Comparison of cw laser performance of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO4 under laser diode pumping

4 crystals under low-power laser diode end-pumping. Output power dependencies on the pump power and the pump wavelength of these diode-pumped solid state lasers were investigated. The high Nd³⁺ concentration of the Nd:KGW samples used in our measurements as well as up-conversion and exited-state absorption processes in Nd:KGW cause the reduced laser output power dependence on the pump wavelength which was experimentally observed. At pump levels up to 270 mW a slope efficiency of η_sl≈46% was reached for the Nd:KGW laser. Nd:KGW microchip laser operation with a slope efficiency of η_sl≈50% was demonstrated. Thermal lensing in Nd:KGW at pump powers up to 3 W was measured. Received: 4 August 1997     

Transient grating measurements of refractive-index changes in intensively pumped Yb-doped laser crystals

Mechanisms of transient light-induced refractive-index changes in Yb-doped crystals Yb:KYW, Yb:KGW and Yb:YVO₄ were studied. Relative amplitudes and lifetimes of the electronic component of phase gratings caused by the polarizability difference of the excited and unexcited Yb³⁺ ions and the thermal component arising from thermalization of the pump energy were measured. Polarizability difference values of the ground ² F _7/2 and the excited metastable ² F _5/2 levels at the testing wavelength of 633 nm were determined for all crystals. The thermal diffusivity of the samples was estimated for selected directions of the thermal gradient. An athermal direction, along which the thermal component of refraction disappears, was found in Yb:KYW crystal under specific experimental conditions. PACS 42.70.Hj; 42.65.An; 66.30.Xj     

Origin of athermal refractive index changes observed in Yb doped YAG and KGW

Time-resolved excited-state absorption and fluorescence measurements were performed in the Yb³⁺ doped YAG and KGW laser crystals. The results indicate that the athermal refractive index changes observed in previous transient interferometric and diffraction grating experiments can be related to polarisability changes caused by the existence of strong UV charge transfer absorption bands.     

Intracavity Raman laser generating a third stokes component at 1.5 μm

The lasing properties of an intracavity Nd:KGW Raman laser which converts the multimode radiation of an Nd:KGW laser operating on the ⁴ F _3/2–⁴ I _11/2 transition into the third Stokes component at a wavelength of 1.5 μm are studied. The energy in the third Stokes component is found to increase essentially linearly with the electrical energy delivered to the flashlamp. Lasing at the third Stokes component begins in the central portion of the Nd:KGW crystal and then propagates to its boundaries. Reducing the geometric aperture of the multimode pump beam in the Raman crystal lowers the divergence of the Stokes emission. For a source pump energy of 6 J, the intracavity Raman laser emits 14.7 mJ pulses of duration 3–4 nsec which are safe to the eyes. The divergence of the Raman laser beam at a level of 86% of the total energy is ≈ 9 mrad.     

Measurement of Wavelength Dependence of Electro-Optic Coefficients r22 of Non-doped and 5% MgO-doped Congruent LiNbO3 Crystals and 1.8% MgO-doped Quasi-stoichiometric LiNbO3 Crystal by Multiple Reflection Interference Method

The electro-optic coefficient r ₂₂ in the wavelength range from 409 to 1580 nm was measured for non-doped and 5% MgO-doped congruent LiNbO₃ crystals and 1.8% MgO-doped quasi-stoichiometric LiNbO₃ crystals in a simple optical system using a multiple reflection interference method capable of producing high-precision results without the application of antireflection film to the end faces of the crystal. The influence of the manufacturing errors of the electrooptic crystals was discussed on the measurement of the r ₂₂ coefficient. The experimental errors are less than approximately 0.5% in the wavelength range from 409 to 1064 nm and approximately 1% from 1340 to 1580 nm. It was further shown that polarizing of the laser along X axis resulted in highly accurate measurement of the r ₂₂ of LiNbO₃ crystals.     

Transformation of optical properties of crystal media (KGW, YVO4) exposed to quasi-continuous laser radiation in the range of the transmission band of the medium

We present data from an investigation of the properties of the blue (475 nm) and green (520–560 nm) emissions that arise from KGW and YVO₄ crystals upon excitation into the transmission band of the medium by continuous infrared laser radiation the power density of which is in the kilowatt range. As distinct from standard luminescence, the two emissions are narrowband, have a structure, and show a nonlinear character. In the case of blue emission, it is noted that, simultaneously with laser radiation, the crystal experiences a significant action of Raman generation. We demonstrate the excitation of the blue emission by the laser radiation alone and present its dynamic characteristics, showing that its intrinsic time differs from the corresponding times of the Raman and laser generations. Our results show that the green emission has an up-conversion nature with the several photons excitation of residual low-concentration erbium ions. We report that the band intensity ratio of the upconversion luminescence of erbium sharply changes with increasing excitation intensity. The nonlinear interaction that is developed in KGW is accompanied by transformation of the transmission. It is concluded that the investigated processes develop in undoped KGW and YVO₄ crystals as a result of a prolonged exposure of the medium to the laser radiation, which leads to a change in its properties.     

Effect of rhodium doping on the photorefractive properties of BCT crystals at 850 nm

We present an experimental investigation of the photorefractive properties of rhodium-doped barium calcium titanate (BCT) crystals of the congruent melting composition Ba_0.77Ca_0.23TiO₃. Considering the results previously obtained on this crystal in the visible region, it should be a good alternative to BaTiO₃. Nevertheless, many applications use infrared light. Therefore we present here a study of rhodium-doped BCT crystals at 850 nm. This wavelength is of special interest as it is in the spectral range of laser diodes. Rhodium doping is expected to enhance the sensitivity of the crystal in the infrared as is the case for BaTiO₃.We first noticed that BCT:Rh crystals are sensitive at this wavelength as expected. Furthermore, the photorefractive properties are interesting in terms of photorefractive gain, with a gain as high as 3 cm^-1 with ordinary polarization. This study has also put forward the fact that rhodium is not the only defect that participates in the photorefractive effect. Indeed, a large quantity of iron seems to be present in the BCT crystals and to participate in the photorefractive effect.An Erratum to this article can be found at     

Characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We present results of the characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO₄)₂ using a z-scan technique over the 800–1600 nm wavelength range. Based on our experimental and theoretical results we conclude that Yb:KGW crystal is a good candidate for efficient Kerr-lens mode locking. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

产生蓝色相干辐射和实现Nd:YAG 1.0642μm倍频90°非临界相位匹配的Nb:KTiPO4

本文报道了用熔盐顶部籽晶法生长Nb浓度(0～13)mol%的Nb:KTP晶体的倍频的Ⅱ型相位匹配的截止波长和Nd:YAG1.0642μm及Nd:YA1O₃1.0795μm激光在这些晶体中倍频的最佳相位匹配角的测量结果.从中可看出,由于Nb⁵⁺的引入使KTP晶体倍频的Ⅱ相位匹配的截止波长有效蓝移,目前已使截止波长蓝移至937nm且有效产生468.5nm的倍频蓝光.同时Nb⁵⁺的引入使Nd:YAG1.0642μm激光和Nd:YA1O₃倍频的最佳相位匹配方向产生很大的变化,目前已使Nb:KTP晶体倍频的最佳相位匹配方向为θ=88.32°、Φ=0°,非常接近于90°非临界相位匹配方向.     

Specific features of absorption and luminescence in CsBr: EuOBr crystals

The specific features of the absorption, photoluminescence, x-ray luminescence, thermally stimulated luminescence, and photostimulated luminescence spectra of CsBr: Eu²⁺ single crystals grown using the Bridgman method are investigated in the temperature range 80–500 K at the highest possible dopant content (0.1–0.4 mol % EuOBr in the batch) required for preparing perfect crystals. It is shown that an increase in the dopant content leads to a broadening of the absorption and photoluminescence excitation bands with maxima at wavelengths of 250 and 350 nm due to the interconfigurational transitions 4f⁷(⁸S_7/2) → 4f⁶5d(e _g , t_2g) in Eu²⁺ ions. The photoluminescence and photostimulated luminescence spectra of CsBr: EuOBr single crystals (0.1–0.4 mol % EuOBr) contain a band at a wavelength of λ_max=450 nm and bands at wavelengths of λ_max=508–523 and 436 nm. The last two bands are assigned to Eu²⁺-V_Cs isolated dipole centers and Eu²⁺-containing aggregate centers, respectively. It is revealed that the intensity of the luminescence associated with the aggregate centers (λ_max=508–523 nm) is maximum at an EuOBr content of less than or equal to 0.1 mol % and decreases with an increase in the dopant content. The possibility of forming CsEuBr₃-type nanocrystals that are responsible for the green luminescence observed at a wavelength λ_max=508–523 nm in CsBr: Eu crystals is discussed. The intensity of photostimulated luminescence in the CsBr: EuOBr crystals irradiated with x-ray photons is found to increase as the dopant content increases. It is demonstrated that CsBr: EuOBr crystals at a dopant content in the range 0.3–0.4 mol % can be used as x-ray storage phosphors for visualizing x-ray images with high spatial resolution.     

Effect of higher order nonlinear optical processes on optical absorption in the photorefractive BSO and BGO crystals

The processes of nonlinear refraction and nonlinear absorption are studied in the photorefractive Bi₁₂SiO₂₀ (BSO) and Bi₁₂GeO₂₀ (BGO) crystals at a wavelength of a picosecond Nd:YAG laser of 1064 nm. The nonlinear refraction in the crystals is shown to be related to the Kerr effect, and the nonlinear absorption at this wavelength, to three-photon absorption. The three-photon absorption coefficients of the BSO and BGO crystals are equal, respectively, to (2.5±0.8) ×10^?20 and (4.4±1.3) ×10^?20 cm³W^?2.     

Dual wavelength operation in diode-end-pumped hybrid vanadate laser

Dual wavelength operation at 1062.8 nm and 1064.1 nm in a diode-pumped hybrid laser comprising of Nd³⁺-doped birefringent YVO₄ and GdVO₄ crystals is demonstrated. A detailed characterization of the laser is performed under CW and pulsed operation. Under Q-switching, 4 W of average power at 5 kHz repetition rate is obtained with 32 ns FWHM pulse duration corresponding to 25 kW of the peak power. The intensity and the polarization of the individual spectral components can be easily controlled by changing the relative gain and the relative orientation of the two crystals. The resulting pulsed dual-wavelength laser has the potential to be used as a source for generating terahertz radiation.     

Structure and magnetic properties of FeCoN films

We obtained crystals of RuS₂ doped with ⁵⁷Fe from a Bi melt and determined the EPR hyperfine structure corresponding to ⁵⁷Fe³⁺ in low-spin configuration. In crystals that were doped with both Fe and Cr an increase of the Fe³⁺ resonance and a simultaneous decrease of the Cr³⁺ resonance occurred by IR irradiation and revealed the same wavelength dependence. Compared with as-grown crystals the iron-doped crystals turned out to have a rather high electrical resistivity of about 10⁴Omega cm at room temperature. For these iron-doped crystals two different activation energies of 0.04 eV and 0.35 eV of the free charge carriers (electrons) were determined from measurements of the electrical conductivity in the range of 94 K and 294 K. Received: 29 July 1996/Accepted: 13 November 1996     

Crystal stability and the theory of ferroelectricity part II. Piezoelectric crystals

In Part I the dielectric properties of cubic crystals with atoms in special positions were discussed, with particular reference to crystals which become unstable against a transverse optic mode of vibration of long wavelength. These considerations are now extended to crystals of general symmetry, and it is shown that the anomalous temperature dependence of certain dielectric, piezoelectric, and elastic constants in the paraelectric phase of ferroelectric crystals can be described and correlated in a very natural way in terms of crystal stability. A formula relating the dispersion frequency of the anomalous dielectric constant to the spontaneous polarization and the displacements of the atoms in a ferroelectric transition, previously derived for barium titanate, is generalized. The transitions in KH₂PO₄ and in NH₄H₂PO₄, and the ordering of the hydrogen atoms in these crystals, are discussed qualitatively in terms of the stability of normal modes of vibration.     

Infrared dielectric dispersion of ZnGeP2 and CdGeP2

The polarised i.r. reflectivity spectra of single crystals of the non-linear chalcopyrite materials ZnGeP₂ and CdGeP₂ have been measured over the spectral range 40–700 cm^?1. Analysis of the spectra using classical dispersion theory and the Kramers-Krönig technique has yielded, in each material, five modes with frequencies between 80 and 385 cm^?1 with the polarisation perpendicular to the optic axis. Parallel to the optic axis, two modes have been observed with frequencies between 300 and 400 cm^?1. The phonon dispersion curves of the isoelectronic analogue GaP have been used, together with a group theoretical analysis, to assign specific atomic displacements to the observed modes.     

New scintillators based on barium fluoride crystals and ceramics

Experimental studies on the optical properties of new crystals and ceramics based on CeF₃- and CdF₂-doped barium fluoride were aimed at producing scintillators with a higher light yield level (particularly in the ultra-short wavelength range with a luminescence time of 0.6–0.8 ns) that could be used in the development of fast and low-cost optical sensors for positron emission tomography (PET). Ceramics manufactured on the basis of BaF₂ single crystals with various concentrations of Ce and Cd are transparent in the 300–600 nm wavelength range; this is comparable to BaF₂ single crystals.     

Frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals

We report on frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals (LiIO₃). The conversion coefficient and the matching angle were measured as a function of the fundamental wavelength in the range 586.3–624 nm. For Type I phasematching the matching angle _m=90° for noncritical matching was found at 586.3 nm. At this wavelength the conversion coefficient reaches the maximum value which was observed to be as high as 1.6×10^–2 W^–1 for a LiIO₃ crystal of 1 cm length. From the dependence of the matching angle on the wavelength the refractive induces were determined for the ordinary as well as for the extraordinary ray in the wavelength range from 293 nm up to 355 nm. The influence of focussing on the conversion coefficient is discussed.     

An initial study of the growth and defect analysis on hydrated 2-benzoyl benzoic acid crystal

In the present study, single crystals of hydrated 2-benzoyl benzoic acid were grown by the slow evaporation solution growth method. Single crystal and powder X-ray diffraction studies confirm the triclinic structure of the grown crystals. The defect in crystalline perfection has been defined by rocking curve analysis. The FTIR spectrum of the crystal displays carbonyl stretching of the carboxylic group at 1685 cm^?1 and of keto group at 1661 cm^?1. It shows a merged medium band around 3450 cm^?1 due to OH stretching and does not distinguish clearly between OH of COOH and H₂O held up in the crystal. The existence of additional bands appears at 1590 cm^?1 due to vibration of H₂O in the compound. The photoluminescence excitation studies were carried out for the grown crystals for wide wavelength range between 300 nm and 700 nm at room temperature. The range of percentage of optical absorption of the crystal has been ascertained by UV?vis spectral characterization. The mechanical strength has been determined by Vickers's microhardness measurement for different loads with constant dwell time.     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

Spectroscopic studies of Nd and Er in KGd(WO4)2 single crystals

Single crystals of KGd(WO₄)₂ doped with Nd³⁺ and Er³⁺ ions were grown by the top seeded solution growth method. Detailed analysis of the room-temperature absorption spectra was performed using the conventional Judd-Ofelt theory and actual dependence of the refractive index on the wavelength. In addition to the intensity parameters Ω_2,Ω_4,Ω₆, the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region. Those transitions which can be potentially used for laser applications have been identified for both ions.