**1. Introduction**

The processing of radio frequency (RF) and microwave signals in the optical domain is an attractive approach to overcome the bottlenecks of bandwidth, power loss, and electromagnetic interference (EMI) encountered in conventional electronic signal processing systems. A wide range of emerging RF signal processing applications require specifically high resolution, ultra-wide bandwidth, wide-range tunability, and fast reconfigurability. While these requirements are difficult to achieve using conventional all-electronic processing, they are feasible with photonics-based signal processing.

Holography is an historic technology that allows the light scattered from an object to be recorded and reconstructed so that the object can reappear when a reference optical beam illuminates a hologram used to record that object. Holography has a wide range of applications such as optical signal storage and retrieval and information processing. The application of reconfigurable phase holograms to realize optical beam steering is an attractive field for either optical engineering or fundamental research. The reconfigurable phase holograms are calculated from the targeted beam steering pattern and implemented using spatial light modulator. Opto-VLSI processors are one of these devices that can dynamically generate phase holograms and perform optical beam steering.

Although full-electronic RF signal processing is very flexible and controllable, it is experiencing the bottlenecks of bandwidth and EMI. Processing microwave signals in the photonic domain can overcome the bottlenecks in the electronic signal processing. However the current technologies of microwave photonics have limited flexibility and reconfigurability. The Opto-VLSI technology is a novel and potential discipline that combine benefits of photonic devices and the intelligence plus processing capabilities of Very-Large-Scale Integrated (VLSI) circuits. It integrates intelligence into photonic systems providing a new foundation for a future information processing and communication systems and networks. This book chapter will discuss a new methodology of expanding the use of Opto-VLSI from the conventional paradigm of optical beam processing to a new frontier of

Photonic Microwave Signal Processing Based on Opto-VLSI Technology 361

<sup>2</sup> sinc *<sup>n</sup> M* π

where *n gM* = + 1 is the diffraction order ( 1 *n* = is the desired order), and *g* is an integer. Thus an Opto-VLSI processor with binary phase levels can have a maximum diffraction efficiency of 40.5%, while a four phase level allow for efficiency up to 81%. The higher diffraction orders (which correspond to the cases g ≠ 0) are usually unwanted crosstalk signals, which must be attenuated or properly routed outside the output ports to maintain

<sup>=</sup> (2)

**Opto-VLSI Processor**

**Address Decoder**

**Data Decoder**

η

Mirror

Adaptive optical beam steering can be achieved by reconfiguring the phase hologram uploaded onto the Opto-VLSI processor. Recent advances in low-switching-voltage nematic LC materials and layer thickness control have allowed the incorporation of a thin quarterwave-plate (QWP) layer between the LC and the aluminium mirror to accomplish polarization-insensitive multi-phase-level Opto-VLSI processors (Manolis *et al.*, 2002), as shown in Fig. 1. In addition, with current 130nm VLSI fabrication processes, VLSI chips featuring 24mm×24mm active area, maximum switching voltage of 3.0 volts, and pixel size of 5 microns, can be realised. Depositing low-switching-voltage electro-optic materials and QWP over such VLSI chips, can realize a polarization-insensitive Opto-VLSI processor that has a diffraction efficiency of 87% (0.6 dB loss) and a maximum steering angle of more than

There have been different algorithms reported for the optimization of Opto-VLSI phase holograms to achieve effective beam steering, including simulated annealing and projection

Fig. 1. Typical 8-phase Opto-VLSI processor and LC cell structure design.

Connection to Mirror

Glass

Silicon Substrate VLSI Layer

**Opto-VLSI Cell Structure**

high signal-to-crosstalk performance.

D0

D2

D1 MUX

8-phase Pixel Architecture

Memory elements

±4.0°.

LC Material

QWP

Aluminium Mirror

ITO

photonics-based RF signal processing, leading to a wide applications such as future broadband RF signal filters and beamformers for wireless systems for many wireless- and mobile-communication-related areas.

The application of holography in processing RF and microwave signals, which is implemented through an Opto-VLSI processor, offers many advantages over traditional microwave photonic methods in terms of flexibility and reconfigurability. The phase hologram is generated according to the applications of RF signal processing and uploaded onto the Opto-VLSI processor.

In this chapter, we discuss the use of holography for the applications of RF and microwave signal processing. This book chapter is organised as follows. In Section 2 a brief background on Opto-VLSI processors is provided. Section 3 discusses the use of holography to realize tunable microwave filters. In Section 4 the adaptive generation of true time delay (TTD) based on uploading appropriate phase hologram onto an Opto-VLSI processor is discussed. In Section 5 architecture of a tunable Opto-VLSI-based beamformer for phased array antennas is discussed. We conclude this chapter in Section 6.

### **2. Opto-VLSI processor**

An Opto-VLSI processor is an array of liquid crystal (LC) cells driven by a Very-Large-Scale-Integrated (VLSI) circuit. Figure 1 shows a typical layout and a cell design of an 8-phase Opto-VLSI processor. It is driven by digital holographic diffraction gratings capable of steering/shaping incident optical beams as illustrated in Fig. 2. The voltage level of each pixel can individually be controlled by using a few memory elements that select a discrete voltage level and apply it, through the aluminium mirror electrode, across the LC cell. A transparent Indium-Tin Oxide (ITO) layer is used as the second electrode, and a quarter-wave-plate (QWP) layer is deposited between the LC and the aluminum mirror to accomplish polarizationinsensitive operation. The ITO layer is generally grounded and a voltage is applied at the reflective electrode by the VLSI circuit below the LC layer. Opto-VLSI processors are electronically controlled, software-configured, polarization independent and are cost effective because of the high-volume manufacturing capability of VLSI chips as well as their capability of controlling multiple fiber ports in one compact Opto-VLSI module; they are also very reliable since beam steering is achieved with no mechanically moving parts.

By driving the Opto-VLSI with blazed gratings of different pitches, as shown in Fig. 3, optical beam steering can be achieved. The diffraction (or steering) angle for an Opto-VLSI processor, αm, is given by (Xiao *et al.*, 2008):

$$
\alpha\_m = \arcsin(\frac{m\mathcal{X}}{d})\tag{1}
$$

where m is the diffraction order (usually only the first order is considered), λ is the light wavelength in vacuum, and d is the grating period. By addressing each pixel independently a phase hologram can be synthesized leading to optical beam steering, beam shaping or multicasting. For example, a 4-phase Opto-VLSI processor having a pixel size of 5 microns can steer a 1550 nm laser beam by a maximum angle of around ±4°. The maximum diffraction efficiency of an Opto-VLSI processor depends on the number of discrete phase levels that the VLSI can accommodate. The theoretical maximum diffraction efficiency is given by (Dammann, 1979):

photonics-based RF signal processing, leading to a wide applications such as future broadband RF signal filters and beamformers for wireless systems for many wireless- and

The application of holography in processing RF and microwave signals, which is implemented through an Opto-VLSI processor, offers many advantages over traditional microwave photonic methods in terms of flexibility and reconfigurability. The phase hologram is generated according to the applications of RF signal processing and uploaded

In this chapter, we discuss the use of holography for the applications of RF and microwave signal processing. This book chapter is organised as follows. In Section 2 a brief background on Opto-VLSI processors is provided. Section 3 discusses the use of holography to realize tunable microwave filters. In Section 4 the adaptive generation of true time delay (TTD) based on uploading appropriate phase hologram onto an Opto-VLSI processor is discussed. In Section 5 architecture of a tunable Opto-VLSI-based beamformer for phased array

An Opto-VLSI processor is an array of liquid crystal (LC) cells driven by a Very-Large-Scale-Integrated (VLSI) circuit. Figure 1 shows a typical layout and a cell design of an 8-phase Opto-VLSI processor. It is driven by digital holographic diffraction gratings capable of steering/shaping incident optical beams as illustrated in Fig. 2. The voltage level of each pixel can individually be controlled by using a few memory elements that select a discrete voltage level and apply it, through the aluminium mirror electrode, across the LC cell. A transparent Indium-Tin Oxide (ITO) layer is used as the second electrode, and a quarter-wave-plate (QWP) layer is deposited between the LC and the aluminum mirror to accomplish polarizationinsensitive operation. The ITO layer is generally grounded and a voltage is applied at the reflective electrode by the VLSI circuit below the LC layer. Opto-VLSI processors are electronically controlled, software-configured, polarization independent and are cost effective because of the high-volume manufacturing capability of VLSI chips as well as their capability of controlling multiple fiber ports in one compact Opto-VLSI module; they are also very

By driving the Opto-VLSI with blazed gratings of different pitches, as shown in Fig. 3, optical beam steering can be achieved. The diffraction (or steering) angle for an Opto-VLSI

*<sup>m</sup>* arcsin( ) *<sup>m</sup>*

where m is the diffraction order (usually only the first order is considered), λ is the light wavelength in vacuum, and d is the grating period. By addressing each pixel independently a phase hologram can be synthesized leading to optical beam steering, beam shaping or multicasting. For example, a 4-phase Opto-VLSI processor having a pixel size of 5 microns can steer a 1550 nm laser beam by a maximum angle of around ±4°. The maximum diffraction efficiency of an Opto-VLSI processor depends on the number of discrete phase levels that the VLSI can accommodate. The theoretical maximum diffraction efficiency is

*d* λ

= (1)

mobile-communication-related areas.

antennas is discussed. We conclude this chapter in Section 6.

reliable since beam steering is achieved with no mechanically moving parts.

α

processor, αm, is given by (Xiao *et al.*, 2008):

given by (Dammann, 1979):

onto the Opto-VLSI processor.

**2. Opto-VLSI processor** 

$$
\eta = \text{sinc}^2 \left( \frac{\pi n}{M} \right) \tag{2}
$$

where *n gM* = + 1 is the diffraction order ( 1 *n* = is the desired order), and *g* is an integer. Thus an Opto-VLSI processor with binary phase levels can have a maximum diffraction efficiency of 40.5%, while a four phase level allow for efficiency up to 81%. The higher diffraction orders (which correspond to the cases g ≠ 0) are usually unwanted crosstalk signals, which must be attenuated or properly routed outside the output ports to maintain high signal-to-crosstalk performance.

Fig. 1. Typical 8-phase Opto-VLSI processor and LC cell structure design.

Adaptive optical beam steering can be achieved by reconfiguring the phase hologram uploaded onto the Opto-VLSI processor. Recent advances in low-switching-voltage nematic LC materials and layer thickness control have allowed the incorporation of a thin quarterwave-plate (QWP) layer between the LC and the aluminium mirror to accomplish polarization-insensitive multi-phase-level Opto-VLSI processors (Manolis *et al.*, 2002), as shown in Fig. 1. In addition, with current 130nm VLSI fabrication processes, VLSI chips featuring 24mm×24mm active area, maximum switching voltage of 3.0 volts, and pixel size of 5 microns, can be realised. Depositing low-switching-voltage electro-optic materials and QWP over such VLSI chips, can realize a polarization-insensitive Opto-VLSI processor that has a diffraction efficiency of 87% (0.6 dB loss) and a maximum steering angle of more than ±4.0°.

There have been different algorithms reported for the optimization of Opto-VLSI phase holograms to achieve effective beam steering, including simulated annealing and projection

Photonic Microwave Signal Processing Based on Opto-VLSI Technology 363

θ

Diffracted Beam

Opto-VLSI Processor

Steering Phase Hologram

M⋅d

θ = λ/(M⋅d)

α α

Photonics-based microwave and millimeter-wave filters offer advantages such as ultra-wide bandwidth, immunity to electromagnetic interference, and lightweight (Seeds&Williams, 2006; Capmany et al., 2006; Minasian, 2006). These advantages open new opportunities in a wide range of potential applications especially when high selectivity, resolution, wide

 In recent years, numerous reconfigurable coherent-free photonic microwave transversal filter structures have been proposed and demonstrated, where multi-wavelength source is employed to suppress the optical interference in conjunction with modifying the optical tap weights or the time-delay increment between taps (Capmany et al., 1999; Polo et al., 2003; Ortigosa-Blanch et al., 2006; Hunter&Nguyen, 2006; Ning et al., 2007; Blals&Yao, 2008). Spectral slicing of an RF-modulated broadband optical source has been employed to generate different wavebands. However, the use of Bragg gratings or arrayed waveguide gratings for realizing spectral slicing results in fixed time-delay increments, which limit the

Fig. 3. Steering and multicasting capabilities of an Opto-VLSI processor.

tunability, and fast reconfigurability characteristics are required.

Incident Beam

**3. Tunable photonic microwave filters** 

methods. In our study, a modified simulated annealing method that can achieve accurate beam steering with low crosstalk is adopted (Yen-Wei Chen *et al.*, 2000).

Fig. 2. Principle of beam steering through variable-pitch blazed grating generation.

methods. In our study, a modified simulated annealing method that can achieve accurate

beam steering with low crosstalk is adopted (Yen-Wei Chen *et al.*, 2000).

**α**

**Horizontal distance (pixels)**

• B

Blazed grating profile

Diffracted Beam

Opto-VLSI Processor

Fig. 2. Principle of beam steering through variable-pitch blazed grating generation.

(iii)

Pixel

Phase hologram

(i) (ii)

**Phase change**

> 2π

0

6π • A

• B

**Horizontal distance (pixels)**

4π

> Beam Steering Principle

**β**

**β**

Incident Beam

• A

0

**Phase change**

> 6π

4π

2π

Fig. 3. Steering and multicasting capabilities of an Opto-VLSI processor.
