Basic Structure of Fiber Optic Communication Systems The basic composition of a fiber optic communication system is shown in Figure 1-1, mainly including three major parts: transmission, reception, and basic fiber optic transmission system:
(1) Transmitting section: The information source converts user information into original electrical signals (baseband signals); the electrical transmitter converts baseband signals into modulated signals suitable for channel transmission (such as FM, PFM, PWM); the optical transmitter modulates and converts electrical signals into optical signals.
(2) Receiving section: The optical receiver converts optical signals transmitted through the fiber into electrical signals; the electrical receiver restores electrical signals to baseband signals; the information sink recovers user information. Note: The electrical signal segments before the optical transmitter and after the optical receiver use the same technology/equipment as cable communication, only replacing cable transmission with "optical transmitter + optical fiber line + optical receiver."
(3) Basic fiber optic transmission system is divided into three parts: optical transmitter, optical fiber line, and optical receiver:
Optical transmitter: The core is the light source (such as LED, semiconductor laser diode, DFB laser, etc.), which needs to meet requirements such as high output optical power, high modulation frequency, narrow spectral line, and stable wavelength; its function is to convert electrical signals into optical signals and couple them into optical fiber.
Optical fiber line: Composed of optical fiber, splices, and connectors (actually using optical cables); its function is to transmit optical signals with low distortion and low attenuation. Optical fiber is cylindrical (core refractive index (n_1) > cladding refractive index (n_2)), utilizing total internal reflection to transmit light; it has 3 low-loss windows: (0.85\mu m) (short wavelength), (1.31\mu m) (long wavelength), (1.55\mu m) (long wavelength); main characteristics are loss (unit: dB/km) and dispersion (unit: (ps/(km·nm)), affecting transmission bandwidth).
Optical receiver: The core is the photodetector (such as PIN photodiode, APD avalanche photodiode), which needs to meet requirements of high responsivity, low noise, and high speed; the most important parameter is sensitivity (reflecting the ability to receive weak optical signals, an important indicator of system quality); its function is to convert optical signals into electrical signals and recover the original signal.
Classification of Fiber Optic Communication Systems Common classification methods are as follows:
(1) Classification by transmission signal type: Divided into fiber optic analog communication systems and fiber optic digital communication systems:
Advantages of fiber optic digital communication systems:
Strong anti-interference capability and good transmission quality (noise only produces bit errors when exceeding threshold);
Regenerative repeating, long transmission distance (eliminating noise accumulation);
Accommodates multiple services with great flexibility (easy to implement integrated services);
Easy to implement high-intensity secure communication (plaintext and key modulo-2 addition);
Uses digital circuits, easy to integrate, miniaturize, low cost, and high reliability.
Disadvantages of fiber optic digital communication systems: Wide occupied bandwidth, low bandwidth utilization, complex equipment, and relatively high cost.
Characteristics of fiber optic analog communication systems: Narrow occupied bandwidth, simple circuits (no need for A/D/D/A conversion), low price, suitable for short-distance communication.
(2) Classification by optical wavelength and fiber type: Divided into short wavelength multimode fiber optic communication systems and long wavelength fiber optic communication systems:
Short wavelength multimode systems: Operating wavelength around (0.85\mu m), rate ≤34Mbit/s, repeater spacing ≤10km.
Long wavelength systems (subdivided into 3 categories):
(1.31\mu m) multimode systems: rate 34/140Mbit/s, repeater spacing ≈20km;
(1.31\mu m) single-mode systems: rate 140/565Mbit/s, repeater spacing 30~50km (at 140Mbit/s);
(1.55\mu m) single-mode systems: rate ≥565Mbit/s, repeater spacing ≈70km.
(3) Classification by digital multiplexing method: Divided into Plesiochronous Digital Hierarchy (PDH) systems and Synchronous Digital Hierarchy (SDH) systems:
PDH: Each hierarchical level bit rate has tolerance and is asynchronous, adopting positive justification to implement plesiochronous multiplexing; rate ≤565Mbit/s.
SDH: Suitable for point-to-point/multipoint network transmission; single wavelength rate can reach 2.5Gbit/s, 10Gbit/s.
(4) Classification by transmission rate: Divided into 3 categories: 1) Low-speed systems: rate 2Mbit/s, 8Mbit/s; 2) Medium-speed systems: rate 34Mbit/s, 140Mbit/s; 3) High-speed systems: rate >565Mbit/s.
(5) Classification by modulation method: Divided into 2 categories: 1) Direct intensity modulation systems (internal modulation): Modulation during the light emission process of the light source; simple equipment, low cost, high modulation efficiency, but spectral broadening affects rate improvement. 2) Indirect modulation systems (external modulation): After the light source emits light, a modulator is added in the output path; minimal impact on the light source spectral line, suitable for high-rate communication.
