Electrically pumped shot-noise limited class A VECSEL at telecom wavelength.

Class A shot-noise limited operation is achieved in an electrically pumped vertical external cavity surface emitting laser (VECSEL), opening the way for integration of such peculiar noiseless laser oscillation in applications where low power consumption and footprint are mandatory. The quantum well active medium is grown on an InP substrate to enable laser oscillation at telecom wavelengths. Single frequency class A operation is obtained by proper optimization of the cavity dimensions, ensuring at the same time a sufficiently long and high-finesse cavity without any intracavity filtering components. The laser design constraints due to electrical pumping are discussed as compared to optical pumping. The intensity noise spectrum of this laser is shown to be shot-noise limited, leading to a relative intensity noise of $-160\;{\rm dB/Hz}$ for 3.1 mA detected photocurrent.

Evaluation of Pollutants Along the National Road N2 in Togo using the AERMOD Dispersion Model.

BACKGROUND: Air pollution has become a major problem around the world and is increasingly an issue in Togo due to increased vehicular traffic. Gaseous pollutants are released by engines and are very harmful to human health and the environment. The fuels used on the major road in Togo, the N2, are adulterated with unknown contents and are of poor quality. Many of the vehicles come from neighboring countries, such as Benin, Ghana and Nigeria. OBJECTIVES: The present study aims to evaluate the pollution rate in Togo through the estimation of the concentrations of sulfur dioxide (SO2), nitrogen oxides (NOx), and particular matter (PM) on the international road, the National Road N2, in Lomé, compared to the World Health Organization's (WHO) standard limit. METHODS: The simulations of pollutant concentration were performed using the Industrial Source Complex Short Term Version 3 model, which is included in the United States Environmental Protection Agency Regulatory Model (USEPA) AERMOD View software. The meteorological averages data were obtained from the local station near the National Road N2 in Togo in 2018. Hourly averages were calculated according to the European Monitoring Evaluation Programme/European Environmental Agency air pollutant emission inventory guidebook 2016 and were processed using AERMET View and a terrain pre-processor, AERMAP. For the model, the sources of pollution were the vehicles traveling on the road segment. The source was a line volume with 20 m of width and 2 m of height. The estimation methodology covered exhaust emissions of NOx, SO2 and PM contained in the fuel. RESULTS: The simulations provided average hourly, daily and annual concentrations of the different pollutants: 71.91 µg/m3, 42.41 µg/m3,11.23 µg/m3 for SO2; 16.78 µg/m3, 9.89 µg/m3, 2.46 µg/m3 for NOx and below the detection limit, 0.62 µg/m3, 0.15 µg/m3 for PM, respectively. These results indicate that on the National Road N2 in Togo, the concentrations of SO2 were high compared to those of NOx and PM. The daily average concentration of SO2 was twice the permissible limits set by the WHO. CONCLUSIONS: Emissions obtained from the AERMOD for NOx and PM were less than the permissible limits set by the WHO, while the rate of SO2 was twice the permissible limit. The fuels used on this road were very rich in sulfur. The sulfur level in fuels must be monitored by stakeholders in Togo. COMPETING INTERESTS: The authors declare no competing financial interests.

Direct measurement of the spectral dependence of Lamb coupling constant in a dual frequency quantum well-based VECSEL.

Spectral dependence of Lamb coupling constant C is experimentally investigated in an InGaAlAs Quantum Wells active medium. An Optically-Pumped Vertical-External-Cavity Surface-Emitting Laser is designed to sustain the oscillation of two orthogonally polarized modes sharing the same active region while separated in the rest of the cavity. This laser design enables to tune independently the two wavelengths and, at the same time, to apply differential losses in order to extract without any extrapolation the actual coupling constant. C is found to be almost constant and equal to 0.84 ± 0.02 for frequency differences between the two eigenmodes ranging from 45 GHz up to 1.35 THz.

High-power flip-chip semiconductor disk laser in the 1.3 µm wavelength band.

We present 6.1 W of output power from a flip-chip semiconductor disk laser (SDL) emitting in the 1.3 µm wavelength region. This is the first demonstration of a flip-chip SDL in this wavelength range with output powers that are comparable to those obtained with intracavity diamond heat spreaders. The flip-chip configuration circumvents the optical distortions and losses that the intracavity diamond heat spreaders can introduce into the laser cavity. This is essential for several key applications of SDLs.

High performance wafer-fused semiconductor disk lasers emitting in the 1300 nm waveband.

We report for the first time on the performance of 1300 nm waveband semiconductor disc lasers (SDLs) with wafer fused gain mirrors that implement intracavity diamond and flip-chip heat dissipation schemes based on the same gain material. With a new type of gain mirror structure, maximum output power values reach 7.1 W with intracavity diamond gain mirrors and 5.6 W with flip-chip gain mirrors, using a pump spot diameter of 300 µm, exhibiting a beam quality factor M(2)< 1.25 in the full operation range. These results confirm previously published theoretical modeling of these types of SDLs.

Control of cavity lifetime of 1.5 µm wafer-fused VCSELs by digital mirror trimming.

Digital chemical etching is used to trim the output mirror thickness of wafer-fused VCSELs emitting at a wavelength near 1.5µm. The fine control of the photon cavity lifetime thus achieved is employed to extract important device parameters and optimize the combination of the threshold current, output power, and direct current modulation characteristics. The fabrication process is compatible with industrial production and should help in improving device yield and in reducing manufacturing costs.

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.

1.56 µm 1 watt single frequency semiconductor disk laser.

A single frequency wafer-fused semiconductor disk laser at 1.56 µm with 1 watt of output power and a coherence length over 5 km in fiber is demonstrated. The result represents the highest output power reported for a narrow-line semiconductor disk laser operating at this spectral range. The study shows the promising potential of the wafer fusion technique for power scaling of single frequency vertical-cavity lasers emitting in the 1.3-1.6 µm range.

1 W at 785 nm from a frequency-doubled wafer-fused semiconductor disk laser.

We demonstrate an optically pumped semiconductor disk laser operating at 1580 nm with 4.6 W of output power, which represents the highest output power reported from this type of laser. 1 W of output power at 785 nm with nearly diffraction-limited beam has been achieved from this laser through intracavity frequency doubling, which offers an attractive alternative to Ti:sapphire lasers and laser diodes in a number of applications, e.g., in spectroscopy, atomic cooling and biophotonics.

8 mW fundamental mode output of wafer-fused VCSELs emitting in the 1550-nm band.

We report record-high fundamental mode output power of 8 mW at 0 °C and 1.5 mW at 100°C achieved with wafer-fused InAlGaAs-InP/AlGaAs-GaAs 1550 nm VCSELs incorporating a re-grown tunnel junction and un-doped AlGaAs/GaAs distributed Bragg reflectors. A broad wavelength tuning range of 15 nm by current variation and wavelength setting in a spectral range of 40 nm on the same VCSEL wafer are demonstrated as well. This performance positions wafer-fused VCSELs as prime candidates for many applications in low power consumption, "green" photonics.

Polarisation stabilisation of vertical cavity surface emitting lasers by minimally invasive focused electron beam triggered chemistry.

Local electron triggered reactions of functional surface adsorbates were used as a maskless, dry, and minimally invasive nanolithography concept to stabilize the polarisation of individual vertical cavity surface emitting lasers (VCSELs) on a wafer in a post-processing step. Using a 30 keV focused electron beam of a scanning electron microscope and injecting volatile organo-metallic (CH(3))(2)Au(tfa) molecules, polarisation gratings were directly written on VCSELs by dissociating the surface adsorbed molecules. The electron triggered adsorbate dissociation resulted in electrically conductive Au-C nano-composite material, with gold nanocrystals embedded in a carbonaceous matrix. A resistivity of 2500 µΩcm was measured at a typical composition of 30 at.% Au. This material proved successful in suppressing polarisation switching when deposited as line gratings with a width of 200 nm, a thickness of 50 nm, and a pitch of 500 nm and 1 µm. Refractive index measurements suggest that the optical attenuation by the deposited Au-C material is much lower than by pure Au thus giving a low emission power penalty while keeping the polarisation stable.

Intra-cavity patterning for mode control in 1.3 µm coupled VCSEL arrays.

We report coupled VCSEL arrays, emitting at 1.3 µm wavelength, in which both the optical gain/loss and refractive index distributions were defined on different vertical layers. The arrays were electrically pumped through a patterned tunnel junction, whereas the array pixels were realized by intra-cavity patterning using sub-wavelength air gaps. Stable oscillations in coupled modes were evidenced for 2x2 array structures, from threshold current up to thermal roll-over, using spectrally resolved field pattern analysis.

3 W of 650 nm red emission by frequency doubling of wafer-fused semiconductor disk laser.

3 W at genuine red wavelength of 650 nm has been achieved from a semiconductor disk laser by frequency doubling. An InP based active medium was fused with a GaAs/AlGaAs distributed Bragg reflector resulting in an integrated monolithic gain mirror. 6.6 W of output power at the fundamental wavelength of 1.3 µm represents the best achievement reported to date for this type of lasers.

Power-scalable 1.57 microm mode-locked semiconductor disk laser using wafer fusion.

We report the first (to our knowledge) wafer-fused high-power passively mode-locked semiconductor disk laser operating at 1.57 microm wavelength. An InP-based active medium was fused with GaAs/AlGaAs distributed Bragg reflector on a 2 inch wafer level, resulting in an integrated monolithic gain mirror. An intracavity wedged diamond heat-spreader capillary bonded to the gain chip provides efficient heat removal from the gain structure without disturbing the spectrum of the mode-locked laser. The laser produces over 0.6 W of average output power at 15 degrees C with 16 ps pulse width. The total output power accounting for all output beams emerging from the cavity was 0.86 W. The results reveal an essential advantage of wafer fusion processing of disparate materials over monolithically grown InP-based gain structures and demonstrate the high potential of this technique for power scaling of long-wavelength semiconductor disk lasers.

10-Gb/s and 10-km error-free transmission up to 100 degrees C with 1.3-microm wavelength wafer-fused VCSELs.

10-Gb/s modulation speed and transmission over 10-km SM fiber with BER < 10(-11) up to 100 degrees C temperature are achieved with optimized wafer-fused GaAs/AlGaAs-InP/InAlGaAs VCSELs incorporating re-grown tunnel junction. These VCSELs operate in the 1310-nm waveband and emit more than 1-mW single mode power in the full temperature range.

1.3-microm optically-pumped semiconductor disk laser by wafer fusion.

We report a wafer-fused high power optically-pumped semiconductor disk laser operating at 1.3 microm. An InP-based active medium was fused with a GaAs/AlGaAs distributed Bragg reflector, resulting in an integrated monolithic gain mirror. Over 2.7 W of output power, obtained at temperature of 15 degrees C, represents the best achievement reported to date for this type of lasers. The results reveal an essential advantage of the wafer fusing technique over both monolithically grown AlGaInAs/GaInAsP- and GaInNAs-based structures.

1.3 microm-wavelength phase-locked VCSEL arrays incorporating patterned tunnel junction.

We report the fabrication and the performance of phase-locked VCSEL arrays emitting near 1310 nm wavelength. The arrays were fabricated using double wafer fusion by patterning a tunnel junction layer, which serves to define the individual single mode array elements. Phase-locking in both one-dimensional and two-dimensional array configurations was confirmed by means of far field and spectral measurements as well as theoretical modeling. CW output powers of more than 12 mW were achieved.

2.6 W optically-pumped semiconductor disk laser operating at 1.57-microm using wafer fusion.

We report a wafer fused high power optically pumped semiconductor disk laser incorporating InP-based active medium fused to a GaAs/AlGaAs distributed Bragg reflector. A record value of over 2.6 W of output power in a spectral range around 1.57 microm was demonstrated, revealing the essential advantage of the wafer fusing technique over monolithically-grown all-InP-based structures. The presented approach allows for integration of lattice-mismatched compounds, quantum-well and quantum-dot based media. This would provide convenient means for extending the wavelength range of semiconductor disk lasers.