Ti:Sapphire Lasers Tyler Bowman April 23, 2015
Introduction Ti:Sapphire lasers are a solid state laser group based on using titanium-doped sapphire (Ti:Al 2O 3) plates as a gain medium. These lasers are very popular for a wide range of applications both commercially and for research due to their broad tunability, high stability, and significant potential output power. [1],[2] Additional research into Ti:sapphire has shown it to have a very short upper carrier lifetime, making it ideal for very high quality time-domain pulses. [3] Ti:Sapphire lasers have been shown to overtake many applications of dye lasers due to having similarly large ranges of frequency tuning in a solid-state device instead of a liquid dye. As such, there have been a wide number of applications that have begun to use Ti:sapphire and other crystal-based gain materials for lasers. The first successfully observed lasing of Ti:sapphire was reported by Moulton in 1986, where it was found that pumping with dye lasers, Nd:YAG lasers, and argon-ion lasers provided a tunable range from 660 nm to 986 nm. [4] The first commercial model for mode-locked time-domain pulsing was released in 1990, while the first continuous wave system was developed in 1998. [3] From that time Ti:sapphire lasers have expanded into a wide number of applications and research areas. Laser Properties The table in [3] gives a good number of properties of the gain medium for Ti:Sapphire systems. Property Value chemical formula Ti 3+ :Al2O3 crystal structure hexagonal melting point 2040 C thermal conductivity 33 W / (m K) thermal expansion coefficient 5 10 6 K 1 thermal shock resistance parameter 790 W/m refractive index at 633 nm 1.76 temperature dependence of refractive index 13 10 6 K 1 Ti density for 0.1% at. doping 4.56 10 19 cm 3 fluorescence lifetime 3.2 μs emission cross section at 790 nm 41 10 20 cm 2 Table 1: Standard properties of Ti:Sapphire [3]
It has been additionally reported that Ti:Sapphire has a good quantum efficiency at room temperature around 0.7 [5], with some estimations going as high as 0.8. [1] It is generally accepted in literature that pulsed Ti:Sapphire systems produce an output power of 0.1 to 3 W of power, while continuous wave systems can maintain a power of several watts. The pulse width of Ti:Sapphire systems is generally on the order of 100 fs, though several research setups of the lasers have approached widths of 5 fs or less. [3] Additionally, a wide range of values for continuous wave Ti:Sapphire lasers were performed in [1], where the laser was approximated as a 4-level system that was then reduced to a 2- level approximation. Within this system the upper state decay rate γ 2 was stated to be 288 khz (with a corresponding decay time of τ 2 = 3.4 µs. This is consistent with the properties reported in Table 1. Additional information found by the experimental analysis of the circular continuous wave cavity are the cavity loss and gain per pass, with the gain defined in terms of the incoming pump power these values are 3.6 % loss per pass in the cavity and 0.707% gain per watt per pass. Thus for that particular setup a pump power of around 5 W was required in order to achieve threshold inversion, though this value would change depending on the intracavity losses. Ti:Sapphire Fabrication The standard method for creating the Ti:Sapphire crystal used in solid state lasers is standard semiconductor processing. For doping sapphire with titanium ions, this generally involves melting down Al 2O 3 (sapphire) crystals in a crucible and infusing the melt with TiO 2 (titanium oxide). The selection of this particular oxide is in order to get the necessary Ti 3+ ions when the oxide bond is broken. A seed crystal of Al 2O 3 is then dipped into the mix and slowly drawn out such that the uniform sapphire crystal solidifies onto the seed. Once a rod of doped Ti:Al2O 3 has been pulled from the melt, it can then be sliced and sectioned into whatever size is needed This particular method is known as the Czochralski growth technique and is commonly used in the semiconductor industry. However, there are many other techniques that are effective in obtaining these crystals as well. [6]
Laser Cavity Construction The construction of the Ti:sapphire laser cavity has a very similar structure to most dye laser systems or other regions using a solid gain medium. A sample figure of the laser cavity is given in Fig. 1. Fig. 1: Basic Ti:Sapphire Laser setup (Image sourced from http://www.bradallica.com/het417/titanium_sapphire_lasers.htm) This image gives an example of a ring cavity Ti:sapphire laser excited by an arbitrary pump beam. This setup can also be realized as a resonant cavity in which the signal passes through the Ti:sapphire crystal gain medium twice per round trip through the cavity. However, the presence of intersecting waves in the gain medium can cause spatial hole burning that exhausts the gain medium. Additionally, a resonant cavity has the risk of sending interfering signals back to the pump laser. Finally there is some need for additional space for other components. On the other hand, aligning the beam waist within the Ti:Sapphire crystal for optimized gain is more feasible in the resonant cavity setup. Thus there are some benefits and drawbacks to either setup such that the type of cavity must be chosen based on the specific needs of the laser being built. Within the sample cavity of Fig. 1 there are several additional components in the optical path other than the gain medium and the mirrors. Each of these is a standard component for solid state or dye lasers in order to address some basic considerations for the design. The first object to be addressed
is the pump lens, which is needed for mode matching inside of the cavity. Since the exact waist position can be difficult to arrange in the center of the gain medium using the cavity mirrors alone, this focusing lens serves the purpose of bringing the pump signal to a focus in the middle of the crystal. This guarantees a strong and stable gain. [1] The second aspect being addressed is the optical diode, which serves the purpose of limiting the lasing to a single direction within the cavity. While it can be loosely assumed that any fluorescence from the crystal will be parallel with the incident pump signal, this is not the case in practice and there will be some fluorescence traveling the opposite direct through the cavity from the primary lasing signal. In order to eliminate this backwards-traveling signal, a birefringence layer or Faraday rotator is used to change the polarization of the waves that travel through it. The signals are then passed through a half wave plate. In this way, waves traveling in one direction can be given a rotation that eliminates the signal when it passes into the half wave plate, whereas the waves traveling in the other direction will simply change polarization and continue. [1] The third component to note is the standalone birefringent tuner in the optical path. This tuner consists of a material that is nonlinear at optical frequencies, and it is arranged in such a way that it is at a Brewster angle for the wavelength of emission being tuned. Given the nature of the nonlinear medium, any frequencies outside of the tuned frequency are either absorbed or redirected outside of the optical path of the system. In this way the desired frequency of the Ti:sapphire laser is isolated while the other frequencies of the laser s gain bandwidth are tuned out. The final additional component of the ring system is the etalon, which isolates the lowest order mode to propagate while absorbing the rest. [1] The functions of the additional components within the system are generally consistent even if the specific devices used vary. Specifically, components for the isolation of a single wavelength, the
enforcement of a single direction of lasing, and the reduction to a single mode are all key aspects of an effective Ti:Sapphire laser cavity. In addition to standard continuous wave activity in the cavity, one of the more attractive aspects of the Ti:Sapphire laser is a very robust ability to be in mode-lock. In short, mode-locking makes use of nonlinear optical systems in order to align the phase components of the signal. This alignment of the signal across a number of frequencies creates a constructive time domain pulse signal. Due to the large gain bandwidth of the Ti:Sapphire emission, these lasers are capable of a very refined pulse with a FWHM as small as 5.5 fs [3],[7]. This mode-lock can be achieved in several ways, but all methods take the role of an effective saturable absorber. [8] These setups generally involve actual saturable absorbers, Kerr lensing, and gain modulator manipulation of the signals, but the final goal is to obtain a group of frequencies within the gain bandwidth that are in phase. The use of Kerr lensing in particular has shown effectiveness in increasing the output power and decreasing the pulse width over saturable absorber reflectors. [9] The use of a mode-locked, pulsing signal opens up a wide number of applications that are not possible with a continuous wave laser. Applications One area of interest uses the wide tunable range of Ti:sapphire lasers in order to pump other laser and optical sources. This technology has been applied for exciting optical-band elements as well as other frequency ranges of interest like terahertz and X-ray signal generation. In particular, the very narrow Ti:Sapphire peak is useful for exciting semiconductor devices and antennas to produce terahertzfrequency pulses representing wide frequency ranges. For increasing the frequency to UV and X-ray frequencies, frequency multiplication of the Ti:Sapphire pulses allow the generation of signals not possible by any other laser system. [8] Other applications include the observance of chemical transitions and other optical properties of materials in very rapid time scales. Additionally, Ti:sapphire lasers have shown particular suitability to multiphonon microscopy due to a high peak power and tight focus. In this
application, a narrow peak and small focal point allow for highly specified fluorescence of chemical dyes. The precision of these measurements allows for higher resolution detection while avoiding potential bleaching of the dyes from overexposure. [10] Finally, Ti:Sapphire pulses have been shown to be useful in detecting chemical transitions on the femtosecond scale by exciting individual molecules. [11] In summary, Ti:sapphire lasers have been shown to be highly suitable to many applications. Conclusion Since their first development, Ti:sapphire lasers have shown a strong potential for a wide array of applications due to having a broad gain bandwidth resulting in a highly tunable continuous wave system or a narrow pulsed system in mode-lock. Due to its high flexibility, Ti:sapphire has become one of the most commonly used laser systems in both continuous wave and pulsed systems. References [1] W. L. Erickson and S. P. Singh. System Design and Relaxation Oscillations of a Titanium-Sapphire Laser. MS Thesis. University of Arkansas. 1992. [2] J. Klein. The Ti:Sapphire Laser: From Research to Industry and Beyond. Photonics Spectra. Published online. Accessed on 2015-04-23. <http://www.photonics.com/article.aspx?aid=42950> [3] R. Paschotta. Titanium-sapphire Lasers. Encyclopedia of Laser Physics and Technology, <http://www.rp-photonics.com/titanium_sapphire_lasers.html>, accessed on 2015-04-23. [4] P.F. Moulton. Spectroscopic and laser characteristics of Ti:Al 2O 3. J. Opt. Soc. Am. B, vol. 3, no. 1, January 1986. [5] P. Albers, E. Stark, and G. Huber. Continuous-wave laser operation and quantum efficiency of titanium-doped sapphire. J. Opt. Soc. Am. B, vol. 3, no. 1, January 1986. [6] R. Uecker, D. Klimm, S. Ganschow, P. Reiche, R. Bertram, M. Roßberg. Czochralski growth of Ti:sapphire laser crystals. Proceedings of SPIE 10. 2005.
[7] K.F. Wall and A. Sanchez. Titanium Sapphire Lasers. The Lincoln Laboratory Journal, vol. 3, no. 3, 1990. [8] G. Steinmeyer. A review of ultrafast optics and optoelectronics. J. Opt. A: Pure Appl. Opt., vol. 5, pp. R1-R15, 2003. [9] C.G. Durfee, T. Storz, J. Garlick. S. Hill, J.A. Squier, M. Kirchner, G. Taft, K. Shea, H. Kapteyn, M. Murnane, and S. Backus. Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser. OPTICS EXPRESS, vol. 20, no. 13, pp. 13677-13683, 18 June 2012. [10] M.D. Young, S. Backus, C. Durfee, and J. Squier. Multiphonon Imaging with a Direct-diode Pumped Femtosecond Ti:sapphire Laser. J. Microsc., vol. 249, no. 2, pp. 83-86, February 2013. [11] A. Talbpour, A.D. Bandrauk, J. Yang, S.L. Chin. Multiphoton ionization of inner-valence electrons and fragmentation of ethylene in an intense Ti:sapphire laser pulse. Chemical Physics Letters, vol. 313, pp. 789-794, 1999.