Variable Optical Attenuator Based on a PDMS-Embedded Microfiber Coupler

We demonstrate a novel fiber-compatible variable optical attenuator (VOA) based on a tunable microfiber coupler, which is assembled by embedding two parallel 2.5- \mu \text{m} -diameter microfibers in a 250- \mu \text{m} -thickness polydimethylsiloxane (PDMS) film with a separation of 2.5~\mu \text{m} and a coupling length of 3 mm. Based on the strain-dependent refractive index of the PDMS, the attenuation of the VOA can be continuously tuned from 0.4 dB to 15.3 dB at 1550 nm wavelength with an insertion loss of 0.4 dB. The VOA presented here has advantages of excellent fiber compatibility, low insertion loss and compact structure.