Far-field Beam Width Research Articles

Experimental demonstration of a silicon nanophotonic antenna for far-field broadened optical phased arrays

Optical antennas play a pivotal role in interfacing integrated photonic circuits with free-space systems. Designing antennas for optical phased arrays ideally requires achieving compact antenna apertures, wide radiation angles, and high radiation efficiency all at once, which presents a significant challenge. Here, we experimentally demonstrate a novel ultra-compact silicon grating antenna, utilizing subwavelength grating nanostructures arranged in a transversally interleaved topology to control the antenna radiation pattern. Through near-field phase engineering, we increase the antenna’s far-field beam width beyond the Fraunhofer limit for a given aperture size. The antenna incorporates a single-etch grating and a Bragg reflector implemented on a 300-nm-thick silicon-on-insulator (SOI) platform. Experimental characterizations demonstrate a beam width of 44°×52° with −3.22 dB diffraction efficiency, for an aperture size of 3.4 μm×1.78 μm. Furthermore, to the best of our knowledge, a novel topology of a 2D antenna array is demonstrated for the first time, leveraging evanescently coupled architecture to yield a very compact antenna array. We validated the functionality of our antenna design through its integration into this new 2D array topology. Specifically, we demonstrate a small proof-of-concept two-dimensional optical phased array with 2×4 elements and a wide beam steering range of 19.3º × 39.7º. A path towards scalability and larger-scale integration is also demonstrated on the antenna array of 8×20 elements with a transverse beam steering of 31.4º.

Subwavelength grating waveguide antenna based on interleaved groove structure

Integrated optical antennas are essential components of optical phased arrays for applications in light detection and ranging technology. To achieve larger detection distances, wider detection ranges, and higher scanning resolutions, it is imperative to employ millimeter-scale or longer effective lengths to achieve a narrow beam width, coupled with high radiation efficiency. In the high refractive index contrast silicon photonics platform, achieving a narrow far-field beam width and high radiation efficiency simultaneously is a formidable challenge. In the article, a subwavelength grating waveguide antenna with interleaved grooved lateral radiating block arrays is proposed. The simulation result shows that an effective length of about 3.3 mm is attained, with a corresponding far-field beam width is 0.02°, and a radiation efficiency of 0.785 at the wavelength of 1550 nm. It is also found that the crosstalk between the adjacent grating antennas designed in the range of 1.49 μm ∼ 1.61 μm is less than −15.6 dB.

Highly efficient ultra-broad beam silicon nanophotonic antenna based on near-field phase engineering

Optical antennas are a fundamental element in optical phased arrays (OPA) and free-space optical interconnects. An outstanding challenge in optical antenna design lies in achieving high radiation efficiency, ultra-compact footprint and broad radiation angle simultaneously, as required for dense 2D OPAs with a broad steering range. Here, we demonstrate a fundamentally new concept of a nanophotonic antenna based on near-field phase-engineering. By introducing a specific near-field phase factor in the Fraunhofer transformation, the far-field beam is widened beyond the diffraction limit for a given aperture size. We use transversally interleaved subwavelength grating nanostructures to control the near-field phase. A Bragg reflector is used at the end of the grating to increase both the efficiency and the far-field beam width. The antenna has a compact footprint of 3.1 µm × 1.75 µm and an ultra-broad far-field beam width of 52° and 62° in the longitudinal and transversal direction, respectively, while the radiation efficiency reaches 82% after incorporating a bottom reflector to further improve the directionality. This unprecedented design performance is achieved with a single-etch grating nanostructure in a 300-nm SOI platform.

Circular Optical Phased Arrays with Radial Nano-Antennas.

On-chip optical phased arrays (OPAs) are the enabling technology for diverse applications, ranging from optical interconnects to metrology and light detection and ranging (LIDAR). To meet the required performance demands, OPAs need to achieve a narrow beam width and wide-angle steering, along with efficient sidelobe suppression. A typical OPA configuration consists of either one-dimensional (1D) linear or two-dimensional (2D) rectangular arrays. However, the presence of grating sidelobes from these array configurations in the far-field pattern limits the aliasing-free beam steering, when the antenna element spacing is larger than half of a wavelength. In this work, we provide numerical analysis for 2D circular OPAs with radially arranged nano-antennas. The circular array geometry is shown to effectively suppress the grating lobes, expand the range for beam steering and obtain narrower beamwidths, while increasing element spacing to about 10 μm. To allow for high coupling efficiency, we propose the use of a central circular grating coupler to feed the designed circular OPA. Leveraging radially positioned nano-antennas and an efficient central grating coupler, our design can yield an aliasing-free azimuthal field of view (FOV) of 360°, while the elevation angle FOV is limited by the far-field beamwidth of the nano-antenna element and its array arrangement. With a main-to-sidelobe contrast ratio of 10 dB, a 110-element OPA offers an elevation FOV of 5° and an angular beamwidth of 1.14°, while an 870-element array provides an elevation FOV up to 20° with an angular beamwidth of 0.35°. Our analysis suggests that the performance of the circular OPAs can be further improved by integrating more elements, achieving larger aliasing-free FOV and narrower beamwidths. Our proposed design paves a new way for the development of on-chip OPAs with large 2D beam steering and high resolutions in communications and LIDAR systems.

Millimeter-Long Silicon Photonic Antenna for Optical Phased Arrays at 2-μm Wavelength Band

Silicon photonic optical phased arrays are promising solutions for solid-state LIDAR. But the nonlinear absorption significantly saturates the optical power in the waveguides at 1.5-μm wavelengths. Besides, the high refractive index contrast of silicon grating exhibits a strong diffraction strength of the antenna, which results in a limited aperture of the device. These ultimately place a limit on the detection accuracy and reach. Here, a shallow etched two-dimensional subwavelength grating structure is proposed to improve the efficiency and aperture of the waveguide antenna. A reduction of diffraction strength is achieved via nanohole structure instead of conventional grating lines. Thus, a millimeter-long optical antenna is demonstrated with a peak efficiency of 71%. The far-field diffracted beam width of 0.035° and vertical field-of-view of 17.5° are achieved within 100 nm wavelengths range from 1.95 μm to 2.05 μm. The operation at 2-μm waveband also guarantees better eye-safety and significantly reduces the nonlinear absorption in silicon waveguide compared with conventional 1.5-μm band.

Highly efficient optical antenna with small beam divergence in silicon waveguides.

Optical antennas are key components in optical phased arrays for light detection and ranging technology requiring long sensing range and high scanning resolution. To achieve a narrow beam width in the far-field region, antenna lengths of several millimeters or more are required. To date, such long antennas have been impossible to achieve in silicon waveguides because currently demonstrated technologies do not allow accurate control of grating strength. Here, we report on a new type of surface-emitting silicon waveguide with a dramatically increased antenna length of L=3.65mm. This is achieved by using a subwavelength metamaterial waveguide core evanescently coupled with radiative segments laterally separated from the core. This results in a far-field diffracted beam width of 0.025°, which is a record small beam divergence for a silicon photonics surface-emitting device. We also demonstrate that by using a design with L-shaped surface-emitting segments, the radiation efficiency of the antenna can be substantially increased compared to a conventional design, with an efficiency of 72% at the wavelength of 1550 nm.

An Octave Bandwidth Coaxial Waveguide Feed Antenna With an Iris Matching Network

The design of a 2.2 to 4.4 GHz coaxial waveguide feed antenna for radio telescopes is described. An iris matching network is designed to improve the unmatched aperture reflection coefficient from $-$ 6.7 to $-$ 12 dB over a 2:1 bandwidth ratio. An efficient method of optimizing the irises over a wideband is presented. The properties of wideband iris matching networks including their behavior as bandpass structures are discussed. The far-field beamwidth of the feed is held constant across the octave band by a corrugated choke. Measured radiation properties agree closely with simulation.

Beam characteristics of the truncated parametric array generated by a panel piston radiator

A theoretical model is presented to describe the difference frequency beam characteristics of the truncated parametric array generated by a panel piston radiator. In this model, a low-pass acoustic filter is used to truncate the virtual-source. The truncated virtual-source can be viewed as a volume distributed cylindrical source and conic source within and beyond the Rayleigh distance, respectively. The difference frequency beam characteristics near the truncated position are significantly changed. Numerical simulation results show that the difference frequency beams near the truncated position are substantially sharpened with the distance between the observation point and the truncated position decreasing. Besides, the difference frequency beamwidth near the truncated position can be used to predict the farfield beamwidth of the truncated parametric array.

Analysis of a Pot-Like Ultrasonic Sensor with an Anisotropic Beam Pattern

AbstractWe investigated the design parameters of a compact pot-like ultrasonic sensor which possesses a highly anisotropic beam pattern. As the sensor size is small due to its application constraint, the parameters are thus highly coupled to one another. We analyzed the respective effects of the parameters in the case where there is a vertical beam width reduction. The parameters investigated include resonant frequency, vibrating plate width-expanded angle, and ratio of thickness discontinuity of the vibrating plate. Numerical models developed by combining finite-element analysis and spatial Fourier transforms were adopted to predict the far-field radiating beam pattern of the various design configurations. The displacement distribution of the vibrating plate was measured using a microscopic laser Doppler vibrometer and the far-field pressure beam patterns were measured using a standard microphone in a semianechoic environment. The three configurations we used to validate the simulation model resulted in an H-V ratio of 2.67, 2.68 and 3.13, respectively which all agreed well with the numerical calculations. We found that by increasing the operating resonant frequency from 40kHz to 58kHz, the vertical far-field beam width of an ultrasonic sensor can be reduced by 31.62%. We found that the vertical beam width can be significantly reduced when the ratio of the thickness discontinuity of the vibrating plate decreases from 1 to 0.4 and is incorporated with its optimal width-expanded angle of the vibrating plate. It appears that an ultrasonic sensor with this type of anisotropic beam pattern can be ideally adopted for today's automotive applications.

High-performance, reliable, 730-nm-emitting Al-free active region diode lasers

Compressively strained InGaAsP quantum well (QW) active (λ=732 nm) diode lasers achieve 2.9 W continuous wave (cw) power from facet-coated (4%/95%) 100-μm-wide apertures, with reliable operation demonstrated at 0.5 W cw power. A broad waveguide structure is used to obtain a large transverse spot size (d/Γ=0.433 μm), resulting in a low internal loss (αi∼2 cm−1) and narrow transverse far-field beam width (θ1/2=38°). Record-high characteristic temperatures for the threshold current and the differential quantum efficiency (T0=115 K and T1=285 K) are obtained by growing on misoriented substrates.

Whispering-gallery laser resonators. I. Diffraction of whispering-gallery modes

Whispering-gallery optics for use at soft X-ray and extreme ultraviolet wavelengths are examined for their potential applications in laser resonators. An examination is made of beam divergence at the exit of a whispering gallery resonator. The properties of whispering-gallery eigenmodes are reviewed, and the features of their Fourier transforms discussed. The Fraunhofer approximation is then used to describe the farfield pattern resulting from the diffraction of a whispering-gallery eigenmode. Explicit formulas are given for the farfield beamwidth and the Rayleigh distance. For modes of high order the results can be understood in terms of geometrical optics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Second Harmonic Generation in an AlGaAs Double-Heterostructure Laser

Second-harmonic radiation emanating from an AlGaAs transverse-mode-stabilized laser diode has been observed under continuous-wave operation at room temperature. The far-field beam width of the second-harmonic radiation has been found to be apprecialy narrower than that of the fundamental laser radiation.

A high-gain multimode dielectric-coated rectangular horn antenna

A broadband high-gain dielectric-coated horn antenna operating in two orthogonal TE10+TE/TM12 and TE01 + TE/TM21 mode sets to generate circularly polarized elliptical shaped beam is described. The aperture E-field distribution can be tapered by virtue of the higher order TE/TM modes to cause the E-plane far-field beamwidth approximately equal to H-plane beamwidth of the other orthogonal set of modes, resulting in low off-axis polarization axial ratio. Because of the tapered aperture field distribution, the radiation patterns will also have low sidelobe levels. The radiation characteristics together with the on-axis gain of the antenna are derived from an electric vector potential on the basis of the vector diffraction formula. Analytical results are established in good agreement with experimental results on radiation pattern and gain measurements. This antenna which can be used as a communication satellite antenna to illuminate efficiently an elliptical zone on the earth surface will offer better directivity, high on-axis gain, low sidelobe levels and low cost.

Narrow-beam five-layer (GaAl)As/GaAs heterostructure lasers with low threshold and high peak power

A variety of (GaAl)As/GaAs heterostructure injection lasers have been fabricated with two pairs of heterojunctions, the inner pair for carrier confinement and the outer pair for optical confinement. Asymmetrical versions of this design have given, at one extreme, threshold current density Jt at 300 K as low as 500 A/cm2 and at the other extreme far-field angular beam widths to the half-power point of 15° with Jt&amp;lt;1000 A/cm2, for 500-μm-long sawn cavity lasers with reflectors on one face. The incremental quantum efficiency from one end of 300-μm-long lasers with reflectors was over 40%. The peak optical power from lasers of 100 μm width was up to 3 W for 200-ns pulses. The temperature variation of threshold over the range 290–355 K was Jt355/Jt290∼1.5. Double-heterostructure lasers with narrow active layers and 20% AlAs confinement steps gave almost comparable performance in Jt and beam width (650 A/cm2 for 33° and 1200 A/cm2 for 15°), but the linearity of light versus current was poorer, the temperature sensitivity of threshold was Jt355/Jt290⩾2, and efficiency also dropped with temperature. The optical distribution has been calculated for various multilayer heterostructure waveguides and related both to the far-field beam width and to the threshold current. A universal gain/current density relation for the active layer has been deduced from the measurements and agrees reasonably with published theory.

A new multimode rectangular horn antenna generating a circularly polarized elliptical beam

A multimode circularly polarized rectangular horn antenna generating an elliptical shaped beam is described. This antenna operates in two orthogonal mode sets, namely the TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</inf> + TE/TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> and TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> + TE/TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</inf> modes. By virtue of the higher order TE/TM modes, the aperture <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> -field distribution can be tapered such that the effective <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> -plane far-field beam width is approximately equal to the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</tex> -plane beam width of the other orthogonal set of modes, resulting in low off-axis polarization axial ratio. Because of the tapered aperture distribution, the radiation patterns also have low sidelobes. The elliptical cross section beam is a direct result of the rectangular shaped aperture. This antenna, used in conjunction with a spacecraft to illuminate an elliptical zone on the earth surface, offers high edge-of-coverage gain, low sidelobes, low edge-of-coverage (EOC) axial ratio, less RF sensitivity to the space environment, and low cost. The performance of this antenna has been evaluated experimentally.

Beamwidth of phased arrays

A formula is developed for estimating the far-field beamwidth of an arbitrary narrow-beam phased array of identical isotropic elements at a single frequency. Equations are developed for the effects of errors in the estimated positions of the array elements.

Surface-wave diffraction on LiNbO3

Radiation patterns from transducers on the Y face of LiNbO3 have been measured for propagation in several directions. It is demonstrated here that simple results may be obtained from a detailed Green&apos;s-function analysis which allow important practical information to be obtained, such as the Fresnel distance and far-field beam width as functions of the orientation of the transducer normal.

Decreasing antenna beamwidth with increasing scan angle

All antennas, including phased arrays, exhibit beam broadening in the near field and also beam broadening in the far field at scan angles off broadside. For scan angles in the near field, the effective aperture is reduced, thereby producing less beam broadening due to the near field. At any scan angle, there is a near-field distance at which the broadside far-field beamwidth plus broadening equals the scan angle far-field beamwidth plus broadening. For larger scan angles, the beamwidth is less than at broadside. Calculations of this phenomenon are made for uniform linear and for tapered circular apertures.