Wafer-fused 1300 nm VCSELs with an active region based on superlattice

The 1300 nm range vertical-cavity surface-emitting lasers with the ac- tive region based on InGaAs/InGaAlAs superlatticeare fabricated using molecular-beam epitaxy and the double wafer-fusion technique. Lasers with the buried tunnel junction diameter of 5 μ m have shown single-mode CW operation with the output optical power of ∼ 6 mW at 20°C. Opened eye diagrams are observed up to 10 Gbps.

1.0% to 1.6%, the D-factor which defines the rising rate of resonance frequency with current, can be increased by 60% [9]. In addition, the modulation bandwidth of InP-based VCSELs with AlGaInAs QWs can be increased up to 11.5 GHz by optimization of the cavity photon lifetime, which in turn enables error-free data transmission rate at 25 Gbps.
An InGaAs/InGaAlAs superlattice (SL) can be used as an alternative approach to enhance differential gain. The use of molecular-beam epitaxy (MBE) technique in comparison with metalorganic vapour-phase epitaxy allows creating sharp heterointerfaces, which made it possible to implement an active region based on SL. In fact, the investigation of 1550 nm range edge-emitting lasers revealed the higher optical gain for the SL-based active region in comparison with the InGaAs QW-based active region [10].
Here, we report on the realization of 1300 nm MBE-grown double wafer-fused VCSELs with an active region based on InGaAs/InAlGaAs SL, which demonstrate the output optical power of 6 mW in a singlemode CW regime. Small and large-signal modulation experiments revealed the possibility of efficient stable operation at 10 Gbps.
Device structure: The VCSEL heterostructure was fabricated by double wafer fusion of AlGaAs/GaAs DBRs grown on GaAs substrate on both sides of the InAlGaAsP optical cavity grown on the InP substrate. The InP-based and GaAs-based heterostructures were grown using the MBE technique. A double intra-cavity contacted the VCSEL design with buried tunnel junction (BTJ) for current confinement which was applied as our basic device design [11]. Figure 1 shows a cross-sectional scanning electron microscope (SEM) image of the completed VCSEL heterostructure in the microcavity region and the corresponding distribution of the electromagnetic field intensity of the cavity mode along with the refractive index profile. GaAs-based VCSEL heterostructure consists of a bottom DBR based on 35.5 pairs of quarter-wave Al 0.91 Ga 0.09 As/GaAs layers, a bottom intra-cavity n-InP contact layer with a thin heavily doped n-InGaAs contact layer, an active region based on SL (24 periods of 0.8 nm-thick In 0.57 Ga 0.43 As/2 nm-thick In 0.53 Ga 0.20 Al 0.27 As layers), a p-In 0.52 Al 0.48 As emitter, an n ++ -In 0.53 Ga 0.47 As/p ++ -In 0.53 Ga 0.47 As/p ++ -In 0.53 Ga 0.31 Al 0.16 As composite-BTJ with an etching depth of ∼25 nm, a top intra-cavity n-InP contact layer with modulated doping profile, and a top DBR based on 21.5 pairs of quarter-wave Al 0.91 Ga 0.09 As/GaAs layers. The wavelength of the photoluminescence peak of the active region emission was about 1280 nm at 20°C. The total thickness of optical microcavity is 3λ (the cavity boundaries are determined by the fused interfaces). The BTJ layers and heavily doped contact layers were placed at the node of the electromagnetic field intensity of the optical cavity mode to reduce optical absorption loss, whereas the active region was placed at its antinode to increase the optical confinement factor.
Experimental results: The CW light-current-voltage (LIV) characteristics for typical VCSELs at 20°C are presented in Figure 2. Despite the use of InGaAs-based BTJ for efficient current confinement and relatively high mirror losses, a threshold current of about 1.25 mA is achieved. The thermal rollover occurs at about 15 mA and limits the maximum output optical power to 6.1 mW. Such high output optical power is due to the high differential quantum efficiency reached for such devices ∼70%. The differential quantum efficiency decreases by  only 11% at 70°C and by 22% at 90°C. The threshold voltage is 1.9 V and the differential resistance, calculated at half rollover current, is ∼85-90 , due to the optimized doping profile and a high quality of the wafer-fused interfaces. The wall-plug efficiency also reaches high values up to ∼30%. Single-mode operation over the entire current range with a side-mode suppression ratio (SMSR) of a least 40 dB is revealed. Inset in Figure 2 shows a spectra at different currents.
To estimate the high-speed performance of the present VCSELs, the small-signal modulation response S 21 (f) was measured using the Keysight N4375D 26.5 GHz lightwave component analyser. The RF signal was combined with the direct current bias through a 45 GHz bias tee and fed to on-wafer VCSELs by a high-frequency ground-source-ground probe head. The results of small-signal modulation analysis for VCSELs at different bias currents are presented in Figure 3. The −3 dB cut-off frequency modulation bandwidth reaches a value of ∼8 GHz at about 10 mA with a modulation current efficiency factor of ∼2.9 GHz/mA 0.5 , then saturates at 8 GHz and drops down to 5 GHz at higher currents (near rollover current). According to the corresponding S 21 (f) fits by the threepole transfer function, the resonance frequency f R significantly exceeds the −3 dB cut-off modulation bandwidth at high currents, while the Kfactor, derived from the dependence of the damping coefficient on the squared resonance frequency, is about 0.4 ns (a further decrease in the value is possible only by reducing the length of the optical cavity). The parasitic cut-off frequency reaches only ∼4 GHz and hence the highspeed performance of the developed VCSELs is limited by an electrical parasitic.
Large-signal modulation experiments were performed at various bit rates to determine the data transmission capacity of the fabricated de-vices. The Keysight M8195A 65 Gbps arbitrary waveform generator was used to generate a non-return-to-zero bit pattern (pseudo-random bit sequence with a pattern length of 2 7 − 1). The Keysight 86100D Infiniium DCA-X wide-bandwidth oscilloscope combined with a 20 GHz optical module was used to record the large-signal modulation. The VC-SEL was biased at 10 mA with 0.7 V peak-to-peak modulation voltage at 20°C. The eye amplitude is weakly changing with bit rate increase up to 10 Gbps, however beyond this rate a decrease in the eye height is observed. The inset in Figure 3 shows a typical eye diagram at 10 Gbps.

Conclusions:
We have studied 1300 nm MBE-grown wafer-fused VC-SELs with the InGaAs/InGaAlAs SL-based active region and a BTJ diameter of 5 μm. Single-mode CW operation with the output optical power of 6 mW and SMSR > 40 dB at 20°C has been obtained. Small-signal modulation more than 8 GHz and clearly opened eye diagrams up to 10 Gbps were observed. We believe that further optimization of the length of the optical cavity, SL design and the photon lifetime, as well as reduction of the parasitic capacitance, would lead to better dynamic characteristics of VCSELs with the SL-based active region compared to the results of InP-based VCSELs with AlGaInAs QWs.