Lately there’s been some hype about the use of blue lasers in optical drives (such as CD and DVD). Blue light has a shorter wavelength than red, so the pits in the disk can be smaller, allowing greater data densities.
Well, great, guys. I could have told you that coming out of high school physics; this is not an earth-shattering advance. So why is it taking so long for shorter wavelengths to be used in optical data storage? Why didn’t they start with the DVD?
And, more importantly, why stop at blue? Surely lasers in even shorter wavelengths would be even more effective, so why not use ultraviolet or shorter? Too high-energy, lack of appropriate materials for the media, or some other reason?
Should I start filling out the patent applications?
The only real difference between CD and DVD is density. The move from red lasers to blue lasers allowed for this greater density. There is no point on using a blue laser in a cdrom drive, but it’d certainly work as DVD players use blue lasers to read CDs. Basically, a blue laser cd-rom is marketing.
Why has it taken so long? Blue lasers are tricky, that’s why. The sorts of lasers used in computer peripherals are often solid-state lasers, based on organic compounds. The wavelength of the laser depends on the energy level structure of the compound.
We’ve just got a lot more compounds that will do red light than blue. We’re finding some - thus the development of DVDs - but it’s taken a lot longer than the commercial exploitation of the red laser technology.
It wasn’t until a couple years ago that blue solid-state lasers became affordable enough to put into mass-market items.
For whatever reason related to chemistry, blue has been an expensive color to produce by solid-state means. 5-10 years ago, blue LEDs were unthinkably expensive. Now, they’re simply quite expensive - $3-4 each, compared to pennies for the red ones.
No optical format currently on the market that I’m aware of uses blue lasers. DVDs quite clearly use red lasers; on some players you can even see the reflection of the beam if you look in at the closed tray at the right angle. CDs use infrared lasers.
gotpasswords is right that blue lasers are difficult and expensive to build. I don’t know the specific reasons why, but I guess there have been some recent breaktrhoughs that make them cheaper.
Sony has announced the first blue laser DVD recorder. It’s very expensive. The device uses the Blu-Ray standard, which was developed by a consortium including Sony, Panasonic, Samsun, Philips (inventors of the CD ) and others. The discs are the same size as CDs and DVDs and can hold 23 GB. That’s nearly five times the amount of a red laser DVD.
It’s not blue, but you can get a green laser here. I don’t know exactly how other colours are made, but it was due to the difficulty of making non-red lasers, not just that no-one thought of it. Heck, I can get a red laser at the dollar store here, batteries included. Can’t get any other colour though.
At the risk of being insulting, Howstuffworks.com has a section with the output ranges of various different types of lasers. I imagine one of the types with output in the red range is dead easy, whereas the others are hard.
Thanks for the replies. I should have predicted cost would be a factor, knowing something about the pricing of different colours of LED.
On to UV lasers, then. Are they not used because of the cost of the laser, the cost of the medium that’ll reflect it, or the potential danger to consumers?
Why is it when you type something a second time it never seems as eloquent as the first?
UV lasers would be very expensive and not very practical. A good approximation of the power required to get a material to continuously lase scales as [symbol]l[/symbol][sup]-5[/sup], thus an infrared (1000nm) semiconductor laser that takes 1 mW of power would take 32 times more power if it operated at 350 nm and 10[sup]5[/sup] more power to lase at 100 nm. The best continuous wave laser we can make operates around 300nm and takes a LOT of power.
The second reason that a UV laser is not pratical is that most materials have resonances at this frequency range that create absorption. Take the vacuum ultra-violet frequency ([symbol]l[/symbol] around 120 nm), the reason is that it is called vacuum ultra-violet is that it interacts strongly with air (and most other materials) so it only really propagates in a vacuum. In every other material it is quickly absorbed and scattered.
Finally, lasers work (simplified version) by creating a population inversion in “meta-stable” energy state. The energy transitions for UV photons are hard to find. Most X-UV lasers work using free-electron gasses (plasmas) to create the population inversion as there are no molecular or atomic transitions that have this energy.
I do not know very much about UV lasers, so hopefully somebody who knows more will come along and enlighten us. I do know a little about the use of ultra-short, ultra-high intensity pulses needed to create coherent x-rays, but this is a whole different kettle of fish. I assume that many of the same problems exist with UV lasers.
One more thing: The x-ray lasers I mentioned and the UV excimer lasers aer all pulsed systems. I do not know if a pulsed laser could be used in a CD or DVD.
) and others. The discs are the same size as CDs and DVDs and can hold 23 GB. That’s nearly five times the amount of a red laser DVD.