1. Introduction

High single-mode (SM) power vertical-cavity surface-emitting lasers (VCSELs) are of great interest for optical fiber communication and sensing applications. Long wavelength SM VCSELs emitting in the 1200-2000nm spectral range are of special interest for long-reach optical fiber telecommunications [1] and gas sensing of particular species [2–4]. For optical fiber communications, at least 1-mW single mode (SM) power in the fiber in the full −20-80°C temperature operation range is required, and VCSELs offer here a particular advantage as their beams can be fiber-coupled much more efficiently as compared with edge emitting lasers [5,6]. For other purposes such as photo-acoustic sensors and spectroscopy, still higher power levels are needed [2–4]. In all these applications, VCSELs offer considerable advantages over edge-emitting lasers, due to their spectral purity, wavelength setting and continuous tuning, lower power consumption, and lower manufacturing costs [6,7].

In the past decade, significant progress in long-wavelength VCSEL technology has been achieved, based on InP/InAlGaAs quantum well (QW) active regions and exploiting tunnel-junctions (TJs) for carrier and photon confinement [8–10]. Constant increase in long-wavelength VCSEL SM power has been accomplished by optimal designs of the transverse optical confinement using regrown (buried) TJ apertures, with reported SM power as high as 6.5 and 7.9-mW for the 1550-nm range [11,12] and 5.4-mW for 1310-nm band at ambient temperature [13]. This is comparable to shorter-wavelength VCSELs; for example, using surface relief structure in 850-nm VCSELs, 6-mW of single-mode power has been obtained [14]. In the case of InP-based VCSELs, higher output power can be achieved by increasing the TJ diameter, but this at the expense of reducing the side mode suppression ratio (SMSR). This is because a larger TJ aperture reduces the radial temperature gradient, which lowers the transverse mode discrimination. Restoring to stronger confinement can be accomplished by shifting the TJ layers away from the optical mode node, but this increases optical absorption and threshold current.

The VCSEL SM power is also sensitive to output coupling and detuning between the QW photoluminescence (PL) peak wavelength and the cavity mode. Through the optimization of these parameters, along with improving the active region quality, we have achieved high performance LW-VCSELs emitting near 1550-nm with record 8-mW SM power at 0°C [11]. Here, we report on record SM room temperature output power from 1310-nm band VCSELs obtained by output coupling optimization via modification of the reflectivity of the top GaAs-based distributed Bragg reflector (DBR). Such fine tuning of the output coupling is necessary for optimizing the VCSEL performance in spite of the very good control on layer thicknesses and compositions during epitaxy, because of the great sensitivity of the device characteristics to the structure details. Devices showing record SM power of 6.8-mW at room temperature and 2.8-mW at 80°C, with more than 30 dB SMSR, are demonstrated.

2. VCSEL design and fabrication

The VCSEL structures were assembled by double wafer fusion using one InP-based active cavity and 2 GaAs-based top and bottom DBRs [15]. The schematic cross section of the completed VCSEL device is presented in Fig. 1. Both the InAlGaAs/InP QW active structure and the GaAs DBRs were grown by low-pressure metallorganic vapor phase epitaxy (MOVPE). The InAlGaAs/InP QW growth was carried out on (100) exactly-oriented InP substrates and was performed in 2 steps. In the first step, the n-InP current spreading layer, 5 strain-compensated InAlGaAs QWs, p-n and p+/n + tunnel junction layers were grown. The InAlGaAs QWs are placed at the peak of the electromagnetic field, while the TJ layers are placed at a node to minimize optical absorption. In the second growth step, an InP/InGaAsP layer over growth of the patterned and etched TJ mesas, ~6-μm in diameter, was carried out.

 

Fig. 1 VCSEL device schematic structure.

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In order to determine the effect of changing the output coupling on the 1.3-μm VCSEL characteristics, two different samples from the same double-fused VCSEL wafer were processed and analyzed. One sample contained a standard top Al_0.92Ga_0.08As/GaAs DBR with 20.5 pairs (sample A). On the second sample, the 3 top pairs were removed by selective etching using solutions of H₂O₂-Citric acid for the GaAs and HCl-H₂O for the AlGaAs layers, respectively (sample B). A similar approach for adjusting the output coupling of 850-nm VCSELs using shallow etching of the top DBR for enhancing the direct modulation speed has been reported [16]. For our wafer-fused, long wavelength VCSELs, so far the pair removal method seems more reproducible in terms of getting uniform reflectivity across the wafer, as compared with shallow etching. On the other hand, pair removal results in smaller variations in DBR reflectivity as compared to shallow etching, for the same etch depth. The next processing steps on both samples were identical: reactive ion etching of the top DBR mesa, selective chemical etching of the InAlGaAs/InP active region, dielectric film deposition, dry etching and electron-beam deposition of metals for electrical contacts, and electroplating for bonding-pads. The VCSEL beam is emitted from the top-side of the device and the P- and N-contact pads are placed on the same surface (Fig. 1). The diode current flows only through the InP layers, InAlGaAs QWs and the TJ, the latter providing both optical and electrical transverse confinement.

3. VCSEL device performance

The Light-Current-Voltage (L-I-V) characteristics of both VCSEL devices, acquired in the 0-90/100°C heat-sink temperature range, are presented in Fig. 2. The L-I-V characteristics on both devices were measured on-wafer under the same conditions without cleaving into separate chips. The emission power was significantly increased for the devices with lower DBR reflectivity up to 80°C. At 90°C the emission power is approximately the same for both devices, whereas at temperatures >90°C the emission power of VCSELs from sample A is higher. A modification in the reflectivity R of the top DBR changes its loss (transmission) rate α_mir. As expected, the increase in the top mirror losses leads to an increase in emission power P_e, as they are related by [17]:

 

Fig. 2 Light-Current (in red) and Voltage-Current (in blue) characteristics in the 0-90/100°C temperature range for VCSEL devices from samples A (a) and B (b).

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In Fig. 3, the threshold current, the SM maximum power (with minimum 30dB SMSR) and the maximum total output power at the L-I thermal roll-over point, measured in the 0-90°C temperature range, are plotted for devices from samples A (a) and B (b). It should be recalled that both structures were fabricated from the same double-fused 2” wafer, for which the photoluminescence peak – cavity mode offset was similar (~50-nm at room temperature). Linear extrapolation of the maximum emission power indicates lasing up to 120°C for structure A and up to 100°C for structure B. Sample A exhibits low threshold currents in the range of 0.8-1.5-mA up to 80°C, attaining a minimum in the 40-50°C range. This sample lases at 100°C with still low threshold of 3-mA and a useful output power of 1.1-mW, well suited for uncooled-laser applications. The SM emission power of these devices is limited to ~3-mW due to the excitation of higher order transverse modes. Reducing the number of top DBR pairs and the consequent increase in output coupling in sample B yields enhancement of both output power and SMSR. Thus, for structure B, the room temperature SM and maximum powers were augmented from 3.2 to 6.8-mW and from 5 to 7.6-mW, respectively. For temperatures higher than 40°C, emission spectra are SM up to roll-over, but for 20°C the maximal SM power is limited to 6.8-mW. As expected, the threshold current also increased from 1 to 1.5-mA to 3-3.5-mA in the 0-80°C range, but these values are still acceptable for many applications.

 

Fig. 3 Threshold current, SM and maximum output power in the 0-90°C temperature range for VCSEL devices from samples A (a) and B (b).

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The room temperature emission spectra, measured at different currents for VCSELs from samples A and B, are presented in Fig. 4. The device with higher top-DBR reflectivity shows SM emission (SMSR>30dB) up to 8-mA of diode current. The influence of the increase in output coupling on the SM performance clearly shows for sample B, where SM emission is maintained up to 15-mA diode current. At this pump current, the highest value of 6.8-mW SM output power is achieved at room temperature, to our knowledge the highest value reported so far for any 13xx VCSEL device.

 

Fig. 4 Room temperature emission spectra at different pumping currents for devices from samples A (a) and B (b).

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4. Conclusions

In this work, we report on the enhancement of the emission of single-mode 1.3-μm VCSELs by reducing the reflectivity of the top GaAs-based DBR for output coupling optimization. Devices with record SM power of 6.8-mW at room temperature and 2.8-mW at 80°C, with more than 30 dB SMSR, have been obtained. Achieving this output coupling optimization with a post-processing adjustment of the number of Bragg periods in the output coupler is compatible with full wafer industrial production of these devices, and should also increase yield and eventually reduce device costs. The steady progress in increasing the SM output power of long wavelength VCSELs has brought their performance in this respect close to that achievable with their short wavelength (850-980nm) counterparts that are based on simpler and more mature technologies. Further increase of the SM power of long wavelength VCSLEs to the 10-mW range is expected with additional improvements in transverse carrier and optical confinement, e.g., using intra-cavity refractive index patterning [18].