Miniaturized optical spectrometer system 

Rainbows are well known phenomena after rain when air humidity is high. They illustrate that normal sunlight consists of different colors. To be more precise, these colors can be called “wavelengths”. When white light shines a surface, we can see the surface of a certain color. This means that the surface absorbed all the colors forming the white color and reflected only the color we see. In other words, the surface absorbed all the incident wavelengths and reflected only the certain wavelengths we see. It has been found that the absorbed and reflected wavelengths depend on the material from which that surface is composed. In addition, every known element has a fingerprint of the wavelengths it absorbs and reflects. Hence, over years, different instruments were belt to measure the absorbed and reflected wavelengths from matter. These instruments are called “Spectrometers”.

Spectrometers are used in various fields e.g. chemistry, biology, medicine, food, agriculture, forensics, production lines, …etc. Driven by these applications, the need of miniaturized spectrometers arose. This requires using suitable optical setups to the limited size. Furthermore, the spectral range of operation should be extended to cover more applications. Hence, covering both visible and infrared regions mainly the wavelengths between 400-1700 nm is important. However, within this range, the response of traditional optical detectors is limited between 400-1100 nm for silicon-based detectors and 900-1700 nm for InGaAs detectors. In addition, some biomolecular and biochemistry applications require very fast detection in time in orders of nanoseconds, but the response of current optical sensors is much less than this range.
In this project, miniaturized spectrometers are built following Fastie-Ebert configuration. A Fastie-Ebert spectrometer consists of an entrance slit, a spherical mirror, a reflection grating and an image sensor. The light enters the system through the entrance slit towards the spherical mirror then gets reflected to the reflection grating which disperses the incident beam to the wavelengths it consists of. The reflected wavelengths fall on again on the spherical mirror which reflects them and focuses them on the image sensor. Hence, different wavelengths fall on different pixels of the image sensor and by reading it out, a spectrum can be created. Fastie-Ebert configuration enables building the optical setup in small sizes and this solves the miniaturization problem. In addition, using only reflective elements in the setup eliminates the effect of chromatic aberrations and hence improves the optical quality.
Using high-speed time-gated sensors enables measuring nanosecond time resolution especially fluorescence emission and emission lifetime. Hence, current-assisted photonic sampler imagers are used with the optical setup. This combination is used to build time-resolved fluorescence spectrometers that can be used to measure the fluorescence lifetime of the same fluorophore at different wavelengths. In addition, more compact spectrometer systems can be build following this concept leading to manufacturing portable and handheld devices. On the other hand, to improve the spectral response of the spectrometer, collaborations are being made with leaders in novel sensor fabrication technologies e.g. graphene-CMOS image sensors. This new sensor family has a promising spectral response between 400-1800 nm.
This is a joint project is done at ETRO and Anteryon B.V. in Eindhoven, Netherlands within the project xCLASS (2017-2021). xCLASS has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 765635. In addition, it is funded in part by the Research Council of the Vrije Universiteit Brussel (SRP 19).