Ultrasensitive fiber sensor with enhanced Vernier effect for simultaneous measurements of transverse load and temperature.

Based on enhanced Vernier effect, a compact fiber sensor with ultrahigh sensitivity is proposed for simultaneous transverse load (TL) and temperature measurements. A single mode fiber (SMF) is spliced with a segment of hollow-core fiber (HCF) coated with polydimethylsiloxane (PDMS), some PDMS is injected into the HCF, forming a Vernier sensor with an air cavity adjacent to a PDMS cavity. It is shown that TL and temperature changes give rise to opposite and remarkable different variations in lengths of the two cavities, thereby enhancing Vernier effect and in favor of simultaneous measurements of TL and temperature. Moreover, the limited sensitivity magnification due to the length mismatch between the two cavities is compensated for by reconstructing the Vernier envelope with a broadened free spectrum range (FSR) from output signal. As a result, the highest TL sensitivity reported so far of -2637.47 nm/N and a good condition number of 69.056 for the sensitivity coefficient matrix have been achieved.

Influence of diffraction distance on image restoration in deep learning networks.

In recent years, significant advancements have been made in the field of computational imaging, particularly due to the application of deep learning methods to imaging problems. However, only a few studies related to deep learning have examined the impact of diffraction distance on image restoration. In this paper, the effect of diffraction distance on image restoration is investigated based on the PhysenNet neural network. A theoretical framework for diffraction images at various diffraction distances is provided along with the applicable propagators. In the experiment, the PhysenNet network is selected to train on diffraction images with different distances and the impact of using different propagators on network performance is studied. Optimal propagators required to recover images at different diffraction distances are determined. Insights obtained through these experiments can expand the scope of neural networks in computational imaging.

Compact Vernier sensor with an all-fiber reflective scheme for simultaneous measurements of temperature and strain.

We propose an all-fiber reflective sensing scheme to simultaneously measure temperature and strain. A length of polarization-maintaining fiber serves as the sensing element, and a piece of hollow-core fiber assists with introducing Vernier effect. Both theoretical deductions and simulative studies have demonstrated the feasibility of the proposed Vernier sensor. Experimental results have shown that the sensor can deliver sensitivities of -88.73 nm/°C and 1.61 nm/µÎµ for temperature and strain, respectively. Further, Both theoretical analyses and experimental results have suggested the capability of simultaneous measurement for such a sensor. Significantly, the proposed Vernier sensor not only presents high sensitivities, but also exhibits a simple structure, compact size and light weight, as well as demonstrates ease of fabrication and hence high repeatability, thus holding great promise for widespread applications in daily life and industry world.

Effect of the optical power factors on the laser-linewidth measurements.

In this paper, the effects of optical power factors like laser power, the powers of the laser beams in the two arms of the optical system, and the power of the photodetector on laser-linewidth measurements are studied. From the experiments, it can be found that when the average optical input power for the photodetector is about 50% of its linear saturation power, the measured laser line width is a minimum. When the optical powers of the laser beams in the two arms are equal in short-delay self-homodyne system, the measured laser line width is narrowest. In the low output power range of the laser, its line width decreases with the increase in optical power. By comparing experiments, it can also be clear that the conventional measurement method is seriously affected by different noise types, which causes the measured line width to become wider and not change even if the laser linewidth changes. However, based on the short-delay coherent envelope method, the measured coherent envelope changes significantly when the laser line width changes slightly, and its corresponding laser-linewidth values are also clearly visible. It confirms the low noise and high resolution of the short-delay self-homodyne coherent-envelope laser-measurement method. The outcomes of this study can provide helpful information for precision ultra-narrow laser-linewidth measurements.

Detection of Missing Insulator Caps Based on Machine Learning and Morphological Detection.

Missing insulator caps are the key focus of transmission line inspection work. Insulators with a missing cap will experience decreased insulation and mechanical strength and cause transmission line safety accidents. As missing insulator caps often occur in glass and porcelain insulators, this paper proposes a detection method for missing insulator caps in these materials. First, according to the grayscale and color characteristics of these insulators, similar characteristic regions of the insulators are extracted from inspection images, and candidate boxes are generated based on these characteristic regions. Second, the images captured by these boxes are input into the classifier composed of SVM (Support Vector Machine) to identify and locate the insulators. The accuracy, recall and average accuracy of the classifier are all higher than 90%. Finally, this paper proposes a processing method based on the insulator morphology to determine whether an insulator cap is missing. The proposed method can also detect the number of remaining insulators, which can help power supply enterprises to evaluate the degree of insulator damage.

Precise laser linewidth measurement by feature extraction with short-delay self-homodyne.

We propose a precision measurement method of laser linewidth based on short-delay self-homodyne, using the second peak-valley difference (SPVD) feature of the coherent power spectrum to fit laser linewidth. The SPVD model of the self-homodyne coherent envelope spectrum was established. One-to-one correspondence among the values of SPVD, the delay length, and the laser linewidth was determined theoretically and through simulations, while the reliability and stability of the method was verified experimentally. By comparing the detected results, it is found that the fitted laser linewidth obtained by the self-homodyne system is closer to its true value than that obtained by the self-heterodyne system. Hence, the simpler structure of the short-delay self-homodyne coherent envelope laser linewidth measurement method proposed is expected to substitute the previous laser linewidth measurement method, including complex short-delay self-heterodyne coherent envelope laser linewidth measurement method and traditional self-homodyne/heterodyne laser linewidth measurement method, to achieve more precise laser linewidth value.

Signal processing assisted Vernier effect in a single interferometer for sensitivity magnification.

The Vernier effect magnifies optical sensitivity by the superposition of two spectra with slightly shifted frequencies from a sensing interferometer (SIM) and a reference interferometer (RIM). In this study, we demonstrate that the Vernier effect can be obtained through a single interferometer, which detects the changed signal and provides an artificial reference spectrum (ARS) to be superposed with the changed signal spectrum. The ARS extracted by spatial frequency down-conversion of one sensing spectrum in the signal processing is not affected by environmental changes and can be detuned at an arbitrarily small amount with the measured signal spectrum. This approach is simpler and accurate and provides ultrahigh sensitivity. To validate the principle, a Mach-Zehnder (MZ) interferometer based on a dual-mode microfiber was designed for sensing the refractive index (RI) change magnification, and a high sensitivity of 71354.58 nm/refractive index unit (RIU) was obtained with good linearity.

Ultrasensitive refractive index sensor based on enhanced Vernier effect through cascaded fiber core-offset pairs.

An ultrasensitive refractive index (RI) sensor based on enhanced Vernier effect is proposed, which consists of two cascaded fiber core-offset pairs. One pair functions as a Mach-Zehnder interferometer (MZI), the other with larger core offset as a low-finesse Fabry-Perot interferometer (FPI). In traditional Vernier-effect based sensors, an interferometer insensitive to environment change is used as sensing reference. Here in the proposed sensor, interference fringes of the MZI and the FPI shift to opposite directions as ambient RI varies, and to the same direction as surrounding temperature changes. Thus, the envelope of superimposed fringe manifests enhanced Vernier effect for RI sensing while reduced Vernier effect for temperature change. As a result, an ultra-high RI sensitivity of -87261.06 nm/RIU is obtained near the RI of 1.33 with good linearity, while the temperature sensitivity is as low as 204.7 pm/ °C. The proposed structure is robust and of low cost. Furthermore, the proposed scheme of enhanced Vernier effect provides a new perspective and idea in other sensing field.

Tens of hertz narrow-linewidth laser based on stimulated Brillouin and Rayleigh scattering.

We proposed and demonstrated a linewidth compression method of a laser based on stimulated Brillouin scattering (SBS) and a Rayleigh backscattering structure (RBS). The relationship between the output SBS laser linewidth and the input pump linewidth was studied theoretically and experimentally. It is shown that the narrower linewidth of the pump laser leads to the narrower bandwidth of the SBS gain and, finally, the bandwidth of the SBS will tend to its intrinsic value as the linewidth of a pump laser narrower than 10 kHz; then the linewidth of an SBS fiber ring laser would tend to 200 Hz. In order to further reduce its linewidth with low cost, RBS and a simple dual-cavity feedback structure were added and, finally, a ∼75 Hz narrow-linewidth laser with a side-mode suppression ratio of 70 dB was obtained.

Tunable dual-wavelength fiber laser with ultra-narrow linewidth based on Rayleigh backscattering.

Dual-wavelength fiber lasers with ultra-narrow linewidth find wide applications in high-speed optical communications, fiber optic sensors, high resolution measurements and medical instruments and microwave or terahertz generation systems. Based on the linewidth compression mechanism due to Rayleigh backscattering, this paper adopts a simple ring structure cooperated with two fiber Bragg gratings centered at 1550 nm and 1530 nm respectively, achieving a dual-wavelength fiber laser with ultra-narrow linewidth, with a 3dB linewidth of ~700 Hz for each wavelength, and the SNR of 60dB. Tuning the center wavelength of one of the two FBGs while the other one keeps unchanged, the fiber laser keeps stable dual-wavelength lasing and the linewidth is still ~700 Hz. It can be seen that the compression for the linewidth based on the Rayleigh backscattering can be used in multi-wavelength laser systems, and because of the characteristic of the Rayleigh backscattering, the method has great potential in the application of wide wavelength range linewidth compression from the ultraviolet to the far infrared.

High spatial resolution distributed fiber system for multi-parameter sensing based on modulated pulses.

We demonstrate a cost-effective distributed fiber sensing system for the multi-parameter detection of the vibration, the temperature, and the strain by integrating phase-sensitive optical time domain reflectometry (φ-OTDR) and Brillouin optical time domain reflectometry (B-OTDR). Taking advantage of the fast changing property of the vibration and the static properties of the temperature and the strain, both the width and intensity of the laser pulses are modulated and injected into the single-mode sensing fiber proportionally, so that three concerned parameters can be extracted simultaneously by only one photo-detector and one data acquisition channel. A data processing method based on Gaussian window short time Fourier transform (G-STFT) is capable of achieving high spatial resolution in B-OTDR. The experimental results show that up to 4.8kHz vibration sensing with 3m spatial resolution at 10km standard single-mode fiber can be realized, as well as the distributed temperature and stress profiles along the same fiber with 80cm spatial resolution.

Experimental Investigations on Subsequent Yield Surface of Pure Copper by Single-Sample and Multi-Sample Methods under Various Pre-Deformation.

Using copper thin-walled tubular specimens, the subsequent yield surfaces under pre-tension, pre-torsion and pre-combined tension-torsion are measured, where the single-sample and multi-sample methods are applied respectively to determine the yield stresses at specified offset strain. The rule and characteristics of the evolution of the subsequent yield surface are investigated. Under the conditions of different pre-strains, the influence of test point number, test sequence and specified offset strain on the measurement of subsequent yield surface and the concave phenomenon for measured yield surface are studied. Moreover, the feasibility and validity of the two methods are compared. The main conclusions are drawn as follows: (1) For the single or multi-sample method, the measured subsequent yield surfaces are remarkably different from cylindrical yield surfaces proposed by the classical plasticity theory; (2) there are apparent differences between the test results from the two kinds of methods: the multi-sample method is not influenced by the number of test points, test order and the cumulative effect of residual plastic strain resulting from the other test point, while those are very influential in the single-sample method; and (3) the measured subsequent yield surface may appear concave, which can be transformed to convex for single-sample method by changing the test sequence. However, for the multiple-sample method, the concave phenomenon will disappear when a larger offset strain is specified.

Precise measurement of ultra-narrow laser linewidths using the strong coherent envelope.

Laser linewidth narrowing down to kHz or even Hz is an important topic in areas like clock synchronization technology, laser radars, quantum optics, and high-precision detection. Conventional decoherence measurement methods like delayed self-heterodyne/homodyne interferometry cannot measure such narrow linewidths accurately. This is because a broadening of the Gaussian spectrum, which hides the laser's intrinsic Lorentzian linewidth, cannot be avoided. Here, we introduce a new method using the strong coherent envelope to characterize the laser's intrinsic linewidth through self-coherent detection. This method can eliminate the effect of the broadened Gaussian spectrum induced by the 1/f frequency noise. We analyze, in detail, the relationship between intrinsic laser linewidth, contrast difference with the second peak and the second trough (CDSPST) of the strong coherent envelope, and the length of the delaying fiber. The correct length for the delaying fiber can be chosen by combining the estimated laser linewidth (Δfest) with a specific CDSPST (ΔS) to obtain the accurate laser linewidth (Δf). Our results indicate that this method can be used as an accurate detection tool for measurements of narrow or super-narrow linewidths.

Dual-cavity feedback assisted DFB narrow linewidth laser.

Single longitudinal mode (SLM) distributed feedback (DFB) lasers with a linewidth lower than a few kHz find applications in many coherent detection systems. In this paper, we proposed and experimentally demonstrated a novel method to compress the linewidth of a SLM DFB laser by utilizing a dual-cavity feedback structure (DCFS). The DCFS first provides optical self-injection feedback to compress the laser linewidth, and then the two feedback lengths are carefully optimized to achieve SLM output via the Vernier principle and the suppression of modes overlapping between two cavities. The linewidthes of 1 MHz and 200 kHz were successfully compressed to ~2.7 and 1.5 kHz with a side mode suppression ratio of 38 and 45 dB, respectively. The stability of the DCFS output power can be controlled within ~0.21%. Our method provides a simple, effective, low cost way to achieve DFB linewidth compression, which will greatly improve the performance of coherent detection systems using DFB laser as sources.

Characteristics of PM2.5 in rural areas of southern Jiangsu Province, China.

To understand pollution level and possible sources of atmospheric fine particulates in rural areas of southern Jiangsu Province of China, samples of PM2.5 were collected and analyzed in Xueyan Town and Taihu Lake Station over three seasons from July 2002 to January 2003. The mass concentrations of PM2.5 and 14 principal component elements were obtained. The results showed that pollution of PM2.5 was serious and the concentration levels of S, Zn, Pb and As were similar to city. There are different seasonal distribution laws of pollutant elements in PM2.5 between two sampling sites, probably due to contribution of local sources, medium or long distance transportation of fine particulates and complicated meteorological conditions. The enrichment levels of S, Zn, Pb, As, K were high, reflecting the influence of anthropogenic activities. Particularly enrichment level of S was much higher in summer, which was probably related to meteorological condition. The result of principal components analysis showed major sources of PM2.5 included crustal resuspension, coal burning, metal processing industry or waste incineration, vehicular emission, which suggests anthropogenic activities is of important influence on PM2.5 in rural areas of southern Jiangsu Province.