Femtosecond optical parametric oscillators based on periodically poled lithium niobate

Abstract

We describe two configurations of collinearly pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate and tunable in the infrared from 975 nm to 4.98 μm. Maximum output powers of 240 mW for the signal and 106 mW for the idler were recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 46% at a wavelength of 1.04 μm and 70% at 1.1 μm were measured. Interferometric autocorrelations of the signal and idler pulses at various wavelengths within the tuning range have been obtained and imply nearly transform-limited pulse durations of about 140fs for the signal and about 190fs for the idler.     

High-power,high-repetition-rate picosecond optical parametric oscillators for the near-to mid-infrared

Abstract

We report high-repetition-rate, singly-resonant, picosecond optical parametric oscillators based on the nonlinear crystals LiB₃O₅ and KTiOAsO₄ which are synchronously pumped by a self-mode-locked Ti:sapphire laser operating at 81 MHz. These devices allow tunable pulse generation from 1·116-3·160 μm to be achieved. The LiB₃O₅ system produces average nearinfrared output powers of 325 mW and is continuous tuning over the wavelength range 1·16-2·26 μm. For 1·8 ps input pump pulses, transform-limited signal pulses with durations of 1-1·2 ps and idler pulses with durations of 2-2·2 ps have been generated over 1·2-2·2 μm, without requirement for dispersion compensation. The KTiOAsO₄ system produces average near-infrared output powers of 403 mW, with the signal tuning over 1·116-1·281 μm and idler tuning over 2·260-3·160 μm. Without dispersion compensation, signal (idler) pulses with durations between 1·01-1·03 (1·61-2·91) ps have been obtained for 1·2 ps input pump pulses.     

A comparative study of borided pure niobium, tungsten and chromium

Pure niobium (Nb), tungsten (W) and chromium (Cr) were boronized at 940 °C for 2, 4 and 8 h. The borided samples were characterized by X-ray diffraction, Scanning electron microscope and microhardness tests. Tribological investigation was conducted. X-ray study showed the presence of NbB₂, WB, and CrB. The hardnesses of boride layers formed on the pure Nb, W and Cr were 2500, 2500 and 1700 HV, respectively, whereas the hardnesses of the pure Nb, pure W and pure Cr were 110, 445 and 115 HV, respectively. Nb boride layers ranged in thickness from 8 to 22 μm, whereas W boride layers ranged in thickness from 10 to 42 μm, and the thickness of Cr boride layer varied from 4 to 12 μm with boronizing time. The boriding of W resulted in thicker boride layer compared to the boriding of Nb and Cr at given time. The frictional behaviour and wear mechanicms differ in modes and scales.     

UV-curable ZnS/polymer nanocomposite for replication of micron and submicron features

In view of the wide interest in high refractive index polymers for microreplication, study was made of UV-curable high refractive index nanocomposite material for microreplication purposes. The refractive index of the nanocomposite was tailored through the addition of surface-modified ZnS nanoparticles to commercial ORMOCOMP^® inorganic–organic hybrid polymer. The refractive index of ORMOCOMP^® was increased linearly from 1.514 (620 nm) to 1.645 (620 nm) by embedding of the nanoparticles (18.6 V%). The nanocomposite showed excellent transparency (T = 89–92%), and increase in the nanoparticle loading shifted the absorption edge from 380 nm to 420 nm. Low scattering of transmitted light (determined by UV–VIS–NIR spectrophotometry) and high dispersion of ZnS (determined by scanning electron microscopy with energy dispersive X-ray spectrometry and transmission electron microscopy) indicated low aggregation of the ZnS nanoparticles. Finally, the nanocomposite was applied to micromolding in capillaries to replicate micrometer-size channels (8 μm × 1.5 μm) with Bragg gratings (period 520 nm and depth 400 nm) on top of the channels. Based on the AFM results the MIMIC molding method was found to be suitable for the replication of microchannels into nanocomposite material.     

TiO2 nanotubes from stirred glycerol/NH4F electrolyte: Roughness, wetting behavior and adhesion for implant applications

Titanium oxide (TiO₂) was anodically formed on titanium from non-aqueous electrolyte containing glycerol and 0.5 wt.% ammonium fluoride (NH₄F). Oxidation was carried out for 30, 60, 120 and 240 min at potentiostatic 30 V with the bath being stirred using magnetic pellet. All the conditions produced amorphous nanotubes. They had an average diameter of 50–130 nm and length in the range of 1.2–1.9 μm. The porosity was in the range of 70–80%. Stirring of the glycerol-based electrolyte has proved to be advantageous in retaining the tubular structure and providing smooth tubes even at 30 V condition.The coatings had surface roughness Ra lower than 0.5 μm, water wetting angles in the range of 58–84°. Increasing pore diameters increased the water wetting angles. All the coatings invariably showed poor tensile pull-off adhesion strengths. This poor adhesion is attributed to the stirring of the electrolyte.     

Multiple-wavelength quasi-phase-matching for efficient idler generation in MgO:LiNbO3 based nanosecond optical parametric oscillator

We present numerical results for optimization of the overall idler conversion efficiency of a nanosecond optical parametric oscillator (OPO), wherein the signal generated in the OPO process is also used as the pump for a difference frequency generation (DFG) process in a quasi-periodic MgO:LiNbO(3) crystal. The phase-matching conditions are considered such that the generated idler frequencies in both the processes (i.e., OPO and DFG) coincide. Optimization for the idler generation has been performed with respect to the different parameters, such as input pump power, pump pulse duration, and the output coupler reflectivity, for quasi-phase-matched interaction in MgO:LiNbO(3). Wavelength of the pump, signal, and idler waves considered in the optimization are 1.064 μm, 1.456 μm, and 3.95 μm, respectively. A maximum overall idler generation efficiency of ≈33% could be obtained in the simultaneous OPO+DFG process for a pump pulse duration of 72 ns and output coupler reflectivity (R(s)) of 90%, whereas for the stand-alone OPO process, the maximum idler generation efficiency was found to be ≈15%. The optimization has been illustrated for an average pump power of 8 W at a pulse repetition frequency (PRF) of 10 kHz. This approach of simultaneous OPO+DFG process can be employed to significantly enhance the idler generation efficiency of nanosecond OPOs.     

Fluorescence property investigations on Er-doped oxyfluoride glass ceramics containing LaF3 nanocrystals

The fluorescence properties of oxyfluoride glass ceramics containing nanosized LaF₃ crystals with different ErF₃ doping level were investigated. The spectroscopy analysis indicated that a dominant fraction of ErF₃ had been incorporated into the crystal phase. Broad 1.5 μm emission spectra with full width at half maximum (FWHM) value ranging from about 40–100 nm were obtained, which increased accordingly with ErF₃ doping level. Noteworthily, intense green and red upconversion emissions from samples with high ErF₃ doping level were observed when excited even with a 30 mW diode laser at 976 nm. An overall increment of the upconversion emissions intensity, and, a relative increase in intensity of the red emission with respect to that of the green one were also identified with increasing ErF₃ concentration. The possible upconversion mechanisms were proposed.     

Highly efficient, widely tunable, 10-Hz parametric amplifier pumped by frequency-doubled femtosecond Ti:sapphire laser pulses

We report a 10-Hz, highly efficient, widely tunable (from the visible to the IR), broadband femtosecond optical parametric generator and optical parametric amplifier (OPA) in BBO, LBO, and CBO crystals pumped by the frequency-doubled output of a regeneratively amplified Ti:sapphire laser at 400 nm. The output of the system is continuously tunable from 440 nm to 2.5 mum with a maximum overall efficiency of ~25% at 670 nm and an optical conversion efficiency of more than 36% in the OPA stage. The effects of the seed beam energy, the type of the crystal and the crystal length, and the pumping energy of the output of the OPA, such as the optical efficiency, the bandwidth, the pulse duration, and the group velocity mismatch between the signal and the idler and between the seeder and the pump, are investigated. The results provide useful information for optimization of the design of the system.     

Effect of equal channel angular pressing and heat treatment on the microstructure of Cu–Al–Be–B shape memory alloy

The combination of equal channel angular pressing (ECAP) and heat treatment was carried out to modify the microstructure of a Cu–Al–Be–B shape memory alloy. Microstructures of the alloy after ECAP and subsequent quenching were investigated by optical microscopy and X-ray diffraction (XRD). The alloy with 8 passes of ECAP at 743 K is characterized with ultra-fine grains (~ 2 μm), but with smaller fraction of martensites which implies the lower shape memory effect (SME). After reheated at 873 K and oil-quenched to room temperature, the grains become coarsen (~ 50 μm) but still finer than that of as-received (100–300 μm), and the fraction and order of martensites were increased simultaneously.     

3D characterization of open porous vacuum plasma sprayed titanium coatings by means of high resolution micro computer tomography

Open porous Ti coatings were applied by vacuum plasma spraying. The overall porosity was determined by light microscopy on cross-section cuts as well as micro computer tomography (μ-CT). μ-CT was additionally used on detached coatings to characterize pore size and interconnectivity as well as the sinter necks between individual Ti particles, as these are important factors for biomedical applications (tissue ingrowth) and coating integrity. The μ-CT measurements with a resolution of 5 μm voxels and the subsequent data treatment showed an excellent pore connectivity and yielded an average pore size of ~ 80–140 µm, pore connection diameter of ~ 50 µm and Ti sinter neck diameter of ~ 30–40 µm.     

Compact KTA-based intracavity optical parametric oscillator driven by a passively Q-switched Nd:GdVO4 laser

We present a compact and efficient KTA-based intracavity optical parametric oscillator (IOPO) driven by a diode-end-pumped Nd:GdVO(4)/Cr:YAG passively Q-switched laser. At the incident diode pump power of 9.2 W, signal (1.53 microm) and idler (3.47 microm) average output powers of up to 744 and 356 mW, respectively, have been obtained. The total (signal+idler) optical-to-optical conversion efficiency is as high as 12%. By using the knife-edge method, near-diffraction-limited signal and idler beams have been detected, and the M(2) factors are well within 1.2. In addition, based on the ABCD matrix theory, the impact of mode matching and the thermal lens effect on the OPO output have been analyzed.     

Tunable continuous-wave doubly resonant optical parametric oscillator by use of a semimonolithic KTP crystal

A temperature-tuned continuous-wave doubly resonant optical parametric oscillator (OPO) consisting of a semimonolithic KTP crystal and a concave mirror has been designed and built. Under single-axial-mode-pair operation, we obtained a combined output power of the signal and idler light fields up to 365 mW at a pump power of 680 mW. The output wavelength of the OPO can be temperature tuned by as much as 9 nm. We achieved 2.8-GHz continuous frequency tuning of the OPO by tuning the pump laser frequency.     

Processing and microstructure of particle stabilized silica foams

Silica foams containing ~ 85% porosity and with different shapes and sizes have been prepared by air entrainment in suspensions of hydrophobized silica + alumina (5 wt.%) powder mixture in aqueous solution of isopropanol and binders, followed by casting, drying and sintering. The silica powder with surfaces modified by presence of long chain amphiphilic molecules has been used successfully for stabilization of air bubbles in suspensions, so that their disproportionation and collapse could not be observed even after 4 weeks. Microstructural examination using optical and scanning electron microscopy as well as measurements by mercury porosimetry has shown trimodal pore size distribution with fine (4–10 μm), medium (50–100 μm) and coarse (~ 680 μm) pores.     

Processing of nanocrystalline 8 mol% yttria-stabilized zirconia by conventional, microwave-assisted and two-step sintering

Manufacturing of near full dense (>97%) 8 mol% yttria-stabilized zirconia (8YSZ) nanopowder (15–33 nm) compacts was manipulated using conventional sintering (CS), two-step sintering (TSS) and microwave-assisted sintering methods. Microwave firing was performed via two different heating rates, i.e. 5 and 50 °C min⁻¹. Although, the lower rate microwave sintering (LMS) was found to yield the higher densities at lower temperatures, this regime ultimately did not provide higher final densities compared to the other methods. The higher rate microwave sintering (HMS) on the other hand managed to suppress the accelerated grain growth and resulted to a finer microstructure (0.9 μm) than LMS (2.35 μm) and CS (2.14 μm). In spite of the great capability of TSS method in fabricating the specimens with ultra-fine grains (0.29 μm), microstructural inhomogeneity and the long total sintering time (>20 h) in comparison with HMS (29 min) set restrictions on the application of TSS method. Based on the effect of grain size on the mechanical properties of ceramics, the specimens produced by TSS exhibited higher fracture toughness (3.16 ± 0.06 MPa m^1/2) than those obtained from CS (1.61 ± 0.07 MPa m^1/2) and LMS (1.9 ± 0.09 MPa m^1/2), due to their finer grain size. The proximity in the fracture toughness values of TSS and HMS (3.17 ± 0.10 MPa m^1/2) samples stems from the higher microstructural homogeneity caused by HMS, while having a larger grain size.     

Diode-end-pumped passively Q-switched 1.34 μm Nd:Gd0.5Y0.5VO4 laser with Co:LMA saturable absorber

In this paper, the output performances at 1.34 μm in continuous wave operation and passive Q-switching regime of a diode-end-pumped Nd:Gd_0.5Y_0.5VO₄ laser have been investigated. The passive Q-switching regime was achieved with Co²⁺:LaMgAl₁₁O₁₉ (Co²⁺:LMA) saturable absorbers crystals. A maximum average output power of 230 mW was recorded with a Co²⁺:LMA with initial transmission of 81%. The minimum pulse duration was 116 ns, which corresponded to a repetition rate of 360 kHz, the single pulse energy of 2.1 μJ and the pulse peak power of 5.5 W.     

Compact, broadly tunable, mid-IR source for the spectroscopic investigation of molecular reference lines in the 27- to 33-THz range

We report on the improvement of a tunable, high resolution, diode laser-based, difference-frequency spectrometer using an AgGaS(2) nonlinear crystal. We use a type-II cut crystal as a part of the improvement compared with a type-I cut, which was used in our preliminary setup. The two tunable laser-diodes are operating near lambda(3)=778 nm (pump) and lambda(2)=842 nm (signal) with a sub-100-kHz linewidth. The high resolution spectrometer is being developed as an alternative to CO(2) laser spectrometers in the 9- to Il-mum range. Using a dual-arm cavity to enhance the two radiation powers, and with 35 mW in front of the 778-nm arm and 100 mW in front of the 842 nm arm, about 70 nW of the tunable 10-mum radiation are generated. This power level is enough to investigate the linear absorption spectroscopy of SF(6). Doppler-limited spectra over 2 GHz, are recorded, showing the wide continuous tunability of the spectrometer.     

20 W CW 589 nm sodium beacon excitation source for adaptive optical telescope applications

Operation of a doubly resonant sum-frequency generation (SFG) ring containing a lithium triborate (LBO) crystal has been demonstrated. It is pumped by 1.064 and 1.319 μm Nd:YAG injection-locked ring lasers and produces a 20 W, diffraction-limited, single-frequency, continuous wave, 589 nm beam. Nearly 60% conversion of pump laser power is obtained. Such a device produced a guidestar in the mesospheric sodium layer that can provide improved sky coverage for adaptive optical telescope applications. “First light” on the sky for the laser source was 21 November 2002 at the Starfire Optical Range, Kirtland AFB, New Mexico.     

Gas permeability of thin polyimide foils prepared by in-situ polymerisation

The entrance windows to the gas detector chambers as well as to the target containers used in high-energy and high-intensity accelerators must be as thin as possible to minimise energy losses of the particles used in astrophysics and nuclear physics studies. Because of their good physical properties, polyimide foils are often considered as suitable material for such windows, but commercially available foils, having a thickness greater than 7–8 μm (>1 mg/cm²), would cause energy losses of particles significant for some nuclear reactions studied. Foils prepared by in-situ polymerisation can, however, be as thin as 0.07 μm (10 μg/cm²). The permeability of 4 μm foils produced by in-situ polymerisation has been measured at room temperature for He and Ar. For He measurements were performed in the pressure range of 4–70 mbar and for Ar in the range of 20–140 mbar and the permeability was found to be in good agreement with the values published for the thicker commercial foils.     

Carbon micro-ribbon and strip polarimeter targets for the AGS and RHIC

Current techniques used to produce carbon micro-ribbon targets 5 μm wide×3.7–4.5 μg/cm²×25-mm long will be described. Developmental emphasis was to provide nearly identical micro-ribbons with the minimum number of atoms per unit of length, and to position them within ±0.5 mm of the desired location on C-shaped frames.The foil strip targets to be described were 200–600 μm wide×3.7–4.5 μg/cm²×51 mm long. These were produced from 25-mm-wide carbon film deposits that were scribed using a jig prior to dissolving the betaine/sucrose release agent under ethanol.Both types of targets required methods and devices that differed significantly from those reported previously for substrate texturing, masking, vacuum deposition, releasing from the substrate, and mounting. Sets of 12–24 of the targets have been made for the 2006 run period at BNL.     

The gain and noise figure of Yb-Er-codoped fiber amplifiers based on the temperature-dependent model

A novel temperature-dependent model for Yb³⁺-Er³⁺-codoped fiber amplifier (EYDFA) based on the energy transfer from Yb³⁺ to Er³⁺ is established. Using appropriate fiber and energy transfer parameters, the coupled rate equations is numerically solved at 25 and 40 °C. The pumping powers are 100 and 200 mW at a pump wavelength of 1060 nm. The signal gain and noise characteristics of a 0.3 m erbium/ytterbium co-doped fiber (EYDF) in a single-pass configuration are investigated by using 1, 10 and 100 μW signals at 1535 nm. A maximum signal gain of 40.5 dB and a corresponding noise figure of 3.65 dB at the temperature of 25 °C are achieved.