Zom-B's blu-ray spectroscope (malfunctioning?)

I made a more precise spectroscope, as my dark-box spectroscope (one you hold to your eye) is not really suited for precise wavelength measurements.

This one projects dots on a wall and the ratio of the distance between the dots and the distance from the apparatus to the wall defines the wavelength.

There are two polarized beam splitter cubes in there, they have a double function:
- Assure the incident beam is parallel to the diffraction grating by forcing the user to aim through a narrow path,
- Assure that the long axis of the (oval) beam is aligned at a right angle to the diffraction angle, so the spots on the wall are as thin as possible.

The drawback is that other colors than violet are not supported

The diffraction grating is a piece op 80min recordable CD with the foil removed. The tracks are spaced 1.48um apart (theoretically). I took a piece near the outer edge to minimize curvature of the tracks.












In this case, I placed the diffraction grating at D=50.0cm from the wall and the direct and first order (n=1) beam are exactly y=14.0 cm apart. Filling this and the spacing of 1.48um in the grating equation:

n*[ch955] = d*sin([ch952]) where [ch952] = tan[sup]-1[/sup](y/D)


gives

1*[ch955] = 1.48 * 15.642° = 399.1nm

This value is a bit too low. Especially considering I have a laser with an even lower wavelength (as seen through my other spectroscope). What could be wrong with the calculation?

How sure are you about the 1.48um? :-/

- Assure the incident beam is parallel to the diffraction

Click to expand...

Don't you mean perpendicular?

Are you sure everything is perfectly alligned and measured?

yeah i mean perpendicular.

I got the 1.48 from this website: http://e-articles.info/e/a/title/CD-ROM-and-CD-R-tracks-and-sectors/
I didn't found info anywhere else so I can't verify it.

Other thing I don't get is 1.48 * 15.642° = 399.1nm , no matter how hard I try I can't get 1.48 * 15.642 to equal anything like 399.1 or something that can pass as a blu-ray wavelength for that matter. :-/ But to tell you the truth, I have no idea about these equations.
The thing is, even if you do have any errors in your calculations or measurements, they must be very small since the result is very close to 405nm.Too bad it doesn't work for green, you could've tested it with a DPSS which is more spectrally stable.Why does it only work for violet? Is it because of the cubes?

Switch said:

Other thing I don't get is 1.48 * 15.642° = 399.1nm , no matter how hard I try I can't get 1.48 * 15.642 to equal anything like 399.1 or something that can pass as a blu-ray wavelength for that matter. :-/ But to tell you the truth, I have no idea about these equations.
The thing is, even if you do have any errors in your calculations or measurements, they must be very small since the result is very close to 405nm.Too bad it doesn't work for green, you could've tested it with a DPSS which is more spectrally stable.Why does it only work for violet? Is it because of the cubes?

Click to expand...

It's actually 1.48 micrometers * sin(15.642), from the equation above it. That'll work out to give you the 399nm, or .399um.

Yeah, I think maybe the diffraction grating spacing isn't precise enough, because it looks to me that Bragg's law is calculated the right way. Everything looks ok, I guess your grating spacing must just be off by some amount.

Oh :-/

And is 15.642 degrees? isn't it supposed to be in Rad? :-/

You can do it either way. 15.642 degrees is .26963 radians. Take the sin of both, it's the same number.

Well, of course, after all , it's the same angle.

You could make sure the beam is perpendicular to the diffraction grating with a green laser, if only..... So is it because the cubes are coated for violet or what?

Yes, cubes are violet polarized beam splitters. I'll have to try shining it with a green laser, and try to get the beam in perpendicular by using another cube or mirror that works for green. That way I can calculate the spacing of the grating.

Maybe you can use tiny mirrors instead of cubes so you can use a green laser to calibrate it so it reads 532nm , then test the blu-rays.Would that work? :-/

Switch said:

Maybe you can use tiny mirrors instead of cubes so you can use a green laser to calibrate it so it reads 532nm , then test the blu-rays.Would that work? :-/

Click to expand...

This sounds like a great idea. From what I know about DPSS, I think they are generally much closer to the central wavelength than diodes are, allowing for a much more precise calibration. I mean, for all we know that diode could actually be 399nm. Some have found "405nm" diodes lasing past 410nm, so why not under 400?