Wavelength-dependent Gain Research Articles

Pulsating dynamics of noise-like pulses in a fiber laser with nonlinear optical loop mirror

We experimentally investigate the nature of pulsating noise-like pulses (NLPs) in a fiber laser based on the nonlinear optical loop mirror. By adjusting the intra-cavity polarization, three types of pulsating NLPs can be obtained in the cavity. By utilizing the dispersive Fourier transformation technique, the real-time evolution dynamics of NLP pulsation have been investigated in detail. Different from the conventional pulsating behavior, the NLPs undergo remarkable and periodical variations in their width with slight changes in pulse peak powers during pulsation process. We speculate that the wavelength-dependent gain saturation is involved in the pulsating NLP evolution. Quasi-periodic energy oscillations are associated with cyclic generation and subsequent walkoff of wavelength-shifted components, resulting in the different pulsating dynamics of NLP. Moreover, during pulsation process, the NLP splitting could happen with the increasing of energy. All these findings will help to complement our understanding of NLP pulsation in a fiber laser.

Building a digital twin of an EDFA for optical networks: a gray-box modeling approach

High-accuracy physical layer models enable intelligent, self-driving optical networks. The dynamic wavelength-dependent gain characteristics of erbium-doped fiber amplifiers (EDFAs) remain a crucial problem in terms of modeling. The gain model directly determines the power spectrum and is therefore important for estimating the optical signal-to-noise ratio as well as the magnitude of fiber nonlinearities. Black-box data-driven models have been widely studied, but they require a large size of data for training and suffer from poor generalizability. In this paper, we derive the gain spectra of EDFAs as a simple univariable linear function; then, based on it, we propose a gray-box EDFA gain modeling scheme. Experimental results show that, for automatic gain control (AGC) and automatic power control (APC) EDFAs, our model built with 8 data samples can achieve better performance than the neural network (NN) based model built with 900 data samples, which means the required data size for modeling can be reduced by at least 2 orders of magnitude. Moreover, in the experiment, the proposed model demonstrates superior generalizability to unseen scenarios since it is based on the underlying physics of EDFAs. With the proposed scheme, building a customized digital twin of each EDFA in optical networks becomes more feasible, which is essential, especially for next-generation multiband network operations.

Open EDFA gain spectrum dataset and its applications in data-driven EDFA gain modeling

Optical networks satisfy high bandwidth and low latency requirements for telecommunication networks and data center interconnection. To improve network resource utilization, machine learning (ML) is used to accurately model optical amplifiers such as erbium-doped fiber amplifiers (EDFAs), which impact end-to-end system performance such as quality of transmission. However, a comprehensive measurement dataset is required for ML to accurately predict an EDFA’s wavelength-dependent gain. We present an open dataset consisting of 202,752 gain spectrum measurements collected from 16 commercial-grade reconfigurable optical add–drop multiplexer (ROADM) booster and pre-amplifier EDFAs under varying gain settings and diverse channel-loading configurations over 2,785 hours in total, with a total dataset size of 3.1 GB. With this EDFA dataset, we implemented component-level deep-neural-network-based EDFA models and use transfer learning (TL) to transfer the EDFA model among 16 ROADM EDFAs, which achieve less than 0.18/0.24 dB mean absolute error for booster/pre-amplifier gain prediction using only 0.5% of the full target training set. We also showed that TL reduces the EDFA data collection requirements on a new gain setting or a different type of EDFA on the same ROADM.

Advanced DSP-Based Monitoring for Spatially Resolved and Wavelength-Dependent Amplifier Gain Estimation and Fault Location in C+L-Band Systems

The development of efficient anomaly detection schemes is a key element for the implementation of autonomous optical networks as they can help telecom operators to automate the location of defective devices and track the overall performance of the network infrastructure. In that regard, the exploitation of receiver based digital signal processing (DSP) for optical performance monitoring has shown to be a promising enabler for detection of spatially resolved and wavelength-dependent properties and anomalies in optical fiber links. In this work, we study the benefits of applying DSP-based longitudinal power estimation on multiple wavelength division multiplexing (WDM) channels allocated in the optical grid to infer wavelength-wise characteristics of a C+L-band optical line system. In that context, we show that the applied scheme can successfully recreate a visualization of the spatial evolution of the gain tilt imposed by in-line optical amplifiers. Additionally, we propose the utilization of advanced DSP tools based on wavelet-denoising to enhance the performance of an anomaly detection approach. The proposed method not only can improve accuracy of the fault location, by reducing positioning uncertainty, but it also delivers more uniform readings of the anomaly signatures.

A Software-Defined Programmable Testbed for Beyond 5G Optical-Wireless Experimentation at City-Scale

Wireless traffic will significantly increase in next-generation networks. Therefore, there is a need for novel optical network front-/mid-/backhaul (x-haul) architectures that can deliver ultra-high bandwidth and ultra-low-latency services to wireless and edge cloud systems, as well as support various cloud radio access network (C-RAN) architectures. Designing and evaluating such architectures requires large-scale experimental platforms. In this article, we present the design and implementation of a unique, remotely accessible, disaggregated, and programmable optical network as the key component of the city-scale PAWR COSMOS advanced wireless testbed. We discuss the design and implementation of a dedicated multi-functional Ryu software-defined networking (SDN) controller for the testbed's optical network wavelength channel assignment and topology reconfiguration. We present an example use of the SDN controller for monitoring and automated measurement of the wavelength-dependent amplifier gain in deployed reconfigurable optical add/drop mul-tiplexer (ROADM) units. We also demonstrate a pilot experiment focusing on a wireless handover scheme for C-RAN architectures in the COSMOS testbed, where high traffic volume can be supported by dynamic wavelength allocation via optical switching in the fronthaul. The field-deployed testbed and SDN controller can be used by the research community to push the envelope in the area of optical networks for beyond 5G and 6G.

High average output power from a backside-cooled 2-µm InGaSb VECSEL with full gain characterization.

We compare the gain and continuous wave lasing properties of two InGaSb-based vertical external cavity surface emitting lasers (InGaSb VECSEL) with different heat management approaches operating at a center wavelength of around 2μ m. To date, intracavity heatspreaders have been required for good average output power, which have many trade-offs, especially for passive modelocking. Here we demonstrate a record high average output power of 810 mW without an intracavity heatspreader using a backside-cooled non-resonant VECSEL chip optimized for modelocking. In addition, we introduce and demonstrate an optical characterization for a wavelength range of 1.9 to 3μ m to precisely measure wavelength-dependent gain saturation and spectral gain. Gain characteristics are measured as a function of wavelength, fluence, pump power and temperature. Small signal gain of more than 5%, small saturation fluences and broad gain bandwidths of more than 90 nm are demonstrated. In comparison to a commercial VECSEL chip with an intracavity heatspreader, we have obtained similar average output power even though our VECSEL chip is designed for antiresonance.

Modeling EDFA Gain: Approaches and Challenges

With the rapid development of virtual/augmented reality and cloud services, the capacity demand for optical communication systems is ever-increasing. To further increase system capacity, current researches focus on efficient and reliable system management, in which the transmission performance should be accurately estimated. The wavelength-dependent gain effects of erbium-doped fiber amplifiers (EDFAs) have a great impact on transmission performance, and therefore a precise EDFA gain model is required. In this paper, we firstly summarize the underlying principles and structures of EDFA, and introduce the gain performance and challenges in modeling. Then, we review the EDFA gain modeling methods. We categorize these researches into analytical modeling methods and machine learning (ML)-based modeling methods, and discuss their feasibilities and performances. In addition, we discuss the remaining problems for applying the models in a system and the possible directions for future investigations.

Machine-learning-based EDFA gain estimation [Invited

Optical transmission systems with high spectral efficiency require accurate quality of transmission estimation for optical channel provisioning. However, the wavelength-dependent gain effects of erbium-doped fiber amplifiers (EDFAs) complicate precise optical channel power prediction and low-margin operation. In this work, we examine supervised machine learning methods using multiple artificial neural networks (ANNs) to build models for gain spectra prediction of optical transmission line EDFAs under different operating conditions. Channel-loading configurations and channel input power spectra are used as an a posteriori knowledge data feature for model training. In a hybrid learning approach, estimated gain spectra calculated by an analytical model are added as an a priori input data feature to further improve the EDFA ANN model performance in terms of prediction accuracy, training time, and quantity of training data. Using these methods, the root mean square error and maximum absolute error of the predicted channel output power can be as low as 0.144 dB and 1.6 dB, respectively.

High Density Multi-Channel Passively Aligned Optical Probe for Testing of Photonic Integrated Circuits

In this work we report the results of high density multi-channel optical multiprobes with pitches of 25 μm and 50 μm that provide edge-coupling used for on-wafer parallel testing of photonic integrated circuits. The probes are fabricated in an oxynitride platform and test demonstrations were carried out of edge-coupled indium-phosphide based photonic integrated circuits (PICs). Thirty-two optical parallel connections are simultaneously, passively aligned between the probe and the PIC by means of integrated guiding channels. The initial placement tolerance is more than 4 μm to give a passive alignment with an optical power variation of less than 1 dB. Multi-port loop-back optical power measurements are reported and the wavelength-dependent net modal gain of integrated semiconductor optical amplifiers was measured to further validate the concept.

Wavelength Switchable Multi-Wavelength Erbium-Doped Fiber Laser Based on Polarization-Dependent Loss Modulation

We propose and demonstrate a novel and simple wavelength switchable multi-wavelength erbium-doped fiber (EDF) laser based on polarization-dependent loss (PDL) modulation with cascaded fiber Bragg gratings (FBGs) as the wavelength comb filter. PDL is generated and controlled by different polarization angles between the polarization state of each lasing line and the optical fiber polarizer in the laser cavity. The PDL modulation balances the difference between the total loss and the wavelength-dependent lasing gain. The relations among gain, loss, and laser output power under different PDL conditions are numerically analyzed on the basis of a simplified Giles model in an EDF laser system. Results show that multi-wavelength lasing lines can be built up by properly tuning the PDLs in different wavelengths. The amount of lasing lines can be switched by changing the PDL intensity in a certain wavelength. In an experiment, the PDL in that certain wavelength is modulated by utilizing four polarization controllable elements, each of which includes an FBG and a corresponding polarization controller (PC). One PC is used to control the polarization state of the reflected lasing line on the center wavelength of the FBG. A four-wavelength individually switchable EDF laser system is demonstrated by utilizing this wavelength switchable method, in which the switching operation for the single-, dual-, triple-, and four-wavelength laser output is verified. The experimental results are consistent with the theoretical results. All wavelength switching operations are conveniently controlled by merely adjusting the PCs. This simple structure and flexible wavelength switchable operations show that this multi-wavelength EDF laser has great potential in practical applications.

Nonlinear Propagation in Optical Fibers With Gain Saturation and Gain Dispersion

There have been many developments in nonlinear propagation models in the past few decades. Especially, a form of such model has been developed to allow a standard ordinary differential equation (ODE) solver to be directly used for its solution. But such a model currently does not consider gain saturation and wavelength-dependent gain, which are very important in high-pulse-energy lasers. In this work, the directly-ODE-integrable nonlinear propagation equation is extended to include gain saturation and gain dispersion, which is then used to study maximum pulse energy limited by amplified spontaneous emission and minimum pulse width limited by gain narrowing in ultrafast fiber lasers in order to demonstrate these new capabilities.

Energy-transfer upconversion and excited-state absorption in KGdxLuyEr1-x-y(WO4)2 waveguide amplifiers

We perform a systematic spectroscopic study in channel waveguides of potassium gadolinium lutetium double tungstate doped with different Er3+ concentrations. Transition cross sections of ground-state absorption (GSA) and excited-state absorption (ESA), as well as stimulated emission (SE) at the pump wavelength around 980 nm are determined. ESA is directly measured by the pump-probe technique. Evaluation of GSA and ESA spectra indicates that ESA may be diminished by an appropriate choice of pump wavelength near 980 nm. Besides, GSA and SE at the signal wavelength around 1.5 µm are measured and the wavelength-dependent gain cross section as a function of excitation density is determined. Non-exponential luminescence decay curves from the 4I13/2 and 4I11/2 levels are analyzed and the probabilities of the energy-transfer-upconversion (ETU) processes (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and (4I11/2, 4I11/2) → (4I15/2, 4F7/2) are quantified. Despite the large interionic distance between neighboring rare-earth sites in potassium double tungstates, the probability of ETU is comparatively large because of the large cross sections of the involved transitions. A rate-equation analysis of the influence of ETU and ESA on gain at ∼1.5 µm is performed, revealing that ETU from the 4I13/2 amplifier level strongly limits the gain when the doping concentration increases above ∼6at.%. The calculated maximum achievable internal net gain per unit length amounts to ∼15 dB/cm for an optimized Er3+ concentration of ∼4 × 1020 cm−3 and a launched pump power of 300 mW at a pump wavelength of 984.5 nm, in reasonable agreement with recent experimental results.

Gain-driven spectral-temporal noise-like pulse dynamics in a passively mode-locked fiber laser.

We study numerically complex noise-like pulse dynamics in a passively mode-locked erbium-doped fiber laser. Wavelength-dependent gain dynamics is modeled as a combination of a three-level and a four-level system, which approximate the gain behavior in the 1530-nm and 1560-nm regions, respectively. The typical deformation of the erbium gain spectrum as it saturates is properly reproduced by this approach. Several puzzling noise-like pulse dynamics that were recently observed experimentally are qualitatively reproduced numerically, in particular slow quasi-periodic energy variations and the emergence and walkoff of wavelength-shifted radiation components. These results clearly reveal that gain dynamics is deeply involved in the onset of such complex temporal and spectral instabilities in these sources.

An All-Fiber Multi-Channel Ultrasonic Sensor Using a Switchable Fiber Bragg Gratings Filter in Erbium-Doped Fiber Laser

We theoretically and experimentally demonstrate an all-fiber multi-channel ultrasonic sensor using a switchable fiber Bragg gratings (FBGs) filter in an erbium-doped fiber laser (EDFL). The FBGs are not only used as the filter to determine the laser wavelength, but also as the sensing element to induce the intensity response of the lasing line via cavity loss modulation from the ultrasound-induced relative spectral shift of the matched FBGs. The switchable FBGs filter consists of a tuning FBG and several sensing FBGs. Multi-channel ultrasound detection is achieved via this filter by tuning an FBG to scan the spectra of multiple sensing FBGs. The ultrasonic-detection channel can be flexibly and efficiently switched via tuning the tunable FBG to match with any one of the sensing FBGs. A novel dynamic response model of the EDFL based on wavelength-dependent gain is proposed and developed to study the effects of different spectrally matched FBG positions and the consistency of the ultrasound response of each detection channel. The theoretical results show that the optimal sensing position is at the overlapping regions of spectra when the slope of the FBGs is greatest and the laser power is relatively high. Meanwhile, the consistency of the ultrasound response for each detection channel is influenced by the laser-resonance condition of the EDFL and can be optimized by adjusting the laser-cavity loss. Such theoretical results are well-verified experimentally. This sensor-laser-integrated system achieves high ultrasonic detection response since cavity-loss modulation is related to the amplifying effect of the laser. Using this method, an eight-channel switchable ultrasonic-sensing system has been demonstrated, covering nearly a 16-nm spectral range. By further controlling the cavity loss in the laser system, the consistency of ultrasonic response of each detection channel was optimized. The proposed all-fiber system has multiple- detection channels in a single laser system and consistent ultrasonic-detection response at each channel; it shows great potential for fiber-optic-ultrasonic-detection applications.

Experimental demonstration of a few-mode Raman amplifier with a flat gain covering 1530–1605 nm

We design and experimentally demonstrate a few-mode Raman amplifier over the C+L band with a flat on-off gain of ∼4 dB, for the first time, to the best of our knowledge. A state-of-the-art gain bandwidth of 75nm (1530-1605nm) is achieved. The wavelength-dependent gain for both LP01 and LP11 modes is about 0.6dB, and the maximum mode-dependent gain is less than 0.3dB. Mode-division multiplexed optical transmissions are performed over 75km few-mode fiber assisted by the proposed Raman amplifier. Due to the low noise Raman amplification, more than 3.3dB optical signal-to-noise ratio enhancement has been obtained with a significantly improved bit-error rate of about two orders.

Vertically Integrated Optical Sensor With Photoconductive Gain > 10 and Fill Factor > 70%

A hybrid active pixel optical sensor for high-resolution and high-sensitivity imaging is proposed and experimentally demonstrated. The sensor vertically integrates an amorphous silicon p-i-n photodiode and a low-temperature polycrystalline silicon readout thin-film transistor (TFT). The vertical integration results in a high fill factor (>70%) and an enlarged photosensing area in the pixel. In the photodiode-gated TFT structure of the sensor, the output current is amplified by operating the TFT in the sub-threshold regime. A weak wavelength-dependent photoconductive gain >10 is obtained at visible wavelengths, enabling large area low-level light detection.

EDFA Wavelength Dependent Gain Spectrum Measurement Using Weak Optical Probe Sampling

Wavelength dependent gain is a critical parameter in erbium-doped fiber amplifiers and is the primary determinant of the channel power divergence and excursions in optical transmission systems, each of which varies with channel loading in wavelength-division-multiplexed (WDM) systems. In an optical transmission system, measurements of the wavelength dependent gain between two locations can be used to obtain estimates of optical power excursions that occur in optical circuit switching. Non-intrusive sampling of just 10% of the WDM channel positions using weak probe signals achieves less than 0.15 dB error for the gain spectrum estimation of the amplifier with 5 dB tilt. In a two-span node-to-node network with a 3 dB peak-to-peak channel power difference, the weak probe approach further shows to provide less than 0.2 dB error in optical channel power excursion prediction for changes in channel loading.

Doping concentration distribution in 2-signal LP-mode ring-core erbium-doped fiber

We simulated an improvement in fiber characteristics, specifically the differential modal gain (DMG) and the LP11 mode wavelength dependent gain (WDG) characteristics. We defined the DMG as the gain difference between the C-band wavelength-division multiplexing signal channels of the LP01 and LP11 modes at the shortest wavelength, and WDG was the gain difference between the signal gain of the LP11 mode at the shortest and longest wavelengths, when the DMG of the LP01 mode was zero. We achieved this improvement by changing the erbium doping distribution in ring-core erbium-doped fiber (RC-EDF), which has been attracting attention as an amplification medium for a 2-mode (LP01 and LP11 modes) erbium-doped fiber amplifier for mode-division multiplexed transmission. We applied an α-th power profile, trapezoid profile, and right and left triangular profiles to the RC-EDF. We found that the DMG value changed from negative to positive despite the value of the LP11 mode WDG being positive, and confirmed that the DMG could be adjusted by changing the distribution. We show level curve maps of the DMG, LP11 mode WDG, pump power, and EDF length on the allowable 2-signal-mode area map for inner and outer radius combinations of the RC-EDF, and clarify that it is possible to realize a region that provides high-efficiency amplification with a low DMG of <|0.1|  dB and a low WDG of <0.1  dB. Furthermore, we show that by varying the distribution of the erbium doping concentration, we can greatly change the position of the region on the map of the allowable 2-signal-mode area.

Development of Gated Pinned Avalanche Photodiode Pixels for High-Speed Low-Light Imaging

This work explores the benefits of linear-mode avalanche photodiodes (APDs) in high-speed CMOS imaging as compared to different approaches present in literature. Analysis of APDs biased below their breakdown voltage employed in single-photon counting mode is also discussed, showing a potentially interesting alternative to existing Geiger-mode APDs. An overview of the recently presented gated pinned avalanche photodiode pixel concept is provided, as well as the first experimental results on a 8 × 16 pixel test array. Full feasibility of the proposed pixel concept is not demonstrated; however, informative data is obtained from the sensor operating under −32 V substrate bias and clearly exhibiting wavelength-dependent gain in frontside illumination. The readout of the chip designed in standard 130 nm CMOS technology shows no dependence on the high-voltage bias. Readout noise level of 15 rms, full well capacity of 8000, and the conversion gain of 75 µV are extracted from the photon-transfer measurements. The gain characteristics of the avalanche junction are characterized on separate test diodes showing a multiplication factor of 1.6 for red light in frontside illumination.

On the Efficiency of Parabolic Self-Similar Pulse Evolution in Fiber Amplifiers with Gain Shaping

We numerically and experimentally demonstrate the influences of gain fiber length, initial pulse center wavelength, duration, chirp, and temporal profile for efficient parabolic self-similar evolution, accounting for the strong gain shaping effect in fiber amplifiers under high gains. A spectrally resolved numerical model allowing for realistic descriptions of the variable wavelength-dependent gain is developed. The results predict the specific regions of the initial center wavelength and the duration for the self-similar evolution, which move toward a longer center wavelength and a broader duration range with the increasing gain fiber length. For short-length high-gain amplifiers, a proper negative initial chirp can not only resist the gain-shaping deformation and facilitate the self-similar evolution but also provide a broadband output. In addition, a triangular initial profile is found to be of minimum sensitivity to the detrimental gain shaping and optimum for efficient self-similar evolution. The experimental results confirm the numerical predictions. The studies of the fast parabolic pulse formation in short-length high-gain amplifiers presented here demonstrate the potential for performance scaling of femtosecond fiber amplifiers, few-cycle pulse sources, high-power frequency combs, and related applications.