Fibre DFB lasers for sensor applications

Summary form only given. Fibre DFB lasers are attractive in sensor applications, both as sensor elements and as sources for interrogation of interferometric fibre optic sensors, due to their very narrow linewidth, small dimensions, and their wavelength settability and multiplexibility. Their applications as sensor elements include high resolution measurements of pressure, temperature and strain, and other measurands which can be converted to strain or birefringence in the fibre, as well as measurements of acoustic signals. A fibre DFB laser consists of one single FBG with a length of 1-10 cm, normally with a 90/spl deg/ optical phase-shift at the centre, written into a single mode fibre doped with at least one of the rare-earths, for example erbium, to provide gain when pumped with a diode laser. The doped fibre is spliced in-between standard single-mode fibres. The FBG provides distributed feedback and single-frequency lasing at the Bragg wavelength, which can be any wavelength within the gain spectrum, ranging from 1520 to 1570 nm in erbium. Lasing will normally occur at two orthogonally polarised wavelengths, where the wavelength separation can be expressed as /spl Delta//spl lambda/=2BA where B is the birefringence in the laser fibre and A is the grating period. The linewidth of the laser modes is /spl sim/10 kHz, 6 orders of magnitude narrower than the reflection bandwidth of the FBG, which allows for ultrahigh resolution measurements both when the laser is used as a sensor and when used as a source for interrogation of interferometric sensors.