Wavelength Metrology Research Articles

Laser wavelength metrology with low-finesse etalons and Bayer filters

We present a wavelength meter with picometer-scale resolution based on etaloning effects of inexpensive glass slides and the built-in color filters of a consumer grade CMOS camera. After calibrating the device to a commercial meter, we tested the device's calibration stability using two tunable visible lasers for a period of over 16 days. The wavelength error over that entire period has a standard deviation of 5.29 parts per million (ppm) about a most probable error of 0.90 ppm. Within 24 hours of calibration, this improves to 0.04 ppm with a standard deviation of 3.94 ppm.

Photonic temperature and wavelength metrology by spectral pattern recognition.

Spectral pattern recognition is used to measure temperature and generate calibrated wavelength/frequency combs using a single silicon waveguide ring resonator. The ring generates two incommensurate interleaving TE and TM spectral combs that shift independently with temperature to create a spectral pattern that is unique at every temperature. Following an initial calibration, the ring temperature can be determined by recognizing the spectral resonance pattern, and as a consequence, the wavelength of every resonance is also known. Two methods of pattern-based temperature retrieval are presented. In the first method, the ring is locked to a previously determined temperature set-point defined by the coincidence of only two specific TE and TM cavity modes. Based on a prior calibration at the set-point, the ring temperature and hence all resonance wavelengths are then known and the resulting comb can be used as a wavelength calibration reference. In this configuration, all reference comb wavelengths have been reproduced within a 5 pm accuracy across an 80 nm range by using an on-chip micro-heater to tune the ring. For more general photonic thermometry, a spectral correlation algorithm is developed to recognize a resonance pattern across a 30 nm wide spectral window and thereby determine ring temperature continuously to 50 mK accuracy. The correlation method is extended to simultaneously determine temperature and to identify and correct for wavelength calibration errors in the interrogating light source. The temperature and comb wavelength accuracy is limited primarily by the linewidth of the ring resonances, with accuracy and resolution scaling with the ring quality factor.

Contemporary x-ray wavelength metrology and traceability

We report recent advances in absolute x-ray wavelength metrology in the context of producing modern standard reference data. Primary x-ray wavelength standards are produced today using diffraction spectrometers using crystal optics arranged to be operated in dispersive and non-dispersive geometries, giving natural-line-width limited profiles with high resolution and accuracy. With current developments, measurement results can be made traceable to the Système internationale definition of the meter by using diffraction crystals that have absolute lattice-spacing provenance through x-ray-optical interferometry. Recent advances in goniometry, innovation of electronic x-ray area detectors, and new in situ alignment and measurement methods now permit robust measurement and quantification of previously-elusive systematic uncertainties. This capability supports infrastructures like the NIST Standard Reference Data programs and the International Initiative on X-ray Fundamental Parameters and their contributions to science and industry. Such data projects are further served by employing complementary wavelength- and energy-dispersive spectroscopic techniques. This combination can provide, among other things, new tabulations of less-intense x-ray lines that need to be identified in x-ray fluorescence investigation of uncharacterized analytes. After delineating the traceability chain for primary x-ray wavelength standards, and NIST efforts to produce standard reference data and materials in particular, this paper posits the new opportunities for x-ray reference data tabulation that modern methods now afford.

Laser wavelength metrology with color sensor chips.

We present a laser wavelength meter based on a commercial color sensor chip. The chip consists of an array of photodiodes with different absorptive color filters. By comparing the relative amplitudes of light on the photodiodes, the wavelength of light can be determined. In addition to absorption in the filters, etalon effects add additional spectral features which improve the precision of the device. Comparing the measurements from the device to a commercial wavelength meter and to an atomic reference, we found that the device has picometer-level precision and picometer-scale drift over a period longer than a month.

First measurement of Lyman alpha x-ray lines in hydrogen-like vanadium: results and implications for precision wavelength metrology and tests of QED

The first measurement of hydrogen-like vanadium x-ray Lyman alpha transitions has been made. The measurement was made on an absolute scale, fully independent of atomic structure calculations. Sufficient signal was obtained to reduce the statistical uncertainty to a small fraction of the total uncertainty budget. Potential sources of systematic error due to Doppler shifts were eliminated by performing the measurement on trapped ions. The energies for Ly α1 (1s-2p3/2) and Ly α2 (1s-2p1/2) are found to be 5443.95(25) eV and 5431.10(25) eV, respectively. These results are within approximately 1.5 σ (experimental) of the theoretical values 5443.63 eV and 5430.70 eV. The results are discussed in terms of their relation to the Lamb shift and the development of an x-ray wavelength standard based on a compact source of trapped highly charged ions.

High precision surface metrology using a phase retrieval method

At wavelength metrology offers a unique opportunity to characterize ultra high quality optics of diffraction limited performance. This technique is based on a coherent interaction of incident wave fields reflected from different surface positions. The mirror surface acts as a phase object which introduces a phase error in the outgoing wave fields. Resultant aberrated wavefront modulates the intensity near the focal plane. The modulated intensity profile can be used to retrieve the mirror shape profile using a numerical phase retrieval method which is developed in the present work. This at-wavelength metrology gives a precise shape information of a mirror having a figure error < 1nm which is extremely difficult to measure with conventional metrology tools.

Broadband precision wavelength meter based on a stepping Fabry–Pérot interferometer

We have constructed a broadband apparatus for wavelength metrology capable of absolute accuracy at a level of better than 2 parts in 109. An evacuated plane-parallel Fabry–Pérot interferometer with continuously adjustable mirror separation is used to compare the wavelength of a single-frequency tunable laser with that of an iodine-stabilized HeNe laser used as a wavelength standard. This work details apparatus construction, a thorough investigation of systematic errors, and data analysis. The wavelengths of five Doppler-free Te2130 transitions in the region from 475.6 to 490.8 nm have been measured and are found to be in excellent agreement with previous measurements. In addition, the wavelengths of five previously unmeasured Te2130 transitions spanning the region from 424.9 to 462.3 nm have been determined for use as new reference wavelength standards.

Measurement of the He Ground State Lamb Shift via the Two-Photon1S1−2S1Transition

We have extended two-photon Doppler-free spectroscopy to the vacuum ultraviolet spectral region, to accurately measure the He $^{1}1S\ensuremath{-}^{1}2S$ transition at 120 nm. Our result is 4 984 872 315(48) MHz. This yields a ground state Lamb shift of 41 104(48) MHz, in fair agreement with theory and other experiments. This approach has the potential for significantly better accuracy once improvements in the laser and the wavelength metrology are implemented.

Absolute spin-flip raman laser frequency measurements with metal-insulator-metal diodes

Infrared frequency synthesis techniques with metal-insulator-metal (MIM) diodes have been extended to include absolute frequency measurement of a spin-flip Raman laser (SFRL). As a result of this extension, spectroscopy in the 5.3 μm region more readily can be put on a frequency rather than a wavelength metrology basis. Additional observations with the diode are in qualitative agreement with recent wirk relating to non-linear tuning over axial SFRL modes.