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The optical interference technique is an tool with high temporal and spatial resolution in the fields of measurement and metering. Combined with laser which has unique temporal and spatial coherence, the interference method has large measurement area (It belongs to a different catalogue from resolution, and not described here). Interference technique is widely used in fiber optic test field. For example, the interference test techniques of PMD and beat length are the typical ones, which base on the same principle.
The PMD test system, PMD-6000B, in Information Transmission Lines Quality Inspect Test Center of M.I.I(the Inspection and Testing Center of No 23 Research Institute, CETC) has a temporal resolution of 0.6 fs, and uses Michelson interferometer. The tested sample is placed on one of the two optical circuit arms. The beams of polarized light, from the two optical circuit arms of the interferometer, are injected into the sample in the directions of two orthogonal characteristic optical axes. By adjusting the optical path of the movable optical circuit arms, the polarized light beams from the two characteristic directions will engender interference at the output end, and we can get the time delay difference between two PDMs obtained from the sample by the adjustment, that is PMD：τ=2d/c. Here d is the quantity of optical path adjustment, and c is optical speed.
According to the definition of beat length, we can get the formula below:
Lp•Δβ=2π或Lp•Δn =λ
两个正交的线偏振模之间，它们的传播相位差达到2π时所经过的空间距离Lp 即为拍长：
When the propagation phase difference of the two orthogonal linearly polarized
modes achieves 2π the spatial distance, Lp, they pass through is called beat length.
Lp=2π/Δβ=λ/Δn
其中Δβ为传播常数差（Δβ=Δn2π/λ）、Δn为模式双折射（modal birefrigence）、λ为测试光波长。而Δn=d/L，其中L为试样长度。
Where Δβ is propagation constant deviation（Δβ=Δn2π/λ）; Δn is modal birefringence; λ is the testing optical wavelength; Δn=d/L, L is the length of sample.

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Optical polarization mode dispersion and patted long measurement
In the measurement of optical interference technique (and) is a kind of time and space of high resolution, With the combination of laser technology, due to its incomparable time and space, and coherence in measuring method of interference with great range (relative to the resolution of different categories, here does not involve). In fiber testing technology, interference technique used widely, such as optical polarization mode dispersion (PMD) and patted long (only consider such as optical polarization keep) of interfering method for testing technology based on a typical, they Shared principles.

6000B PMD - PMD type test system (0.6 fs), time resolution adopted by Michelson interferometer, test sample in one of two optical path interferometer arms. Two optical path from interferometers polarization of light arms with two orthogonal axis direction features samples injected samples. Through the interferometer arms of the optical activity, make the optical path sample output corresponding to the two characteristics of polarized light, the direction of an interference by optical path overshoot available samples of delay difference of two polarization mode, namely, PMD: τ = 2d/c, including d for optical path overshoot, C for speed.
According to the definition, is patted long established formula:
Lp, Δ beta = 2 PI, Δ n = or Lp status
Two orthogonal polarization mode between the lines of communication, which reached 2 PI difference through space for long distance Lp is patted:
Lp = 2 PI/Δ beta = lambda/Δ n
One Δ beta for propagation constant (Δ beta = Δ n2 PI/lambda), Δ n for model for testing birefringence, lambda light wavelengths. And Δ n = d/L, including L for sample length.
From the above formula are visible light interference method, process overshoot d is to realize the common PMD and Lp measurement principle foundation. That way, through the above analysis, interfering method for measuring principle than in Lp, such as common as the modulation technique may more reliable.

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Third-order intermodulation test

Frequency generated by the two third-order intermodulation distortion is a modern communication system common problem. When the system in two (or more) of the carrier frequency signal through a passive device, such as antennas, cables, filters and duplexers, because of their improper material selection, mechanical contact is not reliable, Weld and surface oxidation and other reasons, the junctions of different materials will produce non-linear factors, it is like mixing diodes. 2 carrier frequency signal (F1 and F2) and its second harmonic (2F1 and 2F2) produced by the further the maximum intermodulation product is the third-order intermodulation distortion (2F1-F2 and 2F2-F1). Third-order intermodulation products (IM3) of a typical target is that when two +43 dBm carrier frequency signal at the same time added to the device under test (DUT), its value is not greater than IM3 generated-110dBm, which is-153dBc.

Third-order intermodulation distortion will reduce communication system performance. Excessive emission signals in the third-order intermodulation products interfere with other receivers, the receiver does not work the final result. In the past, designers are more concerned about the active devices intermodulation test. But with the development of communication systems and systems to improve the quality of passive components in the interchange of more and more seriously.

Optical fiber polarization mode dispersion and beat length measurements
Optical interferometry in the measurement (and measurement) in the field is a very high temporal and spatial resolution means; its combination with laser technology, because of its unique time and space coherence, but also to interference measurement method has great measurement range (as opposed to the resolution of different areas, not involved here). In the optical testing technology, a wide range of interference technology, such as optical fiber polarization mode dispersion (PMD) and beat length (only consider the situation, such as polarization maintaining fibers) of the Interferometry Technology shall be a typical example, are based on common principles.

PMD-6000B-type PMD test system (time resolution of 0.6 fs), using Michelson interferometer, the measured samples of two optical interferometer placed in one arm. From the two interferometer arms of the polarized optical beam characteristics of each optical axis of the sample into two orthogonal sample. Activities through the optical interferometer arm optical path adjustment, the sample output corresponds to the direction of the polarized light of two characteristics of interference generated by the process of adjusting the amount of light available to sample the delay difference between the two polarization modes, namely PMD: τ = 2d / c , where d is the optical path adjustment amount; c is the speed of light.
According to the definition of a long shot, set up the following formula:
Lp • Δβ = 2π or Lp • Δn = λ
Two orthogonal linear polarization modes through which the transmission phase to 2π when the distance through which beat length Lp shall:
Lp = 2π/Δβ = λ / Δn
Where Δβ is the propagation constant difference (Δβ = Δn2π / λ), Δn birefringence as a model, λ for the test wavelength. And Δn = d / L, where L is the sample length.
Can be seen from the above formula is given, to interfere in the optical path adjustment method is to realize the amount d and Lp PMD measurement based on the common principle. Incidentally, the above analysis, the interference method for measuring Lp than in principle, such as common external modulation techniques may be more reliable.