Some Great Visual Resources for Your Science and Photonics Facilities

Do you need some posters to decorate the walls of your classrooms, labs and halls? Two of the photonics professional societies, SPIE and the Optical Society of America (OSA), have created some excellent posters that they provide to K-12 schools and colleges, free for the asking. These posters will not only add color and class to your facilities, they provide interesting education and career information about optics and lasers. To view and request copies of these posters, you will need to visit their web sites.

The SPIE web site is http://www.spie.org/ but you can go directly to the page that shows the posters by visiting www.spie.org/x31474.xml. There are many posters shown that you may want. SPIE will send them to you rolled up in a tube. We have had them mounted on foam board for a few dollars. Then they can be hung on walls or positioned on tables, unframed or framed. Some of the ones that I found particularly interesting are:

• Introduction to Popular Applications of Optics & Lasers (new) — This poster shows novel applications that everyone can recognize, but may not know that they were enabled by optics and/or lasers. I think it is particularly useful for elementary and middle school students—and their parents.

• Future of Lasers: Illuminating the Future (new) — This is a futuristic look at new laser applications in healthcare, energy, manufacturing and communications. I think it makes a great addition in either high schools or colleges.

• Invent Your Future (new) — This is my favorite! It explores & encourages photonics careers in science and technology. I think it is particularly appropriate for middle and high schools. We have this one mounted in the entrance of OP-TEC. Others are mounted throughout our Center.

• Posters that relate to photonics applications in other fields include:
 Lithography
 Remote Sensing
 Metamaterials
 Biophotonics
 Sensors
 Nanotechnology
 Energy

The OSA web site is

http://www.osa.org/ but you can go directly to the page that shows posters & other educational materials by visiting http://www.osa.org/Foundation/Youth_Education/Classroom_Materials/default.aspx.

There are two OSA Poster Series:

• Optical Phenomena Posters (four, 11” x 34” — request the set)
 Lasers
 Fiber Optics
 Biomedical Optics
 Spectroscopy

• Make waves—Discover Science Series (new, four, 11” x 34” — request the set)
 Acoustics
 Cross Polarization
 Echolocation
 Lasers

These posters will also be sent to you rolled in a tube; and they are best displayed if they are mounted on foam board. They are printed in five languages, so specify if you want yours in English. We have six of these posters displayed in the OP-TEC office.

Educational Pamphlets to accompany OSA posters: OSA has also developed very high quality educational pamphlets to support the posters described above. These 8.5" x 11", 4-8 page documents contain scientific explanations of the phenomena/equipment, experiments and career profiles of photonics scientists, engineers and technicians. They are available as printable PDFs that you can download from the web site.

Other free educational resources: OSA & SPIE also have several CDs and digital explorations, described in their web sites, that are useful for introducing photonics and careers to young people.

One example that particularly impresses me is Lighten Up! Discovering the Science of Light. This 36 page color booklet, developed through a collaborative effort between OSA Foundation and the Girl Scouts of America, is an exciting educational resource guide for girls, ages 11-15. You can request copies from OSA and GSA, or you can download a PDF and print your own copies.

Someone once said, “The best things in life are free.” I’m not sure they were referring to educational materials in lasers and optics, but the saying sure fits for these resources.

Dan Hull

What would life be like without lasers? Part C - Using Lasers to Burn and Read CDs and DVDs

CDs and DVDs are everywhere these days. Whether they are used to hold music, data or computer software, they have become the standard medium for distributing large quantities of information in a reliable package. Compact discs are now easy and cheap to produce. If you have a computer and CD-R drive, you can create your own CDs, including any information you want.

The Disc
A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of a piece of clear polycarbonate plastic, shaped like a disc. During manufacture, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A cross section of a complete CD looks like this: The Spiral
A CD has a single spiral track of data, circling from the inside of the disc to the outside. What the picture on the right does not even begin to impress upon you is how incredibly small the data track is -- it is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A micron is a millionth of a meter.) And the bumps are even more miniscule...

The Bumps
The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) Looking through the polycarbonate layer at the bumps, they look something like this:The bumps are arranged in a spiral path, starting at the center of the disc. The CD player spins the disc while the laser assembly moves outward from the center of the CD.

CD Player Components
The CD player has the job of finding and reading the data stored as bumps on the CD. Considering how small the bumps are, the CD player is an exceptionally precise piece of equipment. The drive consists of three fundamental components:

• A drive motor spins the disc.
• A laser and a lens system focus in on and read the bumps.
• A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track.

You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps.

The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long! To read something this small you need an incredibly precise disc-reading mechanism. The key element in this mechanism is the pinpoint beam of a laser.

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.

The hardest part is keeping the laser beam centered on the data track. This centering is the job of the tracking system. The tracking system, as it plays the CD, has to continually move the laser outward. As the laser moves outward from the center of the disc, the bumps move past the laser faster. Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.

CDs store music and other files in digital form -- that is, the information on the disc is represented by a series of 1s and 0s. In conventional CDs, these 1s and 0s are represented by millions of tiny bumps and flat areas on the disc's reflective surface.

To read this information, the CD player passes a laser beam over the track. When the laser passes over a flat area in the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as a 1. When the beam passes over a bump, the light is bounced away from the optical sensor. The CD player recognizes this as a 0.

The advent of CD burners marked a huge cultural shift. The technology made it feasible for the average person to gather songs and make their own CDs. Today, writable CD drives (CD burners) are standard equipment in new PCs, and more and more audio enthusiasts are adding separate CD burners to their stereo systems.

CD burners darken microscopic areas of CD-R discs to record a digital pattern of reflective and non-reflective areas that can be read by a standard CD player. Since the data must be accurately encoded on such a small scale, the burning system must be extremely precise.

In addition to the standard read laser, a CD burner has a write laser. The write laser is more powerful than the read laser, so it interacts with the disc differently: It alters the surface instead of just bouncing light off it. Read lasers are not intense enough to darken the dye material, so simply playing a CD-R in a CD drive will not destroy any encoded information.

Questions or comments? E-mail us!

References:

Brain, Marshall. "How CDs Work." 01 April 2000. HowStuffWorks.com. <http://electronics.howstuffworks.com/cd.htm> 02 November 2009.

Harris, Tom. "How CD Burners Work." 01 August 2001. HowStuffWorks.com. <http://computer.howstuffworks.com/cd-burner.htm> 02 November 2009.