Three generations of field-widened interferometer spectrometers have been developed at Utah State University for use in the visible and near infrared spectral regions. Field compensation for these instruments is accomplished using optical wedges, and an instrument throughput of AQ = 1 cm2sr has been achieved with a resolution of 2 cm-1. Measurement time gains compared with conventional interferometers of more than 100 have been demonstrated. Two of these instruments are cooled to 77° K to reduce instrument background and incorporate helium cooled Si:Bi detectors. The latest is a rocketborne interferometer utilizing CaF2 optics operating in the 2-8 I'm spectral region. The design criteria of critical components of the field-widened interferometer are investigated and techniques to fully utilize the large throughput obtained are discussed. Mea-sured gains are compared with theoretical gains, and future possibilities for this type of field-widening are explored. An airglow atlas of the night sky from 4000-10,000 cm-1 at a resolution of 2.5 cm-1 is presented to demonstrate instrument performance capabilities.

The technique of Fourier spectroscopy is instrumental in achieving a high sensitivity for the spectrometry of detecting weak infrared emission sources. In our survey study conducted on the infrared emission of the atmospheric species, this fact is fully appreciated; both the multiplex advantage and the interferometric gain on collecting the infrared photons, together with our emission source of a 36-meter-long electric discharge column, are essential to detect various atmospheric bands and lines, some well known and some little known. Lately, we succeeded to observe the infrared vibration fundamental band of NH which was previously undetected.

A Fourier transform spectrometer was operated at the South Pole. Solar spectra in the 750 - 1350 cm-1 region were obtained with . 015 cm-1 resolution. The spectra are being analyzed to obtain abundances of minor atmospheric species, and will be archived for future reference.

The development of a Michelson interferometer-spectrometer which incorporates a detector array in its focal plane has provided tool for performing simultaneous spectral and spatial measurements. The instrument simultaneously views up to sixteen areas which are defined by a 4 by 4 detector array. Each detector element corresponds to a viewing field of approximately 1.4 by 1.4 degrees square, and the total 16 element array views a 6.1 by 6.1 degree field. Theinstrument was designed to operate in an uncontrolled temperature and pressure environment, which makes it ideal for use on high altitude balloon platforms. No heaters are used to stabilize the alignment of the interferometer cube. As a result, full advantage of cold ambient temperatures can be utilized to reduce the thermal backgrounds emitted from the instrument structures and to increase the detector sensitivity.

The availability of tunable line sources such as diode lasers in the infrared have lead to the development of two new forms of spectroscopy. The first one is passive heterodyne detection, a form of amplitude detection where the spectrum is scanned by tuning a local oscillator, in this case a diode laser. A second development is using a diode laser as a tunable source to scan the absorption spectrum of a sample with detection by the usual power detector. The capability of the first technique will be compared with spectral discriminators of the classic type using power or direct detection. The second technique will be compared with Fourier Transform absorption spectroscopy using a high temperature blackbody as a source.

The throughput of a Hadamard spectrometer is directly proportional to the length of the slits. However, optical aberrations limit the slit length that can be used. The usable length of curved and straight slits are theoretically and experimentally compared for a typical singly-encoded Hadamard spectrometer that has a Czerny-Turner plane-grating mounting. For this Hadamard spectrometer the optical throughput can be increased by a factor of ten by using properly curved slits.

A reverse-optics design for a high-throughput UV-visible spectrophotometer is described and compared briefly to conventional spectrophotometer designs. This new spectrophotometer incorporates diode arrays, an aberration-corrected holographic diffraction grating and beam steering. It can perform a full-spectrum measurement in one second. Design philosophy and certain optical and opto-mechanical considerations are discussed in more detail.

A general method for the measurement of differential absorption spectra using a Fourier transform infrared spectrometer will be described. The method is based on the rapid modulation of the absorption strength of the sample at a frequency that is significantly higher than the Fourier modulation frequencies. Phase sensitive detection of the detector output at the differential modulation frequency yields an interferogram which represents only the differential spectrum, and this can be processed by the fast Fourier algorithm in the usual way, although care must be taken to secure the proper phase correction for the spectrum. If the polaroization of the infrared beam is modulated between alternate states of circular or linear polarization, the resulting differential spectrum represents circular or linear dichroism, respectively. The differential Fourier intensities could also be generated by directly modulating the sample using some external periodic perturbation such as an electric or magnetic field or intense laser radiation. The method lends itself to the study of kinetic phenomena, where transient species could be detected on the microsecond time scale. An example will be given for this method applied to the measurement of vibrational circular dichroism where the differential signal is over four orders of magnitude smaller than the overall absorption strength.

A double-beam interferometer laboratory has been designed and built for use in testing a background optical suppression scheme (BOSS). The interferometer system features a 1 cm -I double-beam interferometer (2 cm beam aperture), a tailored modulation transfer function for the two beams, a servocontrolled drive for the moving retroreflecting mirror, a laser retardation reference system, PbS detector and associated electronics, a 16K fast fourier transform (FFT) analyzer, and a CRT plus X-Y plotter for displaying both interferograms and 1cm-1 spectra in real time. The laboratory facility also includes a test source that projects a point source immersed in a controllable spatial background.

A series of laboratory measurements made with a double-beam interferometer having tailored modulation transfer functions is presented. These measurements demonstrate the capability of this instrument to detect faint point sources in the presence of a structured background on the basis of spatial frequency differences and/or spectral differences between the source and the background. Measurements are presented which indicate that this detection process can be carried out at a level of spectral resolution of 1 cm-1 or better.

A survey of optical Fourier transform techniques is presented. The specific application considered is advanced focal plane Fourier spectroscopy in which multichannel Fourier analysis of long signals of modest band-width are required.

An AFGL interferometer used in airborne work is described in terms of the mechanisms utilized to counteract the influences of the harsh aircraft cabin environment. The problems of vibration are dealt with by reliance upon a command voltage-slaved balanced servo drive and Bendix Corp. Flexure Pivots for the moving mirror transport system. A piezoelectric crystal system maintains auto alignment of the stationary mirror thereby countering the warping effects that arise from aircraft cabin temperature fluctuations. The entire interferometer is further isolated from vibration by suspending it within an outer case on Aeroflex Corp. flexible cable mounts. The result is an instrument that delivers highly accurate and reliable data.

Procedures have been developed for data handling from interferometers used in the high vibration environment produced by an NKC-135 aircraft. These procedures will be described in this paper, covering the data recording, calibration, data processing, storage, and analysis procedures that have been developed and used at the Air Force Geophysics Laboratory.

In order to achieve high signal to noise ratio in spectra obtained via a fourier transform spectrometer, it is necessary to manipulate interferogram information with a minimum of distortion in addition to maintaining a low level of random noise. This requirement leads to the use of constant gain linear phase detection and amplification of the interferogram signal in particular in the presence of inevitable mirror velocity variations. Oversampling of the signal simplifies the implementation of constant gain linear phase response, but leads to large volumes of data when operating at high resolution. A vector processor has been developed which permits real time digital filtering and phase correction of the oversampled interferogram. The output interferogram, which is represented by the lowest number of samples consistent with the bandwidth of interest, is also symmetric. Simple cosine fourier transformation yields the spectrum.

The phase correction algorithm is one of the most critical components of a Fourier Transform infrared spectrometer software package. The implementation of the algorithm is described and the routines developed are applied to measurements with difficult phase problems.

The S/N capability of an Idealab Fourier Transform Spectrometer using a HgCdTe detector was improved by a factor of 30 to a level where system performance with a Nernst glower source was limited only by the extrinsic detector noise. The results reported here constitute a hardware demonstration of an analysis recently performed by Zachor and Aaronson. Our analysis and hardware demonstration are in substantial agreement with the work of Zachor and Aaronson which was carried out independently. Diagnostic data will be presented which illustrate the distortion to the spectrum caused by errors in sampling. The characteristics/specifications for signal and sampling control electronics which provide dynamic compensation for sampling errors will be discussed, and test data showing the 30 fold improvement in S/N will be presented.

New techniques have been developed that permit the unambiguous correction of alignment errors in a scanning Michelson interferometer by means of an electronic feedback system. This dynamic alignment system corrects not only for linearity errors in the mirror drive, but also all misalignments induced by adverse environmental conditions such as vibrations and temperature changes. Long term alignment reproducibility allows precalibration of both the spectral transmission and interferogram phase response. Phase correction of interferograms is done with a-priori phase information resulting in significantly reduced phase ambiguity in spectral regions with little intensity. Resolution to 0.01 cm-1 has been achieved in field instrumentation.

A liquid nitrogen cooled rocketborne near infrared interferometer spectrometer has been developed using compensating wedge prisms to achieve a wide field-of-view and using a gas-lubricated bearing slide for the movable mirror carriage to maintain stable precision align-ment and scanning speeds of 1 second/scan. A nickel manganese iron alloy has been developed which has very nearly the same thermal physical properties of calcium flouride, thus rigid mounting of the calcium flouride is possible which reduces the possibility of fracture on cool down and assures optical alignment after the instrument is subjected to the severe vibration of the rocket flight. An entendue of 0.67 cm2 sr has been achieved and the bearing smoothness stiffness and control are sufficient to yield a resolution of at least 2 cm^-1. A liquid helium cooled bismuth doped silicon detector is used providing a NESR of 1.5 x 10^-12 watt cm-2 sr^-1 at 5 µm when operating at a scan time of 1.3 seconds.

The design optimization of the detector-preamplifier subsystem is critical to the achievement of sensitive infrared spectrometers. The application illustrated is for cryogenically-cooled detectors, but the optimal approach based upon an operational preamplifier is general for detector operation under background limited conditions.

A nitrogen-cooled Michelson interferometer was fabricated using a unique flexural pivot mirror translation system which allows a proportionally large aperture and is ideally suited for operation at cryogenic temperatures. Cooling the entire interferometer yields a sensitivity sufficient to measure weak atmospheric emissions from an electron-gun induced artificial aurora. The spectral range 2.0 to 5.6 µm is scanned at a repetition rate of 1.8 seconds with an apodized resolution of 2 cm-1. Piezoelectric elements in the fixed mirror mount allow realignment at cryogenic temperatures. Laser (sampling) and white light (absolute position) reference channels are run antiparallel to the main channel using the same optics. When launched aboard a Talos Castor rocket as part of the EXCEDE payload the interferometer maintained alignment within 20 percent of launch modulation efficiency.

The assessment of the performance of a Fourier Transform Spectrometer requires an extended source of rad-iance which is nearly flat cagier the spectral range of interest. It must also be sufficiently low in intensity not to saturate sensitive instruments. A collimated blackbody cannot meet these criteria. An integrating sphere will adequately provide the signal characteristics sought. Such a sphere is described for use in the 500 to 5000 cm^-1 spectral range. It is cryogenically cooled to reduce self emissions and is gold coated to provide a high, constant throughput. The output radiance levels are described using a blackbody input. Descriptions of techniques to reduce this level are provided. The unit can also be used in conjunction with a monochromator to provide spectral lines useful for evaluating line profiles throughout the spectral region of use. A brief description is provided of the use of an ambient temperature sphere. This provided useful data in obtaining the spectral calibration of radiometers and in determining the responsivity of various instruments.

The design of the COCHISE facility, a cryogenic apparatus used for laboratory studies of high altitude infrared excitation phenomena, is presented. The basic design concept is the extensive reduction of thermal background radiation through cryogenic (20 K) cooling of the entire reaction volume and detection system. Vibrationally excited molecules (e.g., NO, 03, CO) are formed in a low-pressure ( - 3 mtorr) environment by interaction of a flowing reactant gas with discharge--produced radical and/or metastable species; the resulting infrared radiation is detected in the absence of interference from relaxation and surface effects. A long-path optical system and cryogenic monochromator permit high sensitivity for vibrationally excited species ( - 106 molecules cm^-3) and excellent spectral resolution ( 2 cm ) over the spectral range 2-16 μm. The design and operation of the facility are described in detail. Specific applications of the facility to investigations of infrared atmospheric phenomena are also discussed.