Mode Size Converter Research Articles

Ultra-compact and low-loss silicon nitride optical mode-size converter

Ultra-compact and low-loss silicon nitride optical mode-size converter

Ultra-short and highly efficient metamaterial Fresnel lens-assisted taper.

This paper demonstrates the benefits of leveraging free-space optics concepts in the design of certain integrated photonic components, leading to a footprint reduction without compromising on performance. Specifically, we present ultra-short, highly efficient and fabrication-friendly mode-size converters based on metamaterial Fresnel lens-assisted tapers. This is achieved using a parameterized inverse-design approach, where the metamaterial phase shifters are realized using fabrication-friendly Manhattan geometries, by optimizing the width, length, and position of the phase shifters. This approach overcomes the limitations of the conventional method that uses local periodic approximation, which is not suitable for lenses with a short focal length and high numerical aperture. We also extend the free-space concept of compound lenses and demonstrate a doublet-based taper to further reduce the footprint. The devices are fabricated and experimentally characterized in terms of insertion loss and signal integrity at high data transmission rates, exhibiting high performance. For the singlet, it effectively achieves mode-size conversion from 15 μm to 0.5 μm within a 15 μm distance, leading to ×10 length reduction compared to a linear taper. The insertion loss is under 1 dB over the entire C-band. The doublet achieves the same mode-size reduction within a 10 μm distance, leading to ×15 length reduction compared to a linear taper. The insertion loss is near 1 dB over most of the C-band. In both cases, the signal integrity is maintained for up to 50 Gbit/s.

Ultra-broadband on-chip mode size converter based on metasurface and taper on lithium-niobate-on-insulator platform

Mode size converter enables to match two waveguides with different mode sizes. It has already become an indispensable component in modern integrated photonics. Here, we propose a strategy for implementing an on-chip mode size converter consisted of a gradient geometry metasurface and two cascaded waveguide tapers on the lithium-niobate-on-insulator platform. The metasurface, which is formed by a series of length-graded silica nanostrips embedded into LN waveguide, functions as focusing of incident wave and exaltation of conversion efficiency. A mode size converter that enables to match a fundamental TE00 mode in a 10-μm-wide waveguide with that in a 0.8-μm-wide one was simulated to verify the feasibility of the strategy. We show that the device is not only compact with a length of 22.9 μm, but also shows an ultra-broad operation band as much as 1150 nm (from 1100 to 2250 nm), over which the conversion efficiency keeps >90 %. In addition, we show that the device has a good robustness to fabrication deviations. The strategy proposed here can be flexibly extended to other waveguide cross-section sizes and other guided modes.

Compact integrated mode-size converter using a broadband ultralow-loss parabolic-mirror collimator.

In this Letter, we propose and demonstrate an integrated mode-size converter (MSC) with a compact footprint, low losses, and a broad bandwidth. By exploiting a parabolic mirror, the divergent light from a narrow waveguide (450 nm) is collimated to match the mode size of a wide waveguide (10 µm). The measured insertion loss (IL) is ≈ 0.15 dB over a 100-nm bandwidth. The mode-size conversion is achieved with a footprint as small as ≈ 20 × 32 µm2, which is much shorter than the linear taper length required to attain the same level of losses.

Monolithic integration of 1.3 μm asymmetric lasers grown on silicon and silicon waveguides with tapered coupling

We proposed a design scheme to enable the monolithic integration between a silicon waveguide and a 1.3 μm wavelength band III–V quantum dot laser, which is epitaxially grown on silicon with an asymmetric structure. The III–V laser is grown in a deep trench of a silicon-on-insulator wafer by the selective area epitaxy technique, and a GaAs coupling layer is inserted into the lower cladding layer of the laser, which can make the optical field distribution of the laser shift down. Besides, a mode-size converter with a three-segment tapered structure is designed to couple the output laser into the standard single-mode silicon waveguide. For the laser, the composition and the thickness of AlGaAs cladding layers and AlGaAs transition layer are optimized based on the optical waveguide theory. When the upper cladding layer is 0.6 μm Al0.7Ga0.3As, the lower cladding layer is 1.2 μm Al0.25Ga0.75As, and the transition layer is 20 nm Al0.45Ga0.55As, the optical confinement factors of the active region and the coupling layer are 45.34% and 40.69%, respectively. Then the length of the mode-size converter with a three-segment tapered structure is further optimized by the mode-matching method. When the lengths of the three tapered structures of the mode-size converter are 50 μm, 53 μm and 10 μm respectively, a coupling efficiency of 65% can be obtained between the laser and the Si waveguide. This scheme is expected to realize the efficient optical coupling between the silicon integrated light source and the silicon waveguide, which will promote the development of silicon monolithic photonic integration.

Mode Size Converter Based on Periodically Segmented Liquid Crystal Core Waveguide

We report the numerical and experimental study of a liquid crystal (LC) core periodically segmented waveguide (PSW) based mode size converter, fabricated on an ITO coated glass substrate. The effective index and mode size of the proposed device can be dynamically tuned by changing the applied voltage. The 5CB (4-cyano-4′-pentylbiphenyl) nematic LC has been used as the guiding layer, surrounded by the cladding of negative photoresist AZ15nXT. An equivalent waveguide model has been used to determine the waveguide characteristics, and we have shown that the effective index and mode size of the guided modes change with the duty cycle and the applied voltage. For a fixed duty cycle, we have experimentally demonstrated that the size of the fundamental mode can be tuned by the applied voltage. The advantage of using LC-PSW for mode size transformation is that the effective index and mode size in the fabricated waveguide can be varied by an externally applied electric field. It has also been experimentally demonstrated that the coupling efficiency between the LC-PSW and a single mode fiber can be maximized by appropriate choice of the applied voltage. The proposed mode size converter can be used in LC photonic lightwave circuits to couple the light between optical devices with maximum efficiency.

Ultra-compact dual-mode mode-size converter for silicon photonic few-mode fiber interfaces.

Fiber couplers usually take a lot of space on photonic integrated circuits due to the large mode-size mismatch between the waveguide and fiber, especially when a fiber with larger core is utilized, such as a few-mode fiber. We demonstrate experimentally that such challenge can be overcome by an ultra-compact mode-size converter with a footprint of only 10 µm. Our device expands TE0 and TE1 waveguide modes simultaneously from a 1-µm wide strip waveguide to an 18-µm wide slab on a 220-nm thick silicon-on-insulator, with calculated losses of 0.75 dB and 0.68 dB, respectively. The fabricated device has a measured insertion loss of 1.02 dB for TE0 mode and 1.59 dB for TE1 mode. By connecting the ultra-compact converter with diffraction grating couplers, higher-order modes in a few-mode fiber can be generated with a compact footprint on-chip.

High coupling efficiency, passive alignment setup for visible-range fiber-to-waveguide edge coupling

We present a method to passively edge couple multiple optical fibers with silicon nitride waveguides in the visible wavelengths. We efficiently convert the fiber mode to the waveguide mode using an inverse taper mode size converter and support passive alignment using a U-groove that centers the optical fiber to the inverse taper. In our prototypes, we measure a coupling efficiency of −4.2 dB per facet. To reduce light leakage to the silicon substrate, we use a 6-μm oxide layer, which also eliminates the additional processes of undercutting the silicon substrate underneath the waveguide. Furthermore, the U-groove structure has a polished edge surface for coupling, reducing the steps of edge polishing the die. Fabrication of this visible-range edge coupler is complementary metal–oxide–semiconductor-compatible, making it a highly scalable method for passively packaging multiple visible-range integrated photonics devices.

Nonparaxial Mode-Size Converter Using an Ultracompact Metamaterial Mikaelian Lens

Mode-division multiplexing (MDM) is a promising approach to increase data transmission capacity by using different modes as independent data channels. Intermodal crosstalk and coherent mode interference are detrimental to the performance of an MDM system and should be avoided in the mode converters and mode expanders for MDM. The Mikaelian lens is well known to be free from spherical aberrations and is therefore an excellent candidate for focusing high order waveguide modes which have main mode field distributions located off-axis. Here we proposed an ultracompact mode-independent mode-size converter using an integrated Mikaelian lens formed by refractive index engineering using a subwavelength grating. The Mikaelian lens is fabricated on the silicon-on-insulator (SOI) platform and can be used to convert the size of high order modes at the interface between the silicon waveguide and a multimode fiber in MDM systems. We present a proof-of-concept demonstration of a low loss spot size converter for three transverse electric (TE) modes between a 14 μm wide multimode waveguide grating coupler and a 1.56 μm wide multimode silicon waveguide. The measured insertion loss of the Mikaelian lens mode size converter is 0.21 dB for TE0 mode, 0.26 dB for TE1 mode and 0.28 dB for TE2 mode. 1-dB bandwidth of all the three modes are about 60 nm. The 15 μm long Mikaelian lens has comparable experimental performance as a 450 μm long adiabatic taper.

Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits.

Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10 dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03-0.1dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.

On-chip integration of single solid-state quantum emitters with a SiO2 photonic platform

One important building block for future integrated nanophotonic devices is the scalable on-chip interfacing of single photon emitters and quantum memories with single optical modes. Here we present the deterministic integration of a single solid-state qubit, the nitrogen-vacancy (NV) center, with a photonic platform consisting exclusively of SiO2 grown thermally on a Si substrate. The platform stands out by its ultra-low fluorescence and the ability to produce various passive structures such as high-Q microresonators and mode-size converters. By numerical analysis an optimal structure for the efficient coupling of a dipole emitter to the guided mode could be determined. Experimentally, the integration of a preselected NV emitter was performed with an atomic force microscope and the on-chip excitation of the quantum emitter as well as the coupling of single photons to the guided mode of the integrated structure could be demonstrated. Our approach shows the potential of this platform as a robust nanoscale interface of on-chip photonic structures with solid-state qubits.

Silicon-Nitride-Assisted Edge Coupler Interfacing With High Numerical Aperture Fiber

Reducing the mode size mismatch between high-index-contrast silicon waveguide and single-mode fiber takes an important role in photonic integration. A low-loss and broadband silicon-nitride-assisted edge coupler for the coupling of light between silicon wire waveguides and high numerical aperture fiber is proposed and optimized for TE polarization. The mode size converter is composed of silicon inverse taper waveguides in the core and two silicon nitride interlayers spaced by silica in cladding layers. The edge coupler is characterized by cleaved high numerical aperture fiber (mode field diameter is about $6.5~\mu \text{m}$ ). With the help of refractive index matching oil ( $n =1.46$ ), the coupling loss for TE polarized light input is 0.35 dB at 1570 nm. The 1-dB bandwidth is 95 nm. The 1-dB alignment tolerances are higher than ± 2.5 and $\pm ~1~\mu \text{m}$ in the horizontal and vertical directions, respectively, and higher than $25~\mu \text{m}$ in the axial direction.

Low-Loss Connection of Embedded Optical Fiber Sensors Using a Self-Written Waveguide

This letter reports on a new concept for making a low-loss connection to an optical fiber sensor, which is embedded in a fiber reinforced composite material for structural health monitoring. First, a cross section of the composite is made to reveal the end-face of the embedded optical fiber. Then, an external fiber is aligned and an intermediate polymer-based self-written waveguide (SWW) is fabricated between both fibers. This SWW bridges the gap and serves as mode size converter ensuring a low-loss optical connection even if both fibers have dissimilar mode field diameters. The advantage of this approach is that no special care needs to be taken during the composite production cycle not to damage the sensor fiber in- or egress points, since it will be reconnected afterward.

Periodically-segmented liquid crystal core waveguides

We report the fabrication and characterization of electrically-tunable periodically segmented waveguides (PSWs) with different duty cycles of 0.25, 0.33, 0.50 and 0.76, using the nematic liquid crystal 5CB as the guiding layer, and the negative photoresist AZ15nXT as the cladding. The experimental results show that light diffracts and re-focuses periodically on propagation through the liquid crystal (LC) core PSW, when an external voltage is applied to the periodically segmented electrodes. The performance of the fabricated LC core PSWs are analyzed in terms of effective refractive index, output power and duty cycle. The electrically-tunable LC core PSWs have potential application in the realization of optical filters, polarizers and dynamic mode size converters.

Short and efficient mode-size converter designed by segmented-stepwise method.

We present an efficient segmented-stepwise method to design a short and low-loss mode-size converter. A silicon-on-insulator platform-based converter with 20 μm length and 95.2% conversion efficiency is acquired by taking only 10 optimization generations using 2D-FDTD method. A 3D-FDTD simulation is performed to verify the calculated results, returning an efficiency of 92.1%. The proposed device can be used to connect a 12-μm-wide waveguide and a 0.5-μm-wide single-mode waveguide, with comparable performance of a regular scheme using 150-μm-long linear taper. For demonstration, the converter was fabricated by electron-beam-lithography and inductively-coupled-plasma etching. A conversion loss of -0.62±0.02 dB at 1550 nm was experimentally measured.

Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material.

High-index-contrast (HIC) waveguide such as Si and Si3N4 has small mode size enabling compact integration. However, the coupling loss with single mode fiber is also remarkable owning to the mode mismatching. Therefore, mode size converter, as the interface between HIC waveguide and optical fiber, takes an important role in the field of integrated optics. The material with refractive index (RI) between HIC waveguide and optical fiber can be used as a bridge to reduce the mode mismatching loss. In this letter, we employ silicon oxynitride (SiON) with RI about 1.50 as the intermediate material and optimize the structure of the SiON waveguide to match with cleaved single mode fiber and HIC waveguide separately. Combined with inverse taper and suspended structure, the mismatching loss is reduced and the dependence to the dimension of the structure is also released. The coupling loss is 1.2 and 1.4 dB/facet for TE and TM mode, respectively, with 3 dB alignment tolerance of ± 3.5 μm for Si(3)N(4) waveguide with just 200 nm-wide tip. While for Si waveguide, a critical dimension of 150 nm is applied due to the higher index contrast than Si(3)N(4) waveguide. Similar alignment tolerance is realized with coupling loss about 1.8 and 2.1 dB/facet for TE and TM mode. The polarization dependence loss (PDL) for both platforms is within 0.5 dB.

CMOS compatible integration of Si/SiO_2 multilayer GRIN lens optical mode size converter to Si wire waveguide

We present a CMOS compatible mass manufacturable, compact Si/SiO(2) multilayer GRIN lens mode size converter from standard single mode fiber to 300 nm-thick Si waveguide. The fiber-to-GRIN lens coupling loss is 2.6 ± 0.3 dB (coupling efficiency: 51~60%) with optimized focal length of 11.6~11.8 μm and Si/SiO(2) multilayer thickness of 7.4 μm.

Mode-size converter with high coupling efficiency and broad bandwidth

An ultralow coupling loss and broad bandwidth fiber-to-waveguide mode-size converter is demonstrated for nano-scale waveguides on SOI platform using CMOS technology in this paper. The mode-size converter consists of a cantilevered PECVD SiO(2) waveguide and a-Si nano-tapers by removing the adjacent SiO(2) layer and underlying substrate Si. The a-Si waveguide is located at the center of the cantilevered SiO(2) waveguide. We characterized the cantilevered mode-size converter using cleaved optical single mode fiber with 10.5 µm mode field diameter. With refractive index (1.375) matching oil, the measured coupling efficiencies between the cleaved optical fiber and this converter are higher than 80% per facet and 70% per facet for TE and TM modes at 1600 nm, respectively. The polarization dependent loss and the coupling loss variation of this converter are less than 1.0 dB at the wavelength range of 1520~1640 nm. The 1-dB bandwidths for both TE and TM modes are more than 120 nm. The alignment tolerances for TE and TM modes are ± 2.8 µm and ± 2.1 µm at 1-dB excess loss in horizontal direction and vertical direction, respectively.

Effective plasmonic mode-size converter

Plasmonic mode-size converters (PMSCs) for long-range surface plasmon polaritons (LR-SPPs) at the wavelength of 1.55 μm are presented. The PMSC is composed of an insulator-metal-insulator waveguide (IMI-W), a laterally tapered insulator-metal-insulator-metal-insulator waveguide (LT-IMIMI-W), and an IMIMI-W in series. The mode-intensity sizes of the LR-SPPs for the IMI-W and the IMIMI-W were not only calculated using a finite element method but were also experimentally measured. The propagation losses of the IMI-W and the IMIMI-W as well as the coupling losses between them were analyzed by the cut-back method to investigate the effect of LT-IMIMI-Ws. By using the PMSC with a ~27 ° angled LT-IMIMI-W, the coupling loss between a polarization-maintaining fiber and a 3 μm-wide IMIMI-W was reduced by ~3.4 dB. Moreover, the resulting mode-intensity in the output of the PMSC was squeezed to ~35% of the mode-intensity in the input IMI-W. The PMSC may be potentially useful for bridging micro- to nano-plasmonic integrated circuits.

Slot Waveguide by Using Double High-Mesa Structure for Optical Absorption Sensing

Slot waveguide, of which the optical field profiles mainly in the slot region, is attractive for sensing application by optical absorption because it has a high portion of optical field inside the slot region. A slot waveguide itself can be structured by utilizing buried hetero, ridge, rib, strip, and high-mesa structures. Among them, slot waveguide by using double high-mesa structure is attractive because it offers higher portion of optical field profile inside the slot region compared to the others. One critical issue is its propagation condition, as the slot waveguide structure may allow “directional-coupler” mode, which prevents proper utilization especially to the sensing application. To get rid of the directional-coupler mode propagation, here we propose a scheme by utilizing leaky mode for each high-mesa part. Under the condition of leaky mode for each high-mesa part, it is successfully confirmed that there is a propagation condition of which optical field mainly profiles in the slot region. Based on this propagation condition, more than 85% of the optical field profile can be achieved in the slot region. Moreover, under this condition, it is also confirmed that a very small radius of curvature of 10 µm is available, which is useful for long length integration into a compact area. We have also discussed about a mode-size converter to inject optical field into this double high-mesa slot waveguide from regular single high-mesa waveguide. Only 1.08 dB excess loss has been reported at mode-size converter.