Hyperhemispherical Lens Research Articles

LED-based solar simulator for terrestrial solar spectra and orientations

Light-emitting diodes (LEDs) have shown advantages in simulating complex terrestrial solar spectrum. However, it has difficulty to simulate the geometric characteristics of direct sunlight with one solar constant (100 mW/cm2). In this paper, A method to collect full aperture light with hyper-hemispherical aplanatic lens has been proposed, and a multi-source integrated collimating system was built. The terrestrial solar spectra and orientations under various conditions have been simulated simultaneously. Also, we conducted a control experiment with polycrystalline silicon solar cells under outdoor sunlight conditions. It has good spectral accuracy and orientational consistency. The system meets the Class AAA specifications and achieves one solar constant irradiance of AM1.5G spectrum. The divergence angle of the collimating beam is around ±3°, and the temporal instability of irradiance is less than 0.3%. Within the range of volume space, the simulator achieves uniform illumination and the output beam follows the cosine law. The simulator has widely application in the photovoltaic industry, photochemistry, photobiology and many other fields.

A 670-GHz 4 × 2 Oscillator–Radiator Array Achieving 7.4-dBm EIRP in 40-nm CMOS

A 670-GHz terahertz source array is implemented in 40-nm bulk CMOS technology. To increase the frequency and power of the source, a compact and 1-D scalable oscillator–radiator topology that works on the third harmonic is proposed. In order to avoid the gate transistor node from loading the tank at the output frequency, a filtering feedback network is used to feed through the fundamental signal while filtering the third harmonic. The source consists of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 2$ </tex-math></inline-formula> oscillator cells with different common- and differential-mode coupling mechanisms that are built into the slotline resonant tanks. These slotlines build up an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field pattern that radiates the signal to the far-field. The chip is assembled on a hyperhemispherical lens that suppresses substrate modes and further increases the directivity of the source. The full array and pads have a die size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.75\times 1$ </tex-math></inline-formula> .15 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The source can be tuned in the range of 660.8–676.6 GHz. It has a maximum measured EIRP of 7.4 dBm and −16.1 dBm radiated output power while consuming only 99.7 mW. The source also has a measured phase noise at 1- and 10-MHz offsets of −69 and −93 dBc/Hz, respectively.

Terahertz quantum cascade laser array with spatially-separated beams

A lens-coupled terahertz (THz) quantum cascade laser (QCL) array with multiple spatially-separated beams is presented. The beam divergence and focusing efficiency after lens are both investigated by simulations and experiments. By integrating an appropriate hyper-hemispherical silicon lens with a well-designed six-element distributed feedback (DFB) QCL array inside the cryostat window, excellent emission characteristics of small divergence angles from 4.1°×3.5° to 7.2°×5.8° and multi-wavelength spectra are realized at 77 K. The deflection angle between adjacent beams is ~ 10°. The output intensity after lens is 113 to 25 times that of original beams with a focusing efficiency ranging from ~ 58% to ~ 37% for lasers at different positions. This method provides a simple solution to form a multi-beam THz source with high intensity and low far-field divergence.

Resolution Limits of Hyper-Hemispherical Silicon Lens-Integrated THz Cameras Employing Geometrical Multiframe Super-Resolution Imaging

In this article, we present a comprehensive study to understand the image formation and associated resolution limitations of hyper-hemispherical high-resistivity floating-zone silicon (HRFZ-Si) lens-integrated terahertz (THz) cameras, and we extend this understanding into THz super-resolution imaging applications. Along these lines, design tradeoffs between fixed field-of-view and frequency-dependent angular resolution of such cameras are also discussed. The present study is based on investigations of a CMOS THz camera that is made of a <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">32</b> × <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">32</b> -pixel focal plane array sensor and a 15-mm diameter objective hyper-hemispherical HRFZ-Si lens. Two experiments are performed at 0.652 THz. In the first experiment, THz geometrical multiframe super-resolution imaging is performed to demonstrate that such cameras can always reach the maximum achievable resolution in their entire operating bandwidth. In the other, a full radiation pattern scan is performed to measure aberrations of off-axis displaced pixels in order to give a comprehensive picture of the camera's resolution behavior across the full camera frame. The investigations and discussions are general in nature and should be applicable to any broadband THz camera.

Radiation pattern of planar optoelectronic antennas for broadband continuous-wave terahertz emission.

In future wireless communication networks at terahertz frequencies, the directivity and the beam profile of the emitters are highly relevant since no additional beam forming optics can be placed in free-space between the emitter and receiver. We investigated the radiation pattern and the polarization of broadband continuous-wave (cw) terahertz emitters experimentally and by numerical simulations between 100 GHz and 500 GHz. The emitters are indium phosphide (InP) photodiodes with attached planar antenna, mounted on a hyper-hemispherical silicon lens and integrated into a fiber-pigtailed module. As both packaging and material of the emitter was identical for all devices, similarities and differences can be directly linked to the antenna structure. We found that the feeding point structure that connects photodiode and antenna has a large influence on the radiation pattern. By optimizing the feeding point, we could reduce side lobes from -2 dB to -13 dB and narrow the 6dB beam angle from ±14° to ±9° at 300 GHz.

Vivaldi End-Fire Antenna for THz Photomixers

We propose a broadband end-fire antenna for continuous-wave terahertz (THz) photomixing–based devices working in the frequency range of 0.5–1 THz. A compact Vivaldi antenna is presented that does not require any hyper-hemispherical silicon lens to collect and collimate THz radiation unlike the conventionally used broadside antennas. The antenna is tailored to radiate THz into or receive radiation from a dielectric waveguide placed in close vicinity of it. The antenna is fabricated on an indium phosphide (InP) substrate. A silicon (Si) superstrate is used to improve the directionality of the radiated beam. THz power coupled into Si waveguides is measured using two different techniques between 0.1 and 1.15 THz. Firstly, the waveguide is placed in the optical path of a 1550 nm based continuous-wave THz setup with a commercial broadside emitter, focusing optics, and a detector fabricated on the InP substrate with log-periodic broadside antenna. Secondly, the waveguide is placed in direct contact with the designed Vivaldi antenna–based THz receiver and using the commercial broadside emitter as a source. It is observed that the direct coupling technique using the Vivaldi end-fire antenna outperforms the optically coupled approach at frequencies higher than 668 GHz. Efficient THz photoconductive sources and receivers based on the designed compact Vivaldi end-fire antenna will be suitable for launching THz power into on-chip THz circuitry and for compact THz systems.

Experimental Comparison of UTC- and PIN-Photodiodes for Continuous-Wave Terahertz Generation

We carried out an experimental comparison study of the two most established optoelectronic emitters for continuous-wave (cw) terahertz generation: a uni-traveling-carrier photodiode (UTC-PD) and a pin-photodiode (PIN-PD). Both diodes are commercially available and feature a similar package (fiber-pigtailed housings with a hyper-hemispherical silicon lens). We measured the terahertz output as a function of optical illumination power and bias voltage from 50 GHz up to 1 THz, using a precisely calibrated terahertz power detector. We found that both emitters were comparable in their spectral power under the operating conditions specified by the manufacturers. While the PIN-PD turned out to be more robust against varying operating parameters, the UTC-PD showed no saturation of the emitted terahertz power even for 50 mW optical input power. In addition, we compared the terahertz transmission and infrared (IR) blocking ratio of four different filter materials. These filters are a prerequisite for correct measurements of the absolute terahertz power with thermal detectors.

Compact and sensitive heterodyne receiver at 2.7 THz exploiting a quasi-optical HEB-QCL coupling scheme

We demonstrate a sensitive and compact terahertz heterodyne detection system based on a quantum cascade laser (QCL) as a local oscillator and a hot electron bolometer (HEB) as a mixer. It relies on an original optical coupling scheme where the terahertz (THz) signal to be detected and the local oscillator (LO) signal are coupled to the HEB from both sides of the integrated lens/antenna mixer. The THz signal of interest impinges on the front side through the silicon lens while the LO onto the rear (air) side. This concept allows us to remove the beam splitter usually employed in terahertz heterodyne receivers. The mixer consists of a Niobium Nitride HEB with a log-spiral planar antenna mounted on the flat side of a hyperhemispherical silicon lens. The local oscillator of the heterodyne detector is a low power consumption and low beam divergence 3rd-order distributed feedback laser with single mode emission at the target frequency of 2.7 THz. The coupling between the QCL and the HEB has been further optimized, using a dielectric hollow waveguide that reliably increases the laser beam directivity and permits us to pump the HEB into its most sensitive state through the air side of the planar antenna. We have measured a noncorrected double sideband receiver noise temperature of 880 K at 2.7 THz.

Waveguide coupler for resonant‐tunnelling diode oscillator at 420 GHz

The authors proposed a waveguide coupler that extracts the radiation from a resonant-tunnelling diode (RTD) oscillator at 420 GHz as the fundamental mode of a rectangular waveguide compatible to the WR-2.2 standard. In the proposed structure, an RTD oscillator chip is embedded inside a hyper hemispherical silicon lens to reshape the radiation from the tiny chip into a collimated beam, which is subsequently converted into the waveguide mode using a linear horn. The authors estimate the transmission efficiency of the coupler to be -3 dB. Integrated in one package, this approach enables handy use of RTD oscillators as a platform for various terahertz applications.

Experimental Validation of a Bespoke Lens for a Slot Log-Spiral Feed

In this letter, we experimentally validate the concept of bespoke lenses for the specific design of a log-spiral feed. Bespoke lenses are designed using the concept of quasi-conformal transformation optics (QCTO) to enhance the radiation properties for a given feeding. Our experimental results demonstrate that the antenna with its bespoke lens has higher gain, lower cross polarization, lower sidelobes, and lower axial ratio than that with a conventional hyperhemispherical lens.

In-Band Full-Duplex Multimode Lens-Loaded Eight-Arm Spiral Antenna

A multimode lens-loaded eight-arm spiral aperture is proposed for in-band full-duplex applications. Specifically, the single eight-arm spiral antenna is composed of a four-arm spiral for transmitting (TX) and a 45° spatially rotated four-arm spiral for receiving (RX). The eight-arm spiral antenna is connected into two $4 \times 4$ Butler matrix beamforming networks (BFNs) to differentiate between the TX and RX arms/ports and distinguish their functionalities. In the absence of BFN imbalances and antenna geometry asymmetries, the isolation is theoretically infinite due to full cancellation at the RX antenna ports and RX beamformer ports. This approach also enables diverse co-polarized TX and RX radiation modes while maintaining theoretically infinite isolation. The considered spiral aperture modes are mode 1, with a broadside pattern, and modes 2 and 3, with conical patterns. The aperture is loaded with a hyperhemispherical dielectric lens to improve the far-field performance. The operational principles are discussed first under ideal conditions, followed by the computational and experimental results. Average measured isolation >38 dB is achieved over 3:1, 1.5:1, and 1.3:1 bandwidths for the radiating TX and RX modes 1–3, respectively. Once the TX and RX are driven with identical modes, similar radiation patterns with high envelope correlation coefficient are obtained.

Blue Laser Diode Based Free-space Optical Data Transmission elevated to 18\u2009Gbps over 16\u2009m

Up to 18-Gbps direct encoding of blue laser diode (BLD) is demonstrated for free-space data transmission. By reshaping the orthogonal frequency multiplexed (16-QAM OFDM) stream with sidelobe filtering, the raw data rate expedites from 17.2 to 18.4 Gbps. Employing an ultrafast p-i-n photodiode with smaller active area diameter and lower noise equivalent power significantly enlarges the data rate by 1.6 Gbps or upgrades the signal-to-noise ratio (SNR) by 0.2 dB. Replacing the 80-mW BLD with the 120-mW one essentially increases the received SNR by 0.4 dB under enhanced modulation throughput. Reinforcing the beam collimation and collection by increasing the numerical aperture with a plano-convex hyper-hemispherical lens further improves the SNR by 0.6 dB. After optimization, the 16-QAM OFDM data with and without sidelobe filtering are respectively delivered at raw data rates of 16.4 and 18 Gbps with spectral-density usage efficiency as high as 4 bit/s/Hz over 16 m in free space, wherein the BLD carried QAM-OFDM data stream remains its capacity after reformation with sidelobe filtering as the superior inter-carrier-interference immunity reinforces.

High-Power Growth-Robust InGaAs/InAlAs Terahertz Quantum Cascade Lasers.

We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction. A structure based on a three-well optical phonon depletion scheme was optimized for this bias direction. Depending on the sheet doping density, the performance of this structure shows a trade-off between high maximum operating temperature and high output power. While the highest operating temperature of 155 K is observed for a moderate sheet doping density of 2 × 1010 cm–2, the highest peak output power of 151 mW is found for 7.3 × 1010 cm–2. Furthermore, by abutting a hyperhemispherical GaAs lens to a device with the highest doping level a record output power of 587 mW is achieved for double-metal waveguide structures.

Bespoke Lenses Based on Quasi-Conformal Transformation Optics Technique

In this paper, a systematic method to design a quasi-optimum lens profile based on quasi-conformal transformation optics technique for a given excitation is presented. This method is applied to a number of conventional antennas, such as an aperture and a log-spiral slot. In all these configurations, an increase of the directivity is observed. Furthermore, using this method, a quasi-optimum graded index lens for a broadband enhanced leaky slot excitation is designed and the results are compared with a hyperhemispherical lens with and without matching layers. Our proposed methodology demonstrates to be able to increase the directivity, to reduce the sidelobes and the cross polarization in a broad bandwidth from 20 to 70 GHz. Due to the continuously changed dielectric constant of the lens profile, reflections are also reduced considerably inside the lens.

Design and analysis of composite optical receiver for indoor visible light communication

A novel design of optical receiver for visible light communication system in indoor environment is proposed in this study. The compound parabolic concentrator is coupled with a photo-detector as the receiving unit due to its optical properties. The composite optical receiver is composed of seven receiving units inserted in a hyper-hemispherical lens aligned with geometry configuration based on angle diversity. The composite optical receiver has fields of view of 360 in the horizontal direction and 180 in the vertical direction respectively, while the field of view of each receiving unit is 30. Model of indoor visible light communication is built through measurement in a room of a 5 m 5 m 3 m size. The received power and SNR distribution are acquired through MATLAB scripts. The received power of each receiving unit is treated by different algorithms. At a lower data rate, the sum of the received power from all receiving units is the final received power, while at a higher data rate, the final received power is the highest value of power collected by the each unit. The results show that the received powers of the composite receiver by using two different algorithms increase 11.58 and 7.47 dB, respectively, while the gains of the receiver are 15.31 and 5.98, respectively. The mean values of the signaltonoise ratio are 79.17 dB from the sum algorithm and 72.26 dB from maximum algorithm, respectively. It is concluded that signaltonoise ratio is high and the distribution fluctuation is weak. This usually means a good and stable communication performance. It is proved that the composite receiver designed in this study gives high quality communication performance and presents a wide field of view, thereby avoiding the blind area in communication.

Microstructured gradient-index lenses for THz photoconductive antennas

A new type of substrate lens for photoconductive antennas (PCA’s) based on sub-wavelength microstructuring is presented and studied theoretically by the use of Greens function integral equation methods (GFIEM’s). By etching sub-wavelength trenches into a flat substrate, the effective dielectric constant can be designed to function like a gradient index (GRIN) lens. The proposed GRIN substrate lenses have sub-mm dimension, which is smaller than the dimensions of a typical hyper-hemispherical substrate lens (HSL), and could enable fabrication of arrays of closely packed PCA’s with individual lenses integrated directly into the PCA substrate. The performance of different GRIN lenses is compared to a HSL and shown to be comparable with regards to the terahertz radiation extraction efficiency, and it is shown that the collimating properties of these GRIN lenses can be tailored by changing the parameters used for microstructuring.

Broadband Terahertz Computed Tomography Using a 5k-pixel Real-time THz Camera

We present a novel THz computed tomography system that enables fast 3-dimensional imaging and spectroscopy in the 0.6-1.2 THz band. The system is based on a new real-time broadband THz camera that enables rapid acquisition of multiple cross-sectional images required in computed tomography. Tomographic reconstruction is achieved using digital images from the densely-packed large-format (80×64) focal plane array sensor located behind a hyper-hemispherical silicon lens. Each pixel of the sensor array consists of an 85 μm × 92 μm lithographically fabricated wideband dual-slot antenna, monolithically integrated with an ultra-fast diode tuned to operate in the 0.6-1.2 THz regime. Concurrently, optimum impedance matching was implemented for maximum pixel sensitivity, enabling 5 frames-per-second image acquisition speed. As such, the THz computed tomography system generates diffraction-limited resolution cross-section images as well as the three-dimensional models of various opaque and partially transparent objects. As an example, an over-the-counter vitamin supplement pill is imaged and its material composition is reconstructed. The new THz camera enables, for the first time, a practical application of THz computed tomography for non-destructive evaluation and biomedical imaging.

Hollow metallic waveguides integrated with terahertz quantum cascade lasers.

We present the realization of a compact, monolithically integrated arrangement of terahertz quantum cascade lasers with hollow metallic cylindrical waveguides. By directly mounting a copper pipe to the end facet of a double metal waveguide, it was possible to significantly improve the far field emission from such a sub-wavelength plasmonic mode, while preserving the characteristic performance of the laser. Careful alignment of the quantum cascade laser and the hollow waveguide is required in order to prevent the excitation of higher order/mixed modes as predicted with a high degree of accuracy by a theoretical model. Finally, this approach proved to be a superior method of beam shaping when compared to other in situ arrangements, such as a silicon hyper-hemispherical lens glued to the facet, which are presented.

A Terahertz Detector Array in a SiGe HBT Technology

This paper presents HBT terahertz power detectors implemented in an experimental 0.25- μm SiGe process technology with a peak fT/fmax of 280/435 GHz. Based on the nonlinearity of the HBT base-emitter junction, the detector operates as a square-law down converter, mixing terahertz frequencies directly to dc. Fifteen detectors have been arranged in a 3 × 5-pixel array with differential on-chip ring antennas. The array has been assembled with a hyper-hemispherical silicon lens. Referred to the collecting aperture of the lens, a maximum optical current responsivity RI of 1 A/W and a minimum noise equivalent power (NEP) of about 50 pW/√{Hz} have been measured at 0.7 THz with a 125-kHz chopping frequency. The detectors have been characterized from 0.65 to 1 THz.

A 288-GHz Lens-Integrated Balanced Triple-Push Source in a 65-nm CMOS Technology

A 288-GHz lens-integrated high-power source implemented in a 65-nm CMOS technology is presented. The source consists of two free-running triple-push ring oscillators locked out-of phase by magnetic coupling. The oscillators drive a differential on-chip ring antenna, which illuminates a hyper-hemispherical silicon lens through the backside of the die. An on-wafer breakout of the oscillators core achieves a peak output power of -1.5 dBm with a 275-mW DC power consumption. The radiated power of the packaged source is -4.1 dBm, which is the highest reported radiated power of a single CMOS source beyond 200 GHz. The source die including the antenna occupies only 500 x 570 μ m2.