Abstract
A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.
Highlights
The initial demonstration of quantum cascade laser (QCL) operation at THz frequencies in 2002 has been followed by a number of publications on THz imaging applications using QCL sources
The signal phase stability for a 40-min period, collected by measuring the phase of the signal returned from a stationary dihedron positioned at the center of the transmitter beam at the target location, does not exceed ± 7 degrees and is presented in Fig. 3, which is small enough to create distortion free images because it is much smaller than the phase change generated by a scatterer within the unambiguous cross range (UCR), scanned over a few degrees in azimuth or elevation
The QCL was frequency-locked to the local oscillator (LO) to achieve the required phase stability for a coherent transceiver
Summary
The initial demonstration of QCL operation at THz frequencies in 2002 has been followed by a number of publications on THz imaging applications using QCL sources. At lower frequencies (160 GHz to approximately 600 GHz) harmonics of amplified and multiplied microwave signals have been successfully used [10, 11], and at higher frequencies (above 1.0 THz) CO2 optically pumped molecular gas lasers (OPL) have been employed [9] as transmitter and local oscillator (LO) sources in THz radar ranges These sources have sufficient coherency and frequency tunability to allow 3D imaging using ISAR techniques. The CW frequency-locked THz QCL (the transmitter) radiation with frequency approximately 1.048 GHz below the frequency of the gas laser was split with a 14 μm thick Mylar BS1 and a portion illuminated the Ref. SD while the rest was directed towards the target, which was fully illuminated by the beam with a spherical phase front (near-field configuration measurements). Data Receiver Reference QCL frequency Acquisition section section stabilization section section Ref. signal 35 MHz Ref. signal 62 kHz
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