FBG Fiber Optic Grating Reflectors
Fiber optic reflectors, also known as FBG fiber Bragg grating filter, are usually installed at the front end of ONU of optical network. Combined with OTDR equipment, it can realize point-to-point (PTP) or point-to-multipoint (PTMP) network monitoring and quickly and accurately reflect the network anomalies.
FBGs can reflect nearly 100% of the test pulse sent by OTDR on the OLT side, while wavelengths that don’t satisfy Bragg condition pass through with minimal attenuation.
1. Fiber Bragg Grating (FBG)
FBG is a microstructure etched into an optical fiber. It is made by projecting extremely high-resolution UV patterns into an optical fiber core, which changes the index of refraction in the exposed regions and leaves the rest unchanged. The resulting index variation is a function of the grating period and the wavelength of light propagating through the grating, giving it a unique reflectivity spectrum (see figure below).
Uniform FBGs are sensitive to temperature and strain, but they can’t distinguish between these two factors. To solve this issue, apodized FBGs can be used. Apodized FBGs feature a changing index profile along their length, which helps suppress side-lobes. They can be designed with a range of profiles, including Gaussian and raised cosine functions.
Another type of FBG is the long-period Bragg grating (LPBG). Its longer grating period couples counterpropagating core modes. Like uniform FBGs, LPBGs are insensitive to the outer medium, but they have higher bandwidth and lower FWHM.
FBG sensors can be used in a variety of applications, including optical signal transmission and sensor monitoring. These sensors are nonconductive and immune to EMI-induced noise, making them a valuable alternative to traditional electrical sensor technologies. They are also capable of performing measurements over long distances. In addition, they are easy to use and have a low power consumption. They can also be buried in the wall of an optical fiber to eliminate environmental interference.
2. Dielectric Film
Using the concept of a grating, a multilayer dielectric film can ftth-optical-grating-reflectors be designed to produce high-efficiency polarization-independent reflection. When one incident ray enters the thin dielectric film, it produces many coherent reflected and refracted rays that can interfere constructively or destructively with each other. This results in a broad wavelength range of reflection at near-normal incidence.
The diffraction efficiency of the first order FP modes and the DWG modes depends on the incident wavelength and the thickness of the match layer. The diffraction efficiency of the FP modes increases with the thickness of the match layer, and the diffraction efficiency of the DWG modes decreases with the thickness of the match layer.
These different diffraction effects can be balanced out by choosing the layer materials such that the reflection coefficients at all interfaces have alternating signs. This ensures that the complex amplitudes of the reflected field contributions from each layer interface are added together constructively. For this reason, a high-reflectivity multilayer dielectric is very efficient and can be produced with a relatively small number of layer pairs.
In practice, the diffraction efficiency of the multilayer dielectric is controlled by its insulation resistance (R) and polarization extinction ratio (). The R of the multilayer dielectric is typically higher than that of the metal mirror and can be improved by applying an appropriate coating on the surface.
3. Ceramic Ferrule
In optical fiber communication, ceramic ferrule is the precision device that supports the inserted singlemode optical fiber. The ferrule is made of cordiorite and it consists of many layers and has a complex structure. The ceramic material is mixed with a variety of earth compounds to make what is called green ceramic, which is then pressed into dies and baked to form the ferrule. The raw material used to make the ceramic is mixed in a special way to control shrinkage during processing and ensure it stays together during the manufacturing process.
The ceramic is then polished to form a smooth surface that is free of cracks and abrasions. The surface of the ferrule is ground to a certain structure, which determines the loss performance of the connector. A standard ferrule has an 8 deg angle bevel that improves the physical contact between indoor-duplex-fiber-optic-cables the cladding and the fiber end face, and a smooth surface will reflect light back into the cladding instead of absorbing it, which reduces the overall loss.
The low expansion material 112 212 512 may be sintered and bonded to the higher expansion body 120 220 320 420 520, and the end face 224 324 424 is configured to interface with an adapter or other connector end face. The ferrule is then heated rapidly, preferably in less than 60 seconds, to sinter, bond, and adhere the low expansion material to the high expansion body.
4. Adapter
The adapter design pattern acts as a mediator between two systems or components that have incompatible interfaces. It takes the input from one system, processes it, and then delivers the output in a format that the other system can understand. This allows the two systems to communicate with each other without having to modify their code. This is similar to the bridge mechanism that is used in a computer network to connect different types of hardware or software.
Fiber optic reflector, also known as fiber Bragg grating filter, is an optical passive device installed in the front end of the optical network unit (ONU) and cooperates with the OTDR equipment to realize the network monitoring of point-to-point (PTP) or point-to-multipoint (PTMP) for the FTTx. It can quickly and accurately reflect the network abnormality, which can save the time of locating fault points.
The reflecting wavelength of a fiber reflector is 16451650nm, and the working bands of the passive optical network pass through it without being affected. Therefore, the OTDR can easily determine whether the optical line is normal or not by detecting the intensity of the reflected OTDR test signal. This way, the maintenance of FTTH networks can be realized without disturbing traffic. In addition, the reflector can help to locate the exact position of the optical fault by comparing the reflection loss value of the test signal in each house with the health file.