Operation Manual
POCKELS CELL ALIGNMENT
IN A Q-SWITCHED LASERSEPTEMBER 2017

Gooch & Housego (Ohio)Operation and Installation Manual................................................................................................................................................................
Contents
1: Rough positioning of resonator components

3

2. Alignment of resonator components

3

3. Pockels cell alignment

4

4. Alignment while operating the high power laser

6

For further information or technical support contact:
Gooch & Housego (Ohio)
676 Alpha Drive Highland Heights, Ohio 44143, USA
Email: info@goochandhousego.com
Telephone: +1 216 486 6100

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 2

Operation and Installation Manual................................................................................................................................................................
Warning: Precautions for eye safety must beexercised when operating a high power laser.
Blindness can result from direct exposure to thelaser output. Even indirect exposure such as
viewing a diffuse reflection of an invisible infraredor ultraviolet beam can cause eye damage. Use
precaution avoiding electrical dangers. The highvoltage applied to the Pockels cell is typically
3.5 kV and could cause serious injury. Flash lamp
circuitry can cause electrocution.
An alignment laser such as a small He-Ne laser ora collimated visible diode laser is required for thealignment of a Pockels cell Q-switch. The first stepis to align this laser to the Q-switched-laser
resonator cavity. If the laser resonator is badly outof alignment or the components are being
assembled to form the resonator, it is necessary to
first position the components.

1: Rough positioning of resonator components
A rail on which the components are attached andthe mounts used to hold the components on therail usually determine the optical centerline of thelaser. Some people prefer to use an off-centerspot on the resonator mirrors to allow easy
repositioning to a new spot on the mirror if laser
damage occurs. The alignment laser beam should
be directed through one of the end mirrors into thelaser rod and other cavity components. Some
components such as mirrors may have a wedgethat will deflect the alignment beam and theresonated laser beam. It is necessary to propagate
through these wedged components to accurately
position the following components.

Direct an expanded and collimated alignment laser
beam through the laser rod, and look for reflection
from the side of the rod. These reflections fromgrazing incidence of the rod sides usually appear
as cusps or loops as shown below.

Adjust either the laser rod or the alignment beam
to make these loops and cusps disappear. Now
the alignment beam accurately follows the opticalcenterline of the laser rod. There may be limiting
apertures that prevent the laser beams fromstriking the edges of the laser rod. In that caseadjust the alignment beam and rod to make thebeam pass through the center of the apertures.
The next step is to align the Q-switched laser cavity
mirrors. This is done by obtaining a retro-reflection
from the mirror surface. In doing this it is necessary
to clearly differentiate the front and back surface
reflections of the mirror. The front and backreflections can be discerned by tilting the mirror at alarge angle, as shown in the sketch below.

2. Alignment of resonator components
After the components have been roughly aligned, orwith a laser for which the alignment was not
significantly disturbed, it is next necessary to makethe alignment beam parallel to the laser rod axis.

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 3

Operation and Installation Manual................................................................................................................................................................
Propagating the alignment beam through a small
hole in a piece of cardboard helps in the viewing of
the return reflection. All components in the laser
resonator should now be centered on thealignment beam.
The angular orientation or roll of the polarizer andquarter-wave plate (if one is used) should now beadjusted. Polarization may be accomplished with athin film polarizer or polarizing prism.
The direction of electric field polarization is usually
set either vertical or horizontal. If a quarter-wave
plate is used, the fast and slow axes of thewaveplate should be set at 45° to the direction ofpolarization.

3. Pockels cell alignment
If only the Pockels cell is being replaced in a
Q-switched laser system that was in alignment, it ishighly likely that adjustment of the mirror on the sideof the Pockels cell opposite the laser rod will need tobe adjusted. This is because there may be adeliberate wedge in the cell or a small residualwedge. The mirror alignment is done by transmitting
an alignment laser beam normal to the surface ofthe mirror not being adjusted and adjusting the othermirror for a retro-reflection of the alignment beam.
The alignment beam is also required for adjustmentof the Pockels cell orientation. The Pockels cell ismanufactured to have the electrical connectors
pointing either parallel or perpendicular to thedirection of polarization of the Q-switched laser. The
front-to-back direction in which the cell is placed in
the Q-switched laser makes no difference in mostapplications.

Schematic illustration of a typical Q-switched laser system with components added for Pockels cell alignment.
Two schemes for Pockels cell alignment are shown. Green labels identify the alignment components. The polarizer, analyzer, lenstissue, and alignment beam must be removed before operation of the Q-switched laser.

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 4

Operation and Installation Manual................................................................................................................................................................
The next step in is to align the optic axis of thecrystal within the Pockels cell parallel to theresonator centerline and alignment beam. This isachieved by passing the alignment laser beamthrough the cell with no voltage applied. For thisstep the Pockels cell is placed between crossed
polarizers. Polaroid film can be used for alignment
but must be removed before operation of theQ-switched laser.
If the alignment laser is polarized or the polarizerin the Q-switched laser is effective at thealignment laser wavelength, one or both of theadditional polarizers may be unnecessary. A pieceof lens tissue or similar material placed in thealignment beam before the Pockels cell will
provide some divergent scattering and also allow a
portion of the alignment beam to be directlytransmitted.
After transmission through the crossed polarizers
and Pockels cell, the alignment beam should
impinge on a solid background, such as a piece ofpaper. The isogyre/isochromate pattern, whichlooks like a Maltese cross (isogyre) surrounded bya series of dark rings (isochromates), should bevisible on the paper. Align the pitch and yaw of thePockels cell to center the isogyre/isochromate
pattern on the direct portion of the alignment
beam.

The illustration (bottom left) is a sketch of a slightly
misaligned isogyre/isochromate pattern with thedirectly transmitted alignment laser beam
represented by the dark spot.
It is necessary to determine which Pockels cellparameter is most important for your application,
ICR or VCR. ICR refers to “intrinsic contrast ratio”
and VCR refers to “voltage contrast ratio”. ICR, as
the name states, is the contrast ratio intrinsic to
the material. Again, as the name implies, VCR
refers to the contrast ratio voltage applied to thecell. In most applications, holding off laser
oscillation before Q-switching is of greatest
concern, and the mode of operation determines
the relative importance of ICR and VCR in laser
performance.
As a general rule, if the Q-switch is holding off thebeam with no voltage applied, ICR is yourimportant value. Conversely, VCR is probably yourprimary measurement if the Q-switch holds off thebeam by applying voltage.
We strongly recommend against using theQ-switch with the voltage continuously applied. For
some Pockels cell materials, such as KD*P, there
is a tendency for conduction paths to develop
between the electrodes on the crystal. Pulsing thePockels cell with a small duty cycle (i.e. normallyvoltage-off mode) significantly prolongs thePockels cell lifetime.
Unless you are building or modifying a Q-switchedlaser, the mode of operation is determined by theexisting laser design. Several modes of operation
are common, and it will be necessary to refer tothe Q-switched laser manual to determine themode of operation for your system.

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 5Operation and Installation Manual................................................................................................................................................................
Here are some of the possibilities:(a) A quarter-wave plate is used with the Pockels
cell. The beam passes through the polarizer, thewave plate and the Pockels cell, is reflected at thecavity mirror and is transmitted back through thecell and wave plate to the polarizer. Two passes
through the quarter-wave-retardation plate result in
90° rotation of the polarization and rejection of thebeam at the return pass through the polarizer.
When quarter-wave voltage is applied to thePockels cell the effect of the retardation plate iscancelled, and there is high transmission at thepolarizer.
(b) A variation of the first method is to slightly
misalign the Pockels cell so it acts as a quarterwave plate. Again when voltage is applied theretardation is cancelled and there is hightransmission at the polarizer.
(c) Quarter-wave voltage is applied to the Pockels
cell a short time before pumping of the laser
creating a quarter-wave retardation. Voltage isremoved to Q-switch the laser.
(d) Voltage is continuously applied to the Pockelscell and removed only to Q-switch the laser. AfterQ-switching the voltage again builds up to thequarter-wave voltage.

4. Alignment while operating the high power
laser
After the preliminary alignment with the He-Ne
laser, remove the additional elements such aspolarizing sheet, lens tissue and white screen fromthe cavity. Optimize the alignment of the resonatormirror of the Q-switched laser while operating in
the long-pulse mode, i.e. with the Pockels
cell/polarizer combination set for maximum
transmission and no Q-switching. It is best to startat low pump powers and check to determine that
there are no grossly misaligned components orclipping of the beam.

Next, operate the Pockels cell for maximum loss
and make adjustments of voltage and/or cellalignment to hold off laser oscillation without Qswitching. Obtain the best Q-switchedperformance by adjusting timing and/or voltage
and/or Pockels cell alignment. It may be necessary
to repeat these steps or return to the preliminaryalignment procedure to achieve best Q-switchedperformance.
For operation in the mode of no voltage applied
before Q-switching, there is usually a small rangeof angular adjustment in which the Pockels cell willcompletely hold off laser oscillation. It is best tooperate at the center of that range. Next, thevoltage and delay of voltage application are
adjusted for the best pulse shape (i.e. avoid
multiple pulsing) and maximum output energy. If
you are operating in the voltage on mode, theorder of adjustments should be changed
somewhat. As in the previous procedure, theisogyre should be centered first with the laser off
and no voltage applied.
Next, operating the laser with voltage applied, but
no Q-switching, adjust the voltage for the best holdoff. This is usually the center of the range in which
the laser oscillation is completely held off. The Qswitching delay is then adjusted for best pulse
shape and maximum output energy. At this point,
small iterative adjustments in alignment, voltage,
and delay can be performed to optimize laserperformance.

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 6

Operation and Installation Manual................................................................................................................................................................

For further information
Gooch & Housego (Ohio)
676 Alpha Drive, Highland Heights, OH 44143, USA

T: +1 216 486 6100

goochandhousego.com

Pockels cell alignment in a Q-switched laser
As part of our policy of continuous product improvement, we reserve the right to change specification at any time.

September 2017
Page 7