Optical filter having cascaded long-period gratings

#1. Field of the Invention

#The present invention relates to an optical filter in which long-period gratings (LPGs) are formed in an optical waveguide. Unlike a short-period grating reflecting light of a specific wavelength, the long-period grating is grating converting light of a specific wavelength among core-mode light propagating in a confined state in the core region, into cladding-mode light and radiating the cladding-mode light to the outside of the cladding region, for example, as disclosed in U.S. Pat. No. 5,703,978.

#2. Related Background Art

#FIG. 1 is a structural view of an optical filter according to an embodiment of the present invention.

#FIGS. 2A and 2B are graphs showing transmission characteristics of the second long-period grating and the optical filter in a first example.

#FIGS. 3A and 3B are graphs showing transmission characteristics of the second long-period grating and the optical filter in a second example.

#Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Throughout the description of the drawings the same elements will be denoted by the same reference symbols and redundant description will be omitted.

#First, the reason for the characteristic error as shown in FIG. 17 will be discussed referring to FIGS. 18A, 18B, FIG. 19, and FIG. 20. FIGS. 18A and 18B are graphs showing the transmission characteristics resulting from coupling between core-mode light and cladding-mode light of each mode number in a single body of long-period grating LPG-a, wherein FIG. 18B is an enlarged view of FIG. 18A. FIG. 19 is a graph showing the transmission characteristics resulting from coupling between core-mode light and cladding-mode light of each mode number in a single body of long-period grating LPG-b5. FIG. 20 is an explanatory diagram for explaining mode coupling in an optical filter in which the two long-period gratings LPG-a and LPG-b5 are formed in a unitary optical waveguide. Each of the long-period gratings LPG-a and LPG-b5 herein is of structure similar to that shown in FIGS. 16 and 17.

#As seen from FIGS. 18A and 18B, the sixth cladding-mode light is strongly coupled with the core-mode light in the communication wavelength band (1525 nm to 1565 nm; so-called C-band) in the single body of long-period grating LPG-a, and the loss of the core-mode light reaches the peak value of about 2.8 dB near the wavelength of 1560 nm because of the coupling with the sixth cladding-mode light. As seen from FIG. 19, the fifth cladding-mode light is strongly coupled with the core-mode light in the C-band in the single body of long-period grating LPG-b5, and the loss of the core-mode light reaches the peak value of about 2.3 dB near the wavelength of 1540 nm because of the coupling with the fifth cladding-mode light.