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Bound states in the continuum (BICs) with exceptionally high quality factors ($Q$ factors) have become focal points in photonic device research, finding applications in nonlinear enhancement, laser generation, sensing, and beyond. In this study, we systematically investigate the intricate relationship between the in-plane symmetry groups of photonic crystals and symmetry-protected BICs (SP-BICs) using a rigorous group theory approach. By modifying the spatial distribution of refractive indices to perturb the $C_{2}$ and $C_{3}$ symmetries within a $C_{6v}$ photonic crystal slab, we introduce a novel method to enable precise tuning of BIC resonances, including the BIC frequency, $Q$ factor, and polarization state. The proposed group theory approach and tuning mechanism establish a pioneering pathway for the design and tunability of BICs, opening avenues for the development of advanced photonic devices with tailored functionalities such as tunable lasers and optical sensors with enhanced performance.

Early diagnosis and treatment can reduce the symptoms of Attention Deficit/Hyperactivity Disorder (ADHD) in children, but medical diagnosis is usually delayed. Hence, it is important to increase the efficiency of early diagnosis. Previous studies used behavioral and neuronal data during GO/NOGO task to help detect ADHD and the accuracy differed considerably from 53% to 92%, depending on the employed methods and the number of electroencephalogram (EEG) channels. It remains unclear whether data from a few EEG channels can still lead to a good accuracy of detecting ADHD. Here, we hypothesize that introducing distractions into a VR-based GO/NOGO task can augment the detection of ADHD using 6-channel EEG because children with ADHD are easily distracted. Forty-nine ADHD children and 32 typically developing children were recruited. We use a clinically applicable system with EEG to record data. Statistical analysis and machine learning methods were employed to analyze the data. The behavioral results revealed significant differences in task performance when there are distractions. The presence of distractions leads to EEG changes in both groups, indicating immaturity in inhibitory control. Importantly, the distractions additionally enhanced the between-group differences in NOGO $\alpha $ and $\gamma $ power, reflecting insufficient inhibition in different neural networks for distraction suppression in the ADHD group. Machine learning methods further confirmed that distractions enhance the detection of ADHD with an accuracy of 85.45%. In conclusion, this system can assist in fast screenings for ADHD and the findings of neuronal correlates of distractions can help design therapeutic strategies.

This paper proposes an enhancement of semiconductor optical amplifier (SOA)-based scintillation mitigation by polarization shift on-off keying (PS-OOK) transmission in free-space optical (FSO) communication. The scintillation effect is a critical issue in a vertical FSO link. The issue of the extinction ratio (ER) degradation of the received OOK signal caused by the high dynamic gain frequency of the SOA limits the performance of the turbulence-induced scintillation effect mitigation using a gain-saturated and polarization-independent SOA. Therefore, a PS-OOK signal, which has the characteristics of orthogonal polarization of bits “1” and “0,” is transmitted at the transmitter end and a linear polarizer is deployed after the gain saturated SOA at the receiver end. Based on the polarization orthogonality of PS-OOK, the performance of the scintillation-effect suppression is significantly improved by the mitigation of the ER degradation by the blocking of the polarization of bit “0” of the received optical PS-OOK signal using the linear polarizer. The proposed scheme is experimentally verified utilizing the Mach-Zehnder modulator (MZM)-based fading simulator accommodated various turbulence channels. The experimental results illustrate that the scintillation effect is effectively suppressed by the proposed technique compared to the SOA-based conventional single-polarized OOK detection and balanced PS-OOK detection.

This paper proposes the enhancement of spectral efficiency (SE) by linear polarizer based multilevel linear polarization shift keying (MLPolSK) detection in free-space optical (FSO) communication. At the transmitter end, in order to improve SE of FSO links, the various linear state of polarizations (SOPs) with an equal magnitude-spacing of x- or y-polarization are modulated for MLPolSK signal. At the receiver end, the cascaded polarization-independent semiconductor optical amplifier (SOA) and linear polarizer are introduced to detect MLPolSK signal. Polarization-independent SOA is applied to effectively mitigate the turbulence induced scintillation effect by the gain saturation without the distortion of polarization. Linear polarizer with high extinction ratio (ER) is used to transform various SOPs into the signal with an equal intensity-spacing of x- or y-polarization. Besides, the amplified spontaneous emission (ASE) noises are reduced by the polarization of ASE noises by linear polarizer. Finally, MLPolSK is detected by a single photodiode (PD) and decided by multilevel-thresholdings. In experiments, the performance of the proposed technique is evaluated under turbulence channels, which are emulated using Mach-Zehnder modulator (MZM)-based fading simulator. Experimental results show that the SE is improved up to 3 bit/s/Hz under the effective scintillation mitigation using the proposed technique.

This study investigates the performance of scintillation mitigation for linear polarization shift on-off keying (LPS-OOK) transmission in free-space optical (FSO) communications. Scintillation effect is a critical issue for FSO communication in vertical links. The LPS-OOK signal with orthogonal linear x- and y-polarizations for bits `1' and `0' is modulated and transmitted from the transmitter end. The bits `1' and `0' suffer the scintillation effect in turbulence channel. Based on the orthogonal polarization characteristics of LPS-OOK, linear polarizer (LP) is used to mitigate the scintillation effect on bit `0' by blocking its polarization without the distortion of bit `1'. Further, owing to the nonlinear gain characteristics of logarithmic amplification, it is used to equalize the intensity fluctuation of bit `1'. Finally, the equalized LPS-OOK is distinguished using fixed threshold decision (FTD) instead of a complex symbol-by-symbol estimation of adaptive threshold. The performance of the proposed technique was verified experimentally for various degrees of the turbulence channel. A Mach-Zehnder modulator (MZM)-based turbulence channel simulator was adopted to accommodate turbulence-induced scintillation effect. The experimental results confirmed that the proposed technique that employed FTD has an improved bit-error-rate (BER) performance compared with that of OOK that employed an adaptive threshold decision (ATD) under various degrees of turbulence channels.

We demonstrate a state variable fiber pulse laser based on a graphene-integrated microfiber device. A 3-D microfiber coil integrated with graphene acts as a polarization-sensitive saturable absorber. By adjusting the polarization controllers, stable Q-switched pulses and mode-locked rectangular pulses are observed. To our knowledge, this is the first study conducted to achieve switching between rectangular pulses and Q-switched pulses while keeping the pump power constant. The rectangular pulses have a pulsewidth and repetition rate of ~10 ns and ~940 kHz, respectively, and the Q-switched pulses have a pulsewidth and repetition rate of ~40 μs and ~4.8 kHz, respectively.

A broadband electrically tunable variable polarizer for fiber systems based on a liquid crystal lens is proposed and demonstrated. The polarization selectivity is based on a polarization-sensitive coupling efficiency to the fiber in the fiber system. For an incident ordinary ray, the output beam size remains the same as that of the incident beam, which results in low coupling efficiency. For an incident extraordinary ray (e-ray), the output beam size is close to the size of the fiber core, giving rise to high coupling efficiency because of the lens effect. Moreover, the output beam size of the e-ray can be electrically controlled, thus allowing the optical attenuation to be manipulated. In our experiments, the polarization-dependent loss from the visible to the near-infrared spectral region was approximately 12 dB. Such a broadband electrically variable polarizer may be applied to various designs for fiber-optic sensing devices and polarization-sensitive optical instruments.

A heterodyne optical fiber current sensor based on a nanowire-grid (NWG) in-line polarizer is proposed. The NWG is a one-dimension Au grating fabricated on a fiber tip by focused ion beam machining. Because the grating pitch is far smaller than the wavelength of the incident light, it works like an in-line transflective fiber polarizer with both the transmission and reflection light power being measured simultaneously. Based on this unique feature, an all-fiber optical current sensor is demonstrated. The heterodyne measurement makes it withstand light power variations showing good sensing stability. As an all-fiber sensor, discrete and bulky optical components are saved, making it more reliable in practical applications.

We propose a surface plasmon interferometer based on a wedge metal waveguide. Working principles of the device are introduced through theoretical analyses. The interference performances are investigated by finite-difference time-domain (FDTD) method. Because of the special wedge geometry, the interferometer shows an extremely low coupling loss in comparison with the previous reports. An interesting surface plasmon polariton (SPP) coupling and splitting phenomenon is found at the wedge section. It is then split into two surface plasmon modes in two interference arms. Refractive index sensing applications of this interferometer are also discussed, which shows sensitivity as high as 6 × 103 nm/RIU.

An ~ 12-μm-long Bragg grating was engraved in an ~ 5-μm-diameter optical fiber tip by focused ion beam (FIB) milling. An ~ 10-dB extinction was achieved at 1570 nm with only 11 indentations. The grating was used for temperature sensing, and it exhibited a temperature sensitivity of ~ 22 pm°C.

We propose a photoelastic pressure sensor using a liquid crystal (LC) tunable fiber polarizer. The polarizer contains an etched single-mode fiber sealed in an LC cell. A low-index LC is used as a tunable birefringent cladding of the fiber. Any electric-field-induced LC director reorientation may thus affect the fiber's light-guiding characteristics. Different polarization states experience different LC cladding indices. The transmittance contrast between the x- and y-polarized modes reaches 31.9 dB, which is sensitive enough to monitor any polarization state change in the fiber. A pressure sensor with the sensitivity of 0.25 rad/N is thus demonstrated. The corresponding stress-optic coefficient of the fiber is measured at 1550-nm telecomm band. The advantages and applications of the LC tuning and sensing technologies are discussed.

This paper proposes an enhancement of semiconductor optical amplifier (SOA)-based scintillation mitigation by polarization shift on-off keying (PS-OOK) transmission in free-space optical (FSO) communication. The scintillation effect is a critical issue in a vertical FSO link. The issue of the extinction ratio (ER) degradation of the received OOK signal caused by the high dynamic gain frequency of the SOA limits the performance of the turbulence-induced scintillation effect mitigation using a gain-saturated and polarization-independent SOA. Therefore, a PS-OOK signal, which has the characteristics of orthogonal polarization of bits “1” and “0,” is transmitted at the transmitter end and a linear polarizer is deployed after the gain saturated SOA at the receiver end. Based on the polarization orthogonality of PS-OOK, the performance of the scintillation-effect suppression is significantly improved by the mitigation of the ER degradation by the blocking of the polarization of bit “0” of the received optical PS-OOK signal using the linear polarizer. The proposed scheme is experimentally verified utilizing the Mach-Zehnder modulator (MZM)-based fading simulator accommodated various turbulence channels. The experimental results illustrate that the scintillation effect is effectively suppressed by the proposed technique compared to the SOA-based conventional single-polarized OOK detection and balanced PS-OOK detection. [ABSTRACT FROM AUTHOR]

Metallic grating (MG) with high accuracy is fabricated on the side surface of a D-shaped fiber by using focused ion beam (FIB) machining. The D-shaped fiber is obtained through polishing the cladding of a single-mode fiber away from one side. Then, a thin-film layer of gold is deposited on the flat side surface. A 38- μm-long grating with the pitch of 485 nm is further milled on the gold film. The guiding mode and the surface plasmon polariton mode are thus coupled in the MG through the evanescent field for sensing applications. The refractive indexes of pure water and isopropanol are measured. A sensitivity of 917 nm/RIU is obtained.