Novel Crystal Growth Method Enables Eye-Safe Laser

A compact, eye-safe laser light source has been developed for practical applications in medicine, communications and ranging. The solid-state laser emits in the 1500- to 1600-nm range, and is based on a novel laboratory growth technology for single crystals.

As-grown (Er,Yb):GdAl₃(BO₃)₄ crystal created by using high-temperature solution growth on dipped seeds (a). Courtesy of Nikolay Leonyuk.

Based on yttrium gadolinium borate crystals, the laser has a lifetime of up to 100,000 hours. The plug-and-play system doesn’t require water cooling or generate vibration during operations, said researchers from Lomonosov Moscow State University and the Belarusian National Technical University.

By using a Co²⁺:MgAl₂O₄ crystal as a saturable absorber, Q-switched laser pulses with a duration of 12 ns and a maximum energy of 18.7 μJ at a repetition rate of 32 kHz corresponding to an average output power of 0.6 W were obtained at 1550 nm under CW pumping. In the burst mode of operation, Q-switched laser pulses with the highest energy up to 44 μJ were realized with a pulse repetition rate of 6.5 kHz.

An experimental set-up of the laser operating on a (Er,Yb): GdAl₃(BO₃)₄ crystal. Courtesy of Nikolay Leonyuk.

The light-refracting system of the eye (cornea and crystalline lens) has a sufficiently high absorption coefficient in the 1500- to 1600-nm range, so only a small fraction of the energy reaches the sensitive retina. Radiation in this range also suffers low losses passing through the atmosphere, which makes it advantageous for applications in telecommunications, the researchers said.

Among sources of radiation in this spectral range, the most widely used are solid-state lasers based on phosphate glasses co-doped with Er (erbium), and Yb (ytterbium) ions. Such lasers are also relatively simple, compact and capable of operating in adjusted Q-mode required for producing short impulses. The main disadvantage restricting the usage of erbium phosphate glasses in continuous diode systems is the low thermal conductivity of the matrix.

To overcome this limitation, the team used GdAl₃(BO₃)₄ single crystals co-doped with Er and Yb to improve the efficiency of generation pulse energy and repetition rate, and hence to increase the maximal measurement range, reducing errors and time spending.

The single crystals were characterized by a record value of thermal conductivity and high thermochemical stability (decomposition at temperatures of 1280 °C, resistance to corrosive environments), as well as mechanical strength.

The research was published in the Journal of Crystal Growth (doi: 10.1016/j.jcrysgro.2013.11.100) as well as Optics Letters (doi: 10.1364/ol.41.000918).