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As is well known, a typical measure of the quality of an optical beam is the M2 parameter, but characterizing the beam quality only by M2 is insufficient. A low value of M2 is generally considered to be equivalent to the single-mode operation with a stable beam. However, even when a large amount of power is contained in high-order modes, the existence of a low value of M2is still possible. Hence, a low value of M2 does not guarantee the single-mode operation. Therefore, a new measurement technique, which aims at measuring modal content of high power fiber laser, is proposed and demonstrated in this paper. This method is named spatial and spectral Fourier transform, or F2 transform in short, and it is based on measuring Fourier transform of both spatial domain and spectral domain of output laser. The experimental set is simple in structure and high in robustness. Another advantage of the method is that it requires no prior detailed knowledge of the fiber properties. In this paper, the patterns of the high-order modes between and after Fourier transform are simulated. From the graph it is evident that the energy of spot diffuses outward and is convenient to measure. We also simulate and compare the group delay difference curve of F2 with existing S2, which are well matched with each other. Experimentally, the high-order modes are stimulated by extruding the fiber periodically, which ensures that we can measure it. Firstly, by scanning two-dimensional (2D) pattern of beam after spatial domain Fourier transform and recording the experimental data, and then through the Fourier transform of data in spectral domain, the group delay differences between the high-order modes and the fundamental mode can be obtained. Finally, different modes in spatial domain are reconstructed and the relative power of every mode is calculated. Additionally, we set up an automatic measuring device to verify the effectiveness of the method. The reconstructed modal patterns are presented in the final section of this paper. We can clearly identify the fundamental mode and the high-order modes, such as LP01, LP02, LP03, LP21, LP11, LP12, LP13 and LP14. It reconfirms that this method is feasible. Compared with the S2 method, this method reduces the requirement for precision of mobile platform greatly and thus it is suited to measure the modal content of high power fiber laser output beam. This technique can be effectively applied to a wide variety of measurements, such as dispersion compensator of large-mode-area fiber, bend loss measurement of the high-order modes, refractive index profiles measurement of fiber and mode convertor fiber.
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Keywords:
- fiber modes /
- F2 method based on spatial and spectral Fourier transform /
- group delay difference /
- spectral interference
[1] Liao S Y, Gong M L {2011 Infrar Laser Eng. 40 455 (in Chinese) [廖素英, 巩马理 2011 红外与激光工程 40 455]
[2] Gao L Z {2011 Opt. Commun. Technol. 9 22 (in Chinese) [高林柱 2011 光通信技术 9 22]
[3] Jauregui C, Limpert J, Tnnermann A 2013 Nat. Photon. 7 861
[4] Berdagu S, Facq P 1982 Appl. Opt. 21 1950
[5] Xu M N, Zhou G Y, Chen C, Hou S Y, Xia C M, Zhou G, Liu H Z, Liu J T, Zhang W {2015 Acta Phys. Sin. 64 234206 (in Chinese) [徐闵喃, 周桂耀, 陈成, 侯峙云, 夏长明, 周概, 刘宏展, 刘建涛, 张卫 2015 64 234206]
[6] Sakuma H, Okamoto A, Shibukawa A, Goto Y, Tomita A 2016 SPIE OPTO Pasadena, CA, February 13-18, 2016 p9774
[7] Siegman A E 1993 SPIE OPTO Stanford, California, February 11-16, 1993 p1868
[8] Yoda H, Polynkin P, Mansuripur M 2006 J. Lightwave Technol. 24 1350
[9] Wielandy S 2007 Opt. Eexpress 15 15402
[10] Poole C D, Wiesenfeld J M, Digiovanni D J, Vengsarkar A M 1994 J. Lightwave Technol. 12 1746
[11] Posey R, Phillips L, Diggs D, Sharma A 1996 Opt. Lett. 21 1357
[12] Ramachandran S, Wang Z, Yan M 2002 Opt. Lett. 27 698
[13] Szczepanek P S, Berthold J W 1978 Appl. Opt. 17 3245
[14] Schimpf D, Barankov R, Ramachandran S 2011 Opt. Eexpress 19 13008
[15] Flamm D, Naidoo D, Schulze C, Forbes A, Duparr M 2012 Opt. Lett. 37 2478
[16] Ma Y, Sych Y, Onishchukov G, Ramachandran S, Peschel U, Schmauss B, Leuchs G 2009 Appl. Phys. B 96 345
[17] Nicholson J, Yablon A, Ramachandran S, Ghalmi S 2008 Opt. Eexpress 16 7233
[18] Soldano L B, Pennings E 1995 J. Lightwave Technol 13 615
[19] Wang B, Zhang W, Bai Z, Zhang L, Yan T, Chen L, Zhou Q 2016 Photonics Technol. Lett 28 71
[20] Hu L L, Feng G Y, Dong Z L 2015 Infrar Laser Eng. 44 2517 (in Chinese) [胡丽荔, 冯国英, 董哲良 2015 红外与激光工程 44 2517]
[21] Zheng X J, Ren G B, Huang L, Zheng H L 2016 Acta Phys. Sin. 65 064208 (in Chinese) [郑兴娟, 任国斌, 黄琳, 郑鹤玲 2016 65 064208]
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[1] Liao S Y, Gong M L {2011 Infrar Laser Eng. 40 455 (in Chinese) [廖素英, 巩马理 2011 红外与激光工程 40 455]
[2] Gao L Z {2011 Opt. Commun. Technol. 9 22 (in Chinese) [高林柱 2011 光通信技术 9 22]
[3] Jauregui C, Limpert J, Tnnermann A 2013 Nat. Photon. 7 861
[4] Berdagu S, Facq P 1982 Appl. Opt. 21 1950
[5] Xu M N, Zhou G Y, Chen C, Hou S Y, Xia C M, Zhou G, Liu H Z, Liu J T, Zhang W {2015 Acta Phys. Sin. 64 234206 (in Chinese) [徐闵喃, 周桂耀, 陈成, 侯峙云, 夏长明, 周概, 刘宏展, 刘建涛, 张卫 2015 64 234206]
[6] Sakuma H, Okamoto A, Shibukawa A, Goto Y, Tomita A 2016 SPIE OPTO Pasadena, CA, February 13-18, 2016 p9774
[7] Siegman A E 1993 SPIE OPTO Stanford, California, February 11-16, 1993 p1868
[8] Yoda H, Polynkin P, Mansuripur M 2006 J. Lightwave Technol. 24 1350
[9] Wielandy S 2007 Opt. Eexpress 15 15402
[10] Poole C D, Wiesenfeld J M, Digiovanni D J, Vengsarkar A M 1994 J. Lightwave Technol. 12 1746
[11] Posey R, Phillips L, Diggs D, Sharma A 1996 Opt. Lett. 21 1357
[12] Ramachandran S, Wang Z, Yan M 2002 Opt. Lett. 27 698
[13] Szczepanek P S, Berthold J W 1978 Appl. Opt. 17 3245
[14] Schimpf D, Barankov R, Ramachandran S 2011 Opt. Eexpress 19 13008
[15] Flamm D, Naidoo D, Schulze C, Forbes A, Duparr M 2012 Opt. Lett. 37 2478
[16] Ma Y, Sych Y, Onishchukov G, Ramachandran S, Peschel U, Schmauss B, Leuchs G 2009 Appl. Phys. B 96 345
[17] Nicholson J, Yablon A, Ramachandran S, Ghalmi S 2008 Opt. Eexpress 16 7233
[18] Soldano L B, Pennings E 1995 J. Lightwave Technol 13 615
[19] Wang B, Zhang W, Bai Z, Zhang L, Yan T, Chen L, Zhou Q 2016 Photonics Technol. Lett 28 71
[20] Hu L L, Feng G Y, Dong Z L 2015 Infrar Laser Eng. 44 2517 (in Chinese) [胡丽荔, 冯国英, 董哲良 2015 红外与激光工程 44 2517]
[21] Zheng X J, Ren G B, Huang L, Zheng H L 2016 Acta Phys. Sin. 65 064208 (in Chinese) [郑兴娟, 任国斌, 黄琳, 郑鹤玲 2016 65 064208]
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