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. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Semiconductor lasers and semiconductor optical amplifiers are widely used in optical fiber communication systems. When backward reflections are launched into these devices, their performance degrades, becoming afflicted by instability due to intensity and phase noises. To avoid these problems, optical isolators that allow one-way lightwave transmission are installed, along with active optical devices, in the optical fiber communication systems. Bulktype optical isolators are commercially available and still widely used, but they are difficult to integrate with optical active devices.
Silicon photonic waveguides on a silicon-on-insulator (SOI) wafer are useful for constructing photonic integrated circuits (PICs) with small footprints. Waveguide optical isolators are highly demanded for silicon PICs. Strict optical isolation, independent of light intensity, has been realized with both electro-optical and magneto-optical effects. Electrooptical types use dynamic optical modulators which can be fabricated with CMOS compatible processes. However, they can cause additional losses and power consumption [1][2][3]. On the other hand, magneto-optical materials are not compatible with silicon. Magnetic metals are lossy and growth of magneto-optical garnet on silicon is still challenging [4,5]. So far, magneto-optical isolators fabricated by direct bonding technology have been realized with superior performance [6][7][8][9][10][11]. These isolators are configured as Mach-Zehnder interferometers (MZI) or ring resonators. They work with a fundamental TM mode input since the nonreciprocal function is provided by a magneto-optical phase shift that occurs only for TM modes propagating in a silicon waveguide with a bonded garnet upper cladding layer. On the other hand, most optical active devices operate in the TE mode. In order to realize the desired magneto-optical phase shift while having a TE mode input, a lateral asymmetry in waveguide structure is needed. This makes the fabrication process rather complicated. Waveguide integrated TE-TM mode converters with magneto-optical TM mode isolator represent a straightforward approach to the realization of optical isolators that work with TE mode input [6,10].
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Operation
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Design
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Conclusion
We proposed a novel waveguide optical isolator composed of two tapered TE-TM mode converters and a magneto-optical phase shifter for the TM mode. The device has two main advantages, namely TE mode input operation and ease of magnetization. An optical isolation of 16 dB was demonstrated at a wavelength of 1561 nm for the TE mode input. To obtain higher extinction, splitting ratio of TE to TM mode in the interferometer should be optimized considering their different propagation losses. Very low loss operation was realized because of good light confinement of the TE mode. The Fabry-Perot resonance can be reduced by connecting output ports for radiating light outside the device in order to prevent the destructive interference.