Driven by bandwidth hungry applications, optical broadband access networks have advanced very rapidly in the past few years, becoming the core of new triple-play telecommunication services, which deliver data, video and voice on the same optical fiber right to the user's end. Deep penetration of the optical fiber into the access networks is accompanied with massive deployment of the optical gear that drives the traffic along the fiber links. Specifically, optical transceivers, which receive downstream and send upstream data signals, have to be deployed at every optical line terminal or/and network user interface. Therefore, cost efficiency and volume scalability in manufacturing of such components are increasingly becoming the major requirements for their mass production.
Hence photonic integrated circuits (PICs), in which different functionalities are monolithically integrated onto one photonic chip, are an attractive technology and component solution in that they enable the production of complex optical circuits using high volume semiconductor wafer fabrication techniques. This offers the ability to dramatically reduce the component footprint, avoid multiple packaging issues, eliminate multiple optical alignments and, eventually, achieve the unprecedented cost efficiency and volume scalability in mass production of consumer photonics products.
In the context of applications, the advantages of PIC technology become especially compelling when active waveguide devices, such as laser or photodetector, are combined with the passive waveguide devices and the elements of the waveguide circuitry, to form a highly functional photonic system on the chip with minimal, preferably just one, optical input and/or output port. Since the active devices, which emit, detect or intentionally alter (e.g. modulate) optical signals by electrical means, usually all are made from artificially grown semiconductors having bandgap structures adjusted to the function and wavelength range of their particular application, such semiconductors are the natural choice for the base material of the PICs. For example, indium phosphide (InP) and related III-V semiconductors are the common material system for the PICs used in optical fiber communications, since they uniquely allow the active and passive devices operating in the spectral ranges of interest, e.g. the 1310 nm and 1490 nm (or 1555 nm) bands, to be combined onto the same InP substrate.
As the function of any waveguide device within the PIC made up from epitaxially grown semiconductor heterostructures is pre-determined by its band structure and, more particularly by the bandgap wavelength of the waveguide core layer(s) featuring the narrowest bandgap amongst all the waveguide layers, hereafter referred as the waveguide bandgap wavelength, functionally different devices are made from the different, yet compatible, semiconductor materials. This is a fundamental requirement, and one that has a profound impact both on the design and fabrication of the PIC. Monolithic integration of multiple waveguide devices having different waveguide core regions can be achieved in essentially one of the three following ways:                direct butt-coupling; which exploits the ability to perform multiple steps of epitaxial growth, including selective area etching and re-growth, to provide the desired semiconductor materials, which are spatially differentiated horizontally with a common vertical plane across the PIC die;        modified butt-coupling; which exploits selective area post-growth modification of semiconductor material grown in a single epitaxial growth run to form the regions of required semiconductor material, also spatially differentiated in the common plane of vertical guiding across the PIC die; and        evanescent-field coupling; where vertically separated and yet optically coupled waveguides, grown in a single epitaxial growth step, are employed to provide the desired semiconductor materials, which are now differentiated in the common vertical stack of the PIC die.        
Whereas each of these three major integration techniques has its own advantages and drawbacks, it is only the last one, hereafter referred to as vertical integration, which allows for each waveguide device to be optimized independently while enabling the entire PIC to be manufactured by using only one epitaxial growth step and standard semiconductor fabrication processes, such dry and wet etching. Therefore, a combination of the design flexibility and suitability for a cost-efficient fabrication approach based on commercially available semiconductor processes makes the vertical integration a unique versatile PIC platform for applications aimed at emerging consumer photonics markets.
An example of such a market in the optical telecom domain is the broadband optical access market, where bidirectional optical transceivers for receiving, processing and sending optical signals in different wavelengths are required at a scale infrequently seen in optical component industry and approaching that of electronic consumer products. Therefore, PIC based optical transceivers for the broadband optical access provide an attractive and natural application for the vertical integration platform.
One major challenge faced by PIC designers using this semiconductor platform is in providing an efficient transition of the optical signals between functionally different and vertically separated optical waveguides, thereby providing compliance to the performance requirements and a robust solution to the variations of high-volume manufacturing processes. In fiber-optics transmission system applications, where optical signals in different wavelength ranges often are to be detected, processed, and emitted in varying combinations within the same photonic circuit, these vertical transitions between functionally and structurally different optical waveguides should additionally be of varying degrees of wavelength specificity, with the wavelength specificity being another variable in the design space of the PIC. In particular, there is a need in the art for the vertical integration to provide a waveguide arrangement, hereafter referred to as vertical wavelength (de)multiplexer (VWM), that allows for vertically combining and splitting the optical signals in the different wavelength ranges, such that, in use, signals in each particular wavelength range are transitioned from the wavelength designated (common) input waveguide into the common (this wavelength designated) output waveguide without significantly interacting with the other wavelength designated waveguides.
Despite the core requirements for such a VWM within PIC technology, there is no known generic solution to the VWM presented in the prior art. The most closely related designs found in the art are related to the wavelength-selective directional coupler and are based upon either resonant grating-assistant coupling (e.g. R. C. Alferness, et al., “Grating-assisted InGaAsP InP vertical co-directional coupler filter”, Appl. Phys. Lett., Vol. 55, P. 2011, 1989) or resonant evanescent-field coupling. Resonant evanescent field-coupling is further sub-divided into solutions using planar waveguides (e.g. V. Magnin, et al, “Design and Optimization of a 1.3/1.55-μm Wavelength Selective p-i-n Photodiode Based on Multimode Diluted Waveguide”, IEEE Photon. Technol. Lett., Vol. 17, No. 2, pp. 459-461, 2005), straight ridge waveguides (e.g. C. Wu, et al., “A Vertically Coupled InGaAsP/InP Directional Coupler Filter of Ultra-narrow Bandwidth”, IEEE Photon. Technology Lett., Vol. 3, No. 6, pp. 519-521, 1991), and tapered ridge waveguides (e.g. C.-W. Lee et al., “Asymmetric Waveguides Vertical Couplers for Polarization-Independent Coupling and Polarization-Mode Splitting”, J. Lightwave Technol., Vol. 23, No. 4, pp. 1818-1826, 2005).
Analysis of the resonant grating-assisted designs shows that these are suitable only for narrow wavelength passband applications and require that the grating is formed in the layer(s) separating the vertically integrated waveguides. This precludes the use of a one step epitaxial growth, a significant benefit of the vertical integration platform, which allows for high yield and low cost approach to manufacturing components on III-V semiconductor materials.
In the resonant evanescent-field coupling designs, the transfer between vertically integrated waveguides occurs at pre-determined distance along the propagation axis, this position being specific to the wavelength of the optical signal. This dramatically limits a designers' freedom for designing a circuit but also limits the resonant evanescent-field coupling designs only to the narrow passband applications.
Additionally, any narrow wavelength passband design requires tight fabrication tolerances, as even a minor variation of the epitaxial structure or/and layout of the device may result in a shift of centre wavelength beyond a specified passband and rendering the component useless for the intended application. This may significantly reduce the fabrication yields and, therefore, increase the manufacturing costs of performance compliant PIC components.
It would be advantageous, therefore, to provide a solution removing the constraints of the prior art, by offering increased design, fabrication and utilization flexibilities for the vertical integration approach within III-V semiconductor PIC technologies. It would be further advantageous if the solution was compatible with standard semiconductor materials, exploited an epitaxial semiconductor structure growth approach using only one epitaxial growth step, and supported a plurality of vertically integrated waveguide devices, each waveguide device for operating upon different operating wavelength ranges with the wavelength passband commensurate to the application.
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