single-mode fiber
Optical fibers that can only transmit one mode are called single-mode fibers. Single-mode fibers can only transmit the fundamental mode (lowest order mode), and there is no inter-mode delay difference. Therefore, they have a much larger bandwidth than multimode fibers, which is crucial for high-speed data transmission. The bandwidth of single-mode fibers is typically in the tens of GHz·km range or higher.
The structure of a step-index single-mode fiber is shown in Figure 2-13. Single-mode fiber has a small core diameter to ensure single-mode transmission, but its cladding diameter is more than ten times larger than the core diameter to avoid optical loss.The functions of each part of the single-mode fiber structure are similar to those of multimode fiber. The difference is that the core diameter is expressed using the wavelength-dependent mode field diameter *w*. This type of fiber is widely used in modern fiber optic communication engineering.
Figure 2-13 shows the structure of a step-index single-mode optical fiber.
The dimensional parameters of single-mode optical fibers of types B1.1 and B4 are shown in Table 2-1 and Table 2-2.
| Name | Parameter |
|---|---|
| 1310 nm mode-field diameter | [(8.6 ~ 9.5) ± 0.7] µm |
| Cladding diameter | (125 ± 1) µm |
| 1310 nm fiber concentricity error | ≤ 0.8 µm |
| Cladding non-circularity | ≤ 2% |
| Coating diameter (primary color) | (245 ± 10) µm |
| Coating diameter (colored) | (250 ± 15) µm |
| Cladding / coating concentricity error | ≤ 12.5 µm |
Table 2-1 Size Parameters of B1.1 Single-Mode Fiber
| Name | Parameter |
|---|---|
| 1550 nm mode-field diameter | [(8.0 ~ 11.0) ± 0.7] µm |
| Cladding diameter | (125 ± 1) µm |
| 1550 nm fiber concentricity error | ≤ 0.8 µm |
| Cladding non-circularity | ≤ 2% |
| Coating diameter (primary color) | (245 ± 10) µm |
| Coating diameter (colored) | (250 ± 15) µm |
| Cladding / coating concentricity error | ≤ 12.5 µm |
Table 2-2 Size Parameters of B4 Single-Mode Fiber
Standards and Applications of Single-Mode Fiber
Single-mode fiber, with its advantages of low attenuation, wide bandwidth, large capacity, low cost, and ease of expansion, is an ideal optical communication transmission medium and has been widely used worldwide. Currently, with the development of the information society, researchers have developed fiber amplifiers, time-division multiplexing (TDM), wavelength-division multiplexing (WDM), and frequency-division multiplexing (FDM) technologies, further improving the transmission distance, communication capacity, and transmission rate of single-mode fiber.
It is worth noting that while fiber amplifiers extend transmission distances and multiplexing technologies bring high-speed, high-capacity signal transmission, they also increase the impact of dispersion and nonlinear effects on system transmission quality. Therefore, several types of optical fibers have been specifically researched and developed: dispersion-shifted fiber, non-zero dispersion-shifted fiber, dispersion-flattened fiber, and dispersion-compensated fiber, each with its unique advantages in addressing dispersion and nonlinear effects.
Single-mode optical fibers can be classified into five types based on whether they exhibit zero-dispersion wavelength and cutoff wavelength shift. The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) issued recommendations for four of these types in October 2000: G.652, G.653, G.654, and G.655 fibers. The correspondence between the IEC (International Electrotechnical Commission) and ITU-T naming conventions for single-mode optical fibers is shown in Figure 2-14.
| Chinese Name | ITU-T | IEC |
|---|---|---|
| Single-mode fiber | ||
| Non-dispersion shifted single-mode fiber | G.652 A/B/C | B1.1 and B1.3 |
| Dispersion shifted single-mode fiber | G.653 | B2 |
| Cut-off wavelength shifted single-mode fiber | G.654 | B1.2 |
| Non-zero dispersion shifted single-mode fiber | G.655 A/B | B4 |
| Wideband non-zero dispersion shifted single-mode fiber | - | - |
| Bend-insensitive single-mode fiber | - | - |
Figure 2-14 shows the correspondence between the various single-mode optical fibers named by IEC and ITU-T.
G.652 – Nondispersion-shifted single-mode fiber
The standard text further subdivides G.652 fiber into G.652A, G.652B, and G.652C based on its attenuation, dispersion, polarization mode dispersion, operating wavelength range, and application in SDH systems at different transmission rates. In essence, G.652 fiber can be divided into two types: conventional single-mode fiber (G.652A and G.652B) and low-water-peak single-mode fiber (G.652C).
(1) Conventional Single-Mode Fiber: Conventional single-mode fiber began commercial use in 1983. Its performance characteristics are: zero dispersion at a wavelength of 1310 nm; minimum attenuation coefficient near a wavelength of 1550 nm, approximately 0.22 dB/km, but maximum dispersion coefficient of 17 ps/(nm·km) near 1550 nm; the working wavelength of this fiber can be selected in both the 1310 nm and 1550 nm wavelength regions, with its optimal working wavelength in the 1310 nm region. This fiber is often called "conventional" or "standard" single-mode fiber, and it is currently the most widely used fiber. To date, its cumulative deployment worldwide has reached 7 x 10⁻⁶ km.
(2) Low-water-peak single-mode fiber: To address the challenges faced by metropolitan area networks (MANs) such as complex and variable service environments, a large number of directly supported users, and short transmission distances (typically only 50-80 km), the solution adopted is to use high-density wavelength division multiplexing (HDWDM) technology with tens to hundreds of multiplexed wavelengths. This involves allocating services of different rates and characteristics to different wavelengths and performing routing and demultiplexing on the optical path. Therefore, it is necessary to develop low-water-peak single-mode fiber (ITU-T G.652C) with a wider operating wavelength range to meet the needs of HDWDM MAN development.
G.653 Dispersion-Shifted Single-Mode Fiber
Dispersion-shifted single-mode fiber (ITU-T G.653) was commercialized in 1985. Dispersion-shifted single-mode fiber achieves this by altering the fiber's structural parameters and refractive index distribution to increase waveguide dispersion, thereby shifting the minimum zero-dispersion point from 1310 nm to 1550 nm. This results in the lowest attenuation wavelength at 1550 nm being consistent with the zero-dispersion wavelength, and it operates within the 1530–1565 nm operating wavelength range of ballasted fiber amplifiers. This type of fiber is ideally suited for long-distance, single-channel, high-speed optical amplification systems; for example, a 20 Gbit/s system can be directly implemented on this fiber without any dispersion compensation measures.
The most promising application for dispersion-shifted single-mode fiber is in submarine fiber optic communication systems for long-distance single-channel signal transmission. In addition, a certain number of dispersion-shifted single-mode fibers have also been deployed in terrestrial long-distance wired communication networks.
G. 654-Cutoff Wavelength Shifted Single-Mode Fiber
1550nm cutoff wavelength-shifted single-mode fiber (ITU-T G.654) is a non-dispersion-shifted fiber with a zero-dispersion wavelength around 1310nm. The cutoff wavelength is shifted to a longer wavelength range, resulting in minimal attenuation in the 1550nm wavelength region. Its optimal operating wavelength range is 1500–1600nm.
The method to obtain low-attenuation fiber involves using a pure silica glass core and a fluorine-doped recessed cladding; the long cutoff wavelength reduces the fiber's sensitivity to bending-induced losses.
Because this type of fiber is particularly difficult to manufacture and very expensive, it is rarely used. It is mainly used in repeaterless submarine fiber optic communication systems with long transmission distances where active devices cannot be inserted.
G.655-Non-zero dispersion-shifted single-mode fiber
Non-zero dispersion-shifted single-mode fiber (ITU-T G.655) is a new type of optical fiber designed and manufactured in 1994 by Lucent Technologies and Corning Incorporated specifically for next-generation wavelength division multiplexing transmission systems with fiber amplifiers. This fiber is based on dispersion-shifted single-mode fiber, and by changing the refractive profile structure, the dispersion at a wavelength of 1550 nm is made non-zero; hence the name non-zero dispersion-shifted single-mode fiber.
Dispersion-flat single-mode fiber
In 1988, dispersion-flat single-mode fiber was commercialized. This fiber exhibits low dispersion in the 1310–1550 nm wavelength range and possesses two zero-dispersion wavelengths, namely 1310 nm and 1550 nm. This fiber can be used with lasers having a wider center wavelength and standard lasers operating at 1310 nm and 1550 nm for high-speed transmission with LEDs. However, dispersion-flat single-mode fiber has a complex refractive index profile, making it difficult to manufacture, and its high attenuation limits its practical application. The performance and applications of dispersion-flat single-mode fiber are shown in Table 2-8.
| Performance | Mode Field Diameter (μm) | Cladding Diameter (nm) | Zero Dispersion Wavelength (nm) | Operating Wavelength (nm) | Maximum Macrobend Loss (dB·km⁻¹) | Maximum Polarization Mode Dispersion (ps·√km)⁻¹ |
|---|---|---|---|---|---|---|
| Requirement | 8 (1310 nm) 11 (1550 nm) | ≤ 125 | 1310 and 1550 | 1310 to 1550 | ≤ 0.25 (1310 nm) ≤ 0.30 (1550 nm) | 0 (1310 nm) 0 (1550 nm) |
Application Scenario: This type of fiber is particularly suitable for environments with low internal bending dispersion in the 1310–1550 nm operating wavelength range.
Dispersion-compensated single-mode fiber
With the application of fiber optic amplifiers, attenuation is no longer a significant limitation on the distance of fiber optic communication systems. However, dispersion severely hinders the upgrade and expansion of conventional single-mode fiber operating wavelengths from 1310nm to 1550nm. To address this practical problem, dispersion-compensated single-mode fiber has been developed.
Dispersion-compensated single-mode fiber is a type of single-mode fiber with significant negative dispersion at a wavelength of 1550nm. Current experimental results show that the dispersion coefficient of dispersion-compensated single-mode fiber ranges from 50 to -548 ps/(nm·km), and the attenuation is typically 0.5 to 1.0 dB/km.
When the operating wavelength of a conventional single-mode fiber system is upgraded from 1310nm to 1550nm, its total dispersion is positive. By adding a section of negative dispersion fiber to the system, the positive dispersion at 1550nm in tens of kilometers of conventional single-mode fiber can be canceled, thus upgrading the operating wavelength of the installed conventional single-mode fiber from 1310nm to 1550nm, thereby achieving high-speed, long-distance, and high-capacity transmission. The attenuation introduced by the addition of dispersion-compensating single-mode fiber can be completely compensated by the fiber amplifier.
Multimode fiber
As the name suggests, multimode fiber is an optical fiber that allows multiple modes to be transmitted within it, or in other words, multiple separate transmission modes are allowed to exist in multimode fiber.
Standards and Applications of Multimode Fiber
Graded-mode multimode fiber
G.651 fiber is a graded-index multimode fiber primarily used for analog or digital signal transmission in the 850nm and 1310nm wavelength regions. Its core diameter is 50µm and its cladding diameter is 125µm. In the 850nm wavelength region, the attenuation coefficient is less than 4dB/km and the dispersion coefficient is less than 120ps/(nm·km); in the 1310nm wavelength region, the attenuation coefficient is less than 2dB/km and the dispersion coefficient is less than 6ps/(nm·km).
Gradient-mode multimode fiber
The structure of graded-index multimode fiber is shown in Figure 2-18. This type of fiber includes Ala, Alb, Alc, and Ald types. They can be fabricated using multicomponent glass or doped silica glass. To reduce fiber attenuation, the materials used to fabricate graded-index multimode fibers have a much higher purity than those used in most step-index multimode fibers. It is precisely because of the graded refractive index distribution and lower attenuation that graded-index multimode fibers outperform step-index multimode fibers.
Step-index multimode fiber
The structure of step-index multimode fiber is shown in Figure 2-19. This type of fiber comes in three categories (A2, A3, and A4) and nine varieties. Multicomponent glass, doped glass, or plastic can be used to fabricate the core and cladding. Due to their large core size and large numerical aperture, these multimode fibers can be coupled more effectively to incoherent light sources, such as light-emitting diodes (LEDs). Link connections can be made using inexpensive injection-molded connectors, thus reducing the overall network construction cost. Therefore, step-index multimode fiber, especially A4 plastic fiber, plays an important role in short-distance communication.
