Electromagnetic crystal (EMXT) waveguide band-stop filter

An electromagnetic crystal (EMXT) with a bandgap around 35 GHz is used as the top and/or bottom walls of a rectangular waveguide. Because of its bandgap, such an EMXT waveguide is a very effective band-stop filter with the rejection band centered at 35 GHz. More than 70 dB of isolation for the rejection band and less than 2 dB of insertion loss for the pass band are measured. Finite element simulation of the EMXT band-stop filter agrees very well with measurements. Using tunable EMXT walls, a band-stop filter with adjustable center frequency and bandwidth can also be realized.     

Mutual coupling reduction of low-profile monopole antennas onhigh-impedance ground plane

The mutual coupling effects of flush-mounted (horizontal) monopole antennas on high-impedance ground planes (HIP) have been measured. Experiments of horizontal monopoles on HIP with a bandgap centred at 4.7 GHz show that the interference between two adjacent elements is stronger compared to those on a regular metal ground plane due to special properties of HIP. A technique using a combination of HIP and thin metal strip is proven to reduce the mutual coupling by ~6 dB. Similar results have been obtained for the horizontal monopoles on HIP with a bandgap centre at 17 GHz     

Wave-front adaptive control structure (WACS) for quasi-optical power amplifiers in Intelligent RF front-ends

Tunable electromagnetic crystal (EMXT) surfaces are used as rectangular waveguide sidewalls to provide uniform excitation for quasi-optical power amplifiers. InP hetero-barrier varactors are monolithically integrated into periodic metal-dielectric structures to realize a reconfigurable EMXT surface that has a tunable resonance frequency from 30 to 45 GHz. Real-time reconfigurable (30 - 40 GHz) and low-loss quasi-TEM wave-front control with only 2-mm long EMXT waveguide section is demonstrated. Experimental results are in good agreement with design simulations. Such waveguides can be readily integrated into an intelligent RF front-end with self-assessment and self-adjustment capabilities.     

Application of linear prediction least-squares fitting to time-resolved optical spectroscopy

Linear prediction least-squares fitting is shown to be a powerful tool for the quantitative analysis of pump-probe-type experiments. When used correctly, it is capable of extracting multiple exponential or exponentially damped sinusoidal components from noisy data. Furthermore, it can be used to analyze data that are not simply the response of the system being studied, but rather the convolution of a nonnegligible instrument function with the desired response. In pump-probe experiments, it can be used to deduce time constants that are considerably shorter than the laser pulse itself.     

Practical urea optical parametric oscillator for tunable generation throughout the visible and near-infrared

The optical parametric oscillator (OPO) using the urea crystal is reviewed. This oscillator has demonstrated promise as a new solid-state source of tunable, coherent radiation throughout the visible and the near infrared spectral regions. New experimental results relating to the urea OPO are also discussed. A practical OPO has been constructed utilizing a crystal of relatively short length (13 mm), which can be grown comparatively easily, and this oscillator has been demonstrated to be continuously tunable from 499 nm to 1.23 μm. The possibility of extending the tuning curve still further through the use of an alternate pump source has been examined. Parametric fluorescence in urea pumped by the fourth harmonic of Nd:YAG laser output (266 nm) has been observed for the first time.