Second-harmonic Generation Frequency-resolved Optical Gating Research Articles

Reconstruction of Femtosecond Laser Pulses from FROG Traces by Convolutional Neural Networks

We report on the reconstruction of ultrashort laser pulses from computer-simulated and experimental second harmonic generation-frequency resolved optical gating (SHG-FROG) spectrograms. In order to retrieve the spectral amplitude and phase we use a convolutional neural network trained on simulated SHG-FROG spectrograms and the corresponding spectral-domain fields employed as labels for the network, which is a complex field encompassing the full information about the amplitude and phase. Our results show excellent retrieval capabilities of the neural network in case of the simulated pulses. Although trained only on computer generated data, the method shows promising results regarding experimentally measured pulses.

Ultrashort Pulse Retrieval from Experimental Spectra Transformed in Chalcogenide and Silica Fibers

The characterization of ultrashort optical pulses is a highly requested task. The most popular commercially available hardware/software systems are based on interferometric measurements and second-harmonic generation, leading to some ambiguities and limitations. Here we experimentally test the non-interferometric method of pulse retrieval from three spectra: the fundamental spectrum and two spectra that transformed in an element with Kerr nonlinearity and accumulated different nonlinear phases (different Β-integrals). This method has no ambiguities related to time direction, and allows simple hardware/software implementation. We test a novel simple algorithm for experimental data processing based on the search for a polynomial-approximated spectral phase. Two experimental cases are considered. In the first one, we retrieved 160 fs pulses using a chalcogenide arsenic sulfide glass fiber as a nonlinear Kerr element. In the second case, we retrieved 670 fs pulses with a complex spectrum using a piece of silica-based fiber. The results are confirmed by independent measurements using a standard SHG-FROG technique (Second-Harmonic Generation Frequency-Resolved Optical Gating).

Simultaneous Characterization of Two Ultrashort Optical Pulses at Different Frequencies Using a WS2 Monolayer.

The precise characterization of ultrashort laser pulses has been of interest to the scientific community for many years. Frequency-resolved optical gating (FROG) has been extensively used to retrieve the temporal and spectral field distributions of ultrashort laser pulses. In this work, we exploit the high, broad-band nonlinear optical response of a WS2 monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS2 monolayer enable the simultaneous acquisition of the spectra resulting from both four-wave mixing (FWM) and sum-frequency generation (SFG) nonlinear processes while varying the time delay between the two ultrashort pulses. Next, we introduce an adjusted double-blind FROG algorithm, based on iterative fast Fourier transforms between two FROG traces, to extract the intensity distribution and phase of two ultrashort pulses from the combination of their FWM and SFG FROG traces. Using this algorithm, we find an agreement between the computed and observed FROG traces for both the FWM and SFG processes. Exploiting the broad-band nonlinear response of a WS2 monolayer, we additionally characterize one of the pulses using a second-harmonic generation (SHG) FROG trace to validate the pulse shapes extracted from the combination of the FWM and SFG FROG traces. The retrieved pulse shape from the SHG FROG agrees well with the pulse shape retrieved from our nondegenerate cross-correlation FROG measurement. In addition to the nonlinear parametric processes, we also observe a nonlinearly generated photoluminescence (PL) signal emitted from the WS2 monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

Targeted generation of complex temporal pulse profiles

A targeted shaping of complex femtosecond pulse waveforms and their characterization is essential for many spectroscopic applications. A 4f pulse shaper combined with an advanced pulse characterization technique should, in the idealized case, serve this purpose for an arbitrary pulse shape. This is, however, violated in the real experiment by many imperfections and limitations. Although the complex waveform generation has been studied in-depth, the comparison of the effects of various experimental factors on the actual pulse shape has stayed out of focus so far. In this paper, we present an experimental study on the targeted generation and retrieval of complex pulses by using two commonly-used techniques: spatial-light-modulator (SLM)-based 4f pulse shaper and second-harmonic generation frequency-resolved optical gating (FROG) and cross-correlation FROG (XFROG). By combining FROG and XFROG traces, we analyze the pulses with SLM-adjusted complex random phases ranging from simple to very complex waveforms. We demonstrate that the combination of FROG and XFROG ensures highly consistent pulse retrieval, irrespective of the used retrieval algorithm. This enabled us to evaluate the role of various experimental factors on the agreement between the simulated and actual pulse shape. The factors included the SLM pixelation, SLM pixel crosstalk, finite laser focal spot in the pulse shaper, or interference fringes induced by the SLM. In particular, we observe that including the SLM pixelation and crosstalk effect significantly improved the pulse shaping simulation. We demonstrate that the complete simulation can faithfully reproduce the pulse shape. Nevertheless, even in this case, the intensity of individual peaks differs between the retrieved and simulated pulses, typically by 10–20% of the peak value, with the mean standard deviation of 5–9% of the maximum pulse intensity. We discuss the potential sources of remaining discrepancies between the theoretically expected and experimentally retrieved pulse.

Improving laser standards for three-photon microscopy.

.Significance: Three-photon excitation microscopy has double-to-triple the penetration depth in biological tissue over two-photon imaging and thus has the potential to revolutionize the visualization of biological processes in vivo. However, unlike the plug-and-play operation and performance of lasers used in two-photon imaging, three-photon microscopy presents new technological challenges that require a closer look at the fidelity of laser pulses.Aim: We implemented state-of-the-art pulse measurements and developed innovative techniques for examining the performance of lasers used in three-photon microscopy. We then demonstrated how these techniques can be used to provide precise measurements of pulse shape, pulse energy, and pulse-to-pulse intensity variability, all of which ultimately impact imaging.Approach: We built inexpensive tools, e.g., a second harmonic generation frequency-resolved optical gating (SHG-FROG) device and a deep-memory diode imaging (DMDI) apparatus to examine laser pulse fidelity.Results: First, SHG-FROG revealed very large third-order dispersion (TOD). This extent of phase distortion prevents the efficient temporal compression of laser pulses to their theoretical limit. Furthermore, TOD cannot be quantified when using a conventional method of obtaining the laser pulse duration, e.g., when using an autocorrelator. Finally, DMDI showed the effectiveness of detecting pulse-to-pulse intensity fluctuations on timescales relevant to three-photon imaging, which were otherwise not captured using conventional instruments and statistics.Conclusions: The distortion of individual laser pulses caused by TOD poses significant challenges to three-photon imaging by preventing effective compression of laser pulses and decreasing the efficiency of nonlinear excitation. Moreover, an acceptably low pulse-to-pulse amplitude variability should not be assumed. Particularly for low repetition rate laser sources used in three-photon microscopy, pulse-to-pulse variability also degrades image quality. If three-photon imaging is to become mainstream, our diagnostics may be used by laser manufacturers to improve system design and by end-users to validate the performance of their current and future imaging systems.

Accurate characterization of mid-infrared ultrashort pulse based on second-harmonic-generation frequency-resolved optical gating

We demonstrate a second-harmonic-generation frequency-resolved optical gating (SHG-FROG) apparatus, which can be used to accurately characterize ultrashort mid-infrared laser pulses. An initial guess with constraint both in time and frequency domain is first proposed in principal components generalized projections algorithm (PCGPA), and it outperforms other guesses for the data errors analysis in these SHG-FROG apparatus. A mid-infrared pulse at 3580 nm from our home-built optical parametric amplifier (OPA) system is measured with a good agreement between the retrieved and measured spectrum. Different initial guesses are studied and a further verification is made to compare the measured spectral phase difference with the calculated one by inserting a dispersion plate. The excellent phase agreement verifies the accuracy of the measurement and high reliability of the device.

100% reliable algorithm for second-harmonic-generation frequency-resolved optical gating.

We demonstrate a novel algorithmic approach for the second-harmonic-generation (SHG) frequency-resolved optical gating (FROG) ultrashort-pulse-measurement technique that always converges and, for complex pulses, is also much faster. It takes advantage of the Paley-Wiener Theorem to retrieve the precise pulse spectrum-half the desired information-directly from the measured trace. It also uses a multi-grid approach, permitting the algorithm to operate on smaller arrays for early iterations and on the complete array for only the final few iterations. We tested this approach on more than 25,000 randomly generated complex pulses with time-bandwidth products up to 100, yielding SHG FROG traces to which noise was added, and have achieved convergence to the correct pulse in all cases. Moreover, convergence occurs in less than half the time for extremely large traces corresponding to extremely complex pulses.

Pulse compression of white-light continuum generated at 1053 nm in bulk sapphire: an experimental study

The white-light continuum in optical bulk material has the potential to be used as a seed pulse in optical parametric amplifiers, which entails the compression of seed pulses to temporally overlap with pump pulses. An experimental study of the compression of the white-light continuum is reported in this article. Both chirped mirror compression and prism pair compression methods are used. The 280 fs pulses centered at 1053 nm are employed to generate a white-light continuum in a bulk sapphire plate; the spectrum of a white-light continuum spans from 650 nm to 1000 nm and its second harmonic generation (SHG) is measured. The scanning SHG frequency resolved optical gating technique was used to characterize the compression of the white-light continuum. Prism pair only compression of the (650–1000) nm spectra yielded 190 fs, with chirp mirrors added after leading to 150 fs. Spectral masking of the spectrum below 850 nm inside the prism compressor resulted in a pulse duration of ~120 fs for the 850–1000 nm spectral portion of the pulses.

Complete Field Characterization of Ultrashort Pulses in Fiber Photonics

We report a simple fiber-implemented technique for complete reconstruction of intensity profile and phase of ultrashort laser pulses based on processing only pulse spectrum and two self-phase modulated spectra measured after a short piece of optical fiber. Its applicability is shown on an example of a fiber optical system in the telecommunication range. A retrieval algorithm in a dispersionless approximation and with considering dispersion effects is developed. The obtained results are confirmed by independent measurements using the second-harmonic generation frequency-resolved optical gating technique and by reconstructing purposely introduced signal features. We also provide estimates demonstrating great opportunities for implementing this technique in all-waveguide optical systems ranging from optical communications to nanophotonics with femtojoule pulses as well as to mid-IR photonics, where specialty fibers with huge optical nonlinearities can be used.

Few-cycle optical pulse characterization via cross-polarized wave generation dispersion scan technique.

We demonstrate a dispersion scan (d-scan) pulse characterization scheme employing cross-polarized wave (XPW) generation as a nonlinear optical process. XPW generation is a degenerate four-wave mixing process with no phase-matching limitations. Therefore, its implementation in the d-scan method is a good choice for the characterization of few-cycle pulses in remote spectral regions. We fully characterize 5-10fs pulses delivered through a hollow-core fiber in the near-IR region and compare the results with the second-harmonic generation (SHG) frequency-resolved optical gating and SHG d-scan characterization methods.

X-SEA-F-SPIDER characterization of over octave spanning pulses in the infrared range.

We show a practical implementation of a pulse characterization method for sub-cycle pulse measurements in the infrared spectral range based on spectral shearing interferometry. We employ spatially-encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction with external ancila pulses (X-SEA-F-SPIDER). We show merits and limitations of the setup and an in-depth comparison to another widely used temporal characterization technique - Second-Harmonic Generation Frequency Resolved Optical Gating (SHG-FROG). The X-SEA-F-SPIDER implementation presented in this paper allows measurement of sub-cycle pulses with over one octave wide spectrum spanning the 900-2400 nm range without adding any extra dispersion due to the pulse characterization apparatus.

Ytterbium fiber-based, 270 fs, 100 W chirped pulse amplification laser system with 1 MHz repetition rate

A 100 W Yb-doped, fiber-based, femtosecond, chirped pulse amplification laser system was developed with a repetition rate of 1 MHz, corresponding to a pulse energy of 100 µJ. Large-scale, fused-silica transmission gratings were used for both the pulse stretcher and compressor, with a compression throughput efficiency of ∼85%. A pulse duration of 270 fs was measured by second harmonic generation frequency-resolved optical gating (SHG-FROG). To the best of our knowledge, this is the shortest pulse duration ever achieved by a 100-W-level fiber chirped pulse amplification laser system at a repetition rate of few megahertz, without any special post-compression manipulation.

Reliable multiple-pulse reconstruction from second-harmonic-generation frequency-resolved optical gating spectrograms

Frequency-resolved optical gating (FROG) has become an established method of assessing the amplitude and phase profiles of ultrashort laser pulses. Second-harmonic-generation (SHG) FROG is the most widely used version, because it is relatively simple to set up and operate. However, for some complex pulse shapes it tends to produce false results. Here we discuss the reconstruction of multiple-pulse structures, a situation in which SHG FROG frequently fails. We suggest a modification of the standard procedure and demonstrate that the rate of false results is significantly reduced. We also discuss reconstruction in the presence of noise. A procedure for obtaining error bars is given; these error bars allow us to gauge the quality of the reconstruction.

Unambiguous demonstration of soliton evolution in slow-light silicon photonic crystal waveguides with SFG-XFROG.

We demonstrate the temporal and spectral evolution of picosecond soliton in the slow light silicon photonic crystal waveguides (PhCWs) by sum frequency generation cross-correlation frequency resolved optical grating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. The reference pulses for the SFG-XFROG measurements are unambiguously pre-characterized by the second harmonic generation frequency resolved optical gating (SHG-FROG) assisted with the combination of NLSE simulations and optical spectrum analyzer (OSA) measurements. Regardless of the inevitable nonlinear two photon absorption, high order soliton compressions have been observed remarkably owing to the slow light enhanced nonlinear effects in the silicon PhCWs. Both the measurements and the further numerical analyses of the pulse dynamics indicate that, the free carrier dispersion (FCD) enhanced by the slow light effects is mainly responsible for the compression, the acceleration, and the spectral blue shift of the soliton.

Second-harmonic frequency-resolved optical gating covering two and a half optical octaves using a single spectrometer

We report the measurement of laser pulse shapes covering the range 580–3250 nm using second-harmonic generation frequency-resolved optical gating equipped with a single inexpensive visible-NIR miniature spectrometer and a single pair of homemade broadband beam splitters. Our experimental scheme exploits frequency up-conversion by BBO crystals and appropriate corrections for dispersion, beam splitter filtering and phase-matching efficiency. The signal and idler waves from a commercial optical parametric amplifier pumped by a Ti:Sapphire laser (26 fs, 1 kHz) have been characterized as well as their second harmonic. The pulse shapes out of a commercial difference frequency generation module mixing signal and idler have also been measured up to 3250 nm. The resulting pulses range from 20 to 120 fs, and their chirp characteristics are also exposed. Our approach is demonstrated over most of the doubling crystal transparency range.

Chirp Evolution Algorithm of a Dark-Optical-Comb Injection Mode-Locked SOA Fiber Laser Pulses During Soliton Compression

The chirp evolution of a dark-optical-comb injection harmonic mode-locked (HML) semiconductor optical amplifier fiber ring laser (SOAFL) with a nonlinearly driven electro-absorption modulator is demonstrated. The dispersion compensation of the 10-GHz HML-SOAFL pulse with a 100-m-long dispersion compensation fiber is performed to shorten the pulsewidth from 29 to 2.9 ps with a compression factor of 10, and a time bandwidth product (TBP) of 0.5. The soliton compression with a 10-m-long single-mode fiber is employed to deliver the amplified 10-GHz HML-SOAFL soliton pulse with a peak power of 38.2 W and a pulsewidth of 1.6 ps, while the spectrum is broadened from 1.4 to 2.3 nm with a TBP of 0.45. To monitor the dynamic chirp evolution, a second-harmonic generation frequency-resolved optical gating (SHG-FROG) is used to retrieve the amplitude and phase contour profile of the HML-SOAFL. The phase fitting result distinguishes the linear chirp from the nonlinear chirp associated with the HML-SOAFL pulse, which are 0.016 ps2 and 0.002 ps3. The Schrodinger equation assists to retrieve the phase evolution of the 10-GHz HML-SOAFL with negative group velocity dispersion. Disregarding the linear dispersion compensation and soliton compression, the original 10-GHz HML-SOAFL pulse with a negative GDD of -5 ps2 is quantified.

Compression of CEP-stable multi-mJ laser pulses down to 4 fs in long hollow fibers

Carrier–envelope phase stable 4 fs near-IR pulses with 3 mJ energy were generated by spectral broadening of circularly polarized 8 mJ pulses in a differentially pumped 2 m long composite stretched flexible hollow fiber. The pulses were characterized using both second-harmonic generation frequency-resolved optical gating (SHG-FROG) and SHG d-scan methods.

Fast second-harmonic generation frequency-resolved optical gating using only a pulse shaper

In many ultrafast contexts, a collinear pulse-shaping frequency-resolved optical gating (FROG) technique is desired. Some applicable techniques already exist, but they suffer from one of two issues: either they require many time points to allow for Fourier filtering, or they do not yield a traditional FROG trace. To overcome these issues, we propose and demonstrate a fast new phase-cycled FROG technique using a pulse shaper.

Selective Excitation of Two-pulse Femtosecond Coherent Anti-Stokes Raman Scattering in a Mixture

In this paper, we experimentally study the selective excitation of two-pulse femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture of dibromomethane (CH2Br2) and chloroform (CHCl3) by adaptive pulse shaping based on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) traces indicate that the spectral amplitude and phase of the optimal pulse are both modulated. Finally, we discuss the physical mechanism for the selective excitation of femtosecond CARS based on the retrieved information from SHG-FROG traces.

Experimental study on pulse propagation characteristics at normal dispersion region in dispersion flatted fibers

Pulse propagation characteristics at normal-dispersion region in dispersion-flatted-fibers are experimentally investigated by employing the second-harmonic generation frequency-resolved optical gating (SHG-FROG) method. It is found that the experimental results are consistent with the theoretical prediction. The initial optical pulse with negative chirp is compressed for nonlinear effect in the normal-dispersion fiber, and it evolves into near Gaussian pulse. Temporal width of the optical pulse decreases with the increase of the input power and propagation distance. The output pulse width for small dispersion is less than that for great dispersion at the same input power. The spectrum of the output pulse is still symmetrical about the central wavelength, and is broadened with the increase of input power. The spectral width of the output pulse is much wider than the input spectral width.