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The Ti:Sapphire Laser

From Research to Industry and Beyond

Julien Klein, Spectra-Physics

Since their invention in the early 1980s, titanium-doped sapphire (Ti:Al2O3 or Ti:sapphire) lasers and amplifiers have enabled countless applications in fundamental research in physics, biology and chemistry. Today, they play an important role across a wide range of photonics applications, including multicolor ultrafast spectroscopy, multiphoton deep-tissue imaging, terawatt and petawatt physics, and “cold” micromachining.

The first reported Ti:sapphire laser operation was performed in June 1982 by Peter Moulton at the 12th International Quantum Electronics Conference in Munich, Germany.1 It was the first time Ti3+ was used as the active ion for laser gain. In 1998, Spectra-Physics offered the first commercial Ti:sapphire laser, a broadly tunable continuous-wave model and, in late 1990, the first ultrafast Ti:sapphire laser, a picosecond mode-locked oscillator. Soon thereafter, the ultrafast and tunable laser communities quickly replaced their cumbersome dye lasers with the popular argon-ion-pumped Ti:sapphire systems, resulting in a sudden paradigm shift rarely seen in research.


This Spectra-Physics Tsunami oscillator is being pumped by a Millennia laser at 532 nm. Courtesy of Lund University in Sweden.


During their quick ascension in popularity, these laser systems continued evolving with the introduction in the mid-1990s of tabletop commercial amplified systems based on chirped pulse amplification techniques and, in 1996, with the Millennia, a high-power diode-pumped solid-state (DPSS) 532-nm laser. The use of DPSS lasers over argon-ion lasers as the pump significantly reduced the complexity of ultrafast setups and drastically reduced the amplitude noise, a key improvement for demanding femtosecond spectroscopy experiments.

Since then, a complete set of accessories has been developed to support and complement Ti:sapphire lasers. Optical parametric oscillators and amplifiers can extend wavelength tunability to access the deep-UV (<200 nm) and mid-IR regions (>20 μm). Such spectrally agile tools have proved invaluable for multiwavelength time-resolved spectroscopy.


Shown is a diode-pumped solid-state laser pumping an ultrafast Ti:sapphire laser.


Compared with other competing media, the Ti:sapphire medium is extremely flexible and provides high performance and several advantages. It is unmatched in its characteristics for delivering a combination of broad spectral bandwidth, a range of repetition rates, wide tunability and high-average-power levels. Spectral outputs of Ti:sapphire lasers range from ultranarrow single frequency to several hundred nanometers of bandwidth, resulting in ultrafast pulses as short as a few oscillations of the electric field at 5.5 fs. At the same time, repetition rates can range from single-shot output for maximum energy up to multigigahertz quasi-CW output with tunability of 400 nm and average powers of many watts.

As the push for even shorter, almost single-cycle pulses and high-precision optical metrology gained momentum, an important development was the stabilization of the carrier envelope phase (CEP), which paved the way for the generation of attosecond (10–18 s) pulses in the x-ray spectrum through high-harmonic generation. Attosecond laser sources are extending the frontier in probing dynamic molecular processes with unprecedented time resolution.

CEP stabilization also plays a critical role in optical frequency metrology, where an octave-spanning CEP-stabilized ultrafast oscillator can be used as a frequency comb. Frequency combs are accurate “optical clocks” and can measure optical frequencies with extraordinary precision. This work was honored with the 2005 Nobel Prize in physics, which was awarded to Theodore W. Hänsch and John L. Hall.

In chemistry, Ti:sapphire laser systems are used to study chemical reactions on ultrafast time scales. Ahmed H. Zewail received the 1999 Nobel Prize in chemistry for his pioneering work in the transition states of chemical reactions using femtosecond spectroscopy. Recently, the field of coherent control has grown increasingly sophisticated, and devices to control and measure the spectral phase and amplitude of the ultrafast pulses have been developed.

In biology, Ti:sapphire lasers are instrumental in multiphoton microscopy (MPM), which has developed into the leading noninvasive laboratory tool for studying underlying biological phenomena. It offers high-resolution three-dimensional imaging in thick tissues, including in vivo specimens. Because MPM inherently relies upon a nonlinear process – two-photon absorption – it requires a high peak power, which at first limited the practicality of MPM in the absence of easy-to-use, tunable femtosecond lasers. The intensity of the laser required for an adequate signal would have damaged the sample.


Pictured is the Mai Tai laser from Spectra-Physics.


However, a breakthrough occurred in 1990 when Winfried Denk, James H. Strickler and Watt W. Webb used a femtosecond dye laser in the first successful demonstration of MPM.2 Because of the combination of ultrashort pulses and low duty cycle, these ultrafast lasers offered high peak powers with low average power – the ideal combination because lower average powers eliminated thermal damage issues. The lasers were cumbersome and difficult to maintain, and they required multiple mirror sets and adjustments.

It was not until the recent advent of turnkey, hands-free, commercially available, diode-pumped Ti:sapphire lasers such as the Spectra-Physics Mai Tai that MPM became more practical. The extended tunability of these lasers has enabled the use of various dyes with distinct absorption spectra and chemical properties. Because in MPM the nonlinear signal is generated only in a small volume centered at the focal point of a tightly focused beam, the “out-of-focus” signal is greatly reduced compared with single-photon fluorescence microscopy – giving MPM automatic sectioning capability for deep imaging of live tissue.


This image of a mouse kidney, in vivo, using second-harmonic generation (SHG) imaging for collagen (blue) and autofluorescence signal (green), was taken using a Spectra-Physics Mai Tai eHP DeepSee ultrafast laser. Courtesy of Dr. Claudio Vinegoni, Center for Molecular Imaging Research at Massachusetts General Hospital, Harvard University.


Additionally, the Ti:sapphire laser has been instrumental in fields such as nonlinear physics and terahertz generation. It also is being used for cold micromachining, where the cutting, drilling and scribing are free of undesirable thermal effects.

Indeed, the Ti:sapphire is unsurpassed in its extraordinary breadth of performance and resulting diversity of applications. In particular, its ability to generate ultrafast pulses and wide wavelength tunability enable unprecedented advances across a range of disciplines in science, industry and beyond.

Meet the author

Julien Klein is senior manager of product marketing at Spectra-Physics, a Newport Corporation Brand in Santa Clara, Calif.; e-mail: julien.klein@newport.com.

References

1. P.F. Moulton (1986). Spectroscopic and laser characteristics of Ti:A12O3. J. Opt. Soc. B, Vol. 3, p. 125.

2. W. Denk, J.H. Strickler and W.W. Webb (1990). Two-photon laser scanning fluorescence microscopy, Science, Vol. 248, pp. 73-76.



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