Frequency-resolved Optical Gating Research Articles

Temperature and Pulse-Energy Range Suitable for Femtosecond Pulse Transmission in Si Nanowire Waveguide

We experimentally measured the femtosecond pulse transmission through a silicon-on-insulator (SOI) nanowire waveguide under different temperatures and input pulse energy with a cross-correlation frequency-resolved optical gating (XFROG) measurement setup. The experimental results demonstrated that the temperature and pulse energy dependence of the Si photonic nanowire waveguide (SPNW) is interesting rather than just monotonous or linear, and that the suitable temperature and pulse-energy range is as suggested in this experiment, which will be valuable for analyzing the practical design of the operating regimes and the fine dispersion engineering of various ultrafast photonic applications based on the SPNWs. The research results will contribute to developing the SPNWs with photonic elements and networks compatible with mature complementary metal–oxide–semiconductors (CMOS).

Reconstruction of an ultrashort laser pulse from flexibly shaped FROG traces via ptychography algorithm

Frequency-resolved optical gating (FROG) is a popular technique for characterizing ultrashort laser pulses. This technique comprises two steps. First, a spectrogram, named FROG trace, is constructed for a pulse by measuring process. Second, an efficient algorithm is employed to retrieve the pulse information from its FROG trace. By expressing an arbitrary FROG trace using matrix multiplication, we confirm that the data on all the complex spectral components constituting the relative pulse are, in principle, encoded in the value of each point of the trace. Considering this fact and the characteristics of the measuring devices, flexibly shaped FROG traces are built by simultaneously applying low-pass filtering and up-sampling along the frequency axis and down-sampling along the delay axis to the conventional square ones. Furthermore, these practical traces can be used for the effective reconstruction of ultrashort laser pulses via the ptychography algorithm. Utilizing this specially designed trace will significantly reduce the difficulty and time-consumption in the first step of the FROG process.

Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO4 laser.

We demonstrate sub-100-fs Kerr-lens mode-locking of a Tm:MgWO4 laser emitting at ∼2µm assisted by a single-walled carbon-nanotube saturable absorber. A maximum average output power of 100 mW is achieved with pulse duration of 89 fs at a pulse repetition rate of ∼86MHz. The shortest pulse duration derived from frequency-resolved optical gating amounts to 76 fs at 2037 nm, corresponding to nearly bandwidth-limited pulses. To the best of our knowledge, these are the shortest pulses generated from any Tm-doped tungstate crystal and the first report on saturable absorber assisted Kerr-lens mode-locking of a Tm laser at ∼2µm.

Compensation of optical nonlinearities in a femtosecond laser system in a broad operation regime

We present here a detailed study of a compact and tunable nonlinear phase pre-compensation method based on a tunable chirped fiber Bragg grating (TCFBG) applied to an ultrafast fiber laser system. Frequency resolved optical gating (FROG) measurements reveal a significant variation in the output pulse shape due to the variation in accumulated nonlinear phase for varying pulse energy and repetition rate (even if the pulse energy is kept constant) of the laser system. The TCFBG, used as a stretcher and a pre-compensating element in this chirped fiber amplification (CPA) system, eliminates the need for additional pulse shaping components which greatly simplifies the system for tunable pre-compensation. We demonstrate an efficient pre-compensation of the accumulated nonlinear phase between 0 and 14 rad resulting in pulses with a constant pulse duration and Strehl ratio over a broad operating regime of the laser with repetition rates ranging from 100 kHz up to 30 MHz and pulse energies spanning from 1 µJ up to 70 µJ.

Generation of Intense Sub-10 fs Pulses at 385nm* *Supported by the National Key Research and Development Program of China (Grant No. 2019YFA0307703), the Major Research Plan of NSF of China (Grant No. 91850201), and the National Natural Science Foundation of China (Grant Nos. 11974426, 11974425, U1830206 and 11604386).

We demonstrated the generation and characterization of 9.7 fs, 180 μJ pulses centered at 385 nm via the frequency doubling of few-cycle near-infrared pulses. Both moderate conversion efficiency (9.5%) and broad phase matching bandwidth (20 nm) were achieved by shaping the spectra of the fundamental pulses. The strong intensity dependence of second-order harmonic generation and well controlled material dispersion ensured the inexistence of satellite pulses, which was confirmed by the self-diffraction frequency resolved optical gating measurement.

Highly coherent multi-octave polarization-maintained supercontinuum generation solely based on ZBLAN fibers.

We present a highly stable polarization-maintained supercontinuum (SC) using a setup solely based on ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers. The pumping source consists of a femtosecond master oscillator fiber amplifier based on thulium-doped ZBLAN fibers. It provides multi-watts of output power with the center wavelength of 1920 nm at 1 MHz repetition rate. The SC generated by pumping an elliptical core passive single-mode ZBLAN fiber spans from 350 nm to 4.5 µm and exhibits high stability. We characterized the SC pulse using sum-frequency cross-correlation frequency resolved optical gating.

Single-scan, dual-functional interferometer for fast spatio-temporal characterization of few-cycle pulses.

Accurate and fast characterization of spatio-temporal information of high-intensity, ultrashort pulses is crucial in the field of strong-field laser science and technology. While conventional self-referenced interferometers were widely used to retrieve the spatial profile of the relative spectral phase of pulses, additional measurements of temporal and spectral information at a particular position of the laser beam, however, were necessary to remove the indeterminacy, which increases the system complexity. Here we report an advanced, dual-functional interferometer that is able to reconstruct the complete spatio-temporal information of ultrashort pulses with a single scan of the interferometer arm. The setup integrates an interferometric frequency-resolved optical gating (FROG) with a radial shearing Michelson interferometer. Through scanning one arm of the interferometer, both the cross-correlated FROG trace at the central part of the laser beam and the delay-dependent interferograms of the entire laser profile are simultaneously obtained, allowing a fast three-dimensional reconstruction of few-cycle laser pulses.

Self-Referencing Spectral Interferometric Probing of the Onset Time of Relativistic Transparency in Intense Laser-Foil Interactions

Irradiation of an ultrathin foil target by a high intensity laser pulse drives collective electron motion and the generation of strong electrostatic fields, resulting in ultrabright sources of high-order harmonics and energetic ions. The ion energies can be significantly enhanced if the foil undergoes relativistic self-induced transparency during the interaction, with the degree of enhancement depending in part on the onset time of transparency. We report on a simple and effective approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light, providing a route to optically controlling and optimizing ion acceleration and radiation generation. The scheme involves a self-referencing approach to spectral interferometry, in which coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil. The relative timing of the onset of transmission with respect to the transition radiation generation is determined from spectral fringe spacing and compared to simultaneous frequency-resolved optical gating measurements. The results are in excellent agreement, and are discussed with reference to particle-in-cell simulations of the interaction physics and an analytical model for the onset time of transparency in ultrathin foils.

Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate

The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system.

Encoding the complete electric field of an ultraviolet ultrashort laser pulse in a near-infrared nonlinear-optical signal.

We introduce a variation on the cross-correlation frequency-resolved optical gating (XFROG) technique that uses a near-infrared (NIR) nonlinear-optical signal to characterize pulses in the ultraviolet (UV). Using a transient-grating XFROG beam geometry, we create a grating using two copies of the unknown UV pulse and diffract a NIR reference pulse from it. We show that, by varying the delay between the UV pulses creating the grating, the UV pulse intensity-and-phase information can be encoded into a NIR signal. We also implemented a modified generalized-projections phase-retrieval algorithm for retrieving the UV pulses from these spectrograms. We performed proof-of-principle measurements of chirped pulses and double pulses, all at 400 nm. This approach should be extendable deeper into the UV and potentially even into the extreme UV or x-ray range.

Synthesis of ultrafast waveforms using coherent Raman sidebands

In this work, we implement a scheme to combine six coherent, spatially separated Raman sidebands generated in single-crystal diamond into a collinear beam. With appropriate phase tuning, this results in a pulse much shorter than the generating pump. We elucidate the characteristics of the synthesized pulse by using an interferometric collinear cross-correlation frequency-resolved optical gating setup (ix-FROG). The beating of the synchronized sidebands results in an additional component in the signal, which we use to optimize the relative phases of our sidebands. In this way, we synthesize and measure visible-range, near-single cycle isolated pulses of approximately 5 fs total pulse duration.

Development of single-shot frequency-resolved optical gating for characterizing the instantaneous intensity and phase of LFEX laser pulses

Frequency-resolved optical gating (FROG) is a novel means of measuring the fast motion of a critical density surface during relativistic laser–plasma interaction. Herein, we present a design and demonstration results for a new single-shot FROG system and optical transport system for characterizing the instantaneous intensity and phase at the LFEX (Laser for Fast Ignition Experiment) laser facility at the Institute of Laser Engineering of Osaka University. At LFEX, the laser intensity at the vacuum window is intrinsically high because of two unique properties, namely, the large F-number of the off-axis parabolic mirror and the small radius of the interaction chamber. Consequently, to obtain an accurate FROG trace, attention must be paid to spectrum modulation due to self-phase modulation. The appropriate laser intensity for FROG operation was investigated experimentally, and an optical transport system with an energy attenuator composed of reflective optics was designed to eliminate the concern of spectrum modulation from measurements. A FROG trace recorded at LFEX shot with 161 J energy was reconstructed 100 times using an iterative phase-retrieval algorithm. Despite some differences in structure, the reconstructed spectrum agrees reasonably well with the spectrum obtained by a time-integrated spectrometer. This shows that the developed FROG system and the optical transport system can measure the instantaneous intensity and phase of a laser pulse without spectrum modulation.

Thin plate compression of a sub-petawatt Ti:Sa laser pulses

By extending the concept of thin film compression [Mourou et al., Eur. Phys. J. Spec. Top. 223(6), 1181 (2014)] to a thin plate, nonlinear post-compression from 24 fs to 13 fs of sub-petawatt laser pulses is demonstrated experimentally using a 1 mm-thick silica plate and chirped mirrors with a total anomalous dispersion of −50 fs2. The measurements were implemented with a specially designed dispersionless vacuum frequency-resolved optical gating, which is based on second harmonic generation of tested pulses in a 10 μm β-barium borate crystal glued on a 1 mm fused silica substrate. The used compression scheme is implemented in a geometry compatible with high power on-target experiment realization.

Picosecond frequency-resolved optical gating based on a modified ptychographic-based algorithm for use in a petawatt laser

Accurate temporal characterization both in intensity and phase distribution is important in the diagnosis of the petawatt (PW) class. We present a single-shot picosecond frequency-resolved optical gating (ps-FROG) setup based on an autocorrelator with ps measurement range that is spectrally resolved through a fine grating. The modified ptychographic-based algorithm with a changing update coefficient was used for the reconstruction of the pulse distribution; it can better adapt to the reconstruction of pulse with a large time–bandwidth product. We calibrated and verified the homemade ps-FROG in a 100-μJ ps laser system and used it to characterize the pulse distribution generated by the PW laser system of the Shen Guang II facility. The system shows good performance and high accuracy in reconstructing the intensity and phase distributions of a ps pulse, which provides reference for accurately adjusting the grating pair to acquire the pulse width as a preset.

Few-optical-cycle pulse generation based on a non-linear fiber compressor pumped by a low-energy Yb:CALGO ultrafast laser.

Pulse compression in a short, normal dispersion photonic-crystal fiber is investigated with a Yb:CaGdAlO4 laser pumped by a low-power fiber-coupled single-mode diode that delivers 70-fs pulses at 1050 nm central wavelength, with 45-mW average power at 60 MHz repetition rate. A simple and power-efficient compressor based on a ∼15-cm long, low-cost commercial nonlinear fiber, with normal dispersion at the laser wavelength, produces pulses as short as 14.9 fs, corresponding to ∼4.25 optical cycles, with 29 mW average power after a prism-pair compressor in double pass configuration. Pulse quality was investigated with frequency resolved optical gating (FROG) analysis. Furthermore, a comparative analysis of noise properties of the oscillator, pump laser and compressed pulses has been performed.

Transient grating in a thin gas target for characterization of extremely short optical pulses.

A transient-grating cross-correlation frequency-resolved optical gating (TG XFROG) with a thin gas target toward characterization of sub-femtosecond optical pulses is discussed. For evaluation of the reliability, sub-10 fs near-infrared pulses are characterized, the results of which are compared with those given by the sum-frequency-generation XFROG. The TG XFROG covers the nanojoule energy range or that for the advanced few-cycle UV pulses recently reported. It is also shown that the TG XFROG fails to characterize and heavily underestimates the durations of intense test pulses. The FROG technique sensitively detects the onset of this anomalous behavior, which represents a serious issue for pulse characterizations.

Robustness of the ePIE algorithm for the complete characterization of femtosecond, extreme ultra-violet pulses.

Frequency-resolved optical gating for the complete reconstruction of attosecond bursts (FROG-CRAB) is a well-known technique for the complete temporal characterization of ultrashort extreme ultraviolet (XUV) pulses, with durations down to a few tens of attoseconds. Recently, this technique was extended to few-femtosecond XUV pulses, produced by high-order harmonic generation (HHG) in gases, thanks to the implementation of a robust iterative algorithm: the extended ptychographic iterative engine (ePIE). We demonstrate, by using numerical simulations, that the ptychographic reconstruction technique is characterized by an excellent degree of convergence and robustness. We analyse the effects on pulse reconstruction of various experimental imperfections, namely, the jitter of the relative temporal delay between the XUV pulse and a suitably delayed infrared (IR) pulse and the noise of the measured FROG-CRAB spectrograms. We also show that the ePIE approach is particularly suitable for the reconstruction of incomplete FROG-CRAB spectrograms (i.e., spectrograms with a reduced number of measured time delays) and of spectrograms acquired with a reduced spectral resolution, particularly when relatively high-intensity IR pulses are employed.

Investigation of continuum generation in the non-zero dispersion-shifted fiber pumped by femtosecond nanojoule pulses in 1450–1800 nm spectral range

A synchronously pumped optical parametric oscillator based on periodically poled potassium titanyl phosphate (PPKTP) crystal delivering femtosecond nanojoule energy pulses in the 1.45 μm–1.8 μm spectral range is used as pump source for continuum generation in conventional non-zero dispersion shifted optical fiber. Detailed numerical simulations and experiments employing cross-correlation frequency-resolved optical gating (XFROG) reveal soliton and dispersive wave generation dynamics and can help to estimate limits of how far the pump wavelength can be shifted from zero-dispersion wavelength (ZDW) to still obtain sufficient spectrum broadening. We also demonstrate the applicability of recently introduced fiber dispersion measurement technique based on XFROG trace analysis to conventional optical fibers. Combination of this efficient parametric device with standard non-zero dispersion shifted low cost and low loss optical fiber results in efficient continuum generation with output/input average power ratio greater than 50%.

Measurement of 10\u2009fs pulses across the entire Visible to Near-Infrared Spectral Range

Tuneable ultrafast laser pulses are a powerful tool for measuring difficult-to-access degrees of freedom in materials science. In general these experiments require the ability to address resonances and excitations both above and below the bandgap of materials, and to probe their response at the timescale of the fastest non-trivial internal dynamics. This drives the need for ultrafast sources capable of delivering 10–15 fs duration pulses tuneable across the entire visible (VIS) and near infrared (NIR) range, 500– 3000 nm, as well as the characterization of these sources. Here we present a single frequency-resolved optical gating (FROG) system capable of self-referenced characterization of pulses with 10 fs duration across the entire VIS-NIR spectral range. Our system does not require auxiliary beams and only minor reconfiguration for different wavelengths. We demonstrate the system with measurements of pulses across the entire tuning range.

Generation of 88 as Isolated Attosecond Pulses with Double Optical Gating**Supported by the National Key Research and Development Program of China under Grant No. 2019YFA0307703, the Major Research Plan of the National Natural Science Foundation of China under Grant No. 91850201, and the National Natural Science Foundation of China under Grant No. 11974426.

Isolated attosecond pulses with a duration of 88 as are generated in the spectral range of 29–72 eV using double optical gating technique. The gate width is set to be shorter than half the optical cycle to avoid carrier envelop phase stabilization of the 4.2 fs driving laser pulses centered at 800 nm. The attosecond pulse duration is measured with the technique of frequency resolved optical gating for complete reconstruction of attosecond bursts.