Multi-layer thin film coatings are applied to many optical devices (e.g., lenses, computer monitors, eye glasses, window panes, light bulbs, mirrors, solar cells, detectors, laser gyroscopes, television cameras, prisms, WDM and DWDM filters, VCSEL lasers, etc.) to control the reflected or transmitted light at wavelengths ranging from x-ray to far infrared. A coating's effect is determined by how its layers control the interference and absorption of light. For an example see below (antireflection coating).
Bandpass coating is used in the telecommunications industry to control the transmission of multiple laser lines (i.e., channels) through fiberoptic cables. In Dense Wavelength Division Multiplexing (DWDM) a tighter spacing between laser wavelengths is used that allows the fiber to transmit more information. These filters are usually designed for wavelengths near 1550 nm.
The following table shows the center-to-center spacing of the channels in frequency units and wavelength units. The denser the spacing, the higher the number of channels that can be accomodated. The table also shows that these filters must have an extremely narrow bandwidth. To minimize the interference between channels, it is also required to block the transmission of adjacent channels. This is typically 30 dB (i.e., 0.1%) at the position of the next channel. These can be achieved by designing smart thin-film optical coatings.
| Spacing (GHz) | Spacing (nm) | #
of Channels |
Bandwidth (nm) |
| 400 | 3.2 | 8 | 0.8 |
| 200 | 1.6 | 16 | 0.5 |
| 100 | 0.8 | 32 | 0.2 |
| 50 | 0.4 | 64 | 0.1 |
Fig. 1. a) Design and b) simulation of optical waveguide splitter.
In designing DWDMs the key issues to address are: (i) feeding different wavelengths onto one multimode carrier (multiplexing), (ii) and separation of wavelengths of the carrier signal into component signals (demultiplexing). These are usually accomplished by using either reflection type or holographic mirrors of small dimension. Another way is to design DWDMs with smart material, by adjusting the material refractive index to multiplex and demultiplex the optical signals. A combination of smart design of optical waveguides and smart thin-film optical coating can be deployed to accomplish this.
Fig. 2. Modified design of the splitter in Fig. 1 (analysis not shown).
Many optical devices require maximum possible optical power of a specific wavelength range to be coupled in, and therefore, negligible reflected power. A convenient way of accomplishing this is to antireflection coatings at the required surface. Antireflection coatings consist of thin films of material having predetermined thickness and refractive index lower than that of the component material. For instance, in Fig. 3b the glass slab (n=1.5) is coated with two successive layers, PbF2 (n=1.7, shown in yellow) and MgF2 (n=1.38, shown in green). Each of these coating has a thickness of l/4 so that beams reflected by each layer undergo destructive interference for selective wavelength. However, other wavelengths that stay in phase even after reflection will not be extinct, thus coatings can also be used as optical filters.
Fig. 3. a) Partial reflection (loss) at glass-air interface; b) minimized reflection loss with antireflection coating.
© 1999 Anis Rahman