Chapter 1 Fiber Optic Fundamentals
1.1 Classification of Fiber Optics
The same optical fiber may have different names depending on the classification method and rules.
1.1.1 By Application
Based on application, optical fibers are mainly divided into two categories: communication optical fibers and non-communication optical fibers.
For example: optical fibers inserted into various communication cables are communication optical fibers; those inserted into various light guiding and image transmission cables such as those used in endoscopes are non-communication optical fibers. Some optical fibers can also be used to make optical devices for both communication and sensors, and for non-communication applications.
1.1.2 By Material
Based on the main material, optical fibers can be mainly divided into three categories: silica glass optical fibers, multi-component glass optical fibers, and plastic optical fibers.
Most optical fibers used for communication are composed of a pure silica glass (silicon dioxide) cladding and a doped silica glass core; optical fibers used in endoscopes are mainly made of multi-component glass; and plastic optical fibers are mainly used for short-distance communication and decoration.
1.1.3 According to the Refractive Index Profile of Optical Fibers
Based on the refractive index profile structure of optical fibers, they are mainly divided into three categories: step-index, graded-index (focusing), and complex (triangular, multi-clad) fibers.
1.1.4 According to Transmission Modes
Based on the number of modes transmitted within the optical fiber, optical fibers can be divided into two main categories: multimode and single-mode fibers.
When multiple modes (the movement patterns of the fiber) are transmitted simultaneously within a specified operating wavelength range, this type of fiber is called “multimode fiber.” “Single-mode fiber” can only transmit the fundamental mode (lowest order mode) within a specified operating wavelength range.
1.1.5 According to ITU-T Standards
The International Telecommunication Union (ITU-T) has established unified optical fiber standards (G standards) for communication applications. According to ITU-T recommendations on optical fibers, communication optical fibers are classified into G.651, G.652, G.653, G.654, G.655, and G.656 fiber classes, with some classes further subdivided into several subclasses.
1.1.6 According to IEC Standards
The International Electrotechnical Commission (IEC) has also developed corresponding standards for optical fibers used in communication. According to IEC recommendations on optical fibers, communication optical fibers are classified into categories A1, B1, B2, B3, and B4, with some categories further subdivided into several subcategories.
1.1.7 According to GB/T Standards
National standards (GB/T), etc., adopt IEC standards.
1.2 Optical Fiber Structure and Transmission Principle
1.2.1 Optical Fiber Structure
From the inside out, an optical fiber consists of a core, cladding, primary coating, and coloring layer, referred to as the external structure of the optical fiber, as shown in Figure 1.2.1. The difference between the external structure of multimode and single-mode optical fibers lies only in the size of the core. The internal structure of the optical fiber is the refractive index profile of the core.
1.2.2 Transmission Principle and Transmission Mode of Optical Fibers
(1) Transmission Principle
Figure 1.2.2 simply describes the transmission principle of optical fibers. Figure 1.2.2(a) shows a multimode step-index fiber with a flat core refractive index distribution. Several modes of light propagating within the core undergo multiple reflections at the core-cladding interface based on total internal reflection. The “optical path difference” caused by the different number of reflections of each mode results in a “time delay difference” upon reaching the endpoint, widening the input light pulse and reducing its bandwidth.
Figure 1.2.2(b) shows a multimode graded-index fiber with a core refractive index distribution resembling a parabola, approximating the self-focusing form of a lens. Several modes of light propagating within the core move sinusoidally with a generally consistent focal point, altering the time delay difference of the multimode step-index fiber and thus increasing its bandwidth. However, the collisions caused by the simultaneous propagation of multiple modes result in so-called “intermodal dispersion,” resulting in a significant widening of the input light pulse upon reaching the endpoint.
Figure 1.2.2(c) shows a standard single-mode fiber, although its core refractive index distribution remains flat. However, since only one mode propagates, it avoids the loss and dispersion caused by the coupling between multiple modes during propagation. The loss and broadening are minimized when the input light pulse reaches its destination. Compared to multimode fiber, it has lower transmission loss per unit length and higher bandwidth, but its smaller core size increases the difficulty of coupling and interfacing with the light source.
(2) Fiber Modes
Light waves are electromagnetic waves, and the electromagnetic field distribution in an optical fiber obeys Maxwell’s equations. Solving the wave equations yields the Fiber Mode characteristics, field structure, transmission constant, and cutoff conditions.
In a planar dielectric waveguide, guided waves propagating as light rays travel in a zigzag pattern along the waveguide. All these rays pass through the plane of the waveguide, and these waves are either transverse electric waves (TE) or mode magnetic waves (TM).
Figure 1.2.3(a) shows the electric field distribution of the base film (LP01 mode) in the step-index fiber; Figure 1.2.3(b) shows the electric field distribution perpendicular to the fiber cross-section; and Figure 1.2.3(c) shows the intensity distribution of the LP01, LP11, and LP21 modes perpendicular to the fiber cross-section.
(3) Fiber Mode Number
In a step-index fiber, the transmission mode number is determined by the V parameter (or normalized frequency). The V parameter is related to the structural parameters of the fiber and is expressed by Equation 1.2.1.
(Equation 1.2.1)
n1: Operating wavelength;
n2: Core radius;
n1: Core refractive index;
n2: Cladding refractive index;
n3: Relative refractive index difference between the core and cladding;
n4: Numerical aperture of the fiber.
When V = 2.405, only the base film LP01 transmits through the core. The fiber operates in Single Mode, thus the cutoff wavelength of the fiber is obtained from Equation 2.2.2. 1/2
(Equation 1.2.2)According to Equation 1.2.1, the V value of a standard single-mode fiber with a cutoff wavelength of 1260 nm is 2.31 at an operating wavelength of 1310 nm; and 1.96 at an operating wavelength of 1550 nm, indicating a relatively weak waveguide.
When V = 2.405, the number of transmitted modes increases rapidly. The number of modes N that can be supported in a step-index multimode fiber is represented by Equation 2.2.3.
(Equation 1.2.3)
Figure 1.2.4 shows the relationship between the normalized propagation constant and the normalized frequency for several LP modes. As the V value increases, the number of transmission modes of the fiber also increases.
1.3 Main Technical Characteristics of Optical Fiber
In just over thirty years, from 1970 to the present, optical fiber communication technology has achieved astonishing development. Using extremely wide-bandwidth light waves as the carrier of information, optical fiber communication has advantages such as large communication capacity, long relay distance, good confidentiality, and strong adaptability. However, currently, the actual application of optical fiber communication only accounts for about 2% of its potential capacity, leaving enormous potential for development and utilization. Optical fiber communication is developing towards higher levels and stages. To better understand optical fiber communication technology, we will begin by examining several characteristics of optical fibers.
The characteristics of optical fibers can be divided into three main categories: physical characteristics, transmission characteristics, and environmental characteristics.
The physical characteristics of optical fibers are closely related to coupling loss. (These mainly include refractive index distribution, geometric dimensions, mode field diameter, cutoff wavelength, concentricity, etc.)
The transmission characteristics of optical fibers are related to interruption distance, transmission rate, and transmission capacity. (These mainly include attenuation coefficient, attenuation discontinuity, dispersion, etc.)
The environmental characteristics of optical fibers are related to long-term stability and service life. (These mainly include screening strength, fatigue factor, attenuation temperature characteristics, and time delay temperature characteristics, etc.)
1.3.1 Attenuation Coefficient
The attenuation coefficient is one of the most important characteristic parameters of multimode and single-mode optical fibers, largely determining the relay distance of multimode and single-mode optical fiber communication.
The attenuation factor is defined as the decrease in optical signal power per kilometer of optical fiber. Its expression is:
a = 10 lg Pi/Po (unit: dB/km)
Where: Pi is the input optical power (W watts)
Po is the output optical power (W watts)
If the attenuation factor of an optical fiber is a = 3 dB/km, it means that after transmitting one kilometer of fiber, Pi/Po = 10 + 0.3 = 2, the optical signal power is reduced by half. The total attenuation of an optical fiber of length L kilometers is A = aL.
For single-mode fiber, the attenuation is 0.18 dB/km. For an optical signal, if the output power after EDFA amplification is +5dBm, and the receiver sensitivity is -28dBm, then the amplification gain is 33dB. Dividing this by the attenuation factor, the divisor distance is 33/0.18 = 183 km. Considering aging margins, it can transmit over 120 km.
Many factors contribute to optical fiber attenuation, primarily: absorption attenuation (including impurity absorption and intrinsic absorption); scattering attenuation (including linear scattering, nonlinear scattering, and structural incompleteness scattering); and other attenuation factors, including microbending attenuation.
The most significant attenuation is caused by impurity absorption. Impurities in optical fiber materials, such as hydroxide ions and transition metal ions, have extremely strong light absorption capabilities, making them crucial factors in optical signal attenuation. Therefore, to obtain low-attenuation optical fiber, the raw material silica used in fiber manufacturing must undergo very rigorous chemical purification to reduce its impurity content to below a few ppb.
Scattering loss is typically caused by microscopic variations in the density of optical fiber materials and uneven concentrations of components such as SiO2, GeO2, and P2O5. This results in localized regions with uneven refractive index distribution within the fiber, causing light scattering and diverting some optical power to the outside of the fiber, leading to loss. Alternatively, defects, residual bubbles, and gas traces may appear at the core-cladding interface during fiber manufacturing. These structural defects have geometric dimensions much larger than the light wave, causing wavelength-independent scattering loss and shifting the overAll Fiber loss spectrum upwards. However, this type of scattering loss is much smaller than the former.
Considering all these factors, the attenuation constants of single-mode fiber in the 1310nm and 1550nm wavelength regions are generally 0.3–0.4 dB/km (1310nm) and 0.17–0.25 dB/km (1550nm), respectively. ITU-T G.652 recommends that the attenuation constants of optical fibers at 1310 nm and 1550 nm should be less than 0.5 dB/km and 0.4 dB/km, respectively.
In practical engineering, long-distance transmission of optical signals requires sufficient signal power to offset fiber attenuation. The attenuation coefficient of G.652 fiber in the 1550 nm window is generally around 0.25 dB/km. Considering factors such as optical connectors and fiber redundancy, the overAll Fiber attenuation coefficient is generally less than 0.275 dB/km.
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