by Shubhashish Datta, Abhay Joshi, Roy Howard, Nilesh Soni, and Matthew D’Angiolillo – Discovery Semiconductors
Fiber-optic interconnects have a natural advantage in speed, electro-magnetic immunity, scalability, and weight compared to all-electronic interconnects, and are being increasingly used in avionics platforms to link multiple processors, radars, sensors, and communication terminals. Utilization of star network topology further promises to improve scalability and flexibility in these platforms . Fiber management and related operations, such as fiber splicing, pose a practical challenge for realizing these goals. Therefore, there is a need for high-speed optical components that are free of fiber pigtails that satisfy the stringent temperature and mechanical requirements of avionics platforms. Previously, we have developed fiber-pigtailed ultrafast InP/InGaAs photodiodes that not only satisfy shock and vibration requirements, but have also passed additional tests needed for space qualification . For this work, we modified the input optical interface and related packaging of these photodiodes and manufactured a pigtail-free DC to 22 GHz photodiode module with a Lucent Connector (LC) fiber receptacle having stable operation over a wide range of temperature, namely -40°C to +120°C.
The top-illuminated InP/InGaAs photodiodes used for this work employ the dual-depletion region (DDR) photodiode structure, which has a proven track record for ultra-fast operation, high linearity, and radiation hardness [2,3]. The photodiode module, shown in Figure 1a, contains a 30 µm diameter photodiode with a 50 Ω internal resistive termination in a compact 3-pin microwave package. A commercially available LC receptacle containing a standard single-mode fiber stub was customized for the optical input interface. The design of the photodiode chip as well as the coupling optics enable this module to operate at wavelengths ranging from 1250 nm to 1650 nm.
The packaged photodiode had a coupled DC responsivity of 0.75 A/W at 1550 nm wavelength and a –3 dB bandwidth of 22 GHz (see Figure 1b), which is similar to the fiber pigtailed counterparts. The photodiode was stimulated with a 1550 nm wavelength CW signal with 100% modulation depth and the RF output power was measured for different input optical power levels. The photodiode module produces linear RF output power up to 0 dBm, as shown in Figure 2a.
The dark current of the photodiode module was recorded at temperatures ranging from -40°C to +120°C, as shown in Figure 2b. The dark current approximately doubles for every 10°C increase in temperature due to bandgap shift of the InGaAs absorption layer of the photodiode chip . There was no evidence of thermally induced device failure for operating temperatures up to +120°C needed for several avionics platforms.
This work builds upon our prior experience in manufacturing fiber-pigtailed space-qualified photodiode modules that are resilient to shock and vibration as well as radiation. We have demonstrated a DC to 22 GHz photodiode module with LC fiber receptacle that can deliver up to 0 dBm of RF power and operate over temperatures ranging from 40°C to +120°C, as is needed for several avionics platforms. Such fiber-less modules will significantly improve system scalability by reducing fiber management issues.
This work was partially funded by Lockheed Martin Advanced Technology Laboratories. We thank Roger Karnopp and Greg Whaley for their support.
 G. Whaley and R. Karnopp, “Air Force Highly Integrated Photonics Program: Development and Demonstration of an Optically Transparent Fiber Optic Network for Avionics Application,” Proc. of SPIE, vol. 7700, paper 77000A, 2010.
 A. M. Joshi, F. Heine, and T. Feifel, “Rad-hard Ultrafast InGaAs Photodiodes for Space Applications,” Proc. of SPIE, vol. 6220, paper 622003, 2006.
 A. M. Joshi, “Highly Linear Dual Photodiodes for Ku-Band Applications,” AVFOP Conference 2009, Invited paper TuB2.