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SSY-1 Nd:YAG pulsed laser

   SSY-1 laser is a compact flash lamp-pumped Nd:YAG laser with passive q-switch. This type of laser generates pulses of 1064nm light with duration measured in nanosecond scale. The laser assembly is a part of a rangefinder from the U.S. military. Some useful and interesting information about the SSY-1 laser assembly can be found at the Sam's Laser FAQ: link.


Fig. 1 - SSY-1 laser assembly

   The Nd:YAG rod is side-pumped by a regular flashlight tube, as it is shown in Fig. 2. A passive q-switch installed in the laser, at the HR end of the resonator, is made of Cr3+:YAG. The resonator is actually a plano/plano type which means that the mirror surfaces are completely flat (not like it is drawn in the Fig. 2). The laser rod and flash lamp can be easily taken out of the resonant cavity to access the internal components of the laser (e.g. remove the q-switch). The resonator is therefore permanently aligned and it's not necessary to re-align it after accessing the interior of the laser. Figures 3 to 6 show the laser taken apart.


Fig. 2 - diagram of the assembly construction


Fig. 3

Fig. 4

Fig. 5

Fig. 6

   Since this laser is flash lamp-pumped it uses the same electric circuitry as every flash lamp: it is basically a capacitor charged to a few hundred volts and some kind of triggering circuit. Originally, the flash of SSY-1 laser is powered by 36uF capacitor charged to 900V which gives about 15 joules of stored energy. To power my laser unit I built a very simple power supply running directly off 230V mains (dangerous!). Fig. 7 shows a schematic of my power supply.


Fig. 7 - power supply schematic

   As it is shown in the schematic it is a voltage doubler made of two 150uF capacitors. It doubles the mains voltage which amplitude is actually 325V (230V RMS), thus giving 650V over 75uF of capacitance (2 x 150uF in series). The stored energy can be determined using formula 1 and in this case it is 16J, close enough to the original design.

(1)

   The capacitors should be designed for pulsed applications, but I used regular electrolytic caps and they turned out to work just fine. The triggering circuit uses a car ignition coil to generate high voltage. The laser shoots every time the SW1 switch is pressed.

   For my first attempt I could not detect any laser output even though the lamp was flashing. I looked around me to find some lens to focus the beam and I found old night vision goggles :) I took one of the objectives from it and placed it in front of the laser followed by something to burn a hole in, so that I can be sure if it works or not. It didn't so I started figuring out why it is not lasing. It turned out that the pumping energy was too low to pass the q-switch threshold (I was using smaller caps) so there was no lasing. I increased the capacitors to the 150uF and it started to work! To my surprise, with each laser shot I was spots of ionized air in the lens' focal point! I didn't expect to achieve that effect at all, and suddenly I got it wit the first run of my laser! Amazing. Fig. 8 shows the quick-and-dirty first-run setup. Figs 9 and 10 show the plasma spot in air!


Fig. 8 - first run

Fig. 9

Fig. 10

   After playing for a while with my new toy (maybe 20 shots) I noticed that there's something wrong with the lens I was using. It seemed like the laser beam is so strong that optical coatings the lens (which was not dedicated for pulsed laser applications of course) got burnt. I decided to take the lens out of the objective assembly and I found out that this objective is actually quite complicated optical device. It is made of three lenses, one of which is actually an optical doublet. The damage caused by the laser was between the single lenses in that doublet, probably just burns in the glue. From there on I was using one of the single lenses taken you from the described objective.


Fig. 11 - burnt lens

Fig. 12 - interior of the objective

   To check how the laser really behaves I decided to do some measurements to determine time relation between the pump flash, the laser pulse etc. Fig. 13 shows the measurement setup. What I measured were the flash light tube current, the pumping flash light and the laser light. There are two photodiodes in the picture: one for detecting the pumping light and the other for detecting laser light reflected (scattered) off the laser target (black plastic).


Fig. 13 - timing measurement setup

   This setup allowed me to measure what is the laser behavior at different capacitor energy and also with and without q-switch. Figures 14 to 17 show some charts with experiment results. Top line (yellow) is the flash tube current, the middle (blue) is flash light and the bottom (purple) is laser output. The vertical axis of the charts is not scaled in any units.


Fig. 14 - too low pump energy, no laser pulse


Fig. 15 - 16J pump energy, nice laser pulse detected


Fig. 16 - 32J pump energy (too much), series of pulses at the output


Fig. 17 - 32J pump energy, q-switch removed

   The chart presented in Fig. 15 shows the proper setting of the laser. Pumping energy is high enough to reach the q-switch threshold and we get a nice single laser shot. In Fig. 16 the energy is way too high. The q-switch is triggered and the energy stored in the YAG rod lases out, but then the rod instantly gets pumped again by the same flash of pumping light and the lasers shots again and again, five times in total. In such situation the peak power of the laser beam is not getting higher. Instead, we just have a series of laser pulses. In Fig. 17 a long and relatively flat laser pulse is shown. This is because q-switch was not present in the laser cavity. Without q-switch nothing stops the lasing process and the energy from YAG rod is being constantly lased out during the pumping light pulse. Notice the delay between the laser pulse and the pump light pulse, this is the time for energy stored in the YAG rod to reach lasing threshold (optical amplification of the rod has to exceed the cavity losses).

   The rest of the pictures show some laser burns. Shooting the laser onto something or creating the plasma ball in air is actually quite loud. The sound generated by the laser itself is nothing more than the sound of flashlight, like the camera flash, but burning things generates a nice and loud snap :) In Fig. 18 you can see a stainless steel razor blade shot by a focused laser beam. The metal from the surface is ablated and burns in front of the blade. Figs 19 and 20 show the little crater created on the blade. Unfortunately, the pulse energy is too low to punch a hole through the blade.


Fig. 18

Fig. 19

Fig. 20

   Fig. 21 presents another plasma spot in air. This phenomenon is quite interesting and spectacular. It happens because in the laser beam waist density of the energy creates such a high local electric field (light is in fact an E-M wave) that it exceeds the breakdown voltage of the air. Therefore, the gas gets ionized and we have plasma. Here you can watch a video showing a 3D display based on this phenomenon: www.youtube.com/watch?v=KfVS-npfVuY.

   Fig. 22 and 23 show the laser shooting a focused beam on a piece of black rubber (bicycle's inner tube). The carbon consisted in rubber burns quite spectacularly. In Fig. 24 the beam is not focused but it goes straight from the laser. The beam diameter is about 5 millimeters and it still burns rubber easily.


Fig. 21

Fig. 22

Fig. 23

Fig. 24

   Fig. 26 and 28 show flames bursting out of the rubber after focused shot. Almost like a nuclear mushroom cloud :D Fig. 29 presents a long distance shot with a collimated beam. Since the diameter (waist) of the beam is high it can be (and by default is) collimated really well. The spot diameter in the distance of 50 cm is almost the same as at the laser output.


Fig. 25

Fig. 26


Fig. 27

Fig. 28


Fig. 29 - long distance shot with collimated beam (no focusing lens)

   The SSY-1 laser does not have many applications for a hobbyist, however there are some interesting projects based on this laser unit. One of my favourites is a pulsed holography setup presented on this website:

The author of that project managed to double the frequency of SSY-1 to 532nm with KTP crystal and actually made some holograms with his setup!








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