N231-010 TITLE: Active Low-Voltage Thin-Film Lithium Niobate Electro-Optic Modulator

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics

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 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 an electro-optical device that can practically, and reliably, enable the direct connection between an antenna and a photonic link on a tactical platform.

DESCRIPTION: Radio frequency (RF) photonic systems offer wide bandwidths and unique signal processing that can advance the capabilities of microwave, and millimeter-wave, receivers [Refs 1, 2]. With the addition of the direct antenna to photonic link, proposed in this SBIR topic, RF photonic systems can also increase the sensitivity, dynamic range, and flexibility of microwave, and millimeter-wave, receivers.

With the addition of wavelength division multiplexing (WDM), multiple RF over fiber (RFoF) links can be matched with an antenna array to independently return all the RF signals from an antenna array over a single fiber. Such a system could increase the sensitivity and dynamic range of the array and mitigate the coaxial cabling weight and loss. On one tactical platform, the switch from RF cabling to RFoF would result in an estimated 200 lbs. of weight reduction. With a direct antenna to photonic link, the temperature sensitive elements of the photonic system can be contained in a protected environment within the tactical platform where size, weight, and power (SWaP) and environmental constraints are relaxed.

RF photonic system designs have traditionally been hampered by poor Spurious Free Dynamic Range (SFDR) and high noise figures (NF) due to the performance of the electro-optic modulator that converts RF to RFoF.

The high noise figure is primarily caused by the large half-wave voltage (Vpi) of the electro-optic modulator and low optical power levels [Ref 3]. The high noise figure is typically overcome by the addition of a traditional low noise amplifier (LNA) being placed in front of the modulator. The LNA can lower the noise figure, but it is vulnerable to electromagnetic interference (EMI), and it limits both the bandwidth and the SFDR of the overall system, so that the full capabilities of the RF photonic system cannot be realized.

The dynamic range in an RFoF system is dependent largely on the optical power level. High optical power levels can melt or burn fiber connections due to impurities in the epoxy used to glue fiber pigtails or dirt in the connectors. These challenges typically make high optical power difficult, or even impossible, to use in tactical systems due to concerns of manufacturability, reliability, and maintainability.

An electro-optical device that can directly connect between an antenna and a photonic link without an LNA will need to overcome the high half-wave voltage (Vpi) of the modulator and the high optical power necessary to achieve the required noise figure and SFDR.

Recent advances in thin film Lithium Niobate (LiNbO3) modulators have demonstrated that a Vpi of < 1 V is achievable with low insertion loss within an Integrated Photonic Circuit [Refs 4�6].

An active thin film LiNbO3 modulator is a device that combines a high power optical amplifier with a thin film LiNbO3 modulator on a single photonic integrated circuit. This device achieves both the RF performance and system reliability by isolating the high-power optical elements to within a single photonic integrated circuit.

This device will accept an optical power input from a fiber pigtail in the range of 10 dBm to 20 dBm between 1530�1565 nm. The optical signal will be amplified to 30dBm with a constant output amplifier that is integrated on a photonic integrated circuit to directly feed a thin film LiNbO3 modulator in a Mach-Zehnder configuration. The device will provide a 50 ohm RF input with an RF Vpi = 0.5V @20GHz and a 3dB bandwidth at > 30 GHz. The device will provide a dual fiber pigtail output to enable balanced detection [Ref 7]. The thin film LiNbO3 modulator will have a < 3dB optical insertion loss to be demonstrated by a measured optical power level of 27 dBm out of one of the output fibers in a null or peak bias configuration. The expected RFoF link performance using the device is a Noise Figure of 3db and an SFDR of 116dB/Hz at 20GHz, and a total link bandwidth of 0.1-30Ghz

The SWaP-C of the device must be < 50cm� to enable mounting at the antenna on a tactical platform. The device should demonstrate an amplifier efficiency of 10% or greater with plans to achieve an operating case temperature of -60 �C � +80 �C. Monitoring and control circuitry for the amplifier and modulator should be self-contained within the device requiring only DC power to the device [Refs 8�10].

PHASE I: Develop a chip-level layout and packaging concept for an active thin film-based LiNbO3 modulator with a clear path to meeting the specifications detailed in the Description. Identify key risk areas to realizing the desired modulator performance and packaging constraints, and mitigate these risks using die-level demonstrations and packaging process development. Demonstrate that a modulator can achieve the desired RF performance specifications with a proof-of-principal benchtop experiment. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Optimize the Phase I design. Create, and test a functioning active thin film-based LiNbO3 modulator, and package the modulator. Demonstrate a packaged, fiber-pigtailed prototype for direct insertion into a photonic link. Demonstrate the Vpi, optical power levels, and amplifier efficiency. Demonstrate prototype operation in an RF photonic link showing compliance with the objective noise figure, SFDR, and frequency range. Characterize the temperature sensitivity of the packaged device and develop a packaging concept to meet the full environmental requirements. Show a path to manufacturability up to 5000 devices/year.

PHASE III DUAL USE APPLICATIONS: Support the DoD in transitioning the proposed modulator. This will include working with a program office to develop a final packaging design that meets the platform SWaP and environmental requirements and developing systems specifications for the associated analog photonic links.

Development of these modulators has widespread commercial applications from 5G/6G signal routing to low-power digital telecommunications and data center routing.

REFERENCES:

1.       Urick, V. J., Jr., Williams, K. J., & McKinney, J. D. (2015, February 6). Fundamentals of microwave photonics. John Wiley & Sons. https://doi.org/10.1002/9781119029816

2.       Devgan, P. S. (2018). Applications of Modern RF Photonics. Artech House. https://www.worldcat.org/title/applications-of-modern-rf-photonics/oclc/1029482016

3.       McKinney, J. D., Godinez, M., Urick, V. J., Thaniyavarn, S., Charczenko, W., & Williams, K. J. (2007). Sub-10-dB noise figure in a multiple-GHz analog optical link. IEEE Photonics technology letters, 19(7), 465-467. https://doi.org/10.1109/LPT.2007.893023

4.       Ahmed, A. N. R., Nelan, S., Shi, S., Yao, P., Mercante, A., & Prather, D. W. (2020). Subvolt electro-optical modulator on thin-film lithium niobate and silicon nitride hybrid platform. Optics letters, 45(5), 1112-1115. https://doi.org/10.1364/OL.381892

5.       Yegnanarayanan, S., Kharas, D., Plant, J. J., Ricci, M., Ghosh, S., Sorace-Agaskar, C., & Juodawlkis, P. W. (2021, August). Integrated Microwave Photonic Subsystems. In 2021 IEEE Research and Applications of Photonics in Defense Conference (RAPID) (pp. 1-2). IEEE. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9521455

6.       Wang, C., Zhang, M., Chen, X., Bertrand, M., Shams-Ansari, A., Chandrasekhar, S., Winzer, P., & Loncar, M. (2018). Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages. Nature, 562(7725), 101-104. https://doi.org/10.1038/s41586-018-0551-y

7.       Diehl, J., Nickel, D., Hastings, A., Singley, J., McKinney, J., & Beranek, M. (2019, November). Measurements and Discussion of a Balanced Photonic Link Utilizing Dual-Core Optical Fiber. In 2019 IEEE Avionics and Vehicle Fiber-Optics and Photonics Conference (AVFOP) (pp. 1-2). IEEE. https://doi.org/10.1109/AVFOP.2019.8908161

8.       Department of Defense. (2008, October 31). MIL-STD-810G: Environmental engineering considerations and laboratory tests. Department of Defense. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810G_12306/

9.       DLA Land and Maritime. (2016, April 25). MIL-STD-883K: Test method standard microcircuits. Department of Defense. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-883K_54326/

10.   DLA Land and Maritime. (2015, March 13). MIL-PRF-38534J: General specification for hybrid microcircuits. Department of Defense. http://everyspec.com/MIL-PRF/MIL-PRF-030000-79999/MIL-PRF-38534J_52190/

 

KEYWORDS: Radio-frequency over fiber; microwave photonics; half wave voltage; noise figure; spur free dynamic range; thin film lithium niobate; fiber optic; electro-optic modulator

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