Basic knowledge of optical time domain reflectometry (OTDR)

The development of optical fiber communications plays an important role in my country's economic construction. Optical fiber communication has incomparable advantages: wide transmission frequency and less loss and consumption. The construction of optical fiber communications began in the 1990s and has been developed on a large scale.

As a transmission network carrying a large amount of information, optical fiber communication has certain risks and instability. In order to ensure the smooth operation and safety of optical fiber communication, it is necessary to develop a tool or instrument that can accurately measure the characteristics of optical fiber communication. In order to meet the requirements for optical fiber diagnosis in optical fiber communications, an optical time-domain reflectometer (OTDR) using back Rayleigh scattering as the measurement signal was developed. 

OTDR optical time domain reflectometry technology 

OTDR technology can detect the link loss and health status of optical fiber, because it has the ability to test the loss at different locations of the entire optical fiber link, so that the health status of the optical fiber cable can be evaluated based on the loss at different locations measured by the OTDR.

According to the back-rayleigh scattered light intensity generated by the pulsed light in the optical fiber, the attenuation along the optical fiber can be measured at the single end of the optical fiber; according to the time difference between the arrival time of the scattered light and the emission time of the pulsed light, each attenuation point of the optical fiber can be measured Perform spatial positioning. The two characteristics of single-ended, non-damage optical fiber attenuation measurement and "light radar" effect enable OTDR technology to not only quickly replace conventional methods, but also show unique superior performance in on-site fiber fault point diagnosis and location applications. 

By continuously improving and improving the measurement technology of OTDR, researchers have greatly improved its measurement dynamic range, spatial resolution, signal-to-noise ratio, automatic protection, automatic identification and measurement performance. OTDR systems can go a long way in solving fiber optic communications operational health issues. 

OTDR working principle

OTDR uses the backscattering phenomenon generated when light pulses are transmitted in optical fibers, injects high-power narrow pulse light into the optical fiber to be tested, and then detects the scattered light power returned along the axial direction of the optical fiber at the same end, as shown in the figure below. When the incident light pulse is transmitted in the line, Rayleigh scattered light and Fresnel reflected light will be generated along the way. Most of the Rayleigh scattered light will be refracted into the cladding and then attenuated. Among them, back Rayleigh scattering is opposite to the propagation direction of the light pulse. The light will be transmitted along the optical fiber to the light entrance port of the line. The wavelength of Rayleigh scattered light is the same as the wavelength of the incident light, and its optical power is proportional to the incident optical power at the scattering point. Measuring the back-Rayleigh scattered light power returned along the axial direction of the fiber can obtain information about the transmission loss along the fiber, thereby measuring the attenuation of the fiber. 

Optical Time Domain Reflectometer (OTDR)

Optical time domain reflectometer (OTDR) is an important tool for detecting the integrity of optical cables. It can be used to measure the length of optical cables, measure transmission performance and connection attenuation, and detect fault locations of optical cable links. What is the working principle of optical time domain reflectometer (OTDR)? What are the usage methods and precautions of optical time domain reflectometer (OTDR)?

How optical time domain reflectometry (OTDR) works

In the process of testing optical cables with an optical time domain reflectometer (OTDR), the instrument injects higher-power laser or light pulses from one end of the optical cable and receives the reflected signal through the same side. When light pulses are transmitted through fiber optic cables, some of the scattering and reflections will return to the transmitter. The optical time domain reflectometer (OTDR) will only measure the reflected light signal with higher intensity. By recording the time from transmission to return and the transmission speed of the signal in the glass material, the length of the optical cable can then be calculated using the formula. .

Compared with power supplies and energy meters that can directly measure the loss of optical cable equipment, optical time domain reflectometers (OTDR) work indirectly. The Optical Time Domain Reflectometer (OTDR) is made based on the backscattering and Fresnel inversion principles of light. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information, thereby indirectly measuring optical cable losses and faults. Location.

Optical Time Domain Reflectometry (OTDR) Functions

1. Powerful FastReporter software application. Use the powerful FastReporter software to quickly track data and conduct offline analysis, forming an intuitive graphical interface to help users improve work efficiency.

2. Intelligent trace analysis. The built-in intelligent trace analysis module can quickly and accurately analyze event points and position information in the test curve, and display it in the form of an event table.

3. Blind spot for ultra-short events. OTDR has an ultra-short event blind zone of ≤0.8m, which is especially suitable for testing ultra-short optical fiber links or optical fiber jumpers.

4. Convenient VFL function. The visible red light fault function can easily and quickly find the location of short-distance optical fiber link interruption points or loss points, so that maintenance personnel can take timely measures.

5. Multiple sockets and flexible connections. Complete socket types: RJ-45, USB, power socket, etc., flexible connection. The USB port can be connected to a computer through a data cable to directly export test data.

6. Humanized touch interface. The transmissive color LCD screen allows you to clearly observe the test results even under the sun. With the simple button design, the operation is simple and flexible.

How to use optical time domain reflectometer (OTDR)

When connecting the test pigtail to the Optical Time Domain Reflectometer (OTDR), first clean the test side pigtail, then insert the pigtail vertically into the instrument test jack, and return the raised U-shaped part of the pigtail and the test jack to the U The molded parts are fully connected and properly tightened. During line inspection and repair or cutover, before the optical fiber under test is connected to the OTDR, the maintenance personnel of the opposite end office station of the trunk section should be notified to remove the corresponding connecting pigtail on the optical fiber distribution box to avoid damaging the optical disc.

1. Wavelength selection setting: Select the wavelength required for testing. There are two wavelengths to choose from: 1310nm and 1550nm.

2. Distance setting: First test the fiber in automatic mode, and then set the test distance according to the length of the test fiber, usually 1.5 times the actual distance, mainly to avoid false reflection peaks that affect judgment.

3. Pulse width setting: The pulse widths available for the instrument generally include 10ns, 30ns, 100ns, 300ns, 1μs, 10μs and other parameters. The smaller the pulse width, the shorter the sampling distance and the more accurate the test. On the contrary, the longer the test distance. long, the accuracy is relatively small. According to experience, generally use parameters of 100ns and below for distances below 10KM, and parameters of 100ns and above for distances above 10KM.

4. Sampling time setting: The longer the instrument sampling time, the smoother the curve and the more accurate the test.

5. Refractive index setting: determined according to the different requirements of each transmission line.

6. Event threshold setting: refers to pre-setting the attenuation of the optical fiber connection point or loss point during the test. When an event exceeding the threshold is encountered, the instrument will automatically analyze and locate the event.

Precautions for using optical time domain reflectometer (OTDR)

1. The Optical Time Domain Reflectometer (OTDR) emits high-energy light signals when working. Therefore, it is forbidden to look directly at the port with your eyes during the test to avoid burning your eyes.

2. Keep the optical time domain reflectometer (OTDR) test port and the optical cable port clean to avoid problems such as no data in the test, that is, the optical link cannot work properly or the attenuation test is inaccurate.

3. The optical time domain reflectometer (OTDR) test port has a built-in ceramic core, which is very fragile, so avoid vigorous twisting and bumping.

4. During the testing process of the Optical Time Domain Reflectometer (OTDR), signals other than those emitted by the instrument are not allowed to exist. First, it will interfere with the accuracy of the test, and second, it will damage the optical link equipment.

5. Select the appropriate test distance and pulse width. When you do not know the length of the optical cable, you can first use the automatic test function of the instrument to roughly understand the quality of the optical cable to be tested, and then manually set the reasonable test range, pulse width and other parameters. , used to accurately locate the entire optical cable and the location and loss of each event.

Blind zone solutions for optical time domain reflectometry (OTDR)

The blind area of ​​optical time domain reflectometer (OTDR) originates from the Fresnel inversion principle. There are two types of blind areas, namely event blind areas and attenuation blind areas. The length distance from the starting point of the reflection peak to the saturation peak of the receiver caused by the reflection peak due to the intervention of the movable connector is called the event blind zone; the reflection peak caused by the intervention of the movable connector in the optical fiber, from the starting point of the reflection peak The distance to the point where other events can be identified is called the attenuation dead zone.