Pulse Temporal Profile Research Articles

Experimental evolution of the temporal and spectral profiles of noise-like pulses within the mode-locked regions of a figure-eight fiber laser.

We present an experimental analysis of the pulse profile variability within the mode-locked regions of an erbium-doped figure-eight fiber laser (EDFEFL). The tuning of the mode-locked regions was carried out by varying and recording the values of the angle of the polarization controllers in the ring section and in the nonlinear optical loop mirror (NOLM). Within the mode-locked regions, we obtained a large variability of the temporal profile, specifically amplitude and width of the noise-like pulses (NLPs). Subsequently, we recorded and studied the changes in the spectral domain. We identified the mode-locked regions where the temporal profile of the pulse remains constant (stationary state), and where it expels sub-packets (non-stationary state). Finally, a theoretical analysis of the power transmission through the polarizing in the ring section and in the NOLM switching characteristic as a function of wave plate angles is also performed, which allows an understanding of the existence of the multiple mode-locked regions and pulse profile adjustability. We analyze NLPs with a carrier wavelength of 1560 nm with duration of the order of nanoseconds and a repetition rate of 0.9 MHz.

Shaping of nanosecond pulses in ytterbium fiber lasers by synchronous sine-wave pump modulation

In this work, we discovered the possibility of greatly relaxing requirements to the speed and dynamic range of pump power variation and, thus, of reducing synchronous pumping of ytterbium (Yb)-based fiber lasers to a very simple pump modulation yielding a mode-locked pulsed output. We show that even slow (microsecond-scale) low-index ( ≤ 0.5 ) sine-wave synchronous modulation of the pump power can result in shaping of a regular train of nanosecond laser pulses. It is revealed that the energy-conservative process of laser pulse shortening against the pump modulation period can take place in the quasi–two-level laser active medium owing to mistiming-induced gain discrimination of the temporal laser pulse profile. Thus, nanosecond pulses with energy up to 50 nJ were obtained in our experimental all-fiber Yb-based laser configuration. Our theoretical modeling reveals routes to much stronger pulse shortening through tunable pump modulation parameters. This discovery allows the establishment of more reliable and easy-to-implement high-efficiency alternatives to other types of high-energy ultralong mode-locked fiber lasers.

Generalized temporal synthesis method for a birefringent laser pulse shaper.

We propose a new method for the synthesis of arbitrary temporal output pulse profiles by using a birefringent laser pulse shaper. This shaper contains a series of N birefringent crystals placed between input and output polarizers. The synthesis method starts with the linear time-invariant system theory to determine the optimal amplitudes of the replica pulses at the output polarizer and the time delay between them, and then uses the temporal Jones matrix formalism to obtain the angle of each crystal. The design procedure is demonstrated for an eight-stage birefringent laser pulse shaper. Several examples of predefined output pulse profiles are given to show the potential of the proposed birefringent laser pulse shaper.

High Fidelity Picosecond Pulse Fiber Amplification With Inter-Stage Notch Filter

We propose and demonstrate a high fidelity narrowband picosecond pulse two-stage fiber amplification scheme with inter-stage filtering technique using fiber Bragg grating. This technique can not only narrow the bandwidth of the pulses broadened by self phase modulation in the first-stage fiber amplifier, but also reshape the pulse temporal profile to near Gaussian shape, which reduces the spectral broadening slope of the pulses in the second-stage fiber amplifier and improves the narrowband fiber amplification ability. Using this technique, pulse energy of 10 nJ is achieved from a 0.2-nJ initial pulse by only using standard single mode fiber amplifiers while retaining 10 ps pulse width and 0.4 nm root mean square bandwidth. This technique may provide a simple and effective solution for high fidelity fiber amplification of low-energy ps seed pulse from fiber oscillators.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime.

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP2-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Compact single-shot electro-optic detection system for THz pulses with femtosecond time resolution at MHz repetition rates.

Electro-optical detection has proven to be a valuable technique to study temporal profiles of THz pulses with pulse durations down to femtoseconds. As the Coulomb field around a relativistic electron bunch resembles the current profile, electro-optical detection can be exploited for non-invasive bunch length measurements at accelerators. We have developed a very compact and robust electro-optical detection system based on spectral decoding for single-shot longitudinal bunch profile monitoring at the European X-ray Free Electron Laser (XFEL) for electron bunch lengths down to 200 fs (rms). Apart from the GaP crystal and the corresponding laser optics at the electron beamline, all components are housed in 19 in. chassis for rack mount and remote operation inside the accelerator tunnel. An advanced laser synchronization scheme based on radio-frequency down-conversion has been developed for locking a custom-made Yb-fiber laser to the radio-frequency of the European XFEL accelerator. In order to cope with the high bunch repetition rate of the superconducting accelerator, a novel linear array detector has been employed for spectral measurements of the Yb-fiber laser pulses at frame rates of up to 2.26 MHz. In this paper, we describe all sub-systems of the electro-optical detection system as well as the measurement procedure in detail and discuss the first measurement results of longitudinal bunch profiles of around 400 fs (rms) with an arrival-time jitter of 35 fs (rms).

Ultrafast Plasma Electron Dynamics: A Route to Terahertz Pulse Shaping

Intense ultrafast laser interaction with solid-density plasma can lead to relativistic transient electron dynamics that are favorable for the generation of high-field terahertz (THz) pulses. We investigate the temporal characteristics of such THz pulses. Single-shot electro-optic measurements enable us to record the complete temporal profiles of the THz pulses emanating from planar $\mathrm{Cu}$ and aligned $\mathrm{Cu}$-nanorod-array plasma. The temporal properties of the THz pulses exhibit several transients. Fully relativistic two-dimensional particle-in-cell simulations corroborate experimental observations and reveal that these features originate from electron microbunch emission from the target. Our results demonstrate that target nanostructuring provides a route to control such ultrafast electron dynamics and hence THz-pulse properties in the time domain.

Programmable optical pulse repetition rate multiplication via spectral phase manipulation.

Flexible phase patterns for optical pulse repetition rate multiplication (PRRM) are proposed and experimentally demonstrated via spectral phase-only manipulation. We introduce formulas of the phase condition for power lossless PPRM with arbitrary multiplication factors and undistorted temporal pulse profiles. For some multiplication factors the solution extends PRRM phase patterns from reported phase conditions to more flexible phase patterns, inspiring potentials of further devices available for PRRM. This flexibility also benefits PRRM when we use the reported devices. As a proof of concept, we numerically and experimentally demonstrate PRRM with multiplication factors up to eight by programming the spectral phase using an optical wave-shaper (OWS), involving different phase patterns. In practice, manipulation of the spectral phase induces spectral amplitude variations due to the intrinsic property limitation of the OWS. We quantitatively characterize this limitation and select a suitable phase pattern from our new solution to achieve a uniform temporal pulse train but with no spectral amplitude trimming.

Attosecond-pulse metrology based on high-order harmonic generation

An all-optical method to retrieve the temporal intensity profile of an extreme ultraviolet (XUV) attosecond pulse is proposed based on XUV-assisted high-order harmonic generation (HHG) by an intense infrared (IR) pulse. For a harmonic located on the XUV-induced high-energy plateau (beyond the IR HHG plateau), the measured harmonic yield as a function of the time delay between the XUV and IR pulses is shown to accurately map the temporal intensity profile of the XUV pulse. Single-color and two-color orthogonal, linearly polarized IR pulses are used to demonstrate the method.

High-energy continuously tunable 8–14 μm picosecond coherent radiation generation from BGSe-OPA pumped by 1064 nm laser

Powerful and compact coherent radiation source with wavelength covering the 8–14 µm mid-Infrared spectral range is crucial for a number of applications. Here we present a compact high energy widely and continuously tunable picosecond (ps) coherent radiation source from an optical parametric amplifier (OPA) in BaGa4Se7 (BGSe) crystal, providing hundreds of microjoule energy across 8–14 μm. The BGSe-OPA was pumped by a ps 1064 nm laser at a pulse repetition rate of 10 Hz. With a maximum pump energy of 14 mJ, the idler energy of >140 µJ over the entire tuning range was achieved. The maximum idler energy was up to 230 µJ at 9.5 µm, corresponding to a pump to idler energy efficiency of 1.6% and a photon conversion efficiency of 14.7%. Moreover, the corresponding phase-matching angles for 8–14 μm generation were measured experimentally by optical parametric generation method. Finally, the nonlinear absorption of BGSe crystal under 1064 nm pumping was investigated for the first time. The conversion efficiency, typical pulse temporal shapes and beam spatial profiles of idler light generation were calculated based on a BGSe-OPA numerical model.

Pulsing mechanism based on power adiabatic evolution of pump in Tm-doped fiber laser

We prove an alternative pulsing mechanism based on the power adiabatic evolution of a pump in a Tm-doped fiber laser. A pulsed laser technique, unlike Q-switching and gain switching, is explored under the equilibrium between the stimulated emission and absorption. After the laser reaches a continuous wave (CW) steady state, the temporal fluctuation of pump power is called power adiabatic evolution if it does not break the balance between the stimulated emission and the absorption. Under pump power adiabatic evolution the population densities in the upper and lower laser levels get clamped to their steady-state values, and the temporal shape of the output laser is identical to that of the pump. Based on the power adiabatic evolution of the pump a laser pulse can therefore be generated by a pulsed pump which is added to a CW pump. Moreover, the temporal profile, duration and peak power of the laser pulse can all be precisely controlled. The power adiabatic evolution of the pump can be achieved in the two pumping schemes of the Tm-doped fiber laser, provided that the duration of the pump pulse is broader than a certain value. The study on the pulsing mechanism highlights an alternative pulsed technique possessing several advantages over Q-switching and gain switching.

Passive, Broadband, and Low-Frequency Suppression of Laser Amplitude Noise to the Shot-Noise Limit Using a Hollow-Core Fiber

We use hollow-core fibre to preserve the spectrum and temporal profile of picosecond laser pulses in CBD to suppress 2.6 dB of amplitude noise at MHz noise frequencies, to within 0.01 dB of the shot-noise limit. We provide an enhanced version of the CBD scheme that concatenates circuits to suppress over multiple frequencies and over broad frequency ranges --- we perform a first demonstration that reduces total excess amplitude noise, between 2 - 6 MHz, by 85%. These demonstrations enable passive, broad-band, all-guided fibre laser technology operating at the shot-noise limit.

Programmable optical waveform generation in a mode-locked gain-modulated SOA-fiber laser

In this work, we demonstrate for the first time, to the best of our knowledge, programmable generation of arbitrarily complex optical waveforms in a hybrid synchronously pumped fiber-semiconductor laser. We show that in the mode-locked operation, the temporal profile of the generated pulses may follow that of electrical pulses pumping a semiconductor optical amplifier. Controlling the temporal profile of the electrical pump pulses enables the formation of a desired amplitude–time pattern of laser pulses reproduced over each cavity round trip. The identified availability of stable periodic optical waveforms with a pre-set shape and structure opens up prospects for a new generation of laser radiation sources with a widely controllable pulse shape for research and practical applications.

Prediction of Residual Stress Random Fields for Selective Laser Melted A357 Aluminum Alloy Subjected to Laser Shock Peening

AbstractResidual stress (RS) is a major processing issue for selective laser melting (SLM) of metal alloys. Postprocessing by way of heat treatment or hot isostatic pressing is usually required for acceptable mechanical properties. In this work, laser shock peening (LSP) treatment on both SLM and cast aluminum A357 alloys are compared with regard to the development of beneficial near-surface compressive RS. Experiments are conducted using high energy nanosecond pulsed laser, together with a fast photodetector connected to a high-resolution oscilloscope and high-speed camera to identify detailed temporal and spatial laser pulse profiles to improve numerical predictions. Constitutive modeling for SLM A357 alloy is performed using finite element simulation and data obtained from X-ray diffraction (XRD) measurements. Since XRD-RS measurements are accompanied with significant machine-reported error, an effective method is introduced to quantify the material constitutive model uncertainty in terms of a joint probability mass function. Conventionally, most constitutive behavior research for LSP involves deterministic material modeling. Predicted RS using deterministic approaches fail to reflect real-world variations in the materials, laser treatment, or RS measurements. A discretized Bayesian inference is used to quantify the rate-dependent plasticity material model parameters as a joint probability function. RS are then characterized as random fields, which provides far greater insight into the practical ability to attain desired residual stresses. Moreover, for identical LSP treatments, it is determined that the material models are significantly different for the SLM and the conventional cast A357 aluminum alloys, resulting in much lower magnitude of compressive RS in the SLM alloy.

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.

Highly reconfigurable hybrid laser based on an integrated nonlinear waveguide.

The ability of laser systems to emit different adjustable temporal pulse profiles and patterns is desirable for a broad range of applications. While passive mode-locking techniques have been widely employed for the realization of ultrafast laser pulses with mainly Gaussian or hyperbolic secant temporal profiles, the generation of versatile pulse shapes in a controllable way and from a single laser system remains a challenge. Here we show that a nonlinear amplifying loop mirror (NALM) laser with a bandwidth-limiting filter (in a nearly dispersion-free arrangement) and a short integrated nonlinear waveguide enables the realization and distinct control of multiple mode-locked pulsing regimes (e.g., Gaussian pulses, square waves, fast sinusoidal-like oscillations) with repetition rates that are variable from the fundamental (7.63 MHz) through its 205th harmonic (1.56 GHz). These dynamics are described by a newly developed and compact theoretical model, which well agrees with our experimental results. It attributes the control of emission regimes to the change of the NALM response function that is achieved by the adjustable interplay between the NALM amplification and the nonlinearity. In contrast to previous square wave emissions, we experimentally observed that an Ikeda instability was responsible for square wave generation. The presented approach enables laser systems that can be universally applied to various applications, e.g., spectroscopy, ultrafast signal processing and generation of non-classical light states.

Influences of second-order and third-order dispersion on spectral properties of mid-infrared wavelength conversion in silicon nitride waveguides

The influences of second-order dispersion (SOD) and third-order dispersion (TOD) on spectral properties of wavelength conversion based on FWM at mid-infrared region (mid-IR) are theoretically investigated in a silicon nitride waveguide. It is found that the SOD and TOD can affect the frequency shift and temporal profiles of idler pulses. Moreover, the temporal and frequency spectrum of output signal are also discussed with different SODs and TODs. Meanwhile, the numerical simulation results imply that the efficiency of FWM process will be reduced and the wavelength range of phase-match will be shifted due to the change of SOD or TOD.

Linear Schrödinger equation with temporal evolution for front induced transitions.

The nonlinear Schrödinger equation based on slowly varying approximation is usually applied to describe the pulse propagation in nonlinear waveguides. However, for the case of the front induced transitions (FITs), the pump effect is well described by the dielectric constant perturbation in space and time. Thus, a linear Schrödinger equation (LSE) can be used. Also, in waveguides with weak dispersion the spatial evolution of the pulse temporal profile is usually tracked. Such a formulation becomes impossible for optical systems for which the group index or higher dispersion terms diverge as is the case near the band edge of photonic crystals. For the description of FITs in such systems a linear Schrödinger equation can be used where temporal evolution of the pulse spatial profile is tracked instead of tracking the spatial evolution. This representation provides the same descriptive power and can easily deal with zero group velocities. Furthermore, the Schrödinger equation with temporal evolution can describe signal pulse reflection from both static and counter-propagating fronts, in contrast to the Schrödinger equation with spatial evolution which is bound to forward propagation. Here, we discuss the two approaches and apply the LSE with temporal evolution for systems close to the band edge where the group velocity vanishes by simulating intraband indirect photonic transitions. We also compare the numerical results with the theoretical predictions from the phase continuity criterion for complete transitions.

Generation of sub-microsecond quasi-unipolar pressure pulses

We present a method to generate sub-microsecond quasi-unipolar pressure pulses. Our approach is based on the laser irradiation of a thin copper wire submerged in water. The acoustic waveforms were recorded using two different, well characterized, wideband detection techniques: piezoelectric and optical interferometry. The results show that the irradiated target behaves as an omnidirectional source. Moreover, the peak pulse pressure linearly depends on the laser fluence and the source size. From the results, we propose an empirical equation for the spatial and temporal profile of the pressure pulse. The method has several advantages: ease of implementation, high repeatability, wide ultrasonic bandwidth and quasi-unipolar time profile. These features lead to potential applications of this acoustic source in ultrasonic characterization such as transducer systems, materials or passive devices.

Photon diagnostics at the FLASH THz beamline.

The THz beamline at FLASH, DESY, provides both tunable (1-300 THz) narrow-bandwidth (∼10%) and broad-bandwidth intense (up to 150 uJ) THz pulses delivered in 1 MHz bursts and naturally synchronized with free-electron laser X-ray pulses. Combination of these pulses, along with the auxiliary NIR and VIS ultrashort lasers, supports a plethora of dynamic investigations in physics, material science and biology. The unique features of the FLASH THz pulses and the accelerator source, however, bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles. Here, these challenges are discussed and the pulse diagnostic tools developed at FLASH are presented. In particular, a radiometric power measurement is presented that enables the derivation of the average pulse energy within a pulse burst across the spectral range, jitter-corrected electro-optical sampling for the full spectro-temporal pulse characterization, spatial beam profiling along the beam transport line and at the sample, and a lamellar grating based Fourier transform infrared spectrometer for the on-line assessment of the average THz pulse spectra. Corresponding measurement results provide a comprehensive insight into the THz beamline capabilities.