A Novel System for Quasi-Continuous THz Signal Transmission and Reception.

This paper presents a novel system for generating and receiving quasi-continuous (QC) TeraHertz (THz) waves. A system design and theoretical foundation for QC-THz signal generation are presented. The proposed QC-THz system consists of commercially available photo-conductive antennas used for transmission and reception of THz waves and a custom-designed QC optical signal generator, which is based on a fast optical frequency sweep of a single telecom distributed-feedback laser diode and unbalanced optical fiber Michelson interferometer used for a high-frequency modulation. The theoretical model for the proposed system is presented and experimentally evaluated. The experimental results were compared to the state-of-the-art continuous-wave THz system. The comparison between the continuous-wave THz system and the proposed QC-THz system showed the ability to transmit and receive QC-THz waves up to 300 GHz. The upper-frequency limit is bounded by the length of the used Michelson interferometer. The presented design of THz signal generation has a potential for industrial application because it is cost-efficient and can be built using commercially available components.

SFCW Radar with an Integrated Static Target Echo Cancellation System.

Continuous Wave (CW) radars systems, especially air-coupled Ground-Penetrating Radar (GPR) or Through-Wall Imaging Radar (TWIR) systems, echo signals reflected from a stationary target with high energy, which may cause receiver saturation. Another effect caused by reflection of stationary targets is noticeable as background within a radargram. Nowadays, radar systems use automatic gain control to prevent receiver saturation. This paper proposes a method to remove stationary targets automatically from the received signal. The method was designed for a radar system with a moving platform, with an assumption that the distance between the surface and target is constant. The design is proposed of an SFCW radar with an integrated system for real-time multiple static target Echo Cancellation (EC). The proposed EC system removes the static target using active Integrated Circuit (IC) components, which generate the corresponding EC signal for each frequency step of the SFCW radar and sum it with the received echo signal. This has the main advantage of removing even multiple echoes at any distance, and excludes the need for a high-dynamic-range receiver. Additionally, the proposed system has minimal impact on the radar size and power consumption. Besides static target removal, the antenna coupling can be removed if the signal appears to be constant. The operating frequency was selected between 500 MHz and 2.5 GHz, due to the limitation of the used electronic components. The experimental results show that the simulated target's echo using a cable with a known length could be suppressed to up to 38 dB. Experimental results using a moving radar platform and the real environment scenario with static and dynamic targets, show that the proposed EC system could achieve up to 20 dB attenuation of the static target. The system does not affect any other target of interest, which can even move at any distance during the measurement. Therefore, this could be a promising method for further compact implementation into SFCW radars, or any other radar type that generates CW single frequencies.

Automated Inorganic Pigment Classification in Plastic Material Using Terahertz Spectroscopy.

This paper presents an automatic classification of plastic material's inorganic pigment using terahertz spectroscopy and convolutional neural networks (CNN). The plastic materials were placed between the THz transmitter and receiver, and the acquired THz signals were classified using a supervised learning approach. A THz frequency band between 0.1-1.2 THz produced a one-dimensional (1D) vector that is almost impossible to classify directly using supervised learning. This paper proposes a novel pre-processing of 1D THz data that transforms 1D data into 2D data, which are processed efficiently using a convolutional neural network. The proposed pre-processing algorithm consists of four steps: peak detection, envelope extraction, and a down-sampling procedure. The last main step introduces the windowing with spectrum dilatation that reorders 1D data into 2D data that can be considered as an image. The spectrum dilation techniques ensure the classifier's robustness by suppressing measurement bias, reducing the complexity of the THz dataset with negligible loss of accuracy, and speeding up the network classification. The experimental results showed that the proposed approach achieved high accuracy using a CNN classifier, and outperforms 1D classification of THz data using support vector machine, naive Bayes, and other popular classification algorithms.

THz Signal Generator Using a Single DFB Laser Diode and the Unbalanced Optical Fiber Interferometer.

This paper presents a frequency-modulated optical signal generator in the THz band. The proposed method is based on a fast optical frequency sweep of a single narrowband laser diode used together with an optical fiber interferometer. The optical frequency sweep using a single laser diode is achieved by generating short current pulses with a high amplitude, which are driving the laser diode. Theoretical analysis showed that the modulation frequency could be changed by the optical path difference of the interferometer or optical frequency sweep rate of a laser diode. The efficiency of the optical signal generator with Michelson and Fabry-Perot interferometers is theoretically analyzed and experimentally evaluated for three different scenarios. Interferometers with different optical path differences and a fixed optical frequency sweep rate were used in the first scenario. Different optical frequency sweep rates and fixed optical path differences of the interferometers were used in the second scenario. This paper presents a method for optical chirp generation using a programmable current pulse waveform, which drives a laser diode to achieve nonlinear optical sweep with a fixed optical path difference of the interferometer. The experimental results showed that the proposed signals could be generated within a microwave (1-30 GHz) and THz band (0.1-0.3 THz).

Reply to Comments: A Lightweight and Low-Power UAV-Borne Ground Penetrating Radar Design for Landmine Detection.

In this brief note, we respond to the comments made by Dr [...].

Design of a Low-Cost Ultra-Wide-Band Radar Platform.

This paper proposes an improved design of a pulse-based radar. An improved design of a pulse generator is presented using step recovery diodes and a signal mixer for the received signal. Two-step recovery diodes produce pulses of 120 ps in duration. A pulse generator is improved by removing the negative power supply, resulting in a reduced number of electronic pulses. A sampling mixer at the receiver's site receives the generated signal and stretches it from picoseconds into microseconds. The improved pulse generator is also used in the sampling mixer as a strobe pulse generator, which makes the sampling mixer much simpler. The stretched signal is then sampled by a low sample rate using an analog to digital converter. The proposed radar design achieves up to 8 GHz bandwidth and an equivalent receiving sample rate of about 100 GSa/s. The radar is controlled using a software-defined radio called Red Pitaya, which is also used for data acquisition. The proposed radar design uses widely available commercial components, which makes radar design widely available with low cost implementation.

A Lightweight and Low-Power UAV-Borne Ground Penetrating Radar Design for Landmine Detection.

This paper presents the development of a lightweight and low-power Ground Penetrating Radar (GPR) to detect buried landmines in harsh terrain, using an Unmanned Aerial Vehicle (UAV). Despite the fact that GPR airborne systems have been already used for a while, there has yet been no focus on the UAV autonomy, which depends on the payload itself. Therefore, the contribution of this work is the introduction of a lightweight and low-power consumption GPR system, which is based on the Stepped Frequency Continuous Wave (SFCW) radar principle. The Radio Frequency (RF) transceiver represents an improved implementation of the super-heterodyne architecture, which currently offers higher sensitivity. This is achieved by combining analog and digital processing techniques. The experimental results showed that the developed system can detect both metallic and plastic buried targets. Target detection with a scanning height up to about 0.5 m shows good applicability in an unstructured, harsh environment, which is typical of mined terrain. The proposed system still needs some improvements for a fully operational system regarding different aspects of scanning speeds and soil properties such as moisture content.

All-fiber, thermo-optic liquid level sensor.

This paper proposes an all-optical-fiber sensor for continuous measurements of liquid levels. The proposed sensor utilizes an optically absorbing vanadium doped optical fiber, which is configured as a long-gauge, optically-heated, fiber-optic, Fabry-Perot interferometer that is immersed into the measured liquid. The sensor is excited cyclically by a medium-power 980 nm optical source, which induces periodic temperature variation and, consequently, optical path length modulation within the vanadium doped fiber. The amplitude of this path length variation depends on the liquid level and is measured by an interferometric approach. The relation between the liquid level and the amplitude of optical path length modulation caused by the fiber's temperature variation were investigated analytically, and the theoretical model proved to be in good agreement with the experimental results. Two versions of level sensors are demonstrated experimentally, the first with single-side optical heating power delivery and 0.45 m measurement range, and the second with dual-side power delivery and 1 m of operational measurement span. Experimental measurement level resolutions achieved for 0.45 m and 1m operational measurement span were approximately 2 and 3 mm, respectively. The simple and efficient design of sensor and signal interrogation system, the latter is based solely on a few widely available telecom components, provides straightforward opportunities for use of the proposed system in a variety of industrial applications.

Distance estimation using RSSI and particle filter.

This paper presents a particle filter algorithm for distance estimation using multiple antennas on the receiver's side and only one transmitter, where a received signal strength indicator (RSSI) of radio frequency was used. Two different placements of antennas were considered (parallel and circular). The physical layer of IEEE standard 802.15.4 was used for communication between transmitter and receiver. The distance was estimated as the hidden state of a stochastic system and therefore a particle filter was implemented. The RSSI acquisitions were used for the computation of important weights within the particle filter algorithm. The weighted particles were re-sampled in order to ensure proper distribution and density. Log-normal and ground reflection propagation models were used for the modeling of a prior distribution within a Bayesian inference.

Wavelet-based SAR image despeckling and information extraction, using particle filter.

This paper proposes a new-wavelet-based synthetic aperture radar (SAR) image despeckling algorithm using the sequential Monte Carlo method. A model-based Bayesian approach is proposed. This paper presents two methods for SAR image despeckling. The first method, called WGGPF, models a prior with Generalized Gaussian (GG) probability density function (pdf) and the second method, called WGMPF, models prior with a Generalized Gaussian Markov random field (GGMRF). The likelihood pdf is modeled using a Gaussian pdf. The GGMRF model is used because it enables texture parameter estimation. The prior is modeled using GG pdf, when texture parameters are not needed. A particle filter is used for drawing particles from the prior for different shape parameters of GG pdf. When the GGMRF prior is used, the particles are drawn from prior in order to estimate noise-free wavelet coefficients and for those coefficients the texture parameter is changed in order to obtain the best textural parameters. The texture parameters are changed for a predefined set of shape parameters of GGMRF. The particles with the highest weights represents the final noise-free estimate with corresponding textural parameters. The despeckling algorithms are compared with the state-of-the-art methods using synthetic and real SAR data. The experimental results show that the proposed despeckling algorithms efficiently remove noise and proposed methods are comparable with the state-of-the-art methods regarding objective measurements. The proposed WGMPF preserves textures of the real, high-resolution SAR images well.