Microchip Lasers and Micro-Optics Technologies
Authors: Molva E. , Thony Ph. , Fulbert L. , Marty J. , Rabarot M. , Ferrand B. 
Published: February 2023

Microchip lasers are the most compact (~1mm3) and the simplest diode pumped solid state lasers The mirrors of the optical cavity are directly deposited on the polished faces of a thin (-1mm) laser material. They are fabricated using collective fabrication processes allowing a low cost mass production. A microchip laser has a typical size of 1×1×0.5 mm3. It is very reliable and simple to use. The microchip laser is a kind of optical transformer which transforms a poor quality laser diode beam to a diffraction limited TEM00 and single frequency laser beam Moreover, by active or passive Q-switching , very short pulses (-0.4 to 2 ns) with very high peak power (0.5 to 50 kW) can be obtained. Standard wavelengths are: 1μm or 1.3μm with Nd doped materials such as YAG, YVO4, LMA, etc.; 1.5μm with Er and Yb co-doped phosphate glass; 2μm with Tm doped materials. The laser threshold is generally low, i.e as low as 9mW for 1.5μm microchip laser with a slope efficiency of 38%. Passively Q-switched Nd:YAG microchip laser can be fabricated using a novel process. A thin film (~30-50μm) of Cr4+:YAG, which is a saturable absorber at 1064nm, is epitaxially grown on 25mm diameter and 0.5 to 1mm thick Nd:YAG subtrates. The absorption coefficient of the epitaxial thin film saturable absorber is ~20-25 cm-1, about five times higher than the bulk Cr4+:YAG. The saturable absorption is adjusted by polishing the thin film. This process has the advantage to keep the monolithic structure of the microchip laser without any optical interface between the active and passive materials, and allows a great flexibility for the design of Q-switched lasers. Typical pulse energies are between 0.5 to 5μJ using low power diode pumps, and the pulse width is 0.4 to 2ns. Repetition rates as high 100kHz have been obtained. The pulse energy, width and peak power are independant of the pump power, which acts only on the pulse repetition rate which increases linearly with the diode pump power. The plane-parallel Fabry-Perot cavity of the microchip laser is optically stabilized by thermal effects due to the heating by the pump power. The laser mode is also defined by thermal effects. In order to avoid this problem, stable plano-concave optical cavities are fabricated using photolithography and ion beam etching technologies such as used currently in microelectronics. Small microlenses with 100-200μm in diameter and 1μm thick are directly fabricated with a collective process on laser materials such as Nd:YAG or Er,Yb:glass. The microlense acts as a concave micromirror for the microchip laser cavity, with a radius of curvature of few mm, so that the cavity is optically stable and the laser mode is well defined. The laser threshold is drastically reduced, i.e. less than 2mW incident pump power in CW mode Moreover, this technology allows also the fabrication of micro-optical components, such as microlenses on silica substrates, which can be used for the fabrication of optical microsystems including microchip lasers. For exemple such microlenses are used for the fiber coupling of a stable cavity microchip laser. The low cost, the re lability, the excellent beam quality and the high peak power of the microchip lasers are very useful for several industrial applications such as: time of flight range finding; micro-marking of materials; compact green laser for alignment; injection locking; etc.