Xiao-Tong Guan, Min Hu, Wen-Jie Fu, Yu-Meng Cui, Xiang Fan, Liang Zhang, Ye Yuan,Jing-Yuan Xu, Yuan Li, and De-Wei Zheng

1.Introduction

The terahertz (THz) radiation ranges between farinfrared and millimeter wave on the electromagnetic spectrum, whose frequency ranges from 0.1 THz to 10 THz.For many years, the THz region was called as the “terahertz gap”[1], due to the relative lack of available sources, sensors,and systems for THz radiation.Affected by recent significant advances in the field of THz generation and detection, THz science and technologies have received worldwide attention[2].

As a type of radiation possessing both of the characteristics of microwave and optical wave, imaging with THz radiation has many advantages.Because of its high frequency and short wavelength, imaging using THz radiation has higher resolution than using microwaves;compared to optical and infrared radiation, THz radiation could easily penetrate the materials like clothes, paper,fabric, and plastic; due to its low photon energy (4.2 meV/THz), THz radiation is a non-ionizing radiation and much safer than X-rays, whose photon energy is ranging from 120 eV to 120 keV.It is harmless to materials,especially biological tissues[3].

Therefore, with those unique properties, THz imaging has found high potency of application in many fields,including security screening[4], nondestructive examination[5], biomedical diagnostics[6], chemical recognition[7],and medical analysis[8].

In this article, the raster scan method[9]is adapted into the continuous wave (CW) transmission imaging configuration, to build an essential scanning system which can be later modified into more complicated applications.Two sensors are used to detect the radiation to show the performance of this implementation, while brief analyses upon the speed, scale and quality aspects of raster scan method are discussed after the results.

2.Experiment Setup

In the two systems, an optically pumped far-infrared(FIR) laser, by Edinburg Inc., is utilized as the THz source.This laser system consists of two parts: a tunable CO2laser and an FIR laser filled with low-pressure gas.The diameter of the output THz beam is about 10 mm (intersection) and the working frequency is 2.52 THz.The average output power is above 200 mW, but the power is unstable with fluctuations within the range of 50 mW.

The schematics of the imaging systems are shown in Fig.1.In the array scanning imaging system, the THz beam directly propagates through the object and is collected by an array detector.

The array scanning imaging system employs a Pyrocam III charge-coupled device (CCD) camera as the detector.Its pyroelectric sensor contains 124×124 pixels, and the active area of the camera is 12.4 mm×12.4 mm.The camera has the absorbing wavelength ranging from 1.06 μm to over 1000 μm (0.3 THz to 300 THz).When light illuminates the pixels on the CCD, the energy is converted into a normalized real integer matrix.Therefore, the output is a time-dependent sequence of 124×124 matrices.To operate with CW laser, an internal chopper is set by the manufacturer, which runs at 24 Hz or 48 Hz; the frequency is set so to accommodate the frame-per-second (fps)configuration of general video formats.

Fig.1.Configurations of the two scanning imaging systems: (a) optical path of array scanning imaging system and (b) optical path of point-wise imaging system, the object is placed at the focal point of the lens L1.

In the point-wise scanning imaging system, as shown in Fig.1 (a), the THz beam is focused on the object by a high-density polyethylene (HDPE) lens L1 (focal length 100 mm).The transmitted light is collected by a detector.

The point-wise scanning imaging system, as shown in Fig.1 (b), employs a Golay cell as the detector.Golay cell is a kind of hot electron bolometer, which is suitable for sensing microwave radiations with power above 100 μW.Its input window size is less than 5 mm in diameter and the output is only one sequence of single real number (direct voltage values) indicating the radiation power.In order to operate with a CW laser, a physical chopper is added in the optical configuration in order to generate general video stream.

3.Imaging Results

3.1 Array Scanning Imaging Results

In Fig.2, a paper clip hidden behind a paper cup is imaged by the array scanning system.The dark black circle is the wall of paper cup, which is very thick in the direction of THz wave propagation, and it is easy to recognize the metal paper clip hidden in the center of the cup.The much thinner paper around the paper clip almost has no impact on THz radiation.So THz radiation has strong penetrability for paper, and the penetrability will decrease as the thickness of the paper increases.

The gray disordered patterns that spread through the whole image are caused by the instability of the output power of THz laser.Once the power floats down, the appropriate position would be gray spot.

A common shaver blade enclosed in a paper envelope is imaged by the array scanning imaging system.The result is shown in Fig.3 (b).The envelope is totally invisible in this image.The steel blade is shown as an imperfect dark rectangle.Emanating streaks on the rims are caused by the scattering effect of the metal edge.In fact, most common thin metal plates can lead to this pattern.The bright area in the center corresponds to the fixing slot, of which the details cannot be recognized.

3.2 Point-Wise Scanning Imaging Results

The shaver blade is also scanned by the point-wise system.Fig.3 (c) is the THz image including the fixing slot in the middle.Due to the focusing effect of the convex, the scattering effect is decreased but does not disappear.Through the remaining shadow, the shape of the slot can be clearly recognized.

Fig.2.Imaging result of a paper clip hidden behind a paper cup:(a) white-light image and (b) THz array scanning image.

Fig.3.Photo of a common shaver blade: (a) white-light Image, (b)THz array scanning image, and (c) THz point-wise scanning image.

Table 1: Comparison between the speed of two scanning systems

Fig.4.Photo of hand written words “THz” in different sizes and different ink: (a) smallest size word used 2B pencil, size: 15 mm×5 mm, (b) middle size word used 2B pencil, size: 27 mm×10 mm,(c) largest size word used 2B pencil, size: 46 mm×18 mm, and (d)word written by pen, size: 38 mm×13 mm.

Fig.5.THz point-wise scanning image of hand written words in different size using 2B pencil: (a) THz image of the smallest word,(b) THz image of the middle-sized word, and (c) THz image of the biggest word.

The next sample, shown in Fig.4, hand-written letters in different sizes and with different ink, enclosed by an envelope, is scanned by the point-wise imaging system.

The image of pen ink does not contain any necessary information to identify the word’s characters, while the image of 2B pencil is shown in Fig.5.So THz radiation has different transmittance to different types of writing ink.The common pen ink and printing ink almost have no effect on THz wave propagation.

The pencil strokes which are mainly based on clay and graphite particle have significant effects on the propagation of THz waves.As shown in Fig.5, the image of the largest word is easily recognized as the “THZ” pattern.In the image of the middle-sized word, the noise spots begin to increase and cause some pattern distortion.As for the image of the smallest word, the feature information has been submerged in noise spots and the word is hard to recognize.The smallest word has reached the spatial resolution of the point-wise scanning imaging system.The width of the strokes is about 1 mm, so the spatial resolution of this system reaches the same order of magnitude as 1 mm.

3.3 Comparison between the Two Imaging Systems

Considering of the limits of both the size of illuminated area and the input window of detectors, raster scanning is an effective method in the large-scale imaging progress.Imaging speed and quality are both important to evaluate a raster scanning system.

As shown in Table 1, it just takes 6 min for the array scanning system to scan a 55 mm×55 mm area, while the point-wise scanning system takes almost 20 min to scan an area of 40 mm×11 mm.So the array scanning has a higher scanning speed because of its large imaging plane.

Fig.6.Photo of key: (a) white-light image, (b) THz array scanning image of the whole key, and (c), (d) and (e) are THz point-wise scanning images of different parts of the key.

However, high imaging quality is the key advantage of the point-wise scanning imaging system.For the array scanning system, the limited resolution of Pyrocam camera is about 0.1～1 mm.Moreover, the diffraction of THz radiation makes the resolution much lower than 1 mm.In the point-wise scanning system, with the help of convex lens, the effects of scattering on the image is suppressed.Since only one point per about 1 mm in diameter is measured by the devices, the resolution of point-wise scanning system can be up to 1 mm, as demonstrated in the THz image of hand-written word with 2B pencil.

Above all, the point-wise system has higher image resolution and signal-to-noise ratio (SNR) at the cost of limiting the imaging speed, and the array scanning system has an absolutely advantage on the imaging speed.Therefore, the array scanning system can be used to detect large-scale objects to find the target region.The point-wise scanning system will have a good performance for imaging the local region of interest.As shown in Fig.6, the array system scans the whole key and the point-wise system just scans some fine structures of the same key.

4.Conclusions

Two types of THz raster scan imaging systems are designed based on a 2.52 THz CO2pumped gas laser and two types of detectors.One uses an array pyroelectric detector, Pyrocam III.The other system employs a single-point detector, Golay cell.The array scanning imaging system has higher imaging speed and lower resolution.The point-wise scanning imaging system works slowly but has higher resolution and better image quality.The former system can provide a preview of quick large scale object, while the latter system can present more detailed information about the target.

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