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Experimental study on phase-locking of two high-power all-fiber lasers
Optics Communications 283 (2010) 2390–2393
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Experimental study on phase-locking of two high-power all-fiber lasers Baoyin Zhao a,b, Kailang Duan a,*, Yang Liu c, Wei Zhao a a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Wuhan Ordnance Noncommissioned Officers Academy, Wuhan 430075, China b
a r t i c l e
i n f o
Article history: Received 6 October 2009 Received in revised form 1 January 2010 Accepted 1 February 2010
Keywords: High-power all-fiber laser Mutual injection phase-locking Coherent combining
a b s t r a c t By using a novel mutual injection technique, phase-locking and coherent combining of two high-power all-fiber lasers are realized and experimentally demonstrated. Steady interference strips with high visibility of 46% are observed. The coherent combined 407 W CW output power with a power-combining efficiency of up to 98% is obtained. The laser array works well with excellent stability. In the long time of high-power operation no thermal distortions or damages are observed. The proposed technique can be used to further scale up the coherent combined output power of high-power fiber lasers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction High-power fiber lasers are becoming very competitive in many applications like material processing, marking, range finding, free space communication, and security [1–3] because of their advantages of compactness, reliability, efficiency, and beam quality [4]. The output power of fiber lasers has shown a dramatically increase in output power levels due to the development of pump technology and availability of double-cladding gain fiber and fiber components of excellent properties. In 2004, Jeong et al. reported an Yb-doped large-core fiber laser with 1.36 kW CW output power [3] and in 2005, Bonati et al. demonstrated a 1.53 kW LMA PCF based fiber laser [5]. Recently, IPG Photonics announced the successful test of a single-mode fiber laser with 9.6 kW output power [6]. However, the possible risk of the thermal and optical damages of the fiber and fiber components is always a shadow that looms in the mind of the scientists who are trying to upgrade the output power level of fiber lasers. Combining lots of low power fiber lasers into a high-power one has been put forward as a promising technology to achieve highpower fiber laser [7,8]. Up to now various kinds of combining techniques have been investigated, including both of the wavelength combining technique and the coherent combining techniques. By using the wavelength combining technique [9], several fiber lasers can be arranged to propagate with one same propagating axis, thus the beam quality of the final laser can be maintained as good as each single laser. However, the output power of wavelength * Corresponding author. Tel.: +86 29 88887617. E-mail address: [email protected] (K. Duan). 0030-4018/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.02.004
combining technique is limited by the damage threshold and the limited diffraction orders of the diffraction grating. Many kinds of coherent combining techniques or mechanisms have been investigated principally in experiments. According to their phase-locking methods, they can be classified into two categories: active and passive phase-locking. The active phase-locking involves active phase detection and compensation of phase errors, which is realized by the Master oscillator power amplifier (MOPA) configuration [4,10]. The passive phase-locking can be explained by the self-organized mechanism, and has been realized by different methods such as evanescent wave coupling [11], Talbot cavity [12], self-Fourier transform cavity [13], self-image cavity [14,15], Vernier–Michelson cavity [16,17], and interferometric addition in fiber couplers [18]. Recently, Bellanger et al. demonstrated coherent beam combining of a fiber amplifier array based on self-adaptive digital holography, which can be regarded as between active and passive phase-locking [19]. Despite the differences in these concrete configurations, the final purpose of all these passive phase-locking methods is essentially to realize mutual injection of the fiber lasers. By mutual injection, the combined fiber lasers are mutually injection phase-locked. All these techniques have their inherent merits and defects, but none of them have achieved satisfied outcomes in experiments. The main drawback of the active phase-locking method lies in its complex technology to detect and modulate the phase of each beam, which is a formidable task when the number of fiber lasers is very large or the power of each laser is very high. Unlike the active phase-locking method the self-organized mechanism makes the detection and modulation of the beam phases dispensable, but many of them still faces the same questions as the single laser
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B. Zhao et al. / Optics Communications 283 (2010) 2390–2393
LD×2
LD×2
YDF1
FBG1
C2
C1
~10%@1080nm
Laser 1
EC1
L3 CCD
L1
FBG3
P
~80%@1080nm
L2 EC2
Laser 2 C3 LD×2
FBG2
C4 YDF2
f
~10%@1080nm
LD×2
Fig. 1. Schematic diagram of the experimental setup for phase-locking of two all-fiber lasers.
in the upgrading of the output power. Therefore, exploring new method of coherent combining of fiber lasers is still a challenging and significant job [20]. In this paper, we report the phase-locking of two all-fiber lasers by using a new mutual injection technology. In our experiment, a fiber Bragg grating with a relative low reflectivity of 80% is used as the common cavity reflector of the two fiber lasers. Thus the modes of one fiber laser cavity can partly leak into the other fiber laser cavity, and thereby the two fiber laser can be mutually injection phase-locked. In the experiment steady interference strips with high visibility of 46% are observed. The maximum coherent output power of 407 W with approximate 98% combining efficiency is obtained when the pump power is 657 W. Compared with the previous work [18,21–23], this new injection-locking configuration is very simple and easy to construct. It demonstrates great potential in the scaling-up of output power if there is enough pump power. 2. Experimental setup The experimental setup is shown in Fig. 1. The phase-locking laser array consists of two all-fiber lasers (Laser 1 and Laser 2). YDF1 and YDF2 are two 25 m long Yb3+-doped double clad gain fibers
with absorption coefficient being 1.7 dB/m at 975 nm. Their core and inner cladding diameters are 20 and 400 lm and the number apertures of which are 0.06 and 0.46, respectively. C1, C2, C3, and C4 are four (6 + 1) 1 multimode combiners, each of them is spliced into two diode modules with pigtailed multimode fibers. Each of eight LD modules can provide maximum pump power of 100 W at 975 nm. FBG1 and FBG2 are two fiber Bragg gratings with reflectivity of 10% at 1080 nm. They are used as the output couplers of the two fiber lasers. The fiber Bragg grating FBG3 with reflectivity of 80% is used as the common cavity reflector of Laser1 and Laser 2. Through this common cavity, the modes of one laser cavity can directly leak into resonator cavity of the other, and thereby, this two fiber lasers can be mutually phase-locked. Moreover, in this setup the three fiber Bragg gratings are assembled outside the combiners, so the power of the diode modules can be directly pumped into the gain fiber. This kind of arrangement can greatly relieve the power burden passing through the fiber gratings, and thus protect the fiber gratings from thermal damage. EC1 and EC2 are two fiber endcaps to avoid optical damage. The emitting beams from the endcaps EC1 and EC2 are collimated by two gradient refractive index lenses L1 and L2 with focal length of 11 mm. The two collimated beams are firstly arranged to propagate oppositely and then become parallel after being reflected by a right-angle prism P, which is mounted in the middle of the two lenses L1 and L2. Thus the spacing between the two parallel beams
450
0
100
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400
500
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200
250
300
350
400
Output power (W)
350 300
laser 1 laser 2
250
coherent combined
200 150 100 50 0
0
50
100
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Pump power (W) Fig. 2. Interference pattern of the two phase-locked all-fiber lasers.
Fig. 3. Output power versus pump power for the individual lasers and the all-fiber laser array.
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110
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Efficiency (%)
90
laser 1 laser 2 coherent combined
80 70 60 50 40 30 20
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50
100
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Pump power (W) Fig. 4. Optical–optical conversion efficiency of the individual lasers and the coherent combined efficiency of the laser array versus pump power.
can be changed conveniently by moving the prism forward and backward. A positive lens L3 with the focal length of 50 cm is used to make the two parallel beams convergent, and an infrared charge-coupled device (CCD) camera is placed on the rear focal plane of the lens to record the interference patterns. 3. Results and discussion We examine the superposition field of the two lasers in the farfield with an infrared CCD placed at the rear focus of lens L3. When
both of the fiber lasers are turned on, we observed clear interference strips as shown in Fig. 2, which demonstrates that the two beams have been successfully phase-locked. The distinct interference strips of the two beams shows that the two all-fiber lasers are running in high coherent state. The visibility of the interference strips is about 46%. The contrast of the strips is stable and does not decrease over the long time of observation. Fig. 3 shows the individual output power of the two all-fiber lasers and their coherent combined output power against the pump power. The two fiber lasers have threshold pump power of about 7 and 6 W, respectively, and their own output power increase linearly with increasing their pump power. When the pump power of them are 325 and 332.8 W, the maximum output power of 198.2 and 215 W are obtained correspondingly. In mutual injection phase-locking state, the all-fiber laser array generates 407 W CW coherent combined output power. At this high output power level, the fiber gratings, splice joints, combiners and the gain fibers are cooled by mounting them on active thermal sinks. With this measure, the all-fiber laser array works well with stable output during the long time of the experiment. Fig. 4 shows the optical–optical conversion efficiencies of the two fiber lasers and the coherent combined efficiency versus their pump power. The coherent combined efficiency is defined as g = P/ (P1 + P2), where P is the coherent combined output power, and P1 and P2 are the output power of the two fiber lasers, respectively. When the pump power are near the threshold values, the two fiber lasers have optical–optical conversion efficiencies of 27.5% and 30%, which increase with increasing their pump power, respectively. Under the maximum output power, the conversion efficiencies of the two fiber lasers reach to 61% and 64.6%, respectively. The power coherent combined efficiency of the two all-fiber lasers
Intensity (a.u.)
2500 2000 1500 1000 500 1050
1060
1070
1080
1090
1100
1110
Wavelength (nm) Fig. 5. Combined output spectrum at maximum output power.
LD C 11
HR-FBG
LD
YDF 1
LD
C 12
LD
YDF 2
~8 0%@1 0 8 0n m
FA ~10%@1080nm
C 21
FC
C 22
OC
HR-FBG
LD
~8 0%@1 0 8 0n m
FC
LD
YDF 3 C 31
C 32
Fig. 6. Schematic diagram of experimental setup for phase-locking of an all-fiber laser array.
B. Zhao et al. / Optics Communications 283 (2010) 2390–2393
is around 98%. The combined output spectra, which mainly depend on reflection characteristics of the fiber gratings, is centered at 1081 nm, and the full-width at half-maximum of the output spectra is about 2 nm, as shown in Fig. 5. Fig. 1 can be expanded to phase-lock a fiber laser array. For example, three fiber lasers can be mutually injected by using the scheme as shown in Fig. 6, where FCs are 1 2 fiber couplers and OC is an optical collimator. With this scheme the coherent combined output power of fiber lasers can be further scaled up. 4. Conclusion We have successfully realized phase-locking of two high-power all-fiber lasers by using a novel mutual injection technique. This technique uses one fiber Bragg grating with a reflectivity of 80% as the common cavity reflectors of the two fiber lasers. Thereby, the cavity modes of the two fiber lasers can directly leak into each other and the mutual injection phase-locking of the two fiber lasers can be achieved. Because phase-locking in this configuration is realized by all-fiber components, the overall efficiency of the system is very high. The laser array works stably with high coherent combined power output over the long time of observation, and there are no thermal distortions and fiber damages observed. In the experiment 407 W CW coherent combined power is obtained, which will be updated with more pump sources in our future work. Acknowledgement This work was supported by the Chinese Academy of Sciences under Grant 2006LH02 and the National Key Natural Science Foundation of China under Grant 60537060.
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References [1] V.P. Gapontsev, I.E. Samartsev, OSA Proc. Adv. Solid-State Lasers 6 (1990) 258. [2] G.J. Hu, C.Y. Shan, X.Y. Deng, J. Zhan, Y.Z. Pan, L.J. Wang, Opt. Laser Technol. 37 (1) (2004) 3. [3] Y. Jeong, J.K. Sahu, D.N. Payne, J. Nilsson, Opt. Express 12 (2004) 6088. [4] S. Demoustier, C. Bellanger, A. Brignon, J.P. Huignard, Fiber Integrated Opt. 27 (2008) 392. [5] G. Bonati, H. Voelckel, T. Gabler, U. Krause, A. Tünnermann, J. Limpert, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, Proc. SPIE 5709 (2005) 2a. [6] IPG Photonics, IPG Photonics Successfully Tests World’s First 10 Kilowatt Single-mode Production Laser, (15.06.09). [7] S.J. Augst, J.K. Ranka, T.Y. Fan, A. Sanchez, J. Opt. Soc. Am. B 24 (8) (2007) 1707. [8] T.Y. Fan, IEEE J. Sel. Top. Quantum Electron. 11 (3) (2005) 567. [9] A. Liu, R. Mead, T. Vatter, A. Henderson, R. Stafford, Proc. SPIE 5335 (2004) 81. [10] J. Anderegg, S. Brosnan, E. Cheung, P. Epp, D. Hammons, H. Komine, M. Weber, M. Wickham, Proc. SPIE 6102 (2006) 61020U. [11] Y.M. Huo, P.K. Cheo, J. Opt. Soc. Am. B 22 (11) (2005) 2345. [12] M. Wrage, P. Glas, M. Leitner, Opt. Lett. 26 (13) (2001) 980. [13] C.J. Corcoran, F. Durville, Appl. Phys. Lett. 86 (20) (2005) 201118. [14] L.P. Liu, Y. Zhou, F.T. Kong, Y.C. Chen, K.K. Lee, Appl. Phys. Lett. 85 (21) (2004) 4837. [15] B. He, Q.H. Lou, J. Zhou, J.X. Dong, Y.R. Wei, D. Xue, Y.F. Qi, Z.P. Su, L.B. Li, F.P. Zhang, Opt. Express 14 (7) (2006) 2721. [16] D. Sabourdy, V. Kermene, A. Desfarges-Berthelemot, M. Vampouille, A. Barthelemy, Appl. Phys. B: Lasers Opt. 75 (4–5) (2002) 503. [17] Q.J. Peng, Z.P. Sun, Y.H. Chen, L. Guo, Y. Bo, X.D. Yang, Z.Y. Xu, Opt. Lett. 30 (12) (2005) 1485. [18] A. Shirakawa, T. Saitou, T. Sekiguchi, K. Ueda, Opt. Express 10 (21) (2002) 1167. [19] C. Bellanger, A. Brignon, J. Colineau, J.P. Huignard, Opt. Lett. 33 (24) (2008) 2937. [20] B. Lei, Y. Feng, Opt. Commun. 281 (4) (2008) 739. [21] B. Lei, Y. Feng, Opt. Express 15 (25) (2007) 17114. [22] Z.L. Chen, P. Zhou, X.L. Wang, X. Li, J. Hou, X.J. Xu, Z.F. Jiang, Z.J. Liu, Opt. Laser Technol. 41 (6) (2009) 710. [23] Y. Liu, Y. Chen, L. Xu, R. Zhen, X. Wang, Y. Lu, Y. Guo, Q. Wan, H. Wang, B. Sun, IEEE J. Electron. Lett. 45 (8) (2009) 399.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Experimental study on phase-locking of two high-power all-fiber lasers Baoyin Zhao a,b, Kailang Duan a,*, Yang Liu c, Wei Zhao a a
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Wuhan Ordnance Noncommissioned Officers Academy, Wuhan 430075, China b
a r t i c l e
i n f o
Article history: Received 6 October 2009 Received in revised form 1 January 2010 Accepted 1 February 2010
Keywords: High-power all-fiber laser Mutual injection phase-locking Coherent combining
a b s t r a c t By using a novel mutual injection technique, phase-locking and coherent combining of two high-power all-fiber lasers are realized and experimentally demonstrated. Steady interference strips with high visibility of 46% are observed. The coherent combined 407 W CW output power with a power-combining efficiency of up to 98% is obtained. The laser array works well with excellent stability. In the long time of high-power operation no thermal distortions or damages are observed. The proposed technique can be used to further scale up the coherent combined output power of high-power fiber lasers. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction High-power fiber lasers are becoming very competitive in many applications like material processing, marking, range finding, free space communication, and security [1–3] because of their advantages of compactness, reliability, efficiency, and beam quality [4]. The output power of fiber lasers has shown a dramatically increase in output power levels due to the development of pump technology and availability of double-cladding gain fiber and fiber components of excellent properties. In 2004, Jeong et al. reported an Yb-doped large-core fiber laser with 1.36 kW CW output power [3] and in 2005, Bonati et al. demonstrated a 1.53 kW LMA PCF based fiber laser [5]. Recently, IPG Photonics announced the successful test of a single-mode fiber laser with 9.6 kW output power [6]. However, the possible risk of the thermal and optical damages of the fiber and fiber components is always a shadow that looms in the mind of the scientists who are trying to upgrade the output power level of fiber lasers. Combining lots of low power fiber lasers into a high-power one has been put forward as a promising technology to achieve highpower fiber laser [7,8]. Up to now various kinds of combining techniques have been investigated, including both of the wavelength combining technique and the coherent combining techniques. By using the wavelength combining technique [9], several fiber lasers can be arranged to propagate with one same propagating axis, thus the beam quality of the final laser can be maintained as good as each single laser. However, the output power of wavelength * Corresponding author. Tel.: +86 29 88887617. E-mail address: [email protected] (K. Duan). 0030-4018/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.02.004
combining technique is limited by the damage threshold and the limited diffraction orders of the diffraction grating. Many kinds of coherent combining techniques or mechanisms have been investigated principally in experiments. According to their phase-locking methods, they can be classified into two categories: active and passive phase-locking. The active phase-locking involves active phase detection and compensation of phase errors, which is realized by the Master oscillator power amplifier (MOPA) configuration [4,10]. The passive phase-locking can be explained by the self-organized mechanism, and has been realized by different methods such as evanescent wave coupling [11], Talbot cavity [12], self-Fourier transform cavity [13], self-image cavity [14,15], Vernier–Michelson cavity [16,17], and interferometric addition in fiber couplers [18]. Recently, Bellanger et al. demonstrated coherent beam combining of a fiber amplifier array based on self-adaptive digital holography, which can be regarded as between active and passive phase-locking [19]. Despite the differences in these concrete configurations, the final purpose of all these passive phase-locking methods is essentially to realize mutual injection of the fiber lasers. By mutual injection, the combined fiber lasers are mutually injection phase-locked. All these techniques have their inherent merits and defects, but none of them have achieved satisfied outcomes in experiments. The main drawback of the active phase-locking method lies in its complex technology to detect and modulate the phase of each beam, which is a formidable task when the number of fiber lasers is very large or the power of each laser is very high. Unlike the active phase-locking method the self-organized mechanism makes the detection and modulation of the beam phases dispensable, but many of them still faces the same questions as the single laser
2391
B. Zhao et al. / Optics Communications 283 (2010) 2390–2393
LD×2
LD×2
YDF1
FBG1
C2
C1
~10%@1080nm
Laser 1
EC1
L3 CCD
L1
FBG3
P
~80%@1080nm
L2 EC2
Laser 2 C3 LD×2
FBG2
C4 YDF2
f
~10%@1080nm
LD×2
Fig. 1. Schematic diagram of the experimental setup for phase-locking of two all-fiber lasers.
in the upgrading of the output power. Therefore, exploring new method of coherent combining of fiber lasers is still a challenging and significant job [20]. In this paper, we report the phase-locking of two all-fiber lasers by using a new mutual injection technology. In our experiment, a fiber Bragg grating with a relative low reflectivity of 80% is used as the common cavity reflector of the two fiber lasers. Thus the modes of one fiber laser cavity can partly leak into the other fiber laser cavity, and thereby the two fiber laser can be mutually injection phase-locked. In the experiment steady interference strips with high visibility of 46% are observed. The maximum coherent output power of 407 W with approximate 98% combining efficiency is obtained when the pump power is 657 W. Compared with the previous work [18,21–23], this new injection-locking configuration is very simple and easy to construct. It demonstrates great potential in the scaling-up of output power if there is enough pump power. 2. Experimental setup The experimental setup is shown in Fig. 1. The phase-locking laser array consists of two all-fiber lasers (Laser 1 and Laser 2). YDF1 and YDF2 are two 25 m long Yb3+-doped double clad gain fibers
with absorption coefficient being 1.7 dB/m at 975 nm. Their core and inner cladding diameters are 20 and 400 lm and the number apertures of which are 0.06 and 0.46, respectively. C1, C2, C3, and C4 are four (6 + 1) 1 multimode combiners, each of them is spliced into two diode modules with pigtailed multimode fibers. Each of eight LD modules can provide maximum pump power of 100 W at 975 nm. FBG1 and FBG2 are two fiber Bragg gratings with reflectivity of 10% at 1080 nm. They are used as the output couplers of the two fiber lasers. The fiber Bragg grating FBG3 with reflectivity of 80% is used as the common cavity reflector of Laser1 and Laser 2. Through this common cavity, the modes of one laser cavity can directly leak into resonator cavity of the other, and thereby, this two fiber lasers can be mutually phase-locked. Moreover, in this setup the three fiber Bragg gratings are assembled outside the combiners, so the power of the diode modules can be directly pumped into the gain fiber. This kind of arrangement can greatly relieve the power burden passing through the fiber gratings, and thus protect the fiber gratings from thermal damage. EC1 and EC2 are two fiber endcaps to avoid optical damage. The emitting beams from the endcaps EC1 and EC2 are collimated by two gradient refractive index lenses L1 and L2 with focal length of 11 mm. The two collimated beams are firstly arranged to propagate oppositely and then become parallel after being reflected by a right-angle prism P, which is mounted in the middle of the two lenses L1 and L2. Thus the spacing between the two parallel beams
450
0
100
200
300
400
500
600
700
200
250
300
350
400
Output power (W)
350 300
laser 1 laser 2
250
coherent combined
200 150 100 50 0
0
50
100
150
Pump power (W) Fig. 2. Interference pattern of the two phase-locked all-fiber lasers.
Fig. 3. Output power versus pump power for the individual lasers and the all-fiber laser array.
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110
0
100
200
300
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90
laser 1 laser 2 coherent combined
80 70 60 50 40 30 20
0
50
100
150
Pump power (W) Fig. 4. Optical–optical conversion efficiency of the individual lasers and the coherent combined efficiency of the laser array versus pump power.
can be changed conveniently by moving the prism forward and backward. A positive lens L3 with the focal length of 50 cm is used to make the two parallel beams convergent, and an infrared charge-coupled device (CCD) camera is placed on the rear focal plane of the lens to record the interference patterns. 3. Results and discussion We examine the superposition field of the two lasers in the farfield with an infrared CCD placed at the rear focus of lens L3. When
both of the fiber lasers are turned on, we observed clear interference strips as shown in Fig. 2, which demonstrates that the two beams have been successfully phase-locked. The distinct interference strips of the two beams shows that the two all-fiber lasers are running in high coherent state. The visibility of the interference strips is about 46%. The contrast of the strips is stable and does not decrease over the long time of observation. Fig. 3 shows the individual output power of the two all-fiber lasers and their coherent combined output power against the pump power. The two fiber lasers have threshold pump power of about 7 and 6 W, respectively, and their own output power increase linearly with increasing their pump power. When the pump power of them are 325 and 332.8 W, the maximum output power of 198.2 and 215 W are obtained correspondingly. In mutual injection phase-locking state, the all-fiber laser array generates 407 W CW coherent combined output power. At this high output power level, the fiber gratings, splice joints, combiners and the gain fibers are cooled by mounting them on active thermal sinks. With this measure, the all-fiber laser array works well with stable output during the long time of the experiment. Fig. 4 shows the optical–optical conversion efficiencies of the two fiber lasers and the coherent combined efficiency versus their pump power. The coherent combined efficiency is defined as g = P/ (P1 + P2), where P is the coherent combined output power, and P1 and P2 are the output power of the two fiber lasers, respectively. When the pump power are near the threshold values, the two fiber lasers have optical–optical conversion efficiencies of 27.5% and 30%, which increase with increasing their pump power, respectively. Under the maximum output power, the conversion efficiencies of the two fiber lasers reach to 61% and 64.6%, respectively. The power coherent combined efficiency of the two all-fiber lasers
Intensity (a.u.)
2500 2000 1500 1000 500 1050
1060
1070
1080
1090
1100
1110
Wavelength (nm) Fig. 5. Combined output spectrum at maximum output power.
LD C 11
HR-FBG
LD
YDF 1
LD
C 12
LD
YDF 2
~8 0%@1 0 8 0n m
FA ~10%@1080nm
C 21
FC
C 22
OC
HR-FBG
LD
~8 0%@1 0 8 0n m
FC
LD
YDF 3 C 31
C 32
Fig. 6. Schematic diagram of experimental setup for phase-locking of an all-fiber laser array.
B. Zhao et al. / Optics Communications 283 (2010) 2390–2393
is around 98%. The combined output spectra, which mainly depend on reflection characteristics of the fiber gratings, is centered at 1081 nm, and the full-width at half-maximum of the output spectra is about 2 nm, as shown in Fig. 5. Fig. 1 can be expanded to phase-lock a fiber laser array. For example, three fiber lasers can be mutually injected by using the scheme as shown in Fig. 6, where FCs are 1 2 fiber couplers and OC is an optical collimator. With this scheme the coherent combined output power of fiber lasers can be further scaled up. 4. Conclusion We have successfully realized phase-locking of two high-power all-fiber lasers by using a novel mutual injection technique. This technique uses one fiber Bragg grating with a reflectivity of 80% as the common cavity reflectors of the two fiber lasers. Thereby, the cavity modes of the two fiber lasers can directly leak into each other and the mutual injection phase-locking of the two fiber lasers can be achieved. Because phase-locking in this configuration is realized by all-fiber components, the overall efficiency of the system is very high. The laser array works stably with high coherent combined power output over the long time of observation, and there are no thermal distortions and fiber damages observed. In the experiment 407 W CW coherent combined power is obtained, which will be updated with more pump sources in our future work. Acknowledgement This work was supported by the Chinese Academy of Sciences under Grant 2006LH02 and the National Key Natural Science Foundation of China under Grant 60537060.
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References [1] V.P. Gapontsev, I.E. Samartsev, OSA Proc. Adv. Solid-State Lasers 6 (1990) 258. [2] G.J. Hu, C.Y. Shan, X.Y. Deng, J. Zhan, Y.Z. Pan, L.J. Wang, Opt. Laser Technol. 37 (1) (2004) 3. [3] Y. Jeong, J.K. Sahu, D.N. Payne, J. Nilsson, Opt. Express 12 (2004) 6088. [4] S. Demoustier, C. Bellanger, A. Brignon, J.P. Huignard, Fiber Integrated Opt. 27 (2008) 392. [5] G. Bonati, H. Voelckel, T. Gabler, U. Krause, A. Tünnermann, J. Limpert, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, Proc. SPIE 5709 (2005) 2a. [6] IPG Photonics, IPG Photonics Successfully Tests World’s First 10 Kilowatt Single-mode Production Laser,