We all know there are single mode (SM) and multimode (MM) for optical fiber. So, what’s their differences, and which one to choose when I need a fiber optic cable? HOC (Hone Optical Communications) has 19+ years of experience on optical communication and ODN. Keep reading to know them once for all!

Optical Fiber

Fiber can be divided into single mode and multimode according to the mode of optical transmission.

Diameter

The core diameter of multimode fiber is 50 or 62.5μm, and it has an outer 125μm cladding. So, a multimode optical fiber is often described as 50/125μm or 62.5/125μm. The diameter of the core of single mode fiber is 9μm, outer diameter of cladding is 125μm. A single mode fiber is 9/125μm.

Working Wavelength

At present, the mostly working wavelengths of fiber are 850nm, 1310nm and 1550nm.

Optical loss of fiber is generally reduced with the increase of wavelength. The loss at 850nm is generally 2.5dB/km, at 1310nm is generally 0.35dB/km. While 1550nm has a generally 0.20dB/km optical loss, which is the lowest loss of optical fiber. Loss tends to increase with a wavelength of 1650nm and above.

Because the absorption of OHˉ (water peak), there are optical loss peak in the range of 900nm ~ 1300nm and 1340nm ~ 1520nm. Therefore, these 2 ranges are not fully utilized.

Bandwidth

Table 1: Application of multimode optical cable

Fiber Optical Cable

SM Cable

SM fiber has a very thin core, and only one mode of light can be transmitted. Therefore, the intermodal dispersion is very small, which is suitable for remote communication.

But there are also material dispersion and waveguide dispersion. As a result, single mode fiber has higher requirements for the spectral width and stability of the light source. That is, the spectral width should be narrow and with better stability.

It is found that the total dispersion of the single-mode fiber is zero at 1310nm. From the optical loss characteristics of an optical fiber, 1310nm is just a low loss window. Therefore, 1310nm wavelength region has become an ideal working window. And it is also the main working wavelength of practical optical communication system. The main parameters of 1310nm SM fiber are determined by ITU-T in G652 proposal, so this kind of fiber is also called G.652 fiber.

As mentioned above, due to the absorption of OH, there are optical loss peaks in the range of 900nm ~ 1300nm and 1340nm ~ 1520nm, which are called water peaks. At the same time, as G.652D is the latest index of SM fiber, it is the most stringent index of all G.652 levels and is fully downward compatible. So if only G.652 is specified, it means the performance specification of G.652A, which should be paid special attention to.

The SM fiber only transmits the main mode, that is, the light only transmits along the axis of the fiber, which completely avoids the dispersion and the waste of optical energy. Moreover, SM generally use laser with 1310nm or 1550nm wavelength, which is close to the minimum attenuation wavelength 1550nm of quartz. The transmission distance of SM cable is as shown in below Table 3.

Table 3: Application of multimode optical cable

Selection Suggestion

1. Transmission distance.

If you want to support 10 Gigabit transmission in the future, and it’s a long distance, you should consider using single-mode optical cable.

1. Cost.

Zero water peak optical fiber provides 50% more bandwidth than normal SM optical fiber, but the cost is similar.

MM Fiber Cable

Not like the SM fiber, a multimode (MM) fiber can transmit multiple modes of light at the same time. When multimode optical fiber transmits the main mode or several other modes, the light will continuously reflect along the wall of MM fiber.

However, this also causes a lot of dispersion and waste of optical energy. The large modal dispersion limits the frequency of digital signal transmission. With the distance expands, this situation will be more serious. The MM fiber transmission distance is shown in Table 1.

In the 10GB/s system, using an 850nm VCSEL (Vertical-cavity surface-emitting laser) as light source in high-speed network, OM3 fiber can transmit 300 meters and OM4 fiber can transmit 550 meters.

With the transmission distance of several hundred meters, OM3/OM4 multimode fiber can be widely used in data center, local area network, high performance computing center and storage area network. The IEEE (Institute of Electrical and Electronics Engineers) has proposed parallel solutions for 40GB/s and 100GB/s networks using OM3/OM4 multimode fiber.

The parallel system with OM3 fiber can transmit 100m in 40GB/s and 100GB/s, and parallel system with OM4 fiber can transmit 150m in 40GB/s and 100GB/s. While 85% of the data center connection length doesn’t exceed 100 meters, and almost 100% of the data center connection length will not exceed 150m.

Table 2: Application of multimode optical cable

Another question you might want to ask is that why can’t OM3/OM4 multimode fiber support the same connection length in 40GB/s and 100GB/s as in 10GB/s systems?

This is because the technical specifications of VCSEL are relaxed in 40GB/s and 100GB/s connection. In order to reduce the system cost, IEEE802.3ba standard relaxed the requirement of laser spectrum width, but at the same time it also comes the price of reducing in the network connection length.

Selection Suggestion

Therefore, if you want to choose multimode optical cable, following aspects should be considered:

1. Future Development

In terms of the future development trend, the horizontal cabling network needs 1GB/s bandwidth to the desktop, backbone network of building needs to be upgraded to 10GB/s, and the industrial park backbone network needs to be upgraded to 10GB/s or 100GB/s.

At present, network applications are growing at an annual rate of about 50%. It is estimated that in the next five years, Gigabit to desktop will become as common as the current 100M bandwidth to desktop.

Therefore, the current network planning should be forward-looking. CAT6 network cable can be considered for the horizontal part, and 10 Gigabit multimode optical cable should be considered for the trunk part. In particular, the cost difference between CAT6 copper cable plus 10 Gigabit multimode optical cable and CAT5E copper cable plus 1 Gigabit multimode optical cable is less than 10-20%. From the perspective of long-term application, if the cost is allowed, CAT6 network cable plus 10 Gigabit multimode optical cable should be considered.

1. Invest Perspective

From the perspective of investment, where 10G will not be used in at least 10 years, HOC OM1 (the normal multimode 62.5/125μm fiber) is a good choice. In addition, because OM3 optical cable uses low-cost VCSEL and 850nm optical source equipment, the cost of 10 Gigabit transmission is greatly reduced.

Conclusion

Based on the above analysis, we believe that you should invest in the best performance at the lowest costs from the consideration of application, transmission distance, foresight, and cost.

Now you would have a general idea difference between single mode and multimode, as well as what the optical fiber you want for your project. If you have other doubts on optical communications, make sure you leave us a comment or write to us directly.