User guide

Veloce 50 Operating Manual

6 THEORY OF OPERATION

6.1 FABRY-PÉROT FIBER OPTIC STRAIN GAGES

All of Fiso Technologies' strain gages are designed around a Fabry-Pérot interferometer (FPI). Basically,

an FPI consists of two mirrors facing each other. The space separating the mirrors is called the cavity

length. Light reflected from the FPI is wavelength-modulated in exact accordance with the cavity length.

As shown in Figure 3, the Fabry-Pérot cavity contains mirrors deposited on the tips of two multimode

optical fibers inserted inside a microcapillary. When bonded to a specimen, the strain transferred to the

gage is converted into cavity-length variations; the strain is calculated according to the following equation:

Gauge

Cavity

Strain

(1)

where

Cavity

is the variation of the cavity length and L

Gage

is the gage length, i.e. the distance separating

the spots where the optical fibers are welded to the microcapillary. It is worth noting that the long-term

reliability of the gage length is guaranteed by the quartz-to-quartz welding method which avoids any

creep. The sensing part of the gage is located within the gage length area. The sensitivity of the gage is

adjusted at the factory by varying the gage length, also defined as the scale factor. The key to the

successful use of FPI technology is to find a practical way of obtaining precise cavity-length

measurements. The Fiso Veloce System can measure the cavity length with a typical resolution of 5 nm

(nanometers).

Incoming optical fiber

Gauge length

Fabry-Pérot cavity length

Figure 3 Non-compensated strain gage

It is important to note that the incoming fiber which brings light to the gage is mechanically decoupled from

the strain sensitive optical fibers. This design makes the cavity length of Fabry-Pérot gages—and

consequently, strain measurements taken with them—virtually insensitive to any pulling or manipulation of

the incoming fiber. This design is especially advantageous when the gage is embedded in composite

materials.

Fiso strain gages make use of standard 50/125 µm multimode optical fibers.