Citation: Babichev, A.; Blokhin, S.;
Kolodeznyi, E.; Karachinsky, L.;
Novikov, I.; Egorov, A.; Tian, S.-C.;
Bimberg, D. Long-Wavelength
VCSELs: Status and Prospects.
Photonics 2023,10, 268. https://
doi.org/10.3390/photonics10030268
Received: 31 January 2023
Revised: 24 February 2023
Accepted: 27 February 2023
Published: 3 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
photonics
hv
Review
Long-Wavelength VCSELs: Status and Prospects
Andrey Babichev 1,* , Sergey Blokhin 2, Evgenii Kolodeznyi 1, Leonid Karachinsky 1, Innokenty Novikov 1,
Anton Egorov 3,4, Si-Cong Tian 5,6 and Dieter Bimberg 5,6
1Institute of Advanced Data Transfer Systems, ITMO University, Saint Petersburg 197101, Russia
2Ioffe Institute, Laboratory of Physics of Semiconductor Heterostructures, Saint Petersburg 194021, Russia
3Alferov University, Saint Petersburg 194021, Russia
4Connector Optics LLC, Saint Petersburg 194292, Russia
5Bimberg Chinese-German Center for Green Photonics, Changchun Institute of Optics, Fine Mechanics and
Physics (CIOMP), Chinese Academy of Sciences (CAS), Changchun 130033, China
6Center of Nanophotonics, Institute of Solid State Physics, Technische Universität Berlin,
10623 Berlin, Germany
*Correspondence: [email protected]u
Abstract:
Single-mode long-wavelength (LW) vertical-cavity surface-emitting lasers (VCSELs) present
an inexpensive alternative to DFB-lasers for data communication in next-generation giga data centers,
where optical links with large transmission distances are required. Narrow wavelength-division
multiplexing systems demand large bit rates and single longitudinal and transverse modes. Spatial
division multiplexing transmission through multicore fibers using LW VCSELs is enabling still
larger-scale data center networks. This review discusses the requirements for achieving high-speed
modulation, as well as the state-of-the-art. The hybrid short-cavity concept allows for the realization
of f
3dB
frequencies of 17 GHz and 22 GHz for 1300 nm and 1550 nm range VCSELs, respectively.
Wafer-fusion (WF) concepts allow the realization of long-time reliable LW VCSELs with a record
single-mode output power of more than 6 mW, 13 GHz 3 dB cut-off frequency, and data rates of
37 Gbit/s for non-return-to-zero (NRZ) modulation at 1550 nm.
Keywords:
vertical-cavity surface-emitting lasers (VCSELs); wafer bonding; superlattices; optical
modulation; long-wavelength; short-cavity; 1300 nm; 1550 nm; MBE; MOVPE
1. Introduction
According to Yole Intelligence’s annual ‘VCSEL—Market and Technology Trends 2022
report [
1
], the datacom sector of the vertical-cavity surface-emitting lasers (VCSELs) market
is growing at 22.2%/year, regaining the lead from the mobile and consumer sector, which
are growing at 15.7%/year.
The use of long-wavelength (1300–1550 nm) single-mode (SM) VCSELs makes it pos-
sible to reduce the modal and chromatic dispersion in an optical link and, as a result, to
extend its reach. Moreover, spatial division multiplexing (SDM) transmission by multi-
core fibers using long-wavelength (LW or short-wavelength infrared, SWIR) VCSELs are
enabling many larger-scale data center networks [
2
] than presently possible. A review
of data communication and telecommunication by LW VCSELs can be found in ref. [
3
].
1550 nm SM VCSELs are promising not only for next-generation short-reach optical in-
terconnects but also for SDM transceivers. In fact, the large output power and dynamic
characteristics of 1550 nm VCSELs [
4
] demonstrate their potential for large distance trans-
mission (e.g., 1 km), where high bit rate SM emission is a prerequisite for operating narrow
wavelength-division multiplexing systems (e.g., for 5 km distance). Moreover, 1550 nm
widely tunable micro-electro-mechanical system (MEMS) VCSELs are used for wavelength
DM passive optical network (WDP-PON) systems [
5
] with VCSEL-based SFP+ modules
and THz photomixing [6].
Photonics 2023,10, 268. https://doi.org/10.3390/photonics10030268 https://www.mdpi.com/journal/photonics
Photonics 2023,10, 268 2 of 22
Due to the absence of mode-hopping effects in short cavities, where longitudinal-mode
spacing is larger than the mirror stopband, 1950 nm range MEMS VCSELs can be applied
for tunable diode laser absorption spectroscopy (TDLAS) [
7
]. LW VCSELs (1300–2300 nm)
can also be used for gas sensing, including HF, H
2
O, NH
3
, CH
4
, HCl, H
2
S, CO
2
, CO, N
2
O,
ethylene oxide, and combustibles [8,9].
1300 nm VCSELs are of particularly strong importance for hybrid integration with
silicon photonics, providing integrated modulators and InP- and GaAs-based integrated
photonic circuits [
10
–
13
]. Other emerging markets for 1300 nm VCSELs include eye-safe-
based Laser Imaging Detection and Ranging (LiDARs) [
14
] and light sources for 5G (fifth
generation) mobile and optical wireless applications [
13
]. High-power SM VCSELs operat-
ing uncooled up to large on-chip temperatures are demanded. Modulation bandwidths
>5 GHz are preferable for time-of-flight applications to realize short raise and fall times
<1 ns. Novel proximity sensors may be based on 1380 nm VCSELs, enabling integration
with organic light-emitting diodes (OLEDs). A shift of the VCSEL emission wavelength to
1380 nm will enable using these VCSELs behind OLED displays, replacing the laser diodes
that are being currently used, e.g., in the proximity sensor under the display in the iPhone
14 Pro
©
[
1
]. Therefore, the first large-scale application of LW VCSELs might be proximity
sensors for smartphones. TRUMPF Photonic Components GmbH aims to bring the first
SWIR VCSEL products to the high-volume market in 2025 [15,16].
In this paper, we present a review of the current status of LW VCSEL development
and the relevant technologies. In Section 2, the various distributed Bragg reflectors (DBRs)
that are used for the fabrication of 1300–1550 nm spectral range VCSELs are compared.
In Section 3, we focus on the approaches to realize strong current confinement. The
present status of monolithic LW VCSELs grown on GaAs and InP substrates is presented in
Sections 4and 5, respectively. Hybrid VCSELs with active regions grown on InP wafers
will be discussed in Section 6. Approaches for the fabrication of LW VCSEL by wafer fusion
techniques are described in Section 7.
2. Distributed Bragg Reflectors for the 1300–1550 nm Spectral Range
Monolithic DBRs lattice-matched to InP based on InGaAsP/InP [
17
,
18
], AlInGaAs/
AlInAs [
19
], or AlGaInAs/InP [
20
] heterostructures suffer from very low refractive index
contrast (
∆
n). At 1550 nm, the refractive index contrast is ~0.3. At 1300 nm,
∆
n is only
~0.2 [
21
], since the band gap of the layers must be increased to avoid absorption. Larger
maximum
∆
n values are provided by AlAs/GaAs pairs (
∆
n ~ 0.5) and AlGaAsSb/AlAsSb
pairs (
∆
n ~ 0.49). Growth of, e.g., a bottom mirror with 99.9% reflectivity requires epitaxy
of 63 pairs of InAlGaAs/InP (
∆
n ~ 0.19) [
21
]. DBRs on GaAs substrates provide the same
reflectivity with only 23 AlAs/GaAs pairs. AlAs/GaAs can also be used for the 1550 nm
spectral range. AlGaAsSb-based DBRs combined with aperture formation based on an
air gap fabricated by selective etching of an imbedded tunnel junction (TJ) allowed the
development of LW VCSELs [
22
]. A disadvantage of this approach is the low thermal
conductivity of such mirrors, which leads to early turn-over of the output power with
increasing current and their predisposition to oxidation.
An alternative approach for mirrors with high reflectivity for LW VCSELs is DBRs
based on an InP/Air gap combination [
21
]. The refractive index contrast in such DBRs is 2.2,
and only three pairs of quarter-wavelength layers are required to reach 99.9% reflectivity.
However, the thermal and current conductivities of InP/Air pairs of such mirrors are
extremely low. Only configurations with double intra-cavity contacts and a thick heat
dissipation layer based on InP were reported until now. In addition, a design with air
gaps is quite fragile. Since the thermal resistance of DBRs is directly proportional to the
number of pairs in the DBR and also depends on the composition of the layers [
23
], dielectric
DBRs are also advantageous as compared to DBRs based on AlAs/GaAs while achieving
the same mirror reflectivity. Hybrid DBRs combining several pairs of dielectric DBRs and
a layer of gold enable a compromise from the point of view of minimizing the thermal
resistance of LW VCSELs DBRs. The thermal conductivities of various materials have been
Photonics 2023,10, 268 3 of 22
studied, such as amorphous silica (a-Si), MgF
2
, CaF
2
, Al
2
O
3
, and ZnS [
23
] ranging from
6.83 WK
−1
m
−1
(for ZnS [
23
]) to 2.4 WK
−1
m
−1
(for a-Si [
23
]). Al
2
O
3
/a-Si pairs show the
best thermal behavior, but
∆
n is only 1.6 and high loss is typical. 2.5 pairs of CaF
2
/a-Si
(
∆
n = 1.9) provide a thermal resistance of less than a third of the value of the corresponding
epitaxial mirror (7.4
×
10
3
K/W) [
23
,
24
]. 2.5 MgF/a-Si pairs (
∆
n = 1.9) enable reducing the
thermal resistance down to 5.6
×
10
3
K/W [
25
,
26
]. However, due to significant losses of
a-Si at 1550 nm (400 cm
−1
[
23
,
25
]), 2.5 pairs of CaF
2
/ZnS are used [
2
] to develop hybrid
DBRs for 1550 nm VCSELs with 99.85% reflectivity. The thermal resistance here is only
6.8 ×103K/W
. In general, before p-type contact fabrication, natural oxides are removed
from the contact layer surface by wet etching in diluted hydrochloric acid (HCl) or ammonia
(NH
3
). Moreover, the InP substrate can be removed by wet etching in HCl. Due to the solvent
possibility of CaF
2
in HCl and NH
3
in further works, authors began using DBRs based on
AlF
3
/ZnS for the development of 1550 nm VCSELs. AlF
3
/ZnS DBRs are compatible with
the HCl cleaning process but have a lower
∆
n (about 1.0) in relation to CaF
2
/ZnS [
27
]. The
penetration depth in AlF
3
/ZnS DBR is about 300 nm [
28
]. The use of five AlF
3
/ZnS pairs
provides a reflectivity of 99%. Here, the mirror thickness is about 2.4
µ
m, as compared to
7µm for a semiconductor DBR based on 30 AlGaInAs/AlInAs pairs [29].
For wavelengths at or below 1300 nm, the number of CaF
2
/ZnS pairs is 3.5 [
30
], due
to an
∆
n of 0.85 [
24
]. For 1300 nm VCSELs DBRs based on AlF
3
/ZnS were also used [
31
],
showing a larger difference in the refraction index (∆n = 0.95).
A completely novel approach to largely increases heat dissipation based on circumvent-
ing the mirrors for current injection and heat dissipation is reported by Tian et al. [32,33].
High-contrast gratings (HCG) might well be used in the future for the top
DBRs [34–41]
.
Buried tunnel junction (BTJ) and/or proton implantation are used to provide current con-
finement for HCG at 1300–1550 nm VCSELs. The principal advantages of HCGs in a
VCSEL include transverse mode control, fixed polarization, and reduced manufacturing
cost (reduction of the epitaxial growth time) [
42
]. Lasing via fundamental mode allows
one to realize high SM output power for HCG devices with large apertures. HCGs are
promising to realize widely tunable VCSELs. A review by C. Chang-Hasnain in this
volume reports in depth about HCG-based VCSELs. HCG provides an extraordinarily
wide stopband [
43
]. More than 98.5% reflectivity in the 1120–1620 nm spectral range was
demonstrated [
44
]. The additional advantage is wavelength scalability. By multiplying the
dimensions of HCG (by 6.5), the reflection band center shifts from 1.55
µ
m to 10
µ
m [
45
].
HCG-VCSEL lasing wavelength can be tuned by the HCG period and duty cycle [
43
]. As
a result, monolithic multi-wavelength VCSELs were realized [
46
]. The use of HCG as a
movable mirror allows one to realize the tunable VCSELs with a wide tuning range as well
as rapid tuning speed [
35
,
47
–
49
]. The detailed study of a systematic review of experimental
and modeling results as well as fabrication tolerance can be found in ref. [
43
]. In both
uniform and random-size HCGs, a very large fabrication tolerance in HCG-VCSELs is
demonstrated [
43
]. The main drawback of near-infrared subwavelength HCG VCSELs is
their modest dynamic performance in relation to VCSELs with semiconductor mirrors as
well as the challenging processing related to electron-beam lithography usage [50].
On the other hand, the dynamic performance of SWIR HCG VCSELs is comparable
(about 8 GHz modulation bandwidth for 1500 nm VCSELs [
3
,
34
,
39
]) with the same value
for wafer-fused VCSELs but inferior to the superior modulation performance of hybrid LW
VCSELs. MEMS-tunable 1550 nm VCSELs demonstrate the reliability of Telcordia GR468 [
51
].
A different type of DBR is based on MEMS. Using 11.5 SiO
x
/SiN pairs [
6
] allows one
to realize 7 GHz [
5
] and 9 GHz [
7
] modulation bandwidth for 1500 nm VCSELs. The
∆
n
value for SiO
x
/SiN is about 0.47. Using SiO
x
/SiC
z
DBRs [
52
] allows one to increase the
∆
n value to 1 [
53
]. As a result, the pair number can be decreased to 6.5 instead of 11.5 for
reflectivity of >99.5%. Finally, the stop band width is 243 nm (at a reflectivity of >99.5%)
and is about twice that of the SiO
x
/SiN pair. As a result, the small IR absorption of SiC
z
can
be diminished due to the wider tuning range compared to SiOx/SiN-based MEMS DBRs.
Photonics 2023,10, 268 4 of 22
3. Current Confinement and Tunnel Junctions
Current confinement in InP-based LW VCSELs can be achieved by selective lateral
etching of AlInAs-based TJs (so-called air-gap aperture), which is part of the whole TJ [
54
].
The use of a TJ allows one to inject holes through an n-doped region, thereby reducing the
series resistance and free carrier absorption. Further, the BTJ concept was used for current
confinement in InP-based LW VCSELs. The active region is embedded here between n-InP
and p-AlInAs for effective carrier confinement. Due to the poor electrical, thermal, and
optical properties of AlInAs, it is necessary to reduce their thickness [
3
]. BTJs allow one
to reduce the thickness of AlInAs to one-quarter of the emission wavelength and replace
it with low-resistance n-InP, yielding the desired cavity length [
3
]. BTJ for short-cavity
VCSELs was first developed by Prof. Amann’s group, whereas regrown tunnel junctions
embedded in wafer-fused VCSELs were developed by Prof. Kapon’s group [
55
]. Below, we
use BTJ denotation for both types of TJs.
The main idea of BTJs for LW VCSEL design is to use a laterally structured tunnel
junction inside a p-type layer to:
-
convert most of the current confinement layers in the p-type region from p- to n-type
to get lower electric resistance, reducing self-heating;
- use ohmic contacts from two sides to n-type layers with low contact resistance;
- reduce optical losses in n-type layers used at the former p-side of the laser;
- use intra-cavity contacts with low resistance;
- achieve effective lateral current confinement;
- achieve strong lateral optical confinement [56].
The principal structure and electric circuit of BTJs are presented, e.g., in refs. [
56
,
57
]. To
realize SM lasing, the topology of the overgrown surface relief and the etching depth of TJs
should be considered. The lateral refractive index contrast is qualitatively determined by
the height of overgrown TJ (
∆
L) [
3
]:
∆
n
eff
/n
eff
=
∆
L/L, where n
eff
is the effective refractive
index and Lis a microcavity length. The BTJ consists here [
3
] of highly doped p- and n-type
(InGaAl)As layers. The BTJ is located on the p-side of the diode. The side walls of the
BTJ are formed by selective etching of the n
+
-InGaAs top layer and parts of the bottom p
+
InAlGaAs layers, which contain only a few percent of aluminum [
58
]. The BTJ is overgrown
by a moderately doped n-InP layer. Therefore, when a reverse bias is applied to the BTJ, the
p
+
n
+
junction demonstrates ohmic behavior and extremely low resistance due to tunneling.
The BTJ is located in a node of the optical field aimed at eliminating free-carrier absorption
in the heavily doped TJ layers. p
+
-InGaAlAs/n
+
–In(Al)GaAs TJ can be grown by molecular-
beam epitaxy (MBE), chemical beam epitaxy [
26
,
59
], and metalorganic vapor-phase epitaxy
(MOVPE) [
56
,
60
,
61
]. Using MBE involves an extra hydrogen cleaning procedure [
62
,
63
]
based on the RF source in the chamber [
64
,
65
] to deoxidize the oxidized InGaAlAs surface
before TJ regrowth. Growth of InAlGaAs-based BTJs is difficult in industrial MBE systems.
An alternative approach is using composite n
++
-InGaAs/p
++
-InGaAs/p
++
-InAlGaAs
TJs [
10
,
66
,
67
]. A similar approach to using aluminum-free (n
++
-InGaAs/p
++
-InGaAs) TJs
was previously presented in ref. [
68
]. In both cases, solid-source MBE regrowth, including
the surface planarization effect, is absent [
3
,
10
,
26
,
59
,
61
,
69
] and shows strong index-guiding
results as compared to MOVPE regrowth.
Additional attention is devoted to doping optimization of the overgrown InP layer
at BTJ formation. As previously mentioned for hybrid VCSELs [
27
], the reduction of
overgrown layer doping down to 1
×
10
17
cm
−3
allows one to increase the modulation
bandwidth by 35% (from 6.7 to 9 GHz).
In addition to BTJs, current confinement can be achieved by ion
implantation [34,70–73],
leading to a planar device geometry. A disadvantage is the lack of inherent optical
confinement [74].
To overcome the complicated implementation of high-level p-doping of InGaAs above
1
×
10
19
cm
−3
GaAs
0.51
Sb:C/Ga
0.47
InAs:Si BTJs were suggested to realize 1300 nm VC-
SELs [75] grown by MOCVD.
Photonics 2023,10, 268 5 of 22
Alternatives to BTJs were considered. In contrast to the traditional AlGaAs layer’s
high Al-composition in GaAs-based VCSELs, the use of InP-lattice-matched Al
x
In
1−x
As
layers in InP-based VCSELs is complicated due to low oxidation speeds [
3
]. Selective
oxidation of In
0.78
Al
0.22
As
0.47
P
0.53
layers for the fabrication of 1550 nm oxide-confined
VCSELs was shown in [76,77].
Results were also presented for developing current apertures by undercutting (selec-
tive chemical etching) the quantum wells (QWs) [
21
,
22
,
78
–
80
]. VCSELs with BTJ apertures,
however, show better performance as compared to undercut QWs, implanted apertures, or
selectively oxidized apertures.
4. Monolithically Grown VCSELs on GaAs Substrates in the 1300–1550 nm
Spectral Range
The large refractive index contrasts of mirrors on GaAs substrates enable a reduction
in the number of pairs in DBRs as compared to DBRs on InP substrates. Strained InGaAs,
InGaP, InGaAsSb, GaAsSb, GaInNAs, and GaInNAsSb QWs are usually considered active
regions for monolithic 1300-nm VCSELs grown on GaAs. Implementation of more long-
wavelength VCSELs is possible by using GaInAsNSb/GaAs QWs.
In addition to QWs, quantum dots (QDs) based on InGaAs/AlGaAs were shown to
serve well as active regions already two decades ago. InAs QDs and submonolayer QDs
enable to control of the emission wavelength more flexibly and potentially reach a higher
differential gain and lower threshold currents. The low density of QD arrays makes it nec-
essary to stack rows of QDs to reach the required optical gain [
81
], which at the same time
limited the output power and modulation bandwidth of 1280 nm spectral range devices
to 0.3 mW and 2 GHz (the data transfer rate is 2.5 Gbit/s) at 20
◦
C,
respectively [82]
. Sub-
monolayer QDs circumvented this problem, and 20 Gbit/s open eyes were demonstrated
between 20 and 85 ◦C up to 1250 nm [83].
Using In
0.42
Ga
0.58
As QWs made is possible to demonstrate 10 Gb/s modulation at
both 25 and 85
◦
C at 1280 nm emission wavelength, which is due to a large gain to cavity
detuning, GCD (about 55 nm) (photoluminescence (PL) peak of In
0.42
Ga
0.58
As QWs is
about 1220 nm) [
84
–
90
]. However, implementation of effective VCSELs beyond 1300 nm
wavelength in the framework of this approach turned out to demand more work.
Using InAs
0.44
P
0.56
strain-compensated QWs made it possible to implement multi-
mode lasing at 1300 nm with an output power of about 1.9 mW [
91
]. Another approach
was using GaAs
0.665
Sb
0.335
/GaAs QWs. Operation in continuous-wave (CW) mode was
demonstrated at 1230 nm wavelength with 0.7 mA threshold current [
92
]. Later, the wave-
length could be shifted to 1300 nm [
93
] by using GaAs
0.63
Sb
0.36
/GaAs QWs. CW operation
at 70 ◦C has been demonstrated.
Multiple groups contributed to the development of VCSEL based on diluted nitrides,
including industrial research departments (Infineon, Sandia/Cielo, JDSU/Picolight, Alight
Technologies, Philips U-L-M Photonics, IQE etc.) [
94
–
96
]. Devices in the 1300 nm spectral
range based on GaInNAs QW with 10 Gbit/s operating speed and more than 1.2 mW output
power at 20
◦
C were demonstrated in Ref. [
97
]. The use of TJ and microcavity geometry
with intra-cavity contacts enabled not just an increase in the modulation bandwidth up
to 9.8 GHz at 20
◦
C but also an increase in the output power to 4.2 mW [
98
]. Several
groups demonstrated data transfer rates of 10 Gbit/s [
94
,
97
–
101
]. However, the growth of
GaInNAs QWs is associated with the decomposition of quaternary alloys into microscopic
InGaN and InGaAs regions (phase segregation) and the formation of immiscibility regions,
which requires more detailed research of the epitaxial growth of GaInNAs QWs. Moreover,
the incorporation of N results in an increase in strain and the number of radiation defects.
As a result, for long-term laser reliability, the nitrogen concentration in the QW should
not exceed 1% for indium concentration larger than 30% [
99
], which limits the maximum
VCSEL wavelength to the 1270–1280 nm range.
A partial solution of the phase segregation is associated with the use of Sb, which
makes it possible to increase the critical thickness limit associated with the transition to 3D
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