Broadband amplification of high power 40 Gb/s channels using multimode Er-Yb doped fiber

Broadband amplification of several high power communication channels is demonstrated using a multimode Erbium-Ytterbium doped fiber (EYDF) amplifier. The multimode feature of this amplifier aims at simultaneously enabling wide gain bandwidth and high output power. The amplifier provides a gain bandwidth spanning over the 1535.0 nm-1565.8 nm band. The amplifier also provides a high output power of >30.2 dBm, with ± 2.4 dB natural gain flatness over the bandwidth of interest. The performance of the amplifier is assessed in a 40 Gb/s WDM system, featuring no trace of modal dispersion in the eye diagram and a low power penalty (< 0.4 dB) on the bit error ratio (BER). © 2010 Optical Society of America OCIS Code: (060.2320) Fiber optics amplifiers and oscillators; (060.2330) Fiber optics communications; (060.4510) Optical communications; (060.2410) Fibers, erbium; (060.2280) Fiber design and fabrication. References and Links 1. Y. Jaouën, J.-P. Bouzinac, J.-M. P. Delavaux, C. 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Trepanier, “Gain equalization of EDFA's with Bragg gratings,” IEEE Photon. Technol. Lett. 11(5), 536–538 (1999). #131158 $15.00 USD Received 6 Jul 2010; revised 17 Aug 2010; accepted 21 Aug 2010; published 3 Sep 2010 (C) 2010 OSA 13 September 2010 / Vol. 18, No. 19 / OPTICS EXPRESS 19984


Introduction
High-power (>1 W) optical fiber amplifiers, compatible with high data rate channels spreading over the 1530 nm-1565 nm (C-) band spectrum, are of great interest in applications such as nonlinear signal processing [1][2][3][4] and optical data transport [1,[5][6][7][8][9]. Following the ability of Erbium (Er-) ions to store and emit energy in this wavelength range, the Er-doped fiber (EDF) is a fundamental component of fiber amplifiers. However, conventional EDF amplifiers (EDFAs) have an output power that saturates at a power level ≤ 26 dBm [10], even with double-clad fiber geometry, which rules out their use for applications demanding high power. The co-doping of conventional EDFs with rare-earth materials can enhance the pump absorption, which is followed by a non-radiative energy transfer to Er-ions, thereby increasing the maximum output power of the amplifier. Further increase in output power can be realized by using such co-doped fiber amplifiers in tandem with high power multimode pump diodes [11].
While various materials including Bismuth (Bi) can be used as co-dopants for the power scaling of EDFAs [12][13][14], Ytterbium (Yb)-sensitization of EDFAs is attractive in several ways: (1) It provides amplification over the C-band without significant impairments caused by the nonlinear effects, (2) it helps suppressing the concentration quenching effect, which is the predominant cause of power limitation in conventional EDFAs [15][16][17]11], (3) it allows shortening the required length of fiber [18] to achieve the same amount of gain as that provided by a conventional EDFA -a consequence of an increased dopant concentration offered by the Yb-sensitized EDFAs -and, (4) it reduces the noise originating from pump power fluctuations [19] by allowing an efficient energy transfer from Yb to Er ions via crossrelaxation process.
Recently, cladding pumped EYDF amplifiers (EYDFAs) have attracted considerable attention to achieve high power levels in line with the requirements of applications like LIDAR [20,21], femtosecond pulse amplification [22], and space communications [23,24]. However, experimental details on the performance evaluation of EYDFAs compatible with WDM signals remain scarce in the literature. To date, the reported gain bandwidth of high power EYDFAs is limited to the 1545-1565 nm band [10,25,26], thereby leaving the available 1530 nm-1545 nm region of the C-band with insufficient gain. A straightforward way to increase the gain in this missing band is to increase the population inversion of Er ions in the EYDF [27]. Taking a cladding-pumped, single mode EYDF as a reference, a successful approach to increase the population inversion is to increase the total pump power that overlaps with the doped area of a given EYDF section [28]. Following this, a first option consists in reducing the cladding diameter while maintaining the same doped core diameter [29]. Although it increases the population inversion, this option is currently impractical because it requires using pump laser with non-standard fiber geometry. A second option which has not been investigated yet, involves increasing the EYDF core diameter while keeping the cladding diameter unchanged. This option allows pumping the double-clad EYDF with commercially available, high power laser diodes, but involves a multimode guidance of the signal. However, preferential seeding [30] and/or bending [31] as well as the optimized fiber parameters [32] can allow for a mainly single-mode operation of multimode fibers.
In this paper, we present a multimode EYDFA that exhibits an extended gain spectrum covering the 1535-1565 nm band with an output power of 30.2 dBm. We choose a multimode EYDF to benefit from a strong and wide gain spectrum without measurable negative impact from the multimode propagation. This is achieved by two mechanisms fostering single-mode propagation. The first mechanism is the mode-field diameter (MFD) of the fundamental mode (LP 01 ) in the EYDF designed to match the MFD of the standard single-mode fibers used to feed and extract the signal in and out of the EYDF. The second mechanism is the gain experienced by the LP 01 mode in the EYDF which is much stronger than the one for higher order modes [33]. We show that although the EYDF is in fact multimode, the amplified eye-diagram contains no trace of modal dispersion, as expected from the theoretical analysis. We have also performed the bit error ratio (BER) testing of the EYDFA in a 40 Gb/s WDM system revealing negligible power penalty on the BER.

Fiber composition
The multimode EYDF, as shown in Fig. 1, is a double-clad fiber with a circular core to guide the signal and an octagonal inner cladding to guide the pump. The core is composed of phosphosilicate glass activated with Er and Yb. The phosphorus content (8-10 mol %) in the core as well as the concentrations of Er and Yb (i.e., 2.6 х 10 25 ions/m 3 and 4.4 х 10 26 ions/m 3 , respectively) are optimized for an efficient energy transfer while suppressing the unwanted emission from excited Yb-ions. The circular core has a diameter of 10 ¯m and a numerical aperture (NA) of 0.2, which leads to a fiber that is slightly multimode in the C-band, with four LP modes (LP 01 , LP 11 , LP 21 and LP 02 ) allowed to propagate. There is a close match between the fundamental mode of the multimode EYDF and the guided mode of standard G.652 single-mode fibers. Taking advantage of this, patch cords of G.652 are spliced directly at the input and output of the EYDF using a commercially available, optical fiber fusion splicer (Fujikura, FSM-40S). The splicer allows a manual core alignment, which is guided by maximizing the coupled power observed with a power meter. The inner cladding of the EYDF is made of pure silica with an octagonal diameter of 125 ¯m. The numerical aperture of the inner cladding is 0.46, allowing compatibility with commercially available, high-power multimode pumps.

Calculation of modal gain
Although the multimode EYDF allows four guided signal modes, only the fundamental mode (LP 01 ) significantly benefits of the available gain when the EYDF is fusion spliced to a G.652 single-mode fiber [34]. For an EYDF delivering 30 dBm of output power with a total gain of 15 dB (-in line with the experimental conditions, discussed later in Section 4), the level of amplification of each propagating mode is quantified by taking into account the two processes of higher order mode stripping. The first process arises from the selective mode coupling between the G.652 single-mode fiber and the multimode EYDF that is acting at the input and output of the EYDF. The amount of power transferred from the G.652 fiber to a given LP mode of the EYDF and vice versa, depends on the overlap integral lp η given as where E G.652 is the transverse mode profile of the fundamental mode in the G.652 fiber, E lp is the transverse mode profile of an LP mode in the EYDF, and S denotes the transverse surface area. The value of the overlap integral for higher order modes of EYDF is much smaller than that for the fundamental mode (LP 01 ), which matches closely with the fundamental mode of the G.652 fiber. The second process of higher order mode stripping arises from an increased gain coefficient of the EYDF fundamental mode with respect to the higher order modes due its higher confinement into the EYDF gain medium. The confinement factor of a mode (Γ lp ) in the EYDF core is the ratio of its power confined in the doped core (P core ) to the total power that propagates in the mode (P total ) [35], Once combined, the two processes of higher order mode stripping provide the net gain of an EYDF-LP mode as ( )   Table 1 summarizes the values of lp η , Γ lp , and G lp for each available mode in the EYDF.
We note that the LP 11 and LP 21 modes are odd modes and thus their overlap integral with the LP 01 mode of the G.652 fiber is zero. This prevents the coupling of power to LP 11 and LP 21 modes, of course, provided the fibers are well aligned which can be ensured to a large extent by employing the commercially available splicing machines. Slight core misalignments are considered in the next section. Calculating the values of overlap integral and confinement factor for the other surviving modes, we find that the fundamental LP 01 mode in the EYDF, theoretically experiences ~33 dB more gain than the LP 02 mode. In this way, all the higher order modes are well-suppressed in the fiber, with only the fundamental mode carrying substantial amount of power. No trace of modal dispersion in the eye diagram of amplified signal (discussed further in subsection 4.3) is an experimental proof of higher order modes' suppression.

Effect of finite cleave angle and core offset
In practice, a minimum splice loss of 0.1 dB between the guided mode of a single mode fiber and the fundamental mode of a multimode fiber like the EYDF, must be ensured to avoid significant excitation of higher order modes [34]. Splicing machines are available with lateral alignment accuracy within a small fraction of core diameter ( ± 0.1 µm) and an angular alignment accuracy of < 0.1° (see Ref. 36). Figure 2 shows the dependence of coupling loss between the single mode fiber's guided mode and the fundamental mode of EYDF on the alignment of the two fibers during the fusion splicing process [34]. We note that the coupling loss remains sufficiently low for a considerably wide range of lateral as well as angular misalignments. This shows that most of the power from the single-mode fiber is coupled to the fundamental mode of EYDF. In short, the EYDF essentially operates as a single-mode fiber for the C-band signal and as a multimode fiber for the 975-nm pump. This reduces the pump depletion in the gain medium (EYDF), which leads to an increased population inversion throughout the fiber and hence, a broadened gain spectrum with respect to a single-mode EYDF. The process of population inversion increase in a multimode EYDF with respect to its single-mode counterpart is discussed below in a more quantitative manner.

The origin of population inversion enhancement
The population inversion enhancement provided by the multimode EYDF is explained from the theory of gain media. The pump absorption coefficient is approximated as ( )  [37]. For the pump, Γ p and hence the α p increases with the square of the fiber core diameter due to the pump power being uniformly distributed within the octagonal pump waveguide. On the other hand, Table 1 shows that the signal power in the EYDF is mainly concentrated in the fundamental mode owing to its larger values of overlap integral and confinement factor with respect to those for the higher order modes. As a result, the α s mainly depends on the fundamental mode power confined in the EYDF core. Making the EYDF multimode by increasing the core diameter from 5.9 µm (with V = 2.405) to 10.0 µm results theoretically in a 284% increase in α p , as opposed to a 6.9% increase in α s . The relatively large increase in pump absorption than the signal absorption reduces the depletion of pump, which leads to an increased population inversion in the multimode EYDF relative to that in a single-mode EYDF. This increased population inversion in the multimode EYDF is responsible for the broadening of gain spectrum with respect to a single-mode EYDF, by providing gain at the shorter wavelengths of the C-band [27]. Figure 3 shows a schematic structure of the EYDFA. The pump light from a multimode 3.3 W broad-area laser at 975 nm is coupled in counter-propagation to the EYDF via a 2 + 1→1 multimode pump/single mode signal combiner with only one pump input being used. The isolator (ISO 1) installed at the input side of the EYDF eliminates backward amplified spontaneous emission (ASE), whereas the isolator (ISO 2) at the output side blocks outside back-reflections.

Optimization of the EYDF length
The length of EYDF in the amplifier is optimized based on a standard cutback analysis, with an aim to maximize slope efficiency and gain flatness over the spectral range of 1535-1565 nm. For this purpose, we use a WDM signal carrying 11.8 dBm of power and covering the 1535-1565 nm band. Figure 4 shows the total output power versus launched pump power at different fiber lengths between 1 m and 3 m. The slope efficiency with a 1 m-long fiber is lower than with longer fibers as a consequence of incomplete pump absorption. As the length of fiber is increased, the slope efficiency increases before reaching a maximum value of ~34% for a 2.15 m long fiber. From this optimal length, a further increase in fiber length leads to a reduction in slope efficiency. Figure 4 (inset) shows the gain excursion versus the output power for different lengths of the multimode EYDF over the 1535-1565 nm band. We deduce that the 2.15 m long EYDF is optimum for the amplification of high power, broadband signal − since it provides the highest amount of output power with the lowest gain excursion and a reasonably high slope efficiency.

Multimode versus single-mode design
To show the advantage of a multimode design over a single mode design, a cutback analysis is also performed on a single-mode EYDF prepared with dopant compositions identical to the multimode EYDF. The single-mode fiber has a core diameter of 7 ¯m. Figure 5 shows the gain spectra for different lengths of the single-mode EYDF as well as the gain spectrum for the 2.15 m long multimode EYDF as a reference. The total output power in each case is approximately 33.9 dBm in response to a WDM input signal of 11.8 dBm. Note that the gain at shorter wavelengths increases as the fiber length is being reduced, but it still remains lower than the gain provided by an equivalent length of multimode fiber which allows an equal amount of pump absorption. The better gain of the multimode EYDF at the short wavelengths arises from an increased population inversion [27] with respect to the single-mode EYDF, as discussed previously in Section 2. Figure 5 (inset) also compares the overall gain excursion over the 1535-1565 nm band, for the corresponding lengths of the two fibers. Note that the multimode fiber provides a lower gain excursion with respect to any of the single-mode fibers.  Figure 6 shows the experimental setup used to characterize and test the optimized EYDFA constructed from a 2.15 m long multimode EYDF − in terms of gain, noise figure (NF), eyediagram and BER in a 40 Gb/s WDM system. The EYDFA was used in tandem with a preamplifier. The use of a low-noise preamplifier and a high-power amplifier (EYDFA) in tandem is greatly beneficial in reducing the overall NF of the amplifying system since the NF of the preamplifier dominates in such a system [38]. A WDM signal consisting of 8 channels spreading from 1530.3 nm to 1564.8 nm was used to measure the gain and noise-figure of the EYDFA by using an optical spectrum analyzer (Agilent-86142B). A tunable probe laser with a power level of −10.2 dBm was used to measure the gain and NF at intermediate wavelengths between the saturating tones, in steps of ~0.4 nm. A BER measurement was performed at 40 Gb/s to measure signal degradation caused by the multimode EYDF in the communication link. The probe channel was modulated in a return to zero (RZ) format with 50% duty cycle, following a pseudo-random binary sequence of length 2 15 -1. In order to study any signal degradation caused by the EYDFA, the eye diagram of the probe channel was compared on an oscilloscope (Agilent-86100C) with and without the EYDFA in the setup.  Figure 7 shows the spectrum of the WDM signal before and after the EYDFA. The WDM signal was spectrally equalized to ± 0.4 dB at the output of the pre-amplifier and totalized 15.9 dBm. The total output power after the amplification was 30.2 dBm for a pump power of 3.3 W. The output power could easily be increased to 34.3 dBm by pumping the EYDFA with 6 W of power.  Figure 8 shows the gain and NF of the EYDFA alone as well as that of the dual-stage amplifier. With a total power of 5 dBm at the input of the preamplifier, the natural gain of the EYDFA reaches a value of 18.4 dB with ± 2.4 dB in flatness over a useful spectral range of 1535.0 nm-1565.8 nm. The gain could be flattened even further by using an appropriate filter in the mid-stage of the dual-stage amplifier [39]. The NF of the EYDFA varies between 4.1 dB and 9.7 dB over the spectral range of interest. The most important noise contribution in the EYDFA arises from the amplified spontaneous emission, filling the available modes of the EYDF and partially coupled to the output fiber. Figure 8 shows that the overall gain of dualstage amplifier increases as the power at the input of preamplifier is reduced. However, this gain increase is also accompanied by an increase in gain excursion, with the gain spectrum tilting in favor of the channels at shorter wavelengths. It is also observed that the NF of the dual-stage amplifier approaches that of the preamplifier alone as the power at the input is reduced, as expected from theory [38].  Figure 9 shows the BER measurement curves for the 1546.7 nm channel (with NF = 6.9 dB − the average NF of EYDFA over the 1535.0-1565.8 nm band), with and without the EYDFA in the amplifying chain. The result shows that the power penalty leading from the EYDFA is < 0.4 dB. Also, the eye diagram in Fig. 9 (inset) shows no sign of pulse broadening and/or signal distortion. The small amount of power penalty and the unaltered eye diagram, without any sign of modal dispersion shows the negligible degradation of 40 Gb/s WDM signal caused by the multimode EYDFA. This was expected from the theoretical discussion in Section 2.

Conclusion
We designed, constructed and characterized a multimode EYDF amplifier which enables the amplification of multiple channels over an extended wavelength span from 1535.0 nm to 1565.8 nm. The wide gain bandwidth of the EYDFA is based on an improved population