On Sat, Jan 1, 2011 at 8:27 AM, Mark Lerman <mlerman@...> wrote:
> I've been looking at diode lasers and appears that focusing them is
> not so easily done because they exhibit astigmatism and the beams are
> not round. In fact, some of them are square.
I don't understand what the problem is. According to the Wikipedia
article on CDs:
"CD data are stored as a series of tiny indentations known as "pits",
encoded in a spiral track moulded into the top of the polycarbonate
layer. The areas between pits are known as "lands". Each pit is
approximately 100 nm deep by 500 nm wide, and varies from 850 nm to
3.5 µm in length.
The optical chip extracted from a CD player. The three dark rectangles
are photosensitive, and read the data from the disk. Electronic
tracking keeps the laser beam centered on the middle area.
"The distance between the tracks, the pitch, is 1.6 µm. A CD is read
by focusing a 780 nm wavelength (near infrared) semiconductor laser
through the bottom of the polycarbonate layer."
So CD writers must focus down to approximately 500 nm.
The shape of the beam shouldn't matter, as long as the light rays come
out parallel (as with a laser pointer). Then you can focus the rays
down to a point of a size on the order of the light wavelength.
Under Blu-ray Disks, Wikipedia says:
"While a DVD uses a 650-nanometer red laser, Blu-ray Disc uses a 405
nm "blue" laser. This shorter wavelength allows for over five times
more data storage per layer than allowed by a DVD. Note that even
though the laser is called "blue", its color is actually in the violet
range.
"The diodes are GaN (gallium nitride) lasers that produce 405 nm light
directly, that is, without frequency doubling or other nonlinear
optical mechanisms.[58] Conventional DVDs and CDs use red and
near-infrared lasers, at 650 nm and 780 nm, respectively.
Panasonic Internal Blu-ray ROM notebook drive
"The minimum "spot size" on which a laser can be focused is limited by
diffraction, and depends on the wavelength of the light and the
numerical aperture of the lens used to focus it. By decreasing the
wavelength, increasing the numerical aperture from 0.60 to 0.85, and
making the cover layer thinner to avoid unwanted optical effects, the
laser beam can be focused to a smaller spot, which effectively allows
more information to be stored in the same area.
"For Blu-ray Disc, the spot size is 580 nm. In addition to the optical
improvements, Blu-ray Discs feature improvements in data encoding that
further increase the capacity[citation needed]. (See Compact Disc for
information on optical discs' physical structure.)"
(It's not clear to me why the spot size of Blu-ray disks should be,
apparently, bigger. Maybe the spot size is measured in a different
way, or maybe the 580 nm is actually track pitch or some such error.)