A Terahertz Imaging System with Rotation Mirror

In this article, a THz imaging system with rotation mirror is built. The system considers both imaging speed and cost of hardware. The transmission-mode design realizes miniaturization of the system. With the system, we are able to acquire image size of 60 × 80 mm 2 in 60 seconds, while a raster-scan THz imaging system with the same hardware conditions needs more than 30 mi-nutes. Moreover, internal information of object could be got with the THz imaging system reported in this article.


Introduction
Terahertz (THz) radiation is a kind of electromagnetic radiation located in a specific wave band. The specific wave band, locating between the microwave and infrared frequencies, is from 10 11 Hz to 10 13 Hz [1]. Terahertz radiation has an ability to penetrate many materials, such as foam [2], ceramic [3], magnetic material [4] and polymer composites [5] and so on. Therefore, THz technology has been widely used in non-destructive testing as an established powerful tool [6] [7] [8]. THz applications in such fields as pharmaceutical solid dosage forms [9], dental tissues [10], coating layers [11] [12] [13] [14], glass fiber [15], painting on canvas [16], corrosion under metallic source material [17]  However, imaging speed and cost of hardware are contradictory in THz imaging. While the terahertz camera is expensive, the imaging speed of raster-scan THz imaging system is slow. In this paper, a THz imaging system with rotation the system, we are able to acquire image size of 60 × 80 mm 2 in 60 seconds, while a raster-scan THz imaging system with the same hardware conditions needs more than 30 minutes. Moreover, internal information of object could be got with the THz imaging system.

Diagram of the System
Diagram of the THz imaging system with rotation mirror is shown in Figure 1.
When a sample is scanned, the probe wave transmitted by the 0.3 THz source passes through the shaping lens and then casts on the rotation mirror. As the rotation mirror rotates reflected wave scans the sample. Reflected waves through the sample are converged at the detector by collecting lens.

Source of the System
The source of the system is a THz Impact Ionization Avalanche Transit-Time (IMPATT) diode produced by TeraSense Company. A physical map of the source is shown in Figure 2. Table 1 shows the specifications of the IMPATT diode.

Detector of the System
The detector of the system is a high electron mobility field-effect transistor (FET) based on GaN/AlGaN bow-tie antenna enhancement technique. Physical map of the source can be found in Figure 3 and more specifications of the FET can be found in Table 2.

Electric Control Rotation Mirror of the System
The electric control rotation mirror of the System, including a fast rotation bearing, a slow rotation bearing and a reflector, is produced by OP Mount Instrument Inc. Physical map of the rotation mirror can be found in Figure 4 and more specifications of the FET can be found in Table 3.

Data Acquisition
When the THz imaging system is working, the quick bearing is rotating at 60˚/s and the slow bearing is rotating at 0.2˚/s. The system recorded the signal when the fast bearing angular displacement is round number in angular unit. Diagram of sampling is shown in Figure 5.

Image Reconstruction
Firstly, data collected from the system are mapped to corresponding points in Cartesian coordinates. Then, the points are connected with triangles as shown in Figure 6, using the Delaunay algorithm [20]. Padding triangles mentioned above, final image can be obtained.

Experiment Setup
Photography of the experimental set up is shown in Figure 7. The sample is composed of three layers. As shown in Figure 8, there is a metal layer sandwiched between 2 pieces of cardboard.      samples were named "sample A", "sample B" and "sample C" respectively. The shape of sample A's metal layer can be found in Figure 9(a); the shape of sample B's metal layer can be found in Figure 9(b); the shape of sample C's metal layer can be found in Figure 9(c). Experiment results of the 3 samples are shown in

Conclusion
A THz imaging system with rotation mirror is built in this article. The system based on a two-dimensional rotating scanner has the ability to acquire an image sized at 60 × 80 mm 2 in 60 seconds while a raster-scan THz imaging system with the same hardware conditions needs more than 30 minutes. According to the experiment in chapter 4, it can be found that the system has the ability to detect the information of inner layer of objects effectively.