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We report on the study of a possible first step integration of mode division multiplexed optical component for single-mode fiber networks. State-of-the-art on few-mode erbium-doped fiber amplifiers is used to integrate the amplification function in a single component, which is expected to save energy in comparison to parallelized active components. So as to limit the impact of modal cross-talk, an elliptical-core few-mode erbium-doped fiber has been used to assemble an amplifier sharing setup for different single mode fibers, using non-degenerate modes. With this simple setup, we show the level of performances that can be reached for cross-talk, gain, differential modal gain and losses and discuss the ways to improve them for a possible integration in a real network.
© 2016 Optical Society of America
1. Introduction
Spatial Division Multiplexing (SDM) appears as a promising solution to increase the data transmission rates accessible in a unique fiber and face the so-called capacity crunch [1, 2]. Moreover, even if similar capacity can be reached by using N-Single Mode Fibers (SMFs) instead of a N-mode Few-Mode Fiber (FMF), the integration of specific functions (amplification, for example) in a single component rather than using N individual components is of great interest. For example, by replacing N-Erbium Doped Fiber Amplifiers (N-EDFAs) by a single Few-Mode Erbium-Doped Fiber Amplifier (FM-EDFA) supporting N modes, energy may be saved [3]. This is illustrated by the Fig. 1 that compares electrical power consumption (without using thermoelectric cooling module) as a function of the number of spatial paths for single mode and multimode amplifiers. These graphics have been calculated by maintaining a constant pump intensity (optical power per unit area) in the fiber core. For this study the starting point is a standard Single Mode EDFA, providing 20 dB gain with 50 mW optical pump power assuming no insertion loss. Calculation has been made, on one hand, for an increase of the number of SMFs and, on the other hand, an increase of the number of modes obtained by an increase of the core radius of a Few-Mode Erbium-Doped Fiber (FM-EDF). When more than 10 spatial paths are used, more than 50 % of electrical power could hence be saved by using multimode amplifiers. Furthermore, more economies could be made, especially when the number N of individual EDFA replaced by a single FM-EDFA rises. This concept could hence reduce both operationnal expenditure (OPEX) and also capital expenditure (CAPEX).
Fig. 1 Pump module electrical consumption for Single Mode (dashed line) and Few-Mode (plain line) EDFA, as a function of the number of spatial pathways (i.e. fibers or modes).
This approach can be extended from the case of a single FM-EDFA used in a SMF network, which would represent a first step integration of SDM components in the current network. In this case, it can be imagined that different signals, coming from different SMFs, are converted into different spatial modes, share the same amplifier - and the same pump beam - before being back-converted and re-injected into different SMFs. So as to make this architecture efficient, it is suitable to build a system presenting low-cross-talk with non-degenerated modes in order to simplify signal processing. To reach this goal, spatial multiplexer and demultiplexer with low cross-talk are used in this paper. Those elements are used together with an elliptical-core FMEDF so as to be able to use non-degenerate modes. Indeed, with “classical” FM-EDF (i.e. with circular core) spatial modes belonging to the same group are degenerated. As a consequence, this will increase the digital signal processing at the receiver side for these coupled channels and require 4x4 MIMO that may be inconsistent with the current single mode network. With an elliptical core FM-EDF, each spatial channel could be used independently and this architecture wouldn’t increase digital signal processing complexity. This approach is similar to the one proposed by Riesen et al. [4], and recently used by Ip et al. [5] for a three-mode transmission over a passive few-mode fiber. This choice has the second advantage to relax the constraint on Differential Modal Gain (DMG). Effectively, because each spatial channel could be treated independently, different losses and/or gain flattening filters could be applied to the different channels after back-conversion.
With such a fiber, a shared amplification setup for five SMFs is built. This setup allows us to investigate global cross-talk generated by the system, gain and DMG in amplification regime.
2. Experimental setup of few-mode erbium-doped fiber amplifier
The choice of an elliptical core ensures the lifting of the degeneracy between modes belonging to the same group, which should help to minimize coupling in comparison to a fiber with circular core. In order to draw such a fiber, we ran some numerical simulations (at 1550 nm) of an equivalent step-index fiber which the core shape varies from circular to elliptical (by maintaining a constant core area to keep the number of modes constant), so as to determine the minimum degree of ellipticity required. Results shown on Fig. 2 represent the dependence of the effective index of spatial modes (LP01, LP11a,b, LP21a,b, LP02 and LP31a) as a function of the core ellipticity. With this information, an ellipticity of around 1.3 has been chosen as a target. For such a value, the efficient separation of the effective index between all the modes should enable to use them independently on the relatively short length of the FM-EDF, since an effective index separation of 8 × 10−4 should be sufficient for few-mode fibers [6]. Furthermore, in this case a maximum birefringence of 1.5 × 10−5 is expected between the two polarizations of the same spatial mode. This makes this fiber compatible with dual polarization multiplexing used for the current single mode network. This lifting of degeneracies also leads to modify the intensity profile of modes as shown on Fig. 2, which can result in important changes of the power distribution especially for LP21a, LP02 and LP31a modes.
Fig. 2 Ellipticity dependence of the effective index of the modes of a step-index (Δn = 9.7.10−3) few-mode fiber supporting 5 groups of modes.
Based on these results, an elliptical core FM-EDF (ellipticity=1.3, major radius of 8.8 μm) has been manufactured by Modified Chemical Vapor Deposition (MCVD). With this fiber the effective index separation at 1550 nm (Δneff ≃ 1.2 × 10−3 for both LP11a–LP11b and LP21a–LP21b) is high enough to use them independently. This fiber also presents a ring doping profile for the erbium ions. This erbium doping profile has been successfully used in our previous works because it allows to reach similar gain value for off-centered modes, like LP11a,b and LP21a,b modes [7]. Refractive index profile and erbium doping profile are presented in Fig. 3(a), the 2D refractive index profile of the fiber being shown in Fig. 3(b) (measured using an IFA-100 system). It can be seen that the ring doping has no major influence on the refractive index profile. Note that this fiber also supports LP02 and LP31a,b modes at 1550 nm, but they will not be used in the following.
Fig. 3 a. FM-EDF doping profile and refractive index profile (from preform measurements). b. FM-EDF 2D refractive index profile (from fiber measurement with IFA-100 system)
Using this fiber, a five-mode amplifier has been built and its architecture is schematized on Fig. 4(a). The system uses modal multiplexer (MUX) and demultiplexer (DEMUX) (based on Multi-Plane Light Conversion (MPLC) [8]), for which reflective phase-plates have been specifically designed to generate the profile of the eigenmodes of the FM-EDF. This selective demultiplexer allows us to independently analyze the five different channels, constituted by five SMFs (each channel being attributed to an eigenmode of the FM-EDF). Signal beams were generated by five different tunable fiber lasers coupled to five SMFs whose fundamental mode was then converted into different spatial modes by the MUX. Signal and pump beams were multiplexed in free space and injected into the FM-EDF whose input/output orientation is controlled via fiber rotators. The output modes of the fiber have been imaged on an InGaAs camera: a good agreement can be observed between theoretical and experimental mode profiles [Fig. 4(b)]. The output of the FM-EDF is then injected into the DEMUX that converts the different spatial modes into fundamental modes of different SMFs. These SMFs can be connected, one after another or all together (by using a 5×1 splitter), to a powermeter or an Optical Spectrum Analyzer (OSA). The quality of the modal conversion on the DEMUX side has also been controlled by using counter-propagating signals imaged on the camera. In this particular configuration, all the setup is inverted, MUX becomes DEMUX and so DEMUX becomes MUX. Results similar to those presented on Fig. 3(b) have been obtained.
Fig. 4 a. Few-Mode Erbium-Doped Fiber Amplifier setup. b. Theory/experiment comparison of the 5 signal modes used in the setup.
The procedure for experimental measurements has been divided in two parts called respectively “OFF” and “ON” measurements, one used as reference and the second in amplification regime. So as to facilitate the analysis, different wavelengths have been attributed to the different spatial channels, similarly to the approach proposed by Salsi et al. [9].
Firstly, the five signal channels (wavelengths equally spaced between 1590 nm and 1598 nm) were used at the input of the MUX all together. Each DEMUX channel was successively connected to an OSA and a spectrum was recorded (labelled as “OFF” measurements). This wavelength band has been chosen as reference for signal beams so as to minimize the effect of erbium absorption while working close to the wavelength operating band of MUX and DEMUX. It has been checked that transmission properties for these wavelengths are similar to that of the C-band wavelengths used in amplification regime, which makes it possible to use them as reference for gain measurements. The same control has been done for optical powers provided by the tunable lasers.
Secondly, in amplification regime, pump beam was launched under conditions that allow to maximize the output signal power. Many pump modes among the 18 modes propagating at pump wavelength in the FM-EDF are hence excited. Indeed, whereas centered pump modes can be used in circular core case, ellipticity creates the need to adapt pump beam profile to the core asymmetry for maximizing pump beam absorption. For this second step, just like for the first step, all signals channels were used together at separate wavelengths (this time from 1550 nm to 1558 nm). Spectrum for each DEMUX channel was successively recorded via the OSA (“ON” measurements). For both “OFF” and “ON” procedures, the OSA received signals coming from the tested channel (i.e. same channel for MUX and DEMUX) and from crosstalk signals originating from the other channels at their corresponding wavelengths. With this procedure, both cross-talk and amplification can be investigated. To calculate the modal crosstalk of the whole system, five spectra obtained by “OFF” measurements have been used. This provides a 5×5 matrix where lines correspond to channel from MUX side (input channel) and columns to channels from DEMUX side (output channel). The measured optical powers were used to build the 5×5 cross-talk matrix by using the following formula :
where Pi,j is the optical power measured at wavelength attributed to the i – th input channel at the j – th output channel. The 5×5 matrix obtained with “ON” measurements is corrected in order to take into account Amplified Spontaneous Emission ASE ( ) and then used to calculate cross-talk with the same procedure.The calculation of the gain values is done by comparison of the diagonal elements of the 5×5 power matrix obtained from “ON” measurements to the one deduced from “OFF” measurements :
The gain presented here is different from net gain that compares power entering at MUX side to the one outgoing from DEMUX. This choice is motivated by the fact that the objective was to measure the gain of one channel of the system independently from the other components.3. Results
We now report on the experimental results obtained with an optimized length of 2.25 meters for the FM-EDF. The spectra (for both “OFF” and “ON” measurements) for the five tested output (DEMUX) channels, following procedure described in part 2, are reported on Fig. 5.
Fig. 5 Spectrum for “ON” (blue) and “OFF” (black) measurements for a given DEMUX channel with all input channel on. a) output channel a, b) output channel b, c) output channel c, d) output channel d and e) output channel e.
First, global cross-talk is shown in Table 1 (“OFF” measurements). This cross-talk has been minimized by adjusting phase plates positions, fiber orientation and injection conditions for both fiber sides, with the help of both camera and OSA. It can be observed that the overall “OFF” cross-talk is relatively low (at worst −11.3 dB). It has to be pointed out that low cross-talk is observed between LP11a and LP11b and between LP21a and LP21b (modes that are strongly coupled in a circular core fiber) as expected by the reduced mode coupling favored by the core ellipticity. The highest cross-talk values are attributed to significant mode coupling at the injection and not inside the active fiber itself (LP11b (input channel 3) → LP01 (output channel a), LP21a (input channel 4) → LP01 (output channel a), LP11b (input channel 3) → LP21a (output channel d), LP21b (input channel 5) → LP21a (output channel d).
Table 1. Global cross-talk in dB for “OFF” (top) and “ON” measurements (bottom) of the experimental setup, calculated using Eq. 1 with all input channels on. A 2.25 m length of FM-EDF is used together with a pump power of around 100 mW in amplification regime.
Secondly, “ON” measurements were performed for a pump power of about 100 mW (coupled in FM-EDF). Crosstalk values in amplification regime are reported in Table 1, whereas gain and Noise Figure (NF) for each mode are reported in Table 2. It is observed that amplification doesn’t degrade cross-talk for modes belonging to the same group. The FM-EDFA setup also provides gain, considering LP11a, LP11b, LP21a and LP21b modes, this gain is on average of 17.4 dB for a DMG of 7 dB. As a reminder, the erbium ring doping profile used for this fiber has a low overlap integral with fundamental mode [7], which explains the lower gain of fundamental mode.
Table 2. Gain (dB) calculated using Eq. 2 and Noise Figure (dB) for each channel (mode of the FM-EDF) of the experimental setup, with a 2.25 m length of FM-EDF and a pump power of around 100 mW.
It has to be pointed out that DMG impacts on cross-talk in amplification regime. Indeed, a given input channel generates power on the other channels at injection point and all the modes corresponding to these channels are non-equally amplified in the FM-EDFA if DMG is nonzero. The power distribution at the output is then modified, which changes the cross-talk. The situation is even more complex because cross-talk also occurs at back-conversion. The global cross-talk measured with our setup doesn’t allow to separate the different contributions. However, we believe that the high DMG (DMG=13.1 dB considering all modes) mainly explains the cross-talk degradation observed for input channel corresponding to the less amplified mode (LP01) of the FM-EDF for “ON” measurements.
The core ellipticity helps for reducing cross-talk, because the highest cross-talk values are not attributed to the fiber but to significant mode coupling at the injection or DMG in amplification regime, and permits to use each channel independently. But this core shaping combined to ring doping degrades DMG and average gain, since ring doping profile has been designed for circular core. However, as ellipticity changes intensity profiles of the modes, the overlap integrals of modes (for both signal and pump) with erbium ions are not optimized.
These results could be improved by reducing core ellipticity and/or adapting erbium doping profile and/or using a particular pump shaping in order to reduce DMG and to improve Gaverage while maintaining the lifting of degeneracy between mode belonging to the same group.
4. Conclusion
In this paper, we propose an experimental proof of concept of a SDM-based FM-EDFA for multiple single-mode fiber channels. Using a FM-EDFA based on an elliptical core erbium-doped fiber supporting several non-degenerate modes, the shared amplification of signals from five single-mode fibers is demonstrated with global cross-talk less than −9.5 dB in amplification regime. When the four off-centered modes (LP11a, LP11b, LP21a and LP21b) are considered, cross-talk values below −13 dB off-amplification and below −16 dB in amplification between modes belonging to the same group have been measured. In this condition, an average gain of 17.5 dB and a DMG of 7 dB have been obtained.
This preliminary work highlights the need for improvements in order to envision the integration of such a component in the network. As an example, targeted modal cross-talk maximum level for SDM could be estimated with the approach proposed by Winzer et al. [10] to −16, −24, and −32 dB respectively for QPSK, 16-QAM, and 64-QAM modulations (for 1 dB penalty on OSNR). Efforts have now to be focused on i) erbium doping profile that needs to be re-adapted to the case of an elliptical-core fiber to provide better Gaverage and DMG, ii) global losses of the component and iii) cross-talk reduction.
Acknowledgments
This work was partly supported by the French Ministry of Higher Education and Research, the Nord-Pas de Calais Regional Council and “Fonds Européen de Développement Economique Régional,” the Labex CEMPI, Equipex FLUX through the “Program Investissements d’Avenir” and FUI Modal. We also acknowledge Stéphane Plus, Andy Cassez and Karen Delplace for technical assistance.
References and links
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