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Ultrasonic testing (UT) is widely used for the nondestructive testing (NDT) of composite materials in the aerospace industry. Liquid-coupled piezoelectric ultrasonic transducers are the most common technology used in this field; however, liquid coupling agents are not always practical, economical, or compatible with materials that cannot get wet. Alternative couplant-free technologies such as air-coupled ultrasound or laser ultrasound (LUS) are available, but either lack the required sensitivity and resolution, or are very costly, large, and sensitive to surface condition and properties. In this paper, we introduce a new couplant-free approach using laser excitation and a commercially available optical micro-phone. This technique is termed laser-excited acoustics (LEA) NDT, which combines the advantages of a contact-free ultrasound technology with the potential for improved sensitivity and resolution required for NDT in industrial environments. We will demonstrate the capabilities of LEA on aerospace composite parts made of carbon and glass fiber–reinforced polymer (CFRP/GFRP) under realistic conditions.

A novel fast proximal scanning method, to the best of our knowledge, termed fiber-core-targeted scanning (FCTS), is proposed for illuminating individual fiber cores sequentially to remove the pixelation effect in fiber bundle (FB) imaging. FCTS is based on a galvanometer scanning system. Through a dynamic control of the scan trajectory and speed using the prior knowledge of fiber core positions, FCTS experimentally verifies a precise sequential delivery of laser pulses into fiber cores at a maximal speed of 45,000 cores per second. By applying FCTS on a FB-based photoacoustic forward-imaging probe, the results demonstrate that FCTS eliminates the pixelation effect and improves the imaging quality.

In this paper three case studies of the application of a compact non-contact, non-destructive setup are presented. The herein employed method is based on the air-coupled detection of acoustic waves in the frequency range up to 2 MHz.

We present a dual modality functional optical coherence tomography and photoacoustic microscopy (OCT-PAM) system. The photoacoustic modality employs an akinetic optical sensor with a large imaging window. This imaging window enables direct reflection mode operation, and a seamless integration of optical coherence tomography (OCT) as a second imaging modality. Functional extensions to the OCT-PAM system include Doppler OCT (DOCT) and spectroscopic PAM (sPAM). This functional and non-invasive imaging system is applied to image zebrafish larvae, demonstrating its capability to extract both morphological and hemodynamic parameters in vivo in small animals, which are essential and critical in preclinical imaging for physiological, pathophysiological and drug response studies.

Air-coupled ultrasound (ACU) is already an established method for the non-destructive failure inspection of carbon fiber reinforced polymers (CFRP). In the through-transmission setup, plate-like structures are placed between the ultrasound (US) source and the receiver. The ultrasonic wave propagating through the material is observed; deteriorations inside the material such as defects alter the captured signal. Such defects can be delaminations, cracks, thickness changes or porosity. In the measurement setup chosen, conventional piezoelectric transducers and receivers are replaced by laser-based components. On the excitation side a nanosecond laser pulse, illuminating the plate surface, was used to induce ultrasonic waves (thermal regime) directly into the specimen. On the receiver side a laser-based optical microphone was tested. This membrane-free microphone detects the refractive index changes of the air, when the ultrasound propagates through the miniature Fabry-Perot etalon. Using this new measurement setup, C-scans of CFRP plates were performed containing impact damage, delaminations and blind holes. In comparison to conventional aircoupled testing methods, our method is sensitive over a broader frequency range, has better signal-to-noise ratio (SNR) and a smaller acoustic aperture. This allows obtaining a more detailed image of a specimen including defects.

This publisher’s note corrects an error in the funding section in Opt. Lett.42, 4319 (2017)OPLEDP0146-959210.1364/OL.42.004319.

An Austrian start-up describes how its membrane-free optical microphone technology is being put to good use in ultrasonic non-destructive testing and process control.

We present a novel, optical ultrasound airborne acoustic testing setup exhibiting a frequency bandwidth of 1MHz in air. The sound waves are detected by a miniaturized Fabry-Perot interferometer (2mm cavity) whilst the sender consists of a thermoacoustic emitter or a short laser pulse We discuss characterization measurements and C-scans of a selected set of samples, including Carbon fiber reinforced polymer (CFRP). The high detector sensitivity allows for an increased penetration depth. The high frequency and the small transducer dimensions lead to a compelling image resolution.

This Letter presents a novel dual modality reflection mode optical coherence and photoacoustic microscopy (OC-PAM) system. The optical coherence microscopy modality features a broadband source to accomplish 5 μm axial resolution. The photoacoustic microscopy modality uses a rigid akinetic Fabry-Perot etalon encapsulated in an optically transparent medium, which forms a 2 mm×11 mm translucent imaging window, permitting reflection mode dual modality imaging. After characterization, the OC-PAM system was applied to image zebrafish larvae in vivo, demonstrating its capability in biomedical imaging with complementary optical scattering and absorption contrasts by revealing morphology in the fish larvae.

We present a novel akinetic optical ultrasound sensor, consisting of a rigid, fiber-coupled Fabry-P´erot interferometer (FPI) with a central opening. The sensing principle relies exclusively on the detection of pressure-induced changes of the refractive index in the liquid located between the cavity mirrors. This enables resonance-free, inherently linear signal detection over a large bandwidth. We demonstrate that the sensor allows to realize exceedingly low noise equivalent pressure (NEP) values of 2 Pa over a 20 MHz measurement bandwidth, while maintaining a large full field of view of 2,7mm × 1,3mm as well as a flat frequency response. Imaging tests on phantoms and biological tissue show the suitability of the XARION-sensor for optical resolution photoacoustic microscopy (OR-PAM) applications. Transparent in axial direction, the sensor facilitates the implementation of highly sensitive fast-scanning reflection-mode OR-PAM setups, as well as easy integration with other imaging modalities such as confocal microscopy or OCT.

A novel all-optical akinetic ultrasound sensor, consisting of a rigid, fiber-coupled Fabry-Pérot etalon with a transparent central opening is presented. The sensing principle relies exclusively on the detection of pressure-induced changes of the refractive index in the fluid filling the Fabry-Pérot cavity. This enables resonance-free, inherently linear signal detection over a broad bandwidth. We demonstrate that the sensor achieves a exceptionally low peak noise equivalent pressure (NEP) values of 2 Pa over a 20 MHz measurement bandwidth (without signal averaging), while maintaining a flat frequency response, and a detection bandwidth up to 22.5 MHz (-6 dB). The measured large full field of view of the sensor is 2.7 mm × 1.3 mm and the dynamic range is [Formula: see text] or 63 dB at 20 MHz bandwidth. For different required amplitude ranges the upper amplitude detection limit can be customized from at least 2 kPa to 2 MPa by using cavity mirrors with a lower optical reflectivity. Imaging tests on a resolution target and on biological tissue show the excellent suitability of the akinetic sensor for optical resolution photoacoustic microscopy (OR-PAM) applications.

A novel type of all-optical pressure sensor has been developed. In this context, a vertical-cavity surface-emitting laser (VCSEL) has been modified in its design to provide simultaneous light emission from both facets. One beam serves as measuring signal while the other establishes a reference; and both paths lie on the same optical axis. The VCSELs are based on active InGaAs quantum wells for laser output close to 960 nm wavelength where the GaAs substrate is transparent. From both top and bottom facet, single-polarization and single-mode beams are observed, having a power ratio of 1:2 to 1:4. In this paper we give insight into this new sensing application for VCSELs, describe the laser fabrication and the static operation characteristics as well as the noise properties which have paramount importance for high performance of the sensor. With regard to the sensor application in acoustics, the focus of the noise measurements is put on the low-frequency, i.e. kHz, regime. While laser diode noise performance is readily available for the MHz to GHz frequency range, only very limited data exists in the Hz to kHz domain. The relative intensity noise of both beams is measured and compared and the mutual correlation properties are investigated. The frequency noise is quantified.

A novel all-optical pressure sensor is presented. Based on a rigid Fabry-Perot, the transducer detects refractive index changes induced by pressure fluctuations. This design is so sensitive that the miniaturized device is applicable as microphone.

A Fabry-Perot etalon is used to form a novel type of microphone. Refractive index dependant transmission is proportional to sound pressure over a large dynamic range. The current of the laser diode is tuned to take into account environmental influences.