Methods of spectrally pure two-frequency radiation forming for terahertz carriers generation in optical range

Abstract

One of the promising methods for generating continuous terahertz radiation is the method of photomixing, when the photoconductive material is excited by the emission of two optical continuous lasers with frequencies separated by terahertz frequencies. It should be noted that for effective photomixing the polarization, frequencies and phases of the optical beams introduced into the mixer must be constant, otherwise it is necessary to additionally use a complex system of phase synchronization. In connection with the complexity of such an implementation, variants based on the conversion of single-frequency and broadband laser radiation into two-frequency ones were investigated. The first of these is modulation and consists in the external modulation of single-frequency laser radiation in the electro-optical Mach-Zehnder modulator. The second one is broadband with the allocation of two wavelengths using two rectangular fiber Bragg gratings with a small difference in width. The studies of the first variant showed some boundedness of the obtained difference frequencies at the bottom of the terahertz range. In the claimed embodiment, it is difficult to use methods for obtaining two-frequency radiation with a predetermined difference frequency based on external electro-optical modulation of single-frequency coherent radiation. Since even on InP-modulators it is possible to obtain a difference frequency only in the region of 0.2-0.4 THz. Development of this option can be obtained by using tandem or multiplicative inclusion of several modulators. In the second embodiment, as a pumping source with a dual frequency radiation with a predetermined difference frequency lying in the region of 1.0 THz, a fiber laser with an output driver based on two rectangular fiber Bragg gratings with a small difference in width was investigated. The required ratios of the frequencies and amplitudes of the two-frequency components of the radiation were provided by the technology of recording gratings on an ultraviolet laser, the polarization ratios were determined by the characteristics of the selected laser, the ellipticity, and the quality of the optical fiber. The main factor in the instability of the pump system was determined by the temperature stability in the laser housing, where the two-frequency shaper was also positioned. The increase in the difference frequencies was limited by the group phase delay of the gratings, for the compensation of which the consecutive inclusion of the gratings was used. The studies were carried out to create a terahertz photoconductor, the distinguishing feature of which is the use of heterostructures grown on a GaAs substrate in the low-temperature regime as a photoconductive material in the form of a thick gradient layer In(x)Al(1-x)As (x = 0 → 1) with artificially created local deformations and with photoconductive composite InGaAs/InAs/InGaAs quantum wells.