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Highly efficient Er/Yb-codoped fiber amplifier with an Yb-band fiber Bragg grating Qun Han,1,2,* Yunzhi Yao,1,2 Yaofei Chen,1,2 Fangchao Liu,1,2 Tiegen Liu,1,2 and Hai Xiao3 1

2 3

College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Opto-Electronics Information Technology (Tianjin University), Ministry of Education, Tianjin 300072, China

The Holcombe Department of Electrical and Computer Engineering, Clemson University, Clemson, South Carolina 29634-0915, USA *Corresponding author: [email protected] Received April 6, 2015; revised May 8, 2015; accepted May 11, 2015; posted May 12, 2015 (Doc. ID 237592); published June 1, 2015 In this Letter, a high-power Er/Yb-codoped fiber amplifier (EYDFA) with a high-reflection Yb-band fiber Bragg grating (FBG) at the pump end is experimentally investigated. The FBG was inscribed on a piece of double-clad fiber with a center wavelength of 1032 nm. Due to the selective reflection of the backward Yb-band amplified spontaneous emission (Yb ASE) by the FBG, a co-pump-propagating Yb-band auxiliary signal was generated. Because of the stimulated amplification and reabsorption of the auxiliary signal, the Yb ASE was dramatically suppressed and the pump conversion efficiency (PCE) of the EYDFA was notably improved. An output power of 6.48 W was achieved at a pump power of 16.5 W, which is equivalent to a PCE of ∼39%. The slope efficiency relative to applied pump power was ∼40%. The maximum output power was improved ∼20% because of the introduction of the FBG. © 2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2410) Fibers, erbium; (140.3615) Lasers, ytterbium; (060.3510) Lasers, fiber. http://dx.doi.org/10.1364/OL.40.002634

The amplified spontaneous emission (ASE) in the Yb emission band is a well-recognized problem of highpower erbium-ytterbium codoped fiber lasers (EYDFLs) and amplifiers (EYDFAs) [1–3]. The Yb ASE not only decreases the slope efficiency, but may also cause stability problems or even damage to components of the laser or amplifier due to the onset of parasitic lasing or selfpulsing. In [2], the authors suggested that, instead of suppressing the Yb-band oscillation, introducing a Yb-band cavity in a high-power EYDFL might be helpful to prevent the high Yb-band gain accumulated in the gain fiber and prevent the laser from being damaged by the high-energy pulses generated by self-pulsing. In [3], this idea was introduced to a high-power EYDFA to increase the applicable pump power by purposely introducing an auxiliary signal at 1064 nm. The reabsorption of the auxiliary signal was also surmised as one of the reasons to explain the unaffected slope efficiency of the 1.5 μm signal. In [4] and [5], we systematically investigated the influences of the fiber length, the pump wavelength, and the power and wavelength of the auxiliary signal on the performance of an EYDFA by numerical simulations. It was found that the counter-pump propagating Yb ASE was dominant. By introducing a copump propagating Yb-band auxiliary signal at a proper wavelength, the Yb ASE could be effectively suppressed and the pump conversion efficiency (PCE) could be notably improved in the meanwhile, due to the stimulated amplification and reabsorption of the auxiliary signal. The theoretical predictions have been experimentally verified with several different schemes [6–12]. In [6,10], discrete Yb-band lasers were used to provide the auxiliary signal via a 1064/1550 wavelength division multiplexer (WDM), and the influence of its power, wavelength, and input direction on the performance of the EYDFA were analyzed. Because of the current unavailability of a 1064/1550 WDM based on compatible double-clad fiber, this method is actually limited to all-fiber EYDFAs with a single-mode input and not 0146-9592/15/112634-03$15.00/0

applicable for the power-amplifier stage in a multistage amplifier system with input from a double-clad fiber of a former stage. Otherwise, the inconvenient free-space coupling has to be used [12]. Whereas in [7–9,11], to save the additional Yb-band laser and reduce the cost of the system, a ring resonator oscillating in the Yb-band was constructed. The influence of the power, oscillating wavelength, and oscillating direction on the performance of the EYDFA were investigated. However, for a similar reason, due to the requirement of two 1060/1550 WDMs to construct the ring resonator, this method was limited to EYDFAs based on single-mode Er/Yb codoped fibers. In this Letter, a high-reflection Yb-band double-clad fiber Bragg grating (FBG) was introduced to the pump end of a high-power EYDFA. Due to the selective reflection of the backward Yb ASE, a forward auxiliary signal was autogenerated. The Yb ASE was effectively suppressed because of the stimulated amplification of the auxiliary signal. Reabsorption of the amplified auxiliary signal was demonstrated through numerical simulation. At the highest applied pump power of 16.5 W, the ∼100 mW signal at 1549 nm was amplified to 6.48 W with a slope efficiency of ∼40%. Compared with the 5.41 W output power before the introduction of the FBG, the output power was improved ∼20%. Furthermore, because the FBG can be inscribed on a compatible passive double-clad fiber, or even on the EYDF itself, the aforementioned limitations can be avoided and the cost and complexity of the EYDFA can be reduced in the meanwhile. The schematic diagram of the experimental setup is shown in Fig. 1. The signal from a fiber laser with a power of 100 mW at 1549 nm was injected into the EYDF after successively passing through a high-power isolator (5 W), a 1060/1550 filter wavelength division multiplexer (FWDM), and a 2  1 × 1 pump combiner. Two wavelength-locked 976 nm multimode laser diodes served as the pump. The total pump power measured at the output © 2015 Optical Society of America

June 1, 2015 / Vol. 40, No. 11 / OPTICS LETTERS

Fig. 1. Schematic diagram of the experimental setup. ISO, isolator; PC, pump combiner; CMS, cladding-mode striper.

port of the pump combiner was 16.5 W. The gain fiber was a 5.6 m double-clad EYDF (CorActive DCF-EY-10/ 128). The output end was angle-polished to ∼8° to minimize reflection. The FBG was inscribed on the same kind of fiber as the output pigtail of the pump combiner (CorActive DCF-UN-8/125-14), which is compatible with the EYDF. The transmission spectrum of the FBG is shown in the inset of Fig. 2. The center wavelength, reflectivity, and −3 dB bandwidth of the FBG were 1032 nm, ∼28 dB, and 0.32 nm, respectively. The power and spectra data used in this Letter were, respectively, measured with a power meter (Thorlabs S322C) and an optical spectrum analyzer (Advantest Q8384). It needs to be pointed out that the 1060/1550 FWDM is not an indispensable part of the EYDFA, but only used to make the analysis and comparison of the backward Yb ASE spectra possible in this Letter. The two curves with square and circle markers in Fig. 2 show the output power of the amplified signal from the EYDFA with and without the FBG, respectively, as a function of the pump power. From Fig. 2 we can see that at a lower pump power below ∼6 W, the difference between the two cases is small. However, with the increase of the pump power the difference becomes more and more apparent. When without the FBG, the output of the amplifier began to saturate relative to the pump power at 15 W and the maximum output power was ∼5.41 W at 16.5 W pump power. Although no parasitic lasing was observed even at the maximum pump power, one 1060/1550 FWDM was damaged, due to the highpower backward Yb ASE. From Fig. 2 it can also be seen that after introducing the Yb-FBG, the output power of the amplifier increased linearly with the pump power with a slope efficiency of ∼40%. The slope efficiency did not decrease even at the highest pump power, which

Fig. 2. Comparison of the measured output signal power versus pump power between EDFAs with and without the FBG. The inset shows the transmission spectrum of the FBG.

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means that the output power is currently limited only by the available pump power. The maximum output power improved to 6.48 W after introducing the FBG, which is equivalent to a PCE of ∼39% relative to the applied pump power. The power was improved ∼20% compared to the maximum power before the introduction of the FBG. The forward output spectrum at 16.5 W pump power measured with a resolution of 0.5 nm is show in Fig. 3(a). We can see that the signal-to-noise ratio of the amplified signal is about 45 dB relative to the Er ASE peak at ∼1563 nm and is over 35 dB relative to the peak of the residual pump. The forward Yb ASE spectrum and a small peak at the FBG’s reflection wavelength can also be identified in the spectrum. The backward Yb ASE spectra at various pump powers (2, 4, 6, 10, 14, and 16.5 W) measured from the reflection port of the FWDM are shown in Fig. 3(b). From Fig. 3(b) we can see that before the Yb-band FBG was introduced, the backward Yb ASE (dashed line) had a peak around 1032 nm at a higher pump power. As theoretically proposed in [4] and experimentally verified in [6], the most effective wavelength of the auxiliary signal ought to be around the peak of the backward Yb ASE spectrum, so the center wavelength of the FBG was designed to 1032 nm. In Fig. 3(b), the solid lines show the backward Yb ASE spectra after the introduction of the FBG. A solid line and a dashed line with the same color show the spectra under the same pump power with and without the FBG, respectively. From Fig. 3(b) we can see that the difference between the spectra flows the same trend as the difference between the output powers shown in Fig. 2, i.e., the difference is small at a lower pump power and increases markedly with the pump power. Because of the low Yb-band gain at a lower pump power, the FBG reflection cannot be effectively amplified. Whereas with the increase of the pump power, the stimulate amplification and reabsorption of the auxiliary signal became more and more effective. Because the bottlenecking of the energy transfer between Yb → Er is most severe at the pump end of the gain fiber, the amplification of the auxiliary signal decreased the built-up Yb-band gain there and brought back the energy to the forward direction and

Fig. 3. Measured (a) forward output spectrum and (b) backward Yb-ASE spectra (solid line, with the FBG; dashed line, without the FBG).

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part of the gain fiber, the amplified auxiliary signal is gradually reabsorbed. Thanks to the pumping of the auxiliary signal, the growth of the Er-band signal continues even after the 976 nm pump has long been totally absorbed. In this Letter, an Yb-band FBG was introduced to the pump end of an EYDFA. Due to the reflection, amplification, and reabsorption of the auxiliary signal introduced by the FBG, the backward Yb ASE was effectively suppressed and the efficiency of the amplifier was significantly improved. Because the FBG can be directly fabricated on the double-clad fiber compatible with the gain fiber or even on the gain fiber itself, this method is readily applicable to the power amplifier stages in an all-fiber multistage master oscillator power amplifier system. This work was supported by the National Natural Science Foundation of China under grant 61107035 and is partially supported by the Natural Science Foundation of Tianjin under grant 13JCYBJC16100.

Fig. 4. Simulation results of (a) the Er- and (b) Yb-ASE spectra, and (c) power evolution of the pump, signal, and FBG reflection.

gradually reabsorbed with the fading of the bottlenecking along the fiber. When pumped at a high power, the amplified auxiliary signal actually served as an additional pump to the Er-band signal in the latter part of the gain fiber. So the PCE of the amplifier was improved. To demonstrate the above analysis, an EYDFA with the similar parameters was numerically simulated [13]. Simulation results of the forward and backward Yb ASE spectra, the forward and backward Er ASE spectra, and power evolution of the pump, signal, and FBG reflection are shown in Figs. 4(a)–4(c), respectively. In the simulation, the Yb ASE in 1000–1100 nm and Er ASE in 1500– 1600 nm were divided in to discrete channels with a wavelength resolution of Δλ=1 nm. The center wavelength and reflectivity of the FBG were supposed to be 1032 nm and 99.8%. From Figs. 4(a) and 4(b) we can see that the shape of the ASE spectra agrees qualitatively well with the corresponding ones shown in Fig. 3. As shown in Fig. 4(c) the pump power is quickly absorbed by the fiber. Due to the high Yb-band gain accumulated because of the bottlenecking, the auxiliary signal induced by the FBG reflection is quickly amplified. At the latter

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Yb-codoped fiber amplifier with an Yb-band fiber Bragg grating.

In this Letter, a high-power Er/Yb-codoped fiber amplifier (EYDFA) with a high-reflection Yb-band fiber Bragg grating (FBG) at the pump end is experim...
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