Tunable optical wedge for beam steering

Abstract

This application relates to tunable optical wedges for beam steering. A voltage-controlled optical wedge is formed by creating an adjustable voltage gradient along the length of a relatively large dimension LC cell. A pair of bias voltages (AC voltages) are applied at opposite side terminals of the LC cell, with the RMS values selected to create a continuous phase change along the length of the cell, with a defined phase change associated with a particular beam steering angle. Adjustment of the applied bias voltages (specifically, a change in the RMS values of the bias voltages) results in a change in the beam steering angle, providing active, controllable beam steering as a function of time. The LC cell can be configured to provide a linear or non-linear continuous phase change, as preferred for different beam steering applications.

Description

Technical Field

[0001] This invention relates to providing beam steering in an optical system, and more specifically, to using a voltage-controlled liquid crystal cell as an optical wedge for adjusting the steering angle. Background of the Invention

[0002] Many applications require beam steering. For example, a LiDAR system can use a probe beam that is guided to scan a defined “target” (or, for a vehicle-based system, a traffic lane). Optical communication systems utilize wavelength selective switches (WSS), which direct an input optical signal from an input port to a selected output port out of N output ports.

[0003] Typically, beam steering is provided using glass wedge devices or liquid crystal (LC) cell arrays. Glass wedge devices provide steering by mechanically rotating a wedge to change its refractive angle, while LC cell arrays are controlled unit by unit to form a spatial light modulator with a variable steering angle. Because the physical dimensions of the wedge are fixed, the available range of beam steering cannot be tuned over time. While LC spatial light modulators overcome the physical limitations of glass wedges, LC arrays require a large number of individual LC cells (and associated electrodes), necessitating individual control of each cell in the array to provide the refractive index variation required for beam steering. The segmented nature of multiple cells naturally results in a “stepped” phase distribution in the array. The transitions between steps are related to diffraction scattering that produces the propagating beam, which is problematic for beam steering applications. Invention Overview

[0004] The present invention relates to providing a steering of a free-space beam, and more particularly, to using a voltage-controlled LC-based optical wedge as an active beam steering device that allows the steering angle to change with a change in the applied voltage.

[0005] A voltage-controlled optical wedge is formed by generating an adjustable voltage gradient along the length of a relatively large LC cell (e.g., length L in the range of 10-20 mm). A pair of bias voltages (AC voltages) applied at opposite ends of the LC cell are selected to produce a continuous linear phase change along the length of the cell, wherein the defined phase change is associated with a specific beam steering angle. Adjustment of the applied bias voltages (specifically, variation of the RMS value of the bias voltages) results in a change in the beam steering angle, providing active, controllable beam steering that varies over time.

[0006] Active beam steering using a voltage-controlled LC unit allows sensing applications (such as LiDAR) to modify the available range of beam steering angles as needed over time. In addition to controlling the selection of input / output ports in the WSS, active beam steering according to the principles of this invention also provides the ability to “correct” the alignment between the beam and a designated port by making slight adjustments to the voltage gradient. The ability to use this type of beam steering for alignment purposes is useful in minimizing insertion loss, crosstalk, polarization-dependent losses, and so on.

[0007] An exemplary embodiment of the present invention can employ a tunable optical wedge to adjust the steering angle of the propagating beam. The tunable optical wedge is formed of a liquid crystal cell comprising an LC material disposed between a pair of spaced-apart resistive layers (each resistive layer having its exposed surface covered by a transparent outer plate), the LC material extending along the length of the LC cell. L The system further includes a first pair of metal contacts, a second pair of metal contacts, and an adjustable voltage source. The first pair of metal contacts is positioned at a first end termination of the LC unit (each metal contact in the first pair is disposed along the end region of a single resistive layer in the pair of spaced-apart resistive layers). A first AC bias voltage is applied across the first pair of metal contacts. The second pair of metal contacts is positioned at opposite second ends of the LC unit (similarly, each metal contact in the second pair is also disposed along the opposite end region of a single resistive layer in the pair of spaced-apart resistive layers). A second AC bias voltage is applied across the second pair of metal contacts. The adjustable voltage source is coupled to the first and second pairs of metal contacts. The adjustable voltage source applies a first AC bias voltage and a second AC bias voltage associated with the beam steering angle θ, and the beam steering angle θ is adjusted by changing one or both of the first and second AC bias voltages.

[0008] Other and additional embodiments of the invention will become apparent in the following discussion, and with reference to the accompanying drawings. Brief description of the attached diagram

[0009] Referring now to the accompanying drawings, where similar numbers in multiple views represent similar parts:

[0010] Figure 1 This is a simplified cross-sectional side view of an exemplary tunable optical wedge formed according to the principles of the present invention;

[0011] Figure 2 yes Figure 1 An isometric side view of the tunable optical wedge shown;

[0012] Figure 3 It is a section taken along line 3-3. Figure 1 A planar diagram of the tunable optical wedge;

[0013] Figure 4 It is a graph of the voltage distribution on an exemplary LC when the applied bias voltage is used according to the principle of the present invention;

[0014] Figure 5 It is a graph showing the phase of the beam as a function of the voltage applied to a conventional LC cell, which illustrates the linear response region suitable for use in a tunable optical wedge formed according to the principles of the invention.

[0015] Figure 6 It is a graph showing the linear relationship between the change in beam phase (caused by the change in bias voltage) and the resulting beam steering angle;

[0016] Figure 7 The beam steering characteristics of the tunable optical wedge of the present invention are shown, wherein a change in bias voltage results in a change in steering angle.

[0017] Figure 8 This is a plan view of an exemplary resistive layer within the tunable optical wedge of the present invention, in which the resistive layer has a nonlinear width, associated with a wider beam steering range;

[0018] Figure 9 A comparison is depicted between a typical constant-width resistor layer and an exemplary non-linear-width resistor layer;

[0019] Figure 10 Showing the target Figure 9 A comparison of voltage variations achieved by two different resistive layers (constant resistive layer and variable resistive layer) is shown, illustrating linear variations with respect to constant width layers and nonlinear variations with respect to variable width layers;

[0020] Figure 11 Is with Figure 8 The graph associated with the nonlinear resistive layer shows the ability to extend the range of the applied bias voltage, thereby resulting in a wider beam steering range;

[0021] Figure 12 This illustrates how the phase change of the propagating beam also relates to the cell spacing of the LC unit. d The curve of the function; and

[0022] Figure 13 It is a graph showing the relationship between beam steering angle and phase change, including curves for 1 mm linear phase distribution and 3 mm linear phase distribution. Detailed description

[0023] Figure 1 and Figure 2An exemplary tunable optical wedge 10 based on a pressure-controlled liquid crystal (LC) cell 12 according to the principles of the present invention is shown. Figure 1 A simplified side view of the tunable optical wedge 10 and Figure 2 This is an isometric view; various features of the tunable optical wedge 10 may be better shown in one or another figure. Figure 1 and Figure 2 The diagram shows an LC cell 12 comprising a liquid crystal material 14 contained in a layered structure including an upper transparent plate 16 and a lower transparent plate 18. Accordingly, a pair of resistive (also transparent) layers 20, 22 are disposed between the main surface of the LC material 14 and the transparent plates 16, 18. Although not specifically shown or discussed, a thin layer of polyimide material is typically disposed between the LC material 14 and the resistive layers 20, 22 and is used to generate a “default” alignment of LC molecules without applying a voltage to the LC cell 12.

[0024] In fact, according to the principles of the present invention, as described in detail below, a controlled bias voltage is applied at opposite ends 13 and 15 of the LC cell 12 to produce a specific linear change in the orientation of the LC molecules, thereby providing the desired beam steering angle. Figure 1 As best shown, the orientation of individual molecules M within the LC material 14 is along the length of the LC unit 12 (i.e., when observed). Figure 1 (When, it changes from left to right.)

[0025] Returning to the description of the elements of the optical wedge 10, the voltage-controlled LC unit 12 is shown to also include a first pair of contacts 24, 26 formed at the first side terminal 13 of the LC unit 12 on the ends of the resistive layers 20, 22. A second pair of contacts 28, 30 are similarly formed at the opposite side terminal 15 of the LC unit 12 on the ends of the resistive layers 20, 22. These contacts are typically formed of metal (or heavily doped semiconductor material; in practice, the end regions of the resistive layers 20, 22 may be heavily doped). A first bias voltage V1 is applied across the first contacts to 24, 26, and a second bias voltage V2 is applied across the second contacts to 28, 30. The applied voltages are AC voltages that typically operate at a frequency of approximately 1 kHz. The “values” of V1 and V2 mentioned herein and discussed in detail below should be understood as the RMS values ​​of the AC voltages applied across the side terminals 13, 15.

[0026] As described above and discussed in detail below, the presence of these bias voltages at terminals 13 and 15 generates a continuous linear voltage gradient V(x) across the lateral range of the LC material 14. Based on the resistance exhibited by layers 20 and 22, the presence of the bias voltages produces a phase distribution that acts as a "light wedge" for the propagating beam (as in...). Figure 1The diagram shows optical wedges 10B (V1 > V2) and 10A (V2 > V1). By adjusting the applied voltages V1 and V2, tunable beam steering is provided over a relatively wide range.

[0027] Figure 3 It is along Figure 2 The plan view of the pressure-controlled LC unit 12 is taken from line 3-3. The formation of metal contacts 24 and 28 is shown in particular in this view, which are strips extending from the front surface 12-F to the rear surface 12-R of the LC unit 12. Figure 3 The plan view clearly shows the rectangular shape of the resistor layer 20, where layer 20 is shown as having a length L and a constant width W. Similar to conventional LC cells, layer 20 of the voltage-controlled LC cell 12 typically comprises a material such as indium tin oxide (ITO), which has a strength of 300-500... / Sheet resistance within a certain range. In an exemplary embodiment of the invention, LC cell 12 may be configured to have a length L (also referred to above as its "lateral range") of about 10-20 mm and a constant width W of about 3 mm. Given these parameters, resistive layers 20 and 22 will typically provide about 1.5-2.5 kΩ between contacts 24 and 28 (and between contacts 26 and 30). A constant resistance within a certain range allows for a continuous linear voltage gradient V(x) between cell terminals 13 and 15. This will be combined with the following... Figure 9 and Figure 10 As discussed, a resistive layer with variable width can also be used (and thus create a nonlinear relationship between the applied bias voltage and the beam steering angle). For Figures 4 to 8 The remainder of the discussion assumes that the widths of resistive layers 20 and 22 are constant.

[0028] Figure 4 This is a graph showing the voltage gradient V(x) across the range of LC material 14, which is associated with a first bias voltage V1 applied across the first metal contacts 24, 26 of LC cell 12 and a second bias voltage V2 applied across the second metal contacts 28, 30 of LC cell 12 (remember that V1 and V2 are the RMS values ​​of the applied AC bias voltages). Specifically, the graph shows the voltage variation across the length L of the voltage-controlled LC cell 12, where x=0 corresponds to the end 13 of LC cell 12, and x=L corresponds to the end 15 of LC cell 12. Therefore, the continuous linear variation of the plotted voltage V(x) is defined as:

[0029] .

[0030] Obviously, as Figure 1 and Figure 2 The embodiment of the present invention shown forms an optical wedge 10 having a linear voltage distribution from the first terminal 13 to the second terminal 15 of the voltage-controlled LC unit 12.

[0031] Based on the known properties of LC materials, the presence of a voltage gradient across the width of an LC cell causes a change in the material's refractive index and can be used to control the phase of the reflected / refracted beam (as used in prior art SLMs). For the purposes of this invention, the relationship between the voltage gradient and the phase change is used to provide active, continuous tuning of the beam steering angle for the propagating optical signal. Reference Figure 5 The phase of the beam varies with the amount applied to a conventional (small) LC cell (such as...). Figure 5 The illustration shows a graph of voltage variation, representing the gap between traditional (small) LC cells. d It is approximately 7µm.

[0032] Figure 5 The graph includes phase over a voltage range of approximately 0.5V to 1.45V. The region of linear variation. Therefore, if the bias voltages V1 and V2 are selected such that the voltage gradient V(x) remains within this range, the result is to obtain the desired steering angle by adjusting the applied bias voltage. The ability. In particular, such as Figure 5 The graph shows the linear change of the voltage gradient V(x) with phase. The linear change is related. The steering angle is known. Depends on phase As shown below:

[0033]

[0034] in, The wavelength of the guided light beam, and L The length of LC unit 12. Figure 6 Is this the kind of relationship? The graph shows the relationship between the beam steering angle and the element length. L Inversely proportional. Figure 6 The curves in the graph and L =1mm and L =3mm associated. Typically, the unit length... L It can have any suitable value ranging from tens of micrometers to tens of millimeters. In practice, a specific length can be selected that is best suited for use in arrangements where a small form factor needs to be maintained (and then the bias voltage adjusted to achieve the desired beam steering range). L Alternatively, a relatively long length can be selected that allows for more precise control over the adjustment of the beam steering angle.L (This is preferred when active alignment to a given port is required, as it reduces insertion loss, polarization-dependent loss, crosstalk, etc.)

[0035] Due to phase change It is adjusted by changing the applied bias voltage (e.g.) Figure 5 (as shown), and phase change With steering angle Directly related, therefore, according to the principles of the present invention, beam steering angle adjustment is provided by adjusting the bias voltages V1 and V2 applied to the ends 13 and 15 of the LC unit 12. Again, it should be understood that V1 and V2 are AC voltages with RMS values ​​of V1 and V2 (typically operating at a frequency of 1 kHz).

[0036] Figure 7 This is a simplified diagram of the tunable optical wedge 10, illustrating the ability to change the beam steering angle θ by adjusting the bias voltages V1 and V2 applied to the voltage-controlled LC unit 12. Here, the tunable optical wedge 10 includes a beam steering controller 40 that receives the desired beam steering angle. As input, and providing the beam steering angle that will be generated. A suitable pair of bias voltages V1 and V2 are used as the output. For a given set of parameters ( (W, L), can use the linear relationship shown above. First, determine the phase change. Then, the bias voltages V1 and V2 that produce the desired beam steering angle θ are determined. In many cases, the beam steering controller 40 may include a simple lookup table that pairs the appropriate bias voltages V1 and V2 with the desired beam steering angle θ. This can be for a set of wavelengths that can all pass through the tunable wedge 10. 1, 2, ..., N (A possible scenario in WSS applications) Create additional tables within the controller. For example, Figure 7 Three desired steering angles are shown. a , b and c The following relationship exists:

[0037]

[0038]

[0039]

[0040]

[0041] .

[0042] By using a resistive layer with a variable width W(x) in conjunction with the elements discussed above, phase changes can be extended for tunable optical wedges. The range. Figure 8 This is a plan view of the variable-width resistor layer 80, which can be added to the configuration discussed above to increase phase change. The range, which causes the beam steering angle The scope has increased. (See reference) Figure 8 The lower half of this structure is essentially the same as the structure discussed above; that is, a rectangular resistive layer 20 extending in width W and length L together with the underlying LC material 14 (not shown). However, here an extended-width upper transparent layer 16E (extending relative to the embodiment discussed above) is used, and a variable-width resistive layer 80 is formed on the upper portion 16U of the extended transparent layer 16E. For illustrative purposes, the variable-width resistive layer 80 is shaded differently from the resistive layer 20. However, it should be understood that in a typical manufacturing process, the entire layer including regions 20 and 80 (typically including ITO) is formed as a single deposition process, following curve 82 of W(x) as shown.

[0043] The change in the width W(x) of the resistive layer 80 produces a comparable change in resistance, which can be expressed as follows:

[0044]

[0045] Where Rs is the sheet resistance value of layers 20 and 22 as defined above. The variable resistance R(x) therefore provides a variable (and non-linear) voltage change V(x) on the cell:

[0046] .

[0047] Figure 9 A graph containing an exemplary comparison between a resistor layer 20 with a constant width W and an example resistor layer 80 with a variable width W(x). Figure 10 The graphs showing the voltage variation V(x) for these "constant" width resistive layers and "variable" width resistive layers are presented. In particular, it is clearly shown that using a constant width resistive layer produces a linear voltage variation across the entire LC cell, while using a variable width resistive layer produces a nonlinear voltage variation.

[0048] The nonlinear voltage variation along the length of LC cell 12 can be used to correct any nonlinearity that appears in a particular phase response of LC material 14, or to extend the phase variation into a nonlinear region (where the nonlinear voltage distribution is configured to compensate for the nonlinear phase variation).

[0049] pass Figure 11 The latter capability of the nonlinear resistive layer is shown, exhibiting the same characteristics as described above. Figure 5 The diagram illustrates the same phase change based on the voltage; however, in this case, in addition to the linear shaded region discussed above, the shaded region also includes a nonlinear region N. Here, the phase change is shown. It has been extended to approximately 4.5 radians, with an additional 1.4 radians associated with the nonlinear voltage region N. According to this embodiment of the invention, the addition of a variable-width resistive layer 80 modifies the voltage response in the nonlinear region, thereby providing a linear phase change across the entire LC cell. Therefore, an increase in beam steering angle can be easily provided without requiring a complete reconfiguration of the new LC cell.

[0050] When initially forming a tunable optical wedge from a pressure-controlled LC unit, the appropriate thickness of the LC (defined as the gap) is crucial. d ( ) is a factor to consider because of the gap d There is a direct relationship between this and the range in which linear phase changes may occur. (Reference) Figure 12 It shows phase change and unit gap d This relationship, where for the linear operation region, is defined as:

[0051]

[0052] Use this relationship and refer to Figure 12 This shows the element spacing. d Increasing the gap from 7µm to 14µm (i.e., making the gap...) d "Doubling" doubles the range of the associated phase change (and similarly, doubles the beam steering angle). (The scope is doubled).

[0053] Equally important, the region of linear phase distribution on the tunable wedge of the present invention at least covers the entire dimension of the steering beam. As mentioned above, the range of beam steering angles is inversely proportional to the length of the LC unit. Figure 13The curves illustrate the range of steering angles achievable in two cases: (1) a phase distribution of 1 mm length (curve SA1); and (2) a phase distribution of 3 mm length (curve SA2). The steeper slope of curve SA1 indicates the increase in steering angle range when the phase distribution length is 1 mm compared to 3 mm. For example, when using a 3 mm linear phase distribution, a phase change of 4 radians (rad) produces a steering angle range of approximately 0.20°. As the steering angle range increases to approximately 0.58°, the area of ​​the linear phase distribution decreases to 1 mm.

[0054] When considered in conjunction with the ability to modify the beam steering angle based on the applied bias voltage, the range of variations in element length, gap thickness, and resistive layer width (i.e., constant and variable) results in tunable optical wedges that can be designed to adapt to a wide range of applications. The ability to modify the steering range allows the arrangement of this invention to be used in various system applications, and not only to provide a relatively wide range of beam steering angles, but also to have the ability to continuously adjust the selected steering angle to provide and maintain alignment with a specified coupling port (i.e., to provide “active alignment”).

[0055] The foregoing description is exemplary in nature and not restrictive. Variations and modifications to the disclosed embodiments will likely be apparent to those skilled in the art, and these variations and modifications do not necessarily depart from the spirit of the invention. In fact, the scope of legal protection afforded to this invention can only be determined by examining the appended claims.

Claims

1. A tunable optical wedge for adjusting the steering angle of a propagating light beam, the tunable optical wedge comprising: Liquid crystal unit, the liquid crystal unit comprising: A pair of spaced-apart resistive layers, the length of which is... L It also has a resistance value in the range of 1.5-2.5kΩ, and each resistive layer is defined to include a first end and an opposite second end; An LC material, the LC material being disposed between the pair of spaced-apart resistive layers and along the length of the pair of spaced-apart resistive layers. L extend; A pair of transparent outer plates, the pair of transparent outer plates being configured as exposed surfaces covering the pair of spaced-apart resistive layers, the propagating light beam being introduced through the first transparent outer plate of the pair of transparent outer plates and exiting through the second transparent outer plate of the pair of transparent outer plates; The first pair of contact regions are coupled to the first ends of the pair of spaced-apart resistive layers. A second pair of contact regions, the second pair of contact regions being coupled to the opposite second ends of the pair of spaced-apart resistive layers; and An adjustable voltage source, coupled to the first pair of contact regions and the second pair of contact regions, applies a first AC bias voltage across the first pair of contact regions and a second AC bias voltage across the second pair of contact regions. The first AC bias voltage and the second AC bias voltage are combined with the resistance values ​​of the pair of spaced-apart resistive layers along the length of the LC material. L A voltage gradient V(x) is generated to form a voltage gradient with respect to the beam steering angle. The associated tunable optical wedge adjusts the beam steering angle by changing one or both of the first AC bias voltage and the second AC bias voltage. , The pair of spaced-apart resistive layers have a variable width W(x) along the length of the LC unit, providing a voltage across the length when both the first AC bias voltage and the second AC bias voltage are present. L The nonlinear voltage gradient V(x) is associated with the increase in the beam steering angle range relative to the linear voltage gradient.

2. The tunable optical wedge according to claim 1, wherein, The pair of spaced-apart resistive layers includes a pair of indium tin oxide (ITO) layers.

3. The tunable optical wedge according to claim 2, wherein, The pair of indium tin oxide layers are formed to exhibit an appearance of approximately 300-500. / The resistance of a thin film is on the order of magnitude.

4. The tunable optical wedge according to claim 1, wherein, The length of the LC unit is selected from the range of tens of micrometers to tens of millimeters.

5. The tunable optical wedge according to claim 1, wherein, The tunable optical wedge also includes: A beam steering controller coupled to the adjustable voltage source, the beam steering controller responding to an external command associated with a defined beam steering angle, and determining a first AC bias voltage value and a second AC bias voltage value associated with the defined beam steering angle, the first AC bias voltage value and the second AC bias voltage value being sent from the adjustable voltage source to the first pair of contact regions and the second pair of contact regions.

6. The tunable optical wedge according to claim 5, wherein, The beam steering controller also responds to adjustment signals for modifying the first AC bias voltage and the second AC bias voltage to maintain alignment between the steering beam and the beam receiving port, providing active alignment with the tunable optical wedge.

7. The tunable optical wedge according to claim 1, wherein, The first pair of contact regions and the second pair of contact regions include a first pair of metal contact pads and a second pair of metal contact pads disposed on the opposing facing surfaces of the pair of spaced-apart resistive layers.

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