Download U USE ER MA ANU UAL L

AR
RCop
ptix
Variiable
e Spiiral p
plate
e
W
With LC Drive
D er
U
USE
ER MA
ANU
UAL
L
Arc
coptix SS.A
Malladière 71
1C
200
00 Neuchâ
âtel
Swiitzerland
Maiil: info@a
arcoptix.co
om
Tel: ++41 32 731 04 66
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
Priinciple of the v
variable
e Spiral plate
The
e Spiral pllate (VSP) is a liqu
uid crystall cell with
h transparrent ITO
elec
ctrodes and a non--uniform aligned liquid crystal layer tthat gives
s the
elem
ment its s
special op
ptical prop
perties. A transverrse view in
n Figure 1
give
es an ove
erview of the VSP structure.
s
Glass
substrate
LC Layer
Tran
nsparent
elecctrodes
V
V~
Glass
substrate
Figu
ure 1. Tra
ansverse view of the VSP: L
Liquid cry
ystal layerr between
n two
glas
ss substrrates with
h transparent electrode. Op
ptical axis
s directorr can
be ttilted with
h the help
p of an ap
pplied bias
s V.
o transpa
arent elec
ctrodes it is possib
ble to
By applying a bias on the two
tilt the liquid
d crystal molecule
es and at the same time th
he opticall axis
(an
nd appare
ent birefringence).. This me
echanism permits to adaptt the
reta
ardance ((most cas
ses tuned to Pi) off the varia
able Spira
al plate to
o the
wav
velength of the ligh
ht travers
sing the element.
e
e VSP (als
so caller Q-plate) has a top
pological c
charge off Q= 0,5. This
The
means that the form
mula descrribing the
e orientattion of th
he optical axis
(wh
hen lookin
ng at the
e VSP from
m the top) in pola
ar coordin
nates is g
given
by:
Where q is tthe topolo
ogical cha
arge (0,5 in our ca
ase) and a0 a constant
offs
set angle.. The topo
ological charge
c
of the q–pla
ate is 0.5
5. It is de
efined
at p
production and can
nnot be m
modified. Figure 2 shows th
he opticall axis
disttribution for such a VSP. N
Notice tha
at on dem
mand also
o a topolo
ogical
cha
arge 1 can
n be produced.
-1-
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
Figu
es showin
ure 2.Line
ng the lo
ocal optica
al axis (ffront view
w) of a q
q=0.5
VSP
P.
The
e LG(0,1)) spiral mode
m
and the LG(0,1) radial pol. M
Mode have
e the
sam
me dough
hnut shap
ped inten
nsity distribution but theirr electric field
disttribution (polarization and phase) a
are differrent. The
e spiral p
phase
mode can be
e written in cylindrrical coord
dinates (rr,) as following (rref: G.
Mac
chavariani & al. ”Effect of the spiral phas
se elementt on the ra
adial-polarization
(0,1
1)* LG beam
m”,Opt. com
mm. V.271)):
decompos
sed into two orthog
gonal coh
herent com
mponents
s as follow
wing:
or d
Nottice that tthe null in
ntensity in
n the centter is due
e to the ph
hase
sing
gularity and the ph
hase of th
he beam is helical.
Forr the radia
al polariza
ation mod
de, the fie
eld must be
b defined
d as vecto
ors
and
d can be expressed
e
d as:
Where Ex an
nd Ey are
e given ab
bove and ex and ey are the
e x and y unit
vec
ctors.
-2-
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
Diffferent working mode
es:
The
ere are mainly thre
ee types o
of function
ning modes with th
he VSP:
uthal outp
put polariz
zation
1) Radial or azimu
2 Spiral phase witth constant circula
2)
ar output polarization
3 Neutra
3)
al (no cha
ange)
The
e output b
beam of the VSP w
will be defiined by tw
wo param
meters:
1) The input polarization lin
near or cirrcular
2 The ph
2)
hase retarrdation off the VSP.
n the VSP
P in Pi and
d the inpu
ut polariz
zation
Figure 3: Retarrdation on
is circ
cular (leftt) or line
ear (rightt), giving
g respecttively a s
spiral
phase output orr a radial pol. Outp
put.
The
e table b
below su
ummarize
es all the
e setting
gs necess
sary for four
diffferent functioning m
modes:
MODE
Rettardation
on the VSP
Input
pola
arization
Outp
put
Polariz
zation
Ra
adial pol.
= 
Vertt. linear
Radial
Azim
muthal pol.
=
Horiz. linear
Azimu
uthal
Spiral phase
e
=
Circular
lefft/right
Circu
ular
right//left
N
Neutral
 =n*2
any
sam
me
-3-
User Manual Variable Spiral Plate
ARCoptix Switzerland – www.arcoptix.com
For the first three modes described in the table above the retardation
of the VSP needs to be set to half of the wavelength (or pi). This can
be obtained by simply adjusting the bias on the VSP.
Notice that they are many other possible working modes (spiral,
lemon,…) can be obtain by combining different retardations and input
polarizations. A detailed description of this can be found in
Operating the device:
1) Put the clear aperture of the VSP in the path of your laser beam.
Notice that there is no specific entrance or output aperture, the
device is perfectly symmetric.
2) Make sure that the laser beam is perfectly perpendicular to the
VQP input plane. A small tilt may already cause some
deformations and unwanted effects.
3) The Spiral plate should preferably be attached a x-y positioning
system in order to center the VSP with the laser beam. Notice
that the center of the optical axis structure may not coincide
perfectly
with
the
center
of
the
clear
aperture.
4) Connect the cable of the VSP to one on the inputs of the LC
driver (see electrical driving section).
5) Make sure that the input beam polarization is either Horizontal or
vertical.
6) Vary the bias applied on the VSP (with the LC driver software)
and observe the behaviour of the intensity pattern of the output
beam in the far field (at least 1m for output plane of the VSP).
7) For a phase retardation close to Pi one should observe in the far
field a little hole the laser beam intensity. The hole becomes the
most visible when the phase retardation is Pi.
8) Adjust the bias on the VSP to obtain the most visible hole.
-4-
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
Pic
cture of a intesity distributio
on with th
he to lobe
es
9 Additio
9)
onal contrrol can be
e made kn
now with an extra polarizer
placed just afte
er the VSP
P. A double lobes in
ntensity p
profile sho
ould
be obttained.
1
10) By ro
otating th
he polariz
zer the two lobe
es should
d also ro
otate.
1
11) Adjustt the bias
s on the VSP to obtain
o
the
e most sy
ymmetric
c and
of Pi.
best de
efines lob
bes. This should co
orrespond
d to a reta
ardation o
1
12) Also a
adjust the
e x-y position of th
he Spiral p
plate in o
order to ce
enter
the tw
wo lobes with the beam.
your setu
up is aligned and adjuste
ed in ph
hase (witth pi
Now y
retarda
ation). Th
he differe
ent workin
ng modes
s (as desc
cribed be
efore)
can be
e obtained
d by eithe
er entering the VSP
P with circ
cular left//right
or vertt./Horiz. L
Linear polarization.
Ele
ectrical Driving
g:
1) Connect the LC
C driver tto a PC w
with wind
dows operating sy
ystem
2 Connect the Lem
2)
mo conne
ector attached the VSP to on
ne of the
inputs (out1 or out2) of the LC drriver.
3 The re
3)
etardation
n on the VSP
V
can k
know be a
adjusted b
by varying
g the
outputt voltage of the correspondiing output of the L
LC driver. (see
operattion of the
e lC driver section))
Notice tha
N
at the rettardation of the VS
SP is not linear witth the voltage.
T
The
large
est variatio
on of the retardation is betw
ween 1 an
nd 3,5 Vo
olt.
F
From
0 to
o 1 V therre is almo
ost no varriation of tthe retard
dation.
-5-
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
Op
peration
n of the LC Driv
ver (see
e also lc
c driverr manua
al):
Beffore using
g the LC driver, m
make sure
e that the
e program
m and drrivers
are correctly
y installed
d on your computter. Notice
e that forr the moment
ers from the same computerr.
nott possible to drive ttwo differrent drive
Whe
en starting the progra
am you gett the follow
wing window
w (non-trigg
gered verssion):
The
e output controls (outp
put 1 and output
o
2) co
orrespond to the two outputs on
n the
LC Driver.
One
e can contrrol output vvia
- The rotatio
on button (rough
(
conttrol)
- The edit w
window justt below the
e rotation button.
nd down arrrows of the keyboard
d
- With up an
ouse (if anyy present)
- With the sscroll wheel of the mo
ase of the variable ph
hase
Forr precise sccanning of the voltage (if adjustting the pha
reta
arder or ph
hase compe
ensator forr example) with the up and dow
wn arrows o
or the
mouse scroll w
wheel, one
e can set th
he precisio
on of a sing
gle step witth the volta
age
resolution selection.
ntrolled by the arrowss or the scrroll wheel o
of the mousse:
To select the output con
orresponding rotation
n button.
clicck on the co
The
e maximum
m precision of the LC Driver is 1m
mV. And th
he maximu
um amplitud
de of
the square sig
gnal is abou
ut ±8.8V. T
The constant frequenccy of the output volta
age is
1.6 KHz
ox “same a
as Output 1 is checke
ed then botth outputs a
are identica
al
If the check bo
c
b
by output 1
1.
and are both controlled
minate the program by
b simply p
pressing the red crosss button in the upper right
Term
corn
ner (as usu
ual for wind
dows progrrams).
-6-
User Manual Variable Spiral Plate
ARCoptix Switzerland – www.arcoptix.com
Troubleshooting:
The following question may help to solve eventual functioning
problems of VSP:
1) Is the input polarization either horizontal, vertical or circular?
2) Is the VSP well aligned and perfectly perpendicular to the laser
beam?
3) Do you see two lobes in the intensity profile of your laser beam
when placing a polarizer after the VSP?
4) Do you see any change in the ouput intensity profile when changing
the applied bias on the VSP?
5) Does the LC driver ouput change the voltage when changing the
voltage in the software (check with a multimeter)?
6) Is the VSP correctly centered with the laser beam? What happens if
the VSP is moved in the x-y plane?
-7-
Use
er Manual V
Variable Sp
piral Plate
ARC
Coptix Swittzerland – www.arcop
ptix.com
-8-