by Din » Tue Dec 07, 2021 12:54 pm
Martin wrote: ↑Tue Dec 07, 2021 10:50 am
Din wrote: ↑Mon Dec 06, 2021 9:59 am
If the recording medium is thin, then there are fewer Bragg planes, and the variation of the spacing from the mean may be high (large standard deviation), resulting in a broad range of wavelengths. As the medium gets thicker, the variation from the mean gets smaller (the standard deviation drops), and the resulting wavelength range gets smaller. But 1 - 2 mm (1000 - 2000u) may be a bit excessive however. Bear in mind that the reason the Bragg planes form at all is the absorption of the medium to certain wavelengths. If the medium gets very thick, then the recording beams themselves lose intensity as they travel through the medium.
Right. Finding a recording material adequate for such thick layers isn't trivial. The volume holographic gratings for commercial laser systems are based on photo-thermo-refractive glass. Incidentally and interestingly, some of these systems rely on silver-halide doped glass. It's probably safe to say that such technology is out of reach to the average holographer. These VHGs provide extremely narrow bandwidths, For quite some time I was wondering whether the photo-thermo-refractive glass could be replaced by a slightly more "consumer-friendly" recording material.
It really depends on the bandwidth you want for the application. You're probably right about finding consumer materials for extremely narrow bandwidths for lasers. You can get about 2 or 3 nm with DCG. In the 80's at POC and, before that, at NTS, where I was working on HUDs and laser protection goggles, DCG was considered the material with the highest efficiency (at POC, I got an OD of 4.5, and someone got an OD of 5) and the most flexible material in terms of the ability to tune to any vis wavelength, and control bandwidth up to a point. At Triple take, we did have to get a DCG hologram in the ~5 nm, and I think I could have got it lower. But, getting linewidths on the order of a KHz or so is probably not possible. However, many companies are abandoning DCG for medical reasons, so it no longer seems to be a "consumer-friendly" material.
Now, everyone seems to be using Covestro. Considering that the bandwidth of a hologram results from differential swelling in wet processing, Covestro should be extremely narrow band, since there's no wet processing. But Covestro material does swell a little, so extremely narrow band is probably not possible.
The other problem, as I mentioned earlier, is that the extremely thick materials with a chemical actinic reaction, will absorb, and so the ratio changes along the depth of the material. Therefore, any such material must have an actinic process that does not involve absorption, such as, perhaps, charge transfer. For some years I have considered magneto optical materials.
[quote=Martin post_id=72547 time=1638892256 user_id=2364]
[quote=Din post_id=72540 time=1638802781 user_id=2357]
If the recording medium is thin, then there are fewer Bragg planes, and the variation of the spacing from the mean may be high (large standard deviation), resulting in a broad range of wavelengths. As the medium gets thicker, the variation from the mean gets smaller (the standard deviation drops), and the resulting wavelength range gets smaller. But 1 - 2 mm (1000 - 2000u) may be a bit excessive however. Bear in mind that the reason the Bragg planes form at all is the absorption of the medium to certain wavelengths. If the medium gets very thick, then the recording beams themselves lose intensity as they travel through the medium.
[/quote]
Right. Finding a recording material adequate for such thick layers isn't trivial. The volume holographic gratings for commercial laser systems are based on photo-thermo-refractive glass. Incidentally and interestingly, some of these systems rely on silver-halide doped glass. It's probably safe to say that such technology is out of reach to the average holographer. These VHGs provide extremely narrow bandwidths, For quite some time I was wondering whether the photo-thermo-refractive glass could be replaced by a slightly more "consumer-friendly" recording material.
[/quote]
It really depends on the bandwidth you want for the application. You're probably right about finding consumer materials for extremely narrow bandwidths for lasers. You can get about 2 or 3 nm with DCG. In the 80's at POC and, before that, at NTS, where I was working on HUDs and laser protection goggles, DCG was considered the material with the highest efficiency (at POC, I got an OD of 4.5, and someone got an OD of 5) and the most flexible material in terms of the ability to tune to any vis wavelength, and control bandwidth up to a point. At Triple take, we did have to get a DCG hologram in the ~5 nm, and I think I could have got it lower. But, getting linewidths on the order of a KHz or so is probably not possible. However, many companies are abandoning DCG for medical reasons, so it no longer seems to be a "consumer-friendly" material.
Now, everyone seems to be using Covestro. Considering that the bandwidth of a hologram results from differential swelling in wet processing, Covestro should be extremely narrow band, since there's no wet processing. But Covestro material does swell a little, so extremely narrow band is probably not possible.
The other problem, as I mentioned earlier, is that the extremely thick materials with a chemical actinic reaction, will absorb, and so the ratio changes along the depth of the material. Therefore, any such material must have an actinic process that does not involve absorption, such as, perhaps, charge transfer. For some years I have considered magneto optical materials.