CCD Optical Calibration

1
CCD Optical Calibration

• Steve Malton, UCL
• Laserwire meeting, Oxford
• June 9th 2004

2
Overview

• Optics Layout
• Current Configuration
• New Optical System
• Lens placement
• IP and image spot travel range
• Image spot size
• Knife-edge Scans
• CCD Resolution
• Summary

3
Optics Layout

• Laser focus at IP
• IP spot imaged at CCD1
• ss2 sy2 w2
• CCD2 measures input beam radius
• wIP M2 l f / w0

4
Current Configuration
2f
2f

• PAC 076 imaging lens
• 125 mm focal length
• w0 0.01mm
• Misplacement of imaging lens
• 8 spot size for 10mm offset

Image spot size
Offset from ideal lens position (mm)
5
Travel Range 4f

• Piezo-scanner has full scan range 5mrad at 10V
• 8 in travel range for 10mm offset

Beam translation at CCD per volt applied to
piezo-scanner
Offset from ideal lens position (mm)
6
New Optical System

• 2 lenses
• Collimate
• Refocus
• Lens 1 placement
• Can test collimation using CCDs
• Lens 2 placement
• irrelevent
• CCD placement
• Minimise image
• 11 imaging produces small spot size
• If f1 ? f2, get magnification

f1
f2
7
Collimated Beam
Travel per volt at CCD (mm)

• Assuming we can measure 10µm variation, sampling
  over 1m
• Lens 1, placement to 0.1mm
• lt 0.1 in travel range

Beam radius after collimation (mm)
Beam spot size at CCD (mm)
8
Travel Range - New

• Translation of spot at IP 62.0431 mm per volt
  applied to scanner
• Translation of spot at CCD 62.0239mm/V

IP
Post-IP
9
Knife-Edge Scans

• Measure total intensity at photodiode
• 3 translation stages
• Crossed x-y stages, coarse
• Picomotor stage, fine
• x-position
• Picomotor steps
• Interferometer to calibrate step size

PICOMOTOR
STAGE
10
Example of knife edge

• Knife edge is stepped through the laser by the
  picomotor stage
• Intensity measured by photodiode
• Fit to error function for beam radius wbeam

11
Stage calibration

• Fitted wbeam at points along laser axis
• Measurement with interferometer calibrates stage
  step size

Forward
Beam Spot Size 2 s (microns)
Reverse
Interferometer
Knife-edge position (microns)
12
CCD Resolution

• Current setup at RHUL
• (Splitter CCD) distance matched to (splitter
  knife edge)
• Measure different beam sizes by moving focussing
  lens
• Compare wccd against wknife

13
Summary

• 2 lens imaging system
• Better lens placement
• Image travel range and beam spot size closer to
  IP values
• Testing performance of both systems _at_ RHUL
• Knife edge scans
• Fit for beam radius
• To cross check with CCD beam size
• CCD Resolution
• compare wccd and wknife