Archive for the 'Lasers' Category

Optical Trapping and the Momentum of Light

Published on January 25, 2010 in Lasers. 3 Comments Tags: , , , , , , , , .

Recently, there have been a few posts on LaserPointerForums with regards to a curious little experiment.  A laser is focused as close to the aperture as possible and is pointed straight up at the ceiling.  Then, a piece of very soft black plastic or felt marker tip is burned.  With some luck, a small particle will burn off, and then seem to float in mid-air.  In fact, it looks something like this:

The marker is pointing roughly to the particle in question.  It is not smoke and this fact can be verified because you can actually move the laser around gently and the particle will follow it!  I made a brief write-up in one post on why this works, but I decided to play around a bit and get some better quality pictures and a few videos.

Here’s the essence of why this phenomenon occurs.  A principle called wave-particle duality states that light (but not only light) can be represented as both a wave and a particle; that is, it can have both wavelike and particlelike characteristics.  When you observe Airy disks and other diffraction patterns, you are observing wavelike phenomena; the light behaves just as if you were to take a tank of water and perform the same experiments.  However, light can also behave like a particle–the term for a particle of light being a photon.  Particles have an energy and momentum associated with them and it was shown that photons do indeed have this property.  For more information on wave-particle duality, I recommend the HyperPhysics slides on the subject.

Because photons have an associated energy and momentum, they can impart a force onto other objects.  This is exactly what is occurring in the picture above!  The particle is experiencing a set of force vectors from the laser light which cause it to find an equilibrium position very close to the waist of the beam.  Here is an interesting explanation of this phenomenon, known as optical trapping.  To borrow a diagram from Wikipedia’s excellent optical tweezers article:

When a particle moves out of its equilibrium position, a restoring force will push it back just above the waist.

To illustrate further, I’ve taken a video (720p :D ) and a few pictures:

Edit: It seems as though my video has made it to Reddit and LiveLeak:

http://www.reddit.com/r/science/comments/auh5b/optical_trapping/

http://www.liveleak.com/view?i=995_1264499775

More DIY Greenie Progress

Published on December 24, 2009 in Lasers. 0 Comments Tags: , , , , , , .

Using some crystals generously donated by Kenom, I managed to confirm that the diode I purchased lased at 808nm.   This rules out one theory on why I am not getting output from my original crystals.  These new crystals were used in a cheap laser pointer, so they are bonded together into one package.  As a result, it’s fairly easy to put them in front of a beam and watch the green on the other side.  However, my original crystals are unbonded, which leads me to believe that there is an alignment issue.  The diode that originally pumped those crystals had a countersunk hole for the mounting screw; the diode I’m currently using does not, so it doesn’t quite align as the original diode did.

Yes, I literally stuck a huge chunk of aluminum on the back of the LM317 because it was overheating and the output current was dropping.

Crystal Tests and Module Characterization

Published on November 22, 2009 in Lasers. 2 Comments

I went home for the weekend, so I decided to test out my new 808nm diode with the existing crystal set, which looks like this:

Looking from right to left, there’s a spot for the cmount diode to be mounted onto the heatsink.  The output from this diode goes through a focusing lens which attempts to get as much of the light into the YAG crystal as possible.  After the 1064nm light leaves the YAG crystal, it enters the KTP whereby the frequency is doubled and 532nm light is transmitted.  The black piece on the far left contains a collimating lens with an antireflection coating.  Anyways, I tried mounting the diode and running about an amp through it to get a little IR output (enough to theoretically produce some green).

Uh oh.  No green output.  I tried fiddling around with the alignment quite a bit, but no dice.  There are a few important differences with this replacement diode that I bought and the original: the biggest one is that the original diode was countersunk.  My guess is that the problem has something to do with the alignment of the crystals.  I wonder if the heatsinks were messed around with before I got them; the springloaded screw on the YAG is tightened all the way.  The other possibility for this failure is that the diode I received wasn’t 808nm, because I have no way to test its wavelength.

After the test failed, I started taking the module apart and measuring everything so that I can make a CAD model when I get a chance.  I also took photos of the parts:

Diode Focusing Lens

Front of YAG Heatsink

Front of YAG Heatsink

Back of the YAG

Back of the YAG

KTP

KTP

Collimating Lens

Collimating Lens

Sadly, the crystals are glued to the heatsinks, so I will have to redesign my mounts to account for this.

DIY Green Laser

Published on November 20, 2009 in Lasers. 0 Comments Tags: , , , , , , , , , .

Finally, I have a chance to start discussing a project that has been brewing in the back of my head since July, when I purchased a broken portable laser for peanuts.  My original plan was just to refurbish the beaten up host and replace the damaged diode, but as time passed, my plans grew.  They’ve finally begun to settle down now and I’m well on my way toward finishing plans for a 532nm lab laser constructed pretty much completely from scratch.  My goal is to make the laser as efficient as possible.  Doing this is not just a matter of finding the optimal diode positions and so forth; it requires careful temperature control of the nonlinear optics.

The crystal set (which is the main reason I purchased the laser) is particularly good for three reasons: the crystals are very large, they are unbonded (that is, they weren’t glued together by the factory), and they are already rotationally aligned (they have to be oriented within some small specific relative angle range in order for good efficiency and these crystals have already been cut so that they can lie flat on a heatsink without having to worry about the orientation).

My plan is to design easy-to-machine parts that will fit together nicely while still providing a good level of flexibility for tuning.  I will need to create heatsinks for the crystals, a heatsink for the diode, mounts for the optics, etc.

The laser will be driven by an LM338-based voltage regulator, which will provide the necessary 2.3A or so to the 2W 808nm C-mount pump diode.  The diode and YAG crystal will both have thermoelectric coolers with thermistors embedded into their heatsinks.  A microcontroller will take input from the thermistor and based on some optimal temperature, use PWM to individually control MOSFETs for the diode and crystal TECs.  Furthermore, the KTP crystal will have a resistive heating element attached to it as well as a thermistor embedded into its heatsink; the microcontroller will also try to keep the crystal at its optimal temperature.

From my research, it seems that KTP likes to be very hot; sometimes around 90C.  YAG, on the other hand, likes to be somewhere between 10-20C; these temperatures can vary greatly based on the chemical structure of the crystal.  The microcontroller will try to get the TECs and heaters to keep the crystals at their optimum temperatures.

To determine the optimum temperatures, I might try to build an automatic temperature optimizer using the analog output of my Scientech 372 power meter.  The microcontroller would read this input and watch it as it reaches a maximum.

A few simplified diagrams of the laser:

Because the MOSFETs for the TECs and heater will be generating currents on the order of several dozen amps, a lot of heat will be generated in the rear portion of the laser.  I will place a small exhaust fan on the back for that reason.

The most mechanically complex portion of the laser will be the crystal mounts.  In order to obtain maximum heatsinking, I need to build something with as much contact area as possible.  I came up with the idea above; I’ll use springloaded screws to keep the side portions snug against the crystal as well as more springloaded screws to mount the side portions to a thermally conductive bottom.  This will provide even heat sinking around the crystal.  Furthermore, a thermistor will be embedded into the bottom so that the temperature reading at this location is very close to the temperature of the crystal.

A TEC needs a heatsink on its other side to dissipate heat; a resistive heater, obviously, does not.  However, the lower portion of the heatsink for the KTP crystal will look very similar to that of the YAG.

A few words on the microcontroller: I will add a manual bias function.  Essentially, there will be a three-way switch with states {auto,yag,ktp}.  Choosing auto tells the microcontroller to ignore all biases.  Choosing one of the others allows you to control the temperature bias with the potentiometer.  An LCD screen will display the temperature of the thermistors on the diode, YAG, and KTP.

I’ve already begun  to collect part names as well.  For the AVR, I will use the Teensy development board.  It’s perfect for this application.

The plan is to machine all the parts here at MIT’s Hobby Shop and build the laser itself at home this January.  IAP is going to be pretty fun.  :)

I’m keeping a running page on my work here.

More Laser Stuff (I’ll stop after this)

Published on October 13, 2009 in Lasers. 0 Comments

Okay, most of my recent entries have been laser-related.  I promise that this will be my last one for a while.  In January, I plan to start work on my homemade, TEC-tuned DPSS green laser, where I’ll try to get as much green as I possibly can out of a 2 or 2.5W pump diode and a large crystal set.

First, I got some pretty epic beamshots with my SKY portable and the tripod I recently purchased for it.  I’m finally learning how to tune my camera settings properly for decent quality pictures.

See More >>

Secondly, I finally finished the last laser for my Triple Play set (red laser).  You can see my build log here (in the “Red” section).

Scientech 372 Laser Power Meter

Published on September 27, 2009 in Lasers. 2 Comments Tags: , , , , , .

Recently, I purchased a laser power meter from a very awesome laser hobbyist (Laser_Ben on LPF); it arrived Friday and I’ve been playing with it all weekend.  This meter has a thermopile detector (as opposed to the photoelectric sensor on my Liconix 45PM) and can measure up to 10W.  Last spring, I purchased a large-aperture (100mm!) calorimeter that can tolerate up to 50W; that unit will work with this meter as well.  Thermopiles are becoming very difficult to find nowadays, so I was more than happy to shell out $150 for the unit.  Prices on eBay range dramatically (the meter alone sells for about $150 sometimes, but I’ve seen them for as low as $50).

My Liconix 45PM (top) uses a photoelectric detector and reads up to 100mW. It is pretty effective for watching how power output fluctuates as the laser warms up.

L: 50W thermopile, T: photoelectric detector, R: 10W thermopile

Anyways, after verifying that the meter worked, I started to investigate the meter’s output for an analog recorder (this thing is old…).  I hooked up a multimeter to the output terminals (okay, I didn’t have any banana plugs on me, so I made some with tinfoil).

Then, I could read the voltage drop across the terminals (which was in the 0-100mV range).  I then (manually…) tried to establish a relation between this voltage drop and the reading on my meter.  After taking a bunch of samples, I had data with some pretty decent correlation.

I suspect that the error is the result of an inability to accurately read the meter.  However, the R value is pretty high, so I’m happy with the estimate.  After that, it was just a matter of testing a few lasers.  :)

I’m quite happy with the results.  The green module I purchased for my Triple Play laser set was originally rated to be 50-60mW; I’m glad it exceeded the rating consistently.  The PHR is also doing quite well at around 95mW.

I’m a little curious as to why the readings all have a sharp drop at about 30 seconds in.  Also, the Scientech meters try to compensate for calorimeter warm-up time by overshooting in the very beginning, but this gives very inaccurate readings (my greenie started at like 150mW at one point); so I cut that part out of the readings.

I’m going to test my lasers more thoroughly when I get a chance; I’m excited to see how my SKY operates (my batteries were almost empty when I tried, so I didn’t get very good stability).

You can also see all those pictures (some with more descriptive captions) in my growing Laser Pictures album.  Later, I’ll make a nice interface for viewing the output graphs as well.

Random Laser Pictures

Published on September 8, 2009 in Lasers. 1 Comment

Well, classes start tomorrow, so I thought I’d end the summer off with a few nice laser shots in my dorm.  Here’s one:

The rest of them are at my laser album.

Triple Play: Matching RGV Lasers

Published on August 20, 2009 in Lasers. 0 Comments

I have just about enough portable lasers now: a bunch of green pens, a tiny blu-ray burner with a heatsink I machined myself, and my 300mW SKYlasers portable.  So, I decided to round off my collection with a set of 3 lasers (650nm red, 532nm green, and 405nm blu-ray) in identical hosts by a LPF member named Ehgemus.

To see some nice pictures (plus a somewhat sporadic build-log), take a look here:

http://jwcxz.com/projects/lasers/tripleplay/

I’m still working on the red laser–the driver I’m using isn’t behaving as expected.

Results of a Laser Accident

Published on July 11, 2009 in Lasers. 3 Comments

(Trying to play catch-up here…  I’ve been meaning to blog about a few things).

A few months ago, I was testing a ~60mW laser an had an unfortunate accident.  My selective memory has blotted out most of the event, but I think I was setting up some kind of test when I accidentally turned the laser on, shooting photons into my left eye.

Well, a week ago, after seeing my optometrist for a regular appointment, I went to see another optometrist who specializes in laser safety.  After a couple of tests, it was determined that although I would probably have the spot for the rest of my life, the burn was extremely sharp, indicating that I do not have any chance of developing macular degeneration.

The optometrist agreed to email me the high-resolution picture of my left eye that he had taken, so here it is:

http://jvm.mit.edu/f/lefteye.jpg

(It’s a little creepy, but nothing too gross; and that opinion is coming from an extremely squeamish person, too.)

The blind spot I developed is the little tiny bubble near the center of the dark circular spot.

Improving Cheap Green Laser Pointers

Published on May 5, 2009 in Lasers and Tutorials. 2 Comments

I own several cheap green laser pointers and I’ve noticed that some of them behave very differently from others.

I bought my first green laser pointer back in September for a little over $20.  It was this one.  At that time, I had no clue of what a typical 5mW pointer looked like, but I expected that I received somewhat of an overpowered unit since it could burn through trash bags and had a clearly visible beam (as it turns out, the pointer is about 50-80mW based on a few readings I took).

I then purchased a few more pointers for my family for Christmas a few months later.  The units I received were nothing like what I had originally bought.  First of all, they had different stickers on them and the size of the apertures was much smaller.  Secondly, not only were they extremely weak, but on rechargeable (NiMH) batteries, they needed about 30-45 seconds of warm-up in order to produce a decent dot.  With alkaline batteries, they were a bit better.

A few months ago, I purchased a 50mW pointer from LEDshoppe.  It was also bright on alkalines but miserable on rechargeables.  It occurred to me that the driver seemed to be the problem.  Alkaline batteries probably allow the driver to just cross the threshold voltage required by the IR pump diode.  So, rechargeables, which have a nominal voltage of about 1.2-1.3V, cannot quite reach this barrier and as a result, the laser will barely lase.

The driver used in the first laser I purchased clearly had to be a boost driver; i.e. it boosts the voltage output to compensate for low voltage input.  The other drivers were just linear constant-current drivers.

So, since I don’t really want to pay for new alkalines constantly, I decided to look into another method that was already being used by a few members of LaserPointerForums.com.  10440 batteries are 3.6V lithium cells that are the same size as standard AAA batteries.  Therefore, if I use a single 10440 with a spacer in my laser pointer, the output should be at its intended maximum.

There are a few downsides to this:

  • Assuming that the driver is linear and outputs constant current, a significantly higher input voltage will result in excess heat dissipation.
  • These batteries are typically only around 300mAh, so lots of recharging will be needed, as a typical ~100mW pointer will draw up to 600mA.

While discussing these issues on LPF, member Warske recommended using a diode to lower the voltage going to the driver so that heat would be dissipated in the spacer instead of on the driver board.

I got my 10440s and charger today, so I quickly built a spacer with some cardboard, wire, and a diode.

The results were spectacular.  The laser is drawing its full 600mA or so and is definitely over 100mW (I’ll test it on my LPM when I get home).

Here’s a comparison between my improved laser (on the left) and one that’s doing roughly 50-60mW.

This picture was taken in broad daylight with a limited exposure time, so that's why the lasers aren't particularly bright.

So here’s what you can do if you want to make your green laser pointer more powerful:

  1. Buy a pack of 10440s.  I know it’s tempting, but don’t put both batteries into the pointer because you’ll fry the driver.
  2. Buy a charger.  I used this one, which seems to be the best.  Be careful though, if you’re using your own lithium charger, make sure it is designed to charge 10440s.  They need a significantly lower charging current than CR123As or 18650s.
  3. Buy a regular-ol’-diode from Radioshack or anywhere cheaper.
  4. Get some ~22 gauge wire and make two little coils like I did in the first picture.
  5. Solder the ends of the coils to each end of the diode.  Make sure that the total length is about the same as a standard AAA battery, but err on the side of making it a little longer.
  6. Get some cardboard or thick paper and wrap it around the wire/diode setup so that just the coils stick out on either end.
  7. Finally, make a little paper tube so that you can connect the positive end of the 10440 battery with the coil without the chance of that coil touching the side walls (otherwise, the diode is useless!).  Make sure that the diode is in the correct way; i.e. the white painted stripe should be connected to the coil that touches the end of the barrel.
  8. Put it all together, and enjoy!