Continuous-wave coherent imaging with terahertz quantum cascade lasers using electro-optic harmonic sampling

Abstract

International audienceWe demonstrate a coherent imaging system based on a terahertz (THz) frequency quantum cascade laser (QCL) phase-locked to a near-infrared fs-laser comb. The phase locking enables coherent electro-optic sampling of the continuous-wave radiation emitted by the QCL through the generation of a heterodyne beat-note signal. We use this beat-note signal to demonstrate raster scan coherent imaging using a QCL emitting at 2.5 THz. At this frequency the detection noise floor of our system is of 3 pW/Hz and the long-term phase stability is <3°/h, limited by the mechanical stability of the apparatus

Terahertz imaging using photomixers based on quantum well photodetectors

“…To get stronger and more stabilized IF signal, the THz QCL source should be phase locked to a highly stabilized local oscillator. 33,34 Another issue caused by the unstable THz modes is the temporal power fluctuation. From the "Max-Hold" measurement, we can roughly derive the temporal power fluctuation of the IF signal caused by the longitudinal mode competition.…”

Optical feedback effects on terahertz quantum cascade lasers: modelling and applications

“…[1][2][3][4][5][6][7][8] This makes THz QCLs particularly suited to the development of interferometric THz sensing and imaging systems. [9][10][11] Due to the inherently low penetration depth of THz radiation in hydrated biological tissue, biological and particularly superficial skin imaging are applications to which THz frequency imaging is well suited. [17][18][19][20][21][22][23][24][25][26][27][28][29][30] As such, imaging of superficial skin employing THz QCLs in confocal reflection-mode system geometry is an ideal application for the technique.…”

Three-dimensional terahertz imaging using swept-frequency feedback interferometry with a quantum cascade laser

“…Whilst THz QCLs are particularly suited to coherent sensing approaches due to their continuous-wave narrowband emission with quantum noise-limited linewidths [6], the majority of QCL-based imaging systems reported to date have employed incoherent detection. Nevertheless, coherent detection approaches offer the potential for high dynamic range [7] and detection close to the shot-noise limit [8]. Furthermore, the ability to resolve both the amplitude and phase of the THz field has enabled depth-resolved [three-dimensional (3D)] imaging [8,9] and the spatial mapping of the complex permittivity of targets [10], as well as inverse synthetic aperture radar imaging [11,12].…”

“…Nevertheless, coherent detection approaches offer the potential for high dynamic range [7] and detection close to the shot-noise limit [8]. Furthermore, the ability to resolve both the amplitude and phase of the THz field has enabled depth-resolved [three-dimensional (3D)] imaging [8,9] and the spatial mapping of the complex permittivity of targets [10], as well as inverse synthetic aperture radar imaging [11,12]. Sensing techniques in such systems have used heterodyne mixing between the QCL and a gas laser through a Schottky detector [12], and electro-optic harmonic sampling of the THz field using a near-infrared femtosecond laser comb [8].…”

“…Furthermore, the ability to resolve both the amplitude and phase of the THz field has enabled depth-resolved [three-dimensional (3D)] imaging [8,9] and the spatial mapping of the complex permittivity of targets [10], as well as inverse synthetic aperture radar imaging [11,12]. Sensing techniques in such systems have used heterodyne mixing between the QCL and a gas laser through a Schottky detector [12], and electro-optic harmonic sampling of the THz field using a near-infrared femtosecond laser comb [8]. Both of these methods, however, require electronic stabilization of the QCL to an optically-derived reference frequency, resulting in complex experimental arrangements.…”