1. Introduction

Vertical cavity surface emitting lasers (VCSELs) operating at <1µm wavelengths have established themselves as the ultimate low-power-consumption lasers for an increasing number of applications in data communications, active optical cables, laser mouse and sensing. There are currently increasing needs for similar low-power-consumption laser technology at longer wavelengths, especially for 1310nm and 1550nm optical communications networks, where the huge expansion in data traffic is facing severe limitations due to thermal management problems [1]. However, long wavelength (LW) VCSELs emitting in this spectral range have suffered so far from limited single-mode power outputs, putting them in a disadvantage in comparison with corresponding power hungry edge emitting lasers. Developing such LW-VCSELs with single mode (SM) output power well in excess of 1mW simultaneously with high modulation speed capabilities and accurate emission wavelength setting would allow their incorporation in low-power consumption, high performance modules for broadband optical communications, optical absorption spectroscopy and Bragg-grating sensor applications [2-5].

The technology of LW-VCSELs has lagged behind that of shorter wavelength VCSELs due to the difficulty in producing all-epitaxial structures allowing to obtain high performance devices. Significant progress has been achieved in recent years by introducing hybrid structures combining GaAs/AlGaAs or dielectric distributed Bragg reflectors (DBRs) for high reflectivity with InP/InAlGaAs quantum well (QW) active regions for high optical gain at high temperatures [6-9]. Most importantly, employing tunnel junctions has allowed intra-cavity contacting with low-absorption DBRs, a crucial ingredient for reaching high single mode power and high modulation speed [4,5,9-11]. These developments have yielded high performance LW-VCSELs emitting in the 1310nm wavelength range showing single mode power of 5.4 mW at room temperature and 3.1mW at 75°C with more than 30 dB side mode suppression ratio (SMSR) on devices with wafer-fused AlGaAs/GaAs reflectors [11] and very recently 1550 nm devices with dielectric DBRs that demonstrate 8 mW at −10 °C and 3 mW at 80°C [12].

Among all leading LW-VCSEL technologies, localized wafer fusion [13] offers the greatest flexibility in selecting the emission wavelength and the utilized GaAs/AlGaAs DBRs that provides the best thermal conductivity [14]. Therefore, it has the potential of yielding the highest optical output power without compromising other performance features of the LW-VCSELs. Here we report double wafer-fused LW-VCSELs emitting in the 1550nm waveband with record high SM power of 6.7 mW at 20°C and 8 mW at 0°C. These VCSELs were fabricated using localized double wafer fusion technology on full 2-inch wafers, which makes them compatible with standard industrial manufacturing processes . In addition to the high SM power, this technology demonstrates the possibility of wavelength inventory selection in the range of 40 nm on the same VCSEL wafer and a continuous wavelength tuning over 15 nm of individual devices.

2. VCSEL structure design and fabrication

The VCSEL device structure comprises an InP-based 5/2 λ-active cavity, fused on both sides to undoped AlGaAs/GaAs DBRs, as schematically depicted in Figure 1 . We use top DBRs with either 20 or 18 pairs and bottom DBRs with 35 pairs. The active cavity includes an InAlGaAs/InP multi-QW region with 6 compressively strained quantum wells and a p⁺⁺/n⁺⁺ InAlGaAs tunnel junction. Both the InP-based active cavity and the GaAs-based DBRs were grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) on 2” (100) wafers [15]. Prior to wafer fusion, tunnel junction mesas of 7 μm diameter were defined by photolithography and wet etching and were subsequently re-grown with n-type InP, serving for lateral carrier and photon confinement. The regrworth yields a more planar surface although some non-planarity remains, which is exploited in obtaining good quality of the fused interface in the subsequent localized wafer fusion step [16]. InGaAsP cavity adjustment layers, grown on both sides of the active cavity, allow controlling the cavity length and hence the emission wavelength with nanometer range accuracy on the two-inch wafer level using a simple and effective procedure. Before the first fusion process, the top InGaAsP cavity-adjustment layer d₁ is selectively etched on one half of the wafer. After fusion to the top DBR wafer and InP substrate removal, the exposed bottom InGaAsP layer d₂ is also selectively etched on one half of the wafer rotated by 90 degrees with respect to the first cavity adjustment step, such that 4 quarter wafers with different cavity lengths: D, D + d_1, D + d₂ D + d₁ + d₂, are formed (D is the thickness of the cavity without the cavity adjustment layers). This procedure allows for producing on the same wafer four regions with VCSELs emitting at different, prescribed wavelengths. In the present work, these emission wavelengths were set around λ₁ = 1550 nm, λ₂ = 1560 nm, λ₃ = 1570 nm and λ₄ = 1580 nm.

 

Fig. 1 Schematic cross section of the wafer fused 1550 nm VCSEL. Cavity adjustment layers of thickness d₁ and d₂ are indicated.

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The cavity modes of the patterned wafer were mapped by optically pumping the wafer using a 980nm diode laser source; the result of this wavelength mapping is depicted in Figure 2a . The spectral positions of the cavity modes at the four wafer quarters are compared in Figure 2(b) with the measured photoluminescence (PL) spectra of the QW active region of the same wafer, acquired at different temperatures. At room temperature, the four wafer quarters have cavity emission wavelengths with different off-sets of 30, 40, 50 and 60 nm, respectively, with respect to the QW PL peak, which is set at 1520 nm. The off-sets decrease with increasing temperature and are nearly zero at 70°C for the cavity emitting at 1550 nm, and at 90°C, 100°C and 110°C for the 1560 nm, 1570 nm and 1580 nm cavities. The different cavity-PL peak offsets lead to different performance of the VCSELs, especially at high temperatures, as further discussed below.

 

Fig. 2 (a) Emission wavelength (cavity mode) mapping on a double fused wafer fabricated using the cavity adjustment procedure described in the text; and (b) PL spectra at different temperatures with dashed lines indicating lasing wavelengths that correspond to the 4 cavities depicted on Fig. 2a.

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3. VCSEL characteristics

Light-current-voltage (LIV) characteristics of VCSELs processed from different regions of the wafer were measured by a directly coupled photodiode at different temperatures. Figure 3(a) depicts LIV characteristics measured in the temperature range of 0°C ÷ 70°C of a VCSEL emitting near 1560 nm. These results demonstrate 8 mW output power at 0°C, 6.7 mW at 20°C and 2.8 mW at 70°C. Figure 3(b) presents the LIV characteristics measured in the temperature range of 10°C ÷ 115 °C for a device emitting at 1580 nm, which better fits the gain peak at higher temperatures. These results demonstrate 1.5 mW output power at 100°C and about 3.2 mW at 70 °C. Emission spectra (see in-sets on Figure 3) evidence a fundamental mode emission with side-mode suppression ratio (SMSR) of more than 40 dB.

 

Fig. 3 LIV characteristics of VCSELs emitting at 1560 nm with 18 pairs in the top DBR (a) and 1580 nm with 20 pairs in the top DBR (b) at different temperatures. Insets show the emission spectra at room temperature and 15mA.

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Figure 4 illustrates the spectral evolution of the 1560nm VCSEL with currents up to 22 mA and at 4 different temperatures. One can observe excellent single-mode behavior at any of these operation conditions. In addition, by selecting the right combinations of temperature and driving current the emission wavelength can be precisely adjusted at any value between 1562 nm and 1572 nm. On a full wafer, VCSELs inventories of specified wavelengths can thus be selected in a wide spectral range of 40 nm (1550 nm to 1590 nm).

 

Fig. 4 (a) Evolution of the emission spectra of a 1560 nm VCSEL with current at different temperatures; (b) VCSEL spectral line shape for a 1565 nm, device at 20°C and 2 mW emission power.

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A unique feature of the wafer fused VCSELs is the integration of the GaAs-based DBRs, which exhibit high thermal conductivity. The low thermal resistance of these devices in the range of 1 K/mW, results in an extended region of LIVs well beyond the power roll-over current, which is important for reaching broad wavelength tuning range by means of current variation. Figure 5 depicts the LIV curve of a 1560 nm device measured at 0°C, including the part beyond thermal induced power roll-over. Spectra taken at different currents up to 37 mA demonstrate 15 nm continuous wavelength tunability with preservation of excellent (>40dB) SMSR. Another distinctive feature of these devices is a very narrow line-width of 4.5 MHz at 2 mW output, extracted from self-homodyne line-width measurements (Figure 4b), which linearly decreases at higher power levels. This spectral line-width is the lowest value published so far for any long-wavelength VCSEL.

 

Fig. 5 Light- current characteristic at 0°C (a) and emission spectra (b) of a 1560 nm device at different currents.

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4. Summary

In summary, wafer-fused VCSELs emitting in the 1550 nm band present unique features of record high values of fundamental mode output of 8 mW at 0°C, 6.7 mW at 20°C and 1.5 mW at 100°C. Off-sets between emission wavelengths and the peak of room-temperature photoluminescence spectrum of 30 ÷ 40 nm are favorable for reaching high output power close to room temperatures (important for sensor applications), whereas off-sets of 50 ÷ 60 nm result in high output at elevated temperatures, up to 100°C, which is very important in domains like aerospace optical communications. Single mode behavior with SMSR in excess of 40 dB is preserved in the full operation range. Narrow spectral linewidth of 4.5 MHz at 2 mW are demonstrated for the first time on this type of devices. With the wafer fusion technique, cavity adjustment on the same VCSEL wafer for producing VCSELs inventories with a selection of emission wavelengths in a band of 40 nm is quite straightforward. In addition, as a result of the low thermal resistance of the order of 1 K/mW, a wide tuning range with current, up to 15 nm, can be reached. The results of this study demonstrate that wafer-fused VCSELs emitting near 1550 nm are very close to standard DFB lasers in terms of fundamental output power in the full temperature range and spectral line-width, offer substantial advantages in the flexibility of wavelength control, exhibit 5 ÷ 10 times lower power consumption and a broad continuous wavelength tuning. All these elements are very important for emerging applications in fiber-optic communications like wavelength division multiplexing passive optical networks (WDM PON) and novel sensor spectroscopy techniques.