Edge-Emitting Laser (EEL)
The edge-emitting laser is the original semiconductor laser concept and a mature technology used for several decades. Light propagates in a waveguide structure parallel to the semiconductor surface and is emitted from the edges of the semiconductor chip, which serve as cavity mirrors. With a fairly long active region of typically hundreds of micrometers to a few millimeters, a large amount of gain is provided and high output powers can be achieved. Electrically pumped EELs are compact and cost-effective sources of laser emission, suitable for numerous applications.
Reference: [Huang et al., IEEE J. Quantum Electron. 55, 1-7 (2019)]
Vertical Cavity Surface-Emitting Laser (VCSEL)
In contrast to edge-emitting lasers, the emission from VCSELs is directed perpendicular to the semiconductor surface. The vertical cavity is built by two distributed Bragg mirrors, which consist of alternating layers of high- and low-refractive index material with thicknesses of a quarter of the laser wavelength. Gain is provided by electrically pumped quantum wells or quantum dots located in the active region between the monolithically grown semiconductor mirror, leading to single-longitudinal mode operation. Confining both current and optical field by oxide apertures enables operation in single-transverse mode, making the VCSEL a compact and efficient source of laser emission with excellent beam quality.
Reference: [Jetter, Roßbach, Michler (2013). Red Emitting VCSEL. In VCSELs (pp. 379-401). Springer, Berlin, Heidelberg]
Vertical External-Cavity Surface-Emitting Laser (VECSEL)
The optically-pumped semiconductor disk laser, also called VECSEL, combines the advantageous properties of the thin disk laser concept with the wavelength flexibility of the semiconductor material. The heart of the system is a semiconductor chip consisting of one distributed Bragg mirror and an active region including either quantum wells or quantum dots as active gain medium. The resonator is completed with at least one external mirror and provides space for the implementation of intra-cavity elements.
Besides high output powers with diffraction-limited beam qualities, this is the major benefit of the disk laser concept. Inserting wavelength filters enables linewidth reduction and wavelength tuning over large ranges, while nonlinear crystals allow highly efficient intra-cavity frequency conversion such as second harmonic generation. Furthermore, mode-locked operation can be obtained by replacing one of the external mirrors by a semiconductor saturable absorber mirror, resulting in the emission of ultra-short laser pulses.
Reference: [Mateo et al., Optics Letters 41, 1245 (2016)]
Membrane External-Cavity Surface-Emitting Laser (MECSEL)
Less is more!
For the MECSEL concept, the active region is directly grown on the substrate, then the substrate is removed and the semiconductor gain membrane is bonded between two heat spreaders. What appears to be a step back from the conventional SDL approach actually further expands the wavelength flexibility of optically-pumped surface-emitting lasers and enables an increase in performance.
The epitaxial growth of the membrane is not restricted by the need of finding suitable material compositions for a monolithically fabricated semiconductor mirror, so the full range of possible gain media can be exploited. Additionally, the double-sided contact between membrane and heat spreader maximizes the heat removal, which allows for higher pump powers and boosts the output power.
Reference: [Kahle et al., Optica 3, 1506 (2016)]