Marla L. Dowell

Laser plasma x-ray source for ultrafast time-resolved x-ray absorption spectroscopy.

We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ∼106 photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from >107 laser pulses, we also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.

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 %.

Random testing reveals excessive power in commercial laser pointers

In random testing of 122 commercial laser pointers, the authors observed that 90% of green pointers and 44% of red pointers were not in compliance with the Code of Federal Regulations (CFR), producing laser power in excess of the CFR-allowed limit at one or more laser wavelengths. The measurement results are presented and the authors describe the inexpensive test bed they used. Also, they suggest physical mechanisms that could account for the hazardous levels of laser pointer emissions.

Reflective attenuator for high-energy laser measurements

A high-energy laser attenuator in the range of 250 mJ (20 ns pulse width, 10 Hz repetition rate, 1064 nm wavelength) is described. The optical elements that constitute the attenuator are mirrors with relatively low reflectance, oriented at a 45 degrees angle of incidence. By combining three pairs of mirrors, the incoming radiation is collinear and has the same polarization orientation as the exit. We present damage testing and polarization-dependent reflectance measurements for 1064 nm laser light at 45 degrees angle of incidence for molybdenum, silicon carbide, and copper mirrors. A six element, 74 times (18 dB) attenuator is presented as an example.

Absolute calibration of optical power for PDT: Report of AAPM TG140

This report is primarily concerned with methods for optical calibration of laser power for continuous wave (CW) light sources, predominantly used in photodynamic therapy (PDT). Light power calibration is very important for PDT, however, no clear standard has been established for the calibration procedure nor the requirements of power meters suitable for optical power calibration. The purposes of the report are to provide guidance for establishing calibration procedures for thermopile type power meters and establish calibration uncertainties for most commercially available detectors and readout assemblies. The authors have also provided a review of the use of various power meters for CW and pulsed optical sources, and provided recommended temporal frequencies for optical power meter calibrations and guidance for routine quality assurance procedure.

Laser power-meter comparison at far-infrared wavelengths and terahertz frequencies

We have evaluated the responsivity of seven different thermal detectors compared to an electrically calibrated photoacoustic reference detector at 119 µm (2.5 THz) and 394 µm (0.76 THz) laser wavelengths. Among the thermal detectors is an electrically calibrated thermopile having a vertically aligned carbon nanotube array as the absorber. We document the uncertainty contributions attributable to the photoacoustic reference detector along with a definition of a calibration factor based on the measurement protocol. The expanded relative uncertainty (k = 2) and a calibration factor of each detector are tabulated.

Biophotonic Tools in Cell and Tissue Diagnostics

In order to maintain the rapid advance of biophotonics in the U.S. and enhance our competitiveness worldwide, key measurement tools must be in place. As part of a wide-reaching effort to improve the U.S. technology base, the National Institute of Standards and Technology sponsored a workshop titled “Biophotonic tools for cell and tissue diagnostics.” The workshop focused on diagnostic techniques involving the interaction between biological systems and photons. Through invited presentations by industry representatives and panel discussion, near- and far-term measurement needs were evaluated. As a result of this workshop, this document has been prepared on the measurement tools needed for biophotonic cell and tissue diagnostics. This will become a part of the larger measurement road-mapping effort to be presented to the Nation as an assessment of the U.S. Measurement System. The information will be used to highlight measurement needs to the community and to facilitate solutions.

Accurate, inexpensive testing of laser pointer power for safe operation

An accurate, inexpensive test-bed for the measurement of optical power emitted from handheld lasers is described. The setup consists of a power meter, optical bandpass filters, an adjustable iris and self-centering lens mounts. We demonstrate this test-bed by evaluating the output power of 23 laser pointers with respect to the limits imposed by the US Code of Federal Regulations. We find a compliance rate of only 26%. A discussion of potential laser pointer hazards is included.

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.

New developments in deep ultraviolet laser metrology for photolithography

Current and future laser measurement services at 157, 193 and 248 nm will be reviewed. Laser power and energy measurements at 193 nm will be presented; electrical calibration issues will be reviewed. We report an overall calibration uncertainty of laser power and energy meters of less than 2%. Characterization measurements for ultraviolet materials also will be discussed.