[Laser] dual wavelength laser diodes

[Tim Toast]

I'm not sure if it's been mentioned in here before but
I recently noticed they are using a dual wavelength
laser diode in DVD player and video game unit optical
pickups. Apparently the wavelengths are 650nm (red)
and 780nm (IR). On the net i read the visible red beam
is used for DVD-ROM and the infrared for CD-ROM. 
The diodes I've seen had 4 pins, with a common cathode
for the two lasers and a power monitor diode. The two
laser beams appear to be 'co-aligned', coming from a
single point source inside the diode package. Although
they are probably two separate lasers combined with a
tiny beam splitter or are extremely close together and
both lasers can be ON at the same time. 
Of three diodes i've had a chance to remove from their
pickups, they had threshold currents ranging from
about 25 to 55 milliamps. Two had brewster angled
windows (DVD players) and the other was a flat window
package (Sony PS2 game unit). As far as i can tell,
the power levels are in the 1 to 5 milliwatt class. 

Maybe there is some use for these in our optical
experiments like:
1. Sending two independent channels at the same time
via the two beams. At the receiver, a beamsplitter
with bandpass filters and two photodetectors to
recover the two channels and separate them.

2. Sending the same audio on both channels, then at
the receiver using a 'diversity' mode to take some of
the scintillation out of the signal - since the red
and IR beams might fade at different times.

3. Using the visible laser for alignment, then
switching to the IR laser for communication. 

4. Just being able to switch to another wavelength
without having to re-align anything. (other than
focusing maybe)

As cheap as DVD players have gotten lately, it's
almost worth it to go out and buy one just to get the
diode if it interests you. People give away or throw
out their old ones and as the new video game units
come out, people will be more willing to give up those
as well. Also upgraded computer optical/DVD drives
might be another way to get one for cheap. 

While reading about the dual wavelength thing online,
i saw several that used a blue, green or red combo for
the new generation DVD's and BlueRay disks. They were
using two separate laser diode packages though, with
an outboard beam combiner in the pickup. In order to
play the older CD-ROM disks these will probably have
an IR output as well, either as a separate diode or,
combined with one of the other lasers. There is
something about the dyes used in audio cd's that don't
work well with shorter wavelength lasers.







Tim Toast
http://www.aladal.net/toast/exp.html


 
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