There are two steps to obtain as small as possible fiber Bragg gratings. First, it is to taper the fiber and reduce its diameter. A subwavelength-scale microfiber (MF) is the basic element of miniature fiber devices and sub-systems. Then it is to reduce the grating length. For short fiber grating, strong refractive index modulations are necessary. Strong refractive index modulations can be obtained inducing surface corrugation by alternating layers of different materials, one of which can be air. Several techniques have been proposed for the fabrication of surface-corrugated fiber gratings, including photorefractive inscription using etching, femtosecond lasers, and focused ion beam (FIB). So far, FIB is the most flexible and powerful tool for patterning, cross- sectioning or functionalizing a subwavelength circular MF due to its small and controllable spot size and high beam current density. In past two years, a number of ultra-compact surface corrugated microfiber Bragg gratings (SCMGs) have been successfully fabricated by FIB milling. The length of FIB milled SCGMs can be as small as tens of micrometers. In addition, there are several novel proposals on SCMG including wrapping a microfiber on a microstructure rod or put a microfiber on a surface-corrugated planar grating. In this paper, we will introduce recent advances in these ultra-small SCMGs and their characteristics and applications.
We fabricate a miniature tapered photonic crystal fiber (PCF) interferometer with enhanced sensitivity by a new acid
microdroplets etching method. This method, without elongating the PCF, moving and re-fixing the device during etching
and measuring refractive index sensitivity, is very simple, cost-efficient and highly stable over time. We investigate the
refractive index sensing properties with different PCF diameters both theoretically and experimentally. The size
decreases and the sensitivity increases an order of magnitude after etching the PCF. If we can optimize the etching
process, we can fabricate more uniformly and thinly tapered PCF interferometer with higher sensitivity (~ 100 times)
theoretically in the future.
By modifying the resonant condition of microfiber resonator sensors while taking the coupling effect into account, we
theoretically investigate coupling influence on the resonant wavelength and sensitivity. Numerical calculation shows
significant difference in resonant wavelength and sensitivity with different coupling strength. Tuning the coupling can
shift the resonant position as far as several nanometers and change the sensitivity as large as 30 nm/RIU in an
all-coupling microfiber coil resonator.
We report a miniaturized fiber probe inline reflective interferometer (FPIRI) sensor,
with a several μ-micro-notch cavity fabricated for highly sensitive refractive index
measurement. Its sensitivity in liquid is ~100 nm/RIU (refractive index unit) near the
wavelength of 1550 nm with a high extinction ratio. This probe sensor is very compact, stable,
and cheap, offering great potentials for detecting inside sub-wavelength particles or biocells.
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