2008_基于ybkyw多晶谐振器的高功率脉冲锁模振荡器_h

1、High power cw and mode-locked oscillators based on Yb:KYW multi-crystal resonatorsA.-L. Calendron, K. S. Wentsch, M. J. Lederer*High Q Laser Production GmbH,Kaestle-Areal, 2.OG, Kaiser-Franz-Josef-Str. 61, A-6845 Hohenems, Austria.*Corresponding author max.ledererhighqlaser.atAbstract: We report on

2、a high power diode-pumped laser using multiple bulk Yb:KY(WO4)2 (KYW) crystals in a resonator optimised for this operation. From a dual-crystal resonator we obtain more than 24W of cw- power in a TEM00 mode limited by the available pump power. We also present results for semiconductor saturable abso

3、rber mirror (SESAM) mode-locking in the soliton as well as positive dispersion regime with average output powers of 14.6W and 17W respectively.2008 Optical Society of AmericaOCIS codes: (140.3580) Lasers, solid-state; (140.4050) Mode-locked lasers; (140.3480) Lasers, diode pumped; (320.7090) Ultrafa

4、st Lasers.References and links1.A. Tnnermann, J. Limpert, and S. Nolte, “Ultrafast Fiber Amplifier Systems: Status, Perspectives and Applications,“ in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, 2008 Technic

5、al Digest (Optical Society of America, Washington, DC, 2008), CTuK1J. Neuhaus, J. Kleinbauer, A. Killi, S. Weiler, D. Sutter, and T. Dekorsy, “Passively mode-locked Yb:YAG thin-disk laser with pulse energies exceeding 13 J by use of an active multipass geometry,” Opt. Lett. 33, 726-728 (2008)J. Limp

6、ert, F. Rser, T. Schreiber, and A. Tnnermann, “High-Power Ultrafast Fiber Laser Systems,“ IEEE J. Sel. Top. Quantum Electron. 12, 233-244, (2006)T. Sdmeyer, S. V. Marchese, C R. Baer, S.Hashim oto, A. G. Engqvist, M. Golling, D. J. H. C. Maas, andU. Keller, “Femtosecond Thin Disk Lasers with 10 J Pu

7、lse Energy,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, 2008 Technical Digest (Optical Society of America, Washington, DC, 2008), CFP1S. V. Marchese, T. Sdmeyer, M. Golling, R. Grange, and U. Keller, “Pu

8、lse energy scaling to 5J from a femtosecond thin disk laser,” Opt. Lett. 31, 2728-2730 (2006).P. Rubldt, T. Mans, D. Hoffmann, A. -L. Calendron, M. Lederer, and R. Poprawe, “Compact high Power fs-Oscillator-Amplifier System,” in Conference on Ultrafast Phenomena XVI, Proceedings of the 16th Internat

9、ional Conference (2008), MON4A.2G. R. Holtom, “Mode-locked Yb:KGW laser longitudinally pumped by polarization-coupled diode bars,” Opt. Lett. 31, 2719-2721 (2006)J. M. Eggleston, “Periodic Resonators for Average-Power Scaling of Stable-Resonator Solid-state Lasers,“ IEEE J. Quantum Electron. 24, 182

10、1-1824, (1988)Y.-F. Chen, Y. P. Lan, and S. C. Wang, “Efficient high-power diode-end-pumped TEM00 Nd:YVO4 laser with a planar cavity,” Opt. Lett. 25, 1016-1018 (2000).C. Hoenninger, A. Courjaud, P. Rigail, E. Mottay, M. Delaigue, N. Dguil-Robin, J. Limpert, I. Manek- Hoenninger, and F. Salin, “0.5J

11、Diode Pumped Femtosecond Laser Oscillator at 9MHz,” in Advanced Solid-State Photonics, Vienna 2008, ME2B. Proctor, E. Westig, and F. Wise, “Characterization of a Kerr-Lens mode-locked Ti:sapphire laser with positive group-velocity dispersion,” Opt. Lett. 18, 16541656 (1993).A. Fernandez, T. Fuji, A.

12、 Poppe, A. Frbach, F. Krausz, and A. Apolonski, “Chirped-pulse oscillators: a route to high-power femtosecond pulses without external amplification,” Opt. Lett. 29, 13661368 (2004).X. Zhou, H. Kapteyn, and M. Murnane, ”Positive-dispersion cavity-dumped Ti:sapphire laser oscillator and its applicatio

13、n to white light generation,” Opt. Express 14, 9750-9757 (2006).2.3.4.5.6.7.8.9.10.11.12.13.S. Dewald, T. Lan. Schrter, R. Moshammer, J. Ullrich, M. Siegel, and U. Morgner, “Ionization of14.noble gases with pulses directly from a laser oscillator,” Opt. Lett. 31, 2072 (2006)#100312 - $15.00 USD Rece

14、ived 18 Aug 2008; revised 14 Oct 2008; accepted 14 Oct 2008; published 30 Oct 2008(C) 2008 OSA10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 1883815.V. L. Kalashnikov, E. Podivilov, A. Chernykh, S. Naumov, A. Fernandez, R. Graf, and A. Apolonski, “Approaching the microjoule frontier with femtos

15、econd laser oscillators: theory and comparison with experiment,” New J. Phys. 7, 217 (2005).V. L. Kalashnikov and A. Chernykh, “Spectral anomalies and stability of chirped-pulse oscillators,” Phys. Rev. A 75, 033820(2007).G. Palmer, M. Emons, M. Siegel, A. Steinmann, M. Schultze, M. J. Lederer, and

16、U. Morgner, “Passively mode-locked and cavity-dumped Yb:KY(WO4)2 oscillator with positive dispersion,” Opt. Express 15, 16017-16021 (2007)T. Clausnitzer, J. Limpert, K. Zllner, H. Zellmer, H. J. Fuchs, E. B. Kley, A. Tnnermann, M. Jup, andD. Ristau, “Highly efficient transmission gratings in fused s

17、ilica for chirped-pulse amplification systems,” Appl. Opt. 42, 69346938 (2003).16.17.18.1. IntroductionHigh power ultrashort pulse lasers gain increasing importance as tools in fields as varied as micro and nano processing, bio-medicine and metrology, to name a few. As applications drive the need fo

18、r higher average power and energy per pulse, laser development is aiming to satisfy this need. Generally, power scaling is acknowledged to reach furthest with both the thin-disk and the fibre-amplifier approach which achieve remarkable levels of output power with ps or sub-ps pulses 1-5, albeit with

19、 non-trivial effort. Fibre based systems, for tance, use parabolic or chirped puls amplification (CPA) and several amplification stages employing large mode area fibres to control nonlinearities in order to achieve average powers in excess of 100W at tens of MHz 1,3. High power femtosecond pulse gen

20、eration from an oscillator directly have so far been restricted mainly to the thin disk approach e. g. 2,4,5, with average powers 60W and pulse energies in excess of 10J.In comparison, bulk laser technology has recently been used to amplify fs-pulses from the few-hundred mW level to 77W without CPA

21、using only a single Yb:YAG slab amplifier 6. It is predicted that this amplification scheme is potentially scalable to several hundred watts, indicating that bulk technology is by no means fundamentally limited. The classical approach of using bulk laser crystals in high power femtosecond oscillator

22、s with diffraction limited beam quality is typically constrained by various effects. Firstly, the average power which can be obtained from a typical end-pumped crystal is limited by thermo-mechanical and optical effects (cracking, thermal lens). Secondly, the typically small gain cross sections of f

23、s-laser materials require reasonably tight modes in the medium leading to limitations in the pulse energy due to large nonlinear phase shifts, particularly in the soliton regime, requiring extensive amounts of negative dispersion. The most promising and advanced laser materials for fs-generation in

24、a diode-pumped oscillator available to date are Yb-doped crystals (e. g. YAG, KGW, KYW, etc.). Limitless power scaling using these materials in an end pumped oscillator configuration is additionally hampered by a range of parameters such as diode brightness, reabsorption as well as available crystal

25、 sizes and doping concentration. An example for a laser design in which these parameters have been somewhat optimised using a single laser crystal was shown in 7, where an output power of 10W was achieved using soliton mode-locking.Power scaling using multiple laser crystals in a resonator was descr

26、ibed by Eggleston 8. ummary, one can overcome the thermal effects met with a single crystal by distributingthe load over multiple rods in a periodic resonator structure. Additionally, multiple gain elements increase the small signal gain and hence the extraction efficiency of the resonator for stand

27、ard doping levels and diode brightness. In this work we have followed this approach in a double Yb:KYW crystal setup, allowing the extraction of more than 24W cw in a diffraction limited beam using two pump diodes with up to 30W power each. Using a semiconductor saturable absorber mirror (SESAM) bot

28、h soliton and positive dispersion mode-locking regimes were explored resulting in 14.6W and 17W output power respectively with pulsewidths around 450fs in both cases.#100312 - $15.00 USD Received 18 Aug 2008; revised 14 Oct 2008; accepted 14 Oct 2008; published 30 Oct 2008(C) 2008 OSA10 November 200

29、8 / Vol. 16, No. 23 / OPTICS EXPRESS 188392. High power laser head, cw operationA schematic of the laser design is shown in Fig. 1. Essentially, there is a symmetric short cavity with a length of 440mm, terminated by M5 and containing the two 5% doped, ng-cut Yb:KYW crystals of 2mm length out of foc

30、us. An output coupler, OC 1, with a reflectivity of 90% is used in this configuration. Extensions to this resonator are trivial and are made with the purpose of including a SESAM, dispersion compensating mirrors GTI 1 and 2 for soliton mode-locking as well as further output coupling OC 2. The latter

31、 is merely an aid to achieve positive dispersion in the case of chirped pulse mode-locking. By design this component has a positive dispersion of +250fs2 and a reflectivity of 95%. In this case the reflectivity of OC 1 was increased to 99%. Both Yb:KYW crystals were pumped by up to 30W at 981nm from

32、 fibre coupled laser diodes with a core size of 200m and using an imaging ratio of 1:2. The crystals were mounted in copper holders and cooled to a temperature of 18C using Peltier elements.Fig. 1. Layout of the laser with LD 1 and 2 fiber coupled laser diodes (30W 981nm, 200m core), L collimating a

33、nd focussing lenses, M1 and M2 dichroic mirrors, XTAL 1 and 2 Yb:KYW crystals (l = 2mm, ng-cut, 5% doped), M3 and M4 curved mirrors with R = 200mm, M7 and M9 are curved mirrors with R = 600mm and M8 is a flat turning mirror. M5 is the end mirror of the short cavity. GTI 1 and 2 are Gires-Tournois ty

34、pe dispersive mirrors with -500fs2 negative dispersion. OC 1 had a reflectivity of 90% when operating the symmetric short cavity laser head on its own (M5 in place). For the soliton mode-locked long cavity OC1 had a reflectivity of 85%. In the case of pos. dispersion mode-locking OC 1 was increased

35、to 99% whilst an additional output coupler OC 2 with 95% reflectivity and +250fs2 dispersion was placed in the long cavity.The continuous wave P-curve of the short resonator is shown in Fig. 2. With a pump current of 42A (Ppump = 28.6W per LD), an output power of 23W was extracted from OC 1 whilst t

36、he beam quality was still well below24W output power which is, to our knowledge, the highest power achieved from a Yb:KYW bulk laser to date, comparable with that of a thin disk head. At this power the laser had an efficiency of 40% with respect to the total emitted pump power and nearly 50% with re

37、spect to the absorbed pump power. The slope efficiency of 51% is deduced from a linear fit as depicted in Fig. 2. The laser emission was polarized along the np- direction of the crystal indicatrix with an extinction ratio of 20dB. At the highest pump power the laser operated at a wavelength of 1044n

38、m.M2#100312 - $15.00 USD Received 18 Aug 2008; revised 14 Oct 2008; accepted 14 Oct 2008; published 30 Oct 2008(C) 2008 OSA10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 18840Fig. 2. Output power of the short resonator in CW operation as a function of the total pump power emitted by both fibre

39、coupled pump diodes. The slope efficiency is 51%.3. Soliton mode-lockingIn order to operate the laser in the soliton regime we extended the cavity length to both accommodate a SESAM and provide anomalous dispersion in the form of Gires-Tournois- type dispersive mirrors GTI 1 and 2 (confirm Fig. 1).

40、This resonator had a length of nearly 1.9m, corresponding to a repetition rate of frep = 79.8MHz. Furthermore, output coupling from OC 1 was set to 15% whilst OC 2 was a highly reflecting mirror. The SESAM used to start and stabilize mode-locking had a modulation depth of around 1% and there were 6

41、bounces on each dispersive mirror (GTI 1 and 2), resulting in a total dispersion of -12000fs per round trip. Under these conditions the laser was mode-locked self-starting and with a single pulse per round trip for pump powers between 34W and 50W. At 50W of pump the shortest pulses were emitted at a

42、n output power of Pout = 14.6W, compared to 10W achieved in 7. At this working point the pulsewidth was measured using an intensity autocorrelator. Assuming a sech2 soliton pulse shape the deconvolved pulsewidth is tFWHM = 450fs. Figure 3 depicts the measured autocorrelation function with the et sho

43、wing the corresponding optical spectrum. With a spectral width of l = 2.6nm and a pulsewidth of tFWHM = 450fs one calculates a time- bandwidth product of 0.322, which is essentially at the theoretical limit.Fig. 3. Intensity autocorrelation of the soliton mode-locked laser close to the double-pulsin

44、g limit (Ppump = 50W, Pout = 14.6W). The deconvolved pulsewidth is 450fs, assuming a sech2(t) pulse. The et shows the associated optical power spectrum with a FWHM of 2.6nm, corresponding to a time-bandwidth product of 0.322.#100312 - $15.00 USD Received 18 Aug 2008; revised 14 Oct 2008; accepted 14

45、 Oct 2008; published 30 Oct 2008(C) 2008 OSA10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 18841Although higher output power was possible, particularly in the double pulsing regime, the slightly reduced efficiency of the mode-locked laser mainly stems from the losses on the GTI- mirrors which w

46、ere estimated to be around 3% per round trip. Also, in order to lower the nonlinear phase shift per round trip and to lower the intensity on the SESAM, output coupling was increased to 15% as compared with 10% in cw-mode. The operation of this laser was basically straight forward with mode-locking r

47、eliably self-starting and no degradation observable on the SESAM. pite of operating without cover, the laser displayed RMS fluctuations of below 1% as sampled by a fast photodiode and digital oscilloscope as well as a thermal power meter.4. Positive dispersion mode-lockingMode-locking of cavities co

48、ntaining broadband laser media like Ti:sapphire or Yb-doped crystals and glasses in the presence of positive cavity dispersion has recently received increasing attention 11-17. The basic proposition behind this scheme is to reduce the nonlinear phase incurred during fs-soliton propagation through in

49、tra-cavity elements, including air in long cavities, by lengthening the cavity puls through the action of normal dispersion. Depending on parameters such as gain bandwidth, SESAM modulation and recovery time, amount of positive dispersion and nonlinearity, the pulses from such an oscillator are typi

50、cally chirped and orders of magnitude longer than a soliton of similar energy and spectral width which implies potential for energy scaling. Theoretical and experimental studies were carried out to gain a more fundamental understanding of the physics of such lasers e. g. 15-17.For our experiments wi

51、th the double-crystal oscillator, in order to achieve stable operation, we introduced +500fs2 of positive dispersion per round trip from a 5% output coupler (OC 2) used as a folding mirror (confirm Fig. 1). The positive dispersion of this component was merely a by-product of its particular design bu

52、t was used here to our advantage. The GTI multi-bounce was not present in this configuration and there was an additional 1% output coupler (OC 1). We have used the same SESAM as with the soliton laser. Lacking the availability of a transmission grating with suitable line density and power handling c

53、apability at the time of experimentation, we used output 1 (approximately one tenth of the total laser power) to compress the pulses in a Tracey-type compressor made from a gold reflection grating with 1200 lines/mm. Although this is not ideal, it serves the purpose of showing the compressibility of

54、 the pulses. Highly efficient (95%) transmission gratings have become available leading to less than 20% loss in the compression of the total laser power without discernible thermal effects e. g. 18. Figure 4 shows the spectrum of the pulses at 17W output power (a) and the uncompressed as well as co

55、mpressed pulse autocorrelation(b).Fig. 4. (a) Pulse spectrum of positive dispersion mode-locked oscillator. Total output powerPout= 17W before compression (Ppump = 50W). (b) Corresponding autocorrelation of theuncompressed pulse withets showing the autocorrelation of the compressed pulse. Assuming a

56、 sech2(t) pulse, the compressed pulses have a width of 470fs.#100312 - $15.00 USD Received 18 Aug 2008; revised 14 Oct 2008; accepted 14 Oct 2008; published 30 Oct 2008(C) 2008 OSA10 November 2008 / Vol. 16, No. 23 / OPTICS EXPRESS 18842The shape of the power spectrum is typical for chirped pulses a

57、nd the autocorrelation before compression is around an order of magnitude longer than that of a soliton laser with similar pulse energy and spectral width. Using a grating separation of 100mm, corresponding to a calculated negative dispersion of around -0.8ps2, the pulses were compressed to a minimum autocorrelation