Chia-En Yang

Middle-IR supercontinuum generations and applications

In this paper, the two different mechanisms of supercontinuum generation in single crystal sapphire fibers according to fiber lengths longer and shorter than dispersion length are theoretically and experimentally investigated. When the fiber length is shorter than the dispersion length, self-phase modulation is the dominant factor for supercontinuum broadening. A broad spectrum ranging from near-IR (1.2 μm) to the lower end of mid-IR (2.8 μm) is obtained. But, when the fiber length is longer than dispersion length, soliton-related dynamics with self-phase modulation is the dominant factor for supercontinuum. We further demonstrate that supercontinuum in a sapphire fiber can extend beyond the range of silica fibers by showing the spectrum from 2 μm to 3.2 μm. Also, we successfully apply the supercontinuum source generated from a sapphire fiber to IR spectroscopy. The spectra of pseudo-TNT chemical measured using our own supercontinuum source is in good agreement with those obtained by FTIR. Supercontinuum generation using a sapphire fiber, which has high damage threshold and broad transmission ranges can be used in many applications such as IR spectroscopy, broadband LADAR, remote sensing, and multi-spectrum free space communications.

THz Generation in Multiple Air Plasmas

Because the efficiency of THz generation in air plasma is quite low, the residual power of input beam after THz radiation is generated in air plasma remains almost the same. A new method, multiple air plasmas, is proposed. The residual power can be used to induce other air plasmas and generate THz radiation again. The multiple air plasmas method provides a potential way for the development of the intense THz source. The preliminary experimental results confirm the theoretical prediction. The multiple air plasmas generated THz can be very useful for remote THz generation and standoff detection.

High efficiency IR supercontinuum generation and applications

In this paper, we have reviewed our recent works on IR supercontinuum generation (SCG) and its applications. First, we provide a brief review on the physical mechanism of the supercontinuum generation. Second, the advance of SCG in single crystal sapphire fibers is reviewed and introduced. In particular, we discussed how to fabricate thinned sapphire fiber and use it for high efficiency SCG. Finally, experimental results of chemical analysis with supercontinuum source are reviewed.

Recent advances on IR supercontinuum source and its applications

In this paper, we have reviewed our recent works on IR supercontinuum generation (SCG) and its applications. First, we provide a brief introduction on the motivations of the proposed effort. Second, the work of SCG in single crystal sapphire fibers is reviewed. Third, in addition to single crystal sapphire fibers, the method, the process, and the results of fabricating other IR waveguides are presented. Fourth, a quantitative simulation on the supercontinuum generation with the new IR waveguide is provided, which shows that it is possible to generate SCG beyond 5 microns. To the best knowledge of authors, this is the longest SCG reported so far. Finally, more experimental results of chemical analysis with supercontinuum source are presented.

Light-activated ultrafast magneto-optical modulator

An ultrafast light-activated magneto-optical modulator is demonstrated in this paper. This modulator is capable of 1 ns modulation speed and has a 1 mm clear aperture. The design of the modulator incorporates a photoconductive switch and enables a synchronized and jitter-free operation, which eliminates the need of any electrical or optical delay lines. These features make the current design very attractive in typical free-space pulse laser applications. To the authors knowledge, this is so far the fastest MO modulator with such aperture size that has been reported.

Rapid iron borate (FeBO3) powder fabrication using microwave heating

Microwave assisted synthesis of crystalline FeBO3 powder is investigated in a multimode cavity at 2.45GHz as a possible method for faster synthesis. An Alumina-SiC susceptor enclosure was placed inside the cavity to overcome poor microwave absorption of the precursor at low temperatures. Enhanced diffusion was observed with appropriate precursor viscosity at reaction temperature. Less than 8 hours was found enough to complete FeBO3 synthesis, as compared with more than 20 hours of synthesis using conventional muffle oven. Microwave enhanced diffusion was not obvious with too high viscosity and eventually leveled by thermal diffusion with too low viscosity. The microwave synthesized FeBO3 particles were found more rhombohedral and smaller than conventional furnace synthesized ones, thus are inherently more suitable as optical composite materials.

An experimental investigation on high voltage GaAs photoconductive semiconductor switch

In this paper, we present the design and the fabrication method for high DC bias voltage photoconductive semiconductor switch (PCSS). By employing a low temperature grown molecular beam epitaxial GaAs (LT-MBE GaAs) and a proper protection coating to prevent air breakdown, the DC bias electric field can be significantly increased. Such a PCSS structure can effectively achieve a low DC dark current in a high voltage pulse generation system with smaller PCSS sizes. DC bias capability also eliminates the need of complicated synchronization. The application of high DC bias field PCSS will also be discussed.

Embedded chemicals detection using multiple frequencies excitation

In this paper, recent works of buried chemical detection system by stimulating and enhancing spectroscopic signatures with multi-frequency excitations are discussed. In this detection system, those multiple excitations, including DC electric field, microwave, CO2 laser illumination and infrared radiation, are utilized and each of them plays a unique role. The Microwave could effectively increase the buried chemicals evaporation rate from the source. The gradient DC electric field, generated by a Van De Graaff generator, not only serves as a vapor accelerator for efficiently expediting the transportation process of the vapor release from the buried chemicals, but also acts as a vapor concentrator for increasing the chemical concentrations in the detection area, which enables the trace level chemical detection. Similarly, CO2 laser illumination, which behaves as another type vapor accelerator, could also help to release the vapors adsorbed on the soil surface to the air rapidly. Finally, the stimulated and enhanced vapors released into the air are detected by the infrared (IR) spectroscopic fingerprints. Our theoretical and experimental results demonstrate that more than 20-fold increase of detection signal can be achieved by using those proposed technology.

Investigation of separating the transmitted and scattered light beams in free space by ultrafast optical switches

In this paper, the separation of transmitted and diffused light beams in a scattering medium by a magneto-optical ultrafast switch is investigated. The magneto-optical switch previously developed by the authors is capable of 1 ns switching speed and has a 1 mm clear aperture. The diffused light beams and ballistic beams in a scattering medium are simulated in the lab by two beam paths. One beam is delayed from the other to simulate the diffused light beam and the ballistic beam, respectively. The magneto-optical switch is synchronized with the required delay to the laser pulse to keep only the ballistic beam, acting as an ultrafast light gate. The concept is demonstrated with a 532nm Q-switched pulsed laser.

Biomedical imaging and detection with broadband spatially coherent supercontinuum laser

In this paper, the application of a broadband spatially coherent IR supercontinuum source to the biomedical imaging and detection is presented. New IR material is proposed to generate Mid-IR supercontinuum above 4um, which was previously difficult due to inherent material absorption. Broad Mid-IR supercontinuum is numerically shown to be possible with one single wavelength pump in appropriate fiber structure. Mid-IR broadband sources are very useful in IR Optical Coherence Tomography (OCT) and spectroscopy in biomedical materials, due to the rich absorption structures the Mid-IR region. Broadband Mid-IR source is better than single wavelength tunable source, such as Quantum Cascaded Lasers (QCL), for faster analysis speed, since slow scan is not required.