Nonreciprocal Mode Conversion Research Articles

Review of integrated magneto-optical isolators with rare-earth iron garnets for polarization diverse and magnet-free isolation in silicon photonics [Invited

Passive optical isolators are needed in silicon photonics but unavailable due to challenges in rare-earth iron garnet processing and integration. Material challenges include incompatibility with silicon and high annealing temperatures, and design challenges include a need for polarization diversity and a preference for no external magnetic bias. These challenges have restricted optical isolation to discrete modules that require physical pick and place of bulk garnet pieces. This review presents developments in the processing of magneto-optical garnets on Si and the enhancement of their Faraday rotation that enables small footprint isolators on silicon waveguide structures. For example, seedlayers and/or new garnet compositions have enabled monolithic Si integration, and in some cases, hybrid integration of garnet-on-garnet or transfer-printed garnet nanosheets enable reduced on-chip thermal processing. Integrated isolators that utilize non-reciprocal phase shift (NRPS) or non-reciprocal mode conversion (NRMC) have been demonstrated to have isolation ratios up to 30 dB, insertion loss as low as 9 dB, polarization diversity and magnet-free operation in the desired telecommunication wavelengths. The advances in materials, processing techniques, and isolator designs shown here will pave the way for on-chip isolators and novel multi-lane photonic architectures.

Integrated optical isolators using electrically driven acoustic waves.

We propose and investigate the performance of integrated photonic isolators based on non-reciprocal mode conversion facilitated by unidirectional, traveling acoustic waves. A triply-guided waveguide system on-chip, comprising two optical modes and an electrically-driven acoustic mode, facilitates the non-reciprocal mode conversion and is combined with spatial mode filters to create the isolator. The co-guided and co-traveling arrangement enables isolation with no additional optical loss, without magnetic-optic materials, and with low power consumption. The approach is theoretically evaluated with simulations predicting over 20 dB of isolation and 2.6 dB of insertion loss with a 370 GHz optical bandwidth and 1 cm device length. The isolator uses only 1 mW of electrical drive power, an improvement of 1-3 orders of magnitude over the state of the art. The electronic drive and lack of magneto-optic materials suggest the potential for straightforward integration with drive circuits, including in monolithic CMOS electronic-photonic platforms, enabling a fully contained 'black box' optical isolator with two optical ports and DC electrical power.

Modal Nonreciprocity and Circulator Based on Nonreciprocal Mode Conversion

In this article, we first discover and investigate the pattern diversity of electromagnetic fields within an antisymmetrically biased twin-ferrite-loaded substrate integrated waveguide (SIW) structure. A special scenario is identified in which two separate guided-wave channels can be created exclusively for each direction of wave propagation within the same ferrite-loaded SIW body. A detailed theoretical foundation is formulated and presented to develop the so-called “nonreciprocally dual-channelized waveguide (NDW)” in this article. Next, this NDW structure is used to devise and demonstrate a fully integrated fundamental nonreciprocal building element called “nonreciprocal mode-converting waveguide (NRMCW),” which manifests modal nonreciprocity. In other words, this ferrite waveguide interchangeably converts TE10 and TE20 modes within the SIW, only in one direction. A fundamental approach to design such a unidirectional mode-converting device is proposed through unsymmetrical perturbation loading inside an NDW. Furthermore, a class of four-port circulator is devised and demonstrated for the first time, in which the operation mechanism is based on the one-way mode conversion. The measurement results of the proposed circulator are in agreement with those of the simulation and mathematical models, thereby validating the theoretical foundation and the concept of “NRMCW. ”

High-Gyrotropy Seedlayer-Free Ce:TbIG for Monolithic Laser-Matched SOI Optical Isolators

Monolithic optical isolators that provide polarization diversity, dimensional (500 nm core) matching to on-chip lasers and magnetless operation have been realized with “one step” seedlayer-free garnet in a non-reciprocal mode conversion design.

Electromechanical Brillouin scattering in integrated planar photonics

The exploitation of Brillouin scattering, the scattering of light by sound, has led to demonstrations of a broad spectrum of novel physical phenomena and device functionalities for practical applications. Compared with optomechanical excitation by optical forces, electromechanical excitation of acoustic waves with transducers on a piezoelectric material features intense acoustic waves sufficient to achieve near-unity scattering efficiency within a compact device footprint, which is essential for practical applications. Recently, it has been demonstrated that gigahertz acoustic waves can be electromechanically excited to scatter guided optical waves in integrated photonic waveguides and cavities, leading to intriguing phenomena such as induced transparency and nonreciprocal mode conversion, and advanced optical functionalities. The new integrated electromechanical Brillouin devices, utilizing state-of-the-art nanofabrication capabilities and piezoelectric thin film materials, succeed guided wave acousto-optics with unprecedented device integration, ultrahigh frequency, and strong light-sound interaction. Here, we experimentally demonstrate large-angle (60°) acousto-optic beam deflection of guided telecom-band light in a planar photonics device with electromechanically excited gigahertz (∼11 GHz) acoustic Lamb waves. The device consists of integrated transducers, waveguides, and lenses, all fabricated on a 330 nm thick suspended aluminum nitride membrane. In contrast, conventional guided-wave acousto-optic devices can only achieve a deflection angle of a few degrees at most. Our work shows the promises of such a new acousto-optic device platform, which may lead to potential applications in on-chip beam steering and routing, optical spectrum analysis, high-frequency acousto-optic modulators, RF or microwave filters and delay lines, as well as nonreciprocal optical devices such as optical isolators.

Fabrication Processes of SOI Structure for Optical Nonreciprocal Devices

Fabrication processes of a magneto-optic waveguide with a Si guiding layer for an optical isolator employing a nonreciprocal guided-radiation mode conversion are investigated. The optical isolator is constructed on a silicon-on-insulator (SOI) structure. The magneto-optic waveguide is fabricated by bonding the Si guiding layer with a cerium-substituted yttrium iron garnet (Ce:YIG). The relationship of waveguide geometric parameters is determined at a wavelength of 1550 nm. The results show that larger tolerance for isolator operation can be obtained at smaller gaps between Si and Ce:YIG. Bonding processes including photosensitive adhesive bonding and surface activated bonding are then compared. It is found that the surface activated bonding process is easier to control and more promising than the photosensitive adhesive bonding.

Monolithically-Integrated TE-mode 1D Silicon-on-Insulator Isolators using Seedlayer-Free Garnet

The first experimental TE-mode silicon-on-insulator (SOI) isolators using Faraday Rotation are here realized to fill the ‘missing link’ in source-integrated near infrared photonic circuits. The isolators are simple 1D 2-element waveguides, where garnet claddings and longitudinal magnetic fields produce nonreciprocal mode conversion, the waveguide equivalent of Faraday Rotation (FR). Quasi-phase matched claddings are used to overcome the limitations of birefringence. Current experimental SOI isolators use nonreciprocal phase shift (NRPS) in interferometers or ring resonators, but to date NRPS requires TM-modes, so the TE-modes normally produced by integrated lasers cannot be isolated without many ancillary polarisation controls. The presented FR isolators are made via lithography and sputter deposition, which allows facile upscaling compared to the pulsed laser deposition or wafer bonding used in the fabrication of NRPS devices. Here, isolation ratios and losses of 11 dB and 4 dB were obtained, and future designs are identified capable of isolation ratios >30 dB with losses <6 dB.

Nonreciprocal TE–TM Mode Conversion Based on Photonic Crystal Fiber of Air Holes Filled With Magnetic Fluid Into a Terbium Gallium Garnet Fiber

The nonreciprocal optical effect of Faraday rotation (FR) is widely exploited in optical isolators to suppress back-reflections to protect optical sources and other devices from injection noise, or in optical circulators to route counter-propagating signals in a single physical channel to different ports [1]. The first concept of an integrated isolator was based on nonreciprocal TE-TM mode conversion. However, most proposals of the waveguide type isolators are based on nonreciprocal TE-TM mode conversion [2], the nonreciprocal coupling between these modes is caused by the FR if the magnetization is aligned along the z-axis, parallel to mode propagation. FR effect can be used for magnet sensors, because the Verdet constant of silica fibre is small (∼1.1 rad/(T.m) at 1064 nm) [3]. In this work, we propose to study the nonreciprocal TE-TM mode conversion phenomenon, by the simulation of magneto photonic crystal fibre (MPCF). It consists of a periodic triangular lattice of air-holes filled with magnetic fluids (Fe 3 O 4 ) into a Terbium Gallium Garnet (TGG) fibre which is a kind of synthetic garnet with chemical composition Tb 3 Ga 5 O 12 . This garnet was chosen as a Faraday rotator material which has excellent transparency properties and is very resistant to laser damage. TGG has also a high Verdet constant which results in the FR effect. We simulated the influence of gyrotropy and the wavelength, and calculated the FR and modal birefringence. In this MPCF fibre, the light is guided by internal total reflection, like classical fibres. However, it was shown that they could function on a mode conversion much stronger than conventional fibres.

Design for broadband on-chip isolator using stimulated Brillouin scattering in dispersion-engineered chalcogenide waveguides

We propose a scheme for on-chip isolation in chalcogenide (As₂S₃) rib waveguides, in which Stimulated Brillouin Scattering is used to induce non-reciprocal mode conversion within a multi-moded waveguide. The design exploits the idea that a chalcogenide rib buried in a silica matrix acts as waveguide for both light and sound, and can also be designed to be multi-moded for both optical and acoustic waves. The enhanced opto-acoustic coupling allows significant isolation (> 20 dB) within a chip-scale (cm-long) device (< 10 cm). We also show that the bandwidth of this device can be dramatically increased by tuning the dispersion of the waveguide to match the group velocity between optical modes: we find that 20 dB isolation can be extended over a bandwidth of 25 nm.

A novel phase self-compensation isolator based on TE-TM mode conversion

The intrinsic factors of magneto-optical waveguide isolator based on 45° nonreciprocal mode conversion and 45° reciprocal mode conversion are introduced. A novel phase self-compensation isolator is designed and simulated by the full-vectorial FD-BPM. The insertion loss of this isolator is -12dB and the isolation is-34dB.

Experiments Towards the Realisation of a Monolithically-Integrated Optical Isolator Incorporating Quasi-Phase Matched Magneto-Optical Effects

ABSTRACTExperiments towards the realisation of the necessary components for future integrated waveguide optical isolators have been conducted. The design, fabrication and characterisation of GaAs/AlGaAs reciprocal mode converters, realised through the use of three novel approaches, is reported. These devices have been tested at λ=0.9m and 1.3m, producing TE-TM conversion efficiencies up to 70%. Initial investigations into the inclusion of magneto optic materials with III-V semiconductor structures for non-reciprocal mode conversion are also discussed.

Analysis of nonreciprocal mode‐conversion properties of magnetooptic channel waveguides using the finite element method

AbstractTo evaluate nonreciprocal mode conversion characteristics in a magnetooptic channel waveguide, a finite dement analysis based on the scalar wave approximation is formulated for the first time. A simple iterative calculation method is conceived for solution of the nonlinear eigenvalue equations finally obtained in which the two polarizations are coupled. Specifically, the maximum isolation ratio in the mode‐conversion‐type magnetooptic rib guide was evaluated and the validity of the method is confirmed by comparison with published experimental results.

Semileaky thin-film optical isolator

Two interesting effects have been experimentally demonstrated for the first time: (1) simultaneous reciprocal and nonreciprocal mode conversion to achieve an isolation effect and (2) magneto-optic switching between guided and radiation modes. These effects were observed in connection with the construction of a previously proposed thin-film optical isolator. The isolator consists of a piece of LiNbO3 placed on top of a thin film of yttrium ion garnet (YIG) with a selenium layer to avoid optical contact problems. The isolator, which is 1 cm long, exhibited 10 dB of isolation at λ = 1.15 μm. The observed isolation was better than theoretical predictions and a mysterious isolation direction dependence on mode order was observed. Although the device had 10 dB of insertion loss and required a magnetic field of 40 Oe, with a slight change in wavelength and film composition, it should be possible to reduce the insertion loss and field required to under 1 dB and 0.1 Oe, respectively. These characteristics combined with broad tolerances on film thickness and the length of the isolation region, broadband operation (from λ = 1.1 to 4.5 μm), and easy construction and adjustment make the isolator very attractive for use in integrated optics.

Theory and Applications of Coupled Optical Waveguides Involving Anisotropic or Gyrotropic Materials

Coupled optical waveguides consisting of two isotropic dielectric slab waveguides coupled through anisotropic or gyrotropic materials inserted between them, are treated theoretically in detail. The properties of reciprocal and nonreciprocal TE-TM mode conversion and a nonreciprocal phase shift for TM modes are shown. As an example of application of this type of coupled waveguide, a nonreciprocal optical integrated circuit (IC) mode converter is proposed. It is shown that a circulator and an isolator which require neither mode separators nor mode filters can be constructed by utilizing the proposed nonreciprocal mode converter. The numerical design examples are also given.

Thin−film optical magnetic mode converters

A new type of thin−film magneto−optic mode converter is described which has the potential to perform both reciprocal and nonreciprocal mode conversion. As a reciprocal mode converter, the fabrication of the device is straightforward and presents some features not available in previous designs. The same basic structure can be used as a starting point to attack the problem of nonreciprocal devices.