Chapter 2

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 63

Buried Channel Waveguide 66

Strip-Loaded Waveguide 66

Ridge Waveguide 66

Rib Waveguide 66

Basics of Lasers, Modulators, Detectors, and Wavelength Selective Devices 68

Lasers 68

Basics of Photonic Detectors 70

Detector Characteristics 72

Responsivity 74

Dark Current 74

Noise Characteristics of Photodetectors 74

Modulators: Principles and Mechanisms of Optical Modulation 76

Photonics Switches: Spatial Routing of High-Speed Data Streams 79

Switches 79

Devices for Wavelength Division Multiplexed Systems 81

Devices Based on Spectrally Dependent Interference Effects 83

References 84

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.

Figure 2.1 Schematic diagram of a planar dielectric waveguide in Cartesian coordinate system. nc, nco, and nsub stand for refractive indices of the cover layer, guided wave layer (core), and substrate, respectively. Light is confined in the y direction and generally most of the optical field resides in the core. Light propagation is in the z direction.

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.

Figure 2.2 Nonplanar dielectric waveguide types: (a) buried channel, (b) strip-loaded, (c) ridge, and (d) rib.
Figure 2.3 Electric field distribution of Transverse Electric (TE) mode in a silicon channel dielectric waveguide, the Ex(0, y) and Ex(x, 0) curves express the amplitude distribution in x- and y-axis directions, respectively; the substrate material is SiO2, and the cover is air. The guided layer is made of silicon material with the geometry parameters are height=200 nm and width=450 nm.

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:

• Optical power loss, usually in dB/cm.

• Effective index, usually denoted by N or Neff, which is equal to β/k0, where β is the real part of the propagation constant and k0 is the wave number in vacuum. The effective index is, for guided waves, larger than cladding index but smaller than core index.

• Dispersion, i.e. the variation of the effective index with wavelength. This determines limitations in the propagation length of very short pulses but is normally not so important in PICs due to the small propagation distances. However, for the devices in PICs, such as filters, the so-called group delay dispersion, i.e. the derivative of the group delay with respect to angular frequency can be significant and important.

• Geometrical waveguide and optical field cross-sectional area.

• The useful wavelength range for light transportation. These are characterized by several “bands” between 1260 and 1675 nm for ICT applications.

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.

Table 2.1

Waveguide Parameters for Different Materials

Index difference Δ (%)
=ncore−ncladncore 0.3 0.45 0.75 3.3 7.0 (46) 41 (46) Core size (µm) 8×8 7×7 6×6 3×2 2.5×0.5 (0.2×0.5) 0.2×0.5
0.3×0.3 Loss (dB/cm) <0.01 0.02 0.04 0.1 2.5–3.5 1.8–2.0 Coupling loss (dB/point) <0.1 0.1 0.4 3.7 (2) 5 6.8 (0.8) Waveguide bending radius (mm) 25 15 5 0.8 0.25 (0.005) 0.002–0.005

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.

Figure 2.4 Artists sketch of a monolithic integrated photonics circuit, encompassing single mode waveguides on a planar surface. The waveguides connect the laser source to modulators (see...