Pulse Stretcher Research Articles

Adaptive Nonlinear Phase Compensation in a Femtosecond Hybrid Laser with Varying Pulse Repetition Rate

In this manuscript, an implementation of a tunable nonlinear phase compensation method is demonstrated on a typical femtosecond hybrid laser consisting of a fiber pre-amplifier and an additional solid-state amplifier. This enables one to achieve constant laser pulse parameters over a wide range of pulse repetition rates in such a laser. As the gain in the solid-state amplifier is inversely proportional to the input power, the shortfall in the solid-state gain at higher repetition rates must be compensated for with fiber pre-amplifier to ensure constant pulse energy. This increases the accumulated nonlinear phase and consequently alters the laser pulse parameters such as pulse duration and Strehl ratio. To overcome this issue, the nonlinear phase must be compensated for, and what is more it should be compensated for to a different extent at different pulse repetition rates. This is achieved with a tunable CFBG, used also as a pulse stretcher. Using this concept, we demonstrate that constant laser pulse parameters such as pulse energy, pulse duration and Strehl ratio can be achieved in a hybrid laser regardless of the pulse repetition rate.

Soliton metamorphosis dynamics in ultrafast fiber lasers

The transitions from the incoherent noise to the coherent soliton have been fully revealed in ultrafast lasers. However, the soliton transformation between different coherent states, termed as soliton metamorphosis, remains an attractive yet uncharted territory. Here, we reveal the ultrafast dynamics of the soliton metamorphosis via single-shot spectroscopy in a specially designed fiber laser capable of emitting fast-switchable dissipative solitons and stretched pulses. It is demonstrated that the soliton metamorphosis is a consecutive evolution process including the self-phase modulation stage, pulse split stage, and transient stretched pulse stage. Particularly, the long-period pulse breathing and the spectral period doubling appear in the forepart and middle part of the last stage. The metamorphosis dynamics and soliton properties are substantiated by numerical simulation based on a three-step model. This work not only unveils the transient evolution physics of the pulse in soliton metamorphosis, but also provides a simple and effective way to control operations of ultrafast lasers.

The Adoption of Chalcogenide Glass Fiber as Pulse Stretcher in an All-Fiber Structured 2 μm Chirped Pulse Amplification System

We present the development of an As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> fiber-based reusable inline optical connector; and, for the first time to the best of our knowledge, integrated it into a 2 μm chirped pulse amplification system as the pulse stretcher. By this mechanism, a near 20 times pulse stretching ratio was achieved in 0.5 m-long this As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> fiber and the introduction of insertion loss as low as 2.84 dB while it was connected with silica fiber. After the follow-up fiber amplification and self-compression mechanism, the laser average output power was boosted to 1.97 W and pulse duration was optimized to 2.53 ps. Moreover, to further inspect the laser pulse property, the pulses were then delivered into a 5.3 m-long ZrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -LaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -AlF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -NaF (ZBLAN) fiber which was fusion spliced to SM1950 fiber. Thanks to the 0.4-dB low fusion splice loss between these fibers, a 1.56-W supercontinuum source with bandwidth of 1.93-3.50 μm is obtained. The measured two hours of root-mean-square value of power fluctuation of this laser source is 0.13%, indicating an ultra-high long-term power stability. This work will lay a solid foundation in the realization of the all-fiber structured high-power thulium-doped fiber amplifier with high compactness.

Development of a Femtosecond High-Power Ytterbium-doped Fiber Laser System and Study on Spiky Spectral Modulation in Highly Nonlinear Chirped-Pulse Fiber Amplifiers

We have developed and analyzed a femtosecond high-power fiber laser system based on chirped-pulse amplification. The system consists of a homemade femtosecond ytterbium-doped fiber oscillator, a fiber pulse stretcher, a pulse picker, three ytterbium-doped fiber amplifiers and a grating-based pulse compressor. A single-mode photonic crystal fiber with a large mode-field diameter of 30 µm is used in the final amplifier, and the special homemade mode-field adapter based on a graded refractive index fiber has been developed for the photonic crystal fiber. Pedestal-free pulses of 15 µJ are generated after pulse compression at a repetition rate of 500 kHz, and the pulse width is 300 fs. When small-core-size fibers are used at the final amplifier, the generation of spiky modulations in amplified spectra is observed. We analyze the physical origin of the spiky spectral modulation and investigate the effect of the spectral modulation on the pulse shape and contrast after pulse compression by numerical simulations.

A pulse stretcher for a LINAC-based pulsed slow-positron beam providing a quasi-continuous beam with an energy of 5.2 keV

A pulse stretcher providing a 5.2keV quasi-continuous beam has been developed for use with a pulsed slow-positron beam generated by an electron linear accelerator operated with a repetition frequency of 50Hz. Pulses of positrons, of width 1.2µs, were trapped in a Penning–Malmberg trap and then gradually released downstream, stretching the pulse width to nearly 20ms.

All-fiber short-wavelength tunable mode-locked fiber laser using normal dispersion thulium-doped fiber.

We report an all-fiber high pulse energy ultrafast laser and amplifier operating at the short wavelength side of the thulium (Tm) emission band. An in-house W-type normal dispersion Tm-doped fiber (NDTDF) exhibits a bending-induced distributed short-pass filtering effect that efficiently suppresses the otherwise dominant long wavelength emission. By changing the bending diameter of the fiber, we demonstrated a tunable mode-locked Tm-doped fiber laser with a very wide tunable range of 152 nm spanning from 1740 nm to 1892 nm. Pulses at a central wavelength of 1755 nm were able to be amplified in an all-fiber configuration using the W-type NDTDF, without the use of any artificial short-pass filter or pulse stretcher. The all-fiber amplifier delivers 2.76 ps pulses with an energy of ∼32.7 nJ without pulse break-up, due to the normal dispersion nature of the gain fiber, which marks so far, the highest energy amongst fiber lasers in the 1700 nm-1800 nm region.

Application of Gaussian pulsed beam decomposition in modeling optical systems with diffraction grating.

A diffraction grating is one of the most commonly used components in ultrafast optical systems such as pulse stretchers and compressors. Hence, modeling the temporal dispersion and spatiotemporal distortions associated with the angular dispersion of a diffraction grating is very crucial for wave optical modeling of such systems. In this paper, the Gaussian pulsed beam decomposition (GPBD) method is extended to handle the propagation of ultrashort pulses, with arbitrary spatial and spectral profiles, through complex ultrashort pulse shaping systems containing diffraction gratings. Although the diffraction efficiencies are not rigorously computed, the GPBD method enables modeling of the large angular dispersion of idealized diffraction gratings without running into an impractically large number of spectral samples as in the case of Fourier-transform-based methods. The application of the extended method is demonstrated by performing the wave optical propagation of an ultrashort pulse through a single diffraction grating and then through a Treacy compressor system. By combining the Treacy compressor with the Martinez grating pair stretcher with internal lenses, the pulse shaping through a complete chirped pulse amplification (CPA) setup is modeled. Finally, the effects of using real dispersive lenses in the Martinez stretcher on the output pulse of the CPA setup are presented. For analysis of the output pulses, methods of computing the spatiotemporal and spatio-spectral amplitudes of the output pulse from the phase correct superposition of individual Gaussian pulsed beams are presented.

Investigation of Spatial Chirp Induced by Misalignments in a Parallel Grating Pair Pulse Stretcher

Spatial chirp induced by the misaligned gratings and mirrors in a parallel grating pair pulse stretcher can significantly affect the performance of the output pulses. Firstly, a detailed analysis about the spatial chirp of the stretched pulses caused by the misalignments has been carried out using the ray tracing simulation method. According to the simulation results, an adjustment procedure has been summarized to accurately calibrate these misalignments. The proposed method has been successfully applied in a home-made chirped pulse stretcher. By measuring the output pulse with an imaging spectrometer, the results show the stretched pulse has a good linear temporal chirp and little spatial chirp, which demonstrates the good adjustment of the stretcher.

Jet disintegration in supercritical environments

A N2 jet was injected into quiescent ambient N2. The jet temperature was measured by four T-type thermocouples: one inside the injector and three at downstream positions (0.9d, 10.6d, and 28.1d, respectively; here, d is the inner diameter of the jet). The density was also measured downstream to 15d using two-dimensional Raman scattering with a pulse stretcher. Normally, our observations were consistent with expectations according to classical mixing theory; however, within a certain injection temperature range, there was no constant density region inside the jet core, and the jet temperature was measured to decrease. To explain this behavior of the jet, we considered three effects: classical mixing, thermal disintegration, and relaxation from high to low pressures. The absence of jet potential core can be explained by both thermal disintegration and relaxation, while the decrease of the temperature could be explained only by relaxation.

Numerical Simulation of Three-Core Photonic Crystal Fiber With Large Group-Velocity Dispersion

The three-core photonic crystal fiber (TC-PCF) is proposed to obtain large group-velocity dispersion (GVD) because of the coupling effects between the fundamental modes (FMs) of the central core and higher-order modes (HOMs) of the two side cores. The supermodes of the TC-PCF can provide concave anomalous- and normal-GVD profiles with large peak values at the maximum dispersion wavelengths (MDWs), which can be shifted in three wavelength windows of 1030 nm, 1550 nm and 1900 nm by properly tuning the refractive indexes of the cores or the air-hole diameters. Furthermore, the numerical results show that two segments of the TC-PCFs with large anomalous GVDs and opposite values of the third-order dispersion (TOD) can provide much higher efficiency of pulse compression than one segment of the TC-PCF. Additionally, the TC-PCFs with large normal GVDs can be used to stretch pulses due to their low nonlinearities and short fiber lengths.

112-μJ 180-fs Pulses at 1-kHz Repetition Rate From Yb-Doped Laser Based on Strictly All-Fiber CPA Structure

In this paper, we reported an Yb-doped, femtosecond, chirped pulse amplification laser based on strictly all-fiber structure. It operated at a low repetition rate of 1 kHz and adopted the synchronously pumping technology to suppress ASE and heat accumulation. A chirped fiber Bragg grating providing adjustable dispersion is used as the pulse stretcher. High order dispersion induced by fiber, grating pair in the system was compensated accurately. Laser pulse with duration of 180 fs and energy of 112 μJ has been obtained. In addition, it has a high long-term stability of energy and a good beam quality, corresponding to energy fluctuation of 0.62% in an hour and M2 factor of 1.15, respectively.

Picosecond contrast degradation by surface imperfections in chirped-pulse-amplification stretchers.

In this paper, we propose a study of the picosecond temporal contrast degradation of ultrashort laser pulses by surface defects in pulse stretchers. In a chirped-pulse-amplification stretcher or compressor, dust and damages on the surface of an optical element lead to a spectral amplitude modulation. Furthermore, surface figure errors of optical elements happening where the pulse is spatially dispersed yield a modulation of the spectral phase. The influence of both amplitude and phase noise effects is numerically investigated using a hybrid ray-tracing method that enables treating separately the influence of noise sources, whether noise occurs in the near field or the far field. We show that the main issue in terms of picosecond contrast degradation is a combined effect of surface pattern distortion in the far field and phase-amplitude coupling caused by spatial frequency filters. Temporal domains can be defined, where the temporal contrast is dominated by different noise effects. The algorithm used in this paper is compared to the cross-correlation trace of a pulse. The conclusions emerging from the presented analysis are universally applicable to known grating stretcher geometries.

Time-Stretch Spectroscopy Based on Laser Cavity Tuning With a Dual-Function Delay Line

Time-stretch spectroscopy has established as a viable technique with high resolution and single-shot capability. However, it generally requires ultra-wideband photodetectors and electronics, and its performance is often restricted by a fundamental tradeoff between spectral resolution and signal-to-noise ratio. Here, we report a time-stretch spectrometer based on the concept of optical sampling by laser cavity tuning. A kilometer-scale dual-function fiber-optic link serves as both a pulse stretcher and a long delay line in an imbalanced Mach–Zehnder interferometer. Measurement results based on several device and atomic samples are presented. Theoretical forms of the total spectral range and spectral resolution are derived and compared with experimental data. A spectral resolution as small as 8 GHz has been achieved. Resolution-limiting factors are discussed and possible solutions are presented.

Increasing Laser-Doping Depth of Al in 4H-SiC by Using Expanded-Pulse Excimer Laser

Al doping into 4H-SiC performed by irradiating pulse-width-expanded excimer laser to an Al film deposited on the 4H-SiC surface is investigated. An optical pulse stretcher was constructed to produce the laser pulse whose peak intensity was reduced as half as that of the original pulse and pulse width was expanded from 55 ns to 100 ns. The irradiation of the expanded pulses is found to reduce the ablation of the materials from the surface and enable irradiation of multiple shots. As the result, doping depth of Al is significantly increased. The multiple shots of the expanded pulses is also fund to decrease the sensitivity to spatial non-uniformity of laser intensity and increase the uniformity of doped region.

Single-shot observation of stretched-pulse buildup dynamics in an ultrafast fiber laser

Time-stretch dispersive Fourier transform (TS-DFT) technique offers a powerful way to investigate the non-repetitive formation dynamics of mode locking. Here we have unveiled the entire buildup dynamics of stretched pulse in a passively mode-locked fiber laser using TS-DFT technique. The entire buildup dynamics include relaxation oscillation, transient bound states, beating dynamics, intensity fluctuation and stable stretched pulse finally. These results provide a direct insight into the start-up dynamics of stretched pulse, which could be helpful for the understanding of mode locking formation mechanism.

Mole fraction measurement through a transparent quarl burner using filtered Rayleigh scattering.

A filtered Rayleigh scattering system is developed and applied to measure the mole fraction of methane in a methane-air swirl flow through a transparent conical quartz quarl. Light scattering from the location where the laser beam is incident on the surface of the quarl is orders of magnitudes larger than Rayleigh scattering from the gas mixture of interest. This diffusive scattering is suppressed using molecular absorption by an iodine cell and using spatial filtering by an optical aperture. Residual stray light accounted for up to 5% of the total signal and had to be removed for accurate measurements. The flow consisted of a nonpremixed mixture of methane and air in the central jet surrounded by a strong swirling air flow. Measurements were conducted at a height of 4 mm from the fuel tube's exit for six different conditions of the swirl flow to demonstrate the ability of the instrument to study the effects of swirl strength and fuel flow rate on the mixing process. By using a four-leg pulse stretcher to allow higher laser energies in the probe volume, large collection optics and a reference iodine cell to monitor laser wavelength variations, standard deviations of ∼0.006 in air and ∼0.012 in a laminar methane flow were achieved for mole fraction measurements.

Wide field of view quantitative imaging of cellular ATP release.

Although several mechanical stressors promote ATP secretion from eukaryotic cells, few mechanosensitive pathways for ATP release have been precisely characterized and none have been clearly identified. To facilitate progress, we report here a wide field of view (∼20 × 20 mm sample area) imaging technique paired with a quantitative image analysis to accurately map the dynamics of ATP release from a cell population. The approach has been tested on A549 cells stretched at high initial strain rate (2-5 s-1) or swelled by hypotonic shock. The amount of ATP secreted in response to a series of five graded stretch pulses (5-37% linear deformation, 1-s duration at 25°C) changed nonmonotonically with respect to strain amplitude and was inhomogeneous across the cell monolayer. In a typical experiment, extracellular ATP density averaged 250 fmol/mm2, but the area of detectable signal covered only ∼40% of the cells. In some areas, ATP accumulation peaked around 900 fmol/mm2, which corresponded to an estimated concentration of 4.5 µM. The total amount of ATP released from the combined stretch pulses reached 384 ± 224 pmol/million cells (n = 4). Compared with stretch, hypotonic shock (50%, 30°C) elicited a more homogeneous ATP secretion from the entire cell population but at a lower yield totaling 28 ± 12 pmol/million cells (n = 4). The quantitative extracellular ATP mapping of several thousand cells at once, with this wide field of view imaging system, will help identify ATP release pathways by providing unique insights on the dynamics and inhomogeneities of the cellular ATP secretion that are otherwise difficult to assess within the smaller field of view of a microscope.

Revelation of the birth and extinction dynamics of solitons in SWNT-mode-locked fiber lasers

The dispersive Fourier transform (DFT) technique opens a fascinating pathway to explore ultrafast non-repetitive events and has been employed to study the build-up process of mode-locked lasers. However, the shutting process for the mode-locked fiber laser seems to be beyond the scope of researchers, and the starting dynamics under near-zero dispersion remains unclear. Here, the complete evolution dynamics (from birth to extinction) of the conventional soliton (CS), stretched pulse (SP), and dissipative soliton (DS) are investigated by using the DFT technique. CS, SP, and DS fiber lasers mode locked by single-walled carbon nanotubes (SWNTs) are implemented via engineering the intracavity dispersion map. The relaxation oscillation can always be observed before the formation of stable pulse operation due to the inherent advantage of SWNT, but it exhibits distinct evolution dynamics in the starting and shutting processes. The shutting processes are dependent on the dispersion condition and turn-off time, which is against common sense. Some critical phenomena are also observed, including transient complex spectrum broadening and frequency-shift interaction of SPs and picosecond pulses. These results will further deepen understanding of the mode-locked fiber laser from a real-time point of view and are helpful for laser design and applications.

Generation of coexisting high-energy pulses in a mode-locked all-fiber laser with a nonlinear multimodal interference technique

We demonstrate a passively mode-locked all-fiber laser incorporating a piece of graded-index multimode fiber as a mode-locking modulator based on a nonlinear multimodal interference technique, which generates two types of coexisting high-energy ultrashort pulses [i.e., the conventional soliton (CS) and the stretched pulse (SP)]. The CS with pulse energy as high as 0.38 nJ is obtained at the pump level of 130 mW. When the pump increases to 175 mW, the high-energy SP occurs at a suitable nonlinear phase bias and its pulse energy can reach 4 nJ at a 610 mW pump. The pulse durations of the generated CS and SP are 2.3 ps and 387 fs, respectively. The theory of nonlinear fiber optics, single-shot spectral measurement by the dispersive Fourier-transform technique, and simulation methods based on the Ginzburg–Landau equation are provided to characterize the laser physics and reveal the underlying principles of the generated CS and SP. A rogue wave, observed between the CS and SP regions, mirrors the laser physics behind the dynamics of generating a high-energy SP from a CS. The proposed all-fiber laser is versatile, cost-effective and easy to integrate, which provides a promising solution for high-energy pulse generation.

Proposal for Using DAΦNE as Pulse Stretcher for the Linac Positron Beam

The PADME experiment [1] aims at searching for the dark photon (A’) in the e+e− → Aγ′ process in a positron-on-target experiment, exploiting the positron beam of the DAΦNE Linac at the INFN Frascati National Laboratory (Italy). The Linac can provide a number of positrons in excess of 1010 in a 200 ns long pulse but, in order to keep the pile-up probability in the calorimeter at an acceptable level, the number of positrons for PADME has to be below 105/pulse, in particular by using the attenuating target in the BTF beam-line. The experiment is then essentially limited by the low duty factor (10−5 = 200 ns/20 ms). In this paper, an alternative proposal to use the DAΦNE positron ring as a Linac pulse stretcher, by injecting each pulse into the ring and extracting it by a slow resonant extraction using the m/3 resonance, is presented. This would allow for distributing the positrons of a single Linac pulse over a much longer time interval (0.2-0.5 ms), increasing the duty factor up to ∼1%. The required modifications of the DAΦNE positron transfer line and ring are described. A dedicated lattice for the positron ring has been designed and tracking of the positrons injected in the ring has been performed in order to optimize the extraction parameters and to give a preliminary estimate of the extracted beam characteristics.