Beam alignment and incorporation into optical design

1
Beam alignment and incorporation into optical
design

• Presented by
• Tom Lehecka
• Penn State Electro-Optics Center
• tml15_at_psu.edu

Contributors
Malcolm McGeoch Plex LLC
Bertie Robson RRR Corp.
Graham Flint General Atomics
Ron Korniski SAIC
Presented at High Average Power Laser Program
Workshop Oak Ridge National Lab March 21-22, 2006
2
Outline

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

3
Facility Layout
205 m
Single 25 kJ module

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

4
Single 25 kJ Module Block Diagram
Front End

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

5
Amplifier Area Optical Layout
Beam from front end. 98 beams total, one shown
Input mirror - Convex Rc -14.54 m 8X8 cm beam
Imaging lens Plano-convex Rc -14.54 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 -22.2 m 16X16 cm
beam
To Target
Final Lens Plano-Convex f 17 m 16X16 cm beam
Demultiplex mirror Flat 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

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!
Filename FTFb8sqTILnFL
7
Beam Steering Concept

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

8
Target Chamber Layout
Path from lens to target, Straightened out for
clarity
Dielectric mirrors
Lenses
GIMM, 15 segments
Dielectric mirrors
GIMM
Fast steering mirror (FSM)
Lens
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

9
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. (Rsvtarget/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 ? drinjltdrs
  (Thanks Bertie!)

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Beam 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.

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Steering Mirror Correction Times
Centroid determination 100 ms ? 30 kHz
bandwidth Mirror response Acceleration
300 ms ? 44 mradian 1.5 mm Deceleration
300 ms ? 44 mradian 1.5 mm Settling time
500 ms ? 0.5 level TOTAL 1.2 ms
ts For vtarget100 m/s ? Rs12 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 systems insensitivity to
  vibration
• Vibration levels at 833 Hz and above (1.2 ms) are
  expected to be small

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.

13
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