The substrate material used on an optical integrated circuit (OIC) is dependent primarily on the function performed by the circuit. An optical integrated circuit may consist of sources, modulators, detectors, etc and no one substrate will be optimal for all components, which means that a compromise is needed when building an integrated circuit. There are two main approaches that taken to deciding on a solution to this compromise: hybrid and monolithic approaches.
The hybrid approach attempts to bond more than one substrate together to obtain an optimization for each device in the integrated circuit. This approach allows for a more optimized design for each component in theory, however the process of bolding the various elements together is prone to misalignment and damage from vibration and thermal expansion. For this reason, although the hybrid approach is a theoretically more otpimized approach, it is more common to use the monolithic approach for OIC.
The monolithic OIC uses a single substrate for all devices. There is one complication in this approach which is that most OIC will require a light source, which can only be fabricated in optically active materials, such as a semiconductor. Passive materials, such as Quartz and Lithium Niobate are effective as substrates, however an external light source would need to be coupled to the substrate to use it.
Optically Passive and Active Materials
Optically active materials are capable of light generation. The following are examples of optically passive materials:
- Lithium Niobate
- Lithium Tantalate
- Tantalum Pentoxide
- Niobium Pentoxide
The following are optically active materials:
- Gallium Arsenide
- Gallium Aluminum Arsenide
- Gallium Arsenide Phosphide
- Gallium Indium Arsenide
- Other III-V and II-VI semiconductors
Losses in Substrate due to Absorption
Monolithic OICs are generally limited to the active substrates above. Semiconductors emit light at a wavelength corresponding to their bandgap energy. They also absorb light at a wavelength equal to or less than their bandgap wavelength. It follows then, for example, if a light emitter, a waveguide and a detector are all fabricated in a single semiconductor, there is a considerable issue of light being absorbed into the substrate, meaning that not enough light will be present for the detector. Thus, reducing losses due to absorbtion is one of the main concerns in substrate materials.