Technical Insight
A building block approach to manufacturing monolithic photonic integrated circuits
MetroPhotonics has developed active and passive InP-based components manufactured using the approach of vertical monomode integration, write Bedwyr Humphreys and Adrian O'Donnell.
The demand for lower-cost solutions that provide ever-increasing performance and functionality continues to dominate the optical component landscape. Monolithic integration provides a means for combining multiple functions and devices onto a single chip, enabling orders of magnitude reduction in cost, footprint and power, while at the same time improving overall reliability and performance.
Monolithic integration for DWDM combines separate functions such as (de)multiplexing, monitoring and optical power control on a per-wavelength basis, to produce line-card functionality on a single chip. When an add-drop function is also added to the chip, monolithic integration enables cost-efficient and reliable solutions for bandwidth provisioning in optical networks. Overall, this results in a more flexible, simple and manageable network.
With limited investment in new systems, there is a need to add greater functionality to existing networks through upgrades, for example optical monitoring throughout the network, without increasing incremental costs. Monolithically integrated optical monitors (figure 1) are one example of a fundamental leap to a lower cost curve that changes the entire industry structure, permitting ubiquitous deployment of optical monitors for real-time network monitoring at all optical nodes at a fraction of today s costs.
Component vendors have attempted to meet the demands of system vendors for lower-cost and higher-performance components with hybrid solutions - stitching together multiple discrete components to produce integrated functionality. While this initial level of integration lowers cost to some extent and simplifies design for system vendors, it is a short-term solution that does not significantly address cost and fundamental technology challenges.
In many WDM system components, the incoming multiwavelength signal is demultiplexed, then processed on a per-wavelength basis and, optionally, multiplexed back into a multiwavelength output signal (for example the dynamic channel equalizer in figure 2).
The search for compact and cost-efficient solutions enabling such a functionality has led to the development of monolithically integrated planar components, in which the multiplexer/demultiplexer is a waveguide spectrograph having active devices integrated within its input/output passive waveguides for controlling the optical signals.
The material system of choice for monolithic integration is InP, which allows the design of both the active and passive waveguides for operation in the spectral ranges of interest for WDM systems, such as communication S-, C- and L-bands. Although InP-based waveguide spectrographs, for example those using arrayed waveguide gratings (AWGs) or echelle gratings, have been around for some time, practical realization of these integrated components has been limited due to the absence of a simple and efficient method for integrating the active and passive semiconductor waveguides. This article discusses the building block approach adopted by MetroPhotonics in the design and manufacture of photonic integrated circuits (PICs) that provides a simple and efficient means of integrating active and passive components onto a single chip.
A building block approach
Figure 3 shows the building blocks developed at MetroPhotonics that are used in the design and manufacture of PICs. To date, the building block library consists of passive and active functional elements that can be fabricated on the MetroPhotonics vertical monomode integration (VMI) epitaxy platform. The passive building blocks consist of a multiplexer/demultiplexer function that comprises an echelle grating and polarization compensator, passive shallow waveguide interconnects, turning mirrors and mode size converters. The active building blocks consist of a pin photodetector for optical monitoring and/or an electroabsorption attenuator (EAA)-based variable optical attenuator (VOA) for dynamic channel equalization. In addition to these basic active elements, it may be possible to include a semiconductor optical amplifier (SOA) as part of the building block library, thus providing optical amplification on a per-channel basis. Such a building block element forms part of a future product roadmap at MetroPhotonics, which addresses gain and optical switching elements.
Examples of the building blocks used are shown in figure 4. Adopting a building block approach provides a means to create a library of design and fabrication components that can easily be rearranged on the chip to create any number of different components. Such an approach dramatically reduces new product development cycles and time to market, as well as providing a continuous history of reliability data associated with the overall technology, thus limiting the additional reliability qualification typically required during the introduction of a new product.
Monolithic integration for DWDM combines separate functions such as (de)multiplexing, monitoring and optical power control on a per-wavelength basis, to produce line-card functionality on a single chip. When an add-drop function is also added to the chip, monolithic integration enables cost-efficient and reliable solutions for bandwidth provisioning in optical networks. Overall, this results in a more flexible, simple and manageable network.
With limited investment in new systems, there is a need to add greater functionality to existing networks through upgrades, for example optical monitoring throughout the network, without increasing incremental costs. Monolithically integrated optical monitors (figure 1) are one example of a fundamental leap to a lower cost curve that changes the entire industry structure, permitting ubiquitous deployment of optical monitors for real-time network monitoring at all optical nodes at a fraction of today s costs.
Component vendors have attempted to meet the demands of system vendors for lower-cost and higher-performance components with hybrid solutions - stitching together multiple discrete components to produce integrated functionality. While this initial level of integration lowers cost to some extent and simplifies design for system vendors, it is a short-term solution that does not significantly address cost and fundamental technology challenges.
In many WDM system components, the incoming multiwavelength signal is demultiplexed, then processed on a per-wavelength basis and, optionally, multiplexed back into a multiwavelength output signal (for example the dynamic channel equalizer in figure 2).
The search for compact and cost-efficient solutions enabling such a functionality has led to the development of monolithically integrated planar components, in which the multiplexer/demultiplexer is a waveguide spectrograph having active devices integrated within its input/output passive waveguides for controlling the optical signals.
The material system of choice for monolithic integration is InP, which allows the design of both the active and passive waveguides for operation in the spectral ranges of interest for WDM systems, such as communication S-, C- and L-bands. Although InP-based waveguide spectrographs, for example those using arrayed waveguide gratings (AWGs) or echelle gratings, have been around for some time, practical realization of these integrated components has been limited due to the absence of a simple and efficient method for integrating the active and passive semiconductor waveguides. This article discusses the building block approach adopted by MetroPhotonics in the design and manufacture of photonic integrated circuits (PICs) that provides a simple and efficient means of integrating active and passive components onto a single chip.
A building block approach
Figure 3 shows the building blocks developed at MetroPhotonics that are used in the design and manufacture of PICs. To date, the building block library consists of passive and active functional elements that can be fabricated on the MetroPhotonics vertical monomode integration (VMI) epitaxy platform. The passive building blocks consist of a multiplexer/demultiplexer function that comprises an echelle grating and polarization compensator, passive shallow waveguide interconnects, turning mirrors and mode size converters. The active building blocks consist of a pin photodetector for optical monitoring and/or an electroabsorption attenuator (EAA)-based variable optical attenuator (VOA) for dynamic channel equalization. In addition to these basic active elements, it may be possible to include a semiconductor optical amplifier (SOA) as part of the building block library, thus providing optical amplification on a per-channel basis. Such a building block element forms part of a future product roadmap at MetroPhotonics, which addresses gain and optical switching elements.
Examples of the building blocks used are shown in figure 4. Adopting a building block approach provides a means to create a library of design and fabrication components that can easily be rearranged on the chip to create any number of different components. Such an approach dramatically reduces new product development cycles and time to market, as well as providing a continuous history of reliability data associated with the overall technology, thus limiting the additional reliability qualification typically required during the introduction of a new product.
