The ABSL technique has been already used in the past to measure the absolute length of the interferometer's optical cavities by means of an auxiliary laser source, as described in LIGO-P1200048-v3 and in Alberto Stochino thesis work.

Using the same technique it is possible to measure the g-factor of the power recycling cavity by measuring the cavity Transverse Mode Spacing.

 

The auxiliary laser is set on the POY table and is injected through the ITMY window in way to follow the same path of the POY beam. It hits the AR wedge of ITMY and is reflected back to the BS and the PRM.

 

Since the main beam is P-polarized, all the optics in the central IFO are P-polarization dependent, so it is useful to P-polarize the auxiliary beam before it enters the IFO.  

I made a mode matching calculation with a la mode script, in order to mode match the auxiliary beam waist to the waist of the main laser.

However, before ordering and installing steering optics and mode maching lenses, I'm waiting to know whether someone has an NPRO laser to install on the END table in place of the broken one, otherwise the one I'm using could be taken.

In this case a possibility could be to take the auxiliary beam from the end table with an optical fiber, but it means to use the auxiliary laser alternately to lock the arm or make a measurement of TMS. If so, a new calculation for the mode matching needs to be done.

Anyway, I hope that another laser will be found!

 

In order to phase lock the auxiliary beam with the main beam, the latter will be taken from the PSL table after the PMC through a single mode fiber, which will be brought up to the POY table. This solution results to be more reliable then taking the POY beam to phase lock the two laser, because POY is related to the locking. 

 

The signal with the beat note between the two lasers can be detected by the transmission from PR2 (POP).