A four-channel multi-
wavelength transmitter optical sub
assembly (TOSA) with an integrated
multiplexer is described that includes four lasers, four collimating lenses, a
multiplexer, an
isolator, a focusing lens, and an
optical fiber. The integrated
multiplexer comprises three filters in two stages, with each filter combining a transmission and a reflection beam into one beam. Four collimated beams are combined into one beam and then focused onto the
fiber by the focusing lens. The optical paths of all channels are short and possibly the same length. The invention allows using either
transistor outline (TO) can packaged lasers or lasers in
chip on submount. In some embodiments, four
coupling lenses that focus the light onto
fiber directly can be used instead of the collimating and focusing lenses, providing a more compact
assembly. One or more filters can be beam-splitters to relax
coating and assembling tolerances, thus reducing the cost.