Optical microfibers and nanofibers: A tutorial

Abstract As a combination of fiber optics and nanotechnology, optical microfibers and nanofibers (MNFs) have been emerging as a novel platform for exploring fiber-optic technology on the micro/nanoscale. Typically, MNFs taper drawn from glass optical fibers or bulk glasses show excellent surface smoothness, high homogeneity in diameter and integrity, which bestows these tiny optical fibers with low waveguiding losses and outstanding mechanical properties. Benefitting from their wavelength- or sub-wavelength-scale transverse dimensions, waveguiding MNFs exhibit a number of interesting properties, including tight optical confinement, strong evanescent fields, evident surface field enhancement and large and abnormal waveguide dispersion, which makes them ideal nanowaveguides for coherently manipulating light, and connecting fiber optics with near-field optics, nonlinear optics, plasmonics, quantum optics and optomechanics on the wavelength- or sub-wavelength scale. Based on optical MNFs, a variety of technological applications, ranging from passive micro-couplers and resonators, to active devices such as lasers and optical sensors, have been reported in recent years. This review is intended to provide an up-to-date introduction to the fabrication, characterization and applications of optical MNFs, with emphasis on recent progress in our research group. Starting from a brief introduction of fabrication techniques for physical drawing glass MNFs in Section 2, we summarize MNF optics including waveguiding modes, evanescent coupling, and bending loss of MNFs in Section 3. In Section 4, starting from a “MNF tree” that summarizes the applications of MNFs into 5 categories (waveguide & near field optics, nonlinear optics, plasmonics, quantum & atom optics, optomechanics), we go to details of typical technological applications of MNFs, including optical couplers, interferometers, gratings, resonators, lasers and sensors. Finally in Section 5 we present a brief summary of optical MNFs regarding their current challenges and future opportunities.

Optical microfibers and nanofibers

Bending losses of nanofibers or nanowires with circular 90 degrees bends are simulated using a three-dimensional finite-difference time-domain (3D-FDTD) method. Dependences of bending losses on wavelength and polarization of guided light are investigated, as well as the diameters, refractive indices, and bending radii of nanowires. The acceptable bending losses (approximately 1 dB/90 degrees) predicted in glass, polymer, and semiconductor nanowires with bending radii down to micrometer level may offer valuable references for assembling highly compact photonic integrated circuits or devices with optical nanowires.

Modeling bending losses of optical nanofibers or nanowires

Field-flattening strands may be added to and arbitrarily positioned within a field-flattening shell to create a waveguide that supports a patterned, flattened mode. Patterning does not alter the effective index or flattened nature of the mode, but does alter the characteristics of other modes. Compared to a telecom fiber, a hexagonal pattern of strands allows for a three-fold increase in the flattened mode's area without reducing the separation between its effective index and that of its bend-coupled mode. Hexagonal strand and shell elements prove to be a reasonable approximation, and, thus, to be of practical benefit vis-a-vis fabrication, to those of circular cross section. Patterned flattened modes offer a new and valuable path to power scaling.

Waveguides having patterned, flattened modes

IO N A general tendency in the development of nanotechnology is the miniaturization of electronic and photonic devices. Small-scale light emitters, such as nanolasers, are an important part of integrated nanophotonic devices. Thus, they have been given much research attention. [ 1 ] Currently, there are two independent techniques for the fabrication of nanolasers. One is based on lasing semiconductor nanowires where the nanowire acts as both the emitter and the cavity resonator. [ 2 ] Another is based on a lasing plasmon-assisted semiconductor nanowire, in which the light originates from the nanowire. In this confi guration, the optical modes are confi ned to the metal/semiconductor interface region, which acts as cavity resonator. [ 3 ] Based on the waveguide geometry at the nanometer scale, [ 4,5 ] the large discontinuity of the electric fi eld at the high-index-contrast interfaces can confi ne and enhance the intensity of the light fi eld in the low-index material when light is guided by total internal refl ection. Accordingly, we propose a facile and effective route for the fabrication of small-scale light emitters, which could serve as nanoscopic light sources. The luminescence emission from the nanocolumn is confi ned in the hollow region when a small hole is drilled in a luminescent semiconductor nanocolumn. Meanwhile, the size of the luminescent region of the nanocolumn is greatly reduced from the total nanocolumn to the small hole. Therefore, we can fabricate a smaller light source than would be possible using the nanocolumn alone via this unique strategy. In this paper, using the hollow ZnO nanocolumn as an example, we establish a theoretical model to address the basic physics involved in transport in a hollow luminescent semiconductor nanocolumn. Moreover, we demonstrate that by using such a nanostructure, the luminescence emission originating from the hollow ZnO nanocolumn can be confi ned to a low-index region (hollow region). This intensity is much higher than that which can be achieved at the edge of the ZnO nanocolumn. We then perform experiments to confi rm the validity of these theoretical results by measuring the cathodoluminescence (CL) spectra and images of an individual hollow hexagonal ZnO nanocolumn using scanning electron microscopy (SEM). Finally, computer simulations based on the fi nite element method corroborate our theoretical results and experimental fi ndings.

General Strategy for Nanoscopic Light Source Fabrication

We have investigated the basic properties of subwavelength-diameter hollow optical fiber with exact solutions of Maxwell’s equations. The characteristics of modal field and waveguide dispersion have been studied. It shows that the subwavelength-diameter hollow optical fibers have interesting properties, such as enhanced evanescent field, local enhanced intensity in the hollow core and large waveguide dispersion that are very promising for many miniaturized high performance and novel photonic devices.

Field and dispersion properties of subwavelength-diameter hollow optical fiber.

A refractive index profile design, which can enable us to obtain a flat-top fundamental mode field around the center, is presented. The theoretical method to analyze the four-layer large flattened mode (LFM) fibers is given. The properties of the fiber, including the fundamental and higher order modal fields, effective area, and bending loss, are discussed. The reasons forming the different modal fields are explained. By discussion, the larger effective area of the LFM fiber compared with conventional step-index fiber is proven and the feasibility to suppress the higher order modes via bending is given.

Design guidelines and characteristics of a four-layer large flattened mode fiber

A theoretical method to analyze four-layer large flattened mode (LFM) fibers is presented. The influence of the second cladding on the properties of four-layer LFM fiber, including the fundamental and higher-order modal fields, effective area, bending loss, and dispersion, are studied by comparison. At the same time, the reasons for the different characteristics are considered. The obtained results indicate that the effective area of the four-layer LFM fiber is about 1.6 times larger than that of the conventional standard step-index fiber and the fibers have better bend-induced filtering ability than three-layer LFM fibers. (C) 2007 Society of Photo-Optical Instrumentation Engineers.

Influence of the second cladding on the properties of four-layer large flattened mode fibers

We propose a kind of photonic crystal fiber with flat-top fundamental mode by introducing a depressed central dip into
the core of the conventional index-guiding photonic crystal fiber. The design guidelines and characteristics of the large
flattened mode photonic crystal fiber (LFM-PCF) are discussed in detail. By appropriate design, the effective area of the
LFM-PCF can be increased by a factor of greater than 2 as compared with conventional index-guiding photonic crystal
fiber with the same hole and pitch parameters. The improved effective area, single mode operation and flat-top
fundamental mode output make LFM-PCF an ideal candidate to realize high power, high beam quality fiber amplifiers
and lasers.

Guanghui Chen

Papers

Field and dispersion properties of subwavelength-diameter hollow optical fiber.

Design guidelines and characteristics of a four-layer large flattened mode fiber

Influence of the second cladding on the properties of four-layer large flattened mode fibers

Design guidelines and characteristics of large flattened mode photonic crystal fibers