Gain Excursion Research Articles

A Novel Dynamic Distributed Raman Amplifier for the Gain Excursion Assessment of Real-Time Optically Amplified Web

ABSTRACT In this article, the proposed Distributed Raman Amplifier (DRA) simulator is designed on the MATLAB Simulink platform. 16 channels DWDM in C-band (1544–1559 nm) and 32 channels CWDM system in S-, C- and L band (1460–1625 nm) are executed with data rate of 100 Gb/s. The designed standalone system realizes the various effects that may arise at the receiver output due to reconfiguration of channels in real-time dynamic WDM optical traffic. Gain excursion assessment, and bit error rate (BER) are investigated for 200 km long Fiber cable and 45 km of Raman amplifier in co-pumping, counter pumping and bidirectional pumping propagation.

Erbium-doped circular photonic crystal fiber design for the amplification of 20 OAM modes

We propose a fiber design consisting of ring-shaped erbium-doped core surrounded by circularly arranged holes of varying radius in fused silica glass to amplify higher-order orbital angular momentum (OAM) modes. We numerically demonstrate simultaneous amplification of 20 OAM modes ( l = 5 , 6, 7, 8, 9) with more than 20 dB gain at 1550 nm wavelength while having differential modal gain of 0.12 dB. We also demonstrate amplification of modes over the C-band with gain excursion of less than 2 dB, differential modal gain of less than 0.2 dB, and noise figure less than 4.2 dB. The proposed erbium-doped fiber amplifier would be useful in high-data-rate space-division multiplexing optical-communication systems.

Modal gain equalization of 18 modes using a single-trench ring-core EDFA

The few-mode erbium-doped fiber amplifier (FM-EDFA) is a key component of a few-mode-fiber-based optical communication system to allow the simultaneous amplification of all signal mode groups. This paper proposes the single-trench ring-core FM-EDFA for gain equalization of five signal mode groups—LP01, LP11, LP21, LP31, and LP41. The proposed fiber with ring-core Er+3 doping and fundamental pumping is used to study the gains and differential modal gains (DMGs) of signal mode groups. In this study, we have shown that introduction of a trench increases the modal confinement of higher mode groups and decreases that of lower mode groups in the ring region (or doped region) slightly. This helps in controlling the DMG and also increases the effective index difference Δneff between the adjacent modes. We show that at 1530 nm signal wavelength, the DMG of five mode groups is reduced from 1.32 dB (without trench) to 0.45 dB (with trench) with gain in excess of 20 dB. Also, the mode spacing Δneff increases from 4.9×10−4 to 5.1×10−4. Over the C-band, we have achieved more than 20 dB amplification and nearly 1 dB gain excursion while maintaining sufficient mode spacing Δneff>5.1×10−4 to avoid mode coupling due to macrobending.

Dual-core few mode EDFA for amplification of 20 modes

The capacity scaling beyond Shannon-Limit is a major issue in optical communication system. Space division multiplexing (SDM) employing few mode fiber (FMF) has ability to resolve this issue. Few-mode erbium doped fiber amplifier (FMEDFA) is the key component of FMF based SDM system. In this paper we present a fiber design, which is capable of amplifying 20 modes of 6 mode groups (LP01, LP02, LP11, LP21, LP31 and LP41). Proposed design employs a dual-core doped EDF with many modes pumping. We show amplification of the modes with more than 21 dB gain and 1.75 dB gain excursion over the C-band. The study would be useful in few-mode fiber SDM optical communication system.

Dual-Core Leaky Waveguide-Based Filters for Erbium-Doped Waveguide Amplifier

This article gives the design of a gain-flattening filter for an erbium-doped waveguide amplifier using a dual-core resonant leaky waveguide. The spectral variation of leakage loss of the waveguide has been used for gain equalization of the erbium-doped waveguide amplifier in the conventional (C) band. Leakage loss of the structure has been calculated by using the transfer matrix method. Numerical simulation results show a 17.5-dB gain with a gain excursion of ±1.35 dB in the C-band (1,526 nm–1,566 nm) of a typical erbium-doped waveguide amplifier. The study should be useful in designing integrated optic gain-flattening filters.

A Novel Erbium-Doped Fiber Amplifier Simulator for Gain Excursion Estimation in Multi-Channel Dynamic Optical Network

A novel erbium-doped fiber amplifier simulator designed using the SIMULINK toolbox of MATLAB 7.0 (The MathWorks, Natick, MA, USA) is reported in this article. The present simulator has an ability to incorporate multi-channel amplification simultaneously in both the C- and L-bands. It is realized by defining new FUNCTION block sets and replacing the MATLAB FUNCTION block set reported earlier for multi-channel amplification. Spectral variation of gain for an erbium-doped fiber amplifier simulator is first verified in both the C- and L-bands. Next, the simulator is employed to study gain excursion in a multi-channel dynamic optical network, where the change in the gain excursion by varying the pump power has also been estimated. The present approach to estimate the gain excursion will find applications in quantifying inter-channel cross-talk due to cross-gain saturation among co-propagating multi-channels in a dynamic optical network.

Implementation of PT symmetric devices using plasmonics: principle and applications

The so-called PT symmetric devices, which feature ε((-x)) = ε((x))* associated with parity-time symmetry, incorporate both gain and loss and can present a singular eigenvalue behaviour around a critical transition point. The scheme, typically based on co-directional coupled waveguides, is here transposed to the case of variable gain on one arm with fixed losses on the other arm. In this configuration, the scheme exploits the full potential of plasmonics by making a beneficial use of their losses to attain a critical regime that makes switching possible with much lowered gain excursions. Practical implementations are discussed based on existing attempts to elaborate coupled waveguide in plasmonics, and based also on the recently proposed hybrid plasmonics waveguide structure with a small low-index gap, the PIROW (Plasmonic Inverse-Rib Optical Waveguide).

Assessment and mitigation of Erbium-doped fibre amplifiers (EDFA) gain transients in hybrid wavelength division multiplexing/time division multiplexing passive optical network (WDM/TDM PON) in the presence of packet-based traffic

In this study, the authors present an innovative gain clamping method to mitigate Erbium-doped fibre amplifiers transients under bursty or packetised traffic. This method is based on a lasing technique by means of a continuous wave feedback signal reflected on a fibre Bragg grating. The clamping was assessed using on a range of re-injected powers from -30 to -5-dBm and able to fully mitigate the packet transients. The performance of the system at 2.5 and 10.0-Gb-s was appraised for different inter-packets times being observed in all cases an improvement in the Q factor. Additionally, a study that includes a characterisation of the transients (gain excursion and decaying time) with the inter-packet parameters and a comparison with theory was also presented.

Enhanced optical gain clamping for upstream packet based traffic on hybrid WDM/TDM-PON using fiber Bragg grating

In this paper, we propose a method to mitigate the temporal power transients arising from Erbium doped fiber amplifiers (EDFAs) on packeted/bursty scenario. The technique, applicable on hybrid WDM/TDM-PON for extended reach, is based on a low power clamping provided by a distributed feedback (DFB) laser and a fiber Bragg grating (FBG). An improvement in the data signal Q factor was achieved keeping the clamping control signal with a low power, accompanied by a maximum reduction in the gain excursion of 1.12dB.

Gain dynamics of EDFA in loop buffer switch

This paper studies the gain dynamics of the Erbium doped fiber amplifier (EDFA) in the loop buffer based switch. The gain excursion of the EDFA is measured for different length packets, and in the analysis asynchronous transfer mode (ATM), and the key to all optical packet switching (KEOPS) packet size is considered. The effect of the packet inter arrival time on the gain is also measured. The effect of loop length and data rate is discussed in the gain dynamics of the EDFA. It is shown in the paper that the loop buffer structure acts as an optical cavity where the signal can lase, remain transparent or it can decay with a number of recirculations. In this paper, three gain controlling schemes proposed in recent past, for the loop buffer based architectures are discussed and it is found that none of these schemes can provide a robust solution, as these schemes are developed under simplifying assumptions which are not true once the design aspects of the loop buffer switch are considered.

Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier

We report design and subsequent fabrication of an intrinsically gain flattened Erbium doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ⩾28 dB (gain excursion below ±2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at −20 dBm/channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head.

Novel Gain Spectrum Control Method Employing Gain Clamping and Pump Power Adjustment in Thulium-Doped Fiber Amplifier

In this paper, we propose a novel method for controlling the gain spectrum of a thulium-doped fiber amplifier (TDFA) in the S-band. The conventional gain spectrum control method used for a silica erbium-doped fiber amplifier (EDFA) cannot be applied to TDFAs because of the complicated fluctuation of the TDFA gain spectrum. Our proposed method controls the gain spectrum by a combination of gain clamping and pump power adjustment. The algorithm for the method is as simple as that for the conventional EDFA gain spectrum control method. Furthermore, we describe a function for correcting the gain excursion generated by the incorporation of amplified spontaneous emission (ASE) at a signal monitoring photodiode (PD). We achieved a gain excursion of 0.35 dB against a total input signal power of 32 to 2 dBm.

Dynamic gain induced pulse shortening in Q-switched lasers

We describe a novel mechanism of pulse shortening in a Q-switched laser induced by the gain compression effect under strong pumping conditions. The pulse shortening requires a large variation of the gain excursion during the saturation process and benefits from the large volume of the gain medium. The effect has been experimentally demonstrated using a passive Q-switched Tm/Ho-doped fiber laser that shows gain-induced pulse compression from 800 ns down to 160 ns when the pump threshold is exceeded by 15 times.

Automatic gain control in Raman amplifier with multi-wavelength pumps

Automatic gain control method in Raman amplifier with multi-wavelength pumping scheme is presented. Monitoring of several channel power and feedback pump control is used in the gain control method. The condition to minimize the gain deviation is investigated by numerical simulation. Two monitoring channels are necessary to confine gain deviation in ±0.2dB at two pumps C-band Raman amplifier. In the experiment, gain deviation of 1.5dB is controlled to 0.2dB at 38/40 channels drop, and fast suppression of transient gain excursion is achieved.

Hybrid fiber amplifiers consisting of cascaded TDFA and EDFA for WDM signals

The detailed gain characteristics of hybrid fiber amplifiers that consist of cascaded thulium-doped fiber amplifiers (TDFAs) and erbium-doped fiber amplifiers (EDFAs) are reported. The experimental results showed that the hybrid amplifiers have gains of over 20 dB with the bandwidth of more than 80 nm in the wavelength range between 1460 and 1560 nm. The low noise figure (NF) below 7 dB was obtained in 1460-1540 nm when placing a TDFA in the first stage followed by an EDFA and in 1480-1560 nm when placing amplifiers in a reversed order. The gain of TDFA and EDFA was optimized for minimizing the gain variation ratio (GVR=(maximumgain-minimumgain)/minimumgain: in the unit of decibels) of the hybrid amplifiers, and it could be minimized to less than 0.4 for the amplifiers that have gain in the wavelength region from 1460 to 1537 nm. The gain-equalization technique was applied, and the hybrid amplifier that had an average gain of 20 dB, a gain excursion of less than 2 dB, an output power of 14.5 dBm, and an NF of less than 7 dB in the 77-nm gain band was achieved.

WDM burst mode signal amplification by cascaded EDFAs with transient control

We demonstrate that WDM burst signals are amplified with suppressed gain transients in cascaded erbium-doped fiber amplifiers (EDFAs) with fast electrical feedback control. In a recirculating loop experiment, the gain excursion is suppressed to 5.1 dBp-p after 45 EDFAs while the excursion is 28 dBp-p after 5 EDFAs without the gain control. Spectral hole burning is a main limiting factor for the transient suppression. The effect of the gain control is also verified in a transmission experiment at 10 Gb/s.

Characteristics of low noise hybrid fiber amplifier

The characteristics of hybrid fiber amplifier (HFA) are investigated. HFA is composed of three stages: short-length EDFA pre-stage, DCF Raman amplifier, and power boosting EDFA. HFA has low noise figure, high output power, and also wide input power dynamic range. Gain control method of HFA is presented experimentally, and the transient gain excursion is suppressed to less than 0.5dB at 3dB channel add-drop. HFA can be used as line amplifier in optical transmission link even combined with distributed Raman amplifier due to wide input power dynamic range. The transmission performance of HFA is better than EDFA by more than 1.0dB of Q-factor in 720km SMF transmission.

Semianalytic dynamic gain-clamping method for the fiber Raman amplifier

We present a physically meaningful semianalytic gain-clamping algorithm for multipump Raman fiber amplifier in large signal regime. By utilizing parameters from the steady state conditions, the suppression of gain excursion below 0.5 dB (up to 78/80 channel drop) can be achieved within microsecond/millisecond time scale, depending on the signal power level.

Spectral beam combining of a single-mode 980-nm laser array for pumping of erbium-doped fiber amplifiers

With a view to improve the power level and brightness of semiconductor pump lasers, we have resorted to a scalable implementation and achieved spectral beam combining through a low-quality-factor external cavity. For that purpose, a high-power single-mode laser array emitting up to 2.1 W at 3.2 A at 980 nm has been realized, which delivers 1.5 W in external cavity. A power of 0.66 W was finally coupled into the single-mode fiber, with a coupling efficiency of 44%. Pumping experiments of an erbium-doped fiber amplifier utilizing this laser have demonstrated efficiency and noise characteristics similar to what is usually obtained with conventional sources, with the noteworthy advantage of a lower gain excursion over the C-band.

Measurements of gain cross-saturation and transient response in highly doped TDFAs

We report measurements of cross-saturation bandwidth on signal wavelength in single-pumped Thulium doped fiber amplifiers (TDFA) using two different pump wavelengths (1426 and 1050 nm) and a highly doped (5000 ppm) Thulium-doped fiber module. The 1050 nm pump has a lower cross-saturation effect than the 1426 nm, and an increase in cross-saturation is observed as the signal wavelength decreases within the S band. We also experimentally investigated the transient response of a single-pumped TDFA at 1050 nm, and analyze the amount of surviving channel gain, gain excursion and time-constants or the highly doped fibers.