Abstract : We demonstrate 314 W continuous-wave fiber laser output at 1077 nm from a true single-mode Yb-doped fiber core without any sign of stimulated-Raman-Scattering. The suppression of stimulated-Raman-Scattering was obtained by using a W-type waveguide filtering structure

C Codemard

J K Sahu

J Kim

J Nilsson

Y Jeong

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High power continuous-wave Yb-doped fiber laser with 
true single-mode output using W-type structure. 
 
J. Kim, C. Codemard, Y. Jeong, J. Nilsson, and J. K. Sahu 
Optoelectronics Research Center, University of Southampton, Southampton SO17 1BJ, UK 
Phone : +44(0)23 8059 3143, fax :+44(0)23 8059 3142 Email : Jsk @orc.soton.ac.uk 
 
Abstract : We demonstrate 314 W continuous-wave fiber laser output at 1077 nm from a true 
single-mode Yb-doped fiber core without any sign of stimulated-Raman-Scattering. The 
suppression of stimulated-Raman-Scattering was obtained by using a W-type waveguide 
filtering structure. 
©2006 Optical Society of America 
OCIS codes : (140.3510) Lasers, fiber, (140.3570) Lasers, single-mode, (060.2280) Fiber design and fabrication, 
(060.2320) Fiber optics amplifiers and oscillators 
 
1. Introduction 
High power, high brightness Yb-doped fiber sources at around 1.1µm are of interest for various applications 
such as micro-machining, welding and material processing, and their output power has grown considerably in 
recent years [1, 2]. However, power scaling with a true single mode output beam quality, which requires a small 
core fiber, still remains a technological challenge, in the first instance because of the nonlinear scattering in the 
fiber. In particular, stimulated Raman scattering (SRS) is the main constraint on increasing the output power of a 
fiber based system [3]. In order to mitigate SRS in the fiber, a larger core and a short fiber length are two of the 
obvious choices that work by reducing the optical power density in the doped-core and the interaction length of 
the optical field along the fiber. The increased core size, however, leads to a multimode core and thus a 
significant degradation of the output beam quality. A single mode output can still be obtained using a large core 
fiber with filtering of higher order mode, e.g., with a fiber taper [4] or through bending the fiber [5]. However, 
this approach complicates the fiber laser and remains largely unproven in real-world applications. A true single 
mode core remains an ideal solution in terms of the output beam quality. Recently, a number of groups have 
demonstrated the SRS suppression in a rare-earth doped single mode core by introducing a wavelength-
dependent bend loss in the fiber [6]. However, SRS suppression obtained just by bending a normal step index 
fiber will introduce a significant amount of loss at the signal wavelength too because of the relatively slow 
dependence of the bend loss on the wavelength in such fiber.  
In this paper, we present 314 W of output power from a truly single mode Yb-doped core at 1077 nm without 
any SRS. The SRS in the fiber was suppressed by using a W-type fiber design [7] with fundamental, LP01, mode 
cut-off located between the signal and the 1
st
 order Raman Stokes wavelength, so that the fiber filters out the 
Stokes signal efficiently without introducing any additional loss at the signal wavelength.  
 
2. Experiment and results 
The laser configuration is shown in figure 1. The Yb-doped W-type fiber was cladding pumped by a 972 nm 
diode stack source through a combination of collimating lenses. The maximum pump power launched into the 
fiber was 472 W. A simple laser cavity was formed between perpendicularly polished end facets of the fiber, 
providing 4% Fresnel reflection, and a lens coupled 100% dichroic mirror with a high broadband reflection from 
1030 to 1150nm. 
 
The Yb-doped W-type fiber preform was fabricated in house using a standard MCVD and solution doping 
technique. The preform was further jacketed in order to increase the inner cladding size, for efficient pump 
coupling into the fiber from the diode stack, whilst maintaining the single mode core size in the fiber. Before 
being drawn into the fiber, the preform was milled to a D-shape in order to maintain a uniform pump absorption 
along the fiber. The preform was then drawn to a fiber of 370 µm inner-cladding diameter and coated with a 
Pump  
@ 975 nm 
W-type Yb-doped fiber (37 m) 
Laser output 
Residual pump 
Dichroic mirror 
HT: 975 nm 
HR: 1030 – 1150 nm 
Dichroic mirror 
HT: 975 nm 
HR: 1030 - 1150 nm 
 
Figure 1. Experimental setup. HR: high reflectivity, HT: high transmission. 
Perpendicularly polished 
low-index polymer outer cladding, which provide a nominal inner-cladding NA of 0.5. The inner cladding size 
was chosen for efficient pump coupling from the diode stack. The fiber has a core diameter of 7 µm, with a 
depressed ring in the inner cladding of thickness 7µm. The core and the depressed cladding index differences 
are 0.003 and 0.002, respectively, both with respect to the silica inner cladding. The small signal absorption at 
the pump wavelength was ~0.3dB/m. The fundamental LP01 mode cut-off in the fiber is calculated to be ~1100 
nm. Thus a significant loss at the 1
st
 order Raman Stokes band at around 1120 nm can be expected.  
Figure 2 shows the laser output characteristics. The output power reached 314 W with a slope efficiency of 65% 
with respect to the absorbed pump power at 1077 nm. Based on the Raman threshold calculation using the 
equation from ref [8], the Raman oscillation threshold of a fiber with such a core size is around 140 W. 
However, the W-type fiber we use has the fundamental LP01 mode cut-off at ~1100 nm, and can introduce a 
large loss (leakage), ~5dB/m, for light generated in the core at the 1
st
 order Stokes wavelength. The effective 
area at around 1120 nm is calculated and found to be 45 times larger than the similar core size of a normal step 
index fiber. Therefore, W-type fiber has the potential to increase the Raman threshold to several hundreds of 
kilowatts. In our experiment, even though the output power we achieved is high enough to generate SRS in a 
similar step index core fiber, the suppression of SRS is possible because of the LP01 mode cut-off. The relatively 
low slope efficiency is due to background loss caused by the long length of the fiber. When a 23 m long fiber 
was used, the slope efficiency was 74% with respect to the absorbed pump power and we obtained over 300 W 
of output power at 1077 nm (inset in figure 2).  
The laser wavelength can be tuned to the shorter wavelength region by bending the fiber as shown in figure 3. 
When the bending radius was changed from 15 cm to 7 cm, the slope efficiency was 73.7% with respect to the 
absorbed pump power with the central lasing wavelength of 1058 nm (inset in figure 2).  
  
Figure 2. Laser output characteristics of Yb-doped W-type fiber laser, 
37 m long fiber. Inset: 23 m long fiber with bending radius 15 cm and 
7 cm. 
Figure 3. Optical spectra of Yb-doped W-type fiber laser when 
fiber bending radius is 15 cm and 7 cm at maximum output. 
(resolution 2 nm, fiber length : 23m) 
3. Conclusion 
We demonstrated for the first time a true single mode, small-core, Yb-doped fiber laser that generated 314 W 
output power with 65% slope efficiency with respect to the absorbed pump power. The output power was only 
limited by the pump power. The SRS in the fiber was suppressed efficiently by properly designing a W-type 
waveguide structure. This result shows promising prospects to scale up the output power in a single mode core 
fiber to multi kW level.  
 
4. 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”, 
Opt. Express. 12 (25), 6088-6092 (2004) 
[2] V. P. Gapontsev, N. S. Platonov, O. Shkurihin, and I. Zaitsev, “ 400 W low-noise single-mode ytterbium fiber laser with an integrated 
fiber delivery” , in Proc. CLEO 2003, Baltimore, MD, USA June 1-6, 2003, postdeadline paper CPDB9 
[3] N. S. Platonov, D. V. Gapontsev, V. P. Gapontsev, and V. Shumilin, “135 W CW fiber laser with perfect single mode output”, in Proc. 
CLEO 2002, Long Beach, USA, 19-24 May 2002 postdeadline paper CPDC3 
[4] J.A.Alvarez-Chavez, A.B.Grudinin, J.Nilsson, P.W.Turner, W.A.Clarkson “Mode selection in high power cladding pumped fiber lasers 
with tapered section” CLEO/QELS '99 Baltimore 23-28 May 1999 CWE7 
[5] J. Limpert, A. Liem, H. Zellmer, and A. Tünnermann, “500 W continuous-wave fiber laser with excellent beam quality”, Electron. Lett. 
39, 645-647 (2003) 
[6] P. Dragic, “Suppression of first order stimulated Raman Scattering in erbium-doped fiber laser based on LIDAR transmitters through 
induced bending loss”, Opt. Commun. 250, 403-410 (2005) 
[7] I. V. Neves and A. S. C. Fernandes, “Modal characteristics for W-type and M-type dielectric profile fibers”, Microwave and Opt. 
Technol. Lett. 22, 398-405 (1999) 
[8] J. Au Yeung and A. Yariv, “Theory of cw Raman oscillation in optical fibers”, JOSA 69 (6), 803-807 (1979) 
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High power continuous-wave Yb-doped fiber laser with true single-mode output using W-type structure

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High power continuous-wave Yb-doped fiber laser with true single-mode output using W-type structure

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