Tunable infrared generation in KTA and applications

Noncollinear difference frequency mixing of dye laser and Nd:YAG second harmonic (fundamental) radiation from a commercial laser system is employed for the generation of 2.7–5.3 μm (1.6–1.7 μm) radiations in a flux-grown KTiOAsO₁ crystal. The generated radiation is used to scan the methane absorption in the fundamental (v ₃) and its first overtone (2v ₃) band at pressure 90 torr in a laboratory made single pass gas cell of length 33 cm.     

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.     

Tunable infrared difference-frequency generation with a high-repetition pulsed dye laser

An output from a cresyl-violet dye laser, pumped by the second harmonic of a Q-switched Nd:YAG laser, is mixed with the pumping light in a LiIO₃ crystal to produce an infrared output tuned from 2.8 to 3.4 μm with a repetition rate of 20 pps.     

Tunable infrared generation using third Stokes output from a waveguide Raman shifter

Tunable infrared output has been produced by stimulated Raman scattering of visible dye laser radiation in hydrogen gas using a capillary waveguide to enhance conversion to the third Stokes (S₃). With 568 nm pumping an output energy of 2 mJ is observed near 2 μ m, representing a 4% energy conversion efficiency into S₃). Higher wavelengths yield much lower energies (0.5 mJ at 3 μ m and 0.02 mJ at 4.55 μ m), reflecting the observed dependence of third Stokes output on the inverse of the fifth power of the generated wavelength.     

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.     

Tunable infrared generation using the 6s−5d Raman transition in caesium vapour

Coherent infrared radiation tunable between 1.6 and 3.2 μm was generated with a pulsed dye laser by 6s−5d _5/2 stimulated electronic Raman scattering in caesium vapour. We describe a technique using thermal dissociation to reduce the density of caesium dimers in a heat-pipe oven, which produced a factor of 7 reduction in Raman threshold. The threshold displayed a marked dependence on pump bandwidth.     

Tunable laser-sideband generation in InSb

The resonant third order non-linear susceptability in InSb has been employed to generate a magnetically tunable cw laser sideband near 5 μm.     

Tunable infrared down-conversion in silver thiogallate

A ruby laser beam mixed in AgGaS₂ with the output of various ruby-pumped dye lasers has provided down-converted infrared radiation tuned from 4.6 to 12 μm. The measured phase-matching angles agree well with calculated values. Peak infrared powers of a few hundred milliwatts have been produced.     

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.     

Tunable gamma ray generation

In this note, we wish to point out that utilizing presently available technology it is possible to construct an intense, modulated and polarized γ-ray source which can be used for spectroscopy purposes to good advantage.     

Tunable far-infrared Raman generation

Resonance enhanced stimulated Raman scattering of a tunable laser was observed in the 60–160 μm range for the Q(J) transitions in hydrogen chloride. The resultant far-infrared source was tunable over about 2.3 cm⁻¹ around each transition studied (J = 2 to 7) and gave a peak power around 80 kW corresponding to a photon efficiency of the order of 12%.     

Tunable far-infrared Raman generation

Tunable resonance enhanced far-infrared stimulated Raman scattering was observed in 60–160 μm range for the Q(J) transitions in hydrogen chloride. The tuning range was about 2.3 cm^-1 around each studied transition (J = 2 to 7) and the photon efficiency was measured to be of the order of 12% giving peak power around 80 kW.     

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.     

Phonon-polariton meta-atoms for far infrared range

Polarizabilities of spherical subwavelength silica glass particles are investigated having regard to material dispersion. It is shown that the ranges of the negative electric and magnetic polarizabilities almost coincide. This allows the particles in question to be used for producing media with a negative refractive index.     

Tunable upconversion in Er-doped orthorhombic lead fluoride compound

Near infrared-visible upconversion in Er³⁺ doped orthorhombic PbF₂ compound is investigated. It is experimentally observed that the red emission intensity increases monotonously with Er³⁺ concentration increase, while the green emission intensity first increases and then decreases. Based on the rate-equation, the energy transfers involved in the upconversion processes have been explored. It is shown that due to the different multipolar nature for the energy transfer processes of ²H_11/2 (⁴S_3/2)+⁴I_15/2→⁴I_9/2+⁴I_13/2 and ⁴I_11/2+⁴I_11/2→⁴F_7/2+⁴I_15/2, the green and red upconversion emissions depend on Er³⁺ concentration in different ways. The theoretical results are in good agreement with the experimental observation. It is shown that the upconverted emission bands can be tuned by controlling Er³⁺ concentration in orthorhombic PbF₂ compound, which has many photonic applications under NIR excitation.     

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.     

Classical treatment of the dissociation of hydrogen fluoride with one and two infrared lasers

Classical mechanics is used to show that hydrogen fluoride may be more easily dissociated with two infrared lasers of different frequencies than with just one. It is shown that the two laser dissociation proceeds by a qualitatively different process than the one laser dissociation.