Avago’s 850nm oxide VCSEL for applications requiring modulation at 25-28G has been designed for -3dB bandwidths in excess of 18GHz over an extended temperature range of 0-85C. The VCSEL has been optimized to minimize DBR mirror thermal resistivity, electrical resistance and optical losses from free carrier absorption. The active region is designed for superior differential gain to enable high optical bandwidths. The small-signal modulation response has been characterized and the large-signal eye diagrams show excellent high-speed performance. Characterization data on other link parameters such as relative intensity noise and spectral width will also be presented.
In this paper we will discuss 14 Gb/s 850 nm oxide VCSEL performance and reliability. The device is targeted for the
16G Fibre Channel standard. The 14 Gb/s 850 nm oxide VCSEL meets the standard's specifications over the extended
temperature range to support transceiver module operation from 0C to 85C.
Quantum well (QW) VCSELs have a tendency to switch their polarization from one linearly polarized (LP)
mode to the orthogonal one when changing the operation conditions. As polarization properties of VCSELs are
governed by anisotropies, namely stress-induced birefringence and dichroism, the inherent anisotropy of quantum
dots (QDs) is expected to influence the polarization properties of QD VCSELs. In this paper we summarize our
experimental results on polarization properties of QD VCSELs with the main focus on polarization switching
phenomena. Close to threshold the laser emits linearly polarized light which changes to elliptically polarized
(EP) at some current. The main axes of these states are not aligned and the angle between them increases
with current. As the current is still increased polarization switching accompanied by polarization mode hopping
occurs. Distinctive feature of the observed switching is that the two EP states between which switching occurs are
nonorthogonal. The angle between their major exes is 40 deg. Polarization mode hopping has been characterized
in terms of the dwell time and the current-dependence of this factor examined. Apparently, the dwell time
decreases when the pump current is increased which differs from what has been published for QW VCSELs. The
average dwell time is 20 ns. Similarly to QW VCSELs the distribution of the dwell time is exponential. The
statistics is the same for the two EP states and such symmetric switching is maintained in the whole range of
currents at which the light is elliptically polarized. Large-signal modulation experiments show that the frequency
at which polarization switching disappears is about 100 MHz. This indicates that the switching is of thermal
origin.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10-12) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm2, which is close to the 10 kA/cm2 industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.
We report on the modeling, epitaxial growth, fabrication, and characterization of 830-845 nm vertical cavity surface
emitting lasers (VCSELs) that employ InAs-GaAs quantum dot (QD) gain elements. The GaAs-based VCSELs are
essentially conventional in design, grown by solid-source molecular beam epitaxy, and include top and bottom gradedheterointerface
AlGaAs distributed Bragg reflectors, a single selectively-oxidized AlAs waveguiding/current funneling
aperture layer, and a quasi-antiwaveguiding microcavity. The active region consists of three sheets of InAs-GaAs
submonolayer insertions separated by AlGaAs matrix layers. Compared to QWs the InAs-GaAs insertions are expected
to offer higher exciton-dominated modal gain and improved carrier capture and retention, thus resulting in superior
temperature stability and resilience to degradation caused by operating at the larger switching currents commonly
employed to increase the data rates of modern optical communication systems. We investigate the robustness and
temperature performance of our QD VCSEL design by fabricating prototype devices in a high-frequency ground-sourceground
contact pad configuration suitable for on-wafer probing. Arrays of VCSELs are produced with precise variations
in top mesa diameter from 24 to 36 μm and oxide aperture diameter from 1 to 12 μm resulting in VCSELs that operate in
full single-mode, single-mode to multi-mode, and full multi-mode regimes. The single-mode QD VCSELs have room
temperature threshold currents below 0.5 mA and peak output powers near 1 mW, whereas the corresponding values for
full multi-mode devices range from about 0.5 to 1.5 mA and 2.5 to 5 mW. At 20°C we observe optical transmission at 20
Gb/s through 150 m of OM3 fiber with a bit error ratio better than 10-12, thus demonstrating the great potential of our QD
VCSELs for applications in next-generation short-distance optical data communications and interconnect systems.
Presently VCSELs covering a significant spectral range (840-1300 nm) can be produced based on quantum dot (QD)
active elements. Herein we report progress on selected QD based vertical-cavity surface-emitting lasers (VCSELs)
suitable for high-speed operation. An open eye diagram at 20 Gb/s with error-free transmission (a bit-error-rate < 10-15)
is achieved at 850 nm. The 850 nm QD VCSELs also achieve error-free 20 Gb/s single mode transmission operation through multimode fiber without the use of optical isolation. Our 980 nm-range QD VCSELs achieve error free transmission at 25 Gb/s at up to 150°C. These 980 nm devices operate in a temperature range of 25-85°C without current or modulation voltage adjustment. We anticipate that the primary application areas of QD VCSELs are those that require
degradation-robust operation under extremely high current densities. Temperature stability at ultrahigh current densities,
a forte of QDs, is needed for ultrahigh-speed (> 40 Gb/s) current-modulated VCSELs for a new generation of local and storage area networks. Finally we discuss aspects of QD vertical extended-cavity surface emitting lasers with ultra high power density per emitting surface for high power (material processing) and frequency conversion (display) applications.
980 nm VCSELs based on sub-monolayer growth show for 20 Gbit/s large signal modulation clearly open eyes without
adjustment of the driving conditions between 25 and 120 °C. To access the limiting mechanism for the modulation
bandwidth, a temperature dependent small signal analysis is carried out on these devices. Single mode devices are
limited by damping, whereas multimode devices are limited by thermal effects, preventing higher photon densities in the
cavity.
Low transparency current density and improved temperature stability with a large characteristic temperature
T0 > 650 K up to 80 °C are demonstrated for 1.3 μm MBE grown InGaAs quantum dot (QD) edge emitting
lasers. Digital modulation with an open eye pattern up to 12 Gb/s at room temperature and bit error rate below
10-12 for 10 Gb/s modulation was realized for this wavelength. Semiconductor optical amplifiers based on
InGaAs QD gain media achieved a chip gain of 26 dB. A conventionally doped semiconductor DBR QD-VCSEL
containing 17 p-modulation doped QD layers demonstrated a cw output power of 1.8 mW and a
differential efficiency of 20 % at 20 °C. The maximum -3dB modulation bandwidth at 25 °C was 3 GHz. First
MOCVD-grown QD-VCSELs with selectively oxidized DBRs and 9 QD-layers were realized, emitting at 1.1
μm. A cw multimode output power of 1.5 mW, 6 mW in pulsed operation, and an cw external efficiency of 45 %
were achieved at 20 °C. The minimum threshold current of a device with 2 μm aperture was 85 μA.
We have studied the modulation properties of a vertical cavity surface-emitting laser (VCSEL) coupled to an
electrooptical modulator. It is shown that, if the modulator is placed in a resonant cavity, the modulation of the light
output power is governed predominantly by electrooptic, or electrorefraction effect rather than by electroabsorption. A
novel concept of electrooptically modulated (EOM) VCSEL based on the stopband edge-tunable distributed Bragg
reflector (DBR) is proposed which allows overcoming the limitations of the first-generation EOM VCSEL based on
resonantly coupled cavities. A new class of electrooptic (EO) media is proposed based on type-II heterostructures, in
which the exciton oscillator strength increases from a zero or a small value at zero bias to a large value at an applied
bias. A EOM VCSEL based on a stopband-edge tunable DBR including a type-II EO medium is to show the most
temperature-robust operation. Modeling of a high-frequency response of a VCSEL light output against large signal
modulation of the mirror transmittance has demonstrated the feasibility to reach 40 Gb/s operation at low bit error rate.
EOM VCSEL showing 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths is
realized.
N. Ledentsov, F. Hopfer, A. Mutig, V. Shchukin, A. V. Savel'ev, G. Fiol, M. Kuntz, V. A. Haisler, T. Warming, E. Stock, S. S. Mikhrin, A. R. Kovsh, C. Bornholdt, H. Eisele, M. Dähne, N. D. Zakharov, P. Werner, D. Bimberg
Advanced types of QD media allow an ultrahigh modal gain, avoid temperature depletion and gain saturation effects,
when used in high-speed quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). An anti-guiding VCSEL
design reduces gain depletion and radiative leakage, caused by parasitic whispering gallery VCSEL modes. Temperature
robustness up to 100°C for 0.96 - 1.25 &mgr;m range devices is realized in the continuous wave (cw) regime. An open eye
20 Gb/s operation with bit error rates better than 10-12 has been achieved in a temperature range 25-85°C without current
adjustment. A different approach for ultrahigh-speed operation is based on a combination of the VCSEL section,
operating in the CW mode with an additional section of the device, which is electrooptically modulated under a reverse
bias. The tuning of a resonance wavelength of the second section, caused by the electrooptic effect, affects the
transmission of the system. The second cavity mode, resonant to the VCSEL mode, or the stopband edge of the second
Bragg reflector can be used for intensity modulation. The approach enables ultrahigh speed signal modulation. 60GHz
electrical and ~35GHz optical (limited by the photodetector response) bandwidths are realized.
Nanotechnology is a driver for novel opto-electronic devices and systems. Nanosemiconductors like quantum dots allow controlled variation of fundamental electronic and optical properties by changing the size and shape of the nanostructures. This applies directly to self-organized quantum dots which find a versatile use in many kinds of photonic devices.
Wavelength tunability, decreased laser threshold, scalability of gain by stacking quantum dot layers, low linewidth enhancement factor and temperature stability are consequences of three-dimensional carrier confinement in semiconductor quantum dots. Directly modulated lasers using quantum dots offer further advantages like strongly damped relaxation oscillations yielding low patterning effects in digital data transmission. Quantum dot mode-locked lasers feature a broad gain spectrum leading to ultra-short pulses with sub-ps width and a low alpha factor for low-chirp. Thereby, optical comb generators for the future 100G Ethernet are feasible. Semiconductor optical amplifiers based on quantum dots show advantages as compared to classical ones: broad bandwidth due to the inhomogeneous quantum dot size distribution, ultrafast gain recovery for high-speed amplification and small patterning in optical data transmission. We present our most recent results on temperature stable 10 Gb/s, 23°-70°C direct modulation of lasers, ultrafast 80 GHz and short 710 fs optical pulse combs with mode-locked lasers and semiconductor optical amplifiers showing ultrafast amplification of these optical combs as well as error-free 40 Gb/s data modulation, all based on a quantum dot gain medium.
We report on a miniature solid state emitter structure, which allows electrical pumping of only one single InAs quantum
dot (QD) grown in the Stranski-Krastanow mode. The emitter is based on a single layer of low density (~108 cm-2) QDs
grown by Molecular Beam Epitaxy and a submicron AlOX current aperture defined by selective oxidation of high
aluminium content AlGaAs layers. The device demonstrates strongly monochromatic polarized emission of the single
QD exciton at subnanoampere current pumping. No other emission is observed across a spectral range of 500 nm, proving that indeed just one single QD is contributing. Correlation measurements of the emitted photons show a clear
antibunching behavior.
F. Hopfer, A, Mutig, G. Fiol, M. Kuntz, S. Mikhrin, I. Krestnikov, D. Livshits, A. Kovsh, C. Bornholdt, V. Shchukin, N. Ledentsov, V. Gaysler, N. Zakharov, P. Werner, D. Bimberg
980 nm vertical-cavity surface-emitting laser based on sub-monolayer growth of quantum dots show at 25 and 85°C for 20 Gb/s without current adjustment clearly open eyes and error free operation with bit error rates better than 10-12. For these multimode lasers the small signal modulation bandwidth decreases only from 15 GHz at 25°C to 13 GHz at 85°C. Single mode devices demonstrate at 20°C a small signal modulation bandwidth of 16.6 GHz with 0.8 mW optical output power and a record high modulation current efficiency factor of 19 GHz/mA1/2.
Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.
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