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Few-mode vertical-cavity surface-emitting laser:

Optional emission of transverse modes with different polarizations

Chuyu Zhong1,2, Xing Zhang1*, Werner Hofmann3, Lijuan Yu4, Jianguo Liu4*, Yongqiang Ning1, and Lijun Wang1

1State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, China

2University of Chinese Academy of Sciences, Beijing 100049, China

3Institut für Festkörperphysik und Zentrum für Nanophotonik, Technical University of Berlin, 10623 Berlin, Germany

4State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Beijing 100083, China

*E-mail: zhangx@ciomp.ac.cn; jgliu@semi.ac.cn

Received March 12, 2018; accepted March 29, 2018; published online April 13, 2018

Few-mode vertical-cavity surface-emitting lasers that can be controlled to emit certain modes and polarization states simply by changing the

biased contacts are proposed and fabricated. By directly etching trenches in the p-doped distributed Bragg reﬂector, the upper mesa is separated

into several submesas above the oxide layer. Individual contacts are then deposited. Each contact is used to control certain transverse modes with

different polarization directions emitted from the corresponding submesa. These new devices can be seen as a prototype of compact laser sources

The vertical-cavity surface-emitting laser (VCSEL), a

key component for a variety of applications,1–4)can

intrinsically support multiple transverse modes through

its large transverse dimension, which is usually an undesirable

property in most applications. However, for some applications

where laser sources are employed in the form of an array,5)

such as optical ﬁber communications using multiplexing tech-

nologies, including mode division multiplexing (MDM),6–8)

this unwanted feature could instead be beneﬁcial.

MDM technology, in which information is selectively

loaded on diﬀerent spatial modes and transferred in a single-

core few-mode ﬁber (FMF),9)is expected to solve the problem

that the capacity of commercial single-mode-ﬁber-based

optical networks is reaching its limit. Like other multiplexing

technologies, MDM requires multiplexers (MUXs) to convert

multiple signals from a VCSEL array into a single ﬁber.

Therefore, to ensure applicability to MDM communication

systems, it is meaningful and feasible to develop a new type of

VCSEL with multiple individually controllable modes, which

can remove the need for a VCSEL array and greatly simplify

or even eliminate the need for MUXs. Further, the polar-

ization dynamics of the VCSEL are closely associated with

the transverse modes,10–12)so this type of VCSEL could also

provide orthogonal ﬁber modes for MDM.

Various types of densely packed VCSEL arrays for high-

speed communication have been fabricated. These arrays

must have a small footprint for eﬃcient ﬁber coupling.

However, some of them13,14)are not compact enough to be

coupled into FMFs without sophisticated coupling optics.

Ultrahigh-density VCSEL arrays with pie-shaped ele-

ments15,16)have been fabricated, but in the design, only up

to three mesas can be ﬁtted into one ﬁber core. Photonic

crystal and proton-implanted coherently coupled VCSEL

arrays17)with satisfactory properties have been reported.

However, the need for complex fabrication limits the yield

and potentially results in a high fabrication cost. Similar-

looking devices with deeply etched petals have been

presented,18)but these devices focus on high-power single-

mode output and are not suitable for MDM.

In this work, we present VCSELs with several emission-

controllable transverse modes obtained by directly etching a

few air gaps from the top. The upper mesa is separated into

several submesas acting as waveguides, each of which is

deposited with p-type contacts. Two types of few-mode

VCSEL with three and four submesas are fabricated. Single-

mode emission control of each submesa is achieved under

certain driving current scales. Furthermore, polarization-con-

trollable emission is achieved owing to the transverse-mode-

related polarization of a VCSEL. The few-mode VCSELs

with several pie-shaped submesas in a circle can naturally

emit diﬀerent transverse modes together with diﬀerent polari-

zation directions.

The three-dimensional structures of our few-mode VCSELs

are schematically illustrated in Fig. 1. For example, in the

three-mode VCSEL, the upper mesa is divided into three

Fig. 1. Three-dimensional sketch of three-mode and four-mode VCSELs.

When a current Iais injected through p-contact a, mode a (Ma) is emitted, and

so forth for p-contacts b and c. Lasing region a (ra) corresponds to Maand so

on for other lasing regions and modes. The inset shows the current ﬂow

inside the device, where rtr is the region directly under the trench.

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Applied Physics Express 11, 052702 (2018)

https://doi.org/10.7567/APEX.11.052702

lobes (submesas) by air trenches, and p-contacts are deposited

above them. As mentioned in our previous work on the two-

mode VCSEL,19)the air gaps provide extra current isolation

and carrier-distribution guidance. Therefore, the trenches

enable an inhomogeneous carrier distribution in the active

region, where the laser can emit only if the carrier density is

suﬃcient, because the optical modes in an oxide-conﬁned

VCSEL are governed in part by the relationship between the

optical ﬁeld and carrier distribution.20)In addition, the air

trench aﬀords optical restriction. From the air gap (rtr), only

ﬂuorescence is emitted because of the low reﬂectivity. Laser

light is stimulated from lasing regions ra,rb,andrcwhen

current is injected from the corresponding contact. These

regions are located in the active layer, and their sizes and

shapes are determined directly by the sizes and directions of

the air trenches together with the oxide aperture. Because

of the anisotropy of the oxidation speed of the oxide layer,

the oxide aperture has an elliptical shape. Therefore, the

pie-shaped lasing regions can be nonsymmetrical, as shown in

Fig. 2(a).

To understand how the geometry of the lasing region

aﬀects the transverse mode, mode analysis of submesas with

lasing regions of diﬀerent sizes and shapes was performed

using COMSOL Multiphysics®. The ﬁrst three eigenmodes

of the submesa with a 120°-opening-angle lasing region are

depicted in Figs. 2(d)–2(f). Owing to the region’s symmetry,

they are all x-polarized. In the 95° one shown in Figs. 2(f)

and 2(h), the polarization directions of the fundamental

modes are 6:15oﬀof the x-axis because the shape is set to

be nonsymmetric. In fact, each eigenmode of a certain lasing

region in Fig. 2 has two solutions, both of which have almost

the same intensity distribution but orthogonal polarizations,

and we show only one of them. However, we can still refer to

the simulation results to achieve manipulation of both the

transverse modes and polarization of a single VCSEL device

by properly controlling the size of the oxide aperture and the

direction of the trenches.

The current ﬂow inside our device was also simulated,

and the normalized current density distributions on the active

layer under diﬀerent injection conditions are shown in Fig. 3;

they indicate that current crosstalk between submesas is

small. The carrier density of the active regions under the

unbiased submesas should be far from suﬃcient to excite the

laser, guaranteeing independent emission control of each

p-contact.

The main fabrication process of the few-mode devices is

the same as that of a normal VCSEL, except for an additional

step of air gap etching by inductively coupled plasma. To

control the shapes and sizes of the trenches more precisely,

trench etching was performed in the ﬁrst step. The depth of

the trench is approximately 2.3 µm, whereas the oxide layer

and active layer are 2.65 and 2.79 µm deep, respectively.

Because the trench bottom is so close to the oxide layer, the

width of the air gaps can be designed to be as narrow as 2 and

3 µm, whereas they got about 0.2 µm widened.

All measurements were performed under continuous-wave

(CW) operation at room temperature. Ii(i=a,b,c,ord)

indicates that only contact-iis biased. Iij (i,j=a,b,c,ord,

i≠j) indicates that both contact-iand contact-jare biased, and

so forth for Iijk and Iijkl. The measured near ﬁelds of the three-

mode VCSEL are depicted in Fig. 4. The irregular proﬁles of

the lasing regions and ﬂuorescence from the trenches can be

seen below threshold in Fig. 4(a). Although ﬂuorescence from

the trenches exists under all the types of injection, it is nearly

invisible above threshold because neutral density ﬁlters with

various extinction ratios were used to avoid saturation of the

CCD camera by the laser. The near ﬁelds under Ia,Ib, and Ic

injection are displayed in Figs. 4(b), 4(c), and 4(d), respec-

tively. Iiinjection successfully produced only mode Miemis-

sion coinciding with the near ﬁelds under multicontact injec-

tion, which are shown in Figs. 4(e)–4(h). In summary, the

three-mode VCSEL has seven types of operation: three types

of single-mode operation, three types of two-mode operation,

and one type of three-mode operation.

(a) (b) (c)

=0°

=−6.15°

(d)1

(e)1

(f)1

(g)1

(h)1

(d)

(e)

(f)

(g)

(h)

2=6

.15°

(d)3

(e)3

(f)3

(g)3

(h)3

|E|2|E|2|E|2

x,Ex

y,Ey

lasing region

(un-oxidized aperture)

θ= 120

◦

θ= 120

◦

θ=120

◦

θ=95

◦

θ=95

◦

Fig. 2. Mode analysis of various type of single lasing regions. (a) Infrared

microscope image of the mesa after steam oxidation. Black areas are the

oxide layer, and the three gray regions belong to the un-oxidized aperture.

(b) Simulation geometry. The red pie-shaped area is the lasing region, the

gray area is the AlxO, and the light purple region represents the high-loss area

under the trench. θis the opening angle between trenches. (c) Annotation

of the polarization directions of diﬀerent electric ﬁeld components.

(d1)–(h1) Mode patterns of the total electric ﬁeld E. Intensity distribution of

(d2)–(h2) the major polarization component Eφand (d3)–(h3) the minor

polarization component EφA. Lasing regions in (d), (e), and (f) are symmetric,

whereas those in (g) and (h) are nonsymmetric. φA=φ+ 90°. The white

dashed lines in (d)–(h) indicate the interface between the submesa and

air gap.

Fig. 3. Calculated normalized current density distribution in active layer

under (a) single-contact, (b) two-contact, and (c) three-contact injection.

Appl. Phys. Express 11, 052702 (2018) C. Zhong et al.

The polarization directions at each injection condition are

summarized in Table I. Under single-contact injection, the

main polarization directions of Ma,Mb, and Mcare 50, −40,

and 50°, respectively. Because of the nonsymmetrical geom-

etry, the polarization directions are neither along nor orthog-

onal to the angle bisector of the tip of raand rb. The minor

component of Mais relatively obvious, but its magnitude is

still small compared to the major component, as shown in

Fig. 5(a). Interestingly, it is the second-order mode [see

Figs. 2(c) and 4(c)] rather than the fundamental mode that is

the ﬁrst excited mode of rb. As shown in Fig. 5(b), there are

two peaks in the spectra of Mbunder 5 mA, but we can

observe only the near ﬁeld of the second-order mode, which is

the major polarization component under full current scale.

Further, only Icinjection can achieve single-mode emission

at full current scale, which is evident in Fig. 5(c). Therefore,

every contact can be controlled to achieve single-mode,

single-polarization emission at certain driving currents. Under

multicontact injection, the polarizations of some transverse

modes shifted. Iab injection was unstable, as the major polari-

zation of Mbswitched between 50 and −40° from time to time,

whereas Mahad stable polarization at 50°. Under Iac injection,

Maswitched to stable polarization at −40°, and Mcremained

polarized at 50°. Under Ibc injection, both Mband Mcwere

polarized at 50°. Iac injection resulted in emission with two

controllable orthogonal polarizations.

The CW L–I–Vcharacteristics of the three-mode VCSEL

are shown in Fig. 6. When laser modes propagate from the

active layer to the output window, they suﬀer from scattering

loss from the air trenches. Modes excited from lasing regions

with a larger aspect ratio have a higher ﬁeld intensity at the

edges, resulting in more loss. However, the larger size can

help complement the power. This behavior explains the

diﬀerences among the power curves of ra,rb, and rc.

In agreement with our expectation, mode control of a four-

mode VCSEL was also achieved, as shown in Fig. 7. By

investigating the spectra under single-contact injection, we

demonstrated single-mode emission from three submesas.

Submesas a and b can maintain single-mode operation at full

current scale, as shown in Figs. 7(a) and 7(b). From Fig. 7(c),

we see that single-mode operation remained until the driving

current reached 4.5 mA for submesa c. The largest region,

submesa d, exhibited two-mode emission at full scale, as

illustrated in Figs. 7(d) and 7(h), implying that the direction

of the air gaps can be modiﬁed. The peak of M’0for Md

is higher than the Mφvalues in the spectrum owing to

diﬀerences in the coupling eﬃciency of diﬀerent transverse

modes; that is, the fundamental mode has better coupling

than higher-order modes. The near ﬁelds and the correspond-

ing polarizations are displayed in Figs. 7(e)–7(h). Inde-

pendent control of three polarizations in a single device was

realized, and two of the polarization directions are orthog-

onal. Although it is not demonstrated, the four-mode VCSEL

can operate under 15 injection conditions, that is, in 15

(a) (b) (c) (d)

(e) (f) (g) (h)

Ma a =50°Mb b =−40°Mc c =50°

Fig. 4. Near ﬁelds of a three-mode VCSEL. (a) Near-ﬁeld pattern below

threshold, where lasing regions can be observed and are enclosed in dashed

lines. (b)–(d) Near ﬁelds under Ia,Ib, and Icinjection, respectively. The

arrows show the polarization directions of Mφ. (e)–(h) Near-ﬁeld proﬁles

under Iab,Iac,Ibc, and Iabc injection, respectively.

Table I. Major polarization angle under diﬀerent types of injection.

IaIbIc

Ia50° Ma: 50°, Mb: 50° or −40° Ma:−40°, Mc: 50°

Ib—−40° Ma,Mc: 50°

Ic—— 50°

(a) Iainjectioin

(b) Ibinjectioin

Fig. 5. Spectra of three-mode VCSEL under single-contact injection.

(a) Spectra under Iainjection from 5 to 7 mA. Insets show the fundamental

mode and a high-order mode. (b) Spectra under Ibinjection from 2 to 5 mA.

Current(mA)

100

200

300

400

500

600

Power(μW)

Ia injection

Ib injection

Ic injection

2 4 6 8 10 12

Voltage(V)

Fig. 6. CW L–I–Vperformance of three-mode VCSEL.

Appl. Phys. Express 11, 052702 (2018) C. Zhong et al.

transverse mode combinations. Finally, the L–I–Vcurves are

shown in Fig. 8, from which we can also diﬀerentiate the

lasing regions by their geometries. Improved single-mode

power compared to the three-mode device was achieved.

In summary, VCSELs with several independent contacts

that can control the emission of certain transverse modes

and diﬀerent polarizations independently were demonstrated.

The shape and direction of each lasing region were proven

to have a decisive impact on the transverse mode formation

and polarization direction. The functionality of current

guidance and optical ﬁeld restriction of the air gap enable

the independent emission control. Near-ﬁeld proﬁles and

spectra under single-contact injection and multicontact

injection indicated that the emission status of the few-mode

VCSEL can be manipulated by simply shifting the biased

contact, and the VCSEL can be switched from single-mode

operation to few-mode operation. We believe that our few-

mode VCSEL prototype will contribute to future optical

communication systems.

Acknowledgments This work is supported by the National Basic

Research Program of China (No. 2014CB3400102), the National Natural Science

Foundation of China (NSFC) (Nos. 61434005, 61474118, 11774343, and

11674314), the Youth Innovation Promotion Association, CAS (No. 2017260),

and the Chinese Academy of Sciences President’s International Fellowship

Initiative (No. 2018VTA0005).

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(a) Iainjectioin

(b) Ibinjectioin

(d) Idinjectioin

(e) Ma a =−80°

(f) Mb b =10°

(g) Mc c =−45°

(h) Md d =10°

M M

Fig. 7. Spectra and near-ﬁeld proﬁles of four-mode VCSEL for diﬀerent

single-contact injection conditions. (a)–(d) Spectra for each type of single-

contact injection from 1.5 to 4.5 mA. (e)–(h) Near ﬁelds under Ia–Idinjection.

Inset in (c) shows the minor component of Mcunder 4.5 mA, and insets in

(d) show the major and minor components of Md.

Current(mA)

200

400

600

800

1000

1200

Power(

injection

246810

Voltage(V)

Fig. 8. CW L–I–Vperformance of four-mode VCSEL. Inset shows the

near-ﬁeld proﬁle below threshold.

Appl. Phys. Express 11, 052702 (2018) C. Zhong et al.