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3.15 kW direct diode-pumped near diffraction-limited all-fiber-integrated fiber laser HAILONG YU, HANWEI ZHANG, HAIBIN LV, XIAOLIN WANG,* JINYONG LENG, HU XIAO, SHAOFENG GUO, PU ZHOU, XIAOJUN XU, AND JINBAO CHEN College of Optoelectric Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received 5 March 2015; revised 17 April 2015; accepted 17 April 2015; posted 20 April 2015 (Doc. ID 235713); published 8 May 2015

We demonstrate a direct diode-pumped all-fiber-integrated fiber laser based on master oscillator power amplifier configuration at 1080 nm, producing maximum output power of 3.15 kW with corresponding optical to optical efficiency of 75.1%. Further power scaling is pump-limited and theoretical analysis demonstrates that 4 kW output power can be further achieved without stimulated Raman scattering. Near diffraction-limited beam quality (M 2 ∼ 1.6 in the x and y directions) is also achieved at the maximum output power. This compact prototype laser has excellent stability and reliability, which could benefit many practical applications, such as industrial processing. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3615) Lasers, ytterbium; (140.3280) Laser amplifiers. http://dx.doi.org/10.1364/AO.54.004556

1. INTRODUCTION Fiber lasers fully deserve the attention they are paid by researchers around the world. Benefit from the fiber geometry, fiber lasers are now associated with high average power, excellent beam quality, easy thermal management, superior reliability, and compact size [1,2]. All of these outstanding characteristics make them an ideal source for many industrial and scientific applications. In the last decades, fiber lasers have become one of the most promising laser technologies following the unprecedented power scaling. Since the first kilowatt-level near diffraction-limited fiber laser was established in 2004 [3], a 10 kW level single-mode fiber laser has been accomplished by the IPG with tandem pumping technology [4]. For the direct diode-pumped case, most of the reported single-mode fiber lasers produce limited output power of 1–2 kW level [1,5–9]. Most recently, one 2.5 kW all-fiber master oscillator power amplifier (MOPA) pumped by 976 nm laser diodes (LDs) has been demonstrated [10]. 3 kW output power was achieved based on laser oscillator architecture with bulk spatial optics in 2014 [11]. However, to the best of our knowledge, no strictly all-fiber near diffraction-limited fiber laser based on MOPA configuration with output power beyond 3 kW has been reported. The power scaling potential of direct diode-pumped fiber lasers also lacks theoretical analysis in detail. Here, we implement a reliable all-fiber-integrated fiber laser in MOPA configuration at 1080 nm, which produces maximum 1559-128X/15/144556-05$15/0$15.00 © 2015 Optical Society of America

average power of 3.15 kW with near diffraction-limited beam quality. To the best of our knowledge, this is the highest power ever reported in any direct diode-pumped near diffractionlimited fiber MOPA. The beam quality represented by M 2 factor is measured to be 1.61 and 1.51 in the x and y directions at the maximum output power. No stimulated Raman scattering (SRS) is observed in the output spectrum and further power scaling is only pump-limited. Theoretical analysis demonstrates that our fiber laser can be scaled to above 4 kW level by optimizing fiber length if enough pump power is available. 2. EXPERIMENTAL SETUP The architecture of our all-fiber-integrated fiber laser is shown in Fig. 1. The fiber laser consists of two parts: a laser oscillator and a power amplifier. The laser oscillator serves as a seed laser, which is a linear cavity including a pair of fiber Bragg gratings (FBGs). The high-reflector (HR) and output-coupler (OC) FBGs centered at a wavelength of ∼1080 nm provide 3 dB spectral bandwidths of about 2 nm and 1 nm, respectively. The reflective ratios of HR and OC FBGs at 1080 nm are more than 99% and 10%, respectively. The gain medium utilized in the laser oscillator is double cladding ytterbium-doped fiber (YDF) with a core diameter of 20 μm and inner cladding diameter of 400 μm. Three 50 W fiber-pigtailed 915 nm LDs are used to pump the laser oscillator through a 7:1 home-designed tapered fiber bundle (TFB) (only three pump ports are used).

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Fig. 1. Schematic diagram of the all-fiber-integrated fiber laser and the measuring system (the inset: all of the components are polarizationindependent).

The pigtail fiber of the LDs and the input fiber of the TFB are the same passive fiber with a core/cladding diameter of 105/ 125 μm. The output fiber of the TFB with a core/cladding diameter of 20/400 μm is spliced with the HR FBG directly. Two parts of cladding light strippers (CLSs) are performed to clear the residual pump and high-order signal mode propagating in the fiber cladding, including one part in the cavity. Then, the output laser signal of the oscillator is launched into the power amplifier through the signal port of one 6  1 × 1 home-designed signal/pump combiner. The active fiber utilized in the power amplifier has a core diameter of 30 μm and inner cladding diameter of 400 μm. The cladding absorption coefficient of the active fiber is about 0.7 dB∕m at 915 nm. The length of the active fiber with a value of 26 m is optimized by numerical simulation considering the changing pump center wavelength induced by the increasing temperature when raising the pump power. Totally 4.06 kW pump light at 915 nm is launched into the active fiber to boost the signal power of the laser oscillator to 3 kW level. CLS is performed in the similar way as is done in the laser oscillator. One homemade end cap is spliced to deliver the output signal power and avoid any end-face reflection. All of components are fixed on a water-cooled heat sink with the exception of the end cap and collimator. As shown in the inset of Fig. 1, the measuring system consists of one collimator (CO), one highly reflecting (HR) mirror, one dichroic mirror (DM), one power meter, one wavefront analyzer, one beam analyzer, and one optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C). The HR mirror is used to split the output laser beam. The transmitted laser is used to measure the beam quality (BQ) by the wavefront analyzer. Most of the laser is reflected by the HR mirror and measured by the power meter. The spectra are measured through the light scattered by the power meter.

The seed laser power of 100 W is launched into the power amplifier. The output laser power linearly increases when increasing the pump power and no gain saturation effect is observed. The maximum output power reaches to 3.15 kW when increasing the pump power to 4.06 kW and the corresponding optical to optical efficiency is 75.1%. Owing to the complete absorption and effective cladding light stripping, no unabsorbed pump light is observed in the spectra, as shown in Fig. 3. The 3 dB spectral bandwidth of the seed laser is 0.69 nm, which is broadened with the increase of pump strength due to nonlinear effects, for instance, self-phase modulation [12–14]. The 3 dB spectral bandwidth at maximum output power is broadened to 3.9 nm. The output spectra are clear and no SRS is observed. Output laser power is limited only by the available pump power and can be further scaled to much higher power level by optimization (the theoretical analysis is demonstrated in the next section). The 30/400 μm YDF utilized in the power amplifier supports 4–5 modes, so mode selection techniques are essential to improve the beam quality. Coiling the fiber and cladding mode stripper are adopted in our fiber laser to guarantee the high beam quality. By coiling the fiber, the high-order modes can be leaked into the inner cladding and then stripped by the homedesigned cladding mode stripper. The coiling diameter of the fiber is about 12 cm, based on the analysis of [15]. Finally, near diffraction-limited beam quality is achieved. The beam quality

3. EXPERIMENTAL RESULTS AND THEORETICAL ANALYSIS When the monolithic CW fiber laser system has been established step-by-step, the detailed features are measured, including output power, spectra, stability in time domain, beam quality, etc. Figure 2 shows the measured output power and calculated optical to optical efficiency at different pump power.

Fig. 2. Measured output power (black line) and optical to optical efficiency (blue line) versus the pump power.

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Fig. 3. Seed laser spectrum and output signal spectra at output powers of 1.59 kW and 3.15 kW.

represented by M 2 factors are measured by our wavefront analyzer, which are 1.61 and 1.51 in the x and y directions at the output power of 3.15 kW. The beam profile at laser power of 3.15 kW is shown in Fig. 4. The CW all-fiber-integrated fiber laser has excellent stability and reliability. Benefits from the effective water cooling and perfect splice, all temperatures of the key segments are below the safe temperature under the real-time measurement. The temporal characteristic of the output laser at the maximum output power is shown in Fig. 5. It is found that the fiber laser keeps operating steadily in a temporal range of ∼10 min . The temporal characteristic in a microsecond range is also stable. There is no self-pulsing or mode instability in the time domain.

Fig. 5. Temporal characteristic of the output laser in a long temporal range (measured by a power meter) and microsecond range (the inset, measured by the high-speed photodetector with bandwidth of 5 GHz) at the power of 3.15 kW.

to hamper further power scaling. In order to investigate the potential of our fiber laser, a theoretical model is established based on steady-state rate equation, considering SRS [16], amplified spontaneous emission (ASE), and changing pumping wavelength. The model developed by us can be expressed by the following equations [17]: 

d P pk z dz

 −Γp σ a λpk N 0 − σ a λpk   σ e λpk N 2 P  pk z − αp P  pk z;



d P pk z dz

(1)

 Γs σ a λsk   σ e λsk N 2 − σ a λsk N 0 P  sk z K 1 X g ω − ωk P  si z Aeff i1 R i

4. THEORETICAL ANALYSIS FOR FURTHER POWER SCALING

− αp P  pk z 

The obvious limited factor for our fiber laser is the available pump power at the present power level. However, although no SRS is observed by now, it is the biggest potential limitation

 P −si zP  sk z  2σ e λsk N 2

N2  N0

Γp hcAcore

PK

Γp hcAcore

PK

 k1 λpk σ a λpk P pk z

k1 λpk σ a λpk 

s  P −pk z  hcAΓcore

PK

Γs 1 −  σ e λpk P  pk z  P pk z  τ  hcAcore

Fig. 4. Beam profile of the output laser at the power of 3.15 kW.

 k1 λsk σ a λsk P sk z

PK

k1 λsk σ a λsk 

hc 2 Δλs ; λ3sk

 P −sk z

−  σ e λsk P  sk z  P sk z

(2)

;

(3)

where N 0 is the doping concentration; N 2 is the population of the upper lasing level; λpk and λsk are the kth pump and signal wavelength, respectively; P  pk z is the forward and backward z is the forward and backward signal pump power; P  sk powers; σ a and σ s are the absorption and emission cross sections of pump or signal light; αp is the background loss; Γp and Γs are the power filling factors of pump and signal light; g R ωi − ωk  represents the Raman gain; Δλs is the spontaneous emission noise; Aeff is the effective mode area of the fiber core; Acore is core area of the fiber; τ is the lifetime of upper lasing level; h is Planck constant; and c is the velocity of light.

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Fig. 6. Results of numerical simulation: the optimized active fiber length at the pump power of 4.06 kW.

Fig. 8. Results of numerical simulation: the optimized active fiber length at the pump power of 5 kW.

Numerical simulation is based on the actually measured spectral data of the pump and seed laser. The fiber basic parameters employed in the numerical simulation are in agreement with our fiber laser. We assume that a signal power of 100 W and pump power of 4.06 kW are launched into the main amplifier in correspondence with our experiment. Figure 6 shows the optimized results of active fiber length at a pump power of 4.06 kW and the best value is 25 m. Considering the possibility of multiple splice, we choose an active fiber

length of 26 m in the main amplifier. The simulated results at an active fiber length of 26 m and passive fiber length of 7 m are shown Fig. 7, which are in good agreement with our experimental results. Figure 7(b) shows that the signal power intensity is 34.7 dB higher than the Raman peak, meaning that the Stokes light power is below the Raman threshold (we assume that the one percent of the output power is the Raman threshold). This result further confirms that power scaling is pump-limited at the present power level. However, if the pump power is increased to 5 kW and other parameters are kept unchanged, output laser power of 4 kW is impossible to obtain because of the increased Stokes light power. As shown in Fig. 8, the Stokes light power has exceeded the Raman threshold when the active fiber length is 26 m. Even so, Fig. 8 also shows that the results can be optimized to reach the maximum output power and meanwhile keep the Stokes light power below the Raman threshold by changing the active fiber length. When the active fiber length is 23 m, the output power reaches the maximum value and the Stokes light power contained in the output power is 0.54%. The stimulated results demonstrate that the output power of our fiber laser can exceed 4 kW without SRS by optimizing the fiber length if enough pump power is available. 5. CONCLUSION In summary, we have reported the development of a CW allfiber-integrated fiber laser with 3.15 kW power output at 1080 nm based on MOPA configuration. Near diffractionlimited beam quality is achieved with M 2 factors of ∼1.6 in the x and y directions. The output power is only limited by the available pump power and theoretical analysis demonstrates that 4 kW output power can be achieved without SRS by further optimizing. This compact prototype laser has excellent stability and reliability and then befits industrial processing.

Fig. 7. Results of numerical simulation: (a) the power propagation curves and (b) the output spectra at pump power of 4.06 kW and active fiber length of 26 m.

National Natural Science Foundation of China (NSFC) (61322505); Natural Science Foundation of Hunan Province, China (14JJ3004); Scientific Research Program of the Education Department of Hunan Province, China (YB2013B003).

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The authors want to thank Qiang Wang, Wei Yang, Yunfeng Duan, Kun Zhang, and Xiaoyong Xu for providing helpful assistance in carrying out the experiments. REFERENCES 1. Y. Xiao, F. Brunet, M. Kanskar, M. Faucher, A. Wetter, and N. Holehouse, “1-kilowatt CW all-fiber laser oscillator pumped with wavelength-beam-combined diode stacks,” Opt. Express 20, 3296–3301 (2012). 2. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7, 861–867 (2013). 3. Y. E. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12, 6088–6092 (2004). 4. E. Stiles, “New developments in IPG fiber laser technology,” in Proceedings of the 5th International Workshop on Fiber Lasers (Fraunhofer IWS, 2009), pp. 4–6. 5. F. Becker, B. Neumann, L. Winkelmann, S. Belke, S. Ruppik, U. Hefter, B. Köhler, P. Wolf, and J. Biesenbach, “Multi-kW cw fiber oscillator pumped by wavelength stabilized fiber coupled diode lasers,” Proc. SPIE 8601, 860131 (2013). 6. H. Yu, D. A. Kliner, K. Liao, J. Segall, M. H. Muendel, J. J. Morehead, J. Shen, M. Kutsuris, J. Luu, and J. Franke, “1.2-kW single-mode fiber laser based on 100-W high-brightness pump diodes,” Proc. SPIE 8237, 82370G (2012). 7. Y. Jeong, A. J. Boyland, J. K. Sahu, S. Chung, J. Nilsson, and D. N. Payne, “Multi-kilowatt single-mode ytterbium-doped large-core fiber laser,” J. Opt. Soc. Korea 13, 416–422 (2009).

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3.15 kW direct diode-pumped near diffraction-limited all-fiber-integrated fiber laser.

We demonstrate a direct diode-pumped all-fiber-integrated fiber laser based on master oscillator power amplifier configuration at 1080 nm, producing m...
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