Fibre Connector Polish is crucial, as all fibre optic connections require physical contact. It is at these contact points however that weaknesses and flaws show up that cause the loss of light. So, there are two types of loss that we need to understand; Optical return loss (ORL) and Insertion loss (IL).
Insertion loss in Connector Polish
Insertion loss is when a fibre optic component is inserted into one another. When an optic connector is joined or mated to another some loss of power will present itself. To clarify, this is called insertion loss and is expressed in dB. Refer to AS/NZS/IEC 14763 Standards for current maximum allowable insertion loss for a fibre optic connector.
The main reasons for the insertion loss are:
Absorption
Misalignment
Core mismatch
Air gaps
Optical Return Loss in Connector Polish
Optical Return Loss or back reflection is when light enters into a medium and meets the interface of two materials with different refractive indices and the light is reflected back up the fibre towards the source. This can be at a splice point, connector joins or at a patch panel. It may be fibre to air or fibre to fibre. ORL will always be an issue because of these reasons. measuring how well devices are matched and displayed in negative dB.
Above all, we are trying to ensure that the light source is not impacted by any of the reflected light. So we must be careful. LED systems will not be affected by reflections, this means multimode systems do not have to deal with this problem. But, any systems using Fabry-Perot (FP) and Distributed Feedback (DFB) laser for high precision optical fibre is affected by this. Typically, your Singlemode systems and a high ORL can cause the laser to stop transmitting correctly. This could cause downtime and possible loss of productivity.
We now have an understanding of why ORL is not desired; In short, here are the three types of connector polish styles that try to prevent a high Optical Return loss.
Physical Contact “PC”
Ultra Physical Contact “UPC”
Angled Physical Contact “APC”
Physical Contact Connectors
Physical Contact Connectors (PC Connector) were designed to have a spherical end face.
This reduced the size of the end-face which in turn decreased the air gap, allowing the fibres to come into contact.
This connector is required to achieve a return loss of < -35 dB
Ultra Physical Contact Connectors
Ultra Physical Contact Connectors (UPC Connectors) are designed with a domed shape, smaller end-face surface.
This resulted in even lower back reflections then the aforementioned PC Connectors.
This connector is required to achieve a return loss of < – 55dB
Angled Physical Contact Connectors
Angled Physical Contact Connectors (APC Connector) are designed with a small 8-degree angle to the end-face.
This results in optical high precision performance; As the 8° angle makes the fibre end face tighter.
Reflecting less light from the bevelled angle to cladding, therefore, providing better performance.
This connector is required to achieve a return loss of <- 65dB or higher
Meaning, the higher the return loss is, the lower the reflection and the better the connection performance will be. All of their Insertion Loss should be less than at least 0.25dB, the lower the insertion loss the better the result.
Mismatching and Colour Coding
Single mode fibre cables can have PC, UPC and APC connector polish, whilst multimode fibre connectors do not have APC as an option.
Another thing to note is that you should not pair APC with PC. You would end up with an air gap and the reflectance and directional loss through that connector could be as high as 5dB. An example of this is shown below.
That’s another reason why the connectors are colour coded green for APC and blue for the UPC, so we can see the connector polish and stop this from happening.
APC connectors are great at handling multi-play systems which is why they are deployed in optical passive networks such as a Fibre to the Home (FTTH) optical network.
Let’s have a quick look at an FTTH situation where you might have a 1-32 splitter where not all the connectors are plugged into a patch panel but are all lit (active).
Importance of Understanding Connector Polish
Fresnel reflections are even on the mated ports, but it is critical for them to be on the open ports. Attenuation would be coming upstream, it’s based on the distance as to how much it would affect you. One thing you need to know is if you’re sending an analogue or digital signal; If you’re sending an analogue signal over sensitive digital systems you might have to put terminators on the end of the ports.
Decreasing radiation of the cable signal and preventing possible ingress of strong signals to the cable plant. It really only becomes an issue under specific circumstances (usually something for the service provider to find a solution for).
An interesting fact about the attenuation of light through an optical splitter is that it is completely symmetrical. In other words, It’s identical in both directions. Regardless of if a splitter’s combining light upstream or dividing light downstream, still introducing equal attenuation to optical input signals.
With RF the laser source is an oscillator; Any light fed back into the source introduces noise, affecting the performance of the oscillating source. Therefore, APC connectors are used instead to stop reflected light going back into the laser.
If considering FTTH cross-connects panels, you must note that all connectors being installed are APC. The reason for this is that the service provider does not know what type of signal is going to be sent; For instance, if it will be analogue or digital, that’s the reason why they provide APC connectors, to handle either option.
To sum up, whether UPC or APC is chosen is completely dependent on your specific performance requirements.
