OTDR, short for Optical Time Domain Reflectometer, is a professional instrument for optical fiber line maintenance and detection. OTDR is widely used in the installation, maintenance, repair and monitoring of optical fiber and optical fiber lines. As one of the important instruments for maintenance in the field of optical communication test, OTDR can measure the length, attenuation, connection quality and fault of optical fiber and cable.
Parameter Settings of OTDR
In order to get good and accurate fiber trace analysis, it is suggested to set the following basic data parameters of OTDR.
Different wavelengths is related to different characteristics of light (such as attenuation and micro-bend), so the test wavelength generally follows the principle of corresponding to the transmission wavelength of the system. For example, if the system is to 1550nm wavelength, so should be the test wavelength.
2. Pulse Width
Larger pulse width provides larger dynamic range and can measure longer fiber length, but will also generate a larger blind zone in the OTDR curve waveform. The pulse width duty cycle is usually expressed in ns (nanosecond). The smaller the pulse width, the higher the test accuracy, but requires higher power consumption.
3. Test Range
OTDR test range refers to the maximum distance from which OTDR can obtain data samples. The choice of this parameter determines the size of the sampling resolution. Always choose a test range larger than the target test value. The optimal test range is 1.5 to 2 times the length of the optical fiber to be measured.
4. Averaging Time
Since the back scattering optical signal is extremely weak, statistical averaging method is usually adopted to improve SNR (Signal to Noise Ratio). The longer the averaging time, the higher the signal-to-noise ratio. The longer the averaging time, the higher the test accuracy, but the accuracy does not increase when it reaches a certain level. Suggested averaging time is 30 seconds to 3 minutes.
5. Fiber Parameters
The setting of fiber parameters includes refractive index n, back-scattering coefficient n and back-scattering coefficient η. The refractive index parameter is related to the distance measurement, while backscattering coefficients affect the test results of reflection and return loss. These two parameters are usually provided by fiber manufacturers. Once the setting’s ready, OTDR will emit light pulse, receive light scattered and reflected back from fiber optic link, sample the output of the photoelectric detector and generate OTDR curve, through which the user can analyze the quality and performance of optic fiber.
Notes for Using OTDR
1. Online Test
Before testing, make sure if the OTDR can be tested online. Forced use will burn out the internal components and cause irreversible damage to the instrument.
2. Joint Cleaning
OTDR output port must be kept clean, and the output port needs to be cleaned regularly with absolute ethanol. The dust and particles attached to the connector and output port may cover part of the fiber core of the output fiber, resulting in the performance degradation of the instrument, or even cause damage.