Μm Pulses Research Articles

Laser surface texturing for high friction contacts

A pulsed, nanosecond fibre laser with wavelength of 1064nm was used to texture grade 316 stainless steel and ‘low alloy’ carbon steel in order to generate contacts with high static friction coefficients. High friction contacts have applications in reducing the tightening force required in joints or to easily secure precision fittings, particularly for larger components where standard methods are difficult and expensive. Friction tests performed at normal pressures of 100MPa and 50MPa have shown that very high static friction coefficients greater than 1.25, an increase of 346% over untextured samples at 100MPa, can be easily achieved by single pass laser texturing of both contacting surfaces with the use of low pulse separations. The high static friction coefficients, obtained at 100MPa normal pressure with textures with up to 62.5μm pulse separation (processing speed ∼0.67cm2/s), were found to be associated with a significant amount of plastic deformation caused by the high normal pressures. As a result, higher normal pressures were found to result in higher friction coefficients.

Multi-mJ mid-infrared kHz OPCPA and Yb-doped pump lasers for tabletop coherent soft x-ray generation

We present our recent progress on the development of a mid-infrared (mid-IR), multi-mJ, kHz optical parametric chirped-pulse amplification (OPCPA) system, pumped by a homebuilt picosecond cryogenic Yb:YAG chirped-pulse amplifier, and its application to soft x-ray high-order harmonic generation. The cryogenic Yb:YAG laser operating at 1 kHz repetition rate delivers 42 mJ, 17 ps, 1.03 μm pulses to pump the OPCPA system. Efficient second and fourth harmonic generations from the Yb:YAG system are demonstrated, which provide the pumping capability for OPCPA at various wavelengths. The mid-IR OPCPA system produces 2.6 mJ, 39 fs, 2.1 μm pulses with good beam quality (M2 = ∼1.5) at 1 kHz repetition rate. The output pulses of the OPCPA are used to generate high-order harmonics in both gas cell and hollow-core fiber targets. A photon flux of ∼2 × 108 photon/s/1% bandwidth at 160 eV in Ar is measured while the cutoff is 190 eV. The direct measurements of the photon flux from x-ray photodiodes have confirmed the generation of water-window soft x-ray photons with a flux ∼106 photon/s/1% bandwidth at 330 eV in Ne. The demonstrated OPCPA and Yb:YAG pump laser technologies provide an excellent platform of energy and power scalable few-cycle mid-IR sources that are suitable for high-flux tabletop coherent soft x-ray generation.

Chirped-pulse amplification in a CO_2 laser

Chirped-pulse amplification (CPA) is an integral part of present-day ultra-intense laser systems that normally employ near-infrared (∼1 μm) solid-state lasers. The recently revived interest in expanding the reach of strong-field laser physics into the mid-infrared (mid-IR) spectral domain directs our attention to 9–11 μm carbon-dioxide (CO2) lasers for which progress to reaching high peak intensities has been limited so far. We propose that employing the CPA technique will allow us to realize a new breakthrough toward multiterawatt, ultrafast mid-IR lasers; here we report, to our knowledge, the first implementation of this method for a CO2 laser. Our stretching of a 1 ps, 9 μm pulse to 80 ps improved energy extraction from a regenerative CO2 laser amplifier by 1 order of magnitude. We explain this accomplishment by the reduction in nonlinear absorption and refraction on the amplifier’s optical elements. We consider these findings as being a pivotal step toward establishing next-generation ultra-intense CO2 CPA laser systems for strong-field mid-IR research and its applications.

Mid-infrared passively switched pulsed dual wavelength Ho(3+)-doped fluoride fiber laser at 3 μm and 2 μm.

Cascade transitions of rare earth ions involved in infrared host fiber provide the potential to generate dual or multiple wavelength lasing at mid-infrared region. In addition, the fast development of saturable absorber (SA) towards the long wavelengths motivates the realization of passively switched mid-infrared pulsed lasers. In this work, by combing the above two techniques, a new phenomenon of passively Q-switched ~3 μm and gain-switched ~2 μm pulses in a shared cavity was demonstrated with a Ho3+-doped fluoride fiber and a specifically designed semiconductor saturable absorber (SESAM) as the SA. The repetition rate of ~2 μm pulses can be tuned between half and same as that of ~3 μm pulses by changing the pump power. The proposed method here will add new capabilities and more flexibility for generating mid-infrared multiple wavelength pulses simultaneously that has important potential applications for laser surgery, material processing, laser radar, and free-space communications, and other areas.

Design factors of intravascular dual frequency transducers for super-harmonic contrast imaging and acoustic angiography

Imaging of coronary vasa vasorum may lead to assessment of the vulnerable plaque development in diagnosis of atherosclerosis diseases. Dual frequency transducers capable of detection of microbubble super-harmonics have shown promise as a new contrast-enhanced intravascular ultrasound (CE-IVUS) platform with the capability of vasa vasorum imaging. Contrast-to-tissue ratio (CTR) in CE-IVUS imaging can be closely associated with low frequency transmitter performance. In this paper, transducer designs encompassing different transducer layouts, transmitting frequencies, and transducer materials are compared for optimization of imaging performance. In the layout selection, the stacked configuration showed superior super-harmonic imaging compared with the interleaved configuration. In the transmitter frequency selection, a decrease in frequency from 6.5 MHz to 5 MHz resulted in an increase of CTR from 15 dB to 22 dB when receiving frequency was kept constant at 30 MHz. In the material selection, the dual frequency transducer with the lead magnesium niobate-lead titanate (PMN-PT) 1–3 composite transmitter yielded higher axial resolution compared to single crystal transmitters (70 μm compared to 150 μm pulse length). These comparisons provide guidelines for the design of intravascular acoustic angiography transducers.

High-sensitivity ultrashort mid-infrared pulse characterization by modified interferometric field autocorrelation.

We report on spectral phase retrieval of 43MHz, ∼100 fs, 3.3μm pulses at energies down to 8.9pJ by a modified interferometric field autocorrelation method. The simple setup consists of a Michelson interferometer, a 266μm thick AgGaSe2 crystal, and a homemade spectrometer with an InGaAs point detector, which is readily applicable to measuring a 20fs (1.8 cycles) pulse at 3.3μm. The feasibility is verified by comparing with the results obtained by simulation and frequency-resolved optical gating for the spectral phase modulation because of a 4mm thick germanium plate.

Self-reconstructing spatiotemporal light bullets

We show that spatiotemporal light bullets generated by self-focusing and filamentation of 100 fs, 1.8 μm pulses in a dielectric medium with anomalous group velocity dispersion (sapphire) are extremely robust to external perturbations. We present the experimental results supported by the numerical simulations that demonstrate complete spatiotemporal self-reconstruction of the light bullet after hitting an obstacle, which blocks its intense core carrying the self-compressed pulse, in nonlinear as well as in linear (free-space) propagation regimes.

All-fiber ultralow-energy soliton management at 1.55 µm

A highly nonlinear photonic-crystal fiber (PCF) is shown to enable an efficient coupling of a low-energy picosecond output of an erbium fiber laser into optical solitons. With this fiber, stretched 2.2 ps, 1.2 nJ, 1.55 µm pulses of an erbium fiber laser can be coupled into sub-100 fs optical solitons with an energy slightly below 1 nJ. The peak power and the pulse width of these solitons can be finely tuned by pulse chirp management in a variable-length single-mode fiber, connected to the erbium fiber laser output and cascaded with the PCF.

Mid-infrared source with 02 J pulse energy based on nonlinear conversion of Q-switched pulses in ZnGeP_2

Mid-infrared (3-5 μm) pulses with high energy are produced using nonlinear conversion in a ZnGeP(2)-based master oscillator-power amplifier, pumped by a Q-switched cryogenic Ho:YLF oscillator. The master oscillator is based on an optical parametric oscillator with a V-shaped 3-mirror ring resonator, and the power amplifier is based on optical parametric amplification in large-aperture ZnGeP(2) crystals. Pulses with up to 212 mJ energy at 1 Hz repetition rate are obtained, with FWHM duration 15 ns and beam quality M(2) = 3.

Diode-pumped Er3+:Yb3+:NaCe0.43Gd0.57(WO4)2 pulse laser passively Q -switched with a Co2+:Mg0.4Al2.4O4 saturable absorber at 1.53 μ m

Based on the long fluorescence lifetime of upper laser level 4I13/2 of Er3+ ions and high 1.5−1.6 μm laser operation efficiency, an Er3+:Y b3+:NaCe0.43Gd0.57(WO4)2 crystal was used to generate a Q-switched pulse laser with high energy and low threshold. End-pumped by a 970 nm diode laser, a passively Q-switched 1.5−1.6 μm pulse laser was first realized in an Er3+ and Y b3+ co-doped tungstate crystal, when a Co2+:Mg0.4Al2.4O4 spinel crystal was used as saturable absorber. A 1.53 μm pulse laser with about 22 μJ energy, 76 ns duration and 21 kHz repetition rate was obtained at an absorbed pump power of 12 W in a c-cut 1.7-mm-thick Er3+:Y b3+:NaCe0.43Gd0.57(WO4)2 crystal.

Terahertz wave detection using a low-temperature-grown GaAs photoconductive antenna excited by 1.5µm pulses

Terahertz wave detection using a low-temperature-grown GaAs photoconductive antenna excited by 1.5µm pulses

High Power Supercontinuum Generation in Fluoride Fibers Pumped by 2 $\mu{\rm m}$ Pulses

High power supercontinuum generation in fluoride step-index optical fibers pumped with 2- μm pulses delivered by a gain-switched thulium-doped fiber laser and amplifier system is reported. An output average power of 1.25 W in the ~ 1.8-4.15 μm spectral band and 1.82 W in the ~ 1.8-3.65 μm band was achieved for two fluorozirconate fibers of different parameters. The first demonstration of Watt-level supercontinuum generation in a fluoroindate fiber is also demonstrated. The achieved output SC powers are the highest ones ever reported for fluoride fibers pumped with 2- μm gain-switched optical pulses.

Narrow-bandwidth, ~100 ps seeded optical parametric generation in CdSiP_2 pumped by Raman-shifted pulses at 1198 nm

Low-threshold, efficient optical parametric generation at ~4.64 μm is demonstrated using CdSiP2 nonlinear crystal pumped by 150 ps Raman shifted pump pulses at 1198 nm in noncritical configuration at 1 kHz repetition rate. Maximum single pulse idler energy of 6 μJ and total conversion efficiency of 30% are achieved. Seeding at the signal wavelength with a distributed feedback laser diode enables ~25 fold narrowing of the bandwidths down to ~10 GHz, resulting in a Fourier product of ~1 for the ~100 ps long signal (1615 nm) and idler (4.64 μm) pulses.

In vitro plant regeneration of Albizia lebbeck (L.) from seed explants

Objectives: An efficient and reproducible regeneration protocol for rapid multiplication of Albizia lebbeck (L.) was developed by using intact seed explants.Methods: Murashige and Skoog's (MS) medium supplemented with different hormones (BA, Kn, GA3 and TDZ) was used for the induction of multiple shoots from the seed explants. Ex-vitro rooting was performed by using pulse treatment method in auxins (IBA and NAA) and the complete plantlets were transferred to the field.Results: High frequency direct shoot induction was found in aseptic seed cultures of A. lebbeck on Murashige and Skoog medium supplemented with 5.0 µM TDZ (Thiadiazuron). Seeds were germinated after 7 days of culture and induced maximum 8 shoots from the region adjacent to the apex of the primary shoot of the seedling upto 25 days of incubation. Proliferating shoot cultures with increased shoot length was established by sub-culture of excised sprouting epicotyls on MS medium supplied with reduced concentrations of TDZ. Maximum shoot regeneration frequency (76 %) with highest number of shoots (21) and shoot length (5.1 cm) per sprouting epicotyl was observed in the MS medium supplemented with 0.5 µM TDZ after 8 weeks of culture. Different concentrations of Indole-3-butyric acid (IBA) and α-naphthalene acetic acid (NAA) were tested to determine the optimal conditions for ex-vitro rooting of the microshoots. The best treatment for maximum ex-vitro root induction frequency (81 %) was accomplished with IBA (250 µM) pulse treatment given to the basal end of the microshoots for 30 min followed by their transfer in plastic cups containing soilrite and eventually established in normal garden soil + soilrite (1:1) with 78 % survival rate. In addition, histological study was undertaken to gain a better understanding of the regenerated shoots from the epicotyl region.Conclusion: The findings will be fruitful in getting a time saving and cost effective protocol for the in vitro propagation of Albizia lebbeck.Keywords: Woody legume; TDZ; multiplication; epicotyl segment; shoot stumps.Abbreviations used: GA3 (Gibberellic acid); BA (6-Benzyladenine); Kn (Kinetin); TDZ (Thidiazuron); MS (Murashige and Skoog Medium); IBA (Indole-3-butyric acid); NAA (α-Naphthalene acetic acid).

Second harmonic generation and enhancement in microfibers and loop resonators

We model and experimentally investigate second harmonic generation in silica microfibers and loop resonators, in which the second order nonlinearity arises from the glass-air surface dipole and bulk multipole contributions. In the loop resonator, the recirculation of the pump light on resonance is used to increase the conversion. The effect of the loop parameters, such as coupling and loss, is theoretically studied to determine their influence on the resonance enhancement. Experimentally, microfibers were fabricated with diameters around 0.7 μm to generate the intermodally phase matched second harmonic with an efficiency up to 4.2 × 10−8 when pumped with 5 ns 1.55 μm pulses with a peak power of 90 W. After reconfiguring the microfiber into a 1 mm diameter loop, the efficiency was resonantly enhanced by 5.7 times.

Narrow-pulse-width gain-switched thulium fiber laser

A hybrid-pumped gain-switched Tm3+ fiber laser with an all-fiber configuration is demonstrated to achieve over 10 W 2 μm pulsed laser beams. This laser system is complementarily pumped by a 790 nm continuous-wave diode laser and a 1 μm pulsed Yb3+ fiber laser. Based on the triggering effect of a 1 μm pulse pump and the Rayleigh scattering induced Brillouin scattering effect, the 2 μm laser pulse can be narrowed to less than 20 ns, which is the narrowest pulse width ever achieved in high-power gain-switched thulium fiber lasers. The maximum pulse energy is 2 mJ and the maximum peak power is over 100 kW. Supercontinuum generation spanning over 200 nm can be observed with an average-power spectral density of >30 mW nm−1.

Optical parametric chirped pulse amplifier at 1600 nm with all-optical synchronization

We demonstrate the amplification of 1.6 μm pulses by a KTA optical parametric chirped-pulse amplifier based on an all-optical synchronization scheme as a scalable approach to generation of high power tunable mid infrared.

Asynchronous mid-infrared broadband optical parametric oscillator for dual-comb spectroscopy

Two asynchronous, broadband 3.3+µm pulse trains with a stabilized repetition+rate difference of up to 5+kHz were generated from a single optical parametric oscillator. With additional carrier+envelope+offset stabilization, it could be applied to coherent dual+frequency+comb spectroscopy.

Hybrid Er/Yb fibre laser system for generating few-cycle 1.6 to 2.0 μm pulses optically synchronised with high-power pulses near 1 μm

This paper presents the concept of fibre laser system design for generating optically synchronised femtosecond pulses at two, greatly differing wavelengths and reports experimental and numerical simulation studies of nonlinear conversion of femtosecond pulses at 1.5 μm wavelength in a dispersion-shifted fibre, with the generation of synchronised pulses in the ranges 1.6–2 and 1–1.1 μm. We describe a three-stage high-power fibre amplifier of femtosecond pulses at 1 μm and a hybrid Er/Yb fibre laser system that has enabled the generation of 12 fs pulses with a centre wavelength of 1.7 μm, synchronised with high-power (microjoule level) 250 fs pulses at 1.03 μm.

The generation of a broadband, spectrally flat supercontinuum extended to the mid-infrared with the use of conventional passive single-mode fibers and thulium-doped single-mode fibers pumped by 1.55 μm pulses

Flatly broadened supercontinuum (SC) generation spread toward the mid-infrared region is achieved in telecom-grade passive fibers (SMF-28) as well as in single-mode thulium-doped fibers pumped by 1 ns pulses at a wavelength of 1550 nm. Using only a piece of single-mode fiber (SMF) as a nonlinear medium the SC spread from ∼1300 to 2500 nm with an output power of up to 2 W. A spectrum flatness variation below 5 dB in a span of 640 nm (from 1.6 to 2.24 μm) and a corresponding spectral density of ∼1.1 mW nm−1 were obtained. Using a single-mode silica Tm-doped fiber, the SC cut-off wavelength was extended to 2.65 μm. For this case, broadband mid-IR radiation with a <5 dB spectral flatness in the wavelength interval ranging from 1.95 to 2.51 μm (spectral span of 560 nm) was demonstrated. The output average power was over 1 W out of which ∼85% was in a band of >1.9 μm, which is, to the best of our knowledge, the highest average power of an SC generated directly from a single-mode Tm-doped fiber.