Tunable optical true-time delay devices that would exploit electromagnetically induced transparency (EIT) have been proposed. Relative to prior true-time delay devices (for example, devices based on ferroelectric and ferromagnetic materials) and electronically controlled phase shifters, the proposed devices would offer much greater bandwidths. In a typical envisioned application, an optical pulse would be modulated with an ultra-wideband radio-frequency (RF) signal that would convey the information that one seeks to communicate, and it would be required to couple differently delayed replicas of the RF signal to the radiating elements of a phased-array antenna. One or more of the proposed devices would be used to impose the delays and/or generate the delayed replicas of the RF-modulated optical pulse. The beam radiated or received by the antenna would be steered by use of a microprocessor-based control system that would adjust operational parameters of the devices to tune the delays to the required values.
EIT is a nonlinear quantum optical interference effect that enables the propagation of light through an initially opaque medium. A suitable medium must have, among other properties, three quantum states (see Figure 1): an excited state (state 3), an upper ground state (state 2), and a lower ground state (state 1). These three states must form a closed system that exhibits no decays to other states in the presence of either or both of two laser beams: (1) a probe beam having the wavelength corresponding to the photon energy equal to the energy difference between states 3 and 1; and (2) a coupling beam having the wavelength corresponding to the photon energy equal to the energy difference between states 3 and 2. The probe beam is the one that is pulsed and modulated with an RF signal.
If a properly adjusted probe pulse is sent into the medium in the absence of the coupling beam, the atoms completely absorb the pulse and jump from state 1 to state 3. This absorption is what makes the medium opaque. In the presence of the coupling beam, states 2 and 3 become coupled, preventing the absorption of the probe beam and thereby rendering the medium transparent to the probe beam. At resonance (that is, when the laser wavelengths correspond exactly to the differences between the energy levels of the quantum states of the medium), the index of refraction of the medium is exactly 1. At slightly different frequencies, the degree of cancellation of absorption of the probe beam is less and the index of refraction differs. The nature of the variation of the slope of the index of refraction as a function of wavelength is such as to substantially reduce the group velocity of a probe light pulse.
The reduction in group velocity can be exploited to slow and even stop the pulse. If the coupling pulse is turned off when the probe pulse reaches the middle of the medium, the probe pulse becomes trapped in the quantum states of the medium. The ratio between the numbers of atoms in states 1 and 2 is a measure of the ratio between the electric-field strengths of the probe and the coupling beams immediately before turn off. Also, the quantum state of the RF modulation of a stopped light pulse is stored in the spin quantum state of the medium. After a desired delay, the quantum state of the medium can be interrogated optically: Turning the coupling beam back on causes the regeneration of the optical pulse, complete with the RF modulation.
Suitably prepared clouds of alkali atoms (e.g., clouds of optically cooled sodium vapor) have been observed to exhibit EIT and would be used as the initially opaque media in the proposed devices. Recent experiments in compact cells (<1 cm3 ) of alkali-atom vapors have demonstrated that light pulses can be effectively decelerated to speeds of tens of meters per second and even stopped for controlled amounts of time that can range from microseconds to milliseconds. In contrast, to obtain millisecond delays by use of optical fibers, it would be necessary to make the fibers hundreds of kilometers long, and the delays would not be adjustable because the lengths would not be adjustable.
A simple example of a device according to the proposal is depicted schematically in Figure 2. A probe laser beam would be split into two parts: an undelayed (reference) beam and a beam to be delayed. A single probe pulse would be generated, yielding a single reference pulse. The probe pulse would be trapped in the medium, then released by use of suitably timed coupling pulses. If the coupling pulses were cycled on and off several times with sufficient rapidity while the probe pulse was still inside the medium, then several differently delayed replicas of the probe pulse would be generated.
This work was done by Igor Kulikov, Leo DiDomenico, and Hwang Lee of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-30776