Member: Matteo,Yuefan
This week we increased the height of the crystal instead of lower the miniscope. But after the increase, we still cannot get rid of the higher modes. We guessed it is because of the mismatch of the beam waist size. Since the increase of the crystal was reached with putting a spacer under the 'L' part which used to fix the crystal, the wire of the thermal sensor and the peltier are also connected to this part, we cannot push too much on the crystal,so now when we touch the wire outside the housing, the position of the crystal will change slightly. Another problem is that we found out the first turning mirror is too closed to the beam waist focused by the first lens of the telescope, and this caused the optical damage of the mirror coating, since the power density at the beam waist is too high when we use the full power of the laser. So we decided to move the the easily damaged turning mirror one hole closed the first lens, and do the same change with all the other component of the telescope without changing the distance between them. Then according to the new version optical scheme, we changed the dichroic mirror into the one transmit green and reflect red. In this case, we can use this dichroic mirror and the last mirror of the telescope to adjust the beam, things became easier. As I mentioned before, we guess the reason of the higher mode existence is because of the mismatch of the beam size. So we turned the cavity again,inject the beam from the back side of the housing, tried to lock the cavity and get the exact beam waist size we need. Give the EOM 15.235MHz modulation and use the PD to get the error signal. With the demodulation board found in TAMA, and the Stanford as a low pass filter, we locked the cavity, but cannot get rid of the offset. The lock last for more than two hours until we swtiched off everything and left. During the lock, we measure the beam size and got the beam waist size the cavity need is 54.1 micrometer. So the next step is the adjust the telescope again to get the beam waist size given by this locking.