Commonly, the spectra of high Q-factor microresonators are fixed or only weakly tunable, which limits their versatility. To address this limitation, we demonstrate continuous tunability of the axial Free Spectral Range (FSR) of parabolic microresonators created by bending a 125 μm radius optical fiber segment. By controlling the bent fiber profile with linear stages affixed to its ends, we vary its FSR between 1.9 pm and 2.7 pm for more than 65 equally spaced eigenmodes. We show that the FSR tunability can be achieved with precision better than 0.2 pm. The demonstrated tunability, together with the inherently small FSR of our parabolic microresonators, unlock their potential applications including optical frequency comb generation and frequency conversion.
We show, both theoretically and experimentally, that it is possible to determine a nonuniform temperature distribution along a SNAP microresonator from a single measurement of its spectrum. In our experiment, we use a silica microcapillary containing a SNAP microresonator. The microcapillary is filled with water and locally heated with a moving heating source (light-pumped microfiber) introducing the temperature distribution parameterized as T(z)=T_0 exp(-|z-z_Q+iw|/L), where z is the coordinate along the microcapillary axis, z_Q is the heating source position, and w≪L is the width of the source. At each heating source position z_Q, we restore the parameters of this distribution from the SNAP microresonator spectrum. Our theoretical calculations are in a good agreement with the experimental data.
In this work we present a novel technique to fabricate whispering gallery modes microresonators on the regular optical fiber surface. We use a high-resistance wire heated by constant electrical current up to temperatures of 1100 °C. Due to higher temperature stability, high reproducibility of the microcavities shaping is ensured. Our method makes it possible to reduce the cost and simplify the production of WGM microresonators on the optical fiber surface.
While the optical displacement measurement methods have achieved the picometer precision, the latter is considered elusive for the fabrication methods. The Surface Nanoscale Axial Photonics (SNAP) platform allows to fabricate ultralow loss optical microresonators and resonant circuits at the surface of an optical fiber with the remarkable subangstrom precision. Here, we describe recently proposed SNAP optical signal processors, frequency comb generators, microwave photonic filters, and optical sensors with important potential applications which successful realization requires the picometer fabrication precision. We discuss the developed and potential approaches towards the achievement of this challenging goal.
Lumped-elements models are commonly used for the analysis of optical frequency combs (OFCs) generated by the parametric modulation of optical microresonators. However, these models do not take into account the spatial modulation distribution (SMD) which is critical for the optimization of the OFCs formation. Here, we consider a parabolic SNAP microresonator (SMR) with harmonically modulated parameters and determine the optimum SMDs for the resonant and adiabatic excitation of the SMR parameters. We suggest that the determined optimal SMD can be experimentally realized using piezoelectric, radiation pressure or linear electro-optical excitation of an SMR.
We present an approach to translate and reconfigure SNAP microresonators (SMRs) introduced along a thin-walled hollow silica microcapillary fiber (MCF). First, we demonstrate formation and translation of a train of multiple SMRs induced by a periodic sequence of droplets inside an MCF controlled by air pressure. Next, we reconfigure a permanent SMR introduced at the MCF by a sequence of droplet-induced SMRs controllably translated along the MCF. We believe that the developed approach can become a promising technology for fabrication of reconfigurable low-loss resonant photonic microdevices for optical signal processing applications.
Recent progress in the development of the ultraprecise SNAP platform was recorded at SPIE Photonics West held in San Francisco, California, United States 2022.
The new manufacturing method for fabrication of Surface Nanoscale Axial Photonics (SNAP) structures has been developed. We showed experimentally that the bent fiber can achieve the nanometer-scale variation in the effective fiber radius sufficient for fabrication of SNAP microresonators. The advantage of the demonstrated method is in its simplicity, robustness, and mechanical tunability of the fabricated devices.
We present a novel method based on optical fibre tapering for fabrication of Surface Nanoscale Axial Photonics (SNAP) devices with parabolic profiles with an unprecedentedly large number of axial eigenmodes. Tapering of a commercial 125 μm single-mode optical fibre to a 30 μm diameter waist by laser brushing creates a SNAP bottle microresonator with parabolic radius variation in the centre of the tapered region. Ideal parabolic resonators should demonstrate equal spacing between resonances. Our spectral measurement of the parabolic profile shows spacing of ~6 GHz with 10% deviation over a bandwidth of 2.5 THz containing up to 400 axial eigenfrequencies. This new discovery for the creation of SNAP parabolic microresonator devices is important for fabrication of miniature delay lines, buffers and frequency comb generators. Characterisation of our exemplar microresonators is briefly explored, particularly for broadband frequency comb generators which require equidistant frequency spacing. Further investigations include scaling of the parabolic feature with tapering process parameters, repeatability testing, and the fabrication of more complex shapes.
This Conference Presentation, “Sub-angstrom precise resonant nanophotonic devices at the optical fiber surface,” was recorded at SPIE Photonics West 2018 held in San Francisco, California, United States.
The propagation of whispering gallery modes along an optical fiber is fully controlled by nanoscale variation of the effective fiber radius. In the present work we demonstrate the possibility of the creation, tuning, translation and annihilation of arbitrary-shaped transient photonic elements, such as miniature slow light delay lines, dispersion compensators and dispersionless optical buffers, at the surface of an initially regular optical fiber. This is achieved by means of local heating of the fiber with low-power focused CO2 laser radiation, which introduces nanoscale change to the effective radius of the fiber because of thermo-refractive coupling and thermal expansion. The CO2 laser beam is swept along the fiber, with its position and intensity programmably controlled by an acousto-optical deflector, so that the shape and the settling speed for these structures are constrained only by thermal relaxation processes inside the irradiated fiber. Possible realization of a similar technique on a chip, with laser beam heating substituted by on-chip DC heaters, is analysed. The potential application of this method to the on-the-fly fine tuning of the shape of pre-created Surface Nanoscale Axial Photonics (SNAP) elements, particularly providing gates for switching on and off coupling of optical delay elements to a photonic circuit, is also discussed.
Recently introduced Surface Nanoscale Axial Photonics (SNAP) is based on whispering gallery modes circulating around the optical fiber surface and undergoing slow axial propagation. In this paper we develop the theory of propagation of whispering gallery modes in a SNAP microresonator, which is formed by nanoscale asymmetric perturbation of the fiber translation symmetry and called here a nanobump microresonator. The considered modes are localized near a closed stable geodesic situated at the fiber surface. A simple condition for the stability of this geodesic corresponding to the appearance of a high Q-factor nanobump microresonator is found. The results obtained are important for engineering of SNAP devices and structures.
We investigate high-Q microsphere resonators with whispering gallery modes using a tapered optical microfiber
immersed in a liquid inside a microfluidic platform. The strength of the coupling between the cavity and the
microfiber taper is shown to depend on the contact position of the microsphere along the taper and on the
refractive index contrast between the microsphere and the liquid environment. We demonstrate that barium
titanate glass beads with index around 1.9 are promising candidates for developing sensor and optomechanical
applications of such resonator systems.
Recent progress in the understanding of the transmission properties of optical microfibers and their applications in
photonics are reviewed. An optical microfiber (MF) is usually fabricated from a standard telecom optical fiber by
drawing and has a diameter of ~ 1 micron. The interest in photonic devices fabricated of MFs is basically caused by two
advantages of MFs compared to lithographically-fabricated waveguides: significantly smaller losses for a given index
contrast and the potential ability of micro-assemblage in 3D. Eventually, these properties could make possible the
creation of MF devices, which are significantly more compact than those fabricated lithographically. Furthermore, some
MF-based devices possess functionalities, which are not possible or much harder to achieve by other means. The first
part of this paper discusses methods of fabrication and transmission properties of MFs. The effects of microdeformations
(in particular, the frozen-in microdeformations) and the adiabatically slow deformations of an MF are considered. The
recently developed theory of adiabatic MF tapers is presented and applied to the investigation of transmission loss and
evanescent field structure of MF tapers. The second part of the paper considers applications of MFs in photonics.
Generally, MF devices and circuits can be created by the macro- and micromanipulation (bending, looping, coiling,
twisting, crossing, etc.) of uniform and tapered MFs. The most straightforward application of an MF is using a regular
MF taper as a sensor of an ambient medium. More advanced applications of MFs include MF loop resonators and MF
circuits assembled by wrapping an MF around a cylindrical rod. It is believed that further exploration of MF properties
and possible applications will give rise to the invention and practical realization of innovative MF photonic devices with
unique functionalities.
KEYWORDS: Sensors, Geometrical optics, Near field optics, Optical fibers, Resonators, Nanofibers, Chemical elements, Near field scanning optical microscopy, Refractive index, Optical sensors
An optical microfiber is usually fabricated by drawing a conventional optical fiber and has a diameter of ~ 1 &mgr;m. A
microfiber with a diameter significantly less than one micron is often called a nanofiber. This paper considers the
potential applications of optical microfibers/nanofibers as sensors, which detect changes in the ambient medium by
monitoring changes in the transmission power of light propagating along the MF. These changes may be caused by
variation in temperature, radiation, concentration of chemical or biological species, microparticles, etc.
An optical microfiber with the diameter significantly less than the radiation wavelength ~ 1 micron is often called a
nanofiber (NF). The fundamental mode of a NF consists primarily of an evanescent field propagating in the ambient
medium outside a NF. Any deformation of a NF changes the evanescent field. If a NF is thin enough, even a very small
deformation may cause dramatic changes of the evanescent field structure. The simplest types of deformation of a
uniform NF are bending and tapering. The structure of evanescent field and optics of radiation loss in bent uniform
microfibers is understood quite well. It is determined by an effective potential barrier terminated by a caustic surface,
which separates the tunneling and classically allowed regions. Tunneling through the barrier determines the radiation
loss. Alternatively, for an adiabatically tapered microfiber, called a nanotaper (NT), a similar potential barrier of finite
width cannot be introduced. Instead, the radiation loss in a NT takes place in small neighborhood of focal circumferences
of the evanescent field, while a NT is lossless elsewhere. More specifically, for a NT, the mentioned caustic surface
becomes complex and can intersect real space along certain lines only. These lines are the focal circumferences where
the radiating modes and the guiding mode are split off. As examples, conical and biconical NTs with characteristic
shapes are considered. The theoretically predicted interference between the guiding and radiating components of the
evanescent field are confirmed by the beam propagation method (BPM) numerical modeling. The derived analytical
expressions for radiation loss are in a good agreement with BPM calculations. Finally, a simple estimate formula for
radiation loss of a NT is suggested.
The recently suggested self-coupling microfiber coil optical resonator (COR) is a simplest functional element for the future microfiber-based photonics. It could be created by wrapping a microfiber on a dielectric rod with smaller refractive index. It is feasible that COR, which is produced from a drawn optical microfiber, will not suffer from surface roughness as e.g. the lithographically fabricated 2D microrings. Therefore, COR may have extremely small losses and generate high-Q resonances. In this paper, the theoretical study of the basic electromagnetic properties of the uniform self-coupling COR with N turns is presented. The eigenmodes of COR, which are qualitatively different from the modes of the known types of resonators, exist for the discrete values of the dimensionless coupling parameter K=κS 〉 ½, where κ is the coupling coefficient between adjacent turns and S is the length of a turn. The spatial variation of the mode amplitudes does not have the wavelength scale oscillations and has no correlation with the period of COR, S. For certain series of K, the free spectrum range of COR is independent of the number of turns N and COR behaves similar to a single ring resonator. For N→∞, the microfiber coil optical waveguide (COW) has a simple dispersion relation implying the absence of stop bands. The value K = ½ corresponds to the crossover between two regimes of propagation: with and without zeroing of the group velocity. At the crossover, the dispersion relation of COW has inflexion points wherein the group velocity and the inversed group velocity dispersion simultaneously become zero.
The fiber grating fabrication based on use of the phase masks is the most stable and accurate manufacturing technology. This paper presents a brief overview of holographic methods of phase masks and fiber Bragg gratings (FBGs) writing and characterization with emphasis on the chirped gratings. We discuss the range of FBG parameters enabled by current technological methods, as well as the relation between the accuracy of FBG parameters and the performance of FBG-based dispersion compensators. While holographic phase mask and FBG writing principles have much in common, the phase mask and FBG production is a unified technology where the quality of the FBG is determined by numerous factors in the process of fabrication. As one of the significant factors, we study the effect of mirror non-flatness on the group delay ripple of chirped FBG. The quality of phase masks and FBGs is often important to characterize directly. In this paper we consider holographic side-diffraction methods of their characterization, which are very accurate and provide the information that is not simple to obtain from spectroscopic measurements.
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