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A higher-order-mode Erbium-doped-fiber amplifier

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A higher-order-mode Erbium-doped-fiber amplifier
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  A higher-order-mode Erbium-doped-fiber amplifier J. W. Nicholson 1* , J. M. Fini 1 , A. M. DeSantolo 1 , E. Monberg 1 , F. DiMarcello 1 , J. Fleming 1 , C. Headley 1 , D. J. DiGiovanni 1 , S. Ghalmi 2 , and S. Ramachandran 3   1 OFS Laboratories, 19 Schoolhouse Road, Suite 105, Somerset, NJ 08873, USA 2  Now with Vytran Corp, 1400 Campus Drive West, Morganville, NJ 07751, USA 3  Now with Department of Electrical and Computer Engineering and Photonics Center, Boston University, 8 Saint Mary’s Street, Boston, Massachusetts 02215, US   A *jwn@ofsoptics.com Abstract:  We demonstrate the first erbium-doped fiber amplifier operating in a single, large-mode area, higher-order mode. A high-power, fundamental-mode, Raman fiber laser operating at 1480 nm was used as a  pump source. Using a UV-written, long-period grating, both pump and 1564 nm signal were converted to the LP 0,10  mode, which had an effective area of 2700 μm 2  at 1550 nm. A maximum output power of 5.8 W at 1564 nm with more than 20 dB of gain in a 2.68 m long amplifier was obtained. The mode  profile was undistorted at the highest output power. ©2010 Optical Society of America OCIS codes:  (060.2320) Fiber optics amplifiers and oscillators; (060.2280) Fiber design and fabrication References and links 1. J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen , ― High-power rod-type photonic crystal fiber laser  ,‖ Opt. Express 13 (4), 1055  –  1058 (2005). 2. C. H. Liu , ―Chirally Coupled Core Fibers at 1550-nm and 1064-nm for Effectively Single-Mode Core Size Scaling CLEO 2007 vol. CtuBB3 3. W. S. Wong, X. Peng, J. M. McLaughlin, and L. Dong , ― Breaking the limit of maximum effective area for robust single-mode propagation in optical fibers ,‖ Opt. Lett. 30 (21), 2855  –  2857 (2005). 4. J. R. Marciante , ―Mode -area scaling of helical-core dual- clad fiber lasers and amplifiers,‖ CLEO 2006 vol. 3 1849  –  1851 (2005) 5. J. M. Fini , ― Intuitive modeling of bend distortion in large-mode-area fibers ,‖ Opt. Lett. 32 (12), 1632  –  1634 (2007). 6. J. W. Nicholson, J. M. Fini, A. D. Yablon, P. S. Westbrook, K. Feder, and C. Headley , ― Demonstration of bend-induced nonlinearities in large-mode-area fibers ,‖ Opt. Lett. 32 (17), 2562  –  2564 (2007). 7. S. Ramachandran, J. W. Nicholson, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello , ― Light  propagation with ultralarge modal areas in optical fibers ,‖ Opt. Lett. 31 (12), 1797  –  1799 (2006). 8. S. Ramachandran, J. M. Fini, M. Mermelstein, J. W. Nicholson, S. Ghalmi, and M. F. Yan , ― Ultra-large effective-area, higher-order mode fibers: a new strategy for high-power lasers ,‖ Laser Photonics Rev. 2 (6), 429  –  448 (2008). 9. J. M. Fini, and S. Ramachandran , ―  Natural bend-distortion immunity of higher-order-mode large-mode-area fibers ,‖ Opt. Lett. 32 (7), 748  –  750 (2007). 10. D. Marcuse, Theory of Dielectric Optical Waveguides , 2nd edition (Academic Press). 11. S. Ramachandran, et al  ., in Photonics West, Late Breaking Developments  —  Session 6453  —  9 (San Jose, Calif., 2007) 12. J. W. Nicholson, S. Ramachandran, S. Ghalmi, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, ― Propagation of femtosecond pulses in large-mode-area, higher-order-mode fiber  ,‖ Opt. Lett. 31 (21), 3191  –  3193 (2006). 13. S. Ramachandran , ― Dispersion-tailored few-mode   bers: a versatile platform for in-   ber photonic devices ,‖ J. Lightwave Technol. 23 (11), 3426  –  3443 (2005). 14. Y. Emori, K. Tanaka, C. Headley, and A. Fujisaki, in Conference on Lasers and Electro- Optics ―High -power Cascaded Raman Fiber Laser with 41-W output power at 1480- nm band‖ 2007 Technical Digest (Optical Society of America, 2007), paper CFI2. 15. J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi , ― Spatially and spectrally resolved imaging of modal content in large-mode-area fibers ,‖ Opt. Express 16 (10), 7233  –  7243 (2008). 16. J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. Headley, and D. DiGiovanni , ― Picosecond  pulse amplification in a core-pumped large-mode-area erbium fiber  ,‖ Opt. Lett. 32 (16), 2429  –  2431 (2007). #129079 - $15.00 USDReceived 27 May 2010; revised 21 Jul 2010; accepted 21 Jul 2010; published 2 Aug 2010 (C) 2010 OSA16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17651  17. J. C. Jasapara, M. J. Andrejco, A. DeSantolo, A. D. Yablon, Z. Varallyay, J. W. Nicholson, J. M. Fini, D. J. DiGiovanni, C. Headley, E. Monberg, and F. V. Dimarcello , ― Diffraction-Limited Fundamental Mode Operation of Core-Pumped Very-Large-Mode-Area Er Fiber Amplifiers ,‖ IEEE J. Sel. Top. Quantum Electron. 15 (1), 3  –  11 (2009). 1. Introduction The need to mitigate nonlinearities such as self-phase modulation, Brillouin scattering, and Raman scattering in high-power fiber lasers has led to an increase in the fiber effective area (A eff  ). A number of strategies have been presented to achieve large-mode area (LMA) high  power fiber lasers. One approach, the rod-type fiber, is to reduce the index contrast and make the fiber rigid [1]. However the fiber must be held straight to avoid significant bend losses, which eliminates many of the advantages of the conventional fiber geometry. Another approach, taken by chirally coupled fibers [2], leakage channel fibers [3], and helically coiled cores [4], is to operate the fiber in the fundamental mode and introduce additional structures into the fiber to add differential loss to unwanted higher order modes. Fibers operating in the fundamental mode however suffer from bend induced reductions in A eff  , consequently leading to increased nonlinearities and offsetting the advantage of the LMA design, with the effect  becoming more pronounced as A eff   is increased [5,6]. Recently a new approach to high power fiber lasers was introduced: intentionally operating in a single, large effective area, higher-order mode (HOM) of a specially designed fiber [7,8]. Higher-order modes have the advantage that they are less susceptible than the fundamental mode to bend induced area reduction [9]. At the same time, compared to the fundamental mode, they are more resistant to nearest neighbor mode coupling, which scatters the LP M,N  mode into the LP M ± 1,N  and is the typically the dominant form of mode-coupling in multi-mode fibers [10]. The resistance to nearest neighbor coupling occurs because the difference in effective index between the LP 0,N  and LP 1,N  modes increases with increasing N. Amplification in a cladding-pumped, Yb-doped, higher-order mode amplifier has been demonstrated [11]. In this work we demonstrate, for the first time, amplification in a higher-order-mode, erbium-doped fiber with an A eff    of 2700 μm 2 . ErYb fibers have high absorption  but are difficult to fabricate with large-mode area due to constraints on core composition required for reasonable Yb-Er energy transfer. Furthermore, cladding pumping of an Yb-free, Er-doped fiber is difficult as the low absorption would require a long length of fiber. Instead we use a high power, single-mode (LP 01 ) Raman fiber laser operating at 1480 nm as the pump source. Use of a single-mode pump allows both 1480 nm pump and 1564 nm signal to be converted to the same HOM using a broad-bandwidth, long-period grating (LPG), thus obtaining optimal spatial overlap between pump and signal, which allows for short amplifier lengths. We achieve a clean higher-order mode output with 5.8 W output power and more than 20 dB of gain in a 2.68 m long amplifier. The output power was limited by available  pump power. 2. Higher-order-mode, Erbium-doped fiber The index profile of the HOM, Er-doped fiber, shown in Fig. 1a, had a small inner core and a large outer core, similar in design to the srcinal, passive, HOM fiber [7]. The small inner core was designed such that the LP 01  mode had a mode- field diameter of 9μm, allowing for effective excitation of the LP 01  when the HOM fiber was spliced to single-mode fiber. In contrast, the higher-order modes expanded to occupy the outer core. The diameter of the outer core was chosen to provide an effective area of the LP 0,10   mode of 2700 μm 2  Both inner and outer core were doped with erbium with an absorption of approximately 30 dB/m at 1530 nm; a measurement of the absorption of the fiber is shown in Fig. 1b. #129079 - $15.00 USDReceived 27 May 2010; revised 21 Jul 2010; accepted 21 Jul 2010; published 2 Aug 2010 (C) 2010 OSA16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17652      r  e   f  r  a  c   t   i  v  e   i  n   d  e  x   (  a .  u .   ) radius (a.u.) 1400150016000102030     a   b  s  o  r  p   t   i  o  n   (   d   B   /  m   ) wavelength (nm) (a)(b)  Fig. 1. (a) Index profile of the HOM Er-doped fiber. (b) Measured absorption In a higher-order mode amplifier the choice of which mode order to operate in is constrained by the need for both large A eff   and high modal stability. In contrast to the cladding-pumped HOM-Yb amplifier, the core-pumped HOM-Er amplifier has the additional constraint of requiring both pump and signal to propagate in the same HOM, and consequently the need for broad-bandwidth mode-coupling further limits the available modes that can be used. Note that in certain applications such as ultra-short pulse propagation additional considerations such as dispersion and dispersion slope can also come into play [12]. The difference in effective index between nearest neighbor modes at λ = 1564 nm as a function of their effective area is plotted as points in Fig. 2a for the LP 0,1  through LP 0,10  modes. The points are labeled with the mode order LP 0,N . There is a trade-off between effective area and mode spacing. The LP 0,1  mode has a large mode spacing, but a small effective area similar to single-mode fiber. In contrast, low order modes, such as the LP 0,2  and LP 0,3 , have very large A eff  ’s, but the mode spacing is decreased. As the mode order is increased though, a range of modes is reached where A eff   is still large (in the range of 3000 μm 2 ) and the mode spacing increases. For comparison purposes the mode spacing for a conventional LP 0,1  step index fiber (SIF) with V = 5 is also plotted in Fig. 2a (solid curve).  Note that an SIF in which the core index is adjusted to achieve large mode area with V-number greater than 5 would not have substantially larger mode spacing, but lowering the V-number below 5 can dramatically decrease SIF mode spacing. Therefore with appropriate choice of mode order, mode spacing and stability can be increased in an HOM fiber compared to a best-case conventional LMA fiber. Figure 2b shows calculated intensity profiles at λ = 1564nm for the LP 01  mode and the LP 0,10 . The LP 0,1  and LP 0,10  had calculated A eff  ’s of 55 μm 2   and 2700 μm 2  respectively. 02040600.00.20.40.60.81.0      i  n   t  e  n  s   i   t  y   (  a .  u .   ) radius (  m) LP 0,1  LP 0,10 01000200030004000500060001E-51E-41E-30.011098765432   N=1   m  o   d  e  s  p  a  c   i  n  g   |   L   P    0 ,   N   -   L   P    1 ,   N    |  A eff   (  m 2 ) (b)(a)  Fig. 2. (a) Mode spacing as a function of effective area for the LP 0,N  modes in the HOM fiber (points) compared to a conventional LP 0,1  step-index fiber with V = 5 (solid curve). (b) Intensity profiles of the LP 0,1  and LP 0,10  modes. These calculations were done at a wavelength of 1564 nm. #129079 - $15.00 USDReceived 27 May 2010; revised 21 Jul 2010; accepted 21 Jul 2010; published 2 Aug 2010 (C) 2010 OSA16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17653  LP 0,7 LP 0,8 LP 0,9 LP 0,10 120014001600150200250300     p  e  r   i  o   d   (         m   ) wavelength (nm)  LP 0,7  LP 0,8  LP 0,9  LP 0,10  Fig. 3. Phase matching curves for the long period grating and associated beam profiles. 3. Broad-bandwidth, long-period gratings A UV-written, LPG can be used to convert between modes in an HOM fiber by providing  phase matching between modes. The typical bandwidth of an LPG is usually only a few nanometers as the phase matching curve has a strong wavelength dependence. However, using appropriate waveguide engineering the phase matching curve can be manipulated such that a single grating period can provide phase matching over a broad range of wavelengths [13]. In doing so, LPGs with high conversion efficiency over a bandwidth greater than 100 nm have  been demonstrated. Such a bandwidth is sufficient to convert both the 1480 nm pump as well as the 15xx signal to the same higher-order mode allowing for optimal pump/signal overlap in the HOM gain region. The phase matching curves of the HOM-Er fiber were characterized by writing a series of LPG’s with different grating periods and measuring the transmitted spectra and output modes as a function of wavelength. The results of this measurement along with corresponding beam  profiles are shown in Fig. 3. Experimental measurements are shown as points, and phase matching curves calculated from the index profile are shown as lines. Phase matching curves for the LP 0,7 , LP 0,8 . LP 0,9 , and LP 0,10  are shown. A small vertical offset between theoretical and experimental curves has been removed in this plot, but no other fitting was performed on the theoretical curves. It can be seen that as the mode-order increases the slope of the phase matching curve decreases, and the LP 0,9  and LP 0,10  modes achieve broad bandwidth operation for a single grating period. Given the desired operating wavelengths of 1480 nm pump and 1564 nm signal, the LP 0,10  was chosen as the more appropriate mode. In addition, the LP 0,10  mode also fulfils the desired area and stability requirements shown in Fig. 2a. The transmission spectrum of a broad-bandwidth LPG that converted the LP 0,1  mode to the LP 0,10  mode is shown in Fig. 4. The grating was characterized by splicing the output of the HOM fiber to SMF in order to measure residual light in the LP 0,1  mode after the grating. 13001400150016001700-30-20-100     r  e  s   i   d  u  a   l   L   P    0   1   p  o  w  e  r   (   d   B   ) wavelength (nm)  Fig. 4. Transmission spectrum of the LPG showing measured LP 0,1  light after the grating #129079 - $15.00 USDReceived 27 May 2010; revised 21 Jul 2010; accepted 21 Jul 2010; published 2 Aug 2010 (C) 2010 OSA16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17654  HOM-Er LPGOSA1480nm Raman laser tunable laser SMF  Fig. 5. Experimental setup of the higher-order-mode, erbium-doped fiber amplifier Experiments comparing residual fundamental mode power after the LPG to interferometric measurements of relative mode strength show that the long-period grating transmission spectrum is a good measure of relative mode power and conversion efficiency [15]. The conversion efficiency of the LPG to the LP 0,10  mode was higher than 20 dB over the entire wavelength range of interest, and reached higher than 30 dB at 1564 nm. 4. The HOM-Er amplifier The experimental setup of the higher-order mode amplifier is shown in Fig. 5. A narrow linewidth, external cavity laser was amplified to 50 mW and combined with a high-power, single mode Raman fiber laser at 1480 nm in a single-mode pump/signal combiner. The output of the pump/signal combiner was fusion spliced to the HOM-Er fiber. The length of amplifier fiber after the LPG was 2.68 m. The amplifier fiber was terminated with an angle cleave. For amplification experiments the seed laser was tuned to 1564 nm, where the LPG conversion efficiency was maximized. In these experiments, the maximum available 1480nm  pump power was approximately 16 W, although much higher powers from Raman lasers have  been demonstrated [14]. To measure the output spectrum the beam was imaged and in the image plane, a single-mode fiber (SMF) probe coupled to an optical spectrum analyzer was used to sample different portions of the beam and measure the spectrum, as shown in Fig. 5. By scanning the probe fiber across the beam, the beam profile at different wavelengths could  be obtained. This setup is similar to the recent demonstrated S 2  imaging measurement for analyzing mode content in multi-mode fibers [15]. However, S 2  imaging requires a broad- bandwidth optical source, so the full S 2  data analysis was not performed here. The spectrum measured at full power when the SMF probe was placed at the center of the  beam is shown in Fig. 6a. Signal, unabsorbed pump, and residual Stokes orders from the Raman laser are visible at the output of the amplifier. While the residual Stokes orders appear relatively strong compared to the signal when the spectrum is sampled at the center of the  beam, the Stokes orders do not interact with the LPG and remain in the LP 01  mode, and so the residual Stokes orders are actually weaker than they appear in Fig. 6a. The beam profiles obtained using the scanning SMF probe for various output wavelengths are shown in Fig. 6b. The 1564 nm signal and unabsorbed 1480 nm pump wavelength are clearly in the LP 0,10  mode and show excellent spatial overlap with each other. A calculation of the overlap integral of the electric fields obtained from the measured index profile of the fiber for the LP 0,10  at 1480 nm and 1550 nm shows a spatial overlap of 99.4%. Only one of the representative Stokes orders, 1310 nm, is shown for clarity. All of the wavelengths other than pump and signal were observed to be in the LP 0,1  mode. #129079 - $15.00 USDReceived 27 May 2010; revised 21 Jul 2010; accepted 21 Jul 2010; published 2 Aug 2010 (C) 2010 OSA16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17655
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