Relaxed Fabrication Tolerances Research Articles

Extending the spectral operation of multimode and polarization-independent power splitters through subwavelength nanotechnology

Power splitters play a crucial role in virtually all photonic circuits, enabling precise control of on-chip signal distribution. However, state-of-the-art solutions typically present trade-offs in terms of loss, bandwidth, and fabrication robustness, especially when targeting multimode operation. Here, we present a novel multimode 3-dB power splitter based on a symmetric Y-junction assisted by two regions of subwavelength grating metamaterials with different geometries. The proposed device demonstrates high-performance for multimode and dual-polarization operation with relaxed fabrication tolerances by leveraging the additional degrees of freedom offered by two distinct geometries of subwavelength metamaterials to control mode evolution. Our design achieves, to the best of our knowledge, the widest operational bandwidth reported to date for a nanophotonic multimode silicon power splitter. Simulations for a standard 220-nm-thick silicon-on-insulator platform predict minimal excess loss (< 0.2 dB) for the fundamental and the first-order transverse-electric modes over an ultra-broad 700 nm bandwidth (1300 – 2000 nm). For the fundamental transverse–magnetic mode, losses are less than 0.3 dB in the 1300 – 1800 nm range. Experimental measurements validate these predictions in the 1430 – 1630 nm wavelength range, demonstrating losses < 0.4 dB for all three modes, even in the presence of fabrication deviations of up to ± 10 nm. We believe that this device is suitable for the implementation of advanced photonic applications requiring high–-performance distribution of optical signals, such as programmable photonics, multi-target spectroscopy and quantum key distribution.

Relaxed-tolerance subwavelength grating coupler

Short-wavelength mid-infrared (MIR) silicon photonics at 2–2.5 μm wavelengths has a wide range of applications in optical communications, chemical analysis, and environmental monitoring. In this spectral region, subwavelength grating (SWG) couplers can be fabricated by using commercial multi-project wafer (MPW) services, due to their larger device feature sizes compared with those in the telecommunication band. However, conventional SWG couplers are still sensitive to variations in dimensions produced by errors in fabrication processes, restricting the fabrication reproducibility of the devices. Here, we demonstrated a relaxed-tolerance SWG coupler that has relaxed fabrication tolerance to overcome this limitation. The approach is based on the design of the SWG coupler with dual-hole structures to reduce the influence of random fabrication variations on the grating effective refractive index. Theoretically, the slope of the center wavelength shift of the relaxed-tolerance SWG coupler to the variation of the subwavelength hole size is 0.58, while that is 0.63 for the conventional SWG coupler. We demonstrated the relaxed-tolerance SWG coupler with a peak coupling efficiency of −6.2 dB at a wavelength of 2.038 μm with a 1-dB bandwidth of about 30 nm. Additionally, fabrication reproducibility and optical fiber coupling tolerance are also presented. The study paves a promising way toward developing short-wavelength MIR grating couplers with high performance, excellent repeatability, and cost-efficiency.

3D Integrated Laser Attach Technology on a 300-mm Monolithic CMOS Silicon Photonics Platform

Enabling cost-effective and power-efficient laser source on a silicon photonics (SiPh) platform is a major goal that has been highly sought after. In the past two decades, tremendous effort has been made to develop various on-chip integration techniques to enhance SiPh circuits with efficient light-emitting materials. Here we review our recent advancements in hybrid flip-chip integration of III-V lasers on a 300-mm monolithic SiPh platform. By leveraging advanced complementary metal oxide semiconductor (CMOS) manufacturing processes, we have demonstrated wafer-scale laser attach based on a precisely controlled cavity formed on a silicon-on-insulator (SOI) substrate. The laser integration process is aided by precise mechanical alignment features on the SiPh wafer and high-precision fiducials on the laser. Efficient laser-to-SiPh-circuit butt-coupling with optical power up to 20mW was demonstrated through wafer- and module-level characterizations. Key performance metrics including side-mode suppression ratio, mode-hopping, and relative intensity noise were characterized after laser integration. In addition, early reliability assessments were performed on laser-attached SOI wafers and Si submount assemblies to understand the long-term performance stability of the lasers on the monolithic platform. To further enhance the performance of the laser-integrated chip, we explored alternative spot-size converters that could simultaneously enable improved coupling efficiency and relaxed fabrication tolerance, thus showing great promise over traditional designs.

Highly efficient dual-level grating couplers for silicon nitride photonics

We propose and numerically demonstrate a versatile strategy that allows designing highly efficient dual-level grating couplers in different silicon nitride-based photonic platforms. The proposed technique, which can generally be applied to an arbitrary silicon nitride film thickness, is based on the simultaneous optimization of two grating coupler levels to obtain high directionality and grating-fibre mode matching at the same time. This is achieved thanks to the use of two different linear apodizations, with opposite signs, applied to the two grating levels, whose design parameters are determined by using a particle swarm optimization method. Numerical simulations were carried out considering different silicon nitride platforms with 150, 300, 400 and 500 nm thicknesses and initially employing silicon as the material for the top level grating coupler. The use of Si-rich silicon nitride with a refractive index in the range 2.7–3.3 for the top layer material enabled to obtain similar performance (coupling efficiency exceeding − 0.45 dB for the 400 nm thick silicon nitride platform) with relaxed fabrication tolerances. To the best of our knowledge, these numerical results represent the best performance ever reported in the literature for silicon nitride grating couplers without the use of any back-reflector.

Millimeter-Wave LTSA Array Fed by High-Order Modes With a Low Cross-Polarization Level and Relaxed Fabrication Tolerance

In this article, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 12$ </tex-math></inline-formula> millimeter-wave (mmW) linearly tapered slot antenna (LTSA) array, fed by high-order waveguide modes, is introduced. Due to the antiphase characteristic between adjacent channels of a power divider based on the TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{2n,0}$ </tex-math></inline-formula> mode waveguide, the cross-polarized E-field components can cancel each other out in an endfire direction, even in the case of a relatively thick substrate. Moreover, the electric separations between the adjacent channels employed in a traditional substrate integrated waveguide (SIW) power divider can be removed without affecting the field distributions. This facilitates in improving robustness to avoid fabrication errors. Additionally, a general condition of generating a single high-order mode with symmetrical protection is outlined. By combining two antiphase TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> mode waveguides, an equivalent TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> mode waveguide with six identical output channels is realized. To feed a 12-element array, two equivalent TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> mode waveguides are employed as an equivalent TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12,0</sub> mode-feeding network. Instead of the complex 1-to- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -way power divider and relatively thin substrate featured in previous works, the high-order mode-fed LTSA array introduced in this study is able to operate over 26.5–30 GHz with substrate height of 2 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.19\lambda _{0}$ </tex-math></inline-formula> ), yielding a cross-polarization level of less than −15 dB.

Millimeter-Wave Rectangular Dielectric Resonator Antenna Array With Enlarged DRA Dimensions, Wideband Capability, and High-Gain Performance

In this communication, a rectangular dielectric resonator antenna (DRA) is integrated with a backed cavity to enlarge the DRA side length by 1.8 times. The backed cavity also reduces the sensitivity of the DRA resonance to the DRA size, resulting in a relaxed fabrication tolerance. Based on the enlarged DRA design, a stacked DRA (sDRA) is proposed to improve the bandwidth from 8.7% to 15% and enhance the realized gain by an average of 1.5 dB. The proposed mechanisms are verified by fabricating a $4\times4$ sDRA array with an enlarged DRA side length of 1.8 mm ( $0.40 {\lambda }$ ), an improved bandwidth of 62.7–73.9 GHz (16.4%), and an enhanced measured gain of up to 17.2 dBi. The enlarged DRA dimension permits a higher DRA operating frequency without reducing the DRA size. The proposed millimeter-wave (mmW) sDRA array provides a wide bandwidth and high-gain solution for 67 GHz unlicensed band communications.

Ultrabroadband and fabrication-tolerant mode (de)multiplexer using subwavelength structure

An ultrabroadband and fabrication-tolerant mode (de)multiplexer [(De)MUX] is proposed and optimized by inserting subwavelength gratings (SWGs) into a three-waveguide coupler (TWC). The coupling strength can be dramatically enhanced, benefitting from both the SWG and TWC structures. The proposed mode (De)MUX is optimized using a three-dimensional, full-vectorial finite difference time domain based band diagram and propagation analyses. The simulated results indicate that the optimal mode (De)MUX is with a compact coupling length of 14.75 µm, a low loss of 0.64 dB, and a low mode crosstalk of $ - 31.7\,\,{\text{dB}}$−31.7dB at the operating wavelength of 1.55 µm. With the mode crosstalk $ \lt - 15.0\,\,{\text{dB}}$&lt;−15.0dB, an ultrabroad bandwidth of 312 nm and a relaxed fabrication tolerance of $ - 42.7\,\,{\text{nm}}$−42.7nm to $ + 36.5\,\,{\text{nm}}$+36.5nm can be achieved. The 3 dB bandwidth is over 320 nm and the insertion loss is less than 1.6 dB for a $ \pm 50 \,\,{\text{nm}}$±50nm width change.

Polarization- and wavelength-agnostic nanophotonic beam splitter

High-performance optical beam splitters are of fundamental importance for the development of advanced silicon photonics integrated circuits. However, due to the high refractive index contrast of silicon-on-insulator platforms, state-of-the-art nanophotonic splitters are hampered by trade-offs in bandwidth, polarization dependence and sensitivity to fabrication errors. Here, we present a new strategy that exploits modal engineering in slotted waveguides to overcome these limitations, enabling ultra-broadband polarization-insensitive optical power splitters with relaxed fabrication tolerances. The proposed splitter design relies on a single-mode slot waveguide that is gradually transformed into two strip waveguides by a symmetric taper, yielding equal power splitting. Based on this concept, we experimentally demonstrate −3 ± 0.5 dB polarization-independent transmission for an unprecedented 390 nm bandwidth (1260–1650 nm), even in the presence of waveguide width deviations as large as ±25 nm.

A wideband printed slot antenna with harmonic suppression

AbstractA wideband slot antenna with harmonic suppression is proposed to utilize the environment microwave energy. The antenna element is a printed narrow slot with an offset microstrip‐feed line to widen the operating band by generating the double resonance of similar frequencies. By adding a rectangular notch in the middle of slot element and grounding the microstrip terminal, its second‐order and third‐order harmonics are effectively suppressed. The experimental results of the prototype antenna show that the fractional bandwidth of its fundamental mode can reach 37.4%, while the antenna efficiencies of the second and third harmonics both are less than 23%. In the operating band the prototype antenna has the approximately omnidirectional radiation characteristic. This antenna structure also provides relaxed fabrication tolerances.

Wideband multi-stage CROW filters with relaxed fabrication tolerances.

We present wideband and large free spectral range optical filters with steep passband edges for the selection of adjacent WDM communication channels that can be reliably fabricated with mainstream silicon photonics technology. The devices are based on three cascaded stages of coupled resonator optical waveguides loaded on a common bus waveguide. These stages differ in the number of resonators but are implemented with exactly identical unit cells, comprised of a matched racetrack resonator layout and a uniform spacing between cells. The different number of resonators in each stage allows a high rejection in the through port response enabled by the interleaved distribution of zeros. Furthermore, the exact replication of a unique cell avoids the passband ripple and high lobes in the stopband that typically arise in apodized coupled resonator optical waveguide based filters due to fabrication and coupling induced variations in the effective path length of each resonator. Silicon photonics filters designed for the selection of 9 adjacent optical carriers generated by a 100 GHz free spectral range comb laser have been successfully fabricated with 248 nm DUV lithography, achieving an out-of-band rejection above 11 dB and an insertion loss of less than 0.5 dB for the worst channels.

Multistage second-order microring-resonator filters with box-like spectral responses and relaxed fabrication tolerances * * Project supported by the National High Technology Research and Development Program of China (Nos. 2015AA017001, 2015AA010103) and the Natural National Science Foundation of China (NSFC) (Nos. 61235001, 61575187, 61535002, 61204061, 6157031748, 61377067).

We demonstrate an optical filter based on multistage second-order microring resonators (MRs) with box-like spectral responses. Compared with single-stage high-order optical filters with the same number of MRs, the demonstrated structure has comparable performances in the aspects of passband flatness, rolling-off slope and insertion loss. Moreover, the architecture relaxes the fabrication tolerance, electrical wiring and tuning difficulty since there are only two MRs in each stage. We experimentally demonstrate this kind of optical filter with five stages, which shows a 3-dB bandwidth of ~17 GHz, a rolling-off slope of ~5 dB/GHz and an on-chip insertion loss of ~6 dB.

High-Performance Compact Diplexers for Ku/K-Band Satellite Applications

In this paper, two novel compact diplexers for satellite applications are presented. The first covers the Ku-band with two closely spaced channels (Ku-transmission band: 10.7–13 GHz and Ku-reception band: 13.75–14.8 GHz). The second is wider than the first (overall bandwidth up to 50%) achieves the suppression of the higher order modes, and covers the Ku/K-band with a reception channel between 17.2 and 18.5 GHz. Both diplexers are composed of two novel bandpass filters, joined together with an E-plane T-junction. The bandpass filters are designed by combining a low-pass filtering function (based on $\lambda $ /4-step-shaped band-stop elements separated by very short waveguide sections) and a high-pass filtering structure (based on the waveguide propagation cutoff effect). The novel diplexers show a very compact footprint and very relaxed fabrication tolerances, and are especially attractive for wideband applications. A prototype Ku/K-band diplexer has also been fabricated by milling. Measurements show a very good agreement with simulations, thereby demonstrating the validity and manufacturing robustness of the proposed topology.

Millimetre wave SIW diplexer circuits with relaxed fabrication tolerances

Millimetre wave SIW diplexer circuits with relaxed fabrication tolerances

Fourier plane colorimetric sensing using broadband imaging of surface plasmons and application to biosensing

We demonstrate an optical technique for refractive index and thickness sensing of sub-wavelength-thick dielectric analytes. The technique utilizes the broadband, multimode, directional leakage radiation arising from the excitation of hybrid mode surface plasmons (SP) on low aspect ratio periodic plasmonic substrates with period ≈λ. The approach requires relaxed fabrication tolerances compared to extra ordinary transmission-based sensing techniques, wherein minor shifts in the fabricated dimensions result in a very large change from the designed resonant wavelength. We show that refractive index perturbations due to about 10-nm-thick dielectric can be captured optically by the usage of carefully designed plasmonic substrates, a halogen lamp source, free-space optical components, polarizers, and a low-end, consumer-grade charge coupled device camera. The plasmonic substrates were designed for converting the signature of hybrid mode SP excitation into a transmission peak by utilizing a thin homogeneous metal layer sandwiched between the periodic plasmonic structures and the substrate. The resonance is highly sensitive to the refractive index and thickness of the analyte superstrate. The excitation of hybrid mode SP results in a polarization rotation of 90° of the leaked radiation at resonant wavelength. In order to eliminate the problem of image registration (i.e., placing the same feature in the same pixel of the image, for comparison before and after a change in refractive index) for sensing, we perform the color analysis in the Fourier plane. The change in color of the bright emitted spot with highest momentum, corresponding to the leakage of fundamental SP mode, was used to measure the changes in refractive index, whereas the number and color of spots of lower momenta, corresponding to higher-order Fabry Perot modes, was used to measure the variation in thickness. We further show that the Fourier plane analysis can also be used to sense the index of thicker dielectrics, where real plane image analysis may fail to sense index perturbations, simply due to superposition of different modes in the real plane images of such substrates. Control experiments and analysis revealed a refractive index resolution of 10–5 RIU. The results were correlated with simulations to establish the physical origin of the change in the fundamental mode and higher-order modes due to the refractive index and thickness of analyte. As a demonstration of an application and to test the limits of sensing, the substrates were used to image the surface functionalization using 2-nm-thick 11-mercaptoundecanoic acid and immobilization of 7-nm-thick mouse anti-human IgG antibody. In biological systems, where a priori knowledge about a process step is available, where accurate chemical composition testing is not necessary or possible, the presented method could be used to study the surface changes using a label-free sensing mechanism.

Demonstration of integrated polarization control with a 40 dB range in extinction ratio

Polarization controllers are key elements in many fields of optics, including coherent communications, optical imaging, and quantum applications. Here we present a technology-independent polarization controller scheme based on electrically tunable phase shifters and polarization rotators with largely relaxed fabrication tolerances. Using this scheme, we experimentally demonstrate a fully integrated polarization controller in the silicon-on-insulator platform that is tunable over the complete C-band and achieves a polarization extinction range of 40 dB (±20 dB). These results constitute, to the best of our knowledge, the highest polarization extinction range achieved in a fully integrated device, and overcome the existing limitation in the trade-off between integration and performance in polarization management circuits.

Design of millimetre wave diplexers with relaxed fabrication tolerances

A novel millimetre wave waveguide diplexer topology that can be fabricated using relaxed fabrication tolerances is presented. This is achieved by using a highpass filter/hybrid coupler topology in addition to a low order capacitive iris bandpass filter. By targeting the most sensitive centre resonators in the bandpass filter and increasing their length to full wavelength, the sensitivity to fabrication errors associated with changes to the resonator length can be reduced without the introduction of spurious responses close to the passband. The main benefit of this topology is that only one bandpass filter is required to realise the final diplexing response. This is advantageous when tolerance is considered as it is found that resonating structures associated with bandpass filters contribute to tolerance effects. In addition to this, the authors find that the choice of iris discontinuity used to realise the impedance inverters in the bandpass filter can help to relax fabrication tolerances. By subjecting the proposed circuit to a tolerance analysis, simulated results suggest that the circuit can be fabricated with relaxed fabrication tolerances. These discussions are verified with the fabrication of a Ka-band waveguide diplexer, where the measured and simulated scattering parameters are in very good agreement.

Wideband Excitation Technology of &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm TE}_{20}$&lt;/tex&gt;&lt;/formula&gt; Mode Substrate Integrated Waveguide (SIW) and Its Applications

The higher order mode substrate integrated waveguide (SIW) has advantages in the applications, which are: 1) simplifying the structure with reduced fabrication cost and 2) enhancing the stability of performance by relaxed fabrication tolerance. In this paper, two wideband ${\rm TE}_{20}$ mode excitation structures are presented and investigated for the SIW. A slot along the mid line in the longitudinal direction of the SIW is employed to convert the electromagnetic field pattern between slotline and the ${\rm TE}_{20}$ mode SIW. The second structure is based on the slot aperture coupling and can realize wideband direct transition between the ${\rm TE}_{20}$ mode SIW and microstrip line. Both transitions have a simple and compact structure, as well as broadband characteristics. The wideband transition performance is demonstrated in this paper. As application examples of the transitions, two broadband substrate integrated baluns are presented and fabricated based on the ${\rm TE}_{20}$ mode excitation structures and ${\rm TE}_{20}$ mode characteristics. The 180 $^{\circ}$ out-of-phase and equal magnitude of the balun outputs in a wide frequency range can be achieved inherently. The balun based on the slotline excitation has a fractional bandwidth of 50.2%, from 7.3 to 12.2 GHz, with measured return loss, amplitude imbalance, and phase imbalance better than 10 and 0.45 dB and 3.8 $^{\circ}$ . The balun based on the aperture coupling excitation shows a fractional bandwidth of 50.3%, from 7 to 11.7 GHz, with measured return loss, insertion loss, amplitude imbalance, and phase imbalance better than 10, 1, and 0.27 dB and 2.4 $^{\circ}$ , respectively. Both baluns not only show good performance, but also demonstrate the existence of the ${\rm TE}_{20}$ mode in the SIW.

Design of a Label-Free, Distributed Bragg Grating Resonator Based Dielectric Waveguide Biosensor

In this work, we present a resonant, dielectric waveguide device based on distributed Bragg gratings for label-free biosensing applications. The refractive index sensitive optical transducer aims at improving the performance of planar waveguide grating sensor systems with limited Q-factor and dynamic range by combing the advantages of resonant cavities, such as a multitude of resonance peaks with high finesse, with the manageable complexity of waveguide grating couplers. The general sensor concept is introduced and supported by theoretical considerations as well as numerical simulations based on Coupled Mode Theory. In contrast to a single Bragg grating reflector, the presented Fabry-Pérot type distributed Bragg resonator exhibits an extended measurement range as well as relaxed fabrication tolerances. The resulting, relatively simple sensor structure can be fabricated with standard lithographic means and is independent of expensive light-sources and/or detectors, making an affordable but sensitive device, potentially suitable for point-of-care applications.

Integrated polarization beam splitter with relaxed fabrication tolerances

Polarization handling is a key requirement for the next generation of photonic integrated circuits (PICs). Integrated polarization beam splitters (PBS) are central elements for polarization management, but their use in PICs is hindered by poor fabrication tolerances. In this work we present a fully passive, highly fabrication tolerant polarization beam splitter, based on an asymmetrical Mach-Zehnder interferometer (MZI) with a Si/SiO(2) Periodic Layer Structure (PLS) on top of one of its arms. By engineering the birefringence of the PLS we are able to design the MZI arms so that sensitivities to the most critical fabrication errors are greatly reduced. Our PBS design tolerates waveguide width variations of 400nm maintaining a polarization extinction ratio better than 13dB in the complete C-Band.

Fabrication tolerant polarization splitter and rotator based on a tapered directional coupler

A polarization splitter and rotator (PSR) based on a tapered directional coupler with relaxed fabrication tolerance is proposed and demonstrated on the silicon-on-insulator platform. The device is simply constructed by parallel-coupling a narrow silicon waveguide with a linearly tapered wider waveguide. Compared to previously reported PSRs based on a normal directional coupler, which suffer from stringent requirements on the accuracy of the narrow waveguide width, the introduced tapered structure of the wide waveguide can be used to compensate the fabrication errors of the narrow waveguide. In addition, only a single step of exposure and etching is needed for the fabrication of the device. Similar high conversion efficiencies are experimentally demonstrated for a narrow waveguide width deviation of 14 nm with large tolerance to the coupler length.