Geometrical Approach to Gaussian Beam Profiling and Propagation Lab, Lab Reports of Optics

Download Geometrical Approach to Gaussian Beam Profiling and Propagation Lab and more Lab Reports Optics in PDF only on Docsity! OPTICS 471B – LAB 2 – Geometrical Approach to Gaussian Beam Profiling and Propagation 1 LAB 2 –GAUSSIAN BEAM PROFILING AND PROPAGATION 1. Objectives In this lab, you will get refreshed with Gaussian beam basics, to learn an elegant approach to characterize a laser beam, and learn beam propagation in optical system. Specifically, the lab involves in four parts: (1) Gaussian beam basics from a geometrical perspective; (2) Gaussian beam profiling; and (3) beam propagation through a lens. 2. Contents Specific topics include: • Gaussian beam basics from a geometrical perspective o Basics about Gaussian beam parameters o Beam line—a geometrical perspective o Beam profile and propagation o Beam system • Beam profiling o Two-microscope approach to beam alignment o Measurement of beam profile with an F-edge o Geometrical approach to locate beam waist: using a CAD or other software for geometry drawing 3. Readings Check out the READING folder for a complete reading list and make sure you read the REQUIRED material. 4. Pre-Lab GO TO PRE-LAB folder for your pre-lab assignment, and complete the PRELAB QUESTIONS before coming to the lab. 5. Lab Procedure A. Align the laser beam and a microscope along the optical rail (same as lab 1) o Please find and use the laser that is marked for your team (i.e. laser A or laser B). Also record which laser you used for this lab so that you can reuse the same laser for future labs. In several of the future labs, you will need the laser profile information that you measure in this lab to select appropriate optics and filters for beam expansion and imaging. o Refer to Lab 1 procedure for more detailed instructions to align laser and the microscope. If microscope and laser are well aligned, the laser beam should remain on the center of the crosshair no mater where the microscope is positioned on the rail. o Select an objective lens with an appropriate NA (need enough working distance). For instance, the 50mm objectives are good for this lab. B. Locate the laser aperture and knife edge with the microscope • Move the microscope back near to the laser, and turn off the laser • Look through the microscope, and focus its crosshair on the laser surface • Record this microscope position A on the rail OPTICS 471B – LAB 2 – Geometrical Approach to Gaussian Beam Profiling and Propagation 2 • Move microscope away from laser for an arbitrary distance (e.g. 1000mm) to a new Position B and record this position • Put knife Edge at the location of the virtual crosshair of the microscope. (you should be able to image the knife edge through the microscope, find best focus by moving the knife edge, not the microscope) o You will need a linear stage for the knife edge so that you can scan it across the beam. • Record the location of the Knife edge stand on the rail (E1). • We now know the exact distance of the knife edge surface as referenced from the face of the laser. C. Profile the beam with a knife edge • Place a photodiode immediately after the knife edge. • In a relatively dark room, scan the knife edge across the beam (X-axis, i.e. horizontally) and record the voltage output of the photodiode, take measurements at an increment of 10 microns o Note that some of the translation stages have metric units, and some of them have English units. o Before you perform the measurements, make sure you figure out the amount of translation for each tick on the micrometer (one tick is not one micron!) Check with your TA if you are not sure. o You may take the measurements at a larger increment when the knife edge is approaching the VERY dimmed region of the beam, but make sure to take measurements at a 10-micron increment otherwise. • Move the knife edge downstream along the rail (Z-axis), record the new knife edge location on the rail (E2) and repeat the above steps to take another set of measurements at this new location. o Make sure enough separation from the first location E1. • You may repeat measurements at a few new locations along the rail. D. Beam propagation through a lens system • Place a positive lens with a focal length of 200mm or longer, 400mm away from the laser surface o Record the focal length of your lens choice o Use a microscope to determine lens location (laser off) (Using the method in Step B) • Align the lens with the laser (using the method in Lab 1) • Use a microscope to roughly determine the new beam waist location o If the microscope is focused at the beam waist, the output beam from microscope would be collimated, use a screen to observe if the size of the output beam after microscope varies. • Place the knife-edge and photodiode detector behind the lens, but in front of the new beam waist, and turn the laser on o Keeping a good distance from the estimated beam waist location gives you a better accuracy (i.e. placing the knife edge closer to the lens).