Double-pass configured fibre Mach-Zehnder interferometric optical filter for distributed fibre sensing

A double-pass configured all-fibre Mach-Zehnder interferometric optical filter has been developed and used in a distributed fibre-optic sensor for separation of Rayleigh and Brillouin signals. Its superior performance over a single-pass all-fibre Mach-Zehnder interferometer is highlighted by its 15dB improved rejection of the Rayleigh signal and comparable throughput

Characterisation of strain dependence of the Landau-Placzek ratio for distributed sensing

The authors report on the strain dependence of the Landau-Placzek ratio (LPR). This, together with the known temperature dependence of the LPR and the Brillouin frequency dependence on strain and temperature, may be used to form the basis of a combined distributed strain and temperature sensor

Landau Placzek ratio applied to distributed fibre sensing

It is shown experimentally that the ratio of the intensities of Rayleigh and Brillouin backscattered light (Landau Placzek ratio) in an optical fibre has a temperature dependence which may be used for the basis of a distributed temperature sensor. This result, combined with the known frequency dependence of be Brillouin backscattering on temperature and strain, indicates that spontaneous Brillouin backscatter may be used for the unique determination of either temperature or strain in a distributed optical fibre sensing system

Reduction of coherent noise in the Landau Placzek ratio method for distributed fibre optic temperature sensing

The Landau Placzek ratio method for distributed fibre optic temperature sensing is based on the ratio of the Rayleigh and Brillouin backscattered light. However, because of the coherent nature of Rayleigh scattering. use of the same narrow bandwidth source as required for the Brillouin signal results in significant coherent noise in the Rayleigh signal. A novel technique is demonstrated whereby the amplified spontaneous emission noise and amplifying properties of an erbium doped fibre amplifier may be exploited to reduce the coherent noise on the Rayleigh backscatter signal. This results in a significant improvement in both temperature and spatial resolution over previously reported results

Measurement of Brillouin scattering coherence length as a function of pump power to determine Brillouin linewidth

Measurements of Brillouin scattering coherence length using a fibre Mach-Zehnder interferometer are presented. It is shown experimentally that the SBS lineshape narrows as the Brillouin pump power is increased from below to above threshold and that the coherence length approaches a limiting value

A self-starting passively mode-locked fibre ring laser for distributed sensing

A twelve hundred metre long, passively mode-locked, fibre ring laser has been constructed using polarisation preserving fibre to ensure operational stability. A short length of lo-bi spun fibre has been included to exploit non-linear polarisation evolution (NLPE) as the mechanism for mode-locking the fibre laser. Pulses with durations in the range of 1 to 10ns with intensities of a few Watts centred around the 1.535µm wavelength were achieved. The circulating pulse within the laser cavity generates both Rayleigh and Raman scattered light. The former may be used to measure losses within the cavity, the latter permits the distributed temperature measurements of the fibre forming the cavity

Distributed temperature sensing using the Landau-Placzek ratio

We report a commercially viable high spatial resolution, long range optical fibre distributed temperature sensor based on Brillouin scattering exploiting a low cost detection system. A spatial resolution of 3.5 metres with a Brillouin temperature resolution of 0.9°C at a range of 16km has been achieved

A Raman backscatter distributed temperature sensor based on a self starting passively mode locked fibre ring laser

This paper reports on intracavity Raman backscatter measurements in it passively mode locked 1300m long fibre ring laser for distributed temperature sensing. The high intracavity power leads to a relatively large and easily detected Raman backscatter signal, whereas the short pulse duration (0.7ns) promises high spatial resolution

Experimental measurement of multiple Brillouin Stokes orders in a fibre amplifier under pulsed excitation

The influence of optical gain on SBS is explored experimentally in an Er-doped fibre, and the observation of multiple Stokes orders for pulsed excitation reported

Multiple Brillouin Stokes orders in a 60m erbium-doped amplifier under pulsed excitation

Multiple Stokes orders of stimulated Brillouin scattering in a 60 m long erbium doped fibre amplifier are reported for excitation by 80 ns pulses. The observed threshold power is significantly reduced as a result of optical gain. The points of origin within the fibre of the generated Stokes pulses are located using space-time diagrams