What's the Insertion Loss and Return Loss

  In optical communication, insertion loss and return loss are two important indicators to evaluate the quality of end connection between some optical devices, such as optical connectors, optical patch cords, optical pigtails, etc.

What is insertion loss?

  Insertion loss (often referred to as IL) is a measure of the loss of light between two fixed points in an optical fiber. It can be understood that the loss of optical power in the optical communication system optical link caused by the intervention of optical devices is in dB units.

  A smaller insertion loss, that's better in performance; for example, 0.3dB insertion loss is better than a 0.5dB insertion loss. 

What is return loss?

  When the optical signal enters or leaves an optical device component (such as an optical connector), discontinuity and impedance mismatch will result in a loss of power of the reflected or returned signal, which is called Return Loss (RL). 

  Insertion loss is a measure of the resulting signal value when the optical link encounters loss, while return loss is a measure of the reflected signal loss value when the optical link encounters component access.

  The return loss value is expressed as dB and is usually negative, so the larger the return loss value, the better. for example, -60dB return loss is better than a -50dB return loss. 

  Typical specifications range from -15 to -60 dB. According to industry standards, the return loss of UPC end-face optical connectors should be greater than -50dB, and the return loss of APC end-face optical connectors is usually greater than -60dB. PC type should be greater than -40dB. For multimode optical fibers, typical RL values range from -20 to -40 dB.

What are the influencing factors of insertion loss and return loss?

1. End face quality and cleanliness

  Fiber end defects (scratches, pits, cracks) and particle contamination directly affect the performance of connectors, resulting in poor IL/RL. Even small dust particles on a 5-micron single-mode fiber core may eventually block the light signal, resulting in signal loss.

2. Fiber optic breakage and poor connection

  Sometimes the optical fiber can still guide light through even though it is broken, which can also result in bad IL or RL. As mentioned in the picture at the beginning of the article, APC connectors are connected with PC connectors, one is an angle with an angle of 8 degrees, and the other is the grinding angle of a micro-arc surface. The connection between the two connectors may pass light in a short period of time, but it also causes a large insertion loss and a very low return loss, which may also result in two optical end faces not being precisely docked and the light cannot pass normally.

3. bending radius

  Fibers can be bent, but too much bending can also cause a significant increase in light loss, or direct damage. Therefore, it is recommended to keep the radius as large as possible when the optical fiber needs to be coiled. according to the fiber type. like G657A, That's can bend about 7～10mm, and G652 fiber that's needed about 10~15mm, 

  Generally, doesn't smell 10 times the diameter of the jacket. Therefore, for jumpers with a jacket of 2mm, the minimum bending radius is 20mm.