WO2021022505A1

WO2021022505A1 - Method for fast switching liquid crystal on silicon spatial light modulator

Info

Publication number: WO2021022505A1
Application number: PCT/CN2019/099568
Authority: WO
Inventors: Daping Chu; Mykhaylo Pivnenko
Original assignee: Dchu Ltd
Priority date: 2019-08-07
Filing date: 2019-08-07
Publication date: 2021-02-11

Abstract

A method for fast switching liquid crystal on silicon (LCOS) spatial light modulator, is fast variation of the phase modulation by an LCOS device as a function of driving voltage. The time of the variation of the phase modulation of an LCOS device can be reduced by increasing and adjusting driving voltage. The driving voltages are set as VB ('B' denotes black) and VW ('W' denotes white frame). The time of an LCOS device can be reduced by increasing driving voltage VB from 0.8V to a higher available value 2.2V. Driving voltage VW is set accordingly, to keep the phase depth φ 0 equal π radians for amplitude modulation, or equal 2 π radians for phase modulation in a range from 2.5 to 5.0V.

Description

[Title established by the ISA under Rule 37.2] METHOD FOR FAST SWITCHING LIQUID CRYSTAL ON SILICON SPATIAL LIGHT MODULATOR

FIELD

The present disclosure relates to a fast switching liquid crystal on silicon spatial light modulating system

BACKGROUND

Spatial light modulators (SLM) have seen its applications widely spread in many different industries, including displays, laser processing, optical-tweezers, telecommunications over the past twenty years. The SLM market is growing at an estimated rate of 14-17%annually, and it will be worth USD 550-650 million by 2023. The demand from various applications and usages, together with technological advancements in material, device, and system including modulation schemes are driving the SLM further forward every moment.

Two competing technologies are mostly used as the SLM, namely digital micromirror devices (DMDs) and liquid crystal (LC) devices. DMDs have been often used in projection displays [16, 17] and it can switch at several tens of kilohertz to provide grey scales. Such an amplitude modulation device produces a conjugate image when used for diffraction, the light efficiency of the first diffraction order is low as a result. The LC devices are further divided into transmissive type, liquid crystal display (LCD) , and reflective type, liquid crystal on silicon (LCoS) . The LCD is a very popular choice for large area consumer displays, but for other applications, the LCD pixel circuitry is on the optical pathway, causing the ‘chicken wire effect’ which is not ideal [18] . In addition, the large pixel size results in very limited diffraction angle.

The LCoS on the other hand, is considered as an ultimate choice for SLMs. First of all, the LCoS has pixel circuitry hidden behind the pixel surface, which allows for extremely high image fill factor, excellent for projection and near-eye displays. Secondly, by being the reflective device, the light passes the modulation layer twice and the LC thickness is halved to have the equivalent modulation effect as the LCD. The response of LC switching increases four times as a result. Thirdly, the LCoS can have different configurations to be an amplitude device or a phase device, respectively, or have both modulations with the same device.

One of the most challenging aspect in LCoS SLM is the switching speed, with both a twisted nematic LC for the amplitude modulation and a parallel aligned nematic LC for phase-only modulation, i.e. electrically controlled birefringence (ECB) . The vertically aligned nematic (VAN) LCoS utilising LCs of negative dielectric anisotropy has a high threshold voltage and an even slower response time. A slower switching LCoS results in a colour breakup in a colour sequential operation, and also limits its capabilities to be used in a tiling arrangement, that was often adopted by DMDs, to enlarge field of view and viewing angle. Various LC mixtures, electrode structures and driving scheme have been demonstrated to reduce the switching time to a few and even sub-milliseconds, but much of them are for the amplitude modulation or demonstrated only in glass testing cells.

For the phase modulation, high birefringent or large dielectric anisotropy LCs are developed [36] , and they are often used with a polymer network for a faster response time, still the driving voltages are high because of the polymer existence. Similar for blue phase LCs, although the response time for both the rise and decay processes is in the～100μs range, the high electric voltage is not suitable for the LCoS operation. Ferroelectric LC or chiral smectic LC are used in LCoS to modulate phase and can achieve two orders of magnitude faster switching speed than conventional nematic devices [43, 44] . Unfortunately, the binary phase operation results in a significant loss of incident light to symmetric diffraction orders [45] .

For continuous phase depth modulation in the LCoS device, nematic LC mixtures are still the most practical ones to use. The display industry has developed nematic LCs with high birefringence, low viscosity, low driving voltages and more importantly they are reliable, which are essential for commercial applications. Most recently, high birefringent nematic LC mixtures are reported to achieve a 8.32 ms average phase-to-phase response time at 40℃ in transmissive homogenous glass cells, nevertheless the equivalent response time in LCoS is extrapolated, not demonstrated.

Spatial optical modulating systems that modulate a spatial distribution of a phase, an amplitude, an intensity, and the like of a light are conventionally known. A spatial optical modulating system is provided with, for example, an optical system that includes a spatial light modulator (SLM) , and a controller that controls the spatial light modulator.

A technique for driving voltage range adjustment is described in a patent (Spatial Optical Modulation System, Patent US 10,127,891 ) in order to achieve controllable value of the phase depth step for ultra-high precision wave front capability. There, the driving voltage range reduced from maximum possible value to a smaller value in order to reduce phase change per grey level and, in this way, increase precision of wave front control. Nothing is mentioned about response time. Here, we describe a way to reduce response time considerably by adjustment of driving voltage for specific phase modulation conditions.

As described above, a spatial optical modulating system is provided with a spatial light modulator and a controller. The spatial light modulator is provided with a liquid crystal and a plurality of electrodes arranged along a surface of the liquid crystal. The spatial light modulator is configured to perform phase modulation of a light that passes through the liquid crystal by applying individual voltages to the liquid crystal from each of the plurality of electrodes. The controller is configured to control the voltages applied to the liquid crystal from each of the plurality of electrodes based on phase image data. The phase image data represents values of each pixel corresponding to each of the plurality of electrodes by predetermined gradations.

The controller is provided with a converter that is configured to convert gradation values, which are the values of each pixel represented by the predetermined gradations, into voltages input to the electrodes corresponding to each pixel. According to one or more embodiments, the controller is further provided with a changing circuit that is configured to change a fluctuation width from a minimum value to a maximum value of the input voltages corresponding to a fluctuation width from a minimum value to a maximum value of the gradation values according to an external command.

A phase shift amount of the light by the spatial light modulator changes according to the voltages input to the electrodes. According to the conventional technology, a correspondence relationship between the gradation value and the input voltage is fixed; therefore, a correspondence relationship between the gradation value and the phase shift amount is also fixed. That is, according to the conventional technology, a phase shift amount per one gradation is constant.

CITATION LIST

Patent Literature

[Patent Literature 1] US 10,127,891 B2

References

1. Z. Zhang, A.M. Jeziorska-Chapman, N. Collings, M. Pivnenko, J. Moore, B. Crossland, D. Chu, and B. Milne, "High Quality Assembly of Phase-Only Liquid Crystal on Silicon (LCOS) Devices, " J. Disp. Technol. 7 (3) , 120–126 (2011) .

2. Spatial Optical Modulation System, Patent US 10,127,891 B2

3. Merck Group, Darmstadt, Germany

4. AWAT, Warsaw 00-908, Poland

5. R.

P. Kula and J. Herman, Crystals 2013, 3, 443-482; doi: 10.3390/cryst3030443

SUMMARY

This invention describes how to determine the required range of the driving voltages in use for different phase modulation depths at different switching speeds in order to achieve the fastest possible response.

A Method For Fast switching liquid crystal on silicon spatial light modulator, a way to achieve fast variation of the phase modulation by an LCOS device as a function of driving voltage. The time of variation of the phase modulation ofan LCOS device can be reduced by increasing and adjusting driving voltage.

Driving voltage VB is set in a range from 0.8V to 2.2V. Driving voltage VW is set in a range from 2.5 to 5.0V, accordingly, to keep the phase depth

equal π radians for amplitude modulation, or equal 2πradians for phase modulation. For the low driving voltage total response time, which is a sum of the fall time and rise time, is about 5ms and corresponds refresh rate about 200 frame per second. On the higher voltage side, total response time is about 1.2ms and corresponding refresh rate is about 800 frames per second. In this case driving voltage VB and VW set fixed at low refresh rate and does not change with the refresh rate. Further improvement of the response time can be achieved when driving voltage is adjusted to for each particular refresh rate.

The first part of the invention:

Set VW to the highest available (for the driver) value and then adjust VB to get required modulation phase depth

The value of driving voltage VB is calculated with the following equation:

where F is the function of the phase modulation of driving voltage.

If the period is long enough for phase to reach saturation, both on increase and decrease, the required phase depth

is achieved. For higher frame rate (shorter period) the phase saturation cannot be achieved and total phase depth

is lower than required

However, by adjusting driving voltage VB to a new value V’ B phase depth can be increased back to original

level, figure 3c. Figure 4 explains how the new value V’B is calculated.

The second part of the invention:

Set VW to the highest available (for the driver) value and then adjust V’B to get required modulation phase depth

at given refresh rate.

The new value of the driving voltage V’B for a particular refresh rate can be calculated from the function of the phase modulation of driving voltage with the following equation:

where F is the function of the phase modulation of driving voltage, figure 4, and the value of

is

Where

is the phase depth achieved at this refresh rate for driving voltage VB and VW set at low refresh rate, for example 50 frames per second.

The way to reduce the switching time is applicable to all other LCOS devices (different thickness, different wavelength, different modulation–phase or amplitude) . The numbers are different, they are included in tables in the examples. Overall response time reduction between two and four times.

The searched patent is about OASLM. It mentions voltage adjustment, but not so relevant to our patent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A diagram representing the typical dependence of the phase modulation on driving voltage.

FIG. 2 A graph representing a LCOS response time as function of VB driving voltage.

FIG. 3 A block diagram representing a Phase variation over time for LCOS device with low refresh rate (a) , high refresh rate and fixed driving voltage (b) , high refresh rate and adjusted driving voltage (c) .

FIG. 4 A block diagram representing a Driving voltage adjustment for high refresh rate.

FIG. 5 A graph representing a Normalised diffraction efficiency of a binary grating for different refresh rates.

FIG. 6 A graph representing normalised diffraction efficiency of a binary grating for different wavelengths: 450nm, 543nm, 637nm.

FIG. 7 A graph representing normalised diffraction efficiency of a binary grating for different wavelengths: 1064nm, 1550nm.

FIG. 8 A photograph representing Blur reduction of a fast moving image on a fast LCOS (top) comparing to a reference LCOS device (bottom) .

FIG. 9 A diagram representing Image contrast as a function of the refresh rate and temperature (1-4curves) .

FIG. 10 A graph representing Diffraction efficiency loss due to high refresh rate as a function of temperature.

FIG. 11 A graph representing Normalised diffraction efficiency as a function of the grating pith for two level of the refresh rate of 700fps and 1300fps.

DESCRIPTION OF EMBODIMENTS

LCoS device preparation

The LCoS devices are assembled in the Class 100/1000 cleanroom in house [1] , and they are based on digital-driving Si backplanes from Jasper Display Corp. (Taiwan) . A polyimide alignment layer is spin coated and rubbed to provide initial alignment directions for LC to work in the ECB mode. The LCoS devices have a 2μm thick LC layer, and it has a minimum phase depth of 2π for a phase-only modulation, at the same time it can be used for amplitude modulation while modulating within 1π phase depth.

The Si backplane has a resolution of 1920x1080 and a pixel pitch of 6.4μm. An ASIC driving board (SRK JD7554, Jasper Display Corp. ) is used, which provides a maximum of 3.3 V rms driving voltage and is configured to run a sequential row-by-row scanning driving scheme. As a result, the driving board can achieve 60 Hz refresh rate when scanning all 1080 rows of pixels, 120 Hz when scanning 540 rows, 240 Hz when scanning 270 rows and 480 Hz when scanning 135 rows. The frame rate can be further increased but the active rows are too few and the operating system interface cannot be displayed to carry out the test.

Voltage adjustment method

Accurate adjustment of the driving voltage allows considerable improvement of the refresh rate. These examples describe how to determine the required range of the driving voltages in use for different phase modulation depths at different switching speeds.

Figure 1 shows typical variation of the phase modulation by an LCOS device as a function of driving voltage. The driving voltages are set as the V _B and V _W, subscript ‘B’ denotes black and ‘W’ denotes white frame. Required phase depth

depends on the application, for example π radians for amplitude modulation or phase modulation with binary grating, or 2π for phase modulation with blazed grating and holography. Usually, V _B is set just after the threshold voltage V _th and V _W is set to get required the phase depth

However, considerable improvement of the response time is achieved if driving voltage is set to a higher value–V’ _B and V’ _W, providing that the phase depth

is maintained. The effect of increasing of the driving voltage is shown on figure 2 and details are explained in the first part of the invention.

Figure 2 shows how response time of an LCOS device can be reduced by increasing driving voltage V _B from 0.8V to 2.2V. Driving voltage V _W was set in a range from 2.5 to 5.0V, accordingly, to keep the phase depth

equal π radians for amplitude modulation. For the low driving voltage total response time, which is a sum of the fall time and rise time, is about 5ms and corresponds refresh rate about 200 frame per second. On the higher voltage side, total response time is about 1.2ms and corresponding refresh rate is about 800 frames per second. In this case driving voltage V _B and V _W set fixed at low refresh rate and does not change with the refresh rate. Further improvement of the response time can be achieved when driving voltage is adjusted to for each particular refresh rate, details are explained in the second part of the invention.

The first part of the invention: Set V _W to the highest available (for the driver) value and then adjust V _B to get required modulation phase depth

The value of driving voltage V _B is calculated with the following equation:

where F is the function of the phase modulation of driving voltage, figure 1.

Figure 3a shows phase variation over time for an LCOS device with low refresh rate (long period) . The period is long enough for phase to reach saturation, both on increase and decrease and achieve required phase depth

For higher frame rate (shorter period) the phase saturation cannot be achieved and total phase depth

is lower than required

figure 3b. However, by adjusting driving voltage V _B to a new value V’ _B phase depth can be increased back to original

level, figure 3c. Figure 4 explains how the new value V’ _B is calculated.

The second part of the invention: Set V _W to the highest available (for the driver) value and then adjust V’ _B to get required modulation phase depth

at given refresh rate.

The new value of the driving voltage V’ _B for a particular refresh rate can be calculated from the function of the phase modulation of driving voltage with the following equation:

is

Where

is the phase depth achieved at this refresh rate for driving voltage V _B and V _W set at low refresh rate, for example 50 frames per second.

Example 1.

An LCOS device is made with a backplane with resolution of 1920x1080 and a pixel pitch of 6.4μm (SRK JD7554, Jasper Display Corp. ) . Liquid crystal MDA-98-1602 from Merck ², thickness 2μm.

Figure 5 shows an example of driving voltage V’ _B adjustment and corresponding improvement of the refresh rate. An image of binary diffraction grating with grating period of 16 pixels was applied to an LCOS device. Driving voltage V _W was set to value 5V, the highest available for the driver, and driving voltage V _B=2.48V was calculated from Equation 1 to get modulation phase depth

required to maximise diffraction efficiency for binary grating. Diffraction pattern for 543 nm wavelength was observed and power of the first diffraction order was measured for low refresh rate of 50 frames per second and used as a reference. Diffraction efficiency for fixed driving voltage was measured for refresh rate in the range from 100 to 3500 frames per second and normalised on the reference value, figure 5 (black dots) . For every refresh rate in the range from 100 to 3500 frames per second driving voltage V’ _B was calculated according to Equation 2, figure 5 (blue triangles) . Diffraction efficiency for adjusted driving voltage was measured and normalised on the reference value, figure 5 (red dots) . For the normalised diffraction efficiency of 0.95, maximum refresh rate increased from 850 to 2150 frames per second.

Example 2.

LCOS device from example 1 was tested for three wavelengths: 450, 543 and 637nm with voltage adjustment in the same way as for example 1. Phase depth was set 1π for amplitude modulation applications. Measurements are done for temperature 48C. Driving voltage adjusted V’ _B calculated according to Equation 2 required to achieve the fastest response are presented in table 1. Normalised diffraction efficiency of a binary grating for different wavelengths with adjusted driving voltage is shown on figure 6.

Table 1. Driving voltage required to achieve the fastest response.

Example 3.

Two LCOS devices were made with thickness 4μm and 6μm and were tested for two wavelengths: 1064 and 1550nm, respectively, with voltage adjustment in the same way as for example 1. Phase depth was set 1π for amplitude modulation applications. Measurements are done for

temperature

45C and 60C. Driving voltage adjusted V’ _B calculated according to Equation 2 required to achieve the fastest response are presented in table 2. Normalised diffraction efficiency of a binary grating for different wavelengths with adjusted driving voltage is shown on figure 7.

Table 2. Driving voltage required to achieve the fastest response.

Example 4.

LCOS device from example 1 was tested for three wavelengths: 450, 543 and 637nm with voltage adjustment in the same way as for example 1. Phase depth was set 2π for phase modulation applications. Measurements are done for temperature range from 32 to 60C. The fastest response time in milliseconds achievable for this LCOS device with driving voltage adjusted V’ _B calculated according to Equation 2 is presented in Table 3. Response time for fixed driving voltage V _B calculated from Equation 1 is also presented in Table 3 for comparison.

Table 3. Response time in ms required to achieve phase depth on 2π for three wavelengths in temperature range from 45 to 60C.

Example 5.

Two LCOS devices from example 3 was tested for two wavelengths: 1064 nm and 1550 nm with voltage adjustment in the same way as for example 1. Phase depth was set 2π for phase modulation applications. Measurements are done for temperature range from 45 to 60C. The fastest response time in milliseconds achievable for this LCOS device with driving voltage adjusted V’ _B calculated according to Equation 2 is presented in Table 4. Response time for fixed driving voltage V _B calculated from Equation 1 is also presented in Table 4 for comparison.

Table 4. Response time in ms required to achieve phase depth on 2π for three wavelengths in temperature range from 45 to 60C.

Example 6.

Three reflective cells are made with thickness 2.2μm and filled with liquid crystal materials BL006, BL037 from Merck ³ and 1825 from AWAT ^4, 5, Poland. The cells were tested for wavelength 543 nm with voltage adjustment in the same way as for example 1. Phase depth was set 2π for phase modulation applications. Measurements are done for temperature range from 45 to 60C. The fastest response time in milliseconds achievable for this cell with driving voltage adjusted V’ _B calculated according to Equation 2 is presented in Table 5. Response time for fixed driving voltage V _B calculated from Equation 1 is also presented in Table 5 for comparison.

Table 5. Response time in ms requiredto achieve phase depth on 2π for three different liquid crystals in temperature range from 45 to 60C.

Example 7.

Imaging with high refresh rate

To demonstrate how an LCOS device with fast responding liquid crystal cam improve blur reduction for high frame rates we observed a movie with a fast moving image. In a movie with 240fps refresh rate an image was moving across a frame with velocity 3840 pixels per second and a snapshot was taken with 100μs exposure time to eliminate any blur from the exposure. Figure 8 shows the results for a fast LCOS device from example 1 and a reference LCOS device. Due to slow liquid crystal response in the reference device a long trail is observed after an image corresponding the response time about 30ms. The fast LCOS shows no image blur and response time about 700μs.

The LCOS device from example 1 was tested in the amplitude mode with phase depth 1π radian. A continuous sequence of black and white frames was applied and brightness was measured as a function of time. The maximum contrast between the brightness of the white and black frames at low frame rate of 50 fps was set as reference. Figure 9 shows the waveform for four examples of the result for frame rates from 480 to 2460 fps. The shape changes from rectangular for low frame to sin-like for higher frame rate when LC becomes too slow to “translate” higher harmonics of the signal. For 30C contrast stays higher than 0.94 for frame rate up to 1520fps and higher than 0.67 for frame rate up to 2610fps. Figure 9 shows how the contrast changes with increasing frame rate for four temperatures from 30 to 60C. For 60C the contrast stays at about 1.0 level for frame rate up to 1700fps and higher than 0.707 for 2450fps.

Example 8.

Diffraction efficiency on a single pixel level

The LCOS device from example 1 was tested on diffraction for green light of 543nm. An image of a binary diffraction grating with pitch of 16 pixels, stripes of 8 pixels wide with phase depth of π radians and 0, was applied and intensity of the first diffraction order was measured for a range of frame rates and temperatures. The intensity of the first diffraction order was normalized on its level for low frame rate. Figure 10 shows what frame rate can be achieved for a fixed reduction of the diffraction efficiency between-0.1 and-3.0dB for different temperatures. For example, for 60C fast frame rate does not influence the diffraction efficiency more than-1.0dB for frame rate up to 2400fps (Figure 10) .

Switching LC as a full frame, when all the pixels move uniformly, is very different from switching every pixel individually. To analyse how spatial frequency may influence diffraction efficiency at high frame rates, diffraction on gratings with different grating pitch was measured. Diffraction gratings with pitch from 512 pixels to 2 pixels, horizontal (along rubbing direction) , vertical and “checkerboard” was applied and intensity of the first order diffraction was measured and normalized on its level for low frame rate. For 700fps, figure 11, diffraction efficiency stays at its maximum with the grating pith reduced from 256 to 16 pixels and only then goes down. For grating pitch of two pixels, this means only one pixel switches surrounded with two stationary pixels (or four for the “checkerboard” grating) the diffraction efficiency is about 0.85 of its maximum level. The result does not depend much whether the grating is horizontal, vertical or “checkerboard” . Figure 11 shows the result for 700fps and 1300fps.

Claims

A Method for fast switching liquid crystal on silicon (LCOS) spatial light modulator, fast variation of the phase modulation by an LCOS device as a function of driving voltage. The time of the variation of the phase modulation of an LCOS device can be reduced by increasing and adjusting driving voltage.
The method according to claim 1, wherein time of an LCOS device can be reduced by increasing driving voltage VB from 0.8V to a higher available value (limited by the driver) , for example to 2.2V. Driving voltage VW is set accordingly, to keep the phase depth
equal π radians for amplitude modulation, or equal 2π radians for phase modulation for example in a range from 2.5 to 5.0V.
The method according to claim 2, wherein VW is set to the highest available value and then VB is adjusted to get required modulation phase depth
The value of driving voltage VB is calculated with the following equation:

where F is the function of the phase modulation of driving voltage. When the period is long enough for phase to reach saturation, both on increase and decrease, the required phase depth
is achieved. For higher frame rate (shorter period) the phase saturation cannot be achieved and total phase depth
is lower than required
The method according to claim 3, wherein phase variation over time for an LCOS device with low refresh rate. The period is long enough for phase to reach saturation, both on increase and decrease and achieve required phase depth
For higher frame rate the phase saturation cannot be achieved and total phase depth
is lower than required
However, by adjusting driving voltage VB to a new value V’B phase depth can be increased back to original
level.
The method according to claim 2 and claim 3, wherein adjusting driving voltage VB to a new value V’B phase depth can be increased back to original
level. Driving voltage VW is set to the highest available value and then driving voltage V’B is adjusted to get required modulation phase depth
at given refresh rate.

The new value of the driving voltage V’B for a particular refresh rate is calculated from the function of the phase modulation of driving voltage with the following equation:

where F is the function of the phase modulation of driving voltage, and the value of
is
Where
is the phase depth achieved at this refresh rate for driving voltage VB and VW set at low refresh rate, for example 50 frames per second.
The method according to claim 2 and claim 5, wherein Driving voltage VW was set to value 5V, the highest available for the driver, and driving voltage VB=2.48V was calculated from Equation 1 to get modulation phase depth
required to maximise diffraction efficiency for binary grating. Diffraction pattern for 543 nm wavelength was observed and power of the first diffraction order was measured for low refresh rate of 50 frames per second and used as a reference. Diffraction efficiency for fixed driving voltage was measured for refresh rate in the range from 100 to 3500 frames per second and normalised on the reference value. For every refresh rate in the range from 100 to 3500 frames per second driving voltage V’B was calculated according to Equation 2. Diffraction efficiency for adjusted driving voltage was measured and normalised on the reference value. For the normalised diffraction efficiency of 0.95, maximum refresh rate increased from 850 to 2150 frames per second.
The method according to claim2 and claim 5, wherein analyse how spatial frequency may influence diffraction efficiency at high frame rates, diffraction on gratings with different grating pitch was measured. Diffraction gratings with pitch from 512 pixels to 2 pixels, horizontal, vertical and “checkerboard” was applied and intensity of the first order diffraction was measured and normalized on its level for low frame rate.