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In this paper, a 933 W amplified spontaneous emission (ASE) source with a full width at half maximum (FWHM) of 50.4 nm is presented. The broadband source is generated by a 1271 W ASE amplifier together with 140m ordinary passive fiber for nonlinear transformation. With multi-wavelength rate equations and a nonlinear Schrödinger equation (NLSE), spectral evolution of the ASE source is analyzed theoretically.
© 2016 Optical Society of America
1. Introduction
With the space distribution limitation effect of the optical waveguide, the focus ability of the waveguided light [1,2] could be preserved whatever the time coherent status. High power amplified spontaneous emission (ASE) source has additional advantage of low coherence, wide wavelength coverage and high efficiency. Apart from the traditional applications such as low coherence interferometry [3,4], fiber gyroscopes [5–7] and optical sensing [8], the high power fiber ASE source can be qualified in spectral beam combination [9–13] and material processing [14].
In the past few years, single-stage fiber ASE sources with power around 100 W have been demonstrated. Wang et al. achieved 117 W ASE output with central wavelength of 1063 nm and FWHM of 40 nm in a double-clad Yb-doped multimode-offset-core fiber [15]. With a tapered double-clad fiber (T-DCF), Filippov et al. realized a 74 W ASE source centered at 1079 nm [16]. Xiao et al. realized a 68.3 W all-fiber ASE source with angle-cleaved ends [17]. Using a distributed, side-coupled, cladding-pumped (DSCCP) Yb-doped fiber, An et al. managed to get a combined ASE output power of 102 W centered at 1038nm with FWHM over 10 nm [18].
Recently, kilowatts-level high power ASE sources have been developed with fiber amplifiers. Jiangming Xu et al. realized a 1.01 kW ASE source with central wavelength of 1074.4 nm and FWHM of 8.1 nm with a two-stage fiber amplifier [19]. With polarized-maintained gain fiber, Pengfei Ma et al. generated a 1427 W linearly polarized ASE source with FWHM of 11 nm [20]. High power ASE source with low coherence is applicable in the generation of high power laser with narrow bandwidth for its suppression of Stimulated Brillouin scattering. By filter a narrow range of a broadband ASE source with fiber Bragg gratings, O. Schmidt, et al. get a 679 W ASE source at 1030 nm with 12 pm bandwidth [9]. Pengfei Ma et al. achieved 800 W ASE output with FWHM of 0.2 nm [10]. Jiangming Xu et al. realized 1.87 kW ASE output with FWMN of 1.7 nm with a three-stage fiber amplifier [11].
The bandwidth of the high power broadband sources mentioned above usually decreases while the power increases. As a result, the bandwidth of the ASE source is limited and may be decrease to the level of high power fiber laser source when the power of ASE source rises to a certain point. Besides, the central wavelength of the high power ASE source is confined by the length of gain fiber.
To maintain the broadband property of high power ASE source and find a way to alter central wavelength, we managed to perform nonlinear transformation on the ASE light by using ordinary high power transfer passive double-clad fiber (DCF) with low nonlinearity. We realize a 1271 W ASE output with a two-stage fiber amplifier with FWHM of 11.5 nm. By injecting 1271 W ASE light of the amplifier into 140 m passive DCF, we get 933 W broadband ASE light with FWHM of 50.4 nm. And the central wavelength shifts from 1071.6 nm to 1097.6 nm.
Spectral evolution in the amplification of ASE light is simulated with multi-wavelength rate equations. The results reveals that the pumping effect of short wavelength part to longer wavelength part of ASE light is responsible for the central wavelength shift in the amplification proceed. While in the nonlinear transformation process, the spectral evolution pattern can be attributed to Raman effect and self-phase modulation (SPM).
2. Amplification of high power ASE light
As shown in Fig. 1, the high power ASE amplifier is composed of a ASE seed, a pre-amplification stage and a main amplification stage. The ASE seed source is pumped by a 25 W 975 nm laser diode (LD). The pumping light is coupled into 10 m Yb-doped DCF with a (1 + 1) x1 coupler. The cladding absorption coefficient of the 975 nm pumping light of the gain fiber is 1.26 dB/m. The core/clad size of the gain fiber is 20/400 μm. The end of the backward output is cleaved with an 8° angle to decrease the feedback. The forward output of the seed is connected to a clad stripper (CLS) to filter the remaining pumping light in the clad. Then a high power isolator is placed after the CLS to block the backward ASE light from the next stage. The output of the seed is 4.76 W ASE light with central wavelength of 1049.8 nm and FWHM of 30.3 nm, as shown in Fig. 2(a). The pre-amplification stage uses the same gain fiber with the seed. The fiber length is also 15 m. The pumping source of this stage is a 93 W 975 nm LD. The ASE power under full pumping power is 53.3 W. The central wavelength is 1063.4 nm and the FWHM is 13.7 nm, as shown in Fig. 2(b). A CLS and an isolator is also set after this stage. The main stage is pumped by three 600 W 975 nm LDs. And a (3 + 1) x 1 coupler combines the pumping light and ASE light from the previous stage. The gain fiber is 20 m Yb-doped DCF with the same parameter of the seed. A high power CLS is spliced before the end cap. The output port of end cap is 8° cleaved. With pumping power of 1624 W, the ASE output is 1271 W. The optical-to-optical efficiency of the main stage is 75%, as shown in Fig. 3(a). The central wavelength is 1071.6 nm and the FWHM is 11.5 nm, as shown in Fig. 3(b).
Fig. 2 Output spectra before the main stage. (a) Spectra of the seed. (b)Spectra of the pre-amplifier.
Fig. 3 Output parameters of the main stage. (a) The output ASE power versus the pumping power. (b) Spectra of the main stage.
The amplification process of ASE light can be analyzed with the multi-wavelength rate equations as follows [21],
where p represents pumping light, ase represents ASE light, z is the position along the fiber, P is light power, σa and σe are the emission and absorption cross sections, respectively, n is the population density of doped ions, n2 is the upper lasing level population density, α is the scattering loss, Adope is the doped area, and Γ is the filling factor.These three equations calculate the power of many small wavelength regions which can cover the range of the ASE spectrum in together. We divide the wavelength range into 1000 portions and get the simulated results as shown in Fig. 4(b). To compare the simulated results with the experimental ones, we normalized the experimental results as shown in Fig. 4(a). The spectrum of each stage use the one with the full output power.
Fig. 4 Comparison of the simulation and experimental results. (a) Experimental results. (b) Simulated results.
As we can see from the evolution of the spectrum of the ASE light, the central wavelength of the ASE light increases after each stage. This can be attributed to the phenomenon that the shorter wavelength parts of the ASE light can be the pumping light of the longer ones when the 975 nm pumping power is not high enough, causing the central wavelength shift to longer wavelength. As the calculation of the rate equations Eqs. (1)-(3) includes this effect, the simulation results show good fit with experimental ones. The bandwidth of the ASE light decreases as the light transmit through each stage. We suppose spectral broadening caused by SPM may be exist in the main stage for the bandwidth is broader than expected.
3. Nonlinear transformation of high power ASE light
We spliced 140 m ordinary 20/400 μm Ge-doped DCF used for high power laser transfer between the end cap and the CLS of the main stage to perform nonlinear transformation on the 1271 W ASE light as shown in Fig. 5. The core attenuation of the passive fiber is 1.5 dB/km to 1070 nm light. After propagating through the passive fiber, the power decreases to 933 W while the bandwidth increases to 50.4 nm. And the central wavelength changes from 1071.6 nm to 1097.6 nm, as shown in Figs. 6 and 7.
Fig. 7 Central wavelength and FWHM of ASE light with nonlinear transformation of 140 m passive fiber.
The beam quality of the 1271 W ASE light before injecting into the passive fiber is Mx2 = 1.650, My2 = 1.622 and M2 = 1.638. The beam quality of 933 W output after transmitting in the passive fiber is measured to be Mx2 = 1.695, My2 = 1.667 and M2 = 1.683 with a PRIMES Laser Quality Monitor, as shown in Fig. 8. The beam quality degrades slightly after propagating in the passive fiber for the excitation of new wavelength component caused by nonlinear effect.
Fig. 8 Beam quality measurement results. (a) Beam quality measurement result of the 1271 W ASE light before injecting into the passive fiber. (b) Beam quality measurement result of the ASE light after propagating in the passive fiber.
4. Nonlinear transformation of high power ASE light
Spectral broadening of continuous wave fiber amplifier is mainly caused by SPM [22]. Besides, spectral broadening caused by SPM can be more evident to ASE light for its high incoherence [23–25]. The wavelength of the 933 W ASE light ranges from 1064 nm to 1132 nm with 10 dB, which covers the range of first order stimulated Raman scattering (SRS). Therefore, the spectral evolution can be analyzed by considering both SPM and SRS effect.
Considering Raman effect and SPM, the nonlinear Schrödinger equation (NLSE) can be expressed as follows [25,26],
where A is field amplitude, is nonlinear Kerr parameter, is second order dispersion coefficient, is angular frequency of light, is nonlinear response function, is Raman response function. As to the fiber we use in our experiment, and . We calculate the output spectrum of the after the ASE light propagating through the passive fiber using Eq. (4). The comparison of experimental results and simulated results are shown in Fig. 9. The simulation fits well with the experiments.The spectral evolution is also related to the bandwidth of the original output of the ASE amplifier. Figure 10 shows the simulation results of the spectrum of 1200 W ASE light with different bandwidths after propagating in 140 m passive fiber. The intensity of each bandwidth is moved up to different level intentionally to show the results clearly. The case with narrow bandwidth have discrete peaks while the one with wide bandwidth can generate continuous spectrum. The threshold bandwidth of the continuous spectrum generation is near 8 nm. The discrete peaks in the spectrum of 3 nm original bandwidth corresponds to the first several orders of SRS. The reason why high order SRS is ignited is that the power spectral density gets higher when the bandwidth gets narrower, which means the power of central wavelength is higher. The ASE light with broader bandwidth can generate first order SRS light with broader bandwidth. Together with the spectral broadening caused by SPM, the final spectrum can be continuous. In summary, the broadband ASE light with continuous spectrum achieved in our experiment can be attributed to the combination of first order SRS and SPM effect.
We can foresee that if we lengthen the passive fiber, we may get ASE source with broader bandwidth and longer central wavelength. The simulated spectral evolution of 1200 W ASE light propagating in 800 m passive DCF is showed in Fig. 11. The bandwidth of the ASE light increases with the fiber length of passive fiber until the length gets to 200 m. The bandwidth when the length is 200 m can increase to 60 nm. But when the length is longer than 200 m, with the ASE light energy transferring to higher orders of SRS, the bandwidth will not keep increasing with the fiber length. The bandwidth may even decrease with the fiber length in several parts. When the fiber length is near 500 m and 700 m, the transfer between two orders of SRS can be distinct as the bandwidth has decreased. While the central wavelength always increases with the fiber length. When the fiber length is 800 m, the central wavelength moves to 1373 nm, and the FWHM is 37 nm. Therefore, we can get ASE light with different central wavelength by connecting the source with fiber of different length. This 933 W source can also be the pumping of supercontinuum by connecting to high nonlinear fiber.
5. Conclusion
In conclusion, we realize a 933 W ASE source with FWHM of 50.4 nm by perform nonlinear transformation on a 1271 W high power ASE amplifier with 140 m ordinary passive fiber. Simulation results with multi-wavelength rate equations show that the pumping effect among the ASE light the shift of central wavelength in the fiber amplifier. In the nonlinear transformation part, the spectral evolution is related to SRS and SPM according to the simulation results of NLSE.
Funding
National Natural Science Foundation of China (NSFC) (61307057); Tsinghua University Initiative Scientific Research Program (20151080709).
References and links
1. K. Kang, H. Zhang, and D. Wang, “Thermal-induced refractive-index planar waveguide laser,” Appl. Phys. Lett. 95(18), 181102 (2009). [CrossRef]
2. H. Zhang, X. Shen, D. Chen, C. Zheng, P. Yan, and M. Gong, “High energy and high peak power nanosecond pulses generated by fiber amplifier,” IEEE Photonics Technol. Lett. 26(22), 2295–2298 (2014). [CrossRef]
3. M. Bashkansky, M. Duncan, L. Goldberg, J. Koplow, and J. Reintjes, “Characteristics of a Yb-doped superfluorescent fiber source for use in optical coherence tomography,” Opt. Express 3(8), 305–310 (1998). [CrossRef] [PubMed]
4. A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography principles and applications,” Rep. Prog. Phys. 66(2), 239–303 (2003). [CrossRef]
5. M. J. F. Digonnet, “Status of broadband rare-earth doped fiber sources or FOG applications,” Proc. SPIE 2070, 113–131 (1994). [CrossRef]
6. L. Goldberg, J. P. Koplow, R. P. Moeller, and D. A. V. Kliner, “High-power superfluorescent source with a side-pumped Yb-doped double-cladding fiber,” Opt. Lett. 23(13), 1037–1039 (1998). [CrossRef] [PubMed]
7. Z. C. Hsu, Z. S. Peng, L. A. Wang, R. Y. Liu, and F. I. Chou, “Gamma ray effects on double pass backward superfluorescent fiber sources for gyroscope applications,” Proc. SPIE 7004, 70044M (2008). [CrossRef]
8. S. Martin-Lopes, M. Gonzalez-Herraez, A. Carrasco-Sanz, F. Vanhols-beeck, S. Coen, H. Fernandez, J. Solis, P. Corredera, and M. L. Hernanz, “Broadband spectrally flat and high power density flight source for fibre sensing purposes,” Meas. Sci. Technol. 17(5), 1014–1019 (2006). [CrossRef]
9. O. Schmidt, M. Rekas, C. Wirth, J. Rothhardt, S. Rhein, A. Kliner, M. Strecker, T. Schreiber, J. Limpert, R. Eberhardt, and A. Tünnermann, “High power narrow-band fiber-based ASE source,” Opt. Express 19(5), 4421–4427 (2011). [CrossRef] [PubMed]
10. P. Ma, R. Tao, X. Wang, P. Zhou, and Z. Liu, “High power narrow-band and polarization-maintained all fiber superfluorescent source,” IEEE Photonics Technol. Lett. 27(8), 879–882 (2015). [CrossRef]
11. J. Xu, W. Liu, J. Leng, H. Xiao, S. Guo, P. Zhou, and J. Chen, “Power scaling of narrowband high-power all-fiber superfluorescent fiber source to 1.87 kW,” Opt. Lett. 40(13), 2973–2976 (2015). [CrossRef] [PubMed]
12. J. Liu, H. Shi, C. Liu, and P. Wang, “Widely tunable high power narrow-linewidth thulium-doped all-fiber superfluorescent source,” in Conference on Lasers and Electro-Optics/ QELS Fundamental Science, OSA Technical Digest Series (Optical Society of America, 2015), paper JTh2A.98. [CrossRef]
13. X. Wang, X. Jin, P. Zhou, X. Wang, H. Xiao, and Z. Liu, “High power, widely tunable, narrowband superfluorescent source at 2 μm based on a monolithic Tm-doped fiber amplifier,” Opt. Express 23(3), 3382–3389 (2015). [CrossRef] [PubMed]
14. P. Wang, J. K. Sahu, and W. A. Clarkson, “Power scaling of ytterbium doped fiber superfluorescent sources,” IEEE J. Sel. Top. Quantum Electron. 13(3), 580–587 (2007). [CrossRef]
15. P. Wang, J. K. Sahu, and W. A. Clarkson, “110 W double-ended ytterbium-doped fiber superfluorescent source with M2 = 1.6,” Opt. Lett. 31(21), 3116–3118 (2006). [CrossRef] [PubMed]
16. V. Filippov, Y. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov, “Double clad tapered fiber for high power applications,” Opt. Express 16(3), 1929–1944 (2008). [CrossRef] [PubMed]
17. Q. Xiao, P. Yan, Y. Wang, J. Hao, and M. Gong, “High-power all-fiber superfluorescent source with fused angle-polished side-pumping configuration,” Appl. Opt. 50(8), 1164–1169 (2011). [CrossRef] [PubMed]
18. Y. An, J. Cao, Z. Huang, S. Guo, X. Xu, and J. Chen, “High-power all-fiberized superfluorescent source with distributed side-coupled cladding-pumped fiber,” Appl. Opt. 53(36), 8564–8570 (2014). [CrossRef] [PubMed]
19. J. Xu, L. Huang, J. Leng, H. Xiao, S. Guo, P. Zhou, and J. Chen, “1.01 kW superfluorescent source in all-fiberized MOPA configuration,” Opt. Express 23(5), 5485–5490 (2015). [CrossRef] [PubMed]
20. P. Ma, L. Huang, X. Wang, P. Zhou, and Z. Liu, “High power broadband all fiber super-fluorescent source with linear polarization and near diffraction-limited beam quality,” Opt. Express 24(2), 1082–1088 (2016). [CrossRef] [PubMed]
21. P. Yan, J. Sun, D. Li, M. Gong, and Q. Xiao, “Studies of central wavelength of high-power all-fiber superfluorescent sources with Yb-doped double-clad fibers,” Opt. Commun. 380, 250–259 (2016). [CrossRef]
22. W. Liu, W. Kuang, L. Huang, and P. Zhou, “Modeling of the spectral properties of CW Yb-doped fiber amplifier and experimental validation,” Laser Phys. Lett. 12(4), 045104 (2015). [CrossRef]
23. J. T. Manassah, “Self-phase modulation of incoherent light revisited,” Opt. Lett. 16(21), 1638–1640 (1991). [CrossRef] [PubMed]
24. A. G. Kuznetsov, E. V. Podivilov, and S. A. Babin, “Spectral broadening of incoherent nanosecond pulses in a fiber amplifier,” J. Opt. Soc. Am. B 29(6), 1231–1236 (2012). [CrossRef]
25. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 2012).
26. T. Schreiber, A. Liem, E. Freier, C. Matzdorf, R. Eberhardt, C. Jauregui, J. Limpert, and A. Tünnermanna, “Analysis of stimulated Raman scattering in cw kW fiber oscillators,” Proc. SPIE 8961, 89611T (2014).
