Elimination of cross talk in waveguide intersections

State-of-the-Art and Perspectives on Silicon Waveguide Crossings: A Review

In the past few decades, silicon photonics has witnessed a ramp-up of investment in both research and industry. As a basic building block, silicon waveguide crossing is inevitable for dense silicon photonic integrated circuits and efficient crossing designs will greatly improve the performance of photonic devices with multiple crossings. In this paper, we focus on the state-of-the-art and perspectives on silicon waveguide crossings. It reviews several classical structures in silicon waveguide crossing design, such as shaped taper, multimode interference, subwavelength grating, holey subwavelength grating and vertical directional coupler by forward or inverse design method. In addition, we introduce some emerging research directions in crossing design including polarization-division-multiplexing and mode-division-multiplexing technologies.

Improved broadband performance of an adjoint shape optimized waveguide crossing using a Levenberg-Marquardt update

We derive an adjoint shape optimization algorithm with a compound figure of merit and demonstrate its use with both gradient descent and Levenberg-Marquart updates for the case of SiO₂-buried SOI coplanar waveguide crossings. We show that a smoothing parameter, basis function width, can be used to eliminate small feature sizes with a small cost to device performance. The Levenberg-Marquardt update produces devices with larger bandwidth. A waveguide crossing with simulated performance values of > 60 dB cross power extinction ratio and > -0.08 dB through power over the 1500-1600 nm band is presented. A fabricated device is measured to have a maximum of -0.06 dB through power and a 50 dB cross power extinction ratio.

Supersymmetric transparent optical intersections

Supersymmetric (SUSY) optical structures provide a versatile platform to manipulate the scattering and localization properties of light, with potential applications to mode conversion, spatial multiplexing, and invisible devices. Here we show that SUSY can be exploited to realize broadband transparent intersections between guiding structures in optical networks for both continuous and discretized light. These include transparent crossing of high-contrast-index waveguides and directional couplers, as well as crossing of guiding channels in coupled resonator lattices.

Compact waveguide crossings with a cascaded multimode tapered structure

A compact waveguide crossing constructed by a cascaded multimode tapered structure is numerically presented. By using an eigenmode expansion method, we observe that the phase difference between the two lowest guided modes can be changed after a multimode waveguide junction because of the coupling between these two guided modes. Accordingly, we propose a fourfold symmetric waveguide crossing composed of three cascaded tapered structures with Gaussian profiles on each branch. Analyzed by the finite difference time domain method, this waveguide crossing has the size of 4160  nm×4160  nm, insertion loss of 0.13 dB, cross talk of -43.5  dB, and backreflection of -33.6  dB at the wavelength of 1550 nm and broad transmission spectrum at the wavelength range of 1500-1600 nm.

All-optical diode based on dipole modes of Kerr microcavity in asymmetric L-shaped photonic crystal waveguide

A design of all-optical diode in L-shaped photonic crystal waveguide is proposed that uses the multistability of single nonlinear Kerr microcavity with two dipole modes. Asymmetry of the waveguide is achieved through different couplings of the dipole modes with the left and right legs of the waveguide. Using coupled mode theory we demonstrate an extremely high transmission contrast. The direction of optical diode transmission can be controlled by power or frequency of injected light. The theory agrees with the numerical solution of the Maxwell equations.

Design of a compact silicon-based slot-waveguide crossing

A design scheme for silicon-based slot-waveguide crossing using a slot-to-strip mode converter (at each port) and a strip-multimode-waveguide (SMW) crossing is proposed. The guided modes of the input slot-waveguide are first efficiently transformed into that of the single-mode strip waveguide by using the mode converter, and then enter into the SMW, where fields converge at the center of the intersection due to the multimode interference effect. Consequently, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to the significant reduction of the crosstalk (CT) and radiation loss. The numerical results show that a slot-waveguide crossing operating at a wavelength of 1.55 μm with the insertion loss (IL), CT, and reflection (RT) of 0.086, -35.58, and -27.51 dB, respectively, is achieved. Moreover, the IL, CT, and RT as functions of the structural parameters together with the operating wavelength are analyzed in detail by using a finite-difference time domain method, and their fabrication tolerances are presented. In addition, the evolution of the injected fields along the propagation distance through the slot-waveguide crossing is also demonstrated.

Ultra-low crosstalk, CMOS compatible waveguide crossings for densely integrated photonic interconnection networks

We explore the design space for optimizing CMOS compatible waveguide crossings on a silicon photonics platform. This paper presents simulated and experimental excess loss and crosstalk suppression data for vertically integrated silicon nitride over silicon-on-insulator waveguide crossings. Experimental results show crosstalk suppression exceeding -49/-44 dB with simulation results as low as -65/-60 dB for the TE/TM mode in a waveguide crossing with a 410 nm vertical gap.

Interfacing spins in an InGaAs quantum dot to a semiconductor waveguide circuit using emitted photons

An in-plane spin-photon interface is essential for the integration of quantum dot spins with optical circuits. The optical dipole of a quantum dot lies in the plane and the spin is optically accessed via circularly polarized selection rules. Hence, a single waveguide, which can transport only one in-plane linear polarization component, cannot communicate the spin state between two points on a chip. To overcome this issue, we introduce a spin-photon interface based on two orthogonal waveguides, where the polarization emitted by a quantum dot is mapped to a path-encoded photon. We demonstrate operation by deducing the spin using the interference of in-plane photons. A second device directly maps right and left circular polarizations to antiparallel waveguides, surprising for a nonchiral structure but consistent with an off-center dot.

Photonic crystal waveguides intersection for resonant quantum dot optical spectroscopy detection

Using a finite-difference time-domain method, we theoretically investigate the optical spectra of crossing perpendicular photonic crystal waveguides with quantum dots embedded in the central rod. The waveguides are designed so that the light mainly propagates along one direction and the cross talk is greatly reduced in the transverse direction. It is shown that when a quantum dot (QD) is resonant with the cavity, strong coupling can be observed via both the transmission and crosstalk spectrum. If the cavity is far off-resonant from the QD, both the cavity mode and the QD signal can be detected in the transverse direction since the laser field is greatly suppressed in this direction. This structure could have strong implications for resonant excitation and in-plane detection of QD optical spectroscopy.

Multiply resonant photonic crystal nanocavities for nonlinear frequency conversion

We describe a photonic crystal nanocavity with multiple spatially overlapping resonances that can serve as a platform for nonlinear frequency conversion. We show nonlinear characterization of structures with two resonances nearly degenerate in frequency. We also demonstrate structures with resonances separated by up to 523 nm.

Abrupt coupling between strongly dissimilar waveguides with 100% transmission

We present numerical experiments showing how coupled-mode theory can be systematically applied to join very dissimilar photonic crystal waveguides with 100% transmission. Our approach relies on appropriately tuning the coupling of the evanescent tail of a cavity mode to each waveguide. The transition region between the waveguides may be as short as a few lattice spacings. Moreover, this technique only requires varying a small number of parameters (two for each waveguide in our example) and the tuning to each waveguide may be done separately, greatly simplifying the computations involved.

Fano-type spectral asymmetry and its control for plasmonic metal-insulator-metal stub structures

We use coupled mode theory (CMT) to analyze a metal-insulator-metal (MIM) plasmonic stub structure, to reveal the existence of asymmetry in its transmittance spectra. Including the effect of the near field contribution for the stub structure, the observed asymmetry is interpreted as Fano-type interference between the quasi-continuum T-junction-resonator local-modes and discrete stub eigenmodes. Based on the asymmetry factor derived from the CMT analysis, methods to control transmittance asymmetry are also demonstrated.

Coupled-resonator-induced transparency in photonic crystal waveguide resonator systems

We present an optical coupling system, which consists of waveguide, cavity and waveguide resonator, to investigate coupled-resonator-induced transparency effect. The transmission properties are analyzed theoretically by using coupled-mode theory in time domain. We also numerically demonstrate the effect by simulating the propagation of electromagnetic waves in photonic crystals by finite-difference time-domain method.

Efficient transmission of crossing dielectric slot waveguides

Transmission properties of two crossing dielectric slot waveguides (Si-Air-Si) are investigated using the finite difference in time domain method. Results show that the low transmission of this system mainly results from the reflection and radiation loss rather than the crosstalk. Using a simple method of filling up the crossing slots locally, the reflection, radiation losses and crosstalk are all greatly suppressed. With moderate parameters in this paper, the transmission efficient increases from 35.0% to more than 97% in a wide range of wavelength around 1.55 mm. The results and method presented in this paper may be very useful in the application of slot waveguide in micro and nano photonics.

Ultra-short plasmonic splitters and waveguide cross-over based on coupled surface plasmon slot waveguides

Composite metal-dielectric-metal (MDM) surface plasmon polariton (SPP) structures are first proposed to realize the ultra-short optical splitters with simplified designs. The operation mechanism is based on the contra-directional coupling achieved in composite plasmonic slot waveguides. In certain cases, the switching function can also be realized. It is further shown that based on the same physical mechanism multi-dielectric-core composite MDM structures could serve as a novel plasmonic waveguide crossover component with low cross talk and high throughput.

Ultra high bandwidth WDM using silicon microring modulators

We report 50 Gbit/s modulation capability using four silicon micro ring modulators within a footprint of 500 microm(2). This is the highest total modulation capability shown in silicon using compact micro-ring modulators. Using the proposed techniques, silicon nanophotonic bandwidths can meet the requirements of future CMOS interconnects by using multiple wavelengths to extend beyond single device speeds.

Subwavelength grating crossings for silicon wire waveguides

We report on the design, simulation and experimental demonstration of a new type of waveguide crossing based on subwavelength gratings in silicon waveguides. We used 3D finite-difference time-domain simulations to minimize loss, crosstalk and polarization dependence. Measurement of fabricated devices show that our waveguide crossings have a loss as low as -0.023 dB/crossing, polarization dependent loss of < 0.02 dB and crosstalk <-40 dB.

Resonant-tunnelling-assisted crossing for subwavelength plasmonic slot waveguides

We theoretically investigate properties of crossing for two perpendicular subwavelength plasmonic slot waveguides. In terms of symmetry consideration and resonant-tunnelling effect, we design compact cavity-based crossing structures for nanoplasmonic waveguides. Our results show that the crosstalk is practically eliminated and the throughput reaches the unity on resonance. Simulation results are in agreement with those from coupled-mode theory. Taking the material loss into account, the symmetry properties of the modes are preserved and the crosstalk remains suppressed, while the throughput is naturally lowered. Our results may open a way to construct nanoscale crossings for high-density nanoplasmonic integration circuits.

Elimination of cross-talk in waveguide intersections of triangular lattice photonic crystals

We design an intersection for crossing waveguides in triangular lattice photonic crystals with cross-talk smaller than 10(-5). The cross-talk to the transverse waveguides is suppressed by symmetry mismatch between the cavity mode and the waveguide mode or by the mode-gap effect induced by air hole radius modulation of the waveguides. The transmission behavior of the crossing waveguides are illustrated by numerical simulations through finite difference time domain method.

Photonic bandgap crystal resonator enhanced, laser controlled modulations of optical interconnects for photonic integrated circuits

Ultrafast high-density photonic integrated circuit devices (PICDs) are not easily obtained using traditional index-guiding mechanisms. In this paper, photonic bandgap crystal resonator enhanced, laser-controlled modulations of optical interconnect PICDs were achieved in slab-type mix-guiding configuration - through developed CMOS-compatible processing technologies. The devices, with smallest critical dimensions of 90 nm have footprints of less than 5 x 5 microm(2). Quality-factors an order larger than previously realized was achieved. Through use of effective coupling structures; simultaneous alignment for probing and pumping laser beams, optical measurements of both instantaneous free carriers induced device modulations were obtained together with thermo-optical effects characterizations.

Intersection of nonidentical optical waveguides based on photonic crystals

An intersection based on photonic crystal coupled resonator optical waveguides is proposed and analyzed. The two waveguides are designed to have different transmission bands without overlap, which enables light in the two corresponding bands to propagate through the intersection with no cross talk and with excellent transmission. The MIT Photonic-Bands code is used to calculate the band structures of photonic crystal waveguides. The finite-difference time-domain method is used to simulate the relevant structures.

Tunable photonic crystal circuits: concepts and designs based on single-pore infiltration

We demonstrate that the infiltration of individual pores of certain two-dimensional photonic crystals with liquid crystals and (or) polymers provides an efficient platform for the realization of integrated photonic crystal circuitry. As an illustration of this principle, we present designs for monomode photonic crystal wave-guides and certain functional elements, such as waveguide bends, beam splitters, and waveguide intersections. These devices exhibit very low reflection over broad frequency ranges. In addition, we discuss the inherent tunability of these devices that originates in the tunability of the infiltrated material.

Photonic crystal beam splitters

This work studies two-dimensional photonic crystal beam splitters with two input ports and two output ports. The beam splitter structure consists of two orthogonally crossed line defects and one point defect in square-lattice photonic crystals. The point defect is positioned at the intersection of the line defects to divide the input power into output ports. If the position and the size of the point defect are varied, the power of two output ports can be identical. The beam splitters can be used in photonic crystal Mach-Zehnder interferometers or switches. The simulation results show that a large bandwidth of the extinction ratio larger than 20 dB can be obtained while two beams are interfered in the beam splitters. This enables photonic crystal beam splitters to be used in fiber optic communication systems.

Efficient coupling into and out of high-Q resonators

The temporal-coupled-mode theory is directly applied to the design of devices that feature a resonator with a high quality factor. For the temporal-coupled-mode theory we calculate the decay rate of the resonator to determine the transmission properties of the device. The analysis using the decay rates requires little computational effort, and therefore the optimum device properties can be determined quickly. Two examples, a wavelength filter and a resonator crossing, are presented to illustrate the use of the analysis.

Enhancement of nonlinear effects using photonic crystals

The quest for all-optical signal processing is generally deemed to be impractical because optical nonlinearities are usually weak. The emerging field of nonlinear photonic crystals seems destined to change this view dramatically. Theoretical considerations show that all-optical devices using photonic crystal designs promise to be smaller than the wavelength of light, and to operate with bandwidths that are very difficult to achieve electronically. When created in commonly used materials, these devices could operate at powers of only a few milliwatts. Moreover, if these designs are combined with materials and systems that support electromagnetically induced transparency, operation at single-photon power levels could be feasible.

All-optical transistor action with bistable switching in a photonic crystal cross-waveguide geometry

We demonstrate all-optical switching action in a nonlinear photonic crystal cross-waveguide geometry with instantaneous Kerr nonlinearity, in which the transmission of a signal can be reversibly switched on and off by a control input. Our geometry accomplishes both spatial and spectral separation between the signal and the control in the nonlinear regime. The device occupies a small footprint of a few micrometers squared and requires only a few milliwatts of power at a 10-Gbit/s switching rate by use of Kerr nonlinearity in AlGaAs below half the electronic bandgap. We also show that the switching dynamics, as revealed by both coupled-mode theory and finite-difference time domain simulations, exhibits collective behavior that can be exploited to generate high-contrast logic levels and all-optical memory.

Multichannel optical add-drop processes in symmetrical waveguide-resonator systems

Multichannel optical add-drop processes are studied in a class of symmetric waveguide-resonator systems. With insight gained from group theory, we analyze these systems and show that they can add or drop multiple wavelengths simultaneously, with 100% efficiency. A new mechanism is presented to reduce the remnant light at the dropped wavelengths in the pass-through port. High-order Butterworth filters can also be achieved in these systems. Built upon conventional or photonic-crystal based structures, these systems can be used in optical communication applications.

Nonlinear optical effects in a two-dimensional photonic crystal containing one-dimensional Kerr defects

The nonlinear optical effects induced by a one-dimensional (1D) line defect, made of Kerr material, in a 2D photonic crystal are studied. Comprehensive ab initio numerical simulations based on the finite-difference time-domain method show efficient third-harmonic generation in a photonic crystal waveguide consisting of the 1D defect line. The relationship between the third harmonic generation process and the nonlinear modal properties of the waveguide is discussed. We investigate optical limiting in such a device, that is, control of the transmitted power as a function of the Kerr-induced variation of the refractive index. Power dependent spectral changes in such a device and its use as a frequency selector are also examined.

Broadband waveguide intersections with low cross talk in photonic crystal circuits

We propose a new mechanism for constructing waveguide intersections with broad bandwidth and low cross talk in photonic crystal (PC) circuits. The intersections are created by combination of coupled-cavity wave-guides (CCWs) with conventional line-defect waveguides. This mechanism utilizes the strong dependence of the defect coupling on the field pattern in the defects and the alignment of the defects (i.e., the coupling angle) in CCWs. By properly designing the defect mode, we demonstrate through numerical simulation the establishment of such a waveguide intersection in one of the most useful PCs, which is based on a two-dimensional triangular lattice of air holes made in a dielectric material. The transmission of a 500-fs pulse at ~1.3 microm is simulated by use of the finite-difference time-domain method, showing negligible distortion and low cross talk.

Modeling of discontinuities in photonic crystal waveguides with the multiple multipole method

A method for the simulation of discontinuities in photonic crystal defect waveguides is presented. This frequency domain technique is based on the multiple multipole method. In contrast with other known techniques, spurious reflections (due to the impedance mismatch at the waveguide terminations) are avoided. The absence of spurious reflections allows one to characterize precisely the intrinsic behavior of the sole discontinuity, reducing at the same time the size of the simulation domain. To achieve a perfect impedance matching, the guided modes of infinitely long waveguides corresponding to the input and output channels of the discontinuity are first computed using a supercell approach. Then, the discontinuity is fed with one of the previously computed modes, and the fields transmitted or reflected towards the discontinuity arms are matched to the modal fields corresponding to each output waveguide. This method allows one to compute the intrinsic transmission and reflection coefficients of the discontinuity (i.e., coefficients not altered by additional effects such as finite crystal size, etc.). The procedure is presented in detail using some simple discontinuities as test cases. Then, it is applied to the computation of the coupling from a waveguide to free space and for the analysis of a filtering T junction.

Three-dimensional control of light in a two-dimensional photonic crystal slab

Optoelectronic devices are increasingly important in communication and information technology. To achieve the necessary manipulation of light (which carries information in optoelectronic devices), considerable efforts are directed at the development of photonic crystals--periodic dielectric materials that have so-called photonic bandgaps, which prohibit the propagation of photons having energies within the bandgap region. Straightforward application of the bandgap concept is generally thought to require three-dimensional (3D) photonic crystals; their two-dimensional (2D) counterparts confine light in the crystal plane, but not in the perpendicular z direction, which inevitably leads to diffraction losses. Nonetheless, 2D photonic crystals still attract interest because they are potentially more amenable to fabrication by existing techniques and diffraction losses need not seriously impair utility. Here we report the fabrication of a waveguide-coupled photonic crystal slab (essentially a free-standing 2D photonic crystal) with a strong 2D bandgap at wavelengths of about 1.5 microm, yet which is capable of fully controlling light in all three dimensions. These features confirm theoretical calculations on the possibility of achieving 3D light control using 2D bandgaps, with index guiding providing control in the third dimension, and raise the prospect of being able to realize unusual photonic-crystal devices, such as thresholdless lasers.