Doped Laser Crystals Research Articles

Buried Depressed-Cladding Waveguides Inscribed in Nd3+ and Yb3+ Doped CLNGG Laser Crystals by Picosecond-Laser Beam Writing.

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca3Li0.275Nb1.775Ga2.95O12 (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE3+-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

Local Structure Design and Application of Rare-Earth Doped Alkaline Earth Fluorite Laser Crystal (Invited)

Local Structure Design and Application of Rare-Earth Doped Alkaline Earth Fluorite Laser Crystal (Invited)

Crystal growth and spectral properties of Eu3+-doped La2CaB10O19 single crystals

The invention and commercial application of InGaN/GaN-laser diodes have made it feasible to directly obtain the visible lasing radiation by pumping rare earth (RE) doped laser crystals. Hence searching for new laser crystals with efficient luminescence and applicable optical and physical properties in the visible range is an essential work. Here, the crystal growth and spectral properties of a series of large-size Eu3+ doped La2CaB10O19 (Eu3+-LCB) single crystals with the nominal atomic ratios of 3%, 7%, and 10% have been well studied. High-quality Eu3+-LCB single crystals with the size of 45 × 30 × 17 mm3, was grown by top-seeded solution growth (TSSG) method. The luminescence probe was employed as the tool to identify occupation sites, demonstrating that Eu3+ ions enter into La3+ sites first, then occupy Ca2+ sites after the nominal atomic ratio of Eu3+ ions exceeds 7%. Based on the absorption and emission spectra, the intensity parameters, spontaneous transition probability, radiation lifetime, and fluorescence branch ratio for Eu3+-LCB single crystals are studied with the Judd–Ofelt theory. From the transient spectra, the emission cross-section, fluorescence decay lifetime and luminescence quantum efficiency are deduced, which exhibit the maximum performance at the nominal atomic ratio of 7% Eu3+. Combining the luminescence probe and spectral properties, it can be concluded that Eu3+ doped crystals will have the best spectral properties when Eu3+ occupation at the single La3+ sites reaches the maximum in LCB single crystals, which may be universal for RE-doped crystal with two possible occupation sites. Besides, our work demonstrates that the 7% Eu3+-LCB single crystal may be a promising medium for laser radiation in the visible region in comparison with other Eu3+ doped single crystals.

Temperature dependence of Cr:CYA effective emission cross-section

A high-quality Cr:CYA (Cr:CaYAlO4) single crystal fiber was grown by μ-PD (Micro-Pulling-Down) method. The E//a and E//c polarized excitation spectra of Cr:CYA at room temperature was measured. The crystal field parameters were calculated and the energy level structure of Cr:CYA was analyzed. We also measured the polarization emission spectra and lifetimes at different temperatures and calculated the effective emission cross-section. Based on the above data and Boltzmann law, the thermal distribution between the excited energy levels 2E and 4A2 at different temperatures was analyzed. The present work may be useful for guiding laser research on Cr:CYA and other Cr3+-doped laser crystals above room temperature.

Sub-100 fs Kerr-lens mode-locked femtosecond Yb:CaYAlO<sub>4</sub> laser with GHz repetition rate

Femtosecond lasers with GHz repetition rate play an important role in scientific and industrial applications such as spectroscopy, optical frequency combs and GHz-Burst pulse trains for micro-machining in the ablation-cooled regime. Kerr-lens mode-locked (KLM) technique and passively mode-locking based on semiconductor saturable absorber mirror (SESAM) are the primary methods to generate GHz femtosecond all-solid-state lasers (ASSLs). Kerr-lens mode-locked Ti:Sapphire lasers have made significant progress benefited from the high-power green pump lasers, and repetition rate up to 10 GHz has been obtained with the average power of 1.2 W. In the early 21st century, ytterbium ion (Yb<sup>3+</sup>) doped laser crystals and ceramics with emission wavelengths near 1 μm received attention due to their high conversion efficiency and broad gain-bandwidth. Combining the customized SESAM and high-power multimode fiber-coupled laser diodes (LDs), GHz Yb-doped ASSLs with watt-level average power may be easily attained and have made rapid progress. However, GHz KLM lasers have strict requirements for the cavity design and pump sources. For satisfying mode matching and enhancing the soft aperture effect within the gain medium, a high-brightness pump source with excellent beam quality (<i>M</i><sup> 2</sup> ～ 1) is desired, such as the single-mode fiber coupled LD, however, the maximum pump power of which is only ～1 W. As a result, the average power of GHz KLM femtosecond laser is typically limited to few tens of milliwatts, which limits the further applications. In this work, we report the first GHz high-power KLM Yb:CaYAlO<sub>4</sub> laser by using a high-power single-mode fiber laser instead of the low-power single-mode fiber coupled LDs as the pump source. On the basis of <i>ABCD</i> matrix, a simple four-mirror bow-tie ring cavity is built so that the laser mode can match well with the focused pump spot in the crystal. At the pump power of 8 W, stable unidirectional KLM is achieved, the laser has the average power of 2.1 W with a pulse duration of 88 fs and a repetition rate of 1.8 GHz, corresponding to the peak power of 11.57 kW. The high peak power and extremely short pulse duration are crucial for coherent octave-spanning supercontinuum generation. The powerful GHz KLM laser with sub-100 fs pulse duration provides an attractive source for realizing the optical frequency combs and micro-machining applications.

Photoluminescence and energy transfer mechanisms of Tm3+ doped Y2O3 laser crystals: experimental and theoretical insights.

Rare-earth thulium (Tm3+) doped yttrium oxide (Y2O3) host single crystals are promising "eye-safe" laser materials. However, the mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals are not yet understood at the fundamental level. Here, we synthetize a series of Y2O3:Tm3+ samples by the sol-gel method. Our experimental results show that the most intensive absorption line of the 3H6 → 1D2 transition occurs at 358 nm, and the strongest emission line of the 1D2 → 3F4 transition is located at 453 nm, which are in good agreement with the calculations of 363 nm and 458 nm, respectively. By using the CALYPSO structural search method, the ground state structure of Y2O3:Tm3+ with P2 space group symmetry is uncovered. The complete energy levels, including free-ion LS terms and crystal-field LSJ multiplet manifolds, of Y2O3:Tm3+ are obtained based on our developed WEPMD method. The present findings show that our WEPMD method can be used in experiments to elucidate the underlying mechanisms of photoluminescence and energy transfer in Tm3+ doped Y2O3 crystals, which offer insights for further understanding of other rare-earth doped laser materials.

Investigating the structure-sensitive factors relevant to cryogenic laser operation of Yb3+ ions in oxide crystals

Optical absorption and luminescence spectra of Yb3+ ions in Gd3(Al0.5Ga0.5)5O12, (Lu0.3Gd0.7)2SiO5 and YAl3(BO3)4 hosts crystals were examined as a function of temperature between 80 and 300 K. The analysis of obtained data provided detailed information on the thermally induced change of intensities, wavelengths and bandwidths of Yb3+ transitions that governs the temperature dependence of absorption and emission cross section spectra. This information, combined with structural peculiarities of the systems studied made it possible to propose generalizations regarding the importance of the structural disorder in the Yb3+-doped laser crystals operating at cryogenic temperatures.

The investigations of Dy:YAG and Dy,Tb:YAG as potentially efficient GaN blue LD pumped solid state yellow laser crystals

With the fast development of GaN blue laser diode, Dy3+ doped laser crystals are receiving renewed interests due to their potential yellow laser emission under GaN laser diode pumping. In this study, Dy:YAG and Dy,Tb:YAG with high quality have been grown successfully with Czochralski method. Their spectroscopic properties as well as the energy transfer mechanism between Dy3+ and Tb3+ were discussed in details. The 583 nm emission cross-section of Dy:YAG crystal is improved by co-doping of Tb3+, which indicates the efficient quasi-resonant energy transfer between the Dy3+-6H13/2 state and Tb3+-7F4 state is realized. The thermal properties of Dy:YAG, including specific heat and thermal conductivity, were investigated. The high thermal conductivity suggests the Dy:YAG crystal should be very suitable for high-power laser operation. In addition, considering the high phonon energy of Dy:YAG and Dy,Tb:YAG crystals, the depletion of population on the lower laser level (6H13/2) of Dy3+ could be accelerated. The results suggesting that both Dy:YAG and Dy,Tb:YAG could be highly appraised as excellent competitors for all solid state yellow lasers.

Synchrotron X-ray metrology of dopant distribution and oxidation state in high pressure CVD grown TM2+:ZnSe optical fibers

High pressure chemical vapor deposition (HPCVD) has shown to be a promising method for producing Cr2+:ZnSe and Fe2+:ZnSe mid-IR optical fiber lasers. The oxidation state of the dopants and their spatial homogeneity are critical for the characterization of effective fiber laser performance; however, this is challenging for small fiber cores and small doping concentrations. We demonstrate using synchrotron micro X-ray fluorescence (XRF) mapping to study the distribution of the dopants throughout the fiber cross-sections. Furthermore, we study the local chemical environment of these unique fiber structures using micro X-ray near edge absorption spectroscopy (XANES). Our study reveals that transition metal doped ZnSe deposited using HPCVD has nanoscale dopant aggregation although the material is chemically identical to the commercially produced diffusion doped laser crystals, as indicated by the presence +2 oxidation state of the dopants. This work demonstrates the power of synchrotron-based X-ray techniques for use as metrology tools for improving the performance of laser materials in fiber geometry.

Growth and spectroscopic properties of Yb0.1Gd1.8La0·1SiO5 crystal: A promising new laser material for ultrashort laser

Recently, in the field of femtosecond lasers, an intense interest has been aroused for Yb3+-doped laser crystals which are able to be pumped directly by semiconductor laser. In this work, a novel Yb3+-doped mixed laser crystal Yb0.1Gd1.8La0·1SiO5 (Yb:GLSO) crystal with high-quality was grown successfully by Czochralski method. Raman spectrum confirmed that the Yb:GLSO has highly disordered crystal structure. The large absorption cross section (0.70 × 10−20 cm2) of Yb:GLSO crystal at around 975 nm indicates the crystal can be pumped efficiently by the commercialized InGaAs laser diode. The stimulated emission cross section at 1030 nm was calculated to be 0.7 × 10−20 cm2 and the bandwidth of the emission peak was determined as high as 175 nm. Positive gain cross sections were appeared at 1030, 1045 and 1088 nm when inversion ratio higher than 0.2. All these results suggest that Yb:GLSO crystal is a very promising novel laser material for realizing tunable solid-state and ultra-short pulsed lasers with LD pumping.

Progress of the research on Yb 3+ -doped femtosecond laser crystals

As widely used in military, medicine, communication, processing and other fields, femtosecond laser has become a research hotspot of the whole laser technology in the 21st century. The pump source used by laser diodes (LDS) has become a new development trend of all-solid-state femtosecond lasers owe to the rapid development of LDs. Yb3＋-doped laser crystal materials gradually become important gain media for LD pumping and generate 1.0 μm femtosecond laser due to their unique energy level structures, broad absorption and emission spectra. In this paper, the research progress of Yb3＋-doped femtosecond laser crystals is summarized in detail, the main problems are analyzed, and two directions for the development of femtosecond laser crystals in the future are proposed: high efficiency and low power femtosecond laser, high power and high energy femtosecond laser. The crystal growth, spectra properties, continuous and femtosecond laser performances of the Yb3＋:Sr3Y2(BO3)4 crystal were studied in detail. And the Yb3＋:Sr3Y2(BO3)4 femtosecond laser with the center wavelength of 1060 nm, pulse width of 116 fs, average output power of 1.08 W and optical conversion efficiency of 33.1% were successfully generated. The experimental results indicate that Yb3＋:Sr3Y2(BO3)4 and its corresponding system crystals are excellent femtosecond laser materials with high optical conversion efficiency.

Growth and investigation of Sm3+-doped GLSO crystal for visible laser application

Recently, an intense interest has been aroused for Sm3+-doped laser crystals which are able to realize visible reddish-orange laser directly using semiconductor laser pumping. In this study, a mixed Sm3+-doped Sm0.02Gd1.88La0.1SiO5 (Sm:GLSO) laser crystal was grown successfully by Czochralski method. The structure of the as-grown crystal was determined with X-ray diffraction. The as-grown crystal exhibits large absorption cross section (1.53 × 10−20 cm2) at around 405 nm, indicating that it can be pumped efficiently by the commercialized GaN/InGaN laser diode. Three Judd-Ofelt intensity parameters Ωt (t = 2, 4, 6) were calculated to be 5.177, 3.783 and 2.218 × 10−20 cm2, respectively. The stimulated emission cross section at 614 nm was calculated to be 0.471 × 10−20 cm2 and the fluorescence lifetime for 4G5/2 level was fitted to be 1.90 ms. All these results suggest that Sm:GLSO is very promising for realizing reddish-orange laser operation using the current commercialized GaN/InGaN laser diode pumping.

Compact 999.6 nm Actively Q-Switched Yb3+:LuAlO for Laser-Induced Breakdown Spectroscopy Laser

Compact actively Q-switched diode-pumped lasers based on Yb3+-materials are of practical importance for wide range of scientific, industrial and biomedical applications. The aim of this work was to study the Yb3+:LuAlO3 actively Q-switched laser.One of the most promising crystalline hosts for rare-earth ions are Perovskite-like aluminate crystals. Yttrium aluminate crystal YAlO3 (YAP) is a well-known host with good thermal and mechanical properties (thermal conductivity for undoped crystal is about 11 W/m·K and about 8 W/m·K for Yb(5 at.%):YAP) similar to those of YAG. The reduction in the thermal conductivity of doped laser crystal in comparison with host materials is small in the case of ions with close atomic mass and ionic radii such as for Yb3+ and Lu3+. This feature makes LuAlO3 (LuAP) more promising host crystal for doping by Yb3+ ions in contrast to YAP especially for high output power laser systems. In our work, for the first time to the best of our knowledge actively Q-switching laser operation of Yb3+:LuAP single crystal was demonstrated. The maximum average output power of 4.9 W at 50 kHz pulse repetition frequency (PRF) with opt.-to-opt. efficiency of 21 % was obtained with 30 % OC transmittance. Output power as high as 3.3 W with 333 µJ-laser pulses with duration of about 11.5 ns was demonstrated at 10 kHz PRF the corresponding pulse peak power was 29 kW. 97 µJ second harmonic pulses obtained with 29 % conversion efficiency at 10 kHz PRF.Performed investigations show high potential of Yb3+:LuAP crystals as active elements of compact diode pumped actively Q-switched lasers due to high stimulated emission cross-section (≈ 3.74·10-20 cm2) at 999.6 nm wavelength and significant reduction of heat load on the active element when pumping around 980 nm and generation around 999 nm.

Research Progress of Rare-earth Doped Laser Crystals in Visible Region

Research Progress of Rare-earth Doped Laser Crystals in Visible Region

Czochralski growth and spectral investigations of Er:GSAG laser crystal

Er3+ doped GSAG laser crystal with high optical quality was grown successfully by the Czochralski method for the first time. The structural parameters and the high crystalline quality of this crystal were determined. Judd-ofelt (J-O) parameterization scheme was applied to analyze the absorption spectra at room temperature. Accordingly, the lines strengths, oscillator strengths, transition probabilities, fluorescence branching ratios, and radiative lifetimes were calculated to characterize the laser properties of Er:GSAG crystal. Additionally, the fluorescence spectra and lifetimes of Er:GSAG crystal were measured and studied. The stimulated emission cross-section spectra and gain cross-section spectra were evaluated for 4I13/2 →4I15/2 (1.5–1.6 µm) transitions which indicate high potential of 1 at% Er:GSAG for laser emissions at 1567 and 1605 nm. Furthermore, the possibility of Er:GSAG crystal to realize the visible laser output pumped by the GaN LDs were initially studied and analyzed.

Compact and stable high-repetition-rate terahertz generation based on an efficient coaxially pumped dual-wavelength laser.

A compact and stable terahertz (THz) source is demonstrated based on difference frequency generation (DFG) pumped by an efficient dual-wavelength acousto-optic (AO) Q-switched solid-state Nd:YLF laser with composite gain media (a-cut and c-cut) in the coaxial pumping configuration. Optimal power ratio and pulse synchronization of the orthogonal polarized 1047/1053 nm dual-wavelength laser could be realized by varying the pump focusing depth and/or pump wavelength. The total power of 2.92 W was obtained at 5 kHz pumped by 10-W laser-diode power at 803 nm. Such an efficient dual-wavelength laser demonstrated good stability and inconspicuous timing jitter benefiting from the suppressed gain competition between two resonating wavelengths. An 8-mm-long GaSe crystal was employed to generate THz waves at 1.64 THz by DFG and the maximum THz average output power was about 0.93 μW. This compact coaxial pumping method can be extended to all kinds of neodymium (Nd) doped laser crystals to produce different dual-wavelength lasers for various THz wavelength generation, which have good prospects for portable and costless applications like imaging, non-destructive inspection, etc.

Spectroscopic and thermal characterizations of Yb:LaF3 single crystal

A Yb3+ doped LaF3 laser crystal was detailed investigated by both spectroscopic and thermal measurements. A peak absorption at 974 nm with FWHM broader than 60 nm makes the crystal suitable to InGaAs LDs. Fluorescence spectrum and calculated spectroscopic parameters show potential of Yb:LaF3 crystal to laser operations around 1009 nm. A relatively long fluorescence lifetime of 2.92 ms was detected for Yb3+:2F5/2 manifold. The thermal diffusivity and specific heat capacity in the range of 300–575 K were studied to calculate the thermal conductivity of Yb:LaF3. The results indicate that the Yb:LaF3 crystal is a good candidate for diode-pumped ∼1 μm solid-state laser applications.

Synthesis and photoluminescence properties of Ln3+ (Ln3+=Tb3+, Dy3+, Sm3+, Er3+)-doped Ca2Nb2O7 phosphors

A series of Ln3+ (Ln3+=Tb3+/Dy3+/Sm3+/Er3+) ions doped Ca2Nb2O7 phosphors have been synthesized by high-temperature solid-state reaction. The Ln3+-doped samples are well indexed to the pure Ca2Nb2O7 phase which revealed for the X-ray diffraction (XRD) result. Under the ultraviolet light, the prepared Ca2−xNb2O7:xLn3+ (Ln3+=Tb3+/Dy3+/Sm3+/Er3+) phosphors show the characteristic cyan (Tb3+), green–white (Sm3+), yellowish (Dy3+) and green (Er3+) emissions. The energy transfer mechanisms in Ca2Nb2O7: Tb3+/Dy3+/Sm3+/Er3+ phosphors have been investigated and it deduced to be a resonant type via an electric dipole–dipole interaction. In addition, their critical distances have been calculated by concentration quenching methods. The luminescence properties of Ca2Nb2O7:Tb3+/Dy3+/Sm3+/Er3+ phosphors indicated that the Ca2Nb2O7 is a suitable host for rare earth doped laser crystal and optical materials.

Laser Active Materials for High-Intensity Lasers

Many laser materials have been discovered or rediscovered in the last 30 years but only a few of them have been really considered or could be envisaged in the development of the present and future high-intensity laser chains. It is the case of Ti:Sapphire, because of its exceptional spectroscopic and thermo-mechanical properties, and a number of Yb3+ doped crystals, because of their relatively broad absorption and emission bands allowing for diode pumping and the generation and the amplification of short pulses, but also because of reduced thermal loads resulting from a low quantum defect. It could be the case of other promising laser materials based on Pr3+ - or Cr2+-doped crystals and ceramics which have not been considered yet for this type of applications because of more exotic pump and laser wavelengths. Therefore, without entering into the details of the optical properties and the laser performance of these systems, which can be found in many other articles, a review is made and data presented here on the three important following aspects: our ability of developing ultrahigh quality and large size laser elements of the considered materials, the impact of the thermal loads on their spectral properties, and the multiphoton and nonlinear processes which can be the origin of the degradation or the perturbation of their optical properties at high pump and laser intensities. For that description, the considered laser materials have been divided into three categories: Ti3+:Sapphire, the most important Yb3+-doped laser crystals and ceramics, and some more prospective materials among which Pr3+:LiYF4, Cr 2+ZnSe and Cr2+:ZnS.

Theoretical study of optimal doping concentration in laser ceramics

The ion-doped ceramic crystal is different from the single crystal in the respect of theoretical calculation method, because of the presence of grain boundaries. The populations at the active levels in both CW and pulsed regimes each as a function of the concentration of active impurities are deduced by means of the luminescence decay curve. Taking the Nd3+:YAG laser ceramic for example, we calculate the optimal doping concentration and compare our calculated results with reported experimental data, showing that our theoretical calculation is in line with the trend of the experimental data, and also we compare our results with the Nd3+:YAG single crystal theoretical data. showing that the ceramic crystal is superior to the laser doped laser crystal in the respect of population.