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The label-free detection of SARS-CoV-2 spike protein is demonstrated by using slightly tapered no-core fiber (ST-NCF) functionalized with ACE2. In the fabricated sensor head, abrupt changes in the mode-field diameter at the interfaces between single-mode fiber and no-core fiber excite multi-guided modes and facilitate multi-mode interference (MMI). Its slightly tapered region causes the MMI to be more sensitive to the refractive index (RI) modulation of the surrounding medium. The transmission minimum of the MMI spectrum was selected as a sensor indicator. The sensor surface was functionalized with ACE2 bioreceptors through the pretreatment process. The ACE2-immobilized ST-NCF sensor head was exposed to the samples of SARS-CoV-2 spike protein with concentrations ranging from 1 to 10 4  ng/mL. With increasing sample concentration, we observed that the indicator dip moved towards a longer wavelength region. The observed spectral shifts are attributed to localized RI modulations at the sensor surface, which are induced by selective bioaffinity binding between ACE2 and SARS-CoV-2 spike protein. Also, we confirmed the capability of the sensor head as an effective and simple optical probe for detecting antigen protein samples by applying saliva solution used as a measurement buffer. Moreover, we compared its detection sensitivity to SARS-CoV-2 and MERS-CoV spike protein to examine its cross-reactivity. In particular, we proved the reproducibility of the bioassay protocol adopted here by employing the ST-NCF sensor head reconstructed with ACE2. Our ST-NCF transducer is expected to be beneficially utilized as a low-cost and portable biosensing platform for the rapid detection of SARS-CoV-2 spike protein., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.)

With COVID-19 widespread worldwide, people are still struggling to develop faster and more accurate diagnostic methods. Here we demonstrated the label-free detection of SARS-CoV-2 spike protein by employing a SARS-CoV-2 spike antibody-conjugated phase-shifted long-period fiber grating (PS-LPFG) inscribed with a CO 2 laser. At a specific cladding mode, the wavelength separation (λ D ) between the two split dips of a PS-LPFG varies with the external refractive index, although it is virtually insensitive to ambient temperature variations. To detect SARS-CoV-2 spike protein, SARS-CoV-2 spike antibodies were immobilized on the fiber surface of the fabricated PS-LPFG functionalized through chemical modification. When exposed to SARS-CoV-2 spike protein with different concentrations, the antibody-immobilized PS-LPFG exhibited the variation of λ D according to the protein concentration, which was caused by bioaffinity binding-induced local changes in the refractive index at its surface. In particular, we also confirmed the potential of our sensor for clinical application by detecting SARS-CoV-2 spike protein in virus transport medium. Moreover, our sensor could distinguish SARS-CoV-2 spike protein from those of MERS-CoV and offer efficient properties such as reusability and storage stability. Hence, we have successfully fabricated a promising optical transducer for the detection of SARS-CoV-2 spike protein, which can be unperturbed by external temperature disturbances., (Copyright © 2021 Elsevier B.V. All rights reserved.)

In this work, we experimentally demonstrated an optical fiber sensor capable of performing simultaneous measurement of torsion and temperature using a π -phase-shifted long-period fiber grating (LPFG) inscribed on double-clad fiber (DCF), referred to as a PS-DC-LPFG. The fabricated PSDC- LPFG showed split attenuation bands near its resonance wavelength, and the two dips in these bands were selected as sensor indicators, denoted as Dips A and B, for the simultaneous measurement of torsion and temperature. The torsion and temperature responses of the two indicators were investigated in a twist angle range from -360° to 360° and a temperature range from 30 to 120 °C, respectively. When the twist angle increased from 0° to 360° (clockwise) at room temperature, both Dips A and B showed redshifts. On the contrary, when the twist angle decreased from 0° to -360° (counterclockwise), the two dips showed blueshifts. In terms of temperature responses, both dips showed redshifts with increasing ambient temperature while the sensor head (i.e., the PS-DC-LPFG) remained straight without any applied torsion. Owing to their linear and independent responses to torsion and temperature, the changes in torsion and temperature applied to the PSDC- LPFG could be simultaneously estimated from the measured wavelength shifts and calculated sensitivities of the two indicator dips.

We propose an optical fiber grating sensor capable of simultaneously measuring pH and temperature based on a phase-shifted long-period fiber grating (PS-LPFG) inscribed on high-birefringence fiber (HBF). The PS-LPFG was fabricated on HBF with CO₂ laser pulses, and a phase shift π was induced by inserting a grating-free fiber region (GFFR) between two identical LPFGs with a grating period of ˜510 μ m. The length of the GFFR was set as half of the grating period to induce a π phase shift. With the spectral characteristics of a π -PS-LPFG exhibiting two split attenuation bands, the PS-LPFG written on HBF, which is referred to as the HB-PS-LPFG, can create two polarization-dependent transmission spectra with dual-resonance dips at different wavelengths according to two orthogonal input polarization states, e.g., linear horizontal polarization (LHP) and linear vertical polarization (LVP). For simultaneous measurement of pH and temperature with the fabricated HB-PS-LPFG as a sensor head, the inter-resonance wavelength separation of the dual-resonance dips in each transmission spectrum obtained for an LHP or LVP input signal was exploited as a sensor indicator. By investigating the wavelength changes of the two sensor indicators, which were induced by pH and temperature variations, linear and independent spectral responses to both pH and temperature variations were experimentally confirmed in a pH range from 1 to 11 and a temperature range from 25 to 65 °C. Owing to the unique pH and temperature responses of the fabricated HB-PS-LPFG, ambient variations in pH and temperature could be simultaneously estimated from the measured wavelength changes and sensitivities of the two sensor indicators.

The switching characteristics of a vanadium dioxide (VO₂) thin-film device, in which the current flowing through the device can be switched through the photothermal effect using focused laser pulses, were investigated according to the dimensions of the sapphire substrate on which the VO₂ thin film was deposited through simulation using COMSOL Multiphysics. The physical properties of the VO₂ device, modeled for the simulation, were determined according to the structural and electrical properties and photothermally controlled current-switching characteristics of fabricated VO₂ devices. For a variety of substrate dimensions of the modeled VO₂ device, we explored transient variations in the temperature of some specific regions and the device current switched by laser irradiation. The investigation results revealed that the stability of the bidirectional current-switching operation triggered on and off by laser illumination tends to increase as the area of the substrate increases with its thickness fixed. However, above a certain substrate area, the rate of improvement in the switching stability decreases rapidly and approaches zero.

Here we report an optical fiber sensor capable of performing strain-insensitive simultaneous measurement of bending and temperature using a long-period fiber grating (LPFG) inscribed on doubleclad fiber (DCF) with a CO₂ laser at ˜10.6 μm. The LPFG inscribed on DCF, referred to as a DC-LPFG, was fabricated by scanning CO₂ laser pulses on an unjacketed DCF with a specific period. Due to co-directional mode coupling, the fabricated DC-LPFG has discrete attenuation bands widely distributed over hundreds of nanometers. Among these wavelength-dependent loss dips, adjacent two dips with different resonance wavelengths were selected as sensor indicators for the measurement of bending and temperature. For these two indicator dips designated as dips A and B , their bending and temperature responses were investigated in a curvature range of 4.90 to 21.91 m -1 and a temperature range of 30 to 110 °C. With increasing bending applied to the DC-LPFG at room temperature, dips A and B showed different blue shifts. The bending sensitivities of dips A and B were measured to be approximately -0.77 and 0.51 nm/m -1 , respectively. Unlike the bending response, they showed red shifts of different amounts with increasing ambient temperature, while the sensor head (i.e., the DC-LPFG) remained straight without any applied bending. The temperature sensitivities of dips A and B were measured to be ˜0.094 and ˜0.078 nm/°C, respectively. Owing to their linear and independent responses to bending and temperature, bending and temperature changes applied to the DC-LPFG could be simultaneously estimated from the measured wavelength shifts of the two indicator dips using their pre-determined bending and temperature sensitivities. Moreover, in a strain range of 0 to 2200 με (step: 200 με), strain-induced spectral variations of dips A and B were also measured, and the strain sensitivities of dips A and B were evaluated as approximately -0.028 and -0.013 pm/με, respectively. These strain-induced wavelength shifts were so small that they had little effect on the measurement results of bending and temperature. Thus, it is concluded that the fabricated DC-LPFG can be employed as a cost-effective sensor head for strain-insensitive separate measurement of bending and temperature.

Here we propose an optical fiber sensor capable of simultaneous measurement of liquid level and temperature by utilizing cascaded long-period fiber gratings (LPFGs) inscribed on high-birefringence fiber (HBF) and a Faraday rotator mirror (FRM). Due to the in-fiber Mach-Zehnder interference and birefringence of the HBF, these cascaded LPFGs have polarization-dependent discrete interference spectra, each of which is created within one of the two different attenuation bands obtained in the two orthogonal input polarization states, e.g., linear horizontal polarization (LHP) and linear vertical polarization (LVP). The minimum transmittance dip was selected as a sensor indicator for each interference spectrum obtained for LHP or LVP input signal. To monitor these indicator dips associated with LHP and LVP, referred to as the IDH and IDV, respectively, with one spectral scanning, an FRM was connected to the end of the cascaded LPFGs. Both the IDH and IDV spectrally shifted according to liquid-level or temperature changes and showed very linear responses to them with adjusted R ₂ values greater than 0.997. The liquid-level sensitivities of the IDH and IDV were measured as approximately -37.29 and -121.08 pm/mm in a liquid-level range of 0 to 55 mm, respectively. The temperature sensitivities of the IDH and IDV were measured as ˜28.79 and ˜218.21 pm/°C in a temperature range of 30 to 60 °C, respectively. Owing to their linear and independent responses to liquid level and temperature, our sensor can perform temperature-independent liquid-level measurement using their pre-determined liquid-level and temperature sensitivities, even if both liquid level and temperature change simultaneously.

In this paper, we propose a bend-insensitive optical fiber sensor capable of separately measuring strain and temperature by incorporating a fiber transmission grating (FTG) inscribed on high birefringence photonic crystal fiber (HBPCF) with a CO 2 laser. The FTG was fabricated by exposing unjacketed HBPCF to CO 2 laser pulses using the line-by-line technique. The FTG inscribed on HBPCF, referred to as the HBPC-FTG, has two resonance dips with different wavelengths depending on input polarization. These two resonance dips were utilized as sensor indicator dips denoted by a shorter wavelength dip (SD) and a longer wavelength dip (LD). The strain and temperature responses of the SD and LD were investigated in a strain range of 0 to 3105 μ and a temperature range of 30 to 85 °C, respectively. The measured strain sensitivities of the SD and LD at room temperature (25 °C) were approximately -0.46 and -0.58 pm/μ, respectively. Similarly, the measured temperature sensitivities of the SD and LD without applied strain (0 μ) were ˜5.99 and ˜9.89 pm/°C, respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the HBPC-FTG can be simultaneously estimated from the measured wavelength shifts of the two indicator dips (i.e., SD and LD) using their predetermined strain and temperature sensitivities. Moreover, bend-induced spectral variations of the SD and LD were also examined in a curvature range of 0-4.705 m -1 , and it was observed that both dips showed little wavelength shift due to applied bending. Thus, it is concluded from the experimental results that the fabricated HBPC-FTG can be employed as a cost-effective sensor head for bend-insensitive discrimination of strain and temperature.

In this study, we implemented reversible current switching (RCS) of 100 mA in a two-terminal device based on a vanadium dioxide (VO₂) thin film, which could be controlled by far-infrared (FIR) laser pulses. The VO₂ thin films used for fabrication of two-terminal devices were grown on sapphire (Al₂O₃) substrates using a pulsed laser deposition method. An optimal deposition condition was determined by analyzing the resistance-temperature curves of deposited VO₂ thin films and the current-voltage characteristics of two-terminal devices based on these films, which were suggested in our previous works. The film surface of the VO₂-based device was directly irradiated using focused CO₂ laser pulses, and the insulator-metal transition or metal-insulator transition of the VO₂ thin film could be triggered depending on laser irradiation. Consequently, RCS of up to 100 mA could be accomplished. This on-state current is close to the upper limit of the current flowing through our VO₂ device. The switching contrast, defined as the ratio between on-state and off-state currents, was evaluated and found to be ˜11,962. The average rising and falling times of the switched current were found to be ˜29.2 and ˜71.7 ms, respectively. In comparison with our previous work, the improved heat dissipation structure and the high-quality thin film could maintain the switching contrast at a similar level, although the on-state current was increased by about two times.

Bruton's tyrosine kinase (BTK) is an attractive target for treating patients with B cell malignancies and autoimmune diseases. Many BTK inhibitors have been identified; however, like other kinase inhibitors, they lack diversity in their core structures. Therefore, it is important to secure a novel scaffold that occupies the adenine-binding site of BTK. We screened an in-house library of natural products and their analogs via a biochemical assay to identify a novel scaffold for targeting BTK. A pyranochromenone scaffold, derived from a natural active component decursin, was found to be effective at targeting BTK and was selected for further optimization. A series of pyranochromenone analogs was synthesized through the modification of pyranochromenone at the C7 position. Pyranochromenone compounds with an electrophilic warhead exhibited promising BTK inhibitory activity, with IC 50 values in the range of 0.5-0.9 µM. A docking study of the representative compound 8 provided a reasonable explanation for compound activity. Compound 8 demonstrated good selectivity over other associated kinases and decreased the production of proinflammatory cytokines in THP cells. Moreover, compound 8 presented significant in vivo efficacy in a murine model of collagen-induced arthritis.

In this paper, we propose a wavelength-tunable narrowband fiber multiwavelength filter based on polarization-diversified loop structure. The proposed filter consists of a polarization beam splitter, three half-wave plates (HWPs), two quarter-wave plates (QWPs) and two polarization-maintaining fiber (PMF) segments. The lengths of the two PMF segments are equal with each other. Among the five waveplates within the filter, a pair of an HWP and a QWP and the other pair of an HWP and a QWP are located in front of each PMF segment. The last HWP is placed after the second PMF and used to adjust the effective azimuthal angle of the second PMF. Azimuthal angle sets of the five waveplates, which can give additional phase shifts from 0 to 360° to the narrowband transmittance function derived from the Jones matrix formula, were theoretically found. Narrowband transmission spectra were calculated at eight waveplate angle sets selected among the angle sets derived above, which were designated as sets I, II, III, IV, V, VI, VII, and VIII that induced additional phase shifts from 0 to 315° (step: 45°) in the transmittance function. The calculated multiwavelength spectra clearly show that the narrowband multiwavelength spectrum can be wavelength-tuned by 0.1 nm as the waveplate angle set switches from set I to set VIII. This theoretical prediction was experimentally verified by appropriately controlling the azimuthal angle of each waveplate.

Here we propose a wavelength-switchable erbium-doped fiber ring laser using a temperatureinsensitive spectral polarization-dependent loss (PDL) element and two fiber Bragg gratings (FBGs). The fiber PDL element was fabricated by inscribing a long-period grating (LPG) on polarizationmaintaining photonic crystal fiber (PMPCF) with a 10.6 μ m CO₂ laser. The LPG fabricated on PMPCF, referred to as PMPCF-LPG, has the characteristics of a fiber polarizer at two specific wavelengths due to the birefringence of PMPCF and the co-directional mode coupling of the LPG. The two wavelengths at which the fabricated PMPCF-LPG acts as a polarizer are two resonance wavelengths (~1528.58 and ~1555.90 nm) of the PMPCF-LPG, obtained for orthogonal input polarization states. By considering these two resonance wavelengths of the PMPCF-LPG, the Bragg wavelengths of two FBGs, which determine lasing wavelengths in our wavelength-switchable laser, were selected as ~1527.71 and ~1554.74 nm. As the temperature sensitivity of the PMPCF birefringence is 30 times lower than that of the birefringence of conventional polarization-maintaining fiber (PMF), the fabricated PMPCF-LPG could facilitate more stable switching operation between the two lasing wavelengths in comparison with a previous fiber laser employing an LPG inscribed on conventional PMF as a wavelength-switching filter. The lasing wavelengths of our laser could be switched by controlling input polarization of the PMPCF-LPG with a polarization controller, and temperature-insensitive wavelength switching operation was experimentally demonstrated over a temperature range of 25-100 °C.

Here we report on the simultaneous measurement of bending and temperature, carried out using a long-period fiber grating (LPFG) inscribed on polarization-maintaining fiber (PMF) with a CO₂ laser at ~10.6 μ m. An LPFG written on PMF, referred to as a PM-LPFG, has an input-polarizationdependent resonance dip, and two separated resonance dips, designated as Dips A and B , are obtained with respect to orthogonal input polarization. At the resonance wavelengths of Dips A and B , the core mode is coupled into two different cladding modes that have different bending and temperature sensitivities. The fabricated PM-LPFG whose grating period and length are ~505 μ m and ~14.65 mm, respectively, has two resonance wavelengths, i.e., λ A = ~1479.98 nm and λB = ~1568.78 nm, measured with respect to two orthogonal input polarization states. The bending sensitivities of this PM-LPFG were measured as ~22.23 and ~33.38 nm/m -1 (adjusted R ² values: ~0.9916 and ~0.9810) at λ A and λB , respectively, in a curvature range of 1.41-2.30 m -1 . The temperature sensitivities of the PM-LPFG were measured as ~0.132 and ~0.039 nm/°C (adjusted R ² values: ~0.9929 and ~0.9980) at λ A and λB , respectively, in a temperature range of 30-90 °C. These linear bending and temperature responses of the PM-LPFG at two different resonance wavelengths enable simultaneous measurement of bending and temperature variations applied to the PM-LPFG.

Here we propose a novel optical fiber sensor capable of simultaneous measurement of strain and temperature by utilizing a long-period fiber grating (LPFG) inscribed on polarization-maintaining photonic crystal fiber (PMPCF) as a sensor head. The sensor head was fabricated by irradiating CO₂ laser pulses to one side of PMPCF with line-by-line technique. The LPFG written on PMPCF (referred to as the PMPC-LPFG) exhibits two different wavelength-dependent loss bands, obtained at two orthogonal input polarization states. For two resonance wavelengths of these two wavelength-dependent loss bands, designated as Dips A and B, strain and temperature responses were investigated in a strain range of 0 to 2058 μɛ with a step of 98 μɛ and a temperature range of 30 to 85 °C with a step of 5 °C. Strain sensitivities of Dips A and B were measured and found to be approximately -0.82 and -1.43 pm/ μɛ , respectively, at room temperature (25 °C). Similarly, temperature sensitivities of Dips A and B were measured and found to be ~7.89 and ~4.76 pm/°C without applied strain (0 μɛ ), respectively. Owing to their linear and independent responses to strain and temperature, strain and temperature changes applied to the PMPC-LPFG can be simultaneously estimated from the measured wavelength shifts of the two resonance dips (Dips A and B) using their premeasured strain and temperature sensitivities. The experimental results prove that the PMPC-LPFG can be used as a sensor head for simultaneous measurement of strain and temperature.

Here we report bidirectional current triggering (BCT) with a high repetition rate, achieved in a twoterminal planar device based on a vanadium dioxide (VO₂) thin film by using a laser diode with a center wavelength of 976 nm as an excitation source. The VO₂ thin film was grown on a sapphire (Al₂O₃) substrate using a pulsed laser deposition method, and the two-terminal planar device was fabricated by sawing the grown film into microscale unit devices, each of which was then attached onto a printed circuit board. Current triggering was performed by controlling the output power of the laser beam incident on the device surface. The proposed device allows stable current triggering operation even with laser pulses of higher repetition rate and lower energy because it is designed to have low heat capacity and thermal conductivity. Experimental results showed that a BCT of up to 30 mA was achieved at the maximum repetition rate of 8.0 Hz. The switching contrast between off- and on-state currents was calculated to be ~7295, and average rising and falling times of the current triggering were measured to be ~18.3 and ~22.5 ms, respectively.

Bidirectional current gating was realized in a vanadium dioxide (VO₂) thin film-based two-terminal heterostructure device using a near-infrared laser diode (LD) with a center wavelength of 976 nm. The VO₂ thin film used in the device fabrication was grown through pulsed laser deposition on a Si substrate with an aluminum nitride (AIN) buffer layer. The phase transition temperature of the fabricated VO₂/AIN/Si heterostructure device was ~78 °C, which is higher by ~10 °C than that of the device based on a conventional VO₂ thin film grown on a sapphire (Al₂O₃) substrate. Bidirectional current gating up to 60 mA was realized by directly irradiating the exposed thin film surface with the focused laser beam. The transient responses of the current flowing through the device were investigated for various pulse widths and repetition rates of the focused laser beam. The average switching contrast between off- and on-states was measured as ~9993. The average rise time of the current gating was ~31.5 ms with a much shorter fall time of ~4.0 ms. Our VO₂/AIN/Si heterostructure device could provide a high on-state current and fast response due to a smaller device dimension and higher phase transition temperature compared with previous implementations.

In this paper, we propose an optical comb filter based on a polarization-diversity loop structure, whose two transmission channel spacings (TCS's) can be alternately switched between two principal axes of the filter by controlling waveplates contained in the filter. The proposed filter consists of a polarization beam splitter (PBS), three half-wave plates (HWPs), one Faraday rotator (FR), and two polarization-maintaining fiber (PMF) segments with different lengths ( L and 2L ) and equal birefringence ( B ). As the PMF segments are utilized as birefringence elements, the TCS of the filter is determined by the product of their effective length L e and birefringence B . For an input signal introduced into each principal axis of the filter, L e can be the sum of or difference between the two lengths ( L and 2 L ) of the two PMF segments, that is, L or 3 L , resulting in a TCS of S 1 or S ₂, respectively, according to angular combination of the orientation angles of the three HWPs. At a specific set of the three HWP angles, the proposed filter can create two sinusoidal transmittance functions with different TCS's ( S 1 and S ₂) at the two principal axes, respectively. Also, the two TCS's ( S 1 and S ₂) are switchable with each other depending on three HWP angles for each principal axis. With the fabricated filter, the flexible switching between S 1 and S ₂ could be successfully implemented for two orthogonally polarized input signals aligned along the two principal axes of the filter by properly controlling the three HWPs, without any modification of the filter configuration.

In this paper, we investigated the pressure and temperature dependence of the frequency and amplitude of the field-induced oscillation created in a two-terminal device based on a vanadium dioxide (VO₂) thin film. First, a simple oscillation circuit was constructed using a VO₂-based two-terminal device, a standard resistor, and a DC power supply. Then, the frequency and amplitude variation of the field-induced oscillation was observed for pressure changes applied to the VO₂ device in a pressure chamber. This variation of the oscillation characteristics was also examined for ambient temperature changes applied to the VO₂ device using a plate heater. When the chamber pressure increased from 0 to 5 MPa with a step of 1 MPa at 25 °C, the oscillation frequency increased from ~592 to ~739 kHz, and the oscillation amplitude decreased from ~12.60 to ~11.40 V. Similarly, when the heater temperature increased from 25 to 50 °C (step: 5 °C) without applied pressure, the oscillation frequency increased from ~592 to ~819 kHz, and the oscillation amplitude decreased from ~12.60 to ~7.16 V. Owing to linear pressure and temperature responses of the VO₂ oscillation, the pressure and temperature sensitivities of the oscillation frequency and amplitude could be obtained as four different constant coefficients from the measurement results. These coefficients can be directly utilized for simultaneously measuring pressure and temperature variation applied to the VO₂ device, which can be beneficially applied to localized temperature and pressure sensing at a very small area less than 1 mm².

Here we show 60 mA bidirectional current gating in a two-terminal planar device based on a highly resistive vanadium dioxide (VO₂) thin film by harnessing photothermally induced phase transition occurred in VO₂ when irradiating the VO₂ film with a CO₂ laser oscillating at 10.6 μ m. The VO₂ thin film was grown by a pulsed laser deposition method, and the two-terminal planar device was fabricated using the VO₂ film isolated with sub-millimeter dimensions. The bidirectional current gating between 0 and 60 mA was accomplished by irradiating the VO₂-based device with repetitive pulses of the CO₂ laser. In terms of laser modulation parameters such as the pulse width and repetition rate, their effect on the transient responses of laser-gated currents was also investigated. With a minimum energy per pulse of ~766 mJ, a stable bidirectional current gating of up to 60 mA could be successfully implemented for the repetition rates of 0.5-3.0 Hz in a VO₂ device biased at ~5.4 V, showing a switching contrast between off- and on-state currents of ~11089. This maximum onstate current (60 mA) and switching contrast are the highest values among previous gating results attained in VO₂ devices with a CO₂ laser.

In this paper, we propose a high sensitivity polarimetric optical fiber pressure sensor (OFPS) using a polarization-diversity loop composed of a polarization beam splitter, polarization controllers, and a sensor head. The sensor head consists of 8-cm-long tapered panda-type polarization-maintaining fiber (PMF) and a fiber Bragg grating (FBG) directly spliced with PMF, and the sensor head is located inside a pressure chamber. A pressure-induced birefringence change due to the photoelastic effect can be greatly enhanced at the tapered section of PMF, thereby increasing the pressure sensitivity of the sensor head. The tapered PMF was fabricated using a fusion splicer, and the tapered length and center waist diameter of the tapered PMF segment were ~350 and ~56.82 μ m, respectively. At the polarization-diversity loop, PMF is used as a birefringent element to create an interference spectrum due to polarization interference. A pressure-induced birefringence change of PMF results in a wavelength shift of the interference spectrum. Because the PMF birefringence also has a cross sensitivity to temperature, the FBG is utilized for the compensation of the temperature effect on it. The resonance wavelength of the FBG is sensitive to ambient temperature changes but insensitive to changes in pressure. This spectral response of the FBG can be used to compensate additional ambient temperature changes occurred at the sensor head. The pressure sensitivity of our sensor was measured as approximately -27.70 nm/MPa, and an adjusted R ² value representing the sensor linearity was measured as ~0.9903 in a measurement range of 0-0.5 MPa. Our fabricated sensor exhibits the highest pressure sensitivity among previously reported polarimetric OFPS.

By incorporating a high-power 976 nm laser diode (LD), we demonstrated laser-regulated current switching in a two-terminal planar device based on a vanadium dioxide (VO₂) thin film. The VO₂ thin film was grown by pulsed laser deposition method and etched to sub-millimeter dimension for the fabrication of a two-terminal device. The reversible current switching was implemented by controlling the on/off state of the LD, which illuminates the VO₂-based device. The transient responses of the device currents were analyzed when the device was excited with laser pulses of various repetition rates of up to 5.0 Hz with a pulse width fixed as 75 ms. A switching contrast between off- and on-state currents was calculated as ~9530, and average rising and falling times were measured as ~31 and ~21 ms, respectively.

By incorporating an inline switching filter, which is comprised of a polarization-diversified loop (PDL), two fiber Bragg gratings (FBGs) with different resonances, and three quarter-wave plates (QWPs), we propose a dual-wavelength-switchable erbium-doped fiber (EDF) ring laser that can select and switch between two lasing lines with orthogonal polarization. The proposed laser is composed of EDF, a 980 nm laser diode, a wavelength-division-multiplexing coupler, a rotatable linear polarizer, an optical isolator, a 3 dB optical coupler, and the inline switching filter. At a special combination of the orientation angles of the three QWPs, the inline filter can offer a different transmittance according to input polarization, e.g., reflection spectra of one and the other of the two FBGs for linear horizontally and vertically polarized input light, respectively. At this special combination of the QWPs, one of two different resonances of the two FBGs can be selected by varying laser cavity polarization through the adjustment of the orientation angle of the rotatable linear polarizer. Consequently, switching operation between two laser lines with orthogonal polarization at the two FBG resonance wavelengths could be obtained by properly controlling cavity polarization. The polarization extinction ratio of each lasing line was measured as more than 19.9 dB.

A collimator design was investigated to localize ultrasound stimulation using a flat ultrasound transducer for ultrasound-induced neuromodulation in a mouse model. In brain stimulation, the specific location of stimulation must be specified, as the region responsible for motor or sensory function is clustered in a narrow brain area. To localize ultrasound stimulation, three types of collimator design were simulated to determine the optimal collimator design. The performance of the simulated optimal collimator was compared to that of an unmounted collimator in a transducer in both in vivo and in vitro experiments. Throughout the experiments, the localized ultrasound waveform was shaped using the optimized collimator, which elicited neural spike activity in the targeted motor cortex. The optimized collimator shows potential for controlling a localized ultrasound waveform for ultrasound-induced neuromodulation in a small animal model.

High-sensitivity temperature sensors have been used to validate real-time thermal responses in tissue during photothermal treatment. The objective of the current study was to evaluate the feasible application of a fiber Bragg grating (FBG) sensor for diffuser-assisted laser-induced interstitial thermotherapy (LITT) particularly to treat tubular tissue disease. A 600 - ? m core-diameter diffuser was employed to deliver 980-nm laser light for coagulation treatment. Both a thermocouple and a FBG were comparatively tested to evaluate temperature measurements in ex vivo liver tissue. The degree of tissue denaturation was estimated as a function of irradiation times and quantitatively compared with light distribution as well as temperature development. At the closer distance to a heat source, the thermocouple measured up to 41% higher maximum temperature than the FBG sensor did after 120-s irradiation (i.e., 98.7 ° C ± 6.1 ° C for FBG versus 131.0 ° C ± 5.1 ° C for thermocouple; p < 0.001 ). Ex vivo porcine urethra tests confirmed the real-time temperature measurements of the FBG sensor as well as consistently circumferential tissue denaturation after 72-s irradiation ( coagulation thickness = 2.2 ± 0.3 ?? mm ). The implementation of FBG can be a feasible sensing technique to instantaneously monitor the temperature developments during diffuser-assisted LITT for treatment of tubular tissue structure.