Optical Power Measurements Research Articles

Measurements of Low Optical Power with Cryostat-Based Predictable Quantum Efficient Detector at Liquid Nitrogen Temperature

Abstract We have validated optical power measurements with a Predictable Quantum Efficient Detector (PQED) at liquid nitrogen temperature (77 K) at low optical power from 130 fW to 3.3 pW. Two laser wavelengths at 514 nm and 785 nm were used. The lowest measured optical power corresponds to a photon flux of 0.5·106 photons per second (785 nm). The PQED’s responsivity is linear within the relative measurement uncertainties of 8% at 0.5·106 ph/s and 1.4% at 10·106 ph/s (95% confidence level), which enables the calibration of other low photon flux detectors directly against a primary standard of optical power.

Current Progress of EUV Spectral Responsivity Calibration for EUV Lithography at CMS/ITRI

Abstract To support the development of advanced EUV lithography for the semiconductor industry in Taiwan, a calibration system for photodiodes’ spectral responsivity was established. The current system utilizes the synchrotron radiation light source and the method is traceable to PTB, Germany. The wavelength range is from 10 nm to 15 nm, including the most often used 13.5 nm. Several techniques were studied to compensate for light source fluctuation and to reduce the measurement uncertainty. The relative expanded uncertainty of the spectral responsivity calibration at 13.5 nm is 4.6 % (k=2). A wafer-type EUV radiant power meter designed to be used in EUV lithography chambers is being developed. Our goal is to develop simple and reliable methods for on-site EUV optical power measurement and dose estimation.

Traceable characterisation of fibre-coupled single-photon detectors

Abstract The detection of single photons plays an essential role in advancing single-photon science and technologies. Yet, within the visible/near-infrared spectral region, accurate fibre-based optical power measurements at the few-photon level are not yet well-established. In this study, we report on a fibre-based setup, enabling traceable optical power measurements at the few-photon level in this spectral region. The setup was used to calibrate the detection efficiency (DE) of four single-photon avalanche diode (SPAD) detectors. The relative standard uncertainties on the mean DE values obtained from repeat fibre-to-detector couplings ranged from 0.67% to 0.81% (k = 2). However, the relative standard deviation of DE values, which ranged from 1.38% to 3.20% (k = 2), poses a challenge for the metrology of these devices and applications that require high accuracy and repeatability. We investigated the source of these variations by spatially mapping the response of a detector’s fibre connector port, using a focused free-space beam, allowing us to estimate the detector’s spatial non-uniformity. In addition, we realise a novel calibration approach for fibre-coupled SPADs in a free-space configuration, enabling a direct comparison between the fibre-based setup and the National Physical Laboratory’s established free-space facility using a single SPAD. Finally, we investigated alternative coupling methods, testing the repeatability of different fibre-to-fibre connectors in addition to direct fibre-to-detector couplings: SPADs from three manufacturers were tested, with both single-mode and multi-mode fibre.

Design of a 1 × 3 Power Splitter Based on Multimode Interference in a Parabolic-Type Slot-Waveguide Structure

Multimode interference (MMI) couplers based on silicon slot-waveguide structures have received widespread attention in recent years. The key issues that need to be addressed are the size and loss of such devices. This study introduces a 1 × 3 silicon-based slot-waveguide multimode interference power splitter. The device uses a gallium-nitride slot-waveguide structure to reduce the length of the coupling region and decrease additional losses. To reduce the width of the coupling region, the multimode interference coupling area is designed with a parabolic-shaped structure. The introduction of a tapered structure between the input/output waveguides and the coupling region improves additional losses and non-uniformity. Furthermore, we conducted an analysis of the fabrication tolerances of the coupling region. In this paper, we use mode solution to simulate the design of the device in the 1550 nm optical wavelength range. The eigenmode expansion method is used to simulate and optimize the parameters of the device. The device is simulated using the eigenmode expansion solver. The simulation results show that the total length of the coupling region for the device is only 4 μm. The normalized transmission of the device is 0.992, and its excess loss and imbalance are 0.036 dB and 0.003 dB, respectively. The proposed power splitter can be applied to integrated optical circuit design, optical sensing, and optical power measurement.

Direct implementation of a frequency-programmable Josephson voltage standard to provide an SI traceable optical power scale

We have developed a technique to determine the electrical substitution power of a cryogenic optical radiant power detector, that directly implements a frequency-programmable Josephson voltage standard (FPJVS), thus reducing the traceability chain. The optical power detector and the Josephson voltage reference are combined inside a common cryogenic environment. We demonstrate the practicality of the technique by using a FPJVS to apply a known voltage across the resistive heater of a standard NIST cryogenic planar radiometric detector. The power applied to the detector heater is calculated from a measurement of the heater resistance and the known applied voltage. The FPJVS dc bias current source supplies dc current to the resistive heater. In this demonstration, the standard uncertainty of the substituted electrical power is limited by the uncertainty of the electrical heater four-wire resistance measurement at 4 K. The uncertainty due to the resistance measurement is 1 part in 105 out of a total uncertainty of 1 part in 104 (k = 2) on the 1 mW optical power measurement. We aim to develop the technique, to provide traceability to the International System of Units for the picowatt power measurement of single-photon emitters such as quantum dot sources.

Design and construction of an affordable optical power meter: micro- to milli-Watt in the 400–800 nm range

This study introduces the design, construction, and evaluation of an affordable optical power meter prototype, AYR (Affordable Yet Reliable) version 1.0, which operates effectively within the 400–800 nm range, using a silicon photodiode. Aimed at bridging the gap in accessibility to precise and reliable photonics instrumentation, especially in resource-constrained settings, AYR 1.0 leverages advancements in photodiode technology, additive manufacturing, and do-it-yourself electronics. The device incorporates a custom-built electronic circuit that facilitates accurate optical power measurement by converting light into electrical current. Through rigorous testing against a reliable commercial optical power meter, AYR 1.0 demonstrated exceptional accuracy and reliability. Sensitivity values ranged from ∼13 μA mW−1 at 405 nm to ∼796 μA mW−1 at 805 nm. The operational power range spanned from 0.003 mW to 242.0 mW, with linearity (R 2) values consistently above 0.9981, indicating high fidelity in measurement. Repeatability percentages varied between 99.4% and 99.9%, and response times ranged up to 55 μs, showcasing the prototype’s rapid and reliable response to changes in optical power. The key components include a low-cost silicon photodiode (2DU10), a differential trans-impedance amplifier circuit for signal processing, and a 3D-printed housing for the sensor head and console, contributing to its cost-effectiveness and robustness. The prototype’s total cost was 116 US dollars, highlighting its affordability and potential for widespread adoption.

Light-Field Optimization of Deep-Ultraviolet LED Modules for Efficient Microbial Inactivation

Public awareness of preventing pathogenic microorganisms has significantly increased. Among numerous microbial prevention methods, the deep-ultraviolet (DUV) disinfection technology has received wide attention by using the nitride-based light-emitting diode (LED). However, the light extraction efficiency of DUV LEDs and the utilization rate of emitted DUV light are relatively low at the current stage. In this study, a light distribution design (referred to as the reflective system) was explored to enhance the utilization of emitted DUV from LEDs, leading to successful and efficient surface and air disinfection. Optical power measurements and microbial inactivation tests demonstrated an approximately 79% improvement in average radiation power density achieved by the reflective system when measured at a 5 cm distance from the irradiation surface. Moreover, a statistically significant enhancement in local surface disinfection was observed with low electric power consumption. The reflective system was integrated into an air purifier and underwent air disinfection testing, effectively disinfecting a 3 m3 space within ten minutes. Additionally, a fluorine resin film at the nanolevel was developed to protect the light module from oxidation, validated through a 1200 h accelerated aging test under humid conditions. This research offers valuable guidance for efficient and energy-saving DUV disinfection applications.

Chip-level mass detection for micro-LED displays based on regression analysis and deep learning.

Though micro-light-emitting diode (micro-LED) displays are regarded as the next-generation emerging display technology, challenges such as defects in LED's light output power and radiation patterns are critical to the commercialization success. Here we propose an electroluminescence mass detection method to examine the light output quality from the on-wafer LED arrays before they are transferred to the display substrate. The mass detection method consists of two stages. In the first stage, the luminescent image is captured by a camera by mounting an ITO (indium-tin oxide) transparent conducting glass on the LED wafer. Due to the resistance of the ITO contact pads and on-wafer n-type electrodes, we develop a calibration method based on the circuit model to predict the current flow on each LED. The light output power of each device is thus calibrated back by multi-variable regression analysis. The analysis results in an average variation as low as 6.89% for devices predicted from luminescent image capturing and actual optical power measurement. We also examine the defective or non-uniform micro-LED radiation profiles by constructing a 2-D convolutional neural network (CNN) model. The optimized model is determined among three different approaches. The CNN model can recognize 99.45% functioning LEDs, and show a precision of 96.29% for correctly predicting good devices.

Modelling and Thermographic Measurements of LED Optical Power.

This paper presents a simple engineering method for evaluating the optical power emitted by light-emitting diodes (LEDs) using infrared thermography. The method is based on the simultaneous measurement of the electrical power and temperature of an LED and a heat source (resistor) that are enclosed in the same plastic packaging under the same cooling conditions. This ensures the calculation of the optical power emitted by the LED regardless of the value of the heat transfer coefficient. The obtained result was confirmed by comparing it with the standard direct measurement method using an integrated sphere. The values of the estimated optical power using the proposed method and the integrated sphere equipped with a spectrometer were consistent with each other. The tested LED exhibited a high optical energy efficiency, reaching approximately η ≈ 30%. In addition, an uncertainty analysis of the obtained results was performed. Compact modelling based on a thermal resistor network (Rth) and a 3D-FEM analysis were performed to confirm the experimental results.

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.

Red light instruments for myopia exceed safety limits.

Low-level red light (LLRL) therapy has recently emerged as a myopia treatment in children, with several studies reporting significant reduction in axial elongation and myopia progression. The goal of this study was to characterise the output and determine the thermal and photochemical maximum permissible exposure (MPE) of LLRL devices for myopia control. Two LLRL devices, a Sky-n1201a and a Future Vision, were examined. Optical power measurements were made using an integrating sphere radiometer through a 7-mm diameter aperture, in accordance with ANSI Z136.1-2014, sections 3.2.3-3.2.4. Retinal spot sizes of the devices were obtained using a model eye and high-resolution beam profiler. Corneal irradiance, retinal irradiance and MPE were calculated for an eye positioned at the oculars of each device. Both devices were confirmed to be Class 1 laser products. Findings showed that the Sky-n1201a delivers laser light as a point source with a 654-nm wavelength, 0.2 mW power (Ø 7 mm aperture, 10-cm distance), 1.17 mW/cm2 corneal irradiance and 7.2 W/cm2 retinal irradiance (Ø 2 mm pupil). The MPE for photochemical damage is 0.55-7.0 s for 2-7 mm pupils and for thermal damage is 0.41-10 s for 4.25-7 mm pupils. Future Vision delivers the laser as an extended source subtending 0.75 × 0.325°. It has a 652-nm wavelength, 0.06 mW power (Ø 7 mm aperture, 10 cm distance), 0.624 mW/cm2 corneal irradiance and 0.08 W/cm2 retinal irradiance (Ø 2 mm pupil). MPE for photochemical damage is 50-625 s for 2-7 mm pupils. For both of the LLRL devices evaluated here, 3 min of continuous viewing approached or surpassed the MPE, putting the retina at risk of photochemical and thermal damage. Clinicians should be cautious with the use of LLRL therapy for myopia in children until safety standards can be confirmed.

Optical Power Measurements in Passive Fiber Optic Networks Using the LoRaWAN Protocol

According to the National Telecommunications Agency (ANATEL), in a report released in 2022, it detailed that around 63% of fixed broadband connections used fiber optic networks for connection. This report demonstrates the constant expansion of fiber optic networks in Brazil. The high demand for optical networks means that internet providers face challenges in delivering a quality service that provides a positive customer experience. As a result, companies in the sector have been looking for solutions to solve problems in connecting optical networks. Currently, in the literature and on the market, there are techniques and tools that enable providers to monitor the network connection, carry out speed tests, promote customer service and carry out preventive maintenance. One of these commercial tools is the Optical Power Meter (OPM), a device capable of taking optical measurements in fiber optic networks. Therefore, this work aims to develop an optical power measurement system for passive fiber optic networks, in order to analyze the quality of the signal provided to users, which uses the LoRaWAN communication protocol, thus integrating with the Internet of Things (IoT).

Development of silicon photodetectors for absolute optical power measurement

Traditional methods used to realize the primary standard for the absolute optical power standard rely on expensive equipment and require well-trained personnel for maintenance and measurement activities. Silicon photonics technologies have enabled the development of predictable photodiodes with uncertainty comparable to (or perhaps better than) traditional methods. This work will report these research activities.

Recent advances and perspectives in photometry in the era of LED lighting

Photometry contributes to our understanding how the world is perceived by the human visual system, where a specific example is an early study of photographic plates. Nowadays photometry has an important role in measurement of lighting, particularly in the transition from incandescent lighting to LED lighting. In the context of sustainability and energy efficiency, updated photometric methods are needed that address the spectral, color, geometrical, and temporal characteristics of LED light sources. Highlights of the recent advances in these measurement methods are reviewed. Furthermore, challenges and achievements are described in the measurement of optical power, particularly related to the definition of the photometric SI base unit, the candela. These achievements are based on progress in the development of underpinning optical measurement standards, with a focus on technologies like the electrical substitution radiometer and the predictable quantum efficient detector. As a conclusion, the importance of ongoing research and development in photometry is emphasized considering its crucial role in supporting the advances of LED lighting technology and sustainable development goals of lighting industry.

Single‐mode optical fibre digital decoder based on polarization using a K‐nearest neighbour algorithm

AbstractIt is shown experimentally a new digital optical decoding scheme based on the transmission or polarized light at p polarization planes using a K‐Nearest Neighbour (KNN) algorithm through a single‐mode optical fibre at 633 nm. The optical power signal is sent at p polarization planes which constitute p classes required for signal bit recognition. Results show that it is possible to recognize 32 polarizations planes, 5 bits, using 4 features corresponding to the measurement of optical power at 4 different angles at the photodetector side with an average assertiveness of 99.02%.

2-Dimensional integrated optical power measurement of multiple single-mode optical fibers in wide wavelength range of O to L-band using Fresnel reflection-based optical taps with infrared image sensors

We perform optical power measurements at wavelengths of 1260 nm to 1630 nm for nine single-mode optical fibers integrated in a two-dimensional array with pitch of 500 μm. We employ two optical taps with Fresnel reflection to split parts of the lights from the optical fibers, and two infrared image sensors to measure the optical power of the split lights. We clarify the variation in the optical powers from the output data of the image sensors. The measurable range was −15 dBm to −50 dBm; the mean square error was 0.37 dB. No polarization dependency was observed while changing the polarization condition of the lights in the optical fibers. The measurements were performed while changing the wavelength of the lights in the optical fibers from 1260 nm to 1630 nm and the resulting mean square error was 0.50 dB. The performance of the prototype is sufficient for practical use in the management of optical network facilities in data centers.

Material characterisation for optical wireless communication combining measurement and Monte Carlo simulations

AbstractA method is proposed for optical characterisation of materials, which is a very important input for realistic channel simulation based on Monte‐Carlo Ray‐Tracing algorithms. This original approach consists first of all in carrying out some measurements of the optical power received after propagation in the environment containing the materials sought, using a simple and low‐cost experimental setup. In a second step, this approach is based on an optimization algorithm. It takes as input the optical power measurements made, associated with the parameters of the measurement environment, such as the positions or properties of the sensors. This algorithm searches for the parameters of the material reflection models, minimising the difference between the optical measurement and the simulation. Two cost functions are studied to perform this search and showed that the correlation measure is the more robust one. To avoid uncertainties in the real input data, this approach is discussed using only a virtual configuration with well‐controlled input data and thus a virtual measurement obtained by simulation. The results show that this method produces a correct estimate of the Bidirectional Reflectance Distribution Function (BRDF) albedos, provided that the chosen BRDF models correspond well to the reflection behaviour of the materials, and that the materials have a significant influence on the measured optical power.

High-bandwidth coherent optical communication over 10.3 km of turbulent air.

We demonstrate 111.8Gb/s coherent optical communication throughput over a 10.3km folded free-space laser range. Folded links are low complexity to establish and provide a high uptime for testing equipment. The communication signals were sourced from an un-modified commercial off-the-shelf transceiver intended for long-haul fiber networks. Wavelength dependence was explored by testing 52 optical C-band channels over the course of an evening. In the future, such high-bandwidth communications will be used in feeder links from satellites in geosynchronous orbit. Optical power measurements of the received signal are compared with atmospheric theory to determine the turbulence strength exhibited and therefore the applicability of the laser range to space-to-ground links. We show that the high-uptime, 10.3km laser range is suitable for testing high-bandwidth space-to-ground optical communication systems intended for links to geosynchronous orbit at 20°-50° elevation.

Second-order acceleration for optical power measurement using axial thermopile detectors

A method of accelerating the response of thermopile detectors is proposed for optical power measurements. The physical model of the detectors is established, and the theoretical response is derived. The response is not completely described by a single exponential function. The difference between the conventional latency compensation method assuming a single exponential leads to overshoot and deviation from the equilibrium state. A new method with second-order differential operations is proposed, which is more appropriate for the non-exponential responses. In simulations, the second-order method theoretically shows 67% improvement in the measuring time. Experiments based on fabricated copper-substrate detectors show 43%, 65%, and 56% reduction in the measurement time compared with the conventional method, under the deviation ranges of ± 1%, ±0.5%, and ± 0.3%, respectively. Since the method is based on the adiabatic mechanism that generally exists in the light absorbing process, it is compatible with various types of calorimetric detectors.

Fiber-based optrode with microstructured fiber tips for controlled light delivery in optogenetics

Objective. Optogenetic modulation of neuronal activity requires precise and flexible light delivery to deep brain regions. Flat cleaved optical fibers combined with electrodes are widely used in implantable optogenetic devices for light delivery and electrical monitoring of neural activity. However, the flat fiber tip geometry induces serious tissue damage upon insertion, and makes it difficult to adjust and control the spatial extent of illumination within the brain. With their strongly increased tissue-compatibility and the possibility of spatial illumination control, tapered fibers outperform cleaved fibers in targeted neural photo-stimulation. Approach. In this work, we describe our device concept, and present a novel approach for reproducible fabrication of tapered fiber tips via grinding. Furthermore, we characterize recording electrodes by commenting data obtained from electrochemical impedance spectroscopy (EIS). We also investigate the impact of different cone angles (14°, 30°, 60°, and 90°) on the illumination profile and optical throughput. Main results. We fabricated a fiber-based optrode with cone tip and two deposited electrodes. Custom grinding setup for fabrication of tapered fiber tips with various cone angles is developed as a part of our research. Microscope images showed very good optical quality of cone tips. The results of transmitted optical power measurements performed with integrating sphere suggest that, compared to the flat cleaved optical fiber, transmitted power decreases exponentially with cone angle reduction. Obtained emission profiles (as induced fluorescence in Rhodamine 6G water solution) indicate very strong effect of cone angle on shape and size of illumination volume. Results obtained from EIS show the effect of electrode size on its recording capability. Significance. Compared to optrodes with flat cleaved optical fiber, the demonstrated fiber-based optrode with cone tip allows controlled light delivery with reduced invasiveness. The possibility to fabricate reproducible fiber tips with various cone angles enables control of light delivery in optogenetic experiment. The results presented here give neuroscientists the possibility to choose the appropriate tissue-compatible cone geometry depending on their stimulation requirements.