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Discuss the effects of test light sources, power meters, polarization controllers, and test systems on test accuracy, reliability, and repeatability.
Light source selection:
The test light source is the excitation source of the test system. Because it is used for testing and not for transmission, it generally does not require high power. A laser light source of 0 dBm and a wide spectrum source of -10 dBm/nm are sufficient to meet the test requirements. Also because it is used for testing, the power stability of the light source is quite important, in addition there is a problem of coherence length. In fact, any laser light source has a problem of coherence length. Generally, the coherence length of an FP or DFB laser light source is 1,000 meters or more, and it is about 10 meters after artificially widening the line width of a laser. That is, as long as the test system If the light path is shorter than this length, there will be interference, and the test will be inaccurate or the reliability will be reduced. A tunable laser based on an erbium-doped fiber ring solves this problem very well. The coherence length of the laser is only 15 centimeters, and the device test length is generally 1 to 3 meters, so there must be no coherent influence, so that the test Value stability, repeatability, and reliability are very high, making it a very suitable light source for device testing.
In addition to the coherence length, the signal-to-noise ratio of the laser source is another key parameter. The signal ratio of the laser source to the emitted radiation noise (S/SSE) is a key factor that limits the dynamic range of the test. If the S/SSE is only 60dB, then the filter cannot filter out spontaneous emission noise when testing 65dB filters, so the test can only show 60dB, resulting in test failure. In general, the tunable laser source has 75 dB of S/SSE, so pay attention to the S/SSE value of the source when testing large dynamic range devices.
For a wide spectrum source or ASE light source, the spectral stability is a key parameter, and the stability of the spectrum is a more rigorous and more meaningful parameter than the integrated power stability. It represents the peak-to-peak spectral variation of the broad spectrum source over a period of time. Maximum value. Since the broad spectrum source is generally used with a wavelength selective device such as a spectrometer or a wavelength meter, the integrated power stability does not make much sense for the test.
The material of the power meter detector generally determines the overall performance of the power meter. Generally, there are detectors of materials such as Ge, Si, and InGaAs, in addition to a low-polarization reflectance (PDR) detector. Adding some materials based on the InGaAs detector makes it very insensitive to PDL, so it is very suitable for PDL testing.
In addition to materials, the area of the detector is an important parameter that determines its use. The larger the area of the detector, the stronger the light-receiving capacity, but the lower the sensitivity, and vice versa. Therefore, the area of the optical power meter detector used for calibration is larger than 3mm2. It is used to detect very small optical power such as -100dBm. The area of light energy detector is generally 1mm2. In general, if a bare fiber adapter is used for the optical power meter, the area of the optical power meter detector is required to be greater than 3 mm2. Otherwise, the outgoing light of the optical fiber cannot be sufficiently coupled to the detector, which greatly reduces the repeatability and reliability of the test. In fact, even if a large-area detector is used, the optical fiber in the bare fiber adapter is likely to touch the detector, resulting in aging of the detector and lowering of the test accuracy. Therefore, it is generally recommended to use a fusion method. Although this increases the number of times, it ensures that The long-term stability and reliability of the test.
In addition to the above traditional detector types, there is also a wide-aperture integrating sphere detector technology. The detector area of this detector is equivalent to 7mm2. Due to the use of integrating sphere technology, it does not have the problem of surface non-uniformity of conventional large-caliber detectors, fiber alignment, and the problem that the fiber head easily touches the surface of the detector. The test repeatability is also Traditional detectors cannot be compared.
For a randomly scanned Poincare Ball Polarization Controller (PC), the scan period, the area of the Poincare sphere, the polarization of the light passing through the PC, and the optical power fluctuations due to the PC are some of the key parameters. The meaning of these parameters is easy to understand. Here I only want to focus on the impact of the optical power fluctuations caused by the PC on the test. We know that the PDL test actually detects the maximum value of the optical power that passes through the DUT when the SOP of the transmitted light changes. Therefore, if the optical power changes due to other reasons, the test system will mistakenly think that this Also PDL, resulting in PDL testing too large. So for the PC, the optical power fluctuation will directly affect the accuracy of the test.
The so-called test system mainly refers to more than two test tables or modules work together to form a new interface after the combination, and complete the test equipment for automatic testing. The traditional system construction is through a computer, using the GPIB port to control several optical test instruments. Here we focus on the method of assembling the system through modules. The main idea is that the test host itself is a standard computer, the test host has 5 slots, can insert test modules, form a simple system, can also add expanders for large test systems, between the host and expander Connect through data lines. In this way, there is no difference between the slot on the expansion machine and the slot on the host. There is no difference in the data transmission rate between the module inserted in the expansion machine and the module inserted in the host, so this method of forming the testing system makes the system data The transmission speed is very fast and the operation is very convenient. The expander can also cascade expanders to form a larger system, so the capacity expansion is very good. The WDM test system organically combines a tunable light source, a fast optical power meter, a Muller matrix polarization controller, and a wavelength calibration unit. With a test wavelength accuracy of 5 pm, the IL, ORL, and PDL curves with wavelengths can be measured with a click of the mouse and a crosstalk matrix can be derived. This also demonstrates the advantages of using the host + expander for system setup.