Plano-concave Resonator Research Articles

Evaluation of ultrasound sensors for transcranial photoacoustic sensing and imaging

Photoacoustic imaging through skull bone causes strong attenuation and distortion of the acoustic wavefront, which diminishes image contrast and resolution. As a result, transcranial photoacoustic measurements in humans have been challenging to demonstrate. In this study, we investigated the acoustic transmission through the human skull to design an ultrasound sensor suitable for transcranial PA imaging and sensing. We measured the frequency dependent losses of human cranial bones ex vivo, compared the performance of a range of piezoelectric and optical ultrasound sensors, and imaged skull phantoms using a PA tomograph based on a planar Fabry–Perot sensor. All transcranial photoacoustic measurements show the typical effects of frequency and thickness dependent attenuation and aberration associated with acoustic propagation through bone. The performance of plano-concave optical resonator ultrasound sensors was found to be highly suitable for transcranial photoacoustic measurements.

High power diode end-pumped 1.3 μm Nd:YAG InnoSlab laser

We reported a simple, compact 1.3 μm diode end-pumped 0.6-at.% Nd:YAG InnoSlab laser. The crystal was placed in a stable spherical plano-concave resonator. The laser performance was optimized using output couplers with transmission T = 5% and T = 10% at various cavity lengths. For the cavity length of 40 mm, the maximum output power of 23.2 W was obtained at T = 5%. The laser achieved 17.8% slope efficiency and 11.36% optical conversion efficiency.

Crystal growth, structural characterization and laser operation of an Yb0.19Y0.34Lu0.12Gd0.35Ca4O(BO3)3 mixed crystal

An Yb0.19Y0.34Lu0.12Gd0.35Ca4O(BO3)3 mixed oxyborate crystal was grown with large size and high optical quality, by using the Czochralski method. Its crystal structure was characterized by X-ray diffraction measurements. The continuous-wave laser performance of the mixed crystal was investigated with X-, Y- and Z-cut crystals in a plano-concave resonator. An output power of 17.0 W was produced with a Y-cut crystal, the optical-to-optical and slope efficiencies being respectively 49.3% and 55.1%.

Spectroscopic and lasing properties of a mixed (Yb, Y, Lu, Gd) calcium oxyborate crystal: Yb0.19Y0.34Lu0.12Gd0.35Ca4O(BO3)3

We report on the polarized spectroscopic properties and efficient laser emission of Yb0.19Y0.34Lu0.12Gd0.35Ca4O(BO3)3, a newly developed (Yb, Y, Lu, Gd) mixed oxyborate crystal. The strongest absorption is found to occur at 976.6 nm for E//Y, with a cross-section measured to be 2.05 × 10−20 cm2; the main emission band important to laser action appears to be strongest for E//Z, its maximum cross-section at 1028.8 nm amounts to 0.34 × 10−20 cm2. Efficient laser emission was realized with a Y-cut crystal in a plano-concave resonator, under excitation of either broad-band 937-nm or narrow-band 973-nm pumping radiation. An output power of 7.36 W was produced under 937-nm pumping, with a slope efficiency of 55%; while under 973-nm pumping, the output power achievable could be scaled to 16.80 W, with optical conversion and slope efficiencies being, respectively, 57.6% and 70%.

Polarization switching realized in the continuous-wave and acousto-optic Q-switched pulse Er:Yb:LaMgB5O10 lasers at 1556 and 1568 nm.

By using the natural birefringence of crystalline quartz, the switching of a 1568 nm laser with polarization parallel to the Z optical indicatrix axis to a 1556 nm laser with polarization parallel to the Y optical indicatrix axis was observed in both continuous-wave and acousto-optic Q-switched pulse lasers based on an X-cut Er:Yb:LaMgB5O10 crystal, when the alignment of the output mirror in a 976 nm diode-end-pumped plano-concave resonator was precisely tilted. For the continuous-wave regime, a 1568 nm laser with a maximum output power of 500 mW and slope efficiency of 16%, as well as a 1556 nm laser with a maximum output power of 400 mW and slope efficiency of 13.5% were realized, respectively. For the Q-switched regime, a 1568 nm pulse laser with an energy of 144 μJ and width of 300 ns, as well as a 1556 nm pulse laser with an energy of 168 μJ and width of 270 ns, were obtained respectively by precisely tilting the alignment of the output mirror, when the absorbed pump power was 4.0 W and pulse repetition frequency was 0.5 kHz.

Free-running performance of [formula omitted] mixed crystal laser operating around 1084 nm

Highly efficient high-power laser operation around 1084 nm is demonstrated of the Yb0.14:Y0.77Gd0.09Ca4O(BO3)3 mixed crystal in a compact plano-concave resonator, producing an output power of 18.1 W in free-running mode, with optical-to-optical and slope efficiencies being, respectively, 60% and 70% with respect to incident pump power. In laser action achieved with the high-gain short-wavelength oscillation being suppressed, the highest output power reaches 16.2 W with an optical-to-optical efficiency of 50.3%, while the slope efficiency amounts to 58%. The results presented reveal the superiority of the Yb-ion doped mixed oxyborate crystal to the existing Yb-ion or Nd-ion crystals capable of generating high-power laser radiation around 1084 nm.

Graphene Q-switched 1.4 µm solid state laser

An LD pumped Nd:LuGdAG (Nd:Lu2.9Gd0.1Al5O12) eye-safe laser is reported in this paper for the first time. Using a plano-concave resonator, maximum continuous wave output powers of 1.00 and 1.05 W were obtained under an absorbed pump power of 8.52 W for T = 2%–3% and 5%–7%, corresponding to an optical to optical conversion efficiency of 12.3% and 11.7% and a slope efficiency of 14.5% and 13.6%. With five to seven layers of graphene saturable absorber, passively Q-switched 1444 nm operation was achieved for the first time. The maximum average output power found was 0.29 W with 324 ns as the shortest pulse width and a highest repetition rate of 64 kHz.

Beam propagation in an inhomogeneous medium of a static gas cesium diode pumped alkali laser: three-dimensional wave optics and fluid dynamics simulation

Analysis of beam propagation and kinetic and fluid dynamic processes in static Cs diode pumped alkali lasers (DPALs), using the wave optics model and gasdynamic code, is reported. The analysis is based on a three-dimensional, time-dependent computational fluid dynamics (3D CFD) model. The gas flow conservation equations in the DPAL cell are coupled to a fast-Fourier-transform algorithm for the laser beam transverse modes’ propagation to obtain a solution of the scalar paraxial wave equation where the gain and refractive index in the DPAL medium affect the wave amplitude and phase. Using the CFD and beam propagation models, the gas flow pattern and spatial distributions of the pump and laser intensities and the laser beam phase in a plano-concave resonator were calculated for end-pumped Cs DPAL. The DPAL medium temperature and refractive index, along with the laser power and laser beam quality factor M2, were calculated as a function of pump power. The results of the theoretical model for laser power were compared to experimental results of Cs DPAL. In addition, the pump and laser induced thermal effects in the DPAL cell on the laser beam quality were studied for Cs DPAL with hydrocarbon only as a buffer gas. For methane it was found that the temperature and the resulting refractive index gradients in the DPAL cell are larger than for He rich buffer gas. The large radial gradient of the refractive index in the heated gain medium, achieved for the present resonator and wide-aperture pump beams, along with the fact that in gases the thermo-optic coefficient is negative, results in improvement of M2 as compared to a gain medium with a uniform refractive index. This counterintuitive conclusion is contrary to the behavior of the beam quality of solid-state lasers, which deteriorates when the gain medium is heated by the pump beams.

Thermal, Spectral and Laser Properties of Er3+:Yb3+:GdMgB₅O10: A New Crystal for 1.5 μm Lasers.

A novel laser crystal of Er3+:Yb3+:GdMgB5O10 with dimension of 26 × 16 × 12 mm3 was grown successfully from K2Mo3O10 flux by the top seeded solution growth method. The thermal diffusivity and specific heat capacity were measured to calculate the thermal conductivity of the crystal. The absorption and fluorescence properties of the crystal at room temperature were investigated in detail. The Judd-Ofelt method was used to analyze the polarized absorption spectra. The emission cross-section of the 4I13/2→4I15/2 transition was calculated by the Füchtbauer-Ladenburg formula and the relevant gain cross-sections were estimated. Continuous-wave laser output of 140 mW at 1569 nm with the slope efficiency of 17.8% was demonstrated in a plano-concave resonator. The results reveal that Er3+:Yb3+:GdMgB5O10 crystal is a promising material for 1.5 μm lasers.

Acousto-Optically Q-Switched Operation of Yb:CNGG Disordered Crystal Laser**Supported by the National Natural Science Foundation of China under Grant Nos 11274188 and 11574170.

We report the repetitively Q-switched laser operation of the Yb-doped calcium niobium gallium garnet disordered garnet crystal, achieved with an acousto-optic modulator in a compact plano-concave resonator that is end-pumped by a 935-nm diode laser. An average output power of 1.96W is produced at pulse repetition rate of 50 kHz at emission wavelengths around 1035 nm, with a slope efficiency of 16%. The highest pulse energy of 269 μJ is generated at pulse repetition rate of 1 kHz, with pulse width 12.1 ns and peak power 20.53 kW.

Acousto-optic Q-switching laser performance of Yb:GdCa(4)O(BO(3))(3)crystal.

We report on the active Q-switching laser performance of Yb:GdCa4O(BO3)3 crystal, demonstrated by employing an acousto-optic Q-switch in a compact plano-concave resonator. Stable repetitively Q-switched operation is achieved with pulse repetition rates varying from 30 to 0.2kHz, producing an average output power of 10.2W at 1027.5nm at 30kHz of repetition rate, with an optical-to-optical efficiency of 30%. The maximum pulse energy generated at the lowest repetition rate of 0.2kHz is 4.75mJ, with a pulse width being 11ns, gives rise to a peak power that amounts to 432kW.

Spectroscopic properties and continuous-wave laser operation of Yb:LuPO₄ crystal.

For the first time, and to the best of our knowledge, we report a continuous-wave (cw) laser operation of Yb:LuPO₄ crystal, demonstrated at room temperature in a compact plano-concave resonator end pumped by a diode laser. 1.61 W of cw output power around 1039 nm were generated with 2.40 W of pump power at 976 nm absorbed in a 0.3 mm thick crystal, leading to an optical-to-optical efficiency of 67%, whereas the slope efficiency was 75%. Polarized absorption and emission cross-section spectra are also presented and discussed in detail.

非链式脉冲DF激光器非稳腔设计与实验研究

By employing positive branch confocal unstable resonator, design of cavity parameters and experimental research on non-chain pulsed DF laser are carried out. The advantages of suppressing divergence angle and improving beam quality are shown by comparing plano-concave stable resonator with unstable resonator. To estimate the beam quality, diffraction limit magnification is chosen as evaluating criterion, which can incarnate both the energy focusing degree and divergence angle excursion. 86.5% enveloping energy is used to define spot size, and 90-10 knife edge method is adopted to measure it. Nine sets of mirrors with different magnifications and mode volumes are tested and compared, and optimization method of unstable cavity parameters is concluded. Taking energy, divergence and diffraction limit magnification into account, the optimal parameters are M=1.89, D=40 mm. Laser beam with divergence angle of 0.74 mrad, =1.35, laser energy of 1.86 J, and peak power of 16.4 MW is obtained at these parameters.

Diode-pumped orthogonally polarized dual-wavelength Nd3+:LaBO2MoO4 laser

Polarized spectroscopic properties related to 1.07 μm laser operation of a 1.8 at.% Nd3+:LaBO2MoO4 crystal grown by the Czochralski method were investigated at room temperature. Using a 2.2-mm-thick, Z-cut Nd3+:LaBO2MoO4 crystal as gain medium, orthogonally polarized dual-wavelength laser at 1,068 and 1,074 nm was first realized in a plano-concave resonator end-pumped by a quasi-continuous-wave 795 nm diode laser. A total output peak power of 1.2 W with slope efficiency of 26 % around 1.07 μm was obtained. The influences of resonator length and pump power on output laser wavelength were also investigated.

Analysis of thermal effect and beam wavefront properties for LD-pumped Q-switch Nd:YAG laser

Thermal effects and beam wavefront properties of a diode-pumped Q-switch Nd:YAG laser have been investigated. The influence of thermal lens effect on laser output mode and beam quality is discussed by means of transmission matrix optics. The functional relations between the thermal lens focal length and the beam spot size for a plano-concave stable resonator have been calculated numerically. Further experiment on the output beam quality of a laser diode pumped acousto-optic Q-switched laser has been also performed by using a Hartmann–Shack (H–S) wavefront sensor and Zernike mode reconstruction theory. In this way, the PV and RMS values of the wavefront aberration, the first 35 orders Zernike aberrations, can be acquired, and point spread function (PSF) of the beam and the distribution of circle energy can also be obtained by calculation, so the output beam quality can be known fully. The experimental results show that the wavefront aberration of laser is mainly includes the defocus Z3, the low-order astigmatism Z4 and Z5, the spherical aberration Z10, etc. because of the thermal effect of gain medium, which will lay an experimental foundation for solid-state laser optimum design.

圆形高斯镜平凹腔腔镜倾斜时光场模式分析

By means of boundary finite-element method,the field distributions and eigenvalues of the fundamental mode of the plano-concave resonator with a tilted Gaussian-reflectivity plane mirror are calculated and discussed.The results show that the laser mode moves along the direction of the mirror tilting.When the tilt angle of the mirror is big enough,the mode is distorted,no longer with a Laguerre-Gaussian distribution.Furthermore,following the increasing of the tilt angle,the far field distributions become bad,and the eigenvalue decrease.By decreasing the characteristic radius of the Gaussian-reflectivity,the distortion of the mode that is induced by the mirror tilting is decreased.

圆形高斯镜平凹腔腔镜倾斜时光场模式分析

圆形高斯镜平凹腔腔镜倾斜时光场模式分析

圆形高斯镜平凹腔腔内外激光场研究

By means of boundary finite-element method,the field distribution and eigenvalue of the fundamental mode of the plano-concave resonator with a Gaussian-reflectivity plane mirror are calculated and discussed.The results show that the radius of the mode increase following the decreasing of the characteristic radius of the Gaussian-reflectivity.The radius of the mode is not affected by the changes of the amplitude-reflectivity of mirror center.The characteristic radius of the Gaussian-reflectivity affect the far field distribution of the laser mode.While its value is small,the far field distribution has the diffraction ring around the main peak.In addition,if light is reflected completely at the center of the Gaussian-reflectivity plane mirror,the changes of the far field distribution is not obvious with the characteristic radius of the Gaussian-reflectivity changing.The mode eigenvalue increases with the characteristic radius of the Gaussian-reflectivity increasing.

Efficient diode-pumped Nd:GGG laser operation at 933.6 and 937.3 nm

CW laser emission on the quasi 3-level (4F3/2→4I9/2) transition in Nd:GGG is reported for different pump focusing and resonator dimensions. A nearly hemispherical plano-concave resonator led at 937.3 nm to a maximum output power of 4.1 W for an incident pump power of 21.7 W, thus corresponding to a slope efficiency of about 23.5%. Laser operation was also obtained for the first time around 933.6 nm by using a glass etalon for frequency selection with a maximum output power of 2.3 W and a slope efficiency of 16% with respect to the incident pump power. Dual-wavelength operation (around 933.6 and 937.3 nm), which offers a potential source to generate THz radiation, is also reported.

Efficient Operation of a 400 W Diode Side-Pumped Yb:YAG Planar Waveguide Laser

We present an efficient high-power planar waveguide laser consisting of a 2% doped Yb:YAG core with sapphire claddings. The planar waveguide is double-side pumped at 940 nm using two custom aberration-corrected diode laser stacks which are imaged directly into the waveguide core. A method to measure the pump uniformity has been developed, showing uniform pumping throughout the waveguide core. Laterally resolved gain measurements show a uniform gain of 1 cm-1 across the full width of the device and the onset of pump saturation. Visible emission from 475 nm to 550 nm is observed, which is interpreted as due to a cooperative luminescence process. Using a plano-concave resonator, 400 W output power is extracted with a slope efficiency of 75% with good transverse beam quality but poor lateral beam quality. Using a hybrid unstable resonator, 300 W output power was achieved, with significantly improved lateral beam quality but reduced transverse beam quality due to interference from cladding modes.