Spatial Light Modulator (SLM) Workshop, BFY 2012 Conference Douglas Martin and Shannon O Leary Lawrence University and Lewis & Clark College

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1 Spatial Light Modulator (SLM) Workshop, BFY 2012 Conference Douglas Martin and Shannon O Leary Lawrence University and Lewis & Clark College Briefly, a spatial light modulator (SLM) is a liquid crystal device that acts as a variable waveplate. Each of the 1024x768 pixels can retard a wave by 0 radians with 8 bit precision. The individual pixels are addressable via a VGA cable in essence, the SLM acts as a display driven by a computer graphics card. When sent a black pixel, the SLM acts as a 0 waveplate; when sent a white pixel, the SLM acts as a /2 ( ) waveplate. Gray is in between; color is disregarded. In the workshop, we used the SLM in three different ways: 1. A display 2. A diffractive object 3. A spatial filter in the transform plane of a lens. The overall setup is sketched below.

2 1. SLM as video display. We use a laptop to drive the SLM. The SLM acts as a second monitor, but the SLM itself only changes the phase of the reflected light. To change the amplitude (and see something), a pair of polarizers is necessary. Steps to get the SLM up and running: a) Plug SLM power in (this turns the SLM on) b) Connect the SLM video in to the laptop video out with a VGA cable. c) Setup the laptop to drive a second monitor (in Windows, we extend the desktop onto the 2 nd monitor). d) With the SLM s taped on polarizer in place, you should be able to see the desktop background by directly looking at the SLM (it will be tiny). e) Setup the optical system sketched above, without the object or 200 mm lens on the incoming rail. The beam expander just after the laser should be used to create a collimated beam, parallel to the incident rail and optical bench, that covers (or just overfills) the SLM surface. An iris placed on the rail (on a rail carrier, at the right height), can help align the beam. The beam reflected of the SLM should be parallel to the optical table and the outgoing rail. f) Remove the taped on polarizer (but save). Use the 100 mm lens after the SLM to create an image of the SLM surface on a far screen (we use a wall 5 10 ft away). g) Linearly polarize the incident light (polarized laser or polaroid) and then rotate the linear polarizer in the output beam until the display is as crisp as possible. The incoming polarization matters so you may need to rotate the incoming polarization as well. h) To control what is sent to the SLM, we use PowerPoint. In Windows, under the Slide Show menu is an option Show On use the drop down menu to select second display. Then, a PowerPoint slideshow will be sent to the SLM (the second display). We tend to play around with black and white objects, but gray scale works as well (color is ignored). 2. SLM as diffractive object. The SLM acts as an aperture for incident light a phase aperture if no polarizers are present, or an amplitude aperture if polarizers are in place, as in #1 above. Removing the 100 mm lens will cause the diffraction pattern of the aperture to appear on the far screen (at infinity). That diffraction pattern can be brought onto a screen placed a finite distance away using a lens. In the workshop, we used the SLM to create a 2 slit aperture (two white lines on a black background in PowerPoint), a 4 slit aperture, a quasi crystal aperture, and a computer generated hologram aperture. On the far screen, the diffraction patterns of these apertures were formed. Steps to get use the SLM as a diffractive aperture: a) Remove the beam expansion optics (the microscope objective and 200 mm lens) use of a post collar on the post will help with realignment later. b) Now the unexpanded beam should be incident on a small portion of the SLM surface. With the 100 mm lens in place to image the surface of the SLM, use PowerPoint to put your favorite diffraction aperture on the SLM surface. The far screen should show the aperture. You may need to move the aperture in PowerPoint to ensure it is centered in the incident beam. With the slideshow running, click on an object in PowerPoint (say,

3 the vertical line). Then, use the arrow keys to move it around. The object will be moved around on the SLM surface. To continue the slideshow, be sure to click the small box resume slideshow. c) Remove the 100 mm lens, and the diffraction pattern should appear on the far screen. d) Vary the aperture as desired (we changed the slit width, inserted a quasicrystal, and used a computer generated hologram following Thad Walker, using the MATLAB code in this post e) It is possible to create a sharper diffraction pattern. Re insert the 100 mm lens the diffraction pattern will appear in the transform plane, one focal length downstream. Use a second lens (short focal length, say 50 or 100 mm) to magnify the transform plane onto a far screen. f) Note that the diffraction pattern remains the same with an amplitude aperture (with polarizers in place) or phase aperture (without polarizers in place). Hecht has a nice discussion in section of Optics, 4 th ed. 3. SLM as spatial filter. By inserting the SLM in the transform plane of a lens, the SLM can be used to manipulate the Fourier transform of an object. In the workshop, we used the SLM as a variable amplitude aperture (with polarizers) to remove a kitty from its cage by removing the cage information in the transform plane. Steps to get the SLM working as a spatial filter: a) Re expand the incoming beam, using the microscope objective and 200 mm lens. b) Print your object on a transparency we used a picture of a kitten in a cage with with lines spaced about 0.5 mm apart. The narrower the line spacing, the wider the spots in the diffraction pattern, and vice versa. c) Place the transparency in the beam along the incident rail, more than 200 mm from the SLM. Insert a 200 mm lens on the rail exactly 200 mm before the SLM surface. To ensure the transform plane is on the SLM surface, the following trick may be helpful. First, remove the transparency and the 200 mm lens, and send a crisp image to the SLM. Use the 100 mm lens after the SLM to focus the surface of the SLM onto a far screen. Now, change the PowerPoint slide to a blank, white slide. Re insert the transparency with object and the 200 mm lens into the incident beam. Move the 200 mm lens (not the 100 mm lens) until the Fourier transform of the object, viewed on the far screen, is in crisp focus. Since the far screen is an image of the SLM surface, when the transform is exactly on the SLM, it will also be enlarged and in focus on the far wall. d) Insert your spatial filtering aperture into PowerPoint. You can create a complicated aperture but positioning is difficult (the SLM pixels are about 10 m across). Alternately, as we did in the workshop, black bars can be used to filter an entire row of spots. In our case, we arranged the cage so that the bars were vertical. The transform, therefore, appeared as horizontal line of spots. The spots on either side of the zeroorder were removed with black bars, positioned in PowerPoint (another trick: holding down the control key (Windows) while using the arrow keys to move objects around within PowerPoint allows for very fine control of object position).

4 e) To see the result, remove the 100 mm lens in the outgoing beam path. We use a blank slide immediately after the spatial filter slide to be able to go back and forth between filtered and unfiltered image in PowerPoint. f) A few notes: at 633 nm (red He Ne), the SLM can only delay the phase by 0.8, resulting in an imperfect amplitude aperture. Shifting to green (543 nm He Ne or 532 nm DPSS) allows the phase delay to be increased to closer to (0.95 at 532 nm). This may help improve the aperture filtering. Other fun filters include a low pass filter (simply a small white circle on a black background this removes the high frequency information, resulting in a blurred image; a high pass filter (small black circle on a white background), which removes the DC component (that is, the mean intensity), which tends to make dark regions bright and vice versa; and a razor blade filter, with two large black rectangles surrounding a narrow white slit using this and a square grid object allows either the horizontal or vertical lines of the grid to be removed (in fact, a filter with actual razor blades in the transform plane, no SLM, works fantastically for this spatial filter). Doug Martin douglas.s.martin@lawrence.edu Shannon O Leary soleary@lclark.edu

5 Parts List used in the workshop Part Source Approx Cost. 5 mw polarized HeNe Thorlabs HNL050L $1300 or 5 mw laser pointer ebay, etc. (downside: ugly beam profile) $15 Spatial Light Modulator Cambridge Correlators SDE1024 $ x 4 x 2 breadboard Vere 24482E1FM $930 2 x 24 rails Thorlabs RLA2400 $280 5 x rail carriers Thorlabs RC1 $115 4 x post holder bases Thorlabs BA1S $20 4 x base clamps Thorlabs CL5 $16 6 x 2 post holders Thorlabs PH2 $47 1 x 1 post holder Thorlabs PH1 $7 2 x 3 post holders Thorlabs PH3 $17 1 x 1 post Thorlabs TR1 $5 8 x 2 posts Thorlabs TR2 $42 1 x 1 f=100 mm plano convex Thorlabs LA1509 $20 lens 2 x 1 f=200 mm plano convex Thorlabs LA1708 $40 lenses 3 x lens mounts Thorlabs LMR1 $47 2 x 1 glass linear polarizer Edmund Optics NT $50 2 x rotation mount Thorlabs RSP1 $160 1 x kinematic mirror mount Thorlabs KM100 $40 1 x 1 round mirror Thorlabs ME1 G01 $14 1 x 10x microscope objective Newport L 10X $200 1 x microscope objective mount Thorlabs OMR $25 1 x iris Thorlabs ID25 $53 screws & allen wrenches Thorlabs HW Kit2 & CCHK $140 or McMaster Carr $40 images on transparency Laptop to drive SLM A few notes: ebay is a great source for used optomechanics we ve had good luck with used parts. Used optical breadboards can also save quite a bit of money. Thorlabs offers a new lab discount, typically around 10% ask for it if you decide to set up a new SLM lab. Any laser with good beam quality will work, but the liquid crystal in the SLM is apparently degraded by UV light so 405 nm may not be a wise choice. With just a little more equipment (~$ more), we run an entire optics course of labs.