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Frequency-Equalized Electro-Optic (EO) Phase Modulators for High-Precision Interferometric Inertial Sensors


TECHNOLOGY AREA(S): Materials, Sensors, Electronics 

OBJECTIVE: Develop Electro-Optic (EO) phase modulators with flat frequency response, low switching voltage-length product, and multi-decade environmental lifetime for use in strategic-grade high-precision inertial sensors such as interferometric fiber-optic gyroscopes and accelerometers. 

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. [1]. Another example is the Zero Force Accelerometer (ZFA) developed by Draper Laboratory which relies on a solid-state optical displacement sensor and is projected to exceed strategic-grade performance requirements. A key component in these types of sensors is the integrated optical circuit (IOC), typically comprised of EO phase modulators based on waveguides and electrodes formed on the surface of an electro-optic crystal such as lithium niobate (LiNbO3) [2]. The non-ideal behavior of these phase modulators, particularly their frequency-dependent response and the long-term environmental degradation thereof, is well known, and the precision of the parent inertial sensors is limited by this non-ideal behavior. Various technical approaches have been developed in attempts to both improve the flatness of frequency response at beginning of life as well as the long-term environmental stability of the phase modulators [3-5]. These approaches have ranged from implementation of various different waveguide materials and processing to inclusion of dielectric buffer layers, and often give rise to trade-offs with complexity and other device performance parameters such as switching voltage-length product (Vpi-L), insertion loss, etc. Nevertheless the need remains for new technical approaches to improve EO phase modulator performance for interferometric inertial sensor applications. The objective of this topic relates to advanced EO phase modulators designed for high-precision interferometric inertial sensors. In particular, Y-branch dual phase modulator LiNbO3 IOCs are required with 1550 nanometer operating wavelength, low optical insertion loss, high chip polarization extinction ratio (PER), low Vpi*-L, flat frequency response down to sub-Hertz frequencies, and at least 20-year environmental lifetime. 

PHASE I: Perform a design and materials study aimed at a LiNbO3 phase modulator that achieves improved performance for interferometric inertial sensor applications as compared to the current state of the art via novel designs, materials, and fabrication processes. The study must assess device performance parameters, including beginning-of-life frequency response; of fabricated test structures; consider all aspects of device fabrication (e.g., substrate, waveguides, electrodes, additional features as warranted such as buffer layers, and packaging); include a preliminary assessment of long-term environmental stability based on a materials physics analysis; and justify the feasibility/practicality of the approach. A specific device design must be proposed for fabrication in Phase II of the project based upon this analysis. Also, a plan for Phase II will be developed. 

PHASE II: Based on the Phase I results, design, fabricate and characterize a small lot of prototype Y-branch dual phase modulator IOCs, complete with fiber-optic pigtails and electrical connectorization suitable for incorporation into test beds for interferometric inertial sensors. Characterization must comprise electrical measurements including half-wave voltage (Vpi), frequency response and residual intensity modulation (RIM), and optical measurements including optical insertion loss, split ratio, 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. A proof-of-concept study of one or more prototype IOCs in a suitable IFOG test bed must be performed. The prototypes should be delivered by the end of Phase II. 

PHASE III: Based on the prototypes developed in Phase II, continuing development must lead to productization of Y-branch dual phase modulator 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 that can maintain frequency response over a very wide range of environmental conditions is likely to bring value to many existing commercial applications. 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. 


1: Adams, G. and M. Gokhale. "Fiber optic gyro based precision navigation for submarines," Proceedings of the AIAA Guidance, Navigation and Control Conference, Denver, CO, USA, vol. 1417 (2000).

2:  Wooten, E.L. et al. "A review of lithium niobate modulators for fiber-optic communications systems," IEEE Journal of selected topics in Quantum Electronics, vol. 6, no. 1, pp. 69-82 (2000).

3:  Kissa, K. and J. J. Xu. "Y-branch dual optical phase modulator," U.S. Patent Application No. 13/338,929 (2011).,929%22&OS=%2213/338,929%22&RS=%2213/338,929%22

4:  Feth, J. "Stitched waveguide for use in a fiber-optic gyroscope," U.S. Patent No. 8,373,863 (2013).

5:  Kissa, K. "Optical phase modulator," U.S. Patent No. 8,463,081 (2013).


KEYWORDS: Electro-optic Modulator; Phase Modulator; Lithium Niobate; Waveguides; Inertial Sensor; Fiber-optic Gyroscope 

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