The flexible internal structures of suspended-core photonic microcells (PMCs) enable the possibility of building novel optical fiber tip structures for sensor and device applications. The tip-style structures typically possess a sensitive micro-/nano-wire core protected in the fiber jacket tube, while connected to conventional single mode fiber with low-loss transition and to the environment through openings at the fiber tip. Liquid or gas can flow/diffuse into and interactive with the whole core region of the PMC through the openings. As examples, we will report two types of the tip sensors based on the suspended-core PMCs with, respectively, triangular- and rhombus-like core structures, and introduce the optimization and experiments of the tip structures for fluidic refractive index sensing. Compare to some current fiber tip sensors, the tip sensors based on the PMCs exhibit the advantages of compact, high sensitivity, low optical loss, good structural flexibility and resistance to environmental contamination et al, hence would be reliable light-matter interaction platforms for fiber-integrated optofluidic applications. The potentials and the future trends of the PMC-based tip sensors will also be discussed.

Knowing the exact length of long optical fiber is of great importance to interferometry-based optical sensors, fiber-connected antenna arrays and other microwave photonic applications. This paper reviews recent efforts on long optical fiber length measurement with sub-millimeter resolution, including optical time domain reflectometers, optical frequency domain reflectometers, and optical backscatter reflectometers based on microwave frequency sweeping. Techniques to improve the measurement resolution and to reduce the measurement error are discussed.

We demonstrated the fabrication of long-period fiber gratings (LPFGs) in the few-mode fibers (FMFs) using focused carbon dioxide laser. The mode coupling and characteristics of the FM-LPFGs were investigated experimentally. The mode conversion between the fundamental core mode and high order core modes can be achieved by the gratings written in the FMFs. The FM-LPFGs could have promising application as all fiber mode converters for mode-division-multiplexing optical communications.

Simultaneous spectral unmixing of excitation and emission spectra (ExEm unmixing) has inherent ability to resolve spectral crosstalks, two key issues of quantitative fluorescence resonance energy transfer (FRET) measurement, of both the excitation and emission spectra between donor and acceptor without additional corrections. We recently developed a spectral wide-field microscope by integrating a liquid crystal tunable filter (LCTF) into a wide-field microscope for microscopic spectral imaging for implementing ExEm unmixing-based quantitative FRET measurement (ExEm-spFRET) in single living cells. However, the very low transmittance of LCTF, about 20% in wavelengths range from 450 to 500 nm, made this system inapplicable to single living cells with low expression levels of FPs, and the measured E values by using ExEm-spFRET method on this system were slightly larger than those measured by other’s methods. Considering the high transmittance up to 95% of optical filters, we here set up a filter-based multi-channel wide-field microscope by integrating an emission-wheel containing six different emission filters with a wide-field microscope for quantitative ExEm-spFRET measurement. Furthermore, a system correction factor (f_sc), a constant for a stable instrument, is introduced for modified ExEm-spFRET (m-ExEm-spFRET). We performed m-ExEm-spFRET with four and two excitation wavelengths respectively on our multi-channel wide-field microscope to quantitatively image single living cells expressing FRET constructs, and obtained accurate FRET efficiency (E) and concentration ratio (R_C) of acceptor to donor. We also performed m-ExEm-spFRET microscopic imaging for single living HepG2 cells co-expressing CFP-Bax and YFP-Bax, and found that the E values between CFP and YFP were about 0 for control cells and about 28% for staurosporin-treated cells when R_C were larger than 1, indicating that staurosporin induced significant oligomerization.

Vital signs (including breath) monitoring is a key tool in healthcare. Current monitors need invasive electronic sensors attached to user’s body, which is inconvenient and uncomfortable. We demonstrate photonic smart breath monitoring system based on two types of phase-sensitive optical fiber interferometers: traditional fiber Mach-Zehnder interferometer (MZI) and miniaturized fiber modal interferometer (MMI). Users simply lie/sleep on a sensor mat embedded with optical fiber sensor.  Breath will introduce slight strain changes on the mat and affect the light propagating within the fiber.  Breathing waveforms can be achieved by analyzing the output light with signal processing.  The system can collect user’s signals continuously and remotely to provide big data for health analysis. Our technique is non-invasive, highly sensitive, and immune to electromagnetic interference. The author would like to thank the support of HKPU 1-ZVHA.

Microtubular cavities were fabricated by releasing prestrained nanomembranes via rolled-up nanotech, which naturally provide built-in microfluidic channels for optofluidic applications. In such structure, light can be confined and propagate along the rolled-up dielectric nanomembranes, allowing for efficient interactions with the surrounding media. By coating a noble metal layer onto the microtube surface, hybrid photon-plasmon modes were generated due to the coupling of resonant light and surface plasmons, which leads to an intense surface evanescent field for enhanced light-matter interactions. By trapping a microsphere into the fluidic channel of microtubes, a novel type of photonic molecule was fabricated for the study of resonant mode coupling and tuning. In addition, microtube cavities were monolithically integrated on photonic chips to demonstrate optofluidic functionality, which are well-suited for potential biological/chemical sensing and analysis in a lab-in-a-tube system. As a novel platform, our microtubular cavities imply promising applications for enhanced light-matter interactions, optical tuning, photonic chip integration, and optofluidics. These works were carried out with the efforts of Yin Yin, Jiawei Wang, Abbas Madani, Vladimir Bolaños, Stefan Harazim, Libo Ma, and Oliver G. Schmidt in Leibniz IFW Dresden. 

Optofluidics has recently become a new active research area, which combines the discipline of optics with that of microfluidics and is an exciting platform for exploiting the optical properties of fluids in optics and photonics. On the other hand, magnetic fluid (or ferrofluid) is a kind of attractive fluid materials that possesses several unique magneto-optical properties, such as tunable refractive index, magneto-volume variation and so on. Due to the fluidity of magnetic fluids/ferrofluids, they are easy to be integrated with fibers or infiltrated into microholes/microcavities. Microfiber possesses the characteristic of low dimension and large evanescent field assigned to the unique geometry. Photonic crystal microcavity is a good candidate for sensing because of its high quality factor. Therefore, combining magnetic fluids/ferrofluids with microfiber or photonic crystal structures is promising for realizing novel optofluidic magnetic field sensor. In this talk, we will present the magnetic field sensing with optofluidic techniques. The magnetic fluids/ferrofluids are used as the magnetic field sensitive fluids. The sensing structures include microfiber, microfiber coupler, microfiber Sagnac loop, microfiber knot resonator and photonic crystal microcavity.

This work reports one interesting optical microcavity: self-rolled up optical microcavity with tubular geometry. Several different fabrication processes for the microtubular optical cavities had been reported [1]. Different from transitional fiber-drawing technique [2], the nanomembrane with pre-defined geometries can be bend into a curved structure and forms a three dimensional (3D) tubular structure by predefined strain-engineering via lift-off technology [3, 4]. Our group focus on the research on the optical properties of these self-rolled up optical microcavities [5, 6].

A schematic view of the self-rolled up processes is illustrated in Figure 1 [1]. The nanomembranes released via the lift-off processes from the sacrificial layer on substrate. Then, the freestanding nanomembranes self-rolled up into microtubes and performed as optical microcavities. The typical micro-photoluminescence (μ-PL) spectra of these rolled-up microtubes in various color regions are shown in Figure 2 [5]. Various interesting materials (Figure 2) were used for the fabrication of self-rolled up tubular optical microcavities in difference spectral range [5]. As shown in Figure 3, the μ-PL spectra measured at different positions along the micro-tubes by rolling circular nanomembranes indicates an evident 3D optical confinement. The WGMs shift to lower wavelengths when moving from the middle to the end of the micro-tubular cavity along the z direction. We consider that this interesting phenomenon in WGMs (both emergence of sub-peaks and the shift of modes) should be intimately connected with the geometrical structure of the micro-tubular cavity. Effective surface modification and geometry design will introduce special optical properties in these self-rolled up microcavities [6].

The rolling process has been provides a convenient way to produce a stack of multilayers made from different materials with tunable geometry, demonstrating a new method to prepare optical microdevices. The future of rolled-up microcavities lies in many aspects from fundamental science to practical applications. The corresponding resonance can thus be tuned in terms of wavelength range and Q-factor. Functional materials may also be incorporated to achieve effective coupling between the light and other physical fields, giving birth to special micro devices with good tunability. Meanwhile, rolled-up thin-walled oxide tubular microcavity delivers a new optical component for light coupling and may imply interesting applications in the interaction between light and matter [6]. Considering the on-chip integratability of rolled-up structures, researchers may also be able to produce a sensing device to realize lab-in-a-tube [3].

Microwave photonic filters (MPFs) are of great interest in radio frequency systems since they provide prominent flexibility on microwave signal processing. Although filter reconfigurability and tunability have been demonstrated repeatedly, it is still difficult to control the filter shape with very high precision. Thus the MPF application is basically limited to signal selection. Here we present a polarization-insensitive single-passband arbitrary-shaped MPF with ~GHz bandwidth based on stimulated Brillouin scattering (SBS) in optical fibre. For the first time the filter shape, bandwidth and central frequency can all be precisely defined by software with ~MHz resolution. The unprecedented multi-dimensional filter flexibility offers new possibilities to process microwave signals directly in optical domain with high precision thus enhancing the MPF functionality. Nanosecond pulse shaping by implementing precisely defined filters is demonstrated to prove the filter superiority and practicability.

As a potential technique to further increase the transmission capacity in a single fiber, few mode fibers based mode division multiplexing (MDM) is attracting increasing interest in the worldwide. However, multiple input multiple output (MIMO) digital signal processing (DSP) is required to mitigate the mode crosstalk induced by mode coupling during the transmission, which increase the cost and complexity of the whole system. An effective approach to obtain negligible modal crosstalk in FMFs, so as to eliminate MIMO, is to lift the degeneracy between the adjacent eigenmodes and achieve effective index difference, Δneff values larger than 10⁻⁴. Elliptical core fiber (ECF) and elliptical ring core fiber (ERCF) are designed to achieve a high index difference between adjacent eigenmodes. However, since most of the commercial mode selective multiplexers/demultiplexers (MUX/DEMUX), such as photonic lanterns, are designed for circle core fiber, when connecting ECF/ERCF with commercial mode selective MUX/DEMUX, the mode field mismatch between the ECF/ERCF and the circle core fiber (CCF) leads to large coupling loss and mode crosstalk. In this work, we design and fabricate an asymmetric circle core fiber (ACCF) for MIMO-free MDM systems. Compared with ECF, the proposed ACCF can connect well with the commercial photonics lanterns with reduced the coupling loss and mode crosstalk.