In radio, you multiply the two signals. That can be done with a diode mixer.
Generally you get the sum and difference frequencies, and filter for the one you want. Using the same frequency for both inputs gets you a frequency doubler, and you don't have to filter, since the difference is DC. Digikey has lots of RF mixers. The one I have used the most is the NE602.
A beam splitter will not multiply. Otherwise you could get UV light by putting two beams of green light into a beam splitter. What you need is a medium that multiplies. The common green laser pointers use a frequency doubling crystal to double an infrared laser at 1064 nm into the green at 532 nm.
The photodiode in the article is not there to keep the brightness stable. It is there to detect the Doppler shift in the lidar apparatus he is working with. Light goes out, and is reflected back, and the signal and reflection are mixed, and the difference frequency is in the 10 GHz range for fast aircraft, which is why they need fast diodes.
Almost any non-linear response will work. You can disassemble a cheap green laser pointer to get the lithium niobate or potassium titanyl phosphate crystal out of it, and send any wavelength you want into it to get a doubling. The laser has to have a fairly high power in order to trigger the non-linear response, and the output will be much lower than the input, typically a tenth or less in optical power. There are losses, but the most obvious is the two photons in, one photon out limitation posed by simple physics.
The crystal is inside an optical cavity (a pair of partial mirrors) since you'll want many passes through the crystal to get the most out of it.
Check to see if the crystal and other optics in the green laser pointer operate in the UV ranges you are interested in. They may be opaque at those wavelengths.
Doubling a 200 mw 405 nm violet laser gets you into the 202 nm range, and you still have enough power to be useful (10 to 20 milliwatts). Don't bother with expensive low pass filters, just aim it at a DVD and pick off the high frequency beam -- it will be going in quite a different direction than the low frequency (405 nm) beam, since it has half the wavelength. Filters are for when you need a compact device and you don't care much about price.
Personally, I don't buy laser pointers above 5 mw. It is just too easy to accidentally damage your vision by aiming it at a window or doorknob. You won't even notice you have damaged yourself, because your brain corrects for so much. But you will gradually find that reading gets harder and harder. If you do get one of those lasers, epoxy it to a concrete block, and always wear low-pass goggles.
On Sun, Jan 6, 2013 at 2:53 AM, Nathan McCorkle <nmz787@gmail.com> wrote:
I came across this idea called optical heterodyning, it's the same
concept as RF heterodyning (commonly used for radio) where you have
your antenna signal (SIG) being mixed with a local oscillator (LO)
resulting in two new signals being created with frequencies LO+SIG and
LO-SIG or SIG-LO (whichever is non-negative) called the intermediate
frequencies (IF). With RF the mixer is a dual-input amplifier (I'm not
sure exactly what that is, it would be cool to see a part on mousr or
digikey) but in light it's just a 50/50 beam splitter with a
photodiode on the LO (i.e. laser) to keep the brightness stable.
Doing some algebra with and playing with a wavelength to frequency
calculator, I found that the 650nm red lasers on dealextreme.com would
boost 260nm light to 433.36nm and 280nm to 491.88nm. (The additive
heterodyne signals would be in the 180nm region of the UV)
So does anyone other than me think this could work? I already have a
red laser and a 50/50 beam splitter... I guess I would need at least a
UV low-pass filter for the SIG. Anyone know where to get one of those
cheap?
http://en.wikipedia.org/wiki/Heterodyne
HETERODYNE CHARACTERIZATION OF HIGH-SPEED PHOTOMIXERS FOR THE ULTRAVIOLET
BILLY WAYNE MULLINS - PhD Dissertation - 1989
(experimental setup starts on page 86)
http://www.dtic.mil/dtic/tr/fulltext/u2/a220914.pdf
--
-Nathan
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