**1. Introduction**

Coarse Wavelength Division Multiplexing (CWDM) technologies are being widely deployed internationally in metropolitan and access networks due to the increased demand for delivering more bandwidth to the subscriber, created by the need of enhanced services, (Koonen, 2006). For metro, and mainly for access networks applications, an increment in capacity may be achieved with a cost-effective multiplexing technology without the need for the high channel counts and closely spaced wavelengths typically used in long haul networks. A channel space of 20 nm, as proposed in the G. 694.2 ITU Rec., can be used relaxing the processing tolerances and potentially lowering the cost of components. CWDM technology reaches those requirements and it has been proposed for these applications. It is in this context that holographic optical devices have a potential use.

This chapter describes the theory, design, and experimental results of a generic multipurpose device that can operate as a tunable wavelength filter, wavelength multiplexer and wavelength router. This device could be especially useful in optical network applications based on both Coarse and Dense Wavelength Division Multiplexing technology (CWDM/DWDM). The enabling component is a Ferro-electric Liquid Crystal (FLC) Spatial Light Modulator (SLM) in which dynamic holograms are implemented in real time. As a consequence, the device will be able to carry out different functions according to the hologram recorded on the SLM. The great advantage of this device is polarization insensitivity in the region of operation, allowing low cross-talk and simple handling. As hologram management is the basis for this device, some topics in the Computer Generated Hologram (CGH) design process are commented on and general guidelines are also considered.

Laboratory experiments have demonstrated the capability of a phase FLC-SLM, with the great advantage of polarization insensitivity operation, to diffract the incident light according its wavelength and hologram patterns, for the use in the former applications.

Two typical applications of this technology are described: the first one is a design of an equalized holographic Reconfigurable Optical Add-Drop Multiplexer ( ROADM), where this device can address several wavelengths at the input to different output fibers, according to the holograms stored in a SLM (Spatial Light Modulator), all the outputs being equalized in power; the second one is dealing with the design of an holographic router with loss compensation and wavelength conversion whose main application is in Metro networks in the interconnection nodes. This device uses a SOA (Semiconductor Optical Amplifier), in the non linear region, to do the wavelength conversion and, in addition, to supply the gain in order to compensate for the intrinsic losses of the holographic device.

Application of Holograms in WDM Components for Optical Fiber Systems 259

A

m = +1

2A

**Target Result** 

m = 0

<sup>η</sup>4-phases = 2η <sup>≈</sup> 80%

m = +1

A

m = -1

m = 0

η2-phases ≈ 40%

d = d2-phases

2-phases

white & black bars

4-phases

white, black & grey bars

Fig. 1. two/four-phases bars holograms

**Target Result** 

d = d4-phases

λ.R/2d

λ.R/d

**Phase Holograms**

hologram diffraction

R

R

Fig 1 shows a bars hologram for 2 and 4-phases and their diffraction target in a far field approach. As we can see, the main difference in the holograms is the grey bars in the 4 phases holograms; in this case there is a white bar, a black bar and two different grey bars for addressing the 4-phases (π/4, 3π/4, -3π/4, -π/4); with regard to the diffraction target.

**2-phases 85% eff 4-phases 85% eff**

In Fig. 2 examples of calculated holograms are shown. The program calculates the inverse Fourier transform (F.T.)-1 of the diffraction target (result) by an annealing optimization algorithm. In this case both holograms have a calculated efficiency of 85% and the grey bars are clearly visible in the figure. In the following Section some guidelines about design of

Fig. 2. Examples of 2/4-phases holograms and diffraction targets

holograms by computer are given.

**Hologram**

Another characteristic is the loss of the symmetry for the diffraction orders.
