Bioengineering
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Transmission of Multiple Signals through an Optical Fiber Using Wavefront Shaping
Chapters
Summary March 20th, 2017
We demonstrate the transmission of multiple independent signals through a multimode fiber using wavefront shaping employing a single spatial light modulator. By modulating the wavefront for each signal individually, spatially separated foci are transmitted. Potential applications are multiplexed data transfer in communications engineering and endoscopic light delivery in biophotonics.
Transcript
The overall goal of this procedure is to transmit multiple individual spatially separated light signals through a single multi-mode fiber, while compensating for the light distortion by mode conversion inside the fiber. This novel technique can help to answer key questions in biomedicine and communication engineering via multiple biological cells or data signal channels have to be egressed individually. The main advantage of this technique which enables spacial temporal light modulation is that only a single spatial light modulator is needed to both calibrate and transmit multiple independent signals.
The implications of this technique extends towards noninvasive studies and prospective therapies or neurological dysfunctions like Parkinson's disease. While monitoring and control of neurons is a combination to using spatial temporal light signals. Generally individuals new to this method will struggle because of the high effort for alignment of the optical pass, which is inevitable for high-quality transmission.
The experiment takes place on an optical bench. The optical elements have already been arranged. Some elements are only used for calibration.
This spatial light modulator is among the elements used for data signal transmission on the proximal side of the setup. This CCD camera is the data signal receiver on the distal side. The data signal travels through this multi-mode optical fiber connecting the two sides.
This schematic provides an overview of the apparatus and identifies the proximal and distal elements. A laser provides a collimated coherent light beam which is split by a polarizing beam splitter. Use the half wave plates to keep the two beams at approximately equal power.
The object beam goes to the distal side. For now focus on the reference beam which goes to the proximal side. Split the reference beam in two whith each resulting beam going through an optical modulator.
Then have the two beams follow the same path with the spatial separation. During calibration the beams will be superposed by light from the distal side that is passed through the multi-mode fiber, a microscope objective, and linear polarizer, and eventually interfere. A final beam splitter directs the two beams perpendicularly on to the special light modulator.
In preparing the experiment, be sure to orient the polarizing beam splitter so that polarization of the reference beam aligns with the polarization sensitive spatial light modulator. For calibration purposes the two beams also pass through lenses focused on a CMOS camera. Please note that nothing is displayed on the special light modulator for now.
Therefore the modulator acts like a mirror during the calibration. Work with the two lenses on the bench. Adjust their position and the distance between them.
The goal is to get a sharp image of the special light modulator plane in the CMOS camera. Now focus on the distal side of the setup for calibration. The object beam is transformed by beam splitters and mirrors into two spatially separated beams.
Another beam splitter directs the beams into a microscope objective and from there to a lens focused on a CCD camera. At the bench focus the microscope objective on the distal end of the multi-mode fiber. Check the focus using the lens and CCD camera to observe the back reflection of the light from the multi-mode fiber.
Work with the objective lens on the proximal side as well. Use it to collimate the light exiting the multi-mode fiber. Now view the interference pattern at the CMOS camera and work to change the interference fringe spacing.
This is done by adjusting the mirrors and beam splitters in the reference and object beams. This changes the angle at which the object and reference beams intersect at the spatial light modulator, which should be at less than one degree. Stop when the interference fringe spacing is roughly the size of two pixels.
Finally work with the linear polarizer, adjust its orientation to match the polarization of the object and reference beams. This will produce the maximum contrast in the CMOS camera image which will show distinct fringes. The first calibration sequence involves finding the pixel relation between the spatial light modulator and the CMOS camera.
Begin with the object beam just after the polarizing beam splitter. Block the object beam between the half wave plate and the next beam splitter. Next, work with one of the two reference beams immediately after they have passed through the optical modulators.
Block only one of them so the spatial light modulator is illuminated by the other. Capture an image of the spatial light modulator with the CMOS camera. Use graphic software to identify the coordinates of the upper left corner of the modulator, which will serve as the point of origin for the spatial light modulator.
When done, remove both of the beam blocks to complete finding the pixel relation between the spatial light modulator and the CMOS camera. The next calibration sequence is for the signal paths. At this point these are the beam paths.
The first step is to block object beam two just after two beam splitters in its path. And to block reference beam two after the light modulator. Then, use the CMOS camera to capture an image of the hologram.
From this, calculate the inverted phase in region one of the beam. Continue by removing the blocks from the beams. Next, block object beam one and reference beam one.
Calculate the inverted phase in region two of the beam from the hologram captured by the camera. Be sure to remove all beam blocks before proceeding. The setup for the transmission experiments is simpler.
Signal transmission does not require the CMOS camera and the object beam is blocked. Now the two reference beams are reflected from two regions of the spatial light modulator. From there, the reflected beams pass through the multi-mode fiber to the CCD camera.
At the computer, prepare the image for the spatial light modulator. For the image arrange the region one and two inverted phase images in their corresponding positions. Then, stitch these images together for display on the spatial light modulator using its computer graphics port.
At the bench make the final preparations for the experiment. The reference beams pass through light intensity modulators. Activate these on both reference beams to proceed.
At the computer observe and record the output signals from the CCD camera. These are typical results recorded by the camera on the distal side after the signal has traversed a two-meter long fiber. The images correspond to only signal one being on.
Only signal two being on. And both signals being on. In addition to the desired peaks in each image, there is speckle due to limitations of digital optical phase conjugation.
The peak to background ratio can be increased by using fibers that support a larger number of modes. There is cross talk between the two periodic output signals. The frequency spectrum of signal one shows the expected peak at frequency f1 and a smaller one at frequency f2.
Similarly the frequency spectrum of signal two has the expected peak at f2 and a smaller one at f1. While attempting this procedure it's important to mesh the spatial light modulator and the CMOS camera carefully for the digital optical phase conjugation to work. In order to enhance this procedure model-based calibration techniques can be applied to perform the transmission without access to the distal end of the fiber.
After watching this video you should have a good understanding of how to apply individual wave shaping for the transmission of independent signals through a multi-mode fiber. After its development, this novel technique will pave the way for researchers in optogenetics to explore neurodegenerative diseases in model organisms as well as in human induced pluripotent stem cell-derived neurons or cardiomyocytes.
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