Pulse Characterization Technique Research Articles

Systematic comparison of commercial devices for temporal characterization of few-cycle laser pulses in the 500-1000 nm spectral range

We compare multiple temporal pulse characterization techniques in three different pulse duration regimes from 15 fs to sub-5 fs, as there are no available standards yet for measuring such ultrashort pulses. To accomplish this, a versatile post-compression platform was developed, where the 100 fs near infrared pulses were post-compressed to the sub-two-cycle regime in a hybrid, three-stage configuration. After each stage, the duration of the compressed pulse was measured with the d-scan, TIPTOE and SRSI techniques and the retrieved temporal intensity profiles, spectrum and spectral phases were compared. Spectral homogeneity was also measured with an imaging spectrometer to understand the input coupling conditions of the temporal measurements. Our findings suggest that the different devices give similar results in terms of temporal intensity profile, however they are extremely sensitive to alignment and to beam quality, especially in the case of the shortest pulses. We address specific steps of measurement procedures, which paves the way towards the standardization of pulse characterization in the near future.

Phase retrieval from integrated intensity of auto-convolution

Ultra-fast optical pulses are the most ephemeral sensing paradigm ever devised, examining events over incredibly brief timescales with broadband illumination. A consequence of sensing at timescales lower than a picosecond is that pulse characterization cannot be done with traditional analog-to-digital samplers and must be ascertained from integrating intensity sensors. Techniques for pulse characterization have been constructed using combinations of time-invariant and time-variant filter responses to create non-linear but invertible intensity datasets (Walmsley & Dorrer, 2009). In this paper, we develop a novel high-order phase retrieval technique to perform pulse characterization from a single-pixel integrating sensor measuring integrated intensity of auto-convolution (IIAC). We examine gradient descent’s ability to recover signals as a function of signal dimension and measurement count, and we demonstrate the effective use of iterative hard tensor thresholding as an initializer. Finally, we demonstrate IIAC recovery in a laboratory setting to recover the time profile of a complex laser pulse. We assert that the IIAC recovery solution demonstrated here simultaneously provides the optics community with a pulse characterization technique that scales to low-power microscopy systems and provides the optimization community with a physically motivated high-order phase retrieval problem enhanced by low-rank tensor processing.

Few-cycle laser pulse characterization on-target using high-harmonic generation from nano-scale solids.

We demonstrate high-harmonic generation for the time-domain observation of the electric field (HHG-TOE) and use it to measure the waveform of ultrashort mid-infrared (MIR) laser pulses interacting with ZnO thin-films or WS2 monolayers. The working principle relies on perturbing HHG in solids with a weak replica of the pump pulse. We measure the duration of few-cycle pulses at 3200 nm, in reasonable agreement with the results of established pulse characterization techniques. Our method provides a straightforward approach to accurately characterize femtosecond laser pulses used for HHG experiments right at the point of interaction.

Single-beam technique for ultrashort pulse characterization based on the nonlinear ellipse rotation signal

We present a single-beam method based on the third-order nonlinear-ellipse-rotation (NER) measurement in thick samples to determine the pulse properties, including the duration and linear chirp. Our approach exploits the influence of the intrinsic second-order dispersions of the nonlinear materials, which affect the NER signal as a function of propagation. Two advantages can be highlighted: no delay line is required, and there are no phase-matching issues. To test our method, we characterized ultrafast chirped pulses from a Ti:sapphire amplified laser system and tunable pulses from an optical parametric amplifier using two samples with high dispersions and nonlinearities: SF6 and LaSF-N30 optical glasses.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

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.

Single-shot d-scan technique for ultrashort laser pulse characterization using transverse second-harmonic generation in random nonlinear crystals.

We demonstrate a novel dispersion-scan (d-scan) scheme for single-shot temporal characterization of ultrashort laser pulses. The novelty of this method relies on the use of a highly dispersive crystal featuring antiparallel nonlinear domains with a random distribution and size. This crystal, capable of generating a transverse second-harmonic signal, acts simultaneously as the dispersive element and the nonlinear medium of the d-scan device. The resulting in-line architecture makes the technique very simple and robust, allowing the acquisition of single-shot d-scan traces in real time. The retrieved pulses are in very good agreement with independent frequency-resolved optical grating measurements. We also apply the new single-shot d-scan to a terawatt-class laser equipped with a programmable pulse shaper, obtaining an excellent agreement between the applied and the d-scan retrieved dispersions.

Phase-matching-free pulse retrieval based on transient absorption in solids.

In this paper, we introduce a pulse characterization technique that is free of phase-matching constraints, exploiting transient absorption in solids as an ultrafast optical switch. Based on a pump-probe setup, this technique uses pump pulses of sufficient intensity to induce the switch, while the pulses to characterize are probing the transmissivity drop of the photoexcited material. This enables the characterization of low-intensity ultra-broadband pulses at the detection limit of the spectrometer and within the transparency range of the solid. For example, by using zinc selenide (ZnSe), pulses with wavelengths from 0.5 to 20 μm can be characterized, denoting five octaves of spectral range. Using ptychography, we retrieve the temporal profiles of both the probe pulse and the switch. To demonstrate this approach, we measure ultrashort pulses from a titanium-sapphire (Ti-Sa) amplifier, which are compressed using a hollow core fiber setup, as well as infrared to mid-infrared pulses generated from an optical parametric amplifier (OPA). The characterized pulses are centered at wavelengths of 0.77, 1.53, 1.75, 4, and 10 μm, down to sub-two optical cycles duration, exceeding an octave of bandwidth, and with energy as low as a few nanojoules.

Dynamic power and chirp measurements of electroabsorption and Mach-Zehnder modulator pulse generators and chirp factor extraction using a linear pulse characterization technique

Electroabsorption and Mach-Zehnder modulators are routinely used to generate optical pulses for use in optical transmission systems. Both types of modulator impose dynamic frequency chirp on the generated pulses, which can lead to significant pulse broadening when transmitted in optical fiber. This paper describes the use of a modified linear pulse characterization method to measure dynamic pulse power, phase and chirp. Modulator chirp factors are determined by the use of simple models in conjunction with experimental pulse temporal profiles for 40 and 17.7 ps pulsewidth pulses at 10 and 20 GHz repetition rates, generated by the Electroabsorption and Mach-Zehnder modulators respectively.

Double-pulse characterization by self-referenced spectral interferometry

The reconstruction of ultrashort optical pulses with a complex intensity substructure is demonstrated using the Self-Referenced Spectral Interferometry (SRSI) pulse characterization technique with a modified phase retrieval algorithm. A correction spectral phase term is extracted by the manipulation of the temporal interferogram, allowing the treatment of scenarios with complicated pulse shapes, where the original algorithm fails. The improved SRSI algorithm is verified through the application on two temporally well-separated pulses having the same polarization direction and spectral shape, generated by duplicating 37 fs-long amplified pulses of a Ti:Sa based laser system. The spectral phase of highly chirped double pulses with equal or different amplitude ratios is numerically retrieved. The collinear and achromatic experimental arrangement results in a compact and easy-to-align system.

Strategies for the characterization of partially coherent ultrashort pulses with dispersion scan

Established characterization methods for repetitive pulse trains tend to fail in the presence of degraded coherence properties. Partial coherence is present in several pulse generation schemes but is not yet properly addressed in many pulse characterization techniques. Full coherence is a common assumption in most techniques, and failing to detect and account for it can lead to erroneous measurements that can be easily overlooked. While some techniques can indicate the presence of a coherence degradation, no method is known that can simultaneously retrieve the enveloping pulse width as well as the coherence time within a pulse train. Here we propose different pertinent strategies to deal with partially coherent ultrashort pulses measured using dispersion scan. The effect of partial coherence in dispersion scan is first investigated, and the three strategies, namely fidelity measurement, mixed-states reconstruction, and self-calibrating dispersion scan, are subsequently demonstrated and compared. The demonstrated retrieval algorithms can be readily adapted to existing dispersion scan hardware. Therefore, this study leads the way toward a more holistic approach to pulse characterization, dealing with fewer assumptions on the pulse trains and resulting in more accurate measurements.

Dynamic power and chirp measurements of amplified 19 ps pulses in traveling-wave and reflective semiconductor optical amplifiers using a linear pulse characterization technique

The dynamic chirp and power of amplified pulses in semiconductor optical amplifiers is of importance in the application of these devices as conventional amplifiers and in optical signal processing. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths commonly present in moderate bit rate optical systems. Typical measurements of chirp and power of 20 GHz repetition rate amplified 19 ps pulsewidth pulses in traveling-wave and reflective SOAs are obtained using a linear characterization technique based on small-signal sinusoidal modulation at half the pulse stream repetition rate and post-processing of the resulting optical spectrums. The results show that the amplified pulse dynamic power and chirp can have a complex structure and thereby pulse spectrum, which in turn can influence pulse propagation in optical fiber.

Generation of a single-cycle pulse using a two-stage compressor and its temporal characterization using a tunnelling ionization method

A single-cycle laser pulse was generated using a two-stage compressor and characterized using a pulse characterization technique based on tunnelling ionization. A 25-fs, 800-nm laser pulse was compressed to 5.5 fs using a gas-filled hollow-core fibre and a set of chirped mirrors. The laser pulse was further compressed, down to the single-cycle limit by propagation through multiple fused-silica plates and another set of chirped mirrors. The two-stage compressor mitigates the development of higher-order dispersion during spectral broadening. Thus, a single-cycle pulse was generated by compensating the second-order dispersion using chirped mirrors. The duration of the single-cycle pulse was 2.5 fs, while its transform-limited duration was 2.2 fs. A continuum extreme ultraviolet spectrum was obtained through high-harmonic generation without applying any temporal gating technique. The continuum spectrum was shown to have a strong dependence on the carrier-envelope phase of the laser pulse, confirming the generation of a single-cycle pulse.

Pulse analysis by delayed absorption from a coherently excited atom

In this tutorial, we provide a short review of attosecond pulse characterization techniques and a pedagogical account of a recently proposed method called Pulse Analysis by Delayed Absorption (PANDA) [S. Pabst and J. M. Dahlström, Phys. Rev. A 94, 013411 (2016)]. We discuss possible implementations of PANDA in alkali atoms using either principal quantum number wave packets or spin-orbit wave packets. The main merit of the PANDA method is that it can be used as a pulse characterization method that is free from atomic latency effects, such as scattering phase shifts and long-lived atomic resonances. Finally, we propose that combining the PANDA method with angle-resolved photoelectron detection should allow for experimental measurements of attosecond delays in photoionization from bound wave packets on the order of tens of attoseconds.

Electrochemical Evaluation of the Aging Process for NCA/Graphite Cylindrical Cells Intended for Off-Grid PV Applications

The aging of lithium ion batteries in off-grid photovoltaic (PV) energy systems is evaluated. Off-grid PV systems can improve their reliability and efficiency by storing the excess of energy produced during sunny days and using it when no other source of energy is available. Due to its high energy density, high efficiency, and constantly drop in prices, lithium ion batteries are the most suitable option to be integrated in the system as energy storage [1]. Although the impressive features, lithium ion batteries properties need to be studied further in order to achieve more efficient renewable energy systems [2]. One of the determinant property to be evaluated is the lifetime of the lithium ion battery, which is determined by aging factors [3]. In this work we have studied the capacity fade and impedance increase as aging factors. State of charge (SOC) profiles corresponding to most common applications found in off-grid PV-systems were used to cycle NCA/graphite cylindrical for 8 months in the laboratory. Four SOC ranges were used to cycle the cells; Low ΔSOC (20% to 50%), middle ΔSOC (35% to 65%), high ΔSOC (65% to 95%), and and full ΔSOC (20% to 95%). Electrochemical techniques were used to characterize both capacity fade and impedance increase in full cells. Discharge at C/25 rate was performed to measure the capacity every 100 cycles. Impedance increase due to the solid electrolyte interface (SEI) formation and other unwanted process was determined by performing electrochemical impedance spectroscopy (EIS) measurements along with hybrid power pulse characterization techniques. Half cells and symmetrical cells were built to identify aging process on the NCA and graphite electrodes independently with the same electrochemical techniques described above. From Figure 1-a) we can observe a relatively large capacity fade of the C/25 discharge capacity on cells cycled at high ΔSOC, and almost similar behavior was observed in cells cycled at full ΔSOC. Whereas, for cells cycled at low ΔSOC and middle ΔSOC the capacity fade is small in comparison. Comparison between dV/dQ curves show a shifting and growing of peaks as the number of cycles increase. These changes are more evident in cells cycled at high ΔSOC and full ΔSOC, Figure 1-b). The EIS measurements show a relatively large increase of impedance in the Nyquist plot for the cells cycled at high ΔSOC and full ΔSOC, along with a formation of a second semicircle at the mid frequency range, which is also observed for cells cycled at low ΔSOC and middle ΔSOC, Figure 1-c). Bibliography [1] D. Parra and M. K. Patel, “Effect of tariffs on the performance and economic benefits of PV-coupled battery systems,” Appl. Energy, vol. 164, pp. 175–187, 2016. [2] Y. Zhang, A. Lundblad, P. E. Campana, F. Benavente, and J. Yan, “Battery sizing and rule-based operation of grid-connected photovoltaic-battery system: A case study in Sweden,” Energy Convers. Manag., vol. 133, pp. 249–263, 2017. [3] M. Klett, R. Eriksson, J. Groot, P. Svens, K. Ciosek Högström, R. W. Lindström, H. Berg, T. Gustafson, G. Lindbergh, and K. Edström, “Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis,” J. Power Sources, vol. 257, pp. 126–137, 2014. Figure 1

Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor

In most applications of ultrashort pulse lasers, temporal compressors are used to achieve a desired pulse duration in a target or sample, and precise temporal characterization is important. The dispersion-scan (d-scan) pulse characterization technique usually involves using glass wedges to impart variable, well-defined amounts of dispersion to the pulses, while measuring the spectrum of a nonlinear signal produced by those pulses. This works very well for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much more difficult to measure this way. Here we demonstrate the concept of self-calibrating d-scan, which extends the applicability of the d-scan technique to pulses of arbitrary duration, enabling their complete measurement without prior knowledge of the introduced dispersion. In particular, we show that the pulse compressors already employed in chirped pulse amplification (CPA) systems can be used to simultaneously compress and measure the temporal profile of the output pulses on-target in a simple way, without the need of additional diagnostics or calibrations, while at the same time calibrating the often-unknown differential dispersion of the compressor itself. We demonstrate the technique through simulations and experiments under known conditions. Finally, we apply it to the measurement and compression of 27.5 fs pulses from a CPA laser.

Attosecond transient absorption of a bound wave packet coupled to a smooth continuum

We investigate the possibility of using transient absorption of a coherent bound electron wave packet in hydrogen as an attosecond pulse characterization technique. In a recent work, we have shown that photoionization of such a coherent bound electron wave packet opens up for pulse characterization with unprecedented temporal accuracy—independent of the atomic structure—with maximal photoemission at all kinetic energies given a wave packet with zero relative phase (Pabst and Dahlström Phys. Rev. A 94 13411 (2016)). Here, we perform numerical propagation of the time-dependent Schrödinger equation and analytical calculations based on perturbation theory to show that the energy-resolved maximal absorption of photons from the attosecond pulse does not uniquely occur at a zero relative phase of the initial wave packet. Instead, maximal absorption occurs at different relative wave packet phases, distributed as a non-monotonous function with a smooth shift across the central photon energy (given a Fourier-limited Gaussian pulse). Similar results are also found in helium. Our finding is surprising, because it implies that the energy-resolved photoelectrons are not mapped one-to-one with the energy-resolved absorbed photons of the attosecond pulse.

Impact of AlN layer sandwiched between the GaN and the Al2O3 layers on the performance and reliability of recessed AlGaN/GaN MOS-HEMTs

This work compares the performance and the reliability of recessed-gate AlGaN/GaN MOS-HEMTs with two different gate dielectrics: a single layer of Al2O3 and a bilayer of AlN/Al2O3. Although the normally-OFF operation is well observed in the AlN/Al2O3 device, the other characteristics are remarkable poor and directly prejudice the performance of the device. PBTI phenomenon was studied by using a pulse characterization technique in order to capture the fast charge trapping when a single stress pulse is applied to the gate contact. The physical origin of PBTI is ascribed to electron trapping and de-trapping in defect sites located in the gate dielectrics. The results during the stress phase show that the positive threshold voltage shift (ΔVth) in both devices follows a saturating log-time dependence model. From this analysis, the double stack exhibits a higher saturation threshold voltage and a faster trap time constant compared with the single layer device. The universal relaxation function fits well the experimental data during the relaxation phase at different stress pulse widths and clearly shows a faster defect discharge in the Al2O3 device.

Integrated 16-ps Pulse Generator Based on a Reflective SOA-EAM for UWB Schemes

A flexible pulse shaping photonic structure that is capable of generating train pulses that hold pulsewidths of approximately 16 ps and a Gaussian profile at a repetition rate of 20 GHz is proposed and experimentally demonstrated. The key component is a monolithically integrated reflective semiconductor optical amplifier (SOA)-electroabsorption modulator device that operates within the 1.55-μm frequency band. The generated pulses have low chirp, which is experimentally measured using a novel linear pulse characterization technique. The pulses have the potential to be used as seed pulses for optical ultra-wide band photonic communications systems.

Measurement of the ultrafast lighthouse effect using a complete spatiotemporal pulse-characterization technique

Three of the four spatiotemporal pulse amplitude couplings—spatial chirp, angular dispersion, and pulse-front tilt—are well known for their important roles in optics and, especially, in ultrafast optics. The remaining one, only recently identified, corresponds to the pulse arrival time variation with angle and is known as the “ultrafast lighthouse effect.” This effect has important applications in attosecond science, but its characterization has not yet received much attention. In this work, we generate an ultrafast lighthouse and measure it using a recently developed single-frame complete spatiotemporal pulse-characterization technique called STRIPED FISH. We discuss in great detail the measured couplings in different domains and their roles in generating the ultrafast lighthouse effect. In addition, we display the propagation of the measured ultrafast lighthouse with an intuitive movie plot over space and time. We conclude that STRIPED FISH provides a simple and informative approach for measuring the ultrafast lighthouse effect and also other possible spatiotemporal distortions in the pulse, in both the horizontal and vertical dimensions.