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.
