Norman L. Thomas

UV-shifted durable silver coating for astronomical mirrors

Silver has the highest reflectance of all of the materials, but it tarnishes in the presence of sulfides, chlorides, and oxides in the atmosphere. Also, the silver reflectance is very low at wavelengths below 400 nm making aluminum more desirable mirror coating for the UV region. We have found a way to prevent silver tarnishing by sandwiching the silver layer between two thin layers of NiCrNx, and to extend the metals high reflectance down to 200 nm by depositing the (thin) Ag layer on top of Al. Thus, the uv is transmitted through the thin Ag layer below 400 nm wavelength, and is reflected from the Al layer underneath. This UV-shifted durable coating provides a valuable alternative to the aluminum coating for telescope mirror coatings where high throughput and durability are important considerations. The throughput for a telescope with, say, six reflections from silver coatings is (0.97)6 equals 83% compared to (0.92)6 equals 60% for aluminum coatings, or 28% less. The use of silver coatings allows more photons to be collected by primary mirror. Aluminum also has a reflectance dip at 850 nm caused by inter-band transitions which is eliminated by placing the thin Ag layer on top. This paper describes a non-tarnishing silver coating having high reflectance down into the UV region. The average specular reflectance is 70% - 97% in the near-UV, 95% - 99% in the visible region, and >= 99% in the infrared region covering the total wavelength range 200 nm to 10,000 nm.

Deposition of durable wide-band silver mirror coatings using long-throw, low-pressure, DC-pulsed magnetron sputtering

Sputter deposition at long-throw distances (15-30 in.) and low pressures (<1 mTorr) were developed mainly for the semiconductor industry to deposit metals and dielectrics into trenches or vias on silicon and gallium arsenide wafers. Scientists found that sputter depositions performed at pressures below 1 mTorr (0.13 Pa) results in a virtually collision-free trajectory of the sputtered atoms from the target to the substrate. If the throw distance (source to substrate) is increased at these low pressures, the activated (ionized) gas and target atoms maintain their energy. We used this methodology along with dc-pulsed sputtering to deliver additional energy at the substrate. This allowed us to coat large optics (>21-in. diameter) in a standard box coater using smaller-diameter sputter cathodes. This paper will discuss the process used to successfully coat a 22-in.-diameter optic for the Keck Telescope in Hawaii with a new Wide-Band Durable Silver Mirror. The process uses smaller-diameter sputter cathodes in a 4-ft.-x-4-ft.-x-5-ft. box coater. We will also discuss how the process can be scaled to 36-in. or larger optics for use on terrestrial or space-based platforms.

Protected silver coating for astronomical mirrors

A durable protected silver coating was designed and fabricated for use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light. This coating provides a valuable alternative for telescope mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 mm to 10,000 nm. An alternate coating design extends the high reflectance down to 300 nm while maintaining high reflectance out to 10,000 nm. The specular reflectance is between 95% and 97% in the visible region and 98% or better in the infrared region.

Protected silver coatings for flashlamp-pumped Nd: glass amplifiers

A durable protected silver coating was designed and fabricated for possible use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light. This coating provides a valuable alternative for mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 nm to 10,000 nm. The specular reflectance is between 95 percent and 98 percent in the visible region and 98 percent or better in the IR region.

Prevention of corrosion of silver reflectors for the National Ignition Facility

A durable protected silver coating was designed and fabricated for possible use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light . This coating provides a valuable alternative for mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 nm to 10,000 nm. The specular reflectance is between 95 percent and 98 percent in the visible region and 98 percent or better in the IR region.

Cross-dispersion infrared spectrometry (CDIRS) for remote chemical sensing

The advent of high performance infrared detector arrays together with recent advances in coarse grating fabrication now makes possible the design and fabrication of infrared spectrometers that cover the mid-IR atmospheric windows at high resolution with no moving components. For applications involving small sources at large distances this instrument approach can provide a substantial increase in sensitivity over Fourier transform spectrometers at the same resolution and spectral coverage. We describe the design evolution of the LLNL cross dispersive infrared spectrometer (CDIRS) remote chemical sensors operating in the two atmospheric windows from 2.3 microns to 4.2 microns. The spectral and mechanical performance of the first generation 1 meter focal length prototype echelle grating spectrometer (EGS) is presented. A second generation cryogenic (150 K) spectrometer (mini-EGS) utilizes a high dispersion silicon immersion grating to provide a compact design. This instrument will be flown in the winter of 1995 as part of the instrumentation suite for the DOE airborne multisensor pod system (AMPS) effluent research program.

Echelle grating spectrometer (EGS) for mid-IR remote chemical detection

The availability of high performance two-dimensional InSb detectors enables the design and construction of mid-infrared spectrographs capable of obtaining high resolution spectra over extended spectral regions without moving components. Rugged, stable, cryo-cooled spectrographs suitable for remote field operation are now possible using prism-echelle cross dispersion designs. We discuss the design, fabrication, and performance of a high resolution mid-IR field spectrograph designed specifically for the detection of atmospheric-borne chemicals from airborne platforms. The instrument design provides maximum optical throughput covering the two atmospheric windows at 2.0 - 2.5 micrometers and 3.0 - 4.2 micrometers .

Design of a mid-IR immersion echelle grating spectrograph for remote sensing

We describe the design of a silicon immersion grating spectrograph for the remote detection of chemicals in the atmosphere. The instrument is designed to operate in the two atmospheric windows from 2.3 to 2.5 and 2.8 to 4.2 microns at a resolution of 0.1 cm-1. This is achieved by cross dispersing a high order silicon immersion echelle (13.5 grooves/mm) and a first order concave grating operating in a reflective configuration to generate a 2D spectrum in the image plane with diffraction limited performance.

Design of an immersion echelle-prism mid-IR spectrograph for chemical remote sensing

We have designed an immersion echelle-prism spectrograph for the remote detection of chemicals in the atmosphere. The instrument operates in the 2 and 3 micron atmospheric windows, and provides maximum throughput at 0.1 cm-1 resolution. Crossed dispersers consisting of a high order silicon immersion echelle (13.5 grooves/mm) and 43 degree immersion prism (Rutherford prism) are combined in a Littrow configuration to generate a two-dimensional spectrum in the image plane.

Optical Fiber Chemical Sensing Networks

Many industrial processes or other phenomena of interest cannot be measured with conventional instruments because they are too hot, too cold, highly radioactive, or other-wise inaccessible to direct observation. Nuclear wastes stored in underground repositories, for example, will require in-situ monitoring. A new technology that uses long distance fiber optics to transmit laser-excited fluorescence now makes it possible to remotely monitor such installations via optical fiber cables at distances up to one kilometer. The applications are listed below. - Sampling of remote locations - Multipoint sampling with single instrument - Measurements in inaccessible locations - Coping with aggressive environments - Avoiding contamination - Continuous monitoring