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Gradually detuned coupled waveguides for quantum-like robust light transfer and splitting

Hassan Oukraou

Published May 9, 2018• 0 citations
Publisher

Abstract

The propagation of optical waves in coupled waveguides can be described by the coupled-mode theory. This formalism is mathematically analogous to the one for the quantum dynamics of the population of coupled atomic states, which is described by the Schrödinger equation within the so called Rotating Wave Approximation (RWA). This analogy has been pointed out in recent years. It was applied to coupled waveguide systems with constant effective refractive indices, which are fully analogous to resonant coupled quantum systems. This work extends this kind of studies to optical systems possessing an additional control parameter: the longitudinal modulation of the waveguide propagation constants. This approach is analogous to quantum systems with time dependent non-resonant excitation. As the population transfer between atomic states can be controlled by means of the Rabi frequencies and the laser frequency detunings, the light transfer between coupled waveguides can be controlled by means of the coupling constants and the propagation constants.Several such analogies are studied and exploited in this thesis for the demonstration of robust (broadband) adiabatic light transfer and splitting. The first system is based on the recently introduced quantum process known as two-state STImulated Raman Adiabatic Passage (two-state STIRAP). This is implemented for the first time in classical optics with a set of two evanescently coupled waveguides with proper longitudinal modulation of the mode propagation constants and of the coupling coefficient between them. A broadband 50:50 beam splitter based on two-state STIRAP is theoretically proposed and experimentally demonstrated. The experiments are performed using reconfigurable and tuneable waveguide structures that are optically induced by a lateral illumination technique of a nonlinear photorefractive crystal. This experimental platform provides versatile guiding structures that can be erased and reconfigured to test various systems depending on the considered analogy.A second quantum analogy based on the process of Rapid Adiabatic Passage (RAP) is theoretically studied and experimentally implemented for a set of two waveguides for which the longitudinally varying detuning crosses zero at half propagation distance. The modulation of the detuning and of the coupling constant provide a very robust and highly achromatic mechanism for full light transfer between the waveguides (broadband directional coupler). This is analogous to the RAP-based robust inversion of two-level quantum systems.The above RAP-like transfer applies to systems containing only two waveguides. It is also shown theoretically and numerically that the same functionality can be obtained in systems containing N waveguides with N>2. This relies on a technique called adiabatic elimination. It consists in the formal elimination of the N-2 internal waveguide(s) who reduces the system to an effective two-waveguide system where RAP can be applied. This is relevant because it permits a light transfer between the outer waveguides without excitation of the middle one(s). In contrast to the already known technique based on the conventional STIRAP process, the technique studied here works in a symmetric way and for an odd and even total number of guides N. Adiabatic elimination is achieved by a strong detuning between the two outer waveguides and the remaining one(s). It can be concluded that the analogies of all the classical optical systems studied in this work with corresponding non-resonant quantum systems and processes give powerful tools to design new broadband photonic structures. Moreover, the present studies can pave the way for dealing with future novel functionalities in nonlinear optical waveguide systems, which involve in a natural way a spatial light-intensity-dependent variation of the waveguide propagation constants and detuning.

DOI
10.70675/c58b60b7z47ddz4f6dz924fz30974d6dc9bc

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The propagation of optical waves in coupled waveguides can be described by the coupled-mode theory. This formalism is mathematically analogous to the one for the quantum dynamics of the population of coupled atomic states, which is described by the Schrödinger equation within the so called Rotating Wave Approximation (RWA). This analogy has been pointed out in recent years. It was applied to coupled waveguide systems with constant effective refractive indices, which are fully analogous to resonant coupled quantum systems. This work extends this kind of studies to optical systems possessing an additional control parameter: the longitudinal modulation of the waveguide propagation constants. This approach is analogous to quantum systems with time dependent non-resonant excitation. As the population transfer between atomic states can be controlled by means of the Rabi frequencies and the laser frequency detunings, the light transfer between coupled waveguides can be controlled by means of the coupling constants and the propagation constants.Several such analogies are studied and exploited in this thesis for the demonstration of robust (broadband) adiabatic light transfer and splitting. The first system is based on the recently introduced quantum process known as two-state STImulated Raman Adiabatic Passage (two-state STIRAP). This is implemented for the first time in classical optics with a set of two evanescently coupled waveguides with proper longitudinal modulation of the mode propagation constants and of the coupling coefficient between them. A broadband 50:50 beam splitter based on two-state STIRAP is theoretically proposed and experimentally demonstrated. The experiments are performed using reconfigurable and tuneable waveguide structures that are optically induced by a lateral illumination technique of a nonlinear photorefractive crystal. This experimental platform provides versatile guiding structures that can be erased and reconfigured to test various systems depending on the considered analogy.A second quantum analogy based on the process of Rapid Adiabatic Passage (RAP) is theoretically studied and experimentally implemented for a set of two waveguides for which the longitudinally varying detuning crosses zero at half propagation distance. The modulation of the detuning and of the coupling constant provide a very robust and highly achromatic mechanism for full light transfer between the waveguides (broadband directional coupler). This is analogous to the RAP-based robust inversion of two-level quantum systems.The above RAP-like transfer applies to systems containing only two waveguides. It is also shown theoretically and numerically that the same functionality can be obtained in systems containing N waveguides with N>2. This relies on a technique called adiabatic elimination. It consists in the formal elimination of the N-2 internal waveguide(s) who reduces the system to an effective two-waveguide system where RAP can be applied. This is relevant because it permits a light transfer between the outer waveguides without excitation of the middle one(s). In contrast to the already known technique based on the conventional STIRAP process, the technique studied here works in a symmetric way and for an odd and even total number of guides N. Adiabatic elimination is achieved by a strong detuning between the two outer waveguides and the remaining one(s). It can be concluded that the analogies of all the classical optical systems studied in this work with corresponding non-resonant quantum systems and processes give powerful tools to design new broadband photonic structures. Moreover, the present studies can pave the way for dealing with future novel functionalities in nonlinear optical waveguide systems, which involve in a natural way a spatial light-intensity-dependent variation of the waveguide propagation constants and detuning.
In this paper, optical simulation of double rapid adiabatic passage (RAP) is investigated in three evanescently coupled waveguides with longitudinally varying detunings of the propagation constants. Unlike the time-dependent detuning which is kept linearly increased or decreased when the RAP is applied in two-state atomic systems, here all possible situations of the corresponding space-dependent detunings with or without sign flips are considered. At the maximum of the couplings in the waveguide structure, the detunings tend to become zero and the sign flips are supposed to occur. Theoretical analysis shows that different light evolutions such as light splitting, complete light transfer, or complete light return can be realized adiabatically, depending on the choices of the detunings. Moreover, two of the above three phenomena can be observed simultaneously in the waveguide structure under some circumstances. The theoretical analysis is confirmed by numerical calculations. Due to the robustness of the double RAP, the technique can be applied to design optical devices such as achromatic beam splitters or beam couplers.
Abstract We discuss in this paper the optical analog to rapid adiabatic passage (RAP) and two-state stimulated Raman adiabatic passage (STIRAP) in coupled waveguides with nonlinearity. With the increase of the power of light propagating in waveguides, the nonlinearity can significantly prevent the two adiabatic passages from being successfully simulated. Nevertheless, we find that the negative role of the nonlinearity can be avoided under the assistance of the method of resonance-locked inverse engineering. The method implies redesigning the longitudinally varying detuning of the propagation constants. We further point out that the variation of the detuning does not have to be exactly the same as what the method suggests. As long as the detuning has similar trend to what the method shows, the RAP and two-state STIRAP can still be realized in nonlinear waveguides. This makes the waveguide structure more simpler. Based on our study, the applications of two-state adiabatic passages in designing directional couplers and beam splitters are extended from linear to nonlinear waveguides.
We propose an adiabatic method for the robust transfer of light between the two outer waveguides in a three-waveguide directional coupler. Unlike the established technique inherited from stimulated Raman adiabatic passage (STIRAP), the method proposed here is symmetric with respect to an exchange of the left and right waveguides in the structure and permits the transfer in both directions. The technique uses the adiabatic elimination of the middle waveguide together with level crossing and adiabatic passage in an effective two-state system involving only the external waveguides. It requires a strong detuning between the outer and the middle waveguide and does not rely on the adiabatic transfer state (dark state) underlying the STIRAP process. The suggested technique is generalized to an array of NNN waveguides and verified by numerical beam propagation calculations.