10-Gb/s and 10-km error-free transmission up to 100°C with 13-μm wavelength wafer-fused VCSELs
Transcript of 10-Gb/s and 10-km error-free transmission up to 100°C with 13-μm wavelength wafer-fused VCSELs
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
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).
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infrared,” Chin. Opt. Lett. 6(10), 743–747 (2008).
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MOCVD,” IEEE J. Sel. Top. Quantum Electron. 11(5), 990–998 (2005).
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“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).
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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).
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18(2), 424–426 (2006).
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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
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Rudra, and E. Kapon, “1.5-mW Single-Mode operation of wafer-fused 1550-nm VCSELs,” IEEE Photon.
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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).
#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 12984
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
#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 12985
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