Packaging High-Power Photodiodes for Microwave Photonic Applications

Recently, microwave photonic links have gained broad academic and industrial interests as more high frequency bands are utilized in the 5G era. Comparing to conventional co-ax cables, high-fidelity transmission of analog signals over long distances using fiber is advantageous in size, weight and power (SWaP), bandwidth, and EMI, etc. As one of the key components of a microwave photonic link, the photodiode needs to handle high optical power and generate large photocurrent which is critical for overall link performance. Although advanced photodiode structures have been recently developed to mitigate the space-charge effect at high photocurrents, thermal failure remains a major challenge for high-power photodiodes. To this end, the photodiode chip is often flip-chip bonded to a high thermal conductivity submount to provide a thermal sink, and thus improve the thermal dissipation efficiency. In this paper, we experimentally study the thermal performance of our 40 GHz modified uni-traveling carrier (MUTC) photodiodes with varied flip-chip submount materials. The design, fabrication, packaging, and testing of the MUTC photodiode will be discussed. >600 mW, 90 mA at −7 V applied bias, saturation power has been achieved using an AlN submount and gold bonding pads; >750 mW, 95 mA at −8 V applied bias, saturation power has been achieved using a SiC (6H) submount. To our best knowledge, these results are the best in terms of uncooled power handling for a 40 GHz photodiode. The packaged photodiode is then used in an intensity modulated direction detect (IMDD) link. >0 dB link gain and <20 dB link noise figure (NF) up to 30 GHz are measured at quadrature bias condition, and ∼20 dB link gain and <10 dB link NF up to 40 GHz are measured at low bias condition. The broadband, low NF link results show the great potential of microwave photonic link technology based on our high-power photodiodes.