1.

Introduction

Phase-only liquid crystal on silicon (LCoS) devices are used in a variety of applications including wavelength selective switches (WSS),¹ wavefront correction for optical communications and astronomical observation, and holographic displays for augmented reality/virtual reality (AR/VR).² Since current LCoS devices only modulate a single polarization, they require significant polarization management optics which make the system larger, more expensive, and less efficient.² A necessary improvement is to make LCoS devices polarization-independent, which would reduce cost, size, and complexity of systems while increasing system performance.

As shown in Fig. 1, a typical LCoS phase modulator consists of a silicon backplane circuit with a two-dimensional array of electrically addressable pixels and a glass substrate with a transparent electrode. A liquid crystal (LC) alignment layer is applied to each substrate to pre-align LC molecules, and a layer of LC is sandwiched between the two substrates. By varying the voltage across the LC layer, the effective refractive index for light propagating through the device can be locally changed for the polarization parallel to the pre-alignment direction. Since each pixel on the silicon backplane can be supplied with a defined voltage, light is spatially phase modulated as it passes through the LC layer, reflects off the pixel mirrors, and again passes through the LC layer. For this type of LCoS device, phase modulation will only occur for the light polarized parallel to the LC pre-alignment direction, since the light that is polarized orthogonally to the alignment direction is always subjected to the ordinary refractive index of the LC, regardless of the applied voltage. LCoS devices used in applications, such as WSS, wavefront correction, or holographic display, require a significant degree of polarization management or, if using only a single polarization, are characterized by high optical power loss. Given these significant deficiencies inherent in single polarization modulation, the development of polarization-independent LCoS (PI-LCoS) phase modulators has attracted both industry and academia. Frisken et al.³ outlined many of the proposed methods for making PI-LCoS devices. One concept for fabricating PI-LCoS devices involves using two LC layers that are orthogonally pre-aligned and modulated together.⁴ In this configuration, a single LC layer modulates one input polarization, and the other LC layer modulates the orthogonal polarization. Since any input polarization can be represented by the superposition of these two orthogonal polarizations, the device has the same modulation for light with any polarization resulting in polarization-independent modulation. Using two LC layers in one device, however, requires a higher driving voltage for the same phase delay since the glass electrode is further from the pixel array. Additionally, it is difficult to configure equal phase delay at every pixel for both layers. Rather than using two LC layers, another method proposes making a PI-LCoS device using axially symmetric LCs, but this technology is immature.³

Fig. 1

Structure of a traditional LCoS phase modulator.

A simple method to realize polarization-independent LC devices is to rotate each polarization of light by 90 deg after the first pass through the LC layer. When the light passes through the LC layer a second time, the polarization that was originally modulated by the LCs is now perpendicular to the LC alignment direction and receives no further modulation. At the same time, the polarization that was originally orthogonal to the LC alignment direction on the first pass is now parallel to the LC alignment direction and is modulated by the LC layer. This method of creating polarization-independent LC devices is well developed and widely understood.⁵

One of the simplest methods to achieve the polarization rotation in LC devices is to add a quarter-wave plate (QWP) between the LC layer and the mirror.⁵ Adding a traditional crystal waveplate to an LCoS device, however, is not feasible since the thickness of the waveplate would greatly increase the distance between the electrodes, which means that a significantly higher driving voltage would be required to modulate the LCs.

Frisken et al.³ proposed to fabricate a subwavelength grating on each pixel to achieve the polarization rotation. The grating would be composed of alternating metal and dielectric strips that act as birefringent material when the strips are smaller than the wavelength of light. This method of realizing the polarization rotation is advantageous in that the voltage drop over the grating is very low, since it is made partially of a conductive metal. Therefore, no significantly higher voltages are required to drive the LC layer. The major disadvantage of this method, however, is the difficulty in fabricating high-quality gratings on the LCoS backplane with low cost and high efficiency. In an attempt to simplify the grating fabrication, Anderson et al.⁶ proposed adding a dielectric spacer layer between the pixel array and grating. Even with a simplified fabrication process, however, it is difficult to achieve high efficiency polarization-independent phase-modulation using such subwavelength gratings on the silicon backplane surface.

2.

PI-LCoS Structure and Operation

We propose a practical PI-LCoS phase modulator, as shown in Fig. 2, which uses a thin-film polymer QWP and uses a frame buffer pixel circuit for the silicon backplane to increase the driving voltage.⁷ The significant advantages of this method are that existing silicon chip fabrication processes can be used for the backplane and thin-film polymer waveplate fabrication is a relatively simple process.^8,9 Therefore, it is highly possible to realize low-cost and high-performance PI-LCoS phase modulators. Here we focus on developing a PI-LCoS device for visible light (633 nm), however, the same methods will be used in the future to fabricate PI-LCoS devices that operate in infrared wavelengths (1550 nm). Fabricating the PI-LCoS device will be slightly more difficult for infrared wavelengths as the QWP and LC layers will need to be thicker due to the physically longer wavelength and dispersion, but the methodology and process are the same for both infrared and visible wavelengths. For our PI-LCoS device, the LC layer’s thickness should allow for $2\pi$ modulation in a single pass since each polarization state is modulated one time. This differs from traditional LCoS devices, where the single polarization state is modulated 2 times.

Fig. 2

Structure of the proposed PI-LCoS phase modulator.

The focus of this paper is the development of thin-film polymer waveplate fabrication techniques for use in our PI-LCoS devices. The polymer QWP is fabricated using a layer of photoalignment material to set the optical axis and a layer of liquid crystal polymer (LCP), which acts as the birefringent medium.⁹ The thickness of the LCP layer depends on the operating wavelength and the specific LCP material, but using LCP materials with high birefringence, we can achieve a thin QWP with a low-voltage drop. Therefore, we are exploring LCP materials with high birefringence.

Instead of increasing the voltage to the pixels directly, the frame buffer pixel circuit allows us to use $V_{\mathrm{com}}$ modulation to increase the voltage across the LC layer.^8,10 In general, an alternating current (AC) driving scheme is used in LC phase modulators to prevent the degradation of the LC layer due to charged particles being pulled to one electrode. In traditional LCoS devices, this AC driving is achieved by holding the common voltage on the glass electrode $V_{\mathrm{com}}$ at a mid-voltage between the maximum and minimum voltages supplied by the silicon backplane. By modulating $V_{\mathrm{com}}$ opposite to the pixel mirrors, as described in Ref. 10, we increase the voltage across the LC layer. This $V_{\mathrm{com}}$ modulation is possible because the frame buffer pixel technology updates all pixels simultaneously instead of line by line. We have begun the fabrication process for our first frame buffer pixel silicon backplane design which will be used to demonstrate $64\times 64\text{pixel}$ PI-LCoS phase modulators with up to $2\pi$ modulation for visible light. For this first backplane, the pixels are $5\mu \mathrm{m}$ squares with a pitch of $8\mu \mathrm{m}$.

Our proposed PI-LCoS devices have major advantages compared to traditional LCoS devices for specific applications in WSS, wavefront correction, and holographic display. In telecommunication systems that use WSS, the input light does not have a specified polarization. The polarization-independent operation of our proposed LCoS device eliminates the need for polarization management optics, such as a polarizing beamsplitter and a half-wave plate which would be required if using a traditional LCoS modulator. These optics increase system complexity and are difficult to align, leading to higher polarization-dependent loss. In wavefront correction, a traditional LCoS modulator would require the polarization management optics to be even larger and more complex than WSS applications due to the large diameter of the input beam (e.g., light coming from a telescope for astronomical imaging). Our PI-LCoS modulator eliminates this problem by removing the polarization management altogether. Also our PI-LCoS phase modulator can perform frame at a time wavefront correction that can significantly improve image quality and light efficiency. For holographic display, the frame buffer pixel circuit has major advantages of high image display quality and high light efficiency. This is critically important for color sequential display, because red, green, and blue images need to be displayed with high frame rate and time sequentially. Using a traditional LCoS phase modulator for such display, the light needs to be blocked during data loading time, resulting in low light efficiency. With our frame buffer pixel circuits based LCoS phase modulator, data loading and display can be done in parallel, which can significantly improve the light efficiency compared to traditional LCoS phase modulators.

3.

QWP Development

The QWP is fabricated from two layers, both of which are applied to the substrate via spin coating. The first layer is a thin (10 to 50 nm) photoalignment layer which is made of a material with molecules that are aligned by polarized light. The second layer is thicker (0.5 to $3\mu \mathrm{m}$) and made of a birefringent LCP. Intermolecular forces align the LCP molecules parallel to the photoalignment molecules which sets the optical axis of the QWP. To fabricate the QWP, we selected materials that are widely used in research and are commercially available.

3.3.

Testing

Here we fabricated several QWPs using our developed process and performed waveplate measurements. When developing PI-LCoS devices, it is important to consistently fabricate QWPs with the correct thickness. Therefore, we first measured the samples using an ellipsometer, which takes polarization measurements at many wavelengths and fits the data to a model. Next, we used an optical setup to measure the polarization change induced by the QWP.

The ellipsometer data showed that the thickness of the LCP layer was consistently within 10% deviation of the 884 nm target thickness, and that multiple samples were within 5% deviation of the desired thickness. We will further develop the QWP fabrication process to reduce the thickness error.

The optical setup used to measure the QWP is shown in Fig. 3. In the setup, the beam from a 633 nm HeNe laser first passes through a linear polarizer and is incident on the sample at a small incident angle. The reflected beam then passes through a second linear polarizer, known as the analyzer, and the output power is measured with a photodetector and a power meter. The orientation of the optical axis of the sample is 45 deg from the input polarization. In the ideal case, after the light passes through the QWP twice, the polarization is rotated by exactly 90 deg. Therefore, when the two polarizers are in parallel orientations, very little power should transmit, and when the polarizers are in perpendicular orientations, very high-power should transmit. The ratio of these two measured powers can be used to quantify how effectively the QWP rotates the polarization of light. Here the measured optical power passed through crossed polarizers was typically 10 times the optical power passed through parallel polarizers. These measurements indicate that a double pass through the QWP was effective at rotating the polarization of light by 90 deg.

Fig. 3

Measurement setup used to test QWP effectiveness.

4.

Polarization-Independent Liquid Crystal Cell

After developing the QWP fabrication process, we fabricated and tested a polarization-independent liquid crystal (PI-LC) cell, the structure of which is similar to the PI-LCoS shown in Fig. 2. The PI-LC cell operates upon the same principles as our proposed LCoS device and includes all the same layers as the PI-LCoS device, but instead of a silicon backplane circuit with individually addressable pixels, we used a silicon substrate with a single aluminum layer. Thus the device demonstrates how a single pixel of the PI-LCoS device produces polarization-independent phase modulation.

5.

Results

The fabricated PI-LC cells were viewed through cross polarizers under a microscope. Images of a PI-LC cell, shown in Fig. 4, demonstrate uniform LC alignment with only minor defects from rubbing. After fabricating the PI-LC cell, we used an interferometer to measure the phase modulation as a function of applied voltage for various input polarizations, along with the device transmittance for various input polarizations in order to calculate the polarization-dependent loss (PDL).

Fig. 4

Microscope images of a fabricated PI-LC cell through crossed polarizers. (a) Incident light is polarized parallel to LC alignment and 45 deg from QWP optical axis. (b) Incident light is polarized 45 deg from LC alignment and parallel to QWP optical axis.

5.1.

Interference Measurement

To demonstrate the polarization-independent operation, we measured phase modulation for various input polarizations using the interferometer setup shown in Fig. 5. Here a crystal polarizer and a half wave plate set the polarization of input light. A beamsplitter is then used to create two beams. The first (object beam) goes to the PI-LC cell and is phase modulated by the device; the second (reference beam) passes through a QWP and is reflected back to the beamsplitter. The QWP in the reference beam path compensates for the polarization change caused by the QWP in the PI-LC cell. This way, when interference is measured at the output of the interferometer, the polarization of the reference beam matches the polarization of the object beam. Various voltages are applied to the PI-LC cell using a function generator. The power detector is connected to a power meter which is connected to an oscilloscope to record the data. A curve showing the applied voltage to the cell versus the power output of the interferometer is shown in Fig. 6 for a single polarization. Here we see that as the applied voltage increases, the interferometer shifts between constructive and destructive interference due to the phase change induced by the cell.

Fig. 5

Measurement setup used to test phase modulation.

Fig. 6

Power measured at the output of the interferometer as a function of voltage applied to the PI-LC cell for input polarization 90 deg from LC alignment direction.

The HWP was rotated to five positions to create five unique input polarizations. For each of these polarizations, the power at the detector was measured as a function of voltage applied to the cell. Additionally, the power of the reference beam was measured by blocking the object beam, and the power of the object beam was measured by blocking the reference beam. These measurements were then used to calculate the phase delay of the object beam as a function of voltage.

The power of two interfering waves with powers $P_1$ and $P_2$ can be expressed by

Eq. (1)

$$P=P_1+P_2+2\sqrt{P_1{P}_2}\cos (\varphi ),$$

where $\varphi$ is the phase difference between the two waves. In practice, there is always some misalignment between the beams. Additionally, the fabricated QWP has non-ideal alignment and thickness so there is some component of the polarization that is not rotated by 90 deg. The absence of the same polarization component in the reference beam prevents interference of this unrotated light. To account for light that does not interfere due to either QWP quality or beam misalignment, we include a factor $C$ in Eq. (1) to obtain

Eq. (2)

$$P_V=P_1+P_2+2C\sqrt{P_1{P}_2}\cos ({\varphi }_V),$$

where $C$ assumes a value between 0 (no interference) and 1 (ideal interference). Here the measured power of the reference beam is $P_1$, the measured power of the object beam is $P_2$, the measured power of the interfering beams is $P_V$, and the phase difference between the two beams is ${\varphi }_V$. The subscript $V$ indicates the variables that are a function of applied voltage. Rearranging Eq. (2), we obtain

Eq. (3)

$$\cos ({\varphi }_V)=\frac{P_V-P_1-P_2}{2C\sqrt{P_1{P}_2}}.$$

Here we see that ${\varphi }_V$ is ambiguous, and $C$ is unknown. From the operational theory of the device, we know that ${\varphi }_V$ should be a continuous function of applied voltage spanning at least $2\pi$ and, in general, decrease as the applied voltage increases. We can then assume that $\cos ({\varphi }_V)$ varies between $-1$ and 1 and thus find a value for $C$ along with an offset to apply to the right side of Eq. (3) so that it spans $-1$ to 1. After applying the scaling and offset, we can compute ${\varphi }_V$. Since ${\varphi }_V$ is in the argument of cosine, the specific value of ${\varphi }_V$ is ambiguous, so we choose values such that ${\varphi }_V$ decreases as the applied voltage increases.

Note the similarity of the phase modulations for the various input polarizations in Fig. 7, as well as the computed value for $C$ that is close to 0.75 in all cases. Differences in phase modulation curves may be attributed to a number of measurement errors, such as physical motion in the interferometer or variation in the optical power of the laser between measurements. The results indicate that a similar amount of light is modulated, regardless of input polarization; hence the device is polarization-independent.

Fig. 7

Phase modulation as a function of applied voltages for various input polarizations. The legend shows the angle between the input polarization and the LC pre-alignment direction.

6.

Conclusion

We proposed a PI-LCoS phase modulator that uses thin film QWP technology and frame buffer pixel circuit technology. Unlike traditional LCoS phase modulators, our proposed PI-LCoS phase modulators have significant advantages for use in WSS for fiber networks, wavefront correction for astronomical imaging and free space optical communications, computer-generated holographic display, color sequential display, and AR/VR displays. We demonstrated the feasibility of the proposed LCoS device by fabricating and testing PI-LC cells that demonstrate the polarization-insensitive operation of the device and provide experimental data which can be used to estimate the required driving voltages for future LCoS devices. Future research entails considering more materials to optimize the device. Near future research entails using our developed processes and techniques to fabricate PI-LCoS phase modulators after receiving the silicon backplane chips from the silicon foundry.