Uni-traveling Carrier Photodiode Research Articles

THz Wave Power Enhancement Using a Microstrip Line-Based Combiner Integrated with Arrayed UTC-PDs

Advancements in semiconductor devices, such as Uni-travelling-carrier photodiodes (UTC-PDs), have played a significant role in the development of Terahertz communication technology. However, the persistent challenge is the limited output power from a single UTC-PD required for practical transmission distances. To enhance the output power, we propose and demonstrate a novel Terahertz wave power combining technique using a photomixer device comprising two arrayed UTC-PDs monolithically integrated with a microstrip line-based 2 × 1 Wilkinson power combiner and a patch antenna on a Silicon Carbide (SiC) substrate at 300 GHz. When the two UTC-PDs are activated at photocurrents of 8 mA and 10 mA, the device exhibits a 7.3 dB increase in power relative to the power obtained when only the 8 mA UTC-PD is activated, and a 4.4 dB increase in power relative to the power obtained with the 10 mA UTC-PD. This implies that power can be enhanced by a factor of N2 if the photocurrent is multiplied by N. Additionally, we demonstrate that the UTC-PD output saturation depends on the space charge effect, which modulates the electric field in the depletion region and results from critical charge density of about 80 kA/cm2 for the device in this work.

Secured THz communication in photonic microcell networks based on spatial wave mixing of steered beams

Future 6G communication systems are envisioned to expand their carrier frequency to the THz region, where a broad unexplored region of spectrum is available. With this expansion, THz wireless communication has the potential to achieve ultra-high data transmission rates of up to 100Gbit/s. However, as large amounts of data are transmitted in an open wireless environment, there are significant concerns regarding communication security due to the susceptibility to eavesdropping, interception, and jamming. In this work, we proposed a secure approach for THz wireless communication based on spatial wave mixing and flexible beam steering. To achieve this, two frequency-modulated THz waves, which are generated by photonic THz sources and carry encrypted information with true randomness, are mixed at a THz envelope detector with an exclusive-OR logic operation. We analyzed the possible spatial location for the THz detector to ensure a secure microcell network deployment. Our results demonstrate that the size of the decryptable region is directly dependent on the directivity and width of the emitted THz beam. To address this, we have developed an array antenna with integrated uni-traveling-carrier photodiodes (UTC-PDs), which is capable of generating THz waves while also improving the flexibility of beam pointing, allowing for greater control over the location and size of the decodable region. By controlling fiber-optic delay lines, we successfully demonstrated that the directional gain of a 200GHz wave is increased by 8dB through a 1 × 3 UTC-PD-integrated planar bowtie antenna (PBA) array, together with continuous beam steering from -20° to 10°. Additionally, using a 1 × 4 UTC-PD-integrated PBA array to emulate two encryption transmitters and a Femi-level managed barrier diode to detect spatially mixed THz waves, we successfully achieved a feasibility experiment for real-time 200Mbit/s location-based decryption in the 200GHz band. These results indicate that the proposed scheme is feasible for secured THz communication, and would be a powerful candidate to mitigate security risks in 6G microcell networks.

Enabling cost-effective wireless THz communication through semiconductor optical amplifier’s direct/cross gain modulation for seamless integration of hybrid fiber-coaxial connectivity

We propose a cost-effective THz wireless communication scheme for future femtocell networks that leverages either direct gain modulation (DGM) or cross gain modulation (XGM) of a semiconductor optical amplifier (SOA) to convert coaxially or optically carried data into intensity-modulated THz waves, respectively. This architecture, based on an SOA and a uni-traveling-carrier photodiode (UTC-PD), has advantages over typical THz modulation schemes involving a resonant tunneling diode or an electro-optic modulator combined with a UTC-PD. Consequently, it facilitates the seamless convergence of hybrid fiber-coaxial networks to THz radio access networks with low cost, straightforward monolithic integration, and a more compact footprint. Furthermore, we experimentally verified the possibility of non-return-to-zero on–off keying wireless communication over 0.5 m in the 300 GHz band, achieving real-time transmission of 4 Gbit/s for coaxial access via an SOA’s DGM and 10 Gbit/s for fiber access via the SOA’s XGM. Notably, these data rates were attained by simple non-coherent THz intensity demodulation based on a Fermi-level managed barrier diode while meeting the 7% hard-decision forward error correction (HD-FEC) threshold (3.4×10−3). Finally, we conducted an analysis of the merits and drawbacks of the proposed THz communication scheme based on the combination of an SOA and a UTC-PD, discussed bottlenecks that hinder further rate improvement, and explored possibilities for optimization. In conclusion, this scheme is crucial for realizing future ultra-high-speed wireless networks that seamlessly connect to fiber-coaxial networks.

Terahertz Wave Power Multiplication by Combining Photocurrents from Arrayed UTC-PDs

A terahertz (THz) wave generator for multiplying the output power from arrayed photomixers is developed. The generator is composed of two uni-traveling carrier photodiodes (UTC-PDs) as the photomixers, a [Formula: see text] T-junction power combiner and a microstrip rectangular patch antenna on a silicon carbide (SiC) substrate in which photocurrents of 300[Formula: see text]GHz from the UTC-PDs are combined to be fed to the antenna. Experimental results show that the output power is multiplied by 5.8[Formula: see text]dB (approximately fourfold) compared to that of a single UTC-PD. This advancement has the potential for further multiplication of THz power by cascaded combining of a larger number of UTC-PDs.

Ultrafast evanescently coupled waveguide MUTC-PDs with high responsivity

Novel evanescently coupled waveguide modified uni-traveling carrier photodiodes (MUTC-PDs) employing a thick multi-layer coupling waveguide are reported. To improve the optical-to-electrical (O/E) conversion efficiency, a thick multi-layer coupling waveguide with a gradually increased refractive index from the bottom layer to the absorption layer is utilized. The refractive index profile facilitates the upward transmission of incident light into the absorption region, thereby enhancing the evanescent coupling efficiency. Meanwhile, the coupling waveguide, with a total thickness of 1.75 µm, expands the mode field diameter, thereby reducing the input coupling loss. Additionally, the top layer of the coupling waveguide also serves as the drift layer. This configuration facilitates efficient light absorption within a short PD length, thus ensuring ultrawide bandwidth and high O/E conversion efficiency simultaneously. Without an additional spot size coupler or anti-reflection coating, the measured responsivity is as high as 0.38 A/W for the PD with an active area of 5 × 6 µm2. Meanwhile, an ultrawide 3-dB bandwidth of 153 GHz has been demonstrated.

Terahertz power enhancement by combining photocurrent from arrayed UTC-PDs using Wilkinson power combiner

We designed a novel terahertz wave power combiner which combines HF currents from each photomixer in a micro strip line. The generator is composed of four uni-travelling carrier photodiodes (UTC-PDs) as the photomixers, a 4 × 1 Wilkinson power combiner and a microstrip rectangular patch antenna on a Silicon Carbide (SiC) substrate in which photocurrents of 300 GHz from the UTC-PDs are combined to be fed to the antenna. Experimental results show that the output power is multiplied by 11 dB (approximately 13 times) compared to that of a single UTC-PD. The combined power increases super-linearly with the number of the photomixers, confirms a great advantage of photomixing as a current source.

Demonstration of THz frequency hopping in the 300 GHz band based on UTC-PD and tunable DFB laser array

We explored the potential of an optoelectronic frequency hopping system within the 300 GHz band, leveraging a tunable distributed feedback laser array and uni-traveling carrier photodiode. Our experiments successfully achieved a 10-channel terahertz (THz) frequency hopping, marking a significant advancement in THz secure communication technologies. Notably, the system exhibited a rapid frequency-hopping capability with a short transition time of 0.8 ms between channels, ranging from 288 to 331 GHz. Further, we conducted data transmission tests at a rate of 5 Gbit s−1 across several channels. The results were promising, showing each channel maintained a clear eye pattern and a low bit error rate, crucial factors for reliable and secure data transmission. These findings not only demonstrate the efficacy of our system but also open new avenues for high-speed, secure THz communication.

Design of uni-traveling-carrier photodiode with nanoscale optical microstructures

In this article, we report an uni-traveling-carrier photodiode (UTC-PD) incorporating nanoscale optical microstructures. We have designed a UTC-PD incorporating a core-cladding confinement structure, which incorporates internal optical scattering structures to convert incident light from vertical incidence to lateral propagation, and to as much as possible confine light within the absorption layer of the UTC-PD. In this way, the proposed UTC-PD can have an operating performance like that of a waveguide-type PD under vertical light incidence, and its responsivity will increase. The ideal responsivity of the designed UTC-PD with an absorption layer thickness of 200 nm after optimization can reach 0.43 A/W, which is 48 % higher than that of the traditional structure.

Fast 3D positioning based on a simplified THz photonic stereo ISAR system.

In this work, a THz stereo inverse synthetic aperture radar scheme based on photonics is proposed, and proof-of-concept experimentally demonstrated, achieving high resolution and fast three-dimensional (3D) positioning of multiple targets. An optical frequency comb and a uni-traveling carrier photodiode are employed to photonically generate a linear frequency-modulation signal at 300-320GHz. By two incoherent measurements at different angles with one pair of antennas, 3D positioning of plane reflectors over a distance of 0.6m is successfully accomplished, achieving a resolution of 7.5mm for both range and azimuth dimensions, and a maximum experimental height error of 4mm.

Terahertz photodiode integration with multi-octave-bandwidth dielectric rod waveguide probe.

Photonic integrated circuits play a vital role in enabling terahertz (THz) applications that require multi-octave bandwidth. Prior research has been limited in bandwidth due to rectangular waveguide (WRs) interconnects, which can only support single octave at low loss. To overcome this fundamental limitation, we exploit the ultra-wideband (UWB) near-field coupling between planar waveguides and silicon (Si)-based subwavelength dielectric rod waveguides (DRWs) to interconnect THz bandwidth uni-traveling-carrier photodiodes (UTC-PDs) at 0.08-1.03 THz. In a proof-of-concept experiment, the on-chip integrated UTC-PDs demonstrate a UWB operation from 0.1 THz to 0.4 THz. Furthermore, by employing Si DRWs as probes, multi-octave device-under-test characterization of UTC-PDs integrated with UWB transition is enabled with only one DRW probe. The proposed UWB interconnect technology is distinct from previously used WR-based ground-signal-ground probes or quasi-optical free-space coupling since it can provide multi-octave bandwidth and enable on-chip THz circuit integration.

Circularly Polarized Ultra-Wideband Antenna for Uni-Traveling-Carrier Photodiode Terahertz Source

This paper proposes a circularly polarized ultra-wideband (UWB) antenna for a Uni-Traveling-Carrier Photodiode (UTC-PD) to meet the growing demand for bandwidth and polarization diversity in terahertz (THz) communication. In the design of the UTC-PD integrated antenna, the planar electrodes of the chip are directly integrated with the antenna to simplify the integration process. However, this integration introduces new problems, such as asymmetry inside the spiral antenna, which leads to a deterioration in the corresponding high-frequency performance. To address this issue, the antenna’s structure is optimized, and a lens is integrated to enhance directivity and eliminate surface waves. As a result, the proposed antenna achieves a 100–1500 GHz (175%) impedance bandwidth and a 150–720 GHz (131%) axial ratio bandwidth for the UTC-PD. The maximum gain of the antenna is 21.05 dBi at 1 THz.

Investigation of modified uni-traveling carrier photodiode for cryogenic microwave photonic links

Quantum devices present the potential for unparalleled computing and communications capabilities; however, the cryogenic temperatures required to successfully control and read out many qubit platforms can prove to be very challenging to scale. Recently, there has emerged an interest in using microwave photonics to deliver control signals down to ultracold stages via optical fiber, thereby reducing thermal load and facilitating dense wavelength multiplexing. Photodetectors can then convert this optical energy to electrical signals for qubit control. The fidelity of the quantum operations of interest therefore depend heavily upon the characteristics of the photodiode, yet experimental demonstrations of fiber-coupled photodetection systems at low temperatures are relatively few in number, leaving important open questions regarding how specific detectors may perform in real-world cryogenic settings. In this work, we examine a highly linear modified uni-traveling carrier photodiode (MUTC-PD) under C-band illumination (1530–1565 nm) at three temperature regimes (300 K, 80 K, and ∼4 K) and multiple bias conditions. Our findings of reduced responsivity but preserved bandwidth are consistent with previous studies, while our saturation tests suggest a variety of potential applications for MUTC-PDs in cryogenic microwave photonics with and without electrical bias. Overall, our results should provide a valuable foundation for the continued and expanding use of this detector technology in quantum information processing.

Thermal dissipation enhancement in flip-chip bonded uni-traveling carrier photodiodes.

The thermal properties of modified uni-traveling carrier (MUTC) photodiode flip-chip bonded to AlN and diamond are simulated. The thermal impedance of InGaAs is the primary source of internal heating. An n-down epitaxial structure is designed to improve thermal dissipation. Compared to the conventional p-down configuration, the n-down MUTCs bonded to diamond, or AlN submounts achieved 145% and 110% improvement in dissipated power density at thermal failure, respectively. The improved thermal characteristics presage higher RF output power before thermal failure.

SPICE Modeling in Verilog-A for Photo-Response in UTC-Photodiodes Targeting Beyond-5G Circuit Design

This paper reports the first accurate and physicsbased Verilog-A implementation of the fully analytic form of photocurrent in SPICE compact models for uni-traveling carrier (UTC) photodiodes (PD). To overcome the limitations of single pole network implementations for modeling frequency dependence of the photo-response, especially at frequencies beyond 100 GHz, we explored different solutions for the complete analytic equation of the dynamic photocurrent. A new implementation has been proposed which requires three additional nodes in the UTC-PD electrical equivalent circuit and offers the best trade-off between accuracy and computational efficiency. Model validation has been performed against on-wafer measurements from two UTC-PD technologies depicting very good accuracy over the entire frequency range.

High Speed 1550 nm Indium Gallium Arsenide-Indium Phosphide Photodetector

This paper presents preliminary results of a high speed 1550 nm indium gallium arsenide (InGaAs)-based mesa-type modified uni-traveling carrier photodiode (M-UTC-PD) structure. Conventional UTC-PD refers to P-I-N type photodiodes which selectively use electrons as active carriers. Photons absorbed in the relatively thin P-type absorber create minority carriers which are field accelerated toward a depleted collector thereby establishing high velocity ballistic transport, making these structures applicable for high speed applications. The M-UTC-PD structure presented uses spatially tailored P-type absorber regions to limit minority carrier generation both in the lateral and axial dimensions. Utilizing an otherwise conventional UTC-PD epitaxial structure where the top P-type layers are undoped, the spatially tailored P-type regions are defined by closed ampoule Zinc diffusion techniques. The M-UTC-PD structure presented utilizes a series of nested p-doped rings within a mesa structure to limit dark current and reduce overall capacitance to improve high speed operation. Two photodiode structures will be investigated for this research project, a conventional UTC-PD structure and a modified structure, utilizing similar device designs, epitaxial designs and fabrication processes. The conventional structure will be utilized for fabrication process development, verification of epi quality and development of rapid prototyping approach toward chip-based testing and subsequent high speed RF testing procedures. Conventional UTC-PD device results will be used as a comparison to quantify the performance of the M-UTC-PD structure utilizing Zn-doped defined p-type absorber regions. Results are given for chip tests of UTC-PD chips verifying epitaxial quality and fabrication process, subsequent testing of packaged devices and RF analysis remains. Process development of the Zn-doped devices is underway, once completed, these devices will be compared to the base design to quantify performance enhancement associated with the modified design.

Physically Encrypted Wireless Transmission Based on XOR between Two Data in Terahertz Beams

Future wireless communications require higher security as well as a higher data rate. We have been studying physically secured wireless transmission systems and previously proposed encryption/decryption techniques based on the AND operation caused by coherent detection between two encrypted data sequences on two different terahertz carriers. Furthermore, we suggested that by employing the XOR operation as the decryption, the proposed system can be made more secure because XOR increases the computational complexity for eavesdroppers to recover the plaintext. In this paper, we propose the XOR operation between two data sequences on FSK-modulated terahertz waves. By constructing the XOR encryption transmitters/receivers, which consisted of high-speed wavelength tunable lasers and arrayed uni-traveling-carrier photodiodes (UTC-PDs), we successfully demonstrated the XOR operation between two data sequences on 200 GHz waves from the two transmitters.

Terahertz wireless communication in a 560-GHz band using a Kerr micro-resonator soliton comb

Terahertz (THz) waves have attracted attention as carrier waves for next-generation wireless communications (6 G). Electronic THz emitters are widely used in current mobile communications; however, they may face technical limitations in 6 G with upper-frequency limits. We demonstrate wireless communication in a 560-GHz band by using a photonic THz emitter based on photomixing of a 560-GHz-spacing soliton microcomb in a uni-travelling carrier photodiode together with a THz receiver of Schottky barrier diode. The on-off keying data transfer with 2-Gbit/s achieves a Q-factor of 3.4, thus, satisfying the limit of forward error correction.

Photonic THz InISAR for 3D Positioning With High Resolution

Terahertz (THz) radar has been considered as a new solution to the dilemmas in performing ultra-high-resolution imaging and obtaining highly precise target information. In this work, a photonic THz interference inverse synthetic aperture radar (THz InISAR) scheme is proposed and proof-of-concept experimentally demonstrated, targeting high resolution positioning of multiple targets. We employ an optical frequency comb and a uni-traveling carrier photodiode (UTC-PD) to photonically generate a linear frequency-modulation (LFM) signal at 290-310 GHz. By using the anti-electromagnetic-interference THz photonic signal source, broadband THz transceivers and efficient InISAR algorithms, 3D positioning of corner reflectors over a distance of 2.2 m at the 300 GHz band is successfully achieved, reaching a resolution of 8 mm for both range and azimuth dimensions, and a maximum height error of 0.4 mm in a height range of 11 cm. To the best of our knowledge, our proposed system represents the ever-first accurate 3D positioning at the THz band above 300 GHz, revealing the great potential applications of THz radar systems.

A 262-GHz Wireless IFoF Uplink With Remote Down-Conversion Using Optically Generated Sub-THz LO

We experimentally demonstrate the feasibility of a 262-GHz wireless intermediate frequency (IF)-over-fiber (IFoF) uplink with frequency down-conversion at a remote antenna unit (RAU). To distribute many RAUs for the coverage of terahertz (THz) wireless communications, it is important to simplify the RAU for the THz uplink. The remote down-conversion with a subharmonic mixer and a uni-traveling carrier-photodiode (UTC-PD) is driven by two optical tone signals from the central office (CO). This can reduce the high-frequency and wideband devices of the networks. Instead of coherent detection, the RAU can be simplified with optical transmission and direct detection (DD) by using a conventional optical modulator and a photodiode (PD). We measured the phase noise of the system and compensated for it with blind phase recovery. Then, the error vector magnitudes (EVMs) of the received signals were measured as a function of THz Tx power, optically generated local oscillator (LO) power, and THz wireless link distance. Single-channel 45-and 30-Gb/s data transmission was achieved over a 5-km fiber and 1-m THz wireless link based on soft-(2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-2}$</tex-math> </inline-formula> ) and hard-decision (3.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-3}$</tex-math> </inline-formula> ) forward error correction (FEC) thresholds, respectively.

High overall performance uni-traveling carrier photodiodes for sub-THz wave generation.

Modified near-ballistic uni-traveling-carrier photodiodes with improved overall performances were studied theoretically and experimentally. A bandwidth up to 0.2THz with a 3dB bandwidth of 136GHz and large output power of 8.22dBm (99GHz) under the -2V bias voltage were obtained. The device exhibits good linearity in the photocurrent-optical power curve even at large input optical power, with a responsivity of 0.206A/W. Physical explanations for the improved performances have been made in detail. The absorption layer and the collector layer were optimized to retain a high built-in electric field around the interface, which not only ensures the smoothness of the band structure but also facilitates the near-ballistic transmission of uni-traveling carriers. The obtained results may find potential applications in future high-speed optical communication chips and high-performance terahertz sources.