Design of Rare Earth Doped Multicore Fiber Lasers and Amplifiers

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A home-made computer code has been implemented to design high power ytterbium doped fiber lasers by following different techniques. The goal is to maximize the effective mode area by maintaining a good beam quality. As a preliminary simulation
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  ICTON 2012 We.B6.1978-1-4673-2229-4/12/$31.00 ©2012 IEEE 1 Design of Rare Earth Doped Multicore Fiber Lasers and Amplifiers Michele Surico, Annalisa Di Tommaso, Pietro Bia, Luciano Mescia, Marco De Sario, Francesco Prudenzano  DEE- Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, Via Orabona, 4, 70125 Bari, Italy e-mail: ABSTRACT A home-made computer code has been implemented to design high power ytterbium doped fiber lasers by following different techniques. The goal is to maximize the effective mode area by maintaining a good beam quality. As a preliminary simulation result, a novel aperiodic fiber lattice, very promising in order to construct high power and single mode lasers, has been identified. The laser obtained by employing the aforesaid aperiodic fiber has been compared, via simulation, with three other fiber sections, taken by literature but suitably optimized. Keywords : beam quality, innovative fibers, laser and amplifier modeling. 1.INTRODUCTION The spreading of fiber lasers in a large number of fields is due to their high optical efficiencies and to their geometries allowing very good thermal properties. Nowadays, fiber lasers are definitively considered feasible sources in industrial, material science, medical, and military applications. Therefore, they are frequently employed for the construction of novel optical sources characterized by very high compactness and requiring simple cooling systems. As a consequence of the power up-scaling, recent fiber lasers are kilowatt class lasers in the continuous wave (CW) regime and they allow quasi-Gaussian beam distributions, whereas, in pulsed regime they are slightly less competitive. Fiber laser output beam quality, excellent even at high powers, permits the delivering of the optical beam with small diffraction values and, thus, these lasers result very useful for many applications. As a drawback, the appearance of nonlinear effects as Stimulated Brillouin or Raman scattering, four-wave mixing and photo-darkening, limits the maximum output power. These phenomena can induce both output power degradation and decreasing of spectral beam quality. The power threshold, at which nonlinear effects appear, can be significantly increased by exploiting large mode area (LMA) and double cladding fiber geometries, allowing cladding power pumping schemes. Novel optical fibers, characterized by large cores, have  been investigated with the aim to prevent nonlinearity and to allow beam quality factor M 2  close to the unit [1-5]. The large active core allows the enhancement of the pump absorption and the reduction of the power density, thus the active fiber length can be shortened. This further increases the threshold of the nonlinear effects. The main drawbacks are the decreasing of the beam quality and the increasing of bending sensitivity for single-mode (SM) fibers. Endlessly SM propagation, double cladding section, and LMA behavior can be obtained by employing microstructured optical fiber (MOF) lattices instead of the step index ones (SIF). Therefore, MOF technology, due to its versatility, can allow the right trade-off between the effective mode area and bending sensitivity requirements [6-11]. In [4] the bending characteristics of solid-core LMA photonic  bandgap fibers (PBG) with single-mode operation for high-power Yb-doped fiber lasers and amplifiers were investigated; the solid core PBGF having a 7-cell core achieved sufficient differential bending loss between the fundamental mode and the higher-order modes and a very large effective area limit. Multicore fibers (MCF) are an alternative way to enlarge the effective mode area. In general, the MCF technology allows obtaining larger mode cross-section compared with that achievable via single core and single mode fibers [13-19]. Phase-locked multicore fiber lasers were demonstrated by Wrage et al.  for the first time in [14]. In those structures, the fiber cores provided slight evanescent coupling and phase locking was achieved through diffractive coupling using a Talbot cavity or a structured mirror. MCFs can be employed as amplifiers. As an example, in [12] a high power 19-core Yb-doped fiber amplifier in fundamental in-phase mode operation was demonstrated. A Q-switched single-core fiber laser with single transverse mode was used as seed. In [20] a ring-cavity laser included a multicore fiber amplifier and a single-mode fiber feedback for angular filtering of the beam and for selective seeding. In [3] diffraction-limited fundamental mode amplification was demonstrated in Er-doped fibers with effective area of 1800  m 2 .The amplifier utilized a Raman fiber laser as a pump and a LMA Er  3+  fiber as an active medium. High peak  powers and beam qualities were obtained with conventional fibers. In MCF lasers such as for the amplifiers, multiple rare earth doped cores are separated by undoped regions. The large mode area decreases the thermal stress and nonlinearity drawbacks arising from the high-power levels. However, quasi-Gaussian emission profiles can be obtained via MCF lasers only if the propagation of the different supermodes are taken into account and appropriate techniques of supermode selection or combining or filtering are adopted.  ICTON 2012 We.B6.1 2 In this paper, a home-made computer code is employed to investigate four different fiber lasers, optimized to obtain high brilliance and large mode area. The first two fiber lasers employ a Talbot cavity as a feedback mechanism for mode selection; they are based on i) a multimode (MM) 7 core SIF, and ii) a MM 19 core SIF. The other two fiber lasers are based on iii) a single mode (SM) 19 core SIF and iv) a completely novel SM 1 core aperiodic lattice and single material (silica with air holes) fiber. In the following, the four optical fibers are named MM7CSIF, MM19CSIF, SM19CSIF, SM1CA. 2.THEORY The home-made computer code for simulation of ytterbium doped fiber lasers is implemented by considering the rate equations and the power propagation equations [21-23]. A two level scheme is employed to describe the ytterbium activated glass-system. The pump and the signal wavelengths are 976 nm and 1060 nm, respectively. The electrons in the 4 F 7/2 ground level absorb the pump light and are promoted to level 4 F 5/2 , this is the Ground State Absorption (GSA) phenomenon. The electrons can decay from metastable level 4 F 5/2 to ground level,  because of the signal stimulation. The stimulated emission enhances the signal power. Moreover, some of the electrons of the level 4 F 5/2  can spontaneously decay. The aforesaid mechanism is modeled for each supermode and for each doped cores. The propagation of all the supermodes is accurately taken into account. The supermode electromagnetic field profiles and the propagation constants are calculated by a commercial full vectorial finite element method FEM. The filling factor of the j-th core for the i-th mode is calculated as the ratio of power of the i-th mode inside the j-th core to the total power of the i-th mode [24-25]. All supermodes in MCFs compete with each other and give their contribute to the total population inversion. For the lasers based on: i) the MM 7 core SIF and ii) the MM 19 core SIF, both adopting the Talbot cavity feedback, the far field is calculated by using the Rayleigh-Sommerfeld diffraction equation via the FFT method. Among all supermodes, only the in-phase mode has a good far-field intensity profile; it is selected via the Talbot cavity between the fiber end and the feedback mirror. Self and cross coupling coefficients are calculated as in [24-25]. For SM19CSIF the SM 19 core SIF and SM1CA the novel SM single core aperiodic MOF the Talbot cavity is not required. The beam quality is evaluated via the theory reported in [26]. 3.RESULTS The design of the chosen fiber sections has been performed via a large number of simulations in order to obtain similar effective mode areas A eff  , andto optimize the laser performance. Figure 1 illustrates a sketch of the four fiber sections. For the SM1CA fiber, a quarter of the section is reported. It is a Thue-Morse transversal fiber section which includes eight air hole rings [27]. a)   b)andc)   d)  Figure 1. Laser fiber sections a) MM7CSIF, b) MM19CSIF, c) SM19CSIF, d) SM1CA. The distance between contiguous rings is not constant as in periodic PCF lattices, but it follows the aperiodic Thue-Morse sequence. More precisely, two ring distances, the first pitch ‘ Λ A ’ and second pitch‘ Λ B ’ are used. T  N  is the N order binary sequence of the two pitches ‘ Λ A ’ and ‘ Λ B ’. The T  N+1  is generated from T  N  by replacing ‘ Λ A ’ with ‘ Λ A Λ B ’ and replacing ‘ Λ B ’ with ‘ Λ B Λ A ’. As examples, the first four terms of Thue-Morse sequence are T 0 ={ Λ A }, T 1 ={ Λ A Λ B }, T 2 ={ Λ A Λ B Λ B Λ A }, T 3 ={ Λ A Λ B Λ B Λ A Λ B Λ A Λ A Λ B }. The T 3 lattice (ThMoT 3 ) is considered in the PCF design. In fact, the T 1 and T 2 lattices exhibit high confinement losses [27]. Table I reports the main  parameters of the designed fibers. The emission and absorption cross sections of the ytterbium ions in silica glass and the losses are reported in [22-23]. Ytterbium ion concentration is 5×10 25  ions/m 3  . The distance between the mirror and the fiber end, Z M /2, of the Talbot cavity has been optimized to obtain the in-phase propagation. The fiber SM19CSIF and SM1CA fibers are designed to be single mode at the signal wavelength.  ICTON 2012 We.B6.1 3 Table I. Main laser and optical fiber parameters. Kind of fiber MM7CSIF MM19CSIF SM19CSIF SM1CA  Number of cores 719191Cladding radius  R cl  250 µ m 250 µ m 250 µ m 250 µ mCore radius  R co 5  µ m 4 µ m 2 µ m13  mAir hole radius # # # 0.5  mCentre to centre distance  L co-co 17 µ m 12 µ m 5.5 µ m #  Pitch Λ a  # # # 20  mPitch Λ b  # # # 5   mCladding refractive index n cl   at    s 1.4497 1.4497 1.4497 1.4497Core refractive index n co  at    s  1.4511 1.4527 1.4502 1.4497 Air refractive index n  A  # # # 1Mirror reflectivity  R 1  0.99 0.99 0.99 0.99 Mirror reflectivity  R 2  0.04 0.04 0.04 0.04 Mirror - fiber end distance  Z  M  /2 0.877 mm 1.155 mm # # As an example, Fig. 2 illustrates the variation of a) the self-coupling coefficient and b) the coupling from the in phase coefficient, versus the Talbot distance  Z  M  , for the MM19CSIF. The curves exhibit the behavior reported in similar cases [25] and demonstrate that a selective feedback to obtain a good beam quality is feasible, because most of the power can be coupled to in-phase supermode. 00.511.522.5300. distance Z M  [mm]    C  o  u  p   l   i  n  g   f  r  o  m   i  n  p   h  a  s  e  c  o  e   f   f   i  c   i  e  n   t      η    i   1 119136 00.511.522.5300. distance Z M  [mm]    S  e   l   f  c  o  u  p   l   i  n  g  c  o  e   f   f   i  c   i  e  n   t  s      η    i   i 81816 & 1714 & 15136199&104&52&3111&127 a) b)  Figure 2: a) the self-coupling coefficient and of b) the coupling from in phase coefficient, versus the Talbot distance Z  M   , for the MM19CSIF. Table II reports the main output laser characteristics, i.e. the effective mode area  A eff  , the beam quality M  2  and the slope efficiency η s for the fiber length  L  = 10 m. The SM19CSIF laser exhibits the best beam quality M  2 = 1.05, very close to that of SM1CA laser, M  2 = 1.18 whereas the effective mode area  A eff   of SM1CA is larger than that of SM19CSIF. Table II.Output signal characteristics. Fiber   A eff  [ µ m 2 ] M   2 η s MM7CSIF 830 2.58 0.75 MM19CSIF 957 2.79 0.71 SM19CSIF 703 1.05 0.70 SM1CA 822 1.18 0.88 Figure 3 depicts the variation of the laser output versus the pump power (bidirectional) for the designed lasers with fiber length  L  =10 m. The SM1CA laser allows the best performance in terms of output power level to  parity of pump power.  ICTON 2012 We.B6.1 4 050100150200250050100150200250Pump Power P  p  [W]    L  a  s  e  r   O  u   t  p  u   t   P  o  w  e  r   P   o  u   t    [   W   ] MM7CSIFMM19CSIFSM19CSIFSM1CA  Figure 3.Laser outputs versus the pump power, fiber length L=10 m. 4.CONCLUSIONS Four kind of fiber lasers have been designed with the aim to obtain high power and high quality output beam. They have been compared via simulations. High performance lasers based on the fibers of the same kind of i), ii), iii), but differently optimized, have been reported in recent literature [1][18][20][24-25]. The aperiodic fiber laser iv) is novel and it seems very promising and competitive with respect to the state of the art. In fact, it exhibits the highest slope efficiency η s = 0.88, a high effective mode area  A eff  = 822 µ m 2  and a beam quality M   2 = 1.18 close to the theoretical one. ACKNOWLEDGEMENTS The work has been developed within the COST ACTION MP0702: Towards Functional Sub-Wavelength Photonic Structures and within the PON01_01224 project. REFERENCES [1]M.M. Vogel, M. Abdou-Ahmed, A. Voss, T. Graf: Very-large-mode-area, single-mode multicore fiber, Optics Letters , vol. 34 (18), pp. 2876-2878, Sept. 2009. [2]F. Poli, E. Coscelli, T. T. Alkeskjold, D. Passaro, A. Cucinotta, L. Leick, J. Broeng, S. 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