Integration of grating couplers with a compact
photonic crystal demultiplexer on an InP membrane
Tiziana Stomeo,
1,
*
Frederik Van Laere,
2
Melanie Ayre,
3
Cyril Cambournac,
3
Henri Benisty,
3
Dries Van Thourhout,
2
Roel Baets,
2
and Thomas F. Krauss
1
1
School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, UK
2
Department of Information Technology (INTEC), Ghent University–Interuniversitary Microelectronics Center (IMEC),
B-9000 Ghent, Belgium
3
Laboratoire Charles Fabry de l’Institut d’Optique, CNRS, Campus Polytechnique, Universite Paris-Sud,
RD 128, F-91127 Palaiseau, France
*
Corresponding author: tiziana.stomeo@unile.it
Received January 23, 2008; revised March 12, 2008; accepted March 13, 2008;
posted March 17, 2008 (Doc. ID 91962); published April 14, 2008
We demonstrate the integration of a 30% efficient grating coupler with a compact photonic crystal wave-
length demultiplexer (DeMUX). The DeMUX has seven output channels that are spaced 10 nm apart and is
aimed at coarse WDM applications. The integrated devices are realized on a high-index-contrast InP mem-
brane using a simple benzocyclobutene wafer bonding technique. Cross talks of −10 to −12 dB for four chan-
nels 20 nm apart are obtained without optimization. © 2008 Optical Society of America
OCIS codes: 050.5298, 130.0130, 130.5296.
Compact integration of optical functions on a chip is
a key driver to provide low-cost optoelectronic devices
that are needed in metropolitan optical networks.
High refractive index contrast materials and planar
photonic crystal (PhC) technology enable the large-
scale integration of optical functions, since
waveguides, bends, and core integrated elements can
be very compact. Many recent examples of exploiting
a large refractive index contrast are provided by
silicon-on-insulator (SOI) structures [
1–3] and III–V
materials in membrane configuration [
4]. The SOI
platform, is very well suited for performing passive
functionality, but it is still rather elusive to achieve
active functionality on Si, owing to its indirect band-
gap. An emerging solution is to heterogeneously inte-
grate III–V material (active functions) and SOI (pas-
sive functions) [
5], placing high demands on both.
Another solution is to use indium phosphide 共InP兲
compounds, which have been a workhorse for imple-
menting photonic devices with various active func-
tions, such as photodetection, tunability, and carrier
injection for gain and lasing. Since such an integra-
tion is especially relevant to frequency-selective de-
vices, we use this approach, rather than Si, as the
platform for our investigation. Namely, we use a
simple benzocyclobutene (BCB) wafer bonding tech-
nique [
6] to realize InP-membrane-type devices. Us-
ing this approach, we present the integration of an
efficient grating coupler with a very compact PhC
wavelength demultiplexer (De MUX) that operates in
the 1500–1560 nm wavelength range.
The one-dimensional (1D) grating coupler 共12
m
⫻12
m兲 implemented here on top of an Au mirror
can achieve a coupling efficiency from standard
single-mode fiber to membrane-based waveguides of
⬃56% [
7,8] with relaxed alignment tolerances and
relatively large bandwidth. The DeMUX is a mem-
brane version of the device already described in
[
9–12] for conventional InP heterostructures. It ex-
ploits energy transfer caused by intermodal coupling
at mini-stop-bands (MSBs) [
13,14] of a multimode
PhC waveguide. When polychromatic light impinges
along the axis of such a waveguide, it essentially
feeds the transverse fundamental mode at all fre-
quencies. But, as it experiences propagation along
the periodic PhC waveguide, this fundamental mode
is coupled to a higher-order mode only for the fre-
quency range within the MSBs. The generated sig-
nal, i.e., the WDM signal of interest, is then effi-
ciently and directionally extracted by thinning one of
the PhC walls to a few rows. The wavelength selec-
tivity is introduced by varying the central frequency
of the MSB, which, in practice, relies on tuning (step-
wise or continuously) the PhC waveguide width along
the guide.
A typical layout is shown in Fig. 1(a). The PhC
waveguide is defined by removing five rows of holes
(
Fig. 1. (Color online) (a) Layout of the PhC DeMUX with
seven output channels. (b) Overall test circuit comprising
shallow-etched grating and PhC DeMUX, flip chipped and
BCB bonded to a host substrate.
884 OPTICS LETTERS / Vol. 33, No. 8 / April 15, 2008
0146-9592/08/080884-3/$15.00 © 2008 Optical Society of America
(“W5”) along the ⌫K direction, with the PhC cladding
on one side thinned down to two rows of holes to ex-
tract the higher-order mode. For each channel, a sec-
tion of ⬃30
m in length is sufficient for good extrac-
tion, which results in a compact device that
successfully compares to, e.g., the approaches of
[
15–17]. The PhC period is 540 nm, and the filling
factor is 41%.
Experimentally, we used a heterostructure consist-
ing (from top to bottom) of a 300 nm InP-membrane
layer, a 400 nm InGaAsP etch-stop layer, a 300 nm
InP etch-stop layer, and a 400 nm InGaAsP etch-stop
layer on an InP substrate. All the etch-stop layers are
sacrificial and are removed in the membraning–BCB
bonding process. We used a heterostructure with
multiple etch-stop layers, although simpler designs
are possible. The essential part of the heterostruc-
ture is the 300 nm InP guide layer followed by a
400–600 nm (in our case, 522 nm) thick InGaAs etch-
stop layer. A noteworthy challenge is that the 1D
grating coupler requires shallow etching (⬃90 nm
deep), whereas the wavelength-selective DeMUX has
to be deeply etched (⬃300 nm completely through the
membrane layer). To avoid alignment errors, both the
coupler and the DeMUX are written in a single step
using a Raith Elphy e-beam lithography tool. After
e-beam writing in ZEP520-A positive resist and pat-
tern transfer into the SiO
2
hard mask using reactive
ion etching (RIE) with F chemistry, a window is
opened by optical lithography to etch the DeMUX
first completely through the InP membrane 共300 nm兲.
During the first InP etch, the grating coupler and the
waveguide are covered with photoresist. The photore-
sist is then removed, and the whole pattern is etched
further to a depth of 90 nm. This means that the De-
MUX is etched slightly into the first InGaAsP etch-
stop layer. Since this layer will be removed later,
however, this is of no consequence. Scanning electron
microscope (SEM) pictures of the sample prior to
bonding are shown in Fig.
2. A high-quality grating
coupler (period of 660 nm and duty cycle of 50%) and
PhC DeMUX (hole diameter of 362 nm) with well-
controlled sizes were achieved after the pattern is
transferred to the InP layer. In the next step, a BCB-
buffer layer is spin coated onto the InP die, and Au is
deposited opposite the grating couplers. The die is
then bonded onto a GaAs host substrate (with an-
other BCB layer), aligning the cleaving planes of both
substrates [
6].We chose GaAs as the host substrate in
the BCB bonding process to facilitate facet cleaving,
although other substrates such as SOI or Si are
equally suitable. After curing of the BCB, the InP
substrate is removed using lapping and wet etching.
Finally, all three etch-stop layers are wet etched, the
BCB is removed from the holes by inductively
coupled plasma (ICP) etching to obtain air holes, and
an output facet is cleaved [see optical image in Fig.
1(b)].
To test the integrated device, a single-mode input
fiber connected to a tunable laser is positioned at 10°
off the vertical axis above the grating to avoid reflec-
tions [
7]. The 1D grating couples the light into the
membrane waveguide from where it feeds into the
PhC demultiplexer. The optical signal in the seven
channels, ranging from 1500 to 1560 nm, is collected
at the output facet using a microscope objective lens
and measured with a detector. One channel is mea-
sured at time while scanning the laser frequency. The
demultiplexing operation of the PhC device can
clearly be seen in Fig.
3(b). The Q factor obtained for
the extracted signals as well as the uniform channel
spacing of 10 nm are close to those designed. The out-
put signals are normalized to the input grating cou-
pler spectrum determined from a fiber-to-fiber refer-
ence measurement on the same sample [see Fig.
3(a)]. The cross talk of the device is −8 to −12 dB for
channels spaced by 20 nm. Between adjacent chan-
nels (10 nm spacing) it raises up to −4 dB instead of
the theoretically predicted value of −7.5 dB. Improve-
ment should result from engineering the transfer
function of the MSB, e.g., through fine tuning the size
and number of holes of the PhC cladding through
which the signal is extracted. Figure
3(c) shows the
corresponding coupled-mode modeling [
12], which re-
produces very well all the spectral details related to
the succession of shifted MSBs, for example, the
kinks and shoulders of the rightmost channel spec-
trum. The basis towards further optimization is thus
clearly given and hinges on modestly reducing losses
and optimizing the section lengths for each channel.
In conclusion, we have demonstrated the integra-
tion of two very distinct passive structures on a
bonded InP membrane. We combined a shallow-
etched 1D grating coupler with a deeply etched PhC
DeMUX for operation at the 1500–1560 nm wave-
length window. We achieved a good separation of
seven optical channels with a spectral resolution of
Fig. 2. (Color online) SEM pictures of the devices prior to
bonding. (a) Complete device layout: 1D grating coupler in-
tegrated with the PhC DeMUX. (b) Input grating coupler
(shallow etch). (c) Entrance of the PhC waveguide. (d)
Single-channel view. (e) PhC holes (deep etch).
April 15, 2008 / Vol. 33, No. 8 / OPTICS LETTERS 885
10 nm and −4 dB cross talk, or four channels suitable
for coarse WDM (CWDM) at 20 nm spacing with a
cross talk of −10 to −12 dB. The typical −10 to −12 dB
cross talk for the CWDM 20 nm spacing (every other
channel here) can be further improved, all the more
because the coupled-mode theory underlying the de-
multiplexing operation seems fully validated for this
first implementation on a membrane. We thus dem-
onstrate a remarkable combination of a nanophotonic
strong-index contrast technology and of a design ex-
ploiting modes at such a mesoscopic scale that appli-
cability of a revisited coupled-mode approach is
granted. This extension of tools that have proven so
beneficial to traditional integrated optics in the past
and today, in one of the most compact embodiments
for demultiplexing [
16–18], clearly fosters the per-
spectives of nanophotonic approaches toward devices
of the real world. Along this line, further extension of
this work to include detectors and emitters is readily
possible, given the InP-based material used in our ex-
periments.
This work is carried out within the framework of
the European project FUNFOX under grant IST-
004582.
References
1. M. Loncar, T. Doll, J. Vuckovic, and A. Scherer, J.
Lightwave Technol. 18, 1402 (2000).
2. I. Marki, M. Salt, H. P. Herzig, R. Stanley, L. El
Melhaoui, P. Lyan, and J. M. Fedeli, Opt. Lett. 31, 513
(2006).
3. Y. Akahane, T. Asano, B. S. Song, and S. Noda, Opt.
Express 13, 1202 (2005).
4. A. Xing, M. Davanco, D. J. Blumenthal, and E. L. Hu,
J. Vac. Sci. Technol. B 22, 70 (2004).
5. H. T. Hattori, C. Seassal, E. Touraille, P. Rojo-Romeo,
X. Letartre, G. Hollinger, P. Viktorovitch, L. Di Cioccio,
M. Zussy, L. El Melhaoui, and J. M. Fedeli, IEEE
Photon. Technol. Lett. 18, 223 (2006).
6. G. Roelkens, J. Brouckaert, D. Van Thourhout, R.
Baets, R. Notzel, and M. Smit, J. Electrochem. Soc.
153, G1015 (2006).
7. D. Taillaert, F. Van Laere, M. Ayre, W. Bogaerts, D.
VanThourhout, P. Bienstman, and R. Baets, Jpn. J.
Appl. Phys. Part 1 45, 6071 (2006).
8. F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D.
Taillert, D. Van Thourhout, T. F. Krauss, and R. Baets,
J. Lightwave Technol. 25, 151 (2007).
9. E. Viasnoff-Schwoob, C. Weisbuch, H. Benisty, C.
Cuisin, E. Derouin, O. Drisse, G. H. Guan, L.
Legouezigou, O. Legouezigou, F. Pommereau, S. Golka,
H. Heidrich, H. J. Hensel, and K. Janiak, Appl. Phys.
Lett. 86, 101107 (2005).
10. L. Martinelli, H. Benisty, O. Khayam, G. H. Duan, H.
Heidrich, and K. Janiak, J. Lightwave Technol. 25,
2385 (2007).
11. L. Martinelli, H. Benisty, O. Drisse, E. Derouin, F.
Pommereau, O. Legouezigou, and G. H. Duan, IEEE
Photon. Technol. Lett. 19, 282 (2007).
12. M. Ayre, C. Cambournac, H. Benisty, O. Khayam, H.
Benisty, T. Stomeo, and T. F. Krauss, Photonics
Nanostruct. Fundam. Appl. 6, 19 (2008).
13. S. Olivier, H. Benisty, C. Weisbuch, C. J. Smith, T. F.
Krauss, and R. Houdré, Opt. Express 11, 1490 (2003).
14. S. Olivier, M. Rattier, H. Benisty, C. J. M. Smith, R. M.
De La Rue, T. F. Krauss, U. Oesterle, R. Houdré, and C.
Weisbuch, Phys. Rev. B 63, 113311 (2001).
15. T. Niemi, L. H. Frandsen, K. K. Hede, A. Harpoth, P. I.
Borel, and M. Kristensen, IEEE Photon. Technol. Lett.
18, 226 (2006).
16. Y. Akahane, T. Asano, H. Takano, B.-S. Song, Y.
Takana, and S. Noda, Opt. Express 13, 2512 (2005).
17. J. Brouckaert, W. Bogaerts, P. Dumon, D. Van
Thourhout, and R. Baets, J. Lightwave Technol. 25,
1269 (2007).
18. W. Lijun, M. Mazilu, T. Karle, and T. F. Krauss, IEEE
J. Quantum Electron. 38, 915 (2002).
(a)
(b)
(c)
Fig. 3. (Color online) (a) Grating coupler spectrum control. (b) Channel output normalized to the grating coupler spec-
trum. (c) Coupled-mode theory modeling rendering most spectral details.
886 OPTICS LETTERS / Vol. 33, No. 8 / April 15, 2008