Waveguide Optical Isolator Research Articles

Optical planar and ridge waveguides in terbium scandium aluminum garnet crystal fabricated by ion implantation and precise diamond blade dicing

Terbium scandium aluminum garnet crystal is a promising material which has excellent characteristics for magneto-optical devices. We report on the fabrication of optical planar and ridge waveguides in the terbium scandium aluminum garnet by using combination of helium ion implantation and precise diamond blade dicing. The conditions for the ion implantation are an energy of 400 keV and a fluence of 6 × 1016 ions/cm2. The rotating velocity and cutting speed of the diamond blade were set to be 20,000 rpm and 0.1 mm/s, respectively. The implantation depth and distributions of energy loss were calculated by the stopping and range of ions in matter 2013. The refractive index distribution and the dark-mode spectrum for the implanted planar waveguide were obtained by the reflectivity calculation method and the prism coupling system, respectively. The near-field intensity profiles of both the planar and ridge waveguides were measured by the end-face coupling technique. The optical guiding properties of the terbium scandium aluminum garnet waveguides suggest that they can serve as candidates for optical waveguide isolators.

Laser inscribed waveguide optical isolators in iron-doped lithium niobate.

There is a growing need for optical isolators that do not require a magnetic field, especially for uses such as on-chip optical devices and cold atom physics. As one approach, we propose using waveguides in photorefractive materials, such as Fe:LiNbO3, as optical isolator devices due to their unique asymmetric transmission properties that allow low loss transmission in one crystal orientation and attenuation in the flipped orientation. We utilize ultrafast laser inscription to fabricate photorefractive depressed cladding waveguides in Fe:LiNbO3 along the crystal c axis to demonstrate the operation of Fe:LiNbO3 waveguide optical isolators. We show the ability to write transmission and reflection gratings into these waveguides that provide an isolation ratio of approximately 5000:1 per cm of path length.

Design of a Broadband and Low Loss TM Magneto-Optical Waveguide Isolator Based on Lithium Niobate on Insulator

We proposed a design of a fundamental TM mode waveguide optical isolator using lithium niobate on insulator (LNOI) and deposited magneto-optical film. The optical isolator adopts an asymmetric Mach-Zehnder interferometer (MZI) structure composed of two 1 × 2 MMIs, non-reciprocal and reciprocal waveguides. We used three methods to calculate the non-reciprocal phase shift (NRPS) of the lithium niobate (LN)/ Ce: YIG composite waveguide and got similar results. The device shows about 51.98 nm 30 dB isolation bandwidth and 1.20 dB simulated insertion loss at the wavelength of 1550 nm. The fabrication tolerances and a suitable process are also discussed to meet the state-of-art technology and material properties.

Design, simulation and optimization of a nonreciprocal guided-wave optical isolator based on Mach-Zehnder interferometer

In this paper, an interferometric optical isolator which utilizes a nonreciprocal phase shift was designed, simulated and optimized. The purpose was to develop the waveguide optical isolator which is expected to be low loss, more compatible with integrated optics and also reduces the overall costs as it is integrable to the optical sources. The isolator consists of an asymmetric Mach–Zehnder interferometer with a transverse magnetized nonreciprocal phase shifter in one waveguide and a reciprocal phase shifter in the other waveguide. In the simulation process, the Boundary Mode Analysis was used to compute propagation constants. Furthermore, the Scattering Boundary Condition method was used to make boundaries transparent for dispersed waves. The design parameters, including phase shifter length, bend radius and directional coupler length, were optimized to cut down the insertion loss in the forward direction and increase the return loss in the reverse direction and achieve a 50 dB isolation ratio.

Broadband Bias-Magnet-Free On-Chip Optical Isolators With Integrated Thin Film Polarizers

Most on-chip optical isolators utilize nonreciprocal magneto-optic (MO) garnet-materials that require an external bias magnetic field to operate. The magnetic field is applied through incorporation of either a permanent magnet or an active electromagnet. Permanent magnets add significantly to device bulk and electromagnets increase fabrication complexity and power consumption. Hence, it is highly desirable to reduce or eliminate the magnetizing component in these devices. Here, we experimentally demonstrate a first of its kind bias-magnet-free (BMF) on-chip waveguide optical isolator with integrated Polarcor UltraThin polarizers. The BMF isolator device obviates the need for magnetizing components while possessing superior performance, relative to other on-chip isolators, with ≥25 dB of isolation ratio (IR), and ≤3.5 dB of insertion loss (IL) across the entire infrared optical C-band.

Silicon Waveguide Optical Isolator with Directly Bonded Magneto-Optical Garnet

Silicon waveguide optical isolators were fabricated by direct bonding of magneto-optical (MO) garnet. The technique allowed efficient MO phase shift owing to the use of single-crystalline garnet and negligibly thin interlayer on the silicon core layer. A Mach–Zehnder interferometer (MZI) provided optical isolation utilizing the MO phase shift. High isolation, wide bandwidth, and temperature-insensitive operations had been demonstrated by tailoring the MZI design. Also, transverse electric (TE)–transverse magnetic (TM) mode converters were integrated to control operating polarization. In this paper, we reviewed these progresses on silicon waveguide optical isolators.

Low-loss waveguide optical isolator with tapered mode converter and magneto-optical phase shifter for TE mode input.

We propose and demonstrate a novel low-loss waveguide optical isolator with tapered mode converter and magneto-optical phase shifter. The principle of operation of the isolator is based on the superposition of the TE and TM modes. The two different modes become direction-dependent due to a magneto-optical phase shift affecting the TM mode. We designed a tapered mode converter in order to generate the TE and TM modes with equal amplitude when the waveguide is excited with a TE mode input. We successfully demonstrated that the fabricated device acts as an isolator showing a different transmittance between forward and backward directions. The maximum isolation measured is 16 dB at a wavelength of 1561 nm for a TE mode input.

Optical nonreciprocal devices for silicon photonics using wafer-bonded magneto-optical garnet materials

Abstract

Optimization of figure of merit in magnetoplasmonic waveguides with Fe/Au multilayer for optical isolator based on nonreciprocal coupling on Si waveguides

We report on magnetoplasmonic Si waveguides with a ferromagnetic Fe/conductive metal Au multilayer for realizing a sizable magnetooptic effect with a low propagation loss for integrated optical isolators. By combining the ferromagnetic metal Fe with a highly conductive Au layer, the largest nonreciprocal differences in effective index were estimated for propagation lengths of 1–20 µm. Mode analysis with and without a Au layer clarified that the insertion of a Au layer on an Fe layer improves the optical confinement in the Fe layer with reduced propagation loss and is effective in enlarging the magnetooptic effect for the same propagation length. On the basis of the optimized Fe/Au multilayer structure, we designed waveguide optical isolators based on nonreciprocal coupling by the finite difference time domain (FDTD) method. We estimated an optical isolation of 10.8 dB with a forward insertion loss of 13.4 dB in a 34-µm-long nonreciprocal directional coupler.

Nonreciprocal dielectric-loaded plasmonic waveguides using magneto-optical effect of Fe.

We have implemented the nonreciprocal propagation capabilities into plasmonic waveguides and have simulated the performances. We employed dielectric-loaded surface plasmon polariton waveguide (DLSPPW) and long-range DLSPPW (LR-DLSPPW) configurations, where ferromagnetic-metal Fe is used instead of noble metals in order to obtain nonreciprocal propagations by the transverse magneto-optical (MO) effect. The nonreciprocal performances were characterized by the finite-difference frequency-domain (FDFD) method in terms of the propagation losses in return for the nonreciprocal phase shift (NRPS) and nonreciprocal propagation loss (NRL). The NRPS and NRL of the DLSPPW configuration are larger than those of the previously reported semiconductor waveguide optical isolators owing to the large MO constant of Fe and the field confinement by surface plasmons although the propagation loss for NRL of 1 dB is at least 31 dB and the propagation length is limited to less than 10 μm. To reduce such a large propagation loss, we introduced the LR-DLSPPW configuration composed of Polymethyl methacrylate (PMMA) ridge and Benzocyclobutene (BCB) buffer layer. The Fe layer thickness and width are optimized to 50 nm and 500 nm, respectively, so that sizable MO effect and low propagation loss coexist. The propagation loss for NRL of 1 dB is suppressed to ~10 dB within a waveguide length of ~56 μm. Our comprehensive investigation offers fundamental information on practical magneto-plasmonic waveguides and how much nonreciprocal performances are expected, providing an insight into the integration of magneto-plasmonics with on-chip photonics and electronics.

Silicon Waveguide Optical Isolator Operating for TE Mode Input Light

A waveguide optical isolator with a nonreciprocal phase shift is promising for obtaining high-optical isolation owing to its single-polarization operation. A typical configuration enables operation for only the TM mode input light. A silicon waveguide polarization rotator with asymmetric two parallel waveguides is implemented with the silicon waveguide optical isolator for operation with the TE mode input light. Maximal isolation of 26.7 dB at the wavelength of 1553 nm is demonstrated experimentally.

Design of a compact waveguide optical isolator based on multimode interferometers using magneto-optical oxide thin films grown on silicon-on-insulator substrates

We report the design of a waveguide optical isolator based on multimode interferometer (MMI) structure using silicon on insulator (SOI) and deposited magneto-optical (MO) thin films. The optical isolator is based on a vertical 1 × 2 SOI MMI utilizing the nonreciprocal phase shift (NRPS) difference of different TM modes of the MO garnet thin film/SOI waveguide. By constructing a silicon/MO thin film/silicon structure, we demonstrate that the NRPS of the fundamental and first order TM modes can show opposite signs for certain device dimensions, therefore significantly reduce the device length. For a 310.42 μm long device, 20 dB isolation bandwidth larger than 1.6 nm with total insertion loss of 0.817 dB is achieved at 1550 nm wavelength. The fabrication tolerances and materials losses are also discussed to satisfy the state-of-the-art fabrication technology and material properties.

Athermal Operation of a Waveguide Optical Isolator Based on Canceling Phase Deviations in a Mach–Zehnder Interferometer

Optical isolators play unique roles in photonic circuits. We have developed an optical isolator with silicon waveguides, based on a Mach–Zehnder interferometer. The device characteristics were temperature dependent. We investigated the temperature dependence of the refractive index ( ${\rm d}n/{\rm d}T$ ) and the magneto-optical coefficient ( ${\rm d}\theta _f /{\rm d}T$ ) of a magneto-optical garnet Ce:YIG, and estimated these as $9.1 \times 10^{-5}$ K−1 and 44 ${\rm }^\circ \cdot {\rm cm}^{- 1} \cdot {\rm K}^{- 1} $ , respectively. We discuss the design of an athermal silicon waveguide optical isolator, in which the temperature dependences of the magneto-optical effect and refractive indices are canceled in the backward direction. The high optical isolation, greater than 20 dB, is demonstrated for temperature in the 20–60 °C range.

Compact magnetooptical isolator with cobalt ferrite on silicon photonic circuits

In the telecom wavelength range, the magnetooptical effect of cobalt ferrites is approximately 10 times larger than that of conventional magnetooptical materials such as yttrium iron garnets. In this study, we focus on an application of cobalt ferrite to a magnetooptical isolator that is to be miniaturized and made suitable for integration. First, we prepare polycrystalline cobalt ferrite films deposited on a silicon substrate using a MgO buffer layer. Next, we fabricate a waveguide optical isolator of silicon waveguides by the partial deposition of the cobalt ferrite films. An optical isolation ratio of 5.5 dB is demonstrated.

GaInAsP/InP MZI waveguide optical isolator integrated with spot size converter

We fabricated a waveguide optical isolator with a GaInAsP guiding layer integrated with spot size converters (SSCs) for efficient coupling to optical fibers. The isolator is constructed with a Mach-Zehnder interferometer (MZI), which is composed of multi-mode interference (MMI) couplers, as well as nonreciprocal and reciprocal phase shifters. The nonreciprocal phase shifter is constructed with a magneto-optical cladding layer directly bonded to a semiconductor guiding layer. The performance of the GaInAsP waveguide optical isolator was demonstrated with a maximum optical isolation of 28.3 dB at a wavelength of 1558 nm for the TM mode.

Strip-Loaded Waveguide Optical Isolator Employing Nonreciprocal Guided-Radiation Mode Conversion

An optical isolator employing a nonreciprocal guided-radiation mode conversion has been investigated. The optical isolator is comprised of a strip-loaded magneto-optic waveguide. Only backward-traveling TM modes can couple to TE radiation modes so that the device acts as a TM-mode optical isolator. Fabrication tolerance of waveguide parameters are discussed for a strip-loaded magneto-optic waveguide optical isolator and a rib waveguide optical isolator.

Direct Wafer Bonding and Its Application to Waveguide Optical Isolators.

This paper reviews the direct bonding technique focusing on the waveguide optical isolator application. A surface activated direct bonding technique is a powerful tool to realize a tight contact between dissimilar materials. This technique has the potential advantage that dissimilar materials are bonded at low temperature, which enables one to avoid the issue associated with the difference in thermal expansion. Using this technique, a magneto-optic garnet is successfully bonded on silicon, III-V compound semiconductors and LiNbO3. As an application of this technique, waveguide optical isolators are investigated including an interferometric waveguide optical isolator and a semileaky waveguide optical isolator. The interferometric waveguide optical isolator that uses nonreciprocal phase shift is applicable to a variety of waveguide platforms. The low refractive index of buried oxide layer in a silicon-on-insulator (SOI) waveguide enhances the magneto-optic phase shift, which contributes to the size reduction of the isolator. A semileaky waveguide optical isolator has the advantage of large fabrication-tolerance as well as a wide operation wavelength range.

Waveguide Optical Isolators for Integrated Optics

Waveguide optical isolators are investigated for an application to integrated optics. A nonreciprocal loss shift isolator has the advantage of high compatibility with optical active devices. The interferometric waveguide optical isolator that uses nonreciprocal phase shift brought about by the first-order magneto-optic effect is applicable to a variety of waveguide platforms. In a silicon-on-insulator (SOI) waveguide, the low refractive index of buried oxide layer enhances the magneto-optic phase shift, which contributes to reduce the device size of isolator. We developed the surface activated direct bonding technique to integrate a magneto-optic garnet crystal onto dissimilar crystals. The performance of SOI waveguide optical isolator was demonstrated with an optical isolation of 21 dB at a wavelength of 1559 nm.

Ce:YIG/Silicon-on-Insulator waveguide optical isolator realized by adhesive bonding

A waveguide optical isolator realized by adhesive bonding of a garnet die, containing a Ce:YIG magneto-optic layer, on a silicon-on-insulator waveguide circuit is demonstrated. The die was bonded on top of an asymmetric Mach-Zehnder interferometer using a 100nm thick DVS-BCB adhesive bonding layer. A static magnetic field applied perpendicular to the light propagation direction results in a non-reciprocal phase shift for the fundamental quasi-TM mode in the hybrid waveguide geometry. A maximum optical isolation of 25 dB is obtained.

Operation principle of optical waveguide isolator based on TE-TM mode conversion

According to the perturbation theory, the theory of self-phase compensation is developed for the optical waveguide isolator based on the TE-TM mode conversion. The operation principle of such a device is explained in terms of synchronization of phase and power conversion. The effect of balancing phase mismatches of the two convertors on achieving a proper percentage of mode conversion is revealed. The simulation results confirm the self-phase compensation theory.