Abstract
Thin and flexible sensor foils are very suitable for unobtrusive integration with mechanical structures and allow monitoring for example strain and temperature while minimally interfering with the operation of those structures. Electrical strain gages have long been used for this purpose, but optical strain sensors based on Bragg gratings are gaining importance because of their improved accuracy, insusceptibility to electromagnetic interference, and multiplexing capability, thereby drastically reducing the amount of interconnection cables required. This paper reports on thin polymer sensor foils that can be used as photonic strain gage or temperature sensors, using several Bragg grating sensors multiplexed in a single polymer waveguide. Compared to commercially available optical fibers with Bragg grating sensors, our planar approach allows fabricating multiple, closely spaced sensors in well-defined directions in the same plane realizing photonic strain gage rosettes. While most of the reported Bragg grating sensors operate around a wavelength of 1550 nm, the sensors in the current paper operate around a wavelength of 850 nm, where the material losses are the lowest. This was accomplished by imprinting gratings with pitches 280 nm, 285 nm, and 290 nm at the core-cladding interface of an imprinted single mode waveguide with cross-sectional dimensions 3 × 3 µm2. We show that it is possible to realize high-quality imprinted single mode waveguides, with gratings, having only a very thin residual layer which is important to limit bend losses or cross-talk with neighboring waveguides. The strain and temperature sensitivity of the Bragg grating sensors was found to be 0.85 pm/µε and −150 pm/°C, respectively. These values correspond well with those of previously reported sensors based on the same materials but operating around 1550 nm, taking into account that sensitivity scales with the wavelength.
Highlights
Printing technologies have long been used in electronics owing to their fabrication flexibility, low cost, and short time-to-market, and for photonic applications printing technologies start to find widespread use owing to superior resolution and fabrication quality of emerging printing technologies
We have previously shown that it is possible to realize such Bragg grating sensors in thin flexible foils, making it possible to implement multiple closely spaced sensors in well-defined directions [19], as such enabling realizing an optical variant of electrical strain gage rosettes
The stamp is made from a transparent, soft polymer material so that the Ormocer® can be UV cured while the stamp is in contact thereby permanently fixing the patterns in the material
Summary
Printing technologies have long been used in electronics owing to their fabrication flexibility, low cost, and short time-to-market, and for photonic applications printing technologies start to find widespread use owing to superior resolution and fabrication quality of emerging printing technologies. Printed electrical connection technologies are becoming increasingly precise so that they can be used for alignment-critical (electro-)optical assemblies. Interconnections between electro-optical transmitter/receiver chips and drivers are traditionally provided by wirebonds, but this technology is reaching its limits for very high-speed applications and the wire loops prevent optical fibers from probing features nearby the wirebonds. Aerosol jet printing has recently been proposed as a promising alternative to replace wirebonds for these high-speed applications [5]. Optical sensors, such as ring resonators [6] or liquid crystalline film-based sensors [7] have been inkjet printed
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