What is an absolute frequency reference (Frequency combs)? So I am currently studying for my bachelors thesis which is all about frequency combs. I am reading a lot of articles about them but I just keep coming across things that the people who wrote the articles take as something I should already know.
At the moment I am a bit unsure about the term absolute frequency reference.
In the article I am currently reading they write: 
"The fourth harmonic of a methane-stabilized He-Ne laser that operated with high accuracy was used as an absolute frequency reference for the femtosecond comb, which allowed us to access new targets."
Can someone help me understand what this reference does and why it is necessary? 
 A: Take a really good crystal oscillator for example. I can have an extremely stable oscillator, where I know that it has the same frequency today that it did last year (to within some extremely small uncertainty). But maybe I have no idea what its frequency is in Hertz, or maybe I know it only very approximately. And maybe if I built 10 such oscillators, they would all have kinda different frequencies, even though each individual frequency would be very stable.
That would be an example of a frequency reference which is not an "absolute" frequency reference.
An absolute frequency reference should be tied to some unchanging property of the physical world, like an atomic transition frequency ... ideally a frequency that someone else has previously compared to a caesium atomic clock, which in turn underlies the definition of a second. So you really know what the frequency is in Hertz, now and forever.
A crystal oscillator can become an absolute frequency reference if you calibrate it against a different absolute frequency reference. But that works only temporarily! The crystal oscillator frequency will drift over time, and you'll eventually need to calibrate it again.
