**2. Optical multiplexers and demultiplexers in POF technology**

Multiplexers, combiners and variable optical attenuators are basic elements in POF networks when using the WDM approach but are not widely spread yet on the market due to the aforementioned reasons yielding their associated insertion losses. Nevertheless, reconfigura‐ tion can be an additional feature for those networks but most of them developed in POF technology do not provide such a characteristic. In this section, novel POF devices with reconfigurable characteristics for WDM applications addressing compact, scalable and low consumption solutions and with low insertion losses will be described. They operate at the wavelengths of interest for POF applications and their performance will be compared to current state-of-the-art approaches reported in literature. Some of them will take advantage of the properties of liquid crystal materials.

Several technologies have been reported for implementing optical multiplexers. Arrayed Waveguides Gratings (AWG), based on two multimode interference sections joined through several waveguides of different lengths, are proposed to be used in short distance communi‐ cations [24]. However, this approach is unsuitable for the wavelength range of operation of POF networks with affordable losses and, therefore, cannot be used. On the other hand, in the WDM approach many transmitters with different light colors can carry individual information. In WDM-POF systems, most multiplexers used in POF links are based on N:1 splitting devices which their function is to combine the optical signals from multiple different single-wave‐ length end devices at their inputs onto a single output fiber. For example, red light can be modulated with Ethernet data while blue, green and yellow light can carry image information, radio frequency (RF) and television signal, respectively. There have been many techniques of fabricating POF couplers. These techniques include twisting and fusion, side polishing, chemical etching, cutting and gluing, thermal deformation, molding, biconical body and reflective body [25]. The main drawback of the use of POF couplers as multiplexers are: a) their high associated insertion losses, typically up to 8dB per branch [26] if we consider 3:1 and 4:1 POF couplers; and b) in this kind of multiplexers input ports are not interchangeable and each input port must be excited by a pre-allocated wavelength source. To solve this latter disad‐ vantage, another approaches make use of novel reconfigurable POF multiplexing devices, where inputs are wavelength independent, as they work in the same way for different wavelengths, thus allowing more flexibility in WDM-POF networks. They will be described in the following section.

From the POF demultiplexer perspective, solutions for WDM SI-POF networks reported in literature are based on bulk optics and take advantage of discrete devices such as prisms [27], thin-film filters [28] or diffraction gratings [29, 30]. Thin-film based demultiplexers are easy to implement and are a good choice to design demuxes with low insertion loss and multiple channels. However, they are large, require many elements (typically the number of elements doubles the number of channels) and their channel isolation (crosstalk) is mainly limited by the rejection ratio of the thin-film filters used. Optical filtering at each output channel is usually employed to enhance the crosstalk performance in this type of demultiplexers thus adding complexity into the system. In contrast, prism-based demultiplexers have few elements and are cheaper but usually show a low performance in terms of both insertion loss and crosstalk. Most common proposals are based on concave gratings. These proposals have good expecta‐ tions as they have a small size and because the light spatial separation and its focusing are performed with a single element. However, they require diffractive elements that to date are not easy to manufacture and have not reached large market volumes yet, being a costly solution. Moreover their experimental performance has not yet been tested on a mass basis real scenario. However, it is expected to experience the price reductions accompanying economy-of-scale in a near future.

#### **2.1. Designs of reconfigurable optical multiplexers**

is their costly production, which makes them unsuitable for today's price sensitive mass markets. The underlying reason behind this lack of development is the mismatch between the optimum operating wavelength regions of POFs and the optical devices exploited for tele‐ communications purposes. The latter are developed for a wavelength region (C- and L-bands) totally unsuitable for POF-based transmission over medium-distances (hundreds of meters or greater) due to the high attenuation of PMMA based POF of around 1dB/cm@1550nm. A similar conclusion can be obtained for PF-GIPOFs which attenuation characteristics are not at par with that of standard silica based fibers, but still superior to that of copper based technol‐ ogies and PMMA-POF fibers. Another question to be addressed is the large POF dimensions and NA, which produce beams with high divergence thus being difficult to be routed. In addition, multiplexed systems operating in VIS range for POF networks may need reconfigu‐ ration because they do not have standard channels as well as provide flexibility in the networks

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

In this framework, this chapter is intended to be a progress report and it will focus on the stateof-the art, description and experimental validation of different POF-based key devices that provide an easy-reconfigurable performance for WDM applications. Novel multiplexers/ demultiplexers, variable optical attenuators, interleavers, switches and optical filters to separate and to route the different transmitted wavelengths are described. The main target is to bridge the gap of the WDM POF-based network deployment bottleneck in the final leg of delivering. In addition, a hybrid silica-POF WDM-PON network is analyzed showing the capabilities of novel Fiber Bragg Gratings (FBG) inscribed on microestructured POF devices to be compatible with WDM topologies for both sensing and communication schemes. Moreover, the theoretical capacity for a future WDM-GIPOF deployment is addressed taking advantage of the performance of this recent fiber type. Finally the main conclusions are

**2. Optical multiplexers and demultiplexers in POF technology**

of the properties of liquid crystal materials.

Multiplexers, combiners and variable optical attenuators are basic elements in POF networks when using the WDM approach but are not widely spread yet on the market due to the aforementioned reasons yielding their associated insertion losses. Nevertheless, reconfigura‐ tion can be an additional feature for those networks but most of them developed in POF technology do not provide such a characteristic. In this section, novel POF devices with reconfigurable characteristics for WDM applications addressing compact, scalable and low consumption solutions and with low insertion losses will be described. They operate at the wavelengths of interest for POF applications and their performance will be compared to current state-of-the-art approaches reported in literature. Some of them will take advantage

Several technologies have been reported for implementing optical multiplexers. Arrayed Waveguides Gratings (AWG), based on two multimode interference sections joined through several waveguides of different lengths, are proposed to be used in short distance communi‐

to be developed.

presented.

Among the different technologies used for implementing optical multiplexers (as well as for optical switches) those based on liquid crystals (LC) are very interesting because they do not have mobile parts, need low excitation voltages and have a low power consumption. In the last years, liquid crystal has been widely used in displays applications. Liquid crystals are organic compounds that have properties intermediates between liquid and crystalline solids [31]. They have anisotropic characteristics such as the dielectric constant or the refractive index, like solids, but simultaneously they are fluids. There are mainly two types of LC used in optical multiplexing, Ferroelectric Liquid Crystals (FLC) [32] and Nematic Liquid Crystal (NLC) [33-35], both normally using the structure of a Twisted Nematic Liquid Crystal cell (TN-LC). The first ones have a better response time but they can operate in a smaller wavelength range. The second ones have worst response times (tens of milliseconds in conventional mixtures), but they can operate in a wider wavelength range because they only have to fulfill Mauguin's regime *Δn x d/λ >> 1* in order to obtain the polarization shift, where *Δn* is the birefringence of the LC, *d* the LC cell thickness and *λ* the wavelength, respectively.

In the following, different topologies of optical multiplexers based on liquid crystals are described. The first design is based on Polymer Dispersed Liquid Crystal (PDLC), which is a special case of NLC, while the other devices are based on TN-LC cells. A brief introduction about both LC types is provided within each section for a better understanding.

#### *2.1.1. Optical multiplexer and variable optical attenuator based on polymer dispersed liquid crystal*

The first design of the proposed multiplexers is based on PDLC. PDLC is composed by microdroplets with liquid crystal molecules dispersed in a polymeric matrix. Liquid Crystal molecules have electrical and optical birefringence, which means that the molecules have different dielectric constants and refractive indexes depending on the molecule axes. This mixture is sandwiched between glasses and covered with a transparent conductor [36]. In this way, an electric field can be applied to the mixture allowing the reorientation of the liquid crystal molecules that are inside the microdroplets thanks to the molecule birefringence. The structure of the PDLC cell is shown in Fig. 1.

**Figure 1.** Structure of a Polymer Dispersed Liquid Crystal cell

The principle of operation is the following. At resting state, i.e. there is no voltage applied to the transparent conductors (electrodes), the liquid crystal molecules inside the droplets do not have a predominant orientation and the light that passes through the mixture find different refractive indexes. Therefore, it is highly scattered into different directions. On the other hand, if enough voltage is applied between the transparent conductors, an electric field is created inside the mixture and the liquid crystal molecules are forced to follow the induced electric field. In this case, the mixture has a homogeneous refractive index and the light that passes through the mixture is not refracted and maintains the same propagation direction. In addition to this, there is a gradual transition in the liquid crystal molecules reorientation, thus, if the voltage applied is not high enough, the molecules are not fully oriented but the light that passes through the mixture is less scattered [37].

[31]. They have anisotropic characteristics such as the dielectric constant or the refractive index, like solids, but simultaneously they are fluids. There are mainly two types of LC used in optical multiplexing, Ferroelectric Liquid Crystals (FLC) [32] and Nematic Liquid Crystal (NLC) [33-35], both normally using the structure of a Twisted Nematic Liquid Crystal cell (TN-LC). The first ones have a better response time but they can operate in a smaller wavelength range. The second ones have worst response times (tens of milliseconds in conventional mixtures), but they can operate in a wider wavelength range because they only have to fulfill Mauguin's regime *Δn x d/λ >> 1* in order to obtain the polarization shift, where *Δn* is the birefringence of

In the following, different topologies of optical multiplexers based on liquid crystals are described. The first design is based on Polymer Dispersed Liquid Crystal (PDLC), which is a special case of NLC, while the other devices are based on TN-LC cells. A brief introduction

*2.1.1. Optical multiplexer and variable optical attenuator based on polymer dispersed liquid crystal*

The first design of the proposed multiplexers is based on PDLC. PDLC is composed by microdroplets with liquid crystal molecules dispersed in a polymeric matrix. Liquid Crystal molecules have electrical and optical birefringence, which means that the molecules have different dielectric constants and refractive indexes depending on the molecule axes. This mixture is sandwiched between glasses and covered with a transparent conductor [36]. In this way, an electric field can be applied to the mixture allowing the reorientation of the liquid crystal molecules that are inside the microdroplets thanks to the molecule birefringence. The

about both LC types is provided within each section for a better understanding.

the LC, *d* the LC cell thickness and *λ* the wavelength, respectively.

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

structure of the PDLC cell is shown in Fig. 1.

**Figure 1.** Structure of a Polymer Dispersed Liquid Crystal cell

Due to the possibility for controlling the transmission of light through the PDLC, the latter have been mainly reported for implementing Variable Optical Attenuators (VOA) [38, 39]. A VOA based on a 2 x 2 coupler made of POF is presented in [38]. The idea for implementing a reconfigurable optical multiplexer based on PDLC is to use the PDLC cell with several pixels as the active element [40]. The structure is shown in Fig. 2. The input ports, that are optical fibers, are placed in front of each pixel of the PDLC cell. The light that comes out from the fiber is spreaded according to the numerical aperture of the optical fiber, thus, the lenses placed in front of each input port are required for collimating the light that comes out from the optical fibers. The light from each input port passes through one pixel of the PDLC cell and finally, the lens placed at the output focuses the light into the output fiber. 7

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

2 Fig. 2. Structure of the reconfigurable optical multiplexer based on PDLC **Figure 2.** Structure of the reconfigurable optical multiplexer based on PDLC

14 the order of tenths of milliseconds.

1

4 that comes from the input port is scattered in the PDLC cell and, therefore, it is not focused 5 in the output port. On the contrary, when enough voltage is applied to the PDLC cell, the 6 light can pass through the PDLC cell being focused on the output port. In addition, variable 7 attenuation can be achieved by applying intermediates voltages. Each pixel can be 8 addressed independently from the adjacent ones, so each input port can be switched on/off 9 without affecting the others. 10 The introduced reconfigurable optical multiplexer can also acts as a variable optical 11 attenuator. It can operate in the visible range as POF do. The typical voltage value for By using the proposed structure, when no voltage is applied to the PDLC pixel, the light that comes from the input port is scattered in the PDLC cell and, therefore, it is not focused in the output port. On the contrary, when enough voltage is applied to the PDLC cell, the light can pass through the PDLC cell being focused on the output port. In addition, variable attenuation can be achieved by applying intermediates voltages. Each pixel can be addressed independ‐ ently from the adjacent ones, so each input port can be switched on/off without affecting the others.

3 By using the proposed structure, when no voltage is applied to the PDLC pixel, the light

12 switching the PDLC is tenths of volts. The achieved insertion loss is about 1.6dB, the 13 crosstalk obtained is in the range of 30dB, and finally, the response time of the PDLC is in

16 Other way in which liquid crystal is used is known as Twisted nematic liquid crystal (TN-17 LC). In this kind of devices, the liquid crystal is also sandwiched between two glasses 18 covered with a transparent conductor (electrode). However, the glasses have an additional 19 rubbed alignment film that forces the LC molecules to have an orientation. In a TN-LC the 20 orientation of the LC molecules in one glass is perpendicular to the molecular orientation in 21 the other glass. Thanks to these constraints, the LC molecules inside the cell perform a helix

15 2.1.2. Optical Multiplexers based on twisted nematic liquid crystals (TN-LC)

22 from one glass to the other. The structure of a TN-LC cell is shown in Fig. 3.

The introduced reconfigurable optical multiplexer can also acts as a variable optical attenuator. It can operate in the visible range as POF do. The typical voltage value for switching the PDLC is tenths of volts. The achieved insertion loss is about 1.6dB, the crosstalk obtained is in the range of 30dB, and finally, the response time of the PDLC is in the order of tenths of millisec‐ onds.

#### *2.1.2. Optical multiplexers based on Twisted Nematic Liquid Crystals (TN-LC)*

Other way in which liquid crystal is used is known as TN-LC. In this kind of devices, the liquid crystal is also sandwiched between two glasses covered with a transparent conductor (elec‐ trode). However, the glasses have an additional rubbed alignment film that forces the LC molecules to have an orientation. In a TN-LC the orientation of the LC molecules in one glass is perpendicular to the molecular orientation in the other glass. Thanks to these constraints, the LC molecules inside the cell perform a helix from one glass to the other. The structure of a TN-LC cell is shown in Fig. 3.

**Figure 3.** Structure of a twisted nematic liquid crystal cell.

The principle of operation is the following. If no voltage is applied to the LC, the polarization of the light that passes through the TN-LC cell is ideally rotated 90 degrees. On the other hand, when enough voltage is applied between the transparent conductors of each glass, an electric field is generated inside the cell, and the molecules are reoriented to be perpendicular to the glasses. In this scenario, the polarization of the incident light remains when passes through the LC.

In this way, the polarization of the light that transverse the TN-LC cell can be controlled. Thus, by placing the TN-LC cell between polarizers, the transmission of the incident light can be modified by means of the voltage applied to the cell. A polarizer placed before the TN-LC cell allows passing only one polarization of the incident light. In this way, a polarized light beam enters in the TN-LC cell. The TN-LC controls its polarization stage depending on the voltage applied to the cell. Finally, the polarizer placed after the TN-LC filters, or not, the light that comes out from the TN-LC cell. According to the TN-LC operation, there are two ways of implementing the device for controlling the light transmission: a) putting the TN-LC cell between crossed polarizers, or b) putting it between parallel polarizers. In the first case, the light passes through the device when there is no voltage applied to TN-LC while light is stopped when enough voltage is applied to the LC cell. On the contrary, the input light is blocked by the device when no voltage is applied to the TN-LC cell and the light passes through the device when enough voltage is applied to the TN-LC cell. The procedure described has been mainly used in displays applications [41], but it can also been used for optical multiplex‐ ing as well as for optical switching. The latter will be seen in a following section.

As previously reported, multiplexers and demultiplexers are basic elements in those optical networks where WDM is implemented because they combine different wavelengths in a single fiber. POF fiber has a low attenuation in the visible wavelength region (at 450nm, 550nm and 650nm), for this reason, the optical multiplexers must work in this wavelength range. An example of a reconfigurable multiplexer based on TN-LC cells is presented in Fig. 4 [42]. The introduced structure of the multiplexer can be used in several wavelength ranges depending on the bulk elements used.

**Figure 4.** Structure of the reconfigurable 3x1 optical multiplexer.

The introduced reconfigurable optical multiplexer can also acts as a variable optical attenuator. It can operate in the visible range as POF do. The typical voltage value for switching the PDLC is tenths of volts. The achieved insertion loss is about 1.6dB, the crosstalk obtained is in the range of 30dB, and finally, the response time of the PDLC is in the order of tenths of millisec‐

Other way in which liquid crystal is used is known as TN-LC. In this kind of devices, the liquid crystal is also sandwiched between two glasses covered with a transparent conductor (elec‐ trode). However, the glasses have an additional rubbed alignment film that forces the LC molecules to have an orientation. In a TN-LC the orientation of the LC molecules in one glass is perpendicular to the molecular orientation in the other glass. Thanks to these constraints, the LC molecules inside the cell perform a helix from one glass to the other. The structure of

*2.1.2. Optical multiplexers based on Twisted Nematic Liquid Crystals (TN-LC)*

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

Alignment layer

**<sup>V</sup>** Glass

Glass

The principle of operation is the following. If no voltage is applied to the LC, the polarization of the light that passes through the TN-LC cell is ideally rotated 90 degrees. On the other hand, when enough voltage is applied between the transparent conductors of each glass, an electric field is generated inside the cell, and the molecules are reoriented to be perpendicular to the glasses. In this scenario, the polarization of the incident light remains when passes through

In this way, the polarization of the light that transverse the TN-LC cell can be controlled. Thus, by placing the TN-LC cell between polarizers, the transmission of the incident light can be modified by means of the voltage applied to the cell. A polarizer placed before the TN-LC cell

Transparent Electrode

**Figure 3.** Structure of a twisted nematic liquid crystal cell.

LC

onds.

the LC.

a TN-LC cell is shown in Fig. 3.

The structure is composed by Polarizing Beam Splitters (PBS1 and PBS2), TN-LC cells (NLCa and NLCb), polarizers and lenses. There are three input ports, and a single output port. Each TN-LC cell has three pixels, and each pixel controls the transmission of one input port. 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 focuses each beam into the output port.

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 to the output port.

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

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

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 two possible outputs.

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 if required.

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 pair of inputs and the two outputs.

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 input and its two outputs.

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.

**Figure 5.** Structure of the Advanced Multifunctional Optical Multiplexer. It design also allows switching functionality if required into a single device.
