by Joe Farina » Mon Nov 26, 2018 7:17 pm
Hi John W,
I'm a newbie with spectrometers, but I took a crude screen shot just now (of a red laser diode pointer):
- 0_03.jpg (171.77 KiB) Viewed 6260 times
My calibration is somewhat off, because this is probably a ~640nm pointer. Also, it shows a really wide red spectrum (not accurate of course) but my usb camera is only 640 X 480. My setup is pretty crude, I believe, and I want to build a better spectrometer with much higher resolution with the camera. This particular diode looks rather well behaved, but in another case I had a very ugly double-spiked curve (it looked like an "M" and the peaks were constantly dancing around. I had the HeNe curve going at the same time, and it was rock-solid. The curves refresh at 8 frames per second, I believe. It was the contrast with the HeNe that surprised me, originally I wanted to use the spectrometer to only get a handle on the wavelength.
Wler has done a lot of work with spectrometers, and he did make the comment that a spectrometer is better for checking laser diodes for holography, compared to a scanning Fabry-Perot interferometer:
http://hololaser.kwaoo.me/laser/spectra.html
and he makes the comment: "For lasers with large cavities, the mode spacing is small (a few 100'sMhz) and the best device to measure it is a scanning Fabry-Perot interferometer (SFPI). Most measurements below were done with my home-made SFPI described in the link, and the piezo and mirror data that are sometimes given refer to that page.
On the other hand, for small lasers with small cavities, eg DPSS lasers and especially laser diodes, the mode spacing of many Ghz is too large in order to be well captured by a SFPI (because the display becomes ambiguous as the modes "wrap around" it, and one would need a mirror spacing of a mm or so to prevent this); while one can see whether there are extra modes present or not, it is hard to judge their spacing, and thus, the actual coherence length. For such lasers, a grating based spectrometer is better suitable, and see here for a (partially) home-made device."
I think you will be happy with what you're working on, if you want some insights concerning laser diodes for holography. I was looking into using a linear CCD also, because that's what some of the high-end spectrometers (Ocean Optics, etc.) use. I couldn't find a good source for the CCD, driver board, and also I want to use a USB connection because that's what my software seems to like (Theremino). I think Wler was able to get super-high resolution with his grating-based spectrometer system, something in the pico-meter region, as I recall. I want to do quite a bit more experimenting and work to improve my spectrometer, but even as it stands now, I'm quite excited about it. If you can write your own software, that sounds excellent.
Hi John W,
I'm a newbie with spectrometers, but I took a crude screen shot just now (of a red laser diode pointer):
[attachment=0]0_03.jpg[/attachment]
My calibration is somewhat off, because this is probably a ~640nm pointer. Also, it shows a really wide red spectrum (not accurate of course) but my usb camera is only 640 X 480. My setup is pretty crude, I believe, and I want to build a better spectrometer with much higher resolution with the camera. This particular diode looks rather well behaved, but in another case I had a very ugly double-spiked curve (it looked like an "M" and the peaks were constantly dancing around. I had the HeNe curve going at the same time, and it was rock-solid. The curves refresh at 8 frames per second, I believe. It was the contrast with the HeNe that surprised me, originally I wanted to use the spectrometer to only get a handle on the wavelength.
Wler has done a lot of work with spectrometers, and he did make the comment that a spectrometer is better for checking laser diodes for holography, compared to a scanning Fabry-Perot interferometer:
http://hololaser.kwaoo.me/laser/spectra.html
and he makes the comment: "For lasers with large cavities, the mode spacing is small (a few 100'sMhz) and the best device to measure it is a scanning Fabry-Perot interferometer (SFPI). Most measurements below were done with my home-made SFPI described in the link, and the piezo and mirror data that are sometimes given refer to that page.
On the other hand, for small lasers with small cavities, eg DPSS lasers and especially laser diodes, the mode spacing of many Ghz is too large in order to be well captured by a SFPI (because the display becomes ambiguous as the modes "wrap around" it, and one would need a mirror spacing of a mm or so to prevent this); while one can see whether there are extra modes present or not, it is hard to judge their spacing, and thus, the actual coherence length. For such lasers, a grating based spectrometer is better suitable, and see here for a (partially) home-made device."
I think you will be happy with what you're working on, if you want some insights concerning laser diodes for holography. I was looking into using a linear CCD also, because that's what some of the high-end spectrometers (Ocean Optics, etc.) use. I couldn't find a good source for the CCD, driver board, and also I want to use a USB connection because that's what my software seems to like (Theremino). I think Wler was able to get super-high resolution with his grating-based spectrometer system, something in the pico-meter region, as I recall. I want to do quite a bit more experimenting and work to improve my spectrometer, but even as it stands now, I'm quite excited about it. If you can write your own software, that sounds excellent.