Optical Materials and Processes

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Silk fibroin enabled optical fiber methanol vapor sensor

An optical fiber long period grating coated with silk fibroin is proposed and demonstrated for monitoring methanol vapors. Results show an exponential increase in the wavelength shift of the grating’s attenuation band versus methanol vapor pressure, while reaching a value of 4 nm for 100 mbar of methanol. Results confirm the repeatability of the response of the sensor and show that after multiple exposures to methanol vapors the overlayer retains its functionality 12 days after drop casting.
(left) Long period grating attenuation band negative wavelength shift and (right) maximum wavelength shift for repeated exposures of the same long period grating sensor to 100 mbar of methanol vapor up to 12 days after drop casting of the fibroin overlayer.

References

  • In collaboration with Prof. S. Iannotta’s group CNR-IMEM and Prof. S. Selleri’s group University of Parma
  • M. Konstantaki, D. Skiani, D. Vurro, A. Cucinotta, S. Selleri, A. Secchi, S. Iannotta and S. Pissadakis, Silk fibroin enabled optical fiber methanol vapor sensor, IEEE Photonics Technol. Lett. 32, pp. 514-517 (2020), (Link)

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Optical birefringence in strain tuneable silk fibroin whispering gallery mode cavities

Whispering gallery mode (WGM) silk fibroin resonators, attached on silica glass microfibers, where developed using optical fiber tapers in evanescent tunnelling scheme for modal excitation. The advantage of the current approach lays on the fact that the same cavity is measured in the Silk I and II structure, after the transformation takes place under a controlled thermal annealing process. Silk fibroin, optical micro-resonators of ~32μm diameter were casted onto micrometric diameter silica glass optical fiber tapers (~30 μm diameter), while employing a multi-step preparation process. Silk I and II fibroin WGM micro-cavities were spectrally characterized for TE and TM polarizations under the application of controlled axial strain, while measuring the induced spectral shift of the WGM resonances in transmission mode. Using this approach, optical birefringence of the order of ~2.0x10-4 was measured in Silk I WGM cavities under 3800με axial strain, while this birefringence figure was approximately doubled for the case of Silk II material for the same strain value applied. Our studies depict the paramount importance of the β-sheets segmentation properties in the optical birefringence and mechanical properties of silk fibroin in both states.
(left) Experimental setup for spectrally characterizing and strain exciting the silk fibroin cavities. (right) Transmission spectra of the TM Modes (a) and TE Modes (b) under applied axial strain along the longitudinal axis of the supporting optical fiber for a Silk I structure cavity.

References

  • In collaboration with Prof. S. Iannotta’s group and Prof. S. Selleri’s group University of Parma, Italy
  • N. Korakas, D. Vurro, O. Tsilipakos, A. Cucinotta, S. Selleri, S. Iannotta, S. Pissadakis “ Optical birefringence in strain tuneable silk fibroin whispering gallery mode cavities” CLEO-Europe 2021 CLEO, CE-6.3 (2021)

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Bioresorbable optical fiber Bragg gratings

We demonstrate, for the first time, an inscription and wet dissolution study of Bragg gratings in a bioresorbable calcium-phosphate glass optical fiber. Bragg gratings, with average refractive index changes of 5.8×10−4, were inscribed using 193 nm excimer laser radiation. Results on the dissolution of the irradiated fiber in simulated physiological conditions are presented after immersing a tilted Bragg grating in a phosphate buffered saline solution for 56 h; selective chemical etching effects are also reported. The investigations performed pave the way toward the use of such phosphate glass fiber Bragg gratings for the development of soluble photonic sensing probes for the efficient in vivo monitoring of vital mechanical or chemical parameters.
(Left) Reflected signal evolution of a 1° tilted FBG immersed in PBS solution for up to 56 h. The baseline of the different spectra has been adjusted vertically to assist visualization. (Right) End face (a) and side (b) views of the cladding region of an irradiated optical fiber (dose of 5.2 J/cm2), immersed in PBS. A side section of the fiber was intentionally removed to reveal inner regions. (c) End face view of the FBG region after 5 h in PBS and (d) FDTD simulation (Optiwave-OptiFDTD) of intensity spatial distribution (input beam intensity 3.6 MW/cm2) of a side irradiated optical fiber. The blue arrow in (d) indicates the direction of laser beam illumination.

References

  • In collaboration with Prof. D. Milanese’s group, Politecnico di Torino, Italy
  • D. Pugliese, M. Konstantaki, I. Konidakis, E. Ceci-Ginistrelli, N. G. Boetti, D. Milanese, S. Pissadakis. “Bioresorbable optical fiber Bragg gratings,” Opt. Lett., Volume 43, Issue 4, 671-674 (2018). DOI: 10.1364/OL.43.000671 l(Link to Journal)

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Whispering gallery mode resonances in thermally poled borosilicate glass optical microcavities

The optical characteristics and correspondingly whispering gallery mode (WGM) resonances of cylindrical shape, borosilicate glass cavities are tuned by thermal poling. The idea is to generate an ion depleted zone inside the glass, however, being in the vicinity of the circumference of cylindrical cavity. The light localization through WGM resonation, provides direct access to the ion depleted zone, allowing the investigation of the ion-displacement introduced through the thermal poling into the particular behavior of the WGM resonances. Afterwards the depleted zone is investigated for the first time, while employing whispering gallery mode resonation of light at the wavelength band of 1.5 μm, circulated inside the thin poled region. The cavity used in these experiments is drawn from Duran® Schott AG glass (borosilicate glass), facilitating in the middle a co-centric aluminum wire, thus, constituting a type of metal core, glass optical fiber. The study reveals that the unpoled sample supports a great number of azimuthal WGM modes, together with the higher order, interstitial radial components; Q-factors measured for this cavity are ~2x104 for both polarisations.

(left) Optical microscopy cross-section picture of the 120 μm diameter, Duran glass microcavity. Transmission spectra of WGMs hosted in the Duran glass cavity, for TE (black) and TM ( red lines) polarizations, (middle) for unpoled and (right) the same cavity after being thermally poled.

References

  • In collaboration with Prof. M. Zervas’ and Prof. D. Hewak’ groups, University of Southampton, Southampton, UK and Dr G. Filippidis’ group IESL, FORTH, Greece
  • N. Korakas, V. Tsafas, G. Filippidis, B. Moog, C. Craig, D.W. Hewak, M. N. Zervas, S. Pissadakis “Whispering gallery mode resonances in thermally poled borosilicate glass optical microcavities” CLEO-Europe 2021 CLEO, CE-6.5 (2021)

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Azimuthal alignment method for optimizing Bragg grating inscription in photonic crystal fibers

An accurate (<5o) and straightforward implementation optical method for determining the azimuthal lattice orientation of a microstructured optical fiber with respect to the side illumination beam used for Bragg grating recording is presented. This method is based on the diffraction pattern generated backward of the photonic crystal fiber cladding upon a continuous-wave visible laser light illumination. This optical fiber alignment and coordination method is experimentally demonstrated, describing in detail characteristic light scattering patterns used for the azimuthal orientation of the fiber with respect to the side-coming laser beam. Moreover, this alignment method is theoretically validated using standard ray tracing beam scattering software. Bragg grating recordings performed in this photonic crystal fiber (PCF) using 193-nm excimer laser radiation are also presented to depict the impact of the azimuthal alignment of the fiber with the inscription beam on the photosensitivity refractive index changes induced in the optical fiber.
The distinct diffraction pattern that defines the optical fiber orientation. For every other fiber orientation a more continuous diffraction pattern is observed. The inserts depict the corresponding orientation of the fiber lattice with respect to the side illuminating laser beam.

References

  • N. Korakas, G. Violakis, S. Pissadakis, Azimuthal alignment method for optimizing Bragg grating inscription in photonic crystal fibers, IEEE Photonics Technol. Lett. 31, pp. 857 – 860 (2019) , (Link)

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Light driven optofluidic switch developed in a ZnO-overlaid microstructured optical fiber

A great challenge of Optofluidics remains the control of the fluidic properties of a photonic circuit by solely utilizing light. In this study, the development of a ZnO nanolayered microstructured optical fiber (MOF) Fabry-Perot interferometer is demonstrated, along with its fully reversible optofluidic switching behaviour. The actuation and switching principle is entirely based on the employment of light sources, i.e. UV 248 nm and green 532 nm lasers, while using modest irradiation doses. The synthesized ZnO within the MOF capillaries acts as a light triggered wettability transducer, allowing the controlled water filling and draining of the MOF Fabry-Perot cavity. The progression of the optofluidic cycle is monitored in situ with optical microscopy, while Fabry-Perot reflection spectra are monitored in real time to probe temporal infiltration behaviour. Finally, a first insight on the light triggered switching mechanism, employing photoluminescence and spectrophotometric measurements is presented. Results appear highly promising towards the design of smart in-fiber optofluidic light switching devices, suitable for actuating and sensing applications.
Configuration of the SMF-28/(ZnO nanolayer HNA-5) Fabry-Perot interferometer. (b) Fabry-Perot reflection spectra over 10 nm of pristine and ZnO overlaid HNA-5 cavity with length of 780 μm. (c) SEM scan of the cleaved end face of the HNA-5 MOF, along with details of the ZnO nanolayer formed on the surface of the capillaries.

References

  • I. Konidakis, M. Konstantaki, G.D. Tsibidis, S. Pissadakis, Light driven optofluidic switch developed in a ZnO-overlaid microstructured optical fiber, Opt. Express 23, 31496, (2015) (Link)

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