Constructing an Extended Cavity Diode Laser

An REU project (currently) being done by Ryan Price

Supervisor:  Dr. DePaola

Welcome to my 2008 REU research page. The following is a quick summary of a work currently in progress:

Project: Over the summer, I am tasked with constructing an extending cavity laser (ECL) that will be used within a magneto-optical trapping configuration. For more information until I write a nice general description, see wiki: Magneto-Optical Trap. For the basis of our design, a reference paper concerning an extended cavity diode laser built for Cs laser cooling is used as a starting point. Because the experimental setup here at KSU uses Rb atoms, this requires a re-engineering of the design to fit our specifications. So, essentially the design and construction of the refit laser is the task at hand.

An Extended cavity laser:

tunable external-cavity diode lasers

Research Progress: 


The following is a quick outline of the (presumable) steps involved in the laser construction

1.     Researching / Basics of design

2.     Redesigning for Current Experiment

3.     Parts Acquisition and Construction

4.     Calibration and Testing

During the first week of working on the project, the first steps taken were to approach step one through trying simple redesigns of the model ECL and looking for the parts required for the project. By doing such I have a much better grasp on the requirements and functionality of the laser model I have been tasked with building.


Week two so far has been the creation for detailed designs on the computer. These models created are going to be the reference model used during the machining and construction stage to get the right dimensions and angles (mind you, optical devices are particularly picky about exact geometries). If all goes well, parts will start to be ordered and come in next week. However currently there is a hold up in the process. Two parts required cannot be found easily or cheaply for purchase so alternative sources for such parts may be found to keep the project rolling on.

Oh yeah, and a tornado hit the physics building… how cliché for a summer in Kansas.


Week three is design week! I’m (slowly) learning how to use 3d tools to help model the main casing to my laser design


My first day trying to use this program, or as Dr. Weaver says jokingly, “Something to be proud of (like) in kindergarten”.

And then things have progressed now to where someone in flatland would be proud!


Ok well then let’s try again and try harder…

3.jpg      4.jpg

Now we have 3D! If only this were the end of the story. In the above picture, you can see almost all the parts involved. For example, the big red block on the back is the laser diode mount the green tubes are PZTs and the red thing below them the prism that they control.


So, because of the tornado that hit us, the building had broken windows. And because of broken windows and loose insulation, there is an asbestos fear in the building that caused a building closure for 2 and half days. Kansas seems to be the most eventful place I’ve worked thus far with tornados, power outages and asbestos scares. So all together, not much progress was made in updating the laser design unfortunately.

However, our research group had time for an experiment run using the Kansas Laser Source (KLS) and I got the opportune chance to watch (and help on a few parts!) the setup of the experiment. It should be noted that being able to help out with operation in KLS is quite the honor to me, considering the beam if brought to focus can ionize air at a point (to the non-physics inclined, a familiar form ionized air that you may be familiar with is lightning which has the ability to ionize the air because of the enormous electric field created during discharge). Anyway, as far as the laser goes, purchasing of parts is starting now, so hopefully construction will be underway soon.

7-3-08: Parts, parts, and more parts…

First, the laser design itself is finished and en route to being created in the machine shop. Note that the design has gone under many (minor) revisions. The most important lessoned learned here is: never design in the metric system if submitting items to a machine shop. So, introducing the final design:


Secondly, ordering parts has a downside. Especially when you get a prism that looks like this:

When it should look like this:

And why is this prism a problem? Consider a light beam incident on the hypotenuse face and an arbitrary location:

The purpose of the prism is to act as the retro-reflector in the Litmann ECL design. So with a large chunk of my prism missing (purple above), light that comes in at this location or on the opposite side equidistant from the center of the face is not reflected back properly. This is awful for two reasons:

1.)   The laser power output will be significantly lowered

2.)   Nobody makes prisms in nearly similar dimensions and thus there are no other prisms in the entire land of the internet to be had to replace this one.

So in summary, these prisms were promptly sent back to the manufacturer, and their customer service I’m sure were simply thrilled to talk to me.


Now that the main casing for the laser will be made in the shop soon, I got head to the shop myself to make an accompanying piece. Specifically, the one that holds the laser mount to the case. More specifically, it is a flange that allows the laser mount to be rotated such that the outgoing polarization of the laser light incident upon the diffraction grating can be changed in such a way that it controls the amount of light diffracted versus reflected. This in turn controls how much light is kept in the extended cavity versus sent out of the ECL.

So my project this week was to start with something like this:


Throw it on a lathe, put it through a saw, cut my finger, and drill some holes… and presto:



This piece stands as a testament to my luck in the machine shop.

And because of unfortunate happenstance, our project is somewhat on hold because the specific type PZTs that control the prism rotation cannot be found at a reasonable price. And one of the two companies that do sell them won’t even get back to me with a price quote, even if I call them. Great customer service, I do say.

And speaking of customer service and shameless advertisement, an order from Thor Labs came today and in it, to my surprise, there was a box full of ‘Lab Snacks’ with candy and food of all kinds. Supposedly such a delightful thing comes with all of their shipments. After dealing with other companies and their rotten prisms and not returning phone calls, this really makes one’s day. Thor labs made a future customer out of me by such a gesture, as trivial as it may sound.


Our group had another experiment using the KLS during this week, so for a few days I was in and out of the lab watching/helping to set up parts of the experiment.

And last but not least, the replacement prisms came back and they are exactly as horrid as the first batch. The sales engineer tells me that ‘such deviations are common from original specifications’. I agree, a five percent difference might be ok to slide by, but when the dimensions are off by at least a quarter of the advertised catalogue, then that’s just plain negligence. As it is closing in on my last few weeks here at KSU, there is no way that I can get a replacement prism custom ordered, so what to do is really up in the air.

On the machine shop note, I got to finish a portion of the sub-assembly that holds the prism


Oh yeah, and I have my main assembly too:


Now just to put it all together…


This week has just been finishing up work in the shop and starting work on the electronic sides for the laser diode mount. Thus far I have cables and am ready to go in terms of connecting the cables and mounting the (really expensive) laser diode.



Final Presentation and Report:  An overall summary of the work should be done around the end of the summer (beginning of August)


REU Goes to…

Other Projects and Papers:

Although not the groundbreaking by any means, the following are some papers I’ve had to write for various classes during my undergraduate career at the University of Arizona Physics Department. Perhaps to someone they may provide some value or reference:

Spring 2008:

Measurement of Planck’s constant through Black Body Radiation Analysis

Measurement of the Sodium D Emission Lines Using a Michelson Interferometer

Fall 2007:

            Experimental Determination of the Speed of Light by the Foucault Method

Fall 2006:

            Effect of Matter Density in the Evolution of the Universe

Other Stuff (Lectures and such):

When I get the moment, I’ll try to work on updating this area with other fun times here during the KSU REU

Useful Links: 

World wide internet:

Wikipedia Physics

2008 KSU REU Page

JRM Laboratory (Home sweet home for 2 or so months)

Dr. DePaola’s Page

American Institute of Physics