**3. Optical routing for WDM POF-based applications**

Consequently each input port can be managed independently of the others. The input lenses are required for collimating the light that comes from each input fiber, and the output lens

The operation of the optical multiplexer makes that the light from one input port is guided to the output port when there is no voltage applied to the corresponding pair of pixels of liquid crystal cells. If a voltage is applied to theses pixels, the light from the input port is not guided

In passive optical networks where there is no additional amplification, it is important to have few insertion losses. It could be also interesting to have additional functions in the same device and thus reducing the number of devices in the optical network. In addition to that, reconfig‐ urable optical networks in critical applications where an alternative path is required when

An improvement in terms of flexibility of the 3x1 multiplexer shown in the above figure is presented in Fig. 5 [43]. The structure has two set of three inputs and two outputs, and depending on the configuration each input of the one set of inputs can be guided to one of the

The structure can implement different functionalities only by selecting the inputs, the outputs and modifying the voltage applied to the TN-LC pixels of each cell. It can behave as a 3x1 multiplexer (or combiner) using only three inputs and one of the outputs, each input port can be switched on/off independently of the other three inputs thus also acting as an optical switch

The same device can operate as two complementary 3x1 Multiplexers. Inputs to the device are grouped in pairs, when the *Input Port a* is guided to *Output Port 1*, the other input of this pair, *Input Port b*, is coupled to *Output Port 2*. On the other hand, when the multiplexer is switched, *Input Port a* is directed to *Output Port 2* and the matched *Input Port b* is propagated to *Output Port 1*. As a matter of fact, it can also work as a 2x2 optical switch by using only the adequate

The use of TN-LC cells allows having intermediate values of light transmission by applying lower voltage, so the device can also implement a VOA by using a single input port and only one of its outputs. Finally, it can also implement a variable optical power splitter by using one

The introduced reconfigurable Advanced Multifunctional Optical Multiplexer has fiber to fiber insertion losses when operating as a 2x2 optical switch, in the range from 10dB to 15dB within 200nm wavelength range; with a non-optimized optics for collimation and coupling. Lower losses can be achieved for a smaller wavelength range. The crosstalk measured is better than -15dB at 532nm, 660nm and 850nm. Switching is achieved at voltage levels of 4VRMS.

*2.1.3. Advanced multifunctional optical multiplexer for multimode optical fiber networks*

396 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

focuses each beam into the output port.

there is a failure in the main path would be useful.

to the output port.

two possible outputs.

pair of inputs and the two outputs.

input and its two outputs.

if required.

For WDM routing, key devices such as interleavers, routers and switches are also indispensable to combine, to separate and to re-direct the different transmitted wavelengths. Nowadays, WDM devices are well-established in the IR and near IR (NIR) for silica optical fibers. However, they require a complete re–design for being implemented in SI–POF WDM systems. This is mainly due to their distinct attenuation behavior (compared to silica fiber) and new wave‐ length channels need to be determined. Nevertheless, there is no a widely spread consensus about the characteristics for these WDM channels for POF applications, although some authors have already proposed a spectral grid [44], which includes channels between 400 nm and 700 nm and spectral bandwidths up to 50 nm (LED sources) [45].

Nowadays, optical routers are key components in optical communications and sensor networks. Optical switches allow optical routing without converting the transmitted infor‐ mation into the electrical domain. The elimination of the two required conversions (optical to electrical and electrical to optical) improves the system characteristics, reducing the network equipment and increasing their bandwidth. These devices work by selectively switching optical signals delivered through one or more input ports to one or more output ports, in response to supervisory control signals. Different technologies could be applied to route optical signals, applications of which depend on the topology of the optical network and the switching speed required [46]. Cutting-edge optical switching technologies depending on their principle of operation include micro-electromechanical systems (MEMS) as well as acoustooptical, thermo-optical, opto-optical and electro-optical (EO) devices.

Opto-Mechanical switches are based on the movement of some mechanical devices such as prisms, mirrors or directional couplers. As a subsection of the opto-mechanical technology, MEMS have a great interest in telecommunications applications. MEMS consist of small mobile refractive surface mirrors that route the incident light beams to their destination [47, 48].

Acousto-optic switches are based on the acousto-optic effect of some materials, such as the peratellurite [49] or LiNbO3 [50], where an acoustic wave travelling along the material induces a periodical strain that alters its refractive index. The refractive index modulation induced in the material causes a dynamic phase grating than can diffract light. If the material is isotropic, the diffraction induced by the acousto-optic effect causes beam deflection, and if the material is anisotropic the deflection caused comes along with variation in light polarization.

Other solutions are the thermo-optic switches whose operation consists on the variation of the refraction index of the material by modifying its temperature. This type of switches has a great variety of implementations, but mainly based on using an interferometric mechanism in which the refractive index variation induces a change in the interference condition. This effect facilitates the light switching [51, 52].

Opto-optical switches are based on the intensity-dependent nonlinear effects in optical waveguides, such as the Two-Photon Absorption phenomenon (TPA) [53], the lightwave self action that induces the Self Phase Modulation (SPM) phenomenon and the Kerr Effect that causes the Four Wave Mixing (FWM) and the Cross Phase Modulation (XPM) [54].

Finally, electro-optic switches perform switching by using electro-optics effects. The main technologies are based on Lithium Niobate (LiNbO3) [55], Semiconductor Optical Amplifiers (SOA) [56], Electro-holographic (EH) [57], Bragg Gratings electronically switched [58] and LC. Focusing on the latter, LC switches use different physical mechanisms to steer the light such as polarization management, reflection, wave-guiding and beam-steering (2D or 3D). Main advantages of this technology include no need of moving parts for switch reconfiguration, low driving voltage and low power consumption. In the last years, different devices based on nematic LCs for SI-POF networks have been reported. Some of them are described below.

#### **3.1. Optical Switches based on twisted nematic liquid crystals**

The polarization rotation is the first configuration used in LC switches [59]. A simple example of a 1x2 switch based on TN-LC is presented in Fig. 6 [60]. Depending on the voltage applied to the TN-LC, the light from the input (Port 1) is guided to one of its outputs (Port 2 or Port 3).

#### **Figure 6.** Structure of the 1x2 LC optical switch.

mation into the electrical domain. The elimination of the two required conversions (optical to electrical and electrical to optical) improves the system characteristics, reducing the network equipment and increasing their bandwidth. These devices work by selectively switching optical signals delivered through one or more input ports to one or more output ports, in response to supervisory control signals. Different technologies could be applied to route optical signals, applications of which depend on the topology of the optical network and the switching speed required [46]. Cutting-edge optical switching technologies depending on their principle of operation include micro-electromechanical systems (MEMS) as well as acousto-

Opto-Mechanical switches are based on the movement of some mechanical devices such as prisms, mirrors or directional couplers. As a subsection of the opto-mechanical technology, MEMS have a great interest in telecommunications applications. MEMS consist of small mobile refractive surface mirrors that route the incident light beams to their destination [47, 48].

Acousto-optic switches are based on the acousto-optic effect of some materials, such as the peratellurite [49] or LiNbO3 [50], where an acoustic wave travelling along the material induces a periodical strain that alters its refractive index. The refractive index modulation induced in the material causes a dynamic phase grating than can diffract light. If the material is isotropic, the diffraction induced by the acousto-optic effect causes beam deflection, and if the material

Other solutions are the thermo-optic switches whose operation consists on the variation of the refraction index of the material by modifying its temperature. This type of switches has a great variety of implementations, but mainly based on using an interferometric mechanism in which the refractive index variation induces a change in the interference condition. This effect

Opto-optical switches are based on the intensity-dependent nonlinear effects in optical waveguides, such as the Two-Photon Absorption phenomenon (TPA) [53], the lightwave self action that induces the Self Phase Modulation (SPM) phenomenon and the Kerr Effect that

Finally, electro-optic switches perform switching by using electro-optics effects. The main technologies are based on Lithium Niobate (LiNbO3) [55], Semiconductor Optical Amplifiers (SOA) [56], Electro-holographic (EH) [57], Bragg Gratings electronically switched [58] and LC. Focusing on the latter, LC switches use different physical mechanisms to steer the light such as polarization management, reflection, wave-guiding and beam-steering (2D or 3D). Main advantages of this technology include no need of moving parts for switch reconfiguration, low driving voltage and low power consumption. In the last years, different devices based on nematic LCs for SI-POF networks have been reported. Some of them are described below.

The polarization rotation is the first configuration used in LC switches [59]. A simple example of a 1x2 switch based on TN-LC is presented in Fig. 6 [60]. Depending on the voltage applied to the TN-LC, the light from the input (Port 1) is guided to one of its outputs (Port 2 or Port 3).

is anisotropic the deflection caused comes along with variation in light polarization.

causes the Four Wave Mixing (FWM) and the Cross Phase Modulation (XPM) [54].

**3.1. Optical Switches based on twisted nematic liquid crystals**

optical, thermo-optical, opto-optical and electro-optical (EO) devices.

398 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

facilitates the light switching [51, 52].

As a consequence of the use of the input polarizer for the operation of the TN-LC half of the incoming optical power is filtered. The solution for reducing the insertion losses of the optical switch based on TN-LC cells is by using the polarization diversity method, see Fig. 7. In this technique, the input light is decomposed into its TE (S-Polarized light) and TM (P-Polarized light) components. Both components are treated separately and finally recombined. In this way, the device becomes polarization insensitive, and less insertion losses are expected. The same principle of the reconfigurable multiplexer design reported in the previous section.

Different configurations have been proposed in literature for implementing optical switches based on the polarization diversity method. A polarization insensitive 2 x 2 optical switch based on TN-LC is presented in Fig. 8 [61]. The structure is composed by Polarizing Beam Splitters (PBS1-PBS4), TN-LC cells (NLC1-NLC4), quarter wave plates (Plate 1- Plate 4), and mirrors (Mirrors 1 – Mirror 3). The 2x2 optical switch allows up to three operation modes by applying voltage to the suitable pair of TN-LC cells, see Fig. 9:

8 applying voltage to the suitable pair of TN-LC cells, see **Fig. 9:**

2 **Fig.** 7**.** Structure of the 1x2 LC optical switch.

1

Recent Advances in Wavelength-Division-Multiplexing Plastic Optical Fiber Technologies

3 Different configurations have been proposed in literature for implementing optical switches 4 based on the polarization diversity method. A polarization insensitive 2 x 2 optical switch

7 mirrors (Mirrors 1 – Mirror 3). The 2x2 optical switch allows up to three operation modes by

13

13 **Fig. 8.** Structure of the polarization independent 2x2 LC optical switch. **Figure 8.** Structure of the polarization independent 2x2 LC optical switch.

12

14

1

10

11

14 Optical Fiber

2 Fig. 9. Operation modes of the 2x2 optical switch: (a) Crossed, (b) Direct, (c) Closed. **Figure 9.** Operation modes of the 2x2 optical switch: (a) Crossed, (b) Direct, (c) Closed.

#### 3 3.2. Optical router with output power control **3.2. Optical router with output power control**

5 polarization diversity method following the same principle of operation as in the case of 6 optical multiplexers. The polarization modulation of a TN cell in combination with space 7 polarization selective calcite crystals or polarization beam splitters (PBS) allows optical 8 space-switching. Fig. 10 shows the structures of a typical 1x2 nematic LC−OR (Fig. 10.a) and 9 a 1x2 nematic LC−OR with independent output power control (Fig. 10.b). TN-LCs can also be used in routers (LC−OR) based on the polarization diversity method following the same principle of operation as in the case of optical multiplexers. The polarization modulation of a TN cell in combination with space polarization selective calcite crystals or polarization beam splitters (PBS) allows optical space-switching. Fig. 10 shows the structures of a typical 1x2 nematic LC−OR (Fig. 10.a) and a 1x2 nematic LC−OR with independent output power control (Fig. 10.b).

4 Twisted nematic liquid crystals (TN-LCs) can also be used in routers (LC−OR) based on the

12 Fig. 4. a) Structure of a typical 1x2 nematic LC−OR based on polarization diversity and, b)

14 In the scheme shown in Fig. 10.a, the TN-LC 1 has an input polarizer that changes the 15 polarization state of the transmitted beam depending on the voltage V1. The scheme of Fig. 16 10.b additionally provides the possibility of stabilizing the optical power that is transmitted 17 by each port. This feature was reported in [62]. In that scheme, TN-LC 2 and 3 have both an 18 input and an output crossed polarizer, with the input polarizer parallel to the respective 19 polarization component transmitted by the PBS. Then, in this scheme each LC cell controls 20 the transmitted power depending on the voltages V2 and V3. Fig. 11 shows an example of

13 structure of a 1x2 nematic LC−OR with independent output power control.

21 the power stabilization capacity of the router reported in [62].

Recent Advances in Wavelength-Division-Multiplexing Plastic Optical Fiber Technologies http://dx.doi.org/10.5772/59518 401

Recent Advances in Wavelength-Division-Multiplexing Plastic Optical Fiber Technologies

3 Different configurations have been proposed in literature for implementing optical switches 4 based on the polarization diversity method. A polarization insensitive 2 x 2 optical switch 5 based on TN-LC is presented in **Fig. 8** [61]. The structure is composed by Polarizing Beam 6 Splitters (PBS1-PBS4), TN-LC cells (NLC1-NLC4), quarter wave plates (Plate 1- Plate 4), and 7 mirrors (Mirrors 1 – Mirror 3). The 2x2 optical switch allows up to three operation modes by

9 Direct Mode: NLC2 and NLC4 ON; Port 1 Port 3 & Port 2 Port 4. 10 Crossed Mode: No voltage is applied; Port 1 Port 4 & Port 2 Port 3. 11 Closed Mode: NLC1 and NLC3 ON; Port 1 Port 2 & Port 3 Port 4.

1

12

LC1

LC4

power control (Fig. 10.b).

Plate Plate

Mirror 1 Mirror 3

LC2

Port 2

Mirror 2

Port 1

Plate

14

1

10

11

2 **Fig.** 7**.** Structure of the 1x2 LC optical switch.

8 applying voltage to the suitable pair of TN-LC cells, see **Fig. 9:**

400 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

13 **Fig. 8.** Structure of the polarization independent 2x2 LC optical switch.

14 Optical Fiber

Mirror 1 Mirror 3

LC4

4 Twisted nematic liquid crystals (TN-LCs) can also be used in routers (LC−OR) based on the 5 polarization diversity method following the same principle of operation as in the case of 6 optical multiplexers. The polarization modulation of a TN cell in combination with space 7 polarization selective calcite crystals or polarization beam splitters (PBS) allows optical 8 space-switching. Fig. 10 shows the structures of a typical 1x2 nematic LC−OR (Fig. 10.a) and

TN-LCs can also be used in routers (LC−OR) based on the polarization diversity method following the same principle of operation as in the case of optical multiplexers. The polarization modulation of a TN cell in combination with space polarization selective calcite crystals or polarization beam splitters (PBS) allows optical space-switching. Fig. 10 shows the structures of a typical 1x2 nematic LC−OR (Fig. 10.a) and a 1x2 nematic LC−OR with independent output

12 Fig. 4. a) Structure of a typical 1x2 nematic LC−OR based on polarization diversity and, b)

14 In the scheme shown in Fig. 10.a, the TN-LC 1 has an input polarizer that changes the 15 polarization state of the transmitted beam depending on the voltage V1. The scheme of Fig. 16 10.b additionally provides the possibility of stabilizing the optical power that is transmitted 17 by each port. This feature was reported in [62]. In that scheme, TN-LC 2 and 3 have both an 18 input and an output crossed polarizer, with the input polarizer parallel to the respective 19 polarization component transmitted by the PBS. Then, in this scheme each LC cell controls 20 the transmitted power depending on the voltages V2 and V3. Fig. 11 shows an example of

Plate Plate

Port 4

Mirror 4

LC3

LC1

LC4

Plate Plate

Mirror 1 Mirror 3

LC2

Port 2

Mirror 2

Port 4

Mirror 4

LC3

Port 3

Plate

Port 1

Plate

Port 3

Plate

**Figure 8.** Structure of the polarization independent 2x2 LC optical switch.

Port 4 LC1

Port 2

Mirror 2

**Figure 9.** Operation modes of the 2x2 optical switch: (a) Crossed, (b) Direct, (c) Closed.

9 a 1x2 nematic LC−OR with independent output power control (Fig. 10.b).

13 structure of a 1x2 nematic LC−OR with independent output power control.

21 the power stabilization capacity of the router reported in [62].

2 Fig. 9. Operation modes of the 2x2 optical switch: (a) Crossed, (b) Direct, (c) Closed.

Port 1

Plate

Mirror 4

3 3.2. Optical router with output power control

**3.2. Optical router with output power control**

LC3

Port 3

Plate

13

**Figure 10.** a) Structure of a typical 1x2 nematic LC−OR based on polarization diversity and, b) structure of a 1x2 nemat‐ ic LC−OR with independent output power control.

In the scheme shown in Fig. 10.a, the TN-LC 1 has an input polarizer that changes the polari‐ zation state of the transmitted beam depending on the voltage *V1*. The scheme of Fig. 10.b additionally provides the possibility of stabilizing the optical power that is transmitted by each port. This feature was reported in [62]. In that scheme, TN-LC 2 and 3 have both an input and an output crossed polarizer, with the input polarizer parallel to the respective polarization component transmitted by the PBS. Then, in this scheme each LC cell controls the transmitted power depending on the voltages *V2* and *V3*. Fig. 11 shows an example of the power stabilization capacity of the router reported in [62].

**Figure 11.** Example of the output power control capacity of a 1x2 LC-OR based in polarization diversity. Input power is obtained from a LED source at 650 nm.

LC-OR based on nematic LC cells cannot respond faster than several microseconds. This fact limits its use to telecom and sensor applications for protection and recovery, or optical add/ drop multiplexing which need fewer restrictions about switching time, like WDM transport network restoration [63]. However, in the last years, nematic LCs with response times lower than 3 ms [64] and 2 ms [65], as well as nanosecond response [66] have appeared. And different techniques to reduce the response time below 1 ms [67] have also been reported.

#### **3.3. Broadband LC-OR**

It is a matter of fact that the performance of a twisted nematic LC-OR is optimum only for specific wavelengths (those given by Mauguin Minima) [68]. Besides, the LC birefringence, which defines Mauguin Minima, is very temperature dependent, requiring temperature compensated designs or controllers. These two limitations can be overcome by replacing the twisted nematic cells (see Fig. 10.a) with optimized polarization rotators (PRs) based on structures of stacked LC cells, as reported in [69]. Figure 12 shows an example of the perform‐ ance of the broadband 1×2 LC–OR reported in [69].

**Figure 12.** Scheme of a broadband 1×2 LC–OR for POF networks.

**Figure 13.** Spectral performance of the outputs 1 (dashed lines) and 2 (solid lines) of a broadband 1×2 LC–OR for POF networks in the a) OFF state (*V*<<*Vth*) and b) ON state (*V*>>*Vth*).

The proposed design illustrated above is composed by 3 LC cells and allows a significant improvement of the spectral response of LC optical routers, compared to those previously reported [43, 62, 70] in a broadband range. The proposed router has quite similar insertion loss values in both outputs in the range from 400 nm to 700 nm, as well as crosstalk values lower than –18.7 dB, as shown in Fig. 13. This performance is required for routing channels in SI– POF–WDM networks uniformly, since in these networks the channels may have wide bandwidths and the proposed grid is very wide, as it was aforementioned. In [69] it has been shown that the router performance is quite constant with temperature changes of up to 10 °C. And it was also demonstrated that it is able to control the split ratio of the output power with good uniformity in the range from 400 nm to 700 nm.

### **3.4. LC wavelength selective switch**

than 3 ms [64] and 2 ms [65], as well as nanosecond response [66] have appeared. And different

It is a matter of fact that the performance of a twisted nematic LC-OR is optimum only for specific wavelengths (those given by Mauguin Minima) [68]. Besides, the LC birefringence, which defines Mauguin Minima, is very temperature dependent, requiring temperature compensated designs or controllers. These two limitations can be overcome by replacing the twisted nematic cells (see Fig. 10.a) with optimized polarization rotators (PRs) based on structures of stacked LC cells, as reported in [69]. Figure 12 shows an example of the perform‐

**Figure 13.** Spectral performance of the outputs 1 (dashed lines) and 2 (solid lines) of a broadband 1×2 LC–OR for POF

The proposed design illustrated above is composed by 3 LC cells and allows a significant improvement of the spectral response of LC optical routers, compared to those previously

techniques to reduce the response time below 1 ms [67] have also been reported.

402 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**3.3. Broadband LC-OR**

ance of the broadband 1×2 LC–OR reported in [69].

**Figure 12.** Scheme of a broadband 1×2 LC–OR for POF networks.

networks in the a) OFF state (*V*<<*Vth*) and b) ON state (*V*>>*Vth*).

A 1×M wavelength selective switch (WSS) is an optical device that allows switching any incoming wavelength from its input port to any of the *M* output ports, without the need for optical to electrical conversions. These devices play a key role in protection and reconfiguration tasks of next generation optical networks. A huge number of approaches to implement WSS have been demonstrated. Some are based on gratings that spatially disperse the input channels, on MEMS, or on LC spatial light modulators [70, 71]. Other approaches use silica-based planar lightwave circuits (PLCs) [72] or ring resonators [73].

Some LC reconfigurable devices for the VIS range have been reported, such as tunable filters [74] or a multifunctional device operating as a switch/combiner/variable optical attenuator [43], as well as a 1x2 WSS [74]. The latter is based on an inverted Lyot filter structure. This configuration allows demultiplexing, switching or blocking any channel through any output port using voltages from 0 to 3 VRMS, the same for all the LC cells, with maximum insertion loss of 6 dB, and rejection ratios better than 12 dB. Fig. 14 shows two examples of the eight possible transmission states of the 1x2 LC WSS reported in [74].

**Figure 14.** Performance example of a 1x2 LC WSS considering 2 LED channels at 589nm (solid lines) and 650nm (dash‐ ed lines) with 20 nm full width at half maximum. Where: a) output 1 and b) output 2. V1 = 1.18 VRMS and V2 = 0.21 VRMS.

## **4. Optical filters in POF technology**

Optical filters are basic components as part of routing devices for optical communications networks. Optical interleavers are filters that due to their periodicity: a) separate an incoming spectrum into two complementary set of periodic spectra (odd and even channels), or b) combine them into a composite spectrum. Filters and interleavers play a key role in dense wavelength division multiplexing (DWDM) systems, usually employed in gain equalization, dispersion compensation, prefiltering, and channels add/drop applications.

Literature provides many optical filtering and interleaving devices. Some filters are based on birefringent structures such as the Lyot and Solc filters due to their low dispersion, high reliability, easy fabrication process and low cost [75], with recent applications in VIS for POF networks [69, 74]. However, these solutions mainly operate in DWDM systems (infrared range). In this section, the basic structures of birefringent filters (Lyot and Solc) are presented and compared against birefringent filters designed in the Z transform domain, which can be used in future WDM-POF systems.

#### **4.1. Birefringent filters**

Birefringent filters base their operation on the interference of an input light beam with multiple delayed versions of itself. Typically there are two types of birefringent filters, the Lyot and Solc. An excellent discussion of both types of filters can be found in [76]. In general, a Lyot filter consists of a set of delay stages composed by retarder plates of different widths between polarizers, as the 3-stage Lyot filter shown in Fig. 15.a. The optical axis of the retarder plates are at 45º with respect to the polarizers (azimuth angle) and each stage has twice the delay (Γ) of the previous one. Focusing on the second approach, Solc filters eliminate the need of Lyot filters for using multiple polarizers. Solc filters consist of a stack of M retarder plates between only two linear polarizers. In this case, all the retarder plates have the same delay and each one are at a specific azimuth angles, α1,... αM, e.g. a fan Solc filter has parallel polarizers (αA = 0º) and α1 = α, α2 = 3α, α3 = 5α,..., αM = (2M-1)α, being α = 45º/M, see Fig. 15.b.

**Figure 15.** General structures of: a) Lyot filter of 3 stages and b) Solc Filters

Now, let us compare Lyot and Solc filters. For example, 3-stage Lyot filters require 4 polarizers and the equivalent of 7 retarder plates with delays Γ, as shown in Fig. 15.a. In contrast, a Solc filter with the same number of retarder plates requires only 2 polarizers. From this point of view Solc filters are a more interesting choice than Lyot filters. However, as is shown in Fig. 16, the adjacent side lobes suppression is better for the case of Lyot filters.

**4. Optical filters in POF technology**

used in future WDM-POF systems.

**4.1. Birefringent filters**

Optical filters are basic components as part of routing devices for optical communications networks. Optical interleavers are filters that due to their periodicity: a) separate an incoming spectrum into two complementary set of periodic spectra (odd and even channels), or b) combine them into a composite spectrum. Filters and interleavers play a key role in dense wavelength division multiplexing (DWDM) systems, usually employed in gain equalization,

Literature provides many optical filtering and interleaving devices. Some filters are based on birefringent structures such as the Lyot and Solc filters due to their low dispersion, high reliability, easy fabrication process and low cost [75], with recent applications in VIS for POF networks [69, 74]. However, these solutions mainly operate in DWDM systems (infrared range). In this section, the basic structures of birefringent filters (Lyot and Solc) are presented and compared against birefringent filters designed in the Z transform domain, which can be

Birefringent filters base their operation on the interference of an input light beam with multiple delayed versions of itself. Typically there are two types of birefringent filters, the Lyot and Solc. An excellent discussion of both types of filters can be found in [76]. In general, a Lyot filter consists of a set of delay stages composed by retarder plates of different widths between polarizers, as the 3-stage Lyot filter shown in Fig. 15.a. The optical axis of the retarder plates are at 45º with respect to the polarizers (azimuth angle) and each stage has twice the delay (Γ) of the previous one. Focusing on the second approach, Solc filters eliminate the need of Lyot filters for using multiple polarizers. Solc filters consist of a stack of M retarder plates between only two linear polarizers. In this case, all the retarder plates have the same delay and each one are at a specific azimuth angles, α1,... αM, e.g. a fan Solc filter has parallel polarizers (αA =

dispersion compensation, prefiltering, and channels add/drop applications.

404 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

0º) and α1 = α, α2 = 3α, α3 = 5α,..., αM = (2M-1)α, being α = 45º/M, see Fig. 15.b.

**Figure 15.** General structures of: a) Lyot filter of 3 stages and b) Solc Filters

However, the potential of the structure of Solc filters can be exploited by placing the retarder plates illustrated in Fig. 15.b at arbitrary azimuth angles, in a type of filters called lattice or birefringent filters. Birefringent filters can be designed by using optimization methods or in the Z-transform domain, by using their relation with FIR (Finite Impulse Response) filters. A detailed method for transform FIR filters into birefringent filters can be found in [75]. For example, an arbitrary birefringent filter of seven retarder plates, obtained from a 7th order FIR filter, is presented in Fig. 16. The azimuth angles of the retarder plates are: α1 = 6.07º, α<sup>2</sup> = 15.18º, α3 = 28.65º, α4 = 45.00º, α5 = 61.35º, α6 = 74.81º, α<sup>7</sup> = 83.92º and αA = 0º. This filter performs a uniform suppression of the adjacent side lobes with a maximum value better than both the Lyot and Solc filters. It could even be designed to have a narrow bandpass. Birefringent structures are a versatile solution for designing devices for POF WDM networks due to their reconfiguration capacity, since they can be easily manufactured with LC technology, and flexibility, since any FIR filters synthesis method can be used [75].

**Figure 16.** Transfer functions of different birefringent filters: 3 stages Lyot Filter (dash-dot line), Solc filter with 7 re‐ tarder plates (dashed line) and arbitrary birefringent filter of 7 retarder plates designed in the Z-transform domain (solid line). Retarder plates with Γ = 2π×1.98μm/λ are considered in all the filters.

It should be mentioned that demultiplexer devices may be easily built up by combining the switches and filter schemes described in former sections or by using filter plus the addition of POF splitting devices.
