N21B-T019 TITLE: Tunable Wideband Differential Interferometer for Radio Frequency Photonic Links

RT&L FOCUS AREA(S): Autonomy;General Warfighting Requirements (GWR);Networked C3

TECHNOLOGY AREA(S): Air Platforms

OBJECTIVE: Develop a tunable differential interferometer for wideband phase-to-amplitude conversion to enable wide-dynamic-range radio frequency (RF) photonic links.

DESCRIPTION: Many defense applications require the remoting of antennas at a significant distance from the receiver. At high frequencies, coaxial cables losses are consequential for many applications and require the use of distributed low-noise amplifiers to prevent impacts to receiver performance. In certain applications, the antenna aperture is highly size, weight, and power (SWaP)-constrained, and the implementation of any electronics at the antenna aperture is problematic. Recent advances in RF photonic components show promise in realizing high-frequency antenna remoting with low-noise figure and high-dynamic range. However, most broadband link architectures utilize amplitude modulators at the encoding point that require active bias compensation to ensure linear operation, which can be problematic in SWaP-constrained environments. Many attempts to develop a bias-free modulator have met with limited success [Refs 1, 2], particularly in the harsh environments dictated by most military applications. An alternative amplitude modulation link architecture utilizes phase-to-amplitude conversion devices, such as a differential Mach-Zehnder interferometer (DMZI) to convert a phase-modulated link signal to an amplitude-modulated link signal directly prior to photo detection, thereby removing the need for any bias electronics at the RF encoding point [Refs 3, 4]. Unfortunately, this conversion process results in links limited in bandwidth on the order of one octave due to the details of the conversion process, even though the phase modulators can encode much wider bands. This STTR topic seeks the development of tunable phase-to-amplitude conversion elements, which can take advantage of wideband, bias-free modulation at the remote RF encoding point.

The goals of this effort are to develop a fiber-pigtailed phase-to-amplitude conversion device with a tunable operating frequency range that is compatible with both single and balanced photodiodes. The device must have sufficiently high-optical power handling (> 300 mW) and low loss (< 3 dB excess optical loss) to ensure the creation of low-noise figure, high-dynamic range RF-over-fiber links. The device should operate over a -40�C to +85�C operational temperature range, and be tunable to cover phase-to-amplitude conversion from 1 GHz on the low end to 45 GHz on the high end, with an instantaneous operational bandwidth of at least one octave [Ref 6]. The device should have dimensions no greater than 1 cm height, 10 cm long, and 3 cm wide. Individual devices should be designed to operate in 1 �m wavelength and 1550 nm wavelength RF over fiber links. Tuning speeds over this range on the order of < 10 ms are desired. It is expected that bias control of the device will be necessary to ensure linear operation, but this bias control is performed at the receiver where SWaP constraints are less burdensome. The proposed techniques must provide for closed-loop bias control. Dual-output devices that would be compatible with differential balanced photodiodes are also desirable. Highly accelerated life testing will provide initial device reliability performance [Refs 5, 6].

PHASE I: Develop and analyze a new design. Demonstrate key performance parameters of the proposed phase-to-amplitude conversion approach and simulate component performance. Develop a fabrication process, packaging approach, and test plan. Demonstrate the feasibility that the wideband differential interferometer 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 design and create a functioning tunable phase-to-amplitude conversion prototype device. Demonstrate prototype operation in an RF photonic link. Show compliance of the prototype with the objective power levels, optical losses, tuning range, tuning speed, and temperature performance reached. Demonstrate a packaged, fiber-pigtailed prototype for direct insertion into single-ended and balanced-photonic links.

PHASE III DUAL USE APPLICATIONS: The proposed phase-to-amplitude conversion devices also function for digital-link applications and can be used as quadrature phase-shift keying (QPSK) demodulators for optical communications links. Such a tunable device would enable tunable bit-rate digital demodulators for reconfigurable communications links and would provide a direct dual-use application for telecommunications.

REFERENCES:

1. Fu, Y., Zhang, X., Hraimel, B., Liu, T. and Shen, D. "Mach-Zehnder: a review of bias control techniques for Mach-Zehnder modulators in photonic analog links." IEEE Microwave Magazine, 14(7), 2013, pp. 102-107. https://doi.org/10.1109/MMM.2013.2280332
2. Salvestrini, J. P., Guilbert, L., Fontana, M., Abarkan, M. and Gille, S. "Analysis and control of the DC drift in LiNbO3Based Mach�Zehnder modulators." Journal of Lightwave Technology, 29(10), May15, 2011, pp1522-1534. https://doi.org/10.1109/JLT.2011.2136322
3. Urick, V. J., Bucholtz, F., Devgan, P. S., McKinney, J. D. and Williams, K. J. "Phase modulation with interferometric detection as an alternative to intensity modulation with direct detection for analog-photonic links." IEEE transactions on microwave theory and techniques, 55(9), October 2007, pp. 1978-1985. https://doi.org/10.1109/TMTT.2007.904087
4. Urick, V. J., Williams, K. J. and McKinney, J. D. "Fundamentals of microwave photonics." John Wiley & Sons, 2015. https://doi.org/10.1002/9781119029816
5. AS-3 Fiber Optics and Applied Photonics Committee. "ARP6318 Verification of Discrete and Packaged Photonic Device Technology Readiness." SAE International, August 20, 2018. https://doi.org/10.4271/ARP6318
6. "MIL-STD-810H, Department of Defense test method standard: Environmental engineering considerations and laboratory tests." Department of Defense, US Army Test and Evaluation Command, January 31, 2019. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810H_55998/

KEYWORDS: RF-over-fiber; balanced link; phase modulation; differential interferometer; fiber optic; quadrature phase-shift keying; phase-to-amplitude