1.Basic principles of OTDR

The OTDR is called an Optical Time Domain Reflectometer, and a narrow optical pulse is injected into the end face of the fiber as a detection signal. When the light pulse propagates along the fiber, the backscattered portion of the Rayleigh scattering will continuously return to the incident end of the fiber. When the optical signal encounters a crack, a Fresnel reflection will occur, and the back-reflected light will also return. The entrance end of the fiber.

Rayleigh backscattering and Fresnel reflection occur when light propagates through an optical fiber. The OTDR utilizes the characteristics of light to collect backscattering and reflection of light pulses in the path. High-tech, high-precision photoelectric integrated instrument.

 

By detecting the size and arrival time of the back light at the input end by suitable photocoupler and high-speed response photodetector, the transmission characteristics, length and fault point of the fiber can be quantitatively measured.

 

2.OTDR parameter setting

Fiber optic measurements with OTDR can be divided into three steps: parameter setting, data acquisition, and curve analysis.

The performance parameters of the OTDR generally include: dynamic range, dead zone, distance accuracy, return loss, and reflection loss.

Manually setting measurement parameters includes:

(1) Wavelength selection (λ):

Because different wavelengths correspond to different light characteristics (including attenuation, microbend, etc.), the test wavelength generally follows the principle corresponding to the transmission wavelength of the system transmission, that is, when the system is open at 1550 wavelength, the test wavelength is 1550 nm.

(2) Pulse Width:

The longer the pulse width, the larger the dynamic measurement range and the longer the measurement distance, but the blind zone is larger in the OTDR curve waveform; the short pulse injection light is lower, but the blind zone can be reduced. The pulse width period is usually expressed in ns.

(3) Measurement range (Range):

The OTDR measurement range refers to the maximum distance that the OTDR acquires data samples. The choice of this parameter determines the size of the sampling resolution. The optimal measurement range is between 1.5 and 2 times the length of the fiber to be tested.

(4) Average time:

Since the backscattered light signal is extremely weak, a statistical average method is generally used to improve the signal-to-noise ratio, and the longer the average time, the higher the signal-to-noise ratio. For example, a 3 min acquisition would take a 0.8 dB improvement over the 1 min acquisition. However, the acquisition time over 10 minutes does not improve the signal-to-noise ratio. The average time is usually no more than 3 minutes.

(5) Fiber parameters:

The setting of the fiber parameters includes the setting of the refractive index n and the backscattering coefficient n and the backscattering coefficient η. The refractive index parameter is related to the distance measurement, and the backscattering coefficient affects the measurement of the reflection and return loss. These two parameters are usually given by the fiber manufacturer.

After the parameters are set, the OTDR can send optical pulses and receive the light scattered and reflected by the fiber link. The output of the photodetector is sampled to obtain the OTDR curve. The curve can be analyzed to understand the fiber quality.

Blind zone

(1) definition

After a reflection (Fresnel reflection) is generated by a feature point such as a movable connector and a mechanical joint, a series of "blind spots" caused by saturation of the OTDR receiving end is called a dead zone.

(2) attenuation dead zone

The attenuation dead zone is the minimum distance in which the OTDR can accurately measure continuous event loss after Fresnel reflection. The minimum distance required is from the time the reflection event occurs until the reflection is reduced to 0.5 dB of the backscatter level of the fiber.

(3) event dead zone

The event dead zone is the minimum distance in which the OTDR can detect another event after the Fresnel reflection. In other words, it is the minimum fiber length required between the two reflection events. In order to establish specifications, the most common industry method is to measure the distance between each side of the reflection peak - 1.5dB.

In fact, the main impact on the safety of optical cable is mechanical damage. The large loss of fiber connection does not affect the strength of the connection. Therefore, we found in the acceptance test that some values are indeed too large, and about 1% of the connectors are back to the standard. And after many times of connection can’t be reduced. In this case, it can also be judged qualified. Sometimes it will be required according to the total attenuation of the intermediate stage, so that the acceptance is qualified.