The diverse world of fibre optic connectors!
Connectors offer a mechanical means to terminate optical cables. In other words, they make the connection to other fibres and active devices. The connector’s main role is to hold the end of the fibre precisely in place. By doing this it ensures the maximum light transfer happens.
Fibre optic connectors are referred to as male connectors. This is due to the protruding ferrule that is used to hold and align the fibres.
Ceramic ferrules were introduced in the mid-80s. They were designed to make alignment even more precise. You could now hold the ferrule as it was robust. The ferrule design was a 2.5mm ceramic cylinder which allowed for mix and matching between connectors with that same ferrule size. The most common examples of these are ST, SC, and FC connectors.
Most importantly, this ceramic ferrule design allowed us to connect directly to active devices like LEDs, VCSELs, and detectors.
A curved ferrule end face for physical contact between connectors reduced losses. Most importantly providing loss levels below 0.3 decibels for both Multimode and Singlemode varieties. For more information on polish facts visit this blog!
What does colour have to do with connectors?
Coloured boots used from very early on helped differentiate between multimode fibre and Singlemode fibre. Beige for multimode. Blue for Singlemode connectors and Green for Angled Physical contact connectors.
We also use different colour jackets in Fibre Optic Patch Cables to distinguish the various cable types available.
- Orange cable for Multimode OM1
- Aqua cable for Multimode OM3
- Erika Violet for Multimode OM4
- Lime Green for Multimode OM5
- Yellow for Singlemode.
Most Common connectors
Let’s examine the most common fibre optic connectors.
ST, Straight tip, or Standard Turn Connector (an AT&T Trademark). This connector meets IEC 61754-2 standards. It is a fibre optic connector that uses a half-twist bayonet type of lock. It has a 2.5mm keyed cylindrical ceramic ferrule. The ST connector is spring-loaded for easy mating.
SC, Standard Connector, Square Connector, Standard Click, or also referred to as a Subscriber Connector. This connector meets IEC 61754-4 standards. It is a snap-in fibre optic connector that uses a push-pull latching mechanism. Similar to common audio and video cables. It has a 2.5mm ceramic ferrule and can be mixed with any other 2.5mm ferrule using a hybrid adaptor.
FC, Field assembly connector, Fibre Channel or Ferrule Connector. This connector meets IEC 61754-13 standards. It was the most popular Singlemode connector for many years. The FC connector has a threaded body which is particularly good for high-vibration locations. You can mate it to any 2.5mm ferrule. It screws on firmly and is still provided on test and measurement equipment. SCs and LCs are more popular.
LC, Lucent Connector, Little connector, or Local connector, This connector meets IEC 61754-20 standards. It is a small form factor connector that uses a 1.25mm ferrule. This is half the size of an SC connector. The small size and latch feature make it highly suited for high-density connections and SFP transceiver modules.
MPO Connectors/MTP Connectors
MPO stands for Multi Fibre Push-on and MTP stands for Multi Fibre Termination Push-on. US Conec developed and trademarked the MTP version of the MPO connector.
MPO is a specific interface type that is defined by IEC 61754-7. This connector is a multiple fibre core connector.
These connectors are interchangeable with some of the main differences being that;
- MTP has a removable housing allowing for quick gender change and re-polishing if required.
- The MTP guide pinhole can handle approximately 500 matings before deterioration compared to 50 matings for an MPO connector.
- An MTP has a floating ferrule allowing two mated ferrules to maintain physical contact while under an applied load.
This type of connector combines lots of fibres in one connector. Which means a reduction in the time it takes to connect fibres. Whilst also saving a lot of space in the process. 8, 12 and 24 fibre MPO/MTP connectors are used in today’s communication and data-centre industry.
MTP/MPO connectors are a pin and socket style connector which requires a male side and a female side.
You can identify a male connector by the two alignment pins protruding from the end of the ferrule. The MPO female connectors have holes in the ferrule to accept the alignment pins from the male connector. Male connectors are used at endpoints. Such as in a fibre patch panel or fibre cassette for re-distribution.
Not only do we need to know if the MPO/MTP connector is male or female we use the term polarity with MPO/MTP connectors as well. The positioning of the fibres inside the cable determines what the polarity will be.
There are 3 polarities to consider when designing a network;
1. Type-A MPO Trunk Cable:
Type-A cable has a key up MPO connector on one end and a key down MPO connector on the opposite end. This makes the fibres at each end of the cable have the same fibre position. The following picture shows the fibre sequence of a 12 fibre Type-A cable.
2. Type-B MPO Trunk Cable:
Type-B cable (reversed cable) uses key up connector on both ends of the cable. This type of array mating results in an inversion. Fibre positions are reversed at each end. The following picture shows the fibre sequences of a 12 fibre Type-B cable.
3. Type-C MPO Trunk Cable:
Type-C cable (pairs flipped cable) looks like a Type-A cable with one key up connector and one key down connector on each side. Fibres at one end are flipped at the other end. The following picture shows the fibre sequence of 12 fibre Type-C cable.
Why do we have all these different options?
There is no debate about which of these options is the better one. The reason we have all these different choices is that manufacturers have diverse approaches. Providing many different solutions. Thus, it is merely a matter of which of these options will work the best in the solution chosen.
You fix the Polarity when manufactured. You can then no longer change it. All components within the network will have a component polarity; patch cords (A-B or A-A), MPO cassettes (A, B or C), and MPO trunks (A, B or C). As a result of this, for the link to work correctly all these component polarities must be correct!
In conclusion, You need Fibre Optic Connectors to connect to other fibres and active devices. There are two main Ferrule sizes; one with an OD size φ2.5mm and the other with a Ferrule OD size of φ1.25 mm. Whilst there are many different types of fibre connectors, they still share similar design characteristics. Some are legacy connectors and still required, others provide the latest technology with extremely low losses and back reflections!