Fiber optic connector insertion loss

Optical fiber connectors are widely used in optical fiber transmission lines, optical fiber distribution frames, optical fiber test instruments and meters. So what are the key points of knowledge about the insertion loss of fiber optic connectors? Let's go to find out together.

1. The concept of insertion loss

(1) Definition of fiber optic connector insertion loss (IL): where P1 is the output optical power, and P0 is the input optical power. The unit of insertion loss is dB.

(2) Test method for insertion loss of optical fiber connector

There are generally three methods for testing the insertion loss of optical fiber connectors: benchmark method, substitution method, and standard jumper comparison method. Due to the high-volume production process, the insertion loss test must be fast, accurate and non-destructive. Therefore, most of the current manufacturers adopt the third method, that is, the standard jumper comparison method.

When the single-mode fiber pigtail is less than 50M and the multi-mode fiber pigtail is less than 10M, the loss of the pigtail itself can be ignored, and the measured data at this time is the insertion loss of the 3-terminal relative to the standard connector, and this data available to customers. When the single-mode fiber pigtail is larger than 50M and the multi-mode fiber pigtail is larger than 10M, the loss value of the fiber itself should be subtracted from the measured loss value.

(3) Repeatability

Repeatability refers to the variation range of the insertion loss of the same pair of plugs after multiple insertions and removals in the same adapter. The unit is expressed in dB. The repeatability should generally be less than 0.1dB.

(4) Interchangeability

Since the insertion loss of the fiber optic connector is measured by the standard jumper comparison method, its value is a relative value. Therefore, in any connection, the actual insertion loss value is likely to be greater than the value measured by the standard jumper comparison method, and different connectors and different adapters will have different degrees of influence. Therefore, there is an indicator requirement of interchangeability. Connector interchangeability refers to the variation range of insertion loss between different plugs, or after any conversion of different adapters. It should generally be less than 0.2dB.

2. Main factors of fiber optic connector insertion loss

(1) Loss caused by mismatch of optical fiber structural parameters (different core diameters, different numerical apertures, different refractive index distributions, and other reasons, etc.).

(2) Core alignment error (core dislocation loss)

The loss caused by the transverse misalignment of the fiber core is called misalignment loss. It is an important cause of insertion loss.

The dislocation loss of the multimode graded fiber in the steady state distribution of the mode is:

When the single-mode fiber is connected, when the mode field distribution is approximated by Gauss, the dislocation loss is:

There are many reasons for this loss. It mainly includes the core/cladding concentricity of the optical fiber, the concentricity of the ferrule, and the unsatisfactory parameters of the test adapter. The influence of the above factors on the insertion loss is also related to the external device

(3) Loss due to end face shape and clearance

The main reason for this loss is that the physical parameters of the end faces of the optical fiber connector are not ideal, resulting in non-planar direct contact between the end faces of the two connected optical fibers, leaving a certain gap or non-planar contact. According to the relevant formula, it can be deduced that as long as the end face gap is controlled within 1um, this loss can be ignored.

Of course, there are many other factors that affect the insertion loss besides the above three, such as: the size fit of the external device, the inclination of the end face, the Fresnel reflection of the end face, etc.

3. Production process control elements

(1) The quality of the ferrule

Mainly the inner diameter and concentricity of the ferrule. For multi-mode fiber optic connectors, the concentricity of the ferrule is required to be less than 3um, and for single-mode fiber optic connectors, the concentricity of the ferrule is required to be less than 1um. Factors such as the concentricity of the ferrule, the physical parameters of the polished end face, and the matching dimensions of the outer parts will ultimately affect the concentricity of the fiber core/ferrule, and eventually lead to misalignment loss.

(2) The level of grinding

The standard for measuring the quality of grinding depends on its end face and its physical parameters. There are three main physical parameters: radius of curvature, spherical eccentricity, and fiber sag. For the APC type connector, it also includes two parameters, the end face angle (8 degrees oblique) and the key angle deviation. These parameters can affect the insertion loss. IEC has put forward clear requirements for these parameters and has specific index regulations. To do this, a grinding machine with stable performance is essential. Most of the quality problems in the production process of optical fiber connectors are directly or indirectly related to the stability of the grinding machine.

(3) Dimensional fit of external parts

The dimensional fit of the outer parts will have a direct impact on the repeatability and interchangeability of the connector. Especially for APC type connectors, if the size matching is not ideal, its interchangeability and repeatability may exceed 0.1dB or worse.

4. Test control elements

(1) Accurate and reliable testing equipment

In order to make the test data accurate and reliable, accurate and stable test instruments will provide a reliable guarantee for this.

(2) Standard connector

The standard connector is a set of precision manufactured or selected connectors, which includes two parts: standard jumpers and standard adapters. The insertion loss of the fiber optic connector is actually its loss relative to the standard test line, so the indicators (optical parameters and physical parameters) of the standard test line must be strictly controlled. Similarly, the indicators of the adapter should be strictly controlled according to the principle of selecting standard adapters. In this way, the test results will reflect the true quality of the tested connector to the greatest extent possible. Therefore, the key to reflect the accuracy and reliability of the test is the control of standard test lines and standard adapters.

(3) End face cleanliness

Because the outer diameter of the optical fiber is only 125um, and the light-passing part is smaller, the single-mode optical fiber is only about 9um, and the multi-mode optical fiber has two types: 50um and 62.5um, so the cleanliness of the optical fiber end face is highly required. The end face of the optical fiber must be cleaned before testing to ensure that the end face is highly clean, so as to ensure accurate and reliable test results. If one cleaning is not enough, you can clean it several times.