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Electronic devices such as computers operate by controlling one electron stream with another, to powerful effect. Much research is going into applying this same concept to the photon, with the potential result being new optical communications devices and all-optical computers. Because photons are unaffected by one another, controlling light with light requires that the photons be immersed in a nonlinear medium; one photon stream thus alters the medium's light-transmitting properties in some way, influencing the path of a second photon stream. With its very high brightness, coherent laser light is ideal for inducing nonlinear effects; however, the use of an incoherent source such as an incandescent lamp to induce light-guiding nonlinearities would have the advantage of simplicity.

Researchers at San Francisco State University (San Francisco, CA), Princeton University (Princeton, NJ), Lehigh University (Bethlehem, PA), and Stanford University (Palo Alto, CA) have, for the first time, guided light using incoherent light. Using a phase mask to form a dark strip in an otherwise bright field, the researchers project the stripe into a voltage-biased strontium barium niobate photorefractive crystal, forming a dark soliton waveguide that guides light of another wavelength.

The photorefractive crystal has a dielectric relaxation time of about 1 s. This slow response is important because the intensity pattern of an incoherent light beam consists of speckles that vary randomly in time. If the crystal responded to the instantaneous speckle pattern rather than the time-averaged dark stripe, then no dark soliton would form.

Rather than using an intrinsically incoherent source, the researchers simulated such a source by sending 514-nm light from an argon-ion laser through a spinning diffuser, resulting in a time-averaged incoherence. According to Zhigang Chen, assistant professor at San Francisco State University, this approach was taken because it offers an easily controllable degree of spatial coherence. For the experiment, a 15-µm coherence length was chosen.

The source of the probe light guided by the dark soliton is a 633-nm-emitting helium-neon laser. This wavelength was selected because it has little effect on the photorefractive crystal. With no voltage bias, the probe beam enters the crystal at a width of 20 µm and diffracts to a width of 68 µm (see images on p. 26). With a 950-V/cm bias, an 18-µm-wide incoherent dark soliton forms, guiding the 633-nm light. Neglecting absorption and Fresnel reflection, 80% of the probe beam is guided by the soliton-induced waveguide.

By using an amplitude rather than a phase mask, the researchers can create Y-junction dark solitons. Such a soliton splits as it propagates and results in a Y-junction waveguide useful for evenly splitting the probe beam in two.

Dark solitons are easier to generate than bright solitons, according to Chen. "Besides," he says, "dark incoherent solitons are expected to induce single-mode waveguides, whereas bright incoherent solitons are multimode waveguides." Single-mode waveguides would be required for use in many optical communications devices.

Chen notes that his group is working on using a white-light source to induce dark solitons. "The problem with white-light sources is how to get enough power so as to speed up the waveguide formation," he says. "But it should be possible, as white light has [already] been used for bright solitons."¹

John Wallace

REFERENCE

1. 1. M. Mitchell et al., Phys. Rev. Lett. 77, 490 (1996); M. Mitchell and M. Segev, Nature 387, 880 (1997).

Fri Oct 01 00:00:00 CDT 1999

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