Near-infrared emission from atmospheric OH radicals is known to severely affect astronomical observations. Until now, only complex dispersive instruments were partially capable of removing this unwanted background, which is composed of hundreds of narrow emission lines. Recent development in photosensitive glass and holographic recording technologies now allow the elaboration of filters with a large number of narrow reflecting bands well matched to OH lines. This technology shows promise for removing many tens of lines in the J, H, and K bands. That would result in a many fold increase in imaging and low resolution signal-to-noise ratio. Filters with 10 lines have been tested and show the appealing possibilities of these new devices

Artigau, Étienne

Blais-Ouellette, Sébastien

Havermeyer, Frank

Matthews, Keith

Moser, Christophe

Psaltis, Demetri

Steckman, Gregory J.

Caltech Authors - Main

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Multi-notch holographic filters for
atmospheric lines suppression
Sebastien  Blais-Ouellette, Etienne  Artigau, Frank
Havermeyer, Keith  Matthews, Christophe  Moser, et al.
Sebastien  Blais-Ouellette, Etienne  Artigau, Frank  Havermeyer, Keith
Matthews, Christophe  Moser, Demetri  Psaltis, Gregory J. Steckman, "Multi-
notch holographic filters for atmospheric lines suppression," Proc. SPIE 5494,
Optical Fabrication, Metrology, and Material Advancements for Telescopes,
(24 September 2004); doi: 10.1117/12.552116
Event: SPIE Astronomical Telescopes + Instrumentation, 2004, Glasgow,
United Kingdom
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Multi-Notch Holographic Filters for Atmospheric Lines
Suppression
Se´bastien Blais-Ouellettea,c, E´tienne Artigaua,d, Frank Havermeyerb, Keith Matthewsc,
Christophe Moserb, Demetri Psaltisc, Gregory J. Steckmanb
a Photon etc. inc., 432 boul. des Prairies, Laval, Canada
b Ondax inc. 850 E. Duarte Road, Monrovia, CA, USA
c California Institute of Technology, 1200 E. California blvd., Pasadena, CA, USA
d Universite´ de Montre´al, C.P. 6128, Succ. centre-ville, Montre´al, Que´bec, Canada
ABSTRACT
Near-infrared emission from atmospheric OH radicals is known to severely aﬀect astronomical observations. Until
now, only complex dispersive instruments were partially capable of removing this unwanted background, which
is composed of hundreds of narrow emission lines. Recent development in photosensitive glass and holographic
recording technologies now allow the elaboration of ﬁlters with a large number of narrow reﬂecting bands well
matched to OH lines. This technology shows promise for removing many tens of lines in the J , H, and K bands.
That would result in a many fold increase in imaging and low resolution signal-to-noise ratio. Filters with 10
lines have been tested and show the appealing possibilities of these new devices.
Keywords: astronomy, ﬁlter, near infrared, oh, atmospheric, telluric, lines suppression
1. INTRODUCTION
Near infrared wavelengths (1 to 2.5 micrometers) are increasingly seen as the best window to observe a wide range
of fascinating astronomical phenomena, from the nascent universe to the birth of stars, to direct observations
of exo-planets. These wavelengths are indeed less prone to interstellar extinction, and are easier to correct for
atmospheric perturbations than their visible counterpart. However, NIR observations greatly suﬀer from night
sky emission mainly caused by excited OH radicals in the atmosphere. These narrow emission lines dominates
the inter-line sky emission by a many orders of magnitude. Removing a few tens of these lines can lead to a
twofold or threefold increase in signal-to-noise ratio (SNR). When keeping in mind that SNR is proportional to
the telescope diameter, such ﬁlters could procure a SNR gain equivalent to a 30 m class telescope when used on
a 8-10 m telescope.
Many attempts to remove this background emission have been made over the last decades. They either rely on
expensive dispersion/recombination techniques1 or on very thick multilayer interference ﬁlters.2 None of these
techniques is capable of eﬃciently ﬁltering the narrow airglow lines without severely aﬀecting the astronomical
signal. On the other hand, the projected ability of thick holographic ﬁlters to remove numerous lines with a
spectral width of less than 0.2-0.3 nm with low transmission loss is unprecedented and will lead to a major
evolution in infrared imaging and low resolution spectroscopy.
2. FILTER DESIGN
To understand the unique value of holographic technology for the fabrication of multi-narrow band ﬁlters, it
helps to compare it with the current state of the art ﬁltering technology of thin ﬁlm interference ﬁlters.
The thin ﬁlm ﬁlters available commercially are limited in thickness to a couple of tens of micrometers. Single
band ﬁlters with up to one hundred layers have been fabricated for the telecommunication industry. The FWHM
of these ﬁlters has reached 0.2 nm.3 The deposition of the layers has to be well controlled for every layer. Today,
120 mm wafers with 100 layers have good homogeneity only on 7% of the surface. Due to the limitation on the
Send correspondence to sbo@photonetc.com
Optical Fabrication, Metrology, and Material Advancements for Telescopes, 
edited by Eli Atad-Ettedgui, Philippe Dierickx, Proceedings of SPIE Vol. 5494 
(SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.552116
554
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Figure 1. Dependence of spectral bandwidth on refractive index modulation amplitude for strong volume gratings. The
apodized gratings are recorded in a 10mm thick photosensitive glass (n=1.486).
number of layers, the type of ﬁlter proﬁles required to suppress narrow band lines of OH radicals is simply not
achievable. Rugate ﬁlters, which use a sinusoidal variation of the index rather than a step index, have been
shown theoretically to provide three passbands of width 25 nm.2 For an eﬀective suppression of the OH lines,
at least 10 bands of width 0.2 nm are required. This requirement places a tough challenge for the state of the
art technology.
In contrast, holographic technology inherently provides many thousands of layers that enable the design of
complex ﬁlter proﬁles that is not possible with thin ﬁlm deposition. Holographic recording in photosensitive
glass material of several millimeter thickness provides a technology platform for controlling the ﬁlter proﬁle with
better than 0.1 nm spectral features and ninety percent eﬃciency. The recent advances in glass material with
low absorption and high index contrast at Ondax are enabling the fabrication of these complex ﬁlters.
A volume hologram is a three-dimensional (3D) grating created by recording the interference pattern of
two mutually coherent light beams, the reference and signal, in a volume medium. The 3D nature of volume
holograms oﬀers some desirable properties such as high diﬀraction eﬃciency (close to 100%), high wavelength
selectivity and the ability to multiplex multiple holograms in the same volume. Two diﬀerent theories are
generally used to study the diﬀraction of light from volume holograms. In the weak grating regime, which means
that higher-order scattering by the hologram is negligible, the Born’s approximation is often used to predict the
diﬀraction eﬃciency of a volume hologram. However, in the strong grating regime, the Born’s approximation
breaks down and coupled wave theory4 must be applied.
The bandwidth of highly diﬀracting holograms is proportional to the refractive index change:
∆λ = λ0
∆n
2n
∆n in turn is determined by the recording exposure time. Therefore, the spectral bandwidth of each hologram
can be controlled by carefully choosing the exposure time during the hologram recording. The diﬀraction
eﬃciencies of ﬁve reﬂection gratings with diﬀerent index modulation depths are plotted in Figure 1. It can be
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Figure 2. Experimental measurement of the spectral response of two reﬂection gratings recorded in a 14 mm thick glass
material.
Figure 3. Stability of the central ﬁlter frequency and bandwidth of the holographic ﬁlter at 150 degree Celsius.
seen that the spectral bandwidth starts to increase as ∆n reaches 5 × 10−5. When ∆n is below 5 × 10−5, the
grating is still in the weak grating regime, and the spectral bandwidth is determined by the grating length. Figure
2 shows the experimental result of two reﬂection gratings recorded in a glass material. This result demonstrates
that the bandwidth of single ﬁlters can be manipulated by adjusting the exposure time.
Another inherent advantage of holographic ﬁlters are their relative robustness. No fragile coating prevent
polishing and regular cleaning. To test their lifetime, the ﬁlter in Figure 2 was submitted to prolonged exposure
to elevated temperature. The ﬁlter amplitude has shown no degradation after more than 2000 hours at 150
degrees Celsius. The ﬁlter quality is measured by its center frequency stability and bandwidth. The result of
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Figure 4. Simulation of the gain in integration time and signal to noise when using a multi-notch ﬁlter in a f/16 beam.
this accelerated aging test at elevated temperatures in shown in ﬁgure 3.
3. EXPECTED PERFORMANCE
The center wavelength of the holographic ﬁlter depends on the incidence angle inside the material according
to the relation : λR = Λn
√
2(1 + cos(2α)), where Λ is the grating period and n the index of refraction. The
bandwidth of the ﬁlter needs to be large enough to accommodate for the wavelength shift caused by the oﬀ-axis
angles in the imaging system of the telescope. With a focal ratio of 16, the shift is equal to 0.3 nm at 1.5 µm.
A more rigorous analysis of the ﬁltering response of a telescope beam with a given F-number by the holographic
ﬁlter has been developed. An IDL∗ simulation evaluates the signal-to-noise ratio of infrared imaging of an
extended source in the astronomical J (1-1.25µm), H (1.5-1.8µm), and K (2-2.5µm) band. It takes into account a
real infrared sky spectrum, including OH-lines and thermal emission, a multi-notch ﬁlter with various number of
reﬂecting bands, bandwidths, as well as reﬂection and out-of-band eﬃciencies. It also includes, detector read-out
noise and dark current. The simulation shows the possibility of increasing observing eﬃciency by large factors.
For example, one can half the observing time for full H band imaging by removing 25 OH lines. Figure 4 shows
the gain in integration time and in signal-to-noise against the number of OH emission line removed. As a more
visual example, Figure 5 shows simulated observations of the Infrared Hubble Deep Field with a 10 m telescope,
with and without a 20 line ﬁlter compare to HST images.
The gain is even more dramatic when restricting the observations to a sub-band. Indeed, the challenge in
building holographic ﬁlters reside more in the number of reﬂecting bands than in the band density. It is thus
interesting to explore the possibilities for narrower band than J, H, and K. As an example, Figure 6 shows that
removing 10 lines from the still fairly large 1.49-1.6 µm band reduces the observing time by half to reach a given
signal-to-noise.
∗Research Systems: www.rsinc.com
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Figure 5. Simulations of ground based observation of the Hubble deep ﬁeld with a 10 m telescope, with and without
multi-notch ﬁlter compared to HST images. Angular resolution has been kept the same.
Figure 6. Simulation of the gain in integration time and signal to noise when using a 10 notch ﬁlter in a f/16 beam.
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Figure 7. Eﬃciencies of a 20 line ﬁlter for the 1.45-1.6 µm band against the notches maximum width for various f
numbers. From top to bottom: f/70, f/35, f/16, f/8, f/5.
Because of the wavelength shift due to oﬀ-axis angle, ﬁlter eﬃciency is a function of notch bandwidth. As
shown in Figure 7, a steady eﬃciency increase is seen for increasing bandwidth and reaches a maximum value
that depends on the F number, as expected.
Increasing bandwidth is thus desirable but requires a high modulation of the glass index of refraction. Current
technology allows up to 0.2nm bandwidth but larger bandwidth are expected to be available soon.
4. PRELIMINARY RESULTS
Several experimental ﬁlters have already been produced to demonstrate the applicability of the technique for the
production of a ﬁlter to simultaneously reject multiple infrared OH sky emission lines. Figure 8 shows a ﬁlter
with 10 reﬂection narrow bands (FWHM = 0.1 nm). The ﬁlter was recorded 5 times at diﬀerent place on the
same substrate. All 5 sets of 10 lines fell at the desired wavelength at +0.02nm−0.08nm.
Current recording technology limits the physical width oﬀ the ﬁlter to a few millimeters. Their length on the
other hand is not intrinsically limited. It is however possible to bond together many ﬁlter slices to build large
format ﬁlters suitable for today’s telescope. Figure 9 shows a 18 × 30 mm assembly of 3 × 30 mm slices. In this
conﬁguration, the slice interface need to be blacken to avoid stray light due to total internal reﬂection. These
losses are expected to be greatly reduced in the near future by increasing the slice width to 9 mm.
5. CONCLUSION
Multi-notch volume holographic ﬁlters under developments show promise of substantially increasing eﬃciency of
infrared imaging and low resolution spectroscopy. They have large reﬂection eﬃciencies and narrow bandwidths,
well adapted for atmospheric OH emission lines ﬁltering. Filter with up to 10 lines are currently produced and
their use in near-infrared can reduce the integration time by a large amount. For example, a given signal-to-noise
can be reached in half the time for a 110 nm window in H band.
Improving the ﬁlter performances present important technological challenges. One would like to increase the
number of lines and line width. Both require a improvement in the glass technology which is indeed in constant
evolution. New recording techniques are also explored that can improve the number of lines as well as the size
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Figure 8. Transmission spectrum of a 10 line ﬁlter.
Figure 9. Mosaic of ﬁlter slices.
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of the sample. None of these limitation are intrinsic to the concept and it is foreseeable that large format ﬁlters
of more than 20 lines, adapted to fast beams can be built in a very few years.
ACKNOWLEDGMENTS
This material is based in part upon work supported by the National Science Foundation under Grant No. 0338906
REFERENCES
1. R. Content and J. R. P. Angel, “Design and model demonstration of a camera that removes the infrared
OH background from two-dimensional images,” in Proc. SPIE Vol. 2198, p. 757-762, Instrumentation in
Astronomy VIII, David L. Crawford; Eric R. Craine; Eds., pp. 757–762, June 1994.
2. A. R. Oﬀer and J. Bland-Hawthorn, “Rugate ﬁlters for OH-suppressed imaging at near-infrared wavelengths,”
MNRAS 299, pp. 176–188, Aug. 1998.
3. M. Scobey. OFC2002, 2002.
4. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell System Tech. J. 48(9), p. 2909, 1969.
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Multi-notch holographic filters for atmospheric lines suppression

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Multi-notch holographic filters for atmospheric lines suppression

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