Uni-traveling Carrier Research Articles

Frequency dependent harmonic powers in a modified uni-traveling carrier photodetector.

We use a drift-diffusion model to study frequency dependent harmonic powers in a modified uni-traveling carrier (MUTC) phododetector. The model includes external loading, incomplete ionization, the Franz-Keldysh effect, and history-dependent impact ionization. In three-tone measurements, the bias voltage at which a null occurs (bias null) in the second-order intermodulation distortion (IMD2) is different for the sum frequency and difference frequency. We obtained agreement with the experimental results. The bias null that appears in the IMD2 is due to the Franz-Keldysh effect. The bias voltage at which the bias null is located depends on the electric field in the intrinsic region, and the difference in the location of the bias null for the sum frequency and difference frequency is due to the displacement current in the intrinsic region. When the frequency is large, the displacement current is large and has a large effect on the harmonic powers. We also found that the bias null depends on the recombination rate in the p-absorption region because the electric field decreases in the intrinsic region when the recombination rate in the p-region decreases.

Type-II GaAs0.5Sb0.5/InP Uni-Traveling Carrier Photodiodes With Sub-Terahertz Bandwidth and High-Power Performance Under Zero-Bias Operation

We successfully demonstrate ultrafast uni-traveling carrier photodiodes (PD) with sub-terahertz bandwidth (∼170 GHz) and high-power performance under zero bias and at 1.55-μm optical wavelength operation. By using a type-II (GaAs0.5Sb0.5/InP) absorption-collector interface and inserting an n-type (1 × 1018 cm−3) charge layer in the collector, the current blocking (Kirk) effect can be greatly minimized. A stack of undoped Al x In0.52Ga0.48−xAs layers with different Aluminum mole fractions ( x : 0.2 to 0.08) and bandgaps is adopted as the collector layer. This graded-bandgap design can provide a built-in electric field and further shorten the internal collector transit time. The demonstrated PD structure achieves a 3-dB optical-to-electrical bandwidth of 170 GHz and subterahertz output power −11.3 dBm at 170 GHz, a record among all the reported zero-bias PDs.

High-Power Photodiode-Integrated-Connected Array Antenna

We present a novel optical feeding technique to achieve efficient excitation of an ultrawideband-connected array (CA) antenna. The passive fiber optic feed allows for preservation of the theoretical bandwidth and low profile of elementary connected dipole elements. In order to improve effective radiated power, high-power charge compensated modified unitravelling carrier photodiodes are integrated into an antenna array for the first time. Circuit and full-wave simulations, which include all required antenna and feed components, are conducted for the optimization of the array's performance. A 9 × 12 element CA is populated with a 1-D array of four photodiode-integrated active elements to demonstrate the concept. The optically fed array is confirmed experimentally to have a 3-dB bandwidth of approximately 7–17 GHz, in good agreement with simulations.

120 Gb/s Multi-Channel THz Wireless Transmission and THz Receiver Performance Analysis

A photonic multi-channel terahertz (THz) wireless transmission system in the 350-475 GHz band is experimentally demonstrated. The employment of six THz carriers modulated with 10 Gbaud Nyquist quadrature phase-shift keying baseband signal per carrier results in an overall capacity of up to 120 Gb/s. The THz carriers with high-frequency stability and low phase noise are generated based on photonic photomixing of 25-GHz spaced six optical tones and a single optical local oscillator derived from a same optical frequency comb in an ultrabroadband uni-travelling carrier photodiode. The bit-error-rate performance below the hard decision forward error correction threshold of 3.8×10 -3 for all the channels is successfully achieved after wireless delivery. Furthermore, we also investigate the influence of the harmonic spurs in a THz receiver on the performance of transmission system, and the experimental results suggest more than 30 dB spur suppression ratio in downconverted intermediate frequency signals for obtaining less than 1 dB interference.

Butt-joint design in a uni-traveling carrier photodiode array monolithic with an arrayed waveguide grating by the selective area growth technique

Monolithic integration of an InP-based O-band 4-channel arrayed waveguide grating (AWG) to a uni-traveling carrier photodiode (UTC-PD) array is realized by the selective area growth (SAG) technique. The passive-active buttjoint design is introduced and experimentally proved to ensure both good compatibility between the PD fabrication process and the SAG technique, and high photodiode quantum efficiency under the complex butt-joint geometry. An extended coupling layer is adopted between the AWG output waveguides and the PD mesa. The extended coupling layer length, the regrowth boundary edge position and the AWG etching edge position relative to the heterogeneous butt-joint boundary, and the refractive indices of the PD collector and coupling layer are optically simulated and optimized by a finite-difference time-domain method. It is found that the extended coupling layer, compared with the un-extended situation, ensures a good matched optical field from AWG to PD and could reduce nearly 30% quantum efficiency loss when connecting seamlessly to the regrown InP AWG top cladding layer. A stable high efficiency around 80% is maintained within an extended layer length from 7.5 m to 15.0 m. The regrowth boundary edge into the coupling region will cause a drastic efficiency oscillation up to 20% period with the increase of distance. The efficiency drop is also attributed to the light scattering at the regrowth boundary edge, caused by the optical field mismatch, while the oscillation comes from the alternative light power concentration between the coupling layer and the core layer, for the light scattering is only obvious when the light power is well concentrated in the coupling layer. The AWG etching edge position deviation from the butt-joint boundary, however, exerts little influence on the PD quantum efficiency, which is believed not to bring obvious coupling loss during device fabrication. The higher UTC-PD collector refractive index is proved to be crucial for further better optical coupling from the coupling layer to the PD, with quantum efficiency rapidly increasing from around 0.1 to 0.8 when the index is increased from 3.20 to 3.42. By comparison, the efficiency is little affected by the coupling layer refractive index from 3.34 to 3.42.All things considered, we select a 10 m extended coupling layer, the refractive indices of both PD collector and the coupling layer to be 3.42, and align both the regrowth boundary edge and the AWG etching edge to the heterogeneous butt-joint boundary, and a PD quantum efficiency of 80% is expected. Owing to the extended coupling layer at the butt-joint, the SAG technique facilitates the PD fabrication process. The overgrown AWG top cladding layer ridge stretches out 4.67 m toward the PD, but not over the mesa yet, hence has little influence on the PD fabrication accuracy. The monolithic chip presents a uniform photodiode quantum efficiency of 76%, which accords well with theoretical value and confirms the butt-joint design. Central wavelengths for the four channels are 1347.0 nm, 1325.0 nm, 1308.0 nm, and 1286.5 nm, respectively. The low crosstalk level (below -22 dB) also indicates a good de-multiplexer performance.

单载流子传输光敏晶体管小信号等效电路模型的建立与分析

单载流子传输光敏晶体管小信号等效电路模型的建立与分析

4×25 GHz uni-traveling carrier photodiode arrays monolithic with InP-based AWG demultiplexers using the selective area growth technique

We use the selective area growth (SAG) technique to monolithically integrate InP-based 4-channel arrayed waveguide gratings (AWGs) with uni-traveling carrier photodiode arrays at the O-band. Two kinds of channel spacing demultiplexers of 20 nm and 800 GHz are adopted for potential 100 Gbps coarse wavelength division multiplexing and local area network wavelength division multiplexing systems, with an evanescent coupling plan to facilitate the SAG technique into device fabrication. The monolithic chips in both channel spacings exhibit uniform bandwidths over 25 GHz and a photodiode responsivity of 0.81 A/W for each channel, in agreement with the simulated quantum efficiency of 80%. Cross talk levels are below −20 dB for both channel spacing chips.

Characterization of Amplitude Noise to Phase Noise Conversion in Charge-Compensated Modified Unitravelling Carrier Photodiodes

The phase noise, i.e., amplitude modulation to phase modulation (AM-PM) in the photodetector. In general, AM-PM of charge-compensated modified uni-travelling carrier (CC-MUTC) photodiodes originates from the combined nonlinearities of the detector impedance and carrier transit times, which depend on the electric field distribution and the velocity-field characteristics of carrier transport. In this work, we report measurements of AM-PM in transit-time-limited CC-MUTC photodiodes. A Crosslight-based simulation of the photodetection process is used to quantitatively estimate the AM-PM of CC-MUTC photodiodes. Simulated AM-PM of a 15-GHz microwave signal shows good agreement with experimental results. The measured and simulated AM-PM versus photocurrent exhibit a null point that originates from the competition between hole deceleration due to the collapsed electric field in the depleted absorption region and electron acceleration in the self-induced electric field in the undepleted absorption region.

THz photonic wireless links with 16-QAM modulation in the 375-450 GHz band.

We propose and experimentally demonstrate THz photonic wireless communication systems with 16-QAM modulation in the 375-450 GHz band. The overall throughput reaches as high as 80 Gbit/s by exploiting four THz channels with 5 Gbaud 16-QAM baseband modulation per channel. We create a coherent optical frequency comb (OFC) for photonic generation of multiple THz carriers based on photo-mixing in a uni-travelling carrier photodiode (UTC-PD). The OFC configuration also allows us to generate reconfigurable THz carriers with low phase noise. The multiple-channel THz radiation is received by using a Schottky mixer based electrical receiver after 0.5 m free-space wireless propagation. 2-channel (40 Gbit/s) and 4-channel (80 Gbit/s) THz photonic wireless links with 16-QAM modulation are reported in this paper, and the bit error rate (BER) performance for all channels in both cases is below the hard decision forward error correction (HD-FEC) threshold of 3.8e-3 with 7% overhead. In addition, we also successfully demonstrate hybrid photonic wireless transmission of 40 Gbit/s 16-QAM signal at carrier frequencies of 400 GHz and 425 GHz over 30 km standard single mode fiber (SSMF) between the optical baseband signal transmitter and the THz wireless transmitter with negligible induced power penalty.

Integrated Four-Wavelength DFB Diode Laser Array for Continuous-Wave THz Generation

We demonstrate a four-wavelength distributed feedback (DFB) diode laser array integrated with a semiconductor optical amplifier (SOA) for widely tunable terahertz (THz) mode-beating generation. The InP-based monolithically integrated chip consists of DFB laser diodes, an SOA, and a multimode interference (MMI) coupler. Microheaters and electrode plates are integrated on top of each DFB section for continuous and independent wavelength tuning. By heterodyning the output from the DFB lasers on a Uni-Traveling Carrier Photodiode (UTC-PD) integrated antenna, a continuous THz radiation ranging from 0.1 to 2.25 THz was obtained.

High-responsivity vertical-illumination Si/Ge uni-traveling-carrier photodiodes based on silicon-on-insulator substrate.

Si/Ge uni-traveling carrier photodiodes exhibit higher output current when space-charge effect is overcome and the thermal effects is suppressed. High current is beneficial for increasing the dynamic range of various microwave photonic systems and simplifying high-bit-rate digital receivers in many applications. From the point of view of packaging, detectors with vertical-illumination configuration can be easily handled by pick-and-place tools and are a popular choice for making photo-receiver modules. However, vertical-illumination Si/Ge uni-traveling carrier (UTC) devices suffer from inter-constraint between high speed and high responsivity. Here, we report a high responsivity vertical-illumination Si/Ge UTC photodiode based on a silicon-on-insulator substrate. When the transmission of the monolayer anti-reflection coating was maximum, the maximum absorption efficiency of the devices was 1.45 times greater than the silicon substrate owing to constructive interference. The Si/Ge UTC photodiode had a dominant responsivity at 1550 nm of 0.18 A/W, a 50% improvement even with a 25% thinner Ge absorption layer.

Modelling and measurement of the absolute level of power radiated by antenna integrated THz UTC photodiodes

We determine the output impedance of uni-travelling carrier (UTC) photodiodes at frequencies up to 400 GHz by performing, for the first time, 3D full-wave modelling of detailed UTC photodiode structures. In addition, we demonstrate the importance of the UTC impedance evaluation, by using it in the prediction of the absolute power radiated by an antenna integrated UTC, over a broad frequency range and confirming the predictions by experimental measurements up to 185 GHz. This is done by means of 3D full-wave modelling and is only possible since the source (UTC) to antenna impedance match is properly taken into account. We also show that, when the UTC-to-antenna coupling efficiency is modelled using the classical junction-capacitance/series-resistance concept, calculated and measured levels of absolute radiated power are in substantial disagreement, and the maximum radiated power is overestimated by a factor of almost 7 dB. The ability to calculate the absolute emitted power correctly enables the radiated power to be maximised through optimisation of the UTC-to-antenna impedance match.

Terahertz Frequency-Domain Spectroscopy of Low-Pressure Acetonitrile Gas by a Photomixing Terahertz Synthesizer Referenced to Dual Optical Frequency Combs

A terahertz (THz) frequency synthesizer based on photomixing of two near-infrared lasers with a sub-THz to THz frequency offset is a powerful tool for spectroscopy of polar gas molecules due to its broad spectral coverage; however, its frequency accuracy and resolution are relatively low. To tune the output frequency continuously and widely while maintaining its traceability to a frequency standard, we developed a photomixing THz synthesizer phase-locked to dual optical frequency combs (OFCs). While the phase-locking to dual OFCs ensured continuous tuning within a spectral range of 120 GHz, in addition to the traceability to the frequency standard, use of a broadband uni-traveling carrier photodiode for photomixing enabled the generation of CW-THz radiation within a frequency range from 0.2 to 1.5 THz. We demonstrated THz frequency-domain spectroscopy of gas-phase acetonitrile CH3CN and its isotope CH313CN in the frequency range of 0.600–0.720 THz using this THz synthesizer. Their rotational transitions were assigned with a frequency accuracy of 8.42 × 10−8 and a frequency resolution of 520 kHz. Furthermore, the concentration of the CH3CN gas at 20 Pa was determined to be (5.41 ± 0.05) × 1014 molecules/cm3 by curve fitting analysis of the measured absorbance spectrum, and the mixture ratio of the mixed CH3CN/CH313CN gas was determined to be 1:2.26 with a gas concentration of 1014–1015 molecules/cm3. The developed THz synthesizer is highly promising for high-precision THz-FDS of low-pressure molecular gases and will enable the qualitative and quantitative analyses of multiple gases.

High-bandwidth uni-traveling carrier waveguide photodetector on an InP-membrane-on-silicon platform.

A uni-traveling carrier photodetector (UTC-PD), heterogeneously integrated on silicon, is demonstrated. It is fabricated in an InP-based photonic membrane bonded on a silicon wafer, using a novel double-sided processing scheme. A very high 3 dB bandwidth of beyond 67 GHz is obtained, together with a responsivity of 0.7 A/W at 1.55 μm wavelength. In addition, open eye diagrams at 54 Gb/s are observed. These results promise high speed applications using a novel full-functionality photonic platform on silicon.

3.5-Gbit/s QPSK Signal Radio-Over-Fiber Transmission With 60-GHz Integrated Photonic Array-Antenna Beam Forming

This paper studies the performance of 3.5-Gbit/s quadrature phase shift keying (QPSK) signal radio-over-fiber (RoF) transmission with beam forming of array antennas. An “integrated photonic array- antenna (IPA)” is utilized in this experiment, where uni-travelling carrier photodiodes are fully integrated with 60-GHz-band 4 × 2 array-antenna elements into a single board. The beam forming operation for the IPA is experimentally demonstrated by using a QPSK signal assuming real mobile data traffic, and the QPSK signal quality during the beam forming operation for the IPA is quantitatively evaluated. The expandability for more advanced modulation formats, such as 16-quadratic-amplitude modulation, is also studied. In addition, the required system parameters for practical use are shown by investigating the induced transmission penalty. The obtained results enable us to design next generation radio access networks based on 60-GHz RoF transmission technique.

High-power, high-linearity photodiodes

Microwave photonics and optics-based analog links are technologies that are being developed for a growing number of applications. Photodetectors that operate at high power levels are key components. Additionally, it is important for many of these applications that the photodiodes have millimeter-wave bandwidths and highly linear response. This paper reviews the performance of modified uni-traveling carrier photodiodes with respect to these characteristics.

High Current Responsivity and Wide Modulation Bandwidth Terahertz Detector Using High-Electron-Mobility Transistor for Wireless Communication

A high-current-responsivity terahertz (THz) detector was fabricated using a broadband bow-tie antenna and an InAlAs/InGaAs high-electron-mobility transistor (HEMT) with a short gate length. High-current responsivity can be achieved by using a short gate length; the resulting high transconductance exhibited ballistic transport in the channel. We fabricated the HEMT detector with a 50-nm-long channel; the transconductance was 1.2 S/mm and the subthreshold slope was 120 mV/dec, yielding a high-current responsivity (∼5 A/W) and a cutoff frequency of 460 GHz. We also measured the modulation bandwidth of the THz detector using a heterodyne mixing technique with a uni-traveling carrier photodiode (UTC-PD) for providing the radio frequency (RF) and a frequency multiplier as a local oscillator. The intensity of the intermediate signal (IF) was measured by changing the frequency of the UTC-PD; very high bandwidths of up to 26 GHz were obtained. The experimental results agree well with electromagnetic simulations, which indicate that the bandwidth is determined by the external circuit. The conversion gain from RF to IF was −2 dB in the heterodyne mixing by using the HEMT detector.

Accurate equivalent circuit model for millimetre-wave UTC photodiodes.

We present a comprehensive study of uni-travelling carrier photodiode impedance and frequency photo-response supported by measurements up to 110 GHz. The results of this investigation provide valuable new information for the optimisation of the coupling efficiency between UTC-PDs and THz antennas. We show that the measured impedance cannot be explained employing the standard junction-capacitance/series-resistance concept and propose a new model for the observed effects, which exhibits good agreement with the experimental data. The achieved knowledge of the photodiode impedance will allow the absolute level of power emitted by antenna integrated UTCs to be predicted and ultimately maximised.

Photoresponse characteristics from computationally efficient dynamic model of uni-traveling carrier photodiode

A time domain model of bulk InGaAs/InP uni-traveling carrier photodiode is developed in terms of coupled differential equations of incident photon flux and photo generated carrier density rates. For fast computation of model parameters linear approximation of material absorption coefficient is made with carrier density. Wavelength and bias voltage dependent responsivity is well demonstrated by the model and their values at different absorption layer widths agree well with the experimental results. Optical power induced output photocurrent saturation is also explained. Furthermore, from the temporal variation of output photocurrent, estimation of device bandwidth is shown.

Photonic High-Power Continuous Wave THz-Wave Generation by Using Flip-Chip Packaged Uni-Traveling Carrier Photodiodes and a Femtosecond Optical Pulse Generator

The design, analysis, and demonstration of flip-chip bonding packaged uni-traveling-carrier photodiodes (UTC-PDs) with THz (dc to 315 GHz) 3-dB bandwidth and high-power performance are reported. The high-frequency roll-off (up to 0.4 THz) of the flip-chip bonding structure and device heating under high power operation are both minimized through properly downscaling the area of the bonding pad and minimizing the solder distance to the active area of the miniaturized UTC-PD. In order to suppress the serious space-charge screening effect in miniaturized UTC-PDs under high-current density (~180 kA/cm 2 ) operation, an n-type charge layer is inserted into the collector. The detailed dynamic measurement results of these packaged PD modules indicate that non-equilibrium electron transport plays an important role in determining the maximum speed and THz output power. In addition, a femtosecond (fs) optical pulse train generator with a ~300 fs pulse-width output and repetition rate up to ~0.3 THz is also developed to further boost the photo-generated THz-power. Compared with using an optical signal with a sinusoidal envelope for PD excitation, the short-pulse approach can offer a 3-dB enhancement in output power under the same output photocurrent and operating frequency (around 0.3 THz). By utilizing such an fs light source and our PD module, a continuous wave output power as high as 1 mW at an operating frequency of ~0.3 THz is successfully demonstrated.