Gain Tilt Research Articles

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.

DSP-Based Link Tomography for Amplifier Gain Estimation and Anomaly Detection in C+L-Band Systems

A successful migration from current C-band based optical networks to a multiband scenario primarily depends on the development of solutions that can reliably measure physical properties of optical links over broad spectral transmission windows. Additionally, these solutions must be capable of delivering wavelength-dependent and spatially-resolved indicators that can empower network operators to identify faults before they lead to severe service disruptions. Recently, the exploitation of receiver based digital signal processing as a tool for optical performance monitoring has gained tremendous popularity. One successful example is the so-called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> power profile estimator, which can reconstruct the per-channel longitudinal power profile along the optical fiber link solely processing the received signal samples. In this work, we propose a novel application for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> power profile estimator by harnessing it on multiple wavelengths to accurately estimate the spectral gain profile of C+L-band in-line Erbium-doped fiber amplifiers deployed in a 280-km single mode fiber link. Furthermore, we show how this scheme can be efficiently used to detect amplification-related anomalies, such as gain tilt and narrowband gain compression. In our measurements, we achieved a sub-dB estimation accuracy by comparing the proposed gain extraction approach with the back-to-back characterization obtained from an optical spectrum analyzer.

Automated training dataset collection system design for machine learning application in optical networks: an example of quality of transmission estimation

Applications of machine learning (ML) models in optical communications and networks have been extensively investigated. For an optical wavelength-division-multiplexing (WDM) system, the quality of transmission (QoT) estimation generally depends on many parameters including the number and arrangement of WDM channels; launch power of each channel; number and distribution of fiber spans; attenuation, dispersion, and nonlinearity parameters and length of each fiber span; noise figure; gain and gain tilt of erbium-doped fiber amplifiers; transceiver noise; digital signal processing (DSP) performance; and so on. In recent years, ML-based QoT estimation schemes have gained significant attention. However, nearly all relevant works are conducted through simulations because it is difficult to obtain sufficient and high-quality datasets for training ML models. In this paper, we demonstrate completely automated generation and collection of an ultra-large-scale experimental training dataset for ML-model-based QoT estimation by automation of transceivers and optical link parameters, as well as data transfer and DSP. Implementation details and key codes of automation are presented. Artificial neural network models with one and two hidden layers are trained by the collected dataset, and brief QoT estimation results are evaluated and discussed to verify the performance and stability of the established automated system.

Experimental characterization of an o-band bismuth-doped fiber amplifier.

The recent emergence of bismuth-doped fiber amplifiers (BDFAs) offers the potential to transmit high-speed WDM signals over long distances in the O-band spectral region, thereby greatly enhancing the scope of systems utilizing these wavelengths. In this paper, we present a comprehensive experimental study on several basic characteristics of an O-band BDFA based on a phosphosilicate optical fiber, including the frequency-dependent noise figure, gain tilt (static and dynamic), transient response, and polarization dependent gain. We discuss our findings and their implications on the use of BDFA technology in high bit-rate multichannel systems.

Focus on photonics, spectroscopy, and spectrometry

Focus on photonics, spectroscopy, and spectrometry

Joint channel power and amplifier gain optimization in coherent DWDM systems

In this paper, the channel power and amplifier gain of coherent dense wavelength division multiplexing (DWDM) fiber-optic communication systems are jointly optimized. The objective is to maximize the minimum margin of signal to noise ratio (SNR). The presented optimization problem is non-linear and non-convex, in which the intra- and inter-channel nonlinear interference (NLI) noise of fiber is estimated by using the so-called Gaussian noise (GN) model. Furthermore, the noise and other practical aspects of optical amplifiers such as output power saturation, maximum gain limitation, gain tilt and ripple, and noise figure non-flatness are modeled. Finally, we show that this problem can be reformulated as a convex optimization. Our results reveal that joint power and amplifier gain optimization in a 5-span fiber link with span length of 80 km (100 km) leads to 1 dB (0.47 dB) higher SNR than the conventional power optimization method in which amplifier gain is fixed and equals to span loss. By considering gain ripple and tilt in a 8-span fiber link with span length of 80 km, we observed that our algorithm has 0.72 dB and 1.86 dB higher SNR than the alternative power optimization methods in which amplifier gain is equalized and unequalized, respectively.

Researcher and user interfaces for studies of hearing-aid self-adjustment

The “Goldilocks” explore-and-select protocol of Boothroyd and Mackersie was adapted for field studies of hearing-aid self-fitting and incorporated into the wearable UCSD open-source speech-processing platform (OSP). A researcher interface allows adjustment of hearing aid parameters, including starting response for self-adjustment; compression parameters; and number, consequence, and step size of listener controls. The listener-interface provides for adjustment of level and spectrum using either three controls (overall gain plus low- and high-frequency tilt) or two controls (overall gain and overall spectral tilt). Response selections made in specific acoustic environments can be saved, named, and recalled. Both interfaces are accessible via web-browser on any web-enabled device, including a listener’s smart-phone. Every listener-adjustment is logged and options are available for logging information about the ambient noise plus the listener’s reasons for, and satisfaction with, response modifications. These interfaces were created using HTML, CSS, and Javascript, and use PHP to communicate information with the embedded webserver on the wearable platform. This web framework is faster and easier to customize than is a native application, and it supports the iterative design required in research. Instructions for customizing the interface are included. They vary in challenge, from editing a text document to modifying documented, readable code.The “Goldilocks” explore-and-select protocol of Boothroyd and Mackersie was adapted for field studies of hearing-aid self-fitting and incorporated into the wearable UCSD open-source speech-processing platform (OSP). A researcher interface allows adjustment of hearing aid parameters, including starting response for self-adjustment; compression parameters; and number, consequence, and step size of listener controls. The listener-interface provides for adjustment of level and spectrum using either three controls (overall gain plus low- and high-frequency tilt) or two controls (overall gain and overall spectral tilt). Response selections made in specific acoustic environments can be saved, named, and recalled. Both interfaces are accessible via web-browser on any web-enabled device, including a listener’s smart-phone. Every listener-adjustment is logged and options are available for logging information about the ambient noise plus the listener’s reasons for, and satisfaction with, response modifications. Thes...

Optical variable gain tilt filter with temperature compensation

AbstractBy analyzing the transmission function of a voltage‐controlled electro‐optic transparent ceramic material, OptoCeramic® PMN‐PT based variable gain tilt filter (VGTF), its temperature‐dependent properties are obtained and a temperature‐dependent calculation model is established with the temperature‐dependent coefficients obtained by the combination of experimental and numerical methods. Then through the real‐time temperature surveillance, the control voltages can be dynamically adjusted via a USB interface and the VGTF achieves precise control and remains stable within temperature range from 25°C to 70°C. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1906–1909, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25311

Wavelength switchable L-band erbium-doped fiber laser employing C-band EDFA as gain medium

A new structure for the L-band multiwavelength switchable erbium-doped fiber laser is proposed. Overlapping laser cavities are formed by cascading fiber Bragg gratings as reflectors in a ring structure with a C-band EDFA as a common gain medium. The proposed laser is made to be wavelength-switchable by individually adjusting the loss of each overlapping cavity. Instead of using an L-band EDFA as the gain medium in the L-band fiber laser, we have experimentally demonstrated that by tailoring the gain of the C-band EDFA to have a significant gain tilt in the L-band, a lasing power efficiency of 26% can be achieved over a wide range of switchable lasing wavelengths. Following this optimal design, the side-mode suppression ratio and the minimal separation between two switchable lasing wavelengths were found to be over 42 dB and 0.33 nm, respectively.

Polarization-Insensitive GaInNAs–GaInAs MQW-SOA With Low Noise Figure and Small Gain Tilt Over 90-nm Bandwidth (1510–1600 nm)

rdquoWe developed a broadband polarization-insensitive multiquantum-well (MQW) semiconductor optical amplifier (SOA) for application as an optical switch in broadcast-and-select-type optical packet switching systems. We adopted a GalnNAs-GalnAs MQW structure active layer to decrease the noise figure (NF) around the gain peak wavelength together with a small gain tilt in the C-band. The device exhibited a smaller NF on the shorter wavelength side of the gain peak compared with a GalnNAs strained-bulk SOA. The reduced NF resulted in a large effective gain bandwidth of up to 90 nm (1510-1600 nm). The device also exhibited the small gain tilt (<1.2 dB) and small polarization-dependent gain (<0.8 dB) in the C-band.

Reconfigurable microwave photonic transversal filter using a variable gain tilt filter

A novel and simple technique to realize reconfigurable microwave photonic transversal filter is presented. In the proposed method, a multi-wavelength source and a dispersive medium are applied to achieve multiple taps; and an electro-optic ceramic material-based variable gain tilt filter (VGTF) is utilized to control the tap weights of the filter. The VGTF is computer-controlled via a universal serial bus interface, which enables real-time reconfiguration of the filter. Filter responses with different passband bandwidth and different sidelobe suppression can be achieved. Compared with previous approaches, the given method features in less optical components involved and faster response speed. A three-tap reconfigurable filter with sub-millisecond response speed is experimentally demonstrated.

Response of fibre optic parametric amplifiers to channel add/drop in agile all-photonic networks

Using numerical simulations, we investigate the performance of fibre optical parametric amplifiers (FOPAs) within the context of agile, multi-wavelength photonic networks. In particular, we study the steady-state and dynamic response of FOPAs following changes in network operating conditions. These include changes in the gain spectrum (gain tilt) due to variations in the total input power to the amplifier or as a result of channel add/drop, as well as the characteristics of the gain transients following channel add/drop. Initially, we examine the worst case scenario where all channels are in-phase and exactly equispaced. We compare these results with the ones obtained for random initial phases and inexact channel spacings. We find that the complex nature of the interactions occurring in a FOPA make it very difficult to predict their behaviour a priori and their deployment in WDM systems will require special attention. Moreover, we confirm that unequal spacing can vastly improve the performance of multichannel FOPAs.

Temperature-Dependent Gain Variation Reduction in$C$-Band Erbium-Doped Fiber Amplifier Using Phosphorus-Erbium-Doped Silica Fiber

We propose a new technique to reduce the temperature-dependent gain (TDG) tilt of erbium-doped fiber amplifier in the C-band by supplying a composite optical gain block, conventional aluminum-rich erbium-doped fiber (EDF) serially concatenated with a special EDF composed of phosphorus-doped silica host, which showed opposite TDG coefficients in the C-band. The TDG variation was suppressed to less than plusmn0.45 dB for the saturated gain of 16 dB in 1525-1565 nm, within the temperature range of -40degC to +80 degC

Gain characteristics of a 210 km hybrid Raman/erbium-doped fiber amplified loop

As an alternative to using gain flattening filters or other optical devices that add attenuation to balance the gain in the optical spectrum, we have studied the effects of combining gain from Raman amplifiers and erbium-doped fiber amplifiers (EDFAs) in a fiber recirculating loop. We have shown a parameter range for which the gain profile is flat, and have analyzed the variation of the gain tilt as a function of the individual contributions of the Raman and EDFA amplifiers.

Dynamic gain-tilt compensation using electronic variable optical attenuators and a thin film filter spectral tilt monitor

An all-optical dynamic gain tilt compensator (DGTC) is proposed and experimentally demonstrated. A single wide-band thin film filter and a pair of photodetector allow the DGTC to distinguish band add/drop position. Power fluctuations from EDFA gain tilt were reduced with fast electronic variable optical attenuators.

A highly effective technique for setting the power preemphasis in WDM optical systems

In wavelength-division-multiplexing (WDM) transmission systems, proper preemphasis of the input-channel powers is a key to optimizing the system performance. We present a novel technique for iteratively optimizing the preemphasis when many transmission effects can take place. The technique requires only the knowledge of the transmitted and received per-channel power values and of the uncorrected bit error rates taken from the forward-error-correction (FEC) chips. We first describe the procedure and test it using numerical simulations. We then demonstrate the technique in a realistic environment using a WDM 8 /spl times/ 10-Gb/s laboratory test bed, where power preemphasis is automatically found by means of a common personal computer. Both numerical and experimental tests show that the proposed preemphasis is able to compensate for several impairments, including significant spread of receiver sensitivity, gain tilt of the optical amplifiers, significant third-order uncompensated dispersion, as well as single-channel nonlinear impairments.

Measurement of gain tilt and its contribution to composite second-order distortion in the analog-digital transmission systems with Er/Yb fiber amplifiers.

Fiber-optic-based analog or digital community antenna television systems experience composite second-order (CSO) distortion caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog-digital transmission system with a high-power erbium/ytterbium-codoped fiber amplifier has been experimentally measured and its contribution to the CSO distortion of the system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO distortion of the transmission system with and without the amplifier. The dependence of the gain tilt on the modulation frequency and wavelength of the input light is also investigated.

S -band erbium-doped fiber amplifiers with a multistage configuration-design, characterization, and gain tilt compensation

We report an S-band erbium-doped fiber amplifier (EDFA) with a multistage configuration in terms of its design, gain, and noise characteristics for various pump powers and input signal powers, the temperature dependence of the gain spectra, and gain tilt compensation for changes in input signal power and temperature change. We show that there is a tradeoff between low noise and efficiency in the S-band EDFA and describe the development of an S-band EDFA with a flattened gain of more than 21 dB and a noise figure of less than 6.7 dB. We also show that there is a change in the gain spectra with changes in the pump power and input signal power that is different from that observed in C- and L-band EDFAs, and that our EDFA has a temperature-insensitive wavelength. Furthermore, we develop a gain tilt compensated S-band EDFA that can cope with changes in input signal power and temperature.

A dynamically gain‐flattened erbium‐doped fiber amplifier

AbstractThis paper reports on dynamically gain‐flattened C‐ and L‐band erbium‐doped fiber amplifiers (DGF‐EDFAs), whose dynamic gain ranges are above 20 dB and 24 dB, respectively. This paper gives the detailed test and comparative results for DGF‐EDFAs based on the use of a voltage‐controlled fiber‐based gain tilt controller and a variable optical attenuator. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 39: 8–11, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11110

Wide-dynamic-range and channel-count-free thulium doped fiber amplifiers employing variable slope attenuation compensators designed for photonic networks

This paper presents the unique behavior of thulium doped fiber amplifiers (TDFAs) under circumstantial changes in the multi-wavelength photonic networking systems, namely in the span loss, the channel count, and the environmental temperature. Unlike EDFAs, TDFAs exhibit the dynamic gain-profile variation even when the gain-magnitude is fixed under the automatic gain control (AGC). In addition, it has been shown that the dynamic gain variation is less serious in the up-conversion pumping scheme. As a means to maintain the gain-profile of the up-conversion pumped TDFA, three techniques have been introduced and compared, namely: (i) the auxiliary pump around 1.56 μm, (ii) the mid-stage variable optical attenuator (VOA), and (iii) the mid-stage variable slope attenuation compensator (VASC). Employing the VASC, the output signal level has been successfully maintained within the error of 0.2 dB for the dynamic range of 8 dBp-p and the channel count from 4 to 32. Moreover, it has been shown that the temperature change also induces the gain tilt, which has been successfully compensated by the VASC with the level variation of less than 0.3 dB. In summary, the adjustable slope-compensating device will be useful to control the TDFAs under the various circumstantial changes.