Fibre-optic sensors using long-period gratings and microlens arrays

Abstract

In this project, two fibre-optic sensing systems were developed which use long-period fibre grating and Brillouin scattering in single-mode fibre for temperature and bending measurement. A long-period grating (LPG) has a number of advantages such as simplicity in fabrication and higher temperature and strain sensitivity over the fibre Bragg grating counterparts. Brillouin scattering based distributed fibre-optic sensors also have a number of advantages when compared with other systems based on Raman scattering and optical Kerr effect. These advantages include lower power threshold for Brillouin scattering, higher frequency shift of the scattered wave and simpler detection schemes. In order to realize the LPG grating sensor, two novel LPG fabrication techniques, namely, the microlens array technique and the plano-convex microlens techniques were also developed in this project. These techniques are more efficient and less expensive than the conventional amplitude mask technique in LPG fabrication. The micolens array is characterized by a higher transmission efficiency of UV laser light and higher LPG inscription efficiency than other conventional method, such as the amplitude mask technique. Futhermore, the low cost microlens array can be produced in any laboratory with simple tools only, thus the attractiveness of the technique is further enhanced. By using the same hydrogen loaded germanosilicate fibre and UV laser irradiation, the microlens array technique can produce an LPG rejection band with a peak loss of -11 dB after 50 seconds of UV laser irradiation while using a metal amplitude mask, a -10.9 dB resonant peak can only be produced after 200 seconds of UV laser irradiation. The microlens array technique was further improved by polishing the microlens array to produce the plano-convex microlens array via which the problems of damage to the microlens array and fibre due to internal focusing and excessive power on the inscription plane were eliminated. Also, the new method is capable of selective control of resonant peaks at higher harmonic frequencies by using the plano-convex microlens array with different polishing depths. The plano-convex microlens array technique is extremely efficient using the inscribing laser power to fabricate the LPG. In an experiment, a 17 dB resonant peak was produced by irradiating a hydrogen loaded SMF-28 fibre with a pulsed 193 nm excimer laser beam (intensity = 100mJ/cm2/pulse at 10 Hz) for 100 seconds. In the second stage of the study, a highly sensitive temperature sensor based on a packaged LPG was developed. The temperature sensitivity of the specially packaged LPG sensor, as deduced from the shift of the transmission spectrum of the LPG in response to temperature change is 8.8 nm/C in the range from 16 C to 20 C, which is about 180 times higher than that of an unpackaged LPG and is 800 times higher than that of a fibre Bragg grating. In addition, a low-cost and high return loss fibre-optic switch was implemented with this packaged LPG. For LPG bending sensors, one of the associated problems is the low accuracy for measuring their resonant peak shifts due to their large resonant bandwidths and sometimes it is meaningless to measure the peak shifts because of the irregular changes of transmission spectra of LPGs due to the bending. In this project, a simple LPG bending sensor was developed which is based on the measurement of total transmitted power, instead of the wavelength shift. It has been shown that the total transmitted power from a LPG has a linear response with respect to the bending curvature within the range from 0 to 0.001 mm-1. Therefore, this kind of LPGs can be used as bending sensors for different structures with a simple decoding scheme by measuring the total transmitted power.