N221-078 TITLE: Split Ratio Fine-Tuning Feature for Integrated Optical Circuits in Interferometric Fiber-Optic Gyroscopes

OUSD (R&E) MODERNIZATION PRIORITY: Nuclear

TECHNOLOGY AREA(S): Sensors

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 a new feature for Y-branch dual phase modulator integrated optical circuits (IOC) that enables fine tuning of optical power splitting ratio after assembly.

DESCRIPTION: The performance requirements for strategic-grade inertial sensors based on optical interferometry continue to become more stringent, necessitating continued innovation for optical component technologies. For example, the interferometric fiber-optic gyroscopes (IFOGs) used in inertial navigation systems for fleet ballistic missile (FBM) submarine applications require unprecedented precision, characterized in terms of long-term bias stability, scale factor linearity, angle random walk performance, etc. [Ref. 1]. A key component in these types of sensors is the integrated optical circuit (IOC). The IOC is typically comprised of Y-branch dual phase modulators based on waveguides and electrodes formed on the surface of a crystal such as lithium niobate, and assembled (pigtailed) to optical fiber (one input and two output fiber ports) [Ref 2]. The non-ideal behaviors of these IOCs are well known, and the precision of the parent inertial sensors is limited by this non-ideal behavior. Of particular concern is the optical power splitting ratio between the two output ports. Ideally the split ratio of the IOC should be precisely a 50%/50% even divide, both intrinsic to the Y-branch and after fiber pigtailing. However, manufacturing tolerances typically limit the actual IOC split ratio to a small but nonetheless significant percentage offset, and further limitations on the precision of fiber splicing the IOC into the IFOG optical circuit typically compounds split ratio offset. This SBIR topic relates to advanced lithium niobate IOCs for IFOGs with 1550 nm operating wavelength that shall include a new feature for fine-tuning the split ratio, with precision as good or better than 0.1%, after assembly. The fine tuning shall be achieved by controlling the optical loss of one output port relative to the other, and this control may be implemented by a new feature of the lithium niobate chip, the fiber, or an additional subcomponent. The new feature shall have negligible impact on other IOC design and performance criteria such as overall size, overall optical insertion loss, polarization extinction ratio (PER), and switching voltage-length product (Vpi-L).

PHASE I: Perform a design and materials study aimed at a new feature for fine-tuning the split ratio of a lithium niobate IOC after assembly. The new feature shall be compatible with IOCs having either annealed proton exchange (APE) or reverse proton exchange (RPE) waveguides with 1550 nm operating wavelength. The study must assess performance criteria and consider all aspects of device fabrication. The study shall include a preliminary assessment of long-term environmental stability assuming a design life of 30 years at 50�C based on a materials physics analysis, including Mean Time Between Failure (MTBF), Mean Time to Failure (MTTF) and Failure In Time (FIT) values, along with identification of the assumptions, methods, activation energy, and confidence levels associated with these values. The study shall justify the feasibility/practicality of the approach for achieving split ratio fine-tuning with 0.1% precision with negligible impact on other IOC design and performance criteria including overall size, overall optical insertion loss, polarization extinction ratio, and switching Vpi-L. The study shall estimate the effects of the new split ratio fine-tuning feature on IOC design and performance criteria relative to a control prototype design that does include the new feature. The Phase I Option if exercised, will include the initial design specifications and capabilities description to build prototype solutions in Phase II, as well as a test plan for an accelerated aging study (minimum 5 year real-time equivalent) to be conducted in Phase II.

PHASE II: Based on the Phase I results, design, fabricate, and characterize six (6) prototype IOCs, complete with fiber-optic pigtails and electrical connectorization suitable for incorporation into test beds for interferometric inertial sensors. Characterization must comprise of evaluation of split ratio tunability, as well as electrical measurements including half-wave voltage (Vpi), frequency response and residual intensity modulation (RIM), and optical measurements including optical insertion loss, chip PER, optical return loss (ORL) or coherent backscatter, and wavelength dependent loss (WDL). An accelerated aging study involving IOCs at elevated temperatures under vacuum must be performed to develop a predictive model of long-term environmental stability. The prototypes should be delivered by the end of Phase II.

PHASE III DUAL USE APPLICATIONS: Based on the prototypes developed in Phase II, continuing development must lead to productization of IOCs suitable for interferometric inertial sensors. While this technology is aimed at military/strategic applications, phase modulators are heavily used in many optical circuit applications, including in telecom industry hardware. A phase modulator with split ratio tunability is likely to bring value to many existing commercial applications, such as optical internet, satellite communications, and electric field sensing. Also, technology meeting the needs of this topic could be leveraged to bring IFOG technology toward a price point that could make it more attractive to the commercial markets for land, sea, and aerial systems, including unmanned and autonomous systems.

REFERENCES:

1. Adams, Gary, and Gokhale, Michael. "Fiber optic gyro based precision navigation for submarines." Proceedings of the AIAA Guidance, Navigation and Control Conference, Denver, CO, USA, August 2000: 2�6. https://arc.aiaa.org/doi/pdf/10.2514/6.2000-4384.
2. Wooten, Ed L. et al. "A review of lithium niobate modulators for fiber-optic communications systems," IEEE Journal of Selected Topics in Quantum Electronics 6, January 2000: 69-82. https://ieeexplore.ieee.org/document/826874.

KEYWORDS: Integrated Optical Circuit; Phase Modulator; Lithium Niobate; Waveguides; Inertial Sensor; Fiber-optic Gyroscope