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Journal of the Optical Society of America B

Complex traversal time for optical pulse transmission in a Fabry–Perot cavity

Jae Yong Lee, Hai-Woong Lee, Jae Won HAHN

Published Mar 1, 2000• 17 citations
Publisher

Abstract

We show that the physical clock approach can be applied to the problem of optical pulse transmission in the Fabry–Perot cavity. Our theoretical analysis leads directly to a complex-valued traversal time for the pulse. Real and imaginary parts of the traversal time, referred to as the phase time and the loss time, are associated, respectively, with the rotation angle of polarization and the change in the polarization ellipticity of the outgoing pulse in the presence of a magnetic clock. The physical significance of the phase time and the loss time is discussed in relation to the superluminal group velocity and the spectral shift of the pulse.

DOI
10.1364/josab.17.000401

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We show that the physical clock approach can be applied to the problem of optical pulse transmission in the Fabry–Perot cavity. Our theoretical analysis leads directly to a complex-valued traversal time for the pulse. Real and imaginary parts of the traversal time, referred to as the phase time and the loss time, are associated, respectively, with the rotation angle of polarization and the change in the polarization ellipticity of the outgoing pulse in the presence of a magnetic clock. The physical significance of the phase time and the loss time is discussed in relation to the superluminal group velocity and the spectral shift of the pulse.
The question of how much time a wave packet spends in a traversing region of space has been of both fundamental and practical interest associated with the old problem of quantum particle tunneling. The question was then renewed by the experiment of Steinberg et al. (1993), implying the superluminal tunneling of a photon and began extending to physical systems concerning optical and electromagnetic waves. In this study, the traversal time taken by an optical pulse to off-resonantly transmit in a Fabry-Perot cavity is theoretically investigated as one such extension.
We introduce a magnetic clock approach to measure the traversal and reflection times of an electromagnetic wave through a slab, when multiple reflections are taken into account. The traversal time is a complex quantity whose real component is proportional to the Faraday rotation and whose imaginary component is proportional to the degree of ellipticity. We conclude that the complex traversal time found for an electron by several different methods is not a consequence of the quantum nature of the particle, but due to the wavelike character of the entity involved.
We present a simple and intuitive model based on the impulse response of linear electrical systems for describing the propagation of optical pulses through a dynamic Fabry–Perot resonator whose refractive index changes with time. Our model shows that the adiabatic wavelength conversion process in resonators results from a scaling of the round-trip time with index changes. For pulses longer than the cavity round-trip time, we find that more energy can be transferred to the new wavelength when the input pulses are slightly detuned from the cavity resonance and the refractive index does not change too rapidly. In fact, the optimum duration of index changes scales with the photon lifetime of the resonator. We describe the evolution of the shape and spectrum of picosecond pulses inside a resonator under a variety of input conditions and with the magnitude and duration of index variations. We also apply our general theory to the case of pulses whose widths are shorter than the round-trip time and derive an analytical expression for the output field under quite general conditions. This analysis reveals a shifting of the spectral comb as well as compression of the temporal pulse train that depends on the both the magnitude and sign of the index change. Our results should find applications in the area of optical signal processing using resonant photonic structures.
This work develops a time-domain approach to explain the origin of superluminality, i.e. , an apparent faster-than-light phenomenon in one-dimensional Fabry-Pérot cavities. The interferences between multiple-reflection echoes are tracked and analyzed. Under the quasi-static condition, the destructive interference is found to result in the advancement of the superposed pulse as compared to its parent signals. It is the key to the superluminal effect. On the contrary, constructive interference can lead to the retardation of the superposed pulse. For a cavity operated at the off-resonance state, the competition between roundtrip destructive interference (group advancement) and multiple-roundtrip constructive interference (group retardation) determines how short the group delay can be observed. Manipulating the competition between roundtrip interferences opens numerous opportunities in controlling the signal delays or shaping the pulse waveforms.
We study the use of the Faraday effect as a quantum clock for measuring traversal times of evanescent photons through magnetorefractive structures. The Faraday effect acts both as a phase shifter and as a filter for circular polarizations. Only measurements based on the Faraday phase-shift properties are relevant to the traversal time measurements. The Faraday polarization filtering may cause the loss of nonlocal (Einstein-Podolsky-Rosen) two-photon correlations, but this loss can be avoided without sacrificing the clock accuracy. We show that a mechanism of destructive interference between consecutive paths is responsible for superluminal traversal times measured by the clock.
In the first part the author reviews the classical theory of the Fabry Perot interferometer that is applicable when the interferometer is illuminated by a light beam of coherent vibrations having long duration. In the second part, he studies how a pulse of square form is transmitted through a Fabry Perot interferometer. The two parameters of duration τ and τ' that characterize the instrument were defined. τ represents the duration of light path for a forward and backward motion between the two plates and τ' characterises the duration of the exponential attenuation of the beam due to the successive multiple reflections it suffers. In the case of the pulse having duration Θ 0 > τ the emergent vibrations superpose progressively and the profile of intensity of the output pulse as a function of time is calculated. In case when Θ 0 < τ the successive separated pulses do not overlap. The Fourier analysis of the signal received by a detector has been discussed in this case. In part III, the analogy between the spectrum of the signal and the optical spectrum of the Fabry Perot has been shown. In part IV, the profile of the light reflected by the Febry Perot receiving a light pulse, in the time domain, has been established and in part V the balance of light energy stored in the Fabry Perot has been established.