The performances of two liquid level sensors based on long-period fiber gratings are studied. The long-period gratings (LPGs) have similar characteristics (length and period), but are fabricated with two photosensitive B-Ge co-doped fibers with different dopant concentrations. We investigate the temperature sensitivities of LPGs and exploit their refractive index sensitivity to implement liquid level measurement. By controlling fiber parameters, such as the dopant concentrations, the measurement sensitivity of a LPG-based fiber optic liquid level sensor can be improved.

We describe a novel performance optimization method for all-optical amplitude reshaping via degenerated four-wave mixing (FWM) in highly nonlinear optical fiber. The proposed optimization method is achieved by judiciously configuring the FWM operational condition and exploiting the nonlinear phase shift induced by self- and cross-phase modulations to properly influence the FWM conversion efficiency. Through the proposed scheme, fully functional and prevailing reshaping performance, including significant amplitude jitter suppression and extinction ratio improvement, is obtained within a single FWM stage. Results of the present theoretical calculation and numerical simulation verify the feasibility and advantages of the proposed scheme.

We present a network-level signaling mechanism for user access and service setup in light emitting diode (LED)-based visible light communication (VLC) networks and define the corresponding signaling messages. In this mechanism, lamp selection is an important step for realizing flexible user access and efficient resource allocation. Two basic selection schemes are proposed, and an enhanced bandwidth-based scheme is presented. Simulation results show the different advantages among these schemes.

A simple fiber tip sensor for refractive index (RI) measurement based on Fabry–Perot (FP) interference modulated by Fresnel reflection is proposed and demonstrated. The sensor head consists of an etching-induced micro air gap near the tip of a single-mode fiber. The microgap and the fiber tip function as two reflectors to form a FP cavity. The external RI can be unambiguously measured by monitoring the fringe contrast of the interference pattern from the reflection spectrum. The experimental results show that the proposed sensor achieves temperature-independent RI measurement with good linear response. The proposed sensor achieves a high RI resolution of up to 3.4 \times 10 5 and has advantages of low cost and easy fabrication.

A pulsed master oscillator power amplifier system is constructed using a double-cladding polarized Yb-doped fiber and an all-fiber Q-switched narrow-linewidth pulsed laser used as seed laser. This system has a high repetition rate and provides a nanosecond pulsed laser with a narrow linewidth and linear polarization. Moreover, it generates amplified radiation with up to 14 W of average power, narrow linewidth (full-width at half-maximum is smaller than 0.12 nm), linear polarization at 1 080-nm wavelength, repetition rate of 51 kHz, and pulse duration of approximately 50 ns. Based on this pulsed amplified radiation, 3.5 W of green laser, with an optical-to-optical efficiency of 27.3%, is obtained via single-pass frequency doubling using a noncritical phase matching KTP crystal.

A two-dimensional (2D) distributed feedback (DFB) structure is fabricated on dye-doped sol-gel derived hybrid zirconia films by soft lithography. The Q-switched Nd:YAG laser (\lambda = 532 nm) is used to pump these structures. The lasing emissions of the gain medium doped with Rhodamine 6G (Rh6G) in two perpendicular directions are shown, and the threshold pump energy is measured.

The circular-like sidewall and trench around the periphery of the crater are obtained on Zinc metal surface subjected to fewer than 200 cumulative pulses laser ablation. These patterns can be attributed to the higher secondary heating by regular plasma spherical pressure. The 600 pulses laser ablation, however, results in the formation of undesirable bulk wrinkles of super-heated liquid at the side of the trench. These features may be applied in micro-manufacturing of high localizability and selectivity using laser zone texturing.

We demonstrate a 30-W all-fiberized laser in a master oscillator power amplifier (MOPA) structure with a central wavelength of 1 950 nm. The laser is a continuous-wave (CW) operation with double-clad Tm-doped silica fiber pumped by pigtailed laser diodes with a central wavelength of 793 nm. The optical-to-optical efficiency of the MOPA system is 37.5% with respect to the total 80-W pump power. The maximal slope efficiency of the system is approximately 43%, which belongs to the main amplifier and exceeds the Stoke limit. No obvious amplified spontaneous emitting (ASE) is observed in the experiment, and the power can be scaled straightforwardly by improving the pump power.

Lock-in phenomenon in ring laser gyroscopes is directly related to effective backscattering, which includes both backscattering and nonuniform loss. Effective backscattering often differs in different states and can only be reflected in a working state via online estimation in the working state. Moreover, effective backscattering can result in the intensity modulation of beams in the opposite directions. The effective backscattering parameters can be obtained by measuring the weak modulations in the intensity signals under different rotation rates and by using the curve-fitting method. This letter demonstrates the online estimation of backscattering.

We employ plane-wave with ultrasoft pseudopotential method to calculate and compare the total density of states and partial density of states of bulk-phase GaN, Ga0:9375N, and GaN0:9375 systems based on the first-principle density-functional theory (DFT). For Ga and N vacancies, the electronic structures of their neighbor and next-neighbor atoms change partially. The Ga0:9375N system has n-type semiconductor conductive properties, whereas the GaN0:9375 system has p-type semiconductor conductive properties. By studying the optical properties, the influence of Ga and N vacancy defects on the optical properties of GaN has been shown as mainly in the low-energy area and very weak in high-energy area. The dielectric peak influenced by vacancy defects expands to the visible light area, which greatly increases the electronic transition in visible light area.

The fluorescence power from biological tissue excited by a femtosecond laser pulse compared with excitation power does not appear to obey a simple quadratic relationship given by the steady non-linear theory. A more reliable analysis is developed based on transient two-photon absorption because the response time of two-photon absorption is longer than the width of a femtosecond pulse. Good agreement is obtained between the theoretical analysis and the experimental results of fluorescence power versus excitation power. This letter offers potential value to non-linear optics in biological tissues.

We present a theoretical calculation of the dependence of reflectivity Rpp of the improved fully leaky waveguide geometry, which comprises pyramid, matching fluid, and strongly anchored hybrid-aligned nematic liquid crystal (NLC) cell on the internal angle. The calculation is based on the multi-layer optical theory and the elastic theory of liquid crystals. For different sums of flexoelectric coefficients e11 and e33, the curve of Rpp moves a distance to the left or the right relative to the case of ignoring the flexoelectric effect and the distance of the movement varies with different flexoelectric coefficients. Consequently, the sum of flexoelectric coefficients can be explored by measuring the distance of the movement.

The polarization state is modulated by tilting birefringence component placed in the feedback external cavity. The variation of the polarization state in one period of modulation is found to be similar to sine wave. The periods become increasingly smaller. The maximum of variation in one period decreases against the rotated angle. The experimental phenomenon is subjected to the change of optical path and secondary reflection. The phenomenon is analyzed theoretically based on geometrical optics and crystal optics. High-accuracy measurements of absolute and relative angles can be realized based on the experimental phenomenon. The angle resolution is 0.1 arcsec in theory.

We show that absorbed and stored electromagnetic energy are proportional to the reflection group delay in highly reflective dispersive dielectric mirrors over the high-reflectivity band. Our theoretical considerations are verified by numerical simulations performed on different dielectric mirror structures. The revealed proportionality between group delay and absorbed energy sets constraint on the application of ultrabroadband and/or dispersive dielectric mirrors in broadband or widely tunable, high-power laser systems.

We present an experimental and theoretical study of self-rotation of optical polarization in a rubidium vapor. The atomic vapor is placed in a magnetic shielding cavity to suppress the Faraday rotation effect. In our experiment, Doppler-free spectroscopy configuration is used, and F=2 -> F'=3 transition of 87Rb D2 line is chosen. We observe self-rotation of optical polarization effect at different pump light ellipticities. A theoretical analysis is then provided based on the experimental conditions. Theoretical simulation and experimental results are in good agreement.

We present a design for tunable directional beaming through a subwavelength metallic double slit surrounded by dielectric surface-relief gratings. On-axis and off-axis beaming can be switched by controlling the incident angle to asymmetrically excite surface plasmon polaritons (SPPs) that are subsequently coupled out to propagating beams by the two gratings on the left and the right sides of the double slit. Furthermore, the division of optical power into two off-axis beaming directions can be tuned smoothly by varying the incident angle while keeping the total power almost unchanged. The mechanism of this effect is analyzed theoretically and verified using rigorous numerical simulations.

Chaotic laser radar based on correlation detection is a high-resolution measurement tool for remotely monitoring targets or objects. However, its effective range is often limited by the side-lobe noise of correlation trace, which is always increased by the randomness of the chaotic signal itself and other transmission channel noises or interferences. The experimental result indicates that the wavelet denoising method can recover the real chaotic lidar signal in strong period noise disturbance, and a signal-to-noise ratio of about 8 dB is increased. Moreover, the correlation average discrete-component elimination algorithm significantly suppresses the side-lobe noise of the correlation trace when 20 dB of chaotic noise is embedded into the chaotic probe signal. Both methods have advantages and disadvantages.

An approach to the simultaneous measurement of refractive-index (RI) and temperature changes using optical ring resonators is proposed and theoretically demonstrated. With a liquid-core silica ring resonator as an example, two different-order whispering gallery modes (WGMs) might differ in not only RI but also temperature sensitivities. Thus, a second-order sensing matrix should be defined based on these WGMs to determine RI and temperature changes simultaneously. The analysis shows that the RI and temperature detection limits can be achieved on the order of 10^{-7} RI unit and 10^{-3} K at a wavelength of approximately 780 nm.

A practicable experimental method for measuring scattering on rough surfaces is reported. The scattered patterns are captured on a screen composed of two pieces of ground glass and then imaged using a charge-coupled device. The scattered intensity profiles are extracted by converting the patterns in real space into the wave vector space. Isotropic and anisotropic samples of the rough backsides of silicon wafer are investigated respectively, and their intensity profiles are measured. The profiles of the anisotropic sample are obtained by reading the pixels on the specific orientation curves. The parameters of the samples are extracted using angle-resolved light-scattering schemes and theories. The results well agree with measurements obtained using an atomic force microscope.

This letter proposes and experimentally demonstrates a simple scheme for generating 40-GBaud carrier-suppressed return-to-zero differential quadrature phase shift keying (CSRZ-DQPSK) modulation signals with tunable pulsewidths using two dual-parallel Mach–Zehnder modulators (DPMZMs). The duty cycle of the generated 40-GHz CSRZ-DQPSK pulse train is continuously tuned from 31% to 62%, with the full-width at half-maximum tuned from 7.8 to 15.5 ps, by electrically tuning the delay between the two sine-clock signals in one of the DPMZMs. Error-free performance is achieved after 320-km transmission.

A three-wavelength lidar system is set up. The backscatter signals of 355, 532, and 1 064 nm are measured simultaneously to derive the optical depth, lidar ratio, and backscatter color ratio of cirrus clouds, respectively. The lidar configuration and the data processing are described. The case study shows that the optical depths of cirrus clouds are not dependent on wavelength while the backscatter color ratios are.

Chemical warfare agents (CWAs) are recognized as serious threats of terrorist acts against the civilian population. Minimizing the impact of these threats requires early detection of the presence of CWAs. Cavity ring-down spectroscopy (CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species. In this letter, the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate (DMMP). A limit of DMMP detection of approximately 77 ppb is achieved. The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2 \times 10-7cm-1.