Confocal Fabry-Perot Interferometer Research Articles

Ultrasound-modulated optical microscopy

We demonstrate that microscopic imaging is feasible in ultrasound-modulated optical tomography (UOT) of soft biological tissues, using a high-frequency focused ultrasound transducer with a 75-MHz central frequency. Our experiments in tissue mimicking phantoms show that at an imaging depth of about 2 mm, an axial resolution better than 30 microm can be achieved, whereas the lateral resolution is 38 microm. A long-cavity scanning confocal Fabry-Perot interferometer (CFPI) is used for real-time detection of multiply scattered light modulated by high-frequency ultrasound pulses propagating in an optically scattering medium. We also compare the performances of various high-frequency focused ultrasound transducers with central frequencies of 15 MHz, 30 MHz, 50 MHz, and 75 MHz. The comparison is based on two-dimensional (2-D) images of optically absorbing objects positioned at a few millimeters depth below the surface of both optically scattering phantoms and soft biological tissue samples. Our experimental results show that modulation depth and image contrast decrease with an increase in ultrasound frequency. In addition, we use analytical calculations to show that modulation depth decreases with increasing ultrasound frequency.

Fabry–Perot interferometry for magnetron plasma temperature diagnostics

A confocal Fabry–Perot interferometer was utilized to study Ar–Ti, Ar–Cu and Ar–Cr dc magnetron discharges during sputtering of the corresponding metallic targets. Doppler broadening of Ar and Ti emission lines was measured in the wide range of Ar pressures and at different powers applied to the discharge. Spatial characterization of Ar and Ti line broadening through the discharge volume was also performed. Corresponding temperatures of Ar and thermalized Ti atoms were determined by Doppler broadening of the corresponding emission lines. Results show that the Ar temperature depends mainly on the working pressure in the reactor and it was found to be on average in the range 700–1300 K for the working pressure range 2–240 mTorr. The temperature of Ti was found to be about 600–800 K; it slightly increases with increasing applied power and does not depend on the working pressure in the reactor. Ar temperature measurements were verified by adding nitrogen in the discharge and by the measuring of the N2(C, v′ = 0 − B, v″ = 2) vibrational band shape.

Solar confocal interferometers for sub-picometer-resolution spectral filters

The confocal Fabry-Perot interferometer allows sub-picometer spectral resolution of Fraunhofer line profiles. Such high spectral resolution is needed to keep pace with the higher spatial resolution of the new set of large-aperture solar telescopes. The line-of-sight spatial resolution derived for line profile inversions would then track the improvements of the transverse spatial scale provided by the larger apertures. In particular, profile inversion allows improved velocity and magnetic field gradients to be determined independent of multiple line analysis using different energy levels and ions. The confocal interferometer's unique properties allow a simultaneous increase in both etendue and spectral power. The higher throughput for the interferometer provides significant decrease in the aperture, which is important in spaceflight considerations. We have constructed and tested two confocal interferometers. A slow-response thermal-controlled interferometer provides a stable system for laboratory investigation, while a piezoelectric interferometer provides a rapid response for solar observations. In this paper we provide design parameters, show construction details, and report on the laboratory test for these interferometers. The field of view versus aperture for confocal interferometers is compared with other types of spectral imaging filters. We propose a multiple etalon system for observing with these units using existing planar interferometers as pre-filters. The radiometry for these tests established that high spectral resolution profiles can be obtained with imaging confocal interferometers. These sub-picometer spectral data of the photosphere in both the visible and near-infrared can provide important height variation information. However, at the diffraction-limited spatial resolution of the telescope, the spectral data is photon starved due to the decreased spectral passband.

Spectrum Analysis of an Ultrasonic Signal According to a Surface Status by Using a Robust Laser-Based Ultrasonic System

A laser-based ultrasonic inspection system is a non-contact scanning device with a high spatial resolution and a wide spectrum bandwidth. In this paper, we developed a laser-based ultrasonic inspection system by fabricating a pulse laser and a Confocal Fabry-Perot Interferometer (CFPI) with a dynamic stabilizer. The dynamic stabilizer generates ultrasound by adaptively obtaining the time of the maximum gain of the CFPI whose gain is varied with the time. Then, a computer adaptively corrects the measurement error by using the gain of the CFPI at the measuring time. In this paper, we describe the hardware configurations and the control algorithms to build the stable laser-based ultrasonic inspection system. We have investigated an ultrasonic signal in the time and frequency domain to detect micro cracks through experiments. We experimentally confirmed that the proposed error correction method is effective for improving the performance of the system and the configured ultrasonic inspection system is stable

Construction and calibration of a low cost Fabry-Perot interferometer for spectroscopy experiments

We describe the construction and calibration of a low cost confocal scanning Fabry-Perot interferometer with a free spectral range of 382.3±1.1MHz, which is suitable for use as a frequency reference in laser locking and for spectroscopy experiments. The calibration of the free spectral range is performed using the longitudinal modes of a commercial helium–neon laser.

Development and Verification of a Confocal Fabry-Perot Interferometer for Broad Band Ultrasound Detection

This paper describes the development of a confocal Fabry-Perot interferometer (CFPI) for non-contact and non-destructive detection of broadband ultrasound generated by a pulsed laser. The operation theory of CFPI is introduced. The transmission and reflection modes of operation were investigated theoretically and verified experimentally. For the present study, a CFPI cavity of 50cm with 95.4% reflectivity spherical mirrors was constructed with associated resonant cavity control and signal detection electronics. The design is capable of providing detection frequency bandwidth from 140kHz to 50MHz. For the first step of verification, the input signal simulated by an electro-optical modulator (EOM) was used for verifying the feasibility of surface wave measurement. Signals obtained from an avalanche detector were compared with the results through theoretical analysis of the CFPI transfer function in a transmission mode. The results show a favorable agreement between the two. Furthermore, transmitted ultrasound signals from a 5MHz contact ultrasound transducer were detected and compared between the CFPI system and a Michaelson interferometer. Patterns of ultrasound arrival and reflection were clearly detected by both. Because an intrinsic transfer function is embedded in the operation of CFPI, the output signal will be distorted when measuring surface displacement. A digital filtering process was considered for compensation for the surface displacement signal. From the comparative results, it was further concluded that the present CFPI design has a displacement resolution of 0.05nm. Future studies will be focused on the reflection mode operation for fully utilization of non-contact laser ultrasound generation and detection.

224 nm segmented hollow-cathode silver ion laser

The segmented hollow-cathode discharge arrangement is used the first time to excite the 224 nm Ag II laser transition. Quasi-continuous output power of 45 mW is obtained during the 300 μs current pulses at optimal discharge conditions (10 hPa of He+4% Ar buffer gas) for discharge current of 3 A. No power saturation is observed up to this current value. An average output power of 0.75 mW is reached using pulse repetition frequency of 190 Hz. The longitudinal mode structure of the TEM00 transversal mode is measured by means of a scanning confocal Fabry–Perot interferometer. Two-mode operation is found to be dominant at high current values. Attempts and suggestions are made to prolong the lifetime of the laser tube.

Frequency stabilization of tunable infrared diode lasers for time-resolved monitoring of transient species

Stabilization of an infrared diode laser on the absorption line of a transient species has been developed in a laser flash photolysis/infrared diode laser absorption experiment used for kinetic studies. The stabilization is achieved by modulating the optical pathlength of a confocal Fabry–Perot interferometer instead of the injection current of the laser. It is therefore of prime interest for studies in chemical kinetics or photochemistry, where signals are acquired at fixed frequency and where the timescale of the reactions prohibits modulation of the signal. This method is demonstrated on the detection of HCO radical following 266-nm laser photolysis of acetaldehyde.

High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.

A Structure Nondestructive Evaluation Using Laser-Generated Ultrasonic Application

In the present study, a Nd;YAG Laser (pulse type) was used to emit ultrasonic signals to a test material. In addition, a total ultrasonic investigation system was designed by adopting a Fabry-Perot interferometer, which receives ultrasonic signals without any contact. For non-destructive test SM45C, which contains some flaws was used as a test material. Because it is easy to align light beam in receiver, and the length of the light beam does not change much even if convex mirror leans towards one side, confocal Fabry-Perot interferometer, which has stable frequency, and PI control are used to correct interfered and unstable signals from temperature, fluctuation and time shift of laser frequency. Stable signals are always obtained by the feedback of PI circuit signals in the confocal Fabry-Perot interferometer. The type, size and position of flaws inside the test material were examined by achieving the stabilization of an interferometer. This study presented a useful method, which could quantitatively investigate the fault of objects by using a Fabry-Perot interferometer.

Non Destructive Evaluation Of Heat-damage InAluminium Alloy By Laser-ultrasonic

A Laser-ultrasonic technique has been used as a non-contact evaluation technique to monitor the mechanical property degradation due to the thermal exposure of the materials. Ultrasonic high-energy pulses were generated on a variety of materials, using a Q-switched Nd:YAG laser. The detection of ultrasound waves was performed by a confocal Fabry-Perot interferometer. The effect of exposure of aluminum alloy to high temperatures has been studied using traditional ultrasonic techniques, and a non-contact technique. Measurements of ultrasonic velocity and attenuation were performed to characterize microstructure and to assess the mechanical properties of the aluminum alloy. Ultrasonic wave measurements were used in conjunction with mechanical tests and electrical conductivity measurements to distinguish between phase transformations in aluminum alloy. The results show that ultrasonic wave measurements using laser generation and detection at high

Laser ultrasonic system for on-line steel tube gauging and process control

A laser ultrasonic system has been installed on a seamless tubing production line at The Timken Company and is being used to measure on-line the wall thickness of tubes during processing. The seamless process consists essentially in forcing a mandrel through a hot cylindrical billet in rotation and results in wall thickness variations that should be minimized and controlled to respect specifications. The system includes a Q-switched Nd-YAG laser for the generation of ultrasound by ablation, a long pulse very stable Nd-YAG laser for detection coupled to a confocal Fabry–Perot interferometer. The lasers, data acquisition, and processing units are housed in a cabin off-line and connected to a front coupling head located over the passing tube by optical fibers. The system also includes a fiber-coupled pyrometer to measure tube temperature profile and two fiber-coupled optical velocimeters to measure the coordinates at the probing location on the surface of the passing, rotating hot tube. During the presentation further details of the system will be disclosed, as well as typical results and examples of its diagnostic capability. [Work partially supported by Department of Energy under Award No. DE-FC07-99ID 13651.]

Quantitative Millimetre Wavelength Spectrometry

Quantitative Millimetre Wavelength Spectrometry

A sensitive and stable confocal Fabry-Pérot interferometer for surface ultrasonic vibration detection

A new confocal Fabry-Perot interferometer (CFPI) has been constructed. By using both of the conjugate rays, the sensitivity of the system was doubled. Moreover, the negative feedback control loop of a single-chip microcomputer (MCS-51) was applied to stabilize the working point at an optimum position. The system has been used in detecting the piezoelectric ultrasonic vibration on the surface of an aluminium sample.

The influence of texture and phase distortion on ultrasonic attenuation in Ti-6Al-4V

A high resolution experimental capability has been developed to map the phase and magnitude of ultrasonic waves transmitted in a solid. The advancement presented in this paper is provided by laser detection of the ultrasonic energy over a microscopic aperture of approximately 50 μm. The system is built around a computer controlled scanner and a confocal Fabry-Perot interferometer, which uses a diode pumped, frequency-doubled Nd:YAG laser as a light source. Wave propagation in the axial and radial directions of a 2.5″ diameter bar of textured Ti-6Al-4V was investigated in this study. Measurements were also taken on samples cut with angles between the surface normals and the axis of the bar of 0, 30, 45, 60, and 90 degrees. The work was motivated by the observation of unusually high apparent attenuation in the axial direction of the as-received bar, thought to be associated with phase distortion rather than actual energy loss. The current phase mapping results, using a focused laser spot, show relatively high wavefront distortion and more nonuniform distribution of the transmitted energy in the axial direction. The contribution to attenuation associated with phase cancellation loss was also investigated. These measurements show the laser detected attenuation to be substantially lower than the piezoelectrically measured attenuation. However, even the relative phase insensitivity of focused laser detection approach clearly indicates the attenuation to be strongest in the axial direction. This paper demonstrates the orientation dependence of attenuation stems from scattering effects associated with texturing and the elongated macroscopic grain structure in the mill annealed Ti-6Al-4V bar generated during processing, which may also affect diffraction and beam divergence.

High-sensitivity laser-based ultrasonic C-scan system for materials inspection

A beam-steering C-scan system for laser generation and detection of ultrasound in opaque solids has been developed. For ultrasound detection, the system uses a confocal Fabry-Perot interferometer that is stabilized at its high-sensitivity point by implementing a novel procedure that does not require a reference beam. This stabilization procedure effectively compensates for slowly varying (up to 2 Hz) disturbances and, more important, for any change in the interferometer illumination conditions while probing different scanning points. For the latter reason, the proposed stabilization appears to be more effective than previous approaches for high-sensitivity ultrasound detection in beamsteering scans. Noise performance and sensitivity limit of the system are evaluated. Ultrasonic echoes are detected in aluminum samples having optically rough surfaces with single-pulse generation signal-to-noise ratios on the order of 20 dB. A C-scan inspection is performed where a known defect in the test piece is successfully detected by time-of-flight gating of the ultrasonic signals.

Noncontact ultrasonics for materials nondestructive evaluation

Recent advances in ultrasonic techniques for remote and noncontact nondestructive evaluation of structural materials will be presented. The principles and techniques of the laser generation and detection of ultrasonic waves in opaque solids will be reviewed. A laser-based beam-steering C-scan system will be described, along with a novel stabilization procedure for ultrasound detection by a confocal Fabry–Perot interferometer which appears to be more effective than previous approaches for high-sensitivity measurements in beam-steering scan applications. A hybrid system which is based on laser generation of narrow-band acoustic surface waves and their detection by gas (air) -coupled transduction will also be presented. This system uses fiberoptic light delivery which enhances its flexibility and remoteness from the test piece. The main issues governing the efficient coupling of high-power laser light into optical fibers will be discussed. An improved theory for the laser generation of narrow-band surface acoustic waves by the spatial modulation technique will be outlined and the effect of the number of illumination sources on the bandwidth of the generated wave will be assessed experimentally and theoretically. Finally, the suitability of the hybrid system for the intelligent control of the tow-placement process of advanced thermoplastic composites will be demonstrated.

Ultrasound detection by using a confocal Fabry–Perot interferometer with phase-modulated light

The optical detection of ultrasound with a radio-frequency (RF) sideband type of optical discriminator is described. The sidebands are produced on a laser beam by an optical phase modulator. A confocal Fabry–Perot interferometer with a lens at the mid-point in the cavity has been used to provide frequency discriminator functions. The cavity is tuned to the maximum transmission point, and then the RF beat signal in the back-reflection beam is detected. The ultrasound signals are obtained from the phase changes between the detected RF signal and the modulation source. Several experimental results are presented.

Forward-scattering degenerate four-wave mixing for sensitive absorption detection in microseparation systems: Coupling to micro-column liquid chromatography

The feasibility of forward-scattering degenerate four-wave mixing (F-D4WM) for detection in micro-column (internal diameter 0.3 mm) liquid chromatography (micro-LC) is studied, using the separation of 1-amino-9,10-anthraquinone (I-AAQ) and 2-amino-9,10-anthraquinone (II-AAQ) as a model system and utilizing the 514 nm line of an argon-ion laser. Though the F-D4WM signal can be detected on a virtually dark background, in practice background reduction is of utmost importance. Background reduction by means of a mechanical chopper and a scanning confocal Fabry–Perot interferometer (CFP) are compared both experimentally and theoretically. Using the CFP set-up, an injected concentration limit of detection of 3·10 −8 M could be achieved for I-AAQ (molar absorptivity of 2·10 3 M −1 cm −1 at 514 nm), which is roughly two decades more favourable than obtained when using the mechanical chopper. In comparison to conventional absorption detection in micro-LC at 514 nm, the gain is also about two decades. In view of the fact that the present F-D4WM set-up still suffers from significant band broadening caused by the detector cell, further improvement seems possible.

Forward Degenerate Four-Wave Mixing as a Detection Method in Liquid Separation Systems: Improving Detection Limits by Means of a Fabry—Perot Interferometer

A simple, small, and low-resolution confocal Fabry–Perot interferometer (CFP) has been developed to improve the signal-to-noise ratio (S/N) in forward degenerate four-wave mixing (F-D4WM) detection, coupled to high-performance liquid chromatography (HPLC). Its appropriateness is based on the difference in coherence between the F-D4WM signal and the background stray light. A detailed description of the specially designed CFP and its performance is presented. The improvement in S/N was calculated from the chromatographic peaks recorded after an HPLC separation of 1- and 2-aminoanthraquinone. The concentration limit of detection (LOD) was improved by a factor of 30; for 1-aminoanthraquinone it was 2 × 10−8 M injected (corresponding to about 3 × 10−9 M in the detector cell), which is quite favorable in view of its low molar extinction coefficient being 2000 M−1 cm−1 at the utilized laser wavelength (514.5 nm).