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Research Journal

June 3, 2006

I have proceeded with the experiment using saturation spectroscopy. I spent a few weeks working on the cellophane waveplate experiments but i believe that the allignment is too sensitive for these to be used effectively over a period of time. Additional delays due to my job have been many. An intense couple of weeks in which i was required to work 80 hours a week and a trip to Japan have made it difficult to come to the lab on weekdays, but i have been able to come in steadily on weekends. Thus far i have set up the Sat Spec, achieved a Doppler Broadened Rb signal, and am hoping to have a Doppler free signal by the end of the weekend. Once that is accomplished, the experiment shifts from optics to electronics. I have gained some experience in using a lock-in amplifier but am also considering a Servo-lock circuit. I hope to decide this in the coming week before i depart to England (Univ. of Leeds) for a week.

Febuary 21, 2006

So....I'm back at the LTC to finish my incomplete in Research and Graduate. I started working on locking the diode laser again in december, and have made some progress. I started by looking at 3 different ways of locking a diode laser. These are Saturated absorbtion spectroscopy, Polarization Spectroscopy, and Spatial mode locking. They each have strengths and weaknesses to their use, which i will explain in a future entry. At this time, which i dont have a lot of, i believe only Saturated spectroscopy can be used. I have begun research with Dr. Noe on creating waveplates from cellophane in hopes that we can create a 1/4 waveplate plate which is required for polarization spectroscopy, but i am beginning to doubt that we can achieve this special case. My intentions are to move forward with Sat Spec, lock the laser, and get a grade. I am looking into using this research experience coupled with the waveplate research as a senior writing requirement. Data, pictures, and journal references coming soon!

October 27, 2004

Today I have decided to finally get my life back on track. I dont know whats happened over the past year, but i intend to correct it. Im starting with a jounral entry, dedicated to Dr. Noe who has waited for one for so long. OK, sooo... i guess i will bring everyone up to date. I graduated with a physics degree in May. I worked with Rita over the summer part-time on Sat Spec. and we made a lot of progress. I decided to take some advanced undergraduate classes and contine my research since i didnt have much else to do. Its been slow and difficult getting focused. Tonight I decided to check all the settings and allignment of the laser, and found a strong doppler free sat spec signal. the next step will be to get the locking amplifier working and "lock" the laser. This is supposed to be a difficult and time consuming task, and i have much reading to do on the process and the electronics that will be used. I started a new job today, as a part-time apprentice to a laser engineer at Photonics Industries International, and will be given a full-time Laser Engineer position at the end of this semester. I really dont know as much as i though i did about lasers, so this should be a great learning experience. Now that I have this position I will be able to be in the lab every weekday night for at least a few hours.

November 5th, 2003

Today I met with Mr Lee and he took a look at my new set up. He found that with the current set up the probe beam would be to intense compared to the pump beam. He also noted that since the set up doesn't have a reference beam and hence no signal cancellation that it is possible to have back ground light distorting the signal. In response to these suggestions im going to place a Neutral Density Filter in the the beam path between the Rb cell and the mirror. This will allow the probe beam to pass through it twice. We have a neat NDF disk that can be rotated to increase or decrease the amount of light reduction. The surface has many scratches so it will probably be necessary to order a new one. As for the back ground light Im just going to place a cardboard cylindrical shell in front of the photo diode to block out most of the light. Eventually the entire setup can be placed in a box which will accomplish this and protect the equipment. Another problem exists in that there is optical feedback into the laser, and this is causing mode hops. Some of the light is reflected off of the aperture and the rest off of the beam splitters (microscope slides). We have decided to order an optical isolator to solve this problem. Dr. Noe has been looking at some for awhile now and has emailed me some links about how they work and ordering. I was told by a grad student that the hardest part in building a MOT is getting the Sat. Spec. to work. I hope he's correct.

October 26th, 2003

Ok, its been awhile since I have made a journal entry and that's mainly because i have been bogged down with midterms for the last two weeks. I took apart my entire setup and started over. I now have a beam splitter to give me 10% of the main beam off to the side which i then send through an aperture to reduce the intensity even more and make it circular. Dr. Noe thought it would be a neat idea to measure the amount of absorption in the Rb cell related to the intensity of the beam. In order to do this i added a polarizer after the aperture. I have written many procedures and taken many sets of data only to find flaws with my logic in all. I think I have finally come up with a procedure that will work and I will try it once Sandy is done with her polarization experiments. Ah yes, there's another benefit to working with the reflected part of the main beam and that is that you may place as many beam splitters as you like into the main beam and therefore do many experiments at once. To keep myself busy while I wait to finish my measurements i have built another Sat Spec, this time with a simpler design that should yield good results. I have everything aligned and am currently trying to measure a signal. Everything is more difficult now that the beam intensity is so low. I intend to spend a few more hours today working on it. Also, during this past weekend i reviewed my journal articles and everything i have gathered on constructing a MOT and started to type my proposal. I know exactly what i want its just a matter of figuring out exactly what i need to accomplish it. I have found manufactures for most of the parts i will need to build another diode laser and hope to place an order this week.

October 9th, 2003

Today I met up with Mr. Lee and he showed me his Sat Spec signal. It was very different from what I was getting. He then took a look at my set up and found it to be good. A few problems do exist though. First, the pump beam can be seen if a piece of paper is put in it. This means that the beam is still to intense. In order to avoid power broadening the beam must be so weak that it isn't possible to see it with out an electronic IR viewer. Second, I have two neutral density filters in the beam right after the diffraction grating of the cavity. The two filters are separated by approximately 1cm. Depending on the angles of the filters the pump beam tends to increase in diameter. Because power is proportional to the intensity divided by the area, this changing in diameter can become a problem. The last thing that was noticed is that when we put another filter in the pump beam path and looked at the signal Mr. Lee believed he saw part of a Sat Spec signal. I say part because it appears that the laser was scanning over more than one mode and that a mode hop was directly in the spot where the rest of the signal should be. The plan to correct these is simple, a step by step realignment of the entire system. I will begin by inserting a microscope slide beam splitter in front of the laser and feeding the weaker beam into the Sat Spec. This will dramatically reduce the power. Just incase the power is still too high I will add an adjustable polarizer and an aperture in the beginning of the Sat Spec beam. This will give me the ability to change the intensity and the beam width. I will then align the laser to one mode (hopefully) and proceed through the procedure which should lead to a Sat Spec signal.

September 26th, 2003

I have tried using a diffraction grating and multiple Neutral Density Filters inserted into the laser beam before the beam enters the Sat. Spec apparatus. It reduced the intensity but created new problems. The first new problem is that depending on the angle of the NDF the beam has a tendency to spread out to approximately 4 times its original size. This can be corrected by adjusting the angle of the NDF and watching the beam width on the CCD camera screen. The second is that because the beam intensity is so low it is difficult to align the beam due to the fact you can barely see it with a IR viewing card. Also, with a weaker signal going to the photo detectors back ground light causes a competing signal and causes the signal on the oscilloscope screen to beat. This can be solved by working in the dark, and hopefully in the future by encasing the Sat. Spec. setup. After getting the lower intensity beams to cancel each other I unblocked the pump beam and waited for the nice large signal to appear. Well, nothing at all happened. Not even the little dips I could achieve with the full intensity beam. Hope is quickly fading of ever accomplishing anything, especially a MOT.

I spoke to Mr.. Lee and he is going to contact me when he has his setup working and its generating a signal. He also said he would like to see my setup. Im also going to contact the instructor for Senior Lab and see if they have a Sat. Spec experiment currently being worked on by a student. Perhaps I can work along with one of their students to try and get an idea of what im doing wrong. Lastly, im going to email Bob Azmoun at the end of next week and lay out what's working and ask for advice for what's not.

September 22th, 2003

Last Friday I attended Prof. Metcalf's weekly optics meeting in order to present the problems I was having with the Sat. Spec setup. After some discussion and approximated calculations the consenses is that there is power broadening taking place. Power broadening is when there is so much laser power that it saturates all the atomic transitions allowing the unabsorbed light to continue to the photodetector. Based on "back of the envelope" calculations my power is ten times to much. The calculations requires knowing the intensity of the beam and a more accuart calculation can be down if the Guassian Beam Profile is known. The easiest way to solve this problem is by using a microscope slide to reflect most of the light away or the use of Neutral Density Filters. A diffraction grating can also be used sinse the light emmitted from the laser diode is polarized.

September 16th, 2003

I started today by checking the alignment of the sat spec set up and in the process I knocked the beam splitter out of position. Right now the beam splitter rests on a flat platform and doesnt have a clamp to fold it in position and maintain the correct angle. This is something I will have to fix soon. It actually gave me the opportunity to make it so the probe and reference beam arrive at the photodiode perpendicular to it, instead of at an angle. This made drawing the setup easier and also simplified some calculations.

According to Matt Paltrow one of the reasons I was not achieving Sat Spec was not enough beam over lap between the pump and the probe. This was because the angle between the beams was to large. In this new setup I was able to achieve a 0.6 degree angle between the beams. The result is still uncertain. We can definitly see the effect of the pump beam on the signal (by blocking the pump beam) but it doesnt appear to reveal the Sat Spec peaks we would expect. Dr. Noe suggested that it could be hidden by power broadening. We inserted a 0.8 attenuater into the path of the beam at many different points and found besides the expected signal intensity drop, nothing happened.

I have also begun rereading journal articles in the hope that my experiences in the lab will help me better understand them. I have found greater understanding on many of the aspects of Sat Spec by doing this but unfortuantly nothing that will help move the project beyond its current problems. I intend to grab Prof. Metcalf and/or one of the Matts and get their assistance and advice.

September 9th 2003

It has been a long time since i have made a journal entry. This is a very bad habit to get into because the journal is a place where a researcher can explore their ideas and document progress. My goal is to do better this semester. As a follow up to the summer in the last few days we succeeded in preforming Doppler Broadened Spectroscopy. The REU presentations went well and prgram ended. My work continues in the lab as a 4 credit PHY487 (research) class under Prof. Metcalf.

James Scholtz
Fall 2003
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