N22A-T010 TITLE: Kilowatt Class-k Fiber Optical Isolator for Submarine High Energy Laser Amplifier

OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy (DE)

TECHNOLOGY AREA(S): Weapons

OBJECTIVE: Design and develop a compact and robust fiber optical isolator for kW class fiber lasers/amplifiers.

DESCRIPTION: Optical isolators transmitting light only in one direction while blocking light in the opposite direction have been extensively used to protect laser systems from the influence of the backward light. Fiber lasers have seen significant developments during the last two decades and kW class fiber lasers have been deployed in different platforms for DoD applications. This has created demand for high power compact and robust optical isolators that can be used to protect these kW class fiber lasers. Commercial free-space bulk optical isolators capable of handling optical average powers up to kW level are becoming available. However, the packaging volume, thermal resistance, reliability, and even the power handling cannot meet most DoD applications. Fiber-coupled or fiber-based optical isolators have the advantages of small format, easy operation, and high robustness while exhibiting the promise of high-power handling. Currently, the power handling capability of fiber-coupled isolators is limited to 100 W. This STTR topic seeks innovative device design, advanced Faraday material, new magnet material, and novel power polarizers that can be combined for the development of kW class fiber optical isolators. This topic supports the development of a prototype with the parameters listed below at the end of Phase II:

• Operating Wavelengths: 1�m, 1.55 �m, and 2 �m� Average Power handling: Threshold 3 kW; Objective 5 kW per amplifier
• Bandwidth: Threshold 20 nm; Objective 50 nm
• Insertion Loss: Threshold < 1 dB; Objective < 0.5 dB
• Isolation: Threshold > 30 dB; Objective > 40 dB
• Polarization extension ratio (FER) > 30 dB
• Reliability: Lifetime > 5000 hours
• Thermo Electric (TEC) or Water cooling preferred

Under the Phase II Option II, if exercised, a prototype kW class Fiber optic isolator will be delivered to a Navy lab to evaluate the performance of the system in terms of its optical isolation > 40 dB for HEL system.

PHASE I: Develop a concept that uses the Faraday material, magnet material, and polarizers for a best-performance optical isolator construction that can be used for kW class fiber lasers. Demonstrate the power handling scalability of the new isolator material and device. The isolator concepts will be designed to meet the performance capabilities identified in the Description section. Demonstrate the feasibility of the concept to meet the parameters listed in the Description through modeling, simulation, and analysis.

The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build a prototype solution in Phase II.

PHASE II: Develop and deliver a prototype based on the results of Phase I, supporting the parameters listed in the description. Optimize the design and development of the Phase I kW class optical isolator to a prototype compact and robust fiber optical isolators for kW class fiber lasers.

Deliver a prototype kW class Fiber optic isolator to a Navy lab to evaluate the performance of the system in terms of its optical isolation > 40 dB for HEL system as described in the Phase II SOW. Any test data collected at Navy facilities shall be Government use only.

PHASE III DUAL USE APPLICATIONS: Transition of kW class Fiber optic isolator to Navy use for the purpose of HEL technology integration at 1 to 2 �m MW class laser. Identify the final kW class fiber isolator product and describe how the company will support transition to Phase III. Ultimately, the HEL system will be deployed in a submarine or other Navy platform advancing future Navy warfighting capabilities.

Fiber optical isolators with high power handling capability can be used in various HEL laser systems for DoD and industrial applications such as welding, cutting, soldering, marking, cleaning, and material processing.

REFERENCES:

1. Khazanov, E.A. "Slab-based Faraday isolators and Faraday mirrors for 10-kW average laser power", Applied Optics, Vol. 43, Issue 9, 1907 (2004).
2. Snetkov, I.L.; Voitovich, A.V.; Palashov, O.V. and Khazanov, E.A. "Review of Faraday isolators for kilowatt average power lasers," IEEE Journal of Quantum Electronics, Vol. 50, Issue 6, 434 (2014).
3. Turner, E.H. and Stolen, R.H. "Fiber Faraday circulator or isolator", Optics Letters, Vol. 6, Issue 7, 322 (1981).
4. Sun, L. et al. "Compact all-fiber optical Faraday components using 65-wt%-terbium-doped fiber with a record Verdet constant of -32 rad/(Tm)", Optics Express, Vol. 18, Issue 12, 12191 (2010).
5. Sun, L. et al., "All-fiber optical isolator based on Faraday rotation in highly terbium-doped fiber", Optics Letters, Vol. 35, Issue 5, 706 (2010).

KEYWORDS: Optical isolator; fiber isolator; kW class fiber lasers; Faraday rotator; magneto-optical material; polarizer