Tunable Erbium-doped Fiber Ring Laser with a Polymer Micro Bottle Resonator

A new tunable fiber laser structure based on an erbium-doped fiber ring laser (FRL) and a polymer-based microbottle resonator (PMBR) as the wavelength selective filter is proposed and demonstrated. The tunability of the laser output in response to axial strain of up to 253.6 με applied to the PMBR is demonstrated experimentally. When the strain was applied to the PMBR’s long axis, the central lasing wavelength shifted towards shorter wavelengths in a linear fashion. The laser\u27s strain sensitivity was determined to be 0.69 pm/με. The proposed strain-tunable PMBR laser offers the advantages of simple structure, low cost, robust performance, and has the potential for applications in sensing and tunable micro lasers

Microfluidic Flowmeter Based on Liquid Crystal Filled Nested Capillary

A novel flowmeter composed of a liquid crystal-filled nested capillary is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the nested capillary are excited by a tapered fiber coupled perpendicularly to the nested capillary. The WGM transmission spectrum of the fiber taper was optimized to achieve the highest possible quality (Q) factor by moving the capillary along the axis of the fiber taper. The air flowing through the capillary cools it down, which leads to a temperature-induced change of the refractive index of the nematic liquid crystal. This change in turn leads to a spectral shift of the WGM resonances, which can be linked to the airflow speed in capillary. A sensitivity of 0.242 nm/sccm has been demonstrated in our experiment. The proposed sensor provides a new platform for WGM flowmeters and offers the advantages of high sensitivity and miniature size

One-Shot General Object Localization

This paper presents a general one-shot object localization algorithm called OneLoc. Current one-shot object localization or detection methods either rely on a slow exhaustive feature matching process or lack the ability to generalize to novel objects. In contrast, our proposed OneLoc algorithm efficiently finds the object center and bounding box size by a special voting scheme. To keep our method scale-invariant, only unit center offset directions and relative sizes are estimated. A novel dense equalized voting module is proposed to better locate small texture-less objects. Experiments show that the proposed method achieves state-of-the-art overall performance on two datasets: OnePose dataset and LINEMOD dataset. In addition, our method can also achieve one-shot multi-instance detection and non-rigid object localization. Code repository: https://github.com/qq456cvb/OneLoc

Contrast Enhanced Superharmonic Imaging for Acoustic Angiography Using Reduced Form-Factor Lateral Mode Transmitters for Intravascular and Intracavity Applications

Techniques to image the microvasculature may play an important role in imaging tumor-related angiogenesis and vasa vasorum associated with vulnerable atherosclerotic plaques. However, the microvasculature associated with these pathologies is difficult to detect using traditional B-mode ultrasound or even harmonic imaging due to small vessel size and poor differentiation from surrounding tissue. Acoustic angiography, a microvascular imaging technique which utilizes superharmonic imaging (detection of higher order harmonics of microbubble response), can yield a much higher contrast to tissue ratio (CTR) than second harmonic imaging methods. In this work, two dual-frequency transducers using lateral mode transmitters were developed for superharmonic detection and acoustic angiography imaging in intracavity applications. A single element dual-frequency IVUS transducer was developed for concept validation, which achieved larger signal amplitude, better contrast to noise ratio (CNR) and pulse length compared to the previous work. A dual-frequency PMN-PT array transducer was then developed for superharmonic imaging with dynamic focusing. The axial and lateral size of the microbubbles in a 200 μm tube were measured to be 269 μm and 200 μm, respectively. The maximum CNR was calculated to be 22 dB. These results show that superharmonic imaging with a low frequency lateral mode transmitter is a feasible alternative to thickness mode transmitters when final transducer size requirements dictate design choices

Enhancing the Thermo-Optic Tuning Performance of Whispering Gallery Modes in a Microcapillary Resonator Filled With Nematic Liquid Crystals

We investigated both theoretically and experimentally, the thermo-optic tuning of whispering-gallery modes (WGMs) in a microcapillary resonator filled with nematic liquid-crystal (LC). The tuning of WGMs was realized due to the photo-thermal effect of magnetic nanoparticles (MNPs) on the surface of a fiber half-taper connected to a pump laser source, resulting in temperature-induced refractive index (RI) changes of the LC material. Based on perturbation theory, we analyzed the influence the RI and the wall thickness on the sensitivity of the proposed tuning scheme. Furthermore, we experimentally demonstrated that increasing the thickness of the MNPs coating on the fiber taper surface leads to a stronger photo-thermal effect and to a larger RI change of the LC material within the microcapillary core. Thermo-optic tuning of WGM resonances with a sensitivity of 256.63 ± 5.67 pm/mW to the laser pump power and tuning range of 10.43 nm has been achieved. The developed thermo-optic tuning scheme has many potential applications as tunable devices, optical filters and sensors

Enhancing the Visibility of Vernier Effect in a Tri-Microfiber Coupler Fiber Loop Interferometer for Ultrasensitive Refractive Index and Temperature sensing

In this paper a Vernier effect based sensor is analyzed and demonstrated experimentally in a tri-microfiber coupler (Tri-MFC) and polarization-maintaining fiber (PMF) loop interferometer (Tri-MFC-PMF) to provide ultrasensitive refractive index and temperature sensing. The main novelty of this work is an analysis of parameters of the proposed Tri-MFC-PMF with the objective of determining the conditions leading to a strong Vernier effect. It has been identified by simulation that the Vernier effect is a primary factor in the design of Tri-MFC-PMF loop sensing structure for sensitivity enhancement. It is furthermore demonstrated experimentally that enhancing the visibility of the Vernier spectrum in the Tri-MFC-PMF allows to achieve an ultrahigh refractive index and temperature sensitivity with improved measurement accuracy. Specifically it is shown that small values of the total phase difference (pi/16+Npi)~(pi/4+Npi), where N is an integer, accumulated over the PMF loop and Tri-MFC loop result in a strong Vernier effect. Experimentally an ultrahigh refractive index sensitivity of -20588 nm/RIU and temperature sensitivity of 0.019 nm/C are demonstrated by utilizing the stronger Vernier effect with clear Vernier spectrum. This analysis of the parameters may be useful to future researchers seeking to increase the measurement accuracy of sensors by enhancing the spectral visibility of the Vernier effect in other types of fiber optic interferometers

Design, Fabrication, and Characterization of a Bifrequency Colinear Array

Ultrasound imaging with high resolution and large penetration depth has been increasingly adopted in medical diagnosis, surgery guidance, and treatment assessment. Conventional ultrasound works at a particular frequency, with a −6 dB fractional bandwidth of ~70 %, limiting the imaging resolution or depth of field. In this paper, a bi-frequency co-linear array with resonant frequencies of 8 MHz and 20 MHz was investigated to meet the requirements of resolution and penetration depth for a broad range of ultrasound imaging applications. Specifically, a 32-element bi-frequency co-linear array was designed and fabricated, followed by element characterization and real-time sectorial scan (S-scan) phantom imaging using a Verasonics system. The bi-frequency co-linear array was tested in four different modes by switching between low and high frequencies on transmit and receive. The four modes included the following: (1) transmit low, receive low, (2) transmit low, receive high, (3) transmit high, receive low, (4) transmit high, receive high. After testing, the axial and lateral resolutions of all modes were calculated and compared. The results of this study suggest that bi-frequency co-linear arrays are potential aids for wideband fundamental imaging and harmonic/sub-harmonic imaging

Thermo-optic tuning of a nematic liquid crystal-filled capillary whispering gallery mode resonator

A novel tunable whispering gallery modes (WGMs) resonator based on a nematic liquid crystal (LC)-filled capillary and magnetic nanoparticles (MNPs)-coated tapered fiber has been proposed and experimentally demonstrated. Thermo-optic tuning of the WGM resonances has been demonstrated by varying optical pump laser power injected into the MNPs-coated fiber half-taper inside the capillary. The tuning mechanism relies on the change of the effective refractive index (RI) of the nematic LC, caused by the photo-thermal effect of MNPs on the surface of the fiber half-taper inducing a temperature change inside the capillary. Tuning of the WGM resonances with sensitivities of 101.5 ± 3.5 pm/mW and 146.5 ± 3.5 pm/mW and tuning ranges of 1.96 nm and 3.28 nm respectively for the two types of liquid crystals (MLC-7012, MDA-05-2782) has been demonstrated. In addition, the relationship between the optical power of the pump laser and the local temperature of the nematic LC was investigated and the heating rate is estimated as 1.49 °C/mW. The proposed thermo-optic tuning scheme has many potential applications in tunable photonic devices and sensors

A Micron-Range Displacement Sensor Based on Thermo-Optically Tuned Whispering Gallery Modes in a Microcapillary Resonator

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary’s light coupling point. The sensor’s operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments

Recent Advances in Particle and Droplet Manipulation for Lab-on-a-chip Devices Based on Surface Acoustic Wave

Manipulation of microscale particles and fluid liquid droplets is an important task for lab-on-a-chip devices for numerous biological researches and applications, such as cell detection and tissue engineering. Particle manipulation techniques based on surface acoustic waves (SAWs) appear effective for lab-on-a-chip devices because they are non-invasive, compatible with soft lithography micromachining, have high energy density, and work for nearly any type of microscale particles. Here we review the most recent research and development of the past two years in SAW based particle and liquid droplet manipulation for lab-on-a-chip devices including particle focusing and separation, particle alignment and patterning, particle directing, and liquid droplet delivery