Fabrication Of Nonlinearly Chirped Fiber Gratings And Study Of Nonlinearly Chirped Fiber Gratings Characteristics

Optical fiber communication is the most active technology of communication field. Optical fiber communication, satellite communication and mobile communication are the three main orientations of modern communication technology. At present, plenty of communication business continuously emerges, especially fast development of Internet induces rapid increase of communication capacity. Thus, it is necessary to establish large-capacity and high-speed optical fiber communication system. Fiber gratings provide possibilities for the further development of large-capacity and high-speed optical fiber communication system. Fiber grating is a new kind of all-fiber passive device that results from that the refractive index of optical fiber core is periodicity microperturbation. Because of the excellent properties such as compatible with optical fiber, easy fabrication, little insert loss, small volume and low price, fiber gratings have wide application in optical fiber communication and sensor field.In this paper we first introduced the progress of fiber grating and its applications in optical communication field . Then we introduced the basic structure and function of fiber grating. Chirped fiber grating is one type of fiber gratings whose optical period (product of grating's effective refractive index and refractive index microperturbation's period). Light with different wavelength is reflected at axial different point of chirped fiber grating. So chirped fiber gratings have broad reflection bandwidth. The group delay in chirped fiber grating reflection bandwidth changes gradually. These properties are what other type fiber gratings do not have. It is these unique properties that make chirped fiber gratings have unique applications both in optical communication and sensor field. For example, chirped fiber gratings can be used as reflector in fiber for fabricating many key devices in optical communication such as fiber bandpass filter because of the broad reflection bandwidth. Because gradual group delay, chirped fiber gratings can be used as fiber dispersion compensator for compensating fiber dispersion. Nonlinearly chirped fiber grating is a kind of new fiber gratings, so it has many special properties. The work of this thesis is deeply studying the principle, fabrication properties and applications of nonlinearly chirped fiber gratings.In respect of theory, we strictly deduced couple mode theory that describe the principle of fiber gratings from Maxwell equation, and analysis the properties of chirped fiber grating, obtain the relation between chirped fiber grating's parameters such as change of refractive index, length and chirp as well and the properties such as reflectivity, bandwidth and dispersion. All of these prepare for the design and fabrication of the nonlinearly chirped fiber gratings.We give some possible methods for the fabrication of the nonlinearly chirped fiber gratings. Including the method of using phase mask, the temperature gradient method and the fiber Bragg grating strain gradient method.The first method is using chirp phase mask, which is the normal method for fabricating fiber gratings. After analyzing the relation of the period of mask and the period of fiber gratings, we got the basic method of design phase mask for nonlinearly chirped fiber gratings. We designed to realiz fiber gratings' apodization by a simple method that we name it that fabricating apodization fiber gratings by only once moving fiber together with phase mask. The method of using phase mask has many virtues such as the easy process of fabrication, the good consistency and the easy control in fabricating. The drawbacks of this method are that the mask is too expensive and the bandwidth of fiber gratings is difficult to change.The temperature gradient method is a kind of method that base on the Bragg fiber gratings, and it uses the temperature sensitivity of Bragg fiber gratings. By heating or cooling ,we can gain the temperature gradient in Bragg fiber gratings, so we can change the chirp extent of fiber gratings.