Abstract: The reflective measurement technique of continuous distribution parameters of optical links represented by optical time domain reflectometry (OTDR) is introduced. On the basis of clarifying the basic measurement principle and implementation of OTDR, the characteristics of time domain correlation measurement, frequency domain measurement and interferometry technology and the improvement of measurement accuracy are discussed. Keywords: optical time domain reflectometer, optical frequency domain reflectometer, correlation detection, interference detection I. Overview Optical distribution detection is a technique for measuring characteristic parameters of continuous optical links such as optical fibers. Reflective distribution detection is based on the measurement of optical backscattered signals, which are detected, located, and measured by changes in optical transmission characteristics, resulting in optical performance changes due to splices, connectors, bends, and the like. The optical time domain reflectometer OTDR is a typical application of this technology. The OTDR measures the attenuation of the entire fiber link and provides length-dependent attenuation details. The measurement is non-destructive, the measurement process is quick and easy, and the results are accurate and intuitive. Therefore, it is widely used in the fields of production, research and communication. In order to improve the measurement performance, improved measurement techniques such as time domain correlation measurement, frequency domain measurement and interferometry are proposed based on OTDR. Second, the measurement principle of OTDR [1] When the beam propagates along the fiber, the Rayleigh scattering will continue to occur due to the slight unevenness of the core's refractive index, and some of the scattered light will be reversed back to the input. ![]()
By injecting a laser into the fiber and monitoring the intensity variations of these backscattered light, an attenuation curve along the length of the fiber can be obtained. This technology can be used to detect the scattering coefficient, loss and connection points, coupling points, breakpoints, etc. in the fiber. There are two kinds of backscattered light in the measurement, one is Rayleigh scattered light, and the other is Fresnel reflection generated by fiber cross section or fiber connection. Assuming that the incident light power is P0, the power of the backscattered light from the fiber to the incident end is Ps, and the attenuation coefficient at the fiber l is α(l), then the following formula can be obtained: ![]()
(1) It can be known from equation (1) that the OTDR measurement curve of a good fiber should approximate a straight line with the same slope. The sudden change in the curve shows the change of the light propagation characteristics in the fiber, as shown in Fig. 2. The average attenuation coefficient between l1 and l2 in the fiber can be obtained by equation (2). ![]()
(2) Third, the application prospect of reflection detection on fine structure ![]()
Reflection detection can be applied to a wide range of measurements of optical fibers, and can also play an important role in the small-scale measurement of optical paths and the fine structure measurement of optical devices. Optical chronological correlation detection technology, optical frequency domain detection technology and interference detection technology can be used to improve the measurement accuracy of OTDR. 3.1 Optical Time Domain Correlation Detection The spatial accuracy of OTDR detection refers to the shortest distance between two event points that can be resolved on the detection link, called spatial resolution. The resolution is mainly determined by the width of the probe pulse. The improvement of the detection separation rate can be achieved by reducing the detection pulse width, but under the condition that the laser power is constant, the detection pulse energy is lowered, and the detectable backscattered light signal is very weak. Therefore, improving the detection sensitivity of OTDR is a key issue to be solved for small-scale detection. Correlation detection [2] provides a method to increase the backscattered optical signal-to-noise ratio (SNR) without reducing spatial resolution. The complementary Golay code [3] has a unique autocorrelation property, and uses the complementary code as the excitation pulse sequence. The correlation operation can effectively suppress the noise and improve the detection sensitivity. The definition of the complementary code is: the autocorrelation of the two L-ary sequences and if they are zero for any non-zero shift, then the two sequences are complementary: ![]()
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(3) Complementary codes have good autocorrelation properties. The individual autocorrelation of each complementary code sequence has side lobes in addition to the main peak. The main peak amplitude is the bit length L of the sequence, and the side lobes are about 10% of the main peak, but two The side lobes are eliminated when the autocorrelation of the sequence is added, see Figure 3. The nature of the complementary code can be applied in the correlation of OTDR. If we use the L-bit complementary code to modulate the detection signal for Ak and Bk, we can get the measurement signal as , and perform autocorrelation processing. Using the autocorrelation property of the complementary code, we can get the following final results: ![]()
(4) hk is the response of the unit intensity detection pulse. It can be seen that the measurement signal is increased by 2L times compared with the single pulse mode, and the measurement sensitivity is improved. Considering the negative effects of the correlation detection on the amplification of the noise, the influence of the four measurements of the signal, the SNR of the complementary correlation detection and the single pulse detection can be obtained by theoretical analysis as follows: ![]()
(5) The SNR of the correlation detection is expressed as follows. ![]()
(6) where Pinit is the input optical power, PNE receiver equivalent noise power, Noct is the number of repeated measurements, Loct is the coding length, z is the distance from the incident end, and α is the fiber attenuation coefficient. The correlation detection is compared with the single pulse detection, which effectively improves the sensitivity, and the sensitivity increases as the code length increases.
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