**2. Spatial light modulator (SLM)**

Optical signal processing has been providing relevant solutions to convert data into spatially modulated coherent optical signals with SLM devices, allowing the effective implementation of digital holograms [25]. One of the most useful properties of the hologram is its ability to control phase and amplitude of light in the far field. The Fourier transform describes the relationship between a hologram (near field) and its corresponding replay field (far field). The far field can be formed at the focal point of a positive lens or an infinite distance from the near field plane in free space [25, 26], as depicted in **Figure 1**.

Holograms can reproduce waveforms from an existing object. With digital advances and optical signal processing, it is possible to numerically calculate interference patterns to generate completely synthetic wave fronts of arbitrary form. These interference patterns can have different denominations, such as CGH, diffractive optical elements (DOE), phase/amplitude masks, diffractive grating, etc. [26]. All operate in the principle of diffraction, so it is somehow an arbitrary choice of terminology.

The SLM is a device that can be used to modulate light in accordance with a fixed spatial (pixel) pattern and can be programed electrically. Usually, it can be exploited for incident light phase and/or amplitude control. Subsequently, phase-only, amplitude-only, or the combination of phase-amplitude can be readily realized with SLM. There are a number of modulation mechanisms that can be employed. One of the attractive and widely used ones is electro-optical SLM. The

#### **Figure 1.**

*Diagram of a Fourier transform through a positive lens. A complex design pattern (diffractive optical element (DOE)) is provided to the SLM to generate the expected hologram in the replay field (far field).*

modulation material of electro-optical SLM is liquid crystal. Similarly, a liquid crystal SLM has a microdisplay being employed for the incident light modulation and collection. This can be realized in a transmissive form using a liquid crystal display (LCD) SLM technology or in a reflective form with LCoS SLM technology. One of the leading features of the modulators is in the liquid crystal molecule alignment. Typically, this can be either vertical, parallel, or with twisted formation. Consequently, with suitable polarizing optics, this influences the incident light beam properties that can be effectively changed, i.e., amplitude, phase, or their combination [8, 23, 27].

It should be noted that the common hologram generation techniques cannot arbitrarily modulate the beam phase and amplitude concurrently [26, 27]. Therefore, it is unrealistic to basically address the desired pattern inverse Fourier transform into the far field and replicate the resulting amplitude and phase distribution directly on the SLM. As a consequence, the employment of optimization algorithms is highly recommended for evaluating the best potential hologram within the device constraints, e.g., the best pixel distribution in which each pixel will be able to take only one of two states that correspond to a 0 or π phase shift [23, 26].

The nematic LCoS technology is a type of SLM with phase-only modulation capability. Moreover, it is an electrically addressed reflection modulator category in which a direct and accurate voltage controls the liquid crystal and the light beam wave front can be modulated as well [28, 29]. An example of an LCoS SLM is illustrated in **Figure 2** [23]. The LCoS SLM can be employed as a diffractive device for reconstructing images from CGH [30].

CGH can be employed for different communication purposes and has been gaining application in indoor visible light communication systems [31]. Furthermore, suitable holograms can be readily generated by employing a variety of optimization techniques such as iterative Fourier transform algorithm (IFTA) [5, 32–34]; linear Fourier transform (i.e., linear phase mask) [5, 18, 23, 35]; simulated annealing [36]; and Gerchberg-Saxton algorithm [37]. The employment of the SLM as a diffractive device for reconstructing images from CGH permits the light beam wave front to be modulated [8, 23].

As aforementioned, LCoS displays have been gaining significant recognition as promising microdisplays for various types of SLM applications. Similarly, they possess attractive and significant features like very high spatial resolution and

**119**

performance [40].

*Spatial Light Modulation as a Flexible Platform for Optical Systems*

*LCoS SLM Pluto phase modulator from Holoeye© 2018 Holoeye photonics AG.*

light efficiency [38]. Due to this, they have been applicable in a plethora of optical contexts such as communication, reconfigurable interconnects [39], storage [40], diffractive optics [41], metrology [42], and quantum computing [43]. They are also applicable in the wave shaper technology for optical signal processing and monitoring [44]. In addition, other advantages of the LCoS are that it is highly cost-effective and can as well be flexibly programmed. This helps in supporting a number of additional functions like group delay ripple compensation, wavelength filtering, and chromatic dispersion compensation. Besides, it can aid in ensuring variable attenuation for individual wavelength channels as well as output ports. Consequently, LCoS device offers a cost-effective and promising solution for the

The LCoS microdisplay SLM has a good implementation history in the WSS systems. Its employment in the WSS system core component can be attributed to a number of advantages such as larger spatial bandwidth, more port availability, and enhanced resolution, as well as the system miniaturization. The WSS systems have been exploited in the reconfigurable optical add/drop multiplexers (ROADM) in WDM optical networks. It has been observed that ROADM is one of promising schemes that can be employed to improve on the traffic capacity of the existing and future telecommunication systems [40, 45]. Moreover, in communication networks, the ease of adding or dropping the wavelength is essential. They can ensure effective information access or rerouting to another appropriate path in the network. It should be noted that WSS is the ROADM sub-system that has been extensively employed in optical switch applications. In addition, microelectromechanical systems (MEMS) [46] and WSS based on LCoS [47] have been commercialized for different applications. Also, WSS by means of LCoS operates on the principle of "disperse and select," in which the inward bound WDM channels are dispersed into a distinct wavelength channel and subsequently relayed by LCoS through programmable grating patterns. This is in an attempt to facilitate an add and drop function. It is envisaged that the next-generation ROADM will hold different attractive features such as directionless, colorless, and contentionless in order to improve the system

Furthermore, LCoS technology can also be employed in flex grid that has been considered as the major feature for the next-generation networks [40, 48]. It should be noted that the traditional fixed grid with 50 GHz spacing standardized by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) possesses a number of challenges. The fixed grid has been observed to bring about the optical spectra being inefficiently used. Besides, it constrains the system transmission capacity considerably. On the other hand, the flex grid implementation enables the use of different modulation formats

*DOI: http://dx.doi.org/10.5772/intechopen.88216*

**Figure 2.**

wavelength selective switch (WSS) [40].

*Spatial Light Modulation as a Flexible Platform for Optical Systems DOI: http://dx.doi.org/10.5772/intechopen.88216*

*Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies*

modulation material of electro-optical SLM is liquid crystal. Similarly, a liquid crystal SLM has a microdisplay being employed for the incident light modulation and collection. This can be realized in a transmissive form using a liquid crystal display (LCD) SLM technology or in a reflective form with LCoS SLM technology. One of the leading features of the modulators is in the liquid crystal molecule alignment. Typically, this can be either vertical, parallel, or with twisted formation. Consequently, with suitable polarizing optics, this influences the incident light beam properties that can be effectively changed, i.e., amplitude, phase, or their

*(DOE)) is provided to the SLM to generate the expected hologram in the replay field (far field).*

*Diagram of a Fourier transform through a positive lens. A complex design pattern (diffractive optical element* 

It should be noted that the common hologram generation techniques cannot arbitrarily modulate the beam phase and amplitude concurrently [26, 27]. Therefore, it is unrealistic to basically address the desired pattern inverse Fourier transform into the far field and replicate the resulting amplitude and phase distribution directly on the SLM. As a consequence, the employment of optimization algorithms is highly recommended for evaluating the best potential hologram within the device constraints, e.g., the best pixel distribution in which each pixel will be able to take only one of two states that correspond to a 0 or π phase shift

The nematic LCoS technology is a type of SLM with phase-only modulation capability. Moreover, it is an electrically addressed reflection modulator category in which a direct and accurate voltage controls the liquid crystal and the light beam wave front can be modulated as well [28, 29]. An example of an LCoS SLM is illustrated in **Figure 2** [23]. The LCoS SLM can be employed as a diffractive device

CGH can be employed for different communication purposes and has been gaining application in indoor visible light communication systems [31]. Furthermore, suitable holograms can be readily generated by employing a variety of optimization techniques such as iterative Fourier transform algorithm (IFTA) [5, 32–34]; linear Fourier transform (i.e., linear phase mask) [5, 18, 23, 35]; simulated annealing [36]; and Gerchberg-Saxton algorithm [37]. The employment of the SLM as a diffractive device for reconstructing images from CGH permits the light beam wave front to be

As aforementioned, LCoS displays have been gaining significant recognition as promising microdisplays for various types of SLM applications. Similarly, they possess attractive and significant features like very high spatial resolution and

**118**

modulated [8, 23].

combination [8, 23, 27].

for reconstructing images from CGH [30].

[23, 26].

**Figure 1.**

**Figure 2.** *LCoS SLM Pluto phase modulator from Holoeye© 2018 Holoeye photonics AG.*

light efficiency [38]. Due to this, they have been applicable in a plethora of optical contexts such as communication, reconfigurable interconnects [39], storage [40], diffractive optics [41], metrology [42], and quantum computing [43]. They are also applicable in the wave shaper technology for optical signal processing and monitoring [44]. In addition, other advantages of the LCoS are that it is highly cost-effective and can as well be flexibly programmed. This helps in supporting a number of additional functions like group delay ripple compensation, wavelength filtering, and chromatic dispersion compensation. Besides, it can aid in ensuring variable attenuation for individual wavelength channels as well as output ports. Consequently, LCoS device offers a cost-effective and promising solution for the wavelength selective switch (WSS) [40].

The LCoS microdisplay SLM has a good implementation history in the WSS systems. Its employment in the WSS system core component can be attributed to a number of advantages such as larger spatial bandwidth, more port availability, and enhanced resolution, as well as the system miniaturization. The WSS systems have been exploited in the reconfigurable optical add/drop multiplexers (ROADM) in WDM optical networks. It has been observed that ROADM is one of promising schemes that can be employed to improve on the traffic capacity of the existing and future telecommunication systems [40, 45]. Moreover, in communication networks, the ease of adding or dropping the wavelength is essential. They can ensure effective information access or rerouting to another appropriate path in the network. It should be noted that WSS is the ROADM sub-system that has been extensively employed in optical switch applications. In addition, microelectromechanical systems (MEMS) [46] and WSS based on LCoS [47] have been commercialized for different applications. Also, WSS by means of LCoS operates on the principle of "disperse and select," in which the inward bound WDM channels are dispersed into a distinct wavelength channel and subsequently relayed by LCoS through programmable grating patterns. This is in an attempt to facilitate an add and drop function. It is envisaged that the next-generation ROADM will hold different attractive features such as directionless, colorless, and contentionless in order to improve the system performance [40].

Furthermore, LCoS technology can also be employed in flex grid that has been considered as the major feature for the next-generation networks [40, 48]. It should be noted that the traditional fixed grid with 50 GHz spacing standardized by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) possesses a number of challenges. The fixed grid has been observed to bring about the optical spectra being inefficiently used. Besides, it constrains the system transmission capacity considerably. On the other hand, the flex grid implementation enables the use of different modulation formats and their coexistence on a shared infrastructure. They can also be densely and efficiently multiplexed which aids the optical networks, not only to extend the reach but also the per channel bit rate. It has also been envisaged that implementation of WSS and SDM will significantly help further in extending the network reach and capacity [40].
