Optical fiber test tables include: optical power meter, stable light source, optical multimeter, optical time domain reflectometer (OTDR) and optical fault locator. Optical power meter: Used to measure absolute optical power or relative loss of optical power through a section of optical fiber. In fiber optic systems, measuring optical power is the most basic. Much like a multimeter in electronics, in optical fiber measurement, the optical power meter is a heavy-duty common meter, and optical fiber technicians should have one. By measuring the absolute power of the transmitter or 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 optical fiber links. Stable light source: emit light of known power and wavelength to the optical system. The stable light source is combined with the optical power meter to measure the optical loss of the optical fiber system. For ready-made fiber optic systems, usually 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 terminal. 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 connectors, splice points, and fiber body loss. Optical multimeter: used to measure the optical power loss of the optical fiber link.
There are the following two optical multimeters:
1. It is composed of an independent optical power meter and a stable light source.
2. An integrated test system integrating optical power meter and stable light source.
In a short-distance local area network (LAN), where the end point is within walking or talking, technicians can successfully use an economical combination optical multimeter at either end, a stable light source at one end and an optical power meter at the other end. For long-distance network systems, technicians should equip a complete combination or integrated optical multimeter at each end. When choosing a meter, temperature is perhaps the most stringent criterion. On-site 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): expressed as a function of fiber loss and distance. With the help of OTDR, technicians can see the outline of the entire system, identify and measure the span, splice point and connector of the optical fiber. Among the instruments for diagnosing optical fiber faults, OTDR is the most classic and also the most expensive instrument. Different from the two-end test of optical power meter and optical multimeter, OTDR can measure fiber loss through only one end of the fiber.
The OTDR trace line gives the position and size of the system attenuation value, such as: the position and loss of any connector, splice point, optical fiber abnormal shape, or optical fiber breakpoint.
OTDR can be used in the following three areas:
1. Understand the characteristics of the optical cable (length and attenuation) before laying.
2. Obtain the signal trace waveform of a section of optical fiber.
3. When the problem increases and the connection condition is deteriorating, locate the serious fault point.
The fault locator (Fault Locator) is a special version of the OTDR. The fault locator can automatically find the fault of the optical fiber without the complicated operation steps of the OTDR, and its price is only a fraction of the OTDR. When choosing an optical fiber test instrument, you generally need to consider the following four factors: that is, determine your system parameters, working environment, comparative performance elements, and instrument maintenance. Determine your system parameters. The working wavelength (nm). The three main transmission windows are 850nm. , 1300nm and 1550nm. Light source type (LED or laser): In short-distance applications, due to economic and practical reasons, most low-speed local area networks (100Mbs) use laser light sources to transmit signals over long distances. Fiber types (single-mode/multi-mode) 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 multi-mode fibers (MM) include 50/125, 62.5/125, 100/140 and 200/230 um. Connector types: Common domestic connectors 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 working environment. For users/purchasers, choose a field meter, the temperature standard may be the most stringent. Usually, field measurement must For use in severe environments, it is recommended that the working temperature of the on-site portable instrument should be -18℃~50℃, and the storage and transportation temperature should be -40~+60℃ (95%RH). The laboratory instruments only need to be in a narrow The control range is 5~50℃. Unlike laboratory instruments that can use AC power supply, portable instruments on site usually require more stringent power supply for the instrument, otherwise it will affect work efficiency. In addition, the power supply problem of the instrument often causes instrument failure or damage.
Therefore, users should consider and weigh the following factors:
1. The location of the built-in battery should be convenient for the user to replace.
2. The minimum working time for a new battery or a fully charged battery should reach 10 hours (one working day). However, the battery The target value of 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 battery type, the better, such as universal 9V or 1.5V AA dry battery, etc. Because these general-purpose batteries are very easy to find or buy locally.
4. Ordinary dry batteries are better than rechargeable batteries (such as lead-acid, nickel-cadmium batteries), because most rechargeable batteries have "memory" problems, non-standard packaging, and difficult Buying, environmental issues, etc.
In the past, it was almost impossible to find a portable test instrument that meets all the four standards mentioned above. Now, the artistic optical power meter using the most modern CMOS circuit manufacturing technology uses only general AA dry batteries ( Available everywhere), you can work for more than 100 hours. Other laboratory models provide dual power supplies (AC and internal battery) to increase their adaptability. Like mobile phones, fiber optic test instruments also have many appearance packaging forms. Less than A 1.5 kg handheld meter generally does not have many frills, and only provides basic functions and performance; semi-portable meters (greater than 1.5 kg) usually have more complex or extended functions; laboratory instruments are designed for control laboratories/production occasions Yes, with AC power supply. Comparison of performance elements: here is the third step of the selection procedure, including detailed analysis of each optical test equipment. For the manufacture, 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: an optical power meter can be used to measure the output power of laser light sources and LED light sources; it is used to confirm the loss estimation of optical fiber links; the most important of which is to test optical components (fibers, connectors, connectors, attenuators) Etc.) the key instrument of performance indicators.
To select a suitable optical power meter for the specific application of the user, you should pay attention to the following points:
1. Select the best probe type and interface type
2. Evaluate the calibration accuracy and manufacturing calibration procedures, which are consistent with your optical fiber and connector requirements. match.
3. Make sure that these models are consistent with your measurement range and display resolution.
4. With the dB function of 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 a dedicated connector interface (only one connection type) to input to the probe, or use a universal interface UCI (using 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 selecting an optical power meter is to match the type of optical probe with the expected operating wavelength range. The table below summarizes the basic options. It is worth mentioning that InGaAs has excellent performance in the three transmission windows during measurement. Compared with germanium, InGaAs has flatter spectrum characteristics in all three windows, 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 essential part of 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. Should analyze what causes the uncertainty of the measured value with different connection adapters? It is important to fully consider other potential error factors. Although NIST (National Institute of Standards and Technology) has established American standards, the spectrum of similar light sources, optical probe types, and connectors from different manufacturers is uncertain. The third step is to determine the optical power meter model that meets your measurement range requirements. 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 (determined as +0.8dB for BELLCORE) and resolution (usually 0.1 dB or 0.01 dB) to meet the application requirements. The most important selection criterion for optical power meters is that the type of optical probe matches the expected working range. Fourth, most optical power meters have the dB function (relative power), which can be read directly Optical loss is very practical in measurement. Low-cost optical power meters usually do not provide this function. Without the dB function, the technician must write down the separate reference value and the measured value, and then calculate the difference. So the dB function is for the user Relative loss measurement, thereby improving productivity and reducing manual calculation errors. Now, users have reduced the choice of basic features and functions of optical power meters, but some users have to consider special needs-including: computer data collection, recording, External interface, etc. Stabilized light source In the process of measuring loss, the stabilized light source (SLS) emits light of known power and wavelength into the optical system. The optical power meter/optical probe calibrated to the specific wavelength light source (SLS) is received from the optical fiber network Light converts it into electrical signals.
In order to ensure the accuracy of loss measurement, try to simulate the characteristics of the transmission equipment used in the light source as much as possible:
1. The wavelength is the same and the same light source type (LED, laser) is used.
2. During the measurement, the stability of the output power and spectrum (time and temperature stability).
3. Provide the same connection interface and use the same type of optical fiber.
4. The output power meets the worst-case system loss measurement. When the transmission system needs a separate stable light source, the optimal choice of the light source should simulate the characteristics and measurement requirements of the system's optical transceiver.
The following aspects should be considered when selecting a light source: 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. It also emits several lower peak wavelengths on both sides of the center wavelength. 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 where the loss exceeds 10dB. Avoid measuring multimode fibers with laser light sources as much as possible. 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. The LED light source is much cheaper than the LD light source, but the worst-case loss measurement appears to be underpowered. LED light sources are typically used in short-distance networks and multi-mode optical fiber local area network LANs. LED can be used for accurate loss measurement of laser light source single-mode system, but the prerequisite is that its output is required to have sufficient power. Optical multimeter The combination of an optical power meter and a stable light source is called an optical multimeter. Optical multimeter is used to measure the optical power loss of optical fiber link. These meters can be two separate meters 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 the price is relatively high. To evaluate various optical multimeter configurations from a technical point of view, the basic optical power meter and stable light source standards are still applicable. Pay attention to choosing the correct light source type, working wavelength, optical power meter probe and dynamic range. Optical time domain reflectometer and fault locator OTDR are the most classic optical fiber instrument equipment, which provide the most information about the relevant optical fiber during testing. The OTDR itself is a one-dimensional closed-loop optical radar, and only one end of the optical fiber is required for measurement. Launch high-intensity, narrow light pulses into the optical fiber, while the high-speed optical probe records the return signal. This instrument gives a visual explanation about the optical link. The OTDR curve reflects the location of the connection point, the connector and the fault point, and the size of the loss. The OTDR evaluation process has many similarities with optical multimeters. In fact, OTDR can be regarded as a very professional test instrument combination: it consists 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 to be scanned.
3. Spatial resolution.
Fault locators are mostly handheld instruments, suitable for multi-mode and single-mode fiber optic systems. Using OTDR (Optical Time Domain Reflectometer) technology, it is used to locate the point of fiber failure, and the test distance is mostly within 20 kilometers. The instrument directly digitally displays the distance to the fault point. Suitable for: wide area network (WAN), 20 km range of communication systems, fiber to the curb (FTTC), installation and maintenance of single-mode and multi-mode fiber optic cables, and military systems. In single-mode and multi-mode fiber optic cable systems, to locate faulty connectors and bad splices, fault locator is an excellent tool. The fault locator is easy to operate, with only a single key operation, and can detect up to 7 multiple events.
Technical indicators of 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 expressed in HZ, and are determined by the frequency range of the scanning local oscillator. The frequency range of modern spectrum analyzers usually ranges from low frequency bands to radio frequency bands, and even microwave bands, such as 1KHz to 4GHz. The frequency here refers to the center frequency, that is, the frequency at the center of the display spectrum width.
(2) Resolving power bandwidth refers to the minimum spectral line interval between two adjacent components in the resolving spectrum, and the unit is HZ. It represents the ability of the spectrum analyzer to distinguish two equal amplitude signals that are very close to each other at a specified low point. The spectrum line of the measured signal seen on the spectrum analyzer screen is actually the dynamic amplitude-frequency characteristic graph of a narrow-band filter (similar to a bell curve), so the resolution depends on the bandwidth of this amplitude-frequency generation. The 3dB bandwidth that defines the amplitude-frequency characteristics of this narrowband 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 under a given resolution bandwidth, display mode and other influencing factors, expressed in units such as dBm, dBu, dBv, and V. The sensitivity of a 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 frequency sweep speed. The faster the frequency sweep speed, the lower the peak value of the dynamic amplitude frequency characteristic, the lower the sensitivity and the amplitude difference.
(4) Dynamic range refers to the maximum difference between two signals simultaneously appearing at the input terminal that can be measured with a specified accuracy. The upper limit of the dynamic range is restricted to nonlinear distortion. There are two ways to display the amplitude of the spectrum analyzer: linear logarithm. The advantage of the logarithmic display is that within the limited effective height range of the screen, a larger dynamic range can be obtained. The dynamic range of the spectrum analyzer is generally above 60dB, and sometimes even reaches above 100dB.
(5) Frequency sweep width (Span) There are different names for analysis spectrum width, span, frequency range, and spectrum span. Usually refers to the frequency range (spectrum width) of the response signal that can be displayed within the leftmost and rightmost vertical scale lines on the display screen of the spectrum analyzer. It can be adjusted automatically according to test needs, or set manually. The sweep width indicates the frequency range displayed by the spectrum analyzer during a measurement (that is, a frequency sweep), which can be less than or equal to the input frequency range. The spectrum width is usually divided into three modes. ①Full frequency sweep The spectrum analyzer scans its effective frequency range at one time. ②Sweep frequency per grid The spectrum analyzer only scans a specified frequency range at a time. The width of the spectrum represented by each grid can be changed. ③Zero Sweep The frequency width is zero, the spectrum analyzer does not sweep, and becomes a tuned receiver.
(6) Sweep Time (Sweep Time, abbreviated as ST) is the time required to perform a full frequency range sweep and complete the measurement, also called analysis time. Generally, the shorter the scan time, the better, but in order to ensure the measurement accuracy, the scan time must be appropriate. The main factors related to the scan time are frequency scan range, resolution bandwidth, and video filtering. Modern spectrum analyzers usually 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, both of which are determined by many factors. The absolute amplitude accuracy is an indicator for the full-scale signal, and is affected by the comprehensive effects of input attenuation, intermediate frequency 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, in ideal conditions There are only two error sources, frequency response and calibration signal accuracy, and the measurement accuracy can reach very high. The instrument must be calibrated before leaving the factory. Various errors have been recorded separately and used to correct the measured data. The displayed amplitude accuracy has been improved.
