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Abstract
Optical phased array (OPA) beam scanners for light detection and ranging (LiDAR) are proposed by integrating polymer waveguides with superior thermo-optic effect and silicon nitride (SiN) waveguides exhibiting strong modal confinement along with high optical power capacity. A low connection loss of only 0.15 dB between the polymer and SiN waveguides was achieved in this work, enabling a low-loss OPA device. The polymer-SiN monolithic OPA demonstrates not only high optical throughput but also efficient beamforming and stable beam scanning. This novel integrative approach highlights the potential of leveraging heterogeneous photonic materials to develop advanced photonic integrated circuits with superior performance.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light detection and ranging (LiDAR) technology is extensively employed for measuring the distance of objects in 3D space by calculating the time it takes for laser light to scatter and return from a target. Compared to conventional radar systems, LiDAR provides high-resolution imaging owing to the shorter wavelength of laser light, proving its suitability for applications such as autonomous vehicles, unmanned robots, and drones [1,2]. LiDAR measurements can be accomplished using various techniques, including the time-of-flight (ToF) method [3] and the frequency modulated continuous wave (FMCW) approach [4,5]. The ToF method employs pulsed lasers and highly sensitive single photon avalanche diodes to facilitate long-distance measurements. On the other hand, the FMCW method utilizes continuous wave lasers to simultaneously obtain distance and velocity data by analyzing the beat frequency generated through the interference between the local oscillator and the reflected light. Nevertheless, challenges persist in coupling scattered light into waveguides and addressing vibrations when FMCW LiDAR is mounted on a moving object.
Photonic integrated circuit (PIC) technology is attractive for the development of cost-effective and mass-producible optical phased array (OPA) LiDAR. In OPA devices, horizontal scanning is conducted by controlling the phase distribution of the arrayed waveguide output and vertical scanning is achieved using wavelength dispersion of diffraction gratings [6]. This technology has the potential to be applied for free space communication and imaging as well [7–10].
Silicon photonics has been the leading platform for OPAs offering reduced waveguide dimensions and increased integration levels owing to the high refractive index of Si. Furthermore, well-established CMOS fabrication foundries support the progression of the research area [11–13]. Silicon OPAs enable a wide field-of-view by minimizing the output waveguide pitch and high-resolution scanning by increasing the number of channels or the size of output apertures [14–16]. However, the high refractive index contrast of silicon waveguides leads to phase errors and increased scattering, originating from fabrication errors and sidewall roughness [17]. Moreover, optical nonlinearity in silicon waveguides presents challenges for managing high optical power required by long-distance LiDARs.
Silicon nitride (SiN) based OPAs have emerged as a promising alternative to silicon-based OPAs owing to their low propagation loss and transparency over a wide wavelength band [18–21]. Unlike silicon, SiN materials exhibit less nonlinearities for high optical power, making them suitable for high-power LiDAR systems [22,23]. Nevertheless, the low thermo-optic (TO) effect of SiN renders them inappropriate for constructing the phase modulator (PM) array of OPA [24]. To address this issue, polymer waveguides with superior TO effect could be employed. Polymer waveguides can be fabricated utilizing simple processes such as spin-coating, UV or thermal curing, and oxygen plasma dry etching [25–27]. Polymeric waveguide devices such as variable optical attenuators and tunable wavelength lasers have been successfully commercialized for the WDM optical communication systems satisfying the Telcordia reliability standards [28]. By virtue of the significant TO coefficient and excellent thermal confinement of polymer materials, polymer TO-PM arrays exhibit low power consumption and negligible thermal crosstalk, thus enabling independent phase control of individual channels within the PM array [29,30]. Consequently, OPAs employing polymer TO-PMs have demonstrated rapid and accurate beamforming, while maintaining the initial beamforming state during the beam scanning process [31–33]. However, the limited mode confinement of polymer waveguides constrains the reduction of output waveguide pitch, which restricts the scanning angle of OPA.
Integration of heterogeneous waveguide materials enables the development of advanced PICs with versatile functionality by leveraging the strengths of various photonic materials [34–36]. In the development of OPAs, Si and SiN waveguides have been monolithically integrated using CMOS processes [37–40]. Our previous works demonstrated hybrid OPA devices by connecting chips of polymeric phase modulator array and SiN waveguide arrays [20,33]. In this study, we integrate these two heterogeneous waveguide devices on a single wafer, harnessing the benefits of polymer waveguides with exceptional TO effects and SiN waveguides with tight mode confinement. Adiabatic mode transition tapers are designed to minimize transition losses between the two heterogeneous waveguides. This approach eliminates the difficulty of bothersome chip-to-chip alignment, reduces coupling losses between the different waveguides, and increases the reliability of the OPA. Herein, the mode conversion efficiency is highly enhanced by incorporating a SiO2 thin film between the polymer and SiN waveguides. Moreover, we present an efficient 32-channel OPA beam scanner featuring high optical throughput and effective beamforming and scanning through monolithic integration.
2. Design of the polymer-SiN monolithic OPA beam scanner device
The proposed OPA is designed for a 1550 nm wavelength, ensuring a high permissible power for eye safety and good transmission through the atmosphere. As depicted in Fig. 1, the monolithic integrated OPA comprises a 1×N power splitter, phase modulator array, and narrow-pitch emitter constructed on a single substrate. SiN waveguides are utilized to construct a compact waveguide structure for the power splitter and narrow-pitch emitter, while polymer waveguides compose the phase modulator array with microheaters. The power splitter distributes input light among multiple channels of SiN waveguides before transferring it to the polymer phase modulators via SiN waveguide tapers. Following phase modulation, the light propagates through another taper structure to an output waveguide array with a narrow pitch. When the light emitted from all waveguides is adjusted to in-phase, an outgoing beam propagates in forward direction.
Fig. 1. (a) Schematic of the polymer-SiN monolithic OPA beam scanner consisting of SiN waveguides for 1×N power splitter and narrow pitch emitter along with a polymer waveguide PM array. (b) Top view of the device featuring the two transition tapers between SiN and polymer waveguides.
For the device input, a tapered waveguide with a tip width of 0.2 µm is designed for reducing the coupling loss to an ultra-high numerical aperture (UHNA) fiber as 1.2 dB/facet. By cascading five stages of SiN 1 × 2 multi-mode interference couplers, a 1 × 32 power splitter is constructed.
The taper structure connecting the two heterogeneous waveguides through adiabatic mode coupling is shown in Fig. 2(a). The SiN waveguide has a dimension of 1.0 × 0.5 µm2, and the polymer waveguide has a dimension of 3.0 × 1.5 µm2 as depicted in Fig. 2(b), and they are designed to operate for TE polarization. Although the polymer waveguide can support the first higher-order mode, adiabatic mode conversion in the taper ensures the coupling between fundamental modes. The smallest tip width of the taper is preferred for lower mode conversion loss; however, the tip width is limited by the i-line stepper’s resolution limit of 0.2 µm. An oxide interlayer is adopted as shown in Fig. 2(b) so as to reduce the effective index of the SiN waveguide to be close to that of the polymer waveguide at the end of taper tip.
Fig. 2. (a) Linear tapers adopted to induce adiabatic modal transitions between polymer and SiN waveguides, (b) Cross-section of the transition taper waveguide showing the oxide interlayer between polymer and SiN waveguides, (c) Structures set to perform 3D BPM simulation, Cross-sections of the trapezoidal waveguide structure which reflect the actual fabricated device, and the field distribution for each cross-section along the propagation direction, and (d) 3D BPM results of the adiabatic transition loss according to the taper length for various oxide thicknesses.
3D BPM simulations were performed as a function of taper length (Lt) reflecting the actual trapezoidal geometry of the fabricated waveguide. During beam propagation through the taper, the electric field profile was evolving as shown in Fig. 2(c). A mode transition loss of 0.23 dB is achievable when Lt = 100 µm and tox = 0.3 µm, which was improved by 0.3 dB compared to the case of tox = 0 µm as shown in Fig. 2(d). It was because abrupt changes in effective indices were alleviated by incorporating an oxide interlayer at positions where the two heterogeneous waveguides terminate. The taper length was determined to be sufficiently long as 300 µm to accommodate fabrication tolerances in oxide thickness and alignment errors during photolithography.
The speed of the TO-PM should be fast enough to scan the total scene at frame rates of 10 Hz at least. The TO-PMs had an upper cladding with a thickness of 1.5 µm over the waveguide core, where the absorption loss due to the plasmon absorption became negligible for the TE mode. The microheater was designed as 8 µm wide and 1.5 mm long. From simulations, the power consumption for half-wavelength phase change (Pπ) was calculated to be 20.2 mW, and the response time resulted as 61 µs. Detailed design procedures could be found in our previous publications [29,36]. The response time was improved by 40% by reducing the polymer thickness between the micro-heater and heat-sinking substrate compared to the previous work [36].
At the output emitter, a narrower pitch provides a wider scanning angle, however, optical crosstalk limits the pitch. For an array pitch of 3 µm, optical crosstalk was negligible, and the side lobes appeared at ±31.1°, resulting in a field-of-view (or scanning angle) of ±15.0°. The output beam forms a line shape, then the scattered light from an object will be captured by using a detector array in a vertical direction so as to produce a planar image [19,21,33]. The vertical beam divergence angle can be adjusted by controlling the output mode size. For a SiN waveguide with a 500 nm width, the vertical divergence angle resulted in 32° by the diffraction [33], which could be further reduced using a cylindrical lens.
3. Fabrication of OPA chips and beam scanning experiments
The fabrication procedure for a monolithic OPA device is depicted in Fig. 3. An oxide film with a thickness of 5 µm is formed on an 8-inch silicon wafer to create a lower cladding layer. Subsequently, SiN, with a refractive index of 1.972, is deposited using low-pressure chemical vapor deposition (LPCVD). I-line stepper lithography is used to generate a photoresist pattern, and reactive ion etching (RIE) is used to form the SiN waveguide core. SiO2 layer of 2 µm is deposited over the SiN core pattern through plasma-enhanced chemical vapor deposition (PECVD), followed by etching of the SiO2 layer by 1.7 µm in the area designated for the polymer phase modulator. To fabricate the polymer waveguide core, a fluorinated polyimide (Leomid VSF, PI Advanced Materials Co., Ltd.), which is widely used for flexible OLED displays [41], is adopted. The polyimide with a refractive index of 1.5613 is spin-coated, to have a 1.5 µm thickness. A channel waveguide pattern is formed by contact lithography and etching by inductively coupled plasma (ICP). Subsequently, ZPU-430 polymer (ChemOptics Co., Ltd.), with a refractive index of 1.430 is spin-coated as an upper cladding, followed by ultraviolet curing and thermal curing to produce a thickness of 1.5 µm. After depositing a 10-100 nm thick Cr-Au layer onto the cladding surface, a heater pattern is created using photolithography and wet etching. This metal heater process can be substituted with TiN heaters, which are commonly used in CMOS fabrication processes. As the final step, glass lids are attached to the wafer for dicing and polishing end-facets.
Fig. 3. Schematic diagram of the streamlined fabrication procedures. SiN waveguides are formed on top of the thermally oxidized wafer. PECVD SiO2 is deposited over the SiN waveguide and etched to a desired depth. Polymer waveguides core patterns are defined, and upper cladding polymer is coated. Subsequently microheaters are formed on top of the polymer cladding.
Microscopic images of the fabricated device are presented in Fig. 4. Parts of the beginning and end of the taper were observed as in Fig. 4(a), while Fig. 4(b) and (c) depict clear focuses on polymer and SiN waveguides, respectively. The device was cleaved along the dash-dotted line indicated in Fig. 4(a), and SEM was taken as shown in Fig. 4(d). The oxide interlayer thickness between SiN and polymer waveguides was measured as 223 nm. Cross-sections of the 3-µm-pitch waveguide output are shown in Fig. 4(e) and (f).
Fig. 4. (a) Microscopic photograph of fabricated device exhibiting the taper structure, (b) and (c) are magnified images of the waveguide transition region focused on the polymer and SiN waveguide, respectively. (d) Cross-sectional SEM image of the transition taper at the point indicated as the dash-dotted line of (a) showing the oxide interlayer between polymer and SiN waveguides. (e) Microscopic photograph of the OPA output end facet showing SiN emitter with a pitch of 3 µm, and (f) a magnified SEM image of the SiN waveguide.
The propagation losses of each material were found by measuring the insertion losses of many waveguides with different lengths, which were determined to be 1.1 dB/cm and 0.5 dB/cm for SiN and polymer waveguides, respectively [36]. To assess the transition loss between SiN and polymer waveguides, many straight waveguides with different transition numbers and taper lengths were prepared, as depicted in Fig. 5(a). The insertion losses were measured as shown in Fig. 5(b). Because all the waveguides had the same length of SiN and polymer waveguides, the results provided excess losses due to waveguide transitions, which were found to be 0.70, 0.15, and 0.23 dB/cm for tapers with lengths of 100, 200, and 300 µm, respectively as expected from the design results.
Fig. 5. (a) A schematic layout of the waveguides used for measuring the mode converting transition losses between polymer and SiN waveguides and (b) Insertion losses measured depending on the number of waveguide transitions (Ntrans).
To evaluate the phase modulator, Mach-Zehnder interferometer (MZI) devices were fabricated on the same wafer with the OPA. Figure 6(a) shows the MZI response, revealing a Pπ of 25.7 mW, which was less than 40% of the SiN PMs [19,24]. The response time of the phase modulator was measured to be 69 µs when a 2 kHz rectangular voltage signal was applied, as shown in Fig. 6(b). According to our research milestones of line beam scanning LiDAR, we believe the response time below 100 µs is appropriate for producing 20 frames per second with a scanning range of 100° and resolution of 0.5°.
Fig. 6. Performance of the polymer-SiN monolithic TO-PM: (a) MZ response function exhibiting Pπ of 25.7 mW, and (b) Temporal response of the MZ modulator showing 10–90% rise and fall time of 74 and 69 µs, respectively.
To validate the line beam scanning performance of the OPA, the initial vertical divergence angle was reduced using a cylindrical lens (FAC360, Fisba AG) which was attached to the end facet of OPA through passive alignment with a gap less than 10 µm. The reduced vertical divergence will be controlled depending on the target distances using an additional lens system [33]. The OPA was subsequently mounted on a thermo-electric cooler (TEC), and the electrode pads were wire-bonded to printed circuit boards (PCBs), as depicted in Fig. 7(a). A 1550 nm distributed feedback (DFB) laser light was introduced to the OPA, with the output measured using an optical power meter, revealing a throughput (or optical loss) of −5.0 dB. To conduct the beam scanning experiment, the output beam from the OPA was scattered by a polycarbonate screen and observed with a short-wavelength infrared (SWIR) camera as the setup shown in Fig. 7(b). The TEC was driven to maintain the chip at 60 °C during the beam scanning experiment. To produce the in-phase status of OPA, a fast beam forming algorithm based on the rotating element vector method was used [32]. Subsequently, the beam was scanned horizontally by applying a slopped phase distribution on the polymer PM array. The beam scanning profiles were designed as shown in Fig. 7(c) considering the pitch and the number of output waveguides. During the beam scanning the initial beam forming condition was not altered, and the beam profiles on each angle were observed. As the phase difference between channels was adjusted to π, the beam reached the maximum scanning angle of ±15°, as demonstrated in Fig. 7(d), which corresponded well with the design results of Fig. 7(c).
Fig. 7. Beam scanning demonstration of the monolithic polymer-SiN OPA: (a) Photograph of the OPA chip packaged on an aluminum case, where cylindrical lens was attached to the output end facet of the OPA to form a line beam, (b) Schematic of the experimental setup for beam scanning experiment, (c) Calculated main lobe positions when the phase difference of the waveguide channels changes from −π to +π, with an increment of π/3, and (d) Accumulated image of the scanned beams in horizontal direction for the angles (ψ) within the range of ±15°, with an interval of 5° (see Visualization 1 for real-time video).
To obtain the beam profiles of the far field, the beam power was measured using a slit of 30 µm covering an optical power meter placed on a rotation stage. As shown in Fig. 8(a), after the beamforming, the peak beam intensity was higher than the background noise by 10 dB. As the beam was steered to the maximum scanning angle of 15°, the main beam power dropped by 1.3 dB. The side lobe suppression ratio of the main beam was −7.4 dB. The horizontal and vertical divergence angle of the main beam defined as full width at half maximum was 0.8° and 6.8°, respectively, as shown in Fig. 8(b) and (c). The horizontal beam divergence can be reduced further by scaling up the number of OPA channels to 128.
Fig. 8. Beam profiles measured during the beam scanning using a slit and an optical power meter: (a) Accumulated plot of the scanned beams in horizontal direction, (b) Horizontal and (c) vertical beam profiles of the main beam.
4. Conclusion
By virtue of the excellent TO effect of the polymer waveguide and the high integrability of the SiN waveguide, a monolithic integrated high-performance OPA device has been demonstrated. To minimize the mode transition loss for connecting the two heterogeneous waveguides on a single substrate, a tapered waveguide structure with an oxide interlayer was employed, and a low transition loss of 0.15 dB was achieved. The stable low-power driving characteristics of the polymer phase modulator reduced the burden of beamforming, and the initial beamforming states were maintained during the beam scanning. The monolithic integrated OPA devices could pave the way for low-cost, high-performance beam scanners for a variety of applications.
Funding
Challengeable Future Defense Technology Research and Development Program through the Agency for Defense Development (ADD) funded by the Defense Acquisition Program Administration (DAPA) in 2023 (No.915027201); National Research Foundation of Korea (2020R1A2C2101562).
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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