Backscattering Noise Research Articles

Polarization-Enhanced Underwater Laser Range-Gated Imaging for Subaquatic Applications.

Laser range-gated underwater imaging technology, by removing most of the backscattering noise, can effectively increase image contrast and extend the detection range. The optical signal captured by a range-gated imaging system primarily comprises reflected light from the object and backscattered light from the surrounding water. Consequently, surfaces with low reflectivity or highly turbid water environments substantially constrain the applicability of the range-gated imaging system. To enhance the detection capability of underwater laser range-gated imaging, this paper proposes the incorporation of underwater polarized light imaging technology as an enhancement method. Based on polarization differences, backscattered light and reflected light from an object can be distinguished. Experimental results indicate that, compared to images obtained using a conventional range-gated laser imaging system, those captured with a polarization-enhanced system exhibit an increase of up to 47% for the corresponding Enhancement Measure Evaluation (EME) index. The proposed approach, which integrates polarization imaging with range-gated laser imaging, has the potential to broaden the applicability of underwater laser imaging scenarios, such as deep-sea exploration and military applications.

Study of Millimeter-Wave Fuze Echo Characteristics under Rainfall Conditions Using the Monte Carlo Method

Due to the similarity in wavelength between millimeter-wave (MMW) signals and raindrop diameters, rainfall induces significant attenuation and scattering effects that challenge the detection performance of MMW fuzes in rainy environments. To enhance the adaptability of frequency-modulated MMW fuzes in such conditions, the effects of rain on MMW signal attenuation and scattering are investigated. A mathematical model for the multipath echo signals of the fuze was developed. The Monte Carlo method was employed to simulate echo signals considering multiple scattering, and experimental validations were conducted. The results from simulations and experiments revealed that rainfall increases the bottom noise of the echo signal, with rain backscatter noise predominantly affecting the lower end of the echo signal spectrum. However, rain conditions below torrential levels did not significantly impact the detection of strong reflection targets at the high end of the spectrum. The modeling approach and findings presented offer theoretical support for designing MMW fuzes with improved environmental adaptability.

A 300 km fiber channel mapping using neural networks for Gb/s physical-layer key distribution.

Physical-layer secure key distribution (PLSKD) generally acquires highly correlated entropy sources via bidirectional transmission to share the channel reciprocity. For long-haul fiber links, the non-negligible backscattering noise (BSN) and the challenge of bidirectional optical amplification degrade the key generation performances. Since the channel reciprocity can be precisely mapped using neural networks (NNs), unidirectional PLSKD provides a feasible PLSKD for longer fiber links. Here, a final error-free key generation rate (KGR) in unidirectional PLSKD of 3.07 Gb/s is demonstrated over a 300 km fiber link using NNs. Moreover, the channel mapping is analyzed in terms of fiber distance, chromatic dispersion, the nonlinearity of random source, and BSN.

A receiver-optimized total focusing method for detectability enhancement of small defects in coarse grained materials

The identification and localization of small defects are among the main objectives of ultrasonic non-destructive testing (NDT). In particular, the inspection of polycrystalline materials remains a challenge due to ultrasonic attenuation and backscatter noise caused by grain scattering. An adaptive ultrasonic array imaging algorithm is proposed in this paper, which is capable of enhancing the signal-to-noise ratio (SNR) of small subwavelength defects. The proposed approach adopts an iterative optimization procedure based on sequential backward selection, and is termed receiver optimized total focusing method (ROTFM). ROTFM is formulated in a baseline subtraction (BS) setting, and it is shown to achieve higher defect SNRs compared to TFM results obtained by directly applying the BS method to measured array data. More importantly, the performance of the proposed ROTFM is robust to time and spatial misalignments, and hence, it alleviates the need for complex compensation schemes when performing imaging computations. Compared to conventional TFM, ROTFM achieved an average SNR improvement of 3.41 dB and 6.05 dB in experiments for four test defects (cracks and holes of sizes 0.5 mm and 1 mm) at the frequencies of 5 MHz and 7 MHz, respectively.

A spatially correlated fractional integral-based method for denoising geiger-mode avalanche photodiode light detection and ranging depth images

Geiger-mode avalanche photodiode (GM-APD) light detection and ranging (LiDAR) target echo signals are susceptible to interference from atmospheric backscatter, daylight, and other noise, and extracted target depth images contain a large amount of anomalous noise. Accordingly, this paper devises a method based on a spatially correlated fractional integral for denoising GM-APD LiDAR depth images. First, a multi-scale superpixel fusion algorithm is employed to perform null-pixel complementation on a target depth image extracted using histogram statistics. Next, a pixel neighborhood spatial correlation kernel function is used to optimize the Grünwald–Letnikov fractional integral operator to correct the large amount of anomalous noise and thus achieve GM-APD depth image denoising. Simulation and experimental results demonstrate that the denoising performance of the algorithm on GM-APD LiDAR depth images is significantly better than that of median filtering and bilateral filtering, with at least 22.8 % and 4.8 % improvements in the target reduction degree (K) and peak signal-to-noise ratio (PSNR), respectively. In addition, the K and PSNR reach 91.36 % and 18.0241 dB, respectively, with 25 statistical frames in an outdoor imaging experiment. This demonstrates that the algorithm can effectively denoise GM-APD LiDAR depth images with few statistical frames and thus improve the frame rate of GM-APD LiDAR imaging.

Study on the Influence of Underwater LED Illumination on Bidirectional Underwater Wireless Optical Communication

Underwater wireless optical communication (UWOC) is acknowledged as a useful way to transmit data in the ocean for short-distance applications. Carrying a UWOC device on mobile platforms is quite practical in ocean engineering, which is helpful to exploit its advantages. In application, such a platform needs a camera to observe the surroundings and guide its action. Since the majority of ocean is always dark, active illumination is necessary to imaging. When UWOC works in such an environment, its performance is affected by the illumination light noise. In this paper, we study the influence of underwater LED illumination on bidirectional UWOC with the Monte Carlo method. We simulate forward noise from LED illumination to the opposite receiver in the cooperative terminal, and the backscattering noise on the adjacent receiver in the same terminal. The results show that the forward noise is reduced with the increase of theabsorption coefficient, scattering coefficient, transmitting distance, and separated distance between receiver and the optical axis of LED. However, it becomes greater with the field of view (FOV) of the receiver. The backscattering noise is reduced with the increase of the absorption coefficient and separated distance between receiver and LED. However, it becomes greater with the FOV and scattering coefficient, while it has little relation with transmitting distance. In order to reduce these two kinds of noises, besides inserting an optical filter in the receivers and narrowing their FOV, the optical axis of LED light should keep away from the receivers. The results in this paper are helpful for UWOC application.

Modulation index detection and stabilization technique of phase modulator in resonant fiber optic gyro base on sampling demodulation

A new modulation index detection and stabilization technique (MIDST) based on sampling demodulation is proposed to reduce the influence of phase modulator (PM) thermal instability on resonant fiber optic gyro (RFOG). Firstly, the factors causing the PM thermal instability and the influence on RFOG are analyzed. Then, a phase modulation index detection technique is proposed to detect the fluctuation of modulation index without complicating the optical path configuration of RFOG. Finally, considering the temperature fluctuation, a modulation index stabilization loop based on auto disturbance rejection controller (ADRC) is designed to stabilize the phase modulation index. In experiments, modulation index can be detected in real time and accurately, and the fluctuation is reduced from 10% to 0.3% over 100 °C temperature range (−40 °C to ＋60 °C) with MIDST. The stability of scale factor and the suppression effect of backscattering noise could be improved effectively.

Enriching Capacity and Transmission of Hybrid WDM-FSO Link for 5G Mobility

A dramatic increase in user and capacity demands has been noted after the COVID-19 pandemic. These challenges have damaged the 5G communication system mobility. Therefore, developing mobility and enhancing capacity transmission of 5G advanced services are the focused research gaps in the current era. In this paper, the free space optics (FSO) link is modeled with wavelength division multiplexing (WDM) technology based optical fiber system, purposing to enhance the 5G capabilities in multi-channel, high distance, and bidirectional transmissions. In addition, the presented hybrid FSO-WDM supported optical fiber network is analyzed for 4, 8, and 16 × 10 Gbps downlink and uplink transmission. The paper also includes the mathematical discussion of merged fiber length (SMF = 30 km) and FSO (600 m) with improved mobility management. In another contribution, the tolerance against Rayleigh backscattering (RB) noises is developed through various wavelengths of downlink and uplink channels. Finally, we perform the simulation analysis and reliability of the proposed structure for the 5G advanced communication system.

Three closed loop noise suppression method for resonant micro optical gyroscope

For resonant micro-optical gyroscope (R-MOG), backscattering noise and optical Kerr noise are the main noises that affect the output accuracy of the gyroscope, and it is difficult to suppress them completely. This paper introduces a method to suppress the optical Kerr noise generated by the front end by compensating the optical power difference while suppressing the backscattered noise. Based on a single light source, the system changes the frequency difference between clockwise and counterclockwise optical paths through double-phase modulation, so as to improve the carrier rejection ratio and suppress the backscattered noise. At the same time, a high-speed feedback loop based on intensity modulator is designed, which is added to the optical loop to enhance the optical power, so as to reduce the impact of optical Kerr noise. The experimental results show that this method effectively suppresses the backscattered noise and optical Kerr noise, improves the output accuracy of the gyro by two orders of magnitude, and finally observes the long-term bias stability of 0.982°/h.

A long-reach optically powered multi-band radio-over-fiber network by employing PolM-to-IM converter with enhanced fault-protection ability and less Rayleigh backscattering noise effect

A multi-band radio-over-fiber network is established on polarisation-to-intensity converter with upgraded fault protectivity for transportation of information (10 Gbps/15 GHz, 10 Gbps/60 GHz and 10 Gbps) for end-users over ∼[40 + 30] km single-mode fiber (SMF) link with lessen impairments owing to Rayleigh Backscattering (RB) is proposed and demonstrated. Employment of unalike wavelengths for downstream and upstream results RB minimization. Fault protection capability is uplifted by incorporating the integrated SMF/free-space optics (FSO) link which provides the data continuity condition either through SMF or FSO even when anyone link is in non-workable condition due to damage or environmental limitations. Co-existence of power over fiber (PoF) signal with integrated SMF/FSO scheme was also investigated. Power penalty of 0.74 dB, 0.84 dB, low bit error rate (BER) value (∼10-3for OFDM signal, ∼10-9 for pseudo-random bit sequence (PRBS) signal), <32 dB power budget are achieved by this transmission by employing integrated SMF/FSO scheme and 2.6 dB, 2.1 dB power penalty, 34–35 dB power budget are recorded for transportation of signal in absence of integrated SMF/FSO network. The conjunction of PoF and integrated SMF/FSO scheme enhances the application field areas but causes the transmission distance contraction. Comparatively, lower power penalty than earlier (0.6 dB, 1.7 dB), 29–30 dB power budget, low BER value (∼10-3 for OFDM signal, 10-9 for PRBS signal) are measured for the transmission in presence of integrated SMF/FSO network. Larger power budgets (∼33 dB), ∼2 dB power penalty, expectable BER values are obtained without integrated SMF/FSO network for this transmission.

Discriminating Rayleigh backscattering induced false crosstalk in inline interferometric FBG sensor arrays.

The crosstalk between channels is a main factor restricting the performance of interferometric fiber Bragg grating (FBG) time-division and wavelength-division hybrid multiplexing arrays. The time-division crosstalk caused by multiple reflections and the wavelength-division crosstalk due to insufficient isolation are the main crosstalk problems in the Fabry-Perot (F-P) structure, seriously limiting the number of multiplexing devices and the system's applications. Many theoretical research and suppression scheme designs have been done to solve these problems. We have previously found a new crosstalk phenomenon called the false crosstalk in hybrid multiplexing arrays. This paper focuses on this phenomenon and constructs a theoretical model to analyze its causes and influencing factors. The model demonstrates the influences of Rayleigh backscattering (RB) noise and sensor position on the false crosstalk. Both theoretical and experimental results show that the false crosstalk is induced by parasitic interference in the leading fiber and changes with the leading fiber length. This study quantitatively reveals the crosstalk performance degradation with the change of sensor position in the hybrid multiplexing array and provides key support for optimizing the system array design and expanding the large-scale multiplexing capacity.

A modulated positioning scheme for asymmetric vibration sensor using high-frequency carrier

The asymmetric dual Mach-Zehnder interferometer can be used in the vibration sensing system for the long-distance monitoring. It can effectively suppress the backscattering noises of the system. However, it will also seriously deteriorate the positioning accuracy because of the asymmetry of the interferometer. In this paper, a modulated asymmetric dual Mach Zehnder interferometer (ADMZI) system using a high-frequency carrier scheme has been theoretically proposed and experimentally demonstrated. Field trials have been carried out to verify the effectiveness. Our experimental results show that the standard deviation of the proposed scheme is less than 60 m for a sensing length of 51 km, with an average processing time of 106 ms. Compared to traditional methods, the proposed approach can also be applied to weak signals with low frequencies and small amplitudes. This indicates that the proposed scheme can significantly improve the applicability of the asymmetry based distributed sensing systems.

Lighting the darkness in the sea: A deep learning model for underwater image enhancement

Currently, optical imaging cameras are widely used on underwater vehicles to obtain images and support numerous marine exploration tasks. Many underwater image enhancement algorithms have been proposed in the past few years to suppress backscattering noise and improve the signal-to-noise ratio of underwater images. However, these algorithms are mainly focused on underwater image enhancement tasks in a bright environment. Thus, it is still unclear how these algorithms would perform on images acquired in an underwater scene with low illumination. Images obtained in a dark underwater scene usually include more noise and have very low visual quality, which may easily lead to artifacts during the process of enhancement. To bridge this gap, we thoroughly study the existing underwater image enhancement methods and low illumination image enhancement methods based on deep learning and propose a new underwater image enhancement network to solve the problem of serious degradation of underwater image quality in a low illumination environment. Due to the lack of ready-made datasets for training, we also propose the first dataset for low-light underwater image enhancement to train our model. Our method can be implemented to skillfully and simultaneously address low-light degradation and scattering degradation in low-light underwater images. Experimental results also show that our method is robust against different illumination levels, which greatly expands the applicable scenarios of our method. Compared with previous underwater image enhancement methods and low-light image enhancement methods, outstanding performance is achieved using our method in various low-light underwater scenes.

Plug-and-play QKD architecture with a self-optical pulse train generator.

The commercialization of quantum key distribution (QKD), which enables secure communication even in the era of quantum computers, has acquired significant interest. In particular, plug-and-play (PnP) QKD has garnered considerable attention owing to its advantage in system stabilization. However, a PnP QKD system has limitations on miniaturization owing to a bulky storage line (SL) of tens of kilometers. And, the secure key rate is relatively low because Bob transmits the signal pulses only at the dedicated time slots to circumvent backscattering noise. This study proposes a new method that can eliminate the SL by realizing an optical pulse train generator based on an optical cavity structure. Our method allows Alice to generate optical pulse trains herself by duplicating Bob's seed pulse and excludes the need for Bob's strong signal pulses that trigger backscattering noise as much as the conventional PnP QKD. Accordingly, our method can naturally overcome the miniaturization limitation and the slow secure key rate, as the storage line is no longer necessary. We conducted a proof-of-concept experiment using our method and achieved a key generation rate of 1.6×10-3 count/pulse and quantum bit error rate ≤ 5%.

Performance of a resonant fiber-optic gyroscope based on a broadband source.

A resonant fiber-optic gyroscope (RFOG) based on a broadband source can avoid the fundamental drawback of coherence detection processing while possessing the greater sensitivity afforded by the finesse of the fiber-optic ring resonator. In this paper, the basic operation principle is presented and demonstrated in detail, and various noise sources, as well as the temperature effect encountered in this broadband source-driven RFOG, are studied and analyzed. Then a combined modulation technique is proposed to suppress the residual backscattering noise. To further reduce the effect of temperature transience, an asymmetric fiber ring resonator is designed. In the experiment, a bias stability of 0.01°/h is successfully demonstrated with a 100 m-long fiber ring resonator of 8 cm diameter in a laboratory environment without temperature control.

Suppressing backscattering noise of a resonant fiber optic gyroscope using coherent detection.

This paper provides a novel, to the best of our knowledge, method for suppressing backscattering noise of a resonant fiber optic gyroscope (RFOG) with a coherent detection technique. The light from the fiber ring resonator is mixed with a reference beam rather than being demodulated directly in traditional configurations, generating a coherent signal with a radio frequency. The central frequencies of the two reference lights used for the clockwise and counterclockwise waves are different to avoid the effect of backscattered waves. Besides, a common phase modulation is applied on the two counter-propagating waves to eliminate the parasitic effect due to the residual amplitude modulation in the phase modulator. Two demodulation schemes for the rotation rate detection from the coherent signals are then proposed and demonstrated, with performance on noise suppression tested. One is the beat-frequency demodulation, and the other is the self-mixing demodulation technique. The influence of backscattering intensity is reduced from 270°/h to 3°/h and 0.05°/h with the two demodulation techniques, respectively, showing a full suppression of backscattering noise.

Resonant fiber optic gyroscope using a reciprocal modulation and double demodulation technique.

A resonant fiber optic gyroscope (RFOG) using a reciprocal modulation and double demodulation technique based on a single laser source is proposed and demonstrated. The effect of the residual amplitude modulation of the phase modulator is well suppressed thanks to the reciprocal modulation and demodulation. On this basis, the backscattering noise is also eliminated by the double demodulation process. The long-term bias stability of the RFOG is successfully improved to 0.2°/h for a test time of 45 hours.

Resonant Fiber Optic Gyroscope With HOPLL Technique Based on Acousto-Optic Modulation

Backscattering noise is a key factor to restrict the performance of resonant fiber optic gyroscopes (RFOG). In order to improve the accuracy of RFOG, this paper proposes a scheme with heterodyne optical phase-locked loop (HOPLL) technique based on acousto-optic modulation (AOM) to suppress backscattering noise. In this scheme, the main light source is divided into three output lights, two of which are frequency-shifted by the AOM, and then are phase-locked with the third light, using the HOPLL technique. The two channels of frequency-shifted light are respectively used as the clockwise (CW) and counterclockwise (CCW) light of the RFOG, and are locked on the resonance peak. The transmissive fiber resonator can be regarded as a band-pass filter. By adjusting the reference signal in the HOPLL, the carrier of the two frequency-shifted lights can be moved away from the resonance peak. The carrier light intensity of the frequency-shifted light is suppressed by the cavity itself. The suppression ratio (SPR) reaches 15.83dB. The experimental results illustrate that the bias stability of RFOG is 3.8°/h based on the Allan deviation, strongly proving the effectiveness of this scheme in suppressing backscattering noise in practical applications.

Optimization of the Angular Random Walk in Laser-Driven Fiber-Optic Gyroscopes

This paper investigates whether the angular random walk (ARW) of a fiber-optic gyroscope (FOG) driven by a broadened laser can be minimized, or equivalently the signal-to-noise ratio maximized, by optimizing the phase bias of the FOG, and by how much. Expressions of the ARW&#x2019;s dependence on the phase bias and detected power are derived that take into account the main sources of noise of a FOG (namely detector noise, shot noise, relative intensity noise, and backscattering noise) for both sine-wave and square-wave modulation and demodulation. For backscattering noise, which dominates at usual detected powers, these hitherto unknown dependences are calculated using a published numerical model. For completeness and comparison, the more common case of a FOG interrogated with broadband light is also treated. In a FOG driven by a broadened laser, the answer is that for either modulation waveform the reduction in ARW afforded by biasing optimally is small (&#x007E;1 dB). With broadband light, optimum biasing improves the ARW also only slightly with sine-wave modulation (&#x007E;1 dB), but significantly with square-wave modulation, by up to &#x007E;10 dB (this value increases as the square root of the detected power). These predictions are in excellent agreement with the dependences measured in a 3.24-km FOG biased with square-wave modulation. At the maximum available detected power, an ARW reduction of 3 dB was measured with a broadband light source, and 0.7 dB with a broadened laser.

A Hollow-Core Photonic-Crystal Fiber-Optic Gyroscope Based on a Parallel Double-Ring Resonator.

A novel system structure of resonant fiber optical gyroscope using a parallel double hollow-core photonic crystal fiber ring resonator is proposed, which employs the double closed loop and reciprocal modulation–demodulation technique to solve the problem of the length mismatch between rings. This structure can suppress the residual amplitude modulation noise and laser frequency noise, essentially eliminating the influence of the Rayleigh backscattering noise and dramatically reduce the Kerr-effect-induced drift by three orders of magnitude. Thanks to its excellent noise suppression effect, the sensitivity of this novel system can approach the shot-noise-limited theoretical value of 8.94 × 10−7 rad/s assuming the length of the fiber ring resonator is 10 m.