Titanium Sapphire

For many years, Ti:sapphire was the prototypical example of a solid-state laser material that could not be diode pumped. The rationale for this assessment follows from the laser properties of Ti:sapphire, which demand high brightness pumping in the blue-green region [1]. The development of efficient Gallium Nitride (GaN) based diode lasers [2], however, began to erode this logic and improvements in the spatial brightness of GaN lasers subsequently enabled the first demonstration of a directly diode-laser pumped Ti:sapphire laser in 2009 [3]. This presentation will outline the physics that makes diode-pumping difficult, and the developments that mean, it is, nonetheless, possible. Interestingly, diode-pumping of CW and modelocked Ti:sapphire lasers was achieved not by a radical redesign of the laser, but by careful optimisation of existing approaches that enabled the rapidly improving brightness of GaN diode lasers to be exploited [3-5]. An interesting outstanding question is whether this will remain the right approach in the longer term.
The performance early diode-pumped Ti:sapphire lasers was very modest, in part because the performance of GaN diodes was still marginal for Ti:sapphire pumping. However, this early work hinted at a second issue – that of reduced oscillator efficiency for blue, rather than green pumping [3]. While rigorous subsequent work by Sawada et al. [6] and Moulton et al. [7] has begun to unpick the physics behind this effect the details and implications for Ti:sapphire laser design remain in doubt.
Probably the most significant factor in the subsequent improvements in the performance and functionality of diode-pumped Ti:sapphire lasers has been the emergence of higher power and longer wavelength GaN diode lasers. These diodes have been harnessed by a number of groups world-wide to enable ground-breaking demonstrations. Particular highlights include the demonstration of Kerr-lens modelocking [8], of a frequency comb [9], of a regenerative amplifier [10], of broad tuneability [11], and the announcement of a first commercial system [12]. The most recent demonstrations include GHz repetition-rate modelocking [13] and Ti:sapphire fibre lasers [14]. This progress indicates a bright future for diode-pumped Ti:sapphire lasers, with the potential for further gains in performance and practicality if some of the peculiarities of their operation can be unpicked.

[1]   P. F. Moulton, "Spectroscopic and Laser Characteristics of Ti:Al2O3," J. Opt. Soc. Am. B-Opt. Phys., Article vol. 3, no. 1, pp. 125-133, 1986.
[2]   S. Nakamura, "First III-V-nitride-based violet laser diodes," J. Cryst. Growth, vol. 170, no. 1-4, pp. 11-15, 1997.
[3]   P. W. Roth, A. J. Maclean, D. Burns, and A. J. Kemp, "Directly diode-laser-pumped Ti:sapphire laser," Opt. Lett., vol. 34, no. 21, pp. 3334-3336, 2009. [Online]. Available: http://ol.osa.org/abstract.cfm?URI=ol-34-21-3334
[4]   P. W. Roth, D. Burns, and A. J. Kemp, "Power scaling of a directly diode-laser-pumped Ti:sapphire laser," Opt. Express, vol. 20, no. 18, pp. 20629-20634, 2012. [Online]. Available: http://www.opticsexpress.org/abstract.cfm?URI=oe-20-18-20629.
[5]   P. W. Roth, A. J. Maclean, D. Burns, and A. J. Kemp, "Direct diode-laser pumping of a mode-locked Ti:sapphire laser," Opt. Lett., vol. 36, no. 2, pp. 304-306, 2011. [Online]. Available: http://ol.osa.org/abstract.cfm?URI=ol-36-2-304
[6]   R. Sawada, H. Tanaka, N. Sugiyama, and F. Kannari, "Wavelength-multiplexed pumping with 478- and 520-nm indium gallium nitride laser diodes for Ti:sapphire laser," Appl. Optics, vol. 56, no. 6, pp. 1654-1661, 2017, doi: 10.1364/AO.56.001654.
[7]    P. F. Moulton, J. G. Cederberg, K. T. Stevens, G. Foundos, M. Koselja, and J. Preclikova, "Characterization of Absorption Bands in Ti:sapphire Crystals," in Laser Congress 2018 (ASSL), Boston, Massachusetts, 2018/11/04 2018: Optical Society of America, in OSA Technical Digest, p. AM4A.2, doi: 10.1364/ASSL.2018.AM4A.2. [Online]. Available: http://www.osapublishing.org/abstract.cfm?URI=ASSL-2018-AM4A.2
[8]   C. G. Durfee et al., "Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser," Opt. Express, vol. 20, no. 13, pp. 13677-13683, 2012. [Online]. Available: http://www.opticsexpress.org/abstract.cfm?URI=oe-20-13-13677.
[9]   K. Gürel, V. J. Wittwer, S. Hakobyan, S. Schilt, and T. Südmeyer, "Carrier envelope offset frequency detection and stabilization of a diode-pumped mode-locked Ti:sapphire laser," Opt. Lett., vol. 42, no. 6, pp. 1035-1038, 2017/03/15 2017, doi: 10.1364/OL.42.001035.
[10]  S. Backus et al., "Direct diode pumped Ti:sapphire ultrafast regenerative amplifier system," Opt. Express, vol. 25, no. 4, pp. 3666-3674, 2017/02/20 2017, doi: 10.1364/OE.25.003666.
[11]  J. C. E. Coyle, A. J. Kemp, J.-M. Hopkins, and A. A. Lagatsky, "Ultrafast diode-pumped Ti:sapphire laser with broad tunability," Opt. Express, vol. 26, no. 6, pp. 6826-6832, 2018/03/19 2018, doi: 10.1364/OE.26.006826.
[12]  J. Wallace. (2017, Nov.) KMLabs commercially introduces direct diode pumped Ti:sapphire ultrafast laser. Laser Focus World. 11.
[13]  H. Ostapenko, T. Mitchell, P. Castro-Marin, and D. T. Reid, "Three-element, self-starting Kerr-lens-modelocked 1-GHz Ti:sapphire oscillator pumped by a single laser diode," Opt Express, vol. 30, no. 22, pp. 39624-39630, Oct 24 2022, doi: 10.1364/OE.472533.
[14]  T. Yang et al., "Widely Tunable, 25-mW Power, Ti:sapphire Crystal-Fiber Laser," IEEE Photonics Technol. Lett., vol. 31, no. 24, pp. 1921-1924, 2019, doi: 10.1109/LPT.2019.2950020.