Download Heterogenously integrated InP on Si microdisk lasers

Heterogenously integrated InP on Si microdisk lasers
G. Morthier1, T. Spuesens1, P. Mechet1, N. Olivier2, J.-M. Fedeli2, P. Regreny3, D. Van Thourhout1, G.
Roelkens1
1imec
- Ghent University, Photonics Research Group - Department of Information Technology (INTEC), SintPietersnieuwstraat 41, 9000 Ghent, Belgium
2CEA-LETI,
3Institut
17 Rue des Martyrs, 38054 Grenoble, France
des Nanotechnologies de Lyon INL-UMR5270, CNRS, Université de Lyon, Ecole Centrale de Lyon, 69134 Ecully,
France
ABSTRACT
We review recent theoretical and experimental work on InP membrane microdisk lasers
heterogeneously integrated on SOI and coupled to a Si bus waveguide. After a general introduction on
the fabrication and the operation principles, we will describe various improvements in the fabrication
technology. This includes improvements in the yield of the bonding of the InP die on the SOI die and
in the controllability of the bonding layer thickness, as well as an optimization of the alignment of the
microdisk with respect to the silicon waveguide and some proposals for better heat sinking and loss
reduction. Improvement in the alignment and the bonding has led to interesting results on the
uniformity in device characteristics.
In a second part, unidirectional behaviour and reflection sensitivity will be briefly discussed.
Theoretical, numerical and experimental results will be shown about the unidirectional behavior and it
will be explained how unidirectional microdisk lasers can be a lot less sensitive to external reflections
than other lasers. We will also show how such lasers can be used as optical signal regenerators that
can work with low optical input powers and that have small power consumption.
We will end with a description of demonstrations of optical interconnects based on heterogeneously
integrated microdisk lasers and heterogeneously integrated photodetectors. Optical interconnects on
chip have been demonstrated at 10 Gb/s. An epitaxial layer stack that contains both the laser and the
detector structure has been used for this purpose.
1. INTRODUCTION
Heterogeneous integration of InP lasers on silicon-on-insulator (SOI) waveguides is one of the
possible routes to implement light sources in silicon photonics. Although one can also use external
light sources and distribute the light over the silicon chip, the use of external light sources has the
significant disadvantage of requiring separate packaging and alignment of the laser output with respect
to the SOI waveguide. Applications of silicon photonics are situated to a large extent in short-distance
optical interconnections (going from on-chip, over chip-to-chip to inter-server interconnections in
datacenters) [1]. A minimal power consumption, together with a relatively large external efficiency, is
an important requirement in almost all of these interconnect applications [2].
Heterogeneously integrated microdisk lasers have been studied since almost a decade now and are
were among the first electrically pumped InP lasers integrated on silicon. Because of the high optical
confinement in the InP membranes, the small disk diameters (10m or lower) and the weak,
evanescent coupling to the underlying Si bus waveguide, such microdisk lasers can have relatively low
threshold gain and threshold current and are therefore rather well suited as sources for optical
interconnects [3,4]. A schematic of the microdisk structure is given in Figure 1. The microdisk lasers
are fabricated by bonding an InP die onto an SOI die using adhesive bonding. DVS-BCB
(divinylsiloxane -benzocyclobutene) is used as adhesive, but is also used to planarize the SOI die.
Since DVS-BCB has only marginal adhesion to III-V semiconductors, an intermediate silica or
alumina layer is deposited on the InP prior to bonding. The total bonding layer thickness is controlled
by using a very thin BCB layer and a controllable thickness for the oxide layer. This at the same time
also results in better heat sinking [6]. Our microdisk lasers have typical threshold currents of 0.5 mA
(or lower) with an output power into the SOI waveguide at 5mA of up to a few hundred W. Total
power consumption amounts to a few mW.
In the past years, it has been demonstrated that the heterogeneously integrated microdisk lasers are
quite versatile optical light sources, that can not only be used as transmitters, but also for the
implementation of optical functionalities such as all-optical gating, all-optical flip-flops as well as as
modulators and photodetectors [7-10].
In this paper, we review recent progress that has been made with respect to the design and fabrication
of the microdisk lasers and we also discuss some new applications of microdisk lasers, such as their
use for all-optical 2R signal regeneration.
Figure 1. Schematic view of an InP microdisk laser heterogeneously integrated onto SOI.
2. DESIGN AND FABRICATION
In order to obtain a low threshold and stable single mode operation, it is necessary to have small disk
diameters of 10 m or less. Moreover, since the InP epitaxial structure is around 500nm in our lasers,
any overlap between the top metal contact and the lasing whispering gallery mode has to be avoided.
In addition, the coupling between the whispering gallery mode and the underlying silicon waveguide
must be optimized and depends mainly on the alignment of the microdisk with respect to the silicon
waveguide.
E-beam lithography and optimization of the bonding and etching processes have resulted in very good
fabrication uniformity, which in turn results in uniform laser characteristics of all lasers fabricated on
the same SOI die. Figure 2 shows the uniformity in lasing wavelength that was obtained from 9
nominally identical microdisk lasers fabricated on the same SOI die. The standard deviation in peak
lasing wavelength is 0.37nm. Measurements on 43 microdisk lasers with different diameter gave a
standard deviation in wavelength of 500 pm.
Figure 2. Spectra of 9 nominally identical microdisk lasers processed with electron-beam
lithography and optically pumped at 980nm. The standard deviation in peak lasing
wavelength is 0.37nm.
Figure 3 shows the uniformity in the threshold optical pumping (and the mode hopping threshold) of
the 9 nominally identical microdisk lasers discussed in Figure 2. Six of the seven microdisk lasers start
lasing around a wavelength of 1574.5nm with a standard deviation in pump power of 1.73dBm. As the
pump power increases, single-mode operation around a wavelength of 1600.7nm is triggered in the six
lasers with a standard deviation in pump power of 2.25dBm. Laser 7 is already lasing at a wavelength
of 1600.2nm under low pump power, and remains lasing at this wavelength for higher pump powers.
Figure 3. Laser threshold (LT) and mode hopping threshold (MHT) of nominally identical microdisk
lasers under increasing pump power.
One of the main issues that limits the output power of microdisk lasers is the heating of the microdisk
lasers.
3. UNIDIRECTIONALITY AND REFLECTION SENSITIVITY
Microdisk lasers that are used as optical transmitters have highest efficiency when the lasing is in a
predefined unidirectional mode and all the output power is coupled to the silicon wire waveguide on
one side. This can be achieved by having an asymmetry such that the coupling between clockwise and
counter clockwise mode is different from the coupling between counter clockwise and clockwise
mode. A theoretical analysis of the coupled wave equations [11]:
𝑑𝐸𝐶𝑊
𝑑𝑡
𝑑𝐸𝐶𝐶𝑊
𝑑𝑡
1
1
2
𝜏𝑝
= (1 + 𝑗𝛼) [𝐺 −
] 𝐸𝐶𝑊 + 𝐾1 𝐸𝐶𝐶𝑊
1
1
2
𝜏𝑝
= (1 + 𝑗𝛼) [𝐺 −
(1)
] 𝐸𝐶𝐶𝑊 + 𝐾2 𝐸𝐶𝑊
(2)
Shows that at low power levels (where gain suppression is not important), the ratio of powers in the
clockwise and counterclockwise modes is simply given by the ratio of the amplitudes of the coupling
coefficients, i.e.,
𝜇2 =
𝑆𝐶𝑊
𝑆𝐶𝐶𝑊
|𝐾 |
= |𝐾1 |
(3)
2
While at higher bias levels, where gain suppression is important, one finds
𝜇=
ε′G0 SCW+√(ε′G0 SCW )2 −8|K2 |ε′G0 SCW+16|K2 K1 |𝑐𝑜𝑠(𝜙1 +𝜙2 )
4|𝐾2 |
≈
ε′G0 SCW
2|𝐾2 |
(4)
I.e. the ratio of power levels is determined by the gain suppression (’SCW) and the lowest of the two
coupling coefficients. It can be much larger than the ratio of the coupling coefficients. Figure 4 shows
the calculated ratio of photon numbers in clockwise and counter clockwise mode vs. bias current for a
microdisk laser in which the ratio of coupling coefficients is only 2. At higher bias currents, the power
ratio nevertheless increases to over 100.
Ratio PCW/PCCW
500
400
300
200
100
0
0.5
1.5
2.5
3.5
Current [mA]
Figure 4. Ratio of power in CW and CCW modes, ( ) obtained from a numerical solution of the coupled rate
equations, for a microdisk laser with coupling coefficients K1=6.36 10 8s-1, and 3.18 108s-1, and (__) obtained
from expression (23). = 1 10-18 cm3.
Asymmetry in the coupling coefficients can easily be obtained by including a strong reflector on one
side only of the silicon bus waveguide and such a configuration has been used to experimentally
demonstrate unidirectional behavior in microdisk lasers [11.12,13].
Since there are always several spurious reflections in optical interconnections, it is also important to
analyze and quantify the reflection or feedback sensitivity of optical transmitters. From the same
analysis of the coupled wave equations that gives the unidirectionality, it is also possible to derive the
reflection sensitivity of the microdisk lasers. Where the feedback sensitivity of Fabry-Perot and
DFB/DBR lasers is independent of the bias current, it is strongly decreasing with bias level for
microdisk lasers. This is illustrated in Figure 5, which shows the change in optical pulsation of the
lasing mode of a microdisk laser due to an external reflection as a function of bias current for two
values of the external field reflection, re=10-2 and re = 5 10-3.
.
16
14
 [Grad/s]
12
10
8
6
4
2
0.5
1
1.5
2
2.5
3
3.5
4
Current [mA]
Figure 5. Change in optical pulsation () due to external reflections as a function of bias current.
Black curve: re=10-2, grey curve: re = 5 10-3.
4. OPTICAL SIGNAL REGENERATION WITH MICRODISK LASERS
The undirectionality in microdisk lasers with asymmetrically reflecting bus waveguide can be
switched by injecting light from the side opposite to the largest reflector. In a way, this switches the
asymmetry and the unidirectionality. The microdisk laser only swiches direction if the injected signal
is sufficiently large. There is thus a thresholding behavior and this effect can be used to obtain alloptical signal regeneration [14].
2R regeneration experiments have been done at 10 Gb/s, using a signal degraded by ASE noise and
measuring eye diagrams and BER curves with and without the microdisk regenerator. Figure 6 shows
the measured eye diagrams at input and output for a 10 Gb/s signal.
Figure 6. Eye diagrams of 10 Gb/s NRZ signal regenerated by a microdisk laser, (a) Eye diagram of
the degraded signal. (b) Eye diagram of the regenerated signal.
5. OPTICAL INTERCONNECT DEMONSTRATIONS
Optical interconnects can be implemented using microdisk lases as transmitters, silicon waveguides as
transmission medium and heterogeneously integrated detectors. Although one could also use Gedetectors, we have chosen for an approach with III-V detectors. As explained in [9], one can even use
identical microdisk structures for both the transmitter and the detector, although the detectors are not
wavelength transparant in this case and have to be tuned to the laser wavelength. For this reason, we
have chosen for broadband InGaAs detectors. However, as shown in Figure 7, we have experimented
with a single epitaxial layer stack that contains both the laser and the detector epitaxy, with the
detector layers stacked on top of the laser structure.
Fabricated photodetectors of 40 m and 60 m length gave dark currents (at a reverse bias of 1.5 V) of
22 and 67 nA. Responsivities were 0.69 and 0.73 A/W respectively [14]. The dark current sometimes
increased with detector length due to the increased sidewall surface area, while the increase of the
responsivity with length was marginal. A bandwidth of 12 and 18 GHz was obtained for 0 and -1V
bias respectively. Also direct modulation of the microdisk lasers has been demonstrated with a
bandwidth of 7.8 GHz. By providing InP heaters around the microdisk, thermal tuning of the lasing
wavelength is possible with an efficiency of 0.3nm per mW of heating power [15].
6. CONCLUSION
We have discussed progress in the fabrication of microdisk lasers and their performance for optical
interconnections. We discussed the unidirectional operation of these lasers and their feedback
sensitivity, as well as their modulation bandwidth, which is around 8 GHz, but could be further
increased by optimizing the active layer composition and by increasing the output power. This will be
possible with improved heat sinking. We have also demonstrated that the microdisk lasers can be used
as optical signal regenerator as well. Moreover, it is one of the very few optical regenerators that only
require a minor input power level.
Finally, optical interconnects on chip have been demonstrated at 10 Gb/s. An epitaxial layer stack that
contains both the laser and the detector structure has been used for this purpose.
Figure 7. Schematic overview of the detector design, where (a) shows a 3D impression and (b) shows
the cross section of the detector.
ACKNOWLEDGMENT
This work is supported by the European FP7 ICT-projects HISTORIC, Wadimos and PhotonFab, the
Fund for Scientific Research Flanders (FWO) and the IAP-project “Photonics@be”.
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