What are Alexandrite Rods?

An Alexandrite rod is a solid-state chromium-doped laser useful in dermatology and other medical applications. You’ll find alexandrite gemstones in nature. But for medical uses requiring high crystal quality, the Cr3+:BeAl2O4 crystals are grown using a modified Czochralski method. This chromium-doped beryllium aluminate material can also be used for Lidar or Light Detection and Ranging operations.

The properties that make the alexandrite crystal suitable for its specific uses include:

  • High thermal shock resistance, in fact as much as five times that of Nd YAG lasers.
  • Near the optimal wavelength of 750 nm, you can achieve maximum laser gain and electrical-to-optical power efficiency. 755nm alexandrite has dermatological uses.
  • Alexandrite can be used to make wavelength-tunable lasers with emission ranges between 710 and 800nm
  • It can also be used for narrowband lasing on the R-line.
  • Low symmetry, birefringence and an anisotropic, orthorhombic crystal structure makes it easy to achieve linear polarization with low losses in a resonator due to depolarization
  • The high thermo-mechanical and mechanical strength of alexandrite makes it easy to lamp pump at higher powers than in lamp-pumped Nd: YAG lasers.


Crystal characteristics include:

  • 0.03 to 0.5% Cr concentration. (Higher Cr is associated with lower quality crystal)
  • Transition cross section at 300K (cm2) is 3 * 10 -19
  • Lifetime is 260 x 10 -6 seconds
  • Optical loss is 0.001 to 0.003 -cm
  • The rods appear red with artificial lighting and green in daylight.
  • Refractive index at 750 nm is 1.7346 – 1.7421
  • Thermal conductivity is 0.23 W/cm:K
  • Young’s modulus is 469 GPa

Properties of Alexandrite Rods

Alexandrite has more complicated spectroscopic properties than Nd: YAG crystals. Its gain and absorption properties depend heavily on the direction of light polarization.

The presence of phonons in the lower laser level also further complicates the stereoscopic properties. Phonons are quantized lattice vibrations. There is a strong interaction between the phonons and the electronic states. As a result, there can be transitions where not just a photon but also several phonons are emitted. This ‘vibronic’ interaction or vibrational-electronic interaction leads to large gain bandwidth.

Note that alexandrite can also exhibit non-vibronic transitions. These R-line transitions have lower optical bandwidths.

Also, achievable gain and efficiencies are highest at high temperatures. This is because the upper laser level has slightly higher excitation energy than the lower level right below it. As a result, pulsed alexandrite lasers perform better at higher temperatures of around 100 to 150 degrees C.

However high-temperature performance degrades in continuous-wave lasers since the lifetime in the upper laser level is lower at higher temperatures.

It’s also useful to note that small emission cross-sections and high fluence can cause optical damage and lower efficiency in energy extraction. Even crystals with high optical quality will have an effective optical damage threshold below the intrinsic threshold. Optical damage is usually observed on the surfaces of crystals.


Lamp-pumped alexandrite rods

The Alex rod as it’s also known has been around since the late 1970s. It was one of the first vibronic laser gain media at the time. Back then, continuous-wave Nd: YAG lasers were used to end-pump alexandrite lasers and improve power conversion efficiency.

These days, it’s less cost-effective to use lasers for end-pumping instead of lamps. Powerful xenon flash lamps that fracture Nd: YAG lasers can be used successfully on alexandrite rods. Many applications use rods that are 10cm long and have cylindrical diameters of around a quarter inch.

Simple laser resonators can achieve pulse energies of one to several joules but beam quality tends to be low. Unstable resonators offer a balance between beam quality and power efficiency than laser resonators. However, they work best with low magnification.

Flash lamps offer the best results. Pulse energies of several kilojoules at low pulse repetition rates between 10 and 30 Hz are possible. Free-running long-pulse lasers are commonly used. Q switching can achieve shorter pulses of below 30 ns. Higher peak powers are possible in such cases, though at lower efficiency.

Amplifiers are best for energy extraction. A MOPA (master oscillator power amplifier) can offer good performance at the cost of simplicity.


Applications of Alexandrite Rods

At emissions of 755nm, easily absorbed by melanin, the Alexandrite rod laser is great for removing hair. This makes it suitable for dermatological applications. A long-pulse laser emits light that the hair shafts absorb. The surrounding follicles and hair shafts are damaged and hair production thins or stops. The skin is cooled during the procedure, to limit damage to the surface of the skin.

Because of the shorter emission wavelength, it is better for finer hair removal. However, it tends to cause more damage to dark skin than Nd: YAG lasers.

Alexandrite lasers can also remove or lighten tattoos. The tattoo ink absorbs the laser energy and chemically transforms. The body can then remove the transformed ink. Alexandrite is better than ruby lasers in this application, due to higher pulse repetition rates and higher average power. Q-switched laser and short pulse durations are best for removing tattoo ink.

Alexandrite lasers can also be used to treat pigmented lesions or leg veins that are visible. Shorter treatment times come with the higher pulse energies. free-running lasers are usually used in dermatological applications. Simple lasers work well in this application.

Laser spectroscopy is another area where alexandrite lasers may make the most of their tunability.


In Conclusion

The Alexandrite laser crystal rods are available in the form of translucent violet rods with a melting point of 1870 degrees C. The material has a Vickers hardness of 2000kg/mm2. It is usually available in a range of dimensions and can be customized to requirements. The uniformity of Cr concentration along each rod is made possible by using small portions of the melt, at around 20%.

Besides uses in dermatology and atmospheric lidar, Alexandrite lasers can also be used in spectroscopy, laser medical devices for long pulse, picosecond or Q-switch 755nm lasers. They can also be used in spectroscopy and lithotripsy.

High quality and competitive pricing along with high efficiency and low thresholds give Alexandrite several advantages over other laser gain media.

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