Sub-millimetre wave imaging ROBIN DAHLBÄCK. Thesis for the degree of Master of Science in Wireless and Photonics Engineering - PDF

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[mm] deg [mm] 150 Sub-millimetre wave imaging Thesis for the degree of Master of Science in Wireless and Photonics Engineering ROBIN DAHLBÄCK Terahertz and Millimetre Wave Laboratory Department of Microtechnology and Nanoscience CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2011 Thesis for the degree of Master of Science in Wireless and Photonics Engineering Sub-millimetre wave imaging ROBIN DAHLBÄCK Terahertz and Millimetre Wave Laboratory Department of Microtechnology and Nanoscience Chalmers University of Technology Göteborg, Sweden, 2011 Sub-millimetre wave imaging c ROBIN DAHLBÄCK Terahertz and Millimetre Wave Laboratory Department of Microtechnology and Nanoscience Chalmers University of Technology SE Göteborg, Sweden Phone: +46 (0) Cover: Phase contrast of a laser printed paper imaged at 340 GHz, see section Printed in Sweden TeknologTryck Göteborg, Sweden 2011 Abstract The sub-millimetre wave region withholds many interesting properties that can be used for imaging. Common packaging materials are transparent making package inspection possible without the use of ionising radiation. Contrast can also be seen between healthy and diseased tissue for some common forms of cancer. Many other biomedical applications are still unexplored making it an interesting research topic. A study of common system topologies are presented together with an imaging system built with in-house components. The imaging system is adopted for the use of a microwave tomography image reconstruction algorithm. System characterisation data are presented, the usable bandwidth is 6.5 % centred around 337 GHz. Measurement repeatability and long therm stability is also evaluated, concluding that the system most likely is good enough for use with the image reconstruction algorithm. Furthermore test images are presented. Preface This report is the result of a 60 hec, higher education credits, master thesis carried out at Chalmers University of Technology, Department of Microtechnology and Nanoscience, Terahertz and Millimetre Wave Laboratory. Jan Stake is the main supervisor and examiner and Tomas Bryllert is co-supervisor. The project is part of the SFF sponsored project Charmant Medical THz which also involves Tonny Rubaek, Mikael Persson and Andreas Fhager from the Biomedical Engineering Division, Department of Signals and Systems. Acknowledgements This work would not have been possible without the preceding efforts and guidance of Dr. Biddut Banik. I would like to thank my supervisors professor Jan Stake and Dr. Tomas Bryllert for their help and guidance. Dr. Peter Sobis has been my mentor in the practical work and also a great source of both ideas and components. The entire staff of the Terahertz and Millimetre Wave Laboratory deserves recognition for providing such an inspiring and pleasant working environment. Finally, I would like to thank The Swedish Foundation for Strategic Research, SSF, for funding this research. Contents 1 Introduction Thesis layout Goals and requirements Delimitations Background Passive imaging systems Radiometers Active imaging systems Multiplier based systems FMCW range-finding TPI, terahertz pulsed imaging THz imaging by optical beating Imaging in the sub-millimetre wave region Imaging properties in the sub-millimetre wave region Electromagnetic properties of samples Sample requirements Theory concerning the constructed system SSB up-converter Quadrature demodulation IF reference to enable vector measurements Optics Determining ɛ r by δφ δf Construction of imaging system Preceding 108 GHz scalar system Scalar 340 GHz system Schottky based 340 GHz vector measurement system Quadrature demodulator Frequency divider SSB up-converter HBV based 340 GHz vector measurement system Optics Open waveguide probe Gaussian telescope approach Catadioptric lens Image reconstruction Mechanical components axis mechanical setup Custom WR-2.8 waveguide probe Divider PCB... 36 4.6.4 Lens holder Mounting plate Measurements System characterisation Characterisation of vector measurements Frequency divider measurements GHz power amplifiers Characterisation of catadioptric lens at 340 GHz SSB Up-conversion Quadrature demodulator Results Performance of measurement system Amplitude and phase measurement accuracy SSB up-converter Quadrature demodulator Frequency divider Characterisation of 30 GHz power amplifiers Optics Image reconstruction algorithm Quasi optical focusing Catadioptric lens Imaging GHz scalar imaging GHz imaging Vector imaging Publications Conclusion and discussion 67 8 Future outlook 69 A Appended papers 75 B PCB layouts 79 1 Introduction Terahertz frequency radiation is typically defined to be in the region THz. Since it lies between the infrared and microwave regions in the electromagnetic spectrum it has many interesting properties that can be useful for imaging purposes. Common packaging materials are transparent making non invasive package inspection possible without the use of X-rays. Cancerous tissue show a contrast in permittivity compared to healthy tissue making the wavelength range an interesting candidate for future medical equipment. THz imaging has the potential to have a wide range of medical applications where it can improve and aid the detection and diagnosis of disease. One example is a non-invasive method for detection of skin cancer. Time domain systems have been tested to examine different skin properties, [1], however some of the areas that can be improved are SNR and spectral resolution. Areas where frequency domain methods have an advantage. The system being built operates as a CW heterodyne system thus providing good SNR and spectral resolution. The image reconstruction will be done using a microwave tomography algorithm adopted for use with the sub-millimetre wave hardware. The image reconstruction algorithm provides the possibility to have sub wavelength focusing with a large focal depth, thus eliminating common problems with short focal depth and specular reflections. Other examples of possible areas of implementation is non invasive sensing and inspection in industrial processes or product verification. More general information regarding the wavelength region 100 GHz to 1 THz as well as more specific properties that are interesting for imaging can be found in [2]. The thesis is a part of a collaboration between MC2, S2 and Sahlgrenska in the research centre SSF Charmant. The purpose of the thesis can be split into two tasks. To investigate previous work and technology within the field of THz imaging and to create an imaging system using in-house available components. Cell slides containing cancer samples will be available through the collaboration with Sahlgrenska, making a sub-millimetre wave image of the available samples is one of the goals of the work. 1.1 Thesis layout The following short summary of the different sections is intended to serve as a guide to the reader: Section 1 gives an introduction to the work and presents the goal. Section 2 gives a background and lists several imaging systems constructed by other groups. System topologies are summarised together 1 with some common techniques for image formation. Section 3 lists some of the properties making the sub-millimetre wave region unique. Finally some theory concerning the constructed system is presented. Section 4 describes the evolution of the imaging system being built. Various components included in the system are also described. Section 5 presents measurements done in order to evaluate the constructed system. Section 6 presents the results together with some images created during the work. Section 7 summarises the work and the conclusions. Section 8 presents thoughts about possible future continuation of the work. 1.2 Goals and requirements The goals of the work are listed below: Investigate what can be done with in house available equipment. Test sub millimetre wave imaging using available equipment. Publish a scientific article within the topic. Characterise previously manufactured catadioptric lens at 340 GHz. Image the cancer samples available through the collaboration SSF Charmant. Summarise previous work within the field. Summarise different common technologies. The requirements that needs to be completed are: Test sub-millimetre wave imaging using available equipment. Characterise previously manufactured catadioptric lens at 340 GHz. Image the cancer samples available through the collaboration SSF Charmant. Summarise previous work within the field. Summarise different common technologies. 2 1.3 Delimitations The work should be finished within one academic year corresponding to 60 hec. Available equipment should be used to as large extent possible, new equipment can be bought if the cost is low or if it can be reused in other projects. Concerning the summation of previous work within the field the study will be limited to systems and applications that share some similarities with the constructed system, either in construction or in the field of application. There ere already several excellent articles aiming at summarising the whole field of THz imaging, see for example [3], [4], [5]. 3 4 2 Background This section will give a brief background to the concept of THz imaging and present a number of different THz imaging systems. A more detailed description of the operation of the different systems are given in section 2.1 and 2.2. Imaging can generally be done in passive or active mode where the first relies on detection of an existing signal while the second illuminates the imaged object and records the reflection or transmission. As indicated two common forms of imaging are reflection and transmission imaging. In transmission imaging the signal transmitted through the imaged object is detected while reflection imaging detects the signal that the object reflects. There are a huge number of imaging modalities, in this thesis emphasise will mainly be placed on system used for biomedical examination and hidden objects detection. 2.1 Passive imaging systems This section will give an introduction to passive sub-millimetre wave imaging. Requirements on radiometer systems operating with a high background temperature will also be briefly discussed. When doing passive imaging the contrast in the scene will arise from the different radiometric temperatures in the scene. In a simple view of it the parameters affecting the effective radiometric temperature, T E, are the physical temperature of the object, T O, the emissivity, ɛ, the reflectivity, ρ, and the radiometric temperature of the surrounding illumination, T ILLUMINATOR [6]. The equation for the effective radiometric temperature can be written as: T E = T S + T SC = ɛt O + ρt ILLUMINATOR (1) Where T S is the radiometric temperature and T SC the scattered radiometric temperature. The radiometric temperature is simply the product of the emissivity and the objects physical temperature. The emissivity is a function of the dielectric properties of the material, the angle of observation, the surface properties and is also polarisation dependent. T S = ɛt O (2) The scattered radiometric temperature is the product of the reflectivity and the radiometric temperature of the background illumination. Outdoors the sky is usually the main source of background illumination and the sky radiometric temperature is discussed in section 3.1 but is in general much colder then surrounding objects at sea level. 5 T SC = ρt ILLUMINATOR (3) One common situation in passive sub-millimetre wave imaging is when a smooth metallic object, ɛ 0, is placed in front of something with much higher emissivity i.e a knife hidden under clothes or a vehicle in front of vegetation. For an indoor scene the metallic object will be seen as a shadow on a background with higher radiometric temperature if the object is held close to the skin and specular reflections from objects with higher radiometric temperature is avoided. Outdoors the metallic object is likely to reflect the cold sky thus appearing to be colder in both scenes Radiometers Cryogenic state of the art radiometers operate with equivalent noise temperatures much lower than the warm background seen in ground based imaging situations. Room temperature receivers can therefore be employed in many passive imaging scenarios by allowing a longer signal integration time. An example of a passive image of a human holding a wrench can be seen in Figure 1. The image thermal resolution is 0.25 C and it is acquired using a room temperature Schottky mixer operating at 640 GHz with a 10 ms integration time for each pixel [7]. 10 Temperature (OK) e c.- 0 c 9 W Azimuth ( ) Figure 1: Passive thermal image of a human holding a wrench, acquired with a 640 GHz Schottky mixer[7]. A sensitive passive system built around a hot electron bolometer is presented in [8]. The setup operates by heterodyne detection at 850 GHz and 6 can clearly resolve a 1 K temperature difference between two pieces of absorber. 2.2 Active imaging systems In active reflection imaging the picture is largely dependent on specular reflection, especially when it comes to metal objects with a surface roughness that is small compared to the wavelength. In such cases the the angle of incidence of both the transmitter and receiver is critical and the reflection is only seen in a narrow viewing angle. Transmission imaging on the other hand is mostly limited by attenuation through the sample. Samples containing water or other polar liquids tend to be very lossy while many dielectric materials that are opaque in the optical region show relatively small loss in the THz region, a more detailed discussion of sample properties can be found in section By employing modulation techniques that enables range finding, i.e. the FMCW technique described in section 2.2.2, the dependance of the surface reflectivity can be greatly reduced since the returned signal only needs to be detectable within the instruments dynamic range in order to find the distance to the target Multiplier based systems There are more or less two approaches to sub-millimetre wave imaging. When using microwave techniques the challenge is to increase the frequency. When an optical approach is used the problem is the opposite. Due to the lack of good fundamental electronic oscillators in the region above some GHz a lower frequency signal is typically multiplied in a nonlinear device called a multiplier. The fundamental principle of the multiplier is that if signal is feed to a non linear device the output will contain harmonics of the input signal. By proper filtering the desired harmonic can be extracted an used. Common non-linear devices used in the THz region are Schottky and HBV diodes. As receivers both bolometers and Schottky mixers are common. Depending on the system properties required the receiver topology may vary from direct detection to heterodyning. The receivers are usually configured in single pixel setups due to the difficulty and high cost of multi-pixel arrays. However one emerging technology that might enable cheep focal plane arrays is described in [9]. The simplest form of active imaging is when a CW transmitter is used to illuminate the target and the reflected power level is recorded. An example of such an image is shown in Figure 2 where a person hiding a handgun under a shirt is imaged at 640 GHz. The specular reflection seen from the handgun in the picture is relying on the fact that the image is recorded at 7 close to normal incidence. A slight shift in viewing angle quickly makes the gun unresolvable. Figure 2: Active image showing reflected power from a person with a handgun hidden under a shirt, taken at 640 GHz[2] FMCW range-finding By modulating the output from the measurement equipment as a linear frequency sweep the distance to the target can be found by the frequency offset created by the signals flight path. This technique is commonly known as Frequency Modulated Continuous Wave radar or simply FMCW radar. The FMCW radars range resolution is inversely proportional to its bandwidth according to [10]: r = c (4) 2 F Where c is the speed of light, r the range resolution and F the chirp radar bandwidth. A radar bandwidth of 100 GHz gives a theoretical range resolution of 1.5 mm. The range to the target is found by examination of the IF frequency: f IF = 2KR c Where K (Hz/s) is the chirp rate and R (m) the range to the target. As an example a chirp rate of 250MHz/ms gives f IF =7kHz at a target range of 4 m. With the same chirp rate the difference frequency between two objects separated by 1 cm would be 17 Hz. One of the commercial vendors of THz imaging systems is SynView GmbH who produces imaging units operating in coherent FMCW mode. The high frequency head operates between GHz giving a theoretical 8 (5) range resolution of 2 mm, see section An example of a suitcase scan is shown in Figure 3 [11] THz radiation transmits through ty Figure 3: A package inspection example created using SynViews imaging system [11]. Another FMCW imaging system that uses the range information to improve the image reconstruction is described in [12]. The system used is based on multipliers and operate as a superheterodyne FMCW radar at around 600 GHz. For focusing at standoff distances of 4 m and 25 m large off-axis ellipsoidal reflectors are used, the main reflector is moved by a two axis rotational stage in order to raster scan the scene. The system block diagram is reproduced in Figure 4. The measured spot size and range resolution at 4 m is approximately 1 cm. Since it is possible to distinguish the distance between multiple reflections a multilayered 3D image can be constructed. Figure 5 shows how signal processing has been used to extract the first interface, shirt, and the second reflection, gun and skin. The image is taken at 4 m standoff. Another system that uses the FMCW technique for standoff imaging is found in [13]. Besides using the FMCW approach the quasi-optical configuration is quite interesting since it uses a two axis rotating mirror to direct the beam. This allows a scan speed of 9 seconds per image, the optics are reproduced in Figure 6. Here a frequency divider is used used to create the IF reference instead of a multiplier as in the previous system. A discussion about different IF topologies enabling phase measurements is given in section Figure 4: a) System block diagram showing the multiplier based superheterodyne FMCW radar. b) Image showing the setup with the off-axis ellipsoidal main reflector. c) Optics block diagram [12]. Figure 5: Left: first reflection encountered by the THz beam. Right: First reflection from shirt removed to reveal the reflection from the skin and handgun [12]. 10 Figure 6: System block diagram together with quasi-optical setup[13] TPI, terahertz pulsed imaging Terahertz pulsed imaging, TPI, is probably the most common form of THz imaging. A general THz pulsed time domain transmission measurement system is shown in figure 7. The system works by the principle that short optical pulses excite currents in the semiconductor crystal whereby an electric impulse is radiated with a frequency content in the terahertz region. Figure 7: A typical femtosecond pulsed THz measurement system [14]. One of the first reported THz imaging systems are [15]. A pulsed time domain system was used to image an integrated circuit and map the water 11 content of a leaf. Both images are reproduced in Figure 8 and figure 9. Figure 8: An epoxy packaged IC circuit imaged using a time domain THz imaging system in transmission mode [15]. Figure 9: A leaf imaged with a 48h interval showing the difference in water concentration [15]. One vendor of a commercial systems is TeraView who supply systems for both spectroscopy and imaging. Figure 10 shows the system that probably later evolved to one of TeraView s products. It was used in a study where the skin properties of 20 persons where examined by time doma
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