High Power Photodiodes and their Application in Analog Photonic Links

Analog photonic links (APLs) are promising alternatives to all-electrical coaxial cable systems as they can provide benefits in loss, bandwidth, immunity to electromagnetic interference (EMI), and reduced size and weight. With these advantages over all-electrical coaxial cable links, APLs have been widely investigated in antenna remoting, radio-over-fiber, and phased-array radar systems. Intensity modulation with direct detection (IM/DD) and phase modulation with interferometric demodulation are two candidates for APLs. Both require a high-power high- linearity photodiode to enhance the link performance.
My research focuses on high-power photodiodes and their application in APLs. In my work, I have designed, fabricated, and measured high-power high-linearity modified uni- traveling carrier (MUTC) PDs at wavelengths of 1550 nm and 1060 nm. The dark currents of these devices are typically below 100 nA at -5 V bias. With an optimized anti-reflection coating, the responsivity is as high as 0.65 A/W and 0.62 A/W at 1550 nm and 1060 nm, respectively. A 3-dB bandwidth up to 41 GHz was measured on a 10- μm diameter single PD and 150 mA saturation current was measured on a 28- μm diameter PD. Balanced MUTC photodiodes with old coplanar waveguide (CPW) design had a common mode rejection ration (CMRR) of 20 dB within their bandwidth while a CMRR of 30 dB was measured with new CPW design. A record high 50 dBm third order output interception point (OIP3) was also measured under -6 V bias voltage on our 24- μm diameter PD.
These high-power balanced MUTC photodiodes allowed me to demonstrate an IM/DD APL at 20 GHz with a record-high gain and low noise figure. To the best of my knowledge, this is the first APL with a high-power photodiode that has been demonstrated at a frequency as high as 20 GHz. In my work, I derived an expression for the link gain in an APL with a dual output modulator biased at quadrature point. For this link I measured a link gain of 16 dB and 117.6 dB/Hz2/3 third order spurious free dynamic range (SFDR3) at 20 GHz in the experiment; in good agreement with the calculations.
Furthermore, the performance of a phase modulated APL with a delay-line Mach- Zehnder interferometer (MZI) under different bias conditions and a high-power high-linearity MUTC photodiode was investigated. I derived an expression for the link gain under different bias points of the MZI and compared to the experimental data. Noise and SFDR3 in the phase modulated analog photonic link were analyzed, too. In the experiment, 25 dB RF gain, 18 dB NF and 114 dB/Hz2/3 SFDR3 were obtained at 10 GHz under 130 mA photocurrent with an optimally biased MZI and a 28- μm diameter single photodiode. 16 dB RF gain, 16 dB NF and 118 dB/Hz2/3 SFDR3 were measured at 10 GHz under 100 mA total DC photocurrent with a quadrature biased MZI and a balanced 24- μm diameter photodiode. The measured link gain agrees well with the calculation. For the first time, a positive gain was achieved for this type of APL at modulation frequencies of up to 10 GHz.
I have also developed a 9 GHz balanced photoreceiver by co-packaging an InP-based MUTC balanced 15- μm diameter photodiode pair with a transimpedance amplifier (TIA) built in a 130 nm RF CMOS. 21 V/W optical conversion gain at 1060 nm wavelength, 86 pW/Hz noise equivalent power (NEP), and a CMRR of 20 dB were measured. A signal to noise ratio (SNR) of 15 dB was measured when detecting the beat note of 150 μW and 50 pW optical signals. With the second generation TIA designed by Prof. Steven Bowers’ group, we obtained 30 dB CMRR within 9 GHz and a 2162 V/W peak conversion gain was measured at 3 GHz.
To further improve the APL performance, I designed a balanced traveling wave MUTC photodiode in this work. The traveling wave photodiode has the potential to improve the power handling and, when in balanced configuration, cancel the common mode noise in the APL. I designed and fabricated two kinds of traveling wave devices in this work, one with two pairs of balanced photodiodes and one with four pairs of balanced photodiodes. Preliminary data on those devices has been measured, including dark current of 100 μA at -5 V bias voltage and responsivity of 0.62 A/W at 1060 nm and 0.48 A/W at 1550 nm in the fact that AR coating optimized at 1060 nm.