Optical-resolution photoacoustic microscopy (OR-PAM) can image biological tissues at micrometer level resolution. However, the imaging speed of traditional OR-PAM is often too slow for capturing dynamic information. In this work, we demonstrate a high-speed OR-PAM system using a water-immersible two-axis torsion-bending scanner, in which the fast axis employs the torsion scanning while the slow axis works at the bending mode. The system has achieved a cross-sectional frame rate of 400 Hz, and a volumetric imaging speed of 1 Hz over a field of view of 1.5 × 2.5 mm2. We have demonstrated high-speed OR-PAM of fast hemodynamic changes in vivo.
We present a new functional photoacoustic microscopy system with the highest imaging speed and ultrawide field of view. The high imaging speed is enabled by a 12-facets polygon for fast scanning and a Raman-shifter system for fast dual-wavelength measurement of oxygen saturation in vivo. we imaged the dynamic functions in mouse brains in response to hypoxia challenge, sodium nitroprusside (SNP), and ischemic stroke. The experimental results have demonstrated that the high-speed photoacoustic microscopy system can be a powerful tool for studying the rapid hemodynamics in the mouse brains of a wide range of pathological and physiological models.
The exploration and utilization of marine resources is one of the most important topics in human development. In this paper, a fiber-optic hydrophone was proposed and demonstrated experimentally for submarine ultrasonic detection. The device contained a refractive index modification window in the cladding of tapered fiber-optics tip, making it as a quasi-Michelson interferometer. Moreover, a plastic cone structure was formed and employed to focus the ultrasonic field on the fiber, which highly promote the coupling efficiency of ultrasonic-to-interferometer. Using the spectral-side band filtering technology, the hydrophone was working well for ultrasonic wave detection.
KEYWORDS: Cladding, Fiber Bragg gratings, Femtosecond phenomena, Wave propagation, Single mode fibers, Vibrometry, Sensing systems, Signal to noise ratio, Signal attenuation, Refractive index
A novel orthogonal grating has been proposed and demonstrated. The grating is inscribed over inner cladding of a multi-cladding fiber containing two orthogonally cascaded “cladding” fiber Bragg gratings. Femtosecond laser side-illumination technique is utilized to ensure the two grating inscriptions are precisely positioned and compact in size. With such configuration, two pairs of well-defined cladding resonances have been achieved in reflection, which are originated from the internal cladding (two weakened resonances at shorter wavelength side) and from the fiber core (two stronger resonances at longer wavelength side), respectively. Most importantly, the “cladding” resonances are strongly directional sensitive to the small-vibration.
A highly sensitive gas refractometer based on inverse mode-coupling is proposed and experimentally demonstrated. The sensor has a sandwiched configuration that a tapered four-core fiber is spliced between two standard single mode fibers. In the tapered four-core fiber, inverse mode-coupling from cladding modes to core modes leads to enhanced cladding modes and evanescent fields. The unique waveguide structure (multiple cores arranged close to the fiber cladding) of the four-core fiber enables multiple core modes to sense refractive index change after tapering. The abrupt taper also acts as a bridge between surround refractive index and interference modes (including both cladding and core modes). For the carbon dioxide refractive index (close to 1.0) measurement, the sensor presents a high sensitivity of 922.32 dB/refractive index unit without cross-sensitivity of temperature.
A short fiber Bragg grating (FBG) was successfully written in a multimode fiber (MMF) tube with core and cladding diameters of 105 μm and 125 μm using 800 nm femtosecond laser. A side-illumination technique was utilized to ensure the grating inscriptions regain over the core of MMF. Both fundamental mode and high-order modes of MMF are coupled at the core-mismatch junction and appear as two well-defined resonances in transmission. Femtosecond laserwritten three FBG-types present good thermostability up to 900 °C.
A fiber Bragg grating (FBG)-based gas refractometer is proposed and demonstrated experimentally. The configuration consists of a short section of S-type taper followed with a FBG. The S-taper is capable to couple the core mode to cladding modes into the downstream SMF, and the low-order cladding modes can be reflected back to the fiber core via the FBG, in which the recoupling efficiency is highly dependent on surrounding refractive index (RI) of liquid and gas. Experimental results show that some recoupled cladding modes show high sensitivities to surround RI. This power-referenced RI measurement and wavelength-referenced temperature measurement have been achieved via selective cladding modes monitoring.
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