YAG Microlaser Research Articles

Subnanosecond microlaser pumped fan-out grating design MgO:PPLN optical parametric generator continuously tunable from near- to mid-infrared

An in-depth analysis and description of, to the best of our knowledge, the first subnanosecond pulse duration optical parametric generator (OPG) based on a fan-out grating design MgO-doped periodically-poled lithium niobate (MgO:PPLN) crystal is presented. Pumped by high energy subnanosecond pulse duration Q-switched Nd:YAG microlaser pulses, our OPG provides high output energies ranging from tens to hundreds of μJ with up to 45% conversion efficiency and enables fast and precise wavelength tuning across the near-to-mid infrared spectral regions (1414–4303 nm) just by laterally displacing the fan-out crystal without the need for temperature tuning. Limits of operation of the device were found in terms of the laser-induced damage threshold (LIDT) of the fan-out MgO:PPLN crystal. The OPG was fully characterized in terms of spectral, energy, and spatial properties of the signal wave, and analyzed experimentally and theoretically for the feasibility of producing broadband infrared output through non-collinear quasi-phase-matching and conversion of the fan-out grating structure into a chirped one.

Performance investigation of high-efficiency widely tunable subnanosecond optical parametric generator and amplifier based on MgO:PPLN.

We report on experimental and theoretical studies of widely tunable high-efficiency subnanosecond optical parametric generator (OPG) and amplifier (OPA) based on a 2 cm long multigrating MgO-doped periodically-poled lithium niobate (MgO:PPLN) crystal pumped by a passively Q-switched Nd:YAG micro-laser. Our OPG can be continuously tuned from 1442 nm to 4040 nm with signal wave energies ranging from 33 μJ to 265 μJ and total OPG conversion efficiency up to 46 % that depended on the pump focusing conditions. Characterization of spatial properties of the OPG determine Lorentzian spatial profile of the signal beam with M 2≈2 that was also dependent on the pump focusing conditions. High OPG gain and subsequent pump depletion led to the adjustment of the output signal pulse duration in the range of 242 - 405 ps by varying the incident pump power. By using a distributed feedback (DFB) continuous-wave (CW) 1550 nm wavelength seed laser for the OPA operation we reduced the generation threshold up to 1.6 times, increased maximum conversion efficiency by 4 - 20%, and achieved nearly transform-limited output signal pulses. Experimentally measured characteristics were supplemented by numerical simulations based on the quantum mechanical model for the OPG, and classical three-wave interaction model for the OPA operation.

Weldability of Grade 2 Titanium Sheets with Pulsed Nd:YAG Microlaser Welding Filler and Without Filler

Laser welding method is widely used in the welding of different materials. Deep penetration, low heat input, narrow heat affect zone, low stress-strain, and distortion are important features of this welding method as compared to other joint methods. Today, it is possible to see the applications of laser welding in the repair of precious metals, moulds, and machine parts. The laser welding method is preferred in the manufacture of many parts of precious metals. Titanium and particularly Grade 2 alloys are used in a wide range of applications, from medical applications to the aerospace industry applications. Since titanium is made of precious metals, it is of great use in manufacturing without much scrap. In the joints made by welding, it is estimated that the amount of scrap loss will decrease as a result of the potential to predict the distortion that the material will undergo and to provide more controlled planning of the current production. In this study, the weldability of 0.6 mm sheet materials with laser butt-welding was investigated. The effects of pulsed micro laser welding parameters on the microstructure, mechanical properties, and surface morphology of the fractures were investigated. As a result of the microstructure examination, it was found that cross-section narrowing was seen without filling welding. Fracture of the welded joints occurred in the base metal, showing an ultimate tensile strength of approximately 248 MPa with an elongation of 26.7 %.

Investigation of a widely-tunable subnanosecond BBO-based optical parametric amplifier

We report an experimental realization of a subnanosecond optical parametric amplifier (OPA) system in a beta barium borate (BBO) crystal pumped by the third harmonic of a passively Q-switched Nd:YAG microlaser system and seeded by the continuum generated in a photonic crystal fibre (PCF). It yields broadband continuous signal wavelength tunability in the visible spectrum range from 470 to 660 nm and the idler wavelength from 768 to 1450 nm. Besides the experimental data, the numerical simulation results of the BBO optical parametric amplifier are presented. The maximum output power of the subnanosecond BBO OPA is limited by laser induced damage in the BBO crystal by the pump radiation and seed radiation spectral power density. We also notice and discuss the effect of seed radiation on BBO OPA output radiation characteristics. The numerical simulations qualitatively agree with the experimental data.

Subnanosecond widely-tunable in the visible spectrum range LBO based optical parametric amplifier

We report realization of a widely-tunable subnanosecond optical parametric amplifier system in LBO crystal pumped by third harmonic of a passively Q-switched Nd:YAG microlaser system. It yields continuous signal wavelength tunability in the visible spectrum range from 460 to 680 nm and idler wavelength — from 740 to 1500 nm. We present experimental data and numerical simulations of our optical parametric amplifier and discuss future improvement directions for greater pump to signal conversion efficiency.

High peak-power near-MW laser pulses by third harmonic generation at 355 nm in Ca5(BO3)3F nonlinear single crystals.

In this work, the performance of Ca5(BO3)3F (CBF) single crystals was investigated for the third harmonic generation at 355 nm. A high energy conversion efficiency of 16.9% at 355 nm was reached using a two-conversion-stage setup. First, using a high peak power, passively Q-switched Nd3+:YAG/Cr4+:YAG microlaser based gain aperture in micro-MOPA, the second harmonic at 532 nm was achieved with lithium triborate (LBO) crystal, reaching 1.35 MW peak power. On a second step, laser pulses at 355 nm were generated using a 5 mm-long CBF crystal growth by TSSG method with energy, pulse duration and peak power of 479 µJ, 568 ps and 0.843 MW, respectively. These results are currently the highest reported for CBF material.

Compact optical image amplifier pumped by a sub-nanosecond Nd:YAG microlaser

A compact sub-nanosecond optical image amplifier is proposed and demonstrated. It is pumped by an actively Q-switched diode-pumped Nd:YAG microlaser in virtue of the optical parametric amplification (OPA) technique and quasi-phase-matching scheme. Experimental results show that the optical image gain reaches 15.4 dB under a pump intensity of 15.3 MW/cm2. The influence of parametric fluorescence noise on the amplified image is obvious above 10 dB gain. The signal-to-noise ratio degradation of the amplifier image characterized by the noise figures is in sheer contrast with theoretical expectations. It shows that the experimental noise figure value reaches 2.2 dB, larger than the theoretical result of 0.7 dB at 15 dB gain.

Performance of a Laser Ignited Multicylinder Lean Burn Natural Gas Engine

Market demands for lower fueling costs and higher specific powers in stationary natural gas engines have engine designs trending toward higher in-cylinder pressures and leaner combustion operation. However, ignition remains as the main limiting factor in achieving further performance improvements in these engines. Addressing this concern, while incorporating various recent advances in optics and laser technologies, laser igniters were designed and developed through numerous iterations. Final designs incorporated water-cooled, passively Q-switched, Nd:YAG microlasers that were optimized for stable operation under harsh engine conditions. Subsequently, the microlasers were installed in the individual cylinders of a lean-burn, 350 kW, inline six-cylinder, open-chamber, spark ignited engine, and tests were conducted. The engine was operated at high-load (298 kW) and rated speed (1800 rpm) conditions. Ignition timing (IT) sweeps and excess-air ratio (λ) sweeps were performed while keeping the NOx emissions below the United States Environmental Protection Agency (USEPA) regulated value (brake-specific NOx (BSNOx) < 1.34 g/kW h), and while maintaining ignition stability at industry acceptable values (coefficient of variation of integrated mean effective pressure (COV_IMEP) < 5%). Through such engine tests, the relative merits of (i) standard electrical ignition system and (ii) laser ignition system were determined. A rigorous combustion data analysis was performed and the main reasons leading to improved performance in the case of laser ignition were identified.

Sub-nanosecond periodically poled lithium niobate optical parametric generator and amplifier pumped by an actively Q-switched diode-pumped Nd:YAG microlaser

A sub-nanosecond seeded optical parametric generator (OPG) based on magnesium oxide-doped periodically poled lithium niobate (MgO:PPLN) crystal is presented. Pumped by an actively Q-switched diode-pumped 1 kHz, 1064 nm, Nd:YAG microlaser and seeded with a low power distributed feedback (DFB) diode continuous-wave (CW) laser, the OPG generated an output energy of 41.4 µJ and 681 ps pulse duration for the signal at 1652.4 nm, achieving a quantum conversion efficiency of 61.2% and a slope efficiency of 41.8%. Signal tuning was achieved from 1651.0 to 1652.4 nm by tuning the seed-laser current. The FWHM of the signal spectrum was approximately from 35 nm to 0.5 nm by injection seed laser. The SHG doubled the frequency of OPG signal to produce a output energy of 12 µJ with the energy conversion efficiency of 29.0% and tunanble wavelength near 826 nm.

Temperature stable operation of YCOB crystal for giant-pulse green microlaser.

In this work the performance of two yttrium calcium oxyborate (YCOB) crystals made by Czochralski and Bridgman growth process was measured. By using high peak power, passively Q-switched Nd3+:YAG/Cr4+:YAG microlaser, high conversion second harmonic generation efficiency were obtained. Laser pulses at 532 nm with 1.14 mJ energy and 223 ps duration were obtained with a 15-mm long YCOB crystal that was grown by Bridgman method. The conversion efficiency was 70.2%, comparable with the conversion efficiency of 72.8% that was achieved with 10-mm long lithium triborate (LBO) nonlinear crystal. Also, for the first time, experimental data on temperature tuning in type I YCOB crystal was measured with linear slope in 200°C temperature range equal to -0.057%/°C and -0.064%/°C for the Czochralski and Bridgman grown crystals, respectively. Such YCOB nonlinear crystal can become a serious option for developing laser sources with high-peak power at high repetition rate that can operate in harsh environment.

Visible supercontinuum generation in photonic crystal fiber using various harmonics of subnanosecond Q-switched laser

We report results of supercontinuum generation in highly nonlinear polarization maintaining photonic crystal fiber (PCF) using the first or second harmonics of a subnanosecond passively Q-switched Nd:YAG microlaser. The Nd:YAG microlaser generated 50 μ J energy 300 ps duration pulses at 1064 nm with a kilohertz repetition rate. We demonstrated that we can expand the supercontinuum spectrum to cover the whole visible range and beyond using either first or second harmonics of our pump laser in the case of such pump pulse durations. In the case of a first-harmonic pump ( λ p = 1064 nm ), the supercontinuum extended from 400 to 1300 nm and in the case of a second-harmonic pump ( λ p = 532 nm ), it extended from 400 to 900 nm. In addition, we compared the supercontinuum evolution and dependence on pump pulse energy in both cases. We also performed numerical simulations of supercontinuum generation in PCF using an improved approach. The generalized nonlinear Schrodinger equation was modified so that numerical simulations would yield greater accuracy in the case of longer (300 ps) pulses. The simulated supercontinuum spectra display the same qualitative features as that measured in the experiment.

Two-photon fabrication of three-dimensional silver microstructures in microfluidic channels for volumetric surface-enhanced Raman scattering detection

We report on the nonlinear photochemistry fabrication of three-dimensional silver (Ag) microstructures in microfluidic channels for volumetric surface-enhanced Raman scattering (3D SERS). The fabrication of high resolution 3D Ag microstructures is obtained by a two-photon induced reduction process of silver cations, which is restricted at the focal point of a Q-switched Nd:YAG microlaser (sub-nanosecond pulses at 1064 nm). Firstly, 3D Ag micro-pillars made on cover glass showed a 3D SERS detection limit of Oxazine 720 as low as 10−8 M. Secondly, we directly fabricated 3D microstructures within microfluidic channels, and demonstrated their 3D SERS capability. The micro-cube geometry gave a significantly larger 3D SERS signal than the micro-pillar geometry. This result demonstration is paving the way for further optimization routes by varying the geometry, the size, and the density of complex 3D structures which can be obtained by direct laser writing based on two-photon induced chemistry.

High Peak Power, Passively Q-Switched Yb:YAG/Cr:YAG Micro-Lasers

A high peak power (>MW), passively Q-switched Yb:YAG/Cr:YAG micro-laser end-pumped by fiber-coupled 120-W QCW LDs (Repetition rate <; 100 Hz) is developed. The convex output coupler with a curvature of -2 m successfully enlarges the fundamental mode size in the micro-laser cavity, and the output pulse energy increases to 3.6 mJ at a Cr:YAG initial transmission of 89% without optical damage. The TEM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> transverse mode and the single-frequency oscillation are confirmed. The pulse duration is 1.3 ns, and then the peak power is estimated as 2.8 MW. To our knowledge, these are the highest pulse energy and peak power ever reported in Yb:YAG/Cr:YAG micro-lasers. The Yb:YAG rod with 5 at.% Yb concentration and 4.2-mm thickness keeps more than 80% single-pass pump absorption ranging from 15°C to 45 °C in LD temperature. On the other hand, the characteristic temperatures of Yb:YAG/Cr:YAG and Nd:YAG/Cr:YAG lasers were estimated as 110 and 278 K, respectively. This means that the ratio of the increase in threshold pump energy to temperature of a Yb:YAG/Cr:YAG laser is twice or higher than that of a Nd:YAG/Cr:YAG laser due to a quasi-3-level system of Yb:YAG.

Design and Two-Photon Polymerization of Complex Functional Micro-Objects for Lab-on-a-Chip: Rotating Micro-Valves

Two-Photon Polymerization is a powerful technology that can be used to fabricate complex functional microobjects in lab-on-a-chip platforms. It is a laser-based prototyping technique with full tri-dimensional capability and sub-micron resolution. We report on the development of a computer-assisted design and fabrication process based on Q-switched Nd:YAG microlasers and AutoCAD environment. Microlasers with sub-nanosecond pulses of visible light are efficient and low cost lasers for two-photon microfabrication. Polymerization is easily obtained with sub-milliwatt average powers (12 kHz repetition rate). The microfabrication software has been developed to design the micro-objet models and to calculate their laser trajectories using AutoCAD. It is open platform for the design of 3D solids with full access to entities information, and an efficient slicing command that can cut solids from any orientation plane. As a typical example of complex functional micro-object, we report on the fabrication of laser-driven rotating micro-valves. The design parameters have been optimized to allow a free movement of the valve around its axis using the optical force of a laser tweezers.

Pico- and femtosecond laser-induced crosslinking of protein microstructures: evaluation of processability and bioactivity

This study reports the pico- and femtosecond laser-induced photocrosslinking of protein microstructures. The capabilities of a picosecond Nd:YAG laser to promote multiphoton excited crosslinking of proteins were evaluated by fabricating 2D and 3D microstructures of avidin, bovine serum albumin (BSA) and biotinylated bovine serum albumin (bBSA). The multiphoton absorption-induced photocrosslinking of proteins was demonstrated here for the first time with a non-toxic biomolecule flavin mononucleotide (FMN) as the photosensitizer. Sub-micrometer and micrometer scale structures were fabricated from several different compositions of protein and photosensitizer by varying the average laser power and scanning speed in order to determine the optimal process parameters for efficient photocrosslinking. In addition, the retention of ligand-binding ability of the crosslinked protein structures was shown by fluorescence imaging of immobilized biotin or streptavidin conjugated fluorescence labels. The surface topography and the resolution of the protein patterns fabricated with the Nd:YAG laser were compared to the results obtained with a femtosecond Ti:Sapphire laser. Quite similar grain characteristics and comparable feature sizes were achieved with both laser sources, which demonstrates the utility of the low-cost Nd:YAG microlaser for direct laser writing of protein microstructures.

Composite, all-ceramics, high-peak power Nd:YAG/Cr^4+:YAG monolithic micro-laser with multiple-beam output for engine ignition

A passively Q-switched Nd:YAG/Cr(4+):YAG micro-laser with three-beam output was realized. A single active laser source made of a composite, all-ceramics Nd:YAG/Cr(4+):YAG monolithic cavity was pumped by three independent lines. At 5 Hz repetition rate, each line delivered laser pulses with ~2.4 mJ energy and 2.8-MW peak power. The M(2) factor of a laser beam was 3.7, and stable air breakdowns were realized. The increase of pump repetition rate up to 100 Hz improved the laser pulse energy by 6% and required ~6% increase of the pump pulse energy. Pulse timing of the laser-array beams can by adjusted by less than 5% tuning of an individual line pump energy, and therefore simultaneous multi-point ignition is possible. This kind of laser can be used for multi-point ignition of an automobile engine.

Laser microstructuration of three-dimensional enzyme reactors in microfluidic channels

In this paper, we report on the fabrication of three-dimensional (3D) enzymatic microreactors within polydimethylsiloxane microfluidic channels through a photocrosslinking mechanism mediated by the two-photon absorption process at the focal point of pulse lasers, i.e., a sub-nanosecond Nd:YAG microlaser or a femtosecond Ti:Sapphire laser. This approach allows the building of localized 3D trypsin structures with submicron resolution. The fabrication of two different trypsin structures was successfully demonstrated using Eosin Y and Flavin Adenine Dinucleotide as biological photosensitizers: (i) arrays of 3D cylindrical rows and (ii) 3D woodpile structure. The enzymatic activity of the fabricated structures was evaluated by fluorescence spectroscopy using BODIPY FL casein as fluorogenic substrate. The real time investigation of the peptide cleavage into the microfluidic channel demonstrated that the fabricated trypsin microstructures maintain their catalytic activity. This approach opens up the way to complex multistep enzymatic reactions in well-localized regions of microfluidic devices, with great importance in health screening and biomedical diagnostics.

Enhancing performances of a passively Q-switched Nd:YAG∕Cr^4+:YAG microlaser with a volume Bragg grating output coupler

A diode end-pumped Nd:YAG microlaser passively Q-switched by Cr(4+):YAG with performances controlled by a volume Bragg grating (VBG) output coupler has been developed. Compared with operation at room temperature, the Q-switched laser pulse energy was increased, by a factor of 2 or more, by elevating the Nd:YAG temperature and locking the wavelength of emission with the VBG. Furthermore, the emission wavelength was tuned over 0.7 nm bandwidth by changing the VBG temperature while maintaining laser pulses of millijoule-level energy and short (less than 4 ns) duration.

High Peak Power, Passively $Q$-switched Microlaser for Ignition of Engines

The compact (electric spark plug size), diode-pumped, passively Q-switched Nd:YAG/Cr4+:YAG microlaser was developed for ignition of engines. Output energy of 2.7 mJ per pulse and 11.7 mJ per four-pulse train with a pulsewidth of 600 ps and an M 2 value of 1.2 were obtained at a pump duration of 500 ?s. The optical-to-optical conversion efficiency was 19%. Brightness of the microlaser was calculated as 0.3 PW/ sr-cm2 and optical power intensity was calculated as 5 TW/cm2 at the focal point of ignition. The enhanced combustion by the microlaser ignition was successfully demonstrated in a constant-volume chamber at room temperature and atmospheric pressure. The cross section area of a flame kernel generated by laser ignition is 3 times larger than that by a conventional spark plug at 6 ms after ignition in a stoichiometric mixture (A/F 15.2) of C 3H 8/air, even though ignition energy of the laser is 1/3 of that of the spark plug. Hundred percent ignition was successfully demonstrated in a lean mixture of A/F 17.2 by laser ignition, where electric spark plug ignition failed.

The comparative analysis and optimization of the free-running Tm3+:YAP and Tm3+:YAG microlasers

The thulium microlasers based on yttrium-aluminium perovskite and yttrium-aluminium garnet crystals are considered as coherent light sources at the wavelength near 2 μm. The comparative analysis of these lasers is carried out by means of the lasers’ parameters optimization for each value of the pumping power taking into consideration the cross-relaxation and up-conversion processes as well as heating of the active medium. The optimal values of the active medium length, activator concentration and the output mirror reflectivity are determined both for Tm:YAP- and Tm:YAG-laser. The pulse generation of Tm:YAP- and Tm:YAG-lasers is also considered, the parameters of this regime are determined. The comparison shows the suitability of Tm:YAP laser crystal both for cw and pulse generation. The up-conversion influence on the laser characteristics is also analyzed.