Fang-Wen Sheu

Photorefractive polymeric optical spatial solitons

We predict the formation of optical spatial solitons in photorefractive polymers. The orientational enhancement from the doped chromophores and the dependency of the quantum efficiency of generating mobile holes on the electric field make the polymeric solitons behave differently from other photorefractive solitons.

Stable trapping and manually controlled rotation of an asymmetric or birefringent microparticle using dual-mode split-beam optical tweezers

Inserting a coverslip into half of a Gaussian laser beam at a suitable tilting angle can make the single-mode laser beam become closely spaced dual light spots at the laser focus. In this way, we can reform the conventional single-beam optical tweezers easily and construct a set of dual-mode split-beam optical tweezers, which can be used to manually rotate a trapped and twisted red blood cell around the optical axis. Furthermore, we demonstrate that the split-beam optical tweezers can also stably trap and orient a birefringent polystyrene micro strip particle, which otherwise will self rotate at a varying speed along the structural principal axes, fast spin about the optical axis in a tilting pose, or precess like a gyroscope, in the original linearly polarized single-beam optical tweezers.

Photorefractive polymeric solitons supported by orientationally enhanced birefringent and electro-optic effects

We predict and analyze the formation of photorefractive polymeric solitons, which are supported by both the orientationally enhanced birefringence and the orientationally enhanced electro-optic effects. The formation conditions and characteristics of this new type of optical spatial soliton are discussed.

Using a slightly tapered optical fiber to attract and transport microparticles

We exploit a fiber puller to transform a telecom single-mode optical fiber with a 125 microm diameter into a symmetric and unbroken slightly tapered optical fiber with a 50 microm diameter at the minimum waist. When the laser light is launched into the optical fiber, we can observe that, due to the evanescent wave of the slightly tapered fiber, the nearby polystyrene microparticles with 10 microm diameters will be attracted onto the fiber surface and roll separately in the direction of light propagation. We have also simulated and compared the optical propulsion effects on the microparticles when the laser light is launched into a slightly tapered fiber and a heavily tapered (subwavelength) fiber, respectively.

Fiber cross-sectional imaging by manually controlled low coherence light sources

We couple a variable-coherence light beam into a multimode optical fiber and observe the fiber cross-sectional images. The variation in the fiber imaging is explored as we change the degree of optical coherence of the incident light. Low coherence light is shown to be capable of improving the quality of the fiber images. Various mode patterns of a multimode optical fiber are also shown numerically and experimentally to elucidate the fiber coupling characteristics.

Swinging optical spatial solitons in a biased photorefractive crystal

We have measured and simulated the dynamic evolution of a soliton beam that is launched into a biased photorefractive crystal with a sinusoidally oscillating incident position. Within some frequency range depending on the optical intensity, the swinging amplitude of the soliton at the output face can be larger than that at the input face. This suggests a resonance interaction between the soliton light beam and its induced waveguide. At higher frequency, the swinging soliton beam behaves like a standing wave with nodes. The experiment and the simulation show fairly good agreement.

Capturing a reflective cross-sectional image of an optical fiber with partially coherent laser light to measure the refractive index profile of a multimode optical fiber

We focused partially coherent laser light onto an optical fiber end-face and captured a high-quality reflective cross-sectional image of the fiber. By analyzing the reflected light intensity distribution of the captured fiber image, we can achieve refractive-index profiling of a step-index multimode optical fiber. The measurement error caused by the reflected light from the other fiber end-face positioned in air can be greatly improved by inserting that end of the fiber into water. This simple and easy technique for fiber index profiling by employing reduced-coherence laser light is very useful in determining the refractive index profiles of various multimode optical fibers.

Trapping and Propelling Microparticles at Long Range by Using an Entirely Stripped and Slightly Tapered No-Core Optical Fiber

A stripped no-core optical fiber with a 125 μm diameter was transformed into a symmetric and unbroken optical fiber that tapers slightly to a 45-μm-diameter waist. The laser light can be easily launched into the no-core optical fiber. The enhanced evanescent wave of the slightly tapered no-core optical fiber can attract nearby 5-μm-diameter polystyrene microparticles onto the surface of the tapered multimode optical fiber within fast flowing fluid and propel the trapped particles in the direction of the light propagation to longer delivery range than is possible using a slightly tapered telecom single-mode optical fiber.

Temporal coherence characteristics of a superluminescent diode system with an optical feedback mechanism

We explore the temporal coherence characteristics of the output light of a SLD system with different optical feedback ratios by a Michelson interferometer, and we also observe the long-scan-range interference patterns with the one by one wave packets due to the Fabry-Perot modulation of the SLD device. We can obtain the effective cavity length of the SLD active layer and get more information of the temporal coherence length or spectral width from the long-scan-range interference patterns. This tunable light source system can provide more insights into the optical coherence or lasing phenomena often discussed in the optics course.

Using a wavelength tunable diode laser to measure the beat length of a birefringent fiber

In this report we demonstrated a method for measuring the beat length of a birefringent fiber. In this method the beat length is determined from the wavelength dependence of the phase difference between two orthogonally polarized modes at the output end of a sample fiber. In addition to the mode hopping of the laser diode’s optical wavelength due to the temperature variation, we have also observed the phase hopping of the output light polarization at the end face of the birefringent fiber. It is a simple and precise method to determine the birefringence magnitude of anisotropic materials in an optics laboratory course.