Optical time domain reflectometry (OTDR) is a very important tool for testing and acceptance of optical fiber lines. With the help of OTDR, technicians can see the outline of the whole system, identify and measure the span, connection point and connection head of the optical fiber. OTDR is the most classic and expensive instrument for diagnosing optical fiber fault. Unlike the optical power meter and the optical multimeter, the OTDR can measure the optical fiber loss only through one end of the optical fiber. The OTDR trajectories give the position and size of the system attenuation values, such as the position and loss of any connector, connection point, optical fiber, or fiber breakpoint. OTDR can be used in the following three aspects:
1, before laying, read the characteristics of optical cable (length and attenuation).
2, the signal path line waveform of a section of optical fiber is obtained.
3, locate the serious failure point when the problem is increasing and the connection is getting worse and worse.
Fiber types (single mode / multi mode) and core / coating layer diameter (UM): standard single mode fiber (SM) is 9/125um, although some other special single mode fibers should be carefully identified. Typical multimode optical fibers (MM) include 50/125, 62.5/125, 100/140 and 200/230um.
The choice of OTDR optical time domain reflectometer generally takes into account the following four factors: determining your system parameters, working environment, comparative performance factors, and maintenance of the instrument.
Determine your system parameters
The three main transmission windows of working wavelength (nm) are 850nm, 1310nm and 1550nm.
Clear your work environment
For users , choose a field meter. It is recommended that the working temperature of the field portable instrument should be from -18 C ~50 C, and the storage and transportation temperature is -40~+60 C (95%RH). Laboratory instruments only need to operate at a narrow control range of 5~50 C.
Unlike laboratory instruments that use AC power supply, on-site portable instruments usually require harsh instrument power, otherwise they will affect work efficiency. In addition, the power supply problem of instrument is often an important inducement for instrument failure or damage.
Comparative performance factors
The main performance parameters include the following 4 points:
L dynamic range
L blind area (attenuated blind area and event blind area)
L sampling resolution
The index determines the maximum optical loss that OTDR can analyze, that is, the maximum length of fiber that OTDR can measure. The larger the dynamic range, the farther distance OTDR can analyze. The index of dynamic range must be considered very carefully, mainly due to the following two reasons.
1, OTDR manufacturers may adopt different methods when marking dynamic range, such as defined pulse width, signal to noise ratio, average time, etc. Therefore, we need to have a deep understanding of this index, so as not to cause errors due to different premises and objects when comparing different instruments.
2, if the dynamic range of OTDR is not large enough, it will lead to the failure to test the length of the whole link. In many cases, it will affect the test precision of the whole link loss, the test precision of the attenuation and the test precision of the remote joint. One of the empirical scales is to select OTDR's actual dynamic range value, which is 5 to 8dB higher than the maximum loss that may be encountered.
When evaluating the performance of OTDR, the blind area is a very important index, which determines whether the whole link can be accurately tested. Generally, there are two types of blind areas:
1., the blind area of events: the index is the minimum distance that OTDR can distinguish two reflective events, that is, the ability to distinguish two events. If a reflection event is in the blind area of its previous event, it cannot be detected or correctly tested. The standard value of the industry is from 1 to 5 meters.
2. attenuation blind area: this index refers to how long after a reflection event, after how long the OTDR can accurately test the loss of another reflection event or non reflective event. In short blind area, OTDR can test a short distance optical fiber or find a short optical jumper fault. The shorter the blind area, the better the effect. In industrial standard, the index is generally between 3 and 10 meters.
The degree of linearity refers to the ability of OTDR to suppress noise. The linearity of the instrument is very important because it is directly related to the accuracy of the loss testing of the optical fiber, such as the accurate measurement of the loss caused by the melting point or the optical fiber macro bending. In industrial standard, the linearity index is between 0.03dB/dB and 0.05dB/dB.
Xia Guang XG3110 OTDR series
Sampling resolution, sampling resolution, refers to the minimum distance between two adjacent sampling points. This parameter is also an important parameter, which can affect the accuracy of test distance and the ability of OTDR to determine the failure point. This parameter is related to the pulse width and distance.