25 gbps and beyond: vcsel development at philips [8639-18] · 25 gbps and beyond: vcsel development...

25 Gbps and beyond: VCSEL development at Philips Martin Grabherr, Steffan Intemann, Stefan Wabra, Philipp Gerlach,

Michael Riedl, Roger King

Philips Technologie GmbH U-L-M Photonics, Lise-Meitner-Str.13, 89081 Ulm Germany

Abstract In comparison to widely used InGaAs Quantum Wells (QW) in high speed VCSELs operating at 25 Gbps and beyond, we present an investigation on the use of GaAs QWs, which have proven their ability to serve reliably in 10 Gbps and 14 Gbps VCSEL products and allow for an evolutionary extension of data rates based on mature technology. As data centers continuously increase in size, the demand for longer reach optical links within these data centers is addressed by the proposal of using small spectral width single-mode VCSELs that offer the potential of significantly reduced chromatic dispersion along optical fibers of several 100 m length. Performance and modeling parameters of single-mode VCSELs are being compared to those of typical multi-mode VCSELs built from identical epitaxy and process technology.

Keywords: VCSEL, high speed modulation, optical interconnect,active optical cable,single-mode, 25 Gbps

Introduction In recent years several groups have reported excellent results on high speed data transmission using VCSELs as light sources. Data rates of 25 Gbps up to 55 Gbps have been shown even at elevated temperatures. A list of recent publications can be seen in Table 1. Very low power consumption, as well as extended reach by utilizing the benefits of single-mode VCSELs have been presented8. These results are encouraging in order to meet the demands of next generation data transmission standards like 100GBASE-SR4, FIBRE CHANNEL32, or INFINIBAND EDR. Whereas InGaAs QWs in multi-mode VCSELs are widely used in VCSEL technology for achieving very high data rates at or above 25 Gbps due to their higher differential gain, we would like to focus differently on two crucial aspects of VCSEL developments for these emerging and short term demands. First, we evaluate experimentally the proposed advantage of compressively strained InGaAs Quantum Wells (QWs) against GaAs QWs at 25 Gbps. Second, we compare the performance of GaAs QW based multi-mode VCSELs and single-mode VCSELs at operating speed of 25 Gbps. The presented results are taken into account in design considerations for next generation VCSELs that will enable high-speed active optical cables, transceivers, and interconnects.

Vertical-Cavity Surface-Emitting Lasers XVII, edited by Kent D. Choquette, James K. Guenter,Proc. of SPIE Vol. 8639, 86390J · © 2013 SPIE · CCC code: 0277-786X/13/$18

doi: 10.1117/12.2002029

Proc. of SPIE Vol. 8639 86390J-1

Table 1: Recently published results of high data rate transmission using VCSELs

Requirements on high-speed VCSELs As a general requirement, optical interconnects must offer performance and scalablity, e.g. highly dense large number of ports, at the same time. In order to enable this density, low power consumption and small size is important to achieve. As already mentioned, an additional requirement on longer reach connections while maintaining the use of standard OM3 or OM4 fibers is asking for reduced spectral width lasers. VSR optical interconnects, addressing, e.g., on board chip to chip communication are requiring very low bit error rates (BER) of less than 10-18 at 25 Gbps as error correcting protocols are not acceptable due to their impact on latency.

For all mentioned applications, a longevity greater 10 years at temperatures between 0°C and 70°C is required.It is important to note that the junction temperature within the VCSEL structure will be at 15 and 85°C, respectively. According to roadmaps of IEEE, INFINIBAND association, and FIBRE CHANNEL association, the market introduction of 25/28 Gbps VCSEL based active optical cables is expected latest in 2014.

VCSEL design consideration The basic improvement needed towards 25/28 Gbps is of course increasing the bandwidth. An appropriate approach is to use compressively strained QWs built of InGaAs instead of GaAs, which results in an increase of differential gain. In addition, by reducing the photon lifetime the damped resonator behavior can be optimized. Obviously, parasitic capacitances need to be minimized, which is achieved by the use of semiinsulating substrates that minimize pad capacitance, and the implementation of a second

data rate wavelength QW material

FINISAR1 55 Gbps 850nm InGaAs

TU BERLIN2 49 Gbps 980 nm InGaAs

CHALMERS3 44 Gbps 850 nm InGaAs

VI SYSTEMS4 40 Gbps 850 nm InGaAs

FURUKAWA5 25 Gbps 1060 nm InGaAs

AVAGO6 25 Gbps 850 nm

SUMITOMO7 25 Gbps 850 nm InGaAs

current apperture (see Fig. 1) in adddition to a ssmall mesa ddiameter of oonly 28 µm ffor the multii-mode devvices and 23 µm for the single-modee devices (Fiig. 2), whichh finally resuults in a reduuced mesa cappacitance.


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Commparison oof single-mmode vs. multi-moode VCSELLs Single-mmode VCSELLs are offerinng inherent aadvantages wwhen being ccompared wiith multi-moode VCSELs:

- loower operatinng currents, thus smallerr power conssumption - hiigher photonn densities, thhus expectedd higher moddulation banndwidth

On the otther hand, thhe stronger confinement of current annd optical fieeld may resuult in higher current densities, highher series resistance, andd comparablle reliability has to be prroven.

In the folllowing subcchapters, singgle-mode (3 µm aperturee) and multii-mode (6 µmm aperture) VCSELs are being coompared. Alll devices havve been prodduced on thee same waferr for fair comparisson.


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Figg. 13: Electriical equivalent circuit of the combinaation of VCSEEL driver annd VCSEL.


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Resonance freof laser curre

CSEL at 25°C

imum resonabe even high

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21: Resonanction of laser ce VCSEL at 2

d at 3.0 mA h are not rel

ce frequency current for th25°C and 85°

for the singlevant for the

as a he multi-°C.

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CSEL n due

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Fig. 22: DresonanceVCSEL.

ive current dat about 16 G

the damping given in γ =

be the same

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The 3 dBB small signaal modulationn bandwidthh (S21-parammeter) for thee single- andd multi-modee VCSEL iis plotted in Fig. 24 and Fig. 25, resppectively.

For the siingle-mode VVCSEL the 3dB bandwiidth is foundd to be 20 GHHz for both ttemperatures of 25°C andd 85°C. The 3dB bandwiidth of the mmulti-mode VVCSEL is alsso 20 GHz aat 25°C, but iis slightly reduced to 166 GHz at 85°°C. Both chaaracteristics indicate thatt these VCSELs should be well suiteed for at leasst 25 Gbps ddata transmisssion.


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.

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Small signal mameter) for t

modulation bhe single-mo

behavior ode

Fig. 2(S21-VCSE

25: Small sign-parameter) fEL.

nal modulatifor the multi

ion behaviori-mode

r

In order tpattern ofdiagram itemperatumode VCthe fall ticomparablower out1.0 mW c

Fig. 26: Ecurrent, 0PRBS31 bVCSEL. A

to verify thisf PRBS31, din Fig. 26, reure, present

CSEL at 6.0 mme is slowerble at 4.5 dBtput power ocompared to

Eye diagram 0.52 Vpp voltbit pattern aAmbient tem

s capability, directly driveecorded at 2ts a very opemA at same r and thus th

B and 5.0 dB of the single-o 3.4 mW for

recorded fortage swing, applied to the

mperature is s

both deviceen by a patte.0 mA laser

en eye, low jimodulation

he eye is not for the sing

-mode VCSEr the multi-m

r 2.0 mA biasand 25 Gbpse single-modeset to 25°C.

s are operateern generatorcurrent applitter, and mi conditions aperfectly sy

gle- and multEL the optic

mode VCSEL

s

e

Fig. 2currePRBSVCSE

ed under 25 r without anylied to the siinimum ringalso results i

ymmetric. Exti-mode VCScal modulatioL.

27: Eye diagrent, 0.52 VppS31 bit patteEL. Ambient

Gbps moduly equalizationgle-mode Ving. Operatiin a well opextinction ratiSEL, respecton amplitude

ram recordedp voltage swinern applied tot temperatur

lation with aon. The eye VCSEL at roing the multen eye, althoios are tively. Due te (OMA) is

d for 6.0 mAng, and 25 Go the multi-mre is set to 25

a bit

oom ti-ough

to the only

A bias Gbps mode 5°C.


n

Fig. 28PRBS

Even at 8signal mo4.5 dB an

8: Eye diagrS31 bit patter

85°C ambienodulation annd the OMA

am recordedrn applied to

nt temperaturd the eye dia

A is 0.5 mW.

d for 2.0 mAo the single-m

re, the singleagram is wid

bias currentmode VCSEL

e-mode VCSde open, as c

t, 0.34 Vpp voL. Ambient te

SEL performcan be seen i

oltage swingemperature

ms very well in Fig. 28. E

g, and 25 Gbpis set to 85°C

under large Extinction rat

ps C.

tio is

Conclusion We have investigated different design options for a 25 Gbps VCSEL operating at 850 nm. Compared to InGaAs quantum wells, GaAs quantum wells are still well suited to provide sufficient performance at these datarates. Re-use of well established material compositions implemented in commercial 10 Gbps and 14 Gbps VCSEL platforms may reduce risks in product qualification and allow for a faster time-to-market. In addition we have compared single- and multi-mode VCSELs for the use at 25 Gbps. Although the higher series resistance of single-mode VCSELs of about 250 Ohms is not advantageous and can only partially be compensated by the reduced oxide capacitance of only 150 fF, the modulation characteristics, especially at high temperatures up to 85°C ambient (20 GHz 3dB bandwidth and very open eye diagram) show promising performance in small as well as high signal measurements. It is worth considering single-mode VCSELs for extremely low power, high speed optical links. In addition the small spectral width of single-mode VCSEL may enable extended data transmission over standard multi-mode fibers, as it is required for the increasing size of data centers. The use of single-mode VCSELs in consumer applications like computer mice9 has proven their manufacturability in huge numbers and is an excellent basis to investigate the suitability for datacom applications.

References [1] D. M. Kuchta et al., “A 55Gb/s Directly Modulated 850nm VCSEL-Based Optical Link”,

IEEE Photonics Conference 2012 (IPC 2012) Post Deadline Paper PD 1.5 (2012). [2] W. H. Hofmann et al., “980-nm VCSELs for optical interconnects at bandwidths beyond 40

Gb/s”, Proc. SPIE 8276, pp. 05-1 to 05-9 (2012).


[3] P. Westbergh et al., “High-speed 850 nm VCSELs with 28 GHz modulation bandwidth operating error-free up to 44 Gbit/s”, Electronics Letters 48 (18), pp. 1145-1147 (2012).

[4] S.A. Blokhin et al., “850 nm VCSELs operating at bit rates up to 40 Gbit/s“, Electronics Letters 45, pp. 501-503 (2009).

[5] Toshihito Suzuki et al., “Reliability study of 1060 nm 25 Gbps VCSEL in terms of high speed modulation”, Proc. SPIE 8276, pp. 04-01 to 04-8 (2012).

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Proc. SPIE vol. 8276, pp. 0K-1 to 0K-11 (2012). [9] D. Wiedenmann et al., “High volume production of single-mode VCSELs”, Proc. SPIE 6132,

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