A method of hybrid integration of quantum dot microdisk lasers with silicon wafer is proposed and realized. In addition to the possibility of combining microlasers with various silicon-based electronic and photonic devices, this makes it possible to significantly improve heat removal from the active region of the microlaser. The thermal resistance normalized to the mesa area reaches the level of about 0.002 (K/W)*cm2, which is significantly lower than the corresponding values of QD microlasers on GaAs substrate and monolithically grown on Si. As a result, the threshold current as well as current-induced shift of emission wavelength are reduced in continuous-wave regime.
We report on broad-area lasers, mode-locked lasers (MLLs), and superluminescent light-emitting diodes (SLDs) based on a recently developed novel type of nanostructures that we refer to as quantum well-dots (QWDs). The QWDs are intermediate in properties between quantum wells and quantum dots and combine some useful properties of both. 1.08 μm InGaAs/GaAs QWDs broad area edge-emitting lasers based on coupled large optical cavity waveguides show high internal quantum efficiency of 92%, low internal loss of 0.9 cm-1 and material gain of ~1.1∙104 cm-1 per one QWD layer. CW output power of 14.2 W is demonstrated at room temperature. Superluminescent light-emitting diodes with one QWD layer in the active region exhibit stimulated emission spectra centered at 1050 nm with the maximal full width at half maximum of 36 nm and the output power of 17 mW. First results on mode-locked operation in QWD lasers are also presented. 2 mm long two-section devices demonstrate the pulse repetition rate of 19.3 GHz and the pulse duration of 3.5 ps. The width of the radio frequency spectrum is 0.2 MHz.
We report on optoelectronic devices based on novel type of active region - quantum well-dots (QWD) hybrid nanostructures. This hybrid type of the active region can be described as a quantum well, which has an ultradense array of narrow-gap In-rich regions with the size of 20-30 nm, which serve as the localization centers of charge carriers. Such QWD structures can be formed spontaneously during the MOVPE (metalorganic vapor phase epitaxy) deposition of InxGa1-xAs (0.3<; x<0.5) on GaAs substrate. Optimal average thickness and composition of InxGa1-xAs to achieve maximal PL intensity and photocurrent in QWD structures are determined. Characteristics of edge-emitting lasers based on 5 QWD layers are described. Advantages of using QWD medium in light-emitting and photovoltaic devices are discussed.
A concept of passive cavity surface–emitting laser is proposed aimed to control the temperature shift of the lasing
wavelength. The device contains an all–semiconductor bottom distributed Bragg reflector (DBR), in which the active
medium is placed, a dielectric resonant cavity and a dielectric top DBR, wherein at least one of the dielectric materials
has a negative temperature coefficient of the refractive index, dn/dT < 0. This is shown to be the case for commonly used
dielectric systems SiO2/TiO2 and SiO2/Ta2O5. Two SiO2/TiO2 resonant structures having a cavity either of SiO2 or TiO2
were deposited on a substrate, their optical power reflectance spectra were measured at various temperatures, and
refractive index temperature coefficients were extracted, dn/dT = 0.0021 K-1 for SiO2 and dn/dT = –0.0092 K-1 for TiO2.
Using such dielectric materials allows designing passive cavity surface–emitting lasers having on purpose either positive,
or zero, or negative temperature shift of the lasing wavelength dλ/dT. A design for temperature–insensitive lasing
wavelength (dλ/dT = 0) is proposed. Employing devices with temperature–insensitive lasing wavelength in wavelength
division multiplexing systems may allow significant reducing of the spectral separation between transmission channels
and an increase in number of channels for a defined spectral interval enabling low cost energy efficient uncooled devices.
Spectral and power characteristics of QD stripe lasers operating in two-state lasing regime have been studied in a wide range of operation conditions. It was demonstrated that neither self-heating nor increase of the homogeneous broadening are responsible for quenching of the ground-state lasing beyond the two-state lasing threshold. It was found that difference in electron and hole capture rates strongly affects light-current curve. Modulation p-type doping is shown to enhance the peak power of GS lasing transition. Microring and microdisk structures (D = 4-9 μm) comprising 1.3 μm InAs/InGaAs quantum dots have been fabricated and studied by μ-PL and NSOM. Ground-state lasing was achieved well above root temperature (up to 380 K). Effect of inner diameter on threshold characteristics was evaluated.
Detailed investigation of anomalous modal behavior in fabricated bottom-emitting intra-cavity contacted 960 nm range
vertical cavity surface emitting lasers (VCSELs) have been performed. At low currents the broad-aperture VCSELs show
multi-mode operation at 945 nm via whispering gallery-like modes. Subsequent increase of pump current results in rapid
increase of fundamental mode intensity and switching to a pure single transverse mode lasing regime at 960 nm with the
higher slope efficiency. As a result record single transverse mode output power of 15 mW with a side-mode-suppressionratio
(SMSR) above 30 dB was achieved. The observed phenomena cannot be explained by oxide-index guiding or
changes in current pumping. 2D heat transport simulations show a strong temperature gradient inside the microcavity
due to an effective lateral heat-sinking. This creates an effective waveguide and results in lower optical losses for the
fundamental mode. At fixed pump current in pulsed regime (pulse width < 400 ns) high-order modes dominate, however
the subsequent increase of pulse width leads to a rapid rise of optical power for the fundamental mode and SMSR
increasing. Thus the self-heating phenomena play a crucial role in observed VCSEL unusual modal behavior.
The progressive penetration of optical communication links into traditional copper interconnect markets greatly expands
the applications of vertical cavity surface emitting lasers (VCSELs) for the next-generation of board-to-board, moduleto-
module, chip-to-chip, and on-chip optical interconnects. Stability of the VCSEL parameters at high temperatures is
indispensable for such applications, since these lasers typically reside directly on or near integrated circuit chips. Here
we present 980 nm oxide-confined VCSELs operating error-free at bit rates up to 25 Gbit/s at temperatures as high as 85
°C without adjustment of the drive current and peak-to-peak modulation voltage. The driver design is therefore
simplified and the power consumption of the driver electronics is lowered, reducing the production and operational costs.
Small and large signal modulation experiments at various temperatures from 20 up to 85 °C for lasers with different
oxide aperture diameters are presented in order to analyze the physical processes controlling the performance of the
VCSELs. Temperature insensitive maximum -3 dB bandwidths of around 13-15 GHz for VCSELs with aperture
diameters of 10 μm and corresponding parasitic cut-off frequencies exceeding 22 GHz are observed. Presented results
demonstrate the suitability of our VCSELs for practical high speed and high temperature stable short-reach optical links.
As the density of transistors in CMOS integrated circuits continues to roughly double each two years the processor
computational power also roughly doubles. Since the number of input/output (I/O) devices can not increase without
bound I/O speed must analogously approximately double each two years. In the Infiniband EDR standard (2011) a single
channel bit rate of 26 Gb/s is foreseen. The maximum reliable and efficient copper link length shrinks at bit rates above
10 Gb/s to a few meters at best. At higher bit rates the length of a given multimode fiber link must also shrink, due to
both modal and wavelength dispersions. Although the modal dispersion in modern multimode OM3 and OM4 fibers that
are optimized for 850 nm vertical-cavity surface-emitting lasers (VCSELs) is reduced, the wavelength dispersion
remains a serious issue for standard multimode VCSELs. An ultimate solution to overcome this problem is to apply
single-mode VCSELs to extend and ultimately maximize the link length. In this paper we demonstrate recent results for
single-mode VCSELs with very high relaxation resonance frequencies. Quantum well 850 nm VCSELs with record high
30 GHz resonance frequencies are demonstrated. Additionally single-mode data transmission at 35 Gb/s over multimode
fiber is demonstrated. For comparison we also present specific device modeling parameters and performance
characteristics of 850 nm single-mode quantum dot (QD) VCSELs. Despite a significant spectral broadening of the QD
photoluminescence and gain due to QD size dispersion we obtain relaxation resonance frequencies as high as 17 GHz.
KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Data communications, Oxides, Reliability, Eye, Data transmission, Picosecond phenomena, Photodetectors, Signal to noise ratio
Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and
storage area network (LAN/SAN) optical fiber data communication systems and all other short-reach high-speed data
transfer applications. The intrinsic limitations of copper-based electrical links at data rates exceeding 10 Gbit/s leads to a
progressive movement wherein optical communication links replace traditional short-reach (300 m or shorter) copper
interconnects. The wavelength of 850 nm is the standard for LAN/SAN applications as well as for several other evolving
short-reach application areas including Fibre Channel, InfiniBand, Universal Serial Bus (optical USB), and active optical
cables. Here we present our recent results on 850 nm oxide-confined VCSELs operating at data bit rates up to 40 Gbit/s
at low current densities of ~10 kA/cm2 ensuring device reliability and long-term stability based on conventional industry
certification specifications. The relaxation resonance frequencies, damping factors, and parasitic cut-off frequencies are
determined for VCSELs with oxide-confined apertures of various diameters. At the highest optical modulation rates the
VCSELs' high speed operation is limited by parasitic cut-off frequencies of 24-28 GHz. We believe that by further
reducing device parasitics we will produce current modulated VCSELs with optical modulation bandwidths larger than
30 GHz and data bit rates beyond 40 Gbit/s.
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.
Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and
storage area network (LAN/SAN) optical fiber data communication systems and short-reach computer interconnects. The
continuing rapid increase of serial transmission data rates driven by multi-core microprocessor's bandwidth upgrades
cannot be sustained via conventional copper-based links as bit rates move beyond 10 Gbit/s and distances greater than 1
m. The intrinsic limitation of copper at high single-channel data rates facilitates the need to transition to optical fiberbased
links at ever shorter distances. For LAN/SAN applications the 850 nm wavelength is standard. This same
wavelength is also the standard for several other evolving short-reach application areas including Fibre Channel, CEI,
USB, InfiniBand, and HDMI optical link systems. Herein we present our recent results on 850 nm oxide-confined
VCSELs operating at data bit rates up to 40 Gbit/s. The low operational current density in the range of ~10 kA/cm2
ensures viable device reliability and long-term stability based on well-known industry certification specifications. Key
VCSEL device parameters including the relaxation resonance frequency, damping, and parasitic cut-off frequency are
determined for VCSELs with oxide-confined apertures of various diameters. We find that a parasitic cut-off frequency of
24-28 GHz limits the VCSEL's high speed operation at the highest optical modulation rates. We believe that with some
effort the device parasitics can be further reduced such that current modulated VCSELs can be realized with larger than
30 GHz optical modulation bandwidth and reliable and practical operation beyond 40 Gbit/s.
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