Jyh-Long Chern

Global survey of upper atmospheric transient luminous events on the ROCSAT-2 satellite

Abstract Upper atmospheric transient luminous events (TLEs; sprite, elves, blue jet, etc.) are recently discovered thunderstorm-induced phenomena. Imager of sprites/upper atmospheric lightning (ISUAL) is a scientific payload on the Taiwans ROCSAT-2 satellite that aims primarily to provide crucial observation data on these TLEs from space. The ISUAL payload includes an intensified CCD imager, a six-channel spectrophotometer, and two array photometers. All the instruments are mounted on a common platform and boresighted in the same direction. The imager is equipped with six selectable filters, which have bandpasses covering the visible spectrum. The spectrophotometer contains six photometers, and each photometer is fitted with a special bandpass filter ranging from ultraviolet to red regions. The two array photometers are identical in every aspect, except one is fitted with a blue band filter and another one is equipped with a red band filter. With this set of well-chosen instrument, this project seeks to determine the location and timing of upper atmospheric transient events above thunderclouds, to investigate their spatial, temporal and spectral properties, to obtain a global survey of them, and to perform a global study of aurora and airglow. ISUAL project is an international collaboration supported by the National Space Program Office in Taiwan, with additional contributions from the National Cheng Kung University, Taiwan, the Space Science Laboratory of the University of California at Berkeley, and Tohoku University, Japan. ROCSAT-2, the platform of ISUAL, is scheduled to launch around October 2003.

High-speed picosecond pulse generation in semiconductor lasers with incoherent optical feedback

A novel method for producing high-speed picosecond optical pulses from semiconductor laser diodes that uses incoherent optical feedback is proposed. The incoherent optical feedback provides a successive pulselike modulation in carrier density and drives the system to a self-sustained pulsation state whose repetition rate is determined by relaxation oscillations.

Irradiance formations in hollow straight light pipes with square and circular shapes.

We investigated, both analytically and numerically, the irradiance formation of an asymmetrically located Lambertian light source in hollow straight light pipes with square and circular shapes. The uniform irradiance distribution in a square light pipe and hot-spot localization in a circular light pipe were examined and determined semianalytically. Typical factors of influence, such as light-pipe length, width, and source size, were identified with extensive simulation. When the ratio of light-pipe length and width was less than 0.5, the deviation from uniformity could be more than 20%. But once the source size was large enough (approximately half of the incident port), such that the Lambertian characteristics of the source dominated the irradiance distribution, the uniformity deviation was reduced. Furthermore, a quantity of root-mean-square circular differences was defined in order to identify the shape deformation of the light pipe; it was found that the peak value of the hot spot decreased exponentially with the deformation scale. The influence of nonperfect reflectivity of the pipe wall on irradiance formation was also examined for a square light pipe; when the reflectivity is larger than 90%, the difference in uniformity is less than 10% and uniform irradiance remains, provided that the ratio of light-pipe length and width is larger than 1; even the source is located asymmetrically.

Coexistence of two attractors in lasers with delayed incoherent optical feedback

Abstract Dynamic phenomena that incoherent optical feedback induces in single-mode class-B lasers are examined. Numerical simulations reveal the coexistence of sustained periodic relaxation oscillations and regenerative periodic spiking oscillations leading to chaotic oscillations.

EXPERIMENTAL SUPPRESSION OF CHAOS IN A MODULATED MULTIMODE LASER

Suppressing chaotic behavior by addition of a weak second periodic perturbation signal, which is nearly resonant to a subharmonic of the fundamental system frequency, is observed in a modulated microchip LiNdP(4)O(12) multimode laser by a highly sensitive self-mixing modulation technique. The stabilization of the unstable period-2 orbit embedded in a chaotic attractor is demonstrated in a wide parameter region. The chaos-suppressing experiments are well reproduced by simulations of globally coupled modulated Tang-Statz-deMars [J. Appl. Phys. 34 8289 (1963)] multimode laser equations, including a spatial hole-burning effect.

Electromagnetic resonance in deformed split ring resonators of left-handed meta-materials

Recently, structures called deformed split ring resonators have been proposed to achieve the smallest volume of a unit cell, while retaining negative permeability. With a combination of wire structures, the characteristics of a negative index of refraction, or left-handed meta-materials, can be seen from the transmission spectrum. This electromagnetic resonance, including its orientation tolerance and angular dependence on the incident beam, is explored in detail in the microwave range.

Observation of period-adding in an optogalvanic circuit

Abstract Period-adding is a novel devils staircase structure that, however, is less observed in comparison with the other bifurcation structures, such as period-doubling and intermittency. We show that an optogalvanic system consisting of a neon bulb shone by a modulated green-light HeNe laser exhibits period-adding. This suggests an interesting class of nonlinear systems where period-adding is inherent.

Illuminance formation and color difference of mixed-color light emitting diodes in a rectangular light pipe: an analytical approach

We developed an analytical method of illuminance formation for mixed-color LEDs in a rectangular light pipe in order to derive American National Standards Institute (ANSI) light uniformity, ANSI color uniformity, and color difference of light output using photometry, nonimaging, and colorimetry. The analytical results indicate that the distributions of illuminance and color difference vary with different geometric structures of light pipes and the location of the light sources. It was found that both the ANSI light and the ANSI color uniformity on the exit plane of the light pipe are reduced exponentially with the increase in length of the light pipe. It is evident that a length scale L/A greater than unity assures that the mixed-color LED sources on the entrance plane are uniformly illuminated with acceptable uniform brightness and color on the exit plane of the rectangular light pipe, where L is the length of the light pipe, and A is a constant, which is a geometric parameter for the scale unit of the light pipes input face. Furthermore, the ANSI light uniformity can be minimized, and the ANSI color uniformity can be maximized under the condition of multilight-source locations P=Q= +/- A/4, where P and Q are the coordinates along the long axis and the short axis, respectively, with one being the entrance plane of the light pipe. We can conclude that the optimum form factor of the light pipe is a square shaped cross section, with the length scale L/A being equal to unity and with multilight sources located individually on positions of A/4 in order to achieve very uniform illuminations with the highest light efficiency and compact package for the optical system with mixed-color LEDs, where L is the length of the light pipe.

Irradiance formations of on-axis Lambertian pointlike sources in polygonal total-internal-reflection straight light pipes.

Different geometric shapes of light pipes cause different irradiance distributions. We analytically explore the irradiance distributions of on-axis Lambertian pointlike sources in polygonal total-internal-reflection (TIR) straight light pipes. It is analytically shown that the irradiance of pentagonal light pipes concentrates on the center of the exit plane. Numerical verifications are also provided, and experimental explorations with different shapes of acrylic light pipes are carried out for comparison. We also analyze the influence of light-pipe length on distribution uniformity and deduce the smallest uniform-mixing length/circumradius ratios for polygonal light pipes.

Self-Induced Spiking Oscillations and Associated Instabilities in a Laser-Diode-Pumped Three-Mode Nd:YVO 4 Laser

Self-induced Q-switching-type spiking oscillations and associated instabilities have been observed in a free-running laser-diode-pumped microchip Nd:YVO4 laser operating in three-mode regimes, and dynamical characterizations of spiking pulsations have been demonstrated. Observed instabilities have been reproduced by numerical simulations of the model equations including intensity-dependent cross-gain coupling in three-mode lasers.