Miniature Fiber Research Articles

Realization of high flux daylighting via optical fibers using large Fresnel lens

Due to energy saving and flexible installation, the daylighting system via optical fibers for indoor illumination has attracted a lot of research attention. However, in applications, it is hard to adopt large aperture lens to improve sunlight collection due to the overheating problem caused by high sunlight concentration. The optical fibers generally adopted, Polymethylmethacrylate (PMMA) fiber, often suffers from overheating, and cannot work under a temperature exceeding 70 °C (Ashby, 2005). To solve this problem, this study tested three measures, infrared light filter (Sedki and Maaroufi, 2017), flux homogenization and fiber facet polishing, respectively to control thermal load caused by a large lens with a diameter of 1 m. The experimental data shows that the infrared light filter can reflect infrared light (740–1100 nm) efficiently, meanwhile keeping highly transparent for the visible spectrum (400–700 nm), which produces the cold incident light. A homogenizer was also designed to produce a uniform flux distribution by reflection in a mirror cavity. Analysis based on Monte Carlo rays-tracing method (MCRT) showed that the homogenizer could supply a flat flux distribution of 1500 suns by eliminating the flux peak. Microscopy experiments revealed that fine polish could effectively remove dust off the surface of the PMMA fiber, so to avoid heat spot to protect PMMA fiber. A long time daylighting test on Beijing (40°05′23″N, 116°17′45″E, 97 klux outdoor) demonstrated the feasibility of the large lens with an aperture of 1 m for daylighting via optical fibers, which offered an illumination level of 180 lx on an area of 36 m2. A comparison test on Lhasa, Tibet (29°39′52″N, 91°06′26″E) has also carried out under a higher outdoor illuminance of 125 klux, which showed a daylighting efficiency of 15.1%. The research showed that, as long as there was good thermal load control, PMMA optical fibers can work well and it is feasible to adopt large lens into daylighting system via plastic optical fibers.

Miniature fiber optic acoustic pressure sensors with air-backed graphene diaphragms

Graphene has been known to possess exceptional mechanical properties, including its extremely high Young’s modulus and atomic-layer thickness. Although there are several reported fiber optic pressure sensors using a graphene film, a key question that is not well understood is how the suspended graphene film interacts with the backing air cavity and affects sensor performance. Based on our previously analytical model, we will show that sensor performance suffers due to the significantly reduced mechanical sensitivity by the backing cavity. To remedy this limitation, we will, through experimental and numerical methods, investigate two approaches to enhance the sensitivity of fiber optic acoustic pressure sensors using the graphene film. First, a graphene-silver composite diaphragm is used to enhance the optical sensitivity by increasing the reflectivity. Compared with a sensor with pure graphene diaphragm, a graphene-silver composite can enhance the sensitivity by three-fold, while the mechanical sensitivity is largely unchanged. Second, a fiber optic sensor is developed with enlarged backing air volume through the gap between an optical fiber and a silica capillary tube. Experimental results show that the mechanical sensitivity is increased by 10xfrom the case where the gap side space is filled.

Miniature interrogator for multiplexed FBG strain sensors based on a thermally tunable microring resonator array.

We present a high-resolution and miniature multi-wavelength Fiber Bragg Grating (FBG) interrogator based on a thermally tunable microring resonator (MRR) array. A phase detection method using dithering signals is exploited to generate an antisymmetric error signal curve, which is utilized for the feedback locking of the MRR with the FBG sensor. Dynamic strain sensing of both single FBG and multiple FBGs are experimentally demonstrated, with a dynamic strain resolution of 30 nε/√Hz over 100 Hz to 1 kHz. The proposed interrogator shows the great improvements in both resolution and wavelength accuracy compared with the reported MRR-based interrogators and is promising for scalable multiplexed sensing applications.

Uniform light delivery in volumetric optoacoustic tomography.

Accurate image reconstruction in volumetric optoacoustic tomography implies the efficient generation and collection of ultrasound signals around the imaged object. Non-uniform delivery of the excitation light is a common problem in optoacoustic imaging often leading to a diminished field of view, limited dynamic range and penetration, as well as impaired quantification abilities. Presented here is an optimized illumination concept for volumetric tomography that utilizes additive manufacturing via 3D printing in combination with custom-made optical fiber illumination. The custom-designed sample chamber ensures convenient access to the imaged object along with accurate positioning of the sample and a matrix array ultrasound transducer used for collection of the volumetric image data. Ray tracing is employed to optimize the positioning of the individual fibers in the chamber. Homogeneity of the generated light excitation field was confirmed in tissue-mimicking agar spheres. Applicability of the system to image entire mouse organs ex vivo has been showcased. The new approach showed a clear advantage over conventional, single-sided illumination strategies by eliminating the need to correct for illumination variances and resulting in enhancement of the effective field of view, greater penetration depth and significant improvements in the overall image quality.

Relative Contributions of Bed Load and Suspended Load to Sediment Transport Under Skewed‐Asymmetric Waves on a Sandbar Crest

AbstractA large‐scale laboratory experiment was conducted to evaluate cross‐shore sediment transport and bed response on a sandbar under erosive and accretive field‐scale wave conditions (total of 11 cases). Unprecedented vertical resolution of sediment concentration was achieved through the use of conductivity concentration profilers alongside miniature fiber optic backscatter profilers. Observations were made of intrawave (phase‐averaged) and wave‐averaged cross‐shore sediment flux profiles and transport rates in the lower half of the water column on the crest of a sandbar. The net sediment transport rate was partitioned into suspended sediment (SS) and bed load (BL) components to quantify the relative contributions of SS and BL to the total sediment transport rate. Net SS transport rates were greater than net BL transport rates for the positive (wave crest) half‐cycle in 6 of 11 cases, compared to 100% (11 of 11) for the negative (wave trough) half‐cycle. Net (wave‐averaged) BL transport rates were greater, in magnitude, than net SS transport rates for 7 of the 11 cases. The dominant mode of transport was determined from the ratio of net BL to net SS transport rate magnitudes. The net transport rate was negative (offshore‐directed) when SS dominated and positive (onshore‐directed) when BL dominated. Net BL transport rate correlated well with third moments of free‐stream velocity (r2 = 0.72), suggesting that energetics‐type quasi‐steady formulae may be suitable for predicting BL transport under the range of test conditions.

Transverse Magneto-Optic Error of a Miniature Solid-Core Photonic-Crystal Fiber Optic Gyroscope

The transverse magneto-optic error is a significant nonreciprocal error of fiber optic gyroscopes, particularly for the miniature fiber optic gyroscope. In this paper, the transverse magneto-optic error of a solid-core photonic-crystal fiber optic gyroscope is theoretically analyzed and experimentally measured, as well as compared with that of the conventional fiber optic gyroscopes. The results show that the transverse magneto-optic error per turn of the miniature photonic-crystal fiber optic gyroscope is ~ 3.5 times smaller than that of the conventional fiber optic gyroscopes. A thinner $\mu $ -metal shield can be used in a miniature photonic-crystal fiber optic gyroscope, resulting in less cost and weight. Meanwhile, the experimental and the calculated results agree well with each other, which illustrates that the transverse magneto-optic error derives mainly from the bend-induced asymmetry distribution of the refractive index in fiber. Considering the remarkable properties of the solid-core photonic-crystal fibers, they are the good candidates for the future miniature fiber optic gyroscopes.

Аспекти використання особливостей лазерного випромінювання для оптимізації діагностичних методів при курації пацієнтів з генералізованими захворюваннями пародонта

The aim of the research is to study the possibilities of using laser radiation (biofluorescence, elastic scattering) for diagnostic purposes to optimize and identify periodontopathogenic microorganisms. Materials and methods. For the biofluorescence fixation experiment, we used a 632.8 nm helium neon laser, 3mW; a 668 nm Lika surgeon M semiconductor laser, 30–250 mW; the Ocean Optics USB4000 miniature fiber optic spectrometer; quartz light guides, tripods; reference strains of microorganisms: Escherichia coli, Candida albicans, Klebsiella pneumoniae, Proteus mirabilis, Corynebacterium pseudodiphtheriae, Staphylococcus aureus, Staphylococcus epidermidis cultured on agar; a laptop with SpectraSuite software. The elastic scattering angular distribution was analyzed on the developed experimental platform. The excitation source was a 532 nm continuous-wave (CW) laser (TEM00 mode) with a power of <50 mW. The elastic scattering angular distribution in the anterior and posterior hemispheres for aqueous suspensions of Staphylococcus aureus and Escherichia coli was investigated. Results. During the first part of the experiment, the colonies of the reference microorganisms were alternately irradiated by laser radiation beam first from neon laser helium, then from semiconductor laser. The reflection spectrum was recorded by the Ocean Optics USB4000 spectrometer and processed on a laptop using SpectraSuite software. The samples were irradiated at different angles (90 °, 45 °) in the dark, on a dark substrate. The radiation power of helium neon laser was stable – 3mW, and the radiation power of semiconductor varied in the range of 30-100mW. After irradiation of all the samples of cultures, as described above, and after the spectrograms processing, we did not get confirmation of the theory of biofluorescence: basically the spectrometer recorded only the spectrum of the reflected laser radiation. In the second part of the experiment, we investigated the elastic scattering angular distribution in the anterior and posterior hemispheres for aqueous suspensions of Staphylococcus aureus and Escherichia coli which have different spatial form factors. It was shown that complete integral extinction into the anterior hemisphere is linearly proportional to the bacterial concentration estimated by the McFarland standard on a certified device. A more detailed analysis of the elastic scattering angular distribution showed that the suspension of Staphylococcus aureus had a decrease in scattering in the anterior hemisphere and enhanced scattering in the posterior hemisphere compared with a suspension of bacteria Escherichia coli. The presented method can be applied to solve the problem of classification of microorganisms by the modelling of the elastic scattering angular distribution on the basis of models of elementary scatters with different form factors according to the Mie theory. Conclusion. The experiments on the study of biofluorescence of colonies of bacteria have not confirmed the possibility of recording specific spectra from each of the strains. However, the results of the study of elastic scattering in the suspensions of microorganisms indicated the possibility of determining the structural and concentration features of their content, namely to differentiate the rod shaped flora from the coccoid, and to estimate the relative number of particles in the suspension. The development of this technology can greatly simplify the diagnostics without the use of expensive and technologically difficult research methods in the future Key words: periodontal pathogen, biofluorescence, Mie theory, intensity of elastic optical scattering.

Miniature fiber-optic sensor for simultaneous measurement of refractive index and temperature

A miniature fiber-optic sensor fabricated by fuse splicing a section of hollow silica tube between two sections of single-mode fiber is proposed and experimentally demonstrated for simultaneous measurement of the liquid refractive index (RI) and temperature. The sensitivities of RI and temperature are measured by linear fitting, respectively, and corresponding detecting resolutions are obtained by the sensitivity matrix. The different response sensitivities of two dips to RI and temperature manifest that the structure proposed can be used for simultaneous measurement. The experimental evidence indicates that the miniature fiber sensor with advantages of easy fabrication, ultracompactness, and robustness is insensitive to axial strain.

An optical fiber daylighting system with large Fresnel lens

Because of flexible installation and natural spectrum, the concentrated sunlight transmission systems via PMMA (Polymethyl methacrylate) fibers for indoor illumination are concerned widely in areas of building energy saving. Due to the long-term indoor work, the daily working population in cities lacks of natural light and is susceptible to various diseases. Therefore, from the health point of view, it is necessary to develop deep indoor daylighting. In this work, a sunlight concentration and transmission system was developed and tested, which uses a Fresnel lens with a diameter of 1m to achieve large flux collection. A reflection design of light path was adopted to resolve the problem of long focal length of the Fresnel lens. A hot mirror and a homogenizer were used to protect the optic fiber bundle from burned by high temperature. The results showed that the daylighting efficiency is about 11%-13%. The operability of the equipment was verified through experiments, proving that the equipment can be combined with buildings for daylighting. It is hoped that this research will be of some reference value to the design of optical fiber daylighting systems in buildings.

Imaging of the human tympanic membrane by endoscopic optical coherence tomography

Abstract Endoscopic optical coherence tomography is a non-invasive and contactless imaging technique based on white light interferometry. It enables high-resolution three-dimensional imaging of scattering tissue up to a depth of about 2 mm. In addition, Doppler-OCT can detect sub-resolution movements. These features can be used to examine the tympanic membrane, the surrounding tissue and nearby areas of the tympanic cavity. For this purpose, we present an endoscopic OCT system, which provides access to the tympanic membrane. The design of the endoscope is based on a gradientindex (GRIN) lens system. This allows a broad field of view and a large working distance. An additional VIS beam path allows visual imaging and orientation inside the auditory canal. Therefore, illumination fibers are attached a round the GRIN-system. The resulting endoscope has a length of 55 mm and a diameter of 3.5 mm. By attaching an earphone and a probe microphone, the oscillation of the tympanic membrane can be measured under acoustic stimulation. With the endoscopic OCT system, we provide an examination tool for the diagnosis of a broad number of diseases like conductive hearing loss.

Adjustable photoacoustic tomography probe improves light delivery and image quality

One cause for suboptimal photoacoustic tomography (PAT) penetration depth is attenuation of incident light by soft tissue. To better understand this problem, we investigated the effects of illumination fiber optic bundle geometry on PAT penetration depth and signal-to-noise ratio. An adjustable, motorized PAT probe was used to reduce probe-skin reflection artifacts and improve light distribution in the image acquisition plane by tuning fiber orientation. We validated our motorized PAT probe through Monte Carlo simulations and ex vivo imaging of a tissue mimicking phantom, and in vivo imaging of murine periaortic fat. Overall, our ex vivo results showed a several millimeter improvement in penetration depth and in vivo results showed a >62% increase in lipid signal-to-noise ratio. Our PAT probe also utilized a 7-μm aluminum filter to block in vivo probe-skin reflection artifacts. Together, these findings showed the importance of optimizing illumination geometry to enhance PAT image quality.

Stomach wall structure and vessels imaging by acoustic resolution photoacoustic microscopy.

AIMTo image stomach wall blood vessels and tissue, layer-by-layer.METHODSWe built up the acoustic resolution photoacoustic microscopy (AR-PAM) system for imaging layered tissues, such as the stomach wall. A tunable dye laser system was coupled to a fiber bundle. The fibers of the bundle were placed in nine directions with an incident angle of 45° around a high-frequency ultrasound transducer attached to the acoustic lens. This structure formed a dark field on the tissue surface under the acoustic lens and the nine light beams from the fibers to be combined near the focal point of the acoustic lens. The sample piece was cut from a part of the porcine stomach into a petri dish. In order to realize photoacoustic depth imaging of tumor, we designed a tumor model based on indocyanine green (ICG) dye. The ICG solution (concentration of 129 μM/mL) was mixed into molten gel, and then a gel mixture of ICG (concentration of 12.9 μM/mL) was injected into the stomach submucosa. The injection quantity was controlled by 0.1 mL to make a small tumor model.RESULTSAn acoustic resolution photoacoustic microscopy based on fiber illumination was established and an axial resolution of 25 μm and a lateral resolution of 50 μm in its focal zone range of 500 μm has been accomplished. We tuned the laser wavelength to 600 nm. The photoacoustic probe was driven to do B-scan imaging in tissue thickness of 200 μm. The photoacoustic micro-image of mucosa and submucosa of the tissue have been obtained and compared with a pathological photograph of the tissue stained by hematoxylin-eosin staining. We have observed more detailed internal structure of the tissue. We also utilized this photoacoustic microscopy to image blood vessels inside the submucosa. High contrast imaging of the submucosa tumor model was obtained using ICG dye.CONCLUSIONThis AR-PAM is able to image layer-by-layer construction and some blood vessels under mucosa in the stomach wall without any contrast agents.

Miniature probe for all-optical double gradient-index lenses photoacoustic microscopy.

A novel all-optical double gradient-index (GRIN) lens optical-resolution photoacoustic microscopy (OR-PAM), termed as DGL-PAM, is demonstrated. The miniature probe consists of a single-mode fiber and double GRIN lenses for optical focusing and a miniature fiber Fabry-Perot sensor for ultrasound detection. The new design is simple and realizes high resolution with long working distance (WD) by virtue of the double GRIN lenses. The overall size of the probe is 2.7 mm in diameter. High lateral resolution of 3.7 μm (at 532 nm laser wavelength) and long WD of 5.5 mm are achieved. In vivo OR-PAM of mouse ear demonstrates the imaging ability of DGL-PAM. Since precise alignment of optical and acoustic foci is not needed, the proposed DGL-PAM is relatively easy to implement. It has potential to be developed as a low-cost, disposable OR-PAM probe and for endoscopic applications. The proposed double GRIN lenses for making miniature endoscopic probes can also be applied to other modalities, such as optical coherence tomography and confocal fluorescence microscopy, to enable high resolution and long WD.

Implication of Illumination Beam Geometry on Stray Light and Bandpass Characteristics of Diode Array Spectrometer

This paper describes spectral stray light measurements of diode array spectrometers and, in particular, the dependence of stray light and bandpass properties on the illumination beam geometry. The effects of underfilling and overfilling the nominal field of view of the instrument are presented. Our measurements showed that the line spread function of a commercially available miniature spectrometer module having a fiber optic entrance with 0.22 nominal numerical aperture can depend on fiber illumination cone apex angle up to $40^\circ$ , which is well above the $25^\circ$ nominal acceptance cone apex angle of the fiber. The bandpass width and amount of stray light inside the spectrometer increased proportionally with the illumination cone angle. Spectral stray light measurements with different setups gave consistent and repeatable results for similar illumination geometries. Care should be taken to ensure that illumination geometry during spectral stray light characterization of a spectrometer matches the conditions during target measurements for which the spectral stray light corrections are applied. The impact of illumination geometry during stray light characterization depends on the target signal and particular application. Photochemical reflectance index (PRI) and normalized difference vegetation index (NDVI) of a hemiboreal birch forest are used as contrasting examples in this paper. Compared to natural variability, the relative impact on PRI exceeds that of NDVI by 10 times.

Flexible <inline-formula> <tex-math notation="LaTeX">$3\times3$ </tex-math> </inline-formula> Mueller Matrix Endoscope Prototype for Cancer Detection

In this paper, we research and design a flexible head-modulated polarization endoscope system prototype for early cancer detection in vivo. The new idea of our design is the implementation of the distal tip of the endoscope structure, that is, the distal tip has integrated the polarization state generator (PSG) and polarization state analyzer (PSA), the illumination fiber, and image capture device. A closed-loop motion control system was adopted to accurately rotate the linear polarizers in PSG and PSA. A high-definition image capture and transfer system was used to capture the polarized images. By this design, a $3\times 3$ Mueller matrix could be calculated through such an endoscope head. Furthermore, the measurement process would not be influenced by the polarization disturbance induced to the pathway of light transmission. A state of polarization calibration process was carried out on the PSG and PSA to keep a high degree of accuracy and repeatability of described rotation-based polarization, and the results showed its effectiveness. Finally, our endoscope head was used to measure the nude mice skin with subcutaneous implantation of HeLa cells in vivo. The corresponding Mueller matrix decomposition and Mueller matrix transformation parameters extracted showed that our proposed device can clearly and stably distinguish the cancerous and normal tissues. The results prove that the polarization endoscope system has its potential application in early cancer detection.

Miniature fiber pressure sensor based on an in-fiber confocal cavity

A new type of miniature pressure sensor based on a hybrid Fabry-Perot (F-P) interferometer is proposed. The in-line air cavity of the sensor is a micro-confocal air cavity, which is formed by splicing together a single mode fiber (SMF) and a solid-core photonic crystal fiber (PCF) with the specific splicing parameters. The sensor is fabricated by simple steps involving only fusion splicing, cleaving and polishing. Optical interference principle of the sensor is analyzed and the parameters in sensor design are discussed. Due to low propagation loss, experiments show that the reflection spectrum of the sensor has a fringe contrast of more than 30 dB. By tracing the trough wavelength in the interference spectrum, the pressure can be demodulated. Within the pressure measurement range of 0–1 MPa, the sensitivity is −193 pm/MPa. The experimental results indicate that the deflection is consistent with the theoretical analysis and the sensor has a good linearity. Easy fabrication, low-cost and high-contrast make it appropriate for practical application.

Three dimensional two-photon brain imaging in freely moving mice using a miniature fiber coupled microscope with active axial-scanning

We present a miniature head mounted two-photon fiber-coupled microscope (2P-FCM) for neuronal imaging with active axial focusing enabled using a miniature electrowetting lens. We show three-dimensional two-photon imaging of neuronal structure and record neuronal activity from GCaMP6s fluorescence from multiple focal planes in a freely-moving mouse. Two-color simultaneous imaging of GFP and tdTomato fluorescence is also demonstrated. Additionally, dynamic control of the axial scanning of the electrowetting lens allows tilting of the focal plane enabling neurons in multiple depths to be imaged in a single plane. Two-photon imaging allows increased penetration depth in tissue yielding a working distance of 450 μm with an additional 180 μm of active axial focusing. The objective NA is 0.45 with a lateral resolution of 1.8 μm, an axial resolution of 10 μm, and a field-of-view of 240 μm diameter. The 2P-FCM has a weight of only ~2.5 g and is capable of repeatable and stable head-attachment. The 2P-FCM with dynamic axial scanning provides a new capability to record from functionally distinct neuronal layers, opening new opportunities in neuroscience research.

Air-coupled fiber Fabry-Perot ultrasonic sensor formed by diaphragm for seismic physical model imaging

We presented a miniature fiber ultrasonic sensor based on the principle of Fabry-Perot(F-P) interferometer, which comprises a single-mode fiber(SMF) spliced with a hollow core fiber (HCF), whose end face is adhered with UV glue(NOA68). The interface of NOA68 diaphragm-air as one of the two interfaces of F-P cavity is sensitive to ultrasonic wave due to its low Young’s modulus. Thus, it has the ability to detect weak ultrasonic wave reflected from seismic physical model (SPM) in air. Furthermore, the compact sensor has a characteristics to identify orientation of ultrasonic source because of its perfectly symmetrical structure and is capable of detecting wide ultrasonic frequency range from 300 KHz to 5 MHz. The spectral band-side filter technique is used for the interrogation of ultrasonic signal. As expected, using the designed sensing probe, we can reconstruct the SPM in 2D and 3D by the time-of-flight approach.

ANALYSIS OF IN-SITU SPECTRAL REFLECTANCE OF SAGO AND OTHER PALMS: IMPLICATIONS FOR THEIR DETECTION IN OPTICAL SATELLITE IMAGES

Abstract. We present a characterization, comparison and analysis of in-situ spectral reflectance of Sago and other palms (coconut, oil palm and nipa) to ascertain on which part of the electromagnetic spectrum these palms are distinguishable from each other. The analysis also aims to reveal information that will assist in selecting which band to use when mapping Sago palms using the images acquired by these sensors. The datasets used in the analysis consisted of averaged spectral reflectance curves of each palm species measured within the 345–1045 nm wavelength range using an Ocean Optics USB4000-VIS-NIR Miniature Fiber Optic Spectrometer. This in-situ reflectance data was also resampled to match the spectral response of the 4 bands of ALOS AVNIR-2, 3 bands of ASTER VNIR, 4 bands of Landsat 7 ETM+, 5 bands of Landsat 8, and 8 bands of Worldview-2 (WV2). Examination of the spectral reflectance curves showed that the near infra-red region, specifically at 770, 800 and 875 nm, provides the best wavelengths where Sago palms can be distinguished from other palms. The resampling of the in-situ reflectance spectra to match the spectral response of optical sensors made possible the analysis of the differences in reflectance values of Sago and other palms in different bands of the sensors. Overall, the knowledge learned from the analysis can be useful in the actual analysis of optical satellite images, specifically in determining which band to include or to exclude, or whether to use all bands of a sensor in discriminating and mapping Sago palms.

A Miniature Fiber Collimator for Highly Sensitive Bend Measurements

A Fabry-Perot interferometer (FPI) composed of a miniature fiber collimator is proposed and demonstrated for high-sensitivity bend measurements. The device consists of a quarter-pitch graded index fiber (GIF) spliced with a section of silica tube, which is interposed between single-mode fibers. The divergence angle of the included miniature fiber collimator was 0.058 rad, which was found to enhance the sensitivity of the proposed FPI to -3.353 dB/degree over the applied angle range. In comparison, the divergence angle without the collimator was 0.130 rad and FPI sensitivity was -0.618 dB/degree over the applied angle range. The fiber bend sensors demonstrated in this paper are compact, robust, and highly sensitive, and hence, are promising in various biomedical applications.