28 research outputs found
Kerr-mediated symmetry breaking of counterpropagating light in microresonators
Nonlinear Optics has been a source of surprises for physicists for almost a century
comprising both fundamental physics and real-world applications. To access optical
nonlinearities, significant light intensities are required; thus nonlinear optics often
involves a resonant cavity to amplify the light intensity. Microresonators have proven
to be an ideal platform for this kind of experiment since the cavity mode area can be
as small as a few µm2 and their high Q-factor traps light for many round trips while
more light is coupled in. Also, light interacts with the nonlinear material, of which
the resonator is made, for the whole round trip. However, the interaction between
counter-propagating light in microresonators is still a relatively unexplored field.
This thesis reports on the first observation of Kerr-induced spontaneous symmetry breaking in a microresonator, whereby light can circulate in only one direction
inside the resonator. I develop a theoretical model describing the steady-state solutions and the dynamics of how the symmetry-broken regime responds to the input
changes. I show experimentally how the symmetry breaking can be used to realise all-optical isolator, circulators, memories and logic gates. These devices, based
on the Kerr-nonreciprocity, represent a promising alternative for the realisation of
integrated all-optical passive photonics circuits
Optical memories and switching dynamics of counterpropagating light states in microresonators
The Kerr nonlinearity can be a key enabler for many digital photonic circuits
as it allows access to bistable states needed for all-optical memories and
switches. A common technique is to use the Kerr shift to control the resonance
frequency of a resonator and use it as a bistable, optically-tunable filter.
However, this approach works only in a narrow power and frequency range or
requires the use of an auxiliary laser. An alternative approach is to use the
asymmetric bistability between counterpropagating light states resulting from
the interplay between self- and cross-phase modulation, which allows light to
enter a ring resonator in just one direction. Logical HIGH and LOW states can
be represented and stored as the direction of circulation of light, and
controlled by modulating the input power. Here we study the switching speed,
operating laser frequency and power range, and contrast ratio of such a device.
We reach a bitrate of 2 Mbps in our proof-of-principle device over an optical
frequency range of 1 GHz and an operating power range covering more than one
order of magnitude. We also calculate that integrated photonic circuits could
exhibit bitrates of the order of Gbps, paving the way for the realization of
robust and simple all-optical memories, switches, routers and logic gates that
can operate at a single laser frequency with no additional electrical power.Comment: 10 pages, 10 figure
An All-Optical General-Purpose CPU and Optical Computer Architecture
Energy efficiency of electronic digital processors is primarily limited by
the energy consumption of electronic communication and interconnects. The
industry is almost unanimously pushing towards replacing both long-haul, as
well as local chip interconnects, using optics to drastically increase
efficiency. In this paper, we explore what comes after the successful migration
to optical interconnects, as with this inefficiency solved, the main source of
energy consumption will be electronic digital computing, memory and
electro-optical conversion. Our approach attempts to address all these issues
by introducing efficient all-optical digital computing and memory, which in
turn eliminates the need for electro-optical conversions. Here, we demonstrate
for the first time a scheme to enable general purpose digital data processing
in an integrated form and present our photonic integrated circuit (PIC)
implementation. For this demonstration we implemented a URISC architecture
capable of running any classical piece of software all-optically and present a
comprehensive architectural framework for all-optical computing to go beyond.Comment: 14 pages, 10 figure
Interplay of Polarization and Time-Reversal Symmetry Breaking in Synchronously Pumped Ring Resonators
Optically induced breaking of symmetries plays an important role in nonlinear
photonics, with applications ranging from optical switching in integrated
photonic circuits to soliton generation in ring lasers. In this work we study
for the first time the interplay of two types of spontaneous symmetry breaking
that can occur simultaneously in optical ring resonators. Specifically we
investigate a ring resonator (e.g. a fiber loop resonator or whispering gallery
microresonator) that is synchronously pumped with short pulses of light. In
this system we numerically study the interplay and transition between regimes
of temporal symmetry breaking (in which pulses in the resonator either run
ahead or behind the seed pulses) and polarization symmetry breaking (in which
the resonator spontaneously generates elliptically polarized light out of
linearly polarized seed pulses). We find ranges of pump parameters for which
each symmetry breaking can be independently observed, but also a regime in
which a dynamical interplay takes place. Besides the fundamentally interesting
physics of the interplay of different types of symmetry breaking, our work
contributes to a better understanding of the nonlinear dynamics of optical ring
cavities which are of interest for future applications including all-optical
logic gates, synchronously pumped optical frequency comb generation, and
resonator-based sensor technologies
Counterpropagating light in ring resonators : switching fronts, plateaus, and oscillations
We characterize the formation of robust stationary states formed by light plateaus separated by two local switching fronts in only one of two counterpropagating fields in ring resonators with normal dispersion. Such states are due to global cross coupling and allow for frequency combs to switch from one field to the other by simply tuning the input laser frequency. Exact expressions for the distance between fronts and for plateau powers are provided in excellent agreement with simulations. These demonstrate an unusual high degree of control over pulse and plateau duration in one of the fields upon changes of one of the input laser frequencies. We identify a wide parameter region in which light plateaus are self-starting and are the only stable solution. For certain values of the detunings we find multistable states of plateaus with switching fronts, slowly oscillating homogeneous states and nonoscillating homogeneous states of the counterpropagating fields. Robustness and multistability of these unusual single-field front solutions are provided in parameter ranges that are experimentally achievable in a wide variety of ring resonators
Coherent suppression of backscattering in optical microresonators
As light propagates along a waveguide, a fraction of the field can be reflected by Rayleigh scatterers. In high quality-factor whispering-gallery-mode microresonators, this intrinsic backscattering is primarily caused by either surface or bulk material imperfections. For several types of microresonator-based experiments and applications, minimal backscattering in the cavity is of critical importance, and thus the ability to suppress the backscattering is essential. We demonstrate that introducing an additional scatterer in the near-field of a high-quality-factor microresonator can coherently suppress the amount of backscattering in a microresonator by more than 30 dB. The method relies on controlling the scatterer's position in order for the intrinsic and scatterer-induced backpropagating fields to destructively interfere. This technique is useful in microresonator applications where backscattering is currently limiting the performance of devices, such as ring-laser gyroscopes and dual frequency combs that both suffer from injection locking. Moreover, these findings are of interest for integrated photonic circuits in which backreflections could negatively impact the stability of laser sources or other components
Thermo-optical pulsing in a microresonator filtered fiber-laser: a route towards all-optical control and synchronization
We report on 'slow' pulsing dynamics in a silica resonator-based laser system: by nesting a high-Q rod-resonator inside an amplifying fiber cavity, we demonstrate that trains of microsecond pulses can be generated with repetition rates in the hundreds of kilohertz. We show that such pulses are produced with a period equivalent to several hundreds of laser cavity roundtrips via the interaction between the gain dynamics in the fiber cavity and the thermo-optical effects in the high-Q resonator. Experiments reveal that the pulsing properties can be controlled by adjusting the amplifying fiber cavity parameters. Our results, confirmed by numerical simulations, provide useful insights on the dynamical onset of complex self-organization phenomena in resonator-based laser systems where thermo-optical effects play an active role. In addition, we show how the thermal state of the resonator can be probed and even modified by an external, counter-propagating optical field, thus hinting towards novel approaches for all-optical control and sensing applications
Schedule-dependent cytotoxicity of SN-38 in p53 wild-type and mutant colon adenocarcinoma cell lines
In this study the effects of SN-38 on colon adenocarcinoma cell lines expressing wild-type p53 (LS174T) or mutant non-functional p53 (HT29) have been investigated. On exposure to SN-38, HT29 cells rapidly progressed through G1 and S and arrested in G2/M. Release and concomitant increase in apoptosis after 48 h was concentration- and time-dependent (P < 0.001), being more rapid at higher concentrations, but reaching plateau at 10 ng ml–1 with prolonged exposure. LS174T cells showed only a small increase in apoptosis, and only at high concentrations (50–100 ng ml–1). The main effect of SN-38 in LS174T cells was prolonged cell cycle arrest, which was independent of concentration. Arrest occurred in all phases of the cell cycle, with the distribution depending on concentration (P < 0.001) and not duration (P > 0.05). With increasing concentration, LS174T cells arrested in G2/M, S and G1. Cell cycle arrest was coincident with increased p53 expression in each phase of the cell cycle. Expression in G1 increased with time and concentration (P < 0.001, P = 0.01 respectively), whereas in S and G2/M p53 expression increased only with time (P < 0.001). Dose-dependent p53-associated G1 arrest, in the absence of DNA synthesis indicates an additional cytotoxic mechanism for SN-38, which requires higher concentrations than the S phase mechanism, and detection of which seems to involve p53. For incubations with the same ED (exposure × duration), apoptosis in HT29 cells was significantly higher for prolonged exposure to lower concentrations, whereas in LS174T cells there was a trend towards increased apoptosis with shorter exposures to higher concentrations, indicating a schedule effect of SN-38. Although expression of wild-type p53 leads to a more rapid induction of apoptosis, SN-38 cytotoxicity was generally greater in cells with mutant p53, as wild-type cells escaped apoptosis by p53 associated prolonged cell cycle arrest. Thus, pulsed schedules with higher doses may be more effective in cells expressing wild-type p53, whereas continued exposure with protracted schedules may be more active in cells expressing mutant p53. © 1999 Cancer Research Campaig