Pulse Stretcher Research Articles

Self – assembled aligned single – walled carbon nanotubes as an anisotropic polarizer and saturable absorber in erbium-doped femtosecond fiber laser

We report the production of a saturable absorber (SA) based on aligned single-walled carbon nanotubes SWCNTs by the self-assembly method. The ultrafast saturation behavior of prepared aligned and random nanostructures shows an increase of modulation depth by 6% due to the alignment method. We also observed the dependence of aligned SWCNTs loss on the polarization direction of laser radiation. All-fiber Er-doped hybrid mode-locked fiber laser was demonstrated based on the aligned /random SWCNTs-based SAs. We achieved the generation of stable stretched pulses with a pulse duration of 490/569 fs and an average output power of 20.7/18.25 mW. We also investigated the advantages of using aligned SWCNTs compared to the typical structure with random tube distribution. We show that the use of aligned SWCNTs film increased the SNR by 24.5, decreased the mode-locking threshold power by 42 mW, and decreased the RIN level by (20-25) dB in the frequency range [∼ 0 Hz- 100 kHz] and ∼ 8 dB in the range [100 kHz – 1 MHz].

Wavelength-Tunable Chirped Pulse Amplification System (1720 nm–1800 nm) Based on Thulium-Doped Fiber

Chirped pulse amplification (CPA) has been a commonly used methodology to obtain powerful ultrashort laser pulses ever since its first demonstration. However, wavelength-tunable CPA systems are much less common. Wavelength-tunable ultrashort and intense laser pulses are desirable in various fields such as nonlinear spectroscopy and optical parametric amplification. In this work, we report a 1720 nm–1800 nm tunable CPA system based on Tm-doped fiber. The tunable CPA system contains a seed laser, a pulse stretcher, two cascaded amplifiers and a pulse compressor. The dispersion-managed seed laser cavity emits wavelength-tunable laser pulses with pulse durations of several ps and spectral widths from 25 nm to 34 nm. After being stretched temporally to tens of ps, the laser pulses are then amplified in two-stage amplifiers and compressed in a Treacy-type compressor. At 1720 nm, the maximum average power of 126 mW is obtained with a pulse duration of 507 fs; at 1800 nm, the maximum average power of 264 mW is obtained with a pulse duration of 294 fs. The pulse repetition rates are around 22.7 MHz. We perform an analysis of the system design based on numerical simulations and go on to suggest further steps for improvement. To the best of our knowledge, this is the first demonstration of a tunable CPA system beyond 1.1 μm. Considering the specific wavelength range, this wavelength-tunable CPA system is highly desirable for biomedical imaging, sensing, and parametric amplifiers.

Development of broadband high efficiency Mid-IR gratings for high-energy ultrafast lasers

Broadband high-efficiency diffraction gratings play a crucial role in the pulse stretcher and compressor of high-energy ultrafast lasers. Nevertheless, conventional grating manufacturing techniques, including mechanical ruling and holographic recording, face challenges in creating accurate blazed groove profiles necessary for the fabrication of broadband, high-efficiency mid-infrared gratings. In this work, we utilized combined electron-beam lithography and anisotropic wet etching technology to fabricate nearly perfect blazed grooves, producing high efficiency broadband mid-infrared (IR) grating for 4.3 µm 100 femtosecond laser. Global optimization was performed to achieve a design of > 90% efficiency over spectral range of 3.6 µm – 6.6 µm. Hybrid metal-dielectric coating (Au-Al2O3) is employed and optimized to minimize absorption and to enhance diffraction efficiency and laser-induced damage threshold (LIDT). Prototype gratings undergo testing at a desired application wavelengths of 4.3 µm in a tunable range of 0.2 µm, revealing that the optimal sample achieves a diffraction efficiency of 92%, closely approaching the theoretical value of 94.2%

Simultaneous Application of Raman and Laser-Induced Breakdown Spectroscopy in the Gas Phase with a Single Laser and Detector.

Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) are powerful tools for molecular and elemental analysis, respectively. Their combined application, however, is challenging due to the differences in the signal generation and detection characteristics. This note proposes three experimental schemes for the simultaneous application of Raman and LIBS for gas-phase diagnostics. Ring-cavity optical pulse stretchers facilitate shaping suitable pulse pairs from a Q-switched laser that enables the quasi-simultaneous detection of the Raman and LIBS signals on a single detector.

FURION: modeling of FEL pulses propagation in dispersive soft X-ray beamline systems.

Modern X-ray free-electron lasers (XFELs) can generate pulses with durations ranging from femtoseconds to attoseconds. The numerical evaluation of ultra-short XFEL pulses through beamline systems is a critical process of beamline system design. However, the bandwidth of such ultra-short XFEL pulses is often non-negligible, and the propagation cannot be simply approximated using the central wavelength, especially in dispersive beamline systems. We developed a numerical model which is called Fourier optics based Ultrashort x-Ray pulse propagatION tool (FURION). This model can not only be used to simulate dispersive beamline systems but also to evaluate non-dispersive beamline systems. The FURION model utilizes Fresnel integral and angular spectrum integral to perform ultra-short XFEL pulse propagation in free space. We also present the method for XFEL pulse propagation through different types of dispersive gratings, which are commonly used in soft X-ray beamline systems. By using FURION, a start-to-end simulation of the FEL-1 beamline system at Shenzhen superconducting soft X-ray free electron laser (S3FEL) is carried out. This model can also be used to evaluate gratings-based spectrometers, beam splitters, pulse compressors, and pulse stretchers. This work provides valuable insights into the start-to-end simulation of X-ray beamline systems.

Research on the pre-pulses caused by post-pulses in the optical parametric chirped-pulse amplifier.

Pre-pulses caused by the post-pulses in the optical parametric chirped-pulse amplifier were comprehensively studied for the first time, including the underlying mechanism for the delay-shift of pre-pulses, the intensity variation of pre-pulses affected by the initial delay of post-pulses and the pump energy, and also the nonlinear beat noise. The simulation and measurement confirmed that the high-order dispersion of the pulse stretcher was the main cause for the delay-shift of pre-pulses, which should be similar with the chirped-pulse amplifiers. The intensity of pre-pulses would decrease significantly as the initial delay of post-pulses increased, but would increase with the growth of pump energy. Moreover, the temporal position of the nonlinear beat noise in the experiment was successfully predicted by our simulation. This work could help us better understand the pre-pulses in OPCPA and provide helpful guidance for designing high-contrast laser systems.

Spatiotemporal response of concave VLS grating to ultra-short X-ray pulses.

In soft X-ray free-electron laser (FEL) beamlines, variable-line-spacing (VLS) gratings are often used as dispersive components of monochromators and spectrometers due to their combined dispersion and focusing properties. X-ray FEL pulses passing through the VLS grating can result in not only transverse focusing but also spatiotemporal coupling effects, such as pulse front tilt, pulse front rotation, and pulse stretching. In this paper, we present a theoretical study of the spatiotemporal response of concave VLS gratings to ultra-short X-ray pulses. The theoretical analysis indicates that the tilt angle of the non-zero diffraction orders varies with the propagation distance, and disappears at the focus, where the focal lengths and pulse stretching differ for different diffraction orders. The model demonstrates the pulse duration after the concave VLS grating is the convolution of the initial pulse duration and the stretching term induced by dispersion, while the beam size at the focus in x dimension is the convolution of the geometric scaling beam size and the dispersion term. This work provides a mathematical explanation for the spatiotemporal response of concave VLS grating to ultra-short X-ray pulses and offers valuable insights into the design of FEL grating monochromators, spectrometers, pulse compressors, and pulse stretchers.

Transient behaviors of pulse breaking and recovering in a dispersion-managed mode-locked Yb fiber laser

Various pulse regimes have been discovered in mode-locked fiber lasers with carefully designed cavity setups. However, the transitions between different types of stable pulse solutions are rarely observed in a laser with the same cavity configuration. Here, we report on the experimental observation of the transformation from stretched pulses (SPs) to dissipative solitons (DSs) in an all-polarization-maintaining dispersion-managed mode-locked Yb fiber laser for the first time. By means of the time-stretch dispersive Fourier transform technique, the temporal and spectral dynamics of transient behaviors from pulse breaking to pulse recovering are identified in real time. It is revealed that the overdriven nonlinear effect induced by pump increase triggers the wave breaking of SPs, and the improved net-cavity dispersion originated from wavelength shift helps the self-recovering of DSs. Numerical simulations validate the experimental observations and bring insights to the understanding of transient nonlinear dynamics in ultrafast fiber lasers.

All-Polarization-Maintaining Linear Cavity Fiber Lasers Mode-Locked by Nonlinear Polarization Evolution in Stretched Pulse Regime

Nonlinear polarization evolution (NPE) is among the most advanced techniques for obtaining ultrashort pulses with excellent optical performance. However, it is challenging to design environmentally stable NPE fiber oscillators using only polarization-maintaining (PM) fibers. Here, we use the same PM fiber and non-reciprocal phase shifter to design two different devices, which are capable of acting as effective NPE saturable absorbers (SAs) in two all-PM linear cavity fiber lasers. These two laser setups differ in the position of the non-reciprocal phase shifter, the presence of which is crucial for the proposed fiber lasers to reduce their mode-locking thresholds and achieve high repetition rates above 100 MHz. The mode-locking principle and pulse evolution in the laser cavity are investigated theoretically. The first all-PM fiber oscillator emits sub-200 fs stretched pulses with low peak powers. The second oscillator, with a simpler architecture, directly delivers stretched pulses with high peak powers, the spectral bandwidth greater than 30 nm, and the pulse duration less than 90 fs. To the best of our knowledge, 79 fs achieved in this design is the shortest pulse duration provided by PM fiber lasers using NPE mode-lockers.

Laser pulse train parameters determine the brightness of a two-photon stimulus.

This report presents the results of measurements of the two-photon vision threshold for various pulse trains. We employed three pulsed near-infrared lasers and pulse stretchers to obtain variations of the pulse duty cycle parameter over three orders of magnitude. We proposed and extensively described a mathematical model that combines the laser parameters with the visual threshold value. The presented methodology enables one to predict the visual threshold value for a two-photon stimulus for a healthy subject while using a laser source of known parameters. Our findings would be of value to laser engineers and the community interested in nonlinear visual perception.

Simultaneous measurements of NH2 and major species and temperature with a novel excitation scheme in ammonia combustion at atmospheric pressure

This work reports the first 1D, simultaneous, laser-induced fluorescence (LIF) measurements of the amidogen (NH2) radical concentration and Raman/Rayleigh measurements of major species and temperature in laminar counterflow non-premixed and premixed NH3/H2/N2–air flames. The instrument presents a novel excitation scheme for NH2, which add no complexity to the existing Raman/Rayleigh instrument. The NH2 LIF is excited by three Nd:YAG lasers, temporally stretched to 500 ns using a pulse stretcher, resulting in total energy of ∼ 1 J at 532 nm. LIF spectra collected in NH2 produced from photolysis of NH3, in an oxygen-free environment, match those obtained in the counterflow flames. The NH2-LIF signal is sampled in the 620–630 nm, together with the Raman signal, using the same collection hardware. The weak absorption cross-section at 532 nm is compensated by the high laser energy available, and the long pulse allows repeated excitation and emission from the same molecule, leading to the partial saturation region. A simplified model which neglects quenching, and contributions from other species, but includes Raman crosstalk from H2, N2, and NH3 is proposed and calibrated by comparing measurements and 1D Chemkin simulation of a counterflow flame. Finally, this simplified model was validated in more than 23 flames with varying fuel compositions, strain rates, and non-premixed and premixed flames to assess accuracy and precision. The overall agreement between the measured and simulated NH2 using the same mechanism is within 5%, and the single-shot detection limit is below 100 ppm. NO2-LIF is one of the main interferences to NH2-LIF, and its effects can be negligible on the fuel side, but care only need to be taken on the air side with T < 800 K, in which the maximum interference ∼33 ppm was observed. The accuracy of chemical kinetic models, with an uncertainty of ∼ 20%, limits the accuracy of the proposed NH2 measurements. The technique is best suited to investigate the turbulence-chemistry interaction in turbulent NH3 flames combined with major species and temperature from Raman/Rayleigh scattering, and it can provide useful validation data for the numerical model when the same chemical model is used.

Quasi-continuous time-stretched photon Doppler velocimetry

In transient high-speed scenes such as chemical explosions, magnetic compression, electric explosion, and laser-driven flyer, the target object can accelerate from 0 to tens of km/s in a few nanoseconds. The time-stretched photon Doppler velocimetry uses the time-stretched interferometry to downscale the frequency of the Doppler shift signal in the optical domain, which breaks through the bandwidth limitations of photoelectric detectors and electrical digital to analog converters. A quasi-continuous time-stretched photon Doppler velocimetry (QT-PDV) is proposed in this paper, for the measurement of microsecond high-speed movement events. On the principle of hardware optical path, the velocity signal of the target object is encoded on multiple stretched chirp pulses, avoiding sampling interruptions and trigger condition limitations; in the signal demodulation methods, the error compensation algorithm is used to compensate for the frequency shift signal, which reduces the system error caused by the displacement and increases the effective recording time. This paper adopts the high-power nanosecond laser to drive the metal film to produce a high-speed target, where beat frequency was downscaled to 2.2 GHz from 4.4 GHz.

Stabilizing the carrier–envelope phase of an amplified Ti:sapphire laser pulse to a noise level of sub-100 mrad

To generate an intense femtosecond pulse with an energy of several mJ or more, the pulse must be sufficiently stretched and compressed using grating-based optics in a chirped pulse amplification system. However, the stability of the carrier–envelope phase (CEP) is readily degraded by movement of the diffraction gratings due to vibration caused by pump lasers and other equipment. By suppressing the vibration and acoustic waves, we reduced the CEP noise to much less than 100 mrad (rms). With feedback control using a wedge pair with a single shot f - 2 f interferometer (1 ms integration time), we were able to stabilize the CEP, achieving a CEP noise of 58 mrad (rms) at a pulse energy of 0.5 mJ and 99 mrad (rms) at a pulse energy of 7 mJ and a repetition rate of 1 kHz. We also analyzed the effects of vibration on the pulse stretcher and compressor using a geometrical optics approach and found that roof mirrors were the most influential.

High-dispersive mirror for pulse stretcher in femtosecond fiber laser amplification system

We present a high-dispersive multilayer mirror for pulse stretching in a femtosecond fiber laser amplification system. The designed mirror contains 54 layers with a total physical thickness of 7.3 μm, which can provide a positive group delay dispersion (GDD) of 600 fs2 and a high reflectance over 99.9% from 1010 to 1070 nm. The samples were prepared by dual ion beam sputtering. The measured transmittance matches well with the theoretical result. The GDD characteristics of samples were tested by home-made white light interferometer. The measured GDD is higher than the design results, an average GDD of +722 fs2 from 1010 nm to 1070 nm. The mirrors were employed in a Yb-doped large-mode-area photonic crystal fiber amplification system. An input pulse compressed by the gratings with autocorrelation function of 83 fs is obtained with a stretched FWHM of 1.29 ps after 28 bounces between the dispersive mirrors. The results show that the multilayer dispersive mirror could be an effective and promising technique for pulse stretching in femtosecond amplification systems.

Impact of compression grating phase modulations on beam over-intensities and downstream optics on PETAL facility

Chirped pulse amplification has been widely implemented in high power laser chains. It consists of a set of diffraction gratings used to stretch and compress short laser pulses. In the case of high power laser chains, the compression stage is followed by the transport mirror in order to carry the laser beam to its final target. In such laser chains, laser beams propagate over a complex set of optical components and understanding the propagation of phase noise turns out to be of crucial importance. Phase modulation can induce laser damage on the final optical components. Here, we study the impact of phase modulation induced by the different diffraction gratings of the Petawatt Aquitaine Laser (PETAL) compressor on the downstream over-intensities, in particular on the transport mirror. This work allows us to quantify the impact of phase modulation for every single grating element in the compression stage, and to estimate the quantity of laser induced damage sites on transport optics for a specific laser shot.

A deep-UV picosecond laser for photocathode electron gun

We report a 10 Hz Yb:YAG laser regenerative amplifier with frequency conversion to 257.3 nm. A diode-pumped solid-state Yb:YAG laser oscillator is built as the seed of the system with output power up to 1.75 W. A chirp-volume-Bragg grating (CVBG) is used both as pulse stretcher and pulse compressor. A 10-Hz Yb:YAG regenerative amplifier is built which amplifies the seed pulses to 1.58 mJ. After pulse compression, 1.36 mJ laser pulses with 2.4 ps pulse duration are obtained. Good beam quality is maintained after the compressor. LBO and BBO crystals are cascaded in frequency convertor. At the final output, 120 μJ laser pulses at 257.3 nm are obtained. This system is designed as the driving source for photocathode electron guns in electron accelerator.

How the laser beam size conditions the temporal contrast in pulse stretchers of chirped-pulse amplification lasers

AbstractIn this work, we propose and verify experimentally a model that describes the concomitant influence of the beam size and optical roughness on the temporal contrast of optical pulses passing through a pulse stretcher in chirped-pulse amplification laser systems. We develop an analytical model that is capable of predicting the rising edge caused by the reflection from an optical element in a pulse stretcher, based on the power spectral density of the surface and the spatial beam profile on the surface. In an experimental campaign, we characterize the temporal contrast of a laser pulse that passed through either a folded or an unfolded stretcher design and compare these results with the analytical model. By varying the beam size for both setups, we verify that optical elements in the near- and the far-field act opposed to each with respect to the temporal contrast and that the rising edge caused by a surface benefits from a larger spatial beam size on that surface.

Herriott multi-pass block material stretcher and grism compressor in chirped-pulse amplification

We report a femtosecond chirped-pulse amplification (CPA) system based on block material stretcher and grism compression. An optical material block is employed in Herriott multi-reflection configuration as a pulse stretcher, and a transmission grating is combined with dispersion prism to form grism as a compressor which can provide the negative second and third-order dispersion. By optimizing the prism vertex angle and grating line density, the grism can completely compensate for the third-order material dispersion. We obtain shorter compressed pulses. In the experiment, the stretched pulses are amplified by regenerative amplifier, which amplifies the 800 nm seed pulse to 2.30 W under the 11.4 W, 527 nm, 1 kHz pumping conditions, and the spectral width of the amplified pulse is 26.7 nm. The amplified pulses are compressed to 39.6 fs, which is close to the Fourier transform limit of 35.2 fs. The design of the system simplifies the structure of the conventional chirped pulse amplification system, reduces the space size of the optical path, and improves the operational stability of the laser system. With the block material stretcher and grism compressor, the whole CPA laser system is very compact and can be used as a reliable light source for subsequent amplification as well as ultrafast phenomenon studies.

Use of double-grating Offner stretcher for dispersion control in petawatt level optical parametric chirped pulse amplification systems

The aberration-free characteristic endows the double-grating Offner stretcher with the advantages of good beam quality and perfect-dispersion-match with conjugated Treacy compressor, and therefore makes it an ideal pulse stretcher for optical parametric chirped pulse amplification (OPCPA) systems featuring with low material dispersion. We numerically and experimentally demonstrate the feasibility of dispersion control in OPCPA systems by the use of double-grating Offner stretcher. The numerical results indicate that near Fourier-transform-limited (FTL) pulses can be realized in the double-grating Offner stretcher based petawatt (PW) level broadband OPCPA systems. Moreover, in a proof-of-principle experiment, amplified pulses with ∼210 nm spectral bandwidth and ∼3 ns chirped pulse duration are also compressed to near FTL pulse duration of 15.4 fs just by cooperating a double-grating Offner stretcher with a Treacy compressor. To our knowledge, it is the first time to realize near FTL compression of laser pulses with such a broadband spectrum and such a large chirped pulse duration simultaneously, while without using any extra dispersion compensation methods.

Point-by-point inscription of chirped apodized fiber Bragg gratings for application as ultrashort pulse stretchers

There are special requirements for chirped fiber Bragg gratings used as stretchers in ultrafast fiber lasers. The benefits of fabricating fiber stretchers by the femtosecond point-by-point single-shot technique are described. For the first time, apodization and arbitrary nonlinear chirp are embedded in chirped fiber Bragg gratings point-by-point inscription process. A significant reduction of the amplitude ripple of the grating reflection spectrum is demonstrated.