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• Abstract

SECTION I

## INTRODUCTION

COMPARED with un-cooled 1310 nm distributed-feedback (DFB) lasers, 1310 nm vertical cavity surface emitting lasers (VCSELs) for transmission at 10 Gbps, based on either InGaAlAs/InP [1], [2], [3], or GaInNAs/GaAs [4] active regions offer about 10 times reduction of bias current in the full operating temperature range. Typically, with 1310 nm VCSELs, 10 Gbps transmission over 10 km at 80°C can be performed at 8–9 mA bias current [2], while with DFBs [5] similar performance is reached at currents of 80 mA. This feature becomes especially important when building wavelength-multiplexed transmitter modules like, for example, 4×10 Gbps coarse-wavelehgnth division-multiplexing (CWDM) modules emitting at 1271, 1291, 1311 and 1331 nm according to the recent 40 GbE IEEE 802.3ba standard [6]. Among mentioned active cavity material systems, only VCSELs with InAlGaAs/InP quantum wells have the capability to operate in the full 1310 nm wavelength band, while GaInNAs/GaAs based devices are limited only to wavelengths close to 1270 nm [4].

1310 nm VCSELs with InAlGaAs/InP quantum wells comprise tunnel junction injection of carriers in the active region and 2 distributed Bragg reflectors (DBRs). In recent designs these reflectors comprize one [3], or two [7] as-grown InAlGaAs/InP DBRs, one [3], or two dielectric DBRs [8] and two AlGaAs/GaAs wafer fused DBRs [1], [2]. AlGaAs/GaAs DBRs that proved very effective in short-wavelength VCSELs, are very well suited for 1310 nm devices because of higher refractive index contrast compared with as-grown InAlGaAs/InP DBRs and much better thermal conductivity compared with both as-grown and dielectric DBRs. In addition wafer fusion approach proved to be very effective in emission wavelength setting in the CWDM grid [9], [10].

One important condition for application VCSELs in optical communications consists in demonstrating reliable operation of these devices according to industry standards. So far, only InGaNAs/GaAs based 1310 nm VCSELs have demonstrated the potential for reliable operation [4].

In this letter we present the reliability data of wafer fused VCSELs. These devices have passed all Telcordia mechanical and electrical qualification tests. Accelerated life-time tests result in times to 1% failure at 70°C of 30 years and 18 years at VCSEL driving currents of 8 mA and 9 mA respectively.

SECTION II

## VCSEL DESIGN AND FABRICATION

The VCSEL device structure comprises an InP-based 5/2 $\lambda$-active cavity that is fused on both sides to undoped AlGaAs/GaAs DBRs, as schematically depicted on Fig. 1(a). The active cavity includes an InAlGaAs/InP multi-QW region with 4–6 compressively strained quantum wells and a ${\rm p}^{++}/{\rm n}^{++}$ InAlGaAs tunnel junction. The InP-based active cavity and GaAs-based DBRs are grown by low pressure metal-organic vapour phase deposition (LP-MOVPE) on (001) oriented 2-inch wafers [11]. A mesa-structure of 6–7 $\mu{\rm m}$ in diameter formed in the tunnel junction that is regrown with n-type InP serves for carrier and photon confinement. Electrical contacting is performed by top and bottom intra-cavity n-InP layers. This contacting scheme allows using un-doped top and bottom DBR mirrors. InGaAsP cavity adjustment layers that are located on both sides of the InP-based active region serve to tune the cavity to a specific wavelength in the CWDM grid. Fig. 1(b) shows a scanning electron microscope (SEM) image of a fused VCSEL cross-section produced by focused ion beam (FIB) etching. As one can observe, on this scale the quality of fused interfaces is very similar to that of multiple epitaxially grown interfaces that are present in the VCSEL structure.

Fig. 1. (a) Schematic cross section of the wafer fused 1310 nm VCSEL. (b) SEM image of a 1310 nm VCSEL layer-structure in a cross-section produced by FIB etching.
SECTION III

## VCSEL PERFORMANCE

Figs. 2 and 3depict LIVs and large signal modulation of 1310 nm VCSELs. These devices have low threshold current values close to 2 mA, low operation voltage below 2 V at currents up to 10 mA and output power in excess of 1 mW at 90°C ambient temperature (Fig. 2). Quite good 10 Gbps modulation performance can be achieved at a bias current of 9 mA (Fig. 3).

Fig. 2. LIV characteristics of the wafer fused VCSEL in the temperature range 20°C–70°C.
Fig. 3. 10 Gb/s modulation response of a wafer-fused 1310 nm VCSEL at a bias current of 9 mA.
SECTION IV

## VCSELs RELIABILITY

To perform reliability tests, VCSELs were assembled onto standard, hermetically sealed TO headers. Prior to qualification tests, all devices are exposed to a burn-in (BI) procedure in order to exclude any early failure [12]. Devices that pass the BI procedure normally pass all subsequent reliability tests that we perform according to the GR-468-CORE Telcordia Generic Reliability Assurance Requirements for Optoelectronic Devices [13]. These tests, that include different mechanical tests like shocks, vibrations and die shear, temperature cycling and electrical tests, have demonstrated that wafer-fused VCSELs behave like any other all-grown laser devices. Fig. 4depicts emission power in time of a qualification lot of 11 devices at 10 mA and ambient temperature of 90°C, that correspond to a junction temperature close to 120°C and to a current density through the tunnel junction of 26 ${\rm kA/cm}^{2}$. As one can observe, no visible degradation at these operating conditions is observed after 5000 hours of operation. In addition to the mentioned qualification group, 4 more groups of devices from the same VCSEL wafer were tested for accelarated wearout at higher values of junction temperature and current densities as shown on accelerated aging test matrix on Fig. 5. The performance of devices was periodically tested at room temperature. Fig. 6 depicts evolution in time of emission power at 9 mA (top graph) and threshold current (bottom graph) of a group of 8 devices under aging test at 10 mA and temperature 150°C.

Fig. 4. Emission power in time of 11 devices operating at 10 mA driving current and ambient temperature of 90°C.
Fig. 5. Accelerated aging test matrix.
Fig. 6. Evolution in time of emission power at 9 mA (top graph) and threshold current (bottom graph) of 8 devices under ALT test at 10 mA and temperature 150°C.

Under these test conditions one can observe devices with small changes, for example devices 32, 35, 37 in output power and threshold current and devices that show more pronounced gradual degradation, like devices 30, 31, 33 and 36.

Based on the statistical distributions of the totality of ALT data we have calculaed VCSEL aging parameters: activation energy value of ${\rm E}_{a}= 0.67~{\rm eV}$ and current exponent factor ${\rm N}=3.93$. The value of activation energy of 0.67 eV is close to 0.79 eV-the activation energy of 1550 nm VCSELs with active region in the same (InAlGaAs/InP) material system grown by MOVPE [14], eventhouth devices with undercut apertures investigated in Ref. 14 do not exibit gradual wearout in time during ALT tests.

Predicted operation lifetimes are calculated based on the accelerated life-time data and applying the acceleration factors (activation energy ${\rm E}_{a}=0.67~{\rm eV}$ and current exponent factor ${\rm N}=3.93$) and the pass-fail condition of 2 dB change. The maximum driving current for VCSELs is 9 mA which quite sufficient for 10 Gbps modulation at 8–9 mA bias current and the maximum ambient operating temperature is set to 70°C according to telecom industry requirements. After scaling the lifetime data to these conditions, the data from all ALT conditions are plotted on lognormal plots shown in Fig. 7.

Fig. 7. Lognormal plots of lifetimes (left-25°C, right-70°C) from the totality of test conditions scaled to an operating condition of 9 mA.

From data presented in Fig. 7 one can extract the time to 1% failure of 291 years at 25°C and the time to 1% failure of 18 years at 70°C. With decreasing the driving currents to 8 mA and 7 mA (see Fig. 8), the time to 1% failure at 70°C increases to 30 years and 50 years respectively. These lifetimes values meet the telecom industry requirements for the time to 1% failure of more than 10 years at 70°C for the current generation of 10 Gbps VCSELs that operate at bias currents of 8–9 mA and with larger margins for the new generations of 10 Gbps VCSELs that operate at bias currents below 7 mA. In addition, in real-life applications actual lifetimes are expected to be considerably longer since the devices spend most of the time in less demanding operating conditions.

Fig. 8. Variation of time to 1% failure with driving current at 25°C (blue) and 70°C (red) operating temperature.
SECTION V

## CONCLUSION

10 Gbps wafer fused VCSELs emitting in the 1310 nm wavelength band have successfully passed all mechanical and electrical Telcordia qualification tests. At 9 mA and 8 mA driving currents evaluation of the time to 1% failure at 70°C results in 18 years and 30 years respectively. These lifetimes meet the telecom industry reliability requirements for applications in fiber-optic communications networks.

## Footnotes

A. Sirbu, V. Iakovlev, Z. Mickovic, and E. Kapon are with the Laboratory of Physics of Nanostructures, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland (e-mail: alexei.sirbu@epfl.ch).

G. Suruceanu, A. Mereuta, and A. Caliman are with BeamExpress S.A., Lausanne 1015, Switzerland.

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

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