2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber

Hirotaka Ono; Tsukasa Hosokawa; Kentaro Ichii; Shoichiro Matsuo; Hitoshi Nasu; Makoto Yamada

doi:10.1364/OE.23.027405

1. Introduction

Space-division-multiplexing (SDM) technologies employing multi-core fiber (MCF) and/or few-mode fiber (FMF) as a transmission line have been investigated in recent years to overcome the capacity crunch faced by optical fiber transmission systems using single-core and single-mode fiber (SMF), which has emerged because of the rapid growth in internet traffic [1,2]. A long-haul transmission system using an MCF and/or FMF requires optical amplifiers in the same way as the current single-core and SMF systems to keep the optical signal power level high. Several long-haul FMF transmission experiments using mode-division-multiplexing (MDM) and employing a few-mode erbium-doped fiber amplifier (FM-EDFA) have been reported [3–5]. One issue with FM-EDFAs is the differential modal gain (DMG) needed to minimize the differences between the signal to noise ratios (SNRs) of all the transmitted signals and thus maintain signal quality. To reduce the DMG in FM-EDFAs, it is important to reduce the difference between two overlap integrals, namely that for the excited erbium ion area and the intensity profile of the fundamental mode signal and that for the excited erbium ion area and the intensity profile of higher order signals. For this purpose, the doping of erbium ions with a ring profile and the use of a reconfigurable pump mode have been reported [6–9]. Another approach, which employs a ring-core erbium-doped fiber (RC-EDF) with a ring-shaped index profile [10,11], has been investigated theoretically [12,13], and RC-EDF has been proven to reduce the DMG experimentally [14].

In this paper, we describe in detail the design of the RC-EDF and the amplification characteristics of an FM-EDFA that employs the RC-EDF. In section 2, we describe an RC-EDF designed to reduce the DMG. In section 3, we describe the fabricated RC-EDF and report measured amplification characteristics including the DMG of a 2-LP mode fiber amplifier that employs the RC-EDF. Our main conclusions are summarized in Section 4.

2. Design of ring-core erbium-doped fiber for 2-LP mode amplification

First, we analyze the guided mode of an RC-EDF to determine the inner and outer radii of the ring-core so that only LP₀₁ and LP₁₁ mode lights propagate at the signal wavelength. Figure 1 shows schematics of the cross-section and the refractive index profile of the RC-EDF that we analyze. Here, R_i and R_o are the inner and outer radii, respectively, and n_core and n_clad are the refractive index of the core and cladding, respectively. The inner core and the cladding had the same refractive index and the relative refractive index difference is 1.0%. For the analysis, we calculate the cutoffs using vector mode analysis results [11] to realize a correlation between the allowable modes and the R_i and R_o combinations. In the analysis the signal wavelength is 1550 nm.

Fig. 1 Schematics of the cross-section and the refractive index profile of ring-core erbium-doped fiber.

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Figure 2 shows a map of the allowable modes for R_i and R_o combinations for a 1550 nm signal. The physically impossible area is an area where R_i ≥ R_o. The area shown in yellow allows the LP₀₁ and LP₁₁ modes for the signal wavelength, and a 2-LP mode EDFA is achievable with an R_i and R_o combination in the yellow area. The blue and red lines are LP₂₁ and LP₁₁ cutoffs, respecrively. For combinations of R_i and R_o in the yellow area, we calculate the signal gain in order to optimize the R_i and R_o values and thus obtain a small DMG with a high pump efficiency.

Fig. 2 Map of the allowable modes for the inner and outer radii, R_i and R_o, of the ring-core of a ring-core erbium-doped fiber.

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We select three series of R_i and R_o combinations. In series I and III, the R_o values are set at LP₂₁ cutoff minus 0.2 μm and LP₁₁ cutoff plus 0.2 μm, respectively. In series II, the R_o values are set at an intermediate value between the two cutoffs. Since the R_o values of series I and III become closer to that of series II as the R_i value increases, we set the maximum R_i value in series I and III at 7.0 μm whereas that in series II is 8.5 μm. The gain calculation uses the following simultaneous differential equation for pump, signal and amplified spontaneous emission (ASE) with the dependence of the fiber propagation direction, z, the radius and the azimuth angle of the fiber cross-section, for various modes [12]:

\frac{d P_{p, m} (z)}{d z} = P_{p, m} (z) \int_{0}^{R_{o}} ​ ​ ​ \int_{0}^{2 π} ​ ​ [σ_{e, p} N_{2} (r, θ, z) - σ_{a, p} N_{1} (r, θ, z)] {\bar{ψ}}_{p} (r, θ) r d r d θ - α_{p, m} P_{p, m} (z),

\frac{d P_{s, m} (z)}{d z} = P_{s, m} (z) \int_{0}^{R_{o}} ​ ​ ​ \int_{0}^{2 π} ​ ​ [σ_{e, s} N_{2} (r, θ, z) - σ_{a, s} N_{1} (r, θ, z)] {\bar{ψ}}_{s} (r, θ) r d r d θ - α_{s, m} P_{s, m} (z),

\begin{matrix} \frac{d P_{k, m}^{\pm} (z)}{d z} = \pm P_{k, m}^{\pm} (z) \int_{0}^{R_{o}} ​ ​ ​ \int_{0}^{2 π} ​ ​ [σ_{e, k} N_{2} (r, θ, z) - σ_{a, k} N_{1} (r, θ, z)] {\bar{ψ}}_{k} (r, θ) r d r d θ \\ \pm 2 σ_{e, k} h ν_{k, m} Δ ν_{k, m} \int_{0}^{R_{o}} ​ ​ ​ \int_{0}^{2 π} ​ ​ N_{2} (r, θ, z) {\bar{ψ}}_{k} (r, θ) r d r d θ \mp α_{k, m} P_{k, m}^{\pm} (z) . \end{matrix}

where P_p,m and P_s,m are the pump and signal powers of mode m, respectively, and

P_{k, m}^{\pm}

is the m mode ASE power of the kth frequency slot at ν_k with a Δν_κ bandwidth for the entire erbium ion emission band. σ_e and σ_e are the stimulated emission and absorption cross-section of an erbium ion, respectively. The subscripts p, s, k denote those for the pump, signal and ASE, respectively. α_p,m, α_s,m and α_k,m are the m mode background loss coefficients at the pump, signal and ASE wavelengths, respectively.

{\bar{ψ}}_{p}

,

{\bar{ψ}}_{s}

and

{\bar{ψ}}_{k}

are the normalized power distributions of the pump, signal and ASE, respectively. h is the Planck constant. The power couplings between different modes are neglected in this formula in the same way as [12] which includes a calculation of EDF with a ring-core. N₁ and N₂ are the erbium ion densities of the lower and upper levels, and are described as follows:

N_{2} (r, θ, z) = ρ (r, θ) \frac{σ_{a, p} \frac{P_{p, m} (z)}{h ν_{p}} {\bar{ψ}}_{p} (r, θ) + \sum_{m} (\sum_{s} σ_{a, s} \frac{P_{s, m} (z)}{h ν_{s}} {\bar{ψ}}_{s, m} (r, θ) + \sum_{k} σ_{a, k} \frac{P_{k, m} (z)}{h ν_{k}} {\bar{ψ}}_{k, m} (r, θ))}{1 / τ + (σ_{a, p} + σ_{e, p}) \frac{P_{p, m} (z)}{h ν_{p}} {\bar{ψ}}_{p} (r, θ) + \sum_{m} (\sum_{s} (σ_{a, s} + σ_{e, s}) \frac{P_{s, m} (z)}{h ν_{s}} {\bar{ψ}}_{s, m} (r, θ) + \sum_{k} (σ_{a, k} + σ_{e, k}) \frac{P_{k, m}^{\pm} (z)}{h ν_{k}} {\bar{ψ}}_{k, m} (r, θ))}

N_{1} (r, θ, z) = ρ (r, θ) - N_{2} (r, θ, z)

where ρ(r,θ) is the erbium ion density of the erbium-doped fiber, and τ is the spontaneous emission lifetime of an erbium ion. The input signal is a 40-channel wavelength-division-multiplexing (WDM) signal whose channel allocation is a 100 GHz spacing in the 1530.3 to 1561.4 nm wavelength range for both the LP₀₁ and LP₁₁ modes. We use the stimulated emission and absorption cross-sections of erbium ions at the signal wavelengths shown in Fig. 3, which are obtained for aluminum-codoped erbium-doped fiber, and an absorption cross-section of 1.73 × 10⁻²⁵ m² at the pump wavelength of 980 nm. The erbium ions are assumed to be doped uniformly in the ring-core and its density is 2.0 × 10²⁵ m⁻³. The spontaneous emission lifetime is 10.5 ms. The background losses of the signal and pump are 0.02 and 0.1 dB/m for all the modes, respectively. The normalized scalar power distributions are obtained by using a commercially available mode solver that employs the imaginary-distance beam propagation method with a discretized size of 0.1 μm which is sufficiently small to allow us to calculate and compare the gains of different mode signals. Since higher order modes than LP₁₁ are allowable in the yellow area in Fig. 2 for a pumping wavelength of 980 nm, we chose the LP₀₁, LP₁₁ and LP₂₁ pumping modes. We set the EDF length so that the minimum gain among all the signal channels of both the LP₀₁ and LP₁₁ modes for the pumping modes of LP₀₁, LP₁₁ and LP₂₁ is larger than the value determined in a flat gain operation, which is similar to designing an EDFA for a transmission system. We set the minimum gain at 17 dB. The EDF length for each R_i and R_o combination is plotted in Fig. 4. The difference between the EDF lengths for different R_i and R_o combinations arises from the difference between the overlap integrals of the erbium-doped core area and the signal power mode distribution. The input signal power is −20 dBm/ch/mode (−4 dBm/mode), and the pump powers are set so that the differential channel gain (DCG) of the LP₀₁ mode signal, which is the difference between the maximum and minimum channel gain, is minimized at less than 3 dB for all the calculations. In this pumping condition, although the EDFA operates in a flat gain condition for an LP₀₁ mode signal, the amplifier does not necessarily operate in a flat gain condition for an LP₁₁ mode signal and sometimes has a slight gain tilt. For pumping with higher order modes, we assume the same pump power in the odd (LP_11o or LP_21o) and even (LP_11e or LP_21e) modes to take account of the averaging effect because of the rotation during pump light propagation.

Fig. 3 Emission and absorption cross-sections used in the gain calculation.

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Fig. 4 Ring-core erbium-doped fiber length for each R_i and R_o combination to obtain a minimum gain of 17 dB.

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Figures 5(a) and 5(b), respectively, show the DMG and the pump power for each R_i and R_o combination of the three series. The DMG is defined here as the channel gain of LP₀₁ mode minus the channel gain of LP₁₁ mode when the absolute value of the channel gain difference between the LP₀₁ and LP₁₁ modes is maximized:

DMG = G_{01} (λ) - G_{11} (λ), for max {| G_{01} (λ) - G_{11} (λ) |},

where G₀₁(λ) and G₁₁(λ) are the gain of LP₀₁ and LP₁₁ modes at the channel wavelength λ, respectively. The DMG becomes zero when R_i is about 2.5–4.5 μm and R_o is near the LP₂₁ cutoff, series I. The pump power is minimized when R_i is about 2.0–2.5 μm and R_o is near the LP₂₁ cutoff, series I. The pump power increases when the DMG is large because the gain of one of the signal modes greatly exceeds 17 dB. The pump power also increases for R_i < 6 μm because of the small confinement of the signal and pump modes in the ring-core. These results suggest that the DMG and pump efficiency can be optimized by choosing an R_i of around 2.5 μm and an R_o value near the LP₂₁ cutoff.

Fig. 5 (a) Differential modal gain and (b) pump power for each R_i and R_o combination when the minimum gain is 17 dB.

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Figure 6 shows the relationship between the overlap integral of the signal power distribution and the erbium ion doped area and the absolute value of the DMG. Here, the overlap integral of the signal power distribution of the LP₀₁ mode and the erbium ion doped area is Γ₀₁ and that of the LP₁₁ mode is Γ₁₁. The DMG has a strong correlation with both Γ₀₁ − Γ₁₁ and Γ₁₁/Γ₀₁, and increases as Γ₀₁ − Γ₁₁ increases and Γ₁₁/Γ₀₁ decreases. Therefore, both Γ₀₁ − Γ₁₁ and Γ₁₁/Γ₀₁ are good parameters for designing a few-mode RC-EDF to reduce the DMG.

Fig. 6 Relation between the overlap integral of the signal power distribution and the erbium ion doped area and the absolute value of the differential modal gain.

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3. Fabrication of ring-core EDF and characterization of 2-LP mode EDFA

3.1 Ring-core erbium-doped fiber

When fabricating RC-EDF, we chose the R_i and R_o combination that minimized the DMG and maximized the pump efficiency. Figures 7(a) and 7(b), respectively, show the core cross-section image and the relative refractive index difference (Δ) profile of the RC-EDF we fabricated, along with the calculated intensity profiles of LP₀₁ and LP₁₁ mode signals in the 1.55 μm band. Since the Δ profile was not rectangular as a result of the use of a modified chemical vapor deposition (MCVD) fabrication process, R_i and R_o were defined here as the radii at half maximum and were 2.1 and 4.4 μm, respectively. The inner core radius was slightly different from the optimal value due to a fabrication error. The high-refractive index area was constructed by doping aluminum ions as shown Fig. 8 which shows the radial distributions of the doped aluminum and erbium ions in the RC-EDF measured with an electron probe microanalyzer (EPMA). Although the measured erbium distribution was rather noisy because of the low detection sensitivity, the erbium ion distribution coincided well with that of the aluminum ions, which suggested that erbium ions were doped uniformly in the area of the fiber with a high refractive index. Table 1 summarizes the parameters of the RC-EDF. As shown in Fig. 6(b), both the LP₀₁ and LP₁₁ mode signals overlapped well with the erbium-doped core and the overlap integrals were 0.69 and 0.66, respectively, indicating that the RC-EDF can be expected to exhibit similar gain values. The RC-EDF also exhibited a high erbium absorption of 22.7 dB at 1.53 μm as shown in Fig. 9, and a low background loss of 0.018 dB/m at 1.18 μm, which allowed the use of a short length of fiber for amplification. This RC-EDF has been proven to have a smaller DMG than circle-core EDF (CC-EDF) supporting 2 LP modes in the signal band [14]. In addition to the small DMG, an RC-EDF has the potential to be spliced to an FMF with a small loss although the refractive index profile of the core is different from that of an FMF. For example, we estimate the connection loss between RC-EDF or CC-EDF whose core radius and step-like refractive index difference were 4.0 μm and 1.4%, respectively, and the transmission FMF with a trench-assisted core described in [15] whose core radius and refractive index difference are 6.4 μm and 0.42%, respectively. The connection losses between the RC-EDF and the transmission FMF for 1.55 μm LP₀₁ and LP₁₁ lights, which are estimated from the coupling efficiencies of the two fibers, are 0.38 and 0.91 dB, respectively, while those between the CC-EDF and the transmission FMF are 1.60 and 1.99 dB, respectively. In this connection loss estimation, the radii of both the EDFs were much smaller than that of the FMF and the outer radius of core of the RC-EDF was larger than the core radius of the CC-EDF. In this case, the large amplitude portion of the LP₀₁ mode of the RC-EDF overlaps better with the large amplitude portion of the LP₀₁ mode of the FMF than that of the CC-EDF, which results in a smaller field mismatch in the RC-EDF than in the CC-EDF. This result suggests that the RC-EDF has the potential to be spliced to an FMF with a small splice loss despite the refractive index profile of the core being different from that of an FMF.

Fig. 7 (a) Core cross-section image and (b) relative refractive index difference (Δ) profile of the ring-core erbium-doped fiber along with calculated intensity profiles of LP₀₁ and LP₁₁ mode signals in the 1.55 μm band.

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Fig. 8 Radial distributions of doped aluminum and erbium ions.

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Table 1. Parameters of ring-core erbium-doped fiber.

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Fig. 9 Absorption spectrum for the LP₀₁ mode of the RC-EDF.

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We measured the differential transmission power spectrum of the RC-EDF wound with different bending diameters to estimate the cut-off wavelength as shown in Fig. 10. P₀ and P₁ were the transmitted powers through a 2-m long RC-EDF wound with 1 turn with diameters of 100 and 60 mm, respectively, when the same power was launched into the RC-EDF. The cut-off wavelength was estimated from the wavelength dependence of P₀ − P₁. Since P₀ and P₁ were not measured accurately at around 980 and 1530 nm because of the large absorption of the erbium ions and P₀ − P₁ yielded noise peaks, we eliminated the noise peaks in Fig. 10. The dashed lines show the theoretically calculated cut-off wavelength estimated by taking the refractive index profile of the RC-EDF into account, which coincided well with the peaks of the differential power spectrum. The cut-off wavelength of the LP₂₁ mode was 1435 nm for the 2-m long RC-EDF wound with 1 turn with a diameter of 100 mm, which indicates that only LP₀₁ and LP₁₁ mode signals propagated in the C-band. There was no excess loss increase at wavelengths longer than the C-band, suggesting that there was no bending loss for the LP₁₁ signal even when the RC-EDFA was wound with a diameter of 60 mm.

Fig. 10 Differential transmission power spectrum of the RC-EDF wound with 100 and 60 mm bending diameters.

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3.2 Amplifier configuration

Figures 11(a) and 11(b), respectively, show the configuration of an FM-EDFA that employed the RC-EDF, and the WDM couplers used in the FM-EDFA. The amplifier configuration was similar to that in a previous report [14], and a polarization-multiplexed pump light was used in this measurement to increase the available pump power. A difference from the WDM coupler is that it was also used a phase plate to convert an LP₀₁ mode pump light to an LP₂₁ mode light. Thus, the pumping modes were LP₀₁, LP₁₁ or LP₂₁. The RC-EDF was 3 m long and fusion-spliced to the WDM couplers. Figure 11(a) also shows the beam profiles at the input, RC-EDF and output of the RC-EDF. The LP₁₁ mode signal was observed as a mode group when the same signal powers for the two orthogonal modes (LP_11o and LP_11e modes) were input into the amplifier. It should be noted that since the beam profiles of the amplified signal in the RC-EDF were difficult to measure because of the relatively high pump intensity, we observed them by using a very short unpumped RC-EDF with a length of about 5 cm. The radial distributions of each beam are shown in Fig. 12. Although some imbalances were observed in the beam profiles of the RC-EDF, it was confirmed that the beam profiles of the LP₀₁ and LP₁₁ modes in the RC-EDF were converted to a ring-shaped profile, simply by splicing the RC-EDF to the step-index FMF. The beam profiles of both the LP₀₁ and LP₁₁ modes in the RC-EDF coincided well with the calculated profiles; the intensity near the center of the fiber for the LP₀₁ signal was not zero while that for the LP₁₁ mode signal was almost zero. The beam profiles at the amplifier output were converted again to a step-index-like FMF profile.

Fig. 11 Configuration of (a) few-mode fiber amplifier employing the ring-core erbium-doped fiber, (b) wavelength-division-multiplexing module.

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Fig. 12 Radial distribution of each beam profile in Fig. 11 at the center of the x and y axes of the image area.

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3.3 Amplifier characterization

Figure 13 shows the experimental setup for measuring the gain and the noise figure (NF) of the FM-EDFA that employed the ring-core EDF described in 3.1. An eight-channel WDM signal located from 1531.1 to 1561.4 nm was used as the input signal. The WDM signal was modulated with 10-Gb/s non-return to zero differential phase-shift keying (NRZ-DPSK) and then divided into two lights by using a 3-dB coupler. The two lights were polarization-multiplexed with a polarization beam combiner (PBC) employing a delay between the two polarized signals. The polarization-multiplexed signal was divided into three lights and multiplexed LP₀₁, LP₁₁ odd and even (LP_11o and LP_11e) signals with a mode multiplexer (Mode Mux), and the polarization- and mode-multiplexed signal was input into the FM-EDFA. Using a modulated signal as the input signal made it possible to measure the gain and NF regardless of mode coupling between the LP_11o and LP_11e signals along the fiber [8]. The mode-multiplexed output signals of the FM-EDFA were fed into a mode demultiplexer (Mode Demux), and measured with an optical spectrum analyzer (OSA). The gains and the NFs of the LP₀₁ mode and LP₁₁ mode group signals were measured in the experiment.

Fig. 13 Experimental setup for measuring the gain and the noise figure.

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Figures 14(a), 14(b), and 14(c) show the gains and NFs of an LP₀₁ mode signal with various input signal powers for the LP₀₁, LP₁₁ and LP₂₁ mode pumping, respectively, and (d), (e) and (f) show those of the LP₁₁ mode signals. The pump power was adjusted so that the DCG of the LP₀₁ mode was less than 3 dB for each input signal power. In the higher order mode pumping, the gains and NFs were measured after optimizing the rotational angle of the phase plates to maximize the gain. The gain changes of the LP₁₁ mode signal along with the input signal power change were slightly larger than those of the LP₀₁ mode signals because the pump powers were adjusted based on the LP₀₁ signal gain. These gain changes of the LP₁₁ mode signal were larger with LP₁₁ mode pumping than with LP₀₁ and LP₂₁ pumping. This gain change difference was possibly caused by the azimuthal non-uniformity in the ring-core of the RC-EDF, and it was more noticeable with LP₁₁ mode than LP₀₁ and LP₂₁ mode pumping because the intensity of the LP₁₁ mode had a larger azimuthal dependence than those of the LP₀₁ and LP₂₁ modes.

Fig. 14 Gain and NF for various input signal powers. The signal and pumping modes are (a) LP₀₁ and LP₀₁, (b) LP₀₁ and LP₁₁, (c) LP₀₁ and LP₂₁, (d) LP₁₁ and LP₀₁, (e) LP₁₁ and LP₁₁, (f) LP₁₁ and LP₂₁, respectively.

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Figure 15 plots the dependence of the DMG on the input signal power for the three pumping modes. The DMGs were almost independent of the input signal power and small DMGs of 1.0, 0.8 and 1.1 dB, respectively, were achieved for LP₀₁, LP₁₁ and LP₂₁ pumping modes. The NFs of the amplifier were less than 5.2 dB and less than 5.8 dB for the LP₀₁ and LP₁₁ mode signals for all the pumping modes.

Fig. 15 Dependence of the differential modal gain on the input signal power for LP₀₁, LP₁₁ and LP₂₁ mode pumping.

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Figure 16 shows the input signal power dependence of the pump power used to obtain a flat gain condition. The pump powers for LP₀₁ and LP₁₁ pumping were almost the same, and the pump power for LP₂₁ pumping was about 20–30% larger than those for LP₀₁ and LP₁₁ pumping. The reason for this could be the difference in the propagation and bending losses for different modes.

Fig. 16 Pump power dependence on the input signal power for LP₀₁, LP₁₁ and LP₂₁ mode pumping.

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We also measured the modal crosstalk between different signal modes. Figure 17(a) and (b), respectively, show examples of the output spectra of the LP₀₁ and LP₁₁ mode signals along with the crosstalk spectra of the different mode signals. The spectra in Fig. 17(a) are the output of the LP₀₁ port of the mode demultiplexer in the measurement setup described in Fig. 13. The spectra colored in red and blue are the LP₀₁ mode signal output measured by inputting only the LP₀₁ mode signal into the FM-EDFA and the crosstalk power measured by inputting only the LP₁₁ mode signal, respectively. The spectra in Fig. 17(b) are the total output of the LP_11o and LP_11e ports of the mode demultiplexer. The spectra colored in red and blue are the LP₁₁ mode signal output measured by inputting only the LP_11o and LP_11e mode signals into the FM-EDFA and the crosstalk power measured by inputting only the LP₀₁ mode signal, respectively. In every case, the pump power was adjusted so that a flat gain was obtained. All the spectra were compensated for the insertion loss of the mode demultiplexer and the optical switch. The wavelength resolution of the optical spectrum analyzer was 0.2 nm. The input signal powers were −5 dBm/mode (−14 dBm/ch/mode × 8 ch) in both cases. The nominal maximum crosstalk from the LP₁₁ mode to the LP₀₁ mode was −5.6 dB and that from the LP₀₁ mode to the LP₁₁ mode was −5.7 dB, which become a crosstalk of –6.1 dB for both, taking account of the mode demultiplexer crosstalks of −15.4 and −13.6 dB for LP₁₁ → LP₀₁ and LP₀₁ → LP₁₁, respectively.

Fig. 17 Examples of the output power and crosstalk spectra of the FM-EDFA employing the RC-EDF. (a) LP₀₁ mode signal output and LP₁₁ → LP₀₁ crosstalk, (b) LP₁₁ mode signal output and LP₀₁ → LP₁₁ crosstalk.

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Figure 18 shows the input signal power dependence of the modal crosstalk for LP₀₁, LP₁₁ and LP₂₁ mode pumping. The modal crosstalk was measured in the same way as in Fig. 17(a) and (b) for all the input signal powers and pumping modes. The plotted crosstalk is the maximum for each input signal power and pumping mode. There was no apparent dependence of the modal crosstalk on the input signal power. The observed modal crosstalk was in the −6.7 to −5.0 dB range, which was relatively large because of the large coupling between different signal modes.

Fig. 18 Input signal power dependence of modal crosstalk for LP_01, LP₁₁ and LP₂₁ mode pumping.

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4. Conclusion

We described in detail the design of an RC-EDF for reducing the DMG of a 2-LP mode EDFA. First, we clarified the R_i and R_o combination area of the RC-EDF that allows the propagation only of the LP₀₁ and LP₁₁ mode signals and optimized the R_i and R_o combination to reduce the DMG with a maximized pump efficiency. When designing the RC-EDF, we showed that the DMG is decreased by minimizing Γ₀₁ − Γ₁₁ and maximizing Γ₁₁/Γ₀₁. We successfully fabricated an RC-EDF and constructed a 2-LP mode EDFA with a small DMG of around 1 dB for LP₀₁, LP₁₁ and LP₂₁ mode pumping. The RC-EDFA has been employed in a long-haul multi-core and few-mode transmission experiment [16], and could be a useful amplifier for constructing an SDM transmission system/network in the future.

Acknowledgment

Part of this research is supported by the National Institute of Information and Communications Technology (NICT), Japan under “R&D of Innovative Optical Communication Infrastructure”.

References and links

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Parameter				Value
Core doped ions				Er, Al
Core size	Inner radius, R_i			2.1 μm
	Outer radius, R_o			4.4 μm
	R_i / R_o			0.48
Cladding diameter				124.8 μm
Coating diameter				247 μm
Cut-off wavelength (Calculated)		LP₁₁		2.27 μm
		LP₂₁		1.41 μm
		LP₁₂		0.99 μm
Mode field diameter (LP₀₁ @ 1.55 μm)				8.3 μm
Effective area (LP₀₁ @ 1.55 μm)				82.4 μm²
Er absorption			980 nm	11.3 dB/m
Er absorption			1530 nm	22.7 dB/m
Background loss			1180 nm	0.018 dB/m

2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber

Abstract

1. Introduction

2. Design of ring-core erbium-doped fiber for 2-LP mode amplification

3. Fabrication of ring-core EDF and characterization of 2-LP mode EDFA

3.1 Ring-core erbium-doped fiber

3.2 Amplifier configuration

3.3 Amplifier characterization

4. Conclusion

Acknowledgment

References and links

Cited By

Figures (18)

Tables (1)

Equations (6)

Optics Express