Characterizing Levitating Space Dust with 3D Scatterometry

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"In almost every setup, where precise measurements are needed and complex devices must be controlled, one can find NI technology. The easiest way to combine hardware from different manufacturers is to place LabVIEW at the heart of the system, because every respectable supplier has LabVIEW-based drivers for their hardware."

- Professor Edward Hæggström, University of Helsinki

The Challenge:
Characterizing space dust, by measuring the light scattered in 360 degrees from a levitating cosmic particulate sample, whilst simultaneously controlling its 3D position and recording the large amounts of experimental data.

The Solution:
Using the NI PXIe platform as a base, data from a laser-doppler-vibrometer, a high-speed camera, and several photomultiplier tubes (PMT) were recorded synchronously. A motorized rotation stage precisely controlled the angle of the PMT’s position, along the whole 360°range. High performance LabVIEW algorithms process high-speed camera feeds for post-hoc image stabilization to further improve the fidelity of the scattering calculations.

Author(s):
Professor Edward Hæggström - University of Helsinki
Göran Maconi (Research Assistant) - University of Helsinki
Ivan Kassamakov (Researcher) - University of Helsinki

The Study of Space Dust

ETLA is the home organization for the European Research Council Advanced Grant project "Scattering and Absorption of Electromagnetic Waves in Particulate Media" – SAEMPL. As part of the research, we were required to develop a new-breed of instrumentation, for characterizing space dust. Cosmic samples are typically small, fragile and invaluable. As such the devised experimental setup needed to be carefully considered, and completely non-destructive.

Enabling non-destructive measurements would allow full sample recovery, such that the instrumentation could characterize unique samples returned from Solar System objects (cf. samples from asteroid Itokawa or from the Earth’s Moon). Although characterization would initially take place in an Earth-based clean-room-laboratory, the instrument could be further developed to fly onboard space missions to enable in-situ analyses of Solar System objects.

Scattering theory is a new and promising set of tools for characterizing small particles. By examining the scattered light around a particle, radiation and particle flux can be determined. Scattering theory is already seeing use in the creation of atmospheric models, catalytic agents, next-generation capacitors, medical diagnostic toolkits, and drug carriers. Additionally, scatter-based imaging could provide a pathway towards super-resolution imaging.

Figure 1. The Test Setup

 

Introducing the 3D Scatterometer

Currently there are no experimental setups capable of measuring scattered light from cosmic particles with dimensions between micrometer and millimeter sizes. However, based on ETLA’s experience in ultrasound manipulation and precise optical interferometric devices, we united an interdisciplinary group of scientists to develop a fully automated, 3D scatterometer capable of measuring scattered light at different wavelengths from cosmic samples.

To ensure disturbance-free measurement, with full control over the location and orientation of the scattering object, the 3D scatterometer required an acoustic levitator to provide contactless suspension of the sample, without affecting the light scattering. When the sample is held in the levitator, the sample position is not constant which affects the reconstruction of the scattering event. To compensate for the light intensity variation caused by the sample moving relative to the incident beam, the position of the sample needs to be known with high spatial and temporal resolution.

In the initial prototype of the 3D scatterometer, we used custom built data acquisition circuitry that were patched together. There were always issues with timing and data fidelity. Having used LabVIEW and NI hardware for 25 years in our laboratory, we knew that NI hardware is reliable and, in our opinion, the only commercial solution that can truly enable domain experts to develop complex and precise measurement systems. LabVIEW-based applications are easy debug and modify, which is important during the development stage of any novel technology.

 

How the 3D Scatterometer Works

The system utilises a tunable laser light source that emits collimated and well-polarized light of selectable wavelength. This beam of light is directed at a cosmic sample suspended by the ultrasonic levitator, which scatters the light in 360 degrees.

 

Figure 2. System Overview

 

The scattered light is collected using photo multiplier tubes (PMT). The PMT signal is amplified and captured by an NI PXIe-5171R module, along with the vibrometer output and the camera synchronization signals. The PMT(s) are scanned by a rotational stage controlled via the NI PXIe-8880. We control the speed, angle, and acceleration of a rotational stage using a LabVIEW function library developed by the manufacturer of the rotational stage. The huge variety of 3rd party instrument drivers available for LabVIEW saves a huge amount of time.

A high-speed camera monitors the precise positioning of the sample in the image plane while the vibrometer measures its depth, giving us accurate 3D sample positioning. The camera footage is post-processed and stored on a different computer.

Determining the exact sample position in real-time, which requires vibrometer and camera synchronization was achieved by a specialized LabVIEW-based alogrithm. The ability to seamlessly integrate so many different sub-systems, from many different suppliers, into a single, coherent instrument is a unique and immensely valuable benefit of the NI platform.

The processor is acquiring timing critical data from PMT’s, rotation stage, vibrometer, and camera. The FPGA is averaging the voltage signals from the PMT’s transimpedance amplifier to speed up the data acquisition process. NI technology enabled the precise synchronization between the 3D object position, and the output signal of the PMT’s, which was vital to this project.  From the FPGA-based signal processing to the instrument clean, intuitive GUI, we programmed the system exclusively with LabVIEW.

The LabVIEW-based oscilloscope application worked immediately and together with the card, replaced the external oscilloscope used in the first stage of the component testing and development.

 

The Success of the Scatterometer

The 3D scatterometer allows us to validate and prove theoretical models and collect more data to get new theoretical understanding about the properties of cosmic samples. With this we will fulfill the main goal of the SAEMPLE project.

The device is the first of its kind, measuring controlled spectral angular scattering including all polarization effects, for a levitating object (µm-cm size).  It enables a non-destructive, disturbance-free measurement with full control of the orientation and location of the scattering object.

As a result of this success, we have applied for the European Research Council’s Proof of Concept grant (ERC-PoC), with a goal the full commercialization of the 3D Scatterometer.

 

The Future of the System

In almost every setup, where precise measurements are needed and complex devices must be controlled, one can find NI technology. The easiest way to combine hardware from different manufacturers is to place LabVIEW at the heart of the system, because every respectable supplier has LabVIEW-based drivers for their hardware.

Every successful measurement system depends on reliability and repeatability, which is even more difficult in a scatterometry-based application. Our measurement system achieved these targets with state-of-the-art National Instruments hardware and customized software developed with LabVIEW. This performance has allowed stability in our results, flexibility in the samples under test and a possible opportunity to commercialize the system.

We believe we will succeed with our European Research Council Proof of Concept (ERC-PoC) grant application, which will enable us to further optimize and commercialize the 3D scatterometer. Next, we would like to develop the device into a 4p system (i.e. using the whole area of the spherical polar coordinate system), which would allow the scatterometer to label-free bio-imaging on the nanometer scale, and take us a significant step closer to super-resolution imaging of small objects. The inherent scalability of LabVIEW will play a vital role in the future development of this cutting-edge instrument.

 

Author Information:
Professor Edward Hæggström
University of Helsinki
Finland
Tel: +358 50 3175493
Edward.haeggstrom@helsinki.fi

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