All-fiber polarization interference filters based on 45°-tilted fiber gratings

We report all-fiber polarization interference filters, known as Lyot and Lyot-?hman filters, based on alternative concatenation of UV-inscribed fiber gratings with structure tilted at 45° and polarization maintaining (PM) fiber cavities. Such filters generate comb-like transmission of linear polarization output. The free spectral range (FSR) of a single-stage (Lyot) filter is PM fiber cavity length dependent, as a 20 cm long cavity showed a 26.6 nm FSR while the 40 cm one exhibited a 14.8 nm FSR. Furthermore, we have theoretically and experimentally demonstrated all-fiber 2-stage and 3-stage Lyot-?hman filters, giving more freedom in tailoring the transmission characteristics.     

Analysis of sampled fiber Bragg gratings polarization-maintaining fiber

A novel type of sampled fiber Bragg gratings (FBGs) written in polarization-maintaining fiber (PMF) is proposed.The reflection spectrum,time delay,and group velocity dispersion (GVD) of the gratings are an- alyzed.In addition,the reflection spectrum is optimized by apodization.The scheme of multi-wavelength output based on the gratings is proposed,which could be used as a multi-wavelength polarization filter in the density wavelength division multiplexed (DWDM) system.     

Novel spectrum properties of the periodic π-phase-shifted fiber Bragg grating

A special periodic π-phase-shifted (π-PS) fiber Bragg grating (FBG) is proposed to achieve multichannel reflections. The Fourier analysis and the simulations based on the transfer matrix method show that this simple structure can generate multichannel response with high reflectivity. The reflectivity is much greater than that of the traditional sampled FBG for the same value of κL. Therefore the length of the fiber or the amplitude of refractive index modulation can be greatly reduced if such a periodic π-PS FBG is used instead of the traditional sampled FBG in the applications of comb filtering and optical wavelength division multiplexing. Besides, when the duty cycle is 0.5, this periodic π-PS FBG can create dual wavelength spectrum with high reflectivity.     

Effect of annealing temperature on optical and electrochemical properties of chitosan–ZnO nanostructure

Chitosan–ZnO nanostructures were prepared by chemical precipitation method using different concentration of zinc chloride and sodium hydroxide solutions. Nanorod-shaped grains with hexagonal structure for samples annealed at 300 °C and porous structure with amorphous morphology for samples annealed at 600 °C were revealed in SEM analysis. X-ray diffraction patterns confirmed the hexagonal phase ZnO with crystallite size found to be in the range of ~24.15–34.83 nm. Blue shift of UV–Vis absorption shows formation of nanocrystals/nanorods of ZnO with marginal increase in band gap. Photoluminescence spectra show that blue–green emission band at 380–580 nm. The chitosan–ZnO nanostructures used on surface of a glassy carbon electrode gives the oxidation peak potential at ~0.6 V. The electrical conductivity of chitosan–ZnO composites were observed at 2.1?×?10^?5 to 2.85?×?10^?5?S/m. The nanorods with high surface area and nontoxicity nature of chitosan–ZnO nanostructures observed in samples annealed at 300 °C were suitable as a potential material for biosensing.     

Twist characteristics of the ultraviolet-written long-period fiber gratings

Twist characteristics of the ultraviolet-written long period fiber gratings (UV-LPFGs) are investigated in this paper. It was found experimentally that the resonance wavelength of the UV-LPFG shifts to short side proportional to square of the torsion rate, when it is twisted in both directions of clockwise and counter-clockwise, which is different from that of LPFGs written by CO_2 lasers, but similar with that of the corrugated LPFGs. The phenomena can be explained by mechanisms of static compression in twisting and index change caused by photo-elastic effect.     

Optical properties of ytterbium-doped tandem-pumped fiber oscillatorOptical properties of ytterbium-doped tandem-pumped fiber oscillatorOptical properties of ytterbium-doped tandem-pumped fiber oscillator

With the rapid development of ytterbium-doped fiber lasers, some obtrusive limitations on power scaling appeared. In order to avoid these problems, a scheme called tandem pumping is introduced into the fiber laser field. In this paper, the optical properties of an ytterbium-doped tandem-pumped fiber oscillator are presented. According to the oscillator profile, the proper gain fiber type and pump wavelength range are picked out, under the comprehensive consideration of laser conversion efficiency and beam quality. In addition, the photodarkening performances of tandem pumping lasers and conventional ones are compared based on practical application, with all possible impact parameters taken into account. Moreover, an all-fibered tandem-pumped oscillator centered at 1079.5 nm is built, in the way of clad pumping by a 1030-rim fiber laser. The laser power of the oscillator reaches 7 W, with an opto-optic efficiency of 82.4%.     

Improvement on peak-to-trough ratio of sampled fiber Bragg gratings with multiple phase shifts

Via a cascaded structure,the peak-to-trough ratio is considerably improved for sampled fiber Bragg gratings(SFBGs)based on multiple-phase-shift(MPS)technique.This cascaded filter is composed of two identical SFBGs which are inserted with the increasing or decreasing arrangement of phase shifts. With this inverse arrangement of MPS in grating design,the phase fluctuation of individual SFBG can be compensated,and as a result an excellent phase matching condition is realized.In this way.the peak-to- trough ratio in reflection spectra is improved from 6 to 12 dB when multiplication factor m=4.and from 5 dB to 10 dB when m=8.     

Characteristics of the transmission spectrum of the long period fiber gratings based on the coupling of core mode to the higher order cladding modes

The characteristics of the transmission spectrum of the Long-period fiber gratings (LPFGs) based on the coupling of core mode to a higher order cladding mode (HE mode) are investigated using the coupled mode theory. This kind of LPFGs is different from that based on the coupling of core mode to a lower order cladding mode because of the effect of the coupling of core mode to EH cladding mode. When the cladding mode order is higher, the coupling coefficients of core mode to HE and EH cladding modes are comparable and both of the propagation constants of HE and EH cladding modes approach, so the spectrum has an additional loss peak. The bandwidth of LPFG based on the coupling of core mode to different cladding mode differs greatly. With the change of the mode orders from lower to higher, the transmission spectrum changes from narrow to wide and more narrow.     

Thermal characteristics of Fabry–Perot cavity based on regenerated fiber Bragg gratings

The Letter reports the thermal stability and strain response of Fabry–Perot(FP) cavity under different high temperatures. The FP cavity was made by thermal regeneration of two identical cascaded fiber Bragg gratings(FBGs). It is demonstrated that the FP cavity is capable of measuring temperatures from 300℃ to 900℃ with a temperature sensitivity of 15.97 pm/℃. The elongation of the fiber was observed through the drifted Bragg wavelength at 700℃ or above when weight was loaded. The elongation was further inferred by the slight change in the interference spectra of the FP cavity at 900℃.     

The generation of group delay ripple of chirped fiber gratings

The analysis of the group delay ripple (GDR) generated by chirped fiber gratings is developed based on the Fabry-Perot (F-P) resonator. It shows clearly how the GDR is generated and why the periods of GDR varies along the chirped fiber gratings. It could also explain why apodization could suppress the GDR and how apodization affects the chirp of the grating. The theory could be used to devise the apodization of fiber gratings.     

Influence of polarization and iron content on the transport properties of praseodymium–barium manganite

Polarization and iron effects on the electrical properties of Pr_0.67Ba_0.33Mn_1−xFe_xO₃ have been studied using impedance measurements. When iron is introduced, the insulator–metal transition (MI), observed in free compound, disappears and destroying such transition needs an iron concentration less than 5%. We also found that electrical conductance decreases when increasing Fe content. Such results are attributed to the decrease of Mn³⁺/Mn⁴⁺ ratio. Also, they are ascribed to the high probability of encountering Fe³⁺–O–Fe³⁺ and Mn³⁺–O–Fe³⁺ interactions, which greatly weakens the influence of Mn³⁺–O–Mn⁴⁺ interactions. The AC conductivity studies indicate that different types of hopping are involved. The contribution of hopping mechanism is confirmed by the temperature dependence of the frequency exponent ‘s’. Conductivity analysis shows that small polaron hopping (SPH) and variable range hopping (VRH) models are present in the conduction process. For small iron concentrations (x<0.1), we found that activation energy (E_a) does not changes significantly. Such result is in good agreement with the literature. But, for high iron concentrations (x>0.1), we found that E_a depend strongly in Fe content. We also found in this work that DC-bias does not affect the conduction process but proves its thermal activation. The variation of the conductance with polarization is a proof of an electro-resistance effect.     

Study of fiber-optic current sensing based on degree of polarization measurement

A novel fiber-optic current sensing scheme is proposed by converting the Faraday rotation to the optical signal's degree of polarization (DOP) change. In this scheme, the lightwave passes through a fiber resonant cavity multiply and experiences Faraday rotation simultaneously. Its main merit is immunity from the environment disturbance to the fiber used in ordinary Faraday rotation sensor. Brief theoretical analysis and simulation are given to show its basic characteristics. Experimental results are demonstrated and the feasibility of the proposed method is also shown.     

Broad tunability of erbium-doped fiber ring laser based on few-mode polarization maintaining fiber Sagnac interferometer

A highly stable erbium-doped fiber (EDF) ring laser with broad tunability is proposed and experimentally demonstrated. It is accomplished by using a few-mode polarization maintaining fiber (FM-PMF) Sagnac interferometer, in which simultaneous Sagnac and modal interferences occur. Continuously tunable output is obtained by exerting the longitudinal strain on the FM-PMF in combination with adjusting a polarization controller (PC) next to the FM-PMF in the Sagnac interferometer. In addition, the adverse impact of uneven EDF gain distribution on the laser output power uniformity may be offset by the PC-dependent transmission loss of the FM-PMF Sagnac interferometer and reflected backward C-band amplifier spontaneous emission from the pumped EDF. During the whole tuning process, the tunable span covers more than 40 nm with the output power flatness better than 1 dB, 3 dB spectral linewidth narrower than 0.1 nm and optical signal-to-noise extinction ratio higher than 45 dB.     

Active temperature compensation design of sensor with fiber gratings

A technique for compensation of temperature effects in fiber grating sensors is reported. For strain sensors and other sensors related to strain such as electromagnetic sensors, a novel structure is designed, which uses two fiber Bragg gratings (FBGs) as strain differential sensor and has temperature effects cancelled. Using this technique, the stress sensitivity has been amplified and gets up to 0.226 nm/N, the total variation in wavelength difference within the range of 3-45 °C is 0.03 nm, 1/14 of the uncompensated FBG. The structure can be used in the temperature-insensitive static strain measurement and minor-vibration measurement.     

Effects of rapid thermal annealing on the structural and electrical properties of Na-doped ZnMgO ?lms

Rapid thermal annealing (RTA) is a general approach to improve the crystallinity of thin solid films. In this study, we investigated the effects of RTA on the structural and electrical properties of Na-doped ZnMgO ?lms grown by pulsed laser deposition. X-ray diffraction (XRD) results showed that the crystallinity of the Na-doped ZnMgO ?lms was improved with RTA at 400-700 °C, and the grain size became larger as the annealing temperature increased. Moreover, room-temperature photoluminescence (PL) measurements demonstrated decent optical quality of the as-deposited and annealed Na-doped ZnMgO ?lms. Hall-effect measurements showed that the hole concentration increased from 4.9 × 10¹⁴ to 6.6 × 10¹⁵ cm⁻³ to 1.9 × 10¹⁷ to 8.3 × 10¹⁷ cm⁻³ while the resistivity and the Hall mobility decreased after the RTA treatments. The conduction type of the films converted from p to n when the annealing temperature is higher than 800 °C. Therefore a wide temperature window to obtain reasonable p-type Na-doped ZnMgO films by RTA is achieved. It is important because RTA is generally needed to obtain p-type Ohmic contact in the fabrication processes of light-emitting diodes (LEDs).     

Design and properties study of fiber optic glucose biosensor

A new type of fiber optic glucose biosensor based on fluorescence quenching has been designed and its properties have been studied. Glucose can be oxidized by oxygen when glucose oxidase are used as the catalyst, therefore, the concentration of glucose can be measured by detecting the consumption of oxygen. For the detection of oxygen concentration, the ruthenium(Ⅱ) complex, Ru(bpy)3Cl2, were used as the fluorescence indicator and its fluorescence lifetime were detected by lock-in technology. The detecting range of the sensor is 50 - 500 mg/dl and its response time is 30 seconds, showing that this kind of sensors is possible to be used in clinical diagnosis and detection.     

Wavelength switching in a mixed structure of a long-period and a Bragg fiber gratings

We built a numerical model for evaluating the coupling processes of a mixed structure of a Bragg fiber grat-ing and a long-period grating. From the numerical results, we not only confirmed the wavelength switchingphenomena observed in previously reported experiments, but also discovered a new coupling mechanism,which generated the reflection of a signal with its wavelength longer than the Bragg wavelength. Thedependencies of the wavelength switching behaviors on various parameters of the mixed grating structure     

Invariance of spectrum and polarization of electromagnetic Gaussian Schell-model beams propagating in free space

The propagation of polychromatic electromagnetic Gaussian Schell-model (EGSM) beams in free space is investigated. It is shown that the spectral degree of polarization, spectral degree of coherence, and normalized spectrum change generally on propagation. The conditions of keeping the spectral invariance and keeping polarization invariance for the polychromatic EGSM beams are derived respectively. The results indicate that the constraints on the parameters of EGSM source to keep polarization invariance on propagation are more rigorous than those to keep invariance of the normalized spectrum.     

Measurement of temperature and refractive index based on surface long-period gratings deposited onto a D-shaped photonic crystal fiber

We propose a surface long-period grating (LPG) based on a D-shaped photonic crystal fiber (PCF). The D-shaped PCF is fabricated by a side-polishing technique. The surface LPG based on periodic patterns of photoresist (PR) is formed by using the spin-coating and the standard contact lithography methods. Resonant coupling is created by the surface PR-LPG in the D-shaped PCF. The resonant peak shifts to longer wavelength as the ambient index is increased and shifts to shorter wavelength as the temperature is increased. The total wavelength shift is measured to be 122 nm in the refractive index range from 1 to 1.45 and the temperature sensitivity is measured to be −0.3 nm/°C in the temperature range from 30 to 100°C.     

Calculations on polarization properties of alkali metal atoms using Dirac–Fock plus core polarization method

A semi-empirical atomic structure model method is developed in the framework of a relativistic case. This method starts from Dirac-Fock calculations using B-spline basis set. The core-valence electron correction is then treated in a semiempirical core polarization potential. As an application, the polarization properties of alkali metal atoms, including the static polarizabilities and long-range two-body dispersion coefficients, have been calculated. Our results are in good agreement with the results obtained from ab initio relativistic many-body perturbation method and the available experimental measurements.