Apr 5, 2005
Bragg gratings characterization
The most common and simplest way of the grating characterization is measuring its transmission/reflection spectrum. This method determines only the integral characteristics of the fiber Bragg grating (FBG). To obtain the spatial distribution of local grating properties other techniques such as OLCR , side-diffraction , heat-scan  can be used.
An optical space domain reflectometry (OSDR) method for obtaining the spatial variation of the complex coupling coefficient k(z) of FBG has been proposed in . In  we suggested application of more suitable laser sources (a CO-laser and a frequency-doubled Ar-ion laser) for local induction of a phase perturbation in the OSDR technique. Thorough description of the OSDR and comparison with modern FBG characterization techniques are presented in our paper . It is experimentally demonstrated that IR or UV irradiation of the grating in the technique allows achieving a good sensitivity of measuring index modulation amplitude in a fiber core (~ 10-4) and a high spatial resolution (~100 mm or less). Factors influencing the measurement accuracy are examined, optimal parameters of the measuring setup as well as the method restrictions (technical and methodological) are presented. Present methods of fiber gratings spatial characterization are comparatively analyzed with specifying the ranges of their application.
OSDR technique is based on introducing a local AC phase perturbation scanned along the grating while measuring the transmission variations at a fixed probe wavelength outside the stop band. Mathematical processing of measured results gives an axial distribution of the main grating parameters: induced index, modulation period and phase. OSDR-signal distribution calculated for a sample Bragg grating (reflection coefficient 99.9%, length 4 mm, induced index 14·10-4, induced probe phase shift 10-2) is presented on the following figure:
It is clearly seen that a number of oscillations increases with probe wavelength detuning, while their amplitude decreases. It is shown in  that a probe wavelength should be chosen at one of sidelobes of a tested grating. This fact gives one of the advantages of the OSDR technique: one can measure spatial properties of Bragg gratings with high reflection coefficient.
An example of reconstructed coupling coefficient k(z) for a Bragg grating written by the second harmonic of the Ar-ion laser (244 nm) in the Lloyd interferometer scheme is presented on the next figure:
Coupling coefficient amplitude is proportional to induced index modulation in grating, and such non-symmetrical spatial profile is explained by appropriate UV intensity distribution during writing process. Spatial variations in the phase distribution has 2 sources: induced average index and period of the grating. Importance of phase information is shown on the following figure:
It is seen that taking into account phase information gives better coincidence. Moreover, spatial distributions of coupling coefficient amplitude and phase taken together gives the whole description of the grating structure, making it possible to:
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