Chapter 1 Overview
1.4 Motivation
Compared with conventional bulk solid-sate lasers, fiber lasers have some intrinsic merits and have seen a tremendous growth in both industrial and research markets.
Fiber lasers have excellent thermal properties. Their high ratio of heat-dissipating surface to active volume allows efficient thermal dissipation and usually they do not need active cooling. In addition, the property of wave guide tends to reduce thermal distortion of the beam and achieves excellent beam quality independent of the power.
Fiber lasers can be fabricated robustly with long lifetime stability and reliability. The cladding pump scheme has enabled the coupling of high power LDs with large NA into the fiber lasers. A pulsed laser may be advantageous over a continuous one in some applications, such as remote sensing, material processing, and medical needs [69-71], due to its higher peak power.
Recently there has been considerable interest in studying Yb doped fiber lasers because of their beneficial properties for a number of applications. With a small quantum defect (difference in the energy of the pump photons and the emitted photons), Yb doped fiber lasers are suitable for high power operation with reduced thermal loading. The relatively long upper-state lifetime of Yb enables more efficient pumping from a given diode pump source and storage of a large amount of energy which is of benefit for Q-switching operation. Many actively and passively Q-switched DC Yb fiber lasers have been reported to date [72-77]. The emission spectrum of Yb is also broad which allows wide wavelength tuning or multi-wavelength lasing [78-81].
Passive Q-switching (PQS) is a sophisticated and an efficient technique to create high-pulse-energy and high-peak-power pulses. Besides, PQS lasers are more compact and lower cost than the active Q-switching cause of that they utilize saturable absorbers (SAs) in replace of acoustic-optic or electro-optic modulators as the Q-switch.
Fiber-type SA [82-84] offers the in-line configuration, nevertheless they are restricted by modulation depth to deliver high-pulse-energy laser. Crystal-based and semi- conductor-based SAs are other choices of passive Q-switch. Their high mechanical robustness and well-developed fabrication process make them more common in Q-switched fiber lasers [85-87]. In the spectral region of 1.0~1.1 μm, Cr4+:YAG crystals [85] and InGaAs/GaAs quantum wells (QWs) [87] have been adopted to Q-switch fiber lasers. However, the output pulse energy with InGaAs SESAMs in passively Q-switched lasers are limited by the lattice mismatch with the substrate GaAs
for the spectral region of above 1.0 μm. As a consequence, the output pulse energies and the conversion efficiencies with InGaAs/GaAs QWs in passively Q-switched lasers are generally significantly lower than those with Cr4+:YAG crystals. Recently, an AlGaInAs with a periodic QW/barrier structure has been exploited to be an efficient saturable absorber for a passively Q-switched Nd:YVO4 laser [88]. Compared with InGaAsP materials, the AlGaInAs quaternary alloy with a larger conduction band offset is confirmed to offer a superior electron confinement in the 0.84-1.65 μm spectral region [89-91]. Nevertheless, AlGaInAs/InP QWs have not been employed to passively Q-switching Yb-doped fiber lasers. Therefore, it is interesting to stuy a passively Q- switching Yb-doped fiber laser with an AlGaInAs/InP QWs saturable absorber, and have a comparison between Cr4+:YAG crystal and AlGaInAs QWs.
To achieve higher pulse energy, it is necessary to enlarge the active volume of the gain medium, corresponding to the doped core size of the fiber. However, the conventional double-cladding fibers suffer from mode-quality degradation and their long lengths usually lead to long pulse widths and low peak powers. For improving these deficiencies, photonic crystal fibers have been developed to provide large single-mode cores and high absorption efficiencies. So far passively Q-switching Yb- doped photonic crystal fiber lasers with AlGaInAs QWs or Cr4+:YAG crystal as the saturable absorber have not been investigated, so I have some studies about these.
The configuration of master oscillator fiber amplifier (MOFA) consists of a seed laser and a fiber amplifier for boosting the output power. Because the output performance is affected by the low-power seed laser which can be easily modulated, shorter pulse width can be attained by shortening the cavity length of the seed laser.
Therefore, it is necessary to use the MOFA to have higher-peak-power pulse lasers.
Thus I use the MOFA to have some studies.
Reference
1. E. Snitzer, "Proposed fiber cavities for optical lasers", J. Appl. Phys. 32, 36-39 (1961).
2. E. Snitzer, "Optical maser action of Nd3+ in a barium crown glass", Phys. Rev.
Lett. 7, 444-446 (1961).
3. C. J. Koester, and E. Snitzer, "Amplification in a fiber laser", Appl. Opt. 3, 1182-1186 (1964).
4. J. Stone, and C. A. Burrus, "Neodymium-Doped Silica Lasers in End-Pumped Fiber Geometry", Appl. Phys. Lett. 23(7), 388-389 (1973).
5. Hegarty, J., Broer, M. M., Golding, B., Simpson, J. R., and MacChesney, J. B.,
“Photon Echoes Below 1 K in a Nd3-Doped Glass Fiber,” Phys. Rev. Lett. 51, 2033-2035 (1983).
6. R. J. Mears, L. Reekie, S. B. Poole, and D. N. Payne, "Neodymium-doped silica singlemode fibre laser", Electron. Lett. 21, 738-740 (1985).
7. S. B. Poole, D. N. Payne, R. J. Mears, M. E. Fermann, and R. I. Laming,
“Fabrication and characterization of low-loss optical fibers containing rare earth ions”, J. Lightwave Technol. LT-4 (7), 870-876 (1986)
8. J. E. Townsend et al., “Solution-doping technique for fabrication of rare earth doped optical fibres”, Electron. Lett. 23, 329-331 (1987)
9. E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, and B. C. McCollum, "Double-clad, offset core Nd fiber laser", in Proc. Opt. Fiber Sensors, New Orleans, 1988, post-deadline paper PD5.
10. Welch, D. F., “A Brief History of High-Power Semiconductor Lasers”, IEEE J.
Sel. Top. Quant. Electron. 6, 1470–1477 (2000).
11. H. Injeyan and G. D. Goodno, High-Power Laser Handbook, McGraw-Hill (2011).
12. Information available from DILAS Diodenlaser GmbH: http://www.dilas.com/
13. B. E. A. Saleh and M. C. Teich, Fundamentals of photonics, John Wiley & Sons, Inc. (1991).
14. A. W. Snyder and J. D. Love, Optical Waveguide Theory, Chapman and Hall, London (1983).
15. D. Gloge, “Weakly guiding fibers”, Appl. Opt. 10 (10), 2252 (1971)
16. H. F. Zeng and F. H. Xiao, "The development of Yb-doped double-clad fiber laser
and its application”, Laser Tec. 30(4), 438-441 (2006).
17. K. I. Ueda, "High power fiber lasers", in Proc. Pacific Rim Conference on Lasers and Electro-Optics (CLEO), Chiba, 2001.
18. L. Zenteno, "High power double clad fibre lasers", J. Lightwave Technol. 11(9), 1435-1447, (1993).
19. D. C. Brown and H. J. Hoffman, "Thermal, stress, and thermo-optic effects in highaverage power double-clad silica fiber lasers", IEEE J. Quantum Electron.
37(2), 207 (2001).
20. K. Lu and N. K. Dutta, "Spectroscopic properties of Yb-doped silica glass", J.
Appl. Phys. 91,576–581 (2002).
21. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. 33, 1049-1056 (1997).
22. A. Tunnermann, T. Schreiber, F. Roser, A. Liem, S. Hofer, H. Zellmer, S. Nolte, and J. Limpert, "The renaissance and bright future of fibre lasers," Journal of Physics B-Atomic Molecular and Optical Physics 38, S681-S693 (2005).
23. J. Limpert, F. Roser, S. Klingebiel, T. Schreiber, C. Wirth, T. Peschel, R.
Eberhardt, and A. Tunnermann, "The rising power of fiber lasers and amplifiers,"
Ieee Journal of Selected Topics in Quantum Electronics 13, 537-545 (2007).
24. V. Doya, O. Legrand, and F. Mortessagne, "Optimized absorption in a chaotic doubleclad fibre amplifier", Opt. Lett. 26(12), 872 (2001).
25. P. Leproux, S. Février, "Modeling and optimization of double-clad fibre amplifiers using chaotic propagation of the pump", Opt. Fibre Technol. 7(4), 324 (2001).
26. A. Liu, K. Ueda, "The absorption characteristics of circular, offset, and rectangular double-clad fibers," Opt. Commun. 132(5-6) 511 (1996).
27. G. P. Agrawal, Nonlinear Fiber Optics, Academic, San Diego, Calif. (1995).
28. A. Bjarklev, J. Broeng, A.S. Bjarklev, Photonic crystal fibres, Kluwer Academic Publishers (2003).
29. P. Russell, "Photonic crystal fibers," Science 299, 358-362 (2003).
30. J. C. Knight, "Photonic crystal fibres," Nature 424,847-851 ( 2003).
31. J. C. Knight, T. A. Birks, P. Russell, and D. M. Atkin, "All-Silica Single-Mode Optical Fiber with Photonic Crystal Cladding," Opt. Lett. 21, 1547-1549 (1996).
32. T. A. Birks, J. C. Knight, and P. Russell, "Endless Single-Mode Photonic Crystal Fiber," Opt. Lett. 22, 961-963 (1997).
33. J. C. Knight, T. A. Birks, R. F. Cregan, P. Russell, and J. P. de Sandro,“Large Mode Area Photonic Crystal Fiber,”Electron. Lett. 34, 1347-1348 (1998).
34. J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tunnermann, et al., "Low-Nonlinearity Single-Transverse-Mode Ytterbium-Doped Photonic Crystal Fiber Amplifier," Opt. Express 12, 1313-1319 (2004).
35. K. P. Hansen, J. Broeng, A. Petersson, M. D. Nielsen, P. M. W. Skovgaard, et al.,
"High-power photonic crystal fibers, " Proc. of SPIE, 6102, 61020B-1 (2006).
36. C. C. Wang. "Optical giant pulses from a Q-switched laser," Proc. of the IEEE, 51 (12), 1767-1767 (1963).
37. J. T. Verdeyen, Laser electronics, Prentice-Hall third ed. (1995).
38. R. W. Hellwarth, "Control of Fluorescent Pulsations," Advances in Quantum Electronics, ed J. R. Singer, 334-341 (1961).
39. F. J. McClung and R. W. Hellwarth. "Giant Optical Pulsations from Ruby," J.
Appl. Phys. 33, 828 (1962).
40. W. G. Wagner and B. A. Lengyel, "Evolution of the Giant Pulse in a Laser," J.
Appl. Phys.34, 2040(1963).
41. C. C. Wang, "Optical giant pulses from a Q-switched laser," Proc. IEEE 51(12), 1764 (1963).
42. J.A. Fleck, " Ultrashort-Pulse Generation by Q-Switched Lasers," Phys. Rev. B 1 (1), 84 (1970).
43. J. Limpert, N. Deguil-Robin, S. Petit, I. Manek-Hönninger, F. Salin, P. Rigail, C.
Hönninger, and E. Mottay, "High power Q-switched Yb-doped photonic crystal fiber laser producing sub-10 ns pulses," Appl. Phys. B 81, 19 (2005).
44. O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J.
Limpert, S. Ermeneux, P. Yvernault, F. Salin, and A. Tünnermann, “ Millijoule pulse energy Q-switched short-length fiber laser, ” Opt. Lett. 32, 1551 (2007).
45. J. Dong, P. Deng, Y. Liu, Y. Zhang, J. Xu, W. Chen and X. Xie, "Passively Q-switched Yb:YAG laser with Cr4+:YAG," Appl. Opt. 40, 4303 (2001).
46. G. J. Spuhler, R. Paschotta, M. P. Kullberg, M. Graf, M. Moser, E. Mix, G. Huber, C. Harder and U. Keller, "A passively Q-switched Yb:YAG microchip laser ,"
Appl. Phys. B 72, 285 (2001).
47. W. T. Rhodes, et al., Solid-State Laser Engineering, Springer sixth ed. (2006).
48. P. P. Sorokin and J. R. Lankard, "Stimulated emission observed from an organic
49. F. P. Schäfer, et al., "Organic dye solution laser," Appl. Phys. Lett. 9 (8), 306 (1966)
50. V. A. Buchenkov, A. G. Kalintsev, A. A. Mak, L. N. Soms, A. I. Stepanov, and A.
A. Tarasov, "Characteristics of YAG:Nd3+ lasers passively Q switched by LiF crystals containing color centers," Sov.J. Quantum Electron. 11(10), 1367-1368 (1981)
51. H. Ridderbusch and T. Graf, "Saturation of 1047- and 1064-nm absorption in Cr4+:YAG crystals," IEEE J. Quantum Electron. 43 (2), 168 (2007)
52. Y. Shimony, Z. Burshtein, and Y. Kalisky, "Cr4+ :YAG as passive Q-switch and brewster plate in a pulsed Nd:YAG laser," IEEE J. Quantum Electron. 31, 1738-1741 (1995).
53. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, and M. R. Kokta, "Excited-state absorption studies of Cr4+ ions in several garnet host crystals," IEEE J. Quantum Electron. 34, 292-299 (1998).
54. S. H. Yim, D. R. Lee, B. K. Rhee, and D. Kim, "Nonlinear absorption of Cr4+ :YAG studied with lasers of different pulsewidth," Apl. Phys. Lett. 73, 3193-3195 (1998).
55. N. I. Borodin, V. A. Zhitnyuk, A. G. Okhrimchuk, and A. V. Shestakov,
"Oscillation of a Y3 Al5 O12 : Cr4+ laser in wave length region of 1.34-1.6 μm,"
Izvestiya Akademii Nauk SSSR 54, 1500-1506 (1990).
56. Z. Burshtein, P. Blau, Y. Kalisky, Y. Shimony, M.R. Kokta, "Excited-State Absorption Studie of Cr4+ Ions in Several Garnet Host Crystals," IEEE J. of Quantum Electron. 34, 292–299 (1998).
57. R. Moncorge, H. Manna, F. Deghoul, Y. Guyot, Y. Kalisky, S.A. Pollack, E.V.
Zharikov, M. Kokta, "Saturable and excited state absorption measurements in Cr4+:LuAG single crystals," Optics Commun. 132, 279–284 (1996).
58. Y. Shimony, Z, Burshtein and Y. Kalisky, "Cr4+:YAG as passive Q-switch and Brewster plate in a pulsed Nd:YAG laser," IEEE J. Quantum Electron. 31, 1738-1741 (1995).
59. M. Haiml, R. Grange, U. Keller, "Optical characterization of semiconductor saturable absorbers," Appl. Phys. B 79, 331 (2004).
60. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek, J. Aus der Au, "Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state
lasers," IEEE J. Selected Topics in Quantum Electronics (JSTQE) 2, 435-453 (1996).
61. T. Hakulinen and O. G. Okhotnikov, "8 ns fiber laser Q switched by the resonant saturable absorber mirror," Opt. Lett. 32, 2677-2679 (2007).
62. S. Kivistö, R. Koskinen, J. Paajaste, S. D. Jackson, M. Guina, and O. G.
Okhotnikov, "Passively Q-switched Tm3+, Ho3+-doped silica fiber laser using a highly nonlinear saturable absorber and dynamic gain pulse compression," Opt.
Express 16, 22058-22063 (2008).
63. J.Y. Huang, H.C. Liang, K.W. Su, H.C. Lai, Y.F. Chen and K.F. Huang, "InGaAs quantum-well saturable absorbers for a diode-pumped passively Q-switched Nd:YAG laser at 1123 nm," Appl. Opt. 46, 2, 239-242 (2007).
64. A. Li, S.C. Liu, K.W. Su, Y.L. Liao, S.C. Huang, Y.F. Chen, K.F. Huang,
"InGaAsP quantum-wells saturable absorber for diode-pumped passively Q-switched 1.3-μm lasers," Appl. Phys. B 84, 3, 429-431 (2006).
65. K. Alavi, H. Temkin, W. R. Wagner, and A. Y. Cho, "Optically pumped 1.55-μm double heterostructure GaxAlyIn1-x-yAs/AluIn1-uAs lasers grown by molecular beam epitaxy," Appl. Phys. Lett. 42, 254-256 (1983).
66. S.T. Huxtable, A. Shakouri, C. Labounty, X. Fan, P. Abraham, Y.J. Chiu, J.E.
Bowers and A. Majumdar, "Thermal conductivity of indium phosphide based superlattices," Microscale. Thermophys Eng 4, 197–203 (2000).
67. V. Spagnolo, M. Troccoli, C. Gmachl, F. Capasso, A. Tredicucci, A. M. Sergent, A. L. Hutchinson, D. L. Sivco, A. Y. Cho, and G. Scamarcio, "Temperature profile of GaInAs/AlInAs/InP quantum cascade-laser facets measured by microprobe photoluminescence," Appl. Opt. Lett. 78, 20952097 (2001).
68. S. C. Huang, S. C. Liu, A. Li, K. W. Su, Y. F. Chen, and K. F. Huang, "AlGaInAs quantum-well as a saturable absorber in a diode-pumped passively Q-switched solid-state laser," Opt. Lett. 32, 1480–1482 (2007).
69. S. D. Jackson, and A. Lauto, "Diode-pumped fiber lasers: a new clinical tool?"
Lasers Surg. Med. 30(3), 184–190 (2002).
70. L. Quintino, A. Costa, R. Miranda, D. Yapp, V. Kumar, and C. J. Kong, "Welding with high power fiber lasers – A preliminary study," Mater. Des. 28(4), 1231–1237 (2007).
71. Z. J. Chen, A. B. Grudinin, J. Porta, and J. D. Minelly, "Enhanced Q switching in double-clad fiber lasers," Opt. Lett. 23(6), 454–456 (1998).
72. J. A. Alvarez-Chavez, H. L. Offerhaus, J. Nilsson, W. A. Clarkson P. W. Turner, and D. J. Richardson. "High-energy, high-power ytterbium-doped Q-switched fiber laser," Opt. Lett., 25, 37-39 (2000).
73. C. C. Ranaud, H. L. Offerhaus, J. A. Alvarez-Chavez, C. J. Nilsson, W. A.
Clarkson, P. W. Turner, D. J. Richardson, and A. B. Grudinin. "Characteristics of Q-switched cladding-pumped ytterbium-doped fiber lasers with different high-energy fiber designs," IEEE J. Quantum Electron. 37, 199-206 (2001).
74. A. Piper, A. Malinowski, K. Furusawa, and D. J. Richardson. "High power, high-brightness, mJ Q-switched ytterbium-doped fibre laser. Electron," Lett. 40, 928-929 (2004).
75. M. Laroche, H. Gilles, S. Girard, N. Passilly, and K. At-Ameur. "Nanosecond pulse generation in a passively Q-switched Yb-doped fiber laser by Cr4+ :YAG saturable absorber." IEEE Photon. Technol. Lett. 18,764-766 (2006).
76. J. Y. Huang, H. C. Liang, K. W. Su, and Y. F. Chen. "High power passively Q-switched ytterbium fiber laser with Cr4+:YAG as a saturable absorber," Opt.
Express 15, 473-479 (2007).
77. J. Y. Huang, H. C. Liang, K. W. Su, and Y. F. Chen. "Analytical model for optimizing the parameters of an external passive Q-switched in fiber laser," Appl.
Opt. 47, 2297-2302 (2008).
78. R. Chi, K. Lu, and S. Chen. "Multi-wavelength Yb-doped fiber ring laser.
Microwave Opt," Technol. Lett. 36, 170-172 (2003).
79. W. Guan and J. R. Marciante. "Dual-frequency operation in a shortcavity ytterbium-doped fiber laser," IEEE Photon. Technol. Lett. 19,261- 263 (2007).
80. L. R. Chen and X. J. Gu. "Dual-wavelength yb-doped fiber laser stabilized through four-wave mixing," Opt. Express 15, 5083–5088 (2007).
81. S. L. Hu, J. Yu, C. Q. Gao, G. H. Wei, and F. Y. L. "Dual-wavelength stable nanosecond pulses generation from cladding-pumped fiber laser," Chin. Opt. Lett.
4, 655-657 (2006).
82. A. Fotiadi, A. Kurkov, and I. Razdobreev, "All-fiber passively Q-switched ytterbium laser," CLEO/Europe-EQEC 2005, Technical Digest, CJ 2-3, Munich, Germany (2005).
83. T. Tordella, H. Djellout, B. Dussardier, A. Saïssy, and G. Monnom, "High repetition rate passively Q-switched Nd3+:Cr4+ all-fibre laser," Electron. Lett.
39(18), 1307–1308 (2003).
84. P. Adel, M. Auerbach, C. Fallnich, S. Unger, H.-R. Müller, and J. Kirchhof,
"Passive Q-switching by Tm3+codoping of a Yb3+-fiber laser," Opt. Express 11(21), 2730–2735 (2003).
85. M. Laroche, H. Gilles, S. Girard, N. Passilly, and K. Aït-Ameur, "Nanosecond
pulse generation in a passively Qswitched Yb-doped fiber laser by Cr4+:YAG saturable absorber," IEEE Photon. Technol. Lett. 18(6), 764–766(2006).
86. M. Laroche, A. M. Chardon, J. Nilsson, D. P. Shepherd, W. A. Clarkson, S. Girard, and R. Moncorgé, "Compact diode-pumped passively Q-switched tunable Er-Yb double-clad fiber laser," Opt. Lett. 27(22), 1980–1082 (2002).
87. T. Hakulinen, and O. G. Okhotnikov, "8 ns fiber laser Q switched by the resonant saturable absorber mirror," Opt. Lett. 32(18), 2677–2679 (2007).
88. S. C. Huang, S. C. Liu, A. Li, K. W. Su, Y. F. Chen, and K. F. Huang, "AlGaInAs quantum-well as a saturable absorber in a diode-pumped passively Q-switched solid-state laser," Opt. Lett. 32, 1480-1482 (2007).
89. K. Alavi, H. Temkin, W. R. Wagner, and A. Y. Cho, "Optically pumped 1.55-μm double heterostructure GaxAlyIn1-x-yAs/AluIn1-uAs lasers grown by molecular beam epitaxy," Appl. Phys. Lett. 42, 254-256 (1983).
90. W. T. Tsang and N. A. Olsson, "New current injection 1.5-μm wavelength GaxAlyIn1-x-yAs/InP double-heterostructure laser grown by molecular beam epitaxy," Appl. Phys. Lett. 42, 922-924 (1983).
91. N. Nishiyama, C. Caneau, B. Hall, G. Guryanov, M. H. Hu, X. S. Liu, M.-J. Li, R.
Bhat, and C. E. Zah, "Long-wavelength vertical-cavity surface-emitting lasers on InP with lattice matched AlGaInAs–InP DBR grown by MOCVD," IEEE J. Sel.
Top. Quantum Electron. 11, 990-998 (2005).
Chapter 2
Passively Q-switched
double-cladding fiber lasers
2.1 Passively Q-switched double-cladding fiber laser with AlGaInAs quantum wells
2.1.1 Introduction
Fiber lasers have been confirmed to possess the merits of high efficiency, excellent beam quality, and good heat dissipation. High-pulse-energy Q-switched fiber lasers are practically useful in numerous applications, such as range finding, remote sensing, industrial processing, and coherent lidar systems [1–4]. Passively Q-switched lasers with saturable absorbers have attracted significant attention because of their compactness and simplicity in operation. Several saturable absorbers have been developed to replace the dyes used in solid-state lasers, such as Cr4+-doped crystals [5–
9] and semiconductor saturable absorber mirrors (SESAMs) [10,11]. Currently, Cr4+:YAG crystals are the most recognized saturable absorbers in the spectral region of 0.9–1.2 μm. Passively Q-switched fiber lasers with Cr4+:YAG saturable absorbers have been recently demonstrated [12–14], among which the maximum pulse energy achieved with a large-mode-area Yb-doped fiber was120 μJ.
Alternatively, InGaAs/GaAs quantum wells (QWs) have been used to develop the SESAMs for Nd-doped or Yb-doped lasers. The obtainable absorption change between low and high intensities, however, is hindered by the lattice mismatch for the spectral region of above 1.0 μm. As a consequence, the output pulse energies and the conversion efficiencies with InGaAs SESAMs in passively Q-switched lasers are generally significantly lower than those with Cr4+:YAG crystals. Recently, an AlGaInAs with a periodic QW/ barrier structure has been exploited to be an efficient saturable absorber for a passively Q-switched Nd:YVO4 laser [15]. Compared with InGaAsP materials, the AlGaInAs quaternary alloy with a larger conduction band offset is confirmed to offer a superior electron confinement in the 0.84–1.65 μm spectral region [16–18]. Nevertheless, AlGaInAs/InP QWs have not been employed to passively Q switch Yb-doped fiber lasers.
We demonstrate a high-pulse-energy passively Q-switched Yb-doped fiber laser with an AlGaInAs/InP QWs saturable absorber. With an incident pump power of 7.6 W, an average output power of 3.8 W with a Q-switched pulse width of 30 ns at a pulse repetition rate of 12.5 kHz was obtained; consequently, the maximum pulse energy was up to 300 μJ. More importantly, the overall Q-switching efficiency could exceed 90%
2.1.2 Characteristics of semiconductor saturable absorber
The structure of the semiconductor saturable absorber was essentially similar to that reported in [15]. The previous saturable absorber consisted of 30 groups of two QWs, spaced at half-wavelength intervals by InAlAs barrier layers with a bandgap wavelength around 805 nm. Here we fabricated a saturable absorber with 50 groups of three QWs to increase the modulation strength. The luminescence wavelength of the saturable absorber was designed to be near 1066 nm. An InP window layer was deposited on the QW/barrier structure to avoid surface recombination and oxidation.
The backside of the substrate was mechanically polished after growth. Each side of the semiconductor saturable absorber was antireflection coated to reduce back reflections and the couplecavity effects. Figure 2.1 shows the measured result for the low-intensity transmittance spectrum of the QW saturable absorber. The initial transmission of the absorber at the wavelength of 1066 nm was found to be approximately 26%. The operation bandwidth of the absorber is approximately 8 nm. With the z-scan method, the absorption change between low and high intensities was observed to be approximately 70% in a single pass, and the total nonsaturable losses were lower than 5%. Furthermore, the saturation fluence of the saturable absorber was estimated to be in the range of 1 mJ/cm2, and its relaxation time was on the order of 100 ns.
2.1.3 Experimental setup
Figure 2.2 depicts the schematic of the experimental setup for the passively Q-switched fiber laser, which is composed of a 1.5 m Yb-doped fiber and an external feedback cavity. The external cavity comprises a reimaging lens, a saturable absorber, a highly reflective mirror at 1.06 μm for feedback, and a Fabry–Perot thin film filter (FP filter) for controlling the lasing wavelength. The peak of the FP filter is at 1100 nm with a FWHM bandwidth of 5 nm at normal incidence. The end facets of the fiber were cut to be normal incident. The fiber has a peak cladding absorption coefficient of 10.8 dB/m at 976 nm and a double-clad structure with a diameter of 350 μm octagonal outer cladding, a diameter of 250 μm octagonal inner cladding with an NA of 0.46, and a 25 μm circular core with an NA of 0.07. Note that the robust single-mode output was achieved with a unique low NA feature of the core.
Fig 2. 1 Transmittance spectrum at room temperature for the AR- coated AlGaInAs/InP saturable absorber. Inset, schematic diagram of a periodic AlGaInAs
QW structure.
Fig 2. 2 Schematic diagram of the experimental setup. HR, high reflection; HT, high transmission.
The pump source was a 10 W 976 nm fiber-coupled laser diode with a core diameter of 400 μm and an NA of 0.22. A focusing lens with 25 mm focal length and 90% coupling efficiency was used to re-image the pump beam into the fiber through a dichroic mirror with high transmission (>90%) at 976 nm and high reflectivity (>98.8%) at 1066 nm. The pump spot radius was approximately 200 μm. With launching into an undoped fiber, the pump coupling efficiency was measured to be approximately 80%.
Wavepro 7100; 10G samples/s; 1 GHz bandwidth) and a fast InGaAs photodiode. The laser spectrum was measured by an optical spectrum analyzer with 0.1 nm resolution (Advantest Q8381A).
2.1.4 Results and discussions
Figure 2.3 shows the average output powers with respect to the incident pump
Figure 2.3 shows the average output powers with respect to the incident pump