OS2 vs OM3 vs OM4 vs OM5: Choosing the Fiber Type for MPO Links

OM3, OM4 and OM5 are 50 µm multimode fibers for short links up to about 100–150 m at modern speeds; OS2 is single-mode for 500 m and beyond. OM4 is the default multimode choice for parallel MPO optics. OM5 adds reach only with wavelength-multiplexed (BiDi/SWDM) optics. Choose OS2 for DR4, DR8 and longer runs.

Fiber type is a line on every MPO trunk and patch cord order, next to fiber count, gender, polarity and polish. It decides how far a link can run, which transceivers it supports, and how many upgrades the cabling survives. This guide compares the four fiber grades used in new data-centre cabling, explains what the numbers mean, and shows how to choose for MPO links. New to MPO? Start with what an MTP/MPO cable is.

Two Families: Multimode and Single-Mode

All four grades have a 125 µm glass cladding, so they look the same in a connector ferrule. The difference is the core.

• Multimode (OM3, OM4, OM5) has a 50 µm graded-index core. Light travels in many paths (modes). It pairs with low-cost VCSEL transmitters at 850 nm and connectors with looser alignment tolerances, so multimode links are usually the cheapest option for short distances. The limit is modal bandwidth: the different paths arrive at slightly different times, which caps distance at high speed.
• Single-mode (OS2) has a core of about 9 µm that carries one mode. With laser transmitters at 1310 nm or 1550 nm, reach is set by loss and dispersion, not modal bandwidth, so links run from 500 m to 10 km and beyond. Single-mode optics typically cost more, but the cable itself supports many generations of speed.

Older OM1 (62.5 µm) and OM2 (50 µm) multimode are legacy grades and are not recommended for new high-speed links.

Fiber Grades at a Glance

	OM3	OM4	OM5	OS2
Fiber family	Multimode 50/125 µm	Multimode 50/125 µm	Wideband multimode 50/125 µm	Single-mode 9/125 µm
Min. effective modal bandwidth	2000 MHz·km at 850 nm	4700 MHz·km at 850 nm	4700 at 850 nm + 2470 at 953 nm	Not applicable
Max. cabled attenuation	3.0 dB/km at 850 nm	3.0 dB/km at 850 nm	3.0 dB/km at 850 nm	0.4 dB/km at 1310/1550 nm
Typical jacket colour	Aqua	Aqua (violet in some European markets)	Lime green	Yellow
Fiber standard	TIA-492AAAC	TIA-492AAAD	TIA-492AAAE	ITU-T G.652.D (typical)
Typical use	Existing and short links	New multimode builds, SR4/SR8	BiDi/SWDM multimode	DR4/DR8, FR/LR, backbones

Effective modal bandwidth (EMB) is the key multimode number. OM4 more than doubles OM3's bandwidth at 850 nm, which is why it reaches further at every speed. OM5 has the same 850 nm specification as OM4 and adds a second guaranteed value at 953 nm, so it can carry several wavelengths between about 850 nm and 950 nm on each fiber.

The 3.0 dB/km multimode figure is the ANSI/TIA-568.3-D cable maximum; IEEE 802.3 uses a more conservative 3.5 dB/km at 850 nm when it calculates channel loss limits.

For single-mode, ISO/IEC 11801 defines OS1 (1.0 dB/km, usually tight-buffered indoor cable) and OS2 (0.4 dB/km at 1310, 1383 and 1550 nm). OS2 is normally built with low-water-peak ITU-T G.652.D fiber. In MPO assemblies "OS2" is the common way to specify single-mode. Bend-insensitive versions of both families exist for tight routing.

Reach by Application

The table shows the operating range IEEE 802.3 specifies for common Ethernet applications. Distances assume standards-compliant cabling with a limited number of connections; more or lossier connectors reduce reach.

Application	Fibers / connector	OM3	OM4	OM5	OS2
10GBASE-SR	2, duplex LC	300 m	400 m	400 m	—
25GBASE-SR	2, duplex LC	70 m	100 m	100 m	—
40GBASE-SR4	8, MPO-12	100 m	150 m	150 m	—
100GBASE-SR4	8, MPO-12	70 m	100 m	100 m	—
400GBASE-SR8	16, MPO-16	70 m	100 m	100 m	—
800GBASE-SR8	16, MPO-16	60 m	100 m	100 m	—
400GBASE-SR4.2 (BiDi)	8, MPO-12	70 m	100 m	150 m	—
400GBASE-DR4 / 800GBASE-DR8	8 / 16, MPO APC	—	—	—	500 m
400GBASE-DR4-2 / 800GBASE-DR8-2	8 / 16, MPO APC	—	—	—	2 km
400GBASE-FR4 / 100GBASE-LR4	2, duplex LC	—	—	—	2 km / 10 km

Two patterns stand out. First, each new multimode speed has kept reach at about 100 m on OM4, while OM3 has dropped to 60–70 m. Second, OM5 only beats OM4 in the wavelength-multiplexed row. For parallel MPO optics such as SR4 and SR8, which send one wavelength per fiber, OM5 and OM4 have the same reach.

OM4 vs OM5: Which One for MPO?

OM5 was standardized in 2016 (TIA-492AAAE) for short-wavelength division multiplexing (SWDM): putting four or more wavelengths on one fiber to cut the fiber count. Applications such as 400GBASE-SR4.2 BiDi and SWDM4 transceivers use it, and on these optics OM5 adds distance, for example 150 m instead of 100 m.

Most high-speed multimode links in data centres take the other route: parallel optics on MPO. 100G SR4, 400G SR8 and 800G SR8 use one 850 nm wavelength on each of 8 or 16 fibers. Here OM5 behaves like OM4, so the extra 953 nm bandwidth goes unused. OM5 is typically priced above OM4.

A practical rule:

• Choose OM4 for multimode MPO trunks and cords serving SR4 and SR8 optics. It is the most widely deployed laser-optimized grade and supports about 100 m at 100G, 400G and 800G.
• Choose OM5 if your roadmap includes BiDi or SWDM multimode transceivers, or if site standards require it. OM5 is fully compatible with OM4 optics, so it is never a wrong choice, only a more expensive one when unused.
• Keep OM3 for extending existing OM3 plant, where links are short. For new builds, OM4 gives more margin for little extra cost.

When to Choose OS2 Single-Mode

Choose OS2 when any of these apply:

1. Distance is above about 100 m at 100G per lane, or the link crosses rooms, floors or buildings.
2. The optics are single-mode: 400G DR4, 800G DR4 and DR8, FR4 or LR4. Many hyperscale and AI clusters use single-mode DR optics even inside one hall, because one cable plant supports several speed generations.
3. You want a long-life backbone. Single-mode cabling is not limited by modal bandwidth, so the same trunk can carry future speeds as transceivers change.

On MPO, single-mode means APC 8° polish. Single-mode parallel optics use APC MPO ports, connectors are usually green and adapters must be key-up to key-down, as explained in our APC vs UPC guide. Multimode MPO-12 is normally flat polished, while many multimode MPO-16 ports for SR8 use APC, so check the transceiver datasheet.

Fiber Type on MPO Assemblies: Rules That Avoid Failures

• Never mix multimode and single-mode in one channel. A 50 µm core launched into a 9 µm core loses most of its light, and the connectors, polish and optics differ anyway.
• Do not mix 50 µm with 62.5 µm (OM1). The core mismatch adds loss in one direction.
• OM3, OM4 and OM5 share the same 50/125 µm geometry and can be connected, but the channel performs at the level of the lowest grade. Plan reach to the worst segment, and keep trunks, cassettes and patch cords the same grade where possible.
• Match fiber count to the optic. SR4 and DR4 light 8 fibers; SR8 and DR8 light 16. See our MPO fiber count guide and Base-8 vs Base-12 vs Base-16.
• Use jacket colour as a check, not proof. TIA-598 colours (aqua OM3/OM4, lime OM5, yellow single-mode) help in the field, but the printed jacket legend and the order record are what count.

Fiber type is independent of gender and polarity. Those are separate lines on the order; see male vs female MPO connectors and our MPO polarity guide.

Loss Budgets: Why Fiber Type Is Not the Only Limit

At 400G and 800G the channel loss allowance is small. IEEE allows about 1.8–1.9 dB total insertion loss for 400GBASE-SR8 and 800GBASE-SR8 over OM4, including the fiber and every connection. Multimode fiber loses about 3 dB per km at 850 nm, so 100 m of fiber costs only about 0.3 dB; the rest of the budget goes to connectors. Fiber grade sets reach, but the number and quality of MPO connections often decide whether a link passes. Huawellux MPO and MTP® assemblies are specified at insertion loss ≤0.35 dB for both multimode and single-mode, which helps keep several connections inside the budget. Every Huawellux product receives end-face visual inspection under a 400x video microscope and is tested for insertion loss (IL) and return loss (RL) before it ships. Interferometric end-face geometry (fiber height, coplanarity, end-face angle and radius) is checked by sample-based testing in standard production, and 100% interferometer testing of a batch is available on request. A formal test report is provided on request; state it in your RFQ. Keep connections clean.

How to Specify Fiber Type for an MPO Order

1. Start from the transceivers at both ends: SR4/SR8 (multimode), BiDi/SWDM (multimode, consider OM5) or DR/FR/LR (single-mode).
2. Measure the longest channel, including patching, and compare it with the reach table above, leaving margin.
3. Pick one grade per channel: OM4 for most multimode MPO, OM5 for WDM multimode, OS2 for single-mode.
4. Set the polish to match: OS2 MPO APC 8°; multimode MPO-12 normally flat; MPO-16 check the optic.
5. Write it on the order, for example "MPO-12 female, OM4, flat, Type B, 30 m" or "MPO-12 female, OS2, APC 8°, Type B, 60 m". Our trunk vs patch cable guide covers which assembly to use, and the cable estimator helps with lengths and quantities.

Related reading: fiber type and jacket rating are chosen separately; see OFNP vs OFNR vs LSZH for the jacket.

Beyond the data hall: for OS2 links between buildings, including outdoor cable and 2–10 km loss budgets, see OS2 trunks for DCI and campus links.

Related guides: Single-mode vs multimode fiber

Frequently Asked Questions

What is the difference between OM4 and OM5?

OM4 and OM5 are both 50/125 µm laser-optimized multimode fibers with the same 4700 MHz·km bandwidth at 850 nm. OM5 adds a guaranteed bandwidth at 953 nm, so it supports wavelength-multiplexed BiDi and SWDM optics over longer distances. For parallel MPO optics such as SR4 and SR8, OM4 and OM5 have the same reach.

Is OM5 worth it for MPO cabling?

Only if you plan to use BiDi or SWDM multimode transceivers or your site standard requires it. Parallel MPO optics use one wavelength per fiber, so OM5 gives no extra reach over OM4. OM5 is fully backward-compatible, but it is usually more expensive.

How far can OM4 run at 400G and 800G?

IEEE 802.3 specifies up to 100 m on OM4 for 400GBASE-SR8 and 800GBASE-SR8, compared with 70 m and 60 m on OM3. Real reach depends on the total channel loss, so the number and quality of connections matter.

When should I use OS2 instead of multimode?

Use OS2 single-mode for single-mode optics such as 400G DR4 and 800G DR8 (500 m), FR4 (2 km) and LR4 (10 km), for links longer than about 100 m, and for backbones that must support future speeds. Single-mode MPO connectors are APC 8° polished.

Can I mix OM3, OM4 and OM5 cables?

They share the same 50/125 µm geometry and can be connected, but the channel performs at the level of the lowest grade, so plan reach for the worst segment. Never connect multimode to single-mode fiber or 50 µm to 62.5 µm fiber.

Get MPO Assemblies in the Right Fiber Type

Huawellux builds fiber assemblies at our factory in Shenzhen to the fiber type your optics need: MTP/MPO trunk cables, MPO patch cables, breakout harness cables and standard fiber patch cords. We build MPO trunks and patch cords in OS2, OM3, OM4 and OM5, including violet-jacket OM4 for European projects and bend-insensitive fiber options, with genuine US Conec MTP® or standard MPO connectors. Tell us which transceivers sit at each end and the channel length, and our engineers will check fiber type, polish, gender and polarity before production.

Huawellux's specification for MPO and MTP® assemblies (identical for both) is insertion loss ≤0.35 dB for single-mode and multimode, with return loss >60 dB single-mode (OS2, APC) and >30 dB multimode (OM3/OM4/OM5). LC and SC patch cords are listed at insertion loss ≤0.35 dB and return loss ≥50 dB (single-mode UPC), ≥60 dB (single-mode APC) and >30 dB (multimode). Dust caps are fitted on every connector, and a formal test report is available on request.

Huawellux replies to inquiries within 24 hours. Custom orders typically take 1–5 business days to complete production, with final timing confirmed case by case in the quotation. For special/custom requirements, MOQ is 2 km of fiber length. Standard listed patch cords have no formal MOQ. The sample fee is credited against the bulk order.

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