Waveguide Taper Research Articles

Silica‐Rich Sol‐Gel Ink for Two‐Photon Direct Laser Writing of Microscale Structures and Optical Elements

AbstractInnovative materials enable two‐photon polymerization for precise additive fabrication of intricate optical components. Existing polymeric inks suffer from poor optical performance due to their high organic content. Silica‐rich sol‐gel ink (75 wt%, under ambient conditions) is presented for high‐precision three‐dimensional (3D) printing of microscale structures and optical devices. A photoinitiator with high‐efficiency nonlinear absorption is introduced to initiate two‐photon polymerization. Utilizing a commercial direct laser writing system, 3D‐printing is demonstrated including the optimization of printing parameters and ink characterization. The optical performance of the printed microscale elements using the new sol‐gel ink surpasses commercially available polymeric inks on key metrics: Printed elements exhibit extremely low surface roughness, 3nm; visible and near‐IR light transmission is greater than 90%; Printed elements present enhanced mechanical and chemical resistance to common organic solvents; Structures exhibit low isotropic shrinkage, <10%; Elements withstand continuous‐wave laser intensities exceeding 7 × 105 W cm−2. Direct writing onto an optical fiber tip with no need for surface functionalization is presented, with the demonstration of a tall waveguide taper (≈1:5 aspect ratio) with low losses and modal crosstalk, and a freestanding photonic lantern mode multiplexer. The new ink can be utilized in a variety of applications and across many materials, requiring compact, durable, and complex optical elements.

Interlaboratory Comparison of Power Measurements at Millimetre- and Sub-Millimetre-Wave Frequencies

The aim of this paper is to compare the power measurement capabilities in millimetre- and sub-millimetre-wave frequency bands of several national metrology institutes and one research institute. The first comparison, in WR-6.5 waveguide (110 GHz to 170 GHz), involved NPL, TUBITAK UME and PTB. The second comparison, in WR-1.5 waveguide (500 GHz to 750 GHz), involved NPL, METAS, TUBITAK UME, LNE, WAT, GUM and VDI. Two types of travelling standards were used for these comparisons: a thermoelectric power sensor in the WR-6.5 band and a calorimetric power sensor in the WR-6.5 and WR-1.5 bands. The thermoelectric power sensor was characterised by the participants against their own standards and a generalised effective efficiency was calculated. The calorimetric power sensor operating in the WR-6.5 band was measured to observe its behaviour during the comparison and was also measured in the WR-1.5 band after being fitted with a suitable waveguide taper and used in conjunction with a frequency multiplier. The participants measured the output of the calorimetric power sensor and their own power sensor standard. A normalised power ratio method was used as a comparison parameter for the WR-1.5 band measurements. In addition, a pyroelectric power standard was used by METAS to measure absolute power, and a frequency of 650 GHz was used as a link between the absolute power and the power ratios. Finally, all but two of the measurement points compared between the participants achieved agreement in terms of En scores less than 1. For the first time, an interlaboratory comparison of power measurements at sub-millimetre frequencies has been performed and, overall, good agreement was achieved between the different laboratories.

Optical characterization of mass-productive metal-insulator-metal plasmonic waveguide with a linear taper for nanofocusing.

This paper conducts an experimental evaluation of the optical properties of mass-productive metal-insulator-metal linear taper waveguides for nanofocusing. The vertical linear tapers, with controlled angles in the 12-51 degrees range, were realized with dry etching and mixed gas, while tip-thickness was precisely controlled with atomic layer deposition. The transmission efficiency of the linear taper was measured employing an input grating and a single output slit. The maximum transmission efficiency was estimated at 64% at a taper angle of 30 degrees, which aligned with the calculations. This experimental evaluation provides guidance for the design of practical nanofocusing components.

Integrated Broadband Filter with Sharp Transition Edges Based on SiN and SiON Composite Waveguide Coupler

Broadband filters with sharp transition edges are important elements in diverse applications, including Raman and fluorescence spectral analysis, wideband wavelength-division multiplexing (WDM), and multi-octave interferometry. While the multi-layer thin-film interference broadband filter has been widely applied in various free-space optical systems, its integrated counterpart is still far from mature, which is also highly desired for constructing chip-scale miniature optical modules. In this article, we design, fabricate, and characterize an integrated broadband filter with sharp transition edges. An adiabatic coupler based on silicon nitride (SiN) and silicon oxynitride (SiON) composite waveguide is employed here. Long-pass, short-pass, band-pass, and band-stop filters can be realized in a single design of the composite waveguide coupler for a specific wavelength range, with only a difference in the SiN taper waveguide width. Experimental results with a roll-off value of larger than 0.7 dB nm−1 and a 15 dB extinction ratio (ER) are presented.

Design and experimental demonstration of a silicon five-mode (de)multiplexer based on multi-phase matching condition.

A compact 5-mode (de)multiplexer [(De)MUX] is proposed and experimentally demonstrated based on the principle of multi-phase matching. The proposed device comprises a cascaded asymmetric directional coupler (ADC) based on 3-mode phase-matching, a polarization beam combiner, and a taper waveguide connecting them. The multiple modes in the access waveguides are matched to different modes in the same bus waveguide, which eliminates the need for additional taper structures and results in a total coupling length of only 18.9 µm. Experimental results exhibit that the insertion losses of the five modes are below 3.4 dB, and the mode crosstalks are below -15 dB at the central wavelength. The 3-dB bandwidths of TM0, TM1, TE0, TE1, and TE2 modes are greater than 100 nm, 46 nm, 100 nm, 28 nm, and 37 nm, respectively. The proposed device can serve as a key functional component in highly integrated on-chip mode-division multiplexing systems.

1.55-μm Sidewall Grating DFB Lasers Integrated With a Waveguide Crossing for an Optical Beam Forming Network

By using asymmetric twin-waveguide technology, a 1.55-μm sidewall grating distributed feedback laser monolithically integrated with a passive waveguide crossing was for the first time demonstrated for an optical beam forming network, which needs only one metalorganic vapor-phase epitaxy step. The distributed feedback laser with uncoated facets presents a side-mode suppression ratio of >44 dB and a low 3-dB linewidth of 68 kHz. An elliptical parabolic taper waveguide crossing had a theoretical crosstalk suppression ratio of 40 dB, while the measured cross-talk suppression ratio was at least 20 dB.

Mode division (de)multiplexer based on transverse mode conversion in photonic crystal using asymmetric directional couplers.

Mode division multiplexing technology has the potential to increase the channel capacity of a single wavelength carrier. Attaining cost-effective high-bandwidth-density devices with small footprints is a concern, and photonic crystal based devices are promising for ultra-small on-chip communications. This paper presents a 2D photonic crystal based mode division (de)multiplexer on a silicon on insulator platform. The device comprises two coupling regions of asymmetric directional couplers that perform mode conversion operations between the fundamental mode and higher-order modes. Each coupling section is dedicated to converting a specific mode. Mode conversion is achieved by designing a multimode waveguide to satisfy the phase-matching condition of the desired mode with the single mode waveguide. Two linear adiabatic tapers are introduced for the smooth transition of modes between waveguide sections. The device is designed and simulated for three-channel modes at 1550nm using the finite-difference time-domain technique. The obtained insertion loss and cross talk are <0.41d B and <-20.14d B, respectively. The overall size of the proposed mode division (de)multiplexer is 328.5µm 2. A fabrication tolerance study for the proposed device is performed by varying the rod radius and position in the device structure's taper and bus waveguide regions. The proposed 2D photonic crystal based mode division (de)multiplexer has the potential to be used in large-capacity optical communication systems.

Efficient Fiber‐to‐Chip Interface via an Intermediated CdS Nanowire

AbstractAn efficient fiber‐to‐chip interface via an intermediated CdS nanowire is demonstrated. The fiber mode is firstly squeezed through a fiber taper drawn at the end of a single‐mode fiber, then evanescently coupled into an intermediated CdS nanowire with a longitudinally tapering profile, and finally coupled into an on‐chip silicon waveguide (SiW) via a waveguide taper fabricated on it. Since the fiber‐nanowire‐SiW cascade structure is designed to match effective indices in each coupling area, such a fiber‐to‐chip interface ensures a bidirectional coupling efficiency up to 90% and a 3‐dB bandwidth over 100 nm in experiments. The difference of coupling efficiencies between the TM or TE modes is less than 0.5 dB in the spectral range of 1545–1635 nm. The footprint of the on‐chip coupling structure is about 10 µm in size. The results may provide a compact, efficient, and versatile fiber‐to‐chip interface in applications including optical interconnects, coherent communication, and quantum optical circuitry.

Adiabatic operation slope-loss algorithm for ultrashort and broadband waveguide taper

In this paper, an adiabatic operation slope-loss algorithm (AO-SLA) is developed for designing efficient adiabatic waveguide tapers. The algorithm can compute the efficient adiabatic guided-wave shape of the waveguide taper. A well-studied case of an adiabatic waveguide taper is used to show how it can readily generate a guided-wave shape that gives the same mode-connection power transfer with a much shorter length compared with the linear shape. In the waveguide taper design, it is shown that the device efficiency designed with AO-SLA is better than that based on the parabolic case and the SLA case, and the adiabatic taper designed can give a broad operating bandwidth. Moreover, the adiabatic condition α for AO-SLA can be very large.

Subwavelength structures for taper waveguides

In Photonic Integrated Circuits (PICs) it is often necessary some sort of mismatch adaptation between waveguides of different cross-sections. There are several instances of such a designing constraint, being the vertical coupling between the PIC and an optical fibre probably the most representative of all examples. Here, the beam of electromagnetic energy inside the PIC must be inserted/extracted through/to an optical fibre. Typical core diameters are approximately 10 μm and 5 μm, for single mode optical fibres operating in the near infrared and visible wavelengths, respectively. On the other hand, the optical interconnects linking individual structures in PICs are usually single mode waveguides, 400 to 500 nm wide and a few hundreds of nanometres thick. This presents a bidimensional mismatch between the optical fibre and the single mode waveguide within the PIC, that requires both lateral and longitudinal beam expansions. In this work, we have approached the lateral expansion of the fundamental mode propagating in a single mode waveguide, at the operating wavelength of 1550 nm and being coupled out into an optical fibre, through a grating structure 14.27 μm wide. To this end, we have designed and simulated a subwavelength metamaterial planar structure, which is able to expand laterally the fundamental mode’s profile from 450 nm to 14.27 μm, within 11.1 μm. Furthermore, we will be presenting the results obtained when comparing this structure with several linear inverted taper waveguides, regarding coupling and propagation efficiencies. Namely, we compared the coupling efficiencies of the modes propagating in an 100 μm long waveguide, when being excited by the analytically calculated fundamental mode and the fields obtained at the end of the designed structure. The results obtained for the designed structure 11.1 μm long and the calculated fundamental mode showed a coupling efficiency of -1.53 dB and -1.20 dB, respectively.

A Balanced Frequency Doubler Covering 140–220 GHz With an Efficiency of 6.8%–11.6%

In this letter, a full-band balanced frequency doubler covering 140–220 GHz range is reported. To feed the Schottky barrier diodes (SBDs) with a reverse phase, a novel inline waveguide-to-microstrip (WG-to-MS) transition is adopted, which is used as mode conversion and broadband balun simultaneously. The generated second-harmonic is extracted directly by a taper waveguide, which also serves as broadband output matching. Compared with traditional Erickson-style frequency doublers, the presented doubler not only possesses an inherent inline input–output configuration, but also eliminates the dielectric loss and transition loss at the output circuit with an expanded bandwidth. To verify the proposed concept, the doubler is fabricated and measured, which exhibits a conversion efficiency of 6.8%–11.6% across the whole 140–220 GHz band under 10–20 mW input power.

Optical phase shifter design for single and multi-mode waveguide configurations

In this paper, we introduce simple design equations for optical phase shifter configurations on both single mode and multimode platforms. The design equations are based on approximate analytical expressions describing the phase shift (difference) between two waveguides, one is considered as the reference guide and the second is considered as the phase shifter. Different layout geometrical configurations have been studied: the step width and the linear tapering waveguide. The validity of the design formula is tested using standard Beam Propagation Method BPM. The obtained results are in good agreement with numerical calculations and allow using these closed form expressions in the optical design of such phase shifters.

III–V gain region/Si waveguide hybrid lasers with InP-based two-storied ridge structure by direct bonding technology

We have fabricated III–V gain region/Si waveguide hybrid lasers with an InP-based two-storied ridge structure using direct bonding technology. Continuous-wave operation at a temperature of 20 °C with a threshold current as low as 30 mA was obtained for a hybrid laser consisting of a Fabry–Perot cavity with facets formed on the Si waveguide, and it was achieved thanks to a high optical coupling efficiency of over 80% at the interface between the III–V gain region and the Si waveguide using a two-storied ridge structure and a taper waveguide. The wavelength tunable laser which is comprised of a Si cavity with a loop mirror and double ring filters and III–V gain regions with laser and semiconductor optical amplifier (SOA) sections was demonstrated by utilizing this hybrid structure. It exhibited a wide wavelength tuning range of 48.6 nm and an optical amplification of 10 dB by SOA.

High-performance silicon polarization switch based on a Mach–Zehnder interferometer integrated with polarization-dependent mode converters

Abstract As the key element for optical systems, polarization controllers with versatile functionalities are highly desired. Here, a CMOS-compatible polarization switch is proposed and realized by using a Mach–Zehnder interferometer integrated with two polarization-dependent mode converters (PDMCs) at the input/output ends. The PDMCs, which utilize the mode hybridness and adiabatic mode evolution in a silicon-on-insulator (SOI) ridge waveguide taper, provide a low-loss adiabatic transmission for the launched TE0 mode as well as efficient mode conversion from the launched TM0 mode to the TE1 mode. For the MZI structure, there are two 1 × 2 dual-mode 3-dB power splitters based on a triple-core adiabatic taper, and two thermally-tunable phase-shifters embedded in the arms. The polarization state and the polarization extinction ratio (PER) of the transmitted light can be dynamically tuned by introducing some phase difference between the MZI arms electrically. The fabricated device has an excess loss of ∼0.6 dB for the TE0 and TM0 modes. When the switch is off, the TE0 and TM0 modes go through the device without exchange. In contrast, when the switch is on, the TE0–TM0 conversion occurs and the measured PER is about 20 dB.

Photonic generation of broadband linearly chirped microwave waveform based on a low-loss silicon on-chip spectral shaper.

A silicon on-chip spectral shaper based on a Sagnac loop incorporating a chirped multi-mode waveguide Bragg grating (WBG) for linearly chirped microwave waveform generation is fabricated and demonstrated. The transmission spectrum of the spectral shaper displays low insertion loss characteristic due to the application of edge coupling taper and multi-mode waveguide based grating. An up-chirped microwave waveform with bandwidth as large as 44 GHz is generated by mapping the spectrum profile of the spectral shaper to the temporal domain through a dispersion fiber. The instantaneous frequency of the generated signal shows good linearity benefiting from the weak modulation strength in the multi-mode WBG. The low insertion loss performance as well as the low dispersion value required in our design presents feasibility in further integration with on-chip dispersion.

Orthogonalization of far-field detection in tapered optical fibers for depth-selective fiber photometry in brain tissue.

The field of implantable optical neural interfaces has recently enabled the interrogation of neural circuitry with both cell-type specificity and spatial resolution in sub-cortical structures of the mouse brain. This generated the need to integrate multiple optical channels within the same implantable device, motivating the requirement of multiplexing and demultiplexing techniques. In this article, we present an orthogonalization method of the far-field space to introduce mode-division demultiplexing for collecting fluorescence from the implantable tapered optical fibers. This is achieved by exploiting the correlation between the transversal wavevector kt of the guided light and the position of the fluorescent sources along the implant, an intrinsic property of the taper waveguide. On these bases, we define a basis of orthogonal vectors in the Fourier space, each of which is associated with a depth along the taper, to simultaneously detect and demultiplex the collected signal when the probe is implanted in fixed mouse brain tissue. Our approach complements the existing multiplexing techniques used in silicon-based photonics probes with the advantage of a significant simplification of the probe itself.

Heterogeneously integrated membrane III-V compound semiconductor devices with silicon photonics platform

Silicon photonics is a key technology for constructing large-scale photonic integrated circuits (PICs) because it enables large-scale wafer processes with high uniformity and quality. To further improve device characteristics, heterogeneous integration of III-V compound semiconductors that provide optical gain, a high modulation efficiency, and optical non-linearity is desired. This paper describes the heterogeneous integration of membrane III-V compound semiconductor photonic devices that have a similar structure including thickness and refractive index. These devices provide efficient optical coupling with a Si waveguide using a simple taper waveguide structure. If the total thickness of the film structure is designed to be less than the critical thickness (calculated to be 430 nm for fabrication conditions such as bonding and growth temperatures), high-quality epitaxial layers can be grown on a thin InP layer directly bonded to the Si substrate. Therefore, regrowth techniques are employed on bonded InP layer on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate. We fabricate two kinds of laser-integrated Mach-Zehnder modulators using epitaxial regrowth on Si substrates. One uses Si phase modulators, and the other uses InP-based modulators. A micro-transfer-printing technology is also important when the number of III-V devices is relatively small. Furthermore, the micro-transfer-printing technology enables devices to be selected that meet the required characteristics before integration. For this purpose, we try to integrate a membrane laser on a Si substrate, in which the membrane laser is fabricated on InP substrate. The device shows a threshold current of 0.8 mA when the active region length is 140 μm. Finally, we briefly describe a transmission module, in which directly modulated membrane lasers and electronic drivers are integrated by flip-chip bonding through Au bumps. To reduce power consumption, it is important to design driver circuits that incorporate semiconductor lasers as electronic components. We demonstrate a 2-channel 53-Gbit/s 4-level pulse amplitude modulation (PAM4) transmitter front-end consisting of a 2-channel PAM4 shunt laser driver and 2-channel O-band directly modulated membrane lasers. The total power consumption is only 60.7 mW, resulting in 0.57 mW/Gbit/s.

Numerical investigation of a light delivery device using metal/insulator/metal with a 3D linear taper waveguide and an input grating for heat-assisted magnetic recording.

Heat-assisted magnetic recording (HAMR), a new technique to overcome the data-density limitation, uses a laser to temporarily heat a nanovolume of a recording medium. Thus, this paper proposes a light delivery device that uses the metal/insulator/metal waveguide with a three-dimensional linear taper, and a grating added for an input, for HAMR. Our structure was calculated by finite-element method simulation. By design, 830 nm of light was delivered into a 50nm2 spot area with 63% coupling efficiency, and power intensity was enhanced 930 times. This achievement potential could be applied to the HAMR system in the future.

Ultra-Compact and Ultra-Broadband Polarization-Insensitive Mach–Zehnder Interferometer in Silicon-on-Insulator Platform for Quantum Internet Application

Polarization dependence in integrated silicon photonics has a detrimental effect on the manipulation of quantum state with different polarizations in the quantum technology. Those limits have profound implications for further technological developments, especially in quantum photonic internet. Here, we propose a polarization-independent Mach–Zehnder interferometer (MZI) structure based on a 340 nm-thick silicon-on-insulator (SOI) platform. The MZI facilitates low loss, broad operating bandwidth, and large tolerance of the fabrication imperfection. We achieve an excess loss of &lt;10% and an extinction radio of &gt;18 in the 100 nm bandwidth (1500∼1600 nm) for both transverse electric (TE) and transverse magnetic (TM) modes. We numerically demonstrate an interference visibility of 99% and a polarization-independent loss (PDL) of 0.03 for both polarizations at 1550 nm. Furthermore, by using the principle of phase compensation and self-image, we shorten the length of the waveguide taper by almost an order of magnitude with the transmission of &gt;95% for both TE and TM polarizations. Up to now, the proposed structure could significantly improve the integration and promote the development of monolithic integrated quantum internet.

Polarization-insensitive ultra-short waveguide taper.

Waveguide taper, a key component in the photonic integrated circuit (PIC), enables on-chip mode conversion, but large-footprint tapers are detrimental to the PIC, which desires compact and efficient devices. Polarization sensitivity also limits the tapers in the applications involving orthogonal modes. In this work, we design an efficient polarization-insensitive ultra-short MMI-based waveguide taper, through the mode spreading principle and the self-image principle. The proposed taper is 26.3 µm long, one order of magnitude shorter than the standard linear taper. We fabricate the taper, and experimentally demonstrate that it exhibits a high transmission efficiency of ∼70% and a wide 1 dB bandwidth of >54nm, for both TE and TM polarizations.