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Integrated Spectrometers for Spectral Tissue Sensing

Periodic Reporting for period 3 - InSPECT (Integrated Spectrometers for Spectral Tissue Sensing)

Reporting period: 2017-01-01 to 2018-03-31

"Minimally-invasive image-guided procedures are becoming increasingly important in clinical practice. In a variety of procedures physicians lack reliable feedback on the type of tissue at the tip of their interventional device (needle, catheter, probes, etc…) to ensure they are at the right position before effecting the actual diagnosis or treatment. Spectral tissue sensing using compact photonic probes has the promise to be a valuable tool for screening and diagnosing purposes, e.g. for discriminating between healthy and tumorous tissue. Real-time tissue-characterization feedback to the physician during an intervention can significantly improve the outcome of diagnosis and treatment, and ultimately reduces cost of medical treatment.

The InSPECT project enables the realization of photonic building blocks for low-cost miniaturized spectral tissue sensing devices. This involves the realization of a miniature broad-band solid-state light source and the realization of a miniature low-cost integrated spectrometer, both operating in the VIS/NIR/SWIR from 400 to 1700nm. Two different types of spectrometer devices have been developed: (i) a ""microSpectrometer"" based on the miniaturization and monolithic integration of diffractive dispersive elements and a VIS/NIR + SWIR photodetector in a small volume (cubic inch) device, and (ii) a ""nanoSpectrometer"", in which the spectrometer function is realized on a photonic integrated circuit (PIC) based on transparent SiO/SiN waveguide technology. For the latter multi-mode to single mode fiber coupling structures have been realized in order to couple the output of a photonic tissue sensing probe to the spectrometer PIC. Within InSPECT we realized new key photonics building blocks for low-cost miniature spectrometers, that will drive the widespread adoption of spectral sensing in applications that were not accessible before.

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Within the different work packages of the InSPECT project the following progress has been achieved:

- Broad-band all-solid state based light source:
For our solid state light source new luminescent materials have been synthesized, based on the (Lu,Gd)3-(Ga,Al)5-O12:Ce3+,Cr3+ system and GeO2:Bi glasses, showing light emission upon excitation with blue light. A spectral gap identified at around 900-1000 nm has been covered by synthesizing an additional luminescent material based on Yb as active emitter. In the last year of the project, a compact light source prototype (few cm3) has been manufactured and tested using a high power blue laser diode as excitation source. The light source has been integrated into a photonic needle, capable of doing spectral tissue characterisation experiments. At this moment in time the light source shows clear light emission over the full spectral range from 450 to 1700 nm. Whereas the power output of the source is more than enough (few mW) in the visible and near-infrared range of the spectrum, further device optimisation is needed in order to increase the power output in the SWIR range.

- microSpectrometer:
For the miniature spectrometer based on free-space optics, a modular approach called SpectroBlocks has been followed, incorporating multiple gratings covering several subbands in the VIS-NIR-SWIR spectral range, in combination with a commercial two-dimensional VIS-NIR sensor and and a newly developed miniature 1D SWIR array. During the first two years, functional prototypes of the microSpecrometer have been realized and succesfully tested, paving the way for successfull integration and validation during the last year of the project. Furthermore, a 2nd concept microSpectrometer has been built using a state-of-the-art commercial SWIR sensor and classical spectrometer layout; though having a larger footprint as specified in the system specifications document, also this device has been succesfully functionally tested over the full spectral range. Both miniature microSpectrometer prototypes show excellent technical specifications like bandwidth, resolution and signal-to-noise, and their overall performance is on par with (and even slightly better than) state-of-the-art commercial spectrometers, whereas the form factor has been decreased with more than a factor of 50! During the last phase of the project both microSpectrometer instruments have been connected to a photonic needle device and their performance has been succesfully validated using diffuse reflectance spectroscopy measurements on ex-vivo, non-human tissue.

- nanoSpectrometer:
Creating a broad-band spectrometer on a photonic integrated circuit (PIC) imposes many challenges. The PIC wavelength selective components need to cover the full spectral range from 400-1700 nm, and the PIC's/detectors need to be packaged and connected to the outside world. A new waveguide architecture has been designed and fabricated based on a double-stripe geometry, with higher optical mode contrast. The latter is important for realising shorter waveguide bending radii and allowing for smaller size PIC footprints. As wavelength selective components arrayed waveguide gratings (AWGs) have been manufactured and successfully tested covering the VIS, NIR and SWIR spectral range, finally leading to a fully integrated PIC spectrometer solution covering the whole 400-1700 nm spectral range of interest. First multimode to single mode conversion devices have been made and successfully tested, connecting the single mode PIC's to the multimode outside world. During the last year of the project a fiber-PIC-detector assembly and packaging process has been developed where all nanoSpectrometer building blocks come together and resulted in a fully functional broad-band spectrometer device onto a photonic IC. The device has been tested and validated with respect to spectral resolution, throughput and SNR behavior, and to our knowledge this is the first integrated solution for a broadband spectrometer, covering the visible, near- and shortwave infrared embedded on a photonic IC.
The technology being developed in InSPECT will ultimately have several societal and economical impacts. On the short term new broadband spectrometer modules like the microSpectrometer and solid state based light sources will become available that can be used e.g. in a healthcare setting for diagnostic purposes. Their compact size and cost effectiveness will allow for a more rapid introduction of new healthcare applications not only in the hospital, but also at the point-of-care setting. On the mid to long term, the realization of the nanoSpectrometer device allows for an ultimate size reduction, paving the way for spectral diagnostics in e.g. portable or in-body implantable devices where size reduction is crucial. Furthermore we envision the use of compact broadband solid state light sources in spectral sensing and imaging solutions for personal and professional health monitoring and minimally invasive healthcare applications.
Up to now, no compact solutions are commercially available capable of recording spectra in the VIS/NIR and SWIR spectral range. InSPECT technology allows to extend the spectral range of these sensing devices also beyond the 1 micrometer silicon cutoff wavelength, where additional important spectral fingerprints can be found and thereby increase the application spectrum.