Finn Eichhorn

Terahertz Imaging Systems With Aperture Synthesis Techniques

This paper presents the research and development of two terahertz imaging systems based on photonic and electronic principles, respectively. As part of this study, a survey of ongoing research in the field of terahertz imaging is provided focusing on security applications. Existing terahertz imaging systems are reviewed in terms of the employed architecture and data processing strategies. Active multichannel measurement method is found to be promising for real-time applications among the various terahertz imaging techniques and is chosen as a basis for the imaging instruments presented in this paper. An active system operation allows for a wide dynamic range, which is important for image quality. The described instruments employ a multichannel high-sensitivity heterodyne architecture and aperture filling techniques, with close to real-time image acquisition time. In the case of the photonic imaging system, mechanical scanning is completely obsolete. We show 2-D images of simulated 3-D image data for both systems. The reconstruction algorithms are suitable for 3-D real-time operation, only limited by mechanical scanning.

Optical fiber link for transmission of 1-nJ femtosecond laser pulses at 1550 nm

We report on numerical and experimental characterization of the performance of a fiber link optimized for the delivery of sub-100-fs laser pulses at 1550 nm over several meters of fiber. We investigate the power handling capacity of the link, and demonstrate all-fiber delivery of 1-nJ pulses over a distance of 5.3 m. The fiber link consists of dispersion-compensating fiber (DCF) and standard single-mode fiber. The optical pulses at different positions in the fiber link are measured using frequency-resolved optical gating (FROG). The results are compared with numerical simulations of the pulse propagation based on the generalized nonlinear Schrödinger equation. The high input power capacity of the fiber link allows the splitting and distribution of femtosecond pulses to an array of fibers with applications in multi-channel fiber-coupled terahertz time-domain spectroscopy and imaging systems. We demonstrate THz pulse generation and detection using a distributed fiber link with 32 channels and 2.6 nJ input pulse energy.

Terahertz time-domain spectroscopy of crystalline and aqueous systems

We use ab-initio density-functional perturbation theory together with THz spectroscopy for precise prediction and assignment of vibrational modes in molecular crystals. We show that THz spectroscopy is useful for analysis of liquids and food products.

A mul ti-element THz imaging system

We report on a broadband multi-element THz imaging system based on fiber-coupled, integrated photoconductive emitters and detectors. 32 detectors and 32 emitters are arranged in a planar array. Advanced image reconstruction algorithms are employed to reconstruct an object in the imaging plane.

Efficient coupling of THz radiation to microdisk resonators

We model the coupling of THz radiation between a microdisk (MD) resonator and free-space. This is relevant for a new class of THz emitters based on difference-frequency mixing between two laser modes in an MD resonator. We show efficient coupling between the MD TM mode and the free-space TE mode.

Dual-Color, Power-Scalable DFB Fibre Laser System for kHz-Linewidth Terahertz Generation

We present a dual-wavelength, power-scalable and continuously tunable DFB fibre laser system for continuous wave, narrow-band THz generation. The linewidth of the laser is better than 25 kHz, sufficient for gas sensing applications.

Design of a terahertz CW photomixer based on PIN and superlattice PIN devices

We present the design of a photomixer LO based on standard and superlattice PIN diodes, operating at 1 THz. The design is based on a direct integration of a double slot antenna with the PIN device and a suitable matching circuit. The antenna has been designed together with a dielectric lens using Ansoft HFSS EM simulation. The large-signal PIN diode model employed in the work has been improved compared to our previously developed model presented earlier in a 3 THz design. We demonstrate that the antenna characteristic changes drastically with the device in place.