Laser Power Calibration Research Articles

Calibrating the global network of gravitational wave observatories via laser power calibration at NIST and PTB

Current gravitational wave (GW) observatories rely on photon calibrators that use laser radiation pressure to generate displacement fiducials used to calibrate detector output signals. Reducing calibration uncertainty enables optimal extraction of astrophysical information such as source distance and sky position from detected signals. For the ongoing O4 observing run that started on 24 May 2023, the global GW detector network is employing a new calibration scheme with transfer standards calibrated at both the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). These transfer standards will circulate between the observatories and the metrology institutes to provide laser power calibration traceable to the International System of Units (SI) and enable assessment and reduction of relative calibration errors for the observatory network. The Laser Interferometer Gravitational-Wave Observatory (LIGO) project and the Virgo project are currently participating in the new calibration scheme. The Large-scale Cryogenic Gravitational-wave Telescope project (KAGRA) is expected to join in 2024, with the LIGO Aundha Observatory in India joining later. Before implementing this new scheme, a NIST-PTB bilateral comparison was conducted. It validated the scale representation by both laboratories, with a degree of equivalence of −0.2% and an associated expanded uncertainty of 0.32% (k = 2) which is significantly lower than previous studies. We describe the transfer of power sensor calibration, including detailed uncertainty estimates, from the transfer standards calibrated by NIST and PTB to the sensors operating continuously at the interferometer end stations. Finally, we discuss the ongoing calibration of Pcal-induced displacement fiducials for the O4 observing run. Achieved combined standard uncertainty levels as low as 0.3% facilitate calibrating the interferometer output signals with sub-percent accuracy.

Fabrication of airbridges with gradient exposure

In superconducting quantum circuits, airbridges are critical for eliminating parasitic slotline modes of coplanar waveguide circuits and reducing crosstalks between direct current magnetic flux biases. Here, we present a technique for fabricating superconducting airbridges. With this technique, a single layer of photoresist is employed, and the gradient exposure process is used to define the profile of airbridges. In order to properly obtain the bridge profile, we design exposure dosage based on residual photoresist thickness and laser power calibrations. Compared with other airbridge fabrication techniques, the gradient exposure fabrication technique provides the ability to produce lossless superconducting airbridges with flexible size and, thus, is more suitable for large-scale superconducting quantum circuits. Furthermore, this method reduces the complexity of the fabrication process and provides a high fabrication yield.

Toward Calibration of the Global Network of Gravitational Wave Detectors with Sub-Percent Absolute and Relative Accuracy

The detection of gravitational-wave signals by the LIGO and Virgo observatories during the past few years has ushered us into the era of gravitational-wave astronomy, shifting our focus from detection to source parameter estimation. This has imposed stringent requirements on calibration in order to maximize the astrophysical information extracted from these detected signals. Current detectors rely on photon radiation pressure from auxiliary lasers to achieve required calibration accuracy. These photon calibrators have made significant improvements over the last few years, realizing fiducials displacements with sub-percent accuracy. This achieved accuracy is directly dependent on the laser power calibration. For the next observing campaign, scheduled to begin at the end of 2022, a new scheme is being implemented to achieve improved laser power calibration accuracy for all of the GW detectors in the global network. It is expected to significantly improve absolute and relative calibration accuracy for the entire network.

Fiducial displacements with improved accuracy for the global network of gravitational wave detectors

As sensitivities improve and more detectors are added to the global network of gravitational wave observatories, calibration accuracy and precision are becoming increasingly important. Photon calibrators, relying on power-modulated auxiliary laser beams reflecting from suspended interferometer optics, enable continuous calibration by generating displacement fiducials proportional to the modulated laser power. Developments in the propagation of laser power calibration via transfer standards to on-line power sensors monitoring the modulated laser power have enabled generation of length fiducials with improved accuracy. Estimated uncertainties are almost a factor of two smaller than the lowest values previously reported. This is partly due to improvements in methodology that have increased confidence in the results reported. Referencing the laser power calibration standards for each observatory to a single transfer standard enables reducing relative calibration errors between elements of the detector network. Efforts within the national metrology institute community to realize improved laser power sensor calibration accuracy are ongoing.

Meta-study of laser power calibrations ranging 20 orders of magnitude with traceability to the kilogram**This work of the U.S. Government is not subject to U.S. copyright.

Laser power metrology at the National Institute of Standards and Technology (NIST) ranges 20 orders of magnitude from photon-counting (103 photons s−1) to 100 kW (1023 photons s−1 at a wavelength of 1070 nm). As a part of routine practices, we perform internal (unpublished) comparisons between our various power meters to verify correct operation. Here we use the results of these intercomparisons to demonstrate an unbroken chain tracing each power meter’s calibration factor to the NIST cryogenic radiometer (our lowest uncertainty standard, whose SI traceability is established through the volt and ohm units). This yields the expected result that all the NIST primary standard measurement techniques agree with each other to within their measurement uncertainty. Then, these intercomparison results are re-mapped to describe the agreement of the various techniques with our radiation-pressure-based power measurement approach, whose SI traceability is established through the kilogram. Again, agreement is demonstrated to within the measurement uncertainty. This agreement is reassuring because the measurements are compared with two entirely different traceability paths and show expected agreement in each case. The ramifications of this agreement as well as potential means to improve on it are discussed.We demonstrate SI measurement traceability of our single-photon power measurement through the kilogram with less than 3% relative expanded uncertainty (obtained for a coverage factor k = 2 defining an interval having a level of confidence of approximately 95%).

Direct measurement of radiation pressure and circulating power inside a passive optical cavity.

A mechanical force sensor coupled to two optical cavities is developed as a metrological tool. This system is used to generate a calibrated circulating optical power and to create a transfer standard for externally coupled optical power. The variability of the sensor as a transfer standard for optical power is less than 2%. The uncertainty in using the sensor to measure the circulating power inside the cavity is less than 3%. The force measured from the mechanical response of the sensor is compared to the force predicted from characterizing the optical spectrum of the cavity. These two forces are approximately 20% different. Potential sources for this disagreement are analyzed and discussed. The sensor is compact, portable, and can operate in ambient and vacuum environments. This device provides a pathway to novel nanonewton scale force and milliwatt scale laser power calibrations, enables direct measurement of the circulating power inside an optical cavity, and enhances the sensitivity of radiation pressure-based optical power transfer standards.

Temporal response of laser power standards with natural convective cooling.

Laser power detectors with natural convective cooling are convenient to use and hence widely applicable in a power range below 150 W. However, the temporal response characteristics of the laser power detectors need to be studied in detail for accurate measurement. The temporal response based on the absolute laser power standards with natural convective cooling is studied through theoretical analysis, numerical simulations, and experimental verifications. Our results show that the response deviates from a single exponential function and that an ultimate response balance is difficult to achieve because the temperature rise of the heat sink leads to continuous increase of the response. To determine the measurement values, an equal time reading method is proposed and validated by the laser power calibrations.

Adoption of Six Sigma’s DMAIC to Reduce Complications in IntraLase Surgeries

Purpose: To show how a private eye care center in Turkey initiated Six Sigma principles to reduce the number of complications encountered during and after femtosecond laser-assisted LASIK (IntraLase) surgeries. Method: Data were collected for five years. To analyse the complications among 448 surgeries, main tools of Six Sigma’s Define-Measure-Analyze-Improve-Control (DMAIC) improvement cycle such as SIPOC table, Fishbone Diagram and, Failure, Mode and Effect Analysis were implemented. Sources and root causes of seventeen types of complications were identified and reported. Results: For a successful IntraLase surgery, experience of the refractive surgeon, patient’s anatomy and calibration of laser power were determined to be the “critical few” factors whereas, patient’s psychology, sterilization and hygiene, and suction-ring’s pressure were found to be the “trivial many” factors. The most frequently occurring complication was found to be subconjunctival haemorrhage. Conclusion: The process sigma level of the process was measured to be 3.3547. The surgical team concluded that sixteen complications (out of seventeen) should be significantly reduced by taking the necessary preventive measures.

Flexible Omnidirectional Carbon Dioxide Laser as an Effective Tool for Resection of Brainstem, Supratentorial, and Intramedullary Cavernous Malformations

Lasers have a long history in neurosurgery, yet bulky designs and difficult ergonomics limit their use. With its ease of manipulation and multiple applications, the OmniGuide CO2 laser has reintroduced laser technology to the microsurgical resection of brain and spine lesions. This laser, delivered through a hollow-core fiber lined with a unidirectional mirror, minimizes energy loss and allows precise targeting. To analyze resections performed by the senior author from April 2009 to March 2013 of 58 cavernous malformations (CMs) in the brain and spine with the use of the OmniGuide CO2 laser, to reflect on lessons learned from laser use in eloquent areas, and to share data on comparisons of laser power calibration and histopathology. Data were collected from electronic medical records, radiology reports, operative room records, OmniGuide CO2 laser case logs, and pathology records. Of 58 CMs, approximately 50% were in the brainstem (30) and the rest were in supratentorial (26) and intramedullary spinal locations (2). Fifty-seven, ranging from 5 to 45 mm, were resected, with a subtotal resection in 1. Laser power ranged from 2 to 10 W. Pathology specimens showed minimal thermal damage compared with traditionally resected specimens with bipolar coagulation. The OmniGuide CO2 laser is safe and has excellent precision for the resection of supratentorial, brainstem, and spinal intramedullary CMs. No laser-associated complications occurred, and very low energy was used to dissect malformations from their surrounding hemosiderin-stained parenchymas. The authors recommend its use for deep-seated and critically located CMs, along with traditional tools.

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.

Linear energy control of laser drilling and its application in the repair of TFT-LCD bright pixels

Laser drilling is an energy dependent process in which energy consumption is linearly proportional to the thickness of the material being drilled. Thus, providing linear output power from the laser can provide considerable advantages in the repair of devices such as TFT-LCD pixels. Unfortunately, the non-linear energy characteristics of lasers require compensation to achieve linear power output. Conventional compensation schemes use laser power meters that require repeatedly switching the laser system off and on again. This study developed a software-based energy compensation method to provide optimized energy output and continuous linear laser energy. This software solution enables the measurement of laser energy in a fixed period and its manipulation using a compensation table, which eliminates the need for a laser power meter and enables the system to remain in operation during laser power calibration. The proposed method provides linear output in which the linear energy proportion (R2) reaches 0.9988 and provides a very stable power source. We applied this method to repair bright pixels in LCD panels, achieving a success rate of 86 %. In addition, the proposed method eliminates the need to remove the LCD casing from the fabrication module, thereby increasing the efficiency of production.

Improvements in the AIST Cryogenic Radiometer With Superconducting Thermometer

We are developing a cryogenic radiometer (CR) with a superconducting thermometer for the optical laser power calibration. This device consists of a niobium transition edge sensor and an electrical substitution heater on a silicon substrate. The device temperature is locked at the niobium transition temperature with a superconducting quantum interference device amplifier and a proportional-integral-derivative controller. The device successfully determines the absolute optical power in the range from 20 nW to 600 muW, with a fast response of 3 s. The equivalence measurement shows that the response difference between the substitution power and the simulated optical power is less than 0.0034%. These properties are very attractive for next generation of a high-accuracy CR

A photon pressure calibrator for the GEO 600 gravitational wave detector

Interferometer mirror displacement induced by radiation pressure is used to demonstrate an alternative calibration method for the GEO 600 detector. The photon calibrator utilizes an amplitude modulated laser diode with up to 1.4 W output power at a wavelength of 1035 nm. The achieved accuracy of the strain amplitude calibration is dominated by the laser power calibration error, which is in the range of ±4% for the measurements presented in this Letter.

Thermal characterization of a cryogenic radiometer and comparison with a laser calorimeter

We have developed a finite-element-analysis thermal model to study the temperature distribution in a laser-optimized cryogenic radiometer (LOCR). Our modelling showed that the worst-case inequivalence between electrical and optical power is 0.0004%. We used the thermal model, empirical testing, and research performed at other laboratories, to conduct a detailed uncertainty analysis for laser power calibrations based on the LOCR. Using photodiode trap detectors at laser wavelengths of 633.0 nm and 1550.4 nm as intermediate standards, we showed that the cryogenic radiometer and the NIST C4 laser calorimeter agree within 0.1%.