Vector bend sensor based on an integrated optical fiber coupler with Bragg gratings

We demonstrate a vector bend sensor based on the reflection of light from a coupler with Bragg gratings in each arm, integrated within coreless silica optical fiber, and fabricated in a single femtosecond laser writing step. From the back-reflected Bragg wavelength and power level shifts due to strain-induced coupling ratio change, both bend angle magnitude and direction can be resolved, with a peak wavelength and power level sensitivity of up to 0.68 nm/° and 5.3 dB/°, respectively. The dual grating approach allows issues such as lead-fiber losses to be mitigated by using differential measurement, yielding up to 3.5 dB/° for the peak power difference sensitivity. Moreover, the sensor benefits from straightforward fabrication and compatibility by operating at standard C-band wavelengths.