6.1 Summary
1. Tunable Yb and Er/Yb codoped fiber lasers
In this section I demonstrated efficient narrowband and tunable Yb-doped and Er/Yb-codoped fiber lasers by use of all-dielectric Fabry-Perot thin film filters. The filters have low insertion loss and high transmission at the central wavelength.
Therefore FP thin film filters are suitable for efficiently narrowing and tuning broadband lasers. The performances of the Yb and the Er/Yb fiber lasers are shown as table 6.1.
Tunable
range
∆λnarrowba nd/ ∆λTFF (nm)
∆λna/ ∆λbr
(nm) Pna/ Pbr Power (W) Yb fiber
laser 60 nm 0.36/5 0.36/14 >0.9 14
Er/Yb fiber
laser 28 nm 0.15/0.4 0.15/5 >0.9 3.2
2. Passively Q-switched fiber lasers
In this section I have developed a model and optimized an external Q-switch of fiber lasers Q-switched by Cr4+:YAG crystals. Besides, I also demonstrated that passively Q-switched Yb fiber lasers by a Cr4+:YAG crystal and an AlGaInAs semiconductor saturable absorbers respectively; and Er/Yb fiber lasers by an AlGaInAs semiconductor saturable absorber. The performance of Yb fiber lasers are shown in table 7.2. As seen in the table both the saturable absorbers possess low insertion loss with > 85% Q-switching efficiency. On the other hand the pulse energies obtained in these two experiments are superior to those of others are thanks to the use of a Table 6.1. Performances of the tunable fiber lasers.
LMA fiber with core diameter of 30 µm and saturable absorbers with large modulation depth and low nonsaturable loss.
by AlGaInAs QWs by Cr4+:YAG crystal
Q-switching efficiency 87% 90%
PRR (kHz) 30 38
pulse energy (µJ) 450 350
pulse width (ns) 65 75
pulse stability ±10% ±15%
As for the Er/Yb fiber laser, we have also obtained an efficient Q-switched fiber laser with Q-switching efficiency >85 %. Beside, the generated single pulse owns energy of 100 µJ at pulse repetition rate of 12 kHz, corresponding average power of 1.2 W.
The result manifested the AlGaInAs material are also suitable for Q-switching Er/Yb fiber lasers in the spectral domain of 1.5 µm.
3. Actively Q-switched fiber lasers
In this section we actively Q-switch an Yb fiber laser by use of an acoustic-optic Q-switch. The results reveal that Q-switching by an acoustic-optic Q-switch is efficient and flexible. The pulse repetition rate ranges 53 kHz to 200 kHz at pump power of 24 W and the maximum pulse energy obtained is 250 µJ at repetition rate of 53 kHz. However due to the high gain of the gain fiber it is difficult to prevent the pre-lasing resulted from the feedback of undeflected beam at low pulse repetition rate.
Hence by means of the aid of the saturable absorber or polarization controlling, we have successfully enhanced the loss modulation of the Q-switch. In the hybrid Q-switching experiment, the pulse energy was increased to 550 µJ and pulse width was shortened from 100ns to 50 ns at repetition rate of 22 kHz under pump power of
Table 6.2. Performances of the passively Q-switched Yb fiber lasers
24 W. Besides the hybrid Q-switch reduced the timing jitter at low pump power occurred in the passively Q-switched fiber lasers. In the actively Q-switching with polarization controlling experiment, a polarization beam splitter was inserted in front of the acoustic-optic Q-switch to control the polarization of incident beam. An output pulse of energy of 600 µJ and width of 45 ns was obtained at repetition rate of 20 kHz under pump power of 24 W. The results manifested that it is efficient and simple to directly control the polarization of the resonant beam in the external cavity to enhance the performance of a Q-switched fiber laser by use of an acoustic-optic Q-switch.
6.2 Future works
Applications of nonlinear optical effects can be another interesting research direction.
For example a narrowband and high-peak-power pulsed fiber lasers are suitable for frequency-doubling and OPO experiments. On the other hand, besides the Q-switched fiber lasers we are attempted to generate a mater-oscillation-power- amplifier system by injecting a short pulse with duration of 1 ns into a fiber to generate a high-pulse-energy and short-duration pulse. This object is interesting because it improve the drawback of long-pulse-duration of typical Q-switched fiber lasers.
Another improvement to the drawback is to use a rod-type fiber as the gain medium because their large core size and shorter length can be used to generate a short Q-switched pulse.
Appendix: List of publications Journals publications
¾ 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, 2, 473-479 Jan 22 (2007).
¾ J. Y. Huang, H. C. Liang, K. W. Su, and Y. F. Chen＂Analytical model for optimizing the parameters of the external cavity in passively Q-switched fiber lasers ,＂Appl. Opt. 47, 2297-2302 (2008).
¾ J. Y. Huang, S. C. Huang, H. L. Chang, K. W. Su, Y. F. Chen*, and K. F. Huang,
“Passive Q switching of Er-Yb fiber laser with semiconductor saturable absorber,＂ Opt. Express 16, 3002-3006 (2008).
¾ J. Y. Huang, W. C. Huang, W. Z. Zhuang, K. W. Su, Y. F. Chen, and K. F. Huang,
“High-pulse-energy passively Q-switched Yb-doped fiber laser with AlGaInAs quantum-wells as a saturable absorber,＂accepted by Op. Lett.
¾ 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, 239-242 Jan 10 (2007).
¾ Huang YP, Liang HC, Huang JY, Su KW, Li A, Chen YF, Huang KF,
“Semiconductor quantum-well saturable absorbers for efficient passive Q switching of a diode-pumped 946 nm Nd:YAG laser,＂ Appl. Opt. 46, 25, 6273-6276 Aug 23 (2007).
¾ H.C. Liang, J.Y. Huang, K.W. Su, H.C. Lai, Y.F. Chen, K.F. Huang, Zhang HJ, J.Y. Wang, M.H. Jiang. “Passively Q-switched Yb3+:YCa4O(BO3)3 laser with InGaAs quantum wells as saturable absorbers,＂Appl. Opt. 46, 2292-2296 (2007).
Conferences
¾
黃哲彥, 梁興弛, 蘇冠暐, 陳永富, 林志平, 陳思武,“光纖雷射研製計畫,＂96 年度國防科技學術合作計畫成果發表會 55CS,論文集 B-33,龍潭 (2007).
¾
J.Y. Huang, H.C. Liang, K.W. Su, and Y.F. Chen, “Analytical model for the design of external-cavity passively Q-switched fiber lasers,” Fiber Lasers V, PW2008, Proceedings of SPIE 6873, San Jose, USA (Jan 2008).¾
H.C. Liang, J.Y. Huang, S.C. Huang, K.W. Su, Y.F. Chen, and K.F. Hunag,“High-peak-power flashlamp-pumped passively Q-switched Nd:YAG laser with AlGaInAs quantum wells as a saturable absorber,” Solid State Lasers XVII, PW2008, Proceedings of SPIE 6871, San Jose, USA (Jan 2008).