Comparison of laser
scanning methods
Yong-Gu Lee
Beam steering methods
• Refraction (reflection) type
– tilt platform
• Galvanometer
• Piezo
– linear stage
• motor
• Piezo
• Diffraction type
– Diffraction Optical Element
– Acousto-optical devices
Beam steering (x-y)
Beam steering (x-y)
PI Ultrafast piezo tip/tilt platform
High resolution
Beam steering (x-y)
• An acousto-optics deflector changes the angle of direction of
a laser beam and its angular position is linearly proportional
to the acoustic frequency, so that the higher the frequency,
the larger the diffracted angle.
 
f
Va
• Where λ is the optical wavelength in air, Va is the acoustic
velocity, and f is the frequency
Ref. http://www.brimrose.com/aodefl.pdf
AOD: high scanning rate
Ref. http://www.brimrose.com/aodefl.pdf
Micro-Tetris
Beam steering (x-y)
Ref. http://www.gsilumonics.com/product_data_sheets_en/276.html
Galvanometer: wide scanning range
Beam steering (z)
Beam steering (z)
Motor
•Motorized Actuator: 12V DC Servo Motor
•Actuator Travel: 25mm
•0.05 µm Minimum Incremental Move
•Compatible with Many Standard Controllers/Drivers
•Limit Switches Prevents Accidental Actuator Damage
•High Precision Rotary Encoder (48 counts per revolution)
Ref. http://www.thorlabs.com/
Beam steering (z)
PI PIFOC Nanofocusing positioners
Double optical Trap
+
=
Two orthogonally polarized beams
N-optical trap
Space sharing
Ref. http://www.memsoptical.com/
Scanning optical trap
Time sharing
Basic Steerable Tweezers Setup
The steering mirror must be made “conjugate” to the back aperture of the
objective lens.
Ref. Dholakia and Gabriel C. Spalding, 2004 SPIE course on Optical Tweeezers
Diffractive optical element
•
•
•
•
•
Diffractive optical elements are computer generated holographic devices which
can transform an illuminating laser beam into a specified intensity distribution by
diffraction rather than refraction or reflection. The diffractive surface of a beam
shaping element is split into an array of cells each designed to transform the
phase of the coherent illuminating beam by a specified amount.
In a reflective diffractive optical element the required phase change at each cell is
achieved by accurately producing a sub-wavelength depth profile as shown
below.
Upon reflection each cell is considered to be emitting a spherical wavefront with
a specified phase retardation. These diffracted wavefronts interfere in the
reconstruction plane to produce the required intensity profile.
The computer algorithms used to calculate the diffraction patterns incorporate the
profile of the illuminating laser beam in order to achieve the optimum
reconstruction performance.
A measured intensity profile can be inputted or a mathematical description of the
beam shape may be used. As the number of depth levels reproduced in the
diffractive surface increases so does the efficiency and quality of the
reconstruction. Diffraction efficiencies in excess of 90% are achievable using the
fabrication techniques.
Ref. www.laseroptical.co.uk
Phase only modulation
In fact, phase-only modulation allows for complex
optical “ landscapes”
[Left-hand Image: Oliver Kreylos Right-hand Image: Brian Lent and Dimitrios Mavroidis,
Stanford Computer Graphics Laboratory]
Result of passing light through a
phase modulator