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Abstract
We demonstrate a 1×64 optical phased array (OPA) based on a silicon on insulator (SOI) platform with integrated silicon nitride. The input port of the OPA is fabricated using a silicon nitride waveguide due to its advantage of allowing more optical power. The phase shifter is a silicon waveguide with heater because of the higher thermo-optic coefficient of silicon. And a double layer silicon nitride assisted grating is used in the emitter to reduce the emission strength and then increase the length of emitter to reduce the spot size. The length of the grating emitter is 1.5 mm and the measured field of view of this optical phased array is 35.5°×22.7° with spot size of 0.69°×0.075°.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical phased arrays (OPAs) demonstrated using silicon photonic technology have gained great attention because of its potential to realize solid state light detection and ranging (LiDAR) [1–5]. It is usually necessary to have small divergence angle, high output power and low steering power for LiDAR [6–8]. OPAs implemented in silicon photonic platform have been widely studied [9–13]. However, the third-order nonlinearity of silicon waveguide limits its ability to deal with high optical power [14]. Compared with silicon waveguide, silicon nitride waveguide has much lower nonlinearity allowing it to guide higher optical power [15]. At the same time silicon nitride waveguide has lower phase variation induced by fabrication and much lower propagation loss [15–17]. Recently, a passive large scale silicon nitride OPA with 1024 antennas has been demonstrated and a record small spot size of 0.021°×0.021° is realized [18]. Furthermore, silicon nitride based OPA also realized one dimensional beam steering of 7.4° with 100 nm bandwidth [19] and two-dimensional beam steering of 17.6°×3° [20], in which one dimension is realized via switching the working sub-devices with different grating period and another dimension is achieved by introducing a phase difference utilizing the thermo-optic effect of silicon nitride. However, the power consumption of the silicon nitride phase shifter is much higher than that of silicon due to lower thermo-optic coefficient of silicon nitride.
In this work, we present a 1×64 OPA based on SOI platform with integrated silicon nitride. We use silicon nitride to fabricate input port and multi-mode interferometer (MMI) tree which help to increase the allowing input optical power. The phase shifter is still silicon due to its higher thermo-optic coefficient and a converter is used to connect the silicon nitride and silicon waveguide. The efficiency of the phase shifter is better than 17.5 mW/π much lower than 87.6 mW/π in silicon nitride waveguide. The grating emitter in this OPA is also silicon nitride assisted silicon grating to reduce the emission strength and the length of the grating emitter can be millimeter scale. The measured beam steering range is 35.5°×22.7° with full width at half maximum (FWHM) spot size of 0.69°×0.075°.
2. Design
The OPA is fabricated on SOI integrated with silicon nitride. The SOI has top silicon of 220 nm and buried oxide layer of 2 μm. After silicon waveguide formation, a layer of dioxide is deposited as top cladding and polished to keep 150 nm on silicon, then 300 nm silicon nitride is deposited with low pressure chemical vapor deposition (LPCVD) to reduce the optical loss in C band, followed by silicon nitride waveguide formation. After that, 1 μm dioxide is deposited to protect the waveguide structure and TiN is used as heater with Al for metal connection. The layout of the OPA is shown in Fig. 1(a). The light is divided into 64 channels by cascading 6 stages 1×2 MMI. Figure 1(c) shows the last three stages MMI. The first 4 stages are silicon nitride and the last 2 stages are silicon for the sake of avoiding evanescent crosstalk since the spacing of the MMI in the last two stages is very close. There is a silicon nitride to silicon converter between silicon MMI and silicon nitride MMI. Following the MMI tree is a phase shifter array with spacing of 50 μm and a deep trench between each two-phase shifter is used to reduce thermal crosstalk [21] as shown in Fig. 1(b). The resistance of the TiN heater is 1.42 kΩ. A double layer grating is used as emitter with length of 1.5 mm and the lateral spacing of the grating emitter is 2.5 μm, part of the grating array is shown in Fig. 1(d).
Fig. 1. (a)The layout of the OPA. (b) Zoom-in of the phase shifter array. (c) The last three stages of MMIs. (d) Zoom-in of the grating emitter array.
3. Components
3.1 MMI
The cascaded 1×2 MMI is used to split the light into 64 channels. The first 4 stages use silicon nitride MMI and the last 2 stages use silicon MMI, so two different 1×2 MMIs are used in the splitter. The schematic of the MMI is shown in Fig. 2 and the relative parameters of the two MMI are shown in Table 1. The measured insertion loss of the silicon nitride MMI and silicon MMI are 0.18 dB and 0.04 dB at 1550 nm respectively as shown in Fig. 3. The performance of various state-of-the-art MMIs is listed in Table 2. The performance of our silicon MMI is comparable with the state-of-the-art MMIs. And the performance of the silicon nitride MMI can be further improved in the future.
Table 1. The parameters of the MMI.
Table 2. The performance of the state-of-the-art MMIs.
3.2 Silicon nitride to silicon converter
As mentioned above, the phase shifter is silicon waveguide for higher modulation efficiency, while the input port is silicon nitride waveguide to handle higher optical power, so a silicon nitride to silicon converter is used to convert light from silicon nitride waveguide to silicon waveguide. The schematic of the converter is shown in Fig. 4. The parameters of this structure are marked in the picture. The insertion loss of the converter is measured by concatenating multiple transition structure and the measured insertion loss is 0.07 dB at 1550 nm as shown in Fig. 5.
3.3 Phase shifters
Figure 6(a) shows the test structure of the phase shifter efficiency. The resistance of the heater is 1.42 kΩ and the distance between heater and waveguide is 1 μm. We used an asymmetrical MZI structure to measure the phase change. The result under different voltage is shown in Fig. 6(b) and the calculated modulation efficiency is 17.5 mW/π which is much better than 87.6 mW/π in silicon nitride waveguide [20].
Fig. 6. (a) The test structure of the phase shifter efficiency. (b) The transmission spectrum of the phase shifter under different voltage.
3.4 Grating emitters
A double layer grating emitter is used in the OPA as depicted in Fig. 7(a). The double layer structure was chosen to reduce the emitting strength of the grating emitter, thus the effective emission length can be longer which means a smaller spot size in θ axis. The width of the silicon waveguide is 500 nm and the width of silicon nitride is 700 nm. The period of the double layer grating emitter is 630 nm with duty cycle of 0.5. We fabricated several grating emitters with different length and the far field spot size of those grating emitters are shown in Fig. 7(b). The FWHM spot size decreases as the increasing of the length of the grating emitter. It indicates that the length of the grating emitter can be at least 2 mm which increases the aperture of the OPA. In order to achieve a millimeter emission length with pure silicon emitter, the etching depth of the silicon grating has to be just several nanometer [12], which increases the complexity of manufacturing. While with the silicon nitride assisted silicon grating, the silicon nitride is fully etched so it can be controlled very well with etching end point.
Fig. 7. (a) The schematic of the double layer grating emitter. (b) The measured FWHM spot size of the grating emitter with different length.
4. Beam steering performance
The lateral (Φ) beam steering is realized by controlling the phase difference between adjacent channels. According to the following equation [10, 24]:
where ΔΨ is the phase difference between channels, λ is the wavelength of input light and d is the spacing of the grating emitters.Individual phase shifters for each channel controlled by a multi-port voltage supplier are used to control the phase difference between channels. We use a far field imaging system to measure the beam steering of the OPA. A hill-climbing algorithm is used to correct the phase variation inducing by the fabrication and condense the beam to desired direction. Using the optimization algorithm and the feedback from the infrared InGaAs camera, a set of voltage solutions corresponding to different steering direction are obtained. The diagram of the far field imaging system is shown in Fig. 8.
After phase variation correction, the far field image of Φ=0° is shown in Fig. 9(a), the sidelobe in the both side of center spot is still clear in the image. With the phase difference changing, two light spots with same intensity appear at ±17.76° in the field of view as depicted in Fig. 9(b), which limits the steering range to 35.5° in the Φ axis. Figure 9(c) shows the beam steering along this axis.
Fig. 9. (a)The far field image at 1540 nm after phase correction. (b)The far filed image when two light spots appear in the field of view. (c) The measured beam steering at 1540 nm wavelength along Φ axis.
The longitudinal (θ) beam steering is realized by adjusting the wavelength of injected light. The emission angle θ is given by the equation [16, 25]:
where ${n_{eff}}$ and ${n_c}$ are the effective refractive index of the waveguide fundamental mode and the refractive index of cladding, respectively, and $\wedge$ is the period of the grating emitter. Before testing the beam steering along θ, the beam is firstly condensed at Φ = 0° and θ=-0.16° at wavelength of 1540 nm. There is no need to run the optimization algorithm at other wavelength because the equal path length of all channels is not affected by the wavelength. The longitudinal beam steering is shown in Fig. 10. Figure 10(a) is the far field image at different wavelength captured by an infrared InGaAs camera and the cross section at Φ = 0° is shown in Fig. 10(b). The steering range along θ axis is 22.72° by tuning the wavelength from 1500 nm to 1630 nm. The spot size along Φ axis and θ axis at 1540 nm is 0.69° and 0.075° respectively, as shown in Fig. 11. The spot size along Φ axis can be further decreased by increasing the number of the channels. This OPA is compared with various state-of-the-art OPAs, as shown in Table 3.Fig. 10. (a) The far field image at different wavelength. (b) The measured beam steering along θ axis.
Table 3. Comparisons of state-of-the-art OPAs.
In summary, we demonstrated several advantages of combing the silicon and silicon nitride. Firstly, we can input much higher optical power because we use silicon nitride to fabricate the input port and the first 4 stages MMI, which can sustain much higher optical power due to lower nonlinear effect and double-photon absorption effect in silicon nitride so that higher output power can be realized by increasing the input power for long range applications. Secondly, we still utilize the advantage of the higher thermo-optic effect of silicon to tune the phase, then we can realize the beam steering with lower power consumption. Thirdly, the double layer grating helps to achieve millimeter scale emission length which is difficult to realize with silicon grating. Overall, this work demonstrated the capability of high resolution detection with low power consumption using such an OPA benefiting from the combination of silicon and silicon nitride.
5. Conclusions
In this work, we demonstrated a 1×64 OPA based on SOI platform with integrated silicon nitride. We use silicon nitride to fabricate the input port intended to handling higher input optical power to increase the detection range, the phase shifters are still based on silicon waveguide for higher thermo-optic effect. The grating emitters are also silicon nitride assisted to enlarge the effective emission length. The measured field of view of this OPA is 35.5°×22.7° with spot size of 0.69°×0.075°.
Funding
National Key Research and Development Program of China (2018YFB2200500); Shanghai Municipal Science and Technology Major Project (2017SHZDZX03); Strategic Priority Research Program of Chinese Academy of Sciences (XDB43020500); National Natural Science Foundation of China (61904185).
Disclosures
The authors declare no conflicts of interest.
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