art mot e
2009 E
Fiber Port Clusters for Magneto
Optical
Traps
Fiber-coupled beam delivery systems.
­Postcard size replaces 1 m2 bread­board
constructions. Assem­bled with fiber ­optic
components from Schäfter+Kirchhoff.
The cooling of an atomic cloud
down to a very few micro-Kelvin
in an atomic trap brings thermal
motion to a virtual standstill. To
reach micro-Kelvin temperatures,
a magneto-optical trap produced
from magnetic fields and laser radiation is used (Fig. 1)
A magneto-optical trap (MOT) is
often used for the initial cooling
phase before other super-cooling
mechanisms bring the temperature
down for Bose-Einstein condensation.
Experimental reproducibility when using
cold atoms requires an extremely stable
setup. This is achieved most effectively
by using polarization-main­tain­ing fiber
optics to mechanically decouple the
­vibration and temperature sensitive
optics from the trap.
Efficiency and reproducibility are
the fundamental charac­teristics of
our multi­mo­du­lar fiber delivery system (Fig. 2).
The optical scheme of the fiber port cluster (Fig.
3) shows that the 2 input ports are connected
to their frequency-shifted sources by sing­lemode fibers which maintain laser polarity. The
2 light sources are then combined and are split
between the 6 output ports as required. At each
out­put port, the polarization of both frequencies
is orientated in parallel and coupled into a
polarization-maintaining singlemode fiber.
The six output fibers and their fiber collimators,
optionally fitted with integrated quarter-wave
retarders, are attached to the MOT (Fig. 1).
The delivered fiber port cluster is assembled
and pre-aligned, together with highly detailed
manuals if further adjustment is desired.
The coupling of laser radiation into singlemode
fibers and their correct orientation with the axes
of polarization are performed using computerassisted beam and polarization analysis. This
automation substantially reduces the alignment
effort, especially in com­parison with a more
conventional breadboard configuration.
Laser beam couplers, splitters and combiners,
polarizers and retardation optics can be mixed
together using “multicubes“ to produce almost
any desired system in a postcard size (Fig. 5).
The 1 m2 and bigger breadboard arrangements
are totally superseded by this fully integrated,
ultra-compact, transportable sealed system.
Modular components from Schäfter+Kirchhoff
Atomictrap_Art_ForFiberOptics_09_E.indd • Page 40
1
Laser beam coupler
for Laser IN and OUT
3
Fiber cables: singlemode
and pola­ri­zation-maintaining
2
“multicubes“
with base plate and rods
4
1
6
2
5
3
4
Figure 1: Schematic layout
of a magneto-optical trap
(MOT). 1 - 6 are fiber
­collimators, type 60FC-Q…,
see Fig. 6, Page 40
Custom Design
Innsbruck Configuration
Input: 2 ports for linearly ­polarized
­beams, frequency ­shifted.
Output: 6 ports, both input beams superimposed, with parallel ­orientation
­of linear polarization
Figure 2: Fiber port cluster 2 →6. Manufactured for IQOQI, Austrian Academy of Sciences
Designed
for Isotope Wavelength
Sr
461
Yb
556
Na
589
Li
671
Sr
689
Na
760
K
767
Rb
780
Kr
811
Cs
852
He
1083
Figure 3:
Optical scheme
of configuration
in Figure 2
In global use:
Austria
France
Germany
Italy
U.K.
USA
VR China
India
Figure 4: Example Application
Laser IN
No. 2
Fiber Port Clusters
in Micro-Gravity Experiments
Fiber 1
OUT
Laser IN
No. 1
Figure obtained from
arXiv:­0705.2922v2 [physics.atom-ph]
Fiber 2
OUT
The compactness and ruggedness of the fiber
Fiber 3
port clusters from S
­ chäfter+Kirchhoff have
OUT been
rigorously demonstrated in the demanding microgravity environment
flights.
Fiber 6 of parabolic Fiber
4
The atom-opticsOUT
rack of the atomOUT
interfero­meter
was designed by the French Institut d’Optique in
Fiber 5
Palaiseau and collaborators
OUT (arXiv:­0705.2922v2
[physics.atom-ph]).
1
2
The fiber port ­cluster has fully proven its robust­ness
against the extremes of vibration, pressure and
temperature in these parabolic flights
1 vacuum chamber
2 fiber collimator
3 fiber port cluster
Fiber collimator
with integral λ/4 retarder
3
40
Kieler Str. 212, 22525 Hamburg, Germany
•
Tel: +49 40 85 39 97-0
•
Fax: +49 40 85 39 97-79
•
[email protected]
•
www.SuKHamburg.de
2009 E
Fiber Port Cluster 1→4
Alignment and Beam Analysis using CCD cameras
Fiber port clusters made by Schäfter+Kirchhoff take the optical power
carried by a polarization-maintaining single­mode fiber and split it into
multiple polarization-maintaining singlemode fibers with high ­efficiency.
Additionally, numerous bulk-optical components can be integrated and,
by use of actuating elements, the splitting ratio of the resultant beams
can be adjusted ­arbitrarily and reproducibly.
In principle, the cluster can be cascaded endlessly in order to generate
any desirable number of outgoing ports. (for a fiber port cluster 2→6,
see page 40).
The quality of a fiber port cluster is determined by its polarization maintenance ability and the power efficiency of the coupled laser radiation.
The plano-optics, beam splitters and combiners need a highly precise
alignment. Even the smallest deviations of the beam from the optical
axis can lead to obstruction, diffraction or aberration and, thus, to reduced coupling efficiency at the outgoing ports.
The quality and precision of the connection between a laser beam
coupler and its polarization-maintaining singlemode fiber cable is
­absolutely critical for the functionality of a fiber port cluster.
The laser beam couplers made by Schäfter+Kirchhoff (type 60SMS-...,
0 Laser In
1 Power Monitor
2 3x Retardation Optics
3.1 - 3.3 Beam Splitter
4.1 - 4.4 Laser Out
1
3.2
3.1
4.1
2
4.2
3.3
0
Beam Profile 1 Beam Profile 2 Beam Profile 3
4.3
4.4
Figure 5: Fiber port cluster 1→4 with integrated power monitor, retardation
optics, beam splitter and laser beam couplers for singlemode fibers.
Figure 7: Fiber port cluster 1→4 with cameras attached to ports
for the monitoring of the alignment process.
Figure 5 depicts a 1→4 cluster with the incoming port 0 connected via
a ­laser beam coupler to a polarizer. The polarizer defines the ­incoming
polarization state even in the case of a source with weak ­polarization
stability. A 98/1 beam splitter combined with a ­photo-detector acts as
a power monitor­ 1 . The majority of the radiation passes the first halfwave retardation plate 2 that is mounted in a self-locking bearing free
from backlash. The half-wave retarder rotates the axis of the incoming
polarization in the freely adjustable range from 0:100 to 100:0 and so
defines the splitting ratio reaching the polarization beam splitter 3.1 .
Rotatable half-wave retarders and a further set of polarization beam
­splitters 3.2 and 3.3 are placed at each outgoing port to divide the ­beams
yet again.
There are now a total of four beams passing through the laser beam
couplers 4.1 - 4.4 which act as the outgoing ports.
Polarizing beam splitters produce a high degree of polarization in the
non-deflected beam (typically 1:10,000). Conversely, the low degree of
­polarization in the deflected beam (typically 1:20) necessitates the use
of small ­additional polarizers to increase the degree of polarization of
these deflected beams. Polarization-maintaining singlemode fibers are
connected to the outgoing ports.
Figure 6 depicts the system components and the optical path arrangement schematically.
Fig. 10B) are provided for a large variety of wavelengths and beam diameters. The tilt adjustment and inner focussing mechanism provide all
of the degrees of freedom needed for alignment, while being compact
and insensitive to unintentional displacement. The inclined coupling
axis, provided by the fiber connectors of the FC-APC type, ensures that
back-reflections into the optical path and laser source are avoided.
The laser beam couplers have two different tasks in a fiber port cluster.
One is to collimate the radiation that is emitted from the single­mode
fiber at the incoming port. The other is to couple the split radiation into
the singlemode fibers at the outgoing ports.
The “multicube” system from Schäfter+Kirchhoff is the integrating
element for the fiber port cluster (Fig. 6) and provides a warp-resistant
assembly of laser beam couplers.
To maintain optimal polarization and beam profile characteristics, an
expert and careful selection of the materials is required. Additionally,
we take the utmost care to obviate mechanical stress during fiber
termination in the coupler. Polarization-maintaining singlemode fiber
cables are provided by Schäfter+Kirchhoff in a variety of wavelengths.
Figure 7 shows the fiber port cluster depicted in Figure 5 during alignment adjustment. All beam splitters and half-wave retarders are already
mounted and, instead of laser beam couplers, CCD cameras have been
attached to the outgoing ports 4.1 to 4.3 .
The laser power incident on the sensitive cameras has to be reduced
during the adjustment process. A vignetting aperture is produced by
rotating the half-wave retardation plates, restricting the collimated laser
beam to 5% of its full strength. The computer-assisted image analyzer
determines the quality of the laser beam being coupled and uses a
two-dimensional Gaussian to center the beam.
The screenshot (inset, Fig. 7) reveals the position of the collimated
beam at the sensor of port 4.1 that originates from the input port 0 .
The left-hand beam profile shows that the beam is asymmetrically
obstructed and, so, the laser beam coupler at port 0 , employed as
collimator, has to be realigned using its tilt adjustor. The middle and
right-hand beam profiles show the beam aligned correctly.
The beam positions at the outgoing ports 4.2 and 4.3 are now aligned
by tilting their respective beam splitters 3.1 and 3.2 . Finally, the beam
position at port 4.4 is aligned by tilting its beam splitter 3.3 .
After all adjustments have been completed, the cameras are replaced
by laser beam couplers. The align­ment of the fiber cable at the outgoing
ports is performed using the tilt adjustment and, when needed, by fineadjustment of the focus for each of the four couplers.
The fully aligned fiber port cluster is now stable for transport and can
be utilized without any requirement for a space-consuming optical
breadboard.
Configuration Scheme
1
2
3
4
5
A
B
C
D
D1
E
F
Atomictrap_Art_ForFiberOptics_09_E.indd • Page 41
G
Base Plate 48MB-19.5
“multicube“ 48MB-SO-19.5
3x “multicube“ 48MB-LT-19.5
Spacer 48S-19.5
Cap 48C-19.5
C
1x Laser Beam Coupler
60 SMS (laser IN)
3x Polarizer 48PM-S-...
Beam Splitter 98/1
48BS-19.5...
Photo Detector 48PD
Adapter 48MB-19.5 AC
Retarder 48WP-C...
3x Polarization
Beam Splitter 48BC
4x Laser Beam Coupler
A B
60 SMS (laser OUT)
F
D
4
E
F
3
G
E
D1
B
E
G
F
3
3
5
2
1
G
B
G
Optical Scheme
4.1
­to
4.3
Figure 6: System components for the fiber port cluster shown in Figure 5.
41
Kieler Str. 212, 22525 Hamburg, Germany
•
Tel: +49 40 85 39 97-0
•
Fax: +49 40 85 39 97-79
•
[email protected]
•
www.SuKHamburg.de
2009 E
Polarization Maintainance and Analysis
Figure 10: System Components
Laser beam propagation in polarization-maintaining singlemode ­fibers
enables countless modular and compact systems to be devised for
scientific experimentation and measurements.
The advantages of the substantial flexibility in spatial arrangements are
complemented by the high intensity and Gaussian distribution of the
beam emanating from our singlemode fibers. The potential problem of
polarization fluctuations are totally obviated using modular fiber-optic
system components with specially designed and precise adjustment
mechanisms.
Optical system building blocks for self-assemby and alignment
For examples and applications, see Figures 4 and 5
A
C
The laser beam couplers from Schäfter+Kirchhoff
efficiently launch a collimated laser beam into a
polarization-maintaining singlemode fiber with a
1
mode field diameter as low as 2.5 µm.
Features include:
• Integral coupling lens, from f 2.7 to 18 mm, for
3
highly efficient matching of beam geometries.
1
5
•F
ocus adjustment of integral lens 1 with positive
locking by ‘non-contact’ grub screws 2 .
4
•A
tilt adjustment 3 with carbide inlay for lateral
positioning of the beam focus on the mode field
3
2
of a singlemode fiber in the FC connector 4 .
• FC connectors with either the inclined coupling
axis of the FC-APC type or the coaxial FC-PC type (optionally ST, DIN-AVIO,
F-SMA) couple the singlemode fiber cable. The inclined coupling axis of the
FC-APC connector prevents back-reflections into the laser beam source.
•FC connector fiber cable ferrule positively located with M1.6 grub screw.
•Tightly fitting cylinder system 5 Ø 19.5 mm with a circular V-groove (to align
polarization axis) containing an O-ring (for ‘non-contact’ locking screws).
B
A
“multicubes“ 48MC-... The “multicubes“ 1 and 2 form an integrated
system for assembling laser beam couplers, splitters and combiners B - H . Multiple function
5 6
7
3
components, base plates 3 and mounting plates
4 can all be incorporated, producing a universal
mounting kit. All components have 6 mm precision
2
borings placed in a standardized 30 mm micro1
bench layout.
Hardened steel rods 5 with super-finish surfaces
are used to mount and connect the single modules.
With axially mounted grub screws 6 (M3/WS 1.5
hex allen key), the rods lock the components into
a solid warp-resistant unit. Spacer 7 seals the
4
assembly from dust ingress and light egress.
Laser beam coupler
60SMS-...
The inclined fiber coupling axis
B
Figure 8: Polarization measurement. A SK9782 VIS/NIR Analyzer,
B polarization-maintaining singlemode fiber, C display screenshot
Polarization fluctuations in the fiber output is considered to hinder the
effective replacement of a bulk-optical bread­board by a modern fiberoptical system. While the use of inappropriate polarization-maintaining
singlemode fibers can cause polarization fluctuations, it is usually the
inadequate and suboptimal alignment of the polarization axis with the
axes of the polarization-maintaining ­fibers that causes these problems.
Modular fiber-optical systems also eliminate the need for elaborate
opto-mechanical ­spatial filtering, with its resultant beam intensity loss.
Polarization-maintaining singlemode fibers maintain the oscillation of
the electromagnetic field in two orthogonal axes: one with fast and the
other with slower light-propagating properties. When linearly polarized
radiation is not coupled exactly with one of these axes, the beam is
transformed into an elliptical polarization state because of the dif­ferent
light propagation speeds in the two axes. Temperature changes, fiber
bending and even vibration can affect the ­polari­zation state obtained
at the end of the fiber.
The SK9782-VIS/NIR polarization analyzer (Fig. 8) is a plug&play USB
device developed for ease of use by practitioners in the field. Alignment
measurements and verification are achieved more rapidly than with the
more time-­­con­suming conventional methods.
The state of polarization (SOP) is displayed in real-time as the Stokes
parameter or as a point on a ­Poincaré sphere with azimuth and inclination angle. By maximising a bar display of the extinction ratio, the
axial alignment of a polarization-maintaining fiber can be achieved.
C
Polarizer 48PM-S
D
Beam Splitter 98/1
48BS-CC-PA
E
Photo Diode 48PD
for Power Monitoring
F
Polarization Beam
Splitter PM 48PM-CC
G
λ ⁄2 Retardation Optics
48WP-CA
•Polarisation: linear 10,000:1
•Aperture Ø 3.5 mm
•Mounted in self-locking bearing with rotatable axis
•Application: Suppression of the fractional
p-polari­zation (approx. 5%) falsely deflected
by the polarization beam splitter
•Beam splitting ratio 98:1
•Aperture Ø 10 mm
•1 mm fused silica plate
•Optimized for 45° incident angle and
p-polarization
•Wedge angle of the plate: 0.33° to obviate the
formation of etalons
•Si-diode or Ge-diode
•Aperture Ø 3 mm
•Electrical: BNC socket
•Mechanical: circular V-groove Ø 19.5 mm
to fit adapter 48MB-19.5AC
Polarization beam splitter cube
• Polarization: linear
• In transmission p-polarized, extinction 10,000:1
• In reflection (90° deflection) s-polarized 20:1
• Aperture Ø 6 mm
•S
plitting ratio depending on degree and state of
polarization of the incoming radiation
•Low order quartz retarders
•Aperture Ø 5 mm
•Self-locking adjustment flange
•Rotary axis inclined 3° in order to avoid
back reflection and etalons
•Application: Rotating the axis of linearly polarized
laser radiation
α
H
Dichroic Beam
Combiner 48BC-CC
I
PM Fiber Cable PMC-...
with inclined FC-APC
Connectors
Two laser beams of different wavelengths are
combined to produce a coaxial beam with equal
states of polarization
•1mm fused silica with a wavelength-dependent
coating optimized for 45° incidence
•0.33° plate wedge angle to obviate etalon
formation
Atomictrap_Art_ForFiberOptics_09_E.indd • Page 42
Figure 9: Polarization analyzer software: measuring of extinction ratios
Left When linearly polarized radiation is not coupled exactly to one of the
fiber polarization axes then the state of polarization can fluctuate with
temperature or fiber position. The misaligned polarization axis, indicated by
a ring around H, is realigned by maximizing the extinction ratio using a
continuously updated red-green bar display (inset).
Right A second measurement of the extinction ratio indicates that the
polarization state fluctuation is reduced, the ring is reduced to a small spot
at H on the equator, denoting a stable linear state of polarization.
Singlemode fibers are characterized by their
numerical aperture (NA), the mode field diameter
(MFD) and the cut-off wavelength λCT.
•Polarization-maintaining singlemode fibers with
polarization axis indicated by alignment marker
•Mode field diameter 2.5 – 10 µm
•Wavelength 370 – 1750 nm, usable spectral
bandwidth typically λCT to approx. 1.3 x λCT
The final extinction ratio is displayed on a linear and a logarithmic scale.
42
Kieler Str. 212, 22525 Hamburg, Germany
•
Tel: +49 40 85 39 97-0
•
Fax: +49 40 85 39 97-79
•
[email protected]
•
www.SuKHamburg.de
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