Introduction to visible-light integrated OPAs. Integrated optical phased arrays (OPAs) enable emission and non-mechanical steering of beams emitted from compact, millimeter-scale, fully integrated, planar, solid-state chips [1–10], providing advantages in form factor, simplified assembly, and integrated control over other beam-steering technologies, such as spatial light modulators. Over the years, they have been used in demonstrations for many impactful applications, such as light detection and ranging (LIDAR) and free-space optical data communications [1,2]. However, due to these initial application areas, OPAs have mostly been limited to implementations at infrared wavelengths [1–7]. Visible-light integrated OPAs have been largely underdeveloped.

However, there are many potential wide-reaching applications that require visible-light operation, including image projection and displays [9], neural probes [11], trapped-ion quantum systems [12], 3D printers [13], and underwater wireless communications (UWC) [14–16]. As one example, UWC is a potential application for visible-light OPAs that is vital for maritime applications such as underwater sensor networks. Conventional UWC technologies, based on acoustic or low-radio-frequency systems, are limited in speed and latency. In contrast, emerging optics-based UWC systems enable significantly higher capacities but are based on large mechanical lens-based transceivers [16]. As an alternative to these bulk optical systems, integrated OPAs with non-mechanical beam-steering capabilities have been demonstrated for free-space optical communications in air to enable high-speed, compact, electrically reconfigurable data links [1,2]. However, these demonstrations have been limited to infrared wavelengths, which are suitable for terrestrial communications, but not UWC due to heavy absorption of infrared light through water. For an OPA-based transmitter to be suitable for UWC, it must operate at visible wavelengths to fall within the transparency window of water, where absorption is a few orders of magnitude lower than at infrared wavelengths [14]. However, a visible-light integrated-OPA-based transmitter has yet to be shown.

Recently, visible-light integrated OPAs have been demonstrated in silicon-nitride platforms [8–10]; however, they have been mostly limited to passive systems that are not electrically tunable [8,9], which limits practical use for many applications. An active visible-light OPA was recently demonstrated; however, it utilized long and power-inefficient heater-based modulators [10]. These modulator disadvantages are due to the low thermo-optic coefficient of silicon nitride, which results in traditional heater-based silicon-nitride modulators on the order of hundreds of microns to millimeters long [17,18]. As such, these heater-based modulators are insufficient for OPA systems, which require power-efficient modulators to enable scaling to large aperture sizes and compact modulators to enable intricate routing schemes and tight antenna pitches. To address these modulator limitations, we have recently shown the first integrated visible-light liquid-crystal-based (LC-based) modulators, which provide a compact and power-efficient solution to visible-light modulation on a chip [19].

In this Letter, by leveraging these LC-based phase modulators, we demonstrate the first LC-based integrated OPA and use it to enable visible-light beam forming and steering. We develop an LC-based cascaded OPA architecture and design the corresponding necessary devices, including vertical-layer transitions, LC-based phase modulators, evanescent taps, and grating-based antennas. We fabricate this OPA system and use it to experimentally demonstrate beam steering at a 632.8-nm wavelength with a 0.4°×1.6° power full width at half maximum and 7.2° beam-steering range within ±3.4 V. Furthermore, we apply this OPA to show the first visible-light integrated-OPA-based free-space optical communications transmitter and the first integrated-OPA-based underwater wireless optical communications link. We experimentally demonstrate a 1-Gbps on–off-keying (OOK) link through water and leverage the non-mechanical active beam-steering capabilities of this OPA architecture to establish an underwater point-to-multipoint link with channel selectivity greater than 19 dB.

Liquid-crystal-based cascaded OPA architecture. The integrated OPA consists of an LC-based cascaded-phase-shifter architecture that linearly controls the relative phase applied to an array of antennas, as shown in Fig. 1(a).

 

Fig. 1. (a) Top-view simplified schematic of the LC-based cascaded OPA. (b) Top-view simplified schematic of the vertical-transition escalator from the bottom waveguide to the top waveguide directly underneath the LC region (not to scale). (c) Simulation results showing transmission into the LC region using either a dual-waveguide vertical-transition escalator or a single-waveguide direct interface. (d) Cross-sectional simplified diagram of the phase shifter after the in-house LC packaging steps (not to scale). (e) Simulation results showing the optical mode effective index and the phase gradient applied across the antennas due to the LC-based phase shifter as a function of LC refractive index. (f) Top-view simplified schematic of a cascaded evanescent tap that couples light from the upper bus waveguide to the bottom tap waveguide (not to scale). (g) Simulation results showing transmission into the thru (blue) and tap (green) ports of an evanescent tap as a function of coupler length. (h) Top-view schematic of the grating-based antennas (not to scale). (i) Simulation results showing scattering strength of a grating-based antenna versus perturbation width.

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At the input, an on-chip inverse-taper edge coupler couples light from an off-chip laser into an on-chip single-mode silicon-nitride waveguide. A 100-µm-long escalator device (an adiabatic layer-transition structure) then couples the input light from the single-mode waveguide into a vertically stacked silicon-nitride bus waveguide that is directly below an LC-filled trench (Fig. 1(b)). This vertical-transition escalator is utilized to minimize loss when transitioning into the LC region, since a direct-interface transition would result in increased loss, as seen in simulation results shown in Fig. 1(c). Next, evanescent tap couplers, placed with a pitch of 20 µm and with increasing coupling lengths, uniformly distribute the light from the bus waveguide to 16 vertically stacked and horizontally offset tap waveguides (Fig. 1(f)). This horizontal offset was implemented to minimize the effect of fabrication variation on coupling (with simulation results confirming minimal impact on device performance for ±10 nm variation in waveguide offset). Simulation results for an evanescent tap as a function of coupler length are shown in Fig. 1(g), which we use to determine the required coupler lengths along the LC-based phase-shifting bus region to ensure that uniform light is coupled to each of the 16 tap waveguides. These tap waveguides then route to 16 grating-based 400-µm-long antennas with uniform perturbations and a 2-µm pitch to emit the light out of the surface of the chip (Fig. 1(h)). Simulation results of the scattering strength for a grating-based antenna as a function of perturbation width are shown in Fig. 1(i). We choose a perturbation width of 57 nm, resulting in 98% of the light scattered by the end of the 400-µm-long antenna.

To enable one-dimensional far-field beam steering, the system utilizes the birefringence of LC medium to enable cascaded phase control to the array of antennas. In a nematic LC medium, the index of refraction varies based on the orientation of the LC molecules with respect to the propagation direction of the light. Furthermore, nematic LC molecules self-align to one another, and their directors align to an external electric field. Thus, by applying an electric field across the LC region to orient the molecules in the direction of the field, the index can be actively tuned, resulting in a linear phase shift to the antennas. To enable this functionality, the LC-based phase-shifting region consists of a silicon-nitride bus waveguide to weakly confine and guide the light, LC medium deposited into an oxide trench to enable strong interaction between the optical mode and the LC medium, metal electrodes on each side of the LC-filled trench for applying an electric field across the LC region, and a top glass chip with a mechanical alignment layer on the underside to anchor the LC molecules. A cross-sectional diagram of the phase-shifting region is shown in Fig. 1(d), and additional details on the LC phase shifter are provided in [19]. Simulation results of the optical mode effective index and the phase gradient applied across the antennas as a function of the LC refractive index are shown in Fig. 1(e). The LC medium used here is 5CB (4’-Pentyl-4-biphenylcarbonitrile), which is a common commercially available LC medium with ordinary and extraordinary refractive indices of 1.53 and 1.7, respectively, at the design wavelength of 632.8 nm; in this system, we assume that the maximum refractive index is limited to 1.59, since the waveguide mode becomes poorly confined past this point.

Wafer fabrication and liquid-crystal packaging. The LC-based cascaded integrated OPA was fabricated in a CMOS-compatible 300-mm wafer-scale silicon-photonics process at the State University of New York Polytechnic Institute’s (SUNY Poly) Albany NanoTech Complex.

The final fabricated cross section of the system, as received from SUNY Poly, consists of two 160-nm-thick SiN waveguiding layers separated by 250 nm of silicon dioxide, an 800-nm-deep and 5-µm-wide silicon-dioxide trench vertically spaced 60 nm above the top waveguiding layer, 820-nm-thick and 1-µm-wide metal electrodes on each side of the trench, and a smooth chip facet for edge coupling.

We then diced the fabricated photonic wafer and performed a chip-scale LC-packaging process at MIT. In summary, this packaging process consists of four steps: (i) performing a dry etch to bring the trench closer to the waveguide, (ii) patterning an SU-8 photoresist spacer layer, (iii) epoxying a glass chip with an alignment layer on top of the SU-8 spacer layer, and (iv) injecting LC into the formed cavity, to achieve a final cross section as shown in Fig. 1(d). Further details on wafer fabrication and LC packaging are provided in [19].

OPA-based beam forming and steering results. To experimentally characterize the fabricated integrated OPA, we coupled a 632.8-nm-wavelength helium–neon laser onto the chip via an optical fiber. An optical system, comprised of an objective, a lens to perform a Fourier transform, and a camera, was used to image the far field of the OPA. A photograph of the experimental setup, showing the emitted radiation pattern on a piece of cardstock, is shown in Fig. 2(a). A photograph of the LC-packaged chip is shown in Fig. 2(b), with a micrograph of the fabricated OPA in the inset.

 

Fig. 2. (a) Photograph of the experimental setup, showing the packaged photonic chip and emitted radiation pattern. (b) Photograph of a fabricated and LC-packaged photonic chip with a micrograph of the OPA in the inset. Experimentally measured (c) far-field radiation pattern, showing the main lobe and a grating lobe, (d) cross-sectional cuts of the far-field main lobe in both the array (blue) and antenna (green) dimensions, and (e) electrically controlled beam steering of the main lobe in the array dimension when a square wave with a varying peak voltage is applied across the LC-based phase shifter.

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The experimentally measured far-field radiation pattern is shown in Fig. 2(c). Experimentally measured cross-sectional cuts of the far-field main lobe, in both the antenna and array dimensions, are shown in Fig. 2(d). As expected, the array forms a beam in the far field with a 0.4°×1.6° power full width at half maximum, 8-dB side-lobe suppression, and second-order grating lobes at ±18.8°.

The propagation loss was experimentally measured using two suites of loss test structures and was found to be approximately 0.6 dB/mm for the nominal silicon-nitride waveguides and 1 dB/mm for the LC-based phase shifter. The overall insertion loss from the on-chip input waveguide to the main lobe emitted by the OPA is approximately 25 dB, which can be improved by utilizing strong evanescent taps instead of the weak taps used in this initial demonstration and by reducing the antenna pitch to decrease the power loss to the higher-order grating lobes.

Next, to demonstrate beam steering, we used electronic probes to contact the integrated electrodes. A 10-kHz square wave was applied across the electrodes of the LC phase shifter. We varied the peak voltage of this applied square wave to tune the phase gradient applied across the antennas and, hence, steer the formed beam in the array dimension, θ. As shown in Fig. 2(e), the OPA system enables 7.2° of visible-light beam steering within ±3.4 V.

OPA-based underwater-communications results. As a proof of concept, we use the integrated LC-based OPA to demonstrate visible-light underwater wireless communications. We modulate the current through a 637-nm-wavelength laser diode, which results in an OOK digital signal. This modulated optical signal is coupled onto the photonic chip, and the beam emitted by the OPA propagates through a 75-cm-long glass tank filled with tap water, as depicted in Fig. 3(a). At the far end of the tank, avalanche photodetectors (APDs) are used to recover the transmitted beam.

 

Fig. 3. (a) Photograph of the experimental setup for demonstrating an underwater optical communications link with a vertically mounted chip-based transmitter on the left side of a tank filled with tap water and a photodetector array on the right (inset showing photograph of the OPA chip and probe). (b) Recovered digital eye diagram through the underwater wireless optical channel at 1 Gbps. Signals recovered from CH1 and CH2 APDs, showing spatial channel selectivity for (c) OPA steered to CH1 and (d) OPA steered to CH2. (e) PRBS recovered from CH1 and CH2 APDs, showing electronically switchable time division multiplexing of the two spatially distinct wireless channels.

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We first use the integrated OPA passively (without beam steering) and recover the beam on the far side of the water-filled tank using a single 1-inch lens and a high-speed APD. We use a pattern generator to modulate the laser with a pseudo-random binary sequence (PRBS) at 1 Gbps, resulting in a recovered eye diagram (Fig. 3(b)).

To demonstrate an actively tunable point-to-multipoint link, we apply a 10-kHz square-wave control voltage to the integrated electrodes on the photonic chip and – by varying the peak voltage – steer the beam emitted by the OPA to an array of two separate APDs and matching lenses on the far side of the water-filled tank. These two APDs are vertically stacked, such that the first APD (CH1) is positioned boresight to the OPA (corresponding to a control voltage of 0 V) and the second APD (CH2) is approximately 7 cm below (corresponding to a control voltage of approximately ±2.5 V). The laser diode is modulated with a 100-MHz sinusoid, and both APD voltages are captured with an oscilloscope, resulting in two spatially distinct wireless channels, as shown in Figs. 3(c) and 3(d), with 19-dB and 22-dB channel-to-channel isolation for CH1 and CH2, respectively. To demonstrate the utility of this modality, we replace the 100-MHz test tone with a 100-Mbps PRBS and toggle the control voltage from 0 to ±2.5 V at 1 Hz to multiplex the recovered data between the two APDs, shown in Fig. 3(e). (Slight variations in detected voltage amplitude between experiments are due to APD repositioning and changes in LC driving voltage.)

Conclusions. This work presents the first proposal and demonstration of LC-based integrated OPAs. We developed a cascaded integrated OPA architecture with a compact and power-efficient LC-based phase-shifting region and used it to enable visible-light beam forming and steering. We demonstrated vital device design, including vertical-layer transitions, LC-based phase modulators, evanescent taps, and grating-based antennas. We fabricated this OPA system and used it to experimentally demonstrate beam steering at a 632.8-nm wavelength with a 0.4°×1.6° power full width at half maximum and 7.2° beam-steering range within ±3.4 V. Furthermore, we applied the integrated LC-based OPA to show the first visible-light integrated-OPA-based free-space optical communications transmitter and used it to demonstrate the first integrated-OPA-based underwater wireless optical communications link. We experimentally demonstrated a 1-Gbps OOK link and an electronically switchable point-to-multipoint link with channel selectivity of greater than 19 dB through a water-filled tank.

In the future, this LC-based OPA could be scaled up to a larger aperture demonstration [7,8] and could be implemented using other architecture configurations, such as a splitter-tree-based OPA architecture [1,2], a cascaded architecture with multiple electrodes distributed along the LC region for fine-tuned phase control [3,4], or an advanced architecture to emit circularly polarized light for robustness to turbulent water [20]. Furthermore, the LC-based phase-shifter cross section, driving scheme, and LC compound could be optimized to enable high-speed beam steering. Additionally, the operating frequency of the OPA could be extended from the current red wavelength to green and blue wavelengths.

This OPA system provides a promising solution to the challenge of integrated visible-light beam forming and steering, with many impactful far-reaching applications, including underwater wireless communications, displays, neural probes, and trapped-ion systems.