Two-wave Mixing Interferometer Research Articles

Online Grain Size Measurement by Laser Ultrasonics in a Hot Rolling Mill

Laser ultrasonics (LUS) has shown through numerous studies and in-situ laboratory experiments, its suitability to track microstructural features evolution as well as metallurgical transformation in steels during thermal cycles with temperatures reaching 1250°C. This prior work has motivated the use of this technology to be in applied in an industrial hot strip mill (HSM) in Eisenhüttenstadt, Germany. For this purpose, a transportable trolley which embedded LUS components (Generation and Detection Lasers, Two-Wave Mixing Interferometer, Dedicated Fibered Optical Head) was designed. The industrial investigation was carried on moving strips during normal production at several locations of the HSM, resulting in signals for high-strength lowalloy (HSLA) and advanced high-strength steel (AHSS) grades. The development of a specific data processing tool allowed calculating the grain size from the analysis of the online recorded signals. Such results were then compared to predictions from metallurgical models, finding reasonably close values. LUS appears as a suitable solution to monitor microstructural features and metallurgical transformations occurring in steel in a Hot Strip Mill (HSM) line. This approach could enable in the future the development of more reliable models and control loop of the process. This action was supported by the European commission and led by a consortium composed by ArcelorMittal, Imagine Optic and IMI-NRC.

Differential two-wave mixing interferometer for crack detection in metallic structures based on laser-induced ultrasound

To meet the need of high-sensitivity crack detection in metallic structures with a contactless method, a differential two-wave mixing interferometer based on two nonlinear Bi12SiO20 photorefractive crystals is proposed for laser-induced ultrasound signal measurement. The two Bi12SiO20 crystals are applied with the electric fields of opposite polarities to obtain the dual-path differential signals corresponding to the laser-induced ultrasound under detection, in which the noise is not affected by the polarity of the applied electric field. The signal noise can be effectively suppressed and the sensitivity is doubled by the differential processing of the dual-path signals, by which the high-sensitivity detection of laser-induced ultrasound can be achieved to characterize the crack in metallic structures. The further optimization of system parameters is performed to obtain the optimal detection sensitivity. Both the numerical analysis and experiments have been carried out to demonstrate the feasibility of the proposed method. The proposed system provides a powerful and reliable tool for contactless detection of laser-induced ultrasound and characterization of metal cracks.

Differential Two-wave Mixing Interferometer for Micro-vibration Detection

Differential Two-wave Mixing Interferometer for Micro-vibration Detection

Differential Two-Wave Mixing Interferometer for Crack Detection in Metallic Structures Based on Laser-Induced Ultrasound

Differential Two-Wave Mixing Interferometer for Crack Detection in Metallic Structures Based on Laser-Induced Ultrasound

Study of Lamb Wave Dispersion Characterization Using Multiplexed Two-Wave Mixing Interferometer

Study of Lamb Wave Dispersion Characterization Using Multiplexed Two-Wave Mixing Interferometer

In-situ measurement of Ti-6Al-4V grain size distribution using laser-ultrasonic technique

A laser-ultrasonic system was established combining an 8 ns width pulsed laser for ultrasound generation and a two-wave mixing (TWM) interferometer for detection. Several Ti-6Al-4V samples were heat treated variously to get different grain sizes. Longitudinal wave back wall echoes were used to calculate the frequency dependent attenuation of ultrasound after the signals were denoised using wavelet transform. A model was built to calculate the log-normal distribution of grain size based on the correlation between the frequency dependent attenuation and the volumetric grain size distribution derived from electron backscattering diffraction data (EBSD) using Schwartz–Saltykov method.

Dual demodulation interferometer with two-wave mixing in GaAs photorefractive crystal

A dual demodulation interferometer with two-wave mixing (TWM) in the GaAs photorefractive crystal (PRC) is proposed and experimentally demonstrated. The GaAs PRC has tiny temperature change under high voltage thus not requiring thermoelectric cooler (TEC) to stabilize the temperature, and adaptive to low frequency fluctuation below 200 Hz. The system is an unbalanced TWM interferometer, which could demodulate the phase change both space variation and wavelength shift induced by strain. Two demodulation modes' formulas are provided in theory respectively. Experimental results have been tested and compared with theoretical analysis, demonstrating that it is a practical and flexible system for detection of mechanical vibration or structure health monitoring (SHM) in engineering by selecting different demodulation mode.

Development of laser ultrasonics inspection for online monitoring of additive manufacturing

The quality control of additive manufacturing (AM) parts and the repeatability of AM process are critical issues for the widespread of AM especially for aerospace and healthcare sectors. Due to production costs, there is a strong interest in reducing scrap rates and process monitoring. Laser ultrasonics is a promising technology that fits the constraints of AM online monitoring. This technique shows potential for inspecting the upper cord of the part during manufacturing, as it is a non-contact and nondestructive testing. The generation is produced by a brief laser impulse which heats up the material, inducing constraints that release into ultrasonic waves. Two generation modes can be encountered using lasers: thermoelastic and ablative, depending on the energy deposited on the surface. The generated waves interact with the medium and flaws, thus allowing the detection of defects such as lack of fusion or porosities. The detection is performed using a two-wave mixing interferometer, also contactless. In this paper, we present work carried out in order to evaluate the feasibility and the effectiveness of laser ultrasonics testing for online additive manufacturing process. The influence of key parameters such as laser spot dimensions is highlighted through both experiment and modeling. We present first results obtained on additive manufactured parts containing machined notches.

Comparative assessment of erbium fiber ring lasers and reflective SOA linear lasers for fiber Bragg grating dynamic strain sensing.

Fiber Bragg grating (FBG) dynamic strain sensors using both an erbium-based fiber ring laser configuration and a reflective semiconductor optical amplifier (RSOA)-based linear laser configuration are investigated theoretically and experimentally. Fiber laser models are first presented to analyze the output characteristics of both fiber laser configurations when the FBG sensor is subjected to dynamic strains at high frequencies. Due to differences in the transition times of erbium and the semiconductor (InP/InGaAsP), erbium-doped fiber amplifier (EDFA)- and RSOA-based fiber lasers exhibit different responses and regimes of stability when the FBG is subjected to dynamic strains. The responses of both systems are experimentally verified using an adaptive photorefractive two-wave mixing (TWM) spectral demodulation technique. The experimental results show that the RSOA-FBG fiber linear cavity laser is stable and can stably respond to dynamic strains at high frequencies. An example application using a multiplexed TWM interferometer to demodulate multiple FBG sensors is also discussed.

Nondestructive characterization of thermal barrier coating by noncontact laser ultrasonic technique

We present the application of a laser ultrasonic technique in nondestructive characterization of the bonding layer (BL) in a thermal barrier coating (TBC). A physical mode of a multilayered medium is established to describe the propagation of a longitudinal wave generated by a laser in a TBC system. Furthermore, the theoretical analysis on the ultrasonic transmission in TBC is carried out in order to derive the expression of the BL transmission coefficient spectrum (TCS) which is used to determine the velocity of the longitudinal wave in the BL. We employ the inversion method combined with TCS to ascertain the attenuation coefficient of the BL. The experimental validations are performed with TBC specimens produced by an electron-beam physical vapor deposition method. In those experiments, a pulsed laser with a width of 10 ns is used to generate an ultrasonic signal while a two-wave mixing interferometer is created to receive the ultrasonic signals. By introducing the wavelet soft-threshold method that improves the signal-to-noise ratio, the laser ultrasonic testing results of TBC with an oxidation of 1 cycle, 10 cycles, and 100 cycles show that the attenuation coefficients of the BL become larger with an increase in the oxidation time, which is evident for the scanning electron microscopy observations, in which the thickness of the thermally grown oxide increases with oxidation time.

基于自适应双波混合干涉仪的金属板厚度测量

基于自适应双波混合干涉仪的金属板厚度测量

Quasi-balanced two-wave mixing interferometer for remote ultrasound detection

We present an improved detection scheme for a two-wave mixing interferometer with a Bi12SiO20 crystal. The proposed detection scheme allows quasi-balanced detection of ultrasonic signals whereby electrical disturbances are suppressed. Quasi-balancing is achieved by changing the polarity of the high voltage at the photorefractive crystal, leading to an inversion of the optical interference signal, in combination with inversion of the detector signal using a signal inverter before the data acquisition device. The polarity of the high voltage is changed by utilizing an H-bridge consisting of five high-voltage relays. Microcontrollers are used to synchronize the reversion of the high voltage at the photorefractive crystal and the inversion of the measured signals. We demonstrate remote measurement of ultrasonic waves and shown that electrical disturbances are suppressed using the quasi-balanced mode.

Characteristics of two-wave mixing adaptive interferometer with CdTe:Ge at 106 and 155 μm and improved temporal adaptability with temperature control

We have characterized a two-wave mixing adaptive interferometer based on dc-biased photorefractive CdTe:Ge crystal at 1.06 and 1.55 μm. Excellent performance is shown at both wavelengths by demonstration of high sensitivity for measurement of small displacements and high cutoff frequency at low intensity. We have achieved a considerable reduction of the undesired low-frequency response using controlled heating of the crystal, which ensures depopulation of corresponding traps. The experimental data are used for measurement of the real and imaginary parts of the coupling constant, as well as the dielectric relaxation time of the crystal and the mobility-lifetime product of the free charge carriers.

Photoacoustic imaging using an adaptive interferometer with a photorefractive crystal

In this paper we report on remote three-dimensional photoacoustic imaging without the need for a coupling agent or other aids (e.g. detector foils) by utilizing a two-wave mixing interferometer (TWMI). In this technique the beams coming from sample and reference are brought to interfere in a Bi₁₂SiO₂₀ photorefractive crystal (PRC). Such a setup allows the measurement of ultrasonic displacements on rough sample surfaces. After data acquisition the initial pressure distribution is reconstructed by a Fourier domain synthetic aperture focusing technique (FSAFT). We present three-dimensional imaging of a hair ribbon phantom, on biological samples with embedded artificial blood vessels or pig bristles, and measurements on a human forearm in-vivo.

Remote photoacoustic imaging on solid material using a two-wave mixing interferometer

We report on remote and contactless photoacoustic imaging (PAI) for the inspection of solid materials using a two-wave mixing interferometer. In this Letter, a semitransparent sample was excited with picosecond laser pulses. The local absorption of the electromagnetic radiation led to generation of broadband ultrasonic waves inside the sample. Ultrasonic waves arriving at the sample surface were detected utilizing a two-wave mixing interferometer. After data acquisition, the initial pressure distribution was reconstructed using a Fourier space synthetic aperture technique algorithm. We show the potential of PAI for the inspection of semitransparent solid materials.

Adaptive Fiber Bragg Grating Sensor Network for Structural Health Monitoring: Applications to Impact Monitoring

A passive structural health monitoring (SHM) system for locating foreign-object impact using a network of fiber Bragg grating (FBG) sensors that monitor high frequency dynamic strains is described. The FBG sensor signals are adaptively demodulated using a two-wave mixing (TWM) spectral demodulator. Strains applied on the FBG sensors are encoded as wavelength shifts of the light reflected by the FBG sensor which are then converted into phase shifts and demodulated by the TWM interferometer. The demodulator adaptively compensates for low frequency drifts caused by large quasi-static strain and temperature drift and allows only high frequency signals to pass through. The FBG sensor network is mounted on a plate, and the structure is subjected to artificial impacts generated by dropping small ball bearings. Owing to the directional sensitivity of the FBG sensors, an FBG sensor-pair configuration is used at each sensing location. The impact signals from multiple FBG sensors are simultaneously acquired at frequencies of up to 180kHz. Using time-frequency wavelet analysis, the group velocity dispersion curve of the detected Lamb wave modes is obtained from the measured transient responses of the sensors, and this is used to determine the location of the impact.

Adaptive interferometers with no external field using reflection gratings in CdTe:Ge at 1550 nm

We demonstrate experimentally the operation of an adaptive two-wave mixing interferometer with no field applied to a photorefractive crystal in which the quadrature condition is introduced by an external π/2 phase step. To achieve high signal to noise ratio the counter-propagating geometry for grating recording is used with a CdTe:Ge crystal, ensuring effective recording of reflection holograms. It is demonstrated that the interferometer with a photorefractive semiconductor crystal provides good sensitivity without the adverse effects that are normally associated with an applied field.

Adaptive demodulation of dynamic signals from fiber Bragg gratings using two-wave mixing technology

A two-wave mixing (TWM) interferometer using photorefractive (PRC) InP:Fe crystal is configured to demodulate the spectral shift of a fiber Bragg grating (FBG) sensor. The FBG is illuminated with a broadband source, and any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength shift is converted into phase shift by means of an unbalanced TWM interferometer. TWM wavelength demodulation is attractive for monitoring dynamic strains because it is adaptive and multiplexable. Adaptivity implies that it can selectively monitor dynamic strains without active compensation of large quasi-static strains and large temperature drifts that otherwise would cause system to drift. Multiplexability implies that several FBG sensors can be simultaneously demodulated using a single demodulator. TWM wavelength demodulation is therefore a cost-effective method of demodulating several spectrally encoded FBG sensors.

Two-Wave Mixing Interferometer for Ultrasonic NDT in Defect Detection

Two-Wave Mixing Interferometer for Ultrasonic NDT in Defect Detection

Photo-acoustic imaging of surface acoustic wave slowness using multiplexed, two-wave mixing array interferometry

A method to rapidly measure the group velocities (or their inverse, slownesses) of surface acoustic waves (SAWs) in anisotropic materials is presented. Surface acoustic waves are generated using a pulsed laser focused to a point on a test object. A newly developed multiplexed two-wave mixing (MTWM) interferometer is used to simultaneously detect the SAW displacements along eight different directions using a circular array of detection points. The group velocity slowness curve is obtained directly from the measured signals from the MTWM array. The experimental results are compared to theoretical predictions in aluminum, silicon, and quartz.