Pinghui S. Yeh

Low-loss propagation in Cr 4+ :YAG double-clad crystal fiber fabricated by sapphire tube assisted CDLHPG technique

Cr4+:YAG double-clad crystal fiber with an uniform 10-microm core was fabricated by using a sapphire tube as a heat capacitor to stabilize the power fluctuation of the CO2 laser in the co-drawing laser-heated pedestal growth system. The uniformity of the fiber core showed a factor of 3 improvement compared to that without the use of sapphire tube. The variation of the core diameter is within the +/-1.35-degree adiabatic criterion and has a autocorrelation length of 1.7 mm. The measured propagation loss is only 0.02 dB/cm. The sapphire tube also reduces the vertical temperature gradient during the crystal fiber growth process so the 10-microm crystal core exhibits a smooth perimeter. The sapphire tube assisted system can be applied to the growth of many other optical crystal materials.

Efficient and low-threshold Cr 4+ :YAGdouble-clad crystal fiber laser

A significant advancement of cw lasing in Cr4+:Y3Al5O12 (Cr4+:YAG) double-clad crystal fiber grown by the codrawing laser-heated pedestal growth technique was demonstrated at RT. The optical-to-optical slope efficiency of 33.9% is the highest, to the best of our knowledge, among all Cr4+:YAG lasers, whether they are in bulk or fiber forms. The low-threshold lasing of 78.2 mW and high efficiency are in good agreement with the simulation. The keys to the high laser efficiency are twofold: one is the improved Cr4+ emission cross section and fluorescence lifetime due to release of the strain on the distorted Cr4+ tetrahedron, which also mitigates photobleaching in Cr4+:YAG; the other is the improved core uniformity at long fiber lengths. In addition, because of the low threshold, the impact of excited state absorption of the pump light is significantly reduced. The effects of crystal-orientation, self-selected, and pump-dependent linear polarization states were also addressed.

GaN-Based Resonant-Cavity LEDs Featuring a Si-Diffusion-Defined Current Blocking Layer

GaN-based resonant-cavity light-emitting diode (RCLED) featuring a Si-diffusion-defined confinement structure is reported for the first time. The charge-coupled device images exhibited round, bright spots of sizes corresponding to the diffusion-defined aperture sizes under continuous-wave high-current-density operation and at room temperature. The full widths at half maximum of the electroluminescence spectra were 2 and 1.5 nm for 10- and 5-mu-diameter RCLEDs, respectively. A stable peak wavelength of 406.6 nm was maintained at various injection currents. The results suggest Si diffusion is an effective means to reduce aperture size. The design and fabrication of the devices are described.

Intracavity and resonant Raman crystal fiber laser

We demonstrated an efficient, compact, and continuous-wave Raman crystal fiber laser (RCFL) using an intracavity and resonant χ(3) approach. The gain and nonlinear medium was Cr4+:Y3Al5O12 double-clad crystal fiber grown using the codrawing laser-heated pedestal growth technique. The RCFL threshold was only 50 mW, and the slope efficiency reached 14.3% above a pump power of 350 mW. The result is in good agreement with theory, which indicates a near-100% quantum efficiency of resonant stimulated Raman scattering.

High-Luminance White-Light Point Source Using Ce,Sm : YAG Double-Clad Crystal Fiber

Ce and Sm ions codoped yttrium aluminum garnet (Ce,Sm : YAG) double-clad crystal fibers (DCFs) were successfully fabricated using the codrawing laser-heated pedestal growth method. At a pump wavelength of 446 nm, white-light point source was produced from a 10--core DCF. The amplified spontaneous emissions and the residual pump resulted in a directional high brightness point source. A luminance of 4.5 × 109 cd/m2 was generated. The luminous flux of the point white light was 6.2 lm. When coupled to a 200-core multimode fiber, the output power and coupling efficiency were 4.8 mW and 36.8%, respectively. Point white-light sources are useful for biomedical endoscopic application and long-range directive illumination. The generated broadband yellow emission can also be served as the light source for optical coherence tomography system. Using the 91-nm yellow emission, an axial resolution of 1.5-optical coherence tomography can be achieved for ultrahigh-resolution ocular and industrial applications.

High-Performance InGaN p-i-n Photodetectors Using LED Structure and Surface Texturing

High-performance InGaN-based p-i-n photodetectors (PDs) were fabricated using a typical light-emitting diode (LED) epitaxial structure and surface texturing. A multiple-quantum-well active-layer structure effectively mitigates the indium clustering problem and achieves a high rejection ratio and scalability. Two types of surface texturing for enhancing spectral responsivity were fabricated and compared. High responsivities of 0.21, 0.23, and 0.24 A/W were attained at a wavelength of 388 nm and the biases of 0, -1.5, and -3.0 V, respectively, corresponding to external quantum efficiencies (EQEs) of 67%, 73%, and 78%. And the dark current was below 10 pA at zero bias. To the best of our knowledge, the responsivity reported in this letter is the highest for InGaN-based p-i-n PDs. Moreover, the sample with a surface-textured p-GaN layer has 50%-85% higher EQE at wavelengths between 335 and 365 nm and flatter spectral responsivity than did the other examined sample. High cost performance, low operating voltage, and LED-process-compatible PDs were obtained.

Wavelength conversion using a T-gate laser

Wavelength conversion from 830 nm to 837 nm of a 250 Mb/s NRZ optical signal has been demonstrated at 10/sup -9/ bit-error rate and a detector sensitivity of -29 dBm using a semiconductor power amplifier monolithically integrated at 90/spl deg/ with a GaAs-AlGaAs SQW semiconductor laser. This device offers a high degree of isolation between the input and the output and has the potential for high-speed operation with a wide continuous-wavelength conversion range.

GaN-based vertical-cavity surface emitting lasers with sub-milliamp threshold and small divergence angle

A GaN-based vertical-cavity surface emitting laser (VCSEL) structure featuring a silicon-diffusion-defined current blocking layer for lateral confinement is described. Sub-milliamp threshold currents were achieved for both 3- and 5-μm-aperture VCSELs under continuous-wave operation at room temperature. The vertical cavity was defined by a top dielectric distributed Bragg reflector (DBR) and a bottom epitaxial DBR. The emission spectrum exhibited a single peak at 411.2 nm with a linewidth of 0.4 nm and a side mode suppression ratio of more than 10 dB before device packaging. The full-width-at-half-maximum divergence angle of the 3-μm-aperture VCSEL was as small as approximately 5° which is the lowest number reported. These results implied the 3-μm-aperture VCSEL was in near single-mode operation.

Light-scattering effectiveness of two-dimensional disordered surface textures in thin-film silicon solar cells

To compare the light-scattering effectiveness of surface-textured solar cells of various design parameters such as density, diameter, refractive index, and location, this study used a new parameter, optical path length gain (OPLG), that is more sensitive than Haze. By modeling two-dimensional disordered textures as a structure that comprises many randomly distributed, small, spherical scatterers, ray-tracing simulations of surface-textured thin-film silicon solar cells were performed. The simulation results suggest that: (1) the optimal scatterer diameter for hydrogenated amorphous silicon (a-Si:H) solar cells is ~50 nm, producing an average OPLG of 3.5; and (2) the optimal scatterer diameter for a-Si:H/μc-Si:H (hydrogenated microcrystalline silicon) tandem cells is ~75 nm, producing an average OPLG of 3.4 and an increase in the bandwidth of the absorption spectrum of 14.5%.

Blue resonant-cavity light-emitting diode with half milliwatt output power

GaN-based resonant-cavity light-emitting diode (RCLED) has a circular output beam with superior directionality than conventional LED and has power scalability by using two-dimensional-array layout. In this work, blue RCLEDs with a top reflector of approximately 50% reflectance were fabricated and characterized. An output power of more than 0.5 mW per diode was achieved before packaging under room-temperature continuous-wave (CW) operation. The full width at half maximum (FWHM) of the emission spectrum was approximately 3.5 and 4.5 nm for 10- and 20-μm-diameter devices, respectively. And the peak wavelength as well as the FWHM remained stable at various currents and temperatures.