Commercial Femtosecond Laser Research Articles

An economical single-shot pulse picker without nonlinear effect and dispersion.

An economical and stable single-shot pulse picker design without dispersion, nonlinear effect, and limitation on wavelength is proposed. This design is composed of a periodic pulse blocker (PPB), a control unit, and a mechanical shutter. It has successfully beenapplied to the commercial high-fluence femtosecond laser with 11-mm beam diameter, 2-mJ pulse energy, and 1-kHz repetition rate. Significantly, by incorporating commercial optical choppers equipped with custom-designed chopper blades in the PPB, this design can accommodate lasers with fluences reaching 610 mJ/cm2 and the standard 1kHz repetition rate typical of high-fluence lasers. Furthermore, the proposed design provides a cost-effective substitute compared to using electro-optic modulators or acousto-optic modulators.

Single-laser-based simultaneous four-wavelength excitation source for femtosecond two-photon fluorescence microscopy.

Multicolor labeling of biological samples with large volume is required for omic-level of study such as the construction of nervous system connectome. Among the various imaging method, two photon microscope has multiple advantages over traditional single photon microscope for higher resolution and could image large 3D volumes of tissue samples with superior imaging depth. However, the growing number of fluorophores for labeling underlines the urgent need for an ultrafast laser source with the capability of providing simultaneous plural excitation wavelengths for multiple fluorophores. Here, we propose and demonstrate a single-laser-based four-wavelength excitation source for two-photon fluorescence microscopy. Using a sub-100 fs 1,070-nm Yb:fiber laser to pump an ultrashort nonlinear photonic crystal fiber in the low negative dispersion region, we introduced efficient self-phase modulation and acquired a blue-shifted spectrum dual-peaked at 812 and 960 nm with 28.5% wavelength conversion efficiency. By compressing the blue-shift near-IR spectrum to 33 fs to ensure the temporal overlap of the 812 and 960 nm peaks, the so-called sum frequency effect created the third virtual excitation wavelength effectively at 886 nm. Combined with the 1,070 nm laser source as the fourth excitation wavelength, the all-fiber-format four-wavelength excitation source enabled simultaneous four-color two-photon imaging in Brainbow AAV-labeled (TagBFP, mTFP, EYFP, and mCherry) brain samples. With an increased number of excitation wavelengths and improved excitation efficiency than typical commercial femtosecond lasers, our compact four-wavelength excitation approach can provide a versatile, efficient, and easily accessible solution for multiple-color two-photon fluorescence imaging in the field of neuroscience, biomolecular probing, and clinical applications with at least four spectrally-distinct fluorophores.

Single Longitudinal Mode Lasers by Using Artificially Controlled Backscattering Erbium Doped Fibers

In this work, we propose and experimentally demonstrate a new distributed short linear cavity fiber laser. At one of the cavity ends, fabricated by a commercial femtosecond fiber laser chirped pulse amplifier, an artificially controlled backscattering erbium doped fiber section has been connected. This distributed reflector acts also as a saturable absorber, leading to the generation of tunable and switchable single longitudinal-mode laser emissions. The distributed reflector consists of 9 micro-drilled sections of about 1cm each one and randomly spread throughout 2 meters of highly doped erbium fiber. The total length of the fiber laser is 9.5 m and the laser shows a single mode behavior at all the emitted wavelengths. Using this new kind of reflecting saturable absorber, single and multiple single-mode emissions can be obtained. The achieved laser presents a pump threshold as low as 45 mW and shows up to 8 different single-mode emission lines with an optical signal to noise ratio of 45dB.

The theoretical study of terahertz-streaking photoionization for ultrafast imaging of density matrix in rubidium atom

Terahertz-streaking photoionization can be exploited to resolve ultrafast quantum beating and reconstruct the ultrafast evolution of density matrix. Here, we propose an experimental strategy to implement the method merely with the tabletop femtosecond system and magneto-optical trap reaction microscopy. The probe pulse consists of an ultraviolet pulse with pulse duration of about 30 fs and a strong terahertz pulse with strength of about 2.6 fs in rubidium-atom superposition. The population and coherence terms of the density matrix can be projected into different positions of the photoelectron momentum distribution. The reconstruction algorithm was designed to acquire the ultrafast evolution of density matrix from the time-dependent photoelectron spectrum. The experimental conception can demonstrate the newly proposed transient spectral method only with the commercial femtosecond laser system and magneto-optical trap reaction microscopy, thus preventing the complex laser system, such as extreme ultraviolet free electron lasers and attosecond higher harmonics, allowing a new metrology to explore the coherence dynamics of quantum systems.

Femtosecond laser ablation of dentin and enamel for fast and more precise dental cavity preparation

PurposeThe purpose of the present work was to achieve fast and more precise ablation in dentin and enamel by using a commercial femtosecond laser system with high repetition rate, whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. MethodsWe used fluence of the incident laser pulses which was marginally higher than the ablation threshold for dentin and enamel. The study was based on the hypothesis that femtosecond laser operating with a repetition rate in the range of 100–500 kHz can controllably ablate dental tissue obtaining sufficiently high removal rate whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. ResultsThe ablation yielded the formation of 1 mm3 craters with well-defined precise vertical cavity sides and edges. Advantageous high porosity and numerous interconnected pores were introduced in the ablated zones. Thermal load and hence collateral thermo-mechanical damages were avoided and the crystalline structure of the tooth constituent hydroxyapatite was preserved. ConclusionThe ultrafast femtosecond laser used in our work hold the promise of a significant drilling ability without collateral thermomechanical effects. It achieves high processing efficiency, overcomes disadvantages of other laser systems reported and can be used to develop an instrument for cavity preparation based on fast and precise ablation. Our further aim is to exceed the speed of traditional drilling instruments and thus to reduce the treatment time which in turn will bring comfort to the patient.

Tabletop Femtosecond VUV Photoionization and PEPICO Detection of Microreactor Pyrolysis Products.

We report the combination of tabletop vacuum ultraviolet photoionization with photoion-photoelectron coincidence spectroscopy for sensitive, isomer-specific detection of nascent products from a pyrolysis microreactor. Results on several molecules demonstrate two essential capabilities that are very straightforward to implement: the ability to differentiate isomers and the ability to distinguish thermal products from dissociative ionization. Here, vacuum ultraviolet light is derived from a commercial tabletop femtosecond laser system, allowing data to be collected at 10 kHz; this high repetition rate is critical for coincidence techniques. The photoion-photoelectron coincidence spectrometer uses the momentum of the ion to identify dissociative ionization events and coincidence techniques to provide a photoelectron spectrum specific to each mass, which is used to distinguish different isomers. We have used this spectrometer to detect the pyrolysis products that result from the thermal cracking of acetaldehyde, cyclohexene, and 2-butanol. The photoion-photoelectron spectrometer can detect and identify organic radicals and reactive intermediates that result from pyrolysis. Direct comparison of laboratory and synchrotron data illustrates the advantages and potential of this approach.

A new technique for probing chirality via photoelectron circular dichroism

We present a proof-of-principle approach for discriminating chiral enantiomers based on the phenomenon of multiphoton photoelectron circular dichroism. A novel stereo detection setup was used to measure the number of photoelectrons emitted from chiral molecules in directions parallel or anti-parallel to the propagation of the ionising femtosecond laser pulses. In this study, we show how these asymmetries in the ketones camphor and fenchone depend upon the ellipticity of the laser pulses and the enantiomeric excess of the sample. By using a high repetition rate femtosecond laser, enantiomer excesses with uncertainties at the few-percent level could be measured in close to real-time. As the instrument is compact, and commercial turnkey femtosecond lasers are readily available, the development of a stand-alone chiral analysis instrument for a range of applications is now possible.

Precise femtosecond laser ablation of dental hard tissue: preliminary investigation on adequate laser parameters

This work aims at evaluating the possibility of introducing state-of-the-art commercial femtosecond laser system in restorative dentistry by maintaining well-known benefits of lasers for caries removal, but also in overcoming disadvantages such as thermal damage of irradiated substrate. Femtosecond ablation of dental hard tissue is investigated by changing the irradiation parameters (pulsed laser energy, scanning speed and pulse repetition rate), assessed for enamel and dentin. The femtosecond laser system used in this work may be suitable for cavity preparation in dentin and enamel, due to the expected effective ablation and low temperature increase when using ultra short laser pulses. If adequate laser parameters are selected, this system seems to be promising for promoting a laser-assisted, minimally invasive approach in restorative dentistry.

Tuneable dual-comb spectrometer based on commercial femtosecond lasers and reference cell for optical frequency calibration

Two commercial femtosecond laser sources have been used to implement a dual-comb spectrometer tuneable across a spectral range from 1.5 to 2.2 μm. The optical linewidth of the comb modes was characterized for different time scales in order to estimate the achievable spectral resolution for an optimal acquisition time. The transmission spectra of three different gas samples were recorded, demonstrating good agreement with reference data. Frequency axis calibration was provided via the parallel monitoring of a reference sample. This technique allows an accurate calibration of the frequency axis of the spectrometer, with no need for stabilization or optical referencing of the frequency combs. Our set-up represents a good compromise for a compact and versatile dual-comb spectrometer based on commercially available parts with possible applications in trace-gas monitoring, remote sensing and spectroscopy of short-lived processes.

A tunable femtosecond stimulated Raman scattering system based on spectrally narrowed second harmonic generation

In this work we present a femtosecond stimulated Raman scattering system. The setup is based on a commercial femtosecond laser system supplemented by a pair of travelling-wave optical parametric amplifiers. One of the parametric amplifiers is used to generate the femtosecond actinic pump pulses, whereas the output of the other (high-power) parametric amplifier undergoes a combination of spectrally narrowed second harmonic generation in a long non-linear crystal and subsequent spectral filtering for the generation of narrowband Raman pump pulses. Chirped white light supercontinuum is used as the Raman probe. The setup offers tunability of the Raman pump pulses in the 400–800 nm range, and spectral and temporal resolutions of ca. 6 cm–1 and ca. 70 fs, respectively. We present the basic technical and optical aspects of the system along with data acquisition and signal retrieval techniques. We characterize the system by exploring the time-resolved vibrational dynamics of the S2(11Bu+) and S1(21Ag–) excited states of β-carotene.

Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f THz of CW-THz radiation. Using an f rep-free-running PC-THz comb, the f THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2 × 10−13 and a relative precision of 8.8 × 10−12 to a rubidium frequency standard. Furthermore, f THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.

Femtosecond laser cutting of multiple thin corneal stromal lamellae for endothelial bioengineering.

To assess the feasibility of cutting multiple thin stromal lamellae in human donor corneas using a commercial femtosecond laser (FSL) to provide cell carriers for future endothelial graft bioengineering. Eight edematous organ-cultured corneas not suitable for grafting for endothelial reasons were mounted on a Ziemer anterior chamber and cut with a Z6 FSL with 6 successive parallel cuts, from depth to surface. Target thickness of each lamella ranged from 100 to 150 μm depending on initial corneal thickness. Thickness was measured using anterior segment optical coherence tomography before and after cutting on mounted corneas, and on each stromal lamella after detachment. Scanning electron microscopy observation was performed on 4 lamellae and histological cross sections on 1 cornea before detachment. A median of 5 (minimum 3, maximum 7) lamellae was obtained per cornea. All lamellae still attached were the most posterior ones, suggesting that FSL was less efficient because of light scattering by edematous stroma. Cut precision and postdetachment swelling were correlated with anterior-posterior position within the cornea. Median lamella thickness was 127 μm (56-222 μm) before detachment and 196 μm (80-304 μm) after detachment. Surface state was consistent with previously reported FSL lamellar cuts during Descemet stripping automated endothelial keratoplasty. Up to 7 thin lamellae can be cut in stored corneas with an FSL. This method, once optimized primarily by using deswelled, more transparent corneas, could prove effective for recycling unsuitable donor corneas in corneal bioengineering processes.

Determination of the temporal structure of femtosecond laser pulses by means of laser-induced air plasma

A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the corresponding time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyzing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of ±150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures' effect on the laser-induced ionization or material ablation.

Analysis of microwave leaky modes propagating through laser plasma filaments column waveguide

A plasma column waveguide formed by a bundle of closely spaced plasma filaments induced by the propagation of ultrafast laser pulses in air and revived by a longer infrared laser pulse is shown to support microwave radiation. We consider values of both the plasma electron density and microwave frequency for which the refractive index of plasma is lower than the refractive index of air; therefore, a leaky plasma waveguide can be realized in extremely high frequency band. The guiding mechanism does not require high conductance of the plasma and can be easily excited by using commercial femtosecond laser sources. A theoretical study of leaky mode characteristics of isotropic and homogeneous plasma column waveguides is investigated with several values of plasma and waveguide structure parameters. The microwave transmission loss was found to be mainly caused by the microwave leakage through the air-plasma interface and is weakly dependent on the plasma absorption. In spite of losses of microwaves caused by leakage and plasma absorption, it is shown to be much lower than both that accompanying to surface waves attaching to single conducting plasma wire and the free space propagation over distances in the order of the filament length, which opens exciting perspectives for short distance point to point wireless transmission of pulsed-modulated microwaves.

Comparison of Picosecond and Femtosecond Laser Ablation for Surface Engraving of Metals and Semiconductors

Pico and femtosecond lasers present a growing interest for industrials applications such as surface structuring [1] or thin film selective ablation [2]. Indeed, they combine the unique capacity to process any type of material (dielectrics, semiconductors, metals) with an outstanding precision and a reduced affected zone. We report on results about surface engraving of metals (Al, Cu, Mo, Ni), semiconductor (Si) and polymer (PC) using a picosecond thin disk Yb:YAG-amplifier. The pulse duration of this source can be changed using two different configurations: direct amplification of a 34ps-oscillator on one hand, and 1ps-chirped pulse amplification (CPA) scheme on the other hand. The results obtained with this thin disk laser are compared to ones achieved with two commercial femtosecond lasers respectively based on Yb-doped crystals and fibers, and operating at similar output power levels (up to 15Watt).

Optimal quantum control for conditional rotation of exciton qubits in semiconductor quantum dots

Pulse-shaping protocols for subpicosecond optically controlled quantum gates in semiconductor quantum dots are reported. Our emphasis is the development of shaping schemes for either amplitude or phase control of the pulse that are easily implemented using commercial pulse shapers and femtosecond laser systems. We illustrate the efficacy of our approach through simulations of a controlled-rotation gate in a realistic In(Ga)As quantum dot with electronic structure calculated using eight-band, strain-dependent $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ theory. Our results show that amplitude- and phase-shaping protocols both lead to substantial improvements in fidelity when compared with transform-limited pulses with equivalent gate times. Dephasing was found to have a minimal effect on the gate fidelities due to the ultrafast time scale of the quantum operations.

Performances of 800 nm femtosecond laser ablation on natural and synthetic quartz

A commercial femtosecond laser system operating at its fundamental wavelength (λ = 800 nm, near Infra-Red) was used to ablate both synthetic and natural quartz on polished and unpolished surfaces. Ablation rates and maximum depths were determined using two distinct optical setups: a 25 mm focal length Cassegrain reflecting objective, and a 50 mm focal length convergent coated lens. All samples were ablated with the same laser beam at E = 1 mJ, τ = 60 fs, f = 5 Hz and N = 10–8000 shots. The depth of ablation craters obtained with the lens shows a linear increase with shot number N up to N = 2000 shots. Then the depth increases much less with N and reaches a plateau above N = 3000 shots. Maximum depth was close to 1300 μm for N = 3000 shots. Using the reflecting objective, ablation rate starts from 0.42 μm/shot and decreases rapidly to 0.02 μm/shot at a maximum depth of 350 μm for N =1500 shots. Ablation thresholds ( F th) were calculated for 1 and 10 consecutive shots with energy increasing from E = 0.1–2 mJ/pulse. Threshold values varies from F th=0.1 J.cm−2 (unpolished, 10 shots) to F th = 2.9 J.cm−2 (polished, single shot). The energy penetration of IR-femtosecond laser pulses in quartz has been calculated at l = 271 nm. The low absorption of IR wavelengths in quartz affects the ablation efficiency in the first shots. The associated non-linear effects are visible on a crater FIB foil observed with TEM as progressive high-pressure photomechanical damage developing under the ablation pit. The present study emphasizes the potential of IR-femtosecond laser for ablation of highly transparent material, and provides reliable data for LA-ICP-MS applications in earth sciences.

Temperature increase in porcine cadaver iris during direct illumination by femtosecond laser pulses

To measure the temperature rise in porcine cadaver iris during direct illumination by the femtosecond laser as a model for laser exposure of the iris during femtosecond laser corneal surgery. Department of Ophthalmology, University of California-Irvine, Irvine, California, USA. Experimental study. The temperature increase induced by a 60 kHz commercial femtosecond laser in porcine cadaver iris was measured in situ using an infrared thermal imaging camera at pulse energy levels ranging from 1 to 2 μJ (corresponding approximately to surgical energies of 2 to 4 μJ per laser pulse). Temperature increases up to 2.3 °C (corresponding to 2 μJ and 24-second illumination) were observed in the porcine cadaver iris with little variation in temperature profiles between specimens for the same laser energy illumination. The 60 kHz commercial femtosecond laser operating with pulse energies at approximately the lower limit of the range evaluated in this study would be expected to result in a 1.2 °C temperature increase and therefore does not present a safety hazard to the iris.

Study of the femtosecond fiber comb and absolute optical frequency measurement

An optical frequency comb is built by using a commercial femtosecond laser and signal-noise (S/N) ratio of the fceo signal is obtained to be about 30 dB. Its repetition rate frequency and offset frequency are highly stable and the tracking stability is verified. An absolute frequency measurement of a 1064 nm Nd: YAG laser is realized by using this stabilized optical frequency comb. The tracking stability of the repetition rate frequency is superior to 3.710-14 at 100 s and the absolute frequency of the laser is 281630111757362 Hz. The experimental result is in accordance with the recommended value by International Committee of Weights Measures(CIPM).

Simulation of the temperature increase in human cadaver retina during direct illumination by 150-kHz femtosecond laser pulses

We have developed a two-dimensional computer model to predict the temperature increase of the retina during femtosecond corneal laser flap cutting. Simulating a typical clinical setting for 150-kHz iFS advanced femtosecond laser (0.8- to 1-μJ laser pulse energy and 15-s procedure time at a laser wavelength of 1053 nm), the temperature increase is 0.2°C. Calculated temperature profiles show good agreement with data obtained from ex vivo experiments using human cadaver retina. Simulation results obtained for different commercial femtosecond lasers indicate that during the laser in situ keratomileusis procedure the temperature increase of the retina is insufficient to induce damage.