Difference between single mode fiber and multi-mode fiber

Fiber optics are flexible, transparent fibers made of extruded glass or plastic, slightly thicker than a human hair.  Optical fiber is the most common means of transmitting light at both ends and is widely used in optical fiber communication.  Optical fiber has a longer transmission distance and higher bandwidth than cable.  Optical fibers are usually composed of transparent core with low refractive index and transparent cladding material.  As a light wave conductor, optical fiber causes total reflection of light in the fiber core.

In general, there are two types of optical fibers: those that support multiple propagation paths or transverse modes are called multi-mode fibers (MMF), while those that support a single mode are called single-mode fibers (SMF).  But what's the difference?

What is single-mode fiber?  

In optical fiber communication, single-mode fiber is an optical fiber that transmits optical signals directly in transverse mode.  Single-mode fiber can operate at a data rate of 100M/s or 1 G/s, and the transmission distance can reach at least 5 km.  Typically, single-mode fibers are used for remote signal transmission.  

Single-mode fiber is an optical fiber with only one strand (two strands in most applications) of glass fiber. The core diameter is from 8.3μm to 10μm, and there is only one transmission mode.  Because the core diameter is relatively narrow, single-mode fibers can only transmit optical signals at wavelengths of 1,310 nm or 1,550 nm.  The bandwidth of single-mode fiber is higher than that of multi-mode fiber, but it has higher requirements on the spectrum width and stability of the light source, that is, the spectrum width should be narrow and the stability should be better.  

Single-mode fiber is mainly used in multi-frequency data transmission applications, for example, Wave-division-multiplexing system only needs a single single-mode fiber to realize data transmission.  

Single-mode fiber has a higher transmission rate than multi-mode fiber, and the transmission distance is more than 50 times higher than multi-mode fiber, so the price is also higher than multi-mode fiber.  Compared with multi-mode fiber, the core diameter of single-mode fiber is much smaller. The characteristics of small core diameter and single-mode transmission make the optical signal transmitted in single-mode fiber not distorted due to the overlap of optical pulses.  Single-mode fiber has the lowest signal attenuation rate and the highest transmission speed among all fiber types.

Multimode fiber is another common type of fiber with cores ranging from 50μm to 100μm in diameter that can transmit multiple modes at a given operating wavelength.  Compared with twisted-pair cables, multi-mode optical fibers can support a longer transmission distance. In 10 Mbit/s and 100 Mbit/s Ethernet, multi-mode optical fibers can support a transmission distance of up to 2000 m.  The core diameters of common multimode fibers are 50μm, 62.5μm, and 100μm.  As there are hundreds of transmission modes in multi-mode optical fiber, the propagation constant and group rate of each mode are different, so that the band width of optical fiber is narrow, the dispersion is large, and the loss is large, which is only suitable for the optical fiber communication system with short distance and small capacity.  

What is the difference between single-mode and multi-mode fiber?  

1. Core diameter  

The main difference between multimode and single-mode fibers is that the former have larger diameters, typically 50 or 62.5µm core diameters, while typical single-mode fibers have 8 and 10µm core diameters, both with 125µm cladding diameters.

2, light source

Lasers and leds serve as light sources.  Laser light sources are significantly more expensive than LED light sources because the light they produce can be precisely controlled and has high power.  However, the light produced by LED light sources is more dispersed, and these light sources are mostly used in multi-mode fiber optic patch panels, laser light source is usually used for single mode fiber optic patch panel.  

3, bandwidth

Because multi-mode fiber has a larger core size than single-mode fiber, it supports multiple transmission modes.  In addition, like multi-mode fiber, single-mode fiber also exhibits modal dispersion caused by multiple spatial modes, but the modal dispersion of single-mode fiber is smaller than that of multi-mode fiber.  For these reasons, single-mode fiber has a higher bandwidth than multi-mode fiber.

4. Color of jacket  

The color of the jacket is sometimes used to distinguish multi-mode fiber optic patch cord  from multi-mode fiber optic patch cords.  According to tiA-598C standard definition, non-military using single-mode optical fiber uses yellow jacket, and multi-mode optical fiber with orange or aqua jacket.  Depending on the type, some vendors use purple to distinguish high-performance OM4 fiber from other types of fiber.

5. Modal dispersion  

LED light sources are sometimes used in multi-mode optical fibers to create a range of wavelengths that propagate at different speeds.  This results in multimode dispersion, which limits the effective transmission distance of multimode fiber .  In contrast, lasers used to drive single-mode fibers produce a single wavelength of light.  Therefore, its modal dispersion is much smaller than multi-mode fiber.  Because of modal dispersion, multimode fiber has higher pulse propagation rate than single-mode fiber, which limits the information transmission capacity of multimode fiber.  

6,  price  

Multimode fiber is more expensive than single-mode fiber.  However, in terms of equipment, single-mode fiber equipment is more expensive than multi-mode fiber equipment, because single-mode fiber usually uses solid-state laser diodes.  Therefore, the cost of using multi-mode fiber is much less than the cost of using single-mode fiber.