Today is the Day of Photonics so we would like to take the opportunity and say a few words about the MECSEL laser, developed in our laboratories, and about which we have not written here recently. The word MECSEL is the acronym for Membrane External Cavity Surface Emitting Laser. It’s NOT electrically pumped and is NOT monolithic (this means that the functions of the resonator and the gain medium are not performed simultaneously in the same piece of semiconductor). These features distinguish it from the standard semiconductor laser design.
However, resignation from these two features allows us to achieve a unique feature at the same time: the generation of a beam of good optical quality and high power – of many watts. Excellent beam quality and high power are required in a wide variety of applications, such as eye surgery, biological research, in vivo imaging, micro-technics, semiconductor materials research for telecommunications, etc.
While constructing the MECSEL laser, in order to obtain the perfect beam quality, we make sure that the device has axial symmetry, therefore we use an external linear resonator consisting of two dielectric mirrors, the setting of which is selected so that the laser emits a beam with a circular and Gaussian cross-section power distribution.
Another advantage of using an external resonator is also the optical access to the amplification medium, i.e. the possibility of using optical pumping. The optical arrangement of the pump ensures homogeneous excitation of a very large surface which results in a low optical and thermal power density. Thus, catastrophic degradation of the surface of the reinforcement medium is prevented, as well as the deterioration of the quality of the emitted beam occurring in other constructions as a result of spatially uneven heating of the surface.
As the amplification medium, a semiconductor heterostructure is used, i.e. a system of semiconductor layers – designed to take advantage of quantum phenomena, i.e. the wave nature of the carriers.
Quantum phenomena are observed only in the nano-world, i.e. where sizes are measured in nanometers (1nm = 0.000001mm). For efficient heat dissipation, the heterostructures are separated from the epitaxial substrate and transferred as very thin membranes to transparent coolers. The thickness of the membrane is only a few micrometers (1um = 0.001mm), about 5 times the length of the light wave emitted by MECSEL.
At Łukasiewicz – IMiF we use diamond coolers (made of real diamonds!) due to their good optical properties and excellent thermal conductivity. The use of nanometer layers with strictly controlled chemical compositions in the construction of heterostructures enables the production of lasers with emission at almost any wavelength in the near-infrared range.
In addition, lasers using semiconductor heterostructures have a wide gain band. Consequently, MECSEL lasers can be tuned in the range of many tens of nanometers, thus easily allowing to adjust the wavelength to a specific application. The works carried out so far at Łukasiewicz – IMiF have allowed to demonstrate MECSEL in the spectral range of 980nm and 1550-1780nm. These lasers were tuned in the ranges of 90 and 170 nm, respectively.