Optics and Photonics Journal, 2013, 3, 66-68
doi:10.4236/opj.2013.32B016 Published Online June 2013 (http://www.scirp.org/journal/opj)
Four-wavelength Microdisk Lasers Laterally Coupling to
an Output Bus Waveguide
Ling-Xiu Zou, Xiao-Meng Lv, Yong-Zhen Huang, Heng Long, Qi-Feng Yao, Yun Du
The State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors,
Chinese Academy of Sciences, Beijing, China
Email: zoulingxiu@semi.ac.cn
Received 2013
A multiple-wavelength GaInAsP/InP microlaser with microdisk radiuses from 10 m to 10.6 m laterally coupled into a
bus waveguide are fabricated by standard photolithygraphy and inductively coupled-plasma (ICP) etching techniques.
The lasing wavelengths are 1533 nm, 1541 nm, 1551 nm and 1555 nm at the CW injection current of 10 mA for the
four microlasers. The proposed multiple-microlaser array would be useful for realizing compact wavelength division
multiplexing (WDM) light source for optical interconnects.
Keywords: Optical Resonator; Microdisk Laser; Laser Arrays
1. Introduction
Optical wavelength division multiplexing (WDM) tech-
nology has attracted great attentions for noticeably in-
creasing the bandwidth density and communication ca-
pacity of a single link and makes it more feasible to real-
ize monolithic PICs. To meet the increasing requirement
of optical interconnection link, laser sources which could
emit signals of several wavelengths with equal intervals
are indispensable [1]. This sort of multi-wavelength laser
(MWL) source can be realized by cascading several mi-
crolasers on one SOI bus waveguide as shown in [2]. As
microdisk lasers have long been proposed as attractive
light sources for integrated optical circuits due to their
high Q-factors, low power consumption and cleavage-
free lasing cavities [3,4], multi-wavelength operation has
also been demonstrated by vertically coupling the WG
mode laser from microdisk cavities with different radi-
uses into a bus I/O waveguide [5-7]. However, the fabri-
cation of this kind of microdisk lasers has been highly
limited to the complexity of multi-layer growth process
or bonding technology. In this paper, we proposed a sim-
ple but efficient way to coupling the light out of the mi-
crocavity using laterally coupling method, and the com-
pact integration of four-wavelength microdisk lasers to a
single bus waveguide is also realized.
In this paper, we will introduce the fabrication process
of four-wavelengths microdisk lasers laterally coupling
to a bus waveguide, the voltage and output power inten-
sity versus the injection current are both measured and
discussed. Finally, the lasing spectra of four microlasers
at the same injection current of 10 mA are given and
2. Design and Fabrication
When adjacent microdisk modes, with different radial
distributions or azimuthal mode numbers, are not spec-
trally remote enough from one another, and both compa-
rably close to gain peak wavelength, these modes are
very likely to be involved in a competition process to
acquire gain that results in lasing mode hopping over a
range of currents. To avoid this situation, adjacent modes
should be separated by diverse free spectral range (FSR),
which can be achieved by design different disk sizes. In
this paper, we demonstrate four-wavelength microdisk
lasers consist of four microdisk cavities and one 2-m-
width bus waveguide, with the radiuses of four cavities
regularly ranging from 10 m to 10.6 m. The free spec-
tral ranges (FSR) of the cavity resonance are about 11
nm - 12 nm in this case.
An AlGaInAs/InP laser wafer grown by metal-organic
chemical vapor deposition (MOCVD) is used for fabri-
cating the devices. The active region of the laser wafer
consists of six compressively strained 6-nm-thick Al0.24
GaIn0.71As quantum wells and 9-nm-thick Al0.44GaIn0.49As
barrier layers. The total growth thickness is roughly 2.3
nm without the N-InP buffer layer. The fabrication proc-
esses can be briefly summarized as follows.
*This work was supported by High Technology Development Project
under Grant 2012AA012202, and the National Nature Science Founda-
tion of China under Grants 61235004, 61006042, 61106048, and
61061160502. First, an 800-nm SiO2 layer was deposited by plasma-
Copyright © 2013 SciRes. OPJ
L. X. ZOU ET AL. 67
enhanced chemical vapor deposition (PECVD) on the
laser wafe. Then, the microdisk patterns are transferred
onto the SiO2 layer using standard photolithography and
inductively coupled-plasma (ICP) etching techniques,
and the laser wafer is etched to about 4.7 μm using the
ICP technique subsequently with the patterned SiO2 layer
as hard mask. After the ICP etching process, a 200 nm
silicon nitride (SiNx) layer is deposited by PECVD on the
wafer to prepare a plane with better adhesion for the fol-
lowing DVS-BCB(divinyl siloxane bisbenzocyclobutene)
spin-coating process, and protect the cavities from the
following non-selective BCB etching process at the same
time. The DVS-BCB Cyclotene 3022-46 is coated twice
onto the wafer to create a planar cladding layer and then
experiences soft and hard cure in turn, and then the BCB
film is etched to expose the top of microdisk resonators
by Reactive Ion Etching (RIE) without any mask. After
that, a contact window is opened by ICP etching for cur-
rent injection on top of each resonator buried in BCB, on
which pad-patterned P-electrodes are formed afterward
using lifting off technology. Finally, the laser wafer is
mechanically lapped down to a thickness of about 120
μm, and an Au-Ge-Ni metallization layer is used as n-
type patterned electrode. The microscopic pictures of
fabricated microdisk resonator microlasers with different
radiuses and a bus waveguide are shown in Figure 1,
where the circle patterns on the top of the resonators are
the etched current injection windows. The radiuses for
lasers 1 - 4 are 10.6 μm, 10.2 μm, 10.4 μm and 10 μm,
3. Results and Discussion
The fabricated multi-wavelength MDL is bonded onto a
Cu heat sink and tested at room temperature without
temperature control. Metal needles are utilized to inject
continuous wave (CW) current onto each electrode pad
of the device, while a tapered fiber fixed at a three-di-
mensional stage is used to couple light out of the output
For MDLs with radiuses of 10 μm, 10.2 μm, 10.4 μm
and 10.6 μm, the lasing modes are coupled out using a
2-μm- width bus waveguide and collected by a tapered
multi- mode fiber. As shown in Figure 1, the bus
waveguide is tilted by 7°to avoid the Fabry-perot mode
oscillation. The applied voltage and the output power
versus the CW injection current of laser 1 is shown in
Figure 2. By fitting this V-I curve, we get a series resis-
tance of 16 . The output intensity of laser 1 is also
given and several kinks could be found in the L-I curves,
the first of which indicate the threshold current while the
others could be explained by the mode-jumping. The
threshold current at room temperature is about 3 mA es-
timated from the intersect point at the current axis for the
extended line of the output power curve as shown by the
dashed line in Figure 2. The maximum output powers of
the four microdisk lasers are 0.67 μW, 1.92 μW, 1.09 μW
and 3.89 μW for lasers 1 - 4, respectively. This power
diversity could be explained by the absorption effect of
bus waveguide, as it has the same quantum well struc-
tures with the microdisk cavity, thus microdisk lasers
locating further to the output facet would suffer more
absorption. Besides, the output power could also be op-
timized by improve the coupling efficiency between the
waveguide and microdisk cavities. But it is still chal-
lenging to solve those problems by conventional planar
process technology.
The laser spectra are separately measured by an optical
spectrum analyzer with the resolution of 0.1 nm at room
temperature. In the measurement, clear spectra begin to
be recognized at injection current about 5 - 7 mA for all
the microdisk lasers. The spectra of the four microdisk
lasers measured at the same CW injection current of 10
mA are plotted in Figure 3. The main lasing modes ap-
pear at 1533 nm, 1541 nm, 1551 nm and 1555 nm at the
CW injection current of 10 mA, with a side mode sup-
pression ratio
Figure 1. Fabricated multi-wavelength laser before metalli-
zation, composed of four microdisk lasers and one bus
waveguide. The arrows indicate the directions towards the
grating couplers.
0510 15 20 25 30 35
Power (
Figure 2. The applied voltage and output power versus in-
jection current for laser 1 with a radius of 10.6 μm. The
dashed extended line which reveal the intersect point at the
current axis of L-I curve is used to estimate the threshold
current at room temperature.
Copyright © 2013 SciRes. OPJ
Copyright © 2013 SciRes. OPJ
1520 1530 1540 1550 1560
Intensity (d B)
Wavelength (nm)
L2 L3
Figure 3. The spectra of the four microdisk lasers measured
at the same CW injection current of 10 mA.
of 14.0 dB, 11.5 dB, 14.3 dB and 15.6 dB, respectively.
The peaks of lasers 1 - 4 are marked as L1-L4 in Figure
3, respectively. The spectrum of laser 2 has two adjacent
peaks, resulting from the coupling mode in circular cavi-
ties. The mode intervals between the dominant peaks are
8 nm, 10 nm and 4 nm, which are remote enough but not
quiet well-pro- portioned. The lasing wavelength could
have been controlled more precisely with higher planar
technology fineness. As lasing wavelength could also be
affected by the injection current due to temperature
variation, the mode intervals as well as power intensity
could be optimized by adjusting the injection current of
each microdisk lasers.
4. Conclusions
We have demonstrated the four-wavelength microdisk
lasers buried in BCB and laterally coupling to a bus
waveguide. The lasing wavelengths of the four mi-
crolasers are 1533 nm, 1541 nm, 1551 nm and 1555 nm
at the injection current of 10 mA. However, the output
powers are still limited by the absorption loss of the out-
put waveguide and weak coupling efficiencies between
the microdisks and the output waveguide.
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