Frequency-resolved Optical Gating Research Articles

Simplifying tailored generation of complex structured femtosecond pulses with easily fabricated phase plates

This article presents a novel approach to targeted 4f pulse shaping using phase plates fabricated by single-point diamond turning (SPDT) machining. The manufacturing of the phase plates using SPDT is versatile, cost-effective, fast, robust, and applicable across a wide range of optical materials, spanning from visible to far-infrared spectra (e.g., PMMA, ZnSe). Manufactured profiles can be used for phase manipulation and pulse structuring, analogously to programable spatial light modulators (SLM). We demonstrate that the pulse waveforms can be reproduced with high fidelity by simple simulations based on calculating optical path differences induced by the phase plate for each wavelength and taking into account the finite focal spot. The simulated and reconstructed frequency-resolved optical gating spectrograms featured G errors between 1–2% and intensity errors between 0.02–0.06. Even for complex structured pulses with the rms value of the time-bandwidth product reaching 12, our method maintains high precision, in some cases even reaching lower G error compared to simpler waveforms. Finally, we also show that the phase plate can be used to attain a set of uncorrelated pulse waveforms by moving the plate relatively to the dispersed laser spectrum. Overall, this approach bypasses common limitations associated with pulse shaping using SLMs, such as pixelation, pixel cross-talk, and spectral or laser fluences constraints.

An experimental study of intensity‐phase characterization of femtosecond laser pulses propagated through a polymethyl methacrylate

AbstractThis article presents a comprehensive investigation into the intensity‐phase characteristics of ultrashort laser pulses propagated through a 10 mm thick polymethyl methacrylate (PMMA) using frequency‐resolved optical gating (FROG) and cross‐correlation frequency‐resolved optical gating (XFROG) techniques. The study focuses on discerning characteristic changes that are induced by the PMMA on the laser pulse. The results reveal that the phase variations exhibit a consistent trend and pattern. At the same time, the temporal full width at half maximum (FWHM) of both FROG and XFROG measurements displays only minor changes. The overall analysis indicates that PMMA induces no discernible modifications in the pulse, thus positioning it as a viable optical element. The analysis of reference FROG, FROG, and XFROG with PMMA is compared and discussed, showing that this analysis provides a foundation for optimizing PMMA's utilization in the optical components of ultrafast laser systems. This experimental study strongly affirms that PMMA could be a potential candidate as an optical element for laser pulses.

Dispersion scan frequency resolved optical gating for consistency check of pulse retrieval.

Many methods commonly used to characterize ultrafast laser pulses, such as the frequency-resolved optical gating (FROG) or the dispersion scan (d-scan), face problems when they are used on pulses with a spectrum or phase varying within the laser beam cross section or the acquisition time. The presence of such pulse shape variation leads to discrepancy between the measured FROG trace and its reconstructed counterparts. Nevertheless, it is difficult to reliably discern this shape variation because even the distorted experimental FROG trace can be reasonably reproduced by a realistic pulse shape. In this work, we examine and discern the variation of the pulse shape based on a new method, dispersion-scan FROG (D-FROG), which combines the idea of dispersion scanning with the FROG method. This technique provides a means of careful evaluation of the laser pulse based on a set of FROG traces connected by known dispersion changes. Therefore, this method can disclose seemingly correct pulse retrievals from distorted datasets. The D-FROG method can be used as a simple extension of the FROG technique to provide a consistency check able to identify the shortcomings in the pulse characterization.

Temporal and spectral signatures of the interaction between ultrashort laser pulses and Bloch surface waves

The resonant excitation of Bloch Surface Waves (BSWs) in dielectric one-dimensional photonic crystals is becoming a realistic photonic solution for surface integration in many domains, from spectroscopy to local field management. Bringing BSWs to ultrafast and nonlinear regimes requires a deep knowledge of the effects that the photonic crystal dispersion and the resonant surface wave excitation have on the ultrashort laser pulses. We report on the experimental evidence of spectral and temporal modifications of the radiation leaving a planar one-dimensional photonic crystal after coupling to BSWs. In such a resonant condition, a characteristic long temporal tail is observed in the outgoing pulses. Observations are performed by employing both frequency-resolved optical gating and field cross-correlation techniques.

Vortex plate retarder-based approach for the generation of sub-20 fs light pulses carrying orbital angular momentum

We use a vortex retarder-based approach to generate few optical cycles light pulses carrying orbital angular momentum (OAM) (known also as twisted light or optical vortex) from a Yb:KGW oscillator pumping a noncollinear optical parametric amplifier generating sub-10 fs linearly polarized light pulses in the near infrared spectral range (central wavelength 850 nm). We characterize such vortices both spatially and temporally by using astigmatic imaging technique and second harmonic generation-based frequency resolved optical gating, respectively. The generation of optical vortices is analyzed, and its structure reconstructed by estimating the spatio-spectral field and Fourier transforming it into the temporal domain. As a proof of concept, we show that we can also generate sub-20 fs light pulses carrying OAM and with arbitrary polarization on the first-order Poincaré sphere.

Highly precise timing alignment of multi-wavelength interleaved cavity-less pulse sources with FROG.

Strictly uniform time interval between adjacent channels is a crucial requirement for the multi-wavelength interleaved (MWI) pulse sources, which difficult alignment can be easily solved by what we believe to be our novel method based on frequency-resolved optical gating (FROG). By utilizing highly precise measurements from FROG, which provide fully two-dimensional information of the pulses in time and frequency domain, we can intuitively identify the time mismatches between different channels in the MWI pulse sources. This enables us to directly align the timing of each channel with sub-picosecond resolution at the first time. MWI pulse sources with total repetition rate of 20.8 GHz (four wavelengths) and 41.6 GHz (eight wavelengths) are precisely aligned by the proposed method, this achievement will pave the way for advancements in photonic analog-digital converters (PADC), high-speed optical communications and so on.

Flexible experimental platform for dispersion-free temporal characterization of ultrashort pulses.

The precise temporal characterization of laser pulses is crucial for ultrashort applications in biology, chemistry, and physics. Especially in femto- and attosecond science, diverse laser pulse sources in different spectral regimes from the visible to the infrared as well as pulse durations ranging from picoseconds to few femtoseconds are employed. In this article, we present a versatile temporal-characterization apparatus that can access these different temporal and spectral regions in a dispersion-free manner and without phase-matching constraints. The design combines transient-grating and surface third-harmonic-generation frequency-resolved optical gating in one device with optimized alignment capabilities based on a noncollinear geometry.

The femtosecond structure of extreme contrast, multi-terawatt second-harmonic laser pulses at 400 nm

Ultrahigh intensity contrast and short pulse laser–solid interactions offer an attractive platform for investigating high-energy-density matter, particularly in the context of structured and ultra-thin targets that form hot, dense plasma conditions. Harmonic generation can improve the contrast of laser pulses by several orders of magnitude. In this study, we present the characterization of extreme contrast, relativistic intensity second-harmonic pulses at 400 nm, using the self-diffraction frequency-resolved optical gating technique. The 400 nm pulses were generated at various input intensities using potassium dihydrogen phosphate and lithium triborate crystals. Our observations reveal the presence of spectral broadening, pulse compression, and complex structures at higher input intensities. We see that extreme contrast, few tens of femtosecond pulses can have multiple “prepulses” at the 100s femtosecond scale as large as ten percent of the peak value. These can preionize a solid significantly and may influence the interaction. Simulations based on nonlinear pulse propagation equations reinforce our findings.

Time-Dependent, Two-Photon Rabi Frequency Shift Observed in Multi-Frequency Raman Spectra

It has been observed that when the multi-frequency Raman scattering process is pumped by two frequency chirped pulses in the transient regime, the anti-Stokes orders are spectrally broadened on the lower-frequency side, causing each of the orders to have a broader spectrum with double-peak pattern. In this article, we employ frequency-resolved optical gating to study the spectral shift of the first anti-Stokes Raman order. A double-pulse model is introduced to determine the electric field of the anti-Stokes orders from the experimental FROG spectrogram. With the help from the double-pulse model, we demonstrate that the double-peaked spectrum comes from linear Raman scattering between manifolds in the two-photon dressed states.

Low-threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate.

We report on high average power, low threshold supercontinuum generation in a homogeneous bulk material at 76 MHz pulse repetition rate with amplified as well as unamplified pulses from a Yb:KGW oscillator. An octave-spanning supercontinuum was produced in undoped potassium gadolinium tungstate (KGW), which demonstrated robust, damage-free long-term performance with a total average pump power of 6.4 W. The supercontinuum generation was unambiguously attested by the distinctive features of the phenomenon: beam filamentation visualized via filament-induced luminescence; conical emission and its characteristic angular distribution captured by angle-resolved spectral measurements; and pulse splitting that produced the sub-pulses with well-behaved phases, as retrieved from the measurements employing a second harmonic frequency-resolved optical gating technique.

Direct temporal characterization of sub-3-fs deep UV pulses generated by resonant dispersive wave emission.

We report on the complete temporal characterization of ultrashort pulses, generated by resonant dispersive wave emission in gas-filled hollow-capillary fibers, with energy in the microjoule range and continuously tunable from the deep-ultraviolet to the ultraviolet. Temporal characterization of such ultrabroad pulses, particularly challenging in this spectral region, was performed using an all-in-vacuum setup for self-diffraction frequency resolved optical gating (SD-FROG). Sub-3-fs pulses were measured, tunable from 250 nm to 350 nm, with a minimum pulse duration of 2.4 ± 0.1 fs.

The Temporal Measurement of Infrared Femtosecond Fiber Laser Pulse with Complex Spectra: SPIDER and FROG

Femtosecond fiber laser pulse (FFLP) is very suitable for precision laser processing (PLP) with the advantages of high power, compact integrated structure, and easy installation. However, with the power increasing, nonlinear optical effects (NOE) of fiber, such as self-phase modulation (SPM), will be greatly enhanced, and the spectra of the output pulse will be very complex, which considerably increases the temporal complexity of FFLP and takes an immeasurable influence on PLP, so it is of great significance to accurately measure the temporal property of FFLP for higher accuracy of PLP. This paper presents a self-referencing spectral phase interferometry for direct electric-field reconstruction based on a single grating stretcher (SGS-SPIDER) with the two-step phase shift (TSPS) method for measuring FFLP with complex and infrared spectra. Meanwhile, we present a modified pulse-retrieval algorithm called an adaptive pulse-retrieval algorithm (APRA) of frequency-resolved optical gating (FROG) to reconstruct the measured pulse accurately. The duration of the reconstructed pulse measured by FROG is 162 fs, whose autocorrelation curve is similar to the measured autocorrelation curve including the picosecond background. The error of the reconstructed FROG trace is 0.42%, which is reliable enough according to the literature experience. Finally, the unique advantages of FROG for measuring complex ultrashort pulses such as FFLP are compared with SPIDER; that is, FROG has a wider spectral phase measuring range than SPIDER. These studies provide a new diagnostic technique for the complete temporal characterization of FFLP with complex spectra and extend the measurement range and application of SPIDER at the infrared wavelength band, which is significant for improving the accuracy of PLP using FFLP.

Single-pulse, reference-free, spatiotemporal characterization of ultrafast laser pulse beams via broadband ptychography.

Ultrafast laser pulse beams are four-dimensional, space-time phenomena that can exhibit complicated, coupled spatial and temporal profiles. Tailoring the spatiotemporal profile of an ultrafast pulse beam is necessary to optimize the focused intensity and to engineer exotic spatiotemporally shaped pulse beams. Here we demonstrate a single-pulse, reference-free spatiotemporal characterization technique based on two colocated synchronized measurements: (1) broadband single-shot ptychography and (2) single-shot frequency resolved optical gating. We apply the technique to measure the nonlinear propagation of an ultrafast pulse beam through a fused silica window. Our spatiotemporal characterization method represents a major contribution to the growing field of spatiotemporally engineered ultrafast laser pulse beams.

Construction of a Mirror-Dispersion-Controlled Kerr Lens Mode-Locked Ti:Sapphire Pulse Laser Oscillator

We have successfully constructed a Kerr lens mode-locked (KLM) Ti:sapphire oscillator to generate ultrashort pulses of ~18 fs. The oscillator consists of only 5 elements including a pair of double-chirped mirrors to balance negative group delay dispersion in the cavity. The bandwidth of the mode-locked laser oscillator spectrum is from 600 nm to 950 nm and pulse energy is 1.5 nJ. Both the output spectrum and the output power were stable against environmental disturbance. For the pulse characterization, a second harmonic generation frequency resolved optical gating method was used.

Fast Spectrum Measurements Using Optical Computing

Traditional spectrometers are too slow for the continuous real-time measurement of rapidly varying spectrum. Dispersive Fourier transformation (DFT) technology replaces diffraction grating and detector array in traditional spectrometers with dispersive elements and photodiodes, and therefore reaches MHz-level spectral capture rate. In addition, frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction (SPIDER) are also important ultrafast spectroscopy technologies in relevant fields. However, these approaches have to use mode-locked pulses as probe light, which may cause damages to some samples due to high peak power. Therefore, the requirement for light source restricts the scope of these technologies’ applications. Here, we report a general technique using optical computing for fast spectrum measurements, to overcome the bottleneck of light source in existing ultrafast spectroscopy measurement technologies. The proposed method has no strict requirement on the probe light. It means the common light source could also be applied for fast real-time spectroscopic measurements. In experiments, CW signals with four different optical spectra have been measured at capture speed of 1MHz respectively. The proposed method can overcome the limitations of existing ultrafast spectroscopy technologies in light source, and therefore provide a powerful tool for investigating rapid transient phenomena in many applications.

Pulse Measurement from a Polluted Frequency Resolved Optical Gating Trace Based on Half-Trace Retrieval Algorithm

A half-trace retrieval algorithm based on an extended-ptychographical iterative engine algorithm is proposed to reconstruct the temporal structure of pulse from a polluted and recorded frequency-resolved optical gating (FROG) trace that was modulated by poor spatial profile of output pulses, stray light, or misalignment of the measurement setup. In the proposed algorithm, the probe pulse and the gated pulse were retrieved simultaneously from a recorded FROG trace with a half-delay range, and the measured pulse was obtained by combining the different edges of the probe pulse and the gated pulse. Numerical simulations were carried out to verify the feasibility of the proposed algorithm. A single-shot picoseconds (ps)−THG−FROG setup with a 100-μJ ps laser system and an online ps−SHG−FROG setup in PW laser system were built to test the proposed algorithm experimentally. The results show that the temporal structure of pulses retrieved by the half-trace retrieval algorithm is closer to the real temporal structure than that retrieved by the conventional ptychographical algorithm when the recorded FROG trace is badly polluted.

Study of Ion-Assisted E-Beam Evaporation Coating Process on Chirped Mirrors

A chirped mirror designed with 60 fs2 in group delay dispersion and reflectance large than 97.5% in the bandwidth between 700 and 900 nm by open-sourced thin film filter design software is presented. The chirped mirrors are then fabricated by an ion-assisted deposition coater. First, the optical constants of the thin films deposited by specific process parameters are calibrated by trial runs with single-layer samples measured by variable-angle spectroscopic ellipsometer; then factors are entered to compensate for the settings on film thickness. Afterward, the spectral phase of the coated mirrors is measured by polarization gating frequency-resolved optical gating whereas the reflectance is measured by photo-spectrometer. In conclusion, the measured results indicate a reasonable match with the design goals to show the effectiveness of the proposed method. It is noted that ion-assisted electron gun evaporation coating systems are applicable to the mass production of chirped mirrors employed for ultrafast lasers.

Eliminating local convergences in FROG retrieval algorithms

The complicated electric field structure of ultrashort pulses as characterized by frequency-resolved optical gating (FROG) algorithms often results in local convergence, which does not accurately represent the actual field pulse. We present an efficient and universal test procedure applicable to any FROG retrieval algorithm that allows recognition of the erroneous local convergence. The 100% efficacy of the procedure is demonstrated using Line-Search FROG algorithm, and comparison is given with the performance of a standard extended ptychographic iterative FROG trace retrieval engine.

Reliable determination of pulse-shape instability in trains of ultrashort laser pulses using frequency-resolved optical gating

We describe a reliable approach for determining the presence of pulse-shape instability in a train of ultrashort laser pulses. While frequency-resolved optical gating (FROG) has been shown to successfully perform this task by displaying a discrepancy between the measured and retrieved traces for unstable trains, it fails if its pulse-retrieval algorithm stagnates because algorithm stagnation and pulse-shape instability can be indistinguishable. So, a non-stagnating algorithm—even in the presence of instability—is required. The recently introduced Retrieved-Amplitude N-grid Algorithmic (RANA) approach has achieved extremely reliable (100%) pulse-retrieval in FROG for trains of stable pulse shapes, even in the presence of noise, and so is a promising candidate for an algorithm that can definitively distinguish stable and unstable pulse-shape trains. But it has not yet been considered for trains of pulses with pulse-shape instability. So, here, we investigate its performance for unstable trains of pulses with random pulse shapes. We consider trains of complex pulses measured by second-harmonic-generation FROG using the RANA approach and compare its performance to the well-known generalized-projections (GP) algorithm without the RANA enhancements. We show that the standard GP algorithm frequently fails to converge for such unstable pulse trains, yielding highly variable trace discrepancies. As a result, it is an unreliable indicator of instability. Using the RANA approach, on the other hand, we find zero stagnations, even for highly unstable pulse trains, and we conclude that FROG, coupled with the RANA approach, provides a highly reliable indicator of pulse-shape instability. It also provides a typical pulse length, spectral width, and time-bandwidth product, even in cases of instability.

Random quasi-phase-matching for pulse characterization from the near to the long wavelength infrared.

Experiments requiring ultrafast laser pulses require a full characterization of the electric field to glean meaning from the experimental data. Such characterization typically requires a separate parametric optical process. As the central wavelength range of new sources continues to increase so too does the need for nonlinear crystals suited for characterizing these wavelengths. Here we report on the use of poly-crystalline zinc selenide as a universal nonlinear crystal in the frequency resolved optical gating characterization technique from the near to long-wavelength infrared. Due to its property of random quasi-phase-matching it's capable of phase matching second-harmonic and sum-frequency generation of ultra-broadband pulses in the near and long wavelength infrared, while being crystal orientation independent. With the majority of ultra-fast laser sources being in this span of wavelengths, this work demonstrates a greatly simplified approach towards ultra-fast pulse characterization spanning from the near to the long-wavelength infrared. To our knowledge there is no single optical technique capable of such flexible capabilities.