YAG Crystal Research Articles

Composite Yb:YAG/sapphire thin-disk active elements produced by thermal diffusion bonding

Thermal diffusion bonding of Yb:YAG and sapphire crystals was successfully implemented for the first time, and composite active elements having a geometry of a thin disk with an undoped cap were fabricated. The composites were tested in a continuous-wave thin-disk laser scheme at up to 400 W output average power, confirming that bonding strength is sufficient to operate at high thermal load. The radiation phase and polarization distortions as well as the small signal gain were studied. It is shown that depolarization is a significant problem in the considered composites; however, a proper choice of the parameters allows minimizing this impact.

Intracavity cascade pumped 493.1nm blue laser

In this paper, a continuous-wave(CW) blue laser based on intracavity cascade pumping sum-frequency technology is designed. This system generates quasi-three-level laser through LD end-pumped Nd:YAG crystal, and then intracavity pumps Yb:YAG crystal to achieve 946 nm and 1030 nm laser in the cavity. Finally, a type-I critical phase-matched LBO is used for intracavity sum-frequency to achieve CW blue laser output at 493.1 nm. In the experiment, a compact linear resonator was adopted, and a 493.1 nm blue laser of 426 mW was obtained for a pump power of 25 W. Its optical to optical conversion efficiency is 1.7 %. The beam quality factors M2 in the x and y directions are 1.28 and 1.31, respectively. The power instability of the blue laser output is less than 3.2 % within 2 h.

Polarization ray tracing in thermally loaded solid-state laser crystals

A new ray tracing method for the propagation of electrical fields in inhomogeneous and weakly anisotropic media is presented. With this method, we can efficiently simulate the propagation of laser beams in solid-state laser amplifiers, which suffer from high thermal loads. As a result of this method, we can find optical compensation setups that significantly reduce the depolarization losses in high-power solid-state amplifiers. To the best of our knowledge, we theoretically demonstrate for the first time that [100]-cut YAG crystals reduce the depolarization by more than a magnitude in comparison to [111]-cut crystals while achieving an overall better beam quality at the same time.

Vortex arrays directly generated from an efficient diode-pumped microchip laser

Optical vortex arrays with multiple singularities arranged in distinct lattice structures provide more flexibility in trapping or manipulating microparticles, large-capacity optical communications and high-security information processing, and optical modulation. An efficient high-power compact laser with singularity-tunable vortex-array distribution is crucial for practical applications. Here, we directly generate various vortex arrays with tunable singularities from 1 to 10 in a microchip laser pumped with a tilted annular beam. Formation of vortex arrays in the microchip laser is achieved by manipulating the gain distribution in an Yb:YAG crystal by controlling the tilt angles and pump power. Efficient high-power laser operation with output power of 2.01 W and optical efficiency of 24.5% is obtained for a vortex array with 10 singularities. The pump-power-dependent saturated population inversion distribution plays a key role in generating vortex arrays formed with multi-transverse modes oscillating simultaneously in transverse-mode locking. The good agreement between theoretically simulated transverse patterns, phases and interference patterns for the vortex arrays and experimentally obtained results provides a solid foundation for developing efficient compact microchip lasers for generating vortex arrays by manipulating gain distribution in a thin medium pumped with a tilted annular beam.

Spectroscopic properties of Tb:Y3Al5O12 crystal for visible laser application

Tb3+ doped Y3Al5O12 (Tb:YAG) single crystal has been grown by the Czochralski method. Optical spectroscopic characterization of the crystal was performed in order to evaluate the potential for the visible laser application. Based on the Judd–Ofelt theory, the spectroscopic parameters Ωt (t = 2, 3, 6), spontaneous transition probabilities, fluorescence branch ratio and the radiation lifetime of excited state 5D4 state were calculated. The green emission cross section of 5D4→7F5 transition were calculated to be 1.23 × 10−21 cm2, The yellow emission cross section of 5D4→7F4 transition were calculated to be 7.08 × 10−23 cm2. The decay time of the 5D4 level was measured to be 3.12 ms, which is in reasonable agreement with the radiation lifetime of 3.30 ms. All the results indicate the potential of Tb:YAG crystal as a visible laser material.

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 .

Rheological properties of Y3Al5O12 powder obtained by preceramic organoyttroxanealumoxanes

Submicron powder Y3Al5O12 (YAG) aluminium yttrium garnet was synthesized from an Al-Y-ceramic-forming precursor, according to the method developed in the State Research Institute for Chemistry and Technology of Organoelement Compounds (SRICTO). The advantage of the method is the possibility of obtaining close to the stoichiometric phase and obtaining high purity nanopowder. In this work, we studied the effect of the pyrolysis temperature of the polymer (700°, 800° and 900°C) and grinding time on the rheological properties of the powder. It was shown that after pyrolysis at 700° and 800°C, the powder has an amorphous structure, the ultimate strain, in this case, being 6.1 and 8.1% of the initial size. The compressibility modulus decreases with increasing temperature from 34.5 to 19.4 MPa. Due to the formation of YAG crystals after pyrolysis at 900°C, the compressibility of the powder increases and the compacts become denser.

Comparative study of Al2O3-YAG:Ce composite ceramic and single crystal YAG:Ce phosphors for high-power laser lighting

Recent studies revealed that both the Al2O3–Y3Al5O12:Ce (YAG:Ce) composite ceramic (YAG-CC) and single crystalline YAG:Ce (YAG-SC) possess very high luminescence-saturation thresholds (LSTs), which makes them promising for laser lighting applications. The present study is aimed to conduct a comparative investigation between YAG-CC and YAG-SC on aspects of the laser-pumped luminescence and the associated thermal behavior. The colorimetric and photometric properties (e.g. luminous flux/efficacy, correlated color temperature, and color rendering index) were investigated systematically, by conducting the measurements using a custom-built sphere-spectroradiometer system in the transmission mode. It was found that YAG-SC has a superior LST and it could maintain a high luminous efficacy (106 lm/W) under the irradiation of a 15.33 W blue laser, while the YAG-CC saturated at around 7.76 W. Moreover, by combining the thermal imaging data with the spectral variation of the YAG-CC, novel experimental insights into the origin of luminescence saturation were revealed and proposed as the “thermally-induced absorption saturation”.

Q-Switched and Mode-Locked Nd/Cr:YAG Ceramic Pulse Laser

A mode-locked and Q-switched short pulse laser using the Nd3+/Cr3+:YAG ceramic has been constructed with a SESAM and Cr4+:YAG crystal optical switch based on excite state absorption (ESA). Laser oscillations of the pulse laser were observed experimentally. The Nd/Cr:YAG ceramic laser has a high conversion efficiency from white light (such as lamp light or solar light) to the laser. The Nd/Cr:YAG ceramic has a higher laser gain than the Nd:YAG laser for the same pumping power. The laser oscillation can be obtained very easily. A single-mode-locked laser pulse with fast modulation on the order of 100 ps was obtained in some pump power regimes when using the Cr4+:YAG crystal. The obtained pulse duration of the short pulse was a few hundred ps. A maximum peak power of 60 kW was obtained when using a SESAM. The same level of peak power (60 kW) was also obtained when using the Cr4+:YAG crystal.

Measurement of the optical dielectric properties of thin-film materials by ultrafast time-resolved interferometry

We report a time-resolved optic interferometry (TROI) that enables to straightway probe the optical properties of traditional and special thin-film materials by feat of a phase delayed-interference between double beams of femtosecond laser synchronously traveling through vacuum (air) and sample, respectively. As a verification of measurement accuracy created by the TROI, the group and phase velocities as well as the refractive index of noncrystal thin-film HfO2, the crystal thin-film materials YAG (isotropic cubic system), and LiNbO3 (aeolotropic rigonal system), are measured by femtosecond laser pulse with 800 nm central wavelength. The data exhibit a satisfactory consequence in which even a subtle change of measurand can be ascertained by manipulating the resolution in temporal and spacial domains via the delay-line, whether the films are isotropic or aeolotropic and any different orientations of sample. Unambiguously, the presented TROI is conspicuous to the precision measurements of environment-dependent and phase-sensitive optical parameters especially involved in specific nanoscale optical devices and structured layers or coatings.

Improved Nd distribution in Czochralski grown YAG crystals by implementation of the accelerated crucible rotation technique

Nd:YAG is a well-known laser material typically grown from the melt using the Czochralski (CZ) method, and like many other doped materials grown from the melt, it suffers from dopant segregation, giving an average Nd concentration in the crystal (Nd%) far less than that in the melt, with Nd variation along the growth axis. Here, the Nd segregation issue is addressed by the successful implementation of the Accelerated Crucible Rotation Technique (ACRT) with CZ. Compared to conventional CZ, ACRT provided improved Nd uniformity, higher Nd incorporation, and control of Nd% along the growth axis, even achieving higher Nd% in the crystal than the initial Nd from the overall melt. ACRT also affects the solid/liquid interface shape and thus the radial segregation. It is thus shown that ACRT and the mixing it provides is promising for achieving unconventional dopant profiles in CZ crystals.

Self-Q-switched operation in Tm:YAG crystal and passively Q-switched operation using GaSe saturable absorber

In this paper, we investigated the self-Q-switched (SQS) operation in Tm:YAG crystal. The diode-pumped SQS Tm:YAG laser was operated at a central wavelength of 2006 nm. With the pump power of 4.4 W, we got two sets of data: (1) the optical conversion efficiency of the SQS operation with output coupler (OC) of 10% was 12.50%, and the corresponding output power was 0.55 W; (2) the optical conversion efficiency of the SQS operation with OC of 15% was 19.77%, and the corresponding output power was 0.87 W. Thus, Tm:YAG crystal could be widely used in SQS lasers. In addition, GaSe saturable absorbers (SAs) with broadband nonlinear saturable absorption properties were fabricated and employed as SAs for passively Q-switched Tm:YAG lasers. Attributing to the weak evanescent field and long interaction length, GaSe SAs could work at high-power regime and wide-wavelength range. In summary, GaSe is a good 2D SA material in wide-wavelength range.

Composite Yb:YAG/sapphire thin-disk active elements for high-energy high-average power lasers

Composite active elements (AEs) of thin-disk geometry with an anti-amplified-spontaneous-emission (ASE) cap were created for the first time from dissimilar materials (Yb:YAG and sapphire crystals) using the surface-activated direct bonding method based on etching the surfaces with an Ar ion beam and then bringing them into optical contact. Using a sapphire anti-ASE cap allows the thermal lens optical power and the M 2 factor to be reduced significantly compared with the same AE with a YAG anti-ASE cap, and M 2 compared to standard Yb:YAG disks without an anti-ASE cap. Lasing with 320 W average power and ∼ 50 % slope efficiency was obtained.

Void-free bonding for a large slab laser crystal.

A void-free bonding technique was demonstrated for a large slab Nd: YAG crystal with a bonding surface dimension of ∼160mm×70mm. By using the novel fluxless oxide layer removal technology, the indium-oxide barrier problem was resolved. With the help of electrochemical-polished indium solder and a plasma-cleaned heat sink, the solderability of the indium was enhanced; in particular, the contact angle of the solder was improved from 51° to 31°. With the largest-bonding-size slab, a single-slab laser created a maximum output power of 7.3 kW under an absorbed pump power of 12.8 kW, corresponding to an optical to optical efficiency of 57% and a slope conversion of 67.8%. By detecting the wavefront of the interferometer before and after bonding, the RMS of wavefront was 0.192λ and 0.434λ (λ=633nm), respectively. To the best of our knowledge, this is the largest void-free bonding size for a laser slab and the highest output power achieved from a single-slab crystal laser oscillator.

Theoretical study of eye-safe 2 μm laser directly pumped by sunlight

Solar energy has become one of the new types of energy sources for humanity in the future due to its abundant recourse, clean use and huge reserve. Solar-pumped laser has potential applications in free space optical communications, remote sensing and other fields. However, the research on solar-pumped laser is limited to 1 μm band with neodymium-doped material as a gain medium. To expand the output wavelength range of solar-pumped solid-state lasers, thereby expanding their application fields is one of the goals pursued by researchers in the field. According to the analysis of the absorption spectra of existing solid laser materials, we find that the thulium-doped crystals also have strong absorption peaks in visible light band where solar radiation is strong. Therefore, it is possible that solar-pumped laser could also generate output at 2 μm eye-safe wavelength. In this paper, the absorption spectrum and spectral matching of two common laser crystals—Tm:YAG and Tm:YAP with solar spectrum are analysed and calculated. According to the quasi-three-level transition rate equation of thulium ion and the model of solar-pumped laser system, we obtain the theoretical threshold pump power densities of these two crystals to be 1.14 kW/cm&lt;sup&gt;3&lt;/sup&gt; and 1.434 kW/cm&lt;sup&gt;3&lt;/sup&gt;, respectively. We choose the Tm:YAG crystal with lower threshold pump power density as the gain medium and built a two-stage pumping model with TracePro software. In our model, Fresnel lens is the primary solar light concentrator, and a conical cavity with diffusion reflection surface is used as a secondary concentrator to couple the solar energy to laser crystal. Laser setup parameters such as the distance between the Fresnel lens and the window of conic cavity, length of crystal, taper of conic cavity are optimized with the model. The work in this paper offers a valuable reference for future experimental research of 2 μm solar-pumped laser. Finally, we point out the challenge of the future work. Special attention needs to be paid to the huge thermal effect caused by a large amount of sunlight shining on the Tm:YAG crystal. We could find a new kind of diffuse reflection coolants or use thermally bonded crystals to mitigate thermal effects. It will be the focus of future work.

Two-dimensional tellurene nanosheets as saturable absorber of passively Q-switched Nd:YAG solid-state laser

For the first time, a group-VI single element nanomaterial was used as the optical saturable absorber (SA) to generate laser pulses. With two-dimensional (2D) tellurene as a passive Q-switch, 1.06 μm and 1.3 μm pulse laser operations were realized from a diode-pumped Nd:YAG crystal. The shortest pulse widths were 98 ns and 178 ns, and the highest peak powers were 2.68 W and 2.45 W, respectively. Our research determines that tellurene is an excellent SA material in the near-infrared region.

Thermal lens analysis in a diode-pumped 10 Hz 100 mJ Yb:YAG amplifier

We address the power scaling issue in end-pumped laser rod amplifiers by studying, experimentally and numerically, the magnitude of thermal lensing in a high-energy diode-pumped Yb:YAG crystal. The spatio-temporal temperature profile of the gain medium and the focal length of the induced thermal lens are determined numerically. The influence of the repetition rate and pumping power on the temperature distribution is analyzed. Experimental measurements covered repetition rates between 1 and 10 Hz and up to 4 kW pumping power.

Massive red shift of Ce3+ in Y3Al5O12 incorporating super-high content of Ce.

In light emitting diodes, Y3Al5O12:Ce (YAG:Ce) is used as a yellow phosphor in combination with blue LEDs but lacks a red component in emission. Therefore, considerable efforts have been directed toward shifting the emission of YAG:Ce to longer wavelengths. In this study, a Y3Al5O12 (YAG) crystal incorporating a high content of Ce, (Y1−xCex)3Al5O12 (0.006 ≦ x ≦ 0.21), was successfully prepared by a polymerized complex method in which low-temperature annealing (650–750 °C) was employed prior to sintering at 1080 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicated that the obtained sample was a single phase YAG crystal with x ≤ 0.21. Interestingly, orange-red emission was observed with x ≥ 0.07 with UV-blue light irradiation. With excitation at 450 nm, the emission peak increases from 538 nm (x = 0.006) to 606 nm (x = 0.21). This massive red shift in the high-x region was not observed without the 1st step of low-temperature annealing, which implied that low-temperature annealing was essential for incorporating a high concentration of Ce. The precursor formed by low-temperature annealing was amorphous at x = 0.04, whereas CeO2 nanocrystals were formed in the amorphous material with x ≥ 0.11, based on the XRD and TEM results. CeLIII X-ray absorption edge structure revealed that Ce existed as Ce4+ in the precursor and Ce3+ in the obtained crystal. It was speculated that CeO2 was formed at low temperature, releasing oxygen, with sintering at 1080 °C, leading to the incorporation of Y3+ in the Ce–O framework. The lattice constant increased significantly from 12.024 Å to 12.105 Å with increasing x, but the crystal field splitting did not increase and was constant from x = 0.06 to x = 0.21. Hence, the massive red shift in emission was not explained by the large crystal field splitting, but instead by the Stokes shift.

Temperature tunable lasers with disordered Nd:ABC3O7 crystals

We present the continuous-wave (CW) and passively Q-switched laser operation of disordered Nd:ABC3O7 crystals including Nd:SrLaGa3O7, Nd:SrGdGa3O7, and Nd:BaLaGa3O7. Among the Nd:ABC3O7 group members, Nd:SrLaGa3O7 exhibits superior laser properties. A maximum output power reaches 3.88 W under an absorbed pump power of 29.01 W, which is the highest CW output power using LD as the pump source for Nd:SrLaGa3O7 gain medium. Using a Cr4+:YAG crystal as a saturable absorber, the LD pumped passively Q-switched laser is also realized. With Nd:SrLaGa3O7, the shortest pulse width, the largest pulse energy, and the highest peak power are observed to be 10.6 ns, 60.3 μJ, and 5.3 kW, respectively. In the CW laser operation, wavelength tunings are also reported. The center wavelength of this Nd: SrGdGa3O7 laser shifts from 1062.48 to 1063.91 nm by changing the temperature from 60 °C to 135 °C.