Application of spatial light modulators in optical communication field

In the optical fiber communication system with physical signals to control or change the amplitude, frequency, phase, polarization and other characteristics of the optical carrier parameters of the process is called optical modulation. The role of optical modulation is to allow information to use the characteristics of the light wave itself to achieve high-speed processing and transmission, and can effectively inhibit the interference of external electromagnetic fields, so that the propagation of information is more stable. With the wide application of dense wavelength division multiplexing (DWDM) technology and the huge growth of fiber optic transmission capacity, SDH technology has long been overburdened, based on the wavelength selective switch (WSS) as the third generation of multi-functional reconfigurable optical insertion and multiplexing (ROADM) as a key device to realize the next generation of dynamic all-optical network, in recent years by the optical communication field research institutions attach great importance to, and has been rapid development. 

Advantages of LCOS-based WSS

LCoS-based WSS has profoundly affected the design of ROADM systems. In the past, MEMS-based WSS required the spacing of each channel to be defined in advance (e.g., 100 GHz or 50 GHz) and could not be changed afterwards. However, the millions of pixels on LCoS can change the spacing of each channel arbitrarily, which fully utilizes the frequency resources to improve the spectral efficiency in the ultra 100 Gbit/s era and opens up the era of flexible grids.

LCOS cell structure

The pixel plates that configure the voltage are on the top layer of the control silicon. These plates give each of the millions of pixels a programmable controlled voltage that is used to produce a programmable controlled phase delay in the direction of the primary polarization. Physically, the phase delay is generated by highly polarized liquid crystal molecules. Optically, each liquid crystal molecule can be thought of as a miniature wire with an electron free to move along the length of the wire. When the pixel plate is not charged, these liquid crystal molecules are all lying flat and held in place by a calibration layer and are perpendicular to the light wave and parallel to the oscillating electric field of the light wave. The strong interaction between the quasi-free state electrons in the liquid crystal molecules and the electric field of the light wave temporarily stores energy thereby slowing down the wave transmission. When a voltage is applied between the CMoS chip embedded in the voltage-regulated pixel plate and the indium tin oxide layer on the top glass, the ends of each liquid crystal molecule are pulled in opposite directions. As the voltage is increased, the liquid crystal molecules become more and more aligned with the direction of the light wave and more and more perpendicular to the wave's electric field, resulting in a weaker and weaker interaction between the liquid crystal molecules and the light wave, so the wave is transmitted faster.

Working principle of LCOS-based WSS

Using millions or more pixels on a spatial light modulator LCoS, the relative phase of the incident light wave across the plane can be controlled, and angled virtual mirrors can be fabricated for more complex phase programming. Optical signals with different wavelength channels and varying channel spacing are fed from the top of the fiber array. A diffraction grating splits the optical signal into a “rainbow” of different frequencies over the LCoS. Different angled virtual mirrors are programmed to be assigned to different areas of the LCoS so that they can slightly change the reflection angle for different frequencies. The diffraction grating then recombines the light reflected back from these virtual mirrors at different frequencies, which is then focused by the lens array and transmitted back to the fiber optic array.

Basic structure of LCOS-based WSS

The spatial light modulator of liquid crystal can change the phase of a certain wavelength as needed, and all beam routes are reversible. For example, all light wavelengths from the first fiber input, through the spatial light modulator phase modulation, the other N-1 wavelengths to change the phase of the same, reflected back to re-multiplexed from the second fiber output. And the need for downstream phase can be changed differently, it can be output from the third fiber, the corresponding signal can be transmitted to the downstream branch.

LCOS-based WSS phase change routing schematic

Advantages of LCOS-based WSS

(1) Wavelength-independent:Each upstream and downstream port can be reconfigured to any wavelength;

(2) Direction-independent:Each upstream and downstream port can be reconfigured to any direction;

(3) Non-competitive:The same wavelength in different directions can be flexibly up and down;

(4) Flexible Grid:Better spectral efficiency can be realized;

(5) Flexible bandwidth and low power consumption;

Of course, LCOS-based WSS devices also have their technical difficulties, such as lower diffraction efficiency caused by fringe field effects, noise and crosstalk, etc., but they have been more and more widely used because they are very suitable for the requirements of Colorless, Derectionless, Contentionless and FlexibleGrid of the new generation of all-optical networks. However, it has been more and more widely used because it is very suitable for the requirements of Colorless, Derectionless, Contentionless and FlexibleGrid of the new generation.