What is an OTDR and its basic working principle
                     What is an OTDR and its basic working principle? 
 
OTDR is an Optical Time-domain Reflectometer. It is the main devices in optical fiber 
measurement. It uses to test detect problems that may exist in fiber links. 
 
OTDR is widely used in carrier backbone networks. By analyzing the measurement 
curve, it can quickly detect the fault location of the optical fiber link and the fiber loss. 
The working principle is to use a laser light to send a higher power laser or light pulse 
to the test fiber. OTDR observes the power of the laser signal from each point on the 
fiber and records these results through a trace diagram. It can calculate the distance 
by recording the return time and transmission speed.  
 
In the PON (Passive Optical Network) network, especially the complex point-to-
multipoint PON ODN (Optical Distribution Network) topology applications, rapid 
monitoring, and diagnosis of fiber fault locations is a challenging subject. At present, 
optical time-domain reflectometer (OTDR) tools are widely used for monitoring the 
fiber network. However, OTDR detection may not be so particularly sensitive to the 
attenuation of optical signals at the ends of some ODN branch fibers or ONU fibers. 
The application of FTTx terminal splitter to the main cable signal makes the traditional 
OTDR fiber optical detection techniques unable to achieve on-off detection. So a cost-
effective and wavelength selective fiber optic reflector is a basic method of 
implementation of optical layer monitoring on the whole length of an FTTx network 
from the OLT to the ONT in real-time. 
 
 
 
The working principle of the fiber optic reflector is mainly to use Fiber Bragg Grating 
(FBG) to reflect the test light pulse sent by OTDR with nearly 100% reflectivity. But the 
wavelength of normal passive optical network (PON) will pass with a small attenuation. 
The main purpose is to accurately calculate the return loss value of the reflected event 
at the end of each ONU branch by detecting whether the reflected OTDR test signal 
exists and the intensity of the optical signal. Therefore, it can easy to determine 
whether the optical link from the OLT side to the ONU side is normal. By detecting the 
reflected signal through OTDR, it can accurately grasp the fiber breakage, loss 
anomalies, fault location, and locate the error quickly. 
 
 
 
The 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 
optical fiber reflector is a cost-effective solution to help OTDR detection, and is the 
best way to realize real-time end-to-end (OLT to ONT) monitoring of optical networks 
in FTTx networks. 
 
The optical fiber (FBG) reflector independently developed by HYC has the advantages 
of low insertion loss, high reflectivity, and convenient installation. It is widely used in 
PON network, OTDR testing, central computer room testing, FTTX and other fields. HYC 
offers customized pigtail or adapter type, such as LC, SC, APC/PC and so on. 
 
HYC provides optical passive components OEM/ODM services. We welcome any 
inquiry for customized optical devices solutions for optical communication industry, 
including fiber optic connectors/adapters, WDM, PLC splitter, MEMS optical switch for 
data center, FTTx, Cloud Computing industry, 4G/5G networks . 
www.hyc-system.com