Vibrations in the lead cables is known to cause noise in interferometric sensor systems, through modulation
of the polarization state of the transmitted light, and through Doppler stretching of the interrogating signals.
We demonstrate effective methods for suppression of both these noise sources. The polarization modulation
sensitivity is suppressed by 36 dB by using a polarization resolved interrogation technique. The sensitivity to
Doppler shift is suppressed by 22 dB by using a reference interferometer that is interrogated through the same
lead fiber.
Suppression of crosstalk in an FBG-based serially multiplexed interferometric sensor array is demonstrated by employing the layer peeling inverse scattering algorithm. A polarization resolved impulse response (array reflection Jones matrix versus time) is measured using dual pulse heterodyne interrogation with varying combinations of polarization states in the interrogating pulse pair, and used as input to the layer peeling algorithm. < -40 dB crosstalk is achieved with > 97% confidence in a sensor array with 5 % FBG reflectivity. This is a 15-20 dB improvement compared to interrogation without inverse scattering.
Fiber distributed feedback (F-DFB) lasers have proven to be attractive devices for interrogation of optical sensors with high frequency resolution, due to their very low frequency noise/narrow linewidth, low relative intensity noise (RIN), robust mode-hop free tunability, compact size, and flexible and accurate wavelength setting. It has also been demonstrated that F-DFB lasers can act as sensor elements for high resolution measurements of physical quantities causing strain, refractive index, or birefringence changes in the laser fiber. It has been demonstrated that F-DFB lasers can be used as fast tunable sources for high resolution and high accuracy spectral component characterization. They may also find applications in dense WDM transmission systems utilizing their potentials for accurate wavelength setting, easy wavelength tuning, semiconductor pump redundancy, or multiple wavelength operation. In this paper properties and applications of F-DFB lasers will be discussed, with emphasis on modeling, design and characterization of the devices. In particular, RIN and frequency noise properties, requirements on grating and gain medium quality, the design requirements for achieving singlemoded or (intentionally) multimoded laser operation, and the output characteristics of single- versus multimoded F-DFB laser devices will be treated.
Characterization of the complex relection spectrum and the spatial profile of fiber Bragg gratings using optical frequency domain reflectometry and the layer peeling algorithm is presented. The importance of correct scaling and polarization effects are discussed. The method gives accurate measurement of the spatial profile for grating with reflectivity < 98-99 %. Immunity to spurious reflections and high dynamic range in spectral measurements are achieved.
The tolerance of two fiber distributed feedback lasers to external back-reflection from discrete reflectors and to Rayleigh back-scattering has been investigated. The results show a reduced feedback sensitivity for the longer laser grating.
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