The spectral sensitivity of long period gratings fabricated in elliptical core D-shaped optical fibre

Long period gratings (LPGs) were written into a D-shaped optical fibre that has an elliptical core with a W-shaped refractive index profile and the first detailed investigation of such LPGs is presented. The LPGs’ attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15 nm between the two orthogonal polarisation states. A finite element method was successfully used to model many of the behavioural features of the LPGs. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature and bending. These LPG devices produced blue and red wavelength shifts depending upon the orientation of the bend with measured maximum sensitivities of −3.56 and +6.51 nm m, suggesting that this type of fibre LPG may be useful as a shape/bend orientation sensor with reduced errors associated with polarisation dependence. The use of neighbouring bands to discriminate between temperature and bending was also demonstrated, leading to an overall curvature error of ±0.14 m⁻¹ and an overall temperature error of ±0.3 °C with a maximum polarisation dependence error of ±8 × 10⁻² m⁻¹ for curvature and ±5 × 10⁻² °C for temperature.     

Noise figure and gain temperature dependent of praseodymium-doped fiber amplifier by using rate equations

A theoretical study of the temperature dependent noise effects of praseodymium-doped fiber amplifiers (PEDFAs) has been examined. The Pr³⁺-doped ZBLAN fiber amplifier pumped at 1017 nm and Pr³⁺-doped GeGa-sulfied fiber amplifier pumped at 1028 nm are chosen. The temperature-dependent rate and propagation equation related to four-level system consideration which is based on the population difference among amplification levels has been used. The population difference depends on pump and signal powers, Boltzman factor K_B, cross-sections, noise figure (NF) and Pr³⁺ concentration. The numerical results obtained over the temperature range from −20 °C to + 60 °C are used to present an analytical expression for the signal gain and noise figure effects in PDFAs length and noise figure with input pump power. The amplified spontaneous emission (ASE) has been taken into account.     

Modelling consideration of praseodymium-doped fiber amplifiers for 1.3 μm wavelength applications

To obtain the temperature-sensitive rate equations, a new energy level diagram of Praseodymium ion (Pr³⁺) in a glass host is modelled. By solving the modified rate equations, an analytical expression is presented to investigate the temperature dependence of the signal gain of a praseodymium-doped fiber amplifier (PDFA). It is seen that a change in the signal gain slightly depends on the variation of the distribution of Pr³⁺-ions in transitions ³F₄ ↔ ³F₃ with the temperature. Numerical calculations are carried out for the temperature range which is changing from −20 to +60 °C. Pr³⁺-doped ZBLAN fiber amplifier pumped at 1017 nm and Pr³⁺-doped sulfide fiber amplifier pumped at 1028 nm are selected as an application for the 1.3 μm signal wavelengths. It is also seen that the prediction of the model is in good agrement with their experimental results.     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

Gain clamped L-band EDFA with forward–backward pumping scheme using fiber Bragg grating

The paper proposes a novel two stage L-band erbium doped fiber amplifier with forward–backward pumping scheme for transmission of 32 wavelength division multiplexed (WDM) channels. It is gain clamped with an in-line fiber Bragg grating (FBG) to provide flat gain over 45 nm by restricting and reutilizing amplified spontaneous emission (ASE). We demonstrate that it provides an efficient small signal gain with minimum noise figure of over 20 dB and 5.5 dB, respectively, in the L-band region (1565–1610 nm) by comparing with its forward and backward pumped counterparts with fixed Er³⁺ fiber length of 20 m for −30 dBm/channel input power. We also obtain the gain and noise figure dependence as a function of each of the Er³⁺ fiber lengths, pump power (both 1480 and 980 nm), and temperature. Hence a 10 nm region (1580–1590 nm) has been acknowledged where temperature variations become constricted for 30 °C variations (15–45 °C).     

Research on the temperature characteristics of optical fiber refractive index

The delay of optical signal is determined by the refractive index and length of optical fiber, and temperature would have an intense influence on the index. To establish the relationship between refractive index and temperature, the temperature characteristics of refractive index was analyzed and the thermo-optical coefficient equation was derived according to the polarization of the induced electric dipole moment in SiO₂ optical fiber. A measuring system based on optical fiber delay was carried out to measure the index within the temperature range of −30 °C to 70 °C and the experimental result was compared with the theoretical result. The final result shows that the relationship between refractive index and temperature is linear in the temperature range of discussion.     

Discrimination of strain and temperature based on a polarization-maintaining photonic crystal fiber incorporating an erbium-doped fiber

A simple sensing method for simultaneous measurement of temperature and strain is investigated by using a Sagnac fiber loop mirror composed of a polarization-maintaining photonic crystal fiber (PM-PCF) incorporating an erbium-doped fiber (EDF). Amplified spontaneous emission created by a pumped EDF is transmitted to a Sagnac fiber loop mirror. The interference between two counter-propagating signals in a Sagnac fiber loop mirror generates a periodic transmission spectrum with respect to wavelength. When external temperature is increased, the transmission peak power reduces because the amplified spontaneous emission of the EDF is decreased by the applied temperature change (0.04 dB/°C). The peak wavelength is shifted into the shorter wavelength because of the negative temperature dependence of the birefringence of the PM-PCF (0.3 pm/°C). As the applied strain increases, the peak wavelength of the transmission spectrum of the Sagnac loop mirror incorporating the EDF shifts into a longer wavelength (1.3 pm/με) because the phase change of the proposed sensing probe is directly proportional to the applied strain. The transmission peak power, however, is not changed by the applied strain. Since the source and the sensing probe are integrated, the overall system configuration is significantly simplified without requiring any additional broadband light source. Therefore, it is possible to simultaneously measure temperature and strain by monitoring the variation of transmission peak power and peak wavelength, respectively.     

Femtosecond laser-induced damage of gold films

Single- and multi-shot ablation thresholds of gold films in the thickness range of 31-1400 nm were determined employing a Ti:sapphire laser delivering pulses of 28 fs duration, 793 nm center wavelength at 1 kHz repetition rate. The gold layers were deposited on BK7 glass by an electron beam evaporation process and characterized by atomic force microscopy and ellipsometry. A linear dependence of the ablation threshold fluence F_th on the layer thickness d was found for d ≤ 180 nm. If a film thickness of about 180 nm was reached, the damage threshold remained constant at its bulk value. For different numbers of pulses per spot (N-on-1), bulk damage thresholds of ∼0.7 J cm⁻² (1-on-1), 0.5 J cm⁻² (10-on-1), 0.4 J cm⁻² (100-on-1), 0.25 J cm⁻² (1000-on-1), and 0.2 J cm⁻² (10000-on-1) were obtained experimentally indicating an incubation behavior. A characteristic layer thickness of L_c ≈ 180 nm can be defined which is a measure for the heat penetration depth within the electron gas before electron-phonon relaxation occurs. L_c is by more than an order of magnitude larger than the optical absorption length of α⁻¹ ≈ 12 nm at 793 nm wavelength.     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

Structural and surface properties of Si1−xGex thin films obtained by reduced pressure CVD

This paper investigates the structure and surface characteristics, and electrical properties of the polycrystalline silicon-germanium (poly-Si_1−xGe_x) alloy thin films, deposited by vertical reduced pressure CVD (RPCVD) in the temperature range between 500 and 750 °C and a total pressure of 5 or 10 Torr. The samples exhibited a very uniform good quality films formation, with smooth surface with rms roughness as low as 7 nm for all temperature range, Ge mole fraction up to 32% (at 600 °C), textures of 〈2 2 0〉 preferred orientation at lower temperatures and strong 〈1 1 1〉 at 750 °C, for both 5 and 10 Torr deposition pressures. The ³¹P⁺ and ¹¹B⁺ doped poly-Si_1−xGe_x films exhibited always lower electrical resistivity values in comparison to similar poly-Si films, regardless of the employed anneal temperature or implantat dose. The results indicated also that poly-Si_1−xGe_x films require much lower temperature and ion implant dose than poly-Si to achieve the same film resistivity. These characteristics indicate a high quality of obtained poly-Si_1−xGe_x films, suitable as a gate electrode material for submicron CMOS devices.     

Deep ultraviolet femtosecond laser tuning of fiber Bragg gratings

A deep ultraviolet femtosecond laser operating at wavelength 258 nm was demonstrated to be effective in trimming fiber Bragg gratings in telecommunication fibers. A smooth tunable resonance wavelength shift of up to 0.52 nm has been observed, corresponding to a refractive index change of ∼5 × 10⁻⁴ after an accumulated laser fluence of 63.3 kJ/cm² at a single pulse fluence of 124 mJ/cm². The ultrafast laser enhancement of ultraviolet photosensitivity response and modification of anisotropic index profile in silica fiber is a powerful technique to precise control of the performance of fiber Bragg grating devices for applications in optical filtering and polarization mode dispersion management.     

Continuous-wave Ho:GdVO4 laser pumped by Tm-doped fiber laser with a fiber Bragg grating at room temperature

We reported the Ho:GdVO₄ laser pumped by Tm-doped laser with a fiber Bragg grating. 2.03 W continuous-wave Ho:GdVO₄ laser output power is obtained under 10.5 W incident pump power, with the optical-to-optical conversion efficiency and slope efficiency of 19.3% and 32.3%, respectively, at 7 °C. We can see that, the lower the temperature is, the better the laser output character is. The beam quality factor is M² ∼ 1.29 measured by the traveling knife-edge method.     

Waveband-switchable multiwavelength mixed-cascaded phosphosilicate raman fiber lasers

In this paper, the mixed-cascaded Raman scattering has been developed to investigate multiwavelength phosphosilicate Raman fiber lasers (MRFLs). With a tunable Yb³⁺-doped double-clad fiber laser (YDCFL) as the Raman pump source, we propose a compact and waveband-switchable (from the O- to U-band) MRFL using two- or three-mixed-cascaded Raman scattering of both SiO₂/GeO₂ and P₂O₅ in a P-doped fiber. We also confirm experimentally the feasibility of the proposed mixed-cascaded MRFL. When a 1064 nm YDCFL was used to pump a spool of 1-km P-doped fiber, the compact linear-cavity MRFLs in the O- and L-band operation were obtained, respectively, based on the two- and three-mixed cascaded Raman scattering. Up to 16-wavelength stable oscillation around 1320 nm has been observed with a spacing of 0.40 nm and an extinction ratio >30 dB. 12 lasing lines around 1601 nm have also been achieved with a spacing of 0.58 nm. The multiwavelength output powers as high as 108 and 138 mW were obtained in the O- and L-band operations, respectively. The wavelength spacing of the MRFLs is flexibly adjustable, and the peak wavelength of each lasing line is continuously tunable over the wavelength spacing. In addition, the important characteristics of the mixed-cascaded MRFLs, including the linewidth broadening, the signal-to-noise ratio and the conversion efficiency, are discussed.     

A global design of high power Nd-Yb co-doped fiber lasers

A global optimization method - niche hybrid genetic algorithm (NHGA) based on fitness sharing and elite replacement is applied to optimize Nd³⁺-Yb³⁺ co-doped fiber lasers (NYDFLs) for obtaining maximum signal output power. With a objective function and different pumping powers, five critical parameters (the fiber length, L; the proportion of pump power for pumping Nd³⁺, η; Nd³⁺ and Yb³⁺ concentrations, N_Nd and N_Yb and output mirror reflectivity, R_out) of the given NYDFLs are optimized by solving the rate and power propagation equations. Results show that dividing equally the input pump power among 808 nm (Nd³⁺) and 940 nm (Yb³⁺) is not an optimal choice and the pump power of Nd³⁺ ions should be kept around 10-13.78% of the total pump power. Three optimal schemes are obtained by NHGA and the highest slope efficiency of the laser is able to reach 80.1%.     

Intra-cavity second harmonic generation with Nd:YVO4/BIBO laser at 542 nm

We report, for the first time, an efficient intra-cavity second-harmonic generation (SHG) at 1084 nm in a nonlinear optical crystal, BiB₃O₆(BIBO) at the direction of (θ, ?) = (170.1°, 90°), performed with a LD end-pumped cw Nd:YVO₄ laser. With 590 mW diode pump power, a continuous-wave (cw) SHG output power of 19 mW at 542 nm yellow-green color has been obtained using a 1.5 mm-thick BIBO crystal. The optical conversion efficiency was 3.22%. It was found that the output wavelength could be 532 nm, 537 nm or 542 nm according to regulating the angle of BIBO.     

Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

A broadband ultra flattened chromatic dispersion microstructured fiber for optical communications

A double-cladding microstructured fiber (MF) is proposed in this paper. The inner cladding of this optical fiber is composed of elliptical air holes and silica. The dependence of dispersion on the diameter of the air holes, the pitch, and the axes of the elliptical holes is investigated numerically. The proposed fiber possesses an ultra flattened dispersion curve over a wide wavelength range, and its dispersion value is small. The effective mode area is approaching to 60 μm², and the confinement loss is as low as <0.025 dB/km at 1550 nm. While choosing suitable structure parameters, an ultra dispersion-flattened MF within a broadband from1000 nm to 1900 nm can be achieved. The dispersion fluctuation is 0.6-1.0 ps/(nm·km) in all S, C and L band.     

A tunable narrow-line-width multi-wavelength Er-doped fiber laser based on a high birefringence fiber ring mirror and an auto-tracking filter

A novel multi-wavelength erbium-doped fiber laser operating in C-band is proposed and successfully demonstrated. The wavelength interval between the wavelengths is about 0.22 nm. The 3 dB bandwidth of the laser is about 0.012 nm, and the output power reaches 4.8 mW. By using a high birefringence fiber ring mirror (HiBi-FLM) and a tunable FBG, the laser realizes switchable and tunable characteristic. The mode hopping can be effectively prevented. Moreover, this laser can improve wavelength stability significantly by taking advantage of an un-pumped Er³⁺-doped fiber at the standing-wave section. The laser can operate in stable narrow-line-width with single-, dual-wavelength, and unstable triple-wavelength output at room temperature.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.