Researchers from the University of Wuppertal in Germany have created a THz signal source array for computational imaging, which allows a single-pixel camera – one THz receiver – to form images. This type of computational imaging involves bathing the object to be photographed in varying light fields and measuring the amplitude of light that passes by.

Knowing the various patterns of light used to create the light fields, and the amplitudes they produced, with a lot of computing power it is possible to calculate an image of the object.

“Computational THz imaging can overcome existing signal-to-noise ratio limitations in THz imaging systems as it relaxes the implementation issues of high-power sources and low-noise focal plane arrays,” according to the university’s ISSCC 2020 presentation. “However, measurements must be taken in series rather than all at once, which calls for fast THz system-on-a-chip implementations in silicon.”

They have created a 3x3mm array of 8×8 pixels in 0.13μm SiGe BiCMOS with 350GHz fT, 450GHz fmax heterojunction bipolar transistors, where each pixel includes a free-running balanced fundamental Colpitts oscillator with a tank-isolating cascode stage driving a common-collector doubler and its own circular slot antenna. Between antenna and doubler is an impedance matching network to maximise second-harmonic output power to the capacitive antenna impedance. Radiation is up to 9.2dBm at 0.42THz.

Each pixel can be programmed for a unique chopping frequency (up to 8.33MHz) with a 100% modulation depth. The chopping frequencies show up in the received spectrum and are used in later analysis.

Alongside the array on the same die is built-in self-test, power calibration and pattern repetition logic (4kbit of 64-bit patterns), much of this in 1.2V CMOS. The system’s lenses (diagram above) are 15mm diameter ‘hyper-hemispherical’ types, made from silicon.

The oscillator array sinks up to 1.4A from the supply, requiring a low‑resistance (0.3Ω/mm) thick top metal layer for distribution, and power in the array is up to 6.9W so a copper heatsink is attached. Maximum output from a single pixel is 435μW (-3.6dBm), although not all pixels can be operated at this level simultaneously as output drops once the chip gets to 60°C

A single SiGe heterojunction bipolar transistor‑based THz power detector (22dBi directivity, 8pW/√Hz NEP, 700V/W responsivity at 0.43THz) was used as a receiver in the test rig, viewing a metal stencil hidden behind cardboard.

All pixels were chopped at 1MHz while successive patterns were fed in, and the output read though a spectrum analyser.

The reconstructed image shows a 50dB dynamic range including 9dB loss from focusing optics. Acquisition time (15s) was primarily limited by the spectrum analyser read-out routine (>200ms/pattern).

Pushing switching to 2MHz resulted in a maximum image rate of 31,000frame/s for 64 pattern imaging.

Presented as Paper 29.1, A 0.42THz 9.2dBm 64-pixel source-array SoC with spatial modulation diversity for computational terahertz imaging.