March 21st, 2006 High Average Power Laser Program Workshop 1

PLEX LLC

Beam alignment and incorporation into optical design

Presented by:Tom Lehecka

Penn State Electro-Optics Centertml15@psu.edu

Presented at:High Average Power Laser Program Workshop

Oak Ridge National LabMarch 21-22, 2006

Contributors:

Graham FlintGeneral Atomics

Bertie RobsonRRR Corp.

Malcolm McGeochPlex LLC

Ron KorniskiSAIC

March 21st, 2006 High Average Power Laser Program Workshop 2

PLEX LLCOutline

• System layout• Optical performance• Incorporation of the alignment system• Steering mirror performance requirements• Alignment process definition

March 21st, 2006 High Average Power Laser Program Workshop 3

PLEX LLCFacility Layout

205 m

•Twenty beamline system, one spare amp per side•Modular design with ~ 25 kJ per amplifier

Single 25 kJ module

March 21st, 2006 High Average Power Laser Program Workshop 4

PLEX LLCSingle 25 kJ Module Block Diagram

•Ninety-eight beams per amp, 90 for target interaction, 8 for backlighters/amplifier loading

“Front End”

Oscillator10 Hz Rep

Rate

Beam Smoothing

Pulse Shaping

2.5 ns plus foot pulse

TargetChamber

Amp 42.5 ns 10 Hz~2 J

Amp 338 ns~20 J

15 beam splitter

98 Beam

Splitter

98 beam Multiplexer225 ns

15 beam Multiplexer – 38 ns

Amp 1

Amp 2

98 beam Demultiplexer – 225 ns to 2.5 ns

7X14Convex Mirrors

7X14Convex Mirrors

7X14Flat

Mirrors

Fast Steering Mirrors

Grazing Incidence

Mirrors

TargetInjector/TimingSystem

7X14Lenses

AmpMirrorFixed

AmpMirrorFixed

FinalFocusing

optic/vacuum windows

Coincidence Sensor

TargetTracking System

225 ns, ~1 kJ

225 ns, ~25 kJ

No control

Alignment laser

March 21st, 2006 High Average Power Laser Program Workshop 5

PLEX LLCAmplifier Area Optical Layout

Beam from front end.98 beams total, one shown

Amp 2

Amp 1

Input mirror - ConvexRc = -14.54 m8X8 cm beam

Amp 2 mirrorConvex-Rc = 26.6 m30X30 cm beam

Amp 1 mirrorConvex-Rc = 68.1 m100X100 cm beam

Imaging lens Plano-convexRc = -14.54 m7X7 cm beam Intermediate mirror

Flat13.9X13.9 cm beam

Recollimation mirrorConvexRc = -22.2 m16X16 cm beam

Final LensPlano-Convexf = 17 m16X16 cm beam

Demultiplex mirror Flat16X16 cm beam

•Single lens image relay from amp 2 to amp 1. All powered optics spherical•Tilt of imaging lens and final lens corrects for astigmatism introduced at amplifiers•Output beam size of 16x16 cm yields fluence of 1.1 J/cm2 on optics

To Target

March 21st, 2006 High Average Power Laser Program Workshop 6

PLEX LLCOptical Performance

16:43:25

0.000,0.000 DG 0.00, 0.00

FIELDPOSITION

DEFOCUSING 0.00000

FTF B008 11/18 x,y,z SQape ILnFL tilts

.203E-01 CM

16:42:01

FTF B008 11/18 x,y,z

SQape ILnFL tilts

POSITION 1

SAI 01-Dec-05

0.02249 cm

25

DIFFRACTION INTENSITY SPREAD FUNCTION

FLD( 0.00, 0.00)MAX;( 0.0, 0.0)DEGDEFOCUSING: 0.000000 CM

WAVELENGTH WEIGHT 248.3 NM 1

Code V model of worst case beam: Performance w/ imaging lens and final lens tilted

RMS Wavefront Error (RMS WFE) is 0.004wave

Point Spread Function (PSF), &

RMS WFE: Diffraction based

Spot Diagram: Geometrically

based

Filename: FTFb8sqTILnFL

Airy Disk diameter

Diffraction limited performance with simple spherical optics!

March 21st, 2006 High Average Power Laser Program Workshop 7

PLEX LLCBeam Steering Concept

•Fast steering mirrors control overlap of outgoing laser beam and incoming target glint

Target Chamber

Laser alignment

beam (from laser system)

Target alignment glint beam

Cat’s eye or retroreflector

Laser fast steering mirror

Wedged mirror

Final focusing element

Final mirror(s)

Pellicle/beamsplitter

Target alignment coincidence sensor(Position sensing

diodes/CCD)

March 21st, 2006 High Average Power Laser Program Workshop 8

PLEX LLCTarget Chamber Layout

Lenses

Dielectric mirrorsPath from lens to target, Straightened out for clarity

GIMM, 15 segments

GIMM

Lens

Dielectric mirrors

Coincidence sensor

Wedged mirror

Fast steering mirror (FSM)

•Desire to keep FSM and sensor close to target but well out of neutron irradiation•Potential location for one beam shown

March 21st, 2006 High Average Power Laser Program Workshop 9

PLEX LLCTarget Tracking

•Simple statement of the problem is that we must place the target within the correction zone provided by the fast steering mirror rinj<rs (Thanks Bertie!)

Variable definitions:t0: laser on target timetf: time of flight for target from

injection to chamber center (O~ 80 ms)

tp: laser propagation time from front end to target

ts: time required for steering mirror correction of the beam (O~ 1.2 ms)

Rs: radial distance of target from R0 at time t0-ts. (Rs=vtarget/ts, O~ 12 cm)

R0*: target location at time t0. Will vary each shot

R0: chamber centerrs: radial correction of beam

provided by fast steering mirror (O~ 3 mm)

rinj: radial error of target injector (O~ 2 mm)

vtarget

rinj

rs

Target

Rs

Glint laser strikes target at time t0-ts, at a radial location R0-Rs

Rs not drawn to scale

March 21st, 2006 High Average Power Laser Program Workshop 10

PLEX LLCBeam steering process

1. Alignment laser is maintained near center of the coincidence sensor using steering mirror

2. Target is injected at t0-tf

3. In flight tracking system (Doppler measurement and Poisson spot tracker) determine target trajectory and predict arrival time ts at Rs.

4. Glint laser fires at time t0-ts

5. Glint signal is received at coincidence sensor. Target R0* is predicted based on this signal and the known trajectory from step 3

6. Fast steering mirror is commanded to position alignment laser to predicted R0*. This location will be a position near the center of the coincidence sensor. TBD if move is made based on alignment laser or position sensors on board the FSM.

7. Alignment laser fires to verify laser correction on the coincidence sensor.8. Main laser fires at time t0-tp. Timing is based on predicted arrival at R0* from

measurement of target at Rs and measured velocity.9. Laser and target arrive at R0* at time t0.

March 21st, 2006 High Average Power Laser Program Workshop 11

PLEX LLCSteering Mirror Correction Times

Centroid determination: 100 s ~ 30 kHz bandwidth

Mirror response:Acceleration*: 300 s 44 radian = 1.5 mmDeceleration*: 300 s 44 radian = 1.5 mmSettling time: 500 s 0.5% level

TOTAL 1.2 ms = ts

For vtarget=100 m/s Rs=12 cm

•*Calculated for 1000 radian/s2 mirror acceleration and 17 m focal length•Times are adjustable but an increase in the total time will effect system’s insensitivity to vibration•Vibration levels at 833 Hz and above (1.2 ms) are expected to be small

March 21st, 2006 High Average Power Laser Program Workshop 12

PLEX LLCFast Steering Mirror SystemDescription Symbol Units Value NotesMechanical performanceAngular range Dq milliradians ± 5

Angular resolution q nanoradians 10 Smallest resolvable mirror step (calculated)

Number of active axes 2 Elevation and azimuth

Angular acceleration a radians/s2 1000Closed loop bandwidth Dnclosed Hz 1000 3 dB closed loop bandwidth with optical feedback system

Settling time tsettle ms 0.5 Settling time to 0.5% jitter for a 100 microradian step

Cage mode positioning jitter nanoradians 50Closed loop cage mode jitter using on board position sensing detectors (measured)

Optical performanceClear Aperture DA cm 20 × 20 Based on target design requirementsOptical surface error srms nm 6.0 RMS mirror surface figure

Reflectivity R248 % 99.9 At 248 nm

Laser damage threshold Fdamage J/cm^2 5 Measured at normal incidence, 248 nm, 2 ns pulse duration

"Challenges" highlighted

Fast Steering Mirror System Requirements

•Commercially available mirrors meet and exceed requirements with exception of bandwidth and settling time. Alternatively can we increase ts to 2.5 ms?•Bandwidth and settling time will depend on mirror and control system architecture. Input shaping can provide ~10X improvement in performance. This needs experimental verification.

March 21st, 2006 High Average Power Laser Program Workshop 13

PLEX LLCSummary• Simple optical imaging in amplifier region provided diffraction limited performance

•Potential locations for fast steering mirrors and diagnostics being determined

•Commercially available fast steering mirrors are close to the requirements for beam steering onto target – bandwidth and step & settle time need some improvement

•Work over the past four months has made all team members believe that tracking, alignment & injection based on current technology is achievable