PH 481/581 Physical Optics Winter 2018 Laboratory #1 Week of January 15 Read: Section 5.2 (pp.151-175) of "Optics" by Hecht Do: 1. Experiment I.1: Thin Lenses 2. Experiment I.2: Alignment Project 3. Experiment I.3: Expanding Laser Beams 4. Experiment I.4: Familiarization with Lab Equipment Experiment I.1 Thin Lenses: In this experiment you will study imaging and verify the focal length of a positive (converging) lens. Use a 100-mm lens. A target on a photographic slide is provided for use as the object and white paper or card stock works as an imaging screen. Mount all components on the optical rail on your table. A piece of cardboard with a slide-sized hole cut in it helps to reduce stray light reaching the image plane. Use the desk lamp to illuminate the object, but do not put it so close that the object might melt. Start with the object about 200 mm from the lens. Find the position of the screen where the image is the sharpest. Record the object and image distances (each measured from the lens). Also record the magnification of the image with respect to the object. Reduce the object distance 10 mm and repeat. Do this as many times as you can before the image is too far away to measure. Plot the image distance as a function of the object distance and fit the data to the expected dependence to determine the focal length of the lens. Also plot the magnification and compare to the expected function. Experiment I.2 Alignment project: For many situations, it is convenient if the laser beam is parallel to the top of the optical table and above a line of holes on the table. This will make insertion and adjustment of other components much easier. For convenience and safety our experiments use a "U" shape geometry with an optical rail in the last arm, as shown below. The rail is bolted to the table and has carriages that ride on the rail. Components can be mounted on the carriages so that they can be easily moved along the optical axis defined by the rail. M1 Laser M2 Rail The U geometry is defined by the two mirrors labeled M1 and M2 in the diagram. The goal of this experiment is to ensure that the laser beam (i.e. the optical axis) is parallel to the table top and aligned above the center of the rail. Mount M2 on the optical table and position it close to the beginning of the optical rail such that the center of the mirror is above the center line
of the rail and is at a height greater than 14.5 cm above the table (this height will ensure that the beam passes through the center of components like lenses mounted on the rail check that it does!). Set up two irises on the opposite ends of the optical rail. Adjust the iris heights (again > 14.5 cm) to be as nearly equal as you can by using a ruler. You now want to direct the laser beam through these two holes, which define the optical axis. Close the irises down. Use the knobs on the back of M1 to reflect the beam off M2 and through the first iris. Now adjust M2 so the beam passes through the second iris. This adjustment should do little to perturb your first adjustment because the first iris is so close to M2. That is the trick! You should repeat this procedure until you can open both irises to clip the outside of the beam and see a clean symmetric halo on each iris. You may have to open the irises some to start and iterate a few times to get it just right, but the procedure should converge very rapidly. Note how the beams move and play with this arrangement enough so that you can quickly and correctly determine which knob needs to be adjusted and which direction the knob must be turned to move the beam the correct direction (left, right, up, or down). Note the kinematic construction of the mirror mount; there are two orthogonal pivots about a single fixed point. This procedure can be done with one iris as well simply move the iris between the two positions on the optical rail. Leave the laser beam aligned so that you can use it in Part 3. Do not unscrew the irises from their posts; they can easily be damaged. If you need another post to mount something else, ask and you shall receive. Experiment I.3: Expanding Laser Beams: In this experiment, you will build a Keplerian telescope to expand the size of the laser beam. The Keplerian telescope has two positive lenses as shown below. The objective has a focal length f o and the eyepiece has a shorter focal length f e. The lenses are separated by the sum of the two focal lengths. In astronomy, the starlight enters the objective and ultimately forms an image at the back of your retina. In this lab, the laser enters the eyepiece and exits the objective. The magnification of the diameter of the laser beam is M = f o f e. Build a telescope on your optical rail. Adjust the separation of the two lenses so that the beam exiting the objective is as collimated as you can make it. Then compare the measured separation to the theory. Measure the beam magnification by estimating the beam size on a card, and compare with the expected value. f e f o
Experiment I.4: Familiarization with Lab Equipment: For this section you will need to go to the back room in the lab and collect the following items: USB Camera Camera and USB Cable Connect the camera to the computer through a USB port. On your desktop you will find the camera's program IC Capture 2.1. The program should automatically recognize the camera when you open the program. Do not take pictures of the laser unless you have decreased the laser intensity. With the lens you measured earlier in front of the camera take a picture of an illuminated ruler in focus. Measure the object and image distances to acquire the magnification. Now open your image in the program ImageJ. Using the rectangular highlighter tool, highlight a portion of the image and look at its profile by clicking Analyze à Plot Profile. Explain what this function does. Now use the measure function to determine the size of one pixel. Save this value somewhere special, you will need it for future labs! USB Stages Translational stage, Rotational Stage, Control Box, Power Cable, USB Cable Connect the translational stage to the control box and the control box to the computer using the USB cable. Open the program APT User on the desktop. The program should automatically recognize the stage when you open the program. Play around with the stage. What is the minimum speed at which the will travel? What is the range of the stage? Repeat for the rotational stage. Photodiodes Photodiode, BNC Cable, T-Connector, Resistor Clip, Resistors Connect a photodiode to the oscilloscope with a coaxial cable. Make sure to turn off the photodiode when not in use. Measure the intensity of the room lights when pointing the detector at the lights and at the floor. What is the maximum voltage output of the photodiode? Try this with different resistors attached to the clip. What happens? Equipment needed: Item Qty Source (part #) Helium-Neon Laser 1 Melles Griot 05 LHP 121 Optical Rail 1 Newport PRL-36 Rail carriage 3 Newport PRC-1 Lamp 1 Target slide 1 OSU 100 mm lens 1 Newport KPX094 200 mm lens 1 Newport KPX106 25 mm lens 1 Newport KPX076-25 mm lens 1 Newport KPX043 Index card 2 Filter holder 2 Thor Labs FH2 Al mirror 3 Newport 10D10ER.1 Iris (adjustable) 2 Thor Labs ID12 Meter stick 1
Laser Safety and Care You are responsible for the safety of every student in the class including yourself. Losing control of the laser beam can blind a fellow classmate or the TA! 1. Remove jewelry prior to entering the lab. 2. Never lower your head into the plane of the laser beam. o Unless you cover your eyes (in case you drop something). 3. Always keep the laser beam contained within the plane of the table. o Don t raise or lower the beam drastically. 4. Block the laser beam when adding or removing optics and whenever the laser beam is not in use. 5. Always block the tail end of the laser beam!!! o A stack of razor blades is provided for this purpose. 6. Every surface reflects! Block reflections from every optical component. 7. Do not touch optical surfaces! Fingerprints can permanently damage some components. Handle optics by their mounts. 8. Use latex gloves if it is necessary to handle a component directly or to clean optics. Many cleaning tutorials can be found online, including: https://www.thorlabs.com/tutorials.cfm?tabid=26066 Equipment: Do your best to preserve the instruments for the future. Avoid fingerprint on optical surfaces. The screws used to assemble component are often oily, so clean your hand after using them and before handling optics. Do not over tighten screws. Use the correct screw for the task at hand. Most of the screws you will need are 1/4-20 (1/4" diameter and 20 threads per inch) or 8-32 (#8 screw with 32 threads per inch). However, some threaded holes or screws in the lab may be metric. Do not force a screw if it feels tight. A 1/4-20 screw will go a few turns into an M6 (6 mm diameter with 1 thread per mm) threaded hole and a 8-32 screw will go a few turns into an M4 (4 mm diameter with 1 thread per mm) threaded hole; but don't do it! Be gentle with motors, and cameras. These components are expensive. Turn off photodiodes when finished. The photodiode is off when the circuit is open, and the switch is set to the symbol O o A photodiode tutorial can be found at: http://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=1295
Alignment Techniques Use 2 mirrors and 1 iris to align a beam parallel to the optical rail. 1. Block the laser. 2. Insert the iris into an optical rail carrier - ensuring that all optics to be used can be centered to this height. 3. Slide the iris close to the second mirror and unblock the laser. 4. Align the laser through the center of the iris by adjusting the first mirror. o The best way to align the beam through the iris is to close the iris down until it is slightly smaller than the beam. When the beam is aligned, you should see a faint halo on the iris. The iris is now clipping the beam. 5. Keeping the iris at the same height, slide it to the far end of the optical rail. 6. Align the laser through the center of the iris by adjusting the second mirror. 7. Iterate steps 3 6 until the laser is aligned through the iris at both ends of the optical rail. Your beam is now parallel to the optical rail!