The structure and light transmission principle of optical fiber

The structure and light transmission principle of optical fiber

1. Optical fiber structure and types
1. Optical fiber structure Figure 7-1-1 2. Types

2. Principles of optical fiber transmission
1. Light transmission at the interface between the core and the cladding 2. Light transmission at the end of the optical fiber

3. When light propagates
1° in an optical fiber and satisfies the total reflection condition ( ), the light can continuously produce total reflection at the interface between the fiber core and the cladding, propagating forward in the core in a zigzag route, from one end of the optical fiber to the other end at the speed of light. This is the principle of optical fiber light transmission.
2° Bend the optical fiber: R—radius of curvature of the bent optical fiber d—diameter of the optical fiber
When R>>4d, the total reflection condition is still met, and the optical fiber transmission loss is very small
When R<4d, the total reflection condition is not met, and the optical fiber transmission loss is very large

1. Basic principles of fiber optic sensors
The measured object modulates the light transmitted by the optical fiber, so that the intensity (amplitude), phase, frequency or polarization state of the transmitted light changes with the measured object, and then the modulated optical signal is detected and demodulated to obtain the measured parameter.

2. Classification of fiber optic sensors
Functional type - optical fiber is used as a sensitive element, single-mode optical fiber is commonly used
Non-functional type - optical fiber is used as a sensing element, multi-mode optical fiber is commonly used

3. Modulation methods of transmitted light
1. Types of modulation methods Light intensity modulation - most commonly used
Light phase modulation
Frequency-free modulation
Light wavelength modulation
Light polarization modulation
2. Light intensity modulation methods
① Radiation type
② Fiber displacement type Figure 7-1-5 The incident fiber does not move The output fiber does not move
③ Insertion type Figure 7-1-6
④ Microbend loss type Figure 7-1-7

4. Advantages and disadvantages of fiber optic sensors
Advantages 1. High sensitivity
2. Good electrical insulation performance, anti-electromagnetic interference
3. Corrosion resistance, high temperature resistance
4. Small size, light weight
Disadvantages: Special tools are required to repair broken wires