Optical switching and routing are highly desirable performance in optical communication networks. [1.1] The main role of optical fibers has long distance to transmit high-speed bit streams from point to point between nodes in the network. An optical fiber could carry 4, 8, 16, 32 or more optical channels at different wavelengths. Optical filters are a controlling light technology for wavelength division multiplexing (WDM) systems. The WDM systems require single routing and coupling devices to have large bandwidth and to be polarization insensitive. The most obvious application is for demultiplexing very closely spaced channel waveguides. However, a simplified WDM system showing one direction of signal transmission is outlined in Fig. 1.1. Multiplexing and demultiplexing filters are found in the terminals. A filter referred to as an add/drop filter is required to separate the channel to be dropped from those that pass through unaffected. Node 1 receives the dropped channel and may transmit its own information on a new signal at the same wavelength as that dropped or a new wavelength that does not interfere with those already used by the other channels on the throughput.
Fig. 1.1 Add/Drop filter applications in a simplified WDM system.
An incoming signal is split into a number of parts that are individually split and then recombined. The optical characterization is found in interferometers. Optical filters (interferometers) come in two general classes, although there are many variations of each.
The first class is simply displayed by the Mach–Zehnder interferometer (MZI). This filter consists of a pair of couplers connected by two paths of unequal optical length. i.e., an incoming signal is split equally into two optical paths. The effective group velocity dispersion used for the optical paths of different lengths results that some wavelengths are output to the Out1 and the other wavelengths are output to the Out2. The signals in the two paths are then recombined as shown in Fig. 1.2(a). A partially reflecting mirror indicated by the dashed line, acts as a beam splitter and combiner. A 2x2 waveguides with directional couplers for the splitter and combiner is shown in Fig. 1.2(b). Each implementation has two outputs that are power complementary. The interfering paths are always feeding forward even though the interferometer may be folded such as with a Michelson interferometer. The signal processing used to design this filter type is classified as moving average (MA) or finite impulse response (FIR).
Fig. 1.2 A Mach-Zehnder interferometer: (a) free space propagation (b) waveguide device.
The second class of interferometers is displayed by the Fabry–Perot interferometer (FPI). The FPI consists of a cavity surrounded by two highly reflective parallel mirrors separated by a small distance as shown in Fig. 1.3(a). The frequency response depends on the spacing and index of refraction between the mirrors, at some wavelengths the multiple reflections interfere constructively. At these wavelengths the interferometer’s overall transmission is low and the overall reflectivity is high. The planar waveguide is a ring resonator with two directional couplers as shown in Fig. 1.3(b). There are two outputs: Out2 corresponds to the transmission response of the FPI and Out1 corresponds to its reflection response. The signal processing filters with feedback paths are classified as autoregressive (AR) or infinite impulse response (IIR) filters.
Fig. 1.3 A Fabry-Perot interferometer (a) free-space propagation (b) waveguide device.
The fundamental relationships between optical waveguide and digital filters were developed by Moslehi et al. [1.2] in 1984. Both digital and optical filters consist of splitters, delays and combiners. These parts are identified in Fig. 1.2 and 1.3 for the MZI and FPI, respectively. Thin film and Bragg grating filter design and fabrication are nature, having well established design techniques. [1.3-1.5] Theses filters are important devices because nearly
square bandpass filter response can be achieved with a short filter length.
The first optical waveguide filters were achieved using optical fibers and discrete components such as tapered fused-fiber couplers for splitters and combiners. The FSR was small and the surrounding fluctuations changed the optical path difference significantly. It was advantage of using a source with a coherence length shorter than the unit delay (the optical path lengths are typically integer multiples of the smallest path length difference. the smallest path length is called the unit delay length.) so that the combining functions were linear in intensity instead of the field. In this case, referred to as incoherent processing, the filter operates on the modulated signal on an optical carrier. Only positive filter factors are achievable, limiting the filter response to low pass designs.
Optical MA filters using fiber delay lines were first proposed for high-speed correlators and pulse compression by Wilner et al. [1.6] in 1976. For AR filters, E. A.
Marcatili proposed using an integrated ring resonator for a bandpass filter [1.7] in 1969 is interesting. The fabrication of planar waveguide has been critical for waveguide filters. In particular, low loss and process control, whereby fabricated devices closely approximate the design intent, enable the successful integration of multi-stage filters on a chip. The first coherent MA filter was demonstrated in 1984 using optical fibers. [1.8] The first multi-stage planar waveguide coherent MA filter was demonstrated in 1991 [1.9], and the first coherent multi-stage AR filter was demonstrated in 1996. [1.10]