RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. The development of the terahertz (THz) frequency range reflects recent progress in radiation sources, in particular, THz (submillimeter) lasers. Heterodyne THz devices require dual-frequency lasers capable of emitting two different THz frequencies simultaneously. This work aims to study the process of radiation generation in THz lasers, spurred by a search for dual-frequency laser sources that can produce two frequencies simultaneously and provide options for separate outputs.

Methods and Methodology. The dual-frequency generation is investigated using a three-mirror laser with a resonator formed by a metal mirror on one side of the active element and two output mirrors on the other. The output mirrors are metal gratings arranged so that the conducting bars of one grating are perpendicular to those of the other. Together with the metal mirror, each grating forms a resonator for linearly polarized radiation with the electric field vector directed parallel to the grating conductors. So, the active volume of the laser contains two separate resonators that generate orthogonally polarized radiation beams. Their frequency difference is adjusted within the emission band of the active substance by changing the distance between the gratings. The frequency control employs a custom micrometric mechanism enhanced by the authors. It fine-tunes the mirror position and acts as a wavemeter. The frequency difference is measured using radio frequency devices.

Results. The dual-frequency generation was examined using a gas-discharge 337-μm laser. The mode composition of the laser output was identified, evidencing that the investigated laser can simultaneously generate orthogonally polarized radiation of two frequencies. The simultaneous generation of orthogonally polarized laser beams has been demonstrated, with the frequency difference ranging from 60 kHz to 4.5 MHz. A method has been developed to regulate and measure the frequency difference without resorting to electronic frequency-measuring equipment. Notably, when using modes with the same transverse energy distribution, dual-frequency generation does not appear. Instead, given a specific optical length of the resonator, an instant change in the radiation frequency and polarization occurs, providing opportunities to modulate the THz laser radia- tion frequency and polarization. The possibility of pulse-frequency and pulse-polarization modulations of THz laser radiation has been demonstrated.

Conclusions. The use of dual-frequency THz lasers will enhance various heterodyne systems in THz technologies. The developed polarization and frequency modulation methods will contribute to the THz telecommunication systems.

Keywords: terahertz range; emission band; dual-frequency laser; polarization modulation

Manuscript submitted 29.04.2025

Radio phys. radio astron. 2025, 30(3): 202-210

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