Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/0087—Phased arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
  • H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
  • H01Q3/2676—Optically controlled phased array
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B2207/00—Coding scheme for general features or characteristics of optical elements and systems of subclass G02B, but not including elements and systems which would be classified in G02B6/00 and subgroups
  • G02B2207/117—Adjustment of the optical path length

Definitions

• the present invention is in the field of time delay devices, such as those that may be used for the control of phased-array radars, communication systems, or correlators.
• This invention relates to apparatus for producing true-time delay devices, such as those useful in the control of phased array radars. It is desirable to use a system that produces signals to control the timing of the emission of each of a plurality of electromagnetic radiation beams, delaying each of them in time by some time increment. The delay in each signal should be capable of being controlled independently of the other signals.
• Phased array radars have the advantage that the radar beams can be steered electronically by changing the phase or timing of the signal radiated by the individual elements of the array.
• the present invention includes time delay devices and time delay systems.
• the invention also includes machines and instruments using those aspects of the invention.
• the invention may also be used to upgrade, repair, or retrofit existing machines or instruments, using methods and components known in the art.
• the present invention comprises a true time device that falls into the free-space category but uses a multiple-pass optical cell with refocusing mirrors that has the advantage of avoiding beam-spreading problems.
• This approach differs from previous free-space approaches in that it uses only one optical switch or spatial light modulator instead of one or more switches for each bit.
• the microwave signal for each antenna element may be modulated onto an optical beam. After the individual optical beams are delayed by the desired amount of time, the signals may then be down-converted to microwave signals for further processing. This process may be used in either the transmit or the receive mode of the phased array radar.
• the device for producing optically-controlled incremental time delays of the present invention comprises: (1) an input device selected from the group comprising light sources adapted to generate individual light beams or arrays of light beams from one or several directions, (2) an adjustable input mirror capable of reflecting light from the input device in different directions, (3) a set of optical elements selected from the group consisting of mirrors, lenses, filters and prisms placed in a configuration so as to define a multitude of light paths for each light beam from the input device reflected by the adjustable input mirror, (4) at least one refocusing element to restrict the divergence of a light beam diverted through at least one of the light paths, (5) a spatial light modulator adapted to select a path from among the light paths for each pass through the set of optical elements of an individual light beam from the input device, (6) an output mirror adapted to reflect the light beams emerging from the set of optical elements, and (7) a receiving device capable of responding to the delays in the light beams reflected by the output mirror.
• an input device selected from the group comprising light sources adapted to
• the device for producing optically-controlled incremental time delays of the present invention may also include at least one system of optical transmission lines or waveguides wherein the lengths of the light paths may be varied in a confined space consisting of a subset of the optical elements.
• the spatial light modulator may consist of a polarizing spatial light modulator which changes the polarization of individual light beams directed to the spatial light modulator.
• the use of a polarizing spatial light modulator then may require the addition of a beam-splitting device that can direct light beams through the system of optical elements in multiple directions depending on the polarization of the light beams after passing through the polarizing spatial light modulator.
• the device may alternatively include a micromechanical or deformable mirror device spatial light modulator capable of reflecting the individual light beams in multiple directions, thereby determining the optical path.
• the light sources adapted to generate individual light beams or arrays of light beams from one or several directions may include such devices as lasers, arc lamps, and light emitting diodes.
• the receiving devices capable of responding to the delays may include devices such as photodetectors, pin diodes, photodiodes, and interferometers.
• Figure 1 is a top view of a standard White cell on which the present invention is based.
• Figure 2 is a front elevational view of the spatial light modulator, along with one embodiment of the input and output mirrors in accordance with the present invention.
• Figure 3 is a top view of the dual-arm cell with a beam splitter in accordance with one embodiment of the present invention.
• Figure 4 is a top view of a quadratic cell, where the distances from the spatial light modulator to the White cell mirrors vary, in accordance with one embodiment of the present invention.
• Figure 5 is a perspective view of the dual arm cell with a set of glass blocks and auxiliary mirror in accordance with one embodiment of the present invention.
• Figure 5a is another perspective view of the dual arm cell in accordance with one embodiment of the present invention.
• Figure 6 is a diagram of a White cell using a deformable mirror device spatial light modulator and an appropriate prism in accordance with one embodiment of the present invention.
• Figure 7 is a diagram of a multiple arm version of the deformable mirror device configuration in accordance with one embodiment of the present invention.
• Figure 8 is a perspective view of an alternative cell configuration in accordance with one
• Figure 9 is a side elevational view of a system of lens groups in accordance with one embodiment of the present invention.
• Figure 10 is another side elevational view of the image planes in the optical transmission line in accordance with one embodiment of the present invention.
• Figure 11 is a ray diagram for a spherical mirror in accordance with one embodiment of the present invention.
• Figure 12 is another ray diagram of a spherical mirror/lens system in accordance with one embodiment of the present invention.
• Figure 13 is a plot of path distances in accordance with one embodiment of the present invention.
• Figure 14a is a ray diagram of a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 14b is another ray diagram in accordance with one embodiment of the present invention.
• Figure 15 is a ray diagram for a small angle prism in accordance with one embodiment of the present invention.
• Figure 16a is a diagram showing spot location on a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 16b is another diagram showing spot location on a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 16c is another diagram showing spot location on a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 17 is another diagram showing spot location on a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 18 is another diagram showing spot location on a deformable mirror device in accordance with one embodiment of the present invention.
• Figure 19a is a diagram of reflected planes in accordance with one embodiment of the present invention.
• Figure 19b is another diagram of reflected planes in accordance with one embodiment of the present invention.
• Figure 20a is a diagram of a light beam incident on a DMD in accordance with one embodiment of the present invention.
• Figure 20b is another diagram of a light beam incident on a DMD in accordance with one embodiment of the present invention.
• Figure 1 is a diagram of the path of a light beam passing through a White cell.
• the cell comprises three identical spherical mirrors, all of the same radius of curvature.
• the first mirror 12 is separated from the second 13 and third 14 mirrors by a distance equal to their radii of curvature.
• the center of curvature 15 of the first mirror lies on the centerline or optical axis 16 and falls between the second and third mirrors.
• the second and third mirrors are aligned so that the center of curvature 20 of the second mirror 13 and the center of curvature 19 of the third mirror 14 land on the first mirror, for example an equal distance from the optical axis. Light from the second mirror is imaged onto the third mirror, and vice versa.
• Light is input onto a spot 18 in the plane of but off the edge of the first mirror; the light beam is prepared so that it expands as it goes to the third mirror.
• the third mirror refocuses the beam to a point on the first mirror.
• the beam is then reflected to and expanded at the second mirror.
• the second mirror refocuses the light beam to a new spot 17 on the first mirror.
• the light may either exit the cell if the spot is off the edge of the first mirror, or continue to traverse the cell.
• the beam may traverse the cell a predetermined number of times, depending on the locations of the centers of curvature of the second and third mirrors.
• the angle of the input beam may be controlled by an input turning mirror 21, as shown in
• the angle of the output beam may similarly be controlled by an output turning mirror 22.
• Each bounce of a light beam is shown by a spot 23 on the turning mirrors or the first mirror 12.
• a spatial light modulator or other appropriate device may alternatively replace the first mirror.
• a beam of light may be reflected off the input turning mirror into the White cell, and may traverse the cell until the beam is directed to the output turning mirror, at which point it may exit the cell.
• Figure 3 shows a first modification to the White cell to adapt it to variable time delay applications.
• a first modification is to change the first mirror 12 from a curved mirror to a flat one and to add a lens 27 of focal length such that the lens-mirror combination is optically equivalent to the mirror it replaces.
• the flat mirror may be replaced with a spatial light modulator.
• This particular spatial light modulator may be configured to rotate the direction of polarization of the reflected beam by ninety degrees at any particular pixel that is activated.
• a polarizing beam splitter 28 may be added, and the distances to the second 13 and third 14 mirrors may be adjusted to maintain imaging.
• the input light may be polarized in the plane of the paper.
• the beam splitter may reflect light polarized in the plane perpendicular to the paper but transmit light polarized parallel to the plane of the paper.
• a better photonic device may be implemented by next adding a fourth 24 and fifth 25 mirror, where these mirrors are identical but have a focal length different than that of the second and third mirrors.
• a lens 26 of different focal length may be added to the other output side of the beam splitter. The focal lengths of the lenses are chosen to compensate for the new mirror locations.
• auxiliary mirror 29 may be added in the plane of the spatial light modulator 12.
• a second auxiliary mirror may be placed in the image plane of the spatial light modulator.
• a time delay mechanism such as a set of glass blocks 30 may be substituted for this second auxiliary mirror, as shown in Figure 5a.
• the blocks are reflective on the sides furthest from the lens 41.
• the glass blocks could be replaced by optical fibers or an array of fibers.
• the optical axis is between the spatial light modulator and the auxiliary mirror.
• the first lens 27 has been made larger to cover them.
• the thicknesses of the glass blocks may be chosen so that the additional time required for the beams to go through successive blocks increases as powers of two times the initial thickness.
• the operation is comparable to that of the dual cell with the plane of the spatial light modulator enlarged and additional time delays due to the addition of the glass blocks or equivalent transparent materials.
• a simple White cell can be constructed as shown in Figure 6.
• a prism 32 may be used to direct the light beam through a focusing lens 33 onto the appropriate mirror 34 off the optical axis.
• Figure 7 also shows that another prism 37 may be introduced to direct light from the deformable mirror device spatial light modulator 31 through a refocusing lens 38 onto the other off-axis mirror 39 in the dual-arm configuration.
• FIG 8. Another possible configuration of the dual arm cell is shown in Figure 8.
• the second and third mirrors of the first arm of the original device are replaced by the second 35 and third 39 mirrors of the new configuration.
• the fourth and fifth mirrors that comprised the second arm of the original device are then replaced by the second mirror 35 again, along with the first mirror 34 of the new configuration.
• light beams may bounce from the second or third mirrors to the first or second mirrors, then back to the second or third mirrors, mimicking the operation of the original dual-arm cell.
• An additional lens 40 may be used to image the spatial light modulator onto an auxiliary mirror 29, and a lens 41 may be used to image the spatial light modulator onto a delay mechanism such as glass blocks 30.
• a prism such as 32 and its adjacent lens such as 33 may be replaced with a single lens that is appropriately tilted or decentered or both.
• Figure 9 shows such a lens waveguide.
• Three lenses 42 form a lens group 43.
• the lens groups may then be placed along a common optical axis to form an optical transmission line or lens waveguide. The light comes into the optical transmission line from the right.
• the input plane 44 is coincident with an auxiliary mirror plane.
• At the left of each lens group is an additional plane conjugate to the auxiliary mirror plane.
• a transparent material may be placed at these conjugate planes, as shown in Figure 10.
• Each sheet of transparent material 45 may have a reflective strip 46 on a portion of its surface. This permits light beams incident on different areas of the waveguide to propagate through different lengths of the optical transmission line. Materials and Methods
• curvature of Mirror C is a distance ⁇ below the optical axis. To the left of the White cell mirrors
• optical ray matrices are used. These matrices operate on a column vector where y, n, and p y refer to the projection of a ray on the y - z plane.
• the vector element y represents the displacement of the ray from the optical (z) axis at some value of z.
• the element /? > represents the slope of the ray at that point and n is the refractive index in the region.
• the third matrix element "1" is used in representing a tilted spherical mirror as will be shown later.
• a similar analysis could be used with y replaced by x andp y replaced by p x for the projection of the ray on the x - z plane.
• 3 x 3 ray matrices are used because they will be useful in representing the tilted spherical mirrors.
• Three ray matrices are used. The first is the matrix T(d, ⁇ ), representing a translation through a material of refractive index n by a distance d in the axial direction.
• the second is the matrix L(f) representing a thin lens of focal length
• the thin lens matrix is identical with that of a spherical mirror of focal length /with its center of curvature on the axis.
• a last matrix represents a spherical mirror tilted so that a line from the intersection of the
• Line CCP makes angle cc ⁇ with the incoming ray and angle ⁇ with the reflected ray, as shown in
• The-center of curvature is a distance ⁇ above the optical axis
• point P is a distance y above the optical axis and a distance y ' above point CC.
• the first requirement is that Mirror B be imaged onto Mirror C so that no light will be lost by rays starting from Mirror B and missing Mirror C.
• the second requirement is that Mirrors E and F be images of each other.
• the image of the center of curvature of mirror B on the SLM can be called the center of curvature point.
• _v 2 -vi + 2 ⁇ d.
• y ⁇ is the location of a point source on the SLM and > is the location of its image after the light from the source has passed through lens fl , been reflected off Mirror B and passed back
• the SLM 12 is shown, along with a polarizing beam splitter 28, lens f2 27, the White cell mirrors (13, 14, 24, 25) and Auxiliary Mirror I 29 and a group of glass blocks 30.
• the size of the polarizing beamsplitter is d ⁇ and its refractive index is n ⁇ .
• the focal length of cell lens f2 is f 2 and is separated from the White cell mirror by a distance dgF.
• the distance, _J 2 between lens fl and Auxiliary Mirror I is divided into two regions, one of thickness d" 2 filled with air or other material, and the other of thickness d' 2 filled with material of refractive index n 2 .
• d min 61.46cm, 1 mm/sec.
• Deformable Mirror Device SLM To derive a ray matrix for a particular situation two equations are needed, one showing how the distance of a ray from the axis changes as the ray moves through the object, and the other showing how the ray slope changes.
• DMD are oriented with their normals at +#and some at - ⁇ , as shown in Figure 14a, where for one
• the surface of the DMD may be defined as a vertical line (y direction) intersecting the center of each pixel so that part of the pixel is behind the surface and part is in front of it.
• a ray can enter from the right with an angle p 0 , as shown in Figure 14b, and intersects the pixel at a distance y above the center of the pixel and is reflected off the pixel.
• the matrix equation also applies to a tipped plane mirror if the tip angle is small. If the tip angle is not a small angle, however, then the approximation does not hold and there will be an increase in distance from the axis.
• a prism with a small angle is considered, as shown in Figure 15.
• a prism with its apex pointing down can be considered.
• the refractive index of the prism material is n.
• a dual White cell is shown in Figure 3 connected by a polarizing prism beamsplitter.
• the distances between the SLM and Mirrors B and C are the same, and the distances for light reflected off the polarizing beamsplitter going to Mirrors E and F are the same.
• the distance from the SLM to Mirrors E and F is greater than the distance from the SLM to Mirrors B and C. In operation, a light beam bounces from the SLM to one of Mirrors B, C, E and F and back again on each traverse of the cell.
• the polarizing beamsplitter and the SLM determine which cell the beam goes to on each pass.
• the polarizing beam splitter transmits light of one polarization, say the plane of the figure, and reflects light of the polarization perpendicular to the plane of the figure. If the light starts out going to Mirror B with polarization in the plane of the figure and the SLM does not change the polarization, it is then reflected back and forth between the SLM and Mirrors B and C. Conversely, if the light starts towards Mirror E with polarization perpendicular to the plane of the figure and the SLM does not change the polarization, it will continue to reflect between the SLM and Mirrors E and F.
• the path of a beam can be changed from one cell to the other by using the SLM to rotate the plane of polarization as the beam bounces off the SLM.
• the present disclosure discusses a set five possible imaging conditions.
• First, the focal length of lens fl is chosen to image Mirror B onto Mirror C and vice versa.
• Second, similar to the first condition, the focal length of lens f2 is chosen to image Mirror E onto Mirror F and vice versa. This requirement may be met by placing Mirrors B and C in the right hand focal plane of lens fl and by placing Mirrors E and F in the focal plane of lens f2.
• the third condition is that Mirror B should be imaged onto Mirror F, and Mirror C should be imaged onto Mirror E.
• the requirement that Mirrors B and C be in the focal plane of lens fl together with the requirement that Mirrors E and F be in the focal plane of lens f2 also satisfies this condition.
• the last two imaging conditions are also comparable.
• the fourth condition is that the focal lengths of Mirrors B and C are chosen so that, in conjunction with lens fl, Mirrors B and C image a small spot of light on the SLM back onto another small spot on the SLM.
• the last condition is that the focal lengths of Mirrors E and F are chosen so that, in conjunction with lens f2, a small spot of light on the SLM is again imaged back onto the SLM.
• a point of light starts on a small mirror next to the SLM called a turning mirror.
• the light is directed towards Mirror B.
• Mirror B images the spot light onto the SLM.
• the light is reflected off the SLM and imaged by lens fl onto Mirror C, which images it to a different spot on the SLM. It then goes to mirror B, which again images it onto the SLM.
• the light bouncing back and forth forms a sequence of spots on the SLM.
• the polarization is changed by the SLM to be perpendicular to the plane of the figure, the light bounces in a similar fashion back and forth between Mirrors E and F and the SLM.
• the plane of polarization of the light can be changed at any bounce off the SLM so that any combination of paths in cells one and two can be chosen.
• the quantity of interest, the transit time through the cell is the number of bounces off Mirrors B and C times the transit time from the SLM to Mirror B and back, plus the number of bounces off Mirrors E and F times the transit time from the SLM to Mirror E.
• Figure 2 is a view of the SLM looking from lens fl , showing the most traditional spot configuration.
• the SLM is assumed to have a square shape. Also shown are two long thin mirrors, the input and output turning mirrors respectively below and above the SLM. For this
• the turning mirrors are centered at distances of ⁇ 2m ⁇ where m is an integer related to the number of times the light is re-imaged onto the SLM and the SLM is taken to have dimension
• the light is then reflected back and re-imaged by Mirror C.
• the point image is opposite the center of curvature of Mirror C and an equal distance from it.
• v # + ⁇ is the location of the point midway between the two centers of curvature.
• the equation still gives two columns of spots parallel to the line between the centers of curvature, the y-axis. The spots alternate from one column to the other as n increases. In general, a distance 4 ⁇ separates the spots in a given column. The vertical positions of the spots in one column are,
• the spot size be ⁇ . Since the spots are separated by a
• ⁇ 2 ⁇ , i.e. the distance between the centers of curvature of mirrors B and C should be equal to the spot size.
• the spots may also be separated
• the number of bounces on the SLM was taken to be equal to the number of spots from top to bottom on the SLM. This may well not be the case. In many situations the number of spots from top to bottom on the SLM may be of the order of many hundreds but only tens of bounces may be desired. In that case, groups of spots can bounce back and forth in synchronization. Two examples can be considered. There can be sets of spots arranged in columns being reflected, or other array of spots being reflected. In both cases the set of spots can be reflected five times off the SLM. The use of spot arrays allows one to make most effective use of the SLM capabilities.
• the time delays possible with the dual cell are also considered. As described, there are a number of beams, each executing n bounces. Each beam can go to either Cell I, which includes Mirrors B or C, or go to Cell II, i.e. Mirrors E or F.
• D BC is defined to be the optical distance in Cell I, i.e. from the SLM to Mirror B or C and back.
• D EF is defined to be the optical distance in cell II, i.e., from the SLM to Mirror E or F and back.
• dec is the distance from the cell lens to either Mirror B or Mirror C, and the corresponding
• n ⁇ and d ⁇ are the refractive index and size of the prism respectively.
• T The total time delay
• Tc (li ) mD B c in this device.
• the second term is proportional to i and is variable.
• the time increment, AT can be expressed in terms of design parameters, d B c, d ⁇ F , f ⁇ , and f ⁇ .
• D B c and D EF can be replaced using previous equations.
• d ⁇ can then be eliminated in each cell using a previous imaging condition, written with d replaced by dec for Cell I and by d ⁇ F for cell II. The result is
• the dual arm cell can again be extended.
• the distances d B c and d ⁇ F to the pairs of White Cell mirrors are made unequal.
• the optical distance from the SLM to Mirror F is made greater than that from the SLM to Mirror E.
• Figure 4 is identical to the configuration of Figure 3 with the exception that Mirror F has been replaced with Lenses Gl 25c and G2 25b and Mirror G3 25a.
• Lens Gl is chosen so that, in conjunction with Lens F2, the SLM is imaged onto Lens G2.
• Lens G2 is conjugate with the SLM.
• Lens G2 is chosen to image the plane of Lens Gl onto Mirror G3 with unit magnification, so that Mirror G3 is conjugate with the plane of Mirror Gl, which is also conjugate to Mirror E.
• Mirror G3 is chosen to have its center of curvature on Lens G2. The image of its center of curvature then also lies on the SLM, and is located so that the spots bounce as mentioned previously. The imaging conditions of the dual arm cell are still satisfied.
• An alternative configuration with lenses Gl and G2 replaced by mirrors is also included in the present disclosure. Further, Gl and G2 may be combined into a single lens.
• n the number of bounces
• n the number of bounces.
• the difference in transit times between the SLM and Mirrors B or C and Mirror E is then set equal to the smallest desired time increment, AT.
• the difference in transit times between Mirrors B or C and Mirror G3 is set to
• the spot starts on the turning mirror next to the SLM and goes first to Mirror B and back to the SLM. From the SLM there are two choices, towards either Mirror C or Mirror G, depending on the polarization of the light leaving the SLM. Upon return the light can either go to Mirror B or Mirror E. After odd-numbered bounces off the SLM, the light can go to either Mirrors C or G. After even-numbered bounces off the SLM, the light can go to either Mirrors B or E. The light bounces half the time off Mirrors B or E and half the time off Mirrors C or G. The shortest transit time occurs when the light always goes to Mirrors B and C and the longest transit time occurs when the light always goes to Mirrors E and G.
• the transit time for a given sequence of bounces can then be expressed by letting i be the number of bounces off Mirror E and be the number of bounces off Mirror G. 0 ⁇ , j ⁇ (n/2). Then the number of bounces off Mirrors B and C will be ((n/2) - i) and ((n/2) -j) respectively.
• T( ⁇ j) ( ⁇ lc) ⁇ ((n/2)- ⁇ ) D BC + ⁇ D EF + ((n/2) -j) D BC +jD EG ]
• the design of the dual armed TTD unit can additionally be modified to improve the number of potential time delays, where the number of delays can be proportional to 2" rather than to n 2 .
• the modification can be done in two parts: first by adding Auxiliary mirrors in the plane of the SLM, and then adding time delay mechanisms in conjunction with the auxiliary mirrors.
• the dual cell with auxiliary mirror is shown in Figure 5 as a three-dimensional rendition of the dual cell. Added to it are two mirrors, one over the SLM called Auxiliary Mirror I, and an extra mirror or other reflective material over the edge of the beam-splitting cube, called Auxiliary Mirror II.
• the optical axis is between the SLM and Auxiliary Mirror I.
• Lens fl has been made larger to cover the SLM and Auxiliary Mirror I.
• the turning mirrors are at the left of Auxiliary Mirror I and the SLM.
• the operation is comparable to that of the dual cell with the plane of the SLM enlarged.
• the light starts on the Input Turning Mirror and goes first to Cell I.
• Mirrors B and C image the Input Turning Mirror spot onto the SLM. If the SLM does not change the polarization the light is imaged onto Auxiliary Mirror I and again onto the SLM. If the SLM changes the polarization, the light that is reflected off the beam-splitting cube is imaged onto Auxiliary Mirror II. Light leaving Auxiliary Mirror II is similarly re-imaged back onto the SLM.
• Other configurations satisfying the same requirements but having various advantages are also presented in the present disclosure.
• the SLM surface is then divided into m areas such that each beam falls once into each area. Thirty-six input spots are shown on the turning mirror in Figure 18. Only the images associated with the upper left hand turning mirror spot are shown on the SLM and Auxiliary Mirror for simplicity.
• the length of the path traveled by the beam can be changed in Cell II on the traverse in which it strikes each of the different areas.
• Extra path length can be placed in front of or in place of each area of Auxiliary Mirror II.
• the methods of increasing the path length will be presented shortly.
• the distance associated with the increase in path length is chosen to be a binary function of some minimum distance, AL, and AT is the minimum non-zero time delay.
• T 2mD BC + AT ( ⁇ x + 2 ⁇ x + 4 ⁇ , + ... + 2 (m ⁇ ) ⁇ m ) where the describe whether the i-th delay is
• T - 2mD B c- The factor of two in front of D B c occurs because the distance from the SLM to Auxiliary Mirror I and back required for this case is twice the distance from the SLM to Mirror B or Mirror C and back as required for the Dual Arm cell.
• AL can be implemented in many ways. For small time increments, blocks of material such as glass can be added next to the auxiliary mirror. For larger time increments an optical transmission line of the desired length may be added.
• the method of adding blocks of glass or other transparent material is shown in Figure 5a, where blocks of glass of different thickness are shown. The blocks are oriented to replace Auxiliary Mirror II. The thickness of the blocks are chosen so that the additional time required for the beams to go through successive blocks increases as powers of two times the initial thickness. In operation, the light in a given beam goes either to each section of Auxiliary Mirror II and receives the associated delay, or goes to Auxiliary Mirror I and receives no delay.
• the beam's polarization is such that it passes through the beamsplitter. This beam goes to Mirror B or C, from which it goes to Auxiliary Mirror I, encounters no glass blocks, and receives no delay. If on that pass the beam's polarization has been changed, the beam goes to Mirrors E or F and thence to Auxiliary Mirror II where it passes through the associated extra optical distance of the glass block.
• d ⁇ and n ⁇ are the thickness and refractive index of the beamsplitting prism.
• the lens transmission line provides another method of generating time delays that may be appropriate when the delays are much longer than those allowed by the glass block method.
• the situation is shown in Figure 9.
• the plane on the right 44 is the input or object plane and is intended to be coincident with and replace the plane of Auxiliary Mirror II.
• Light that was reflected off Auxiliary Mirror II now proceeds to the left into the lens system.
• Figure 10 shows the plane of Auxiliary Mirror II and the five conjugate planes in three dimensions. As all the groups may operate identically, only one group will be considered. Recall that at the input to a group and the output to a group there is a plane conjugate to Auxiliary Mirror II (and therefore to the SLM). There are three imaging tasks performed by a group. The first task is basic to the operation and will be considered immediately. The other two tasks deal with light conservation.
• the first possible task of a lens group is to image the input conjugate plane onto the output conjugate plane.
• the lens in the center performs that operation.
• the focal length of the lens, / can be separated from both conjugate planes by a distance 2/ This can produce the desired imaging.
• the input and output conjugate planes are related by a magnification of -1.
• each conjugate plane In operation, a portion of the areas of each conjugate plane are replaced by vertical strip mirrors. These areas correspond to the areas of the glass blocks in the previous design. This is seen in Figure 11 where the shaded areas correspond to the mirrors.
• Light incident in Area I of the plane of Auxiliary Mirror II passes through it and Group Gl, and is reflected at conjugate plane 45a by the vertical strip mirror placed to cover the image of Area I.
• Light passing through Area II of the plane of Auxiliary Mirror II passes through Lens Groups 43a and 43b, and is reflected by a mirror placed in Conjugate Plane 45b at the image of Area II.
• light in areas III, IV, and V is reflected by mirrors strategically placed in Conjugate Planes 45c, 45d, and 45e.
• the length of the lens groups determines the time delays.
• the length of the first lens group may be chosen so that the light that travels through Lens Group 1 , reflects from the strip mirror in Conjugate Plane 45a, and travels back has the shortest desired time, AT.
• the length of the second group is equal to that of the first group so that the transit time through groups 43a and
• the second task involves conserving optical throughput.
• the left-hand lens in each group is chosen so that when the plane mirror is placed next to it, it then images the center lens onto itself. This may be accomplished by letting the focal length of the left-hand lens be equal to the distance of that lens from the center lens.
• the lens and plane mirror combination will have a focal length of half the center-to-left hand lens distance and there is a magnification of -1 so that the edges of the center lens are indeed imaged onto themselves.
• Another way of considering the operation of the left- hand lens arises because its focal point is on the center- imaging lens.
• the left-hand lens collimates light leaving any point on the center-imaging lens. It is still collimated after being reflected by the plane mirror so upon return it is refocused by the left-hand lens back onto the center-imaging lens.
• the left-hand lens is in actuality a field lens placed next to the output conjugate plane. Since it is next to the conjugate plane it does not affect the imaging of the center lens onto that plane.
• the third task performed by a lens group is again devoted to conserving light. It is to assure that all the light entering the center imaging lens of one group left the center imaging lens of the proceeding group. To do this, the center lens of one group can be made the image of the center lens of the proceeding lens. This can be accomplished by properly choosing the right hand lens of the group so that, when combined with the left-hand lens of the preceding group, the desired imaging is produced. This can be accomplished by making the focal length of the right hand lens equal to the distance between the right hand lens and the center lens, so that the focal point of the right hand lens is on the center lens.
• the segmented mirrors can optionally be replaced with gratings that reflect one wavelength and pass all others, such as a Bragg grating. Then for a beam bouncing through the White cell, the delay it experiences would depend on its wavelength.
• DMD Deformable Mirror Device spatial light modulator
• the DMD has the potential advantages of higher information density and faster speed. But it also has some associated problems that have to be addressed.
• the DMD is a pixilated spatial light modulator. That is, the reflecting surface is divided into incremental image areas.
• Each image element has a mirror surface that can be independently rotated to two positions, for example making angles of ⁇ 10° with the surface.
• the elements can modulate the direction of the reflected light by changing the input direction to one of two output directions. It does this by individual image element.
• the direction change can be transformed into an amplitude change by directing the reflected light through an aperture or directing it to something blocking it. Pulsing the mirror between transmitting and blocked states, at a rate faster than eye response, can also change the average observed amplitude.
• the angle, ⁇ . of tip is ⁇ 10 ° on presently available devices so that light incident normal
• elements or pixels are currently square, ⁇ 6 ⁇ m on a side with a spacing of 17/_/m between centers.
• each image element There is a hole in the center of each image element roughly 6/ ⁇ in diameter. The pixels rotate
• the DMD presents an interesting pattern on reflection. To see this, compare it to a flat mirror 47 as shown in Figure 19a. The intersection of the mirror surface with the x - z plane
• this average plane is not perpendicular to the direction of propagation of the reflected light.
• the fact that the elements of the DMD do not lie in one continuous surface makes it more difficult at times to image the DMD in reflected light.
• the difficulty is shown in Figure 20a.
• a DMD 49 at the left is normally illuminated with a beam from the right.
• the reflected light is imaged with a lens 50.
• the object plane nor the image plane is perpendicular to the direction of propagation of the light leaving the DMD.
• the object plane and image plane are parallel only if the magnification is unity or if the lens is rotated to be parallel to the object plane.
• the main problem is that the object and image planes are not perpendicular to the direction of propagation.
• One way to remedy the problem of the object plane and image plane not being perpendicular to the direction of propagation is to use an associated prism to change the direction, as shown in Figure 6.
• the DMD is illuminated with light normal to its surface as before, and a prism is placed in the reflected beam.
• the directions and the angles of the prism have been adjusted to remove the angular offset of the DMD.
• the lens is then used in a normal fashion.
• the first of the three linear equations represented by the matrix equation shows that upon reflection the position of the ray remains constant and the second linear equation shows that the
• the prism compensates nicely for the angular deflection of the DMD.
• the equivalent of the Dual White Cell with unequal arms using the DMD is shown in Figure 7.
• the DMD 31 is at the left and to the right of the DMD are lens /l 36 and spherical Mirror C 35.
• lens /' 2 33 and Mirror M 34 The prisms counteract the angular effects of the DMD as described.
• the SLM-imaging conditions, and the light- conserving conditions it is simpler to consider the light-conserving conditions first.
• the light-conserving conditions are that Spherical Mirrors B, C, and M be imaged onto each other and no light is lost going around the outsides of Mirrors B, C or M. This is accomplished by placing Mirrors B, C, and M in the focal planes of Lenses/, /, and/ 2 respectively.
• the curvatures of Mirrors B, C, and M are all chosen so that in conjunction with lenses/, /, and 2 the DMD is imaged back onto itself.
• images of the centers of curvature of Mirrors B, C and M through Lenses/, /, and/ 2 lie on the DMD.
• the result is the equivalent of the dual cell in Figure 3 with equal arms.
• the light can go from Mirror C to Mirror B and back or from Mirror C to Mirror M and back depending on the state of a given pixel.
• the DMD decides between the two paths on any particular bounce.
• Lens/ can be chosen to image the DMD onto Lens/ and the radius of curvature of Spherical Mirror D chosen so that its center of curvature is on Lens /.
• the center of curvature of Spherical Mirror lies on Lens /, it is imaged by Lens/ onto the DMD as required.
• the distance from the DMD to the lenses /, /, and / 2 is designated d 0 , and the focal lengths of Lenses/, /, and/ 2 can be taken to be equal.
• light travels a distance 4(d 0 +/) to Spherical Mirror B and back and then to Spherical Mirror C and back to get "into the system".
• the light can go either to Mirror B and back and to Mirror C and back, a distance of 4(d 0 +/) or it can go to Mirror D and back and then to Mirror C and back, a distance of 4( ⁇ sO +/) + 8 t .
• the light goes to Mirror D and to the turning pixel, a distance of 2(d 0 +/) + 8/. Then if there are m bounces. m 2 of which are switched to Mirror D, the expression for the transit time through the cell is
• the time increment is (l/c)8w 2 / and there are m 2 choices, as before.
• FIG. 5 The binary cell of Figure 5 is considered next, with auxiliary mirrors and a means for extending distances.
• the equivalent of Figure 5 with the auxiliary mirrors is shown in Figure 8.
• Figure 8 is derived from the equal arm cell made with the DMD. The difference is that Spherical Mirrors B and M have been realigned so that the DMD is imaged onto the auxiliary mirrors rather than back onto itself. The transition to DMD-based optics has been made. All that remains is to add the either the glass blocks or the optical waveguide. The area of Auxiliary Mirror II can then be divided into strips. Auxiliary Mirror II can now be removed and replaced with the glass blocks or the lens waveguide. In Figure 1 1 , Auxiliary Mirror II has been removed and replaced with the entrance to the lens waveguide. The operation is the same as described in the dual arm binary device.

Priority Applications (4)

Applications Claiming Priority (4)

Publications (3)

Family

ID=26848129

Family Applications (1)

Country Status (5)

Cited By (7)

Families Citing this family (23)

Citations (3)

Family Cites Families (15)

• 2000
  • 2000-08-25 EP EP00961366A patent/EP1210641A4/en not_active Withdrawn
  • 2000-08-25 KR KR1020027002528A patent/KR20020062625A/ko not_active Application Discontinuation
  • 2000-08-25 WO PCT/US2000/023361 patent/WO2001014924A1/en not_active Application Discontinuation
  • 2000-08-25 JP JP2001519225A patent/JP2003536090A/ja active Pending
  • 2000-10-16 US US09/688,904 patent/US6266176B1/en not_active Expired - Lifetime
  • 2000-10-16 US US09/688,478 patent/US6525889B1/en not_active Expired - Lifetime

Patent Citations (3)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events