Four-wavelength Microdisk Lasers Laterally Coupling to an Output Bus Waveguide

doi:10.4236/opj.2013.32B016

Paper Menu >>

Journal Menu >>

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: [email protected]

Received 2013

ABSTRACT

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

compared.

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-

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,

respectively.

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

waveguide.

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 Ω. 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

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Current(mA)

Voltage(V)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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.

L. X. ZOU ET AL.

1520 1530 1540 1550 1560

-90

-85

-80

-75

-70

-65

Intensity (d B)

Wavelength (nm)

L2 L3

10mA

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.

REFERENCES

[1] H. Debregeas-Sillard and C. Kamierski, “Challenges and

Advances of Photonic Integrated Circuits,” Comptes Ren-

dus Physique, Vol. 9, No. 9-10, 2008, pp. 1055-1066.

doi:10.1016/j.crhy.2008.10.004

[2] Y. C. Shi, X.F. Chen, Y.T. Zhou, S.M. Li, L.L. Lu, R. Liu,

et al., “Experimental Demonstration of Eight-Wavelength

Distributed Feedback Semiconductor Laser Array Using

Equivalent Phase Shift,” Optics Letters, Vol. 37, No. 16,

pp. 3315-3317, 2012. doi:10.1364/OL.37.003315

[3] S. J. Wang, J. D. Lin, Y. Z. Huang, Y. D. Yang, K. J. Che,

J. L. Xiao, et al., “AlGaInAs-InPMicrocylinder Lasers

Connected With an Output Waveguide,” IEEE Photonics

Technology Letters, Vol. 22, 2010, pp. 1349-1351.

[4] X. M. Lv, L. X. Zou, J. D. Lin, Y. Z. Huang, Y. D. Yang,

Q.F. Yao, et al., “Unidirectional-Emission Single-Mode

AlGaInAs-InP Microcylinder Lasers,” IEEE Photonics

Technology Letters, Vol. 24, No. 11, 2012, pp. 963-965.

doi:10.1109/LPT.2012.2190892

[5] S. J. Choi, Z. Peng, Q. Yang, S. J. Choi and P. D. Dapkus,

“Eight-Channel Microdisk CW Laser Arrays Vertically

Coupled to Common Output Bus Waveguides,” IEEE

Photonics Technology Letters, Vol. 16, No. 2, 2004, pp.

356-358. doi:10.1109/LPT.2003.821249

[6] J. Van Campenhout, L. Liu, P. R. Romeo, D. Van Thour-

hout, C. Seassal, P. Regreny, et al., “A Compact

SOI-Integrated Multiwavelength Laser Source Based on

Cascaded InP Microdisks,” IEEE Photonics Technology

Letters, Vol. 20, No. 13-16, 2008, pp. 1345-1347.

doi:10.1109/LPT.2008.926857

[7] A. Bennecer, K. A. Williams, R. V. Penty, I. H. White, M.

HamacherH. Heidrich, “Directly Modulated Wave-

Length-Multiplexed Integrated Microring Laser Array,”

IEEE Photonics Technology Letters, Vol. 20, No. 13-16,

pp. 1411-1413, 2008.

doi:10.1109/LPT.2008.927880