High-speed Laser Doppler Research Articles

Silicon wafer crack detection using nonlinear ultrasonic modulation induced by high repetition rate pulse laser

In this study, a high repetition rate laser ultrasonic system is developed for noncontact detection of silicon wafer crack. A high repetition rate pulse laser is used to generate two narrowband ultrasonic waves on a target silicon wafer, whereas a high-speed laser Doppler vibrometer is used to measure the corresponding ultrasonic responses. Nonlinear interaction occurs between the two generated ultrasonic waves due to crack formation, producing nonlinear ultrasonic modulation at the sum and difference of the two excitation frequencies. Outlier analysis is performed on the amplitudes of the nonlinear modulation components, and a fatigue crack is identified when the amplitudes of the nonlinear modulation components exceed a user specified threshold. The uniqueness of this study is as follows. (1) A noncontact, high-speed, and scanning laser ultrasonic system is developed using a high repetition rate laser for ultrasonic generation and a laser Doppler vibrometer for sensing as well as scanning the two laser beams simultaneously in a pitch-catch mode. (2) Two narrowband ultrasonic waves up to 1 MHz are simultaneously generated from a single laser pulse. (3) Micro cracks with widths of less than 10 µm are successfully detected by extracting the crack induced nonlinear ultrasonic modulation components. (4) The probability of detection of the proposed crack detection technique is evaluated. The performance of the proposed technique is experimentally validated using eight silicon wafer specimens with varying crack lengths. Results show that the proposed technique yields a 90% probability of detection in silicon wafers when the crack length is over 3.77 mm.

An explanation for the effectiveness of the ‘Draijer’ algorithm for high speed laser Doppler perfusion imaging

Laser Doppler flowmetry relies upon the use of the first moment of the power spectrum of the photocurrent and numerous methods of providing an estimate of this have been implemented. One, operating in the time domain and using only a few, simple processing steps, is claimed to be both fast and accurate (Draijer etal. in Med Biol Eng Comput 47(10):1103-1109, 2009). However, it is acknowledged that the theory underpinning its operation is intuitive. Here it is shown that this algorithm mimics the use of the analytic representation of the photocurrent and although capable of approximating the required value is certainly not equivalent mathematically.

Evaluation of a new high-speed laser-doppler camera for early burn depth assessment

Evaluation of a new high-speed laser-doppler camera for early burn depth assessment

The application of a new laser Doppler imaging system in planning and monitoring of surgical flaps

Despite the latest technical improvements in reconstructive flap surgery, the operative success may still be jeopardized by ischemia. Therefore, there is a demand of technologies enabling to assess flap microcirculation quantitatively and reliably. We tested a new high-speed laser Doppler imaging (LDI) system developed for 2D high-definition imaging of superficial blood flow in skin. We assessed the potential of the new LDI system in the use of preoperative flap design and intra- and postoperative monitoring.

High-speed wave-mixing laser Doppler imaging in vivo

An interferometric method for parallel optical spectroscopy in the kilohertz range is reported, as well as its experimental validation in the context of high-speed laser Doppler imaging in vivo. The interferometric approach enables imaging in the low light conditions of a 2 kHz frame rate recording with a complementary metal-oxide semiconductor camera. Observation of mice craniums with near-infrared (lambda=785 nm) laser light in reflection configuration is reported. Doppler spectral images allegedly sensitive to blood flow are sequentially measured at several optical frequency detunings, to shift the spectral range of analysis in the radio-frequency spectrum.

High-speed laser Doppler perfusion imaging using an integrating CMOS image sensor

This paper describes the design and the performance of a new high-speed laser Doppler imaging system for monitoring blood flow over an area of tissue. The new imager delivers high-resolution flow images (256x256 pixels) every 2 to 10 seconds, depending on the number of points in the acquired time-domain signal (32-512 points). This new imaging modality utilizes a digital integrating CMOS image sensor to detect Doppler signals in a plurality of points over the area illuminated by a divergent laser beam of a uniform intensity profile. The integrating property of the detector improves the signal-to-noise ratio of the measurements, which results in high-quality flow images. We made a series of measurements in vitro to test the performance of the system in terms of bandwidth, SNR, etc. Subsequently we give some examples of flow-related images measured on human skin, thus demonstrating the performance of the imager in vivo. The perspectives for future implementations of the imager for clinical and physiological applications are discussed.