Wafer-fused Vertical-cavity Surface-emitting Lasers Research Articles

20-Gbps 1300-nm range wafer-fused vertical-cavity surface-emitting lasers with InGaAs/InAlGaAs superlattice-based active region

1300-nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated by wafer fusion (WF) technique and studied. The active region based on InGaAs/InAlGaAs superlattice was grown by molecular-beam epitaxy (MBE). Current and optical confinement was provided by composite n ++ -InGaAs / p ++ -InGaAs / p ++ -InAlGaAs buried tunnel junction (BTJ) realized by selective etching and overgrowth by n-InP. AlGaAs/GaAs distributed Bragg reflectors grown by MBE were applied on both sides of the cavity by WF and substrate removal techniques. The devices with BTJ diameter of 5 μm demonstrated a stable single-mode lasing with threshold current <1.3 mA and output optical power >6 mW and operation in a wide temperature range. The measured −3 dB bandwidth was more than 8 GHz at 20°C and about 5.5 GHz at 85°C, the eye diagrams were open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard at 20°C. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps was demonstrated up to 5 km single-mode fiber at 20°C. The developed VCSELs represent a platform for further significant performance improvement.

Effect of saturable absorber in 1.5 μm wafer-fused vertical cavity surface-emitting lasers

The static characteristics of 1.5 μm wafer-fused VCSEL with the thin n++-InGaAs/p++-InGaAs/p++-InAlGaAs tunnel junction (TJ) were investigated. The devices with the 8 μm diameter of the buried tunnel junction (BTJ) demonstrate effective single-mode lasing. A rapid increase in the threshold current, accompanied by an abrupt change in the output optical power is observed for devices with BTJ-diameter less than 7 μm. The observed behavior can be explained by increasing in the overlap of the optical mode with the unpumped (passive) part of the active region, which leads to the additional optical absorption. On the one hand, since the high-order transverse modes have the higher optical losses due to larger overlap with the unpumped part of the active region, then lasing via fundamental mode is more preferable even at large size of BTJ region. On the other hand, the reduction of effective index step between the BTJ region and the regions outside BTJ (caused by using small TJ etching depth) and the lateral smoothing of the difference in the height of the surface relief after TJ regrowth results in the significant drop of the transverse optical confinement factor and rise of absorption. Moreover, the absorption effect can be also strongly enhanced by increasing the heatsink temperature due to decreasing in the gain-to-cavity detuning.

The fabrication technology of VCSELs emitting in the 1.55 μm waveband

The paper presents the results on fabrication technique of a 1,55 μm wafer-fused vertical-cavity surface-emitting lasers (VCSELs) based on the InAlGaAsP/InP optical cavity and AlGaAs/GaAs distributed Bragg reflectors (DBRs) grown by solid-source molecular beam epitaxy. The optical cavity InAlGaAsP/InP containes an active region based on multiple InGaAs quantum wells, n++/p++-In (Al)GaAs buried tunnel junction (BTJ), InGaAsP intracavity contact layers and n-InP spreading layers. The top and bottom AlGaAs/GaAs DBRs were grown on GaAs substrate, while the optical cavity InAlGaAsP/InP was grown on InP substrate. The main fabrication proceseses of 1,55 μm VCSELs such as: BTJ fabrication, dry etching of first mesa on top DBR, wet etching of second VCSEL mesa, forming of the ohmic contacts and passivation of the VCSEL structure were described in details.

Semiconductor light sources for near- and mid-infrared spectral ranges

1550 nm band wafer-fused vertical-cavity surface-emitting lasers (VCSELs) and 5-10 μm band multi-stages quantum-cascade lasers (QCL) grown by molecular beam epitaxy (MBE) were fabricated and studied. VCSELs show high output optical power up to 6 mW in single-mode regime (SMSR > 40 dB) and open-eye diagrams at 30 Gbps of standard NRZ at 20°C. QCL heterostructures show high structural quality (fluctuations of composition and thickness < 1%). 20-μm-stripe width QCLs mounted on copper heatsinks show lasing at ∼ 6, 7.5 and 9 μm.

Polarization mode control of long-wavelength VCSELs by intracavity patterning.

Polarization mode control is enhanced in wafer-fused vertical-cavity surface-emitting lasers emitting at 1310 nm wavelength by etching two symmetrically arranged arcs above the gain structure within the laser cavity. The intracavity patterning introduces birefringence and dichroism, which discriminates between the two polarization states of the fundamental transverse modes. We find that the cavity modifications define the polarization angle at threshold with respect to the crystal axes, and increase the gain anisotropy and birefringence on average, leading to an increase in the polarization switching current. Experimental measurements are explained using the spin-flip model of VCSEL polarization dynamics.

Effect of Cavity Lifetime Variation on the Static and Dynamic Properties of 1.3-μm Wafer-Fused VCSELs

We investigate experimentally the impact of photon cavity lifetime variations on the static and dynamic performance of high-speed 1.3-μm wavelength wafer–fused vertical-cavity surface-emitting lasers (VCSELs). The photon lifetime is modified by decreasing the top mirror reflectivity either by shallow-surface etching or by Bragg-reflector layer-pair removal. Significant improvements in both static and dynamic VCSEL performance as well as the extraction of internal VCSEL device parameters are achieved.

Numerical Analysis of Mode Discrimination by Intracavity Patterning in Long-Wavelength Wafer-Fused Vertical-Cavity Surface-Emitting Lasers

This paper presents an extensive numerical analysis of 1.3-μm wavelength wafer-fused vertical-cavity surface-emitting lasers (VCSELs) incorporating intracavity patterning. Using a 3-D, self-consistent model of the physical phenomena in VCSELs, supported by experimental results used for parameter calibration, we investigate the influence of arch-and ring-shaped intracavity features with a broad range of geometrical parameters on the modal behavior of the VCSEL. To design and optimize the devices, we used intracavity patterning that provides very strong discrimination of higher order modes, pushing them out from the active region. This mechanism makes possible single mode operation under a broad range of currents and could potentially enhance the single-mode output power of these devices.

Near-infrared wafer-fused vertical-cavity surface-emitting lasers for HF detection

1310nm Wavelength VCSELs produce single mode emission in excess of 1mW in the temperature range of 20−70°C with a very low relative intensity noise of −148dB/Hz. These devices have a broad tuning range of ~50cm−1 with variable temperature and current, which cover multiple absorption lines of methane, ammonia and hydrogen fluoride. With these VCSELs, we report optical absorption spectroscopy of several gases near 1310nm wavelength. In particular, we report for the first time detection of HF using the absorption line in the vicinity of 7568cm−1 with an estimated low-pressure detection limit at the level of several ppm.

Transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning

Transverse mode discrimination is demonstrated in long-wavelength wafer-fused vertical-cavity surface-emitting lasers using ring-shaped air gap patterns at the fused interface between the cavity and the top distributed Bragg reflector. A significant number of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing in particular the identification of a design that reproducibly increases the maximal single-mode emitted power by about 30 %. Numerical simulations support these observations and allow specifying optimized ring dimensions for which higher-order transverse modes are localized out of the optical aperture. These simulations predict further enhancement of the single-mode properties of the devices with negligible penalty on threshold current and emitted power.

MOVPE growth of InGaAsP/InP-based vertical-cavity structures for wafer-fused VCSELs

We have successfully grown InGaAsP/InP-based vertical-cavity structures with precisely adjusted resonant-cavity wavelengths by metalorganic vapor phase epitaxy (MOVPE). These structures have been used to create vertical-cavity surface emitting lasers (VCSELs) that emit exactly 1.55 μm through wafer fusing to GaAs/AlAs distributed Bragg reflectors (DBRs). With such growths, high lateral uniformity and reproducibility in InGaAsP layer thickness and composition are essential to obtain well-defined resonant cavity and MQW PL peak wavelengths.