CD/DVD laser wavelength and track widths

In summary, the spot size for reading a track on a CD is twice the wavelength, and this rule comes from the fact that smaller spot sizes can result in light diffraction and blurred reflections. The DVD standard uses a 640nm laser, but its track separation is 7.4 microns due to a safety margin to avoid reading neighboring tracks and potentially causing destructive interference. The calculation of 2x the wavelength for spot size also ensures a larger margin for accurate reading.
  • #1
AndyW
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I have read that that 'spot size' for reading a track on a CD is twice the wavelength, e.g. on a CD the track separation is 15 microns which can be read by a 700nm laser. But where does this physical rule come from?

Also, the DVD standard uses a 640nm laser. That would imply a track separation of about 13 microns, but in actual fact it is 7.4 microns. Why is this?

Thanks
 
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  • #2
I would also like to know this.
 
  • #3
Check your math. 2x 640nm is 1.28um not 13um.
Track separation is different thing from spot size. Both are on safe side to avoid reading neighborig tracks.

Twice the wavelength size rule comes from fact that with smaller spot light diffraction goes nuts, and you can't get exact reading, reflection gets blurred.
 
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  • #4
Maybe it insures substantially more than one wavelength for reading, thus less destructive interference.
 

What is the laser wavelength used in CDs and DVDs?

The laser wavelength used in CDs and DVDs is approximately 780 nm (nanometers).

Why is a specific laser wavelength used in CDs and DVDs?

The specific laser wavelength used in CDs and DVDs allows for precise tracking and reading of data stored on the discs. This wavelength is chosen because it falls within the range that can be easily focused and detected by the optical components of the disc drive.

How does the laser wavelength affect the storage capacity of a CD or DVD?

The laser wavelength does not directly affect the storage capacity of a CD or DVD. However, it does play a role in determining the size of the data "pits" that are used to store information on the disc. Smaller pits can be created with shorter wavelengths, allowing for more data to be stored in a given space.

What is the difference between the laser wavelengths used in CDs and DVDs?

The laser wavelength used in CDs is 780 nm, while the laser wavelength used in DVDs is 650 nm. This difference in wavelength allows for DVDs to have a higher storage capacity than CDs, as the smaller pits created by the shorter wavelength can hold more data.

What is the track width of a CD or DVD?

The track width of a CD or DVD is approximately 0.74 microns (μm). This is the distance between adjacent tracks on the disc, and it is a key factor in determining the storage capacity of the disc.

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