Optically pumped far infrared lasers

Abstract

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.