1. Introduction

In recent years the concept of fiber lasers and amplifiers has found the way to industrial application. Its unique properties like single transversal mode operation independent of output power, high optical efficiency, high power capability and finally, its compact and robust design have made such systems a real competitor to canonical laser systems like CO₂ lasers or Nd:YAG lasers, especially in the field of material processing. With the invention and technical realization of low-loss large-mode-area rare-earth-doped fibers and high brightness semiconductor based pump sources at the beginning of the decade a tremendous increase of continuous-wave output power from single-mode fiber lasers has been witnessed [1-4].

Nowadays, systems delivering near diffraction limited beam quality at power levels well beyond the 1 kW level are commercially available. Not only that fiber lasers and amplifier systems are about to replace conventional laser systems (like CO2 or Nd:YAG), they also drive the development of new machining techniques, which have not been possible so far with other laser systems. Despite this impressive development the demand for higher power is still increasing. Although fiber lasers have shown a tremendous power scaling potential, they are facing limitations. Due to the fiber geometry most challenging are the onset of detrimental fiber nonlinearities, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). In principle, fiber nonlinearity can be reduced by mode size scaling and shortening the interaction length, hence, the concept of large-mode-area fibers has been introduced in conventional step-index fibers [5, 6]. More recently, new fiber designs based on photonic crystal structures have been investigated, which offer a significant increase of the mode area due to the precise and flexible control of index profiles while suppressing higher order transversal modes [7, 8]. Besides nonlinearity, power restrictions due to finite pump brightness, thermal issues and damage have to be considered. A detailed study of power scalability, which has been published recently, came to the conclusion that “These effects interact to create hard limits on the output power of broadband fiber lasers at around 36 kW and narrowband fiber lasers at around 2 kW” [9].

Further power scaling beyond these given limits appears to be only feasible by the technique of beam combination. The combination of multiple beams to one single beam with near diffraction limited beam quality is subject of extensive research. Different approaches are investigated and can be classified in coherent and incoherent techniques. Coherent combining requires a rather precise control of the phase of each channel emitting single-frequency radiation [10]. Following the approach on coherent beam combination power levels in the range of several 100 W have been reported [11, 12]. In contrast, incoherent beam combining is a more straightforward way to combine multiple beams of different wavelength in near- and far-field via one or more wavelength selective elements without any phase control [13, 14], but suffers from the loss of spectral purity of the output beam. So far power levels up to ~750 W have been demonstrated employing conventional fibers as well as photonic crystal fibers as gain medium [15, 16].

In this contribution we report on spectral beam combining of four narrow-linewidth ytterbium-doped photonic crystal fiber amplifier chains using a highly efficient reflective diffraction grating to a power level of 2 kW. To the best of our knowledge, this is the highest output power ever reported for a laser beam combination experiment. A linewidth control applied to the narrowband emission of the seed laser avoids the onset of stimulated Brillouin scattering (SBS) in the main amplifiers, but also sets limits to the dispersion induced degradation of beam quality. In the presented configuration the beam quality at the highest power level has been characterized to quality of M² _x = 2.0 and M² _y = 1.8, respectively. The configuration possesses significant scaling potential in terms of power of the individual emission and number of combined chains due to the potentially close channel spacing.

2. Experimental setup and results

The setup of the high power spectral beam combination experiment consists of four individual channels, each seeded by a narrowband diode laser emitting at a specific wavelength followed by a two-stage pre-amplifier and a power amplification stage. The individual beams are overlapped in near- and far-field by a reflective diffraction grating. Figure 1 shows the experimental setup (one amplifier chain is highlighted).

 

Fig. 1. Experimental setup for spectral beam combining of four photonic crystal fiber amplifier channels. A single channel is highlighted and consists of a seed source (1), a first (2) and second pre-amplifier (3), the main amplifier (4), the folding mirrors (5) and the grating (6).

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2.1 Seed source and Pre-amplifier stages

The seed signal (~20 mW of optical power) is provided by a fiber coupled wavelength tunable external cavity single-frequency diode laser (ECDL). The emission wavelength of these diode lasers can be tuned from 1025 nm to 1075 nm. To enhance the SBS threshold a linewidth control is applied, which employs a modulation of the ECDL’s driver current with a noise generator (not shown in Fig. 1), resulting in an increased seed-signal bandwidth. The non-modulated emission bandwidth is about 1 MHz, which is increased by the imposed current modulations to about 60 pm measured with a high resolution optical spectrometer.

The seed signal is launched into the first amplification stage, which uses a polarization-maintaining ytterbium-doped large-mode-area double-clad fiber with 20 μm signal core diameter and 125 μm pump core diameter and ~ 2 m length, pumped by a laser diode at a wavelength of 976 nm to amplify the seed-signal to a linearly polarized beam with a power of ~500 mW. This signal is launched into the second pre-amplifier stage using a 1.6 m long, polarization-maintaining photonic crystal fiber having a 40 μm signal core and a 200 μm pump core [17]. An output power of ~ 20 W in a linearly polarized and diffraction-limited beam is obtained, what constitutes the seed signal of the main amplification stage. The preamplifier is protected against back reflection from the main amplification stage by optical isolators.

2.2 Main amplifier and combining section

The main amplifier-stage consists of a 15 m long ytterbium-doped photonic crystal fiber. The active core of the fiber has a measured mode field diameter for the fundamental mode of ~25 μm. The pump core is defined by an air-cladding region [18] and has a diameter of 550 μm and a numerical aperture of 0.5. The stage is pumped at 976 nm through one fiber facet in a counter propagating configuration by a fiber coupled diode laser.

The four amplifier chains cover the spectrum from 1050 nm to 1065 nm with 5 nm spectral spacing between adjacent channels. This spacing results in an angular dispersion that provides enough spatial separation between adjacent beams [19] at a distance of 3.2 m between the final folding mirrors and the dispersive element (see Fig. 1). The collimated beams had an diameter of ~3 mm in front of the grating.

The grating, which is used as combining element, is a highly efficient reflective diffraction grating (binary grating), optimized for both TE- and TM-polarization, hence, for non-polarized light [21]. Thus, no polarization control of the main amplifier is required. The line density of the grating is 960 lines/mm. The grating was characterized after fabrication at a wavelength of 1064 nm in 1^st-order Littrow- configuration at an angle of 30.7 degree by measurement of the power in the 0^th order. The measured diffraction efficiency at 1064 nm is > 99% for TE-polarization and > 98% for TM-polarization uniformly across the grating as shown for different positions on the gratings in Fig. 2.

 

Fig. 2 (a) Reflective diffraction grating and (b) measured efficiency at 1064 nm.

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2.3 Experimental results of high power incoherent beam combination

Figure 3 shows the slope efficiency and the output spectrum of the combined output beam. An optical power of 2065 W is reached at a total launched pump power of 3395 W. The total slope efficiency including the diffraction efficiency is as high as 61%. The combining efficiency, determined experimentally by the ratio of optical power of the combined beam and the power diffracted in the zeroth order, was measured commonly for all four channels to 99%. That proves the high efficiency of the grating for both polarizations measured before (Fig. 2(b)) but also that this efficiency is obtained across a large bandwidth and slightly off-Littrow given by the distinct wavelength of the channels from 1050 nm to 1065 nm.

Figure 4 shows the beam quality characterization of the combined output beam at 2 kW optical power. The beam quality factor M² of each individual channel at ~500 W after the grating is characterized to M² _x ~ 1.6 and M² _y ~ 1.4. The larger value in horizontal (x-) direction can be attributed to additional divergence imposed by the diffraction grating on the ~60 pm bandwidth optical beam. The combined beam shows a beam quality of M² _x = 2.0 and M² _y = 1.8 at the power of 2 kW (Fig. 4). The slight beam quality degradation of the combined beam compared to the quality of the individual channels can be attributed to the inaccuracy of the spatial overlap of the four output beams on the grating and the far field and to statistical pointing instabilities of the four individual high power beams propagating over long paths in a standard laboratory environment. Due to the all reflective nature of the combining element thermo-optical issues are not expected at that power level, what is confirmed by FEM-analysis published earlier [3].

 

Fig. 3. Slope efficiency (a) and emitted optical spectrum (b) of the combined output beam

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Fig. 4. Near-field beam profile (a) and beam quality measurement (b) of the combined output beam at optical power of 2 kW (M² _x = 2.0, M² _y = 1.8).

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The approach possesses significant scaling potential. We expect the onset of SBS in a single main amplifier at power levels above 1 kW, as already experimentally demonstrated [21]. The channel spacing can be reduced by revising the combining stage, i.e. employing a grating with a smaller grating period, so that at the same time the number of individual channel can be increased. The useful Yb-gain bandwidth covers more than 50 nm around 1060 nm and the diffraction efficiency of the reflective dielectric grating is high over a comparable wavelength range around the design wavelength [3]. Hence, a further scaling by additional and more powerful channels appears to be feasible in a straightforward manner.

5. Conclusion

We realized a spectral beam combining scheme of four narrow-linewidth photonic crystal fiber amplifier chains at an intermediate power level of ~500 W per channel. The four output beams are combined via a highly-efficient dielectric polarization-independent diffraction grating to an output beam of 2 kW optical power with good beam quality (M² _x = 2.0, M² _y = 1.8). A slight degradation of beam quality due to the additional angular dispersion imposed by the grating is induced. No thermo-optical effects on the grating are anticipated at this power level. Other influences on beam quality are subject of further investigations and are expected to be reduced in a revised setup.