This work describes fabrication and characterization of anisotropic leaky mode modulators for holographic video.
Holovideo displays are based on light-bending spatial light modulators. One such spatial light modulator is the anisotropic leaky mode modulator. This modulator is particularly well suited for holographic video experimentation as it is relatively simple and inexpensive to fabricate1-3. Some additional advantages of leaky mode devices include: large aggregate bandwidth, polarization separation of signal light from noise, large angular deflection and frequency control of color1. In order to realize these advantages, it is necessary to be able to adequately characterize these devices as their operation is strongly dependent on waveguide and transducer parameters4. To characterize the modulators, the authors use a commercial prism coupler as well as a custom characterization apparatus to identify guided modes, calculate waveguide thickness and finally to map the device’s frequency input and angular output of leaky mode modulators. This work gives a detailed description of the measurement and characterization of leaky mode modulators suitable for full-color holographic video.
Most holographic display technologies, such as pixelated light valves as well as MEMs devices and bulk wave acousto-optic modulators, are too complex to allow for broad participation in their development. Pixelated modulators, especially those with filter layers and active back planes may require dozens of patterning steps to build5 and may be limited by fan-out6. The greater the number of patterning steps the higher the device complexity, and the tighter the fabrication protocol must be to achieve reasonable device yield7. Bulk-wave acousto-optic modulators do not lend themselves to wafer based processes8,9. Anisotropic leaky mode modulators, however, require only two patterning steps to fabricate and utilize relatively standard microfabrication techniques10,11. The accessibility of these processes make it possible for any institution with modest fabrication facilities to participate in the development of holographic video display technology12.
The simplicity of device fabrication can be beguiling, however, as the proper function of the devices is strongly dependent upon waveguides which must be carefully measured and adjusted to achieve the desired device characteristics. For example, if the waveguide is too deep, the device's operational bandwidth will be narrowed13. If the wave guide is too shallow, the device may not work for red illumination. If the waveguide is annealed too long, the shape of the waveguide's depth profile will be distorted, and the red, green and blue transitions may not sit adjacent in the frequency domain14. In this work the authors present the tools and techniques to perform this characterization.
The leaky mode modulator consists of a proton exchanged waveguide indiffused on the surface of a piezoelectric, x-cut lithium niobate substrate15,16. At one end of the waveguide is an aluminum interdigital transducer, see Figure 1. Light is introduced into the waveguide using a prism coupler17. The transducer then launches surface acoustic waves which interact contralinearly with light in the waveguide along the y-axis. This interaction couples guided light into a leaky mode which leaks out of the waveguide into the bulk and finally exits the substrate from the edge face18,19. This interaction also rotates the polarization from TE polarized guided light to TM polarized leaky mode light. The surface acoustic wave pattern is the hologram, and it is capable of scanning and shaping the output light to form a holographic image.
The waveguide is created by proton exchange. First, aluminum is deposited on the substrate. Then the aluminum is patterned photo-lithographically and etched to expose regions of the substrate to become waveguide channels. The remaining aluminum acts as a hard mask. The substrate is immersed in a melt of benzoic acid which alters the surface index in the exposed regions. The device is removed, cleaned and annealed in a muffle furnace. The final depth of the waveguide determines the number of leaky mode transitions. The waveguide depth also determines the frequency of each guided-to-mode transitions for each color4.
The aluminum transducers are formed by liftoff. After waveguides are formed, an E-beam resist is spun onto the substrate. An interdigital transducer is patterned with an electron beam to form a chirped transducer designed to respond to the 200 MHz band responsible for controlling color in waveguide devices. The finger period is determined by Λƒ = v where, Λ, is the finger period, v, is the velocity of sound in the substrate and, ƒ, is the radio frequency (RF). The transducer will have an impedance that must be matched to 75 ohms for efficient operation20.
The guided to leaky mode interaction occurs at different frequencies for different wavelengths of illumination light and as a result red, green, and blue light can be controlled in the frequency domain. The surface acoustic wave pattern is generated by an RF signal sent to the interdigital transducer. The RF of the input signal translate to spatial frequencies on the surface acoustic wave pattern. The waveguide can be fabricated so that low frequency signals control the angular sweep and amplitude of red light, while middle frequencies control green light and high frequencies control blue light. The authors have identified a set of waveguide parameters that allow all three of these interactions to be separate and adjacent in the frequency domain so that all three colors can be controlled with a single 200 MHz signal which is the maximum bandwidth of commodity graphics processing units (GPUs).
By matching the bandwidth of a GPU channel to that of a leaky mode modulator, the system becomes fully parallel and highly scalable. By adding bandwidth matched pairs of GPUs and leaky mode modulator channels, one may construct holographic displays of arbitrary size.
After the device is created, it is carefully characterized to verify that the frequencies for guided-to-leaky mode transition are appropriate for frequency control of color. First, the location of the guided modes are determined by a commercial prism coupler to confirm that the waveguide has the appropriate depth and the correct number of guided modes. Then, after the devices are mounted and packaged, they are placed in a custom prism coupler which maps the input frequencies of the scanned output light. The resulting data gives the frequency input response and the angular output response for red, green, and blue light for the device to be tested. If the device has been fabricated correctly, the device input response will be separated in frequency and the output response will be overlapping in angle. When this is confirmed, the device is ready for use in a holographic video display.
The first measurements take place before the device has been packaged. The waveguide depth is determined by a commercial prism coupler. This can be accomplished with just one illumination wavelength (typically 632 nm red) but authors have modified their commercial prism coupler to allow it to gather mode information for red, green and blue light. After packaging, the device undergoes a second measurement in a custom prism coupler which records deflected output light as a function of input RF. A detailed description of these measurements follows. Fabrication steps are also given.
The design of each device has two critical steps, proton exchange and development of the LOR. Of the two, proton exchange time determines the depth of the waveguide, which in turn determines the number of guided to leaky mode transitions, the controllable frequency bandwidth, and every key design parameter for each color of light. Two guided modes in red is desired. If more exist then bandwidth is sacrificed. If less exist then no guided to leaky mode transition is guaranteed. Follow the note in step 2.2.1 to correct pro…
The authors have nothing to disclose.
The authors gratefully acknowledge financial support from Air Force Research Laboratory contract FA8650-14-C-6571 and from DAQRI LLC.
X-Cut Lithium Niobate | Gooch and Housego | 99-00630-01 | Lithium Niobate 3″ Diameter X-CUT Wafer 1mm Polish/Polish |
Positive Photo Resist 1 | EMD Performance Materials | AZ 3330 F Photoresist | Used in the creation of the proton exchange mask. |
Photoresist Developer | EMD Performance Materials | AZ MIF 300 | Develops AZ3330 and LOR 3A |
Aluminium | International Advanced Materials | AL13 | 99.999% Pure |
Aluminium Etch | Transene | Type A Aluminum Etchant | |
Benzoic Acid | Sigma Aldrich | 109479-500G | 99% Pure |
Acetone | Fisher Chemical | UN1009 | |
IPA | Fisher Chemical | UN1219 | 99.5% pure Isopropyl Alcohol |
Acidic Piranha etch | Cyantek Corperation | Nanostrip | |
Under Layer Resist | Micro Chem | LOR 3A | Bottom layer used for liftoff. |
Positive Photo Resist | Micro Chem | 950 PMMA A9 | Top layer used for liftoff |
Anisole | Micro Chem | A Thinner | |
Conductive polymer aqueous solution | Mitsubishi Rayon Company | AquaSAVE | |
MIBK (4-Methyl-2-pentanone) | Sigma Aldrich | 360511 | Develops PMMA |
NMP (1-methyl-2-pyrrolidone) | Sigma Aldrich | 328634 | Used for liftoff |
Name of the Equipment | Company | Catalog Number | Comments/ Description |
E-beam Evaporator | Denton Vacuum | Integrity 20 | Any equivalent equipment would suffice. |
Thin Film Spinner | Laurell Technologies Corporation | WS-400A-6NPP-LITE | Any equivalent equipment would suffice. |
Mask Aligner | Karl Suss America Inc. | MA 150 CC | Any equivalent equipment would suffice. |
Automatic Dicing Saw | Disco Corperation | Disco Dad 320 | Any equivalent equipment would suffice. |
Muffle Furnace | Thermo Scientific | FB1415M | Any equivalent equipment would suffice. |
Electron Microscope | FEI | XL30 ESEM | Any equivalent equipment would suffice. |
Dehydration Oven | Lab-Line Instruments | Ultra-Clean 100 (3497M-3) | Any equivalent equipment would suffice. |
Hot Plate | Thermo Scientific | SP131325 | Any equivalent equipment would suffice. |
Polisher | Ultra Tec Mfg., Inc. | Ultrapol End & Edge Polisher | Any equivalent equipment would suffice. |
Class IIIb 12V RBG Lasers: Wavelengths(nm): 638, 532, and 445 | Bought second-hand. Probably pulled from a laser projector. Any equivalent equipment would suffice. | ||
Signal Generator | Agilent | 8648D | Now found at Keysight. Obsolete. Any equivalent equipment would suffice. Needed Frequency sweep 9 KHz-1000 MHz. |
Signal Amplifier | Mini-Circuits | TB-17 | Necessary only to overcome the limitations of the signal generator. |
Power Meter Controller | ThorLabs | PM100D | With power meter model S130C. Any equivalent equipment would suffice. Needed sensitivity 500pW |
Linear Actuator Controller | Newport | ESP7000 | With linear actuator model MFN25PP. Any equivalent equipment would suffice. Needs 0.1mm accuracy. |
AutomatedDeviceCharacterization.vi | LabView | Experimental Control Software by BYU | Found in the appendix |
CompareWDMmodes.m | MATLab | Analytical Software by BYU | Found in the appendix |