Generation and optical parametric amplification of picosecond supercontinuum

We have demonstrated the output characteristics of optical parametric amplification (OPA) seeded by a supercontinuum (SC) generated in deionized water excited by 1064 nm. The SC spectrum (from 426 to 943 nm) overlaps well with the tuning range of a 355 nm pumped OPA for both the signal and idler. The tunable range covers from 430 to 2035 nm, with a maximum overall energy conversion efficiency of 32% in the OPA stage. A FWHM bandwidth of 54 nm near the degeneracy point from 658 to 760 nm can be generated by using a collinear OPA. Chirping of the SC pulses is also investigated with the OPA technique. The time delay between the 430 nm component and the 567 nm component of the generated SC due to chirp is measured to be -10.72 ps, increasing almost linearly with the wavelength.     

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.     

All-solid-state, high-power, deep-UV laser system based on cascaded sum-frequency mixing in CsLiB6O10 crystals

We report on an efficient use of CsLiB(6)O(10) (CLBO) crystals employed for an all-solid-state deep-UV laser system operated at 5 kHz. We obtained greater than 3 W of UV radiation around 242 nm by mixing the 349-nm third harmonic of a Nd:YLF laser with the tunable output from a Ti:sapphire laser in a CLBO crystal. This UV radiation was subsequently mixed with the residual 1047-nm output from the Nd:YLF laser in a second CLBO crystal. The system produced 1.5 W of deep-UV radiation at 196.3 nm, which is, to our knowledge, the highest deep-UV power below 200 nm generated in a nonlinear optical crystal. Additionally, the bandwidth of both outputs was estimated to be less than 200 MHz.     

Nd:YAG-pumped periodically poled LiNbO3 optical parametric generator seeded with the narrowband output of a 532-nm pumped optical parametric generator

We present a simple scheme to generate a continuously tunable pulsed narrow-bandwidth infrared wave. An Nd:YAG-pumped periodically poled lithium niobate optical parametric generator (OPG) is seeded with the output of another OPG pumped by the second harmonic (0.532 microm) of the Nd:YAG laser. A tunable idler wave from the 0.532-microm pumped OPG, operated away from the degenerate point, provides a narrow linewidth seed source for the 1.064-microm pumped broadband OPG. Seeding forces the second OPG to operate in a narrowband operation comparable with that of the seed source. Linewidth of the YAG-pumped OPG is 5-25 nm, in the tuning range of 1.61-1.83 microm (signal), narrowed to 1.48-1.05 nm. Methods of further reduction of linewidth also have been discussed.     

Broadband Tuning of a Continuous-Wave, Doubly Resonant, Lithium Triborate Optical Parametric Oscillator from 791 to 1620 nm

We report the use of extreme broadband, high reflectivity >99.5%, optical parametric oscillator (OPO) cavity mirrors. A continuous-wave, doubly resonant, OPO demonstrated tuning over a range of 791-1620 nm with a single mirror set. Wavelength tuning was performed by temperature tuning the nonlinear material of lithium triborate. Narrow linewidth oscillation was confirmed throughout the tuning range, and threshold pump power increased gradually from 50 mW near the degeneracy to 800 mW at the tuning band edge in a double-pass pumping configuration.     

Wavelength dependence of repetitive-pulse laser-induced damage threshold in beta-BaB2O4

The dependence on wavelength of repetitive-pulse (10 Hz, 8-10 ns) laser-induced damage on beta barium metaborate (BBO) has been investigated. The thresholds of dielectric breakdown in bulk crystal have been found to be 0.3 GW/cm(2) at 266 nm, 0.9 GW/cm(2) at 355 nm, 2.3 GW/cm(2) at 532 nm, and 4.5 GW/cm(2) at 1064 nm. Results indicate two-photon absorption at 266 and 355 nm, which helps to produce an avalanche effect that causes breakdown at each of the four wavelengths tested. Neither the BBO refractive indices nor the absorption spectrum change until breakdown occurs.     

Seeded optical parametric generator with efficiency-enhanced mid-wave infrared beam output

A novel device with a simple architecture for high-power mid-wave infrared beam generation is proposed and analyzed using a realistic model that takes the diffraction of the beams into account. The device is a seeded efficiency-enhanced optical parametric generator based on an aperiodically poled MgO-doped LiNbO₃ grating in which two optical parametric amplification (OPA) processes are simultaneously phase matched. When pumped by a high-repetition-rate nanosecond-pulsed laser operating at 1064 nm, power conversion efficiency enhancement of the mid-wave infrared output at a wavelength of 3.8 μm (compared to what is achievable with a single OPA process) occurs. Also, a difference-frequency beam is generated. Multiple aperiodic gratings with varying relative strengths of the two optical parametric amplification processes are designed. The developed model is used for determining the optimum relative strengths of the two processes and input pump power levels for achieving the maximum mid-wave infrared conversion efficiency and output power for various crystal lengths.     

Cavity ring-down spectroscopy for detection in liquid chromatography: extension to tunable sources and ultraviolet wavelengths

In earlier studies, it was demonstrated that the sensitivity of absorbance detection in liquid chromatography (LC) can be improved significantly by using cavity ring-down spectroscopy (CRDS). Thus far, CRDS experiments have been performed using visible laser light at fixed standard wavelengths, such as 532 nm. However, since by far most compounds of analytical interest absorb in the ultraviolet (UV), it is of utmost importance to develop UV-CRDS. In this study, as a first step towards the deep-UV region, LC separations with CRDS detection (using a previously described liquid-only cavity flow cell) at 457 and 355 nm are reported for standard mixtures of dyes and nitro-polyaromatic hydrocarbons (nitro-PAHs), respectively. For the measurements in the blue range a home-built optical parametric oscillator (OPO) system, tunable between 425 and 478 nm, was used, achieving a baseline noise of 2.7 x 10(-6) A.U. at 457 nm, improving upon the sensitivity of conventional absorbance detection (typically around 10(-4) A.U.). An enhancement of the sensitivity can be seen at 355 nm as well, but the improvement of the baseline noise (1.3 x 10(-5) A.U.) is much less pronounced. The sensitivity at 355 nm is limited by the quality of the UV-CRDS mirrors that are currently available: whereas the ring-down times as obtained at 457 nm are around 70-80 ns for the eluent, they are only 20-25 ns at 355 nm. Critical laser characteristics for LC-CRDS measurements, such as pulse length and mode structure, are given and prospects for going to shorter wavelengths are discussed.     

Second-order optical effects in seed-mediated grown palladium nanoparticles

Palladium nanoparticles deposited on SnO₂-doped In₂O₃ substrate show substantial optical second harmonic generation (SHG) in the spectral range (λ?=?120–160?nm) which is a part of the vacuum ultraviolet (VUV) spectrum. A single crystalline Li₂ B₄ O₇ optical parametric generator (OPG) pumped by nanosecond xenon–fluorine excimer laser (EMG 500/218 (Lambda Physics)) with the wavelength 218?nm, pulse duration about 6–8?ns; pulse rate about 80?hz, average pulse power about 0.2?MW and beam diameter varying within the 1.3–7.5?nm was used to form the fundamental beam. The OPG Li₂ B₄ O₇ single crystal was cut in the XZ optical plane. We have tuned the fundamental wavelengths within the 250–320?nm spectral range varying the angle of the plane with respect to the incident pumping beam. Maximal SHG output (in the reflected SHG geometry) is observed for the incident angles 75^○–80^○ with respect to the surface normal and p-polarized incident fundamental ultraviolet beams. Spectral separation between the vacuum ultraviolet (VUV) SHG intensities and the fundamental beams was performed using a VUV Seya-Numioka vacuum monochromator with spectral resolution 6?nm in the investigated spectral range. We have found that decreasing mean average palladium nanoparticle sizes favour substantial enhancement of the output SHG within the 120–160?nm spectral range. A layer of platinum nanoparticles coated on a layer of palladium nanoparticles suppresses the SHG effect indicating a quenching of the surface plasmon excitation originating from the palladium nanoparticles. The observed effect allows utilizing the palladium nanoparticles as an efficient material for frequency transformation of the UV nanosecond pulses (spectral range 240–310?nm) into the nanosecond laser pulses with wavelengths 120–160?nm.     

A novel optical device with wide-bandwidth wavelength conversionand an optical sampling experiment at 200 Gbit/s

This paper reports the characteristics of our proposed prototype optical parametric diffuser (OPD). An OPD is based on the theory of four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). However, to improve the conversion bandwidth and FWM efficiency, the gain bandwidth is spread and the gain peak wavelengths are set to a wavelength near the FWM light on the short-wavelength side by combining different MQW active layers. We measured the optical gain characteristics; the fiber-to fiber gain was 16.1 dB and the gain bandwidth over 8 dB was 117 nm when driven at 200 mA dc, and 190 nm when driven by an 800 mA pulse current. In a wavelength-conversion experiment, a high conversion efficiency of ⩾-20 dB was obtained across a detuning wavelength bandwidth of 43 nm. A clear waveform was obtained in an optical sampling experiment to measure 200 Gbit/s optical data sequences     

Laser-induced bulk damage in various types of vitreous silica at 1064, 532, 355, and 266 nm: evidence of different damage mechanisms between 266-nm and longer wavelengths

Laser-induced-damage thresholds (LIDT's) with various types of vitreous silica at 1064, 532, 355, and 266 nm are investigated. At 1064 nm no difference in the LIDT was observed in any sample. At 1064-355 nm the wavelength dependence of the LIDT of synthetic fused silica (SFS) can be described well by the relation I(th) = 1.45lambda(0.43), where I(th) is the LIDT in J/cm(2) and lambda is the wavelength in nanometers. At 266 nm, however, LIDT's were smaller than half of the calculated value from the relation above. This difference can be explained by the damage mechanism; at 266 nm two-photon absorption-induced defects lower the LIDT as in the case of KrF-excimer-laser-induced defects, whereas at longer wavelengths the two-photon process does not occur. LIDT's of fused quartz (FQ) at 532 and 355 nm and that of SFS containing ~1000 ppm of Cl and no OH at 355 nm were a little lower than those of the other SFS's. This lower LIDT may be related to the absorption of metallic impurities in FQ and dissolved Cl(2) molecules in SFS. At 266 nm, on the other hand, LIDT's of FQ's were higher than those of most SFS's.     

Development of a fiber-optic laser delivery system capable of delivering 213 and 266 nm pulsed Nd:YAG laser radiation for tissue ablation in a fluid environment

Ultraviolet (UV) lasers have the capability to precisely remove tissue via ablation; however, due to strong absorption of the applicable portion the UV spectrum, their surgical use is currently limited to extraocular applications at the air/tissue boundary. Here we report the development and characterization of a fiber-optic laser delivery system capable of outputting high-fluence UV laser pulses to internal tissue surfaces. The system has been developed with a view to intraocular surgical applications and has been demonstrated to ablate ocular tissue at the fluid/tissue boundary. The fifth (213?nm) and fourth(266?nm) harmonics of a Nd:YAG laser were launched into optical fibers using a hollow glass taper to concentrate the beam. Standard and modified silica/silica optical fibers were used, all commercially available. The available energy and fluence as a function of optical fiber length was evaluated and maximized. The maximum fluence available to ablate tissue was affected by the wavelength dependence of the fiber transmission; this maximum fluence was greater for 266?nm pulses (8.4?J/cm2) than for 213?nm pulses (1.4?J/cm2). The type of silica/silica optical fiber used did not affect the transmission efficiency of 266?nm pulses, but transmission of 213?nm pulses was significantly greater through modified silica/silica optical fiber. The optical fiber transmission efficiency of 213?nm pulses decreased as a function of number of pulses transmitted, whereas the transmission efficiency of 266?nm radiation was unchanged. Single pulses have been used to ablate fresh porcine ocular tissue. In summary, we report a method for delivering the fifth (213?nm) and fourth (266?nm) harmonics of a Nd:YAG laser to the surface of immersed tissue, the reliability and stability of the system has been characterized, and proof of concept via tissue ablation of porcine ocular tissue demonstrates the potential for the intraocular surgical application of this technique.     

Tunable high-power narrow-spectrum external-cavity diode laser at 675 nm as a pump source for UV generation

High-power narrow-spectrum diode laser systems based on tapered gain media in an external cavity are demonstrated at 675 nm. Two 2 mm long amplifiers are used, one with a 500 μm long ridge-waveguide section (device A), the other with a 750 μm long ridge-waveguide section (device B). Laser system A based on device A is tunable from 663 to 684 nm with output power higher than 0.55 W in the tuning range; as high as 1.25 W output power is obtained at 675.34 nm. The emission spectral bandwidth is less than 0.05 nm throughout the tuning range, and the beam quality factor M(2) is 2.07 at an output power of 1.0 W. Laser system B based on device B is tunable from 666 to 685 nm. As high as 1.05 W output power is obtained around 675.67 nm. The emission spectral bandwidth is less than 0.07 nm throughout the tuning range, and the beam quality factor M(2) is 1.13 at an output power of 0.93 W. Laser system B is used as a pump source for the generation of 337.6 nm UV light by single-pass frequency doubling in a bismuth triborate (BIBO) crystal. An output power of 109 μW UV light, corresponding to a conversion efficiency of 0.026% W(-1), is attained.     

Multimodal coherent anti-Stokes Raman spectroscopic imaging with a fiber optical parametric oscillator

We report on multimodal coherent anti-Stokes Raman scattering (CARS) imaging with a source composed of a femtosecond fiber laser and a photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). By switching between two PCFs with different zero dispersion wavelengths, a tunable signal beam from the FOPO covering the range from 840 to 930 nm was produced. By combining the femtosecond fiber laser and the FOPO output, simultaneous CARS imaging of a myelin sheath and two-photon excitation fluorescence imaging of a labeled axons in rat spinal cord have been demonstrated at the speed of 20 μs per pixel.     

Multisection optical parametric–Raman hybrid amplifier for terabit+ WDM systems

We demonstrate flat-gain wide bandwidth Raman-Fiber optical parametric hybrid amplifier for wavelength division multiplexed systems (WDM). Raman-parametric amplifiers exploit system non-linearities which are otherwise inevitable in evolving WDM systems. Investigations show that the pumps of parametric amplifier and Raman amplifier can be carefully selected at wavelengths, to give gain over complementary bandwidth regions, resulting in wide bandwidth with low gain ripple. Results show a flat gain of 24.3 dB for 12 × 100 Gbps WDM system with lowest ripple of less than 2.78 dB reported over 220 nm bandwidth for Raman-FOPA hybrid.     

Nanosecond pulsed laser deposition of TiO2: nanostructure and morphology of deposits and plasma diagnosis

Nanostructured TiO₂ films on Si (100) substrates have been grown by nanosecond pulsed laser deposition at the wavelengths of 266, 355 and 532 nm using a Q-switched Nd:YAG laser and TiO₂ sintered rutile targets. The effect of laser irradiation wavelength, the temperature of the substrate and the presence of O₂ as background gas on the crystallinity and surface structure of deposits were determined, together with the composition, expansion dynamics and thermodynamic parameters of the ablation plume. Deposits were analyzed by X-ray photoelectron spectroscopy, X-ray diffraction, environmental scanning electron microscopy and atomic force microscopy, while in situ monitoring of the plume was carried out and characterized with spectral, temporal and spatial resolution by optical emission spectroscopy. Stoichiometric, crystalline deposits, with nanostructured morphology were obtained at substrate temperatures above 600 °C. Microscopic particulates were found overimposed on the nanostructured films but their size and density were significantly reduced by operating at short wavelength (266 nm) and upon addition of a low pressure of oxygen (0.05 Pa). The dominant crystalline phase is rutile at 355 and 532 nm. At the short irradiation wavelength, 266 nm, the preferred phase in the presence of oxygen is rutile, while anatase is preferably observed under vacuum. The narrowest size distribution and smallest nanoparticle diameters, of around 25 nm, were found by deposition at 266 nm under 0.05 Pa of oxygen.     

An all solid-state narrow bandwidth optical parametric oscillator and its applications to the high resolution spectroscopy of free radicals

The paper describes the design, performance and illustrative spectroscopic applications of a high power, pulsed, injection seeded, narrow bandwidth and continuously tuneable optical parametric oscillator (OPO) based on two β-barium borate (BBO) nonlinear crystals. A ring cavity OPO was injection seeded with an external cavity diode laser in the spectral range 755–855 nm. The OPO generated high power transform limited nanosecond light pulses in this range and, simultaneously, at 606–667 nm. Spectral parameters of the OPO were investigated as a function of pump power for two different cavity lengths. A bandwidth of ～120 MHz (0.004 cm^-1) in the visible has been achieved with a 40-cm-long cavity. The potential of the OPO system is demonstrated by recording high resolution sub-Doppler spectra in the UV of PF and C₃ radicals in a pulsed supersonic jet expansion.     

Simple, high-performance type II ?-BaB 2 O 4 optical parametric oscillator

A visible /near-IR optical parametric oscillator (OPO) based on type II phase matching in ?-BaB2 O4 (BBO) is described. Pumped at 355 nm, this OPO covers 410 -2500 nm completely with a single set of standard Nd:YAG cavity optics. The output efficiency is >25 %, the linewidth of the OPO is narrower than 1 -2 cm-1 without the use of gratings or etalons, and the signal-beam divergence is <400 ?rad. Three type I BBO doubling crystals are used to extend the tuning range from 208 to 415 nm. Doubling efficiencies as high as 40 % are easily obtained. The reasons for the high doubling and overall system efficiency are discussed.     

The influence of laser wavelength on the structure,morphology, and photoluminescence properties of pulsed laser deposited CaS: Eu2+ thin films

Europium-doped calcium sulfide thin films were grown on Si (100) substrates using the pulsed laser deposition technique. The influence of the laser wavelength on the structure, morphology, and photoluminescent properties of the films was studied. The X-ray diffraction data showed that the films were amorphous, except for the (200) diffraction peak observed from the films deposited at the wavelengths of 266 and 355 nm. The atomic force microscopy and Rutherford backscattering spectroscopy data showed that the film deposited at 355 nm was rougher and thicker than those deposited at 266 and 532 nm. As a result, the highest photoluminescence emission intensity around 650 nm was observed from the film deposited at 355 nm. This emission was attributed to the 4f⁶5d¹ → 4f⁷Eu²⁺ transitions of Eu²⁺ observed. These films were evaluated for blue light-emitting diodes.     

Fast fabrication of silicon based microstructures using 355 nm UV laser

Abstract

In this study, a 355 nm UV Nd:YAG laser is used to process silicon wafers. In order to obtain microstructures with high aspect ratio, a dual prism optical system is set up to control the cutting linewidth of the UV laser beam. During the laser beam propagation through the prisms, the two prisms are rotated with the same angular velocity, which results in the focal spot of the laser beam moving in a circular path on the silicon substrates. When the laser beam moves relative to the holder (workstation), a laser cutting process can be carried out. With this laser system, the cutting linewidth is controllable ranging from 10 μm to 1 mm by adjusting the initial phase difference in the two prisms. The experimental results show that arbitrary shaped silicon based microstructures with high aspect ratio can be fabricated by this 355 nm UV laser system, and the aspect ratio over 10 can be obtained.