Miniaturised Spectrometer

About

As PCs, mobile phones and smart devices continue to shrink, miniaturisation of sensing technology is playing a growing role in our lives. However, the performance of miniaturised and smartphone-integrated spectrometers falls significantly behind that of their lab-based counterparts. This is largely due to the technical challenges associated with achieving compactness and ruggedness whilst maintaining high detection performance and affordability. Fourier transform (FT) spectrometers are known for their high spectral resolution and high light throughput, making them powerful analytical instruments. The spectral resolution of FT spectrometers can be significantly increased at the expense of only a moderate reduction in spectral range by using an optical scheme known as spatial heterodyning, whereby diffractive optical elements (DOEs) are introduced in the interferometric design. Efforts to miniaturise spatial heterodyne spectrometers (SHS) have previously fallen short due to the complexity of instruments or shortcomings in their optical arrangements. This spectrometer combines a conventional interferometric optical arrangement with the spatial heterodyne scheme based on diffractive optical elements to increase the miniaturisation and light-throughout performances over conventional SHS designs. The instrument offers high light throughput and high spectral resolution, as well as ruggedness due to a lack of moving parts. The required optical components are available across wavelengths ranging from ultraviolet (UV) to mid-infrared (MIR), making the system fully configurable. The ambition is to develop a full instrument with a footprint of approximately 1cm3, with the potential to make further reductions in size. It is likely that the next generation of smart devices will contain spectral analysers due to the wide range of potential applications. This instrument design has the potential to meet the miniaturisation requirements for integration with these devices. Current stage of development - A benchtop prototype system based on free-space optics has been modelled and built, successfully demonstrating proof of principle: • The prototype has used primarily commercial-off-the-shelf (COTS) components, except for the custom components. Its footprint was bigger (8 times) than what is potentially achievable (~1 cm3) by using all-custom components bonded together to make the system quasi-monolithic. • The performance of the prototype has been assessed by using standard calibration sources. Preliminary results at visible wavelengths show that high levels of spectral resolution were achieved (0.39 nm), in line with the theoretical values expected for this design (0.45 nm). These values are more than one order of magnitude higher that commercially available pocket-sized spectrometers for consumer market (typically >10nm, not based on Fourier transform SHS principles). Next Steps for Development: • Reduce size (from benchtop prototype system) by using all-custom components bonded together to make the system quasi-monolithic • There are currently high tolerance design requirements as the quality of the spectral output appears to be very sensitive to the precision of the optical alignment. We therefore plan to carry out a comprehensive tolerance analysis of the design and to explore design variants which may relax the tolerance requirements