CORM annual conference, NRC, Ottawa, CANADA June 1, 2012 New automated laser facility for detector calibrations Yuqin Zong National Institute of Standards and Technology Gaithersburg, Maryland USA
Overview Introduction Existing spectral calibration facilities New 1 khz optical parametric oscillator (OPO) based calibration system Conclusion
Spectral calibration of optical sensors Light source (Watt) Detector i o (λ) (Ampere) Spectral POWER responsivity (Underfilled geometry) (Ampere/Watt) Light source (Watt.m -2 ) i o (λ) (Ampere) SI base unit -luminous intensity: candela SI base unit radiance temperature: Kelvin Colorimetry and radiometry Spectral irradiance scale (FEL lamps) Spectral IRRADIANCE responsivity (Overfilled geometry) (Ampere/(Watt.m -2 ))
Existing NIST spectral calibration facilities NIST POWR Cryogenic Radiometer (Power resp.) (0.01 %) NIST SIRCUS facility Trap detectors (Irrad resp.) (0.025 %)* NIST SCF Trap detectors (Irrad resp.) (0.025 %)* Traps to photodiodes Irradiance responsivity (Ampere/(Watt.m -2 )) ( 0.05 %)* NIST SCF Working std. photodiodes (Power resp.) * BEST expanded uncertainty (k=2) POWR: primary optical watt radiometer SIRCUS: Spectral irradiance and radiance responsivity calibrations using uniform sources SCF: Spectral comparator facility Trap detector: specially configured, multi-element silicon photodiodes detector with high performance. Power resp. (Ampere/Watt) ( 0.20 %)*
The NIST SIRCUS facility Standard Trap Detector integrating sphere Test Detector Continuous spectral coverage from UV to NIR Continuous wave (CW) or quasi CW tunable lasers based research facility high power (e.g., 100 mw), narrow bandwidth (<0.01 nm) Used for realization of SI base units: Kelvin and candela Provide calibrations for primary radiometric standards and for remote sensing instruments Difficult to automate & high-cost
NIST SCF Double monochromator Working standard detector Fully automated Lamp-monochromator based calibration facility, no fringe problem Major facility to disseminate NIST Scale to industry Low radiant power (µw level), broad bandwidth (4 nm) Designed for power responsivity Large uncertainties to acquire irradiance responsivities (mapping method does not work well!) Test Detector
Spatial uniformity of a Si photodiode 205 nm 230 nm 250 nm Acknowledgement to Ping-shine Shaw, NIST 10 10 10 8 8 8 6 4 Y (mm) 6 4 Y (mm) 6 4 Y (mm) 0.995-1.000 0.990-0.995 0.985-0.990 2 2 2 0.980-0.985 0.975-0.980 0 0 2 4 6 8 10 X (mm) 0 0 2 4 6 8 10 X (mm) 0 0 2 4 6 8 10 X (mm) 0.970-0.975 0.965-0.970 0.960-0.965 265 nm 10 320 nm 10 500 nm 10 0.955-0.960 0.950-0.955 0.945-0.950 8 8 8 0.940-0.945 0.935-0.940 6 4 Y (mm) 6 4 Y (mm) 6 4 Y (mm) 0.930-0.935 2 2 2 0 0 2 4 6 8 10 0 0 2 4 6 8 10 0 0 2 4 6 8 10 X (mm) X (mm) X (mm)
Fully automated tunable OPO-based laser sources OPO: optical parametric oscillators Fully automated Large tunable range Portable Much lower cost pump mirror Low repetition rate (10 Hz to 1000 Hz) Narrow pulse width, extremely low duty cycle (e.g., 10-6 ) Pulse to pulse variation and difficult to stabilize Trans-impedence amplifiers don t work well Have not been used as calibration source yet nonlinear crystal mirror
Key questions Can pulse lasers be used for calibration of detectors with small uncertainties? How to overcome fluctuation of a pulsed laser and obtain repeatable results? Will detectors be saturated? Is a pulse laser equivalent to a CW laser for detector calibrations?
Schematic of the new automated calibration system integrating sphere baffle Monitor Detector Standard Trap Detector Electrometers (2) Optical fiber Ultrasound bath shutter Test Detector 1000 Hz OPO Computer Shutter controller R test ( λ) = R ( λ) Q standard M test ( λ) / Q M standard ( λ)
The automated OPO system 350 300 Average power (mw) 250 200 150 100 50 0 200 700 1200 1700 2200 Wavelength, λ (nm) 210 nm to 2400 nm tunable range 1 khz repetition rate 5 ns pulse width 5 8 cm -1 bandwidth ( 0.2 nm in visible range)
OPO Pulse waveforms Rel. laser power 1.2 1.0 0.8 0.6 0.4 0.2 OPO laser Rel. laser power 1.2 1.0 0.8 0.6 0.4 0.2 After 5 m MM fiber 0.0 0.0-0.2-20 -10 0 10 20 Time, t (ns) -0.2-20 -10 0 10 20 Time, t (ns) Rel. laser power 1.2 1.0 0.8 0.6 0.4 0.2 0.0 After 50 mm sphere -0.2-20 -10 0 10 20 Time, t (ns)
OPO spectra 1.2 1 600 nm 0.8 Relative SPD 0.6 0.4 0.2 1.2 1 0 598 599 600 601 602 Wavelength, λ (nm) 1.2 350 nm 1 1100 nm 0.8 0.8 Relative SPD 0.6 0.4 Relative SPD 0.6 0.4 0.2 0.2 0 348 349 350 351 352 Wavelength, λ (nm) 0 1098 1099 1100 1101 1102 Wavelength, λ (nm)
The electrometers Pulse train i(t) C T Q = i( t) dt = C V 0 Detector - + V DMM Charge measurement function from 2 nc to 2 μc using a charge amplifier < 3 fa bias current < 20 μv burden voltage High performance multichannel switching card
Measurement Timing Electrometer s synchronized charge measurement start end Laser shutter open close
Measurement repeatability 1 s integration time for each point 1.000050 1.000050 1.000030 1.000030 Relative charge ratio 1.000010 0.999990 0.999970 Relative charge ratio 1.000010 0.999990 0.999970 0.999950 0 5 10 15 20 25 30 0.999950 0 5 10 15 20 25 30 Measurement No., i Measurement No., i two Hamamatus S2281 silicon photodiodes (PD) standard deviation = 7 ppm! one 3 silicon PD trap and one S2281 Si PD standard deviation = 12 ppm!
Detector non-linearity test Relative responsivity 1.0002 1.0000 0.9998 0.9996 0.9994 0.9992 Obtained by normalizing the charge ratio r(p i ) of the test detector (S2281 PD) to reference detector (S2281 PD with 2 orders of magnitude lower signal). 0.9990 0.9988 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 Averaged photocurrent, I (A) OPO at 450 nm. Saturation starts at peak=100 ma, averaged=1 µa. 1) Nonlinearity depends on the detector and laser wavelength. 2) The instantaneous photocurrent without causing nonlinearity is several orders of magnitude higher than the threshold nonlinear DC photocurrent (0.1 1 ma typically). 3) The level of allowed averaged photocurrent is several orders of magnitude lower than the threshold nonlinear DC photocurrent.
Validation: charge amp vs trans-impedance amp Relative Difference ΔR(λ) (%) 0.05 0.04 0.03 0.02 0.01 (a) charge amplifiers vs. trans-impedance amplifiers 0.00 450 500 550 600 650 Wavelength, λ /nm CW laser + charge amplifiers vs. CW laser + trans-impedance amplifiers Difference in measured responsivity is 0.02 %.
Validation: 1 khz pulsed OPO vs CW lasers Relative Difference ΔR(λ) (%) 0.05 0.04 0.03 0.02 0.01 (b) pulsed OPO vs. CW laser 0.00 450 500 550 600 650 Wavelength, λ /nm Pulsed OPO + charge amplifiers vs. CW laser + trans-impedance amplifiers Difference in measured responsivity is also 0.02 %
Comparison of Results Relative Difference ΔR(λ) (%) 0.05 0.04 0.03 0.02 0.01 (a) charge amplifiers vs. trans-impedance amplifiers (b) pulsed OPO vs. CW laser 0.00 450 500 550 600 650 Wavelength, λ (nm) Replacing CW laser with pulsed OPO for charge amplifiers does not make difference in measured responsivity. Pulsed OPO CW laser.
Uncertainty budget Relative standard unc. (%) Uncertainty component Type A Type B Reference trap detector 0.020 OPO wavelength (0.02 nm) 0.005 Sphere source irradiance non-uniformity 0.005 Detector reference plane 0.010 Detector non-linearity 0.005 Transfer to test detector 0.005 Electrometer (range to range gain error) Combined uncertainty (%) 0.025 Expanded uncertainty (k=2) (%) 0.05 0.005
Conclusions A fully automated laser facility using a khz pulsed OPO for calibration of detectors and instruments has been developed and validated. The estimated uncertainty is 0.05 % (k=2). The new facility is to be used for calibration of photometers, colorimeters, and spectroradiometers with significantly reduced uncertainty. The developed method may be used in other applications: - transmittance and reflectance - surface color and appearance - optical medical imaging -
Acknowledgements Keith Lykke Steve Brown George Eppeldauer Yoshi Ohno and many other colleagues
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