Alexey M. Nadtochiy

Frequency response of large aperture oxide-confined 850 nm vertical cavity surface emitting lasers

Small and large signal modulation measurements are carried out for 850 nm vertical cavity surface emitting lasers (VCSELs). The resonance frequency, damping factor, parasitic frequency, and D-factor are extracted. Small signal modulation bandwidths larger than 20 GHz are measured. At larger currents the frequency response becomes partially limited by the parasitics and damping. Our results indicate that by increasing the parasitic frequency, the optical 3 dB bandwidth may be extended to ∼25 GHz. A decrease in the damping should enable VCSEL bandwidths of 30 GHz for current densities not exceeding ∼10 kA/cm2 and ultimately error-free optical links at up to 40 Gbit/s.

22-Gb/s Long Wavelength VCSELs

1.55-microm vertical cavity surface-emitting low-parasitic lasers show open eyes up to 22-Gb/s modulation speed. Uncooled error-free operation over a wide temperature range up to 85 degrees C under constant bias conditions is demonstrated at 12.5-Gb/s data rate. At these fixed bias conditions the laser characteristics are practically invariant with temperature. These are the highest data-rates reported from a long-wavelength VCSEL structure to date.

Electro-optical resonance modulation of vertical-cavity surface-emitting lasers

Optical and electrical investigations of vertical-cavity surface-emitting lasers (VCSEL) with a monolithically integrated electro-optical modulator (EOM) allow for a detailed physical understanding of this complex compound cavity laser system. The EOM VCSEL light output is investigated to identify optimal working points. An electro-optic resonance feature triggered by the quantum confined Stark effect is used to modulate individual VCSEL modes by more than 20 dB with an extremely small EOM voltage change of less than 100 mV. Spectral mode analysis reveals modulation of higher order modes and very low wavelength chirp of < 0.5 nm. Dynamic experiments and simulation predict an intrinsic bandwidth of the EOM VCSEL exceeding 50 GHz.

1.55 µm high-speed VCSELs enabling error-free fiber-transmission up to 25 Gbit/s

We present 1.55-µm short-cavity VCSELs with modulation bandwidths beyond 17 GHz and extremely low parasitics. These devices are the first long-wavelength VCSELs enabling error-free data-transmission at 25-Gb/s over 4.2 km standard single-mode fiber.

Highly temperature-stable modulation characteristics of multioxide-aperture high-speed 980 nm vertical cavity surface emitting lasers

We present multioxide-aperture 980 nm-range vertical cavity surface emitting lasers (VCSELs) with highly temperature stable modulation characteristics operating error-free at 25 Gbit/s at 25 and 85 °C. We perform small signal modulation experiments and extract the fundamental physical parameters including relaxation resonance frequency, damping factor, parasitic cut-off frequency, D-factor, and K-factor, leading to identification of thermal processes and damping as the main factors that presently limit high speed device operation. We obtain very temperature-insensitive bandwidths around 13–15 GHz. Presented results clearly demonstrate the suitability of our VCSELs for practical and reliable optical data transmission systems.

AlGaAs/GaAs Photovoltaic Cells with InGaAs Quantum Dots

We studied the different carrier kinetic mechanisms involved into the interband absorption of quantum dots (QDs) by photocurrent spectroscopy. It was shown that in vertically coupled InGaAs QDs an effective carrier emission, collection and separation take place due to minizone formation. The possibility for the incorporation of vertically-coupled QDs into solar cells (SC) without any deterioration of structural quality of the p-i-n-junction has been shown. Due to the additional absorption of solar spectrum in QD media and the subsequent effective separation of photogenerated carriers, an increase (~1%) in short-circuit current density (Jsc) for the QD SC-devices has been demonstrated. However the insertion of QDs into intrinsic region reduced the open circuit voltage (Voc) of such devices. Moving the QD array in the base layer as well as including the Bragg reflector (BR) centered on 920 nm resulted in increase of the Voc. Moreover an improved absorption in the QD media for SC with BR led to further increase of Jsc (~1%). The efficiency for QD SCs at the level of 25% (30 suns AM1.5D) has been demonstrated.

Hybrid InGaAs quantum well–dots nanostructures for light-emitting and photo-voltaic applications

Hybrid quantum well-dots (QWD) nanostructures have been formed by deposition of 7-10 monolayers of In0.4Ga0.6As on a vicinal GaAs surface using metal-organic chemical vapor deposition. Transmission electron microscopy, photoluminescence and photocurrent analysis have shown that such structures represent quantum wells comprising three-dimensional (quantum dot-like) regions of two kinds. At least 20 QWD layers can be deposited defect-free providing high gain/absorption in the 0.9-1.1 spectral interval. Use of QWD media in a GaAs solar cell resulted in a photocurrent increment of 3.7 mA cm(-2) for the terrestrial spectrum and by 4.1 mA cm(-2) for the space spectrum. Diode lasers based on QWD emitting around 1.1 μm revealed high saturated gain and low transparency current density of about 15 cm(-1) and 37 A cm(-2) per layer, respectively.

Modulation Characteristics of High-Speed and High-Temperature Stable 980 nm Range VCSELs Operating Error Free at 25 Gbit/s up to 85 °C

High-speed oxide-confined 980 nm vertical cavity surface emitting lasers (VCSELs), exhibiting very temperature stable static and dynamic characteristics, are presented, which are well-suited for future optical data link and high-performance computing applications. Error-free operation at data rates of 25 Gbit/s in a wide temperature range from 25 to 85 °C is achieved without the need to adjust any of the electrical driving parameters. Small-signal modulation of VCSELs with different oxide aperture diameters is analyzed to identify the primary mechanisms that control device modulation rate by extracting the standard two-pole rate equation parameters.

Optical components for very short reach applications at 40 Gb/s and beyond

Just as the density of transistors on a silicon chip about doubles with each new generation, processor bandwidth also about doubles. Consequently the speed of input-output (I/O) devices must grow and today we find processor I/O speed approaching or slightly surpassing 10 Gb/s (G) per channel for 100G Ethernet server applications. Similarly Storage Area Networks are supported by Fibre Channel FC16G transceivers operating at the newly standardized serial signaling rate of 14 Gbaud. Further upgrades will require within only a few years links at 25, 28 and 40 Gbaud, speeds that are barely feasible with copper cabling, even for very short reach distances. Thus the role of optical interconnects will increase dramatically as the data transfer rates increase. Furthermore an increased bandwidth demand necessitates an equal or greater demand for low cost and highly power efficient micro-laser and -detector components along with their associated driver and transimpedance amplifier (TIA) integrated circuits (ICs). We summarize our recent achievements in vertical cavity surface emitting lasers (VCSELs) and PIN photodetectors suitable for very short reach multimode fiber links that enable bit rates up to and beyond 40 Gb/s. We address achievements in current modulated VCSELs, electrooptically modulated VCSELs, top illuminated PIN photodiodes, TIA and driver ICs, and packaging solutions.

Control of emission spectra in quantum dot microdisk/microring lasers

Focused ion beam is applied to quantum dot based microresonators to form pits or groove on their surface. The emission spectra of the resonators based lasers are significantly thinned out after the ion beam milling, and one or two modes become dominant instead of a group of modes having comparable intensities. The linewidth of the lasing mode is kept unchanged, whereas the lasing threshold demonstrates an insignificant growth.