Gunnar Spickermann

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.

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.

New approach for an electrooptic THz-detector array using photonic mixing device camera

We present a new 2D electrooptical THz detector using a photonic mixing device (PMD) camera. The combination of PMD-cameras with electrooptic THz sensing increases sensitivity drastically, enabling the use of non amplified fs laser sources for high resolution real-time THz electrooptic imaging.

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.

Multi-pixel continuous-wave THz-imaging by electro-optic sampling using a photonic-mixer-device camera

Multi-pixel terahertz (THz) detection based on the electro-optic principle is highly interesting for fast THz imaging. In the past, a CCD camera was used to measure the THz-induced-birefringence in an electro-optic crystal [1]. Due to the relatively weak birefringence, small modulation signals have to be detected on a large optical background. High THz field amplitudes are required to achieve an acceptable signal-to-noise ratio (SNR).

Development of a hybrid THz camera using synchronized two-color laser radiation

We demonstrate a hybrid system for THz raster scan imaging using frequency-stabilized DFB lasers for electro-optic detection. The system combines a 0.65-THz micro-electronic narrow-band emitter with two synchronized diode lasers, providing two-color laser radiation. The functional principle promises three-dimensional real-time imaging capabilities.