We demonstrate an ytterbium-doped single-trench fiber ensuring a high losses ratio (~1000) and low power fraction (~0.7) between the higher-order-modes and fundamental-mode with excellent bend robustness and 85% laser efficiency at a wavelength of 1040nm

Baskiotis, C.

Jain, D.

Kim, Jae-Sun

Sahu, J.K.

English

Southampton (e-Prints Soton)

First demonstration of single trench fiber for delocalization of higher order modes  Deepak Jain1, Catherine Baskiotis1, Jaesun Kim2, and Jayanta K. Sahu1 1. Optoelectronic Research Center, University of Southampton, Southampton SO17 1BJ, UK 2. SPI Lasers, Southampton, UK dj3g11@orc.soton.ac.uk Abstract: We demonstrate an ytterbium-doped single-trench fiber ensuring a high losses ratio (~1000) and low power fraction (~0.7) between the higher-order-modes and fundamental-mode with excellent bend robustness and 85% laser efficiency at a wavelength of 1040nm. OCIS codes :(060.2280) Fiber design and manufacturing; (140.3510) Fiber lasers. 1. Introduction  Novel large mode area (LMA) fiber designs have led to significant progress in the development of fiber lasers in the past years. For high power applications, a LMA fiber design should offer characteristics like high suppression of the higher order modes (HOMs), low bending losses of the fundamental mode (FM), and easy fabrication. Several fiber designs such as low NA step index fibers (SIFs) [1], Bragg fibers [2], 2-D all solid photonic bandgap fibers (2D-ASPBGFs) [2-3], and photonic crystal fibers (PCFs) based designs like leakage channel fibers (LCFs) [4] have been proposed to scale the effective area (Aeff) of the FM. However, most of these designs require refractive index of the core to be equal to the cladding which poses serious challenge in fabricating rare earth ions doped core having refractive  index  equal  to  the  cladding.  Moreover,  PCFs  and  2D-ASPBGFs  require  stack  and  draw  technique. Recently,  we  proposed  multi  trench  fibers  (MTFs),  which  are  easy  to  fabricate  and  can  provide  very  high suppression of the HOMs [5]. In this paper, we demonstrate for the first time, a ytterbium (Yb)-doped single trench fiber which is having refractive index of the core higher than the cladding and offers very high suppression of the HOMs with easy fabrication requirements. The fiber exhibited a laser efficiency of ~85% at a wavelength of 1040 nm whilst retaining good beam quality. 2. Numerical Simulations  Fig. 1(a) Schematic cross-section of the proposed single-trench fiber. (b) Computed bend losses and power fraction of the FM and LP11 mode for different bend radius. Inset shows the images of the FM and LP11 mode at 7cm bend radius.  Fig. 1(a) presents the schematic of single trench fiber. Fig. 1(b) shows the numerically computed bend losses of the FM and LP11 mode (here LP11 has lowest losses and highest power fraction among all other HOMs). The fiber parameters considered in our simulations are rc=10µm, t=1µm, Δn=0.001, and d=2µm. The bending losses of the LP11 mode are larger than 18dB/m and for the FM mode it is lower than 0.012dB/m at 7cm bend radius, ensuring a losses ratio of the HOMs to FM larger than 103 and the Aeff is ~340µm2. Moreover, power fraction ratio of the HOMs to the FM in core maintains a good ratio of ~0.7 irrespective of bend radius. It is important to note that, in a double clad fiber laser, where all the effective losses are zero, low power fraction of the HOMs in core compared to the FM is an important  factor to ensure single  mode operation. Due to this  low power  fraction of  the  HOMs compared to the FM in core, a single mode output can be ensured irrespective of the bend radius thanks to the delocalization of the LP11 mode. Moreover, numerical simulations show that two and three trench fibers can achieve much larger Aeff (600-700µm2) with similar level of suppression, although a compromise with bend radius has to be made [6]. The delocalization of the LP11 mode is due to presence of the resonant ring surrounding the core. The best suppression of the HOMs that can be achieved from a SIF with the same core radius (rc=10µm) and refractive index contrast (Δn=0.001) is only a losses ratio of ~280 and power fraction ratio larger than 0.82 with Aeff of ~340um2 [1, 6]. Moreover, in case of single trench fiber the level of HOMs suppression and the operational range of bending radius is better than other fiber designs like PCFs, W-type fibers, Parabolic fiber etc. [5-7].  3. Experimental Realization of Yb-doped Single Trench Fiber Fig.  2(a)  shows  the  measured  refractive  index  profile  (RIP)  of  Yb3+  and  Al3+  doped  preform  fabricated  using conventional  MCVD  and  solution  doping  technique.  Preform  was  milled  to  D-shaped  and  drawn  to  an  outer diameter of 240µm (core diameter 20µm) with low index polymer coating. The fiber shows 2.5dB/m absorption at 975nm.  Fig. 2 (b) shows the measured Multi-Path Interference (MPI) versus group delay using S2 measurement. LP11 and LP21 modes suffer high losses and their power level is 32dB lower than the FM. A significant amount of power for the HOMs is observed outside the core which ensures delocalization of the HOMs. A 6.17m of fiber was used in a 4%-4% linear laser cavity to measure the laser efficiency. Fig. 3 (a) shows the output spectrum of fiber laser at the bend radius of 5cm and 7.5cm. Inset shows the captured output beam image for 5cm bend case. Both lasers show lasing at a wavelength 1040nm wavelength. Fig. 3(b) shows the output power of the fiber laser for different bend radius. The slope efficiency measured is approximately 85% with respect to the launched pump power with M2 ~ 1.3.   Fig. 2(a) Measured RIP of fabricated preform (b) MPI (dB) versus relative group delay (ps) for fabricated fiber. Inset images show the beam profiles of obtained modes.   Fig. 3 (a) Measured spectrum of the output beam (b) slope efficiency for two different bend radius of fiber. Inset shows the output beam for fiber laser bent at 5cm radius (shown in red colour). 4.  Conclusion We have demonstrated, for the first time, an efficient Yb-doped single trench fiber in the MTF family. The fiber ensures a single mode operation by offering large losses and poor power fraction of the HOMS in the core thanks to the delocalization of HOMs. The laser yields output at 1040nm with ~85% slope efficiency from a 20µm core fiber. Our proposed fiber design is suitable for mass production using conventional low-loss fiber fabrication techniques.   5.  References  [1] M.-J. Li et. al., “Limit of effective area for single-mode operation in step-index large mode area laser fibers,” J. Lightw. Technol., vol. 27, pp. 3010–3016, August 2009.  [2] E. M. Dianov et. al., “Solid-Core photonic bandgap fibers for high-power fiber lasers,” IEEE J. Sel. Top. Quantum Electron., vol. 15, 2009. [3] F. Kong et. al., “Mode area scaling with all-solid photonic bandgap fiber,” Opt. Exp., vol. 20, no. 24, pp. 26363–26372, Nov. 2012.  [4] L. Dong et. al, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding”, Opt. Exp., vol. 17, pp. 8962-8969, May 2009.  [5] D. Jain et. al., “Bending performance of large mode area muli-trench fibers,” Opt. Exp. vol. 21, pp. 26663–26670, Oct. 2013.  [6] D. Jain et. al., “Large mode area multi-trench fiber with delocalization of higher order modes [Invited],” to be published in  Special issue on  high power fiber lasers, IEEE J. Sel. Top. Quant. Elect., Sep. 2014. [7] J. M. Fini, “Bend-resistant design of conventional and microstructure fibers with very large mode area,” Opt. Exp., vol. 14, pp. 69-81, Jan. 2006. Acknowledgement The work is supported by the EPSRC Centre for the Innovative manufacturing in Photonics EP/HO2607X/1. 

Bend-resistant design of conventional and microstructure fibers with very large mode area,”

Bending performance of large mode area muli-trench fibers,”

Large mode area multi-trench fiber with delocalization of higher order modes [Invited],” to be published in Special issue on high power fiber lasers,

Limit of effective area for single-mode operation in step-index large mode area laser fibers,”

Mode area scaling with all-solid photonic bandgap fiber,”

Solid-Core photonic bandgap fibers for high-power fiber lasers,”

Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding”,

First demonstration of single trench fiber for delocalization of higher order modes

First demonstration of single trench fiber for delocalization of higher order modes

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