Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasers [original]

Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361 97, 151101

Highly temperature-stable modulation

characteristics of multioxide-aperture

high-speed 980 nm vertical cavity surface

emitting lasers

Cite as: Appl. Phys. Lett. 97, 151101 (2010); https://doi.org/10.1063/1.3499361

Submitted: 09 June 2010 • Accepted: 20 September 2010 • Published Online: 11 October 2010

A. Mutig, J. A. Lott, S. A. Blokhin, et al.

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Highly temperature-stable modulation characteristics of multioxide-

aperture high-speed 980 nm vertical cavity surface emitting lasers

A. Mutig,1,a兲J. A. Lott,2S. A. Blokhin,1,b兲P. Wolf,1P. Moser,1W. Hofmann,1

A. M. Nadtochiy,1,b兲A. Payusov,1,b兲and D. Bimberg1

1Institut für Festkörperphysik und Zentrum für Nanophotonik, Technische Universität Berlin,

Hardenbergstrasse 36, 10623 Berlin, Germany

2VI Systems GmbH, Hardenbergstrasse 7, 10623 Berlin, Germany

共Received 9 June 2010; accepted 20 September 2010; published online 11 October 2010兲

We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers 共VCSELs兲with

highly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and

85 °C. We perform small signal modulation experiments and extract the fundamental physical

parameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency,

D-factor, and K-factor, leading to identification of thermal processes and damping as the main

factors that presently limit high speed device operation. We obtain very temperature-insensitive

bandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELs

关doi:10.1063/1.3499361兴

Human society’s constantly increasing demand for more

information, the on-going scaling of silicon-based integrated

circuits to ever smaller dimensions, and the intrinsic limita-

tions of copper-based electrical data links have led to our

current need for lower cost, power efficient, readily manu-

facturable, reliable photonic devices. The unstoppable and

progressive penetration of optical communication links into

traditional copper interconnect markets greatly expands the

applications of vertical cavity surface emitting lasers 共VC-

SELs兲for next-generations of board-to-board, module-to-

module, and chip-to-chip interconnects.1,2The continued

down-sizing of integrated photonics systems and quest for

broader bandwidths leads naturally to interest in very short-

reach 共VSR兲data transmission systems residing, e.g., among

microprocessors, where the high temperature stability of the

lasers is indispensable. In the recent years, great progress

was achieved in increasing both the data rate and the tem-

perature stability of VCSELs. At room temperature bit rates

of 32 共Ref. 3兲to 38 Gbit/s 共Refs. 4and 5兲共850 nm兲,35

Gbit/s 共980 nm兲,6and 40 Gbit/s 共1100 nm兲7have been dem-

onstrated. While 850 nm is the current standard wavelength

for local and storage area networks and other short-reach

optical link systems, potential competitive standards at 980

and 1100 nm have many critical advantages for VSR systems

including: smaller operational voltages, deeper potential

wells, and transparency of the GaAs substrate, which is im-

portant for bottom-emitting VCSELs. Hereby the wavelength

of 980 nm is preferable for Si-based photodetectors, since

the absorption coefficient of Si is much larger at 980 nm as

compared to longer wavelengths.

Because the free carrier absorption coefficient increases

with the wavelength, tolerances for adjustment of doping

levels in the epitaxial structure for 980 nm are more strictly

as compared to 850 nm. Additionally the application of

highly strained materials for the active region and thus the

proper strain handling creates new challenges, which are nor-

mally absent in 850 nm devices. While at elevated tempera-

tures of 85 °C or even above bit rates of 25 Gbit/s have been

demonstrated for VCSELs operating at 850 共Ref. 3兲and

1100 nm,7however, at 980 nm the highest bit rate at 85 °C

was still limited to 20 Gbit/s,8where VCSELs with InGaAs

layers grown in the submonolayer growth mode by molecu-

lar beam epitaxy were applied. Here we indeed present 980

nm-range oxide-confined VCSELs operating error-free at

data bit rates of up to 25 Gbit/s at temperatures as high as

85 °C, which were grown by metal-organic chemical-vapor

deposition 共MOCVD兲with conventional multiple quantum

wells 共QWs兲, thus applying the mature and well approved

large-scale mass production techniques. To facilitate our

analysis of the limiting physical processes inside the VC-

SELs we perform small and large signal modulation experi-

ments at various temperatures enabling us to determine the

dominant factors that control the VCSEL’s speed, notably at

elevated temperatures.

The VCSEL structure is grown on an 兵001其-oriented 2°-

off n-doped GaAs-substrate using MOCVD. Doped

Al0.12Ga0.88As/Al.90Ga0.10As distributed Bragg reflectors

共DBRs兲with 20 nm thick linear graded interfaces with 24/37

periods for the top/bottom mirror are employed. The active

region contains five 4.2 nm thick compressively strained

In0.21Ga0.79As QWs with 6 nm thick GaAs0.88P0.12 tensile

strained barrier layers. The 4.7% smaller lattice constant of

GaAs0.88P0.12 as compared to GaAs partially compensates the

larger lattice constant of In0.21Ga0.79As and the correspond-

ing lattice constant mismatch of 1.55%. Carefully designing

of the layer thicknesses leaves both the local and the global

strain well below values critical for the high quality growth.

To improve the temperature stability of the gain and differ-

ential gain optimized detuning of 15 nm between the QW

photoluminescence peak and the cavity resonance was ap-

plied, helping to compensate the gain decrease at elevated

temperatures and leading to the very temperature insensitive

a兲Electronic mail: [email protected].

b兲On leave from the St. Petersburg Physical Technological Centre for Re-

search and Education RAS, Khlopina 8/3 and Ioffe Physical-Technical

Institute RAS, Polytekhnicheskaya 26, Saint-Petersburg, 194021 Russian

Federation.

APPLIED PHYSICS LETTERS 97, 151101 共2010兲

threshold current behavior. To facilitate electrical and optical

confinement and to reduce electrical parasitics two 30 nm

thick Al0.98Ga0.02As oxide aperture layers3with tapered fin-

gers are formed by selective wet oxidation. The VCSELs are

planarized with ⬃10

␮

m thick bisbenzocyclobutene to re-

duce the parasitic capacitances. Ground-source-ground metal

contact pads are evaporated for ease of on-wafer high-

frequency probe characterization.

We show in Fig. 1the measured output power-current-

voltage characteristics for a VCSEL with a 10

␮

m diameter

oxide aperture and the corresponding values for differential

efficiency and threshold current. The maximum output power

is 4.3 mW at 20 °C and decreases little to 2.6 mW at 85 °C.

The maximum differential efficiency decreases by only 18%

from ⬃0.34 W/Aat20°Cto⬃0.28 W/A at 85 °C. The

applied redshifted detuning of nominally 15 nm between the

QW photoluminescence peak wavelength 共⬃965 nm兲and

the cavity resonance wavelength 共⬃980 nm兲resulted in a

minimum threshold current appearing at a temperature

around 50 °C as seen in Fig. 1共b兲. The threshold current of

⬃584

␮

A at 20 °C, decreasing to ⬃504

␮

A at 50 °C, and

again increasing to ⬃579

␮

A at 85 °C. The variation in the

threshold current across the whole range from 20 to 85 °C is

less than ⬃16% of the minimum value. The detuning effect

decisively contributes to the improved temperature stability

of the device by keeping the threshold current density to a

minimum. The small threshold current density helps to main-

tain a large differential gain9and thus high speed VCSEL

operation. The differential resistance of the VCSEL is

⬃45 ⍀at 12 mA, practically perfectly matched to the stan-

dard impedance of 50 ⍀. The thermal resistance is

⬃2.2 K/mW, which can be further improved in the future

by using, e.g., binary alloys in the bottom DBR.

To determine the factors that control high speed opera-

tion, we measure the small signal modulation response 共S21兲

and input impedance 共S11兲using a network analyzer and a

calibrated photodetector at three temperatures of 20, 50, and

85 °C. These data are used to extract the relaxation reso-

nance frequency fr, damping factor

␥

,D-factor, K-factor,

parasitic cut-off frequency fp, and –3 dB frequency f−3 dB.

These key physical parameters are commonly used to char-

acterize the high speed properties of VCSELs and are based

on the standard laser diode rate equation formalism.9

Small signal modulation response for different currents

at 20 and 85 °C with corresponding curve fits are shown in

Fig. 2. All fits show excellent agreement with the measured

data. The maximum parasitic cut-off frequency extracted

from the measured S11 data is at all temperatures larger than

22 GHz. In Fig. 3we present the extracted values of the

relaxation resonance frequency frand the –3 dB-frequency

f−3 dB as a function of the forward bias current for 20, 50,

and 85 °C. The maximum bandwidth is 15.3 GHz at 20 °C

and decrease to 14.5 GHz at 50 °C. At 85 °C the maximum

bandwidth is still 13.2 GHz, which is only by ⬃2 GHz

lower than the value at 20 °C. The overall change in the

maximum bandwidth within the investigated temperature

range is only ⬃2 GHz, which is a result of the gain-cavity

detuning and the highly temperature stable multi-QW active

region. The maximum relaxation resonance frequency is

12.9 GHz at 20 °C and decreases to ⬃9.8 GHz at 85 °C.

This decrease corresponds to higher temperatures inside the

VCSEL at higher ambient temperatures 共internal temperature

at 12 mA at 85 °C is ⬃140 °C兲, leading to saturation of the

relaxation resonance frequency at lower currents.

The K-factor that characterizes the damping limitation

can be extracted by fitting a linear function to a plot of the

damping factor as a function of the squared relaxation reso-

nance frequency,9as shown in Fig. 3共b兲. The K-factor is in

the range of 0.4 ns between 20 and 85 °C and is temperature

insensitive. One of the main reasons for the relatively large

K-factor is a relatively high mirror reflectivity, since the

K-factor increases with the photon life time.9On the other

hand, the high mirror reflectivity reduces the mirror losses

and thus the threshold carrier density, which increases the

differential gain and thus the relaxation resonance frequency.

Since the relaxation resonance frequency is lower than the

bandwidth 关Fig. 3共a兲兴, especially at 85 °C, a proper compro-

mise between the acceptable high damping and the reason-

FIG. 1. 共Color online兲Output power-current-voltage 共L-I-V兲characteristics

of a 980 nm double oxide-aperture VCSEL at temperatures from 20 to

85 °C 共a兲, and the corresponding extracted values for the maximum differ-

ential efficiency and threshold current 共b兲.

FIG. 2. 共Color online兲Magnitude of the small signal modulation response

共2-port scattering parameter S21兲for different applied forward bias currents

and the corresponding curve-fits at 20 °C 共a兲, and at 85 °C 共b兲.

151101-2 Mutig et al. Appl. Phys. Lett. 97, 151101 共2010兲

able high relaxation resonance frequency has been found.

The extracted values of the K-factor correspond to a maxi-

mum damping limited bandwidth of ⬃21–23 GHz, clearly

demonstrating its noticeable limiting effect.

The important D-factor can be obtained by fitting a lin-

ear function to the extracted squared relaxation resonance

frequencies as a function of the current above threshold.9The

D-factor is practically constant over the whole temperature

range, and is ⬃3.5 GHz/sqrt共mA兲. The high temperature

stability of the D-factor is caused by the gain—cavity detun-

ing, which prevents the decrease in the differential gain at

higher temperatures.

Next, we performed data transmission experiments using

a nonreturn to zero 共NRZ兲data format with a 共27−1兲pseu-

dorandom binary sequence 共PRBS兲in a standard back-to-

back measurement configuration 共BTB兲共⬃3 m fiber兲.In

Fig. 4we show the measured bit error rates 共BER兲at the data

bit rate of 25 Gbit/s and temperatures of 25 and 85 °C. The

drive current and the peak-to-peak modulation voltage Vp-p

are held constant in both data transmission experiments at 12

mA and 0.8 V, respectively. This makes the expensive driver

electronics redundant, reducing the application costs. At both

temperatures error-free data transmission with BERs smaller

than 10−12 is demonstrated. Received optical power for the

lowest BER is at both temperatures smaller than –1 dBm,

well below 1 mW. The power penalty for the lowest BER for

the higher temperature of 85 °C in respect to 25 °C is

smaller than 0.8 dBm and is caused by the reduction of the

output power at higher temperatures 关Fig. 1共a兲兴. These results

clearly demonstrate the maturity of the presented devices for

future high speed VSR interconnects, where high tempera-

ture stability is indispensable.

To summarize and conclude, 980 nm VCSELs with

highly temperature stable modulation characteristics are pre-

sented. We demonstrate maximum bandwidths around 13–15

GHz that are very temperature insensitive. By using a stan-

dard small signal analysis, we identify a saturation of the

relaxation resonance frequency, due to the limits of heat dis-

sipation in our current VCSELs, and damping as the stron-

gest factors that limit high speed operation. We measure

error-free data transmission at data bit rates of up to 25

Gbit/s at temperatures of up to 85 °C, and thus demonstrate

the suitability of our VCSELs for practical high speed and

high temperature stable short-reach optical link applications.

We acknowledge funding by the EU FP7 program under

Agreement No. 224211, the Deutsche Forschungsgemein-

schaft 共Sfb 787兲, the State of Berlin 共100⫻100 Optics兲and

grants of St. Petersburg Government and Ministry of Educa-

tion and Science of Russia.

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FIG. 3. 共Color online兲Relaxation resonance frequency frand –3 dB fre-

quency f−3 dB vs VCSEL forward bias current 共a兲, and damping factor

␥

fr-squared with corresponding linear fits 共b兲, both at temperatures of 20, 50,

and 85 °C.

FIG. 4. 共Color online兲Bit error rates at 25 Gbit/s at 25 and 85 °C at the

corresponding VCSEL drive current of 12 mA and VP-P of 0.8 V for a BTB

measurement configuration with a 共27−1兲PRBS in the conventional NRZ

excitation scheme.

151101-3 Mutig et al. Appl. Phys. Lett. 97, 151101 共2010兲