active frequency shift的用法和样例：
- The earth's rotation and its motion around the sun add an appreciable frequency shift.
- According to the laser gain change is linear related to the frequency shift, the Doppler frequency is extracted by the signal processing.
- Among all the passive and active islanding detection method,the active frequency drift method is pretty attractive.
- The Brillouin frequency shift can be precisely measured by changing the local frequency and adopting narrow filter.
- The scheme of measuring Brillouin frequency shift in fiber sensor based on edge optical filter is proposed.