MIT Lincoln Laboratory999999-1

XYZ 10/13/2010

Holographic Optical Beam-Steering

Demonstration

Group 66 – Advanced Lasercom Systems and Operations

MQP Final Presentation

Gabriel Ayers

Michael Ciampa

Nicholas Vranos

13 October 2010

This work was sponsored by the Department of the Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations,

conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.

MIT Lincoln Laboratory999999-2

XYZ 10/13/2010

Presentation Outline

• Optical Beam-Steering Background

• Project Goals

• System Design

• Characterization of holographic gratings

• Pointing and Beam-Steering measurements

• Conclusions and Future Work

MIT Lincoln Laboratory999999-3

XYZ 10/13/2010

Beam Steering Applications

• Free-space laser communications ("lasercom")

– High Bandwidth

– High Security

– Point to Point laser communication

• Infrared Countermeasures

– Possibly used to ‘blind’ sensors of airborne projectiles

• Laser Radar

MIT Lincoln Laboratory999999-4

XYZ 10/13/2010

Beam Steering Examples

Gimbaled Mirrors Risley Prisms

BAE Systems Agile Eye

Infrared Countermeasure

Optra 2” diameter clear

aperture compact beam

steering system

MIT Lincoln Laboratory999999-5

XYZ 10/13/2010

• Conformal intrudes less into an aircraft’s air stream

– Less impact to flight dynamics

– Less drag induced to aircraft

– Less optical distortions to beam

Conformal vs. Nonconformal Beam Directors

BeamDirector

Optical module

Window Interface

Beam Director

Optical module

Turret Interface

MIT Lincoln Laboratory999999-6

XYZ 10/13/2010

Holographic Optical Diffraction Gratings

What is a Holographic Optical Diffraction Grating?

In each optical element there is a periodic structure, which modulates the refractive index. This structure uses Bragg diffraction to deflect the beam.

Properties:

• Reflection or Transmission mode

• Multi-Wavelength

• High Efficiency

• High Power

• Thermally Stable

HOBS Gratings:

• Square 50 mm

• Blazed for two wavelengths

• Transmission Mode

MIT Lincoln Laboratory999999-7

XYZ 10/13/2010

Single Degree of Freedom

• Gratings diffract light at constant angle

• Steer laser beam by rotating gratings around the optical axis

MIT Lincoln Laboratory999999-8

XYZ 10/13/2010

Two Degrees of Freedom

• Second diffraction grating and motor pair is positioned normal to the diffracted beam of the first grating

• Steering range is dependent on diffraction angle

MIT Lincoln Laboratory999999-9

XYZ 10/13/2010

HOBS Goal

• Construct a Holographic Optical Beam-Steering

(HOBS) prototype capable of steering two

wavelengths to transmit and receive

• Develop a steering algorithm

• Characterize the optical properties of the system

• Deliver a prototype demonstration and evaluation

to Lincoln Laboratory

MIT Lincoln Laboratory999999-10

XYZ 10/13/2010

HOBS Opto-Mechanical System

• Designed and machined

motor and grating mounts

• Motor mount secures two

rotational motors at a fixed

angle of 25°

• Gratings are secured and

aligned with micrometry

adjustments within each

mount

MIT Lincoln Laboratory999999-11

XYZ 10/13/2010

Holographic Optical Beam-Steering: Concept

and Realization

• Simulation verifies beam-steering closed form solution using

numerical analysis

• Realization of the system shows ballistic trajectory using closed

form algorithm

Realization of SystemSimulation of System

MIT Lincoln Laboratory999999-12

XYZ 10/13/2010

Optical Characterizationof Holographic Diffraction Gratings

• Diffraction Efficiency

• Power throughput of diffracted laser beam

• Measured diffraction efficiency over varying power level,

wavelengths, beam size and shape

• Wavefront Error

• Quantify phasefront distortion caused by grating

• Measured wavefront error over varying wavelengths, beam size and shape

• Polarization State Changes

• Quantify changes in polarization state caused by grating

• Measured degree of linear and circular polarization state over various wavelengths

• Material Losses

• Excess power loss due to scattering and absorption

• Examined quality of antireflection coating

MIT Lincoln Laboratory999999-13

XYZ 10/13/2010

Diffraction Efficiency

• Examined relationship between diffraction efficiency and

beam size

• Comparison of data shows ~8% variation between gratings

• Based on this data we used the 12mm collimator for

subsequent measurements

Measurement Setup

MIT Lincoln Laboratory999999-14

XYZ 10/13/2010

Wavefront Error

Wavelength: 1544.5 nm

Effect on phase front of incident beam traveling through diffraction

gratings

MIT Lincoln Laboratory999999-15

XYZ 10/13/2010

Summary of Results

Characteristic Beam

Size

Requirement Grating A Grating B

Diffraction

Efficiency

3.4 - 24

(mm)92.8% 89.4% 92.8%

High Power

Efficiency11 (mm) 92% 92% 93%

RMS Wavefront

Error38 (mm) 0.05 waves 0.04 waves 0.07 waves

Polarization State

Changes12 (mm) Best Effort 0.16% 0.15%

Material Efficiency 38 (mm) 97.9% 98.5% 99.3%

MIT Lincoln Laboratory999999-16

XYZ 10/13/2010

Scan Pattern Simulation

• Demonstrated with a green laser for

visualization purposes

• Scan patterns are often used in applications

such as lasercom to find a target terminal

MIT Lincoln Laboratory999999-17

XYZ 10/13/2010

Field of Regard Demonstration

• Demonstrates the system’s ability to point to various locations

surrounding the optical axis

• The widest elevation shown is 11º from the optical axis due to camera

limitations

• System is capable of reaching 50º from the optical axis

11º

0º

MIT Lincoln Laboratory999999-18

XYZ 10/13/2010

High Resolution Precision Demonstration

• The two points are separated by 1680 μrad

• Camera has an angular resolution of approximately 50 μrad per

pixel

• Demonstrates the system’s ability to precisely point to a location

on a small angle scale

MIT Lincoln Laboratory999999-19

XYZ 10/13/2010

Motor Alignment Demonstration

• Trace of a one grating system, clocked for one rotation

• The varying beam intensity indicates imperfections in the

alignment of the system to the incident beam

MIT Lincoln Laboratory999999-20

XYZ 10/13/2010

Diffraction Angle Wavelength Dependence Demonstration

• Demonstrates how the diffraction angle of the incident beam varies

with wavelength

• There is a boresight misalignment between the two wavelengths of

175 μrad

λ = 1544.5 nm

nmλ = 1564.5 nm

MIT Lincoln Laboratory999999-21

XYZ 10/13/2010

Summary

• We have presented a semi-conformal method of beam steering using 50 mm holographic diffraction gratings

• The optical characterization and system pointing tests show that the HOBS approach holds promise for future beam steering systems

• Limitations

Boresight misalignment between wavelengths

Inability to accurately align mechanical system

• Future work

Determine tolerance of alignment to incident beam

Redesign mechanical mounting system

Characterize thermal stability

MIT Lincoln Laboratory999999-22

XYZ 10/13/2010

Acknowledgements

Jeffrey M. Roth

Timothy H. Williams

Edward A. Clancy

Steven Michael

William E. Wilcox

George A. Nowak

Patrick T. Cable

Special Thanks to:

Group 66