Name: Characterization of Anisotropic Leaky Mode Modulators for Holovideo (Video) | JoVE Journal
Uploaded: 2016-03-19T15:00:00.000+00:00
Duration: 9 min 36 s
Description: Learn to characterize anisotropic leaky mode modulators for holographic video by mapping frequency response to angular output. This method provides vital data for electroholography optimization.

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5 minutes

The authors gratefully acknowledge financial support from Air Force Research Laboratory contract FA8650-14-C-6571 and from DAQRI LLC.

1. Initial Preparation
Note: Begin with a new X-cut lithium niobate wafer. It should be optical grade, 1 mm thick, clean, with nothing deposited on the surface, both sides polished, and the top side marked.
<ol>
	<li>Using an Electron Beam Evaporator or equivalent machine at a vacuum of 50 &#181;Torr, evaporate 200 nm of aluminum on the wafer at 5 &#197;/sec. To replicate the presented results, position the wafer constellation 65 cm above the aluminum crucible.</li>
	<li>Spin on 30 drops of a positive photo resist, such as AZ3330, at 3,000 rpm for 60 sec. Softbake the resist at 90 &#176;C for 60 sec. Note: For a detailed description of the mechanics of spinning polymer films see the work by C. J. Lawrence21.</li>
	<li>Using the proper mask, such as the &#34;Mask 1. Proton Exchange Mask.dxf&#34; file supplied in the appendix, expose the wafer using a mask aligner with a 350 W mercury bulb or its equivalent for 10 sec as per machine specifications. Ensure that the wafer is aligned so that the waveguides are parallel to the y-axis.</li>
	<li>Develop the resist in a positive photoresist developer for 60 sec. Hard bake the wafer for 60 sec at 110 &#176;C. Etch away the exposed aluminum completely by submerging it for 2 min in a 1 L solution aluminum etch heated to 50 &#176;C. 
	CAUTION: Aluminum etch is toxic, corrosive and harmful. See MSDS for proper handling and storage of this chemical. Use proper personal protective equipment for acid when handling this chemical.</li>
	<li>Remove the photoresist mask with a rinse of acetone followed by isopropyl alcohol (IPA).</li>
	<li>Using a 0.016 in. thick diamond blade with an exposure depth of 0.165 in. on an automatic dicing saw, cut the wafer into 10 x 15 mm2 devices with the long dimension parallel to the y-axis. 
	Note: The blade will not cut all the way through the substrate. To separate each device, simply stress each cut made by the dicing saw. Each 10 x 15 mm2 device will individually go through the remaining steps of the protocol.</li>
</ol>
2. Proton Exchange
<ol>
	<li>Place an individual device in a test tube with a small hole ground into the bottom to allow interaction between the device and all liquid baths.</li>
	<li>Proton exchange the device by immersing it in a 1 L melt of 99% pure benzoic acid at 240 &#176;C. Use an immersion time of 10 min and 10 sec in order to achieve the target depth of 0.4504 &#181;m. 
	Note: The proton exchange immersion time is dictated by the diffusion coefficient, D, which for the authors&#39; melt is currently D = 0.2993. The proton exchange immersion time is calculated using the relation T = d2 / (4D). In this equation, T is the exchange time in hours, d is the waveguide depth in microns, and D is the diffusion coefficient. For a detailed description of the mechanics of proton exchange see the work by J. L. Jackel15.</li>
	<li>Remove the device and allow to cool for 5 min or until cool to the touch. Clean off any benzoic acid residue with a rinse of acetone then IPA.</li>
</ol>
3. Anneal
<ol>
	<li>Place device in a regular test tube and wrap the tube in aluminum foil. Place the tube in a muffle furnace for 45 min at 375 &#176;C. Remove the device and allow to cool for 5 min or until cool to the touch.</li>
</ol>
4. Clean
<ol>
	<li>Clean the aluminum mask from the device using aluminum etch for roughly 2 min at 50 &#176;C. Clean the device in acidic piranha etch to remove any organic residues. 
	CAUTION: Acidic piranha etch is toxic, corrosive and harmful. See MSDS for proper handling and storage of these chemicals. Use proper personal protective equipment for acid when handling these chemicals.</li>
	<li>Rinse the device in acetone, then IPA, and dry with compressed nitrogen.</li>
</ol>
5. Waveguide Measurements
<ol>
	<li>Using any commercial waveguide analyzer measure the characteristics of the proton exchanged waveguide. 
	Note: A good device will have 2 guided modes using a 633 nm laser. See Figure 2 for an example of desired results. If the device shows more than two guided modes for red illumination then the exchange time in step 2.2 should be reduced. Likewise if the device shows less than two guided modes the exchange time should be increased.</li>
</ol>
6. Add Resist
<ol>
	<li>Spin on 4 drops of a Lift Off Resist (LOR) at 3,000 rpm for 60 sec and then bake at 200 &#176;C for 1 hr. Remove and allow the device to cool for 5 min or until cool to the touch. Spin on 4 drops of a 3:1 solution of polymethyl methacrylate (PMMA) and Anisole at 3,000 rpm for 60 sec and then bake at 150 &#176;C for 15 min.</li>
	<li>Remove and allow the device to cool for 5 min or until cool to the touch. Spin on 2 drops of a conductive polymer at 1,000 rpm for 60 sec, then spin at 6,000 rpm for 4 sec to remove any excess.</li>
</ol>
7. Pattern
<ol>
	<li>Use an electron microscope enhanced with a beam blanker to enable writing or an equivalent machine to expose the device.
	<ol>
		<li>Under a vacuum of 50 &#181;Torr, expose the conductive layer to an electron beam with an area dose of 30 &#181;C/cm2 that scans the pattern of the interdigital transducers. To replicate the results use a measured beam current of 410 pA.</li>
		<li>Write the pattern from a .dxf or equivalent file on the electron microscope as per machine specifications. 
		Note: For a detailed description of the E-Beam lithography process see the work done by R. E. Fontana22.</li>
	</ol>
	</li>
</ol>
8. Develop
<ol>
	<li>Remove the conductive layer by rinsing the device in a continuous stream of deionized water for 5sec. Remove the exposed PMMA by dipping the device into 1:3 solution of methyl isobutyl ketone (MIBK) and IPA for 45 sec.
	<ol>
		<li>Remove from the 1:3 solution of MIBK:IPA and rinse with IPA for 5 sec. Dry the device with compressed nitrogen.</li>
	</ol>
	</li>
	<li>Repeat steps 8.1-8.1.1 as necessary to fully develop the PMMA. 
	Note: However expose the device to the solution of MIBK:IPA in 5sec increments only. Complete development should reveal the LOR underneath the PMMA and can be identified by uniform coloration throughout the developed area surrounded by crisp edges and corners. 
	Note: Over development of PMMA leads to small feature blowout and can completely erase the interdigital transducer fingers leaving a single large developed block. Likewise under development leaves non-uniform residues that will decrease the effectiveness of the liftoff process that follows.</li>
	<li>Remove LOR in exposed region by dipping the device into a 1:1 solution of an appropriate developer and deionized water for 25 sec. Remove from the 1:1 solution of an appropriate developer and deionized water. Rinse with IPA for 5 sec.
	<ol>
		<li>Dry with compressed nitrogen. Repeat steps 8.3 as necessary to fully develop the LOR. 
		Note: However expose the device to the solution of an appropriate developer and deionized water in 2 sec increments only. Complete development should reveal the surface of the substrate underneath the LOR. It can be identified by uniform white coloration throughout the developed area while maintaining crisp edges and corners. Failure to develop the LOR properly also leads to the problems discussed in 8.2.3.1. See Figure 3 for an example LOR development process. 
		Note: Switching to a lower ratio of an appropriate developer to deionized water such as 1:2 or 1:3 is helpful as the device nears complete development to allow the fine features to develop without blowing out the device. However, it is not advantageous to start with these doses as total time increases and exceeds the optimal time in the developer.</li>
	</ol>
	</li>
</ol>
9. Deposit Aluminum
<ol>
	<li>Using an Electron Beam Evaporator or equivalent machine at a vacuum of 50 &#181;Torr, evaporate 200 nm of aluminum on the wafer at 5 &#197;/sec.</li>
</ol>
10. Liftoff Aluminum
<ol>
	<li>Fill a large glass dish with 750 ml of water on a hot plate at 90 &#176;C. Insert a plastic buffer into the water dish. In a separate small glass container submerge the device in a 100 ml solution of N-methyl-2-pyrrolidone (NMP).</li>
	<li>Place the container of NMP solution containing the device onto the plastic buffer ensuring that the water level does not exceed the height of the container of NMP. Cover and let sit 3 to 4 hr or until aluminum liftoff is complete. Remove the device from NMP. 
	Note: It is advantageous to clean off large sections of aluminum from the device before removing it from the NMP bath. Do this by using a pipette filled with NMP to squirt the device and knock off any remaining large chunks of unwanted aluminum.</li>
	<li>Rinse the device in IPA and dry with compressed nitrogen. Under a microscope, verify that liftoff is complete. If unwanted residual aluminum remains, wet the device with acetone and very gently brush with a cleanroom swab coated in acetone to remove.</li>
	<li>Rinse in IPA, dry with compressed nitrogen, and recheck under the microscope. Repeat 10.3 and 10.4 as needed.</li>
</ol>
11. Polish the End
<ol>
	<li>Coat the device in a protective film such as a layer of positive photoresist. Clamp the device so that the end with the transducers is exposed for polishing. Using proper polishing procedures23, slowly polish the end of the device to a surface roughness of less than 100 nm, so that no surface defects interfere with the light exiting the device.</li>
	<li>Remove the device from the clamp and clean off the protective film. If photoresist was used as a protective film, a generous rinse in acetone and then IPA will remove it. Dry the sample as necessary with compressed nitrogen.</li>
</ol>
12. Mount on a Breakout Board
<ol>
	<li>If any assembly is required for the RF breakout board, assemble the breakout board according to its specifications.</li>
	<li>Build, out of glass slides, a mounting platform to hold firmly both the RF breakout board and the device. Note: The mounting platform is built in a U shape out of three glass slides: one 75 x 50 x 1 mm3 and two 75 x 25 x 1 mm3.
	<ol>
		<li>Place a generous bead of superglue over the leftmost fourth of the large slide. Place one of the smaller slides over the bead of superglue so that leftmost edge and the bottom edge align with the corresponding edges on the large slide.</li>
		<li>Apply firm and equal pressure to the two slides until the superglue sets, about 15 sec. Repeat the process for the rightmost fourth of the large slide.</li>
	</ol>
	</li>
	<li>Mount the device to the top of the mounting platform with double-sided tape. Make sure the end of the device overhangs the end of the mounting platform so that the mounting platform does not interfere with light exiting the end of the device.</li>
	<li>Mount the RF breakout board to the mounting platform so that it is not in the beam path of the light exiting the device. A simple way to do this is to elevate the breakout board with thick tape so that the bottom of the breakout board is above the top of the device.</li>
	<li>Wire bond the pads on the device to their respective locations on the RF breakout board. Use a 27 nH series inductor to impedance match each transducer to the breakout board inputs.</li>
</ol>
13. Prism Coupling
<ol>
	<li>Select a rutile prism to couple light into the device. The polarization of the light (Transverse Electric) should be parallel to the optical axis of the rutile the optical axis (Z-axis) of the X-cut lithium niobate.</li>
	<li>Clean the contact surfaces of both the device and the prism thoroughly with IPA. Position the prism so that it is centered on the channel to be tested.</li>
	<li>Press the bottom of the prism firmly against the top of the device with a clamping mechanism. Note: Do not overtighten as excessive pressure will crack the substrate and damage the coupling prism.</li>
	<li>If successful, observe a wet spot will appear. 
	Note: A wet spot is a region of frustrated total internal reflection at the interface between the prism and the sample. For an example of proper prism coupling see Figure 4.</li>
</ol>
14. Mount in the Characterization Apparatus
<ol>
	<li>Mount the device on the rotating platform of the Frequency division color characterization apparatus for anisotropic leaky mode light modulators discussed by A. Henrie4. 
	Note: A schematic of the Characterization Apparatus is supplied in Figure 5.</li>
</ol>
15. Align in the Characterization Apparatus
<ol>
	<li>Turn on the laser. To replicate the results presented in this paper use 5 V for 638 nm, 5.5 V for 532 nm, and 6.5 V for 445 nm.</li>
	<li>Attenuate the beam until the intensity of the scattered light is comfortable to the eye. Verify the laser polarization.
	<ol>
		<li>Place a polarizer in the beam path after the half wave plate so that it blocks horizontally polarized light. Rotate the half wave plate to achieve maximum attenuation of the laser light. Remove the polarizer.</li>
	</ol>
	</li>
	<li>Manually rotate the platform so that the angle between the laser and the top surface of the device is set to the proper entrance angle. 
	Note: The proper angle can be found in Table 1 according to the desired testing wavelength and mode.</li>
	<li>Align prism using the linear translation stages when the focal point of the laser passes through the 90&#176; corner of the prism. Note: Increased laser scatter caused by the corner of the prism can sometimes be seen.
	<ol>
		<li>At this point, light should be coupling into the device which can be verified either by the characteristic streak of light caused by scattering in the waveguide or by the characteristic mode lines coming out of the end of the device24 (see Figure 6). 
		Note: If using mode lines to verify coupling, it is helpful to remove the power meter from the beam path. Instead insert a uniformly scattering object, such as a sheet of white paper, into the beam path.</li>
		<li>If no coupling is detected, slowly rotate the device while maintaining the coupling edge of the prism in the focal point of the laser. If after five degrees rotation in either direction no coupling can be detected, remove the device from the rotating platform, remove the prism and return to step 13.</li>
	</ol>
	</li>
	<li>Once coupling is detected, fine tune the rotational platform and linear translation stages to maximize the coupling of the light.</li>
</ol>
16. Attach the RF Input and Enclose the Device
<ol>
	<li>Replace the power meter that was removed during alignment. Also remove any obstructions to the beam path used for alignment purposes.</li>
	<li>Attach the RF input to the device breakout board and turn on the RF signal generator. Make sure the amplifier is powered. Note: To protect the device from burnout, the electrical power of the signal reaching the device should not exceed 1 W.</li>
	<li>Remove any attenuation used for safety during alignment. The laser is now at the optical power levels used for testing. Enclose the entire system in an optically isolating box.</li>
</ol>
17. Run the Provided Testing Program
<ol>
	<li>Obtain a laboratory equipment manager to run the characterization device, such as the LabView file AutomatedDeviceCharacterization.vi provided in the appendix.</li>
	<li>Insert all user parameters into the testing software on the control computer. Note: Figure 7 is supplied for those using the provided experiment control file. It indicates with a yellow box the fields that must be updated before each automated test is run in order for the provided analytical program to run properly in step 19.
	<ol>
		<li>To replicate the results presented in this paper use the following testing parameters: Initial Frequency: 100 MHz, Final Frequency: 800 MHz, Frequency Step: 10, Rough Initial Position: 0, Rough Final Position: 25, and Position Step: 1. Make sure the &#34;Output to File&#34; button is pressed.</li>
	</ol>
	</li>
	<li>Run the testing program. 
	Note: The provided program drives a power meter along a linear track at user defined intervals. At each position the RF input signal is swept through a set of chosen frequencies and power measurements are made. A measurement is also made with the RF input at its lowest frequency setting and lowest output power which has been experimentally determined as equivalent to no input signal4. These measurements are then graphed in real time in a 3D interactive chart.
	<ol>
		<li>Observe the four output files: *config.csv describes the experiment, *data.csv contains the power reading at each frequency, *no_stim.csv contains the background noise reading, and *graph.jpeg contains a copy of the graph on the user interface of the program as it was when the program ended. See Figure 8.</li>
	</ol>
	</li>
	<li>Repeat sections 15-17 for each wavelength and the TE1 mode described in Table 1.</li>
</ol>
18. Analyze the Frequency and Angular Output Profiles
<ol>
	<li>Obtain a statistical analysis program or download the CompareWDMmodes.m MATLab code provided in the appendix.</li>
	<li>In the folder (where the program is located), create a subfolder, &#34;Sample Number&#34; enter the &#34;Sample Number&#34; into the testing program. The sample number is the device identification number.</li>
	<li>In this folder, &#34;Sample Number,&#34; create three subfolders. Name each folder as follows, &#34;Sample Number&#34;_ &#34;Color&#34;_M1_ &#34;Transducer&#34;. The names in &#34;bold and italics&#34; are values entered into the testing program by the user. (e.g. A16_BLUE_M1_T1, C5_RED_M1_T13, or D35_GREEN_M1_T18).</li>
	<li>Into each subfolder, copy the four files created by the testing software that correspond with that particular wavelength, mode, and transducer.</li>
	<li>Open the analytical program and change the user defined variables at the top to reflect the user defined values input into the testing software. 
	Note: If using the provided analytical program and the user defined values in the testing program are &#34;Sample Number&#34;=A16, &#34;Guided Mode&#34;=1, &#34;Transducer&#34;=1 the analytical code would be modified to the following: 
	&#160;&#160;&#160;&#160; % User Defined Variables 
	&#160;&#160;&#160;&#160; series=&#39;A&#39;; 
	&#160;&#160;&#160;&#160; sample=16; 
	&#160;&#160;&#160;&#160; modes=[1]; 
	&#160;&#160;&#160;&#160; transducer=&#39;T1&#39;;</li>
	<li>Run the analytical program. 
	Note: If using the provided analytical code, among other things it creates a figure that compares the normalized frequency response and the angular output for red, green, and blue light. The file it creates is located in the &#34;Sample Number&#34; subfolder. See Figure 9 for an example of the output .</li>
</ol>

<ol><li>Smalley, D., Smithwick, Q., Bove, V., Barabas, J., Jolly, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Anisotropic+leaky-mode+modulator+for+holographic+video+displays%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Anisotropic leaky-mode modulator for holographic video displays.</a> Nature. 498 (7454), 313-317 (2013).</li>
<li>Smalley, D., Smithwick, Q., Bove, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Holographic+video+display+based+on+guided-wave+acousto-optic+devices%5BTitle%5D)+AND+%22Proc.+SPIE%22%5BJournal%5D)">Holographic video display based on guided-wave acousto-optic devices.</a> Proc. SPIE. 6488, 64880L-64880L-7 (2007).</li>
<li>Smalley, D. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=%22Holovideo+on+a+stick%3A+integrated+optics+for+holographic+video+displays%22">Holovideo on a stick: integrated optics for holographic video displays</a>. , MIT. MASS. (2013).</li>
<li>Henrie, A., Haymore, B., Smalley, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Frequency+division+color+characterization+apparatus+for+anisotropic+leaky+mode+light+modulators%5BTitle%5D)+AND+%22Rev+Sci+Instrum%22%5BJournal%5D)">Frequency division color characterization apparatus for anisotropic leaky mode light modulators.</a> Rev Sci Instrum. 86 (2), (2015).</li>
<li>Lawes, R. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aLawes%2C+R+%22MEMS+Cost+Analysis%3A+Basic+Fabrication+Processes%22">MEMS Cost Analysis: Basic Fabrication Processes</a>. , Pan Stanford. Boca Raton. (2014).</li>
<li>Pearson, E. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aPearson%2C+E+%22Mems+spatial+light+modulator+for+holographic+displays%22">Mems spatial light modulator for holographic displays</a>. , (2001).</li>
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<li>Chang, I., Lee, S. Efficient Wideband Acuosto-Optic Bragg Cells. Ultrasonics Symposium. , 427-430 (1983).</li>
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<li>Ito, K., Kawamoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((An+optical+deflector+using+collinear+acoustooptic+coupling+fabricated+on+proton-exchanged+LiNbO+3%5BTitle%5D)+AND+%22Jpn.+J.+Appl.+Phys%22%5BJournal%5D)">An optical deflector using collinear acoustooptic coupling fabricated on proton-exchanged LiNbO 3.</a> Jpn. J. Appl. Phys. 37 (9R), 4858(1998).</li>
<li>Smalley, D., Smithwick, Q., Barabas, J., Jolly, S., DellaSilva, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Holovideo+for+everyone%3A+a+low-cost+holovideo+monitor%5BTitle%5D)+AND+%22J+Phys+Conf+Ser%22%5BJournal%5D)">Holovideo for everyone: a low-cost holovideo monitor.</a> J Phys Conf Ser. 415 (1), 012055(2013).</li>
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<li>Monneret, S., Flory, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((M-lines+technique%3A+prism+coupling+measurement+and+discussion+of+accuracy+for+homogeneous+waveguides%5BTitle%5D)+AND+%22J+Opt+A-Pure+Appl+Op%22%5BJournal%5D)">M-lines technique: prism coupling measurement and discussion of accuracy for homogeneous waveguides.</a> J Opt A-Pure Appl Op. 2 (3), 188(2000).</li>
</ol>

The authors have nothing to disclose.

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...

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&#39;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&#39;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 &#923;&#402; = v where, &#923;, is the finger period, v, is the velocity of sound in the substrate and, &#402;, 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.

<table><tbody><tr><td>X-Cut Lithium Niobate</td><td>Gooch and Housego</td><td>99-00630-01</td><td>Lithium Niobate 3&amp;Prime; Diameter X-CUT Wafer 1&amp;nbsp;mm Polish/Polish</td></tr><tr><td>Positive Photo Resist 1</td><td>EMD Performance Materials</td><td>AZ 3330 F Photoresist</td><td>Used in the creation of the proton exchange mask</td></tr><tr><td>Photoresist Developer</td><td>EMD Performance Materials</td><td>AZ MIF 300</td><td>Develops AZ3330 and LOR 3A</td></tr><tr><td>Aluminium</td><td>International Advanced Materials</td><td>AL13</td><td>99.999% pure</td></tr><tr><td>Aluminium Etch</td><td>Transene</td><td>Type A Aluminum Etchant</td><td></td></tr><tr><td>Benzoic Acid</td><td>Sigma Aldrich</td><td>109479-500G</td><td>99% pure</td></tr><tr><td>Acetone</td><td>Fisher Chemical</td><td>UN1009</td><td></td></tr><tr><td>IPA</td><td>Fisher Chemical</td><td>UN1219</td><td>99.5% pure isopropyl alcohol</td></tr><tr><td>Acidic Piranha etch</td><td>Cyantek Corperation</td><td>Nanostrip</td><td></td></tr><tr><td>Under Layer Resist</td><td>Micro Chem</td><td>LOR 3A</td><td>Bottom layer used for liftoff</td></tr><tr><td>Positive Photo Resist</td><td>Micro Chem</td><td>950 PMMA A9</td><td>Top layer used for liftoff</td></tr><tr><td>Anisole</td><td>Micro Chem</td><td>A Thinner</td><td></td></tr><tr><td>Conductive polymer aqueous solution</td><td>Mitsubishi Rayon Company</td><td>AquaSAVE</td><td></td></tr><tr><td>MIBK (4-methyl-2-pentanone)</td><td>Sigma Aldrich</td><td>360511</td><td>Develops PMMA</td></tr><tr><td>NMP (1-methyl-2-pyrrolidone)</td><td>Sigma Aldrich</td><td>328634</td><td>Used for liftoff</td></tr><tr><td>E-beam Evaporator&amp;nbsp;</td><td>Denton Vacuum&amp;nbsp;</td><td>Integrity 20</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Thin Film Spinner</td><td>Laurell Technologies Corporation</td><td>WS-400A-6NPP-LITE</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Mask Aligner&amp;nbsp;</td><td>Karl Suss America Inc.</td><td>MA 150 CC</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Automatic Dicing Saw&amp;nbsp;</td><td>Disco Corperation</td><td>Disco Dad 320</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Muffle Furnace</td><td>Thermo Scientific</td><td>FB1415M</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Electron Microscope</td><td>FEI</td><td>XL30 ESEM</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Dehydration Oven</td><td>Lab-Line Instruments&amp;nbsp;</td><td>Ultra-Clean 100&amp;nbsp; (3497M-3)</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Hot Plate</td><td>Thermo Scientific</td><td>SP131325</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Polisher</td><td>Ultra Tec Mfg., Inc.</td><td>Ultrapol End &amp;amp; Edge Polisher</td><td>Any equivalent equipment would suffice.</td></tr><tr><td>Class IIIb 12&amp;nbsp;V RBG Lasers: Wavelengths (nm): 638, 532, and 445</td><td></td><td></td><td>Bought second-hand. Probably pulled from a laser projector. Any equivalent equipment would suffice.</td></tr><tr><td>Signal Generator</td><td>Agilent</td><td>8648D</td><td>Now found at Keysight. Obsolete. Any equivalent equipment would suffice. Needed Frequency sweep 9 kHz-1,000 MHz.</td></tr><tr><td>Signal Amplifier</td><td>Mini-Circuits</td><td>TB-17</td><td>Necessary only to overcome the limitations of the signal generator.</td></tr><tr><td>Power Meter Controller</td><td>ThorLabs</td><td>PM100D</td><td>With power meter model S130C. Any equivalent equipment would suffice. Needed sensitivity 500&amp;nbsp;pW.</td></tr><tr><td>Linear Actuator Controller</td><td>Newport</td><td>ESP7000</td><td>With linear actuator model MFN25PP. Any equivalent equipment would suffice. Needs 0.1&amp;nbsp;mm accuracy.</td></tr><tr><td>AutomatedDeviceCharacterization.vi&amp;nbsp;</td><td>LabView</td><td>Experimental Control Software by BYU</td><td>Found in the appendix</td></tr><tr><td>CompareWDMmodes.m</td><td>MATLab</td><td>Analytical Software by BYU</td><td>Found in the appendix</td></tr></tbody></table>

characterization of anisotropic leaky mode modulators for holovideo

Characterization of Anisotropic Leaky Mode Modulators for Holovideo

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.

The principle results of the protocol above are the guided mode measurement from the commercial prism coupler shown in Figure 2, the single frequency, raw input/output data gathered from the customized prism coupler shown in Figure 8 and the multicolor curves shown in Figure 9. In the following paragraphs we discuss the actionable information produced by each of these outputs.
T...

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 ...

The overall goal of this procedure is to reproducibly characterize spatial-light modulators by mapping their frequency response to angular output. This method helps to provide vital data required to answering key questions in the electroholography field, such as identifying guided leaky mode transitions and optimization requirements. The main advantage of this technique is that it clearly separates leaky mode transitions and allows us to quickly obtain repeatable information on their linearity, relative intensity, angular spread, and operational frequency.A visual demonstration of this process is critical as the alignment and the prism coupling both rely on visual cues that are hard to explain. To characterize the device, first prepare to mount it on a radio frequency breakout board. Have ready a device, an RF breakout board, and three glass slides to make a mounting platform.One slide is larger than the other two. It will form the base of the U-shaped platform. Begin to work with the largest slide.Place a generous bead of superglue over the left-most quarter of a slide's longest dimension. Next, orient a smaller slide so that its longest dimension is perpendicular to that of the first slide. Align the left edges of the two slides so that their lower-left corners overlap.Put them in contact and apply firm and equal pressure to the slides until the glue sets. Repeat the analogous steps for the right-hand side. This will result in a U-shaped structure.To mount the device, apply double-sided tape to the platform at the center of the U.Now, work with the leaky mode modulator to be characterized. Check that the device has polished ends and is ready for use. Next, place the device on top of the tape that is already on the platform.Mount it so the end of the device overhangs the end of the mounting platform to avoid interfering with the light path. At this point, mount the RF breakout board. Mount the breakout board so that it is not in the beam path of the light exiting the device.The next step is wire bonding. This is the device and breakout board after they have been wire bonded. Now, select an appropriate prism to cut the light into the device and use isopropyl alcohol to clean the surface that will be in contact with the device.In addition, clean the contact surface of the device. Then, place the prism on the device so that it is centered on the device channel to be tested. Continue by using a clamping mechanism to press the bottom of the prism firmly against the top of the device, coupling the elements.The clamping mechanism should press the bottom of the prism firmly against the top of the device and successful coupling will produce a wet spot at the interface. When viewed at the proper angle, the wet spot will reflect a rainbow of color. The next step is to make use of the characterization apparatus.The apparatus has three laser sources, red, green, and blue, at one end. Light from the lasers first passes through a variable attenuator, then a half-wave plate, followed by a variable aperture, and finally, a focusing lens. The focused light falls upon the prism on the sample which will be mounted on this rotation stage.This schematic provides an overview of the optical elements, the rotation stage, and the electronics. Once light has entered the device, input of a radio frequency signal generates surface acoustic waves. These cause light to exit at a frequency controllable angle and fall on a power meter.Configure the instruments to collect data over a range of frequencies and positions. Mount the device with the prism and holder on the rotation platform. Place the assembly so that light from the focusing lens first encounters the prism.To align the device, first turn on the laser, and adjust the attenuator until the intensity of the scattered light is comfortable to the eye. Next, place a polarizer in the beam path after the half-wave plate. Orient it so that it blocks horizontally polarized light.Rotate the half-wave plate to achieve maximum attenuation of the laser light. One this is achieved, remove the polarizer. Now, return to the rotation platform to manually rotate it.Adjust it so the laser light is at the proper entrance angle with respect to the top surface of the device. Align the prism using the linear translation stage on top of the rotation stage. Adjust the alignment until the focal point of the laser light passes through the 90-degree corner of the prism.At this point, make fine adjustments to the rotation stage to achieve coupling. Monitor the device. As the wave guide begins to couple, a characteristic streak of light appears in the wave guide from scattering.Another way to verify coupling is to have light exiting the device fall onto a back plane. On the back plane, confirm the presence of characteristic mode lines of the light. These are various transverse electric modes.Once coupling is detected, fine tune the rotation and translation stages to increase the evanescent coupling. Next, ready the cable that connects the breakout board to the amplifier and signal generator. Make the connection to the signal input of the breakout board.Continue by turning on both the radio frequency signal generator and the amplifier. Here it is useful to do a preliminary test of the device. Sweep the frequency from 400 megahertz to 600 megahertz and check for deflected light.Before continuing, clear the beam path and ensure the power meter is in place. Then, return to the attenuator in the optical path. There, undo any attentuation that was implemented for safety during alignment.Finally, use an optically isolating box to cover the entire characterization apparatus for the duration of the experiment. Make use of instrument control software to run the characterization apparatus. This experiment uses lab view, running a custom testing program.After entering the testing parameters, run the program. The script should take less than five minutes to run. During testing, it will produce a plot which can be manipulated.Both the plot and the data will be saved. This data collected before the device was packaged, is for a commercial thin film analyzer. The vertical axis is laser intensity.The horizontal axis is a measure of the rotation of the device. The two dips correspond to angles at which a guided mode allows light to enter the wave guide and exit at the end of the device, thus avoiding reflection into the power meter. This optical power data, collected after packaging, is from the characterization apparatus.The plot is the result of scanning the radio frequency input in megahertz and power meter location in millimeters. The projection of the data on the Y-axis gives the frequency response of the device. Projection on the X-axis gives the span of the diffracted light output.The slope of the data in the XY plane provides a sense of the scan's linearity. This plot combines raw data from several experiments in all three wavelengths for TE 1 guided modes. If the response for each color is adjacent in frequency and overlapping in angle, the device is appropriate for frequency control of color.Once mastered, a full characterization in red, green, and blue light for a single channel takes 30 minutes. Of course, high resolution images take more time. After its development, this technique paved the way for researchers in the field of electroholography to explore frequency division multiplexing in wave guiding spatial light modulators.After watching this video, you should have a good understanding of how to characterize spatial modulators in a repeatable way. This includes proper prism coupling, alignment and testing procedures.

characterization-of-anisotropic-leaky-mode-modulators-for-holovideo

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Learn to characterize anisotropic leaky mode modulators for holographic video by mapping frequency response to angular output. This method provides vital data for electroholography optimization.

Characterization of Anisotropic Leaky Mode Modulators for Holovideo

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