59 research outputs found
Complete polarization control for a nanofiber waveguide using directional coupling
Optical nanofiber waveguides are widely used for near-field delivery and
measurement of light. Despite their versatility and efficiency, nanofibers have
a critical drawback - their inability to maintain light's polarization state on
propagation. Here, we design a directional coupler consisting of two crossed
nanofibers to probe the polarization state at the waist region. Directionality
of coupling occurs due to asymmetric dipolar emission or spin-locking when the
evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber
system. We demonstrate that, by monitoring the outputs from the directional
coupler, two non-orthogonal polarization states can be prepared at the
nanofiber waist with a fidelity higher than 99%. Based on these states, we
devise a simple and reliable method for complete control of the polarization
along a nanofiber waveguide.Comment: 8 pages, 8 figure
Bandpass transmission spectra of a whispering-gallery microcavity coupled to an ultrathin fiber
Tapered fibers with diameters ranging from 1-4 micron are widely used to
excite the whispering-gallery (WG) modes of microcavities. Typically, the
transmission spectrum of a WG cavity coupled to a waveguide around a resonance
assumes a Lorentzian dip morphology due to resonant absorption of the light
within the cavity. In this paper, we demonstrate that the transmission spectra
of a WG cavity coupled with an ultrathin fiber (500-700nm) may exhibit both
Lorentzian dips and peaks, depending on the gap between the fiber and the
microcavity. By considering the large scattering loss of off-resonant light
from the fiber within the coupling region, this phenomenon can be attributed to
partially resonant light bypassing the lossy scattering region via WG modes,
allowing it to be coupled both to and from the cavity, thence manifesting as
Lorentzian peaks within the transmission spectra, which implies the system
could be implemented within a bandpass filter framework.Comment: 4pages,6figure
All-Optical Nanopositioning of High-Q Silica Microspheres
A tunable, all-optical, coupling method has been realized for a
high-\textit{Q} silica microsphere and an optical waveguide. By means of a
novel optical nanopositioning method, induced thermal expansion of an
asymmetric microsphere stem for laser powers up to 171~mW has been observed and
used to fine tune the microsphere-waveguide coupling. Microcavity displacements
ranging from (0.612~~0.13) -- (1.5 0.13) m and nanometer scale
sensitivities varying from (2.81 0.08) -- (7.39 0.17) nm/mW, with
an apparent linear dependency of coupling distance on stem laser heating, were
obtained. Using this method, the coupling was altered such that different
coupling regimes could be explored for particular samples. This tunable
coupling method, in principle, could be incorporated into lab-on-a-chip
microresonator systems, photonic molecule systems, and other nanopositioning
frameworks.Comment: 6pages,4figure
Thermal noise reduction in soliton microcombs via laser self-cooling
Thermal noise usually dominates the low-frequency region of the optical phase noise of soliton microcombs, which leads to decoherence that limits many aspects of applications. In this work, we demonstrate a simple and reliable way to mitigate this noise by laser cooling with a pump laser. The key is rendering the pump laser to simultaneously excite two neighboring cavity modes from different families that are respectively red and blue detuned, one for soliton generation and the other for laser cooling
Polarisation control for optical nanofibres by imaging through a single lens
We present a simple method for controlling the polarisation state of light at
the waist of a single-mode optical nanofibre. The method consists of complete
polarisation compensation based on imaging scattered light from inherent
inhomogeneities both on the fibre surface and in the glass material itself. In
contrast to the recently reported protocol exploiting two imaging systems
oriented at 45 degrees to each other, our method requires only one lens and a
video camera. It is particularly useful for nanofibre-based applications with
severe geometric constraints, such as inside vacuum chambers for experiments
with cold atoms. The measured fidelity of the achieved control is about 98\%
using lenses with moderate numerical apertures
Polarization-Controlled Cavity Input-Output Relations
Cavity input-output relations (CIORs) describe a universal formalism relating
each of the far-field amplitudes outside the cavity to the internal cavity
fields. Conventionally, they are derived based on a weak-scattering
approximation. In this context, the amplitude of the off-resonant field remains
nearly unaffected by the cavity, with the high coupling efficiency into cavity
modes being attributed to destructive interference between the transmitted (or
reflected) field and the output field from the cavity. In this Letter, we show
that, in a whispering gallery resonator-waveguide coupled system, in the
strong-scattering regime, the off-resonant field approaches to zero, but more
than 90% coupling efficiency can still be achieved due to the Purcell-enhanced
channeling. As a result, the CIORs turn out to be essentially different than in
the weak-scattering regime. With this fact, we propose that the CIOR can be
tailored by controlling the scattering strength. This is experimentally
demonstrated by the transmission spectra exhibiting either bandstop or
bandpass-type behavior according to the polarization of the input light field.Comment: 6 pages, 4 figure
Integrated, Ultra-Compact High-Q Silicon Nitride Microresonators for Low-Repetition-Rate Soliton Microcombs
Multiple applications of relevance in photonics, such as spectrally efficient coherent communications, microwave synthesis or the calibration of astronomical spectrographs, would benefit from soliton microcombs operating at repetition rates <50GHz. However, attaining soliton microcombs with low repetition rates using photonic integration technologies represents a formidable challenge. Expanding the cavity volume results in a drop of intracavity intensity that can only be offset by an encompassing rise in quality factor. In addition, reducing the footprint of the microresonator on an integrated circuit requires race-track designs that typically result into modal coupling losses and disruptions into the dispersion, preventing the generation of the dissipative single soliton state. Here, we report the generation of sub-50GHz soliton microcombs in dispersion-engineered silicon nitride microresonators. In contrast to other approaches, the authors\u27 devices feature an optimized racetrack design that minimizes the coupling to higher-order modes and reduces the footprint size by an order of magnitude to approximate to 1mm(2). The statistical intrinsic Q reaches 19 million, and soliton microcombs at 20.5 and 14.0 GHz repetition rates are successfully generated. Importantly, the fabrication process is entirely subtractive, meaning that the devices can be directly patterned on the silicon nitride film
Enhanced Directional Coupling of Light with a Whispering Gallery Microcavity
Directional coupling of light in nanophotonic circuits has recently attracted increasing interest, with numerous experimental realizations based on broken rotational or mirror symmetries of the light–matter system. The most prominent underlying effect is the spin–orbit interaction of light in subwavelength structures. Unfortunately, coupling of light to such structures is, in general, very inefficient. In this work, we experimentally demonstrate an order of magnitude enhancement of the directional coupling between two nanowaveguides by means of a whispering gallery microcavity. We also show that both transverse magnetic and transverse electric modes can be used for the enhancement
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