Several terahertz-wave radar techniques based on a resonant-tunneling-diode (RTD) oscillator are under investigation. First, an amplitude-modulated continuous-wave (AMCW) radar able to measure the absolute distance was achieved by demodulating the wave returning from the target and measuring its phase, either by determining the time delay with an oscilloscope, or by using an IQ demodulator to determine the sine and cosine of the phase. Our current record with this type of radar is a ranging error of 0.063 mm; however, it can only measure the distance up to a single reflecting surface. Second, a subcarrier frequency-modulated continuous-wave (FMCW) radar has been demonstrated to work and is expected to allow the simultaneous ranging of several targets. The ranging error for this radar was found to be 0.73 mm. Additionally, a radar technique based on the swept source optical coherence tomography (SS-OCT) principle is proposed. Terahertz-wave 2D imaging can be combined with these radar techniques to obtain a 3D imaging system.
This paper reviews advances in sub-THz photonic frequency conversion using optoelectronic transistors for future fully coherent access network systems. Graphene-channel field effect transistors (G-FETs) and InP-based high electron mobility transistors (inP-HEMT) are experimentally examined as photonic frequency converters. Optoelectronic properties and three-terminal functionalities of the G-FETs and InP-HEMTs are exploited to perform single-chip photonic double-mixing operation over the 120 GHz wireless communication band. A single transistor can photomix the optical subcarriers to generate LO and mix down the RF data on the sub-THz carrier to the IF band.
In this paper we discuss a study and application of the optical properties of liquids in the terahertz range and a possible technique for identifying liquids. The optical properties are the reflection and transmission of the radiation at various interfaces between known media and between a known medium and the unknown medium and the complex refractive index which is to be determined from measurements of the change in radiation signal after multiple optical interactions inside a silicon prism. For this purpose we use an attenuated total reflection experimental arrangement. We show that one can identify liquids placed on the top of the prism turned upside down using this method. Terahertz waves are especially useful in this context because they allow the determination of the liquid content of containers that are difficult to open or that simply cannot be opened, such as luggage in airports. Some explosives are liquids and the identification of the liquids using terahertz waves may help detecting hidden explosives.
We report on active imaging with CMOS transistors at 300 GHz and 1.05 THz. Two basic focal plane arrays consisting
of nMOS transistors and wide-band bow-tie antennas have been implemented in a low-cost 130 nm CMOS technology.
Raster scan imaging of objects concealed in a paper envelope has been achieved at 300 GHz with a commercial radiation
source. The images clearly reveal the concealed objects with a dynamic range of 35 dB and a resolution of 3 mm. At
1.05 THz, the pixels achieve a responsivity of 50 V/W and a noise equivalent power of 900 pW/Hz0.5.
Recent progress in the field of terahertz (THz) imaging is overviewed. First, various THz-wave sources developed and recently improved in our group are described. Second, imaging of samples can be achieved in different modes, of which we discuss here the transmission mode and the reflection-scattering mode. An emphasis in placed on the latter, which can be used to detect and determine for example the distribution of powders inside THz-transparent containers and packages. One-frequency or wide-spectrum imaging can be extended to chemical imaging, a technique by which images acquired at different THz frequencies can be combined to allow the identification of the chemical composition of the target at each spatial position. Other THz imaging applications are also discussed.
A method to detect production faults in plastic packages using terahertz (THz) radiation is presented. Relying on the large difference between the absorption coefficients of plastic and water (for water-filled defects), and on the refraction index difference between plastic and air (for air-filled defects), our technique consists of focusing and scanning a terahertz beam on the sealed area of the package, followed by the detection of the transmitted signal. Compared to previous methods such as visual and ultrasound inspection, our technique can be applied for optically opaque packages and does not require immersion in a matching liquid. We tested our terahertz system on defects simulated by water-filled and air-filled round channels imbedded in polyethylene films, with diameters ranging from 10 to 100 µm. The results show that detection is possible down to 30 µm for water-filled and 40 µm for air-filled channels. The results are the same for both transparent and opaque packages.
Our paper provides analytical expressions for the statistical errors related to statistical processing of digitally recorded Newton's rings interference patterns by least squares fitting. These results completes some of our previous papers concerned with Newton's rings fringe patterns processing, which well describe an iterative numerical algorithm that we commonly use for fringe processing.
Our paper concerns with statistical processing of digitally recorded straight equispaced fringe patterns. We determine the highest degree of accuracy that can be achieved in estimating fringe parameters by statistical processing in given statistical fluctuation conditions affecting the recorded image.
Our paper concerns with statistical processing of digitally recorded straight equispaced fringe patterns by a numerical method based on discrete Fourier transforming (DFT) of the input data, which has the advantage of faster computation than the usual least square fitting method, that we have presented in a previous paper. This new method leads to the same accuracy as the least square fitting method and it is more convenient to use for processing fringe pattern with high harmonic order features.
A numeric algorithm for processing Newton's rings fringe patterns is presented. The interference images of this type have a characteristic appearance which can be described mathematically by a function depending on a set of parameters. The algorithm consists in finding the parameters of this mathematical expression by means of fitting the pattern using the least squares method, specially implemented with an iterative procedure. Unlike other processing methods which also use statistical calculus, this algorithm efficiently utilizes the whole information contained in the image and ensures the highest degree of accuracy, in given statistical fluctuation conditions affecting the image.
The phenomenon of phase retardation in the waist region of a gaussian beam (known as the Guoy effect) is described and demonstrated in the visible wavelength range. Two gaussian beams, originating from the same laser, are made to interfere in a region around the waist of one of them and far from the waist of the other. The relative phase is measured by processing the interference patterns recorded at different locations on the axis. A comparison with the theoretical results is carried out.
Wavelength measurement is a critical topic in many applications. Stationary interferometers, such as Fizeau and Murty, can be successfully used, considering the proportionality between the wavelength and the fringe spacing in the interference pattern.In this work we present a 1D algorithm for the calculation of the fringe spacing and error sources. The final accuracy that can be achieved is also assessed. The experimental data are taken from fringe pattern recorded with a Murty interferometer.
Speckle interferometry for non destructive testing of out of plane or in-plane stresses or deformations of rough mechanical parts is a powerful and modern technique. Basics of speckle phenomena and interferometry in specked light are reviewed. Electronic speckle pattern interferometry for vibration analysis and a Duffy-Young digital camera for in- plane measurement are presented.
A new method of processing Newton's rings fringe patterns is presented. After the center of the circular rings is found, a special type of pixel intensity spectrum is calculated, in which the 2D pattern is reduced to a 1D profile showing a periodic structure of fringes. By further processing the parameters of the initial interferogram can be easily extracted. The statistical nature of the method leads to a higher accuracy and a better immunity to noise.
The method and experimental results of circularity measurements by optical triangulation are presented. The measuring system is a laser based one, using a bi-cell detector with rectangular active surface shape and diagonal gap. The measured data were fitted with a parametric function depending on the position of the tested object center, relative to the center of the rotating table. A resolution of 2.5 micrometers /mW is reported. This sensitivity is satisfactory for the majority of industrial applications-- circularity measurement and centering.
A new self-calibration principle for phase shift interferometry is introduced, involving a whole-field consideration of the information contained by the interferograms. The principle is illustrated for the three-sample case, which was previously known as not having a self-calibration capability. Three related self- calibrating algorithms for phase shift interferometry are proposed, all of them based on this principle. The input of the algorithms is the set of three interferograms, and the output consists in both the correct phase shifts and the phase map of the wavefront being analyzed. No information on the actual phase shifts has to be supplied.
An interferometric set-up able to measure angles as large as +180 degree(s) is presented. The principle of the method is to measure a linear displacement (translation) produced by a crank-gear mechanism which converts the angular movement of a rotating table. The optical scheme and consideration on the accuracy of the method are presented.
The measurement of large angles is still an actual problem in metrology. Interferometric methods based on Michelson interferometer have high sensitivities, precision and accuracy but the angular range is limited. Two methods are presented: hollow roof prism on a rotating table and a plane-parallel plate in the interferometer's arm, as systems for angular measurements.