Although large amounts of “dark fiber” — that is, unused fiber cables laid across cities, nations and oceans — still exist in the world today, the demand for capacity is constantly increasing. To satisfy this demand, telecom scientists are searching for techniques to expand the capacity of existing fibers. Many laboratories are trying to cram more information onto each wavelength in the fiber by increasing the data rate. Elsewhere, scientists are squeezing more and more distinct wavelengths into the narrow C-band — between 1530 and 1565 nm — where optical fibers and other telecom components perform optimally. Complementing these efforts, other scientists are examining systems that operate outside the C-band, particularly in the S- and L-bands, as a means to expand the spectrum available to wavelength division multiplexing.A less-investigated approach to increase a given fiber’s capacity is mode division multiplexing. This entails launching several propagation modes, each carrying different information, into a multimode fiber and demultiplexing them at the other end to obtain a separate data stream from each. A few laboratories have described successful breadboard demonstrations of such systems and their components, but one shortcoming has been the lack of flexibility in launching the multiple modes into the fiber.Recently, researchers at the Korea Advanced Institute of Science and Technology in Daejeon designed and demonstrated a variable optical-mode generator capable of converting the 1.55-μm radiation propagating in a single-mode waveguide into any of several high-order modes in a multimode waveguide.The mode generator comprises an incoming single-mode waveguide; a power-divider section that splits the incoming power into four phase-retarding, single-mode arms; and a junction where the power in the arms is combined (Figure 1). The modes that are created in the multimode waveguide at the combining junction depend on the relative phase differences introduced in the four arms.Figure 1. The optical mode generator converts power entering from the single-mode waveguide at the bottom into the desired high-order mode in the multimode waveguide at the top. Different high-order modes are generated by changing the phase retardation in the four arms. (L1 = 1.5 cm, L2 = L4 = 0.475 cm, L3 = 0.5 cm, L5 = 2 cm, d = 25 μm, W = 20 μm, w = 5 μm, θ = 0.3°.) Reprinted with permission of IEEE Photonics Technology Letters.The scientists adjusted the phase retardation of each arm thermo-optically by applying voltage to a heater on each. The phase difference among the four arms at the junction depends on the device’s geometry, the length of the arms and the voltage applied to the heater on each arm. The scientists arbitrarily defined the “initial condition” as occurring when applied voltages create the E×31 mode in the multimode waveguide (Figure 2a). Then, by applying the appropriate voltage to the heater on each arm, they could convert the single-mode input into any desired multimode output (Figures 2b, 2c and 2d). Figure 2. The “initial condition” is defined as having the heater voltages adjusted to produce the situation in (a). In these pictures, the bottom-most curve represents the single-mode power entering the device from the bottom. The second-from-bottom curve shows how the power is divided among the four arms. The four curves above the arms represent the power emerging from each arm, and the single curve at top shows how these four combine to generate a high-order mode in the multimode waveguide. In (b), (c) and (d), the Y or N on each arm indicates whether or not that arm’s heater has applied a π-phase change (relative to a).They observed mode conversion efficiencies varying from 88 percent to greater than 99 percent for the four modes illustrated in the figure. The unconverted light propagated as unwanted modes in the multimode waveguide.In a mode division multiplexed system, it can be desirable to convert one high-order mode to another. The scientists noted that a pair of their mode generators, fabricated back-to-back, could serve as an effective mode converter.IEEE Photonics Technology Letters, Oct. 15, 2006, pp. 2084-2086.