Seungin Baek

Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors

The characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors are discussed. This sensor is able to detect macro bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. Since an incident ray experiences different variations of tilt angles with respect to bending direction, the blazed fiber Bragg grating has different coupling efficiencies of the first side mode, which can be reduced considerably in the case of twisted blazed fiber Bragg gratings.

A 980-nm Yb-doped fiber MOPA source and its frequency doubling

A continuous-wave (CW) master oscillator-power amplifier (MOPA) fiber source, tunable around 978 nm, was frequency-doubled to 488.7 nm. Both the laser and the amplifier were made with cladding-pumped jacketed air-clad Yb-doped fibers. The MOPA generated up to 2.7 W of power in an output beam with an M/sup 2/ value of 1.8. This was frequency-doubled in a periodically poled potassium titanyl phosphate crystal at room temperature, in a single-pass configuration. The generated blue light had a CW power of 18.1 mW, a nearly Gaussian spatial intensity profile, and an M/sup 2/ value of 1.7.

Neodymium-doped cladding-pumped aluminosilicate fiber laser tunable in the 0.9-/spl mu/m wavelength range

A tunable high-power cladding-pumped neodymium-doped aluminosilicate fiber laser is demonstrated. The maximum power reached was 2.4 W with a slope efficiency of 41% and a threshold pump power of 1.68 W, both with respect to launched pump power, when cladding pumped by two 808-nm diode pump sources at both fiber ends. The dependence of the tuning range on the fiber length is investigated. The tuning range changed from 922 to 942 nm for a 25-m-long fiber to 908-938 nm with a 14-m-long fiber, because of reabsorption effects. The output linewidth was 0.26 nm in a diffraction-limited beam. Operation on the challenging 0.9-/spl mu/m three-level transition in neodymium-doped double-clad fiber laser was facilitated by a W-type core refractive index profile. This filtered out the unwanted and competing strong transition at 1.06 /spl mu/m while guidance of 0.9 /spl mu/m remained intact.

Modal power decomposition of beam intensity profiles into linearly polarized modes of multimode optical fibers.

We calculate the modal power distribution of a randomly and linearly polarized (LP) multimode beam inside a cylindrical fiber core from knowledge of spatial-intensity profiles of a beam emitted from the fiber. We provide an exact analysis with rigorous proofs that forms the basis for our calculations. The beam from the fiber end is collimated by a spherical lens with a specific focal length. The original LP-mode basis is transformed by the spherical lens and forms another orthogonal basis that describes the free-space beam. By using this basis, we calculate the modal power distribution from the mutual-intensity profile. This is acquired by adopting a well-known mutual-intensity-profile-retrieving technique based on measurements of the intensity patterns several times after two orthogonal cylindrical lenses with varying separation. The feasibility of our decomposition algorithm is demonstrated with simulations.

Single-mode plane-polarized ytterbium-doped large-core fiber laser with 633-W continuous-wave output power

We demonstrate a cladding-pumped single-mode plane-polarized ytterbium-doped fiber laser generating 633 W of continuous-wave output power at 1.1 microm with 67% slope efficiency and a polarization extinction ratio better than 16 dB. The laser is end pumped through both fiber ends and shows no evidence of roll-over, even at the highest output power, which is limited only by the available pump power.

Polarization-isolated electrical modulation of an etched long-period fiber grating with an outer liquid-crystal cladding

An etched long-period fiber grating with an outer liquid-crystal cladding capable of polarization-isolated electrical modulation, is de- scribed. A 5-dB transmission-dip modulation with 4-dB polarization iso- lation can be obtained at 225 V. The reduction in fiber diameter by a chemical etching process leads to an improvement in the long-range ordering of liquid-crystal molecules and a reduction in the controlling voltage. In addition, the experimental results are supported by numerical simulation.

Experimental demonstration of enhancing pump absorption rate in cladding-pumped ytterbium-doped fiber lasers using pump-coupling long-period fiber gratings

A long-period fiber grating (LPFG) was inscribed in a single-mode fiber and was spliced with a ytterbium (Yb)-doped double-clad fiber in order to couple pump radiation of the inner cladding into the core in a cladding-pumped fiber laser. The use of an LPFG permits a partial core-pumping scheme in a cladding-pumping fiber laser because a portion of the pump radiation can be coupled to the core by LPFG. The enhancement of the pump absorption of a Yb-doped cladding-pumped fiber laser as the result of pump coupling by LPFG was 35%, and the maximal output power increased by up to 55% when a 20-W pump source is used.

Macrobend sensor via the use of a hollow-core splice fiber: theory and experiments

The characteristics of the use of a hollow-core splice scheme for macrobend measurements are discussed both theoretically and experimentally. A perturbation theory for the modes of a bent hollow-core fiber and its loss characterization are developed so as to better understand the characteristics of the scheme. The maximal detection range of fabricated sensors with the proposed scheme is experimentally determined to be as large as a few hundred millimeters relative to the radius of curvature. In addition, the numerical estimation of the loss characteristics using the scheme shows modal trends which are in good agreement with experimental data.

Electrically controllable in-line-type polarizer using polymer-dispersed liquid-crystal spliced optical fibers

The polarization-dependent transmission of light through an electrically controllable in-line-type polarizer that is made from polymer-dispersed liquid-crystal spliced optical fibers is discussed experimentally and theoretically. This in-line-type optical splicing method has the advantage of low transmission loss when it is applied in optical fiber communication systems. An anomalous diffraction approach is used to compute the scattering cross section of polymer-dispersed liquid-crystal droplets. The experimental results are supported by a theoretical analysis. This device can be employed in electrically controllable in-line-type polarizers and has the potential to yield electrically controllable polarization-dependent loss compensators.

A cladding-pumped fiber laser with pump-reflecting inner-cladding Bragg grating

A fiber Bragg grating (FBG) was inscribed in the inner cladding of a double-clad fiber, in order to reflect, and thus, double-pass, the pump radiation in a cladding-pumped fiber laser. The effective pump reflectivity of the inner-cladding FBG was /spl sim/46.4%. This increased the slope efficiency of a Yb-doped cladding-pumped fiber laser by up to 8.5%, and the output power increased by 45% with a 3-W pump source. The FBG provides stable and robust pump reflection, while leaving the fiber end free, for example, so that a signal transmission fiber can be spliced to the double-clad fiber.