CN117597619A - Phase combining waveguide multiplier for optical phased arrays in solid state lidar applications - Google Patents

Abstract

Integrated optical phased array devices are a good potential solution for solid state laser radar (LIDAR) technology in autopilot technology applications. However, the development of Optical Phased Array (OPA) devices is still limited by some of the difficulties, one of which is the contradiction between the need for fewer units in the phase tuning system and the need for more components in the transmit system. The present disclosure provides a method of phase-combining waveguide multipliers (PCWD) to solve this problem. This arrangement can double the number of waveguides without any phase mismatch. It has the ability to control 2N-1 transmit elements using N phase shifters. The device is competitive with any grating coupler array-based or end-fire-based emission method, which can potentially meet sub-wavelength spacing requirements.

Description

Phase combining waveguide multiplier for optical phased arrays in solid state lidar applications

Cross Reference to Related Applications

The present application claims priority from U.S. patent application Ser. No. 17/730,930, filed on month 4, 2022, and also claims the benefit of U.S. provisional application Ser. No. 63/182,314, filed on month 4, 2021, 30. The entire disclosure of the above application is incorporated herein by reference.

Technical Field

The present disclosure relates to optical phased arrays, and more particularly to a phase-combining waveguide multiplier (phase-combining waveguide doubler, PCWD) for an optical phased array in solid state laser radar (LIDAR) applications.

Background of the inventiondescription of the invention

This section provides background information related to the present disclosure, which is not necessarily prior art. This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

As autopilot technology has evolved, many companies in the automotive industry have turned their emphasis toward laser radar (LIDAR) (light detection and ranging (light detection and ranging)). LIDAR can provide more accurate three-dimensional (3D) images over a sufficiently long distance compared to conventional radar, enabling the system to distinguish humans over typical braking distances (e.g., 200 meters).

In most cases, the optical portion of the LIDAR system includes three components, including a light source, a light-redirecting device, and a light detector. Conventional LIDARs typically turn the entire device body to steer the beam. However, such a mechanized approach results in drawbacks such as high cost, relatively low steering speed, large body size, and relatively low reliability. These shortcomings are overcome by the solid state beam steering system and method of the present teachings.

Further areas of applicability will become apparent from the description provided herein. The descriptions and specific examples in this summary are for illustrative purposes only and are not intended to limit the scope of the present disclosure.

Drawings

The drawings described herein are for illustration purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure.

Fig. 1 is a diagram of a solid state beam steering device based on an integrated optical phased array (optical phased array, OPA).

Fig. 2 is a diagram of a PCWD device disposed between a phase tuning system and a launch system.

Fig. 3 is a simulation result of three devices with main lobes of 0 °. The intensities are normalized according to the main lobe (0 °).

Fig. 4A to 4C showAt [0 °,90 ]]Simulation results for three devices within range. The data were normalized as in fig. 3.

Fig. 5A and 5B show the optical path in a Scanning Electron Microscope (SEM) picture and measurement setup of a PCWD structure before cladding.

Fig. 6A and 6B show the following experimental results: in the experimental results, two lobes may be clearly distinguished at a relatively large distance in the presence of the PCWD device, whereas two lobes may be very clear without the PCWD device, whereas more than two lobes may be roughly distinguished, with a relatively small distance between the lobes.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Detailed Description

Example embodiments will now be described more fully with reference to the accompanying drawings.

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that the example embodiments may be embodied in many different forms without the use of specific details, and neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known techniques have not been described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having" are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein should not be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It should also be understood that additional steps or alternative steps may be employed.

When an element or layer is referred to as: in the case of another element or layer being "on," "joined to" or "connected to" another element or layer, the element or layer may be directly on, joined directly to, connected directly to, or coupled directly to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," or "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a similar fashion (e.g., "between … …" and "directly between … …", "adjacent … …" and "directly adjacent … …", etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms are used herein without implying order or sequence unless the context clearly indicates otherwise. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially relative terms (e.g., "inside … …," "outside … …," "below … …," "below … …," "below … …," "above … …," and "above … …," etc.) may be used herein to describe one element or feature's relationship to another element or feature as illustrated for ease of description. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below … …" may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

Solid-state lidar products with various steering mechanisms based on micro-electro-mechanical systems (MEMS) and flash methods have appeared in the market, but these solid-state lidar products have been limited in performance and application. Despite the attempts made, there are not yet sufficiently mature and mass-producible products on the market due to design and manufacturing difficulties. In contrast, the integrated Optical Phased Array (OPA) device of the present teachings presents great potential to overcome the limitations of the prior art—in some embodiments, the present teachings employ solid state solutions or systems compared to MEMS, and the present teachings provide longer detection distances (on the order of 200 meters) compared to flash methods. Furthermore, in some embodiments, the present teachings provide a field of view (FOV) of approximately 180 °, fast steering speed, high resolution, and potentially lower cost.

In accordance with the present teachings, a phase combining waveguide 106 multiplier (PCWD 12) for an optical phased array in solid state lidar applications is provided to enable control of 2N-1 transmit elements using N phase shifters. The present teachings can accommodate any phase tuning method and most emission methods, and the associated structures are not limited by materials or other detailed structural dimensions, and are therefore nearly compatible with every OPA-based beam steering device.

Referring to the drawings, an OPA-based beam steering apparatus for LIDAR is contemplated. Generally, as shown in fig. 1, the OPA-based beam steering device 100 may include a single light source 102, typically in the near-infrared (NIR) range. The output of the light source 102 is input to a beam splitter 104 to split the light energy into a plurality of waveguides 106 106 —a star coupler or a Y-splitter tree may be used.

After the energy from the light source 102 is evenly distributed to the individual waveguides 106, the energy enters the phase tuning system 108. One simple way to tune the phase is to use the dispersion effect, and by designing suitable delay lines in different waveguides 106, the phase can be tuned to different results when different wavelengths are applied. The U-shaped delay line region is used to obtain the proper dispersion phase tuning results. This principle was further developed to obtain the additional advantage of large pore size. In some embodiments, a phase shifter may be applied to each waveguide 106, allowing the phase of each waveguide 106 to be individually tuned. Such a configuration would eliminate the need for a tunable laser. Furthermore, the response speed of the thermo-optic phase shifter (thermal phase shifter, TPS) is typically faster than the wavelength scanning speed of the tunable laser. However, it should be noted that this approach also brings about two disadvantages: first, the total power required to tune each waveguide 106 is typically high; second, a complex electronic system is typically required to control a large number of waveguides 106 individually.

In some embodiments, once the phase in each waveguide 106 is properly tuned, the light is ready to be emitted and may be directed to the emission system 110. In some embodiments, the emission system 110 may be configured in any one of at least three configurations. First, in some embodiments, the transmitting system 110 may include a grating coupler array (grating coupler array, GCA) capable of achieving beam steering by phase tuning in one direction and dispersion in another direction. This approach is employed in many works-one major limitation of this approach is that grating couplers generally provide only a limited steering range over a range of wavelengths.

Second, in some embodiments, the transmitting system 110 may include a grating Antenna Array (AA). This configuration is arranged in an array and provides similar apertures between the two directions, but typically results in grating lobes due to the relatively large spacing. Grating lobes may be suppressed by using non-periodically arranged arrays in a two-dimensional (2-D) non-periodic antenna array designed by genetic algorithms. However, non-periodic arrangements are very likely in terms of highlighting the main lobe, but do not help in terms of energy. The energy in the grating lobes (in the absence of non-periodic arrangements) is actually distributed over the entire 180 deg. range, rather than being transferred into the main lobes, which look like normal noise with slightly higher intensity. However, it is not truly "noisy" and cannot be further suppressed by apodization (which is a method of dealing with real noise).

Third, in some embodiments, the emission system 110 may include coupling the light direct end to free space (EF). The advantage of this approach compared to the first two approaches is the high emission efficiency, while the difficulty of this approach is the generation of a 2D converging beam in case the surface has to be phase tuned. It has been found that a direct write method of converting a one-dimensional (1D) waveguide array to a 2D waveguide array can successfully produce a 2D converging beam. However, it may also result in large pitches of grating lobes.

One of the most critical obstacles between OPA devices and successful products on the market is the contradiction between the phase tuning system 108 and the transmitting system 110. In phase tuning system 108, it is generally desirable to use fewer components to avoid overly complex electronic hardware; while in the transmission system 110, a large number of waveguides 106 are more desirable to meet both a large aperture (typically greater than 1 cm) and a small pitch (preferably having a half wavelength pitch). To overcome this contradiction, it may be desirable to attempt to increase the complexity of the electronic hardware or to use non-periodic arrangements to suppress grating lobes. Furthermore, fewer phase tuning units may be used to control more transmit elements. In some embodiments, 2N phase shifters may be used to control N ² A plurality of emissive elements; however, this concept can only be applied to the following antenna arrays: the antenna array does not meet the small pitch requirements and requires a cross waveguide design that typically includes large insertion loss.

Thus, in some embodiments of the present disclosure, as shown in FIG. 2, a system is provided that uses N phase tuning units to control 2N-1 transmit elements. When light comes from the phase tuning system 108, the light is finely tuned so that the phase of the modes in each waveguide 106 follows the desired output-that is, if they are directly into the emission system 110, they are already able to form a converging beam, but the small number of waveguides 106 determines that it can only meet one of a large aperture and a small pitch. Thus, the value of the device we propose is here.

The structure of the device 10 is shown in fig. 2. In some embodiments, the apparatus 10 may include a cascaded Y-beam splitter pair 12 disposed between the phase tuning system 108 and the transmission system 110. In some embodiments, all Y-splitters 12 in the same cascade layer are identical and symmetrical, so the phase change on both sides is also identical. The waveguide mode just before the first Y-splitter 12 can be denoted as E (1, x) and therefore the following equation is available regardless of the time variation.

Wherein,and->Is the tuning result from phase tuning system 108. Assuming that the insertion phase variation of the Y-beam splitter 12 is +.>Is +.>We then get the waveguide 106 mode at the output as follows.

The method comprises the steps of,

we can observe from fig. 2 that for each output of the device there is aIf we use +.>Then we can get equations 1 and 2 as follows.

Thus, we have the output of each odd operation similar to equation 3 and the output of each even operation similar to equation 4. From these equations, we can see that the modes in each odd output are identical in phase and half of the corresponding input in amplitude as compared to the corresponding input. On the other hand, the even outputs are additional parts of the PCWD 12 generation that are centered between two adjacent odd outputs and whose phases are also equal to the average of the two adjacent odd outputs, which perfectly meets the phase requirement of the OPA. The only imperfection is that each has a different factorThe intensity differences in the additional outputs of (a) are all different. However, when we studied this different factor, we could actually find that it is less important for the following reasons: in OPA-based LIDAR, the target detection FOV is a range centered at 0 °, which requires the same phase in each transmit element, so the intensities are different by a factor +.>Equal to 1, which means no difference. On the other hand, to meet the intensity uniformity, it is necessary to discard the light in the waveguide 106 on at most two sides, which is labeled E in FIG. 2 _Discarding . Thus, 2/N of the energy is wasted, where N is the number of inputs corresponding to how many individual phase shifters are in the phase tuning system 108. The most obvious improvement for a greater number of radiating elements is smallerAs can be seen from fig. 2, the waveguide spacing after the PCWD 12 is halved from the waveguide spacing before the PCWD 12.

From theoretical analysis, it can be found that this mechanism is based on phase combination only, and therefore, the PCWD 12 structure has no requirements on the material or specific waveguide dimensions, as long as the waveguide 106 is single-mode. The PCWD 12 may potentially be designed with output pitches as small as 1.3 μm or with output pitches in the range of 1.3 μm to 1.5 μm in view of crosstalk between the waveguides 106.

Thus, with this configuration of the PCWD 12, we can control 2N-1 transmit elements using N phase shifters. It is expected that a device having this structure will perform perfectly when emitting in the normal direction and will remain good in a certain range around 0 whenCloser to pi, the performance of the PCWD 12 device would become worse, but not always worse than using N phase shifters alone.

Time domain finite difference (finite difference time domain, FDTD) simulation methods have been used to study PCWD 12. To better understand this structure, we simulated three devices: the first device contains PCWD 12 and 6 inputs from phase tuning system 108, so it has 11 outputs to transmit system 110; the second and third devices were used for comparison and they did not have PCWD 12, where one of them contained 11 inputs and 11 outputs and the other contained 6 inputs and 6 outputs.

Two important assumptions were employed in our study: first, we assume that the phase tuning system 108 can individually tune the phase in each waveguide 106 to a range of 0,2 pi]The method comprises the steps of carrying out a first treatment on the surface of the Secondly, we assume that the Y-beam splitter can be optimized to near adiabatic conditions in practical experiments. According to previous studies, these two assumptions are realistic. Thus, the splitter and phase tuning system 108 is not included in the simulation, and phase tuning is accomplished by directly changing the phase parameters of the light source 102. After PCWD 12, the light is end-shot to free space (rix=1) to obtain the far field. In SiO ₂ Environment (environment)In which the waveguide 106 material is set to Si ₃ N ₄ As this is consistent with the experiment. A sinusoidal pulse with a wavelength of 1550nm and Transverse Electric (TE) polarization were used as the light source for each waveguide 106, and all other parameters were set to be the same in the simulation of the three devices. The pitch of the input sections is set to 8 μm (see also fig. 5A for 6 inputs) and the pitch of the output sections is set to 4 μm (see also fig. 5A for 11 outputs), so that all three devices have the same emission aperture of 40 μm. We have chosen this large pitch in order to obtain a good contrast between grating lobes (see explanation of fig. 3), which can easily be applied to smaller pitches once the principle has been verified.

FIG. 3 shows whenSimulation results for three devices when set to 0, i.e., main lobe at 0 °. In this figure, the black dot dashed line is for a device without PCWD 12 and comprising 6 inputs and 6 outputs. It can be seen that since only 6 elements cover 40 μm and the pitch is 8 μm, there are Xu Duoshan lobes, with 4 distinguishable grating lobes in both positive and negative directions, the two grating lobes closest to the main lobe being at + -11.5 deg.. Blue dot-dash lines are for devices without PCWD 12 and comprising 11 inputs and 11 outputs, with an emission pitch of 4 μm, so the nearest grating lobe is located at ± 22.81 °, corresponding to the second closest grating lobe in the black curve, while the intensity is much smaller than the black curve. The red line is for the proposed device with PCWD 12 and comprising 6 inputs and 11 outputs, and the results clearly show that the nearest grating lobes are at-22.73 ° and 22.72 °, which is consistent with the theoretical analysis that the device with PCWD 12 works well in the normal direction, comparable to the device with PCWD 12 with a 2N-1 phase shifter.

Fig. 4A to 4C showAt-180 DEG, 180 DEG]Simulation results for three devices in range, note that where the data is normalized in the same manner as in FIG. 3And (5) managing. It can be seen that with +.>The angle of each lobe in all three devices is uniformly offset and the three main lobes are offset at a steering angle of 5.3 deg. which coincides with the (phase change)/(distance) value, it being noted that this value can be easily increased by using a smaller pitch as discussed in section II. FIG. 4A is a graph showing the performance of the PCWD 12 device as between FIG. 4B and FIG. 4C, consistent with theoretical analysis, showing that ≡>When close to 0, the performance of the PCWD 12 device is closer to that of FIG. 4B, and when +.>Close to pi, the performance of the PCWD 12 device is closer to that of fig. 4C.

For the proof of concept we have made one sample. Notably, the principles of the PCWD 12 architecture are applicable to any phase tuning method and are more valuable when a single phase shifter is used, as the function of the PCWD 12 is to allow for the use of fewer phase shifters to control more transmit elements. However, due to the limitations of our experimental platform, we have chosen a wavelength tuning mechanism because it can completely eliminate the need for electronic components, which greatly reduces the complexity of the experiment. We fabricated two devices, each using a single light source, a 1 x 16 splitter tree, and an omega-shaped delay line region with a delay length difference of 10 μm between the individual waveguides 106, set the spacing between the individual waveguides 106 to 8 μm, consistent with the simulation parameters. After the delay line region, one device included the PCWD 12 structure to double the number of waveguides 106 from 16 to 31 such that the emission pitch of the PCWD 12 device was 4 μm, while the other device did not include the PCWD 12, which still had an emission pitch of 8 μm.

The fabrication was done in the Lurie nano-fabrication factory of Annagao, michigan. The sample is based on a silicon wafer, and is first prepared by low pressure chemical vapor phaseDeposition (LPCVD) deposition of substrate SiO ₂ And waveguide 106Si ₃ N ₄ Layers, which are to achieve better layer uniformity, thereby reducing waveguide 106 losses. Then, the Si is subjected to optical lithography ₃ N ₄ Patterning followed by deposition of cladding SiO using Plasma Enhanced Chemical Vapor Deposition (PECVD) ₂ A layer. Coating the substrate with SiO ₂ The thickness of the layer is set to 2 μm to avoid potential leakage to the bottom silicon wafer or air, and Si is ₃ N ₄ The waveguide 106 is provided 600nm thick and 650nm wide, note that the dimensions of the waveguide 106 need not be exact, as there is virtually no requirement for it. The final step is to cut the sample and polish the sidewalls so that light can be extracted from the Si ₃ N ₄ Waveguide 106 ends up in free space. Fig. 5a is an SEM image of the PCWD 12 device before cladding, the structure in which a cascade Y-beam splitter is seen in a yellow block, with 16 waveguides 106 spaced 8 μm before PCWD 12 and 31 waveguides 106 spaced 4 μm after PCWD 12.

For measurement setup, light is provided by a tunable laser (TLX 1, thorlabs), followed by a fiber polarization controller (FPC 030, thorlabs), a cleaved fiber end is used to butt-couple light into the sample. At the emission system 110, one lenticular lens is used for the near field (blue dashed line in fig. 5B), while the two lenses together are used to create a fourier optical plane (orange dashed line in fig. 5B) for far field measurement.

Because only one layer of waveguide 106 has an end-fire emission, the emitted beam is in effect a fan-shaped beam that converges horizontally but diverges vertically. In fig. 6A and 6B, far field measurements are shown, in which only a screenshot of the far field on a normal plane in the vertical direction is collected. Fig. 6A is the result of a device with a PCWD 12, and it can be seen that there are two distinct lobes, both of which are offset to the left as the wavelength increases. On the other hand, fig. 6B is for a device without PCWD 12, also having two clear lobes, and some other lobes may be roughly distinguished from the figure, all of which are offset to the left with increasing wavelength.

Based on a review of fig. 6A and 6B, we can see that (a) has a greater lobe distance than (B) (as indicated by the arrows in fig. 6A and 6B). In particular, the distance indicated by the arrow in fig. 6A is approximately twice the distance indicated by the arrow in fig. 6B. This is consistent with theoretical analysis and simulation results, and in most cases, the PCWD 12 device may effectively halve the transmission spacing, thereby doubling the distance between the lobes. Thus, experimental results support the conclusion that: the PCWD 12 structure provides the benefits described herein.

Second, from the simulation results, we can know how much the lobe distance of two devices should be: they should be 11.15 ° (for 4 μm) and 22.73 ° (for 8 μm). With such a reference, we can read the steering angle from the result. The two lobes of the PCWD 12 device in fig. 6A are turned 1.58 ° over a wavelength range of 1535nm to 1563 nm; the two clear lobes of the PCWD 12 device in fig. 6B are turned 2.07 °. This difference may be due to slight waveguide 106 thickness variations, as they are located at two different locations on the wafer. Notably, the device uses a wavelength tuning method, so in principle, the steering angle can be easily increased by using a smaller pitch or using a larger delay length.

In this work we have proposed a phase combining waveguide 106 multiplier (PCWD 12) to achieve control of 2N-1 transmit elements using N phase shifters. We attach importance to the value of this structure, analyze the mechanism of this structure in theory, study numerically and demonstrate the concept of the device experimentally; this result is expected to be used for the concept verification. This structure can meet any phase tuning method and most emission methods, while it has no requirements on materials and any detailed structural dimensions, it is compatible with almost all OPA-based beam steering devices.

The foregoing description of the embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. The individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used with selected embodiments, even if not specifically shown or described. This can also be varied in a number of ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims (3)

1. An optical phased array system comprising:

a phase tuning system that receives optical energy and outputs optical energy;

a transmitting system; and

a phase combining waveguide multiplier having a cascaded Y-splitter pair disposed between the phase tuning system and the emission system, the phase combining waveguide multiplier receiving the optical energy output from the phase tuning system and directing the optical energy via the Y-splitter pair to double optical energy output to the emission system, each Y-splitter being located at the same cascade layer and being symmetrical such that phase variations of the optical energy are the same on opposite sides of the optical energy.

2. The optical phased array system of claim 1, wherein the phase combining waveguide multiplier is single-mode.

3. The optical phased array system of claim 1, wherein the output pitch of the phase combining waveguide multipliers is in the range of 1.3 μιη to 1.5 μιη.