Optical fiber test charts include: optical power meters, stable light sources, optical multimeters, optical time domain reflectometers (OTDRs), and optical fault locators. Optical power meter: Used to measure absolute optical power or relative loss of optical power through a length of fiber. In fiber optic systems, measuring optical power is essential. Much like the multimeter in electronics, in optical fiber measurement, the optical power meter is a heavy-duty common watch, and the fiber technician should be one. By measuring the absolute power of the transmitter or optical network, an optical power meter can evaluate the performance of the optical equipment. Using an optical power meter in combination with a stable source, it is possible to measure connection loss, verify continuity, and help evaluate fiber link transmission quality. Stable light source: Light that emits a known power and wavelength to the light system. The stable light source is combined with an optical power meter to measure the optical loss of the fiber system. For off-the-shelf fiber optic systems, the system‘s transmitter can often be used as a stable source. If the terminal is not working or there is no terminal, a separate stable light source is required. The wavelength of the stabilized source should be as consistent as possible with the wavelength of the system end. After the system is installed, it is often necessary to measure the end-to-end loss to determine whether the connection loss meets the design requirements, such as measuring the loss of the connector, the connection point, and the loss of the fiber body. Optical Multimeter: Used to measure the optical power loss of a fiber link.
There are two types of optical multimeters: 1. It consists of an independent optical power meter and a stable light source. 2. Integrated test system combining optical power meter and stable light source.
In short-range local area networks (LANs) where the endpoint distance is within walking or talking, the technician can successfully use the economical combined optical multimeter at either end, with a stable light source on one end and an optical power meter on the other end. For long-haul network systems, technicians should be equipped with a complete combination or integrated optical multimeter at each end. Temperature is perhaps the most stringent standard when choosing a meter. Field portable equipment should be at -18 ° C (no humidity control) to 50 ° C (95% humidity) optical time domain reflectometer (OTDR) and fault locator (Fault Locator): as a function of fiber loss and distance. With the help of OTDR, technicians can see the entire system profile, identify and measure fiber spans, splices, and connectors. Among the instruments that diagnose fiber faults, OTDR is the most classic and most expensive instrument. Unlike the optical power meter and the optical multimeter‘s two-end test, the OTDR can measure the fiber loss only through one end of the fiber. The OTDR trace gives the location and magnitude of the system‘s attenuation value, such as the position of any connector, splice, fiber profile, or fiber breakpoint and its loss.
The OTDR can be used in the following three aspects: 1. Understand the characteristics (length and attenuation) of the cable before laying. 2. Obtain a signal trajectory waveform of a piece of fiber. 3. Position the serious fault point when the problem increases and the connection status deteriorates.
The Fault Locator is a special version of the OTDR. The fault locator can automatically detect the fault of the fiber without the complicated operation steps of the OTDR. The price is only a fraction of the OTDR. To select a fiber optic test instrument, generally consider the following four factors: determine your system parameters, working environment, comparative performance factors, instrument maintenance, determine your system parameters, operating wavelength (nm), three main transmission windows are 850nm , 1300nm and 1550nm.
Type of light source (LED or laser): In short-range applications, most low-speed LANs (100Mbs use laser sources to transmit signals over long distances for economical reasons. Fiber types (single mode / multimode) and core / coating Diameter (um): Standard single mode fiber (SM) is 9/125 um, although some other special single mode fibers should be carefully identified. Typical multimode fibers (MM) include 50/125, 62.5/125, 100/140 and 200/230 um. Connector types: Common connectors in China include: FC-PC, FC-APC, SC-PC, SC-APC, ST, etc.
The latest connectors are: LC, MU, MT-RJ, etc. The maximum possible link loss. Loss estimation / system tolerance. Clarify your work environment For users/purchasers, choose a field instrument, the temperature standard may be the most stringent. Generally, field measurements must be used in severe environments. It is recommended that the operating temperature of the on-site portable instrument should be from -18 ° C to 50 ° C, while the storage temperature is -40 to +60 ° C (95% RH). Laboratory instruments need to operate at a narrow control range of 5 to 50 °C. Unlike laboratory instruments that can be powered by AC, on-site portable meters are often more demanding on instrument power supplies, otherwise they can affect work efficiency. In addition, the power supply problem of the instrument is often an important cause of instrument failure or damage.
Therefore, users should consider and weigh the following factors: 1. The location of the built-in battery should be easy for the user to replace. 2. The minimum working time of the new battery or fully charged battery should reach 10 hours (one working day). However, the target life of the battery should be 40 to 50 hours (one week) or more to ensure the best working efficiency of technicians and instruments. 3, the more common the battery model, such as the general 9V or 1.5V No. 5 dry battery, because these universal batteries are very easy to find or buy on the spot. 4, ordinary dry batteries are better than rechargeable batteries (such as: lead-acid, nickel-cadmium batteries), because rechargeable batteries mostly have "memory" problems, packaging is not standard, not easy to buy, environmental issues. Previously, it was almost impossible to find portable test instruments that met all four of the above criteria.
Nowadays, the artistic optical power meter using the most modern CMOS circuit manufacturing technology can work for more than 100 hours with only the general No. 5 dry battery (available everywhere). Other laboratory models offer dual power supplies (AC and internal batteries) to increase their adaptability. Like a mobile phone, fiber optic test instruments are also available in a variety of form factors. Handheld watches of less than 1.5 kg generally do not have many illusions, only provide basic functions and performance; semi-portable instruments (greater than 1.5 kg) usually have more complex or extended functions; laboratory instruments are designed to control the laboratory / production Designed for occasions, with AC power supply. Comparing performance factors: This is the third step in the selection step, including a detailed analysis of each optical test equipment. Optical Power Meter Optical power measurement is essential for the manufacture, installation, operation, and maintenance of any fiber optic transmission system.
In the field of fiber optics, there is no optical power meter, and no engineering, laboratory, production workshop or telephone maintenance facilities can work. For example, an optical power meter can be used to measure the output power of a laser source and an LED source; it is used to confirm the loss estimate of a fiber link; the most important of these is the test optics (fiber, connector, splicer, attenuator) And so on) the key instrument of performance indicators.
For the specific application of the user, to choose the appropriate optical power meter, you should pay attention to the following points: 1. Select the optimal probe type and interface type 2, evaluate the calibration accuracy and manufacture the calibration procedure, and your fiber and connector requirements range match. 3. Make sure these models match your measurement range and display resolution. 4. dB function with direct insertion loss measurement.
Almost all of the performance of an optical power meter, the optical probe is the most carefully selected component. An optical probe is a solid-state photodiode that receives coupled light from a fiber optic network and converts it into an electrical signal. You can use a dedicated connector interface (one connection type only) to input to the probe, or use the universal interface UCI (with a screw connection) adapter. UCI accepts most industry standard connectors. Based on the calibration factor of the selected wavelength, the optical power meter circuit converts the probe output signal and displays the optical power reading in dBm (absolute dB equals 1 mW, 0 dBm = 1 mW) on the screen. Figure 1 is a block diagram of an optical power meter. The most important criterion for selecting an optical power meter is to match the type of optical probe to the expected operating wavelength range. The table below summarizes the basic choices. It is worth mentioning that InGaAs performs well in all three transmission windows when measured. InGaAs has flatter spectral characteristics in all three windows compared to 锗, and has higher measurement accuracy in the 1550 nm window. At the same time, it has excellent temperature stability and low noise characteristics. Optical power measurement is an integral part of the manufacture, installation, operation, and maintenance of any fiber optic transmission system. The next factor is closely related to the calibration accuracy. Is the power meter calibrated in a way that is consistent with your application? That is: the performance standards for fiber and connectors are consistent with your system requirements. What should be analyzed for the uncertainty of measurement values with different connection adapters? It is important to consider other potential error factors. Although NIST (American National Institute of Standards and Technology) has established US standards, the spectrum of light sources, optical probe types, and connectors from different manufacturers is uncertain. The third step is to determine the model of the optical power meter that meets your measurement range requirements. Expressed in dBm, the measurement range (range) is a comprehensive parameter that includes determining the minimum/maximum range of the input signal (so that the optical power meter guarantees all accuracy, linearity (BELLCORE is determined to be +0.8dB) and resolution (usually 0.1 dB or 0.01 dB) Whether the application requirements are met. The most important selection criterion for optical power meters is that the optical probe type matches the expected operating range. Fourth, most optical power meters have dB function (relative power) and can be read directly. Optical loss is very useful in measurement. Low-cost optical power meters usually do not provide this function. Without the dB function, the technician must write down the individual reference and measurement values and then calculate the difference. So the dB function gives the user Relative loss measurement, thus increasing productivity and reducing manual calculation errors. Now, users have reduced the basic characteristics and functions of optical power meters, but some users have to consider special needs - including: computer data collection, External interface, etc.
Stable Light Sources During the measurement of loss, a Stable Light Source (SLS) emits light of known power and wavelength into the optical system. An optical power meter/optical probe that calibrates a specific wavelength source (SLS) receives light from a fiber optic network and converts it into an electrical signal.
In order to ensure the accuracy of the loss measurement, the characteristics of the transmission equipment used for the simulation of the light source are as follows: 1. The same wavelength and the same type of light source (LED, laser). 2. Stability of output power and spectrum (time and temperature stability) during measurement. 3. Provide the same connection interface and use the same type of fiber. 4. The output power size satisfies the measurement of the system loss in the worst case. When the transmission system requires a separate stable light source, the optimal choice of the light source should simulate the characteristics and measurement requirements of the system optical transceiver.
The choice of source should consider the following: Laser tube (LD) Light from the LD, with a narrow wavelength band, almost monochromatic, ie single wavelength. Compared to LEDs, lasers passing through their spectral bands (less than 5 nm) are not continuous, and on the two sides of the center wavelength, several lower peak wavelengths are also emitted. Although the laser source provides more power than the LED light source, it is more expensive than the LED. Laser tubes are commonly used in long-haul single-mode systems where losses exceed 10 dB. Measurement of multimode fiber with a laser source should be avoided as much as possible.
Light Emitting Diodes (LEDs): LEDs have a broader spectrum than LD, typically ranging from 50 to 200 nm. In addition, the LED light is non-interfering light, and thus the output power is more stable. LED light sources are much cheaper than LD light sources, but they are underpowered for worst-case loss measurements.
LED light sources are typically used in short-distance networks and LANs of multimode fiber. LEDs can be used in laser source single-mode systems for accurate loss measurements, but only if they are required to deliver enough power. A light multimeter combines an optical power meter with a stable light source and is called an optical multimeter. The optical multimeter is used to measure the optical power loss of the fiber link. These meters can be two separate meters or a single integrated unit.
In summary, the two types of optical multimeters have the same measurement accuracy. The difference is usually cost and performance. Integrated optical multimeters are often mature, have a variety of performances, but are more expensive. From a technical point of view to evaluate various optical multimeter configurations, the basic optical power meter and stable light source standards still apply. Be careful to choose the right source type, operating wavelength, optical power meter probe, and dynamic range. The Optical Time Domain Reflectometer and Fault Locator OTDR is the most classic fiber optic instrumentation that provides the most information on the fiber associated with the test. The OTDR itself is a one-dimensional closed-loop optical radar that measures only one end of the fiber. A high-intensity, narrow light pulse is emitted into the fiber while a high-speed optical probe records the return signal. This instrument gives a visual explanation of the optical link. The continuation point, the position of the connector and the fault point, and the magnitude of the loss are reflected on the OTDR curve. The OTDR evaluation process has many similarities with the optical multimeter. In fact, the OTDR can be thought of as a very specialized test instrument combination: consisting of a stable high-speed pulse source and a high-speed optical probe.
The OTDR selection process can focus on the following attributes: 1. Confirm the working wavelength, fiber type and connector interface. 2. Expected connection loss and range of scans required. 3. Spatial resolution. Fault locators are mostly hand-held instruments for multimode and singlemode fiber systems. Using OTDR (Optical Time Domain Reflectometry) technology, it is used to locate the fault of the fiber, and the test distance is mostly within 20 km. The instrument directly displays the distance to the point of failure by number. Suitable for: Wide Area Network (WAN), 20 km range communication systems, fiber to the roadside (FTTC), single mode and multimode fiber optic cable installation and maintenance, and military systems. In single-mode and multimode fiber optic cable systems, fault locators are an excellent tool for locating faulty connectors and bad splices. The fault locator is easy to operate and can detect up to 7 multiple events with a single button operation.
Specifications of the spectrum analyzer (1) Input frequency range refers to the maximum frequency range in which the spectrum analyzer can work normally. The upper and lower limits of the range are indicated by HZ, which is determined by the frequency range of the scanning local oscillator. The frequency range of modern spectrum analyzers can usually range from low frequency bands to radio frequency segments, even microwave segments, such as 1 kHz to 4 GHz. The frequency here refers to the center frequency, which is the frequency at the center of the display spectrum width. (2) Resolution bandwidth refers to the minimum spectral line spacing between two adjacent components in the resolved spectrum, in units of HZ. It means that the spectrum analyzer is capable of distinguishing two equal-amplitude signals that are close together to each other at a specified low point. The line of the signal under test seen on the spectrum analyzer screen is actually a dynamic amplitude-frequency characteristic of a narrow-band filter (like a bell-shaped curve), so the resolution depends on the bandwidth of this amplitude-frequency. The 3dB bandwidth defining the amplitude-frequency characteristic of this narrow-band filter is the resolution bandwidth of the spectrum analyzer. (3) Sensitivity refers to the ability of the spectrum analyzer to display the minimum signal level given a resolution bandwidth, display mode and other influencing factors, expressed in units of dBm, dBu, dBv, V, etc. The sensitivity of the superheterodyne spectrum analyzer depends on the internal noise of the instrument. When measuring small signals, the signal lines are displayed above the noise spectrum. In order to easily see the signal line from the noise spectrum, the general signal level should be 10 dB higher than the internal noise level. In addition, the sensitivity is also related to the sweep speed, the sweep speed is fast, and the lower the peak of the dynamic amplitude frequency characteristic, the lower the sensitivity and the difference in amplitude. (4) Dynamic range means the maximum difference between two signals occurring at the input simultaneously with the specified accuracy. The upper limit of the dynamic range is loved by the constraints of nonlinear distortion. There are two ways to display the amplitude of the spectrum analyzer: linear logarithm. The advantage of the logarithmic display is that a large dynamic range is obtained over a limited range of effective screen heights. The dynamic range of the spectrum analyzer is generally above 60dB, and sometimes even above 100dB. (5) Frequency scan width (Span) There are different expressions such as spectrum width, span, frequency range, and spectrum span. Usually refers to the frequency range (spectral width) of the response signal that can be displayed in the leftmost and right vertical tick marks of the spectrum display. Automatically adjusted according to test needs, or artificial settings. The scan width indicates the range of frequencies displayed by the analyzer during a measurement (ie, a single frequency sweep) that can be less than or equal to the input frequency range. The spectrum width is usually divided into three modes. 1 Full Sweep The spectrum analyzer scans its effective frequency range at a time. 2 Sweep per division The spectrum analyzer scans only one specified frequency range at a time. The width of the spectrum represented by each cell can be changed. 3 zero sweep frequency width is zero, the spectrum analyzer does not sweep, becomes a tuned receiver. (6) Sweep Time (simply ST) is the time required to perform a full frequency range scan and complete the measurement, also called analysis time. Usually, the shorter the scan time, the better, but to ensure measurement accuracy, the scan time must be appropriate. The factors related to the scan time mainly include frequency sweep range, resolution bandwidth, and video filtering. Modern spectrum analyzers typically have multiple scan times to choose from, and the minimum scan time is determined by the circuit response time of the measurement channel. (7) Amplitude measurement accuracy There are absolute amplitude accuracy and relative amplitude accuracy, which are determined by many factors. Absolute amplitude accuracy is an indicator for the full-scale signal, which is affected by the input attenuation, IF gain, resolution bandwidth, scale fidelity, frequency response and the accuracy of the calibration signal itself. The relative amplitude accuracy is related to the measurement method. There are only two sources of error in the frequency response and calibration signal accuracy, and the measurement accuracy can be very high. The instrument is calibrated before leaving the factory, and various errors have been recorded and used to correct the measured data. The displayed amplitude accuracy has been improved.
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