Basics of Integrated Photonics

In this chapter, the basic of photonic components including different waveguide design, modulators, lasers, and detectors is presented.

Keywords

Waveguides; modulators; laser; detectors; general

Outline

General

Integrated photonics devices, and indeed most photonics systems, include sources (lasers, LEDs), light detectors, and an optical waveguide (or possibly free space) based “fabric” or network in between to transport light in some shape. In the waveguided version, this fabric can include optical modulators, changing amplitude, phase, and/or polarization of the light, as well as switches, to redirect light, optical amplifiers, and wavelength selective structures for filtering, wavelength multiplexing and demultiplexing, and other operations involving wavelengths or light frequency. Integrated photonics has developed at a considerably slower pace than integrated electronics, as a matter of fact, it was a subject of a joke stating that “integrated photonics is the technology of the future and will remain the technology of the future.” However, this state of affairs has altogether been changed by progress in material technology in III–V compounds (GaAs, InP systems, etc), ferroelectrics (LiNbO3), silicon, polymers, and metal optics.

The basic structure of an integrated photonics circuit is the optical waveguide (Figure 2.1). In most photonics integration applications and in all such applications where highest performance is sought, these waveguides are single mode, by which is meant that only one spatial mode can propagate, and other modes are evanescent or cut off [1]. However, the waveguides are not strictly single mode and normally support two orthogonal polarizations, a fact that has caused a number of problems in the past and present. The reason is that the standard single mode fiber does not preserve light polarization, and hence photonic elements in a fabric of such fibers need to work independently of the state of polarization of the input light. Such polarization independence normally means that compromises in the device performance have to be made.

A so-called channel waveguide confines light in two dimensions (in the so-called core) while it propagates in the third dimension. In the more exotic so-called plasmonic waveguide, light can be guided along a single plasmonic, usually metal-dielectric interface, as will be briefly discussed below. The confinement of light in the two dimensions orthogonal to the direction of light propagation is accomplished by total internal reflection [1], just like in an optical fiber, by having a core with higher refractive index than the surrounding. This surrounding is called cladding in a fiber and is in general partly the substrate in a PIC.

Figure 2.2 shows some basic structures of integrated photonic waveguides, with a central core of higher refractive index than the surrounding medium, cladding, or substrate. The optical field is also shown in Figure 2.3.

Buried Channel Waveguide

Figure 2.2(a) shows a buried channel waveguide, and it consists of a high-index waveguiding core buried in a low-index cladding. The optical wave can be confined in two dimensions due to differences of refractive index between the core and the cladding.

Strip-Loaded Waveguide

Figure 2.2(b) is the geometry of a strip-loaded waveguide, which is composed of three dielectric layers: a substrate, a planar layer, and then a ridge. The planar waveguide (without the strip) already provides optical confinement in the vertical direction (y-axis), and the additional strip can offer localized optical confinement under the strip, due to the local increase of effective refractive index.

Ridge Waveguide

Figure 2.2(c) is the ridge waveguide, which is a step-index structure. The difference between dielectric layers at the sides of the guide, as well as the top and bottom faces, can confine the optical wave in two dimensions.

Rib Waveguide

Figure 2.2(d) is the cross-section of a rib waveguide. The guiding layer basically consists of a slab with a strip (or several strips) superimposed onto it, which has a similar structure as the strip-loaded waveguide, and the strip is part of the waveguiding core.

The waveguides are characterized by the following:

Waveguide parameters and propagation characteristics for waveguides fabricated with different material compositions are presented in Table 2.1. SOI is silicon on insulator, usually quartz (SiO2). Small waveguide bending radii are desirable for dense integration.

The waveguides connect different functional elements—lasers, modulators, switches, optical amplifiers, wavelength selective devices, detectors, etc.—and are generally also used to create these device structures, as will be described in the following section. Figure 2.4 shows one of the first publications introducing the concept of integrated photonics.