Speeding up spectral analysis

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The optical set-up of the new dual-comb spectrometer. (Image: David R. Carlson, NIST and the University of Colorado Boulder)

                             

 

Matthew Dale speaks with the developers of the world’s first dual-comb spectrometer with an acquisition speed of 10 gigahertz

Spectroscopy is a well-established branch of photonics that enables the colour of light absorbed or emitted from a substance to be measured. This has applications ranging from chemical analysis in the biomedical and pharmaceutical fields, to measuring the output of horticultural lighting or flat-panel displays, and even studying the spectral emission lines of distant galaxies.

Much of the development in the field of spectroscopy in recent years has been focused on miniaturising the technology – for example, so that it may either be mass-produced or more easily wielded out in the field.

However, recent work by researchers from the US’ National Institute of Standards and Technology (NIST) and the University of Colorado Boulder has been aimed at dramatically increasing the acquisition speed of spectroscopy. This is because long data acquisition times are still typically required, particularly for complex and detailed measurements, when using existing spectrometers.

The team has developed an advanced dual-comb spectrometer that can acquire data with exceptionally high speed, which shows promise for a variety of applications including remote sensing, realtime biological imaging and machine vision.

Our new system can measure a spectrum in mere microseconds,’ said Scott Papp, leader of the research team. ‘This means it could be used for chemical studies in the dynamic environment of power plants or jet engines, for quality control of pharmaceuticals or semiconductors flying by on a production line, or for the video imaging of biological samples.’

According to the researchers, the new spectrometer, reported in Optics Express, is the first dual-comb spectrometer with a pulse repetition rate of 10 gigahertz. The researchers demonstrated the system by carrying out spectroscopy experiments on pressurised gases and semiconductor wafers.

A two-combed approach

Papp explained that frequency comb technology is emerging to fill a gap in the market for laser spectroscopy instruments offering both wide spectral coverage and fast acquisition speed. Existing solutions include tunable lasers, which according to Papp have a limited spectral measurement range, and traditional Fourier spectrometers, which have a limited acquisition speed.

‘Our research paper demonstrates a frequency comb technology with a record combination of performance with regard to these metrics,’ he said. ‘Our dual-comb spectrometer, based on electrooptics, offers extremely fast measurements, high spectral resolution, and wide spectral coverage. This combination comprehensively enables spectrometer applications in dynamic environments.’

Dual-comb spectroscopy uses two optical sources, known as optical frequency combs that emit a spectrum of colours – or frequencies – perfectly spaced like the teeth on a comb. Frequency combs are useful for spectroscopy because they provide access to a wide range of colours that can be used to distinguish various substances.

‘Frequency combs are already known to be useful for spectroscopy,’ said David Carlson, lead author on the Optics Express paper. ‘Our research is focused on building new, high-speed frequency combs that can make a spectrometer that operates hundreds of times faster than current technologies.’

A close-up of the chip-based nanophotonic nonlinear waveguide used in the new spectrometer. (Image: David R. Carlson, NIST and the University of Colorado Boulder)

To create a dual-comb spectroscopy system with extremely fast acquisition and a wide range of colours, the researchers brought together techniques from several different disciplines, including nanofabrication, microwave electronics, spectroscopy and microscopy.

The frequency combs in the new system use an optical modulator driven by an electronic signal to carve a continuous laser beam into a sequence of very short pulses. These pulses of light pass through nanophotonic nonlinear waveguides on a microchip, which generates many colours of light simultaneously. This multi-colour output, known as a supercontinuum, can then be used to make precise spectroscopy measurements of solids, liquids and gases. The chip-based nanophotonic nonlinear waveguides were a key component in the researchers’ new spectrometer. These channels confine light within structures that are a centimetre long but only nanometres wide. Their small size and low light losses combined with the properties of the material they are made from allow them to convert light from one wavelength to another very efficiently to create the supercontinuum.

‘The frequency comb source itself is also unique compared to most other dual-comb systems because it is generated by carving a continuous laser beam into pulses with an electro-optic modulator,’ said Carlson. ‘This means the reliability and tunability of the laser can be exceptionally high across a wide range of operating conditions, an important feature when looking at future applications outside of a laboratory environment.’

Analysing different states of matter

To demonstrate the versatility of the new dual-comb spectrometer, the researchers used it to perform linear absorption spectroscopy on gases of different pressure. They also operated it in a slightly different configuration to perform nonlinear Raman spectroscopy on semiconductor materials. This technique, which uses pulses of light to characterise the vibrations of molecules in a sample, has not previously been performed using an electro-optic frequency comb, according to the researchers.

The high data acquisition speeds that are possible with electro-optic combs operating at gigahertz pulse rates are ideal for making spectroscopy measurements of fast and nonrepeatable events.

‘It may be possible to analyse and capture the chemical signatures during an explosion or combustion event,’ confirmed Carlson. ‘Similarly, in biological imaging the ability to create images in real time of living tissues without requiring chemical labeling would be immensely valuable to biological researchers.’

Practical application via miniaturisation

The researchers are now working to improve the system’s performance to make it practical for applications such as real-time biological imaging and to simplify and shrink the experimental setup so that it could be operated outside of the lab.

Papp gave a breakdown of the three main photonics components of the new spectrometer and discussed whether or not each of these can be improved upon: a frequency-comb generator, an optical amplifier, and a nonlinear waveguide.

‘The optical amplifier is already mature erbiumfibre technology, borrowed from the telecom industry,’ he said. ‘In our current experiments, we use mature electro-optics also borrowed from the telecom industry for frequency-comb generation. But frequency-comb technology is rapidly changing through innovation with integrated photonics. Our own research shows how to dramatically reduce power consumption and cost with integrated microresonators that directly convert a continuous-wave laser into a frequency comb. Such microresonator frequency combs or “microcombs” are certain to replace the electrooptics technology that we used to demonstrate the dual-comb spectrometer.’

From a technological perspective, the biggest innovation associated with demonstrating the dual-comb spectrometer has been with the nonlinear waveguide that creates a wide measurement spectrum, according to Papp: ‘For this device, we already use advanced integrated photonics to realise breakthrough performance. The next step would be to develop suitable photonics packaging to interface with erbium-fibre optical amplifiers.’

The leveraging of telecom industry components by the new spectrometer will likely make it possible for the technology to compete with commercially available spectrometers on cost, according to the researchers. ‘Therefore, perhaps the key is for a company to work on building a commercially viable instrument,’ Papp concluded. ‘That of course requires a detailed market analysis that is beyond the scope of our fundamental research work.’

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Reference

Paper in Optics Express: D. Carlson, D. Hickstein, S. Papp, Broadband, electro-optic, dual-comb spectrometer for linear and nonlinear measurements, Optics Express, 28, 20, 29148 –29154 (2020). DOI: https://doi.org/10.1364/ OE.400433.

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