Smaller Mirror Segments Research Articles

Creating a large aspheric primary mirror using spherical segments

The use of aspheric mirrors is a common practice to design astronomical telescopes with a few optical elements. In the most preferred optical design Ritchey Chretien (RC), both primary and secondary mirrors are hyperboloid. Nowadays large telescopes are being built using small mirror segments, however, making aspheric off-axis mirror segments is still a challenge. We have conducted a study in which, we explored the possibility to mimic an aspheric hyperbolic primary mirror by making use of smaller spherical mirror segments. Three different methods have been used to form a large segmented aspheric primary of nearly 12m aperture. In the first method, fixed ROC(radius of curvature) spherical mirror segments are reconfigured by a piston, tip, and tilt (PTT). In the other two methods, in addition to PTT, ROC of the segments are also varied. We further attempted to reduce the telescope wave-front error by varying the segment size and the F ratio of the primary. We found out that none of these three methods provided acceptable image quality unless we incorporate the warping harness in the segment support. The use of the warping harness emulated by Zernike coefficient correction, remarkably reduced the wave-front error and delivered a decent image quality over a large field of view. In this paper, we present the results of our study on designing an RC type optics for a 12m class optical-NIR(Near Infrared) telescope using spherical mirror segments.

Radiation trapping in laser diodes

This paper is concerned with the problem of cavity geometries which exhibit anomalously low radiative efficiencies. While the ideas presented apply to any type of cavity, the experiments and discussion refer specifically to the type of cavity of importance in commercial injection lasers, a Fabry-Perot cavity with cleaved end facets and sawed sidewalls. In this study, it is shown that the lowering of the external efficiency is probably associated with the excitation of internally circulating modes, analogous to those found in lasers with four cleaved sides. A dual-cavity diode structure is employed to demonstrate that it is the sidewall loss which controls the excitation of the internally circulating modes. To determine the geometric conditions for internal mode excitation in single and double heterojunction diodes, an extensive study was made of the combination of length, width, and facet reflectivity which lead to efficiency decreases. It is assumed that the internal modes are excited when the distributed sidewall loss for the internal modes is less than the distributed Fabry-Perot end loss for the externally radiated modes. The conditions for internal mode excitation and accompanying efficiency decrease are predicted by either assigning an empirically determined attenuation factor to the sawed side or assuming that the loss is associated with diffraction from small mirror segments at the sidewall which have been formed during the sawing operation. These predictions may be used to design Fabry-Perot sawed sidewall injection lasers so as to avoid efficiency loss through the excitation of internal modes.