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 [email protected]
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