摘要:

Infrared spectroscopy has many applications, and the Fourier transform spectrometer (FTS) has become an important analytical tool for these purposes. In addition to its use in benign laboratory environments, the FTS has found great acceptance for the difficult tasks of remote sensing of the Earth from satellites, and for space exploration by probes to other planets. This paper provides an overview of the characteristics and applications of many of the spaceborne FTS systems used in the past as well as those on the drawing boards for future use. A discussion of critical design issues is included in addition. A review of Fourier transform spectroscopy principles; a brief history of FTS instrument development; and a discussion of a possible future direction are also provided. An extensive list of references is contained as well. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146154

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We describe a liquid helium‐cooled absolute reference load for the calibration of a 2.5 GHz radiometer. This calibration system provides an absolute reference near the liquid helium boiling point. The accuracy of the reference source is better than 100 mK. We use this cold load to perform absolute temperature measurements of the cosmic microwave background radiation. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146155

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The mode comb of a synchronously pumped mode‐locked dye laser is stabilized in frequency by locking one of its modes to a transmission maximum of a reference Fabry–Perot interferometer. The frequency error signal, obtained by a conventional modulation–demodulation technique, is fed back to permanently readjust the cavity length of the laser via an integrating servo amplifier and a piezo‐positioned end mirror. Insertion of an appropriate loop filter prevents loop oscillations on the dominant mechanical resonance of the transducer–mirror combination. A special ramp automatic reset function is introduced to avoid saturation of the servo amplifier. To verify the improvement in laser stability the emission spectrum was scanned with a high‐resolution optical spectrum analyzer. A laser linewidth of 3 MHz has been achieved. Extended linear sweeps of the stabilized mode comb for spectroscopic measurements were realized by simultaneously tuning the reference cavity and tilting the intracavity etalon. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146156

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report on the operation of a high voltage high current switch in which command resonant charging capability is built in. Elimination of switch latch up problems do not require any extraneous and complex circuitry. The indigenously developed switch delivers power in excess of 3 kW into a dummy load typical of a TE laser at a pulse repetition rate of 200 Hz. The test on a real TEA CO2laser load was, however, limited up to 50 Hz repetition rate beyond which the accumulation of dissociation products within the active volume led to arcing. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146157

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This article reports a microwave spectrometer with a novel technique of locking a frequency ‘‘scanning window’’ [F1,F3] in the radiation source to a microwave rotational absorption peak. The center (F2) of the scanning window is initially locked to the resonant frequency (Fr) of a tunable resonant cavity so that the center of the scanning window may be tuned by changing the resonant frequency of the cavity. Microwaves are then input to a nonresonant gas cell containing the analyte gas at low pressures and by means of Stark modulation the lower sideband (F1,F2) and upper sideband (F2,F3) of the scanning window are amplitude modulated at different phases. After demodulation of the two sidebands, an error signal is derived through addition of the two demodulated signals (being zero when the scanning window exactly overlaps with the absorption peak) and this is used to lock the resonant frequency (Fr) of the tunable cavity (and thus the center (F2) of the scanning window) to the center (F0) of the absorption peak of the analyte gas. For quantitative measurement, a signal is derived through subtraction of the two demodulated signals (being maximum when the center of the scanning window is approximately at the center of the absorption peak). This spectrometer system has been used to measure ethylene oxide gas (absorption line 23.123 GHz) at concentrations up to 100% and water vapor (absorption line 22.235 GHz) at concentrations from −40 °C dewpoint (127 ppm) to 50 °C dewpoint (138,736 ppm). The detect limit of the spectrometer has been tested and found to be approximately 3×10−9cm−1(signal‐to‐noise ratio equal to 2). ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146525

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A long‐base‐line (90 m) multiple reflection cell has been constructed to carry out spectroscopic measurements on gases for atmospheric sciences purposes. The cell is installed in the Laboratori Nazionali del Gran Sasso at Assergi (L’Aquila). The optical path ranges from about 180 m up to about 1500 m in the UV and up to about 2100 m in the visible region. Tests of the instrument by measuring the Rayleigh cross section of molecular nitrogen have been performed. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146158

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Recent developments in thermoacoustics have shown that the quality factor,Q, of an acoustic resonator can be controlled by establishing a temperature gradient across properly positioned thermoacoustic elements. Quite separate from thermoacoustics, acoustic resonators are used in photoacoustic spectroscopy, where a laser beam, modulated at the acoustic resonance frequency, is partially absorbed, thereby producing sound. The photoacoustic signal is typically measured with a microphone, and is proportional to the laser power, to the absorption coefficient, and to the resonatorQ, among other factors. The acoustic signal to noise ratio is proportional toQ1/2. Thermoacoustics can be used to enhance the signal to noise ratio of photoacoustic spectrometers by increasingQ. Measurements and theory are reported for the signal to noise ratio of a photoacoustic cell, with thermoacoustic enhancement, as a function of both resonatorQand the bandwidth of the lock‐in amplifier. Regimes where thermoacoustic enhancement is useful are identified. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146159

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In order to judge the potential fluorescent lamp applications of various low‐pressure positive column discharges it is necessary to measure the absolute power emitted in the ultraviolet region of the spectrum. For rare‐gas discharges the principle emission occurs in the vacuum ultraviolet so that it is difficult to measure the radiant emittance (power per unit area) of the resonance radiation by standard methods. Two independent techniques are discussed for measuring the radiant emittance of positive column discharges in the vacuum ultraviolet. These techniques are used to study xenon positive column discharges at the resonance wavelength of 147 nm. The first method relies on the measurement of the resonance level density by absorption techniques. The effective decay rate of the resonance level is then determined by the simulation of resonance radiation transport. These two quantities are combined to yield the radiant emittance at 147 nm without implementing vacuum ultraviolet techniques. The second method uses a measurement of the resonance radiation normal to the positive column axis made with a calibrated vacuum ultraviolet detector. The angular distribution of the resonance radiation leaving the tube is determined by the simulation of resonance radiation transport. The detector measurement places the angular distribution of the radiance on an absolute scale, which can then be integrated to yield the radiant emittance. These two techniques are compared for pure xenon discharges at various pressures and currents. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146160

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146161

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray multilayer supermirrors for the energy range up to 20 keV have been theoretically studied and experimentally measured with synchrotron radiation. A multilayer mirror with 50 W/Si bilayers with different thicknesses on the Si substrate has a smooth reflectivity of up to 32% in the whole energy range from 5 to 22 keV at a grazing incidence angle of 0.32° which is considerably larger than using total external reflection. ©1995 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1146162

出版商:AIP    

年代:1995

数据来源: AIP