Waveguide image-slicers for ultrahigh resolution spectroscopy

Abstract

Waveguide image-slicer prototypes with resolutions up to 310.000 for the fiber fed PEPSI echelle spectrograph at the LBT and single waveguide thicknesses of down to 30 pin have been manufactured. The waveguides were macroscopically prepared, stacked Lip to an order of 7 and thinned back to square stack cross sections. A high filling ratio was achieved by realizing homogenous adhesive gaps of 4.6 mu m, using index matching adhesives for TIR within the waveguides. The image-slicer stacks can be used in immersion mode and are miniaturized to be implemented in a set of Four, measurements indicate an overall efficiency of above 80% for them.

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Waveguide Image-Slicers for ultrahigh resolution spectroscopy

Erik Beckert*a, Klaus G. Strassmeierb, Manfred Wocheb, Ramona Eberhardta, Andreas
Tünnermanna, Michael Andersenc
aFraunhofer-Institute for Applied Optics and Precision Engineering, Albert-Einstein-Strasse 7,
D-07745 Jena, Germany;
bAstrophysical Institute Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany;
cUniversity of Copenhagen, Juliane Mariesvej 30, DK-2100 Kobenhavn, Denmark
ABSTRACT
Waveguide image-slicer prototypes with resolutions up to 310.000 for the fiber fed PEPSI echelle spectrograph at the
LBT and single waveguide thicknesses of down to 30 µm have been manufactured. The waveguides were
macroscopically prepared, stacked up to an order of 7 and thinned back to square stack cross sections. A high filling ratio
was achieved by realizing homogenous adhesive gaps of 4.6 µm, using index matching adhesives for TIR within the
waveguides. The image-slicer stacks can be used in immersion mode and are miniaturized to be implemented in a set of
four, measurements indicate an overall efficiency of above 80 % for them.
Keywords: Image-slicers, Spectrographs, LBT

1. INTRODUCTION
Potsdam Echelle Polarimetric and Spectroscopic Instrument PEPSI [1] is the fiber fed high-resolution echelle spectro-
graph device for the 11.8 m Large Binocular Telescope (LBT) on Mount Graham in Arizona. It was designed to utilize
the two 8.4 m apertures of the LBT in a unique, combined spectropolarimetric mode. With independent, but identical
polarimeters it enables simultaneous observation of circularly and linearly polarized light with high spectral and
temporal resolution, while non-polarized light is injected to the standby spectrograph (fig. 1) by two permanent focal
stations. The fiber-fed optical integral-light spectroscopy device works in a wavelength range from 390 nm to 1050 nm
with spectral resolving powers, R = λ⁄∆λ, of 40000 (2.2" aperture), 130000 (1.5" aperture) and 310000 (0.75" aperture).

Fig. 1. Spectrograph optical design [1], based on exchangeable, optionally immersed image-slicers (located in the circle) for
100 µm, 200 µm and 300 µm fibers. The grating is a 80 x 20 cm2 R4 echelle “F” in Littrow mode, a Maksutov main
collimator “M1” is used in double pass while an immersed field lens with two dichroids for a blue (390 nm to 570 nm)
and a red (570 nm to 1050 nm) arm is located in an intermediate focus. Each arm consists of a blue or red optimized
Maksutov transfer collimator (“M2” and “M3”) , a set of three cross dispersers (combinations of a volume phase
holographic grating and two prisms), an optical 20 cm f/3 camera with 11 lenses and a monolithic 10k CCD camera
(“CAM B” and “CAM R”).
*erik.beckert@iof.fraunhofer.de; phone +493641807-338; fax +493641807-604; www.iof.fraunhofer.de
Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, edited by Eli Atad-Ettedgui,
Dietrich Lemke, Proc. of SPIE Vol. 7018, 70182J, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.787756
Proc. of SPIE Vol. 7018 70182J-1
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The requirement of reaching a high efficiency for the high resolution modes (R > 100000) on the large telescope of the
LBT implies that the use of image-slicers becomes important, if not even mandatory considering the fiber coupled
spectrograph. Alternatives such as adaptive optics have been found [2] to yield only marginal gain in efficiency due to
limited correction in the visible part of the spectrum. In contrast, feeding light to the spectrograph by fibers has the
advantage that the spatial profile on the detector can be made stable and that the spectrograph itself can be used in a
stable environment [3]. In order to reach an acceptable combination of throughput and resolution it is necessary to use
large core diameter fibers together with image-slicers. These image-slicers must have a high efficiency and be near
parafocal in order to retain the required resolution, which is not feasible in case of a larger number of slices for the
classical Bowen-Walraven image-slicer design. One alternative, the waveguide image-slicer [4], has been adopted for
PEPSI.

2. IMAGE-SLICER CONFIGURATION FOR PEPSI
In order to combine acceptable throughput and resolution the use of large core fibers with core diameters of 300 µm,
200 µm and 100 µm and the F-ratio of the corresponding transfer optics (Maksutov collimator) determine possible
layouts for the image-slicers, which are linked to each core diameter by their individual slice geometries. Fig. 2 shows
the layout in principle.

Fig. 2. Waveguide image-slicer layout, shown in principle for a number of seven individual slices. The entrance window is a
square, its geometry results from the required number of slices, their uniform thickness and the adhesive gaps between
the slices that ensure total internal reflection TIR within each slice for a respective incoupling NA. The back end of
each slice contains a polished, protected Ag coated 45° mirror that reflects light towards the output window. In order to
arrange the output windows in a row with certain offsets (ca. 60 µm) each slice is required to have a different length,
defined by a constant pitch.
The complete instrument requires a set of 4x4 image-slicers. These will be used for the R = 40000, 130000 and 310000
non-polarimetric modes and for the R = 130000 polarimetric mode. Each mode uses four fibers (one object and one sky,
or two polarization states from each of the two telescopes) and requires individual image-slicers for each fiber. Limiting
factors for the image-slicer layout are the availability of thin glass preforms for individual slices (thickness 30 µm and
above, resulting in an F-ratio of ≈F/5 and higher) and the achievable F-ratio degradation. While it was initially believed
that sufficient throughput is not possible for F/14.5 (Maksutov main collimator F-ratio) later experiments [3] using test
setups for the image-slicers suggested that F-ratios for the image-slicers of >F/5 might possibly be adopted.
45° mirrors,
protected Ag
slice lengths l
d x d µm2
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R = 40000
3
500 µm
1506 x 1506 µm2
1506 x 500 µm2
25 mm
R = 130000
5
175 µm
887 x 887 µm2
887 x 175 µm2
25 mm
R = 310000
7
70 µm
508 x 508 µm2
508 x 70 µm2
25 mm
number of slices
slice thickness
entrance window
single output window
length of image-slicer (= longest slice)
As indicated several restrictions apply to the layout of the image-slicers. Using thin glass preforms made of Schott®
Borosilicate glass D263T (n = 1.5231 @ 587.6 nm), initially developed for LCD-panels, and concerning the manu-
facturing technology described in chapter 3 the slice thickness to be used is determined by available preforms. D263T is
available in thicknesses of 30 µm, 50 µm, 70 µm, 100 µm, 145 µm, 175 µm, 210 µm and above. To ensure TIR within
the individual slices for the given incoupling NA the slices need to be surrounded by a medium with n < 1.47. It was
found that an UV-curable adhesive with n = 1.451 (Epo-Tek OG134) matches this requirement and thus not only fulfills
boundary condition for TIR but also enables to fix the stacked slices in order to assemble the image-slicer.
Boundary condition for the stacking technology is to create adhesive layers with a sufficient thickness ( > 1µm) that
covers penetration depth for the required wavelength range while maintaining an as small and homogeneous adhesive
gap as possible in order to reach a high fill factor of waveguiding sections within the entrance window. OG134, being a
low-viscous adhesive, therefore was combined with spacing elements (micro-pearls from Sekisui Corp. [5], sphere
diameters 3 µm) to carry out an investigation for adhesive gap thickness that is feasible for the manufacturing of
waveguide image-slicers. Fig. 3 shows such a typical adhesive gap between two slices made of D263T. During the
manufacturing of test-slicers it was determined that the achievable adhesive gap and its reproducibility is 4.6±1 µm, with
corresponding wedge errors of < 40“ for 15 mm long slices. In terms of 30 µm and 100 µm slices this leads to fill factors
of 88 % and 96 %.

Fig. 3. Test-slicer with 100 µm individual slices (left panel) and close-up (right panel) on a typical OG134 adhesive gap
with a thickness of 3.87 µm.
Investigations during the manufacturing of test-slicers also led to the conclusion that the required homogeneous and
small adhesive gap can only be maintained for slice thickness to slice length aspect ratios of < 300 to 1000, at least for
the current state of manufacturing technology (chapter 3). This limits the overall length for waveguide image-slicer
(determined by the longest single slice) to approx. 30 mm and thus results in major restrictions for the “scrambling”
behavior of the waveguiding slices. Depending on the given incoupling NA on slice lengths less than 30 mm only a few,
if not only one TIR happens to the propagating beam, rendering any beam scrambling almost non-existent and thus
making the spectrograph detector focal plane illumination dependant from the seeing illumination. Future work will have
to address either to increase slice lengths dramatically (which, on the other hand, would have implications on a compact
design) or to implement a hybrid scrambling element in front of the entrance window, ideally a glass cube with a cross
section matching that of the image-slicer. The current setup for the PEPSI image-slicers without scrambling functionality
is summarized in table 1.
Table. 1. Image-slicer layouts for each resolution mode.




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3. MANUFACTURING TECHNOLOGY
Since thickness and width of the individual slices of each image-slicer configuration is in the sub-millimeter range it
becomes inefficient to build up the image-slicer assembly from completely finished individual parts. Instead, a
manufacturing approach has been successfully investigated that makes favor of macroscopically prepared slices. These
macroscopic slices can be assembled with the required accuracy, before post processing reduces the slicer assembly to its
final dimensions. Fig. 4 shows the CAD-model of the final image-slicer design. The stacked slices are surrounded by
cover plates to create mechanical stability, the top cover plate leaves open all output windows for air or immersion
outcoupling from the image-slicer into the Maksutov main collimator.

Fig. 4. Complete image-slicer assembly for a three slice configuration.
The complete manufacturing process chain contains the following steps:
•
Slice preparation: Preforms of the desired slice thickness are available in substrate scale format, e.g. 100 x
100 mm2, with sheet surfaces already manufactured in optical quality and thus being sufficient for TIR. From
these substrates slices with dimensions in the millimeter range, which makes them easy to handle by means of
conventional vacuum grippers, are separated using a conventional wafer saw. The slice geometry already
consists of the rough shape for the 45° mirror. Temporarily stacking these macroscopic slices together enables
for polishing the end faces of the mirrors, resulting in optical quality of these surfaces for reflection, too. This
polishing process is followed by DC magnetron sputtering of the protected backside Ag mirror layer onto the
polished 45° end face of the slice. Fig. 5 shows the measured reflectance of the backside Ag-mirror,
demonstrating a good bandwidth behavior for the required wavelength range.
•
Slice stacking: The most challenging task for stacking the macroscopic slices is to arrange the mirror ends of
the individual slices in parallel and with the desired pitch (output window width plus offset) while maintaining a
small and homogeneous adhesive gap between the slices. A FINEPLACER® device commonly used in electro-
nics manufacturing for a micron precise assembly of chips on printed circuit boards [6] was adapted to fulfill
this task. Planar gripping of single slices, microscopically guided parallel and pitch alignment of the mirror end
faces by means of fine translation and rotation movement of the slice to be stacked and finally cardanic guidan-
ce of the planar gripper in order to ensure parallel adhesive gaps can be integrated the FINEPLACER. In this
way a slice stack is assembled onto one cover plate (fig. 6) and completed on top by the second cover plate.
•
Post processing: The stacked assembly can be grinded and polished down to its required final dimensions now
before the third and fourth cover plates are applied, creating the TIR surfaces on the remaining sides of the sli-
ces. An advantage of this post processing is that it can take into account the result of the stacking process, in
particular the realized adhesive gaps and consequently the overall height of the slice stack. For the grinding and
polishing process it now becomes possible to create a best fit square entrance window with respect to the manu-
factured stack height.
Entrance window
3 output windows
4 cover plates
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Fig. 5. Measured reflectance (lower wavelength region) of the used backside Ag mirror


Fig. 6. View onto the partially assembled stack (two slices on base cover plate) of individual, macroscopic slices.

4. PROTOTYPE MANUFACTURING RESULTS
Several prototypes of image-slicers in different configurations have been manufactured in order to investigate the
manufacturing technologies as well as to establish a reproducible process chain, since the final spectrograph design
requires several sets of image-slicers with the same performance. For evaluation purposes the image-slicers were
integrated into an immersion tank that contains a planar window for outcoupling and an adjustable ball lens / fiber ferrule
assembly for incoupling from the respective fiber. Fig. 7 shows the mechanical layout and a view on the outcoupling
window of the immersion tank during illumination tests using laser light.
Backside mirror
Wavelenght in nm
% Reflectance
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Fig. 7. CAD-Layout (left panel) of the immersion tank for a single image-slicer with an adjustable fiber ferrule / ball lens
assembly that is mounted onto the immersion tank after alignment. The output window is a planar element, with the
possibility for integrating a cylindrical lens for refocusing or a prism for beam twisting. In the right panel the output
window of the immersion tank is shown with the output windows of a three slice image-slicer being illuminated by a
laser.

Of particular interest was to demonstrate that it is possible to handle even the thinnest available preforms (30 µm) of
D263T glass and to stack them onto each other up to a number of seven to ten. Also the achievable accuracies for
adhesive gap thickness and homogeneity, fill factor and output window arrangement were under investigation. The
results can be summarized as following:
•
Preform thickness deviation: Tolerance for the D263T preform thickness depends on the actually chosen
thickness and is given by 30±8 µm, 50±10 µm, 70±10 µm, 100±15 µm, 145±15 µm, 175±15 µm and
210±20 µm from the manufacturer. When only one substrate is used the deviation nevertheless is much less
and was measured to be in the uncertainty range (±3 µm) of the tactile length measurement system (fig. 8). A
low thickness deviation is a necessary prerequisite for a homogeneous image split.

25
26
27
28
29
30
31
32
33
34
35
147 10 13 1619 2225 28 3134 374043 4649 5255 58
Sample
Thickness [Mikrons]

Fig. 8: Thickness deviation of slices (target thickness 30 µm) separated from one single substrate.

•
Adhesive gap deviation: For a three slice configuration the adhesive gap accuracy was found to be 4.6±1 µm
(target value: 3 µm, determined by the chosen micro pearl spacers). Earlier experiences from a seven slice con-
figuration indicate that this is a commonly achievable value.
Fiber ferrule / ball lens
assembly
Immersion tank with
filling equipment
Tank outcoupling
window with image-
slicer beneath
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C,
0

•
Stacking order: Stacks of up to seven slices (30 µm thickness) were demonstrated (fig. 9) as well as stacks of
three slices with single thicknesses of 100 µm and 210 µm. There is no practical limit to the number of slices to
be stacked.


Fig. 9. Entrance window of a seven slice image-slicer with 30 µm thick individual slices. The calculated ideal cross section
of 238 x 238 µm2 is matched in real by 207 x 226 µm2.

•
Entrance window: Despite of the theoretical dimensions of the entrance window the achieved stack height de-
termines the target value for its square dimensions, since both incoupling and outcoupling optics are capable of
adapting to minor changes from the ideal values, while for homogeneous illumination reasons a square window
is favorable. For a three slice configuration it was found that by the final grinding and polishing step the width
of the entrance window can be approximated to the given height of the window within typically ±5 %.
•
Output windows pitch: A certain pitch of the output windows is necessary to arrange their images in row
within the entrance slit of the spectrograph. Part of the pitch is an offset (60 µm) between each slice image that
is required in order to allow a distinction between each image at the detector plane. The given pitch of 696 µm
for a three slice configuration with a single slice thickness of 210 µm was achieved with an accuracy of better
±10 µm (fig. 10).


Fig. 10. Entrance window (left panel) of a 3 slice image-slicer with 210 µm thick individual slices and a cross section of
636 x 636 µm2. The output windows (cross section 210 x 636 µm2, spacing 60 µm) are shown in the right panel.
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5. SUMMARY AND OUTLOOK
Waveguide image-slicers are a promising and miniaturized alternative to classic Bowen-Walraven designs for slit filling
of fiber fed spectrographs. It has been shown that these image-slicers can be manufactured in a reproducible way with
thinnest waveguiding sections of 30 µm height and a slicing order up to seven (with larger numbers possible) while
maintaining a high filling ratio (88 % to 96 %) of the sliced image. First optical tests indicate that an overall efficiency of
> 80 % is achievable with such a waveguide image-slicer layout, while beam scrambling has to be addressed separately.
The current manufacturing limit is the length of the longest individual slice, while slice height is only restricted to
available preform geometries. There is no practical limit to the number of slices.
Several sets of four waveguide image-slicers are to be integrated into the PEPSI spectrograph, according to the fiber
diameter in use. Within one set the image-slicers have to be arranged with a defined pitch in dispersion and cross
dispersion direction in order to arrange all output window images in row within the entrance slit of the spectrograph. It is
planned to use one mount for all four image-slicers that provides precise mechanical stops for an alignment of the
individual assemblies. The mount also serves as a hermetic tank in case immersion mode is chosen for incoupling from
the respective fiber and for outcoupling into the collimator of the spectrograph.


Fig. 11. Planned set of four image-slicers, arranged with a pitch in dispersion direction and perpendicular to it in one
immersion tank.
REFERENCES
[1] Strassmeier, K. G. et al.: PEPSI: The Potsdam Echelle Polarimetric and Spectroscopic Instrument for the LBT.
2008, SPIE, this volume
[2] Sacco, G.; Pallavicini, R.; Spano, P. et al.: Can we use adaptive optics for UHR spectroscopy with PEPSI at the
LBT? In: Ground-based Instrumentation for Astronomy. Proceedings SPIE 5492
[3] Andersen, M. I.; Spano, P.; Woche, M.; Strassmeier, K.G.; Beckert, E.: Optical design of the PEPSI high-resolution
spectrograph at LBT. In: Ground-based Instrumentation for Astronomy. Proceedings SPIE 5492.
[4] Suto, H.; Takami, H.: Waveguide image slicers. Applied Optics 36, 1997, p. 4582.
[5] www.sekisui-corp.com
[6] www.finetech.de

4x Fiber ferrule / ball
lens assemblies
1x Immersion tank
4x image-slicers
Proc. of SPIE Vol. 7018 70182J-8