Enhancing resolution and contrast in fibre bundle-based fluorescence microscopy using generative adversarial network

Fibre bundle (FB)-based endoscopes are indispensable in biology and medical science due to their minimally invasive nature. However, resolution and contrast for fluorescence imaging are limited due to characteristic features of the FBs, such as low numerical aperture (NA) and individual fibre core sizes. In this study, we improved the resolution and contrast of sample fluorescence images acquired using in-house fabricated high-NA FBs by utilising generative adversarial networks (GANs). In order to train our deep learning model, we built an FB-based multifocal structured illumination microscope (MSIM) based on a digital micromirror device (DMD) which improves the resolution and the contrast substantially compared to basic FB-based fluorescence microscopes. After network training, the GAN model, employing image-to-image translation techniques, effectively transformed wide-field images into high-resolution MSIM images without the need for any additional optical hardware. The results demonstrated that GAN-generated outputs significantly enhanced both contrast and resolution compared to the original wide-field images. These findings highlight the potential of GAN-based models trained using MSIM data to enhance resolution and contrast in wide-field imaging for fibre bundle-based fluorescence microscopy. Lay Description: Fibre bundle (FB) endoscopes are essential in biology and medicine but suffer from limited resolution and contrast for fluorescence imaging. Here we improved these limitations using high-NA FBs and generative adversarial networks (GANs). We trained a GAN model with data from an FB-based multifocal structured illumination microscope (MSIM) to enhance resolution and contrast without additional optical hardware. Results showed significant enhancement in contrast and resolution, showcasing the potential of GAN-based models for fibre bundle-based fluorescence microscopy.

Broadband optical vortex beam generation using flat-surface nanostructured gradient index vortex phase masks

We developed a new kind of compact flat-surface nanostructured gradient index vortex phase mask, for the effective generation of optical vortex beams in broadband infrared wavelength range. A low-cost nanotechnological material method was employed for this work. The binary structure component consists of 17,557 nano-sized rods made of two lead-bismuth-gallium silicate glasses which were developed in-house. Those small rods are spatially arranged in such a way that, according to effective medium theory, the refractive index of this internal structure is constant in the radial direction and linearly changes following azimuthal angle. Numerical results demonstrated that a nanostructured vortex phase mask with a thickness of 19 μm can convert Gaussian beams into fundamental optical vortices over 290 nm wavelength bandwidth from 1275 to 1565 nm. This has been confirmed in experiments using three diode laser sources operating at 1310, 1550, and 1565 nm. The generation of vortex beams is verified through their uniform doughnut-like intensity distributions, clear astigmatic transformation patterns, and spiral as well as fork-like interferograms. This new flat-surface component can be directly mounted to an optical fiber tip for simplifying vortex generator systems as well as easier manipulation of the generated OVB in three-dimensional space.

Polarization dynamics of vector solitons in a fiber laser

We investigate the polarization dynamics of vector solitons in a fiber laser mode-locked by a saturable absorber (SA). Three types of vector solitons were obtained in the laser, including group velocity locked vector solitons (GVLVS), polarization locked vector solitons (PLVS), and polarization rotation locked vector solitons (PRLVS). Their polarization evolution during intracavity propagation is discussed. Pure vector solitons are obtained from the continuous wave (CW) background by soliton distillation, and the characteristics of the vector solitons without and with distillation are analyzed, respectively. Numerical simulations suggest that the features of vector solitons in a fiber laser could be assemble to those generated in fibers.

Characterization of sidebands in fiber lasers based on nonlinear Fourier transformation

Phase evolution of soliton and that of first-order sidebands in a fiber laser are investigated by using nonlinear Fourier transform (NFT). Development from dip-type sidebands to peak-type (Kelly) sidebands is presented. The phase relationship between the soliton and the sidebands calculated by the NFT are in good agreement with the average soliton theory. Our results suggest that NFT can be an effective tool for the analysis of laser pulses.

Transmission of an optical vortex beam in antiresonant fibers generated in an all-fiber system

We report an experimental study on transmission of orbital angular momentum mode in antiresonant fibers generated with a dedicated all-fiber optical vortex phase mask. The vortex generator can convert Gaussian beam into vortex beams with topological charge l = 1. Generated vortex beam is directly butt-coupled into the antiresonant fiber and propagates over distance of 150 cm. The stability and sensitivity of the transmitted vortex beam on the external perturbations including bending, axial stress, and twisting is investigated. We demonstrate distortion-free vortex propagation for the axial stress force below 0.677 N, a bend radius greater than 10 cm.

Optimization of the nanostructured weakly coupled few-mode fiber for mode-division-multiplexed systems

The objective of the study is to optimize the optical fiber structure for mode-division multiplexing systems using nanostructurization. The nanostructuring technique allows to fabricate fibers with arbitrarily designed (free-form) refractive index distribution based on two glasses. Three optimization schemes have been proposed. The nanostructuring method allows for designing fibers with optical properties similar and even better parameters impossible to produce by other methods. In this proposal, we examined four linearly polarized (LP) few-mode fibers. We report a high effective refractive index difference between modes while maintaining other important parameters for the weakly coupled approach.

Two octave supercontinuum generation in a non-silica graded-index multimode fiber

The generation of a two-octave supercontinuum from the visible to mid-infrared (700-2800 nm) in a non-silica graded-index multimode fiber is reported. The fiber design is based on a nanostructured core comprised of two types of drawn lead-bismuth-gallate glass rods with different refractive indices. This yields an effective parabolic index profile and ten times increased nonlinearity when compared to silica fibers. Using femtosecond pulse pumping at wavelengths in both normal and anomalous dispersion regimes, a detailed study is carried out into the supercontinuum generating mechanisms and instabilities seeded by periodic self-imaging. Significantly, suitable injection conditions in the high power regime are found to result in the output beam profile showing clear signatures of beam self-cleaning from nonlinear mode mixing. Experimental observations are interpreted using spatio-temporal 3+1D numerical simulations of the generalized nonlinear Schrödinger equation, and simulated spectra are in excellent agreement with experiment over the full two-octave spectral bandwidth. Experimental comparison with the generation of supercontinuum in a silica graded-index multimode fiber shows that the enhanced nonlinear refractive index of the lead-bismuth-gallate fiber yields a spectrum with a significantly larger bandwidth. These results demonstrate a new pathway towards the generation of bright, ultrabroadband light sources in the mid-infrared.

Heat treatment and fiber drawing effect on the luminescence properties of RE-doped optical fibers (RE = Yb, Tm, Ho)

We investigate the influence of various optical fiber fabrication processes on the fluorescence decay of RE ions commonly used in fiber lasers and amplifiers, i.e. Yb³⁺, Tm³⁺ and Ho³⁺. Optical fiber preforms were prepared using the MCVD method combined with Al₂O₃ nanoparticle doping and subjected to subsequent heat treatment processes such as preform elongation and fiber drawing. The fluorescence decay of RE ions was measured in multiple stages of optical fiber preparation: in an original preform, in an elongated preform (cane), in a standard fiber, and in an overcladded fiber. It was found that heat treatment processing of the preforms generally leads to a faster fluorescence decay, which can be explained by the diffusion of dopants and clustering of RE ions. The fiber drawing exhibited a greater effect compared to preform elongation, which was ascribed to a faster cooling rate of the process. In general, the heat treatment of RE-doped silica glass preforms leads to the decline of fluorescence decay.

Development of gradient index microlenses for the broadband infrared range

The development of gradient index free-form micro-optic components dedicated to the mid-infrared range is challenging due to the lack of appropriate technology. We propose a method for developing gradient index components for broadband infrared range beyond the transmission window of silicate glass based on nanostructurization using a stack-and-draw fiber drawing technique. A proof-of-concept microlens is developed and verified experimentally in the wavelength range 1.5-4.3 µm. The microlenses are composed of a set of nanorods with a diameter of 940 nm made of a pair of SiO₂-PbO-Bi₂O₃-Ga₂O₃ based glasses ordered into the preliminary calculated binary pattern. The pattern forms effectively continuous parabolic refractive index distribution for infrared range according to Maxwell-Garnett effective medium model. The development of individual microlenses with a diameter of 118 µm and focal length of 278 µm at the wavelength of 3.75 µm are reported. A large array of 737 microlenses with an individual diameter of 125 µm and focal length of 375 µm is also presented and analyzed.

All-solid polarization-maintaining silica fiber with birefringence induced by anisotropic metaglass

We report the development of a silica glass single-mode polarization-maintaining fiber with birefringence induced by artificial anisotropic glass in the circular core without any external stress zones or structured cladding. The fiber core is composed of silica and germanium-doped silica nanorods ordered in submicrometer interleaved layers. The fiber has a measured cut-off wavelength at 1113 nm, phase birefringence of 0.3×10^-4, and an effective mode diameter of 10.5 µm at the wavelength of 1550 nm. The polarization extinction ratio in the fiber is 20 dB at 1550 nm. The fiber is compatible with the standard SMF-28 fiber and can be easily integrated using standard fusion splicing with losses of 0.1 dB.

Silica-based photonic crystal fiber infiltrated with 1,2-dibromoethane for supercontinuum generation

This study proposes a photonic crystal fiber (PCF) made of fused silica glass with the core infiltrated with 1,2-dibromoethane (C₂H₄Br₂) as a new source of supercontinuum light pulses. Due to the modifications of the PCF's structure geometry, a number of computer simulations investigating their optimized structures has been carried out. This aimed at achieving flat near-zero dispersion and zero dispersion wavelength matching of the pump wavelength for efficient spectral broadening. Based on the obtained results, the structural geometries of two C₂H₄Br₂-core PCFs were optimized using numerical modeling for broadband supercontinuum (SC) generation. The first fiber structure with a lattice constant 1.5 µm and filling factor 0.4 has all-normal dispersion profile. The SC with a broadened spectral bandwidth from 0.64 to 1.70 µm is generated by pump pulses centered at a wavelength of 1.03 µm, 120 fs duration, and energy of 1.5 nJ. The second proposed structure-with lattice constant 1.5 µm and filling factor 0.65-has anomalous dispersion for wavelengths longer than 1.03 µm. We obtained high coherence of the SC pulses in the anomalous dispersion range over wavelengths of 0.7-2.4 µm with the same pump pulse as the first fiber and with input energy of 0.09 nJ. These fibers would be interesting candidates for all-fiber SC sources operating with low-energy pump lasers as cost-effective alternatives to glass core fibers.

Reconstruction and modeling of the complex refractive index of nonlinear glasses from terahertz to optical frequencies

The linear complex refractive index of a set of borosilicate and tellurite as well as heavy metal oxide silicate, germanate and fluoride glasses has been determined using the Kramers-Kronig analysis on combined data from terahertz time domain (THz-TD) and Fourier transform infrared (FTIR) spectrometers in the ultrabroadband range of 0.15 THz to 200 THz. Debye, Lorentz and shape language modeling (SLM) approaches are applied. Far-infrared absorption power-law model parameters are determined via searching for the largest frequency range that minimizes the root mean squared error (RMSE) of a linear least squares fit for the set of glasses and other glass literature data. Relationships between the absorption parameters, glass properties and compositions are explored.

Toward highly birefringent silica Large Mode Area optical fibers with anisotropic core

We test the development of a silica all-glass optical fiber with a highly birefringent large mode area (HB-LMA). In the fiber, the birefringence and single mode operation are independent of bending and results from the internal nanostructuring of the core, which makes the glass anisotropic. Taking into account technological limitations of the doped silica glasses, we optimized the HB-LMA fiber properties by appropriate selection of germanium and fluorine doping level of silica used in the fiber core and cladding. We demonstrated that the anisotropic glass can be successfully used as a core material in large core area fibres in C-band for polarization components of the fundamental mode. We obtained phase birefringence of 1.92 × 10^-4 in the fiber with the core diameter of 30 µm and the effective mode area equal to 573 µm² and 804 µm², for x- and y-polarization, respectively. The same approach was applied to designing a single mode fiber with 40 µm core diameter and effective mode area over 1000 µm², which supports only single polarization.

Heterodyne photothermal spectroscopy of methane near 1651 nm inside hollow-core fiber using a bismuth-doped fiber amplifier

We present laser-based methane detection near 1651 nm inside an antiresonant hollow-core fiber (HCF) using photothermal spectroscopy (PTS). A bismuth-doped fiber amplifier capable of delivering up to more than 160 mW at 1651 nm is used to boost the PTS signal amplitude. The design of the system is described, and the impact of various experimental parameters (such as pump source modulation frequency, modulation amplitude, and optical power) on signal amplitude and signal-to-noise ratio is analyzed. Comparison with similar PTS/HCF-based systems is presented. With 1.3 m long HCF and a fiber amplifier for signal enhancement, this technique is capable of detecting methane at single parts-per-million levels, which makes this robust in-fiber sensing approach promising also for industrial applications such as, e.g., natural gas leak detection.

Nanostructured active and photosensitive silica glass for fiber lasers with built-in Bragg gratings

A nanostructured core silica fiber with active and photosensitive areas implemented within the fiber core is demonstrated. The photosensitivity, active and passive properties of the fiber can be independently shaped with this new approach. We show that discrete local doping with active ions in form of nanorods allow to obtain effective laser action as in case of continuous distribution of the ions in the core. Co-existing discrete photosensitive nanostructure of germanium doped silica determine single-mode performance and allow inscription of highly efficient Bragg grating over the entire core area. Each nanostructure do not degrade performance of other one since physical interaction between active and photosensitive areas are removed. As a proof of concept, we have designed and fabricated the nanostructured, ytterbium single-mode silica fiber laser with the Bragg grating inscribed in the entire core area. We demonstrated fiber laser with good quality of generated laser beam (M²=1.1) with lasing efficiency of 44% and inscribed Bragg grating with 98.5% efficiency and -18 dB contrast.

Numerical analysis of optical vortices generation with nanostructured phase masks

We study the theoretical formation of optical vortices using a nanostructured gradient index phase mask. We consider structures composed of spatially distributed thermally matched glass nanorods with high and low refractive indices. Influence of effective refractive profile distribution, refractive index contrast of component glasses and charge value on the quality of generation of vortices are discussed. A trade-off between waveguiding and phase modulation effects for various refractive index contrast is presented and analysed.

Light field camera based on hexagonal array of flat-surface nanostructured GRIN lenses

In this paper we present a light field camera system where a flat-surface hexagonal array of nanostructured gradient index lenses was used as a lens matrix. In our approach we use an array of 469 gradient index microlenses with a diameter of 20 µm and 100% fill factor. To develop the single lens and the lenslet array we used a modified stack-and-draw technology. In this technique, variation of refractive index is achieved by using quantized gradient index profiles and rods from different types of glasses. We show experimental results of using this type of lenses for imaging in a system of two kinds of light field cameras. In the first one, the microlens array is located in the focal plane of the main lens. The image is reconstructed, in this case using a Fourier slice photography algorithm. This allowed a partial reconstruction of a 3D scene with spatial and depth resolution of 20 µm and field of view of 500×500×500 µm. In the second configuration, the microlens array is located between a sample and a microscopic objective, thus allowing for superresolution 3D reconstruction of a microscopic image. The scale-invariant feature transform method was used for image reconstruction and obtained a partial 3D reconstruction with a field of view of 150×115×80 µm and a spatial resolution of 2 µm and depth resolution of 10 µm.

Development of large diameter nanostructured GRIN microlenses enhanced with temperature-controlled diffusion

Nanostructured GRIN components are optical elements which can have an arbitrary refractive index profile while retaining flat-parallel entry and exit facets. A method of their fabrication requires assembly of large quantities of glass rods in order to satisfy subwavelength requirement of the effective medium theory. In this paper, we present a development of gradient index microlenses using a combination of methods: nanostructurization of the preform and controlled diffusion process during lens drawing on a fiber drawing tower. Adding a diffusion process allows us to overcome limits of the effective medium theory related to maximum size of nanorods in the lens structure. We show that nanorods are dissolved during the fiber drawing process in high temperature and glass components are locally quasi-uniformly distributed. To demonstrate feasibility of the proposed approach, we have developed and experimentally verified the performance of a nGRIN microlens with a diameter of 115 µm composed of 115 rods on the diagonal, and length of 200 µm devoted to work for the wavelength over 658 nm.

Inscription of Bragg gratings in nanostructured graded index single-mode fibers

We report on efficient inscription of fiber Bragg gratings (FBGs) in a new type of single mode fiber with nanostructured core and with an effective parabolic graded index profile, using the standard phase mask method and a 248 nm pulsed laser. A nanostructured core allows to obtain high concentration of GeO₂ in subwavelength glass rods and simultaneously to maintain low average germanium dopant level of silica similarly to standard single mode fibers. We showed that in a nanostructured core fiber, a factor of 3 better efficiency in gratings inscription was achieved, although the fiber has 20% lower average concentration of GeO₂ with respect to SMF-28. In the nanostructured fiber we obtained a significant improvement in temperature sensitivity, while the strain sensitivity of FBG in nGRIN optical fiber is the same as in case of standard single-mode fiber (SMF-28). We have measured the strain sensitivity of 0.72 × 10^-6 1/με (1.11 pm/με@1.53μm), and the temperature sensitivity is about 30% higher than for FBG in SMF-28 and equals to 10.2 × 10^-6 1/K (15.6 pm/K@1.53μm).

Achromatic nanostructured gradient index microlenses

We present a development of microlenses achromatically corrected in near-infrared spectral windows. We show that the standard fiber drawing technology can be successfully applied to the development achromatic gradient index microlenses by means of internal nanostructurization. These gradient index microlenses can achieve similar performance to standard aspheric doublets, while utilizing a simpler, singlet element geometry with flat surfaces. A nanostructured lens with a parabolic profile was designed using a combination of the simulated annealing method and the effective medium approximation theory. Measurements on the fabricated lenses show that the microlenses have a nearly wavelength-independent focal plane at a distance of about 35 μm from the lens facet over the wavelength range of 600-1550 nm. The successful design and fabrication of achromatic flat-parallel rod microlenses opens new perspectives for micro-imaging systems and wavelength-independent coupling into optical fibers.

Fabrication and characterization of large numerical aperture, high-resolution optical fiber bundles based on high-contrast pairs of soft glasses for fluorescence imaging

Fabrication and characterization of flexible optical fiber bundles (FBs) with in-house synthesized high-index and low-index thermally matched glasses are presented. The FBs composed of around 15000 single-core fibers with pixel sizes between 1.1 and 10 μm are fabricated using the stack-and-draw technique from sets of thermally matched zirconium-silicate ZR3, borosilicate SK222, sodium-silicate K209, and F2 glasses. With high refractive index contrast pair of glasses ZR3/SK222 and K209/F2, FBs with numerical apertures (NAs) of 0.53 and 0.59 are obtained, respectively. Among the studied glass materials, ZR3, SK222, and K209 are in-house synthesized, while F2 is commercially acquired. Seven different FBs with varying pixel sizes and bundle diameters are characterized. Brightfield imaging of a micro-ruler and a Convallaria majalis sample and fluorescence imaging of a dye-stained paper tissue and a cirrhotic mice liver tissue are demonstrated using these FBs, demonstrating their good potential for microendoscopic imaging. Brightfield and fluorescence imaging performance of the studied FBs are compared. For both sets of glass compositions, good imaging performance is observed for FBs, with core diameter and core-to-core distance values larger than 1.6 μm and 2.3 μm, respectively. FBs fabricated with K209/F2 glass pairs revealed better performance in fluorescence imaging due to their higher NA of 0.59.

Optical fibers with open side channel by wet etching

The paper presents a new approach to developing exposed-core fibers. We designed a new asymmetric structure of suspended core fibers with series of additional air holes in the cladding. Using the standard wet etching method we removed a part of glass, demonstrating that the method allows to open a selected air hole surrounding the suspended core. Such modified of fibers can be used to build sensors and devices dedicated to chemical and biological studies and based on the interaction of light with liquids. We used the developed fiber to develop an interferometric sensor that measures changes in the refractive index with a high accuracy. As a proof of concept, we present the experimental measurement results of the ethanol concentration in water.

Fused silica optical fibers with graded index nanostructured core

The ability to shape the index profile of optical fibers holds the key to fully flexible engineering of their optical properties and future applications. We present a new approach for the development of a graded index fused silica fiber based on core nanostructurization. A graded index core is obtained by means of distribution of two types of subwavelength glass rods. The proposed method allows to obtain arbitrary graded distribution not limited to the circular or any other symmetry, such as in the standard graded index fibers. We have developed a proof of concept fiber with parabolic refractive index core and showed a perfect match between its predicted, designed and measured properties. The fiber has a core composed of 2107 rods of 190 nm of diameter made of either pure fused silica or Ge-doped fused silica with 8.5% mol concentration. The proposed method breaks the limits of standard fabrication approaches used in fused silica fiber technology.

Integrating Free-Form Nanostructured GRIN Microlenses with Single-Mode Fibers for Optofluidic Systems

We present both a theoretical and an experimental study of a novel compact lensed fiber system utilizing a nanostructured GRIN lens. The lens can be integrated with an optical fiber, which ensures a unique and efficient focusing in any high index medium, such as a liquid. We use the effective medium approach to design lenses with arbitrary refractive index. To fabricate lenses, we utilize a discrete array of nano-sized rods made of two types of glasses, and apply a standard stack-and-draw fiber drawing technology. The fabricated nanostructured GRIN lenses have a parabolic refractive index profile with a diameter of a standard fiber, very short working distances (55 µm in the air) and a high numerical aperture (NA = 0.16). As a proof-of-concept of the new micro-lensed fiber system, we demonstrate an experiment on optical trapping of micrometer-sized glass beads. We also show that our method is compatible with optical fiber technology and allows for any shape of the refractive index distribution in 2D. Thanks to that a new functionality could be achieved by replacing the GRIN lens with an axicon lens, vortex type elements, micro-lenses arrays or diffraction elements.

Formation of optical vortices with all-glass nanostructured gradient index masks

We report the development of microscopic size gradient index vortex masks using the modified stack-and-draw technique. The vortex mask has a form of flat surface all-glass plate. Its functionality is determined by an internal nanostructure composed of two types of soft glass nanorods. The generation of optical vortices with charges 1 and 2 is demonstrated.

Dispersion characteristics of a suspended-core optical fiber infiltrated with water

In this paper we present a study on the dispersion characteristics in the suspended-core optical fibers made of borosilicate of NC21A glass infiltrated with water. Replacement of air with water results in dramatic improvement of the dispersion characteristics in the fibers, valuable in the process of supercontinuum generation. A near-zero flat dispersion can be achieved in the anomalous or normal dispersion range for various diameters of the core.

Diffractive optics development using a modified stack-and-draw technique

We present a novel method for the development of diffractive optical elements (DOEs). Unlike standard surface relief DOEs, the phase shift is introduced through a refractive index variation achieved by using different types of glass. For the fabrication of DOEs we use a modified stack-and-draw technique, originally developed for the fabrication of photonic crystal fibers, resulting in a completely flat element that is easy to integrate with other optical components. A proof-of-concept demonstration of the method is presented-a two-dimensional binary optical phase grating in the form of a square chessboard with a pixel size of 5 μm. Two types of glass are used: low refractive index silicate glass NC21 and high refractive index lead-silicate glass F2. The measured diffraction characteristics of the fabricated component are presented and it is shown numerically and experimentally that such a DOE can be used as a fiber interconnector that couples light from a small-core fiber into the several cores of a multicore fiber.

Large elliptical nanostructured gradient-index microlens

We demonstrate the feasibility of the development of a gradient-index elliptical microlens with a size of 75×125  μm using nanostructured glass technology. The gradient index is obtained by means of a discrete internal structure composed of two glasses with feature sizes much smaller than the wavelength of the incident light. A modified photonic crystal fiber-drawing technique is used for the lens fabrication. The elliptical shape of the lens is obtained by a novel final drawing stage where the spherically symmetric lens preform is drawn into an elliptical form by collapsing two large air holes placed in the preform during assembly. The effective focal lengths of 160 and 260 μm for the orthogonal axes are obtained experimentally for the fabricated lens, and show good agreement with those predicted by the effective medium theory and the full-wave beam propagation simulations.

Nanostructured gradient index microaxicons made by a modified stack and draw method

We report the design and fabrication of nanostructured gradient index microaxicons suitable for integration with optical fibers. A structure with the effective refractive index decreasing linearly from the center to the edges (i.e., an axicon) was designed using a combination of a simulated annealing method and the effective medium theory. The design was verified numerically with beam propagation method simulations. The axicons were made by the modified stack and draw method and integrated with optical fibers. The optical properties of the fabricated elements were measured and showed good agreement with the numerical simulations. The fabricated axicons produced an extended line focus at a distance from about 70 to 160 μm from the lens facet with a minimum FWHM diameter of 8 μm at 90 μm. At smaller distances, an interference pattern is observed both in the experiment and in simulations, which is attributed to the uneven effective refractive index profile at the structure.

Broadband infrared supercontinuum generation in hexagonal-lattice tellurite photonic crystal fiber with dispersion optimized for pumping near 1560 nm: reply

We respond to the comment submitted by Xian Feng on our recent Letter, Opt. Lett.38, 4679 (2013). The comment addressed the attenuation of our oxide tellurite glass labeled TWPN/I/6. We provide the originally measured absorbance spectrum of the glass and correct values of its mid-infrared attenuation.

Broadband infrared supercontinuum generation in hexagonal-lattice tellurite photonic crystal fiber with dispersion optimized for pumping near 1560 nm

We report on supercontinuum generation (SG) in a hexagonal lattice tellurite photonic crystal fiber (PCF). The fiber has a regular lattice with a lattice constant Λ = 2 μm, linear filling factor d/Λ = 0.75, and a solid core 2.7 μm in diameter. Dispersion, calculated from scanning electron microscope (SEM) image of drawn fiber, has zero dispersion wavelength (ZDW) at 1410 and 4236 nm with a maximum of 193 ps/nm/km at 2800 nm. Under pumping with 150 fs/36 nJ/1580 nm pulses, supercontinuum spectrum in a bandwidth from 800 nm to over 2500 nm was observed in a 2 cm long PCF sample, which is comparable to results reported for suspended core tellurite PCFs pumped at wavelengths over 1800 nm. Measured spectrum is analyzed numerically with good agreement, and observed spectral broadening is interpreted. To our best knowledge, tellurite glass, regular lattice PCFs for successful SG in this bandwidth have not been reported before.

Numerical characterization of an ultra-high NA coherent fiber bundle part II: point spread function analysis

Straightforward numerical integration of the Rayleigh-Sommerfeld diffraction integral (R-SDI) remains computationally challenging, even with today's computational resources. As such, approximating the R-SDI to decrease the computation time while maintaining a good accuracy is still a topic of interest. In this paper, we apply an approximation for the R-SDI that is to be used to propagate the field exiting a Coherent Fiber Bundle (CFB) with ultra-high numerical aperture (0.928) of which we presented the design and modal properties in previous work. Since our CFB has single-mode cores with a diameter (550 nm) smaller than the wavelength (850 nm) for which the CFB was designed, we approximate the highly divergent fundamental modes of the cores with real Dirac delta functions. We find that with this approximation we can strongly reduce the computation time of the R-SDI while maintaining a good agreement with the results of the full R-SDI. Using this approximation, we first determine the Point Spread Function (PSF) for an 'ideal' output field exiting the CFB (identical amplitudes for cores on a perfect hexagonal lattice with the phase of each core determined by the appropriate spherical and tilted plane wave front). Next, we analyze the PSF when amplitude or phase noise is superposed onto this 'ideal' field. We find that even in the presence of these types of noise, the effect on the central peak of PSF is limited. From these types of noise, phase noise is found to have the biggest impact on the PSF.

Numerical characterization of an ultra-high NA coherent fiber bundle part I: modal analysis

Advances in fiber optics and CCD technology in the last decades have allowed for a large reduction in outer diameter (from centimeters to submillimeter) of endoscopes. Attempts to reduce the outer diameter even further, however, have been hindered by the trade-off, inherent to conventional endoscopes, between outer diameter, resolution and field of view. Several groups have shown the feasibility of further miniaturization towards so called micro-endoscopes, albeit at the cost of a very reduced field of view. In previous work we presented the design of an ultra-high NA (0.928) Coherent FiberBundle (CFB) that, in combination with proximal wave front shaping, could be used to circumvent this trade-off thus paving the way for even smaller endoscopes. In this paper we analyze how the modal properties of such an ultra-high NA CFB determine the required input field to achieve any desired output field. We use the periodicity of the hexagonal lattice which characterizes a CFB, to define a unit cell of which we analyze the eigen-modes. During the modal analysis, we also take into account realistic variations in lattice constant, core size and core shape due to the limitations of the fabrication technology. Realistic values for these types of fabrication-induced irregularities were obtained via SEM images of a CFB fabricated according to the aforementioned design. The presence of these irregularities results, for a desired output, in the required input to be different from the required input for a defect-free CFB. We find that of the different types of fabrication-induced irregularities present in the CFB, variations in core ellipticity have the biggest impact on the required input for a given desired output.

Large diameter nanostructured gradient index lens

In this paper we report on the development and optical properties of nanostructured gradient index microlenses with good chromatic behavior. We introduce a new fabrication concept for the development of large diameter nanostructured gradient index microlenses based on quantized gradient index profiles and the use of nanostructured meta-rods. We show a dependence of the quality of performance on the number of refractive index levels and the lens diameter. Measurements carried out at 633 and 850 nm show good optical properties and similar focal lengths for both wavelengths.

Nanostructured elliptical gradient-index microlenses

We show theoretical and experimental characterizations of a nanostructured gradient-index lens. The elliptical lens is a nonguiding element fabricated using the mosaic method, which is widely used for the fabrication of photonic crystal fibers. For the first time we show experimental data in the optics regime that confirm the effective medium approximation for discrete mosaic structures with subwavelength feature size. This opens the door for the development of general asymmetric gradient-index materials.

Low-loss propagation and continuously tunable birefringence in high-index photonic crystal fibers filled with nematic liquid crystals

Experimental investigations of microstructured fibers filled with liquid crystals (LCs) have so far been performed only by using host fibers made of the silica glass. In this paper, the host photonic crystal fiber (PCF) was made of the PBG08 high-refractive index glass (approximately 1.95) that is much higher than silica glass index (approximately 1.46) and also higher then both ordinary and extraordinary refractive indices of the majority of LCs. As a result, low-loss and index-guiding propagation is observed regardless of the LC molecules orientation. Attenuation of the host PCF was measured to be approximately 0.15 dB/cm and for the PCF infiltrated with 5CB LC was slightly higher (approximately 0.19 dB/cm), resulting in a significant reduction to approximately 0.04 dB/cm of the scattering losses caused by the LC. Moreover, an external transverse electric field applied to the effective photonic liquid crystal fiber (PLCF) allowed for continuous phase birefringence tuning from 0 to 2.10(-4).

Design and fabrication of nano-structured gradient index microlenses

We present a novel fabrication technology for nano-structured graded index micro-optical components, based on the stack-and-draw method used for photonic crystal fibres. These discrete structures can be described with an effective refractive index distribution. Furthermore we present spherical nano-structured microlenses with a flat facet fabricated with this method and designed using an algorithm based on the Maxwell-Garnett mixing formula. Finally we show theoretical verification by using FDTD simulations for a nano-structured lens as well as experimental data obtained in the microwave regime.

Highly birefringent silicate glass photonic-crystal fiber with polarization-controlled frequency-shifted output: A promising fiber light source for nonlinear Raman microspectroscopy

A highly birefringent silicate glass photonic-crystal fiber (PCF) is employed for polarization-controlled nonlinear-optical frequency conversion of femtosecond Cr: forsterite laser pulses with a central wavelength of 1.24 mum to the 530--720-nm wavelength range through soliton dispersion-wave emission. The fiber exhibits a modal birefringence of 1.2.10(-3) at the wavelength of 1.24 mum due to a strong form anisotropy of its core, allowing polarization switching of the central wavelength of its blue-shifted output by 75 nm. Polarization properties and the beam quality of the blue-shifted PCF output are shown to be ideally suited for polarization-sensitive nonlinear Raman microspectroscopy.

Imaging phase objects with square-root, Foucault, and Hoffman real filters: a comparison

Methods of imaging phase objects are considered. First the square-root filter is inferred from a definition of fractional-order derivatives given in terms of the integration of a fractional order called the Riemann-Liouville integral. Then we present a comparison of the performance of three frequency-domain real filters: square root, Foucault, and Hoffman. The phase-object imaging method is useful as a phase-shift measurement technique under the condition that the output image intensity is a known function of object phase. For the square-root filter it is the first derivative of the object phase function. The Foucault filter, in spite of its position, gives output image intensities expressed by Hilbert transforms. The output image intensity obtained with the Hoffman filter is not expressed by an analytical formula. The performance of the filters in a 4f imaging system with coherent illumination is simulated by use of VirtualLab 1.0 software.

Mathematical morphology operations with a comparator array processor

We present a demonstrator photonic system for mathematical morphology image processing. Optically interconnected arrays of differential pairs of optoelectronic transceivers realize morphological erosion and dilation in an 8x8 pixel size image.

Cross-sensitivity effect in temperature-compensated sensors based on highly birefringent fibers

We analyzed the influence of the measurand-temperature cross-sensitivity effect on temperature stability in fiber-optic cross-spliced sensors that employ highly birefringent fibers. We show that the ratio of the measurand-temperature cross-sensitivity coefficient to the measurand first-order sensitivity determines the physical limit for temperature stability in cross-spliced sensors. Employing polarimetric as well as white-light interferometric methods, we experimentally determine a hydrostatic pressure-temperature cross-sensitivity coefficient in York bow-tie 800 fiber. From this we estimate the achievable limit for temperature stability of cross-spliced pressure sensors under environmental temperature changes.