Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-05-01
2003-09-30
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C385S016000
Reexamination Certificate
active
06628453
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for optical switches based on a dark resonance induced two-photon coherence swapping.
DESCRIPTION OF THE PRIOR ART
An optical switch is generally used to drop, add, multiplex, convert to different frequencies, or route a data in fiber optic communications. As data traffic increases, the bandwidth of the optical switch is needed to be wider. Several optical switching methods have been developed for semiconductor-optical-amplifier (SOA) based optical switches utilizing resonance transitions of the medium and polymer based optical switches utilizing nonresonance transitions of the medium. These optical switches are based on refractive index change induced by applied electric current. In these switching methods, the time needed for the refractive index change is absolutely constraint of the switching time. Here, the refractive index change is restricted by carriers' redistribution time for the resonance transitions such as in the SOA-based optical switching method. The refractive index change based on the resonance transitions is more efficient than the method based on the nonresonance transitions, even though the switching time for the resonance transitions is a limiting factor. Therefore, the switching time is fundamentally limited by the carriers' lifetime in the optical switching technologies based on the resonance transitions, so that ~10 GHz is the upper limit of the switching bandwidth in heterostructures of semiconductors.
On the other hand, it is well known that two-color electromagnetic fields can induce refractive index change in a nonlinear optical medium composing three energy levels. The energy level structure of the optical medium can be two-closely spaced ground states and an excited state, or two-closely spaced excited states and a ground state, or arbitrarily spaced states. The refractive index change is induced to either the optical transition of the medium or to the closely spaced level transition via two-photon resonance of the applied electromagnetic fields. This refractive index change can result in absorption cancellation at line center, while strong coherence is induced on the closely spaced states. This phenomenon is called dark resonance or electromagnetically induced transparency (EIT) in the context of optically dense medium. Here, the origin of the dark resonance is the existence of dark state, which is a coherent superposition state, composed of the two-closely spaced ground states.
In the case of dark resonance or EIT, the time needed for the refractive index change is, however, not limited by the carriers' lifetime or population relaxation time, but may be limited by the phase decay time. Generally, the phase decay time is faster than the carrier's lifetime at least twice in most atomic gases and hundreds times in most ion-doped crystals such as Pr
3+
doped Y
2
SiO
5
. The detection of the two-photon coherence induced on the closely spaced ground states is optically made by nondegenerate four-wave mixing processes. The optical intensity of the generated nondegenerate four-wave mixing signal can be stronger than that of the original input laser. This signal amplification in the nondegenerate four-wave mixing processes based on dark resonance was already demonstrated experimentally in atomic vapors and ion-doped solids.
Now, suppose that the nonlinear optical medium has four energy levels composing closely spaced three-ground states and an excited state. When three-color electromagnetic fields interact with the four-level optical system, however, the optical medium can experience disruption on the induced refractive indices. The two-photon coherences induced on the ground states can, therefore, be controlled by a third laser field. This phenomenon produces two-photon coherence swapping among the three ground states when all three lasers interact with the optical system. Of course, the two-photon coherence induction can be optically detected via the nondegenerate four-wave mixing processes. Therefore, an entirely different all-optical switching method whose switching bandwidth is not limited by the population decay time or carrier's life time can be achieved when the dark resonance phenomenon is used in a four-level optical medium interacting with three-color lasers.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method of an optical switch based on dark resonance or EIT, where the switching mechanism is based on the quantum coherence swapping produced by three-color lasers interacting with a four-level optical medium, and the switching bandwidth is not limited by the population relaxation time or carrier's lifetime.
It is another object of the present invention to provide a quantum switch, wherein the optical switching is based on dark resonance.
It is another object of the present invention to provide a method and apparatus of the quantum switch for all-optical field driving, ultrawide bandwidth, parallel processing, time- and space-division superimposed, signal amplifiable, and line narrowed switching device.
In accordance with one aspect of the present invention, there is provided a method of quantum switch using a nonlinear optical medium composing three closely spaced ground states |a>, |b> and |c> such that the transition among the ground states is dipole forbidden, and an excited state |d> such that two-photon transitions between the ground state |a> and the |c>, the |b> and the |c>, and the |a> and the |b> via the excited state |d> are allowed, the method comprising following steps: applying a first laser beam to the nonlinear optical medium as an input beam through an optical fiber, waveguide, or free space at a frequency of &ohgr;
1
corresponding to a first transition between the ground state |a> and the excited state |d>; applying a second laser beam to the nonlinear optical medium through an optical fiber, waveguide, or free space at a frequency of &ohgr;
2
corresponding to a second transition between the ground state |b> and the excited state |d>; applying a third laser beam to the nonlinear optical medium through an optical fiber, waveguide, or free space at a frequency of &ohgr;
3
corresponding to a third transition between the ground state |c> and the excited state |d>; adjusting the intensities of the first laser beam &ohgr;
1
, the second laser beam &ohgr;
2
, and the third laser beam &ohgr;
3
to produce a strongly driven superposition state composed of the ground state |a> and the |c> creating two-photon coherence induction Re&rgr;
ac
; adjusting the frequency of the first laser beam &ohgr;
1
to produce a strongly driven superposition state composed of the ground state |b> and the |c> creating two-photon coherence induction Re&rgr;
bc
, whereas making the two-photon coherence induction Re&rgr;
ac
suppressed; applying a fourth laser beam &ohgr;
p
through an optical fiber or a free space corresponding to a fourth transition between the ground state |c> and the excited state |d> for nondegenerate four-wave mixing or phase conjugation geometry with either the first laser beam &ohgr;
1
, the &ohgr;
3
, and the &ohgr;
p
to produce nondegenerate four-wave mixing signal &ohgr;
1d
, or the second laser beam &ohgr;
2
, the &ohgr;
3
, and the &ohgr;
p
to produce nondegenerate four-wave mixing signal &ohgr;
2d
; and connecting the nondegenerate four-wave mixing signals &ohgr;
1d
and the &ohgr;
2d
to optical fibers physically separated.
In accordance with one aspect of the present invention, there is provided an apparatus for quantum switch using a nonlinear optical medium composing three ground states |a>, |b>, |c> such that the transition between the ground states |a> and |b>, |a> and &ve
Blakely & Sokoloff, Taylor & Zafman
Electronics and Telecommunications Research Institute
Lee John D.
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