Tm 3 +-doped CW fiber laser based on a highly GeO 2-doped dispersion-shifted fiber

A novel all-fiber laser based on a highly GeO2-doped dispersionshifted Tm-codoped fiber, pumped at 1.56 μm wavelength and lasing at 1.862 μm wavelength with a slope efficiency up to 37% was demonstrated. The single-mode Tm-doped fiber with the 55GeO2-45SiO2 core was fabricated for the first time by MCVD technique. The laser produces spectral side bands, resulting from the four-wave mixing owing to the shift of the zero-dispersion-wavelength of the fiber to the laser wavelength, thus, making it potentially particularly attractive for dispersion management and ultrashort pulse generation. ©2011 Optical Society of America OCIS codes: (060.2290) Fiber materials; (060.2320) Fiber optics amplifiers and oscillators. References and links 1. “Rare-Earth-Doped Fiber Lasers and Amplifiers,” M. J. E. Digonet, ed. Marcel Dekker, Inc., New York-Basel, 2001. 2. S. D. Jackson, and T. A. King, “High-power diode-cladding-pumped Tm-doped silica fiber laser,” Opt. Lett. 23(18), 1462–1464 (1998). 3. S. D. Jackson, “Midinfrared holmium fiber lasers,” IEEE J.Quantum Electronics 42(2), 187–191 (2006). 4. A. S. Kurkov, V. V. Dvoyrin, and A. V. Marakulin, “All-fiber 10 W holmium lasers pumped at λ=1.15 microm,” Opt. Lett. 35(4), 490–492 (2010). 5. H. Osanai, T. Shioda, T. Moriyama, S. Araki, M. Horiguchi, T. Izawa, and H. Takata, “Effect of Dopants on Transmission Loss of Low-OH-Content Optical Fibers,” Electron. Lett. 12(21), 549–550 (1976). 6. S. H. Wemple, “Material dispersion in optical fibers,” Appl. Opt. 18(1), 31–35 (1979). 7. T. Sun, G. Kai, Zh. Wang, Sh. Yuan, and X. Dong, “Enhanced nonlinearity in photonic crystal fiber by germanium doping in the core region,” Chin. Opt. Lett. 6(2), 93–95 (2008). 8. J. Wang, J. R. Lincoln, W. S. Brocklesby, R. S. Deol, C. J. Mackechnie, A. Pearson, A. C. Tropper, D. C. Hanna, and D. N. Payne, “Fabrication and optical properties of lead-germanate glasses and a new class of optical fibres doped with Tm,” J. Appl. Phys. 73(12), 8066–8075 (1993). 9. J. Wu, Z. Yao, J. Zong, and S. Jiang, “Highly efficient high-power thulium-doped germanate glass fiber laser,” Opt. Lett. 32(6), 638–640 (2007). 10. V. M. Mashinsky, V. B. Neustruev, V. V. Dvoyrin, S. A. Vasiliev, O. I. Medvedkov, I. A. Bufetov, A. V. Shubin, E. M. Dianov, A. N. Guryanov, V. F. Khopin, and M. Yu. Salgansky, “Germania-glass-core silica-glasscladding modified chemical-vapor deposition optical fibers: optical losses, photorefractivity, and Raman amplification,” Opt. Lett. 29(22), 2596–2598 (2004). 11. V. L. Kalashnikov, “Effective refractive indexes and dispersion characteristics of the tapered fibers, ” http://info.tuwien.ac.at/kalashnikov/TFmodes.html. 12. G. Sansone, G. Steinmeyer, C. Vozzi, S. Stagira, S. De Silvestri, K. Starke, D. Ristau, B. Schenkel, J. Biegert, A. Gosteva, U. Keller, and M. Nisoli, “Mirror dispersion control of a hollow fiber supercontinuum,” Appl. Phys. B 78(5), 551–555 (2004). 13. E. Sorokin, V. L. Kalashnikov, J. Mandon, G. Guelachvili, N. Picqué, and I. T. Sorokina, “Cr: YAG chirpedpulse oscillator,” N. J. Phys. 10(8), 083022 (2008). 14. V. L. Kalashnikov, E. Podivilov, A. Chernykh, S. Naumov, A. Fernandez, R. Graf, and A. Apolonski, “Approaching the microjoule frontier with femtosecond laser oscillators: theory and comparison with experiment,” N. J. Phys. 7, 217 (2005). 15. A. Chong, J. Buckley, W. Renninger, and F. Wise, “All-normal-dispersion femtosecond fiber laser,” Opt. Express 14(21), 10095–10100 (2006). #141330 $15.00 USD Received 19 Jan 2011; revised 12 Mar 2011; accepted 2 Apr 2011; published 11 Apr 2011 (C) 2011 OSA 25 April 2011 / Vol. 19, No. 9 / OPTICS EXPRESS 7992 16. S. Lefrançois, K. Kieu, Y. Deng, J. D. Kafka, and F. W. Wise, “Scaling of dissipative soliton fiber lasers to megawatt peak powers by use of large-area photonic crystal fiber,” Opt. Lett. 35(10), 1569–1571 (2010). 17. V. L. Kalashnikov, Energy scalability of mode-locked oscillators: completely analytical approach to an analysis, Europhysics Conference Abstracts Volume 34C (4th EPS-QEOD EUROPHOTON Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources held in Hamburg, Germany, 29 August 3 September 2010), p. TuP4.


Introduction
The long-wavelength spectral edge of the lasing wave-length range of conventional rare-earth-doped silica fiber lasers is known to be around 1.9 µm [1].Even longer-wavelength range up to 2.15 µm is covered now by Hodoped fiber lasers [2].Further spectral expansion is expected through the involvement of nonlinear effects, like e.g.Raman scattering.Development of high energy ultrashort pulsed Tm-laser sources as well as super-continuum sources in the long-wavelength spectral range above 2 microns is also currently a very hot topic.
Keeping all these applications in mind we have developed and are presenting here a new type of highly nonlinear germanium fiber with the zero-dispersion-wavelength shifted towards the lasing wavelength of Tm 3+ .We have verified the fiber in laser experiments, having observed manifestation of the nonlinear effects due to the dispersion shifted nature of the fiber.We also present the results of an extensive theoretical analysis, explaining the experimentally observed phenomena.This analysis provides us further outlook and design criteria for this type of fibers used for generation of high power ultrashort pulses from Tm-fiber laser, operating in the positive dispersion regime [3] (similarly to what has been demonstrated at the shorter wavelengths [5]), as well as for using undoped version of this fiber for supercontinuum generation in the mid-infrared (>2 µm).

Fiber fabrication and experimental results
Fiber preform was fabricated by MCVD method [4].Thulium was incorporated to the preform core from the vapor phase.The concentration of GeO 2 in the fiber core detected by X-ray microanalysis was 55 mol.%.Then a singlemode fiber with a cut-off of 1.43 µm was drawn.Optical absorption has a typical shape of a Tm-doped silica glass, the optical loss minimum at 1900 nm amounted to 0.12 dB/m.
To build a fiber laser cavity, a Tm-doped single-mode fiber piece was spliced from both ends to fiber Bragg gratings (FBGs) by an electric fusion splicer.The FBGs were UV-written in a germanosilicate fiber with 20% mol concentration of GeO 2 in its core and a cut-off at 1200 nm wavelength.One FBG was highly reflective (HR) at the 1862 nm wavelength but the other one has a reflection of 20% only.Two fiber pieces of 2.5 and 4.5 m lengths were studied.The fibers were pumped through HR FBG by an Er-doped fiber laser with a pump power up to 210 mW.The smallsignal pump absorption amounted to 6 dB/m.Core-excitation of the fiber with an Er-doped fiber laser operating at 1.56 µm wavelength produced characteristic emission of Tm 3+ in the range of 1.8-2 µm, corresponding to 3 F 4 → 3 H 6 transition.The emission observed from the output end of the 10 m long fiber had a noticeable dependence on pump power.Its increase led to rise of the emission intensity and narrowing of its band-width .At the same time the emission maximum experienced blue-shift to 1820 nm.
The dependence of the output power on the pump power is shown in Fig. 1 for the both fiber lasers.The slope efficiency is 26 and 37% for the short and long fiber lasers, respectively.The threshold was 40 and 25 mW for the short and long fiber lasers.The line-width of the lasers emission was less than 1 nm.The output spectrum of both lasers also OSA / ASSP 2011 ATuB17.pdfcontained two spectral components at wavelengths of 1840 and 1884 nm (Fig. 1, inset).Their intensities increased by a square law with the optical power of the central component at 1862 nm.These symmetrical components were attributed to the four-wave mixing.Fig. 1.The dependence of the output fiber laser power on the launched pump power for the two lasers of 2.5 and 4.5 m length of the active fiber and a typical output spectrum of the lasers (inset).
The calculation of GVD has been based using the finite-element vectorial modelling of an effective refractive index of zero-order mode.The wavelength dependence of the bulk material refractive index for different concentrations of germanium has been approximated by Sellmeier's formula in correspondence with [3].
The GVD corresponding to the geometry shown by inset in Fig. 6 is presented by the solid curve with the grey area defining the uncertainty limits.It has been found, that the wavelength of zero GVD shifts into a longer wavelength region with the decreasing core redius and that this wavelength is quite sensitive to the core size.Such behaviour corresponds to the domination of the waveguiding effects in the vicinity of the second zero point of the GVD.This domination results from the comparatively small core radius and very high N.A. of the fiber, resembling the tapered and microstructured fibers.The calculated zero dispersion wavelength equals to 1.87 µm for the given Ge distribution which explains the experimental observation of four-wave mixing in this spectral region.

Conclusion and outlook
The newly developed fiber type has shown to be promising for laser applications, in particular for ultrashort pulse generation in the positive dispersion regime, although its lasing efficiency is yet moderate.The latter can be due OSA / ASSP 2011 ATuB17.pdf to a mismatch between the mode field diameters of the active fiber and the FBGs.We thus expect to achieve a noticeable improvement in laser efficiency by writing FBG directly in the active fiber.
On the other hand, the fiber demonstrated attractive properties for its further use in ultrashort pulsed systems.The high nonlinearity of the fiber make the nonlinear effects visible even at low optical power less than 100 mW and a small fiber length (2.5 m).The small group velocity dispersion also favors the four-wave mixing processes visible in our experiment.The latter is very important as for the nonlinear interaction, so for the mode-locking.We hope that such type of fiber allows us to develop in future ultrashort pulse lasers and super-continuum sources in the vicinity of 2 µm.

Fig. 6 .
Fig. 6. dispersion of the fiber.The inset shows corresponding Ge distribution.