Chi-Feng Chen

Fabrication of an antireflective polymer optical film with subwavelength structures using a roll-to-roll micro-replication process

An antireflective optical film with subwavelength structures replicated by use of a roll-to-roll micro-replication process (RMRP) is investigated. Firstly, a single layer of a nanostructure on a polymer film is designed for an antireflection purpose by the finite difference time domain method in the visible light spectrum. Structures of a conical cylinder array, with spatial period of 400 nm, diameter of 200 nm and height of 350 nm, are numerically obtained. Then, such structures are fabricated by RMRP combining originated structure fabrication realized by deep ultra-violet lithography and dry etching, Ni mold electroplating and replication by using the roll-to-roll process imprinting into the flexible polyethylene terephthalate substrate. A nanostructure roller mold bonded with Ni molds has been successfully fabricated and coated with the self-assembly monolayer process for the purpose of fabricating an anti-adhesion film and improving the lifetime of the Ni molds. The duplicated nanostructure films show a good optical quality of antireflection (AR ≤ 2.45% in a 400–700 nm spectral range) and are in good agreement with the theoretical predictions. The experimental results show that the developed process is a promising and cost-effective method for the continuous duplication of flexible devices with nano-scaled feature sizes used in nanophotonics by RMRP.

A dual-directional light-control film with a high-sag and high-asymmetrical-shape microlens array fabricated by a UV imprinting process

A dual-directional light-control film with a high-sag and high-asymmetric-shape long gapless hexagonal microlens array fabricated by an ultra-violent (UV) imprinting process is presented. Such a lens array is designed by ray-tracing simulation and fabricated by a micro-replication process including gray-scale lithography, electroplating process and UV curing. The shape of the designed lens array is similar to that of a near half-cylindrical lens array with a periodical ripple. The measurement results of a prototype show that the incident lights using a collimated LED with the FWHM of dispersion angle, 12°, are diversified differently in short and long axes. The numerical and experimental results show that the FWHMs of the view angle for angular brightness in long and short axis directions through the long hexagonal lens are about 34.3° and 18.1° and 31° and 13°, respectively. Compared with the simulation result, the errors in long and short axes are about 5% and 16%, respectively. Obviously, the asymmetric gapless microlens array can realize the aim of the controlled asymmetric angular brightness. Such a light-control film can be used as a power saving screen compared with convention diffusing film for the application of a rear projection display.

Experimental Study of a Silver Layer on an Antireflection Subwavelength-Structured Surface

The optical characteristics of a silver layer deposited on an antireflection subwavelength-structured surface are investigated. The experimental results of the reflectance and the transmittance of several different thicknesses of silver layer on the subwavelength-structured surface are carried out. A subwavelength structure with the spatial period and diameter of about 230 nm and height of about 150 nm on polyethylene terphthalate film is fabricated by microreplication process. It is shown that such an element with the suitable silver layer deposited on an antireflection subwavelength-structured surface has high transmittance and low glare in the visible spectral range and high reflectance in the infrared range. Obviously, the optical film with silver layer can not only obtain the high performance of heat insulation but also give an application in automobile and house windows.

Femtosecond Soliton Propagation in an Optical Fiber

An accurate wave equation beyond the slowly varying envelope approximation for femtosecond soliton propagation in an optical fiber is derived by the iterative method. The derived equation contains higher nonlinear terms than the generalized nonlinear Schrodinger equation obtained previously. For a silica-based fiber, we have shown that the higher nonlinear terms in the derived equation are negligible for optical pulses in the single cycle regime. The propagations of a 5 fs fundamental soliton and 10 fs and 50 fs second-order solitons in an optical fiber are numerically simulated. We have found that when a 5 fs fundamental soliton propagates in a fiber, it becomes asymmetric with an oscillatory structure near its trailing edge and the main pulse broadens. We have also found that, for the 10 fs second-order soliton, the soliton decay is dominated by the third-order dispersion, and for the 50 fs second-order soliton, the soliton decay is dominated by the delayed Raman response.

The Optimization of Directly-Under-Light Type Backlight Module Structure for Brightness Uniformity

The optimization of the directly-under-light type backlight module structure for brightness uniformity is investigated. All structure parameters of the 32-inch LCD-TV backlight module are chosen by the ray-tracing method for the optimal brightness uniformity. It is shown that the brightness uniformity without optical sheets can be made as high as 87.3%.

A Highly Directional Light Guide Plate Based on V-Groove Microstructure Cell

A new light guide plate (LGP) with highly directional emission properties based on V-groove cells has been investigated. To achieve high uniformity at normal luminance distributions for LCD backlight units (BLUs), we present a cell composed of multiple V-groove microstructures, and demonstrate its performance in a 10.1-inch BLU. We have found the density of the V-grooves to affect luminance distribution, and the directionality of emissions increases with number of V-groove microstructures in a cell. A cell composed of five V-groove microstructures delivered a 1.9-fold increase in optical efficiency when compared with a conventional BLU: the central luminance was 2.3 times greater, the main viewing angle was at -1°, and distribution of viewing luminance was almost symmetrical. In conclusion, our V-groove cell LGP for BLUs is thin and simple, providing high efficiency and high uniformity at normal luminance distributions.

Femtosecond soliton propagating in slow group-velocity fiber

Summary The slow group-velocity pulse in fiber described by nonlinear Schrodinger equation were demonstrated and investigated extensively. We derive a more generalized nonlinear Schrodinger equation as the superposition of monochromatic waves and numerically study the propagations of 2.5-fs fundamental and 5-fs second-order solitons. It is found that, for a slow-group velocity fiber, the magnitude of time shift is related with the group velocity and the more generalized NLSE is more suitable than the conventional generalized NLSE. When the pulse is slow down to 50% of normal group velocity ( c / n 0 ), the effect of the higher nonlinear terms is significant.

Design of Direct LED Backlighting Based on an Analytical Method of Uniform Illumination

A direct LED backlight unit (DLBLU) with extended sources based on an analytical design method of uniform illumination is investigated. We developed an analytical calculation method of the light spread function (LSF) for achieving highly uniform illumination into the diffuser plate (target plane) of a DLBLU. The effect of the rays reflected by the sides of the backlight box was also considered and calculated using the LSF. When comparing the results calculated using the LSF method at y = x and y = 0 with the results from the optical design software, the mean absolute percentage error (MAPE) were 0.74% and 0.46%, respectively. To demonstrate the effectiveness of the LSF method, a DLBLU model with 36 batwing sources was designed using the LSF method and assembled. The measurement result for DLBLU uniformity was 96.68%. When comparing the measurement result with the results of the LSF method at y = x and y = 0, the MAPEs were 0.92% and 0.46%, respectively. Thus, the proposed design method is an effective method for designing a DLBLU.

Non-degenerated Four-Wave Mixing Generated Power Ratio between Co-Propagated Pump Fabry--Perot Modes and Probe Signal in a Non-zero Dispersion Shifted Fiber

We theoretically and experimentally study of the four-wave mixing (FWM) that exists between the co-propagated pump Fabry–Perot (FP) modes of a FP laser diode (FPLD) and the single wavelength probe signal in a 25 km of non-zero dispersion shifted fiber (NZDSF) with zero dispersion wavelength of 1495 nm. The 100-mW co-propagated FP spectra centered at 1440 nm are reproduced via FWM around the 1-mW single wavelength signal around 1552 nm. The maximum FWM generated power ratio is found to have the first condition in matching of the group indices of the pump and signal to obtain the maximum FWM efficiency. And the second condition is mapping the signal wavelength to the center wavelength of the FP pump modes to have the adjacent FP modes with larger powers. The zero-dispersion wavelength of NZDSF can also be determined at the middle of the signal and the center wavelength of pump to have the maximum FWM generated power ratio.

New Weighting Function Designed for Low-Crosstalk, Small Length Mismatched Optical Coupler

A new weighting function we developed called the broadband weighting function is reported. It is numerically shown that excellent results are obtained compared with those obtained using the Blackman and Hamming weighting functions for application to the waveguide structure of mismatched optical couplers that satisty the demand for small length, C+L-band, and low crosstalk level. It is found that a coupler with a coupling length of 2.5 mm and a crosstalk of -35 dB can reach bandwidths within 1.360–1.700 µm. Using the same crosstalk and a coupling length of 3.7 mm, the broadband-weighting-function-weighted mismatched optical couplers can achieve broader bandwidths within 1.230–1.700 µm. Considering the tolerances of structure parameters, the coupler with a coupling length of 3.4 mm which is much smaller than those obtained using the Blackman and Hamming weighting functions (namely, 5.0 and 4.9 mm, respectively) satisfies the demands for the C+L-band and a crosstalk level of -35 dB.