M. A. Bobrov

Reliability performance of 25 Gbit s−1 850 nm vertical-cavity surface-emitting lasers

Multimode 850 nm vertical cavity surface-emitting lasers (VCSELs) suitable for high bit rate operation are studied. VCSELs with oxide aperture diameters of 5–7 µm show a high −3 dB modulation bandwidth (~20 GHz) and D-factor (~ 8 GHz mA−1/2). To allow low capacitance a multiple layer oxide-confined aperture design was applied. Eye diagrams are clearly open up to 35 Gbit s−1 at the temperature of 25 °C. Using 35 µm diameter PIN photodiodes and 6 µm oxide aperture diameter VCSELs error-free 25 Gbit s−1 (defined as a bit error ration of ≤1 × 10−12) optical fiber communication links were tested over 100 m of standard OM3 multimode optical fibers at 25 °C and 85 °C. The received optical power for error-free operation was below −4 dBm at both temperatures. A VCSEL reliability study at 95 °C was performed at the high current densities (~18 kA cm−2) needed for error-free 25 Gbit s−1 operation at elevated temperatures. After 6000 h a slight increase (less than 5%) of the output optical power at a constant current was observed and most likely due to an ohmic contact burn in effect within the first 2000 h of the study. The results clearly indicate that 25 Gbit s−1 850 nm oxide-confined VCSELs with a complex AlGaO multilayer aperture design and with step-graded Al-compositions have the potential for reliable operation.

Impact of a large negative gain-to-cavity wavelength detuning on the performance of InGaAlAs oxide-confined vertical-cavity surface-emitting lasers

Vertical-cavity surface-emitting lasers (VCSELs) based on the InGaAlAs-materials system on GaAs substrates are the key component for short-reach data and computer communications systems. Several different modulation schemes have been developed to realize high data bit rates based on various oxide-confined near-infrared VCSEL designs operated under direct current modulation. However, one open question to resolve is the optimal gain-to-cavity wavelength detuning to employ for temperature-stable high-speed performance. We investigate the static and dynamic characteristics of 850 nm high-speed oxide-confined VCSELs with different negative gain-to-cavity wavelength detunings. Our oxideconfined 850 nm VCSELs with a more common ~10 nm negative gain-to-cavity detuning demonstrate the conventional optical mode behavior with a classical single-resonance frequency response. With a larger (≥ 20 nm) negative detuning, our devices with large oxide-aperture size (>6 μm) show an anomalous start of lasing via higher order modes with a subsequent switching to lasing via the lowest order modes at higher currents. At intermediate currents, co-lasing via two types of transverse modes and a two-resonance modulation response is observed. The increase of operation temperature as well as the reduction in the oxide-aperture area resulted in classical lasing of index-guided VCSELs. The observed optical mode behavior can be attributed to the specific index guiding profile caused by the oxide-apertures, low internal optical losses, and the large gain-to-cavity detuning. Moreover, one can suggest that the complex shape of the modulation response results from the mode competition for the available gain during an interesting co-lasing operating regime.

A Design and New Functionality of Antiwaveguiding Vertical-Cavity Surface-Emitting Lasers for a Wavelength of 850 nm

Vertical-cavity surface-emitting lasers (VCSELs) with an aperture limited by an oxide and a resonance cavity based on GaAlAs with high Al content provide a maximum γ factor (λ/2 design) and suppression of optical power beyond the aperture. A VCSEL with two coupled cavities provides additional sharp growth of the loss of high-order lateral modes by leakage to the oxidized region and provides single-mode laser generation for an aperture diameter of up to 5 μm. Single-mode antiwaveguiding VCSELs provide ultrafast data transmission with a rate of up to 160 Gbit/s. The structure in which the active medium is placed in the lower distributed Bragg reflector and the cavity and the upper distributed Bragg reflector are dielectric, reducing the temperature shift of the radiation wavelength by a factor of 2 (to ∼0.03 nm/K).

The Influence of Cavity Design on the Linewidth of Near-IR Single-Mode Vertical-Cavity Surface-Emitting Lasers

The studies of the emission linewidth for single-mode near-IR vertical-cavity surface-emitting lasers with an active region based on InGaAs/AlGaAs quantum wells and different optical microcavity design. For low mirror loss, lasers with a 1λ cavity and carrier injection through distributed Bragg reflectors demonstrate a linewidth of 70 MHz and its growth to 110 MHz with increasing mirror loss (corresponding differential of efficiency ∼0.65 W/A). The design of the optical cavity with carrier injection through intracavity contacts and low-Q composition Bragg lattices reduces the linewidth to 40 MHz in spite of high mirror loss (corresponding differential efficiency of ∼0.6 W/A).

High-Speed Semiconductor Vertical-Cavity Surface-Emitting Lasers for Optical Data-Transmission Systems (Review)

The main problems of providing a high-speed operation semiconductor lasers with a vertical microcavity (so-called “vertical-cavity surface-emitting lasers”) under amplitude modulation and ways to solve them have been considered. The influence of the internal properties of the radiating active region and the electrical parasitic elements of the equivalent circuit of lasers are discussed. An overview of approaches that lead to an increase of the cutoff parasitic frequency, an increase of the differential gain of the active region, the possibility of the management of mode emission composition and the lifetime of photons in the optical microcavities, and reduction of the influence of thermal effects have been presented. The achieved level of modulation bandwidth of ∼30 GHz is close to the maximum achievable for the classical scheme of the direct-current modulation, which makes it necessary to use a multilevel modulation format to further increase the information capacity of optical channels constructed on the basis of vertical-cavity surface-emitting lasers.

Emission-Line Width and α-Factor of 850-nm Single-Mode Vertical-Cavity Surface-Emitting Lasers Based on InGaAs/AlGaAs Quantum Wells

The emission-line width for 850-nm single-mode vertical-cavity surface-emitting lasers based on InGaAs/AlGaAs quantum wells is studied. The width of the emission line for a laser with a 2-μm oxide current aperture attains it minimum (~110 MHz) at an output power of 0.8 mW. As the optical output power is further increased, anomalous broadening of the emission line is observed; this is apparently caused by an increase in the α-factor as a result of a decrease in the differential gain in the active region under conditions of increased concentration of charge carriers and of high internal optical losses in the microcavity. The α-factor is estimated using two independent methods.

Molecular-beam epitaxy of InGaAs/InAlAs/AlAs structures for heterobarrier varactors

The molecular-beam epitaxy of InGaAs/InAlAs/AlAs structures for heterobarrier varactors is studied and optimized. The choice of the substrate-holder temperature, growth rate and III/V ratio in the synthesis of individual heterostructure regions, the thickness of AlAs inserts and barrier-layer quality are critical parameters to achieve the optimal characteristics of heterobarrier varactors. The proposed triple-barrier structures of heterobarrier varactors with thin InGaAs strained layers immediately adjacent to an InAlAs/AlAs/InAlAs heterobarrier, mismatched with respect to the InP lattice constant at an AlAs insert thickness of 2.5 nm, provides a leakage current density at the level of the best values for heterobarrier varactor structures with 12 barriers and an insert thickness of 3 nm.

Lasing of metamorphic hybrid 1300nm spectral band VCSEL under optical pumping up to 120 °C

The ability to create metamorphic hybrid heterostructure of 1300 nm spectral band VCSEL is demonstrated. Metamorphic semiconductor part of heterostructure with GaAs/AlGaAs DBR and InAlGaAs/InGaAs QW active region has been grown by molecular beam epitaxy (MBE) on GaAs (100). Top dielectric SiO2/Ta2O5 DBR is made by the magnetron sputtering method. VCSEL has been studied under optical pumping (λ = 532 nm, diameter of the focused laser beam of ~ 1 μm) by using micro-PL setup in the range of optical pump power 0 – 70 mW at room temperature. Presence of the superlinear PL intensity growth having threshold-like dependence of PL integral intensity together with the PL peaks narrowing and mode composition modification with the pumping density increasing could be attributed to lasing behavior of the structure. Obtained results indicate the opportunity to use metamorphic growth on GaAs substrates for the 1300 nm range VCSEL manufacturing.

1.3 μm InAs quantum dot semiconductor disk laser

Vertical-external-cavity surface-emitting lasers (VECSEL), or semiconductor disk lasers (SDL), are attractive laser source for a wide range of applications owing to unique possibility to combine high output power with an excellent beam quality [1]. The intrinsic features of InAs quantum dots (QD) can offer low threshold, broad wavelength tunability, fast carrier dynamics and low temperature sensitivity. Recently, continuous wave (CW) operation of QD-based VECSEL emitting at 1.25 μm with output powers reaching multi-watt levels were achieved at room temperature [2]. However, extending the emission wavelength to 1.3 μm and beyond becomes more challenging. To date, QD-based VECSEL with optical power greater than 0.5 mW at 1305 nm has been demonstrated [3]. Here, we present a record-high power InAs/InGaAs QD-based VECSEL operating at the wavelength of 1.3 μm.

VCSEL polarization control by rhomboidal selectively-oxidized current aperture

Vertical-cavity surface-emitting lasers (VCSELs) are low-cost high-performance light sources for high-speed data communication systems, optical interconnects and different sensors. New VCSEL applications (spectroscopy, compact atomic clock) require single-mode operation with stable linear polarization combined with high temperature stability. While conventional GaAs-based VCSELs with small selectively-oxidized current apertures demonstrate stable fundamental transverse mode operation they have unstable polarization due to cylindrical symmetry and isotropic gain. Currently the most popular method for VCSEL polarization control is based on precise etching of sub-wavelength grating in output distributed Bragg reflector. Drawbacks of this approach are relatively complicated fabrication technology and limited output power. In this work we discuss alternative approach for single-mode polarization-stable VCSELs based on rhomboidal selectively-oxidized current aperture combined with intracavity contacts.