What’s the difference between OM3, OM4 and OM5 Multimode Fiber?

We understand all the advantages we can get from using Fiber Optics as means for data transmission such as low attenuation, infinite bandwidth, and non EMI/RFI … etc.

For Long Distance (Kilometers) we choose for Singlemode Fibers and using Lasers as transmitter whereas for building networks (meters) we go for Multimode Fibers that uses VCSEL transmitter.

Multimode optical fiber accepts many modes of light because it has a large numerical aperture.  It supports wavelength of 850nm and 1300nm.

Singlemode optical fiber accepts only the axial mode of light for propagation and it has small core diameter of about 7 to 9 micron.  There is no modal dispersion and hence this provides the largest bandwidth and ability to go the longest distance.

OM3, OM4 and OM5 are all laser optimized multimode fiber.

OM3 fiber was introduced way back in 1999 and it supports 300 meters for 10 Gb/s Ethernet. OM3 fiber with and effective modal bandwidth of 2000MHz/Km

OM4 fiber can support 550 meters for 10 Gb/s Ethernet with effective modal bandwidth of 4700MHz/Km

For 100 Gb/s Ethernet, OM3 can support 100 meters while OM4 is 125 meters.

Why OM5?

OM5 is also known as WBMMF –  Wideband multimode fiber.  

This means that OM5 can support WDM whereas OM3 and OM4 are for supporting only 1 wavelength at 850nm.

WDM is short for wavelength-division multiplexing.  WDM is technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light.  This enables 4 different transceivers to be multiplex into single fiber for Tx and another fiber for Rx. 

Next generations Terabit Ethernet (TbE)  200 and 400 Gigabit Ethernet uses WDM for multimode fiber and may use OM5 which has a band of 850nm to 953nm for the wavelength of transmitters.

Here is a look at WDM:

WDM had been applied extensively in singlemode fiber optics cables.

Basic (BWDM), Coarse (CWDM) and Dense (DWDM) are different methods employed for singlemode fiber optical systems.

BWDM uses 1310nm and 1550nm on 1 fiber.

CWDM provides 16 channels over 1530nm to 1565nm while DWDM can provide 40 channels using 100Ghz spacing or 80 channels using 50 GHz spacing.

WDM can be applied in Multimode fiber optics cables which can support 4 channels using 30nm wavelength spacing from 850nm to 953nm.

This is the new OM5 specified Multimode fiber optics cables,

OM5 Multimode fiber optics cables can support multiple ‘short’ wavelengths of between 850nm and 953nm that can be aggregated for higher bandwidth applications.  For 4 wavelength of light that can transmit 25Gps will results in 100Gps in a single fiber. Using a pair of fiber for DUPLEX therefore will allow the 100Gps transmission using only 2 fibers compared with 20 fibers via the 10Gbps X 10 lanes also known as Parallel Optics.

OM3 and OM4 is primarily designed to support a single wavelength of 850nm.

OM5 also has core of 50um and cladding of 125um which is the same as OM3 and OM4 and hence are backward compatible to these fibers.

For any applications / Systems operating at a single 850nm wavelength OM5 fiber provides no greater bandwidth than OM4.