A Practical and Portable Solids-State Electronic Terahertz Imaging System

An image of the measured transmission or reflection properties of the sample is generated by raster scanning the sample through the fixed focused point as illustrated in Figure 3. The THz signal generated by the AMC is collimated and focused onto the sample and then the transmitted or reflected signal is collected and focused on the detector by another pair of lenses. The sample is mounted and scanned in the X and Y planes with two moving linear translation stages. A lock-in amplifier synchronized with an optical chopper (set at 1 kHz speed) is used to acquire the detector voltage responses, which are processed by a computer to produce an image using an in-house developed LabVIEW (http://wwwni.com/labview) program.

The resolution of the system is determined by the spot size of the beam on the sample. The collimation and focusing lenses help in maximizing the beam strength from the source and minimizing the spot size on the sample. The theoretical spatial resolution can be estimated using the Rayleigh criterion ∆x = 1.22 λf/D assuming the perfect optical alignment, where the wavelength λ = 3 × 108 ms−1/614 GHz = 488 μm, f and D are the focal length and diameter of the focusing lenses. A THz signal of 614 GHz was used in the experimental results. The estimated theoretical spatial resolution is 1.19 mm. In the experiments, compromise must be made between the resolution and total scanning time. For the images shown in this paper, a typical resolution of 0.5 mm was used for a scanning area of 50 mm × 50 mm which gave an image size of 100 × 100 or 10,000 pixels. It took about 10 min to obtain the images.

We examined the experimental resolution of our imaging system by plotting out 1D line scans of the image data corresponding to the image shown in Figure 4. Figure 4 is a transmission THz image of a computer floppy disk (Figure 4a) showing sharp metal edges obtained with above mentioned scanning conditions and a photograph (Figure 4b) showing the floppy disk; flipped to correspond to scanned image as the data is received on the rear of the sample. The signals blocked by the metallic parts are shown as blue colour in image (Figure 4) and correspond to zero signal level in single line scans (Figure 5). The maximum transmitted signal is shown in red colour in the image. Note that the blue section on right side was caused when the signal was blocked by a metal sample holder.

The black lines indicate the single line scans at vertical line 17 and horizontal line 23 shown in Figure 5. The imaging system scans in the vertical Y plane and steps in the horizontal X plane to build an image. A slight blurring at the metal edges is visible, which is caused by the scattering effect of the metal edge. This effect is minimised in our system due to the focusing effect of the convex lens; only one point of ~1 mm (theoretic prediction is 1.19 mm) in diameter is exposed to the beam and measured by the device. Therefore although the metal edges still scatter the beam this only causes slight signal dilution (green colour regions adjacent to the blue colour edges). From the image in Figure 4, the visible blurred regions (green colour) along the metal edges (vertical or horizontal edges) are approximately 2 pixels, i.e., around 1 mm. The single line scans in Figure 5 correspond to the vertical line 17 across the rectangular shape metallic frame on the left side of the image and the horizontal line 23 across the small hole in the circular centre of the metallic part shown in Figure 4. The THz beam signal is completely blocked by the metallic parts resulting in zero signal level (blue in the image) and reaches the maximum signal level (red in the image) in the plastic parts of the disk which are more transparent to the THz beam. The marked widths in the line scans shown in Figure 5 show good agreement with the dimension-marked parts in Figure 4 photograph (red arrows). It can be seen that 10%–90% rise in the transmitted signal level occurs between 1.0 and 1.5 mm transition at each sharp metal edge, consistent with the visual colour change region (green) in the image (Figure 4). This provides a measure for the spatial resolution of our imaging system. The experimental results showed that our system has diffraction-limited resolution close to the theoretical spatial resolution (~1.19 mm).

Reflection mode is suitable for many practical applications, such as security screening and non-destructive testing in industrial environments [10]. Reflection mode is particularly useful where the sample is bulky and cannot be placed at transmission focal point or where the THz radiation cannot penetrate or is largely attenuated through the sample to obtain enough signal level at the detector for quality imaging. Reflection mode is also suitable for imaging of highly reflective surfaces or for imaging the surface features or sub-surface interface of a sample; for example the detection of rust and corrosion under paint [12] and non-destructive testing of valuable historical or cultural artefacts [20].

Figure 6 shows imaging of a 50 cent coin and a kangaroo key ring in the reflection mode where a beam splitter was used to deflect the THz beam reflected from the sample surface to the detector perpendicular to the axis of the lens and original THz beam. The image obtained with the reflected THz beam is shown in the inset. It clearly shows the surface topography features of the coin and the key ring.

THz imaging has been explored for security applications such as detection of concealed weapon or dangerous objects due to its ability to penetrate through clothes and packaging materials [2,3]. Figure 7 shows another example measurement of a hidden object in a shoe using the reflection mode. The main photograph shows the setup with the shoe mounted on the XY scanner. The top inset shows a visual image of the hidden object, a scalpel inside the shoe lining material, and the lower inset shows the terahertz image of the hidden object. In the terahertz image blue corresponds to weak reflection of the THz waves and red shows strong reflection. The hidden object is clearly visible in the image demonstrating the ability of THz wave to penetrate common non-metallic materials such as the canvas cloth lining of the shoe. The result indicates the suitability of our THz imager for security applications, such as a shoe scanner in airports, for example.

Imaging in transmission mode can be used for applications such as non-destructive testing of or inspection through opaque materials, including determining the fibre density or consistency of papers and cardboard, the detection of contaminants or inclusions in plastics and polymers. Transmission mode can also be used to quantify the water content taking advantage of terahertz frequencies’ sensitivity to water and other polar liquids. It offers a non-contact method of monitoring water content and distribution in both living things such as plants and non-living things such as wood polymers or paper [21].

Figure 8 shows an example measurement taken in transmission mode of a fresh leaf imaged at 614 GHz; the image clearly demonstrates the key feature of the THz radiation, i.e. sensitivity to water. The THz image shows how the attenuation of the signal corresponds to water content and distribution particularly in the vein structure of the leaf, both large and fine vein structure can be observed. Blue colour represents the largest absorption of the THz signal and thus the highest water content. This presents a non-invasive way of monitoring water content and distribution within biological structures such as leaves. As the measurement is non-contact it can be used on live plants and repeated in situations where observing changes in water content over time are required. This example has further implication for THz imaging being applied to medical imaging examinations, with potential applications in the areas of detection of skin and breast cancers or monitoring the hydration of corneal grafts [5,22].

The above results were obtained using an optical chopper to modulate the THz source signal at the rate of 1 kHz. The ACM solid-state THz source has a logic input that allows the THz beam to be TTL modulated at up to 5 kHz. This function allows further simplification of the THz system by replacing the optical chopper with a small, low cost, logic signal source. An experiment was carried out to compare the system performance using the TTL electronic signal modulating the THz beam to that using the optical chopper modulation. Figure 9 compares the real-time detector voltage outputs (note that a different detector was used in this measurement) acquired using both modulation methods at the same rate of 1 kHz. The optical chopper modulated THz beam response shows some distortion due to the non-instantaneous nature of the optical chopping of the beam. In comparison, the TTL electronic signal modulated THz beam results in a near ideal response due to the fact that the electronic chopper can switch the RF power more quickly. The computer disk shown in Figure 4 was also imaged using the TTL electronic modulation at the same speed, 1 kHz, as that of the optical chopper, and the result is shown in Figure 10. There are no observable differences in the visual quality of these two images (Figure 4 and Figure 10), leading us to the conclusion that it is possible to replace the function of the optical chopper with TTL logic signal without compromising the system performance. Implementation of electronic modulation provides us an opportunity of developing a software-based “virtual lock-in” amplifier instead of the stand-alone analogue lock-in amplifier, which will further simplify the system and reduce the overall system cost. The development of a virtual lock-in amplifier and new signal processing method is underway.