Thomas Scott

Nonlinearity Measurements of High-Power Laser Detectors at NIST

We briefly explain the fundamentals of detector nonlinearity applicable to both electrical and optical nonlinearity measurements. We specifically discuss the attenuation method for optical nonlinearity measurement that the NIST system is based upon, and we review the possible sources of nonlinearity inherent to thermal detectors used with high-power lasers. We also describe, in detail, the NIST nonlinearity measurement system, in which detector responsivity can be measured at wavelengths of 1.06 µm and 10.6 µm, over a power range from 1 W to 1000 W. We present the data processing method used and show measurement results depicting both positive and negative nonlinear behavior. The expanded uncertainty of a typical NIST high-power laser detector calibration including nonlinearity characterization is about 1.3 %.

Power measurement standards for high-power lasers: comparison between the NIST and the PTB

We report the results of the first laser high-power measurement comparison between the Physikalisch-Technische Bundesanstalt (PTB, Germany), and the National Institute of Standards and Technology (NIST, USA). Laser power transfer standards were calibrated at both national standards laboratories between 82 W and 127 W at 1.06 µm and between 85 W and 554 W at 10.6 µm. Relative agreement between the standards of the two laboratories was demonstrated to lie between 5 × 10−3 and 7 × 10−3, which is well within the combined uncertainties.

Heat Transfer Analysis and Modeling of a Cryogenic Laser Radiometer

This study investigates the laser optimized cryogenic radiometer (LOCR) recently acquired by the National Institute of Standards and Technology in Boulder, Colorado, to calibrate laser power meters and detectors. The objectives are to evaluate potentially significant sources of uncertainty in the radiometric measurements and to develop transient models that efficiently and accurately predict the behavior of this radiometer. The analysis suggests that radiation from the Brewster window assembly may cause the total power entering the radiometer to drift more than 130 nW for a room temperature variation of 0.2 K. Steady-state modeling of the LOCR with finite element analysis software indicates a relative inequivalence between optical and electrical heating of 4 × 10 -6 at the 1-mW power level. A new tbermal model has been developed to simplify transient predictions by combining lumped parameter and one-dimensional elements. This model outperforms single-time-constant exponential models and can be expanded to simulate the complete radiometer system.

Error propagation in laser beam spatial parameters

We have performed a propagation-of-errors analysis on two methods used to determine the spatial parameters of a laser beam. We measured diameters of a diode laser beam focused by a 993 mm focal length lens. Measurement uncertainties of less than 1% can result in uncertainties greater than 200% in locating the beam waist of the laser. We compare the inherent uncertainties in the spatial parameters as obtained by the two methods. Longer focal length lenses and lens position can reduce this magnification of uncertainty, but would require large propagation distances.

Comparison of reference standards for measurements of optical-fibre power

National reference standards for determining optical-fibre power maintained by the National Institute of Standards and Technology (NIST, USA) and the All-Russian Research Institute for Optophysical Measurements (VNIIOFI, Russian Federation) were compared at wavelengths near 1300 nm and 1550 nm at a power level of 0.5 mW. Instruments from both laboratories are based on thermal detectors capable of electrical calibration. The comparisons indicate relative differences of 2 parts in 103 or less, which is easily within the combined uncertainty of the two laboratories.

Nbs Laser Power And Energy Measurements

The National Bureau of Standards (NBS) maintains a set of electrically calibrated calorimeters designed and built specifically for laser energy measurements. These calorimeters are used as national reference standards for the calibration of optical power and energy meters. NBS offers laser measurement services based on the standard calorimeters to the public at a variety of laser wavelengths and power ranges. The uncertainties associated with these measurements have recently been re-evaluated.

Automated measurement of nonlinearity of optical fiber power meters

We have developed a system for measuring the nonlinearity of optical power meters or detectors over a dynamic range of more than 60 dB at telecommunications wavelengths. This system uses optical fiber components and is designed to accommodate common optical power meters and optical detectors. It is based on the triplet superposition method. The system also measures the range discontinuity between neighboring power ranges or scale settings of the optical power meter. We have developed an algorithm to treat both the nonlinearity and the range discontinuity in a logically consistent manner. Measurements with this system yield correction factors for powers in all ranges. The measurement system is capable of producing results which have standard deviations as low as 0.02%. With slight modification the system can operate over a 90 dB dynamic range at telecommunications wavelengths. This measurement system provides accurate determination of optical power meter or detector nonlinearity; the characterized detectors then can be used for such applications as absolute power and attenuation measurements.

Optical detector nonlinearity: a comparison of five methods

We derived a set of unified equations for five methods to evaluate nonlinearity of power meters and detectors. We performed computer simulations of these methods. The simulations assist in design of a measurement system to meet a target accuracy. Measurements verified the simulations.<<ETX>>

Deep ultraviolet laser metrology for semiconductor photolithography

Recent improvements in calibration procedures have led to reductions in overall uncertainties of laser power and energy calibrations to ±1%. We plan to extend these services to include laser dose measurements and angular response. Deviations from cosine behavior for angular response can introduce measurement errors when dose meters, calibrated with parallel laser beams, are employed in stepper systems. These measurement errors will become important as variable numerical aperture systems become commonplace. Current and future laser measurement services at 193 and 248 nm will be reviewed.

Deep-UV excimer laser measurements at NIST

The National Institute of Standards and Technology has designed and built two electrically calibrated laser calorimeters as primary standards for absolute energy measurements at the wavelength of 248 nm. Under the sponsorship of SEMATECH, NIST developed the calorimeters to improve measurement of dose energy in excimer laser based microlithography. The calorimeter system can be used to calibrate transfer standards which in turn can be used to calibrate detectors employed for energy measurements of semiconductor wafer exposure. The excimer calorimeter uses a glass filter which functions as a volume absorber that allows collection of nanosecond pulses of laser radiation without suffering damage. The measurement range of the calorimeters is 0.3-25 J, but can be extended to 1 mJ with beamsplitters. Electrical calibration of the calorimeters shows a standard deviation in the calibration factor of less than 0.5% for entire energy range. The total uncertainty of typical power and energy meter calibrations is approximately 2%.