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Abstract
As an indispensable component in the photonic integrated circuits, the design and fabrication of optical isolators, particularly in the transverse electric (TE) polarized mode, is a long-standing challenge. Herein, we present a TE mode magneto-optical isolator using adiabatic tapered waveguides to realize conversions between designated modes. The isolator exhibits an ultranarrow structure of 1.27 μm × 1498 μm. We demonstrate that the device functions under a TE mode input with a maximum isolation ratio of 15 dB and an insertion loss of 5 dB at a wavelength of 1537.3 nm.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical isolators, which are typically placed between a laser source and subsequent devices, are widely used in optical communication systems. While ensuring the forward transmittance of light, they reduce signal noise and maintain system stability by obstructing the reflected light from reaching the laser cavity [1,2]. The completion of silicon-based integration of lasers, optical modulators, and other optical devices [3–5] has rendered the integration of optical isolators challenging for enabling more complex photonic integrated circuits (PICs) functions.
Although dynamic modulation [6,7] and optical nonlinear effects [8,9] can break on-chip reciprocity, isolators based on the magneto-optical (MO) effects offer advantages of high performance and control circuit free [10,11]; thence, they are the most suitable option for the design and fabrication of integrated optical isolators.
Most MO isolators reported hitherto for transverse magnetic (TM) polarized light [12–21]. This is because an in-plane asymmetric distribution of the MO material is required to achieve a nonreciprocal phase shift (NRPS) of the transverse electric (TE) mode, which renders fabrication difficult [22,23]. Therefore, a TE mode MO isolator can be achieved by depositing MO materials on sidewalls [24,25] or serial polarization rotators [26,27] to convert the input TE mode to the TM mode. However, the MO materials deposited on the sidewalls typically do not crystallize well, resulting in poor device performances, and the polarization rotator will result in additional insertion loss and on-chip footprint.
The configuration proposed herein is inspired by the half-mode-converter we have reported previously [28]. Using mode evolution, a tapered waveguide without an abrupt structure was designed to significantly reduce insertion loss. Simultaneously, a polarization rotator and an MO waveguide were combined to create a more compact device structure. Since separate polarization rotators were not required, the device size and the number of couplers and connecting structures was reduced. The device structure is ultranarrow, and much smaller than the polarization-rotation-based isolator in the width dimension, which significantly reduced the on-chip footprint. Furthermore, the isolator can function under a unidirectional magnetic field, thereby reducing the complexity of magnetic field integration. Finally, the device, which has a high isolation ratio and a low insertion loss with a simple structure, was demonstrated experimentally.
2. Device design
2.1 Structure and operation principle
Figure 1(a) shows the structure of the proposed isolator, which comprises two side-by-side tapered MO waveguides on a silicon-on-insulator (SOI) wafer with a bonded MO garnet Ce:YIG as the top cladding. The lightwave propagated along the z-axis. The cross section of the MO waveguide is shown in Fig. 1(b). The isolator was designed with a 220-nm-thick Si layer on the SOI wafer. Because of the asymmetry in the vertical direction, after an in-plane magnetic field was applied along the x-axis direction, the TM modes or hybrid modes containing TM components in the MO waveguide possessed different propagation constants between forward and backward transmission, i.e., an NRPS. The value of the NRPS $\Delta \beta (TM)$ is calculated as
Fig. 1. Device structure sketch. (a) Illustration of device structure. (b) Cross section of magneto-optical waveguide. (c) Tapered waveguide structure of input side. W1 (W2) refers to width of wider (narrower) waveguide, Li the length of Parti (i = 1, 2, and 3). (d) Zero- and first-order mode field distributions at specific locations.
To function as a TE mode isolator, the device was designed to divide the input TE mode energy into two modes, i.e., zero- and first-order mode, mode0 and mode1, respectively, and convert mode1 into the TM mode to acquire the required NRPS of $\pi $. Such mode conversions were realized by the tapered waveguide structure, as shown in Fig. 1(c), where only the input side is shown because of the symmetry of the device structure. Based on different types of mode conversions, the tapered waveguide can be categorized into three parts (Parts1, 2, and 3), and the mode field distribution at the specific positions is shown in Fig. 1(d). The two waveguides have the same width (W1 = W2) at the input port. Therefore, the input TE0 mode from any port is divided into TEeven and TEodd modes equally. Along the propagation direction, W1 (W2) gradually became wider (narrower). Based on the mode evolution, different parts of the tapered waveguide were designed as follows: Part1 converts the TEeven and TEodd modes into TE0 modes in the wider and narrower waveguides, respectively; Part2 does not involve any mode conversions and is only intended for smooth connections; Part3 converts the TE0 mode in the narrower waveguide into the TM0 mode in the wider waveguide and accumulates an NRPS of $\frac{\pi }{2}$. After propagating in the remaining second-half waveguide on the output side, the same mode evolution occurred in the opposite direction. The TEeven and TEodd modes interfered, and the output was obtained at a designated port. The device can be designed to render the interference between the two modes constructive in forward transmission. This results in transmission in the through port, e.g., from Port 1 to Port 3. Because of the NRPS, destructive interference occurred between the two modes in the backward transmission, thereby resulting in transmission in the cross port, e.g., from Port 3 to Port 2. Hence, the device functions as an optical isolator for the TE mode input.
2.2 Structural parameter design
The width of the input and output single-mode waveguides was designed to be 500 nm. The sum of W1 and W2 in any cross section was 1 μm, and the gap between them was 150 nm. As shown in Fig. 2(a), the effective refractive index (neff) of the modes was simulated at a wavelength of 1550 nm with different W1 and W2 values. The material parameters used in the simulation were ${n_{Si}} = 3.48$, ${n_{Si{O_2}}} = 1.45$, and ${n_{Ce:YIG}} = 2.2$. When W1/W2 was approximately 500 nm/500 nm or 600 nm/400 nm, the neff of the modes were similar, and TE0-TE0 mode coupling or TM0-TE0 mode hybridization occurred. At these points, mode conversion occurred via mode evolution in an adiabatic tapered waveguide.
Fig. 2. Structural parameter design of each part of device. Dotted lines indicate starting and ending positions of each part of tapered waveguide. (a) Simulated effective refractive index as a function of waveguide width. (b) Energy distribution and (c) TE ratio and NRPS of mode1 as a function of waveguide width. (d) Mode conversion efficiency as a function of tapered waveguide length of each part.
First, we designed the starting and ending waveguide widths for each part. The starting width W1/W2 of Part1 was 500 nm/500 nm. Figure 2(b) shows the distribution of the energy of mode1 in the two waveguides as W1 (W2) changes. When 95% of the energy was confined to the narrower waveguide, it was assumed that the mode conversion in Part1 had completed. Waveguide widths W1 and W2 at this point were 525 nm and 475 nm, respectively. Figure 2(c) shows the simulated TE mode ratio and NRPS of mode1 around the hybrid point. We assumed that the Faraday rotation of Ce:YIG was -4500 °/cm to calculate the NRPS. As the TE ratio decreased, the NRPS of mode1 increased gradually. We determined the starting and ending waveguide widths of the mode conversion range in Part3, where the TE ratio changed from 95% to 5%. The starting and ending waveguide widths, W1/W2, were 595 nm/405 nm and 615 nm/385 nm, respectively.
Subsequently, we designed the length of each part to satisfy the adiabatic condition for realizing high-efficiency mode conversion. The mode conversion efficiency is shown in Fig. 2(d) as a function of the length of each part. Lengths L1 and L2 were designed to ensure that the mode conversion efficiency reached 99% (∼0.04 dB), i.e., 200 μm and 50 μm, respectively. Length L3 was designed to be 499 μm by integrating the NRPS corresponding to different W1 (W2), which yielded an accumulated NRPS of $\frac{\pi }{2}$ (red line in Fig. 2(c)). The mode conversion efficiency reached ∼95% (∼0.22 dB), as shown in Fig. 2(d) for this length.
The structural parameters of the devices are listed in Table 1. The overall optical isolator with side-by-side tapers measured 1498 μm. The insertion loss caused by the mode conversion was approximately 0.6 dB.
Table 1. Structural Parameters
3. Fabrication
Figure 3 illustrates the device processing flow. The device was fabricated on an SOI wafer with a 220-nm thick Si layer on a 3-μm thick SiO2 layer. A 200 nm SiO2 layer was pre-deposited using P-CVD as a mask layer. Electron-beam (EB) resist ZEP520A was patterned via EB lithography after coating. Next, using CF4 gas to etch the SiO2 layer via reactive ion etching (RIE), the waveguide pattern was transferred to a SiO2 layer. Subsequently, using SiO2 as a mask, SF6 gas was used to etch Si via RIE. Finally, the top SiO2 was removed with HF solution to complete the fabrication of the Si core layer.
We performed surface-activated direct bonding to integrate Ce:YIG (500 nm)/SGGG on the SOI wafer. After using plasma to treat the surface of Si and Ce:YIG, they were aligned and pressurized at 200 °C to bond them. The device was fabricated after dicing.
4. Performance
Figures 4(a) and (b) show micrographs of the reference waveguide and the device at the same magnification. As shown in Fig. 4(a), the reference waveguide was designed to resemble the isolator such that the insertion loss of the isolator can be measured correctly. The green square area in Fig. 4(b) represents the Ce:YIG chip. Owing to the limitation of dicing accuracy, the Ce:YIG chip measured 1500 μm × 1500 μm. To increase the tolerance in the wafer bonding process, we designed the device layout along the diagonal direction of the Ce:YIG chip. The blue arrow indicates the direction of the external magnetic field applied by the permanent magnet.
Fig. 4. Micrograph of (a) reference waveguide and (b) isolator; blue arrow indicates direction of applied magnetic field. (c) Transmission spectra of reference waveguide and isolator between Ports 2 and 4.
Bending waveguides with a radius of 50 μm were used to increase the distance between the ports to facilitate measurement. The reference waveguide and device ports with the lowest coupling loss between the fiber and the chip are selected to evaluate the performance of the device to reduce the estimation error caused by the difference in coupling efficiency. The measured transmission spectrum of the reference waveguide and the forward and backward transmission spectra of the isolator between Ports 2 and 4 are shown in Fig. 4(c) for the TE mode input. At a wavelength of 1537.3 nm, a maximum isolation ratio (IR) of ∼15 dB and an insertion loss (IL) of ∼ 5 dB were achieved with a 10-dB isolation bandwidth of ∼2 nm. The fluctuation observed in the backward transmission spectrum around the center wavelength was caused by noise in the measurement system.
5. Discussion
5.1 Analysis of device performance
We analyzed the breakdown of the insertion loss, the loss contributions to the isolator as shown in Fig. 5(a) and Table 2. In addition to the 0.6 dB loss caused by the mode conversion mentioned above, the loss was caused by the coupling loss due to the mode mismatch at the edge of the Ce:YIG chip and the Ce:YIG absorption. In addition, insufficient NRPS reduced the transmittance in forward propagation.
Fig. 5. (a) Insertion loss contributions to the isolator. (1) Mode conversion (2) Ce:YIG edge coupling (3) Insufficient NRPS (4) Out-device MO loss (5) In-device MO loss. (b) Confinement factor in Ce:YIG of zero- and first-order modes as a function of waveguide width.
Table 2. Loss Breakdown
At the edge of the Ce:YIG chip, the cladding layer changed from air to Ce:YIG. Therefore, a mode mismatch occurred between the air- and Ce:YIG-cladded silicon waveguides. Although the mode mismatch loss reduced significantly in the TE mode compared with in the TM mode, it was simulated to be approximately 0.2 dB on one side. When the NRPS value was lower than that of the design, the constructive interference became insufficient. This resulted in an increase in the insertion loss, which was estimated to be 0.8 dB at the center wavelength. The MO loss caused by the light absorption of Ce:YIG originated from the additionally covered input and output of the single-mode out-device waveguide measured ∼0.55 dB. Based on the confinement factors in the Ce:YIG of modes0 and mode1 in the device shown in Fig. 5(b), we calculated the MO losses of mode0 (TE) and mode1 (TE→TM→TE) and obtained 1.6 dB and 3.83 dB, respectively, and the in-device MO loss was ∼2.65 dB after interference. The difference was due to the different confinement factors in Ce:YIG for the TE and TM modes. This difference in MO loss between mode0 and mode1 contributed primarily to the failure of the device in achieving sufficient light extinction in backward transmission. Based on calculations, the maximum IR of the isolator with such an MO loss imbalance was 17.9 dB, which was similar to the experimental result. The theoretical analysis of IR also proved the accuracy of the tested IL of the device. The isolation bandwidth of the device was limited by the intermode dispersion between mode0 and mode1 (Supplement 1, Section 1).
Optimized NRPS values and bonding accuracy will enable the NRPS to be the designed value and the absorption of the out-device MO material to be minimized. Theoretically, the minimum IL and the maximum IR are expected to be 3.65 dB and 17.9 dB, respectively, based on the current design and MO material. If the MO loss is decreased, not only the minimum IL, but also the maximum IR will be improved because of efficient interference with less imbalance.
5.2 Four-port circulator
This device functions as a four-port circulator as well. The measured transmission between the port pairs of the circulator is presented in Fig. 6. As shown, the device completes a four-port circular operation with ∼10 dB crosstalk. The difference in the transmittance level among the four paths was caused by the uneven coupling efficiency between the fiber and chip. In addition, fabrication errors resulted in different waveguide widths and hence performance differences. Jagged edges of Ce:YIG chips and the uneven bonding strength also made the transmission loss of ports appeared different.
6. Conclusions
We presented a TE mode MO isolator with an ultranarrow structure that had a footprint of 1.27 μm × 1498 μm. Mode conversion was realized by mode evolution in an adiabatic tapered waveguide. While significantly reducing the on-chip footprint, the insertion loss of the device was significantly lower as no abrupt changes occurred in the device structure. We demonstrated a device fabricated via wafer bonding for integrating Ce:YIG on a silicon waveguide. The isolator exhibited an isolation ratio of 15 dB, insertion loss of 5 dB, and a bandwidth of 2 nm for a 10-dB isolation ratio at a wavelength of 1537.3 nm. The device can be used as a four-port circulator in a transceiver for optical sensor applications [29,30]. The device demonstrates the possibility of using mode evolution to fabricate low-loss MO nonreciprocal devices on a silicon-based platform.
Funding
Core Research for Evolutional Science and Technology (JPMJCR15N6, JPMJCR18T4); Japan Society for the Promotion of Science (19H02190); New Energy and Industrial Technology Development Organization; China Scholarship Council.
Disclosures
The authors declare no conflicts of interest.
Data Availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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