Insertion Loss and Return Loss in Fiber Connectors

Fiber connectors are crucial components in fiber optic networks that enable the transfer of optical signals from one fiber to another. The quality of the connectors plays a significant role in the overall performance of the network. Two key parameters that are used to assess the performance of fiber connectors are insertion loss and return loss. In this blog post, we will delve into the concept of insertion loss and return loss in fiber connectors, their measurement methods, and their importance in fiber optic networks.

What Is Insertion Loss and Return Loss for Fiber Connectors?

 1. Insertion Loss

Insertion loss is the loss of optical power that occurs when a fiber connector is inserted into a fiber optic link. It is the difference between the input power and the output power of the link, expressed in decibels (dB). The insertion loss is caused by various factors, such as the misalignment of the fibers, the reflectivity of the connector, and the absorption and scattering of light in the connector.

Insertion loss is a critical parameter in fiber optic networks because it directly affects the signal quality and transmission distance. High insertion loss leads to a decrease in signal strength, which can result in signal distortion, errors, and reduced transmission distance. Therefore, it is essential to minimize the insertion loss of fiber connectors to ensure reliable and efficient signal transmission.

2. Return Loss

Return loss is the amount of optical power that is reflected in the source due to a mismatch between the connector and the fiber. It is expressed in decibels (dB) and is caused by factors such as surface imperfections, fiber core diameter variations, and axial misalignments. Return loss can lead to signal interference, distortion, and reduced transmission distance.

Return loss is an important parameter in fiber optic networks because it measures the ability of the connector to minimize signal reflections and maintain signal integrity. High return loss indicates that the connector is effectively matching the fiber and minimizing reflections, which is essential for reliable and efficient signal transmission.

Insertion Loss Measurement

Insertion loss is the amount of optical power that is lost when a signal is transmitted through a fiber optic connector. The most common method for measuring insertion loss is to use an optical power meter in conjunction with a calibrated reference cable. Here are the steps involved in measuring insertion loss:

1. Calibrate the Optical Power Meter:

   The optical power meter must be calibrated before the measurement process. This ensures that the measurements are accurate and consistent. The calibration process involves connecting the power meter to a known light source and adjusting it to read the correct power level.

2. Connect the Reference Cable:

   A reference cable is a length of fiber optic cable with connectors on both ends. It is used as a benchmark to measure insertion loss. Connect one end of the reference cable to the optical power meter and the other end to the connector being tested.

3. Measure the Input Power: Measure the input power using the optical power meter. This is the power level at the beginning of the reference cable.

4. Measure the Output Power:

   Measure the output power using the optical power meter. This is the power level at the end of the reference cable.

5. Calculate the Insertion Loss:

   Subtract the output power from the input power to calculate the insertion loss. The result is expressed in decibels (dB).

Return Loss Measurement

Return loss is the measure of the amount of light that is reflected from the connector due to impedance mismatches. The most common method used to measure return loss is the Optical Time Domain Reflectometer (OTDR). Here are the steps involved in measuring return loss using an OTDR:

1. Connect the OTDR:

   Connect the OTDR to the fiber optic connector being tested. The OTDR sends a pulse of light down the fiber and measures the reflections that are produced.

2. Measure the Reference Signal:

   Measure the reference signal by connecting the OTDR to a reference cable. The reference cable should have a known length and be properly calibrated.

3. Measure the Test Signal:

   Measure the test signal by connecting the OTDR to the connector being tested. The OTDR sends a pulse of light down the fiber, and the reflections are analyzed.

4. Analyze the Results: The OTDR produces a trace of the reflected light. Analyze the trace to determine the amount of return loss. The result is expressed in decibels (dB).

What Causes Bad Insertion Loss and Return Loss?

Ideally, if the fiber patch cord is not connected, there will be a minimal loss - continuous, straight-through glass fiber from point A to point B with no interruptions. However, fiber optic networks require connectors for modularity. The required low IL and high RL performance is thus reduced due to three factors:

1. End Face Quality and Cleanliness

Fiber end-face defects such as scratches, pits, cracks, and particle contamination will have a direct impact on performance, resulting in poor insertion/return loss. Any irregularity that prevents the transmission of light from one fiber to another will negatively affect IL and RL.

2. Misalignment Between the Two Cores

The main task of the connector is to precisely clamp the fiber, ensuring that the core of one fiber is aligned neatly and accurately with the core of the other fiber so that each connector mates with the other with a precise core Alignment and core-to-core contact. Generally speaking, the smaller the ferrule bore diameter, the more accurate the positioning of the fiber in the ferrule. If the ferrule hole is not perfectly centered, the fiber it holds will never be perfectly centered. Therefore, when fibers with loaded cores are not perfectly aligned with each other, misalignment between the two cores often occurs, resulting in poor IL/RL.

3. Poor Contact with the Iron Core

To achieve the desired low IL and high RL, an optimized core-to-core contact must be achieved and maintained. Optical fiber connectors with different polishing methods have different core-to-core contact performances in terms of insertion loss and return loss of the connector. Usually, the insertion loss of PC, UPC, and APC connectors is less than 0.3 dB. However, UPC connectors have the lowest IL due to the smallest air gap, while APC connectors have the highest RL due to the beveled fiber end face. PC vs UPC vs APC connectors will help you choose the right type of fiber optic connection.

How to Reduce the Loss of Fiber Optic Connectors?

Using well-tested, high-quality connectors can help network installers provide high-speed connections that perform well over the long term. Here are some tips for optimizing insertion/return loss values:

1. Keep all fiber optic connectors clean, especially before and after installation and testing. Use an appropriate tool to clean the connector ferrule.
2. Minimize the number of tight bends, coils, splices, and connectors that can cause light to refract through the fiber cladding. If coiled fibers are required, keep the radius as large as possible.
3. Use factory-terminated cables. These terminations are made according to strict guidelines and usually include a manufacturer's warranty.
4. Make an informed budgetary decision: your "power loss" budget or your cable inventory budget. Buying cheap fiber optic cables of poor quality can add to the cost even further.

Conclusion

High insertion loss and return loss lead to reduced signal strength, interference, and signal distortion, which can result in errors and reduced transmission distance. Therefore, it is essential to minimize the insertion loss and return loss of fiber connectors to ensure reliable and efficient signal transmission.

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