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Fiber-optic Beam Homogenizer

Description:

TECHNOLOGY AREA(S): Battlespace 

OBJECTIVE: Develop a high-efficiency, lightweight optical beam homogenizer that can operate in the mid-infrared spectrum. 

DESCRIPTION: Fiber-optic cables can be used as a flexible, effective means for delivering high-power infrared (IR) laser radiation from a central high-power laser source and distributing it to dispersed locations on an aircraft platform. Such beam delivery systems are highly desirable and critical for current and future Navy applications. The typical intensity distribution from standard multi-mode fiber optics contains spatial frequency components that are undesirable for many applications. Such applications require, or benefit significantly from, a “Top Hat”/“Flat-top” far-field intensity distribution, wherein uniform illumination can enable advanced system capabilities. The Navy desires development of a fiber optic beam homogenizer that can be attached to the output of a standard multi-mode fiber operating throughout the IR spectrum (.900 micron thru Mid-IR) and that operates with 95 – 97% optical efficiency to produce a “Top Hat” illumination profile in the far field. The proposed equipment must be able to withstand the environment as specified in MIL STD 810G [Ref 5] experienced by high-performance, maritime aircraft such as the H-60 or F/A-18, while maintaining the ability to operate at multi-Watt optical power levels. A uniform intensity distribution of a laser beam is usually required by many applications such as optical information processing, precise measurements, laser radar, additive manufacturing, etc. Laser beams from most sources are Gaussian-like in nature, which limits their ability to form a uniform irradiation pattern. A Diffraction Optical Element (DOE) is desired for transforming a rotationally symmetrical Gaussian-like beam into a nearly diffraction-limited flat-top beam profile. Shaping a laser beam into a small flat-top with steep edges and no sidelobes is desired (e.g. Top-Hat). A beam homogenizer that could accommodate laser energy from a spectral window of 0.900 microns to less than 6.0 microns would be ideal; otherwise technical solutions for separate spectral regions will be considered that combine the capability in a single form factor. This design as a proof-of-concept should be optimized to handle greater than 10 Watts of IR laser radiation, with a spectral range of .9 to 6 microns. Respondents should detail their methods for accommodating the full spectral range. Power handling capabilities of the beam homogenizer should accommodate tens of Watts of optical power from either a Continuous Wave (CW) or a modulated laser source. Sources are coupled to the homogenizer by either fiber or free space. The aggregate loss of the beam homogenizer should be less than 10%. Respondents should describe loss modalities of the beam homogenizer. The laser source may be of multiple varieties, including but not limited to fiber, Quantum Cascade, Vertical Cavity Surface Emitting Laser (VCSEL). The laser sources will be linearly polarized with a linewidth that can be less than .001 microns. Respondents should describe how they will measure the beam profile and how to specify the flatness to top hat beam. The respondent should discuss the capability of its homogenizer to accommodate multiple spectral lines in a specific range, and the effects of changing the laser spectral input ±5% from a central wavelength with .001 linewidth. The proposed homogenizer should also be able to take variations in laser power input, and should be able to work under high power without damage during operation. 

PHASE I: Design, develop and demonstrate feasibility of an optical beam homogenizer that meets the requirements and specifications as outlined in the Description section. The Phase I effort will include the development of prototype plans for Phase II. 

PHASE II: Further develop and fabricate the prototype optical beam homogenizer designed in Phase I. Perform a demonstration of a prototype system in a test or lab environment that can measure and validate the requirements and specifications listed in the Description including, but not limited to power, linewidth, spectral range, and illumination profile. 

PHASE III: Perform final testing and update the design according to results obtained from lab and field testing; incorporate findings from test results gathered in an operational environment, if available. Transition the optical beam homogenizer to appropriate Navy platforms and for commercial use. The commercial sector can use fiber-optic beam delivery with engineered illumination for several applications, including but not limited to advanced chemical sensors, environmental monitoring, communications, material cindering, additive manufacturing, and cutting. 

REFERENCES: 

1: Dickey, F. and Holswade, S. "Nearly Diffraction-Limited Size Flat-top Laser Beam." Proceedings of SPIE, Laser Beam Shaping, Volume 4095, 2 August 2000. http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=916060

2:  Hendriks, A., et al. "The generation of flat-top beams by complex amplitude modulation." Proc. of SPIE, 2012, Vol. 8490 849006-1. https://webcache.googleusercontent.com/searchq=cache:Sz9eIgQTBbEJ:https://researchspace.csir.co.za/dspace/bitstream/handle/10204/6219/Hendriks_2012.pdf%3Fsequence%3D1+&cd=2&hl=en&ct=clnk&gl=us

3:  Linang, J. "1.5% Root-Mean-Square Flat-Intensity Laser Beam Formed using a Binary-Amplitude Spatial Light Modulator." Applied Optics, 01 April 2009, Vol. 48, No. 10. https://www.osapublishing.org/ao/abstract.cfm?uri=ao-48-10-1955

4:  Voelkel, V. and Weible, K. "Laser Beam Homogenizing: Limitations and Constraints." Proceedings of SPIE, 25 September 2008, Volume 7102. http://spie.org/Publications/Proceedings/Paper/10.1117/12.799400

5:  MIL-STD-810G, Environmental engineering considerations and laboratory tests. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810G_12306/

KEYWORDS: Flat-Top; Top-Hat Beam Homogenizer; Mid-Infrared; Laser; Additive Manufacturing; Beam Shaping 

CONTACT(S): 

Glenn Marshall 

(301) 342-6735 

glenn.marshall@navy.mil 

Chandraika (John) Sugrim 

(301) 757-7970 

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