In an optical coating, the thin film engineer must control film thickness and index of refraction to achieve high optical coating performance. While film thickness can generally be controlled with sweeps, sources, and distribution shields, index can be a tougher problem to solve. Often times, contaminants affect the index of refraction of a thin film and cause variations within different portions of the coating chamber leading to poor yield. Presented here is a methodology and case study used to diagnose index of refraction non-uniformity as well as identify the potential sources of contaminants in the system using a combination of Residual Gas Analysis and Secondary Ion Mass Spectrometry.
We report on progress in developing low-cost methods for shaping thin-foil glass x-ray optics. Such optics might serve as substrates for reflection gratings or as foil mirrors in high-throughput missions such as Constellation-X. Novel thermal shaping to lithographically defined pin chucks leads to the desired shape with high accuracy, thereby avoiding the need for replication. To demonstrate this method we have produced 200 micron-thick glass sheets with sub-micron flatness and half power diameter below 10 arc seconds. We also present a process for depositing low-stress metallic coatings that provides high x-ray reflectivity without significant foil distortion.
Semiconductor lasers with tapered gain regions are well suited for applications requiring high output powers and good spatial mode quality. In this paper, the development of 1.5-micrometer InGaAsP/InP quantum well (QW) material suitable for this type of device will be discussed and initial results on high-power tapered lasers fabricated in this material presented. Several different 1.5-micrometer QW laser structures grown by metalorganic chemical vapor deposition are being evaluated. Structures containing three compressively strained QWs have shown transparency current densities JT as low as 170 A/cm2 and net gains of approximately equal 40 cm-1 at less than 800 A/cm2. With 5QWs, these parameters were JT approximately equals 275 A/cm2 and net gain approximately 40 cm-1 at 600 A/cm2, respectively. Self-focusing at high current densities and high intensity input into the taper section has been identified as a fundamental problem in these devices that has to be dealt with. Tapered devices with a 0.6-mm-long single-mode gain section coupled to a 1.4-mm-long tapered region fabricated in 5QW material have shown CW output powers of greater than 1.0 W at 3.8 A. Approximately 80% of the 1 W is in the near- diffraction-limited central lobe of the far field-pattern.
GaSb-based and InAs-based semiconductor gain media with band-edge wavelengths between 3.3 to 4 micrometers were used in grating-tuned external cavity configuration. Output wavelength was tuned up to approximately 9.5% of the center wavelength; and power from few tens of mW to 0.2-W peak, 20- mW average was achieved at 80 K operation. The tuning range is approximately 2 - 3 times wider than those of near-IR semiconductor lasers, as expected for mid-IR semiconductors which have smaller electron masses. The external cavity laser had a multimode linewidth of 1 - 2 nm, which was approximately 10 to 20 times narrower than that of a free running laser. Analysis of the gain/loss spectral properties indicates that the tuning range is still severely limited by facet anti-reflection coating and non-optimal wafer structure. Model calculation indicates a tuning range a few times larger is possible with more optimal wafer design.
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