Several single-mode fiber optic applications benefit from the use of depolarized light. Examples are suppression of polarization hole burning (PHB) in erbium-doped fiber amplifiers (EDFA),{sup}(1-3) reduction of polarization-dependent loss arising from fiber components (e.g., couplers and wave-division multiplexers),{sup}4 reduction of the signal fading in fiber optic sensors,{sup}(5～7) and coherent detection systems.{sup}(8-9) Depolarizers are also used for fiber optic component testing. Two major techniques are employed to depolarize the light: polarization modulation and spectrum averaging. The former makes use of one (or more) polarization modulators{sup}(3,10) which are able to change the output state of polarization (SOP) in such a way that the time average of the corresponding Stokes parameters vanishes. The light appears to be depolarized if it is averaged over a time period which is large compared to the modulation period. Because of the active components involved, these devices are often referred to as active depolarizers. The second technique works by introducing a differential group delay between orthogonal components of the electric field, so as to make them uncorrelated. Several devices implementing this technique have been described in the literature.{sup}(11-14)
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