Generation and application of high power femtosecond pulses in the vibrational fingerprint region

We present a novel high power femtosecond infrared laser source, based on a three-stage chirped-pulse amplification scheme. Owing to the high power output of the Ti:sapphire amplifiers, it becomes routinely possible to produce femtosecond infrared laser pulses in the wavelength region of 2.6–20 μm with minimum pulse energies of 15 μJ, to our knowledge roughly an improvement of an order of magnitude. With such pulses we have performed femtosecond second-order nonlinear optical surface spectroscopy in the fingerprint region. We have probed the skeletal modes of the first few monolayers of a polymer/air interface in a femtosecond vibrational sum frequency generation experiment. This development opens up new possibilities to investigate surface structures and dynamics of, e.g., organo-metallic compounds, proteins, and peptides. PACS 42.65.-k; 68.35.-Ja; 82.35.Ps     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm     

Femtosecond laser direct writing of embedded optical waveguides in aluminosilicate glass

Using tightly focused femtosecond laser pulses to irradiate lines in aluminosilicate glass, embedded lines with increased refractive index, which function as optical waveguides were observed. The pulse energy (4.5–11.2 μJ) and writing speed (50–700 μm/s) were shown to affect the resultant optical properties of the waveguides such as the magnitude of refractive index change, core diameter and propagation mode. At pulse energies above 5 μJ, two types of structures were observed, namely an inhomogeneous void-like structure and a cross-sectional crack-like structure. These structures were found to affect significantly the resultant waveguiding properties of the irradiated lines. Using pulse energy of 5 μJ or below, single mode waveguides were fabricated. Raman spectroscopy showed that the fs laser pulses generated structural changes to the aluminosilicate glass. The fabrication of a 1×4 splitter was also demonstrated. PACS 42.62.-b; 42.82.-m; 81.05.Kf     

A CW seeded femtosecond optical parametric amplifer

A CW seeded femtosecond optical parametric amplifier, a compromise between OPG/OPA and strongly seeded OPA configurations, has been demonstrated. An independent CW Nd: YAG laser with sub-watt power decreases the pump threshold of the OPA and improves the stability of the output pulses considerably. Output signal energies as high as 30 μJ with pulse durations less than 250 fs near 1 μm are reported. from this two-stage device.     

$90^{\circ}$ phase-matched parametric frequency conversion in AgGa1-xInxS2

AgGa_1-xIn_xS₂ with x=0.14±0.01 was found to be 90° phase-matchable for type-I difference-frequency generation (DFG) by mixing the dual-wavelength pulses emitted from an electronically tuned Ti:sapphire laser. Infrared radiation continuously tunable over the range of 4.80–6.98 μm was generated by independently varying the two wavelengths in the 705–932 nm spectral range, and 4.04 μm radiation by mixing a Nd:YAG laser with the Ti:sapphire laser. In addition, this material was found to be noncritically phase-matchable for the second harmonic generation (SHG) of CO₂ laser radiation at 10.591 μm at 203 °C. Sellmeier equations that reproduce well these experimental data are presented. PACS  42.65.-k; 42.65.ky     

The generation of intense, transform-limited laser pulses with tunable duration from 6 to 30 fs in a differentially pumped hollow fibre

We have investigated experimentally the energy transmission and spectral broadening of 30-fs, 700-μJ laser pulses in a neon-filled, 250-μm inner diameter hollow fibre. We implement a differentially pumped fibre, where a vacuum is maintained at the fibre entrance, and compare this to a statically filled fibre. We obtain significantly higher transmission and increased spectral broadening in the differentially pumped case due to a reduction of ionisation defocusing at the fibre entrance. This arrangement provides a method for the generation of near-transform- limited pulses with smoothly varying pulse duration whilst maintaining constant pulse energy, by simple adjustment of the gas pressure. Compression of ∼450-μJ pulses from the differentially pumped fibre to a duration of 6.5 fs has been achieved for pulses with spectra spanning 650–900 nm, by use of negatively dispersive chirped mirrors. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Investigation of time characteristics of photodetectors based on Ge/Si nanoheterostructures

Results of investigations into the time characteristics of photosensitive layers based on Ge/Si nanoheterostructures excited by femtosecond laser pulses with a wavelength of 1.55 μm are given. It is demonstrated that the leading front duration of the photoresponse pulse for the examined specimens excited by laser pulses of 120 fs duration does not exceed 30–40 пs.     

Pulsed optical parametric oscillators with intracavity optical parametric amplification: a critical study

It is known that the idler conversion efficiency of optical parametric oscillators (OPOs) can be increased by adding a second nonlinear crystal in the cavity. This crystal is pumped by the signal and acts as an optical parametric amplifier (OPA) for the idler. However, this technique unavoidably increases the oscillation threshold because of additional losses and increased build-up time due to cavity lengthening. In this paper, we investigate both theoretically and experimentally the benefits and drawbacks of this so called OPO–OPA configuration versus the singly resonant OPO (SRO) configuration. Calculations are found to be in agreement with an experimental study of a SRO and an OPO–OPA operating near 3.4 μm both pumped by a 90-mJ 27-ns Nd:YAG laser. Our study reveals that the OPO–OPA needs to be driven at least two times above threshold to produce more idler energy than the SRO. In addition, near 3 μm the OPO–OPA is particularly efficient given that the difference frequency wave generated in the second crystal is also output coupled. PACS 42.65.Yj; 42.65.Sf     

Deep drilling on a silicon plate with a femtosecond laser: experiment and model analysis

The ablation rate when drilling fine holes having large aspect ratios in silicon substrates with femtosecond laser pulses was estimated from mechanically ground cross sections of the ablated holes. The ablation rate shows a dramatic change at the depth at which the laser pulse reaches a certain fluence, which is nearly constant when the initial laser fluence was varied from 14.5 to 59.4 J/cm². The ablation rate, threshold fluence, in three fluence domains, and the transition fluences at which the ablation rate shows a dramatic change, were derived. However, when a pulse energy of 200 μJ was used a much greater ablation rate was obtained, suggesting that another fluence domain for larger ablation rates exists. The experimentally obtained hole depths as a function of shot numbers were reproduced by a theoretical model, which incorporates laser pulse attenuation in the holes that is the same as that in waveguides for some attenuation coefficient and ablation rates for three fluence domains. PACS 42.62.-b; 42.65.Re; 78.40.Fy; 78.47.+p; 81.20.Wk     

A comprehensive study of Nd:YAG,Nd:YAlO<Subscript>3</Subscript>, Nd:YVO<Subscript>4</Subscript> and Nd:YGdVO<Subscript>4</Subscript> lasers operating at wavelengths of 0.9 and 1.3 μm. Part 2: passively mode locked-operation

This paper reports on the generation of picosecond (ps) laser pulses by self-phase-adjusting additive-pulse-mode-locking (PSA) at wavelengths of 0.9 and 1.3 μm. The main objective of this work was to investigate and compare the characteristic optical properties of ps lasers based on different Nd-doped laser crystals like Nd:YAG, Nd:YAlO₃, Nd:YVO₄ and Nd:GdVO₄. As a result of these investigations a mode-locked Nd:YVO₄ laser for example, generated, ps pulses at 1.3 μm with a duration of 7 ps, a repetition rate of 160 MHz and an average power of 4.7 W. At 0.9 μm pulses with a duration of 1.9 ps were obtained at a repetition rate of 158 MHz and an average power of 2.8 W. PACS  42.70.Hj; 42.65.Re; 42.65.Ky     

Surface relief diffraction gratings written on <Emphasis Type="BoldItalic">β</Emphasis> -BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal by femtosecond pulses

Surface relief diffraction gratings were written at the entrance surface of a β-BaB₂O₄ (BBO) crystal under irradiation with femtosecond laser pulses. Probe–beam diffraction and atomic force microscopy (AFM) were employed to characterize the diffraction properties and the microstructures of the gratings. The maximal first-order diffraction efficiency of these gratings is approximately 63%. The surface periodic structure of the material induces noncollinear propagation of the fundamental input beam in the crystal. Using this diffractive optical element, efficient generation of multiple second-harmonic beams was achieved. PACS 42.79.Dj; 42.40.Lx; 42.65.–k     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

Temperature-dependent Sellmeier equation in the MIR for the extraordinary refractive index of 5% MgO doped congruent LiNbO<Subscript>3</Subscript>

We present a temperature-dependent Sellmeier equation for the extraordinary refractive index of 5 mol % MgO doped congruent lithium niobate. This equation is adapted for wavelengths in the range of 1.3–5 μm and temperatures between 40 °C and 200 °C. The calculation of the appropriate Sellmeier coefficients is based on the wavelengths of the signal and idler radiation measured for quasi-phase-matched optical parametric generators excited by 10-ns-long, 1064-nm pulses of a Q-switched Nd:YVO₄ laser. PACS 42.65.Ky; 42.70.Mp; 42.65.Yj     

Mode-locked and Q-switched laser operation of the Yb-doped Li<Subscript>6</Subscript>Y(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystal

We report for the first time pulsed laser operation of the Yb-doped Li₆Y(BO₃)₃ (Yb-LYB) crystal in the nanosecond as well as in the femtosecond regime. Pulse durations as short as 355 fs have been obtained in passively mode-locked operation and pulse energies up to 140 μJ in Q-switched operation. PACS 42.70Hj; 42.55Xi; 42.60Gd; 42.65Re     

Filamentary cavity formation in poly(methyl methacrylate) by single femtosecond pulse

A single femtosecond laser pulse creates a filamentary structural change along the optical axis inside bulk poly(methyl methacrylate). The filamentary structural change was revealed to be a cylindrical cavity based on scanning electron microscope examination, the presence of capillary action, and analysis of diffraction by an embedded diffraction grating. The cavity had a diameter of 0.8 μm and a length of 125 μm. PACS 42.65.Jx; 42.70.Jk     

Fabrication of polaritonic structures in LiNbO<Subscript>3</Subscript> and LiTaO<Subscript>3</Subscript> using femtosecond laser machining

Fabrication of patterned materials in ferroelectric LiNbO₃ and LiTaO₃ crystals using femtosecond laser micromachining is presented and discussed. Damage feature sizes in the 10–100 μm range were achieved using 800-nm, 50-fs (FWHM) ultra-fast laser pulses with energies ranging from 10 μJ up to 350 μJ. Fabrication of polaritonic devices such as waveguides, resonators, focusing reflectors, diffractive and dispersive elements, photonic band gap materials, and other microstructures is demonstrated. PACS 77.84.Dy; 42.62.Cf; 71.36.+c     

A subnanosecond pulsed laser-source for mid-infrared LIDAR

A new subnanosecond pulsed laser-source for mid-infrared (MIR) LIDARbased on difference-frequency generation (DFG) in periodically poled LiNbO₃ (PPLN) is developed. An output pulse energy of up to 36 pJ is achieved by amplification of the pump and signal laser. A pulse duration of Δτ=538 ps has been measured using silver thiogallate as a non-linear device for an upconversion cross correlation experiment. The mid-infrared pulse duration corresponds to a spatial resolution of Δs≈8 cm in the LIDAR system. PACS 07.07.Df; 42.60.Da; 42.65.Ky     

Pulse-parameter dependence of the configuration characteristics of a micro-structure in fused SiO2 induced by femtosecond laser pulses

The fabrication of a micro-structures in fused SiO₂ by femtosecond laser pulses was studied in the range of pulse duration 150∼500 fs, pulse energy 60 nJ to ∼7 μJ and wavelengths 400 nm and 800 nm. The characteristics of the cross section of the micro-structure were explained with the multiphoton-ionization and avalanche-ionization processes in the femtosecond laser-induced breakdown. The characteristics of the longitudinal length of the micro-structure relating to the direct-focusing and self-action effects are also discussed. Received: 10 January 2001 / Accepted: 7 April 2001 / Published online: 23 May 2001     

基于单个BBO晶体载波包络相位稳定的高效率光参量放大器

基于单个BBO非线性晶体,利用非共线光参量放大技术,研究了载波包络相位稳定的高效率可调谐近红外脉冲产生.以载波包络相位稳定的飞秒激光放大系统产生的白光作为种子光,注入一个二类匹配的二级光参量放大器,在1350 nm波段获得抽运-信号光34%的转换效率.利用f—2f光谱相干测量技术,放大脉冲载波包络相位的抖动30 min内小于137 mrad.该方法提供了一种简单高效的载波包络相位稳定的红外脉冲产生技术.