There's not really much you can do to make a definitive assessment of a laser's condition, other than to rely on experience and substitution. Unless it is part of a manufacturers service setup procedure - Pioneer CD players for instance - the laser power pots should NEVER be adjusted from their factory set, and usually sealed, positions. Even a laser power meter won't tell you a lot about a laser's condition. Even if it indicates that the emitted power is within spec, plenty of laser problems are caused by defective pickup chips ( the clear plastic chip usually at the bottom, which has the individual photo diodes on it ) or dust on the internal optics, diffusing the reflected beam from the play surface, before it gets to the pickup chip.
The difference between a CD and a DVD laser, is wavelength and power. A DVD laser is in the visible red portion of the spectrum, and of higher emitted power than a CD laser, which is in the infra red part of the spectrum. DVD lasers do not read CDs or vice versa, in general. Rather, there are two laser emitter chips, for the two different wavelengths, and usually a single pickup chip, although these items are all integrated together in a single pickup head assembly.
If your player handles CDs ok, but struggles with DVDs, theres a better than
I believe a dvd laser can in fact read a cd.. I know a lot of early dvd players had two lasers to save wear on the more costly dvd laser (and they do wear out with time), but I thought most modern players used only a red laser for all media.
It was my understanding that the IR laser in a cd player is just too "fat" in terms of wavelength to resolve the tin
It's the reverse. Older players which had the red lasers were able to read factory cd's with a reduced level of efficiency. They could not read CD-R's because the discs were essentially "invisible" to the red laser. Some older red-laser players, like Sony's could read CD-RW's but not CD-R's. Others would not, but that was a firmware issue. Newer players don't use two separate lasers - they use a special "double" laser diode, which emits from one side or the other depending on how it's biased; that is, for an audio cd or CD-R, or a DVD. Ocassionally, one will malfuction in such a way that DVD's can't be read anymore, but cd's can.
That was where firefox crashed. I was going to say something about tinier pits on DVD's..
When you say emits from one side or the other, do you mean that both ends of the crystal are used as emitters? Or are there multiple diodes in a single package? I never heard of this; it sounds interesting.
I just assumed that it was a single laser, but I have definitely observed that a much more powerful looking beam is used to read DVD's than CD's.
I actually tore apart a dual-wave diode a couple years ago, but I don't remember exactly how it was built. I do remember it was roughly rectangular in shape with tiny wires going to it. I only "presume" it was three connections total, either 3 wires or two wires with a center common connection.
I just had a quick look at a few scrap lasers which were hanging around the workshop, and they have six connections to the laser diode unit. Looking at a few schematics, these are one each for the anodes and cathodes of the laser diodes ( one red, and one infra red for DVD and CD respectively ) and one each for the feedback photo diode integrated with each laser diode. The other connection of the feedback diode is commoned with one of the laser diode connections in each case. The feedback diode is to allow for automatic power control of the laser output. Laser diodes with only three connections appear on the regular CD only laser heads. Here, the connections are again anode and cathode of the laser diode, and one leg of the feedback diode, the other leg again being common with one of the laser diode legs.
The general sequence of events is that the laser homes and then the red DVD laser is switched on to burn. The focus sequence is then initiated and, once focus has been obtained, the disc is spun up, and an attempt is made to read data from the disc. Sometimes, the disc is spun up at random speed prior to or during the focus search.
As Mark correctly says, the shorter wavelength red DVD laser is actually able to read the longer pits on a CD, although not very efficiently. Thus, once enough data has been gathered to evaluate whether the disc that's being read is a DVD or a CD ( or even a VCD ), the system control processor will make an evaluation as to whether the red DVD laser should be left burning, or the infra red CD one, switched on instead. Other subtle changes are made as well at this time, including altering servo time constants, and disc base rotational speed etc. In the case of a DVD, the focus has to be adjusted to ensure that it's reading layer zero at this time also ( assuming that it has been informed by some of the early data that's collected, that it's a dual layer disc that's in there. It's a complicated business, to be sure ...
To verify that the laser is operating remove any clamps or items that would block your direct view of the lens and glance at it when it should be reading a disc. The laser is focused to read a disc that is close to the lens, so it will not be focused on your eye's retina. While I would not advise staring at a CD/DVD laser for any length of time, a passing glance will not cause eye damage or injury even if you do it a thousand times!
However this will not verify the correct output power or the sensor's ability to read the reflected signal from the disc.
Whilst it is probably true that the laser will not cause any eye damage, all manufacturers recommend that if you must look at it, do it obliquely from an angle of 45 degrees or so. Also remember that a CD laser is basically in the infra red spectrum, and emits very little visible light, so can be difficult to see, encouraging people to try to look closer and directly in the top, to check if its burning.
The DVD laser can, of course, be seen actually through the disc, so in a lot of cases where the centre of the disc is not obscured by a large clamp holder, you can see if the laser is burning with the disc in place.
Personally, I would never look directly into a laser, no matter what the physics dictates, or how low the power is.
I do know the difference between a laser diode and an entire pickup, so I was also referring to the laser diode, as I thought that I had carefully explained, when I said that a typical one, used in a DVD player pickup, has
6 connections, as opposed to one used in a CD only pickup, which has only 3 connections. A complete pickup, either CD or DVD, has many more connections, than 6.
A CD laser diode has two components in it - a laser diode and a feedback diode, both integrated onto the same chip. A DVD laser diode, has the same thing, but times two.