What are Ti:Sapphire Lasers?

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- GoPhotonics

Jan 16, 2023

The Ti:Sapphire laser (Ti:Al2O3 laser) is an efficient tunable solid-state laser that emits light in the visible and near-infrared region ranging from 650 to 1180 nm. This laser was first demonstrated by Peter Moulton in 1982 at MIT Lincoln Laboratory. A Sapphire crystal doped with Ti3+ ions act as the active medium for these lasers. The quantity of titanium ions within the host material is 0.1% and they substitute the aluminum atoms in the crystal. The laser employs optical pumping in a wavelength range of 514 to 532 nm. This laser operates both in continuous mode and pulsed mode. For continuous wave operation, argon-ion lasers (514.5 nm) are used to pump the laser and to operate in pulsed mode,  frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) are used. Ti:Sapphire lasers operate at wavelengths near 800 nm very efficiently. Ti:Sapphire lasers are widely used in the laser industry due to their outstanding mode-locking properties, which make it possible to produce femtosecond pulses with great ease and stability. The passive mode locking method, usually in the form of Kerr lens mode locking is used to produce ultrashort pulses from these lasers. They are mainly used in scientific research applications.

Structure of a Ti:Sapphire Laser

Figure 1: Schematic of a Ti:Sapphire Laser

A titanium-doped sapphire crystal is the lasing medium for this laser. This medium is placed in a resonator cavity consisting of two mirrors. The output is obtained through the output coupling mirror. Optical pumping is provided for this laser. A diffraction grating that selectively chose the required wavelength is used for the wavelength tuning or a birefringent filter or prism installed within the cavity at Brewster’s angle can be rotated for wavelength tuning. Figure 1 represents the basic structure of a Ti:Sapphire laser. 

Figure 2: CW Ti:Sapphire X fold configuration

A continuous wave Ti:Al2O3 laser uses an x-cavity design containing an astigmatically compensated cavity for the laser crystal. Depending on the dopant level, the crystal in such a cavity design typically has a length of 2 to 10 mm and is arranged with its output faces at Brewster angle. The longer length crystal lengths with lower doping concentrations are used with higher pumping flux intensities in order to obtain higher power output. Generally, either a continuous wave argon ion laser or a doubled Nd:YAG laser is used as a pumping source. The beam enters the cavity from the left. Wavelength can be tuned by rotating the birefringent filter installed within the cavity at Brewster’s angle. Figure 2 shows the continuous wave Ti:Sapphire X fold configuration.

Figure 3: Femtosecond mode-locked Ti:Sapphire Laser

The modified version of the cavity is used to produce mode-locked pulses. It contains two prisms for intracavity dispersion compensation and uses the Kerr lens mode-locking (KLM) technique. The method of mode-locking lasers via a nonlinear optical process known as the optical Kerr effect is Kerr-lens mode-locking (KLM). With this technique, it is possible to produce light pulses with durations as short as a few femtoseconds. The necessary aperture within the Ti:Al2O3 crystal to produce KLM is provided by a separate aperture located next to the crystal or simply by the aperture effect associated with the small diameter of the pump beam. Extremely precise adjustments and alignment of the mirror and cavity dimensions are essential to maintain a stable mode-locked output for this laser. Figure 3 shows the schematic of a Femtosecond mode-locked Ti:Sapphire laser.

Energy level diagram of Ti:Sapphire laser

Figure 4: Energy level diagram of Ti:Sapphire Laser

The ground state 2T2 contains a sequence of overlapping vibrational or vibronic levels. The 2E state is the first excited state that extends upwards with a series of overlapping vibronic levels like the ground state. 

When the laser is pumped, the excitation occurs from the lowest vibronic levels of the 2T2 to the vibronic levels of the 2E excited state. The atoms or ions pumped to these vibronic levels of the broadband excited state will rapidly relax to the lowest level of the 2E state. And then they decay back to any one of the vibronic levels of the ground state by emitting laser radiation. When they reach the excited vibronic levels of the ground state, rapid relaxation to the lowest lying levels occurs. 

In the case of gain media with high density, the energy level arrangement of the laser is effectively a four-level system. Level i of the four-level system will be the higher-lying vibronic levels of the 2E state and level u will be the upper laser level. The atom or ion decay to any of the excited vibrational levels I of ground state 2T2  will cause laser emission. These rapidly relax to the lowest level of 2T2 which serves as ground 0. Figure 4 shows the energy level transitions in Ti:Sapphire laser.

Parameters of Ti:Sapphire Laser

Laser Wavelengths660 - 1180 nm
Laser transition probability2.6 x 105/s
Upper laser level lifetime3.8 µs
Stimulated emission cross-section3.4 x 10-23 m2
Spontaneous emission linewidth and gain bandwidth, FWHM1 x 1014/s
Inversion density6 x 1023/m3
Small signal gain coefficient20/m
Laser gain medium length0.1 m
Single pass gain7-10
Doping density3.3 x 1025/m3
Index of refraction of gain medium1.76
Operating temperature300 K
Thermal conductivity of laser rod3.55 W/m-K
Thermal expansion coefficient of laser rod5 x 10-6/K
Pumping methodOptical (flashlamp or laser)
Pumping bands380 - 620 nm
Output powerup to 50 W (CW), 1012 W for 100-fs pulse
ModeSingle-mode or multi-mode

Advantages of Ti:Sapphire Laser

  • Wide wavelength tunability
  • Efficient and reliable
  • Large stimulated emission cross-section
  • Broad gain bandwidth
  • High saturation energy

Disadvantages of Ti:Sapphire Laser

  • High cost
  • Large and complex
  • Not useful for industrial and mobile applications

Applications of Ti:Sapphire Laser

The primary applications for Ti:Sapphire lasers are in research labs, specifically spectroscopy. These lasers are suitable for producing tunable sub-picosecond pulses at low wavelengths due to their broad tuning range. NASA's Lidar Atmospheric Sensing Experiment uses Ti:Sapphire lasers to measure water vapour and aerosols and their effect on atmospheric dynamics. In order to examine chemical reactions on ultrafast time scales, Ti:Sapphire laser systems are used. Terahertz generation and nonlinear physics are two areas where sapphire lasers have proven to be useful.

Ti:Sapphire lasers are used in the medical field for applications like Photodynamic therapy which is used to destroy cancer cells or diseased tissues. Multiphoton microscopy, which has become the top non-invasive laboratory tool for studying underlying biological phenomena, depends on Ti:Sapphire lasers. High-resolution three-dimensional imaging in thick tissues, including in vivo specimens, is offered by this technology. 

They are also used in laser radar, range finders, and remote sensing. Ti:Sapphire lasers are used in the production of nanoholes and laser-assisted micro-injection of exogenous substances into living cells.