We propose a novel fiber Bragg grating (FBG) sensor system using a linear-cavity fiber laser scheme with a distributed Raman amplifier as a gain medium. The inhomogeneous broadening property of the distributed Raman amplifier is used for multiwavelength operation. The experiment shows that such a linearcavity fiber Raman laser can provide a stable output with an optical signal-to-noise ratio over 50 dB even if the FBG is located at a 25-km remote sensing position. The feature of our proposed fiber laser can facilitate a long-distance or a large-scale fiber sensor system and can be easily extended for multipoint sensing applications.
from Fig. 3 that the change in the effective refractive index has a powerlaw dependence on UV fluence. It shows distinctly different behaviours for different dose regions, indicating that there is interplay between at least two different mechanisms of photosensitivity In the low dose range (<20 Jjcm'), the slope is approximately 0.27 and it is virtually independent of pressurc. Beyond a transition region where both mechanisms operate (20-800 J/cm'), the refractive index change also exhibits power-law dependence. Hawever, in this high-dose region, the slope is pressure dependent, increasing from 0.39 at 26 bar to 0.68,at 160 bar. nuence m. J c d Fig. 3 Phorosensitivitp of SMF-28 fibre loaded at diferent hydrogen pressure.Y Inset: Pressure dependence of exponent 1 at high fluence Conclusion: We have presented a novel highly accurate technique to measure photosensitivity in optical fibre. Enpenmcntal data on photosensitivity measurements of different fibres are presented. The experimental data indicate that two mechanisms of photosensitivity are likely to operate in hydrogen-loaded germanosilicate fibres.
We theoretically show that a self-induced transparency (SIT) soliton and a Bragg soliton can coexist in a nonlinear photonic band gap (PBG) medium doped uniformly with inhomogeneous-broadening two-level atoms. The Maxwell-Bloch equations for the pulse propagating through such a uniformly doped PBG structure are derived first and further reduced to an effective nonlinear Schrödinger equation. This model describes an equivalent physical mechanism for a Bragg-soliton propagation resulting from the effective quadratic dispersion balancing with the effective third-order nonlinearity. Because the resonant atoms are taken into account, the original band gap can be shifted both by the dopants and the instantaneous nonlinearity response originating from an intense optical pulse. As a result, even if a SIT soliton with its central frequency deep inside the original forbidden band, it still can propagate through the resonant PBG medium as long as this SIT soliton satisfies the effective Bragg-soliton propagation. An approximate soliton solution describing such coexistence is found. We also show that the pulse width and group velocity of this soliton solution can be uniquely determined for given material parameters, atomic transition frequency, and input central frequency of the soliton. The numerical examples of the SIT soliton in a one-dimensional As2S3-based PBG structure doped uniformly with Lorentzian line-shape resonant atoms are shown. It is found that a SIT soliton with approximately 100-ps width in such a resonant PBG structure can travel with the velocity being two orders of magnitude slower than the light speed in an unprocessed host medium.
and P 4 ϭ P 1 sin 2 ͓͑a ϩ c͒z͔,where c ϭ Ϫbs and z is the coupling length. Figure 7 shows the output power versus wavelength for a fiber coupler. Solid lines represent the theoretical outputs (assumed to be exact), and the dotted lines are the outputs calculated using the linear approximation. In this example calculation, the coupling length is fixed at 4 cm and the spacing is set at 4 m. From Figure 7 it is seen that the linear approximation tracks the theoretical result very well. For designing WDM couplers or calculating the output of optical couplers, the simple relationship presented here is easier to use (and often more insightful) than the more precise techniques cited.
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