at the system output. We show by computer simulation that the second-harmonic autocorrelator is able to trace quadratic (linear chirp), fourth and other even orders of phase distortions. Computed images directly reproduce the phase distortions and no mathematical algorithm is required for phase retrieval.

Measurement of ultrafast optical pulses with two-photon interference

We introduce a new two-photon interference scheme for measurement of the coherence time and pulse width of ultrafast pulses. Experiments were performed with the output pulses of a cw mode-locked dye laser, with results in good agreement with the conventional second-harmonic autocorrelation measurement.

Femtosecond relaxation of localized plasma excitations in Ag islands.

We report the first direct measurement of the relaxation of plasma oscillations in a solid. Via the second-harmonic autocorrelation of femtosecond laser pulses, the lifetimes of localized plasma oscillations in silver island films were investigated. Decay times of 40\ifmmode\pm\else\textpm\fi{}7 fs were obtained, which indicate that single-particle scattering is mainly responsible for the energy relaxation of the plasma oscillations.

First-order autocorrelations and domain-model mode structure of a synchronously pumped mode-locked dye laser

First-order field autocorrelation measurements of high dynamic range and sensitivity have been performed on the pulse train of a synchronously pumped mode-locked dye laser with a two-plate birefringent filter as a tuning element. A double-phase-modulation detection technique developed by us for time-resolved stimulated Raman gain spectroscopy was employed. These measurements yield information about the mode structure of the dye-laser pulses. Our experimental results lend support to the domain model as a viable description of the mode structure of imperfect mode locking. Moreover, second-harmonic autocorrelation and cross-correlation widths and shapes are shown to be reconcilable with this model.

Passively mode-locked continuous-wave Rhodamine 110 dye laser

The passive mode locking of a cw Rhodamine 110 dye laser is reported. In a simple uncompensated linear cavity, subpicosecond pulse generation was achieved over the spectral range 553–570 nm. Pulses as short as 150 fsec were recorded using standard second-harmonic autocorrelation measurement techniques. Two saturable-absorbing species were found to operate successfully in this region.