YAG Microchip Laser Research Articles

Development of a radiation tolerant laser-induced breakdown spectroscopy system using a single crystal micro-chip laser for remote elemental analysis

ABSTRACT For the development of the remote elemental analysis method in a radiation environment based on the laser-induced breakdown spectroscopy (LIBS), the radiation effects on the laser oscillation properties of the single crystal (SC) Nd:YAG microchip laser (MCL) were investigated and compared with those of ceramic Nd:YAG MCL. The laser oscillation properties were measured under gamma ray irradiation as a function of dose rate. The effects on the SC MCL properties were very small compared to those on the ceramics, indicating minimal radiation effects on the LIBS signal when using SC MCL. Pulse energy and oscillating build-up time (BUT) were measured for cumulative dose exceeding 1400 kGy. The pulse energy remained stable under irradiation. The BUT also remained stable, demonstrating negligible optical loss accumulation that could affect laser properties even at the demonstrated cumulative dose. The results indicate that the effects of dose rate and cumulative dose on SC MCL laser properties are minimal. The SC MCL was then integrated into the LIBS system, and the gadolinium signal was successfully measured at a dose rate of 5 kGy/hr. These findings highlight the radiation tolerance of SC MCL for remote LIBS applications in harsh radiation environments.

An efficient Yb:YAG microchip laser for generating high-power cylindrical vector beams with four topological charges

High-power cylindrical vector beams (CVBs) in the form of Laguerre-Gaussian (LG0,l) with fixed topological charge and desirable state of polarization (SoP) have been widely needed for various applications such as quantum communication, optical trapping, manipulating microparticles, laser machining, and high-resolution imaging. Highly efficient high-order CVBs with fixed topological charge and SoP generated in solid-state lasers are desirable for various applications. Here, by adjusting annular pump beam waist and distance between focus spot and gain medium, we directly generated LG0,l (l = 2, 3, 4) CVBs with SoP in the form of E0,l(ϕ,r) = - Eϕ ×sin[(l + 1)ϕ]+ Er ×cos[(l + 1)ϕ] in a highly efficient Yb3+:Y3Al5O12 (Yb:YAG) microchip laser. The LG0,4 CVB is obtained when the input pump power is higher than 2.9 W and keeps as the input pump power further increases. The output power of the LG0,4 cylindrical vector laser is 1.3 W at the input pump power of 6.1 W. The optical conversion efficiency is as high as 21.3%. Polarization purity is over 93% for LG0,4 CVB generated in the Yb:YAG microchip laser.

Short-Pulse Nd:YAG/Cr:YAG Microchip Laser With Pulse Duration of < 200 ps

A passively Q-switched Nd:YAG/Cr:YAG microchip laser with the pulse duration of < 200 ps has been developed. To obtain such short pulses from Nd:YAG/Cr:YAG microchip lasers, a numerical model based on rate equations was introduced and a design strategy of microchips was discussed. Since the pump bleaching effect in Cr:YAG leads to an increase of output pulse duration, a method of small-spot pumping was employed to reduce the effect and shorten the pulse duration. In the experiments, 1064-nm laser pulses with the duration of 153 ps and the energy of 18 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{J}$ </tex-math></inline-formula> at 4.5 kHz was achieved. A good beam quality of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M^{2} =1.15$ </tex-math></inline-formula> was also measured.

Development of a portable laser peening device and its effect on the fatigue properties of HT780 butt-welded joints

Laser peening (LP) is a well-established technique for introducing compressive residual stress (RS) near the surface of metal components, to improve their high-cycle fatigue properties. The authors have developed a compact LP device with a thumb-sized Nd:YAG microchip laser mounted on a collaborative robot arm. The device was applied to 9-mm-thick HT780 high-strength steel plate samples with irradiated pulse energies of 7.5−8.0 mJ, spot sizes of 0.42−0.58 mm and pulse densities of 100−1,600 pulses/mm2. X-ray diffraction showed that the maximum compressive RS was over 500 MPa near the surface, and the LP effect reached a depth of approximately 0.1 mm from the surface. Butt-welded HT780 samples were laser-peened with a pulse energy of 7.7 mJ, spot size of 0.49 mm and pulse density of 800 pulses/mm2. Then, the samples were subjected to a uniaxial fatigue test with a stress ratio of 0.1. The results showed that the fatigue strength at 107 cycles was improved by at least 50 MPa, comparable to the improvement attained by LP in a previous study with a pulse energy of 200 mJ from a conventional Nd:YAG laser.

>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).

Effects of Laser Peening with a Pulse Energy of 1.7 mJ on the Residual Stress and Fatigue Properties of A7075 Aluminum Alloy

Laser peening without coating (LPwC) using a palmtop-sized microchip laser has improved the residual stresses (RSs) and fatigue properties of A7075 aluminum alloy. Laser pulses with a wavelength of 1.06 μm and duration of 1.3 ns from a Q-switched Nd:YAG microchip laser were focused onto A7075 aluminum alloy samples covered with water. X-ray diffraction revealed compressive RSs on the surface after irradiation using laser pulses with an energy of 1.7 mJ, spot diameter of 0.3 mm, and density of 100–1600 pulse/mm2. The effects were evident to depths of a few hundred micrometers and the maximum compressive RS was close to the yield strength. Rotation-bending fatigue experiments revealed that LPwC with a pulse energy of 1.7 mJ significantly prolonged the fatigue life and increased the fatigue strength by about 100 MPa with 107 fatigue cycles. The microchip laser used in this study is small enough to fit in the hand or be mounted on a robot arm. The results may lead to the development of tools that extend the service life of various metal parts and structures, especially outdoors where conventional lasers are difficult to apply.

Study on properties of Nd:YAG ceramic microchips with different concentration

Polycrystalline Nd-doped Y3Al5O12 ceramic is a competitive candidate for solid-state laser material. Here, attenuation loss, optical absorption, fluorescence lifetime and laser oscillation have been investigated for different concentration ceramic microchips. A 1064 nm laser is generated for 0.1 at.%, 0.5 at.%, 0.8 at.%, 1 at.% and 2 at.% Nd:YAG samples. The maximal output power of 17.4 W is achieved on a 4-mm thick 0.8% Nd-doped ceramic. The measured conversion efficiency is as high as 55%, which represents, to our knowledge, the highest result demonstrated to date for Nd:YAG microchip lasers. Based on a theoretical model of continuous-wave end-pumped four-level system, the effects of optical properties for different concentration ceramics on the laser output performance are also analytical and numerical solutions, where the simulated results agree well with the experimental data.

Generation and amplification of a multi-beam sub-nanosecond laser

In this paper, a methodology to produce a multi-beam sub-nanosecond laser is proposed. Laser pulses with a pulse energy of 0.14 mJ and a pulse width of 490 ps are generated in a YAG/Nd:YAG/Cr4+:YAG microchip laser at a repetition rate of 200 Hz. After amplification with a laser diode (LD) side-pumped Nd:YAG module, four laser beams are generated because of the thermally induced birefringence. With a double-pass LD side-pumped amplifier, the single pulse energy of the four laser beams is amplified to 5.23 mJ with a peak power of ∼10.67 MW, and air breakdown with four points is achieved with a 2 × 2 lens array.

High-pulse-energy passively Q-switched sub-nanosecond MOPA laser system operating at kHz level.

A passively Q-switched sub-nanosecond master oscillator power amplifier (MOPA) laser system at 1064 nm has been reported in this paper. The master oscillator was a passively Q-switched YAG/Nd:YAG/Cr4+:YAG microchip laser, yielding a pulse energy of 0.14 mJ and a pulse width of ∼490 ps at repetition rates of 500 Hz and 1 kHz. After passing a double-pass side-pumped Nd:YAG amplification system, the pulse energy reached 7.6 mJ and 1.7 mJ at 500 Hz and 1 kHz, respectively. The spatial beam deformation caused by the thermally induced birefringence was investigated numerically and experimentally.

Highly efficient Yb:YAG microchip laser for direct generation of radially polarized vector vortices

High beam quality, highly efficient radially polarized vector vortex beam has been generated in a Yb:YAG microchip laser pumped with an annular beam formed with a hollow focus lens. The output power of 2.15 W has been achieved at the absorbed pump power of 3.53 W. The optical-to-optical efficiency is 60.9%. The radially polarized vector vortex lasers working at 1030 nm, 1050 nm, 1030/1050 nm dual-wavelength depending on applied pump power. High polarization purity with degree of polarization of over 91% has been achieved for radially polarized vector vortex lasers. Our work provides an effective method for developing compact wavelength tunable, radially polarized vector vortex lasers.

Single-longitudinal-mode-operated, passively Q-switched Nd:YAG/Cr4+:YAG microchip laser with >100 kHz repetition rate and

We report a comprehensive design procedure for passively Q-switched monolithic Nd:YAG/Cr4+:YAG microchip lasers to realize operation conditions of the pulse repetition rate (PRR) >100kHz and pulse width (PW) <400ps, simultaneously. Crucial parameters including effective pump power and waist diameter of the pump laser, doping concentration and thickness of the Nd:YAG crystal, initial transmittance (T0) of the Cr4+:YAG crystal, as well as the reflectivity of the output coupler are all considered during the design process. Two single-longitudinal-mode-operated lasers are designed and constructed according to the numerical results. The lengths and doping concentrations of Nd:YAG for both microchips are optimized to be 0.5 mm and 2%, respectively. A PRR up to 170 kHz and PW of ∼370ps are measured under the pump power of 2.6 W for a microchip with T0 of 0.85. A slightly lower PRR of 118 kHz with a shorter PW of ∼320ps is also achieved under the same pump power for another microchip with T0 of 0.8. The related pulse energies for the two microchips are ∼2.1µJ and ∼1.8µJ, respectively. To the best of our knowledge, these results are among the highest PRRs achieved for passively Q-switched Nd:YAG/Cr4+:YAG microchips with PWs <400ps.

Single-longitudinal-mode-operated, passively Q-switched Nd:YAG/Cr4+:YAG microchip laser with &gt;100 kHz repetition rate and &lt;400 ps pulse width

We report a comprehensive design procedure for passively Q -switched monolithic Nd : YAG / Cr 4 + : YAG microchip lasers to realize operation conditions of the pulse repetition rate (PRR) &gt; --> 100 kHz and pulse width (PW) &lt; 400 ps , simultaneously. Crucial parameters including effective pump power and waist diameter of the pump laser, doping concentration and thickness of the Nd:YAG crystal, initial transmittance ( T 0 ) of the Cr 4 + : YAG crystal, as well as the reflectivity of the output coupler are all considered during the design process. Two single-longitudinal-mode-operated lasers are designed and constructed according to the numerical results. The lengths and doping concentrations of Nd:YAG for both microchips are optimized to be 0.5 mm and 2%, respectively. A PRR up to 170 kHz and PW of ∼ 370 ps are measured under the pump power of 2.6 W for a microchip with T 0 of 0.85. A slightly lower PRR of 118 kHz with a shorter PW of ∼ 320 ps is also achieved under the same pump power for another microchip with T 0 of 0.8. The related pulse energies for the two microchips are ∼ 2.1 µ J and ∼ 1.8 µ J , respectively. To the best of our knowledge, these results are among the highest PRRs achieved for passively Q -switched Nd : YAG / Cr 4 + : YAG microchips with PWs &lt; 400 ps .

High-order cylindrical vector beams with tunable topological charge up to 14 directly generated from a microchip laser with high beam quality and high efficiency

Large topological charge optical vortex beams carrying orbital angular momentum have potential applications on optical trapping, optical communication with high capacity, quantum information processing. However, the beam quality is degraded in vortex beams generated with spiral phase plates or resonator mirrors with defect spots and optical conversion efficiency in solid-state lasers is sacrificed by controlling the loss of resonator. It is a big challenge for generating high beam quality, high-order cylindrical vector beams with large topological charge in compact solid-state lasers. Here, high-order cylindrical vector beams [Laguerre-Gaussian (LG) modes with zero degree and order of l, LG0,l] with tunable topological charges up to 14 have been generated in an annular beam pumped Yb:YAG microchip laser by manipulating the pump power-dependent population inversion distribution. Efficient performance with optical efficiency of 17.5% has been achieved. The output power is 1.36 W for a vector-vortex laser with 14 topological charges. The pump power dependent wavelength tunable and dual-wavelength laser oscillation in vector-vortex beams has been observed by controlling the reabsorption loss at 1030 nm. Wavelength tunable, dual-wavelength (1030 and 1050 nm) laser oscillation has been achieved for vector-vortex beams with topological charges of 8, 9, and 10. The laser beam quality factor M2 close to the theoretical value (l + 1) has been achieved for LG0,l vector-vortex beams with tunable topological charges up to 14. This work provides a new effective method for generating large topological charge high-order cylindrical vector beams in solid-state microchip lasers with high efficiency and high beam quality.

Vector vortices with tunable polarization states directly generated in a microchip laser

High beam quality vector vortices with tunable polarization states have been produced in a Yb:YAG microchip laser under focused annular beam pumping. Radial, anti-radial, and hybrid polarization are obtained in vector vortices by adjusting the offset distance (Δz) between the focus spot and the Yb:YAG crystal. Output powers of over 1 W for vector vortices with three polarization states are achieved within a wide range of Δz (e.g., 1.25 mm). The efficient, high-power Yb:YAG microchip laser is demonstrated to generate vector vortices with an output power of 1.7 W and an optical efficiency of 22%.

Generation of controllable Ince – Gaussian modes in a passively Q-switched microchip laser under truncated decentred Gaussian beam pumping

We report a passively Q-switched (PQS) Nd : YAG/Cr4+: YAG microchip laser pumped by a truncated decentred Gaussian beam for controllable Ince – Gaussian (IG) mode generation. IGe(n, n) modes (n = 0, …, 4) are obtained experimentally by choosing the appropriate truncation parameter, which is defined as a ratio of the radius of the aperture to the beam waist. The IGe(4, 4) mode is produced in the PQS Nd : YAG/Cr4+: YAG microchip laser under decentred Gaussian beam pumping at a pump power of 2.6 W, the mode selection being achieved by reducing the truncation parameter. The threshold pump powers for different IGe(n, n) modes are theoretically calculated to illustrate the mode selection process. The pulsed IGe(n, n) (n ≥ 1) mode generation with a peak power exceeding 1 kW and a nanosecond pulse width is obtained in the PQS microchip laser, which is a potential laser source for forming flexible vortex arrays for optical tweezers.

Effects of Cr4+ ions on forming Ince–Gaussian modes in passively Q-switched microchip solid-state lasers

The effects of Cr4+ ions on forming Ince–Gaussian (IG) laser modes in a tilted pumped Cr4+:YAG passively Q-switched (PQS) Nd:YAG microchip lasers has been studied experimentally and theoretically. The formation of Cr4+-ion domains in Cr4+:YAG crystal has been proposed based on the spatial distribution of Cr4+ ions in Cr4+:YAG crystal. The spatial modulation effect of Cr4+-ion domains on selecting a transverse mode has been investigated based on their periodic distribution and nonlinear absorption of the Cr4+ saturable absorber. The formation of IG modes in the Nd:YAG/Cr4+:YAG PQS microchip laser under tilted pumping has been demonstrated theoretically with the spatial modulation of the Cr4+-ion domains in Cr4+:YAG crystal and the inversion population distribution, by taking account of the thermal lens effect. The IG modes selected theoretically in the Nd:YAG/Cr4+:YAG PQS microchip laser are in good agreement with experimentally obtained IG modes. This work provides a theoretical model of the selection of IG modes with Cr4+-ion domains in a tilted beam pumped Nd:YAG/Cr4+:YAG PQS microchip laser to produce efficient, stable and controllable IG laser modes.

High beam quality and high peak power Yb:YAG/Cr:YAG microchip laser.

The prospect for developing a passively Q-switched Yb:YAG/Cr:YAG monolithic microchip laser that operates at cryogenic temperature is theoretically analyzed. It is concluded that such a system has the potential to deliver laser pulses with improved energy and increased peak power in comparison with composite Yb:YAG/Cr:YAG or Nd:YAG/Cr:YAG devices that are operated at room temperature. Consequently, a cryogenically cooled Yb:YAG/Cr:YAG system is built and the emission performances are investigated. Laser pulses with 3.2 mJ energy, 6.1 MW peak power and high beam quality of M2 = 1.8 are achieved. By increasing the pump beam diameter, laser pulses with higher energy 32 mJ are obtained at 25 MW peak power with M2 = 5.4. To our knowledge, these are the best results obtained from passively Q-switched composite Yb:YAG/Cr:YAG monolithic microchip lasers.

160 ps Yb:YAG/Cr:YAG microchip laser

By cryogenically cooling the Yb:YAG/Cr:YAG medium, one can break through the damage limit of Yb:YAG/Cr:YAG passively Q-switched microchip lasers at room temperature and thus achieve a shorter minimum pulse duration. In the proof of principle experiment we carried out, a 160.6 ps pulse duration was obtained. To the best of our knowledge, this is the first realization of sub-200 ps pulse operation for an Yb:YAG/Cr:YAG microchip laser.

PCSEL pumped coupling optics free Yb:YAG/Cr:YAG microchip laser.

The passively Q-switched operation of a cryogenically cooled Yb:YAG/Cr:YAG microchip laser was demonstrated with end pumping by a photonic crystal surface emitting laser (PCSEL). This laser generated 70μJ/1.7ns/3.2kHz pulses with near diffraction limited beam quality (M2=1.1) at 1029.4nm. There were no coupling optics between the microchip laser crystal and PCSEL, which made the system simple and compact.

Sub-nanosecond Nd:Lu0.61Gd0.39VO4 microchip laser

We report the fabrication and operation of a single-to-three-mode, sub-nanosecond passively Q-switched Nd:Lu0.61Gd0.39VO4/Cr4+:YAG microchip laser, which exhibits changes in mode structure with increasing incident pump power. The laser exhibits longitudinal mode oscillations with a partial transverse mode overlap. The shortest pulse duration, highest pulse energy and peak power observed are 646 ps, 8.7 μJ and 13.5 kW, respectively.