Laser Power Measurements Research Articles

Multi-kilowatt cw laser power measurement comparison between national standards

We present here the first comparison between National Metrology Institutes of high accuracy continuous wave optical power measurements in the kilowatt regime. The National Institute of Standards and Technology (NIST) performed measurements with a power meter relying on photon momentum. The Physikalisch-Technische Bundesanstalt (PTB) performed measurements with a modified off-the-shelf thermal power meter. The non-absorbing photon momentum measurement approach permits the two power meters to measure the same laser beam optical path simultaneously, resulting in a direct comparison of the meters supported by an optical system to accommodate differences in instrument settling times. The results show agreement within the expanded uncertainties for each instrument. NIST and PTB illustrate a degree of equivalence of 0.49% with an expanded uncertainty of 1.37% (k = 2) for an average result across all power levels.

Achieving 1-µg Weighing Resolution of Light Pressure by Drift Analysis and Compensation for Precision Sub-Kilowatt Laser Power Measurement

Achieving 1-<i>µ</i>g Weighing Resolution of Light Pressure by Drift Analysis and Compensation for Precision Sub-Kilowatt Laser Power Measurement

Evaluation of laser power stability of repeatedly used SubCyclo probe in micropulse transscleral cyclophotocoagulation for glaucoma: A step towards sustainable ophthalmic surgery

Purpose To evaluate the laser power stability of the SubCyclo probe for micropulse transscleral cyclophotocoagulation after repeated use. Materials and methods This experimental study involved 6 new probes. Each probe was connected to the SubCyclo mode (2,000 mW power, 31.3% duty cycle, and 100 seconds duration) of the Vitra 810 laser delivery system (Quantel Medical, France). Laser power measurements were taken using a calibrated laser power meter (Nova, Ophir Optronics Solutions, Israel) every 10 seconds from 10 to 90 seconds during each of the 40 cycles. Intra-rater reliability was assessed using intraclass correlation (ICC). A linear mixed model for repeated measures and pairwise comparisons with Bonferroni adjustment were used for the analysis. Results The mean (SD) power outputs of all probes for the first cycle and all cycles were 421.9 (19.7) mW and 436.7 (16.1) mW, respectively. During the first cycle, the mean (SD) laser power gradually decreased from 444.3 (13.4) mW at 10 seconds to 407.3 (17.0) mW at 90 seconds (Fig 3). For all cycles, the power was 446.0 (13.6) mW at 10 seconds and gradually declined to 426.8 (21.0) mW at 90 seconds. Pairwise comparisons revealed significant differences in mean laser power outputs after 16 cycles of repeated use compared to the first cycle. The ICC estimate (95% CI) for intra-rater reliability was 0.96 (0.89, 0.99). Conclusions The SubCyclo probe maintains stable laser power outputs throughout repeated use for up to 16 cycles, with a significant increase observed after 16 cycles.

Optimized design and experimental study of a macroscopic mirror to achieve linear amplification of optical force-induced displacement.

A new thin plane mirror with an Archimedes spiral structure (Archimedes-structure thin plane mirror - ATPM) that implements an elastic support boundary is proposed in this study. An optimal structure of ATPM is developed to achieve a linear displacement response with respect to optical forces. The displacement response of the optimized ATPM is analyzed by considering the combined effects of optical force and gravity. The distribution of the optical force density is calculated based on a tilted Gaussian laser beam. Experimental results demonstrate that the optimized ATPM can produce a steady-state displacement of 24.18 nm on average in a normal-gravity environment when subjected to an average optical force of 132.17 nN. When the optical force exceeds 133 nN, the nonlinearity of the displacement response of the optimized ATPM is less than 6.28%. An amplification of the optical force-induced displacement is achieved by more than 15 times compared with that for an unstructured mirror of the same size. The results of this study can assist the development of a miniaturized macroscale optical force platform based on an ATPM for practical applications including the in-situ laser power measurement and nN level force source in the atomic and close-to-atomic scale manufacturing.

Microfabricated sensor device for CW and pulsed laser power measurements.

On-line measurement is a trend of development toward laser-based applications. We present a fiber-integrated force sensor device for laser power measurement with both CW mode and pulse mode based on laser radiometric heat and radiation force sensing simultaneously. The sensor device is fabricated using a standard microfabrication process. Laser intensity is determined through the displacement of a movable mirror measured by an integrated Fabry-Perot interferometer. Compared with the performance of the device in the ambient condition, a non-linearity error of 0.02% and measurement uncertainty of 2.06% is observed in the quasi-vacuum condition for CW laser illumination. This device can measure a CW laser power with a 46.4 μW/Hz1/2 noise floor and a minimum detection limit of 0.125 mW. For a pulsed laser, a non-linearity error of 0.37% and measurement uncertainty of 2.08% is achieved with a noise floor of 1.3 μJ/Hz1/2 and a minimum detection limit of 3 μJ.

In vivo imaging of the human eye using a 2-photon-excited fluorescence scanning laser ophthalmoscope.

BackgroundNoninvasive assessment of metabolic processes that sustain regeneration of human retinal visual pigments (visual cycle) is essential to improve ophthalmic diagnostics and to accelerate development of new treatments to counter retinal diseases. Fluorescent vitamin A derivatives, which are the chemical intermediates of these processes, are highly sensitive to UV light; thus, safe analyses of these processes in humans are currently beyond the reach of even the most modern ocular imaging modalities.MethodsWe present a compact, 2-photon-excited fluorescence scanning laser ophthalmoscope and spectrally resolved images of the human retina based on 2-photon excitation (TPE) with near-infrared light. A custom Er:fiber laser with integrated pulse selection, along with intelligent postprocessing of data, enables excitation with low laser power and precise measurement of weak signals.ResultsWe demonstrate spectrally resolved TPE fundus images of human subjects. Comparison of TPE data between human and mouse models of retinal diseases revealed similarity with mouse models that rapidly accumulate bisretinoid condensation products. Thus, visual cycle intermediates and toxic byproducts of this metabolic pathway can be measured and quantified by TPE imaging.ConclusionOur work establishes a TPE instrument and measurement method for noninvasive metabolic assessment of the human retina. This approach opens the possibility for monitoring eye diseases in the earliest stages before structural damage to the retina occurs.FundingNIH, Research to Prevent Blindness, Foundation for Polish Science, European Regional Development Fund, Polish National Agency for Academic Exchange, and Polish Ministry of Science and Higher Education.

High-power laser power measurement based on principle of light pressure

大功率激光功率测量常用量热法，但溯源复杂。介绍了具有较高测量精度的基于光压原理的大功率激光功率测量方法，设计了利用1/10⁵精度天平大功率激光测量实验，测试了基于GaAs半导体材料制作的反射镜的反射率及损伤阈值，确定了基于GaAs半导体材料反射镜的相关性能。得到了普通实验室条件下的功率测量重复性及线性，验证了1/10⁵精度天平用于大功率激光测量的可行性。通过实验结果结合理论计算，得出利用1/10⁵精度天平的光压测量功率的测量上限可以达到3×10⁴ W以上。

Absolute radiation pressure detector using a diamagnetically levitating test mass

We have developed an absolute radiometer to measure laser power by means of radiation pressure in a closed control-loop configuration using diamagnetically levitated pyrolytic graphite as the soft spring and gravity as a contactless restoring force. Our approach takes advantage of an extremely low spring stiffness (2.95 mN/m) afforded by diamagnetic levitation above a permanent magnet array and a lock-in amplifier in the feedback loop to attain a 30 m W r m s noise. A horizontally directed laser beam impinges on a highly reflective dielectric stack mirror mounted on the levitating pyrolytic graphite shuttle. The light’s momentum displaces the shuttle, and its position is restored by tilting the magnet array with a piezoelectric actuator. The closed-loop configuration avoids the need for accurate knowledge of the system spring stiffness or environmental dependencies of the diamagnetic levitation. Absolute laser power measured is traceable to the mass of the diamagnetically levitating shuttle system, the local gravitational acceleration, and the interferometrically measured tilt angle of the magnet array. The system is demonstrated with a laser power of 2.7 W, yielding a combined relative expanded uncertainty ( k = 2 ) of 1.9%. The upper limit of measurable power is determined by the range of the piezoelectric actuator, and the minimum power is currently limited by the noise floor that is constrained by air currents.

Development of a versatile, high-temperature, high-throughput ion irradiation system

Ion irradiation has long been used as a surrogate for neutron irradiation experiments which are comparatively slower, more expensive, and often can leave materials activated and difficult to handle post-irradiation. However, with the growing use of combinatorial synthesis methods and high-throughput, automated characterization techniques, ion irradiation facilities will need to upgrade their capabilities to be able to accommodate these new material platforms. To address this challenge, a high-throughput ion irradiation system has been developed at the University of Wisconsin-Madison Ion Beam Laboratory (IBL). The system is capable of accommodating large sample arrays of arbitrary geometries and heat samples individually using an infrared laser to enable high-temperature, high-throughput ion irradiation while mitigating the risk of annealing the irradiation damage in nearby samples. To control stage motion, laser power, and beam current measurement, among other tasks, the program Chronos was developed which enables automated high-throughput irradiation experiments.

High amplification laser-pressure optic enables ultra-low uncertainty measurements of optical laser power at kilowatt levels

We present the first measurements of kilowatt laser power with an uncertainty less than 1%. These represent progress toward the most accurate measurements of laser power above 1 kW at 1070 nm wavelength and establish a more precise link between force metrology and laser power metrology. Radiation pressure, or photon momentum, is a relatively new method of non-destructively measuring laser power. We demonstrate how a multiple reflection optical system amplifies the pressure of a kilowatt class laser incoherently to improve the signal to noise ratio in a radiation pressure-based measurement. With 14 incoherent reflections of the laser, we measure a total uncertainty of 0.26% for an input power of 10 kW and 0.46% for an input power of 1 kW at the 95% confidence level. These measurements of absolute power are traceable to the SI kilogram and mark a state-of-the-art improvement in measurement precision by a factor of four.

A bilateral comparison of NIST and PTB laser power standards for scale realization confidence by gravitational wave observatories

The gravitational wave (GW) observatories calibrate interferometer displacement using photon momentum, with laser power serving as the measurand. These observatories are traceable to the International System of Units through a primary standard maintained by the US’s National Metrology Institute (NMI), the National Institute of Standards and Technology (NIST). The bilateral degree of equivalence of laser power measurements for various NMIs indicated in the 2010 EUROMET.PR-S2 supplementary comparison reveals scale realization uncertainty unacceptably large for GW event parameterization. We offer here an analysis to identify the source of the discrepancy between the Physikalisch-Technische Bundesanstalt (PTB) and NIST results. Using an improved transfer standard in a bilateral comparison, with representatives of the Laser Interferometer Gravitational-Wave Observatory (LIGO) receiving results prior to their comparison, NIST and PTB demonstrated a degree of equivalence of −0.15% with an uncertainty of 0.95% (k = 2) for combined 100 mW and 300 mW comparison results.

METHODS FOR MEASURING LASER POWER

Today we are witnessing the rapid development of the laser industry. Laser sources with new wavelengths, higher powers and energies, different modes of operation (generation of laser pulses), as well as various applications in industry, medicine, environmental protection etc. are emerging. This requires the development of new physical methods and principles for accurate measurement of power, energy, and other parameters of laser sources. In different types of laser technological processes, accurate measurement of laser power is extremely important in terms of quality, repeatability, and validation of the process. In most cases, accurate laser power measurement is a difficult task, especially when working with high-power lasers or having to perform real-time measurements.The report analyses and systematizes the physical principles and methods on which existing laser measuring instruments are based. This research also aims to help researchers and technologists find and develop new approaches to solving this challenging measurement problem.

Design and implementation of photoacoustic based beam dump-average optical power meter for fast and ultrafast lasers

We reported a new method for measuring the optical power of fast and ultrafast laser pulses. This type of measurements usually encounters some technical difficulties in field of experimental practice. The method is based on investigating the laser-induced photoacoustics (PA) inside a non- lambertian integrating sphere made of stainless steel. The measured laser beam is introduced inside the sphere, and then absorbed after a series of specular reflections on its reflective inner surface. These reflections induced a similar PA signal propagated through the spheres wall. A contact piezoelectric detector fixed outside the sphere is used to detected the resultant thermo-elastic vibrations. The study of the PA signals intensity lead to the measurement of laser power. A complete theoretical modeling, design, test and calibration procedure of such power meter are inclusive. This design is suitable for checking the output power of pulsed high power laser sources directly without splitting or damping.

Axial force radiometer for primary standard laser power measurements using photon momentum

We have demonstrated operation of a new photon momentum radiometer for measuring laser power levels above 1 kW by use of radiation pressure. The ‘axial’ design allows the input and output laser beams to remain collinear and the force sensing can be carried out with any analytical balance having a vertical sensing direction and sufficient readability (sensitivity). The design also provides for a parasitic fast photodiode power measurement that is calibrated in situ by the slower radiation pressure-based power measurement. This permits a short-time-constant measurement limited primarily by the photodiode bandwidth, rather than the response time of the force balance. Uncertainty contributions yield a relative expanded uncertainty of 2.1% for (1–2) kW and 1.2% above 2 kW (for laboratory environmental conditions). We perform a direct comparison between this new power meter and the existing primary standard radiation pressure power meter. We find the two techniques agree with an average inequivalence of 0.12% for continuous wave (CW) optical powers from (1–10.8) kW. This disagreement is well-explained by the quadrature-summed uncertainties of the two techniques.

Design of an electrostatic balance mechanism to measure optical power of 100 kW.

A new instrument is required to accommodate the need for increased portability and accuracy in laser power measurement above 100 W. Reflection and absorption of laser light provide a measurable force from photon momentum exchange that is directly proportional to laser power, which can be measured with an electrostatic balance traceable to the SI. We aim for a relative uncertainty of 10-3 with coverage factor k = 2. For this purpose, we have designed a monolithic parallelogram 4-bar linkage incorporating elastic circular notch flexure hinges. The design is optimized to address the main factors driving force measurement uncertainty from the balance mechanism: corner loading errors, balance stiffness, stress in the flexure hinges, sensitivity to vibration, and sensitivity to thermal gradients. Parasitic rotations in the free end of the 4-bar linkage during arcuate motion are constrained by machining tolerances. An analytical model shows this affects the force measurement less than 0.01 percent. Incorporating an inverted pendulum reduces the stiffness of the system without unduly increasing tilt sensitivity. Finite element modeling of the flexures is used to determine the hinge orientation that minimizes stress which is therefore expected to minimize hysteresis. Thermal effects are mitigated using an external enclosure to minimize temperature gradients, although a quantitative analysis of this effect is not carried out. These analyses show the optimized mechanism is expected to contribute less than 1 × 10-3 relative uncertainty in the final laser power measurement.

Final report on supplementary comparison COOMET.PR-S7: Laser power responsivity (COOMET 599/RU/13)

Main textThe National Institute of Standards and Technology (NIST), USA and the All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI), Russia agreed in February 2013 to conduct a comparison on the laser power responsivity at wavelengths of 532 nm, 1.064 μm and 10.6 μm. The aim of this comparison is to assess the equivalence of the laser power responsivity between two laboratories. The comparison was conducted within the COOMET regional metrological organization (COOMET 599/RU/13) and was registered in the BIPM Key Comparison DataBase as a supplementary comparison with the identifier COOMET.PR-S7. The comparison was carried out using one detector head for measuring laser power. The detector head was supplied by VNIIOFI (OPHIR 10A). The results of this comparison essentially demonstrated agreement between the results obtained at the two participating laboratories.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Metrological Characteristics of Measuring Laser Power Divider

The study analyzes the errors of the measuring optical power divider of laser radiation. This divider is used to transmit a unit of average power of high-intensity laser radiation and is structurally designed as a wedge. The error of the division factor, which affects the accuracy of the unit transmission, is determined. The random component of the division factor error estimation is assessed. A determination method based on the rearrangement of the measuring instruments used is proposed to provide the most probable estimate. The error caused by the nonlinearity of the measuring instruments used to determine the division factor is investigated, and its largest interval is estimated. In case of invariability of the conversion coefficients of the measuring instruments used on the laser radiation power, the method excludes the dependence of the error in determining the division factor on the said coefficients and reduces their influence on the transmission error. The possibility of controlling the divider division factor by determining the control parameter and its confidence interval when transmitting a radiation power unit is substantiated, and a formula for calculating this interval in each measurement cycle is obtained. The parameter value and the change in confidence interval can be used to control the reliability of the transmission of a power unit to the calibrated measuring tool in real time.

Miniature force sensor for absolute laser power measurements via radiation pressure at hundreds of watts.

We present a small power meter that detects the radiation pressure of an incident high-power laser. Given its small package and non-destructive interaction with the laser, this power meter is well suited to realizing a robust real-time, high-accuracy power measurement in laser-based manufacturing environments. The incident laser power is determined through interferometric measurement of displacement of a 20 mm diameter high reflectivity mirror, mounted at the center of a dual element spiral flexure. This device can measure laser power from 25 W to 400 W with a 260 m W/H z noise floor and ≤ 3.2% expanded uncertainty. We validate our device against a calibrated thermopile with simultaneous measurements of an unpolarized 1070 nm laser and report good agreement between the two systems. Finally, by referencing to an identical mechanical spring that does not see the incident laser, we suppress vibration noise in the power measurement by 14.8 dB over a 600 Hz measured bandwidth. This is an improvement over other radiation pressure based power meters that have previously been demonstrated.

Metrological characteristics of measuring laser power divider

An analysis of the errors of the measuring optical power divider of laser radiation, which is used to transmit a unit of average power of high-intensity laser radiation and is structurally made in the form of a wedge, is carried out. The error of the division ratio, which affects the accuracy of the unit transmission, is determined. The estimation of the random component of the division coefficient error is carried out and it is shown that the proposed method for its determination, based on the rearrangement of the used measuring instruments, gives the most probable estimate. The error caused by the nonlinearity of the measuring instruments that determine the division coefficient is investigated, and its largest interval is estimated. It is shown that in the case of invariability of the conversion coefficients of the used measuring instruments on the laser radiation power, the method makes it possible to exclude the dependence of the error in determining the division coefficient on the mentioned coefficients, and in another case, to reduce their influence on the transmission error. The possibility of controlling the division factor of the divider by determining the control parameter and its confidence interval when transmitting a unit of radiation power is substantiated; a formula for calculating this interval in each measurement cycle is obtained. It is shown that the value of the parameter and the change in the confidence interval can be used to control the reliability of the transmission of the unit of power to the calibrated measuring instrument in real time.

Light-induced reversible phase transition in polyvinylidene fluoride-based nanocomposites

We report switching behavior between the ferroelectric β phase and the paraelectric α phase in polyvinylidene fluoride–multiwalled carbon nanotube (PVDF–CNT) nanocomposite. The PVDF–CNT nanocomposite thin films are prepared on Au/Si substrates using spin coating technique. Various characterization techniques reveal the interaction of PVDF with CNTs as a nucleating agent for nucleating ferroelectric β phase in PVDF. A wavelength-independent reversible phase transformation behavior is observed for the PVDF–CNT nanocomposite during the measurement of laser power modulated micro-Raman spectroscopy (533 and 632 nm lasers). This observation opens up an innovative route where we can control the interactions between polar nanocarbons and PVDF using visible light.