Photodiodes for Applications in Optic Links and Microwave Photonics

In recent years, analog photonic links (APLs) have emerged as promising alternatives to all-electrical coaxial cable systems as they can provide low loss, broad bandwidth, immunity to electromagnetic interference (EMI), and reduced size and weight. With these advantages, APLs have been widely investigated in broadband wireless access networks, radio-over-fiber, and phased-array radar systems. The microwave range is the primary operating frequency for these systems due to its broad bandwidth and high data transmission rate. Photonic generation of continuous wave (CW) and pulsed microwave signals can provide widely tunable carrier frequency range with low phase noise and reduced system cost by eliminating the expensive electronic components. In these systems, the photodiode must be able to deliver very high photocurrent and, thus high radio frequency (RF) output power to improve link gain and signal-to-noise ratio, while maintaining high linearity. However, at present, the bottleneck in increasing the dynamic range and signal-to-noise ratio in these optical links is the power handling capabilities of the photodiodes.
Based on that, the primary focus of this work is to address some of these issues by demonstrating high-power photodiodes and their performance in microwave analog photonic links. High-power V-band charge-compensated modified uni-travelling-carrier (CC-MUTC) photodiodes at 1550 nm were developed and characterized. The devices were flip-chip bonded to diamond with high-thermal-conductivity for improved heat dissipation. The output power of 18 μm- and 24 μm- diameter devices was 22.3 dBm and 24.3 dBm at 50 GHz and 41 GHz, respectively. Compared to previous results with active cooling, the output RF power improved by more than 57% and the failure power density increased by more than 36%.
I also demonstrated photonic microwave generation of high-power pulsed signals in the X-, Ku- and K-band using the CC- MUTC photodiodes. The impulse photoresponse without modulation showed a maximum peak voltage of 38.3 V and full-width at half-maximum (FWHM) of 30 ps. High-power pulsed microwave signals at 10, 17 and 22 GHz with peak power up to 44.2 dBm (26.3 W), 41.6 dBm (14.5 W) and 40.6 dBm (11.5 W) were achieved, respectively. The record high peak power is desirable in radar and wireless applications.
In addition to work at 1550 nm, I have also designed, fabricated, and characterized high-power and high-linearity photodiodes at 1064 nm. Two groups of devices were designed and investigated. For the 1st design, the photodiodes with 15 µm and 18 µm diameters achieved 18.6 dBm RF output power at 55 GHz and 19.4 dBm RF output power at 41 GHz, respectively. The dark currents of these devices were typically below 10 nA at -8 V bias. The bandwidth was analyzed with parameters obtained from S-parameter fitting. There was good agreement between measured and calculated bandwidth. Based on the analysis, the redesigned 2nd group showed 48% higher bandwidth efficiency product (BEP). The RF output powers of photodiodes with diameters of 10 µm and 20 µm were 15 dBm at 60 GHz and 21.7 dBm at 39 GHz, respectively. The measured third-order intercept point (OIP3) showed a high linearity of 33 dBm at 40 GHz. A circuit analysis based on Z-parameter extraction indicates that voltage-dependent and photocurrent-dependent capacitance components are the primary nonlinear mechanisms for these photodiodes at high frequency.
Finally, using the same epitaxial wafer, I designed balanced photodiodes that delivered powers of 17.7 dBm, 19.8 dBm, 20.7 dBm and 22 dBm at 38 GHz, 29 GHz, 27 GHz and 24.5 GHz, respectively. High common-mode rejection ratio (CMRR) of 25 dB and good linearity with an OIP3 of 34 dBm were measured at 38 GHz for 10 µm-diameter balanced photodiodes. I demonstrated two analog photonic links with different noise reduction techniques without electronic amplification. In the first link, I used balanced detection with a quadrature-biased Mach–Zehnder modulator (MZM) for noise cancellation. Link gain of 6.8 dB, noise figure (NF) of 25.8 dB and spurious free dynamic range (SFDR3) of 114 dB·Hz2/3 at 38 GHz were achieved. The second link worked at low bias modulation for improved signal-to-noise ratio with a single photodetector. Record-high gain (19.3 dB at 26 GHz and 17 dB at 38 GHz) and low noise figure (14.5 dB at 26 GHz and 17 dB at 38 GHz) were demonstrated.