POL Basics - Optical Fiber

One of the key distinguishing features of Passive Optical LAN (POL), compared to the traditional switch based LAN implementation is that almost all cabling in the LAN deployment is optical fibre.

For those who have been in a mostly copper based world for most of their career, I wanted to share a short introduction on optical fibre.

The type of fibre that is being used for POL is Single Mode Fibre (SMF), standardised in ITU-T G.652. It's called that way as there is only one mode allowed, the so-called transverse mode. However, multiple wavelengths are possible within the same fibre at the same time (which is the case for POL).

Just to by 100% clear, there is no Multi Mode Fiber (ITU-T G.651) used in POL.

The structure of a typical SMF as per the schematic above:

1. Core 8 – 9 µm diameter: this is where the optical signal is transmitted
2. Cladding 125 µm dia.: optical material to keep the optical signal within the core
3. Buffer 250 µm dia.
4. Jacket 900 µm dia.

Refraction and reflection

Due to the extreme small size of the core, the source of the optical signal for SMF is always a laser. Both core and cladding are optical material (basically made of glass), but with a different refraction index. This allows for the signal to remain within the core, the photons bounce off the cladding and are reflected back into the core. That is if the angle at which the signal hits the boundary between core and cladding is not too steep.

A practical way to illustrate this is by comparing it to when you look at a lake (when there is no wind and the water surface is perfectly still). During the morning and in the afternoon, you will find the lake to reflect the light in a way that makes it almost like a mirror. However, at noon, you will see the surface to be transparent and depending on the clarity of the water you can see under the surface. The air and the water in the lake have different refraction indexes. At noon, the light from the sun hits the water surface at a very steep angle and does not reflect off it.

This is Snell's Law, and it illustrated below:

Refraction of light at the interface between two media of different refractive indices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium is closer to the normal.

This means that bending an optical fiber beyond certain limits can create attenuation (more refraction and less reflection). This is referred to as bending-loss.

However, ITU-T standard G.657 has defined the characteristics of a type of SMF that is highly insensitive to bending-loss.

Wavelengths

As mentioned earlier, multiple wavelengths or colours of light can be transmitted over a SMF, at the same time. Wavelengths can be compared with frequencies over a copper cable. The wavelengths used in POL are in the Infrared spectrum, which means they are invisible to the human eye.

Connectors

The start and finish of an optical fiber will be a connector that will be inserted into a transceiver at the active equipment side. A transceiver, most typically in the form of an SFP (Small-Form-factor Pluggable unit) and contains both a transmitter (laser) and an optical receiver.

SFPs typically are attuned to specific wavelengths.

The connectors used in POL are SC or Squared Connectors. Two types are used:

UPC: Ultra Polished Connector, Blue colour

APC: Angled Polished Connector. Green colour; due to the angle, it generate less return loss

Warning: Be aware of which of the two types is required on which end of the SMF before deployment so you end up with the right equipment!

There are two ways to connect section of a fiber:

• connectors: used when the connection might have to be disconnected, which typically introduce a loss of around 0.3 dB
• splicing: used for permanent connections, the loss depends on the quality of the splice. With modern technique and done by an experienced installer, the loss is consider to be 0.1 dB (or less).

Below you can see an example of a device used for fusion splicing of optical fiber (from Wikipedia)

With the small size of the core, the most critical issue with splicing is that the cores of both ends are aligned perfectly to prevent any additional loss.

Benefits of optical fiber

These are the main benefits of optical fibre as compared to traditional copper based cabling:

1. Extremely high bandwidth

It's hard to pinpoint the limits of optical fiber when it comes to its capacity to transmit data. Since the first deployments of fiber-optic communication systems three decades ago, the capacity carried by a single-mode optical fiber has increased by a staggering 10 000 times.

The latter part of the 1990s saw dramatic increases in system capacity brought about through the use of wavelength division multiplexing (WDM) enabled by optical amplifiers. This technological revolution ignited massive investment in system development both from traditional vendors as well as new entrants, and the capacity of commercial lightwave systems increased from less than 100 Mb/s when they debuted in the 1970s to roughly 1 Tb/s by 2000 (Terabit per second = 10^12 bits per second).

WDM has been introduced in GPON recently (GPON is the base technology behind POL and will be discussed in the next article).

2. Smaller and Lighter Cables

SMF cables for installation (for installation in risers) typically come as multicore, with 12 being the minimum amount. This makes the relative weight and diameter per core even more advantageous. This has a positive impact on the structural cost of laying fiber. 

3. No crosstalk between parallel cables

Transmission of data on a copper cable is done by sending an electrical current over a copper wire. This creates an electro-magnetic field. One of the issues with having a bundle of copper cables is that the EM fields of the different cables interfere with each other, this is referred to as crosstalk.

Optical fiber however is made of glass and the data transmission is nothing more than laser light been sent through the core. There is no electro-magnetic field generated.

4. Immune to inductive interference

For the same reason as mentioned above, optical fiber is immune to external interference. This interference can come from power lines that run close to the data cables.

In the case of fiber optical, there is no issue to have both data cables and power cables in the same trench. The main reason they remain separated even today is since power lines can only be managed by a certified electrician.

5. High quality transmission

On top of being immune to any interference, there is little to no attenuation on a SMF, roughly 0.4dB per km of fiber. This in sharp contrast to the strict relationship between attenuation and the length of copper cables (let's not forget the limit of 100m).

6. Low installation (CAPEX )and operating costs (OPEX)

With the vast amount of SMF being produced today (which was helped by the many FTTH – Fiber To The Home roll-outs worldwide), the price has gone down to a point where it is at the same level or less than Cat 6 cables.

Installation time for a POL deployment compared to traditional cabling is less time consuming, which reduced the labour cost significantly.

In order to transmit data over a SMF, a much smaller amount of energy is required. Based on research done at the University of Melbourne, POL consumes over 50% less power per Ethernet port than traditional LAN.

References and standards

https://www.foa.org/ : The Fiber Optic Association Inc.

ITU-T G.652: describes the geometrical, mechanical and transmission attributes of a single-mode optical fibre and cable

https://www.itu.int/rec/T-REC-G.652

ITU-T G.657: Characteristics of a bending-loss insensitive single-mode optical fibre and cable.

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

https://en.wikipedia.org/wiki/Snell%27s_law

Capacity Trends in Optical Networks: https://www.icg.isy.liu.se/courses/optical/Capacity_Trends.pdf