Lock Acquisition




Lock Acquisition Issues:


  • What is the length sensing and control scheme?
    Its important for lock acquisition studies
    that we figure out how we will read out each of the 5 length DOFs. The first scheme used to
    lock the 40m prototype was tuned to make a nice sensing matrix but was not optimized for
    noise.
  • What is the lock acquisition sequence?
    What states do we travel through to get from all DOFs uncontrolled to all under control
    and then to full resonance of all cavities? In the past people have used the 'Blind'
    approach (turn on all the servos and wait for awhile), the 'Bang-Bang' approach (e.g. in
    LIGO-I where the digital matrix is dynamically inverting the optical plant), and there
    is the 'Low-Finesse' slow approach taken by Virgo.
  • Can we actuate the laser wavelength to lock L+ ?
    Since we expect the L+ DOF to be noisier than L-, it is attractive to consider adjusting the
    laser wavelength to lock L+. In principal, the wavelength can be slewed alot faster than
    a 30 kg mass with a weak electro-static actuator can. Its important that the overall laser
    stabilization system be designed with enough range and robustness that it can be actuated
    during lock acquisition. In particular, the PSL's FSS needs to be able to handle large
    transients.
  • Suspension Point Interferometer.
    Since there will always be low frequency (f < 0.2 Hz) motion of the seismic isolation
    platforms (especially during storms and earthquakes) it seems prudent to have a global
    feedback system which will stabilize the relative motions between the optics tables.
    By making a few low power, low finesse interferometers attached to the optics table,
    we can interferometrically lock the tables together and, in principal, suppress the low
    frequency motion by > 40 dB. A detailed analysis of this idea should be done including:
    scattered light issues, optical layout, controls modeling, and cost estimate.
  • Single Cavity Time Domain Simulations.
    The Advanced LIGO arm cavity mirrors are supported by quad suspensions. This can lead to
    complicated dynamics during lock acquisition. However, if we believe that its possible
    to acquire lock by just driving the ESD on the test mass, then the problem becomes the
    just one of locking single Fabry-Perot cavities with a simple pendulum (in the low power
    limit where the optical spring is not important). In reality, if we acquire lock with 1 W
    of power into the interferometer, the optica spring frequency is ~8 Hz. I believe this is
    not substantially different from the LIGO-I case, albeit with lower available forces, lower
    seismic noise, higher finesse arm cavities, and a higher effective pendulum frequency.




    • More to add:

    - Useful numbers (fringe velocities, actuation forces, spring frequencies, optic mass, cavity finesse)

    LA Related Documents
  • L. Barsotti's Thesis
  • M. Evans' Thesis
  • R. Ward's Thesis
  • LIGO-I LA Paper
  • LA Related Current Events