You often mention photon sensing. What’s the difference between photons and electrons traveling in an optical fiber?
In the fields of optical communication and photonic sensing, photons and electrons are two distinct physical carriers. When they ‘run’ in their respective channels, the physical characteristics, transmission mechanisms, and sensing performance exhibit fundamental differences.
Here’s a detailed comparison of photons and electrons operating in optical fibers/wires from the perspectives of physical principles, engineering applications, and sensing characteristics:
1. Fundamental Differences in Physical Carriers and Transmission Media
- Electrons: Are microscopic elementary particles with a negative electric charge and rest mass (m_e \approx 9.11 \times 10^{-31}\ \text{kg}). Electrons can only transmit energy and electrical signals within conductive media (such as copper, aluminum, and other metal cables) through the directional drift of free electrons or electromagnetic wave guidance. Since glass is an excellent insulator, electrons cannot flow directionally in optical fibers.
- Photons: Are quanta (energy packets) of electromagnetic waves, carrying no electric charge and having zero rest mass. Photons primarily propagate as electromagnetic waves in transparent media (such as quartz glass optical fibers). For example, the standard OFSCN® G.652D Optical Fiber, mainly composed of high-purity silicon dioxide (\text{SiO}_2) insulating material, achieves total internal reflection through the refractive index difference between the core and cladding, thereby efficiently confining and guiding photons to propagate within the optical fiber.
2. Speed of ‘Running’ and Microscopic Motion Behavior
While the signal transmission speeds of both are macroscopically close to the speed of light, their microscopic operating mechanisms are entirely different:
- Microscopic Differences in Propagation Speed:
- Photon speed in optical fiber: Follows the formula v = \frac{c}{n}. Here, c is the speed of light in a vacuum (approximately 3 \times 10^8\ \text{m/s}), and n is the refractive index of the fiber medium. For the standard OFSCN® G.657 Optical Fiber, with a refractive index n \approx 1.468, the operating speed of photons in the fiber is approximately 2 \times 10^8\ \text{m/s} (about 200,000 kilometers per second).
- Electron speed in wires: Requires distinguishing between the directional drift velocity of electrons themselves and the propagation speed of the electrical signal (electromagnetic field). The directional movement speed of individual free electrons in a metal conductor is extremely slow, typically only from several micrometers to millimeters per second; however, when a voltage is applied, the speed at which the electric field is established (i.e., the signal propagation speed) is very fast, generally reaching 2/3 to 9/10 of the speed of light in a vacuum in copper wires (approximately 2 \times 10^8\ \text{m/s} to 2.7 \times 10^8\ \text{m/s}).
- Collision and Heat Loss:
- Electrons, while moving within the metal lattice, undergo frequent collisions, impeding charge movement and generating resistive loss (Joule heating), which causes the cables to heat up. Furthermore, as signal frequency increases, losses become more severe due to the