This articles describes criteria to evaluate FBG based waveguide sensors like multiplexing ability, linearity, cross sensitivity, shock resistance, vulnerability to thermal drift, saturation effects, mechanical complexity
An optical waveguide is similar to an optical fiber. It typically consists of three layers of silica with the same refractive index. The center layer is doped with germanium. UV light increases the refractive index of the core layer but not the layers above and below. Due to this effect, optical path can be written into the waveguide structure. Bragg gratings can also be written into the waveguide in the same process step with two interfering UV beams.
The oscillating structure in the niddle can be cut out for example with a CO2 laser with high wavelength or with a femto second pulse laser.
Description of criteria
An acceleration sensor based on waveguide technology might be a cost efficient and easy to produce sensor. The main problem this sensor concept faces is the stiffness of glass. The tensile strength of glass of 30 MPa is too small to survive the mechanical deformation of the rcquired acceleration loads. The damage can be avoided with mechanical guidance and way limitations of the seismic mass but the performance is not increased by this step. The following picture shows a possible solution for the waveguide based acceleration sensor with FBGS.
The thermal drift of the sensor can be compensated mathematically. The thermal elongation of the FBGs is equal on both sides of the seismic mass. The wavelength difference due to applied acceleration is the same. Due to this effect, the FBG waveguide sensor can also be used as a temperature sensor.
The mechanical complexity of this sensor is low. Production technologies are widely available, but also very expensive. Higher quantities of this sensor will reduce the costs. The size of a waveguide based sensor is small, which leads to a low weight (approximately < 10 gram).
Trade criteria (properties and behavior of the sensor)
- Multiplexing ability: Yes
- Linearity: Good expectation (<1%). This sensor uses a cantilever based measurement concept.
- Cross sensitivity: Good expectation (<2%). This parameter depends on the mechanical layout and can be minimized for example by mechanical guidance of the moving parts.
- Resistance to shock load: No good expectation. A FBG based waveguide sensor isn´t able to survive the mechanical stress caused by the bending moments.
- Vulnerability to thermal drift: Low. The thermal drift is a problem for all optical sensor concepts. But this drift can be compensated mathematically.
- Saturation effects: None. Optical sensor concepts are not concerned about the problem of saturation effects, contrary to electrical acceleration sensors (see the page saturation effects).
- Mechanical complexity, producibility: This sensor concept is similar to a cantilever sensor. The mechanical complexity is low. Production technologies are available but expensive.
- Size: Small, the dimension are about 30mm * 20mm * 6mm.
- Weight: Light, it weighs less than 10g.
Sensor performance requirements
- Acceleration, which implies a Measurement range of 14g, a Sensor Bandwidth > 1000 Hz and a Resolution >= 9 Bit: No. The FBG based waveguide sensor cannot afford this combination of parameters that the acceleration requires.
- Vibration 1, which implies a Measurement range of 50g, a Sensor Bandwidth > 1000 Hz and a Resolution >= 9 Bit: No. The FBG based waveguide sensor cannot afford this combination of parameters that the vibration 1 requires.
- Vibration 2, which implies a Measurement range of 20g, a Sensor Bandwidth > 3000 Hz and a Resolution >= 9 Bit: No. The FBG based waveguide sensor cannot afford this combination of parameters that the vibration 2 requires.
- Shock load, which implies a Measurement range of 5000g, a Sensor Bandwidth > 3000 Hz and a Resolution >= 9 Bit: No. The FBG based waveguide sensor cannot afford this combination of parameters that the shock load requires.
These criteria can also be used evaluated other sensors using optic fiber.
- FBG based Fibersensing GS6500 – COTS
- FBG – Cantilever Sensor
- FBG tube sensor
- FBG with pivot coupling of the seismic mass