N212-104 TITLE: Multimode-Coupled High-Frequency Photoreceiver
RT&L FOCUS AREA(S): Autonomy;General Warfighting Requirements (GWR);Networked C3
TECHNOLOGY AREA(S): Air Platforms
The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.
OBJECTIVE: Develop and package a multimode, pigtailed, high-frequency, high-current, optical photodetector operating at a wavelength near 1.5 µm for radio-frequency (RF) photonic link applications on military platforms.
DESCRIPTION: Current airborne military communications and electronic warfare systems require ever-increasing bandwidth, while simultaneously requiring reductions in space, weight, and power (SWaP). The replacement of the coaxial cable used in various onboard radio frequency (RF)/analog applications with RF/analog fiber-optic links will provide increased immunity to electromagnetic interference, reduction in size and weight, and an increase in bandwidth. However, it requires the development of high-performance, high-linearity optoelectronic components that can meet extended temperature range requirements (-40 °C to 100 °C). Additionally, avionic platforms pose stringent requirements on the SWaP consumption of components for avionic fiber communications applications [Ref 4]. To meet these requirements, new optical component technology will need to be developed.
Typical microwave photonic links for 20 GHz and higher frequencies utilize single-mode fiber between transmitter and receiver. Single-mode fiber eliminates modal dispersion associated with multimode fiber, which reduces bandwidth in long fiber lengths [Ref 1]. Many links on airborne platforms however are short in length and can utilize multimode fiber, yielding installation, maintenance, and durability advantages. A standard 50 µm core multimode fiber is easily coupled to photodiodes larger than 50 µm in diameter, but 50 µm photodiodes are limited to bandwidths below 20 GHz due to capacitive limitations. If the light from a 50 µm multimode fiber could be focused onto high-current photodiodes with diameters of 25, 15, or even 10 µm using a high-numerical aperture optical system [Ref 2], while still capturing a majority of the light in the fiber, link bandwidths can be pushed to over 50 GHz. With a typical 1 GHz-Km graded index 50 µm fiber, this allows for link lengths upwards of 50 and 20 m at 20 and 50 GHz, respectively, when all modes contain energy. This will negate standard butt-coupling approaches and will require optical lenses to be utilized with higher numerical apertures. This, combined with the fact that many high-current photodiodes are rear illuminated through the substrate, leads to long optical path lengths, a challenge for compact packaging. One additional challenge is the speckle pattern that will appear within the image of the output of the fiber on the photodiode surface. If too large of a fraction of this reduced image falls outside the photodiode active area, speckle variations will result in variations in the detected photocurrent that will be unavoidable and cause deleterious link gain variations. These current variations must be kept below +/- 5% to limit link gain variations to +/- 0.5 dB.
The packaged photoreceiver must perform over the specified temperature range and maintain hermeticity and optical alignment upon exposure to typical Navy air platform vibration, humidity, thermal shock, mechanical shock, and temperature cycling environments [Ref 3], and which can include unpressurized wingtip or landing gear wheel well (with no environmental control) to an avionics bay (with environmental control).
PHASE I: Develop, demonstrate feasibility, and package (non-hermetic) a sub-35 µm diameter photodiode (> 12 GHz bandwidth) with a 50 µm graded-index multimode fiber pigtail. The detected photocurrent should maintain +/- 5% current variation as the input modal pattern is varied. Provide calculations to support the development of 20, 35, and 50 GHz versions for development during Phase II. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Develop a prototype design and package hermetic prototypes at 15, 20, 35, and 50 GHz. Test prototype receivers to meet the above photocurrent variation specification over input modal conditions, and over temperature, to meet design specifications in a Navy air platform representative of a relevant application environment, which can include unpressurized wingtip or landing gear wheel well (with no environmental control) to an avionics bay (with environmental control). The packaged receivers should be tested in an RF photonic link over temperature with the objective performance levels reached. Demonstrate a prototype fully packaged receiver for direct insertion into analog fiber optic links.
PHASE III DUAL USE APPLICATIONS: Finalize the prototype. Perform extensive operational reliability and durability testing, while optimizing manufacturing capabilities. Transition the demonstrated technology to naval aviation platforms and interested commercial applications.
Commercial sector telecommunication systems, fiber optic networks, and data centers could benefit from the development of a Multimode Coupled High Frequency Photoreceiver.
KEYWORDS: Photodetector; Photodiode; Radio Frequency; Wideband; Fiber Optic; Multimode.
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