Interferometers for stability measurements Gauge block Interferometry using phase stepping algorithms combined with CCD sensors is well suited for the measurement of long term stability, CTE and compressibility. Due to lateral resolution readjustment of the samples is easily possible after long times by masks. 0.1 nm stability over many years is proven using a silicon sample. Publications on translation interferometry with interpolation nonlinearities below 10 nm makes it attractive to investigate the long time stability of such interferometers to further increase the accuracy of stability and thermal dilatation measurements on a shorter time frame together with the possibility to get a high measurement bandwidth up to 100 khz.but you loose the high lateral resolution of a CCD. With segmented Diode up to 8x8 measurement points planned. AG 5.21 Length Graduations
Homodyne Heterodyne (for long time stability) Signal to Noise Ratio : Theoretically Heterodyn can reduce noise but today amplifiers are also very good at DC Interpolation Nonlinearities : Homodyne: due to multiple Amplifiers and nonideal polarisation stages. Correction possible only with signal knowlege over a signal period Heterodyne: due to frequency mixing in the interferometer arms. Feeding by spacially seperated beams prevents frequency mixing. Fiber Coupling to reduce thermal load on the experiment Homodyn: fibers to feed the interferometers in regular use. No fiber coupling to the detection unit. Suspicion that fiber vibrations change light properties too fast for nonlinearity compensation (0.1 nm range) Homodyne: Seperate incoming beams allows for fiber feeeding using two fibers. Fiber coupling to the detection unit under investigation. AG 5.21 Length Graduations
Phase Measurement Lock-In Amplifier for the demodulation of the heterodyne signals SIGNAL INPUT FILTER STAGE A A B LPF CH1 à SIN1 INTERNAL OSCILLATOR B LOW-PASS FILTER A A B LPF CH2 à COS1 REFERENCE INPUT PHASE SHIFTER +90 B REFERENCE TRIGGER traditional analog lock-in principle Phase determination by φ = atan(sin/cos) AG 5.21 Length Graduations
FPGA Implementation Windows based processing Minimizing FPGA load Synthetic Reference used for measurement and reference signals Allows for easy generation of 90⁰ shifted signals Σ Mixer ADC Values MEASUREMENT ARM REFERENCE ARM ADC CH 1 ADC CH 2 REFERENCE TABLE Φ=90 Reference MAC X REFERENCE TABLE Φ=0 MAC X SIN1 COS1 SIN2 DATA PROCESSING FPGA SIN COS FRINGE COUNTER MAC X MERGE DDR2 MAC COS2 φ2 RAM X φ1 Lowpass φ1 - φ2 ~ ATAN Σ EXTERNAL TRIGGER ORDER FRACTION ADC Values LOGIC EXTRAPOLATION VIA LINEAR REGRESSION LENGTH VALUE Reference table PC TRIGGER- VME- Bus 100 MHz AG 5.21 Length Graduations 50 khz
Heterodyne vwith seperate incoming beams Tanaka 1989 AG 5.21 Length Graduations
Differential Interferometry No compensation of Dilatation No compensation of angular motion Fully differential AG 5.21 Length Graduations
Abweichung von einer Geraden / pm Fully differential Interferometer with separated incoming beams Mikro Inductosyn JRP Nanotrace frequency doubled Nd:YAG laser /2 PBS Pump diode current Monitor diode Power stabilization detectors D /4 P D P j1 /4 j2 channel 1 channel 2 ADC FPGA board j P /2 PBS /2 AOM AOM 78,4375 MHz P 80 MHz 2channel function generator polarization maintaining fibers /2 NPBS P P /2 PBS /4 1 2 3 4 Common mirror 10 5 0-5 mittlere Abweichung von Geraden der 6 Messreihen y NL = 4.25 pm *sin(2* /(133,04 nm)* x + 0.16) -10 0 50 100 150 200 250 Verfahrweg des Röntgeninterferometers / nm AG 5.21 Length Graduations
Phase variation / pm Phase variation / pm Heterodyne Interferometer stability test (Mach-Zehnder) Phase difference between the two detectors at the Zehnder setup 40 20 0-20 50 khz -40 0 20 40 60 80 Time / ms 6 Hz Time / h
Heterodyne Interferometer stability test (common mirror) phase variation / pm 250 200 150 100 50 0-50 -100-150 -200 0 48 96 144 192 240 288 time / h Phase variation of 360 pm over 13 days 80 60 40 20 0-20 -40-60 -80 temperature variation / mk -100 1040 1030 1020 1010 1000 Correlation with temperature variation with about 2 pm/mk but with a delay of 8 h 990 980 air pressure / hpa
Phase variation / pm Fiber coupling Interferometer - Detektor Differenz zu Freistrahlanordnung am parallelen Interferometerausgang Mittelung auf 16 Hz Datenrate AG 5.21 Length Graduations Time /h
Setup of the VSL Picodrift Interferometer J.D. Ellis, K.-N. Joo, J.W. Spronck, and R.H. Munnig Schmidt, Balanced interferometric system for stability measurements, Appl. Opt. 48(9), 1733-1740 (2009).
VSL Picodrift Interferometer Phase measured of the incoming beams added. This allows for the independent measurement of the refractometer and the sample beam path.
Environmental shielding
Environmental shielding
Environmental shielding Gradient after 50 hours: 0.45 mk/h RMS after 50 hours: 0.22 to 0.11 mk/h
Laser diode laser (DBR / external grating) o 10 mw / 4 mw o PDH lock using EOM o low expansion cavity Separation of forward and backward propagating paths using optical isolator FSR = 1076.93 MHz, N = 100 1 ms: 160 khz 10 ms: 90 khz 100 ms: 40 khz 1 s: 22 khz
Phase detection hardware and software FPGA: NI PXIe-7962R Virtex 5 500 MHz 512 MB > 800 MB/s ADC: NI 5734 4 channels 120 MS/s 16 bit 10 MHz reference Labview programmable
distance [nm] Phase measurements for unfilled refractometer and measurment pathes after temperation 10 8 ADC channel 1-3 ADC channel 2-3 6 4 2 0 Additional reference signal (channel 3) -2 50 52 54 56 58 60 62 time [h]
Virtual gauge block Possible extensions: Measurement of CTE and stability of parts without parallel optical surfaces. Compensation of mirror thermal expansion by prior measurement without sample - Force actor (e.g. voice coil) for test of creep with interferometric tracebility - Position measurement systems for reproducible realignment of samples (long term investigations)