In the January 2004 Connector Specifier article, "At the Core, how scratches, dust, and fingerprints affect optical connector performance," the reference to return loss is in error. Return loss is defined as the ratio of the reflected energy to the input energy and is generally a very small number (<1). Thus, it is stated in -dB rather than the +dB shown in the graphs.
Your article states that a contaminated optical connector exhibits a reduction of return loss compared to a clean connector. In reality, a dirty optical connector exhibits an increase in return loss (reflection from the dirt). This is because the return loss is a ratio and is measured in a logarithmic scale (dB).
The definition of return loss is: 10 log (Reflected signal/input signal). For a reflection of 1%, RL(db) = 10 log (0.01/1) = –20.
Regards,
Bill Kornrumpf
Electronics Engineer
GE Global Research
The author replies:
Thank you very much for reviewing our article and highlighting the importance of a proper return loss definition. According to our references, usually the definitions of optical reflectance and return loss in dB are differentiated as follows:
Reflectance (dB)= –10 Log (incident power, Pi/reflected power, Pr)
Return loss (dB)= 10 log (Pincident/Preflected)
Return loss (dB)= -Reflectance (dB)
In other words, the reflectance is a negative number and the return loss is positive. You referred to a different definition of the return loss, "10* log (Reflected signal/input signal)," which is our definition of the reflectance.
Our apologies that we didn't provide the explanation of how the return loss was calculated due to the limited size of the article. Of course, the dirty connector exhibited degradation of performance (return loss).
Thank you very much for your help.
With best wishes,
Tatiana Berdinskikh, Ph.D.
Celestica Fiber Optics Components
