Microfluids tune optical signals

Microfluids tune optical signals
Air-silica microstructure fibers (ASMF) are attracting interest because of their unique optical properties and ability to manipulate light. These fibers typically are all-silica optical fibers with air holes introduced in the cladding region that runs along the length of the fiber. The distribution as well as the size of the air holes can be designed to change the optical properties in the transmission of these fibers. For example, light can be guided by a photonic bandgap, or propagate as an endlessly single mode, or experience a regime of enhanced nonlinearity.

We have taken two approaches for attaining interaction between light propagating in the fiber and the fluids. First, a long-period grating (LPG) is written in the core of the fiber, which couples the core mode into a mode whose field distribution is in the cladding and hence is sensitive to the index change in the air holes. A second approach is tapering the fiber into a small-diameter, where the mode field leaves the core and spreads into the cladding to extend into the silica/air-hole interface. In addition, tunability is obtained by actively controlling the motion and position of fluids by thermal expansion of air in the channels using capillary tube heaters. The fluids then move in the channels along the fiber until they overlap with the optical fields.

The fiber comprises six cylindrical air holes in the cladding. These holes form an inner cladding region, 34 microns in diameter, to allow for the infusion of microfluids. The core is germanium-doped with a diameter of 8 microns; the outer diameter is 125 microns.

To displace the fluid along the air channels, a single-mode fiber is spliced to provide light coupling to the core and as a hermetic seal for the channels. A capillary heater expands the air in the channels, which induces pressure on one side of the fluid plug. This pushes the microfluid in the desired direction. When the heater is deactivated, pressure from the opposing air segment drives the fluid to its original equilibrium position.

An LPG is written in the core of the fiber. Light propagating in the core incident on the grating will couple to forward-propagating higher-order modes in the cladding region. Coupling to these "cladding" modes manifests as sharp resonance loss in the transmission spectrum of the fiber.

The core mode of the fiber is not affected by the presence of the air holes or materials infused in them. On the other hand, cladding modes have their energy field distribution spread into the cladding and are sensitive to any change in the refractive index at the cladding air-hole interface. This promotes efficient interaction between the mode field and active materials infused in the air holes. As a result, the wavelength at which the resonance occurs, which also depends on the refractive indices, will be shifted to higher wavelength.

Another method to obtain mode field interaction with the fluid is by adiabatically tapering the fiber. The fiber is heated and stretched such that the fiber diameter is reduced while preserving its cross-section index profile along the taper. If the fiber is decreased to a very small diameter, the core also reduces to a point where it does not support light. In this case, the mode field spreads into the cladding where its guidance depends on the cladding/air-hole interface. In the case where the fluid is in the non-tapered region, the mode in the core is unaffected by the air holes. In the tapered region, it is also guided by total internal reflection because the refractive index of air is lower than that of silica and propagates back into the core with loss lower than 0.1 dB.

Heating injects fluid into the waist of the fiber. This changes the boundary conditions at the channel-fluid interface. Hence, the mode field will be refracted into the high index material. This system behaves as a broadband attenuator with a switching speed of about 10 seconds. These experiments demonstrate that positioning fluids inside the fiber enables modulation at narrow or broad bandwidths.

Other contributors to this article include P. Mach, M. Dolinski and J.A. Rogers, Bell Labs, Optical Fiber Solutions, Lucent Technologies Inc.

The complete paper is being presented at the Optical Fiber Communications Conference.