A simple linear cavity dual-wavelength fiber laser using AWG as wavelength selective mechanism

In this paper, a simple design of linear cavity dual-wavelength fiber laser (DWFL) is proposed. Operating in the C-band region stretching from 1538.3 nm to 1548.6 nm, an arrayed waveguide grating (AWG) is used to generate the dual-wavelengths output together with a broadband fiber Bragg grating as a back reflector and an optical circulator with a 10% output coupling ratio which acts as a front mirror. The measured average output power of the DWFL is about −5.66 dBm and with a side mode suppression ratio (SMSR) of 53.1 dB. The spacing between the two output wavelengths can be varied from 0.8 nm to 10.3 nm with a stable output and minimum power fluctuations.     

Temperature-sensitive dual-segment polarization maintaining fiber Sagnac loop mirror

A new temperature-sensitive fiber Sagnac loop mirror (FLM) is proposed using two segments of polarization maintaining fiber (PMF). The proposed dual-segment FLM provides greater temperature sensitivity in the spectral spacing detuning compared to the conventional single-segment configuration. Besides, the proposed configuration also enables both efficient positive and negative spectral spacing detuning by rising the temperature of one of the PMF segments. The experimental results show that the proposed configuration has achieved a great improvement in increasing spectral spacing variation range by 6.6 times and an increment of temperature sensitivity as much as 337.6% as compared to the conventional configuration.     

A theoretical study of double-pass thulium-doped fiber amplifiers

An efficient fluoride-based thulium-doped fiber amplifier (TDFA) is theoretically demonstrated using a double-pass scheme. A reflector is incorporated in the double-pass TDFA to allow double propagation of the test signal in the gain medium and thus improve the gain of the TDFA. The small signal gain improvement of more than 15 dB is obtained in the 1465 nm region. A gain as high as 42 dB is obtained in this region with 300 mW of 1050 nm pump using 20 m of thulium-doped fiber. However, a noise figure penalty of approximately 1 dB is also obtained in this wavelength region. Differential equations are solved using the Runge-Kutta method in the theoretical analysis. The theoretical result is in agreement with the experimental result.     

Gain-clamping techniques in two-stage double-pass L-band EDFA

Two designs of long-wavelength band erbium-doped fiber amplifier (L-band EDFA) for gain clamping in double-pass systems are demonstrated and compared. The first design is based on ring laser technique where a backward amplified spontaneous emission (ASE) from the second stage is routed into the feedback loop to create an oscillating laser for gain clamping. The gain is clamped at 18.6 d B from -40 to -80 dBm with a gain variation of less than ±0.1 dB and a noise figure of less than 6 dB. Another scheme is based on partial reflection of ASE into the EDFA, which is demonstrated using a narrowband fiber Bragg grating. This scheme achieves a good gain clamping characteristic up to -12 dBm of input signal power with a gain variation of less than ±0.3 dB from a clamped gain of 22 dB. The noise figure of a 1580 nm signal is maintained below 5 dB in this amplifier since this scheme is not based on lasing mechanism. The latter scheme is also expected to be free from the relaxation oscillation problem.     

Directed assembly of copolymer materials on patterned substrates: Balance of simple symmetries in complex structures

The self-assembly of a lamella-forming blend of a diblock copolymer and its respective homopolymers on periodically patterned substrates is investigated by a concerted experimental and theoretical approach. The substrate pattern consists of square arrays of spots that preferentially attract one component of the blend. The mismatch between the lamellar equilibrium morphology of the copolymer material and the substrate pattern results in the formation of a bicontinuous morphology. At the substrate, a quadratically perforated lamella (QPL) assembles in perfect registry with the substrate pattern. From this, QPL necks emanate and reach the top surface of the film. The detailed structure of these cylindrical nanochannels is analyzed using Voronoi tessellation, orientation correlation functions, and the structure factor of the neck positions on the top surface. The surface morphology is dictated by the antagonism of the square symmetry of the substrate pattern and the tendency of the necks to locally pack in a hexagonal arrangement. The analogy and differences to a system of adsorbed monolayer on corrugated substrates is explored by comparing the arrangement of the necks on the film's top surface with the structure of a soft disk model on a quadratically corrugated substrate. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2589–2604, 2006     

Modeling of flow in a polymeric chromatographic monolith

The flow behavior of a commercial polymeric monolith was investigated by direct numerical simulations employing the lattice-Boltzmann (LB) methodology. An explicit structural representation of the monolith was obtained by serial sectioning of a portion of the monolith and imaging by scanning electron microscopy. After image processing, the three-dimensional structure of a sample block with dimensions of 17.8 μm × 17.8 μm × 14.1 μm was obtained, with uniform 18.5 nm voxel size. Flow was simulated on this reconstructed block using the LB method to obtain the velocity distribution, and in turn macroscopic flow properties such as the permeability and the average velocity. The computed axial velocity distribution exhibits a sharp peak with an exponentially decaying tail. Analysis of the local components of the flow field suggests that flow is not evenly distributed throughout the sample geometry, as is also seen in geometries that exhibit preferential flow paths, such as sphere pack arrays with defects. A significant fraction of negative axial velocities are observed; the largest of these are due to flow along horizontal pores that are also slightly oriented in the negative axial direction. Possible implications for mass transfer are discussed.     

Temperature sensor based on fluorescence measurement of Cerium Ytterbium doped fiber

An investigation of the characteristics of temperature-dependent fluorescence of a Cerium Ytterbium (Yb³⁺)- doped silica fiber (CYDF) is presented. A 0.3-m long CYDF is diode-pumped at 976 nm and the fluorescence intensity is measured against the fiber temperature. It is observed that the fluorescence intensity decreases linearly with the increase in temperature due to the enhancement of non-radiative transition as well as the quenching of the Ytterbium luminescence. The sensitivity of the sensor is obtained at 0.0024 mW/°C with a linearity of more than 98% in the range of room temperature to 250°C. The proposed sensor is compact, low-cost and has long-life. It can also be combined with a wide range of existing fiber-optic multiplexing schemes that can simultaneously detect multiple physical parameters.     

High-resolution Fresnel zone plate fabrication by achromatic spatial frequency multiplication with extreme ultraviolet radiation

We used an approach based on the self-imaging property of gratings to fabricate high-resolution Fresnel zone plates (FZPs). Under certain conditions, the illumination of a parent ZP with a wideband EUV beam produces a radially oscillating intensity distribution with double the spatial frequency of the ZP. This intensity distribution is observed in a certain distance range, given by the local zone width, the focal length of the ZP, and the spectral bandwidth of the illuminating beam. This phenomenon has been used to lithographically record daughter ZPs that have approximately half the zone width, thus twice the resolution, of the parent ZP. FZPs with zone widths as low as 30 nm have been fabricated in this way. Use of this technique in the extreme ultraviolet (EUV) region has the potential for high throughput production of FZPs and similar high-resolution diffraction optics with variable spatial frequency for the EUV and x-ray regions.     

Q-Switched Fiber Laser with a Hafnium-Bismuth-Erbium Codoped Fiber as Gain Medium and Sb2Te3 as Saturable Absorber

Journal of Russian Laser Research - We successfully demonstrate Q-switched fiber laser with a 20 cm long Hafnium-Bismuth-Erbium codoped fiber (HBEDF) as an active medium in conjunction with...