For better communication to take place between two ends, there should be a proper transmission of information from one end to another. There has been a tremendous evolution of the communication systems to make the communication process faster, reliable, and safer. It has evolved from analog to digital, wired to wireless. One of the modes of communication that are highly used nowadays is Fiber Optic Communication. In this mode, using infrared ray pulses information is transmitted from one place to another through an optical fiber. But as with every system, fiber optic communication system also faces certain challenges and requires proper maintenance and regular testing. One of the equipment used for the testing of a fiber-optic communication system is an Optical Time-Domain Reflectometer (OTDR).
What is Optical Time-Domain Reflectometer or (OTDR)?
Optical Time-Domain Reflectometer is also known as OTDR. It is a testing device used for determining the characteristics of the optical fiber cable. OTDR is known to be similar to the electronic time-domain reflectometer. In fiber-optic communication, a signal is transmitted inside an optical fiber. During transmission, if there are some impurities or defects in the cable, a part of the signal is reflected back causing distortion and signal attenuation. This is where OTDR is used. It transmits a laser signal into an optical fiber cable then captures the reflected signal from the same end of the cable.
The power of the reflected signal is measured and integrated as a function of time. Timed measurement of the reflected signal is performed. The scattering of signal/light from optical fiber occurs due to a phenomenon known as Rayleigh backscatter.
Working Principle of OTDR
Optical Time-Domain Reflectometer, OTDR, works on the same principle as that of Radar. Radar is a detection system, that uses radio waves to determine the range, position, or velocity of the objects. The transmitter generates the radio waves towards the object. These waves when come across the object get reflected back towards the transmitter. A receiver gathers these reflected waves and extracts information about the object’s position and speed.
OTDR works on the same principle as radar. Here, instead of radio waves, a laser is used. Laser is transmitted inside the optical fiber whose characteristics have to be determined. Some of the laser signals get reflected back due to defects present in the fiber. A receiver present at the same side of the transmitter gathers the reflected rays and extracts information about the defects present in the optical fiber. This is also known as the Backscatter measurement method. The power of the reflected rays is calculated and plotted against a time graph to know about the defects or distortions present in the cable.
OTDR Block Diagram
The different types of equipment used in the design of the OTDR are shown in the figure below.
OTDR Laser: As the name suggests, optical i.e. light source is used in the construction of OTDR. Here the light source used is a ” “Laser “. These laser pulses are applied to the fiber optic cable under test through a coupler. Laser pulses are short and of the intense beam.
Fiber Coupler: A fiber coupler is a fiber device used for coupling light from one or more input fibers with one or more output fibers. These are also used to direct the light from free space into a fiber cable. But coupler can cause wastage of transmitted signal. To avoid this one can opt for a circulator. Circulators provide good directional property which can increase the dynamic range of the OTDR. One of the drawbacks of using a circulator is it increases the overall cost of the OTDR system.
Photodetector: The photodetector connected to the circuit detects the reflected signals and provides information about the power of signals to the next stage. The signal passed through the fiber cable can undergo reflection either due to Rayleigh scattering, splicers inside fiber bend in fiber, or variations in the refractive index of light.
Display: Here, the display is used to display the time-based plot of OTDR which gives information about the type of defect causing the reflection of transmitted light in the fiber.
Optical Time-Domain Reflectometer Specifications
OTDR Trace and OTDR Dead Zone are two main specifications of OTDR used to understand the characteristic graph.
On the graph, the optical power of the reflected waves is plotted against the distance of the fiber. Different power levels determine different types of causes for the reflection of light in the fiber. The positive spikes present in the graph represent imperfections of fiber, fresnel reflection.
The presence of joints in a cable can be detected by looking at the shifts in the curve. One of the major reasons for the distortion of signals in fiber is Rayleigh scattering. This can be detected by looking for a deteriorated tail in the curve.
OTDR Dead Zone
The dead zone in an OTDR graph is the distance in the cable at which it is unable to determine the defect. This is caused when the power of the light received at the photodetector is much greater than the backscattered power level. This causes OTDR to enter into the saturation zone.
Hence, it stops detecting the incoming reflected signals until it gets out of saturation state. Due to this a dead zone is created in the trace of the OTDR curve. Once out of saturation zone it again starts working as normal.
Parameters that determine the quality of OTDR equipment are Accuracy, Measurement range, and Instrument resolution.
Accuracy: The difference between the measured value and the true value of the measured event gives the accuracy of the device.
Measurement Range: It is defined as the maximum amount of attenuation or noise that can be placed between instrument and event such that the instrument can still be able to provide accurate results.
Instrument Resolution: It is a measure of how close two events can be calibrated and still be measured as two separate events. Shorter the pulse duration, shorter the sampling interval, better the instrument resolution.
Advantages and Disadvantages
Some of the advantages of OTDR are as below:
- These are easily available for measurements over an entire wavelength range.
- OTDR instruments can be used to carry out tests for single or dual wavelengths for single mode as well as multimode optical fiber links.
- These are also used for measuring optical loss, dispersion measurements, etc…
- OTDR due to its various applications is referred to as Universal network test systems.
The main disadvantage of OTDR is the occurrence of a dead zone during measuring the defects.
Please refer to this link to know more about fiber-optic communications MCQS
Optical Time-Domain reflectometer is a crucial instrument used by manufacturers and other organizations to determine the quality of new optics links and detect problems in old optics links. These are used for maintaining fiber plant performance. It helps in detecting problems along the length of the channel that may affect the reliability of the system over the long term. OTDR also detects the sharp bends caused in fiber cable during installation of the cable. Which specification present in the curve indicates distortion due to sharp bends present in the cable?