the present invention provides modular optical amplifier constituents suitable for constructing optical amplifiers of various designs and modular fiber optic cassettes . each stage of a multiple stage optical amplifier is housed in a separate optical cassette . optical pump are separately packaged in pump modules to further simplify amplifier design and enhance amplifier manufacturability . in an exemplary embodiment , a modular optical amplifier is constructed comprising a first amplifier housing including a first optical cassette for holding a first amplifier stage . cassette regions are provided for receiving one or more passive optical components used with the first stage of the optical amplifier . a first length of rare - earth doped optical fiber is retained by cassette retaining projections and optically communicates with a pump interconnection element . a second amplifier housing is provided comprising a second optical cassette for holding a second amplifier stage in a substantially similar manner as the first optical cassette . at least one optical pump is retained in an optical pump housing separate from the first and second optical cassettes for supplying pump energy to the optical amplifier . the cassettes are optionally mounted vertically or adjacent one another on a circuit board . the circuit board may include electrical circuits for monitoring amplifier performance and for passing the monitoring information to a network management system .