h a l f b a k e r yPoof of concept
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A CD player could run a lot longer on batteries if it didn't have to spin up the mass of that disk. So let's imagine a way to get a moving laser beam to read a STATIONARY disk.
First, we need a really small-diameter laser beam. Normally the laser is generated by a specialized LED, and fiber optics
transmit it to a lens that focuses the beam into the data layer of the CD. However, lasers being what they are, able to shine a long distance with little divergence, it should be possible to create a specialized laser diode that emits a beam having the same diameter as a normal already-focused laser. Having this ultra-fine beam will be beneficial in several ways, one of which is an obvious lessened need for power.
Now let's cover the CD with a specially curved mirror. This will make for a portable CD player that perhaps is a little bulkier than what you might be used to, but perhaps not, and maybe the optics can be improved later. Right now I just want to present the basic Idea. At the center of this mirror, and actually located at the level of the hole in the CD, is a spinning mirror. Our laser beam is aimed upward to bounce off of first the spinning mirror and then the curved mirror. It then bounces STRAIGHT down to the surface of the CD and straight back up again. The reflected beam returns to the spinning-mirror spindle and its signal is caught and processed in the normal way. A precision tilting device is part of the spinning mirror spindle, so that the beam can be reflected off the big curved mirror toward both the inner and outer parts of the CD.
That's all.
CD with no spin
CD_20Spin_20Off
halfbaked before, although less thought through [neilp, Jun 03 2006]
Reading laser media with a scanner.
Optical_20Disk_20Sc...edia_20Obsolescence
[spidermother, Jun 07 2006]
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I am hoping you remembered that the disc has data stored in a spiral track. I am assuming that the tilt and rotation take care of this? If not, build an invert of todays existing CD player - place the disc on the bottom and spin the laser assembly. No mirrors, tilt hardware, etc. Besides, if i were to drop your player, i have to get a new one (the mirrors break). todays existing technologies are shock proof. Dropped one while riding a bike, still works fine ( it needed paint on the lid to fix scratches and the batteries put back in ). |
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I think if you thought this through a little more you would find that 2 properly shaped mirrors would allow for no spinning at all. A micro gimbaled LED would be able to see any part of the disk just by changing the angle at which is is aimed. |
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I dont really think its the spinning of the disc so mach as the audio amplification that consumes all the energy. Once that disc is spinning it takes vey little power to keep it moving. |
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clever. gj for overcoming the "spinning record" paradigm. |
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I knew I'd read about this idea on the HB before, although I'd forgotten it was one of mine (see link). |
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"How about a really high resolution CCD scanner the length of the disk radius and one element wide? One rotation of the disk and you're done. A might slow to start playing though, I'd venture to say. |
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I wonder if it [not spinning the disk] could be done digital-optically without the laser: an actual CCD array taking a sequence of pictures of a very small area, still using the rotating mirror. Machine vision processing to find the track blah, blah, blah... |
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Some crystals refractive properties when change when energy is applied (I forget if it's electrical or acoustic energy). It might be possible to move the laser beam with no moving parts. |
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[wolstech], not all mirrors are made of glass. Yes, I'm aware that the precision tilting mirror has to accommodate the track structure on the CD. |
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[jhomrighaus], yes, I was aware that it might be possible to eliminate the spinning component, yet even the gimballing you suggested counts as a kind of spinning component. But (not certain), I think I've read about crystals that, under the influence of electric or magnetic fields, can affect the direction of a beam of light that passes through them. If I'm remembering correctly, then this would let us "spin" the beam without any moving parts. I just wasn't confident enough about that memory to suggest it as part of the main Idea here. |
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[neilp], sorry I confess I didn't look to see if this had already been posted. However, since this Idea has a different focus than yours (low power), perhaps it can be allowed to persist. |
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[half], you still need to illuminate the area you are planning on sensing (takes energy). In an ordinary CD player, of course, the laser does that. |
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[yppiz], looks like we posted about the crystals at almost the same time. Thanks for the additional evidence! :) |
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fascinating idea, but as [UnaBubba] pointed out in the related idea, CD's are simply old news at this point. |
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Folks, please remember that this IS the HalfBakery.... |
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[Ian Tindale], I think I've read that a hologram of a lens, when illuminated with a laser, can work like a lens when ordinary light passes through the hologram --but I might be misremembering some of the details of what it was I actually read. |
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//specially curved mirror// The required curve is a parabola. With the small mirror in the same plane as the surface of the CD (and at the focus of the parabolic mirror) and the parabolic mirror just touching the outer edge of the CD, the aspect ratio of the parabolic mirror will be 1:4; in other words, this mirror's apex will be (as least) 1/4 the CD's diameter away from the small mirror. So the smallest possible player will still be somewhat bulky, but will have a funky parabolic curve on one side. |
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(edit) [xaviergisz] mentioned the parabolic mirror in 2004 in an anno on CD Spin Off. |
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What about this as a set up: |
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1. A light source.
2. A light reciever next to it
3. A (very) high-resolution LCD screen between the disk and the light source/reciever. |
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Rapidly switch one pixel to transparent, until you build up the entire grid. |
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I don't know whether the reflection from the CD will vary in such a way as to indicate a 1 or a 0....but it would need to for this to work. |
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Also, because of orientation problems, the LCD would have to be twice (estimate) the 'resolution' of the CD.... |
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Or.... a variation of a flat-bed scanner. |
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One scan across the CD 'xeroxs' the entire CD is one movement. |
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Play back the CD from internal memory. |
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Still has moving parts, but posssibly less energy needed than spinning the CD constantly.... |
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Sorry 'half' - stole you're idea there..... |
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//CD's are simply old news at this point" /// |
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yeah there are better technologies out there...but I think the old CD is going to be around for while yet....(just like you can still buy VHS and Vinyl).... |
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The dots on a CD are really, really tiny. Trying to keep a laser accurately focused with a method such as described here would be basically impossible. As for scanning, that would require a resolution of over 100,000 dots per inch. Scanners have improved over the years, but I don't think they're there yet. |
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[supercat], the Idea did not say to focus the laser; it said to use a very-narrow-diameter laser beam, equivalent to one that had been focussed. This should be possible because the "color" of a CD Laser is, if I recall right, infrared, so a modern red laser can indeed in theory be narrower than the wavelength of a focussed infrared beam. Thus it WILL go straight to a single dot and reflect from it in a way that can be be "read" as data. The biggest technical concern I had with this idea is the fact that the rotating mirror won't be in the same orientation when the beam returns, that it was in when it went out toward the disc. This MIGHT not be a problem due to the extreme speed of light (and a CD player is only 1x, after all, not 52x like most computer CDROM drives). It might even actually be an advantage, allowing a natural offset in location of the detector from the laser-source. |
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Unfortunately, laser beams cannot be made both arbitrarily narrow and also parallel. Small solid state lasers, for example, produce a diverging beam due to diffraction of light at the small aperture. Given that the pits on a CD are about 1 micrometer in size and near-parallel lasers are of the order of 1mm, the discrepancy in area is about 10^6. And you can't just go using shorter wavelength lasers, because the depth of the pits on a pressed CD are about 1/4 the wavelength of the IR laser used to read them (causing destructive interference). This is a neat idea, but I'm pretty sure it won't work. |
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The HB link I posted earlier discusses using a scanner to read optical disks. For that to work, 'scanner' needs to be replaced with 'scanning electron microscope', or at least 'ultraviolet scanner'. I don't think an optical scanner can, even theoretically, resolve objects on the order of the wavelength of light, and that's how small those pits are. |
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[spidermother], if a tiny lens is placed at the spot where the laser beam leaves the diode, cannot it be designed to compensate for diffraction, thus resulting in a straight beam? |
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If not, well, it won't be the first time some notion here at the Half-Bakery proved to be unworkable. |
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The problem will not go away that easily. As far as I know, it's not so much the way the beam is constrained (pinhole, lens, whatever) but the very fact that it is constrained that results in diffraction. You can focus a laser to a spot on the order of a few wavelengths (this is what current CD players do) but it will diverge much as if it were constricted to the same size by being passed through a hole. At this point I'm a bit out of my depth, but I'm guessing that due to symmetry the focal point cannot be both very far from the source - there is no electromagnetic arrow of time, so the beam will diffract in both directions from the focal point. |
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If the focal point could be far enough away, maybe you could mount above the mirror a lens which is radially polyfocal, i.e. compensates for the varying path length such that the focal point always lies at the level of the CD's pits. You would probably still need a fairly narrow laser so the focal length doesn't change too much over is width. |
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Polyfocal lense:
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I was considering a lens to cancel out the diffraction, not intended to converge at all. |
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Although lasers are much closer to being colinear light sources than are other forms of light-generating apparatus, there are still some pretty severe limits in that regard. The only way to get really small spot sizes with lasers is to focus them. |
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Is there some way of exploiting the rainbow shimmer you get off the disk when illuminated with full-spectrum light? |
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[BunsenHoneydew], look up "diffraction grating". The regular spacing of tracks/data-pits on a CD qualify that surface as a diffraction grating. |
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Optical disc drives do not use either LEDs or fiber optics.
They use laser diodes, typically with conventional lenses.
Take some unwanted ones apart; it's fun and you get cool
parts. |
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