Uni-traveling Carrier Photodiode Research Articles

In brief

PAGE 560 Turkish researchers propose a novel mixture model for visible light communication systems that use multiple sources. The model provides sufficient flexibility to accurately model the received signal levels, enabling better system designs. The validity of the model has been empirically verified using real-life measurements. PAGE 542 Researchers from Nanjing, China, have developed an initial rotor alignment method for the position sensorless start-up of doubly salient electromagnetic machines. The method, “align and go”, uses electromagnetic pulses to align the rotors, looking for the most energetically favourable starting conformation. The findings are validated by experiment, showing the flexibility and effectiveness of the proposed method. SolidWorks plot of the measurement setup. Each PD shows the position respectively P1, P2 and P3. PAGE 531 A collaboration of researchers from China and Australia propose an independent path-based process recommendation algorithm to speed up biomedical process recommendation, whilst guaranteeing accuracy. The experimental results show the improvement of the proposed algorithm over current state-of-the-art. The full experiment result of “align and go” with the initial alignment pulse A+B+C−. PAGE 562 Researchers from Japan propose a novel THz-wave autocorrelator which is based on an optical delay line (ODL) and unitravelling-carrier photodiodes (UTC-PDs) for easy observation of optical waveforms. The waveform is delayed in the optical domain whilst it is autocorrelated in the THz domain. Demonstrations show the system working with sinusoidal optical waveforms with frequencies up to 300 GHz. Process models presented as a Petri net. PAGE 527 Researchers from China present a novel, 3-D printed waveguide step-twist integrated with a bandpass filter. The device has a centre frequency of 32 GHz and a bandwidth of 1 GHz. Measurements show good agreement with simulation, with insertion loss of 0.84 dB and a return loss better than 15 dB across the passband. Experimental configuration. Structural illustrations of the Ka-band step-twist.

Heterogeneous photodiodes on silicon nitride waveguides.

Heterogeneous integration through low-temperature die bonding is a promising technique to enable high-performance III-V photodetectors on the silicon nitride (Si3N4) photonic platform. Here we demonstrate InGaAs/InP modified uni-traveling carrier photodiodes on Si3N4 waveguides with 20 nA dark current, 20 GHz bandwidth, and record-high external (internal) responsivities of 0.8 A/W (0.94 A/W) and 0.33 A/W (0.83 A/W) at 1550 nm and 1064 nm, respectively. Open eye diagrams at 40 Gbit/s are demonstrated. Balanced photodiodes of this type reach 10 GHz bandwidth with over 40 dB common mode rejection ratio.

High-Speed and Cost-Effective Reflective Terahertz Imaging System Using a Novel 2D Beam Scanner

Nondestructive THz imaging technology is a promising technology in several industrial applications such as process monitoring and quality control. To further popularize the application of this technique in the industry, we developed photonics-based devices such as a dual-mode laser, uni-travelling carrier photodiode, and Schottky-barrier diode that enable the realization of a compact, low-cost, multifunctional continuous wave (CW) THz system. To develop CW THz reflective imaging systems with these devices, a telecentric f-θ lens and novel two-dimensional (2D) beam scanner were developed. A high-speed, low-cost 2D reflector with a 4-sided polygon mirror, a brushless DC (BLDC) motor rotating at approximately 2,990 revolutions per minute (RPM) and a stepping motor was designed to ensure that a THz image sized 500 × 400 pixels could be scanned in approximately 2 s. However, it is difficult to maintain a constant speed in high-speed rotating BLDC motors. The measured maximum deviation is approximately 0.7%, which results in distorted images. To compensate for this distortion, a sync signal reflector was designed to monitor the real-time RPM of the BLDC motor. Using the novel 2D beam scanner and sync signal reflector, the distorted images could be corrected simply, enabling the development of a 2D THz reflective imaging system using a single emitter and detector; this system could exhibit a higher speed and was more cost effective than the existing system. Although further improvements to photonics-based THz technologies are necessary, we believe that these efforts will begin an era of THz technologies as a widely used industrial technique.

Uni-traveling-carrier photodiodes

The uni-traveling-carrier photodiode (UTC-PD) is a kind of pin junction photodiode that selectively uses electrons as active carriers. The diode structure has a relatively thin p-type absorber where electrons are generated as minority carriers, and then they diffuse and/or field-accelerate toward the collector. Since the electrons travel in the depleted collector at a ballistically high velocity, the photoresponse performance of a UTC-PD is superior to that of a conventional pin-PD. In this tutorial, the basics of the current response in a UTC-PD, the electron transport in the p-type absorber, and the performance of a terahertz-wave UTC photomixer, as a representative, are described.

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.

Minority-electron transport through atomic-diffusion-bonded InGaAs/a-Ge/InGaAs structure studied by photodiode characterization

We experimentally investigated minority-electron transport through a wafer-bonded InGaAs/a-Ge/InGaAs structure fabricated by using atomic-diffusion-bonding technology. The transport properties of minority electrons were examined in a uni-traveling-carrier photodiode (UTC-PD) structure. The C–V characteristics and the DC and O/E responses were compared with those of a conventional UTC-PD, which revealed that the bonded region behaved as an n-type semiconductor and the electric field was concentrated in the vicinity of the bonding interface when a bias voltage was applied to the PD. From the O/E response, it was found that the minority-electron transport through bonded region is similar to that in a conventional PD without wafer bonding under a relatively high bias condition.

High performance waveguide uni-travelling carrier photodiode grown by solid source molecular beam epitaxy.

The first waveguide coupled phosphide-based UTC photodiodes grown by Solid Source Molecular Beam Epitaxy (SSMBE) are reported in this paper. Metal Organic Vapour Phase Epitaxy (MOVPE) and Gas Source MBE (GSMBE) have long been the predominant growth techniques for the production of high quality InGaAsP materials. The use of SSMBE overcomes the major issue associated with the unintentional diffusion of zinc in MOVPE and gives the benefit of the superior control provided by MBE growth techniques without the costs and the risks of handling toxic gases of GSMBE. The UTC epitaxial structure contains a 300 nm n-InP collection layer and a 300 nm n++-InGaAsP waveguide layer. UTC-PDs integrated with Coplanar Waveguides (CPW) exhibit 3 dB bandwidth greater than 65 GHz and output RF power of 1.1 dBm at 100 GHz. We also demonstrate accurate prediction of the absolute level of power radiated by our antenna integrated UTCs, between 200 GHz and 260 GHz, using 3d full-wave modelling and taking the UTC-to-antenna impedance match into account. Further, we present the first optical 3d full-wave modelling of waveguide UTCs, which provides a detailed insight into the coupling between a lensed optical fibre and the UTC chip.

Design guidelines for edge‐coupled waveguide unitravelling carrier photodiodes with improved bandwidth

This study presents experimental and simulation results for edge-coupled waveguide unitravelling-carrier (UTC) photodiodes based on an InGaAs/InP heterostructure. Experimental results are used to calibrate the numerical device simulator. The authors study how different aspects of the UTC photodiode epistructure and contacts impact on the overall device bandwidth, calculating the photoresponse for different structural parameters and doping concentration profiles. The effect of these parameters on the 3-dB cut-off frequency is studied, and design guidelines for UTC photodiodes with improved performance are presented. The UTC photodiode simulated using authors’ design guidelines has a 3 dB cut-off frequency of 49 GHz, a factor of 2 larger than the 25 GHz cut-off of the fabricated UTC photodiode.

P.139 Dopaminergic modulation of reinforcement learning in stimulant drug addiction

We present a cost-effective photonics-based THz wireless delivery system using a directly modulated distributed feedback laser diode (DFB-LD). A DFB-LD with non-return to zero (NRZ) modulation up to 25 Gbps and a uni-traveling-carrier photodiode (UTC-PD) with a horn antenna is employed to transmit a THz wave into free space in the photonics-based THz signal transmitter. A Schottky barrier diode (SBD) with a horn antenna is used to directly detect the THz wave from free space at the THz signal receiver. To achieve the best bit error rate performance, we varied the optimal THz carrier frequency in a photonics-based THz wireless delivery system. After THz carrier frequency optimization, bit error rates were measured by varying THz link operating conditions, such as the data rate, photocurrent of UTC-PD, and wireless transmission distance. We successfully transmitted a 25 Gb/s NRZ signal over 1.6 and 2.2 m wireless transmission distances before and after managing the adiabatic chirp characteristic of directly modulated DFB-LD using the laser-to-filter detuning effect, respectively, while satisfying the 7% hard decision-forward error correction (HD-FEC) threshold (3.4×10−3).

Terahertz wave generation using a soliton microcomb.

The Terahertz or millimeter wave frequency band (300 GHz - 3 THz) is spectrally located between microwaves and infrared light and has attracted significant interest for applications in broadband wireless communications, space-borne radiometers for Earth remote sensing, astrophysics, and imaging. In particular optically generated THz waves are of high interest for low-noise signal generation. Here, we propose and demonstrate stabilized terahertz wave generation using a microresonator-based frequency comb (microcomb). A unitravelling-carrier photodiode (UTC-PD) converts low-noise optical soliton pulses from the microcomb to a terahertz wave at the soliton's repetition rate (331 GHz). With a free-running microcomb, the Allan deviation of the Terahertz signal is 4.5×10-9 at 1 s measurement time with a phase noise of -72 dBc/Hz (-118 dBc/Hz) at 10 kHz (10 MHz) offset frequency. By locking the repetition rate to an in-house hydrogen maser, in-loop fractional frequency stabilities of 9.6×10-15 and 1.9×10-17 are obtained at averaging times of 1 s and 2000 s respectively, indicating that the stability of the generated THz wave is limited by the maser reference signal. Moreover, the terahertz signal is successfully used to perform a proof-of-principle demonstration of terahertz imaging of peanuts. Combining the monolithically integrated UTC-PD with an on-chip microcomb, the demonstrated technique could provide a route towards highly stable continuous terahertz wave generation in chip-scale packages for out-of-the-lab applications. In particular, such systems would be useful as compact tools for high-capacity wireless communication, spectroscopy, imaging, remote sensing, and astrophysical applications.

Tunable THz Signal Generation and Radio-Over-Fiber Link Based on an Optoelectronic Oscillator-Driven Optical Frequency Comb

We propose and demonstrate experimentally a photonic THz signal generation technique based on a tunable optoelectronic oscillator (OEO), and its application in a radio over fiber (RoF) link. The OEO's frequency is tuned by varying the bandwidth of a tunable optical bandpass filter (TOBF) that is cascaded with a phase modulator (PM). The resulting tunable microwave photonic filter is used to generate OEO oscillations from 6.58 GHz up to 18.36 GHz (with a phase noise of ≤−103 dBc/Hz at 10 kHz offset from the carrier frequency). The OEO is subsequently used to drive an optical comb, generating 22 comb lines with a frequency spacing of 17.33 GHz covering a bandwidth of 360 GHz within a 20 dB envelope. By selecting two optical comb lines with a wavelength selective switch (WSS) and beating them in a uni-traveling carrier photodiode (UTC-PD), THz signals are generated at 101.5 GHz and 242.6 GHz with phase noise of −90 dBc/Hz and −78 dBc/Hz, respectively at 10 kHz offset from carrier frequency. Tunable mm-wave and THz signals can be generated either by changing the OEO oscillation frequency or the selected comb lines. Using the OEO driven OFCG, we implemented a RoF link at 242.6 GHz with a data rate of 24 Gbps over a wireless distance of 30 cm and with a bit error rate (BER) below the hard decision forward error correction (FEC) limit of 3.8 × 10−3. This method allows the creation of an all-photonic frequency reconfigurable THz signal generator and RoF system.

Microwave Photonic Link With Improved SFDR Using Two Parallel MZMs and a Polarization Beam Combiner

This article presents a linearized microwave photonic link by means of two parallel Mach-Zehnder modulators (MZMs) and a polarization beam combiner (PBC). The proposed link exhibits significant suppression in third-order nonlinearity, which results in fifth order limited performance. Experiments are conducted and measured results demonstrate 17-dB improvement in spurious-free dynamic range (SFDR), resulting in a fifth order limited SFDR of 124 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4/5</sup> . An Erbium Doped Fiber Amplifier (EDFA) and a high-power Modified Uni-Traveling Carrier (MUTC) photodiode are used to realize a link gain of 6 dB at 60 mA photocurrent for operation at 1 GHz. LTE signals with various instantaneous bandwidths centered at 1 GHz are delivered by the linearized link where spectral regrowth is dramatically suppressed.

Linear transfer function estimation using the photodiode impulse response.

In this Letter, we implement a model-based auto-regressive estimator of the frequency response for a high-speed photodiode (PD). A transfer function is developed, and the associated coefficients representing the PD model and frequency response are solved for using a two-stage least-squares approach. The model is implemented for a modified uni-traveling carrier PD and experimentally compared to measured impulse response data.

High-Speed Characteristics of Uni-Traveling-Carrier Photodiode Under Bias-Free Operation

In this letter, we have conducted experiments on a traditional uni-traveling carrier photodiode (UTC-PD) and a previously modified UTC-PD under the zero-bias operational condition and make a comparison between them. The experimental results demonstrate that the modified UTC-PD is benefit to high speed, and the 3-dB bandwidth can achieve 40 GHz even when the active diameter of ${14}~{\mu }\text{m}$ under the zero-bias operation. Besides, based on the modified UTC-PD structure, we make some improvements in the collector to restrain the trend in which the bandwidth attenuated by the increase of photocurrent under the zero bias operation.

High-Speed and High-Power MUTC Photodiode Working at 1064 nm

We demonstrate low-dark-current, high-speed and high-power back-illuminated flip-chip-bonded modified uni-traveling carrier photodiodes operating at 1064 nm. The photodiodes with $15~\mu \text{m}$ and $18~\mu \text{m}$ diameters achieve 18.6 dBm RF output power at 55 GHz and 19.4 dBm RF output power at 41 GHz, respectively. The bandwidth is analyzed with parameters obtained from S-parameter fitting. There is good agreement between measured and calculated bandwidths.

Optimized uni-traveling carrier photodiode and mushroom-mesa structure for high-power and sub-terahertz bandwidth under zero- and low-bias operation

In this paper, physically-based simulations are carried out to investigate and design uni-traveling carrier photodiode (UTC-PD) for high-power sub-terahertz wave generation at zero- and low-bias operation. The reliability of the physically-based simulation is demonstrated by comparing with our experimental result. Both the bandwidth and RF output power of the proposed UTC-PD is significantly improved by careful design the built-in electric field distribution under high-power input. For the optimized UTC-PD with the mesa diameter of 5 μm, its 3dB bandwidth large than 100 GHz even if the photocurrent reaches 6 mA under zero-bias operation. The device can reach a high bandwidth of 92.4 GHz, 105 GHz, and 119.5 GHz under the reverse bias of 0.5 V, 1 V, and 2 V, respectively, even the input photocurrent as high as 18.2 mA. The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero- or low-bias operation. Besides, a novel design of mushroom-mesa UTC-PD (MM-UTC-PD) is proposed which with 4.3% improved high-speed performance. The MM-UTC-PD can trade-off between the external quantum-efficiency and bandwidth when miniaturized junction size is required.

Terahertz for high‐speed communications

terahertz for high‐speed communications

High-speed uni-traveling carrier photodiode for 2 μm wavelength application

Current optical communication systems operating at the 1.55 μm wavelength band may not be able to continually satisfy the growing demand on data capacity within the next few years. Opening a new spectral window around the 2 μm wavelength with recently developed hollow-core photonic bandgap fiber and a thulium-doped fiber amplifier is a promising solution to increase transmission capacity due to the low-loss and wide-bandwidth properties of these components at this wavelength band. However, as a key component, the performance of current high-speed photodetectors at the 2 μm wavelength is still not comparable with those at the 1.55 μm wavelength band, which chokes the feasibility of the new spectral window. In this work, we demonstrate, for the first time to our knowledge, a high-speed uni-traveling carrier photodiode for 2 μm applications with InGaAs/GaAsSb type-II multiple quantum wells as the absorption region, which is lattice-matched to InP. The devices have the responsivity of 0.07 A/W at 2 μm wavelength, and the device with a 10 μm diameter shows a 3 dB bandwidth of 25 GHz at −3 V bias voltage. To the best of our knowledge, this device is the fastest photodiode among all group III-V and group IV photodetectors working in the 2 μm wavelength range.

Enhancement in speed and responsivity of uni-traveling carrier photodiodes with GaAs0.5Sb0.5/In0.53Ga0.47As type-II hybrid absorbers.

We demonstrate a top-illuminated high-speed uni-traveling carrier photodiode (UTC-PD) with a novel design in the p-type absorber, which can effectively shorten the photon absorption depth at telecommunication wavelengths (1.31~1.55 μm) and further enhance the bandwidth-efficiency product of UTC-PD. In our proposed new UTC-PD structure, the p-type In0.53Ga0.47As absorption layer is replaced by the type-II GaAs0.5Sb0.5 (p)/In0.53Ga0.47As (i) hybrid absorber. Due to the narrowing of the bandgap and enhancement of the photo-absorption process at the type-II interface between the GaAs0.5Sb0.5 and In0.53Ga0.47As layers, our device shows an over 16.7% improvement in the responsivity compared with that of UTC-PD with the same thickness of pure In0.53Ga0.47As absorber (0.7 μm) and a zero optical coupling loss. Our demonstrated device with a simple top-illuminated structure offers a large active mesa (25 μm), a wide optical-to-electrical (O-E) bandwidth (33 GHz), a high responsivity (0.7 A/W), and a high saturation current (>5 mA) under 1.31 µm optical wavelength. These promising results suggest that our proposed PD structure can fundamentally overcome the trade-off among bandwidth, efficiency, and device active diameter of high-speed PDs.

300 GHz Optoelectronic Transmitter Combining Integrated Photonics and Electronic Multipliers for Wireless Communication

THz communications systems at carrier frequencies above 200 GHz are the key to enable next-generation mobile communication networks with 100 Gbit/s wireless data rates. One of the key questions is, which carrier frequency generation technique will be the most suitable. This is currently addressed by two separate approaches, electronics-based and photonics-based. We present in this paper a truly microwave photonic approach that benefits from the main key features of each, bandwidth, tunability, stability and fiber compatibility from photonics and power handling capability from the electronics. It is based on a Photonic Local Oscillator (PLO), generating a 100 GHz frequency, fed into an electronic frequency multiplier. A high speed uni-travelling carrier photodiode (UTC-PD) provides the 100 GHz PLO for Schottky tripler diodes, generating 300 GHz signal. To feed the UTC-PD, we present a photonic integrated mode locked laser source. According to the simulations and measurements, the developed transmitter can produce a maximum of 12 μW of THz power at 280 GHz.