Commonly used optical fiber test tables are: optical power meter, stable light source, optical multimeter, optical time domain reflectometer (OTDR) and optical fault locator.
Optical power meter: It is used to measure the relative loss of absolute optical power or optical power through a section of optical fiber. In optical fiber systems, measuring optical power is the most basic. Much like a multimeter in electronics, in optical fiber measurement, the optical power meter is a commonly used heavy-duty meter, and the optical fiber technician should have one. By measuring the absolute power of the transmitter or the optical network, an optical power meter can evaluate the performance of the optical device. Using an optical power meter in combination with a stable light source can measure connection loss, check continuity, and help evaluate the transmission quality of fiber links.
Stable light source: emits light of known power and wavelength to the light system. The stable light source is combined with the optical power meter to measure the optical loss of the fiber system. For off-the-shelf fiber optic systems, the transmitter of the system can also be used as a stable light source. If the terminal cannot work or there is no terminal, a separate stable light source is required. The wavelength of the stable light source should be as consistent as possible with the wavelength of the system end machine. 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 fiber body loss.
Optical multimeter: used to measure the optical power loss of optical fiber links. There are the following two types of optical multimeters:
1. It consists of an independent optical power meter and a stable light source.
2. An integrated test system that combines an optical power meter and a stable light source.
In a short-range local area network (LAN), where the end point distance is within walking or talking, technicians can successfully use an economical combination optical multimeter at either end, with a stable light source at one end and an optical power meter at the other end. For long-distance network systems, technicians should equip each end with a complete combination or integrated optical multimeter.
When choosing an instrument, temperature is perhaps the most stringent standard. On-site portable equipment should be between -18 ° C (without humidity control) to 50 ° C (95% humidity)
Optical time domain reflectometer (OTDR) and fault locator (Fault Locator): performance as a function of fiber loss and distance. With the help of OTDR, technicians can see the entire system outline, identify and measure the span, splice point and connector of the fiber. Among the instruments for diagnosing fiber faults, OTDR is the most classic and the most expensive instrument. Unlike the optical power meter and optical multimeter, the OTDR can measure fiber loss only through one end of the fiber. The OTDR trajectory gives the location and size of the system attenuation value, such as: the location of any connector, splice point, fiber profile, or fiber breakpoint and its loss. OTDR can be used in the following three aspects:
1. Understand the characteristics (length and attenuation) of the optical cable before laying.
2. Obtain the signal trace waveform of a section of optical fiber.
3. When the problem increases and the connection condition deteriorates, locate the serious fault point.
Fault Locator (Fault Locator) is a special version of OTDR. Fault Locator can automatically find the fault of optical fiber without the complicated operation steps of OTDR, and its price is only a fraction of OTDR.
When choosing a fiber-optic test instrument, you generally need to consider the following four factors: namely, determine your system parameters, working environment, comparative performance factors, and instrument maintenance to determine your system parameters. Operating wavelength (nm) The three main transmission windows are 850nm , 1300nm and 1550nm.
Type of light source (LED or laser): In short-distance applications, due to economic and practical reasons, most low-speed local area network LAN (<100Mbs) usually use LED light source. Most high-speed systems> 100Mbs use laser light sources to transmit signals over long distances.
Fiber type (single-mode / multimode) and core / coating diameter (um): Standard single-mode fiber (SM) is 9 / 125um, although some other special single-mode fibers should be carefully identified. Typical multimode fiber (MM) includes 50/125, 62.5 / 125, 100/140 and 200/230 um.
Types of connectors: Common connectors in China include: FC-PC, FC-APC, SC-PC, SC-APC, ST, etc. The latest connectors are: LC, MU, MT-RJ and other possible maximum link loss.
Loss estimation / system tolerance.
Clarify your working environment. For users / buyers, choose a field instrument, the temperature standard may be the most stringent. Generally, field measurement in the field must be used in a severe environment. It is recommended that the operating temperature of the field portable instrument should be from -18 ℃ ~ 50 ℃, and the storage and transportation temperature is -40 ~ +60 ℃ (95% RH). Laboratory instruments only need to work in a narrow control range of 5 ~ 50 ℃.
Unlike laboratory instruments that can be powered by AC, on-site portable instruments usually have stricter requirements for instrument power, otherwise they will affect work efficiency. In addition, the power supply 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 users to replace.
2. The minimum working time of new batteries or fully charged batteries must reach 10 hours (one working day). However, the target value of battery working life should be more than 40 ~ 50 hours (one week) to ensure the best working efficiency of technicians and instruments.
3. The more common the model of the battery, the better, such as the universal 9V or 1.5V No. 5 dry cell battery, etc., 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 and nickel-cadmium batteries), because most of the rechargeable batteries have "memory" problems, non-standard packaging, not easy to buy, and environmental protection issues.
Previously, it was almost impossible to find portable test instruments that met all four of the above standards. Now, the state-of-the-art optical power meter using the most modern CMOS circuit manufacturing technology can work for more than 100 hours using only ordinary No. 5 dry batteries (available everywhere). Other laboratory models provide dual power supplies (AC and internal battery) to increase their adaptability.
Like the mobile phone, the optical fiber test instrument also has many appearance packaging forms. Hand-held watches under 1.5 kg generally do not have many fakes, and only provide basic functions and performance; semi-portable instruments (greater than 1.5 kg) usually have more complicated or extended functions; laboratory instruments are designed for controlling the laboratory / production Designed for occasions with AC power supply.
Comparing performance elements: This is the third step of the selection step, including a detailed analysis of each optical test device.
Optical power meter
For the production, installation, operation and maintenance of any optical fiber transmission system, optical power measurement is essential. In the field of optical fiber, without an optical power meter, no engineering, laboratory, production workshop or telephone maintenance facility can work. For example: optical power meter can be used to measure the output power of laser light source and LED light source; used to confirm the loss estimation of fiber link; the most important thing is that it is the test optical components (optical fiber, connector, splice, attenuator Etc.) the key instruments for performance indicators.
For the user's specific application, to select the appropriate optical power meter, you should pay attention to the following points:
1. Choose the best probe type and interface type
2. Evaluate the calibration accuracy and manufacturing calibration procedures to match your fiber and connector requirements.
3. Make sure that these models are consistent with your measurement range and display resolution.
4. With dB function for direct insertion loss measurement.
In almost all the performance of the optical power meter, the optical probe is the most carefully selected component. The optical probe is a solid-state photodiode, which receives the coupled light from the optical fiber network and converts it into an electrical signal. You can use the dedicated connector interface (only one type of connection) to input to the probe, or use the universal interface UCI (use screw connection) adapter. UCI can accept most industry standard connectors. Based on the calibration factor of the selected wavelength, the optical power meter circuit converts the output signal of the probe and displays the optical power reading in dBm (absolute dB equals 1 mW, 0dBm = 1mW) on the screen. Figure 1 is a block diagram of an optical power meter.
The most important criterion for choosing an optical power meter is to match the type of optical probe to the expected operating wavelength range. The following table summarizes the basic options. It is worth mentioning that InGaAs has excellent performance in the three transmission windows during measurement. InGaAs has a flatter spectral characteristic in all three windows compared to germanium, and has higher measurement accuracy in the 1550nm window At the same time, it has excellent temperature stability and low noise characteristics.
Optical power measurement is an indispensable part in the manufacture, installation, operation and maintenance of any optical fiber transmission system.
The next factor is closely related to calibration accuracy. Is the power meter calibrated in a manner consistent with your application? That is, the performance standards of optical fibers and connectors are consistent with your system requirements. It should be analyzed what causes the measurement values ​​with different connection adapters to be uncertain? It is important to fully consider other potential error factors. Although NIST (National Institute of Standards and Technology) has established American standards, similar light sources, optical probe types, and connector spectrum from different manufacturers are uncertain.
The third step is to determine the type of optical power meter that meets the needs of your measurement range. Expressed in dBm, the measurement range (range) is a comprehensive parameter, including determining the minimum / maximum range of the input signal (so that the optical power meter can guarantee all accuracy, linearity (BELLCORE is determined to be + 0.8dB) and resolution (usually 0.1 dB or 0.01 dB) whether it meets the application requirements.
The most important selection criterion for an optical power meter is that the type of optical probe matches the expected operating range.
Fourth, most optical power meters have a dB function (relative power), and direct reading of optical loss is very practical in the 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. Therefore, the dB function allows users to measure relative losses, thereby increasing productivity and reducing manual calculation errors.
Now, users have reduced the choice of the basic features and functions of optical power meters, but some users have to consider special needs ---- including: computer acquisition data records, external interfaces, etc.
Stable light source
In measuring loss, a stable light source (SLS) emits light of known power and wavelength into the optical system. An optical power meter / optical probe calibrated to a specific wavelength light source (SLS) receives light from the fiber optic network and converts it into an electrical signal. In order to ensure the accuracy of loss measurement, as far as possible, the characteristics of the transmission equipment used for light source simulation:
1. The wavelength is the same, and the same light source type (LED, laser) is used.
2. During the measurement, the output power and the stability of the spectrum (time and temperature stability).
3. Provide the same connection interface, and use the same type of fiber.
4. The output power meets the measurement of the system loss in the worst case.
When the transmission system needs a separate stable light source, the optimal choice of light source should simulate the characteristics and measurement requirements of the system's optical transceiver. The selection of light source should consider the following aspects:
Laser tube (LD) The light emitted from the LD has a narrow wavelength bandwidth and is almost monochromatic light, that is, a single wavelength. Compared with LEDs, the laser light passing through its spectral band (less than 5nm) is not continuous, and on both sides of the center wavelength, it also emits several lower peak wavelengths. Compared with LED light sources, although laser light sources provide more power, they are more expensive than LEDs. Laser tubes are often used in long-distance single-mode systems with losses exceeding 10dB. Try to avoid using laser light source to measure multimode fiber.
Light-emitting diode (LED):
LED has a wider spectrum than LD, usually in the range of 50 ~ 200nm. In addition, LED light is non-interference light, so the output power is more stable. LED light sources are much cheaper than LD light sources, but the worst-case loss measurement appears to be insufficient power. LED light sources are typically used in short-distance networks and multimode fiber local area networks. LED can be used in laser light source single-mode system for accurate loss measurement, but the prerequisite is that it requires sufficient output power.
Optical Multimeter
The combination of an optical power meter and a stable light source is called an optical multimeter. Optical multimeters are used to measure the optical power loss of optical fiber links. These instruments can be two separate instruments or a single integrated unit. In short, the two types of optical multimeters have the same measurement accuracy. The difference is usually cost and performance. Integrated optical multimeters usually have mature functions and various performances but are relatively expensive.
From a technical perspective to evaluate various optical multimeter configurations, the basic optical power meter and stable light source standards are still applicable. Pay attention to choose the correct light source type, working wavelength, optical power meter probe and dynamic range.
Optical time domain reflectometer and fault locator
OTDR is the most classic fiber optic equipment, it provides the most information about the relevant fiber during the test. OTDR itself is a one-dimensional closed-loop optical radar, measuring only one end of the fiber. High-intensity, narrow optical pulses are emitted into the fiber, and the high-speed optical probe records the return signal. This instrument gives a visual explanation of the optical link. OTDR curve reflects the position of connection points, connectors and fault points and the size of loss.
The OTDR evaluation process has many similarities with the optical multimeter. In fact, OTDR can be regarded as a very professional test instrument combination: consisting of a stable high-speed pulse source and a high-speed optical probe. The selection process of OTDR can focus on the following attributes:
1. Confirm the working wavelength, fiber type and connector interface.
2. Expected connection loss and the range to be scanned.
3. Spatial resolution.
Fault locators are mostly hand-held instruments, suitable for multimode and single-mode fiber optic systems. OTDR (Optical Time Domain Reflectometer) technology is used to locate the point of fiber failure, and the test distance is mostly within 20 kilometers. The instrument directly displays the distance to the fault point in numbers. Applicable to: Wide area network (WAN), communication system in the range of 20 km, fiber to roadside (FTTC), installation and maintenance of single-mode and multi-mode fiber optic cables, and military systems. In single-mode and multi-mode optical cable systems, fault locators are an excellent tool to locate faulty connectors and bad splice points. The fault locator is easy to operate, requiring only a single key operation, and can detect up to 7 multiple events.
The technical index of the spectrum analyzer (1) The 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 represented by HZ, which is determined by the frequency range of the scanning local oscillator. The frequency range of modern spectrum analyzers can usually be from the low frequency band to the radio frequency section, and even the microwave section, such as 1KHz ~ 4GHz. The frequency here refers to the center frequency, that is, the frequency at the center of the width of the displayed spectrum.
(2) Resolution bandwidth refers to the minimum spectral line interval between two adjacent components in the resolution spectrum, in HZ. It indicates the ability of the spectrum analyzer to distinguish two equal-amplitude signals that are close to each other at a specified low point. The spectrum line of the measured signal seen on the spectrum analyzer screen is actually a dynamic amplitude-frequency characteristic graph of a narrow-band filter (similar to a bell-shaped curve). Therefore, 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 in a given resolution bandwidth, display mode and other influencing factors, expressed in dBm, dBu, dBv, V and other units. The sensitivity of the superheterodyne spectrum analyzer depends on the internal noise of the instrument. When measuring small signals, the signal spectrum is displayed above the noise spectrum. In order to easily see the signal spectrum from the noise spectrum, the general signal level should be 10dB higher than the internal noise level. In addition, the sensitivity is also related to the sweep frequency. The faster the sweep frequency, the lower the peak value of the dynamic amplitude-frequency characteristic, resulting in lower sensitivity and resulting amplitude difference.
(4) Dynamic range refers to the maximum difference between two signals that can appear at the input at the same time with the specified accuracy. The upper limit of the dynamic range loves the constraints of nonlinear distortion. There are two ways to display the amplitude of the spectrum analyzer: linear logarithm. The advantage of logarithmic display is that within a limited effective height range of the screen, a large dynamic range can be obtained. The dynamic range of the spectrum analyzer is generally above 60dB, and sometimes even above 100dB.
(5) Frequency sweep width (Span)
There are also different methods for analyzing spectral width, span, frequency range, and spectral span. Usually refers to the frequency range (spectrum width) of the response signal that can be displayed in the left and right vertical scale lines of the spectrum analyzer display screen. According to the test needs automatic adjustment, or artificial settings. The sweep width indicates the frequency range displayed by the spectrum analyzer during one measurement (that is, one frequency sweep), which can be less than or equal to the input frequency range. The spectrum width is usually divided into three modes.
â‘ Full-scan frequency spectrum analyzer scans its effective frequency range at one time.
â‘¡Sweep frequency spectrum analyzer only scans a specified frequency range at a time. The spectrum width expressed in each division can be changed.
â‘¢ The frequency width of the zero-sweep frequency is zero, and the spectrum analyzer does not sweep, and becomes a tuned receiver.
(6) Scan time (Sweep Time, abbreviated as ST)
That is, the time required to perform a full frequency scan and complete the measurement is also called the analysis time. Generally, the shorter the scan time, the better, but in order to ensure measurement accuracy, the scan time must be appropriate. The factors related to the scan time mainly include the frequency scan range, resolution bandwidth, and video filtering. Modern spectrum analyzers usually have multiple scan time options. The minimum scan time is determined by the circuit response time of the measurement channel.
(7) The amplitude measurement accuracy is divided into absolute amplitude accuracy and relative amplitude accuracy, which are determined by various factors. Absolute amplitude accuracy is an indicator for full-scale signals, which is comprehensively affected by input attenuation, IF gain, resolution bandwidth, scale fidelity, frequency response, and the accuracy of the calibration signal itself; relative amplitude accuracy is related to the measurement method, in an ideal situation There are only two sources of error, frequency response and calibration signal accuracy, and the measurement accuracy can be very high. The instrument has to be calibrated before leaving the factory. Various errors have been recorded and used to correct the measured data, and the displayed amplitude accuracy has been improved.
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