Beat-frequency adjustable Er3+-doped DBR fiber laser for ultrasound detection.

A compact low beat-frequency dual-polarization distributed Bragg reflector (DBR) fiber laser whose beat frequency can be varied, for high-frequency ultrasound detection has been proposed and experimentally demonstrated. The laser was fabricated in small birefringent commercial erbium-doped fiber. It operated in a robust single-longitude mode with output power of more than 1 mW and high signal-to-noise ratio better than 60 dB. Induced birefringence to the fiber during the UV inscription process is small (~10(-7)) and consequently the laser beats at a low frequency of ~20 MHz which is at least one order of magnitude smaller than previously reported results, making frequency down-conversion unnecessary. The beat frequency can be adjusted by controlling the side-exposure time of the UV light irradiating the gain cavity, providing a simple approach to multiplex a large number of DBR fiber lasers of different frequencies in series using frequency division multiplexing (FDM) technique. The proposed DBR fiber laser is also temperature insensitive, making it a good candidate for hydrophone applications.

Single tilted Bragg reflector fiber laser for simultaneous sensing of refractive index and temperature.

A type of fiber laser, called tilted Bragg reflector fiber laser (TBR-FL), is proposed and its application in simultaneous sensing of surrounding refractive index (SRI) and temperature is demonstrated. This FL is formed by a pair of wavelength and tilt-angle matched tilted fiber Bragg gratings (TFBGs) that acted both as a resonant cavity and sensing element. A unique spectral feature of the TBR-FL is the presence of grating tilt-induced cladding modes spectrum that does not appear in other type of FL, which provides an extra sensing mechanism. By employing a simple experimental setup with the discrete wavelet transform as the demodulation technique, simultaneously sensing of SRI and temperature are achieved by measuring and analyzing the wavelet coefficients shifts of the laser output and averaged cladding modes.

Fourier analysis for hydrostatic pressure sensing in a polarization-maintaining photonic crystal fiber.

We measured the hydrostatic pressure dependence of the birefringence and birefringent dispersion of a Sagnac interferometric sensor incorporating a length of highly birefringent photonic crystal fiber using Fourier analysis. Sensitivity of both the phase and chirp spectra to hydrostatic pressure is demonstrated. Using this analysis, phase-based measurements showed a good linearity with an effective sensitivity of 9.45 nm/MPa and an accuracy of ±7.8 kPa using wavelength-encoded data and an effective sensitivity of -55.7 cm(-1)/MPa and an accuracy of ±4.4 kPa using wavenumber-encoded data. Chirp-based measurements, though nonlinear in response, showed an improvement in accuracy at certain pressure ranges with an accuracy of ±5.5 kPa for the full range of measured pressures using wavelength-encoded data and dropping to within ±2.5 kPa in the range of 0.17 to 0.4 MPa using wavenumber-encoded data. Improvements of the accuracy demonstrated the usefulness of implementing chirp-based analysis for sensing purposes.

Multiplexing technique using amplitude-modulated chirped fiber Bragg gratings.

We propose a new multiplexing technique using amplitude-modulated chirped fiber Bragg gratings that have an identical center Bragg wavelength. Each grating is inscribed with a unique amplitude modulation that allows them to be multiplexed with complete overlapping within a certain bandwidth. To demodulate the multiplexed signal, the discrete wavelet transform is employed. Concurrently, a wavelet denoising technique is used to reduce the noise. This proposed multiplexing technique has been verified through strain measurements. Experimental results showed that for strains applied up to 1250 microepsilon the absolute error and cross-talk are within +/-20 microepsilon and 16 microepsilon, respectively. A strain resolution of 4 microepsilon is obtained.

Simultaneous demodulation technique for a multiplexed fiber Fizeau interferometer and fiber Bragg grating sensor system.

We propose a demodulation technique for a multiplexed fiber Fizeau interferometer (FFI) and fiber Bragg grating (FBG) sensor system using the discrete wavelet transform with signal processing enhancements. This simple and flexible demodulation technique determines the cavity length of FFI and the Bragg wavelength of FBG simultaneously and is especially suited for quasi-static measurements. We demonstrate this demodulation technique by performing some strain measurements, and a strain resolution of 1.0 microepsilon and an accuracy of 2.6 microepsilon are obtained. The maximum cross-talk of the sensor system is 6% of the applied strains.