White Papers

KW-Power Fiber Lasers with Single Transverse Mode Output

September 22, 2005

By: A. Galvanauskas, C-H. Liu, S. Heinemann, B. Ehlers, A. Carter, K. Tankala, J. Farroni

Introduction

Recent developments in fiber laser technology have led to a true technological breakthrough expressed as a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Indeed, due to the introduction of large large-mode-area (LMA) fibers as well as continuing advances in high power and high brightness diodes CW single-transverse-mode powers from Yb-doped fiber lasers have increased from 100-W in 2001 to approximately ~1 kW at present [1-3]. One of the most important practical features of this power revolution is that such previously unattainable powers can be achieved with commercially available off-the-shelf fibers and components. As a result, emergence of this high power and high beam quality fiber laser technology is expected to have a profound effect on a broad variety of industrial applications in the nearest future.

In this paper we describe kW-level CW fiber lasers with single transverse mode output beams based on standard low-NA 20-µm core/400-µm clad Yb-doped DC fibers. Experimental demonstrations have yielded powers of more than 800-W. Our numerical analysis of Raman and thermal limitations in these fibers indicate that these achieved powers are purely pump-power limited and significantly higher powers of > 5 kW can be reached using these off-the-shelf components. Practical significance of these findings lies in the fact that 20-µm core low-NA fibers are compatible with a number of standard components, such as monolithic MM-fiber pump-beam combiners [4] and fiber gratings, thus enabling the development of completely monolithic, reliable and, therefore, highly practical multi-kW power fiber lasers.

Large Mode Area Fibers

There are three main advantages of increasing mode area for high power generation:

• Nonlinearity in the core decreases with the mode area A_mode increase. Maximum CW fiber amplifier power limited by the Stimulated Raman Scattering (SRS) threshold:
  P_α = 16 A_mode / L_eff—gR
• Pump power, which can be coupled into the cladding, increases with the clad size. For double-clad (DC) fibers, pump absorption is determined by the ratio between core and clad areas. Increasing core size allows the same increase in clad size for fixed pump absorption value:
  α_Clad = A_core / A_clad α_Core
• For the same reason, pump absorption length can be reduced with larger-core fibers, leading to shorter fiber lasers (further reduction in nonlinearity):
  L_absorption ≈ 1/α_Clad => L_fiber ≈ 3.2 L_absorption

Increase in core size for low-NA fibers can be achieved using mode-filtering through optical tunneling effect in a curved core [5]. There is a certain trade-off, however, which manifests as a diminishing mode discrimination in curved fibers with the increase in the core size. This means increasing difficulty of splicing such fibers without significant mode perturbation, thus restricting use of other fiber components, such as MM-fiber pump couplers and fiber gratings, conventionally used with single-mode fibers for building monolithic laser cavities. Table I below compares two standard Yb-doped LMA fibers, illustrating the fact that 20-µm LMA core fibers are already compatible with monolithic fiber components, while the 30-µm LMA core fibers are still much more technologically challenging.

It is important to stress, however, that splicing of 30-µm core fibers while preserving mode quality is feasible, although difficult.

810-W Single-Transverse-Mode Fiber Laser

The configuration of a 810-W single-transverse-mode fiber laser is shown in Figure 1. Fiber lasers were built using low NA (NA=0.06) 20-µm core double-clad fiber with 400-µm diameter and NA=0.46 hexagonal cladding. One version of a fiber laser cavity was formed by 3.5% Fresnel reflections from each straight-cleaved fiber end. In this version (shown in Figure 1), two equal-power beams were emitted from both ends of the laser. We also used another version (not shown in the figure), which had an HR mirror at one end, thus providing all of the output into one direction.

A double-clad fiber laser was end pumped from both sides using six diode-bar lasers operating at three different wavelengths of 915-nm, 936-nm, and 976 nm, providing a total of 1.6-kW of combined pump power. Each diode pump was separately coupled into 800-µm diameter, 0.22 NA delivery fibers. Each set of three different-wavelength diodes was wavelength-combined and coupled into 400-µm cladding from each fiber laser end using 2:1 imaging optics. The total coupled pump power into double-clad fiber was 1.16-kW. A total signal power of 810W at 1092nm has been achieved, as shown in Figure 2 (a), with 70% slope efficiency and the threshold for coupled pump of 3-W. The laser spectrum is shown in the insert of the figure, indicating rather broad band (> 10 nm) emission. Such a broad band completely suppresses Brillouin scattering.

Measured output-beam quality is shown in Figure 3. The measured value of M² = 1.27 indicates single transverse mode performance, achieved by coiling fiber onto a 15-cm spool for LP₁₁ mode suppression.

Power Limits due to Nonlinear Effects

We also developed a numerical model to compare with the experimental results and to predict the further power scalability of the 20-µm core fibers. Comparison between the calculated and experimental results is shown in Figure 2 (b). The model predicts laser behavior quite accurately. It also predicts a Raman threshold at 900 W of a signal power, for the particular configuration of the experimental setup shown in Figure 1. In this setup, straight fiber cleaves provide equal 3.5% feedback at both signal and Raman wavelengths. It is quite obvious that reducing or, preferably, eliminating feedback at Raman wavelength would reduce effective nonlinear interaction distance and, therefore, would increase the Raman threshold. Indeed, our simulations for such a configuration confirm that Raman threshold can be increased to more than 5-kW of signal power for 50-dB reflection suppression at the Raman wavelength. Such level of reflection suppression is available in angle-cleaved fiber lasers, where feedback at the laser wavelength can be achieved using in-fiber gratings. This result is consistent with the above given simple formula for a single-pass fiber amplifier Raman threshold, which predicts even higher threshold powers. The conclusion is that for a properly optimized CW fiber laser design, nonlinear interactions can be eliminated. The remaining principal question is whether thermal dissipation can be sufficiently effective in multi-kW fiber lasers.

Figure 3: Measured beam quality of M² = 1.27 from 810-W fiber laser.

Thermal Issues

The significant advantage of a fiber for generating high powers is its geometry: large surface-to-volume ratio facilitates good thermal dissipation, thus reducing fiber susceptibility to thermal effects compared to bulk solid-state lasers by orders of magnitude. However, at very high pumping powers the effect of fiber heating is not negligible. Nevertheless, the advantageous thermal geometry of the fiber together with a suitable fiber cooling scheme allows avoiding fiber overheating even at the highest pump powers. Figure 4 shows temperature rise at different positions along 1-kW and 10-kW pumped fiber lasers. Indeed, even for 10-kW pumping, the maximum temperature rise (occurring at the pumped fiber ends) is only 80°C. Such a temperature rise is not expected to be an obstacle. The calculation has been performed for fibers immersed in water: heat conduction through water heat-sink is much more efficient than heat convection through air.

Figure 4: Temperature rise distribution along a 30-m long 1-kW and 10-kW pumped DC Yb-doped 20-µm core and 400-µm clad fiber.

Summary

In conclusion, we have demonstrated a high power CW single transverse mode beam from a 20-µm core fiber laser. Such advancement constitutes an important technological step in developing other types of fiber-based laser systems, since such low-NA core size is compatible with other fiber components (couplers, gratings, etc.). Extremely high output powers from such lasers enable a variety of practically significant applications which were not possible previously. It is likely that the demonstrated powers do not represent the limit for the fiber technology and can be scaled further towards multi-kW power levels from a single fiber emitter.

References

1. J. Limpert, A. Liem, H. Zellmer, A. Tünnermann, Electr. Lett. 39, 645 (2003).
2. Y. Jeong, J.K. Sahu, D.N. Payne, J. Nilsson, ASSP 2004, Santa Fe, NM, post-deadline paper PD1.
3. C. Liu, A. Galvanauskas, B. Ehlers, F. Doerfel, S. Heinemann, A. Carter, K. Tankala, ASSP 2004, Santa Fe, NM, post-deadline paper PD2.
4. F. Gonthier et al., Proc. SPIE vol. 5335, paper 5335-48 (2004).
5. J.P. Koplow, D.A.V. Kliner, L. Goldberg, Opt. Lett. 25, 442 (2000).