Analysis on dependence of phase matching angle on temperature in KDP crystal

The dependence of the phase matching (PM) angle on temperature in type-I second harmonic generation (SHG) and type-II third harmonic generation (THG) of KDP is experimentally investigated. The experimental results show that as the crystal temperatures vary by 1 °C, the external PM angles of the KDP doubler and tripler change 100 μrad and 220 μrad, respectively. The PM angle of KDP tripler is more sensitive to temperature variation than is that of the KDP doubler. Comparing the numerical calculations and the previous reports, a discrepancy has been found among the different reported results. Based on the analytical solution of the temperature dependence of the SHG process it is shown that a small error in the thermo-optic coefficients of the crystal is the main factor source of the discrepancy.     

60 W yellow laser at 561 nm by intracavity frequency doubling of a diode-pumped Q-switched Nd:YAG laser

A high-power yellow laser was achieved by intracavity frequency doubling of a diode-pumped Nd:YAG rod laser. A solid etalon was inserted into the resonator to implement efficient operation of the low-gain single line at 1123 nm transition. By using a LBO crystal inside the cavity as the frequency doubler, the maximum output power of the 561 nm yellow laser was obtained to be as high as 60.3 W with a pulse repetition frequency of 6 kHz, corresponding to an optical-to-optical conversion efficiency of about 6.1%. The output power fluctuation of the yellow laser was measured to be better than 3% in half an hour.     

大口径KDP晶体折射率非均匀性检测

KDP晶体的折射率不均匀性将导致光束的空间分布存在不同程度的相位失配,从而使得三倍频系统的转换效率下降。为了得到KDP晶体折射率非均匀性的高精度检测结果,基于正交偏振干涉法,采用ZYGO MST大口径干涉仪,测量得到了大口径KDP晶体折射率非均匀性分布,其测量精度达到10-7,并通过实验研究了晶体面形对测量结果的影响。对晶体e光折射率非均匀性的高精度检测,为大口径晶体材料生长工艺、加工工艺等改进和提高提供了定量的检测依据。     

A compact continuous-wave green laser with line beam for laser projection

An efficient and compact continuous-wave green laser with line beam using LBO crystal was developed. The maximum output power was 6.5 W. The optical-to-optical conversion efficiency was as high as 31%. The compact size of the line beam green laser was 2 cm×5 cm×8.8 cm. Reliability and stability of the green laser were evaluated.     

Pump tuned wide tunable noncritically phase-matched ZnGeP2 narrow line width optical parametric oscillator

A line tunable singly resonant noncritically phase matched narrow band width ZnGeP₂ (ZGP) optical parametric oscillator pumped by the output idler radiation from a KTA OPO based on a 20 mm long KTA crystal pumped from a Q-switched Gaussian shaped Nd:YAG laser beam with a grating having grooves density 85 lines/mm has been demonstrated in the spectral ranges of 3–7 μm. The measured threshold of oscillation energy was 10 μJ. The conversion efficiency was 20.5% and slope efficiency of the ZGP OPO was 20% using a 23 mm long ZGP crystal at 26 mm cavity length. Line width of the generated infrared radiation from ZGP OPO was 37–60 nm.     

High-temperature Raman spectra of KDP:Ni crystal

High-temperature Raman scattering measurements of KDP doped with Ni (1% weight) were performed over the spectral range 50–1200 cm^?1. The X-ray powder diffraction patterns taken at room temperature and analyzed by Rietveld refinement showed that doped samples of KDP:Ni have the same tetragonal structure of a pure KDP crystal, but with a contraction of the crystalline cell. The behavior of the high-temperature Raman spectra indicate that two phase transitions occur, one at 413 K and another one at 443 K. These phase transitions occur at temperatures below the thermal dehydratation point, 488 K. DTA and TGA measurements reveals that KDP:Ni and pure KDP have the same thermal dehydratation temperature.     

Submicron-scale spatial compression of light beam through two-stage photonic crystals spot-size converter

Photonic crystals spot-size converter that achieved the controlling of the spot-size through two-stage conversions was proposed. The pre-conversion depended on the efficient coupling between the high quality factor resonator and photonic crystal waveguide. Nearly unity transmission efficiency of the pre-conversion can be achieved through optimizing the radii of the rods located surrounding the resonator. Nanowire waveguide with width of 0.14 μm at a distance 1.05 μm from the resonator was introduced to realize the second stage conversion. Through two-stage conversions, the light beam width was converted to 0.16 μm. The transmission efficiency and conversion ratio reached to 94.6% and 14.875 respectively in theory.     

Dynamics of plasma formation and permanent structural transformation in ZBLAN excited by tightly focused femtosecond laser pulses

Time-resolved dynamics of plasma formation and bulk refractive-index modification in fluoride glass (ZBLAN) excited by a tightly focused femtosecond (130 fs) Ti:sapphire laser (λ_p=790 nm) was observed in situ. The femtosecond time-resolved pump–probe measurement with perpendicularly linear polarized beams was used to study the dynamics of both plasma formation and induced permanent structural transformation with refractive-index change. In the refractive-index domain, the lifetime of induced plasma formation is ～35 ps and structural transition time for forming the refractive-index change is ～80 ps. In the optical damage domain, however, the lifetime of induced plasma formation is ～40 ps and structural transition time for forming the optical damage is ～140 ps. We found that the process of refractive-index bulk modification is significantly different from that of optical cracks. From the diffraction efficiency of Kogelnik's coupled mode theory, the maximum value of refractive-index change (Δn) was estimated to be 1.3×10^?2. By the scanning of fluoride glass on the optical X–Y–Z stages, the fabrication of internal grating with refractive-index modification was demonstrated in fluoride glass using tightly focused femtosecond laser.     

KDP晶体折射率不均匀性对三倍频转换效率的影响

采用正交偏振干涉测量法获得了典型的快速(RG)和传统(CG)生长的KDP晶体折射率空间分布,数值模拟了倍频晶体固定失谐角分别为0和220 mrad时晶体折射率不均匀性对高功率三倍频光转换效率的影响。结果表明：快速生长晶体的折射率不均匀性的均方根约为传统生长晶体的6倍;三倍频转换效率在低功率密度下对折射率不均匀性不敏感,在高功率密度下尤其是转换效率较高时很敏感;当混频过程中的二倍频光不过剩时,在晶体折射率变化对三倍频效率的影响方面,倍频晶体比混频晶体严重;目前国产的传统生长晶体可以满足高功率三倍频实验要求。     

Synthesis and characterisation of glycine sodium nitrite crystals having non linear optical behaviour

Glycine sodium nitrite (GSNi) having non linear optical efficiency more than KDP crystal has been obtained from solutions by slow cooling technique. The crystals are found to be chemically and thermally stable with orthorhombic unit cell. The presence of wide transparency window in UV–Visible region makes GSNi suitable for opto-electronic device applications. It has second harmonic generation efficiency of 1.273 times that of standard KDP crystal. GSNi crystal belongs to soft category of materials with work hardening index of 2.33. Crystals introduced in timer circuit gave a stable output with the capacitance value of 236 pF. I–V characteristic indicates a safe operating range of 300 V/cm. GSNi is seen to behave as a normal dielectric with applied frequency and crystals exposed to light exhibit negative LDR characteristic which is attributable to space charge formation.     

Crystal growth,optical and thermal studies of nonlinear optical γ-glycine single crystal grown from lithium nitrate

Single crystals of the nonlinear material γ-glycine have been grown in the presence of lithium nitrate by slow-evaporation method. Structural characterization of the grown crystals was carried out by powder and single crystal X-ray diffraction (XRD) methods and it is observed that the samples crystallize in hexagonal system with non-centrosymmetric space groups. The modes of vibrations of different molecular groups present in glycine have been identified by spectral analyses. UV–visible transmittance study was performed to analyze optical transparency of γ-glycine crystals and found that the crystal was transparent in the entire visible-NIR region. Second harmonic generation (SHG) conversion efficiency has been estimated as 62 mV and the output power by the crystal was 1.72 times that of potassium dihydrogen phosphate (KDP) crystal. The thermal stability and decomposition of the sample have been studied by thermal analysis and it is observed that the γ-glycine crystal have good thermal stability.     

High-power eye-safe KTA-OPO driven by YVO4/Nd:YVO4 composite laser

We present a high-power 1.53 μm laser based on intracavity KTA-OPO driven by diode-end-pumped acousto-optical Q-switched YVO₄/Nd:YVO₄ composite. The composite crystal was utilized for reducing the thermal effect, and the mode mismatch compensating OPO cavity was designed for efficient OPO conversion. The output power of eye-safe laser at 1535 nm was up to 4.4 W with the pump power of 27 W, corresponding to a diode-to-signal conversion efficiency of 16.3%. To our knowledge, this is the highest output power in diode-end-pumped circumstances. In the experiment, the strong yellow light generated by Raman conversion and frequency doubling in the KTA crystal was observed.     

Spectroscopy and efficient laser emission of Yb3+: LuAG single crystal grown by μ-PD

A LuAG shaped rod crystal, doped with Yb³⁺, has been grown by μ-PD technique. The crystal diameter was about 3 mm and the length around 130 mm. A complete spectroscopic investigation in the temperature range 10–300 K is reported and data has been utilized to model the laser behavior. In the laser experiment the Yb:LuAG sample was placed in an X cavity and pumped longitudinally obtaining an efficient CW laser emission. The Yb:LuAG laser yielded a maximum output power of 23 mW with a slope efficiency of 32% and a threshold around 35 mW, at lasing wavelength of 1030 nm. No significant depolarization effects were observed, indicating a crystal growth with negligible stress. The output beam profile was investigated, yielding M² ≈ 1.0 in both directions, further confirming the good optical quality of the sample.     

KDP晶体折射率不均匀性对三倍频转换效率的影响

 采用正交偏振干涉测量法获得了典型的快速(RG)和传统(CG)生长的KDP晶体折射率空间分布，数值模拟了倍频晶体固定失谐角分别为0和220 mrad时晶体折射率不均匀性对高功率三倍频光转换效率的影响。结果表明：快速生长晶体的折射率不均匀性的均方根约为传统生长晶体的6倍；三倍频转换效率在低功率密度下对折射率不均匀性不敏感，在高功率密度下尤其是转换效率较高时很敏感；当混频过程中的二倍频光不过剩时，在晶体折射率变化对三倍频效率的影响方面，倍频晶体比混频晶体严重；目前国产的传统生长晶体可以满足高功率三倍频实验要求。     

Synthesis,spectral and thermal characterization of Bis(diethylammonium) tetrachloromercurate(II)-An nonlinear optical material

A new semiorganic compound, bis(diethylammonium) tetrachloromercurate(II) was grown by slow evaporation solution growth technique at ambient temperature from its aqueous solution. The crystal system and the cell parameters have been identified from the powder X-ray diffraction pattern. The UV–visible absorption of the compound shows absorption at 246 nm and there is absorption observed in the entire visible region indicates that the compound can be used as a nonlinear optical material. The UV–visible transmittance spectrum of grown crystal shows a lower cut-off wavelength at 275 nm and it was found that the crystal is suitable for optoelectronic applications. Thermal studies were carried out to find out the thermal stability and confirm the stoichiometric ratio of the compound. The thermal anomalies in DSC study indicate the occurrence of first order transition in the compound at low temperatures. The FTIR spectrum of the compound characterizes various functional groups. The various kinds of protons and carbons were assigned through NMR (¹H and ¹³C) spectroscopic techniques. The SHG efficiency of the compound was studied by Kurtz-Perry power technique and observed that it has SHG efficiency 1.5 times greater than that of potassium dihydrogen phosphate (KDP). The dielectric constant and dielectric loss of the compound decreases with increase in frequency.     

A high-power continuous-wave diffraction-limited Nd:GdVO4 laser operating at 1.34 μm

High-power and high beam quality continuous-wave (CW) Nd:GdVO₄ lasers operating at 1.34 μm were experimentally demonstrated. The lasers consisted of either one or two crystals, which were both end-pumped by high-power fiber-coupled diode lasers. With one crystal, the maximum CW output power generated was 8.4 W. When two crystals were used, a maximum output power of 15.7 W was achieved with the incident pump power of 76.2 W, showing a slope efficiency of 26.2% and an optical-to-optical efficiency of 20.6%. The beam divergence at an output power of 15 W was measured to be about two times that of the diffraction limit.     

High efficiency diode-pumped slab oscillator and amplifier for space-based application

The design and performance of conduction-cooled, laser diode-pumped oscillator and amplifier slab laser featuring high efficiency, high pulse energy and high beam quality for space-based application are reported. The oscillator was a diode-pumped Q-Switched Nd:YAG slab laser using unstable resonator, and the amplifiers were two zig-zag Nd:YAG slabs based on a side-pumped slab geometry. A near diffraction-limited output of 450 mJ in a 10 ns pulse at a repetition rate of 20 Hz was obtained, corresponding to an optical-to-optical conversion efficiency of over 20%.     

Dual-wavelength laser polarimeter and its performance capabilities

A dual-wavelength optical polarimetric approach has been proposed as a means of elimination of the systematic errors and estimation of the optical anisotropy parameters for a single DKDP crystal. Our HAUP-related polarimeter uses two semiconductor lasers with the neighbouring wavelengths of 635 nm and 650 nm. Based on the temperature dependence analysis of small characteristic azimuths of light polarization with respect to the axis of the sample, we found the parameters of imperfections of polarization system. We acquired eigen waves ellipticities in a DKDP crystal and found perpendicular to the optic axis value of the optical rotatory power. Our results correlate positively with previously measured data for KDP crystals.     

Growth,structural, optical and mechanical studies on acid mixed glycine metal salt (GABN) crystal as potential NLO material

Transparent crystals of α-glycine with ammonium nitrate and barium nitrate (GABN) have been grown from aqueous solution by slow evaporation technique at room temperature. Crystals of size 11 × 7 × 4 mm³ have been obtained in about 3–4 weeks time. The solubility of GABN has been determined in water. The grown crystal belongs to orthorhombic system with cell parameters a = 7.317 A.U, b = 12.154 A.U and c = 5.468 A.U with a unit cell volume 486.35 (A.U)³. The presence of chemical components/groups has been identified by CHN, EDAX and NMR analysis. Comparative IR and Raman studies indicate a molecule with a lack of centre of symmetry. A wide transparency window useful for optoelectronic applications is indicated by the UV Studies. Using a Nd-YAG laser (1064 nm), the optical second harmonic generation (SHG) conversion efficiency of GABN is found to be 1.406 times of that of standard KDP. On exposure to light the GABN crystals are found to exhibit negative photoconductivity. I–V characteristics, SEM studies, dielectrics studies, and Vickers micro hardness measurement have been carried out.     

Effect of ammonium acetate addition on crystal growth,spectral, thermal and optical properties of glycine single crystals

Gamma glycine single crystal is a potential organic nonlinear optical (NLO) material. It has been grown from a mixture of aqueous solutions of glycine and ammonium acetate by slow evaporation solution growth technique at room temperature for the first time. Powder X-ray diffraction studies are carried out and the results are compared. CHN analysis confirms the non-inclusion of ammonium acetate species in to the solution. FTIR studies are performed to identify the presence of various functional groups in the grown crystal. Differential scanning calorimetry analysis (DSC) was carried out to study the thermal stability and phase transition of the grown crystal. The optical analysis shows that UV cut-off of γ-glycine is at 190 nm and it has a wide transparency window. The second harmonic generation relative efficiency of the grown crystals was observed to be 1.72 times that of potassium dihydrogen orthophosphate (KDP).