Tight focusing of axially symmetric polarized vortex beams

Tight focusing of axially symmetric polarized vortex beams is studied numerically based on vector diffraction theory. The mathematical expressions for the focused fields are derived. Simulation results show that the focused fields and phase distributions at focus are largely influenced by both the polarization order and topological charge of the incident beams. Moreover, focal spots with flat-topped or tightly-focused patterns can be flexibly achieved by carefully choosing the polar- ization order and the topological charge, which confirms the potential of such beams in wide applications, such as optical tweezers, laser printing, lithography, and material processing.

Experimental verification on tightly focused radially polarized vortex beams

The theoretical and experimental results of tightly focused radially polarized vortex beams are demonstrated. An auto-focus technology is introduced into the measurement system in order to enhance the measurement precision, and the radially polarized vortex beams are generated by a liquid-crystal polarization converter and a vortex phase plate. The focused fields of radially polarized vortex beams with different topological charges at numerical apertures （NAs） of 0.65 and 0.85 are measured respectively, and the results indicate that the total intensity distribution at focus is dependent not only on the NA of the focusing objective lens and polarization pattern of the beam but also on the topological charge l of the beam. Some unique focusing properties of radially polarized vortex beams with fractional topological charges are presented based on numerical calculations. The experimental verification paves the way for some practical applications of radially polarized vortex beams, such as in optical trapping, near-field microscopy, and material processing.

Design and characteristics of a non-contact rotational sensor based on a fiber Bragg grating

We propose a novel non-contact rotational sensor based on a fiber Bragg grating(FBG) packaged in a core of a magnetic head, which converts the introduced strain from the circular magnetic railings ruler into the rotational information. A mathematical model is built for processing the data obtained by an interrogator, and the accuracy and resolution of the measurements are analyzed by altering the radius and period of the circular magnetic railings ruler, as well as the dimension of the sensor. The experimental results show that it is in good accordance with the theoretical analysis on rotational angle, and the fitting results indicate that the results obtained from the rotational sensor match very well with the real rotational velocity with a linearity of 0.998 and a standard error of about 0.01.

Performance enhancement of paper-based SERS chips by shell-isolated nanoparticle-enhanced Raman spectroscopy

Paper-based flexible surface-enhanced Raman scattering(SERS) chips have been demonstrated to have great potential for future practical applications in point-of-care testing(POCT) due to the potentials of massive fabrication, low cost, efficient sample collection and short signal acquisition time. In this work,common filter paper and Ag@Si O2 core-shell nanoparticles(NP) have been utilized to fabricate SERS chips based on shell-isolated nanoparticle-enhanced Raman spectroscopy(SHINERS). The SERS performance of the chips for POCT applications was systematically investigated. We used crystal violet as the model molecule to study the influence of the size of the Ag core and the thickness of the Si O2 coating layer on the SERS activity and then the morphology optimized Ag@Si O2 core-shell NPs was employed to detect thiram. By utilizing the smartphone as a miniaturized Raman spectral analyzer, high SERS sensitivity of thiram with a detection limit of 10^-9 M was obtained. The study on the stability of the SERS chips shows that a Si O2 shell of 3 nm can effectively protect the as-prepared SERS chips against oxidation in ambient atmosphere without seriously weakening the SERS sensitivity. Our results indicated that the SERS chips by SHINERS had great potential of practical application, such as pesticide residues detection in POCT.

Transformation of Laguerre-Gaussian beam by a ring-lens

The propagation property of Laguerre-Gaus- sian （LG） beams passing through a diffractive ring-lens （RL） was studied, where the RL was generated by a liquid crystal spatial light modulator （LC-SLM）. It was found that the LG beam was transformed into a sharp ring at the focal plane first, and then a Bessel-similar beam was formed behind the focal plane but the beam size was enlarged with the increase of propagation distance. With the help of a group of lenses, the beam was further collimated into a Bessel beam. Finally, the ＂non-diffractive＂ and self- reconstruction properties of the generated Bessel beams were experimentally verified.

Optical fiber sensor by cascading long period fiber grating with FBG for double parameters measurement

An optical fiber sensor for strain and temperature measurement based on long period fiber grating(LPFG) cascaded with fiber Bragg grating(FBG) structure has been proposed and realized both theoretically and experimentally. Theoretical analysis shows that two microstructures with similar sensitivities cannot be used for double parameters measurement. The LPFG is micromachined by the CO_2 laser, and the FBG is micromachined by the excimer laser. For the validation and comparison, two FBGs and one LPFG are cascaded with three transmission valleys, namely FBG1 valley at 1 536.3 nm, LPFG valley at 1 551.2 nm, and FBG2 valley at 1 577.3 nm. The temperature and strain characteristics of the proposed sensor are measured at 45—70 °C and 250—500 με, respectively. The sensitivity matrix is determined by analyzing wavelength shifts and parameter response characterization of three different dips. The proposed optical fiber sensor based on LPFG cascaded with FBG structure can be efficiently used for double parameters measurement with promising application prospect and great research reference value.

A switchable dual-wavelength fiber laser based on phase-shifted fiber Bragg grating combined with Sagnac loop

A switchable dual-wavelength erbium-doped fiber(EDF) laser is proposed and demonstrated.The interference filter is achieved by employing a phase-shifted fiber Bragg grating(PSFBG) combined with Sagnac loop structure.By adjusting polarization controller(PC) states, the birefringence effect is introduced to weaken mode competition, then stable and switchable dual-wavelength fiber laser can be realized.Based on coupled-mode theory and transmission matrix, the Sagnac loop transmission characteristics are studied.The experimental results show that the proposed fiber laser can operate in switchable dual-wavelength output mode at room temperature just by simply adjusting PC.The output wavelength range of fiber laser is 1 556.128—1 556.384 nm, the side mode suppression ratio(SMSR) is over 45 dB, and dual-wavelength spacing as small as 0.048 nm is achieved, which can be used in high-fineness dense wavelength division multiplexing(DWDM) systems and similar structures.