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Experimental study on transverse-mode controlling with coated concave mirror for large-mode-area fiber laser
Optik 124 (2013) 3723–3725
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Experimental study on transverse-mode controlling with coated concave mirror for large-mode-area fiber laser Dong Xue ∗ , Yang Liu Zaozhuang University, Zaozhuang 277100, PR China
a r t i c l e
i n f o
Article history: Received 7 July 2012 Accepted 12 November 2012
Keywords: Fiber laser Large-mode-area Mode control
a b s t r a c t A new mode control method is introduced in the linear cavity large-mode-area (LMA) fiber laser. In our experiment, the cavity mirror was replaced by a coated concave mirror which only reflects the lower order lasers propagating along the fiber axis. And the higher order modes of the multimode LMA fiber are suppressed according to the mode competition theory. The beam quality gets better at the expense of slope efficiency. Output power of 12 Ws is obtained with M2 of 1.20, which is much better than the uncontrolled laser beam with M2 of 2.65. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction
2. Experiment and results
Fiber lasers are now mature products and have numerous applications in medical, military, industrial processing and modern telecommunication because of some unique advantages including high conversion efficiency, excellent beam quality, less thermal effect, small volume and weight, etc. Double-clad fiber laser (DCFL) overcome the problem of power launching and made high power fiber lasers possible [1,2]. Whereas some nonlinear processes like stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), self-phase modulation and amplified spontaneous emission (ASE) still limited the power scaling [2,3]. Therefore large-modearea (LMA) fiber with low numerical aperture (NA) was applied to overcome the nonlinearity [4,5]. However, such LMA fiber is multimode and can guide some higher order transverse modes, which debase the beam quality. In order to suppress the higher order mode and obtain near diffraction-limited beam quality, many methods are designed in the previous work [3,6–9]. In our group, we tried several methods like bending, coiling, tapering the fiber to suppress the higher mode. They all do well in mode controlling except that the fiber, especially the thick LMA fiber, is easy to get broken off or damaged. The concave mirror method demonstrated in this paper is more safe and simple in operation.
The scheme of the experimental setup is shown in Fig. 1. The LMA double clad fiber (DCF) used in this experiment was designed and fabricated by Fiberhome Telecommunication Tech Co. Ltd., China. The fiber has a Yb-doped core with diameter of 43 m and a D-shaped inner cladding with diameter of 450/400 m for the longer/shorter axis. The NA of the Yb-doped core is ∼0.10 (V ≈ 12.7 at 1064 nm). The length of the fiber is 20 m. The experiments setup is shown in Fig. 1. Because cutting and grinding a fiber end with certain angle is not reliable and feasible, both ends of the fiber were perpendicularly cleaved relative to the fiber axis. The fiber was cladding-pumped by two diode stacks emitting at ∼975 nm with 5 nm spectral width. The spatial filter was used to improve the beam quality of the pump light and an aspheric was specially designed to focus the pump beams into inner cladding. In order to pass the pump light and reflect the laser, dichroic concave mirror (975 nm, T ∼ 95%, 1080–1150 nm, R > 99.8%, radius of 10 mm) which is coated only in the center area (with diameter D of 1 mm, 2 mm, and 3 mm respectively) is placed in the cavity working as the feedback mirror. Another dichroic mirror is employed as output mirror and filter to the residual pump light. The beam quality of the output laser is measured by a Beam Progagation Analyzer (Spiricon, stepI-IIB, M2 -200) with knife edge method [9,10]. The output power with respect to the launched pump power and the measured results of beam quality factor M2 is illustrated in Fig. 2. By using a concave mirror with coating area diameter of 1 mm, the output laser beam quality factor M2 is 1.20, which is much better than M2 of 2.65 with regular plane mirror.
∗ Corresponding author. E-mail address: [email protected] (D. Xue). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.02.023
3724
D. Xue, Y. Liu / Optik 124 (2013) 3723–3725
25 20
Output power (W)
2
M =2.65
plane 3mm 2mm 1mm
2
M =2.12 2
15
M =1.73 2
M =1.20
10
Fig. 4. The mode patterns observed in the experiment.
5 0 25
30
35
40
45
Launched power (W) Fig. 1. Scheme of the experimental setup.
Aspheric lens Dichroic
LD Yb-doped DCF Spatial filter
Output
Dichroic concave
Fig. 2. The slope efficiency and M2 factor with different concave mirror.
Fig. 5. 3D display of the fiber mode before (a) and after (b) controlled. (a) Beam profile with regular plane mirror. (b) Beam profile with concave mirror D = 1 mm.
in Fig. 5(b). The beam profile of uncontrolled laser is also displayed in Fig. 5(a) for comparison. The hollow ring pattern results from the overlaying of many higher-order modes. Fig. 3. The cutoff condition and power distribution of several LP modes.
4. Conclusions 3. Discussions According to the mode theory of circular waveguides, the E&M fields in cylindrical coordinates can be express as Bessel function with variable parameters U and W [11,12]. U2 2 = k0 n1 2 − ˇ2 , a2
W2 2 = ˇ2 − k0 n2 2 a2
where ˇ is the propagation constant of electromagnetic waves in fiber. The U and W is real number only if: k0 n2 < ˇ < k0 n1 With the weakly guided approximation (n1 ≈ n2 ), the cutoff condition and power distribution of several lower LP mode is calculated and drawn in Fig. 3. As shown in the picture, the lower modes according to the smaller U, or smaller propagation angle , or concentrate more power in the center, which can be judged from the mode pattern. If the cavity mirror is designed to feedback only the lower order light propagating along the central axis of fiber, the higher order mode will not be oscillated. The working mechanism is somewhat alike the tapering fiber. With the Beam Progagation Analyzer, some mode patterns were observed in the experiment, which is shown in Fig. 4. They are corresponding to the LP01 , LP11 , and LP21 respectively. Obviously, the lower order modes carry with the smaller propagation angle. By carefully adjudging the concave mirror, reflecting only the LP01 mode back into the cavity, raising the loss of high-order transverse mode, the multimode LMA fiber laser was well modecontrolled .The power distribution with Gauss profile is displayed
In summary, we have demonstrated a new mode control method with coated concave mirror on LMA Yb-doped fiber laser, which is used to only reflect lower order mode back into the cavity, so the beam quality of the multimode fiber laser gets better. Because the concave is placed out of the cavity, the fiber is untouched and with no risk of damage. Nevertheless, the slope efficiency is dropped slightly since only part of the power is feed backed. Finally, we demonstrate 12 Ws output with M2 = 1.2. The experimental result agrees well with the theoretically result. Acknowledgments The authors gratefully acknowledge the support from the China Foundation for Outstanding Young Scientist in Shandong Province (No. 2008BS0121). References [1] H.M. Pask, R.J. Carman, D.C. Hanna, A.C. Trpper, C.J. Mackechnie, P.R. Barber, J.M. Dawes, Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 pm region, IEEE J. Select. Top. Quant. Electron. 1 (1995) 2–13. [2] V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P.S. Yeh, E. Zucker, Operating of cladding-pumped Yb3+-doped silica fibre lasers in 1 m region, Electron. Lett. 35 (1999) 1158–1160. [3] J.P. Koplow, D.A.V. Kliner, L. Goldberg, Single-mode operation of a coiled multimode fiber amplifier, Opt. Lett. 25 (2000) 442–444. [4] J. Limpert, F. Roser, T. Schreiber, A. Tunnermann, High-power ultrafast fiber laser systems, Quantum Electron. 12 (2006) 233–244;
D. Xue, Y. Liu / Optik 124 (2013) 3723–3725 J. Limpert, F. Roser, T. Schreiber, A. Tunnermann, High-power ultrafast fiber laser systems, IEEE J. Select. Top. Quant. Electron. 12 (2006) 233–244. [5] D. Xue, Q. Lou, J. Zhou, Comparison of one-sided and double-sided pumping configuration, Opt. Laser Technol. 39 (2007) 871–874. [6] H.L. Offerhaus, N.G. Broderick, D.J. Richardson, R. Sammut, J. Caplen, L. Dong, Picosecond pulse amplification in a core pumped large-mode area erbium fiber, Opt. Lett. 23 (1998) 1683–1685. [7] J.M. Sousa, O.G. Okhotnikov, Multimode Er-doped fiber for single-transversemode amplification, Appl. Phys. Lett. 74 (1999) 1528–1530.
3725
[8] O.G. Okhotnikov, J.M. Sousa, Fibre optic bandpass transversal filter employing fibre grating arrays, Electron. Lett. 35 (1999) 1010–1011. [9] L. Li, Q. Lou, J. Zhou, J. Dong, Y. Wei, J. Li, High power low-order modes operation of a multimode fiber laser, Chin. Opt. Lett. 5 (2007) 221–222. [10] T.F. Johnston Jr., Beam propagation (M2) measurement made as easy as it gets: the four-cuts method, Appl. Opt. 37 (1998) 4840–4850. [11] G.P. Agrawal, Nonlinear Fiber Optics, Academic Press Inc., New York, USA, 1989. [12] A.K. Ghatak, K. Thyagarajan, An Introduction to Fiber Optics, Cambridge University Press, Cambridge UK, 2000.
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Experimental study on transverse-mode controlling with coated concave mirror for large-mode-area fiber laser Dong Xue ∗ , Yang Liu Zaozhuang University, Zaozhuang 277100, PR China
a r t i c l e
i n f o
Article history: Received 7 July 2012 Accepted 12 November 2012
Keywords: Fiber laser Large-mode-area Mode control
a b s t r a c t A new mode control method is introduced in the linear cavity large-mode-area (LMA) fiber laser. In our experiment, the cavity mirror was replaced by a coated concave mirror which only reflects the lower order lasers propagating along the fiber axis. And the higher order modes of the multimode LMA fiber are suppressed according to the mode competition theory. The beam quality gets better at the expense of slope efficiency. Output power of 12 Ws is obtained with M2 of 1.20, which is much better than the uncontrolled laser beam with M2 of 2.65. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction
2. Experiment and results
Fiber lasers are now mature products and have numerous applications in medical, military, industrial processing and modern telecommunication because of some unique advantages including high conversion efficiency, excellent beam quality, less thermal effect, small volume and weight, etc. Double-clad fiber laser (DCFL) overcome the problem of power launching and made high power fiber lasers possible [1,2]. Whereas some nonlinear processes like stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), self-phase modulation and amplified spontaneous emission (ASE) still limited the power scaling [2,3]. Therefore large-modearea (LMA) fiber with low numerical aperture (NA) was applied to overcome the nonlinearity [4,5]. However, such LMA fiber is multimode and can guide some higher order transverse modes, which debase the beam quality. In order to suppress the higher order mode and obtain near diffraction-limited beam quality, many methods are designed in the previous work [3,6–9]. In our group, we tried several methods like bending, coiling, tapering the fiber to suppress the higher mode. They all do well in mode controlling except that the fiber, especially the thick LMA fiber, is easy to get broken off or damaged. The concave mirror method demonstrated in this paper is more safe and simple in operation.
The scheme of the experimental setup is shown in Fig. 1. The LMA double clad fiber (DCF) used in this experiment was designed and fabricated by Fiberhome Telecommunication Tech Co. Ltd., China. The fiber has a Yb-doped core with diameter of 43 m and a D-shaped inner cladding with diameter of 450/400 m for the longer/shorter axis. The NA of the Yb-doped core is ∼0.10 (V ≈ 12.7 at 1064 nm). The length of the fiber is 20 m. The experiments setup is shown in Fig. 1. Because cutting and grinding a fiber end with certain angle is not reliable and feasible, both ends of the fiber were perpendicularly cleaved relative to the fiber axis. The fiber was cladding-pumped by two diode stacks emitting at ∼975 nm with 5 nm spectral width. The spatial filter was used to improve the beam quality of the pump light and an aspheric was specially designed to focus the pump beams into inner cladding. In order to pass the pump light and reflect the laser, dichroic concave mirror (975 nm, T ∼ 95%, 1080–1150 nm, R > 99.8%, radius of 10 mm) which is coated only in the center area (with diameter D of 1 mm, 2 mm, and 3 mm respectively) is placed in the cavity working as the feedback mirror. Another dichroic mirror is employed as output mirror and filter to the residual pump light. The beam quality of the output laser is measured by a Beam Progagation Analyzer (Spiricon, stepI-IIB, M2 -200) with knife edge method [9,10]. The output power with respect to the launched pump power and the measured results of beam quality factor M2 is illustrated in Fig. 2. By using a concave mirror with coating area diameter of 1 mm, the output laser beam quality factor M2 is 1.20, which is much better than M2 of 2.65 with regular plane mirror.
∗ Corresponding author. E-mail address: [email protected] (D. Xue). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.02.023
3724
D. Xue, Y. Liu / Optik 124 (2013) 3723–3725
25 20
Output power (W)
2
M =2.65
plane 3mm 2mm 1mm
2
M =2.12 2
15
M =1.73 2
M =1.20
10
Fig. 4. The mode patterns observed in the experiment.
5 0 25
30
35
40
45
Launched power (W) Fig. 1. Scheme of the experimental setup.
Aspheric lens Dichroic
LD Yb-doped DCF Spatial filter
Output
Dichroic concave
Fig. 2. The slope efficiency and M2 factor with different concave mirror.
Fig. 5. 3D display of the fiber mode before (a) and after (b) controlled. (a) Beam profile with regular plane mirror. (b) Beam profile with concave mirror D = 1 mm.
in Fig. 5(b). The beam profile of uncontrolled laser is also displayed in Fig. 5(a) for comparison. The hollow ring pattern results from the overlaying of many higher-order modes. Fig. 3. The cutoff condition and power distribution of several LP modes.
4. Conclusions 3. Discussions According to the mode theory of circular waveguides, the E&M fields in cylindrical coordinates can be express as Bessel function with variable parameters U and W [11,12]. U2 2 = k0 n1 2 − ˇ2 , a2
W2 2 = ˇ2 − k0 n2 2 a2
where ˇ is the propagation constant of electromagnetic waves in fiber. The U and W is real number only if: k0 n2 < ˇ < k0 n1 With the weakly guided approximation (n1 ≈ n2 ), the cutoff condition and power distribution of several lower LP mode is calculated and drawn in Fig. 3. As shown in the picture, the lower modes according to the smaller U, or smaller propagation angle , or concentrate more power in the center, which can be judged from the mode pattern. If the cavity mirror is designed to feedback only the lower order light propagating along the central axis of fiber, the higher order mode will not be oscillated. The working mechanism is somewhat alike the tapering fiber. With the Beam Progagation Analyzer, some mode patterns were observed in the experiment, which is shown in Fig. 4. They are corresponding to the LP01 , LP11 , and LP21 respectively. Obviously, the lower order modes carry with the smaller propagation angle. By carefully adjudging the concave mirror, reflecting only the LP01 mode back into the cavity, raising the loss of high-order transverse mode, the multimode LMA fiber laser was well modecontrolled .The power distribution with Gauss profile is displayed
In summary, we have demonstrated a new mode control method with coated concave mirror on LMA Yb-doped fiber laser, which is used to only reflect lower order mode back into the cavity, so the beam quality of the multimode fiber laser gets better. Because the concave is placed out of the cavity, the fiber is untouched and with no risk of damage. Nevertheless, the slope efficiency is dropped slightly since only part of the power is feed backed. Finally, we demonstrate 12 Ws output with M2 = 1.2. The experimental result agrees well with the theoretically result. Acknowledgments The authors gratefully acknowledge the support from the China Foundation for Outstanding Young Scientist in Shandong Province (No. 2008BS0121). References [1] H.M. Pask, R.J. Carman, D.C. Hanna, A.C. Trpper, C.J. Mackechnie, P.R. Barber, J.M. Dawes, Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 pm region, IEEE J. Select. Top. Quant. Electron. 1 (1995) 2–13. [2] V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P.S. Yeh, E. Zucker, Operating of cladding-pumped Yb3+-doped silica fibre lasers in 1 m region, Electron. Lett. 35 (1999) 1158–1160. [3] J.P. Koplow, D.A.V. Kliner, L. Goldberg, Single-mode operation of a coiled multimode fiber amplifier, Opt. Lett. 25 (2000) 442–444. [4] J. Limpert, F. Roser, T. Schreiber, A. Tunnermann, High-power ultrafast fiber laser systems, Quantum Electron. 12 (2006) 233–244;
D. Xue, Y. Liu / Optik 124 (2013) 3723–3725 J. Limpert, F. Roser, T. Schreiber, A. Tunnermann, High-power ultrafast fiber laser systems, IEEE J. Select. Top. Quant. Electron. 12 (2006) 233–244. [5] D. Xue, Q. Lou, J. Zhou, Comparison of one-sided and double-sided pumping configuration, Opt. Laser Technol. 39 (2007) 871–874. [6] H.L. Offerhaus, N.G. Broderick, D.J. Richardson, R. Sammut, J. Caplen, L. Dong, Picosecond pulse amplification in a core pumped large-mode area erbium fiber, Opt. Lett. 23 (1998) 1683–1685. [7] J.M. Sousa, O.G. Okhotnikov, Multimode Er-doped fiber for single-transversemode amplification, Appl. Phys. Lett. 74 (1999) 1528–1530.
3725
[8] O.G. Okhotnikov, J.M. Sousa, Fibre optic bandpass transversal filter employing fibre grating arrays, Electron. Lett. 35 (1999) 1010–1011. [9] L. Li, Q. Lou, J. Zhou, J. Dong, Y. Wei, J. Li, High power low-order modes operation of a multimode fiber laser, Chin. Opt. Lett. 5 (2007) 221–222. [10] T.F. Johnston Jr., Beam propagation (M2) measurement made as easy as it gets: the four-cuts method, Appl. Opt. 37 (1998) 4840–4850. [11] G.P. Agrawal, Nonlinear Fiber Optics, Academic Press Inc., New York, USA, 1989. [12] A.K. Ghatak, K. Thyagarajan, An Introduction to Fiber Optics, Cambridge University Press, Cambridge UK, 2000.