Efficient Light Coupling Research Articles

Spectroscopy using a visible photonic lantern at the Subaru Telescope: Laboratory characterization and the first on-sky demonstration on Ikiiki (α Leo) and ‘Aua (α Ori)

Context. Photonic lanterns (PLs) are waveguide devices enabling high-throughput single-mode spectroscopy and high angular resolution. Aims. We aim to present the first on-sky demonstration of a PL operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects. Methods. We used the SCExAO instrument (a double-stage extreme adaptive optics system installed at the Subaru Telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations. Results. The best averaged coupling efficiency over the PL field of view was measured at 51% ± 10%, with a peak at 80%. We also investigated the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the adaptive optics regime a PL offers a better coupling efficiency performance than an SMF, especially in the presence of low-spatial-frequency aberrations. We observed Ikiiki (α Leo – mR = 1.37) and ‘Aua (α Ori – mR = −1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in the H band) and large tip or tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% ± 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star’s spectra, containing various absorption lines. Conclusions. The successful demonstration of this device opens new possibilities in terms of high-throughput single-mode fiber-fed spectroscopy in the visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip or tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). This emphasizes the enhanced resilience of PL technology to such atmospheric disturbances. The additional capabilities in high angular resolution are also promising but still have to be demonstrated in a forthcoming investigation.

Efficient Light Coupling and Propagation in Fiber Optic Systems

This study explores the propagation of light in optical fibers, focusing on the fundamental principles and practical implications for fiber optic technologies. By analyzing the wave equation, the research demonstrates that light propagates as cylindrical waves within the fiber, contrasting with spherical waves in free space. The study highlights the significance of Gaussian beams, particularly from helium-neon lasers, finding a beam waist radius of approximately 12.6 μm and its position about 1.25 μm from the focus. These parameters are critical for optimizing laser beam coupling into the fiber. The research also measures the light transit time through a 100-meter fiber, revealing a light speed of approximately 2×108 m/sec, which is influenced by the fiber's refractive index. Additionally, the relationship between diode laser output power and injection current was investigated, demonstrating a linear correlation crucial for practical applications. The findings emphasize the importance of accurate measurements and configuration in improving fiber optic communication and laser performance. This comprehensive analysis provides valuable insights into the design and optimization of optical fiber systems, contributing to advancements in communication and laser technologies.

Anomalous reflection for highly efficient subwavelength light concentration and extraction with photonic funnels

Abstract Photonic funnels, microscale conical waveguides that have been recently realized in the mid-IR spectral range with the help of an all-semiconductor designer metal material platform, are promising devices for efficient coupling of light between the nanoscales and macroscales. Previous analyses of photonic funnels have focused on structures with highly conductive claddings. Here, we analyze the performance of funnels with and without cladding, as a function of material properties, operating wavelength, and geometry. We demonstrate that bare (cladding-free) funnels enable orders-of-magnitude higher enhancement of local intensity than their clad counterparts, with virtually no loss of confinement, and relate this phenomenon to anomalous reflection of light at the anisotropic material–air interface. Intensity enhancement of the order of 25, with confinement of light to wavelength/20 scale, is demonstrated. Efficient extraction of light from nanoscale areas is predicted.

Chalcogenide Ge20Sb5Se75 photonic crystal fiber with seven air-hole rings for ultraflat mid-infrared supercontinuum generation

Supercontinuum (SC) generation in solid-core circular photonic crystal fibers (PCFs) made of Ge20Sb5Se75 is numerically analyzed. A large core is projected to increase light coupling efficiency into selected PCFs as well as raise coupling to standard silica fibers. High nonlinear coefficient and near-zero flat dispersion allow ultraflat SC spanning 1.5–4.6 μm in an all-normal dispersion regime. This requires 3 kW of peak power with 180 fs of pulse duration. The fiber with one zero-dispersion wavelength (ZDW) generates SC bandwidth in the range of 1.54–7.39 μm at 3.5 μm using peak power of 10 kW. For the same input power, the SC spectral covers from 1.39 to 7.36 μm in 10 cm of fiber with two ZDWs. These are wider SC bandwidths than those of previous chalcogenide fibers reached with lower peak powers. Therefore, the proposed Ge20Sb5Se75 PCFs are excellent candidates for the broadband ultraflat mid-infrared SC spectra used in high-speed nonlinear imaging and frequency measurement.

Citrate polymer optical fiber for measuring refractive index based on LSPR sensor

Fiber optic localized surface plasmon resonance (LSPR) sensors have become an effective tool in refractive index (RI) detection for biomedical applications because of their high sensitivity. However, using conventional optical fiber has caused limitations in implanting the sensor in the body. This research presents the design and construction of a new type of polymer-based LSPR sensors to address this issue. Also, finite element method (FEM) is used to design the sensor and test it theoretically. The proposed polymer optical fiber (POF) based on citrate is biocompatible, flexible, and degradable, with a rate of 22% and 27 over 12 days. The step RI structure utilizes two polymers for light transmission: poly (octamethylene maleate citrate) (POMC) as the core and poly (octamethylene citrate) (POC) as the cladding. The POF core and cladding diameters and lengths are 700 µm, 1400 µm, and 7 cm, respectively. The coupling efficiency of light to the POF was enhanced using a microsphere fiber optic tip. The obtained results show that the light coupling efficiency increased to 77.8%. Plasma surface treatment was used to immobilize gold nanoparticles (AuNPs) on the tip of the POF, as a LSPR-POF sensor. Adsorption kinetics was measured based on the pseudo-first-order model to determine the efficiency of immobilizing AuNPs, in which the adsorption rate constant (k) was obtained be 8.6 × 10–3 min−1. The RI sensitivity of the sensor in the range from 1.3332 to 1.3604 RIU was obtained as 7778%/RIU, and the sensitivity was enhanced ~ 5 times to the previous RI POF sensors. These results are in good agreement with theory and computer simulation. It promises a highly sensitive and label-free detection biosensor for point-of-care applications such as neurosciences.

An evanescent waves sensor based on polydopamine-gold coated chalcogenide tapered fiber for enzyme activity profiling

In this research, we demonstrate a high-sensitivity tapered fiber (TF) sensor made of As2S3 chalcogenide glass coated with polydopamine (PDA) and gold nanoparticles (AuNPs). The presence of in-situ grown AuNPs on the PDA-coated TF surface via the reduction of Au ions by catechol groups in PDA, facilitated efficient light coupling with evanescent waves, thereby improving the IR absorption of molecules. The sensor was utilized to measure the activity of the glucose oxidase enzyme as a point-of-concept analyte. This process of enzyme catalyzing the conversion of glucose to hydrogen peroxide and Gluconic acid can be recorded by the sensor. In investigating the enzyme activity and D-glucose oxidation kinetics, we monitored the intensity of the Gluconic acid band at 5.764 μm, which arises from the C=O stretching vibration, as a function of time. The estimated enzyme activity using the PDA/AuNPs-coated TF sensor was 103.66 ± 2.87 Umg−1 when the enzyme activity of the sample was 100 Umg−1 which is consistent with standard value. The Lineweaver–Burk plot yielded the following kinetic constants: Km = 16.10 ± 1.63 mM and Vmax = 93.72 ± 12.71 μmol/min. Results suggest that this sensor has great application potential in various enzyme activity applications.

Genetic algorithm optimized microstructure to enhance waveguide light coupling efficiency at normal incidences

The recent trend to place well-designed photonic structures on waveguides is capable of effectively enhancing waveguides properties. One typical example is a nanostructure-empowered waveguide targeted for efficient light coupling. However, conceiving the high-freedom structures is a time-consuming and labor-intensive process, where an ineffective workflow limits the development of photonic microstructures. To address this issue, we deploy a genetic algorithm to customize structures in order to improve the coupling coefficients under predetermined situations (i.e., normal incidence combined with two polarizations). Three types of micropatterns are first conceived in the periodical model and then fully characterized on the real waveguide sizes. The simulated data reveal that the 550–1650 nm average coupling efficiencies of structure-enabled waveguides are raised by about 2% in contrast to the bare case, and the lineshapes are also flattened thanks to the grating modifications. In short, our solution underlines the role of an algorithm-developed nanostructure to lift waveguide coupling coefficients. By integrating well-engineering patterns, the waveguide-based probes may find a multitude of usages for weak signal detection and communication systems.

Packaging of micro-lens arrays to photonic integrated circuits using beam shape evaluation

We propose a method for aligning and attaching micro-lens arrays to photonic integrated circuits (PICs). Unlike the conventional approach of assessing power coupled to a fiber directly, our method utilizes a beam profiler. This profiler allows us to optimize the lens position by analyzing the transmitted beam shape from the PIC edge coupler through the lens. In conjunction, we employ grating couplers to introduce external light, acting as a ‘beacon’ for optimization. The use of grating couplers enables efficient coupling of external light into the PIC, providing a reference point for alignment. Importantly, our method accommodates both regular waveguide-side-up and upside-down (through-Silicon) orientations of the PIC. This versatility allows us to reproduce coupling results across a 6-channel array, demonstrating robust performance. This innovative approach not only ensures precise alignment and attachment but also opens up new possibilities for photonic packaging. The flexibility to work in different orientations is likely to lead to advancements in the design and assembly of photonic devices.

A temperature sensor based on multi-beam capture and interference

In this study, we proposed and validated a temperature fiber sensor based on optical signal capture Mach-Zehnder Interferometer (OSC-MZI) with high-precision temperature measurement capabilities. By creating three air cavities on a continuously offset-spliced no-core fiber (NCF), the optical signal experiences reflection and divergence within these cavities, which enhances the optical path of propagating light. Gradually, filling these NCF cavities with polymethyl methacrylate (PMMA) microspheres to increase the light coupling efficiency and as a sensing unit. Due to the directly contact between the optical fiber waveguide and PMMA, the state of incident lights is easily influenced by the refractive index (RI) variations of PMMA. When the environmental temperature changes, RI of the PMMA with high thermal-optic coefficient shows marked change, which finally leads to the obvious wavelength shift of interference dips in the transmission spectrum. Experimental results show that MZI coated with the polymer materials achieves a temperature sensitivity of up to 2.04 nm/°C within the temperature range of 25–50 °C. Furthermore, as an interference-based MZI sensor, its total length is only 4.75 mm. The proposed OSC-MZI provides a new approach for combining optical fiber sensors with polymer materials.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics.

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber-chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of -1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber-chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.

Toward Complete Optical Coupling to Confined Surface Polaritons.

Optical coupling between propagating light and confined surface polaritons plays a pivotal role in the practical design of nanophotonic devices. However, the coupling efficiency decreases dramatically with the degree of mode confinement due to the mismatch that exists between the light and polariton wavelengths, and despite the intense efforts made to explore different mechanisms proposed to circumvent this problem, the realization of a flexible scheme to efficiently couple light to polaritons remains a challenge. Here, we experimentally demonstrate an efficient coupling of light to surface-plasmon polaritons assisted by engineered dipolar scatterers placed at an optimum distance from the surface. Specifically, we fabricate gold disks separated by a silica spacer from a planar gold surface and seek to achieve perfect coupling conditions by tuning the spacer thickness for a given scatterer geometry that resonates at a designated optical frequency. We measure a maximum light-to-plasmon coupling cross section of the order of the square of the light wavelength at an optimum distance that results from the interplay between a large particle-surface interaction and a small degree of surface-driven particle-dipole quenching, both of which are favored at small separations. Our experiments, in agreement with both analytical theory and electromagnetic simulations, support the use of optimally placed engineered scatterers as a disruptive approach to solving the long-standing problem of in/out-coupling in nanophotonics.

Coherent control of scattering and absorption in organic microresonators

We study coherent perfect absorption in organic microcavity resonators and extend these principles and our findings to more complex microresonator systems that, beyond absorption, also possess additional cavity energy dissipation mechanisms. The experimental approach uses laser interferometry to closely monitor the energy fluxes within the system at all device ports as a function of the device geometry and the phase relationships of the incident beams. A particular focus is on optical systems based on 2nd order Bragg gratings, which are crucial for the operation of organic distributed feedback (DFB) lasers or as light incouplers in optical waveguiding films. Coherent control allows the diffraction efficiency of the underlying grating to be tuned over a wide range of values. This strategy allows significant optimisation of resonator structures for high efficiency light coupling in optical waveguides and fine tuning of grating parameters for the most efficient optical mode conversion.

Reconfigurable phase change chalcogenide grating couplers with ultrahigh modulation contrast

In photonic integrated circuits, efficient coupling of light between fibers and waveguides is challenging due to mode area mismatch. In-plane grating couplers (GC) have become popular for their low cost, easy alignment, and design flexibility. While most GC designs have fixed coupling efficiencies, with a view to emerging adaptive neuromorphic and quantum integrated circuits and interposers that need ultra-compact memory/modulation components, we introduce a CMOS-compatible GC based on phase-change chalcogenide alloy germanium antimony telluride. The GC design optimized utilizing inverse design techniques achieves over 50% coupling efficiency at 1550 nm when amorphous, and near-zero efficiency when switched to a crystalline state. This design is non-volatile, reversible, and provides ultra-high transmission modulation contrasts of up to 60 dB. While the operational range can be adjusted across the telecommunication band by modifying the GC's etch depth or thickness. We show that such devices do not need global switching of their entire phase change volume and can achieve maximum modulation contrasts through switching precisely positioned phase change inclusions hinting at low-power and ultrafast modulation potential.

Effective Kerr lens mode locking in Er3+:ZBLAN fiber laser with phase mismatched periodically poled lithium niobate

We theoretically introduce an effective Kerr lens mode locking concept to a polarization maintaining (PM) Er3+:ZBLAN fiber laser and generate a stable femtosecond pulse train at 2.76 µm. Phase-mismatched periodically poled lithium niobate (PPLN) is utilized to implement cascaded second order nonlinearities (CSN) for the effective Kerr lens mode locking. Furthermore, the CSN in the PPLN can be tuned to balance the huge fiber anomalous dispersion for femtosecond soliton formation in the mid-infrared regime. High coupling efficiency of light into fiber, shorter effective focal length of PPLN and larger effective nonlinear refractive index areessential for enhancing intracavity peak power and shortening pulse duration. The pulse could be compressed down to 311 fs with output pulse energy of 1.36 nJ and peak power of 3.6 kW. This mode locking method provides deep insight into direct generation of femtosecond fiber laser in mid-infrared regime.

Silicon-tapered waveguide for mode conversion in metal-insulator-metal waveguide-based plasmonic sensor for refractive index sensing.

In this study, we have undertaken a comprehensive numerical investigation of a refractive index sensor designed around a metal-insulator-metal (MIM) plasmonic waveguide. Our approach utilizes the finite element method to thoroughly analyze the sensor's performance. The sensor's configuration utilizes a ring resonator design, which has been slightly modified at the coupling segment. This modification enhances the efficiency of light coupling between a bus waveguide and the ring resonator, particularly at the resonance wavelength. This strategic adjustment significantly improves the device's extinction ratio, a critical factor in its functionality. Remarkably, the sensitivity of this sensor is determined to be approximately 1155.71nm/RIU, while it possesses a figure of merit of 25.9. Furthermore, our study delves into the intricate mechanism governing the injection of light into the nanoscale MIM waveguide. We achieve this through the incorporation of silicon-tapered waveguides, which play a pivotal role in facilitating the transformation of a dielectric mode into a plasmonic mode, and vice versa. Ultimately, the findings of this research hold significant promise for advancing the field of plasmonic sensing systems based on MIM waveguide technology. The insights gained here pave the way for the practical realization and optimization of highly efficient and precise plasmonic sensors.

Enhanced performance of solution processed OLED devices based on PFO induced TADF emission layers

The internal quantum efficiency (IQE) of the OLED devices based on thermally activated delayed fluorescence (TADF) materials was thought to nearly 100 % by assuming the light outcoupling efficiency of 20–25 %. However, the external quantum efficiency (EQE) is still limited by the ηout. One crucial factor that affects the light output coupling efficiency is the molecular transition dipole moment of the emission molecule. In this study, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) was chosen as an orientation inducer to enhance the horizontal orientation ratio of a TADF emitter, 2-Phenyl-4,6-bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl] pyrimidine (DMAc-PPM), prepared using a solution method. With doping PFO in weight ratios ranging from 1 wt% to 9 wt%, the orientation factor of TADF emitter increased, which testified the inducing function of PFO. The TADF-OLED devices using PFO as an inducer exhibited an EQE increasing with the PFO doping weight ratio. The maximum EQE of 7.3 % was achieved at the doping weight ratio of 7 wt%, which increase about 1.7 times compared to the device without the PFO inducer. This observed variation of EQE differs from the trend observed in maximum luminance intensity. The increase of EQE can be attributed to the improvement of light output efficiency caused by the variation of orientation factor. The lower carrier mobility lead to the lower maximum luminance intensity. The synergistic effect of light output efficiency and carrier mobility enhanced the performance of OLED devices.

A spatially uniform illumination source for widefield multi-spectral optical microscopy.

Illumination uniformity is a critical parameter for excitation and data extraction quality in widefield biological imaging applications. However, typical imaging systems suffer from spatial and spectral non-uniformity due to non-ideal optical elements, thus require complex solutions for illumination corrections. We present Effective Uniform Color-Light Integration Device (EUCLID), a simple and cost-effective illumination source for uniformity corrections. EUCLID employs a diffuse-reflective, adjustable hollow cavity that allows for uniform mixing of light from discrete light sources and modifies the source field distribution to compensate for spatial non-uniformity introduced by optical components in the imaging system. In this study, we characterize the light coupling efficiency of the proposed design and compare the uniformity performance with the conventional method. EUCLID demonstrates a remarkable illumination improvement for multi-spectral imaging in both Nelsonian and Koehler alignment with a maximum spatial deviation of ≈ 1% across a wide field-of-view.

Optically pumped Milliwatt Whispering-Gallery microcavity laser

Whispering-gallery-mode microcavity lasers possess remarkable characteristics such as high Q factors and compact geometries, making them an essential element in the evolution of microlasers. However, solid-state whispering-gallery-mode lasers have previously suffered from low output power and limited optical conversion efficiency, hindering their applications. Here, we present the achievement of milliwatt laser emissions at a wavelength of 1.06 µm from a solid-state whispering-gallery-mode laser. To accomplish this, we construct a whispering-gallery-mode microcavity (with a diameter of 30 µm) using a crystalline Nd: YAG thin film obtained through carbon-implantation enhanced etching of a Nd: YAG crystal. This microcavity laser demonstrates a maximum output power of 1.12 mW and an optical conversion efficiency of 12.4%. Moreover, our unique eccentric microcavity design enables efficient coupling of free-space pump light, facilitating integration with a waveguide. This integration allowed for single-wavelength laser emission from the waveguide, achieving an output power of 0.5 mW and an optical conversion efficiency of 6.18%. Our work opens up new possibilities for advancing solid-state whispering-gallery-mode lasers, providing a viable option for compact photonic sources.

C- and O-Band Dual-Polarization Fiber-to-Chip Grating Couplers for Silicon Nitride Photonics.

Highly efficient coupling of light from an optical fiber to silicon nitride (SiN) photonic integrated circuits (PICs) is experimentally demonstrated with simple and fabrication-tolerant grating couplers (GC). Fully etched amorphous silicon gratings are formed on top of foundry-produced SiN PICs in a back-end-of-the-line (BEOL) process, which is compatible with 248 nm deep UV lithography. Metallic back reflectors are introduced to enhance the coupling efficiency (CE) from -1.11 to -0.44 dB in simulation and from -2.2 to -1.4 dB in experiments for the TE polarization in the C-band. Furthermore, these gratings can be optimized to couple both TE and TM polarizations with a CE below -3 dB and polarization-dependent losses under 1 dB over a wavelength range of 40 nm in the O-band. This elegant approach offers a simple solution for the realization of compact and, at the same time, highly efficient coupling schemes in SiN PICs.

On-Demand Waveguide-Integrated Microlaser-on-Silicon

The integration of high-quality III–V light sources on the Si platform has encountered a challenge that demands a highly precise on-demand addressability of single devices in a significantly reduced integration area. However, simple schemes to address the issue without causing major optical losses remain elusive. Here, we propose a waveguide-integrated microlaser-on-silicon in which the III–V/Si integration requires only a small micron-sized post structure with a diameter of <2 µm and enables efficient light coupling with an estimated coupling efficiency of 44.52%. Top-down fabricated high-quality microdisk cavities with an active gain medium were precisely micro-transferred on a small Si-post structure that was rationally designed in the vicinity of a strip-type Si waveguide (WG). Spectroscopic measurements exhibit successful lasing emission with a threshold of 378.0 µW, bi-directional light coupling, and a propagation of >50 µm through the photonic Si WG. Numerical study provides an in-depth understanding of light coupling and verifies the observations in the experiment. We believe that the proposed microlaser-on-Si is a simple and efficient scheme requiring a minimum integration volume smaller than the size of the light source, which is hard to achieve in conventional integration schemes and is readily applicable to various on-demand integrated device applications.