1. Beam alignment and incorporation into optical design
Presented by:
Tom Lehecka
Penn State Electro-Optics Center
Contributors:
Graham Flint
General Atomics
Bertie Robson
RRR Corp.
Malcolm McGeoch
Plex LLC
Ron Korniski
SAIC
Presented at:
High Average Power Laser Program Workshop
Oak Ridge National Lab
March 21-22, 2006
March 21st, 2006
High Average Power Laser Program Workshop

2. High Average Power Laser Program Workshop
Outline
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. High Average Power Laser Program Workshop
Facility Layout
205 m
Single 25 kJ module
Twenty beamline system, one spare amp per side
Modular design with ~ 25 kJ per amplifier
March 21st, 2006
High Average Power Laser Program Workshop

4. Single 25 kJ Module Block Diagram
“Front End”
Ninety-eight beams per amp, 90 for target interaction, 8 for backlighters/amplifier loading
March 21st, 2006
High Average Power Laser Program Workshop

5. Amplifier Area Optical Layout
Beam from front end.
98 beams total, one shown
Input mirror - Convex
Rc = m
8X8 cm beam
Imaging lens
Plano-convex
Rc = m
7X7 cm beam
Amp 2 mirror
Convex-Rc = 26.6 m
30X30 cm beam
Amp 2
Intermediate mirror
Flat
13.9X13.9 cm beam
Amp 1 mirror
Convex-Rc = 68.1 m
100X100 cm beam
Amp 1
Recollimation mirror
Convex
Rc = m
16X16 cm beam
To Target
Demultiplex mirror
Flat
16X16 cm beam
Final Lens
Plano-Convex
f = 17 m
16X16 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
March 21st, 2006
High Average Power Laser Program Workshop

6. Optical Performance
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
Airy Disk diameter
Diffraction limited performance with simple spherical optics!
March 21st, 2006
High Average Power Laser Program Workshop
Filename: FTFb8sqTILnFL

7. High Average Power Laser Program Workshop
Beam Steering Concept
Fast steering mirrors control overlap of outgoing laser beam and incoming target glint
March 21st, 2006
High Average Power Laser Program Workshop

8. High Average Power Laser Program Workshop
Target Chamber Layout
Path from lens to target,
Straightened out for clarity
Dielectric mirrors
Lenses
GIMM, 15 segments
Dielectric mirrors
GIMM
Lens
Fast steering mirror (FSM)
Coincidence sensor
Wedged mirror
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. High Average Power Laser Program Workshop
Target Tracking
Variable definitions:
t0: laser on target time
tf: 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 center
drs: radial correction of beam provided by fast steering mirror (O~ 3 mm)
drinj: radial error of target injector (O~ 2 mm)
drs
drinj Rs
Target
vtarget
Glint laser strikes target at time t0-ts, at a radial location R0-Rs
Rs not drawn to scale
Simple statement of the problem is that we must place the target within the correction zone provided by the fast steering mirror  drinj<drs (Thanks Bertie!)
March 21st, 2006
High Average Power Laser Program Workshop

10. High Average Power Laser Program Workshop
Beam steering process
Alignment laser is maintained near center of the coincidence sensor using steering mirror
Target is injected at t0-tf
In flight tracking system (Doppler measurement and Poisson spot tracker) determine target trajectory and predict arrival time ts at Rs.
Glint laser fires at time t0-ts
Glint signal is received at coincidence sensor. Target R0* is predicted based on this signal and the known trajectory from step 3
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.
Alignment laser fires to verify laser correction on the coincidence sensor.
Main laser fires at time t0-tp. Timing is based on predicted arrival at R0* from measurement of target at Rs and measured velocity.
Laser and target arrive at R0* at time t0.
March 21st, 2006
High Average Power Laser Program Workshop

11. Steering Mirror Correction Times
Centroid determination: ms  ~ 30 kHz bandwidth
Mirror response:
Acceleration*: ms  44 mradian = 1.5 mm
Deceleration*: ms  44 mradian = 1.5 mm
Settling time: 500 ms  0.5% level
TOTAL 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. Fast Steering Mirror System
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. High Average Power Laser Program Workshop
Summary
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
March 21st, 2006
High Average Power Laser Program Workshop