Calibration of wavelength shift for a fibre Bragg grating using a single mode laser source

Abstract

Although coordinate metrology has reached a very high state of development concerning versatility and accuracy for common engineering parts, a high precision capability with nano scale resolution and accuracy is often hard to achieve when it is required to measure very small parts and features. The limiting component is the bulky probing system of traditional coordinate measuring machines (CMMs). In order to satisfy increasing demand for highly accurate geometrical measurements on small parts and small structures, a new measuring probe of high sensitivity and small geometrical dimension with low contact forces needs to be developed. In this paper, a probing system, which combines a Fibre Bragg grating (FBG) embedded optical fibre tactile probe with an optical sensing technique, has been used. A novel simple wavelength shift demodulation system is tested which incorporates using the single mode light launched from a laser diode (LD) forming an external cavity between the LD and the FBG sensor to detect the Bragg wavelength shift induced by the strain on the FBG sensor. This demodulation method can be used to detect the strain-induced wavelength shift of the FBG. A strain resolution of 0.6 με is achieved. With the sensor elements integrated into the probe tip directly, the system sensitivity can be increased significantly.