Single-mode vs multimode fiber
The fundamental classification of optical fiber: single-mode supports only the LP01 fundamental mode, multimode supports many guided modes. The distinction drives core size, application class, source compatibility, and cost.
Optical fibers divide into two fundamental classes based on how many guided modes they support at the operating wavelength:
- Single-mode fiber (SMF): supports only the fundamental LP01 mode. Pulse propagation has well-defined group delay; modal dispersion is zero.
- Multimode fiber (MMF): supports many guided modes (10 to 1000+). Each mode propagates with slightly different group velocity, producing modal dispersion.
The transition between regimes is set by the V-number criterion: a step-index fiber is single-mode when .
Geometric distinction.
| Property | Single-mode | Multimode |
|---|---|---|
| Core diameter | 4 – 10 μm | 50 – 200 μm |
| Cladding diameter | 125 μm (standard) | 125 μm (standard) |
| NA | 0.10 – 0.16 | 0.20 – 0.30 |
| Index difference | 0.002 – 0.008 | 0.005 – 0.025 |
| Color identification | Yellow jacket (typical) | Orange (OM1/OM2), aqua (OM3), erika violet (OM4), lime green (OM5) |
Application distinction.
| Application | Fiber choice |
|---|---|
| Telecom long-haul (10+ km) | Single-mode |
| Datacom short reach ( 100 m) | Multimode (cost-driven) |
| Datacom medium reach (100 – 500 m) | Single-mode (with cheap DR/FR optics) |
| Local area network (legacy) | Multimode |
| Coherent / DWDM | Single-mode required (modal dispersion incompatible) |
| Sensors (strain, temperature) | Single-mode |
| Microscopy illumination | Multimode (high power, broad spectrum) |
| Fiber lasers | Both (specialty MMF for high-power; SMF for output) |
| Test instruments | Single-mode (modal-stable) |
Source compatibility.
| Source | Compatible fiber | Why |
|---|---|---|
| 850 nm VCSEL | Multimode (small spot, broad spectrum) | Spot diameter 5 – 10 μm matches MMF core, spectral linewidth too broad for SMF |
| 1310 nm DFB | Single-mode | Narrow spot, narrow linewidth |
| 1550 nm DFB | Single-mode | Standard telecom |
| 1310 nm Fabry-Perot | Multimode (typically) | Broad linewidth not ideal for SMF chromatic-dispersion budget |
| Fiber laser (1064 nm) | Both | Single-mode pigtail standard; high-power output through large-mode-area MMF |
| LED (broadband) | Multimode only | Cannot efficiently couple to SMF |
Cost distinction. SMF and MMF cable themselves cost roughly similar amounts per meter ($0.20 – $1.00 per meter for jacketed cable). But the dramatic cost difference is in optoelectronic transceivers:
| Transceiver | Cost (2024 – 2026) |
|---|---|
| 10G-SR (MMF, 850 nm VCSEL) | $20 – $50 |
| 10G-LR (SMF, 1310 nm DFB) | $80 – $200 |
| 100G-SR4 (MMF, 4× 850 nm VCSEL) | $100 – $300 |
| 100G-LR4 (SMF, 4× 1310 nm DFB + WDM) | $300 – $800 |
| 400G-SR4 (MMF, 4× 50G VCSEL PAM4) | $400 – $800 |
| 400G-DR4 (SMF, 4× 1310 nm DFB PAM4) | $800 – $2000 |
The cost gap drives the persistence of multimode fiber in data centers despite SMF's superior performance characteristics.
Bandwidth-distance product.
| Fiber | BLP @ 850 nm | BLP @ 1310 nm | BLP @ 1550 nm |
|---|---|---|---|
| OM1 MMF | 200 MHz·km | 500 MHz·km | — |
| OM3 MMF | 2000 MHz·km | — | — |
| OM4 MMF | 4700 MHz·km | — | — |
| OM5 MMF | 3500 (850) + 1850 (953) MHz·km | — | — |
| SMF-28 | — | THz·km | THz·km |
| NZ-DSF (G.655) | — | — | THz·km |
Single-mode fiber is limited by chromatic dispersion (not modal); typical reaches are 10 – 1000 km depending on source linewidth and dispersion compensation.
Cutoff wavelength. Single-mode operation requires . Below cutoff, the LP11 mode is also guided and the fiber is effectively multimode. SMF-28 has nm — operating below this (e.g., at 850 nm) would make SMF-28 multimode. This is why short-wavelength applications (HeNe at 633 nm, GaAs at 850 nm) require specialty SMF (S630-HP for 633 nm, HI-1060 for 1060 nm) with smaller core and proportionally shorter cutoff.
Mixed-fiber connectivity. Direct coupling between SMF and MMF is asymmetric:
- MMF → SMF: only the LP01 mode in the MMF couples efficiently; high-order MMF modes are lost. Loss = 10 log₁₀(N) dB where N is the number of MMF modes; typically 8 – 15 dB.
- SMF → MMF: SMF mode field is small enough to fit within the MMF core; coupling is nearly lossless if launched at the center.
This asymmetry is why mixed-fiber links use launch conditioning: deliberate mode-mixing fibers convert SMF input to a controlled MMF launch pattern that maximizes coupling.
References: Saleh & Teich, Fundamentals of Photonics (3rd ed., 2019), Ch. 9; ITU-T G.652 (SMF), G.651 (MMF); IEEE 802.3 Ethernet specifications for source-fiber compatibility requirements.