What are Alexandrite Lasers?

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- GoPhotonics

Jan 16, 2023

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 4Tstate. 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

Laser Wavelengths700 - 820 nm
Laser transition probability3.8 x 103/s
Upper laser level lifetime260 µs at 298 K
Stimulated emission cross-section1 x 10-24 m2
Spontaneous emission linewidth and gain bandwidth, FWHM2.6 x 1013/s
Inversion density6 x 1024/m3
Small signal gain coefficient4 - 20/m
Laser gain medium length0.12 m
Single pass gain1.6 - 11
Doping density1.75 to 10 x 1025/m3
Index of refraction of gain medium1.74
Operating temperature500 K
Thermal conductivity of laser rod23 W/m-K
Thermal expansion coefficient of laser rod6 x 10-6/K
Pumping methodOptical (flashlamp or laser)
Pumping bands380 - 630 nm, with peaks at 410 nm and 590 nm
Output powerup to 1.2 J/pulse
ModeSingle-mode or multi-mode


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


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


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.