Subwavelength Grating Couplers Research Articles

Wideband-Efficient SOI Uniform Subwavelength Grating Couplers by Effective-Index and Leakage-Factor Matching at Multiple Wavelengths

Wideband-Efficient SOI Uniform Subwavelength Grating Couplers by Effective-Index and Leakage-Factor Matching at Multiple Wavelengths

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.

Ultra-Broadband Subwavelength Grating Coupler for Bound State in Continuum

Grating couplers (GCs) are important for light to couple between optical fibers and photonic integrated circuits (PICs). The Bound State in Continuum (BIC) waveguides offer new functionalities for PIC. High-Q resonators, electro-optical modulator, and photodetectors can be built on it without etching steps. Due to the special waveguide geometry of BIC waveguides, a high efficiency GC is still missing. In this work, we demonstrated a subwavelength grating (SWG) GC for BIC waveguide with a coupling efficiency of 8 dB and a 1 dB bandwidth >70 nm. The device shows ultra-broadband behavior which is extremely useful for BIC-based PICs.

Dual-Polarization Bandwidth-Bridged Bandpass Sampling Fourier Transform Spectrometer from Visible to Near-Infrared on a Silicon Nitride Platform.

On-chip broadband optical spectrometers that cover the entire tissue transparency window (λ = 650–1050 nm) with high resolution are highly demanded for miniaturized biosensing and bioimaging applications. The standard spatial heterodyne Fourier transform spectrometer (SHFTS) requires a large number of Mach–Zehnder interferometer (MZI) arrays to obtain a broad spectral bandwidth while maintaining high resolution. Here, we propose a novel type of SHFTS integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the bandwidth and resolution of the SHFTS without having an outrageous number of MZI arrays or adding additional active photonic components. By applying the bandpass sampling theorem, the continuous broadband input spectrum is divided into multiple narrow-band channels through tuning the phase-matching condition of the SWGC with different polarization and coupling angles. Thereby, it is able to reconstruct each band separately far beyond the Nyquist criterion without aliasing error or degrading the resolution. We experimentally demonstrated the broadband spectrum retrieval results with the overall bandwidth coverage of 400 nm, bridging the wavelengths from 650 to 1050 nm, with a resolution of 2–5 nm. The bandpass sampling SHFTS is designed to have 32 linearly unbalanced MZIs with the maximum optical path length difference of 93 μm within an overall footprint size of 4.7 mm × 0.65 mm, and the coupling angles of SWGC are varied from 0° to 32° to cover the entire tissue transparency window.

Compact silicon-based polarization beam splitter using directional couplers assisted with subwavelength gratings

A compact silicon-based polarization beam splitter (PBS) using directional couplers assisted with subwavelength grating (SWG) is proposed and analyzed in detail, where a conventional directional coupler at the input/output end and an SWG coupler in the middle together with SWG-based transitions between them are used. The introduced SWG structures can significantly improve the coupling strength for TE mode, whereas that for TM mode is hardly affected. Subsequently, by carefully choosing structural dimensions, the total coupling length for TM mode can be exactly twice that of TE mode, thus the two polarization states can be effectively separated so that a PBS is realized. Results show that a PBS with a coupling length of 10 μm is achieved, and the extinction ratio, insertion loss, and reflection loss are 23.38/31.78, 0.136/0.104, and −22.03 / − 26.91 dB, respectively, for TE/TM mode. In addition, fabrication tolerance to the key structural parameters and field evolution through the present device are both presented.

On-chip silicon dual-mode multiplexer via a subwavelength grating-based directional coupler and a mode blocker.

An on-chip silicon dual-mode multiplexer (MUX) is proposed via a subwavelength grating (SWG)-based directional coupler and a mode blocker. The proposed mode MUX is optimized by utilizing the three-dimensional full-vectorial finite-difference time-domain-based band structure and propagation calculations. The simulated results indicate that an ultralow mode cross talk of $ - {40}\,\,{\rm dB}$-40dB at the operating wavelength of 1550 nm can be achieved by introducing a ${{\rm TE}_0}$TE0 mode blocker. Benefiting from the triple SWG coupler, the mode-coupling strength can be significantly enhanced inside the proposed mode MUX, which can achieve a compact coupling length of 8.75 µm, a low insertion loss of 0.39 dB at 1550 nm, an ultrabroad bandwidth of 310 nm for the mode CT $ \lt - {15.0}\,\,{\rm dB}$<-15.0dB, and insensitivity to fabrication errors.

Dual-wavelength-band subwavelength grating coupler operating in the near infrared and extended shortwave infrared.

We show that dual-wavelength-band (DWB) subwavelength grating couplers (SWGCs) can be designed for simultaneous coupling of the near-infrared and extended shortwave-infrared (SWIR) fundamental transverse electric polarized light at the same diffraction angle. Numerical simulations predict coupling efficiencies (CEs) larger than 34% for the DWB SWGCs operating in the S/C band (1.48/1.55μm) and extended SWIR band (1.8-2.8μm) with a widely and continuously tailorable peak wavelength separation between 250 and 1250nm. The fabricated DWB SWGCs with peak wavelengths of (1.56, 2.255)μm and (1.487, 2.331)μm respectively obtain CEs of (20.2, 25.8)% and (20.6, 26.9)%, 1-dB bandwidths of 38nm and 54nm at a diffraction angle of 2°.

Broadband and polarization flexible SOI grating coupler based on sub-wavelength gratings with low back reflections

Optical couplers play an integral role in coupling of the guided mode from an optical fiber into a Photonic Integrated Circuit (PIC). Apart from the edge couplers, grating couplers are the most extensively used optical I/O's. Grating couplers have limited application owing to their polarization selectivity and a small coupling 3-dB bandwidth of approx. 70 nm. By engineering the average effective index of the grating coupler using sub-wavelength gratings, we increased the coupling bandwidth while introducing polarization insensitivity. With the a uniform sub-wavelength grating coupler designed in this thesis, we attained 3-dB bandwidth of 105nm and 121 nm for the TE and TM polarizations, respectively. The proposed device attains a simulated coupling efficiency of -4.88dB for both TE and TM polarization at 1550 nm, resulting in zero polarization dependent loss. To the best of our knowledge, the proposed SWGC offers the best reported performance in bandwidth and polarization sensitivity.

Ultracompact and broadband silicon-based TE-pass 1 × 2 power splitter using subwavelength grating couplers and hybrid plasmonic gratings.

An ultracompact TE-pass 1 × 2 power splitter, using subwavelength grating (SWG) couplers and hybrid plasmonic gratings (HPGs), is proposed and analyzed in detail. As the input strip waveguide is tapered to a narrow wire, where TE mode is cutoff, the launched TE-mode can be evenly divided with high efficiency with the help of the SWG couplers, which are placed between the central silicon wire and two nearby output branches. The injected TM-mode will be perfectly reflected by the carefully designed HPG, whose periodically varied metal layer is located above the bottom strip and SWG waveguides. Consequently, a single device combining both the functions of polarization selection and power division can be realized. This is valuable for highly dense integrated circuits. Results show that, with a period number of 4 in HPG, the present device is only ~6.2-μm-long with an extinction ratio (ER) and an insertion loss (IL) of 25.4 and 0.53 dB at 1.55 μm, respectively, and its bandwidth of ER > 20 dB is ~180 nm with the IL < 0.8 dB, showing a broadband property. Besides, fabrication tolerances to the key dimensions are analyzed and modal field evolution through the device is also presented.

Efficient and broadband subwavelength grating coupler for 3.7 μm mid-infrared silicon photonics integration.

A grating coupler is an essential building block for compact and flexible photonics integration. In order to meet the increasing demand of mid-infrared (MIR) integrated photonics for sensitive chemical/gas sensing, we report a silicon-on-insulator (SOI) based MIR subwavelength grating coupler (SWGC) operating in the 3.7 μm wavelength range. We provide the design guidelines of a uniform and apodized SWGC, followed by numerical simulations for design verification. We experimentally demonstrate both types of SWGC. The apodized SWGC enables high coupling efficiency of -6.477 dB/facet with 3 dB bandwidth of 199 nm, whereas the uniform SWGC shows larger 3dB bandwidth of 263.5 nm but slightly lower coupling efficiency of -7.371 dB/facet.

Ultracompact and high efficient silicon-based polarization splitter-rotator using a partially-etched subwavelength grating coupler.

On-chip polarization manipulation is pivotal for silicon-on-insulator material platform to realize polarization-transparent circuits and polarization-division-multiplexing transmissions, where polarization splitters and rotators are fundamental components. In this work, we propose an ultracompact and high efficient silicon-based polarization splitter-rotator (PSR) using a partially-etched subwavelength grating (SWG) coupler. The proposed PSR consists of a taper-integrated SWG coupler combined with a partially-etched waveguide between the input and output strip waveguides to make the input transverse-electric (TE) mode couple and convert to the output transverse-magnetic (TM) mode at the cross port while the input TM mode confine well in the strip waveguide during propagation and directly output from the bar port with nearly neglected coupling. Moreover, to better separate input polarizations, an additional tapered waveguide extended from the partially-etched waveguide is also added. From results, an ultracompact PSR of only 8.2 μm in length is achieved, which is so far the reported shortest one. The polarization conversion loss and efficiency are 0.12 dB and 98.52%, respectively, together with the crosstalk and reflection loss of −31.41/−22.43 dB and −34.74/−33.13 dB for input TE/TM mode at wavelength of 1.55 μm. These attributes make the present device suitable for constructing on-chip compact photonic integrated circuits with polarization-independence.

Compact and high extinction ratio polarization beam splitter using subwavelength grating couplers.

A compact and high extinction ratio polarization beam splitter using subwavelength grating (SWG) couplers is proposed and characterized, where the SWG couplers are located between the two input/output strip waveguides, including SWG-based transitions combined at both ends. The TM mode can be confined well in the strip waveguide and transmits along it with nearly neglected coupling, while the TE mode undergoes a strong coupling and is transferred to the adjacent waveguide with the help of SWG couplers due to dissimilar modal characteristics and cutoff conditions between these two polarizations. To further enhance the performance, an additional tapered waveguide is added in the lateral end of the input SWG-based transition. Results show that a total length of 6.8μm with an insertion loss of 0.08 (0.36) dB, extinction ratio (ER) of 32.19 (20.93) dB, and reflection loss of -34.76 (-32.59) dB for TE (TM) mode is obtained at 1.55μm; its bandwidth can be enlarged to ∼81 nm for an ER>18 dB. In addition, fabrication tolerances and mode-field evolution are also presented.

Design of broadband subwavelength grating couplers with low back reflection

We present a methodology to design broadband grating couplers using one-dimensional subwavelength gratings. Using the presented method, we design subwavelength grating couplers (SWGCs) with 1-dB bandwidths ranging from 50 to 90 nm. Our designed SWGCs have competitive coupling efficiency, as high as -3.8 dB for the fundamental TE mode, and state-of-the-art back reflections, as low as -23 dB.

Apodized Focusing Fully Etched Subwavelength Grating Couplers

We experimentally demonstrate apodized focusing subwavelength grating couplers for both the fundamental transverse electric (TE 00 ) mode and the fundamental transverse magnetic (TM 00 ) mode. A measured insertion loss of 3.2 dB with a 1-dB bandwidth of 36 nm has been obtained for the TE 00 mode, and a measured insertion loss of 3.3 dB with a 1-dB bandwidth of 37 nm has been obtained for the TM 00 mode. Back reflections of -24 dB and -21 dB have been obtained for the TE 00 and TM 00 modes, respectively.

Efficient light coupling into in-plane semiconductor nanomembrane photonic devices utilizing a sub-wavelength grating coupler

We report a subwavelength grating (SWG) coupler for coupling light efficiently into in-plane semiconductor nanomembrane photonic devices for the first time. The SWG coupler consists of a periodic array of rectangular trenches fabricated on a silicon nanomembrane (SiNM) transferred onto a glass substrate. At a wavelength of 1555.56 nm, the coupling efficiency of the fabricated 10 µm wide, 17.1 µm long SWG is 39.17% (-4.07 dB), with 1 dB and 3 dB bandwidths of 29 nm and 57 nm, respectively. Peak efficiency varies by 0.26 dB when measuring 5 fabricated grating pairs. Coupling efficiency can further be improved with an improved SiNM transfer process. Such high efficiency couplers allow for the successful realization of a plethora of hybrid photonic devices utilizing nanomembrane technology.

Direct Side Pumping of Double-Clad Fiber Laser by Laser Diode Array Through the Use of Subwavelength Grating Coupler

An electron-beam-fabricated subwavelength grating coupler for direct side coupling of light emission from a high-power laser diode array (LDA) was studied theoretically and implemented experimentally. A gold-embedded silica-based design for grating coupler was employed to minimize the thermal expansion caused by the accumulated heat from light absorption by metal part of the grating coupler. In addition, with the consideration of the backward diffraction loss and the groove wall nonverticality caused by fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. According to the experimental results, the grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave-operated LDA into the inner clad of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, an LDA side-pumped ytterbium-doped DCF laser by using the grating coupler was demonstrated. By fine tuning the slow-axis collimation lens array, the laser-pumping scheme can easily be switched between bidirectional pumping and unidirectional pumping. Compared with the unidirectionally pumped fiber laser of the same gain fiber length, the laser slope efficiency of the bidirectionally pumped fiber laser was increased by 18% because of a better gain distribution over the fiber length. Finally, the signal output power of 10 W with a slope efficiency of 61% was achieved for the bidirectional side-pumped fiber laser.

Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide

A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.

Subwavelength waveguide grating for mode conversion and light coupling in integrated optics

We propose a new method for mode conversion and coupling between an optical fiber and a sub-micrometer waveguide using a subwavelength grating (SWG) with a period less than the 1st order Bragg period. The coupler principle is based on gradual modification of the waveguide mode effective index by the SWG effect that at the same time frustrates diffraction and minimizes reflection loss. We demonstrate the proposed principle by two-dimensional Finite Difference Time Domain (FDTD) calculations of various SWG structures designed for the silicon-on-insulator (SOI) platform with a Si core thickness of 0.3 microm. We found a coupling loss as small as 0.9 dB for a 50 microm-long SWG device and low excess loss due to fiber misalignment, namely 0.07 dB for a transverse misalignment of +/-1 microm, and 0.24 dB for an angular misalignment of +/-2 degrees. Scaling of the SWG coupler length down to 10 microm is also reported on an example of a 2D slab waveguide coupling structure including aspect ratio dependent etching and micro-loading effects. Finally, advantages of the proposed coupling principle for fabricating 3D coupling structures are discussed.