Field testing of fiber optic communication links

With the rapid development of fiber optic communication technology, broadband networks based on FTTH will surely become a new hot spot in fiber optic communication. Fiber optic is the best transmission medium so far. Compared with other access technologies (such as copper twisted pair and coaxial cable), the biggest advantage of fiber optic access technology is the large available bandwidth. The fiber optic access network also has the characteristics of good transmission quality, long transmission distance, strong anti-interference ability, high network reliability, and saving pipeline resources, which is the driving force for the development of FTTH.

The application of optical fiber communication technology is becoming more and more extensive. The variety of raw materials for manufacturing optical fiber is increasing, and the process technology for optical fiber manufacturing has also made breakthrough developments. New varieties and new structures of optical fiber continue to emerge, and product quality continues to improve. However, the performance of a complete optical fiber link depends not only on the quality of the optical fiber itself, but also on the quality of the connector, the construction process and the on-site environment. Therefore, it is very necessary to conduct on-site testing of optical fiber links.

1. Purpose of field testing of optical fiber links

Fiber link field testing is an essential part of installing and maintaining fiber networks, and is an important way to ensure that cables support network protocols. Its main purpose is to follow specific standards to detect the quality of fiber system connections, reduce failure factors, and find out the fiber failure point when a failure exists, so as to further find the cause of the failure.

2. Fiber optic link field test standards

At present, the field test standards for optical fiber links are divided into two categories: optical fiber system standards and application system standards. (1) Optical fiber system standards: Optical fiber system standards are field test standards for optical fiber links that are independent of applications. For different optical fiber systems, its test limit values ​​are not fixed. It is a variable standard based on cable length, adapters and joints. Currently, most optical fiber link field tests use this standard. The standards recognized worldwide mainly include: EIA/TIA-568-B standards in North America and ISO/IEC11801 standards of the International Organization for Standardization. (2) Optical fiber application system standards: Optical fiber application system standards are field test standards for optical fiber links based on specific applications for installing optical fiber. The test standards for each different optical fiber system are fixed. Commonly used optical fiber application systems include: 100BASE-FX, 1000BASE-SX, etc.

3.Fiber optic link field testing

What needs to be ensured for the fiber optic system is that the signal received at the receiving end should be large enough. Since the fiber optic uses light signals to transmit data, it does not generate a magnetic field and is not subject to electromagnetic interference and RF interference. It does not need to test parameters such as NEXT, so the test of the fiber optic system is different from the test of the copper wire system.

In the application of optical fiber, there are many types of optical fiber itself, but the basic test parameters of optical fiber and its system are roughly the same. In the field test of optical fiber link, the optical properties and transmission characteristics of optical fiber are mainly tested. The optical properties and transmission characteristics of optical fiber have a significant impact on the working wavelength, transmission rate, transmission capacity, transmission distance, signal quality, etc. of the optical fiber communication system. However, since the dispersion, cutoff wavelength, mode field diameter, baseband response, numerical aperture, effective area, microbend sensitivity and other characteristics of optical fiber are not harmfully affected by the installation method, they should be tested by the optical fiber manufacturer and do not need to be tested on site.

EIA/TIA-568-B specifies the single performance parameter required for field testing of fiber optic communication links as link loss (attenuation).

(1) Optical power test: The most basic test for fiber optic projects is the optical power test defined in the EIA's FOTP-95 standard, which determines the strength of the signal transmitted through the optical fiber and is also the basis for loss testing. During the test, the optical power meter is placed at one end of the optical fiber and the light source is placed at the other end of the optical fiber. (2) Optical connectivity test: The optical connectivity of the optical fiber system indicates the ability of the optical fiber system to transmit optical power. The optical connectivity of the optical fiber system is a basic requirement for the optical fiber system, so testing the optical connectivity of the optical fiber system is one of the basic tests. By connecting a light source at one end of the optical fiber system and an optical power meter at the other end, the optical connectivity of the optical fiber system can be determined by the detected output optical power. When the ratio of the optical power measured at the output end to the actual input optical power at the input end is less than a certain value, the link is considered to be optically disconnected. When testing optical connectivity, red laser or other visible light is usually injected into the optical fiber and the light output is monitored at the end of the optical fiber. If there is a break or other discontinuity in the optical fiber, the optical power at the output end of the optical fiber will drop or there will be no light output at all. (3) Optical power loss test: The term optical power loss, which is commonly used in the field of optical fiber, represents the attenuation of the optical fiber link. Attenuation is an important transmission parameter of the optical fiber link, and its unit is decibel (dB). It indicates the transmission loss (conduction characteristics) of the optical fiber link to the light energy, and plays a decisive role in the evaluation of the optical fiber quality and the determination of the relay distance of the optical fiber system. When the optical signal propagates in the optical fiber, the average optical power decreases exponentially along the length of the optical fiber. In an optical fiber network cable, the greater the attenuation between the transmitter and the receiver, the shorter the maximum possible transmission distance between the two. Attenuation has a negative impact on the transmission speed and transmission distance of all types of network cable systems, but because there is no crosstalk, EMI, RF I and other problems in optical fiber transmission, optical fiber transmission is particularly sensitive to attenuation. (4) Optical fiber link budget (OLB): The optical fiber link budget is the maximum signal loss allowed in the network and application. This value is calculated based on the actual network conditions and the loss specified by international standards. A complete optical fiber link includes optical fiber, connectors and fusion points, so when calculating the maximum loss limit of the optical fiber link, all these factors must be taken into account. The causes of optical energy loss in optical fiber communication links are composed of three parts: the loss of the optical fiber itself, the loss caused by the connector, and the loss caused by the fusion splice. However, due to the uncertainty of the length of the optical fiber, the number of connectors and fusion splices, the test standard of the optical fiber link is not fixed like the twisted pair cable. Therefore, the test standard of each optical fiber link must be calculated. [page]

4. Fiber optic link field test tool

(1) Light source: Currently, there are two main types of light sources: LED (light-emitting diode) light source and laser light source. Although LED light source is relatively cheap, it is widely used in short-distance local area networks due to its power and scattering performance defects. Traditional laser light sources are used in long-distance local area network backbones, but laser light source equipment is expensive. In order to solve the defects of these two light sources, a new type of light source has been developed in the past two years, which is the VCSEL light source. VCSEL refers to vertical cavity surface emitting laser, which is a semiconductor type of micro laser diode. It is different from the traditional edge emitting technology currently used in communication equipment. It emits light vertically on the chip. Compared with traditional laser light source devices, VCSEL laser light source has many advantages: high manufacturing efficiency on the chip; can be manufactured together with other components using standard manufacturing methods (no pre-manufacturing is required); packaging and testing are completed on the chip; high transmission speed and low energy consumption, and is less affected by temperature. In short, VCSEL is a new type of laser light source with good performance and low manufacturing cost. Due to these characteristics of VCSEL light source, it has been increasingly widely used, especially in Gigabit networks. Currently, many network interconnection devices, such as switches and routers, can provide ports for VCSEL light sources, thereby reducing the prices of routers and switches. The most widely used VCSEL multimode laser light source is the 850nm VCSEL multimode laser light source. (2) Optical power meter: An optical power meter is a device that measures the signal strength transmitted on an optical fiber. It is used to measure the absolute optical power or the relative optical power loss through a section of optical fiber. In an optical fiber system, measuring optical power is the most basic. The principle of an optical power meter is very similar to a multimeter in electronics, except that a multimeter measures electrons, while an optical power meter measures light. By measuring the absolute power of a transmitter or optical network, an optical power meter can evaluate the performance of an optical terminal device. An optical power meter combined with a stable light source to form an optical loss tester can measure connection loss, verify continuity, and help evaluate the transmission quality of an optical fiber link. (3) Optical time domain reflectometer: An OTDR is made based on the principle of light backscattering. It uses the backscattered light generated when light propagates in an optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, joint loss, locate optical fiber fault points, and understand the loss distribution along the length of the optical fiber. In a sense, the role of an optical time domain reflectometer (OTDR) is similar to that of a time domain reflectometer (TDR) used in cable testing, except that while TDR measures signal reflections caused by impedance, OTDR measures signal reflections caused by backscattering of photons. Backscatter is a phenomenon that affects all optical fibers and is caused by the reflection of photons in the fiber.