Kuei-Chu Hsu

Tunable Er 3+ -doped fiber amplifiers covering S and C+L bands over 1490-1610 nm based on discrete fundamental-mode cutoff filters

We demonstrate thermo-optically tunable Er3+-doped fiber amplifiers covering S and C+L bands (1490-1610 nm) using fundamental-mode cutoff filters discretely located in a 17.5 m long standard Er3+-doped fiber. The material dispersion and waveguide structure of the tapered fibers are locally modified to produce high-cutoff-efficiency filters that make the unwanted amplified spontaneous emission highly attenuated at the longer wavelengths, and the optical gain is thus moved toward the shorter wavelengths. The maximal signal gains are measured to be 18.92, 37.18, and 15.19 dB with 980 nm pump power of 135 mW in the S, C, and L bands, respectively.

Spectral Response of Long-Period Fiber Grating Based on Tapered Fiber With Side-Contacted Metal Grating

A new long-period fiber grating structure based on a tapered fiber side contacted with a metal grating is proposed and demonstrated in this paper. The temperature/index tuning sensitivity can be enhanced, and the unusual spectral response of decreasing phase-matching wavelength with respect to increasing grating period and temperature is observed.

Analysis of Thermo-Optic Tunable Dispersion-Engineered Short-Wavelength-Pass Tapered-Fiber Filters

Thermo-optic tunable short-wavelength-pass tapered-fiber filters based on fundamental-mode cutoff mechanism are realized experimentally and analyzed theoretically. The effects of material and waveguide dispersion are investigated and the optimal tapered fiber structures for attaining high-spectral cutoff slope and high-rejection efficiency are determined.

Influence of depressed-index outer ring on evanescent tunneling loss in tapered double-cladding fibers

A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (-1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers.

Phase separation and pattern instability of laser-induced polymerization in liquid-crystal-monomer mixtures

Directly written by a ultra-short femto-second laser pulse, we report the phase separation and pattern formation induced by polymerization in a liquid-crystal-monomer mixture. By varying the scanning speed of optical fields along a line, pattern transitions of photon-induced polymer structures are illustrated in shapes of double-humped, single-humped, and broken stripes. The experimental data collected by optical microscopic images are in a good agreement with numerical simulations based on a modified set of coupled 2 + 1 dimensional diffusion equations. The demonstration in this work provides a step for controlling phase separation morphologies as well as transferring patterns in polymer-dispersed liquid crystals.

Single-longitudinal-mode linear-cavity fiber laser using multiple subring-cavities

A single-longitudinal-mode selection is realized in a linear-cavity fiber laser construction using a loop-back optical circulator and a partial reflectance fiber Bragg grating as the cavity ends. At 1551 nm wavelength, the measured signal-to-noise-ratio is 56 dB and with a line width less than 1 MHz obtained using multiple subring cavities as mode filters. The pumping efficiency is 10% improved by recycling the residual pump power to gain medium.

Bandpass Filter With Variable Bandwidth Based on a Tapered Fiber With External Polymer Cladding

A thermo-optical bandwidth-variable fiber-based bandpass filter is experimentally and theoretically demonstrated by simply utilizing a tapered standard single-mode fiber covered with a dispersive polymer layer. The 3-dB spectral bandwidth can be tuned from 250 to 500 nm with the filter center wavelength around 1 mum.

Tunable pattern transitions in a liquid-crystal-monomer mixture using two-photon polymerization

Through two-photon lithographic processes, we report experimentally and numerically a series of photoinduced tunable polymerization patterns in shapes from straight channel, serpentine curve, to periodic grating when an ultrashort femtosecond laser pulse directly writes in a liquid-crystal-monomer mixture along a line for different scanning speeds. Laser beams with polarization perpendicular to the direction of writing and the alignment of liquid crystals, produce snake-shaped patterns at an intermediate scan rate.

Linear-cavity fiber laser using subring-cavity incorporated saturable absorber for single-frequency operation

By using a subring cavity incorporated with a saturable absorber, a stable single-frequency selection is realized in a linear-cavity fiber laser, which is constructed using a broadband fiber mirror and a partial reflectance fiber Bragg grating as the cavity ends. At 1550.33 nm, the laser has an optical signal-to-noise-ratio of >53.28 dB and with linewidth less than 1 MHz. The pumping efficiency is 25% improved by recycling the residual pump power to gain medium. The power stability and wavelength stability have also been studied.

A cost-effective fast frequency-hopped code-division multiple-access light source using self-seeded fabry-Pe/spl acute/rot laser with fiber Bragg grating array

We propose a novel light source for the optical fast frequency-hopped code-division multiple-access scheme using a self-seeded gain-switched Fabry-Pe/spl acute/rot laser diode (FP-LD) with the reconfigurable fiber Bragg grating array (FBGA). The embedded FBGA in the external cavity loop encodes the input pulses both temporally and spectrally. Before feeding back into the FP-LD, the encoded optical pulses propagate the FBGA with reverse direction and are again temporally combined. With the corporation of the variable fiber delay line, the limitation of the pulse rate by the fixed external cavity length is removed and makes the scheme more flexible. The output with sidemode suppression ratio of more than 20 dB has been obtained. With three fiber Bragg gratings on the FBGA, we demonstrate the encoded chip rate of 1.425 GHz under the driving radio frequency of 475 MHz. It is proved that a different codeword can be generated by simply reconfiguring the encoder without changing the rest of the experimental setup.