Abstract
In recent years, analog photonic links have demonstrated the potential to improve the performance of broadband wireless access networks, cable television signal distribution networks, and remote microwave antenna systems owing to its large instantaneous bandwidth, the low attenuation of optical fibers, continuous spectral coverage, enhanced signal processing capabilities, as well as size, weight, and power consumption benefits. High-performance analog photonic links can also be achieved by using optoelectronic components whose characteristics are optimized for these applications, namely, high-power and low-noise lasers, highly linear modulators that operate at high input power with high conversion efficiency, and high-power, high-frequency and high-linearity photodiodes. For those links that employ a high power and low noise laser and a linearized modulator, power handling capability of the photodiodes sets an upper limit on the link performance.
In this dissertation, photodiodes with high power handling capability are demonstrated. One of the primary limitations on photodiode output power is thermal failure. Self-aligned flip-chip bonding to high-thermal-conductivity submounts has been used to improve heat dissipation. In this work, I have extended previous reports that utilized Si or AlN submounts to diamond heat sinks. These photodiodes have achieved record RF output power of 1.8 W at 10 GHz and saturation current of 300 mA. This is 80% higher than the previous “champion” result [1]. These CC-MUTC photodiodes also achieved record power conversion efficiency of 50.7% to 60% at 6 to 10 GHz with ~ 27.8 dBm RF output power, which compares favorably with previously reported efficiencies of < 40 % for the same frequency range.
I have used these photodiodes to build an analog photonic link with record performance. This link achieved gain of 24.5 dB at 12 GHz, noise figure of 6.9 dB at 12 GHz, and SFDR with >120 dB·Hz2/3 in the frequency range 6 -12 GHz. In addition, optical generation of high-power pulsed microwave signals has been demonstrated. The impulse response without RF modulation at -35 V bias voltage, yielded pulses with 33.5 V peak voltage and full width at half maximum (FWHM) of 50 ps. The peak power levels for gated modulation at 1 GHz and 10 GHz were 41.5 dBm (14.2 W) and 40 dBm (10 W), respectively.
Currently most microwave photonic systems that consist of discrete components suffer from large size, poor reliability and high cost. In order to address these limitations, microwave photonics systems integrated on SOI have been proposed and demonstrated [2, 3]. In this dissertation, a high-performance CC-MUTC photodiode has been integrated on SOI waveguides using a heterogeneous wafer-bonding technology. This is a key enabler for silicon-based on-chip microwave photonics systems. By optimizing the epitaxial layer structure, single CC-MUTC waveguide photodiodes with 210 µm2 area have achieved 48 GHz bandwidth and 10×35 μm2 photodiodes exhibited 0.95 A/W internal responsivity and output >12 dBm RF power at 40 GHz modulation frequency. A similar approach has been used to fabricate balanced detectors with 0.78 A/W internal responsivity, 14 GHz bandwidth, 17.2 dBm RF output power at 10 GHz, and 15.2 dBm RF output power at 20 GHz. These are the highest RF output power levels reported at for any waveguide photodiode technology including native InP, Ge/Si, and heterogeneous integration.
The power handling capability of the CC-MUTC waveguide photodiodes on SOI is limited by thermal failure. In order to improve thermal dissipation and increase the RF power of these photodiodes, a chemical-vapor-deposited (CVD) diamond with high thermal conductivity is used to replace the silicon dioxide in SOI. The dark current of photodiodes integrated with these SOD (silicon on diamond) waveguides is ~10 nA at -5 V bias and the bandwidth can reach up to 25 GHz bandwidth.
