Benefits of fibre-optic patch cords with 'A' grade connectors

Modern systems are exponentially increasing the amounts of data being transmitted, requiring ultrahigh transportation speeds while also transmitting over long distances.

With the advances in fibre-optic technology and transmission systems, reliable cabling systems are becoming even more important.

Active optical equipment, often worth hundreds of thousands of dollars, is connected into the network via the humble fibre-optic patch cord or patch lead. Substandard patch cords will affect the performance and reliability of the network and are often the most common source of failure within a network. The risk of network downtime due to unreliable cabling should be avoided.

The new-generation ultrahigh-speed terabit per second (Tbps) DWDM networks transporting data over 100 km+ require high-performance connectivity capable of handling high input optical power (+27 dBm), very low return loss (≥55 dB) and low insertion connection loss (avg 0.07 dB) in order to operate efficiently and ensure reliable transmission over long distances.

If the quality of the connectors is not of a high standard, ie, the end face has small blemishes and specks of dust and is connected to a high power laser (+27 dBm) output, the connector and the fibre can be badly burnt, sometimes for up to 1 km down the fibre.

It is imperative that telecom companies upgrade their connector specifications in order to step up to the long-distance, super high data transportation speeds across the country. Therefore, these types of networks along with many other data centre and high-speed commercial networks require reliable cabling infrastructure in order to maximise performance and to ensure long-term reliability.

Differences in fibre-optic connector grades

IEC standards dictate the connector performance requirement for each grade of fibre-optic patch cord connector. These standards guide end users and manufacturers in ensuring compliance to best practices in optical fibre technology. The IEC standard 61753 has not been ratified but guidelines that refer to the connector performance on the fibre-optic patch cord have been provided. According to IEC 61753 and IEC 61300-3-34 Attenuation Random Testing Method, ‘C’ Grade connectors have the following performance characteristics: attenuation: ≤0.25 dB mean, ≤0.50 dB max, for >97% of samples; return loss: ≥35 dB.

According to IEC, ‘B’ Grade connectors have the following performance characteristics: attenuation: ≤0.12 dB mean, ≤0.25 dB max, for >97% of samples; return loss: ≥45 dB.

The ‘A’ Grade connector (that is yet to be officially ratified by IEC) has the following performance characteristics: average insertion loss of 0.07 dB (randomly mated IEC Standard 61300-3-34) and a maximum insertion loss of 0.15 dB max, for >97% of samples. While the return loss using IEC 61300-3-6 Random Mated Method is >55 dB (unmated - only angled connectors) and >60 dB (mated), this performance level is generally available for LC, A/SC, SC and E2000 interfaces.

Warren & Brown Technologies is one of the few global manufacturers that have developed a process of quality manufacturing and inspection to meet the stringent specification of the ‘A’ Grade optical fibre connectors. For Warren & Brown angled SC and LC connectors the return loss is ≥65 dB. To be able to measure accurately very low insertion loss of connectors, test equipment needs to be highly stable and accurate to measure losses <0.1 dB.

The IEC 61755 testing method defines an estimation of insertion loss in respect to concentricity and fibre angular alignment in respect to fibre core and ferrule diameter. The insertion loss correlation between the two testing methods (random mated, using high stability test equipment, and concentricity measurements) is extremely high.

Not a lot been said about optical fibre patch cords with ‘A’ Grade connectors in the market and many are unaware that this type of performance is available.

How are ‘A’ Grade connectors on optical fibre patch cords identified?

‘A’ Grade fibre-optic patch cords are identified with the letter ‘A’ printed on the connector side. The symbol is actually the letter ‘A’ enclosed within a triangle (). This identification marker is proof that you are using a high-quality fibre-optic patch cord.

Grade A connectivity is also available for optical fibre through adaptors. The same rule applies for A grade fibre-optic Thru Adaptors, which also have the letter () clearly marked.

What makes a quality fibre-optic patch cord - ‘A’ Grade patch cord

Firstly, a high-quality ‘A’ Grade fibre-optic patch cord begins with using high-quality zirconia ferrules and high-quality optical fibre cable. However, the manufacturing and testing process must be first class. In order to meet the stringent performance criteria of ‘A’ Grade connectors on patch cords, high-quality manufacturing, inspection, testing and quality assurance procedures are required. Without proper expertise in optical fibre technology, many other manufacturers are unable to meet these requirements.

Quality is all about consistency. A quality process ensures consistency, where every patch cord meets the high standards set. To consistently achieve ‘A’ Grade performance, high accuracy testing using state-of-the-art test equipment as well as constantly assessing testing methods are all required. 'A' Grade. Physical attributes giving rise to good IL, RL include:

Radius of curvature: 10 to 25 µm
Pole offset: <50 µm
Fibre undercut: <0.05 µm
Concentricity of ferrule: <0.3 µm (new)
Concentricity, fibre MFD: <0.8 µm (IEC) Typical <0.3 µm
Surface finish and final polish
Combined fibre and ferrule concentricity: 0.3+0.3 = 0.6 µm max

There are many factors and processes involved in manufacturing an optical fibre ‘A’ Grade patch cord. One of the important physical attributes is the concentricity. Concentricity of the hole and fibre is important for a good repeatable result. Concentricity defines how central an object is. When aligning two connectors the cores must match perfectly for good IL and RL. Concentricity error allows area mismatch, which in turn causes loss of optical power. In order to optimise performance and achieve the desired results the concentricity must be measured and then tuned to perform at the highest level. Other end face limits that affect performance that are all tested and tuned as required include: the fibre undercut, radius of curvature as well the final finish and polish.

The effects of insertion loss

One of the key factors that affect the performance of optical fibre networks is insertion loss. Insertion loss refers to the reduction in optical power across the link caused by applying a connector or splice. Insertion loss measurement involves measuring optical power through a length of optical fibre, cutting the fibre, applying connectors and then remeasuring. The power transmitted through the optical fibre will be lower because the interconnection causes some loss of optical power.

In addition, when joining or terminating an optical fibre link, fusion splices are generally considered to provide lowest IL, ensuring an accurate and acceptable optical loss budget. Fusion splicing is the process of melting or fusing two glass fibres together. Fusion splicing permanently joins the optical fibre - losses for fusion splices can generally range from 0.01 to 0.08 dB. ‘A’ Grade connectors offer virtually the same IL performance as a fusion splice, with the added benefit of a physical contact that can be connected, disconnected and moved when required.

Therefore, low insertion connection loss is important for two main reasons:

1. Accumulative low connection loss means that the budget for longer network links can be achieved.
2. In the past, connection or patching to a variety of high power DWDM equipment was carried out with a splice through connection. Now, this can be achieved using clean, low loss and high return loss Grade ‘A’ connectors. This means greater flexibility and time to connect to differing DWDM equipment. This also means that the connector inspection and patching practices to connect to the new equipment have to be high standard.

Testing methods for insertion loss

• Factory tests are typically performed against a reference cord, with known concentricity offset and end face profile.
• Random mating increases the probability of a mismatch in critical dimension alignment for a number of reasons: different optical fibre types in the field, different ferrule types and errors, impractical test due to the sheer volumes of connectors needed to obtain enough data.
• Excellent random mating results are obtained with good ferrules, fibre and tuning.
• Random mating results achieve <0.06 dB average @1310 nm.
• Results against a reference connector are a maximum of 0.15 dB due to the precision of the reference connector.

As highlighted above, it is extremely important to verify the randomly mated performance results of the fibre patch cord due to the unpredictable nature of field installations. As there are many different fibre and ferule types in the field, having a good quality patch cord with excellent randomly mated results will ensure that the patch cord will perform with any other compatible interface.

Importance of proper field use of patch cords

Cleaning and inspection of the contact surface is important for good long-term performance. In fact, one of the major problems in the field that affects connector performance is lack of cleanliness. Even though patch cords may come directly from the manufacturer fully tested and inspected, poor handling and installation practices may cause the connector face to become compromised. While the damage is invisible to the naked eye, it can be seen with a microscope. Proper handling and cleaning procedures must be followed.

Conclusion

It is important to understand the benefits of using reliable, good quality optic fibre patch cords and connectivity. A reliable and high-performing connector ensures link integrity over the long term.

Good quality connectors with low insertion loss will meet large bandwidth and high-speed requirements of the latest active optical equipment, allowing large streams of data to be transmitted reliably over long distances. Good quality connectivity begins with an excellent manufacturing, testing and inspection process.