Speedy and Colorful Terahertz Scanning
Real-time imaging technique permits investigation of objects in motion.
When it comes to nondestructive screening and testing, terahertz time-domain spectroscopic imaging offers significant advantages but one considerable drawback. Lying between infrared and microwave wavelengths, terahertz waves can penetrate a wide variety of nonconducting materials to reveal hidden objects. Thanks to distinct terahertz spectral fingerprints, the composition of these items also can be determined. The drawback is that image acquisition typically takes too long for successful scanning of moving objects.
Images acquired with a real-time terahertz time-domain spectroscopy line scanner show a four-segment metal hole array moving past a point. The images, taken at four frequencies (inset in each image) differ because the holes act as bandpass filters in the terahertz range. The metal plate was moving at 1 mm/s. Each image measures 200 × 232 pixels. Reprinted with permission from Optics Express.
Now researchers from Osaka University in Japan and from Université Bordeaux I in Talence, France, have demonstrated a terahertz time-domain spectroscopic line scanner that has an imaging rate of more than 2300 pixels per second for moving objects. That is potentially fast enough to spot explosives in luggage or to inspect pills during pharmaceutical manufacturing.
Modifications to the terahertz pulse generation technique could produce still faster scan rates, said team member Takeshi Yasui, an assistant professor in the graduate school of engineering science at Osaka University. “I believe it is possible to achieve 10- to 100-times-higher pixel rates.”
Yasui said that the new technique is based on an approach that was developed at Bordeaux for another application. It exploits the fact that objects moving past a point at a constant speed can be imaged in two dimensions from consecutively acquired line images.
In their demonstration, the researchers used a femtosecond Ti:sapphire regenerative amplifier from Spectra-Physics of Mountain View, Calif., to generate and detect terahertz pulses. To generate the terahertz burst, they sent 150-fs pulses centered at an 800-nm wavelength into a zinc telluride crystal. As a result of this, the crystal emitted a terahertz beam.
At the upper left is an image of a sliced tooth sample, and the other images are the corresponding view at the terahertz frequencies in the inset. The images were acquired with a real-time terahertz time-domain spectroscopy line scanner. Terahertz imaging could be an important tool for biomedical research and assessment. Courtesy of Takeshi Yasui, Osaka University.
They blocked the residual optical light with a white polyethylene plate and line-focused the terahertz pulses onto a moving sample. After interacting with and emerging from the sample, the pulses struck a second zinc telluride crystal that was set at an angle to the beam. That same crystal also was illuminated with a probe light that was produced by the amplifier and arrived with a delay set by the positioning of a movable reflector.
The interaction of the two beams created a signal that the researchers captured with a Hamamatsu high-speed CMOS camera that was synchronized to the pump light. Fourier transformation converted the data into a one-dimensional terahertz time-domain spectroscopy image.
They tested their setup on a metal plate with 2-D periodic arrays of circular holes, each with a diameter less than the wavelength of the terahertz pulses. They used a 300-μm-thick stainless steel plate with holes of four different sizes and spacings, with the arrays arranged in quarter circles. The largest holes were 0.75 mm in diameter, arranged in an array with center-to-center spacing of 1.5 mm. The smallest were 0.1 mm in diameter, with a spacing of 0.19 mm. The holes acted as terahertz bandpass filters, with the various sizes yielding different frequency responses. The researchers moved the plate while capturing the color terahertz line image in real time with their scanner. They observed that the imaging rate was 2320 pixels per second with a resolution near the diffraction limit of the optics — performance good enough to be considered for real-world applications.
According to Yasui, the group is working to improve the signal-to-noise ratio of the technique. He also noted that it might be possible to boost the scan rate by using a tilted pulse wavefront for excitation of the crystal for terahertz generation, which would reduce the image acquisition time by increasing the electric field of the terahertz pulse.
“An ultimate goal is to develop a compact terahertz color scanner system as small as the color scanner used in offices.”
Optics Express, Jan. 21, 2008, pp. 1208-1221.
- The successive analysis or synthesizing of the light values or other similar characteristics of the components of a picture area, following a given method.
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