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arXiv:2002.02954 · Optics Express

Optical memories and switching dynamics of counterpropagating light states in microresonators

Leonardo Del Bino, Niall Moroney, Pascal Del’Haye

Published Dec 21, 2020• 35 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/oe.417951

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A contrasting technique

anonymous

Researchers at the Rochester Institute of Technology in New York have increased the switching contrast of a particular kind of all-optical flip-flop by 28 dB, resulting in a switching contrast of 36.6 dB. This could provide a huge leap in the performances of a range of photonic techniques, such as all-optical packet switching, all-optical label addressing, and square-wave clock generation, as well as other photonic devices housing semiconductor optical amplifiers or even passive nonlinear media. Lead author, Claudio Ippolito, investigating optical flip-flops in the Photonic Systems Laboratory at the Rochester Institute of Technology All-optical flip-flops provide a means of sequential signal processing – as opposed to combinational signal processing – in the optical domain. The sequential-processing nature of all-optical flip-flops provides a latchable control signal for devices based on combinational processing. Drew Maywar, one of the authors of the Letter, told us that “advances in optical-packet switching performed at the University of Ghent, the University of Tokyo, and Eindhoven University of Technology, for example, use optical flip-flops to drive data-wavelength converters to achieve wavelength routing of optical packets.” However, Maywar explained, the switching performance of some previous optical flip-flops is poor – control signals operate with only specific polarisation states, over a narrow wavelength range of operation, or with high optical power. “The cycle switching time of some flip-flops is longer than 1 ns,” he said, “which is not ideal for some applications, and the switching contrast is also poor ( < 10 dB) for some demonstrated flip-flops.” In order to realize their 28 dB contrast enhancement, the team exploited two disparate physical processes within a resonant-type nonlinear medium: optical dispersive bistability and nonlinear polarisation rotation. The resulting bistable polarisation rotation (BPR), in conjunction with a linear polariser, then led to the increased switching contrast. In essence, Maywar explained, the team have realised a latchable polarisation switch: “whereas the traditional polarisation switch (or Kerr shutter) is a combinational device, acting only when on present control signals, our device is sequential, exhibiting memory of past control pulses, and these two polarisation states can be switched back and forth by control signals whose polarisation states are themselves arbitrary.” Another interesting aspect of the enhancement of the switching contrast is that it outperformed the standard method for measuring the contrast. For switching contrasts around 5 dB (the previous standard), a photodiode and oscilloscope were sufficient but, as the contrast improved, this technique set a false upper bound of around 15 dB. “After some tinkering and reflection,” explained Maywar, “it was realised that a more sensitive diagnostic was needed, which provided a nice, educational-enriching hurdle that challenged the students’ notions of the actual signal vs the measured signal.” Maywar expects their work to be easily extended to other resonators, such as loops, rings, distributed feedback structures, and vertical cavities, and to other nonlinear media, such as passive InP and SiO2. Furthermore, enhanced-contrast flip-flops are building blocks and he hopes to make use of such devices in larger functional photonic subsystems. Contrast enhancement is fuelled by bistable polarisation rotation. The latchable state of polarisation exiting the all-optical flip-flop is controlled by arbitrarily polarised set and reset pulses This integration would be achieved through the miniaturisation of the team's all-optical flip-flops. “Such an effort is expected to increase the cycle-switching time, and lower energy consumption and heat dissipation,” said Maywar “and this work is aligned with our team's wider research goals of developing photonic devices and demonstrating optical-switching applications.” Looking to the future, Maywar told us that he thinks research in all-optical flip-flops will continue to improve switching performance and attack the major issues of drive power (energy consumption and heat dissipation), and device size: “I would like to see demonstration of miniaturised all-optical flip-flops, at least 100 times smaller than the millimetre-length devices used in present-day demonstrations. As such, nanophotonics may provide a path forward in keeping control-signal power reasonable with such small, nonlinear components.”

Electronics Letters•Oct 1, 2014•0 citations
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.
A consequence of the Kerr effect in ring microresonators is that light can not always circulate simultaneously in both directions. Instead, under the appropriate conditions of input power and detuning, light can couple into the resonator only in one direction while the other continues on the coupling waveguide [1] – [3] . The two resulting light states, clockwise (CW) and counterclockwise (CCW), can be interpreted as the digital states 1 and 0 as shown in Fig. 1 (c) . Microresonators have been theorized to be a suitable solution for on-chip logical processing and switching [4] . Here we present an all-optical bistable device based on the difference of the Kerr effect for co-propagating and counterpropagating light. The device does not require any electrical power, and relies only on a single laser input for its operation. The optical state is set by the dominant input power direction. The system exhibits hysteresis ( Fig. 1 (d) ) hence the optical state is maintained until an significant input in the opposite direction is received. The hysteresis provides resilience to fluctuations in the inputs up to half of the total input power.
Spontaneous symmetry breaking is a concept of fundamental importance in many areas of physics, underpinning such diverse phenomena as ferromagnetism, superconductivity, superfluidity and the Higgs mechanism. Here we demonstrate nonreciprocity and spontaneous symmetry breaking between counter-propagating light in dielectric microresonators. The symmetry breaking corresponds to a resonance frequency splitting that allows only one of two counter-propagating (but otherwise identical) states of light to circulate in the resonator. Equivalently, this effect can be seen as the collapse of standing waves and transition to travelling waves within the resonator. We present theoretical calculations to show that the symmetry breaking is induced by Kerr-nonlinearity-mediated interaction between the counter-propagating light. Our findings pave the way for a variety of applications including optically controllable circulators and isolators, all-optical switching, nonlinear-enhanced rotation sensing, optical flip-flops for photonic memories as well as exceptionally sensitive power and refractive index sensors.
We demonstrate, for the first time to our knowledge, optical bistability on a highly integrated silicon device, using a 5-microm-radius ring resonator. The strong light-confinement nature of the resonator induces nonlinear optical response with low pump power. We show that the optical bistability allows all-optical functionalities, such as switching and memory with microsecond time response and a modulation depth of 10 dB, driven by pump power as low as 45 microW. Silicon optical bistability relies on a fast thermal nonlinear optical effect presenting a 500-kHz modulation bandwidth.
We review our recent progress on all-optical switching and bistability using ultrahigh-Q photonic crystal nanocavities. We also discuss all-optical logic gate operations. Because photonic crystal nanocavities can strongly confine photons in a small area for a long time, light can efficiently interact with dipoles. As a result, optical nonlinearities occur at a very low input. Optical nonlinearities are essential as regards all-optical switching and bistability, because they dynamically change such material properties as refractive index. We demonstrate all-optical switching at an operating energy of less than 100 fJ, and bistable operation at less than ~100 μW. In addition, these devices are small and so can operate at a reasonably high speed (~100 ps).
A possible device concept for photonic switching is the crossed resonator in which two Fabry-Perot resonators with perpendicular cavity axes enclose a nonlinear medium. Light entering one of the cavities can modify the light exiting the other cavity by altering the refractive index of the medium. This configuration has been proposed for use as an optical logic gate.1 In this work we also demonstrate its potential as an optical switch and transistor. We model the device following the single Fabry-Perot cavity work for steady-state fields of Miller2 and for pulsed inputs following Bischofberger and Shen.3 We assume the medium has a Kerr nonlinearity with an intensity-independent background absorption. Using parameters for the organic polymer polydiacetylene (PTS) etched into an optical waveguide, we find differential gains of greater than three for steady-state input fields of reasonable powers and that this device in certain regimes behaves like an optical transistor. For the time-dependent case, we model the medium temporal response by the Debye equation using the medium response time for PTS. We present results for input pulses of varying intensity and duration.
Researchers at the Rochester Institute of Technology in New York have increased the switching contrast of a particular kind of all-optical flip-flop by 28 dB, resulting in a switching contrast of 36.6 dB. This could provide a huge leap in the performances of a range of photonic techniques, such as all-optical packet switching, all-optical label addressing, and square-wave clock generation, as well as other photonic devices housing semiconductor optical amplifiers or even passive nonlinear media. Lead author, Claudio Ippolito, investigating optical flip-flops in the Photonic Systems Laboratory at the Rochester Institute of Technology All-optical flip-flops provide a means of sequential signal processing – as opposed to combinational signal processing – in the optical domain. The sequential-processing nature of all-optical flip-flops provides a latchable control signal for devices based on combinational processing. Drew Maywar, one of the authors of the Letter, told us that “advances in optical-packet switching performed at the University of Ghent, the University of Tokyo, and Eindhoven University of Technology, for example, use optical flip-flops to drive data-wavelength converters to achieve wavelength routing of optical packets.” However, Maywar explained, the switching performance of some previous optical flip-flops is poor – control signals operate with only specific polarisation states, over a narrow wavelength range of operation, or with high optical power. “The cycle switching time of some flip-flops is longer than 1 ns,” he said, “which is not ideal for some applications, and the switching contrast is also poor ( < 10 dB) for some demonstrated flip-flops.” In order to realize their 28 dB contrast enhancement, the team exploited two disparate physical processes within a resonant-type nonlinear medium: optical dispersive bistability and nonlinear polarisation rotation. The resulting bistable polarisation rotation (BPR), in conjunction with a linear polariser, then led to the increased switching contrast. In essence, Maywar explained, the team have realised a latchable polarisation switch: “whereas the traditional polarisation switch (or Kerr shutter) is a combinational device, acting only when on present control signals, our device is sequential, exhibiting memory of past control pulses, and these two polarisation states can be switched back and forth by control signals whose polarisation states are themselves arbitrary.” Another interesting aspect of the enhancement of the switching contrast is that it outperformed the standard method for measuring the contrast. For switching contrasts around 5 dB (the previous standard), a photodiode and oscilloscope were sufficient but, as the contrast improved, this technique set a false upper bound of around 15 dB. “After some tinkering and reflection,” explained Maywar, “it was realised that a more sensitive diagnostic was needed, which provided a nice, educational-enriching hurdle that challenged the students’ notions of the actual signal vs the measured signal.” Maywar expects their work to be easily extended to other resonators, such as loops, rings, distributed feedback structures, and vertical cavities, and to other nonlinear media, such as passive InP and SiO2. Furthermore, enhanced-contrast flip-flops are building blocks and he hopes to make use of such devices in larger functional photonic subsystems. Contrast enhancement is fuelled by bistable polarisation rotation. The latchable state of polarisation exiting the all-optical flip-flop is controlled by arbitrarily polarised set and reset pulses This integration would be achieved through the miniaturisation of the team's all-optical flip-flops. “Such an effort is expected to increase the cycle-switching time, and lower energy consumption and heat dissipation,” said Maywar “and this work is aligned with our team's wider research goals of developing photonic devices and demonstrating optical-switching applications.” Looking to the future, Maywar told us that he thinks research in all-optical flip-flops will continue to improve switching performance and attack the major issues of drive power (energy consumption and heat dissipation), and device size: “I would like to see demonstration of miniaturised all-optical flip-flops, at least 100 times smaller than the millimetre-length devices used in present-day demonstrations. As such, nanophotonics may provide a path forward in keeping control-signal power reasonable with such small, nonlinear components.”