Spectrometer

Holography related topics.
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

Spectrometer

Post by Joe Farina »

I finally set up a decent (and inexpensive) spectrometer, and was surprised to see that it showed laser diode instabilities. I compared a HeNe to a laser diode. The HeNe had a nice stable, smooth curve on the spectrometer, while the laser diode had a rough, double-spiked curve which was constantly in motion. A lot of information was published by forum member Wler on his website, particularly at http://hololaser.kwaoo.me/laser/gratingOSA.html I found this most interesting.

The software is available free at http://physicsopenlab.org/resources-downloads/ under software downloads, Theremino Spectrometer. I bought the $70 spectrometer on ebay https://www.ebay.com/itm/Mini-USB-Spect ... :rk:2:pf:0 These types of spectrometer are easy to build, though, from a usb webcam ($20 to $50), a diffraction grating ($5 ebay, 1000 lines/mm) a slit (can be made from two razor blades), and a light-tight enclosure http://physicsopenlab.org/wp-content/up ... ro_ENG.pdf and also http://renaud.schleck.free.fr/spectrometre.php
John W
Posts: 3
Joined: Fri Jan 09, 2015 4:39 pm

Re: Spectrometer

Post by John W »

Hi Joe

Thanks for reporting on this. I am trying to build the same thing but from component parts...
similar to what you mentioned:
a linear (one dimensional) CCD
and a reflection grating
software I wrote myself...

but I am not sure if I could resolve the fine splitting of modes from the diode I was testing... so I was thinking of trying a scanning Fabry Perot spectrometer instead, like this DIY version:
http://repairfaq.cis.upenn.edu/Misc/sale/sfpiins1.htm

Could you provide some screen shots of the output from the spectrometer you have built?

Many Thanks

John
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

Re: Spectrometer

Post by Joe Farina »

Hi John W,

I'm a newbie with spectrometers, but I took a crude screen shot just now (of a red laser diode pointer):
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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.
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

Re: Spectrometer

Post by Joe Farina »

Attached below is another screen shot taken just now, the HeNe is on the right, and an Osram PL-450 laser diode is on the left. The diode is being run at 51mA for this picture. Although only a hologram will tell the story, it's interesting to see how these curves change during warm-up and changes in current for the diode.

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John W
Posts: 3
Joined: Fri Jan 09, 2015 4:39 pm

Re: Spectrometer

Post by John W »

Hi Joe

It was a while ago now... but there was an open source project, by a group called "myspectral"

https://labrigger.com/blog/2012/06/25/i ... spectrino/

I just purchased the linear CCD and did the software myself. It was a struggle to prevent the CCD from saturating. I also refered to wler's laser page, it was very comprehensive.

I am keen to hear later if you can use your spectrotometer to judge the suitability of a diode for holography....

How are you driving the diode? Is it a homemade driver or did you purchase one?

John
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

Re: Spectrometer

Post by Joe Farina »

Hi John,

Thanks for the link. The driver was bought from Wavelength Electronics, it's a constant current driver (model # LDD200-2M). Below is a picture of my crude setup, next to the computer. The blue diode is mounted in a large bar of copper (which measures 1" X 1" X 12"). I soldered the base of the diode to the copper using Wood's Metal (71C melting point).
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