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18 \title{Upgrade of the 40M Interferometer}
19 \author{R. Adhikari, Y. Aso, S. Ballmer, R. Bork, J. Miller, S. Vass, R. Ward, A. Weinstein}
20 %\date{${}$Date: 2007/09/30}
21 \ligodccnumber{T}{08}{0074}{00}{R} \ligodistribution{ISC Group}
22 %\ligodraft
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24 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
25 \begin{document}
26
27 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
28 \section{Overview}
29 \label{sec:I}
30
31 This document describes the proposed upgrade of the 40m prototype interferometer
32 at Caltech. Detailed background and motivation is given in Appendix~\ref{app:motive}.
33
34 The purpose of the upgrade is to match the properties of the 40m interferometer
35 to the recent design of the Advanced LIGO interferometer~\cite{ISC:CDD}
36 (T070247-00) and to enable more faithful prototyping of the real thing.
37
38 The following is a list of the major changes:
39 \begin{itemize}
40
41 \item Arm Cavity Finesse. The ITM transmission will be changed to match the
42 AdvLIGO finesse choice of 450 (T = 1.4\%)
43
44 \item Lightweight ITMs. In order to increase the effects of radiation pressure
45 we will replace the 1.25 kg MOS's with the 0.25 kg SOSs.
46
47 \item Modulation Frequencies. The 40m frequencies ($f_1$ = 33 MHz and $f_2$ = 166 MHz)
48 will be changed to match the AdvLIGO frequencies ($f_1$ = 9 MHz and $f_2$ = 45 MHz).
49
50 \item Recycling Cavities. The PRC and SRC will be made longer to accommodate
51 the lower modulation frequencies. This will be done by folding the cavities
52 using passively damped ANU tip/tilt suspended optics.
53
54 \item Mach Zehnder. The CDD shows that it is not necessary to have a Mach Zucker
55 so we will remove the 40m Mach Zucker and go back to using serial modulation.
56
57 \item Controls Computers. All of the front end processors, ADCs, and DACs will be
58 replaced with AdvLIGO style hardware. There will be one or two central
59 multi-core computers with fiber links to remote ADC/DAC "blue-boxes". Custom
60 interface connectors will be made to connect the new digital controls with
61 the existing analog electronics.
62
63 \item Multi wavelength locking. Investigate techniques to improve mean time to
64 lock: Multi-wavelength locking locking using dichroic optics (green, blue),
65 frequency shifted PSL light injected through ETM or pick-off ports, PRN
66 techniques.
67
68 \item Optical Levers. Test out fiber based OL distribution scheme? Not really necessary.
69 We should make sure to include whitening of all Oplev systems.
70
71 \item Wavefront Sensing. The 40m will continue to only have wavefront sensing for
72 the IMC. The full interferometer alignment will continue to be done by angular
73 dither demodulation.
74
75 \item Adaptive Noise Cancellation. There will be a few adaptive noise cancellation
76 machines to study the technique. This will require some new computers,
77 PEM sensors, shakers, etc.
78
79 \end{itemize}
80
81
82
83 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
84 \clearpage
85 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
86 \section{Descriptions of the Changes}
87 \label{sec:II}
88
89 \subsection{Modulation Frequencies}
90 The current modulation frequencies for the main interferometer are
91 (f$_1$ = 33~MHz \& f$_2$= 166~MHz). The modulation frequency for the
92 input mode cleaner is (f$_{MC}$ = 29.5~MHz). There are many practical
93 difficulties associated with using such high frequencies: small
94 photodiodes to reduce diode capacitance, increased dielectric losses
95 in the RF cables, incompatibility with the existing RF board layouts,
96 and incompatibility with the LIGO standard op-amp collection.
97
98 This experience led to a design choice (f$_1$ = 9~MHz and f$_2$ =
99 45~MHz) for Advanced LIGO utilizing a much lower frequency pair than
100 the previous design (f$_1$ = 9~MHz and f$_2$ = 180~MHz).
101
102
103 \subsection{Recycling Cavity Lengths}
104 The recycling cavity lengths will be made much longer than is current
105 (see Tables \ref{tab:params} and \ref{tab:oldparams}). This will
106 require the installation of 2 folding mirrors per cavity. Since the
107 40m stacks do not give much seismic isolation in the control band
108 (reference?) we have chosen to install a variant of the ANU Tip-Tilt
109 mirrors to provide some isolation. In order to not have the added
110 complexity of controlling 4 more suspended optics, we will take
111 advantage of the inherent eddy current damping in the cages (reference
112 - add tip/tilt plots). We will also not install BOSEMs or coils in
113 these. They will be passively damped, uncontrolled folding
114 mirrors. The residual excitation at the Tip-Tilt free body modes is
115 small (the Q is ~2). Some basic Looptickle modeling shows that the 1/f
116 isolation provided by the Tip-Tilt mirrors is enough to bring the
117 noise from the recycling cavities to a level where it is below the
118 thermal noise in the arm cavity suspensions.
119
120 Figure~\ref{fig:RC2} and \ref{fig:RC3} show two possible layouts for the recycling
121 cavities that are consistent with the
122 cavity length requirements (see section \ref{sec:III}).
123
124 \begin{figure}[htbp!]
125 \begin{center}
126 \includegraphics[width=1\linewidth,angle=0]{40m_opt_layout_Mark1_Pa-crop.pdf}
127 \caption[layout1]
128 {Possible way to fold both recycling cavities with 2 folding mirrors per cavity. The
129 power recycling cavity is shown in red, the signal recycling cavity in green.}
130 \label{fig:RC2}
131 \end{center}
132 \end{figure}
133
134 \begin{figure}[htbp!]
135 \begin{center}
136 \includegraphics[width=1\linewidth,angle=0]{40m_opt_layout_Mark2_Pa-crop.pdf}
137 \caption[layout2]
138 {Possible way to fold both recycling cavities with 3 folding mirrors for the power
139 cavity. The power cavity is shown in red, the signal cavity in green.}
140 \label{fig:RC3}
141 \end{center}
142 \end{figure}
143
144
145
146 \subsection{Mode Cleaner}
147 It will be made longer. The existing Input Mode Cleaner (IMC) length
148 is 13.548 m \cite{Kirk:Length} and is resonant with an 11.064~MHz RF
149 sideband. The new length will be 16.65 m, requiring a 3.1~m extension.
150 This can be accommodated in the 40m lab (see figure~\ref{fig:MC2}) by
151 installing an extension tube between the existing tube and the
152 MC2 chamber. This is expected to be a fairly simple operation.
153
154 We would have qualitatively the same prototyping experience whether we
155 used a 9 or 11 MHz sideband but since it is relatively easy and
156 inexpensive we feel that it is worth it to use the exact frequencies
157 in case there is something surprising to be learned about components,
158 RFI, etc.
159
160 \begin{figure}[htbp!]
161 \begin{center}
162 \includegraphics[height=20cm]{mc2p.jpg}
163 \caption[MC2 Tank]
164 {View of the MC2 tank from around the MC1 chamber.}
165 \label{fig:MC2tank}
166 \end{center}
167 \end{figure}
168
169
170 \subsection{Arm Cavity Finesse}
171 The arm cavity finesse will be changed from 1200 to 450 to match the
172 new design~\cite{ArmFinesse}. This will require repolishing and
173 recoating the ITMs. We place no requirements on the thermal noise or
174 the polish and so we can afford to go with the cheapest / fastest
175 vendors (within reason). For the coating, we will require the
176 transmission of the ITM HR surface to be 0.014 +/- 0.002 and that the
177 differential transmission (T$_X$ - T$_Y$) be less than 0.001. This is
178 to prevent there being excess noise couplings which are qualitatively
179 beyond what we expect in Advanced LIGO.
180
181
182 \subsection{ITM Masses}
183 For the new ITMs we have the choice of either installing 2 MOSs or 2
184 SOSs.
185
186 There are 2 ITM MOS spares on hand and more than enough MCFM spares
187 that we can choose between them.
188
189 Installing SOSs will increase the effect of radiation pressure and
190 therefore require developing controls techniques which will be useful
191 in Advanced LIGO. We estimate that with SOS-ITMs, the longitudinal
192 optical spring can be made as high as 200 Hz in the detuned RSE
193 case. This seems, at this stage, like a potentially useful thing to
194 prototype and so we are planning to install SOS ITMs.
195
196
197 \subsection{Borkspace Revolution}
198
199 The current CPU power for the SUS and LSC FEs is undersized at the
200 40m. These processors frequently run over their time limit and fall
201 out of sync, requiring reboots of several systems. We therefore would
202 like to replace several of these with a single multi-core computer
203 (Sun Fire X4600 M2 AMD; 8 x 2.8 GHz Dual Core Opteron). Changing
204 from the VME crate style Pentiums to the commodity FE processor will
205 make the upgrade path easier in the future. There is also the significant
206 benefit of producing $\sim$7 spare VME processors for the observatories
207 during the Enhanced LIGO science runs.
208
209 We should put a table in here of all the FE computers, what they are,
210 how much time they take up, and how many ADC/DAC connections they
211 have, and how many signals they pass to where across the RFM net.
212
213 Figure~\ref{fig:SENDS} shows an example block diagram of what would
214 be prototyped at the 40m lab. This is also the proposed new DAQ
215 structure for Enhanced LIGO.
216
217
218 \subsection{Wavefront Sensing}
219
220 The 40m is the only place outside of the actual Advanced LIGO
221 interferometers that the full ASC-WFS system could be prototyped. The
222 iLIGO experience was difficult because of difficult to diagnose
223 problems in the WFS heads, demod boards, whitening boards, and
224 ADCs. On the optical side the WFS sensing matrix was often degenerate
225 and singular due to ill understood properties of the unstable power
226 recycling cavity and the Sigg-Sidles torque instabilities.
227
228 It will certainly be valuable to test out the full electronics chain
229 with realistic optical signals but the 40m optical plant is unlikely
230 to be a good prototype of the Advanced LIGO interferometer since the
231 40m arms will not be changed to approach the nearly unstable, confocal
232 geometry of Advanced LIGO.
233
234 Unless something unforeseen comes up, we will simply make the 40m
235 available as a lab to test the WFS electronics chain if it proves to
236 be convenient (e.g. by installing an Advanced LIGO WFS system on the
237 IMC). We will not setup a full IFO RF WFS system.
238
239 On the other hand, we will continue to commission the dither based
240 alignment system so as to have a useful bootstrap system when
241 commissioning the WFS in Advanced LIGO.
242
243
244 \subsection{Optical Levers}
245
246 Currently the 40m optical lever system serves to stablize the low
247 frequency (< 6 Hz) motion of the suspended optics. The lever lengths
248 are 1-3 meters long and use HeNe lasers. Due to geometrical
249 constraints the beams bounce off of fixed mirrors on the stacks on the
250 input and output paths. It is clear that the sensing noise of the
251 system is limited by bouncing off of the stack (f < 10 Hz) and from a
252 lack of whitening before the ADCs (f > 10 Hz).
253
254 The new design will use existing viewports, port mounted steering
255 mirrors, and simple telescopes with 2 inch optics to make 40m long
256 optical levers.
257
258 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
259 %\newpage
260 \section{Looptickle Modeling}
261 \label{sec:III}
262 This section contains the results from a Looptickle / Optickle model.
263 It focuses on the zero-detuned mode that is going to be the main mode of AdvLIGO.
264
265 \subsection{Design parameters}
266 Table \ref{t:IFOConfig} summarizes the key design parameters. All cavity length are
267 optical path length numbers, i.e. they have not been corrected for the BS thickness.
268
269 \begin{table*}
270 \begin{center}
271 \begin{tabular}{lc}\hline\hline
272 %\multicolumn{2}{l}
273 %{{\bf Interferometer configuration}}\\
274 \textit{Quantity} & \textit{Value} \\ \hline
275 Input power & $1~{\rm Watt}$ \\
276 Finesse & $446$ \\
277 ITM transmission & $0.014$ \\
278 PRM transmission (unchanged) & $0.07$ \\
279 SRM transmission (unchanged) & $0.07$ \\
280 Schnupp asymmetry & $0.050$ \\
281 $\rm l_{PRC}$ & $8.328~{\rm m}$ \\
282 $\rm l_{SRC}$ & $6.662~{\rm m}$ \\
283 Distance BS to ITMX & $2.025~{\rm m}$ \\
284 Distance BS to ITMY & $1.975~{\rm m}$ \\
285 Distance BS to PRM & $6.328~{\rm m}$ \\
286 Distance BS to SRM & $4.662~{\rm m}$ \\
287 $\rm l_{IMC}$ (round trip) & $33.310~{\rm m}$ \\
288 $\rm l_{EX}$ & $38.55~{\rm m}$ \\
289 $\rm l_{EY}$ & $38.55~{\rm m}$ \\
290 Lower mod. frequency & 9~{\rm MHz} \\
291 Upper mod. frequency & 45~{\rm MHz} \\
292 \hline\hline
293 \end{tabular}
294 \caption[Interferometer configuration]{
295 Basic interferometer parameters. }
296 \label{t:IFOConfig}
297 \end{center}
298 \end{table*}
299
300
301 \subsection{Loop Designs}
302 No magic in this section. we aimed for a DARM UGF of 350~Hz, and auxiliary loop UGF's
303 of 100~Hz. But neither of them are critical.
304
305 \subsection{Power / Signal Levels}
306 The table below shows the optical power at each port. No additional attenuators were
307 included. The POP
308 beam assumes a PRC folding mirror with 1000~ppm transmission. A DARM offset of 40~pm
309 was assumed. This gives $1.5~{\rm mWatt}$ of power on the DC detector. The offset can be increased
310 more, but the sensitivity will go down.
311
312 \begin{tabular}{lcccccccccccc}
313 \multicolumn{13}{l}{{\bf Optical Port Power in mWatt}}\\ \hline
314 FREQ (MHz)& -54.0& -45.0& -36.0& -28.8& -9.0& 0.0& 9.0& 28.8& 36.0& 45.0& 54.0& Total \\ \hline
315 REFL & 0.0& 0.2& 0.0& 0.0& 2.0& 26.8& 2.0& 0.0& 0.0& 0.2& 0.0& 31.2 \\
316 AS & 0.0& 0.0& 0.0& 0.0& 0.0& 1.5& 0.0& 0.0& 0.0& 0.0& 0.0& 1.5 \\
317 POP & 0.0& 0.0& 0.0& 0.0& 0.1& 17.6& 0.1& 0.0& 0.0& 0.0& 0.0& 17.9 \\
318 \end{tabular}
319
320 And the following table shows the RF power at each port in mWatt (both quadratures).
321
322 \begin{tabular}{lccccccc}
323 \multicolumn{8}{l}{{\bf RF power at ports in mWatt}}\\ \hline
324 FREQ (MHz)& 0.0& 9.0& 18.0& 36.0& 45.0& 54.0& 90.0 \\ \hline
325 REFL &31.17& 0.02& 4.01& 0.82& 0.01& 0.83& 0.35 \\
326 AS & 1.48& 0.01& 0.00& 0.00& 0.11& 0.00& 0.00 \\
327 POP2 &17.91& 0.00& 0.24& 0.19& 0.00& 0.19& 0.03 \\
328 \end{tabular}
329
330 \subsection{Noise Subtraction}
331 Since the suspension thermal noise is dominant, we only need a modest amount of
332 MICH subtraction to suppress the MICH noise below thermal noise. However, the
333 couplings are the same as for AdvLIGO, and there is no reason not to implement
334 the same subtractions.
335
336 \subsection{DARM Noise Budget}
337
338 The Looptickle model included the following noise sources:
339 \begin{itemize}
340 \item {\bf Quantum} or {\bf Shot Noise} from the loop itself, calculated by
341 injecting vacuum noise at every open optical port.
342
343 \item {\bf Auxiliary length}: Shot Noise from the other loops, calculated by
344 propagating the quantum noise in the other loops through the control system.
345
346 \item {\bf Seismic Noise}, This is an estimate (power law) for the noise at the 40m.
347 It includes coupling through the new Eddy-current damped recycling cavity folding mirrors.
348
349 \item {\bf Mirror Thermal} noise, estimate for 40m.
350
351 \item {\bf Suspension Thermal} noise, estimate for 40m.
352
353 \item {\bf Frequency Noise} incident on the input mode cleaner (i.e. input
354 mode cleaner and common mode sensing noise are counted as shot noise from
355 those length loops). See figure \ref{fig:LaserNoise} for the assumed PSL noise level.
356
357 \item {\bf Intensity Noise} incident on the IMC. The measured out-of-loop
358 noise from the 40m ISS is $3 \times 10^{-8}$ from 80-3000~Hz.
359 See figure \ref{fig:LaserNoise} for the assumed PSL noise level.
360
361 \item {\bf Oscillator Phase Noise}. See figure \ref{fig:LaserNoise} for the assumed PSL noise level.
362 The coupling does include the light passing through the IMC.
363 Currently the estimate does not include potential noise added after the EOM/LO split.
364 The 40m lab currently uses IFR (Marconi) 2023 generators instead of Wenzel crystals for
365 signal generation.
366
367 \item {\bf Oscillator Amplitude Noise} See figure \ref{fig:LaserNoise} for the assumed PSL noise level.
368 Measurements of the iLIGO RF AM Stabilization box at the 40m~\cite{Valera:AM} show that the
369 AM noise stabilized EOM drive can be as low as $5 \times 10^{-8} \rm /\sqrt{Hz}$ above 10~Hz.
370
371 \end{itemize}
372
373 Figure \ref{fig:nbDARM} shows a noise budget for DARM.
374
375 \begin{figure}[htbp!]
376 \begin{center}
377 \includegraphics[width=17cm,height=20cm]{DARM_noisebudget.pdf}
378 \caption[nbDARM]{DARM noise budget.}
379 \label{fig:nbDARM}
380 \end{center}
381 \end{figure}
382
383 \begin{figure}[htbp!]
384 \begin{center}
385 \includegraphics[width=17cm,height=20cm]{LaserNoise.pdf}
386 \caption[nbDARM]{Assumed noise at the IMC input. N = RIN for laser intensity noise,
387 radians for oscillator phase noise, dA/A for oscillator amplitude noise, and
388 Hz for laser frequency noise.}
389 \label{fig:LaserNoise}
390 \end{center}
391 \end{figure}
392
393
394 \subsection{Optickle Sensing Matrices}
395 Tables~\ref{t:SensingMatrix100} and \ref{t:SensingMatrix1000} show the full sensing
396 matrix at 100~Hz and 1~kHz.
397 We intend to used the same error signals as AdvLIGO, namely REFL\_I1 for CARM,
398 AS\_DC for DARM, POP\_I1 for PRCL, POP\_Q2 for MICH and POP\_I2 for SRCL.
399 The 3-f signals for lock acquisition are easy to add, but were not yet modeled.
400 \input{sensingMatrix40m100Hz}
401
402 \input{sensingMatrix40m1kHz}
403
404 % ---------------------- IFO Diagram
405 \begin{figure}[!ht]
406 \begin{center}
407 \includegraphics[width=15cm]{IFO_diagram.pdf}
408 \caption{Def of LSC DOFs}
409 \end{center}
410 \label{fig:lsc-dia}
411 \end{figure}
412 % -----------------------------------------------------------------------
413
414 % =========================================================
415 \section{Lock Acquisition}
416 The duration of the lock acquisition process should be short enough
417 to not significantly impact the Detector availability for science
418 mode operation. In Acquisition mode there are no requirements on the
419 noise in the sensing systems (length or angle) in the GW band other than
420 what is required to prevent saturations during acquisition.
421
422 The low frequency (control band) angular fluctuations must be
423 consistent with small gain fluctuations in all LSC and ISC loops.
424
425 The present lock acquisition path at the 40m involves first locking
426 the interferometer with a non-negligible common arm offset which is
427 subsequently reduced to zero in a controlled manner. The first stage
428 of this technique is statistical in nature, and is the most
429 significant contributor to the uncertainty in time required to acquire
430 lock. This technique also introduces complications due to the
431 changing optical plant as the CARM offset is reduced.
432
433 We envisage developing an improved version of this path for
434 application to a broadband recycled 40m. Our approach will mirror that
435 covered in section 9 of \cite{ISC:CDD}. However, we do not plan to
436 implement a SPI at the 40m. We shall instead explore the possibilities
437 described in section~\ref{sec:auxsig}.This will involve two parallel
438 investigations:
439
440 \begin{itemize}
441 \item Control signals which are not affected by common arm offsets.
442
443 \item Independent control the arm cavity length at the nanometre level
444 - allows one to hold the arms both on or off resonance and smoothly
445 transition between these states.
446 \end{itemize}
447
448 We address each in turn:
449
450 \subsection{Harmonic Demodulation}
451
452 Simulations by L.~Barsotti show that error signals for the central
453 interferometer (PRCL,SRCL \& MICH) are available by demodulating the
454 reflected light at 3$f_1$ (PRCL, MICH) and 3$f_2$
455 (SRCL)\cite{ISC:CDD}. These signals are attractive as they present
456 little sensitivity to CARM detuning, while avoiding the complexity
457 introduced by doubly demodulating PD outputs.
458 \begin{figure}[!ht]
459 \begin{center}
460 \includegraphics[width=7.5cm]{3fsidebands.pdf}
461 \caption{The reflected signal demodulated at $3f_1$ is dominated by
462 the beat between first and second order sidebands.}
463 \end{center}
464 \label{fig:3fsidebands}
465 \end{figure}
466
467 Consider the case of the $3f_1$ demodulation. These $3f$ signals arise
468 from the beat between the first and second order sidebands
469 ($f_1SB1_{\pm}$ and $f_1SB2_{\mp}$) and between the carrier and third
470 order sidebands ($CR$ and $f_1SB3_{\pm}$) (figure~\ref{fig:3fsidebands}).
471 The signal from the beat between sidebands is
472 generally much larger than that due to the carrier/$SB3$ beat as the
473 interferometer reflectivity for the third order sidebands is low. In
474 contrast, the second order sidebands are non-resonant in the recycling
475 cavity and hence provide a stable local oscillator in reflection which
476 is almost independent of optical parameters, interferometer resonances
477 and CARM offset. Figure~\ref{fig:3fsignals} shows a
478 comparison between $1f$ and $3f$ signals for various CARM offsets.
479
480 \begin{figure}[!ht]
481 \begin{center}
482 \includegraphics[width=17cm]{3fsignals.pdf}
483 \caption{A comparison between (simulated) science mode and lock
484 acquisition signals for various CARM offsets. Left: PRCL, centre -
485 MICH, right - SRCL. Parameters used are representative of AdvLIGO,
486 we expect similar results at the 40m. Reproduced from p. 60 of
487 \cite{ISC:CDD}}
488 \end{center}
489 \label{fig:3fsignals}
490 \end{figure}
491
492
493 Preliminary tests at the 40m using existing electronics have shown that MICH and
494 PRCL can be held on REFL31 (Reflected light demodulated at
495 $3f_1$=99MHz) I \& Q, respectively.
496
497 \subsection{Auxiliary signals}
498 \label{sec:auxsig}
499
500 To manipulate the arm cavities independently of the `standard' control
501 structure we require some means of separating the chosen readout
502 method from the usual PSL light. There exist a number of
503 options:
504
505 \begin{itemize}
506 \item {\bf Orthogonal polarizations}
507
508 It is possible to inject orthogonally polarized light through any of
509 the auxiliary ports and distinguish these signals by means of a
510 polarizing beam splitter. To get a wide tuning range with this scheme
511 we would need to use an auxiliary laser and tune its frequency
512 by several~MHz. This scheme, as well as some of the following, would
513 require the frequency stabilization of an external laser with respect to
514 the PSL beam.
515
516 \item{\bf Frequency shifted PSL light}
517
518 Better control of the arm length is achievable using a frequency
519 shifted PSL pick-off. The shift would be in the hundreds of MHz
520 regime and would likely be introduced by double-passing an
521 AOM. Adjustment of the frequency shift via a VCO would give a wide
522 tuning range and allow the arm to be swept to a resonance of the PSL
523 light. A very similar scheme would be to use an external, tuneable
524 laser as above.
525
526 \item {\bf Other wavelengths}
527
528 The use of other wavelengths represents another valid option. A
529 simple choice might be to double the PSL light to 532~nm. This beam
530 benefits from the stabilisation of the PSL pump beam whereas other
531 light sources must be independently stabilised.
532
533 Use of a second wavelength would require dichroic coatings (both HR
534 and AR). Recent explorations by S. Ballmer suggest that this type of
535 coating can be achieved through small deviations from a 1/8, 3/8
536 design.
537
538 We should also take advantage of the dispersion in the bulk to achieve
539 a split of this other color with respect to the 1064~nm beam.
540
541 \item{\bf Digital interferometry/ PRN}
542
543 Phase modulated light, imprinted with pseudo random code is, when
544 suitably demodulated, able to determine the positions of optics over
545 a wide range without requiring any resonant phase
546 shifts\cite{Bram:WhitePaper}\footnote{We do not discuss the
547 suspension point interferometer mentioned in this reference as
548 there are no plans to study this idea at the 40m}. We do not
549 expect to investigate this technology independently but would like
550 to host outside experiments (ANU).
551
552
553 \end{itemize}
554
555 Having a modulated beam one must decide where to inject it to create
556 useful signals\footnote{We are presently considering an actively
557 stabilised fibre distribution system for transporting any
558 secondary light source.\cite{Ye:fibres}}. Two ideas have been proposed:
559
560 \begin{itemize}
561
562 \item{\bf Injecting through ETM} From the ETM side the arm cavities
563 are massively under coupled. This leads to small signal levels which
564 are could easily be dominated by technical noise. Additionally, the
565 light from the arm will enter the central cavities and efforts must
566 be taken to minimize the coupling between the arm signal and other
567 degrees of freedom. In this respect a second wavelength would be
568 advantageous as the dispersion of fused silica may allow the
569 separation of beams from the recycling cavities and the arm if the
570 wedges are of appropriate size and orientation. Note: this wedge
571 trick will not work for AdvLIGO since the beam sizes there are larger;
572 we would have to use a beam reducer to split those beams.
573
574 \item{\bf Injecting through pick-offs} Similar to the above. Injecting
575 through POX/POY gives an appreciable improvement in the signal to
576 noise ratio since the arm cavities become over coupled with this
577 topology. Coupling to the central interferometer is reduced. POP is
578 also under consideration but is dependent on the layout of the
579 stable recycling cavities.
580
581 \end{itemize}
582 % =========================================================
583
584 \section{Noise Diagnostics}
585 \label{sec:diag}
586 The following is a non-exhaustive list of diagnostic tests that we
587 will do in order to verify our models of a DRFPMI with DC Readout.
588 As in the past, we expect the most useful parts of the noise coupling
589 measurements to be where we do not find agreement with the Optickle
590 model. Performance requirements for the AdvLIGO system are listed
591 in Section XXX of the ISC CDD~\cite{ISC:CDD}.
592
593
594
595 \subsection{Sensing and Control System Noise}
596 \subsubsection{Beam jitter coupling}
597 Band-limited noise injections on all of the interferometer optics
598 as well as the input beam to the interferometer and the OMC tip/tilt
599 steering mirrors. We will apply this noise in conjunction with low
600 frequency misalignments of the core optics in order to verify our
601 models of bilinear jitter coupling~\cite{Sigg:MM10}.
602
603 \subsubsection{Auxiliary Length Degrees-of-freedom}
604 The sensing and control of each auxiliary length degree-of-freedom
605 must be done in such a way that limits the coupling of their sensing
606 noise into the GW readout. We will check via standard swept sine
607 tests that the modeled transfer functions are correct and also do a
608 set of Monte-Carlo offset/noise tests as above to check our estimates
609 of allowable loop offsets.
610
611 \subsubsection{Frequency Stabilization}
612 Due to the short arm cavity lengths, we expect there not be a significant
613 frequency noise coupling in the 40m, even with low light levels on the
614 CARM/CM sensing detector(s). This can be compromised by scattering
615 and clipping noise in the MC/CM sensing chains but this also seems
616 controllable. There are frequency noise injection points available on these
617 servos.
618
619 \subsubsection{Laser Intensity Noise}
620 Laser intensity stabilization at the $5 \times 10^{-8} \rm /\sqrt{Hz}$ level
621 which is currently achieved seems copacetic and so we will not pursue an
622 AdvLIGO style Intensity Stabilization Servo (ISS). The existing system has
623 an injection point which can hooked up to the AWG.
624
625 \subsubsection{Modulation Source Noise}
626 Modulation/Oscillator noise is often tricky to model correctly and diagnose. The
627 baseline plan is to continue with the Marconi (IFR) generators even though they
628 have a much higher phase noise than the Wenzel OCXOs employed in iLIGO and
629 GEO600. The Marconi and the RF AM Stabilization circuit allow high bandwidth
630 modulation and straightforward wideband tuning range which are the {\it sine qua non}
631 of oscillator noise characterization.
632
633 \section{DC Readout System}
634
635 As Advanced LIGO will use the technique known as \emph{DC Readout}~\cite{pf:Talk03} for
636 detection of the gravitational wave channel it is a crucial part of the global length
637 sensing and control system, and thus needs to be tested in an integrated manner. The current
638 40\,m has a DC Readout system which consists of:
639
640 \begin{itemize}
641 \item A pair of piezo-electric tip-tilt steering mirrors (from Piezo-Jena) at the output
642 port of the interferometer.
643 \item A length adjustable (via New Focus picomotor), fixed mount, mode-matching telescope
644 with spherical mirrors.
645 \item A monolithic, four-mirror mode cleaning cavity with a finesse of 210 and a transmission
646 of approximately 90\%. The body is made of copper. The beam waist is 370 microns.
647 \item A pair of bare photodiodes (exposed to the vacuum) with an electronics amplification
648 package housed in a separate canister, all installed in the vacuum system atop a
649 seismic isolation stack.
650 \end{itemize}
651
652 As of this writing, the DC readout has not been demonstrated with a
653 DR system. The program to study the PRFPMI with DC Readout was extended
654 to support noise modeling for Enhanced LIGO. The system was designed to
655 work with the DRFPMI and it is expected to also work well with the
656 upgraded system. The HOM susceptibility analysis was done assuming the
657 old system with the higher frequency sidebands and so it remains to be
658 determined if the existing low finesse OMC will be sufficient in isolating
659 the DC Readout photodiodes.
660
661 As the location and size of the arm cavity beam waist is not expected to change, all the
662 installed components will function in the upgraded 40\.m interferometer. However, desirable
663 upgrades include, in order of priority:
664
665 \begin{itemize}
666 \item{\bf Photodetectors} The currently installed vacuum-compatible DC Readout photodetectors
667 are a first prototype, and improvements have been made to the design for Enhanced LIGO. The
668 40\,m DC Readout system should be updated with a new photodetection chain with electronics
669 similar to that being installed into Enhanced LIGO in order to lower the front end electronics
670 noise. This will require laying out a new board to fit inside the in-vacuum can which currently
671 contains the PD pre-amp.
672
673 \item{\bf OMC} A new output mode cleaner can be built and installed which matches the finesse
674 of the projected Advanced LIGO output mode cleaner, to provide a more faithful
675 representation. Another option is to swap mirrors to up the finesse of the existing
676 copper OMC. We will wait to do some supporting calculations before going down
677 that road.
678
679 \item{\bf Tip-Tilts} The piezo-electric steering mirrors can be replaced with suspended \
680 tip-tilt mirrors, to more faithfully reproduce the situation in advanced LIGO.
681 Equipping these with curved mirrors could allow the removal of the fixed
682 mode-matching telescope. The baseline plan is to stay with the PZT mirrors.
683
684 \end{itemize}
685
686
687 \section{Squeezed Light Injection}
688 The 40m upgrade will allow for squeezed light/vacuum injection through the dark port via
689 the AS port Faraday Isolator. At this time we do not include the design of such a squeezer
690 but just leave the port open in case future squeezed light research indicates a need for
691 this kind of work.
692
693 \section{Adaptive Noise Cancellation}
694 Recent work~\cite{Pepper:SURF} has shown that there is some
695 promise in exploring active cancellation of environmental noise sources. The concomitant
696 technique of adaptive filtering has recently~\cite{Matt:AdaptiveTalk} been demonstrated
697 on a real time system in the 40m lab.
698
699 In order to pursue this we will have a dedicated front end system acquire many signals
700 including the PEM and SUS/ASC sensors. Some of the envisioned development includes:
701
702 \begin{itemize}
703 \item {\bf Seismic} Using the ultra-low frequency Wilcoxon accelerometers as well as a few
704 seismometers (Ranger SS-1, Guralp CMG-40T, etc.) we will send signals to the
705 longitudinal (POS) input of the suspensions
706 to minimize fringe velocities for ease of lock acquisition. This will not lower the overall
707 control signal amplitudes at the 40m nor reduce the amount of angular fluctuation but
708 should ease locking efforts during the daytime.
709
710 From our modeling we expect that optic angular fluctuations at the 40m and in AdvLIGO
711 are dominated by length to angle cross-coupling in the suspensions and so it is not likely
712 to be useful to attempt feed-forward cancellation of tilt at the 40m (although it should be
713 useful for the AdvLIGO SEI system).
714
715 \item {\bf Acoustic} To explore higher frequency noise cancellation and reduce the noise in the
716 100-1000~Hz band, we will install a set of microphones in the LSC sensing tables and the
717 PSL and attempt to cancel some of the observed scattering/clipping noises. All of the
718 ISC beam paths in AdvLIGO will be in-vacuum to avoid this effect but the technique may
719 be useful to reduce unexpected couplings.
720
721 \item {\bf Magnetic} A set of magnetometers (Bartington or otherwise) will be installed to attempt
722 subtraction of power line harmonics picked up in the sensing channels and from pickup
723 in the test mass magnets. In AdvLIGO this technique may be useful to cancel pickup in the
724 PUM (PenUltimate Mass) of the quad suspensions and the actual mirror of the triple
725 suspensions of the beamsplitter and recycling cavity suspensions. Direct hardware based
726 subtraction may reduce the bilinear noise which masks the Crab pulsar signal in iLIGO.
727
728 \item {\bf RF} A more speculative source of excess noise is direct coupling of RFI at the LSC
729 modulation frequencies into the readout electronics. RF antennae with
730 modulators/demodulators will be installed to
731 explore noise cancellation via this mechanism.
732
733 \item {\bf LSC Corr} A potentially significant noise limit in AdvLIGO may be feedthrough of
734 sensing noise from the aux. length loops. In order to achieve large cancellation factors
735 of this noise we may be forced to use adaptive noise cancellation. We will build this into
736 the LSC system at the 40m either directly or by hooking in a parallel process running the
737 adaptive code.
738 \end{itemize}
739
740 The total list of sensors is still being determined but is likely to include a seismometer, several
741 accelerometers, microphones, magnetometers, and RF antennae. We will use the existing PEM
742 and PSL/IOO ADCU systems to acquire the slow channels. For all of the $\sim$100~Hz BW
743 cancellation we will require ADCs with less delay than the ICS-110B and therefore would hook
744 into the new acquisition system with General Standards ADCs.
745 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
746
747 \begin{figure}[h]
748 \begin{center}
749 \includegraphics[width=17cm,angle=90]{SENDS.pdf}
750 \caption[DAQ]{DAQ and Controls Diagram}
751 \end{center}
752 \label{fig:SENDS}
753 \end{figure}
754
755
756 % =================================================================
757
758
759 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
760
761 \newpage
762 \begin{thebibliography}{99}
763
764 \bibitem{aLIGO:Wiki}
765 LIGO, ``Advanced LIGO Wiki'',
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767 http://ilog.ligo-wa.caltech.edu:7285/advligo/AdvLigo}
768
769 \bibitem{ISC:DRD}
770 ISC~Group, "Interferometer Sensing and Control Requirements",
771 \href{http://www.ligo.caltech.edu/docs/T/T070236-00.pdf}
772 {\tt http://www.ligo.caltech.edu/docs/T/T070236-00.pdf}
773
774 \bibitem{ISC:CDD}
775 ISC~Group, "Interferometer Sensing and Control Conceptual Design",
776 \href{http://www.ligo.caltech.edu/docs/T/T070247-00.pdf}
777 {\tt http://www.ligo.caltech.edu/docs/T/T070247-00.pdf}
778
779 \bibitem{ArmFinesse}
780 R.~Adhikari, S.~Ballmer, M.~Evans, P.~Fritschel, "Arm Cavity Finesse For
781 Advanced LIGO",
782 \href{http://www.ligo.caltech.edu/docs/T/T070303-01.pdf}
783 {\tt http://www.ligo.caltech.edu/docs/T/T070303-1.pdf}
784
785 \bibitem{40mcdr01}
786 B.~Abbott {\it et al.}, ``Conceptual Design of the 40 meter Laboratory Upgrade
787 for prototyping a Advanced LIGO Interferometer'', T010115,
788 \href{http://www.ligo.caltech.edu/docs/T/T010115-00.pdf}
789 {\tt http://www.ligo.caltech.edu/docs/T/T010115-00.pdf}
790
791 \bibitem{40m-sidebands}
792 B.~Barr {\it et al.},
793 ``Control sideband generation for dual-recycled laser
794 interferometric gravitational wave detectors'',
795 Class. Quantum Grav. 23 (2006) 5661,
796 \href{http://www.ligo.caltech.edu/docs/P/P060022-02}
797 {\tt http://www.ligo.caltech.edu/docs/P/P060022-02}
798
799 \bibitem{40m-DRFPMI}
800 O.~Miyakawa {\it et al.},
801 ``Measurement of Optical Response of a Detuned Resonant Sideband Extraction Interferometer'',
802 Phys.Rev.D74, 022001 (2006),
803 \href{http://www.ligo.caltech.edu/docs/P/P060007-01.pdf}
804 {\tt http://www.ligo.caltech.edu/docs/P/P060007-01.pdf}
805
806 \bibitem{40m-DCreadout}
807 R.~Ward {\it et al.},
808 ``DC Readout Experiment at the Caltech 40m Prototype Interferometer'',
809 \href{http://www.ligo.caltech.edu/docs/ScienceDocs/P/P070125-00.pdf}
810 {\tt http://www.ligo.caltech.edu/docs/ScienceDocs/P/P070125-00.pdf}
811
812 \bibitem{SRD}
813 A.~Lazzarini and R.~Weiss,
814 ``{LIGO} Science Requirements Document'',
815 \href{http://www.ligo.caltech.edu/docs/E/E950018-02.pdf}
816 {\tt http://www.ligo.caltech.edu/docs/E/E950018-02.pdf}
817
818 \bibitem{pf:Talk03}
819 P.~Fritschel, ``DC Readout for Advanced LIGO'',
820 \href{http://ilog.ligo-wa.caltech.edu:7285/advligo/AdvLigo} {\tt
821 http://ilog.ligo-wa.caltech.edu:7285/advligo/AdvLigo}
822
823 \bibitem{wiki:nonlinearfringe}
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825 {\tt http://lhocds.ligo-wa.caltech.edu:8000/mLIGO/Nonlinearity\_of\_the\_DC\_readout\_signal}
826
827 \bibitem{Rana:Thesis}
828 R.~Adhikari, ``Sensitivity and Noise'',
829 \href{http://www.ligo.caltech.edu/docs/P/P040032-00.pdf} {\tt
830 http://www.ligo.caltech.edu/docs/P/P040032-00.pdf}
831
832 \bibitem{Ballmer:Thesis}
833 S.~Ballmer, ``LIGO Interferometer Operating as a Radiometer'',
834 \href{http://www.ligo.caltech.edu/docs/P/P060043-00.pdf} {\tt
835 http://www.ligo.caltech.edu/docs/P/P060043-00.pdf}
836
837 \bibitem{Kirk:Length}
838 K.~McKenzie, "Mode Cleaner Length Measurement",
839 \href{http://tinyurl.com/2ubeac}
840 {\tt http://tinyurl.com/2ubeac}
841
842 \bibitem{Valera:AM}
843 V.~Frolov, "RFAM of the RF stabilization box is measured",
844 \href{http://tinyurl.com/4rkq6d}
845 {\tt http://tinyurl.com/4rkq6d}
846
847 \bibitem{siggsidles}
848 D.~Sigg and J.~Sidles, ''Optical Torques in Suspended Fabry-Perot Cavities",
849 \href{http://www.ligo.caltech.edu/docs/P/P030055-C/}
850 {\tt http://www.ligo.caltech.edu/docs/P/P030055-C/}
851
852 \bibitem{LSC:FDD}
853 ISC~Group, ``Length Sensing and Control Final Design (iLIGO)'',
854 \href{http://www.ligo.caltech.edu/docs/T/T980068-00.pdf}
855 {\tt http://www.ligo.caltech.edu/docs/T/T980068-00.pdf}
856
857 \bibitem{pf:S5}
858 P.~Fritschel, "LIGO",
859 \href{http://arxiv.org/abs/0711.3041}{\tt
860 http://arxiv.org/abs/0711.3041}
861
862 \bibitem{Matt:Optickle}
863 M.~Evans, "Optickle",
864 \href{http://www.ligo.caltech.edu/docs/T/T070260-00.pdf}{\tt
865 http://www.ligo.caltech.edu/docs/T/T070260-00.pdf}
866
867 \bibitem{Matt:OptickleTalk}
868 M.~Evans, "Optickle (LSC talk)",
869 \href{http://www.ligo.caltech.edu/docs/G/G070728-00}{\tt
870 http://www.ligo.caltech.edu/docs/G/G070728-00}
871
872 \bibitem{Bram:WhitePaper}
873 B.~Slagmolen et al., ``Adv. LIGO Arm Cavity Pre-Lock Acquisition System'',
874 AdvLIGO Wiki, SPI page,
875 \href{http://lhocds.ligo-wa.caltech.edu:8000/advligo/}
876 {\tt http://lhocds.ligo-wa.caltech.edu:8000/advligo/}
877
878 \bibitem{Osamu:MachZender}
879 O.~Miyakawa et. al., "Mach-Zehnder interferometer for Advanced-LIGO optical
880 configurations to eliminate sidebands of sidebands",
881 \href{http://www.ligo.caltech.edu/docs/T/T040119-00.pdf}
882 {\tt http://www.ligo.caltech.edu/docs/T/T040119-00.pdf}
883
884 \bibitem{Ye:fibres}
885 S.M.~Foreman et al, ``Remote transfer of ultrastable frequency references
886 via fiber networks'', Review of Scientific Instruments 78, 021101/1-25 (2007).
887
888 \bibitem{Sigg:MM10}
889 D.~Sigg et. al., "Modal Model Update 10: Noise Coupling and
890 Random Imperfections",
891 \href{http://www.ligo.caltech.edu/docs/T/T980001-00.pdf}
892 {\tt http://www.ligo.caltech.edu/docs/T/T980001-00.pdf}
893
894 \bibitem{Matt:AdaptiveTalk}
895 M.~Evans, R.~Adhikari, "Adaptive Noise Cancellation (LSC talk)",
896 \href{http://www.ligo.caltech.edu/docs/G/G080213-00/}
897 {\tt http://www.ligo.caltech.edu/docs/G/G080213-00}
898
899 \bibitem{Pepper:SURF}
900 K.~Pepper, "Newtonian Noise Simulation and Suppression for
901 Gravitational Wave Interferometers (SURF Report)",
902 \href{http://www.ligo.caltech.edu/docs/T/T070192-00/}
903 {\tt http://www.ligo.caltech.edu/docs/T/T070192-00/}
904
905 \bibitem{Weiss:QPR}
906 R.~Weiss, "Electromagnetically Coupled Broadband Gravitational Antenna",
907 \href{http://www.ligo.caltech.edu/docs/P/P720002-01}
908 {\tt http://www.ligo.caltech.edu/docs/P/P720002-01}
909
910 \end{thebibliography}
911
912 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
913 \newpage
914 \appendix
915
916 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
917 \section{New Interferometer Parameters}
918 \label{app:params}
919
920 \renewcommand{\arraystretch}{1.4}
921 \begin{table}[!h]
922 \begin{center}
923 \begin{tabular}{lc}
924 \hline\hline
925 ITM Transmission & 0.014 \\
926 %
927 PRM Transmission & 0.07 \\
928 %
929 SRM Transmission & 0.07 \\
930 %\hline
931 ITM imbalance ($T_{ITMX} - T_{ITMY}$) & $T_{ITM} /50$\\
932 %\hline
933 Average round trip arm loss & 160 ppm \\
934 %\hline
935 Differential arm loss & 50 ppm \\
936 %\hline
937 Beamsplitter R/T imbalance & $\pm5 \times 10^{-3}$ \\
938 %\hline
939 Differential arm offset from dark fringe & 40 pm \\
940 %\hline
941 Arm Cavity finesse & 446 \\
942 %\hline
943 Output mode cleaner finesse & 250 \\
944 %
945 Power Recycling cavity length & 8.328 m \\
946 %
947 Signal Recycling cavity length & 6.662 m \\
948 %
949 Schnupp Asymmetry & 0.05 m \\
950 \hline\hline
951 \end{tabular}
952 \caption{Parameters used in the Optickle modeling. Shown are those
953 parameters that determine the laser and modulation source noise
954 couplings.}
955 \label{tab:params}
956 \end{center}
957 \end{table}
958 \renewcommand{\arraystretch}{1}
959 % ----------------------------------------------------------------------------------------
960 \newpage
961 \section{Current (2007) Interferometer Parameters}
962 \label{app:oldparams}
963
964 \renewcommand{\arraystretch}{1.4}
965 \begin{table}[!h]
966 \begin{center}
967 \begin{tabular}{lc}
968 \hline\hline
969 ITM Transmission & 0.005 \\
970 %
971 PRM Transmission & 0.07 \\
972 %
973 SRM Transmission & 0.07 \\
974 %
975 ITM imbalance ($T_{ITMX} - T_{ITMY}$) & $T_{ITM} /11$\\
976 %
977 Average round trip arm loss & 160 ppm \\
978 %
979 Differential arm loss & 20 ppm \\
980 %
981 Beamsplitter R/T imbalance & $\pm5 \times 10^{-3}$ \\
982 %
983 Differential arm offset from dark fringe & 25 pm \\
984 %
985 Output mode cleaner finesse & 210 \\
986 %
987 Input mode cleaner finesse & 1500 \\
988 %
989 Input mode cleaner length & 13.547 m \\
990 %
991 Power Recycling cavity length & 2.15 m \\
992 %
993 Signal Recycling cavity length & 2.257 m \\
994 %
995 Schnupp Asymmetry & 0.452 m \\
996 %
997 \hline\hline
998 \end{tabular}
999 \caption{Measured parameters of the 40m interferometer as of March 2008. Shown are those
1000 parameters that determine the laser and modulation source noise couplings.}
1001 \label{tab:oldparams}
1002 \end{center}
1003 \end{table}
1004 \renewcommand{\arraystretch}{1}
1005 % ----------------------------------------------------------------------------------------
1006 \clearpage
1007 \section{Interferometer Displacement Noise Spectrum}
1008 \label{app:straintarget}
1009
1010 \begin{figure}[!ht]
1011 \begin{center}
1012 \includegraphics[width=17cm,height=16cm]{40.pdf}
1013 \label{fig:lsc-dia}
1014 \caption{Displacement noise from the 40m Bench model (in the ISC Modeling CVS).
1015 The quantum noise is shown for broadband and detuned RSE cases.}
1016 \end{center}
1017 \end{figure}
1018
1019
1020 % #####################################################################
1021 % ####################################################################
1022 \clearpage
1023 \section{Background and Motivation}
1024 \label{app:motive}
1025
1026 Throughout the 1990's,
1027 the 40m interferometer was used for developing and testing
1028 elements of the Initial LIGO optical configuration and control scheme.
1029 It was upgraded in 2001 - 2004 in order to test elements of the
1030 (then conceived) Advanced LIGO optical configuration and control scheme,
1031 as described in the ``Conceptual Design of the 40 meter Laboratory Upgrade
1032 for prototyping a Advanced LIGO Interferometer'', T010115~\cite{40mcdr01}.
1033 The main parameters of that upgrade are given in Appendix
1034 \ref{app:oldparams}.
1035
1036 The main goals of that upgrade were to learn how to control
1037 a power-recycled Fabry-Perot Michelson (PRFPMI) suspended mass interferometer
1038 with a signal recycling cavity (SRC) operated in
1039 detuned resonant sideband subtraction mode (RSE, forming a
1040 dual-recycled FPMI, DRFPI)
1041 (see Appendix C of \cite{40mcdr01}).
1042 The Advanced LIGO design at the time called for the following
1043 innovations in optical configuration and control:
1044 (a) High finesse arm cavities ($\sim$1200) in order to reduce
1045 the power required to circulate in the power recycling cavity.
1046 (b) RSE to recover the high-frequency signal response
1047 of the detector, ``detuned''
1048 to optimize it for detection of binary neutron star
1049 inspirals in the presence of other technical noise sources.
1050 (c) A signal extraction scheme for controlling the
1051 DRFPMI that employed a moderate Schnupp asymmetry and
1052 two pairs of RF sidebands
1053 with very different frequencies, in order to provide the
1054 best contrast between the PRC and SRC length degrees of freedom
1055 (9~MHz and 180~MHz for AdvLIGO, and 33~MHz and 166~MHz for the 40m testbed,
1056 employing ``double-demod'' signals derived from the sidebands alone
1057 to control the DRMI).
1058 (d) A Mach-Zehnder to apply both pairs of sidebands in parallel,
1059 eliminating ``sidebands on sidebands'' \cite{40m-sidebands}.
1060 (e) DC readout for homodyne detection of the GW signal using
1061 arm-filtered carrier light as the local oscillator.
1062
1063 By the end of 2005, the 40m team was able to acquire lock and control
1064 all five degrees of freedom of the DRFPMI, in detuned RSE \cite{40m-DRFPMI}.
1065 By 2007, DC readout was demonstrated on a PRFPMI~\cite{40m-DCreadout}.
1066
1067 In the process, several issues were encountered:
1068 (a) lock acquisition and control of high-finesse arm cavities
1069 is very difficult, requiring the development of many tricks
1070 to get robust signals during lock acquisition.
1071 (b) The detuned signal cavity formed an optical spring
1072 which complicated the dynamics of lock acquisition.
1073 (c) High-frequency RF (greater than say, 50 MHz)
1074 is hard to work with.
1075 (d) Double-demod signals have poor SNR and the demod phases seem to
1076 drift around excessively without obvious explanations.
1077 (e) The Mach-Zehnder solution proved to have its own problems: intensity
1078 noise and non-stationary RFAM.
1079 (f) The VME-based control electronics architecture is a liability just as
1080 it is for initial LIGO.
1081
1082 Partially in response to these observations, the AdvLIGO ISC
1083 group reconsidered high-finesse arm cavities \cite{ArmFinesse},
1084 signal extraction schemes that make use of lower RF
1085 sideband frequencies \cite{ISC:CDD}, and continuing with serial
1086 modulation as in initial LIGO.
1087
1088 It will be important to test this new scheme with a realistic,
1089 suspended interferometer. The 40m lab is a near-ideal place to do this.
1090
1091 % :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
1092 \begin{table}
1093 \begin{center}
1094 \begin{tabular}{|l|r|r|r|r|c|}\hline
1095 Parameter &40m (1998)& LIGO 4K & 40m(2002) & Adv LIGO & units \\ \hline
1096 Carrier $\lambda$ & 514.5 & 1064. & 1064. & 1064. & nm \\
1097 Transmissivity T(ETM) & 1.2E-5 & 1.5E-5 & 1.0E-5 & 1.0E-5 & \\
1098 Transmissivity T(ITM) & 0.00565 & 0.02995 & 0.005 & 0.005 & \\
1099 Transmissivity T(RM) & 0.1375 & 0.0244 & 0.07 & 0.07 & \\
1100 Mode cleaner length & 1.0 & 12.255 & 13.542 & 16.655 & m \\
1101 FSR$_{MC}$ & 150. & 12.23 & 11.07 & 9.00 & MHz \\
1102 RF freq1 $f_1 = n_1 fsr_{mc}$ & 32.7 & 24.46 & 33.207 & 9.00 & MHz \\
1103 Arm Cavity $L_{arm}$ & 38.25 & 3999. & 38.55 & 3999. & m \\
1104 PR Cavity $L_{PRC}$ & 2.294 & 9.191 & 2.257 & 8.328 & m \\
1105 PRM--BS length & 0.25 & 4.396 & 0.30 & 4.000 & m \\
1106 BS--ITMinline length & 2.315 & 4.877 & 2.183 & 4.536 & m \\
1107 BS--ITMperpin length & 1.773 & 4.599 & 1.731 & 4.119 & m \\
1108 Schnupp Asymmetry length & 0.542 & 0.278 & 0.451 & 0.416 & m \\
1109 Arm cavity pole freq & 1814 & 91 & 1578 & 15 & Hz \\
1110 Arm Cavity Finesse & 1080 & 205 & 1235 & 1235 & \\
1111 Rec Cavity Finesse & 24 & 138 & 47 & 47 & \\
1112 Arm Cavity power gain & 670 & 130 & 775 & 775 & \\
1113 Rec Cavity power gain & 9 & 48 & 16.5 & 16.5 & \\
1114 mirror diameter & 10.16 & 25.0 & 12.5 & 31.4 & cm \\
1115 mirror length & 8.89 & 10.0 & 5.0 & 13.0 & cm \\
1116 mirror mass & 1.58 & 10.8 & 1.35 & 40.0 & kg \\
1117 PRM ROC & flat & 8700 & 348 & 8700 & m \\
1118 ITM ROC & flat & 14540 & flat & 14540 & m \\
1119 ETM ROC & 61 & 7400 & 57.375 & 7400 & m \\
1120 $n_1$ & - & 3 & 3 & 1 & \\
1121 $n_2$ & 0 & 1 & 0 & 0 & \\
1122 $n_3$ & 7.84 & 652.13 & 8.05 & 239.61 & \\ \hline
1123 $n_4$ & & & 0 & 0 & \\
1124 $n_5$ & & & 5 & 21 & \\
1125 RF freq2 $f_2 = n_4 f_1$ & & & 166.033 & 180.0 & MHz \\
1126 SR Cavity $L_{SRC}$ & & & 2.151 & 9.148 & m \\
1127 SRM--BS length & & & 0.200 & 3.821 & m \\
1128 RSE peak frequency & & & 4000 & 300 & Hz \\
1129 Signal Cavity tune & & & 0.235 & 0.038 & rad/($\pi/2$)\\
1130 SRM ROC & & & 365 & 9000 & m \\
1131 \hline
1132 \end{tabular}
1133 \caption{
1134 Historical comparison of several interferometers' design parameters:
1135 the 40m in 1998 (recycling experiment),
1136 the Initial LIGO 4K interferometers,
1137 the 40m in 2002 (dual recycling for Advanced LIGO),
1138 and the Advanced LIGO design circa 2002.
1139 }
1140 \end{center}
1141 \label{tab:ancientparams}
1142 \end{table}
1143 % #######################################################################
1144 % #######################################################################
1145
1146
1147 %-----------------------------------------------------------------------------------------
1148
1149 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
1150
1151
1152
1153 \end{document}
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