Fiber segment interferometry (FSI) is a proprietary measurement technique by Kistler in the field of fiber optic measurement technology. Fiber optical sensors (FOS) use light transmitted through an optical fiber to measure physical or chemical changes. The light is contained within the optical fiber and is reflected or transmitted to an interrogator where it is measured and processed.
Fiber segment interferometry (FSI)
What is fiber segment interferometry (FSI)?
FSI is an optical signal conditioning and processing technique that uses a series of inscribed gratings which act as reflectors and create segments of optical fiber. Fiber segment interferometry sensors measure physical changes such as strain, temperature, shape (2D and 3D) and acoustic emission by detecting phase shifts in light in each of the segments.
Which are the key measurands of fiber segment interferometry?
- Strain
- Temperature
- Shape 2D and 3D
- Vibration
- Torsion
- Acoustic Emission
- Pressure
- Acceleration
How does fiber segment interferometry (FSI) work?
A coherent light source, typically a diode laser, emits light that is launched into an optical fiber cable. Along the optical fiber, reflectors, implemented as inscribed gratings, reflect portions of the light back toward the interrogator. Two reflectors along an optical fiber define a sensing segment with a length described by the physical separation of the two reflectors along the path of the fiber.
Signals from the two reflectors recombine at a photodetector in an interference pattern. When a segment is subjected to external physical effects like strain or temperature change, the interference pattern changes as a result of phase changes in the segment.
The opto-electric interrogator processes the photodetector signal to compute the phase changes and translate them into precise, actionable measurements. By demodulation, interference patterns from multiple combinations of reflectors can be evaluated at once.
Fiber segment interferometry (FSI) key terms – definitions:
- Coherent light: light with a single wavelength that maintains its physical properties over extensive distances
- Reflector: a structure such as a grating written into the optical fiber to reflect light
- Sensor segment: sensing region bounded by two reflectors
- Reference: a stronger reflector used to mark the beginning of the sensitive region of the sensor
- Interference: the pattern formed by the recombination of reflected light from two or more reflectors
- Demodulation: the process of evaluating segments independently from one another
- Opto-electric Interrogator: the hardware and electronics contained in a packaged unit to capture and process data from fiber optical sensors
What are the benefits of using FSI measurement technology?
Fiber segment interferometry ensures high measurement performance
FSI offers exceptional measurement precision and speed. It provides high resolution down to the sub-nanometer range and supports high sampling frequencies, reaching several hundred kilohertz depending on the specific electronics and system configuration
Fiber segment interferometry enables measuring in demanding environments
Since fiber optical sensors rely on light transmission rather than electrical signals, they are inherently immune to electromagnetic and radio-frequency interference. Fiber segment interferometry sensors can be safely operated in high-voltage areas and are compatible with explosive or flammable atmospheres, as they do not generate sparks or electrical noise. The materials used in optical fibers are resistant to corrosion and chemical attack, allowing long-term operation in harsh or corrosive environments.
Fiber segment interferometry allows for high flexibility in application
A single FSI sensor can support multiple sensing segments and measure different physical quantities within one system, allowing a distributed sensing approach along the same sensor path. This capability enables simultaneous monitoring at several locations while reducing the need for additional sensors or complex wiring layouts.
Their small size with a diameter down to 155 µm and flexibility allow installation in confined or complex structures, minimizing space requirements.