Signal Capture and Processing for Multichannel THz Tomography with Room Temperature Detectors

John S. Young, Paul Wright, Krikor B. Ozanyan

School of Electrical and Electronic Engineering, The University of Manchester, Manchester M13 9PL, United Kingdom

ABSTRACT

Because of the complex technology employed, THz Tomography has been limited to single channel operation, with imaging experiments requiring hours of data collection, suitable for static subjects only. We are currently developing fast (millisecond) THz Absorption Tomography, allowing the imaging of dynamic objects (e.g. flames obscured by soot particles), employing narrow­line tunable THz sources pumped through optical fibres. This requires the deployment of a substantial number of room temperature, compact and portable THz detectors for simultaneous multichannel measurements. Based on performance, availability, size and cost, the technology of choice is pyroelectric detectors (PED).

We report here on the development and performance of an integrated THz receiver module based on an 8 element pyroelectric linear array, with 4 such receivers providing readings of a total of 32 line integrals grouped in 4 projection angles.

The main limitations in the performance of a single PED are studied with the aim to reconcile the short­pulse character of the THz emitter source and low frequency ~ 10 Hz and very low bandwidth ~ 10-2 – 10-­1 Hz of the PED, required for a reasonable signal­to­noise ratio. The noise floor of the detector is analysed and estimated to be approximately 1 nW, based on a 100GΩ feedback in the front­end amplifier. The relatively low modulation frequency facilitates the use of standard microcontrollers; the design is based on 4 ultra high speed Dallas 89C450 family controllers each serving 8 PEDs via an LTC1867 16 bit sequentially multiplexed Analog-to-­Digital Converter. All data is gathered using serial access via a master processor which routes the data stream to either a standard (e.g. Matlab environment) or customized reconstruction algorithm. The above design has been demonstrated and the 1 nW sensitivity proven in a single channel version, which is currently being scaled up for the full system. The component and materials cost of the complete 8 element THz receiver module is below £3,000 and it measures less than 200x<Φ60mm beyond the coupling flange.

We apply signal processing algorithms based on the lock­in principle, whereby a periodic modulation is applied to the fibre sources and the signal is extracted using standard techniques identical for each channel. Furthermore, a contingency is in place to reference the outputs ratiometrically to a single source detector managed in the same way; this removes the effects of drift which is always potentially a problem with very low bandwidth measurement.

PEDs, because of their thermal nature, have flat spectral sensitivity down to the visible region; therefore the developed signal capture and processing, as well as the complete receiver module for the THz case, is directly applicable as an alternative for broadband MidIR and NearIR systems, where better sensitivity is achievable by other room­temperature technology only in a limited spectral range.

Keywords THz Tomography, pyroelectric array, multichannel measurements