Typically, sensor sensitivity is determined by the wavelength shift divided by the parameter range to be measured. So units can be pm/°C (temperature sensor), nm/Mpa or MPa^-1 (pressure sensor), and nm/g (accelerometer)... Also, we can also determine the sensitivity of the sensor through the linear experimental line y = ax + b where a is the sensitivity of the sensor.

I recommend you some research articles where you can get an idea to find sensitivity of an optical fiber sensor in unit of wavelength or power or voltage per unit measurand( temperature, pressure, vacuum etc.)

1. 10.1088/1402-4896/acc619

2. 10.1016/j.vacuum.2022.111566

3. 10.1088/1361-6501/acd01c