Tag Archives: Photonic Integrated Circuits

Optical Waveguides

Just as a metallic strip connects the various components of an electrical integrated circuit, optical waveguides connects components and devices of an optical integrated circuit. However, optical waveguides differ from the flow of current in that the optical waves travel through the a waveguide in a spatial distribution of optical energy, or mode. In contrast to bulk optics, which guide optical waves through air, optical waveguides guide light through dielectric conduits.

Bulk Optical Circuit:


Optical Waveguides:


The use of waveguides allows for the creation of optical integrated circuits or photonic integrated circuits (PIC). Take for example, the following optical transmit and receive module:


Planar Waveguides

A planar waveguide is a structure that limits mobility in only one direction. If we consider the planar waveguide to be on the x axis, then the waveguide may limit the travel of light between two values on the x axis. In the y and z directions, light may travel infinitely. The planar waveguide does not serve many practical uses, however it’s concept is the basis for other tpyes of waveguides. Planar waveguides are also referred to as slab waveguides.Planar waveguides can be made out of mirrors or using a dielectric with a high refractive index slab. See also, Planar Boundaries, Total Internal Reflection, Beamsplitters.


Rectangular Waveguides

Rectangular waveguides can also be built either from mirrors or using a high refractive index rectangular waveguide.


The following are useful waveguide geometries:


Various combinations of waveguides may produce different and useful configurations of waveguides:





Optoelectronic Integrated Circuit Substrate Materials

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.


Hybrid Approach

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.


Monolithic Approach

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:

  • Quartz
  • Lithium Niobate
  • Lithium Tantalate
  • Tantalum Pentoxide
  • Niobium Pentoxide
  • Silicon
  • Polymers

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.