A single-mode or multi-mode duplex fibre optic patch cable a simple enough item, commonly you’ll find these with an LC connector on the end.
What are the types?
The default type is typically a UPC (Ultra Physical Contact) type connector, if you were to ask a supplier for a fibre patch cable this is normally what you’d get. The other type is the APC (Angled Physical Contact) type connector.
With a UPC type connector when the insertion is made, both connectors but up to each other, at the interface light can reflect directly back down the fibre at the point of interface this means a higher return loss because some of the light is reflected back toward the transmitter.
With an APC type connector when the insertion is made the angle of the end (8 degrees) means at the point of insertion the light passing the interface and is also reflected but due to the angled end it is not reflected directly back down the fibre, meaning less interference and therefore a lower return loss. This makes the APC type connector more appealing for longer distance applications where the accumulation of loss over the distance and a number of interfaces (connections) eats into your budget of tolerable loss end to end.
There’s plenty of sites that will go into the real details of the physics of it but essentially if you are using a UPC connector you must be plugging this into another UPC connector to ensure both parts of the interface meet. The same is true for APC, that being said its not that it won’t work, you’ll just get greater loss than you should.
Colours are typically the identifier of either an APC or UPC type connector, but these can differ so best to check the cable markings (or ask the supplier if you are unsure), but typically the following is the case for Single Mode fibre optic cables:
Blue or White LC Connector = UPC (Ultra Physical Contact)
Green LC Connector = APC (Angled Physical Contact)
You can get cables that are UPC type connectors at both ends, or cables that are APC type connectors at both ends, you can also get cables that have UPC at one end and APC at the other for a situation where you are patching from an APC type connector to a UPC type or terminating into equipment where the transceiver is of UPC type.