Calibrate a Spatial Light Modulator (SLM) for Phase Delay (Viewer Inspired) | Thorlabs Insights

[@zyzzrespector]

I don't have the background to understand half of this but the Stud on screen really helped me out!

[@danieldahl]

Incredible work, very clear and concise. Another video idea could be to show people how to measure a flatness profile for an SLM. Keep these amazing videos coming people in the optics community really appreciate them.

[@thorlabs]

Thanks! We really enjoy making the Video Insights, and it’s great to hear that these lab demonstrations are helpful! Thanks also for your topic suggestion 😊 we love to hear ideas for future demos - they have inspired a lot of our videos. Please keep the topic suggestions coming!

[@TheWaffleburglar]

Very helpful, Jack’s a great teacher!

[@FrancescoFersini-q9j]

At 8:46, how did you create the gray block and binary grating simultaneously? Can you explain, please?

[@thorlabs]

We used the SLM’s software to upload gray-scale image files of those custom gray-block-and-binary-grating phase-delay patterns. The software configured the SLM’s panel by reading the patterns in those image files. The SLM panel is divided into a grid of pixels, and the phase delay of each pixel can be set independently using the SLM’s software. The software provides a few different options for specifying the phase delay to applied to each pixel. These options include some built-in patterns generated by the software, as well as the option to upload custom patterns. Since none of the software-generated patterns fulfilled our needs, we used a separate application to generate images of the required gray-scale patterns and then uploaded those image files. If you have any questions about the process, please reach out to Tech Support ( [e-mail] ).

[@FrancescoFersini-q9j]

Ok fine, I created the patterns myself with Python, but how did you decide that 0-100 gives the best results?

[@thorlabs]

We first calibrated the phase delay vs. grayscale value across the 0 - 255 range and then determined that a grayscale value difference of about 125 would provide a pi phase delay. (With a pi phase delay, the majority of light incident on the binary phase grating will be diffracted into the ±1 diffracted orders. The interference fringes between the diffracted light and the light reflected from the mirrored side of the panel will have better contrast when more light is diffracted into the ±1 orders. For more information, see 2:40.) We then configured the SLM’s panel so that one side was mirrored and the other side was a binary phase grating, with grayscale values of 0 and 125. We located the interference fringes created by the diffracted and reflected beams and evaluated the fringe contrast. Keeping the 0 grayscale value the same, we varied the other grayscale value until the fringe contrast was optimized. In our case, it was optimized for a value of 100.

[@BingOuyang-n2x]

Would you also provide the link to the camera lens you used to capture the fringe? Thank you!

[@thorlabs]

@user-bt2xz3li4t We did not use a lens between the beam expander and the camera, so the light incident on the camera was collimated. However, we did remove the window that the factory installs in front of the camera sensor for these measurements. There is a risk involved with removing the window, because it helps protect the camera sensor from dust, contamination, and damage, but we wanted to eliminate any interference effects that could be created by the window, like additional fringes.

[@giammi56]

7:55 is tan(phi), not tan-1(phi). Great video!

[@thorlabs]

@giammi56 Thank you, twice! You are correct, we typo-ed at 7:55. The equation at the top of the diagram should have tan(phi) in the denominator, but the calculated value (~29 cm) is correct.

[@ShashankJoshi-m4n]

how did you calculate the distance of the camera from slm, can you explain this? What is 6.25 mm term in Y= 6.25 mm/tan phi = 29 cm come from? Is that slm panel width? or beam diameter? Can you explain this please. Also can you explain the grating period calculation, how did you get 25.6 microns?

[@thorlabs]

@ShashankJoshi-m4n The distance between the camera and SLM is estimated using the laser's wavelength (532 nm), our chosen grating periodicity (25.6 µm), the reflected and diffracted beam widths (6.25 mm), and the grating equation. To customize the reflected and diffracted beam characteristics, we used the SLM's software to control individual pixel phases across the 12.5 mm wide and 7.1 mm tall panel. Half of the panel's width (i.e. 6.25 mm) was configured to be a mirror (i.e. flat phase profile) and the other half to be a binary phase grating. The incident beam filled the entire panel, so the reflected and diffracted beams widths were the same (Y = 6.25 mm). When configuring the grating, we chose a period equal to 4 pixels for reasons discussed at 10:00. Since this panel has a 6.4 µm pixel pitch, we calculated a grating period of 25.6 µm (i.e. 4 pixels * 6.4 µm/pixel). We wanted to place the camera within the region of total beam overlap, which depends on the relative angle (phi) between the reflected and -1 order diffracted beams. Since the reflected and 0 order diffracted beams are parallel, phi is the angle between the 0 and -1 order diffracted beams. The equation for phi shown at 7:55 was derived using the grating equation. The distance from the SLM to the region of total beam overlap was calculated using phi and assuming normal incidence for the input beam, instead of using the actual 10° angle of incidence. In addition, we assumed negligible beam divergence. These assumptions had the benefit of allowing us to use the convenient right-angled triangle formula ( tan(phi) = Y/X ) to calculate the estimated distance to total overlap (~29 cm) without introducing significant error. Click the link for the SLM ( EXULUS-HD1/M ) included in the description text to find the manual providing the specifications we used in our calculations.

[@ShashankJoshi-m4n]

@@thorlabsThis was helpful. Kudos to you.

[@jamespaulnadien5126]

goated with the sauce

[@lucasbottino6148]

Gold

[@lavleshpensia5195]

Dear Sir/Mam. Can you make a video on all resolution targets, like Sector Star Targets, 1951 USAF Targets, and NBS 1963A Targets? It will be helpful for us and as well as publicity your company products.

[@thorlabs]

@lavleshpensia5195 Thank you very much for your suggestion! Are there particular measurements you would like us to highlight in the video or are you primarily interested in resolution?

[@kaushikgupta1410]

So slow this video is!! but great explanation.

[@briannaafranz]

Virtual Insanity