TECHNOLOGY AREA(S): Electronics, Ground Sea, Air Platform
OBJECTIVE: Develop and package a heterogeneously integrated optical transmitter operating at a wavelength near 1 micrometer for balanced radio-frequency (RF) photonic link applications on air platforms.
DESCRIPTION: Current airborne military communications and electronic warfare systems require ever-increasing bandwidths while simultaneously requiring reductions in space, weight, and power (SWaP). The replacement of the coaxial cable used in various onboard 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. Typically, RF-to-optical transmitters are made by integrating many discrete components into a single large module that routinely exceeds 300 cm^3. However onboard RF/analog applications require the development of high performance, high linearity optoelectronic components that can operate over extended temperature ranges. Additionally, avionic platforms pose stringent SWaP requirements on components such as optical transmitters for avionic fiber communications applications. New optical component and packaging technology is needed to meet future requirements. Current analog optical transmitter technology typically consists of discrete lasers and modulators operating at 1550 nanometers (nm), with active cooling for operation in military environments. To meet avionic requirements, the transmitter should integrate a laser and modulator into a compact uncooled package that can maintain performance over full avionic temperature range (minimum -40 to +85 Celsius). It is envisioned that a laser emitting at approximately 1 micrometer wavelength can serve as the laser source in the transmitter. Innovative Lithium Niobate modulator design including heterogeneous packaging is necessary to integrate a wide-band dual-output (1X2) intensity modulator with the laser and a dual-core single mode fiber output. Recently low relative intensity noise (RIN) lasers and small form factor modulators have become commercially available. However, the challenges posed by integrating both components together in a package less than 150 cm^3 via heterogeneous integration has yet to be accomplished for high performance wideband RF over fiber links, as typically the laser and modulator are of differing materials. Some work has been done to integrate optical components monolithically [Ref 1], and heterogeneously [Ref 2], but researchers have yet to demonstrate an integrated laser and modulator design with the low noise figures (sub-15dB) needed for practical RF/analog photonic links operating over extended temperature ranges.The optical transmitter component is to be based on integration of a dual-output analog transmitter with a dual-core single mode optical fiber [Ref 3] pigtail with a multicore fiber connector at the end of the pigtail. Simultaneously, the transmitter must have performance requirements that support high-performance balanced RF link specifications such as RF noise figures below 25 dB (no RF or optical amplification) when connected directly to a separate balanced high current photodiode pair (0.7 Amp/Watt responsivity); and spur free dynamic ranges (SFDR) above 110 dB-Hz^2/3. The laser source must have a linewidth of <100 kHz, a wavelength of around 1,000 nm, and an output power greater than 200 mW, with RIN spectrum of -165 dBc/Hz from 50 MHz to 20 GHz. The optical modulator is required to operate at up to 20 GHz, and have dual output configuration for applications requiring noise cancellation utilizing balanced detection. The modulator’s power output and modulation efficiency should be optimized to meet the 25 dB noise figure target utilizing both modulator outputs with the above photodiode specifications operated in a balanced detection configuration [Refs 4, 5].Ideally, the transmitter should operate uncooled over a minimum temperature range of -40 to +85 degrees Celsius while maintaining RIN and linewidth performance. A dual output optical transmitter including an integrated optical intensity modulator packaged in a ruggedized package is envisioned. It is desirable for this transmitter module to have a package dimension no greater than 17.5 × 65 × 115 mm when both the bias control circuits for the modulator and the low noise CW laser power supply are contained in the module. The packaged transmitter 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 6].
PHASE I: Develop and analyze a new design and packaging approach for an uncooled 1 micrometer optical transmitter that meets the requirements outlined in the Description section. Develop fabrication process, packaging approach, and test plan. Demonstrate the feasibility that the optical transmitter can achieve the desired RF performance specifications with a proof of principle bench top experiment or preferably in an initial prototype. The Phase I effort will include prototype plans to be developed under Phase II.
PHASE II: Optimize the Phase I transmitter and package design and develop a prototype. Test prototype transmitter to meet design specifications in a Navy air platform representation of a relevant application environment [Ref 6], which can include unpressurized wingtip or landing gear wheel well (with no environmental control [Ref 7]) to an avionics bay (with environmental control). The prototype transmitter should be tested in a balanced RF photonic link over temperature with the objective performance levels reached. Demonstrate a prototype fully packaged transmitter for direct insertion into balanced analog fiber optic links.
PHASE III: Perform extensive operational reliability and durability testing [Refs 8, 9], as well as optimize manufacturing capabilities. Transition the demonstrated technology to Naval Aviation platforms and interested commercial applications.Commercial sector data centers, industries utilizing local area networks, and telecommunication systems, as well as companies that install networks and telecommunications systems would benefit from the development of this transmitter technology.
KEYWORDS: Multicore Fiber, Connector, 1 Micrometer, Responsivity, Avionics, Wide Band Dual-Output
1. Pappert, S., Esman, R. and Krantz, B. “Photonics for RF Systems.” IEEE Avionics Fiber Optics and Photonics Conference, 2008. https://ieeexplore.ieee.org/document/46531482. Novak, D, and Clark, T. R. “Broadband adaptive feedforward photonic linearization for high dynamic range signal remoting.” IEEE Military Communications Conference, 2007. https://ieeexplore.ieee.org/document/44548983. Diehl, J., Nickel, D., Hastings, A., Singley, J., McKinney, J. and Beranek, M. “Measurements and Discussion of a Balanced Photonic Link Utilizing Dual-Core Optical Fiber.” Proc. IEEE Avionics Fiber- Opt. Photon. Technol. Conf., 2019. https://ieeexplore.ieee.org/document/89081614. McKinney, J.D., Godinez, M., Urick, V.J., Thaniyavarn, S., Charczenko, W. and Williams, K.J. “Sub-10-dB Noise Figure in a Multiple-GHz Analog Optical Link.” IEEE Photonics Technology Lett., vol. 19, no. 7, April 2007, pp. 465-67. https://ieeexplore.ieee.org/document/41265645. Williams, K.J., Nichols, L.T. and Esman, R.D. “Externally-Modulated 3 GHz Fibre Optic Link Utilising High Current and Balanced Detection.” Electronics Letters. vol. 33, no. 15, 1997, pp. 1327-1328. https://ieeexplore.ieee.org/document/6060856. “MIL-STD-810H, DEPARTMENT OF DEFENSE TEST METHOD STANDARD: ENVIRONMENTAL ENGINEERING CONSIDERATIONS AND LABORATORY TESTS (31-JAN-2019).” http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810H_55998/7. “DO-160F Environmental Conditions and Test Procedures for Airborne Equipment.” http://www.rtca.org/store_product.asp?prodid=759-8. “MIL-HDBK-217F, Reliability prediction of electronic equipment. http://everyspec.com/MIL-HDBK/MIL-HDBK-0200-0299/MIL-HDBK-217F_NOTICE-2_14590/9. “ARP 6318 “Verification of Discrete and Packaged Photonic Device Technology Readiness.” https://www.sae.org/standards/content/arp6318/