Narrow Band-Pass Transmission Filter Consisting of Two Fiber Bragg Gratings with Different Periods

We developed a narrow band-pass transmission filter by connecting in series two fiber Bragg gratings (FBGs) with slightly different periods. The filter was fabricated by writing two FBGs directly onto the core of a single optical fiber. The center wavelengths of the element FBGs are 1549.71 nm and 1550.05 nm, their bandwidths are 0.36 nm and 0.42 nm, and their reflectances are 99.8% and 99.9%, respectively. The measured bandwidth (full width at half maximum, FWHM) of the fabricated band-pass filter is 0.052 nm, and the maximum transmittance is -12.5 dB. In order to reduce the transmission loss, we adjusted the period difference between the two element FBGs by temperature control, and achieved a maximum transmittance of -0.3 dB although the bandwidth slightly increased up to 0.092 nm. To obtain the narrower bandwidth of the transmission peak, we fabricated a series-type FBG filter using FBGs with higher reflectance, whose measured band-pass bandwidth was as narrow as 0.033 nm.