Neodymium concentration dependence of 0.94-, 1.06- and 1.34-μm laser emission and of heating effects under 809- and 885-nm diode laser pumping of Nd:YAG

Laser emission in the 0.94-, 1.06- and 1.34-micron ranges in diluted and concentrated Nd:YAG crystals longitudinally pumped by a 885-nm diode laser on the ⁴ I _9/2→⁴ F _3/2 transition is investigated. Continuous-wave operation at watt level in all these wavelength ranges is demonstrated with a 1.0-at. % Nd:YAG crystal; however, the laser performance is impeded by the low pump absorption efficiency. Improved output power and overall efficiency were obtained with a highly doped 2.5-at. % Nd:YAG crystal: 5.5 W at 1.06 μm and 3.8 W at 1.34 μm with 0.38 and 0.26 efficiencies, respectively. Comparative results with traditional pumping at 809 nm into the highly absorbing ⁴ F _5/2 level are presented, showing the advantage of the direct ⁴ F _3/2 pumping. The influence of the lasing wavelength and of the Nd concentration on the thermal effects induced by the optical pumping in the laser material is discussed. A clear relation between the heat generated in the Nd:YAG crystals in lasing and non-lasing regimes, a function of the Nd doping, is demonstrated. PACS 42.55.Rz; 42.60.By; 42.60.Da     

Optical and waveguiding properties of Nd:KGW films grown by pulsed laser deposition

 Nd: KGd(WO₄)₂ thin films were deposited by KrF laser ablation on MgO, YAP, YAG and Si substrates at temperatures up to 800 °C. Film crystallinity, morphology, stoichiometry (WDX, RBS and PIXE), excitation spectra, fluorescence, refractive index and waveguiding properties were studied in connection with deposition conditions. The best films were crystalline and exhibited losses of approximately 5 dB cm^-1 at a wavelength of 633 nm. Received: 8 January 2001 / Accepted: 7 November 2001 / Published online: 11 February 2002     

Optical properties and highly efficient laser oscillation of Nd:YAG ceramics

Optical absorption, emission spectra have been measured for polycrystalline Nd-doped Y₃Al₅O₁₂ ceramics. Fluorescence lifetimes of 257.6 μs, 237.6 μs, 184.2 μs and 95.6 μs have been obtained for 0.6%, 1%, 2% and 4% neodymium-doped YAG ceramics, respectively. For the first time, highly efficient laser oscillation at 1064 nm has been obtained with this kind of ceramics. Slope efficiency of 53% has been achieved on a uncoated 4.8-mm thick 1% Nd:YAG ceramics sample. Optical to optical conversion efficiency is 47.6%. Laser oscillation has also been obtained with a 2% Nd:YAG ceramics. The optical properties and laser output results have been compared with that of Nd:YAG single crystal grown by the Czochralski method. Almost identical results have been achieved including laser experiments results. But fabrication of Nd:YAG ceramics is much easier compared to the single-crystal growth method. And also large size (now of about 400 mm diameter×5 mm is available) and high-concentration (>1%) Nd:YAG ceramics can be fabricated. The results show that this kind of Nd:YAG ceramics is a very good alternative to Nd:YAG single crystal. Received: 20 April 2000 / Published online: 16 August 2000     

Comparative laser study of Nd:KGW and Nd:YAG near 1.3 μm

A comparative study of Nd:KGW and Nd:YAG laser crystals pumped by flashlamp has been conducted near 1.3 μm with output energy up to 1 J and at a repetition rate up to 50 Hz. An average power of 23 W for KGW in free-running mode was achieved with a total efficiency better than 2.8 % for the Nd:KGW and 1.8 % for the Nd:YAG. Received: 9 December 1996 / Revised version: 10 February 1997     

Accurate spectroscopic wavelength determination of pulsed lasers

We present a simple, easy, and straightforward spectroscopic approach for the determination of the wavelength of pulsed laser systems with an accuracy of ≤0.006 cm^-1 (180 MHz). This is better than currently realised by the best commercially available wavemeters for pulsed laser systems. As an example, the temperature dependence of the emitted wavelength of a pulsed Nd:YAG laser has been studied in detail. Our measurements indicate that the passively Q-switched monolithic nonplanar ring-oscillator Nd:YAG laser with a free spectral range of the resonator of about 5.8 GHz can be tuned without mode hops over a range of 3.8 GHz. The concept is generally applicable to any other wavelength regions of interest. Received: 9 April 2001 / Revised version: 9 July 2001 / Published online: 19 September 2001     

High-peak power,passively Q-switched,composite, all-poly-crystalline ceramics Nd:YAG/Cr<Superscript>4+</Superscript>:YAG laser and generation of 532-nm green light

Output performances of passively Q-switched, composite Nd:YAG/Cr⁴⁺:YAG lasers that consisted of bonded, all-poly-crystalline ceramics Nd:YAG and Cr⁴⁺:YAG are reported. Laser pulses at 1.06 μm with 2.5-mJ energy and 1.9-MW peak power are obtained from a 1.1-at % Nd:YAG/Cr⁴⁺:YAG ceramics that was quasi-continuous-wave (quasi-CW) pumped with a diode laser. Single-pass frequency doubling with LiB₃O₅ (LBO) nonlinear crystal at room temperature yielded green laser pulses at 532 nm of 0.36-mJ energy and 0.3-MW peak power, with a conversion efficiency of 0.27.     

High-power diode-end-pumped Nd:YVO4 laser: thermally induced fracture versus pump-wavelength sensitivity

We report two kinds of compact and efficient diode-end-pumped TEM₀₀ lasers with output power >25 W at ≈50 W of incident pump power. One laser consists of a single 0.3 at. % Nd:YVO₄ crystal in a V-type cavity, the other laser includes two 0.5 at. % Nd:YVO₄ crystals in a linear cavity. Experimental results show that lowering Nd³⁺ concentration can be beneficial in extending the fracture-limited pump power but it also increases the sensitivity of the pump wavelength due to the overlapping efficiency. Received: 19 February 2000 / Revised version: 30 May 2000 / Published online: 20 September 2000     

Diode-pumped passively Q-switched Nd:GdVO4 lasers operating at 1.06 μm wavelength

With a 10-W diode laser to pump Nd:GdVO₄ crystal in a folded cavity, we demonstrated Cr⁴⁺:YAG passively Q-switched Nd:GdVO₄ lasers at 1.06 μm. The maximum average output power of 2.1 W and the highest peak power of 625 W were, respectively, obtained when the initial transmissions of the Cr⁴⁺:YAG crystals were 90% and 80%. Received: 8 September 1999 / Revised version: 30 December 1999 / Published online: 8 March 2000     

Efficient generation of blue light by frequency doubling of a Nd:YAG laser operating on 4 F 3/24 I 9/2 transitions

Efficient room-temperature operation of ⁴ F _3/2⁴ I _9/2 transitions in diode-end-pumped Nd:YAG lasers at 946 nm and 938.5 nm is reported. 7.0-W continuous-wave output power at 946 nm and 3.9 W at 938.5 nm have been obtained. An analytical model has been developed for the quasi-three-level laser including the influence of energy-transfer upconversion. Frequency doubling of these transitions in periodically poled KTP generated blue light at 473 nm and 469 nm. Both single-pass extra-cavity as well as intracavity schemes have been investigated. Received: 31 July 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +46-8/750-5430, E-mail: stefan.bjurshagen@acreo.se     

Temperature characteristics of small signal gain for Nd/Cr:YAG ceramic lasers

Thermal dependence on the small signal gain of Nd/Cr:YAG (yttrium aluminum garnet) ceramics was observed experimentally. Usually, Nd:YAG crystal and ceramics have remarkable gain reduction when optical pumping is performed and the temperature of the laser media is upped to 373 K. A CW laser light generated in a 1064 nm Nd:YAG laser oscillator was amplified by Nd/Cr:YAG ceramic amplifier, and the output power was measured at non-saturation level. Laser small signal gain of the ceramic disk was kept to 470 K. This property was remarkably different from one of Nd:YAG crystals or ceramics. The peak shift of the fluorescence was observed experimentally when the temperature is high.     

Preparation and characterization of transparent Nd:YAG ceramics

Transparent Nd:YAG ceramics were produced by solid.state reaction of high.purity (4N) nanometric oxides powders, i.e., Al₂O₃, Y₂O₃ and Nd₂O₃. After sintering, mean grain sizes of 2% Nd:YAG samples were about 20 μm and their transparency were a bit worse than that of 0.9% Nd:YAG single crystal. Two types of active elements: rods and slabs were fabricated and characterized in several diode pumping schemes. In end pumping configuration as a pump source 20.W fiber coupled laser diode operating in low duty cycle regime (1 ms pump duration/20 Hz) was deployed. In the best case, 3.7 W of output power for 18 W of absorbed pump power, M² < 1.4 were demonstrated for uncoated ceramics Nd:YAG rod of ϕ 4×3mm size in preliminary experiments. For the ceramics of two times lower Nd dopant level above 30% slope efficiency was achieved. In case of Nd:YAG ceramic slab side pumped by 600.W laser diode stack above 12 W was demonstrated with slope efficiency of 3.5%.     

Synthesis and microstructure analysis of composite Nd:YAG/YAG transparent ceramics

Transparent Nd：YAG/YAG composite ceramics are synthesized by solid-state reaction method using high- purity Y203, A1203, and Nd203 powders as raw materials. The mixed powder compacts are sintered at 1780 ℃ for 10 h under vacuum and annealed at 1450 ~C for 20 h in air. The Nd：YAG/YAG ceramics exhibit a pore free structure with an average grain size of about 30 μm. The microstructure of the Nd：YAG/YAG composite transparent ceramics is studied and there is no interface between Nd：YAG and YAG ceramics. The Nd ion distribution in one grain is also studied, which shows that there is no segregation of Nd ions as in Nd：YAG crystals.     

Laser-diode pumped self-Q-switched microchip lasers

Optical properties of Cr,Yb:YAG, Cr,Nd:YAG crystals, and composite Yb:YAG/Cr:YAG ceramics self-Q-switched solid-state laser materials are presented. The merits of these self-Q-switched laser materials are given and the potentials of such lasers can be chosen by the applications. Cr,Yb:YAG and composite Yb:YAG/Cr:YAG ceramics self-Q-switched laser are conducted. Although several tens of kW peak power can be obtained with a monolithic microchip Cr,Yb:YAG laser, the experimental results show that the performance of this laser is limited by the absorption of Cr⁴⁺ ions at a pump wavelength of 940 nm and strong fluorescence quenching at high Cr concentration. Composite Yb:YAG/Cr:YAG ceramics are more suitable to realize high pulse energy and peak power (up to MW level) with optimized lasing and Q-switching parts. In addition, the instabilities induced by the multi-longitudinal mode competition in Cr,Nd:YAG and Cr,Yb:YAG microchip lasers are addressed. The different gain bandwidths of Yb:YAG and Nd:YAG play an important role in the instability of the output laser pulse trains. Stable laser pulses from the Cr,Yb:YAG microchip laser were obtained due to the antiphase dynamics. For the Cr,Nd:YAG microchip laser, the instability caused by the multi-longitudinal mode competition is an intrinsic property. Different transverse patterns were observed in Cr,Nd:YAG microchip lasers when a pump beam with larger diameter was used. Saturated inversion population distribution inside the gain medium plays an important role in the transverse pattern formation. Different transverse patterns were reconstructed by combining different sets of the Hermite-Gaussian modes.     

Photorefractive properties of LiNbO3:Zn crystals related to the defect structure

LiNbO₃:Zn single crystals and powders were studied by precise X-ray diffraction methods. Structural refinement yielded new models of the intrinsic defect structure valid for different Zn concentration ranges. For concentrations up to 7 at. % in the crystal, Zn ions incorporate onto Li sites; at higher concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of LiNbO₃:Zn are discussed in the context of the deduced defect models. A smooth increase in the photoconductivity up to 7 at. % Zn is accounted for by a decrease of Nb antisites. Steeper growth of the photoconductivity at higher Zn concentrations is related to vanishing Li vacancies. Received: 17 November 2000 / Revised version: 23 January 2001 / Published online: 20 April 2001     

Low-energy ion emission from a xenon gas-puff laser-plasma X-ray source

We have measured low-energy ion emission from a gas-puff laser-plasma X-ray source. The ions may cause the degradation of the condenser mirror of the extreme ultra-violet projection lithography system. A 0.7 J in 8 ns Nd:YAG laser at 1.06 μm was focused onto the xenon gas-puff target with an intensity of ∼10¹² W/cm². The silicon (111) plates, placed at a distance of 32 mm from the laser-interaction region, were exposed with the xenon ions. The average ion energy was measured to be less than 50 eV with a Faraday-cup detector placed close to the silicon plates. The xenon deposition occurred in the silicon plates with a depth of less than 40 nm. The deposition density was measured with a quadrupole secondary ion mass spectrometer to be 10²¹ /cm³ after 1500 laser shots. The energy-conversion efficiency from the laser energy into the ions is ∼0.1%/4 π sr/shot. For the lithography system, if we can remove such ion bombardment completely using novel techniques such as electro-magnetic devices or gas flow curtain techniques, the lifetime of the condenser mirror will be extended significantly. Received: 20 November 2000 / Published online: 9 February 2001     

Diode-end-Pumped Nd:YAG Ceramic and Crystal Operation at 1,123 nm

We present a high-power diode-end-pumped continuous-wave Nd:YAG laser operating at 1,123 nm. Laser operation was carried out and compared using high optical quality Nd:YAG ceramics fabricated in-house and commercial Nd:YAG single crystals. At the absorbed diode pump powers of 23.2 and 28.0 W, output powers of 10.7 and 12.5 W at 1,123 nm were achieved for the employed ceramics and crystals as the laser material, which correspond to conversion efficiencies of 46.1% and 44.6%. For high-power lasers, the Nd:YAG ceramic has the advantage of a higher destructive threshold than that of commercial crystals.     

Structural and optical characterization of Er3+/Yb3+-doped LiNbO3

We report the dependence of the unit-cell parameters and the extraordinary and ordinary refractive indices of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals. Both properties depend in a non-monotonic manner on the Er³⁺/Yb³⁺ content. A singularity was observed at concentrations of 1.1-1.2 mol. % in the crystal (0.6-0.7 mol. % in the melt). In the same way the Er and Yb concentration influences the periodically poled lithium niobate formation. The observed behavior of refractive indices and unit-cell parameters of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals could be explained in terms of the RE³⁺-ion concentration affecting the Li-vacancy concentration and the RE³⁺-ion positions in the crystal. Received: 21 May 2001 / Revised version: 22 August 2001 / Published online: 23 October 2001     

Laser and thermo-optical investigations of Nd:YAG ceramics

The ‘wet’ technology was implemented for processing of Nd:YAG ceramics. The samples of disk, rod, slab shapes with 1–2% Nd dopant were fabricated and tested. Several method of optical characterization were applied. Near 80% transmission and scattering losses <0.2 cm⁻¹ were demonstrated. The laser action with 34% slope efficiency was obtained for the best case. To characterize the spatial inhomogeneities of output parameters “half-microchip configuration” with output coupler formed by uncoated output facet of plane parallel ceramic sample was implemented. Thermally induced aberrations and birefringence observed in Nd:YAG ceramic of disk samples under high heat load were examined and compared with numerical models.     

Interaction of laser radiation with TNT with respect to the laser neutralisation of AP mines

According to UN estimations there are between 80 and 115 million activated landmines worldwide. These mines, or other unexploded ordnance (UXO), can be triggered accidentally and kill or injure more than 2000 civilians per month. The most common explosive in these mines is trinitrotoluene (TNT). In this paper, the potential of some of the most promising lasers for mine neutralisation is investigated, namely an ArF laser, a KrF excimer laser and a Nd:YAG solid-state laser. We have studied the interaction between laser beams emitting at λ=193 nm, 248 nm and 1060 nm and a bare solid sample of TNT of approximately 15 mg. Using pulsed excimer radiation at λ=193 nm, with an energy density up to 1 J/mm², ablation of the TNT without any deflagration has been achieved. At λ=248 nm, using the KrF excimer laser with a pulse duration of 30 ns and a repetition rate of 5 Hz, the TNT sample started melting and burning after an irradiation of 10 s. Preliminary results with the Nd:YAG solid-state laser operating in cw emission have shown that the irradiated sample exhibits the desired burning behaviour even after the exposure is stopped. Received: 14 December 2000 / Accepted: 18 December 2000 / Published online: 20 June 2001     

Efficient and compact intracavity-frequency-doubled Nd:GdVO4/KTP laser end-pumped by a fiber-coupled laser diode

A compact and efficient diode-pumped intracavity-frequency-doubled Nd:GdVO₄/KTP green laser is demonstrated with a flat–flat cavity design. With a 1.3 at. % Nd³⁺-doped GdVO₄ crystal and pumped at the weak-absorption peak of 806 nm, the second-harmonic output power at 532 nm was measured to be 1.95 W at an incident pump power of 8.4 W, corresponding to an optical conversion efficiency of 23.2%. The output characteristic at the fundamental wavelength of 1.063 μm was investigated with two different pump wavelengths. More than 4.5-W output power was generated when the laser was pumped at 806.2 nm. Received: 26 July 2000 / Revised version: 18 September 2000 / Published online: 7 February 2001