About the luminescence properties of YAG∶Nd,Ce and YAG∶Nd single crystals and their relation to laser properties

Laser excited luminescence studies of various YAGNd, Ce and YAG:Nd (with an excess of yttrium) single crystals together with a testing of laser properties of rods made from the same crystals have been investigated in this paper. It was observed that laser pulse energies increase with increasing halfwidths of the luminescence spectral bands. This dependence and other observations indicate that local structure changes or Nd³⁺ nonequivalent centres are present in the studied crystals. Various mechanisms leading to the formation of Nd³⁺ nonequivalent centres are discussed and it seems that the more probable mechanism is oxygen segregation and diffusion.     

Laser properties of coactivated YAP: Nd free of colour centres

YAP:Nd, Cr grown under Ar-H₂ or Ar-He-H₂ atmosphere possesses good energy transfer from Cr³⁺ to Nd³⁺ but suffers from the colour centre formation. The centre formation was completely prevented using further admixture of Ce³⁺ and 10^–4–10^–3 wt. % Fe. Small luminescence quenching of Cr³⁺ or Nd³⁺ due to iron ions is negligible in the presence of Ce³⁺. The crystals may be also heavily doped with Nd³⁺ because the increased pumping efficiency compensates the shortening of the luminescence lifetime. YAP: Nd, Ce, Cr, Fe is advisible active laser material particularly for all the types of pulsed lasers.     

Luminescence sensitization in Nd-Cr-Ce doped yttrium aluminates

Energy transfer from Ce to Cr in YAG is radiative and relatively inefficient. If excited below 500 nm, the opposite transfer is dominant. YAP: Ce, Cr showed relatively efficient CeCr transfer only. The sensitization of Nd³⁺ luminescence by Ce³⁺ ions depends on the overlap of the Ce³⁺ emission band with Nd³⁺ absorption lines. Cr³⁺Nd³⁺ transfer is characterized by an expressive non-radiative portion. It is inefficient in YAG but very efficient in YAP. No UV-induced colour centres were found in YAG: Nd, Cr grown under Ar-H₂ atmosphere and doped with 10^–3 wt. % Cr, but at a higher Cr concentration anomalous absorption between UV absorption edge and 650 nm was stabilized. Ce³⁺ admixture in YAG:Nd, Cr and/or reducing treatment of the crystals facilitate the decomposition of the centres. The decomposition is accompanied with a strong Nd³⁺ luminescence. Therefore, YAG: Nd, Ce, Cr is an advisable active laser material. On the other hand the same centre in YAP: Nd, Cr seemed to be more stable even in the presence of cerium ions.     

Colour centre-free perovskite single crystals

Yb³⁺:YAlO₃ (YAP) and Yb³⁺:GdAlO₃ (GAP) are interesting 1 μm high-power laser media thanks to their very good thermo-mechanical properties. However, as-grown perovskite single crystals exhibit colour centres. Parasitic thermal load generated by these centres is deleterious for high-power laser action and can lead to crystal damages. Moreover these defects decrease Yb³⁺ lifetime. They are related to trapped holes on the oxygen network. In the present work, several schemes to remove colour centres are presented. Attention is focused on cerium codoping, thermal annealing under reducing atmosphere and growth of non-stoechiometric compounds.     

A single wavelength 1339 nm Nd:YAP pulsed laser

The laser oscillating at a weak line of Nd:YAP around 1.3-μm realized though selecting polarization is described. The energy level transitions of Nd:YAP crystal and their polarization properties were analyzed. A thin-film polarizer was adopted to restrain the oscillating of the c-axis strong polarized spectral lines and a reasonable transmittance was designed to suppress the a-axis polarized 1064 nm strong line lasing, and then a-axis polarized 1339 nm pulse laser of 336 mJ for free running mode and 64 mJ for electro-optic Q-switched mode were successfully achieved, corresponding to pulse widths of 180 μs and 35 ns, respectively. This method of selecting polarization to realize weak line oscillating is significant for anisotropic laser crystals doped with Nd³⁺ ions to select the particular transitions.     

Quasi-three level laser based on diode-pumped Nd<Superscript>3+</Superscript>:YAlO<Subscript>3</Subscript> crystal

We describe the output performances of the 930 nm ⁴ F _3/2 → ⁴ I _9/2 transition in Nd³⁺:YAlO₃ (Nd:YAP) under in-band pumping with diode laser at the 803 nm wavelength. An end-pumped Nd:YAP crystal yielded 1.13 W of continuous-wave (CW) output power for 17.8 W of incident pump power. Moreover, intracavity second-harmonic generation has also been achieved with a power of 172 mW at 465 nm by using a LiB₃O₅ (LBO) nonlinear crystal. The blue beam quality factor M ² was less than 1.3. The blue power stability was less 3% in 60 min.     

Diode-pumped continuous-wave Nd:YAP laser emitting at 1073 nm based on the 4F3/2-4I11/2 transition

We describe the output performances of the 1073 nm ⁴ F _3/2 → ⁴ I _11/2 transition (generally used for a 1064 nm transition) in Nd³⁺:YAlO₃ (Nd:YAP) under in-band pumping with diode laser at the 803 nm wavelength. An end-pumped Nd:YAP crystal yielded 390 mW of continuous-wave (CW) output power for 17.8 W of incident pump power. Moreover, intracavity second-harmonic generation has also been achieved with a power of 38 mW at 536 nm by using a LiB₃O₅ (LBO) nonlinear crystal. The green beam quality factor M ² was less than 1.33. The blue power stability was less 3.5% in 4 h.     

Laser properties of YAG: Nd,Ti

YAG: Nd containing 10^–3 wt% Ti showed slightly increased losses at 1·064 m but a substantially increased gain coefficient if compared with a material containing no Ti. The increased losses may be attributed to the increased absorption near 1·064 nm and the increased gain to the better energy coupling among Nd³⁺ ions occupying nonequivalent sites due to the presence of Ti³⁺ ions. YAG: Nd, Ti may be used as a high performance pulsed laser.     

Effect of Nd3+ concentration on CW and pulsed performance of fiber-coupled diode laser pumped Nd:YVO4 laser at 1064 nm

The effect of Nd³⁺ concentration on the CW and Q-switched laser performances at 1064 nm from Nd: YVO₄ has been studied under diode laser pumping in identical laser configuration. The Nd³⁺ concentrations used were 1, 2 and 3 at.% in YVO₄ crystals. Under the CW operations we have compared the thermal lensing effect, slope efficiencies and also the beam quality at the fourth-order degeneracy configuration. Q-switching was done with the help of an acousto-optic modulator and we have compared the pulses obtained from Nd: YVO₄ laser with different doping concentration. It was found that the 1 at.%-doped crystal is the best, offering highest optical-to-optical conversion efficiency (55%), lowest fractional heat load (24%), highest pulse energy (80 μJ) and shortest pulse width (20 ns). It was also found that there was not much difference in performances for 2 and 3 at.%-doped crystals both in CW and Q-switched configurations.     

The optical properties and laser characteristics of Cr and Nd co-doped Y3Al5O12 ceramics

We have fabricated Cr³⁺ and Nd³⁺ co-doped YAG (Cr;Nd:YAG) ceramics, and investigated their optical properties and laser characteristics. The Cr;Nd:YAG has two broad absorption bands at around 440 nm (⁴A₂→⁴T₁) and 600 nm (⁴A₂→⁴T₂) respectively, caused by Cr³⁺ ions. In the case of pumping at 440 nm, the maximum effective lifetime of the Cr;Nd:YAG was 737 μs with a 0.1 at% Cr³⁺ and 1.0 at% Nd³⁺ co-doped YAG sample. Cr³⁺ ions take a role of an effective sensitizer to convert the UV light of flashlamp. For single-shot laser operation, a 10.4 J output energy at 1064 nm was obtained with 0.1 at% Cr³⁺ and 1.0 at% Nd³⁺ co-doped YAG ceramic rod with a laser efficiency of 4.9%. The laser efficiency was found to be more than twice that of a 1.0 at % Nd³⁺:YAG ceramic rod.     

Spectral and laser properties of YAP:Nd grown in reducing atmosphere

YAP:Nd crystals grown from the melt in molybdenum crucible under reducing (He-Ar-H₂) protective atmosphere contain growth colour centre (brown appearance) due to surplus of metal ions. The colouration may be irreversibly removed by annealing at 1200 °C. Oxygen annealed crystals are improper as a laser active material because of colour centre formation (reddish appearance) which is substantially strenghtened by the pumping light. This phenomenon is connected with the presence of molybdenum (5 ppm) in a higher valence state. On the other hand the same crystals treated in vacuum or hydrogen need a limited filtration of the pumping light (up to 280–320 nm), to exclude detrimental role of iron impurity and they may be succesfully used in continuous and particularly in pulsed lasers. No ageing or erratic performance was found in the lasers using such crystals.     

Energy transfer in YAlG:Nd codoped with Ce

Laser excited luminescence has been used to study an energy transfer between Ce³⁺ and Nd³⁺ ions in YAlG:Nd,Ce laser crystal with varying small Ce concentrations. Radiative Ce³⁺Nd³⁺ energy transfer dominates in the studied single crystal samples but this process can also be accompanied by multistep transfer mechanisms (for example by (Ce³⁺Ce³⁺)_n Nd³⁺ transfer mechanism). The Ce³⁺Nd³⁺ radiative energy transfer improves pumping of green and yellow excitation lines in YAlG:Nd,Ce laser rods because the contribution of the additional transfer pumping of Nd³⁺ ions from Ce³⁺ ions is more than three times greater than should be expected from concentration differences between Nd and Ce. This favourable behaviour is explained from high quantum efficiency of Ce³⁺ emission and higher Ce³⁺ absorption cross sections in comparison with the Nd³⁺ ones (more than one order of magnitude difference).This work was done in cooperation with Monokrystaly Turnov, Research Institute for Single Crystals. The author is grateful to Jos. Kvapil and J. Kubelka for supplying him with samples used in this work. He would like to acknowledge many fruitful discussions with J. Kvapil and K. Blaek of Monokrystaly Turnov.     

Cooperative emission study in ytterbium doped NdVO4

The infrared and visible cooperative emissions of ytterbium ions are studied in Yb-doped NdVO₄ single crystals. The absorption of optical phonons allows the emissions at room temperature when a Nd:YAG laser is used. Low temperature emissions are observed due to the Nd³⁺→Yb³⁺ energy transfer following an argon ion laser excitation of the Nd³⁺ ions. Analysis of the cooperative emission at low doping concentration (1%) indicates that it is generated by distant pair forming Yb³⁺ ions while at high doping concentration (≥ 5%) close ions magnetically coupled and superexchange mechanisms prevail in the emitting process.     

Fluorescence decays and lifetimes of Nd3+, Ce3+ and Cr3+ in YAG

The present paper summarizes results of detailed studies of fluorescence decays and lifetimes of Nd³⁺, Ce³⁺ and Cr³⁺ carried out on a large group of YAC.: Nd, Ce and YAC.: Nd, Ce, Cr single crystals. The average and local lifetimes (mainly for⁴F_3/2 state of Nd³⁺) of these ions are given and an influence of various processes (energy transfers, cross-relaxation, nonequivalent centres etc.) on their decays are qualitatively discussed.The authors are grateful to J. Kvapil of Monokrystaly Turnov for supplying them with the samples and for many fruitful discussions.     

Different processes responsible for blue pumped, ultraviolet and violet luminescence in high-concentrated Er:YAG and low-concentrated Er:YAP crystals

Ultraviolet and violet upconverted luminescence in high-concentrated (8.4 at%) Er³⁺:YAG and low-concentrated (1 at%) Er³⁺:YAP crystals has been investigated under blue laser excitation of the ⁴F_7/2 multiplet. The upconversion mechanisms were studied in detail based on upconversion luminescence intensity dependence and decay curves. Upconversion luminescence was attributed to energy transfer processes in Er³⁺:YAG and excited state absorption processes in Er³⁺:YAP, respectively.     

Doubly doped BaY2F8:Er,Nd VUV scintillator

Doubly doped BaY₂F₈:Er,Nd scintillation crystals were grown by modified micro-pulling-down method. The Er co-doping was chosen to enhance the energy transfer from the host lattice to the Nd³⁺ luminescence center via the 5d-levels of Er³⁺, which can be enabled by the overlap of Er³⁺ 5d-4f emission spectrum with the Nd³⁺ 4f-5d absorption. The energy transfer was clearly evidenced in the BaY₂F₈:Er,Nd. The processes are complicated by energy migration to killer centres and/or cross-relaxation processes. The luminescence and energy transfer mechanism are discussed.     

Diode pumped Nd:GSAG and Nd:YGG laser at 942 and 935 nm

Two new Nd³⁺ doped crystals show laser activity between the ⁴F_3/2 and ⁴I_9/2 levels. Diode pumped emission of Nd:GSAG at 942 nm wavelength with 950 mW output power and 14% optical to optical efficiency as well as laser emission of Nd:YGG at 935 nm with 700 mW output power and 11% efficiency have been obtained. These wavelengths are useful for water vapour absorption measurements.     

Diode-pumped continuous-wave Nd<Superscript>3+</Superscript>:YAlO<Subscript>3</Subscript> intracavity-doubled red laser

It is reported that efficient continuous-wave (CW) red laser generation at 670 nm in a LBO crystal at type-I phase matching direction performed with a diode-pumped Nd³⁺:YAlO₃ (Nd:YAP) laser. With incident pump power of 15.6 W, output power of 273 mW at 670 nm has been obtained using a 10 mm-long LBO crystal. At the output power level of 273 mW, the output stability is better than 3.7%.     

Diode-pumped CW Nd<Superscript>3+</Superscript>:YAlO<Subscript>3</Subscript> laser at 1339 nm

A diode-pumped Nd³⁺:YAlO₃ (Nd:YAP) laser emitting at 1339 nm is described. At the incident pump power of 17.8 W, as high as 3.4 W of continuous-wave (CW) output power at 1339 nm is achieved. The slope efficiency with respect to the incident pump power was 23.6%. The output power stability over 60 min is better than 3.5%. The laser beam quality M ² factor is 1.33.     

CW laser properties of Nd:GdYAG,Nd:LuYAG,and Nd:GdLuAG mixed crystals

Three mixed crystals, Nd:GdYAG, Nd:LuYAG, and Nd:GdLuAG, were grown by Czochralski method. We report the continuous-wave (CW) Nd:GdYAG, Nd:LuYAG, and Nd:GdLuAG laser operation under laser diode pumping. The maximum output powers are 4.11, 5.31, and 7.47 W, with slope efficiency of 73.0, 55.3, and 57.1%, respectively. With replacing Lu³⁺ or Y³⁺ ions with large Gd³⁺ ions, the pump efficiency increases.