Consideration of Stimulated Raman Scattering in Yb:Sr(5)(PO(4))(3)F Laser Amplifiers

The stimulated Raman-scattering (SRS) gain coefficient has been measured quantitatively for the first time to our knowledge in Yb:Sr(5)(PO(4))(3)F to be 1.23 ? 0.12 cm/GW at 1053 nm. These data, along with surface and bulk losses, feedback that is due to surface reflections, gain saturation, and bandwidth, have been applied to a quantitative model that predicts the effects of SRS within a laser amplifier system where the laser gain media show SRS gain. Limitations and impact to the laser amplifier performance are discussed, along with possible techniques to reduce SRS loss.     

Passively Q-switched quasi-three-level laser and its intracavity frequency doubling

A passively Q-switched quasi-three-level Nd:YAG laser is intracavity frequency doubled to generate a blue laser. The 473-nm blue laser has a peak power of 37 W and a pulse width of 23 ns at a pumping power of 1.6 W. To model this laser numerically, we developed rate equations by taking into consideration both the quasi-three-level nature of the gain medium and the four-level nature of the saturable absorber. Good agreement was achieved between experimental and simulated results for both the fundamental and the second-harmonic output. The reabsorption loss of the gain medium is estimated under pulsed operation.     

Gain saturation measurements of ytterbium-doped Sr5(PO4)3 F

We report on the experimental measurement of the saturated gain of Yb(3+):Sr(5)(PO(4))(3)F at the 1047-nm laser line as a function of pump fluence and probe energy. The emission line was accurately modeled as a single homogeneous extraction, yielding values of 6.2 x 10(-20) cm(2) for the emission cross section and 3.3 J/cm(2) for the saturation fluence.     

High-power cw 671 nm output by intracavity frequency doubling of a double-end-pumped Nd:YVO4 laser

We report on a high-power (cw) red laser at 671 nm by intracavity frequency doubling of a double-end-pumped 1342 nm Nd:YVO4 laser based on the nonlinear crystal LiB3O5. A red output power of 3.38 W is obtained for a pump power of 27 W, with corresponding optical-to-optical efficiency of 12.5%. The 671 nm beam is nearly diffraction limited.     

Saturable Absorber Cr(4)+:(Sr, Ca)Gd(4)(SiO(4))(3)O Crystals as Q Switches for a Diode-Pumped Nd(3)+:KGd(WO(4))(2) Laser

Passive Q switching of a diode-pumped Nd(3+):KGd(WO(4))(2) laser is demonstrated by use of Cr(4+):SrGd(4)(SiO(4))(3)O and Cr(4+):CaGd(4)(SiO(4))(3)O crystals as saturable absorbers. An average output power of 40 mW was obtained with a pulse repetition rate of ~0.4 MHz.     

Synthesis and photoluminescence of Sr5(PO4)3Cl:Mn nanorods

Mn-doped Sr₅(PO₄)₃Cl nanorods were synthesized by a simple precipitation method without any controlling agents. The obtained nanorods were uniform and had diameters of 15–35 nm and lengths of 200–300 nm observed by transmission electron microscopy (TEM). A red emission band centered at 695 nm corresponding to ⁴T₁–⁶A₁ transition of Mn²⁺ ion was seen from the photoluminescence spectra. The optimum concentration for Mn²⁺ luminescence in Sr₅(PO₄)₃Cl host is about 1.5 mol%.     

Intracavity frequency doubling and Q switching in diode-laser-pumped Nd:YVO(4) lasers

A highly efficient and compact Nd:YVO(4) laser is proposed. In cw operation, a single-longitudinal-mode output of 95 mW and a multilongitudinal-mode output of 435 mW have been observed at 1.06 μm with a 1-W diode laser. Using a KTP crystal in the short laser cavity, a green output of 105 mW was generated. A Q-switched pulse with a peak power of 230 W and a pulse width of 8 ns was obtained with the intracavity KTP crystal, which was used as both an electro-optic Q switch and a frequency doubler.     

Low cytotoxicity porous Nd(2)(SiO(4))(3) nanoparticles with near infrared excitation and emission

Porous Nd(2)(SiO(4))(3) nanoparticles were successfully synthesized by a controlled route. This kind of silicate nanoparticle could be excited by near-infrared (NIR) radiation (808 nm) and triggered a NIR emission (1066 nm) at room temperature. By monitoring the 1066 nm emission, the long-lived luminescent lifetime was determined to be 19.5 μs. These NIR nanoparticles with appropriate diameters (<100 nm) were suitable for cell assays. MTT assays showed that the cytotoxicity of the porous Nd(2)(SiO(4))(3) nanoparticles was very low. Therefore, these porous silicate nanoparticles are potential biosafe high-performance NIR biolabeling materials.     

Mechanical and thermal properties of hot pressed neodymium-substituted britholite Ca9Nd(PO4)5(SiO4)F2

Neodymium-substituted britholite, a phosphate–silicate apatite Ca₉Nd(PO₄)₅(SiO₄)F₂, is considered as a potential host matrix for specific immobilization of radionuclides. Complementary investigations have been carried out to complete the database concerning this compound. The aim was to establish mechanical and thermal properties of dense britholite. Hot pressing was used to nearly fully densify the material. Low values of mechanical properties were found: 0.75 MPa m^1/2 for the fracture toughness and 95 MPa for the flexural strength. The Young's modulus and the Poisson's ratio were 108 GPa and 0.30, respectively. The specific and the thermal conductivity at 298 K were Cp=0.75 J g⁻¹ K⁻¹ and λ=1.15 W m⁻¹ K⁻¹. The average coefficient of thermal expansion in the 20–1000 °C temperature range was α=21×10⁻⁶ K⁻¹.     

Low thermal expansion materials: a comparison of the structural behaviour of La0.33Ti2(PO4)3, Sr0.5Ti2(PO4)3 and NaTi2(PO4)3

Variable temperature neutron powder diffraction data have been used to examine the structural mechanism of unusual thermal expansion behaviour in La_0.33Ti₂(PO₄)₃ (LaTP). These data are compared to those we have recently reported for the related systems NaTi₂(PO₄)₃ (NaTP) and Sr_0.5Ti₂(PO₄)₃ (SrTP), which differ in the extent of occupation of the extra-framework MI (La, Sr, Na) cation sites. The root cause of the unusual behaviour is ascribed to the differing expansivities of the occupied and vacant MI sites. This leads to cooperative rotations of TiO₆ and PO₄ polyhedra, which can be used to quantify and rationalise the overall anisotropic thermal expansion behaviour.     

Multipass pumped Nd-based thin-disk lasers: continuous-wave laser operation at 1.06 and 0.9 microm with intracavity frequency doubling

The laser performances of the 1.06 microm (4)F(3/2) --> (4)I(11/2) four-level transition and of the 0.9 microm (4)F(3/2) --> I(9/2)4 quasi-three-level transition were investigated using multipass pumped Nd-based media in thin-disk geometry. When pumping at 0.81 microm into the (4)F(5/2) level, continuous-wave laser operation was obtained with powers in excess of 10 W at 1.06 microm, in the multiwatt region at 0.91 microm in Nd:YVO(4) and Nd:GdVO(4), and at 0.95 microm in Nd:YAG. Intracavity frequency-doubled Nd:YVO(4) thin-disk lasers with output powers of 6.4 W at 532 nm and of 1.6 W at 457 nm were realized at this pumping wavelength. The pumping at 0.88 microm, which is directed into the (4)F(3/2) emitting level, was also employed, and Nd:YVO(4) and Nd:GdVO(4) thin-disk lasers with ~9 W output power at 1.06 microm and visible laser radiation at 0.53 microm with output power in excess of 4 W were realized. Frequency-doubled Nd:vanadate thin-disk lasers with deep blue emission at 0.46 microm were obtained under pumping directly into the (4)F(3/2) emitting level.     

Four watt long-term stable intracavity frequency-doubling Nd:YVO(4) laser of single-frequency operation pumped by a fiber-coupled laser diode

The longitudinal-mode stability of a single-frequency intracavity frequency-doubling laser can be enhanced by increasing the ratio between the nonlinear spectral bandwidth and the gain bandwidth of the laser. A 4 W long-term stable cw single-frequency green laser is obtained using an etalon inside the laser cavity as a spectral filter. No mode hopping is observed when the laser operates for 6 h and the power fluctuation is less than +/-1.2%. In contrast, in the situation of no etalon inside the laser cavity, mode hopping can be observed at irregular intervals and the power fluctuation is +/-3.8% when the laser operates for 4 h.     

Reaction of chlorapatite,Ca5(PO4)3(Cl,F) with sodium carbonate and silica

The conversion of insoluble phosphorous minerals such as apatite, Ca₅(PO₄)₃(F, OH, Cl), to phases containing phosphorous in soluble form is an essential step in the production of fertilizer grade phosphates. Developing countries lack the capital-intensive industrial base necessary to create a phosphate industry. We examine a process suitable for use in Sri Lanka which uses mainly indigenous raw materials. In this process, a chlorine-rich apatite is fused with Na₂CO₃ and SiO₂ at 900° C and 1300° C for 1 to 2 h to yield a product having >90% available phosphorous. Data necessary for the technological evaluation of the process are presented. Phase equilibria in the lime-rich portions of the system CaO-Na₂O-P₂O₅-SiO₂ have been studied. The section Ca₃(PO₄)₂-Ca₂SiO₄-CaNaPO₄ is shown to be a ternary system, and phase relations on the 1100° and 1300° C isothermal sections are presented. A reconnaisance of the system CaO-Ca₃(PO₄)₂-Ca₂SiO₄-CaNaPO₄ has been made. These data, combined with studies of the kinetics of the reaction, point to regions of composition and reaction conditions favouring high yields of available phosphorous.     

Cw industrial rod YAG:Nd(3+) laser with an intracavity active bimorph mirror

A special type of resonator with an intracavity wide-aperture active mirror was built, and a concave spherical bimorph active corrector was investigated. An increase of laser beam quality by a factor of 2-2.5 was achieved in a multimode regime of laser generation with an intracavity-controlled mirror. It was shown that various radiation mode structures could be formed at the laser output and in the far-field zone.     

Near shot-noise-level relative frequency stabilization of a laser-diode-pumped Nd:YVO(4) microchip laser

A laser-diode-pumped Nd:YVO(4) microchip laser was built and actively frequency stabilized relative to a Fabry-Perot cavity with the frequency-modulated sideband technique. The error signal reaches the shot-noise level of 7.4 mHz/√Hz around 1 kHz. Excess intensity noise sets a lower limit of16.5 mHz/√Hz for the relative frequency noise, corresponding to a spectral linewidth of 860 μHz. We discuss the method for reconstructing the actual frequency deviation from the observed error signal.     

Luminescence properties of Eu2+-activated Sr5(PO4)2(SiO4) for green-emitting phosphor

A green-emitting phosphor of Eu²⁺-activated Sr₅(PO₄)₂(SiO₄) was synthesized by the conventional solid-state reaction. It was characterized by photoluminescence excitation and emission spectra, and lifetimes. In Sr₅(PO₄)₂(SiO₄):Eu²⁺, there are at least two distinguishable Eu²⁺ sites, which result in one broad emission situating at about 495 nm and 560 nm. The phosphor can be efficiently excited in the wavelength range of 250–440 nm where the near UV (～ 395 nm) Ga(In)N LED is well matched. The dependence of luminescence intensities on temperature was investigated. With the increasing of temperature, the luminescence of the phosphor shows good thermal stability and stable color chromaticity. The luminescence characteristics indicate that this phosphor has a potential application as a white light emitting diode phosphor.     

Noise characteristics of a Nd:YVO(4) laser pumped by laser diode modulated at high frequency

We report, for the first time to our knowledge, on the noise characteristics of a Nd:YVO(4) laser pumped by a laser diode modulated at high frequency. We have investigated noise characteristics of a Nd:YVO(4) laser pumped by a laser diode that is oscillating in a stable multilongitudinal mode because of modulation by a high-frequency (several hundred megahertz) current. As a result, low-noise operations of -130 dB/Hz above frequencies of 1 MHz have been achieved. This noise level is comparable to the level obtained when pumping a Nd:YVO(4) laser with a laser diode that is oscillating in a single longitudinal mode. However, a noise peak corresponding to relaxation oscillations of the Nd:YVO(4) laser has appeared around a frequency of several hundred kilohertz.