Fiber Bragg gratings with micro-engineered temperature coefficients

Fiber Bragg gratings (FBGs) are ubiquitous as sensors for a range of parameters and also as optical components in telecommunications systems. However, their temperature dependence of around +10 pm/°C is a limiting factor, making it challenging for sensors to discriminate strain from temperature, while telecommunications components require additional thermal stabilization. We micro-engineer the temperature dependent properties of optical fiber by creating microchannels within the cladding using femtosecond laser assisted etching. These channels are filled with low loss refractive index liquids which have thermooptic coefficients that are around 50 times greater magnitude than silica and the opposite sign. We microfabricated low loss FBGs in standard single-mode fiber, with wide control over their temperature coefficient between +10 pm/°C and -55 pm/°C. We also fabricated a temperature insensitive FBG which is stable to ±12.5 pm over a 17 to 45°C range, which is an order of magnitude reduction in sensitivity. It has only ±3.5% reflectivity variation from the mean over this range and only 1.29 dB insertion loss.