Investigation of Hollow Laser Beams Generated by Coherent Fiber Bundles
Prepared for the RAIRE Summer Fellowship Program, 2001
It's sometimes said that the phrase most often heard before scientific breakthrough is not "Eureka!" but "That's strange". If anything in Stony Brook's Laser Teaching Center has caused folks to say the latter, their coherent fiber optic cable has. Professor Yuri Polyakov donated the cable to the center last year. Karl Fey, a Simons Fellowship Student during the summer of 2000, discovered that if red laser light was shone into the cable at an angle, a circle of light emerged from the other side. No one, faculty or student, could explain why this occurred. To date, no one can explain this phenomenon, including visiting scholars.
Coherent fiber optic cables differ from other fiber optic cables by their ability to transmit high-resolution images, while most optic cables are called upon to transmit light pulses. The behavior of the teaching center's cable may or may not be something all coherent fibers can do. Given that the hollow laser it emits surprises even experts, we are probably witnessing either an unexplored aspect of coherent cables, or a cable that is defective in some unusual way.
Since Karl's discovery, research of the cable has been ongoing. When laser light is shone directly into the cable, as opposed to at an angle, the output is a gaussian beam with its peak intensity at its center, instead of a hollow beam. The intensity of this beam has been measured and identified by Alia Sabur and Dhruv Bansal as two overlapping gaussian distributions (i.e. bell curves), which lends the question: Why? It's also been noticed that some laser light reflects backwards from the cable and exhibits a very strange image. The reason for this odd image is also unknown.
Additional questions have been raised. The intensity measurement was done for red laser light, but it's not known if a different frequency of light would yield different results. It's also not known if the cable exhibits consistent behavior when the experiment is run in reverse (If a hollow laser beam converges on the cable, does a narrow beam of coherent laser light exit from the other side?). In preparation for the writing of this report, I examined the cable with Dr. Noé, who co-runs the teaching center with Dr. Metcalf. We discovered that both the resulting hollow laser beam as well as the back reflection is depolarized. This came as quite a surprise, and warrants additional investigation.
Typically, hollow laser beams are used to trap atoms into a small space so they can be more accurately studied. Atomic traps are way beyond the scope of any undergraduate research project, but a more advanced researcher may find the generation of hollow beams with a fiber optic cable to be of interest.
It's difficult to say exactly where this project will lead and what kind of knowledge it could ultimately help to generate. Presently, hollow laser beams are generated with axicons, which are cone-shaped lenses. A recent paper by Matthew Terraciano describes this as a difficult process. It is possible that the Laser Teaching Center's fiber optic cable offers a new and genuinely easier way of creating these types of lasers.
III. Proposed Experiments
The Teaching Center's optical cable provides excellent opportunities for undergraduate research. The questions it poses are unanswered, but basic enough to place their comprehension within the grasp of an undergraduate. Yet answering these questions will require a greater understanding of optics and the scientific process be built along the way. Furthermore, as the nature of research is such that discoveries lead to more questions, this research project will lead to more projects by other students regarding the cable.
Since the most concrete research done on this cable has concerned itself with beam intensity, further experimentation with intensity seems a likely place to begin. When a laser is shined directly into the cable, the result is a gaussian beam. As the angle between the laser and the cable grows, the resulting beam becomes increasingly hollow. I expect that the two overlapping bell curves in the intensity plot will separate as the angle between laser and cable grows. As a result, the initial focus of this project will be on intensity plotting.
Intensity differences between input and output lasers may offer insight regarding the geometric workings of the cable. Laser light generally bounces through the inside of a cable until exiting out the other side. Some intensity is lost with each bounce. If the intensity of the input and output beams are measured, then compared with their frequencies and input angles, it may be possible to generate some kind of picture of what's happening inside.
Should intensity plotting prove to be a cul-de-sac, there are other avenues to explore. The back reflection from the cable looks complicated, but very well ordered. It seems to have evenly spaced points and appears to be unpolarized. By analyzing it as different wavelengths and angles, perhaps something can be learned about what causes the back reflection.
Even if back reflection analysis hits a brick wall, there are other experiments to be performed. It's been established that when laser light enters the one side of the cable, a hollow beam exits the other side. But what would happen if a hollow beam were generated by other means, then focused into one end of the cable? Would a regular laser beam emerge from the other side, or would another hollow beam be emitted instead? What would the back reflection look like? Would an intensity plot deviate from plots taken earlier in the project?
The possibility of funding for this project places additional experiments within reach. The introduction of a new coherent fiber bundle allows it to be determined if the circle of light phenomenon observed in the teaching center's cable is unique to just that cable, or common to coherent bundles in general. Edmund Scientific sells optical tapers, which are optical plates that magnify or reduce images they receive.
IV. Location and Faculty
The cable is presently located in the Laser Teaching Center (S-202, Grad Physics, SUNY Stony Brook), and I see little reason to move it. The teaching center is an ideal place to conduct this research. It is roomy and contains numerous pieces of optical equipment. The project will be under the supervision of Dr. John Noé and Dr. Harold Metcalf.