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Spatial Light Modulator (SLM) Workshop, BFY 2012 ConferenceDouglas Martin and Shannon O’LearyLawrence University and Lewis & Clark CollegeBriefly, a spatial light modulator (SLM) is a liquid crystal device that acts as a variablewaveplate. 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 displaydriven 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 isdisregarded.In the workshop, we used the SLM in three different ways:1. A display2. A diffractive object3. A spatial filter in the transform plane of a lens.The overall setup is sketched below.

1. SLM as video display.We use a laptop to drive the SLM. The SLM acts as a second monitor, but the SLM itself onlychanges the phase of the reflected light. To change the amplitude (and see something), a pairof 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 ontothe 2 nd monitor).d) With the SLM’s taped‐on polarizer in place, you should be able to see the desktopbackground 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 theincoming rail. The beam expander just after the laser should be used to create acollimated 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), canhelp align the beam. The beam reflected of the SLM should be parallel to the opticaltable and the outgoing rail.f) Remove the taped‐on polarizer (but save). Use the 100 mm lens after the SLM to createan 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 linearpolarizer in the output beam until the display is as crisp as possible. The incomingpolarization 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 “SlideShow” menu is an option “Show On” – use the drop‐down menu to select seconddisplay. Then, a PowerPoint slideshow will be sent to the SLM (the second display). Wetend to play around with black and white objects, but gray scale works as well (color isignored).2. SLM as diffractive object.The SLM acts as an aperture for incident light – a phase aperture if no polarizers are present, oran amplitude aperture if polarizers are in place, as in #1 above. Removing the 100 mm lens willcause the diffraction pattern of the aperture to appear on the far screen (at infinity). Thatdiffraction 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 blackbackground in PowerPoint), a 4‐slit aperture, a quasi‐crystal aperture, and a computergenerated hologram aperture. On the far screen, the diffraction patterns of these apertureswere formed. Steps to get use the SLM as a diffractive aperture:a) Remove the beam expansion optics (the microscope objective and 200 mm lens) – useof 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 putyour favorite diffraction aperture on the SLM surface. The far screen should show theaperture. You may need to move the aperture in PowerPoint to ensure it is centered inthe incident beam. With the slideshow running, click on an object in PowerPoint (say,

the vertical line). Then, use the arrow keys to move it around. The object will be movedaround 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, andused a computer generated hologram following Thad Walker, http://wwwatoms.physics.wisc.edu/papers/holo.pdf,using the MATLAB code in this posthttp://tinyurl.com/d662n7o).e) It is possible to create a sharper diffraction pattern. Re‐insert the 100 mm lens – thediffraction 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 planeonto a far screen.f) Note that the diffraction pattern remains the same with an amplitude aperture (withpolarizers in place) or phase aperture (without polarizers in place). Hecht has a nicediscussion in section 11.3.3 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 theFourier transform of an object. In the workshop, we used the SLM as a variable amplitudeaperture (with polarizers) to remove a kitty from its cage by removing the cage information inthe 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 withlines spaced about 0.5 mm apart. The narrower the line spacing, the wider the spots inthe diffraction pattern, and vice versa.c) Place the transparency in the beam along the incident rail, more than 200 mm from theSLM. Insert a 200 mm lens on the rail exactly 200 mm before the SLM surface. Toensure 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 transparencywith object and the 200 mm lens into the incident beam. Move the 200 mm lens (notthe 100 mm lens) until the Fourier transform of the object, viewed on the far screen, isin crisp focus. Since the far screen is an image of the SLM surface, when the transformis 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 complicatedaperture – 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 ofspots. 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 zeroorderwere removed with black bars, positioned in PowerPoint (another trick: holdingdown the control key (Windows) while using the arrow keys to move objects aroundwithin PowerPoint allows for very fine control of object position).

e) To see the result, remove the 100 mm lens in the outgoing beam path. We use a blankslide immediately after the spatial filter slide to be able to go back and forth betweenfiltered 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 532nm DPSS) allows the phase delay to be increased to closer to (0.95 at 532 nm). Thismay 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‐frequencyinformation, resulting in a blurred image; a high‐pass filter (small black circle on a whitebackground), which removes the DC component (that is, the mean intensity), whichtends to make dark regions bright and vice versa; and a “razor‐blade” filter, with twolarge black rectangles surrounding a narrow white slit – using this and a square‐gridobject allows either the horizontal or vertical lines of the grid to be removed (in fact, afilter with actual razor blades in the transform plane, no SLM, works fantastically for thisspatial filter).Doug Martindouglas.s.martin@lawrence.eduShannon O’Learysoleary@lclark.edu

Parts List used in the workshopPart Source ApproxCost.5 mW polarized HeNe Thorlabs HNL050L $1300or 5 mW laser pointer eBay, etc. (downside: ugly beam profile) $15Spatial Light Modulator Cambridge Correlators SDE1024 $10002’ x 4’ x 2” breadboard Vere 24482E1FM $9302 x 24” rails Thorlabs RLA2400 $2805 x rail carriers Thorlabs RC1 $1154 x post holder bases Thorlabs BA1S $204 x base clamps Thorlabs CL5 $166 x 2” post holders Thorlabs PH2 $471 x 1” post holder Thorlabs PH1 $72 x 3” post holders Thorlabs PH3 $171 x 1” post Thorlabs TR1 $58 x 2” posts Thorlabs TR2 $421 x 1” f=100 mm plano‐convex Thorlabs LA1509 $20lens2 x 1” f=200 mm plano‐convex Thorlabs LA1708 $40lenses3 x lens mounts Thorlabs LMR1 $472 x 1” glass linear polarizer Edmund Optics NT54‐926 $502 x rotation mount Thorlabs RSP1 $1601 x kinematic mirror mount Thorlabs KM100 $401 x 1” round mirror Thorlabs ME1‐G01 $141 x 10x microscope objective Newport L‐10X $2001 x microscope objectivemountThorlabs OMR $251 x iris Thorlabs ID25 $53screws & allen wrenches Thorlabs HW‐Kit2 & CCHK$140or McMaster‐Carr$40images on transparencyLaptop to drive SLMA few notes:eBay is a great source for used optomechanics – we’ve had good luck with used parts. Usedoptical 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 anew SLM lab.Any laser with good beam quality will work, but the liquid crystal in the SLM is apparentlydegraded by UV light – so 405 nm may not be a wise choice.With just a little more equipment (~$500‐1000 more), we run an entire optics course of labs.