Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

Spectral narrowing and manipulation in an optical parametric oscillator using periodically poled lithium niobate electro-optic polarization-mode converters

We report a unique spectral narrowing and manipulation technique in an optical parametric oscillator (OPO) realized by an integrated periodically poled lithium niobate comprising an optical parametric gain medium sandwiched by two electro-optic polarization-mode converters (EO PMCs). We achieved a manipulation of the gain spectrum of the OPO via EO and/or temperature control of the EO PMCs, in which we obtained single to multiple signal spectral peaks from the OPO with a spectral width reduced by up to 10 times and peak intensity increased by up to 6 times in comparison with the original signal. Fast EO tuning of the narrowed signal spectral peak has also been demonstrated.     

Nanostructured gradient-index antireflection diffractive optics

We describe the fabrication and characterization of a nanostructured diffractive element with near-zero reflection losses. In this element, subwavelength nanostructures emulating adiabatic index matching are integrated on the surface of a diffractive microstructure to suppress reflected diffraction orders. The fabricated silicon grating exhibits reflected efficiencies that are suppressed by 2 orders of magnitude over broad wavelength bands and wide incident angles. Theoretical models of the fabricated structure based on rigorous coupled-wave analysis and effective medium theory are in agreement with the experimental data. The proposed principles can be applied to improve the performance of any diffractive structures, potentially leading to more efficient Fresnel lenses, holographic elements, and integrated optical systems.     

Highly sensitive miniature photonic crystal fiber refractive index sensor based on mode field excitation

A highly sensitive miniature photonic crystal fiber refractive index sensor based on field mode excitation is presented. The sensor is fabricated by melting one end of a photonic crystal fiber into a rounded tip and splicing and collapsing the other end with a single-mode fiber. The rounded tip is able to induce cladding mode excitation, which resulted in an additional phase delay. Linear response of 262.28 nm/refractive index unit in the refractive index range of 1.337 to 1.395 is obtained for the physical length of a 953 μm sensor. The sensor is also shown to be insensitive to environmental temperature.     

A rehabilitation training system with double-CCD camera and automatic spatial positioning technique

This study aimed to develop a computer game for machine vision integrated rehabilitation training system. The main function of the system is to allow users to conduct hand grasp-and-place movement through machine vision integration. Images are captured by a double-CCD camera, and then positioned on a large screen. After defining the right, left, upper, and lower boundaries of the captured images, an automatic spatial positioning technique is employed to obtain their correlation functions, and lookup tables are defined for cameras. This system can provide rehabilitation courses and games that allow users to exercise grasp-and-place movements, in order to improve their upper limb movement control, trigger trunk control, and balance training.     

Investigation of surface plasmon biosensing using gold nanoparticles enhanced ellipsometry

We present a label-free, nondestructive and high sensitivity biosensor by using the phase information of a gold nanoparticles enhanced ellipsometry signal. The refractive index (RI) resolution from ellipsometric phase information is of the order of 1.6×10(-6) RI units. Furthermore, spectroscopic and dynamic measurements show substantial change in the phase signal when biomolecules are coated on gold nanoparticles. The detection limit of our proposed technique is up to ～18?pM concentration of the target biomolecules.     

Optical phase modulators using deformable waveguides actuated by micro-electro-mechanical systems

An optical phase modulator is presented by using micro-electro-mechanical systems to actuate deformable silicon waveguides. Via mechanically stretching the waveguide length, the optical path is extended, resulting in a phase shift. The experimental results show that a phase shift of near 0.4π is achieved at 200 V for both TE- and TM-polarized waves by cascading six phase modulation units, agreeing well with the theoretical prediction. The power consumption is estimated to be smaller than 0.2 mW at 200 V, mainly resulting from the leakage current.     

Dual-mode operation of a twisted nematic liquid crystal cell by switching between dynamic and memory modes

We propose a twisted nematic liquid crystal device that can be operated in dynamic or memory mode, based on the information content to be displayed at that time. +90°-twisted and -90°-twisted states are used as two stable states for operation in the memory mode. A vertical electric field is applied to realize gray levels for operation in the dynamic mode. The proposed device has a memory retention time of over a month for the memory mode and a response time of 12 ms for the dynamic mode. Contrast ratios of over 500∶1 can be obtained in both the dynamic and memory modes.     

Stationary-fiber rotary probe with unobstructed 360° view for optical coherence tomography

A side-scanning fiber probe is a critical component for optical coherence tomography in medical imaging and diagnosis. We propose and fabricate an on-axis rotating probe that performs in situ, circumferential scanning that is shadow-free (not susceptible to shadow effects caused by the motor's wires). A miniature motor that incorporates a bored-out shaft for the optical fiber is located at the distal end of the probe, which results in a more stable and uniform circumferential scan, free from wire-shadow interference effects. More importantly, this design, novel to our knowledge, compared to other probes avoids the insertion losses introduced by optical coupling components and the multitude of optical interfaces, which is very important for sensing weak signals backscattered from structures deep in the tissue.     

Hybrid cable television and orthogonal-frequency-division-multiplexing transport system basing on single wavelength polarization and amplitude remodulation schemes

A hybrid community antenna television (CATV) and orthogonal-frequency-division-multiplexing (OFDM) transport system is proposed and experimentally demonstrated to transmit multiple CATV channels and bi-directional radio frequency signals on a single optical carrier. By polarization remodulating an optical CATV signal with downstream OFDM signals and then amplitude remodulating upstream OFDM signals with the hybrid CATV/OFDM signals, this architecture can efficiently utilize only one optical carrier to support optical analog/digital CATV transmission and bi-directional wireless broadband services for each client. Good experimental results prove that this architecture provides a proper wavelength utilization scheme for future multiwavelength optical transport systems.