Patent Application
20190025666
Acousto-optic modulators (AOMs) show promise as components of light display systems for generating light fields as are required for holographic displays and other applications. One class of AOMs are termed surface acoustic wave (SAW) optical modulators. These modulators can provide controllable sub-holograms from which a light field can be constructed.
One type of SAW modulator is the guided-to-leaky-mode device fabricated using lithium niobate as described, for example, in Hinkov et al., Collinear Acoustooptical TM-TE Mode Conversion in Proton Exchanged Ti:LiNbO3 Waveguide Structures, J. Lightwave Tech., vol. 6(6), pp. 900-08 (1988), Smalley et al., Anisotropic leaky-mode modulator for holographic video displays, Nature, vol. 498, pp. 313-317 (2013), herein after “Smalley”; McLaughlin et al., Optimized guided-to-leaky-mode device for graphics processing unit controlled frequency division of color, Appl. Opt., vol. 54(12), pp. 3732-36 (2015), Qaderi et al., Leaky-mode waveguide modulators with high deflection angle for use in holographic video displays, Opt. Expr., vol. 24(18), pp. 20831-41 (2016), hereinafter “Qaderi”; and Savidis et al., Progress in fabrication of waveguide spatial light modulators via femtosecond laser micromachining, Proc. of SPIE Vol. 10115, 2017.
In these SAW modulators, surface acoustic waves (SAWs) diffract light propagating in the modulators' waveguides and cause at least some of the light to change from guided modes to leaky modes that exit the waveguides at angles dictated in part by the frequency of the light and the frequency of the SAWs.
Currently proposed SAW modulator-based holographic display systems generate holographic images from stored or computed representations of a 3D scene. Such systems can project still holographic images, or holographic video by translating each frame into electronic control signals for the display systems. The holographic images are typically encoded as one or more views of the scene in each frame, with a 2D image (brightness of each color component of each pixel) for each view. Each view corresponds by definition to a different angle of light emission from the display. As a result, a view dictates the exit angle of light from the display, and an observer will see one or more views of different pixels depending on the location of their pupils relative to the display, in a way that mimics some or all of the depth cues of a real 3D object.
In any event, the pixel brightness information for all the views in any one frame is encoded into light signals provided to the SAW modulators and/or radio frequency drive signals that are used to generate the SAWs in the modulators. Holographic display systems using SAW modulators sometimes require wave propagation cancellation between the light signals and the SAWs that co-propagate or counter propagate along the length of the waveguides. Wave propagation cancellation is required because the light signals and SAW are traveling waves that move through or adjacent to the waveguide, whereas the systems require an image that is stationary or moving in an arbitrary way. For example, a displayed point in a view may cover only half of a waveguide, such that the left side of the waveguide might need to emit light while the right side remains dark. However, the same SAW that would cause light to scatter from the left side of the waveguide will then travel to the right side of the waveguide, where it would cause undesirable scattering.
One undesirable outcome of unmitigated SAW propagation is image motion, which is usually perceived as image blur by an observer. For example, the acoustic velocity of a typical SAW in a lithium niobate substrate having an x-cut, y-propagating waveguide is 3,909 meters per second (m/s). Current approaches for accomplishing wave cancellation include descanning of the modulated light signals using spinning mirrors, and a “traditional strobe” modality that applies pulsed light signals to the SAW modulator.
The spinning mirror descanning is analogous to that used in 1930s era scophony television displays. See H. W. Lee, “The Scophony Television Receiver,” Nature, 142, 59-62 (9 Jul. 1938). When scophony-type scanning is applied to electro-holographic display, the light signals are applied as a continuous wave (CW) and a spinning polygonal mirror continually shifts the apparent location of the AOM modulator at an equal and opposite speed from the SAWs. While this works, it also typically requires thick form-factors and moving parts. Examples of descanning-based holographic displays include the “MIT Mark” series of prototypes. See St Hilaire, “Scalable optical architecture for electronic holography,” Optical Engineering 34(10), 2900-2911 (October 1995), and Smalley, Smithwick, and Bove, “Holographic video display based on guided-wave acousto-optic devices”, Proc. SPIE 6488, 64880L, 2007.
When employing the traditional strobe modality, the SAWs are created as one would in a descanning display (i.e. the ideal desired optical phase modulation pattern), but strobed light is used instead of continuous-wave (CW) illumination to accomplish the wave propagation cancellation. The pulses of the strobe light are timed consistent with the repetition rate of the SAW so that the SAWs appear to be stationary. For example, see Jolly et al., “Near-to-eye electroholography via guided-wave acousto-optics for augmented reality”, Proc. SPIE 10127, 101270J (2017) and references therein.
The present invention provides improvements over current light field generators, such as holographic display systems, using AOMs such as SAW modulators. It can be used to limit the peak power required from the optical source, such as a laser. It also concerns multiple approaches for wave propagation cancellation among the light signals and SAWS within SAW modulators.
In general, according to one aspect, the invention features a light field projection system. This system comprises an array of surface acoustic wave (SAW) modulators for projecting a light field, an optical source generating light, and a directional switch for dividing the light among the SAW modulators of the array.
In embodiments, the directional switch divides the light for two or four or more groups of SAW modulators.
This division can be serial in time. Additionally, the different groups being associated with distinct quadrants of the array of SAW modulators or interlaced groups of SAW modulators.
The directional switch could be implemented as one or more Mach-Zehnder interferometers.
In general, according to another aspect, the invention features a light field generation method comprising generating light, dividing the light, and delivering the light to an array of surface acoustic wave (SAW) modulators for projecting a light field.
In general, according to another aspect, the invention features a light field projection system. This system comprises an array of surface acoustic wave (SAW) modulators for projecting a light field and a light modulator for generating light signals for the SAW modulators that encode brightness information for different views.
Preferably, a radio frequency (RI) drive circuit that generates the same RF signal for multiple SAW modulators. These RF signals can that determine the views. The light signals can be pulsed signals or continuous wave signals and encode brightness information.
In general, according to another aspect, the invention features a light field generation method. This method comprises generating light signals for an array of SAW modulators that encode brightness information and generating RF signals for the array of SAW modulators that encode views.
In general, according to another aspect, the invention features a light field generation system comprising one or more acousto-optic modulators, such as SAW modulators, in which scene-specific information is conveyed in optical signals provided by light modulators to the acousto-optic modulators.
In general, according to another aspect, the invention features an acousto-optic modulator for a light field generation having a continuous waveguide for light re-circulation.
In general, according to another aspect, the invention features a SAW modulator, including a SAW substrate and one or more light emitting chips bonded to the SAW substrate.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:
The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present.
The projector module 400 includes two electro-holographic light field generator devices 300-1 and 300-2. They are located in the projector module 400 side by side with their proximal faces 160 extending parallel to the plane of the figure.
The two electro-holographic light field generator devices 300-1, 300-2 are mounted to a common module board 402. of the projector module 400. An RE connector 404 is installed on the module board 402 and interfaces with a ribbon umbilical cable 420, for example that provides one or more RE drive signals produced by an RF drive circuit 25 for this module 400 and other modules modules of a display system. At the common module board 402, the module RF connector 404 then distributes the RF drive signals via an RE feed line network 406.
Each electro-holographic light field generator device 300-1, 300-2 comprises an array 202 of SAW devices or modulators 200. The SAW devices 200 are fabricated in piezoelectric, crystalline, SAW substrates 120-1 and 120-2, respectively. The longitudinal axes of each of these SAW devices 200 extend parallel to each other, across each light field generator device 300. In the specific illustrated embodiment, each light field generator device 300-1, 300-2 includes an array 202 of three (3) SAW devices 200-1, 200-2, 200-3.
Of course, in other embodiments, usually larger numbers of SAW devices 200 are provided in each light field generator device 300 and/or in each SAW substrate 120. In a preferred embodiment, there are at least ten (10) such SAW devices 200 per each light field generator device 300/SAW substrate 120. Even higher levels of integration are envisioned.
Each SAW substrate 120 may be made, for example, of lithium niobate. In the current embodiment, the SAW substrates 120 are x-cut, y-propagating, measuring 5 millimeters (mm) (in the direction of the waveguides 102)×10 mm (in a direction perpendicular to the waveguides 102, but in the plane of the figure)×1 mm (substrate 120 thickness). Many other materials and design choices are available, however, including other piezoelectric materials and crystallographic orientations, and waveguide architectures such as planar, ridge, rib, embedded, immersed, and bulged. Doping such as MgO-doped lithium niobate may be useful, in some cases.
Each SAW modulator 200 includes an in-coupling device 106 (e.g., in-coupling grating or prism), a waveguide 102 and a SAW transducer 110 (e.g., an interdigital transducer or IDT, for example).
In the illustrated embodiment, the in-coupling device 106 of each SAW modulator 200 is an in-coupling grating. The grating receives input light 101 carried by a respective optical fiber pigtail 122 that terminates above the respective grating 106. This input light is provided from a light modulator 30 that supplies this light to this module 400 and the other modules in the system.
There are, of course, other ways to couple light into the waveguides 102 of the substrates 120, however. These include butt-coupling to the pigtails 122, free-space illumination, and fiber or free-space coupling into an in-coupling prism.
In a typical design, the waveguides 102 provide confinement of the input light in a TE (transverse electric, E-field in the plane of the device) guided mode. In a current embodiment, the waveguide 102 is 100 micrometers wide (in the plane of the figure) and 1 micrometer thick (perpendicular to the plane of the figure).
The SAW transducers 110 are driven by an RF input signal that creates a corresponding surface acoustic wave (SAW) 140. The surface acoustic wave 140 counter-propagates collinearly with the light in the waveguide 102. The SAW interacts with the guided mode light in the waveguides 102 to convert or diffract part of the light to a transverse magnetic (TM) polarization, leaky mode.
Here, the SAW transducers are interdigital transducers that are approximately 1 mm long (i.e., in the direction of the waveguide 102) and have features on the order of 1-3 micrometers. IDT pads 128A, 128B are each roughly 300 micrometers×300 micrometers.
Birefringence of the waveguide 102 and the SAW substrate 120 causes the TM leaky mode portion to leak out of the waveguide 102 into the SAW substrate 120 when guided mode light interacts with the SAW. The leaky mode portion of the light enters the substrate 120 as diffracted light 162, which travels within the substrate 120 towards an exit face. Here, the exit face is an end face 170 of each SAW substrate 120 of each light field generator device 300-1, 300-2.
In different embodiments, the IDTs 110 can occupy a variety of specific locations and specific orientations with respect to the waveguides 102. For example, in the illustrated embodiment, the transducers 110 are located near the end face 170 so that the surface acoustic waves 140 will propagate in a direction opposite the propagation of the light in the waveguides 102. In other embodiments, however, the transducers 110 are located near the in-coupling devices 106 so that the surface acoustic waves 140 will co-propagate in the direction of the light in the waveguides 102.
Also, there could be multiple SAW transducers 110 for each in-coupling device 106/waveguide 102. In such an implementation, each SAW transducer 110 might be responsible for a different specific bandwidth around a given center frequency (e.g.: 100-200 MHz, 200-300 MHz, and 300-400 MHz).
Moreover, additional transducers could be added to provide more than one beam-fan axis, such as by adding a transducer oriented at an angle to SAW transducers 110, for scanning along different axes.
in a specific embodiment, the array 202 of SAW optical modulators 200 may be packed relatively tightly with a waveguide separation 206 of between 10 μm-400 μm, for example, 50 μm. The waveguide length WL may be less than a centimeter to several centimeters (e.g., 1 cm) long.
In terms of the SAW modulator operation, the input light signal 101 is carried to the device via the optical fiber pigtail 122. In the illustrated embodiment, end 122-E of the optical fiber pigtail 122 is polished at an angle and preferably metallized or coated with another reflective coating. Thus, the optical signal 101 transmitted by the pigtail 122 is reflected at the end 122-E toward the in-coupling grating 106 of the SAW modulator device 200. As a result, the optical signal is coupled into the waveguide 102 via the grating 106.
In some examples, the optical fiber pigtails 122 are arranged on and bonded to the surface of the substrate 120. In other cases, the pigtails are placed such that they lie on or within trenches formed into the proximal face 160 of the SAW substrate 120. Still another option involves a focused fiber beam at the modulator's entry face that has been polished at an angle.
At the other end of the SAW modulator device 200, the IDT 110 generates the surface acoustic wave (SAW) 140 that counter propagates with the light in the waveguide 102. When they interact along the length of the waveguide, as illustrated at point I, the surface acoustic wave 140 diffracts the optical signal 101 to create the diffracted light 162 that leaks out of the waveguide 102.
In the illustrated embodiment, the diffracted light 162 exits the substrate 120 via end face 170 as the exit face, i.e., edge-fire.
It should be noted, however, that in other embodiments, the exit face might alternatively be the distal face 168 or the proximal face 160, to create a face-fire configuration. One technique for creating a face-fire configuration is to mirror-coat the end face 170 and pick a different edge cut angle β (beta) for the end face 170. Another technique is to extend the length of the modulator so that the diffracted light has an opportunity to reach the distal face and possibly 1) add a reflective element, e.g. a reflective diffraction grating, to the distal side so that it redirects the light out towards the proximal face or 2) add a transmissive element, e.g. a transmissive diffraction grating, to the distal side so that it directs the light out the distal face.
In the specific illustrative example, the edge cut angle β is polished into the end face 170. The edge cut angle β is measured from a plane 126 of the proximal face 160, to the end face 170. Here, the edge cut angle β is preferably about 100 to 140°, or about 120°. As a. result, when the diffracted light 162. exits the substrate 120 into air, for example, the edge cut angle β in combination with the refraction at this interface causes the exit light 150 to propagate in a direction that is generally parallel to the longitudinal axes of the SAW devices 200 and parallel to the plane 126 of the proximal faces 160 of those devices 200. Preferably, the exit light is controlled to have wavefront curvature, such as pixels with corresponding focus.
Exit optics are typically further used. Their purpose includes angle magnification, polarization, and elliptical diffusing. The optics can be separate from the substrate 120 or fabricated on the end face 170, in examples.
In terms of the construction of this specific example projector module 400, the SAW substrate 120 is attached to a top face 412 of the module board 402. In the illustrated implementation, the rear end of the substrate 120 can be separated from the top face 412 of the module board 402 via an optional rear standoff block 408. On the other hand, the front end of the substrate 120 is separated from the top face 412 of the module board 402 via a series of front conductive blocks or pads 410.
In addition to supporting the front end of the substrate 120, the front conductive blocks 410A, 410B are also utilized in the delivery of the RF signals to the IDTs 110 of the SAW devices 200. In more detail, the RF signals from the RF connector 404 are routed over the top face 412 or through layers of the module board 402 in the RF feed line network 406 of the module board 402 and to the front standoff blocks 410, which are electrically conducting. Pairs of conformal RF traces 124A and 124B electrically connect to respective front standoff blocks 410A, 410B. The conformal RF traces 124A and 124E then extend forward, on the distal face 168 of substrate 120 and then wrap around the edge to the end face 170, and extend over the end face 170 to the proximal face 160. On the proximal face, the conformal RF traces 124A, 124B run rearward to make contact with respective IDT bond pads 128A, 128B that connect with the IDT 110.
Here, a single SAW substrate 120 is attached to the top of the module board 402, rather than two as depicted in
More details are shown concerning the RF feed line network 406. The feedlines include an array of traces that run on or through the board 402 and carry separate RF signals. In this way, the module board 402 has an array of RF teed lines 406 for providing RF signals to the substrates.
Also shown is a specific implementation of the module RF connector 404. A ribbon umbilical cable 420 plugs into a ribbon-style connector 404 as the module RF connector. The connector 404 is attached to the top face 412 of the module board 402.
The optical fibers 122 run in groups and connect to provide the optical signals to the separate SAW devices 200.
In other embodiments, however, the ribbon-style connector 404 is replaced with Pogo pins, press-fit, conductive adhesives, wire-bonding, or ZEBRA-brand (Fuji Polymer Industries) elastomeric connectors.
In this way, a two-dimensional array of SAW modulators is implemented that can be controlled to project a light field that will enable one or more views of different pixels depending on the location of the viewer relative to the system 10. The projected light field will mimic some or all of the depth cues of a real 3D object.
The holographic display system 10 also includes a controller module 60, and the RF drive circuit 25, the light modulator 30, and the optical source module 40.
In one example, the RF drive circuit 25 can generate hundreds of RF signals 15 that are distributed to the projector modules 400 of the system 10.
The optical source module 40 will often include one or more lasers. These lasers will each generate light signals of different wavelengths (colors). In one example, the optical source module 40 includes three separate lasers that each generate light signals 101 of different visible wavelengths, such as red, green, and blue light. Each wavelength of light signals 101 is preferably provided as the input to each separate SAW modulator 200, according to one implementation.
In operation, the controller module 60 receives frames 82 of holographic image data 80. These frames 82 might represent “still” images of a scene. The frames 82 will further encode one or more views 84 and brightness information 88 for each view 84.
The desired light field within each frame 82 (e.g. views 84 and brightness information 88 for each view 84) can be represented many ways. Typically, the views 84 are 2-D representations of pixels such as in a bitmap, though other implementations are possible. The pixels within each view 84 are encoded at the same angle of illuminated light.
In conjunction with the SAW signals, the controller module 60 also controls the optical source 40 and/or light modulator(s) 30 to create light signals 101 with appropriate time, intensity and duration to create the desired output light field. For this purpose, in one example, the controller module 60 can control the generation of the light signals from the source 40 to have the appropriate time, intensity and duration by strobing/electronically pulsing the optical source 40.
The surface gratings output couplers 410 can be fabricated via standard photolithography or laser writing processes.
Here, the diffracted light travels through the substrate and is reflected by gratings 410 to exit the proximal face 160 at potentially three or more emissive regions or pixels corresponding to exit light beams 150-1 to 150-3. In other examples, the gratings 410 could be transmissive optics or gratings such that light exits via the distal face 168.
Each of the projector modules 400 receives input light 101 generated by illumination source 40 and modulated by modulator 30. From this light, it produced different views 84 in different directions.
It is also important to note that display or light field generator systems 10, though described in the specific context of 3D display systems, also can usefully be applied to other applications such as optogenetics, 3D printing, cloaking, and near-eye displays for augmented reality/virtual reality (AR/VR).
When these signals are applied to SAW modulators, the arrangement will provide for wave propagation cancellation. Since the light signal is simply pulsed and distributed in common the SAW modulators, holographic display systems of this type do not necessarily require the light modulator.
In the “traditional strobe” modality, the controller module directs the RF drive circuit to provide modulated RF signals, and directs the optical source to “strobe” the light signals that are synchronized with the modulated RF signals. When illuminated by a strobe light, the SAW signals induced in the SAW modulators by RF drive signals can be made to appear stationary, or can be changed in a controllable way, rather than appear to travel along the waveguide.
In the “traditional strobe” modality, the waveform of the RF signals is typically different for each SAW modulator, and different for each frame. In contrast, the intensity-vs-time profile of the light signal is typically the same for each SAW modulator in the system, and the same for each frame.
As a consequence of the traditional strobe modality scheme, the RF signals and thus the SAW signals encode image and view information, particularly how much brightness information is included into which views. The brightness information of each view (e.g. the brightness of each pixel within a 2-D map of pixels forming a view, in one example) is controlled by the SAW waveform. If the SAWs have a strong Fourier component at a certain frequency, the SAW modulator sends a significant amount of light in the corresponding direction. The light signals provide brightness in the trivial sense that light is bright, but do not encode the brightness information. The light signals in the traditional strobe modality are generally the same regardless of the view of the scene, i.e. the light signals encode no information whatsoever.
The traditional strobe modality has disadvantages. Each SAW modulator requires a different RF waveform/RF signal. This poses challenges for the hardware and software which calculate the waveforms, the RF chain for generating, frequency-converting, and amplifying the RF signals, the controller module, and the RF cabling/waveguide(s), and ultimately to the fingers of the IDTs of the SAW modulators. Finally, the RF drive circuit approaches a 100% duty cycle for generating RF signals for images of bright scenes and the RF signal amplitude must also be carefully controlled to control pixel brightness.
With reference to a waveguide 102 having pixels, or spatial columns 102-1, 102-2, 102-n at different locations along the length of the waveguide 102, the controller module can provide wave propagation cancellation between the SAW 140 and the light signals propagating within and along the waveguide 102, relative to an eye-integration time of individuals. In this modality, because the SAW 140 that carry the views 84 and brightness information 88 for each view 84 complete their propagation along the entirety of the waveguide 102 according to a sound propagation time of each SAW 140, the controller module 60 typically signals the light source 40 to provide its light signals 101 to the waveguide 102 only once per sound propagation time of the SAW 140.
The SAW is represented as a collection of smaller SAW subsignals, which are individual labeled “1,” “2,” “3,” . . . “20”, each of which are positioned at a different spatial column 102 along the length of the waveguide at a given snapshot in time.
For illustration purposes, in this example, the waveguide 102 is 20 millimeters (mm) long, while the desired spatial resolution of the holographic image formed by the modulated emitted light signals is only 1 mm. Therefore, there are twenty (20) pixels 102-p along the waveguide 102, each of which has a desired profile of brightness for each view 84, and a corresponding SAW sub-signal which can generate this brightness-vs-view profile. These SAW sub-signals are labeled “1” through “20”. An operator of the holographic display system typically has flexibility in the assignment of a SAW sub-signal to its instantaneous or time-averaged diffractive purpose; typical examples from the literature include a pixel, a hogel and a wafel.
It is important to note that
The controller module directs the optical source to generate pulsed light signals, where the pulses are timed to the SAW propagation time through the waveguide, which is generally much less than the eye-integration times of users. Eye-integration times of users is typically 1/60 of a second (0.17 sec), which corresponds to an eye “refresh rate” of 60 Hz. The controller module can provide a number of hologram refreshes per eye-integration time based upon the waveguide length, SAW velocity, and perhaps temporal multiplexing configuration within each frame.
In the illustrated example, the controller module might utilize a 260 nanosecond (ns) periodic pulse to the light source, thereby producing a modulated light signal having a period of (20)×(260 ns)=5.2 microseconds. As a result, the controller module can provide 3205 holograms per eye integration time: (0.17 seconds per eye-refresh integration time)×(5.2 microseconds per hologram)=3205 holograms per eye integration time.
The modulated light signals are created according to the timing diagram of
In the illustrated example, a pixel is illuminated (e.g. “turned on”) when the SAW is positioned such that each of its sub-signals 89 is at least half overlapping its corresponding waveguide sub-section. When the same SAW 140 has traveled such that its sub-signals are less than half overlapping their corresponding waveguide sub-section at a later point in time, according to the pulse width (here, 260 nanoseconds), the illumination is removed from the waveguide.
In more detail, the light modulator 30 is implemented as a directional switch that multiplexes the light signals 101 from the optical source module 40 among different groups of SAW modulators 200 of the light field generator system 10. The modulator/directional switch 30 might be a cascade of Mach-Zehnder interferometers as in
In more detail, in one implementation, four exemplary groups of SAW modulators A through D are part of a holographic display system 10. In other implementations, there may be only two groups of SAW modulators. However, in other embodiments, there might be eight (8) or more groups.
The different groups associated with distinct quadrants of the two-dimensional array of SAW modulators of the system 10 in one example. In other embodiments, the different groups might be alternating projector modules 400 within the two-dimensional array of SAW modulators of the system 10, however.
Unlike the traditional strobe approach to synchronization, however, not all groups of the SAW modulators A-D are strobed at the same time. The controller module 60 instructs the modulator/directional switch 30 to alternate the strobing of the light signals 101 serially among the four different groups of SAW modulators A-D. The modulator/directional switch 30 directs the light signals sequentially to the first group during time slot t1, to the second group during time slot t2, to the third group during time slot t3, and the fourth group during time slot t4, then the process repeats. As a result, in this specific example, the peak power of the optical source 40 can effectively be reduced by a factor of 4.
The reduction in the peak power required by the optical source module 40 has several benefits. Various component requirements/specifications of the system 10 can be relaxed, which includes selection of the laser optical sources 40 and the current drivers that power the laser optical source module 40, choice of external waveguides, fiber optic cabling, and/or couplers that the various light signals 101 pass through before being split into the individual waveguides 102 of the SAW modulators 200.
In more detail, exemplary RF signal 15 is shown and the same RF signal is distributed to multiple SAW modulators 200. In practice, the same RF signal might be distributed to different groups of SAW modulators in a sequential fashion or to perhaps all of the SAW modulators 200 of the system 10, in parallel.
On the other hand, different modulated light signals 101-1 and 101-2 are generated for each of the SAW modulators 200 of the system 10. These light signals 101 might be in the form of continuous wave (CW) signals 101-1 or pulsed signals 101-2 as shown.
In this approach, the SAW signals 140 generated in each SAW modulator 200 encode the views that are to be projected, and the light signals 101-1 or 101-2 encode/carry the brightness-vs-position information 88.
This approach might still be characterized as a variant of the traveling pulse modality. Different traveling pulses are generated by the RE drive circuit 25. These different traveling pulses of different RE frequencies are required for each view 84 because the traveling pulse frequency encodes the view 84.
In operation, each view 84 can he repeated multiple times because there is enough time in one 60 Hz frame to do so, and because repeating each view reduces the peak power requirements upon the optical source module 40 and other components. In certain time-multiplexing scenarios, however, it may be advantageous to provide only one traveling pulse per view 84 of each frame 82.
The modulated light signals 101 generated by the light modulator 30 are different for each SAW modulator 200 within the two-dimensional array of SAW modulators of the system 10, however.
Either continuous wave (CW) 101 or pulsed 101 modulated light signals can be used. In examples, the pulsed 101 modulated light signals could be applied if micro-lenses are utilized to spread the light, to avoid scattering at the boundary between pixels, and for locating the pulse at the appropriate part of the pixel to get the desired view direction. The continuous wave (CW) 101 modulated light signals are typically applied when the brightness carrying modulated light signals 101 propagating within the waveguide 102 of SAW modulator 200 require a change in brightness/intensity at an appropriate time in synchronization with the view-information-carrying SAW signals 140 that are also propagating within or near the waveguide 102.
One possible approach to time synchronization is to have a butler for each SAW modulator 200, update the buffers in a time-multiplexed way, but then read the buffer values for each SAW modulator 200 simultaneously, to match the simultaneous SAW signal waves 140. Another possibility is to have different SAW modulators 200 switch intensities at different times, but compensate for that by time-offsetting the SAW modulators 200, output optics, or other components between different SAW modulators 200.
In
In the traveling pulse modality, in
Additionally or alternatively, the optical source 40 peak power can be reduced, which prolongs the life of the laser source 40 and reduces damage to the components of the system 10 caused by the laser source 40.
For bright scenes, the light signals 101 diffract off the SAW signals 140 all along the length of waveguide 102 within the SAW modulators 200 of the array of the system 10. Therefore, if the outcoupling efficiency of the guided modes into the modulated emitted light signals 150 is high, then there is less light at the end of the waveguide 102 than at the beginning. Addressing this problem requires a combination of low outcoupling efficiency, use of short waveguides 102, software pre-compensation for this effect, and additional headroom in the display brightness budget, all of which are undesirable. In the traveling pulse modality, in contrast, light is only emitted from a small part of the waveguide at a time, even in a very bright scene, so this issue does not arise.
In
In the traveling pulse modality, typically, the pulses of the SAW signals 140 are relatively narrow-band (compared to the total bandwidth used in the display), with different-frequency pulses used for different views. The length of each pulse of the SAW signals 140 is comparable to the desired spatial resolution of the display, for example 250 ns (if the speed of sound is 4 km/s and the desired spatial resolution is 1 mm), and the bandwidth of the pulse is comparable to the reciprocal of that number, for example 4 MHz and is typically between 2 and 6 MHz.
As in the traditional strobe modality, the traveling pulse modality allows long waveguides 102 (longer than the pixels or unit of spatial resolution/view information) and has no moving parts. However, the most fundamental difference is that in the traveling pulse modality, the scene-specific brightness information 88 is conveyed via the light modulators 30 rather than via the RE drive circuit 25.
In a variation of the traveling pulse modality, the pulses of RP signals 22 applied to the SAW transducers 110 all have the same frequency, and the wavelength of the laser optical source 40 is changed instead.
In
In
As the SAW pulse 140 propagates within and long the waveguide 102, the controller module 60 directs the light modulator 30 to make the light signals 101 of the optical source brighter and dimmer depending on the desired light in that view of the pulse's current location as shown in
To create a quasi-continuum of different brightness levels, one or more lasers of the optical source module 40 can be modulated in intensity, and/or switched on and off quickly (faster than the desired spatial resolution divided by the speed of sound) with modulated duty-cycle, and/or, if the same type of pulse is transmitted multiple times within the same visual frame 82. The laser intensity could also be set to more than one level during the various repetitions of each pixel-view, where the eye of the observer 99 averages the brightness level to an intermediate brightness level.
When the SAW pulse 140 has completed passing through the waveguide 102, a new pulse of the RF signal 15-2 excites a corresponding new pulse of the SAW signals 140 within the waveguide 102 that encodes the next view 84 of the frame 82. This process is repeated for the remaining views for the current frame 82 and for all subsequent frames. Thus in a system that can projected 8 different views, the frequency of the pulses are selected to access each of those views using 8 different frequencies.
As a variation, the pulse of the SAW signals could also encode a certain angle and certain focal plane, which has some benefits as explained in Smithwick et al., “Real-time shades rendering of holographic stereograms”, Proc. SPIE 7233, 723302 (2009), if this is compatible with other aspects of the optical design.
A SAW modulator 200 of a holographic display system 10 is tasked with recreating the exemplary pattern of brightness information 88 in the table. In one example, for the traveling pulse modality, the controller module 60 first accesses the leftmost column, with label “View 1” for view 84-1. The controller module 60 directs the RE control circuit 25 to send a modulated RF signal 15 that induces a pulse of the SAW signals 140 in the waveguide 102 that is appropriate to encode the view 84-1 for that that column, and additionally sends control signals to the light modulator 30 to have bright light, then dim light, then no light, then bright light, and so on as the pulse 140 passes through locations 1, 2, 3, 4 (spatial columns 102-1 through 102-4) and to 102-N. After that pulse 140 has completed propagating, the controller module 60 induces a new pulse of the SAW signals 140 which is appropriate to the next column (“View 2”), and additionally controls the light modulator 30 as appropriate for the brightness information 88 for the pixels of view 84-2.
The controller module 60 iterates through all columns until the final column is processed, repeats as necessary until it is time to access the next frame 82, and then repeats the process again. In one example, with 4 km/s speed of sound, 3 cm long waveguide 48, 60 Hz video rate, and 100 different columns of views 84 per frame 82, the controller module 60 can cycle through all the columns as many as 22 times each within a single display frame. In another example, the controller module 60 could process the brightness information 88 by iterating through each column 22 times before processing the next column (e.g. processing the brightness information 88 for View 184-1 22 times, then processing the brightness information 88 for View 284-2 22 times, etc)
In contrast, a SAW modulator using the traditional strobe modality recreates this entire pattern every time the strobe light modulator 30 turns on, and this is executed once per transit time of the SAW signals.
In
In general, the light modulator 30 comprises an input waveguide 208, which is preferably a single mode waveguide. This waveguide has been formed in a lithium niobate substrate, which could be the same substrate 120 as the modulators 200. The input waveguide 208 branches between a first arm 210 and a second arm 212. These two arms later merge into the output waveguide 214. In the illustrated embodiment, there are electrodes 21 on the first arm 210.
Depending on the voltage difference applied to the two metal electrodes 21, the transmission to the SAW modulator(s) 200 can vary from near 100% to near 0%. In more detail, the electric field generated by the electrodes 21 causes a phase shift in the corresponding first arm 210 relative to the second arm 212. This leads to constructive and destructive interference in the output waveguide 214. Optionally, a phase shifter can be included on the other arm of the interferometer.
One advantage of this light modulator 30 is that it is relatively broadband, so the same voltage setting could potentially be used for red, green, and blue light, for example.
The switches 59 can be arranged into a tree or branching arrangement as shown in
In more detail, in the illustrated embodiment, a first level switch 59-1 receives the input optical signal from the optical source 40. This first level switch 59-1 divides and controls the division of the input optical signal between two waveguides to two second level switches 59-2-A and 59-2-B. In turn, these second level switches 59-2-A and 59-2-B divide their received light between four third level switches 59-3-A, 59-3-B, 59-3-C, and 59-3-D. The various output waveguides 214-1, 214-2 . . . 214-8 then provide the input optical signals 101 to either groups of SAW modulators 200 or individual SAW modulators 200 of the display system 10.
More detail, each switch 59 is implemented as a Mach-Zehnder interferometer fabricated in a lithium niobate substrate, which could be the same substrate 120 as the SAW modulators 200. Specifically, two input waveguides 208-1, 208-2 receive input light and then merge at a combiner 216, before dividing again between the first arm 210 and the second arm 212.
As before, the first arm includes two electrodes 21. After the electrodes, the first arm 210 and the second arm 212 combine and then divide again into two output waveguides 214-1 and 214-2. By controlling the voltage applied to the electrodes 21, the phase shift in the first arm 210 can be controlled to thereby control the amplitude of the optical signal appearing on the two output arms 214-1, 214-2.
Comparing the embodiments of the light modulator 30 in
The precise timing of the laser optical source 40 amplitude modulation provided by the light modulator 30 and controlled by the controller module 60 should be synchronized to the RF drive circuitry 25. The timing system may account for subtle factors like frequency-dependent speed of sound as the pulses travel, frequency-dependent time offset (e.g. from chirped IDTs 100 which effectively emit different pulses from different locations), and temperature-dependent speed of sound.
Each light modulator 30 typically operates by controlling the voltage of a certain metal electrode lead 21. Typically, the voltage needs to change each 50-500 nanoseconds (specifically, the desired spatial resolution, divided by the propagation velocity of the SAW signals 140). This is true even when applying very short laser pulses to the SAW signals as in the traveling pulse modality, since the pulse shuttering can happen at the (common) laser source 40, rather than at the individual light modulators 30.
A holographic display system 10 might contain hundreds or thousands of these light modulators 30-1/30-2, which must be individually controlled to convey scene-specific information. Through well-known techniques such as time-multiplexing and latching, only one or a few signal lines of the controller module 60 can pass digital or analog signals to each of hundreds of light modulators 30-1/30-2. Each light modulator 30-1/30-2 can store either just the present voltage level, or can store an array of voltage levels to be cycled through repeatedly.
In
This increases the fraction of the light diffracted by the SAW modulator 200. Note that there are no SAW modulators 200 in the return path of the laser optical source 40. While this configuration can be applied to all the embodiments of the holographic display system 10 described hereinabove and can operate under all wave propagation cancellation modalities, the racetrack configuration works especially well for the traveling pulse modality and within the single-pixel SAW modulator embodiment. This is because the single-pixel SAW modulator embodiment of the holographic display system 10 and any of the previous holographic display systems 10 that apply the traveling pulse modality generally have short light-SAW modulator interaction lengths, and thus generally only a small fraction of the light is outcoupled in a single pass.
The low numerical aperture (NA) of these waveguides 102 could limit the turn radius in a SAW modulator 200. Although at ‘some’ radius this will be possible it may be worth mentioning that end optical interconnects/mirrors or some such optical element may be required to recirculate the light in a real device,
Additionally and/or alternatively, a second laser optical source 40 input can be applied to the system 10 that transmits light signals 101 in a counterclockwise fashion in
Here, a separate VCSEL chips 40-V-1, 40-V-2, 40-V-3 are bonded to each grating input coupler 106 on the SAW substrates 120 of each light field generator devices 300-1 and 300-2. These VCSELs generate light that is coupled into the guided mode of the respective waveguide 102.
Thus, in this example, the light modulations is performed electronically by an electrical drive light modulator 30-E. Each of these laser or LEI) light emitting chips together function as the optical source 40. Then, the input lights to each SAW modulator is controlled by modulating the drive current of each laser or LED via the controller module 60.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 62/534,866, filed on Jul. 20, 2017, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62534866 | Jul 2017 | US |
