21-12-2012, 03:22 PM
TERAHERTZ IMAGING TECHNIQUE
[attachment=44776]
INTRODUCTION
The major driving force for development of terahertz imaging systems today originates from security applications and, in particular, stand-off imaging of persons and hidden objects for security enforcement, including illicit drug and explosives detection. Bonding flaw and defect detection at stand-off distances for space applications requires imaging systems with similar performance as those for security applications. Hence, system concepts can be discussed jointly for both applications. By terahertz, we mean a frequency range covering 300–3000 GHz, the lower part of which (100–1000 GHz) is sometimes referred to as the sub millimeter-wave frequency region. Conventional imaging systems utilize optical and X-ray imaging for stand-off detection for security and space applications.
This paper has two goals. The first one is to review imaging system and algorithms at millimeter-wave and terahertz frequencies, subsequently called terahertz cameras. The second goal is to present two new imaging systems- Photonic and Electronic Imaging systems.
The system is based on electronic and photonic principles and provide a short description of imaging techniques and algorithms reported in the literature. As to eletronic imaging systems, we mean those systems relying entirely on electronic components, whereas photonic imaging systems are those where a terahertz signal is generated or received through transformation of an optical signal into the terahertz range.
WORKING PRINCIPLE
By terahertz, we mean a frequency range covering 300–3000 GHz. One scenario considered in this paper is illustrated schematically in Fig. 1 and consists of an active array of transmitters and receivers illuminating an object at stand-off distance. The receivers are place in the center part of the array, while the transmitters are situated at larger distances. The major imaging task here is to detect an object at a distance and classify the object with regards to its potential threat or impact. One would like to detect all possible materials (i.e., solid, fluid, or gaseous) with many different properties (e.g., metal or dielectric).
An imaging system in such a scenario should be able to detect the object with high spatial resolution and should classify this object with high success rate or low false alarm rate. The imaging should be close to real time in security applications with somewhat relaxed demands for industrial applications. The terahertz camera considered here should operate at stand-off distances of a few meters to tens of meters, image man-sized scenes and produce images close to real time, and detect objects with a minimum size of a few centimeters. The bandwidth of the terahertz camera should therefore be large for good range resolution. In the case of terahertz imaging systems based on photonic continuous-wave (CW) or pulsed techniques, the power is not sufficient to operate at such large distances, but the resolution is much better. This is due to the high operating frequencies.
ELECTRONIC IMAGING SYSTEMS
Active systems can operate in monostatic, bi-static, or multistatic mode. An advantage of active systems is the possibility for bi-static or multistatic measurements, providing information on the reflection and transmission properties of the object, which can be fully exploited in image processing algorithms. Passive systems operate only in an observation mode or with a broad-band incoherent source, which then acts as a signal source.
However, powerful broadband noise sources are very difficult to realize. In principle, both passive and active imaging systems are feasible for the applications considered. Heterodyne millimeter-wave systems at room temperature require local oscillator (LO) signal sources, which can be used for object illumination.
Passive Imaging Techniques
A passive imaging system is essentially a radiometer, which detects the radiation emitted from an object in view. One can employ either direct-detection focal plane arrays or a heterodyne receiver with a scanning system and single element receiver . Direct-detection receivers have been using millimeter-wave monolithic microwave integrated circuit (MMIC) technology with good noise performance however, without dual-polarization and phase-detection capabilities. However, inter ferometric imaging requires the determination of the amplitude and phase of the detected signal. .
For adequate spatial resolution at large stand-off distances, apertures need to be large, i.e., of the order of 1-m diameter. Recently, focal plane arrays at sub millimeter-wave frequencies have been demonstrated operating at room temperature and using a standard CMOS process. It is anticipated that, in the near future, MMIC technology will become the technology of choice from a cost and performance point of view, providing a natural path towards multichannel imaging systems.
. Scanners
A number of mechanically scanned optical systems have been developed, such as the spherical scanning antenna, the Lewis scanner, the Schwarzschild scanner, and the Rotman lens scanner. Also, there is the scanning spherical tri reflector antenna, the two-mirror scanning system and the millimeter-wave monopulse twist reflector scanning antenna. The oscillating motion of the mirrors used in these later designs is too slow to produce real-time scanning. A rotating polygon scanner, however, can scan at rates up to 50 kHz with scan angles approaching 180. A high-speed scanning mirror has formed the basis for a millimeter-wave line-scan system for use in airborne applications. Other imagers achieve a raster scan by motion of both the antenna and receivers.
PHOTONIC IMAGING SYSTEMS
Photonic terahertz imaging systems have predominantly been employing time-of-flight measurements by raster scanning the imaging object. Recently, more advanced proposals for terahertz synthetic aperture and inter ferometric imaging methods have been proposed by several groups and published both in the scientific literature and filed as patents. In practice, until today, most systems employ one Tx/Rx pair by moving the transmitter and/or receiver around the imaging object, acquiring data from various positions and subsequently reconstructing an image. Having more than one transmitter and/or receiver at different positions enables illumination of an object from more than one location and, hence, employs tomography. Below we will demonstrate a possible implementation of a multi-channel imaging system using fiber-coupled sensor heads.
Several research groups have explored the analogy of X-ray computer tomography (CT) with terahertz radiation and pioneered by Zhang et al. In contrast to X-ray CT, terahertz CT resolves both the amplitude and the phase information of the scattering object. As a consequence, the terahertz CT image contains more information than the X ray CT about the target, such as the frequency-resolved refractive index. The wide-aperture reflection tomography allows tomographic reconstruction of a series of slices measured at different view angles. This technique works best with strong reflectors such as metals. The algorithms used for the reconstruction of the images are in general filtered back-projection algorithms.
PHOTONIC TERAHERTZ MULTIELEMENT IMAGING
SYSTEM REALIZATION
As discussed above, most photonic imaging systems today utilize a single element with the mechanical according scanning, resulting in long acquisition times. The photonic terahertz imaging system presented here is based on an advanced dual-femtosecond fiber laser with an electronic delay stage unit, an all-fiber ultrashort pulse distribution system with dispersion control and a 2-D array of 32 individual terahertz emitters and 32 individual terahertz detectors. The advantage of an all-fiber design is a system that is very stable with respect to external vibrations and temperature fluctuations, without need for further alignment. In addition, we have developed ultra low-noise amplifiers, directly integrated into the individual terahertz receivers. Fig. shows an outline of the principal components of the photonic imaging system. The first experimental results for this system are described below.
[attachment=44776]
INTRODUCTION
The major driving force for development of terahertz imaging systems today originates from security applications and, in particular, stand-off imaging of persons and hidden objects for security enforcement, including illicit drug and explosives detection. Bonding flaw and defect detection at stand-off distances for space applications requires imaging systems with similar performance as those for security applications. Hence, system concepts can be discussed jointly for both applications. By terahertz, we mean a frequency range covering 300–3000 GHz, the lower part of which (100–1000 GHz) is sometimes referred to as the sub millimeter-wave frequency region. Conventional imaging systems utilize optical and X-ray imaging for stand-off detection for security and space applications.
This paper has two goals. The first one is to review imaging system and algorithms at millimeter-wave and terahertz frequencies, subsequently called terahertz cameras. The second goal is to present two new imaging systems- Photonic and Electronic Imaging systems.
The system is based on electronic and photonic principles and provide a short description of imaging techniques and algorithms reported in the literature. As to eletronic imaging systems, we mean those systems relying entirely on electronic components, whereas photonic imaging systems are those where a terahertz signal is generated or received through transformation of an optical signal into the terahertz range.
WORKING PRINCIPLE
By terahertz, we mean a frequency range covering 300–3000 GHz. One scenario considered in this paper is illustrated schematically in Fig. 1 and consists of an active array of transmitters and receivers illuminating an object at stand-off distance. The receivers are place in the center part of the array, while the transmitters are situated at larger distances. The major imaging task here is to detect an object at a distance and classify the object with regards to its potential threat or impact. One would like to detect all possible materials (i.e., solid, fluid, or gaseous) with many different properties (e.g., metal or dielectric).
An imaging system in such a scenario should be able to detect the object with high spatial resolution and should classify this object with high success rate or low false alarm rate. The imaging should be close to real time in security applications with somewhat relaxed demands for industrial applications. The terahertz camera considered here should operate at stand-off distances of a few meters to tens of meters, image man-sized scenes and produce images close to real time, and detect objects with a minimum size of a few centimeters. The bandwidth of the terahertz camera should therefore be large for good range resolution. In the case of terahertz imaging systems based on photonic continuous-wave (CW) or pulsed techniques, the power is not sufficient to operate at such large distances, but the resolution is much better. This is due to the high operating frequencies.
ELECTRONIC IMAGING SYSTEMS
Active systems can operate in monostatic, bi-static, or multistatic mode. An advantage of active systems is the possibility for bi-static or multistatic measurements, providing information on the reflection and transmission properties of the object, which can be fully exploited in image processing algorithms. Passive systems operate only in an observation mode or with a broad-band incoherent source, which then acts as a signal source.
However, powerful broadband noise sources are very difficult to realize. In principle, both passive and active imaging systems are feasible for the applications considered. Heterodyne millimeter-wave systems at room temperature require local oscillator (LO) signal sources, which can be used for object illumination.
Passive Imaging Techniques
A passive imaging system is essentially a radiometer, which detects the radiation emitted from an object in view. One can employ either direct-detection focal plane arrays or a heterodyne receiver with a scanning system and single element receiver . Direct-detection receivers have been using millimeter-wave monolithic microwave integrated circuit (MMIC) technology with good noise performance however, without dual-polarization and phase-detection capabilities. However, inter ferometric imaging requires the determination of the amplitude and phase of the detected signal. .
For adequate spatial resolution at large stand-off distances, apertures need to be large, i.e., of the order of 1-m diameter. Recently, focal plane arrays at sub millimeter-wave frequencies have been demonstrated operating at room temperature and using a standard CMOS process. It is anticipated that, in the near future, MMIC technology will become the technology of choice from a cost and performance point of view, providing a natural path towards multichannel imaging systems.
. Scanners
A number of mechanically scanned optical systems have been developed, such as the spherical scanning antenna, the Lewis scanner, the Schwarzschild scanner, and the Rotman lens scanner. Also, there is the scanning spherical tri reflector antenna, the two-mirror scanning system and the millimeter-wave monopulse twist reflector scanning antenna. The oscillating motion of the mirrors used in these later designs is too slow to produce real-time scanning. A rotating polygon scanner, however, can scan at rates up to 50 kHz with scan angles approaching 180. A high-speed scanning mirror has formed the basis for a millimeter-wave line-scan system for use in airborne applications. Other imagers achieve a raster scan by motion of both the antenna and receivers.
PHOTONIC IMAGING SYSTEMS
Photonic terahertz imaging systems have predominantly been employing time-of-flight measurements by raster scanning the imaging object. Recently, more advanced proposals for terahertz synthetic aperture and inter ferometric imaging methods have been proposed by several groups and published both in the scientific literature and filed as patents. In practice, until today, most systems employ one Tx/Rx pair by moving the transmitter and/or receiver around the imaging object, acquiring data from various positions and subsequently reconstructing an image. Having more than one transmitter and/or receiver at different positions enables illumination of an object from more than one location and, hence, employs tomography. Below we will demonstrate a possible implementation of a multi-channel imaging system using fiber-coupled sensor heads.
Several research groups have explored the analogy of X-ray computer tomography (CT) with terahertz radiation and pioneered by Zhang et al. In contrast to X-ray CT, terahertz CT resolves both the amplitude and the phase information of the scattering object. As a consequence, the terahertz CT image contains more information than the X ray CT about the target, such as the frequency-resolved refractive index. The wide-aperture reflection tomography allows tomographic reconstruction of a series of slices measured at different view angles. This technique works best with strong reflectors such as metals. The algorithms used for the reconstruction of the images are in general filtered back-projection algorithms.
PHOTONIC TERAHERTZ MULTIELEMENT IMAGING
SYSTEM REALIZATION
As discussed above, most photonic imaging systems today utilize a single element with the mechanical according scanning, resulting in long acquisition times. The photonic terahertz imaging system presented here is based on an advanced dual-femtosecond fiber laser with an electronic delay stage unit, an all-fiber ultrashort pulse distribution system with dispersion control and a 2-D array of 32 individual terahertz emitters and 32 individual terahertz detectors. The advantage of an all-fiber design is a system that is very stable with respect to external vibrations and temperature fluctuations, without need for further alignment. In addition, we have developed ultra low-noise amplifiers, directly integrated into the individual terahertz receivers. Fig. shows an outline of the principal components of the photonic imaging system. The first experimental results for this system are described below.