Characterization of a Sagnac loop mirror-based hybrid passive variable optical coupler/attenuator

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Abstract
The implementation and performance of a unidirectional all-single mode fiber hybrid passive variable optical coupler/attenuator based on a Sagnac loop mirror with a continuous variable coupling ratio using off-the-shelf optical sub-components are discussed. Parameters of two output ports of the unidirectional hybrid passive variable optical coupler/attenuator, such as maximum coupling ratio, insertion loss, excess loss, and wavelength- dependent return loss over the C-band at room temperature are reported. The reflectivity of the Sagnac loop mirror continuously varies from 0.1% to 99.9% by adjusting retarders of a polarization controller. The 3-dB coupling ratio (i.e., 50:50) between the two output ports of the hybrid passive variable optical coupler/attenuator is achieved when the reflectivity of the Sagnac loop mirror is set at 73%. The attenuation and coupling ratio range of 1.7 - 45.5 dB and 0.13 - 99.87% is achieved, respectively, from the implemented unidirectional hybrid passive variable optical coupler/attenuator. The availability of the off-the-shelf optical sub-components and achieved easy control of continuous variable coupling ratio makes the proposed unidirectional hybrid passive variable optical coupler/attenuator a cost-effective optical device for several applications including optical system testing, and general laboratory experiments.

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