Uni-traveling Carrier Research Articles

Characterization of uni‐traveling carrier photodiodes for high‐linearity and high‐SNR applications

AbstractA typical uni‐traveling carrier photodiode (UTC‐PD) and a commercial PIN‐PD are compared in the context of the analog performance at 10 GHz in a realistic noise environment.The behavior of the carrier‐to‐noise ratio (CNR), the linearity, represented by the third‐order output intercept point (OIP3), and the spurious free dynamic range are studied as a function of photocurrent. We have found that the increment of the CNR is only minor because of excess noise of the amplifier, as an optical amplifier is used to reach high optical power. However, the OIP3 of the UTC‐PD is substantially higher and increases with photocurrent, in contrast to the PIN‐PD. Thus, even though the responsivity is lower, it is beneficial to use a UTC‐PD to have superior dynamic range. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:984–987, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25055

Design and Characterization of Evanescently Coupled Uni-Traveling Carrier Photodiodes with a Multimode Diluted Waveguide Structure

A new evanescently coupled uni-traveling carrier photodiode (EC-UTC-PD) is designed, fabricated and characterized, which incorporates a multimode diluted waveguide structure and UTC active waveguide structure together. A high responsivity of 0.68A/Wat 1.55-μm without an anti-reflection coating, a linear photocurrent responsivity of more than 21 mA, and a large −1 dB vertical alignment tolerance of 2.5 μm are achieved.

Measurement of intermodulation distortion in high-linearity photodiodes

Accurately characterizing third order intermodulation distortion (IMD3) in high-linearity photodiodes is challenging. Two measurement techniques are evaluated-a standard two-tone measurement and a more complicated three-tone measurement technique to measure IMD3. A model of the measurement system is developed and used to analyze the limitations of the two techniques in determining the distortion of highly linear photodiodes. Experimental validation is provided by comparing the simulation trends with IMD3 results measured on two types of waveguide photodiodes: 1) an InP based uni-traveling-carrier (UTC) photodiode and 2) a Ge n-i-p waveguide photodetector on Silicon-on-Insulator (SOI) substrate.

Theoretical analysis and experimental study of the space-charge-screening effect in uni-traveling-carrier photodiode

A new optimized structure of an UTC (uni-traveling-carrier) photodiode is developed and epitaxied by metal-organic chemical vapor deposition. We fabricated a UTC photodiode of 30 μm in diameter. Theoretical simulation based on drift-diffusion model was used to analyze the space-charge-screening effect in UTC photodiode primarily in two aspects: the carrier concentrations and the space electric field. The simulation results were generally in agreement with the experimental data.

Optically power supplied Gbit/s wireless hotspot using 1.55 [micro sign]m THz photomixer and heterodyne detection at 200 GHz

A wireless transmission system with a remote-supplied hotspot is demonstrated working up to gigabit data rates. The emission uses a 200 GHz carrier frequency, generated using an 1.55 µm photomixer composed of an unitravelling carrier photodiode monolithically integrated with a wide bandwidth antenna. The receiver is based on a heterodyne mixing and an AM detector. Eye diagram analysis at 1.0625 Gbit/s shows a BER<10−9 for a 2.6 m free space transmission distance and a 0.32 mA current in the photomixer.

Extremely High Saturation Current-Bandwidth Product Performance of a Near-Ballistic Uni-Traveling-Carrier Photodiode With a Flip-Chip Bonding Structure

In this study, we demonstrate near-ballistic uni-traveling carrier photodiodes (NBUTC-PDs) with an optimized flip-chip bonding structure, wide 3-dB optical-to-electrical (O-E) bandwidth (> 110 GHz), and extremely high saturation current-bandwidth product performance (37 mA, > 110 GHz, > 4070 mAmiddot GHz). NBUTC-PDs with different active areas (28-144 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) are fabricated and flip-chip bonded with coplanar waveguides onto an AlN-based pedestal. The overshoot drift velocity of the electrons in the collector layer of the NBUTC-PD means that both the thicknesses of the collector layer and active areas of our device can be increased to reduce the density of the output photocurrent, compared to that of the traditional UTC-PD. This improves the high power performance without seriously sacrificing the speed performance. According to the measured O-E frequency responses, devices with even a large active area (144 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) can still have a flat O-E frequency response, from near dc to 110 GHz. A three-port equivalent circuit model for accurately extracting the 3-dB bandwidth of the devices is established. The extracted 3-dB O-E bandwidth of a device with a small active area (28 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) can be as high as 280 GHz under a load of 25 Omega . In addition, the saturation current measurement results indicate that after inserting a center bonding pad on the pedestal (located below the p-metal of the NBUTC-PD for good heat sinking), the saturation current performance of the device becomes much higher than that of the control device (without the center bonding pad), especially for the device with a small active area (28 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The measurement and modeling results indicate that a device with a 144 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active area and optimized flip-chip bonding pedestal can achieve an extremely high saturation current-bandwidth product (6660 mA-GHz, 37 mA, 180 GHz).

Telecommunications technology-based terahertz sources

A review is presented of continuous-wave terahertz sources based on the optical heterodyne generation (photomixing) technique, that make use of optical components and fabrication techniques originally developed for the 1550 nm optical fibre telecommunications window. The uni-travelling carrier photodiode is identified as a key component for conversion of optical to terahertz power, and the state of the art is summarised in terms of terahertz power generated at various frequencies. An approach based on phase locking the heterodyned lasers is described, which enables terahertz signals to be generated with extremely high spectral purity and frequency accuracy, and progress in developing the photonic integrated circuits required for its implementation is reported. Finally, possible future developments in the field are discussed.

High-Power Monolithically Integrated Traveling Wave Photodiode Array

A novel traveling wave photodiode array (TWPDA) based on back-illuminated InGaAs-InP charge compensated modified uni-traveling carrier photodiodes is demonstrated. A two-element TWPDA with integrated termination resistor has a responsivity of 0.55 A/W and bandwidth of 17 GHz. The saturation photocurrent and the available electrical output power are 114 mA and + 13 dBm at - 3.5-V bias voltage, respectively.

Measurement and modeling of hiigh-linearity modified uni-traveling carrier photodiode with highly-doped absorber

The third-order intermodulation distortions of InGaAs/InP modified uni-traveling carrier photodiodes with a highly-doped p-type absorber are characterized. The third-order local intercept point is 55 dBm at low frequency (< 3 GHz) and remains as high as 47.5 dBm up to 20 GHz. The frequency characteristics of the OIP3 are well explained by an equivalent circuit model.

A High-Linearity Modified Uni-Traveling Carrier Photodiode With Offset Effects of Nonlinear Capacitance

Third-order intermodulation distortion in an InGaAs/InP charge compensated modified uni-traveling carrier photodiode is characterized using a two-tone setup. Plots of the third-order local intercept point (IP3) versus photocurrent exhibit peaks. A simple analytical model is developed which explains that the peaks are due to the interaction of voltage-dependent and photocurrent-dependent capacitance effects.

A high-efficiency high-power evanescently coupled UTC-photodiode

The effects of the multimode diluted waveguide on quantum efficiency and saturation behavior of the evanescently coupled uni-traveling carrier (UTC) photodiode structures are reported. Two kinds of evanescently coupled uni-traveling carrier photodiodes (EC-UTC-PD) were designed and characterized: one is a conventional EC-UTC-PD structure with a multimode diluted waveguide integrated with a UTC-PD; and the other is a compact EC-UTC-PD structure which fused the multimode diluted waveguide and the UTC-PD structure together. The effect of the absorption behavior of the photodiodes on the efficiency and saturation characteristics of the EC-UTC-PDs is analyzed using 3-D beam propagation method, and the results indicate that both the responsivity and saturation power of the compact EC-UTC-PD structures can be further improved by incorporating an optimized compact multimode diluted waveguide.

Mode-Locked InP-Based Laser Diode With a Monolithic Integrated UTC Absorber for Subpicosecond Pulse Generation

Future optical transmission systems and signal processing circuits will require optical pulse sources capable of producing subpicosecond (sub-ps) pulses with low timing jitter at repetition rates of tens of gigahertz. In this paper, we present the theory, design, and measurements of a novel InP-based hybrid mode-locked laser diode (MLLD) structure with an ultrafast monolithically integrated, reverse-biased, uni-traveling-carrier (UTC) absorber. The necessity of an ultrafast absorber to obtain sub-ps pulses is analyzed and explained with our advanced time-domain rate-equation model. The realized MLLD demonstrated clean sub-ps pulses of 900 fs at 42-GHz repetition rate and the potential in an optimized device to reach values around 600 fs.

High-Power High-Linearity InGaAs/InP Photodiodes

The third-order intermodulation distortions of an InGaAs/InP modified uni-traveling carrier photodiode (MUTC-PD) are characterized using a two-tone setup. At 0.3 GHz modulation frequency and photocurrents above 40 mA the third order local intercept point (IP3) reaches 52 dBm and remains above 35 dBm up to the photodiode's 3 dB bandwidth. A simple equivalent circuit model is used to explain the frequency characteristics of the intermodulation distortions. Based on experimental data, we find that the frequency characteristics of the IP3 can be attributed to the change of both the responsivity and device capacitance induced by the photocurrent and the transient voltage drop at the load and series resistors. Further investigations of the non-linear phenomena suggest that the Franz-Keldysh effect and impact ionization are the primary reasons for voltage-dependent responsivity, while low doping concentration in the p-type InGaAs absorber and space charge screening are responsible for the voltage dependent capacitance and photocurrent dependent capacitance, respectively.

The Frequency Behavior of the Intermodulation Distortions of Modified Uni-Traveling Carrier Photodiodes Based on Modulated Voltage Measurements

The effect of modulated voltage on the nonlinear response of an InGaAs-InP charge-compensated modified uni-traveling carrier photodiode is characterized using a modulated bias measurement setup. Based on experimental results for the dependence of responsivity and photodetector capacitance on bias voltage, we use a simple equivalent circuit model to explain the frequency characteristics of the intermodulation distortions caused by the modulated bias. Good agreement has been achieved between the simulation and measurement.

InP/InGaAs uni-travelling carrier heterojunction phototransistors

A novel heterojunction phototransistor using a uni-travelling carrier photodiode structure in the base and collector layers is proposed and fabricated. A 5×5 µm optical detection area device shows an optical gain cutoff frequency of 52 GHz and an optical gain of 37 dB at 1 GHz.

Linearity of Modified Uni-Traveling Carrier Photodiodes

The third-order intermodulation distortions of an InGaAs/InP charge compensated modified uni-traveling carrier (UTC) photodiode are characterized using a two-tone setup. At 0.3-GHz modulation frequency and photocurrents above 40 mA, the third-order local intercept point (IP3) reaches a record-high of 52 dBm and remains above 35 dBm up to the photodiode's 3-dB bandwidth. A simple equivalent circuit model with a voltage-dependent responsivity and a voltage- and photocurrent-dependent junction capacitance is used to explain the frequency characteristics of the intermodulation distortions.

Characterization of Sub-THz Photonic-Transmitters Based on GaAs–AlGaAs Uni-Traveling-Carrier Photodiodes and Substrate-Removed Broadband Antennas for Impulse-Radio Communication

A novel photonic transmitter for wireless terahertz (THz) impulse-radio (IR) communication is realized by monolithic integration of a GaAs-AlGaAs-based uni-traveling-carrier (UTC) photodiode with a substrate-removed broadband antenna. The device can radiate strong sub-THz pulses (20-mW peak-power) with a narrow pulsewidth (<2 ps) and wide bandwidth (100 ~ 250 GHz). The maximum average power emitted by our device, under the same THz time-domain spectroscopic setup, is around ten times higher than that of the low-temperature-grown GaAs-based photoconductive antenna, while with a much lower dc bias (9 versus 35 V). The bias-dependent peak output powers of our devices suggest their suitability for application as a data modulator/emitter for photonic THz IR communication.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

Measurement and Modeling of a High-Linearity Modified Uni-Traveling Carrier Photodiode

The third-order intermodulation distortions of an InGaAs-InP charge compensated modified uni-traveling carrier photodiode (PD) are characterized using a two-tone setup. At 310-MHz modulation frequency, the third-order local intercept point (IP3) reaches 52 dBm and remains above 35 dBm up to 21GHz. Based on experimental results for the dependence of responsivity and PD capacitance on bias voltage and photocurrent, we use a simple equivalent circuit model to simulate the frequency characteristics of the intermodulation distortions.

Design of novel three port optical gates scheme for the integration of large optical cavity electroabsorption modulators and evanescently-coupled photodiodes

This paper presents a novel scheme to monolithically integrate an evanescently-coupled uni-travelling carrier photodiode with a planar short multimode waveguide structure and a large optical cavity electroabsorption modulator based on a multimode waveguide structure. By simulation, both electroabsorption modulator and photodiode show excellent optical performances. The device can be fabricated with conventional photolithography, reactive ion etching, and chemical wet etching.