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We report on the design, fabrication and characterization of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) emitting at 1470 nm. We demonstrate 6.2 mW of CW output power, which represents the highest power value reported so far for EP-VECSELs in the 14XX nm and 15XX nm wavelength bands.
©2013 Optical Society of America
1. Introduction
It has been shown recently that optically pumped 14XX nm -band wafer fused vertical external cavity surface emitting lasers (VECSELs) can produce multi-Watt fundamental mode emission that can be efficiently coupled into single mode fibers for pumping Raman-fiber lasers [1,2]. In addition, 14XX nm lasers cover in part the fiber-optic coarse wavelength division-multiplexing (CWDM) grid [3]. The wafer fusion approach allows the independent optimization of the gain medium and the distributed Bragg reflector (DBR) of the gain mirror in these devices, without compromising interface quality or manufacturability [4,5]. In addition, VECSELs in general have the advantage of emitting light with very low relative intensity noise (RIN) [6,7] as a result of a longer photon life-time as compared to the carrier life-time in the gain mirror, which is not the case in edge-emitting and vertical cavity surface emitting laser (VCSEL) devices. On the other hand, existing 14XX and 15XX nm band VECSELs employ optical pumping, which increases cost, size and power consumption of these devices. Thus, development of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) emitting in these wavelength bands may benefit new applications in areas such as microwave photonics, radio over fiber, gas spectroscopy, Raman amplifiers, etc., in which the combination of low noise, emission wavelength compatible with existing low-loss single mode fibers, small size and low cost are important requirements [8,9]. Compared with existing 15XX nm VCSELs [10], besides the potential for reaching higher brightness, EP-VECSELs have an important advantage that consists in the possibility of introducing intra-cavity elements like intra-cavity Fabry-Perrot filters for reaching single frequency operation [11].
Short-wavelength EP-VECSELs, operating close to 980 nm with reasonable efficiency, have been demonstrated in the GaAs/AlGaAs material system [12,13]. Output power levels around 0.5 W coupled into single mode fiber were reported in [12]. On the other hand, EP-VECSELs emitting in the long-wavelength band reported so far at 15XX nm [14] exhibit quite low continuous wave (CW) power levels of 0.5 mW.
In this paper, we report on the design and performance of the first 14XX nm electrically pumped VECSEL produced by the wafer fusion technique, exhibiting 6.2 mW output power in continuous wave operation.
2. Design and fabrication of 1470 nm electrically pumped gain mirrors
Demonstrated short-wavelength EP-VECSELs, described in [12] and [13], are based on gain mirrors consisting of a p-type bottom DBR and an n-type partial DBR that are placed on opposite sides of a GaAs-based active region incorporating InGaAs/GaAs quantum wells (QWs). The device aperture in these structures is defined by the diameter of the electrical contact applied to the p-DBR. This approach cannot be applied to long-wavelength vertical cavity devices, which typically employ tunnel junction carrier injection in the active region and dielectric, or undoped/n-type semiconductor DBRs [10,15]. In these devices, the aperture size is defined by the diameter of the tunnel junction that injects carriers into the active region. In the 1470 nm wavelength electrically pumped gain mirrors developed in the current work (see Fig. 1), the diameter of the n++/p++ InAlGaAs tunnel junction aperture is set at 35µm. The InP-based, 3.5 lambda active region, which is grown by low pressure metal-organic vapour phase epitaxy (MOVPE) on (100) InP substrate, includes one group of 6 undoped InAlGaAs strained QWs placed in an antinode of electric field distribution, with room-temperature photoluminescence spectra centered at 1440 nm wavelength, an InAlGaAs p-n junction, a top n-InP-spacer, and a bottom n-InP current spreading layer. By applying the wafer fusion technique [5], the active region is sandwiched between a top low reflectivity 8 pairs-AlGaAs/GaAs n-type DBR (intermediate DBR) and an undoped bottom high reflectivity 34 pairs- AlGaAs/GaAs DBR, both grown by MOVPE on (100) GaAs substrates. Before fusing to the active region, the AlGaAs/GaAs DBRs are transferred by wafer fusion onto n-type InP (100) substrates. This procedure limits defect formation in the fused gain mirror as described in [16] and, because of the high electron mobility in n-InP, provides better current spreading as compared with n-GaAs substrate emitting devices. Efficient current spreading through the 150 µm-thick n-InP substrate is important for decreasing the current non-uniformity through the tunnel junction resulting from the window in the top electrical contact.
Fig. 1 Schematic cross-section of the wafer-fused EP-VECSEL gain-mirror. Dashed lines represent the injected current paths.
To enhance current spreading on the bottom side of the structure we rely on an n- InP current spreading layer with increased thickness of about 1 µm (2 λ optical length). Insertion of the intermediate top DBR allows for compensating additional optical losses in the gain mirror arising as a result of optical absorption in the doped regions (which do not exist in optically pumped structures). On the other hand, the sub-cavity between the two DBRs may introduce a spectral filtering effect that could disturb the mode selection by the curved external mirror. In our structure, we use a low reflectivity intermediate DBR with reflectivity of about 70%, as seen from the cavity, that has a minor effect on mode selection, but which is quite efficient in compensating for the optical losses in the doped regions for reaching threshold at lower pump currents. Both bottom and intermediate GaAs/AlxGa1-x As (x = 0.9) DBRs have a high reflectivity stop band centred at 1470 nm. An electric contact and electroplated copper heat-spreader on the bottom as well as an antireflection coating and electrical contact on the top side complete the gain mirror.
The near-field electroluminescence intensity profiles of a 35-μm diameter TJ aperture gain mirror for different injection currents are depicted in Fig. 2. As one can see, the uniformity of current injection is rather good even though some non-uniformity due to “carrier crowding” can be observed near the TJ periphery, which is most probably due to lateral current flow in the n-type InP current spreading layer on the bottom mirror side of the structure.
Fig. 2 Electroluminescence intensity profiles of the wafer-fused EP-VECSEL for different injected currents. The threshold current for this device was 39 mA.
3. EP-VECSEL performance
After having been soldered on a Cu-submount and fixed to a Peltier element, the gain mirror assembly was aligned with a highly reflective spherical (radius of curvature of 50mm) dielectric mirror, to form a stable planar-concave Fabry-Perot external cavity, as shown in Fig. 3. The selected cavity length of about 49 mm allows for the largest variation of cavity mode size on chip. For this cavity length, calculated beam radius (1/e2) at chip surface is close to 50 µm. In our EP-VECSELs assembly we have tested different output couplers (OCs) with transmission values of 10%, 5%, and 3% in the 1230–1730nm wavelength range.
Fig. 3 Schematic representation of the wafer-fused EP-VECSEL assembly in a linear cavity configuration.
Continuous wave (CW) light-current-voltage characteristics (LIVs) measured with a 3% output coupler (OC) at 5, 10 15 and 20°C are depicted in Fig. 4. We reach the maximum power of 6.2 mW at 15°C at driving current and voltage values of 240 mA and 3 V.
As one can observe in Fig. 4, the threshold current practically does not change with temperature and equals c.a 40 mA, while quite similar maximum output power values are reached both at 10 and 15 °C at around 240 mA. The voltage values at threshold and maximum output power correspond to 1.9 V and 3 V, respectively. Decreasing operating temperature should allow reaching higher output power at larger supply current. However,, it is apparently not the case in our results. This implies, that temperature tuning of the QW emission and µ-cavity resonance is not the mechanism which is responsible for the observed behaviour. The reason for lower output power achieved at 5°C is not clear at the moment. As depicted in Fig. 5, the output coupler of 3% produces the highest output power compared with OC value of 5% at both 15 and 20 °C. At 10% output coupling, the output mirror losses prevent reaching lasing threshold.
Fig. 5 Maximum output power and threshold current density of the wafer-fused EP-VECSEL versus the transmission of the output coupler (OC).
In addition, the minimum threshold current density also corresponds to 3% output coupling. Reaching maximum output power at relatively low output coupling of 3% for low reflectance intermediate DBRs of about 70% is consistent with data for shorter wavelength EP-VECSELs presented in [13]. Increased gain enhancement by introducing higher reflectivity intermediate DBRs affects the mode selection capacity of the output coupling mirror.
Emission spectra of the EP-VECSELs, measured at 15 °C at 39, 45, 80, 150 and 240 mA are depicted in Fig. 6.
Fig. 6 Emission spectra of the EP-VECSELs at 15 °C under CW conditions for 3% output coupling and different currents.
Even though near threshold the EP-VECSEL emission is singlemode, at higher currents one can observe a second mode. The observed mode structure might represent two longitudinal modes filtered by the intermediated DBR.
The wavelength shift with temperature estimated from spectra recorded at different heat-sink temperatures is 0.09 nm/K. Based on this shift we have determined the temperature rise in the active region at 15 °C to be 1 K and 18 K at threshold and maximum output power, respectively. The thermal impedance of this gain mirror is thus estimated as13 K/W and 25 K/W for the threshold and maximum output power respectively. These values are close to the thermal impedance value determined in 1550 nm optically pumped VECSELs with similar heat dissipation approach based on electroplated copper heat-spreader [17]. Even though with the current E-VECSEL design the maximum output power is inferior to the maximum power obtained with VCSELs in Ref. [10], we expect demonstrating a further improvement of device performance by introducing a n-type doped bottom AlGaAs/GaAs DBR to reach a more uniform current distribution through the tunnel junction at all operating currents and at larger apertures.
4. Conclusions
This paper presents the design and performance of the first electrically pumped VECSELs emitting in the 14XX nm band fabricated by the wafer fusion technique. For improved current uniformity in the patterned tunnel junction with increased lateral dimensions, we introduced an n-type InP substrate emitting design in combination with increased thickness of the bottom n-InP current spreading layer. Low reflectivity (70%) intermediate DBR is applied for compensating absorption losses in the doped regions of the structure. Electroplated copper deposited directly on the bottom DBR produces efficient heat-sinking with a measured low values of thermal impedance of 13-25 K/W in the full operation range. In our results we demonstrate 6.2 mW continuous wave EP-VECSEL operation, which represents a considerable improvement compared with previously published reports. Even though the results of this work are obtained for 1470 nm, there are no fundamental limitations in reaching the same performance at any wavelength in the 15XX wavelength range. Further improvements in the device performance are expected by replacing the bottom un-doped AlGaAs DBR with a n-type doped one that will allow reaching a uniform current density through the tunnel junction at all operation currents.
Acknowledgments
This work was supported by the Swiss Nanotera Program. The authors acknowledge the technical contribution of M. Wasiak from Technical University of Lodz, Poland and J. Lyytikainen from the Optoelectronics Research Centre, Tampere University of Technology.
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