Terahertz interferometric and synthetic aperture imaging

Field of view for near-field aperture synthesis imaging

Aperture synthesis techniques are increasingly being employed to provide high angular resolution images in situations where the object of interest is in the near field of the interferometric array. Previous work has showed that an aperture synthesis array can be refocused on an object in the near field of an array, provided that the object is smaller than the effective Fresnel zone size corresponding to the array-object range. We show here that, under paraxial conditions, standard interferometric techniques can be used to image objects that are substantially larger than this limit. We also note that interferometric self-calibration and phase-closure image-reconstruction techniques can be used to achieve near-field refocusing without requiring accurate object range information. We use our results to show that the field of view for high-resolution aperture synthesis imaging of geosynchronous satellites from the ground can be considerably larger than the largest satellites in Earth orbit.

Continuous-wave terahertz interferometry with multiwavelength phase unwrapping

Continuous-wave (cw) terahertz (THz) phase imaging can accurately and noninvasively present the depth information of an object's surface and interior. However, a 2π ambiguity limits the measurement of a sample with a thickness larger than the detection wavelength of THz waves. A multiwavelength phase unwrapping method is introduced to the cw THz phase imaging to reconstruct the exact phase map of the object. By using this method, three different types of high-density polyethylene samples were measured, and their phase profiles were well extracted. The result shows that this method is effective in cw THz phase imaging and has the potential to improve the applications of cw THz imaging.

Two-dimensional interferometric and synthetic aperture imaging with a hybrid terahertz/millimeter wave system

We have developed an interferometric synthetic aperture incoherent imaging system at 94 GHz, in which a high-power electronic millimeter wave source (Gunn Oscillator) is integrated with a continuous-wave terahertz (THz) photomixing detection system to achieve a high signal-to-noise ratio. Imaging of a point source located 10?m away from the detector array is presented. Two-dimensional THz reflective images of an extended object with different shapes are reconstructed with only four detectors by use of rotational synthesis.

Passive interferometric array optimization based on redundant spacing calibration

An optimization of passive interferometric circular arrays for redundant spacing calibration (RSC) is advanced to eliminate phase errors of the array system. The principle of RSC is presented to solve corresponded constraints for passive interferometric circular arrays. The simulated annealing algorithm (SAA) is introduced to settle the array optimization with a criterion of maximizing the distance between u-v points. The optimized circular arrays with element numbers of 8 to 16 antennas are laid out, and RSC is used for the optimized ten-element passive interferometric circular array.

Video-rate terahertz interferometric and synthetic aperture imaging

Experimental results from a video-rate terahertz interferometric imaging system are presented. The source emits continuous narrow bandwidth radiation at 0.1 THz. The 2D image of a point source is reconstructed at a rate of 16 ms per frame with a four-element detector array. The image resolution and quality are affected by the number of detectors, the configuration of the detection array, and how well the baselines are calibrated. Details of the hardware system and video-rate terahertz image are presented.

Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.

Terahertz imaging with compressed sensing and phase retrieval

We describe a novel, high-speed pulsed terahertz (THz) Fourier imaging system based on compressed sensing (CS), a new signal processing theory, which allows image reconstruction with fewer samples than traditionally required. Using CS, we successfully reconstruct a 64 x 64 image of an object with pixel size 1.4 mm using a randomly chosen subset of the 4096 pixels, which defines the image in the Fourier plane, and observe improved reconstruction quality when we apply phase correction. For our chosen image, only about 12% of the pixels are required for reassembling the image. In combination with phase retrieval, our system has the capability to reconstruct images with only a small subset of Fourier amplitude measurements and thus has potential application in THz imaging with cw sources.