Open Access
Add to BibSonomyAbstract
High-power narrow-linewidth photonic bandgap fiber amplifier was demonstrated. In order to suppress stimulated Brillouin scattering, the seed linewidth was broadened by applying a random phase noise with an electro-optical modulator. A factor of 15 in terms of Brillouin gain suppression can be theoretically expected. An 87 W linearly-polarized (11 dB PER) and narrow-linewidth (780 MHz FWHM) output was obtained.
© 2015 Optical Society of America
1. Introduction
Owing to the unique characteristics, photonic bandgap fiber (PBGF) attracts a great deal of attention as the engineerable gain medium for laser applications. Recently, many interesting demonstrations of PBGF sources have been reported in literatures by using PBGF for: dispersion compensation [1, 2], core area scaling [3–5], and spectral filtering [6–11]. PBGF is the optical fiber that has the core surrounded by a microstructured cladding typically incorporated with a periodical structure, e.g., the hexagonal honeycomb structure, by embedding high index inclusions in a lower index background. The core confinement is based on the antiresonant reflection [12]. The solid core PBGF can be used as an active gain medium by doping rare earth ions in the core [1, 4, 5, 7–11]. We have investigated PBGFs which are based on distributed spectral filtering (DSF). By use of the DSF it is possible to control the amplified spontaneous emission (ASE) of a fiber laser/amplifier and thus we can obtain high optical to optical conversion efficiency in a low gain region of the gain medium. The PBGF doped with Yb3+ ions has shown the excellent performance on ASE suppression and we have demonstrated high-power laser [10] and amplifier [9,11] operations at 1178 nm. We have reported a record output of 167 W power (broadband) and 15 dB gain by the Yb-doped PBGF (Yb-PBGF) amplifier at 1178 nm [9]. The wavelength is at the long wavelength region of the ytterbium emission spectrum and is related to the application of laser guide star (LGS). LGS is a key device in the astronomical science for compensating the atmospheric distortion to improve the resolution of a ground-based telescope [13]. Thus, one can obtain stars in diffraction-limited quality through blurring of the atmosphere. For next generation large aperture telescopes, a LGS source requires some particular properties, e.g., high optical power (a total power of 150 W has been proposed for the TMT project [14]), continuous wave (CW) or quasi-CW pulsed operation at 589 nm, narrow linewidth (the specific linewidth need to be optimized for each telescope but at least narrower than the Doppler broadened sodium absorption linewidth ~2.7 GHz), diffraction limited beam quality (M2 < 1.2), wavelength tunability, long term stability, and so on [15]. Currently, a fiber source is considered to be the preferred candidate for the LGS source because of its excellent inherent properties: good beam quality, high heat dissipation capacity, long gain length, low loss, and compactness and robustness, which are beneficial for obtaining a high CW output power. So far the Yb-PBGF amplifier [8, 9, 11] and the fiber Raman amplifier (FRA) [16–18] are two of the major candidates among the fiber sources. At 1178 nm an output power of more than 100 W CW has been achieved only in these two formats [9,18]. In comparison with FRA, the Yb-PBGF amplifier has an advantage in the simple LD pumped setup and the potential for pulsed operation owing to the energy storage due to the longer upper state lifetime (0.84 ms). The main obstacle for the development of the LGS source is stimulated Brillouin scattering (SBS), because of the narrow linewidth. SBS is stimulated backward scattering and thus causes a huge attenuation to the signal propagation when the signal power exceeds the SBS threshold. The threshold power is quite low in the case that the signal linewidth is comparable or less than the Brillouin gain bandwidth of the fiber, typically several tens MHz in an optical fiber [19]. Therefore, power scaling is challenging, because typically the output power, in terms of gain, of an ytterbium doped fiber amplifier is scaled by the length of gain fiber but the fiber length is limited by SBS in the case of narrow linewidth. We have reported a 24.6 W single-frequency Yb-PBGF amplifier [11]. However, because of the very narrow linewidth (320 kHz), the Yb-PBGF amplifier suffered from a significant power limitation by SBS in case that the fiber length and/or the seed power were increased in order to obtain a higher output power. In this paper, we report power scaling of the single-frequency output of our YB-PBGF amplifier at 1178 nm by introducing seed linewidth broadening by electro-optical (EO) phase modulation. An 87 W power was obtained with linearly polarization and narrow linewidth. The output power of the Yb-PBGF amplifier was limited by pump absorption saturation, while we obtained no sign of SBS in backward scattering. Further power scaling is discussed based on a numerical amplifier model in the last part of this paper.
2. Design of the photonic bandgap fiber
For an efficient amplifier operation, the injected seed power should be higher than the saturation power. The saturation power Psat is given by
where Isat is the saturation intensity, h is Plancks constant, ν is the frequency, σem is the emission cross section, τ is the upper state lifetime, and Acore is the core area. Because of the small emission cross section at 1178 nm, 0.009 × 10−20 cm2, the saturation intensity is as high as 2.2 × 106 W/cm2. The value is two orders of magnitude higher than that at 1064 nm, 8.2 × 104 W/cm2. Although core size scaling is possible while maintaining single-mode guidance by the PBGF [3–5] which is beneficial for suppression of nonlinearity, a small-core design is important for saturated amplification at 1178 nm. The cross section of the Yb-PBGF is shown in Fig. 1(a). The corresponding illustration of the fiber cross section is shown in Fig. 1(b). The Yb-doped core at the center of the inner cladding is surrounded by a periodic hexagonal honeycomb structure, composed of high index Ge rods. The inner cladding structure is similar to that used in [9–11]. The pitch (center to center spacing) of the Ge rods is 10.1 μm. The mode field diameter was obtained to be 10.3 μm. The Psat at 1178 nm is about 1.8 W. The propagation loss of the core mode was measured to 0.09 dB/m at 1178 nm by cut-backing. Boron-doped rods replace the Ge-doped rods on both sides of the core and provide stress-induced birefringence on the order of 10−4 to the core region. Because of the lower refractive index (−0.006 relative to silica) of the boron rods, the light confinement is given by total internal refraction for the boron axis. The numerical aperture (NA) of 0.57 of the inner cladding was provided by the airclad. The diameter of the inner cladding is 230 μm. The pump absorption is about 1.1 dB/m at 976 nm. Since fiber twisting reduces the steepness of the cutoff slope, two parallel flat surfaces were imposed for the coil control on the outer cladding of the PBGF, which prevents fiber twisting of the fiber and fixes the tilt through the length of the fiber. The tilt angle of the PBGF was 77 degrees.3. Experimental setup
The experimental setup is shown in Fig. 2. The master oscillator was an in-house external cavity laser diode (ECLD) [11]. The seed signal was coupled to a single-mode (SM) polarization maintaining (PM) fiber. The signal linewidth broadening was obtained by phase modulation using an in-line electro-optical modulator (EOM). The EOM was driven by a low-pass filtered white noise. The noise signal bandwidth was 280 MHz and the noise power was about 29 dBm. The seed linewidth was broadened from 340 kHz up to 780 MHz. The linewidth was measured by use of a delayed self-heterodyne interferometry. A delay fiber with the length of 9 km was used with the corresponding resolution of 23 kHz. The broadened linewidth is beneficial for the SBS suppression [20, 21]. The signal linewidth dependence of the Brillouin gain coefficient is approximately given by
where gB is the Brillouin gain coefficient and ΔνB and Δνs are the Brillouin gain bandwidth and the signal linewidth, respectively. The SBS threshold enhancement factor (EF) is obtained to EF=1+Δνs/ΔνB [20]. Since the power of the ECLD (<150 mW) is much lower than the Psat of the Yb-PBGF (1.8 W), before the amplification by the Yb-PBGF, a backward pumped FRA was used to boost the seed power. The FRA was composed of all PM fiber components. The Raman fiber was a 300 m commercial SM-PM fiber (PM980-XP from Nufren). The acoustic properties of the fiber were characterized by measuring the Brillouin gain spectrum (BGS) by using a pump probe method at 1064 nm. The Brillouin gain parameter CB (CB = gB/Aao [22, 23]) and the linewidth (FWHM) ΔνB were measured to 0.43 m−1W−1 and 63 MHz, respectively. From the λ−2 dependence of ΔνB, the linewidth is estimated to 51 MHz at 1178 nm [19]. Thus the EF of 16 was gained in the FRA. The backscattered power of the FRA was monitored by using a 20 dB tap coupler. 1120/1178 nm WDM couplers were used for pump light coupling and removing. The pump source of the FRA was an in-house ytterbium doped fiber laser (YDFL) at 1120 nm. The maximum power was 31 W. In the free space between the FRA and the PBGF amplifier an isolator (ISO) was placed, which was also used for the SBS monitoring of the Yb-PBGF amplifier. The Yb-PBGF amplifier was a simple end-pumped fiber amplifier. The length of the Yb-PBGF was 32 m and was coiled on a fiber spool with the diameter of 32 cm. Both fiber ends were polished by 10 degrees to avoid feedback by Fresnel reflection. The fiber was cladding pumped by a 300 W fiber coupled LD at wavelength of 976 nm. The BGS of an Yb-PBGF with a similar design has been measured at 1178 nm [11]. The CB and the ΔνB were 0.12 m−1W−1 and 56 MHz, respectively. Thus, the EF of 15 was gained in the Yb-PBGF amplifier.
Fig. 2 Schematic of the 1178 nm fiber MOPA. HWP: half wave plate, FR: Faraday rotator, PBS: polarization beam splitter.
4. Results and discussion
By suppressing SBS we obtained an output power of 10 W from the FRA. The polarization extinction ratio (PER) was 20 dB. In Fig. 3(a) the output power properties of the FRA are shown. The slope efficiency was 54%. From a seed power into the FRA of about 5 mW, a gain as high as 33 dB was obtained. The output power was limited by available pump power. However rising backward power suggests that the output power level was close to the SBS threshold. The backward power as a function of output power is shown in Fig. 3(b). In the figure FRAs with and without linewidth broadening are compared. In the case without linewidth broadening, the backward power increased drastically due to the onset of SBS and the output power saturated at 400 mW.
Fig. 3 (a) Output power property of the 300 m long FRA. (b) Backscattering vs. forward output power. The cases with and without linewidth broadening were compared in the same FRA setup.
The output power property of the Yb-PBGF amplifier is shown in Fig. 4. The seed power was about 5 W. We obtained the maximum output power of 87 W at the maximum pump power of 280 W. The optical to optical conversion efficiency was 30% for the launched pump power and for the absorbed pump power the conversion efficiency was 40%. The increasing slope of the residual pump power shows the saturation of pump absorption. Hence the roll off from the initial slope of forward output was mainly caused by the pump absorption saturation. Besides that induced additional loss by the photodarkening is also one of considerable reasons because of the high inversion ratio in the amplifier. Since the slope of the backward scattering shows a similar shape to that of the forward output, it suggests the absence of SBS. The backward signal was mostly caused by the reflection at the output fiber facet. The reflected light coupled to the cladding and then appeared at the SBS monitoring port.
Fig. 4 Output power property of the Yb-PBGF amplifier: forward (circle), backward (square), and residual pump (triangle). The solid line stands for the linear fit of the initial slope of the forward output. The backward power was scaled by 10 times.
The seed and amplifier output spectra measured by use of an integrating sphere are shown in Fig. 5(a). The white light transmission spectrum of a 1.5 m Yb-PBGF coiled on a 32 cm spool for one round is also shown in the figure. The signal spectra show effective ASE suppression by the spectral filtering. The spectrum measured by coupling the signal into a SM fiber for high SNR is shown in Fig. 5(b). The spectra show a SNR of >40 dB.
Fig. 5 (a) Spectra from the Yb-PBGF amplifier measured by use of an integrating sphere. Gray curve shows the seed and red curve shows the amplifier output at the maximum pump power. Black curve shows a white light transmission spectrum of the 1.5 m Yb-PBGF coiled on a 32 cm spool for one round. (b) Spectra measured by coupling the signal into a SM fiber.
The linewidths of the seed and the output of the amplifier at 87 W were measured by the aforementioned delayed self-heterodyne interferometry. The beat spectra are shown in Fig. 6. No linewidth broadening was measured in the Yb-PBGF amplifier.
Fig. 6 Beat spectra of the seed (gray) and the output at 87 W (red) measured by delayed self-heterodyne interferometry using a 9 km delay fiber.
Steady state rate equations for two energy levels transition [24, 25] were solved numerically for the modeling of the amplifier. The geometrical properties corresponding to the actual fiber were used. The absorption and emission cross sections were 3.0 × 10−20 cm2 and 3.1 × 10−20 cm2 at the pump wavelength of 976 nm and 0 cm2 and 0.009 × 10−20 cm2 at the signal wavelength of 1178 nm, respectively. The doping concentration was 0.38 × 1020 cm−3. The lifetime of 0.84 ms and the overlap factor of 0.8 for the signal with the core were used. In this simulation, we neglected the effect of spontaneous emission and backscattering loss and assumed a homogeneous broadening. The output power as a function of the seed power and the pump power is show in Fig. 7. We also calculated the SBS limit. The total Brillouin gain in the amplifier is given by
where PS(z) is the signal power at the longitudinal position z in the Yb-PBGF. The Stokes wave amplification by the ytterbium gain was neglected. The red curves in the figure correspond to the SBS threshold gain GSBS ≈ 21 [19] multiplied by the EF.Fig. 7 Calculated output power of the 32 m Yb-PBGF amplifier as a function of seed power and pump power. The SBS-limited output power is indicated in red curves for different EFs. The pink circle indicates the estimated output power with the seed power of 5 W and the pump power of 280 W.
From a seed power of 5 W and a pump power of 280 W, the expected output power was about 110 W and was indicated by the pink circle in the figure. The calculated output power is higher than the actual output power because of no ASE and backscattering loss taken into account. The calculated result shows that the amplifier was operated in an unsaturated amplifier regime at high pump power. It suggests a high inversion ratio in the amplifier and saturated pump absorption which caused the roll off of the output power. The figure also shows the sufficient SBS suppression by EF=15 for our setup. It is obvious that the output power scaling is still possible by increasing the seed power as well as the fiber length. Using a tripled seed power the amplifier can be operated in the saturated amplifier regime up to a maximum pump power of 280 W and an output power of 170 W can be expected.
5. Conclusion
A narrow-linewidth and linearly-polarized fiber MOPA at 1178 nm has been successfully demonstrated by seed linewidth broadening. As high as 33 dB gain and 10 W power were obtained in the FRA as the preamplifier. From the Yb-PBGF main amplifier we obtained 87 W output, 12.4 dB gain, 40 dB SNR, and 11 dB PER without suffering from ASE as well as SBS. The linewidth was still less than 1 GHz, which means the light source is suitable for LGS applications. However, the output power was limited by pump absorption saturation. The numerical modeling suggests the possibility of the power scaling up to >200 W by increasing the seed power and the pump power. The developed source can be used as the seed of a large mode area PBGF (core diameter >30 μm) for the further power scaling [26].
Acknowledgments
This research was supported by JSPS KAKENHI Grant Number 25247067 and by the Photon Frontier Network Program of Ministry of Education, Culture, Sports, Science and Technology of Japan.
References and links
1. A. Isomki and O. G. Okhotnikov, “Femtosecond soliton mode-locked laser based on ytterbium-doped photonic bandgap fiber,” Opt. Express 14, 9238–9243 (2006). [CrossRef]
2. J. Riishede, J. Laegsgaard, J. Broeng, and A. Bjarklev, “All-silica photonic bandgap fibre with zero dispersion and a large mode area at 730 nm,” Pure Appl. Opt. 6, 667–670 (2004). [CrossRef]
3. O. N. Egorova, S. L. Semjonov, A. F. Kosolapov, A. N. Denisov, A. D. Pryamikov, D. A. Gaponov, A. S. Biriukov, E. M. Dianov, M. Y. Salganskii, V. F. Khopin, M. V. Yashkov, A. N. Gurianov, and D. V. Kuksenkov, “Single-mode all-silica photonic bandgap fiber with 20-μ m mode-field diameter,” Opt. Express 16, 11735–11740 (2008). [CrossRef] [PubMed]
4. T. T. Alkeskjold, M. Laurila, L. Scolari, and J. Broeng, “Single-mode ytterbium-doped large-mode-area photonic bandgap rod fiber amplifier,” Opt. Express 19, 7398–7409 (2011). [CrossRef] [PubMed]
5. G. Gu, F. Kong, T. Hawkins, J. Parsons, M. Jones, C. Dunn, M. T. Kalichevsky-Dong, K. Saitoh, and L. Dong, “Ytterbium-doped large-mode-area all-solid photonic bandgap fiber lasers,” Opt. Express 22, 13962–13968 (2014). [CrossRef] [PubMed]
6. A. Wang, A. K. George, and J. C. Knight, “Three-level neodymium fiber laser incorporating photonic bandgap fiber,” Opt. Lett. 31, 1388–1390 (2006). [CrossRef] [PubMed]
7. V. Pureur, L. Bigot, G. Bouwmans, Y. Quiquempois, M. Douay, and Y. Jaouen, “Ytterbium-doped solid core photonic bandgap fiber for laser operation around 980 nm,” Appl. Phys. Lett. 92, 061113 (2008). [CrossRef]
8. R. Goto, E. C. Mgi, and S. D. Jackson, “Narrow-linewidth, Yb3+-doped, hybrid microstructured fibre laser operating at 1178 nm,” Electron. Lett. 45, 877–878 (2009). [CrossRef]
9. C. B. Olausson, A. Shirakawa, M. Chen, J. K. Lyngs, J. Broeng, K. P. Hansen, A. Bjarklev, and K. Ueda, “167 W, power scalable ytterbiumdoped photonic bandgap fiber amplifier at 1178 nm,” Opt. Express 18, 16345–16352 (2010). [CrossRef] [PubMed]
10. X. Fan, M. Chen, A. Shirakawa, K. Ueda, C. B. Olausson, J. K. Lyngs, and J. Broeng, “High power Yb-doped photonic bandgap fiber oscillator at 1178 nm,” Opt. Express 20, 14471–14476 (2012). [CrossRef] [PubMed]
11. M. Chen, A. Shirakawa, X. Fan, K. Ueda, C. B. Olausson, J. K. Lyngs, and J. Broeng, “Single-frequency ytterbium doped photonic bandgap fiber amplifier at 1178 nm,” Opt. Express 20, 21044–21052 (2012). [CrossRef] [PubMed]
12. N. M. Litchinitser, S. C. Dunn, B. Usner, B. J. Eggleton, T. P. White, R. C. McPhedran, and C. M. de Sterke, “Resonances in microstructured optical waveguides,” Opt. Express 11, 1243–1251 (2003). [CrossRef] [PubMed]
13. C. E. Max, S. S. Olivier, H. W. Friedman, J. An, K. Avicola, B. V. Beeman, H. D. Bissinger, J. M. Brase, G. V. Erbert, D. T. Gavel, K. Kanz, M. C. Liu, B. Macintosh, K. P. Neeb, J. Patience, and K. E. Waltjen, “Image Improvement from a Sodium-Layer Laser Guide Star Adaptive Optics System,” Science 277, 1649–1652 (1997). [CrossRef]
14. C. Boyer, B. Ellerbroek, M. Gedig, E. Hileman, R. Joyce, and M. Liang, “Update on the TMT laser guide star facility design,” Proc. SPIE7015, 70152N (2008). [CrossRef]
15. N. Saito, K. Akagawa, M. Ito, A. Takazawa, Y. Hayano, Y. Saito, M. Ito, H. Takami, M. Iye, and S. Wada, “Sodium D2 resonance radiation in single-pass sum-frequency generation with actively mode-locked Nd:YAG lasers,” Opt. Lett. 32, 1965–1967 (2007). [CrossRef] [PubMed]
16. Y. Feng, L. R. Taylor, and D. Bonaccini Calia, “25 W Raman-fiber-amplifier-based 589 nm laser for laser guide star,” Opt. Express 17, 19021–19026 (2009). [CrossRef]
17. I. Dajani, C. Vergien, C. Robin, and B. Ward, “Investigations of single-frequency Raman fiber amplifiers operating at 1178 nm,” Opt. Express 21, 12038–12052 (2013). [CrossRef] [PubMed]
18. H. Zhang, P. Zhou, H. Xiao, X. Wang, and X. Xu, “536 W 1178 nm Yb-Raman Amplifier Feed by Three-Tone Seed,” in Advanced Solid State Lasers, paper ATu3A.6 (2014). [CrossRef]
19. G. P. Agrawal, “Nonlinear fiber optics, 4th ed,” (Academic Press, 2007).
20. V. R. Supradeepa, “Stimulated Brillouin scattering thresholds in optical fibers for lasers linewidth broadened with noise,” Opt. Express 21, 4677–4687 (2013). [CrossRef] [PubMed]
21. B. Anderson, C. Robin, A. Flores, and I. Dajani, “Experimental study of SBS suppression via white noise phase modulation,” Proc. SPIE8961, 89611W (2014).
22. A. Kobyakov, S. Kumar, D. Q. Chowdhury, A. B. Ruffin, M. Sauer, and S. R. Bickham, “Design concept for optical fibers with enhanced SBS threshold,” Opt. Express 13, 5338–5346 (2005). [CrossRef] [PubMed]
23. V. Lanticq, S. Jiang, R. Gabet, Y. Jaoun, F. Tailladde, G. Moreau, and G. P. Agrawal, “Self-referenced and single-ended method to measure Brillouin gain in monomode optical fibers,” Opt. Lett. 34, 1018–1020 (2009). [CrossRef] [PubMed]
24. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-Doped Fiber Amplifiers,” IEEE J. Quantum Electron. 33, 1049–1056 (1997). [CrossRef]
25. I. Kelson and A. Hardy, “Strongly pumped fiber lasers,” IEEE J. Quantum Electron. 34, 1570–1577 (1998). [CrossRef]
26. S. R. Petersen, T. T. Alkeskjold, F. Poli, E. Coscelli, M. M. Jrgensen, M. Laurila, J. Lgsgaard, and J. Broeng, “Hybrid Ytterbium-doped large-mode-area photonic crystal fiber amplifier for long wavelengths,” Opt. Express 20, 6010–6020 (2012). [CrossRef] [PubMed]
