Coherent Fiber Research Articles

Kramers-Kronig Optical OFDM for Bandlimited Intensity Modulated Visible Light Communications

Visible light communication (VLC) operates on optical intensity channels that are inherently limited in bandwidth. Though Kramers-Kronig (KK) receivers have been considered as a lower complexity alternative for coherent fiber optic communication (FOC) systems, in this paper we extend this concept to bandlimited IM/DD VLC channels and propose KK optical OFDM (KKO-OFDM). In KKO-OFDM the optical power of a low-cost LED is directly modulated by the double-sideband (DSB) squared modulus of a minimum phase single-sideband (SSB) signal. This results in a real-valued, non-negative and strictly bandlimited transmit signal which is suitable for VLC channels. At the receiver, the phase of the transmitted SSB signal is reconstructed via the KK relations. The required DC bias, average electrical signal-to-noise ratio (SNR), bit error rate (BER), and capacity are analyzed with approximate closed-forms and through simulation. Numerical results show that KKO-OFDM achieves the same spectral efficiency as the existing DC biased optical OFDM (DCO-OFDM), however, realizes approximately 1 dB optical SNR gain at a BER <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$=10^{-4}$</tex-math></inline-formula> while simultaneously having a small peak-to-average power ratio.

Depixelation of coherent fiber bundle imaging by fiber-core-targeted scanning.

A novel fast proximal scanning method, to the best of our knowledge, termed fiber-core-targeted scanning (FCTS), is proposed for illuminating individual fiber cores sequentially to remove the pixelation effect in fiber bundle (FB) imaging. FCTS is based on a galvanometer scanning system. Through a dynamic control of the scan trajectory and speed using the prior knowledge of fiber core positions, FCTS experimentally verifies a precise sequential delivery of laser pulses into fiber cores at a maximal speed of 45,000 cores per second. By applying FCTS on a FB-based photoacoustic forward-imaging probe, the results demonstrate that FCTS eliminates the pixelation effect and improves the imaging quality.

Adaptive multi-layer filters incorporated with Volterra filters for impairment compensation including transmitter and receiver nonlinearity.

We propose a receiver-side signal processing to compensate for nonlinearity that occurs in transmitter (Tx) and receiver (Rx) components of coherent optical fiber transmission systems. Nonlinear effects in transmission systems are not mutually commutative with any linear effects in general. Considering the order in which all the relevant impairments occur, we adopt a multi-layer (ML) filter architecture. The ML filters consist of strictly-linear and widely-linear filter layers to compensate for relevant linear impairments that occur in a transmission system and two Volterra filter layers to compensate for Rx and Tx nonlinearity. The coefficients of the ML filters including Volterra filter layers are adaptively controlled by using a gradient calculation with back propagation, which is similar to that used in the learning of neural networks, from the last layer and stochastic gradient descent to minimize a loss function that is composed of the last layer outputs. We evaluated the compensation performance of Tx and Rx nonlinearity using the proposed adaptive ML filters including Volterra filter layers both in simulations and experiments of the transmission of a 23 Gbaud polarization-division-multiplexed 64-quadrature amplitude modulation signal over a 100-km single-mode-fiber span. The results demonstrated that the Volterra filter layers in the ML filter architecture could compensate for the nonlinearity that occurs in Tx and Rx simultaneously and effectively even when other impairments such as chromatic dispersion coexist.

From spectral broadening to recompression: dynamics of incoherent optical waves propagating in the fiber

Interplay between dispersion and nonlinearity in optical fibers is a fundamental research topic of nonlinear fiber optics. Here we numerically and experimentally investigate an incoherent continuous-wave (CW) optical field propagating in the fiber with normal dispersion, and introduce a distinctive spectral evolution that differs from the previous reports with coherent mode-locked fiber lasers and partially coherent Raman fiber lasers [Nat. Photonics 9, 608 (2015).]. We further reveal that the underlying physical mechanism is attributed to a novel interplay between group-velocity dispersion (GVD), self-phase modulation (SPM) and inverse four-wave mixing (IFWM), in which SPM and GVD are responsible for the first spectral broadening, while the following spectral recompression is due to the GVD-assisted IFWM, and the eventual stationary spectrum is owing to the dominant contribution of GVD effect. We believe this work can not only expand the light propagation in the fiber to a more general case and help advance the physical understanding of light propagation with different statistical properties, but also benefit the applications in sensing, telecommunications and fiber lasers.

Coherent Fiber-Optic Sensor for Ultra-Acoustic Crack Emissions.

A coherent optical fiber sensor with adequate sensitivity for detecting the acoustic emission (AE) during the propagation of a crack in a ferrous material is presented. The proposed fiber optic sensor is successfully compared in terms of the SNR (Signal to Noise Ratio) and detectable AE energy levels to commercially available AE piezo-transducers sensors and is proven to be an effective and advantageous alternative for sensing and monitoring fatigue damage in structural applications.

Nonlinear Coherent Optical Systems in the Presence of Equalization Enhanced Phase Noise

Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN.

Higher-Order Airy Patterns and Their Application in Tailoring Orbital Angular Momentum Beams with Fiber Laser Arrays

Airy patterns, formed by the Fraunhofer diffraction of plane waves via circular apertures, have been widely utilized as criterions in areas such as imaging optics and energy transmission. With deeper insights into the spatial structure of light, the use of Airy patterns as criterions today becomes inadequate in many scenarios. Here, a new concept of higher-order Airy patterns is proposed by introducing the orbital angular momentum (OAM) of light, namely the Fraunhofer diffraction pattern of a plane wave via the defined annular aperture with helical wavefront tailoring. For the m-order Airy pattern, the tailored helicity is m, and the scaling factor of the annular aperture and is determined by the m-related equation set, which is derived based on the paraxial diffraction theory. As a potential application of the proposed concept, a coherent fiber laser array system capable of customizing OAM beams is presented and analyzed. Using the higher-order Airy pattern to define the performance metric of combined beam, the fiber laser array system is optimally designed and the properties in energy concentration and high mode purity of the generated OAM beams could be realized. This work could be beneficial to the theoretical study and applications of OAM beams, especially the generation and manipulation of high-power, fast switchable OAM beams with fiber laser arrays.

Fast tracking of polarization impairments using DSP algorithms in fiber for a coherent optical communication system

AbstractIn a 100G coherent optical fiber communication system, the equalization algorithm of polarization effects in fibers is generally integrated in digital signal processing (DSP) using a multiple input/multiple output (MIMO) equalizer to overcome the impairment due to the polarization effects in fiber. However, it was found in some extreme scenarios, such as the lightning strikes near the fiber cables, the Kerr effect and Faraday effect would induce very fast time‐varying state‐of‐polarization (SOP) and transient variation of principal‐state‐of‐polarization (PSP) of polarization mode dispersion (PMD) which would result in the coherent receivers unlocking. Faced with the difficulty, we propose a 3‐parameter RSOP model and design a window‐split structured Kalman filter equalization scheme instead of conventional MIMO equalizer to cope with this extreme scenario of large PMD combined with ultra‐fast rotation of SOP in presence of residual chromatic dispersion (RCD). The proposed equalization method has an excellent performance and can cope with RCD in the range of ±820 ps/nm under RSOP from 100 krad/s to 3 Mrad/s and DGD = 200 ps.

A Comparative Survey of Coding, Multiplexing, and Equalization Techniques Used in Coherent Optical Fiber Communications

As the world advances into 5G networks, significant scientific research accomplishments are being conducted for a communication system that could further enhance the current limit of data transmission capacity. Currently, the communication systems with the highest data rate are optical fiber systems. Due to the recent advancement of coherent optical fiber communications by exploiting time, wavelength, phase, amplitude, polarization, and space, optical engineering can break the petabit barrier data rate. Thus, coherent optical fiber communications is a hot topic due to its very high data rate that could be applied or a requirement in 5G and big data analytics. This paper focuses on a comparative survey of the current applied fundamental techniques in fiber communication channels. These fundamental techniques that could be further studied and exploited to increase the bandwidth performance, decrease the error rate and energy consumption are coding, multiplexing, and equalization. At the end of this paper, a comparative result is discussed to explain the difference among the current techniques in the literature for the optical engineering community to improve collective coding, multiplexing, and equalization in coherent fiber systems.

Coherent Optical Fiber Sensing Based on a Frequency Shifting Loop

In this paper, the proof-of-concept of an optical fiber sensing technique based on a frequency shifting loop (FSL) is proposed. The system combines both a refreshing rate of the sensing measurement in the MHz range and a centimetric spatial resolution. The FSL is used as a versatile source of broadband optical chirped waveforms, with controllable chirp rate. Digital pulse compression is applied to optimize the system's signal-to-noise-ratio. The sensor system is used to monitor the phase perturbation applied to a localized transducer (an electro-optic phase modulator) placed in an optical fiber. A preliminary and promising sensitivity <4 μrad/Hz^(1/2) in a 20 kHz–2 MHz bandwidth is obtained without any stabilization of the FSL. The sensor system is also tested on an optical fiber containing two transducers, emulating a quasi-distributed antenna. The system enables the accurate measurement of a phase shift between the two modulators, making it suitable for geolocation operation.

Binarization for eliminating calibration in fiberscope image processing

High-density coherent fiber bundles, mainly used for high-resolution fiberscopes, consist of non-circular cores and have various optical noises such as auto-fluorescence, crosstalk, and honeycomb artifacts that may require excessive image processing. Binarization was applied to post-image processing to eliminate a calibration used as pre-image processing for traditional fiberscope image reconstruction. It was able to significantly reduce the complexity of implementing core peak detection and effectively reduce the overall processing time. For binarization, the global thresholding suggested by Otsu and the local thresholdings suggested by Bernsen, Niblack, and White and Rohrer were used. By applying a dynamic method to our method, we can further simplify the process and apply it to image sequencing. When evaluating the results comprehensively, the local thresholding proposed by Bernsen was best suited for interpolating our fiberscope images. By introducing binarization combined with local thresholding in post-image processing, we were able to successfully eliminate the preliminary calibration process and reduce the burden of overall image processing for image reconstruction in a high-resolution fiberscope.

Applicability of an interferometric optical fibre sensor for shallow landslide monitoring – Experimental tests

Abstract Optical fibre-based sensors have found a variety of applications throughout engineering and science in the measurement of strain and temperature. Current commercially available sensing technologies are able to provide strain measurements in the order of microstrain across tens of kilometres of range. The strong demand for reliable sensing tools in engineering geology allowed for the advancement of the optical fibre technology which has settled in this field during the past decade. Among the variety of sensing approaches, interferometric fibre optic sensors have rarely been tested for detection of ground movement. In this work we present the results of six laboratory scale experiments in which simulated rainfall induced shallow landslide model made up of non-cohesive soil is equipped with a newly developed coherent fibre optic sensor. Results indicate that a phase of early precursors of instability could be identified by the sensors in all six experiments, two to eight minutes before slope failure. The sensing system proved to be reliable in detecting the evolution of slope stability at shallow depths of up to a few meters depending on the sensors' burial depth, hence stating the emergence of a simple and cost-effective approach for detailed strain monitoring. The proposed sensing technique could therefore provide a viable and cheaper alternative to commercially available optical fibre interrogators and furnish quantitative monitoring data for shallow landslide monitoring in soil-like materials. In order to be efficient for the detection of such failures with a significant precursory window, sensors should ideally be placed in the vicinity of a sliding surface or landslide boundary, which is an achievable task given the high sensing range and multiplexing capabilities of optical fibre sensors.

Pulse interactions in weakly nonlinear coherent optical communication links

The intrachannel interaction of pulses in weakly nonlinear coherent optical fiber lines is theoretically investigated. It is shown that the main contribution to the perturbation of the optical field comes from resonant interactions of ordered triplets of pulses. The structure of triplets is determined. The weight contributions of such interactions are calculated. A classification of interactions using Löschian numbers is proposed. Using computer simulation, the dependence of the average energy of the optical field perturbations on the distance is shown. Based on the performed analysis, an effective algorithm is proposed for assessing the perturbations resulting from intrachannel interaction.

Joint OSNR and Frequency Offset Estimation Using Signal Spectrum Correlations

We propose an efficient and modulation-format-transparent method to jointly estimate optical signal-to-noise ratio (OSNR) and frequency offset (FO) by using signal spectrum correlations for coherent optical fiber communication systems. Based on the signal spectrum correlation analysis, a coarse FO estimation (FOE) and the corresponding compensation can be conducted after chromatic dispersion compensation (CDC). Then, OSNR monitoring can be accurately operated without deterioration caused by FO. Meanwhile, to realize fine FOE, down-sampling process of signal can be used to reduce the complexity of fast Fourier transform-based FOE (FFT-FOE) without losing FOE resolution. Simulation results show that when the OSNR is in the range of 10 to 30 dB, the proposed scheme presents an absolute OSNR estimation error lower than 0.18 dB. Finally, we experimentally demonstrate our scheme in an optical back-to-back (B2B) transmission link using 28 Gbaud dual-polarization (DP)-4/16/32-quadrature amplitude modulation (QAM) formats, and an absolute OSNR estimation error lower than 0.66 dB is achieved for OSNR ranging from 15 to 30 dB.

Hybrid Dy-NFIS &amp; RLS equalization for ZCC code in optical-CDMA over multi-mode optical fiber

For long haul coherent optical fiber communication systems, it is significant to precisely monitor the quality of transmission links and optical signals. The channel capacity beyond Shannon limit of Single-mode optical fiber (SMOF) is achieved with the help of Multi-mode optical fiber (MMOF), where the signal is multiplexed in different spatial modes. To increase single-mode transmission capacity and to avoid a foreseen “capacity crunch”, researchers have been motivated to employ MMOF as an alternative. Furthermore, different multiplexing techniques could be applied in MMOF to improve the communication system. One of these techniques is the Optical Code Division Multiple Access (Optical-CDMA), which simplifies and decentralizes network controls to improve spectral efficiency and information security increasing flexibility in bandwidth granularity. This technique also allows synchronous and simultaneous transmission medium to be shared by many users. However, during the propagation of the data over the MMOF based on Optical-CDMA, an inevitable encountered issue is pulse dispersion, nonlinearity and MAI due to mode coupling. Moreover, pulse dispersion, nonlinearity and MAI are significant aspects for the evaluation of the performance of high-speed MMOF communication systems based on Optical-CDMA. This work suggests a hybrid algorithm based on nonlinear algorithm (Dynamic evolving neural fuzzy inference (Dy-NFIS)) and linear algorithm (Recursive least squares (RLS)) equalization for ZCC code in Optical-CDMA over MMOF. Root mean squared error (RMSE), mean squared error (MSE) and Structural Similarity index (SSIM) are used to measure performance results.

Comparative Evaluation of Diffusion Kurtosis Imaging and Diffusion Tensor Imaging in Detecting Cerebral Microstructural Changes in Alzheimer Disease

Comparative evaluation of diffusion kurtosis imaging (DKI) and diffusion tensor imaging (DTI) using a whole-brain atlas to comprehensively evaluate microstructural changes in the brain of Alzheimer disease (AzD) patients. Twenty-seven AzD patients and 25 age-matched controls were included. MRI data was analyzed using a whole-brain atlas with inclusion of 98 region of interests. White matter (WM) microstructural changes were assessed by Fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD), Kurtosis fractional anisotropy (KFA), mean kurtosis (MK), axial kurtosis (AK) and radial kurtosis (RK). Gray matter (GM) integrity was evaluated using KFA, MK, RK, AK and MD. Comparison of the DKI and DTI metrics were done using student t-test (p ≤ 0.001). In AzD patients widespread increase in MD, AD and RD were found in various WM and GM region of interests. The extent of abnormality for DKI parameters was more limited in both GM and WM regions and revealed reduced kurtosis values except in lentiform nuclei. Both DKI and DTI parameters were sensitive to detect abnormality in WM areas with coherent and complex fiber arrangement. Receiver operating characteristic curve analysis for hippocampal values revealed the highest specificity of 88% for AK <0.6965 and highest sensitivity of 95.2% for MD >1.2659. AzD patients have microstructural changes in both WM and GM and are well-depicted by both DKI and DTI. The alterations in kurtosis parameters, however, are more limited and correlate with areas in the brain primarily involved in cognition.

Handheld macroscopic Raman spectroscopy imaging instrument for machine-learning-based molecular tissue margins characterization.

.Significance: Raman spectroscopy has been developed for surgical guidance applications interrogating live tissue during tumor resection procedures to detect molecular contrast consistent with cancer pathophysiological changes. To date, the vibrational spectroscopy systems developed for medical applications include single-point measurement probes and intraoperative microscopes. There is a need to develop systems with larger fields of view (FOVs) for rapid intraoperative cancer margin detection during surgery.Aim: We design a handheld macroscopic Raman imaging system for in vivo tissue margin characterization and test its performance in a model system.Approach: The system is made of a sterilizable line scanner employing a coherent fiber bundle for relaying excitation light from a 785-nm laser to the tissue. A second coherent fiber bundle is used for hyperspectral detection of the fingerprint Raman signal over an area of . Machine learning classifiers were trained and validated on porcine adipose and muscle tissue.Results: Porcine adipose versus muscle margin detection was validated ex vivo with an accuracy of 99% over the FOV of in using a support vector machine.Conclusions: This system is the first large FOV Raman imaging system designed to be integrated in the workflow of surgical cancer resection. It will be further improved with the aim of discriminating brain cancer in a clinically acceptable timeframe during glioma surgery.

An Eye-Safe, SBS-Free Coherent Fiber Laser LIDAR Transmitter with Millijoule Energy and High Average Power

We report on an eye-safe, transform-limited, millijoule energy, and high average power fiber laser. The high gain and short length of the NP phosphate-glass fibers enable the SBS-free operation with kW level peak power. The output energy is up to 1.3 mJ, and the average power is up to 23 W at an 18 kHz repetition rate with 600 ns pulses (peak power &gt; 2.1 kW). The PER is ≈16 dB and the M2 of the beam is 1.33 × 1.18. The coherent LIDAR Figure Of Merit (FOM) is 174 mJ*sqrt(Hz), which to our knowledge is the highest reported for a fiber laser. We also report 0.75 mJ energy and &gt;3.7 kW peak power with down to 200 ns pulses and up to 1.21 mJ energy with a 3–5 kHz repetition rate operation of the current system.

Ultra-thin 3D lensless fiber endoscopy using diffractive optical elements and deep neural networks

Minimally invasive endoscopes are indispensable in biomedicine. Coherent fiber bundles (CFBs) enable ultrathin lensless endoscopes. However, the propagation of light through a CFB suffers from phase distortions and aberrations that can cause images to be scrambled. The correction of such aberrations has been demonstrated using various techniques for wavefront control, especially using spatial light modulators (SLMs). This study investigates a novel aberration correction without SLM for the creation of an efficient and compact system. The memory effect of CFBs enables a paradigm shift in the use of static diffractive optical elements (DOEs) instead of dynamic modulation with SLM. We introduce DOEs produced by 2-photon polymerization lithography for phase conjugation on a CFB for focusing, raster scanning, and imaging. Furthermore, a DOE with random patterns is used to encode the three-dimensional (3D) object information in a 2D speckle pattern that propagates along the ultra-thin CFB. Neural networks decode the speckles to retrieve the 3D object information using single-shot imaging. Both DOE methods have compact low-cost concepts in common, and both pave the way for minimally invasive 3D endomicroscopy with benefits for optical imaging in biomedicine.

Core crosstalk in ordered imaging fiber bundles.

Coherent fiber bundles are used widely for imaging. Commonly, disordered arrays of randomly sized fiber cores avoid proximity between like-cores, which would otherwise result in increased core crosstalk and a negative impact on imaging. Recently, stack-and-draw fiber manufacture techniques have been used to produce fibers with a controlled core layout to minimize core crosstalk. However, one must take manufacturing considerations into account during stack-and-draw fiber design in order to avoid impractical or unachievable fabrication. This comes with a set of practical compromises, such as using only a small number of different core sizes. Through characterization of core crosstalk patterns, this Letter aims to aid the understanding of crosstalk limitations imposed by such compromises in the core layout made for ease of fabrication.