Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

在线超声检测系统中螺旋线圈换能器的应用

针对在线车轮超声检测系统的工程应用,研究了圆形螺旋线圈电磁超声换能器由洛伦兹力换能机制在钢中产生的辐射声场.给出了涡流、等效表面力源和辐射声场的计算方法,分析了圆形螺旋线圈换能器在钢试块上的辐射指向性.结果表明,辐射的圆周径向偏振横波为两边瓣中空指向性,对理解圆形螺旋线圈换能器的辐射声场和在重载货车车轮轮辋在线辋裂缺陷...     

Spectral-linewidth broadening in synchronous Raman fiber amplification caused by cross-phase modulation effects and its suppression

When coherent light is amplified by a synchronous Raman fiber amplifier pumped by output pulses from a high-power erbium-doped fiber amplifier operating in an unsaturated gain regime, the detection efficiency with the heterodyne method is degraded. This degradation is caused mainly by cross-phase modulation effects. It is confirmed that using power-flattened pump pulses can suppress these effects.     

Detection of shear picosecond acoustic pulses by transient femtosecond polarimetry

Femtosecond transient polarimetry has been applied for the detection picosecond acoustic pulses generated in a thin film of zinc oxide (ZnO) deposited on an anisotropic zinc crystal surface. The measurements of the variations of the azimutal and the ellipticity angles describing the ultrafast variations of the state of polarisation (SOP) of the reflected light have been obtained. These polarimetric signals, interpreted as a SOP path on the Poincaré sphere, provide a sensitive method to detect acoustic echoes, particularly for shear acoustic pulses.     

Multi-mJ carrier envelope phase stabilized few-cycle pulses generated by a tabletop laser system

A compact system for the generation of few-cycle multi-mJ Carrier Envelope Phase (CEP) stabilized pulses is presented. At the output 1.9?mJ, 5.7?fs pulses were achieved after hollow fiber compression (HFC) of 5?mJ, 25?fs circularly-polarized pulses from a Ti:sapphire multipass chirped pulse amplifier (CPA). Polarization control of the generated pulses was done using all reflective phase retarders which can be nearly arbitrarily scaled for increasing energies. The CEP noise from the amplifier system is shown to be 190?mrad rms over a period of more than 7?hours. The full system, i.e., oscillator, amplifier, CEP stabilization, and HFC is compact enough to fit on a standard optical table.     

Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier

An Yb:glass regenerative amplifier directly side pumped by four 20-W diodes is demonstrated. By use of a novel pumping scheme and introduction of cylindrical optics into the cavity, a free-running average output power as great as 4 W with a TEM(00) -like mode was achieved from the bare cavity, with a 0.56 pump duty cycle. When the regenerative amplifier injected, 1-mJ 200-fs FWHM pulses were obtained following compression by use of 2-ms pump pulses and up to a 150-Hz repetition rate.     

Spectral broadening of femtosecond pulses in an nonlinear optical fiber amplifier

A promising method for the generation of a supercontinuum with a high spectral power density based on the spectral broadening of ultrashort pulses in a fiber amplifier is considered. The advantage of the method, as compared to the conventional way of the supercontinuum generation in a microstructure fiber, is a lower pulse spectral broadening rate, which allows one to achieve higher SC spectral power densities. The initial stage of the supercontinuum generation in an ytterbium fiber amplifier (a fiber core diameter of 7 μm) with side pumping from an array of laser diodes with a total power of 8 W at a wavelength of 976 nm is experimentally studied. Yb:KYW laser pulses with a duration of 250 fs, a central wavelength of 1046 nm, and an average power of 150 mW have been supplied to the input of the amplifier. In this case spectrally broadened radiation with an average spectral power density of higher than 65 mW/nm and a spectrum width of 50 nm has been obtained.     

1-mJ, sub-5-fs carrier–envelope phase-locked pulses

We report the routine generation of sub-5-fs laser pulses with 1-mJ energy and stable carrier–envelope phase at 1-kHz repetition rate, obtained by compressing the multi-mJ output from a phase-locked Ti:sapphire amplifier in a rare-gas-filled hollow fiber. The dual-stage amplifier features a hybrid transmission grating/chirped mirror compressor providing 2.2-mJ, 26-fs pulses at 1 kHz with standard phase deviation of 190 mrad rms. We demonstrate hour-long phase stability without feedback control of grating position or rigorous control of the laser environment, simply by using small pulse stretching factors in the amplifier, which minimize the beam pathway in the compressor. The amplifier also integrates a versatile AOPDF (acousto-optic programmable dispersive filter) for closed-loop spectral phase optimization. The various factors influencing the overall phase stability of the system are discussed in detail. Using the optimized output, 1-mJ, 4.5-fs pulses are generated by seeding the neon gas filled hollow fiber with a circularly polarized input beam. A standard phase deviation of 230 mrad after the HCF is obtained by direct f-to-2f detection and slow-loop feedback to the oscillator locking electronics without any additional spectral broadening.     

Detection of scattered light pulses at femto-Joule level by using a picosecond BBO optical parametric amplifier

We report a technique for detecting weak scattered light pulses based on a 532 nm pumped picosecond β-barium-borate collinear optical parametric amplifier. The measured maximum slope gain factor of the amplifier was found to be around 10⁷, and the energy detection limit was of the order of fJ/pulse for the signal of 730 nm and the idler of 1.5 μm at a pumping intensity of 2.83 GW/cm². The linearity of the gain for this amplifier was found to be excellent for a seeding level of lower than 420 fJ per pulse. The maximum gains and the energy detection limits for the scattered light pulses from various scattering targets were found to be poorer than that from the reflective mirror, owing to the degraded beam quality and the depolarization of the scattered light. A reduction of the maximum gain for the scattered light with the increase of the angle of incidence, which causes pulse broadening and reduces the photon flux of the signal, was investigated. The feasibility of detecting weak scattered light in the infrared by using idler-to-signal frequency up-conversion was also demonstrated, in which the infrared seeder located in the idler branch of the amplifier was injected as the seeding beam and was then parametrically up-converted into the visible signal branch, with an even higher gain.     

Laser picosecond acoustics in isotropic and anisotropic materials

We present experimental results concerning the laser generation of picosecond acoustic pulses and their propagation in isotropic and anisotropic materials. We make use of a conventional reflectance detection technique as well as interferometric detection to probe the real and imaginary changes in reflectance. We also demonstrate the detection of transverse acoustic waves by mode conversion at an interface between an isotropic polycrystalline film and an anisotropic substrate.     

The analysis of adhesive bonds using electromagnetic acoustic transducers

The paper presented here outlines a technique for examining aerospace adhesive bonds using electromagnetic acoustic transducers (EMAT). The main restriction on the use of bonded structures is the lack of a reliable, applicable non-destructive test. Simple acoustic theory shows that a shear wave at normal incidence to an interface should be a more sensitive probe of interfacing coupling than a longitudinal wave. Conventional piezoelectric shear transducers require a very viscous couplant which makes scanning problematic. The EMAT described here consists of a pancake coil, and a permanent magnet behind the coil provides a static magnetic field normal to the surface of the sample and the plane of the coil. The EMATs used have the advantage of generating broadband radially polarized shear waves, while requiring no acoustic couplant. They are also comparable in size to typical piezoelectric transducers. The broadband nature of the transducer gives it a high spatial resolution in the direction of wave propagation. Experiments performed on plate-like samples have successfully detected deliberately constructed defects, while monitoring the adhesive thickness. Defects have been identified using a C-scan technique using a single EMAT in send-receive mode from either side of the bond.     

Pulse-resolved measurement of quadrature phase amplitudes of squeezed pulse trains at a repetition rate of 76 MHz

We report pulsed homodyne detection of squeezed pulses at a repetition rate of 76 MHz. Measurement of individual pulses, which were interrogated by the correlation coefficient between adjacent pulses, was realized. A homodyne detector was constructed using a low-noise, high-speed operational amplifier; it had a usable bandwidth of 250 MHz. We observed 2.3 dB of squeezing in both the time and frequency domains.     

Electrooptical sampling of ultra-short THz pulses by fs-laser pulses at 1060 nm

Electrooptical sampling of ultra-broadband terahertz (THz) radiation with the help of ultra-short 1060-nm pulses is reported for the first time. The THz pulses are generated by exciting a surface emitter (InAs) with a parabolic fiber laser amplifier delivering 100-fs pulses at a repetition rate of 75 MHz and an average power of 10 W. ZnTe and GaP crystals are used for detection and their velocity mismatch is compared at 800 nm and 1060 nm. PACS 42.55.Wd; 42.70.Nq; 42.72.Ai     

Testing a thermoacoustic sensor of high-power microwave pulses

We present the results of testing a new thermoacoustic sensor designed to detect microwave pulses having durations from 3 to 120 ns at wavelengths of 0.8 and 3 cm. Operation of the sensor is based on the effect of generation of acoustic signals during absorption of microwave pulses in a radiotransparent substrate–absorber–liquid layered structure . A thin nanometer-thick film deposited on a substrate is used as an absorber. Microwaves are converted to an acoustic pulse in the film and the adjacent liquid. The pulse is received by a wideband acoustic receiver and then recorded by a digital oscilloscope. It is shown that for a pulse duration of 120 ns, the shape of the signal recorded by the thermoacoustic sensor completely corresponds to the signal of a tube-diode detector of microwave pulses. The response of the thermoacoustic sensor to shorter pulses (3 and 5 ns long) is a pulse with a duration of 18 ns which is determined by a limited frequency band of the acoustic receiver.     

A high spectral brightness Fourier-transform limited nanosecond Yb-doped fiber amplifier

In this paper, we present a frequency-doubled pulsed Yb-doped fiber amplifier exhibiting Fourier-transform limited 10-ns pulses at a repetition rate of up to 5 kHz. The amplifier is seeded by an external cavity diode laser whose output is chopped into light pulses of 10- to 1200-ns duration. The first stage is operated with a single-mode fiber, while the other two stages employ multi-mode double clad fibers. Output energies of up to 516 μJ per pulse have been recorded. The output power is limited by the onset of stimulated Brillouin scattering. As an example of the possible applications, we demonstrate the temperature measurement of water by Brillouin scattering employing the frequency-doubled radiation from the amplifier.     

Parametric amplification of few-cycle carrier-envelope phase-stable pulses at 2.1 microm

We demonstrate an optical parametric chirped-pulse amplifier producing infrared 20 fs (3-optical-cycle) pulses with a stable carrier-envelope phase. The amplifier is seeded with self-phase-stabilized pulses obtained by optical rectification of the output of an ultrabroadband Ti:sapphire oscillator. Energies of -80 microJ with a well-suppressed background of parametric superfluorescence and up to 400 microJ with a superfluorescence background are obtained from a two-stage parametric amplifier based on periodically poled LiNbO3 and LiTaO3 crystals. The parametric amplifier is pumped by an optically synchronized 1 kHz, 30 ps, 1053 nm Nd:YLF amplifier seeded by the same Ti:sapphire oscillator.     

在小型制冷机中测量薄片交流磁化率以决定超导转变温度的装置

本文介绍了一种利用电磁感应原理和超导磁效应,在小型制冷机中测量超导体转变温度的装置.本装置包括密闭的真空室、压缩制冷机、真空泵、真空计、锁相放大器、温控仪、计算机、线圈绕组.其中,线圈绕组置于真空室内,由初级线圈和次级线圈组成,初级线圈和次级线圈分别绕制在两个线圈骨架上;被测超导薄片材料放置于初级线圈和次级线圈之间;压缩制冷机用来为超导材料制冷;真空泵用来对真空室抽真空;温控仪用来测量和控制真空室内的温度;锁相放大器为初级线圈提供交流电压信号,并测量次级线圈的电信号以得到交流磁化率值;计算机记录温控仪测得的温度数据和锁相放大器测得的次级线圈的电压信号,并显示锁相放大器测得的次级线圈的电压信号随温度变化的曲线.实验证明该装置可以通过测量超导体交流磁化率的变化测得超导转变温度,具有自动化测量及测试成本低等特点.     

Carrier–envelope phase stabilization of 5-fs, 0.5-mJ pulses from adaptive phase modulator

We compressed high-power laser pulses to 5 fs and 0.5 mJ using a neon-filled hollow-core fiber and a high-throughput 4f system. The carrier–envelope phase of the two-cycle pulses was measured directly by using their over-one-octave output spectrum. The carrier–envelope phase was stabilized by feedback controlling the grating separation of a stretcher in a chirped pulse amplifier. To the best of our knowledge, this is the first time that carrier–envelope phase locked few-cycle pulses were generated by using an adaptive phase modulator.     

Sensitivity improvement of a pump-probe set-up for thin film and microstructure metrology

This investigation deals with various new aspects of the sensitivity improvement of a pump-probe laser based acoustic method. A short laser pulse is used to excite a mechanical pulse thermo-elastically. Echoes of these mechanical pulses reaching the surface are causing a slight change of the optical reflectivity. The surface reflectivity is scanned versus time with a probe pulse. Thus the time of flight of the acoustic pulse is measured. The quantity to be measured i.e. the optical reflectivity change deltaR caused by acoustic pulses, is rather small. A set-up having an estimated sensitivity deltaR/R of about 10(-5) has shown to be sufficient to detect up to the fifth echo in a 50 nm aluminum film on sapphire substrate. A key challenge is the reduction of optical and electrical cross-talk between the excitation and the detection. Therefore the concepts of double-frequency modulation, cross-polarization, and balanced photodetection are implemented. Practical aspects like beam guiding, modulation techniques, beam focus minimization, and beam focus matching are discussed. Measurements for single- and multi-layer metallic films demanding higher sensitivity are presented.     

Acoustic positioning using a tetrahedral ultrashort baseline array of an acoustic modem source transmitting frequency-hopped sequences

Acoustic communications and positioning are vital aspects of unmanned underwater vehicle operations. The usage of separate units on each vehicle has become an issue in terms of frequency bandwidth, space, power, and cost. Most vehicles rely on acoustic modems transmitting frequency-hopped multiple frequency-shift keyed sequences for command-and-control operations, which can be used to locate the vehicle with a good level of accuracy without requiring extra signal transmission. In this paper, an ultrashort baseline acoustic positioning technique has been designed, simulated, and tested to locate an acoustic modem source in three dimensions using a tetrahedral, half-wavelength acoustic antenna. The position estimation is performed using the detection sequence contained in each message, which is a series of frequency-hopped pulses. Maximum likelihood estimation of azimuth and elevation estimation is performed using a varying number of pulse and various signal-to-noise ratios. Simulated and measured position estimation error match closely, and indicate that the accuracy of this system improves dramatically as the number of pulses processed increases, given a fixed signal-to-noise ratio.