What is an optical connector? A simple explanation of the basics of structure, types, and polishing methods

Optical connectors are important connection components that support high-speed, large-capacity optical communications, and play a role in transmitting optical signals efficiently and stably. Because they are extremely precise components that determine the quality of optical communications, understanding their types, structures, and handling methods is essential when working with optical communications systems.

This article explains the basic structure, types and features of typical single-core and multi-core connectors. In addition, we will introduce in detail the differences and characteristics of PC/UPC/APC polishing, which is an important factor that affects communication quality, as well as the polishing procedures for single-core and multi-core. We hope you will find this article useful in deepening your knowledge of optical connectors.

What is an optical connector?

Optical connectors are important components that precisely connect optical fibers and transmit optical signals with as little loss as possible.

Its major features are that it is less susceptible to electromagnetic interference and can handle high-speed, large-volume communications. It can handle a wider frequency band than conventional metal cables, making it suitable for large-volume communications such as video data and cloud services.

However, even a slight misalignment of the connection surface can cause light leakage or reflection, reducing communication quality. For this reason, the processing precision of the part called the ferrule and the finish of the polishing are extremely important.

If installed correctly, it can achieve highly reliable optical transmission in a wide range of applications, from large-scale networks to internal wiring in medical devices.

Basic structure of optical connector

Optical connectors are precision components that protect the tips of optical fibers and connect them in the correct position, and are primarily made up of three main parts: the ferrule, the connector body, and the mating mechanism.

These components work together to provide a stable connection while minimizing optical signal loss.

Let's take a closer look at each of these components below.

• Ferrules
• Connector body
• Interlocking mechanism

Ferrules

The ferrule is the heart of an optical connector, and its role is to precisely secure and protect the tip of the optical fiber.

Zirconia ceramic is commonly used as a material, due to its high dimensional precision, excellent mechanical strength, and stability against temperature changes.

This is because the optical fiber core (the central part through which the optical signal is transmitted) is extremely thin and even the slightest misalignment can result in significant optical loss, so the fiber must be held with extremely high precision.

The tip of the ferrule is precision ground into a spherical shape to ensure accurate alignment of the optical fibers.

The precision of this polishing is a key factor that greatly affects performance such as connection loss and return loss of optical connectors.

Connector body

The connector body serves to hold the ferrule and optical fiber together as a single unit. In many cases, the exterior is designed to protect the inside from impacts and at the same time ensure the bending radius of the cable.

Materials such as engineering plastics and metals are used depending on the application and required strength.

The connector body usually has a key (protrusion) or key groove to prevent connection in the incorrect orientation, and is designed so that it can only be mated in the correct orientation.

In addition, some connectors, such as the SC connector and LC connector, have an outer part called the housing that slides or is equipped with a latch mechanism, which enables both a secure connection and easy removal.

The design of the connector body can greatly affect ease of handling and connection reliability.

Interlocking mechanism

The mating mechanism is a mechanism for physically connecting and fixing optical connectors together or between an optical connector and an adapter on the equipment side. This mechanism ensures that the optical fibers at the tips of the ferrules are accurately butted together, maintaining a stable optical connection.

Typical mating methods include the "push-pull method" used in SC and LC connectors, the "screw method" used in FC connectors, and the "bayonet method" used in ST connectors.

The push-pull method is easy to use, since it locks in place just by pushing in the connector and can be easily removed by pulling the housing. The screw method secures the connector by tightening it with a screw, so it is resistant to vibration and provides a stable connection. The bayonet method secures the connector by pushing it in and rotating it.

These mating mechanisms differ depending on the type of optical connector, and the usage environment and application are taken into consideration according to the characteristics of each. In general, a certain spring pressure is applied to the mating mechanism so that the core wires of the optical fibers face each other firmly.

This pressure seals the ferrules together, minimizing light loss. If the mating area is dirty, the spring pressure can be uneven, so it is essential to keep the area around the connector clean.

Types and features of optical connectors

There are many different types of optical connectors, differing by the number of optical fiber cores to be connected, the mating method, and the polishing method.

Each connector is designed for specific applications and environmental conditions, and selecting the right connector is key to maximizing the performance of your optical communications system.

Here, we will look at the features of typical single-core and multi-core connectors.

<Typical single-core connectors>

• SC Connector
• FC Connector
• ST Connector
• LC Connector

<Typical multi-core connectors>

• MPO/MTP Connectors

Representative single-core connectors

A single-fiber connector is an optical connector designed to connect a single fiber optic cable.

Because its structure is relatively simple and easy to handle, it is used for a wide range of purposes, such as LAN (Local Area Network) wiring, FTTH (Fiber To The Home) in-house wiring, and connecting various optical communication devices.

In terms of physical characteristics of single-fiber connectors, the ferrule diameter is generally 2.5mm or 1.25mm, and they can be classified as SC connectors, FC connectors, and ST connectors being 2.5mm, and LC connectors being 1.25mm.

They have different characteristics such as connection stability, ease of connection and disconnection, and durability, and can be used for different purposes. Understanding the advantages and disadvantages of each makes it easier to make the optimal choice in network design.

SC Connector

The SC connector is a single-core optical connector that has a square housing and mates using the push-pull method.

Developed by NTT, it is widely used in optical communication systems such as in-house cabling, FTTH (Fiber To The Home), and CATV (cable television) due to its excellent operability and reliability.

The connector can be easily connected by simply pushing it in, and can be easily removed by pulling the housing, making it easy to work with. In addition, the connection density is relatively high, making it used in many devices and panels.

On the other hand, since it is slightly larger in size than the LC connector, LC connectors tend to be chosen in situations where higher density mounting is required.

FC Connector

The FC connector is a single-core optical connector with a circular metal housing and a screw-type mating mechanism.

Since it is fixed securely with screws, it is resistant to vibration and shock and is suitable for environments that require stable connection characteristics. It is a long-established connector that has been around since optical fiber communications first began to spread, and is often seen in the outdoor facilities of telecommunications carriers.

The ferrule has high positioning accuracy, and the connection direction is uniquely determined by the key (protrusion), allowing for stable, low-loss connections. However, since it is difficult to attach and remove, it is not very suitable for environments that require frequent maintenance. The peace of mind that comes from knowing that it will be firmly fixed once attached is a major advantage.

ST Connector

The ST connector is a single-core optical connector with a circular housing and a bayonet locking mechanism (a method of pushing in and rotating to secure).

It has been mainly used in LAN (Local Area Network) systems using multimode fiber, industrial networks, etc. The bayonet method allows for relatively easy attachment and detachment, while still providing a reliable connection.

However, since the appearance of SC and LC connectors, the number of opportunities for adoption in new systems has been declining. This is because SC connectors are easier to operate, while LC connectors are smaller and more suitable for high-density mounting. Currently, they are often used for limited purposes such as maintenance of existing facilities and specific industrial equipment.

LC Connector

The LC connector is a single-core optical connector that is approximately half the size of an SC connector and features a push-pull mating mechanism.

Developed by Lucent Technologies (now Nokia), its small size makes it widely used for connections inside data centers and telecommunications equipment, where high-density packaging is required.

It is also used as a standard in optical modules such as SFP (Small Form-factor Pluggable) transceivers, and plays a very important role in modern optical communication systems.

Its greatest feature is that it is small yet has the same excellent optical performance and ease of use as an SC connector. This contributes to the miniaturization of devices and an increase in the number of ports, making it possible to improve the integration level of the entire communication system.

Representative multi-core connectors

A multi-fiber connector is an optical connector that can connect multiple optical fibers together at once.

Because it can greatly improve the efficiency of cable wiring work, it is increasingly being used in trunk wiring in data centers that handle many optical fibers, and inside communication equipment that requires high-density implementation, etc. Compared to using multiple single-core connectors, it has the great advantage of significantly reducing the time and space required for connections.

Representative examples of multi-fiber connectors are MPO and MTP.

The MPO (Multi-fiber Push-On) connector is a multi-fiber connector that can connect multiple optical fibers (usually 4-fiber, 8-fiber, 12-fiber, 16-fiber, 24-fiber, 32-fiber, etc.) arranged in a ribbon shape together.

The connectors have a push-pull mating mechanism and can achieve high-density connections in a small space, making them indispensable for high-speed, large-capacity communications between servers and switches in data centers in particular.

The MTP connector is a high-performance version of the MPO connector manufactured by USConec, with more precise components and construction providing superior optical properties and mechanical durability.

These connectors are equipped with keys to prevent incorrect connection and guide pins (attached to the male connector) for accurate alignment.While they contribute greatly to the efficiency and high density of wiring work, they require more care than single-core connectors when handling and cleaning.

How to polish optical connectors

The polishing method of the ferrule end face is a very important factor that determines the performance of optical connectors, especially the connection loss and return loss. The transmission efficiency of optical signals changes greatly depending on how precisely and without gaps the tips of the optical fibers are brought into contact with each other.

There are several types of polishing methods, each of which produces different end face shapes and characteristics.
There are three main polishing methods:

● PC polishing
● UPC polishing
● APC polishing

PC polishing

PC polishing is a basic polishing method aimed at physically contacting the ferrule end faces of optical connectors. It is designed to ensure that the tips of the optical fibers make contact by polishing the tip of the ferrule into a slightly convex spherical shape.

This spherical shape allows the center of the fibers to make contact first and exert pressure on the periphery, preventing air pockets from forming between the fibers.

It has a proven track record in many general communication environments, and its merit is that it is relatively inexpensive and easy to implement. For short distances or less strict requirements for reflection characteristics, PC polishing performance is often sufficient. Since it is offered as a standard product by various connector manufacturers, it is widely distributed and is an easy-to-obtain polishing shape.

UPC polishing

UPC polishing is an advanced polishing method that improves on PC polishing to achieve even lower return loss (the ability to reduce light reflection).

The basic principle is the same as PC polishing, in that the ferrule end face is polished into a convex spherical shape to create physical contact, but more precise polishing technology and surface finishing create a smoother, more uniform spherical surface. This improves the adhesion between the optical fibers, and the return loss is significantly improved compared to PC polishing, typically achieving an excellent value of -50dB or better.

UPC polishing is used in applications with high signal quality requirements, such as high-quality digital signal transmission and systems with a mix of analog and digital signals, such as CATV (cable television). Although it is physically compatible with PC polished connectors, it is recommended that connectors with the same polishing method be mated together to achieve optimal performance.

APC polishing

APC polishing is a special polishing method in which the tip of the ferrule is polished at an angle (usually about 8 degrees).

This oblique angle is its biggest feature, and is designed so that reflected light generated at the end of the optical fiber does not return to the core of the optical fiber (the central part through which the optical signal is transmitted) but escapes toward the cladding (the part that surrounds the core).

This allows the return loss to be kept extremely low, typically achieving an excellent value of -60dB or better.

APC polishing is widely used in analog signal transmission (e.g. some CATV systems) which are particularly susceptible to the effects of reflected light, systems which use high-power laser light sources, and FTTH (Fiber To The Home).

Because the end face shape is different from PC and UPC polishing, only APC polished connectors can be connected together. If you mix and connect them, there is a possibility of large losses, so care must be taken.

Optical connector polishing procedure

To maximize the performance of optical connectors, precise polishing of the ferrule end face is essential. The polishing process aims to give the tip of the optical fiber a clean and smooth finish and minimize loss of optical signals.

The polishing procedures differ slightly between single-fiber and multi-fiber connectors in terms of the polishing tools and processes used. In either case, the ferrule end face must be uniformly finished to the desired shape.

Here we explain the process of each polishing step.

• Polishing of single-core connectors
• Multi-fiber connector polishing

Polishing of single-core connectors

Polishing of single-core connectors focuses on one ferrule, making it easier to minimize misalignment, but the work process is divided into small steps.

The general steps are as follows:

1. Attach the connector to the polishing holder
2. Roughly smooth the end shape with a rough abrasive sheet
3. Switch to a fine abrasive sheet to make the surface smoother
4. Finally, the end surface is finished with an ultrafine particle sheet to thoroughly remove scratches and dirt.
5. Check the polishing condition with an end-face observation microscope, and repolish if there is a problem.

Starting with a coarse abrasive film, excess adhesive and fiber protrusions are removed, and then gradually changing to a finer film to create a smooth, scratch-free spherical or beveled surface.

It is important to maintain a constant polishing pressure at each step, as uneven pressure can cause the end face to tilt or become misaligned.

After polishing, it is essential to clean the cable using an ultrasonic cleaner or air blower to remove any fine dust or polishing debris. Even the slightest bit of dirt or damage can seriously impair transmission quality, so the final inspection must be carried out carefully and over a long period of time.

The polishing process is complete once it has been confirmed that the required optical characteristics (connection loss and return loss) have been obtained.

Multi-fiber connector polishing

Polishing multi-fiber connectors, especially MPO/MTP connectors, requires the end faces of multiple optical fibers to be finished simultaneously and uniformly, which requires more advanced technology and specialized equipment than polishing single-fiber connectors.

During polishing, multiple connectors are set on the polishing machine at the same time using a special fixture, and the polishing pressure, time, polishing trajectory, etc. are precisely controlled.

The general steps are as follows:

1. The multi-core connector is fixed in a special holder and aligned so that all ferrules are parallel.
2. Rough grinding to adjust the height of the core and the large step on the end surface
3. Finish polishing by using abrasive sheets of different grain sizes in sequence.
4. After polishing, the end face is cleaned and checked under a microscope for scratches and dirt.
5. Regrind and reclean as necessary, and repeat until final acceptance criteria are met.

Post-polishing inspection has also become more sophisticated, not only checking for scratches or dirt on each fiber end face, but also using an interferometer to precisely measure 3D shape parameters such as fiber height, curvature radius, and apex offset to ensure that they meet specified standards. For large-scale cabling, work efficiency and quality control are important issues as many multi-fiber connectors must be polished and inspected.

Summary

Optical connectors are essential components in supporting modern high-speed, large-capacity optical communication networks. Their basic structure, the ferrule, connector body, and mating mechanism, are precisely designed to transmit optical signals efficiently and stably.

From single-fiber connectors such as SC connectors and LC connectors to multi-fiber connectors such as MPO/MTP connectors, various types are used depending on the application and environment. Furthermore, differences in polishing methods such as PC, UPC, and APC are important factors that affect communication quality, so it is necessary to understand the characteristics of each.

The knowledge about the types, structures, and polishing techniques of optical connectors explained in this article will be useful in building and maintaining a stable optical communication system.