Lock acquisition scheme

1

• Lock acquisition scheme
• for the Advanced LIGO
• optical configuration
• Amaldi conference
• June24, 2005
• O. Miyakawa, Caltech
• and the 40m collaboration

2
Caltech 40 meter prototype interferometer

• Objectives
• Develop lock acquisition procedure of detuned
  Resonant Sideband Extraction (RSE)
  interferometer, as close as possible to AdLIGO
  optical design

• Characterize noise mechanisms
• Verify optical spring and optical resonance
• Develop DC readout scheme
• Extrapolate to AdLIGO via simulation

BS
SRM
PRM
Bright port
Dark port
X arm
Y arm
3
AdLIGO signal extraction scheme
ETMy

• Mach-Zehnder will be installed to eliminate
  sidebands of sidebands.
• Only f2 is resonant on SRC.
• Unbalanced sidebands of /-f2 due to detuned SRC
  produce good error signal for Central part.

4km
f2
ITMy
ETMx
PRM
ITMx
BS
4km
f1
SRM

• Single demodulation
• Arm information

• Double demodulation
• Central part information

• Arm cavity signals are extracted from beat
  between carrier and f1 or f2.
• Central part (Michelson, PRC, SRC) signals are
  extracted from beat between f1 and f2, not
  including arm cavity information.

4
5 DOF for length control
Signal Extraction Matrix (in-lock)
ETMy
Phase Modulation f133MHz f2166MHz
Ly38.55m Finesse1235
ITMy
L?( Lx? Ly) / 2 L? Lx? Ly l?( lx? ly) /
2 2.257m l? lx? ly 0.451m ls( lsx?
lsy) / 2 2.15m
Common of arms Differential of arms Power
recycling cavity Michelson Signal recycling
cavity
GPR14.5
lsy
ETMx
Laser
ly
ITMx
BS
lx
Lx 38.55m Finesse1235
PRM
lsx
T 7
SRM
T 7
PO
SP
AP
5
Differences betweenAdvLIGO and 40m prototype

• 100 times shorter cavity length
• Arm cavity finesse at 40m chosen to be to
  AdvLIGO ( 1235 )
• Storage time is x100 shorter.
• Control RF sidebands are 33/166 MHz instead of
  9/180 MHz
• Due to shorter PRC length, less signal
  separation.
• LIGO-I 10-watt laser, negligible thermal effects
• 180W laser will be used in AdvLIGO.
• Noisier seismic environment in town, smaller
  stack
• 1x10-6m at 1Hz.
• LIGO-I single pendulum suspensions are used
• AdvLIGO will use triple (MC, BS, PRM, SRM) and
  quad (ITMs, ETMs) suspensions.

6
Lock acquisition procedure towards detuned RSE
Ideas for arm control signal
Off-resonant arms using DC lock
DRMI using DDM
RSE
ETMy
Shutter
ITMy
ITMx
PRM
ETMx
BS
Shutter
SRM
Done
Done
In progress
7
DRMI lock using double demodulation with
unbalanced sideband by detuned cavity

• August 2004
• DRMI locked with carrier resonance (like GEO
  configuration)
• November 2004
• DRMI locked with sideband resonance (Carrier is
  anti resonant preparing for RSE.)

Typical lock acquisition time 10sec Longest
lock 2.5hour
8
Struggling lock acquisition for arm cavities

• Problems
• High recycling gain of 15 produces 94 coupling
  between two arms.
• Very high coupled finesse of 18000.
• Slow sampling rate of 16kHz for direct lock
  acquisition.
• So, we have two steps
• Each arm lock using transmitted light with DC
  offset. done
• Reduce offset to have full resonance of carrier.
  in progress

9
Off-resonant DC lock scheme for arm cavity

• Error signal is produced by only transmitted
  light as
• Smaller coupling
• Wider linear range than RF error signal.

Resonant Lock
Off-resonant Lock point
10
All 5 degrees of freedom under controlledwith DC
offset on L loop

• Both arms locked with DRMI
• Lock acquisition time 1 min
• Lasts 20 min
• Can be switched to common/differential control
• L- AP166 with no offset
• L TrxTry with DC offset

Have started trying to reduce offset from L
loop But
11
Peak changing due to reduction of offset

• Peak started from 1.6kHz and reached to 450H.
• This peak introduces phase delay around unity
  gain frequency.

12
CARM at 218 pm offset (locking point)
Unity Gain Frequency
13
CARM at 118 pm offset
Unity Gain Frequency
14
CARM offset at 59 pm (losing lock)
Unity Gain Frequency
15
L- optical gain with RSE peak

• Optical gain of L- loop
• DARM_IN1/DARM_OUT,divided by pendulum transfer
  function
• No offset on L- loop
• 150pm offset on L loop
• Optical resonance of detuned RSE can be seen
  around the design RSE peak of 4kHz.
• Q of this peak is about 6.

Design RSE peak 4kHz
16
DARM, operating point
17
Summary

• All 5DOFs are locked with some offset on L.
• RSE detuning peak was seen.
• L loop has a detuning peak with DC offset and it
  introduces phase delay around unity gain
  frequency with reducing offset.
• Fortunately, Advanced LIGO will not have L peak
  problem !
• Cavity pole of single arm is around 16Hz from the
  beginning, far below from unity gain frequency of
  L loop around 300Hz.
• Fast frequency control or proper compensative
  digital filter may fix this problem for 40m.

Hope we succeed in locking full RSE very soon!
18
DC Readout at the 40m

• DC Readout eliminates several sources of
  technical noise (mainly due to the RF sidebands)
  
• Oscillator phase noise
• Effects of unstable recycling cavity.
• The arm-filtered carrier light will serve as a
  heavily stabilized local oscillator.
• Perfect spatial overlap of LO and GW signal at
  PD.
• DC Readout has the potential for QND
  measurements, without major modifications to the
  IFO.
• We can use a 3 or 4-mirror OMC to reject RF
  sidebands.
• Finesse 500, In-vacuum, on a seismic stack.
• The DC Detection diode
• an aluminum stand to hold a bare photodiode, and
  verified that the block can radiate 100 mW
  safely.

from SRM
From SRM