we present a novel online fabrication scheme of helical long-period fiber gratings (h-lpfgs) by directly twisting a standard single-mode fiber (smf) in a microheater. this is done by taking advantage of the inherent core-cladding eccentricity in smf. we adopt a fiber optic rotary joint to eliminate the accompanying twisting spiral for real-time spectral monitoring and a stepping mechanical system to accurately control the twisting length in fabrication. as a consequence, low-cost and high-quality h-lpfgs can be readily fabricated. meanwhile, by using this kind of h-lpfg, we design a simple and low-cost wavelength-interrogated liquid-level sensor with a high sensitivity of 0.1 nm/mm. (c) 2016 optical society of america