We propose a depressed clad hollow optical fiber with fundamental (LP01) mode cut-off suitable for high power short-wavelength, especially three-level, fiber laser operation by introducing highly wavelength dependent losses at longer wavelengths. The cut-off characteristic of such fiber structure was investigated. A Yb-doped depressed clad hollow optical fiber laser generating 59.1W of output power at 1046nm with 86% of slope efficiency with respect to the absorbed pump power was realised by placing the LP01 mode cut-off at ~1100nm

Codemard, C.

Dupriez, P.

Jeong, Y.

Kim, J.

Nilsson, J.

Sahu, J.

Soh, D.B.S.

Yoo, S.

English

Southampton (e-Prints Soton)

Depressed clad hollow optical fiber with the fundamental LP01 mode cut-off  J.Kim, P.Dupriez, D.B.S.Soh, C.Codemard, S.Yoo, Y.Jeong, J.Nilsson and J.K.Sahu Optoelectronics Research Center, University of Southampton, SO17 1BJ UK Tel : +44(0)23 8059 3143, Fax :+44 (0)23 8059 3142, Email : jsk@orc.soton.ac.uk  ABSTRACT  We propose a depressed clad hollow optical fiber with fundamental (LP01) mode cut off suitable for high power short wavelength,  especially  three level,  fiber  laser  operation  by  introducing  highly  wavelength  dependent losses at longer wavelengths. The cut off characteristic of such fiber structure was investigated. A Yb doped depressed clad hollow optical fiber laser generating 59.1W of output power at 1046nm with 86% of slope efficiency with respect to the absorbed pump power was realised by placing the LP01 mode cut off at ~1100nm.   Keywords : Hollow optical fiber, fiber laser, Ytterbium, Fundamental LP01 mode cut off.  1.  INTRODUCTION  Fiber based lasers have been extensively researched during the past decade because of their high efficiency, compactness and excellent beam quality. The output power of fiber lasers has been increased significantly over the last few years by adapting the cladding pumping technology, and is now competing with conventional bulk solid state laser in applications such as micro machining, welding and material processing. In particular, the cladding pumped Yb doped fiber laser has already reached kW levels at around 1.1 m with a nearly diffraction limited output beam [1,2]. In contrast, the power scaling of a three level laser still remains a technological challenge because of the competing unwanted four (or quasi four) level laser transition which has the lower threshold [3], and becomes even more difficult in the case of the cladding pumped fiber laser owing to the relatively low pump absorption which requires a relatively long device length as compared to core pumped fiber laser. For example, Nd doped alumino silicate fibers have two strong emission bands; a three level around 930 nm and a four level one centered at 1060 nm. While Nd doped 1060 nm lasers are relatively easy to realize due to the absence of ground state absorption (GSA), the interest in this is rather modest because of the superiority of Yb doped fiber lasers emitting at the same wavelength. However, the 0.9 m Nd doped fiber laser is still attractive for applications such as blue generation by frequency doubling [4] and water sensing [5]. Similarly, the Yb doped fibers consist of two broad emission bands at 0.98  m and 1.03 – 1.1 m respectively. The band at ~ 0.98  m corresponds to a three level laser transition and requires a much higher level of the population inversion and also suppression of the other laser transitions between 1.03 m and 1.1 m. Increasing the output power of such fiber lasers is of interest as they have applications as a pump source for erbium doped fiber amplifiers and fiber DFB lasers [6].  Waveguide  design  for  three level  fiber  lasers  that  produces  a  distributed  loss  at  longer  wavelength,  was suggested by some groups [7, 8]. In particular, W type fiber can efficiently filter out unwanted transition (four level emission) by the fundamental LP01 mode cut off, in contrast to the normal step index fiber. In the Nd doped alumino silicate system, the lasing at 0.9 m was demonstrated, by suppressing the emission at 1.06 m with 47%  slope efficiency and 9W of  maximum output  power [7]. However, the limited controllability of doped core size, for the LP01 mode cut off to be at the desired wavelength, still puts constraints on improving the pump absorption in cladding pumped W type fiber lasers. In practice, a large and single mode core is an important design feature for high power cladding pumped fiber lasers, since it allows the inner cladding size, and thus the pump power, to be scaled whilst maintaining acceptable pump absorption.   In this paper, we propose a depressed clad hollow optical fiber (DCHOF), which is composed of a ring shaped core, an air hole in the center, and a low index ring in the cladding. The low refractive index (nair = 1) of the hole region produces a negative dielectric volume [9] and the depressed cladding plays a role to increase the mode confinement in the doped core region. These features bestow the DCHOF with the fundamental mode cut off required for good suppression of unwanted emission at longer wavelengths, whilst the ring shaped doped core  will  provide  the  improved  pump  absorption  in  the  cladding  pumped  configuration.  In  a  preliminary experiment we have obtained ~ 60W of output power at 1046 nm from a Yb doped DCHOF with 85% of slope efficiency with respect to the absorbed pump power by filtering of the amplified spontaneous emission (ASE) at longer wavelengths.    2. Depressed clad hollow optical fiber  A schematic index structure of the DCHOF is shown in figure 1. It is composed of a ring shaped doped core, whose refractive index is nco and outer radius  co r , around an air hole of radius  air r  (nair = 1), and a low index (dip n ) ring in the cladding with an outer radius of dip r .  Finally, the silica cladding surrounded the low index ring whose refractive index ( clad n ) is 1.4571.     It is proposed to assume that the modes in this index structure are weakly guided and linearly polarised (LP), because of the relatively large index differences between the air and doped core. However, as far as the mode cut off is concerned, the difference between the full vectorial calculation and the LP approximation is negligible. Actually, the differences between results from our mode solver assuming LP modes and commercial software (FEMLAB) for full vectorial calculation were within 1% in most cases. Therefore, the weakly guiding and LP mode  approximation  was  used  for  simplicity.  In  this  case,  the  radial  dependence  of  the  transverse  field component is represented by [10]  ≤< ≤ +< ≤ +<=r r sr K Ar r r wr K A wr I Ar r r ur Y A ur J Ar r vr I Ar Edip mdip co m mco air m mair m), (), ( ) (), ( ) (), () (54 32 10                                    (1)  Here, r is the radial position,  ) 5 , 4 , 3 , 2 , 1 , 0 ( = i Ai  is constant, and  ) ( ), ( m m m m K I Y J  are Bessel functions of first (second) kind and modified Bessel functions of first (second) kind respectively. Mode parameters ( w u v , , and s ) are  defined  as  , , ,202 2 2 202 202 k n w k n u k v dip co − = − = − = β β β and202 2 k n s clad − = β ,  where  β  is  the propagation constant of each guided mode and  0 k  is the vacuum wave number. The modal effective index ( eff n ) is determined by the relation of  0 / k neff β = . According to the boundary condition, the field and its radial derivative must be continuous on the three boundaries, air r r = , co r r =  and  dip r r = . From these conditions, the characteristic equation can be written as follows:   0  rair  rco  rdip ndip nclad nco Radial position Refractive index Figure1. A schematic index structure of DCHOF 0) ( ) ( ) ( 0 0 0) ( ) ( ) ( 0 0 00 ) ( ) ( ) ( ) ( 00 ) ( ) ( ) ( ) ( 00 0 0 ) ( ) ( ) (0 0 0 ) ( ) ( ) (1 1 11 1 1 11 1 1=−−− − − −− −− −− −dip m dip m dip mdip m dip m dip mco m co m co m co mco m co m co m co mair m air m air mair m air m air msr sK wr wK wr wIsr K wr K wr Iwr wK wr wI ur uY ur uJwr K wr I ur Y ur Jur uY ur uJ vr vIur Y ur J vr I  (3)  Where  r ur I ur I mlm ∂ ∂ = / ) ( ) (  and  •  represents  the  determinant  of  a  matrix.  We  have  solved  equation  3 numerically. The parameters for the fundamental  LP01 mode cut off  wavelength at the desired location are determined by a combination of an air hole size ( air r ), core thickness ( air co r r − ), the width of depressed clad ( co dip r r − ) and refractive indices of core ( co n ) and depressed clad ( dip n ). Figure 2(a) shows the changes of effective indices for different modes for the fiber parameters as listed in table 1. The parameters were chosen in order to obtain the LP01 mode cut off wavelength at ~ 1.03  m. Figure 2(b) shows the guided LP01 core mode at 0.93  m whereas the figure 2(c) shows that the LP01 guided mode is no longer supported by the core after the fundamental mode cut off wavelength at 1.03 m.    Fiber Dia. (       m) Dip thicknesss (       m) Core OD (       m) Hole Dia (       m) Core thickness (       m) Core NA  Dip NA 125  7.82  10.82  3.7  3.6  0.09  0.078 Table 1. Designing parameters for the fundamental LP01 mode cut off at ~1.03 m    Thus DCHOF can produce LP01 mode cut off characteristics and good suppression of the unwanted emission is possible in such waveguide structure by controlling the location of the LP01 mode cut off. Using DCHOF, we have  demonstrated  both  high  power  Nd doped  and  Yb doped  fiber  lasers  operating  at  0.9 m  and  0.98 m respectively [11, 12].  It is worth mentioning that inspite of the ring shaped doped core in DCHOF, we obtained a true (Gaussian) single mode output through collapsing the air hole at the output end of the fiber.  3. High power Yb-doped DCHOF laser operating at 1.046       m   Wavelength (       m)0.4 0.6 0.8 1.0Effective index (a.u.)1.45751.45801.45851.45901.45951.4600LP01 modeLP11 modeLP21 modeLP31 modeLP02 mode(a) (c) (b) LP01 mode cut-off LP01 mode guide Figure 2. (a) effective index change of different LP modes in DCHOF, (b) the guided LP01 core mode field at 0.93 m, and (c) the LP01 mode field at 1.06 m which is filtered out of the core. In most high power Yb doped alumino silicate fiber lasers, the output is shifted towards the long wavelength side of the emission band, typically ~1.1 m [2]. However, a fiber laser operating at the short wavelength side of the Yb emission band would offer an increased scope of  further power scaling in the kW regime because of the reduced  thermal  loading  in  the  fiber  (due  to  the  low  quantum  defect  between  the  pump  and  the  emission photons) as compared to the long wavelength one. DCHOF has the potential of offering short wavelength lasing, ≤ 1.05 m, by suppressing the ASE at long wavelength.  We fabricated the Yb doped DCHOF using a standard MCVD and solution doping technique. At the final collapsing stage, a hole of around 1.0mm diameter was left in the preform. Before being drawn to fiber, the preform was milled to a double D shape in order to improve the pump absorption in the doped core. This perform was then drawn to a fiber with 170  m inner cladding diameter and coated with a low index polymer outer cladding, which provided a nominal inner cladding NA of ~ 0.45. The core comprised of a 6.2 m Yb doped ring (NA ~ 0.07) around an air hole of 18 m diameter, and a depressed ring in the inner cladding of thickness 14 m (NA ~ 0.08). From these fiber parameters, we numerically analyzed the modal characteristics using equation (3). Figure 3(a) shows the effective index change of two LP modes depending on wavelength. The fundamental LP01 mode cut off was ~1.15 m. Experimentally, the fundamental mode cut off wavelength was verified from the white light transmission measurement, as shown in figure 3 (b), and found to be in good agreement with our modeling. Figure 3(b) also shows that the induced loss at 1100 nm was ~ 10 dB/m, while the loss at 1040nm was not affected by the LP01 mode cut off.    The laser configuration is shown in figure 4. A 5m long Yb doped DCHOF was pumped by a 975nm multimode diode stack through a combination of collimating lenses and dichroic mirrors. A simple laser cavity was formed between perpendicularly cleaved end facets of the fiber, providing 4% Fresnel reflections. Dichroic mirrors (high reflection at 1030 – 1150 nm, high transmission at 975 nm) were used to separate the signal from the pump beams. The operational pump absorption was 2dB/m.    Figure 5 shows the laser output characteristics. The output power reached 59.1W with a slope efficiency of 81% with respect to the launched pump power (85% with respect to the absorbed pump power). The central lasing Pump  @ 975 nm Yb doped DCHOF (5m) Laser output Residual pump Dichroic mirror HT: 975 nm HR: 1030 – 1150 nm Dichroic mirror HT: 975 nm HR: 1030   1150 nm  Figure 4. Laser configuration for Yb doped DCHOF . HR: high reflectivity, HT: high transmission. Perpendicularly cleaved  Laser output 0.7 0.8 0.9 1.0 1.1 1.21.45721.45741.45761.45781.4580Effective index (a.u.)Wavelength (       m) LP01 mode LP11 mode800 900 1000 1100 1200-90-80-70-60800 1000 1200-80-60Wavelength (nm)Transmitted power (dBm)Figure 3. (a) Effective index change of LP01, LP11 guided modes in DCHOF and (b) white light transmission through 1m long Yb doped DCHOF. Not bent Bending radius : 5cm LP01 mode cut-off LP01 mode cut-off (b)  (a) wavelength  was  1046nm.  The  emission  at  longer  wavelength  was  suppressed  successfully  by  adopting  the significant loss induced by the fundamental mode cut off.  Moreover, as shown in figure 3(a), only the LP01 mode is supported in the core at a laser wavelength of 1046nm. In DCHOF, because of the ring shaped core, the fundamental mode is expected to have a ring shaped output field distribution. In order to obtain the exact Gaussian output the holes in the output ends of the fiber have to be collapsed, as described in ref. [11, 12].               4. Conclusion  We have investigated the guiding characteristics of DCHOF. The LP01 mode cut off in this waveguide structure can act as a waveguide filter to suppress the undesired emissions. An efficient, short wavelength, Yb doped DCHOF laser generating 59.1W of output power at 1046nm, with 85% of slope efficiency with respect to the absorbed pump power has been demonstrated. The output beam is expected to be a single mode based on our modal calculation. This initial result shows promising prospects to scale up the output of cladding pumped fiber lasers in a single mode core to kW power level.   5. References  [1] Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbium doped large core fiber laser with 1.36 kW continuous wave output power”, Optics Express, 12(25), 6088 6092 (2004). [2] J.Hecht, “Photonic frontiers : High power fiber laers: Pumping up the pumping power “ Laser Focus World,  http://lfw.pennnet.com/Articles/Article_Display.cfm?Section=ARTCL&ARTICLE_ID=234077&VERSION_NUM=4&p=12. [3] J. Nilsson, J. D. Minelly, R. Paschotta, D. C. Hanna, and A. C. Tropper, “Ring doped cladding pumped single mode three level fiber laser”, Optics Letters, 23 355 357 (1998). [4] E. F. Wassermann, et al., “Fabrication of large scale periodic magnetic nanostructures”, Journal of Applied Physics 83, 1753 1757 (1998). [5] M. A. Noginov, et al, “Development of Nd doped solid state laser materials for 944 nm operation”, IEEE. Journal of Quantum Electronics 37, 469 481 (2001). [6]  L.B.Fu,  R.Selvas,  M.Ibsen,  J.K.Sahu,  J.N.Jang,  S.U.Alam,  J.Nilsson,  D.J.Richardson,  D.N.Payne, C.Codemard, S.Goncharov, I.Zalevsky and A.B.Grudinin, “Fiber DFB laser array pumped with a single 1W CW Yb fiber laser” IEEE Photonics Technology Letters 15(5), 655 657 (2003). [7] T. J. Kane, G. Keaton, M. A. Arbore, D. R. Balsley, J. F. Black, J. L. Brooks, M. Byer, L. A. Eyres, M. Leonardo, J. J. Morehead, C. Rich, D. J. Richard, L. A. Smoliar, and Y. Zhou, “3 Watt blue source based on 914 nm Nd:YVO4 passively Q switched laser amplified in cladding pumped Nd:fiber”, in Proc. ASSP 2004, Santa Fe, 2 5 Feb. 2004. 1000 1020 1040 1060 1080-80-60-40-20Output (dBm)Wavelength (nm)Figure 5 (a) Laser output characteristics of Yb doped DCHOF at 1046nm, and (b) Laser output spectrum (OSA resolution : 2nm) (b) 0 10 20 30 40 50 60 700102030405060Output power (W)Absorbed pump power (W) Measured data Linear fit(a) [8] L.A.Zenteno, J.Wang, D.T.Walton, B.A.Ruffin, M.J.Li, S.Gray, A.Growley, and X.Chen, “Suppression of Raman gain in single transverse mode dual hole assisted fiber” Optics express, 13(22) 8921 8926 (2005). [9]  V.Neves and A.S.C.Fernandes, “Modal characteristics for W type and M type dielectric profile fibres”, Micro. & Opt. Tech. Lett. 22 398 405 (1999). [10] A.W.Snyder and J.D.Love, Optical Waveguide Theory, Chapman & Hall, 1983. [11] J.Kim, P.Dupriez, D.B.S.Soh, J.K.Sahu, J.Nilsson, and D.N.Payne, “Nd:Al doped depressed clad hollow fiber laser at 930nm” in Proc. ASSP 2005 Vienna 6 9 Feb 2005. [12] J.Kim, D.B.S.Soh, C.Codemard, S.Yoo, Y.Jeong, J.Nilsson, and J.K.Sahu “Yb:Al doped depressed clad hollow optical fiber laser operating at 980nm” CLEO/IQEC Pacific Rim Tokyo 11 15 July 2005 CTuI4 5. 

Development of Nd doped solid state laser materials for 944 nm operation”,

Fabrication of large scale periodic magnetic nanostructures”,

M.Ibsen, J.K.Sahu, J.N.Jang, S.U.Alam, J.Nilsson, D.J.Richardson, D.N.Payne, C.Codemard, S.Goncharov, I.Zalevsky and A.B.Grudinin, “Fiber DFB laser array pumped with a single 1W CW Yb fiber laser”

Photonic frontiers : High power fiber laers: Pumping up the pumping power “ Laser Focus World,

Ring doped cladding pumped single mode three level fiber laser”,

Ytterbium doped large core fiber laser with 1.36 kW continuous wave output power”,

Depressed clad hollow optical fiber with fundamental LP01 mode cut-off

Depressed clad hollow optical fiber with fundamental LP01 mode cut-off

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