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Given the breadth of technical and application areas that have been significantly touched by optics, experts with different backgrounds were invited to give a snapshot of the field from their individual vantage points.

The fields of optics and photonics have experienced dramatic technical advances over the past several decades and have cemented themselves as key enabling technologies across many different industries. This paper explores past milestones, present state of the art, and future perspectives of several different topics, including: lasers, materials, devices, communications, bioimaging, displays, manufacturing, and industry evolution.

All the ideas essential to making a laser were known before 1930, but there was no operating laser before 1960. So why didn't the laser come sooner? There are several reasons. One important impedim...

The size and variability of the continuum photosphere of o Ceti have been measured with 11 μm heterodyne interferometry to an accuracy of about 1%. Narrow bandwidths (~0.17 cm-1) were used to avoid spectral lines and measure continuum only. The resulting 11 μm diameter of o Cet is larger than the previously measured visible and near-infrared sizes. In addition, variations in the diameter with phase and a possible elongation have been observed. Visibilities were also measured at wavelengths known to contain strong H2O spectral contamination, giving larger apparent stellar sizes and information on the distribution of hot H2O gas.

Adaptive optics provides a method for improving telescope imaging affected by atmospheric seeing distortions, but the differences in path length fluctuations through the atmosphere to two or more separate telescopes continues to limit the quality of stellar interferometry, and unfortunately is not ameliorated by adaptive optics. Some corrections to such fluctuations can be made by atmospheric density measurements near the ground, particularly since in some cases a substantial fraction of path length fluctuations occur in the atmosphere within 20-30 m of ground level. It is shown that more extensive corrections can be made by measurements of backscattered lidar radiation sent along the direction of the telescopes' observation. It is proposed that Rayleigh or Raman backscattering be used to measure changes in atmospheric density or index of refraction. Rayleigh scattering by molecules provides more intensity than does Raman and can allow path length corrections over distances of a few hundred meters to somewhat more than a kilometer with a fractional accuracy as good as 10-6 of the total atmospheric delay. Such measurements can substantially assist stellar interferometry. Details of how this might be done, and of likely errors and difficulties, are discussed quantitatively.

A detailed description is given of the Infrared Spatial Interferometer (ISI), developed at the Space Sciences Laboratory of the University of California at Berkeley, which is a high spatial resolution interferometer for mid-infrared wavelengths. The instrumentation, its capabilities and performance, data analysis, science program, and future plans are all discussed. The system's use of heterodyne detection, analogous to that of a modern radio interferometer, is also compared with the homodyne or direct methods more commonly encountered in the visible and infrared. The ISI has been operating productively on Mount Wilson for the past 10 years measuring materials immediately surrounding stars and their changes as well as some stellar diameters. The new spectral capabilities described here, a recent increase in baseline length, and the upcoming expansion to a closure-phase imaging array provide important additional types of measurements.

We review the classical and the quantum aspects of the noise behaviour of heterodyne detection, making connections among quantum noise in heterodyne receivers, the uncertainty principle, and the concept of electromagnetic zero-point fluctuations as a description of that noise.

The University of California Infrared Spatial Interferometer (ISI) for the 10 μm wavelength region is briefly described and results obtained on 13 prominent stars and on atmospheric phenomena are discussed. The interferometer has two movable telescopes of 1.65 m aperture and operates in principle like a modern radio interferometer, using heterodyne detection, CO 2 laser local oscillators, RF delay lines, and lobe rotation to maintain a fixed-frequency fringe rate. Extensive measurements have been made on atmospheric pathlength or phase fluctuation characteristics. These show substantial deviations from the Kolmogorov-Taylor model, fortunately in a direction favoring adaptive optics, long baselines, and the use of infrared wavelengths. Outer scales as small as about 10m occur under good seeing conditions. Visibility results on the 13 stars show that six of them have dust shells rather far from the star and provide evidence for episodic emission of gas with times between emissions of a number of decades. Other stars also vary with time, but are characterized by cyclic variations typical of long-term variables and more continuous emission with dust formation near the stars at temperatures as high as 1300K.

Recent upgrades of the Infrared Spatial Interferometer are described. These provide improved sensitivity, precision and convenience of measurement. Analysis of phase fluctuations within the interferometer as well as in the atmosphere above indicates that popular simple atmospheric models need to be refined. Results of stellar observations are presented in another paper (cf. Danchi et al., in these Proceedings).

Radio waves and light for a long time seemed quite separated, one primarily a field of engineering, and the other more a field of science. Now they have come together, with engineering and basic science strongly involved in both. This change originated largely through the development of microwave physics, which led to laser oscillators and amplifiers. The whole electromagnetic spectrum, from radio waves to the ultraviolet and beyond, now looks like a continuous field from both a scientific and engineering point of view. The development of microwave physics, an essential part of this story, will be discussed here with its impact on masers, lasers, and optics.

The Infrared Spatial Interferometer (ISI) is a three telescope interferometer system that operates near 11 microns wavelength using heterodyne detection with CO 2 lasers as local oscillators. Stellar measurements have been made using consistent instrumentation for 20 years, allowing comparisons of stellar sizes of red giant and Mira stars over time intervals which are long in comparison to stellar luminosity periods. Recent visibility and clos ure phase measurements of the star Betelgeuse have been fitted to simple image models and these results have been added to the 17 year record of stellar observations. A new area of investigation of stellar properties at very high spectral resolution will begin in the 2010-2011 observing season. The design of a new digital spectrometer-correlator system is discussed. This system will obtain visibility measurements on-and-off individual spectral lines and the continuum, simultaneously. Keywords: stellar interferometry, in terferometry instrumentati on, mid-infrared astronomy

FIFI, an imaging spectrometer with two or three Fabry-Perot interferometers in a series for astronomical observations in the FIR range, is described. Spectral resolutions of 2 km/s can be obtained with FIFI. Design considerations are discussed as well as optics, the detector array, the transimpedance amplifier array, signal demodulation, data acquisition, and instrument control.

The ISI (Infrared Spatial Interferometer) involves three movable telescopes of Pfund design, each in trailers, each with a radiation collecting mirror 65 inches in diameter. The telescopes can be moved, and then set down on concrete pads for stability. They are located on Mt. Wilson, where pads have been arranged to give baselines varying from 4 to approximately 75 meters and oriented in various directions. Initial work, between 1988 and 2002, was done with two telescopes; the third telescope was added in 2003.

Aperture masking interferometry and Adaptive Optics (AO) are two of the competing technologies attemptingto recover diraction-limited performance from ground-based telescopes. However, there are good argumentsthat these techniques should be viewed as complementar y, not competitive. Masking has been shown to deliversuperior PSF calibration, rejection of atmospheric noise and robust recoveryof phase information through the useofclosure phases. However, this comes at the penalty of lossof ”ux at the mask, restricting the technique to brighttargets. Adaptive optics, on the othe r hand, can reach a fainter class of objects but suers from the dicultyof calibration of the PSF which can vary with observational parameters such as seeing, airmass and sourcebrightness. Here we present results from a fusion of these two techniques: placing an aperture mask downstreamof an AO system. The precision characterization of the PSF enabled by sparse-aperture interferometry can nowbe applied to deconvolution of AO images, recovering structure from the traditionally-dicult regime within thecore of the AO-corrected transfer function. Results of this program from the Palomar and Keck adaptive opticalsystems are presented.Keywords: Optical Interferometry, Adaptive Optics

We present a comparative study of aperture-masking on the Keck-I telescope and adaptive optics with the Keck-II telescope. Recent results from an aperture-masking program at the Keck Observatory in the near-infrared amply demonstrate that this method occupies an important niche in achieving diffraction-limited images despite the many advances in adaptive optics technology. Examples of the efficacy of aperture-masking are the images of the persistent dust-producing Wolf-Rayet star WR 104 and the massive young star with IR excess, MWC 349A. Both these objects were resolved, providing fundamental new insights into their nature. Here we present images of these objects made using adaptive optics in the same wavelength band. These provide a unique opportunity for a direct comparison of two important and competing techniques for ground-based high-resolution imaging. From the AO images, we are unable to recover the gross morphology or detail seen in the aperture-masking results. We note that the AO program might have been hindered by less than ideal observing conditions.

The U.C. Berkeley Infrared Spatial Interferometer (ISI), previously described by Hale et al. as a two-telescope stellar interferometer operating in the mid-infrared regime of 9-12 μm, is now testing a recently constructed third telescope and centralized laser local oscillator and beam combining facilities for operation of a three-telescope system with phase closure. This new system will allow the ISI Array to make measurements of stellar photospheric and dust asymmetries in the mid-IR, measurements which have previously been lacking. The new facilities and instrumentation required for three-way heterodyne beam combination and fringe detection, together with techniques for measuring the closure phase, are described. Both hardware and software methods of beam combination are used for producing three simultaneous fringe visibilities and a closure phase measurement. Additionally, the phase closure signal-to-noise of pairwise and of all-in-one beam combination are contrasted.

The U.C. Berkeley Infrared Spatial Interferometer is a two telescope stellar interferometer operating in the 9-12 micron atmospheric window, utilizing heterodyne detection with CO2 laser local oscillators. Science with the ISI has been focused on the measurements of the spatial distribution of dust and molecules around mass-losing late type stars, and more recently precision measurements of stellar diameters in the mid-infrared avoiding molecular lines. During the past few years, a National Science Foundation sponsored program of expansion from two to three telescopes has been underway. This expansion will allow the ISI to make visibility observations on three simultaneous baselines and a measure a closure phase. The third telescope was completed last year and shipped to Mt. Wilson, and more recently a Central Control Facility and Master Laser Oscillator Facility were also completed and recently shipped to Mt. Wilson. In this paper we report progress on this program and highlight some of the most recent astrophysical results.

The U. C. Berkeley Infrared Spatial Interferometer has been outfitted with a filterbank system to allow interferometric observations of mid-infrared spectral lines with very high spectral resolution (R ~ 10^5). This paper describes the design, implementation, and performance of the matched 32-channel filterbank modules, and new spectral line observations of Mars and IRC+10216 are used to demonstrate their scientific capability. In addition, observing strategies are discussed for accurate calibration of fringe visibilities in spectral lines, despite strong atmospheric fluctuations encountered in the infrared., 20 Pages, 7 Figures, Accepted in Astrophysical Journal Supplement Series

A novel, fast-switching filterbank system has been incorporated into the Infrared Spatial Interferometer to allow high spectral resolution observations of mid-infrared spectral lines, while precisely calibrating atmospheric fluctuations. This system allows the complex visibility in a spectral line to be measured relative to the continuum with high precision, about 1% in visibility amplitude and a few degrees in visibility phase. The high spectral resolving power (R approximately 100000) and spatial resolution (1/10 arcsecond) allow the formation radii of various polyatomic molecules (e.g., ammonia and silane) to be directly measured. Initial results for carbon star IRC + 10216 and red supergiant VY CMa are presented.

Recent upgrades of the 10 micron interferometer have during 1997 been fully programmed, tested, and used. They provide improved tracking and stronger signals, and have been extensively used for new stellar measurements. Multiple shells around several stars have been found and analyzed, and movements or other changes with time are seen in the dust shells around some stars. Methods for compensating atmospheric fluctuations have been examined, and measurements of the vertical distribution of turbulence have been made with acoustic radar.

The UC Berkeley Infrared Spatial Interferometer (ISI) is a heterodyne stellar interferometer which operates in the mid-infrared between 9 and 12 microns. This wavelength range makes the ISI particularly well-suited to high resolution study of late-type stars and other objects surrounded by relatively cool dust, the emission from which peaks in the mid-infrared. Unfortunately, this same dust tends to reduce the amount of visible light available from the sources, making many interesting infrared objects too faint for the original telescope guiding system, which used a silicon CCD camera. This system has been replaced by a guiding and tip-tilt correction system based on an InSb IR camera and a fast, controllable tilting mount for one of the mirrors in the signal path. The new system has improved the quality of ISI fringe visibility data on bright sources and allowed the study of previously inaccessible objects.

The Infrared Spatial Interferometer (ISI) is an interferometer installed on Mt. Wilson and operating in the 10 μm wavelength region, using heterodyne detection and two movable 1.65 m telescopes. Its general technology and characteristics, recent changes, and observational results are broadly discussed. Some compensation for atmospheric path length fluctuations is demonstrated. Stellar observations show, among other characteristics, that many stars emit gas and dust episodically with times of 10-100 years between events, and that stellar diameters measured in the mid-infrared region are about 10 percent larger than those measured with interferometry using visible light.

The far infrared is an important region for the study ofgalaxies. These infrared wavelengths easily penetrate the clouds ofdust and molecules found in many galaxies which tend to hide their intaiors. In addition, the temperatures and densities ofgalacticmolecular clouds fall in a gieral range which easily excites far infrared spectra, but which do not produce much radiation at short wavelengths. Hence spectroscopic measuremaits in this region are of great value in providing measures of temperature, density and velocity. Howeva, it is only within the last ten or fifteen years that much galactic spectroscopy has been done in this region partly because technology for sensitive detectors and for some of the other opticai components has only recently been developed. In addition, the opacity of our atmosphere requires special platforms, generally above most ofthe atmosphere, for astronomical observations at these wavelengths. Fig. 1 shows the transparency ofour atmosphere at various wavelengths in the visible and infrared from the ground, from the height of the observatories on Mauna Kea, and from the height of NASA's aircraft observatory, the Kuiper Observatory, which generally operates at 40,00045,000 feet. Clearly, between about 40 and 300 microns wavelength, essentially no signals are transmitted to the top ofMauna Kea, while thae is substantial transmission to the high flying Kuiper Observatoiy, a C141 four engine aircraft carrying a 36 inch telescope. The telescope is gyrostabihzed and at altitude looks out ofthe side ofthe aircraft with no obstructing windows. Fig. 2 shows a somewhat more detailed picture ofthis transmission. Even at 14 kilometas transmission is by no means perfect the are many regions where spectral lines are well transmitted but also other regions whe radiation is absorbed by atmospheric spectral lines, particularly those ofwater vapor. From the point of view of transmission, an ideal base from which to observe the far infrared is a spacecraft or satellite, such as the Infrared Astronomical Satellite (IRAS), which flew a 60 inch cooled telescope for several years in orbit. However, its function was primarily to obtain systanatic information ofa broad-band nature in the infrared, which it did well, ratha than spectra. Up to the present, most ofthe spectral work done in the far infrared has been carried out in the Kuipa Observatory already mentioned. There is also the possibility ofdoing useful far infrared work from the South Pole. The land mass at the South Pole is actually below sea level, but a pile of about 10,000 feet of snow and ice on top of it gives the structures built atthe South Pole an altitude of about 9,300 feet. This altitude, combined with vy low temperatures which freeze out wat vapor, provide enough transparency in certain localized frequency regions that the South Pole can also be quite useful for particular spectral lines which fall in these atmospheric windows.1 Balloon borne telescopes, which fly considerably higher than the Kuiper Observatoiy, can also be useful. However, they do not carry human attendants to adjust or modil3r the equipment, whereas the airplane can take a sizable crew of passengers who work with the telescopes and spectrometers while observing. The plane also characteristically lands 'and takes offagain for further observations with a turnaround lime ofabout two days, and hence with it the experimental apparatus can be tried, modified and repaired, and flown again rather efficiently. There are three rather different types of spectrometers presently in use for astronomical measurements in the far infrared. The first is a grating systan constructed by Erickson and his associates at the NASA-Ames Research Center, which is shown in Fig. 3 . Ituses a linear array ofdctectors, with three different types ofdetectors for various wavelength ranges and as many as 13 detectors responding at any one frequency. It has excellent sensitivity and a resolving powerof about 5,000. A second type of spectrometer involves the use of Fabry-Perots, and is shown schematically in Fig. 4. Such a Fabiy-Perot system has the advantage that it can examine spectral lines in a number of geometric positions at once. The system shown in Fig. 4, which has been built jointly by groups at the University of California at Berkeley and at the Max Planck Institute in Garching, Gennany, has a 5 x 5 array of detectors and hence maps spectral lines over sizable regions rather rapidly. With three Fabiy-Perots in series, it has a resolving power as high as about 100,000. Its NEP approaches i0 waUs/sec'. The third type of spectrometer, shown in Fig. 5, uses heterodyne detection, with far infrared gas lasers as local oscillators. At infrared wavelengths as long as 300 run, harmonics ofGunn diodes have also been used for a heterodyne detection spectrometa. Hetaodyne detection can achieve exceedingly high spectral resolution and thus resolve any spectral line. For a narrow spectral region it has somewhat higher sensitivity than direct detection, but it is somewhat less saisitive for rather broad lines. The heterodyne system shown was built by Betz3, who is now at the University of Colorado.

Infrared astrometry at the 10-milliarcsecond (mas) level is applicable to experiments in stellar evolution astronomy, solar system dynamics, relativistic gravitation, and deep space laser tracking. We are pursing astrometry with the U. C. Berkeley Infrared Spatial Interferometer (ISI) on Mt. Wilson to demonstrate a 10-mas capability for tracking stellar and solar system objects. Astrometric data from the ISI, taken and analyzed over the last 5 years, have shown that instrumental and atmospheric effects limit current demonstrations. The ISI data show that point-to-point interferometric phase fluctuations due to tropospheric and quantum noise, for optimal integration times of 0.2 seconds, are approaching the 0.1-cycle level needed to reliably connect the phase. Modeling the ISI data suggests that atmospheric fluctuations on Mt. Wilson, during the best seeing, are dominated by a low-lying component, within the first 25 meters above the ISI, which, in the future, may be minimized with in situ calibration. A calculation of atmosphere-limited astrometric accuracy shows that the ISI will soon be able to achieve 10-mas astrometry, on a 13-m baseline in a single observing session, employing current ground-based laser distance interferometer calibrations to minimize atmospheric effects.

Recent upgrades of the Infrared Spatial Interferometer are described. These provide improved sensitivity, precision and convenience of measurement. Analysis of phase fluctuations within the interferometer as well as in the atmosphere above indicates that popular simple atmospheric models need to be refined. Results of stellar observations are presented in another paper (cf. Danchi et al., in these Proceedings).

The Infrared Spatial Interferometer (ISI) is a high-resolution aperture synthesis imaging system for the 10-micron region. As in many radio interferometers, heterodyne signal detection and lobe rotation for fringe tracking are employed. A Helium-Neon laser metrology system is used for monitoring critical distances within the telescope optics. Although designed for baselines up to 1000 m, initially the interferometer will have baselines ranging from 4 to 34 m, yielding angular resolutions as fine as 0.030 arcsec. First fringes were detected in June 1988. Since then the system has been successfully operated on 4 m and 13 m baselines.

The development of an imaging Fabry-Perot spectrometer for the wavelength range 40–200µm employing a 5x5 detector array is reported.

The extension of high quality astronomical observations towards larger apertures, adaptive optics, and infrared wavelengths leads to extrapolation of present knowledge of astronomical «seeing» by means of theoretical models, such as Kolmogorov turbulence combined with Taylor's «frozen atmosphere» swept past the observer by winds. Observations of path length fluctuations from a star to a two-telescope spatial interferometer at 11 μm wavelength, and also measurements of path length fluctuations 3 m above the ground by laser distance interferometers, show substantial deviations from such a model. Intermittent turbulence may be involved and relatively short outer scales are frequently indicated

Les mesures de la visibilite interferometrique dans la region 10 μm sont presentees pour l'etoile carbonee IRC +10216 a des frequences spatiales dans l'etendue 1.4 - 12 × 10 5 rad -1 . Un modele ajuste aux courbes de visibilite indique que la poussiere se forme plus pres de l'etoile et a des temperatures plus elevees que les resultats indiques par des mesures anterieures.

The potential of high-resolution spatial interferometry for detailed mapping and precision astrometry in the mid-infrared region, somewhat analogous to interferometry now done in the microwave region, is discussed from an instrumental point of view. Some results from a prototype system and from tests of atmospheric properties are given. The design of a more advanced two-telescope system now under construction is outlined. This involves movable telescopes of 1.65 m aperture and of high precision, using heterodyne detection of infrared in the 10 Μm atmospheric window.

A K-band ruby traveling-wave maser (TWM) has been developed, which has provided a decrease in system noise temperature compared to other front ends presently used in radio telescopes at K-band by an order of magnitude. The maser uses a new type of photoetched slow-wave structure, integral with the ruby rod, especially suitable for millimeter-wave masers. It also employs a new type of built-in isolator configuration, which guarantees stable net gain of typically 30 dB over the tunable bandwidth, which is about 20 percent. Its phase stability, both short and long term, is excellent, making it highly suitable for use on an interferometer for radio astronomy.

Under certain operating conditions, magnetrons emit harmonics of their fundamental frequency of oscillation. With the use of the generating, filtering, and detecting equipment described here, harmonics as high as the tenth (1.25‐mm wavelength) have been obtained from K‐band tubes and the third (1.1 mm) from a 3.3‐mm tube. The peak power at these shortest wavelengths is estimated as a few hundred microwatts, which is adequate for a variety of physical measurements. Data are given on the harmonic spectra of various tubes and tube types.

Light pulses self steepening resulting from propagation in medium with intensity dependent refractive index

The design and operating characteristics of a maser radiometer for use in radio astronomy at 3-cm wavelength are discussed. The operating system which is described has a bandwidth of 5.5 mc and an input noise temperature, including background radiation into the antenna, of about 85°K. An rms fluctuation level of about 0.04°K is attained using an averaging time of 5 seconds. A discussion of the factors determining the sensitivity of such devices is presented.

A type of device is described which can be used as a microwave amplifier, spectrometer, or oscillator. Experimental results are given. When operated as a spectrometer, the device has good sensitivity, and, by eliminating the usual Doppler broadening, a resolution of 7 kc/sec has been achieved. Operated as an oscillator, the device produced a frequency stable to at least 4 parts in ${10}^{12}$ in times of the order of a second, and stable over periods of an hour or more to at least a part in ${10}^{10}$. The device is examined theoretically, and results are given for the expected sensitivity of the spectrometer, the stability and purity of the oscillation, and the noise figure of the amplifier. Under certain conditions a noise figure approaching the theoretical limit of unity, along with reasonably high gain, should be attainable.

Steric effects and compressibility appear to be important in determining the effective Kerr constant of liquids for short laser pulses, and also the saturation of the Kerr constant. Their influence may explain the relatively high threshold power and large diameters of self-trapped light filaments which are observed in many liquids.

Science and religion are often viewed as necessarily separate aspects of our beliefs and understanding. But I see religion as an attempt to understand the purpose of our universe and science as an attempt to understand its nature and characteristics, so that the two are necessarily closely related. The so-called anthropic principle for the physical constants and recent discoveries in cosmology such as the “Big Bang” are at least suggestive of such a relationship. We furthermore try to understand each of these fields with all our human resources: intuition, observations, logic, and esthetics, with science and religion having different emphasis on these resources yet nevertheless using all of them. Science has undergone revolutions in the post, which have rather completely changed our views, and yet science of the past has often maintained an important validity. It still faces many inconsistencies, and we must be open to new changes with deeper understanding and yet the continued validity of present science as an approximate model. Can we expect similar changes and deepening of our human understanding of religion? I discuss the parallelism and increasingly strong interaction of science and religion, which I visualize, along with the possibility of their ultimately merging into a more unified understanding of both the purpose and the nature of our universe.

The techniques of quantum electronics allow interesting new ways to generate and explore most of the acoustic spectrum and much of the electromagnetic domain. We can look forward to another decade of rapid development in this field From the time when man first saw the sunlight until very recently, the light which he has used has come predominantly from spontaneous emission, like the random emission of incandescent sources. So have most other types of electromagnetic radiation — infrared, ultraviolet, or gamma rays. The maximum radiation intensities, or specifically the power radiated per unit area per unit solid angle per unit frequency bandwidth, have been controlled by Planck's black-body law for radiation from hot objects. This sets an upper limit on radiation intensity — a limit which increases with increasing temperature, but we have had available temperatures of only a few tens of thousands or possibly a few millions of degrees.

Optical masers of useful performance will probably be available to a number of research workers in the course of the next one or two years. Their characteristics have already been sufficiently tested to make them predictable with considerable confidence from theoretical expectations. Potential research applications of the coherent form of light produced by optical masers seem interesting enough to warrant pointing out here the characteristics of masers to biologists and biophysicists. An optical maser can either amplify light waves or produce coherent oscillations at optical frequencies by use of stimulated emission from excited atoms or molecules. The process of stimulated emission as a result of interaction between electromagnetic waves and excited atoms is just the reverse of absorption, and dominates when there are more atoms in an excited state than in a lower state to which transitions may occur. If a coliection of suitably excited atoms are placed, for example, between two parallel reflecting plates, a light wave traveling perpendicularly to the plates will be amplified between each successive reflection. If the amplification is greater than the loss on reflection, the light wave will build up into a coherent oscillation, and some of it may be drawn off into a beam if one of the reflecting plates is partially transparent. There are many ways of providing excited atoms, and many different detailed arrangements of the optics involved, which will not be discussed here. Rather, we shall point out briefly the characteristics of the waves produced. The resulting radiation is extremely monochromatic; masers oscillating in the near infrared region at a frequency of about 3 x 1014 cycles per second have been shown to produce radiation with a frequency width, over short times, as small as a few thousand cycles per second. The path length over which such a wave would show interference phenomena is about 100 Km, as compared with a path length of about 1 meter for the most monochromatic light produced by other methods. We may expect from theory that light which is even more monochromatic can be produced by masers. The emitted light is remarkably directional. The wave emitted from the surface of one of the parallel plates may be coherent over the entire surface of the plate. Hence it will be almost a plane parallel wave, and will have as an angle of divergence

Light waves traveling backward and forward in very small filaments of optical material result in standing waves producing visible spatial beats, allowing detection of changes in refractive index