3D-printed facet-attached optical elements for beam shaping in optical phased arrays

We demonstrate an optical phased-array (OPA) equipped with a 3D-printed facet-attached element for shaping and deflection of the emitted beam. The beam shaper combines freeform refractive surfaces with total-internal-reflection (TIR) mirrors and is in-situ printed to edge-emitting waveguide facets using high-resolution multi-photon lithography, thereby ensuring precise alignment with respect to on-chip waveguide structures. In a proof-of-concept experiment, we achieve a grating-lobe free steering range of $\pm 30{\deg}$ and a full-width-halfmaximum (FWHM) beam divergence of approximately $2{\deg}$. The concept opens an attractive alternative to currently used grating structures and is applicable to a wide range of integration platforms.


Introduction
Optical phased-arrays (OPA) open a promising path towards compact robust beam scanners that do not contain any mechanically moving parts and that are, e.g., key to advanced light detection and ranging (LiDAR) sensors [1][2][3]. In general, OPA rely on rather large photonic integrated circuits (PIC) that are usually mounted into densely packed assemblies, thus making edge emission difficult and rendering light emission perpendicular to the chip surface the only practical option [1,4]. This requires efficient coupling of light guided in the planar PIC to a well-defined free-space beam propagating in an out-of-plane direction. In this context, grating structures etched into the in-plane waveguide array have become a mainstay for high indexcontrast silicon photonics, which can rely on high lithographic resolution [1][2][3][4][5][6][7][8]. However, such grating couplers require tight process control, especially when well-defined beam emission profiles need to be maintained. Moreover, the efficiency of grating couplers is impaired by the fact that light is diffracted both to the top and to the bottom, unless more complex structures such as bi-layer waveguides [5] are used. If the downward-radiation is redirected to the top, e.g., by a back-side mirror [6], unwanted intensity fringes may appear in the emitted beam along the scanning direction. It should also be noted that emission perpendicular to the chip surface is difficult to achieve by grating structures, since this would unavoidably lead to unwanted coupling of power to the counterpropagating in-plane waveguide mode. Some integration platforms suited for implementation of OPA do not even contain grating structures and hence have to rely on edge emission [9,10].
In this paper, we demonstrate an alternative approach for beam shaping and for deflecting light from a planar integrated OPA to an out-of-plane direction. The concept relies on 3D-printed facet-attached elements that combine freeform refractive surfaces with total-internalreflection (TIR) mirrors. These elements are in-situ printed to edge-emitting waveguide facets using high-resolution multi-photon lithography, thereby ensuring highly precise alignment with respect to on-chip PIC structures. In a proof-of-concept experiment, we design, implement, and characterize an edge-emitting silicon photonic (SiP) OPA with a facet-attached beam-shaping structure, offering a scanning range of 30 .
 The full width at half maximum (FWHM) beam divergence is 2.1 along and 1.9 perpendicular to the scanning direction. The emission efficiency of our current 3D-printed beam-shaping elements amounts to 72 % (1.4 dB loss) and can well compete with best-in-class Si3N4-based dual-level gratings [11], which occupy considerable on-chip footprint and which rely on rather complex sequences of patterning and deposition steps. In contrast to conventional grating structures, the emission direction of 3Dprinted beam-shaping elements can be adapted to any angle, and the concept can be widely applied to practically any integration platform that offers edge-emitting facets.

Concept and design of 3D-printed beam shapers
Figure 1(a) shows the concept of an integrated OPA-based beam scanner that relies on 3Dprinted facet-attached optical elements for beam shaping, see Inset (1). The optical part of the assembly comprises a laser-diode (LD) chip, a photonic integrated circuit (PIC) containing the optical phased-array (OPA), and the 3D-printed optical beam shaper. The beam shaper matches the emitted beam to different application requirements and is printed to the device facet in-situ using high-resolution multi-photon lithography. The phases in the branches of the OPA are thermally tuned by on-chip heaters. A printed circuit board (PCB) carries peripheral electronics such as the LD driver, digital-to-analog converters (DAC) that feed the heaters, and microcontrollers (µC) that adjust the phases required for beam steering. The PCB is electrically connected to the PIC by metal wirebonds. The PIC and the LD chip are mounted on a common metallic submount serving as a heat sink and can be efficiently connected by 3D-printed photonic wirebonds (PWB) that eliminate the need for active alignment during the assembly process [12,13].
The tight integration of the PIC into the optoelectronic assembly renders light emission to a surface-normal direction the most practical option. This is accomplished by the 3D-printed beam shaper at the output facet of the OPA, which comprises a cylindrical lens to collimate the emitted light perpendicular to the scanning direction and a total-internal-reflection (TIR) mirror to redirect the beam to a surface-normal direction, see Inset (1) of Fig. 1(a). The implementation of the 3D-printed beam shaper is illustrated in more detail in Fig. 1(b), which shows a sideview of the cylindrical collimation lens and the redirecting mirror. In the following,  and  denote the far-field angles along and perpendicular to the scanning direction, respectively, both measured with respect to the surface normal of the PIC.
To demonstrate the viability of 3D-printed beam shapers for OPA, we implement a proofof-concept integrated system relying on an OPA with 32 N = output channels, fabricated on the silicon photonics (SiP) platform of IMS CHIPS (Stuttgart, Germany) [14]. The layer stack of the photonic chip is illustrated in Fig. 1(b). The silicon-on-insulator (SOI) device layer has a standard thickness of 220 nm and is optically isolated from the silicon handle wafer by a 3 µm thick buried oxide layer (BOX). The waveguides are overclad by a 2.1µm -thick layer of sputtered silicon dioxide, and the light-emitting facets are created by etching deep trenches into the top oxide. Note that the light-emitting tips of the SiP waveguides (WG) lie entirely within the top oxide, approximately fac 1µm d  behind the deep-etched sidewalls of these trenches to safely avoid unwanted uncovering of the waveguide tips during trench formation. For a precise lens design, this spacing has to be taken into account, since the refractive index of the SiO2 layer surrounding the tip ( 1.44 n = @ 0 1.55 µm)  = deviates slightly from that of the 3D-printed lens body ( 1.53 n = @ 0 1.55 µm),  = leading to slight refraction of light at the interface.
Heaters and contact pads for metal wirebonds are implemented by a AlSiCu metal layer that features a thickness of 100 nm and that is deposited on top of the cladding. The standard trace width for on-chip electrical connections amounts to 20 µm, while the trace width of the heater sections is only 1µm, thus leading to an increased electrical resistance and therefore to heating of the silicon waveguides below. (a) Artist's impression of a beam scanner system consisting of an OPA on a silicon photonic integrated circuit (PIC), a laser-diode (LD) chip, and control electronics such as LD drivers, digital-to-analog converters (DAC), or microcontroller (µC). All optical components are assembled on a common metallic submount that also functions as a heat sink. Laser and OPA are optically connected via a photonic wirebond (PWB). The OPA waveguide facets emit a beam, which is then shaped by a 3D-printed cylindrical collimation lens and a total-internal reflection (TIR) mirror that redirects the light to the top, see Inset (1) and Subfigure (b). Phase shifters in the OPA waveguides are used to steer the beam in the chip plane (( , ) xz -plane), which, after the beam shaper, results in a steering along angle .    when steered to 0  =  [15]. Note that the steering range could be further increased by using sub-wavelength spacing of the edge-emitting waveguide facets as demonstrated in [16,17]. Note also that the chip footprint shown in Fig. 2(a) can be significantly reduced as most of the chip area is occupied by partially unnecessary electrical routing, designed for simplified packaging via metal wirebonds.
To avoid optical cross-talk between the parallel sections of the tightly spaced OPA output waveguides, two different waveguide widths WG w of 300 nm and 400 nm are alternated in this area, see Fig. 2(e). This leads to different propagation constants in neighboring waveguides, therefore minimizing optical coupling [16,18]. Towards the edge of the chip, the waveguides are first up-tapered to a common width of 480 nm, before being down-tapered to a final tip width of 350 nm, thereby guaranteeing identical far-field radiation characteristics (element factors, EF) of all OPA waveguides facets. The associated vertical and horizontal 3 dB beam divergences of a single emitter amount to EF 32  =   and EF 43 ,  =   respectively, thereby covering the targeted 30  scanning range in  -direction. To exclude cross-talk in the tapered waveguide sections as a relevant source of distortion in our devices, we assess the residual coupling between neighboring OPA waveguides both through simulations and through measurements. We find that the guided light remains confined to the respective waveguide without any leakage into neighboring ones. Note also that the OPA output waveguides end about 1µm before the facet inside the chip, which needs to be taken into account for the lens design. We measure an approximately circular spot size with a 1/e²-diameter of the intensity distribution of fac,meas 2 2.4 µm r = at the facet of the chip, well in line with finite-difference time-domain (FDTD) simulations of the optical field emitted from the waveguide tip.
The overall structure of the 3D-printed beam shaper has been introduced in Fig. 1(b). As a key element of this structure, the shape of the cylindrical lens was designed and optimized using a home-made implementation of a wave-propagation algorithm based on a theory described in [19], that has previously been applied for design of 3D-printed freeform structures [20][21][22]. The optimized shape of the cylindrical lens shown in Fig. 2(f) is described by a y -dependent position z of the lens surface, see also Fig. 1(b). This shape is parametrized by an even-order polynomial with four free coefficients 0 , a 2 , a 4 , a 6 , a 2 4 6 0 2 4 6 () z y a a y a y a y = + + + The ( , ) yz -origin of the coordinate system is at the center of the emission facet, see Fig. 1

(b).
After emitted by the lens, the beam passes through a prism-like redirection element that consists of a plane input facet pointing to the left, a plane TIR mirror surface at a 45 inclination, and a plane output facet pointing to the top. These plane facets only redirect the beam, but do not have any influence on the beam shape. For the optimization of the lens shape, we consider a two-dimensional field distribution in the plane x = 0 and assume that each of the OPA waveguide tips emits a Gaussian beam having a waist at the waveguide tip, where the 1/e²diameter of the intensity distribution amounts to tip 2 1.6 µm. r = Using the Gaussian mode field in the beam waist as an excitation for the wave propagation at fac zd =− and assuming a fixed value of 0 85 µm, a = we vary the coefficients 2 , a 4 , a and 6 a in Eq. (1) to maximize the overlap of the resulting emitted field with a two-dimensional Gaussian beam having a FWHM beam waist diameter of 25 µm, corresponding to a FWHM divergence angle of refers to the waist of the Gaussian beam right after the waveguide tip and is thus slightly smaller than the spot size of fac,meas 2 2.4 µm, r = that was measured at the chip facet, i.e., fac 1µm d = away from the taper tip.

Device fabrication and characterization
The beam-shaping element is 3D-printed to the facet of the silicon photonic (SiP) chip in-situ, similar to the structures presented in [20,22,23]. We rely on an industry-grade two-photon lithography system (Sonata 1000, Vanguard Automation GmbH, Germany), which is also capable of printing photonic wirebonds (PWB) [12,13]. It is hence possible to fabricate package-level chip-chip and fiber-chip connections along with the 3D-printed beam shaper in a common, fully automated step of the underlying assembly process. A microscope image of the fabricated beam-shaping element is shown in Fig. 2(f). The structure consists of a photoresist (VanCore B, Vanguard Automation GmbH, refractive index 1.53 n = at 0 1.55 µm)  = optimized for printing of micro-optical structures with high shape fidelity. In the 3D-printing process, the exact positioning of the cylindrical lens relative to the chip facet is crucial as it has a large influence on the collimation properties of the lens and on the direction of the collimated beam. Our machine achieves a placement accuracy of 30 nm such that positioning inaccuracies do not play a significant role. The working distance is slightly larger than the focal length, L1 1 , f d  leading to a highly magnified image on the camera sensor. (b) Measurement of the far-field intensity. Each position u in the Fourier plane of Lens 1 is associated with a specific angle  of a plane wave incident on Lens 1. Lens 2 is inserted between Lens 1 and the IR camera to image the intensity distribution in the Fourier plane of Lens 1 onto the camera sensor. Lens 2 also de-magnifies the field in the Fourier plane of Lens 1 to match the size of the sensor. For calibration of the setup, we use a single-mode fiber emitting a beam at an angle  (not shown). The fiber is rotated around its facet while recording the position x of the center of the associated intensity distribution on the camera sensor.
Beside the cylindrical lens, the 3D-printed beam shaper comprises a TIR mirror that redirects the light to the top and that is held by support structures at each side, see Fig. 2

(f).
After assembly of the system and 3D-printing of the beam-shaping element, the far-field intensity distribution of the light emitted by the optical phased-array (OPA) is characterized using the setup illustrated in Fig. 3, which similar to the one reported in [7]. Figure 3(a) shows the setup with a single lens between the OPA and the infrared (IR) camera sensor. The scanning angle  lies in the drawing plane of Fig. 3, whereas the out-of-plane direction corresponds to the angle ,  see Fig. 1(a) for the definition of the angles. Lens 1 is a microscope objective which maps the aperture field of the OPA to the camera sensor. The distance L1 d between the lens and the OPA is slightly larger than the focal length 1 f, L1 1 f, d  leading to a greatly 7 magnified image of the aperture on the IR camera sensor. This setup helps in finding the emission aperture of the OPA and allows to investigate the intensity distribution in the aperture plane. For a subsequent measurement of the far-field intensity, an additional lens (Lens 2) is inserted, which images the intensity distribution in the back focal plane of Lens 1 to the camera sensor. Since each position u in the back focal plane of Lens 1 is associated with a certain emission angle  with respect to the optical axis of the setup, the camera image directly reveals the angle-dependent intensity distribution of the OPA emission. Note that the field distribution in the back focal plane of Lens 1 would directly correspond to the Fourier transform of the aperture field if the OPA is precisely positioned in the front focal plane of Lens 1. However, since the distance L1 d between the lens and the OPA slightly exceeds the focal length 1 f of Lens 1, only the modulus of the Fourier transform of the aperture field appears in the Fourier plane, but not its phase [24,. Since we are only interested in the intensity distribution, this deviation does not play a role. In our setup, the intensity distribution in the back focal plane of Lens 1 can be captured within an area which has a length of 20 mm along the horizontal direction (  -direction) in Fig. 3 and a length of approximately 16 mm in the direction perpendicular to it. This area is subsequently de-magnified to fit to the 9.5 mm 7.6 mm  camera sensor. The relationship between the emission angles  and  and the corresponding position on the camera chip can be extracted by a calibration measurement in which the aperture of the OPA is replaced by the end-face of a single-mode fiber, which can be rotated in  -direction while recording the position x of the center of the associated intensity distribution on the camera sensor as a function of .  Due to manufacturing tolerances of the high-index-contrast silicon photonic waveguides, the optical phases of the fields emitted at the various facets of the OPA are subject to random variations. These variations come on top of the deterministically distinct phase shifts that the signals in the various channel experience on their ways through the 2x2 MMI splitters and the various waveguide sections with different design widths. To this end, we perform a one-time calibration process, in which we experimentally determine the phase shifter currents for every scan angle .
 This calibration procedure is similar to the one described in [8] and relies on maximizing the intensity at the targeted angle  by tuning all 32 N = phase shifters subsequently. The procedure is repeated for all steering angles, and the resulting heater currents are stored in a look-up table. In our experiments, we chose a steering angle increment of 2.
  =  Figure 4(a) shows far-field intensity distributions of the calibrated OPA, measured for different steering angles  and then stacked on top of each other. The spot is steered across full range from 30 30 , −      which is limited by the occurrence of secondary main lobes (grating lobes) due to higher diffraction orders, see Fig. 4  Far-field intensity measurement of the optical phased-array (OPA) with 3D-printed beam shaper in a setup according to Fig. 3(b). The elevation angle  and the steering angle  are indicated in Fig. 1. (a is smaller than 1.9  for all steering angles  and depends slightly on  since the employed cylindrical lens is traversed a lateral angle for 0.  main lobes occur at directions in which all emitted partial waves of the OPA interfere constructively in analogy to higher-order diffraction of conventional gratings. The directions are dictated by the waveguide pitch of 1.5 µm, d = which is slightly smaller than a free-space wavelength 0 1.55 µm.

 =
All other side lobes within the scan range 30 ...30  = −   are down by more than 10 dB compared to the main lobe across the entire scan range, see blue line in Fig. 4(b). The measured suppression ratio is slightly smaller than its theoretically predicted counterpart of approximately 13.2 dB. We attribute this deviation to residual roughness of the chip facet that still occurred in our devices due to non-optimum reactive-ion etching (RIE) processes for forming deep trenches along the dicing lanes of the wafer. Note that this roughness is not a fundamental problem, but a very specific issue of the fabrication processes used for our devices. Optimized etching processes can greatly reduce roughness and should hence lead to better radiation characteristics. These imperfections lead to different radiation characteristics, also called element factors (EF) of the various apertures, which, together with their spatial arrangement characterized by the so-called array factor (AF), form the total radiation pattern EF × AF. Figure 4(c) depicts the FWHM divergences along the and  -direction as a function of steering angle .
 The FWHM divergence  in direction is better than 1.9 for all steering angles and shows a slight systematic dependence on the steering angle .
 This is caused by the fact that the effective focal length of the cylindrical lens changes when then lens is traversed at a steering angle 0.  The rather random variations of the FWHM divergence  in  -direction is attributed to the aforementioned roughness of the chip facet.
We also estimated the emission efficiency of the 3D-printed beam shaper and the associated on-chip interface. To this end, we couple light into the input waveguide of our OPA using a lensed fiber, see Fig. 2(a). We steer the OPA to an emission direction of 0  =  and use an integrating sphere (Thorlabs S145C) with a round input aperture to measure the optical power emitted by the 3D-printed beam shaper into a circular cone with an opening half-angle of 15 (full angle 30 )  , centered about the beam axis. Stray light from the chip is shielded by an appropriate cover. From the lensed fiber to the integrating sphere, we measure, after a calibration step, a power loss of 20.8 dB. Coupling loss from the lensed fiber to the chip amounts to 7.3 dB, and approximately 3.3 dB of loss are added by an MMI that is included into the on-chip input waveguide for test purposes. This MMI couples 50% of the incoming light to a reference port and features an excess insertion loss of 0.27 dB. Similarly, the subsequent five stages of the MMI-based 1:32 splitter tree contribute excess losses of approximately 0.27 dB per stage, which leads to a total loss of 1.35 dB, obtained by relating the power at the input of the splitter tree to total power at all its outputs. The average waveguide attenuation amounts to 0.43 dB/ mm, which leads to a total loss of approximately 4.7 dB along each of the 11 mm-long signal paths. Reflection losses at the waveguide tips and emission into unwanted secondary main lobes (grating lobes) add another loss of approximately 2.7 dB. Disregarding the unknown roughness loss of the radiating facets, this leads to an OPA loss of approximately 19.4 dB, leaving approximately 1.4 dB of loss (72% emission efficiency) for the current implementation of the 3D-printed beam shaper. The Fresnel reflections at the three interfaces of the 3D-printed beam shaper including the prism-like redirection element amount to approximately 0.6 dB. We attribute the remaining losses of approximately 0.8 dB to fabrication inaccuracies of the 3D-printed elements, to the aforementioned roughness of the chip facet, and to scattering of light from the waveguide tips to directions outside the capture range of the 3D-printed beam shaper. Still, the 1.4 dB of loss measured for the current structures is significantly lower than the 5 dB of loss that were measured for single-layer grating couplers [25] and can well compete with the losses of 1.33 dB (0.97 dB directionality + 0.36 dB loss due to incomplete radiation of power) and 2.06 dB (1.04 dB directionality + 1.02 dB loss due to incomplete radiation of power) that were obtained for best-in-class Si3N4-based dual-level fishbone and chain gratings [11]. In contrast to such gratings, our 3D-printed beam-shaping elements neither require excessive onchip area nor do they rely on complex platform-specific sequences of patterning and deposition steps. Based on the sub-1 dB loss demonstrated for beam shapers printed to edge-emitting InGaAsP lasers [23], we believe that further optimization of the emission interface, using, e.g., sub-wavelength grating (SWG) waveguides [26][27][28], in combination with smooth chip facets and improved fabrication processes for the 3D-printed beam shaper will eventually permit emission losses below 1 dB.

Summary and outlook
We demonstrate 3D-printed facet-attached beam shapers as an attractive approach to beam forming and out-of-plane deflection of light in integrated optical phased-arrays (OPA). The elements are in-situ printed to the chip facet using high-resolution multi-photon lithography, making the concept applicable to any integration platform that offers edge-emitting facets. We demonstrate the viability of the scheme in a proof-of-concept experiment using an edgeemitting silicon photonic OPA. Within the steering range of 30 ,  we achieve a full-width-half-maximum beam divergence of less than 2.1 along the steering direction and of 1.9 in the direction perpendicular to it. The steering range is limited by the 1.5 µm pitch of the edgeemitting waveguide facets and can be further increased by using smaller pitches. Our current device generation exhibits emission efficiencies of 72 % (1.4 dB loss) and can well compete with best-in-class Si3N4-based dual-level gratings [11], which occupy considerable on-chip footprint and which rely on rather complex sequences of platform-specific patterning and deposition steps. We believe that losses below 1 dB will become feasibly with further optimization of designs and fabrication processes.

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.