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Abstract
A novel optical antenna for optical phased arrays is proposed and simulated. A high-contrast grating structure is used to achieve extremely efficient emission. The emission efficiency is as high as 93.94% at 1.55 μm, which exceeds 50% in a range of wavelength from 1.48 μm to 1.62 μm. The antenna can achieve a perfect grating lobe suppression with background suppression of 28.4 dB when the phase difference between adjacent waveguides is 0. A 16-wire optical phased array can easily achieve a scan range of ± 22.8° × 20.2° with a beam width of 2.4° × 2.5°, by employing the optical antenna proposed.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of automatic driving and remote sensing technology, light detection and ranging (LIDAR) has attracted great attention. The optical phased arrays based on silicon photonics have become one of the major solutions for LIDAR due to its low material cost, high integration, and compatibility with CMOS technology [1–11]. However, most of the optical antennas applied for LIDAR are second-order gratings directly etched on the array waveguides, which makes the emission efficiency very low, and a large part of the optical energy is not fully exploited due to the side lobes and grating lobes [1–6]. Although some new antenna technologies have been reported recently [12–17], for example in-plane radiation with dielectric antennas, plasmonic antennas coupled to dielectric waveguides, and plasmonic antennas. These antennas have some excellent characteristics such as high directivity, low loss, and high signal-to-noise ratio (SNR). However, these antennas currently only been verified the on-chip point-to-point beam steering, and the feasibility of chip-to-free space beam steering still needs a lot of work to be verified.
To date, the optical antennas used in phased array LIDAR are basically gratings fabricated on the waveguides, which makes it difficult to avoid diffracted light towards the substrate, resulting in very low emission efficiency of the optical antenna that usually doesn’t exceeding 50% [1–6]. The emission efficiency is defined as the ratio of the energy emitted to free space by the antenna to the energy inject into the antenna from the input waveguides. Some literatures mentioned that an optical antenna using double SiN layer structure can achieve a unidirectional emission efficiency of more than 90% [18], but this antenna is complicated and the tolerance to the fabrication process is quite narrow. And almost all optical antennas use the operating mechanism of emitting first and interfering with each other then in free space. It is difficult to eliminate the second-order peak generated by interference, which means the grating lobes. The energy of the grating lobes is useless in LIDAR application, which makes it difficult to increase the energy efficiency of the antenna. Therefore, improving the emission efficiency and energy utilization of grating antennas has become the key solution to improve the performance of phased-array LIDAR systems.
According to reports, the high contrast grating (HCG) structure can achieve good light transmittance regulation [19–26], but it has not been fully applied to optical antennas yet. The HCG consists of a periodic high refractive index material, which is surrounded by a low index material. Although there are reports in the literature [27] on the use of HCG structured optical antennas to optimize antenna characteristics, it mainly optimizes the divergence angle of the spot in the emission direction without further optimizing other performance, without further exploiting the performance of the HCG antenna.
In this paper, we adopt a new type of optical antenna with HCG structure, which greatly improves the antenna performance. By optimizing the structural parameters of the antenna, the leakage light can be well suppressed and the emission efficiency is greatly improved. In addition, we use a new working mechanism in which the light interferes with each other in a flat interference zone and then emits. This allows only the main lobe to meet the phase requirements of the emission, so that the grating lobes are well suppressed. The proposed antenna works as a whole device to achieve beam steering without crosstalk issues, and there is no need to optimize the width of the waveguide at the antenna to reduce crosstalk [28]. For the proposed HCG antenna, when the input waveguides are very close (sub-wavelength spacing), it can achieve a extremely large scan range, and the input waveguides can be bent to quickly increase the waveguide spacing to connect with other devices on the chip. Since the waveguide width is different in [28], the effective refractive index is different for the same wavelength, so it is necessary to etch different periods gratings to alleviate the divergence of the far-field spot for 2-dimensional (2D) scanning. Etching different gratings in such a small area is also a great challenge to the manufacturing process. However, our antenna does not suffer from this issue.
2. Structure
As shown in Fig. 1, the proposed optical antenna is based on an SOI substrate. The substrate is a conventional CMOS substrate with a 2 μm or 3 μm buried oxide and a 0.22 μm top silicon thickness. The substrate is also used to be compatible with the CMOS process, which can greatly reduce the production cost.
Fig. 1 Schematic of the HCG optical antenna for integrated LIDAR with optical phased arrays on SOI substrate.
The antenna mainly includes three parts. The first part is the array waveguide input port, the second part is the taper connector, and the third part is the HCG region. The HCG region includes, from the bottom to the top, a plate interference coupling region, a low refractive index gap layer, a high refractive index grating layer, and a low refractive index cap layer.
In the input port, the phase of each waveguide mode is adjusted by the front-end phase modulator. W_Wg is the waveguide width, d is the spacing width between the waveguides, and N is the number of the waveguides. The waveguide width W_Wg is set to 0.5 μm in this paper.
The array waveguide and the plate interference coupling region are connected by tapers so as to reduce the reflection due to the difference of effective refractive index. The HCG is located directly above the plate and is separated by a thin layer of low-refractive-index material. Here, the low-refractive-index gap material is SiO2 with a thickness g, and the HCG material is poly-Si.
As described in the literature [26], when the HCG period Λ satisfies
and the spacer thickness g is optimal, the light in the waveguide layer will be almost entirely coupled into the HCG, where n0 is the refractive index above the incidence plane, neff is the effective refractive index of a waveguide mode, θi is the angle of the incident light and λ is the wavelength. By optimizing the period Λ and duty cycle Dc, the HCG can be operated in a dual mode area. If the thickness of the HCG H_HCG is optimal, these two modes can be emitted with a very high efficiency, satisfy the Fabry-Perot(FP) resonance condition and arrive at the exit interface in phase (with a phase difference of an even multiple of π) [25,26,29]. W_HCG is the width of the HCG region. The value of it is determined by the number N and spacing d of the front-end array waveguides and can be expressed asThe length of the HCG region is set to 80 μm. The low refractive index cap layer is also made from SiO2 with a thickness H_cap.
3. Simulation results
3.1 Emission characteristics
Through the simulation and optimization by using 3-dimensional (3D) finite-difference in time-domain (FDTD) method [30], the proposed optical antenna achieves a unidirectional emission. First, we create a physical model of the antenna and place the mode source at the same distance from the antenna in each input waveguide, and the TE fundamental mode is calculated of each mode source from the waveguide structure. When we simulate the emission efficiency, each source is set to the same phase. Then, the power monitors are placed in different directions around the antenna, including its top, bottom, start and end. The transmittance T of these monitor characterizes emission efficiency, leakage efficiency, input efficiency, and remaining efficiency, respectively.
As shown in Fig. 2, the light input by the waveguide successfully transmits to the free space unidirectionally in the HCG region, and the radiation toward the substrate is well suppressed.
First, we determine the period of the HCG by Eq. (1). The simulation results are shown in Fig. 3(a). By varying the wavelengths of both the excitation and the monitor, we can obtain the emission spectrum of the antenna. When Λ is 1.228 μm, the antenna emission efficiency is the highest at 1.55 μm. Then, we simulated the effect of different spacer thickness g on the antenna emission efficiency. The result is shown in Fig. 3(b), when g = 1.1 μm, the emission efficiency is the highest at 1.55 μm.
Fig. 3 (a) Emisson spectrum with different Λ; (b) Emisson spectrum with different g; (c) Emission efficiency with different H_HCG and duty cycle at 1.55 μm; (d) Light transmission efficiency in different directions, the black line indicates the input efficiency, the red line indicates the upward emission efficiency, the blue line indicates the remaining efficiency through the optical antenna, and the pink line indicates the downward leakage efficiency. (The negative sign indicates that the radiation direction is downward.)
Finally, we determine the optical antenna structure by sweeping the thickness and the duty cycle of the HCG. As shown in Fig. 3(c), when H_HCG is 0.43 μm and Dc is 0.4, the emission efficiency is high up to 93.94% at 1.55 μm.
As shown in Fig. 3(d), when the L_HCG is very long, the energy dissipated when passing through the interference coupling region of the flat plate is almost 0, and about 6% of the energy leaked into the substrate at 1.55 μm. The upward emission efficiency exceeds 50% in a range of wavelength from 1.48 μm to 1.62 μm. This is very favorable for the LIDAR working mechanism of scanning by adjusting the wavelength. This structure can also be used for optical interconnection to greatly improve optical coupling efficiency.
The optimization results of various parameters of the proposed optical antenna are shown in Table 1.
Table 1. Optimization Results of the HCG Optical Antenna
3.2 Far field characteristics
In addition to the significant increase in emission efficiency of the optical antenna, the far field characteristics have also been greatly improved. The far field pattern in this paper is calculated from the near-field emission using the near-to-far-field transformation. For a 16-wire optical phased array LIDAR, the far field of the energy emitted by the optical antenna is shown in Figs. 4(a) - 4(c), where Ψ is the emission angle perpendicular to the direction of the waveguide and θ is the emission angle along the waveguide.
Fig. 4 (a) Simulated far field radiation of the 16-wire HCG optical antenna, showing 22.8° radiation angle to the normal of the chip surface; (b) and (c) indicate the far field profile of 4 wires, 8 wires, 16 wires and 32 wires optical antenna in Ψ axis and θ axis, respectively. The maximum value of the simulation results is normalized.
Figure 4(a) shows the far field of a 16 wires optical antenna where spacing d is set to 1.5 μm. In combination with Figs. 4(b) and 4(c), there is only one main lobe in the far-field range and the beam width is small. This is because that the grating lobe emits at a certain angle with the main lobe, the declination angle [1] is
and the light emitted by this angle does not satisfy the phase relationship of dual-mode FP oscillation emission in the HCG antenna, where λ0 is the free-space wavelength. Therefore, the background suppression of this antenna is very good, as high as 24.87 dB. Since the phase difference of each input waveguide is set to 0, the main lobe is located at a position where Ψ is equal to 0°. For the wavelength of 1.55 μm, the diffraction angle θ of the optical antenna is 22.8°, and the beam width is 2.4° × 2.5°.As can be seen from Fig. 4(b), the more wires the optical phased array consists of, the smaller the beam size of the far field can be in the lateral direction and the higher the peak electric field intensity can achieve. There are no grating lobes over the entire Ψ axis, which is not possible with a grating antenna of a general structure. In addition, as shown in Fig. 4(c), the beam width in the longitudinal direction hardly varies with the number of wires, and the difference in far-field peaks of antennas with different numbers of wires is also caused by the different divergence angles in Ψ axis.
3.3 Sweeping characteristics
Lateral sweeping characteristics. If the phase difference between the waveguides is not zero, and the phase difference between the first and Nth waveguides is changed by an equal difference from one to next one, lateral scanning can be realized. The lateral sweeping angle Ψ is given by [1]
with Δφ being the uniform phase difference between the waveguides. As shown in Figs. 5(a) -5(b), a 16-wires phased array with a 2 μm waveguide spacing can achieve a ± 22.8° sweep range in the horizontal direction when Δφ changes from 0° to 360°. With the increase of the scanning angle, the peak electric field intensity in the positive direction decreases and the electric field intensity in the opposite direction increases. When Δφ reaches 180°, the electric field strengths in the positive direction and in the opposite direction reach the same level, and the two spots will not be distinguishable. The position of the two spots at this time is the max scanning range of the antenna.Fig. 5 (a) and (b) indicate beam profiles at 1.55 μm wavelength as the beam was swept in the Ψ axis by changing the phase difference between adjacent waveguides Δφ from 0° to 360°; (c) The maximum scanning angle of the optical antenna in the Ψ axis when d = 0.5 μm, 1 μm, 1.5 μm and 2 μm; (d) Antenna emission efficiency when Δφ changes from 0° to 360° when d = 1.5um.
When the waveguide spacing d is different, the maximum scan angle will also be very different. As shown in Fig. 5(c), the smaller the d, the larger the maximum scan angle will become, but the beam width also increases. From Fig. 5(d), it can be seen that the antenna emission efficiency is at the minimum when Δφ = 180°, and the antenna emission efficiency will decrease by nearly 13%. This is because the scanning angle is the largest when Δφ = 180°, and the light emitted by the antenna is also the least compatible with the antenna resonance condition, resulting in a decrease in the efficiency of the upward emission.
Longitudinal sweeping characteristics. In the longitudinal direction, different emission angles are achieved by adjusting the working wavelength. The longitudinal emission angle θ is given by [1]
The proposed HCG-type optical antenna has a large bandwidth, which enlarges the adjustable wavelength range. As shown in Fig. 6, the wavelength changes from 1.62 μm to 1.48 μm, and the emission angle in the longitudinal direction is deflected from 17.25° to 37.45°, achieving a 20.2° scanning range. Moreover, during the entire wavelength adjustment process, the background suppression of the antenna is very good, which is not achievable by the currently reported antenna.
Fig. 6 Normalized optical output profile in the far field as the beam was swept in the θ axis by changing the wavelength from 1.48 μm to 1.62 μm.
4. Conclusion
We have proposed a new type of optical antenna and optimized its performance through simulation. The optical antenna achieves a unidirectional emission efficiency up to 93.94% at 1.55 μm by HCG structure, and the emission efficiency is higher than 50% at the wavelength range from 1.48 μm to 1.62 μm. In addition, the optical antenna employs a mechanism of first interference and subsequent emission, which makes the grating lobes suppressed without satisfying the antenna emission conditions. When the phase difference Δφ between adjacent waveguides is 0, the background suppression of the antenna is 28.4 dB. We also discussed the far-field scanning characteristics of the proposed antenna, with which a 16-wire optical phased array can easily achieve a scan range of ± 22.8° × 20.2° with a beam width of 2.4° × 2.5°. The proposed antenna works as a whole device, which allows the input waveguide spacing of it to be of the sub-wavelength level without crosstalk issues, thereby it can achieve a significant large scanning range.
Funding
National Key Research and Development Program of China (2017YFB0405301); Frontier Science Research Project of CAS (QYZDY-SSW-JSC021); National Natural Science Foundation of China (61604144, 61504137); National Key Research and Development Program of China: Design and Optimization of Nanoheterostructure optoelectronic Devices (2018YFA0209001).
Acknowledgments
Pengfei Wang and Guangzhen Luo contributed equally to this work.
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