When an engineer specifies optical fiber for a network installation, the first and most consequential decision is the fundamental architecture: single-mode or multi-mode. This choice cascades through every subsequent design decision—equipment selection, power budgets, distance capabilities, upgrade paths, and total cost of ownership.

These aren't merely different 'grades' of the same technology. Single-mode and multi-mode fibers represent fundamentally different approaches to light propagation, each optimized for distinct operating regimes. Understanding their differences—not just superficially, but at the level of physics and engineering tradeoffs—is essential for any network professional.

This page provides comprehensive coverage of both fiber types: their construction, propagation characteristics, performance capabilities, and the decision frameworks for selecting between them.

The defining characteristic that separates single-mode from multi-mode fiber is precisely what the names suggest: the number of propagation modes supported.

Recall from the previous page that light in an optical fiber propagates as specific electromagnetic field patterns called modes. The V-number determines how many modes can exist:

V = (π × d × NA) / λ

• When V < 2.405: Only the fundamental mode (LP₀₁) propagates—single-mode fiber
• When V > 2.405: Multiple modes propagate simultaneously—multi-mode fiber

The Engineering Consequence:

Since V depends on core diameter, numerical aperture, and wavelength, achieving single-mode operation requires a small core. Multi-mode fibers use larger cores that support hundreds or thousands of modes.

This seemingly simple distinction creates vastly different transmission characteristics:

• Single-mode fiber: No modal dispersion → higher bandwidth × distance product → longer reach → lower attenuation
• Multi-mode fiber: Modal dispersion limits distance → easier light coupling → lower equipment cost → simpler installation

Single-mode fiber (SMF) represents the highest-performance solution for optical transmission. By supporting only the fundamental mode, it eliminates modal dispersion entirely, enabling extreme bandwidth × distance products measured in terabits per second × kilometers.

Physical Construction:

The SMF core must be small enough that V < 2.405 at operating wavelengths. For 1550 nm operation with NA ≈ 0.13:

d < (2.405 × λ) / (π × NA)
d < (2.405 × 1.55 × 10⁻⁶) / (π × 0.13)
d < 9.1 µm

Standard single-mode fiber cores are 8-10 µm in diameter—roughly 1/8 the width of a human hair. This extreme precision demands sophisticated manufacturing and careful handling.

Refractive Index Profile:

Unlike the simple step-index profile of basic multi-mode fibers, single-mode fibers often employ matched cladding or depressed cladding designs:

• Matched cladding: Core doped with GeO₂ to raise n above pure silica cladding
• Depressed cladding: Inner cladding doped with fluorine to lower n below pure silica, creating a 'trench' around the core

Depressed cladding designs improve bend performance and are the basis for modern bend-insensitive fibers (G.657).

Mode Field Diameter:

In single-mode fiber, the light doesn't remain perfectly confined to the core—the electromagnetic field extends into the cladding as an evanescent wave. The Mode Field Diameter (MFD) describes the effective extent of this light distribution.

Typical MFD values:

• At 1310 nm: 9.0-9.5 µm
• At 1550 nm: 10.1-10.5 µm

MFD increases at longer wavelengths because more light extends into the cladding. This has practical implications:

1. Splicing and connector loss depends on MFD matching between fibers
2. Bend sensitivity increases with larger MFD (more light in cladding = more vulnerable to bending)
3. WDM systems experience wavelength-dependent losses if MFD varies significantly

Cut-off Wavelength:

Below a certain wavelength, the fiber becomes multi-mode—the V-number exceeds 2.405. The cut-off wavelength (λc) is the shortest wavelength at which single-mode operation is guaranteed.

For G.652 fiber: λc < 1260 nm (cable cut-off)

This ensures single-mode operation across the entire 1260-1625 nm operating range.

Multi-mode fiber (MMF) takes the opposite approach: a larger core that supports many propagation modes simultaneously. This creates fundamental tradeoffs—easier light coupling and lower equipment costs, but reduced reach due to modal dispersion.

Physical Construction:

Multi-mode fiber cores are either:

• 50 µm (modern standard for high-speed data centers)
• 62.5 µm (legacy standard, still prevalent)

The larger core provides:

1. Easier alignment — Greater tolerance for connector and splice imperfections
2. Higher coupling efficiency — More light enters from sources (especially LEDs)
3. Lower equipment cost — VCSEL lasers and simple photodiodes suffice

The Modal Dispersion Problem:

With hundreds of modes propagating simultaneously, each travels a slightly different path. Higher-order modes reflect at steeper angles, traveling longer paths than lower-order modes that travel nearly straight through the core.

For a 1 km step-index multi-mode fiber:

Modal dispersion ≈ (n₁ - n₂) / c × L
                ≈ (1.48 - 1.46) / (3×10⁸) × 1000
                ≈ 67 ns/km

This 67 ns pulse spread over 1 km would destroy any signal at rates above ~15 Mbps—utterly impractical for modern networking.

Graded-Index Profile:

The ideal graded-index profile follows a power law:

n(r) = n₁ × √(1 - 2Δ(r/a)^α)

where:

• n₁ = refractive index at core center
• Δ = relative refractive index difference
• a = core radius
• r = radial position
• α = profile exponent (ideally ~2.0 for parabolic)

When α ≈ 2 (parabolic profile), modal dispersion is minimized. Manufacturing precision matters enormously—deviations from the optimal profile increase dispersion.

Modern laser-optimized multi-mode fibers are manufactured with α values precisely tuned for VCSEL sources, achieving modal bandwidths of 3500-4700 MHz·km at 850 nm.

Effective Modal Bandwidth (EMB):

Traditional bandwidth measurements (overfilled launch, OFL) don't accurately characterize laser-optimized fiber performance. The Effective Modal Bandwidth (EMB) uses a laser source that replicates actual VCSEL launch conditions.

OM3/OM4 specifications include both OFL bandwidth and EMB:

• OM3: 2000 MHz·km (EMB)
• OM4: 4700 MHz·km (EMB)

Differential Mode Delay (DMD):

To verify graded-index quality, manufacturers measure Differential Mode Delay (DMD)—the arrival time differences between modes excited at different radial positions. A well-optimized fiber shows minimal DMD variation across the core, indicating that all modes travel at similar group velocities.

The practical differences between single-mode and multi-mode fiber become starkly apparent when examining achievable distances at various data rates.

Multi-mode Fiber Reach:

Multi-mode reach is limited primarily by modal dispersion, which causes pulse spreading proportional to fiber length. The bandwidth × distance product is essentially constant for a given fiber grade.

Single-mode Fiber Reach:

Single-mode fiber, unencumbered by modal dispersion, achieves vastly greater distances. The primary limitations become attenuation and chromatic dispersion, both of which can be addressed through amplification and dispersion compensation.

The choice between single-mode and multi-mode involves significant cost trade-offs that vary depending on the application scale, distance requirements, and time horizon.

Fiber Cost:

Contrary to common assumption, single-mode fiber itself is often cheaper than multi-mode fiber. The precision graded-index profile required for laser-optimized multi-mode fiber is more difficult to manufacture than the simpler step-index profile of single-mode fiber.

Typical cable costs (per meter, installed):

• OM3: $0.50-1.00
• OM4: $0.60-1.20
• OM5: $1.00-2.00
• OS2 (G.652.D): $0.30-0.80

However, fiber cost is typically a small fraction of total link cost. The dominant factors are transceivers and installation labor.

Transceiver Cost Comparison (approximate, 2024):

The choice of light source is intimately connected to fiber type. Multi-mode systems predominantly use VCSELs (Vertical-Cavity Surface-Emitting Lasers), while single-mode systems require edge-emitting lasers (DFB, FP, or external cavity designs).

VCSEL Technology:

VCSELs emit light perpendicular to the semiconductor surface, from a small circular aperture. Key characteristics:

• Circular beam profile — Excellent coupling efficiency to multi-mode fiber
• Low threshold current — Lower power consumption, easier driving
• Array packaging — Multiple VCSELs integrated on single substrate (for 40/100/400G)
• Very low cost — Simplified manufacturing, high-volume wafer testing
• Wavelength — Primarily 850 nm; emerging 940 nm for SWDM

Limitations:

• Broad spectrum (~0.6 nm) limits reach due to chromatic dispersion
• Modal noise issues with single-mode fiber (not recommended)
• Power output limited (~10 mW peak)

Edge-Emitting Lasers:

Fabry-Pérot (FP) Lasers:

• Lower cost edge-emitters
• Multiple longitudinal modes (2-10 nm spectrum)
• Suitable for shorter single-mode reaches (<10 km at 1310 nm)

Distributed Feedback (DFB) Lasers:

• Integrated Bragg grating selects single longitudinal mode
• Extremely narrow spectrum (<0.1 nm)
• Wavelength-stable with temperature control
• Required for DWDM, long-haul, and high-rate systems

External Cavity Lasers (ECL):

• External grating provides wavelength selection
• Widest tuning range (50+ nm)
• Used in coherent transceivers and tunable devices

OM5 represents the latest evolution in multi-mode fiber, specifically designed to support Short Wavelength Division Multiplexing (SWDM)—a technique that uses multiple wavelengths in the 850-950 nm range to multiply capacity over a single fiber.

The Challenge OM5 Addresses:

As data center bandwidth demands exceed what 850 nm VCSELs alone can provide, engineers explored higher lane counts (SR4, SR8 with multiple fiber pairs) and parallel optics. But more fibers mean more cables, more connectors, and higher installation costs.

SWDM offers an alternative: use 4 wavelengths (850, 880, 910, 940 nm) over a single fiber pair, achieving 4× capacity without additional fiber count.

The Technical Challenge:

Standard OM3/OM4 fibers are optimized for 850 nm. Their graded-index profiles produce higher modal dispersion at longer wavelengths (880-940 nm), drastically reducing bandwidth. OM5 fiber is manufactured with a wideband optimized profile that maintains high bandwidth across the entire 850-950 nm range.

SWDM Applications:

• 100GBASE-SWDM4: 4×25G over duplex OM5 (100m reach)
• 200GBASE-SWDM4: 4×50G over duplex OM5 (planned)
• Future: 400G+ using 8 wavelengths or higher rates per wavelength

OM5 Identification:

OM5 cables are jacketed in lime green (aqua for OM3/OM4) to prevent accidental mixing. All OM5 specifications meet or exceed OM4 at 850 nm, ensuring backward compatibility with existing SR equipment.

Is OM5 Worth It?

The value proposition depends on deployment scale:

• Large-scale new builds: May benefit from reduced fiber count and future SWDM migration
• Existing OM4 installations: SWDM adoption remains limited; upgrade rarely justified
• Single-mode alternative: Many operators skip OM5 entirely, deploying single-mode for ultimate future-proofing

Choosing between single-mode and multi-mode fiber requires evaluating multiple factors. Here's a structured decision framework based on real-world engineering practice.

We've comprehensively examined the two fundamental optical fiber architectures—each with distinct physics, performance characteristics, and optimal application domains.

What's Next:

With fiber types understood, the next page examines the connectors that terminate optical fibers—the mechanical interfaces that enable reliable light coupling between fibers, transceivers, and patch panels. We'll cover SC, LC, MTP/MPO, and other connector types, along with their specifications and applications.