Update: Someone translated this post to Russian.
Update: I have confirmed that the output is still well within the spontaneous emission range and I am doubtful I can build an exciter with enough power. The paper uses a 532 nm Nd:YAG laser with a 7ns pulse duration and samples were performed at energies of 1.6mJ and 3.7mJ. This means that their source had an approximate power output between 229kW and 529kW.
There was an interesting article a while back over at new scientist describing research into a laser with a tunable output wavelength, after tracking down the original paper over at arXiv.org and looking at there experimental setup the I decided the device should be pretty straight forward to build.
The principal behind the design is you suspend a bunch of reflective balls in a laser dye and shake them really fast so the balls behave as though they are suspended. The laser dye is then excited by an external source and by changing the vibration frequency you can filter out different wavelengths of laser light emitted by the dye.
The overall design only requires six components to build:
- A laser dye, in this case Rhodamine B
- Lots of small ball bearings
- A small glass container to hold the ball bearings and laser dye
- A speaker to serve as the source of vibrational energy
- A laser to excite the dye
- An adjustable frequency generator
Laser module & exciter
The ball bearings and borosilicate sample vials were the easiest parts to source however the Rhodamine B was a little more challenging. A fellow Protospace member was kind enough to prepare three different solutions of Rhodamine B in methanol for me: 0.001mol/L, 0.01mol/L, and 0.1mol/L. After cleaning the oil off the ball bearings with acetone and letting them dry it was then just a matter of adding the ball bearings to the vial and then filling them with Rhodamine B.
The next step was to find a suitable pump source, something powerful enough to push the Rhodamine past it’s lasing threshold. I ended up settling on a 30mW 532nm source and decided not to go higher because of safety concerns about to taking it to Maker Faire YYC and not damaging anyone’s eyes. I’m still trying to figure out if that was enough to successfully pass the dyes lasing threshold or if the yellow light being produced by the Rhodamine is in fact spontaneous instead of stimulated emission.
Initially I was going to just glue the module mount directly to the speaker. However after I successfully destroying the first speaker I was going to use by epoxying the voice coil to the magnet I settled on a more modular design that would let me swap laser modules out more easily.
The final design had a small rare earth magnet mounted to the voice coil of the speaker and I attached a small metal washer to the bottom of the module mount. You can see the magnet mount between the speaker and the module mount in picture. I also made sure there was enough space between the magnet and the vial to make sure the ball bearings would not clump together.
In an effort to replicate the source paper as accurately as possible I included an accelerometer to measure the intensity of the vibrations. I ended up using an old 6DOF Razor IMU I had around from another project (which has since been replace by a newer more compact model), five of those degrees of freedom are completely unnecessary.
I built the function generator around EXAR’s XR2206 monolithic function generator with the circuit being identical to the one presented in the chips datasheet. Most of the work here involved choosing an appropriate timing resistor and capacitor to get the frequency range I wanted with the parts I had on hand. The end result was a fairly simple circuit capable of outputting both sine and triangle waves with a frequency range of ~35-45Hz.
The output of the function generator is then fed into 1:1 audio transformer to make the output signal relative to circuit ground instead of my virtual ground and then passed into a LM386 audio amplifier in the simplest 20dB of gain configuration.
I did encounter one issue with the XR2206 sensitivity to temperature changes. While outside at Maker Faire YYC in direct sunlight the frequency drifted up 15Hz over the course of 2 hours.
Maker Faire YYC
To take it to Maker Faire YYC I really wanted the whole thing to be interactive instead of just a fixed display. I ended up laser cutting some nice acrylic enclosures with controls for wave select (sine or triangle), amplitude adjust, and frequency adjust. This was great because it allowed my to demonstrate how frequency and amplitude effected the motion of the ball bearings in the liquid.
Faby Martin took a whole bunch of pictures of the event here including several of my laser and me giving the general public a very hand wavy introduction to basic laser physics.
I am putting this on the shelf for a while due to a bit of project burnout leading up to Make Faire. I have a whole slew of experiments I still want to perform and improvments I would like to make.
First I want upgrade to a more powerful pump laser, something in the 200mW or even 1W range so that I can be absolutely sure I have passed the lasing threshold of the Rhodamine. At this point I’ll be sticking the whole unit in a closed box so I can’t see it while I am running the experiments so I don’t bake my eyes.
I also want to get my hands on a proper spectrometer so I can properly monitor the light output and better “see” the wavelength shift described in the original paper. I suspect I may also need to invest in an accelerometer with a higher tolerance, the peak acceleration of the speaker cone is somewhere in the magnitude of 30g.
A properly fabricated PCB for the control circuitry would be really nice, I have run into the typical breadboard issues where wires come loose or there is excessive signal noise. Once I have those issues sorted out and my basic experiments complete to have both the laser module and control circuitry mounted in the same case would be nice.