High frequency operation of an integrated electro-absorption modulator onto a vertical-cavity surface-emitting laser [original]

J. Phys.: Photonics 1 ( 2019 ) 02LT01 https: // doi.org / 10.1088 / 2515-7647 / aaf5af
LETTER
High frequency operation of an integrated electro-absorption
modulator onto a vertical-cavity surface-emitting laser
L Marigo-Lombart
1 , 3
, A Rumeau
1
, C Viallon
1
, A Arnoult
1
, S Calvez
1
, A Monmayrant
1
,
O Gauthier-Lafaye
1
, R Rosales
2
, JA Lott
2
, H Thienpont
3
, K Panajotov
3
and G Almuneau
1
1
LAAS-CNRS, Université de Toulouse, CNRS, Toulouse, France
2
Center of Nanophotonics, Institute of Solid-State Physics, Technische Universität Berlin, Berlin D-10623, Germany
3
Department of Applied Physics and Photonics ( TW-TONA ) , Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
E-mail: [email protected]
Keywords: vertical cavity surface emitting lasers, modulator, high frequency characterization
Abstract
We present in this paper the vertical integration of an electro-absorption modulator ( EAM ) onto a
vertical-cavity surface-emitting laser ( VCSEL ) . We discuss the design, fabrication, and measured
characteristics of the combined VCSEL and EAM. We previously demonstrated a standalone EAM
with an optical bandwidth around 30 GHz. In this paper we present for the ﬁ rst time an optical
bandwidth of 30 GHz for an EAM integrated onto a VCSEL. This device exhibits single-mode
operation and a very low chirp, below 0.1 nm, even with a modulation depth of 70% which makes
this device very competitive for high-speed communications in data centers.
1. Introduction
Nowadays, vertical-cavity surface-emitting lasers ( VCSELs ) receive a lot of attention for their versatility in
different emerging applications requiring massively expanding data transmission rates. In particular, in very
recent years, new applications including internet of things, virtual reality, cloud computing and storage, have
driven the demand for a signi ﬁ cant expansion in capacity of short-reach links in data centers [ 1 ] , favouring
solutions with low power consumption. In that context, VCSELs bring many advantages to optical transceivers,
such as a high density of integration and an easy coupling to optical ﬁ bers, making them ideal as light sources for
very high-frequency and high-capacity data links.
Direct modulation on short wavelength VCSELs ( 850, 940 or 980  nm ) has been greatly improved through
many device design strategies, but the performances are restricted by different intrinsic physical limitations
including carrier-photon dynamics, parasitic electrical losses, and self-heating. Albeit, VCSELs with impressive
performances have been demonstrated in the past years reaching modulation frequencies in excess of 25 GHz
[ 2 – 4 ] . In particular, VCSELs with very small oxide con ﬁ nement apertures ( down to few micrometers in
diameter ) have been found to increase the modulation bandwidth to above 30 GHz [ 5 , 6 ] , but at the expense of a
very stringent fabrication and a potentially-compromised reliability [ 7 ] . Nevertheless, using directly-modulated
devices with complex but more ef ﬁ cient modulation formats such PAM-4 or Discrete MultiTone have enabled
optical data systems to reach transmission rates in the range of 100  Gbps on single channel links [ 8 – 10 ] .
In order to reach even higher communication speeds, the con ﬁ guration based on externally modulated laser
( EML ) is more adapted and advantageous as established in long and medium-range links with systems exploiting
distributed feedback lasers that are in-wafer-plane integrated with an electro-absorptive modulator ( EAM ) . The
implementation of such EML with VCSEL sources would allow their respective inherent advantages to be
combined and should facilitate very high speed data transfers with low power consumption to be obtained.
Indeed, in the case of the vertically integrated EAM-VCSEL, the frequency modulation bandwidth is limited
by the EAM frequency response, and no longer by the optoelectronic dynamics and the thermal effects of the
VCSEL. As early demonstrated in [ 11 ] , the asymmetric Fabry – Perot modulator ( AFPM ) , used in our proposed
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RECEI VED
28 August 2018
REVISED
16 November 2018
ACCEPTED FOR PUBLICATION
3 December 2018
PUBLISHED
15 January 2019
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EAM-VCSEL structure, is primarily limited in frequency bandwidth by the RC electrical characteristics, and not
by the carriers escape rate. This limit is inherent to the quantum con ﬁ ned Stark effect which only relies on the
ﬁ eld switching time delay. The highest modulation frequency demonstrated so far for AFPM is 37 GHz [ 11 ] .
The chirp is dependent on the spectral spacing between the AFPM resonance and the modulated signal
wavelength [ 11 ] . This corresponds for EAM-VCSEL device to the detuning between the VCSEL and the EAM
cavity resonances, which can be optimized to tailor the device chirp within a given system.
The asymmetric Fabry – Perot EAMs are also very advantageous regarding their energy consumption. As
shown in [ 12 ] , this type of vertical modulator can operate with a consumption as low as 10 fJ bit
− 1
,t ob e
compared with 100 fJ bit
− 1
for oxide-con ﬁ ned directly-modulated VCSELs [ 2 ] .
Despite these potential bene ﬁ ts, such a vertical EML con ﬁ guration has been scarcely studied and developed
to-date. Practically, the co-integration of a VCSEL with a modulator has been achieved using either a lateral [ 13 ]
or a vertical integration scheme.
Nevertheless, the lateral coupling presents a low extinction ratio below 4 dB and moreover, the suitable
coupling regime for enabling high modulation rate may be dif ﬁ cult to achieve while preserving well-controlled
transverse optical properties of the VCSEL beam. Also, the optical feedback can be detrimental regarding the
wavelength chirp.
In the vertical con ﬁ guration, the emitter amplitude modulation is introduced by an electro-refractive ( ER )
( phase ) modulation embedded in an interferometer or by electro-absorption modulation. The former exploits
quantum wells ( QW ) positioned in the top Distributed Bragg Re ﬂ ector ( DBR ) mirror of the VCSEL, converting
the voltage-driven modulation of the QW refractive index into an amplitude modulation through the change of
the mirror effective re ﬂ ectivity [ 14 – 16 ] . If modulation bandwidths greater than 10 GHz have been obtained
[ 15 , 16 ] , the prime limitation of this approach is its inherent large chirp resulting from the signi ﬁ cant variation of
the device effective cavity length [ 17 , 18 ] .
To avoid the latter drawback, a more promising approach to implement the modulation relies on using the
electro-absorption phenomenon as done in [ 19 ] . In this case, the EML structure consists of three DBR mirrors
de ﬁ ning two weakly-coupled cavities, each of which containing QWs, where the bottom cavity is current-driven
to operate as the emitter and the top cavity is voltage-driven to induce the modulation. A modulation at 20 GHz
was demonstrated by proper detuning of the EAM and VCSEL cavity resonances [ 20 ] .
Higher bandwidths could be achieved if the parasitic capacitances and resistances are further reduced [ 21 ] .
Temperature control is also a challenge to preserve low-power consumption and a reduced chirp size. The
development of these two last points have been the main items of our work and will be presented in this paper.
We present in this paper a device based on the proposed concept of Van Eisden et al [ 20 ] , the vertical
integration of an electro-absorption modulator onto a VCSEL. We describe in the ﬁ rst section the structure and
the fabrication of this three-electrode device. Then we demonstrate the ef ﬁ ciency of this device with static
characterization. Finally, the high-frequency modulation results will be discussed.
2. Structure description and fabrication
The EAM VCSEL structure was grown, with an N – P – N doping pro ﬁ le, monolithically in a single epitaxial run
on an N-doped GaAs substrate using a RIBER 412 molecular beam epitaxy ( MBE ) system.
Above the substrate, the structure combines a conventional 850  nm VCSEL structure stacked with an AFPM
structure.
The VCSEL part is formed by a 35.5 period Al
0.9
GaAs / Al
0.15
GaAs N-type bottom DBR, a half-wave thick
optical cavity including three GaAs / Al
0.3
GaAs quantum well / barrier layers ( QW ) , and a 12.5 periods P-type
DBR embedding an Al
0.98
GaAs 30  nm thick layer for oxide con ﬁ nement in the ﬁ rst period above the cavity.
Above that, a P-type 3 λ / 4 thick layer is grown to serve commonly as the top-electrode of the VCSEL and the
bottom-electrode of the EAM. Above, the AFPM structure is grown composed of a 10.5-period P-doped DBR
( with the same composition as used in the VCSEL ʼ s DBRs ) ,a2 λ thick undoped cavity embedding 25
GaAs / Al
0.3
GaAs QWs, and ﬁ nally capped by an N-doped 6.5-period DBRs and a 50  nm GaAs cap as an
N-contact layer.
The common intermediate DBR was deliberately chosen to be thick enough ( 23 periods overall ) to minimize
the coupling between the VCSEL cavity and the EAM cavity to reduce the impact of the coupled vertical cavities
on optical feedback that may impact the VCSEL performance while enabling high-depth modulation. Also, a not
too high top mirror re ﬂ ectivity as seen by photons in the VCSEL cavity have to be kept to not decrease the output
power.
The second crucial point is the wavelength detuning between the VCSEL cavity ( respectively AFPM cavity )
resonance and the VCSEL gain ( resp. AFPM QWs absorption tail ) to ensure operation over a large temperature
range. Typically a redshift room temperature cavity detuning of about 15  nm with respect to the QW
2
J. Phys.: Photonics 1 ( 2019 ) 02LT01

photoluminescence peak leads to a perfect gain-to-cavity alignment around 40 ° C – 50 ° C, when the integrated
AFPM undergoes about the same heating as the VCSEL.
Furthermore the VCSEL and AFPM cavity resonances are slightly offset to avoid optical feedback between
both cavities, as shown in [ 19 ] .
Following the epitaxial growth of the EAM VCSEL structure, the respective wavelength detunings were
measured by top re ﬂ ectivity and photoluminescence ( PL ) of the global epitaxial structure and after successively
etching this structure down to the top of the EAM and laser cavity respectively. The measured cavity to peak PL
offsets are + 23 and + 22  nm respectively for the VCSEL and the EAM, while the VCSEL cavity resonance is
redshifted by 5  nm with respect to the EAM cavity resonance wavelength.
Subsequently, the devices were fabricated with triple contact electrodes to independently bias the VCSEL
and the EAM. Also, to facilitate high frequency ( HF ) operation, the whole 6  μ m thick EAM VCSEL was
planarized with BCB, and a low-loss microstrip injection scheme was implemented to drive the modulator. The
details of the HF pad design and processing will be published elsewhere [ 22 ] .
A schematic of the ﬁ nal device is shown in ﬁ gure 1 . The vertical and horizontal scaling factors are deliberately
chosen to differ to give a clear view of the device.
3. EAM VCSEL static characterization
Figure 2 presents the light-current ( LI ) response of the VCSEL at room temperature while applying different
static voltages to the modulator.
In ﬁ gure 2 ( a ) the modulation effect can clearly be seen on the LI curve with insigni ﬁ cant change of the lasing
threshold current demonstrating uncoupled operation of the two cavities. As shown on ﬁ gure 2 ( b ) , the
modulation depth reaches 70% for a voltage change of 10 V across the EAM and a VCSEL operating just above
threshold. We can note that a modulation depth surpassing 40% can be reached all over the LI curve, showing
that the modulation can be applied for any values of the output power from the VCSEL.
The optical spectra have also been measured with an optical spectrum analyser as shown in ﬁ gure 2 ( c ) , for
different VCSEL currents and different voltages on the EAM. A redshift of the laser emission is observed when
the driving current rises according to the inherent temperature increase in the structure. In contrast, at a
given laser injected current, the wavelength remains essentially unaffected by the EAM applied voltage ( see
ﬁ gure 2 ( d )) . The maximum wavelength chirp reaching 80  pm is observed to occur when the modulation depth
is maximal.
4. EAM VCSEL HF modulation
Standalone EAM structures were ﬁ rst characterized by measuring the top re ﬂ ectivity at a ﬁ xed wavelength using
a single-mode ﬁ ber optical injection / collection and driving the modulator with the HF voltage signal injected
using a cascade In ﬁ nity 67 GHz GSG-150 probe. A tunable laser source was used as a monochromatic
illumination source on the top of the EAM, and a ﬁ ber coupler enabled the collection of the re ﬂ ected signal.
Figure 1. Global view of the fabricated EAM VCSEL device with microstrip line access for RF signal injection.
3
J. Phys.: Photonics 1 ( 2019 ) 02LT01

A FieldFox Keysight was used to extract the EAM frequency response through a high-speed photodiode. Details
of this measurement setup is given in [ 23 ] . The EAM standalone vertical modulators exhibit modulation
bandwidths in the range of 28 – 32 GHz, for modulator diameters ranging from 26 to 18 μ m, respectively.
Also for HF characterizations of both the EAM and the EAM VCSEL structures, the injection losses due the
different RF elements in the setup were carefully characterized and compensated for in order to retrieve the real
input and output powers to / from the studied modulator devices [ 23 ] . In particular, the frequency response of
the New Focus 1434 InGaAs photodetector used for the measurements was measured by a heterodyne technique
for an 850  nm wavelength giving a f
− 3 dB
of 27 GHz, and hence the measured response was included to extract
the effective modulation amplitude of the EAM up to 40 GHz.
The EAM VCSELs were next characterized by applying the HF voltage generated by an Agilent PNA-X
vectorial network analyzer with a range extending up to 67 GHz. The presented results were measured on a
71 μ m diameter VCSEL, with an oxide aperture diameter smaller than 5 μ m, topped by a 24 μ m EAM. The
normalized frequency response of the EAM VCSEL devices is shown in ﬁ gure 3 with an applied current of
2.9 mA on the VCSEL section and a DC bias of 8.5 V on the EAM. The injected modulated voltage amplitude is
estimated to be < 1 V in the full range of frequencies due to the limited power delivered by the VNA and the high
impedance of the EAM itself. In agreement with the previous results on a standalone EAM, a − 3dB modulation
bandwith as high as 29 GHz has been measured. This result shows clearly the advantage of decoupling the
modulator section from the VCSEL emitter, and indeed the potential gain compared to directly-modulated
VCSELs. For comparison, these results show a signi ﬁ cant improvement compared to the results from [ 19 ] on a
similar EAM VCSEL structure. This last structure contains a thicker intermediate DBR separating the two
cavities, and multiple double QWs which serves as the modulated absorbing region. Regarding the HF injection
con ﬁ guration, our design seems to be more optimized in terms of reduced HF injection losses, including the use
of a 2- λ thick cavity for reducing the device capacitance compared to a 1.5- λ thick cavity.
The frequency response of the EAM VCSEL device was also studied for different EAM DC bias, in order to
check the in ﬂ uence of the working point on the absorption curve on the dynamics ( in ﬁ gure 4 ) . As already
reported in [ 20 ] , the DC bias applied on the modulator noticeably modi ﬁ es the shape of the frequency response.
In particular, a strong resonance peak arises around 3 GHz, and is strongly dependent on the bias voltage. In the
Figure 2. ( a ) L-I curve of the EAM VCSEL structure at room temperature. The output power is plotted for different EAM voltages.
( b ) is the modulation depth function of the applied EAM voltage. ( c ) Emission spectra under different EAM bias voltages.
( d ) Extraction of the wavelength shift when the VCSEL is modulated.
4
J. Phys.: Photonics 1 ( 2019 ) 02LT01

inset of ﬁ gure 4 we plotted this relaxation frequency against the bias voltage on the EAM. When increasing the
bias, this peak goes to a minimum frequency at 2.5 GHz while decreasing in intensity, until it vanishes for a bias
of 8.5 V, for which the frequency response is ﬂ at up to 20 GHz. For higher values of the bias, the resonance peak
arises again, and its frequency increases again towards the same frequency observed at low biases.
The frequency response can be understood as the modulator stand-alone response combined to an
undesired though often-present loss-modulated laser response originating from the coupling between the
modulator and laser cavities. Indeed, as explained in [ 20 ] , the EAM modulation also induces an additional
mirror re ﬂ ectivity change which, in-turn, impacts the photon density in the laser cavity and gives rise to the
observed relaxation resonance frequency. Nevertheless, this resonance can be avoided by tuning the EAM bias
( uncoupling the cavities ) , enabling a signi ﬁ cant increase in the modulation bandwidth but at the expense of a
reduced power in modulated signal.
We also observe on standalone EAM devices that, even though the modulation contrast changes with the
temperature, the 3 dB frequency remains constant in the investigated range between 15 ° C and 55 ° C.
5. Conclusions
We ha ve demo nst r at e d th e fab r ic at io n an d t he c ha rac t er iz at io n of a n ew mo n ol it hic con ﬁ gu ra ti on of a n EAM
VCS EL , in whic h th e ga in and th e mo d ul a tion cavi t y re so nan c es are op ti ca ll y de co u pl ed. The eff e ct iv e mo d ul ati on
thr ou g h an AF PM ba se d on ele c tr o- abs or pt io n a ll ow s us to ac hi ev e a lar ge mo dul a ti on co n tra st wh il e e nsu r in g a
ve ry s ma ll wa vele ngt h chi rp com pare d to devi ces that op er at e via the ER ef fec t. Th e fab ric at io n of t he EAM V CSEL
de vice ha s bee n rea li zed wi th a lo w lo ss RF in jec tion micr ostr ip li ne acc ess a nd prop erly plan ari zed thic k BCB la yer .
The high -fre que nc y modu la tio n on th is E AM V CS EL devic e has been s ucce ssfu lly de mon st rat ed up t o 29 GH z
Figure 3. Modulation frequency response at room temperature of the EAM VCSEL. The VCSEL is powered at 2.9  mA and the EAM at
8.5  V.
Figure 4. Modulation frequency response at room temperature of the EAM VCSEL under different DC bias on the EAM section.
Inset: frequency of the resonance peak around 3  GHz as the function of the EAM bias.
5
J. Phys.: Photonics 1 ( 2019 ) 02LT01

wi th a ﬂ at r espon se t hrou gh t he adj ustm ent of the a ppli ed bi as on the mo dula tor . This modu la tion ba nd width is to
ou r know ledg e the be st ac hieve d on t hi s typ e of ele ctr o-ab sor ptio n modu late d VCSE L.
Acknowledgments
The authors gratefuly acknowledge the technological support of RENATECH ( French Network of Technology
Platforms ) within the LAAS-CNRS cleanroom infrastructure and the ﬁ nancial support of the Methusalem
foundation, Belgium.
ORCID iDs
L Marigo-Lombart https: / / orcid.org / 0000-0003-2241-5162
C Viallon https: / / orcid.org / 0000-0002-5331-6880
S Calvez https: / / orcid.org / 0000-0001-7788-5909
H Thienpont https: / / orcid.org / 0000-0003-0483-0960
K Panajotov https: / / orcid.org / 0000-0002-5215-1228
G Almuneau https: / / orcid.org / 0000-0003-1340-3629
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