Page "Optical amplifier" Paragraph 37

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 ( VCSELs ).

The major difference when comparing VCSOAs and VCSELs is the reduced mirror reflectivities 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 reflectivities 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.