Optical microfiber coil delay line

The feasibility of a broadband, miniature, and low loss optical delay line composed of a coiled microfiber with several-micron radius is demonstrated theoretically. Under the introduced low-loss condition, the fundamental mode of the microfiber is shifted away and does not scatter from the interfaces with the central rod and adjacent turns. Dimensions of the designed 100 ns feedforward microfiber delay line containing 20 m of microfiber are 5 mm x 5 mm x 20 mm at the radiation wavelength 1.5 microm and can be much smaller for the recirculating loop delay line. These dimensions can be further optimized by varying the radii of the microfiber and coil. The predicted insertion loss of this device is approximately 0.004 dB/ns, which is two orders of magnitude smaller than the loss achieved presently for the miniature delay lines. A curved microfiber taper is proposed as a compact, low-loss, and broadband connection to this optical delay line. The taper adiabatically converts the input fundamental mode of a straight microfiber into the shifted mode of the coiled microfiber.