77 research outputs found
Selective Near Perfect Light Absorbtion by Graphene Monolayer Using Aperiodic Multilayer Microstructures
We investigate 1D aperiodic multilayer microstructures in order to achieve
near total absorption in preselected wavelengths in a graphene monolayer. Our
structures are designed by a genetic optimization algorithm coupled to a
transfer matrix code. Coupled mode theory (CMT) analysis, in accordance with
transfer matrix method (TMM) results, indicates the existence of a critical
coupling in a graphene monolayer for perfect absorptions. Our findings show
that the near-total-absorption peaks are highly tunable and can be controlled
simultaneously or independently in wide range of wavelengths in the
near-infrared and visible. Our proposed approach is metal free and does not
require surface texturing or patterning, and can be applied for other two
dimensional (2D) materials.Comment: 12 pages, 5 figure
Stabilizing Multimode Hopping Oscillations and Reducing Associated Noise in Long Wavelength Laser Diode Using External Optical Feedback
We report on converting the multimode hopping oscillation (MHO) in
long-wavelength semiconductor laser into single-mode oscillation (SMO) by
applying external optical feedback (OFB). We characterize and compare the noise
performance of the laser when supporting SMO and multimode oscillations. The
study is based on a modified time-delay multimode rate-equation model of the
laser that includes mechanisms of spectral gain suppression along with OFB
induced due to multiple reflections by an external reflector. The study is
applied to 1.55um-InGaAsP laser that exhibits multimode hopping in its solitary
version and supports wide bandwidth. The noise is evaluated in terms of the
relative intensity noise (RIN). We show that when OFB synchronizes with the
asymmetric gain suppression (AGS), it enhances the gain of one longer
wavelength mode and supports SMO. In this case OFB improves the noise
performance of the laser. On the other hand, when OFB works against AGS, it
sustains hopping multimode oscillation (HMMO) and deteriorates the side-mode
suppression ratio (SMSR) and the noise performance.Comment: 15 pages, 7 figure
Transverse Coupled Cavity VCSEL: Making 100 GHz Bandwidth Achievable
Enhancing the modulation bandwidth (MBW) of semiconductor lasers has been the
challenge of research and technology to meet the need of high-speed photonic
applications. In this paper, we propose the design of vertical-cavity
surface-emitting laser integrated with multiple transverse coupled cavities
(MTCCs) as a promising device with ultra-high 3-dB bandwidth. The laser
features high modulation performance because of the accumulated strong coupling
of the slow-light feedback from the surrounding lateral TCCs into the VCSEL
cavity. Photon-photon resonance (PPR) is predicted to occur at ultra-high
frequencies exceeding 145 GHz due to the optical feedback from short TCCs,
which achieves 3-dB MBW reaching 170 GHz. The study is based on the modeling of
the VCSEL dynamics under multiple transverse slow-light feedback from the
surrounding TCCs. We show that the integration of the VCSEL with four or six
feedback cavities is advantageous over the TCC-VCSEL in achieving much higher
MBW enhancement under weaker coupling of slow-light into the VCSEL cavity. We
also characterize the noise properties of the promising MTCC-VCSEL in the
regime of ultra-high bandwidth in terms of the Fourier spectrum of the relative
intensity noise (RIN)
Attojoule-Efficient Graphene Optical Modulators
Electro-optic modulation is a technology-relevant function for signal keying,
beam steering, or neuromorphic computing through providing the nonlinear
activation function of a perceptron. With silicon-based modulators being bulky
and inefficient, we here discuss graphene-based devices heterogeneously
integrated. This study provides a critical and encompassing discussing of the
physics and performance of graphene modulators rather than collecting relevant
published work. We provide a holistic analysis of the underlying physics of
modulators including the graphenes index tunability, the underlying optical
mode, and discuss resulting performance vectors of this novel class of hybrid
modulators. Our results show that the reducing the modal area, and reducing the
effective broadening of the active material are key to improving device
performance defined by the ratio of energy-bandwidth and footprint. We further
show how the waveguides polarization must be in-plane with graphene such as
given by plasmonic-slot structures. A high device performance can be obtained
by introducing multi- or bi-layer graphene modulator designs. Lastly, we
present recent results of a graphene-based hybrid-photon-plasmon modulator on a
silicon platform, requiring near Boltzmann approximation (100mV) low drive
voltages. Being physically compact this 100 aJ/bit modulator opens the path
towards a new class of attojoule opto-electronics.Comment: arXiv admin note: text overlap with arXiv:1502.04672 by other author
Plasmonic Optical Modulator based on Adiabatic Coupled Waveguides
In atomic multi-level systems, adiabatic elimination is a method used to
minimize complicity of the system by eliminating irrelevant and strongly
coupled levels by detuning them from one-another. Such a three-level system,
for instance, can be mapped onto physical in form of a three-waveguide system.
Actively detuning the coupling strength between the respective waveguide modes
allows modulating light propagating through the device, as proposed here. The
outer waveguides act as an effective two- photonic-mode system similar to
ground- and excited states of a three-level atomic system, whilst the center
waveguide is partially plasmonic. In adiabatic elimination regime, the
amplitude of the middle waveguide oscillates much faster in comparison to the
outer waveguides leading to a vanishing field build up. As a result, the middle
waveguide becomes a dark state and hence a low insertion-loss of 8 decibel is
expected to keep when achieving the modulation depth as high as 70 decibel
despite the involvement of a plasmonic waveguide in the design presented here.
The modulation mechanism relies on switching this waveguide system from a
critical coupling regime to adiabatic elimination condition via
electrostatically tuning the free-carrier concentration and hence the optical
index of a thin ITO layer residing in the plasmonic center waveguide. This
alters the effective coupling length and the phase mismatching condition thus
modulation in each of outer waveguides. Our results show a modulator energy
efficiency as low as 40 atto-joule per bit and an extinction ratio of 50
decibel. Given the minuscule footprint of the modulator, the resulting
lumped-element limited RC delay is expected to exceed 200 giga hertz. This type
of modulator paves the way for next-generation both energy-and speed conscience
optical short-reach interconnects.Comment: 11 Pages,5 figure
Influence of Asymmetric Gain Suppression on Relative Intensity Noise Properties of Multimode Semiconductor Lasers
We introduce modeling and simulation of the noise properties associated with
types of modal oscillations induced by scaling the asymmetric gain suppression
(AGS) in multimode semiconductor lasers. The study is based on numerical
integration of a system of rate equations of 21-oscillating modes taking
account of the self- and cross-modal gain suppression mechanisms. AGS is varied
in terms of a pre-defined parameter, which is controlled by the linewidth
enhancement factor and differential gain. Basing on intensive simulation of the
mode dynamics, we present a mapping (AGS versus current) diagram of the
possible types of modal oscillations. When the laser oscillation is hopping
multimode oscillation (HMMO), the spectra of relative intensity noise (RIN) of
the total output and hopping modes are characterized by a sharp peak around the
relaxation oscillation (RO) frequency and a broad peak around the hopping
frequency. The levels of RIN in the regimes of single-mode oscillation (SMO)
are much lower than those under HMMO, and the mode-partition noise is two order
of magnitudes lower
Extra Loss-free Non-Hermitian Engineered Single Mode Laser Systems
In a laser system non-Hermitian methods such as Parity-Time (PT) Symmetry and
Supersymmetry (SUSY) have shown and demonstrated the ability to suppress
unwanted lasing modes and, thus, achieved single mode lasing operation through
the addition of lossy passive elements. While these approaches enable laser
engineering versatility, they rely on the drawback of adding optical losses to
a system tasked to produce single mode gain. Unlike PT and SUSY lasers, here we
show an extra loss-free non-Hermitian laser engineering approach to realize
single mode lasing operation for the first time. By selectively enhancing the
fundamental modes quality factor, we obtain single mode operation with higher
output power per cavity since all cavities in this system contribute to the
laser output, in contrast to other non-Hermitian approaches. Furthermore, we
show that this approach interestingly allows reducing the number of
to-be-designed cavities in super-partner array as compared with, for example,
the SUSY approach, thus leading to reduced design complexity upon coupled
cavity scale up of laser arrays. In summary, the ability to engineer coupled
laser systems where each laser cavity contributes to coherent light
amplification opens up a new degree of laser-design freedom leading to
increased device performance and simultaneous reduced design and fabrication
complexity
Coupling-controlled Dual ITO Layer Electro-Optic Modulator in Silicon Photonics
Electro-optic signal modulation provides a key functionality in modern
technology and information networks. Photonic integration has enabled not only
miniaturizing photonic components, but also provided performance improvements
due to co-design addressing both electrical and optical device rules. However
the millimeter-to-centimeter large footprint of many foundry-ready photonic
electro-optic modulators significantly limits scaling density. Furthermore,
modulators bear a fundamental a frequency-response to energy-sensitive
trade-off, a limitation that can be overcome with coupling-based modulators
where the temporal response speed is decoupled from the optical cavity photo
lifetime. Thus, the coupling effect to the resonator is modulated rather then
tuning the index of the resonator itself. However, the weak electro-optic
response of silicon limits such coupling modulator performance, since the
micrometer-short overlap region of the waveguide-bus and a microring resonator
is insufficient to induce signal modulation. To address these limitations, here
we demonstrate a coupling-controlled electro-optic modulator by heterogeneously
integrating a dual-gated indium-tin-oxide (ITO) phase shifter placed at the
silicon microring-bus coupler region. Our experimental modulator shows about 4
dB extinction ratio on resonance, and a about 1.5 dB off resonance with a low
insertion loss of 0.15 dB for a just 4 {\mu}m short device demonstrating a
compact high 10:1 modulation-to-loss ratio. In conclusion we demonstrate a
coupling modulator using strongly index-changeable materials enabling compact
and high-performing modulators using semiconductor foundry-near materials
Hexagonal Transverse Coupled Cavity VCSEL Redefining the High-Speed Lasers
The vertical-cavity surface-emitting lasers (VCSELs) have emerged as a vital
approach for realizing energy efficient, high speed optical interconnects in
the data center and supercomputers. As of today, VCSEL is the most suitable for
mass production in terms of cost-effectiveness and reliability. However, there
are still key challenges for higher speed modulation above 40 GHz. Here, a
hexagonal transverse coupled cavity VCSEL adiabatically coupled through the
center cavity is proposed. A 3-dB roll-off modulation bandwidth of 45 GHz is
demonstrated, which is five times greater than a conventional VCSEL fabricated
on the same epi-wafer structure. While a parity time (PT) symmetry approaches
add loss to engineer the topological state of the laser system, here, a radical
paradigm shift with gain introduces symmetry breaking. This idea, then enables
a single mode operation with a side-mode suppression-ratio (SMSR) of > 30
decibels and signal-to-noise ratio (SNR) of > 45 decibels. The energy
distribution inside the coupled cavity system is also redistributed to provide
a coherent gain in a spatially separated system. Consequently, throughput power
is three times higher than that of the conventional VCSEL.Comment: 14 pages, 4 figure
Hybrid Photonic-Plasmonic Non-blocking Broadband 5x5 Router for Optical Networks
Photonic data routing in optical networks overcomes the limitations of
electronic routers with respect to data rate, latency, and energy consumption
while suffering from dynamic power consumption, non-simultaneous usage of
multiple wavelength channels, and large footprints. Here we show the first
hybrid photonic-plasmonic, non-blocking, broadband 5x5 router. The compact
footprint (<250 {\mu}m2) enables high operation speed (480 GHz) requiring only
82 fJ/bit (1.9 dB) of averaged energy consumption (routing loss). The router
supports multi-wavelength up to 206 nm in the telecom band. Having a
data-capacity of >70 Tbps, thus demonstrating key features required by future
high data-throughput optical networks.Comment: 12 pages, 4 figure
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