  We present highly sensitive refractometers based on a long-period grating in
a large mode area PCF. The maximum sensitivity is 1500 nm/RIU at a refractive
index of 1.33, the highest reported for any fiber grating. The minimal
detectable index change is $2\times 10^{-5}$. The high sensitivity is obtained
by infiltrating the sample into the holes of the photonic crystal fiber to give
a strong interaction between the sample and the probing field.

Optics