Multimode-Coupled High-Frequency Photoreceiver Using InGaAs UTC Photodiode

Current airborne military communications and electronic warfare systems require higher bandwidth as well as reduced size, weight, and power (SWaP). By eliminating bulky coaxial cable used in current radio frequency (RF)/analog applications with fiber-optic links, increased immunity to electromagnetic interference, reduced size and weight, and higher bandwidth are realized. To achieve this, a high-performance, high-linearity optoelectronic photoreceiver capable of extended temperature range operation (-40°C to 100°C) is required. Historically, Single-Mode Fiber (SMF) has been used for RF over Fiber (RoF) links, but the chromatic dispersion problem limits the link length. Multimode Fiber (MMF) operating in a RoF link up to 50 GHz has been studied and considered to be feasible in mm-wave communication systems. More recent applications that make use of MMF, have been analyzed by researchers at the Naval Research Lab (NRL) that include coherent summation of multiple RF signals for signal processing, including filtering and use in receive-mode Phase-Array Antennas (PAAs). Modal noise for a graded-index MMF link with speckle contrast has been studied and found that the MMF behaves as an imperfect transversal microwave photonic filter. It was concluded that the performance of high-frequency RoF transmission through MMF links for short reach distances is not substantially degraded by modal noise. Freedom Photonics has developed 1550-nm high-power photodiodes capable of over 100 GHz bandwidths, utilizing single-mode fiber, which are ideal for long span fiber applications. The Navy is interested in photonic links for airborne platforms with short fiber lengths where multimode fiber can be used, yielding installation, maintenance, and durability advantages. A standard 50-µm graded-index core multimode fiber is typically butt coupled to a photodiode slightly larger in diameter, but the photodiode’s bandwidth for a device with that diameter is on the order of 10 GHz due to capacitive limitations. For our 100-GHz photodiodes, focusing optics have been used to achieve full illumination of a small diameter photodiode. The Phase I of this program will address how to achieve a robust, compact, high responsivity, and wide bandwidth package with longer optical path lengths required for focusing optics and a multimode fiber. By focusing the light from a 50-µm multimode fiber onto our high-power photodiodes with small diameters using a high-numerical aperture lens, we may be able to achieve bandwidths of 20, 25, 67 or even 100 GHz without sacrificing responsivity. , the speckle pattern of the light output from the multi-mode fiber will be studied to assure that all the light falls on the photodiode active area, thus minimizing deleterious link gain variations caused by variations in the detected photocurrent from the speckle variations.