Photothermal Common-Path Interferometer - LIGO

PCI Technique for ThermalAbsorption MeasurementsAshot Markosyan, Roger Route, HelenaArmandula, Martin Fejer and many othersfrom the LIGO teamAbsorption Loss Measurementson LIGO OpticsLIGO-G080315-00-Z

Outline• Photothermal Common-Path Interferometer• Surface Absorption Measurements on CoatedOptics• Transversal scans through mirror: point data• Longitudinal scans: surface inspection• Time scans

Outline• Photothermal Common-Path Interferometer• Surface Absorption Measurements on CoatedOptics• Transversal scans through mirror: point data• Longitudinal scans: surface inspection• Time scans

Outline• Photothermal Common-Path Interferometer• Surface Absorption Measurements on CoatedOptics• Transversal scans through mirror: point data• Longitudinal scans: surface inspection• Time scans

Outline• Photothermal Common-Path Interferometer• Surface Absorption Measurements on CoatedOptics• Transversal scans through mirror: point data• Longitudinal scans: surface inspection• Time scans

Outline• Photothermal Common-Path Interferometer• Surface Absorption Measurements on CoatedOptics• Transversal scans through mirror: point data• Longitudinal scans: surface inspection• Time scans

PCI Method(by Alex Alexandrovsky)Probe beam (low power)Pump beam (high power)Intensity distortionL RPhase distortion

PCI MethodThe probe phase is distorted due to heating:Δϕ transforms into an intensity distortion.At the Rayleigh length the intensitymodulation is:∂ nΔϕ= kL ΔT∂ TΔI/I 0 = kδ = (2π/λ)ΔnLΔn = (∂n/∂T)ΔTThe simplified formula for α with a “chopped” pump beam of power W Pis:α ≈2C(∂n/ ∂T)( λ / L)(w / WP )( I / I )0Δf

PCI MethodPDProbe beamHe-Ne, 632.8 nmRIR-imaging SystemRIL 1PD PhotodetectorsL iLensesPDSampleL 2Pump beamMMirrorMChopper1 Lock-in AmplifierSR830 DSP22 Chopper ControllerSR340PC with GPIB-connection(LabVIEW software)1PC

PCI MethodSampleProbePumpPhotodetectorΔΙΔϕRayleighrangeZ rayleighDistanceΔI/I ∝ Δϕ

Measurement technique: transversal z-scan(through surface)Start position Maximum Signal End positionCalibrated absorption signalScanning direction: z-axis→ signal reading point (the focus of the optical system)

Measurement technique: longitudinal x-scan(across surface)y-axisThe heated point is positioned at the coated surfacex-axisCalibrated absorption signalScanning direction: x-axis

Measurement technique: time scanThe heated point is focused at the coated surface.The calibrated signal is recorded vs. time elapsedy-axisx-axisCalibrated absorption signalThe calibrated signal isrecorded vs. the time elapsedt

HfO 2 -SiO 2 /SiO 2 coating, ∅ = 1 inchAbsorption [ppm]8.07.06.05.04.03.02.01.00.0#11251, W pump = 8.7 W, λ pump = 1064 nmα = 6.6 ppm0 0.5 1 1.5 2Distance scanned along z -direction [mm]VDC [V]Phase [Degree]2.3702.3652.3602.3552.3500 0.5 1 1.5 2Distance [mm]200150100500-50-100-150-2000 0.5 1 1.5 2Distance [mm]A transversal z-scan of sample #11251.α = 6.6 ppm.Additionally monitoredparameters : probe beamstability, phase delaywith respect to chopper

HfO 2 -SiO 2 /SiO 2 coating, ∅ = 1 inch#11251, W pump = 8.7 W, λ pump = 1064 nmAbsorption [ppm]8.07.06.05.04.03.02.01.00.00 500 1000 1500 2000 2500Time [sec]A characteristic time dependent 45 min t-scan of sample #11251.α shows a “bump” (7.3 ppm) at about 5 min.

Tabulated data on series of HfO 2 -SiO 2 /SiO 2 coated samplesSample #Absorption (ppm)3 mm above Center 3 mm below 3 mm left 3 mm right Average014-01 7.2 7.4 7.1 7.1 7.2 7.2018-01 8.0 8.0 7.9 8.1 7.9 8.0024-01 6.9 6.7 6.9 6.8 6.9 6.911251 6.8 6.4 6.6 6.6 6.6 6.6Up (y)Position of measured pointsLeft (x)14 2 53RightDown

Method’s resolution: down to 0.1 ppm with W pump < 10 WHR, 30 layers0.25Absorption [ppm]0.200.150.100.05α≈0.19 ppmλ = 1.064 μ0.000 1 2 3 4Distance [mm]A transversal z-scan of a 30 layer HR mirror on a FS substrate.W pump = 8.8 W, λ = 1.064 μ. Absorption measured = 0.19 ppm.

A coated 3 inch disk marked #17, α = 0.65 ppm.Covered by a First Contact strip for 1 week1 2 3#17, complete x-scan 9000 - 28000 through the strip covered byFirst Contact (1 min after it was opened)Absorption [ppm]1.00.80.60.40.21 2 3Testing the “FirstContact” effect.α = 0.65 ppm.0.00 5 10 15Distance [mm]

Study of the UV illumination effect on LIGO coated mirrorsSample "No", x-scan unfiltered, in vacuum chamber, 2008-02-13,Absorption [ppm]1.000.900.800.700.600.500.400.300.200.100.000 2 4 6 8 10Distance [mm]Characteristic x-scans beforeand after UV illumination (seeKe-Xun Sun’s talk for details)Absorption [ppm]3.002.502.001.501.000.500.00Sample "No", x-scan unfiltered, in vacuum chamber,2008-02-22,exposed 112 hrs0 2 4 6 8 10 12Distance [mm]

Study of the UV illumination effect on LIGO coated mirrors2.52.0Sample # "No"Experimental DataAbsorption (ppm)1.51.00.50.00 50 100 150 200 250 300Exposure Time (Hours)Variation of the absorption loss vs. Exposure Time

SummaryThe PCI method is capable to measure thermal absorptionlosses as low as 0.1 ppm with 8 W incident powerPoint by point as well as continuous scan across the coatingare possibleThe lowest Absorption value measured on LIGO mirrors tillnow were 0.19 ppm