Frequency-modulated Continuous-wave Reflectometry Research Articles

Theoretical and Experimental Investigation of the Effect of Pump Laser Frequency Fluctuations on Signal-to-Noise Ratio of Brillouin Dynamic Grating Measurement with Coherent FMCW Reflectometry.

Signal-dependent speckle-like noise has constituted a serious factor in Brillouin-grating based frequency-modulated continuous-wave (FMCW) reflectometry and it has been indispensable for improving the signal-to-noise ratio (S/N) of the Brillouin dynamic grating measurement to clarify the noise generation mechanism. In this paper we show theoretically and experimentally that the noise is generated by the frequency fluctuations of the pump light from a laser diode (LD). We could increase the S/N from 36 to 190 merely by driving the LD using a current source with reduced technical noise. On the basis of our experimental result, we derived the theoretical formula for S/N as a function of distance, which contained the second and fourth-order moments of the frequency fluctuations, by assuming that the pump light frequency was modulated by the technical noise. We calculated S/N along the 1.35 m long optical fiber numerically using the measured power spectral density of the frequency fluctuations, and the resulting distributions agreed with the measured values in the 10 to 190 range. Since higher performance levels are required if the pump light source is to maintain the S/N as the fiber length increases, we can use the formula to calculate the light source specifications including the spectral width and rms value of the frequency fluctuations to achieve a high S/N while testing a fiber of a given length.

Auxiliary interferometer in an optoelectronic swept-frequency laser and its application to the measurement of the group refractive index.

An optoelectronic swept-frequency laser (SFL) is an optoelectronic feedback system that includes an auxiliary interferometer that can exert precise control over the optical frequency sweep. The arm-length difference (ALD) of the auxiliary interferometer directly affects the performance of the whole system. We established a theoretical model to choose the optimal ALD of an auxiliary interferometer in an optoelectronic SFL system using a frequency-modulated continuous-wave reflectometry experimental setup. The experimental results indicated that, based on our system, the optimal ALD was 7 m, which agreed with the theoretical analysis. As an example application, we implemented the proposed system for measurement of the group refractive index of a glass sample. A minimum measurement error of 0.12% was obtained with the ALD of 7 m.

Guided Reflectometry Imaging Unit Using Millimeter Wave FMCW Radars

Frequency-modulated continuous wave (FMCW) radar systems in the millimeter and submillimeter range are technologically mature for many applicative fields such as automotive and aerospace industries for imaging and nondestructive testing. This work reports on a new implementation of a guided FMCW radar reflectometry unit for sensing and imaging applications. Only a terahertz dielectric waveguide is used for signal transmission between the transceiver module and the sample, thus drastically simplifying the experimental setup. Compared to continuous wave guided systems, one of the main advantages granted by the use of FMCW radars in combination with waveguides, is the differentiation capability between the reflected signals generated along the wave guide as parasitic signals or at its probing end as sensing information and therefore improving the expected signal-to-noise ratio. This innovative approach is demonstrated by using a dielectric hollow-core waveguide integrated with two different radar transceivers; the high-performance, III–V based 100 GHz SynView unit as a reference system and a compact, low-cost, printed circuit board (PCB)-integrated, 122 GHz transceiver developed by silicon-radar GmbH. Both three-dimensional electromagnetic simulations and raster scans are performed to investigate quantitatively the propagation behaviors including the coupling capabilities, dynamic range limitations, beam profile, and induced artefacts of the guided FMCW reflectometry system. The feasibility of a simplified guided terahertz FMCW reflectometry probing unit is proven. The integration of a solid immersion lens at the end of the waveguide is also demonstrated for imaging resolution improvement.

Beat Noise Reduction by Fast Optical Switching in Brillouin Grating-Based FMCW Reflectometry

We demonstrated an optical switching method for deriving a Brillouin reflectogram from a 1-m-long optical fiber without the use of a polarization diversity technique or a least-squares procedure in coherent frequency-modulated continuous wave reflectometry. We reduced the intrinsic beat noise by blocking one pump light from entering the balanced mixer for $120~\mu \text{s}$ with an acousto-optic modulator while sweeping the laser wavelength at a rate of 0.5 nm/s. The variations in the derived reflectograms at the center of the Brillouin spectrum were within ±14 %, and these were governed mainly by the multiplicative noise.

Design of Q-band FMCW reflectometry for electron density profile measurement on the Joint TEXT tokamak

The Q-band (33–50 GHz) fast sweep frequency modulated continuous wave (FMCW) reflectometry has been recently developed for electron density profile measurement on the Joint TEXT tokamak. It operates in ordinary mode (O-mode) with a 20 μs sweeping period, covering the density range from 1 × 1019 m−3 to 3 × 1019 m−3. On the bench test, a Yttrium Iron Garnet (YIG) filter is used for the dynamic calibration of the voltage controlled oscillator (VCO) to obtain a linear frequency sweep. Besides, the use of a power combiner helps to improve the side-band suppression level of the single side-band modulator (SSBM). The reconstructed density profiles are presented, which demonstrate the capability of the reflectometry.

Coherent frequency-modulated continuous wave reflectometry for measuring stationary Brillouin grating induced under uniform pumping by counterpropagating nonmodulated light waves.

We describe theoretically and experimentally how valuable information on the distributed Brillouin spectra of an optical waveguide is derived from the stationary Brillouin grating measurement under uniform pumping over the waveguide by using the coherent frequency-modulated continuous wave reflectometry. We upconvert the frequencies of the probe and pumping light waves by the Brillouin frequency with one modulator and detect the Stokes light in the same way that we detect the Fresnel and Rayleigh backreflections in the fiber. The intrinsic coherent spike is reduced by using the lock-in detection and the least squares method to reveal the distributed Brillouin spectra of a short optical fiber consisting of two different fibers spliced together.

Coherence enhancement of a chirped DFB laser for frequency-modulated continuous-wave reflectometry using a composite feedback loop.

We demonstrate efficient coherence enhancement of a chirped distributed feedback (DFB) laser for frequency-modulated continuous-wave (FMCW) reflectometry. Both sweep nonlinearity and broadband stochastic frequency noises during the laser chirp are efficiently suppressed by a composite feedback loop. The residual frequency error relative to a perfect linear chirp is shown to be about 89 kHz for a laser chirp of 50 GHz in 100 ms, compared with 44 MHz with the loop open. The broadband frequency noise suppression of the frequency-swept laser greatly improves its coherence, leading to a higher signal-to-noise ratio and a significantly extended measurement range in FMCW reflectometry ranging. We demonstrate a 2 mm transform-limited spatial resolution at a range window of 50 m and a 17.5 cm spatial resolution at an extended measurement range of 750 m, which is about 15 times the intrinsic laser round-trip coherence length.

Time-frequency analysis for microwave reflectometry data processing in the HL-2A tokamak

The Choi-Williams distribution (CWD) technique is introduced as a time-frequency tool for processing data measured from the new developed homodyne and the fixed frequency reflectometry in the HL-2A tokamak. The comparison between spectrogram and CWD for the simulated signal is presented. It indicates that the CWD can greatly improve the representation of the time-frequency content of the multi-components signal. Its effectiveness is demonstrated through two applications in HL-2A, which are the extraction of beat frequencies from the frequency modulated-continuous wave reflectometry (FM-CW) and the characterizing of the fluctuations. The density profile inversed from the group delay of the FM-CW and the density fluctuations deduced from the fixed-frequency reflectometry would be more reliable and accurate by using the CWD technique.

Ultrafast millimeter-wave frequency-modulated continuous-wave reflectometry for NSTX

The millimeter-wave frequency-modulated continuous-wave (FM-CW) reflectometer on NSTX is a multichannel system providing electron density profile measurements with a frequency coverage of 13–53GHz [corresponding O-mode density range of (0.21–3.5)×1013cm−3]. Recently, this system has been modified to allow ultrafast full-band sweeps for repetition intervals down to 10μs. For this system to function as a fluctuation diagnostic it is crucial to eliminate artifacts in the phase derivative caused by nonlinearities in the frequency sweep; we introduce a simple hardware technique for reducing these artifacts to ≃0.3%. For NSTX, the additional bandwidth (⩽100kHz) greatly enhances the capability of the FM-CW reflectometer as a diagnostic for low frequency magnetohydrodynamics instabilities (e.g., internal kinks, resistive wall modes, neoclassical tearing modes, as well as fast-particle driven fishbones and low frequency toroidal Alfvén eigenmodes).

Automatic profile reconstruction for millimeter-wave frequency-modulated continuous-wave reflectometry on NSTX

UCLA operates a set of millimeter-wave/microwave reflectometers on the National Spherical Torus Experiment (NSTX) for routine measurements of electron density profiles and fluctuations. The system has a combined frequency coverage of 12 to 50 GHz (in the bands 12–18, 20–32, and 33–50 GHz) or a corresponding ordinary-mode cutoff range of 1.8×1012 to 3.1×1013 cm−3 to cover both the plasma core and edge. Profile measurements via frequency-modulated continuous-wave operation are typically made in O-mode reflectometry, with sweep times down to 50 μs over the full band. Automated profile analysis of the reflectometry data is available with limited between-shot analysis and full batch analysis capabilities. The reconstruction algorithm uses complex demodulation with the short-time Fourier transform for signal processing. The unknown portion of the edge profile below the lowest cutoff density is modeled by fitting a family of polynomial density profiles to the experimental data. Uncertainties due to edge profile modeling and comparisons to Thomson scattering measurements are discussed. The reconstructed profiles have documented fast events such as L–H transitions and edge-localized modes in NSTX.

Laboratory experiments with an FM-CW reflectometry system proposed for detecting and monitoring bridge scour in real time

Thousands of bridges throughout the United States have been identified as being scour critical (i.e., susceptible to failure from pier and (or) abutment scour). Scour occurs during times of rapid river flow when sediments, including rocks, gravel, and silt, are transported by the currents, undermining bridge pier foundations and similar structures. It can be increased by the presence of an ice cover. The scour process is dynamic; erosion and deposition can occur during the same high-energy river event, so the worst-case and the net effect cannot be easily predicted or dynamically monitored using previously available equipment. Herein, a technique and system (U.S. Patent #5,790,471) employing frequency modulated - continuous wave (FM-CW) reflectometry are discussed. This system is proposed for continuous monitoring of the extent of scour around riverine structures. A bench scale version of the system with a 490-MHz linearly swept bandwidth was implemented and tested in the laboratory, where sediments were incrementally added to a water-filled plastic barrel containing an 86-cm-long scour probe. Reflectometer data were taken after each increment of sediment was added. The data indicating the sediment boundary were plotted in a waterfall format that clearly shows the progressive sedimentation. This system has the potential for continuous round-the-clock operation and accuracy to within 5 cm of sediment depth. Key words: reflectometry, real-time monitoring, frequency modulated continuous wave, scour, sediment transport.

A study on degradation of spatial resolution in frequency‐modulated continuous wave reflectometry due to nonlinear optical frequency sweep

AbstractThe degradation of the spatial resolution in frequency‐modulated continuous wave (FMCW) reflec‐tometry due to the nonlinearity in the optical frequency sweep are discussed both experimentally and theoretically. The response lag in the optical frequency change of a laser diode behind the injection current change was experimentally measured, showing that there were several time constants in the optical frequency response. The interference signals in the FMCW reflectometry were theoretically calculated by taking account of the response lag. From these calculations, the beat frequency becomes low just after the turning point of the optical frequency sweep; this is due to the response lag in the optical frequency response. As a result, the spectrum of the interference signal is spread out toward the low frequency region and spatial resolution is degraded. Moreover, the spatial resolution is seriously degraded with increasing the diagnosing distance due to the nonlinearity in the optical frequency sweep. The calculated results agree well with the measured results.

Loss measurement in optical waveguide devices by coherent frequency-modulated continuous-wave reflectometry

We describe experimental results of loss measurement in optical waveguide devices by coherent frequency-modulated continuous-wave reflectometry composed of a frequency-swept semiconductor laser and a two-beam interferometer. The backscattered light in a potassium-ion-exchanged single-mode optical waveguide is successfully detected, and the loss along the optical waveguide is evaluated to be 1.0 dB/cm.

Coherent FMCW reflectometry using a temperature tuned Nd:YAG ring laser

A long-coherence-length, diode-pumped, monolithic Nd:YAG laser operating at 1.32 mu m is used to perform coherent FMCW (frequency-modulated continuous wave) reflectometry measurements. Compared with semiconductor-based lasers, the low phase noise of the Nd:YAG source offers greatly increased distance range combined with increased sensitivity to optical reflections. Measurement of Rayleigh backscatter from 50 km of fiber is demonstrated with over 60 dB of two-way optical dynamic range. Two-point spatial resolution is demonstrated to be better than 10 cm. This source and coherent measurement technique may have potential for both short and long-haul reflectometry applications. >