High-index-contrast waveguides and devices

Abstract

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.

Transformation relationship of directional coupler with multimode interference coupler and two mode interference coupler

“…So the coupling coefficient for DC with n 3 close to n 1 , satisfies also the coupling coefficient formula for MMI coupler. The beat length of MMI coupler with n = 5% and width ~ 6 m is obtained as 74 m. [6,15] matching well with theoretical curves. It is seen that as index contrast ( n) increases, the beat length decreases and the rate of decrease of L with n becomes slower in lower values of h. As h becomes closer to zero coupling gap (h=0), the curves become closer and closer.…”

Holographic planar lightwave circuit for on-chip spectroscopy

“…These losses are reduced by adiabatically widening the RWG linewidths up to 5 mm over long straight paths. 42,43 The total output/input ratio is around 215 dB, mostly due to material losses and the low coupling efficiency measured to be around 20% for wavelengths from 600 to 800 nm. The stray light due to imperfect coupling of the fiber with the input RWG is visible in Figure 5a.…”

Design of Polarization-Independent Coarse Wavelength Splitters Based on Ridge-Waveguide Directional Couplers

“…The PDL is very sensitive to the waveguide width. For the InP waveguide with w = 0.4 μm, previously reported results at λ = 1.54 μm are about 9 dB/cm and 7 dB/cm for TE and TM, respectively [3]. The PDL is not excessive if the device is sufficiently short, which is another reason to keep L small.…”

“…In a drive towards high-density photonic integration, waveguides with strong lateral confinement, such as the ridge waveguide shown in Figure 2, have been developed in recent years because they permit sharper waveguide bends and smaller devices. Such waveguides also have different optical properties from conventional rib waveguides [3]. The lateral confinement in a ridge waveguide is strongly dependent on the waveguide width.…”

“…Then in Section 3, the birefringent nature of the waveguides will be briefly described, in Section 4, the dependence of the DC characteristics on waveguide parameters will be discussed, and in Section 5, the design approach will be illustrated for both InP/InGaAsP and Si/SiGe material platforms. Finally, in Section 6, the device performance is discussed in terms of crosstalk, optical bandwidth, and sensitivity, which was not discussed in [3].…”