Tag Archives: single-mode fibre

Optical Fibre Selection for Network Interconnection

The emergence of Data Centres, Setorage Area Networks and other computing applications drives the needs for ultra-high speed data interconnections and structured cabling. The interconnect media choices include wireless technology, copper cable and optical fibre cable. Fibre cable offers the highest bandwidth and supports the highest data rates. There are single-mode and multimode fibre types. Different types of fibre connect with fibre optic transceivers resulting in different performances and costs. So it’s important for the network designers to understand the fibre types and select the right fibre and corresponding fibre optic transceivers for network interconnection.

Optical Fibre Types

There are three main types of optical efibre suitable for network interconnection use:
9/125μm Single-mode fibre
50/125μm multimode fibre
62.5/125μm multimode fibre

optical-fiber-types

The above numbers respectively mean the diametre of the glass core where the light travels and outside glass cladding diamere which is almost the same to most fibre types. So the difference of each fibre type is caused by the core diametre. It has great impact on system performance and system cost when balanced against network application needs. Two primary affected factors are attenuation and bandwidth.

Factors Affected by the Fibre Core Diametre

Attenuation is the reduction of signal power, or loss, as light travels through an optical fibre. Fibre attenuation is measured in decibels per kilometre (dB/km). The higher the attenuation, the higher rate of signal loss of a given fibre length. Single-mode fibres generally operate at 1310 nm (for short range) while multimode fibres operate at 850 nm or 1300 nm. Attenuation is not usually considered to be the main limiting factor in short rang transmissions. But it can cause big differences in high speed network such as 100Gb/s.

Bandwidth means the carrying capacity of fibre. For single-mode fibre, the modal dispersion can be ignored since its small core diametre. Bandwidth behavior of multimode fibres is caused by multi-modal dispersion during the light traveling along different paths in the core of the fibre. It has an influence on the system performance and data rate handling. Multimode fibre uses a graded index profile to minimize modal dispersion. This design maximizes bandwidth while maintaining larger core diametres for simplified assembly, connectivity and low cost. So manufacturers start to develop higher-performance multimode fibre systems with higher bandwidth.

System Costs: Single-mode and Multimode Fibres

A fibre optic transceiver usually consists the optical light sources, typically LED–light emitting diode and optical receivers. Since the core diametre size and primary operating wavelengths of single-mode fibre and multimode fibre are different, the associated transceiver technology and connectivity will also be different. So is the system cost.

To utilize the single-mode fibres generally for long distance applications (multi-kilometre reach), transceivers with lasers such as SFPP-10GE-LR (an SFP+ 1310nm 10 km transceiver supporting single-mode fibres) that operate at longer wavelengths with smaller spot-size and narrower spectral width. But these kinds of transceivers need higher precision alignment and tighter connector tolerance to smaller core diametres. Thus, it causes higher costs for single-mode fibre interconnections. To lower the cost, manufacturers produce transceivers based on VCSEL (vertical cavity surface emitting laser), for example, 10G-SFPP-SR (an SFP+ 850nm 300m transceiver supporting multimode fibres), which are optimised for use with multimode fibres. Transceivers applying low cost VCSEL technology to develop for 50/125μm multimode fibres, take advantage of the larger core diametre to gain high coupling efficiency and wider geometrical tolerances. OM3 and OM4 multimode fibres offer high bandwidth to support data rates from 10Mb/s to 100Gb/s.

Conclusion

Optical fibre is an easily-installed medium that is immune to electromagnetic interface and is also more efficient in terms of power consumption. What’s more, fibre optic cable can save space and cost with higher cabling density and port density over copper cabling. For single-mode fibre and multimode fibre, each one has its advantages and disadvantages. Network designers should better select the right fibre type and related fibre optic transceivers according to specific situations for higher system performance. Of course, cost is another important factor to be considered.

Link Budget Evaluation Over SMF and MMF

Evaluating a link budget is equivalent to calculating the total loss suffered by a transmitted signal along fibre channels with the minimum receiver power to maintain normal operation. Calculating the link budget helps network architects to identify the feasibility of a physical-layer deployment.

Optical fibres come in several different configurations, each ideally suited to a different use or application. Early fibre designs that are still used today include single-mode fibre (SMF) and multimode fibre (MMF). And the most common optical communication data links include point-to-point transmission, WDM and amplified transmission. This article depicts the rules to be applied in order to evaluate link budget of these optical transmissions over SMF and MMF.

Link Budget for Point-to-Point Transmissions over Multimode Fibres

In this first case, the rule is fairly simple. A few parametres need to be taken into account:
● The minimum transmit power guaranteed (minTx), expressed in dBm
● The minimum receive power required (minRx), expressed in dBm
● The loss of optical connectors and adapters (L), expressed in dB
● The number of connectors and adapters (n)
● The normalized fibre loss (FL), expressed in dB/km
● The reach or distance to be achieved (d), expressed in km
With these parametres, the link budget (LB) expressed in dB is given as follows:
● (LB) = (minTx) – (minRx)
This value needs to be compared to the total loss (TL) suffered by the transmitted signal along the given link, and expressed in dB:
● (TL) = n*(L) + d*(FL)
If LB is greater than TL, then the physical deployment is theoretically possible.
In these calculations n is at least equal to 2 since there are a minimum of 2 connectors at each end, L is typically around 0.5 to 1 dB, and FL is of about 1 to 1.5 dB per km.

Link Budget for Point-to-Point Transmissions over Single-mode Fibres

At first, you need to know that the lasers deployed in optical communications typically operate at or around 850 nm (first window), 1310 nm (second window), and 1550 nm (third and fourth windows). In this second case, the calculations are exactly similar to the previous case. Only the numerical values will differ. For single-mode point-to-point transmissions, n is at least equal to 2, L is typically around 0.3 to 0.5 dB, and FL is of about 0.4 dB per km in the second window and about 0.25 dB per km in the third window.

The following drawings show the power budget of a 2km hybrid multimode/singlemode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle.

power budget

Link Budget for WDM and Amplified Transmissions over Single-mode Fibres

In the case of WDM transmissions, passive modules are used to multiplex and demultiplex various wavelengths respectively before and after the signal propagates along the fibre channel. These passive modules introduce additional insertion losses suffered by the signal transmitted.

Additionally, the signal may be amplified and compensated for dispersion, and in this case, the amplifier gain and the dispersion compensation unit’s loss need to be taken into account. Dispersion and OSNR (optical signal noise ratio) penalties suffered by the receiver shall be considered as well.

Therefore all the parametres needed for a proper link budget evaluation are:
● The minimum transmit power guaranteed (minTx), expressed in dB/m
● The minimum receive power required (minRx), expressed in dB/m
● The loss of optical connectors and adapters (L), expressed in dB
● The number of connectors and adapters (n)
● The normalized fibre loss (FL), expressed in dB/km
● The reach or distance to be achieved (d), expressed in km
● The loss of passive add/drop modules (A and D), expressed in dB
● The gain of the amplifier (G), expressed in dB
● The penalty suffered by the receiver (P), expressed in dB
● The loss of a dispersion compensation unit (DCU), expressed in dB With these parametres, (LB) is given as for previous cases:
● (LB) = (minTx) – (minRx)
And the total loss is expressed as follows:
● (TL) = n*(L) + d*(FL) + (A) + (D) – (G) + (DCU) + (P)
Here again, if LB is greater than TL, then the physical deployment is feasible. Please note that for simplicity, only one amplifier, one dispersion compensation unit, and one set of add/drop modules are considered in this example. If more devices are planned to be deployed, their loss or gain should be added or subtracted accordingly in order to calculate TL.

Link budget is a way of quantifying the link performance. And the performance of any communication link depends on the quality of the equipment being used. Thus, when evaluating a link budget, you are supposed to consider the types of applications, the reach to be achieved, as well as the types of optical fibres deployed. For more information about fibre optical link products, please visit FS.COM.