X-FROG Technique Research Articles

High repetition rate flat coherent optical frequency comb generation based on the normal dispersion tantalum pentoxide optical waveguide

In this paper, high repetition rate flat coherent optical frequency comb generation based on the normal dispersion tantalum pentoxide (Ta2O5) optical waveguide is proposed and numerically investigated. The 1.2 meters long normal dispersion Ta2O5 integrated nonlinear waveguide through dispersion engineering is used to generate a flat optical frequency comb of about 50 nm span based on self-phase modulation and optical wave breaking near 1550 nm. The time-spectrum evolution during the pulse propagation process is analyzed by the X-Frog technique. The effects of various parameters on the broadening comb spectra bandwidth and flatness are considered, and the spectral coherence of the optical frequency comb is also studied. The simulation results show that the Ta2O5 integrated nonlinear waveguide has a great prospect in broadband high repetition rate flat coherent optical frequency comb generation.

Ultrabroadband XFROG of few-cycle mid-infrared pulses by four-wave mixing in a gas

Cross-correlation frequency-resolved optical gating (XFROG) based on four-wave mixing (FWM) in a gas medium is shown to enable dispersion-free characterization of few-cycle mid-infrared (mid-IR) pulses tunable within a spectral range of more than two octaves. The FWM XFROG technique is used to measure spectra and pulse shapes, as well as to retrieve the phase of a few-cycle output of difference-frequency generation (DFG) tunable from 3 to 11 μm. With Ti:sapphire laser pulses used as a reference, this FWM process maps the entire tunability range of the DFG source, spanning over more than two octaves, onto a wavelength region of only 50 nm in the visible, allowing convenient XFROG measurements and revealing the reshaping of few-cycle mid-IR field waveforms by molecular rovibrational modes.

Sub- and superluminal velocity of supercontinuum pulses propagating in scattering media

The XFROG technique has been applied to measuring temporal shifts of supercontinuum pulses propagating in scattering media consisting of spherical beads. Negative (subluminal) and positive (superluminal) changes in the group velocity have been measured. This behaviour can be explained in terms of spectral scattering losses and is directly related to the complex time formulation used in other phenomena where tunnelling times are used to describe the velocity changes. A numerical simulation of the XFROG measurement is consistent with the experimental results.

Ultrafast All Optical Switching using Pulse Trapping by Ultrashort Soliton pulse in Birefringent Optical Fiber

Ultrafast all optical switching by use of pulse trapping in birefringent optical fiber is demonstrated both experimentally and numerically. The wavelength of the control soliton pulse and the trapped soliton pulse is shifted to satisfy the condition of group-velocity matching. Furthermore, the energy of trapped pulse is increased through Raman gain of control pulse. Only a signal pulse among three pulses with temporal separation of about 1.2 ps is successfully picked off. The repetition frequency corresponds to 0.83THz. The spectrogram of the optical switching is directly observed using the X-FROG technique.

Ultrafast all optical switching by use of pulse trapping across zero-dispersion wavelength

Ultrafast all optical switching by use of pulse trapping across zero dispersion wavelength in optical fiber is demonstrated both experimentally and numerically for the first time. Only an arbitrary single pulse among four pulses with temporal separation of about 1.5 ps is successfully picked off with almost perfect extinction ratio. The spectrogram of the optical switching is directly observed using the X-FROG technique. The characteristics of all optical switching are analyzed numerically by the use of strict coupled nonlinear Schrödinger equations and the numerical results are in agreement with the experimental ones. It is interesting to note that although the other pulses are also overlapped with the soliton pulse, they are not trapped.

Few-cycle mid-infrared pulse generation, characterization, and coherent propagation in optically dense media

The generation of four- to five-cycle mid-infrared pulses using a single-stage potassium niobate optical parametric amplifier (OPA) is demonstrated. The OPA is pumped by a gain-switched Ti:Sapphire regenerative amplifier and is seeded with the near-infrared portion of a white light continuum. The OPA is continuously tunable from 2700 to 4700 nm, and maintains sub-65 fs pulses that are fully characterized using cross-correlation frequency-resolved optical gating (XFROG). These are the shortest near-transform-limited pulses reported over this large infrared spectral range. This apparatus is used to measure ultrafast vibrational responses, specifically, pulse profiles modified by the free-induction decay (FID) of O–H oscillators. Both the intensity and instantaneous phase of these pulses are determined after traversing samples of isotopically diluted water (HDO in D2O) using XFROG, representing a new application of the XFROG technique to the study of (dipolar) molecular responses. Pronounced beating of the trailing edges of FID-modified pulses is observed even below optical densities of one, and the details of these features are found to depend on the chirp of the field used to generate the FID. These results indicate that discretion should be used in the selection of sample conditions for nonlinear infrared spectroscopic measurements.