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Graded-index optical fiber probes suitable for focusing or collimating the output of an optical fiber at a wavelength of 1.3 µm have become an enabling technology in optical coherence tomography imaging applications for in vivo bioimaging. Such fiber tips however remain uncommon in other photonics applications. This paper provides the first numerical study of graded-index fiber tips covering a broad range of wavelengths spanning from the UV to short-infrared. The wavelength dependency and the influence of probe geometry on performance characteristics such as far-field divergence angle, spot size and working distance are analyzed. The paper yields easily accessible design guidelines for the fabrication of collimating or focusing fiber tips. Fiber collimators have considerable potential for use in free-space systems and could benefit a range of devices such as variable attenuators, dynamic wavelength equalisers and large 3D optical cross-connect switches, whereas focusing fiber tips have applications in high-resolution imaging. Refereed/Peer-reviewed

Refereed/Peer-reviewed Whispering gallery modes (WGMs) in micro-resonators are of interest due to their high Q-factors. Ultra-thin fiber tapers are widely deployed to couple light into micro-resonators but achieving stable and practical coupling for out-of-lab use remains challenging. Here, a new WGM coupling scheme using an exposed-core silica fiber (ECF) is proposed, which overcomes the challenge of using fragile fiber tapers. Microspheres are deposited onto the exposed channel for excitation via the evanescent field of the fiber's guided modes. The outer jacket of the ECF partially encapsulates the microspheres, protecting them from external physical disturbance. By varying the mode launching conditions in this few-mode ECF, in combination with a Fano resonance effect, we demonstrate a high degree of tunability in the reflection spectrum. Furthermore, we show multi-particle WGM excitation, which could be controlled to occur either simultaneously or separately through controlling the ECF mode launching conditions. This work can bring value towards applications such as optical switches and modulators, multiplexed/distributed biosensing, and multi-point lasing, integrated in a single optical fiber device that avoids fiber post-processing.

This paper describes the design and characterization of miniaturized optofluidic devices for sensing based on integrating collimating optical fibers with custom microfluidic chips. The use of collimating graded-index fiber (GIF) tips allows for effective fiber-channel-fiber interfaces to be realized when compared with using highly-divergent standard single-mode fiber (SMF). The reduction in both beam divergence and insertion losses for the GIF configuration compared with SMF was characterized for a 10.0 mm channel. Absorption spectroscopy was demonstrated on chip for the measurement of red color dye (Ponceau 4R), and the detection of thiocyanate in water and artificial human saliva. The proposed optofluidic setup allows for absorption spectroscopy measurements to be performed with only 200 mu L of solution which is an order of magnitude smaller than for standard cuvettes but provides a comparable sensitivity. The approach could be integrated into a lab-on-a-chip system that is compact and does not require free-space optics to perform absorption spectroscopy. Refereed/Peer-reviewed

We propose a two-dimensional distributed sensing scheme using few-mode exposed-core fibre and optical frequency domain reflectometry. This scheme aims to measure the size and position of particles using the evanescent field of different guided modes. © 2020 The Author(s)

The increase in Internet data demand has resulted in the development of novel optical fibers. Ultrafast laser inscription is a powerful tool to create 3D waveguide circuits that can interface with these new fiber types. Refereed/Peer-reviewed

To meet future data traffic demand, the telecommunications industry is developing optical fiber network architectures that exploit space division multiplexing (SDM). Femtosecond laser written photonics is a promising platform for SDM end-of-fiber components. Refereed/Peer-reviewed

Fiber-optic Fabry-Perot interferometers developed using collimating graded-index fiber tips are shown to allow for dramatic improvements in fringe visibility of up to 10-fold for long pathlength, small FSR, cavities

The tracking of small particles is an important but challenging task for biological applications such as disease diagnostics and medical research. Current methods are limited to the use of bulky instruments such as flow cytometers and microscopes. Here, a novel technique for the detection and measurement of micron-scale optical scatterers using a few-mode exposed-core microstructured optical fiber is proposed. Through selective mode launching combined with optical frequency domain reflectometry, scatterers located on the fiber core surface can be simultaneously mapped with both longitudinal and transverse information. This technique is demonstrated by detecting the two-dimensional positions of several femtosecond-laser-inscribed micron-scale ablations written at different locations on the fiber core surface. Due to the compact nature of the optical fiber and its local sensitivity to scatterers that are in close proximity to it, this technique has the potential for the measurement and detection of micron-scale particles in difficult to reach biological environments for in vivo applications. Refereed/Peer-reviewed

Space-division multiplexing (SDM) in few-mode multi-core fibers (FM-MCF) allows the transmission of multi-petabit/s data-rates over a single optical fiber. Recently, transmission of more than 1.2 Pbit/s was demonstrated in a FM-MCF with a cladding diameter of 160 μm, compatible with the standard coating diameter of 250 μm [1]. When increasing the core density in FM-MCF or the transmission distance, inter-core crosstalk can limit the achievable data rates. Inter-core crosstalk is strongly wavelength-dependent and higher-order modes in FM-MCF are expected to show a larger crosstalk penalty compared to the fundamental mode, as their mode fields extend farther into the cladding [2]. In this paper, we present an experimental investigation of the crosstalk-induced Q-factor degradation of 16-Quadrature amplitude modulation (16-QAM) signals, transmitted over a four-core three-mode fiber Refereed/Peer-reviewed

The exponential demand on the internet is producing a capacity crunch in legacy short haul communication networks based on multimode fibre, and is foreshadowed to do the same for long haul single mode fibre networks. Femtosecond lasers have been used to produce a range of compact lightwave circuits that are compatible with optical fibres. These include 3D lightwave chips that offer mode selectivity. Recent developments in plug-and-play devices produced by the fs laser direct-write technique have enabled the modes of few mode fibre (FMF) to be individually addressed with high fidelity (20dB mode extinction). 10G has also been demonstrated over 2 kms of conventional multimode fibre (OM1 and OM2) with fs laser written photonics. As a result, this platform can extend the service life of existing multimode fibre networks and enable next generation networks exploiting mode division multiplexing.

We transmit 33 WDM channels over 3500-km three-mode fiber at record post-FEC spectral-efficiency-distance product of 31500 bit/s/Hz · km. This was facilitated by building a DGD compensated fiber span with low MDL and coherent 6×6 MIMO processing. Refereed/Peer-reviewed

With the capacity limits of standard single-mode optical fiber fast approaching, new technologies such as space-division multiplexing are required to avoid an Internet capacity crunch. Few-mode multicore fiber (FM-MCF) could allow for a two orders of magnitude increase in capacity by using the individual spatial modes in the different cores as unique data channels. We report the realization of a monolithic mode-selective few-mode multicore fiber multiplexer capable of addressing the individual modes of such a fiber. These compact multiplexers operate across the S + C + L telecommunications bands and were inscribed into a photonic chip using ultrafast laser inscription. They allow for the simultaneous multiplexing of the LP01, LP11a and LP11b modes of all cores in a 3-mode, 4-core fiber with excellent mode extinction ratios and low insertion losses. The devices are scalable to more modes and cores and therefore could represent an enabling technology for practical ultra-high capacity dense space-division multiplexing.

The demonstration of a three-dimensional tapered mode-selective coupler in a photonic chip is reported. This waveguide-based, ultra-broadband mode multiplexer was fabricated using the femtosecond laser direct-write technique in a boro-aluminosilicate glass chip. A three-core coupler has been shown to enable the multiplexing of the LP01, LP and LP spatial modes of a multimode waveguide, across an extremely wide bandwidth exceeding 400 nm, with low loss, high mode extinction ratios and negligible mode crosstalk. Linear cascades of such devices on a single photonic chip have the potential to become a definitive technology in the realization of broadband mode-division multiplexing for increasing optical fiber capacity.

Tapered mode-selective couplers are shown to allow for ultra-broadband mode-division multiplexing of few-mode optical fiber. Using appropriate three-core configurations, modes of arbitrary spatial-orientation can be demultiplexed. The successful fabrication of these wavelength-insensitive couplers would represent the realization of compact low-loss mode-multiplexers for use in high-bandwidth few-mode fiber networks. Refereed/Peer-reviewed

Optical fibres based on multi-core and few mode formats are being developed to meet the growing demand for data. The development of compatible end-of-fibre components based on laser written 3D photonics will be reviewed. Refereed/Peer-reviewed

Although whispering gallery mode (WGM) biosensors have shown tremendous potential, they are still yet to find practical use as biomedical diagnostic tools. This is primarily due to the nature of the interrogation mechanism itself which relies on indirect measurement of the binding of a specific biomolecule onto the sensor through the associated refractive index change. Since nonspecific binding cannot be differentiated from the specific interaction of interest, this can result in a high rate of false positive readings when the detection is performed in complex biological samples. Here we show that this inherent limitation can be solved using a relatively simple approach. This approach involves the development of a self-referenced biosensor consisting of two almost identically sized dye-doped polystyrene microspheres placed on adjacent holes at the tip of a suspended core optical fiber. Here self-referenced biosensing is demonstrated with the detection of Neutravidin in undiluted human serum samples. The fiber allows remote excitation and collection of the WGMs of the microspheres in a dip sensing setting. By taking advantage of surface functionalization techniques, one microsphere acts as a dynamic reference, compensating for nonspecific binding events, while the other microsphere is functionalized to detect the specific interaction. The almost identical size allows the two spheres to have virtually identical refractive index sensitivity and surface area. This ensures their responses to nonspecific binding and environmental changes are almost identical, whereby any specific changes such as binding events, can be monitored via the relative movement between the two sets of WGM peaks. Refereed/Peer-reviewed

We report transient spectral hole-burning studies of the R-2(+/- 1/2) line in 130, 20 ppm and 0.6 ppm ruby between 2.4 and 70 K. The homogeneous line width Gamma(h) is found to be 142 +/- 6 MHz at 2.4 K for both 20 and 130 ppm crystals; this value is in agreement with the T-1 value of 1.1 ns (Gamma(h) = 1/2 pi T-1 = 140 MHz) for the 2 (A) over bar state obtained by Rives and Meltzer who used pulsed, non-selective laser excitation. The line width is dominated by a direct one-phonon relaxation process 2 (A) over bar -> (E) over bar up to 50 K whereas at higher temperatures two-phonon Raman scattering becomes dominant. Cross-relaxation and spin-lattice relaxation times significantly affect pseudo-CW experiments. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) have been widely studied over the past 20 years for various applications, including biological sensing. While the WGM-based sensing approaches reported in the literature have shown tremendous performance down to single molecule detection, at present such sensing technologies are not yet mature and still have significant practical constraints that limit their use in real-world applications. Our work has focused on developing a practical, yet effective, WGM-based sensing platform capable of being used as a dip sensor for in-vivo biosensing by combining WGM fluorescent microresonators with silica Microstructured Optical Fibers (MOFs).We recently demonstrated that a suspended core MOF with a dye-doped polymer microresonator supporting WGMs positioned onto the tip of the fiber, can be used as a dip sensor. In this architecture the resonator is anchored to one of the MOF air holes, in contact with the fiber core, enabling a significant portion of the evanescent field from the fiber to overlap with the sphere and hence excite the fluorescent WGMs. This architecture allows for remote excitation and collection of the WGMs. The fiber also permits easy manipulation of the microresonator for dip sensing applications, and hence alleviates the need for a complex microfluidic interface. More importantly, it allows for an increase in both the excitation and collection efficiency compared to free space coupling, and also improves the Q factor.In this paper we present our recent results on microstructured fiber tip WGM-based sensors and show that this sensing platform can be used in clinical diagnostics, for detecting various clinically relevant biomarkers in complex clinical samples. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) allow for remarkable refractive index sensing performance with extremely low detection limits, and thus have found use in various emerging label free biosensing applications. Among the different types of resonators which have been studied, microcapillaries have the unique property of having the evanescent fields extend into and sample the medium inside the resonator, which is particularly interesting because the resonator itself serves as a microfluidic channel. Here, lasing of the WGMs in fluorescent microcapillaries is demonstrated for the first time, and their application to refractive index sensing is investigated. The laser gain medium used here is embedded inside a high refractive index polymer coating deposited onto the inner surface of the capillary. Lasing can only be realized for thick polymer coatings (in this case ≥ 800 nm), with higher Q factor but also stronger confinement of the propagating wave, which lowers the refractive index sensitivity compared to non-lasing capillaries which can have thinner polymer coatings. We however find that the large improvement in signal-to-noise ratio and Q factor realized upon lasing more than compensates for the reduced sensitivity, resulting in an order-of-magnitude improvement in the detection limit for refractive index sensing.

Whispering-gallery modes have been studied extensively for biosensing applications. Whilst the vast majority of work undertaken has focused on high Q factor resonators, with the main improvement being a reduction of the resonator size to improve sensitivity, we have chosen a different pathway by starting with resonators that exhibit extremely high refractive index sensitivity but low Q factor. A way forward to overcome this limitation is to introduce a gain medium and operate the resonator above its lasing threshold. This has been shown to result on average in a 5 fold increase in the Q factor. With the lasing threshold itself being dependent on the Q factor, amongst other parameters, the Q factor enhancement can be exploited to either reduce the lasing threshold or alternatively enable smaller resonators to be operated above their lasing threshold. As a demonstration we present a 10 μm diameter polystyrene microsphere lasing in aqueous solution for refractive index sensing applications, which to the best of our knowledge is the smallest polystyrene microsphere laser ever demonstrated in these conditions.

We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (∼1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in modedivision multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for highperformance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber. Refereed/Peer-reviewed

usc Refereed/Peer-reviewed Three dimensional lightwave circuits, fabricated using femtosecond laser direct-write techniques, are enabling new applications in quantum optics, astronomy and telecommunications. We review those and highlight our recent work on 3D compact, passive mode selective couplers.

Asymmetric mode-selective couplers for multiplexing the LP11 modes of an optical fiber fabricated by the femtosecond laser direct-write technique are demonstrated. The devices feature high coupling ratios, excellent mode extinction ratios and low insertion losses. Refereed/Peer-reviewed

The excitation and separation of individual modes in a few-mode optical fiber network can be realized using mode-selective couplers. For excitation at the beginning of the fiber, two-core mode-selective couplers can be used, while at the end of the fiber, either two- or three-core mode-selective couplers are required for demultiplexing of the field symmetric or field asymmetric modes, respectively. Both analytical and numerical solutions are presented to quantify the mode-selective functionality. Refereed/Peer-reviewed

The theory of mode-sorting in bimodal asymmetric Y-junctions is extended to multimode asymmetric Y-junctions with multiple output arms. This theory allows for the optimization of these mode-sorting planar structures. Asymmetric Y-junctions provide unique opportunities for spatial mode division multiplexing (MDM) of optical fiber. Spatial MDM is considered paramount to overcoming the bandwidth limitations of single-mode fiber. The design criteria presented in this paper facilitate their design. Refereed/Peer-reviewed

Dispersion equalisation of the modes of cylindrically symmetric few-mode optical fibres is investigated using equivalent planar waveguide representations. A numerical analysis based on the Characteristic Matrix Method has identified what we believe to be the first practical three-mode fibre with zero intermodal dispersion. This implies that the group velocities of each of the fibre's three linearly polarized modes are equivalent. Dispersion equalisation is also confirmed in a power-law profile, as well as a novel two-mode fibre which operates within the third telecommunications windows. These few-mode fibres could potentially be used for long-haul telecommunications data links. In the case where the modes are not distinguished and are equally excited, the few-mode fibres could be used as larger core-radius alternatives to traditional single-mode fibres. If the modes are distinguished, each mode could in theory be considered an independent data channel. All modal data channels would be equivalent for such dispersion-free few-mode fibres, notwithstanding differences in intramodal dispersion and mode crosstalk. Refereed/Peer-reviewed

This paper investigates the Q-factor limits imposed on the far-field detection of the whispering gallery modes of active microspherical resonators. It is shown that the Q-factor measured for a given active microsphere in the far-field using a microscope is significantly lower than that measured using evanescent field collection through a taper. The discrepancy is attributed to the inevitable small asphericity of microspheres that results in mode-splitting which becomes unresolvable in the far-field. Analytic expressions quantifying the Q-factor limits due to small levels of asphericity are subsequently derived. Refereed/Peer-reviewed

Directional mode coupling in an asymmetric holey fiber coupler is demonstrated both numerically and experimentally for the first time. The holey fiber mode couplers have interesting spectral characteristics and are also found to exhibit increased dimensional tolerances. Following a design based on numerical investigations, a dual-core polymer holey fiber coupler for LP01 and LP11 mode multiplexing was fabricated via a drilling and drawing technique. The measurements are compared with the simulation results. Refereed/Peer-reviewed

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported. Refereed/Peer-reviewed