High-resolution Frequency-modulated Continuous-wave Research Articles

Long distance high resolution FMCW laser ranging with phase noise compensation and 2D signal processing.

A long distance high resolution frequency-modulated continuous wave (FMCW) laser rangefinder with phase noise compensation and two-dimensional (2D) data processing skills is developed. Range-finding ladar consists of a continuously chirped laser source, an auxiliary reference interferometer, and a monostatic optical transceiver for target illumination and return photon collection. To extend the range unambiguity and lower the electronic processing bandwidth, a two-step laser frequency chirping scheme is adopted, where a long pulse width, small frequency bandwidth laser chirping signal are used in step 1 for coarse distance estimation, and a short pulse width and large frequency bandwidth laser chirping signal are applied afterwards for step 2 high resolution distance realization. An auxiliary reference interferometer is to record the phase noise originated from the laser source to compensate for phase errors induced in the target return photons. The 2D data processing skill helps to coherently sum up all the phase noise removed echo photons to achieve high resolution range peak extraction with high detection sensitivity. Experimental demonstration shows that the proposed FMCW ladar at 1550 nm wavelength with a laser chirping bandwidth of 10 GHz and electronic processing bandwidth of 200 MHz can measure a corner cube test target in an outdoor atmospheric environment, and the measurement results are 12013.905 m with a 2.4 cm range resolution under strong return photon levels and 12013.920 m with a 2.5 cm range resolution under weak return photon levels.

Comparison of Signal Processing Algorithms for High-Resolution FMCW Terahertz Thickness Measurements

When using the frequency-modulated continuous-wave (FMCW) approach for layer thickness determination of dielectrics, such as multilayer pipe walls, the resolution is usually limited by the bandwidth. For thinner layers, we compare the measurement signal with potential model signal outputs. Since our approach results in high computation efforts by using brute force optimization, spectral estimation may present an alternative for high resolution distance and hence thickness determination. In this contribution, we compare results of a parametric algorithm (modified covariance method) and a subspace method (multiple signal classification, MUSIC) with the results of our approach.

A Scalable 79-GHz Radar Platform Based on Single-Channel Transceivers

This paper presents a scalable E-band radar platform based on single-channel fully integrated transceivers (TRX) manufactured using 130-nm silicon–germanium (SiGe) BiCMOS technology. The TRX is suitable for flexible radar systems exploiting massive multiple-input-multiple-output (MIMO) techniques for multidimensional sensing. A fully integrated fractional-N phase-locked loop (PLL) comprising a 39.5-GHz voltage-controlled oscillator is used to generate wideband frequency-modulated continuous-wave (FMCW) chirp for E-band radar front ends. The TRX is equipped with a vector modulator (VM) for high-speed carrier modulation and beam-forming techniques. A single TRX achieves 19.2-dBm maximum output power and 27.5-dB total conversion gain with input-referred 1-dB compression point of −10 dBm. It consumes 220 mA from 3.3-V supply and occupies $3.96~\mathrm {mm^{2}}$ silicon area. A two-channel radar platform based on full-custom TRXs and PLL was fabricated to demonstrate high-precision and high-resolution FMCW sensing. The radar enables up to 10-GHz frequency ramp generation in 74–84-GHz range, which results in 1.5-cm spatial resolution. Due to high output power, thus high signal-to-noise ratio (SNR), a ranging precision of 7.5 $\mu \text{m}$ for a target at 2 m was achieved. The proposed architecture supports scalable multichannel applications for automotive FMCW using a single local oscillator (LO).

Calibration of High-Resolution Synthetic Aperture Radar (SAR) Image Features by Ground-Based Experiments

ABSTRACTResults of recent experiments reported in the literature with frequency-modulated continuous wave (FMCW) mode of synthetic aperture radar (SAR) imaging are enabling high-resolution, day and night remote sensing of ground terrain. Also, FMCW-based SAR techniques are accepted for ground-based SAR (GB-SAR) instrumentation for early warning of fragile slopes such as in coal mines, avalanche detection. Features of images generated by FMCW SAR sensors need to be calibrated for interpretation purposes as like other forms of microwave imaging. In this paper, we demonstrate a ground-based method to calibrate high-resolution FMCW SAR image features by experimenting with a long railSAR testbed. Signal processing algorithms are derived to support high-resolution SAR image formation according to the requirement of the experiment. Trihedral corner reflectors (CRs) of submeter dimensions are placed on terrains with different normalized radar cross-section (RCS) coefficient (σ0) to measure backscattering properties from targets and surrounding clutter from different heights and varied look angles. Calibration of the features of CRs is performed with respect to their physical dimensions by measuring the impulse response functions and spatial resolution in the SAR images. Target RCS and speckle properties derived from single-look and multilook processing of experimental data closely follows target and scene parameters such as radiometric and geometric measures of RCS of the CRs and σ0 of surroundings such as tar road and concrete road. By this experiment the features of high-resolution FMCW SAR images are measured and calibrated without flying the radar sensor on airborne platform.

Research on the fiber dispersion and compensation in large-scale high-resolution broadband frequency-modulated continuous wave laser measurement system

The influence of a fiber dispersion calibration interferometer on the measurement results for a large-scale high-resolution broadband frequency-modulated continuous wave (FMCW) measurement system was studied. A model was constructed to simulate the influences of fiber dispersion on the measurements when using a frequency sampling method that corrects the tuning nonlinearity. The results indicated that a broadband external cavity tunable laser, in comparison with a semiconductor laser, causes linear variations in the measurement results because of the effect of the fiber dispersion in the calibration interference path for large-scale high-resolution measurements, and these variations decreased the resolution of the measurements. A method that combines chirp slope calibration and phase compensation to reduce the effects of the fiber dispersion was proposed. A gauge block with a height difference of 200 μ m at a distance of 2.43 m was measured during the experiments. Before calibrating the fiber dispersion, the frequency spectrum showed false peaks, and it was difficult to distinguish the peaks of the targets. After compensating for the dispersion, the peaks of the targets could be clearly distinguished, and a height difference of 199.6 μ m was measured. Using this model and the method to compensate for the dispersion will provide a reference for large-scale high-resolution broadband FMCW laser measurements.

Multi-dimensional, non-contact metrology using trilateration and high resolution FMCW ladar.

Here we propose, describe, and provide experimental proof-of-concept demonstrations of a multidimensional, non-contact-length metrology system design based on high resolution (millimeter to sub-100 micron) frequency modulated continuous wave (FMCW) ladar and trilateration based on length measurements from multiple, optical fiber-connected transmitters. With an accurate FMCW ladar source, the trilateration-based design provides 3D resolution inherently independent of standoff range and allows self-calibration to provide flexible setup of a field system. A proof-of-concept experimental demonstration was performed using a highly stabilized, 2THz bandwidth chirped laser source, two emitters, and one scanning emitter/receiver providing 1D surface profiles (2D metrology) of diffuse targets. The measured coordinate precision of <200 microns was determined to be limited by laser speckle issues caused by diffuse scattering of the targets.

High-Resolution FMCW Reflectometry Using a Single-Mode Vertical-Cavity Surface-Emitting Laser

High-resolution frequency-modulated continuous-wave (FMCW) reflectometry is realized by using a single-mode vertical-cavity surface-emitting laser (VCSEL) as a frequency-swept light source for the first time. The optical frequency of the VCSEL is swept by the injection current modulation. The experimental spatial resolution of 250 μm was achieved, which is the best value when an injection current tuned laser diode is used as a frequency-swept light source.

Sub-surface melting in a seasonal snow cover

AbstractThe ability of solar radiation to penetrate into a snow cover combined with the low thermal conductivity of snow can lead to a sub-surface temperature maximum. This elevated sub-surface temperature allows a layer of wet snow to form below the surface even on days when the air temperature remains sub-freezing. A high-resolution frequency-modulated continuous wave (FMCW) radar has been used to detect the onset of sub-surface melting in a seasonal snow cover. The experimental observation of sub-surface melting is shown to be in good agreement with the predictions of a one-dimensional mass- and energy-balance model. The effects of varying snow characteristics and solar extinction parameters on the sub-surface melt characteristics are investigated using model simulations.

Sub-surface melting in a seasonal snow cover

AbstractThe ability of solar radiation to penetrate into a snow cover combined with the low thermal conductivity of snow can lead to a sub-surface temperature maximum. This elevated sub-surface temperature allows a layer of wet snow to form below the surface even on days when the air temperature remains sub-freezing. A high-resolution frequency-modulated continuous wave (FMCW) radar has been used to detect the onset of sub-surface melting in a seasonal snow cover. The experimental observation of sub-surface melting is shown to be in good agreement with the predictions of a one-dimensional mass- and energy-balance model. The effects of varying snow characteristics and solar extinction parameters on the sub-surface melt characteristics are investigated using model simulations.