Yanlu Li

BACKGROUND: Carotid-femoral pulse wave velocity (cfPWV) is the gold standard for noninvasive arterial stiffness assessment, an independent predictor of cardiovascular disease, and a potential parameter to guide therapy. However, cfPWV is not routinely measured in clinical practice due to the unavailability of a low-cost, operator-friendly, and independent device. The current study validated a novel laser Doppler vibrometry (LDV)-based measurement of cfPWV against the reference technique. METHODS: In 100 (50 men) hypertensive patients, cfPWV was measured using applanation tonometry (Sphygmocor) and the novel LDV device. This device has 2 handpieces with 6 laser beams each that simultaneously measure vibrations from the skin surface at carotid and femoral sites. Pulse wave velocity is calculated using ECG for the identification of cardiac cycles. An ECG-independent method was also devised. Cardiovascular risk score was calculated for patients between 40 and 75 years old using the WHO risk scoring chart. RESULTS: LDV-based cfPWV correlated significantly with tonometry (r=0.86, P LDV_ECG ] and r=0.80, P LDV_w/oECG ] methods). Bland-Altman analysis showed nonsignificant bias (0.65 m/s) and acceptable SD (1.27 m/s) between methods. Intraobserver coefficient of variance for LDV was 4.7% (95% CI, 3.0%–5.5%), and interobserver coefficient of variance was 5.87%. CfPWV correlated significantly with CVD risk (r=0.64, P P =0.003; and r=0.37, P =0.006 for tonometry, LDV-with, and LDV-without ECG, respectively). CONCLUSIONS: The study demonstrates clinical validity of the LDV device. The LDV provides a simple, noninvasive, operator-independent method to measure cfPWV for assessing arterial stiffness, comparable to the standard existing techniques. REGISTRATION: URL: https://clinicaltrials.gov/study/NCT03446430 ; Unique identifier: NCT03446430.

Grating couplers are essential in photonic systems, facilitating efficient light coupling between optical fibers and on-chip waveguides and allowing for wafer-level testing. With the increasing integration of photonics in communication systems like the internet, there is a growing need for grating couplers to be utilized on the backside of the chip, in addition to the top side. This paper presents a novel design for backside grating couplers by optimizing Si overlay on conventional grating couplers on silicon photonics circuits. The designed backside grating coupler simulation shows the coupling efficiency from the waveguide toward the back side is around 90% with reflection less than 2 %.

Optical coherent detection is widely used for highly sensitive sensing applications, but nonlinearity issues pose challenges in accurately interpreting the system outputs. Most existing compensation methods require access to raw measurement data, making them not useful when only demodulated data are available. In this study, we propose a compensation method designed for direct application to demodulated data, effectively addressing the 1st and 2nd-order nonlinearities in both homodyne and heterodyne systems. The approach involves segmenting the distorted signal, fitting and removing baselines in each section, and averaging the resulting distortions to obtain precise distortion shapes. These shapes are then used to retrieve compensation parameters. Simulation shows that the proposed method can effectively reduce the deviation caused by the nonlinearities without using the raw data. Experimental results from a silicon-photonics-based homodyne laser Doppler vibrometry prove that this method has a similar performance as the conventional Heydemann correction method.

In this paper, we introduce a novel method to realize a multi-beam optical frequency shifting component for photonic integrated circuits, utilizing an array of parallel optical modulators and a free-propagation region (FPR), such as a slab waveguide-based star coupler. This component generates multiple optical beams with different frequency shifts, making it suitable for various systems, such as multi-beam laser Doppler vibrometry (LDV). We thoroughly elaborate on the working principle of the component through theoretical analysis and demonstrate that by applying periodic wave-like modulation in the modulator array, the discrete harmonic content of the light can be selectively directed to different outputs based on the delay between consecutive modulators. A design comprising a 16-element modulator array and 5 outputs will be presented. Simulations show that this design can generate and collect 5 different harmonics (−2, −1, 0, +1, +2) in the different outputs with a side band suppression ratio of 20 dB to 30 dB for each output. Our proposed design is just one possibility and the component can be modified and optimized for specific applications.

Pulse-wave velocity (PWV) can be used to quantify arterial stiffness, allowing for a diagnosis of this condition. Multi-beam laser-doppler vibrometry offers a cheap, non-invasive and user-friendly alternative to measuring PWV, and its feasibility has been previously demonstrated in the H2020 project CARDIS. The two handpieces of the prototype CARDIS device measure skin displacement above main arteries at two different sites, yielding an estimate of the pulse-transit time (PTT) and, consequently, PWV. The presence of multiple beams (channels) on each handpiece can be used to enhance the underlying signal, improving the quality of the signal for PTT estimation and further analysis. We propose two methods for multi-channel LDV data processing: beamforming and beamforming-driven ICA. Beamforming is done by an SNR-weighted linear combination of the time-aligned channels, where the SNR is blindly estimated from the signal statistics. ICA uses the beamformer to resolve its inherent permutation and scale ambiguities. Both methods yield a single enhanced signal at each handpiece, where spurious peaks in the individual channels as well as stochastic noise are well suppressed in the output. Using the enhanced signals yields individual PTT estimates with a low spread compared to the baseline approach. While the enhancement is introduced in the context of PTT estimation, the approaches can be used to enhance signals in other biomedical applications of multi-channel LDV as well.

Laser Doppler vibrometry (LDV) is a non-contact vibration measurement technique based on the Doppler effect of the reflected laser beam. Thanks to its feature of high resolution and flexibility, LDV has been used in many different fields today. The miniaturization of the LDV systems is one important development direction for the current LDV systems that can enable many new applications. In this paper, we will review the state-of-the-art method on LDV miniaturization. Systems based on three miniaturization techniques will be discussed: photonic integrated circuit (PIC), self-mixing, and micro-electrochemical systems (MEMS). We will explain the basics of these techniques and summarize the reported miniaturized LDV systems. The advantages and disadvantages of these techniques will also be compared and discussed.

A variety of mechanisms can induce distortions in the output signals of a homodyne laser Doppler vibrometer (LDV). In this paper, the nonlinear LDV distortions caused by a strong second-order ghost reflection originating from lens flares are theoretically explained and analyzed. We propose a simple compensation method to mitigate this distortion. The performance and limitations of this method are also explained both in simulation and in experiment.

Pulse wave velocity (PWV) is a reference measure for aortic stiffness, itself an important biomarker of cardiovascular risk. To enable low-cost and easy-to-use PWV measurement devices that can be used in routine clinical practice, we have designed several handheld PWV sensors using miniaturized laser Doppler vibrometer (LDV) arrays in a silicon photonics platform. The LDV-based PWV sensor design and the signal processing protocol to obtain pulse transit time (PTT) and carotid-femoral PWV in a feasibility study in humans, are described in this paper. Compared with a commercial reference PWV measurement system, measuring arterial pressure waveforms by applanation tonometry, LDV-based displacement signals resulted in more complex signals. However, we have shown that it is possible to identify reliable fiducial points for PTT calculation using the maximum of the 2^nd derivative algorithm in LDV-based signals, comparable to those obtained by the reference technique, applanation tonometry.

We present a scanning average method used in laser Doppler vibrometry systems for mitigating the noise induced by dynamic speckles. In this method, the measurement beam is scanned over the target surface within the area of interest at a relatively high frequency. Then an averaging operation (e.g., low-pass filtering) is applied to the acquired photocurrent signals to remove the impacts of the scan. Movement signals recovered from the averaged photocurrents turn out to have lower speckle-induced noise. We report the experimental demonstration of this technique through the use of a silicon-based photonic integrated circuit.