Singlemode refers to light rays traversing the same path through the length of a singlemode fiber cable, also known as the fundamental mode. Singlemode fiber can be further classified as OS1 and OS2 singlemode fiber. This article will compare OS1 with OS2 singlemode fiber, list the pros and cons, and illustrate why we recommend you to replace OS1 with OS2 singlemode fiber cable.
Overview of singlemode fiber cable
Singlemode fiber dispersion occurs because of group velocity dispersion (GVD)—a combination of material and waveguide dispersion. Typically, total dispersion is zero near 1320 nm wavelength.
Signal attenuation in optical fibers occurs due to material absorption, scattering, bending, etc. The minimum loss in silica fiber is around 1550 nm wavelength.
Single-mode fibers have greater bandwidth and lower losses, while multimode fibers are cheaper and easier to use.
Figure 1: singlemode fiber ITU/TIA specification
Figure Credit: Tarsem Lal Singal, Optical Fiber Communications: Principles and Applications.
Advantages of Single-mode Step-index Fibers
- Minimum dispersion: All light rays traveling within the single-mode step-index fiber traverse almost the same path, arriving simultaneously.
- Higher information transmission rates: As a result, it is possible to obtain higher information transmission rates (bps) due to available wider bandwidths.
Disadvantages of Singlemode Step-index Fibers
- Singlemode fiber core size is tiny, which implies that the optical source-to-fiber aperture is significantly less. Thus, the coupling of light becomes quite challenging.
- Due to the tiny fiber core size, a highly focused optical source such as a laser for coupling light into a singlemode step-index fiber has to be used.
- The manufacturing process of singlemode step-index fibers is quite complex.
- Singlemode step-index fibers are relatively expensive.
What are OS1 and OS2 singlemode fiber cables?
As shown in the figure above, OS1 is designed for indoor applications and was installed mainly in the 1980s. It is tight buffered, meaning the fiber is manufactured into a solid medium, coated with a bundle of flexible fibrous polymer strength members like aramid, suitable for data centers, campuses, etc. OS2 is intended both for indoor and outdoor applications. It is loose-tubed, meaning the fiber is not held in medium but blown or pushed into a duct.
The difference between OS1 and OS2
Besides the already mentioned cable construction difference, there’re other aspects to compare.
Large vs. small attenuation
OS1 and OS2 are standard singlemode fiber optic cables. OS1 has a maximum attenuation of 1dB/km for 1310nm and 1550nm, whereas OS2 has a maximum attenuation of 0.4dB/km.
OS1 can support up to 10 km effective transmission distance, while OS2 can support up to 200 km effective transmission distance.
Legacy vs. standard
OS1 cable can work at 1310nm and 1550nm wavelengths. Therefore, applications using OS1 can accommodate OS2 with ITU-T G.652D and G.657A1 specifications.
OS1 is already a legacy singlemode fiber cable; meantime, OS2 is de facto the standard singlemode fiber cable.
OS1 complies with ITU-T G.652, including G.652A, G.652B(conventional), G.652C, and G.652D(low-water-peak). However, OS2 only conforms to ITU-T G.652C or G.652D standards developed to reduce the water peak at the 1383nm wavelength range and use an additional operating E-band window.
The Singlemode fiber(SMF) attenuation coefficients comparison of the four ITU-T G.652 standards can be seen in the figure below.
The G.657.A1 standard is the de facto standard for bend-insensitive singlemode fiber cable for premises cabling with optimized performance and cost balance. Many OS2 cables are compliant with G.652D and G.657A1 standards.
Our Recommendation: choose OS2, not OS1.
When it comes to system upgrades, though data center managers are unwilling to invest capital in new OS2 cables and are inclined to reuse the existing OS1 cabling system, a wise decision providing long-term sustainability and system scalability has to be made.
As stated previously, OS1 is already legacy and should not be used as the default SMF cable; otherwise, you are at potential risk:
- OS1 has limited bandwidth capacity compared to OS2, as it cannot operate at 1383nm wavelength; on the contrary, OS2 supports all wavelength bands, from 1260nm to 1625nm.
- OS1 has a much higher attenuation coefficient than OS2, or more specifically, the coefficient is <1.0 dB/km vs. <0.4 dB/km.
Yet, OS1 also has its advantage, especially for price-sensitive users. It is much cheaper than OS2.
So, which one to select? OS1 or OS2?
In a word, if your budget permits, always choose a new bend-insensitive OS2 (ITU-T G.657A1) SMF cabling system. It will perform much better than legacy OS1 and is ideal for connecting 10G/40G/100G Ethernet connections, meeting high-performance requirements with longer distances.
However, if you are short of budget, you can deploy OS1 for indoor applications like internal buildings and campuses where transmission distance is beyond the reach of multimode fiber cables. If your applications only require a data transmission distance of less than several hundred meters, then multimode is always the way to go.
Tarsem Lal Singal, Optical Fiber Communications: Principles and Applications (Delhi, India: Cambridge University Press, 2016)