Wavelength demultiplexer using the spatial dispersion of multilayer thin-film structures

Abstract

Abstract-We demonstrate that a single 66-layer nonperiodic thin-film stack can be used to separate four wavelength channels by spatial beam shifting. This device uses group velocity effects similar to the superprism effect observed in photonic crystals, but shows larger and more controlled shifts including constant dispersion allowing for equidistant channel spacing. Experimentally, a nearly linear 100-m shift is achieved between 827 and 841 nm. System considerations are discussed.Index Terms-Dispersive media, nanotechnology, nonhomogeneous media, wavelength-division multiplexing. W AVELENGTH-DIVISION-MULTIPLEXING (WDM) systems create a strong need for compact, cost-effective wavelength multiplexing and demultiplexing devices. We focus here on thin-film structures, since they are easy to fabricate with well-known technology. In contrast to typical dielectric interference filters though, we use group velocity effects to separate multiple beams of different wavelengths with a single multilayer structure. These group velocity effects are similar to the superprism effect observed in one [4], and three-dimensional [5] photonic crystals. The multilayer thin-film stack is designed such that different wavelengths propagate at different effective group velocity angles as shown in Fig. 1, thus demultiplexing different wavelengths by spatial beam shifting. We have previously discussed that nonperiodic thin-film stacks offer superior wavelength splitting characteristics due to the larger design freedom [6]. Here, we present experimental results demonstrating a four-channel wavelength demultiplexer. Furthermore, we discuss the maximum number of channels theoretically possible for the given design.We designed and experimentally tested a 66-layer nonperiodic thin-film stack consisting of alternating layers of SiO ( at 830 nm) and Ta O with a total stack thickness of 13.4 m for operation at an incidence angle of 54 [6]. We choose to operate the stack in reflection as shown in Fig. 1. To prevent reflections off the front of the stack, we used a "tapered" Bragg stack as the starting design [7]. In such a Bragg stack, the periodicity is slowly "turned on" by increasing the amount of high index material in each period. This is equivalent to tapering a surface corrugated structure on the tapered Bragg stack to achieve a design with a linear shift upon reflection as a function of wavelength. A detailed discussion of the physics and design of this structure is given in [6]. Since we operate the dielectric stack at an angle, the device is polarization sensitive. This particular design only works for p-polarization. The designed stack exhibits an 18-m linear shift along the exit interface between 822 and 842 nm for a single reflection [6]. The effective group propagation angle changes from 17 to 46 corresponding to a dispersion of 1.5 /nm. Depending on the application, nonperiodic stacks with very different dispersion values can be designed [6], such that the dispersion of a stack is not on its own a good figure of merit to judge the pe...

Device Requirements for Optical Interconnects to Silicon Chips

“…[41], pp. [82][83], and that power management is now the primary issue across most application segments. The inability to handle higher powers limits the performance of chips.…”

Nanostructured Optics and Optoelectronics for Dense Optical Interconnects

“…[3]). If, however, we use non-periodic structures, we can achieve larger wavelength separations in smaller structures [4 -8], and can engineer specific forms of separation, such as linear [5] or even step-like dispersion [6]. Such non-periodic structures have many more degrees of freedom in their design, hence giving greater performance and flexibility of design.…”

Tunable Wavelength Demultiplexer for DWDM Application Using 1-D Photonic Crystal

“…This multiplexer contains 184 layers of low and high refractive index materials. Also, Gerken and Miller fabricated a wavelength division demultiplexer using the spatial dispersion of multilayer thin film structures [14]. They use a single 66-layer non-periodic thin film stake to separate four wavelength channels by spatial beam shifting.…”