10-Gb/s and 10-km error-free transmission up to

100°C with 1.3-µm wavelength wafer-fused

VCSELs

Alexandru Mereuta,1,*

Grigore Suruceanu,2

Andrei Caliman,1 Vladimir Iacovlev,

2 Alexei Sirbu,

1 and Eli Kapon

1,2

1Laboratory of Physics of Nanostructures, Ecole Polytechnique Fédérale de Lausanne (EPFL)1015 Lausanne,

Switzerland 2BeamExpress S.A., 1015 Lausanne, Switzerland

* alexandru.mereuta@epfl.ch

Abstract: 10-Gb/s modulation speed and transmission over 10-km SM fiber

with BER < 10−11

up to 100°C temperature are achieved with optimized

wafer-fused GaAs/AlGaAs-InP/InAlGaAs VCSELs incorporating re-grown

tunnel junction. These VCSELs operate in the 1310-nm waveband and emit

more than 1-mW single mode power in the full temperature range.

©2009 Optical Society of America

OCIS codes: (140.0140) Lasers and laser optics; (140.7260) Vertical cavity surface emitting

lasers.

References and links

1. E. Kapon, A. Sirbu; “Long-Wavelength VCSELs: Power - efficient answer,” Nat. Photonics 3(1), 27–29 (2009).

2. Markus-Christian Amann and Werner Hofmann, “Single-mode and tunable VCSELs in the near- to mid-

infrared,” Chin. Opt. Lett. 6(10), 743–747 (2008).

3. N. Nishiyama, C. Caneau, B. Hall, G. Guryanov, M. H. Hu, X. S. Liu, M.-J. Li, R. Bhat, and C. E. Zah, “Long

Wavelength Vertical Cavity Surface Emitting Lasers on InP with Lattice Matched AlGaInAs/InP DBR Grown by

MOCVD,” IEEE J. Sel. Top. Quantum Electron. 11(5), 990–998 (2005).

4. A. Mircea, A. Caliman, V. Iakovlev, A. Mereuta, G. Suruceanu, C. A. Berseth, P. Royo, A. Syrbu, and E. Kapon,

“Cavity Mode-Gain Peak Tradeoff for 1320-nm Wafer-Fused VCSELs With 3-mW SM Power and 10-Gb/s

Modulation Speed up to 70°C,” IEEE Photon. Technol. Lett. 19(2), 121–123 (2007).

5. A.Sirbu, A.Mereuta, V. Iakovlev, A. Caliman, P. Royo, and E. Kapon. “10 Gbps VCSELs with high single mode

output in 1310-nm and 1550-nm wavelength bands”. OFC/NFOEC-2008, paper OThS2, 1–3, (2008).

6. N. Nishiyama, C. Caneau, J. D. Downie, M. Sauer and C.-E. Zah. “10-Gbps 1.3 and 1.55-µm InP-based

VCSELs: 85°C 10-km Error-free Transmission and Room Temperature 40-km Transmission at 1.55-µm with

EDC”. OFC-2006, paper PDP23, (2006).

7. W. Hofmann, N. H. Zhu, M. Ortsiefer, G. Böhm, J. Rosskopf, L. Chao, S. Zhang, M. Maute, and M.-C. Amann,

“10-Gb/s Data Transmission Using BCB Passivated InGaAlAs–InP VCSELs,” IEEE Photon. Technol. Lett.

18(2), 424–426 (2006).

8. W. Hofmann, M. Müller, G. Böhm, M. Ortsiefer, and M.-C. Amann. “1.55µm InP-based VCSEL with Enhanced

Modulation Bandwidht >10 GHz up to 85°C”. OFC/NFOEC-2009, paper OTuK5 (2009).

9. A. Mereuta, A. Syrbu, V. Iakovlev, A. Rudra, A. Caliman, G. Suruceanu, C. A. Berseth, E. Deichsel, and E.

Kapon, “1.5 µm VCSEL structure optimized for high-power and high-temperature operation,” J. Cryst. Growth

272(1-4), 520–525 (2004).

10. A. Syrbu, A. Mircea, A. Mereuta, A. Caliman, C.-A. Berseth, G. Suruceanu, V. Iakovlev, M. Achtenhagen, A.

Rudra, and E. Kapon, “1.5-mW Single-Mode operation of wafer-fused 1550-nm VCSELs,” IEEE Photon.

Technol. Lett. 16(5), 1230–1232 (2004).

11. A. Baecker, S. Odermatt, R. Santschi, F. Roemer, B. Witzigmann, P. Royo, V. Iakovlev, A. Caliman, A.

Mereuta, A. Syrbu, and E. Kapon, “Transverse Optical Mode Analysis of Long-wavelength VCSELs for High

Single-Mode Power Operation”. 8-th International Conference on Numerical Simulation of Optoelectronic

Devices (NUSOD), paper WB2 (2008).

1. Introduction

Vertical cavity surface emitting lasers (VCSELs) operating in the 1310-nm waveband at

multi-Gb/s speeds and high temperatures are highly desirable for applications in local area

fiber networks. Such devices would offer reduced power consumption, uncooled operation

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and flexible multi-wavelength configurations, all of which are essential features for the

rapidly expanding, dense optical communication infrastructure. Significant progress has been

achieved in recent years with such long wavelength VCSELs, using several device

technologies [1]. In particular, 1-2-mW single mode (SM) power and 10Gb/s modulation up

to 85°C have been reported for such devices [2–8]. In addition, error-free transmission over

10-km at room temperature for both 1.3 and 1.5-µm wavelengths VCSELs [3,4,6] and at 85°C

for 1.3-µm VCSELs [2,3,7] have been demonstrated.

Among the successful relevant VCSEL technologies, the wafer-fused approach provides

particularly high SM power due to efficient heat dissipation [1] and ease of construction of

multi-wavelength VCSELs, e.g., for application in coarse wavelength division multiplexing

(CWDM) transmitters [10]. In this paper we report 10-Gb/s operation and transmission with

wafer-fused VCSELs emitting in the 1310-nm band with improved design. Devices with 6-µm

apertures show single fundamental mode operation in the 2-10 mA current range with an

output power exceeding 1-mW up to 100°C. Small signal modulation curves show > 6-GHz

bandwidths for currents above 5-mA. Large signal modulation at 10-Gbps shows clear open

eye diagrams up to 100°C. In addition, the 10-Gb/s transmission experiments in back-to-back

configuration and through a 10-km SM fiber with bit error rate (BER) better than 10−11

at both

room temperature and 100°C, and with power penalties less than 0.5 and 1dB, respectively,

were demonstrated. The obtained results have the potential to reduce both the power

consumption and cost in datacom and GPON transceivers.

2. VCSEL structure and fabrication

Figure 1 shows a schematic cross-section of the device. The VCSEL comprises an

InAlGaAs/InP-based quantum well (QW) active cavity and a re-grown tunnel junction (TJ)

with 6-µm diameter aperture, fused on both sides to AlGaAs/GaAs un-doped distributed

Bragg reflectors (DBRs), as described in [4,9,10]. Both the InP-based active cavity and the

GaAs-based DBRs are grown by low pressure metal-organic vapor phase epitaxy (LP

MOVPE).

Fig. 1. Schematic cross-section of the wafer-fused VCSEL.

The growth of the active cavity was performed in two steps. In the first step, the n-InP

current spreading layer, the InAlGaAs QWs, and the p-n and tunnel junction layers were

grown. The TJ aperture was then defined using photolithography and wet chemical etching. In

the second step, the over-growth of the patterned TJ mesa was performed. The re-grown TJ

serves at the same time as a source of holes injection and introduces a refractive index profile

that supports the optical cavity modes. The current flow is confined to the TJ disc by a reverse

biased p-n junction. In the present work, the VCSEL structure is designed for reaching high-

speed modulation characteristics by optimizing the QW active structure, the gain-cavity

wavelength off-set and by reducing the electrical parasitics. First, the number of QWs was

increased to six for obtaining higher differential gain. Second, the off-set between of the peak

#112216 - $15.00 USD Received 1 Jun 2009; revised 3 Jul 2009; accepted 7 Jul 2009; published 14 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12982

in the photoluminescence (PL) spectrum at room temperature and the VCSEL cavity mode

was set at ~50-55-nm for high temperature operation. Third, low index polyimide passivation

was used under the bond pads in order to decrease the parasitic bond pad capacitance. Finally,

the area of the reverse biased p-n junction was reduced to decrease the parasitic capacitance.

3. Static device characteristics

The single-mode operation of VCSELs depends on the device design and on the size and

lateral index control introduced by the re-grown tunnel junction [9–11]. Increasing the size of

optical aperture is the common way to increase the maximum output power, but with

increasing it - higher second transversal modes appears. Figure 2 depicts the light-current-

voltage characteristics of a nominal device with 6-µm TJ aperture, measured in the

temperature range of 20-100°C. The maximum output power at room temperature is 3-mW

and as high as 1-mW at 100°C. The threshold current is ~1 mA and is almost constant up to

70°C, which reflects the device optimization for high temperature operation. At ~60°C the

threshold exhibits a minimum of ~0.9-mA, which reflects the ~50-nm wavelength off-set

between the room-temperature PL peak and cavity mode at 20°C. The maximum output

power, limited by the thermal roll-over, decreases linearly with temperature and reaches a

value close to 1-mW at 100°C.

Fig. 2. Light-current-voltage characteristics of a 6-µm aperture device in the 20-100°C

temperature range (the current-voltage curves are only for 20 and 100°C, respectively).

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Fig. 3. VCSEL spectra at different currents at room temperature (a) and at 100°C (b).

The room temperature and 100°C emission spectra of the wafer fused VCSEL, measured

at different diode currents, are shown in Fig. 3. The room temperature spectra are single mode

at currents up to 10 mA, with side-mode suppression ratio (SMSR) in excess of 40 dB (Fig.

3a). At 12-mA the SMSR is lower than 20-dB. With increasing temperature, the SMSR

remains higher than 40-dB for higher currents; at 100°C, such high SMSR is maintained up to

16-mA, at which thermal role over takes place. Thus, with this VCSEL design single mode

operation with more than 1-mW output power is obtained for the entire temperature range of

20-100°C.

4. Modulation characteristics and transmission experiments

The relative intensity noise (RIN) spectra of the 1310-nm band VCSELs at 20°C and at 85°C,

measured using the HP 71400C Lightwave Signal Analyzer system, are presented in Fig. 4. A

resonance frequency of 10-GHz is reached at a bias current of 10-mA for both test

temperatures. The extracted value of the modulation current efficiency factor (MCEF) from

the RIN plots yields the values of 3.8-GHz/mA1/2

(T = 20°C) and 3.5-GHz/ mA1/2

(T = 85°C).

One can see that at elevated temperatures the MCEF value is just slightly lower compared to

20°C. This is an indication of well-optimized design of the VCSEL structure, with good

modulation potential within a large temperature range.

Fig. 4. Relative intensity noise (RIN) spectra of a 1310-nm band VCSEL measured at different

currents (from 2 to 10-mA) at room temperature (a) and at 85°C (b).

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Small signal modulation characteristics (s21) of the 1310-nm band VCSELs were

measured on-wafer using a probe station with high frequency (HF) probes and data

acquisition set-up based on the signal analyzer system Agilent 8702E with 6-GHz test

bandwidth. In Fig. 5a and 5b, the s21 spectra at both 20°C and 85°C temperature in the 2-10-

mA current range are plotted. The 3-dB cut-off is higher than 6-GHz for currents above 5-mA

at both temperatures. The extracted values for MCEF using the s21 plots yields 3.34-

GHz/mA½ (T = 20°C) and 3.18-GHz/mA

½ (T = 85°C), respectively. These values are only

slightly lower than those extracted from the RIN spectra. This discrepancy is related to the

fact that the RIN-based data indicate the intrinsic modulation capabilities of the VCSEL and

are not influenced by external parasitic factors (capacitance, resistance, etc), which is the case

for S21 measurements. Based on the s21 measurements, one can predict the modulation

capabilities of the VCSEL device at 10-Gb/s data rates for bias currents higher than 5-mA, for

which the 3-dB modulation bandwidth exceeds 6-GHz.

Large signal modulation tests of the 1310 nm band VCSELs were performed on- wafer

using a probe station with HF contacting probes and the 12.5 Gb/s Agilent N4903A J-BERT.

As expected from the RIN and s21 measurements, the devices show good large signal

modulation response as well. The tests, performed at 10.3125 Gb/s data rates (10G Base

Ethernet standard) with a Pseudo Random Binary Sequence (PRBS) of “231

-1” input signal

and electrical modulation amplitude extinction ratio in excess of 5-dB, show open eye

diagrams for “Back to Back” (B2B) transmissions (3 m of SMF patch-cord), which conforms

to the mask tests with 20% margins at 20°C and 100°C as well (see insets in Fig. 6a and 6b).

For a bias current of 8 ÷ 9 mA the rise/fall time values are within the 35 ÷ 40-ps range and are

compatible with 10-Gb/s operation.

Fig. 5. Resonance frequency (ROF) spectra of a 1310-nm band VCSEL measured at different

diode currents at room temperature (a) and 85°C (b).

The transmission tests of optical data at 10-Gb/s through 10-km of single mode fiber

(SMF) line have been performed at both room temperature and up to 100°C. The VCSEL

emission was coupled into SM fiber patch-cord with a special mini-coupling lens termination

w/o optical isolator. Next, the patch-cord was connected to the 10-km SMF line via standard

FC/PC adapter. The tests were performed for VCSEL device temperatures of 20°C and

100°C. As depicted in Fig. 6 (a) and (b), the transmission through 10-km of SMF shows open

eye diagrams with applied 20% margins test mask. A few violations of the mask margins are

rather related to the receiver sensitivity than to the VCSEL modulation performances

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Fig. 6. B Back-to Back and 10-km 10-Gb/s eye diagrams (insets) measured at 20°C (a) and

100°C (b).Back-to-back and 10-km BER measured at 20°C (a) and 100°C (b) (see text for

details).

and transmission phenomena through the fiber. Modulation voltage was 0.7-Vpp and bias

current was 9-mA. A BER better than 10−11

was measured for such tests at both room

temperature and 100°C with power penalties of less than 0.5 and 1-dB, respectively.

6. Summary

By design optimization of the device structure of wafer-fused 1310-nm VCSELs we

demonstrate an improvement of the high-speed performance maintaining high output single

mode power in excess of 1-mW throughout the 20-100°C temperature range. Modulation

bandwidth exceeding 6-GHz at bias currents > 5-mA and 10-Gb/s modulation up to 100°C

have been achieved. 10-Gb/s transmission, back-to-back and through the 10-km SM fiber,

with BER better than 10−11

at both 20°C and 100°C and with power penalties below 0.5 and 1-

dB, respectively, was demonstrated.

Acknowledgement

This work was partly supported by the Swiss innovation promotion agency (CTI) and by the

European Commission via the project MOSEL.

#112216 - $15.00 USD Received 1 Jun 2009; revised 3 Jul 2009; accepted 7 Jul 2009; published 14 Jul 2009

(C) 2009 OSA 20 July 2009 / Vol. 17, No. 15 / OPTICS EXPRESS 12986