The Alexandrite laser is a tunable solid-state laser that uses a gemstone called alexandrite. Alexandrite is a common name for chrysoberyl (BeAl₂O₄) doped chromium (Cr³⁺) ions and is used as the active medium in this laser. The fundamental emission wavelength of this laser is 752 nm. The laser output can be tuned from 710 nm to 820 nm under optimum operating conditions. The output power of the laser beam can be up to 20 watts. Pulses with a duration of 100 µs and 1-3 J energy can be obtained. This laser is optically pumped with flashlamps, continuous arc lamps, or laser diodes.

Alexandrite lasers can achieve frequency doubling with the use of nonlinear crystals. By using a barium beta-borate (BBO) crystal, the frequency can be doubled resulting in the wavelength going from 752 nm to 376 nm. At this wavelength, water absorption is very good. The laser can be operated in continuous as well as in pulsed mode by employing both Q-switching and mode-locking. The vibrational levels which are a result of strong coupling between Cr³⁺ ions and lattice vibrations help in the tunability of this laser.

Structure of Alexandrite laser

Figure 1: Schematic of Alexandrite Laser

The gain medium of the laser is an alexandrite rod kept in between two resonator mirrors: a dichroic back mirror (BM) and an output coupler (OC). The dichroic back mirror is a highly reflective mirror. The pumping is provided optically by means of laser diodes or flash lamps. The beam-shaping optics are used to focus the pump beam into the laser cavity. The laser output comes out of the output coupler. They use Pockels cells for Q-switching purposes. For tuning, a feedback grating element is used that selectively reflects the wavelength needed. A plano-convex lens (PCX) is used to focus this feedback light beam into the alexandrite rod. Figure 1 represents the schematic of the alexandrite laser.

Figure 2: Wavelength tuning in Alexandrite Laser

Sometimes, a prism or tunable filter with a narrow bandwidth is used inside the optical resonator for wavelength tuning as shown in figure 2. The filter causes losses at all the wavelengths other than the required wavelength. 

Energy level diagram of Alexandrite laser

Figure 3: Energy level diagram of Alexandrite Laser

Laser transitions occur within closely packed energy levels in an alexandrite laser. This laser lases at both four-level and three-level. The ⁴A₂ ground state consists of a series of vibrational levels that are populated according to Boltzmann distribution. In the three-level state, when strong pump radiation occurs, the transition happens from ⁴A₂ which is the ground state to the upper laser level ⁴T₂ which is a band of vibrational levels. Then rapid relaxation to the lowest levels of ⁴T₂ occurs and then the electrons relax faster to the ²E state, which is coupled to ⁴T₂. ⁴T₂ has a relatively short lifetime of 6.6 µs, but it is strongly coupled to the longer-lived ²E state located 0.1 eV below the ⁴T₂ state. Most of the decay from the ⁴T₂ state occurs to the ²E state. Lasing occurs when the transition happens between the ²E state and the ground state ⁴A₂. The laser output obtained will be fixed (680.4 nm at room temperature) and will have relatively low efficiency. The threshold is very high at this level of transition.

In the four-level mode, ⁴T₂ is the absorption state. The lasing takes place between the ⁴T₂ state and excited vibronic states within ⁴A₂, which is the ground state. Laser wavelength depends on the vibrationally excited terminal. The energy that is not released as a laser photon will be carried away by a vibrational phonon leaving the chromium ion at its ground state. Figure 3 shows the energy level transitions in the alexandrite laser.

Laser Parameters of Alexandrite laser

Advantages 

• They are used for high-power-density applications.
• Efficiency is very high.
• Q-switching can be done for pulsed or continuous wave operation.

Disadvantages 

• Production is slow for thicker materials
• Not suitable for materials with moderate thickness
• Radiation absorption is low for lighter materials

Applications

Alexandrite lasers operate using a process called Photothermolysis. The high-energy light of a certain wavelength emitted by the laser is converted to heat energy and it damages a particular area. This technique is used to destroy tissues in the medical field. Therefore, the important application of alexandrite laser is in the medical field. They are used for dermatological procedures like tattoo removal, leg vein treatment, hair removal, vascular lesions, cancer therapy, kidney stone removal, etc. 

In the hair removal procedure, a suitable wavelength from the laser that falls in the midrange of the melanin absorbing spectrum is targeted to the melanin of the hair follicle. This damages the hair shafts and the surrounding follicles.  

The high pulse repetition rates and average power of this laser are suitable for tattoo removal applications. The tattooed ink absorbs the laser light and undergoes chemical transformation and thereby removing the tattoo from the body. Q-switched alexandrite lasers that heat the ink particle more are favorable for this treatment.

The wavelength tunability due to the wide emission spectrum of the alexandrite laser is used for laser spectroscopy. But since Ti:Sapphire lasers offer broader tunability, alexandrite lasers are not much used.