This paper presents a brief introduction to optically pumped submillimeter (SMM) lasers: the wavelengths available; the differences between CW and pulsed operation; SMM laser polarization; SMM laser theory; SMM laser efficiency and a summary of the advantages and limitations of optically pumped submillimeter lasers.
During the last few years, the application of intense, relativistic-electron beams to the generation of electromagnetic radiation at wavelengths ranging from 10 cm down to fraction of a millimeter has enabled significant advances to be made in peak power capabilities. The purpose of this review is to summarize the status of these advances and to describe briefly the nature of the several mechanisms involved.
Heterodyne radiometry with blackbody sources was used to determine the sensitivities of GaAs Schottky diode submillimeter receivers. At 500 μm an NEP of 10-18 watts/Hz was measured for the system. Application of these receivers to the characterization of radiation from high power pulsed submillimeter lasers has been demonstrated. Examples of the extension of this effort to planar devices, suitable for large area coherent detectors and imaging, are also presented. Recent studies have indicated that GaAs Schottky diodes could be used to construct extremely sensitive submillimeter heterodyne receivers. By carefully tailoring every phase of fabrication from material selection to optimization of the mixer environment, some of this potential performance has now been realized. In this paper we discuss heterodyne radiometry measurements which show more than two orders of magnitude improvement over our initial devices. An application of these low-capacitance, small-junction-area, Schottky diodes so frequency and linewidth measurements of a high power pulsed CH3F laser is discussed. Finally, our first test results and projections for all-planar diodes and arrays are presented. The diodes used in our studies have been recently developed at Lincoln Laboratory and are less than 1 micron in diameter and have cut-off frequencies approaching 10,000 GHz. In order to minimize waveguide losses at submillimeter wavelengths these diodes have been packaged in short sections of overmoded, reduced height, N-guide. Stripline filters, incorporated as an integral part of the coaxial studs of these diodes, supply dc bias to the diode and provide for an intermediate-frequency output. At the same time, these filters present an almost perfect reflecting circuit element at the signal and local-oscillator frequencies. The development of the complete diode package has required both theoretical calculations and experimental evaluation of scaled models. In order to determine the sensitivities of these diodes we have made heterodyne radiometric measurements of radiation from a blackbody. Three different receiver configurations were used in these experiments. The first used a horn-lens (TPX) system to couple submillimeter radiation into the diodes. A germanium beam splitter was then used to introduce local oscillator power from an optically pumped CH3F laser. The complete test setup is shown schematically in Fig. 1. Our second system replaced the hornlens with a focusing mirror and the third system used an additional orthogonal mirror to alleviate the need for the beam splitter.
Near infrared detector technology in the 10-30 μm band have been developed for use in liquid helium cooled telescopes both for celestial background measurements and for target identifications. Focal plane arrays using multidetector elements with charge coupled devices are being developed for infrared imagery. Research and development technology are now in progress for long wavelength (30 μm - 125 μm) detectors to be used in cryogenically cooled infrared telescopes to be used in satellite infrared experiments.
Quasi-optical devices are used to perform the functions of waveguide components in the submillimeter region. A quasi-optical local oscillator injection scheme utilizing a novel folded Fabry-Perot resonator is described and compared to the usual ring coupler waveguide system. A quasi-optical mixer utilizing a biconical antenna type radiating element is discussed and compared to a typical waveguide geometry.
The Cosmic Background Explorer Satellite (COBE) is now under study by a NASA-appointed team of R. Weiss (MIT), J. Mather and M. Hauser (GSFC), G. Smoot (Berkeley), S. Gulkis (JPL), and D. Wilkinson (Princeton). The COBE applies existing infrared and microwave techniques to achieve a major advance in our knowledge of the very early Universe. Three instruments will cover the spectral range from 8μ to 13mm, determining both the spectrum and the angular distribution of the large scale background radiation fields. A cryogenic polarizing Michelson interferometric spectrometer will measure the spectrum of the 3°K relic radiation from the Big Bang with high precision, to probe conditions at the very earliest times. Four differential microwave radiometers will map the sky from 23 to 90 GHz in a search for anisotropy of the universe. A broadband cryogenic infrared photometer will map zodiacal dust emission, galactic dust, and an extragalactic residual component. Study of these instruments has already produced designs for ultralow sidelobe flux collectors and horn antennas over the millimeter and microwave range.
The application of far-infrared lasers and detection systems to the measurement of plasma parameters is discussed. Potential applications include Thomson scattering determination of the ion temperature in fusion plasmas and holographic imaging of plasmas.
The submillimeter background in tokamaks will be produced by cyclotron emission from the plasma electrons. Two classes of electrons contribute to the radiation; the electrons which are part of the thermal distribution having energies of ≈1 keV and secondly electrons which have "runaway" from the thermal distribution and have energies of ≈1 MeV. In this paper we present the results of computations of the specific intensity at 500 μ, emitted by both classes of electrons and the results of some experimental measurements done on the PLT tokamak. We conclude that at the present operating parameters of PLT, the "runaway" electrons are the dominant source of 500 μ radiation and that the measured specific intensity at 500 μ is ≈10-13 in MKS units.
High resolution submillimeter interferometry systems for measurement of electron densities in the range 1013 cm-3 ≤ne ≤ 2 x 1015 cm-3 have been developed for use in Tokamaks. For the HCN laser interferometer, a mechanical modulation technique is used. The optically pumped CH3 OH lasers, which operate on the 118.8 μm line, employ phase modulation at ≈1 MHz which is accomplished by difference frequency mixing of two cavity tuned laser oscillators. These lasdrs feature a novel output coupling design which permits good mode quality and low beam divergence. The beat signals are detected using a newly developed Ge:Ga photoconductor and a direct measurement of the phase shift is obtained from the time lag between probe and reference signals. The sensitivity of the resulting phase measurement is independent of the instantaneous phase, and unaffected by fluctuations in the amplitude or in the frequency of the modulation.
A high power optically-pumped submillimeter oscillator for use in plasma diagnostics has been developed which employs an unstable resonator cavity. This oscillator has been operated using CH3I(447 μm), CH3F (496 μm) and D20(385/361 μm). The peak power output varied from approximately 40 kW with CH3I to 200 kW with D20.
The design and operation of pulsed high-power narrow-line FIR oscillator-amplifier configurations in the 0.2 - 0.6 mm region for Thomson scattering is described. Narrow-line power outputs in the neighborhood of 200 kW at 496 μm have been obtained. Proposed scattering experiments on the UCLA tokamaks are also discussed. Additional efforts are directed toward the development of high-power sources in the 0.75 mm to 2.0 mm region for use in nonlinear plasma instability studies.
The DoD system applications of submillimeter and for infrared radiation are considered. The list of possible applications are reduced to, Target detection, range and direction; Obstacle avoidance; Fire control and Guidance; Countermeasures and intelligence; Communications; Fusing; Imaging; Radiation damage; and others, which are discussed in detail.
The US Army Missile Research and Development Command has set up an instrumented propagation range at Redstone Arsenal, Alabama, for the purpose of systematically identifying and exploring the effects of the atmosphere and materials reflectance (natural and cultural) on high angular resolution radar systems. The emphasis will be on propagation and reflectance measurements in the water vapor absorption windows around 730, 870, and 1,200 micrometers. This paper describes the techniques, apparatus, and goals, and provides detail about some Army interest in this spectral region.
Typical of new applications of the rapidly developing submillimeter wave (SMMW) technology is the detection and sounding of upper atmospheric constituents. Before discussing this application it is appropriate to review coherent receiver concepts as they pass from the microwaves and mm-waves through the infrared. This is necessary because SMMAI is being approached from both ends of the spectrum conceptually and with experimental apparatus.
Most plastic materials are transparent to FIR laser radiation, which can therefore he used to inspect and to control the quality of industrial plastic products. This paper will descrihe a FIR laser imaging system for the detection of voids and contaminants in dielectric insulation materials for high voltage power cables.