Cross-correlation Frequency-resolved Optical Gating Research Articles

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.

Observation of Ultrashort Laser Pulse Evolution in a Silicon Photonic Crystal Waveguide

Using the sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) measurement setup, we observed the soliton evolution of low energy pulse in an Si photonic crystal waveguide, and it exhibited the pulse broadening, blue shift, and evident pulse acceleration. The soliton evolution was also investigated by nonlinear Schrödinger equation (NLSE) modelling simulation, and the simulated results agreed well with the experimental measurements. The effects of waveguide length on the pulse evolution were analyzed; the results showed that the pulse width changed periodically with increasing waveguide length. The results further the understanding of the ultra-fast nonlinear dynamics of solitons in silicon waveguides, and are helpful to soliton-based functional elements on CMOS-compatible platforms.

Investigation of supercontinuum generation in photonic crystal fiber using bursts of femtosecond pulses

Results of experimental and numerical investigation of supercontinuum generation in polarization maintaining photonic crystal fiber using a burst of two orthogonal polarization femtosecond pump pulses are presented. The initial pump pulse source was a mode-locked Yb:KGW laser generating 52 nJ energy 90 fs duration pulses at 1033 nm with 76 MHz repetition rate. Analysis of cross-correlation frequency-resolved optical gating traces and numerical simulation results reveal that there are slight changes in supercontinuum generation when temporal delay between pump pulses in the burst is changed. Weak dependence of supercontinuum characteristics on temporal distance between the pump pulses is related to supercontinuum generation dynamics which is analyzed in detail.

Measuring an ultrashort, ultraviolet pulse in a slowly responding, absorbing medium.

Frequency-resolved optical gating (FROG) is a common technique for measuring ultrashort laser pulses using an instantaneous, nonlinear-optical interaction as a fast time-gate to measure the pulse intensity and phase. But at high frequencies, materials are often absorbing and it is not always possible to find a medium with a fast nonlinear-optical response. Here we show that an ultrashort, ultraviolet (UV) pulse can be measured in a strongly absorbing medium, using the absorption as the nonlinear-optical time-gate. To do this, we build on our recent implementation of FROG, known as induced-grating cross-correlation FROG (IG XFROG), where an unknown, higher-frequency pulse creates a transient grating that is probed with a lower-frequency, more easily detectable reference pulse. We demonstrate this with an 800 nm reference pulse to characterize 400 nm or 267 nm pulses using ZnS as the nonlinear-optical medium, which is absorptive at and below 400 nm. By scanning the delay between the two UV pulses which create the transient grating, we show that the phase-sensitive instantaneous four-wave-mixing contribution to the nonlinear signal field can be detected and separated from the slower, incoherent part of the response. Measuring a spectrally-resolved cross-correlation in this way and then applying a simple model for the response of the medium, we show that a modified generalized projections (GP) phase-retrieval algorithm can be used to extract the pulse amplitude and phase. We test this approach by measuring chirped UV pulses centered at 400 nm and 267 nm. Since interband absorption (or even photoionization) is not strongly wavelength-dependent, we expect IG XFROG to be applicable deeper into the UV.

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).

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.

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.

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.

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%.

Femtosecond pulse delivery around 1560 nm in large-core inhibited-coupling fibers

Transmission of ultrashort laser pulses under 100 fs at a central wavelength of 1560 nm from a mode-locked laser is investigated over meter-scale lengths of an inhibited-coupling fiber with 65 µm core diameter. Performance of the fiber in this application is evaluated experimentally using cross-correlation frequency-resolved optical gating, either with the fiber loosely coiled up to 3 m of length or bent over one full loop down to 6 cm radius. Experimental results on observed dispersive stretching of the pulse up to around 200 fs are compared with non-linear propagation simulations using the generalized non-linear Schrodinger equation parametrized with measured characteristics of the fiber. Dechirping of the pulse to its original shape in the fiber under bending is observed and related to suppression of higher-order modes. We show that the proposed fiber can be used to transmit around 90 fs long laser pulses without spectral or temporal distortions, if a 6 cm radius loop is applied.

Generation and Characterization of Mid-Infrared Supercontinuum in Bulk YAG Pumped by Femtosecond 1937 nm Pulses from a Regenerative Amplifier

We have demonstrated the generation of supercontinuum (SC) pulses in bulk, using femtosecond pulses directly derived from a Tm:YAP regenerative amplifier. The SC spans from 380 nm–4 μ m. Such an SC combined with the applied 1937-nm pump pulse can be used in a mid-infrared optical parametric amplifier (MIR-OPA). A full characterization of the phase and stability of the SC provides important insight into the performance of such MIR-OPA systems. Using cross-correlation frequency-resolved optical gating (XFROG), we characterized the properties of the MIR section of the SC that can be used as a seed pulse in an MIR-OPA system. The pulse-to-pulse instability rooted in the intrinsic properties of the self-phase modulation process was observed in the retrieved trace and spectrum of the SC. In addition, the effect of the pump pulse quality was studied on the obtainable SC pulses.

Generation and characterization of mid-infrared supercontinuum in polarization maintained ZBLAN fibers.

We present mid-infrared (MIR) supercontinuum generation in polarization-maintained ZBLAN fibers pumped by 2 µm femtosecond pulses from a Tm:YAP regenerative amplifier. A stable supercontinuum that spreads from 380 nm to 4 µm was generated by coupling only 0.5 µJ pulse energy into an elliptical core ZBLAN fiber. The supercontinuum was characterized using cross-correlation frequency-resolved optical gating (XFROG). The complex structure of the XFROG trace due to the pulse-to-pulse spectrum instability have been fixed by reducing the length of the applied fibers or improving the quality of the incident pulse spectrum.

Nonlinear Compression of Intense Optical Pulses at 1.55 μm by Multiple Plate Continuum Generation

Intense ultrashort laser pulses at 1.55 μ m are generated by a home-built light source and compressed to 20 fs by multiple plate continuum (MPC) and Fourier pulse shaping. The entire system consists of three building blocks, responsible for seed generation, optical parametric chirped pulse amplification (OPCPA), and nonlinear compression. A commercial Yb:KGW amplifier pumps a difference frequency generator and an optical parametric amplifier (OPA) to generate 1 kHz, 44 fs, 10 μ J, carrier-envelope phase stable seed pulses at 1.55 μ m. A grating–lens pair stretches the seed pulses to 50 ps by introducing a great amount of normal group delay dispersion (GDD). After that, the chirped pulse energy is boosted to 5 mJ by two OPA stages pumped by a commercial Nd:YAG amplifier (30 mJ, 85 ps). To compress the amplified pulses, a grating pair compressor providing an adequate amount of anomalous GDD is employed. The OPCPA system eventually delivers 3 mJ and 80 fs pulses, limited by the phase-matching bandwidth of nonlinear crystals. To further compress the pulse duration, pulses are sent to an MPC stage, which consisted of nine pieces of 200- μ m-thick quartz plates. The OPCPA output spectrum is broadened by four times and compressed to 20 fs (3.6 carrier cycles) by a femtosecond Fourier-transform pulse shaper. The pulses are characterized by polarization gating cross-correlation frequency-resolved optical gating. Due to the high pondermotive force, an intense and long-wavelength laser system like this is useful in seeding a MeV-level electron accelerator and generating coherent soft X-ray high-harmonic generation.

Improving techniques for diagnostics of laser pulses by compact representations.

We propose and demonstrate, numerically and experimentally, use of sparsity as prior information for extending the capabilities and performance of techniques and devices for laser pulse diagnostics. We apply the concept of sparsity in three different applications. First, we improve a photodiode-oscilloscope system's resolution for measuring the intensity structure of laser pulses. Second, we demonstrate the intensity profile reconstruction of ultrashort laser pulses from intensity autocorrelation measurements. Finally, we use a sparse representation of pulses (amplitudes and phases) to retrieve measured pulses from incomplete spectrograms of cross-correlation frequency-resolved optical gating traces.

Control of soliton self-frequency shift dynamics via Airy soliton interaction.

We investigate Airy-soliton interaction in a nonlinear fiber with Raman effect. We find that Airy solitons may fuse upon interaction at a position that can be controlled by a proper engineering of the Airy tail direction. This control allows us to generate Airy solitons with varying deceleration. At variance with the case of two solitons interaction, Raman-induced soliton self-frequency shift (SSFS) is strongly enhanced when the leading soliton is replaced with the accelerating Airy pulse and slightly suppressed for the decelerating one. These notable features are ascribed to the unique properties of asymmetrical Airy pulses with a switchable direction of the oscillatory tails. We show the way these processes are uncovered unambiguously by cross-correlation frequency resolved optical gating. We also investigate the impact of chirp imposed on the input pulse on the SSFS dynamics. Our results not only provide a new way to manipulate the SSFS, but may help to improve the control of soliton fusion events during supercontinuum generation, optical rogue waves and giant dispersive waves formation.

Ultrafast mid-infrared spectrochronography of dispersion near molecular absorption bands.

Quantitative characterization of dispersion near molecular resonances is difficult both conceptually and technically, as it requires systematic measurements of rapidly varying dispersion profiles across the edges of molecular absorption bands. Here, we show that this challenge can be confronted with an ultrafast spectrochronography technique that combines time-resolved four-wave mixing and cross-correlation frequency-resolved optical gating. We demonstrate that, with the spectrum of an ultrashort mid-infrared laser probe stretching across the edge of a molecular absorption band, the Wigner chronocyclic maps of such a probe reveal universal signatures of anomalous dispersion and help quantify such dispersion anomalies.

Extremely broadband single-shot cross-correlation frequency-resolved optical gating using a transient grating as gate and dispersive element.

A cross-correlation frequency-resolved optical gating (FROG) concept, potentially suitable for characterizing few or sub-cycle pulses in a single shot, is described in which a counter-propagating transient grating is used as both the gate and the dispersive element in a FROG spectrometer. An all-reflective setup, which can operate over the whole transmission range of the nonlinear medium, within the sensitivity range of the matrix sensor, is also proposed, and proof-of-principle experiments for the ultraviolet and visible-to-near-infrared spectral ranges are reported.

Estimation of photonic crystal fiber dispersion by means of supercontinuum generation.

We present a technique for photonic crystal fiber dispersion measurement. We demonstrate that investigating supercontinuum using cross-correlation frequency resolved optical gating (XFROG) technique can be used for quantitatively characterizing dispersion and observing orthogonal polarization modes in polarization maintaining photonic crystal fibers. In addition, an XFROG trace of supercontinuum generated in a polarization maintaining photonic crystal fiber reveals complex behavior of orthogonal polarization modes that is different in normal and anomalous dispersion regions of the photonic crystal fiber.