According to the measurement done by Aidan and me, there are two beat-note state.
One gave us a small beat signal and the other gave us a bigger signal by approximately 20 dB.
A possible reason for this phenomenon is that the end laser is operating at a special temperature that somehow drives the laser with two different modes at the same time.
So that it permits the laser sometimes locked with one of the two modes and sometimes with the other mode.
For the first step we will change the temperature such that the laser can run with a single stable mode.
Then for investigating it we will put a scanning cavity on the X end table to see if it really exhibits a two modes or not.
The attached table shows the amplitude of the green beat note when the end laser was in various states. We can increase the beat note amplitude dramatically by switching to a different states.