The long tail of the distribution of waiting times for acquiring initial lock shown in figure8.4moti- vated attempts to develop an alternate lock acquisition scheme which would be largely deterministic.
Without continual, complete knowledge of the microscopic state of the interferometer, a truly deter- ministic lock acquisition process is impossible (and, moreover, unnecessary). Deterministic locking is thus a process which appears deterministic on human timescales–any statistical variations in pro- cess length are on the order of seconds rather than minutes. Such a process is a holy grail of lock acquisition, sought by gravitational wave interferometer scientists and commissioners the world over.
It has many benefits, including reduced commissioning time (and consequently greater observation time), easier debugging of hardware and software, and much reduced frustration and boredom of detector operators. As discussed in section 8.6.5 some effort was expended at the 40 m to find a deterministic locking process for a power-recycled Fabry-P´erot Michelson interferometer (LIGO and Enhanced LIGO) configuration; promising candidates were found but not pursued, as the straight- forward application of the procedure in section8.6 was already quite fast. Furthermore, significant effort was expended at the 40 m to find a deterministic locking process for a dual recycled Fabry- P´erot Michelson interferometer, as such a technique could be very useful for Advanced LIGO. No
Start
ARM >
threshold wait
ARM >
threshold wait
NO YES
YES NO kill aqstep
kill cmstep
report wait time
report locked time
aqstep script
down script
Figure 8.13: The flow of the main watch script used for locking.
c1_watch
Watches the transmitted arm powers and digital filter states to determine IFO status (locked, in common mode, etc). Runs appropriate scripts, and kills subscripts if lock is lost.
operator
cm_step Initiates transition to laser frequency based common mode control by handing off CARM control to MC_L, engaging moving zero, reducing CARM offset, switching CARM sensing from TR DC to PO DC, starting MC_F feedback, reducing CARM offset, switching CARM sensing from PO DC to PO RF.
Script-based slow servo to offload signal on MC by pushing ETMs, which prevents MC_F from saturating
offload_MCF Gently pushes on ETMs to
swing them through fringes tickle Engages boosts, ramps
gains, transitions short DOF signals to DDM, switches XARM+YARM to DC CARM and DC DARM, then transitions DARM to RF.
aqstep
Returns all IFO settings to acquisition state
down
Transitions control of short DOFs to DDM signals
DD_handoff
Resets short DOF loops to acquisition state
DD_reset
Begins control via frequency actuation of a DOF. The actuator is the MCL, which the laser frequency follows as part of the MC loop.
ontoMCL
Figure 8.14: The hierarchcy of scripts used for automating lock acquisition of the 40 m.
promising candidates were found, which serves to underline the added complexity in the controls plant introduced by the technique of signal recycling.
The failure to find a deterministic locking procedure for the 40 m has prompted the serious consideration of significant auxiliary hardware, such as secondary interferometers (operating in a different color or polarization) which are dedicated to providing signals for lock acquisition. The goal is to be able to sense robustly each single DOF without interference from the other DOFs, and without causing interference to the other DOFs. Such an approach, which strives to reduce the dimensionality of the initial, statistical stage of lock acquisition, can greatly reduce the MTTL.
Essentially, the goal is to reduce the problem of inverting ann×nmatrix to invertingn1×1 matrices.
A promising candidate strategy is described in the next section, and it will soon be prototyped at the 40 m lab.
8.7.1 The Future: It’s easy being green
The primary difference between the scheme proposed below and the scheme described in8.6involves replacing step A with a version which is deterministic in nature.
The 40 m experience has demonstrated that it is not difficult to lock a 3 DOF system such as the DRMI, provided there is an appropriate selection of signals and the recycling cavities are of sufficiently low finesse, which is the case for the current Advanced LIGO design. In order to have an essentially deterministic locking scheme for Advanced LIGO, it should thus be sufficient to improve the locking time of the arm cavities, which can be accomplished with the addition of some auxiliary hardware dedicated to lock acquisition.
The basic idea is to quadruple the number of laser fields. For a primary laser (i.e., the one that will be used during interferometer operation) operating at 1064 nm, we would add
• A laser field frequency doubled from the main (PSL) laser (so at 532 nm—green).
• Two additional 532 nm lasers at the transmitted ports of the two arm cavities.
• Dichroic coatings for the ETMs (T ∼0.9 for 532 nm).
• Dichroic coatings for the ITMs (T ∼0.99 for 532 nm).
The benefits of this additional hardware include (more details in the following)
• Multiple lasers allow truly independent sensing of the two arm cavities.
• Independent velocity damping of arm cavities can be done when they are not resonant for the main carrier field.
• Dichroic coatings break the degeneracy of acquisition and detection finesses.
• Green light + wedged optics allows excellent isolation of the arm cavities from other degrees of freedom, and helps to isolate the arms from each other.
With these benefits, the basic plan would be to lock the arms first using the green laser, then lock the short degrees of freedom in the same manner as is already done at the 40 m.
8.7.1.1 Envisioned lock acquisition procedure
At each end station, the green laser will be phase modulated, and the field injected into the arm cavity through the ETM. With the ITM having a highly reflective coating for green light, this system now forms a low-finesse, overcoupled cavity, and a standard PDH based length sensing and control scheme can be easily applied, with the feedback to the green laser frequency, which must be a fast actuator (a PZT). This fast actuator combined with a low-finesse cavity will make for tremendously simple lock acquisition. The feedback loop should only be limited by the range of the frequency actuator, which should be at least several tens of MHz. Such a range corresponds to many free-spectral ranges of the arm cavity.
Now the green laser is resonant in the arm cavity; the transmitted field (exiting the ITM toward the BS) can be picked off and directed to a photodetector, where it can be heterodyned with the frequency-doubled PSL light. The beat frequency is then exactly twice the difference between the resonant frequency of the arm cavity and the optical frequency of the PSL; this is exactly what we want for lock acquisition. The linear range of this signal will be limited only by the bandwidth of the photodetector used for heterodyning. The linear range, expressed as the number of wavelengthsn, should be the PD bandwidth (∆f) divided by the cavity free-spectral rangeνF SR, which of course is much larger than the linear range of a PDH signalλ/4F:
n= ∆f νF SR 1
4F, (8.8)
where we are consideringF the finesse for the PSL (λ= 1064 nm) light.
With this broad linear signal, the arm length can simply be velocity damped with gentle length actuation until it is no longer fluctuating with respect to the PSL light. Once the arm is under length control, it can be held stably at a point near the carrier resonance. With the arm lengths held such that they fluctuate much less than a fringe, they will not disturb the signals for the short DOFs (MICH, PRCL, SRCL), control of which can be easily acquired using the techniques described in section8.6.
Once control of the short DOFs has been achieved, the full IFO is under control, and techniques similar to those described in Step B and onward in section 8.6 can be used to bring the IFO to the operating point. Alternatively, it may also be possible to use the green light sensing chain to control the arm cavities independently while the offset is reduced; this might be possible if the initial
lock stage is done at low laser power, thus avoiding significant radiation pressure effects. Radiation pressure effects can also be avoided by appropriately locking the SRCL away from its own nominal operating point (detuning it), and adjusting this offset in concert with the arm offsets to eliminate any optical spring in the DARM degree of freedom. Such a procedure is conceivable as the sensing scheme for the SRCL in Advanced LIGO has been designed to allow the SRC to be operable at multiple detunings, and to be able to switch between these detunings in lock.
8.7.1.2 Advantages of this technique
With an understanding of the technique, the advantages relative to the procedure in Step A of8.6 become apparent:
• Using green light: breaks the degeneracy of cavity finesse for lock acquisition and GW detection; highly reflective ITMs isolates arm cavities from flashing in the short DOFs; highly reflective ITM plus the optic wedge angle serves to isolate arm cavities from flashing in the other arm cavity.
• A secondary laser: allows the creation of a length signal with a broad linear range. This would permit effective velocity damping of the cavity, eliminating the need for actuators that can acquire lock before the cavity has swept through a single fringe. With this the concerns outlined in section 8.2can be sidestepped.
• Two secondary lasers: allow the two arms to be locked independently, both with a frequency control technique followed by velocity damping. Moreover, independent signals for the two arms allows the choice of locking to any point in the two dimensional CARM + DARM configuration space.
8.7.1.3 Alternative Technique
It is possible that this technique will be adversely affected by the green light transmitted through the ITM resonating in the short degrees of freedom. In that case, nearly the same technique can be used, by installing optical fibers that can transmit the main carrier field to the end stations. The lasers at the end station are phase locked to the PSL light, then frequency doubled. The resulting green light can then be locked to the arm cavity in the same way, with the error point of the phase-locked loop now taking the place of the frequency actuator. The feedback to the phase-locked loop error point now is the sensing signal for the arm cavity length; it can be used in the same way to velocity damp the arm cavity. In this case, all the sensing for the arm takes place at the end station, and so the high reflectivity (for green light) of the ITM will effectively isolate the arm from the short degrees of freedom. The main drawback of this scheme is the need for a 4 km optical fiber, which
can introduce substantial noise. The fiber system would thus require a setup to cancel phase noise induced by the fiber, such as the system devised in [95].