What are the characteristics and principles of fiber-optic transmission?

I. Background of Optical Communication

In today's world, optical fiber communication has become the primary means of communication.

Fiber-optic transmission systems offer the following significant advantages: large capacity and long transmission distance. The theoretical bandwidth of optical fiber can reach 20000 GHz, and the repeaterless transmission distance can reach 50-80 km. Made of glass, it is immune to electromagnetic interference and provides high transmission quality, making it suitable for strong electromagnetic environments such as power grids and substations. Optical fiber is lightweight, flexible, and easy to install. It saves precious metals and has strong corrosion resistance. The raw materials for manufacturing optical fiber are abundant, and with process improvements and scale expansion, costs have further decreased, resulting in lower overall system costs.

II. Introduction to Fiber-Optic Transmission Systems

Fiber-optic transmission is a communication method that uses light waves as the carrier frequency and optical fiber as the transmission medium. It offers advantages such as wide transmission bandwidth, low signal loss, strong anti-interference capability, and light weight. Optical fiber communication has developed rapidly over the past two decades.

1. Structure of Optical Fiber

Bare optical fiber generally consists of three layers: a central high-refractive-index glass core (core diameter typically 50 or 62.5 μm), an intermediate low-refractive-index silica glass cladding (diameter typically 125 μm), and an outermost reinforcing resin coating.

2. Classification of Optical Fiber

1. By transmission mode:

Single-mode fiber
Single-mode fiber transmits only the fundamental mode. Since modal dispersion is completely avoided, single-mode fiber has a very wide transmission bandwidth, making it suitable for large-capacity, long-distance transmission systems.

Multi-mode fiber
Multi-mode fiber supports multiple modes propagating in the fiber. Due to dispersion and phase differences, its transmission performance is inferior, with narrower bandwidth, smaller capacity, and shorter transmission distances.

2. By refractive index profile:

Multi-mode fiber can be divided into: step-index (abrupt) and graded-index (gradient, self-focusing) types.
Single-mode fiber is of the step-index type.

3. Common Optical Fiber Specifications

Single-mode: 8/125 μm, 9/125 μm, 10/125 μm
Multi-mode: 50/125 μm (European standard); 62.5/125 μm (US standard)
Industrial, medical, and low-speed networks: 100/140 μm, 200/230 μm
Plastic: 98/1000 μm for automotive control.

4. Fiber Attenuation

The main factors causing fiber attenuation include: intrinsic, bending, squeezing, impurities, inhomogeneity, and splicing.

Intrinsic: inherent loss of the fiber, including Rayleigh scattering, intrinsic absorption, etc.

Bending: when the fiber is bent, part of the light within the fiber is lost due to scattering, causing loss.

Squeezing: micro-bending caused when the fiber is squeezed, resulting in loss.

Impurities: impurities within the fiber absorb and scatter the light propagating in the fiber, causing loss.

Inhomogeneity: loss caused by non-uniform refractive index of the fiber material.

Splicing: loss generated during fiber splicing, such as axial misalignment (coaxiality requirement for single-mode fiber is less than 0.8 μm), end faces not perpendicular to the axis, uneven end faces, mismatched core diameters, and poor fusion quality.

5. Principles of Fiber-Optic Transmission Systems

An optical transmission system consists of three parts: a light source (optical transmitter), a transmission medium, and a detector (optical receiver).

By transmission signal type, systems can be divided into digital transmission systems and analog transmission systems.

In analog transmission systems, the input signal is converted into continuous variations in the amplitude (frequency or phase) of the transmission signal. Analog fiber-optic transmission systems perform analog modulation of light intensity, where the modulation power of the light source varies with the amplitude of the modulating signal. However, due to the severe nonlinearity of the light source, the signal-to-noise ratio, transmission distance, and transmission frequency are all quite limited.

Digital transmission systems convert the input signal into pulse signals represented by "1" and "0" as the transmission signal. At the receiving end, it is restored to the original information. In this way, the nonlinearity of the light source has minimal impact on the digital code stream. Additionally, digital communication can employ coding and error-correction methods and is easy to implement multiplexing. Therefore, digital transmission systems hold significant advantages and are widely applied in many fields.

III. Introduction to Digital Video Optical Transmission

Currently, optical terminals for video images, audio, data, Ethernet, and telephone are being widely and extensively applied in fields such as highways, transportation, electronic police, surveillance, security, industrial automation, electric power, customs, water conservancy, and banking. In the early stages, analog optical terminals using frequency modulation, amplitude modulation, and phase modulation were predominant, but digital replacing analog is the development trend of optical fiber communication technology. Since digital optical terminals offer high transmission signal quality and do not suffer from the severe intermodulation interference during multi-channel simultaneous transmission, susceptibility to environmental influences, poor transmission quality, and poor long-term operational stability of analog FM/PM/AM optical terminals, many large key projects have widely adopted digital optical terminals.