Due to their flexibility and robustness, polymer optical fibers represent a promising platform for the development of brain-compatible implantable devices with reduced risk of tissue inflammation. Furthermore, by combining different biocompatible materials it is possible to integrate multiple functionalities in a single hybrid optical fiber. This approach allows the fabrication of soft brain interfaces able to support multiple modalities of neural interrogation. Such interfaces capable of simultaneous light delivery and recording of neuronal activity with minimal tissue damage are currently lacking for infrared wavelengths in the strong water absorption region. This spectral region, in particular, is crucial for infrared neuromodulation, a promising technique for direct light-induced control of neural activity without genetic manipulation. Here we present novel infrared fiber-based neural interfaces developed by thermal drawing of soft, biocompatible optical polymers, which are able to simultaneously modulate and record neural activity, as validated experimentally in vivo.
UV supercontinuum based on gas filled anti resonant hollow core fibers is demonstrated to have passed a major milestone by providing spectral properties comparable to those of plasma arc lamps, namely a broad, flat, low noise, and stable spectrum. The primary advancement is the use of pump modulation which flattens the spectrum by more than 20dB. As proof of concept, results from scatterometry measurements, with both UV supercontinuum and plasma arc lamps are shown to produce comparable results. However, UV supercontinuum can meet additional requirements making it suitable for many cutting-edge UV metrology applications such as imaging and spectroscopy.
Infrared neurostimulation has emerged in recent years as a promising technique for controlling neuronal activity without genetic manipulation. Having high absorption of the employed wavelengths as its fundamental mechanism, it requires implantable platforms to deliver light in brain regions deeper than the first cortical layers. Due to the spatial confinement of the stimulation, electrodes integrated in close proximity to the illumination spot are desirable to verify the effects of the stimulation by extracellular electrophysiology. Here we developed and validated in vivo a multifunctional neural interface based on a soft, biocompatible polymer optical fiber that allows simultaneous infrared neurostimulation and electrophysiology.
Implantable optical fibers have been widely used for optical neuromodulation in deep brain regions. Polymer fiber-based neural devices have natural advantages over silica fibers since their high flexibility would lead to a less inflammatory response in chronic in vivo experiments. Using three kinds of polymer materials: polycarbonate (PC), polysulfone (PSU), and fluorinated ethylene propylene (FEP), we present multifunctional soft polymer fiber (POF)-based brain implants with an Ultra-High Numerical Aperture (UHNA) and integrated Microfluidic Channels (MCs) for wide illumination and drug delivery, respectively. The flexibility of the proposed fiber devices has been found to be 100-fold reduced compared to their commercially available counterparts. Biofluids delivery can be controllably achieved over a wide range of injection rates spanning from 10 nL/min to 1000 nL/min by the structured MCs in the fiber cladding. The illumination area of the UHNA POFs in brain phantom has been increased significantly compared with the commercially available silica fibers. A fluorescent light recording experiment has been conducted to demonstrate the proposed UHNA POFs can be used as optical waveguides in fiber photometry. The limited illumination angle of the optical fiber imposed by current technology has been enlarged by the proposed UHNA POFs and we anticipate our work to pave the way toward more efficient multifunctional neural probes for neuroscience.
Unraveling the scientific and technological importance of the mid-infrared (mid-IR) region remains yet a long-standing challenge. Despite the significant efforts on mid-IR light sources, development of high-energy, narrow-linewidth and compact lasers still constitutes the main obstacle towards novel spectroscopic, imaging and sensing devices. Photoacoustic modality is known as one of the most powerful tools enabling high signal-to-noise ratio gas detection and albeit its wide use in the mature near-infrared (near-IR) region, further research has to be carried out in the mid-IR in order to “unlock” its full potential. In this work, we aim on tracing CO2 based on the innovative combination of the emerging gas-filled mid-IR silica anti-resonant hollow-core fiber (ARHCF) Raman laser technology with the powerful photoacoustic modality. The laser source adopts the stimulated Raman scattering effect of H2 filled in a piece of ARHCF, to enable the generation of first-order vibrational Raman Stokes from a 1533 nm Er-doped fiber laser pump. With this configuration, a nanosecond laser pulses with micro-joule level pulse energy is achieved at ~ 4.25 μm wavelength, which is located within the strongest absorption band of CO2. The laser’s linewidth is estimated to be tens GHz level. This laser source is used to drive an in-house developed photoacoustic sensor, revealing a 1.78 ppm level CO2 detection limit in laboratory condition. This work provides a valuable reference for the development of high-sensitivity gas detectors.
Gas-filled anti-resonant hollow-core fiber (ARHCF) constitutes an efficient route towards development of high-energy fiber lasers in the near-infrared (NIR) and mid-infrared (MIR) spectral region. We will present our recent work on developing both vibrational and rotational Raman-active gas-filled fiber lasers spanning from the NIR up to around 4.3 μm wavelength. We will also show how such fiber lasers can be used for high-resolution photoacoustic gas sensing and imaging.
Here, we present a high pulse energy Raman laser at 1946 nm wavelength pumped with a 1533 nm linearly polarized fiber laser, with ∼92 μJ pulse energy, ∼60 pm linewidth, 8 kHz repetition rate, and 7 ns pulse duration. The Raman laser is based on the stimulated Raman scattering (SRS) effect in an 8-meter carbon dioxide (CO2) filled nested anti-resonant hollow-core fiber (ARHCF). The nested structure contributes to the significant reduction of the fiber loss caused by light leakage, surface scattering and bend, therefore allowing coiling the gas-filled ARHCF with a relatively small bend radius of just ~5 cm. When the pressure in the CO2-filled ARHCF increases from 1 to 17 bar, the pulse energy first reaches the maximum pulse energy level of 16.3 μJ (corresponding to 28 % quantum efficiency) at only 1.2 bar, and then rapidly decreases due to the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at 2 μm spectral range. The relative intensity noise (RIN) of the Raman laser reaches a minimum level (4%) when the pulse energy exceeds ∼8 µJ. Due to the low amount of heat release during the SRS process, the laser has a good long-term stability without significant drift. Our results constitute a novel and promising technology towards high-energy 2 μm lasers.
We will present our recent work using noble and Raman-active gas-filled anti-resonant hollow-core fiber (ARHCF) technology. First, we will present the generation of supercontinuum spanning from 200 nm to 4000 nm based on nonlinear effects of soliton self-compression and phase-matched deep‑ultraviolet (DUV) dispersive wave (DW) emission in Argon (Ar)-filled ARHCFs pumped at 2.46 μm wavelength with 100 fs pulses and ~8μJ pulse energy. Then we will discuss our recent work on stimulated Raman scattering (SRS) effect in a hydrogen (H2)-filled ARHCF, to achieve near- and MIR Raman lasers. By employing the single-stage vibrational SRS effect, a 4.22 μm Raman laser line is directly converted from a linearly polarized 1.53 μm pump laser. A quantum efficiency as high as 74% was achieved, to yield 17.6 µJ pulse energy. The designed 4.22 μm wavelength is well overlapped with the strongest CO2 absorption, therefore constituting a promising way for CO2 detection. In addition, we report a multi-wavelength Raman laser based on the cascaded rotational SRS effect. Four Raman lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm are generated, with pulse energies as high as 18.25 µJ, 14.4 µJ, 14.1 µJ, and 8.2 µJ, respectively. The energy of these Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar.
Silica anti-resonant hollow-core fiber (ARHCF) is a promising platform for optofluidic applications because it can act as fluid-cell, permitting intense fluid-light interaction over extended length with low optical loss from ultra-violet to midinfrared region. For this kind of applications, an all-fiberized and compact structure is necessary. However, a prerequisite for this purpose is that micro-channels must be created on the side of the fiber, to provide access for the diffusion of fluids (i.e. liquid or gas) into the core. Several attempts based on femtosecond laser micro-machining technology have been made to create micro-channels through the silica cladding, but significant loss could be induced due to the damage of the cladding capillaries of ARHCF. Here, we report a high-precision and repeatable micro-machining technique using focused ion beam (FIB) milling on a nodeless ARHCF. Ga+ ion beam is employed to bombard a 43 μm thick outer cladding of ARHCF for 30 minutes, to create a 50 μm deep fluidic channel. The micro-channel in the silica cladding is precisely drilled at the middle position of two adjacent capillaries with a 2.8 μm gap, providing direct access for liquid/gas to diffuse into the hollow-core region, while avoiding the damage of the capillaries. Corroborating results from simulation of such a structure are presented to demonstrate that no additional loss is induced by the milled structure.
In this presentation, we will introduce some of the promising applications where mid-infrared supercontinuum lasers covering from 2-10 μm can have an advantage over competing technologies, such as multi-species gas spectroscopy, fiber evanescent wave spectroscopy, waveguide characterization, and photo-acoustic imaging. One novel application using mid-IR SC is non-destructive testing using optical coherence tomography. By combining broadband supercontinuum and upconversion detection, it is possible to penetrate deeper into materials while maintaining high resolution and real-time imaging capabilities. To demonstrate this, we show results with sub-surface inspection of marine and automotive paints and coatings for evaluation of thickness, uniformity, and defects.
Supercontinuum (SC) lasers are of high interest for applications like multispectral photoacoustic imaging (MSPAI), where the wide optical bandwidth of the SC laser system facilitates functional investigations on top of the structural information of various endogenous agents inside the human body. The current work addresses a promising attempt at devising high pulse energy SC laser source using telecom-range diode laser systems and few meters of standard single-mode fibers for various MSPAI applications in near-infrared (NIR) and extended-NIR wavelength regions.
We demonstrate a robust photoacoustic medium for measuring the concentration of ammonia in an aqueous solution. We target the near-infrared (NIR) overtone absorption band (~1540 nm) of ammonia with a supercontinuum (SC) laser-based excitation system and an immersion-based acoustic transducer as the detection system. We further present how such a simple system can be used to perform effective in-situ measurements of ammonia over a range of concentrations with a sensitivity of parts per million (ppm) by volume and linearity of <96%. We demonstrate how the sensing system can be readily tailored to monitor the concentrations of other miscible gases in the aqueous solution.
Detection of ammonia based on an all-fiber configuration is reported. The system consists of a hollowcore photonic-bandgap (HC-PBG) fiber with 20μm core diameter and transmission window from 1490 to 1680 nm. Absorption bands of ammonia at ~1538 nm are targeted using a supercontinuum source with central wavelength at 1550 nm. We present the method of achieving a complete fiber system while addressing the gas entry/exit path through the HC-PBG. Analysis of the ammonia absorbance in the fiber with respect to fiber length and response time is investigated. By operating in the near infrared, we demonstrate how the proposed system addresses several challenges associated with fiber-based gas-sensing, using readily available commercial components.
In this work, we report a cost-effective supercontinuum (SC) laser-based multispectral photoacoustic spectroscopy (MSPAS) system for studying spectral characteristics of various analytes. We demonstrate an in vitro label-free monitoring of the analytes in the extended near-infrared (NIR) (1540–1840 nm) spectral range. We further demonstrate how a simple ratiometric analysis in conjunction with linear regression can be used for accurate prediction of glucose over commonly encountered physiological levels inside the human body (0–400 mg/dL). Looking forward, the proposed SC-based MSPAS system provides a framework for the development of label-free and non-invasive monitoring multiple bio-analytes accurately, with potential translation to clinical in vivo applications.
One of the main advantages of photonic crystal fibers (PCFs) is their ability to host novel functional materials in the airholes of the cladding. Here, we demonstrate a unique post-processing method which allows the integration of materials with significantly different thermo-mechanical properties inside the voids of silica PCF. We first present the material properties of silica, As2Se3 and polydimethylsiloxane (PDMS) in terms of their refractive indices and viscosity profile. The latter suggests that the proposed materials are not suitable for direct fiber drawing and thus we present the development of a multi-material As2Se3/PDMS/Silica PCF based on a solution-processed and pressure-assisting method. The integration of both As2Se3 chalcogenide glass films and PDMS was made in ambient conditions using a costeffective approach. The deposition of the high-index chalcogenide glass films revealed distinct resonances in the visible and near-infrared region while the high thermo-optic coefficient of PDMS provides the ability to thermally control the intensity of the antiresonant bands. The proposed method opens new directions towards multimaterial silica-based PCFs for novel tunable devices and sensors.
A microstructured polymer optical fiber (mPOF) Bragg grating sensor system for the simultaneous measurement of temperature and relative humidity (RH) has been developed and characterized. The sensing head is based on two in-line fiber Bragg gratings recorded in a mPOF. The sensor system has a root mean square deviation of 1.04 % RH and 0.8 °C in the range 10 to 90% RH and 20 to 80 °C. The proposed sensor system is easy to fabricate, cheap and compact.
We describe the fabrication of a polycarbonate (PC) micro-structured polymer optical fiber (mPOF) and the writing of fiber Bragg gratings (FBGs) in it to enable strain and temperature measurements. We demonstrate the photosensitivity of a dopant-free PC fiber by grating inscription using a UV laser. We further show that PC Bragg gratings can be extended up to at least 3% without affecting the initial functionality of the micro-structured fiber. The response of PC FBGs to temperature up to 125°C is also investigated. Polycarbonate has good mechanical properties and its high temperature resistance might extend the range of application of polymeric FBGs.
We have fabricated and characterised a humidity insensitive step index(SI) polymer optical fibre(POF) Bragg grating sensors. The fibre was made based on the injection molding technique, which is an efficient method for fast, flexible and cost effective preparation of the fibre preform. The fabricated SIPOF has a core made from TOPAS with a glass transition temperature of 134°C and a cladding from ZEONEX with a glass transition temperature of 138°C. The main advantages of the proposed SIPOF are the low water absorption and good chemical resistance compared to the conventional poly-methyl-methacrylate (PMMA) based SIPOFs. The fibre has a minimum loss of ~6dB/m at 770nm.
We report the fabrication and characterization of a hybrid silica photonic crystal fiber (PCF) with integrated chalcogenide glass layers and we show how the bandgaps of the fiber can be thermally tuned. The formation of the high index chalcogenide films on the inner surface of the PCF holes revealed resonances as strong as ~35 dB both in the visible and infrared regime. Temperature measurements indicate that the transmission windows can be tuned with a sensitivity as high as ~3.5 nm/°C. The proposed fiber has potential for all-fiber filtering and temperature sensing.
Lately the demand for in situ and real time monitoring of industrial assets and processes has been dramatically increased. Although numerous sensing techniques have been proposed, only a small fraction can operate efficiently under harsh industrial environments. In this work the operational properties of a proposed photonic based chemical sensing scheme, capable to monitor the ageing process and the quality characteristics of coolants and lubricants in industrial heavy machinery for metal finishing processes is presented. The full spectroscopic characterization of different coolant liquids revealed that the ageing process is connected closely to the acidity/ pH value of coolants, despite the fact that the ageing process is quite complicated, affected by a number of environmental parameters such as the temperature, humidity and development of hazardous biological content as for example fungi. Efficient and low cost optical fiber sensors based on pH sensitive thin overlayers, are proposed and employed for the ageing monitoring. Active sol-gel based materials produced with various pH indicators like cresol red, bromophenol blue and chorophenol red in tetraethylorthosilicate (TEOS), were used for the production of those thin film sensitive layers deposited on polymer's and silica's large core and highly multimoded optical fibers. The optical characteristics, sensing performance and environmental robustness of those optical sensors are presented, extracting useful conclusions towards their use in industrial applications.
Fiber Bragg grating (FBG) writing in PMMA microstructured Polymer Optical Fibers (mPOFs) using the UV Phase Mask technique is a time consuming process requiring about 40 minutes to inscribe a grating in an undoped fiber. Here we demonstrate the FBG inscription with the writing times shorter than 10 min. By careful alligning and increasing the beam intensity in the core of the fiber, writing times as short as 6 minutes and 50 second were achieved. The FBGs were written in a 125 μm PMMA mPOF having 3-rings of holes, the reflection peaks were centred at 632.6 nm and have a reflectivity as high as 26 dB. We also demonstrate how the writing dynamics depends on the intensity of the writing beam.
The potential of a new class of multifunctional photonic circuits is demonstrated by femtosecond laser micro-machining and inscription of micro-optical structures, such as ring and disk resonators, Mach-Zehnder interferometers, and microfluidic devices, in a flexible flat-fibre chip. Additional filling of surface channels with functional materials is employed towards sensing applications. Furthermore, direct write femtosecond-laser inscribed Bragg gratings were written in the Ge-doped core of flat-fibre enabling a number of relevant applications. The flat-fibre platform offers a unique degree of freedom by allowing surface and sub-surface devices to be integrated onto an a single optical chip with the potential for straightforward incorporation into integrated photonic circuits or optofluidic devices.
A low cost and low complexity optical detection method of proteins is presented by employing a detection scheme based on electrostatic interactions, and implemented by sensitization of a polymer optical fibers' (POF) surface by thin overlayers of properly designed sensitive copolymer materials with predesigned charges. This method enables the fast detection of proteins having opposite charge to the overlayer, and also the effective discrimination of differently charged proteins like lysozyme (LYS) and bovine serum albumin (BSA). As sensitive materials the block and the random copolymers of the same monomers were employed, namely the block copolymer poly(styrene-b-2vinylpyridine) (PS-b- P2VP) and the corresponding random copolymer poly(styrene-r-2vinylpyridine) (PS-r-P2VP), of similar composition and molecular weights. Results show systematically different response between the block and the random copolymers, although of the same order of magnitude, drawing thus important conclusions on their applications' techno-economic aspects given that they have significantly different associated manufacturing method and costs. The use of the POF platform, in combination with those adaptable copolymer sensing materials could lead to efficient low cost bio-detection schemes.
We demonstrate femtosecond laser micro-machining and inscription of micro-optical structures, such as ring and disk resonators, Mach-Zender interferometers, and complex microfluidic devices, in a novel optical flat-fibre chip. In addition, we fill the channels with functional materials that can be used for sensing applications. Furthermore, direct write femtosecond-laser inscribed Bragg gratings are written in the Ge-doped flat-fibre core at 1547.8 nm. The flat-fibre chip offers a unique degree of freedom by allowing surface and sub-surface devices to be integrated onto an optical platform with the potential for straightforward incorporation into integrated photonic circuits or optofluidic devices.
In this paper we present our latest work on Fiber Bragg Gratings (FBGs) in microstructured polymer optical fibers
(mPOFs) and their application as strain sensing transducers in devices, such as accelerometers and microphones. We
demonstrate how the cross-sensitivity of the FBG to temperature is eliminated by using dual-FBG technology and how
mPOFs fabricated from different grades of TOPAS with glass transition temperatures around 135°C potentially allow
high-temperature humidity insensitive operation. The results bring the mPOF FBG closer to being a viable technology
for commercial applications requiring high sensitivity due to the low Young's Modulus of polymer.
We have numerically investigated the guiding properties in a hybrid polymer/silica photonic crystal fiber (PCF). We
considered poly-dimethylsiloxane (PDMS) as the infused polymer into the air-holes of PCF and we present how the
modal properties of the fiber are affected due to PDMS inclusions. We numerically calculated the guiding and thermal
properties of the hybrid structure in terms of the effective index, single-mode operation, confinement loss, numerical
aperture (NA), effective modal area (EMA) and fraction of power into the polymer-filled cladding for different relative
hole sizes, d/Λ (0.35-0.75) of the hybrid PCF whereas direct comparison with a conventional air-filled PCF is also
shown. Further investigation of EMA, NA and fraction of power in the cladding with respect to thermal variations is also
reported for a range of temperatures from 0°C to 100°C.
In this work, we demonstrate the formation of Poly-dimethylsiloxane (PDMS) films inside the holes of conventional
silica photonic crystal fiber. The index guiding properties of the new PDMS-layer/Silica structures were investigated and
optimized numerically using FDTD analysis. Films with thicknesses ranging from 100nm to 1μm were formed using
different PDMS solution concentrations and different solution movement speeds. The thickness of films was very
uniform along almost all the deposition length as indicated by Scanning Electron Microscopy (SEM) images and micro-
Raman mapping.
In this work, we demonstrate numerically and experimentally the temperature dependence of a photonic crystal fiber
(PCF) infiltrated with PDMS elastomer. We investigated the guiding properties of the PDMS-filled PCF and we present
the variation of the effective index and effective modal areas of the fundamental guiding mode at 633 and 1550 nm, for a
range of temperatures from 20°C to 75°C. Experimental measurements have shown an up to ~6% power recovery of the
bend-induced loss for a 6-cm long PDMS-filled PCF at 4 cm bend diameter.
In this work, we demonstrate a highly birefringent (Hi-Bi) photonic crystal fiber (PCF) infiltrated with PDMS elastomer
in order to enhance the sensitivity of the fiber to external temperature variations. Index guiding mechanism of the new
PDMS/Silica structure and birefringent properties were investigated numerically and experimentally. We investigated
the temperature dependance of birefringence from 20-120°C. For the particular design of Hi-Bi PCF, the cut-off
operating wavelength of the hybrid fiber was found to be around 750 nm. We also experimentally demonstrate the effect
of the elastomer inclusions to the polarization of the fiber. The sensitivity of the PDMS/Silica Hi-Bi fiber was found to
be ~ 0.37 rad/K/cm for temperatures ranging from 20 to 80°C. The total length of the hybrid PCF examined was about
1.4 cm.
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