1.5 kW, near-diffraction-limited, high-efficiency, single-end-pumped all-fiber-integrated laser oscillator Hailong Yu, Xiaolin Wang,* Rumao Tao, Pu Zhou, and Jinbao Chen College of Optoelectric Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received 31 July 2014; revised 20 October 2014; accepted 27 October 2014; posted 29 October 2014 (Doc. ID 220131); published 24 November 2014
We report a monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. The laser scheme design and fiber parameter selection are based on our theoretical analysis through one rate equation model consisting of changing pump wavelengths. The laser oscillator is pumped by 49 fiber-pigtailed 915 nm laser diodes with power of 50 W each through two stage combiners. The measured laser power is 1520 W at the pump power of 2054 W, and the corresponding optical-tooptical efficiency is 74%, in agreement with numerical simulation. Stimulated Raman scattering is observed when the laser power reaches 1030 W and the power ratio of Raman power is 4% at maximum output power. The experimental results show that the beam qualities represented by the M 2 factor are less than 1.2 at all pump power levels. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3615) Lasers, ytterbium; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/AO.53.008055
1. Introduction
High-power fiber lasers have a bright future in many industrial, scientific, and defense applications due to their high efficiency, high beam quality (BQ), compactness, and superior reliability [1,2]. Power scaling of fiber lasers has shown a remarkable increase in the last decade [3], reaching ∼20 kW laser output with single-mode operation [4] and 100 kW laser output with multimode operation [5]. Most of the fiber lasers with output power beyond 1.5 kW are based on the master oscillator power amplifier (MOPA) configuration. Recent advances have shown that mode instability, a new type of limitation, restricts the maximum average power that can be achieved by a fiber laser [3,6], especially a fiber amplifier. Laser oscillators have the ability to suppress mode instability owing to the high cavity loss of high-order modes, and to date there have been no experimental 1559-128X/14/348055-05$15.00/0 © 2014 Optical Society of America
observations of mode instability reported in laser oscillators. The fiber laser oscillator has reach to the output power of 3 kW [7], but the reported output power of the laser oscillator with an all-fiber architecture is limited to the 1 kW level [1,8]. The all-fiber architecture is more reliable than the spatial structure. Moreover, the all-fiber laser oscillator is simpler and more compact than the MOPA configuration, which provides a monolithic solution for kW-level fiber lasers. In this paper, we established a monolithic singleend-pumped all-fiber laser oscillator operating at 1070 nm with single-mode operation based on two stage pump combining. 1520 W laser output power with optical-to-optical efficiency of 74% is achieved, which is in agreement with numerical simulation. To the best of our knowledge, this is the highest power ever reported in a single-end-pumped allfiber-integrated laser oscillator. Stimulated Raman scattering (SRS) is observed when the laser power reaches to 1030 W and the power ratio of Raman power is 4% at maximum output power. The 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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experimental results show that the M 2 factors of output beam are less than 1.2 at all pump power levels. By further optimizing the laser oscillator parameters to suppress SRS, the output power can be scaled to a much higher level. 2. Scheme of the All-Fiber Laser Oscillator
The architecture of the all-fiber laser oscillator is shown in Fig. 1. The resonant cavity consists of a high-reflector (HR) fiber Bragg grating (FBG) and an output-coupler (OC) FBG. The HR FBG provides a spectral bandwidth of 2.03 nm at center wavelength of 1069.93 nm and reflective ratio of 99.64%. The OC FBG with reflective ratio of 10.42% provides spectral bandwidth of 1.04 nm at a center wavelength of 1069.98 nm. Double cladding Yb-doped fiber (YDF) with core diameter of 20 μm and inner cladding diameter of 400 μm serves as the gain medium with length of 41 m optimized by the numerical simulation, whose absorption coefficient at 915 nm is 0.4 dB∕m. The pump light directly inserts into the resonant cavity through a 7∶1 China-made tapered fiber bundle (TFB) with power of ∼300 W at each port. The 300 W pump light is combined by another sort of 7∶1 China-made TFB with 50 W fiberpigtailed 915 nm laser diodes (LDs) spliced at each port. In total, 49 fiber-pigtailed LDs with 105/125 μm pigtail fiber are used to pump the laser. The first stage TFBs have input fiber with core/cladding diameter of 105/125 μm and numerical aperture (NA) of 0.15 and output fiber with core/cladding diameter of 220/242 μm and NA of 0.22. The second stage TFB’s input fiber has the same parameters as the output fiber of the first stage TFB and the output fiber with core/cladding diameter of 20/400 μm and core/cladding NA of 0.06/0.46. After the OC FBG, about 2 m passive fiber with core diameter of 20 μm and inner cladding diameter of 400 μm is spliced to deliver the output signal power to the end cap. Meanwhile, passive fiber is used for the cladding light
strippers (CLSs). Four parts of CLSs including one part in intracavity are performed to strip the residual pump and high-order signal modes, and the CLS in intracavity can also reduce the amplified spontaneous emission (ASE), ensuring reliability for high-power operation. Except the end cap and the collimator, all the devices are fixed on a watercooled plate with a special design. The measuring system consists of one highly reflecting (HR) mirror, one dichroic mirror (DM), one collimator, one 10 kW 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 small transmitted laser is used to measure the BQ by the wavefront analyzer and analyze the laser beam by the beam analyzer. Most of the laser is reflected by the HR mirror and measured by the 10 kW power meter. The spectra are measured through scattered light by the OSA. 3. Experimental Results and Theoretical Analysis
First, we measure the entire spectrum of the pump light after the combiner, as shown in Fig. 2(a). The center wavelength of spectra shifts to the long wavelength when increasing the pump power. Moreover, the spectral bandwidth is so broad that the traditional rate equation model only considering a single pump wavelength cannot achieve accurate results to guide the design of this laser oscillator. So we established one rate equation model consisting of fiber bending loss [9], mode coupling [10], changing pump wavelengths, and cladding light stripping. In order to simplify the numerical calculation, we assume that the various modes of different signal wavelength have the same absorption loss and scattering loss coefficients. The model developed by us can be expressed by the following steady-state rate equations [10,11]:
Fig. 1. Architecture of the all-fiber laser oscillator. 8056
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Fig. 2. (a) Spectra of pump light at different pump powers and (b) signal/pump power along the longitudinal direction of the fiber at the pump power of 2054 W.
N 2 z N ×
Γp
PN s as s s s p ap p p p p− p s− s m1 λm σ m λm Pm λm ; z Pm λm ; z Γs n1 λn σ n λn Pn;k λn ; LPk ; z Pn;k λn ; LPk ; z PN s as s p ap p ep p p p p− p ℏcAeff s s es s s− s m1 λm σ m λm σ m λm Pm λm ; z Pm λm ; z τ Γs n1 λn σ n λn σ n λn Pn;k λn ; LPk ; z Pn;k λn ; LPk ; z
PM
Γp
PM
(1.1)
p dPp m λm ; z p ap p p p p p p p Γp σ ep m λm N 2 z − σ m λm N 1 zPm λm ; z − αm λm Pm λm ; z dz
(1.2)
s dPs n;k λn ; LPk ; z s s s as s s s sRS s Γs σ es n λn N 2 z − σ n λn N 1 zPn;k λn ; LPk ; z − αn λn αn λn dz s s s αbl k λn ; LPk ; zPn;k λn ; LPk ; z s − Ps n;k λn ; LPj ; z
K X j≠k
N N 1 z N 2 z;
dkj
K X j≠k
(1.4)
where N is the doping concentration; N 1 and N 2 are the populations of the lower and upper lasing levels, respectively; λpm and λsn are the mth pump wavelength and the nth signal wavelength, respecp tively; Pp m λm ; z represents the forward and backs ward pump power; Ps n;k λn ; LPk ; z represents the forward and backward signal powers of different ep transverse modes; σ ap m and σ m are the absorption and emission cross sections of pump light and σ as n and σ es n represent the absorption and emission cross sections of signal light; αsn λsn , αsn RS λsn , and s αbl k λn ; LPk ; z are the absorption loss, Rayleigh scattering, and bending loss coefficients, respectively [9]; dkj is the mode coupling coefficient between the kth and jth modes [10]; Γp and Γs are the power filling factors of pump and signal light [12]; Aeff is the effective mode area of the fiber core; and τ is the lifetime of upper levels.
s s dkj Ps n;j λn ; LPj ; z 2Γs σ e λn ; LPk N 2 z
ℏc2 Δλs λsn 3
(1.3)
Numerical simulation is based on the actually measured spectral data of pump light. The simulation results are shown in Fig. 2(b). It can be seen that the YDF with length of 40 m is enough to absorb the pump light of 2054 W and the signal power can reach a maximum value without obvious re-absorption. In fact, the fiber length selected in our laser oscillator is 41 m due to the possibility of multiple fusions. After the all-fiber laser oscillator is established, we measure its features with details, such as power, spectra, and BQ. As shown in Fig. 3, the measured experimental results are in good agreement with the numerical simulation, confirming the accuracy of our theoretical model. The maximum output power reaches to 1520 W when increasing the pump power to 2054 W, and the corresponding optical-tooptical efficiency is 74%, as shown in Fig. 3(a). The optical-to-optical efficiency at different pump power levels is shown in Fig. 3(b). The measured optical-tooptical efficiencies are higher than the simulated results at low power level, but the two curves come 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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Fig. 3. Measured and simulated (a) output power and (b) optical-to-optical efficiency versus pump power.
Fig. 4. Laser spectra with (a) log and (b) linear scale at different output powers.
Fig. 5. (a) Measured BQ represented by M 2 factor at various output powers and (b) beam profile at maximum output power of 1520 W.
close to each other when the power is increased. The output spectra are shown in Fig. 4 with log and linear scale. There is no unabsorbed pump light in the spectra owing to the effective cladding light stripping. The spectra are broadened with the increase of pump strength due to nonlinear effects, for instance, selfphase modulation and the Raman gain process, and the 3 dB spectral bandwidth at maximum output power is 4.5 nm. When the output power reaches 8058
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1030 W, SRS is observed, as clearly shown in Fig. 4(a). The Raman power level increases a lot when further increasing the pump power, but its power ratio is only 4% at maximum output power, as shown in Fig. 4(b), which is not detrimental to applications that have much greater demand on the power level. The YDF used in the laser cavity supports two signal modes. In order to improve the BQ, mode
selection techniques, such as coiling the fiber and cladding mode stripper, are adopted in our laser oscillator to guarantee single-mode operation. The BQ represented by M 2 factors is measured by our wavefront analyzer. Figure 5(a) shows the neardiffraction-limited BQ of the laser oscillator at various laser power levels. Both of the M 2 factors in the X and Y directions are measured to be below 1.2. The beam profile at 1520 W is shown in Fig. 5(b), which is near Gaussian beam. In our experiment, we monitor the temperature of all splices and CLSs with real-time measurement. Usually, the most important splice is the one between the HR FBG and YDF because of the highest thermal load, whose temperature is below 57°C and remains stable in our experiment. Therefore, the limited factor for further power enhancement is not the thermal load but the SRS. Our following work will focus on optimizing the oscillator parameters, such as the fiber length, to suppress SRS. Further power scaling to 2 kW will hopefully be achieved. 4. Conclusion
In summary, we have reported the development of a reliable monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. When the pump power is 2054 W, the laser output power is 1520 W with optical-to-optical efficiency of 74% in agreement with numerical simulation. The theoretical model can be used to guide the design of the fiber laser with high accuracy. By coiling the fiber and cladding mode stripper, neardiffraction-limited BQ is achieved with M 2 factors less than 1.2 in the X and Y directions. The SRS is the main limitation to hamper further power scaling, and will be suppressed by effective methods in our following work, such as reducing the fiber length by using a higher doping concentration LMA fiber. This prototype laser can be applied in the industrial processing area.
The authors thank Qiang Wang, Shui Zhao, and Hanwei Zhang for providing helpful assistance in carrying out the experiments. The research leading to these results has received funding from the Projects of Scientific Research Foundation of National University of Defense Technology (Grant No. JC1207-03) and the Scientific Research Program of Education Department in Hunan Province (Grant No. YB2013B003). 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. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27, B63–B92 (2010). 3. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7, 861–867 (2013). 4. http://www.ipgphotonics.com/index.htm. 5. E. Shcherbakov, V. Fomin, A. Abramov, A. Ferin, D. Mochalov, and V. P. Gapontsev, “Industrial grade 100 kW power CW fiber laser,” in Advanced Solid State Lasers Congress, OSA Technical Digest (online) (Optical Society of America, 2013), paper ATh4A.2. 6. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Laegsgaard, “Theoretical analysis of mode instability in high-power fiber amplifiers,” Opt. Express 21, 1944–1971 (2013). 7. V. Khitrov, J. D. Minelly, R. Tumminelli, V. Petit, and E. S. Pooler, “3 kW single-mode direct diode-pumped fiber laser,” Proc. SPIE 8961, 89610V(2014). 8. 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 highbrightness pump diodes,” Proc. SPIE 8237, 82370G (2012). 9. R. T. Schermer and J. H. Cole, “Improved bend loss formula verified for optical fiber by simulation and experiment,” IEEE J. Quantum Electron. 43, 899–909 (2007). 10. M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15, 3236–3246 (2007). 11. I. Kelson and A. A. Hardy, “Strongly pumped fiber lasers,” IEEE J. Quantum Electron. 34, 1570–1577 (1998). 12. O. G. Okhotnikov, Fiber Lasers (Wiley-VCH, 2012).
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We report a monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. The laser scheme design and fiber parameter selection are based on our theoretical analysis through one rate equation model consisting of changing pump wavelengths. The laser oscillator is pumped by 49 fiber-pigtailed 915 nm laser diodes with power of 50 W each through two stage combiners. The measured laser power is 1520 W at the pump power of 2054 W, and the corresponding optical-tooptical efficiency is 74%, in agreement with numerical simulation. Stimulated Raman scattering is observed when the laser power reaches 1030 W and the power ratio of Raman power is 4% at maximum output power. The experimental results show that the beam qualities represented by the M 2 factor are less than 1.2 at all pump power levels. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3615) Lasers, ytterbium; (140.3570) Lasers, single-mode. http://dx.doi.org/10.1364/AO.53.008055
1. Introduction
High-power fiber lasers have a bright future in many industrial, scientific, and defense applications due to their high efficiency, high beam quality (BQ), compactness, and superior reliability [1,2]. Power scaling of fiber lasers has shown a remarkable increase in the last decade [3], reaching ∼20 kW laser output with single-mode operation [4] and 100 kW laser output with multimode operation [5]. Most of the fiber lasers with output power beyond 1.5 kW are based on the master oscillator power amplifier (MOPA) configuration. Recent advances have shown that mode instability, a new type of limitation, restricts the maximum average power that can be achieved by a fiber laser [3,6], especially a fiber amplifier. Laser oscillators have the ability to suppress mode instability owing to the high cavity loss of high-order modes, and to date there have been no experimental 1559-128X/14/348055-05$15.00/0 © 2014 Optical Society of America
observations of mode instability reported in laser oscillators. The fiber laser oscillator has reach to the output power of 3 kW [7], but the reported output power of the laser oscillator with an all-fiber architecture is limited to the 1 kW level [1,8]. The all-fiber architecture is more reliable than the spatial structure. Moreover, the all-fiber laser oscillator is simpler and more compact than the MOPA configuration, which provides a monolithic solution for kW-level fiber lasers. In this paper, we established a monolithic singleend-pumped all-fiber laser oscillator operating at 1070 nm with single-mode operation based on two stage pump combining. 1520 W laser output power with optical-to-optical efficiency of 74% is achieved, which is in agreement with numerical simulation. To the best of our knowledge, this is the highest power ever reported in a single-end-pumped allfiber-integrated laser oscillator. Stimulated Raman scattering (SRS) is observed when the laser power reaches to 1030 W and the power ratio of Raman power is 4% at maximum output power. The 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
8055
experimental results show that the M 2 factors of output beam are less than 1.2 at all pump power levels. By further optimizing the laser oscillator parameters to suppress SRS, the output power can be scaled to a much higher level. 2. Scheme of the All-Fiber Laser Oscillator
The architecture of the all-fiber laser oscillator is shown in Fig. 1. The resonant cavity consists of a high-reflector (HR) fiber Bragg grating (FBG) and an output-coupler (OC) FBG. The HR FBG provides a spectral bandwidth of 2.03 nm at center wavelength of 1069.93 nm and reflective ratio of 99.64%. The OC FBG with reflective ratio of 10.42% provides spectral bandwidth of 1.04 nm at a center wavelength of 1069.98 nm. Double cladding Yb-doped fiber (YDF) with core diameter of 20 μm and inner cladding diameter of 400 μm serves as the gain medium with length of 41 m optimized by the numerical simulation, whose absorption coefficient at 915 nm is 0.4 dB∕m. The pump light directly inserts into the resonant cavity through a 7∶1 China-made tapered fiber bundle (TFB) with power of ∼300 W at each port. The 300 W pump light is combined by another sort of 7∶1 China-made TFB with 50 W fiberpigtailed 915 nm laser diodes (LDs) spliced at each port. In total, 49 fiber-pigtailed LDs with 105/125 μm pigtail fiber are used to pump the laser. The first stage TFBs have input fiber with core/cladding diameter of 105/125 μm and numerical aperture (NA) of 0.15 and output fiber with core/cladding diameter of 220/242 μm and NA of 0.22. The second stage TFB’s input fiber has the same parameters as the output fiber of the first stage TFB and the output fiber with core/cladding diameter of 20/400 μm and core/cladding NA of 0.06/0.46. After the OC FBG, about 2 m passive fiber with core diameter of 20 μm and inner cladding diameter of 400 μm is spliced to deliver the output signal power to the end cap. Meanwhile, passive fiber is used for the cladding light
strippers (CLSs). Four parts of CLSs including one part in intracavity are performed to strip the residual pump and high-order signal modes, and the CLS in intracavity can also reduce the amplified spontaneous emission (ASE), ensuring reliability for high-power operation. Except the end cap and the collimator, all the devices are fixed on a watercooled plate with a special design. The measuring system consists of one highly reflecting (HR) mirror, one dichroic mirror (DM), one collimator, one 10 kW 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 small transmitted laser is used to measure the BQ by the wavefront analyzer and analyze the laser beam by the beam analyzer. Most of the laser is reflected by the HR mirror and measured by the 10 kW power meter. The spectra are measured through scattered light by the OSA. 3. Experimental Results and Theoretical Analysis
First, we measure the entire spectrum of the pump light after the combiner, as shown in Fig. 2(a). The center wavelength of spectra shifts to the long wavelength when increasing the pump power. Moreover, the spectral bandwidth is so broad that the traditional rate equation model only considering a single pump wavelength cannot achieve accurate results to guide the design of this laser oscillator. So we established one rate equation model consisting of fiber bending loss [9], mode coupling [10], changing pump wavelengths, and cladding light stripping. In order to simplify the numerical calculation, we assume that the various modes of different signal wavelength have the same absorption loss and scattering loss coefficients. The model developed by us can be expressed by the following steady-state rate equations [10,11]:
Fig. 1. Architecture of the all-fiber laser oscillator. 8056
APPLIED OPTICS / Vol. 53, No. 34 / 1 December 2014
Fig. 2. (a) Spectra of pump light at different pump powers and (b) signal/pump power along the longitudinal direction of the fiber at the pump power of 2054 W.
N 2 z N ×
Γp
PN s as s s s p ap p p p p− p s− s m1 λm σ m λm Pm λm ; z Pm λm ; z Γs n1 λn σ n λn Pn;k λn ; LPk ; z Pn;k λn ; LPk ; z PN s as s p ap p ep p p p p− p ℏcAeff s s es s s− s m1 λm σ m λm σ m λm Pm λm ; z Pm λm ; z τ Γs n1 λn σ n λn σ n λn Pn;k λn ; LPk ; z Pn;k λn ; LPk ; z
PM
Γp
PM
(1.1)
p dPp m λm ; z p ap p p p p p p p Γp σ ep m λm N 2 z − σ m λm N 1 zPm λm ; z − αm λm Pm λm ; z dz
(1.2)
s dPs n;k λn ; LPk ; z s s s as s s s sRS s Γs σ es n λn N 2 z − σ n λn N 1 zPn;k λn ; LPk ; z − αn λn αn λn dz s s s αbl k λn ; LPk ; zPn;k λn ; LPk ; z s − Ps n;k λn ; LPj ; z
K X j≠k
N N 1 z N 2 z;
dkj
K X j≠k
(1.4)
where N is the doping concentration; N 1 and N 2 are the populations of the lower and upper lasing levels, respectively; λpm and λsn are the mth pump wavelength and the nth signal wavelength, respecp tively; Pp m λm ; z represents the forward and backs ward pump power; Ps n;k λn ; LPk ; z represents the forward and backward signal powers of different ep transverse modes; σ ap m and σ m are the absorption and emission cross sections of pump light and σ as n and σ es n represent the absorption and emission cross sections of signal light; αsn λsn , αsn RS λsn , and s αbl k λn ; LPk ; z are the absorption loss, Rayleigh scattering, and bending loss coefficients, respectively [9]; dkj is the mode coupling coefficient between the kth and jth modes [10]; Γp and Γs are the power filling factors of pump and signal light [12]; Aeff is the effective mode area of the fiber core; and τ is the lifetime of upper levels.
s s dkj Ps n;j λn ; LPj ; z 2Γs σ e λn ; LPk N 2 z
ℏc2 Δλs λsn 3
(1.3)
Numerical simulation is based on the actually measured spectral data of pump light. The simulation results are shown in Fig. 2(b). It can be seen that the YDF with length of 40 m is enough to absorb the pump light of 2054 W and the signal power can reach a maximum value without obvious re-absorption. In fact, the fiber length selected in our laser oscillator is 41 m due to the possibility of multiple fusions. After the all-fiber laser oscillator is established, we measure its features with details, such as power, spectra, and BQ. As shown in Fig. 3, the measured experimental results are in good agreement with the numerical simulation, confirming the accuracy of our theoretical model. The maximum output power reaches to 1520 W when increasing the pump power to 2054 W, and the corresponding optical-tooptical efficiency is 74%, as shown in Fig. 3(a). The optical-to-optical efficiency at different pump power levels is shown in Fig. 3(b). The measured optical-tooptical efficiencies are higher than the simulated results at low power level, but the two curves come 1 December 2014 / Vol. 53, No. 34 / APPLIED OPTICS
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Fig. 3. Measured and simulated (a) output power and (b) optical-to-optical efficiency versus pump power.
Fig. 4. Laser spectra with (a) log and (b) linear scale at different output powers.
Fig. 5. (a) Measured BQ represented by M 2 factor at various output powers and (b) beam profile at maximum output power of 1520 W.
close to each other when the power is increased. The output spectra are shown in Fig. 4 with log and linear scale. There is no unabsorbed pump light in the spectra owing to the effective cladding light stripping. The spectra are broadened with the increase of pump strength due to nonlinear effects, for instance, selfphase modulation and the Raman gain process, and the 3 dB spectral bandwidth at maximum output power is 4.5 nm. When the output power reaches 8058
APPLIED OPTICS / Vol. 53, No. 34 / 1 December 2014
1030 W, SRS is observed, as clearly shown in Fig. 4(a). The Raman power level increases a lot when further increasing the pump power, but its power ratio is only 4% at maximum output power, as shown in Fig. 4(b), which is not detrimental to applications that have much greater demand on the power level. The YDF used in the laser cavity supports two signal modes. In order to improve the BQ, mode
selection techniques, such as coiling the fiber and cladding mode stripper, are adopted in our laser oscillator to guarantee single-mode operation. The BQ represented by M 2 factors is measured by our wavefront analyzer. Figure 5(a) shows the neardiffraction-limited BQ of the laser oscillator at various laser power levels. Both of the M 2 factors in the X and Y directions are measured to be below 1.2. The beam profile at 1520 W is shown in Fig. 5(b), which is near Gaussian beam. In our experiment, we monitor the temperature of all splices and CLSs with real-time measurement. Usually, the most important splice is the one between the HR FBG and YDF because of the highest thermal load, whose temperature is below 57°C and remains stable in our experiment. Therefore, the limited factor for further power enhancement is not the thermal load but the SRS. Our following work will focus on optimizing the oscillator parameters, such as the fiber length, to suppress SRS. Further power scaling to 2 kW will hopefully be achieved. 4. Conclusion
In summary, we have reported the development of a reliable monolithic single-end-pumped all-fiber laser oscillator with 1.5 kW power output operating at 1070 nm. When the pump power is 2054 W, the laser output power is 1520 W with optical-to-optical efficiency of 74% in agreement with numerical simulation. The theoretical model can be used to guide the design of the fiber laser with high accuracy. By coiling the fiber and cladding mode stripper, neardiffraction-limited BQ is achieved with M 2 factors less than 1.2 in the X and Y directions. The SRS is the main limitation to hamper further power scaling, and will be suppressed by effective methods in our following work, such as reducing the fiber length by using a higher doping concentration LMA fiber. This prototype laser can be applied in the industrial processing area.
The authors thank Qiang Wang, Shui Zhao, and Hanwei Zhang for providing helpful assistance in carrying out the experiments. The research leading to these results has received funding from the Projects of Scientific Research Foundation of National University of Defense Technology (Grant No. JC1207-03) and the Scientific Research Program of Education Department in Hunan Province (Grant No. YB2013B003). 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. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27, B63–B92 (2010). 3. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7, 861–867 (2013). 4. http://www.ipgphotonics.com/index.htm. 5. E. Shcherbakov, V. Fomin, A. Abramov, A. Ferin, D. Mochalov, and V. P. Gapontsev, “Industrial grade 100 kW power CW fiber laser,” in Advanced Solid State Lasers Congress, OSA Technical Digest (online) (Optical Society of America, 2013), paper ATh4A.2. 6. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Laegsgaard, “Theoretical analysis of mode instability in high-power fiber amplifiers,” Opt. Express 21, 1944–1971 (2013). 7. V. Khitrov, J. D. Minelly, R. Tumminelli, V. Petit, and E. S. Pooler, “3 kW single-mode direct diode-pumped fiber laser,” Proc. SPIE 8961, 89610V(2014). 8. 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 highbrightness pump diodes,” Proc. SPIE 8237, 82370G (2012). 9. R. T. Schermer and J. H. Cole, “Improved bend loss formula verified for optical fiber by simulation and experiment,” IEEE J. Quantum Electron. 43, 899–909 (2007). 10. M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15, 3236–3246 (2007). 11. I. Kelson and A. A. Hardy, “Strongly pumped fiber lasers,” IEEE J. Quantum Electron. 34, 1570–1577 (1998). 12. O. G. Okhotnikov, Fiber Lasers (Wiley-VCH, 2012).
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