The use of the laser for the measurement of inertial rotation was recognized by Adolf Rosenthal in August 1961, only a few months after Javan's announcement of the first cw laser. Rosenthal showed that the sensitivity of the Sagnac rotation interferometer could be improved by many orders of magnitude by the use of laser principles to convert path length changes to frequency changes. He proposed a ring-type laser resonator which can support two counter rotating laser beams; any path difference between the clockwise and counter-clockwise cavity length introduced by inertial rotation would cause a frequency difference between the counter-rotating laser beams that is proportional to the rate of rotation.
A seminar-in-depth on the Impact of Lasers in Spectroscopy was featured at the SPIE 18th Annual Meeting in San Diego in August, 1974. The overall intent of the seminar was to review the current state-of-the-art and the future of laser spectroscopy with emphasis on recent developments in laser technology and on the impact of the laser on spectroscopic methods and applications. Moreover, the seminar was designed to provide an environment where practical applications of laser spectroscopy could be discussed, reviewed, and evaluated.
The theme to be explored in this meeting is that the laser is far more than just another light source for spectroscopists; it will be amply demonstrated that the laser has been the basis of a qualitative revolution in this branch of science, just as the development of NMR has profoundly altered the practice of organic chemistry, or as the digital computer has made modern systems analysis possible. In this paper we will consider some of the unique features of the laser that have made this revolution possible, and some of the applications of these features. The most striking aspect of the laser is the enormous quantity of optical power and/or energy that is capable of being generated. This feature combines with the spectral purity and beam collimation which are consequences of the coherence of the laser output to produce a spectral surface brightness which may be 16 or more orders of magnitude greater than for conventional light sources. The phase coherence of the laser output is another unique feature, and improvement in time resolution over conventional sources by five or more orders of magnitude can also be obtained. These features have already been exploited in a number of ways. Selective excitation of molecular energy levels has opened new possibilities in fluorescence spectroscopy, not only in the visible, but also in the hitherto inaccessible infrared region. New double-resonance techniques have been developed, and Raman spectroscopy has experienced a renaissance. Selective excitation is now likely to become very important in chemical processing as well. Saturated-absorption techniques have made possible Lamb-Dip spectroscopy and the development of optical frequency standards. Measurements can now be made on molecular beams, providing information about exotic reaction conditions and the details of scattering events. The use of coherent light sources opens the possibility of a wide variety of optical analogues of magnetic-resonance phenomena, such as pulse and echo effects, coherent transient decays, etc. A
Holography can be used to record the Fourier transform of the source spectrum directly onto a photographic emulsion. Coherent optical processing of the hologram can be used to display the spectrum as intensity versus wave number (spatially displayed). Analogies with ordinary Fourier transform spectroscopy and with Fourier transform holography are explored in detail. The advantages and disadvantages of holographic spectroscopy are discussed.
Opto-acoustic spectroscopy refers to the measurement of optical absorption by a sample using acoustic methods to detect the degree to which the sample has been heated by the absorbed radiation. This technique has numerous applications, including monitoring trace contaminants in the atmosphere. It is especially useful in situations requiring the quantitation of weak absorption spectra or circumstances where a large dynamic range is required. This paper summarizes the results of recent experiments in which acoustic resonance has been used to enhance signal levels and improve detectivity. Scaling laws are given which allow the dynamic range and detectivity of this technique to be determined. Consideration is given to the use of both coherent and incoherent light sources, and comparisons are made to conven-tional techniques such as absorption spectroscopy. Finally, the application of optoacoustic spectroscopy to the measurement of collision-broadened absorption lineshapes is discussed.
Three methods of kinetic spectroscopy using lasers are described. In the first, the infrared double resonance technique using two CO2 lasers has been used to examine the relaxation of both 11BC13 and 10BC13. The relaxation of the initially excited state to a distribution of excited states proceeds by a very fast V-V process at a rate of at least 2 x 10 6 sec-1 torr-1. The rates of deactivation of BC13 by the rare gases, HC1, DC1, H2, HD, D2, N2, CH3F, and CHC13 have been measured and indicate that rotational degrees of freedom in the collision partner play an important role in deactivation. Two other applications of lasers to kinetic spectroscopy will be described. A Q-switched CO laser is used to excite CO to its first vibrational level and subsequent relaxation of CO by D2CO is monitored by following the CO fluorescence. In another application, a nitrogen laser is used to dissociate H2 CO and the time-evolution of specific states of the CO produced is monitored by the absorption of CO laser light.
Studies made of the temporal behaviour of laser-induced fluorescence as a function of emission wavelength for a variety of materials, such as crude oils, refined petroleum products, fish oils, and rock and mineral samples, lead us to believe that this information represents a new kind of spectral signature. The specificity of this "fluorescence decay spectrum" appears to be somewhat superior to that associated with the normal fluorescence spectrum. Several examples are presented to illustrate the improved identification capability of this new approach. We believe that a significant improvement to the ground truth evaluation capability of the new form of environmental probe currently under development, called a laser fluoresensor, might result from this advance.
The feasibility of remotely measuring acidity of aqueous solution is demonstrated. Profile changes in the Raman band for water, which occurs over the Raman shift range between 2800 cm-1 and 3800 cm-1, are attributed to modifications of the hydrogen bonding by acid protons. It is shown that these profile changes are directly related to acid concentrations. Measurements have been made for aqueous solutions of HC1, HBr, and H2SO4. In addition, Raman spectra of an acid aerosol are shown and estimates of the detectivity of acidity by remote Raman spectroscopy made.
The separation of isotopes by photochemical processes employing high power lasers has received much attention during the past years. The method promises very efficient and economically competitive separation processes for a large number of isotopes, most notably uranium. Although no laser separation experiment has as yet produced weighable quantities of isotopic materials to date, a number of very recent experiments have shown the viability of the fundamental concepts. We shall review the basic concepts and the existing experimental work. In the course of this review we shall try to point out a number of unresolved problems open to research and discuss the requirements for the further development of high-power lasers for isotope separation. Finally, we will discuss possible applications of isotopes other than uranium and investigate the uses and the economics of titanium-50 as a large scale construction material for nuclear reactors.
Tunable infrared lasers have been used in research laboratories for ultrahigh resolution spectroscopy measurements of molecular gases. While these lasers offer the potential for greatly improving instrument (e.g., laser spectrometer) performance, implementation of the instrument function is complicated by the nature of the laser source. Laser characteristics which must be considered include factors such as tuning mechanisms, spectral purity, stability and power. This talk reviews these characteristics for a variety of tunable infrared lasers including diode lasers (TDL), optical parametric oscillators (0P0), and high pressure gas lasers (HPG). Particular attention is paid to recent developments in the TDL including continuous operation at 8.5 gm wavelength at temperatures above liquid nitrogen (77°K) and high power efficiency in the 4-5 um region with output powers ~0.3 W. These developments reduce the operating complexity of TDL's and significantly increase their application potential. Spectroscopic techniques used for obtaining high resolution spectra in the 3.5 to 20 um region will be discussed and illustrated with spectra of NO, CO2, and H2O.
The use of one or more intracavity etalons for the selection of a single longitudinal mode of a cw dye laser is described. The interaction of the etalon with the main cavity and the broadband tuning elements is discussed and criteria for selecting the proper etalon free spectral range and mirror reflectivity are given. Synchronization of the etalon and main cavity tuning to obtain large deviation frequency scans is discussed. Factors influencing the stability and ultimate linewidth are briefly mentioned.
A computer compatible digital scanning system has been developed to simplify the controls of a narrowband pulsed tunable dye laser so that operation is analogous to scanning a spectrometer. The system is an accessory to the nitrogen laser-pumped DL-Series dye laser which incorporates both grating and etalon as the wavelength selecting elements. The grating is rotated with a mechanical sine drive so that wavelength response is linear. Linewidth of the laser without etalon is 0.1A and scanning is achieved with a stepping motor. Inserting an etalon into the cavity reduces the linewidth to 0.01A and tuning is achieved by tilting the etalon. However, since the etalon wavelength change depends on the grating wavelength and decreases as the square of the tilt angle, wavelength synchronization to the grating becomes difficult. In our system a digital computation accurate to 0.01% linearizes the etalon response to match that of the grating. Single scans of 3A and multiple scans of 30A can be achieved without loss of wavelength synchronization. Analog and digital wavelength readout to 0.01A and computer compatibility are included. System details and sample data are presented.
In order to perform ultrahigh resolution spectroscopy, a tunable laser with a narrow spectral width is required. In this paper we describe a single-frequency jet stream cw dye laser that is carefully designed to minimize high frequency laser jitter. The residual low-frequency jitter is reduced to 200 kHz rms by locking the laser frequency to an external reference cavity. The stability of the laser has been verified by observing extremely narrow resonances (700 kHz FWHM) in an iodine molecular beam. The laser frequency can be linearly and smoothly tuned by varying the length of the external cavity. In certain precision spectroscopic applications long-term laser stabilization is needed for the generation of secondary wavelength standards throughout the visible region. By locking the dye laser to a hyperfine transition in a molecular beam of I2, we have demonstrated a long-term stabilization of 6 parts in 1013.
A large aperture diffraction limited off-axis optical system adaptable to duplexed transmitter and receiver operation has been designed. The system employs tilted stigmatic telescopes consisting of paraboloidal primary and secondary mirrors. The magnifications and tilts are selected to remove focal plane tilt.
Mertz and Young introduced the idea of using a Fresnel zone plate as a shadow-casting reticle, or coded aperture, in X-ray astronomy. More recently, considerable progress has been made toward using the zone-plate aperture for gamma-ray imaging in nuclear medicine. The most successful configuration has used an off-axis section of a zone plate in conjunction with a halftone screen. In this paper, we discuss a variety of closely related coded apertures, including an annulus, an inverted zone plate, a spiral zone plate, and the Girard grill. In most cases, the technique of grid-coded subtraction is used to suppress the zero-order (DC) background light usually associated with zone-plate imaging. The first application of this technique, reported by Stoner et al., used a sequence of two to four on-axis zone plates. In the present paper it is shown that the method can be extended to other apertures and is also very useful in synthesizing the spatial filters for optical decoding.
The catadioptric magnifier* provides inherently larger viewing space than conventional lens magnifiers. This makes it more comfortable and convenient to use for tasks requiring both high magnification and wide field of view, such as reading microfilm with 28 cm X 36 cm computer printout recorded at 48-fold linear reduction.
A survey is given on the principle, the theory and the practical performance of an instrument that measures transmittance modulation over an extremely wide range of spatial frequencies. In a conventional microdensitometer, a microscope lens magnifies a small section of the sample onto a narrow single slit in front of the light detector. The Multiple-Sine-Slit Microdensitorrteter MSSM uses laser two-beam interference to project a sinusoidal intensity distribution of a great number of periods directly onto the sample. The total light flux transmitted while interference fringes scan across the transmittance pattern is collected by an integrating sphere in contact with the sample. The absence of any imaging elements gives the MSSM a constant instrument MTF over its entire working range, which at present spans from 4 to 1500 cycles/mm. By the large scanning area and the great number of periods in the scanning function a narrowband characteristic is achieved that efficiently suppresses grain noise and improves signal-to-noise ratio. Due to these features, MTF's could be determined up to 300 cycles/mm for coarse-grained emulsions and up to 1500 cycles/mm for high resolution emulsions as they are used for holography, optical data storage and integrated circuit manufacture.