RESUMEN
A photonics-based scheme is proposed to generate wideband linear frequency modulation pulses with broadly tunable carrier frequencies for coherent radars. The approach integrates the concept of the microwave-photonic multiplication and coherent beating to enable reconfiguration of the bandwidth and carrier frequency of the generated pulses. The phase fluctuation between two beating arms is suppressed by a stabilization technique based on an optical phase-locked loop to maintain the pulse-to-pulse phase coherence. Further, we also demonstrate a coherent radar system, including the generation, wireless transmission and detection of the radar echo signal in the Ka-band. The coherent integration of several echo signals is achieved. About 8 dB of signal-to-noise ratio improvement is obtained with every ten-fold of integration times. The range resolution of the radar system is â¼3.75 cm, which is close to the theoretical prediction.
RESUMEN
A photonics-based scheme is presented for generating wideband and phase-stable chirped microwave signals based on two phase-locked combs with fixed and agile repetition rates. By tuning the difference of the two combs' repetition rates and extracting different order comb tones, a wideband linearly frequency-chirped microwave signal with flexible carrier frequency and chirped range is obtained. Owing to the scheme of dual-heterodyne phase transfer and phase-locked loop, extrinsic phase drift and noise induced by the separated optical paths is detected and suppressed efficiently. Linearly frequency-chirped microwave signals from 5 to 15 GHz and 237 to 247 GHz with 30 ms duration are achieved, respectively, contributing to the time-bandwidth product of 3×108. And less than 1.3×10-5 linearity errors (RMS) are also obtained.
RESUMEN
We proposed and experimentally demonstrated a short-delayed self-heterodyne method with 15.5m delay to get a large-frequency-range laser frequency-noise spectrum over 10Hz to 50 MHz, and an averaging approach to extract the intrinsic frequency noise of a frequency-swept laser. With these two techniques, dynamic frequency-noise spectrum of a frequency-swept DFB laser when free running and servo-controlled are both measured. This measurement method permits accurate and insightful investigation of laser stability.
RESUMEN
We proposed a precise and simple method to estimate the laser linewidth from its frequency power spectral density, which is termed as power-area method (PAM). We applied this method to determine the full width at half-maximum (FWHM) of white-frequency noise and flicker-frequency noise, and the error was less than 7%. Then we successfully estimated the FWHM of the beat note of delayed self-homodyne/heterodyne interferometry with this method. Lastly we investigated the selection of loop gain and loop bandwidth using PAM to achieve a better result in linewidth compression with servo-loop control.
RESUMEN
Serial assessment of the biomechanical properties of tissues can be used to aid the early detection and management of pathophysiological conditions, to track the evolution of lesions and to evaluate the progress of rehabilitation. However, current methods are invasive, can be used only for short-term measurements, or have insufficient penetration depth or spatial resolution. Here we describe a stretchable ultrasonic array for performing serial non-invasive elastographic measurements of tissues up to 4 cm beneath the skin at a spatial resolution of 0.5 mm. The array conforms to human skin and acoustically couples with it, allowing for accurate elastographic imaging, which we validated via magnetic resonance elastography. We used the device to map three-dimensional distributions of the Young's modulus of tissues ex vivo, to detect microstructural damage in the muscles of volunteers before the onset of soreness and to monitor the dynamic recovery process of muscle injuries during physiotherapies. The technology may facilitate the diagnosis and treatment of diseases affecting tissue biomechanics.