I think everyone who reads this article played with laser pointers. Recently, the Chinese have been raising radiation power ever higher - and we will have to take care of safety ourselves.

On Habré already wrote about spectroscopy (on the kickstarter , and on the knee ), as well as about the green DPSS lasers ( 1 , 2 ).

Recently, I had the opportunity to check whether copper laser can be cut on a 1W PCB with a green laser (the answer is no), but I didn’t want to risk checking it without specific information about parasitic IR radiation and how well the goggles work.
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In addition to this - it also turned out on the knee to see the laser emission spectrum - whether it generates at one frequency, or several at once. This may be necessary if you want to try to record a hologram at home.

Recall the design of green DPSS lasers

An 808nm infrared laser diode shines on a Nd: YVO4 or Nd: YAG crystal neodymium laser that emits light already at a wavelength of 1064nm. Then, in a nonlinear KTP crystal, a frequency doubling occurs, and we get 532nm green light.


The obvious problem here is that the 808nm and 1064nm radiation can come out of the laser (if there is no output filter, or it is of poor quality) at an unknown angle, and it is unnoticeable for us to do artistic cutting on the retina. The human eye does not see 1064nm at all, and the 808nm radiation is very weak, but you can see it in the dark (it is not too dangerous only with scattered radiation at low power!).

Unfocused spurious radiation

To begin, take a look at the radiation of a green laser camera without IR filter:

The ring around the point is the scattered radiation of the 808 nm laser pump diode. If, due to the imperfection of the laser design, it is too powerful, 1064nm and 532nm may appear there. With high power - this radiation can be dangerous, especially if you do not guess about its existence.

However, what is the radiation in the focused part of the laser radiation? Let's try to find out.

First approach: a sheet of paper and a CD

The idea is simple - light a laser through a hole in a sheet of A4 paper onto the surface of a stamped CD . The grooves on the surface of the disk - in the first approximation, work as a diffraction grating, and spread out the light in the spectrum.


Each wavelength generates several images at once - several positive and several negative orders.

As a result, the eye and the usual camera will see the following:


However, if you look at a piece of paper with a camera without an infrared filter, we notice a strange purple dot between the first and second points from the center:


This is just parasitic, not filtered 808nm radiation. Unfortunately, this way you can’t see the point of 1064nm radiation - it ideally exactly coincides with the second order of 532nm radiation. What to do?

Second approach: dispersion prisms

A prism also spreads light into a spectrum, but the difference in angles of refraction for different wavelengths is much less. That is why this option was far from being immediately realized by me - I continued to see one point. The situation was aggravated by the fact that I had prisms from ordinary glass, which in the spectrum decompose the light twice as poorly as specialized.

As a result, I had to take 2 prisms, and increase the distance to the screen up to 2 meters. A sheet of cardboard with a hole between the laser and the prisms - in order to filter out the parasitic unfocused radiation from the laser.


The result was achieved: the points of 808nm, 1064nm and green 532nm are clearly visible. The human eye in the place of the IR points does not see anything at all.


On a 1W green laser, using a “finger high-precision power meter” (abbreviated to PMIM), it was possible to find out that in my case the overwhelming part of the radiation is 532 nm, and 808 nm and 1064 nm, although detectable by the camera, but their power is 20 or more times less than the limit detection of pvim.

The turn has come to check points

The Chinese promise that the attenuation is 10 thousand times (OD4) for the ranges of 190-540nm and 800-2000nm. Well, let's check, the eyes are not public.

We put the glasses on the camera (if we put on the laser, we will melt the hole, they are plastic), and we get: 532nm and 808nm are weakened very much, from 1064nm it remains a little, but I think it is not critical:


Out of curiosity, I decided to check colored anaglyph glasses (with red and blue glass). The red half of the green lingers well, but for infrared light they are transparent:


The blue half has almost no effect at all:

Does the laser generate at one frequency or several?

As we remember, the main structural element of a DPSS laser is a Fabry-Perot resonator , which consists of 2 mirrors, one is translucent, the second is ordinary. If the wavelength of the generated radiation does not fit the length of the resonator an integer number of times - due to interference, the waves will dampen themselves. Without the use of special tools, the laser will simultaneously generate light at once at all permissible frequencies.

The larger the dimensions of the resonator, the greater the possible wavelengths at which the laser can generate. In the most low-power green lasers, a neodymium laser crystal is a thin plate, and often only 1 or 2 wavelengths are possible there for generation.

When the temperature changes (= resonator size) or power - the generation frequency may change smoothly, or abruptly.

Why is it important? Lasers generating light at a single wavelength can be used for home holography, interferometry (super-precision distance measurement), and other fun pieces.

Well, check. We take the same CD-ROM, but this time we will watch the spot not from 10 cm, but from 5 meters (since we need to see the difference in wavelengths of the order of 0.1 nm, and not 300 nm).

1W green laser: Due to the large size of the resonator - the frequencies go with a small interval:


10mW green laser: The dimensions of the resonator are small - only 2 frequencies fit in the same spectrum range:


With a decrease in power - only one frequency remains. You can write a hologram!


Let's look at other lasers. 650nm red 0.2W:


Ultraviolet 405nm 0.2W:

Summary

• 532nm green lasers - abundantly illuminate all with infrared light: a broad unfocused beam of 808nm (and with a high power of parasitic radiation, there will be 1064 and 532nm there), and in a focused spot there is both 808 and 1064.
• Hoping for random colored glasses for protection is criminal. They transmit this infrared radiation, and you can fry the retina unnoticed.
• In the field, having only a CD-ROM, it’s quite realistic to look at the laser emission spectrum on a 0.1nm scale and see if the laser works in single-frequency mode.

So be careful when playing with laser pointers, do not buy excessive power unless absolutely necessary.
Let there be coherent light!