Kejia Li

High-Power and High-Linearity Photodetector Modules for Microwave Photonic Applications

We demonstrate hermetically packaged InGaAs/InP photodetector modules for high performance microwave photonic applications. The devices employ an advanced photodiode epitaxial layer known as the modified uni-traveling carrier photodiode (MUTC-PD) with superior performance in terms of output power and saturation. To further improve the thermal limitations, the MUTC-PDs were flip-chip bonded on high thermal conductivity substrates such as Aluminum Nitride (AlN) and Diamond. Modules using chips with active area diameters of 40, 28, and 20 μm were developed. The modules demonstrated a 3-dB bandwidth ranging from 17 GHz up to 30 GHz. In continuous wave mode of operation, very high RF output power was achieved with 25 dBm at 10 GHz, 22 dBm at 20 GHz, and 17 dBm at 30 GHz. In addition, the linearity of the modules was characterized by using the third order intercept point (OIP3) as a figure of merit. Very high values of OIP3 were obtained with 30 dBm at 10 GHz, 25 dBm at 20 GHz and more than 20 dBm at 30 GHz. Under short pulse illumination conditions and by selectively filtering the 10 GHz frequency component only, a saturated power of >21 dBm was also measured. A very low AM-to-PM conversion coefficient was measured, making the modules highly suitable for integration in photonic systems for ultralow phase noise RF signal generation.

Recombination lifetime of free polarons in polymer/fullerene bulk heterojunction solar cells

The recombination lifetime of free polarons was measured using three different methods: electrical field–dependent photoresponse, transient photoconductivity, and forward-to-zero bias transient-current response. The average free polaron recombination lifetime is estimated to be a few microseconds for poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells. The competition between sweep-out by the internal field and the loss of photogenerated carriers by recombination is analyzed. The short-circuit free polaron collection efficiency for P3HT:PCBM bulk heterojunction material was determined to be in the range of 80% to 90%.

High-Power Flip-Chip Bonded Photodiode With 110 GHz Bandwidth

Back-illuminated flip-chip-bonded charge-compensated modified uni-traveling-carrier photodiodes (PDs) with bandwidths in excess of 110 GHz are demonstrated. PDs with 10- and 6-μm-diameters deliver RF output power levels as high as 9.6 dBm at 100 GHz and 7.8 dBm at 110 GHz, respectively. An analytical model based on parameter extraction from S-parameter fitting was used to assess the bandwidth limiting factors.

Photonic Generation of High-Power Pulsed Microwave Signals

Photonic generation of high-power pulsed microwave signals is demonstrated using charge-compensated modified unitraveling-carrier photodiodes. The impulse response without RF modulation achieved unsaturated peak voltage of 33.5 V and full-width at half-maximum of 50 ps. The peak power levels for gated modulation at 1 and 10 GHz were 41.5 dBm (14.2 W) and 40 dBm (10 W), respectively.

High-Power Photodiode Integrated With Coplanar Patch Antenna for 60-GHz Applications

A high-power V-band photodiode integrated with a coplanar patch antenna is demonstrated. Modified unitraveling carrier photodiodes with broad 3-dB bandwidth (60 GHz) and high-output power up to 16.7 dBm at 50 GHz were integrated with antennas. The radiation pattern characteristics of the coplanar patch antenna were investigated at V-band. The experimental results agree well with simulations. The effective radiated power of the photodiode-integrated antenna is ~20 dBm at 60 GHz.

SiC avalanche photodiode array with microlenses

A simple technique to incorporate microlenes with small photodiode arrays is demonstrated and analyzed. Using this method, the fill factor was increased from 2.6% to 22.4% for a two by two array. Simulation results are also shown. The photocurrent with microlens was approximately 8.6 times larger than without the microlens, which is consistent with simulation results.

High-Gain, Low-Noise-Figure, and High-Linearity Analog Photonic Link Based on a High-Performance Photodetector

We report a high-gain, low-noise-figure, and high-linearity analog photonic link. The link combines a high-power and low-noise fiber laser, a Mach-Zehnder modulator, a high-gain erbium-doped fiber amplifier (EDFA), and a high-performance charge-compensated modified unitraveling-carrier photodiode. The critical characteristics of the photodiode include 16-GHz bandwidth, 200-mA saturation current, and >35 dBm third-order intercept point. In the link, low biasing is utilized to reduce the noise and maximize the signal-to-noise ratio, and an EDFA is used to provide sufficient optical input to the MUTC photodiode to ensure that the link can achieve high gain and high output RF power. The combination of high-power low-noise fiber laser and high-bandwidth high-power photodiode, we have enabled high gain (27.8 dB at 6 GHz and 24.5 dB at 12 GHz), low noise figure (4.7 dB at 6 GHz and 6.9 dB at 12 GHz), and high spur free dynamic range (>120 dB · Hz2/3) in the frequency range 6-12 GHz.

Determination of free polaron lifetime in organic bulk heterojunction solar cells by transient time analysis

A transient response technique that is widely used to measure the minority carrier lifetime in inorganic semiconductors is proposed to measure the lifetime of free polarons in a polymer:fullerene bulk heterojunction (BHJ) solar cell. A numerical model that can be used to describe the transient behavior of BHJ devices has been developed. Using the proposed method, the lifetime of free polarons in poly (3-hexylthiophene) and [6, 6]-phenyl C61-butyric acid methyl ester blend film is estimated to be in the range of 300–400 ns.

High-power, high-linearity photodiodes

This talk will describe modified uni-traveling carrier photodiodes (MUTCs) that have achieved high RF output power and high saturation current. Discrete photodiodes, 4×1 phase matched arrays, and balanced detectors will be discussed.

High Power 20-GHz Photodiodes With Resonant Microwave Circuits

We report the design, fabrication, and characterization of resonant microwave circuits integrated with InGaAs/InP modified unitravelling-carrier photodiodes. The photodiodes were flip-chip bonded on AlN substrates with coplanar waveguide circuits. The RF output power levels of 23 and 21.6 dBm at 20 GHz are demonstrated for microwave open stubs and shorted stubs, respectively.