Editing Optical amplifier (section)

===Vertical-cavity SOA===
A recent addition to the SOA family is the vertical-cavity SOA (VCSOA). These devices are similar in structure to, and share many features with, vertical-cavity surface-emitting lasers ([[VCSEL]]s). The major difference when comparing VCSOAs and VCSELs is the reduced mirror reflectivity used in the amplifier cavity. With VCSOAs, reduced feedback is necessary to prevent the device from reaching lasing threshold. Due to the extremely short cavity length, and correspondingly thin gain medium, these devices exhibit very low single-pass gain (typically on the order of a few percent) and also a very large [[free spectral range]] (FSR). The small single-pass gain requires relatively high mirror reflectivity to boost the total signal gain. In addition to boosting the total signal gain, the use of the [[resonant cavity]] structure results in a very narrow gain bandwidth; coupled with the large FSR of the optical cavity, this effectively limits operation of the VCSOA to single-channel amplification. Thus, VCSOAs can be seen as amplifying filters.

Given their vertical-cavity geometry, VCSOAs are resonant cavity optical amplifiers that operate with the input/output signal entering/exiting normal to the wafer surface. In addition to their small size, the surface normal operation of VCSOAs leads to a number of advantages, including low power consumption, low noise figure, polarization insensitive gain, and the ability to fabricate high fill factor two-dimensional arrays on a single semiconductor chip. These devices are still in the early stages of research, though promising preamplifier results have been demonstrated. Further extensions to VCSOA technology are the demonstration of wavelength tunable devices. These MEMS-tunable vertical-cavity SOAs utilize a microelectromechanical systems ([[Microelectromechanical systems|MEMS]]) based tuning mechanism for wide and continuous tuning of the peak gain wavelength of the amplifier.<ref>{{cite web|url=http://www.engineering.ucsb.edu/~memsucsb/Research/MT-VCSOA/MT-VCSOA.html|title=MEMS-Tunable Vertical-cavity SOA|website=Engineering.ucsb.edu|access-date=10 August 2017|archive-date=11 March 2007|archive-url=https://web.archive.org/web/20070311043610/http://www.engineering.ucsb.edu/~memsucsb/Research/MT-VCSOA/MT-VCSOA.html|url-status=dead}}</ref> SOAs have a more rapid gain response, which is in the order of 1 to 100 ps.