Tapered fiber gratings and applications

#The present invention relates to the communication of signals via optical fibers, particularly to tapered optical fibers and couplers. More particularly, the invention relates to wavelength selective tapered fiber gratings, add/drop filters and optical circuits for wavelength division multiplexed systems.

#Low loss, wavelength selective filters are important components for optical fiber communication networks based on wavelength division multiplexing (WDM). WDM enables an individual optical fiber to transmit several or many channels simultaneously, at different spaced apart center wavelengths. As the art has developed, standardized wavelength band parameters have been set, for the number of channels, the total wavelength band, and the separation between channels. As a basic example, 30 nm may be devoted to a band of 4 channels having 10 nm separations between the center wavelengths.

#Transmission of multiplexed signals without intervening wavelength signals or interference between them is complicated by the need for low loss both above and below the selected center wavelengths. In-fiber (e.g., erbium) amplifiers have a relatively wide window (e.g., 35 nm) and unwanted off-wavelength signals affect network reliability. Optical wave energy interacts with active and passive components which can have an adverse effect on insertion loss and signal attenuation, as well as introducing undesired backreflection. Devices such as fiber Bragg gratings are widely used in WDM systems. Most of such filters are used in a reflection mode, which provides higher wavelength selectivity than transmission mode, and write a Bragg grating in the core of the fiber (dopant in the core enables photosensitization and subsequent grating recording). Some of these filters, and also couplers made by fusing two fibers along a short length, are tapered to a reduced diameter waist. Whether tapered or not, the reflected wavelength, which is typically called a drop wavelength, is accompanied by signals of shorter wavelength that arise from lossy cladding modes.

#A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which;

#FIG. 1 is an idealized and somewhat simplified representation of a tapered fiber grating in accordance with the invention;

#FIG. 2 is a pair of idealized sectional views, FIGS. 2a and 2b, of dopant distributions in the waist of a tapered fiber grating, including extensive distribution (FIG. 2a) and more limited distribution (FIG. 2b);

#Optical fibers carry signals in the form of modulated light waves from a source of data, the transmitter, to a recipient of data, the receiver. Once light enters this fiber, it travels in isolation unless an optical coupler is inserted at some location along the fiber. Optical couplers allow light signals to be transferred between normally independent fibers. If multiple signals at different wavelengths travel down the same fiber, it is desirable to reflect a signal at only a predetermined set of wavelengths within this fiber. A desirable attribute of such a tapered fiber grating is that it remains transparent to all wavelengths other than those to be backreflected. That is, it should exhibit no excess loss.

#FIG. 1 illustrates the structure of this device. The tapered doped-cladding fiber grating consists of a fiber 12 tapered to form a waist region 32 wherein an index of refraction grating has been impressed. If the input mode 31 with propagation vector β1 and output mode 23 with propagation vector −β1 satisfy the Bragg law for reflection from a thick index grating of period λg at a particular wavelength, say λi:

#2β1(λi)=2π/Λg,