Title LIGO-G1100174 DC Readout in Enhanced LIGO 1 Enhanced LIGO - - PowerPoint PPT Presentation

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Title

DC Readout in Enhanced LIGO

LIGO-G1100174

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Enhanced LIGO

More Power Output Mode Cleaner DC Readout

• Try out Advanced LIGO technologies
• Bet that increased sensitivity outweighs the downtime

exposure = time * (range)^3 New Laser New input optics New Thermal Compensation New Alignment Control

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Interferometer

?

Gravitational waves produce phase modulation in the arm cavities How to detect it?

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DC Readout: sideband view

• ptical frequency
• rf

+rf

• gw

+gw

• ptical frequency
• rf

+rf

• gw

+gw HETERODYNE (RF) HOMODYNE (DC) laser carrier

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DC Readout: fringe view

power at AS port DARM length

10 picometers

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DC Readout

DARM_ERR Looks pretty simple...

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Junk Light

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DC Readout with OMC

DARM_ERR

Clean up the light at the AS port with an

• utput mode cleaner.

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DC Readout promises

• fundamental improvement in SNR
• technical improvement in SNR
• perfect overlap of local oscillator and signal beams
• junk light removal by OMC
• independence from RF oscillator noises
• exploit the amazing filtering ability of the interferometer
• Easier platform for squeezed light injection
• Easier to handle higher power

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DC Readout: phasor view

• ptical gain:

How do we choose the DARM offset?

• Must be much greater than residual DARM displacement
• Must overcome contrast defect and electronics noise
• But not excessively detrimental to power recycling

In practice: turn the knob to get the best sensitivity

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OMC design

Sam Waldman et al beam from interferometer two DC photodiodes two quadrant photodiodes (QPDs) monolithic, suspended, in-vacuum bowtie cavity fast and slow length actuators beam steering mirrors

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Dither Locking

in phase 180 degrees

• ut of phase
• 1. put in small dither sinusoid
• 2. demodulate output at same freq

== error signal! no first-order response at maximum

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OMC Length Control

DARM

Cavity length dithered at ~10 kHz via PZT actuator PZT offloaded onto slow, long-range thermal actuator

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OMC Alignment Control

Initial idea: maximize transmission through the OMC The mode cleaner will clean the modes if you can identify what mode you want to keep.

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Junk light confuses simple servo

00 mode 01 mode 00 + 01

transmission versus beam pointing

01 mode leads the servo astray

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Drumhead Beacon Dither

Excite the test-mass drumhead mode (9 kHz) M.Evans/Nicolás(LHO) Dither the "tip tilt" mirrors at low frequency (~3 Hz) Idea: Tag the photons in the arm by modulating the ETM detect power in drumhead mode demodulate at dither frequency

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Optical Gain vs Jitter

But if we optimize alignment for

• ptical gain...

...and best optical gain does not correspond to maximum transmission ...then we introduce a beam jitter coupling

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Beam Jitter Noise

Most significant new noise source 130 Hz, one of the more prominent jitter peaks

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A closer look at 130 Hz

bilinear coupling linear coupling 0.87 Hz 1.6 Hz 130 Hz 130 Hz ± 0.875, ± 1.6

due to gain modulation due to HG 01 mode

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10

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L1:OMC−QPD4_SUM_IN1_DAQ arbitrary units frequency [Hz]

125 126 127 128 129 130 131 132 133 134 135 10

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L1:OMC−QPD3_Y_OUT_DAQ arbitrary units frequency [Hz]

125 126 127 128 129 130 131 132 133 134 135 10

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L1:LSC−DARM_ERR arbitrary units frequency [Hz]

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A closer look at 130 Hz

Quick check: overlay the spectra

125 126 127 128 129 130 131 132 133 134 135 10

−9

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−8

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−7

10

−6

arbitrary units frequency [Hz]

DARM QPD YAW QPD SUM (shifted)

0 Hz 129.9 Hz 129.9 Hz mirror image of qpd sum spectrum

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Can we predict the timeseries?

(bilinear wiener filtering?)

have to be careful to not subtract DARM from DARM

QPD sum QPD yaw

125 126 127 128 129 130 131 132 133 134 135 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

frequency coherence

DARM

ASD (arbitrary)

125 126 127 128 129 130 131 132 133 134 135 0.5 1 1.5 2 2.5 3 3.5 4 4.5 x 10

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QPD YAW SUMHP(YAW)

One of several possible bilinear couplings.

• Yes. Good coherence

shows this is a real coupling.

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Beam jitter mitigation

• Remove the offending resonance
• r increase isolation

i.e. tip-tilt blade springs, no fixed mirror

• Reduce sensitivity to the motion

i.e. clever telescope design

• Cancel the motion (feedback/forward)

i.e. 60 Hz magnetometer FF

• Reshape the output beam

i.e. use WFS1 to push on the ASC

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Noise Couplings

* Oscillator amplitude * Oscillator phase * Laser intensity * Laser frequency

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Oscillator Amplitude noise

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Oscillator Phase noise

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Anatomy of intensity noise coupling

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−1

10 10

1

10

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−120 −100 −80 −60 −40 −20 frequency [Hz] Watts per Watt [dB] carrier + sidebands carrier only sidebands only

sideband AM carrier AM cancellation

recycling gain carrier Michelson transmission

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Anatomy of intensity noise coupling II

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−1

10 10

1

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2

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3

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−120 −100 −80 −60 −40 −20 frequency [Hz] Watts per Watt [dB] carrier + sidebands carrier only sidebands only

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Laser intensity noise

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2

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−15

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−14

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−13

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−12

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−10

Laser intensity noise to DARM m/RIN frequency [Hz]

MODELS

warm colors = positive DARM offset cool colors = negative DARM offset

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Laser frequency noise

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−18

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−16

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−14

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−12

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−10

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Laser frequency noise to DARM (DARM meters) per (CARM Hz) frequency [Hz]

seems high - calibration?

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Noise Couplings To-Do

• Wrap control loops around Optickle results

(Pickle) and around measured results to see effect of cross-coupling.

• T

une parameters to fit results as well as possible

• Be informed by / compare with other methods:
• Zach's mode tracking absorption msmts
• Mode scans
• Arm cavity pole (ringdown) measurements

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Digital Gotchas

* Not new! * But we keep 'rediscovering' them!

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Digital Gotchas: Synchronization

Jul2009 Oct2009 Jan2010 Apr2010 −5 5 10 15 20 25 30 35 40 microseconds duotone delay history LSC OMC

* synchronization of communication * ADC timestamp synchronization Duotone tracking shows nondeterministic ADC startup and drift. OMC LSC

analog reference signal (two sines) recorded by independent ADCs phase of timestamped digital signals compared (in plot to the left)

OMC LSC

reflected memory

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Digital Gotchas: Other

* Quantization noise in DACs

• - "dark noise" not enough

* Floating point dynamic range/oddities

• - don't add small numbers to big numbers
• - use double precision
• - beware denormalized numbers (slow)

* Nondeterministic runtimes

• - need to leave headroom in cycle time

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Commissioning

DECEMBER 2009 MAY 2008

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Shot noise

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−20

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−18

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Frequency (Hz) Displacemnet (m/ Hz) L1 DARM Noise. SensMon 19.8 Mpc. DARM (13.7 W) Shot Noise (13.7 W) Shot Noise (25 W) iSRD (~8 W) eSRD (~25 W)

V.Frolov

May 2010

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Last slide

Thanks for listening!