Fourier transform spectroscopy made easy

Gemma Church finds out more about the Gemini interferometer from Nireos and its new uses


pectroscopy is commonly used to identify materials by measuring the colours

of light that have interacted with a sample. But there’s still plenty of room for improvement in this established field. The Gemini interferometer

from Nireos is one example. It’s highly versatile and can be added to any optical setup, and used with a range of light sources. Fabrizio Preda, CEO at Nireos, said: ‘Gemini is a tool that enables the measurements of the spectrum of any kind of light, either a coherent light source, such as a laser, or an incoherent source, such as a lamp, or a fluorescence signal emitted by a sample.’ That’s not all. Gemini is compact and lightweight, providing broad spectral coverage from 400 to 2,300nm (with an ultra-broadband option of coverage from 250 to 3,500nm) across continuous spectra. It’s compact, light and robust. Thanks to the 10mm

clear aperture and the absence of entrance slits or gratings, it provides a high throughput, unaffected when the spectral resolution is changed via the system’s software by varying the maximum scan delay. We’ll look at each of

those benefits. First, the technology behind the Gemini interferometer. This is based on Fourier transform (FT) spectroscopy, which uses the interference of light, rather than dispersion, to measure spectra. Using FT spectroscopy, light is split into two collinear time-delayed replicas, whose interference pattern is measured by a detector as a function of their delay. The FT of the resulting interferogram yields the continuous-intensity spectrum of the waveform. ‘While common

interferometers (such as Michelson or Mach-Zehnder interferometers) are typically limited to the IR range, Gemini also provides ultra-broadband spectral coverage and can

The ground-breaking NIREOS’ core technology is represented by the GEMINI Interferometer: a compact and ultra-stable common-path interferometer, which unlocks all the advantages of the Fourier-Transform approach (such as light throughput and wavelength accuracy) also in the UV-Visible and Near Infrared spectral region. The GEMINI can be used in many experiments in linear and non-linear spectroscopy, such as fluorescence and pump-probe spectroscopy

deliver all the benefits of FT spectroscopy across the spectrum, down to the ultraviolet spectral region. For example, I am referring to the well-known multiplex or Fellgett’s advantage, the étendue or Jacquinot’s advantage, and high wavelength accuracy or Connes’ advantage,’ Preda added. In fact, thanks to Nireos’

patented common-path geometry, Gemini is insensitive to external vibrations. It can guarantee extremely high delay reproducibility and stability of better than one attosecond, which is around a thousand times smaller than the optical cycle of visible radiation. FT spectrometers have

many prominent benefits over those instruments that rely on dispersion. ‘The typical way to operate with gratings-based devices is to focus the light on the small entrance slit, and then collimate the light coming out of the device. This can be extremely difficult, especially with incoherent light beams, such as fluorescence,’ Preda explained. ‘Instead, with the Gemini, a

The Gemini interferometer 26 Electro Optics June 2021

collimated beam is used. The light coming in is the light coming out. You have no spatial distortion to the beam, and it is very easy to focus the light, even on the small active areas of single photon avalanche diode (Spad) detectors. This is a great practical advantage,’ he said. The Gemini interferometer

also provides many benefits when compared with monochromators. Preda explained: ‘It overcomes the main drawbacks of monochromators, in terms of footprint, low throughput, fixed spectral resolution and limited spectral coverage.’ Finally, Gemini is very easy to

set-up and use. Preda explained: ‘It’s a portable, plug-and-play device. The user just needs to position the interferometer in the beam path, between sample and detector (or between light source and sample, depending on application). Due to its compactness, it’s simple for users to integrate into existing systems without any technical difficulties.’ Nireos can also provide plug

and play software, allowing the user to easily control the device, acquire data and retrieve the calibrated spectrum of light. ‘So, the user has a tool which is plug-and-play from a hardware and software perspective,’ Preda said.

Growing applications Nireos has been in business since 2018 and its founders are former researchers, specialising in ultrafast spectroscopy. ‘The company started off as a spin-out from the Politecnico di Milano, and Gemini (also known as the Twins interferometer in previous scientific publications) is the core technology on which our other products are based,’

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