# Ultrashort Pulse Laser Technology

Ultrashort pulse lasers are an enabling technology for a whole range of fundamental and applied research areas as well as industrial applications. The pulse width typically ranges from picoseconds down to few femtoseconds in duration. To generate such short pulses typically multiple longitudinal modes in a laser resonator are excited, such that a pulse much shorter than the cavity roundtrip time is created inside the laser resonator. The shortest pulse that can build up is then limited by the gain bandwidth of the amplifying medium. Fig. 1 shows the progress in generating short optical pulses from various laser materials. Over the last decades, our group has made major contributions to both the understanding of the pulse generation mechanisms involved in the various types of lasers and the emerging technologies leading to shorter and shorter pulse durations.

The progress of pulse shortening culminated in the generation of the shortest pulses directly from a laser oscillator approaching a single optical cycle at 800 nm from Kerr-lens mode-locked Ti:sapphire lasers, developed in our group. Fig. 2 shows the generic setup of such a laser, which is strikingly simple.

The laser is made up of an optically-pumped laser crystal (Ti:sapphire), an additional material (BaF2) for precise dispersion-compensation in each arm of the laser and broadband mirrors, which compensate for the dispersion in the laser crystal, other

intra-cavity optical components and the air in the laser resonator. The broadband mirrors are designed in pairs (green and blue), called double-chirped mirror pairs. They have a custom designed group delay over one octave of bandwidth as well as a high reflectivity over the same bandwidth and one of them has a pump transmission window to transmit the pump light into the cavity (see Fig. 3).

Fig. 4(a) shows a up-converted spectral interferogram obtained using the two-dimensional spectral shearing (2DSI) pulse characterization technique developed in our group. Figure 4(b) shows a typical spectrum of an octave-spanning sub-two-cycle laser pulse and Fig. 4(c) the reconstructed electric field from the 2DSI data. Fig. 4(d) verifies the effectiveness of the 2DSI technique by comparing the computed interferometric autocorrelation (IAC) from the reconstructed pulse with the directly measured IAC. Laser systems similar to the one discussed here are widely used in many research projects in our group, such as femtosecond laser frequency combs, optical clocks and attosecond science.

For more information related to recent works in this area, see selected key references from our group below.

**References:**

1. C-H. Li, A. Benedick, P. Fendel, A. G. Glenday, F. X. Kärtner, D. Phillips, D. Sasselov, A. Szentgyorgyi and R. Walsworth, “Femtosecond laser frequency comb for precision astrophysics spectroscopy,” Nature 452, 610-612 (2008).

2. H. M. Crespo, J. R. Birge, E. L. Falcao-Filho, M. Y. Sander, A. Benedick, and F. X. Kärtner, “Non-intrusive phase-stabilization of sub-two-cycle pulses from a prismless octave-spanning Ti:sapphire Laser,” Opt. Lett. 33:(8), 833-835 (2008).

3. H. M. Crespo, J. R. Birge, M. Y. Sander, E. L. Falcao-Filho, A. Benedick, and F. X. Kärtner, “Phase-stabilization of sub-two-cycle pulses from prismless octave-spanning Ti:sapphire Lasers,” JOSA B 25:(7), pp. B147-B154 (2008).

4. S. Rausch, Th. Binhammer, A. Harth, N. Meiser, R. Ell, F. X. Kärtner, and U. Morgner, “Controlled Waveforms on the Single-cycle Scale from a Femtosecond Oscillator,” Vol. 16 Issue 13, pp.9739-9745 (2008)

5. J. Chen, J. W. Sickler, T. Wilken, P. Fendel, R. Holzwarth, T. W. Hänsch, E. P. Ippen and F. X. Kärtner, “Generation of Low Jitter Femtosecond Pulse Trains with 1~2 GHz repetition rate via external Fabry-Perot Cavities, Opt. Lett. 33:(9), pp.959-961 (2008).

6. U. Demirbas, A. Sennaroglu, A. Benedick, A. Siddiqui, F. X. Kärtner, and J. G. Fujimoto, “Highly efficient, low-cost femtosecond Cr3+:LiCAF laser pumped by single-mode diodes,” Opt. Lett. 33:(6), pp.590-592 (2008) .

7. J. Chen, J. W. Sickler, E. P. Ippen and F. X. Kärtner, “High Repetition Rate, Low Jitter, Low Intensity Noise, Fundamentally Mode-locked 167fs Soliton Er-fiber Laser,” Opt. Lett. 32:(11), pp. 1566-1569 (2007).

8. A. Sennaroglu, F. X. Kärtner, and J. G. Fujimoto, “Low-threshold, room-temperature femtosecond Cr4+:forsterite laser,” Opt. Express 15, 13043-13048 (2007).

9. U. Demirbas, A. Sennaroglu, A. Benedick, A. Siddiqui, F. X. Kärtner, and J. G. Fujimoto, “Diode-pumped, high-average power femtosecond Cr3+:LiCAF laser,” Opt. Lett., 32:(22), pp.3309-3311 (2007).

10. L. Matos, O. D. Mücke, J. Chen, and F. X. Kärtner, “Carrier-envelope phase dynamics and noise analysis in octave-spanning Ti:sapphire lasers,” Opt. Express 14, 2497 (2006).

11. J. R. Birge, and F. X. Kärtner, “Efficient analytic computation of dispersion from multilayer structures,” Applied Optics, 45, 1478 (2006).

12. J. R. Birge, R. Ell and F. X. Kärtner, “Two-dimensional spectral shearing interferometry for few-cycle pulse characterization” Optics Letters, 31, 2063 (2006).

13. T. Binhammer, E. Rittweger, U. Morgner, R. Ell, and F.X. Kärtner, “Spectral phase control and temporal superresolution toward the single-cycle pulse,” Optics Letters, 31: (10), 1552-55 (2006).

14. T. M. Liu, F. X. Kärtner and J. G. Fujimoto, “Multiplying the repetition rate of passive mode-locked femtosecond lasers by an intracavity flat surface with low reflectivity,” Opt. Lett. 30, 439-441, 2005.

15. O. D. Mücke, R. Ell, A. Winter, J. W. Kim, J. R. Birge, L. Matos, and F.X. Kärtner, “Self-Referenced 200 MHz Octave-Spanning Ti:Sapphire Laser with 50 Attosecond Carrier-Envelope Phase Jitter,” Optics Express 13, 5163-5168, 2005.

16. T. Binhammer, E. Rittweger, R. Ell, F.X. Kärtner, and U. Morgner, “Novel prism-based pulse shaper for octave spanning spectra,” IEEE Journal of Quantum Electronics, JQE-41, 1552-1557 (2005).

17. J. Kim, J. R. Birge, V. Sharma, J. G. Fujimoto, F. X. Kaertner, V. Scheuer, and G. Angelow, “Ultrabroadband beam splitter with matched group-delay dispersion”, Opt. Lett. 30, 1569 (2005).

18. L. Matos, O. Kuzucu, T. R. Schibli, J. Kim, E. P. Ippen, D. Kleppner and F. X. Kärtner, “Direct frequency comb generation from an octave-spanning, prism-less Ti:sapphire laser,” Optics Letters 29, pp. 1683-5, 2004.

19. Y. Takushima, H. A. Haus and F. X. Kärtner, “The noise of ultrashort pulse mode-locked lasers beyond the slowly-varing envelope approximation,” J. Opt. B: Quantum Semiclass. Opt. 6, 746-756, 2004.

20. P. C. Wagenblast, T. H. Ko, J. G. Fujimoto F. X. Kärtner, and U. Morgner, “Ultrahigh-resolution optical coherence tomography with a diode-pumped broadband Cr3+:LiCAF laser”, Optics Express 12, 3257-3260, 2004.

21. S. N. Tandon, J. T. Gopinath, H. M. Shen, G. S. Petrich, L. A. Kolodziejski, F. X. Kärtner, and E. P. Ippen , “Large Area Broadband Saturable Bragg Reflectors using Oxidized AlAs,” Opt. Lett. 29, 2551-53, 2004.

22. F. O. Ilday, F. W. Wise, and F. X. Kärtner, “Possibility of self-similar pulse evolution in a Ti:sapphire laser,” Optics Express 12, 2732, 2004.

23. O. D. Mücke, O. Kuzucu, F. N. C. Wong, E. P. Ippen, and F. X. Kärtner, S. M. Foreman, D. J. Jones, L.-S. Ma, J. L. Hall, and J. Ye “Experimental implementation of optical clockwork without carrier-envelope phase control,” Opt. Lett. 29, p.2806-09, 2004.

24. J. R. Birge and F. X. Kärtner, “Analysis and mitigation of systematic measurement errors in spectral shearing interferometry of few-cycle Pulses,” JOSA B, 25:(6), pp.A111-A119 (2008).

25. T. R. Schibli, O. Kuzucu, J. Kim, E. P. Ippen, J. G. Fujimoto, and F. X. Kärtner, V. Scheuer, G. Angelow, “Towards Single-Cycle Laser Systems,” Invited paper IEEE J. Selected Topics in Quantum Electronics 4, pp. 990-1001, 2003.

26. A. Sennaroglu, A. M. Kowalevicz, F. X. Kärtner, and J. G. Fujimoto, “High performance, compact, prismless, low-threshold 30 MHz Ti:Al2O3 laser,” Opt. Lett. 28, pp. 1674-76, (2003).

27. A.M. Kowalevicz, A. T. Zare, F. X. Kärtner, J. G. Fujimoto, S. Dewald, U. Morgner, V. Scheuer, and G. Angelow, “Generation of 150-nJ pulses from a multiple-pass cavity Kerr-lens modelocked Ti:Al2O3 oscillator,” Opt. Lett., 28, 1507-09, 2003.

28. P. Wagenblast, R. Ell, U. Morgner, F. Grawert and F. X. Kärtner, “10-fs, diode-pumped Cr3+:LiCAF laser,” Opt. Lett. 28, pp. 1713-15, 2003

29. D. J. Ripin, J. T. Gopinath, H. M. Shen, A. A. Erchak, G. S. Petrich, L. A. Kolodziejski, F. X. Kärtner, E. P. Ippen, “Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashort-pulse Cr4+:YAG laser,” Opt. Communications 214, 285-289, 2002.

30. P. Wagenblast, U. Morgner, F. Grawert, V. Scheuer, G. Angelow, M. J. Lederer, and F. X. Kärtner, “Generation of sub-10-fs pulses from a Kerr-lens modelocked Cr3+:LiCAF laser oscillator using third order dispersion compensating double chirped mirrors,” Opt. Lett. 27, 1726-9, 2002.

31. A. M. Kowalevicz, Jr., T. R. Schibli, F. X. Kärtner, and J. G. Fujimoto, “Ultralow-threshold Kerr- lens mode-locked Ti:Al2O3 lasers,” Opt. Lett. 27, 1-3, 2002.

32. U. Morgner, R. Ell, G. Metzler, T. R. Schibli, F. X. Kärtner, J. G. Fujimoto, H. A. Haus, and E. P. Ippen, “Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,” Phys. Rev. Lett. 86, 5462-5465, 2001.

33. R. Ell, U. Morgner, F. X. Kärtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, and T. Tschudi, “Generation of 5fs pulses and octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26, 373-375, 2001..