How does FBG (Fiber Bragg Grating) Optical Reflector work?

FBG fiber optic reflector, also known as Fiber Bragg Grating Filter, is usually installed on the front end of the ONU. Combined with OTDR equipment, it can realize the point-to-point (PTP) or point-to-multipoint (PTMP) network monitoring of optical link, which can reflect the network abnormality quickly and accurately.

What is FBG? FBG is Fiber Bragg Grating. A fiber grating is a type of diffraction grating. Fiber Bragg Gratings (FBG) is a simple and low-cost way to build a grating into a specific wavelength range of fiber. When broad-spectrum light is incident on the fiber with built-in grating, the wavelength matching the grating will be reflected back to the input end, and the rest of the light will be passed through to the other end. Fiber gratings have the characteristics of small size, good wavelength selectivity, good compatibility, etc. The manufacturing process for fiber gratings is mature, with good practicality and low cost, they are widely used in the field of communication and sensing and is also ideal important device in optical network.

The FBG optical reflector utilizes the wavelength selectivity of the fiber grating, and the fiber grating is embedded in the adapter. It can be conveniently installed on the front end of an optical network (ONU), and it can quickly and accurately implement optical network fault detection with the help of an optical time domain reflector (OTDR). Due to the excellent wavelength selection characteristics of fiber gratings, anti-electromagnetic interference, the center wavelength drift is small with the change of external temperature, and when the optical network coverage and depth continue to increase, the entire optical network fault point can be detected quickly and accurately.

With the application of optical splitters in passive optical network (PON) networks, the complexity of the ODN network deployment environment has been increased. The splitters at the end of the FTTx will split the trunk optical signals and increase the difficulty to identify attenuation events on ODN branch fibers. The application of fiber grating reflector is a wavelength selective reflector based on this environment, which aims to improve the fast and accurate detection of optical link failure events by OTDR.

The fiber optic reflector is usually installed on the ONU side. The OTDR detects the intensity of the optical signal reflected by this reflector and compares the return loss between the normal line and the failed link to determine whether the optical fiber in the link is damaged or not. The normal PON system working wavelength does not meet the reflector conditions then will pass through the reflector with a small attenuation. While the reflector achieves the monitoring function, but without disturbing the transmit. 1650nm fiber grating reflector, the grating reflection wavelength is 1645 ~ 1650nm. The optical signal in this Bragg wavelength range will return along the input path, and all other wavelengths of light (1260 ~ 1625nm) can pass through. If the input optical signal segment can detect the fiber grating reflection peaks in this band, it can be judged that the transmission path is normal. Otherwise, it is abnormal.

End-to-end OTDR measurement from the OLT to the ONT is usually difficult because the splitter brings high losses and complex networks. The application of fiber optic reflector is a cost-effective solution to help OTDR detection, and it is the best way to implement real-time end-to-end (OLT to ONT) monitoring of optical networks in FTTx networks.

HYC’s self-developed FBG fiber optic reflector has the advantages of low insertion loss, high reflectivity, and easy installation. It is widely used in PON networks, OTDR testing, central computer room testing, FTTX and other fields. It can be customized as special pigtail or adapter type, and the connector interface is LC, SC APC / PC.

HYC Innovative Product, 1×48 MEMS Optical Switch

The upcoming 5G application promotes the upgrade of the all-optical network (AON). As a key part of the all-optical network, the ROADM market is expected to usher in rapid growth, especially in metropolitan area networks.

MEMS optical switch is one of the core devices in ROADM. Optical switch is a kind of optical path controlling device, with the ability to control and switch the optical path. MEMS fiber optic switches are based on micro-electro-mechanical system (MEMS) technology, which achieved low insertion loss and highly repeatability by rotating the mirror of MEMS chip.

Fiber Optic Patch Cable PC, APC or UPC

To make better contact between the ends of the two fibers, the ferrule end faces of the fiber optic patch cable is usually polishing into different structures. Standard polishing methods are PC, APC, and UPC. PC/APC/UPC represents the front end structure of the ceramic ferrule.

Different fiber end face finishes
PC (Physical Contact).The two end faces are polished to be slightly curved or microspherical, and the fiber core is at the highest point of the bending. This eliminates the air gap and forces the fibers into physical contact.

UPC (Ultra Physical Contact) is based on the PC to optimize the end face polishing and surface finish, the end face looks more dome-shaped. The end face of the UPC connector is not entirely flat, and there is a slight arc to achieve more accurate connecting.

APC (Angled Physical Contact). The end face of APC is usually polishing into an 8-degree angle. The 8° angled bevel makes the fiber end face tighter and reflects light through its beveled angle to the cladding instead of returning directly to the source, providing better connection performance.

Different physical appearance
The APC fiber optic connector is usually green. UPC/PC connectors are easily identified by their blue color on the connector boot.

Insertion loss and return loss
Different polishing styles determines the quality of optical fiber, which results in different performances regarding the connector's insertion loss and return loss. Insertion loss refers to the signal loss caused by the connector or cable. In general, the typical insertion loss of PC, UPC, and APC connectors should be less than 0.3dB. Compared with APC connectors, UPC connectors are usually easier to achieve low insertion loss due to the smaller air gap. Insertion loss can also be caused by a tiny particle of dust trapped between the connector end-faces. A single dry debris particle could even damage the fibers and ferrules.

Return loss, also known as reflection loss, is a parameter representing the signal reflection performance. Usually expressed in negative dB value, the higher the value of the setting, the better. The end faces of APC connectors are beveled, so the return loss of APC connectors is usually better than UPC connectors. In general, the return loss of the PC fiber cable is -40dB. UPC return loss is higher relative to PC, generally at -55dB (or even higher). APC industry-standard return loss is -65dB. With the UPC connector, any reflected light is directly reflected back to the light source. The beveled end face of the APC connector allows the reflected light at an angle into the cladding instead of reflecting it directly to the light sources. This is the main factor that causes the return loss to differ.

Applications
PC is the most common grinding method for optical fiber connectors, which is widely used in telecommunication operator equipment. UPC is commonly used in Ethernet network equipment (such as ODF fiber distribution frames, media converters and fiber switches, etc.), digital, cable television and telephone systems. APC is generally used in optical radio frequency applications such as CATV, and also in passive optical applications, such as PON network structures or passive optical local area networks.

Connector connections need to be in the same end face structure; for example, APC and UPC cannot be mated together , because doing so will resulting in poor connector performance. However, the end faces of PC and UPC fibers are flat, and the difference is in the quality of grinding. Therefore, the mixed connection of PC and UPC will not cause permanent physical damage to the connector.

HYC has 19 years of experience in research and development of fiber optic component manufacturer , supplying all kinds of fiber connectors and fiber patch cords for various specifications.