Fabian Friederich

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

Abstract. The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

Phase-locking of the beat signal of two distributed-feedback diode lasers to oscillators working in the MHz to THz range

We present difference-frequency stabilization of free-running distributed-feedback (DFB) diode lasers, maintaining a stable phase-lock to a local oscillator (LO) signal. The technique has been applied to coherent hybrid THz imaging which employs a high-power electronic radiation source emitting at 0.62 THz and electro-optic detectors. The THz radiation of the narrow-band emitter is mixed with the difference frequency of the DFB diode laser pair. The resulting intermediate frequency is phase-locked to the LO signal from a radio-frequency generator using a fast laser-current control loop. The stabilization scheme can be adapted readily to a wide range of applications which require stabilized laser beat-notes.

High signal-to-noise-ratio electro-optical terahertz imaging system based on an optical demodulating detector array

We present a 64x48 pixel 2D electro-optical terahertz (THz) imaging system using a photonic mixing device time-of-flight camera as an optical demodulating detector array. The combination of electro-optic detection with a time-of-flight camera increases sensitivity drastically, enabling the use of a nonamplified laser source for high-resolution real-time THz electro-optic imaging.

Hybrid Continuous-Wave Demodulating Multipixel Terahertz Imaging Systems

We present an electrooptic (EO) terahertz imaging technique providing a demodulating detector array for phase-sensitive multipixel terahertz detection. The terahertz radiation from a quartz-stabilized microelectronic emitter is mixed with the synchronized laser beat signal of a continuous-wave distributed-feedback diode laser pair. A fast laser current control loop provides stable phase locking between the terahertz emitter and the laser difference frequency, whereby a demodulating near-infrared photonic-mixer-device camera is used for depth-resolving EO terahertz imaging. Alternatively, a femtosecond laser is used for the EO read-out.

Conception and realization of a semiconductor based 240 GHz full 3D MIMO imaging system

Multiple-input multiple-output (MIMO) imaging systems in the terahertz frequency range have a high potential in the field of non-destructive testing (NDT). With such systems it is possible to detect defects in composite materials, for example cracks or delaminations in fiber composites. To investigate mass-produced products it is necessary to study the objects in close to real-time on a conveyor without affecting the production cycle time. In this work we present the conception and realization of a 3D MIMO imaging system for in-line investigation of composite materials and structures. To achieve a lateral resolution of 1 mm, in order to detect such small defects in composite materials with a moderate number of elements, precise sensor design is crucial. In our approach we use the effective aperture concept. The designed sparse array consists of 32 transmitters and 30 receivers based on planar semiconductor components. High range resolution is achieved by an operating frequency between 220 GHz and 260 GHz in a stepped frequency continuous wave (SFCW) setup. A matched filter approach is used to simulate the reconstructed 3D image through the array. This allows the evaluation of the designed array geometry in regard of resolution and side lobe level. In contrast to earlier demonstrations, in which synthetic reconstruction is only performed in a 2D plane, an optics-free full 3D recon- struction has been implemented in our concept. Based on this simulation we designed an array geometry that enables to resolve objects with a resolution smaller than 1mm and moderate side lobe level.

Continuous wave terahertz inspection of glass fiber reinforced plastics with semi-automatic 3-D image processing for enhanced defect detection

The increasing industrial application of fiber reinforced plastics demands developments of new techniques for non-destructive testing. The sub-terahertz frequency band has proved to be a noteworthy option for this task. Composite structures or laminates can be inspected for foreign inclusions, delaminations, debonds, etc., using sub-terahertz sensors during the manufacturing process or maintenance. In this contribution we present our results using a frequency modulated continuous wave terahertz imaging system in comparison with conventional NDT measurements on several different GFRP samples. Thereby a semi-automatic terahertz image processing approach for enhanced defect detection is applied.

Application of the phase coherence method for imaging with sparse multistatic line arrays

Sparse multistatic array concepts can offer cost-effective millimeter-wave and terahertz imaging solutions, while greatly reducing the system complexity. Despite the high design flexibility of sparse arrays the imaging quality could strongly be limited by unavoidable violations of the Nyquist-Shannon sampling criterion, which lead to significant grating lobes artifacts. Within this contribution we investigate the application of the phase coherence method to enhance the imaging quality. Simulations show a significant increase of the integrated side lobe ratio along the arrays axis, which holds for undersampled effective apertures. Experimental results, which have been achieved with a factor 4 undersampled sparse line array operating in the W-band, demonstrate the potential of this approach not only in reducing grating lobes but also side lobes and clutter.

A transfer matrix modification for accurate terahertz FMCW thickness measurements

Millimeter and terahertz waves offer novel solutions for nondestructive testing such as imaging and layer thickness measurements within dielectrics. For the thickness measurement of plastics, we use the frequency modulation continuous wave technique within fully electronic terahertz systems. The available modulation bandwidth inherently restricts the resolution to several millimeters. Our correlation approach, which compares the acquired measurement data with a signal model, overcomes this limit for predefined measurement conditions. However, to obtain high measurement accuracies — especially in the case of compact multilayer structures and dielectric coatings on conductive substrates — beam propagation aspects such as multiple reflections between the boundary surfaces of t he l ay ers h ave to be considered. Therefore, we adapt the Transfer Matrix method to our measurement scheme with optimizations with regard to computation complexity.

Coherent terahertz imaging with synchronized distributed-feedback diode lasers

We present a heterodyne hybrid terahertz imaging system, which combines electronic narrow-band emitters, operating at 0.2 THz and 0.62 THz respectiveley, with a continuous-wave two-color laser system for electro-optic detection. The laser system employs two distributed-feedback laser diodes, providing a tunable difference frequency which is phase-locked to the emitted terahertz frequency with an offset of 10 MHz.