Can multi-mode fiber and single-mode fiber be distinguished by the naked eye? Why is its transmission distance so much shorter than single-mode?
I. Can the difference between multimode and single-mode fiber be seen with the naked eye?
In terms of physical structure and appearance, whether the naked eye can distinguish depends on whether you are observing bare fiber or a finished fiber optic patch cord:
1. Directly observing bare fiber (extremely difficult to distinguish the core with the naked eye)
For both single-mode and multimode fiber, the standard outer diameter of the outermost glass cladding is the same, usually 125\ \mu\text{m} (approximately the thickness of a hair). The outer diameter of the outermost plastic coating is also typically the same (e.g., 245\ \mu\text{m} or 250\ \mu\text{m}).
The core difference lies in the diameter of the core inside the cladding:
- Single-mode fiber (SMF): The core is extremely narrow, usually only about 9\ \mu\text{m}.
- Multimode fiber (MMF): The core is thicker, commonly 50\ \mu\text{m} (like OM2, OM3, OM4) or 62.5\ \mu\text{m} (like OM1).
Since the core is encased within a glass cladding of the same size and is completely transparent, it is impossible to distinguish the core size by simply looking directly at it with the naked eye.
2. Through the outer jacket color of the fiber optic patch cord (most intuitive and easiest to distinguish with the naked eye)
If the fiber has been processed into a fiber optic patch cord with a plastic outer jacket, international standards (such as TIA-598-C) define very clear color codes that the naked eye can easily distinguish:
- Single-mode fiber patch cord: The outer jacket is usually Yellow.
- Multimode fiber patch cord:
- Orange: Often used for older OM1 or OM2 multimode fibers (62.5\ \mu\text{m} or 50\ \mu\text{m});
- Aqua: Often used for 10 Gigabit OM3 or OM4 multimode fibers;
- Erika Violet: Used for OM5 wideband multimode fiber.
3. With the help of a fiber end-face microscope (professional microscopic observation)
If a fiber end-face microscope with 200x or 400x magnification is used to observe the end face of the connector ferrule, due to the several-fold difference in core size (9 vs. 50), the naked eye can clearly see the difference in the size of single-mode and multimode cores in the central area within the eyepiece or on the display.
II. Why is the transmission distance of multimode fiber so much shorter than single-mode fiber?
The transmission distance of multimode fiber is typically only a few hundred meters (even only tens of meters at high 10 Gigabit speeds), while single-mode fiber can easily transmit tens or even hundreds of kilometers. This is mainly determined by the following two core physical mechanisms:
1. Modal Dispersion (Modal Dispersion) — The Core Limiting Factor
Because the core of multimode fiber (e.g., 50\ \mu\text{m}) is much larger than the wavelength of the working light, it allows light to propagate simultaneously through hundreds or thousands of different paths (called