Optical: CD-R/CD-RW
That is how a normal CD works, which is great for prepackaged software, but no help at all as removable storage for your own files. That's whereCD-recordable(CD-R) andCD-rewritable(CD-RW) come in.
CD-R works by replacing the aluminum layer in a normal CD with an organic dye compound. This compound is normally reflective, but when the laser focuses on a spot and heats it to a certain temperature, it "burns" the dye, causing it to darken. When you want to retrieve the data you wrote to the CD-R, the laser moves back over the disc and thinks that each burnt spot is a bump. The problem with this approach is that you can only write data to a CD-R once. After the dye has been burned in a spot, it cannot be changed back.
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CD-RW fixes this problem by usingphase change, which relies on a very special mixture of antimony, indium, silver and tellurium. This particular compound has an amazing property: When heated to one temperature, it crystallizes as it cools and becomes very reflective; when heated to another, higher temperature, the compound does not crystallize when it cools and so becomes dull in appearance.
cd - rw驱动器have three laser settings to make use of this property:
- Read- The normal setting that reflects light to the optoelectronic sensor
- Erase- The laser set to the temperature needed to crystallize the compound
- Write- The laser set to the temperature needed to de-crystallize the compound
Other optical devices that deviate from the CD standard, such asDVD, employ approaches comparable to CD-R and CD-RW. An older, hybrid technology calledmagneto-optical(MO) is seldom used anymore. MO uses a laser to heat the surface of the media. Once the surface reaches a particular temperature, a magnetic head moves across the media, changing the polarity of the particles as needed.