This PDF file contains the front matter associated with SPIE Proceedings Volume 1576, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Using the "MAGIC" code, computer simulation is performed to design a cathode structure which provides a radial distribution of electron beam density matching the field distribution of the TM₀₅ mode. Thus, very narrow band TM₀₅ mode microwave radiation can be generated in an open end waveguide. The simulation results show that the vircator can indeed be operated at the selected frequency 10.4 GHz with a very narrow bandwidth(∆ω/ω<1%).

The magnicon is a new scanning beam (deflection-modulated) microwave amplifier related to the gyrocon. We discuss the design of a high perveance, magnetic-field-immersed frequency-doubling magnicon amplifier at 11.4 GHz. This magnicon will operate with a field-emission diode, but is intended to be a prototype for future thermionic devices with potential application to advanced linear accelerators and to space power beaming.

The recirculating hub charge causes nonlinear feedback between the input and the output in Crossed-Field Amplifiers. By varying some control parameters, such as the secondary emission coefficient or the recirculating charge fraction, the operation exhibits transitions from steady state, to limit cycles and finally to unpredictable (chaotic) behavior. The amplifier dynamics is described by a truncated system of fluid equations following guiding center trajectories¹. Numerical integration of these equations yields very good agreement with experimental results, while reducing the computation time by three orders of magnitude compared to particle simulations. A pass-to-pass map is employed to investigate whether the chaotic transitions in CFAs follow the 'universal' rout to chaos for nonlinear-dissipative systems.

It is known that the working regime of the magnetron is defined by thwo generalized parameters, namely H and J. H=H/H_cr - is the parameter of the supercritical state (where H - is the strength of working magnetic field, H_cr is the strength of the critical magnetic field), J=J_e/J_e* - is the parameter characterizing the proximity to the regime of the emission limitation by the space charge field...

The interest evinced to high stability sources of radiation is explained by their broad application both in a physical investigation and a technical supplements. Herewith, the peculiar attention is spared to the elaboration and research of millimeter wave range electron-vacuum devices of the middle and larger power.

Recently, two-dimensional (2D) plasmons in thin semiconduc. tor electron layers have been widely studied /1/. 2D plasmons are well-defined excitations at the frequencies ω in the far-infrared range where the condition uτ≫1 can be satisfied for the realistic relaxation time τ∼10^-12s.

Electron synchrotrons can serve as infrared sources with special characteristics which are particularly well matched to certain types of experiments. The electrons emit from a small area into a narrow solid angle, resulting in a source several orders of magnitude brighter (and at very long wavelengths, greater overall power) than a blackbody source. The NSLS infrared beamline at Brookhaven National Laboratory realizes these advantages, and the source has been used to explore, among other things, the properties of high-T_c copper oxide superconductors.

Very high magnetic fields are required to determine accurately the resonance position, width and shape of broad magneto-optical absorptions. A strip-line was developed for use in pulsed magnetic fields. This was used for sensitive measurements in ferromagnetic 3d metals. Paramagnetic resonances were measured in Gd-doped high T_c superconductors; an additional resonance was observed at low temperatures which was sensitive to the oxygen content.

The photoresponse of granular Bi₂Sr₂CaCu₂O₈ and of epitaxial YBa₂Cu₃O_7-δ thin films has been investigated at wavelengths between 10 μm and 944 μm. In addition to a bolometric signal we observe for the granular Bi-based films a nonbolometric response at temperatures T <80 K. The dependence of the signal on intensity, applied magnetic field and wavelength is consistent with an ac josephson effect occurring at the grain boundaries.

In the past four years there have been many reports of detection of far- infrared radiation in High Temperature Supercondutors^1,2. We report here some additional observations of this phenomena, in particular we have measured a threshold of bias-current below which no response is observed to radiation in the range 85-150 cm^-1.

We present experimental results of far-infrared reflectivity investigations of RBa₂Cu₃O_7-δ (R=Er, Tm, Dy) and YBa₂Cu₄O₈ superconducting materials in the temperature range 10-300 K. In order to determine the phonon parameters such as frequency, half-width and oscillator strength, we performed a Kramers-Kronig transform of the reflectivity spectra and than fitted the bands in the conductivity spectra to lorentzian lineshapes.

Two YBCO oriented epitaxial films on SrTiO₃ substrates prepared by laser ablation technique were used for mid-infrared optical Investigations. Both ac-ori rented (i.e. a^→ (or b^→) and c^→ lattice vectors lie in the substrate plane) and a^→b^→-oriented films were deposited and annealed at the same technological cycle and had thickness about 4000A. Special attention was paid to the determination of quality and orientation. X-rays difraction showed only presence of 00n peaks for (ab)-orientation and 110, 220 peaks for (ac)-film. Lattice constants from X-rays measurements were used to calculate the oxigen contents of films by empyrical formula 7−δ=76.4−5.95⋅c^→ [A], which gives δ≈0.08+0.12.

Ellipsometry is based on exploiting the polarization transformation that occurs as a beam of polarized light is reflected from or transmitted through an interface or film. With this method optical constants can be determined very fast, very precisely and without further assumptions (no Kramers-Kronig analysis). We have developed a rotating analyzer ellipsometer for the far-infrared wavelength range from 30 to 1200 μm. Applications are in the characterization of semiconductors and also, together with a cryogenic reflection unit, of high-T_c superconductors.

The investigation of thin film features on high reflectivity materials by optical methods is complicated due to small useful signal. Surface electromagnetic waves (SEW) is considered to be very perspective method for analysis of such materials. For SrTiO₃ SEW exist in FIR region (ν=88 - 807cm^-1), except narrow gaps, and its propagation distance is about a few millimeters [1,2]. So, it can be easy measured. In the present paper we report the measurements of the optical features of YBa₂Cu₃O_7-x films on SrTiO₃(100) substrate by FIR SEW amplitude and phase spectroscopy at temperatures 80 - 300K.

Far-infrared and submillimeter-wave astronomy are relatively new branches of astronomical research. This is due to the poor transmission of the terrestrial atmosphere in this spectral range. A restricted number of narrow spectral windows permit the observation of astronomical objects directly from the ground. Reliable observations have therefore to be performed from satellites [1], stratospheric-balloon gondolas [2,3] or aircraft [4].

Far infrared emission spectroscopy is a powerful technique for the measurement of a number of molecules that are important in stratospheric (i.e., ozone layer) chemistry. The technique is particularly applicable to polar molecules with large, hydrogenic rotational constants [Chance et al., 1985]. These molecules include diatomics (HCl, HF, HBr, and OH), and nearly symmetric prolate molecules with large perpendicular components of their dipole moments (HOCl, H₂O₂, and HO₂). They have strong rotational spectra in the far infrared that persist to sufficiently high energy (80 cm^-1 and above) that their spectra do not suffer great interference from the rotational lines of H₂O and O₃ which dominate the far infrared atmospheric spectrum. Thus, this type of molecule can be measured with great sensitivity from about 80 to 200 cm^-1.

Ozone is the most important minor constituent of the upper atmosphere. It plays a very important role in the chemistry and photochemistry of the stratosphere. For this reason, a large number of studies have been performed on this molecule. In particular, pure rotational and vibrational-rotational spectra have been recorded with several techniques (1).

Lineshape parameters of ozone rotational transitions in the submillimeter wave region are required for an accurate determination of mixing ratio of this molecule in stratosphere. In order to obtain very accurate concentration profiles, it is very important to have a. good knowledge not, only of lineshape parameters, but also of their temperature dependence.

Since 1978, the Microwave Sounding Unit (MSU) onboard the NOAA american satellites provides the meteorological community with an enhanced temperature profiling capability. In addition to the 60GHz oxygen band observations, the future generation of these polar orbiting satellites will also include a humidity sounder up to 190GHz : AMSU-B.

To help understanding the radiative transfer in the atmosphere at these high millimeter wave frequencies, the UK Meteorological Office and the Laboratoire de Météorologie Dynamique (France) have jointly developed a microwave receiver at 89 and 157 GHz, frequencies corresponding to the AMSU-B window channels. The sounder has been flown in two international experiments in 1990. In this paper, we will focus on the gaseous absorption problem.

The melting layer is assumed to be composed of spherical melting snow particles. Size distribution and average dielectric constant are used to characterize the melting snow particles. Both the size distribution and the dielectric constant are found to be connected with the physical and meteorological parameters. The cross sections, the albedo, and the effective reflectivity factor are computed in a frequency range of 1-100 GHz by using the Mie scattering for five size distributions and rain rates below 12.5 mm/h. Numerical results show that the difference between the melting layer attenuation and the attenuation of an equal path length in rain with the drop size distribution that results from the melting is at maximum around 25 GHz. It has also been seen that the peak of the effective reflectivity factor is disappeared in the melting layer when the frequency is higher than 70 GHz.

A measurement system for the observation of the influences of the local climate on the transmission behaviour at millimeter waves has been developed and tested. Because of the required high resolution for path attenuation changes the coherent measurement principle has been selected, thereby simultaneosly avoiding a biologically relevant power density anywhere in the open transmission path. Both transmitter and receiver antennae have been placed at the same end of the bidirectional transmission path to facilitate the coherent measurement, whereas the other end consists of a metallic reflector.

There are many atmospheric trace gas species involved in the three catalytic cycles that contribute to stratospheric ozone depletion. The OH molecule is central to one of this cycles yet its concentration as a function of altitude is very difficult to measure in the stratosphere. The stronger rotational transitions lie at frequencies greater than 1500 GHz, significantly above the realm, of traditional submillimeter heterodyne instruments. Both Fabry-Perot and Fourier transform spectrometers have been used to observe OH in the atmosphere. Due to their inherent lower resolution these instruments can obtain atmospheric profiles only in a limb viewing geometry, thus they must be lofted above the stratosphere via balloons. This significantly constrains their seasonal and geographical survey capabilities. In this paper we Investigate the feasibility of utilizing a far infrared laser heterodyne instrument, operating aboard an aircraft, for the detection of OH and other relevant trace gas species in the stratosphere.

For some years the main aim of investigations of gyrotrons is the creation of sources ECRH of fusion plasmas at frequenceies over 100 GHZ with outputs of 1 MW - level in pulses with duration of several seconds (up to CW). Nowadays the fabloration of gyrotrons with average output from 10 to 100 kW at frequencies ∼30 GHz and higher has been begun for technological applications. Nevertheless the basic investigation of cyrotrons are still bound with problems of increase in output power. Pulse duration and frequency.

A recent multimode simulation[1] of a high power gyrotron[2] has suggested that mode competition with backward waves is responsible for the lower than expected efficiency at high current. The present effort is an attempt to compare the predicted and measured features of parasitic modes in a particular cavity.

We present a simulation study of mode competition in a highly overmoded gyrotron cavity, modeled after the approximate design parameters of a 280 GHz, 1 MW TE_+42,7-mode gyrotron at the Massachusetts Institute of Technology (MIT). This study addresses 1) the problem of achieving gyrotron operation in the design mode, and at a value of magnetic detuning sufficient to achieve high efficiency operation, and 2) the mode purity of the final state.

On the basis of a generalized theory we compare the cold-cavity and the self-consistent methods of calculation of mode competition in gyrotrons. This comparison enables us to determine conditions, when the cold cavity approximation is justfied. A specific example is presented.

The output power from a gyrotron is usually extracted through a window which is not perfectly transmitting. Thus, a fraction of the power is reflected back into the interaction region. This reflection can affect the operation of a gyrotron in several ways¹. First, the reflection modifies the Q factor of the cavity and changes the operating characteristics such as start current and efficiency. Second, due to the so-called long line effect the spectrum of stationary modes in the cavity becomes dense with a spacing equal to the reciprocal of the round trip travel time from the cavity to the window and back. A third consequence is the effect of the variation of reflection coefficient with frequency on mode competition. We will examine these effects with the aid of simple models and time dependent multimode numerical simulation. One result of our studies is that even for small reflection coefficients if the delay time is sufficiently long the time dependence of the radiation field in a single transverse mode can become quite complicated.

Gyrotron design is almost exclusively based on a time-domain analysis of the tube performance. Such a procedure is cumbersome mainly with respect of taking also device/circuit interactions into account. We present a network-theoretical approach to the gyrotron oscillator analysis which circumvents such difficulties.

In a preceding paper (Jensen, Schtinemann: "Network-Theoretical Approach to the Gyrotron Oscillator", 16th Conf. IR & MMW, Lausanne 1991) we have developed a network-theoretical approach to gyrotron oscillator analysis. As in normal microwave oscillator theory, it formulates an oscillation equation in frequency-domain in which both the device and the load admittance are generalized now to a nonlinear (and mainly frequency- insensitive) and to a linear admittance matrix, respectively. The latter describes the frequency-dependent passive circuit.

There is more interest in electrostatic electron cyclotron resonance maser (EECRM) ,which is similar to Orbitron, because of its attractive advantages to gyrotron^[1-4], e. g. , it may be voltage tunable, it may operate at high cyclotron harmonic and nonrelativistic case, it requires no magnetostatic field. By now, most articles for EECRM and Orbitron dealed with an ideal model, i. e. , the space-charge effects were neglected. Whereas, one can see from the results given below that the space-charge effects have a important role in EECRM, and may change parts of EECRM characteristics.

Bending a short bunched electron beam from the linac by the magnetic field, we have observed coherent synchrotron radiation, which is a new source of far-infrared and millimeter waves with high brightness. Spectra of coherent Cherenkov and rf wake field radiation have also been observed. This paper is a review of our experiments by S- and L-band electron linacs at Tohoku University and Kyoto University.

The idea behind this particular Gunn oscillator-frequency multiplier combination which is a follow up on an earlier project (1), was that in combination with SIS receivers which have ℒO requirements in the 100 nW range,efficiency and maximum output power of a source can be traded for convenience and economy. Instead of cascading carefully optimized varactor doubler and triple stages, this frequency source only rectifies the radiation from the Gunn oscillator and filters for the desired harmonic output frequency.

Measurements of the dielectric and optical properties of materials (complex refractive indices, dielectric constants, magneto and electro-optic coefficients etc.) over wide ranges of frequency above 300 GHz are made by Fourier transform spectrometry and dispersive Fourier transform spectrometry. These techniques utilise mercury arc lamps as broadband noise sources with noise temperatures of about 2000 K. For frequencies below 300 GHz the spectrometric methods are still applicable but the power available from mercury arc lamps is very low (< 2000 K) and below about 100 GHz spectrometric measurements require long integration times to reach acceptable signal-to-noise ratios.

Waveguide mounted noise diodes are used for power calibration of microwave systems at frequencies below 60 GHz. They have fairly high noise temperatures but are limited in frequency coverage to a single waveguide band.

The low-noise high efficiency Q-band MESFET oscillators are developed. Their noise, power and temperature characteristics are investigated. The possibility of the 2th and 3th harmonic power generation are demonstrated.

We have modified the frequency of a propagating R.F. signal using a pulsed plasma discharge. In our experimental work, we have utilized two frequency shifting mechanisms. The first is to use the plasma as an nonphysical plunger to upshift the frequency of the signal.

A structure of active layer for relieble efficient gunn diodes is suggested. Such layer was grown by MBE and good gunn diodes were achieved.

One of the means of lowering of noise of sources of electromagnetic oscillation is the use in generator high Q-quality osciallation system. This way especially actual in elaboration solid-state generators of extremely high-frequency band (E,F,G, band), where the using of the method of outer synchronisation and other methods stabilisation is rather problematically. The open resonator (OR) with the echelette is one of the versions of the dispersion open oscillating system. The diffraction reflection grating echelette has brightly pronounced angle dispersion and this helps to realize the selection of spectrum as an transversal as an longitudinal modes and this is of a great importance. The last property does these gratings very attractive from the point of view their use in the open oscillation system of the solid-state and electron-vacuum tube devices.

The difficulties connected with the development of MM-wave generators of middle and large power in with the shortening of the wavelength. The main cause of this is the decreasing of the geometrical sizes of the elements of slow wave system (SWS) of TWT and, accordingly, the decreasing of transversal cross-section of its floating-drift channel and the increasing of heat loads. The improvement of heat-physics characteristics of TWT is achieved by the using slow wave system with the extensive surface of type of coupled cavity circuit or modification of ribbed structures, working at spatial harmonics. The increasing of the geometrical sizes of the elements of slow wave circuit entails the increasing of the operating voltage, RF-loss, the decreasing of electron flow perveance and electron efficiency. The increasing of the beam perveance and the efficiency in TWT is achieved by the using multi-beam electron flows /1/. At this the increasing of output power, the decreasing of operating voltage and the broadening of operating bandwidth take place. The possibility of the realization of TWT with staircase, slow wave structure to obtain middle and large output power in UHF-region was shown in /1,2/.

Creation and investigation of Gann-diode low-noise comb spectrum oscillator (CSO) are discussed. Considered oscillator can build the base for coherent frequency synthesizers forming oscillations directly in millimeter band.

After four years of world wide research on high T_c, superconductors of the cuprate type a variety of different material families is known, with T_c values up to 125 K. Progress in material preparation and in the development of experimental and theoretical methods has lead to a basic understanding of various material properties, however, the nature of the pairing mechanism is yet unknown. In this paper some basic properties will be discussed. A second point will be concentrated on important optical properties from microwave to u.v. frequencies. Condensation of charge carriers can be accompanied with anomalous phonon behavior; at T_c anomalous changes of oscillator strengths of infrared active phonons and anomalous shifts of resonance frequencies of some phonons are observed. Condensation of charge carriers leads, furthermore, to a behavior of the (anisotropic) optical properties that may be described by superconducting energy gaps. For excitations along the copper oxide planes the optical properties can be described as arising from a superconducting energy gap 2∆(0)/kT_c≈7 . However, there remains at low temperatures intra-gap absorption that extends from microwave to far-infrared frequencies. In a third part of the paper, microwave and far-infrared applications of high T_c superconductors will be discussed, namely microwave antennas, Fabry-Perot filters and

We report on the application of high-T_c superconductors as reflectors for far-infrared Fabry-Perot resonators. At low temperatures and frequencies smaller than the superconducting gap frequency the optical properties of YBa₂Cu₃O_7-δ are mainly determined by inductive currents. Therefore YBa₂Cu₃O_7-δ thin films on MgO, which is transparent in the far-infrared, are suitable for Fabry- Perot resonators with high peak-transmissivity. We report on a resonator with first order resonance at a frequency of 80 cm^-1, a quality factor of 55 and a peak transmissivity of 0.16.

The concentration of the electromagnetic field inside WG dielectric resonators [1] has been emphatized as a cause of sensitivity enhancement in the techniques of ESR spectroscopy [2]. This work is an experimental study dedicated to the detection of optical and infrared radiation utilizing as a thermal detector a small polycristalline sample of high-T_c superconducting material placed inside a WG resonator and using the techniques of microwave measurements [3]. The experimental set-up, as depicted in Fig 1, consisted essentially in a simple transmission-cavity spectrometer operated in the frequency range from 18 to 26.5 GHz (K-band). The resonator was a 20 mm alumina disk, 3 mm thick, excited in the WGH modes of resonance with dielectric rod waveguides, and supported by a liquid nitrogen cold finger in high vacuum conditions. The transmitted power was detected by a crystal receiver. A few tens of μg of YB₂C₃O_7-δ powder [4], was pressed at the bottom of a .9 mm hole, in a region of high electromagnetic field intensity. The hole was drilled in radial direction into the curved surface of the disk and was about 1.5 mm deep. A microwave sweep oscillator of 10 dBm of output power was fixed at the resonant frequency of the loaded resonator, the temperature being sligthly lower than the critical temperature of the superconducting material. The transmitted power level was recorded while a red-laser beam was directed inside the hole. The recorded signals, as reported in Fig 2, give indications about the response time, and the sensitivity attainable. The upper trace represents the transmitted signal variation obtained illuminating the resonator hole with a non focalized 1 mw CW red laser beam. The rise time of the signal was of τ=.5 s , and it was found to be proportional, within limits, to the sample mass. The voltage signal was proportional to the incident microwave power. The lower trace represents the transmission curves of the resonator as a function of time: with "laser on" (the first two peaks), and with "laser off" (the three peaks to the rigth) . These curves were obtained by sweeping repeatedly the microwave source +/−10 MHz around the frequency of 24.4 MHz, i.e. the resonant frequency of the WG disk resonator with laser "off'. The resonant frequency shift under "laser on" conditions was of 4 MHz and the relative Q-factor variation was of about 17%. Both the resonant frequency and the Q-factor diminished as the sample temperature raised over T_c. The observed signals are attributable to the phase transition of the superconducting sample under the heating effect of light. Chopped laser signals up to 10 Hz could be recorded in a straigth video- detection scheme, with a signal to noise ratio of 10. The loaded Q-factor was always of the order of 10³. The quasi black-body absorption characteristic of the powder charged hole, acting much as a light-trap, makes the device capable of detecting infrared radiations in a wide range of wavelengths. The maximum voltage "responsivity" (at 0 hertz) was of about 200 V/W , with a voltage noise less than of .1 mV/(Hz)^1/2. Experiments with higher Q-factor dielectric resonators and epitaxially deposited thin superconducting films for characterization purposes at microwave frequencies are in project.

The work introduced below has extended previous reports of far infrared (FIR: 10 cm^-1 < ω < 100 cm^-1) responses from granular Y-Ba-Cu-O [1] and Tl-Ba-Sr-Cu-O [2] films to the Bi₂Sr₂CaCu₂O₈ system for the first time. More importantly, the photosignal was found to depend on √Power over a two orders of magnitude range of incident powers. The detection mechanism is therefore believed to arise from the interaction of current biased intergranular Josephson junctions with radiation induced screening currents. A non bolometric response is also observed at low temperatures at a mid infrared frequency of 903 cm^-1.

We report on the use of high temperature superconductor material for fabricating of a resonator of a far-infrared solid state laser. In our experiment the active medium consisted of a p-germanium crystal at 4.2 K in crossed electric and magnetic fields. We used a SrTiO₃ plate as highly reflecting mirror and a YBa₂Cu₃O_7-δ thin film on MgO plate as output coupling mirror. The spectra of the laser with the superconducting mirror was measured. We demonstrate also highly reproducible operation of the laser.

Since the discovery of high-Tc superconducting cuprates, the interest in envisioning various radiation detecting mechanisms has been considerably revived. If the implementation of Josephson micro-junctions is greatly inhibited by difficulties to setup multilayer processes, simpler devices - that involve a single superconducting film deposition step - are technologically more accessible.

We have fabricated HTSC bolometers with a responsivity that is essentially wavelength independent in the range from λ ∼ 0.6 μm to 450 μm. They consist of a thin film of YBaCuO on a SrTiO₃ substrate and were patterned by photolithography in the form of a micro-bridge (20 x 20 μm²). A maximum responsivity of S = 0.1 V/W and an NEP = 2.10^-7 W/√Hz have been obtained when the element was operated near the transition temperature of 90°K. The frequency response varies like S ≈ 1/√f which indicates a strong thermal coupling of the micro-bolometer to the substrate. Higher responsivities (S ≈ 800 V/W and an NEP of 2.10^-11 W/√HZ) have been obtained using a second type of detector element in the form of a meander covering an area of 1 mm². For these elements a frequency response S ≈ 1/f and a cut-off frequency of 0.1Hz have been found.

A microwave phase shifter has been made by using Bi-Sr-Ca-Cu-O bulk superconductor. In this paper we describe the fabrication of microwave phase shifter and present the expermental results on the relation between phase and frequency, also phase and bias current.

The generation of ultrashort far- infrared laser pulses is of interest for applications in solid state physics, plasma physics and also for the understanding of fundamental processes occuring on a short-time scale in far-infrared gas lasers. However, far-infrared lasers are generally low-pressure narrow-band systems which implies that in this wavelength range short pulses cannot be produced by conventional means. We therefore investigate the generation of ultrashort far-infrared pulses with laser gases which emit by coherent processes such as superradiance or Raman emission.¹ In the process of superradiant emission² the atoms are coupled together by their common radiation field, and thus decay cooperatively. For a number N of emitting atoms or molecules the pulse intensity is therefore proportional to N² and the pulse duration to 1/N. Since N can be large, this process offers the possibility of producing intense short pulses. In the case of Raman emission³ the pulse can be shorter than the inverse linewidth of the transition if the pump pulse is broad band. Hence, the pulse duration is determined by the larger of either the pump-pulse bandwidth or the laser linewidth.

We report operation of a passively mode-locked tunable high-pressure CO₂ laser and its use for generation of far-infrared laser pulses. Mode-locking of the CO₂ laser was performed with p-doped Germanium as saturable absorber. We obtained tunable radiation consisting of trains of short pulses of about 1 ns duration. By optical pumping of CH₃F gas superradiant emission resulted in generation of subnanosecond far-infrared laser pulses at a frequency of about 35 cm^-1.

Using a continuously tunable high pressure CO₂ laser as pump source, we obtain laser action at 58 lines, 48 detected for the first time, from D₂O gas. 53 of these lines, covering the frequency region from 26 cm^-1 to 236 cm^-1, were found to occur by stimulated Raman scattering with a maximum bandwidth of the Raman gain regions of 17 GHz. We made use of these broad amplification profiles for generation of subnanosecond far infrared pulses.

In this paper, an overview is presented on the frequency tunable methylfluoride Raman laser. By use of a continuously tunable high pressure CO₂ laser as pump source, tunable far-infrared radiation can be obtained from all the stable isotopes of methylfluoride which have symmetric top structure. Using these gases, ¹²CH₃F, ¹³CH₃F, ¹²CD₃F and ¹³CD₃F, as FIR media about half of the frequency region from 8 cm^-1 to 73 cm^-1 can be covered with tunable intense pulsed radiation. We studied quantum effiencies and bandwidths of the tuning intervals for the different gases and also for different pump schemes.

The para-H₂ Raman laser has been developed intensively as a high intensity tunable laser source in the wave-length range of 13-18μm. The successive conversion to the second Stokes wave can expand wavelength up to 46μm. However, the increase of wave-length is accompanied by the substantial decrease of Raman gain. Practically, the wavelength which is obtained by the second Stakes conversion re- mains below 30μm even with the use of a liquid-nitrogen-cooled Raman cell¹⁾. In this report, we generate a high intensity far-infrared pulse from a para-H₂ Raman laser being operated at room temperature as the second Stokes wave. The longest wavelength extended to 42.5μm. Powers of the pump pulse and the first Stokes pulse were fully depleted and the converted energy was estimated to be more than 100 mJ with a pulsewidth of about 50 ns.

Utilization of active media of different isotopic composition enables to expand significantly the spectral range of optically pumped FIR Raman lasers. In this work CD₃F FIR emission was investigated using grating tunable TEA-CO₂ laser. Experimental investigation of possibility to obtain superradiant emission on different FIR lines under relatively low pump intensities (up to 5 MW) was the main objective of this work. We used multi pass cell with total optical length of about 15 m in order to diminish the emission thresholds. The experimental conditions were analogous to /1/, where the output characteristics of ¹²CH₃F, ¹³CH₃F mixture FIR Raman laser were investigated. The pressure of the active media was not optimized.

Theoretical studies of the efficiency of cw optically pumped far infrared lasera (OPFIRL) tend to neglect transversal variations of the pump-intensity (see e.g. passes through the low absorbing cavity. Realistic pump field distributions appear to be different due to the common practice of focussing the pump beam through a small coupling hole, thus introducing large intensities of the "primary beam" that will always dominate the overall intensity profile /2/. The specific consequences of radially varying pump fields will be discussed in two steps.

Very little research on optically pumped FIR-ringlasers has been reported in the past, despite the fact that such lasers have proven to be more stable FIR-monochromatic sources than the usual standing wave resonator types (Heppner, Weiss 1978).

The ringlaser offers much better isolation between the pump and the FIR-laser due to the elimination of interaction between the two. This is similar to pumped dye ringlasers in the visible part of the EM-spectrum.

Unlike most ringlasers in the visible range unidirectional emission is a general feature of FIR-ringlasers. Hence an optical diode is not required to suppress simultaneous bidirectional operation.

A theoretical model was set up to calculate numerically the output power of far infrared lasers with an active medium where vibrational-translational relaxation predominates, e.g. formic acid HCOOH, in dependence of the pump beam. The calculation is based on an actual waveguide ring laser system similar to that described in [1] with a pump power of 15W.

Recent results will be presented on high frequency (100-300 GHz) megawatt gyrotron experiments being conducted at MIT. Two gyrotron oscillators are being investigated: one operating at 148 GHz in the TE_16,2,1 mode, and the other at 280 GHz in the TE_42,7,1 mode. These gyrotrons operate with 3μsec pulses, but can be scaled to cw operation. The highest power at 148 GHz has been achieved with a two-section cavity. Powers up to 1.2 MW with an efficiency of 32% were measured at 48 A. The 280 GHz experiment will begin shortly, and will use a single cavity with a diameter of 23 wavelengths. This will be a severe test of the gyrotron's ability to excite a single mode and suppress competing modes. Measurements of the electron beam spatial and velocity characteristics for the 140 GHz gun will also be presented.