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Abstract
Optical phased array can be widely used in many fields benefiting from its superior performance. We designed and fabricated chirped grating antennas and uniform grating antennas for the optical phased array. The effective aperture of the chirped grating antennas is about twice that of the uniform grating antennas. The chirped grating optical phased array can receive the reflected signal of the object at a distance of 100 m, while the uniform grating optical phased array can only receive 50 m under the same conditions. Additionally, a ranging distance of 25 m is achieved when two chirped grating optical phased arrays are set as the transmitter and receiver.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical phased array (OPA) has broad application prospects in such areas as light detection and ranging (LiDAR) [1–4], free-space optical communication [5–7], laser imaging [8,9], and biosensor [10], because of its excellent performance, including all-solid-state architecture, high scanning speed, high stability, high resolution, and low cost.
Effective aperture is one of the most important parameters of OPA, which determines divergence angles of the OPA. Additionally, the smaller divergence angles mean the more concentrated far-field spot energy, and the larger maximum ranging distance. Hence, improving the effective aperture of the OPA is a significant research direction. It has been reported that multiple channels [11] and chirped gratings [12] are the main schemes to achieve large apertures. For instance, Analogphotonics achieved 0.01° divergence by expanding the number of elements to 8192 [13]. However, they chose polysilicon with a high refractive index as antenna material, which leads to high phase noise in the large-aperture antenna array. As a result, the light spot will deteriorate seriously. High-performance OPA is still a research hotspot.
OPA can be made of Si [14–16], Si3N4 [4,17–19], InP [20], GaAs [21], liquid crystals [22,23], and ceramic materials [24]. Among them, the Si3N4 OPA has the best-ranging performance because of its weak nonlinear optical effect, large transmissible wavelength range, low phase noise, and high damage threshold [25].
In this paper, we demonstrate 512-channel Si3N4 OPA chips and compare the chirped grating antenna (CGA) with the uniform grating antenna (UGA). The CGA successfully realizes the uniform near-field intensity distribution and a large aperture of 3.16 mm × 2.05 mm which is about 2 times that of the UGA. Benefiting from the large aperture, the CGA’s main lobe has ultra-small divergence angles of 0.04° × 0.05° and high side-mode suppression ratios (SMSR) of 11.52 dB × 12.03 dB. Additionally, we achieved 100 m coherent ranging by using the CGA. In contrast, the effective aperture of the UGA is smaller, and only 50 m coherent ranging can be realized under the same conditions. The maximum ranging distance of 25 m can be achieved when two chirped grating optical phased arrays are set as the transmitter and receiver. To the best of our knowledge, we are the first to prove the superiority of the Si3N4 CGA through the coherent ranging experiment.
2. Design of antennas and OPA
For long-distance ranging, we designed and fabricated OPA chips using Si3N4 with a thickness of 200 nm for low phase noise. As shown in Fig. 1(c), the designed OPA chip contains an edge coupler, a 1 × 2 MMI tree, and a side-wall grating antenna array. The side-wall grating antenna array transmits the beam into the free space. Figure 1(a) and Fig. 1(b) are the schematic diagrams of the antenna design. We designed and compared two different types of side-wall gratings: uniform grating and chirped grating. The uniform grating design makes the perturbation strength of the antennas consistent everywhere, which means that the light intensity decays e-exponentially along the antennas. The near-field intensity of the UGA is concentrated in the starting part of the antennas. On the contrary, the perturbation strength of the CGA increases gradually along the antenna to keep the intensity of the output light consistent, and each part of the antenna has contributed equally to the effective aperture. As a result, the effective length of the CGA will be larger than that of the UGA.
Fig. 1. (a) Diagram of the single antenna. (b) Diagram of the antenna array. (c) Photo of the OPA chip.
The exponential profile emitted by antennas follows e-β(x)x, where x is the direction of propagation along the antennas and β(x) is the perturbation strength at location x. In order to make the near field intensity of the chirped grating antenna uniform, β(x) should meet Eq. (1) [26].
Meanwhile, β(x) is related to the effective refractive index of the antennas. Therfore, uniform light output can be realized by adjusting the structural parameters of the antennas. In this paper, the principle of realizing uniform light emission is to optimize the perturbation value p and the length of the perturbed regions (Lp) so that the perturbation strength of the grating increases gradually along the antennas. We first determined the other parameters except for p and Lp in antenna design. The length of a single antenna (L) is 3.16 mm, the waveguide width (Ww) is 1.5 µm, and the length of the non-etched area at side-wall grating (Lg) is 500 nm. Considering crosstalk between antennas, the center spacing of the two antennas (d) was set as 4 µm and the width of the 512-channel OPA is about 2.05 mm. Based on the above parameters, we constructed the functional relations of position - p and p - Lp to realize uniform light distribution. The results of the above two models are shown in Fig. 2(a) and Fig. 2(b), respectively. Specifically, p was designed to gradually increase from 75 nm at the beginning to 550 nm at the end, and Lp was designed to increase from 528 nm to 565 nm. Additionally, p and Lp of the UGA were set as 130 nm and 530 nm to make the beam emit vertically. Correspondingly, the grating period (Λ) of the CGA increases from 1.028 µm to 1.065 µm, while that of the UGA is 1.03 µm (Table 1).
Table 1. Design Parameters of the CGA and the UGA
Fig. 2. (a) Relationship between position and p. (b) Relationship between p and Lp. (c) Near-field distribution simulation results of the CGA and the UGA.
We simulated the near-field distributions of the two antennas based on the finite difference time domain method. Limited by server performance, we simulated the CGA and the UGA with a length of 1.5 mm, but we can also infer that the near-field intensity distribution of the CGA almost uniform from the results shown in Fig. 2(c), by comparison, that of the UGA fit the e-exponent attenuation function. Additionally, the emission efficiency of the CGA is 22.9%, while that of the UGA is 24.4%. In the future work, we will use double-layer Si3N4 grating antenna to improve the emission efficiency [18].
3. Experiment
3.1 Chip performance characterization
We have manufactured the CGA and the UGA stated above. Si3N4 was grown by PECVD, and its refractive index is 1.95. From Fig. 3(a) - (d), it can be found that process consistency and error control are commendable, for example, p and Lp of the CGA are in good agreement with designed values.
Fig. 3. (a) The beginning of the CGA. (b) The end of the CGA. (c) The beginning of the UGA. (d) The end of the CGA.
We observed the near-field of the CGA and the UGA through an infrared camera to preliminarily evaluate the performance of the two OPA chips. Figure 4 shows the near-field distributions of two chips with input power of -40 dBm and -30 dBm, which corroborates that the CGA successfully achieves the almost uniform distribution and the distribution of the UGA diminishes exponentially. The near-field distributions obviously tallies well with the simulation results in Fig. 2(c). Moreover, it can be seen from Fig. 4(d) - (f) that although the vertical length of the UGA was designed to be 3.16 mm, the actual effective length is only about 1.6 mm, which means that the CGA is about 2 times the effective aperture of the UGA.
Fig. 4. (a) Near-field of the CGA under the input power of -40 dBm. (b) Near-field of the CGA under the input power of -30 dBm. (c) Near-field distribution of the CGA. (d) Near-field of the UGA under the input power of -40 dBm.
(e) Near-field of the UGA under the input power of -30 dBm. (f) Near-field distribution of the UGA.
We characterized the far-field properties of the two chips with a wavelength of 1550 nm. Generally, the orientation of the antennas is called the vertical direction (θ), and the orientation perpendicular to it is called the horizontal direction (ψ). Meanwhile, the divergence angles are recorded as ΔθFWHM and ΔψFWHM, respectively [2].
We tested the intensity distribution of the spot as shown in Fig. 5. The vertical and horizontal divergence angle of the CGA is 0.04° × 0.05° with SMSR of 11.52 dB ×12.03 dB; meanwhile, the vertical and horizontal divergence angle of the UGA is 0.07° × 0.05° with SMSR of 11.07 dB × 10.05 dB. In the vertical direction, the CGA and the UGA can achieve 0.063 °/nm and 0.064°/nm scanning, respectively.
Fig. 5. (a) Divergence angles of the CGA. (b) Vertical SMSR of the CGA. (c) Horizontal SMSR of the CGA. (d) Divergence angles of the UGA. (e) Vertical SMSR of the UGA. (f) Horizontal SMSR of the UGA.
According to Fraunhofer diffraction, the far-field distribution in the horizontal direction of the CGA and the UGA conforms to Eq. (3). From the simulation result shown in Fig. 6(b), it can be found that there will be four grating lobes, and the angle between the main lobe and the grating lobe is 22.79°. Meanwhile, the ratio of main lobe intensity to the adjacent grating lobe intensity is 1.6 [27].
Fig. 6. (a) Beam spots of the CGA irradiated to the wall at a distance of 1 m. (b) Calculated far-field distribution of the CGA.
In Fig. 6(a), we exhibited the far-field of the CGA in a real environment. When distance is short, the divergence angle can be ignored, and the far-field of the UGA which is not given is almost consistent with that of the CGA. There are two grating lobes near the main lobe and two grating lobes which are not shown in the figure because the angles between them and the main lobe are too large. The angle between the main lobe and the adjacent grating lobe is 22.93°, and the ratio of intensity is about 1.7, which are all close to the simulation result. Additionally, it can be found that the far-field interference results of the proposed Si3N4 OPA are excellent without external phase modulation because of very low phase noise and optical crosstalk.
3.2 Coherent ranging
To further compare the performance between the proposed antennas, we carried out coherent ranging experiments. The architecture schematic diagram of coherent ranging is shown in Fig. 7. The input light passes through the 1:9 optical branching device (OBD) and is divided into two parts, of which 10% is directly input into the 2 × 2 mixers as the local oscillator signal (LO) and 90% is used as the transmitted signal (TS) and shifted by the acousto-optic modulator (AOM). The frequency-shifted beam will be amplified by an erbium-doped fiber optical amplifier (EDFA), then transmitted into free space by a fiber collimator (FCM) or an OPA chip. The received signal (RS) will be accepted by the OPA chip and input into the 2 × 2 mixers. Finally, the balance detector (BPD) and the digital signal processor (DSP) are used to calculate the frequency difference between the LO and the RS. In this paper, the frequency shift of the AOM is 80 MHz, which means that the coherent beat signal appears at 80 MHz.
Generally, when the signal-to-noise ratio (SNR) in the ranging test is greater than 5 dB, it is considered credible [28–30]. According to the principle of digital signal processing, the SNR can be divided into two separate parts: signal intensity and noise intensity. The signal intensity is the electrical signal output strength of the detector, which is proportional to the power of the RS. Meanwhile, the noise can be divided into the shot noise and thermal noise of the BPD, the noise of trans-impedance amplifier, and the noise of analog-to-digital converter. If the influences of the above equipment on the SNR is described by a coefficient kSP, the SNR meets Eq. (4) [31]:
where PTX is the transmitted optical power, PLO is the power of the LO, Ar is the receiving aperture and R is the target distance.Based on Eq. (4), when R changes, the attenuation of SNR can be deduced:
We selected a 1550 nm narrow linewidth laser as the input light. At the same input power, the chirped grating OPA has achieved 25 m, 50 m, and 100 m coherent ranging, and the uniform grating OPA only has achieved 10 m, 20 m, and 50 m coherent ranging. Comparing the results shown in Fig. 8(a) – (f), it can be found that the values of the ΔSNR of the two chips with the distance are roughly consistent with the theoretical values calculated by Eq. (5). Inevitably, there will be some deviation in the value due to the jitter of noise, when the ranging distance is too long.
Fig. 8. (a) - (c) SNR of the chirped grating OPA in 25 m, 50 m, and 100 m coherent ranging. (d) - (f) SNR of the uniform grating OPA in 10 m, 20 m, and 50 m coherent ranging.
Additionally, we used the chirped grating OPA instead of the FCM, which means that two OPA chips were set as the transmitter and receiver, respectively. Coherent ranging experiments from 5 m to 25 m were carried out and the results are shown in Fig. 9. It can be found that the relationship between the ΔSNR and the R is also in line with the theory. However, the loss of the current OPA chip is large, totaling about 17 dB, including 8 dB of edge coupler, 3 dB of emission efficiency, and 6 dB of grating lobe and side lobe. If the loss of edge coupler reduces to 2 dB through special design and end face treatment, the transmission loss and reception loss will decrease by 12 dB in total, and 100 m chip transmit-receive ranging will be realized. The loss of the OPA chip will be further reduced if the CGA is structured by double-layer Si3N4 [32].
Fig. 9. (a) - (c) SNR of the chirped grating OPA in 10 m, 20 m, and 25 m chip transmit-receive coherent ranging.
The above discussion on the maximum detection range of the chirped grating chip verifies its great application prospect in LiDAR. Moreover, the difference between the coherent ranging and FMCW is that the frequency of the beam has not been linearly modulated. If the OPA chip can realize coherent ranging, it is also competent in FMCW ranging and we will use the chirped grating OPA to test the FMCW ranging in the future.
4. Conclusion
We demonstrated two types of 512 channel Si3N4 OPA chips with the chirped grating and the uniform grating. The CGA successfully realizes uniform light output, a large effective antenna aperture of 3.16 mm × 2.05 mm, and small divergence angles of 0.05° × 0.04°. From the results of the near-field observation and the divergence angle measurement, it can be inferred that the chirp design makes maximum use of the actual aperture of the antennas. In the ranging test, the chirped grating OPA can achieve 100 m coherent ranging which is 2 times what the uniform grating OPA can achieve. Additionally, we have realized a maximum range of 25 m by setting two chirped grating OPA as the transmitter and receiver, respectively. In the future, we will further increase the antenna aperture and reduce the OPA loss to obtain a larger ranging distance.
Funding
National Natural Science Foundation of China (62090053, 61934007).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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