Spot-size converter (SSC) is an important building block of InP-based photonic integrated circuits since it allows a standard single-mode fiber with a large and symmetric mode spot to be efficiently coupled with high displacement tolerance to a semiconductor waveguide with a small and asymmetric mode spot. Having an on-chip SSC integrated with a semiconductor waveguide is practically advantageous since such an element greatly simplifies the packaging process while increasing its reliability. In this paper, a SSC utilizing two-step lateral tapering is proposed for converting the semiconductor waveguide device mode into that suitable for fiber coupling without compromising the designs of the device and coupling waveguides. This is achieved by inserting a transient taper between the device and coupling waveguide as an impedance matcher.
This paper describes the design principles and characterization results for such a two-step SSC, compatible with earlier reported InP-based photonic integrated circuits for WDM. Transmission, Fabry-Perot fringes and photoresponsivity measurements (the last - by using an on-chip waveguide photodetector monolithically integrated with the SSC) show excellent performance of the two-step SSC. In good agreement with simulations, it was experimentally demonstrated that using this integrated component for fiber coupling can reduce the coupling loss to below 1dB, which includes the taper radiation loss of ~0.2 dB.
In WDM optical networks, signals have to be manipulated and monitored on a per wavelength basis. This requires that (de)multiplexing and wavelength processing functions are combined in the same optical component. The most efficient solution from a footprint size, cost and reliability point of view would be a combination of both functions in one photonic integrated circuit (PIC), fabricated by photolithography. Given that processing / detecting of individual wavelengths is an active function, in S, C and L communication bands naturally realized by InP and related semiconductor materials, InP-based PICs are widely agreed to be the superior choice.
This paper reports a practical design of such InP-based PICs. It is based on a building block approach, which allows a large variety of WDM optical components to be built from relatively few monolithically integrable elements, by using standard semiconductor fabrication technologies. These include: (i) (de)multiplexer based on a planar echelle diffractive grating with polarization compensation in the slab waveguide region; (ii) single-mode vertically integrated waveguide active devices with detecting, attenuating, amplifying and switching features, inserted in every (in)output waveguide channel of a planar (de)multiplexer, and (iii) passive waveguide circuitry required for coupling the light to and directing it through the InP-based photonic chip. Design and characterization examples of the building blocks and PICs for channel monitoring, variable attenuation / equalization and spatial switching are presented.
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