Single-mode Fiber vs. Multimode Fiber: Principles, Performance, and Selection

Single-mode optical fiber uses a solid laser as a light source; multi-mode optical fiber uses a light-emitting diode as a light source; single-mode optical fiber has a wide transmission bandwidth and a long transmission distance, but because it requires a laser source, the cost is relatively high; multi-mode optical fiber has a low transmission speed and a short distance, but its cost is relatively low; single-mode optical fiber has a small core diameter and dispersion, allowing only one mode of transmission; multi-mode optical fiber has a large core diameter and dispersion, allowing hundreds of modes of transmission.

I.Background: The “Dual Track System” of Fiber Optic Communications

Since its commercial introduction in the 1970s, fiber-optic communications have rapidly replaced copper cables as the backbone of telecommunications due to their high bandwidth, low attenuation, and strong anti-interference capabilities.

In practical applications, optical fiber is divided into two major categories: single-mode fiber (SMF) and multi-mode fiber (MMF).

The core difference between these two types lies not simply in color or price but in the physical differences in the propagation modes of light within the fiber, which determine transmission distance, speed, cost, and application scenarios.

II. Core Principle: Light Propagation Mode in the Fiber Core

A. What is a Mode?

• An optical fiber is a waveguide structure consisting of a core and cladding. The different refractive indices create a condition of total internal reflection.
• A mode refers to the distribution of paths along which light propagates within an optical fiber.

                   Single-mode: Only one propagation mode (the fundamental mode, LP01) is permitted.

                   Multimode: Multiple modes (multiple paths) are permitted to propagate simultaneously.

In the field of optical fiber data transmission, the term “mode” is used to describe the way a light signal propagates within the glass core of an optical fiber—that is, a mode is the path along which light propagates.

Thus, in single-mode fiber, light propagates along a single path; in multimode fiber, light propagates along multiple paths.

B. Physical differences between single-mode and multi-mode

The sheath of single-mode fiber optic patch cords is generally yellow, while the sheath of multi-mode is generally orange or so-called aqua (a color between blue and green); in terms of fiber core diameter, multi-mode is generally slightly thicker.

III. In-depth analysis of optical principles

A. Single-mode Fiber

• The fiber core is thin (allowing only one mode), allowing light to travel in a nearly straight line, eliminating inter-mode delays.
• It is primarily affected by dispersion (chromatic dispersion and waveguide dispersion), but overall, this is minimal, allowing for longer transmission distances.
• Common Standards:

                   OS1: Indoor, low attenuation ≤1 dB/km (1310/1550 nm).

                   OS2: Outdoor, low attenuation ≤0.4 dB/km (1310/1550 nm).

B. Multimode Fiber

• The fiber core is thick (50/62.5 μm). Light has multiple reflection paths, resulting in different propagation times along different paths, causing modal dispersion and limiting transmission distance.
• Common Standards:

OM1 (62.5 μm): An older standard with a bandwidth of 200 MHz/km at 850 nm.

OM2 (50 μm): 500 MHz/km at 850 nm.

OM3: Laser-optimized, 2000 MHz/km @ 850 nm, suitable for 10G transmission distances ≤ 300 m.

OM4: Higher bandwidth, 4700 MHz/km @ 850 nm, 10G transmission distances ≤ 550 m.

OM5: Supports SWDM (short wavelength division multiplexing) and multiple wavelengths in parallel.

Single-mode fiber has a transmission distance of at least 5 km and is generally used for long-distance communications. Multimode fiber can only reach approximately 2 km and is suitable for short-distance communications within buildings or on campuses.

Because LED light sources are more dispersed and can produce multiple modes of light, they are often used in multi-mode optical fibers; while laser light sources are close to a single mode, so they are usually used in single-mode optical fibers.

IV. Performance and Application Comparison

V. Analysis of Typical Application Scenarios

1. Data Center

• Short-reach intra-rack/inter-rack connections: OM4/OM5 multimode + LC/MPO, 10/40/100/400G speeds.

• Long-reach connections across computer rooms or campuses: OS2 single-mode + LC, 10G/40G/100G-LR/ER/ZR.

2. Carrier Backbone

• Mostly uses OS2 single-mode. Wavelength Division Multiplexing (DWDM/CWDM) systems can carry 100G-400G ultra-long-haul links.

3. Industrial and Healthcare

• Factory Automation: Multimode short-reach sensor networks.
• Medical Imaging Transmission: Single-mode is used for high-resolution, long-distance image backhaul.

VI. Selection and Engineering Recommendations

1.Depend on the distance

• 2 km: Single-mode
• <550 m: Multimode is feasible

2.Depend on the speed

• High-speed (≥400G) long-distance: Single-mode

• High-speed short-distance: Multimode (OM4/OM5) 

3.Budget

• Expensive optical transceivers/few links: Single-mode
• Many links/short distance: Multimode

VII. Conclusion

Single-mode and multimode fiber are not a matter of “one is better or worse,” but rather a trade-off between different technologies:

• Single-mode: Long distance, high bandwidth, and ample room for future upgrades, but expensive optical transceivers.

• Multimode: Short distance, simplified cabling, and inexpensive optical transceivers, but limited by modal dispersion.

In network construction, a proper combination of the two can achieve the optimal balance between performance and cost.

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