Firstly, a general description about the development of the optical frequency metrology is given in this paper. Secondly,comprehensive details about the theories of femto-second frequency comb and the NIM femto-second optical comb are introduced.And the system offset frequency f0, one of the important parameters ofthe NIM femto-second optical comb,is deeply studied. Based on the work above, according to the hardness of accessing the system offset f0, its low S/N ratio anddisturbance from the outside, a feedback system is designed in order to make compensation for the drop of its S/N ratio, by changing the position of the three-axis stage, on which the PCF fiber is located. In the condition of not locking the frequency of f0,f0 keeps a high S/N ratio for a longer time effientlly. Based on thecharacteristics of f0. A digital PFD which is suitble for locking f0 is made, the frequency stabillity of f0 is in the level of formal system of Menlosystem. |