Coherent tunable far infrared radiation

Tunable, cw, far infrared (FIR) radiation has been generated by nonlinear mixing of radiation from two CO₂ lasers in a metal-insulator-metal, (MIM) diode. The FIR difference-frequency power was radiated from the MIM diode antenna to a calibrated indium antimonide bolometer. Two-tenths of a microwatt of FIR power was generated by 250 mW from each of the CO₂ lasers. Using the combination of lines from a waveguide CO₂ laser, with its larger tuning range, with lines from CO₂, N₂O, and CO₂ isotope lasers promises complete coverage of the entire far infrared band from 100 to 5000 GHz (3–200 cm^–1) with stepwise-tunable cw radiation.Contribution of the National Bureau of Standards, not subject to copyright     

The far infrared helium laser

Two far i.r. lines from a pulsed helium laser have been accurately measured with a metal grid Fabry-Perot interferometer at wavelengths of 95.763 ± 0.006 μm and 216.12 ± 0.01μm. The laser output critically depends on the plasma conditions in the laser during the excitation phase. The rapid increase in collision frequency at this time limits the duration of the laser emission.     

Optically pumped far infrared lasers

The far-infrared (FIR) region of the electromagnetic spectrum is commonly thought of as the wavelength region ≈ 30μm-≈2 mm. Thus, the FIR wavelength region is located between the more familiar areas of microwaves and optics. Primarily due to the lack of FIR sources and detectors, the FIR region is difficult to access and therefore relatively unexplored and unused. The FIR source problem is presently under attack from neighbouring disciplines; from the microwave side by extending the frequency operating range of classical electron tube oscillators (e.g. backward wave oscillators) and semiconductor devices (e.g. IMPATT and quantum well oscillators) and from the optical side primarily by optically pumped molecular gas lasers.The FIR technology evolution accelerated in the mid 60's with the discovery of the discharge pumped hydrogen cyanide laser, lasing at a handful of lines located at about 330μm wavelength. However, the most important step towards a useful coherent FIR source was the discovery of the optically pumped FIR laser in 1970. In optically pumped FIR lasers a molecular gas (e.g. methyl fluoride methyl alcohol, formic acid) is pumped by an external laser, usually a carbon dioxide laser. The FIR laser transitions typically takes place between adjacent rotational levels in an excited vibrational state. Today, optically pumped FIR lasers cover the full FIR region by more than one thousand discrete laser lines observed in hundreds of FIR laser media. FIR output powers on the order of 1–100 mW are available from a vast number of laser transitions.Despite the rapid development of semiconductor FIR oscillators the optically pumped FIR laser is still the only practical unit that bridge the full frequency-gap between microwaves and optics. The fact that FIR lasers are considered as local oscillators in space born applications, indicate that FIR laser technology has matured considerably.This survey paper discusses optically pumped FIR lasers from the engineer's point of view: principles of operation, design and characteristics.     

Reference reflection cavity for the far infrared1

A simple non-resonant cavity system has been built and tested for use in the far-i.r. (4–700 cm⁻¹). It allows for detection of small percentage changes in sample reflectivity by use of a matched reference cavity.     

A corner-reflector mixer mount for far infrared wavelengths

A new type of corner-reflector mixer mount, which has the advantages of ease of fabrication and assembly as well as frequency versatility, has been designed and constructed. The mixer works with arbitrary antenna lengths > or = 4 lambda with the reflector to antenna spacing adjusted to give a strong and symmetric central lobe. The predicted response patterns have been experimentally verified for various antenna lengths and operating frequencies between 800 and 2000 GHz. An important design feature is the incorporation of a microstrip matching network which eliminates IF impedance mismatch and provides mechanical isolation of the whisker antenna.     

A circular reflector interferometer for the far infrared

The construction of a Fourier transform spectrometer to study magnetic resonances in solids is described. The spectrometer operates in the Far Infrared (maximum frequency of 500 cm^–1) and is based on a wavefront dividing interferometer with circular geometry. To illustrate the performance, a water vapour spectrum is given.     

Noncollinear folded mixing geometries for difference-frequency far infrared generation

Noncollinear folded geometries for far infrared generation by difference-frequency mixing of CO₂ lasers are analyzed, and are shown theoretically to provide orders of magnitude increase in the far infrared output compared with the simple noncollinear geometry used previously. The basic principle of the folded geometry has been verified experimentally.     

Hybrid output mirror for optically pumped far infrared lasers

A hybrid metallic mesh multilayer dielectric coating mirror has been developed for use as an output mirror for optically pumped far infrared lasers. The metallic mesh provides a high reflectance in the far infrared while the multilayer dielectric coating is chosen to provide a maximum reflectivity at the pump wavelength (10 μm). This hybrid mirror has increased the output power of a CH₃F waveguide laser at 496 μm by a factor of 350 over that obtained with a hole coupling mirror. In addition, this mirror results in a far infrared output beam which has a minimum angular divergence limited only by the particular oscillating transverse waveguide mode (the EH₁₁ mode for this experiment).     

Rate equations for an optically-pumped,far infrared laser

Rate equations for an optically-pumped, far infrared laser are developed and solved for the case of a high intensity, pulsed pump beam. Conditions for saturated absorption and for saturated stimulated emission are established. A comparison is made to experimental data for absorption of 9.55 μm, CO₂ laser radiation in CH₃F. Generalized requirements for efficient, high power, FIR laser oscillator-amplifier systems are discussed.     

Mechanism for far infrared absorption of small metallic particles

A mechanism for far infrared absorption of small particles of noble metals is described. The electric double layer formed at the particle surface allows excitation of the resonant low frequency mode in which the metal core oscillates rigidly with respect to the shellof adsorbed atoms. The model is capable of explaining the large magnitude and the anomalous frequency dependence recently reported for Pt particles.     

New possibilities for frequency stabilization of far infrared lasers

All spectrometers based on the use of a far infrared laser present the drawback of the laser frequency uncertainty. Even if the knowledge of the laser frequency is very precise, a shift of the laser cavity length is always present, limiting the spectrometer frequency accuracy to 500 kHz.The goal of this paper is to give a solution to this drawback in demonstrating the feasibility of a frequency stabilization of the far infrared laser.     

Considerations on balloon-borne far infrared telescopes

The design of a telescope devoted to infrared and millimetric differential measurements is described. The optics are well corrected for aberrations, either in the on-axis or in the off-axis configurations, for tilt angles ∼1° of the secondary mirror. The diffraction pattern at large angles is calculated as a function of the surface quality and the in flight background at ballon altitude is estimated.     

Subnanosecond optical switching of far infrared radiation

Switching of 119 μm CH₃OH laser radiation in less than 1 ns was achieved by irradiating high resistivity Si (>10⁴ Ω cm) and GaAs (>10⁷ Ω cm) samples with 1.4 ns, 150 μJ pulses from a 337 nm TEA nitrogen laser. Modulation degrees of up to 100% were observed both in reflection and transmission.     

Thermography: Imaging in the far infrared

Thermography, the art of visualizing and interpreting thermal patterns, is a versatile new tool for science, medicine and technology. It is developing rapidly and spreading into widely diverse fields. Although its origins are more than 130 years old, the first practical applications (in military reconnaissance) were achieved only 15 years ago. Today, clinical thermography offers new hope in the fight against cancer, and has many other uses; it is a completely passive diagnostic method and absolutely safe. In industry, thermography has potential value whenever there are problems in measuring temperature over extended areas, where point contact methods are insufficient, tedious, or impossible (e.g. in inaccessible places). Thermographic microscopes and telescopes offer great possibilities which are only just beginning to be explored. The design of thermographic equipment presents problems which do not arise in most electro-optical systems, including television, and which more nearly resemble the design problems of radio telescopes. This article reviews the basic principles and the design optimization of thermographic scanners.     

The far infrared complex reflectivity of GaTe

The room temperature polarized complex (amplitude and phase) infra-red reflection spectra of single crystal monoclinic GaTe have been measured between 20 and 400 cm^–1 using a dispersive Fourier transform spectrometer. The measurements allow the experimental zone centre transverse and longitudinal phonon modes and optical constants to be determined without recourse to oscillator fit models or Kramers-Kronig analysis. Infrared power transmission measurements have allowed all fifteen predicted modes to be identified and the results are compared to recent Raman work.     

A 3-field differential photometer for far infrared anisotropy measurements

We describe a differential photometer developed for ground based observations of the Cosmic Background Radiation anisotropy and of diffuse galactic dust emission at millimetric wavelengths. Preliminary tests, which demonstrate the advantages of the 3 fields modulation with respect to the more conventional single subtraction, are reported. Both atmospheric fluctuations and instrumental offset are significantly reduced.     

The far infrared optical constants of GaP

The FIR optical constants of GaP have been determined on either side of the Reststrahlen band by single pass transmission dispersive Fourier transform spectroscopy. These results extend the range of measured refractive index data for GaP, and reveal the dispersion in the refractive index in regions of weak absorption. Features in the spectrum are assigned as phonon combination bands with the aid of a critical point analysis based on phonon frequencies calculated using an 11-parameter rigid ion model.     

Tunable far infrared generation in hydrogen fluoride

The use of resonantly enhanced stimulated Raman scattering for Q(J) transitions in hydrogen flouride permits a broad coverage of the far infrared spectrum. The tunability was about 5 cm^-1 around several transitions (J = 0 to 5). The output power was measured up to 300 kW corresponding to a photon efficiency of 18%.     

Optical properties of barite in the infrared and far infrared ranges

Far infrared and infrared reflectivity was measured for barite single crystal using polarized light. Twenty six oscillators were observed. The real and imaginary parts of the complex dielectric function were obtained by a fitting procedure for three directions (E a;E b andE c). Group theory analysis has been done in detail and the number and position of experimentally observed infrared active modes were compared with the theoretical predictions.