OTDR YYDS, it is said that communications people can understand it!

Optical fiber is an important part of communication transmission, and it shoulders the important mission of transmitting network signals. The performance of transmission optical fiber directly affects the reliability of the communication system, and the probability of problems with transmission optical fiber is relatively high, such as bending and breaking of the optical cable.

Regardless of the cause, it is necessary to quickly locate the fault location, quickly repair the optical fiber, and restore normal communication. However, it is impossible to rely on people to find dozens or hundreds of kilometers of optical fiber bit by bit in the live network. At this time, the "radar" technology in the optical fiber industry - OTDR is needed to locate the fault point in minutes, saving time, effort and worry.

1. What is OTDR?

OTDR (Optical Time Domain Reflectometer) technology is a precise optoelectronic integrated device made by using backward Rayleigh scattering and Fresnel reflection phenomena to measure the length, loss, joint attenuation and breakpoint location of optical fiber.

OTDR Instrument

Let's talk about Rayleigh scattering first. In fact, Rayleigh scattering is very common in life. For example, the reason why we can see a beam of light when we turn on a flashlight at night is because the dust and fog in the air cause light scattering. Only when part of the light enters our eyes can we see it, and all the particles scatter to form a beam of light. When the fog is thick, the beam of light will appear dense, otherwise it will appear sparse. If it is in a vacuum, the beam of light will not be visible.

Back to optical fiber, the density and composition of SiO2 (silicon dioxide), the main component of optical fiber, are uneven. In addition, there are impurities, bubbles and micro-bend structures, similar to dust and fog in the air. When we inject a beam of light pulse into the optical fiber link, when the light energy of this pulse encounters uneven deposition points during the forward transmission process, extremely weak energy will be scattered in all directions. This phenomenon is called Rayleigh scattering, and part of it will be completely reflected back in the direction of the origin. This part is called backward Rayleigh scattering.

Let's look at Fresnel reflection. I wonder if you have noticed that when we stand by the lake and look down at the water under our feet, the water we see is transparent and the reflection is not particularly strong. But when we look at the lake surface in the distance, the lake water we see is not transparent, but a reflection of the surrounding landscape, and the reflection is very strong. This is the "Fresnel effect". Simply put, when the line of sight is perpendicular to the surface, the reflection is weak, and when the line of sight is not perpendicular to the surface, the smaller the angle, the more obvious the reflection.

In optical fiber, Fresnel reflection is a discrete reflection caused by individual points in the entire optical fiber, such as the gap between glass and air. At these individual points, strong backscattered light will be reflected back, which is called Fresnel reflection. The maximum reflected energy can reach 4% of the forward transmitted light energy. Fresnel reflected light is usually much stronger than Rayleigh scattering and can be easily distinguished.

2. OTDR equipment classification

OTDR equipment can be roughly divided into two categories: OTDR instruments and OTDR boards:

• OTDR instrument: OTDR instrument is not associated with WDM equipment. Maintenance personnel need to carry the instrument to the site to manually scan the optical fiber to locate the fault. OTDR instruments are of two types: desktop and handheld.
• OTDR board: The OTDR board integrated in the optical communication equipment has a similar structure to the instrument, but without a display. The measurement results can be presented through the network management. The fiber/cable monitoring function is integrated into the wavelength division equipment. There are three advantages, so this method is increasingly used.

3. OTDR working principle

The working principle of OTDR is similar to that of optical "radar". OTDR sends an optical pulse signal to the optical fiber under test, and then observes and analyzes a small amount of light returned from the optical fiber under test to OTDR (the returned light comes from backward Rayleigh scattering and Fresnel reflection), thereby obtaining information such as the fault point of the optical fiber under test. The specific process is:

(1) The light waves emitted by the laser in the OTDR are coupled into the optical fiber under test through a directional coupler.

(2) As light waves propagate forward in the optical fiber, they continuously generate Rayleigh scattering, and may also generate reflection events and non-reflection events:

Reflection events: If there are active connectors, mechanically fixed joints and break points in the optical fiber, it will cause optical power attenuation and cause Fresnel reflection, which is called reflection event.

Non-reflective events: The fusion points and bending points in the optical fiber will cause optical power attenuation, but there is no reflection phenomenon at these points, which is called non-reflective events.

(3) The Fresnel reflected and back-Rayleigh scattered light is coupled by a directional coupler and transmitted to the detector. After being photoelectrically converted by the detector, it is sent to the signal processor. Finally, the measurement curve of the optical fiber to be tested is displayed on the display.

What does the displayed curve mean? How to use this curve to locate the fault?

The corresponding relationship between the OTDR curve and the optical fiber link is shown in the figure below. These curves reflect the status of the optical fiber link. The curves can be used to observe and measure the loss, end face, breakpoint, connector loss, etc., realize optical fiber monitoring, and locate the optical fiber fault location.

If you think it is too troublesome to analyze this curve, it doesn't matter. The results presented by the network management will give possible reasons for reference and positioning. The following figure shows the measurement results obtained by using the OTDR function after two 25km long optical fibers are connected. It can be seen that the possible reasons such as the splice point and connector are given for the change points of the curve, and the location information is given. Isn't it very intuitive?

If the two 25km optical fibers are disconnected, the OTDR measurement results shown in the figure below show that only the optical fiber signal at 25km can be measured, and an optical fiber end event is reported, indicating that the optical fiber is broken at 25km.

Well, that's the end of the introduction to OTDR. In summary, through this article we know that OTDR is a device that uses Rayleigh scattering and Fresnel reflection to locate fiber faults. OTDR has two types: instrument and single board. We also learned how to obtain the fiber link status from the OTDR curve. In this era of increasingly frequent information exchange, OTDR, as the "radar" in the fiber optic industry, plays a vital role in monitoring the working status of transmission fibers, locating fiber faults, and ensuring the smooth operation of optical networks.