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Abstract
The reliability and small size of solid state scanners makes them ideal for LIDAR. We fabricated and demonstrated the successful operation of an optical scanner using silicon photonics integrated circuit technology. The scanner comprises a ring resonator multiplexer and a number of grating arrays, and employs a beam switching method, which means that the scanner is movement-free. The multiplexer determines the optical path and light is emitted from the selected grating. The scanning angle obtained was 6 degrees. LIDAR sensors can be used in automotive applications for automated cruising.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The advent of automated cruising has intensified the competition in developing the sensor technologies required for this. LIDAR sensors are important perimeter monitoring sensors.
LIDAR sensors based on CMOS-single photon avalanche diodes are reported as being highly sensitive [1]. Sensors for automotive applications need to be compact because the components used in these applications are densely packed. They also need to be cheap. Thus SPADs produced by CMOS processes are suitable for automotive applications.
One approach is to use a LIDAR sensor based on an optical pre-amplifier [2]. One such sensor with a thumb-sized sensor head has been reported. The measurement range of this was over 200 m, though the diameter of the receiving lens was 3 mm. However, a mechanical scanning mirror was used and the size of this mirror determined the size of sensor head. We consider the use of LIDAR based on silicon photonics integrated circuit technology to be a promising approach for automotive applications. The scanner, detector, controller and signal processor can all be included on a single chip.
LIDAR sensors based on silicon photonics integrated circuit technology have previously been reported [3–8]. A key device for chip-scaled LIDAR is the optical scanner. The current leading chip-based LIDAR sensor, reported by Watts’ group, uses 1D or 2D phased array antennas to bidirectionally steer a coherent beam. Moreover, Hajimili’s group has demonstrated detection of a target using a photonics integrated circuit. The challenge, however, is to develop an industrial process for optical phased array antennas.
We fabricated an optical scanner using silicon photonics integrated circuit technology [9]. Two beneficial features of this device are that it is movement-free and is small in size.
In Section 2, we explain the principle of and the theory behind this scanner, and in section 3 describe its structure. In section 4 we describe the experimental setup used to demonstrate operation of this device and present the results. Section 5 is devoted to a discussion. Finally, we present a summary and give our conclusions in Section 6.
2. Principle and theory of the scanner
2.1 Principle of scanner
As shown in Fig. 1, the scanner consists of an imaging lens and a photonics integrated circuit, which comprises an array of optical switches and a number of grating arrays. Light is fed into the input port and the light path is selected by the optical switches. The light is then emitted from the grating selected by the optical switches. The gratings are arranged in front of the lens, which provides an image of the emitted light. The position of the grating with respect to the optic axis of the lens determines the direction of the output.
2.2 Design method
The lens and optical antenna array work as imaging optics (Fig. 2).The position of the antenna determines the direction in which the beam of light is sent. The grating array is placed in the focal plane of the lens, and light emitted from the gratings is redirected by the lens. The principle is similar to that of a camera comprising a lens and an image sensor, but in reverse, with the light passing from the grating array through the lens to the outside world.
Fig. 2 (a) Relationship between the angular resolution, the focal length of the lens and the pitch of the gratings and (b) Relationship between the scanning angle, the focal length of the lens and the pitch of the gratings.
We can calculate the angular resolution by tracing the paths from adjacent gratings as shown in Fig. 2(a).The dot-dashed lines show the axes of the light paths, and the dotted lines show the waists of the light beams. Here we define points A, O and B as in Fig. 2(a). The angular resolution of the scanner is expressed by the angle AOB. We can approximate the angular resolution, θres, by,
where f is the focal length of the lens and p is the distance between the centers of the gratings, not the period of the gratings.Usually the beam spot diameter on the grating is smaller than the size of the grating. In this case, a gap between the beams arises.Next, we calculate the scanning angle by tracing the paths of the outermost gratings. We define points P and Q as shown in Fig. 2(b). The scanning angle is equal to the angle POQ. We can approximate the scanning angle, θscan, by,
where ldev is the distance between the centers of the outermost gratings.The divergence of the gratings has no effect on the angular resolution and the scanning angle. However, the divergence of the gratings can have an effect on the transmittance of the scanner if the diameter of the lens is insufficient. If the divergence causes light to be directed outside the lens, vignetting occurs and the transmittance of the optics degenerates. To avoid this
where θdiv is the divergence of the grating, and D is the diameter of the lens.We used the following parameters for the design; p = 0.016 mm, ldev = 0.32 mm and f = 3 mm. Thus, the resolution and scanning angle of the scanner should be 0.3 degrees and 6.1 degrees, respectively.
3. Device structure
3.1 Structure of silicon photonics integrated chip
Figure 3 shows the structure of the silicon photonics integrated circuit. The thickness of the BOX layer is 3 μm. Waveguides and gratings are formed in the SOI layer. There is a 3 μm thick layer of silicon oxide with a refractive index of 1.51 on top of the patterned silicon layer, and then a 3 μm thick layer of silicon dioxide on top of that. Heater electrodes are formed in a thin film of tantalum deposited on top of the silicon dioxide layer. Wiring lines and bonding pads are formed in Au deposited on the heater electrodes.
3.2 Design of the grating
The gaps between the beam spots need to be as small as possible. These gaps generate gaps in the near field pattern. Thus, the gratings should be densely packed. We designed the shape of the gratings so that they could be densely packed [10].
The gratings are based on focusing gratings. The original shape of the gratings is shown in Fig. 4(a).The pitch and duty cycle are 0.62 μm and 0.5 respectively. The structure is the same as shown Fig. 3. The wavelength of light is 1.55 μm. We calculated the near field pattern (NFP) of the light emitted from this original grating using the finite-difference time-domain method (FDTD). Figure 4(b) shows the result of this calculation. We can see that the intensity is low at the edges of the grating. We reduced the size of grating by cutting off the four corners of the grating to form a diamond shaped grating as shown in Fig. 5(a). We calculated the NFP of the light emitted from this diamond shaped grating using FDTD. Figure 5(b) shows the results of calculations of the NFP from the diamond grating. Compared to Fig. 4(b), Fig. 5(b) shows the calculated NFP from the diamond grating to be acceptable. We calculated the effect of cutting off different amounts from the four corners. The area factor is defined as the area remaining after cutting the four corners from the original area. The different amounts cut off are shown in Fig. 6(a). The emission efficiency as a function of grating area is shown in Fig. 6(b), which shows that the efficiency remains at 0.35 even though the area of the grating has been halved. Following the FDTD calculations, we chose the size of the grating to be 14 μm.
Fig. 4 (a) Bird’s eye view of the original grating, (b) numerical calculation of the NFP of the original grating.
Fig. 5 (a) Diamond shaped grating for dense packing, and (b) calculated NFP of the diamond shaped grating.
Fig. 6 (a) Diagram showing the areas used in the numerical calculations used to examine the effect of removing the four corners from the grating, and (b) result of the numerical calculations, showing the dependence of the emission efficiency on the fraction of the area remaining.
3.3 Silicon photonics integrated chip
Figure 7(a) shows the layout of the devices, and Fig. 7(b) shows a microscope image of the silicon photonics integrated circuit. Light is fed to the input port of the optical scanner using an efficient spot-size-converter. The light signal is then delivered via a waveguide to the optical switch array, which then sends the signal to one of the antennas, from which light is emitted.
Fig. 7 (a) The layout of the device, and (b) microscope image of the silicon photonics integrated circuit.
The spot size converter consists of a tapered waveguide and a silicon oxide waveguide. The optical switch comprises a ring resonator, a waveguide, and a thin film heater. The ring resonator is a closed waveguide. When the resonant wavelength of the ring resonator matches that of the input, the light signal passes through the output port of the switch. When the resonant wavelength of the ring resonator is different to that of the input light, the signal is directed through the bus waveguide. Current is injected into the thin film heaters installed on the waveguide to control the switching states. The optical antenna comprises a number of gratings. As described above, the shape of the gratings was optimized so that they could be densely packed.
The foot print of a single device is 1.3 mm × 0.3 mm, and the size of the chip is 7 mm × 12 mm. We had to leave 40 μm of clearance between ring resonator devices for thermal isolation. We consider that it is able to shrink the footprint of the optical switch array by using an extra fabrication process.
3.4 Scanner
Figure 8 shows a prototype of the optical scanner. For the input port, an optical fiber is attached to the chip, which is mounted on a printed circuit board with gold wires attached. A connecter is used to connect this module to a control circuit. A lens is mounted in front of the chip. We tried two kinds of lens, one with a focal length of 3 mm, and the other with a focal length of 4 mm. We fed light at a wavelength of 1551 nm from a tunable laser source (HL-200, Alnair) into the optical fiber.
The resonant frequencies of the switches are sensitive to temperature and fabrication error, so we tuned the operating current conditions of the ring resonator switches to manually test the prototype. The bandwidth of the ring resonators was 0.3 nm.
4. Experimental results
4.1 NFP and FFP of diamond shaped grating
We measured the NFPs and FFPs of the fabricated gratings. We measured the NFPs using a microscope and an infrared (IR) camera. Figure 9(a) shows the experimental result for the NFP of the original grating, and Fig. 9(b) shows that for the diamond shaped grating. The light is input from the upper side in the Figs.
Fig. 9 (a) The NFP of the original grating, and (b) the NFP of the diamond shaped grating (experimental results).
To measure the FFPs we used an F-theta lens and the IR camera. The image captured by the IR camera was converted into the FFP. Figure 10(a) shows the experimental result for the far field pattern of the original grating, and Fig. 10(b) shows that for the diamond shaped grating. Despite reducing the area of the grating, the diamond shaped grating has almost the same NFP and FFP as the original one.
Fig. 10 The FFPs of the original grating and the diamond shaped grating. (a) FFP along the x direction, (b) FFP along the z direction. (experimental results).
4.2 Scanning angle
Figure 11 shows the far field pattern emitted from the scanner. This shows that the resolution is 0.3 degrees and the scanning angle is 6 degrees. We replaced the lens with one with a focal length of 4 mm and again measured the resolution and scanning angle. With the new lens, the resolution and scanning angle were 0.23 degrees and 4.6 degrees, respectively. Thus, the experimental results provide verification for the use of the design method using Eqs. (1) and (2).
Measurements of the response time of the optical switch for this scanner at room temperature gave a value of 0.3 μs.
5. Discussion
5.1 Beam shape
There is some distortion in the output light pattern as shown in Fig. 11. We consider the reasons for this distortion. The output from the grating is at an angle of 45 degrees to the plane of the grating as shown in Fig. 10. Thus the angle of incidence at the lens is 45 degrees. This large angle of incidence causes distortion. Ideally, the output is vertical, but our priority was on the output power of the scanner and the output angle had lower priority.
In addition to the output angle, we used a single lens. A doublet lens would improve the distortion. Using a doublet lens and designing the device such that the output angle was closer to vertical should reduce the distortion.
5.2 Feasibility of a 2D scanner
We carried out a feasibility study for a 2-dimensional scanner based on the beam switching method. The gratings were arranged in a 2D lattice as shown in Fig. 12. A bus line was laid between the gratings and the same number of directional couplers as the number of gratings was added. Light was fed into the bus line and distributed to the gratings by the directional couplers. We placed a 6 mm focal length lens in front of the grating array, and observed the FFPs emitted from the gratings using an IR camera. As shown Fig. 13, the patterns are arranged in a 2D square lattice. This result demonstrates that we can expand this method to a 2D scanner.
Fig. 12 The layout of the silicon photonics integrated chip for the feasibility study for a 2D scanner.
5.3 Comparison with other methods
As stated above, an important feature of this device is that the resolution and scanning angle of scanner can be reconfigured by changing the lens. When the specification needs to be changed, we can save time and money, simply by minor changing the lens.
Optical phased array antennas are sensitive to phase differences between the elements. When fabricating an optical phased array antenna using a 90 nm process, phase differences between the elements may be generated. However, this is not a serious issue for our scanner. No grating lobes occur with our scanner. This is an advantage of the beam switching method when we want to apply the scanner at shorter wavelength.
It is better to use high output power for LIDAR to increase the sensitivity and reduce the measurement time. In our method, the width of the waveguide can be expanded by using another material. The width of a single-mode waveguide in SiOx is 3 μm. The cross sectional area of the SiOx waveguide is 100 times greater than that of the silicon waveguide. The width of the waveguide determines the maximum output power.
MIT reported a scanner based on a beam switching method with a planar lens [11]. A feature of their device is the on-chip lens, which enables them to reduce the cost of the lens. On the other hand, the wavelength of the laser needs to be tuned for 2D scanning. With our method the operating wavelength need not be changed.
On the other hand, as the number of gratings increases, the number of electrodes or driver circuits increases. If we need a large number, say thousands, of emitters, it will be necessary to simplify the driver circuit. We consider that monitoring of the ring resonator switches will be necessary in the future. With such monitoring, using photodiodes, the total pad number would double and extra control circuitry would be needed to manufacture an optical scanner.
6. Summary and conclusions
We fabricated an optical scanner using silicon photonics integrated circuit technology. The scanner comprises a ring resonator multiplexer and a number of grating arrays, and employs a beam switching method. The beam switching method means that the scanner is movement-free. We demonstrated the successful operation of this scanner. The multiplexer determines the optical path and one of the gratings is selected for the emitter. The scanning angle obtained was 6 degrees. It is a feature of this method that the resolution and the scanning angle can be reconfigured simply by changing the lens used in the scanner.
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