A reciprocal vector theory for analysis of the Talbot effect of periodic objects is proposed. Using this method we deduce a general condition for determining the Talbot distance. Talbot distances of some typical arrays (a rectangular array, a centered-square array, and a hexagonal array) are derived from this condition. Further, the fractional Talbot effect of a one-dimensional grating, a square array, a centered-square array, and a hexagonal array is analyzed and some simple analytical expressions for calculation of the complex amplitude distribution at any fractional Talbot plane are deduced. Based on these formulas, we design some Talbot array illuminators with a high compression ratio. Finally, some computer-simulated results consistent with the theoretical analysis are given.
We describe a reciprocal-lattice vector method for analysis of the diffractive self-imaging (or Talbot effect) of a two-dimensional periodic object. Using this method we analyze the fractional Talbot effect of a hexagonal array and deduce a simple analytical expression for calculation of the complex amplitude distribution at any fractional Talbot plane. Based on this new formula, we design a hexagonal array illuminator (HTAI) with a high fractional parameter. A computer simulation for demonstration of the HTAI is also given.
A compact passively Q-switched Yb:YAG microchip laser is demonstrated. Featuring a semiconductor saturable-absorber mirror (SESAM), the laser yields pulses of 219 ps when the length of the microchip Yb:YAG crystal is 100 µm and the beam quality is M 2 < 1.3. To the best of our knowledge, pulses from the proposed laser are the shortest Q-switching pulses obtained from Yb:YAG microchip lasers currently available.
The design of a terahertz short-slot coupler with curved waveguide is proposed. A traditional short-slot coupler uses a step-like structure in order to suppress higher order modes and improve bandwidth. It becomes difficult to control the fabrication of tiny steps with the incensement of frequency especially in terahertz band. The designed coupler is composed of two curved waveguides overlapping in the middle to realize a specific coupling coefficient. Then the step-like structure can be replaced with a curved structure which is much easier to fabricate. The coupling coefficient of the coupler is 3 dB, and the variation is less than 1 dB around the center frequency. The phase difference between two output ports is 90 •. The isolation is greater than 10 dB in the whole working band. Measured results show high agreement with simulation predictions. The designed coupler can be widely used as feed networks of horn antenna array.
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