The integration of conventional silica optical fibers with optoelectronic materials and electrical structures opens the door to miniaturized and multifunctional all-fiber optoelectronic systems with ...actively controllable capabilities. The flat endface of an optical fiber tip offers a naturally light-coupling microscopic platform for optoelectronic integration using optimized micro- and nano-fabrication techniques. In recent decades, the increasing demand for active fiber devices has created opportunities to develop various optoelectronic structures on fiber tips, from integrating high-performance single modules for specific applications to integrating multi-function multiple modules for more complex applications. In this paper, we review the rapid development of fiber-compatible fabrication technologies for building optoelectrical circuits on fiber endfaces. Then, we discuss the fast-progressing field of active fiber tips along with its wide applications, including photodetection, modulation, lasing, sensing, and light-emitting. Finally, we discuss challenges and opportunities for future development in this field. We believe that this scheme of active fiber tips will pave the way for next-generation ultracompact optical fiber systems with diverse optical and optoelectronic applications.
Electrostriction in an optical fiber is introduced by interaction between the forward propagated optical signal and the acoustic standing waves in the radial direction resonating between the center ...of the core and the cladding circumference of the fiber. The response of electrostriction is dependent on fiber parameters, especially the mode field radius. We demonstrated a novel technique that can be used to characterize fiber properties by means of measuring their electrostriction response under intensity modulation. As the spectral envelope of electrostriction-induced propagation loss is anti-symmetrical, the signal to noise ratio can be significantly increased by subtracting the measured spectrum from its complex conjugate. We show that if the transversal field distribution of the fiber propagation mode is Gaussian, the envelope of the electrostriction-induced loss spectrum closely follows a Maxwellian distribution whose shape can be specified by a single parameter determined by the mode field radius.
Fusion splicing of solid-core microstructured silica fibers has been one of the key enablers which opened practical applications of these structures in ultrafast light sources or fiber-based sensors. ...Anti-resonant hollow core fibers are special in this context, because their optical properties critically depend on single-micron or sub-micron thin wall membranes. This corresponds to high thermal isolation of their cladding structure and makes thermal processing of these fibers challenging. We investigate fusion splicing feasibility of a single capillary ring anti-resonant hollow core fiber made of silica glass. We begin by splicing pairs consisting of standard single mode and hollow core fibers, followed by pairs of polarization maintaining and hollow core fibers. Splice loss is determined within the 1450-2000 nm transmission window of the fiber at around 1-2 dB, depending on wavelength along the transmission window, in the transmission direction from the step index fiber into the hollow core fiber. Maintenance of polarization is verified and polarization extinction ratio of no less than 10 dB is recorded for the ARF spliced with a polarization maintaining fiber. Mechanical robustness of the fabricated splices is verified with standard pull tests returning damage thresholds of 140 g (32 kpsi) and 100 g (24 kpsi) for hollow core fibers spliced to single mode and polarization maintaining fibers, respectively.
We report on a new phenomenon of light guidance in a fiber core created by an arrangement of holes making a partially open ring. In such structure there is no complete refractive index barrier to ...confine the light and therefore it cannot guide light if untwisted. However, if the open ring of holes is shifted off the symmetry axis of the twisted fiber then the mode confinement and low loss propagation is possible due to purely geometrical effect related to the increase in the optical path of light following the helical route. Properties of such structures, including confinement loss, modal field distribution, birefringence of fundamental modes were investigated both numerically and experimentally. We also studied the effect of bending, which leads to periodic modulation of the propagation characteristics of the twisted fiber. Furthermore, we demonstrate a possibility of displacement measurements based on bend-induced loss using some of the fabricated fibers, in which the sensitivity to bend is controlled by a twist rate and core structure.
In this paper, we describe recent progress in space-division multiplexed (SDM) transmission, and our proposal and demonstration of dense space-division multiplexing (DSDM), which offers the ...possibility of ultra-high capacity SDM transmission systems with high spatial density and spatial channel count of over 30 per fiber. We introduce the SDM transmission matrix, which cross indexes the various types of multi-core multi-mode transmissions according to the type of light propagation in optical fibers and how the spatial channels are handled in the network. For each category in the matrix, we present the latest advances in transmission studies, and evaluate their transmission performance by spectral and spatial efficiencies. We also expound on technologies for multi-core and/or multi-mode transmission including optical fiber, signal processing, spatial multi/demultiplexer, and amplifier, which will play key roles in configuring DSDM transmission systems, and review the first DSDM transmission experiment over a 12 core × 3 mode fiber.
We demonstrate birefringence enhancement in an optical fiber with artificial core anisotropy. A nanostructured fiber core in the form of a glass slab with layers of germanium-doped silica interleaved ...with the layers of fluorine-doped silica enabled an increase of the birefringence of 0.54×10 -4 without any stress zones. The experimental result confirms the numerical prediction of birefringence enhancement over previously presented fibers by increasing the refractive index contrast between the two core base glasses. Birefringence improvement has been achieved only by core material engineering and not by adding any stress zones or other approaches involving cladding shaping and core geometry design. The modification introduced to core had no impact on the guiding performance of the realized fiber, which remained perfectly matched to the SMF-28 standard in terms of its numerical aperture NA = 0.11 and the effective mode-field diameter MFD = 10.1 μm at the wavelength of 1550 nm.
We propose an optical coupler insertion technique based on fiber side-polishing that does not interrupt service. We clarify that fiber side-polishing with an insertion loss of 0.64 dB or less is ...possible by polishing while monitoring the loss and halting the polishing when the loss reaches the target value. Experiments on fabricated couplers confirm coupling efficiencies of up to 95% when another side-polished fiber is set close to the polished in-service fiber.
The radiation-induced attenuation (RIA) levels and kinetics depend on several parameters, including silica-based optical fiber composition, operation wavelengths, and preirradiation treatments. We ...studied in this work how the <inline-formula> <tex-math notation="LaTeX">{X} </tex-math></inline-formula>-ray RIA in the visible and near-infrared domains, up to a total ionizing dose (TID) of 100 kGy(SiO2), of a standard telecom Ge-doped fiber, changes when this optical fiber is subjected to different preirradiation treatments such as an H2-loading, with or without an additional thermal treatment at 50 °C. The performed systematic study, at room temperature (RT), allows to better characterize the influence of the presence of either molecular H2 or bonded atomic hydrogen on the generation and bleaching mechanisms of radiation-induced point defects. Our results show that if the hydrogen can passivate point defects at lower doses (< a few kGy), it globally negatively impacts the visible and near-IR RIA at doses larger than a few kGy. This hardening solution can then not be employed for this class of optical fibers (OFs).
Multiple cladding modes can exist in a small-core optical fiber unaccompanied by core modes, yet this fact has not been sufficiently explored in the literature to date. In this paper, we study the ...self-imaging of cladding modes in small-core optical fiber interferometers. Our analytical and numerical simulations and experiments show that unlike the self-imaging of core modes, self-imaging of cladding modes only appears at a set of discrete positions along the interferometer axis with an equal spacing corresponding to some discrete values of fiber core radius. This is the first observation of the discrete self-imaging effect in multimode waveguides. More strikingly, the self-imaging period of cladding modes grows exponentially with fiber core radius, unlike the quadratic relationship in the case of core modes. The findings bring new insights into the mode propagation in an optical fiber with a core at micro/nanoscale, which may open new avenues for exploring multimode fiber technologies in both linear and nonlinear optics.
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