Polarization-independent Coupler Research Articles

Grating couplers for efficient integration of surface-emitting blue lasers and silicon nitride waveguide

We report here a theoretical investigation on high efficiency grating coupling for surface emitting blue lasers integrated with silicon nitride (Si3N4) waveguide. DBRs (distributed Bragg reflectors) were introduced into the design which located at the upper and one side of the Si3N4 grating coupler region, and oxide buffer layers was also incorporated simultaneously. By optimizing the grating coupler parameters and the DBR structure, the configuration can be adapted for single polarization or TE/TM simultaneous coupling. The coupling efficiency for TE polarization can reach 72.4 %, and the polarization-independent coupling efficiency of 48 % at vertical incidence was also achieved. This work presents a PIC (photonic integrated circuits) on III-V laser strategy for visible light PICs and innovative applications based on blue laser integrated devices.

Low-loss and broadband polarization-diversity edge coupler on a thin-film lithium niobate platform.

Fiber-to-chip coupling is an essential issue for taking high-performance integrated photonic devices into practical applications. On a thin-film lithium niobate platform, such a high-performance coupler featuring low loss, large bandwidth, and polarization independence is highly desired. However, the mode hybridization induced by the birefringence of lithium niobate seriously restricts a polarization-independent coupling. Here, we propose and experimentally demonstrate a high-performance and polarization-diversity cantilever edge coupler (EC) with the assistance of a two-stage polarization splitter and rotator (PSR). The fabricated cantilever EC shows a minimal coupling loss of 1.06 dB/facet, and the fully etched PSR structure shows a low insertion loss (IL) of -0.62 dB. The whole polarization-diversity cantilever EC exhibits a low IL of -2.17 dB and -1.68 dB for TE0 and TM0 mode, respectively, as well as a small cross talk of <-15 dB covering the wavelength band from 1.5 µm to 1.6 µm. A polarization-dependent loss <0.5 dB over the same wavelength band is also obtained. The proposed fiber-to-waveguide coupler, compatible with the fabrication process of popular thin-film lithium niobate photonic devices, can work as a coupling scheme for on-chip polarization-diversity applications.

One-dimensional grating coupler on lithium-niobate-on-insulator for high-efficiency and polarization-independent coupling.

A metal-based one-dimensional grating coupler on an x-cut lithium-niobate-on-insulator wafer structure for a polarization-independent fiber interface is designed and demonstrated. By using a metal-based plasmonic mode, the diffractive angle for the two polarized modes in the lithium niobate ridge waveguide can be tuned to be the same. The polarization dependence of the grating coupler therefore can be effectively reduced. The fabricated device exhibits -3.56-dB and -4.08-dB peak coupling losses per coupler at 1573 nm for the TE and TM modes, respectively. The polarization-dependent losses are less than 0.69 dB in a 44-nm wavelength range. The demonstrated grating coupler can serve as a polarization-independent optical fiber interface on lithium-niobate-on-insulator and facilitate on-chip polarization diversity applications.

Low loss and polarization-insensitive coupling length for a terahertz fiber directional coupler with symmetric dual-suspended core structure

In this work, a terahertz (THz) fiber directional coupler with a symmetric dual-suspended core structure has been proposed for realizing low loss and polarization-insensitive coupling length in the THz frequency region. The fiber core of the directional coupler consists of two round solid dielectric rods supported by two intersecting thin dielectric layers. Numerical simulations performed using the full-vector finite element method show that a short polarization-independent coupling length can be obtained by adjusting the geometric parameters of the fiber. Moreover, a low-loss THz fiber directional coupling performance can also be achieved at a central frequency of 1.00-THz and approximately 0.16 THz of bandwidth. For example, a coupler with a length of 0.421 cm has a transmission loss of 0.30 dB and 0.26 dB for the x- and y-polarization modes, respectively.

Polarization-independent, lithium-niobate-on-insulator directional coupler based on a combined coupling-sections design.

We have proposed a polarization-independent directional coupler (DC) by using sections with optimal coupling strength designs on the lithium-niobate-on-insulator platform. With this design, arbitrary polarization-independent coupling ratio ranging from 0% to 100% can be achieved by tuning the length of the identical coupling region. The DC exhibits ultralow excess losses (<0.1dB) and polarization-dependent losses (<0.05dB for the complete coupling) over a bandwidth of 100 nm. Moreover, the proposed DC is compact in size, simple in structure, and easy to fabricate.

Polarization-independent one-dimensional grating coupler design on hybrid silicon/LNOI platform.

We report here on one-dimensional (1D) grating couplers based on hybrid silicon/LNOI platform for polarization-independent and high-efficient single-polarization coupling efficiencies. A low index oxide buffer layer was introduced in between the top silicon high index grating coupler and bottom LNOI waveguide. With optimal design of the buffer layer thicknesses, modal and index matches can be tuned for either single polarization or both TE/TM polarization coupling applications. Over 70% coupling efficiency can be achieved for single polarization based on the basic uniform 1D grating coupler design without any bottom reflectors incorporated. Polarization independent coupling efficiency of 51% was also achieved. The spectral bandwidth is over 50 nm with polarization dependent loss of 0.1 dB. The proposed structure is simple to fabricate. Detailed modal and loss analysis suggest different dominant loss mechanisms in the proposed hybrid structure, where the introduction of the bottom mirror may not result in significant improvement in coupling efficiency, as the dominant loss mechanism arises from the top reflection loss.

Two-dimensional apodized grating coupler for polarization-independent and surface-normal optical coupling

A four-port grating coupler with etched square holes is proposed and demonstrated for surface-normal and polarization independent coupling. Benefiting from the perfectly vertical coupling and 1 × 4 power splitting/combing scheme, efficient polarization independent operation was achieved. The coupling efficiency was further improved by grating apodization. According to the simulations, the proposed 2D apodized grating coupler can achieve a coupling efficiency (CE) of 64.5% (−1.9 dB), a low upward back-reflection of 8.5% (−11 dB), and polarization-dependent loss (PDL) lower than 0.04 dB across the wavelength range of 1520–1620 nm. A CE of 56.3% (−2.5 dB) was experimentally measured, with PDL below 0.3 dB within the C-band and lower than 0.5 dB within the L-band. Waveguide delay lines were used to compensate the waveguide channel phase differences at the output port.

Low polarization-dependent-loss double-layer grating coupler for three-dimensional photonic integration

Dense integration of silicon photonic devices is the key enabling technology for next-generation high-performance computers. In this paper, we propose a double-layer grating coupler for the future multi-layer photonic integrated system. With a pair of one-dimensional (1-D) gratings orthogonally vertical-stacked, the device can couple light between single-mode fiber and two separate waveguide layers, realizing polarization splitting and rotation in different layers simultaneously. Benefiting from the perfectly vertical coupling scheme and optimal fiber position (OFP) overlapping design, the coupling efficiency peak of the upper and lower grating are calculated to be 28% and 25% respectively. Assisted with a DBR bottom mirror, the coupling efficiency of the two gratings can be further enhanced to 33.2% and 28.9% respectively. For real application, the misalignment tolerance on grating overlapping and the incidence angle are carefully investigated. To achieve power balance between the two grating layers, the impact of top cladding thickness is analyzed and an optimized thickness of 0.6μm is chosen. From the simulation result of a back-to-back DLGC circuit, the coupling performance shows considerable robustness to the input polarization state and the PDL is under 0.5 dB within the wavelength range from 1536 nm to 1558 nm. Such a device can be a very promising solution to achieve polarization-independent coupling in the future three-dimensional (3-D) high density photonic integrated circuits.

Multi-layer silicon nitride-on-silicon polarization-independent grating couplers.

A polarization-independent grating coupler is proposed and demonstrated in a 3-layer silicon nitride-on-silicon photonic platform. Polarization independent coupling was made possible by the supermodes and added degrees of geometric freedom unique to the 3-layer photonic platform. The grating was designed via optimization algorithms, and the simulated peak coupling efficiency was -2.1 dB with a 1 dB polarization dependent loss (PDL) bandwidth of 69 nm. The fabricated grating couplers had a peak coupling efficiency of -4.8 dB with 1 dB PDL bandwidth of over 100 nm.

Double inverse nanotapers for efficient light coupling to integrated photonic devices.

Efficient light coupling to integrated photonic devices is of key importance to a wide variety of applications. "Inverse nanotapers" are widely used, in which the waveguide width is reduced to match an incident mode. Here, we demonstrate novel "double inverse" tapers, in which we reduce both the waveguide height and width. We demonstrate >45% chip-through coupling efficiency for both the transverse electric and transverse magnetic polarizations in Si3N4 tapers of >500 nm width, in comparison to regular inverse tapers that necessitate <100 nm width. The double inverse tapers show polarization-independent coupling and allow the fabrication using photolithography, relevant for applications at near-infrared and visible wavelengths, e.g.,supercontinuum and soliton microcomb generation.

Polarization-Independent Mode-Evolution-Based Coupler for the Silicon-on-Insulator Platform

We demonstrate a polarization-independent mode-evolution-based coupler for the silicon-on-insulator platform. The measured coupler has negligible insertion loss over a bandwidth of about 100 nm, i.e., from 1500 to 1600 nm. The measured maximum power imbalances for the polarization-independent coupler are 1.2 and 0.2 dB for the fundamental transverse electric (TE $_\text{00}$ ) mode and the fundamental transverse magnetic (TM $_\text{00}$ ) mode, respectively. Our coupler also has a compact design footprint with mode-evolution region not more than ${75}{-\mu {\rm m}}$ long.

Modeling Effective Anisotropy of Substrate Integrated Dielectric Waveguides for Polarization Diversity

A field-based analytical procedure is presented and developed for accurate modeling of the effective parameters of substrate integrated nonradiative dielectric (SINRD) waveguide. This modeling strategy is based on the eigenmode analysis of a dielectric substrate material perforated with air holes and enclosed between two horizontal metallic plates. From this model, it is found that the resulting effective dielectric constant of the periodic geometry of the SINRD waveguide is dependent on the direction of an electric field vector with respect to the material periodicity. In other words, the guiding parts of the SINRD waveguides act as anisotropic materials. A full demonstration for the proposed analysis is supported with simulation results using a full-wave finite-element method. The proposed model provides a scheme for the isotropization of anisotropic substrates. This is in addition to clarifying the exact regimes of operation inside the SINRD waveguide based on the guiding mechanism. Validation examples are studied based on polarization-selective coupling and effective polarization-independent coupling. A prototype is realized and measured where good agreement is achieved between simulation and measurement results.

Design of a barcode-like waveguide nanostructure for efficient chip–fiber coupling

A barcode-like waveguide nanostructure with discretized multilevel pixel lines is designed and optimized by a nonlinear search algorithm. We obtain the design of a one-dimensional multilevel nanostructure with −1.04 dB efficiency for surface normal coupling to a standard single-mode fiber. Another design is achieved from the automatic optimization process, which enables polarization-independent coupling to a single-mode fiber. The optimum coupling efficiency is simulated to be −2.83 dB for TE and −3.49 for TM polarization centered near the 1550 nm wavelength. Polarization-dependent loss of less than 1 dB over 45.3 nm is achieved.

Polarization-independent nonuniform grating couplers on silicon-on-insulator.

Grating couplers are proposed for polarization-independent coupling of light between a single-mode fiber and a 220-nm-thick channel waveguide on silicon-on-insulator. The grating couplers have nonuniform grating periods that are composed of the intersection or union of a set of two near-optimal TE- and TM-grating periods. The proposed grating couplers have a coupling efficiency greater than 20% and polarization dependent loss (PDL) lower than 0.5 dB within 3-dB bandwidth in design. For the evaluation of the design concept, a fabricated intersection grating coupler has the PDL of less than 0.8 dB within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is ∼18%.

T-shaped dielectric slot waveguides for efficient control of birefringence and polarization independent directional coupling

A novel dielectric slot waveguide supporting strongly confined field in a T-shaped low-index slot region for both TE and TM polarizations is proposed and analyzed. Numerical simulations have demonstrated that quite different birefringent modal properties are achievable with tight optical confinement in the slot by tuning key geometrical parameters of the waveguide. Based on such a slot structure, the characteristics of directional couplers are investigated and the conditions for polarization independent coupling are also given. The presented T-shaped slot waveguide might be employed in integrated photonic systems as important building blocks enabling a number of potential applications.

Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides

We propose the use of subwavelength structures in a waveguide grating to achieve polarization-independent coupling of light between an optical fiber and a silicon-on-insulator (SOI) optical waveguide. The subwavelength structure allows the mode effective indices of the TE and TM modes in the grating section to be precisely engineered. We calculate that coupling efficiency of over 64% is possible using the proposed design for polarization-independent coupling between single-mode optical fibers and SOI nanophotonic waveguides.

Design of polarization-insensitive ring resonators in silicon-on-insulator using MMI couplers and cladding stress engineering

A novel silicon-on-insulator (SOI) ring resonator design is described that uses a 2 /spl times/ 2 multimode interference coupler to achieve polarization-independent coupling, and cladding stress induced birefringence control to eliminate the difference in round-trip phase accumulation between the transverse-electric and transverse-magnetic polarized modes. The design parameters are determined for polarization-independent SOI ring resonators with couplers having a 50 : 50 or 15 : 85 splitting ratio, and 1.5-/spl mu/m ridge height and width. As designed, the resonators offer a polarization-independent free-spectral range of 0.5 nm for a ring radius of 200 /spl mu/m, and a quality factor Q as high as 55 000.

Asymmetric waveguides vertical couplers for polarization-independent coupling and polarization-mode splitting

This paper presents a coherent approach to the design of compact vertical coupler (VC) mode-size converters with variable polarization dependence. As a polarization-independent (PI) coupler, the VC is shown to transfer light with more than 90% efficiency for both transverse-electric (TE) and transverse-magnetic (TM) modes over a transfer length of only 150 /spl mu/m. As a polarization-mode splitter (PMS), the VC is shown to preferentially couple TE or TM modes with a contrast ratio of up to 20 dB. This versatility makes the VC a compact and useful input-stage device that not only maximizes input/output coupling efficiency to small active devices but also provides a degree of polarization control before the actual device. The design principle is based on resonant coupling between the top tapered active waveguide and the underlying uniform large and passive waveguide. For the PI coupler, the taper is designed so that resonant transfer occurs near the critical width where the upper waveguide is nonbirefringent. For the PMS, the resonant transfer for TE and TM are designed to occur at widely separated points far from the critical width. Resonant coupling is sensitive to exact dimensions and the refractive index of the waveguides. The taper design is the key in ensuring that the VC performance is robust as well as compact.

편광에 관계없이 매우 높은 소멸비와 짧은 길이를 가지는 수직 방향성 결합기 스위치

We propose a novel vertical directional coupler switch using a vertical directional coupler with polarization independent coupling lengths employing the doublesided deep-ridge waveguide structure. This switch is composed of a switching operation induced section with symmetric structures and an extinction ratio enhanced section with asymmetric structures. We present design methods and examples for this switch with very short lengths and very high extinction ratios larger than 30 dB for both TE and TM modes in cases of both cross and bar states.

Double-Sided Deep-Ridge 도파관 구조 수직 방향성 결합기의 날개구조부 폭과 두께가 편광 특성에 미치는 영향

Double-Sided Deep-Ridge(DSDR) 수직 방향성 결합기의 날개구조부 폭과 두께가 편광에 따른 결합길이에 미치는 영향에 관하여 연구하였다. 날개구조부가 존재하지 않는 DSDR 수직 방향성 결합기는 편광에 무관한 결합길이를 가질 수 없음을 볼 수 있었다. 일정한 날개구조부 두께에 대하여 날개구조부 폭이 특정 길이 이상이 되면 TE 모드와 TM 모드의 결합길이의 변화는 거의 없으며 또한 두 모드의 결합길이의 차이도 거의 발생하지 않음을 볼 수 있었다. 따라서 편광에 무관한 결합길이를 가지는 DSDR 수직 방향성 결합기를 구현하기 위한 최소 날개구조부 폭이 존재함을 볼 수 있었다. 이러한 최소 날개구조부 폭은-같은 코어 두께에 대해서는 날개구조부 두께가 증가할수록, 같은 날개구조부 두께에 대해서는 코어 두께가 감소할수록-증가함을 볼 수 있었다. 또한 최소 날개구조부 두께는 코어의 두께에 의하여 결정됨을 볼 수 있었고 코어의 두께가 증가할수록 최소 날개구조부 두께는 감소함을 볼 수 있었다. We investigate the effect of the wing width and thickness of a Double-Sided Deep-Ridge(DSDR) vertical directional coupler on the coupling length dependent on the polarization, We have found that the DSDR vertical directional coupler without a wing does not have polarization independent coupling lengths. The variation of the coupling length of TE and TM modes and the difference between the coupling lengths of the two modes are negligible as the wing width increases beyond the specific wing width for the same wing thickness. Thus, we can see that a DSDR vertical directional coupler has a wing width larger than the minimum wing width to obtain the polarization independent coupling length. The minimum wing width increases as the wing thickness increases for the same core thickness and as the core thickness decreases for the same wing width. Also, we have found that the minimum wing thickness is determined by the core thickness and the minimum wing thickness decreases as the core thickness increases.