Couplers:

-Couplers are used to divide and combine multiple optical signals.

example on page 1(pdf)

WDM (wavelength-division multiplier):

-WDMs are more complex couplers that split optical signals of different wavelengths into different fibers. They may also converge signals of differing wavelengths into a single fiber.

example on page 2(pdf)

Attenuators:

-Attenuators reduce the strength (amplitude) of an optical signal