Research on reconfigurable waveguide devices based on liquid crystal on silicon

Due to the limitation of materials, PIC cannot achieve large magnitude phase modulation in a small size, and lacks the reconfigurable ability. Because of its birefringence characteristic, liquid crystal can be used in the liquid crystal on silicon (LCoS) device to achieve a large amount of phase modulation in a small size and has the capacity of reconstruction. Based on the experimental results of LCoS devices and basic liquid crystal waveguide devices, the liquid crystal waveguide device is prepared. While retaining the LCoS spatial optical phase modulation capability, the liquid crystal waveguide and phase modulation capability are realized. The beam splitter, optical switch, and Mach–Zehnder interferometer structure are constructed, which verifies the feasibility of the liquid crystal waveguide phase modulator.


| INTRODUCTION
The concept of photonic integrated circuits (PIC) was first proposed by Miller in 1969, describing an integrated optoelectronic device that can achieve the generation, modulation, routing, conversion, and other functions of optical signals in a single chip. 1 With the progress of nanotechnology, the realization of reconfigurable capability is important for PIC.As a fundamental component in PIC, the implementation of optical waveguide reconfigurable function is an important foundation for the reconfigurable control of light in PIC.And there are some problems existing in optical waveguide devices.When optical waveguide devices involve optical structures with amplitude and phase modulation, if only silicon-based materials are used, large size is required to achieve the required modulation range, and it is difficult to integrate into the micron-level system.For example, the phase modulation structure realized by the thermal-optical modulation principle requires at least millimeter device size to realize the modulation function.These problems lay obstacle on the implementation of reconfigurable function and the further integration of PIC.
The liquid crystal on silicon (LCoS), as a display device, can load different patterns and can adjust the voltage applied to pixel electrodes based on the loaded pattern, which can control the tilt of liquid crystal (LC) molecule.The structure diagram of LCoS is shown in Figure 1.The structure consists of three layers, namely, the glass and conductive layer at the top, the LC layer in the middle, and CMOS chip at the bottom.In this situation, the refractive index distribution of LC conforms to the refractive index distribution of waveguide, which means the structure of waveguide could be established.The structure of waveguide can be changed by changing the loaded pattern, which theoretically realizes the reconfigurable capability.
At present, in order to achieve the reconfigurable function and further integration of waveguide, materials that can change its crystalline state or molecular arrangement have received attention, including phase change material (PCM) and LC.PCM has been introduced into waveguides due to its high refractive index contrast and reversible nonvolatile properties.In the article reported by Lu et al., the waveguide by controlling the crystalline state of Sb 2 S 3 achieves reconfigurable control of light. 2 However, this reconfigurable waveguide requires high modulation conditions.The high-power laser (90 MW) is needed to ensure fast modulation, which is not conducive to integration and control.The function of the waveguide can only be achieved according to the predesigned waveguide structure and cannot be controlled through programming, lacking flexibility.
LC, as a birefringent material with various anisotropy, not only has excellent optical performance, but its unique electro-optical characteristics can achieve the reconfigurable function of waveguides and can make progress in integration.4][5][6] Therefore, it is necessary to introduce LC as a new material to realize further integration of devices. 7enerally, there are three different solutions for introducing LC into waveguide structure, which using LC as core material, LC as cladding material, and LC as cladding and core material at the same time.At present, the most widely used method is to use LC as the core layer. 8This kind of structure needs enough size to ensure enough LC in the middle as the core layer of the waveguide, which has some restrictions on the size design of the waveguide.However, the LC waveguide has a fixed form, which limits the size and capability of modulation.In this paper, the structures of different waveguide are simulated, and the director of LC is simulated to verify the feasibility of the LCoS to established the waveguide structure and analyze the tilt of LC.And several LCoS samples are prepared for experiment of waveguide establishment.substrate glass, the distribution of refractive index is basically the same as the refractive index distribution in the waveguide.As the results show, the structure of waveguide can theoretically be established in LCoS.And the data of LC director tilt angle are exported for calculation of the refractive index distribution.

| SIMULATION OF LC DIRECTOR AND WAVEGUIDE STRUCTURE
When the direction of the incident light is determined, the distribution of LC layer refractive index under different voltages is calculated according to Equation (1).
in which θ is the angle between the incident direction and the optical axis direction and n o and n e are the refractive index corresponding to ordinary light and extraordinary light, respectively.The effective refractive index distribution on different voltages is shown in Figure 3.In Figure 3, the horizontal axis represents the position of the electrodes.According to the result, the core layer of waveguide structure can be obtained when the LC molecule tilts completely, and the LC molecule without deflection can be regarded as the cladding layer to form a rectangular waveguide structure.This part of simulation proves that the LC refractive index distribution can be controlled by voltage applied on the pixel electrodes so that the basic structure of waveguide can be established under enough voltage.

| Simulation of waveguide structure
After proving that the structure of waveguide can be established, the transmission of light in the waveguide should be simulated.The rectangular waveguide structure is designed with software Comsol.The X-Z section view of its structure is shown in Figure 4A, and the Y-Z section view is shown in Figure 4B.According to the result in Figure 3, the tilt of the LC director can be simplified to change the refractive index of the core layer, and the transmission of the beam in the waveguide is simulated.In Figure 4A, the LC core layer formed by voltage modulation is fixed in the middle rectangular part, and n eff of this part will change.The two sides are the LC cladding layer without voltage modulation, whose refractive index maintains on n o , and the top and bottom are filled with glass.In this simulation, n o and n e are the same as LC material E7(n o = 1.56, n e = 1.73), and light source uses a Gaussian beam.The X-Z section view is used to observe whether the beam can be bounded in the core when n eff is changing.In Figure 4B, optical fibers are used to simulate coupling situation.This section view is used to observe the transmission in the waveguide with different n eff .
The results are shown in Figure 5. Figure 5A-C shows the results of the X-Z section view.In Figure 5A, the beam cannot be bounded in the core, while the refractive index of the core layer is maintained at n o .While n eff of the core increases, the beam is gradually bounded into the core, as shown in Figure 5B.When n eff is set to n e , the beam finally can be bounded in the core, as the result shown in Figure 5C.gradually bounded in the core and can transmit in the waveguide when n eff is set to n e .
Based on the results of the simulation of the waveguide, it can be concluded that LCoS can construct a rectangular waveguide structure and enable the beam to transmit in the waveguide theoretically.The change of the core refractive index not only affects the intensity of the beam in the core but also affects its phase.
The simulation of Mach-Zehnder interferometer (MZI) is designed as well, whose structure is shown in Figure 6.This structure consists of two couplers and two optical channels.By modulating the refractive index of the reference arm and the measuring arm, the phase of the beam can be modulated.And the modulation of the phase can be observed by the light intensity change of the outgoing light.
As Figure 7 shows, when there is a refractive index difference between the two branches, it can be observed that the transmissivity of the output changes, and the corresponding light intensity changes with the change of refractive index difference.Because there is bending waveguide structure existing in the MZI and the form of the pixel electrode is square, the bending waveguide will have a break corner.It is a kind of reason that causes loss, and it is necessary to simulate a simple break corner, whose diagram is shown in Figure 8A.The 3 Â 3 array is designed by Techwiz, and the voltage is applied on the middle pixel electrode.The result is shown in Figure 8B.This view is perpendicular to the pixel.When the voltage is applied on the pixel electrode, LC molecule in this area tilts completely.But LC molecules around the pixel applied the voltage are also affected by the electric field and tilt slightly.If the bending waveguide exists in the waveguide structure, the corner may contribute to the loss of the light.According to the simulation results, the waveguide structure can be established by LCoS, and the intensity and phase modulation of the LC waveguide can be achieved.The feasibility of the reconfigurable LC waveguide established by LCoS is proved according to the simulation.

| Preparation of the LCoS device
The LCoS device has been prepared, whose structure diagram is shown in Figure 1.The device consists the spacer of 6.2 μm, a glass with ITO, LC material E7, the chip whose size of pixel is 15 Â 15 μm, and control system.PI with frictional orientation is used for making LC molecules to align in ECB mode.The prepared LCoS device is shown in Figure 9.
The process of preparing the LCoS device is shown in Figure 10.The cell gap and the size of the pixel electrode determine the size of the waveguide, so the cell gap is needed to measure before injecting LC, even if the size of spacer is certain.If the uniformity of the cell gap is not well enough, the glass on the top will tilt up, which will make adverse effect in the structure of rectangular waveguide.
Shown in Figure 11, the optical system that makes use of the interference effect of light is established to measure the cell gap.The wavelength of the laser source is 632 nm.After measuring, the thickness of the cell gap is 6.269 μm, and the uniformity of the cell gap performs well.This uniformity proves the accuracy of the voltage control and the quality of waveguide structure.After bonding operation and PCB substrate welding, the LCoS device for LC waveguide test can be obtained, and the sealant will be removed before the experiment.
The optical system in Figure 11 is also used in modulation capacity measure.When the two polarizers are set at 90 and 45 from the orientation direction of the LC molecule, the light intensity is measured to estimate the modulation capacity.The result is shown in Figure 12.There are five to six peaks in the curve, which means the phase modulation is larger than 8π.When the input voltage is between 0 and 2.6 V, the curve changes too frequently.Compared to this voltage range, the change of light intensity is more smooth in 2.6-7 V, and in this voltage range, the tilt of the LC director can be controlled accurately.Therefore, the voltage range 2.6-3.5 V is chosen to control the tilt of LC director.

| Establishment of rectangular waveguide
According to the result of simulation and the curve in Figure 3, the rectangular waveguide structure is established in the device, whose structure diagram is shown in Figure 13.To ensure that the waveguide can work, the voltage applied on the pixel is near 3.3 V, which ensures the LC director tilts completely.
The optical fiber couples the light from the reserved test port into the device.Because the LCoS can realize the patterned display, the optical fiber probe does not need to be aligned at a specific position at this time but only needs to be parallel to the orientation direction and  as close as possible to the reserved test port.After the specific position of the optical fiber is aligned, the straight line is loaded on the device, and different voltages are applied on the pixel to control n eff of the core.The results under the polarization microscope is shown in Figure 14.
With the increase of voltage, the LC director of the core layer gradually tilts up, and the n eff of the core layer increases.When the refractive index difference Δn of core and cladding is large enough, the light is bounded in the core layer and can transmit in the waveguide.These results also prove the validity of the simulation.

| Establishment of the optical splitter
Based on the rectangular waveguide experiment, change of the waveguide structure is realized.The 1 Â 2 optical splitter is designed and can separate part of the energy into the branch, whose structure diagram is shown in Figure 15.
In Figure 16A, a diagonal branch is established.Because the form of the pixel is square, the serrated edge of the waveguide can be observed, which is caused by the break corner in Figure 8A.When the light goes in this structure, the light intensity in the branch is weakened.The direction of the branch waveguide suddenly changes, which leads to the efficiency of the splitting position coupling end is low.And according to the simulation result in Figure 8, serrated edge may lead to the loss of energy.The pattern of the splitter structure needs to be adjusted to relieve the large loss caused by the serrated edge.In Figure 17A, the two bending branches have been established as the adjusted splitter to relieve the light scattering caused by break corner.The adjusted splitter structure distribute the light intensity evenly in two branches.But the serrated edge still exists, which contributes to the loss of the light.Because the light from the laser source is not linearly polarized, only extraordinary light can be modulated in the waveguide, while ordinary light will scatter in the waveguide.In Figure 17B, the light is leaking between two branches.Even if the loss still exists, this structure still improves the performance of the splitter.
Another rectangular branch is added to establish the 1 Â 3 optical splitter, and the structure is shown in Figure 18A.Three branches is numbered in Figure 18A.When No.2 branch is opening, the light can transmit in all branches.Changing voltage applied on the pixel to close No.2 branch, the light intensity in No.2 branch is decreasing, while the light intensity in No.1 and No.3 branch is increasing.Finally, the result is similar with that the Figure 17B shows, when No.2 branch is completely closed.Comparing all branches of the 1 Â 3 optical splitter, because bending waveguide has serrated edge caused by break corner, which contributes to the loss, the performance of rectangular waveguide is better than bending waveguide.

| Establishment of MZI
Because the serrated edge in the bending waveguide cause the large loss of the light intensity, the curvature should be suitable for the experiment.Two bending branches will make the low coupling efficiency and the large loss.Therefore, MZI with a rectangular branch and a bending branch is established in the device, which is shown in Figure 19.The problem of uneven light intensity distribution still exists.The light in the rectangular branch is brighter than that in the bending branch, and it causes the problem that the output will not be completely dark.The performance of MZI is not good enough with the noise and large light loss.But the light still can be modulated, and this result proves that the LCoS device can modulate the phase in the reflective working state, as well as in the waveguide working state.

| CONCLUSION
LCoS, as a display device, can load patterns in the device, and the waveguide can be established, combining with the electro-optical property of LC.There are still existing some problems to be solved.Large loss and noise lead to the difficulty on measure of output light intensity.Therefore, input and output coupling method needs to be improved, and materials used for preparing the LCoS device can be changed to reduce the loss.And LCoS with higher resolution may relieve the effect of the break corner, which may have a better performance than LCoS used in this paper.But different waveguide structures are established in the LCoS device.It proves that reconfigurable waveguide can be realized with the characteristics of LC in LCoS.It means that different kinds of waveguide can be integrated in the area of display provided by LCoS.The integrated size of the device is controlled in micron level, and the condition of modulation can realize with simple modulation condition.It lays the foundation for the realization of integrated reconfigurable waveguide devices.

2. 1 |
Simulation of LC directorIn order to prove the feasibility of the waveguide in LCoS, several simulations have been conducted.The tilt of the LC director under different voltages has been simulated to observe how voltage affects the LC director.The distribution of LC director is simulated by software Techwiz.In this simulation model, five-pixel electrodes are set and from right to left are numbered No.1 to No.5.LC material E7 is used as LC layer in electrically controlled birefringence (ECB) mode, and the cell gap is set as 6 μm.The size of pixel electrode is 15 Â 15 μm.Applying voltage on No.2 to No.4 electrodes, the distribution of LC director is shown in Figure 2. The LC molecule begins to tilt when the electrodes start to apply the enough voltage.When the voltage is large enough to make the LC molecule tilt completely, which is perpendicular to the F I G U R E 1 The structure diagram of liquid crystal on silicon (LCoS).
Figure 5D-F displays the results in the Y-Z section view.In this section view, the light F I G U R E 2 The distribution of the liquid crystal director under different voltages in electrically controlled birefringence (ECB) mode.F I G U R E 3 The effective refractive index distribution at different voltages.

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I G U R E 4 The section view of rectangular waveguide structure.(A) The X-Z section view of rectangular waveguide structure.(B) The Y-Z section view of rectangular waveguide structure.

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I G U R E 5 The transmission of beam under different n eff in the rectangular waveguide.F I G U R E 6 The structure of Mach-Zehnder Interferometer.F I G U R E 7 Change of transmissivity with refractive index.F I G U E 8 Simulation of the break corner.(A) Diagram of break corner.(B) Simulation result of the break corner.

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I G U R E 9 The liquid crystal on silicon (LCoS) device.F I G U R E 1 0 The process of preparing the sample.

F I G U R E 1 1
The optical system for measure experiment.F I G U R E 1 2The curve of light intensity.

F I G U R E 1 5
The structure diagram of 1 Â 2 optical splitter.F I G U R E 1 6 The experiment of the 1 Â 2 optical splitter.(A) The 1 Â 2 optical splitter in the device.(B) Light in the 1 Â 2 optical splitter.F I G U R E 1 3 The structure of rectangular waveguide.F I G U R E 1 4 The light in the rectangular waveguide under different voltages.

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I G U R E 1 7 The experiment of the adjusted 1 Â 2 optical splitter.(A) The adjusted 1 Â 2 optical splitter in the device.(B) Light in the adjusted 1 Â 2 optical splitter.I G U R E 1 8 The experiment of 1 Â 3 optical splitter.(A) The 1 Â 3 optical splitter in the device.(B) Light in the 1 Â 3 optical splitter (No.2 branch opening).(C) Light in the 1 Â 3 waveguide (No.2 branch closing).